High performance TPVs for long term high temperature applications.

Thermoplastic elastomers (TPE) exhibit rubber-like properties similar to conventional thermoset elastomers and can be processed on thermoplastic processing equipment. Thermoplastic vulcanizates (TPV) consist of a continuous thermoplastic phase and a dispersed cured rubber phase. These TPVs are prepared by a dynamic vulcanization step (ref. 1). A significant improvement in the properties of these blends was achieved by Coran, Das and Patel (refs. 2 and 3). They fully vulcanized the rubber phase while mixing and maintaining thermoplasticity of the blends. Coran and Patel (ref. 4) also reported that smaller particles and higher crosslink density of rubber in the thermoplastic plastic matrix gave higher tensile strength and elongation.

The morphology of elastomeric alloys was investigated by Abdou-Sabet and Patel (ref. 5). They found that the main variables affecting morphology of a TPV were molecular weight of EPDM and polypropylene (PP), the ratio of EPDM to PP, the degree of crosslinking and the type of crosslinking. General features of dynamically vulcanized thermoplastic elastomers were reviewed by Coran (ref. 6) and by Abdou-Sabet and his coworkers (ref. 7). More recently, the effects of dynamic vulcanization on morphology and mechanical properties of TPVs were studied by Chung and Coran (ref. 8). They reported that dynamic vulcanization converted co-continuous morphology to a fixed rubber particulate morphology. The higher ultimate tensile properties of a TPV compared to the unvulcanized blend are largely due to enhanced rubber/plastic adhesion and uniquely dispersed particulate rubber morphology.

In general, TPVs can provide compositions that are very elastomeric in their performance characteristics (i.e.. reduced permanent set, increased oil resistance, low die swell for improved profile extrusion, etc.). And the same TPV can be rapidly fabricated into finished parts and can be readily reground and recycled.

Recently, Zeon Chemicals developed a new type of TPV based on dynamic vulcanized polyacrylate (ACM) and nylon (ref. 9). This new type of TPV has good hot oil resistance compared to the current generation of TPVs (e.g., EPDM/PP) and copolyester (COPE) at high temperature.

DuPont reported that their engineering TPV (ETPV) based on COPE (ref. 10) had a unique performance profile at elevated temperature. They claimed that their ETPV had higher heat resistance, and better oil and chemical resistance. For instance, their 60 durometer A ETPV can withstand prolonged exposure to a 170[degrees]C environment while maintaining its tensile properties at greater than 80% for up to 2,000 hours.

In addition, Tasaka and his coworkers (ref. 11) reported that TPVs of PP/polystyrene-block-poly(ethylene-co-propylene)-block-polystyrene (SEEPS or SEPS) had good oil resistance and compression set by generating intermolecular crosslinking of poly(ethylene-co-propylene) (PEP) in the block copolymer.

We have developed a new type of TPV based on a modified hydrogenated styrene block copolymer with excellent elastic recovery (lower compression set) and high solvent resistant properties. A comparative study will be discussed herein with conventional TPVs (PP/EPDM) and the newly developed TPV.

Experimental

Material

The styrenic TPV developed herein was based on modified hydrogenated styrene block copolymer (HSBC). Detailed structural information was not disclosed. This HSBC is believed to have reactive groups in the main chain. Commercially available TPV (PP/EPDM) compounds were acquired and used for the comparative study. The different TPV grades are shown in table 1, where STPV and CTPV are representing the styrenic TPV and conventional TPV, respectively. The detailed information of curatives for the rubber phase in the TPV system is classified.

Standard factorial design

Design of experiments for compositional formulations was carried out using FastR&D software from Quality Sciences. The effect of each experimental formulation on the STPV's performance was investigated. Finally, the resulting experimental design (DOE) made it possible to optimize the STPV's performance.

Sample preparation

The styrenic TPV was prepared using a twin screw extruder (L/D = 44, D = 40 mm). The temperature profile was from 150[degrees]C to 200[degrees]C with a screw speed of 150 rpm. The styrenic TPV was produced by dynamically vulcanizing the rubber phase of the rubber/plastic blends. The typical TPV formulations were as follows: HSBC, polypropylene, processing oil, curatives and additives.

Mechanical properties

After dynamic vulcanization, test specimens were injection molded. The injection molding machine used had a clamping force of 250 tons. The temperature for injection molding was 200[degrees]C for all three zones.

Hardness was determined using ASTM D2240 and expressed in durometer A units. Specific gravity of samples was measured using ASTM D792. The tensile stress-strain properties according to ASTM D412 were measured lit 23[degrees]C using injection molded dumbbell shape specimens. The testing was performed on an Instron tensile machine according to ASTM D 412 with a cross-head speed of 500 mm/min. Tear properties (die C) were also determined according to ASTM D 624.

