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Ageing and degradation of paint film media
Progress in Orgmic Comings Elsevicr Sequoia S.A., Lausannc Printed in Eklgium
AGEING EMIL
AND DEGRADATION
KREJCAR
AtGD
OTAKAR
Research Institute for Synthetic
OF PAINT FILM
MEDIA
KOLAR
Resins and Lacquers,
Pardubice (CSSR)
CONTENTS
1 Inteoduction, 249 2 Chemical processes during the ageing of paint films, 250
2.1 EfFect of U.V. radiation, 251 2.2 The determination of chemical changes, 253 2.2.1 Infrared spectroscopy, 253 2.2.2 Study of volatile products, 253 2.2.3 Methods of chemical analysis, 256 3 Physical processes during the ageing of paint films, 256 3.1 Paint tilms as organic glasses, 257 3.2 Volume changes in paint films, 258 3.3 Adhesion and internal stresses, 259 3.4 Material constants and practical mechanical tests, 259 3.5 Methods for the determination of changes in physical and mechanical properties of paint films during ageing and degradation, 260 3.5.1 Determination of glass transition temperature, 260 3.5.2 Determination of internal stress, 261 3.5.3
Determination
of adhesion,
261
3.6 The relevance of physical measurements to the selection of film-farmers, 261
4 Electron microscopy, 263 5 Conclusions, 264 References, 264 I. INTRODU~ION
The problems of ageing and degradation of macromolecular substances are usually considered in relation to some kind of final product made from them, e.g. a moulding, a sheet or a paint film. It is the last of these with which the present paper is concerned. The use of paint lilms to protect a substrate against the influence of various media raises the question of service life, i.e. the period during which the substrate is protected before the film deteriorates. The ageing of a paint film embraces all the physical, physico-chemical and chemical processes occurring in the paint film after its application to the substrate, its drying and hardening, up to and including its deterioration. The important criteria are the rate at which a hard, homogeneous film exhibiting optimum properties is formed and the period during which these properties Progr. Org. Coatings,
Z (1972/73)
249
are preserved before the film deteriorates. This period is usuahy referred to as the “service life” of the film. Talen has classified film-forming processes into a number of groupsl. Physical film formation by solvent evaporation yields a l3m which already exhibits optimum properties typical of the resin employed. On the other hand, when chemical or physico-chemical processes are involved in drying, a film is obtained which, after solvent evaporation or, in the case of solventless coating materials, after initial setting, does not yet possess the required properties; to develop these a certain length of time is necessary for reaction to occur. Oxidative polymerisation and polycondensation at higher temperatures are examples. Until optimum properties of the paint film are developed, various factors are exerting either favourable or unfavourable influences on film formation. Favourable factors include, among others, elevated temperature, low atmospheric humidity, abundance of sunlight, calm air and absence of dust. Unfavourable factors include low temperature, high humidity, lack of sunlight, wind, dust and rain. These factors are, of course, most relevant when application occurs under normal weather conditions and when the films dry chemically.
Assuming the film to have attained optimum properties, the reasons for its ageing and deterioration are, in the first place, effects of the surroundings2. Predominant among these are temperature and sudden temperature changes, intense sunlight, oxygen, ozone, water in whatever form (atmospheric humidity, dew, rain or snow) and certain microorganisms (mildew, fungi). Atmospheric contaminants like sulphur dioxide and hydrogen sulphide are harmful, while air-borne solids, particularly in combination with wind, cause erosion of the paint film. A marked influence is also exerted by thermal, mechanical and electrical strains which may exacerbate the unfavourable effects of the surroundings. These unfavourable effects of the surroundings can cause chemical changes which, in turn, can change the physical properties of the paint film and may additionally cause detachment from the substrate. Surface cracks, in particular, speed up chemical changes, e.g. oxidation and hydrolysis3. 2. CHEMICAL
CHANGES
Basic chemical divided into’
DURING
processes
THE
occurring
AGEING
FILMS
during ageing of paint films can be sub-
(a) chemical processes isothermal processes,
taking place regardless
(b) chemical
involving the surroundings.
processes
OF PAINT
of the surroundings,
i.e. spontaneous
From the practical point of view, weathering of paint films is of primary importance, but sometimes also chemicals, higher temperatures, etc. are effective. Some of these factors affecting paint films may, of course, predominate while others may be suppressed, but it should be made clear that the effects of separate factors are not 250
Progr. Org. Coorings, 1 (1972173)
simply additive, because in combination they can work both against and with each other. The service life of a paint film is naturally influenced, above all, by the chemical structure of the binder’. Many binders based upon drying oils give films possessing high weather resistance. ?‘hey are, however, susceptible to oxidation and, moreover, by ultraviolet radiation, peroxides are formed. These are unstable6 and their decomposition yields free radicals which cause, on the one hand, break-up of molecules and, on the other, intermolecular reactions accompanied by formation of three-dimensional networks. The mechanism of formation and decomposition of hydroperoxides has been described in a series of papers7 ,‘. Crosslinking in binders evidently leads to increasing hardness and brittleness of films. Weathering causes an increase in the content of degradation products, some of which escape into the air. At the same time, the film becomes progressively more hydrophilic so that it can absorb increasing quantities of water which cause, in turn, a higher degree of swelling and increased mechanical stress. Depending upon the relationship between deformation forces, adhesion and cohesion, the paint film may crack and peel off either at the surface or down to the very substrate’. Naturally, also, the weight of the film will change, because gaseous products escape, and low molecular degradation products are leached out by water. These low molecular compounds are formed by chain scission. Gaseous products arise most frequently through scission in the neighbourhood of end-groups. 2.1 Eflect of U.V. radiation In
studying the effects of ultraviolet
radiation,
two factors have to be considered,
namely the wavelength and the intensity of the radiation_ At a given wavelength the effect is proportional to the energy absorbed, which, in turn, depends upon the intensity of incident radiation, and upon the absorptivity of the medium”. The radiation will affect the whole thickness of the film only in completely transparent, non-absorbing pr very thin films. As far as the polymer absorbs the radiation of a given wavelength range, a gradient of changes related to the varying degree of crosslinking, chain scissioning and impaired mechanical properties can be expected to appear across the thickness of the film. For example, the study of degradation of films from drying oils by ultraviolet radiation has revealed’ ’ that gaseous products of lower H/C ratio are liberated more readily from the surface of the film than from its interior. A high hydrogen content of the volatile products (particularly water) has been taken to indicate, predominantly, polycondensation reactions leading to crosslinking of the polymer. Carbon is present mainly in the skeleton of the polymer so that, with some reservation, the presence of carbon oxides in volatile products points to reactions leading to the break-up of chains. Deterioration of paint films, particularly of the drying oil type, changes the behaviour of films towards water and other solvents. Ultraviolet radiation causes functional groups to be formed at the surface of the frim, and their hydrophilic nature diminishes the contact angle with water so that the surface is more readily Progr. Org. Coalings, I (1972/73)
251
wetted; thus the erosive effect of water is enhanced. The analysis of leached products suggests that water itself does not participate to a significant extent in the degradation reactions but it leaches soluble products of the oxidative degradation process. Thus, after the water has evaporated, formation of mechanical tensions in the film or in the film-substrate boundary is favoured. In this way, among other effects, the swellability of surface layers in the film is increased, while the reverse may be true of the interior due to network formation within the film. The influence of wavelength has been clearly demonstrated in studies of the degradation of oil-modified alkyd afilms”. At wavelengths below 250 run, it is the aromatic part of the alkyd that is degraded very quickly and preferentially; at wavelengths above 295 nm, the aromatic structures are much more stable than the aliphatic structures (Figs. 1, 2 and 3). At the higher wavelengths, the degradation of aromatic nucIei is a secondary process which is evidently caused by decomposition of attached
120 -
Fig.
1. Disappearance
I
I
20
I
40
Fig. 2. Disappearance
252
of functional
I
60
groups during exposure
,
80
,jOzrs c
of functional
120
to unfiltered
light12.
I
740
groups during exposure to wavelengths Progr.
~250
nm A * 2.
Org. Coatings,
I (1972/73)
20 t
1
I
40
20
Fig.
3. Disappearance
aliphatic
60
80 100 HOUt-5
of functional
I
120
I
140
groups during exposure
to wavelengths
>?95
nm .&I *.
groups.
As cured films contain few double bonds, oxidation is likely to hydrogen of crosslinked units. This mechanism is suggested by comparison with an alkyd resin modified by stearic acid, which is much more resistant towards photo-oxidation not only in the aliphatic, but also in the aromatic part of start at the tertiary
the molecule. 2.2
Tile determinaiion of chemical charlges
In the study of chemical changes occurring during the ageing of paint films, photometric methods have mostly been employed, and infrared techniques have been preferred to the study of the ultraviolet or visible regions. Classical analytical methods have also been applied in special cases. Paper and column chromatography, and gel permeation chromatography may be used to study soluble polymers. Degradation products, particularly those formed at higher temperatures, can readily be studied by gas chromatography alone or in combination with mass spectroscopy. Radical reactions occurring during the ageing of soluble polymers may be studied by electron paramagnetic resonance. Wide-band nuclear magnetic resonance spectrozcopy is useful for the study of the network formation. 2.2. I
hfrared
spectroscopy
The main advantage of infrared spectroscopy is its ability to detect a wide variety of chemical changes occurring during the ageing of films. In Fig. 4 changes in the infrared spectrum are shown when an alkyd resin film was exposed to U.V. irradiation. The appearance or disappearance of some functional groups in macromolecular substances can be followed either directly or using differential techniques’ 3. The latter have been employed for investigating the influence on ethyl cellulose films of the surrounding atmosphere during ultraviolet irradiation at various wavelengths. In order to determine the depth of effect of individual weathering factors, infrared spectra of successively stripped layers of the film can be examined, or the attenuated total reflection (ATR) technique can be applied, which enables changes in both the exposed Progr.
