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Protection of polymers against light irradiation
European Polymer Journal-Supplement, 1969, pp, 105-132. Pergamon Press. Printed in England.
PROTECTION OF POLYMERS AGAINST LIGHT IRRADIATION * H.
J.
HELLER
J. R. Geigy A. G., Basel, Switzerland
INTRODUCTION PHOTODEGRADATION is an old phenomenon. Particularly eyecatching is the bleaching of dyes in direct sunlight and most funspoiling is a good sunburn. Considering its energy content, such effects of light are not surprising. Potentially the energy of visible and particularly of U.v. light suffices to dissociate almost every chemical bond. It is important to know, however, that the most damaging range of u.v., i.e, the radiation below approximately 280-290 urn, is completely missing in Colour
~
nm
/I
E
Ideal separation energies
ern" keal
mOl
X=J/X - C(Ell z Ph -X - CH(EtlCH=CH z
IR
30- AeO-OAe MeO-OMe Red---730--13700- - - - - - - -- - -- -- - - - - Yellow
50-
Green
Br-Br
X=C
X-CHEt z
X=f
X-Ph
Me-Me 90-
~ ~ --~;~~~;~:~ X - CO A l k Y I
Violet 70X=W -----400--25000-----MeS-SMe ------
UV
~
X-C(MeIEtz
CL-CL Me-CHzPh
Blue
""'-X-COPh ~X-C(EtlzOR "X-CH(CHzCLlz
Me-Ph
Ph-Ph - - - - 280--:35710----- -H-OEt--110FIG. 1. Ideal separation energies vs. frequency of light. (The separation energies are calculated according to H. Preuss. Quantentheoretische Chemie I (Bibliographisches Institut Mannheim. B-I-Hochschultaschenbuch 43.) The use of real bond dissociation energies would certainly result in some crossovers in the lines connecting the bars representing the energies of the F-, H-, Cl- and J-bonds.)
natural daylight. This short-wave length light is emitted by the sun but completely absorbed in the outer atmosphere. In Fig. I the ideal separation energies ofseveral specific bonds important in polymers are confronted with the energy of various wave lengths.
* Main
lecture.
POLYMER-H
105
106
H. J. HELLER
It is seen that only the most stable bonds-such as the C-F-bonds in alkyl- and arylfiuorides and the C-H bonds in aromatic and simple aliphatic hydrocarbonscould withstand the radiation energy of the natural daylight's photons if all this energy could be concentrated on one specific bond in a molecule. Luckily such localization of radiation energy is a very rare occurrence. Otherwise, life as we know it could not have developed on earth at all. To produce chemical effects, light has to be absorbed. Most commercially important polymers should absorb very little or no radiation at all down to 290 nm if only their usually cited structural moieties would contribute to their real structure. Pure polypropylene, for instance,
should not be influenced by daylight, even if it contained vinyl groups or similarly unsaturated structures as end groups. The same holds true for PVC, polyamides, polyester resins and the homo- and copolymers of styrene. However, all these well known polymers tend to degrade upon light exposure. Very frequently the damage is mainly aesthetic in nature, like discolouration and formation of surface craze and cracks. In the case of thin structures like films and especially fibres, these damages are further accompanied by a breakdown of the mechanical and electrical properties as evidenced by an increase in brittleness and in the electrical loss factor.
2. PHOTOINDUCED PROCESSES Responsible for the photosensitivity of most polymers are impurities and/or abnormal structural moieties which are introduced during polymerization, workup or shaping processes. In the case of polystyrene, for instance, the light sensitivity increases with residual monomer content. The exact natures of these photoactive impurities or structural moieties are not known with certainty. However, ketones and/or aldehydes and also peroxides are generally accepted as photoactive species. Simple aliphatic ketones exhibit a forbidden, i.e. low intensity transition, in the region of 270-290 nm (€ approx. 10-16). Aromatic-aliphatic ketones such as acetophenone have an analogous, but somewhat more intense band at slightly longer wavelengths: 310-330 urn (€ approx. 40-50). In purely aromatic ketones such as benzophenones, these shifts are even more pronounced: "-max approx. 360 urn (€ approx. 120). These bands correspond to an excitation of nonbonded electrons of the carbonyl-oxygen which is called an n--?7T* excitation. The intense absorption of ketones and aldehydes below 290 nm is of no concern to us here, since such radiation is not contained in daylight. Four types of reactions contribute most to the photoinduced processes arising from ketone irradiation. The radicals produced in these reactions are the source of further polymer degradation, as we will see later. In purely aliphatic ketones, the two "Norrish Type"-splits are most important.
Protection of Polymers Against Light Irradiation
-CH-C-C +CO-
107
"Norrish Type I"
o II
o II [-CH-C-C-C-] * -
"Norrish Type II"
-C=C- + -CH-COH I
---+
-C-CI
intramolecular reduction
I
C-C OH
RH
I
-CH-C-C-C + R·
abstraction of substrate atoms
The relevant quantum yields!'? for a simple ketone representative of a hydrocarbon polymer structure are:
(2)
~-CHz-CHz'
(4)
CH2 =CH2
+ j-
BuCO
CH3
+
-,
/H-
CH2C OCH3
¢. - 0 -06
CH3
The excited aliphatic ketones break down into roughly equal amounts of radicals and molecules, with the exception of higher methylketones for which the nonradical Norrish split II seems to be preferred by about one order of magnitude. A similar situation exists for araliphtic ketones like butyrophenone. However, in this case a new phenomenon becomes apparent. The influence of subsituents in the phenyl group is eminent. Strongly electron donating substituents, like hydroxyand aminogroups in 0- and p-position to the carbonyl group, almost completely inhibit the ethylene formation. We will come back to this phenomenon later. The intramolecular reduction of the carbonyl group becomes another possible photoreaction in unsaturated aliphatic ketones of specificstructure. It is related to the Norrish Type II olefine production. In both cases the intermed iate assumes the configuration of a 6 membered ring which, however, collapses differently. (See Formula overleaf.) On the other hand, irradiation of a, ,B-unsaturated ketones often leads to photodimers. Pure aryl ketones, exemplified by benzophenone, do not dissociate but are powerful oxidizing agents in their excited state. They are capable of abstracting hydrogen atoms from almost any kind of substrate apart from purely aromatic hydrocarbons like benzene. The reactive species is the triplet [3(n~1T*)] formed at high efficiency
H. J, HELLER
108
\/'j(\L _ I\~
/
HO
\
rf~
R C-C /\/\
+ \-
Norrish Type II
II C C
I \ 1\
~=H. alkyl
\Cl O~ C1\
/
R C-C
1\/ \
~=Yinyl
\.
HO
\.
C C<, /\ / /C=C C-C \ 1\/ \
intramolecular reduction \
~.
HO\
OH
c''r:-I -~ I C
c-cI;
<, .
,/
intersystem crossing from the primary excitation product. Thus benzophenones are photo sensitizers of very high efficiency, whereas aliphatic ketones-prone to undergo non radical dissociation-abstract hydrogen atoms from the substrate with lower quantum yields. The hydrosensitizer radicals themselves are frequently of low reactivity and dimerize harmlessly. In the case of benzophenone two ketylradicals form benzpinakole:
2
Ph Ph -, -, +2RH C' C=O-----? 2 /
Ph
-2R'
Ph
/""
Ph
OH
-----?
""c-c/
/1
PhOH
Ph
I""
OHPh
On the other hand the R' radicals derived from the substrate as well as those formed by direct photo-dissociation of aliphatic carbonyl compounds are capable of initiating autoxidation chains with the formation of hydroperoxides. Dissociation of these hydroperoxides leads to new free radicals. The original chain branches in a typical "snow-balling" manner. Termination of the chains is possible only by dimerization or disproportionation of active free radicals. Due to their high reactivity, such events are effective only at relatively high concentrations of free radicals, i.e. after the induction period of the autoxidation, when the polymer degradation is well advanced. At room temperature the chainbranching, i.e, production of free radicals from hydroperoxides, is in itself a very slow process but is greatly accelerated by light and/or metal impurities in the polymer.
Protection of Polymers Against Light Irradiation
109
An especially bad catalytic influence is exerted by transition metals having stable valence states which differ by one electron such as Co , Cu, Fe and the like. Such metals are in principle capable of decomposing hydroperoxides catalytically, i.e. with regeneration of their original valence state. Furthermore, numerous metal ions absorb quite strongly in the u.v. and can act as photosensitizers, especially for hydroperoxides. This latter type of mechanism is, however, not truly catalytic. Besides this metal sensitized photodecomposition of peroxides, direct photolysis also plays an important role. Peroxides as well as hydroperoxides absorb below 340 nm. This absorption represents the very weak tail end of a band, the maximum ofwhich is below 260 nm . In the case of di-rerr-butylperoxide, the molar extinction coefficient at 300 nm is approximately two. This absorption might be due to the promotion of a non-bonding electron of the oxygen to an antibonding 1T* orbital. The separation energy ofthe-O-O-bond in peroxides and hydroperoxides is around 35 kcaljmol, whereas the absorbed energy of a photon of 300 nm is 95·2 kcaljmol. Thus, the alkoxyradicals formed by -0-0- bond cleavage carry off together an excess energy of approximately 60 kcal. These "hot" radicals are therefore prone to break; apart before they can abstract a hydrogen-atom from the substrate to form an alcohol. The breakdown products are mainly free alkyl radicals and carbonyl compounds, i.e. ketones and aldehydes. The structures causing the primary attack in the light induced autoxidation are hence reproduced and augmented. In this sense, the light degradation of polymers is autocatalytic. 3. INHIBITING ADDITIVES From the above it is evident that a single photon absorbed by a photosensitizer can cause considerable degradation of the polymeric substrate, particularly when atmospheric oxygen is present. In this , it is quite immaterial whether this sensitizing group is part of the macromolecule or of a low molecular weight impurity. To prevent such degradation, the follow ing protective measures can be taken (cf. Fg. 2) : (1) Prevention of light absorption by sensitizers. For this "u.v.-absorbers" are used. Li g hl injury of substrcte RH Type of reacfion Primary processes
Secondory processes
Protective alienI
cha in sc iss ion e.g. u.v. -absorber CH-C-C- Co- ~C=C +CH-COrad ic ol-production u.v, - obsor ber quencner X- X ·-- R' (decctivot ion of inter medi ate)
ouloxidalion
ontioxidan!
RH---y-ROO·--,. ROOH-A- R· ----.-A-b 2 Perox ide dissociotion
ROOR'- RO· However. e.g .
ROOR' + M,"n-
"synerqist "
+ R'O· RO·
+ M(OR'tn
rneto ldecc t jvct or
is energel icolly preferred
FIG.
2. Summary of light degradation mechanisms and the corresponding remedial additives.
H.J. HELLER
110
(2) Deactivation of light excited sensitizers. Such energy transfer is effected by "quenchers". (3) Inhibition of the propagation of autoxidation. To effect this, the highly reactive chain propagating free hydroperoxy-radicals are transformed into harmless radicals by "antioxidants". (4) Prevention of the initiation of secondary autoxidation chains by hydroperoxides. The effectivityof hydroperoxides in starting new chains is minimized by (a) ionically decomposing them by means of "synergists", (b) suppressing the metal catalysis in the production of free radicals with the
help of "metal deactivators". To illustrate the interplay of u.v.-absorbers and antioxidants, the time for unoriented polypropylene samples to reach the brittle-point in outdoor-weathering is given in Fig. 3. The stabilizers used are 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole as u.v.-absorber and pentaerythritol tetra 3-(4-hydroxy-3,5-di-tertbutylphenyl) propionate as antioxidant at the indicated levels. Lightstability of polypropylene
150
20
0·2"10 uv-absarber
Antioxydant, "10
FIG. 3. Lightdegradation of polypropylene. (l Langley (Ly) = 1 cal/em"; 1 year outdoorweathering in Basel corresponds to approximately 70 kLy (70,000 Ly). Further explanations see text.)
It is seen that the antioxidant used, while improving the resistance toward autoxidation by orders of magnitude, conveys only a limited light protection. Light irradiation produces so many autoxidation chains that locally they cannot be stopped by the antioxidant. Antioxidants, having similar ability to suppress autoxidation as measured by a specific test method, vary widely in their response to light. Two main factors are operative: (1) the diffusion coefficientof the antioxidant in the polymer, i.e. the velocity with
which the antioxidant is replenished by diffusion into critical spots and
Protection of Polymers Against Light Irradiation
III
(2) the photochemical behaviour of the antioxidant and especially its reaction and degradation products. In the above example a very effective polypropylene antioxidant has been chosen, which, however, is photochemically neutral. Due to the slow diffusion of its bulky molecule, it provides little light protection; due to its structure, it does not form sensitizing degradation products. The two upper curves demonstrate how the effect of the u.v.-absorber is improved quite drastically by the antioxidant. Near the surface of the testpiece, the intensity of the light not yet absorbed by the u.v.-absorber is still high. Autoxidation chains are still induced, but their number is so small that the "snow-balling" effect can be checked by the antioxidant. 4. LIGHT STABILIZERS After this general discussion of the basis of light stabilization of polymers, let us turn more specifically to light protective agents of which, as pointed out above, there exist two classes: viz. u.v-absorbers and quenchers. (0) Quenchers
Typical quenchers, in contrast to absorbers, need not have high absorptions at the wavelengths critical for polymer degradation. In most cases, light excitation from the ground singlet to the lowest triplet is more or less forbidden and the resulting absorption is only weak. For energy transfer, these selection rules do not hold and hence a quencher can deactivate sensitizertriplets Sz1) at energy levelsat which it is quite transparent to light. According to Wigner's spin conservation rule, the quencher has to change its multiplicity when deactivating sensitizer-triplets. If its ground state is a singlet l(QO)' a triplet will be produced 3(Ql):
e
3(Szl)+1(QO)
-+
1(SZ3)+3(Ql)'
If the groundstate of the quencher is a triplet, as in the case of oxygen, a singlet is produced: 3(SZl)+3(QO) -+ I(Szo)+l(Ql)' Since the quencher is raised to an excited state, the mere fact that a compound quenches a photosensitizer does not of itself mean that this compound is a light protective agent. Only if the excited quencher can dissipate its accumulated energy harmlessly, has it a chance to reduce photodegradation. A typical example of a very efficient, but non-protective quencher, is oxygen. Its reaction product, singlet oxygen, is very reactive and forms hydroperoxides readily. Quenching is a bimolecular process and is very fast if exothermic. In other words, quenching is a diffusion controlled process and is effectivein polymer protection only if the sensitizer triplets have a long half-life and if the quencher is freely diffusible. So far, only nickel compounds have found an industrial application as typical quenchers, and this mostly in polyolefines. As is so often the case, their discovery as light stabilizers was more or less accidental since they were originally suggested as dyesites in polypropylene.
