PVC additives (Part 11)

Impact Modifiers

Unmodified PVC-U has relatively poor impact strength at, and below, ambient temperatures and the inclusion of an impact modifier significantly improves its performance, in particular the notch sensitivity (sensitive to stress concentrations at the notch).

Impact modification is achieved by incorporating rubber domains into the polymer matrix, which, of course, must fit well within the matrix. Good dispersion and adhesion to the PVC matrix is essential.

The mechanism for impact improvement depends on the rubber component being able to absorb impact energy without the propagation of crazes and cracks.

Examples of the different generic types of impact modifier that can be added to PVC formulations are:

? Methacrylate–butadiene–styrene (MBS) terpolymer

? Acrylate–polymethacrylate copolymer (acrylic)

? Chlorinated polyethylene (CPE)

? Ethylene–vinyl acetate copolymer (EVA)

? Acrylonitrile–butadiene–styrene terpolymer (ABS)

Addition levels are typically 5–12 phr and the incorporation of impact modifiers can influence gelation and melt viscosity. Having limited compatibility with PVC, it is believed that the impact improvement is achieved via the interruption of the homogeneous morphology into a heterogeneous structure.

Acrylic modifiers have good processing characteristics linked to reduced die swell and good outdoor weathering stability. Their use in window profile and siding applications is well documented. Based on a crosslinked butylacrylate elastomer core and polymethacrylate shell technology, produced by emulsion polymerisation, the elastic core of the modifier has a hard shell to keep the product free flowing and couple the modifier to the PVC matrix. Much work has been carried out covering various

aspects of their processing and performance characteristics. Important criteria are:

? Influence on weld strength in window frame manufacture.

? The relationship with gelation level on impact strength and morphology.

? The effect on impact strength of varying crosslinking levels in the rubber component of the

impact modifier.

? Relationships between melt flow and mechanical properties.

New and updated acrylic impact modifier grades are regularly being developed to meet demands for higher output with a broader processing window.

CPE impact modifiers contain around 35% chlorine and are thermoplastic in nature. With a similar melting point to PVC, they initially form a network structure which changes to a particulate structure on processing. They also have good processability and excellent weathering performance consequently they are also used for window profiles. CPE also acts as an internal lubricant providing improved flow characteristics to the PVC polymer melt. The incorporation of relatively small amounts has been shown to improve the strength and toughness of PVC pipes.

MBS impact modifiers also depend on core shell technology, with a polybutadiene/styrene core and a polymethyl methacrylate/styrene shell. They have a wide use in PVC applications, particularly for clear packaging (bottles, film, and sheet), as the refractive indices of PVC and the modifier are similar.

They are also used in pipes and fittings. They are particularly efficient for low-temperature impact improvement but are not suitable for outdoor applications.

EVA copolymers mixed with PVC are also available for impact modification with good weathering resistance. They also generally confer a lubricating action requiring modification to the lubricating system of the formulation.

Specific ABS terpolymer modifiers also process well in clear applications giving low crease whitening and excellent chemical resistance. The heat distortion temperature of ABS-modified compounds can be higher in comparison to MBS-modified componds. They are not suitable for outdoor applications.

Impact performance can be measured by different techniques:

? Falling weight impact – consists of a dart-shaped mass, of a specific weight, falling from a standard height onto the extruded material. The temperature at which this test is carried out is usually –10 °C. The results depend on formulation, gelation level, and profile shape.

? Tensile impact – this is a high-speed tensile test using a pendulum weight. This test is linked to the intrinsic material formulation.

It is also important to recognize the way a specimen breaks. Ductile fracture (hinge break) shows stretching or elongation until failure. There is also whitening around the broken area. Brittle fracture (complete break) occurs with little deformation happening during failure. The difference can be due to temperature, where brittle fracture occurs at lower temperatures. Alternatively, brittle fracture can also occur if insufficient impact modifier has been used or dispersion is poor.