Org. Coatings, I (1972/73)
253
--BEFORE
I
I
2
3
4
5
Fig. 4. I.R. spectrum
6
of drying
7
15 h U.V. A > 3000
---AWER
135 h
.
6 Micrgons
EXPOSURE
..-.-..AFTER
U.V.
I
10
I
I
11
12
13
14
oil alkyd before and after exposure”.
and protected film layers to be studied r4*’ 5. Infrared spectroscopy can also be used for the determination of volatile products of paint film photodegradation. In the photodegradation of cellulose nitrate, vinyl polymers and alkyd resins, evolved carbon monoxide and dioxide, formic acid and possibly also water can be determined’ 6. In presence of rutile titanium dioxide the composition of the volatile components does not change, but the rate of formation is reduced.
TABLE
1
INFRARED
PEAKS
The per cent absorption Peak (cm-
1)
OF THE
VOLATILE
is given for the products
Peak assignmenl
Alkyd ffg
721 730 912 950 992 1034
1105 1271 1307 1370
1750= 1777 1792 2120 2160 2360 2960
Carbon dioxide Acetylene Vinyl group Ethylene Vinyl group Methanol
of 6-hour
Xe
-
FILMSz3
3.2
6.2 1.8 2.0
-
Vinyl-alkyd Xe
Hg
0.6 1.4
0.8
VEHICLE
arc and %I-hour xenon arc irradiations
Oil vehicle
oehicle
-
FROM
mercury
-
15.4 0.8 2.2 -
2.0
1.0
1.6
Formic acid 2.8 Acetone Methane 0.6 Acetyl group 0.6 Carbonyl group 6 Vinyl ester 2.8 (Unassigned) 2.4 Carbon monoxide 6.4 Carbon monoxide 7.8 Carbon dioxide >34 Carbon-hydrogen linkages 2.4
1.0 0.6
1.8
3.0 0.6
0.8 4.6 -7 3.2 2.2 8.2 9.4 >78 4.8
4.8 14 -6 8.0 9.8 97 7.2
- No apparent for the products
354
2.6 0.3
PRODUCTS
-6 -3.4 5.8 6.4 >61 4.0
-
peak. “Partially hydrolysed vinyl chloride-vinyl from the polyvinyl asetate and for the products
Xe
Hg
1.6 0.8
-
0.6 -
-
vehich
4.2 1.6 b -
4 ~2.6 -2.0 5.4 6.4 ~-23 1.6
-
6.4 0.6 1.2 0.6 0.4
-6 -5.6 3.2 4.6 78 4.4
acetate copolymer. bIndetinite because of from the xenon arc irradiation of the other Progr.
Org.
Coatings,
I (1972/73)
2.2.2 Stud]) of volatile products The examination of volatile products of photodegradation can be carried out directly in a special gas cell with silica walls’ 7. This method was employed in examining the photodegradation caused by the radiation of mercury and xenon lamps in 6lms from alkyd resins, oils, pclyvinyl acetate, epoxide resins and other vehicles. A survey of results is given in Table I. In the spectruin of the volatile products, carbon monoxide and dioxide, acetone, formic acid, methane, ethylene and acetylene, as well as compounds containing carbonyl and vinyl groups, have been identified. In the volatile products of all binders, carbon dioxide was predominant, but was relatively more abundant in those of media drying by oxidation rather than by polymerisation or evaporation_ In binders drying by oxidative reactions, more ethylene was found in the volatile products when the xenon rather than the mercury lamp was used. Acetylene and methane, on the other hand, were found only during radiation with the mercury lamp. These results clearly demonstrate the influence of wavelength distribution on the course of the photo-oxidation reactions. Volatile products which are formed during photo-oxidation and thermodegradation can readily be investigated by gas chromatography. This method has proved particularly useful in studyin, 0 the thermal degradation of epoxide resins” and some Gas chromatography is also recommended as a rapid hydrocarbon polymers’g.
Vinvivehicle
Vin_yI vehicle (hydrolyses)
Xe
Hg
Jfg
0.8
-
2.4
3.8 11.6 0.6 2.4 -
5.2
0.6
2.4 7.2 4.0
2.4 1.4 0.4
11.4 -
5.0 1.6
7.6
-
17.4 -3 -5 -29
9 14 -
-9
3.2
-13 5 5 56 -8
1.8 11.0 -
4.6
14 -9.8
-
3.0
12 -5 -3
7.6
18 -
5.8 7.8 >20
Hg
4.2
-
11.8 -
16 -
-10
Xe
3.6 4.2 >25
7.6
9.2
Epoxy celricie (anline-cared)
Po!_winyl acelaie
-
3.0 5.6 1.8 6.8 4.4
-11 -3 -12 -37
-20 -17 9.4 10.5 68 12.2
Xe
M3 2.0
0.4 -
4.0 8.8
-
13.2
2.2 0.8
-
-
9.4
0.4 0.6 > 8.0 10.8
-
5.2 4.6 0.6 3 8
2.4
5.4
2.4 3.4
-
3.0 -5 -
6.2
-
4.8
-
0.4 2.0
-
2.0 1.6
.-3 -
6.0 7.0
0.6 0.8
>21
3.6 2.4 0.6 2 4
3.0
-
0.6
4.0
1.0
-
-
1.8
-
7.4
>51 w-8
0.4
-
Xe
Hs
-
2.2
7.8
15.0 -
Xe
Epos_~. cchiclc (polyamide-cured)
>34 -6
1.6 2.0 >23 3.0
experimental conditions. vinyl films, the carbonyl Progr.
Org.
Coatings,
‘This peak generally had a maximum within the range 1745-1755 peak had a maximum at 1738 cm- I.
I (: 972/73)
cm-‘.
However,
355
method for determining the resistance of polymers to irradiation2’. In this case the quantity of decomposition products is determined after a given period of ultraviolet irradiation. The peak area serves as a measure of the degradation rate of the polymer. 2.2.3 Methods of chemical analysis Chemical methods have not been extensively applied to the ageing due to the general insolubility of dried films unless first saponified. Various end-groups which arise in the course of degradation can, however, be determined, e.g. ionisable endgroups and groups which can be transformed into ionisable ones”. In such cases the polymer with its end-groups acts as the moiety of a cationic or anionic soap and is caused to react with a suitable dye molecule so that the resultant coloured compound passes into a solvent in which the dye itself is not soluble. In this way carboxyl groups can directly be detected and determined’ 1*22. The hydroxyl functionality2 1*23 can be replaced by carboxyl by treatment with maleic anhydride or phthalic anhydride, or it can even be replaced by a sulphonic acid group by treatment with chlorosulphonic acid. Similarly, bromo compounds can be determined by the formation of quaternary base with pyridine using the reaction with bromophenol blue2’. These methods are very sensitive and have so far been used in the study of the ageing of methyl acrylate, methyl methacrylate and styrene polymers. Carboxyl groups can also be determined by infrared spectroscopy after reaction with sulphur tetrafluoridez4. The method has been applied to oxidised polyethylenes and polypropylenes, even in presence of aldehyde or ketone groups. In polyesters, polyvinyl chloride and certain other polymers, carbonyl groups are formed on the surface of the film during ageing. They can be determined by reaction with diamines since the coloured condensation products are suitable for is the most suitable calorimetric measurement 25 . N , iV-dimethyl-p-phenylenediamine reagent. If a sample of the film is exposed to this reagent, yellow to brown reaction products are formed, either on the surface of the sample or in the reagent solution. The residual amine is then determined spectrophotometrically in the solution. lt is also possible to evaluate quantitatively the coloration of the film itself. 3. PHYSICAL
PROCESSES
DURING
THE
AGEING OF PAINT FILMS
Adhesion, hardness, flexibility, deformability, abrasion resistance, swelling, colour, gloss, hiding power, water absorption, reflectance, resistance to low and high temperatures, thermoplasticity, electrical resistivity and dielectric strength are all basic physical properties of the film which are subject to change during ageing. The relationship between such changes and the lifetime of the paint film is obviously a complicated one 26 . Naturally, interest has centred on the mechanical properties of the paint film and their changes during ageing. First of all, mechanical properties are determined by the relationship behveen various mechanical stresses (a) and deformations or strains brought about in the film27. This relationship is unambiguously expressed by the modulus of elasticity E 256
Progr. Org. Coalings, I (1972173)
or the compliance Q=E.E=-.&
I and the elongation
coefficient
E:
1 I
A discussion now be useful.
of stress-strain
relationships
at the film-substrate
interface
will
3. I Paintfilms as orgariicglasses Paint films from macromolecular vehicles may conveniently be described as organic glasses whose properties do not change abruptly with temperature. There is no melting point, and softening takes place over a range of temperature. The temperature at which the displacement of chain segments within a macromolecule is restricted is called the glass transition temperature (T,). This temperature is very dependent on the structure of the molecule and is the temperature at which the transition between rubbery and glassy states occurs (Fig. 5). At room temperature many resins used in varnishes and paint media are amorphous, glassy solids. Others, typically the cellulose derivatives, have a fibrous or flaky structure and appear in the glassy state only after dissolution and evaporation of the solvents. Other resins are more or less viscous
I
+ Standoil 1:3 I
I I
-40
-20
P
I
0 20 40 60 Temperature -
80
lOO=‘C
Fig. 5. Illustration of the glass transition temperature (T,) of non-pigmented lacquer films from the temperature dependence of the refractive index. Fi!ms stored I year at room PemperatureJg.
