Table IV. Performance of Radiation-Cured Acrylic Paint
Test
RadiationCurable Acrylic
Commercial A 1kyd
Pass Pass Pass
Pass Pass Pass
5000 hours ASTM No. 8 checking
4700 hours ASTM No. 3
American Plywood
Association Boil (25 cycles) Soak (25 cycles) Freeze (10 cycles) Weatherometer Xenon lamp
Acknowledgment
checking Severe yellowing
Carbon arc
2000 hours No change
upon cured coating properties are discussed. The degree of unsaturation influences all of the properties of the films, while the polymer composition has lesser effect on hardness and flexibility, but affects solution properties and longer term performance properties. These acrylics can be pigmented and applied by conventional procedures and equipment.
2000 hours Severe yellowing
room temperature water, and 10 freeze-thaw cycles) show the ability of the painted surface to withstand the dimensional changes of the substrate. These data, then, reflect the feasibility of utilization of electron-cured acrylic coatings for exterior applications. This type of paint is, in fact, now in service on factorypainted exterior wood product substrates.
The authors acknowledge the valuable assistance of W.
K. Okamoto, who prepared many of the reaction products and conducted a great deal of the film evaluation. literature Cited
Brown, W. H., Miranda, T. J., Ofic. Dig. Federation SOC. Paint Technol. 36, Part 2, 92 (1964). Chapiro, A., Adoan. Chem. Ser., No. 66, 22 (1967). Charlesby, A., “Atomic Radiation and Polymers,” Chap. 3, Pergamon Press, New York, 1960. Charlesby, A., Wycherley, J., J . Appl. Rad. Isotopes 2, 26 (1957). Fox, T. G., Bull. Am. Phys. SOC.1, 123 (1956). Miranda, T. J., Huemmer, T. F., J . Paint 7’echrzol. 41, 118 (1969). Tawn, A. R. H., J . Oil Colour Chemists’ Assoc. 51, 182 (1968).
Summary
Reported here, for the first time, are structures and SOme properties Of unsaturated acrylic copolymers by electron beam radiation. The effects of two important parameters in the design of unsaturated acrylic copolymers
RECEIVED for review July 10, 1969 ACCEPTED October 13, 1969 Symposium on Radiation Curing, Division of Organic Coatings and Plastics Chemistry, 157th Meeting, ACS, Minneapolis, Minn., April 1969.
Electron Radiation Curing of Styrene-Polyester Mixtures Effect of Backbone Reactivity and Dose Rate Allan S. Hoffman’, John T. Jameson’, Warren A. Salmon3, Donald E. Smith3,and David A. Trageset Massachusetts Institute of Technology, Cambridge, Mass. 02139
T H E USE of electron radiation to cure coatings on metal or wood substrates has several advantages over the conventional catalytic curing process. The major advantages include high speed curing, use of solvent-free (“100% solids”) mixtures, and elimination of the need for large, long ovens for curing and/or removing solvent residues. However, since radiation curing of coatings mixtures involves mechanistic considerations different from conventional curing systems, special new compositions have to be “engineered” for this new curing process. The work reported here is based on this consideration. Results are presented of the effect of backbone composi-
’ Present address, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139 Present address, U. S. Army Present address, High Voltage Engineering Corp., Burlington, Mass. 158
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9 , No. 2, 1970
tion, molecular weight, and concentration on the radiation curing of styrene-unsaturated polyester solutions. The ICT-500 electron accelerator was used. I t is one of a series of electron accelerators, developed specifically for thin film, coatings, or fabric applications. Electrons are accelerated to energies of 500 kev a t beam currents up to 20 Ma. The beam may be spread out or “scanned” up to 72 inches in width. The 500-kev electrons penetrate to a depth of 50 mils in an absorber of unit density such as water. The ICT-500 electron processing system consists of three major assemblies: power supply, electron accelerator, and control consoles. Electrical power is transmitted to the accelerator by means of a flexible EHV cable leading from the power supply. Up to three electron accelerator assemblies may be accommodated by any one power supply. This arrangement makes it practical t o irradiate several production lines simultaneously, or frac-
Mixtures of styrene monomers with unsaturated polyesters of varying composition and molecular weight were irradiated with a 500-kv electron accelerator. The extent of cure was evaluated as a function of styrene content, polyester composition, and molecular weight by measuring volatile monomer and sol and gel content of the irradiated mixtures. Density of crosslinking in the cured material was qualitatively estimated from swollen gel measurements. The effects of dose rate on curing efficiency was also studied. All results were used to infer the mechanism of gel formation in the different solutions and under the various irradiation conditions. The applicability of the results to the design of coatings mixtures for high speed radiation curing i s discussed.
