150 [degrees]C capable TPVs for demanding polyamide and polyester over-molding.

Over the past several years a new high-performance thermoplastic vulcanizate (TPV) has been making inroads into demanding under-the-hood automotive and industrial applications. With improved heat (150 [degrees]C) and oil resistance (exceeding 3,000 hours), this material comprises a dynamically vulcanized polyacrylate (ACM) thermoset phase dispersed within a continuous polyamide (PA) thermoplastic matrix. This ACM//PA TPV can easily be incorporated into two-shot molding processes where the polyamide in the TPV provides excellent adhesion to both polyamide and polyester rigid thermoplastic substrates. The polyacrylate thermoset phase imparts improved heat and oil resistance when compared to many conventional TPE/V materials.

Today's material requirements

Conventional materials, such as EPDM//PP, copolyesters and thermoset rubber, are being stretched further and further in both performance and cost. Service environments are becoming more harsh, with increasing temperatures and more aggressive fluids. Temperatures of 150 [degrees]C and higher, as well as more aggressive fluid additive packages, are pushing the limits of conventional materials. In addition, longer service life expectations are fast becoming the rule rather than the exception. Cost-down pressures are considerable and often mandated as a condition of doing business.

With the numerous materials available today, one might ask why consider TPVs? Several reasons are obvious. TPVs do not require compound preparation (mixing, prepping, etc.), have very fast cycle times (seconds versus minutes), provide recyclability with little or no scrap, are very flexible with regard to design (numerous choices in over-molding and blow molding), and finally, offer significant part cost savings versus thermoset rubber. For all their merits, though, conventional thermoplastic vulcanizates (EPDM//PP, COPE, silicone TPVs) do have limitations. High engine temperatures and harsh fluids can destroy most conventional systems. As a result, thermoset rubbers (e.g., ACM, AEM, CR) have prevailed where more robust materials are needed.

There is an exception, however. Z-100-series (ACM//PA), the world's first commercialized 150 [degrees]C capable TPV, fills the gap not only between conventional TPVs and high-temperature service, but also across the entire range of traditional thermoset service temperatures from -40 [degrees]C to 175 [degrees]C. They provide superior long-term heat and oil resistance versus olefin based TPVs and copolyesters. As illustrated by the microphoto (from AFM) that is shown in figure 1, the morphology of ACM//PA lends itself to such a challenge. The polyacrylate rubber phase provides heat, oil and low-temperature resistance. The polyamide plastic phase contributes to heat resistance, ease of processing and excellent over-mold adhesion. Available in several durometer A hardness ranges, typical original physical properties are illustrated in table 1.

[FIGURE 1 OMITTED]

The case for two-shot molding with ACM//PA TPVs

There are several reasons to consider the use of an ACM//PA TPV in two-shot (2K) and insert over-molding applications. These include replacing metal cases, part consolidation, material performance upgrades and obtaining a chemical bond with polyamide. Replacement of metal cases is possible in dynamic sealing applications, for example. These parts are subjected to sustained 150 [degrees]C temperatures, as well as ATF oils. They typically consist of a metal case and thermoset ACM or AEM compound. The metal cases and thermoset stocks require preparation steps, which are not required with a material like ACM//PA TPV. The metal case must be cleaned, phosphatizcd and coated with an adhesive. Just prior to molding, the case must be exposed to heat, which activates the adhesive. With ACM//PA, an efficient two-shot type over-mold process can be used where a polyamide substrate is molded and then immediately covered with the high-performance ACM//PA TPV material. This not only eliminates the need for all the prep work associated with metal cases, but also can provide a true chemical bond with the substrate.

The elimination of multiple component systems is also a good opportunity for ACM//PA TPVs. Polyamide based, high-temperature air intake and turbo ducts are often assembled from multiple materials and can require purchasing rubber parts and mating these using securing clamps in assembly. Conventional materials may consist of AEM or silicone cuffs clamped to a blow molded polyamide and copolyester duct. A polyamide based high-performance TPV, such as ACM//PA, can be over-molded to form cuffs without the need for clamps or the introduction of purchased rubber cuffs. ACM//PA hardness values are significantly lower than COPE, which can be an important consideration for vibration isolation.