Progr. Org. Cuotings, I (1972/73)
257
Tempemture
Fig. 6. Schematic illustration dependence of the moduluszs.
T,, inflection
point
of the glass transition T,, T,, glass transition
of the curve;
AT,
softening
region as determined by the temperature temperature; Tb, Ts, softening temperature;
region.
liquids which do not enter the glassy state until the film is formed as a result of polymerisation. The glass transition temperature is approximated28 by the softening temperature of amorphous polymers such as are used in paint films (Fig. 6). Deformation of the film in the glassy state is associated with the extension of covalent bonds and distortion of bond angles in the polymer chains, together with the movement of side chains and short-range chain dislocations. Chemical bonds are strong and, therefore, high stresses are necessary to cause a relatively small elongation of a chemical bond. E-Ience stresses applied to films in the glassy state cause only small elongations; in this state the paint film is hard and may even be brittle.
Various degrees of applicability of paint films can be expected both in the glassy and rubbery states, if the paint film remains in a state of persistent tension”. Only those tensions which arise in the paint film after drying and curing are considered here. The formation of films by solvent evaporation can be likened to glassy solidification during the cooling of a melt. Chemical film formation is essentially a process of polymerisation associated with viscosity increase. Both forms of drying involve a decrease in the volume occupied by the film. The shrinkage associated with crosslinking in particular reduces the mobility of the polymer chains. Because the film adheres to the substrate the volume decrease causes a build-up of stresses which K6nig26 has called “shrinkage tension”. This shrinkage continues throughout the ageing of the film so that the stresses in the film increase during its life (Fig. 7). In addition to this monotonic shrinkage, other volume changes may occur in the paint film, such as temporary swelling as a result of water absorption. These tot cause stresses to arise at the film-substrate interface. Volume changes in paint films may also be caused by varying temperarure. Paint films and substrates usually have different coefficients of thermal expansion which again leads to stress at the film-substrate interface. 258
Progr. Org. Coaiings, I (1972/73)
C 0 ._
1.0
E d aJ F * _f
0.5
5
20
40
Ageing
60
Days
Fig. 7. Influence of amount of plasticiser (per cent tricresyl phosphate upon the formation of internal stress in cellulose nitrate filmsz6.
related
to cellulose
nitrate)
3.3 Adhesion and internal stresses Adhesion is characterised essentially by two parameters, the work of adhesion (erg cm - 2, and the adhesive strength (kg cm- ‘). Numerous authorszgP3 ‘, particularly in the USSR, have studied the origin and change of internal stresses in the paint film after drying and ageing. As a consequence of the volume contraction, a force (P) arises. If this force is divided by the cross-section of the film F= 1-d (length x thickness), the internal stress cV is obtained, P a, = F The internal stress developed may be high, and if it is greater than the adhesive strength the paint film will part from the substrate. If, however, it is smaller than the adhesive strength, but greater than the strength of the paint film, cracking will occur. As long as the stress is homogeneously distributed through.out the paint film, the resultant defects are uniformly distributed over the surface and within the film. However, if it is not homogeneous, surface (or local) cracking, or local loss of adhesion will occur. 3.4 Material constants and practical mechanical tests From what has been said so far, it follows that the study of mechanical properties by colmmonly used methods permits no more than approximate evaluation of the properties of paint films. Such methods are the determination of surface hardness using pencils or 2 pendulum instrument, determination of the internal stress of paint Pfogr. Org. Coatings, I (1972/73)
259
films by using chaik paper, and determination of the film resistance against deep drawing, bending, shock and abrasion_ For reliable work, tests are required which give sufficient objective information on mechanical properties of paint films, or of the basic character and behaviour of paint films 38 . Therefore such methods comprise direct or indirect measurement of the physical constants of the material. Jmportant material constants are the shear modulus (G), Young’s modulus (E) and Poisson’s number (lpi, which denctes the ratio of relative contraction of the crosssection to relative elongation. The relationship between these three material constants is of special importance3’ as it may help to explain factors which influence test methods like the determination of hardness of paint films with pendulum instruments26*38, and impact tests. If their dependence upon temperature is measured, these properties can even explain relationships with molecular structure of the paint film28. Although these material constants are significant in relation to ageing and degradation, practical mechanical tests are still important, the most popular ones being tensile strength, impact strength and stress relaxation. Bosch et a/.39 have studied the mechanical properties of paint films by measuring elongation and tensile strength. The values thus obtained could be related to the properties of the paint film. Low elongation and tensile strength mean that the paint film is hard and fragile and tends to early destruction. A low elongation and a high tensile strength mean that the paint film is hard, tough and resistant to abrasion_ A high elongation and a low tensile strength are typical of soft, elastic paint films. If both values are high, paint films are both tough and elastic. From what has previously been said, it follows that the behaviour of the paint film, first of all, depends on the state of the film-former at the temperatures of exposure (e-9. - 10 to f35 “C). Secondly, it will be affected by the adhesion to the substrate, which may be influenced by volume contraction of the film or movement of the substrate, which bring about internal stresses. At the same time it will also depend upon the tensile strength and elongation of the film. If it is possible to relate all these factors to each other both before and after ageing, one should probably be able to select the most suitable film-former for a given set of ambient conditions. 3.5 Methods for the determination qf changes in pilysicai aud tnecha~~icaf properties of pain1 fiIm~ during ageitlg atld degradatiotl 3.5.1 Determinatiort of glass transition tenzperature Determination of the glass transition temperature can generally be carried out by following the dependence of some physical property, e.g. refractive index, density, specific volume26, or G and E moduli4’, upon temperature. Usually the temperature range between -40 and -i- 100°C is covered. It must be stressed that, at the glass transition temperature, many physical properties undergo a sharp change which profoundly affects both deformability and the results of deformation. Progr.
Org.
Coatings,
I (1972173)
._
3.5.2
Deterrnitzation
Various methods
of internal stress
are employed
to determine
changes
of internal
stress in paint
films. K&rig2 6 mentions an optical method for amorphous isotropic substances, like glass. The film-formers of paints, lacquers and varnishes become anisotropic and show double refraction due to deforming forces. Detached films may be inspected under polarised light. Soviet authors 2.g-34 have employed various methods for the determination of internal stress in coatings, one being based upon the deflection of coated plates of various substrate materials during drying and ageing of the paint film. The deflection of the plate (typically a metal strip 90 x I5 x 0.2 or 0.3 mm) caused by internal stresses in the applied paint film is measured microscopically (the so-called “console method”) or with a tensometric indicator on the coated side method”2g*“0). Zubov3’*37 uses a of the metal plate (the so-called “tensometric polarising microscope for measuring the internal stress. The principle of the method is the double refraction of light in the undercoat at the boundary surface with the paint film. Paint films are applied on a glass prism (10 x 20 x 20 mm) previously calibrated over a stress range of JO-120 kg cmm2. The given methods2g-33 were used for measuring internal stresses in films of paints comprising various binders and their dependence on the formation and ageing of the films at varying thicknesses. temperatures and times. 3.5.3 Determitlation of adhesion Many methods exist for the determination of adhesion, but most of them are not very accurate and do not allow a comparison of the results. Among the methods used are those dependent upon the application of tensile stress4’43, deformation33, scraping, scratching or stripping”“,45, and acceleration4648.
3.6 T/le releoance of physical nieasrtremer~rs to the seiectioir of ,Ciiin-forniers The objective test methods mentioned above can shed much light on the changes occurring in paint films, both during and after drying. There follows an indication of the authors’ views on how they may be used and interpreted. First, the glass transition temperature of the film should be determined, as it is important to know whether it lies below, within or above the typical ambient range of, of the temperature say, - 10°C to +3O”C. Suitable methods include measurement dependence of refractive index or of the torsional modulus of detached films. Such measurements should be made after various periods of ageing and the influence of exposure to U.V. radiation and heat may be studied separately and conjointly. The most satisfactory films would be those showing least change in glass transition temperature during ageing and the least abrupt change in mechanical properties over the typical exposure range of - 10 “C to -+-30°C. Secondly, tensile strength and elongation of the paint film should be determined before and after ageing. This should enable a film-former to be chosen which yields Prow.
Org.
Coatings,
I (1972/73)
261
a paint film which is firm, hard and tough within the ambient temperature range. These properties should not change appreciably during ageing. Tensile strength and elongation can readily be determined on free paint fihms by means of, several commercially available instruments. The adhesion and the internal stress of the films should be determined under the above-mentioned conditions. The best vehicle would again be that giving paint films exhibiting high adhesion and low internal stresses and no considerable change of these properties during ageing under the relevant conditions_ It is suggested that adhesion to the substrate may best be measured by the “pull-off” method using a tensile testing apparatus. The “console method” would probably be the most suitable for the determination of the internal stresses. It permits calculation of the internal stress from the elasticity modulus. TABLE
2
GLASS TRANSITION LACQUER RESINS,
TEMPERATURE T, (“C) OF VARIOUS PAINT FILMS AND PLASTICSfig
Anrhor’s measurements Rosin Ester gum Albert01 I1 1 L Albert01 209 L Maleic resin Terpenphenol resin Albert01 111 L-bodied oil varnish I:1 Albert01 11 I L-bodied oil varnish I:3 Bodied linseed oil Alkyd resin 58% linseed oil content Cyclicised rubber-Clophen A 60 Taken
melting melting
film
point point
after
105” 73”
I year
ageiog
at room
temp.
