Abrasion resistance radiation curable coating

The invention relates to a radiation curable coating which provides superior abrasion and chemical resistance and excellent adhesion properties. The coating is produced by radiation curing. A first monomer selected from the group consisting of triacrylates and tetracrylate mixed with a second monomer having an N-vinyl imido group, preferably an N-vinyl lactam, such as vinyl pyrrolidone or vinyl caprolactam. The monomer mixture is substantially oligomer-free and if the radiation is ultraviolet light, a photoinitiator, preferably p-phenoxydichloro acetophenone or dimethoxyphenyl acetophenone, is included in the mixture.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to radiation curable coating which is produced from 
a polyacrylate monomer and a vinyl monomer having an N-vinylimido grouping 
and, more particularly, to a tri- or tetraacrylate monomer in combination 
with an N-vinyl lactam monomer. 
2. Description of the Prior Art 
It is known that superior abrasion resistant coatings can be produced by 
thermal curing systems, but these suffer disadvantages, including the 
requirement to remove solvents and provide heat to process the coating 
which results in high costs for energy and air pollution control and 
eliminates their use on heat-sensitive materials. Thermal curing systems 
also do not lend themselves to rapid, continuous processing, as opposed to 
slow, batch processing, because of the requirement for heat and dwell time 
in the ovens to complete the cure and develop the superior abrasion 
resistance. 
One hundred percent solids, radiation curing systems overcome the 
disadvantages of energy costs, solvent emissions, high temperatures, and 
slow batch processing associated with thermal curing systems. However, 
most radiation curing systems for abrasion resistant coatings developed up 
to this time have incorporated reactive polymers such as an urethane 
acrylate, together with various reactive monomers. These systems do not 
show the superior abrasion and chemical resistance properties of the best 
thermal curing systems. 
Radiation curing systems using reactive monomer ingredients, without 
significant amounts of reactive polymers, are known which produce the 
desired superior abrasion and chemical resistance but these coatings are 
too brittle and produce too much curl for use on flexible substrates. 
Also, they exhibit poor adhesion on many substrates which, combined with 
their brittleness, results in undesirable cracking and peeling off from 
the substrate. In those cases where the existing monomer-based radiation 
curing coatings can be made to adhere to a rigid substrate without some 
type of adhesion failure, they exhibit slow radiation curing speeds and 
high viscosities which make rapid processing and smooth coating 
application difficult. 
SUMMARY OF THE INVENTION 
It has now been found that the disadvantage of the prior art systems can be 
overcome through the use of a radiation curable coating which provides 
superior abrasion and chemical resistance and excellent adhesion 
properties. 
In accordance with the present invention, a first monomer selected from the 
group consisting of triacrylates and tetraacrylates is mixed with a second 
monomer having an N-vinyl imido group, preferably an N-vinyl lactam and 
subjected to radiation until cured. The monomer mixture is substantially 
oligomer-free and can include a photoinitiator such as 
p-phenoxydichloroacetophenone and dimethoxyphenylacetophenone. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Crosslinked coatings of tri- and tetraacrylate monomers, such as 
pentaerythritol triacrylate (PETA) are recognized as providing extremely 
high abrasion resistance under severe test conditions such as the DuPont 
Steel Wool Rotary Test which involves subjecting the coating to five 
revolutions of a 1.25 square inch pad of commercially available 0000 grade 
steel wool which has been loaded with appropriate weights to give either 
12 or 24 p.s.i. pressure. Abrasion resistance is rated according to the 
increase in the level of haze from rubbing with the steel wool. 
However, these coatings, especially with PETA, exhibit shrinkage causing 
curl in the care of thin substrates and, further, exhibit cracking when a 
thick coating is applied on any substrate and subjected to bending. 
Generally, copolymerization of PETA with low Tg yielding monomers and/or 
oligomers enhances the resistance to cracking due to bending and curl due 
to shrinkage when the copolymers are applied to various substrates. 
However, curing speed and abrasion resistance are sacrificed, particularly 
in the care of resistance to severe abrasion. 