Rheology and dynamic mechanical properties

Dynamic mechanical properties were measured using an Advanced Rheometer AR 1000. The tests were performed over temperature sweeps from -100[degrees]C to 200[degrees]C at 1 Hz with a ramp rate of 5[degrees]C/min. and frequency sweep at 200[degrees]C under a constant strain of 0.25%. The sample, torsion rectangle, had dimensions of 63.5 x 12.7 x 3.18 mm (LxWxT).

Long term compression set and oil resistance

Compression set was determined at 25% deformation at 125[degrees]C according to ASTM D 395. Oil resistance was performed using IRM 903 oil lit 125[degrees]C and was measured according to ASTM D471, which was based on weight change.

Morphology

The samples were microtomed with an RMC Powerome XL with an RXL cryo attachment at a temperature of -60[degrees]C. The resulting microtomed sections were approximately 1,000 angstroms thick. The sections were exposed to ruthenium tetraoxide (Ru[O.sub.4]) for [5 minutes. The samples were analyzed with a Philips EM 400 TEM interfaced with a Gatan Model 673 TV camera.

Results and discussion

Standard factorial design

Figure 1 shows the effect of polypropylene and oil on the tensile strength (a) and compression set (b). As the level of polypropylene increased and the level of oil decreased, the tensile strength increased. On the other hand, there is no effect of oil levels on compression set, but the compression set increased proportionally as polypropylene levels increased. These contour graphs are examples of the kind of information a standard factorial design can generate. Based on this standard factorial design, one can optimize the TPV formulation to obtain target properties as long as the target properties can be generated in the compositional space being evaluated. The styrenic TPVs (STPV-65, STPV-74 and STPV-80) were designed based on a standard factorial design DOE.

[FIGURE 1 OMITTED]

Comparison with conventional TPV

Figure 2 shows the comparison of tensile strength (a) and teal strength (b) for several TPV grades in the same hardness range. The STPV shows a 10% to 20% higher tensile strength relative to the conventional TPV. Figure 2 (b) shows that the tear strength of STPV is at the same level as the peroxide cured conventional TPV (CTPVI). The conventional TPV with either a phenolic resin cured (CTPV2) or siloxane cured system (CTPV3) shows a higher tear strength relative to the STPV. One may conclude that tear strength is affected by the different cure systems and the degree of cure.

[FIGURE 2 OMITTED]

In general, TPVs are a sub-category material of the broader thermoplastic elastomer (TPE) family of materials that have the properties of thermoset elastomers and the processability of thermoplastics (ref. 9). TPVs have a vulcanized rubber phase dispersed in a thermoplastic matrix, resulting in improved heat and oil resistance when compared with unvulcanized TPEs. However, in order to compete with thermoset materials, higher oil resistance and better elastic recovery (lower compression set) are required.

In figure 3, compression set and weight gain are shown as a function of hardness. The compression set after 22 hours at 125[degrees]C (figure 3a) and weight gain after 24 hours at 125[degrees]C in IRM 903 oil (figure 3b) of various TPVs are shown. The compression set of the STPV, relative to the various CTPVs, is lower by approximately 20%, even at 125[degrees]C. For IRM 903 oil at 125[degrees]C (figure 3b), the STPV has a much lower weight gain by approximately 20% than conventional TPVs.

[FIGURE 3 OMITTED]

Figure 4 represents the long term compression set at 125[degrees]C. The long-term compression set of the STPV at 125[degrees]C has almost the same value as the initially measured compression set. All of the conventional TPVs show increased compression set with time. In particular, the compression set of CTPV2, a phenolic resin cured system, showed an increase of 50%. The compression set of CTPVI, a peroxide cured system, and CTPV3, a siloxane cured system, increased by approximately 20%.

[FIGURE 4 OMITTED]

It was noticed that STPV had excellent long term elastic recovery properties, based on compression set, that increased by only 5%.

Figure 5 represents long term solvent resistance (IRM 903 oil) of various TPVs. It is noted that the STPVs have the lowest weight gain in IRM 903 oil and also show a very small effect feet with time. Conventional TPVs, particularly CTPV2-74, showed increased weight gain up to 500 hours. It is concluded that STPVs have a superior long-term elastic recovery and solvent resistance relative to conventional TPVs based on EPDM/PP blends.

[FIGURE 5 OMITTED]

The overall performance comparison of various TPVs is shown in figure 6. The properties of TPVs were compared in the same hardness range. It is observed that the properties of the STPV were better balanced than those of the CTPVs, even though the STPV had slightly lower elongation at break. In particular, tensile strength, compression set and solvent resistance of the STPV were better properties than those of the CTPVs.

[FIGURE 6 OMITTED]

Dynamic mechanical properties of various TPVs

Storage modulus (G') and loss tangent (tan [delta]) versus temperature are shown in figure 7. This figure shows that the HSBC has two glass transition temperatures (Tg). The lower Tg is -44.6[degrees]C and represents the robber phase (soft block), while the second Tg is 139[degrees]C and is associated with the polystyrene phase (hard block). Both peaks were well defined. In the STPV, the Tg of the soft segment was -52[degrees]C and the glass transition peak for polystyrene broadened at around 118[degrees]C.

