R E I N F O R C E D P L A S T I C S SYMPOSIUM
MONOMERS ARTHUR L. SMITH
Choice of monomer f o r curing o f unsaturated polyesters f o r reiizforced plastics is a principal factor in determining the utility of the end product n the reinforced plastics field the largest volume resin is unsaturated polyester resin. I t consists of an unsaturated polyester, discussed in the following paper, dissolved in a polymerizable monomer solvent. The main functions of the monomer are two :
I base used with glass reinforcement -To
act as a solvent carrier for the unsaturated polyester -To provide a rapid means of reacting with the unsaturation in the polyester to yield a completely reacted cross-linked copolymer The major monomer in use in polyester resins todayand, in fact, since the start of the reinforced plastics industry in the 1940’s-is styrene. I t dissolves most unsaturated polyesters very well, and it has a high inherent tendency to copolymerize with fumarate unsaturation ( 7 , 2, 9 ) . This has made it an ideal monomer for use with unsaturated polyesters. Its ready availability and resulting low price are also key factors in favor of its use in unsaturated polyester resins. Styrene Copolymerization
Styrene contributes substantially to the final properties of the cross-linked, cured resin network because of its fast copolymerizing capability; this is the main factor in the characterization of its performance in unsaturated polyester resins. Work by Parker (72)and Fisher (7)shows, for example, that water absorption properties of polymers do not vary significantly with styrene content over the 20-807, styrene range. The level of water absorption reported was an order of magnitude higher than that of polystyrene. I n these and other studies, the composition of the polyester is a far more important factor. If, for example, the polyester is not condensed to a sufficiently high molecular weight, Church (5) found that water 50
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
absorption and other polyester properties are more sensitive to styrene content. Other studies (75, 76) pertaining to weathering characteristics in copolymer systems using methyl methacrylate indicate the importance of the copolymerizing characteristics of the whole resin system. These studies involve a system of unsaturated polyester with methyl methacrylate to which styrene is added to improve weatherability. This anomaly is explained later in this paper on the basis of the copolymerizing characteristics of styrene. TVhat is the nature of the finish-cured copolymerized unsaturated polyester1 st)-rene network? From data regarding relative reactivity of monomer pairs ( 7 ) , one might expect a copolymer of styrenejfumaric unsaturation of 1 :1 to result from polymerization of a resin of this type. However, work of Parker (72) and Fisher (7)’ as well as others, based on coreactivity exotherm and performance property data, suggests an optimum molar ratio of styrene:fumaric of 2 : l . Boenig ( Z ) , summarizing the work of several authors, has added significantly to the understanding of the nature of this cross-linked structure. Several workers have detected no polystyrene, as such, in a cured styrene :unsaturated polyester resin system unless the molar ratio of styrene: fumarate is greater than 9 :1, O n the other hand, residual fumarate unsaturation has been found in cured resins (8) of low styrene content where styrene : fumarate was 1 :l. The low degree of isomerization of maleic to fumaric unsaturation in
Arthur L. Smith is a Private Consultant in t h e j e l d of reinforced plastics. He has been active in this$eld for 20 years, is a professional member of the Society of Plastics Industry, and a member of the Socieh of Plastics Engineers. T h i s paper, prepared for the ACS Organic Coatings and Resins Division, is one of many he has published over the years. AUTHOR
TABLE I.
MONOMERS USED I N UNSATURATED POLYESTER RESINS
Monomer Styrene
Molecular Weight 104
Boiling eint, C. 145
Specific Gravity 0.902
Refractive Index 1.5439
Vinyl toluene
118
171
0.892
1.5395
Chlorostyrene
139
187
1.095
.5599
Methyl styrene
118
166
0.906
.5359
...
Divinyl benzene
131
195
0.913
.5585
*..
Diallyl phthalate
246
290
1.12
...
ia
Triallyl cyanurate
249
162
1.113
...
...