I
from IireraIrrre 30 25 36
Rosin Mastic Sandarac Dammar Shellac
Glyptal
(glycerol phthalate resin) Phenol-Novolak Polystyrene Polyvinyl chloride Polyvinyl acetate Polymethyl acrylate Polymethyl methacrylate Polybutyl methacrylate Cellulose triacetate Ethyl cellulose Ethyl cellulose-tricresyl phosphate Ethyl cellulose-tricresyl phosphate Nitrocellulose Nitrocellulose-tricresyl phosphate Nitrocellulose+tricresyl phosphate
262
AND
Nofes
TO 34 40 77 95 70 38 30 8 -17 12 17
VARNISH
80:20 6040 80:20 60:40
57 39 85
I
83 21 77 28 3 75-85 17 55 65 -2 -28 49 -10 -30
1
according
to Tammann
according
to Jenckel
according
to ilberreiter
Progr. Org. Coatings,
I (1972/73)
In general, optimum performance is shown by films whose physical properties remain substantially constant during ageing and which show no sharp changes over the ambient temperature range. Appropriate modifications of film-formers may be achieved by a variety of means well known to the polymer chemist. Glass transition points may be lowered by external or, preferably, internal plasticisation26*4g. Table 2 shows the glass transition temperatures of various coating resins according to Kanig. Suitable plasticisers can also improve tensile strength and elasticity, and reduce internal stress with but little effect on the stability of physical properties during ageing. The nature of the pigment and the pigment volume concentration naturally affect internal stress, and their effects are usually extended to tensile strength, eiongation, hardness and adhesion. 4.
ELECTRON
MICROSCOPY
The first signs of deterioration of paint films are most readily observed by means of electron microscopy, using either the method of surface replicas or the study of paint film sections. For the study of the surface and the inner structure of paint films, as well as for following changes during ageing, electron microscopy is of considerable importance. The changes brought about during ageing can be detected at first by the appearance of micro-cracks and micro-bubbles, the origin of which can be followed up only with the electron microscope. Failures at the surface of the film can therefore be spotted much earlier than is possible by visual inspection or optical micros-
copyg”oT The most common method of detecting these changes is the replica method. Preparation and evaluation of the replica is rather difficult. Essential requirements are that replicas are reproducible, that they show characteristic structures, distinguishable from artefacts, and that they permit unequivocal identification of the microstructure of the original sample. The most convenient procedure for sample preparation’ 1 is shadowing with a heavy metal in LWCIIO.Aluminium is then deposited on this layer, and the replica is removed on a film of methyl celiulose or gelatin. The difficulties of this procedure are avoided by use of the scanning electron microscope”, which yields photographs clearly showing the three-dimensional topography of the surface at magnifications of 20,000-50,000. The degree of homogeneity before ageing and structural changes in the paint films caused by individual ageing factors can be investigated by electron microscopy of sections of thickness 0.05-0.20 Jtm, cut in an ultramicrotomes3. The preparation of these sections is difficult, particularly in pigmented films containing pigment particles of diameter greater than the thickness of the section itself. Choice of a suitable Pmbedding medium as well as the choice of a suitable knife are important factors in achieving successful results.
Progr. Org. Coatings,
I (1972/73)
263
5. CONCLUSlONS
From this review of the complex factors contributing to the ageing of paint films the following broad conclusions may be drawn. (1) Successive coats should be based on film-formers of a kind appropriate to the conditions of exposure and having similar characteristics in terms of glass transition temperature, tensile strength, elongation, internal strain and adhesion. (2) Successive coats should not differ greatly in their ratios of pigment volume concentration to critical pigment volume concentration54. (3) Successive coats and the paint system as a whole should have coefkients of thermal expansion as close as possible to that of the substrate. (4) Over the ambient temperature range expected during the lifetime of the coating, the film should be hard and tough and such properties as glass transition temperature, tensile strength elongation, internal stress and adhesion should be substantially invariant with time. (5) In terms of chemistry, the coating should above all be resistant to oxidation and photodegradation. If this cannot be achieved by selection of primary Mm-former, resort must be had to anti-oxidants and U.V. absorbers. REFERENCES 1 W. H. Talen, J. Oil Coforrr Chemists’ Assoc., 45 (1962) 387; 46 (1963) 2 M. Dyck, Farbe Lack, 68 (1962) 543, 608. 3 H. A. Stuart, Angem. Chem., 79 (1967) 877. 4 W. Kern, Chemiker-Zig., 91 (1967) 255. 5 6 7 8
9 10
11 12
13 14
R. W. S. Burell, Paint Mamrf., 36 (2) (1966) 32. R. R. Myers, Ofic. Dig., 34 (1962) 575. J. L. Bolland and G. Gee, Trans. Faraday Sot., 42 (1946) 236. E. H. Farmer, J. Chem. Sot., (1943) 541; (1946) 1022. J. D. Nye and J. S. Mackie, Ojic. Dig., 34 (1962) 816. A. R. Schultz, J. Chem. Phys., 29 (1958) 2Op. C. D. Miller, J. Am. Oil Chemists’ Sot., 36 (1959) 596. C. D. Miller, I& Eng. C’hem., 50 (1958) 125. K.-H. Reichert and R. Sattelmeyer, Farbe Lwk, 73 (1967) 512. K.-H. Reichert, Alterung und Korrosion vofi Kunststoffen, Korrosion,
940.
20 (1966)
15 K.-H. Reichert and E. Gulbins. Proc. V/llth Congr. FAT’PEC, Scheveninyen, 16 S. V. Yakubovich. Lakokrasoch. Mater. Ikh Primen., (1) (1964) 56. 17 P. J. Hearst, J. Paint Techno!.. 39 (1967) 119 18 M. A. Keenan and D. A. Smith, J. Appl. Polymer Sci., II (1967) 1009. 19
V. S. Papkov and G. L. Slonimskiy,
33.
1966, p. 406.
Y_wokor;zol&rl. Soedin., f0 (1968) 1204.
Anal. Chem.. 50 (1960) 513.
20 H. C. Su and J. L. Cameron,
33 (1967) 949.
21 S. R. Palit, Ktmstsfqfe, 22 S. R. Palit and P. Ghosh, J. Poiymer Sci., 58 (1962) 1225. 23 S. R. Palit and A. R. Mukherjee, J. Poiymer Sci., 58 (1962) 1243. 24 J. F. Heacock, J. AppL Poiymw Sci., 7 (1963) 2319. 25 V. E. Gray and J. R. Wright, J. Appl. Polymer Sri., 7 (1963) 2161.
26 W. Kijnig,
Deut. Farben-Z.,
27 P. Fink-Jensen, Farbe 28 U. Zorll, Farbe Lack,
16 11962) 379.
Lack, 65 (1954) 75 (19691 I II.
121.
29 A. T. Samharovskiy and G. 30 A.
T. Sanzharovskiy,
I. Epifanov, V.vsokomolekul. Soedin., .? (1960) 1703. G. I. Epifanov and A. T. Lomatin, tikokrasoch. Mater. fkkhPrimen., (3)
(1962) 21.
264
Progr. Org. Caolings, I (1972/73)
31 P. I. Zubov and L. A. Lepilkina, Kolfoidn. Zh., 24 (1962) 30. 32 P. I. Zubov and L. A. Lepiikina, Kolloidn. Zh., 23 (1961) 418. 33 V. A. Kargin, T. I. Sogolova and M. I. Karyakina, Khim. Prom., (7) (1955) 392. 34 M. I. Karyakina, V. A. Kargin and T. I. Sogolova, Khim. Prom., (7) (1957) 365. 35 W. Simpson, .i. Oil Colour Chemists’ Assoc., 45 (1963) 331. 36 S. V. Yakubovich and N. L. Maslenikova, Lukokrasoch. Mater. Iklz Primen., (5) (1961) 27. 37 P. 1. Zubov and L. A. Lepilkina, Lukokrusoch. Mater. Ikh Primen., (5) (1961) 19. 38 U. Zorli, Proc. IXth Congr. FATZPEC, Brussels, 1968, p. Pldn. 3. 39 W. Bosch, F. J. Macekonis and P. H. Martin, Ofic. Dig., 26 (1954) 1219. 40 U. Zorli, Forschungsber. Landes Nordrhein- Westfilen, (136 1) (1964). 41 F. Kollman, Technologie des Holzes, Berlin, 1936. 42 E. G. 0. Schmidt, Furben-Ztg., 37 (1932) 376. 43 R. Quadrion, Paint Manuf., 20 (1950) 470. 44 N. A. Krotonova and L. P. Morosova, DAN SSSR, I7 (1959) 302. 45 H. Danneberg, J. Polymer Sci., 33 (1958) 509. 46 A. M. Malloy and W. Soller, Pain1 Oil Chem. Reu., (1953) 14. 47 Anon., Paint Manrrf., 28 (1958) 273. 48 S. Moses and R. K. Witt, Ind. Eug. Chem., 41 (1949) 2334. 49 W. KBnig, paper on VIrh Congr. FATZPEC, Wiesbaden, i962. 50 J. Mariott, J. Oil Coiour Chemisrs’ Assoc., 41 (1958) 363. 51 S. H. Bell. 3. Oil Colorrr Chemists’ Assoc., 43 (1960) 466. 52 J. Fairless, J. Oil Cofour Chemists Assoc., 52 (1969) 491. 53 U. Zorll, Furbe Luck, 71 (1965) 273. 54 0. Kolai and B. Svoboda, Furbe Lack, 75 (1969) 31.
