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Materials: hydrolysis-resistant thermoplastic polyesters.

New polybutylene terephthalate materials show significantly improved resistance to hot water and steam while maintaining the mechanical properties and processability of unmodified PBT resins.

Thermoplastic polyesters have been used in a variety of applications for many years because of their useful balance of properties. However, because they are condensation polymers, they can be hydrolyzed under certain conditions. Hydrolysis results in chain cleavage (Equation 1) and, with sufficient loss of molecular weight, deterioration of mechanical properties.

The specific nature of polyester hydrolysis has been widely studied.[1-3] In general, thermoplastic polyesters have good retention of properties when exposed to water below their glass-transition temperature ([T.sub.g]).[4] For polybutylene terephthalate (PBT), this is about 50 [degrees] C to 60 [degrees] C. However, at temperatures above the [T.sub.g], accelerated hydrolysis may occur.

In addition to temperature, polyester hydrolysis depends upon factors such as pH, relative humidity (RH), sample type and preparation, mode of exposure, and property to be measured.[5] The reproducibility and sensitivity of the test can also be important to the relative ranking of materials.[6] Polyester resin variables such as molecular weight (MW), end-group concentration, polymerization catalyst residues, additives, colorants, and impurities may also play a role in hydrolysis.

Hydrolysis testing, especially accelerated hydrolysis testing, should be undertaken with attention to all these possible variables. As with all accelerated testing, the closer one comes to testing actual parts under actual use conditions, the more meaningful the results. The data presented in this article are only relative guidelines to part performance under actual use conditions.

Experimental

Hydrolysis testing at 85 [degrees] C was conducted on injection-molded Type I (7 x 1/8 inch) or Type V (2-1/2 x 1/8 inch) tensile bars. Bars were suspended above a saturated aqueous potassium sulfate solution in a closed container at an estimated 94% RH. Accelerated hydrolysis testing (110 [degrees] C/100% RH) was performed on Type V bars suspended above deionized water in a pressure steam sterilizer. After exposure, samples were removed and held at ambient temperature and humidity for at least 24 hrs before testing, except as noted in the text.

Tensile strength at yield and percent elongation at break were measured as per ASTM D638. Crosshead speed was 2.0 inch/min for Type I bars and 0.5 inch/min for Type V bars. Data are reported as averages of four to six samples. Carboxylic acid end-groups (CEGs) were measured by titration and reported as mequiv/kg.

Differential scanning calorimetry (DSC) was performed with a Perkin-Elmer 7 Series Thermal Analysis System. Samples weighing 10 mg were heated from 40 [degrees] C 280 [degrees] C at 20 [degrees] C/min, and then cooled to 40 [degrees] C at 20 [degrees] C/min. After holding for 5 min. samples were again heated to 280 [degrees] C at 20 [degrees] C/min.

Discussion

Polyester hydrolysis is known to be dependent on the number of carboxylic acid end-groups.[7] In general, the lower the carboxylic acid content of the polymer, the better its resistance to hydrolytic degradation. While lowering CEG content does have some benefit, it also has some limitations. In addition to the CEGs initially present, melt processing inevitably increases acid content, and because polyester hydrolysis is autocatalytic, increases in acid content can give rise to accelerated degradation, especially above the polyester [T.sub.g].

We addressed the hydrolysis issue by modification of the PBT resin. Modification results in a significant increase in hydrolytic stability, as indicated by the improved retention of melt viscosity and tensile elongation vs. those of a standard PBT at 85 [degrees] C/94% RH. Further, under the very harsh conditions of the accelerated hydrolysis test (110 [degrees] C/100% RH), the modified PBT showed a five-fold improvement in property retention vs. a standard resin.

Besides the improvement in hydrolytic resistance, the mechanical properties of the modified resin are almost identical to those of the control. The melting temperatures ([T.sub.m]) of the two materials are very similar; however, the crystallization temperature ([T.sub.c]) of the modified PBT is higher.