An extremely abrasion resistant coating with good curl and 
cracking-resistance properties is obtained by copolymerizing a tri- and/or 
tetraacrylate monomer and a monomer having an N-vinyl imido group, such as 
a vinyl lactam monomer. The acrylate is preferably pentaerythritol 
triacrylate (PETA) or trimethylolpropane triacrylate (TMPTA) and the 
preferred lactams are N-vinyl pyrrolidone (VP) and N-vinyl caprolactam 
(VC). The structural formulas for these materials are: 
##STR1## 
The lactams tend to have a greater shelf-like stability than open chain, 
N-vinyl imido monomers are preferred. 
The following table shows the curing rate in air under a high power (two 
200 w/in. lamps) UV source (PPG-QC 1202 A/N) of four triacrylate 
formulations containing 79.3 parts by weight of triacrylate, 20.7 parts by 
weight of the vinyl lactam, 5 parts by weight of the photoinitiator sold 
under the Trademark Sandoray 1000 by Sandoz. For evaluation, each 
formulation was coated on 3 MIL polyester film, sold by ICI as Melinex 505 
film, using a no. 3 wire wound rod. The coating thickness was 
approximately 0.5 MIL. For comparison purposes, the same procedure was 
used to coat formulations of 100 parts by weight of triacrylate monomer 
(PETA or TMPTA) with 5 parts per weight of Sandoray 1000. 
TABLE I 
______________________________________ 
Steel Wool 
Viscosity (cps.) 
Curing Abrasion 
@25.degree. C. 
Rate Resistance 
Coating (Brookfield) 
(ft/min) (.DELTA. Haze) 
______________________________________ 
Melinex 505 
Polyester Film (ICF) 26.8 
Lucite AR (DuPont) 3.1 
PETA (100) 717 150 4.4 
100 0.5 
PETA/VP (79.3/20.7) 
99 175 3.1 
125 0.2 
PETA/VC (79.3/20.7) 
180 75 1.4 
50 0.7 
TMPTA (100) 82 17 3.9 
TMPTA/VP (79.3/20.7) 
28 25 1.0 
TMPTA/VC (79.3/20.7) 
39 25 3.7 
17 1.8 
______________________________________ 
The curing rate of PETA/VP was actually faster than for PETA alone, even 
though it is known that VP alone does not cure readily with either UV or 
electron beam radiation. It would appear that the enhanced cure rates 
observed with VP or VC as a diluent monomer are due to a charge transfer 
complex formed between VP or VC and acrylates. The N-vinyl imido 
##STR2## 
grouping in the vinyl monomers might be involved in this complex formation 
and, thus, be responsible for the substantial curing rate enhancement. 
None of the commonly used diluent monomers have the same effect and, in 
fact, substantially slow the curing rate of PETA alone. These diluent 
monomers include ethylhexyl acrylate, diethoxyethyl acrylate, phenoxyethyl 
acrylate, and dicyclopentadienyloxyethyl acrylate. Thus, the function of 
diluent monomers has been limited to viscosity reduction, since they tend 
to have adverse effects on the curing rate. 
It can be seen from Table I that the abrasion resistance of the coating 
depends on the curing rate. Within the limit of complete curing, the 
longer the exposure, the greater will be the abrasion resistance. To 
obtain an abrasion resistance with a change in percent haze of nearly 
zero, required a longer exposure than to obtain an abrasion resistance 
with an increase in percent haze of 3.1. The curing rates described are 
based on the use of two 200 w/in. lamps and for a higher rate of 
production, if necessary, more lamps can be installed. The use of more 
than two 200 w/in. lamps, or their equivalent, is not uncommon in 
commercial applications. 
It is noted that while TMPTA does not have a high shrinkage coefficient and 
can be used alone without a diluent monomer, it needs VP or VC to enhance 
the curing rate. 
The usefulness of VC, besides curing rate enhancement, is its low 
volatility (less volatile than VP), low moisture sensitivity (more 
hydrophobic than VP), and low Tg (a better impact modifier than VP). 
Low viscosity formulations are, generally, favored from the rheological 
point of view, especially in making coatings of less than 0.5 MIL. Low 
viscosity formulations provide better wetting and faster leveling and 
contribute to higher productivity. Thus, the viscosity reduction from the 
use of VP or VC, as shown in Table I, contributes to the effectiveness of 
the systems of the present invention. 
