Sealable films

Details Sealable films Sealable films

1988-03-17

1990-05-01

Ives, P. C.

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

428516, 428349, 428910, 428518, 4284763, B32B 2708, B32B 2732, B32B 3130

Patent

active

049217496

DESCRIPTION:

BRIEF SUMMARY
This invention relates to films based on polypropylene including those which can be sealed by the application of heat.
Various polypropylene films have been described, e.g. in GB No. 2028168A, EP No. 135178 and DE No. 3247988 where the core or substrate of the film is polypropylene. Various additives are added for example certain modulus improvers, certain antistatic media, siloxanes and pigments.
However, none of these documents teach that films may be obtained having a seal strength which is significantly improved compared with previously known polyolefin films. We have devised films having a seal layer needing only one component which show improved seal strength properties over those previously known having only one component in the seal layer, i.e. either the seal strength is much improved at the same temperature of sealing or the same seal strength can be obtained at lower temperatures.
According to this invention a film, particularly a sealable film comprises a base layer comprising 70 to 97 wt. %, preferably 70 to 90 weight % of a polyolefin and 3 to 30, preferably 10 to 30 weight % of a resin of lower molecular weight than that of the polyolefin, said layer having on at least one surface thereo 1 to 20 weight % based on the weight of the base layer, of a film layer comprising a random copolymer of 80 to 99 weight % of propylene and 1 to 20 weight % of ethylene.
These films are capable of being used in the packaging industry and can meet the stringent criteria which are required for such films, such as sufficiently high modulus and excellent clarity. In addition, they have good barrier properties by which we mean low permeability to water vapour and gases, particularly oxygen. The films also have superior seal strength properties and are especially useful in heat seal packaging although they are useful in other packing outlets where cellophane is traditionally used, such as twist wrapping.
The base layer comprises a polyolefin and a low molecular weight resin. The polyolefin is preferably a polymer of a mono-alpha olefin containing from 2 to 8 carbon atoms and particularly 2 to 4 carbon atoms per molecule. Thus the polymer may be a homopolymer of one of the following olefins: ethylene, propylene, butene-1 and 4-methyl pentene-1, or hexene or a copolymer of two or more of these olefins. A particularly suitable material for the base layer of the film is polypropylene, especially a high molecular weight stereoregular predominantly crystalline polymer of propylene. Alternatively, a copolymer of propylene with up to 20 weight percent of another olefin, e.g. ethylene, could be used. An especially preferred polyolefin is an isotactic polypropylene with a density of from 0.86 to 0.92 g/cc measured at 23.degree. C. according to ASTM D 1505 and a melt flow index of from 1 to 15 g/10 min as determined according to ASTM D1238 (conditions 230.degree. C. and 2.16 kg). It maybe made by Ziegler polymerisation methods using for example AlCl.sub.3 and TiCl.sub.4 as catalyst.
The other component of the base layer is the low molecular weight resin, preferably a hydrogenated resin. This resin has a molecular weight below that of the polyolefin and is usually less than 5000, preferably less than 1000, for example, 500 to 1000. The resin can be a natural or synthetic resin and can have a softening point as measured by ASTM E 28 of from 60.degree. to 180.degree. C., e.g. from 80.degree. to 150.degree. C., most preferably from 100.degree. to 140.degree. C., especially 120.degree. C. to 140.degree. C.
Suitable resins which can subsequently be hydrogenated are hydrocarbon resins, ketone resins, polyamide resins, colophonium, coumarone resins, terpene resins, chlorinated aliphatic or aromatic hydrocarbon resins. Examples of hydrocarbon resins are polymers of coke oven gas, cracked naphtha, gas oil and terpene oil.
Particularly preferred hydrogenated resins are hydrogenated petroleum resins. These are usually prepared by catalytically hydrogenating a thermally polymerised steam cracked petroleum distillate fraction, especially

REFERENCES:
patent: 4230767 (1980-10-01), Isaka et al.
patent: 4590125 (1986-05-01), Balloni et al.

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