Radiation curing of organic coatings

71

Progress in Organic Coqtings, 11 (1983) 71 - 103

RADIATION CURING OF ORGANIC! COATINGS R. DOWBENKO, C. FRIEDLANDER, PPG Industries Inc., Coatings Abloom Park, Pa 15101 (U.S.A.)

G. GRUBER, P. PRUCNAL and M.~SMER

and Resins LXuision Research

Center,

Rosanna

Drive,

CO&?& 1 Introduction.

71

.............................................

2 Scope of the review .........................................

72

3 Mechanisms of UV cure. ......................................

73

4 Mechanisms of electron initiation ................................

77

5 Comparison of UV and electron cure ..............................

78

6 Oligomers and polymers for radiation cure ..........................

78

7 Pigmented radiationcurable

84

coatings ..............................

84

8 Pigmented EB coatings ....................................... 9 Pigmented UV coatings ...................

; ...................

85

10 White UV-cured basecoats .....................................

89

11 Inks ....................................................

91

12 Other opacification

93

methods

...................................

13 Radiation-curable coatings - Applications .......................... 13.1 Vinyl film substrates. .................................... 13.2 Floor coverings ........................................ 13.3 Paper ............................................... 13.4 Wood ............................................... 13.5 Textiles ............................................. 13.5.1 Non-woven fabric bonding. ........................... 13.5.2 Coated fabrics .................................... 13.5.3 Pigment printing .................................. 14 Equipment for UV cure. ...................................... 15 Electron beam curing equipment. References.

................................

96 96 96

97 97 98 98 98 98 99 101

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

1. Introduction The subject of UV/EB cured coatings is, in many of its implications, a uniquely current topic. Ecological concern in both the public and private sectors together with an impending energy crisis will, in a few short years, force a change in the complexion of the entire coatings industry. 0033-0655/83/010071-33/$10.15

0 Elsevier Sequoia/Printed in The Netherlands

72

The technology of UV/EB cure addresses itself to both of these problems. In an idealized sense, it attempts to transfer a significant portion of the chemistry and mechanics of the plant directly to the substrate being coated with attendant savings of energy and ecological benefit. Inherent in the (idealized) concept of the radiative process is the elimination of volatile byproducts not only at the application level but also as far as possible in the manufacturing process itself. This could be accomplished with coating vehicles consisting of low viscosity, non-volatile fluids which require little or no added solvents for processing or application. Keeping the use of volatiles to a minimum in both processing and application ensures a reduction in solvent pollution. One of the significant advantages that can be realized in the radiation process is the reduced space requirement inherent in a high application rate coating line. Hence, large plant areas to house long oven lines are no longer a requisite for curing at high space rates. Perhaps les obvious, but nonetheless real as regards energy utilization, is the fact that with the UV/EB cure process the idealized monomeric or oligomerit vehicle polymerizes to high molecular weight and dries to the finished coating in one and the same rapid step. With most conventional coatings the polymerization and drying steps are separate processes, each with its own considerable energy/time requirement. Also inherent in the above considerations are reduced labor costs accruing from increased productivity and reduced maintenance requirements. As yet, many of the idealized objectives of this technology are unrealized. Nevertheless, the very newness of the technology spurred by everincreasing ecological and energy requirements ensures that progress toward the ideal will be rapid and of a high technological caliber. The intent of the present review is to critically evaluate this progress from the perspective of the coatings technologist and polymer chemist and, where possible, to point out those signals of science which may lead to a rapid maturation of this new technology.

2. Scope of the review This discussion will consider the use of radiation primarily as a means to form crosslinked coatings directly on the substrate to be coated, as opposed to its use (as an initiator) to prepare polymeric coating vehicles for subsequent application by conventional means. The coatings thus formed may be non-pigmented or may contain inorganic or organic pigments and colorants in addition to soluble dyes. Printing inks, being a type of highly pigmented coating, are also considered. Details of cure mechanisms, chemical structure and equipment design (process innovations) are discussed insofar as these are germane to the coating process, and specific industrial applications are also considered.

13

The nature of the radiation used to obtain cured coatings is for all practical purposes limited to that of ultraviolet light and accelerated electrons. Other curing sources which may be broadly described as radiation, such as infrared, microwave and induction curing, are essentially alternate means for thermally curing otherwise conventional coatings and are, thus, excluded from this discussion.

3. Mechanisms of W cure The component of a UV-curable coating system which is commonly present in the smallest amount but yet often requires the most sophisticated degree of technological development is the photoinitiator. A photoinitiator may be broadly defined as any compound which can be raised to an excited electronic energy state by the absorption of electromagnetic radiation in the form of ultraviolet or visible light; and which through either intramolecular or intermolecular interaction, can result in the formation of a reactive intermediate. These reactive intermediates can then react further to initiate polymerization and cause curing of a coating film. The most commonly utilized photoinitiators are various types of aromatic ketones. The photochemistry of these types of compounds is shown in Reactions 1 - 6. K K” K*

hg

> K*(Singlet)

( Singlet

1 w isc

( Singlet)

K*(Triplet

1 +

K* (Triplet

)+a

K*(

1 ->

Triplet

K+“*

l K*(Triplet)

K+hv” -

Q*+K Fi.

Absorption

(1)

Fluorescence

(2)

I

Intersystem

Crossing

(3)

Phosphorescence

(4)

Quenching

(5)

Radical

Production

(6)

The terms singlet and triplet refer to the electronic spin configurations of a molecule in its excited states. A configuration with no unpaired electrons (as in the ground state of most compounds) is a singlet state and that with two unpaired electrons is a triplet state. Spectroscopic selection rules forbid the direct formation of a triplet state from a singlet ground state, therefore population of the triplet state usually occurs only through the spin inversion process known as intersystem crossing. The same spectroscopic rules also disfavor decay of the triplet excited state back to the ground state, resulting in a lifetime for the excited triplet which is several orders of magnitude longer than that of the corresponding singlet. Since the triplet state is of lower energy than the singlet

74

due to its greater multiplicity (Hund’s rule), the triplet to singlet reverse reaction does not occur. The net result is that the triplet excited state is the most favorable for the production of useful photoproducts. Perhaps the most efficient photoinitiators are acetophenone derivatives which undergo a-cleavage photofragmentation reactions (Reaction 7). OMe

0

OMe

0*

(7)

ET-

Bond

Energy

kcal

0

OMe

0

71

=

61

kcal/mole

For cleavage to occur, the energy of the excited electronic state of the compound must be larger than the energy of one of its chemical bonds and the site of excitation should be localized very near the weak bond. Benzoin ethers meet both these requirements and their rates of a-cleavage are sufficiently great that the quenching reaction indicated in the general scheme does not effectively compete and radical production is extremely efficient. If a ketone is not able to undergo a photofragmentation process, then the nature of its lowest excited triplet state will largely determine its ability to produce useful radicals. These types of ketones may produce radicals through bimolecular reactions in the presence of other compounds which can serve as hydrogen donors. The differences in reactivity of T--K* and n--a* triplet states can be understood by examination of the electron distribution in the excited state carbonyl group (Fig. 1). The net effect of the n-x* configuration is to remove electron density from the oxygen atom and give the carbonyl group a diradical character with hydrogen atom abstracting ability, as shown in Reaction 8, similar to alkoxy radicals. H

;,.o+-A-x n,n*

X=OH

I

or

NRp

-;i:-OH+-b-X

(8) I

76

rncrrasad

density

eon oxygen

Fig. 1. Carbonyl photochemistry.

The P-K” state, however, tends to increase the electron density on the oxygen, thereby deactivating the carbonyl group toward hydrogen abstraction reactions. Ketones with T--K* lowest triplets can produce radicals by means of electron transfer reactions which are followed by proton transfer. This process (Reaction 9) is favorable only with compounds such as tertiary amines which have a heteroatom with a relatively low ionization potential.

