The present invention relates generally to polyolefin resin compositions containing a hindered amine light stabilizer (HALS) additive. A HALS additive can be used to prolong the stability of a polymer, or an article produced from the polymer, during exposure to sunlight and ultraviolet (UV) radiation.
These polyolefin compositions also can contain a polymer processing aid (PPA) additive to reduce or eliminate melt fracture. Melt fracture is a flow instability of a polymer during processing which often manifests as a sharkskin or orange-peel appearance on the surface of an article produced from the polymer. Melt fracture is generally more problematic with linear polyolefins having higher molecular weights or narrower molecular weight distributions, and during melt processing via narrow die gaps, at lower processing or melt temperatures, or at higher output or production rates.
PPA additives are used to improve the melt processing of polyolefins, for instance, to reduce or eliminate melt fracture. Generally, a PPA additive coats the metal surface that the polymer is in contact with during melt processing, for example, during extrusion through a metal die. By coating the metal surface of the die, the PPA additive can reduce the extrusion pressure and the shear stress on the polymer at the metal interface, resulting in a fabricated article with a smooth surface.
The ability of a PPA additive to reduce or eliminate melt fracture can be adversely affected by the presence of other additives. For instance, the addition of both a HALS additive and a PPA additive has a negative, or antagonistic, effect on melt fracture. See e.g., B. V. Johnson and J. M. Kunde, “The Influence of Polyolefin Additives on the Performance of Fluorocarbon Elastomer Process Aids,” Society of Plastics Engineers Annual Technical Conference (ANTEC) Proceedings, April 18-21, Atlanta, Ga. (1988). More specifically, additional PPA is needed to eliminate melt fracture than that required in the absence of a HALS additive, or a longer processing time is needed to completely eliminate melt fracture when a HALS additive is present. Further, the presence of colorants or pigments, such as titanium dioxide, can also have an antagonistic effect on the ability of a PPA to reduce or eliminate melt fracture.
Therefore, there is a need in the polymer industry for a polyolefin composition containing a HALS additive and a PPA additive which has improved resistance to melt fracture. Such a composition can contain both a HALS and a PPA additive, but beneficially, where the HALS additive does not negatively impact the ability of the processing aid to eliminate melt fracture during extrusion or other melt processing operation. Accordingly, it is to these ends that the present invention is directed.