The preparation of the formulation is very simple because no chemical 
reaction is involved. Since both PETA and VC are solids at ambient 
temperature, a gentle warming (40.degree. C.) is preferred to melt the 
material before mixing. In a typical preparation, after the comonomers are 
mixed, a silicone surfactant DC-193 (Dow Corning), in the amount of 0.5%, 
and the photoinitiator (Irgacure 651 or Sandoray 1000) in the amount of 
3-5% are mixed thoroughly into the solution. Both materials are very 
soluble in the comonomers. Because of its fast curing rate with UV 
radiation, the formulation with photoinitiator present should be kept in 
the dark before use and exposure to light source. 
The coatings of the instant invention not only provide superior abrasion 
and chemical resistance and excellent adhesion properties but also have 
other desirable features for a protective layer including stability to 
discoloration and degradation of properties by ultraviolet light, optical 
quality transparency including (non-yellow color) and good printability. 
Among the applications for the coating is the coating of plastic lenses 
made from materials, such as cellulose acetate butyrate, cellulose acetate 
propionate, cellulose acetate, polycarbonate, polystyrene, methyl 
methacrylate, copolymer of styrene and methyl methacrylate, and allyl 
diglycol carbonate. 
Additionally, the coatings can be used on flexible or rigid plastic 
materials, including sheets, foams and various shapes, such as associated 
with cast forms. The coatings can be applied and cured either before or 
after the molding operation. Additional plastic substrates include 
polyolefins, such as polypropylene and polyethylene, polycarbonate, 
polyvinyl chloride (PVC), polyethylene terephthalate (PET), polybutylene 
terephthalate (PBT), polystyrene, methyl methacrylate, polyamide (nylon), 
polymethyl pentene and polyethersulfone. 
Protective Top Coatings over other coatings-wide variety including 
ratiation cured coatings, pigmented coatings, varnishes, and the like. 
Additional substrates include: 
Wood 
Metal, such as aluminum, steel and copper 
Paper, including impregnated and surface coated paper 
Glass, including fiberglass and glass fiber optics 
Tile, such as ceramic, vinyl and vinyl/asbestos and 
Textiles, including various natural and synthetic fibers. 
To achieve functional coating properties in addition to superior abrasion 
and chemical resistance and excellent adhesion, additives known in the 
art, such as pigments for gloss control, wetting agents for surface 
uniformity and flatness, and dyes or colored pigments to produce colored 
coatings, can be added. 
The coating can be done by conventional techniques including dip, spin, 
spray, curtain coat, gravure, and roller. Where dirt contamination is 
undesirable, such as with plastic lenses, the coating should be done in a 
suitable dust-free atmosphere. 
Although photoinitiators, in general, can be used in tri- and 
tetraacrylate/N-vinyl imido UV polymerizable systems, dramatic differences 
were experienced in respect to abrasion resistance. 
Dialkoxy acetophenones, such as dibutoxy acetophenone and alkoxy phenyl 
acetophenones, such as methoxy-phenyl acetophenone did not give the 
desired results, whereas p-phenoxy dichloro acetophenone and 
dimethoxyphenyl acetophenone gave extremely good results in all respects. 