* 1

(9)

Compounds such as benzophenone which have lowest WE* triplet states can react with either alcohol or amine groups, but molecules like 2-ch~orothiox~~one (CTX) can only react with amines fl, 21. The nature of this initial ebetronie transition has im~o~nt implications in UV curing technology. Due to spectrosoupic selection rules, the x-z* position is more allowed and thus its absorption coefficient is much larger. This effect is seen in the absorption spectrum of Fig. 2. This spectrum is representative of both benzophenone and the benzoin alkyl ethers, The absorption coefficient at 250 nm (corresponding to a IYE* transition) is two orders of magnitude larger than that at 330 nm (corresponding to the n-x* transition). As mentioned previously, either transition will result in eventual population of the same lowest excited triplet state. Figure 3 illustrates the effect of absorption coefficient as a function of concentration of the absorbing species in a film.

76

Benzophenone

Solvent.

2000

2200

2400

Fig. 2. Absorption (1945) 2127).

h

R. R.

350 "In 10 mil 90X

R.

2600

2600 Wavelength

3000 m Angstroms

spectrum of benzophenone

02

O2 250 "I" 1 ml1 90::

rfhonol

R.

R.

R.

O2 Il.

R.

R.

R.

02

2

It.

R.

R.

0 R.

R.

R.

R.

R.

O2 R.

R.

R.

R.

3200

3

(R. N. Jones, J. Am.

0

2

O2

R. R.

R.

0

R. R.

10 Chem.

Sot.,

-t 3600

67

2 R.

R.

R.

R.

R.

R.

R.

R.

R.

R.

!YbBenzophenone rn non-absorbing film

Fig. 3. Effect of absorption coefficient as a function of concentration of absorbing species. Light of 250 nm wavelength will only penetrate the film to a depth of 1 mil before 90% of its intensity has been absorbed, whereas light of 350 nm will penetrate to a depth of 10 mil before it has been 90% absorbed.

77

Light of 250 nm wavelength will only penetrate the film to a depth of 1 mil before 90% of its intensity has been absorbed, whereas light of 350 nm will penetrate to a depth of 10 mil before it has been 90% absorbed. This results in a very high localized co~cen~ation of radicals at the surface (250 nm absorption) where they are needed to overcome oxygen inhibition and a lower concentration of radicals throughout the film (350 nm absorption) where oxygen inhibition is less important. This allows coatings to be cured in air by the use of a light source which has output in both the 250 nm and 350 nm region (medium pressure Hg lamps).

4. Mechanisms of electron initiation Electron cure represents a rather major departure from ultra~olet cure in terms of mechanism of initiation. This is probably best represented by contrast. Actinic radiation is of course electromagnetic energy. Electrons are particles. UV energies are of the order of 3 - 6 eV, whereas electron accelerators for coatings operate at five orders of magnitude greater energy in the 150 - 500 000 eV range. When a charged particle passes close to a molecule, electrostatic interactions strongly polarize the molecular electrons. If the energy added to a molecular electron in such an interaction is greater than the binding energy of that electron, the molecular electron escapes. Thus the incident electron produces a second free electron. The two electrons can then produce further ionization if they have sufficient energy. The actual binding energy of electrons in organic molecules is in the range 10 - 15 eV. Input energies are of the order of 100 000 - 1000 000 eV, thus each electron causes multiple ionizations. The key initiation step usually occurs subsequent to ionization. Usually the radical cation which results from ionization fragments to form a radical and a positive ion. The radical can then initiate, while the ion will typically trap a moderate electron or a negative ion. A second potentially important electron initiation mechanism occurs when the energy transferred to a molecular electron, upon interaction with an accelerated electron, is insufficient to ionize the molecule. Under these conditions, the molecule can be raised to an electronically excited state just as if it had absorbed a photon. Thus photoche~s~ is also possible. It has been pointed out [3] that about 50% of the energy absorbed during electron-molecule interactions can be accounted for in terms of ionization energy. It is reasonable to assume that much of the remaining energy is used for electronic excitation. Thus the number of electronic excitations per electron is possibly as large as the number of ionizations. These excitattions are of course subject to optical selection rules, but occur randomly, since the excitations result from electron-molecule interaction. Thus a maleate double bond has as much chance of excitation as a

78

benzoin ether. Unfortunately, maleate excited states do not undergo reactions which are beneficial in a polymerization context, and the energy is wasted as heat.

5. Comparison of W and electron cure For reasons of chemistry, one can sometimes choose between the alternatives of UV and electron beam curing. For example 300 keV electrons will easily penetrate a 10 mil paper and resin laminate, whereas UV photons will not. UV coatings can be cured in air, but in general an electron-curable coating requires an inert atmosphere. A 5 mil, highly opaque coating is no problem for electrons, but a major challenge for UV. A l/2 inch clear casting requires an electron of several MeV energy for penetration, whereas UV will do the same job without difficulty. Thus UV and electron cure are complementary methods, as summarized in Table 1. A choice between the two requires a chemical and economic analysis specific to the desired product. TABLE 1 UV - Electron technical comparison Product

Electron cure 300 kv

UV cure 200 w/in

10 mil paper laminate 10 mil opaque 100 mil casting 1 mil opaque, air

Yes Yes No No

No No Yes Yes

6. Oligomers and polymers for radiation cure The more prominent recent objective of research as related to radiationcurable oligomers has been to increase cure rates and reduce or eliminate the oxygen sensitivity of the commonly used unsaturated polyesters and acrylates. Obtaining the desired physical properties has been achieved by using the principles defined for conventional coatings, i.e., crosslink density, modification of the oligomer backbone and variation of the diluting monomer used for formulation. We will discuss here the modifications of the radiation-sensitive oligomers themselves, as differentiated from sensitizer technology, to change the mechanism of cure for enhanced free-radical processes and thereby improve cure rate and reduce oxygen sensitivity. 30th of these approaches, however, are ~~rde~ndent. Polyesters initially received most attention, and reduced oxygen sensitivity was achieved by the use of wax which rises to the surface during cure,

79

or by the incorporation of ally1 groups into the polyester backbone. Cure rates were also enhanced by increasing the amount of unsaturation in the polyester backbone. Aerylates have been a more difficult problem and early attempts to improve cure included the use of very highly functional monomers, such as pentaerythritol triacrylate, combined with high levels of initiators, e.g., combinations of benzoin ethers and halogenated paraffins at levels as high as 10% or more. For acrylates, and to a lesser degree for polyesters, the use of aryl ketones in combination with reducing agents has significantly advanced the potential applications of radiation-cured coatings. A means of reducing the oxygen inhibition of radiation-curable oligomers is to incorporate amine groups into the oligomer sites which compete effectively with the crosslinkable unsaturation for oxygen. One class of oligomers which function exceptionally well in air is the betahydroxy esters of acrylates [4] . These are readily prepared from epoxides and acrylic acid and yield -CHOC-groups. From several lines of evidence it is reasonable to propose that the hydrogen on the hydroxyl-bearing carbon enters into the polymerization. First, transfer constants for secondary alcohols are respectably high. Secondly, the concentration of such carbinyl hydrogens is high in acrylic acidepoxy adducts. Thirdly, the geometry in B-hydroxy-alkyl acrylates is favorable for intramolecular hydrogen transfer (Reaction 10). Fourthly, CHZ

OH I

HEA+R’

-

0 \

,

0

Cl-l ‘H3

\

HOkHCH20:CH2CH2R

(10)

/

b-c=0 H

RCHP-

Bisphenol A derivatives A and B have relative cure rates in air of A = 5.5 and B = 1.0 when initiated by UV in the presence of a benzoin ether. Of course,

Me.

OH

0

(A)

C(-@-O-CH&H-CH@-&CH=CH2)2

Me’

OH

I

Me. C(-@-0-CH2Me’

structure

0

II

C-CHyO-DCH=CH2 I Me

B is incapable

of carbinyl

(B)

j2

hydrogen

abstraction.

So, if it can be

OH assumed that a carbinyl radical (-C .-) is frequently formed, then what advantage does it have? A number of possibilites can be envisioned, as shown in Reactions 11 - 14.

