New thermoplastic vulcanizates.

Thermoplastic vulcanizates (TPVs) are dynamically vulcanized alloys of rubber and plastic first discovered in 1958 (ref. 1). Since then, there have been several significant TPV developments. Fisher's development work led to one of the first commercial thermoplastic vulcanizates (refs. 2 and 3). The most significant work in the field of thermoplastic vulcanizates came through the work of Coran, Das and Patel with the advent of fully crosslinked TPVs (ref. 4).

There are several TPVs available, varying in their rubber and plastics phase combinations. Ethylene propylene diene (EPDM) rubber, natural rubber and butyl rubber are examples of the rubber phase in TPVs. Polypropylene and polyethylene are the examples of the plastics phase. EPDM- and polypropylene (PP)-based TPVs have gained the most commercial interest and are the subject of this article.

There are also several known crosslinking systems for dynamic vulcanization in EPDM/PP TPVs. Each of these curing systems has advantages and disadvantages. Peroxide was the first system used for dynamic vulcanization. The main drawback with this system is that it degrades polypropylene. Sulfur was the first system successfully used in making fully crosslinked TPVs. This system has the disadvantages of odor and unstable crosslinks at high temperatures. Phenolic cure is the basis of most commercially successful TPVs. This cure system has the disadvantages of yellowish color and hygroscopicity. More recently, there have been new developments toward overcoming the problems of colorability and hygroscopicity (ref. 5).

It is the objective of this work to compare the properties of a new TPV based on proprietary technology with the conventional TPV (phenolic cure). It is shown that the new TPV has the advantages of non-hygroscopicity and colorability while maintaining the elastomeric properties of conventional TPVs. Another fully crosslinked TPV, also without the problems of hygroscopicity and colorability, is included for comparison.

Experimental

Commercially available pellets of the new thermoplastic vulcanizates, designated DVA 7100-64 and DVA 7100-73, were used to conduct all the tests. These TPVs are not fully cured but highly cured in the rubber phase as determined by the percent of insolubles in hydrocarbon solvent.

Two other commercially available thermoplastic vulcanizates, TPV A and TPV B, were used for comparative analysis. TPV A is fully crosslinked but is known to have problems with hygroscopicity and color. TPV B is also fully crosslinked but does not have the drawbacks of TPV A.

All the testing was conducted on injection molded plaques. The dimensions of the plaques were 3 x 2 x 0.125 in. TPV A was dried prior to injection molding, while others were not. Samples were conditioned for at least 24 hours prior to testing.

Results and discussion

Due to limitations of sample availability, the TPV A and TPV B samples used in this study differed slightly in hardness. Such small differences have very little bearing on properties such as moisture absorption and colorability, but have a significant effect on physical and elastomeric properties. Thus, physical and elastomeric properties are compared for equivalent hardness.

Colorability

Whiteness Index and Yellowness Index were used as measurements of colorability. These measurements were done on injection molded plaques using a Chroma CS-5 color spectrophotometer.

White titanium dioxide pigment was mixed with the natural pellets using a lab internal mixer. Concentration of the white pigment was varied from 0 to 3% in 1% intervals. Whiteness index was measured at different pigment loadings. The results are shown in figure 1. DVA 7100 has significantly higher whiteness index as compared to TPV A, even without any pigment. At all levels of pigment, DVA 7100 has whiter color compared to TPV A. The whiteness index of TPV B is very similar to DVA 7100 at all pigment levels.

[Figure 1 ILLUSTRATION OMITTED]

The results of yellowness index are shown in figure 2. Again, TPV A has a more yellowish color as indicated by a higher yellowness index. DVA 7100 and TPV B have a very similar yellowness index.

[Figure 2 ILLUSTRATION OMITTED]

The yellowish color of conventional TPVs (TPV A) is a hindrance in making light colors. Also, the yellowish color requires use of a high amount of pigment in making light colors. Use of very high pigment loading is detrimental to the properties of the finished product.

Moisture absorption

Figure 3 shows the data on moisture pickup of different samples. It is clearly evident that DVA 7100 absorbs insignificant levels of moisture, very much less than conventional thermoplastic vulcanizates (TPV A). The moisture absorbed by DVA 7100 is well below the maximum of 0.08% generally cited as the level above which drying is required. Like DVA 7100, TPV B absorbs low levels of moisture.

[Figure 3 ILLUSTRATION OMITTED]

The low moisture pickup by DVA 7100 enables the material to be processed without pre-drying. Besides eliminating an extra step, this also saves on capital cost by eliminating the need to invest in dryers.

Compression set

An important measurement of the elastic properties of thermoplastic elastomers is compression set. It is a measure of the ability of the material to recover after being compressed for a specified time at a specified temperature. The compression-set measurements for this study were performed using the protocols of ASTM test method D 395 B (ref. 6).

Figures 4 and 5 compare the room-temperature (23 [degrees] C) compression-set value of DVA 7100 with that of TPV A and TPV B. Compression set is plotted as a function of time in hourg. It can be seen that the compression set of DVA 7100-73 is practically the same as that of TPV A. A similar interpretation can be drawn by comparing DVA 7100-64 with TPV B. Thus the room-temperature compression set of DVA 7100 is very similar to that of TPV A and TPV B.