Progr. Org. Coatings,
I (1972/73)
265
AGEING EMIL
AND DEGRADATION
KREJCAR
AtGD
OTAKAR
Research Institute for Synthetic
OF PAINT FILM
MEDIA
KOLAR
Resins and Lacquers,
Pardubice (CSSR)
CONTENTS
1 Inteoduction, 249 2 Chemical processes during the ageing of paint films, 250
2.1 EfFect of U.V. radiation, 251 2.2 The determination of chemical changes, 253 2.2.1 Infrared spectroscopy, 253 2.2.2 Study of volatile products, 253 2.2.3 Methods of chemical analysis, 256 3 Physical processes during the ageing of paint films, 256 3.1 Paint tilms as organic glasses, 257 3.2 Volume changes in paint films, 258 3.3 Adhesion and internal stresses, 259 3.4 Material constants and practical mechanical tests, 259 3.5 Methods for the determination of changes in physical and mechanical properties of paint films during ageing and degradation, 260 3.5.1 Determination of glass transition temperature, 260 3.5.2 Determination of internal stress, 261 3.5.3
Determination
of adhesion,
261
3.6 The relevance of physical measurements to the selection of film-farmers, 261
4 Electron microscopy, 263 5 Conclusions, 264 References, 264 I. INTRODU~ION
The problems of ageing and degradation of macromolecular substances are usually considered in relation to some kind of final product made from them, e.g. a moulding, a sheet or a paint film. It is the last of these with which the present paper is concerned. The use of paint lilms to protect a substrate against the influence of various media raises the question of service life, i.e. the period during which the substrate is protected before the film deteriorates. The ageing of a paint film embraces all the physical, physico-chemical and chemical processes occurring in the paint film after its application to the substrate, its drying and hardening, up to and including its deterioration. The important criteria are the rate at which a hard, homogeneous film exhibiting optimum properties is formed and the period during which these properties Progr. Org. Coatings,
Z (1972/73)
249
are preserved before the film deteriorates. This period is usuahy referred to as the “service life” of the film. Talen has classified film-forming processes into a number of groupsl. Physical film formation by solvent evaporation yields a l3m which already exhibits optimum properties typical of the resin employed. On the other hand, when chemical or physico-chemical processes are involved in drying, a film is obtained which, after solvent evaporation or, in the case of solventless coating materials, after initial setting, does not yet possess the required properties; to develop these a certain length of time is necessary for reaction to occur. Oxidative polymerisation and polycondensation at higher temperatures are examples. Until optimum properties of the paint film are developed, various factors are exerting either favourable or unfavourable influences on film formation. Favourable factors include, among others, elevated temperature, low atmospheric humidity, abundance of sunlight, calm air and absence of dust. Unfavourable factors include low temperature, high humidity, lack of sunlight, wind, dust and rain. These factors are, of course, most relevant when application occurs under normal weather conditions and when the films dry chemically.
Assuming the film to have attained optimum properties, the reasons for its ageing and deterioration are, in the first place, effects of the surroundings2. Predominant among these are temperature and sudden temperature changes, intense sunlight, oxygen, ozone, water in whatever form (atmospheric humidity, dew, rain or snow) and certain microorganisms (mildew, fungi). Atmospheric contaminants like sulphur dioxide and hydrogen sulphide are harmful, while air-borne solids, particularly in combination with wind, cause erosion of the paint film. A marked influence is also exerted by thermal, mechanical and electrical strains which may exacerbate the unfavourable effects of the surroundings. These unfavourable effects of the surroundings can cause chemical changes which, in turn, can change the physical properties of the paint film and may additionally cause detachment from the substrate. Surface cracks, in particular, speed up chemical changes, e.g. oxidation and hydrolysis3. 2. CHEMICAL
CHANGES
Basic chemical divided into’
DURING
processes
THE
occurring
AGEING
FILMS
during ageing of paint films can be sub-
(a) chemical processes isothermal processes,
taking place regardless
(b) chemical
involving the surroundings.
processes
OF PAINT
of the surroundings,
i.e. spontaneous
From the practical point of view, weathering of paint films is of primary importance, but sometimes also chemicals, higher temperatures, etc. are effective. Some of these factors affecting paint films may, of course, predominate while others may be suppressed, but it should be made clear that the effects of separate factors are not 250
Progr. Org. Coorings, 1 (1972173)
simply additive, because in combination they can work both against and with each other. The service life of a paint film is naturally influenced, above all, by the chemical structure of the binder’. Many binders based upon drying oils give films possessing high weather resistance. ?‘hey are, however, susceptible to oxidation and, moreover, by ultraviolet radiation, peroxides are formed. These are unstable6 and their decomposition yields free radicals which cause, on the one hand, break-up of molecules and, on the other, intermolecular reactions accompanied by formation of three-dimensional networks. The mechanism of formation and decomposition of hydroperoxides has been described in a series of papers7 ,‘. Crosslinking in binders evidently leads to increasing hardness and brittleness of films. Weathering causes an increase in the content of degradation products, some of which escape into the air. At the same time, the film becomes progressively more hydrophilic so that it can absorb increasing quantities of water which cause, in turn, a higher degree of swelling and increased mechanical stress. Depending upon the relationship between deformation forces, adhesion and cohesion, the paint film may crack and peel off either at the surface or down to the very substrate’. Naturally, also, the weight of the film will change, because gaseous products escape, and low molecular degradation products are leached out by water. These low molecular compounds are formed by chain scission. Gaseous products arise most frequently through scission in the neighbourhood of end-groups. 2.1 Eflect of U.V. radiation In
studying the effects of ultraviolet
radiation,
two factors have to be considered,
namely the wavelength and the intensity of the radiation_ At a given wavelength the effect is proportional to the energy absorbed, which, in turn, depends upon the intensity of incident radiation, and upon the absorptivity of the medium”. The radiation will affect the whole thickness of the film only in completely transparent, non-absorbing pr very thin films. As far as the polymer absorbs the radiation of a given wavelength range, a gradient of changes related to the varying degree of crosslinking, chain scissioning and impaired mechanical properties can be expected to appear across the thickness of the film. For example, the study of degradation of films from drying oils by ultraviolet radiation has revealed’ ’ that gaseous products of lower H/C ratio are liberated more readily from the surface of the film than from its interior. A high hydrogen content of the volatile products (particularly water) has been taken to indicate, predominantly, polycondensation reactions leading to crosslinking of the polymer. Carbon is present mainly in the skeleton of the polymer so that, with some reservation, the presence of carbon oxides in volatile products points to reactions leading to the break-up of chains. Deterioration of paint films, particularly of the drying oil type, changes the behaviour of films towards water and other solvents. Ultraviolet radiation causes functional groups to be formed at the surface of the frim, and their hydrophilic nature diminishes the contact angle with water so that the surface is more readily Progr. Org. Coalings, I (1972/73)
251
wetted; thus the erosive effect of water is enhanced. The analysis of leached products suggests that water itself does not participate to a significant extent in the degradation reactions but it leaches soluble products of the oxidative degradation process. Thus, after the water has evaporated, formation of mechanical tensions in the film or in the film-substrate boundary is favoured. In this way, among other effects, the swellability of surface layers in the film is increased, while the reverse may be true of the interior due to network formation within the film. The influence of wavelength has been clearly demonstrated in studies of the degradation of oil-modified alkyd afilms”. At wavelengths below 250 run, it is the aromatic part of the alkyd that is degraded very quickly and preferentially; at wavelengths above 295 nm, the aromatic structures are much more stable than the aliphatic structures (Figs. 1, 2 and 3). At the higher wavelengths, the degradation of aromatic nucIei is a secondary process which is evidently caused by decomposition of attached
120 -
Fig.
1. Disappearance
I
I
20
I
40
Fig. 2. Disappearance
252
of functional
I
60
groups during exposure
,
80
,jOzrs c
of functional
120
to unfiltered
light12.
I
740
groups during exposure to wavelengths Progr.
~250
nm A * 2.
Org. Coatings,
I (1972/73)
20 t
1
I
40
20
Fig.
3. Disappearance
aliphatic
60
80 100 HOUt-5
of functional
I
120
I
140
groups during exposure
to wavelengths
>?95
nm .&I *.
groups.