80

I

-Poo.+c

-

-POOH+i:

1

(Reinitiation

‘I’ OH

‘I’ OH

(11)

H

,F\

w* I

i C+O*-C ‘I’ OH

+& /I\

/I\

‘I’ OH

OH

OH

(12)

OH

OH

.L

R-d-R+*OH

R-L?--+

L,

OOH

OH;

Modest Heat

~Reinitiation)

(13)

A

Li

OH R-:+I?. :

OH l?.+kH

OH -3

i I’\

4(f?einitiation)

‘Idead\’ polymer

Thus, not only does available oxygen react at a site other than the acrylyl group, but such products probably further initiate polymerization [51Another approach is to modify the oligomer with a known reducing agent (Reactions 15 - 17). Thus, primary and secondary amines have been 0

0 (R)2NCHsCH2;0-

0t)2NH+CH2=CH&O--

(15)

0

W , ~R2~NC”*C”2~0-+~0)*c=0

(16)

0 (*),c-OH+(R,)NcH-CH,2-0-

(17)

Dimers

added to a portion of the available acryloyl groups in an oligomer via the Michael reaction. The resulting product, therefore, has abstractable hydrogens alpha to the available nitrogen for reaction with oxygen and photosensitizer such as benzophenone. Another means of incorporating amine groups is the reaction of isocyanate-containing acrylates with tertiary alkanol amines.

81

Other synthetically useful methods can be readily envisioned for the inclusion of amine functionality into oligomers. Other nitrogen-containing groups which are reportedly useful in improving the air curability of acrylics are the urethane, amide [ 61 and triazine [ 71 moieties. An oligomer modification which is essentially the inverse of introducing groupings with abstractable hydrogens is the inclusion of photoreactive groups which abstract hydrogen. One method which has been used is to introduce carboxyl-substituted benzophenones into the oligomers [8 - 111. These benzophenone moieties may also be substituted with halogens or tertiary amine groups. Large amounts of such groups could, in addition to abstracting hydrogens, also increase the surface adsorption of UV light and further enhance the surface cure of the oligomer. Presumably incorporating large amounts is accomplished without deleterious effect upon the coating, as is often the case when large amounts of ‘free’ photoinitiator are added to a coating composition to enhance cure rate at the surface. That the initiator is bound to the prepolymer is apparently responsible for the reduced amount of chain transfer, better through-cure and lack of plasticization as compared with unbound photoinitiator. An interesting variant of the free radical mechanism is the combination of di-, tri- and higher thiols with ethylenically unsaturated oligomers [12 141. A typical composition contains an allyl-terminated oligomer and a trithiol such as trimethyolpropane his-( 3-mercaptopropionate). Acrylyl, methacrylyl, ally1 and maleic types of unsaturation are also used. Reduced oxygen sensitivity and high cure rate are possible by employing the concept of charge-transfer assisted photopolymerization [16 - 191. Specific examples are blends of acrylates, maleates and N-vinylpyrrolidone, and certain of these compositions cure at 50 - 100 ft/min/lamp in air [ 151. Possible pathways for polymerization are shown in the accompanying equations (Reaction 18).

cx +

/kc:

’

N

I

(18)

AH.CH2

w

[

y

0%o N

LH-CH1:

0

Copolymer

0; / \

* 1

*

1 and

N-VP

Homopolymbr

82

If the acceptor is maleate, N-VP homopo~yme~zation and zation of N-V~~rna~ea~ are primary pathways. ff acrytate is initial charge-transfer complex could initiate both acryfate and polymerization as well as copolymerization. The donor-acceptor interactions shown (Reaction 19)

eopolymeripresent, the N-VP homoreduce

the (19)

ion

radicsfs

energy required to activate the rnQ~ornar pair and induce their polymeriza-

tion. The significance of these processes for radiation-cured coatings is that if the proper choice of donor and acceptor monomers and oligomers is made, rapid photopolymerizations with reduced oxygen sensitivity are possible in the presence of conventional free radical photoinitiators [ 153 . Oxygen inhibition is reduced as both cationic and free radical propagation of the donor monomer can occur simultaneously [16,17], Copolymerization of the donor with acceptor occurs by the free radical process and no anionic polymerization by the acceptor has been detected in recent investigations j16, 173. The choice of donor and acceptor is critical in that the ground state interaction should not be strong enough to produce ion-

radicals and spon~eo~~y induced po~yme~zation. Upon pho~excitation, however, rapid charge transfer should occur from the excited-state complex. Acceptable donor monomera for consideration in the preparation of practical coatings systems are at present restricted primarily to the N-vinyl class of compounds. Acceptors include the maleate polyesters and acrylates, the former being the better acceptor, Other donors would include the vinyl ethers, arrd other acceptors include the &cyano-vinyl compounds, e.g., acrylonitrile. Photocrosshnking of unsaturated polymers solely by charge-transfer initiation is also a possibility. An example is the photocrosslinking of poly(Zpheny~bu~d~ene) in the presence of small amounts of ~~acy~oe~y~ene f’i8f. A weak ground state interaction exists between these two components and crosslinking of the unheard poiymer is situate by the photoexcited charge-transfer complex. As charge transfer can be inhibited by impurities [19], the use of this process as the sole means of initiating the photopolymerization of coatings has so far been restricted. Noa-free radical initiation of polymerization is also employed [ 20,211. It is a ring-opening polymerization initiated by species which produce cations upon UV exposure. The monamers used are primarily OX~UH?S, but blends of the epoxides with larger cycle ethers or esters, such as tetrahydrofuran or c-caprolactone, are reportedly useful, and blends of the epoxides with unsaturated compounds, such as vinyls, have also been ~v~stiga~d 121 - 233 0

83

Catalysts are aryldiazonium salts, the preferred ones having hexafluorophosphate as the anion [ 241 (Reaction 20), and aryl iodonium and sulfonium

(20)

R-‘c-Ye+ R-\C-\C/+PF+_a \ \/

I

0

0

PF5 \

\I

R-C-C \

I

\\/

/-

C-R

R-C-C-O,

\

/

0

\

I

cs+

0

/\

I

PF5

salts with anions of PF;; or SbG [25]. Other catalysts which have been claimed to be effective are sulfonate esters and nitro toluene derivatives which undergo phototropism (Reactions 21 and 22).

@K~

,

R_S06”

(21)

(22)

NcO

-0 Neutral

Acid

H+

Inhibitors used to reduce viscosity drift in the case of the diazonium compounds are sulfoxides, nitriles, amides and other compounds which have readily available electron pairs [ 26 - 291. Small amounts of solvent are required to dissolve these catalysts, the preferred one being propylene carbonate at levels of 1 - 2 wt.% of sample. The cure rates of these systems upon UV exposure are quite rapid, speeds in the range of 400 - 1200 fpm/lamp being reported [26 - 291. As the initiation is ionic, oxygen inhibition is not a problem. Such speeds are attainable not only because of the inherently fast initiation and

84

propagation of the systems and lack of oxygen sensitivity, but also due to the fact that curing continues after completion of UV exposure. Having discussed the mechanisms of initiation and polymerization in UV and EB, we will now turn our attention to the discussion of radiationcured coatings. The subject may be broadly divided into two parts, one dealing with clear coatings and one concerned with pigmented coatings.

7. Pigmented m~~on~umble

coatings

Although some of the very first commercial radiation applications involved pigmented coatings, perhaps the greatest remaining challenge for radiation research - especially in the field of UV coatings - involves the development of commercially viable pigmented coatings analogous to conventional coatings. For one thing, many of the difficulties (e.g., flow, wetting, flatting) attendant to the introduction of this new, high-solids coating technology are magnified when applied to pigmented coatings. For example, application viscosity becomes a particularly difficult problem in pigmented systems where conventional solvents cannot be used. Moreover, pigment/vehicle interactions arising from the higher reactivity of the radiation vehicle often lead to poor shelf stability or color problems not encounted with conventional vehicles. Finally, the very reason for using the pigment - its opacity - becomes a barrier to commercialization in the case of UV coatings. Commercial applications of pigmented UV coatings that are well established include crosslinkable wood fillers and various UV inks. The former application employs moderately high loadings of inexpensive extender pigments that are substantially transparent to UV light. This permits ready UV cure, albeit at relatively low line speeds. Pigments of this general class include calcium carbonate, talcs, silicas and clays. Vehicles, traditionally of slow curing polyester/sty~ne, are now giving way to the four-cu~g acrylic vehicle or combinations of the two types. Viscosity is much less a problem in this application since such products are normally applied with equipment capable of handling paste-like consistencies. Sanding and subsequent finish coats upgrade product appearance significantly. 8. Pigmented EB coatings Electron cure of even very thick pigmented coatings is relatively easy. Beyond such a generality, there is a distinct lack of published information concerning the formulation of pigmented EB coatings. So fundamental a question as “What is the outdoor durability of EB cured coatings?” goes largely unanswered - even at radiation conferences. Other than patent samples and a scattering of papers alluding in the most general way to pigmentation, virtually the only definitive paper to date on the subject appeared in 1972 t30].

