Blends of Unsaturated Polyesters with High-Molecular-Weight

21 Blends of Unsaturated Polyesters with High-Molecular-Weight Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0222.ch021

Elastomers Bearing Reactive Functional Groups Richard F. Grossman Synthetic Products Company, Stratford, CT 06497

Blends were prepared throughout the range of composition of a typ ical unsaturated polyester with acrylonitrile-butadiene copolymer elastomers(NBR),ethylene-propylene-diene-monomer copolymers (ΕΡDΜ), carboxylated NBR, carboxylated EPDM, and amino-func tional NBR elastomers. The elastomers hearing reactive functional groups show sufficient compatibility that their blends are suitable for further compounding. Blends with amino-functional NBR show im proved fracture surface energy in glass-filled compositions.

BLENDS OF ELASTOMERS WITH THERMOSETTING RESINS

have been in com mercial use for many years in several applications. Blends with phenolic resins have been used to increase the hardness of rubber compounds. Blends with about equal weights of each component were popular many years ago in hard-rubber applications and, more recently, phenolic compounds have been modified with minor additions of various elastomers (1). The use of depolymerized rubber to modify the brittleness of epoxy resins is almost as old as the resins themselves (2). In addition, epoxy resins are common ingredients used by the rubber compounder, most often to improve the stability of halogenated elastomers (3). The toughening of epoxy resins by minor addition of liquid low-molecular-weight elastomer precursors has proven highly fruitful and led to considerable experimentation with elasto mer modification of a variety of thermosets (4). 0065-2393/89/0222-0415$06.00/0 © 1989 American Chemical Society

Riew; Rubber-Toughened Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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Elastomers in Unsaturated Polyesters

Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0222.ch021

Similar results with unsaturated polyesters are desirable, as increased tough ness would be of great commercial significance (5). Breakage of parts during demolding and finishing is a significant factor in the molding industry. In addition, availability of tougher compositions would enable wider use in automotive and other applications. Although some degree of success has been reported (6), and at least one low-molecular-weight elastomer is pro moted in this area, improvements in toughness in actual working compounds are minor compared to those found with epoxy resins. Present commercial use is very small. For example, the most recent text on polyester-molding technology devotes a half page to the subject (7). Explanations of the relative unresponsiveness of unsaturated polyesters to elastomer toughening have been diverse. One school of thought holds that matrix toughening is not significant in typical fiber-reinforced compo sitions (i.e., that failure occurs primarily at the resin-reinforcement inter face). In this case, only toughening specifically at this locus would be effective (8). Others have suggested that overall matrix toughness probably is signif icant, but that poor elastomer-resin compatibility during the initial phase of cross-linking prevents development of a suitably distributed rubber phase of useful particle size (9). With epoxy resins, considerable control of molecular architecture is possible. The molar mass, size, and mobility between cross-links are readily varied, as well as the same factors in the cross-link itself. The cross-linking reaction affects only certain groups present in the elastomer, whose numbers and frequency may also be varied. The importance of these considerations in developing toughened epoxy resins is well appreciated. The converse, that these controls are not readily available in the case of unsaturated poly esters, has not been stressed. In theory, special-purpose polyester resins might be prepared with sites for peroxide-initiated free-radical cross-linking disposed so as to augment toughening with elastomers. Instead of a low-cost monomer, typically sty rene, unsaturated oligomers or other specific cross-linking agents might be employed to enhance toughening. With epoxy resins, this type of approach would be normal compounding procedure. In the case of unsaturated poly esters, economies and past practice dictate against such an approach. The compounder in this field is normally limited to off-the-shelf polyester resins, low-cost monomers, and the range of fillers and additives promoted for this use. To date the inclusion of minor fractions of elastomers in this compound ing framework has not generated improvements more significant than could be achieved by manipulation of the proportions of standard ingredients. The present study has proceeded with two premises: that practices in the unsaturated polyester molding industry are likely to continue, and that, despite this, it may be possible to find elastomers that have a toughening