The following table compares the results obtained using various 
photoinitiators with pentaerythritol triacrylate/vinyl pyrrolidone in a 
79.3/20.7 weight ratio. The UV source was a PPG system designated QC 1202 
A/N employing two lamps. The substrated was primed polyester film and the 
curing rate was 50 ft./min./2 lamps. Primed polyester films are films 
treated for adhesion enhancement and sold under the designation 
clear-055-primed by ICI Corporation and 4561-primed by Celanese 
Corporation. In the following table, the (p-phenoxy)dichloro acetophenone 
is sold under the Trademark Sandoray 1000 by Sandoz Corporation and the 
(dimethoxy phenyl) acetophenone, is sold under the Trademark Irgacure 65, 
by Ciba-Geigy Corporation. Their structures are as follows: 
##STR3## 
TABLE II 
______________________________________ 
Steel Wool 
Abrasion 
Acetone Resistance 
Photoinitiator (3%) 
Resistance 
(Change in Haze) 
______________________________________ 
p-phenoxydichloro acetophenone 
Good 0.3 
dimethoxyphenyl acetophenone 
Good 0.5 
dibutoxy acetophenone 
Good &gt;5 
chlorinated benzophenone 
Good " 
aryl ketone Good " 
benzophenone/dimethyl ethanol 
amine Good " 
Good " 
diethoxy acetophenone 
Poor &gt;10 
a-isobutoxy-a-phenyl 
acetophenone Poor " 
a-methoxy-a-phenyl 
acetophenone Poor " 
______________________________________ 
The chlorinated benzophenone is sold under the Trademark Trigonal P-1 by 
Noury Corporation and Eastman FI-4 by the Eastman Corporation. The aryl 
ketones are sold under the Trademarks EM-1173 and EM-1176 by Merck 
Corporation. The a-isobutoxy-a-phenyl acetophenone is sold under the 
Trademark Vicure 10 by Stauffer Chemical Corporation and is 
2-chlorothioxanthone sold under the Trademark Sandoray 1050 by Sandoz 
Corporation. 
It is, thus, seen that through the use of a specific photoinitiator, high 
abrasion resistant UV curable coatings can be attained in air cure systems 
without the expected sacrifice in curing speed and without the need to use 
a photoinitiator concentration level above those normally employed. 
Where equipment limitations do not preclude the use of a controlled 
nitrogen atmosphere or a slow curing rate is of no consequence, suitable 
photoinitiators can include vicinal ketaldonyl compounds (i.e., compounds 
containing a ketone group and an aldehyde group) such as diacetyl, benzil, 
2,3-pentanedione, 2,3-octanedione, 1-phenyl-1,2-butanedione, 
2,2-dimethyl-4-phenyl-3,4-butanedione, phenyl-glyoxal, diphenyl-triketone; 
aromatic diketones, such as anthraquinone; acyloins, such as benzoin, 
pivaloin acryloin ethers, such as benzoin-methyl-ether, 
benzoin-ethyl-ether, benzoin-butyl-ether, benzoin-isobutyl-ether, 
benzoin-phenyl-ether; alpha-hydrocarbon substituted aromatic acyloins, 
including alpha-methyl-methylbenzoin, alpha-alkyl-benzoin, as in U.S. Pat. 
No. 2,722,512, and phenylbenzoin; diaryl ketones, such as benzophenone and 
dinaphthyl ketone; and organic disulfides, such as diphenyldisulfide. The 
photoinitiator can also include a synergistic agent, such as a tertiary 
amine, to enhance the conversion of photo-absorbed energy to 
polymerization initiating free radicals. Dimethoxyphenylacetophenone such 
as IRCACURE 651 available from Ciba-Geigy or Sandoray 1000 are preferred. 
The photoinitiator is present in the coating composition in an amount 
sufficient to initiate the desired polymerization under the influence of 
the amount of actinic light energy absorbed. The coating composition 
generally contains from 0.01 to 5 weight percent of photoinitiator based 
on the weight of the coating composition. 
The coating composition can also contain an additional polymerization 
inhibitor to prevent undesirable auto-polymerization of the coating 
composition in storage prior to use. Examples of suitable addition 
polymerization inhibitors include, among others, di(1,4 secbutylamino) 
benzene available from the DuPont Company under the trade name 
"Anti-Oxidant 22" and Monomethyl Ether of Hyroquinone and Hydroquinone 
phenothiazine available from Tefenco Chemical Co. The additional 
polymerization inhibitor is present in an amount sufficient to prevent 
auto-polymerization and is generally present in an amount from 100-300 PPM 
based on the weight of the coating composition. 
The coating composition can also contain a surfactant. The preferred 
surfactants are silicone surfactants such as that available from the Dow 
Corning Corporation under the trade name "DC-193". The surfactant is 
present in an amount necessary to reduce the surface tension of the 
coating composition and reduce its viscosity to the desired level. The 
surfactant generally comprises from 0.01 to 2 weight percent based on the 
weight of the coating composition. 
