The present invention relates to a fine particle for modifying polyolefine. More particularly, it relates to a fine particle which is excellent in affinity with polyolefine, and modifies polyolefine without accompanying damage on the transparency of the polyolefine.
When polyolefine films are overlapped, they adhere to each other and result in a so-called blocking phenomenon, which causes many problems. To restrain the blocking phenomenon, there are known methods in which the properties of polyolefine are modified by adding fine particles to polyolefine. Particularly for polyolefine films, such methods are commonly conducted with inorganic fine particles such as silica particles or titanium oxide particles, or polymeric crosslinked particles such as polystyrene particles or polymethylmethacrylate particles added to polyolefine, to prevent the films from blocking.
For example, Japanese Patent Application Laid-Open No. 57-64522 (JP-A-57-64522) discloses an oriented polypropylene film obtained in the following manner. That is, polymeric fine particles having a particle size of 3 to 40 .mu.m with a crosslinked structure and having no melting point are mixed with a polypropylene resin in a specific amount, the mixture is molten and extruded, followed by cooling, and then the resulting resin is stretched uniaxially or biaxially to obtain the oriented polypropylene film. The application also discloses that when the difference in solubility coefficient between the non-melting crosslinked polymer particles and the polypropylene resin is small, the affinity between the crosslinked polymer particles and the polypropylene resin is good, resulting in higher transparency of the oriented film. The fine particles of crosslinked polymers which are mainly made from polyethylene, polypropylene, polystyrene, polymethylmethacrylate, polycarbonate, polyethylene terephthalate or the like, are exemplified.
Japanese Patent Application Laid-Open No. 6-107868 (JP-A-6-107868) discloses a polyolefine film made of a composition obtained by mixing polyolefine with crosslinked particles which are made from a copolymer mainly composed of a (meth)acrylic monomer and a styrene monomer, and has an average particle size of 0.4 to 7 .mu.m.
In addition, Japanese Patent Application Laid-Open No. 7-228729 (JP-A-7-228729) discloses a polyolefine film obtained by dispersing crosslinked particles having a core-shell form into a polyolefin polymer, wherein the crosslinked particles (core-shell particles) are prepared by polymerizing polymerizable monomers containing a crosslinkable monomer in an amount of 10% by weight or more to give crosslinked polymer particles (A), followed by coating the surface of the crosslinked particles (A) with a thermoplastic resin (B), and are composite crosslinked polymer particles having an average particle size of 0.01 to 10 .mu.m.
As described above, many kinds of fine particles such as the inorganic particles or the crosslinked particles have been known and used for modifying polyolefine. However, these conventional fine particles can not keep the high transparency of polyolefine or can not show the high affinity with polyolefine, when the particles are mixed with polyolefine.
For example, fine particles such as silica particles or crosslinked polymethylmethacrylate particles have a refractive index close to that of polyolefine, and hence the transparency of polyolefine containing the particles is high. However, the affinity between polyolefine and the particles is insufficient and, therefore, the particles tend to separate from polyolefine when a polyolefine film containing the particles is process (ex. stretched or bent), causing problems such that voids are produced and the particles fall out of the polyolefine film.
The crosslinked polystyrene fine particles have high affinity with polyolefine, and hence the problems, such that voids are produced and the particles fall out of the polyolefine film, do not tend to occur. However, the refractive index of the crosslinked polystyrene fine particles does not be close to that of polyolefine and, therefore, the transparency of polyolefine containing the particles deteriorates.
When the copolymer fine particles mainly composed of a (meth)acrylic monomer and a styrene monomer are used for modifying polyolefine, both properties of the affinity and of the transparency are somewhat high. However, the transparency is not as high as that achieved by the crosslinked polymethylmethacrylate fine particles, and the affinity is not as high as that between the crosslinked polystyrene fine particles and polyolefine and, therefore, the generation of voids and the separation of particles and polyolefine are not restrained so much compared with the crosslinked polystyrene fine particles, either.
As described above, the composite crosslinked polymer particles having a core-shell form composed of core layer with a high crosslinking density and a shell layer with a low crosslinking density are used in the JP-A-7-22879, and in the examples thereof, the composite crosslinked polymer particles of which shell layer is mainly made from acrylic monomers are used. However, the affinity of these exemplified particles with polyolefine is insufficient and, therefore, the problems such as the generation of voids and the separation of particles and polyolefine have not been solved completely when the polyolefine to which these fine particles have been added is subjected to a stretching process.