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Flexible PVC doesn't have many friends these days. A number of materials suppliers and compounders would like to replace it with chlorine-free formulations based on polyolefins. However, Teknor Apex Co. of Pawtucket, R.I., is taking a different route. As a PVC compounder, Teknor isn't trying to oust chlorine but instead to reduce or eliminate liquid plasticizers and thereby fix some of the problems they cause.

Liquid plasticizers make up as much as a third of conventional flexible vinyls, and they leach out slowly over time, leaving behind brittle products and causing problems like window fogging in car interiors. Other problems associated with liquid plasticizers are flame propagation, poorer heat aging, and poorer dielectric properties.

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Teknor has patented developmental vinyl materials that dispense with some or all of the liquid plasticizer. Instead, a metallocene-based polyolefin elastomer (POE) is alloyed with suspension-grade PVC resin to provide flexibility. Teknor's patent (U.S. Patent No. 6063846, issued May 16, 2000) notes that Engage 8150 from DuPont Dow Elastomers in Wilmington, Del., and Exact 3035 from Houston-based ExxonMobil Chemical Co. are the preferred POE ingredients. Various compatibilizers can be used to alloy the POE and PVC, including Tyrin 3623A, a chlorinated polyethylene from DuPont Dow Elastomers. Teknor is the only vinyl compounder pursuing use of POE to replace liquid plasticizers, says Robert Brookman, Teknor's v.p. for ����ý development.

No liquid plasticizer

Test results on an array of PVC compounds containing 20% to 35% POE and 0 to 60 phr of liquid plasticizer were presented by Vishal Kadakia, product-development engineer, at the Society of Plastics Engineers' Vinyltec 2001 conference in Iselin, N.J., last September. The data show that properties improve steadily as liquid plasticizer levels fall and POE levels rise.

At least one developmental grade, designated Alloy 3 in the accompanying table, has no liquid plasticizer at all. Its mechanical properties are dramatically better, especially at extremes of heat and cold, than those of conventional flexible PVC. The all-solids compound shows complete retention of elongation after high-temperature aging, while elongation retention is negligible for the conventional liquid-plasticized vinyl.

Teknor still has to use some liquid plasticizer to create soft vinyls, but less than in conventional compounds. In its SPE paper, the softest compound Teknor could achieve with only POE as a plasticizer was a semi-rigid grade with 94 Shore A hardness. Since then Teknor has achieved 88 to 89 Shore A without liquid plasticizers, says Philip Morin, industry manager for flexible PVC. 

Unusual benefits

Developmental Alloy 1 in the table, containing 20% compatibilized POE and 60 phr of DEHP plasticizer, achieves a Shore A hardness of 70. Surprisingly, the brittle point of Alloy 1 actually drops 20° C after seven days of oven aging at 121 C. Likewise Alloy 2 (25% to 35% POE, 20 phr DEHP) also displays a lower brittle point after heat aging at 136 C than after aging at 121 C. These unexpected phenomena apparently reflect the loss of plasticizer during heat aging, which increases the concentration of the POE in the aged alloy sample and forms a more compatibilized alloy, Teknor sources suggest. The result is better low-temperature flexibility.

"Another unexpected result is that the harder compound, Alloy 2 (94 Shore A), has a lower brittle point than the softer Alloy 1 (70 Shore A)," the paper's authors observe. Typically, the softer material would be less brittle. The same theory of POE compatibility would appear to explain this anomaly.

"All three of the POE/PVC alloys showed higher viscosity than conventional flexible PVC at the same hardness," the paper notes. "Based on capillary rheometer data, the PVC/POE alloy duplicated the viscosity curve of a conventional flexible PVC with either a 10° C increase in processing temperature or slightly higher shear rates." And the plasticizer-free Alloy 3 has much higher viscosity than Alloys 1 and 2. Because of its higher viscosity, Alloy 3 is more suited to extrusion applications than to injection molding, notes Dr. Dexi Weng, technical manager at Teknor.

Slightly more expensive

Flexible PVC formulated with the new PVC/POE alloy, with or without liquid plasticizer, is expected to be only 10% to 15% more expensive than conventional PVC, says Teknor's Morin. However, all the POE/PVC alloys have lower specific gravity than conventional PVC, which improves their economics.

Teknor plans to sell the PVC/POE alloys only as complete formulations. The firm has offered a couple of precommercial grades for eight months. These materials will target premium markets where leaching of liquid plasticizers is a problem, such as medical films and automotive interiors. The new materials also offer improved dielectric properties for wire and cable jacketing. One early application in development is a gasket to be used in contact with ABS in appliances, where leaching of liquid plasticizers can mar the ABS.