A new class of renewable and very efficient multifunctional modifiers for PVC has been developed at Metabolix. This series of biobased PHA (polyhydroxyalkanoate) copolymers offers toughening, plasticization, and improved processing of all types of PVC compounds. They also have good UV stability and transparency, do not promote biodegradation, and are fungi resistant.

Because of their unique miscibility with PVC, the PHA modifiers do not migrate, extract, or volatilize, and they are easy to handle and process at the same conditions as PVC. They offer a new, simplified approach to PVC formulation in which multiple processing and modifying additives could be replaced with a single product, eliminating both undesirable additive interactions and the need to stabilize unstable ingredients.

The centerpiece of the Metabolix plant-based resin technology is PHB (polyhdroxybutyrate), the simplest member of the broad PHA family of biopolymers. Metabolix’s current range of Mirel PHA copolymers are produced by bio-fermentation of sugars in which the water-insoluble inert polymer accumulates inside specially engineered microorganisms. The polymer is then extracted and purified and can be compounded and processed using conventional plastics converting equipment.

In our recent study, we used three different PHA copolymers as modifiers for PVC: I6001, a semi-crystalline PHB copolymer with a nominal crystallinity of about 25% and glass-transition temperature (Tg) of about -8 C; I6002, an amorphous PHB copolymer with a Tg of about -21 C; and a developmental, next-generation PHA grade for rigid PVC. The PHA copolymers can contain hard crystalline and soft rubbery segments, which is very important for impact modification. Both phases have good miscibility with PVC.

PVC resin with a K-value of 70 was used for blending with PHA. For additional and comparative plasticization we used DIDP as the less extractable monomeric phthalate, as well as DINA (diisononyl adipate). For heat stabilization we used liquid BaZn carboxylates. The PVC/PHA blends are quite transparent when stabilized with liquid carboxylates. Commercially available impact modifiers of MBS, ABS, ASA, and CPE types were used for comparison.

TOUGHENING PLUS PROCESSING EASE

PVC has a well known propensity for brittle fracture, particularly in the presence of sharp notches or cracks and at low temperatures or high deformation rates. One way to enhance fracture toughness under these conditions is to incorporate a rubbery phase. Less polar elastomers, such as polybutadiene, natural rubber, butyl rubber, and styrene-butadiene rubbers are not useful for PVC modification due to their incompatibility and phase separation. The usual way to solve the incompatibility issues is to use grafted core-shell impact modifiers, such as MBS, AIM (all-acrylic impact modifiers), or ABS, whereby one phase—e.g., methyl methacrylate—is miscible with PVC and provides good adhesion with the matrix, and another phase—e.g., n-butyl acrylate or butadiene rubber—absorbs and dissipates the energy of an impact.

The performance of toughened PVC in this case depends strongly on the type of rubber and its dispersion quality, structure, size, and particle-size distribution. In addition, the rubber’s glass-transition temperature (Tg) must be lower than the intended service temperature of the PVC compound, and the rubber particles must be thermally stable at the processing conditions.

Metabolix’s newly developed PHA copolymer “rubbers” are miscible with PVC, have a Tg range from -8 to -28 C, and are thermally stable at PVC processing conditions.

It is generally accepted that a blend with a pseudo-network morphology has a higher toughening efficiency than a blend with a morphology of well-dispersed particles. At Metabolix, we have tried using this approach to create PHA-based toughening modifiers for PVC. Our expectation was that the entangled PVC/PHA networks would be less sensitive to processing, as they would not require the high-shear conditions needed to produce the required melt viscosity for the core-shell particle dispersion.

Along with their ability to promote shear melting of PVC, PHA modifiers also reduce the fluxing time and improve the overall thermal stability of PVC. We used a typical PVC two-roll mill for compounding at 330 F (165 C). The PHA copolymers added at up to 40 phr presented no processing problems. In fact, all PVC/PHA rigid and semi-rigid blends released nicely from the rolls and promoted much easier fusion of PVC particles than with other toughening additives, especially of CPE and MBS types. All the tested impact modifiers (CPE, MBS, ASA, and ABS) took two to five times longer to properly melt PVC resin than with PHA and required additional processing aids. Due to their wall-adhering nature and miscibility with PVC, PHA polymers promote shear melting, reduce the processing time, and do not require processing aids.

The milled sheet was then compression molded at 340 F into testing samples for tensile, flexural, and notched Izod impact performance, as well as Shore D hardness, and Tg (measured using a Dynamic Mechanical Analyzer).

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About the Author

Yelena Kann joined Metabolix, Inc., Cambridge, Mass., as a senior polymer scientist focusing on PHA technologies for polymer compounds, sheet, fibers, foam, and extrusion coating. She earned her Ph.D. in Chemistry in 2003 from the University of Sussex, U.K. Contact: (978) 513-1819; kann@metabolix.com; metabolix.com.

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