Polymerization Kinetics and Technology

13 One-Step Synthesis of Cured Polyester RUDOLPH D. DEANIN and VITTORINO G. DOSSI

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0128.ch013

Plastics Department, Lowell Technological Institute, Lowell, Mass. 01854

Unsaturated polyesters were polymerized and cured in a single simple step by mixing propylene oxide, maleic anhydride, phthalic anhydride, styrene, lithium bromide, and benzoyl peroxide in crown-cap bottles, heating and mixing to melt and dissolve, and casting to produce cured polyesters. Lithium bromide catalyzed an ionic ring-opening polyesterification, while benzoyl peroxide initiated styrenemaleic free-radical copolymerization to produce the cured polyester. Reaction times were comparable with commercial polyesterification and hand lay-up cycles. Cured properties were adequate though not equal to commercial polyesters. Problems of volatility, reaction rates, and higher cure would have to be solved before such a one -step process could be considered for commercialization. lass-fiber-reinforced polyesters were first used commercially in 1944 and have now reached a market volume of 1.2 billion pounds per year (J). Prospects are good for continued growth, primarily in boats, building, and autos (2). At the same time, however, the base cost of these commodity resins has leveled off at about 18-22 cents per pound, keeping them out of the reach of still larger potential markets (3). For continued large-scale growth, it would be desirable to make their synthesis simpler and more economical to advance to a lower base cost and thus enter a greater variety of larger applications and markets. These polyesters are primarily copolyesters of propylene glycol with maleic and phthalic acids, dissolved in styrene monomer, and cured by peroxides ± activators (1, 4, 5). In a typical general-purpose polyester, a 10% excess of propylene glycol is cooked with an equimolar mixture of maleic and phthalic anhydrides for eight or more hours at 150200° C to produce a viscous liquid polyester of 1000-5000 molecular weight. This is then partially cooled, diluted about 60/40 with styrene monomer, cooled to room temperature, and stored. It is later mixed 176 In Polymerization Kinetics and Technology; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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177

Cured Polyester


with peroxides ± activators, impregnated into glass mat or fabric, gelled at room temperature, and finally hot-molded or oven-cured. These successive steps form a complex process involving considerable time, heat, power, and handling, all of which contribute to increasing the cost of the final product. If all of these separate steps could be combined into a single integrated process, the resulting saving in time, heat, power, and labor should significantly reduce the base cost of the polyester resin (Table I). Table I. Base Cost of Polyester Resin Ingredients Propylene Glycol Maleic Anhydride Phthalic Anhydride Styrene Raw Material Cost Polyester Resin Price

Cents Cents/Lb. Lb. Monomer/ Monomer (6) Monomer/ Lb. Polymer Lb. Polymer 2.83 10.5 0.269 0.157 14 2.20 0.238 8 1.91 0.400 8 3.20 10.14 18-22

The first difficulty is the long time, high temperature, and high viscosity involved in present condensation polymerization reactions to produce the initial polyester. A number of previous studies have indicated that alkylene oxides should react directly with dibasic acid anhydrides, by ionic ring-opening addition polymerization, to produce polyesters under milder conditions than current commercial practice (7-14). Specifically, propylene oxide has reacted directly with maleic and phthalic anhydrides to produce unsaturated polyesters under these milder conditions (15, 16). This would certainly be a major first step toward simplifying the process and lowering the cost. Incidentally, use of propylene oxide in place of propylene glycol would also result in an additional saving of 1 cent per pound in total raw material cost as well (6). After polyesterification, the separate steps of cooling, dilution with styrene, catalysis, impregnation, gelation, and cure are a distinct operational and economic liability. The idea put forth here is that these steps might be telescoped into one by running the ionic ring-opening polyesterification reaction in styrene as solvent and, simultaneously, running the free-radical copolymerization of styrene and maleic double bonds to produce crosslinking and cure at the same time. In commercial operation, all the ingredients (Table II) would be mixed to form a solution; this solution would be impregnated into the glass mat or fabric in the mold, and heat and pressure would be applied to produce polymerization and cure.

In Polymerization Kinetics and Technology; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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POLYMERIZATION KINETICS A N D T E C H N O L O G Y

Table II.

