Silicone TPV offers high performance solutions.

In a growing number of applications, thermoplastic elastomers (TPEs) are used as alternatives to thermoset rubber because of their ability to reduce the total fabrication cost and increase the design flexibility (as an example, the combination of soft and hard materials by overmolding). Like traditional thermoset rubbers, TPEs possess elastomeric properties. However, TPEs can be processed by conventional thermoplastic techniques such as injection molding, extrusion and blow molding. Thermoplastic vulcanizates (TPVs) are an alloy type of TPE, consisting of cured rubber particles suspended in a continuous thermoplastic phase. The most common commercially available TPVs are based on the combination of polypropylene (PP) and cured EPDM rubber. As compared to thermoplastics, these materials offer some unique properties due to the cured rubber phase, such as lower compression set, increased resilience and improved chemical resistance. However, the possible applications for TPVs are limited by the temperature extremes these materials can sustain. For example, a PP-EPDM based TPV is limited to a maximum service temperature in the range of 100 to 135[degrees]C (dependent on product hardness).

Silicone TPV (STPV), a thermoplastic silicone vulcanizate under the trade name of TPSiV, has been invented by Dow Corning (refs. 1-4) and most recently, commercialized by Multibase, a Dow Corning subsidiary (ref. 5). The material is an alloy type TPE consisting of fully cured silicone rubber particles dispersed in a continuous thermoplastic phase.

As an alloy, TPSiV combines the many unique attributes of silicone rubber, such as excellent low and high temperature performance, and selected engineering thermoplastics. The unique combination of properties results in a high performance material solution to applications where high temperature elastomeric properties and chemical resistance are required. For example, STPV materials are expected to find their utility in automotive under-the-hood applications, such as fluid systems, connector and gasket seals, boots and air ducts. STPV materials are also expected to find applications in the sports and leisure, furniture, appliance and communication industries.

Four TPSiV products have been recently commercialized by Multibase. The product designations are 1180-50D, 3010-50A, 3010-60A and 3011-60A. 1180-50D is a TPV of polyamide and silicone rubber. The 3000 series (3010-50A, 3010-60A and 3011-60A) are alloys of a proprietary thermoplastic and silicone rubber. The key properties and applications of each product are further discussed in this article.

1180-50D

The key properties of 1180-50D, including its heat aging and fluid immersion performance, are summarized in table 1. The combination of the continuous polyamide phase and dispersed silicone rubber domains results in a flexible material with good mechanical properties and excellent resistance to oils and chemicals. 1180-50D has excellent melt strength and drawing characteristics that make it ideal for extrusion applications. It has a continuous use temperature (CUT) of 140[degrees]C as tested according to SAE J2236 (i.e., less than 50% loss of initial properties after aging for 1,008 hrs. at the selected temperature in air). This combination of properties makes it well suited for automotive and industrial hose applications where high temperature, oil and chemical resistance are required.

3000 series

The 3000 series are soft elastomeric TPEs in the hardness range of 50 to 65 durometer A. As presented in table 2, the materials offer low compression set, low tensile set and excellent abrasion resistance. The properties are further compared with commonly available TPEs, such as thermoplastic urethane (TPU), copolyamide (CoPA) and PP-EPDM TPV in table 3. The 3000 materials offer the lowest compression set, especially at elevated temperatures. The most important distinguishing property for STPV is its excellent high temperature performance, and all 3000 materials have a CUT of 150[degrees]C (as defined by SAE J2236). The comparative TPEs exhibit catastrophic property loss after heat aging in air under the same conditions (CoPA melts at this temperature within one hour). The 3000 series materials can be injection molded and extruded similar to conventional thermoplastics. The melt strength of the 3000 series makes them ideal candidates for free form extrusion, which is the preferred extrusion method due to the tactile nature of the material to metal (i.e., formers, calibrators, etc.) during fabrication.

3010-50A and 3010-60A are designed for applications that require elasticity, resilience, fatigue resistance and noise reduction. In addition to excellent high temperature performance in air, T3010-50A also exhibits good hydrolytic stability as indicated by retention of its mechanical properties and hardness (less than 5% change) after immersion in water at 100[degrees]C for 1,000 hrs. 3010-60A is designed to offer improved sealing performance, in particular, sealing force retention at elevated temperatures. Figure 1 compares the compression stress relaxation (CSR, resisting force monitored for a constant strain of 25%) at 120[degrees]C of 3010-60A, a TPU of similar hardness, and two grades of liquid silicone rubber (LSR) that are typically used in electrical connector applications. 3010-60A exhibits a sustained sealing force similar to LSR, while TPU rapidly loses its sealing force at this temperature.