Many times material upgrades are required due to more aggressive under-the-hood environments. For instance, ACM//PA TPV was recently evaluated in the replacement of PA11 and olefin-based TPV in a two-shot molded diesel fuel line sensor. Superior polyamide adhesion and resistance to diesel fuel were needed. With no other material available to do the job, ACM//PA was pressed into service and provided excellent results.

Objective and criteria

The objective of this article is to show the performance of ACM//PA TPV with regard to service in common temperature ranges and fluids. We will focus on the merits of utilizing the material to enhance the performance of rigid thermoplastic two-shot molded applications while providing excellent adhesion to polyamides and polyesters.

Important criteria for evaluating these materials are:

* Retention of at least 50% tensile and elongation properties from 1,000 hours to 3,000 hours;

* no significant (more than 15 points, durometer A) change in hardness;

* low temperature flexibility to -40 [degrees]C; and

* over-mold adhesion values no less than 1 KN/m.

Experimental

Material performance

In previous work (refs. 1 and 2), it has been shown that ACM//PA TPVs outperform many currently used TPE/V materials, and work as well as many current thermoset materials in service. Serviceability is often quantified by the amount of change to original physical properties. ASTM D2000 and SAE J2236 standards provide guidelines in this regard. Generally speaking, this is gauged by no more than a 50% change in tensile and elongation properties, and by no more than a 15-point change in hardness.

As was discussed earlier, one benefit of ACM//PA TPV materials is long-term performance over a temperature range of 100 [degrees]C to 150 [degrees]C. To demonstrate this, injection molded plaques of ACM//PA 100-80B were aged in a hot air oven at 100 [degrees]C, 125 [degrees]C, 135 [degrees]C and 150 [degrees]C. As a general rule, after 200 hours across this temperature range, key physical properties level off and show good serviceability to 1,000 hours. This is illustrated in figures 2, 3 and 4, where hardness, tensile and elongation changes are noted after aging for a total of 1,000 hours in hot air.

[FIGURES 2-4 OMITTED]

During more aggressive aging tests, ACM//PA TPV showed equally good service. For example, in figures 5-7 results are plotted for hardness, tensile and elongation changes after aging at 150 [degrees]C for 3,000 hours in hot air and ASTM reference oil (SF105). In a hot air environment, the material was observed to change very little after 1,000 hours. The material was equally serviceable after aging for 3,000 hours in a hot oil environment. In addition to hot air and oils, ACM//PA TPV held up quite well in other under-the-hood media. Figures 8-10 demonstrate results in Evolube 232 grease (used in prop-shaft boots), as well as Dexron III ATF fluid after aging 1,000 hours at 150 [degrees]C. After exposure to the Evolube 232 grease, ACM//PA leveled off with very little change in hardness, tensile or elongation after 500 hours. At 1,000 hours, the material still meets service life criteria.

[FIGURES 5-7 OMITTED]

In ATF, the hardness climbed by 10 points until reaching 500 hours, where it leveled off. The tensile declined until about 300 hours, after which it was level or slightly decreasing to 1,000 hours. The elongation changes were level or slightly in the positive direction from 500 to 1,000 hours. In both cases, the ACM//PA material was still serviceable, per SAE J2236, after 1,000 hours at 150 [degrees]C in grease and ATF fluid.

Since ACM//PA is based on high-temperature polymer systems, testing at elevated or spike temperatures is of equal interest. In figures 11-13, the material was aged for one week at elevated service temperatures of 165 [degrees]C and 175 [degrees]C. When hardness, tensile and elongation changes were measured after exposure to this aggressive environment, the new TPV material still maintained its functionality.

[FIGURES 11-13 OMITTED]

Of equal importance in developing a proper service profile of any material are low-temperature properties. ACM//PA has a Tg of -40 [degrees]C and Gehman T100 of -60 [degrees]C, as illustrated in figure 14.

