Dry or semi-dry cold rolling of polyolefin sheet - for enhanced mechanical and optical properties without subsequent drying

摘要

Thermoplastic sheet is cold rolled with a thickness redn. of 5-95% by passing between rollers having tangential velocities matched to the sheet speed and where the haul-off stress is close to the elastic limit for the material leaving the roll nip. The roll surfaces are soaked with a fluid coolant, but the relationship ZN/P (where Z is the viscosity of the coolant, N the sheet offtake speed and P the roll nip pressure) is maintained at values which result in partial or zero lubrication of the sheet surface, i.e. the sheet leaving the roll nip has partially or completely dry surfaces. Used for producing cold rolled polyethylen or polypropylene sheet or tape having advanced mechanical properties and transparency and free from voids or tears, e.g. for fruit packaging film. Higher roll nip pressures and haul-off stresses can be sustained than when cold rolling sheet with an interfacial lubricant film, resulting in greater strength, transparency, etc. Little or no liq. has to be recovered from teh drawn sheet or tape. Process condition variations are less critical than rolling with a complete lubricant film between roll and sheet surfaces.