Ingredients of One-Step Preparation of Cured Polyester


Propylene Oxide Maleic Anhydride Phthalic Anhydride Styrene Catalyst for Ionic Ring-Opening Addition Polyesterification Initiator for Free-Radical Styrene/Maleic Ester Copolymerization (Crosslinking Reaction) Experimental To test this proposal in the laboratory, unreinforced castings were made by charging all the ingredients (Table III) into crown-cap glass bottles, tumbling in a water bath at polymerization temperature to melt and dissolve the ingredients and initiate the reactions, and then standing in an oven at polymerization temperature to complete the polymerization, gelation, and cure. Generally, small exploratory runs used 18- or 36-gram charges in small soft drink bottles; larger runs for more thorough evaluation of properties were 54 or 90-gram runs in medium-sized soft-drink bottles. After polymerization and cure, each bottle was broken, and the cylinder of cured polyester was removed and used for testing. In exploratory studies, the cure reaction was followed simply by measuring Barcol hardness. Several larger runs were then evaluated more thoroughly. When test pieces of specific size and shape were required, they were obtained by machining them out of the cyclindrical castings. Most properties were measured by ASTM standard test methods. Swelling was measured by immersing a 3-gram sample of cured polyester in 50 ml of acetone for 24 hours at room temperature. The sample was then removed from the acetone, wiped, and weighed as rapidly as possible. Results Series of runs were made to explore the effects of temperature, catalyst, and initiator on the cure rate (Figures 1 through 3). Taking a Barcol hardness of 63 as a reasonable level for comparison, standard formulations reached this level in 48 hours at 75°C, 24 hours at 85°C, Table III.

Standard Experimental Formulation

Propylene Oxide Maleic Anhydride Phthalic Anhydride Styrene LiBr Benzoyl Peroxide Reaction Temperature

20.5 15.7 23.8 40.0 0.258 1.0 85°C


13.

Cured Polyester


40

60

80

Reaction Time, Hours

Figure 1. Effect of reaction temperature 100 r : 75 ! 50 § 25

0.258%

0.516% 15

30

45

60


Figure 2. Effect of LiBr concentration 100 V»

a> c •o ο

orco


20

179

75

2%

1%

50 25

CO

0

15

30

45

60


Figure 3. Effect of benzoyl peroxide concentration and 18 hours at 95°C. These times are comparable with some poly esterification reactions and hand lay-up cycles, but do not approach the high speed of matched metal molding used in large-quantity production. Lithium bromide was a satisfactory ionic catalyst for the ring-open ing polymerization reaction (Figure 2). The standard concentration of 0.258% gave good reaction. A lower concentration gave a slower reac-



180

POLYMERIZATION KINETICS A N D T E C H N O L O G Y

tion, but higher concentration was no further help. This suggests either that a small amount was sufficient for the catalytic effect or that insolubility of the inorganic salt in the organic medium limited the amount that could be used effectively. Other catalysts reported in the literature (7-16) deserve evaluation to see if they could give faster reaction or higher cured properties. Benzoyl peroxide initiator was more effective at a higher concentration (Figure 3). Other peroxides, and especially activators such as cobalt soaps or tertiary amines, could undoubtedly produce much faster cures. This would probably however, have to be balanced against the parallel rate of polyesterification. Several formulations were prepared in larger quantity, to reasonable cure, for a greater variety of tests, and compared with a well-cured typical commercial general-purpose polyester, W. R. Grace Marco G R 941 (Table IV). At reasonably similar cure, reaction temperature had no significant effect on final properties. Low LiBr catalyst concentration gave a low Vicat softening point, suggesting incomplete polyesterification. High benzoyl peroxide initiator concentration gave a high Vicat softening point, suggesting more thorough styrene-maleic ester crosslinking. Table IV. Formulation Standard 95°C 75°C 1/2 LiBr 2X LiBr 2Z Bz 0 Commercial 2

2

Properties of Cured Polyesters

Barcol Rockwell Compressive Vicat Swelling HardR Strength, Softening in ness Hardness psi Point Acetone °C % 64 60 65 65 65 63 75

118

13,200 15,100 14,300 24,000

98 102 100 67 105 140 210

44 50 45 56 44 40 1

Cured properties of the one-step polyesters were generally inferior to conventional commercial polyester. The general belief is that, during conventional high-temperature commercial polyesterification, most of the maleic isomerizes into fumaric in the final polyester, and that the fumaric ester reacts much more readily with styrene by free-radical copolymerization, producing more thorough crosslinking. At the low temperatures used in our one-step study, such isomerization probably did not occur. The relative sluggishness of the styrene/maleic ester free-radical copolymerization would account for the lower final proper-


13.


