| Also for high temperatures |
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Frieder Vielsack Jšrg SŠnger Markus Beitzel
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The growth rates for thermoplastic elastomers (TPE) are, on average, about 6.2% worldwide. This indicates that TPEs are replacing other elastomeric materials in new fields or are being discovered as an innovative material to optimize the function and design of products and components. Above-average growth opportunities are being observed especially for TPE-V, that is, compounds with a chemically cross-linked elastomer phase. These are found traditionally in the automotive sector, with its demanding requirements regarding resistance of compounds to heat and oil. Here, there is a trend to encounter TPE-V in the well-established markets for rubber formulations based on EPDM, CR or HNBR and even ACM rubber. These so-called Super TPE-Vs are intended for use at service temperatures of up to 175℃ while retaining excellent oil resistance and can be viewed as predestined for applications in the engine compartment.
For some time, Kraiburg TPE, Waldkraiburg/Germany, has been working on innovative compound formulations and production processes that, with improved long-term temperature resistance, would open up new opportunities to expand into new fields of application. With the aid of novel concepts to raise the service temperatures of styrene block copolymers, it was possible to create a new product class of thermoplastic elastomers specifically for use at elevated temperatures. This was the first step on the path to successful development of the Super-TPV.
Selective chemical cross-linking
The softening temperature of the styrene domains represents the Achilles heel of the temperature resistance of TPEs based on hydrated styrene block copolymers (HSBC). This cannot be raised further even by increasing the molecular weight of the HSBC employed. Numerous attempts to improve the temperature resistance of HSBC via subsequent cross-linking have failed, since all known cross-linking agents react only with the elastic middle block. This, however, only has an adverse effect on its elastic qualities. The softening temperature of the styrene domains that limits the temperature resistance is not affected. A way out of this dilemma is to incorporate during production of the HSBC via anionic polymerization a comonomer into the styrene end blocks that permits selective chemical cross-linking of the end blocks. As a consequence of this new approach, cross-linking of HSBC is accompanied for the first time by a noticeable increase in the stiffness of the end blocks. The results are the following benefits versus the usual HSBC:
● Because of the selective chemical cross-linking of the end blocks, the restoring force of the elastic phase is retained even beyond the glass-transition of the end blocks and
● the service temperature range shifts to considerably higher temperatures and is limited solely by the onset of melting of the polypropylene.
Production of such novel, chemically crosslinked HSBC (TPES-V) is accomplished through dynamic vulcanization in a twin-screw extruder.
The new TPES-V compounds from Kraiburg TPE are based on cross-linked SEPS in a PP matrix. By chemically cross-linking the modified poly-styrene (PS) end blocks, it is possible to improve the strength of the PS domains significantly. The result is outstanding thermal stability and excellent hysteresis behavior combined with unusually little loss of sealing force when testing the compressive stress relaxation as well as unusually low long-term compression set at high temperatures. The new family of materials is especially well-suited for demanding applications that require a high level of weatherability and design flexibility. This makes the material extremely suitable for a multitude of applications in the automotive sector and industry in general, e. g. grommets for hose and cable feed-through openings, air ducts, cable clamps, seals, window encapsulation, housing gaskets, vibration-absorbing elements, tool handles and sealing profiles.
 | Fig.1.The chemical cross-linking of the
PS end blocks shifts the service temperature
to considerably higher values |
Controlling the morphology
The objective during production of TPE-V by means of dynamic vulcanization is a morphology with a continuous thermoplastic phase and the finest possible distribution of thermoplastic and elastomer phases. If, however, the thermoplastic phase is distributed discontinuously in the elastomer phase, a filled rubber that cannot be processed as a thermoplastic results. The mechanical properties of a TPE-V depend directly on the size of the vulcanized elastomer phase distributed in the thermoplastic phase. If the distribution of thermoplastic and elastomer phases is not fine enough, acceptable mechanical properties and processing characteristics are not obtained. During production of the new Thermolast V family of TPES-V products by means of dynamic vulcanization in a twin-screw extruder, a phase morphology forms that - in addition to thermoplastic and elastic phases as a substructure of the elastic phase on the nanometer scale - exhibits phase separation between the rigid, cross-linked styrene end blocks and the soft, elastic middle blocks of the HSBC.