85

While generally supporting the oft-repeated contention that pigmentation of EB paints presents no great problems, some interesting and sometimes unexpected effects are described. For example, while it is true that increasing density reduces electron penetration and, one would suppose, the through-cure of a given coating, Huemmer et al. surprisingly found that higher film densities (e.g., higher pigment levels) actually promote through cure [ 301. Also somewhat surprising is the finding that organic colorants (phthalocyanine and azo types) do not change significantly in spectral properties over a practical dose range (2.5 - 10 Mrad). Higher doses did cause spectral change (darkening) of the colorants both neat and in combinations with other pigments. No study was made of light fastness of organic colorants in outdoor exposure, which might be expected due to subtle changes caused by even low radiation doses. Phtbalocyanine pigments of all the organic colorants had limited can stability with the particular vehicle used ~thio-u~th~e acrylate ) .

Black pigments and metallics as classes were the least stable in radiation systems. Iron oxide and oxidized blacks had the greatest s~bility, while all metallics caused gelation with the thioureth~e as well as with a more conventional polyacrylate. In most other respects the effects of pigmentation on other film properties, e.g., hardness, brittleness, permeability etc., are analogous to those found with conventional coatings. While this also extends to the flattening ability of silica, higher leveling must be used in the solventless systems to achieve a given level of flatting. Because of high silica/vehicle interactions, a significant degree of flatting could give a total increase in viscosity of the system - an often encountered problem in radiation-cured coatings. An answer to the flatting/viscosity problem in EB (and UV) coatings is described in a U.S. patent [ 311. A curing arrangement termed ‘dual cure’ greatly increases flatting efficiency at low silica - and thus, low viscosity levels. In this process the pigmented coating is EB (or UV) cured in an atmosphere containing oxygen to give a coating with an incompletely cured surface. This has the effect of increasing the silica exposure at the surface of the coating through shrinkage or microevaporation. The amount of protrusion of pigment is sensitive to total volatile loss and shrinkage during and after first-cure stage. Subsequent cure (EB or UV) in an inert atmosphere gives a completely cured surface, which is almost ind~~n~shable in properties (except for gloss) from a surface cured only in an inert atmosphere. The method is the basis of gloss control in co~erci~ EB processes, and products ranging from dead flat to nearly full gloss are affable. The sequence of this process is shown schematic~ly in Fig. 4. 9. Pigmented W coatings Several years ago the suggestion of UV curing of highly opaque TiOz pigmented base coats would have been met with some skepticism. This, of

A

B

^

0

Fig. 4. Stages in the curing of a pigmented film. (A) Wet coating. (B) After first cure in atmosphere containing oxygen - incomplete curing at surface. (C) Before second curing state - pigment particles protrude. (D) After curing in an inert atmosphere --low gloss, abrasion resistant surface.

course, derives from the apparent contradiction of passing light through a body of some thickness that is opaque to that very light. UV inks, of course, are a commercial reality with hundreds of operating lines throughout the world. The successful UV cure of inks is easy to rationalize on the basis that, although highly pigmented, they are applied as extremely thin films and most often comprise organic and inorganic pigments and colorants that are substantially transparent in the UV region. The UV cure of much thicker coatings which contain up to 50% titanium dioxide is not so easily understood, much less accomplished. Some of the technical details will be considered later in this discussion. First, it would seem that one simple answer to the problem of delivering UV energy throughout the film is to choose pigments that have substantial UV transparency but still scatter efficiently in the visible region. Pigments in this general class include zinc sulfide, antimony oxide, zinc oxide, anatase and lithopone. Some of these have been specified, usually in combination with yet other pigments such as r-utile, for improved UV cure at good hiding levels. While the above pigments do in fact permit effective UV cure at fast line speeds, the hiding power, i.e., visible light scattering ability, is too low compared to r-utile. This accrues mostly from the relatively low refractive indices of such pigments. Hence thick films or very high pigment loadings must be used. This creates yet other problems, since several of these pigments also show pronounced vehicle interactions such that viscosities are prohibitive at even low levels. Finally, most of these alternate pigments

87

either have poor inherent whiteness or discolor during UV cure/post-bake, a result of pigment-vehicle interactions, making them unsuitable for a truly white product. For UV metal decorative applications requiring non-white colors, some of the above will no doubt be useful in pigment blends. Since r-utile TiOz seems to be the pigment of choice in UV white systems, full advantage of its spectral qualities must be taken to achieve the most efficient UV cure. While it is true that rutile absorbs strongly in the UV region (Fig. 5), it is also as efficient a scatterer of UV light as of visible light. REFLECTION

ABSORPTION

70

_

---

Rutile Anatase

I 320

340

360

380

400

420

440

“In

Fig. 5. Absorption

spectrum of rutile and anatase.

Hence, in a film of moderate thickness (i.e., 0.3 - 1.0 mil) some scattered UV light will succeed in penetrating through the film to the substrate. This effect may be demonstrated by cure of rutile pigmented film containing a photoinitiator absorbing in the region of maximum TiOz absorption, given a long enough exposure to UV (Fig. 6). The fact that rutile has been reported to act as a photosensitizer at low levels (1 - 3%) in clear UV coating is reportedly due to its UV scattering ability [32] . UV

HEAVILY PIGMENTED FILM

SUBSTRATE

Fig. 6. Penetration

of UV at high TiOz concentration.

Non-Piwnted Coatlng Substrate

Lightly Pigmented Coating

Substrate

Fig. 7. Enhancement

of UV cure at low TiOz concentration.

The effective dose of UV is increased through an increase in the mean free path of the incident light (Fig. 7). The strong UV absorption of r-utile falls from 85% at 380 nm to less than 10% at 420 nm and results in a region where a great deal more a&ink energy will penetrate the film, even after some attenuation by multiple scattering. Hence, the choice of photo~itiato~ with absorption maxima in this region will do much to improve the UV cure. Unfortunately, many of the commonly used photoinitiators (e.g., benzophenone, benzoin ethers) have their absorption maxima below this region (i.e., at 320 - 350 nm). There seem -to be two major approaches to this problem. For photoinitiation to occur in the presence of pigment, the photoinitiator must: 1. Absorb light in the same region as the pigment. light absorbed by photoinitia~r K = function of concentration and absorption coefficient This requires a. high E b. high concentration

low penetration of light i 2. Absorb light in the region where pigment absorption and reflection properties are minimal. higher a. lower e penetration of light b. lower concentration