Methyl methacrylate
100
100
0.939
the polyesters used in this study casts some doubt on its complete applicability to commercial resins in which such isomerization is essentially complete. But the conclusion drawn by the authors was that a molar ratio of styrene:fumarate of 2 : 1 is required to obtain a high degree of conversion of fumarate unsaturation ; this is confirmed with studies of commercial resins (7, 72). I n these studies all properties were not superior at this optimum molar ratio of 2 :l. But those depending on degree of cross-linking, such as heat distortion point, elevated temperature strength, and exotherm, were best a t a ratio of 2 :l. Properties such as tensile or flexural strength and modulus were not optimized near this ratio. I t is postulated that these properties may have been more subject to unrelaxed stresses from the curing of the cast resins used in measuring these properties. For example, Parker found that high tensile strengths were obtained in resins containing excess of either fumarate or styrene. Highest tensile strengths were obtained with highlest styrene : fumarate ratios. I n commercial practice, “general purpose” resins are used with a styrene:fumarate ratio of about 2 : l . This does not imply that they are not used successfully a t other ratios; many factors other than properties of the resins per se influence the product performance-i.e., viscosity, shrinkage, thermal coefficient, and temperature of cure (3, 4). Resins in commercial use have styrene: fumarate ratios of 6 :1 in rigid resins, and as high as 40 :1 in flexible resins, but longer cure times are required when the ratio varies significantly from 2 :l. Other Monomers
There are monomers other than styrene used in unsaturated polyester resins. Table I includes these monomers and some properties pertinent to their use.
1.412
Volume Shrinkage,
%
15 12.5 12
20
Characteristics Offers low cost, fast coreaction Provides lower volatility, low shrinkage Provides lower volatility, low shrinkage, very fast coreactivity Controls exothermic heat Provides extra crosslinking Exhibits low volatility, fast cure Provides high hot strength Provides weathering or translucency
Vinyl Acetate. I n the early years, unsaturated polyester resins using vinyl acetate (10) as monomer were offered and used to some extent. Vinyl acetate, too, is a relatively low cost monomer. But because of its higher volatility, poorer coreacting capability with fumarate unsaturation, and lack of outstanding performance characteristics, it is no longer a monomer of significant usage in unsaturated polyester resins. Vinyl Toluene. The need for lower volatility than that exhibited by styrene has led to significant use of several monomers. Vinyl toluene is being used in increasing amounts because its higher boiling point permits cure at higher temperatures in less time. Because it is less volatile (vapor pressure at 7 7 ” F. is 2 mm. of H g us. 6.6 mm. for styrene), it also gives less loss of monomer during mixing and storage of “premix” molding compounds. These compounds are formulated putty-like molding compounds, catalyzed and ready for use by the molder. Low polymerization shrinkage also shows as an advantage in that less surface pattern results from reinforcements incorporated in the compounds. Chlorostyrene. Recently (13) chlorostyrene has been suggested for use with unsaturated polyester resins. High boiling point, low polymerization shrinkage, and high heat distortion are cited as advantages. Polyesters using monochlorostyrene have higher strength properties in both cast resin and laminate forms than styrene, as well as better surface smoothness and greater resistance to burning. High cost and specific gravity are drawbacks. I n this study the authors show that higher chlorostyrene ratios give superior pcrformance in all properties over the range investigated. This is different from what previous studies with styrene have indicated. It further supports the importance attached to copolymerization. VOL. 5 8
NO. 4
APRIL 1 9 6 6
51
This monomer provides faster copolymerization (cure) rates than styrene in polyester systems at moderate cost. This has been demonstrated in both resin cure tests in the laboratory and at the practical level. Molding cycles, already fast when styrene is used as monomer, have been substantially reduced by use of this monomer. This faster molding is suggested as partial economic justification to overcome its higher price. a-Methyl Styrene. The use of a-methyl styrene in unsaturated polyesters is relegated to enhancing storage stability or controlling exothermic heat of polymerization in large section castings. Because of its chain transfer behavior, use of more than 1.5-2.0y0 detracts from physical properties (2). Its use as a ready copolymerizing monomer with other vinyl monomers has been suggested. Divinyl Benzene. This compound has been suggested to increase the cross-linking density with polyesters; it effects faster cure rates and higher temperature resistance than styrene, but its commercial use is not significant today. Diallyl Monomers. These compounds were some of the first used with unsaturated polyesters. Both allyl diglycol carbonate and, more significantly, diallyl phthalate modified unsaturated polyester resins were used during