As cured films contain few double bonds, oxidation is likely to hydrogen of crosslinked units. This mechanism is suggested by comparison with an alkyd resin modified by stearic acid, which is much more resistant towards photo-oxidation not only in the aliphatic, but also in the aromatic part of start at the tertiary
the molecule. 2.2
Tile determinaiion of chemical charlges
In the study of chemical changes occurring during the ageing of paint films, photometric methods have mostly been employed, and infrared techniques have been preferred to the study of the ultraviolet or visible regions. Classical analytical methods have also been applied in special cases. Paper and column chromatography, and gel permeation chromatography may be used to study soluble polymers. Degradation products, particularly those formed at higher temperatures, can readily be studied by gas chromatography alone or in combination with mass spectroscopy. Radical reactions occurring during the ageing of soluble polymers may be studied by electron paramagnetic resonance. Wide-band nuclear magnetic resonance spectrozcopy is useful for the study of the network formation. 2.2. I
hfrared
spectroscopy
The main advantage of infrared spectroscopy is its ability to detect a wide variety of chemical changes occurring during the ageing of films. In Fig. 4 changes in the infrared spectrum are shown when an alkyd resin film was exposed to U.V. irradiation. The appearance or disappearance of some functional groups in macromolecular substances can be followed either directly or using differential techniques’ 3. The latter have been employed for investigating the influence on ethyl cellulose films of the surrounding atmosphere during ultraviolet irradiation at various wavelengths. In order to determine the depth of effect of individual weathering factors, infrared spectra of successively stripped layers of the film can be examined, or the attenuated total reflection (ATR) technique can be applied, which enables changes in both the exposed Progr.
Org. Coatings, I (1972/73)
253
--BEFORE
I
I
2
3
4
5
Fig. 4. I.R. spectrum
6
of drying
7
15 h U.V. A > 3000
---AWER
135 h
.
6 Micrgons
EXPOSURE
..-.-..AFTER
U.V.
I
10
I
I
11
12
13
14
oil alkyd before and after exposure”.
and protected film layers to be studied r4*’ 5. Infrared spectroscopy can also be used for the determination of volatile products of paint film photodegradation. In the photodegradation of cellulose nitrate, vinyl polymers and alkyd resins, evolved carbon monoxide and dioxide, formic acid and possibly also water can be determined’ 6. In presence of rutile titanium dioxide the composition of the volatile components does not change, but the rate of formation is reduced.
TABLE
1
INFRARED
PEAKS
The per cent absorption Peak (cm-
1)
OF THE
VOLATILE
is given for the products
Peak assignmenl
Alkyd ffg
721 730 912 950 992 1034
1105 1271 1307 1370
1750= 1777 1792 2120 2160 2360 2960
Carbon dioxide Acetylene Vinyl group Ethylene Vinyl group Methanol
of 6-hour
Xe
-
FILMSz3
3.2
6.2 1.8 2.0
-
Vinyl-alkyd Xe
Hg
0.6 1.4
0.8
VEHICLE
arc and %I-hour xenon arc irradiations
Oil vehicle
oehicle
-
FROM
mercury
-
15.4 0.8 2.2 -
2.0
1.0
1.6
Formic acid 2.8 Acetone Methane 0.6 Acetyl group 0.6 Carbonyl group 6 Vinyl ester 2.8 (Unassigned) 2.4 Carbon monoxide 6.4 Carbon monoxide 7.8 Carbon dioxide >34 Carbon-hydrogen linkages 2.4
1.0 0.6
1.8
3.0 0.6
0.8 4.6 -7 3.2 2.2 8.2 9.4 >78 4.8
4.8 14 -6 8.0 9.8 97 7.2
- No apparent for the products
354
2.6 0.3
PRODUCTS
-6 -3.4 5.8 6.4 >61 4.0
-
peak. “Partially hydrolysed vinyl chloride-vinyl from the polyvinyl asetate and for the products
Xe
Hg
1.6 0.8
-
0.6 -
-
vehich
4.2 1.6 b -
4 ~2.6 -2.0 5.4 6.4 ~-23 1.6
-
6.4 0.6 1.2 0.6 0.4
-6 -5.6 3.2 4.6 78 4.4
acetate copolymer. bIndetinite because of from the xenon arc irradiation of the other Progr.
Org.
Coatings,
I (1972/73)
2.2.2 Stud]) of volatile products The examination of volatile products of photodegradation can be carried out directly in a special gas cell with silica walls’ 7. This method was employed in examining the photodegradation caused by the radiation of mercury and xenon lamps in 6lms from alkyd resins, oils, pclyvinyl acetate, epoxide resins and other vehicles. A survey of results is given in Table I. In the spectruin of the volatile products, carbon monoxide and dioxide, acetone, formic acid, methane, ethylene and acetylene, as well as compounds containing carbonyl and vinyl groups, have been identified. In the volatile products of all binders, carbon dioxide was predominant, but was relatively more abundant in those of media drying by oxidation rather than by polymerisation or evaporation_ In binders drying by oxidative reactions, more ethylene was found in the volatile products when the xenon rather than the mercury lamp was used. Acetylene and methane, on the other hand, were found only during radiation with the mercury lamp. These results clearly demonstrate the influence of wavelength distribution on the course of the photo-oxidation reactions. Volatile products which are formed during photo-oxidation and thermodegradation can readily be investigated by gas chromatography. This method has proved particularly useful in studyin, 0 the thermal degradation of epoxide resins” and some Gas chromatography is also recommended as a rapid hydrocarbon polymers’g.
Vinvivehicle
Vin_yI vehicle (hydrolyses)
Xe
Hg
Jfg
0.8
-
2.4
3.8 11.6 0.6 2.4 -
5.2
0.6
2.4 7.2 4.0
2.4 1.4 0.4
11.4 -
5.0 1.6
7.6
-
17.4 -3 -5 -29
9 14 -
-9
3.2
-13 5 5 56 -8
1.8 11.0 -
4.6
14 -9.8
-
3.0
12 -5 -3
7.6
18 -
5.8 7.8 >20
Hg
4.2
-
11.8 -
16 -
-10
Xe
3.6 4.2 >25
7.6
9.2
Epoxy celricie (anline-cared)
Po!_winyl acelaie
-
3.0 5.6 1.8 6.8 4.4
-11 -3 -12 -37
-20 -17 9.4 10.5 68 12.2
Xe
M3 2.0
0.4 -
4.0 8.8
-
13.2
2.2 0.8
-
-
9.4
0.4 0.6 > 8.0 10.8
-
5.2 4.6 0.6 3 8
2.4
5.4
2.4 3.4
-
3.0 -5 -
6.2
-
4.8
-
0.4 2.0
-
2.0 1.6
.-3 -
6.0 7.0
0.6 0.8
>21
3.6 2.4 0.6 2 4
3.0
-
0.6
4.0
1.0
-
-
1.8
-
7.4
>51 w-8
0.4
-
Xe
Hs
-
2.2
7.8
15.0 -
Xe
Epos_~. cchiclc (polyamide-cured)
>34 -6
1.6 2.0 >23 3.0
experimental conditions. vinyl films, the carbonyl Progr.
Org.
Coatings,
‘This peak generally had a maximum within the range 1745-1755 peak had a maximum at 1738 cm- I.
I (: 972/73)
cm-‘.
However,
355
method for determining the resistance of polymers to irradiation2’. In this case the quantity of decomposition products is determined after a given period of ultraviolet irradiation. The peak area serves as a measure of the degradation rate of the polymer. 2.2.3 Methods of chemical analysis Chemical methods have not been extensively applied to the ageing due to the general insolubility of dried films unless first saponified. Various end-groups which arise in the course of degradation can, however, be determined, e.g. ionisable endgroups and groups which can be transformed into ionisable ones”. In such cases the polymer with its end-groups acts as the moiety of a cationic or anionic soap and is caused to react with a suitable dye molecule so that the resultant coloured compound passes into a solvent in which the dye itself is not soluble. In this way carboxyl groups can directly be detected and determined’ 1*22. The hydroxyl functionality2 1*23 can be replaced by carboxyl by treatment with maleic anhydride or phthalic anhydride, or it can even be replaced by a sulphonic acid group by treatment with chlorosulphonic acid. Similarly, bromo compounds can be determined by the formation of quaternary base with pyridine using the reaction with bromophenol blue2’. These methods are very sensitive and have so far been used in the study of the ageing of methyl acrylate, methyl methacrylate and styrene polymers. Carboxyl groups can also be determined by infrared spectroscopy after reaction with sulphur tetrafluoridez4. The method has been applied to oxidised polyethylenes and polypropylenes, even in presence of aldehyde or ketone groups. In polyesters, polyvinyl chloride and certain other polymers, carbonyl groups are formed on the surface of the film during ageing. They can be determined by reaction with diamines since the coloured condensation products are suitable for is the most suitable calorimetric measurement 25 . N , iV-dimethyl-p-phenylenediamine reagent. If a sample of the film is exposed to this reagent, yellow to brown reaction products are formed, either on the surface of the sample or in the reagent solution. The residual amine is then determined spectrophotometrically in the solution. lt is also possible to evaluate quantitatively the coloration of the film itself. 3. PHYSICAL
PROCESSES
DURING
THE
AGEING OF PAINT FILMS
Adhesion, hardness, flexibility, deformability, abrasion resistance, swelling, colour, gloss, hiding power, water absorption, reflectance, resistance to low and high temperatures, thermoplasticity, electrical resistivity and dielectric strength are all basic physical properties of the film which are subject to change during ageing. The relationship between such changes and the lifetime of the paint film is obviously a complicated one 26 . Naturally, interest has centred on the mechanical properties of the paint film and their changes during ageing. First of all, mechanical properties are determined by the relationship behveen various mechanical stresses (a) and deformations or strains brought about in the film27. This relationship is unambiguously expressed by the modulus of elasticity E 256
Progr. Org. Coalings, I (1972173)
or the compliance Q=E.E=-.&
I and the elongation
coefficient
E:
1 I
A discussion now be useful.
of stress-strain
relationships
at the film-substrate
interface
will
3. I Paintfilms as orgariicglasses Paint films from macromolecular vehicles may conveniently be described as organic glasses whose properties do not change abruptly with temperature. There is no melting point, and softening takes place over a range of temperature. The temperature at which the displacement of chain segments within a macromolecule is restricted is called the glass transition temperature (T,). This temperature is very dependent on the structure of the molecule and is the temperature at which the transition between rubbery and glassy states occurs (Fig. 5). At room temperature many resins used in varnishes and paint media are amorphous, glassy solids. Others, typically the cellulose derivatives, have a fibrous or flaky structure and appear in the glassy state only after dissolution and evaporation of the solvents. Other resins are more or less viscous
I
+ Standoil 1:3 I
I I
-40
-20
P
I
0 20 40 60 Temperature -
80
lOO=‘C
Fig. 5. Illustration of the glass transition temperature (T,) of non-pigmented lacquer films from the temperature dependence of the refractive index. Fi!ms stored I year at room PemperatureJg.
