New thermoplastic vulcanizates (TPVs). (Cover Story).

Thermoplastic vulcanizates (TPVs) are alloys of dynamically vulcanized rubber and plastic. TPVs were first discovered in 1958 (ref. 2). Since then, there have been several significant developments in the field of thermoplastic vulcanizates. Fisher's development work led to one of the first commercial thermoplastic vulcanizates (refs. 3 and 4). 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 thermoplastic vulcanizates (ref. 5).

There are several TPVs available, which vary in the rubber and plastic phase combinations (ref. 6). Ethylene propylene diene (EPDM) rubber, natural rubber, butyl rubber and nitrile rubber are some of the examples of the rubber phase in TPVs. Polypropylene, polyethylene, polyester and polyamide are the examples of the plastic phase in TPVs. EPDM and polypropylene (PP) based TPVs have gained the most commercial interest and are the subject of this article.

There are several known crosslinking systems for dynamic vulcanization of EPDM/PP blends. Each of these curing systems has advantages as well as disadvantages. Peroxide was the first system used for dynamic vulcanization (refs. 3 and 4). The main drawback with this system is that it degrades polypropylene or crosslinks polyethylene. Sulfur was the first system successfully used in making fully crosslinked TPVs (ref. 5). This system has the disadvantages of odor and unstable crosslinks at high temperatures. Phenolic cure is the basis of most commercially successful TPVs (ref. 7). This cure system has the disadvantages of yellowish color and hygroscopicity. More recently, there have been new developments towards overcoming the problems of colorability and hygroscopicity (refs. 8 and 9).

It is the objective of this work to compare the properties of a new TPV based on proprietary technology with the conventional TPV. It is shown that new TPVs are nonhygroscopic, have no yellowish color and provide better colorability while maintaining the elastomeric properties of conventional TPVs.

Experimental

Pellets of the new TPV, TPV N, which are commercially available, were used in this study. These new TPVs are highly crosslinked in the rubber phase as determined by weight % insolubles in hydrocarbon solvent. A commercially available thermoplastic vulcanizate, TPV A (Santoprene from Advanced Elastomer Systems [ref. 7]) was used for comparison. This TPV is also highly crosslinked, but is known to have problems with hygroscopicity and color consistency. All comparisons were made on similar hardness compounds.

All the testing was conducted on injection molded plaques molded using Arburg injection molding equipment. The dimensions of the plaques were 3" x 2" x 0.125". Mechanical properties were measured using the Instron testing machine Model 4464. Rheological data were measured using the Kayeness capillary rheometer. Weathering of the TPVs was measured using an Atlas Ci65 Weather-O-Meter. A 300g capacity lab internal mixer was used for mixing purposes. The moisture content was analyzed using an Aquatest 10 coulometric moisture analyzer. The dimensions of the plaques were 3" x 2" x 0.125". 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

Colorability

Yellowness and whiteness indices were measured to examine colorability. These measurements were done on injection molded plaques using a Chroma CS-5 color spectrophotometer. The yellowness index for the new TPV and TPV A are compared in figure 1. The new TPV does not exhibit a yellow color as compared to TPV A. The yellowish color of conventional TPVs (TPV A) is a hindrance in making light colors. Also, the yellowish color requires the use of high amount of pigment in making light colors. Use of a very high pigment loading is detrimental to the properties of the finished product. Besides low yellowness index, the new TPV also exhibits consistency in color from batch to batch.

Figure 1 -- yellowness index of TPV N and TPV A

TPV N       20
TPV A       42

Note: Table made from bar graph.

Whiteness index was measured at different Ti[0.sub.2] pigment loadings. The results are shown in figure 2. The new TPV has a higher whiteness index as compared to TPV A, even without any pigment; and at all levels of pigment, it has whiter color compared to TPV A.

[FIGURE 2 OMITTED]

Moisture absorption

Dynamic vulcanization is a rather fast and sophisticated process that usually occurs in an open system. Moisture can not only be a result of the vulcanization process, but can also be picked up from the atmosphere or during under-water pelletizing. The presence of moisture is usually catastrophic to processing. Figure 3 shows the data on moisture pickup on the new TPV and TPV A. The new TPV absorbs an insignificant amount of moisture as compared to TPV A, and is well below the maximum of 0.08% cited before drying is required.

Figure 3 -- % moisture absorbed by TPV N and
TPV A

TPV N     0.03
TPV A     0.21

Note: Table made from bar graph.

