Inorganic Impact Modifier and Processing Aid for PVC Pipes

For most pipe applications impact modification is not required. In certain cases however, in cold climates, for pressure or thin-walled pipes, high impact strength is essential. The general understanding of impact improvement of thermoplastics is that the presence of a second – elastomeric – phase is essential to absorb the mechanical energy of the impact stress. In PVC for instance, this second phase either forms a honeycombed network encapsulating the PVC primary particles, like CPE and EVA, or spherical particles with an average size of about 0.2 µm are evenly dispersed in the thermoplastic matrix, like MBS or polyacrylates (1). However, it is also known that certain inorganic particles can have a positive influence on impact strength, like nanoclays or fine talc in PP or ultrafine-coated precipitated calcium carbonate in PVC (2). We have developed an inorganic material based on synthetic amorphous silicon dioxide with an average primary particle size of 150 nm and a specific surface area of 20 m2/g, called SIDISTAR® (Picture 1). These spherical particles can be optimally dispersed throughout the polymer matrix and will dissipate the impact energy over their particle surface so that in any direction the transmitted energy value is smaller than necessary for crack initiation or propagation. TEM (transmission electron microscopy) investigations prove that on impact the particles create a space between them and the surrounding polymer matrix thus avoiding energy transmittance. In Addition, due to their spherical particle shape, the silica particles have a positive effect on processing, improving melt flow and reducing internal friction during extrusion or injection molding. Other properties such as stiffness Vicat softening point and fire retardancy are positively affected. Picture 1 Transmission Electron Micrograph of SIDISTAR primary particles (magnification 41k x)