Based on the electron microscope image of the TPES-V compound Thermolast V TV7LVZ where a sample was taken from material produced under series production conditions with a lot size of 3 ton a twin-screw extruder with a barrel diameter of 110 mm, the HSBC phase appears dark; the PP phase, white. The image shows that the cross-linked HSBC domains with their substructure are embedded in a uniformly distributed, continuous PP matrix. The size of the cross-linked HSBC particles is typically in the range of 1mm or below. The objective of a very fine distribution between the elastic and thermoplastic phases is achieved. This is the prerequisite for the very good mechanical properties described in the following as well as the favorable long-term and processing characteristics of the TPES-V. The comparison is between the TPES-V compound Thermolast V TV7LVZ with a Shore A hardness of 70 and commercially available EPDM/PP compounds with Shore A hardnesses of 64 and 73.
Benefits in the properties
Table 1 shows the mechanical properties of Thermolast V TV7LVZ. The very good tensile values confirm the statement, based on the morphological examination, regarding the outstanding control of the morphology during production of the compound. Based on the comparison of the short-time compression set over 24 and 72 h (at 23℃) for the TPE classes TPES-V and conventional TPE-V, it can be seen that the performance of the materials is similar in this test at elevated temperatures. The conventional TPE-V compounds exhibit somewhat better values at 70℃, while at 100 and 125℃ the materials are identical when the hardness differences are taken into account. At low test temperatures such as - 25 and 23℃, the compression set values for the TPES-V are significantly below those for the conventional TPE-V. With increasing test duration, the results for long-time tests at elevated temperatures are shifted more in favor of the TPES-V.
Comparison of the long-time material performance of the TPE material classes TPES-V and conventional TPE-V indicates that the TPES-V compound is considerably better suited to demanding requirements than the EPDM/PP TPE-V that was also tested. The long-time compression set values as well as the compressive stress relaxation at various service temperatures and for different durations have also been compared. Regarding measurement of the longtime compression set, it has been noted that, with increasing test duration, at 125℃ TPES-V offers increasingly significant performance benefits versus conventional TPE-V compounds of similar hardness, while at 50℃ both materials are equivalent. Testing the compressive stress relaxation is an especially useful way of evaluating the long-time sealing action of an elastomeric material, since (in contrast to the compression set) it provides information on the remaining sealing force of the material. The test of the compressive stress relaxation shows that the new TPE class TPES-V is clearly superior to conventional TPE-V compounds both at room temperature and at elevated temperatures such as 125℃. This becomes increasingly obvious with increasing temperature as well as increasing test duration.
 | Table 1.Mechanical properties of the
TPES-V product Thermolast VTV7LVZ |
Comparing the hysteresis behavior of the two material classes TPES-V and conventional TPE-V, it is seen that the new TPES-V compound exhibits less setting behavior in combination with a lower energy loss. Thus, the material reacts more elastically in general, which can be a benefit for seals subject to dynamic loads. This is confirmed by hysteresis curves for the TPES-V compound TV7LVZ and the conventional TPE-V compound EPDM/PP 73 in the stress strain mode at room temperature over 5 cycles under constant force. Compared to EPDM/PP 73, the energy loss and residual strain are lower in the TPES-V compound TV7LVZ. As consequence, the TV7LVZ feels Wivelier than its competitor EPDM/PP 73.
In summary, the TPES-V compounds are a new class of TPE-V. They combine the good processing characteristics of TPE-S (HSBC) materials with the good performance of TPE-V (EPDM/PP) at elevated temperatures. A comparison of the long-term performance of conventional TPE-V on the basis of EPDM/PP with the long term performance of TPES-V on the basis of SEPS shows that the TPES-V compounds exhibit further significant benefits. This combination of properties makes TPES-V compounds an indispensable component in the material portfolio not only for the automotive industry. With TPE-V materials based on TPES-V, new fields of application that were previously not open to TPEs can also be developed. It can be expected that in this regard the development of TPES-V compounds will not remain stagnant, but rather that the basis for a broad TPES-V portfolio similar to that for HSBC compounds is being established.
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