H. J. HELLER
112
Of the numerous products claimed in the patent literature, the following products are distributed today:
Ferro AM 101 (Ferro Corp.) (2) Me
Me
M.-j-CH,-j---fc5\-o
9 /\. M.-j-CH,-j\Q;-~ Me
Me
Me
Me
Me
Me
d
-c
S----N,---NH 2
H 4
9
Cyasorb 1084 (Am. Cyanamid) (3)
>-<8] [BU Bu
S
Ni
Nickeldibutyldithiocarbamate (various sources)
2
Negopex A (ICI) Bu (t)
HO
>cJ Y
CH,
Bu (t)
A63'99 (experimental Geigy) (4)
Protection of Polymers Against Light Irradiation
In
Some of these nickel compounds have a shortcoming. They have a tendency to decompose to black nickel sulphide at the high temperatures (up to 300°) needed for a good throughput in the costly modern extruding and spinning equipment for polyolefines. The necessary sulphur stems from the ligand itself or from the thiodipropionates used as synergists. (b) u.o-absorbers: general requirements
Obviously a u.v-absorber can always take part in energy transfer processes and it needs special investigations to determine the contribution of quenching to the action of a u.v.-absorber under specific conditions. In practice, however, the quenching action of u.v.-absorbers is only of importance in very thin structures such as fibres. The question is, how much light is left to be absorbed by a photosensitizer in the presence of a u.v.-absorber. In Fig. 4 the situation is exemplified by the calculated Transmission vs. depth at penetration
Residual absorption of acetophenone vs. absorber concentration
. ~
.
I--
6
8
10
mm
FIG.
4. (a) Transmission of compositions of acetophenone and 2-(2-hydroxy-5-methylphenyl)benzotriazole at 330 nm. (b) Residual absorption of the acetophenone in these compositions.
absorptions of acetophenone as sensitizer and 2-(2-hydroxy-5-methyl-phenyl)benzotriazole as u.v-absorber at 330 urn at various absorber concentrations and sample thicknesses. Acetophenone has been chosen because similarly absorbing species participate in the degradation of homo- and copolymers of styrene. In Fig. 4(a) the transmission of samples containing 20 ppm acetophenone and various amounts of absorber are shown as a function of the sample thickness. It is evident that the acetophenone contributes very little to the total absorption particularly at absorber concentrations above 100 ppm. Consider the lowest level of practically used concentrations of u.v.-absorbers; at 0·1 per cent only 50 per cent of the incident light (330 nm) reaches a depth of 49p. and 10 per cent reaches 154p., while at O·3 per cent concentration of the benzotriazole 50 per cent penetrates to 16p. and 10 per cent to 51p..
H. J. HELLER
114
The amount of radiation absorbed by acetophenone in the presence ofu.v.-absorber as a fraction of the acetophenone absorption in the absence of absorber is a measure of the protective power of the absorber by absorption. We will call it "residual absorption". The higher its numerical value, the poorer the effect of the absorber. The calculation of this residual absorption (R) is straightforward if Beer-Lambert's law is valid. The following formulae apply l_lo-xakek
(la)
At
I_Io-x:!:aie i
(lb)
Ak
akck A· L.ai ci '
(I c)
A kO
wherein x denotes the sample thickness (in em), a; the absorptivity* (in cm 2 /g) and c, the concentration (in gil) of the ith-species of a multicomponent absorber system, /0 the intensity of the incident light, At the absorption of the system (as a fraction of /0) and A ko the absorption of the kth-species, if present alone, while A k stands for its actual absorption as a fraction of /0 in the system. Hence the residual absorption for the kth-species (R k ) is: Ak
akck
A ko
L.alCI
R k= - = - -
(2)
In Fig. 4(b) the residual absorption for acetophenone is given under the conditions of Fig. 4(a). At the concentration of the triazole absorber used in practice, there is very little absorption of the sensitizer left in thick samples, whereas in thin samples an appreciable residual absorption is found, necessitating high absorber concentrations. The question now arises as to the effect of the initial sensitizer absorption. In the insert of Fig. 5 the residual absorption (R s ) of a sensitizer is given as a function of its initial absorption at various levels of total absorption of the resulting composition. A surprisingly small effect is noted and the curves are easily extrapolated to initial sensitizer absorptions of zero-value. Hence the concept of a lower residual absorption limit becomes practical. For small absorption (xascs-+O) by the light sensitizer, (2) can be rewritten: I_Io-x:!:aie l Rs (log e + txascs +.... ) XL.aICI
l_lo-x:!:ale i XL.aiCI
log e (1 + tAso
+.... )
(3)
Hence the lower limit of the residual absorption (L s ) l_lo-xxaiei L, = limR, =
XL.alCi
log e
(4)
xascs-+O
depends only on the absorption of the u.v.-absorber. '" With e = molar decadic extinction coefficient and w = molecular weight, the relationship is e =
aiW.
Protection of Polymers Against Light Irradiation
115
Residual absorption vs. Optical density of Initiol obsorption uv - obsorber of sensitizer 100 50
100c:=====:=:::===,
.. >!!
~80
-.e
60
~
40
~
50
~
90%- - - - - j ~~~
99·999% total absorption
~O
80
'-95
90
95
20
10
0
'~8 .....99 98 " 99
As
20
=50%
<, 0__
Aso= 0 %
99·9 ..... _ 99'9 -
o
---
.....:..::...::..::....:::.::..:::.= -----
__
5
6
0.0'0
FIG. 5. Residual absorption vs. optical density of added u.v-absorber. Solid line lower limit of residual absorption (A so = 0) Dashed curve residual absorption if the initial absorption of the sensitizer is 50 per cent of the incident light. The total transmission of the resulting compositions are indicated on the curves. Insert: Residual absorption vs. initial sensitizer absorption at various levels of total absorption.
The solid curve (marked Aso=O per cent) in Fig. 5 gives this lower limit as a function of the optical density of an added u.v.-absorber. As a comparison, the dashed curve has been calculated for an initial sensitizer absorption of 50 per cent, a value which is very rare in practice. It is evident that, for practical purposes, the lower limits L, of the residual absorption are good enough to judge the effect of a u.v.-absorber based upon its absorption. In a composition in which the added u.v.-absorber has an optical density of I (i.e. absorbs 90 per cent of the incident light), the L s is about 39 per cent. If the u.v.absorber were perfectly stable and nonvolatile it should prolong the useful life of the substrate under these conditions by about a factor of 2·5. At an optical density of 2 TABLE
I
Polymer
Polyesters (various formulations) Polystyrenes Polyethylene Polypropylene (non-heat stabilized) Polyvinyl chloride Polyvinyl chloride copolymer with vinyl acetate Polycarbonate Polymethyl methacrylate Polyformaldehyde
Wavelength of maximum sensitivity 325 318 300 310 310
om nm om om nm
322 and 364 nm 295 om 290 to 315 nm 300 to 320 nm
H.I. HELLER
116
(99 per cent absorption), the corresponding values of L, and the improvement factor are 22 per cent and 5 respectively. If an absorber provides a better protection, it must act by an additional mechanism such as quenching. The above derivations obviously hold strictly only for monochromatic light. As the molar extinction coefficients and thus the absorptivities of the involved species vary differently with wavelength, the picture gets rather complex in the case of polychromatic light. In practice, however, it should suffice to consider the wavelengths of maximum sensitivity of the polymer in question. In Table 1(5) such values for some polymers are given. Which practical requirements have to be fulfilled by a technical u.v.-absorber and what kind of structures are useful for such purpose? (1) u.v.-absorption There are two distinctly different groups: (i) Cosmetic absorbers. Human skin has a very distinct maximum of sensitivity at 297 nm. In healthy persons, however, light of longer wavelengths stimulates the protective tanning. Sunlotion or suncream should therefore selectively filter out light below approximately 320 nm. A large number of u.v-screeners have been suggested such as 0- and p-aminobenzoic acid derivatives, p-aminosulphonamides and benzoxazoles. Most satisfactory also from a toxicological point of view are, however, the esters of salicylic acid. (ii) Absorbers for technical polymers. The ideal absorber for this purpose absorbs all U.V.- but no visible light. In practice, as usual, a compromise has to be found. Either absolute freedom of colour is insisted upon and a certain transparency in the long range u.v. is allowed, or if good absorption in the long range u.v. is needed, such as for 66-nylon, a certain yellow discolouration of the final composition has to be "areot density· concentration x length, optical density:obsorptivity x areal density, I gIL at I cm : 10 g/m 2 : 10 gIL at I mm
Influence of "oreol density" ond "steepness" upon uv-obsorption and visual colour 100
r"
/
80
/
60
/
\
. I-
/
\
\ 20
o
-
\ r:»> \/
\.../---
/
,
/
2
. ......... _--
.,...-', 0'1 g/m
/
I 300
. ./
.·15
-
;/
/
350
/'
.II
1/
'/
~
,/
I \
40
•
/
/
0'1
/'/
j
f
50 g/m
t·
// /J
2
I
400
nm
FIG. 6. Transmission of 2-hydroxy-4-methoxybenzophenone (- -) and 2-(2-hydroxy-5methylphenyl)-benzotriazole (. -'). "area density" is the product of concentration (in gIl.) and sample thickness (in ern), (It has the dimension ml- Z ) .
Protection of Polymers Against Light Irradiation
117
accepted. The sum of the oscillator strengths over all absorptions cannot exceed one for a given compound. If the absorption were equal in the range from 280 to 400 nm and zero below and above (i.e, 10279=10401=0; €280=€400=i) theoretically the maximum molar extinction coefficient over this region would be about 2 .10+4 (i). Obviously, such an ideal "square wave" absorption band does not exist, but it is important to find u.v.-absorbera approaching it as closely as possible. Figure 6 gives the transmission of the solution of two-absorbers at different "area densities". It is evident that one absorber covers the range much better at low "area density", i.e, low concentration and/or thin samples. Nevertheless its solution is colourless even at high "area density" while the other absorber causes a visually detectable yellow discolouration. It is an empirical rule that compositions with monotonically increasing transmission curves appear yellow if their transmission at 420 nm is less than 85-90 per cent. (2) Protective power Obviously a u.v-absorber should not be photoactive, i.e. it should dissipate the absorbed energy in a harmless manner. This is actually the problem to be solved in the synthesis of new absorbers. Only very few of the many u.v.-absorbing molecules are really light stabilizers. In general, light stabilizers should not fluoresce but transform the energy into heat. So far, we have not encountered fluorescent substances such as optical brightening agents which are real light stabilizers, in spite of their similar u.v.-absorption. (3) Polymer compatibility In order to absorb at all, u.v.-absorbers have to be compatible with the polymer to be protected. They have to be in true solution. This condition is not always easy to fulfil in the case of highly crystalline polymers. The chance that the low molecular weight light stabilizers crystallize isomorphously with the polymer is very low, so that they accumulate in the amorphous phase or at lattice imperfections. Therefore, additive doses of a few tenths of a per cent based on the total weight of the polymer represent rather high concentrations if the amorphous phase accounts for less than 10 per cent. (4) Light fastness A high permanence of the absorption of a u.v-absorber is essential but very difficult to meet. The fastness properties required are much more stringent than for instance with the normal textile dyestuffs. In general practice the combination of light stability and nonvolatility of an absorber determines its performance. (5) Specific properties
Depending upon its intended use, u.v-absorbers for polymers have to fulfil a series of further conditions. They have to be stable to heat (up to 300°) under a variety of conditions, in acidic or basic and reductive or oxidative media. Especially stringent are fibre uses. Besides the afore-mentioned properties, all classical fastness properties of a textile dyestuff should be met-such as resistance to sublimation, washing, dry cleaning etc.
118
H. J. HELLER
All these technological problems are hard enough to solve. Tougher to meet, however, are the toxicological requirements. Since the end users of commercially available u.v.-absorbers want to export their polymers to various countries, all the diverse laws have to be respected. This necessitates rather elaborate and very costly toxicological investigations and extraction studies.