Progr. Org. Cuotings, I (1972/73)
257
Tempemture
Fig. 6. Schematic illustration dependence of the moduluszs.
T,, inflection
point
of the glass transition T,, T,, glass transition
of the curve;
AT,
softening
region as determined by the temperature temperature; Tb, Ts, softening temperature;
region.
liquids which do not enter the glassy state until the film is formed as a result of polymerisation. The glass transition temperature is approximated28 by the softening temperature of amorphous polymers such as are used in paint films (Fig. 6). Deformation of the film in the glassy state is associated with the extension of covalent bonds and distortion of bond angles in the polymer chains, together with the movement of side chains and short-range chain dislocations. Chemical bonds are strong and, therefore, high stresses are necessary to cause a relatively small elongation of a chemical bond. E-Ience stresses applied to films in the glassy state cause only small elongations; in this state the paint film is hard and may even be brittle.
Various degrees of applicability of paint films can be expected both in the glassy and rubbery states, if the paint film remains in a state of persistent tension”. Only those tensions which arise in the paint film after drying and curing are considered here. The formation of films by solvent evaporation can be likened to glassy solidification during the cooling of a melt. Chemical film formation is essentially a process of polymerisation associated with viscosity increase. Both forms of drying involve a decrease in the volume occupied by the film. The shrinkage associated with crosslinking in particular reduces the mobility of the polymer chains. Because the film adheres to the substrate the volume decrease causes a build-up of stresses which K6nig26 has called “shrinkage tension”. This shrinkage continues throughout the ageing of the film so that the stresses in the film increase during its life (Fig. 7). In addition to this monotonic shrinkage, other volume changes may occur in the paint film, such as temporary swelling as a result of water absorption. These tot cause stresses to arise at the film-substrate interface. Volume changes in paint films may also be caused by varying temperarure. Paint films and substrates usually have different coefficients of thermal expansion which again leads to stress at the film-substrate interface. 258
Progr. Org. Coaiings, I (1972/73)
C 0 ._
1.0
E d aJ F * _f
0.5
5
20
40
Ageing
60
Days
Fig. 7. Influence of amount of plasticiser (per cent tricresyl phosphate upon the formation of internal stress in cellulose nitrate filmsz6.
related
to cellulose
nitrate)
3.3 Adhesion and internal stresses Adhesion is characterised essentially by two parameters, the work of adhesion (erg cm - 2, and the adhesive strength (kg cm- ‘). Numerous authorszgP3 ‘, particularly in the USSR, have studied the origin and change of internal stresses in the paint film after drying and ageing. As a consequence of the volume contraction, a force (P) arises. If this force is divided by the cross-section of the film F= 1-d (length x thickness), the internal stress cV is obtained, P a, = F The internal stress developed may be high, and if it is greater than the adhesive strength the paint film will part from the substrate. If, however, it is smaller than the adhesive strength, but greater than the strength of the paint film, cracking will occur. As long as the stress is homogeneously distributed through.out the paint film, the resultant defects are uniformly distributed over the surface and within the film. However, if it is not homogeneous, surface (or local) cracking, or local loss of adhesion will occur. 3.4 Material constants and practical mechanical tests From what has been said so far, it follows that the study of mechanical properties by colmmonly used methods permits no more than approximate evaluation of the properties of paint films. Such methods are the determination of surface hardness using pencils or 2 pendulum instrument, determination of the internal stress of paint Pfogr. Org. Coatings, I (1972/73)
259
films by using chaik paper, and determination of the film resistance against deep drawing, bending, shock and abrasion_ For reliable work, tests are required which give sufficient objective information on mechanical properties of paint films, or of the basic character and behaviour of paint films 38 . Therefore such methods comprise direct or indirect measurement of the physical constants of the material. Jmportant material constants are the shear modulus (G), Young’s modulus (E) and Poisson’s number (lpi, which denctes the ratio of relative contraction of the crosssection to relative elongation. The relationship between these three material constants is of special importance3’ as it may help to explain factors which influence test methods like the determination of hardness of paint films with pendulum instruments26*38, and impact tests. If their dependence upon temperature is measured, these properties can even explain relationships with molecular structure of the paint film28. Although these material constants are significant in relation to ageing and degradation, practical mechanical tests are still important, the most popular ones being tensile strength, impact strength and stress relaxation. Bosch et a/.39 have studied the mechanical properties of paint films by measuring elongation and tensile strength. The values thus obtained could be related to the properties of the paint film. Low elongation and tensile strength mean that the paint film is hard and fragile and tends to early destruction. A low elongation and a high tensile strength mean that the paint film is hard, tough and resistant to abrasion_ A high elongation and a low tensile strength are typical of soft, elastic paint films. If both values are high, paint films are both tough and elastic. From what has previously been said, it follows that the behaviour of the paint film, first of all, depends on the state of the film-former at the temperatures of exposure (e-9. - 10 to f35 “C). Secondly, it will be affected by the adhesion to the substrate, which may be influenced by volume contraction of the film or movement of the substrate, which bring about internal stresses. At the same time it will also depend upon the tensile strength and elongation of the film. If it is possible to relate all these factors to each other both before and after ageing, one should probably be able to select the most suitable film-former for a given set of ambient conditions. 3.5 Methods for the determination qf changes in pilysicai aud tnecha~~icaf properties of pain1 fiIm~ during ageitlg atld degradatiotl 3.5.1 Determinatiort of glass transition tenzperature Determination of the glass transition temperature can generally be carried out by following the dependence of some physical property, e.g. refractive index, density, specific volume26, or G and E moduli4’, upon temperature. Usually the temperature range between -40 and -i- 100°C is covered. It must be stressed that, at the glass transition temperature, many physical properties undergo a sharp change which profoundly affects both deformability and the results of deformation. Progr.
Org.
Coatings,
I (1972173)
._
3.5.2
Deterrnitzation
Various methods
of internal stress
are employed
to determine
changes
of internal
stress in paint
films. K&rig2 6 mentions an optical method for amorphous isotropic substances, like glass. The film-formers of paints, lacquers and varnishes become anisotropic and show double refraction due to deforming forces. Detached films may be inspected under polarised light. Soviet authors 2.g-34 have employed various methods for the determination of internal stress in coatings, one being based upon the deflection of coated plates of various substrate materials during drying and ageing of the paint film. The deflection of the plate (typically a metal strip 90 x I5 x 0.2 or 0.3 mm) caused by internal stresses in the applied paint film is measured microscopically (the so-called “console method”) or with a tensometric indicator on the coated side method”2g*“0). Zubov3’*37 uses a of the metal plate (the so-called “tensometric polarising microscope for measuring the internal stress. The principle of the method is the double refraction of light in the undercoat at the boundary surface with the paint film. Paint films are applied on a glass prism (10 x 20 x 20 mm) previously calibrated over a stress range of JO-120 kg cmm2. The given methods2g-33 were used for measuring internal stresses in films of paints comprising various binders and their dependence on the formation and ageing of the films at varying thicknesses. temperatures and times. 3.5.3 Determitlation of adhesion Many methods exist for the determination of adhesion, but most of them are not very accurate and do not allow a comparison of the results. Among the methods used are those dependent upon the application of tensile stress4’43, deformation33, scraping, scratching or stripping”“,45, and acceleration4648.
3.6 T/le releoance of physical nieasrtremer~rs to the seiectioir of ,Ciiin-forniers The objective test methods mentioned above can shed much light on the changes occurring in paint films, both during and after drying. There follows an indication of the authors’ views on how they may be used and interpreted. First, the glass transition temperature of the film should be determined, as it is important to know whether it lies below, within or above the typical ambient range of, of the temperature say, - 10°C to +3O”C. Suitable methods include measurement dependence of refractive index or of the torsional modulus of detached films. Such measurements should be made after various periods of ageing and the influence of exposure to U.V. radiation and heat may be studied separately and conjointly. The most satisfactory films would be those showing least change in glass transition temperature during ageing and the least abrupt change in mechanical properties over the typical exposure range of - 10 “C to -+-30°C. Secondly, tensile strength and elongation of the paint film should be determined before and after ageing. This should enable a film-former to be chosen which yields Prow.
Org.