the early 1940’s. They have.not enjoyed the phenomenal growth of styrene-based resins. But they have enjoyed a steady increase in use. Low volatility coupled with difunctionality are the chief reasons for their use. These characteristics have led to use with unsaturated polyesters with inorganic fillers and/or reinforcement in preparation of compounds or (‘prepreg” materials ready for use by the molder ( 7 7 ) . Known as alkyd molding compounds, these have enjoyed significant commercial success. They also yield improved electrical performance characteristics. These are in someways similar to the diallyl phthalate prepolynier compounds also offered commercially; the latter are outside the scope of this paper. Because of their low reactivity and nonvolatility at room temperatures, these resins using diallyl phthalate permit formulation into finished catalyzed compositions with acceptable room temperature storage stability as prepreg materials or molding compounds. LVhen heated to molding temperatures, which are higher than those used with styrene-copolymerized resins, they cure very fast ; 15-second molding cycles are not uncommon. Triallyl Cyanurate. This compound has been investigated for use at elevated temperatures. Of all the monomers tested for elevated temperature strength with unsaturated polyesters, triallyl cyanurate gives the best results. I t is suitable for use at temperatures u p to 500 F., at which temperature it maintains good strength properties and ages well. Its trifunctionality and resulting extremely rapid copolymerization have made it difficult to use, but continued effort has alleviated these troubles at the commercial level ( 9 , 17). Methyl Methacrylate. This compound, referred to as MMA, is used at significant levels with unsaturated polyesters today. As might be expected, use of this 52
INDUSTRIAL AND ENGINEERING CHEMISTRY
monomer enhances the ability to resist weathering of the copolymers. Work which started in 1954 (15) showed the details of exterior performance as related to monomer composition. I n the system of polyesterMMA-styrene, optimum weathering was not found as anticipated at the high MMA content copolymers. Resistance to weathering showed an optimum at a composition containing equal molar ratio of styrene and MMA. Smith and Lowry related this to copolymerizing characteristics of the two monomers with fumarate unsaturation and with each other. Physical and chemical properties were also found to optimize at this composition. Microscopic studies of weathered panels substantiated this anomalous behavior. Transparency when used with glass fibers has also been investigated (6, 14). Because of their low refractive index, methyl methacrylate (1.4120) and methyl acrylate (1.4003) can be used to control refractive index in a polyester-styrene resin. Typical resins have refractive indices somewhat higher than those of glass fibers (1.548). These monomers are used to lower the refractive index to match that of glass. The lower boiling point of methyl acrylate (80’ C.), coupled with the MMA’s better performance in weathering, has kept the acrylic ester from becoming commercially significant. The relatively low boiling point of MMA (100’ C.) is itself considered a disadvantage. Special U s e Monomers. Ethyl acrylate and 2ethylhexyl acrylate are used to some extent for their softening or plasticizing effect in unsaturated polyesters, but their use cannot be considered commercially extensive at this time. 2-Vinyl pyridene and hT-vinylpyrrolidone are also cited (2) as being of commercial importance for use in unsaturated polyesters. Their prime purpose is to increase strength properties when used with glass. I t is evident that styrene will continue to dominate monomer use with unsaturated polyesters for a long time to come. T o compete with styrene for even a portion of the market, a monomer must have one or more of the following characteristics : It must impart unusual properties to the finished copolymer or provide low volatility, low shrinkage, and fast coreactivity with polyester unsaturation, and all at low cost.
LITERATURE CITED (1) Alfrey, T., Bohrer, Mark, H., “Copolymerization,” Interscience, New York, 1952. (2) Boenig, H. V., “Unsaturated Polyesrers,” Elsevier, Amsterdam, 1964. (3) Boenig, H. V., \Valker, N., M o d . Plastics 38, No. 6 , 123 (February 1961). (4) Carson, W‘.G. Smith, A . L., Plasiics Tech. 4, 805 (1958). (5) Church, T. M., Berenson, C., IKD.ENO.CHEM.47,2456 (1955). (6) Crenshaw, J. B., Smith, D., Plastics Tech. 5 , No. 3, 42 (March 1959). (7) Fisher, J. J., American Cyanamid, private communication. (8) Funke, W., Hamann, K., M o d . Plastics 39, No. 9, 147 (May 1962). (9) Lawrence, J. R., “Polyester Resins,” Reinhold, New York, 1960. (10) Marco Chemical Corp., industrial lirerature, 1942. (11) Oleesky, S., Mohr, G., “Handbook of Reinforced Plasrics,” Reinhold, h’ew York, 1964. (12) Parker, E. E., Ivfoffett, E. W., IND.ENG.CHEM.46, 1615 (1954). (13) Rubens, L. C., et al., Preprints of SPI Reinforced Plastics Div., Sect, 2-c (February 1965). (14) Slone, M. C., SPEJournal, p. 1123 (October 1960). (15) Smith, 4.L., Lowry, J. R., Plastics Tech. 5 , KO. 6 , 42 (June 1959). (16) Smith, A . L., Lowry, J , R., Preprints of SPI Reinforced Plastics Div., Sect. 10-b (February 1960).