(c) u.ii-absorbers: chemical classes (1) Absorbers with carbonyl oxygen acceptors The first technically used light protective agents were salicylates of which salol
c6r
coo ,"
and its alkylated derivatives are still on the market because of their generally low prices. Due to their low initial absorption, such compounds like Dow Light absorber TBS (p-tert-butylphenyl salicylate) and Eastman Inhibitor OPS (p-tert-octylphenyl salicylate) are only utilized in substrates of slight photosensitivity. Next Dow Compo used simple o-hydroxybenzophenones like 2-hydroxy-5-methyl and -5- chlorobenzophenone to stabilize polyvinylidene chloride. Since such benzophenones cause a distinct yellow discolouration, a blue pigment had to be added to produce the neat Saran®-foils. The practically colourless resbenzophenone and its monoalkyl ethers proved to be a step forward in this respect. It was a rather unexpected discovery of the General Anilin & Film Corp. that the introduction of a second auxochrome into the benzophenone molecule has a hypsochromic effect. Figure 7 gives the spectra of some hydroxy-benzophenones. The p-hydroxyderivative exhibits only one strong band with the normal red shift in polar solvents. The two o-hydroxybenzophenones on the other hand exhibit a double headed band of which the peak at the longer wavelength shows a blue shift in polar solvents. The visual hypsochromic effect of the methoxy group in p-position to the carbonyl group' is phenomenologically due to a reduction of the band-width of the very flat longwave length peak of the simple o-hydroxybenzophenone, The short wavelength peak exhibits the normal bathochromic effect expected from the introduction of an auxochrome. As pointed out before, the longest wavelength absorption of benzophenone itselfthe n-7'TT* excitation-has a maximum around 345 nm (e approx. 120). In the hydroxybenzophenone this absorption is partially buried in the strong 7T-'»-7T* band, because the auxochrome does not shift it as much towards longer wavelengths. To demonstrate the behaviour of the two hydroxybenzophenone isomers, their effect in chlorine containing polyester resin has been determined (see Fig. 8). This substrate is easy to work with, since it allows a quantitative follow-up of the protective potence of an additive by simple spectrophotometry of the photoindiced discolouration of the test specimens. The difference between the two isomers is striking: the o-hydroxybenzophenone
119
Protection of Polymers Against Light Irradiation
=
R
OCO-R
-o-OH
CD
-0
®
20.000
HO
yOMe@
E
HO
- - I n chloroform - -In dioxane/water
nm
FIG. 7. Spectra of hydroxybenzophenones in chloroform and water/dioxane. (1) p-hydroxybenzophenone (2) o-hydroxybenzophenone (3) 2-hydroxy-4-methoxybenzophenone. 100
Oh 80
, I
./
...- 180 h
./
/
/
/ /
'I 60
-
---...---
...----
/
1000h /
/
I"
/
/
/D
"10
/
U:lJ
t
/
40
/
o
/ /
OH
I In polyesterresin 180 h
20
--Controls
500
600
nm
FIG. 8. Transmissions of polyester resin plates containing hydroxybenzophenones (0' 25 per cent) after 180 hr. Fadeometer irradiation. o-hydroxybenzophenone (1) p-hydroxybenzophenone (2) controls after irradiation of indicated duration (- - )
is a good light stabilizer, whereas the p-hydroxyderivative acts as a photosensitizer, since its test specimen is more discoloured after 180 hr than the control after 1000 hr. The failure of the p-isomer to provide protection is a consequence not of the presence
H. J. HELLER
120
of a p-hydroxy group, but of the absence of a o-hydroxy group. 2,4-dihydroxybenzophenone is a very good light stabilizer. The influence of other substituents in the c-hydroxybenzophenone system has been the subject of many investigations. Technically, however, only compounds like those in Table 2 have been successful. 2
TABLE
(a)
a (b)
=
lOR
1
RO
Cyasorb 9 Uvinul M 40 Anti UV-A Avastab 45 Merk 4588 Riedel HMB ASL24 Cyasorb 531 Uvinul410 Eastman Inhibitor DOBP
(Cyanamid) (Antara, Ward Blenkensop) (Organo-Synthese) (Deutsche Advance)
Cyasorb 24 Avastab 47
(Cyanamid) (Deutsche Advance)
C sH 17 :
Cyasorb 315
(Cyanamid)
R=H:
Uvinul D 50
(Antara)
CH 3 :
Uvinul D 49 Uvinul490
(Antara) (Antata, mixture)
CH 3 :
C SH 1 7 : C lOH2 1 : C UH 2 S : R=CH 3 :
(Shonan Jap.) (Cyanamid) (Antara)
::::--..
OR OH
HO
(c)
(Antara) (Deutsche Advance) (Organo-Synthese) (Japan)
co~
&oot& 0-.-
Uvinul400 Avastab 48 Anti UV-D Alichte-90
R=H:
OH
hoo~
OR
R= OCO-R
20,000
In chloroform
E
-oOM. HO Alk
CD
-oOM. HO
®
-Q-OM. HO Alk
@
nm
FIG. 9. Spectra of benzophenones in chloroform. 2-hydroxy-4-methoxybenzophenone (1) 2-hydroxy-4-methoxy-5-ethylbenzophenone (2) 2-hydroxy-3-methyl-4-methoxybenzophenone (3).
Protection of Polymers Against Light Irradiation
121
Among other o-hydroxybenzophenones there are many good light stabilizers which, however, are not easily accessible and/or cause undue yellow initial colour in the substrate. As an example for initial discolouration, the spectra of two simple a1kylderivatives of Uvinul M 40 are shown in Fig. 9. The hydroxyxanthonesv'?
resemble the o-hydroxybenzophenones in their light fastness and protective power. Unfortunately, the absorption of these heterocyclic compounds, which are stable even in nitrocellulose, is not very suitable. A rather weak band is found close to the visible, which renders the products yellowish in spite of a poor absorption in the long wave u.v.-range. From the above, it should not be concluded that all aromatic ketones with chelatable hydroxy groups are lightfast. For instance, the l-hydroxy-3-phenyl-fluorenone-(9) I and the I-hydroxypleiadene-7,12-dione II are quite sensitive to light.
o
I
II
(2) Absorbers with nitrogen acceptors Besides carbonyl groups, nitrogen functions are known to be acceptors of hydrogen bonds. The question arises which kind of nitrogen-containing structures would be useful. Open chain derivatives such as Schiff bases, hydrazones and azines of salicylaldehyde have at one time or another been suggested as u.v-absorbers. They do absorb but unfortunately a little too much of the visible light so that they are quite yellowish. Due to this and also on account of their poor light fastness, they have never reached any technical importance. Among the ring compounds, cyclic imino ethers and amidine derivatives have been suggested as u.v-absorbers,
(i) Benzoxazoles, benzimidazoles and benzothiazoles Representative compounds of this structural type'"?
z=o, S, NR3 absorb nicely. POLYMER-I
122
H. J. HELLER
The benzoxazoles are quite lightstable but provide no protection, while the benzimidazoles are less lightfast and act as photosensitizers. The benzothiazoles finally are themselves quite light sensitive. (ii) Oxdiazolesand triazoles
1,3,4-oxdiazoles (III)<8) and the different isomers of the 1,2,4-triazoles (Ill,IV) are not suitable as light stabilizers, because N-N
OH
R,)lZ~
Z=O; NR3
R2
III
IV they sensitize the discolouration of polyester resin in spite of a quite remarkable light fastness. Why are the above structural analogues containing a -OH.. .N= hydrogen-bond so different from the o-hydroxybenzophenones? Is this correlated to the fact that nitrogen functions are much more basic than oxygen functions of the same hybridization? Qualitatively, it is found that the benzophenones really are the least basic of the compounds mentioned. A connection between this finding and the lightstabilizing effect of a compound is provided by the spectra. In Fig. 10 the molar extinctions of 2-(2-hydroxyphenyl)-benzoxazole and 2-hydroxy-4-methoxybenzophenone are given. The difference in the fine-structure of these two compounds is evident. The benzophenone derivative has to be quite a flexible molecule, as is indicated by the complete absence of a spectral fine structure even in chloroform in which the internal hydrogen bond should help to restrict the vibrational mobility of the molecule. In contrast thereto, the benzoxazole compound seems to possess a rather rigid molecule even in methanol which has a tendency to disrupt internal hydrogen bonds. This difference in the fine structure of the two spectra could be explained by the assumption that the hydrogen bond in the benzoxazole is strong enough to force the two ring systems of the molecule into a fixed coplanar position, while a certain freedom of rotation about the bond connecting the resorcinol moiety to the carbonyl carbon still exists in the benzophenone derivative. In this way, the hydrogen bond would successively be stretched beyond the breaking point and reformed again. This would not only explain the absence of fine structure in the spectrum, but also the capacity to dissipate energy harmlessly by rotation of the two ring systems and the extreme vibrations of the proton connected therewith. This idea can be tested by checking the behaviour of other known and new u.v.absorbing light stabilizers. Heterocycles with very weakly basic trigonal nitrogen
Protection of Polymers Against Light Irradiation
123
HO
CD
CX:X> HO
20,000
®
O?rD-oMe 0
CD
In methanol
®
In chloroform
320
nm
FIG. 10. Spectra of 2-(2-hydroxyphenyl)-benzoxazole (I) in methanol and 2-hydroxy-4methoxybenzophenone (2) in chloroform.
atoms, in the o-position of which an o-hydroxyphenylrest is placed, should in principle yield useful u.v.-absorbers. (iii) pyrimidines and chinazolines
These two classes of compounds have not yet yielded technically satisfactory light stabilizers. Though in both cases judicious substitution leads to lightfast products, the pyrimidines suffer from insufficient protective ability and the chinazolines are too strongly coloured. (iv) s-triazines It is known that s-triazines are less basic than analogous pyrimidines. This class of compounds has therefore been carefully investigated by American Cyanamid, (9) Ciba(10) and ourselves. (11) The tris-o-hydroxyphenyl-s-triazines are extremely lightfast, but quite yellow and of insufficientcompatibility in most polymers. Applicationally better suited are asymmetric compounds with at least one of R 1 and R 2 being a non-aromatic substituent.
Spectrum, lightfastness and protective power of these s-triazines are influenced mostly by (') the number of the e-hydroxyphenyl groups and
124
H.J. HELLER
(") the basicity of the ring nitrogens, i.e. by the nature of R 1 and R2 (aryl and alkyl groups bound directly or by way of a heteroatom). The following rules apply as a first approximation: Derivatives with a higher number of o-hydroxyphenyl radicals have better protective power and stronger absorption in the long wavelength u.v. than analogues with fewer of such moieties. The more the basicity of the s-triazine ring is lowered by the substituents, the higher is the lightfastness of the resulting compounds. Thus the 2-(2-hydroxyphenyl)-4,6-dialkylamino-s-triazines are quite light-unstable and tend to accelerate the discolouration of polyester resins; the 2-(2-hydroxyphenyl) -4,6-dialkyl-s-triazines are of mediocre lightfastness and weak stabilizing effect; the 2-(2-hydroxyphenyl)-4,6-diphenyl-s-triazines are quite lightstable and provide a mediocre light protection. In the case of 2,4-bis-(2-hydroxyphenyl)-s-triazines, the nature of the third substituent in the 6-position is less pronounced. Alkylamino groups still yield unsatisfactory products. Alkyl- and arylmercaptogroups and, even more so, alkyl- and aryloxygroups lead to compounds of good stability and protective power. Finally, 2,4-bis(2-hydroxyphenyl)-6-phenyl-s-triazines are incredibly lightfast. In thin acetyl cellulose films, they are not changed measurable after 500 hr of irradiation in the fadeometer. Under these conditions, they are degraded only in nitrocellulose. Dr. Brunetti of our laboratories has worked out a convenient method for the synthesis of these compounds
lH
o
~~ 19lo/
Rr-l \
N,.Hz
R
where R stands for an arylradical, in particular a o-hydroxyphenyl group, and R 1 for a hydrocarbon radical or an aryl- or alkoxy- or alkyl- mercaptogroup. Since the analogous o-hydroxynaphtyl compounds are easily made and have also been suggested as lightstabilizers, we have tested them in non-flammable polyester resin. As is indicated by Fig. 11, both the 2-(1-hydroxy-naphth-2-yl)- and the 2-(3hydroxy-naphth-2-yl)-4,6-dimethyl-s-triazinestrongly accelerate the photolysis whereas the 2-(2-hydroxyphenyl)-analogue provides a weak, but distinctly measureable, stabilization. (v) Benzotriazoles So far, we have dealt with compounds formally derived from salicylaldehyde or salicylic acid, i.e. compounds in which the o-hydroxyphenylrest is bound to a carbon atom. This is, however, not absolutely essential. In dyestuff chemistry, a number of carbon-free chromophores are known. The most common in this connection is the azo group but the open chain e-hydroxyphenylazo-compounds, which frequently are of quite good light fastness and possess even light stabilizing effects, are all yellow to red dyestuffs.
Protection of Polymers Against Light Irradiation
125
100
I
1 1_ -
-
-
/l
I
80
I I
/
I
/
I I
/
I
60
I
R
Y
%
NJ. N
J.. )....