[FIGURE 1 OMITTED]

3011-60A is a plasticizer-free TPV designed to offer good mechanical properties, such as high tear strength, excellent high temperature performance (CUT of 150[degrees]C) as well as a dry, silky feel. To systematically evaluate the unique soft-touch properties of 3011-60A, a trained panel of 16 individuals was used to compare the sensory attributes with competitive TPEs. Using methodology developed to compare the sensory attributes of fabrics, the panelists were trained to rank a series of materials according to their tackiness, slipperiness and the noise it generates when rubbed against itself. Figure 2 presents an example of the ranking results for a series of materials including 3011-60A, a plasticized ester TPU, a non-plasticized ether TPU, a CoPA (copolyamide), and a PP-EPDM TPV. For each attribute, the panelists rank the materials according to a one to five scale (one is for the material exhibiting the least amount of the attribute and five is for the material exhibiting the most amount of the attribute). The panel score for each material is then calculated according to the equation:

[FIGURE 2 OMITTED]

Panel score = (1)*(number of panelists ranking a material as 1) + (2)*(number of panelists ranking a material as 2) + ... + (5)*(number of panelists ranking a material as 5)

In addition to comparing the panel scores, statistical analysis of the series of five materials, as well as between each pair of materials, can be completed to identify significant differences. As presented in figure 2, the soft-touch attributes of 3011-60A are clearly distinguished from the competitive TPEs as it is identified as the least tacky, most slippery and least noisy material. Thus, 3011-60A is a novel TPE solution for applications requiring a combination of soft touch and engineering performance (i.e., good mechanical properties, high temperature performance, chemical and oil resistance, etc.).

Over-molding and co-molding

During the last decade, there has been a growing trend in the marketplace to mold or extrude a soft rubber-like material over a more rigid substrate. This type of fabrication has been successful in applications such as tool grips, toothbrushes, footwear, electronic devices, PDA enclosures and automotive under-the-hood constructions. Often, adhesives are used to achieve a strong bond between the soft material and rigid substrate. Due to the continuous thermoplastics phase, STPV products offer the flexibility of bonding to a variety of engineering thermoplastic substrates without adhesives. Generally speaking, 1180 bonds to polyamide and its blends; the 3000 series bonds to PC, ABS, PVC, PU and their blends.

To quantitatively evaluate the bonding strength of STPVs on various engineering thermoplastic substrates, a mold (figure 4) was designed to overmold TPSiV onto a pre-molded plastic substrate. As depicted in figure 4, the plastic substrate is first molded using the indicated portion of the mold (the runner shutoff is positioned to allow the flow only in the direction of the first shot). The substrate is then placed into the position indicated in figure 4, and TPSiV is overmolded onto the substrate via the flow path of the second shot (the runner shutoff is now positioned to allow flow only in this direction). The resulting part is a "T-bar" of TPSiV molded onto the plastic substrate, with a bonding area of one square inch between the materials. The bonding strength of the T-bar is tested according to a modified version of ASTM D1876 (peel strength of adhesives) using a conventional tensile tester. The ultimate (i.e., at break) bonding strength of 3010-60A and 3011-60A on various plastic substrates are compared in table 4 (T-bar generated by molding TPSiV onto a "cold" substrate). Excellent bonding to PC and ABS is achieved, with a cohesive bond failure observed during the test. For PA6 the bond failure is adhesive, which results in a lower bonding strength. The overmoldability of TPSiVs allows for greater design flexibility in many applications, such as overmolded seals using 3010-60A.

[FIGURES 3-4 OMITTED]

Closing comments

Combining the benefits of engineering thermoplastics and high performance silicone rubber, TPSiV materials expand the performance scope of current TPV. These new materials exhibit thermoplastic behavior during processing and many thermoset rubber characteristics during end use applications. Their capability of withstanding oils, chemicals, temperatures and UV radiation will find them many under-the-hood applications in automotive and in certain industrial environment.

Table 1 - properties of TPSiv 1180-50D

Property                                   Testing method

Initial
Hardness (durometer D)                     ASTM D2240
Tensile strength (MPa)                     ASTM D412, Die C
Elongation at break (%)                    ASTM D412, Die C
Tear strength (N/mm)                       ASTM D412, Die B
Flex modulus at 40[degrees]C (MPa)         ASTM D4065

Change after 1,008 hrs. in air at
  140[degrees]C                            ASTM D573-99 for
Tensile strength (%)                       heat aging; same methods as
Elongation at break (%)                    above for testing

Change after immersion in
  DOT4 brake fluid at 150[degrees]C
  for 168 hrs.                             ASTM D471-98 for
Tensile strength (%)                       fluid immersion; same
Elongation at break (%)                    methods as above for testing
Volume swell (%)

Change after immersion in automatic
  transmission fluid at 127[degrees]C
  for 1,008 hrs.                           ASTM D471-98 for
Tensile strength (%)                       fluid immersion; same
Elongation at break (%)                    methods as above for testing
Volume swell (%)

Change after immersion in reference
  fuel C at 23[degrees]C for 3,024 hrs.    ASTM D471-98 for
Tensile strength (%)                       fluid immersion; same
Elongation at break (%)                    methods as above for testing
Volume swell (%)

Change after immersion in water at
  100[degrees]C for 1,008 hrs.             ASTM D471-98 for
Tensile strength (%)                       fluid immersion; same
                                           methods as above for testing

Property                                     TPSiV 1180-50D

Initial
Hardness (durometer D)                                  52
Tensile strength (MPa)                                  24
Elongation at break (%)                                 207
Tear strength (N/mm)                                    138
Flex modulus at 40[degrees]C (MPa)                      319