The coating compositions of the present invention can also contain other 
conventional additives, such as flow control and leveling agents, organic 
and inorganic dyestuffs and pigments, fillers, plasticizers, lubricants, 
and reinforcing agents, such as alumina, silica, clay, talc, powdered 
glass, carbon black and fiberglass. 
The coating compositions of the present invention can be cured by applying 
them as a film on the substrate. Typical coating thicknesses are 1-25 
microns. Curing can be done under air or under an inert atmosphere of 
nitrogen. The coating composition may be applied as a thin film in any 
conventional manner such as by spraying, brushing, dipping, roll coating 
and the like. 
Conventionally, the film on the substrate is positioned to travel on a 
conveyor or some other film handling equipment and pass under a source of 
a free radical generator, such as radiation. The coated side of the 
substrate is exposed to the radiation for a time sufficient to effect 
polymerization and convert the film into an adherent, tough, flexible 
coating. 
As used herein the term radiation refers to any radiation source which will 
produce free radicals and induce additional polymerization of vinyl bonds. 
The actinic radiation is suitable in the wave length of 2000-7500 A, 
preferably 2000 to 4000. A class of actinic light useful herein is 
ultra-violet light and other forms of actinic radiation are from the sun, 
artificial sources such as Type RS sunlamps, carbon arc lamps, Xenon arc 
lamps, mercury vapor lamps, tungsten halide lamps, lasers, fluorescent 
lamps with ultra-violet light emitting phosphors. 
Ultra-violet curing rates greater than 20 ft/min 200 w./in. lamp must be 
obtained in order to be commercially acceptable in most applications. With 
a reasonable coating thickness (about 0.5 MIL), the coating compositions 
with this invention can be cured at rates of 25-100% ft/min 200 w./in. 
lamp. 
The preferred electron beam system contains a wide curtain of electrons 
directly from a linear cathode. A curtain of electrons from the gun's 
cathode, accelerated to a high velocity by a 200 KV potential, emerges 
from the chamber through a foil window into the coated substrates 
(Electron-curtain .sup.TM by Energy Sciences, Inc.). 
The electron beam curing of the coating compositions as described above is 
cured at less than 5 Mrads and generally at between 1 and 2 Mrads. Curing 
at greater than 8 Mrads is usually deemed unacceptable because of the 
higher cost. 
For evaluation, each formulation was cast on the substrate by means of a 
wire rod. A No. 3 rod was used for flexible polyester films and a No. 12 
rod was used for rigid substrates, such as PVC and polycarbonate (Lexan). 
These will approximate a coating thickness of 0.5 mils and 1.5 mils, 
respectively. 
The wet films were cured under air with three different UV sources: PPG-QC 
1202 A/N (consisting of two 200 watts/in. Hg lamps), Linde DBHG3M13-14 
(consisting of three 100 w/in. Hg Lamps) and Fusion 208V K-520Q357-358 
(consisting of one 300 w/in. Hg lamp). The film was cured to a condition 
in which it is abrasion resistant under DuPont's Steel Wool Rotary test. 
An increase in "haze" of less than 4 was considered to be abrasion 
resistant by the commercial standard. 
Haze is measured in percent and determined in accordance with ASTM D1003. 
The Steelwool Rotary Test is a severe abrasion test using 1.25 inch square 
pad of commercially available 0000 grade steelwool. The wool is loaded 
with appropriate weights to give either 12 or 24 p.s.i. pressure and 
revolved five times. The results are reported as an increase in percent 
haze (Delta Haze) using the ASTM D1003 test procedure for measuring haze 
before and after the test. The steelwool rotary test described herein 
employed sufficient weights on the steelwool pad to produce a 12 p.s.i. 
pressure. 
Lucite AR with commercial abrasion resistant glazing undergoes a haze 
change of 3.1, whereas the uncoated polyester film undergoes a haze change 
of 26.8. 