/1
Me N
/
40
Me
/ / /
R=
/
CD ~
20
@ 0
HO-b HoB HO~ 600
nm
11. Transmission of polyester resin plates containing s-triazines (0 ' 25 per cent) after 250 hr fadeometer irradiation. 2-(2-hydroxyphenyIH,6-dimethyl-s-triazine (l), 2-(l-hydroxynaphth-2-yl)-4,6-dimethyl-s-triazine (2), 2-(3-hydroxynaphth-2-yl)-4,6-dimethyl-s-triazine (3), and cont rols (--) of which the upper curve corresponds to the sample before irradiation. FIG.
A way to lessen the chromophor-effect of the azo group is also known from dyestuff chemistry viz.: transformation into a vic-triazole ring which represents some kind of an internal shortcircuit:
-'~'Y -
·-
H2N
This group of compounds has been investigated in our laboratories. (12) Their structure is best represented by the following limiting structures N
N
G
©:~>y - ©:>y
0"\
/ "0
a>>G'> HO
of which the quinoid form probably contributes the least.
126
H.J. HELLER
As in the systems mentioned before, the o-hydroxyphenyl group is essential for lightfastness and stabilizing potency. So far, no equivalent has been found. o-Acylaminophenyl-, 2-hydroxynaphth-l-yl- and l-phenyl-5-hydroxypyrazole-3-yl-benzotriazoles all are not useful u.v-absorbers. In Fig. 12 the spectra of the simplest hydroxyphenylbenzotriazoles are given. The introduction of an auxochrom into the p-position of the phenyl ring of 2-phenylbenzotriazole leads to a blue shift. This effect of a p-auxochrome can be cancelled
nm
FIG. 12. Spectra of benzotriazoles in chloroform. 2cphenylbenzotriazole (1), 2-(4-hydroxy-3-methylphenyl)-benzotriazole (2), 2-(2-hydroxy-5methylphenyl)-benzotriazole (3), 2-(4-hydroxy-2-methylphenyl)-benzotriazole (4).
by introduction of a further substituent into the phenyl ring in o-position to the triazol ring (curve 4). Obviously, the steric hindrance by an o-substituent is big enough to twist the phenyl ring so strongly out of the plane of the benzotriazole ring that conjugation between the two systems is substantially killed. The hydroxy group being of similar size as the methyl group should also force the phenyl ring out of plane. On the other hand, the energy of a hydrogen bond can only be gained of the two halves of the molecule are coplanar. It seems that the two extreme positions-the sterically preferred conformation with the planes of the phenyl and triazole rings vertical, and the hydrogen bonded coplanar conformation-differ little in energy and are comparably populated. Intermediate conformations are less preferred and hence less populated. According to the FrankCondon principle, 2-(2-hydroxyphenyl)-benzotriazoles should exhibit two absorption peaks, the one at short wavelengths corresponding to the out-of-plane conformation. This assumption is seeminglyborne out by the spectrum (curve 3, Fig. 12). In this connection it might be worth-while to recall the fact that o-hydroxybenzophenones also show a double peak and p-hydroxybenzophenones only a single one. A first test of this assumption is provided by the solvent effect upon the spectrum. A weakening of the hydrogen bond should lessen the intensity of the long wavelength peak. In Fig. 13 this is demonstrated. By far the least intensity of the long wavelength
Protection of Polymers Against Light Irradiation
127
CD in Heptane ® in Dioxanel
water @in Dioxanel NaOH t::. in Methanol
15,000
nm
FIG. 13. Spectra of 2-(2-hydroxy-5-methylphenyI)-benzotriazole in heptane (1), dioxane-water (2) and in dioxane-IN NaOH (3). The long wavelength maximum in methanol is indicated by a triangle .
peak is found in the anion which cannot form a hydrogen bond. The surprising phenomenon that the deprotonation of the hydroxygroup to the much more efficient auxochrom -0 9 does not lead to an increase of the absorption is typical for o-hydroxyphenyl light stabilizers. Naturally, the quantitative aspect of this anomaly depends upon the steric requirements of the particular molecular species. From Fig. 14, it is evident that the s-triazines are somewhat less sensitive to steric hindrance than the benzotriazoles. The enormous difference between the behaviours of the 0- and pisomers illustrates the importance of this effect even in this class of compounds.
In dioxane - water In dioxane-sodiumhydroxide
20,000
10,000
28 0
300
34 0
320
nm
FIG. 14. Spectra of 2-(4-hydroxyphenyIH,6-dimethyl-3-triazine (1) and 2-(2-hydroxyphenyIH, 6-dimethyl-s-triazine (2) in dioxane water (---) and dioxane-IN NaOH (-).
128
H. J. HELLER
Bulky substituents like tert.alkyl groups in o-position to the hydroxygroup protect the hydrogen bond and lessen the influence of hydrogen bond breaking solvents. This is illustrated in Fig. 15.
15,000
10,000 E
Bultl
o:N~-Q N HO
CD In ® In
Heptane Dioxane-water 6 In Methanol
Bult)
nm
FIG. 15. Spectra of 2-(2-hydroxy-3,5-di-tert-butyIphenyl)-benzotriazole in heptane (1) and dioxane-water (2). The long wavelength maximum in methanol is indicated by a triangle.
A second test of our assumption is demonstrated in Fig. 16. A second steric hindrance by a group in the other o-position ofthe phenyl ring cannot be compensated anymore by the energy of the hydrogen bond: the long-wavelength peak disappears almost completely (curve 1). A second hydroxy-group in this position stabilizes the
nm
FIG. 16. Spectra of 2-(2-hydroxy-3-tert-butyl-5,6-dimethylphenyl-benzotriazole (1), 2-(2hydroxy-3,5-di-tert-butylphenyl)-benzotriazole (2) and 2-(2,6-dihydroxy-3,5-di-tert-butylphenyl)-benzotriazole in chloroform.
129
Protection of Polymers Against Light Irradiation
coplanar configuration strongly without stiffening the molecule to such an extent as to produce spectral fine structure (curve 3). Besidesthese steric effects, conjugation can also strongly influence the energy difference between the two conformations. Strong electron donators in p-position to the triazole acceptor increase the stability of the coplanar configuration by resonance. This leads to an increase of the intensity of the long wavelength peak and at the same time to a decrease in its band-width. The same resonance can, however, diminish the electron withdrawing power of the acceptor so that this intensified peak appears at a shorter wavelength. This effect has been found in the benzophenone-series (Fig. 7), but is also pronounced with the s-triazines (Fig. 17). In the case of the benzotrazoles the "capacity" of the acceptor is big enough, so that a small bathochromic effect of the auxochrome is retained (Fig. 18). The decrease of the short wavelength peak is very pronounced also in this case. Me
)-N
N
rN>-R Me
R=
-0-0 Alkyl (])
HO
-b
HO
20,000
®
-o0Alkyl@ QAlkyl
-0
280
@
300
nm
FIG. 17. Spectra of s-triazines in chloroform (-) and dimethylformamide-water (--) 2-(4-methoxyphenyl)-4,6-dimethylos-triazine (1) 2-(2-hydroxyphenyl)-4,6-dimethyl-s-triazine (2) 2-(2-hydroxy-4-methoxyphenyl)-4,6-dimethyl-s-triazine (3) 2-(2-methoxyphenyl)-4,6-dimethyl-s-triazine (4).
What is then the effect of substituents in other positions? A decrease in the acidity of the hydroxy group lessensits ability to form stable hydrogen bonds. This produces a decrease of the long wavelength extinction. The concomitant increase in electronegativity of the phenyl ring system lowers the charge-transfer energy which leads to a bathochromic shift of this weakened peak. Figure 19 demonstrates this effect. Acidifying substituents in the benzotriazole moiety provoke similar effects (Fig. 20, curves 1, 2, 3). The influence upon the extinction coefficients is not so cleancut, because the influence of such acidifying groups upon electronegativity and proton basicity is not quite parallel. Basifying substituents in the acceptor moiety increase the stability of the hydrogen bond and promote the long wavelength absorption (Fig. 20, curves 5 and 6). The bathochromic effect of the methoxygroup is, however, not clear.
130
H.J. HELLER
O:Np-yR N HO
20,000
In chloroform
280
300 nm
FIG. 18. Spectra of 2-(2-hydroxy-4-methoxyphenyl)-benzotriazole (1), 2-(2-hydroxyphenyl)-benzotriazole (2), and 2-(2-methoxy-5-methylphenyl)-benzotriazole (3) in chloroform.
in Chloro form
/
280
30 0
340
320
36 0
nm
FIG. 19. Spectra of benzotriazoles in chloroform. 2-(2-hydroxy-5-methoxyphenyl)-benzotriazole (1) 2-(2-hydroxy-5-acetylaminophenyl)-benzotriazole (2) 2-(2-hydroxy-5-phenylphenyl)-benzotriazole (3) 2-(2-hydroxy-5-methylphenyl)-benzotriazole (4) 2-(2-hydroxy-5-chlorphenyl)-benzotriazole (5) 2-(2-hydroxyphenyl)-benzotriazole (6) 2-(2-hydroxy-5-carbobutoxyphenyl)-benzotriazole (7)
In Fig. 21 the effects of basifying substituents, exemplified by the methoxygroup , are summarized. The influence of such a group in o-position to the hydroxygroup is interesting. It competes with the nitrogen for the hydrogen and hence lessens the longwavelength peak.
Protection of Polymers Against Light Irradiation
0
20,000
R
13l
CH 3
>-o HO
In chloroform
E
10,000
nm
FIG. 20. Spectra of benzotriazoles in chloroform. 2-(2-hydroxy-5-methylphenyl)-5-phenoxysulfonyl-benzotriazoIe (1) 2-(2-hydroxy-5-methylphenyl)-5-carbobutoxy-benzotriazole (2) 2-(2-hydroxy-5-methylphenyl)-benzotriazole (3) 2-(2-hydroxy-5-methylphenyl)-5-chloro-benzotriazole (4) 2-(2-hydroxy-5-methylphenyl)-5-methyl-benzotriazole (5) 2-(2-hydroxy-5-methylphenyl)-5-methoxy-benzotriazole (6).
CXN~_(-r~OCH3 (if 20,000
N HO~J
in Chloroform
10,000
360
nm
FIG. 21. Spectra of benzotriazoles in chloroform. 2-(2-hydroxy-5-methylphenyl)-5-methoxybenzotriazole (1) 2-(2-hydroxy-3-methoxy-5-methylphenyl)-benzotriazole (2) 2-(2-hydroxy-4-methoxyphenyl)-benzotriazole (3) 2-(2-hydroxy-5-methoxyphenyl)-benzotriazole (4).
(3) Alkali insensitive u.v.-absorbers All these o-hydroxyphenyl-u.v.-absorbers have one common drawback: they are sensitive to alkali and heavy metal ions. In plastic applications this is not a severe limitation. In the few cases in which metals lead to discolourations, such as in cobaltcures for polyester resins, the use of sterically hindered products (cf. Fig. 15) produces
132
H.J.HELLER
good results. For textile application, however, the alkali sensitivity is a severe shortcoming. A search for compounds free of phenolic groups has therefore been under way in many laboratories. However, only one suitable class of compounds has been uncovered so far, viz. the cinnamic acid derivatives. The most lightfast and oxidation resistant members of this class are
Ph
CN
"'-C=C/ '/ "'Ph R
R=CN
(Uvinul N 38)
COOEt (Uvinul N 35)
The sole drawback of these products is their low absorbancy. Somewhat less efficient are partially aliphatic analogues:
Ph
CN
"'C=C - / / "'-COOR CH
R=Me (Bayer 318) Bu (Bayer 317)
3
In conclusion, it can be said that u.v.-absorbers are yet to be found having the alkali insensitivity which allows their textile application and the high absorptivity which makes them commercially acceptable. REFERENCES (1) J. G. Calvert and J. N. Pitts Jr. Photochemistry, Wiley (1966) (Ketone no. 42). As this book excellently covers the pertinent literature, only very specific citations outside the realm of this book will be given in the following. (2) U.S. Pat. 2 971 940; U.S. Pat. 2971 941 (priority date 20.3.59), Ferro Corp. (3) U.K. Pat. 943 081 (18.10.62), American Cyanamid. (4) U.S. Pat. 3 310575 (21.3.67), Geigy. (5) 1968 Modern Plastics Encyclopedia, Intemat. edn. p. 407, Vol. 45/No. lA, McGraw-Hill, New York (1967). (6) FR Pat. 1 279690 (priority date 18.1.60), Geigy. FR Pat. 1 346787 (priority date 9.2.62), ICI. (7) Swiss Pat. 350763 (priority date 14.10.58), Ciba. Belg. Pat. 583 552 (priority date 13.10.59), Ciba. (8) FR Pat. 1 127243 (priority date 5.11.54), Ciba. (9) Belg. Pat. 614'726 (priority date 6.3.61), American Cyanamid. (10) Belg. 639'329 (priority date 30.10.62), 639'330 (30.10.62), 642'767 (25.1.63), 643'432 (7.2.63), 643'898 (18.2.63), 650'932 (26.7.63), 658'233 (14.1.64), 661'225 (18.3.64), 663'345 (4.5.64), 667'044 (17.7.64),679'733 (20.4.65), Ciba. (11) Belg. Pat. 661'186661'187661'188661'189661'190 (priority date 24.1.63), Geigy. (12) FR. Pat. 1 195207 (priority date 14.12.56), FR. Pat. 1 324897; FR. Pat. 1 324898; FR. Pat. 1 324899; FR. Pat. 1 324900; FR. Pat. 1 325403; FR. Pat. 1 325404; FR. Pat. 1 325405; FR. Pat. 1 330378; FR. Pat. 1 330379 (all priority date 16.6.61), Geigy.
