The disclosure relates generally to silicone-acrylate impact modifiers and their use in resin molding compositions, particularly those comprising thermoplastic resins. Furthermore, the disclosure also relates to an emulsion polymerization method for making the silicone-acrylate impact modifiers.
Butadiene-based impact modifiers, such as acrylonitrile-butadiene-styrene (also called ABS) copolymers and methyl methacrylate-butadiene-styrene (also called MBS) copolymers have been previously used to improve the impact performance of thermoplastic materials. However, due to the presence of unsaturation, these butadiene-based copolymers respond poorly to weathering. Weathering is a phenomenon where the combined effect of several natural elements, particularly oxygen in air and sunlight act upon the polymer thereby causing the material to degrade. Generally, this degradation is observable by a yellowing and loss of surface gloss of the polymer material. Impact modifiers based on acrylonitrile-styrene-acrylate (also called ASA) copolymers avoid the issues faced by the butadiene-based polymers. However, these materials only have room temperature ductility. Acrylate rubbers are widely used for impact modification of thermoplastic materials where weathering is a concern. However, the impact strength of acrylate rubber-modified thermoplastic materials at low temperatures, such as 0° C. or below, is substantially reduced as compared to thermoplastic materials containing other organic blends, such as the butadiene-based polymers. Efforts have been made to use silicone-based materials to improve low temperature impact. For example, silicone-polycarbonate copolymers show good ductility at minus 40° C., but they can be used only in polycarbonates and polycarbonate blends. Efforts to improve the low temperature impact of thermoplastic polymer compositions by using silicone rubber-based impact modifiers, such as Mitsubishi Rayon's S 2001 are known. However, the low temperature impact and ductility performance, as measured for example by the ductile-to-brittle transition temperature (hereinafter referred to as “DBTT”), is in some cases not up to the desired mark. Therefore, there is a continued need for impact modifiers that can afford superior impact properties, lower ductile-to-brittle transition temperatures (hereinafter sometimes referred to as DBTT's), and outstanding weatherability performance, that is, a slowing down or prevention of yellowing and loss of surface gloss, in polymer compositions and articles comprising these polymer compositions. Such impact modifiers, when incorporated into polymer resin systems are expected to find a wide variety of applications, especially outdoor applications.