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effect. Search for such species was limited to elastomers of current com mercial availability and those well along in development. Such an elastomer, it was thought, should have the following attributes: 1. The greater part of the elastomer should be thermodynamically in compatible with unsaturated polyesters.. This feature is needed to prevent formation of a phase in which the elastomer acts as a plasticizer, thus to improve impact strength only by lending ductility, not through actual tough ening, and consequently to reduce the physical and thermal properties of the composition. 2. Nonetheless, the elastomer should contain sufficient polar groups to permit solubility in the uncured resin-monomer mixture. These should be distributed so as to provide a large molar mass of nonpolar component between the polar groups so that there is sufficient mobility to form a dis tribution of discrete incompatible elastomeric particles during cross-linking. A necessary corollary is that the elastomer should have intrinsically high mobility (i.e., low glass-transition temperature). 3. The polar groups on the elastomer should have strong affinity not only for the unsaturated polyester matrix, but also for reinforcements such as treated glass fibers. 4. The rate of cross-linking of the elastomer via thermal or induced decomposition of peroxide initiators should be slow compared to that of the unsaturated polyester matrix, to permit formation of a distribution of discrete elastomer particles during the cross-linking phase. 5. The elastomer should be of relatively high molecular weight. It should possess elastomeric properties rather than be a liquid polymer that, under other conditions, could be chain-extended and cross-linked to yield a rubbery product. In respect to toughening brittle plastics in general, high molecular weight is not necessarily required, depending on whether the load-bearing ability of the elastomer particle enters into a specific toughening mechanism. In the case at hand, high molecular weight should be a positive factor because of its contribution of mass and size between polar groups, and because of its contribution to the work of adhesion needed to disrupt bonds to the matrix and to reinforcements.

Experimental Details Unreinforced Compositions. Blends were made from 0-100% unsaturated polyester (Owens-Illinois OCF-RP-325), and the remainder elastomer. To each blend was added 50 parts per 100 total polymer offine-particleamorphous silica, 35 parts of styrene monomer, 4 parts of zinc stéarate, and 1.5 parts of dicumyl peroxide. No low-profile or other polymeric additives were used. Rubber-rich blends were mixed in an internal mixer (Farrel Midget Banbury) and polyester-rich blends in a sigma blade mixer (Baker-Perkins). Compositions were press cured at 160 °C according to ASTM D 3182. Optimum cure time was


418

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taken as the oscillating disk rheometer i90 value at this temperature, following A S T M D 2804. Tensile properties were measured by using a tensile tester (Instron M o d e l 1130) to perform A S T M D 638, with a crosshead speed of 5 m m / m i n on Type IV samples, compression molded directly to dumbbell shape, as indicated previously.


Reinforced Compositions. Blends consisted of 0-100% unsaturated polyester (Owens-Illinois O C F - R P - 3 2 5 ) , and the remainder elastomer. T o each was added 200 parts per 100 total polymer of fine-particle calcium carbonate, 35 parts of styrene, 4 parts of zinc stéarate, 1.5 parts of dicumyl peroxide, and 100 parts of 6-mm chopped fiberglass strand (Owens-Corning 847GE). Double-beam compact tension specimens were compression molded at 160 °C according to the dimensions given by Kinloch (10). Cracks were initiated with a razor blade and the samples loaded at a rate of 1 m m / m i n crosshead travel in a tensile tester, to calculate stress-intensity factors and then fracture energies (II). Specimens were also compression molded at 160 °C, 1.6 m m thick, 25 X 50 mm for measurement of flexural modulus, according to A S T M D 790, Method I, by using a tensile tester with crosshead speed of 0.8 m m / m i n . The elastomers amorphous ethylene-propylene-diene-monomer ( E P D M , E p syn 4506), carboxylated E P D M , (Epsyn DE203), acrylonitrile-butadiene copolymer (NBR, Nysyn 33-3), and amino-functional N B R (Nysyn DN508-14A) were provided by Copolymer Rubber Chemical Corp. Carboxylated N B R (Krynac 221) was provided by Polysar, Inc.

Discussion Amorphous E P D M was chosen as a control elastomer because it does not contain polar groups and is known to have low affinity for both glass and polyester. In blends with unsaturated polyester (Figure 1), tensile strength

40

τ

NBR-COOH 30 + Tensile Strength, M Pa 20 +

10 +

0

0

NBR, EPDM

Wt% Polyester

—

Figure 1. Tensile strength versus composition.