PROTECTION OF POLYMERS AGAINST LIGHT IRRADIATION * H.
J.
HELLER
J. R. Geigy A. G., Basel, Switzerland
INTRODUCTION PHOTODEGRADATION is an old phenomenon. Particularly eyecatching is the bleaching of dyes in direct sunlight and most funspoiling is a good sunburn. Considering its energy content, such effects of light are not surprising. Potentially the energy of visible and particularly of U.v. light suffices to dissociate almost every chemical bond. It is important to know, however, that the most damaging range of u.v., i.e, the radiation below approximately 280-290 urn, is completely missing in Colour
~
nm
/I
E
Ideal separation energies
ern" keal
mOl
X=J/X - C(Ell z Ph -X - CH(EtlCH=CH z
IR
30- AeO-OAe MeO-OMe Red---730--13700- - - - - - - -- - -- -- - - - - Yellow
50-
Green
Br-Br
X=C
X-CHEt z
X=f
X-Ph
Me-Me 90-
~ ~ --~;~~~;~:~ X - CO A l k Y I
Violet 70X=W -----400--25000-----MeS-SMe ------
UV
~
X-C(MeIEtz
CL-CL Me-CHzPh
Blue
""'-X-COPh ~X-C(EtlzOR "X-CH(CHzCLlz
Me-Ph
Ph-Ph - - - - 280--:35710----- -H-OEt--110FIG. 1. Ideal separation energies vs. frequency of light. (The separation energies are calculated according to H. Preuss. Quantentheoretische Chemie I (Bibliographisches Institut Mannheim. B-I-Hochschultaschenbuch 43.) The use of real bond dissociation energies would certainly result in some crossovers in the lines connecting the bars representing the energies of the F-, H-, Cl- and J-bonds.)
natural daylight. This short-wave length light is emitted by the sun but completely absorbed in the outer atmosphere. In Fig. I the ideal separation energies ofseveral specific bonds important in polymers are confronted with the energy of various wave lengths.
* Main
lecture.
POLYMER-H
105
106
H. J. HELLER
It is seen that only the most stable bonds-such as the C-F-bonds in alkyl- and arylfiuorides and the C-H bonds in aromatic and simple aliphatic hydrocarbonscould withstand the radiation energy of the natural daylight's photons if all this energy could be concentrated on one specific bond in a molecule. Luckily such localization of radiation energy is a very rare occurrence. Otherwise, life as we know it could not have developed on earth at all. To produce chemical effects, light has to be absorbed. Most commercially important polymers should absorb very little or no radiation at all down to 290 nm if only their usually cited structural moieties would contribute to their real structure. Pure polypropylene, for instance,
should not be influenced by daylight, even if it contained vinyl groups or similarly unsaturated structures as end groups. The same holds true for PVC, polyamides, polyester resins and the homo- and copolymers of styrene. However, all these well known polymers tend to degrade upon light exposure. Very frequently the damage is mainly aesthetic in nature, like discolouration and formation of surface craze and cracks. In the case of thin structures like films and especially fibres, these damages are further accompanied by a breakdown of the mechanical and electrical properties as evidenced by an increase in brittleness and in the electrical loss factor.
2. PHOTOINDUCED PROCESSES Responsible for the photosensitivity of most polymers are impurities and/or abnormal structural moieties which are introduced during polymerization, workup or shaping processes. In the case of polystyrene, for instance, the light sensitivity increases with residual monomer content. The exact natures of these photoactive impurities or structural moieties are not known with certainty. However, ketones and/or aldehydes and also peroxides are generally accepted as photoactive species. Simple aliphatic ketones exhibit a forbidden, i.e. low intensity transition, in the region of 270-290 nm (€ approx. 10-16). Aromatic-aliphatic ketones such as acetophenone have an analogous, but somewhat more intense band at slightly longer wavelengths: 310-330 urn (€ approx. 40-50). In purely aromatic ketones such as benzophenones, these shifts are even more pronounced: "-max approx. 360 urn (€ approx. 120). These bands correspond to an excitation of nonbonded electrons of the carbonyl-oxygen which is called an n--?7T* excitation. The intense absorption of ketones and aldehydes below 290 nm is of no concern to us here, since such radiation is not contained in daylight. Four types of reactions contribute most to the photoinduced processes arising from ketone irradiation. The radicals produced in these reactions are the source of further polymer degradation, as we will see later. In purely aliphatic ketones, the two "Norrish Type"-splits are most important.
Protection of Polymers Against Light Irradiation
-CH-C-C +CO-
107
"Norrish Type I"
o II
o II [-CH-C-C-C-] * -
"Norrish Type II"
-C=C- + -CH-COH I
---+
-C-CI
intramolecular reduction
I
C-C OH
RH
I
-CH-C-C-C + R·
abstraction of substrate atoms
The relevant quantum yields!'? for a simple ketone representative of a hydrocarbon polymer structure are:
(2)
~-CHz-CHz'
(4)
CH2 =CH2
+ j-
BuCO
CH3
+
-,
/H-
CH2C OCH3
¢. - 0 -06
CH3
The excited aliphatic ketones break down into roughly equal amounts of radicals and molecules, with the exception of higher methylketones for which the nonradical Norrish split II seems to be preferred by about one order of magnitude. A similar situation exists for araliphtic ketones like butyrophenone. However, in this case a new phenomenon becomes apparent. The influence of subsituents in the phenyl group is eminent. Strongly electron donating substituents, like hydroxyand aminogroups in 0- and p-position to the carbonyl group, almost completely inhibit the ethylene formation. We will come back to this phenomenon later. The intramolecular reduction of the carbonyl group becomes another possible photoreaction in unsaturated aliphatic ketones of specificstructure. It is related to the Norrish Type II olefine production. In both cases the intermed iate assumes the configuration of a 6 membered ring which, however, collapses differently. (See Formula overleaf.) On the other hand, irradiation of a, ,B-unsaturated ketones often leads to photodimers. Pure aryl ketones, exemplified by benzophenone, do not dissociate but are powerful oxidizing agents in their excited state. They are capable of abstracting hydrogen atoms from almost any kind of substrate apart from purely aromatic hydrocarbons like benzene. The reactive species is the triplet [3(n~1T*)] formed at high efficiency
H. J, HELLER
108
\/'j(\L _ I\~
/
HO
\
rf~
R C-C /\/\
+ \-
Norrish Type II
II C C
I \ 1\
~=H. alkyl
\Cl O~ C1\
/
R C-C
1\/ \
~=Yinyl
\.
HO
\.
C C<, /\ / /C=C C-C \ 1\/ \
intramolecular reduction \
~.
HO\
OH
c''r:-I -~ I C
c-cI;
<, .
,/
intersystem crossing from the primary excitation product. Thus benzophenones are photo sensitizers of very high efficiency, whereas aliphatic ketones-prone to undergo non radical dissociation-abstract hydrogen atoms from the substrate with lower quantum yields. The hydrosensitizer radicals themselves are frequently of low reactivity and dimerize harmlessly. In the case of benzophenone two ketylradicals form benzpinakole:
2
Ph Ph -, -, +2RH C' C=O-----? 2 /
Ph
-2R'
Ph
/""
Ph
OH
-----?
""c-c/
/1
PhOH
Ph
I""
OHPh
On the other hand the R' radicals derived from the substrate as well as those formed by direct photo-dissociation of aliphatic carbonyl compounds are capable of initiating autoxidation chains with the formation of hydroperoxides. Dissociation of these hydroperoxides leads to new free radicals. The original chain branches in a typical "snow-balling" manner. Termination of the chains is possible only by dimerization or disproportionation of active free radicals. Due to their high reactivity, such events are effective only at relatively high concentrations of free radicals, i.e. after the induction period of the autoxidation, when the polymer degradation is well advanced. At room temperature the chainbranching, i.e, production of free radicals from hydroperoxides, is in itself a very slow process but is greatly accelerated by light and/or metal impurities in the polymer.
Protection of Polymers Against Light Irradiation
109
An especially bad catalytic influence is exerted by transition metals having stable valence states which differ by one electron such as Co , Cu, Fe and the like. Such metals are in principle capable of decomposing hydroperoxides catalytically, i.e. with regeneration of their original valence state. Furthermore, numerous metal ions absorb quite strongly in the u.v. and can act as photosensitizers, especially for hydroperoxides. This latter type of mechanism is, however, not truly catalytic. Besides this metal sensitized photodecomposition of peroxides, direct photolysis also plays an important role. Peroxides as well as hydroperoxides absorb below 340 nm. This absorption represents the very weak tail end of a band, the maximum ofwhich is below 260 nm . In the case of di-rerr-butylperoxide, the molar extinction coefficient at 300 nm is approximately two. This absorption might be due to the promotion of a non-bonding electron of the oxygen to an antibonding 1T* orbital. The separation energy ofthe-O-O-bond in peroxides and hydroperoxides is around 35 kcaljmol, whereas the absorbed energy of a photon of 300 nm is 95·2 kcaljmol. Thus, the alkoxyradicals formed by -0-0- bond cleavage carry off together an excess energy of approximately 60 kcal. These "hot" radicals are therefore prone to break; apart before they can abstract a hydrogen-atom from the substrate to form an alcohol. The breakdown products are mainly free alkyl radicals and carbonyl compounds, i.e. ketones and aldehydes. The structures causing the primary attack in the light induced autoxidation are hence reproduced and augmented. In this sense, the light degradation of polymers is autocatalytic. 3. INHIBITING ADDITIVES From the above it is evident that a single photon absorbed by a photosensitizer can cause considerable degradation of the polymeric substrate, particularly when atmospheric oxygen is present. In this , it is quite immaterial whether this sensitizing group is part of the macromolecule or of a low molecular weight impurity. To prevent such degradation, the follow ing protective measures can be taken (cf. Fg. 2) : (1) Prevention of light absorption by sensitizers. For this "u.v.-absorbers" are used. Li g hl injury of substrcte RH Type of reacfion Primary processes
Secondory processes
Protective alienI
cha in sc iss ion e.g. u.v. -absorber CH-C-C- Co- ~C=C +CH-COrad ic ol-production u.v, - obsor ber quencner X- X ·-- R' (decctivot ion of inter medi ate)
ouloxidalion
ontioxidan!
RH---y-ROO·--,. ROOH-A- R· ----.-A-b 2 Perox ide dissociotion
ROOR'- RO· However. e.g .
ROOR' + M,"n-
"synerqist "
+ R'O· RO·
+ M(OR'tn
rneto ldecc t jvct or
is energel icolly preferred
FIG.
2. Summary of light degradation mechanisms and the corresponding remedial additives.
H.J. HELLER
110
(2) Deactivation of light excited sensitizers. Such energy transfer is effected by "quenchers". (3) Inhibition of the propagation of autoxidation. To effect this, the highly reactive chain propagating free hydroperoxy-radicals are transformed into harmless radicals by "antioxidants". (4) Prevention of the initiation of secondary autoxidation chains by hydroperoxides. The effectivityof hydroperoxides in starting new chains is minimized by (a) ionically decomposing them by means of "synergists", (b) suppressing the metal catalysis in the production of free radicals with the
help of "metal deactivators". To illustrate the interplay of u.v.-absorbers and antioxidants, the time for unoriented polypropylene samples to reach the brittle-point in outdoor-weathering is given in Fig. 3. The stabilizers used are 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole as u.v.-absorber and pentaerythritol tetra 3-(4-hydroxy-3,5-di-tertbutylphenyl) propionate as antioxidant at the indicated levels. Lightstability of polypropylene
150
20
0·2"10 uv-absarber
Antioxydant, "10
FIG. 3. Lightdegradation of polypropylene. (l Langley (Ly) = 1 cal/em"; 1 year outdoorweathering in Basel corresponds to approximately 70 kLy (70,000 Ly). Further explanations see text.)
It is seen that the antioxidant used, while improving the resistance toward autoxidation by orders of magnitude, conveys only a limited light protection. Light irradiation produces so many autoxidation chains that locally they cannot be stopped by the antioxidant. Antioxidants, having similar ability to suppress autoxidation as measured by a specific test method, vary widely in their response to light. Two main factors are operative: (1) the diffusion coefficientof the antioxidant in the polymer, i.e. the velocity with
which the antioxidant is replenished by diffusion into critical spots and
Protection of Polymers Against Light Irradiation
III
(2) the photochemical behaviour of the antioxidant and especially its reaction and degradation products. In the above example a very effective polypropylene antioxidant has been chosen, which, however, is photochemically neutral. Due to the slow diffusion of its bulky molecule, it provides little light protection; due to its structure, it does not form sensitizing degradation products. The two upper curves demonstrate how the effect of the u.v.-absorber is improved quite drastically by the antioxidant. Near the surface of the testpiece, the intensity of the light not yet absorbed by the u.v.-absorber is still high. Autoxidation chains are still induced, but their number is so small that the "snow-balling" effect can be checked by the antioxidant. 4. LIGHT STABILIZERS After this general discussion of the basis of light stabilization of polymers, let us turn more specifically to light protective agents of which, as pointed out above, there exist two classes: viz. u.v-absorbers and quenchers. (0) Quenchers
Typical quenchers, in contrast to absorbers, need not have high absorptions at the wavelengths critical for polymer degradation. In most cases, light excitation from the ground singlet to the lowest triplet is more or less forbidden and the resulting absorption is only weak. For energy transfer, these selection rules do not hold and hence a quencher can deactivate sensitizertriplets Sz1) at energy levelsat which it is quite transparent to light. According to Wigner's spin conservation rule, the quencher has to change its multiplicity when deactivating sensitizer-triplets. If its ground state is a singlet l(QO)' a triplet will be produced 3(Ql):
e
3(Szl)+1(QO)
-+
1(SZ3)+3(Ql)'
If the groundstate of the quencher is a triplet, as in the case of oxygen, a singlet is produced: 3(SZl)+3(QO) -+ I(Szo)+l(Ql)' Since the quencher is raised to an excited state, the mere fact that a compound quenches a photosensitizer does not of itself mean that this compound is a light protective agent. Only if the excited quencher can dissipate its accumulated energy harmlessly, has it a chance to reduce photodegradation. A typical example of a very efficient, but non-protective quencher, is oxygen. Its reaction product, singlet oxygen, is very reactive and forms hydroperoxides readily. Quenching is a bimolecular process and is very fast if exothermic. In other words, quenching is a diffusion controlled process and is effectivein polymer protection only if the sensitizer triplets have a long half-life and if the quencher is freely diffusible. So far, only nickel compounds have found an industrial application as typical quenchers, and this mostly in polyolefines. As is so often the case, their discovery as light stabilizers was more or less accidental since they were originally suggested as dyesites in polypropylene.