[FIGURE 7 OMITTED]

Figure 8 shows a plot of complex viscosity and loss tangent vs. frequency at 200[degrees]C of various TPVs. This figure shows that the complex viscosity of STPV-74 and CTPV2-64 have a similar value at high frequency. The loss tangent of STPV-74 has the lowest value when compared with the conventional TPVs. In other words, STPV-74 has a much higher elasticity than conventional TPVs at 200[degrees]C. This is possibly explained by morphology, which will be further discussed in the next section.

[FIGURE 8 OMITTED]

Morphology

TEM micrographs of STPV-65 are shown in figure 9 at two different magnifications. The observed morphology in figure 9(a) shows dark psuedo-spherical domains that are of the dispersed micro-crosslinked phase with dimensions of approximately 0.5 [micro]m to 2 [micro]m in size. This kind of morphology is similar to other conventional TPVs (PP/EPDM) (ref. 12). Figure 9(b) shows that the STPV also has nano scale domains approximately 30 nm in size. These nano domains are observed within the micro domains dispersed throughout the matrix. It is believed that these stained nano domains are the polystyrene hard segments. It is well known that polystyrene block copolymers have hard block domains that act as multifunction junction points that act like crosslinks (ref. 13).

[FIGURE 9 OMITTED]

After dynamic vulcanization, the morphology on a micro scale for a STPV is similar to that of a CTPV. However, the STPV has the subtle difference of nano phases (PS multi-junction) dispersed within the micro domains. This dual rubber phase network morphology with micro and nano scale rubber phases is likely to increase crosslink density and elasticity of the rubber phase. This dual network morphology could explain the excellent long term compression set consistency (elastic recovery) and solvent resistance. One can conclude that the nano domain is a fairly unique feature of the STPV morphology when compared with a conventional TPV.

Conclusions

The development of a styrenic TPV (STPV) as shown in this article had great success. The STPV was shown to have excellent stable long-term compression set and improved hot oil resistance at 125[degrees]C compared to conventional PP/EPDM TPVs. Also, the developed STPV in comparison to the conventional TPV had a well-balanced mechanical profile that can be readily molded and recycled. These excellent long-term properties are explained by the dual rubber phase network generated by a modified HSBC.

Table 1--material

Polymer     Type of TPV                          Grade

STPV-65     Thermoplastic vulcanizate of
STPV-74     HSBC, PP and processing
STPV-80     oil/peroxide cure; 65, 74 and 80
            durometer A hardness

CTPVI-64    Thermoplastic vulcanizates of        Uniprene
CTPVI-73    EPDM, PP and processing              Teknor Apex
CTPVI-80    oil/peroxide cure; 65, 73 and 80
            durometer A hardness

CTPV2-64    Thermoplastic vulcanizate of         Santoprene 201-64
            EPDM, PP and processing              Advanced Elastomer
            oil/phenolic resin cure; 64            Systems
            durometer A hardness

CTPV3-70    Thermoplastic vulcanizate of         Santoprene 8201-70
            EPDM, PP and processing oil/         Advanced Elastomer
            siloxane cure; 70 durometer A          Systems
            hardness

References

(1.) A.M. Gessler and W.H. Haslett, U.S. patent 3,037,954 (1962).

(2.) A.Y. Coran, B. Das and R. Patel, U.S. patent 4,104,210 (1978).

(3.) A.Y. Coran and R. Patel, U.S. patent 4,271,049 (1981).

(4.) A.Y. Cotan and R. Patel, Rubber Chem. and Technol., 53, 141 (1980).

(5.) S. Abdou-Sabet and R. Patel, Rubber Chem. and Technol., 64, 769 (1991).

(6.) A.Y. Coran, Rubber Chem. and Technol., 68, 351 (1995).

(7.) S. Abdou-Sabet, R.C. Puydak and C.P. Rader, Rubber Chem. and Technol., 69, 476 (1996).

(8.) O. Chung and A.Y. Coran, ANTEC (2002), vol. 3, 3,192.

(9.) B. Cail and R.D. DeMarco, ANTEC (2003), 3,022.

(10.) DuPont Engineering Polymers, TOPCON 2003, Akron, OH (2003).

(11.) M. Tasaka, A. Tamura and R. Mori, ANTEC (1998), vol. 3, 3,196.

(12.) A.Y. Coran, ch. 7 in Thermoplastic Elastomers, N.R. Legge. G. Holden and H.E. Shroeder (eds.) Hanser Publishers, NY (1987).

(13.) G. Holden and N.R. Legge, ch. 3 in Thermoplastic Elastomer, N.R. Legge, G. Holden and H.E. Shroeder (eds.), Hanser Publishers, NY (1987).