Change after 1,008 hrs. in air at
  140[degrees]C
Tensile strength (%)                                    -24
Elongation at break (%)                                 -43

Change after immersion in
  DOT4 brake fluid at 150[degrees]C
  for 168 hrs.
Tensile strength (%)                                    -26
Elongation at break (%)                                +0.4
Volume swell (%)                                       +7.3

Change after immersion in automatic
  transmission fluid at 127[degrees]C
  for 1,008 hrs.
Tensile strength (%)                                   -6.8
Elongation at break (%)                                 +38
Volume swell (%)                                        -23

Change after immersion in reference
  fuel C at 23[degrees]C for 3,024 hrs.
Tensile strength (%)                                    -34
Elongation at break (%)                                 -18
Volume swell (%)                                        +23

Change after immersion in water at
  100[degrees]C for 1,008 hrs.
Tensile strength (%)
                                                        -35

Table 2 - properties of X3000 series

Property                         Testing method

Mechanical
Tensile strength (MPa)          ASTM D412, Die D
Elongation at break (%)         ASTM D412, Die D
Tear strength (N/mm)            ASTM D624, Die C

Physical
Hardness (durometer A)             ASTM D2240
Compression set (%)                ASTM D395
 23[degrees]C                       Method B
 120[degrees]C (annealed)
Tensile set (%)                 ASTM D412, Die D
 100%
 300%
Abrasion resistance              ASTM D1044 (H-
 (mg loss)                     18, 1,000 g, 1,000
                                    cycles)

Property                    3010-50A   3010-60A   3011-60A

Mechanical
Tensile strength (MPa)        7.1         16         12
Elongation at break (%)       473        500        720
Tear strength (N/mm)          23.5        33         44

Physical
Hardness (durometer A)         52         65         65
Compression set (%)
 23[degrees]C                  14         12         21
 120[degrees]C (annealed)      53         41         66
Tensile set (%)
 100%                         5.5        6.2         10
 300%                          20         20         37
Abrasion resistance
 (mg loss)                     21         21         37

Table 3 - comparison of X3000 with competitive TPEs

Property                                Testing method

Mechanical
Tensile strength (MPa)                 ASTM D412, Die D
Elongation at break (%)                ASTM D412, Die D
Tear strength (N/mm)                   ASTM D624, Die C

Physical
Hardness (durometer A)                    ASTM D2240
Compression set (%)                        ASTM D395
 23[degrees]C                              Method B
 120[degrees]C (annealed)
Tensile set (%)
 100%                                  ASTM D412, Die D
 300%
Abrasion resistance                       ASTM D1044
 (mg loss)                       (H-18, 1,000 g, 1,000 cycle)

Change after 1,008 hrs. in
  air at 150[degrees]C
Hardness (durometer A units)      ASTM D573-99 for heat aging
Tensile strength (%)               Same methods as above for
Elongation at break (%)                     testing

Property                       3010-50A   3011-60A   TPU

Mechanical
Tensile strength (MPa)           7.1         12      23
Elongation at break (%)          473        720      870
Tear strength (N/mm)             23.5        44      51

Physical
Hardness (durometer A)            52         65      68
Compression set (%)
 23[degrees]C                     14         21      14
 120[degrees]C (annealed)         53         66      85
Tensile set (%)
 100%                            5.5         10      9.3
 300%                             20         37      44
Abrasion resistance
 (mg loss)                        21         37      22

Change after 1,008 hrs. in
  air at 150[degrees]C
Hardness (durometer A units)     -13         -3      -35
Tensile strength (%)             -44        +0.8     -92
Elongation at break (%)          -36        -42      -57

Property                             Co-PA         PP-EPDM
                                                     TPV
Mechanical
Tensile strength (MPa)                 33            5.5
Elongation at break (%)               640            660
Tear strength (N/mm)                   39            20

Physical
Hardness (durometer A)                 75            58
Compression set (%)
 23[degrees]C                          20            32
 120[degrees]C (annealed)              85            100
Tensile set (%)
 100%                                  16            13
 300%                                 120            34
Abrasion resistance
 (mg loss)                            161            103

Change after 1,008 hrs. in
  air at 150[degrees]C
Hardness (durometer A units)          CoPA          >+40
Tensile strength (%)                 melted          -60
Elongation at break (%)         at 150[degrees]C     -98

Table 4 - bonding strength of TPSiV to thermo-plastic
substrates (reported as ratio of peak
force and bonding width)

              Ulitmate bond strength (N/mm)

Material          PC       ABS      PA6
3010-60A         22.0     23.1     6.05
3011-60A         15.9     18.2     12.0

References

1. "Thermoplastic silicone elastomers," U.S. Patent 6013715, 2000.

2. "Thermoplastic silicone elastomers formed from nylon resins," U.S. Patent 6362287, 2002.

3. "Thermoplastic silicone elastomers from compatibilized polyamide resins," U.S. Patent 6362288, 2002.

4. "Thermoplastic silicone elastomers formed from polyester resins," U.S. Patent 6417293, 2002.

5. M. Rosenzweig, Modern Plastics, December 2001, p. 48.