Lack of moisture pickup provides the flexibility of processing without drying, which eliminates the need to install dryers. This is not only a capital cost saving, but also an elimination of an extra step in processing. It is also important to point out that upon heating/drying the new TPV, no changes in color are noticed, whereas TPV A's yellowness index increases with drying as shown in figure 4.

[FIGURE 4 OMITTED]

Compression sets

An important measurement of elastic properties of thermoplastic elastomers is compression set. It is a measure of the ability of the material to recover after being compressed for the specified time at the specified temperature. The compression set measurements for this study were performed using the protocols of ASTM test method D 395 B (ref. 10).

Compression set values were conducted on both extruded tapes and injection molded plaques. Figures 5 and 6 compare the compression set of the new TPV and TPV A at different temperatures. This particular data is for 22 hours. For both extruded tapes and injection molded plaques, the compression sets are similar or better than TPV A. Similar results are also obtained at higher temperatures.

Figure 5 -- 22 hr. compression set on injection
molded plaques

Temperature ([degrees] C)     TPV N     TPV A

-29                              13        14
23                               25        25
50                               33        33
70                               34        32

Note: Table made from bar graph.
Figure 6 -- 22 hr. compression set on extruded
tapes

Temperature ([degrees] C)     TPV N     TPV A

-29                              13        19
23                               27        30
50                               36        38

Note: Table made from bar graph.

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

Compression set of annealed samples

The above compression sets were tested as prescribed by the protocol of ASTM D 395. Occasionally, samples are annealed to eliminate orientation effects in TPEs. Samples of the new TPV and TPV A were aged for 4 hours at 125 [degrees] C, and compression sets were measured before and after aging. Compression sets were measured for 70 hours at 125 [degrees] C, and the results are shown in figure 7. It is evident that compression sets decrease markedly with annealing. This is an indication of reduction in orientation effects.

Figure 7 -- effect of annealing on compression
sets at 125 [degrees] C and 70 hrs.

              Without annealing     Annealed for 4 hrs.
                                    at 125 [degrees] C

TPV N                60                     49
TPV A                55                     47

Note: Table made from bar graph.

Viscosity

Figure 8 shows the effect of shear rate and temperature on the viscosity of the new TPV and TPV A. A 16/1 L/D die was used to minimize the entrance effects. Shear thinning behavior of the new TPV is very similar to TPV A. Also, temperature has little effect on viscosity of both the materials.

[FIGURE 8 OMITTED]

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 [Delta] E < 3.0, which is a measurement of color change, is considered acceptable. Also, any surface defects constitute a failure. The change in color was recorded for two different exposure levels, as shown in figure 9. The [Delta] E for both natural and black samples was well below 3.0. No surface defects were observed in either sample.

[FIGURE 9 OMITTED]

Another test was conducted as per SAE J 1885. For four different colors, the [Delta] E remained well below 3.0 (figure 10).

Figure 10 -- color change for four different colors
as per SAE J 1885

Automotive specification [DELTA] E =3.0

Natural             0.95
BJCA
medium
parchment           0.43
BJAA
light
parchment           0.54
ZUBA
light
graphite            0.64

Note: Table made from bar graph.

Tensile properties

Figure 11 compares the tensile behavior of the new TPV with TPV A as a function of temperature. Although both show excellent tensile properties, a slight improvement in the tensile behavior of the new TPV can be noticed. The data shown are for 73A durometer hardness.

Figure 11 -- comparison of the tensile strength of
TPV N and TPV A

        TPV N     TPV A

-29     2,787     2,666
23      1,239     1,196
50        989       960

Note: Table made from bar graph.

Conclusions

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

References

(1.) J. Batra, K. Saunders, L. Wallace, S. Swaminathan and J. Andries, Paper # 66 presented at the 154th Rubber Division, ACS Meeting, September 1998.

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

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

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

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

(6.) A.Y. Coran and R.P. Patel, Paper # 41 presented at the 142nd Rubber Division Meeting, November 1992.

(7.) S. Abdou-Sabet and M.A. Fath, U.S. Patent 4,311,628 (1982).

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

(9.) R.E. Medsker and R.P. Patel, U.S. Patent 5,672,660 (1997).

(10.) ASTM D385, Method B, Annual Book of ASTM Standards, Vol. 09.01.

This article is based on a paper given at the October, 2000 Rubber Division meeting