89

The fiit involves essentially a competitive absorption process wherein the photoinitiator can effectively compete for the light in’a wavelength region where the pigment absorbs very strongly. This requires that the photoinitiator has a very high absorption coefficient. The second approach involves the use of a ketone which absorbs light strongly in the region where TiO? absorption properties are minimal (> 380 nm). Thioxanthone and Bchlorothioxanthone have coefficients of 4000 6000 in this region and are very commonly used in combination with tertiary amines. An alternative to the use of amines which has been reported is the incorporation of sulfonyl chloride or chloroalkane groups into the thioxanthone molecule. Such groups contain a sufficently labile bond to allow photofragmentation to occur. The concentration of photoinitiator is more critical in opaque coatings than in clear ones. An optimum level must be used which will be high enough to compete with pigment absorption, yet low enough to allow maximum light penetration into the film. This optimum concentration will decrease with increasing film thickness. While there is not a great deal of published information concerning the best photoinitiators for UV white systems, some examples include methylanthroquinone [ 321, phenanthrenequinone [ 331, arylketone derivatives of diphenyl sulfides [ 341, etc. Published information also indicates that certain aryl ketone/amine combinations are effective for cure of TiOz coatings [33] . Likewise, aryl ketone derivatives of diphenyl sulfide groups are said to promote cure without a yellowing of the white coating [ 341. Michler’s ketone in combinations with chlorinated polynuclear hydrocarbons and aryl ketones is also specified [35]. We will now discuss the problems encountered in achieving a practical commercial product. 10. White UVeured base coats Table 2 summarizes the principal requirements for a UV white base coat for metal, which constitutes one of the largest potential applications for a UV white coating. Of these requirements, the most persistently difficult to TABLE 2 Requirements of UV white base coat Cure speed Adhesion Color Application viscosity Scuff resistance Total weight loss Hiding

2 25 fpmb (5 1 s exposure) Aluminum, tin-free steel, tin plate Non-yellowing (UV and thermal) 120 - 260 cps o withstand can rubbing T - 2.5% (UV and thermal) > 76% Reflectance (cap y)

90

achieve in combination have been adhesion (~uminum, tin-free steel and tin plate) non-ye~owing (both initial UV cure and post-bake) and application properties (good flow and leveling). Any approach to successful UV base coats necessitates a choice of vehicle components (i.e., oligomer, monomers, etc.) with maximum reactivity (for high line speeds) and with at least the potentiaI for wetting the various substrates. Major American can manufacturers insist on air-curable coatings. Information concerning specific vehicle components used in UV base coats is sparse, limited to relatively few patent examples and some of raw material suppliers. in these, two or three basic formulation types emerge. One type is based on oligomeric urethane acrylates (isocyanate p~polymer reacted with HEA) blended with smaller amounts of epoxidized oil-derived acrylate and low viscosity mono- (e.g., methyl carbamyl acrylate) and diacrylates (e.g., neopentyl glycol diacrylate) [33]. With such vehicles, chlorothioxanthone/alkanol amines are the photoinitiators of choice. Another vehicle type is built around the diacrylate of ‘bisphenol A diglycidylethers with smaller amounts of pentaerythritol acrylate, a polyether diacrylate and, in some cases, an unsaturated polyester/bisphenol dlglycidyl ether adduct [ 341. In another paper [32], blends of higher molecular weight epoxy diacrylate are combined with trifunctional (TMP triacrylate) or difunctional monomers (hexane diol or neopentyl glyeol diacrylates) and monofunctional acrylate (e.g., cyclohexylbenzyl or butyl acrylate). A ratio of 60/20~20 oligomer~polyfunction~~mono~nction~ seems preferred, while a benzoin e~er~me~yl~t~qu~one is used. Pigment levels @utile) may range from 20 - 50% by weight for the various vehicles described above. The adhesion problem of UV whites and several other problems (cure speed, abrasion resistance and gloss) are directly a result of UV light attenuation in the pigmented fiim. While the light scattering phenomenon permits some light to penetrate an otherwise strongly absorbing pigment, as mentioned above, it also makes it difficult for a significant amount of radiation of wavelen~hs that are not absorbed to penetrate the depth of the film. This results in much lower concentrations of initiating radicals and incomple~ polyme~zation at practical UV exposure times (Le., one second or less). Consequently, even where cure is not so reduced as to cause gross wrinkling, weak boundary layers and regions of poor cohesive strength may exist, giving rise to poor adhesion and a tendency to softness in the freshly cured film. Fortunately, may metal base coat applications require the UV-cured product to be exposed to a thermal bake when interior can liners are cured. During this post bake (300 - 400 OF, polymerization is further advanced and volatile residues are driven out of the film. The result of this post bake is a coating with significantly upgraded adhesion and film integrity. Unfortunately, weight losses may occur in improperly formulated coatings, detracting from economics and creating a pollution problem. Although the thermal cycle upgrades total film properties, its use

91

greatly limits the choice of vehicle components and photoinitiators for white base coats because of a tendency of many toward thermal yellowing. Hence each component must be carefully chosen for non-yellowing properties, both alone or in combination with other constituents of the coating. Another source of post-bake yellowing is from vehicle components which do not yellow per se, if completely polymerized, but which do cause yellowing as residual, unpolymerized species. In some cases yellowing is proportional to the UV dose. In such cases, if the dose is minimized by operating at maximum line speeds, color will be acceptable. There is some indication that color problems are related to source output distribution and photopolymerization in the presence of air. In the absence of definitive studies, the mechanisms of such yellowing are open to speculation at this time. The final test qualifying a UV base coat is that of application characteristics. Good flow and leveling properties for roll coat application involve a considerable number of interacting parameters, some of which are definable. Basically the white composition must be a viscosity of 200 - 400 cps (35 100 s No. 4 Ford cup) and, in addition, display Newtonian rheology. The presence of thixotropy or shear-dependent viscosity behavior invariably deteriorates application properties. While it is fairly easy to achieve a product that meets all requirements with a viscosity of = 100 cps, it is much more difficult to maintain that performance at 200 cps. Many of the reasons for this are related to the cure attenuation problems discussed above. It seems that the most effective viscosity-reducing species are invariably the least efficiently polymerized, creating the possibility for trapped volatile or, at least liquid, constituents which cause many secondary problems. In many cases a UV white composition may tolerate low levels of volatile solvents better than reactive monomers, since virtually all the solvents in this film are lost on exposure to UV lamps. This solution is of course, unacceptable to industry for a variety of reasons. An alternative solution to the viscosity/performance quandary is to apply the coating hot or use different application techniques. There is considerable resistance in U.S. industry to either approach, based largely on economic considerations. The search for a low-viscosity reactive diluent that has both a low volatility and high copolymerizability is a subject of great interest among many coatings manufacturers and coatings suppliers. Thus far it is not apparent that such a material has been found. 11. Inks Another important area of application of pigmented UV coatings is in printing inks. A typical UV ink that may provide many of these advantages consists polymer, multifunctional of 50 - 70% reactive vehicle (i.e., unsaturated acrylate or methacrylate, monofunctional acrylate), 5 - 20% initiator, 10 30% colored pigments and dyes and inert fillers, waxes, crosslinking agents and additives [ 361.

92

The fact that UV inks contain no solvent introduces one of the major problem areas with these new inks, namely viscosity [ 371. Typical viscosities for various applications are given in Table 3. TABLE 3 Typical viscosities

of printing inks [ 371

Printing process

Viscosity (poke 1

Letterpress Lithography News ink Flexography Gravure

10 100 2 0.5 0.3

at 25 “C

500 800 10 -5 -2

The problem occurs when the 35 - 60% solvent of the conventional ink is replaced with reactive materials which are normally of higher viscosity and poorer solvency. Further dilution with these materials now affects the optimum pigment volume concentration (PVC) necessary to yield the correct color strength. It must be emphasized here that UV inks must already carry two to three times the amount of pigment of conventional inks to obtain the same PVC in the dry film. High pigment loadings obviously exaggerate flowout problems and it is exactly this phenomenon with which the UV ink formulators must deal. Possibly, new methods of application for UV inks may have to be developed to obtain appearances comparable to conventional systems [38]. High pigment concentrations, or for that matter any pigment at all, introduces problems in curing due to the interference of the pigment, as mentioned in the previous discussion. Carbon black pigments amplify this problem extensively and are the subject of considerable investigation [39, 401. Some relief can be found in increased sensitizer concentrations and judicious choice of black pigments [40] , but increased sensitizer concentration leads to poor shelf stabilities. In multicolor printing, the fact that black UV inks (also white inks using TiOz) are the most difficult to cure necessitates these applications early in the process sequence [41] . An interesting means of curing that employs two separate stages may find application to this problem. The process, as mentioned above, utilizes a metal halide lamp with a dominant wavelength between 3800 A and 4200 A, followed by a high- or medium-pressure mercury lamp [42]. Reportedly this process can cure TiOz pigmented films of thickness greater than 80 microns. A problem relevant only to UV inks intended for lithographic printing processes is the fact that acrylics are chemically quite polar. Since the principles in lithography necessitate that the ink be constrained by an