Table III illustrates the effect ov varying the PETA/VP ratio (weight 
percent) on abrasion resistant coatings. In each case, the UV source was a 
PPG-quartz crystal 1202 A/N unit employing two lamps. The substrate was 
primed polyester film. The photoinitiator, in each case, was a 
dimethoxyphenolacetophenone sold under the trademark Irgacure 651 and 
employed in an amount of 3% by weight of the monomers. The use of 
increased concentrations of VP decreased the viscosity of the system, 
thereby making the application of the coating material easier. It is noted 
that the pure PETA system gave good acetone resistance and adequate haze 
change at modest curing rates. However, the high viscosity significantly 
interferred with processing procedures. The use of as little as about 10% 
VP significantly decreased the viscosity thereby facilitating the coating 
operation without significantly affecting the change in haze for the 50 
feet per minute curing rate. At VP concentration of over about 41%, a 
curing rate decrease was required to stay within the acceptable haze 
change rate. 
Table IV relates to the use of the procedure of Table III employing 
pentaerythritol triacrylate with vinyl caprolactam and employing 5% of 
Irgacure 651. Table IV shows that VC significnatly reduces the viscosity 
of the PETA formula. The use of up to about 31% VC produced traumatic 
viscosity reduction without resulting in a haze change of greater than 3 
at a 17 foot per minute curing rate. It is further noted that a greater 
energy input is necessary to provide the same curing rate using VC as that 
which is obtained with VP. Since VC is more hydrophobic than VP, its 
copolymer is less water sensitive. 
Table V relates to the test procedure employed with materials of Table III 
using pentaerythritol triacrylate in combination with other monomers. 
Three percent Irgacure 651 was employed while otherwise following the 
prior noted procedure. The table shows that none of the other monomers 
evaluated were as effective as vinyl pyrrolidone in producing the desired 
result. Phenoxyethylacrylate and dicyclopentadienyloxyethyl imparted 
significantly higher viscosity. In each case, the curing rate was from 4 
to 6 times slower than with the systems noted in Examples I and II to 
approximate the same abrasion resistance. 
Table VI shows that VP was fastest in reactivity and VC next highest, when 
Sandoray 1000 was employed in a 5% concentration. The use of the 
difunctional acrylate hexindiodiacrylate and the mono functional 
acrylates, ethylhexoacrylate, diethoxyethylacrylate, phenoxyethylacrylate, 
dicyclopentadienyloxyethylacrylate required substantially slower curing 
rates to achieve results comparable to those attainable with either VP or 
VC. 
In the examples of Table VI, the Sandoray 1000 photoinitiator was used in a 
5% concentration with trimethylpropanetriacrylate/vinyl pyrrolidone films 
using the aforenoted procedures of Tables V and VI. It is evident from 
Table VII that TMPTA homopolymer alone was abrasion resistant but required 
much more energy to cure than the PETA homopolymer. TMPTA, by itself, has 
a lower viscosity than PETA. An enchanced curing rate and reduced 
viscosity was attained with the addition of VP. A haze change of under 3 
could be obtained with VP concentrations below about 40%. 
Table VIII relates to TMPTA/VC films using 5% of the Sandoray 1000 
photoinitiator. A VC concentration of up to about 30 weight percent could 
be employed to obtain optimum viscosity reduction, maintained curing rate 
and maintained the Haze change below about 3. 
Table IX relates to TMPTA/monomer films using 5% of the Sandoray 1000 
photoinitiator in accordance with the aforenoted procedure. In Table I it 
is evident that VP yielded improved curing rates and decreased viscosity. 
The use of vinyl pyrrolidone gave a viscosity decrease and a curing rate 
increase without a sacrifice and abriasion resistance as measured through 
the change in haze. The vinyl caprolactam gave improved viscosity 
properties without a sacrifice in curing rate or haze change. The 
phenoxylethylacrylate gave an improved viscosity with no curing rate 
decrease but a loss in abrasion resistance, thus, a curing rate decrease 
would have been required to maintain roughly the same haze change quality. 
Table X illustrates the abrasion resistance of PETA/VP films, 1.2 mils in 
thickness cured using a PPG-QC1202 A/N unit and employing a PETA/VP weight 
percent ratio of 79.3 to 20.7. In the case of a polyvinyl chloride 
substrate, the Sandoray 1000 and Irgacure 651 gave roughly comparable 
results. Using a substrate of a polycarbonate sold under the trademark 
Lexan by General Electric Corportion, Sandoray gave better results than 
those obtained with the Irgacure 651.