Film made of thermoplastic polymer is extensively used in particular as packaging material, covering for goods and plant beds, and--combined with other films--as a laminate. The starting materials are homopolymers, for example those polyethylenes of low, medium and high density, or copolymers of ethylene with one or more monomers, such as mono-alpha-olefins, alkenecarboxylic acids, alkenecarboxylic esters, or vinyl acetate.
The key to their versatility is a number of properties, among which mechanical, thermal, and optical characteristics are of particular importance. Accordingly, the polymerization conditions and the qualitative or quantitative composition of the monomer mixture being polymerized have been varied in numerous ways to meet different practical film requirements.
According to EP-B-82,502, ethylene copolymers are prepared by continuous polymerization of a monomer mixture comprising 95.9% to 99% by weight of ethylene; 0.3% to 4.0% by weight of vinyl acetate and/or vinyl propionate; 0.1% to 3.5% by weight of saturated aliphatic aldehydes having 2 to 5 carbon atoms in the alkyl moiety, saturated aliphatic ketones having 1 or 2 carbon atoms in the alkyl moiety, alkenes having 3 to 6 carbon atoms, and/or cyclohexene. The reaction is carried out in a tubular reactor at a pressure of 100 to 400 MPa and 190.degree. and 270.degree. C. by introducing 30% to 70% by weight of the monomer mixture at the start of the tubular reactor and the remainder at two or more points downstream thereof. The film obtained from the copolymers is suitable in particular for manufacture of large bags.
DE-A-1,924,823 describes, in Example 4, a terpolymer of ethylene, 5.0% by weight of vinyl acetate, and 1.5% by weight of propylene. In the form of a packaging and wrapping film 0.025 mm in thickness, this terpolymer exhibits excellent optical properties, stiffness, and breaking strength.
DE-B-2,018,718 concerns a process for producing modified high pressure polyethylenes wherein polymerization of ethylene, and optionally of other compounds which are copolymerizable with ethylene, is carried out in the presence of oligomers of isobutylene having 4 to 100 isobutylene units. The polymers obtained by this process are suitable for producing film that is free of surface tackiness which is responsible for blocking.