93

aqueous medium, the polarity of these UV inks can lead to problems termed ‘tinting’ and ‘scumming’ and a critical water/ink control. The UV inks of today must therefore find the appropriate balance of acrylic functionality (degree of polarity) and cure speed [associated with degree of ~nc~on~~ty) 137,431. V~derhoff [44, 451 records the major groups of today’s UV ink compositions as follows: 1. Esters of un~tu~~d acids and polyhy~c alcohols plus a photoinitiator. 2. Acrylate {or methacrylate) ester derivatives of conventional ink vehicles plus a photoinitiator. 3. Acrylate (or methacrylate) ester derivatives of epoxy resins combined with a photoinitiator. 4. Unsaturated polyester prepolymers plus a photoinitiator. Proper handling of these materials warrants some comment. Although no regulatory problems are known for UV inks, it is realized that some of the more volatile acrylate monomers are toxic and hazardous chemicals [ 38, 43 1, and some skin and/or eye irritation may result from extended exposures. However, with adequate ventilation, proper protective clothing and adherence to manufacturers’ suggested handling procedures, worker safety should not be a problem. The ultimate question to be answered about UV inks is that of cost. For the most part UV inks now cost 30 - 100% more than conventional inks f.451, However, the consensus is #at the economics will become more favorable both as usage increases and the cost of operating conventional lines with added pollution controls increases.

12. Other opac~cation

methods

Any efficient ~~~a~v~ to the use of UV-opaque pigments in order to get completely hiding UV coatings a& worthy of consideration. A recent patent [46] describes the extension of conventional microvoid technology to UV cure for the purpose of obtaining coatings with a high degree of hiding. This technology involves the use of initially compatible solvents which are rapidly precipitated into a matrix of polymerized resin during UV cure. As the entrapped solvent leaves the film, a high degree of light scattering is induced by the entrapped microvoids. Although practical hiding levels can easily be achieved using this method, the substantial amounts of solvents required tend to offset the low pollution, higb solids aspect of radiation cure. Fortunately, in many cases exempt solvents may be used. More important are the implications of solvent escape to physical properties of cured films. Figure 8 and 9 show the surface of a microvoided film produced by solvent precipitation. The myriad of small crater-like structures are the openings of interconnected channels through which solvent has escaped. Unfortunately the application of low-viscosity liquids reverses the process,

Fig. 8. Surface of solvent microvoided film, SE&l (X7000).

Fig. 9. Surface of solvent microvoided film, SEM ( x3500).

filling the channels with complete loss of opacity. A closed surface is obtamed in many cases if a smaller amount of a good solvent for the matrix is used with the normal solvent. When the good solvent has a lower evaporation rate, sealing of the surface is achieved. At moderate microvoid levels certain properties such as impact adhesion and flexibility are improved over the same coating without microvoids. This phenomenon may be understood in terms of reduced shrinkage stress and the greater ability of the microvoid film to deform under stress. When the microvoid concentration is carried to the extreme, however, pronounced friability and loss of film integrity occurs.

95

In related work at PPG [473 UV-cured coatings are produced wherein void structures result from dendritic microfracture rather than solvent precipitation. The mechanism in this case entails microfracture along weak boundary layers in a reticulated structure of precipitated thermoplastic and crosslinked UV resin. The driving forces for microfracture are differential thermal expansioncontraction and shrinkage (from both polymerization and volatile loss), which provide induced stress (Fig. 10). The uneven distribution of film stress gives rise to a microvoid density gradient (see Fig. 11) that gives the appearance and many of the properties of an opaque layer overcoated with a clear

Wet

hv

Film

(transparent,

homogenious

Gel

stress (thermal,

(phase

1

-

Scattered

-

Microfracture (stress

retease)

Fig. 10. Mechanism of UV fracture.

Relative Microvoid Concentration

\

1

2

-

separation)

Microvoids

-

(haziness)

shrinkage)

-

Structure

3

4

5

Film Thickness (stress)

Fig. 11. Microvoid distribution

in a thick film.

Opaque

Cured

Film

96

film. That is to say, coatings produced by this method are impermeable to applied liquids and otherwise behave very much like clear thermoset coatings. As in the case of solvent precipitation, improved flexibility, adhesion and impact properties are often obtained, since much of the stress that governs those properties in conventional coatings has been in the generation of microvoids. As a consequence, zero shrinkage or net expansion on cure of such films is often observed. Normally, relatively thick films (3 - 5 mils) are necessary to generate maximum hiding for practical applications. However, recent work demonstrates that special compositions and curing techniques may be used to develop a high degree of hiding in coatings as thin as 0.4 mils. In a practical coating with high hiding, multiple layers of dendritic structures combine to provide a high degree of light scattering. KubelkaMonk scattering coefficients of greater than 10 have been measured for such coatings. Some evidence of pigment-microvoid synergism is observed in UV coatings containing both TiOz and microvoids, which may signal yet a new class of UV-curable opaque coatings with properties not attainable using pigment alone. One practical consequence of reducing or eliminating TiOz in metal basecoat applications is that film thickness is greater at the same coating weight (e.g., 0.9 mil microvoided uersus 3.0 mil TiOz pigmented). Besides reduction in viscosity in the absence of pigment, more precise application of thin coatings is possible with a lower density of the coating. These are only a few of the advantages to be gained if this new technology can be developed to the performance level of conventional coatings. 13. Radiationeurable

coatings - Applications

13.1 Vinyl film substrates One of the newer applications of UV curing of clear coatings is in the vinyl film industry. A U.S. company has recently introduced a UV-coated vinyl laminating film which solves the characteristic burnishing problem associated with vinyl films. The important factor in this line is PPG’s coating. The UV-curable coating provides a product of excellent abrasion, scratch and strain resistance which can be laminated in a continuous process and used in miter grooving operations [ 481 . Another application is the manufacture of vinyl laminated panels for use in the furniture industry. In 1974, a high performance UV-coated vinyl laminate product was introduced. The UV coating can be applied to the laminated panels either by precision coater or curtain coater to yield a textured or smooth finish. Cure is accomplished with a dual-cure processor and was the first such ‘dual-cure’ UV processing line in the United States [ 31, 481. 13.2 Floor coverings The alternative to UV would be moisture cure urethane coating. Moisture cures are of course being used for sheet goods, but for several reasons

97

they are not used on vinyl asbestos tile. Probably one of the more crucial problems is the dimensional stability of vinyl asbestos flooring in the presence of a moisture cure system. This problem results from attack on the tile by the moisture cure coating solvent and from the cure cycle. These problems are obviated with U.V. In summary, UV-coated vinyl asbestos tile is a new product made possible by the process advantages inherent in the UV method. The analysis extends to vinyl sheet floor producers as well. At least three producers of vinyl floor products have opted for UV coatings. In this case dimensional stability is less of a problem. However, with a moisture cure it is difficult to apply more than two mils of dry coating, the cure ovens are very large and some properties of U.V. coatings, such as stain resistance, are superior to those of the conventional systems. In addition, the overall economics probably favor UV, particularly for a new line. 13.3 Paper Folding cartons have for years been printed, spray starched and then overcoated with a conventional varnish or a clear laminate. The major problems with this approach are spray starch and lamination. Spray starch is messy, but required to prevent offset. One wonders what press maintenance costs might be attributable to starch. In terms of quality, a process which uses print, starch and coat produces a product with a somewhat gritty feel and without exceptionally high gloss. An alternative to the above is to laminate a clear sheet over the package, but this is expensive and is usually done off line. It was these problems which provided an opportunity for UV inks. UV inks can, of course, be dried in line and as a result no spray starch is required to prevent offset. Gloss is increased and rub resistance is improved. Often no varnish is required, but UV inks are expensive and there are some press stability-related problems which provide incentives for a third alternative. In this third process, wet conventional inks are trapped in line with a UV- or EB-curable clear coating. Once again, spray starch and lamination are not required. 13.4 Wood UV fillers are used essentially on composition board. Conventional fillers are used on natural wood. On a cost per gallon basis, the choice would be conventional, but the high solids of UV gives it the advantage in filling capability. Consequently, the applied cost ($/ft2) usually favors the UV filler. UV topcoats for wood lend themselves to print lines with reduced ink and base coat disturbance and the finished product is typically more stain and abrasion resistant. Recently, electron-cured opaque topcoats for wood have been commercialized. In these cases the product can have the abrasion, stain and scratch resistance of high pressure laminates without employing the pressure process [48] .

