COEXTRUSION PROCESSES FOR MAKING MULTILAYER FILMS AND FILMS MADE THEREBY

摘要

The invention relates to a blown film coextrusion process. The process involves melting and extruding a core layer and a pair of core contact layers adjacent both sides of the core layer through a die exit at a linear speed V_o cm/s and cooling, orienting and crystallizing the multi-layer film formed at a deformation rate E_avg=(Vf-Vo)/ FLH of at least 0.6 s^-1 until a the frost line is reached at a distance FLH from the die exit by withdrawing film at a take up speed V_f in cm/s at the frost line. The core and core contact layers are of a film forming polymer composition comprising at least 90 wt% of ethylene based polymer. The ethylene based polymer of the core layer has an average density of ethylene based polymers equal to or higher than that of ethylene based polymer of the core contact layers. The invention specifically concerns the feature that the core layer composition contains at least 5 wt% of LDPE having an I2.16 _ldpe-core of less than 1.5 g/10 min and the composition of the core contact layers contains at least 75 wt% of a linear polyethylene so as to make the core layer more resistant to deformation than the skin layers, all I2.16 values being determined according to ASTM D 1238 procedure A condition 190°C/2.16 kg.