tionate the total dbse into two or three doses in series for one production 1~ 2. A minimum of local radiation shielding is require+ ,nly for the compact accelerator assembly; the power supply does not require shielding. Hoffman and Smith (1966) reported on the electron radiation-curing of solutions of two unsaturated polyesters with varying concentrations of styrene (S),ethyl acrylate (EA), or methyl methacrylate (MMA). Although the polyesters were of unknown composition, mixtures with styrene cured most effectively. A maximum was noted in the curve of per cent gel us. concentration of styrene a t about 20 to 25% monomer. As dose increased, the estimated gel compositions approached the values calculated for a copolymer of styrene and diethyl fumarate, based on known reactivity ratios. This indicates an average of about 2 to 3 moles of styrene copolymerized with each mole of polyester unsaturation. A posteffect was observed in this system, even after monomer had been removed, suggesting the occurrence of slow termination reactions of growing polyester-grafted monomer chain radicals. Additional irradiation of the monomer-free mixture was very effective in producing additional gel. The efficiency of gel formation was somewhat lower in EA mixtures, and much lower in MMA mixtures. The lower reactivity of these monomers with the polyester unsaturation and the tendency of PMMA to disproportionate on termination and to degrade upon irradiation tend to produce long grafts or crosslinks between these molecules. Thus, less gel is produced per unit dose of radiation, despite their greater radiation sensitivity. Two mixtures of EA/S (12.5%/12.5%) and M M A / S (12.5%/ 12.5%), respectively, with the polyester backbone (75%) cured much more effectively than either of the acrylic monomers by itself. These mixtures are pertinent, since they combine the low cost and high reactivity of the styrene with the age resistance and clarity of acrylic coatings systems. There have been several other publications on radiation curing of vinyl monomer-unsaturated polyester mixtures (Ballantine and Manowitz, 1956; Burlant and Hinsch, 1964, 1965; Callinan, 1956; Charlesby and Wycherley, 1957) but only the recent work of Burlant and Hinsch (1964, 1965) presents pertinent data on electron radiation curing. I n one the these studies, Burlant and Hinsch (1965) copolymerized a 65% styrene-35% unsaturated polyester mixture under nitrogen with 300-kev electrons (estimated to be 40 kev at the mixture surface) over a wide range of dose rates. They noted very high initial rates of disap-
pearence of monomer and/ or polyester unsaturation which were proportional to the dose rate up to a dose rate of about 20 Mrads per minute. Above this radiation intensity, the initial rate of copolymerization leveled out and appeared to be independent of intensity up to 100 Mrads per minute. They concluded that, at these high intensities, the polymerization occurred in discrete volume elements with chain propagation by hot radicals and that the average molar ratio of styrene-unsaturated group in the polyester backbone was about 3.6, independent of dose rate. This ratio is higher than tke 1.5 expected from copolymerization composition calculations, and they conclude that the discrepancy may be due to the fact that a large extent of the polymerization is occurring in localized regions within a rigid glassy matrix. The work reported here extends the work of Hoffman and Smith (1966) to the study of the effect of the polyester backbone composition and molecular weight on the curing process. Styrene monomer was utilized solely in the study reported here. Dose rates were also varied. Materials a n d Procedures
Styrene monomer was obtained from Matheson, Coleman, and Bell, reagent grade quality, and used directly with no further purification. All the polyesters but one were obtained from the Chevron Research Co., Richmond, Calif. This last polyester (designated H) was supplied by the Pittsburgh Plate Glass Co., Springdale, Pa. Table I presents the compositions, acid numbers, and intrinsic viscosities of these polymers. The resins are compared and contrasted with each other as shown in Table I, on the following bases: Compare
Basis of Comparison
Molecular weight (also possibly composition, IP us. PA) D us. E Molecular weight A us. B us. C. Backbone reactivity or degree of unsaturation D us. B Backbone composition (DEG us. PG, assuming all maleate isomerized to fumarate during polyesterification) (Leavitt et al., 1957)
C us. H
Each polyester was dissblved in styrene to the approximate concentration desired by shaking a mixture of powdered polyester in styrene at room temperature until the mixture flowed smoothly down the side of the glass container. About 1 gram of the solution was added to a tared aluminum cup (to yield a layer about 20 to 30 mils in depth) and then covered with an aluminum foil, which was pinched over the lip. Air was not excluded Ind. Eng. Chem. Prod. Res. Develop., Vol. 9 , No. 2, 1970
159
Table II. Effect of Added Hydroquinone on Cure of Styrene-Polyester C Mixtures
Table 1. Characterization of Polyester Backbones
Resin Designation C
H D
E A
B C
Acid No.