98

13.5 Textiles

The textile industry provides a relatively new area for ra~ation~~ble systems. Here the textile finisher imparts the functional properties of durability, permanent press, soil resistance, flame resistance, water repellency and lubricity to the various fabrics by adding from 0.1% to 25% of the fabrics’ weight of some chemical. Additionally resins can be used to bond composite fabrics, decorate non-wovens and to flock fabrics. The textile applications under investi~tion are non-woven fabric bonding, coated fabric, pigment printing and crimp stabilization. 13.5.1 Non-woven fabric bonding

Conventionally, emulsions or latex binding resins are applied to nonwoven webs or fiber bats and dried/cured thermally. At North Carolina State University [49, 50] radiation~u~ble systems were evaluated as binders for this application. Results indicated the more flexible systems were superior in web-breaking strength to the conventional latexes for uncompressed webs, but were poorer when the webs were compressed to give thin paper-like fabrics. Needling increased fabric strength, due in part to a more even binder distribution. Present evidence indicates that the physical properties of the binder are the key to improving fabric strength. 13.5.2 Coated fabrics

At NCSU, 100% solids radiation-curable monomer-oligomer mixtures are being evaluated as substitutes for conventional latexes used for yarn/ fabric s~bilization, and for the polyu~~~es of the smoother, more continuous leather-like products. In the former instance (back coating) a commercially available UV composition was compared to a thermally cured sprayable latex. At lower add-ons the UV-cured coating performed better than the latex and was generally comparable at add-ons of about 0.5 oz/yd2. The feasibility of md~tion~u~ble leather substitutes was demonstrated with a highly flexible polyester-acrylic-urethane oligomer. Since conventional urethanes presently serve this marketplace, a switch to radiation-cured systems would be economically competitive. 13.5.3. Pigment printing Printed fabrics, convention~ly produced via rotary screen presses using water dispersion of pigments, resin bonding agents, thickener, catalyst and wetting agent, have been simulated with radiation curables by using a gravureoffset printing application method to minimize add-ons. Very good fabric properties were achieved on both 50/50 cotton/polyester and 100% polyester with the highest mol. wt. oligomer tested (4600). Additional inroads of radiation cure applications in the textile industry include a 50 in wide commercial flocking adhesive electro-curtain curing line at Bixby International Inc. [49,50] .

99

This installation produces material for counters and box toes in shoes. United Merchants and Manufacturers have also installed an 18 in wide coater/electro-curtain pilot facility at their Langley Research Center [ 511. Though considerable effort is being expended by various organizations and individuals in the field of radiation curing for textiles, the success of radiation here is thought to depend upon both the ‘total systems development approach for an individual process’ and on making radiation curing cheaper and more durable.

14. Equipment

for UV cure

Curing with ultraviolet radiation normally involves a polymerizable resin, a photoinitiator and a UV source. In principle, any UV source which emits light of an energy which can be absorbed by a photoinitiating molecule will cause polymerization. In practice, air inhibition, substrate heating, pigmentation and film thickness affect the performance of a given source. Early UV processing of coatings involved the use of low-pressure mercury lamps for curing styrenated polyesters. Because these lamps emit only a watt or less of ultraviolet per inch of arc length, they soon gave way to the so-called medium pressure mercury arcs. In general these lamps operate at 200 W in-’ input power with UV output being in the region of 30 - 50 W [52] . This type of lamp is offered by several manufacturers and it has become the U.S. industry standard. Despite this fact, medium pressure mercury arcs probably do not represent the optimum UV source. These arcs have a somewhat unattractive lifetime of at most a few thousand hours. Table 4 shows the performance parameters of these lamps. TABLE 4 Mercury arcs Lifetime Energy

Cooling

2000+ hours _ 18% Ultraviolet _ 63 % UV-Vis-IR _ 37% Non-radiant losses Electrodes 300 “C Envelope 700 “C

Much of the input power emerges as infrared, and excessive substrate heating can be a problem. The arc must operate at greater than 700 “C, but at the same time the arc electrodes must be cooled to below 300 “C. Clearly temperature control around the arc is a serious problem. It has been stated that about 1000 CFM of air is required to cool three forty-inch lamps [53] . The above discussion points out what is probably the major reason why the industry has developed as it has. Temperature control around a mercury arc is necessary and an obvious way to do this is to cool with air. Because

oxygen inhibits free radical polymerizations, air, cure does not afford the superior properties required for many end uses. Much progress has been made in this area and indeed most of the 200-odd commercial UV curing lines operate in air atmospheres; however, new developments in UV hardware are now providing alternatives. Water cooling is advantageous in an air atmosphere, and has allowed the commercialization of a system where nominal amounts of nitrogen are used to exclude oxygen. A high intensity, electrodeless ultraviolet source has recently been introduced. This UV source is distinguished from other commercially available UV lamps by the fact that there are no electrodes inside the lamp. Electrical energy is supplied in the form of radio frequency power which is coupled into an evacuated quartz tube containing mercury and other additives. As compared to conventional medium pressure mercury arcs with electrodes, the electrodeless system has several attributes which make it unique [ 541. These are summarized in Table 5. One of the disadvantages of electrode lamps is that they take about 3 min to warm up from a cold start and about 5 min to restart after being turned off. Electrodeless lamps need no more than 9 s to come to full power from a cold start or 15 s from a hot start. A unique characteristic of the electrodeless system is the modular lamp construction. The basic lamp is 10 in in length, and since there are no electrodes it emits from the entire length. Thus a 40 in web can be cured by putting four 10 in modules next to each other. The elimination of glass-metal seals results in a simpler, less stressed bulb which can be uniformly cooled. As a result, potential lifetimes of the electrodeless lamp should be superior to those of the electrode type. Because there are no electrodes in the radio frequency (r.f.) discharge lamp, less care need be taken to avoid fill materials which interact unfavorably with them. Additives can be used in small or large quantities to vary the spectral output distribution. The same basic lamp system and power supply can be used with different fills to provide a wide range of spectral characteristics, including sources with enriched high- or low-energy photon outputs. There is also a wider range of mercury pressures easily available with electrodeless lamps, so that various ratios of line to continuum radiation can

TABLE 5 Radio-frequency

discharge lamp

-Rapid on-off (9 - 15 s) -Electrodeless 10 inch modules -Increased lifetime (3000+ hrs.) -Variable spectrum -Improved energy conversion 117/320 W = 36% UV