Initially, commercial development of this technology focused on a high-viscosity product for extruded optical-fiber buffer tubes. In optical-fiber cables, the extruded PBT tube must have a very long service life. Testing of the modified polyester indicated a considerably longer service life than standard polyesters.[8]

The hydrolysis-resistant (HR) polyester technology was extended to a lower-viscosity resin. Again an improvement was observed over standard resin. In this comparison between a low-viscosity resin and a higher-viscosity extrusion-grade standard resin, it is interesting to note that improved hydrolysis resistance is not solely dependent upon having a high-molecular-weight polymer. The mechanical properties of this low-viscosity HR resin are very similar to those of a standard PBT of comparable MW.

Shear rate vs. viscosity measurements for hydrolysis-resistant and standard PBTs show that the HR PBT resins can be processed under standard thermoplastic polyester conditions. The high-viscosity grade is especially useful TABULAR DATA OMITTED for melt-extrusion applications.

A 30% fiberglass-filled modified PBT also showed improved performance. After an initial drop in tensile strength, the modified resin showed a fivefold increase in hydrolysis resistance at 85 [degrees] C/94% RH vs. a standard 30% glass-filled PBT. Under the accelerated hydrolysis conditions, the modified resin eventually begins to lose MW and ultimately mechanical properties. However, performance is still considerably improved.

The initial drop in tensile strength, thought to be due to a loss of adhesion between the glass fiber and PBT matrix, can be reversed by drying the sample. Eighty percent of the initial strength can be recovered by allowing the sample to equilibrate at room temperature. It may be that under less stressful conditions, the initial drop may not be as severe as seen at 85 [degrees] C/94% RH. In any event, the overall retention of properties is far superior to standard glass-filled thermoplastic polyesters. The mechanical properties of the glass-filled HR PBT are comparable with (and perhaps even better than) those of a standard PBT using the same fiber.

The HR polyesters also show improved property retention under normal "dry" oven aging. After 28 days at 140 [degrees] C, the part melt-volume index of a standard unfilled PBT doubled, indicating degradation. In contrast, a similar HR PBT showed no change in viscosity under identical aging. Likewise, a 30% glass-filled standard PBT showed a 22% loss of tensile strength after 63 days at 170 [degrees] C while the glass-filled HR product showed no loss of initial tensile strength. While these studies are not complete, we expect a side benefit of the HR technology to be improved resistance to long-term thermal aging under dry as well as humid conditions.

Conclusion

New technology has been demonstrated in extrusion, injection molding, and glass-filled PBT grades that can significantly expand the capabilities of these materials to resist the loss of properties that results from hydrolysis. In some cases a fivefold improvement in property retention has been observed. A side benefit is an improvement in the dry thermal aging. The mechanical properties of HR grades are equivalent to or slightly better than those of standard PBT grades. While no thermoplastic polyester is impervious to hydrolysis, these new materials should extend performance, giving PBT resins significantly longer lifetimes under conditions of mild hydrolysis.

References

1. W.F.H. Borman, Polym. Eng. Sci., 22, 883 (1982).

2. R.J. Gordon and J.R. Martin, J. Appl. Polym. Sci., 25, 2353 (1980).

3. L.H. Buxbaum, Angew. Chem., 7, 182 (1968).

4. C. Bastioli, I. Guanella, and G. Romano, Polym. Compos., 11, 1 (1990).

5. P.G. Kelleher, R.P. Wentz, and D.R. Falcone, Polym. Eng. Sci., 22, 260 (1982).

6. D.R. Parris, Proc. Int. Wire and Cable Syrup., 105 (1989).

7. S. Sawada, K. Kamiyama, S. Ohgushi, and K. Yabuki, J. Appl. Polym. Sci., 42, 1041 (1991).

8. L. Baccaro, R. Black, and L. Nelson, Proc. 61st Wire Assoc. Int. Meeting (Interwire), 145 (1991).
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Author:Gallucci, Robert R.; Dellacoletta, Brent A.; Hamilton, Douglas G.
Publication:Plastics Engineering
Date:Nov 1, 1994
Words:1321
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