Intrinsic Viscosity, [ q ] Dl!G
Estimated Mol. wt.,
IPIMAIPG = 11112 P A / M A / P G = 11112
15.6 47.2
0.087c 0.04lC
2400 800
in Original Mixture
Hydroquinone Added, PPM
I P / F A / D E G = 21113 IPIFAIDEG = 21113
16 25
0.083* 0.066*
2300 1700
15
I P / M A / P G = 31114 I P / M A / P G = 21113 I P / M A / P G = 11112
11 15.1 15.6
0.087' 0.079' 0.087'
2400 2100 2400
0 100 500 1000
95.6 93.4 92.0 85.8
28
0 100 500 1000
53.7 50.2 48.8 43.9
38
0 100 500 1000
14.8 16.4 13.8 17.6
Composition"
Dose = 4 Megarads
M,b
% Styrene
I P isophthalic acid, P A phthalic anhydride, M A maleic anhydride, F A fumaric acid, PG propylene glycol, DEG diethylene glycol. ' Calculated from follouing (determined for polyester resin of composition I P I M A I P G = l i 2 : 3 in chloroform at 30" C.) (Cantou: et al., 1964) [?] = (4.8 x lK4) MBGi. 'Measured i n benzene, 23°C. *Measured in chloroform, 23" C.
from the cup, despite the foil. The cups were irradiated by passing under the ICT-500 electron accelerator beam; doses were delivered in multiple passes of 2 Mrads per pass using 500-kev electrons a t a current of 5 Ma scanned over an area of 18 x 6 inches (perpendicular and parallel to the conveyor, respectively). The conveyor moved a t 20 feet per minute, 5 inches below the scanner window, and the samples received an approximate dose rate of 0.4 Mrad per second. The samples were weighed accurately after irradiation (and after removal of the foil), then left open in a hood for 96 hours to remove volatile monomer. The samples were weighed after this to obtain per cent volatiles removed. Then they were placed in an excess of fresh 2-butanone (methyl ethyl ketone) a t room temperature for 16 hours, with only occasional stirring. The swollen gel weights were estimated by draining the excess solvent as well as possible and then weighing quickly. The gels were dried under vacuum overnight a t room temperature and weighed the next day to obtain per cent gel. The difference between swollen gel weight and dry gel weight represents solvent sorbed into the gel and is used to calculate the volume fraction of crosslinked polymer in swollen gel, u2,
Results
Radiation Curing Data. All of the radiation curing data for the resins studied here are presented in Table IV.
Table 111. Effect of Evacuation Time and Extraction Time and Procedure on Accuracy of Results
Sample. 26% styrene, 74% polyester A. Dose = 8 Megarads
Time
w1,
wz
= weight fractions solvent and polymer, respec-
tively, in swollen gel P I , p~ = density of solvent and polymer, assumed to be 0.8 and 1.2 grams per cc, respectively Accuracy of D a t a
Relatively small amounts of free radical inhibitors are usually added to unsaturated polyester resin kettles to inhibit gelation reactions during the cook or during the subsequent blending operations with styrene monomer. Since the polyesters used in this study probably contained these minor but unknown quantities of inhibitors, additional known amounts of hydroquinone were added to determine their effect on the rate of gel formation. Table I1 presents these data; there is a real but relatively small 160
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
Gel
reduction in total gel formed, with increasing amounts of inhibitor up to the maximum amount added, 1000 ppm. Therefore, it can be concluded that the presence of unknown amounts of inhibitor in the polyesters did not reduce the rate of gel formation greatly over that which would have been observed in the absence of such inhibitors. The effectiveness of the evaporation and extraction techniques was investigated by subjecting one gelled sample (-60% gel) to a much more elaborate sequence of volatilization and extraction (Table 111). I t can be concluded that the techniques employed as "standard procedures" were acciirate within a few per cent of the actual volatiles and solubles contents in the samples.
Procedures after Irrad.
where
cc
A! Open cup in hood B: Evacuate, room temp. C. Repeat B D. Grind up sample and repeat B E. Evacuate, room temp. F! Immerse in MEK room temp. G. Repeat F H. Repeat F I. Immerse in hot xylene (-75°C.) J. Evacuate, room temp. a
Standard procedures.
%
%
%
Volatiles Solubles Error in Step, Hr Calculated Calculated Technique of Each
... ... ...
96
21.6
16
22.2
16 16
22.4 22.6
64
22.8
...
16
...
58.5
20 72 2
...
56.6 56.7
120
... ...
...
...
59.1
2.6
3.2
Table IV. Radiation Curing Data
Comoositions
5% volatile % gel
Dose, Mmds
5% sol
styrene
9% volatile
70 volatile 5% gel
% sol
styrene
% gel
so1
styrene
E
D
H
70
0 2 4 8 0 2 4 8
0 0 1 5 0 32 60 97
100 100 99 95
0 0 0 0
0 27 41 58
100 73 59 42
0 0 0 0
0 16 30 48
100 84 70 52
0 0 0 0
88 61 38 2
12 7 2 1
0 36 72 98
83 50 20 1
17 14 8 1
0 34 51 91
83 52 39 7
17 14 10 2
0
0 30
71 43
29 27
73 55 43 24
27 26 24 18
0 9 25 52
73 65 52 30
27 26 23 18
55 53 47 42
45
0 5 11 20
55 51 45 38
45 44 < 44 42
2 4 8
...
...
56
24
20
0 19 33 58
0 2 4 8
0 21 27 33
51 31 27 22
49 48 46 45
0 2 9 17
...
2
44 41
A
B
C
0