100


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decreases rapidly as the composition curve is entered from either end, to levels far below the average of the individual components. Figure 2 shows that ultimate elongation falls sharply as polyester is introduced into the elastomer. In Figure 3, the tensile modulus of the polyester falls similarly sharply with introduction of the elastomer. This pattern of observations is typical of mixtures of incompatible polymers. These experiments were repeated with carboxylated E P D M , essentially the same polymer except for the presence of a low level of pendant - C O O H groups. This polymer is much closer to the criteria presented for useful toughening. The polar groups are of a useful type, widely disposed, and present at a level low enough not to interfere with overall mobility. Further, the rate of cross-linking with organic peroxides is very slow. It was found (Figures 1, 2, and 3) that tensile strength, elongation, and tensile modulus show relatively smooth curves from one extreme of composition to the other. A high level of elongation was obtained with approximately equal proportions of the two polymers. Addition of 400 parts per 100 total polymer of calcium carbonate filler to the 50:50 blend produced a vulcanizate retaining ultimate elongation of 75-100%. Addition, instead, of 250 parts of hydrated alumina led to the same level of ultimate elongation. These compositions were easily compres sion molded and retained more than 50% of their original elongation after 7 days at 150 °C in a circulating-air oven. This level of heat-aging resistance is considerably better than that of typical hard-rubber compounds (I). Acrylonitrile-butadiene rubber (NBR), 33% acrylonitrile (ACN) content,

Figure 2. Ultimate elongation versus composition.



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Figure 3. Tensile modulus versus composition. yielded tensile, elongation, and modulus results in blends with unsaturated polyester very similar to those found with amorphous E P D M , as indicated in Figures 1, 2, and 3. This was the case despite a solubility parameter estimated to be similar to that of unsaturated polyesters (12). N B R may be considered another control polymer that does not meet the suggested cri teria. The experiments were repeated with a similar N B R containing a low level of pendant amino groups (Nysyn DN508-14A). This was considered another potential candidate to meet the criteria. Unlike the parent N B R , the cross-linking reaction with organic peroxides is very slow. The pattern of tensile, elongation, and modulus data was similar to that of carboxylated E P D M . When these data are plotted on the same scale as polyester, which has much greater tensile strength and modulus and much lower elongation than any of the elastomers, the differences tend to vanish because of dis tortions of scale. The same is true of the N B R and E P D M controls also given in Figures 1, 2, and 3. The scale needed to accommodate the pure polyester compound obscures the relatively minor differences between the two elas tomers. Compositions based on equal parts of unsaturated polyester and aminofunctional N B R exhibited strong adhesion to polar substrates such as metal mill rolls and glass plates. Analogous compounds made with carboxylated E P D M , on the other hand, appeared to have easy release from polar sub strates, compared to unmodified polyester compounds.


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Carboxylated N B R (Krynae 221, medium A C N ) was also investigated, despite its relatively rapid cross-linking by organic peroxides. This compound proved to be an intermediate case, yielding tensile strength at some points in the composition curve (Figure 1) lower than either component, but with out the catastrophic loss noted with E P D M and N B R . The curves of tensile strength, elongation, and tensile modulus versus composition are similar to those of elastomer blends (e. g., N B R with E P D M ) that are less than optimum but nonetheless often used in practice. Although compositions containing equal fractions of carboxylated N B R and polyester have lower ultimate elongation than analogs that use carboxy lated E P D M or amino-functional N B R , useful products may still result. F o r example, addition of 300-400 parts of hydrated alumina to the 50:50 test compound yielded vulcanizates having 10-20% elongation, with Izod impact strengths in the range of 5-10 J / c m . It was therefore decided to proceed with carboxylated N B R , as well as carboxylated E P D M and amino-functional N B R , in typical glass-reinforced polyester formulations. Unmodified by elastomer, the glass-reinforced polyester compound yielded a flexural modulus in the range of 13.5-14 GPa. As shown in Figure 4, replacement of up to 20% of the polyester with carboxylated E P D M provided at most a 10% increase in fracture surface energy. At the 20% level, a flexural modulus of 13-13.5 G P a was retained. When the mix was increased to 30% carboxylated E P D M , the flexural modulus dropped to 8.5 GPa, indicating loss of the toughening effect and merely conversion from brittle failure.