H. J. HELLER
112
Of the numerous products claimed in the patent literature, the following products are distributed today:
Ferro AM 101 (Ferro Corp.) (2) Me
Me
M.-j-CH,-j---fc5\-o
9 /\. M.-j-CH,-j\Q;-~ Me
Me
Me
Me
Me
Me
d
-c
S----N,---NH 2
H 4
9
Cyasorb 1084 (Am. Cyanamid) (3)
>-<8] [BU Bu
S
Ni
Nickeldibutyldithiocarbamate (various sources)
2
Negopex A (ICI) Bu (t)
HO
>cJ Y
CH,
Bu (t)
A63'99 (experimental Geigy) (4)
Protection of Polymers Against Light Irradiation
In
Some of these nickel compounds have a shortcoming. They have a tendency to decompose to black nickel sulphide at the high temperatures (up to 300°) needed for a good throughput in the costly modern extruding and spinning equipment for polyolefines. The necessary sulphur stems from the ligand itself or from the thiodipropionates used as synergists. (b) u.o-absorbers: general requirements
Obviously a u.v-absorber can always take part in energy transfer processes and it needs special investigations to determine the contribution of quenching to the action of a u.v.-absorber under specific conditions. In practice, however, the quenching action of u.v.-absorbers is only of importance in very thin structures such as fibres. The question is, how much light is left to be absorbed by a photosensitizer in the presence of a u.v.-absorber. In Fig. 4 the situation is exemplified by the calculated Transmission vs. depth at penetration
Residual absorption of acetophenone vs. absorber concentration
. ~
.
I--
6
8
10
mm
FIG.
4. (a) Transmission of compositions of acetophenone and 2-(2-hydroxy-5-methylphenyl)benzotriazole at 330 nm. (b) Residual absorption of the acetophenone in these compositions.
absorptions of acetophenone as sensitizer and 2-(2-hydroxy-5-methyl-phenyl)benzotriazole as u.v-absorber at 330 urn at various absorber concentrations and sample thicknesses. Acetophenone has been chosen because similarly absorbing species participate in the degradation of homo- and copolymers of styrene. In Fig. 4(a) the transmission of samples containing 20 ppm acetophenone and various amounts of absorber are shown as a function of the sample thickness. It is evident that the acetophenone contributes very little to the total absorption particularly at absorber concentrations above 100 ppm. Consider the lowest level of practically used concentrations of u.v.-absorbers; at 0·1 per cent only 50 per cent of the incident light (330 nm) reaches a depth of 49p. and 10 per cent reaches 154p., while at O·3 per cent concentration of the benzotriazole 50 per cent penetrates to 16p. and 10 per cent to 51p..
H. J. HELLER
114
The amount of radiation absorbed by acetophenone in the presence ofu.v.-absorber as a fraction of the acetophenone absorption in the absence of absorber is a measure of the protective power of the absorber by absorption. We will call it "residual absorption". The higher its numerical value, the poorer the effect of the absorber. The calculation of this residual absorption (R) is straightforward if Beer-Lambert's law is valid. The following formulae apply l_lo-xakek
(la)
At
I_Io-x:!:aie i
(lb)
Ak
akck A· L.ai ci '
(I c)
A kO
wherein x denotes the sample thickness (in em), a; the absorptivity* (in cm 2 /g) and c, the concentration (in gil) of the ith-species of a multicomponent absorber system, /0 the intensity of the incident light, At the absorption of the system (as a fraction of /0) and A ko the absorption of the kth-species, if present alone, while A k stands for its actual absorption as a fraction of /0 in the system. Hence the residual absorption for the kth-species (R k ) is: Ak
akck
A ko
L.alCI
R k= - = - -
(2)
In Fig. 4(b) the residual absorption for acetophenone is given under the conditions of Fig. 4(a). At the concentration of the triazole absorber used in practice, there is very little absorption of the sensitizer left in thick samples, whereas in thin samples an appreciable residual absorption is found, necessitating high absorber concentrations. The question now arises as to the effect of the initial sensitizer absorption. In the insert of Fig. 5 the residual absorption (R s ) of a sensitizer is given as a function of its initial absorption at various levels of total absorption of the resulting composition. A surprisingly small effect is noted and the curves are easily extrapolated to initial sensitizer absorptions of zero-value. Hence the concept of a lower residual absorption limit becomes practical. For small absorption (xascs-+O) by the light sensitizer, (2) can be rewritten: I_Io-x:!:aie l Rs (log e + txascs +.... ) XL.aICI
l_lo-x:!:ale i XL.aiCI
log e (1 + tAso
+.... )
(3)
Hence the lower limit of the residual absorption (L s ) l_lo-xxaiei L, = limR, =
XL.alCi
log e
(4)
xascs-+O
depends only on the absorption of the u.v.-absorber. '" With e = molar decadic extinction coefficient and w = molecular weight, the relationship is e =
aiW.
Protection of Polymers Against Light Irradiation
115
Residual absorption vs. Optical density of Initiol obsorption uv - obsorber of sensitizer 100 50
100c:=====:=:::===,
.. >!!
~80
-.e
60
~
40
~
50
~
90%- - - - - j ~~~
99·999% total absorption
~O
80
'-95
90
95
20
10
0
'~8 .....99 98 " 99
As
20
=50%
<, 0__
Aso= 0 %
99·9 ..... _ 99'9 -
o
---
.....:..::...::..::....:::.::..:::.= -----
__
5
6
0.0'0
FIG. 5. Residual absorption vs. optical density of added u.v-absorber. Solid line lower limit of residual absorption (A so = 0) Dashed curve residual absorption if the initial absorption of the sensitizer is 50 per cent of the incident light. The total transmission of the resulting compositions are indicated on the curves. Insert: Residual absorption vs. initial sensitizer absorption at various levels of total absorption.
The solid curve (marked Aso=O per cent) in Fig. 5 gives this lower limit as a function of the optical density of an added u.v.-absorber. As a comparison, the dashed curve has been calculated for an initial sensitizer absorption of 50 per cent, a value which is very rare in practice. It is evident that, for practical purposes, the lower limits L, of the residual absorption are good enough to judge the effect of a u.v.-absorber based upon its absorption. In a composition in which the added u.v.-absorber has an optical density of I (i.e. absorbs 90 per cent of the incident light), the L s is about 39 per cent. If the u.v.absorber were perfectly stable and nonvolatile it should prolong the useful life of the substrate under these conditions by about a factor of 2·5. At an optical density of 2 TABLE
I
Polymer
Polyesters (various formulations) Polystyrenes Polyethylene Polypropylene (non-heat stabilized) Polyvinyl chloride Polyvinyl chloride copolymer with vinyl acetate Polycarbonate Polymethyl methacrylate Polyformaldehyde
Wavelength of maximum sensitivity 325 318 300 310 310
om nm om om nm
322 and 364 nm 295 om 290 to 315 nm 300 to 320 nm
H.I. HELLER
116
(99 per cent absorption), the corresponding values of L, and the improvement factor are 22 per cent and 5 respectively. If an absorber provides a better protection, it must act by an additional mechanism such as quenching. The above derivations obviously hold strictly only for monochromatic light. As the molar extinction coefficients and thus the absorptivities of the involved species vary differently with wavelength, the picture gets rather complex in the case of polychromatic light. In practice, however, it should suffice to consider the wavelengths of maximum sensitivity of the polymer in question. In Table 1(5) such values for some polymers are given. Which practical requirements have to be fulfilled by a technical u.v.-absorber and what kind of structures are useful for such purpose? (1) u.v.-absorption There are two distinctly different groups: (i) Cosmetic absorbers. Human skin has a very distinct maximum of sensitivity at 297 nm. In healthy persons, however, light of longer wavelengths stimulates the protective tanning. Sunlotion or suncream should therefore selectively filter out light below approximately 320 nm. A large number of u.v-screeners have been suggested such as 0- and p-aminobenzoic acid derivatives, p-aminosulphonamides and benzoxazoles. Most satisfactory also from a toxicological point of view are, however, the esters of salicylic acid. (ii) Absorbers for technical polymers. The ideal absorber for this purpose absorbs all U.V.- but no visible light. In practice, as usual, a compromise has to be found. Either absolute freedom of colour is insisted upon and a certain transparency in the long range u.v. is allowed, or if good absorption in the long range u.v. is needed, such as for 66-nylon, a certain yellow discolouration of the final composition has to be "areot density· concentration x length, optical density:obsorptivity x areal density, I gIL at I cm : 10 g/m 2 : 10 gIL at I mm
Influence of "oreol density" ond "steepness" upon uv-obsorption and visual colour 100
r"
/
80
/
60
/
\
. I-
/
\
\ 20
o
-
\ r:»> \/
\.../---
/
,
/
2
. ......... _--
.,...-', 0'1 g/m
/
I 300
. ./
.·15
-
;/
/
350
/'
.II
1/
'/
~
,/
I \
40
•
/
/
0'1
/'/
j
f
50 g/m
t·
// /J
2
I
400
nm
FIG. 6. Transmission of 2-hydroxy-4-methoxybenzophenone (- -) and 2-(2-hydroxy-5methylphenyl)-benzotriazole (. -'). "area density" is the product of concentration (in gIl.) and sample thickness (in ern), (It has the dimension ml- Z ) .
Protection of Polymers Against Light Irradiation
117
accepted. The sum of the oscillator strengths over all absorptions cannot exceed one for a given compound. If the absorption were equal in the range from 280 to 400 nm and zero below and above (i.e, 10279=10401=0; €280=€400=i) theoretically the maximum molar extinction coefficient over this region would be about 2 .10+4 (i). Obviously, such an ideal "square wave" absorption band does not exist, but it is important to find u.v.-absorbera approaching it as closely as possible. Figure 6 gives the transmission of the solution of two-absorbers at different "area densities". It is evident that one absorber covers the range much better at low "area density", i.e, low concentration and/or thin samples. Nevertheless its solution is colourless even at high "area density" while the other absorber causes a visually detectable yellow discolouration. It is an empirical rule that compositions with monotonically increasing transmission curves appear yellow if their transmission at 420 nm is less than 85-90 per cent. (2) Protective power Obviously a u.v-absorber should not be photoactive, i.e. it should dissipate the absorbed energy in a harmless manner. This is actually the problem to be solved in the synthesis of new absorbers. Only very few of the many u.v.-absorbing molecules are really light stabilizers. In general, light stabilizers should not fluoresce but transform the energy into heat. So far, we have not encountered fluorescent substances such as optical brightening agents which are real light stabilizers, in spite of their similar u.v.-absorption. (3) Polymer compatibility In order to absorb at all, u.v.-absorbers have to be compatible with the polymer to be protected. They have to be in true solution. This condition is not always easy to fulfil in the case of highly crystalline polymers. The chance that the low molecular weight light stabilizers crystallize isomorphously with the polymer is very low, so that they accumulate in the amorphous phase or at lattice imperfections. Therefore, additive doses of a few tenths of a per cent based on the total weight of the polymer represent rather high concentrations if the amorphous phase accounts for less than 10 per cent. (4) Light fastness A high permanence of the absorption of a u.v-absorber is essential but very difficult to meet. The fastness properties required are much more stringent than for instance with the normal textile dyestuffs. In general practice the combination of light stability and nonvolatility of an absorber determines its performance. (5) Specific properties
Depending upon its intended use, u.v-absorbers for polymers have to fulfil a series of further conditions. They have to be stable to heat (up to 300°) under a variety of conditions, in acidic or basic and reductive or oxidative media. Especially stringent are fibre uses. Besides the afore-mentioned properties, all classical fastness properties of a textile dyestuff should be met-such as resistance to sublimation, washing, dry cleaning etc.
118
H. J. HELLER
All these technological problems are hard enough to solve. Tougher to meet, however, are the toxicological requirements. Since the end users of commercially available u.v.-absorbers want to export their polymers to various countries, all the diverse laws have to be respected. This necessitates rather elaborate and very costly toxicological investigations and extraction studies.