Coatings,
I (1972/73)
261
a paint film which is firm, hard and tough within the ambient temperature range. These properties should not change appreciably during ageing. Tensile strength and elongation can readily be determined on free paint fihms by means of, several commercially available instruments. The adhesion and the internal stress of the films should be determined under the above-mentioned conditions. The best vehicle would again be that giving paint films exhibiting high adhesion and low internal stresses and no considerable change of these properties during ageing under the relevant conditions_ It is suggested that adhesion to the substrate may best be measured by the “pull-off” method using a tensile testing apparatus. The “console method” would probably be the most suitable for the determination of the internal stresses. It permits calculation of the internal stress from the elasticity modulus. TABLE
2
GLASS TRANSITION LACQUER RESINS,
TEMPERATURE T, (“C) OF VARIOUS PAINT FILMS AND PLASTICSfig
Anrhor’s measurements Rosin Ester gum Albert01 I1 1 L Albert01 209 L Maleic resin Terpenphenol resin Albert01 111 L-bodied oil varnish I:1 Albert01 11 I L-bodied oil varnish I:3 Bodied linseed oil Alkyd resin 58% linseed oil content Cyclicised rubber-Clophen A 60 Taken
melting melting
film
point point
after
105” 73”
I year
ageiog
at room
temp.
I
from IireraIrrre 30 25 36
Rosin Mastic Sandarac Dammar Shellac
Glyptal
(glycerol phthalate resin) Phenol-Novolak Polystyrene Polyvinyl chloride Polyvinyl acetate Polymethyl acrylate Polymethyl methacrylate Polybutyl methacrylate Cellulose triacetate Ethyl cellulose Ethyl cellulose-tricresyl phosphate Ethyl cellulose-tricresyl phosphate Nitrocellulose Nitrocellulose-tricresyl phosphate Nitrocellulose+tricresyl phosphate
262
AND
Nofes
TO 34 40 77 95 70 38 30 8 -17 12 17
VARNISH
80:20 6040 80:20 60:40
57 39 85
I
83 21 77 28 3 75-85 17 55 65 -2 -28 49 -10 -30
1
according
to Tammann
according
to Jenckel
according
to ilberreiter
Progr. Org. Coatings,
I (1972/73)
In general, optimum performance is shown by films whose physical properties remain substantially constant during ageing and which show no sharp changes over the ambient temperature range. Appropriate modifications of film-formers may be achieved by a variety of means well known to the polymer chemist. Glass transition points may be lowered by external or, preferably, internal plasticisation26*4g. Table 2 shows the glass transition temperatures of various coating resins according to Kanig. Suitable plasticisers can also improve tensile strength and elasticity, and reduce internal stress with but little effect on the stability of physical properties during ageing. The nature of the pigment and the pigment volume concentration naturally affect internal stress, and their effects are usually extended to tensile strength, eiongation, hardness and adhesion. 4.
ELECTRON
MICROSCOPY
The first signs of deterioration of paint films are most readily observed by means of electron microscopy, using either the method of surface replicas or the study of paint film sections. For the study of the surface and the inner structure of paint films, as well as for following changes during ageing, electron microscopy is of considerable importance. The changes brought about during ageing can be detected at first by the appearance of micro-cracks and micro-bubbles, the origin of which can be followed up only with the electron microscope. Failures at the surface of the film can therefore be spotted much earlier than is possible by visual inspection or optical micros-
copyg”oT The most common method of detecting these changes is the replica method. Preparation and evaluation of the replica is rather difficult. Essential requirements are that replicas are reproducible, that they show characteristic structures, distinguishable from artefacts, and that they permit unequivocal identification of the microstructure of the original sample. The most convenient procedure for sample preparation’ 1 is shadowing with a heavy metal in LWCIIO.Aluminium is then deposited on this layer, and the replica is removed on a film of methyl celiulose or gelatin. The difficulties of this procedure are avoided by use of the scanning electron microscope”, which yields photographs clearly showing the three-dimensional topography of the surface at magnifications of 20,000-50,000. The degree of homogeneity before ageing and structural changes in the paint films caused by individual ageing factors can be investigated by electron microscopy of sections of thickness 0.05-0.20 Jtm, cut in an ultramicrotomes3. The preparation of these sections is difficult, particularly in pigmented films containing pigment particles of diameter greater than the thickness of the section itself. Choice of a suitable Pmbedding medium as well as the choice of a suitable knife are important factors in achieving successful results.
Progr. Org. Coatings,
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263
5. CONCLUSlONS
From this review of the complex factors contributing to the ageing of paint films the following broad conclusions may be drawn. (1) Successive coats should be based on film-formers of a kind appropriate to the conditions of exposure and having similar characteristics in terms of glass transition temperature, tensile strength, elongation, internal strain and adhesion. (2) Successive coats should not differ greatly in their ratios of pigment volume concentration to critical pigment volume concentration54. (3) Successive coats and the paint system as a whole should have coefkients of thermal expansion as close as possible to that of the substrate. (4) Over the ambient temperature range expected during the lifetime of the coating, the film should be hard and tough and such properties as glass transition temperature, tensile strength elongation, internal stress and adhesion should be substantially invariant with time. (5) In terms of chemistry, the coating should above all be resistant to oxidation and photodegradation. If this cannot be achieved by selection of primary Mm-former, resort must be had to anti-oxidants and U.V. absorbers. REFERENCES 1 W. H. Talen, J. Oil Coforrr Chemists’ Assoc., 45 (1962) 387; 46 (1963) 2 M. Dyck, Farbe Lack, 68 (1962) 543, 608. 3 H. A. Stuart, Angem. Chem., 79 (1967) 877. 4 W. Kern, Chemiker-Zig., 91 (1967) 255. 5 6 7 8
9 10
11 12
13 14
R. W. S. Burell, Paint Mamrf., 36 (2) (1966) 32. R. R. Myers, Ofic. Dig., 34 (1962) 575. J. L. Bolland and G. Gee, Trans. Faraday Sot., 42 (1946) 236. E. H. Farmer, J. Chem. Sot., (1943) 541; (1946) 1022. J. D. Nye and J. S. Mackie, Ojic. Dig., 34 (1962) 816. A. R. Schultz, J. Chem. Phys., 29 (1958) 2Op. C. D. Miller, J. Am. Oil Chemists’ Sot., 36 (1959) 596. C. D. Miller, I& Eng. C’hem., 50 (1958) 125. K.-H. Reichert and R. Sattelmeyer, Farbe Lwk, 73 (1967) 512. K.-H. Reichert, Alterung und Korrosion vofi Kunststoffen, Korrosion,
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15 K.-H. Reichert and E. Gulbins. Proc. V/llth Congr. FAT’PEC, Scheveninyen, 16 S. V. Yakubovich. Lakokrasoch. Mater. Ikh Primen., (1) (1964) 56. 17 P. J. Hearst, J. Paint Techno!.. 39 (1967) 119 18 M. A. Keenan and D. A. Smith, J. Appl. Polymer Sci., II (1967) 1009. 19
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26 W. Kijnig,
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121.
29 A. T. Samharovskiy and G. 30 A.
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264
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31 P. I. Zubov and L. A. Lepilkina, Kolfoidn. Zh., 24 (1962) 30. 32 P. I. Zubov and L. A. Lepiikina, Kolloidn. Zh., 23 (1961) 418. 33 V. A. Kargin, T. I. Sogolova and M. I. Karyakina, Khim. Prom., (7) (1955) 392. 34 M. I. Karyakina, V. A. Kargin and T. I. Sogolova, Khim. Prom., (7) (1957) 365. 35 W. Simpson, .i. Oil Colour Chemists’ Assoc., 45 (1963) 331. 36 S. V. Yakubovich and N. L. Maslenikova, Lukokrasoch. Mater. Iklz Primen., (5) (1961) 27. 37 P. 1. Zubov and L. A. Lepilkina, Lukokrusoch. Mater. Ikh Primen., (5) (1961) 19. 38 U. Zorli, Proc. IXth Congr. FATZPEC, Brussels, 1968, p. Pldn. 3. 39 W. Bosch, F. J. Macekonis and P. H. Martin, Ofic. Dig., 26 (1954) 1219. 40 U. Zorli, Forschungsber. Landes Nordrhein- Westfilen, (136 1) (1964). 41 F. Kollman, Technologie des Holzes, Berlin, 1936. 42 E. G. 0. Schmidt, Furben-Ztg., 37 (1932) 376. 43 R. Quadrion, Paint Manuf., 20 (1950) 470. 44 N. A. Krotonova and L. P. Morosova, DAN SSSR, I7 (1959) 302. 45 H. Danneberg, J. Polymer Sci., 33 (1958) 509. 46 A. M. Malloy and W. Soller, Pain1 Oil Chem. Reu., (1953) 14. 47 Anon., Paint Manrrf., 28 (1958) 273. 48 S. Moses and R. K. Witt, Ind. Eug. Chem., 41 (1949) 2334. 49 W. KBnig, paper on VIrh Congr. FATZPEC, Wiesbaden, i962. 50 J. Mariott, J. Oil Coiour Chemisrs’ Assoc., 41 (1958) 363. 51 S. H. Bell. 3. Oil Colorrr Chemists’ Assoc., 43 (1960) 466. 52 J. Fairless, J. Oil Cofour Chemists Assoc., 52 (1969) 491. 53 U. Zorll, Furbe Luck, 71 (1965) 273. 54 0. Kolai and B. Svoboda, Furbe Lack, 75 (1969) 31.
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