30

/

T

NBR-NH Fracture Surface Energy, KJ/m*

2

EPDM-COOH 20

NBR-CO OH

10

100

90

80

70

Wt% Polyester

Figure 4. Fracture surface energy versus composition.


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PLASTICS


Replacement instead with amino-functional N B R led to more significant improvement in fracture surface energy. The original level of flexural mod ulus is maintained to about 25% rubber content with, as shown in Figure 4, an increase of almost 30% in fracture surface energy. Although not dra matic, this improvement is greater than most previous reports with glassreinforced compounds. It is tempting to ascribe this result to increased interface adhesion provided by the amino-functional N B R . At levels above 25%, rubber flexural modulus again declines sharply. The remainder of the fracture surface energy versus composition curve no longer indicates tough ening, but conversion from brittle failure. Carboxylated N B R did not appear to provide any toughening effect. Minor additions resulted in a loss of fracture surface energy, as shown in Figure 4. Addition of 10% of this elastomer lowered the flexural modulus to 10.5 GPa, with further loss as the concentration was increased. It should be regarded as a reactive plasticizer for unsaturated polyesters. Improve ments in brittleness will be attended by corresponding losses in properties. Compounding polyesters with high-molecular-weight elastomers intro duces several peripheral effects. As the fraction of elastomer is increased, there is a notable increase in apparent viscosity. Therefore thixotropic agents such as magnesium oxide must be reduced or eliminated. This viscosity increase is in contrast to the use of low-molecular-weight liquid elastomer precursors, which may provide the opposite effect (7). As the level of elas tomer is increased, the proportion of thermoplastic modifier (low-profile additive) should be decreased, as part of its function will be provided. C o m positions using a major fraction of elastomer may be conveniently processed by dry-rubber techniques. Such compositions, however, should be molded with tooling appropriate for solid thermosets rather than rubber, unless the elastomer content is very high.

Conclusions The experiments described suggest that in typical glass-fiber-reinforced bulkmolding compounds (BMC) or sheet-molding compounds (SMC), elasto meric carboxylated N B R polymers will function only as reactive plasticizers, without true toughening. Carboxylated E P D M appears to have a minor toughening effect, but probably will not be of great use in B M C or S M C formulations because of poor adhesion to polar substrates. Amino-functional N B R elastomer exhibits toughening in reinforced compositions and appears to have good adhesion to polar substrates.

References 1. Rubber Compounding Formulary; Phillips Petroleum: Bartlesville, OK. 2. Flick, E. W. Adhesive and Sealant Compound Formulations; Noyes: Park Ridge, NJ,

1984.


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3. Maynard, J. T.; Johnson, P. R. Rubber Chem. Tech. 1963, 36, 963. 4. Rubber-Modified Thermoset Resins; Riew, C . K.; Gillham, J. K., Eds.; Advances in Chemistry 208; American Chemical Society: Washington, DC, 1984.

5. Siebert, A . R.; Rowe, Ε. H.; Riew, C . K. Toughening Polyester Resins with Liquid Rubbers; 27th A n n . Conf. SPI, 1972. 6. M c G a r r y , F. J.; Rowe, E. H.; Riew, C . K . Polym. Eng. Sci. 1978, 18, 2.

7. Meyer, R. W . Handbook of Polyester Molding Compounds and Molding Tech nology; Chapman and Hall: New York, 1987; p 111.

8. McGarry, F. J . Rubber in Crosslinked Glassy Polymers; Rubber D i v . , A C S , 1986, Spring Meeting, New York.

9. Korb, J. Proc. Annu. Conf. SPE 1984, 660. 10. Kinloch, A . J.; Young, R. J. Fracture Behavior of Polymers; Applied Science: London, 1983.


11. Kinloch, A . J.; Shaw, S. J.; Tod, D . A . In Rubber-Modified Thermoset Resins; Riew, C . K.; Gillham, J. K., E d s . ; Advances in Chemistry 208; American C h e m ical Society: Washington, DC, 1984.

12. Barton, A. F. M. Handbook of Solubility Parameters; C R C : Boca Raton, FL, 1983. RECEIVED 1, 1988.

for review February 11, 1988.

ACCEPTED

revised manuscript September


Blends of Unsaturated Polyesters with High-Molecular-Weight

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