(c) u.ii-absorbers: chemical classes (1) Absorbers with carbonyl oxygen acceptors The first technically used light protective agents were salicylates of which salol
c6r
coo ,"
and its alkylated derivatives are still on the market because of their generally low prices. Due to their low initial absorption, such compounds like Dow Light absorber TBS (p-tert-butylphenyl salicylate) and Eastman Inhibitor OPS (p-tert-octylphenyl salicylate) are only utilized in substrates of slight photosensitivity. Next Dow Compo used simple o-hydroxybenzophenones like 2-hydroxy-5-methyl and -5- chlorobenzophenone to stabilize polyvinylidene chloride. Since such benzophenones cause a distinct yellow discolouration, a blue pigment had to be added to produce the neat Saran®-foils. The practically colourless resbenzophenone and its monoalkyl ethers proved to be a step forward in this respect. It was a rather unexpected discovery of the General Anilin & Film Corp. that the introduction of a second auxochrome into the benzophenone molecule has a hypsochromic effect. Figure 7 gives the spectra of some hydroxy-benzophenones. The p-hydroxyderivative exhibits only one strong band with the normal red shift in polar solvents. The two o-hydroxybenzophenones on the other hand exhibit a double headed band of which the peak at the longer wavelength shows a blue shift in polar solvents. The visual hypsochromic effect of the methoxy group in p-position to the carbonyl group' is phenomenologically due to a reduction of the band-width of the very flat longwave length peak of the simple o-hydroxybenzophenone, The short wavelength peak exhibits the normal bathochromic effect expected from the introduction of an auxochrome. As pointed out before, the longest wavelength absorption of benzophenone itselfthe n-7'TT* excitation-has a maximum around 345 nm (e approx. 120). In the hydroxybenzophenone this absorption is partially buried in the strong 7T-'»-7T* band, because the auxochrome does not shift it as much towards longer wavelengths. To demonstrate the behaviour of the two hydroxybenzophenone isomers, their effect in chlorine containing polyester resin has been determined (see Fig. 8). This substrate is easy to work with, since it allows a quantitative follow-up of the protective potence of an additive by simple spectrophotometry of the photoindiced discolouration of the test specimens. The difference between the two isomers is striking: the o-hydroxybenzophenone
119
Protection of Polymers Against Light Irradiation
=
R
OCO-R
-o-OH
CD
-0
®
20.000
HO
yOMe@
E
HO
- - I n chloroform - -In dioxane/water
nm
FIG. 7. Spectra of hydroxybenzophenones in chloroform and water/dioxane. (1) p-hydroxybenzophenone (2) o-hydroxybenzophenone (3) 2-hydroxy-4-methoxybenzophenone. 100
Oh 80
, I
./
...- 180 h
./
/
/
/ /
'I 60
-
---...---
...----
/
1000h /
/
I"
/
/
/D
"10
/
U:lJ
t
/
40
/
o
/ /
OH
I In polyesterresin 180 h
20
--Controls
500
600
nm
FIG. 8. Transmissions of polyester resin plates containing hydroxybenzophenones (0' 25 per cent) after 180 hr. Fadeometer irradiation. o-hydroxybenzophenone (1) p-hydroxybenzophenone (2) controls after irradiation of indicated duration (- - )
is a good light stabilizer, whereas the p-hydroxyderivative acts as a photosensitizer, since its test specimen is more discoloured after 180 hr than the control after 1000 hr. The failure of the p-isomer to provide protection is a consequence not of the presence
H. J. HELLER
120
of a p-hydroxy group, but of the absence of a o-hydroxy group. 2,4-dihydroxybenzophenone is a very good light stabilizer. The influence of other substituents in the c-hydroxybenzophenone system has been the subject of many investigations. Technically, however, only compounds like those in Table 2 have been successful. 2
TABLE
(a)
a (b)
=
lOR
1
RO
Cyasorb 9 Uvinul M 40 Anti UV-A Avastab 45 Merk 4588 Riedel HMB ASL24 Cyasorb 531 Uvinul410 Eastman Inhibitor DOBP
(Cyanamid) (Antara, Ward Blenkensop) (Organo-Synthese) (Deutsche Advance)
Cyasorb 24 Avastab 47
(Cyanamid) (Deutsche Advance)
C sH 17 :
Cyasorb 315
(Cyanamid)
R=H:
Uvinul D 50
(Antara)
CH 3 :
Uvinul D 49 Uvinul490
(Antara) (Antata, mixture)
CH 3 :
C SH 1 7 : C lOH2 1 : C UH 2 S : R=CH 3 :
(Shonan Jap.) (Cyanamid) (Antara)
::::--..
OR OH
HO
(c)
(Antara) (Deutsche Advance) (Organo-Synthese) (Japan)
co~
&oot& 0-.-
Uvinul400 Avastab 48 Anti UV-D Alichte-90
R=H:
OH
hoo~
OR
R= OCO-R
20,000
In chloroform
E
-oOM. HO Alk
CD
-oOM. HO
®
-Q-OM. HO Alk
@
nm
FIG. 9. Spectra of benzophenones in chloroform. 2-hydroxy-4-methoxybenzophenone (1) 2-hydroxy-4-methoxy-5-ethylbenzophenone (2) 2-hydroxy-3-methyl-4-methoxybenzophenone (3).
Protection of Polymers Against Light Irradiation
121
Among other o-hydroxybenzophenones there are many good light stabilizers which, however, are not easily accessible and/or cause undue yellow initial colour in the substrate. As an example for initial discolouration, the spectra of two simple a1kylderivatives of Uvinul M 40 are shown in Fig. 9. The hydroxyxanthonesv'?
resemble the o-hydroxybenzophenones in their light fastness and protective power. Unfortunately, the absorption of these heterocyclic compounds, which are stable even in nitrocellulose, is not very suitable. A rather weak band is found close to the visible, which renders the products yellowish in spite of a poor absorption in the long wave u.v.-range. From the above, it should not be concluded that all aromatic ketones with chelatable hydroxy groups are lightfast. For instance, the l-hydroxy-3-phenyl-fluorenone-(9) I and the I-hydroxypleiadene-7,12-dione II are quite sensitive to light.
o
I
II
(2) Absorbers with nitrogen acceptors Besides carbonyl groups, nitrogen functions are known to be acceptors of hydrogen bonds. The question arises which kind of nitrogen-containing structures would be useful. Open chain derivatives such as Schiff bases, hydrazones and azines of salicylaldehyde have at one time or another been suggested as u.v-absorbers. They do absorb but unfortunately a little too much of the visible light so that they are quite yellowish. Due to this and also on account of their poor light fastness, they have never reached any technical importance. Among the ring compounds, cyclic imino ethers and amidine derivatives have been suggested as u.v-absorbers,
(i) Benzoxazoles, benzimidazoles and benzothiazoles Representative compounds of this structural type'"?
z=o, S, NR3 absorb nicely. POLYMER-I
122
H. J. HELLER
The benzoxazoles are quite lightstable but provide no protection, while the benzimidazoles are less lightfast and act as photosensitizers. The benzothiazoles finally are themselves quite light sensitive. (ii) Oxdiazolesand triazoles
1,3,4-oxdiazoles (III)<8) and the different isomers of the 1,2,4-triazoles (Ill,IV) are not suitable as light stabilizers, because N-N
OH
R,)lZ~
Z=O; NR3
R2
III
IV they sensitize the discolouration of polyester resin in spite of a quite remarkable light fastness. Why are the above structural analogues containing a -OH.. .N= hydrogen-bond so different from the o-hydroxybenzophenones? Is this correlated to the fact that nitrogen functions are much more basic than oxygen functions of the same hybridization? Qualitatively, it is found that the benzophenones really are the least basic of the compounds mentioned. A connection between this finding and the lightstabilizing effect of a compound is provided by the spectra. In Fig. 10 the molar extinctions of 2-(2-hydroxyphenyl)-benzoxazole and 2-hydroxy-4-methoxybenzophenone are given. The difference in the fine-structure of these two compounds is evident. The benzophenone derivative has to be quite a flexible molecule, as is indicated by the complete absence of a spectral fine structure even in chloroform in which the internal hydrogen bond should help to restrict the vibrational mobility of the molecule. In contrast thereto, the benzoxazole compound seems to possess a rather rigid molecule even in methanol which has a tendency to disrupt internal hydrogen bonds. This difference in the fine structure of the two spectra could be explained by the assumption that the hydrogen bond in the benzoxazole is strong enough to force the two ring systems of the molecule into a fixed coplanar position, while a certain freedom of rotation about the bond connecting the resorcinol moiety to the carbonyl carbon still exists in the benzophenone derivative. In this way, the hydrogen bond would successively be stretched beyond the breaking point and reformed again. This would not only explain the absence of fine structure in the spectrum, but also the capacity to dissipate energy harmlessly by rotation of the two ring systems and the extreme vibrations of the proton connected therewith. This idea can be tested by checking the behaviour of other known and new u.v.absorbing light stabilizers. Heterocycles with very weakly basic trigonal nitrogen
Protection of Polymers Against Light Irradiation
123
HO
CD
CX:X> HO
20,000
®
O?rD-oMe 0
CD
In methanol
®
In chloroform
320
nm
FIG. 10. Spectra of 2-(2-hydroxyphenyl)-benzoxazole (I) in methanol and 2-hydroxy-4methoxybenzophenone (2) in chloroform.
atoms, in the o-position of which an o-hydroxyphenylrest is placed, should in principle yield useful u.v.-absorbers. (iii) pyrimidines and chinazolines
These two classes of compounds have not yet yielded technically satisfactory light stabilizers. Though in both cases judicious substitution leads to lightfast products, the pyrimidines suffer from insufficient protective ability and the chinazolines are too strongly coloured. (iv) s-triazines It is known that s-triazines are less basic than analogous pyrimidines. This class of compounds has therefore been carefully investigated by American Cyanamid, (9) Ciba(10) and ourselves. (11) The tris-o-hydroxyphenyl-s-triazines are extremely lightfast, but quite yellow and of insufficientcompatibility in most polymers. Applicationally better suited are asymmetric compounds with at least one of R 1 and R 2 being a non-aromatic substituent.
Spectrum, lightfastness and protective power of these s-triazines are influenced mostly by (') the number of the e-hydroxyphenyl groups and
124
H.J. HELLER
(") the basicity of the ring nitrogens, i.e. by the nature of R 1 and R2 (aryl and alkyl groups bound directly or by way of a heteroatom). The following rules apply as a first approximation: Derivatives with a higher number of o-hydroxyphenyl radicals have better protective power and stronger absorption in the long wavelength u.v. than analogues with fewer of such moieties. The more the basicity of the s-triazine ring is lowered by the substituents, the higher is the lightfastness of the resulting compounds. Thus the 2-(2-hydroxyphenyl)-4,6-dialkylamino-s-triazines are quite light-unstable and tend to accelerate the discolouration of polyester resins; the 2-(2-hydroxyphenyl) -4,6-dialkyl-s-triazines are of mediocre lightfastness and weak stabilizing effect; the 2-(2-hydroxyphenyl)-4,6-diphenyl-s-triazines are quite lightstable and provide a mediocre light protection. In the case of 2,4-bis-(2-hydroxyphenyl)-s-triazines, the nature of the third substituent in the 6-position is less pronounced. Alkylamino groups still yield unsatisfactory products. Alkyl- and arylmercaptogroups and, even more so, alkyl- and aryloxygroups lead to compounds of good stability and protective power. Finally, 2,4-bis(2-hydroxyphenyl)-6-phenyl-s-triazines are incredibly lightfast. In thin acetyl cellulose films, they are not changed measurable after 500 hr of irradiation in the fadeometer. Under these conditions, they are degraded only in nitrocellulose. Dr. Brunetti of our laboratories has worked out a convenient method for the synthesis of these compounds
lH
o
~~ 19lo/
Rr-l \
N,.Hz
R
where R stands for an arylradical, in particular a o-hydroxyphenyl group, and R 1 for a hydrocarbon radical or an aryl- or alkoxy- or alkyl- mercaptogroup. Since the analogous o-hydroxynaphtyl compounds are easily made and have also been suggested as lightstabilizers, we have tested them in non-flammable polyester resin. As is indicated by Fig. 11, both the 2-(1-hydroxy-naphth-2-yl)- and the 2-(3hydroxy-naphth-2-yl)-4,6-dimethyl-s-triazinestrongly accelerate the photolysis whereas the 2-(2-hydroxyphenyl)-analogue provides a weak, but distinctly measureable, stabilization. (v) Benzotriazoles So far, we have dealt with compounds formally derived from salicylaldehyde or salicylic acid, i.e. compounds in which the o-hydroxyphenylrest is bound to a carbon atom. This is, however, not absolutely essential. In dyestuff chemistry, a number of carbon-free chromophores are known. The most common in this connection is the azo group but the open chain e-hydroxyphenylazo-compounds, which frequently are of quite good light fastness and possess even light stabilizing effects, are all yellow to red dyestuffs.
Protection of Polymers Against Light Irradiation
125
100
I
1 1_ -
-
-
/l
I
80
I I
/
I
/
I I
/
I
60
I
R
Y
%
NJ. N
J.. )....
/1
Me N
/
40
Me
/ / /
R=
/
CD ~
20
@ 0
HO-b HoB HO~ 600
nm
11. Transmission of polyester resin plates containing s-triazines (0 ' 25 per cent) after 250 hr fadeometer irradiation. 2-(2-hydroxyphenyIH,6-dimethyl-s-triazine (l), 2-(l-hydroxynaphth-2-yl)-4,6-dimethyl-s-triazine (2), 2-(3-hydroxynaphth-2-yl)-4,6-dimethyl-s-triazine (3), and cont rols (--) of which the upper curve corresponds to the sample before irradiation. FIG.