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There are several classes of TPEs available today which have excellent room-temperature compression set. The most prominent example is that of TPEs based on styrenic block copolymers (SBCs). The biggest drawback of most TPEs, including the SBCs, involves their elevated-temperature compression-set values. Even at moderately high temperatures, most of the TPEs have values that are very high. This is where TPVs distinguish themselves from other classes of TPEs. The high level of crosslinking in the rubber phase is the reason for the excellent elevated-temperature compression set of TPVs.

To have a better assessment of the elastic properties of TPEs, it is necessary to compare the elevated-temperature compression-set values. Figures 6 and 7 compare the compression sets of DVA 7100 with TPV A and TPV B at 121 [degrees] C, which could be an extreme temperature for most TPEs. Compression set values are given as a function of test time in hours. This temperature was chosen because some of the automotive TPE specifications require compression set measurements at 121 [degrees] C (ref. 7).

[Figures 6-7 ILLUSTRATION OMITTED]

Compression set values of DVA 7100-73 are slightly lower than TPV A at 22 hrs. but there is no difference at 168 hrs. DVA 7100-64 has slightly better compression set resistance than TPV B at all times. Nevertheless, these differences in compression set are very small.

Compression set of annealed samples

The above compression set values were determined in tests as prescribed by the protocol of ASTM D 395. Occasionally, samples are annealed to eliminate orientation effects in TPEs. Samples of DVA 7100-73 and TPV were aged for four hrs. at 125 [degrees] C, and compression set was measured, before and after aging, for 70 hrs. @ 125 [degrees] C. The results are shown in figure 8. It is evident that compression set decreases markedly with annealing. This is an indication of reduction in orientation effects.

[Figure 8 ILLUSTRATION OMITTED]

Viscosity

Figures 9 and 10 show the effect of shear rate and temperature on the viscosity of DVA 7100-73 and TPV A. Measurements were done using the Kayeness Model 6052M capillary rheometer. A 16/1 L/D die was used to minimize the entrance effects. Shear thinning behavior of DVA 7100-73 is very similar to TPV A. Also, temperature has little effect on viscosity of both the materials.

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Thermal aging

Samples were aged for 168 hrs. at 150oC in a circulating air oven. The results, in terms of elongation percentage retained after aging, are shown in figure 11. DVA 7100-73 shows minimal change in elongation on aging in both flow and cross-flow direction. TPV A shows slightly higher loss in elongation in both the flow and x-flow directions. This indicates that TPV A is somewhat inferior in thermal aging to DVA 7100-73.

[Figure 11 ILLUSTRATION OMITTED]

Weathering

Due to the length of testing, only samples of DVA 7100-73 were tested for weathering. Samples in natural and black (2% black concentrate) were exposed to UV radiation in the exterior cycle. The test was conducted as per SAE J 1960. The test requires measurement of color change, and ideally no color change is desirable. Nevertheless, a AE (a measurement of color change) of less than 3.0 is considered acceptable. Any surface defects constitute a failure.

The change in color was recorded for two different exposure levels as shown in figure 12. The AE for both natural and black samples was well below 3.0. Also, no surface defects were observed in either sample.

[Figure 12 ILLUSTRATION OMITTED]

Conclusions

DVA 7100 has advantages of non-hygroscopicity and better colorability when compared to conventional thermoplastic vulcanizates. DVA 7100 has a slightly better elevated-temperature compression set when compared to other commercially available non-hygroscopic and improved-colorability TPVs. Thus, DVA 7100 provides the best combination of low compression set, excellent colorability and non-hygroscopicity. The shear thinning behavior of DVA 7100 is similar to conventional TPV. The hot air aging of DVA 7100 is better than TPV A. Also, the new TPVs meet the requirements of automotive accelerated exterior weathering.

References

(1.) A. Gessler and W.H. Hasslet (to Exxon Chemical Co.), U.S. Patent 3,037,954 (1962).

(2.) W.K. Fisher (to Uniroyal Chemical Co.), U.S. Patent 3,758,643 (1973).

(3.) W.K. Fisher (to Uniroyal Chemical Co.), U.S. Patent 3,806,558.

(4.) A.Y. Coran, B. Das and R.P. Patel (to Monsanto Co.) U.S. Patent 4,130,535 (1978).

(5.) R.E. Medsker, D.R. Hazelton, G.W. Gilbertson and J.E. Pjeiffer, "New, non-hygroscopic thermoplaslic vulcanizates for extrusion," SPE-ANTEC 1997.

(6.) ASTM D385, Method B, Annual Book of ASTM Standards, vol. 09.01.

(7.) General Motors Engineering Standards, GMP. LTP. O02, September 1986.

This article is based on a paper presented at the September, 1998 meeting of the Rubber Division.

Jitender Batra is a Senior Development Process Engineer at Teknor Apex. He has been with Teknor Apex for the last four and a half years. John Andries has been with Teknor Apex for 15 years and currently serves as Vice President of Technology. Keith G. Saunders has been with Teknor Apex for eight years and is currently Technical Manager, Thermoplastic Elastomers. Larry Wallace works for Teknor Apex as Technical Manager - PVC. He has been with Teknor Apex for four years. Suresh Swaminathan holds the position of Manager of Business Development at QST Inc., a wholly owned subsidiary of Teknor Apex.