[FIGURE 14 OMITTED]

Overmolding and adhesion

To illustrate the performance of ACM//PA TPVs in overmolding applications, the bond type and strength were evaluated with many common polyamide and polyester materials such as PA 6; PA6,6; PA4,6; PA11; PA12 and PBT. Besides the neat resin, glass filled, toughened, plasticized, fire retardant and heat stabilized grades were evaluated. Test coupons having a width of 2.54 cm and a typical length of 15.24 cm were cut from injection molded plaques. A 2.54 cm length at one end was covered with heat resistant tape to keep a tab clear for later testing. The remaining exposed bond targeted area was washed with iso-octane. After the iso-octane evaporated, the coupon was placed in a hot air oven for five minutes at 25 [degrees]C below the manufacturer's specified melt temperature. After five minutes, the coupon was immediately transferred into an injection mold located in an HPM-110 injection press. The ACM//PA was over-molded on the coupon.

The over-molded coupons were conditioned on a lab bench top at 23 [degrees]C for 24 hours with a relative humidity of 50%. They were then tested according to ASTM D429 Method B, with a 90 [degrees] peel at a rate of 5.08 cm/min. An adhesion value in KN/m was reported, as was the actual mode of failure, whether adhesive (bond) or cohesive (material tear).

The first materials evaluated were based on a polyamide 6 substrate. The 33% glass filled substrate exhibited adhesive (bond) failure, yielding a strength of 1.37 KN/m. All other samples exhibited cohesive failure where the bond stayed intact but the TPV material tore. These values were as high as 2.4 KN/m (figure 15).

[FIGURE 15 OMITTED]

Over-molded adhesion to polyamide 4,6 did not show any sign of adhesive (bond) failure. Instead, as shown in figure 16, the mode was cohesive, with values as high as 3.96 KN/m.

[FIGURE 16 OMITTED]

In the case of polyamide 6,6, the heat stabilized, impact resistant grade indicated bond or adhesive failure. The other materials in this class showed good strength, with cohesive failure only. The highest value obtained (figure 17) was 3.08 KN/m with a 33% glass filled type.

[FIGURE 17 OMITTED]

When over-molding to polyamide 11 and 12 types, both modes of peel failures were observed. The PA11 showed a cohesive type with a value of 3.13 KN/m. Conversely the PA 12 type exhibited adhesive failure with a value of 1.44 KN/m (figure 18).

[FIGURE 18 OMITTED]

Polyesters were also evaluated. All types displayed an adhesive failure mode, with values ranging from 0.86 KN/m with an unfilled type to 1.24 KN/m in a 33% glass filled material (figure 19).

[FIGURE 19 OMITTED]

Conclusions

There are numerous potential applications where polyamide adhesion is required and where no material can equal ACM// PA's chemical bond for polyamides. Numerous materials and technologies have failed to achieve bonding to polyamides. Rubber to plastic adhesives and the addition of small amounts of polyamide to thermosets as alloy type systems have been tried with limited success. ACM//PA is a genuine permanent solution to what was once a complex, expensive bonding problem.

One of the many technical merits of a ACM//PA TPV system is that it will, unlike conventional TPVs, provide an intimate chemical bond with polyamide and polyesters without the need for primers, adhesives or mechanical interlocks. Cost-down scenarios include the elimination of preparation and processing steps associated with conventional thermoset and metal case bond systems. The ACM//PA material provides performance upgrades for harsh, high-temperature and hot oil environments over conventional TPE/V.

References

(1.) Brian J. Cail, et al., "High performance 150[degrees]C capable thermoplastic vulcanizates (TPVs)--long term aging behavior and processing, "paper #116 Rubber Division, ACS, October 2003 and Rubber World, Vol. 229, No. 5, February, 2004.

(2.) Jeffrey E. Dickerhoof, et al., "150[degrees]C heat- and oil-resistant TP Vs--long-term fluid and spike temperature comparison, " paper #405, ANTEC 2004.

Table 1--ACM//PA typical properties

                                  ACM//PA   ACM//PA   ACM//PA
                                    70A       80A       90A

Durometer A (15 sec.), points       76        85        96
Tensile strength at break, MPa       7         9        10
Elongation at break, %              198       190       174
Stress at 100% elongation, MPa       4         6         8
Density--specific gravity, g/cc    1.13      1.15      1.15