A way to lessen the chromophor-effect of the azo group is also known from dyestuff chemistry viz.: transformation into a vic-triazole ring which represents some kind of an internal shortcircuit:
-'~'Y -
·-
H2N
This group of compounds has been investigated in our laboratories. (12) Their structure is best represented by the following limiting structures N
N
G
©:~>y - ©:>y
0"\
/ "0
a>>G'> HO
of which the quinoid form probably contributes the least.
126
H.J. HELLER
As in the systems mentioned before, the o-hydroxyphenyl group is essential for lightfastness and stabilizing potency. So far, no equivalent has been found. o-Acylaminophenyl-, 2-hydroxynaphth-l-yl- and l-phenyl-5-hydroxypyrazole-3-yl-benzotriazoles all are not useful u.v-absorbers. In Fig. 12 the spectra of the simplest hydroxyphenylbenzotriazoles are given. The introduction of an auxochrom into the p-position of the phenyl ring of 2-phenylbenzotriazole leads to a blue shift. This effect of a p-auxochrome can be cancelled
nm
FIG. 12. Spectra of benzotriazoles in chloroform. 2cphenylbenzotriazole (1), 2-(4-hydroxy-3-methylphenyl)-benzotriazole (2), 2-(2-hydroxy-5methylphenyl)-benzotriazole (3), 2-(4-hydroxy-2-methylphenyl)-benzotriazole (4).
by introduction of a further substituent into the phenyl ring in o-position to the triazol ring (curve 4). Obviously, the steric hindrance by an o-substituent is big enough to twist the phenyl ring so strongly out of the plane of the benzotriazole ring that conjugation between the two systems is substantially killed. The hydroxy group being of similar size as the methyl group should also force the phenyl ring out of plane. On the other hand, the energy of a hydrogen bond can only be gained of the two halves of the molecule are coplanar. It seems that the two extreme positions-the sterically preferred conformation with the planes of the phenyl and triazole rings vertical, and the hydrogen bonded coplanar conformation-differ little in energy and are comparably populated. Intermediate conformations are less preferred and hence less populated. According to the FrankCondon principle, 2-(2-hydroxyphenyl)-benzotriazoles should exhibit two absorption peaks, the one at short wavelengths corresponding to the out-of-plane conformation. This assumption is seeminglyborne out by the spectrum (curve 3, Fig. 12). In this connection it might be worth-while to recall the fact that o-hydroxybenzophenones also show a double peak and p-hydroxybenzophenones only a single one. A first test of this assumption is provided by the solvent effect upon the spectrum. A weakening of the hydrogen bond should lessen the intensity of the long wavelength peak. In Fig. 13 this is demonstrated. By far the least intensity of the long wavelength
Protection of Polymers Against Light Irradiation
127
CD in Heptane ® in Dioxanel
water @in Dioxanel NaOH t::. in Methanol
15,000
nm
FIG. 13. Spectra of 2-(2-hydroxy-5-methylphenyI)-benzotriazole in heptane (1), dioxane-water (2) and in dioxane-IN NaOH (3). The long wavelength maximum in methanol is indicated by a triangle .
peak is found in the anion which cannot form a hydrogen bond. The surprising phenomenon that the deprotonation of the hydroxygroup to the much more efficient auxochrom -0 9 does not lead to an increase of the absorption is typical for o-hydroxyphenyl light stabilizers. Naturally, the quantitative aspect of this anomaly depends upon the steric requirements of the particular molecular species. From Fig. 14, it is evident that the s-triazines are somewhat less sensitive to steric hindrance than the benzotriazoles. The enormous difference between the behaviours of the 0- and pisomers illustrates the importance of this effect even in this class of compounds.
In dioxane - water In dioxane-sodiumhydroxide
20,000
10,000
28 0
300
34 0
320
nm
FIG. 14. Spectra of 2-(4-hydroxyphenyIH,6-dimethyl-3-triazine (1) and 2-(2-hydroxyphenyIH, 6-dimethyl-s-triazine (2) in dioxane water (---) and dioxane-IN NaOH (-).
128
H. J. HELLER
Bulky substituents like tert.alkyl groups in o-position to the hydroxygroup protect the hydrogen bond and lessen the influence of hydrogen bond breaking solvents. This is illustrated in Fig. 15.
15,000
10,000 E
Bultl
o:N~-Q N HO
CD In ® In
Heptane Dioxane-water 6 In Methanol
Bult)
nm
FIG. 15. Spectra of 2-(2-hydroxy-3,5-di-tert-butyIphenyl)-benzotriazole in heptane (1) and dioxane-water (2). The long wavelength maximum in methanol is indicated by a triangle.
A second test of our assumption is demonstrated in Fig. 16. A second steric hindrance by a group in the other o-position ofthe phenyl ring cannot be compensated anymore by the energy of the hydrogen bond: the long-wavelength peak disappears almost completely (curve 1). A second hydroxy-group in this position stabilizes the
nm
FIG. 16. Spectra of 2-(2-hydroxy-3-tert-butyl-5,6-dimethylphenyl-benzotriazole (1), 2-(2hydroxy-3,5-di-tert-butylphenyl)-benzotriazole (2) and 2-(2,6-dihydroxy-3,5-di-tert-butylphenyl)-benzotriazole in chloroform.
129
Protection of Polymers Against Light Irradiation
coplanar configuration strongly without stiffening the molecule to such an extent as to produce spectral fine structure (curve 3). Besidesthese steric effects, conjugation can also strongly influence the energy difference between the two conformations. Strong electron donators in p-position to the triazole acceptor increase the stability of the coplanar configuration by resonance. This leads to an increase of the intensity of the long wavelength peak and at the same time to a decrease in its band-width. The same resonance can, however, diminish the electron withdrawing power of the acceptor so that this intensified peak appears at a shorter wavelength. This effect has been found in the benzophenone-series (Fig. 7), but is also pronounced with the s-triazines (Fig. 17). In the case of the benzotrazoles the "capacity" of the acceptor is big enough, so that a small bathochromic effect of the auxochrome is retained (Fig. 18). The decrease of the short wavelength peak is very pronounced also in this case. Me
)-N
N
rN>-R Me
R=
-0-0 Alkyl (])
HO
-b
HO
20,000
®
-o0Alkyl@ QAlkyl
-0
280
@
300
nm
FIG. 17. Spectra of s-triazines in chloroform (-) and dimethylformamide-water (--) 2-(4-methoxyphenyl)-4,6-dimethylos-triazine (1) 2-(2-hydroxyphenyl)-4,6-dimethyl-s-triazine (2) 2-(2-hydroxy-4-methoxyphenyl)-4,6-dimethyl-s-triazine (3) 2-(2-methoxyphenyl)-4,6-dimethyl-s-triazine (4).
What is then the effect of substituents in other positions? A decrease in the acidity of the hydroxy group lessensits ability to form stable hydrogen bonds. This produces a decrease of the long wavelength extinction. The concomitant increase in electronegativity of the phenyl ring system lowers the charge-transfer energy which leads to a bathochromic shift of this weakened peak. Figure 19 demonstrates this effect. Acidifying substituents in the benzotriazole moiety provoke similar effects (Fig. 20, curves 1, 2, 3). The influence upon the extinction coefficients is not so cleancut, because the influence of such acidifying groups upon electronegativity and proton basicity is not quite parallel. Basifying substituents in the acceptor moiety increase the stability of the hydrogen bond and promote the long wavelength absorption (Fig. 20, curves 5 and 6). The bathochromic effect of the methoxygroup is, however, not clear.
130
H.J. HELLER
O:Np-yR N HO
20,000
In chloroform
280
300 nm
FIG. 18. Spectra of 2-(2-hydroxy-4-methoxyphenyl)-benzotriazole (1), 2-(2-hydroxyphenyl)-benzotriazole (2), and 2-(2-methoxy-5-methylphenyl)-benzotriazole (3) in chloroform.
in Chloro form
/
280
30 0
340
320
36 0
nm
FIG. 19. Spectra of benzotriazoles in chloroform. 2-(2-hydroxy-5-methoxyphenyl)-benzotriazole (1) 2-(2-hydroxy-5-acetylaminophenyl)-benzotriazole (2) 2-(2-hydroxy-5-phenylphenyl)-benzotriazole (3) 2-(2-hydroxy-5-methylphenyl)-benzotriazole (4) 2-(2-hydroxy-5-chlorphenyl)-benzotriazole (5) 2-(2-hydroxyphenyl)-benzotriazole (6) 2-(2-hydroxy-5-carbobutoxyphenyl)-benzotriazole (7)
In Fig. 21 the effects of basifying substituents, exemplified by the methoxygroup , are summarized. The influence of such a group in o-position to the hydroxygroup is interesting. It competes with the nitrogen for the hydrogen and hence lessens the longwavelength peak.
Protection of Polymers Against Light Irradiation
0
20,000
R
13l
CH 3
>-o HO
In chloroform
E
10,000
nm
FIG. 20. Spectra of benzotriazoles in chloroform. 2-(2-hydroxy-5-methylphenyl)-5-phenoxysulfonyl-benzotriazoIe (1) 2-(2-hydroxy-5-methylphenyl)-5-carbobutoxy-benzotriazole (2) 2-(2-hydroxy-5-methylphenyl)-benzotriazole (3) 2-(2-hydroxy-5-methylphenyl)-5-chloro-benzotriazole (4) 2-(2-hydroxy-5-methylphenyl)-5-methyl-benzotriazole (5) 2-(2-hydroxy-5-methylphenyl)-5-methoxy-benzotriazole (6).
CXN~_(-r~OCH3 (if 20,000
N HO~J
in Chloroform
10,000
360
nm
FIG. 21. Spectra of benzotriazoles in chloroform. 2-(2-hydroxy-5-methylphenyl)-5-methoxybenzotriazole (1) 2-(2-hydroxy-3-methoxy-5-methylphenyl)-benzotriazole (2) 2-(2-hydroxy-4-methoxyphenyl)-benzotriazole (3) 2-(2-hydroxy-5-methoxyphenyl)-benzotriazole (4).
(3) Alkali insensitive u.v.-absorbers All these o-hydroxyphenyl-u.v.-absorbers have one common drawback: they are sensitive to alkali and heavy metal ions. In plastic applications this is not a severe limitation. In the few cases in which metals lead to discolourations, such as in cobaltcures for polyester resins, the use of sterically hindered products (cf. Fig. 15) produces
132
H.J.HELLER
good results. For textile application, however, the alkali sensitivity is a severe shortcoming. A search for compounds free of phenolic groups has therefore been under way in many laboratories. However, only one suitable class of compounds has been uncovered so far, viz. the cinnamic acid derivatives. The most lightfast and oxidation resistant members of this class are
Ph
CN
"'-C=C/ '/ "'Ph R
R=CN
(Uvinul N 38)
COOEt (Uvinul N 35)
The sole drawback of these products is their low absorbancy. Somewhat less efficient are partially aliphatic analogues:
Ph
CN
"'C=C - / / "'-COOR CH
R=Me (Bayer 318) Bu (Bayer 317)
3
In conclusion, it can be said that u.v.-absorbers are yet to be found having the alkali insensitivity which allows their textile application and the high absorptivity which makes them commercially acceptable. REFERENCES (1) J. G. Calvert and J. N. Pitts Jr. Photochemistry, Wiley (1966) (Ketone no. 42). As this book excellently covers the pertinent literature, only very specific citations outside the realm of this book will be given in the following. (2) U.S. Pat. 2 971 940; U.S. Pat. 2971 941 (priority date 20.3.59), Ferro Corp. (3) U.K. Pat. 943 081 (18.10.62), American Cyanamid. (4) U.S. Pat. 3 310575 (21.3.67), Geigy. (5) 1968 Modern Plastics Encyclopedia, Intemat. edn. p. 407, Vol. 45/No. lA, McGraw-Hill, New York (1967). (6) FR Pat. 1 279690 (priority date 18.1.60), Geigy. FR Pat. 1 346787 (priority date 9.2.62), ICI. (7) Swiss Pat. 350763 (priority date 14.10.58), Ciba. Belg. Pat. 583 552 (priority date 13.10.59), Ciba. (8) FR Pat. 1 127243 (priority date 5.11.54), Ciba. (9) Belg. Pat. 614'726 (priority date 6.3.61), American Cyanamid. (10) Belg. 639'329 (priority date 30.10.62), 639'330 (30.10.62), 642'767 (25.1.63), 643'432 (7.2.63), 643'898 (18.2.63), 650'932 (26.7.63), 658'233 (14.1.64), 661'225 (18.3.64), 663'345 (4.5.64), 667'044 (17.7.64),679'733 (20.4.65), Ciba. (11) Belg. Pat. 661'186661'187661'188661'189661'190 (priority date 24.1.63), Geigy. (12) FR. Pat. 1 195207 (priority date 14.12.56), FR. Pat. 1 324897; FR. Pat. 1 324898; FR. Pat. 1 324899; FR. Pat. 1 324900; FR. Pat. 1 325403; FR. Pat. 1 325404; FR. Pat. 1 325405; FR. Pat. 1 330378; FR. Pat. 1 330379 (all priority date 16.6.61), Geigy.