101

be obtained. It would seem that these variables would be of considerable value in promoting the cure of titanium dioxide pigmented coatings. Because the electrodeless lamp can be cooled uniformly, it can be made to run cooler than the electrode type, but at the same time it can be driven at higher power. This allows lamp input powers up to 600 W in-’ as compared to the conventional 200 W in-’ of electrode lamps, without any serious degradation of lamp lifetime. Because the plasma inside has a higher electron temperature, the spectrum is shifted further towards the ultraviolet. With the same fill, the same lines and continuum appear as with an electrode lamp, but the output in the UV is much greater. The actual UV output from a 320 W in-’ electrodeless lamp is 117 W in-‘. This represents a 36% energy efficiency as compared to about 13% for 200 W in-’ electrode arcs. Of course, it must be added that considerable energy losses occur in converting a.c. to r.f. In summary, the r.f. discharge source appears to offer some significant advantages such as variable spectra, reduced infrared and increased efficiency. The question of emission spectrum has been alluded to in the r.f. discussion. Over the years a fair amount of evidence has been accumulated that photon energy and intensity are of utmost importance. It is, of course, obvious that the incident energy must be absorbed before it can cause chemical changes and there is then merit in matching the lamp output to the photoinitiator absorption spectrum. Perhaps less well appreciated is the need for high energy photons for air cure. In 1971 we at PPG also learned that ozone-free quartz lamp jackets resulted in reduced cure in air. Others have since published data to this effect [ 391. A number of specialized UV curing devices have appeared. One of these is the Dual-Cure processor, which has been highly successful [31]. This technology employs medium-pressure mercury arcs in a two-stage cure as discussed in Section 8. Nippon Paint Co. has published information relating to UV curing of automotive coatings. To do this, they employ a two-stage cure in a reflective tunnel. The first stage uses metal halide lamps for through cure. The second stage then reportedly provides the surface cure. A prototype tunnel is said to be under development [42]. Union Carbide has also promoted a two-stage cure in which the first stage consists of short wavelength low-pressure lamps which operate in an inert atmosphere. This stage is said to seal the surface of the coating and preclude air inhibition. Bulk cure is then accomplished in air with mediumpressure mercury arcs. 15. Electron beam curing equipment As is known, EB curing equipment using the swept beam design has been available for some time and is offered in models which provide a wide range of electron energies. Companies offering this equipment include S3, Hi Voltage Engineering, RPC and ESI.

192

A new generation of electron accelerators employs linear cathodes which need not be, scanned, and is specifically designed for coatings processing. As a result the energy at the workpiece is of the order to 100 - 235 keV and the unit density coating can be 4 - 8 mils and have optimum energy coupling. The linear cathode allows for reduced dose rates at the same total dose. This has clear mechanical advantages, while in principle the lower dose rate should have chemical advantages. The compact linear cathode devices reduce the shielding requirements such that the total elec&on cure package is compatible with many existing curing lines. Figure 12 summ~~es the differences between these two types of EB accelerators.

Fig. 12. Two types of electron beam curina; equipment. (Dose D = (I/A) (d&f&) (l/u) (l/e), dose rate = D/t, I = intensity, A = beam area, dE/bc = energy loss per unit depth, (t/u) = t = time increment of exposure, i = effective width of cure zone, v = velocity of traversal of cure zone, e = density factor).

References 1 S. G. Cohen

and R. J. Baumgarten, 6, Am. Chem. Sot., 87 (1968) 2996. 2 C. L. &born and R. J. Trecker (to Union Carbide), U.S. Pat. 3 759807 (1973). 3 A. Chapiro, ~adi&~ion Chemistry of Polymeric Systems, High Polymers, XV, Inter science, New York, 1962, p. 41. 4 S. B. Radlove et al. (to Continental Can Co.), U.S. Pat. 3 872 162 (1975). 5 G. W. Gruber, unpublished results, 6 G. W. Gruber et al. (to PPG Ind.), Br. Pat. 2002-374 (1979). 7 R. Hall (to SCM Corp.), U.S. Pat. 3 899 611 (1975). 8 G. Rosen et al. (to Sun Chemical Corp.), U.S. Pat. 3 926 639 (1976). 9 G. Rosen et al. (to Sun Chemical Corp.), U.S. Paf. 3 926638 (1975). 19 G. Rosen et al. (to Sun Chemicai Corp.), U.S. Pat. 3 926 640 (1975).

103 11 12 13 14 15 16 17

18 19 20

21 22 23 24 25 26 27

28 29 30 31 32 33 34 35

36 37

G. Rosen (to Sun Chemical Corp.), U.S. Pat. 3 926 641 (1975). F. M. Magnotta (to W. R. Grace Co.), U.S. Pat. 3809633 (1974). C. L. Kehr and W. R. Wszolek (to W. R. Grace Co.), U.S. Pat. 3898349 (1975). J. L. Guthrie and F. J. Rendulic (to W. R. Grace Co.), US. Pat. 3 900594 (1975). P. Prucnal and C. Friedlander, (to PPG Ind.), U.S. Pat. 3 874 906 (1975). S. Takzake, Adu. Polym. Sci., 6, Springer-Verlag, New York and Berlin, 1969, p. 321. K. Tada, Y. Shirota and H. Mikawa, Macromolecules, 6 (1973) 9. K. Kato, S. Okamara and H. Yamaoka, d: Polym. Sci., 14 (1976) 211. R. Foster, Organic Charge Transfer Complexes, Academic Press, New York, 1969. S. Schlesinger (to American Can Co.), U.S. Pat. 3 708 296 (1973). W. Watt (to American Can Co.), U.S. Pat. 3 794576 (1974). W. Watt (to American Can Co.), U.S. Pat. 3 816278 (1974). H. M. Rosenbaum (toEsso Research and Engineering Co.), U.S. Pat. 3816379 (1974). J. Feinberg (to American Can Co.) U.S. Pat. 3 829 369 (1974). J. V. Criveiio, in S. P. Pappas, (ed.) U.V. Curing: Science and Technology, Technology Marketing Corp., Stamford, Conn., 1978, p. 24. S. 5. Schlesinger (to American Can Co.), U.S. Pat. 3 816 279 (1974). J. Feinberg (to American Can Co.), U.S. Pat. 3 711391 (1974). 6. Feinberg (to American Can Co.), US. Pat. 3 817845 (1974). J. Feinberg (to American Can Co.), U.S. Pat. 3 817850 (1974). T. F. Huemmer, L. A. Wasewski and R. J. Plooy,J. Paint Technol., 44 (1972) 61. E. Hahn (to PPG Ind.), U.S. Pat. 3 918 893 (1975). Tioxide International, Ltd., Some aspects of the pigmentation of UV curable systems, OCCA Symposium Paper, Durham Univ., April, 1975. C. Osborne and H. Trone (to Union Carbide), U.S. Pat. 3 840448 (1974). Ger. Put. 2 504 335 (to Continenta Can Co.) (1975). V. McGinnis (to SCM Corp.), U.S. Publication Pat. Appl. R346350. I. K. Shahida and T. M. Powanda, Am. Inkmaker, 53 (1975) 20. A. J. Bean, Sot. of Manufacturing Engineers Technical Paper Preprint #FC75-310

(1975). 38 R. B, Mesrobian, 301 (1975).

Sot.

of Manufacturing Engineers Technical Paper Preprint #FC75-

39 R. W. Bassemir and A. J. Bean, Parameters of Wprinting inks, presented at the 26th Annual TAGA Meeting, May 1974. 40 M. D. Garrett, Am. Inkmaker, 54 (1976) 16. 41 J. F. Ackerman, Sot. of Manufacturing Engineen Technical Paper Preprint #FC75312 (1975). 42 S. Nakabayshi, Sot. of Manufacturing Engineers Technical Paper Preprint #$FC75-322

(1975). 43 H. E. Pansing, 44 45 46 47

48 49 50 51

52 53

Sot. of Manuf~turing Engineers Technical Paper Preprint ~FC75-309 (1975). J. W. Vanderhoff, Coatings and PlasticsPreprints, 35 (1975) 124. J. W. Vanderhoff, J. Radiation Curing, 1 (1974) 7. M. Wismer et al. (to PPG Ind.), U.S. Pat. 3 823 027 (1974). M. Wismer, et al. (to PPG Ind.), U.S. Pat. 4 005 244 (1977). PPG Coatings and Resins Magazine, 8/l (Spring 1976). W. Walsh, 3rd Semi-Annual Report, 1975, North Carolina State University. W. Walsh, 4th Semi-Annual Report, 1976, North Carolina State University. Anonymous, Radiation Cure, 2 (1975) 4. 0. E. Lienhard, Sot. of Manufacturing Engineers Technical Paper Preprint #FC75-335 (1975). R. W. Pray, Sot. of Manufact~~ng Engmeers Technical Paper Preprint #FC74-513

(1974). 54 B. J. Eastlund, M. J. Ury and C. H. Wood, Sot. of Manufacturing Engineers Technical Paper Preprint #~~75-336 (1975).