Cured Polyester

181

ties (17). Analysis for fumaric acid content would help in studying this possibility further (18).


Conclusions Unsaturated polyesters are made and cured commercially by a series of difficult steps, resulting in fairly high cost and limited markets for the resulting glass-fiber-reinforced polyester products. The work reported here demonstrates that similar chemical compositions can be made in a single simpler step, potentially offering considerable savings both in raw material and processing costs. A number of major problems remain, however, before this process could be considered for commercialization: ( a ) The volatility of propylene oxide would require processing in a closed pressurized system. Alternatively, a less volatile epoxide would have to be used. (b) Reaction rates in this study were comparable with commercial polyesterification and hand lay-up cures of polyesters, but much slower than the high speed of matched metal molding used in large-quantity production. Reaction rates might be accelerated by screening of other polyesterification catalysts and could certainly be speeded greatly by using other peroxides and, especially, conventional activators such as cobalt soaps or tertiary amines. (c) Low reactivity of maleic ester with styrene probably accounts for both slow reaction and low final properties. Higher temperatures and higher catalysis could certainly improve cure somewhat. If temperatures were high enough, they might even produce isomerization to fumarate ester, as in present commercial processes. Alternatively, other methods might have to be considered to improve cure and final properties. Glass-fiber-reinforced polyesters have grown at a good rate to a large volume of commercial utilization, and this growth will undoubtedly continue even with modest improvements in present technology. If the above problems could be solved, however, the resulting one-step process could offer distinct savings in overall costs, producing faster and further growth into a greater variety of larger applications and markets. Literature Cited

1. Lubin, G., "Handbook of Fiberglass and Advanced Plastics Composites," Van Nostrand Reinhold, New York (1969). 2. Rosato, D. V., Fallon, W. K., Rosato, D. V., "Markets for Plastics," Van Nostrand Reinhold, New York (1969). 3. Fullmer, G. E., Soc. Plastics Eng. J. (1968), 24, 2, 22. 4. Billmeyer, Jr., F. W., "Textbook of Polymer Science," 2nd Ed., WileyInterscience, New York (1971). In Polymerization Kinetics and Technology; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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5. 6. 7. 8. 9. 10. 11.


12. 13. 14. 15. 16. 17. 18.

P O L Y M E R I Z A T I O N KINETICS A N D T E C H N O L O G Y

Brydson, J. Α., "Plastics Materials," Van Nostrand, New York (1966). Oil, Paint&Drug Reporter ( 1971) 200, 35-40. Hayes, R. Α., U.S. Patent 2,779,783 (1957). Hayes, R. Α., U.S. Patent 2,822,350 (1958). Fischer, R. F., Ind. Eng. Chem. (1960) 52, 321. Fischer, R. F., J. Polym. Sci. (1960) 44, 155. Schwenk, E., Gulbins, K., Roth, M., Benzing, G., Maysenholder, R., Hamann, K., Makromol. Chem. (1962) 51, 53. Tsuruta, T., Matsuura, K., Inoue, S., Makromol. Chem. (1964) 75, 211. Hedrick, R. M., U.S. Patent 3,257,477 (1966). Kern, R. J., Schaeffer, J.,J.Am. Chem. Soc. (1967) 89, 6. Waddill, H. G., Milligan, J. G., Peppel, W. J., Ind. Eng. Chem., Prod. Res. Develop. (1964) 3, 53. Levine, L., ACS Div. of Org. Coatings&Plastics, Preprints (1971) 31, 623. Levine, L., private communication. Funke, W., Adv. Polym. Sci. (1965) 4, 157.

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April 1, 1972.


Polymerization Kinetics and Technology

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