Blends of Low Density Ethylene Polymers With Certain Polyesters

Low density ethylene polymers are widely employed in the manufacture of polymer films. In preparing bags, pouches, and the like from such films, the films are heat sealed together under pressure. It would be desirable to seal such films together at lower temperatures than are presently required.

The applicants have discovered that blends of a low density ethylene polymer with minor quantities of either poly(ethylene adipate) or poly(trimethylene adipate) are compatible and melt at lower temperatures than the low density ethylene polymer. As a consequence, films prepared from such polymer blends can be heat sealed at significantly lower temperatures than those required to heat seal films of the low density ethylene polymer contained in such blends.

The polymer blends of the invention consist essentially of 100 parts by weight of the low density ethylene polymer and about 0.2 to 3, preferably about 1 to 2.5, and more especially about 2 parts by weight of the poly(ethylene adipate) or poly(trimethylene adipate). The polymer blends can be prepared by any of the common techniques employed in the art to prepare blends of two polymers. This conventionally is done by malaxating the polymers on a two-roll rubber mill, in Banbury Mixers, or by extruding the mixture, or by other like techniques.

The low density ethylene polymers included in the blends will be the conventional film forming grades of ethylene polymers which have a density of less than about 0.945, preferably less than about 0.94, and more especially less than about 0.93. It is preferred to employ polymers having melt indexes in a range of about 0.5-20, preferably about 0.5-10, and more especially about 0.5-5.0. The low density ethylene polymers can be prepared by the well known high pressure processes, or by lower pressure processes by copolymerizing ethylene with C.sub.3 and higher alpha-olefins.

The poly(ethylene adipate) and poly(trimethylene adipate) employed in the invention can be prepared by techniques known and reported in the art. Such polyesters can be prepared by a simple esterification of adipic acid with either ethylene glycol or trimethylene glycol (1,3-propylene glycol in alternate nomenclature). In such esterifications, after the polymerization has been carried to the point of producing low molecular weight polymers, the further esterification and polymerization are carried out by heating the reaction mixture under reduced pressure to aid in removing water. Alternatively, the polyesters can be prepared by a transesterification technique in which a lower ester of adipic acid, such as dimethyl adipate, is transesterified with ethylene glycol or trimethylene glycol.

The molecular weight of the poly(ethylene adipate) and the poly(trimethylene adipate) has an effect upon the compatibility of such polyesters with the ethylene polymers. In general, the higher the molecular weight of the polyester, the larger the quantity of polyester that can be incorporated into the ethylene polymer without exudation occurring. It is noted, however, that as the molecular weight of the polyester increases, the effect of the polyester in lowering the seal temperature of the polymer blend decreases. Accordingly, the applicants prefer to employ polyesters having molecular weights in a range from as low as approximately 300 to the order of about 3,000.

All of the polymer blends employed in the work subsequently reported were prepared by adding comminuted polymers to a Brabender Plasticorder operated at approximately 130.degree. C. and 40 rpm. The mixing time was 20 minutes in all cases. Films of each blend having a range of thickness of 2-4 mils were compression molded in a press. Tensile and elongation values were measured for each film.

The films were cut into 1".times.2" strips and heat sealed at different temperatures over a range from 235.degree. F. to 325.degree. F. To minimize errors and improve reproducibility, a set of at least three films was sealed at each temperature. The seal strength at each temperature was measured by pulling the samples to the break point in an Instron Tester. Plots of Seal Strength (in lbs/inch) vs. Seal Temperature in .degree.F. were made for each polymer blend and the ethylene-alkyl acrylate copolymer contained therein. The Seal Temperature required to obtain a Seal Strength of 2.75 lbs/inch was noted for each sample.

The following examples are set forth to illustrate more clearly the principle and practice of the invention to those skilled in the art. Where parts or percentages are mentioned, they are parts or percentages on a weight basis unless otherwise noted.

A polymer blend was prepared from 98 parts of a commercially available low density ethylene polymer having a density of about 0.924 and a melt index of about 0.8. The ethylene polymer had a tensile strength of 2,100 lbs/psi and an elongation at break of 441%. The poly(ethylene adipate) was purchased from a specialty laboratory supply house and had a glass transition temperature of -50.degree. C., a melting temperature of 55.degree. C., a density of 1.183, and a molecular weight of approximately 1,000. The blend prepared from the two polymers had a tensile strength of 2,200 psi and an elongation at break of 546%.

Following the experimental procedures previously described, the seal strength of the polymer blend and the ethylene polymer were determined at sealing temperatures from 235.degree. to 325.degree. F. With the ethylene polymer, a seal temperature of approximately 260.degree. F. was required to obtain a seal strength of 2.75 lbs/inch. With the polymer blend, a seal strength of 2.75 lbs/inch was obtained at a seal temperature of approximately 235.degree. F. It is thus seen that the polymer blend has a seal temperature approximately 25.degree. F. lower than the seal temperature of the ethylene-methyl acrylate copolymer contained therein.

Example 1 was repeated, except that the poly(ethylene adipate) of Example 1 was replaced with a poly(trimethylene adipate) sample which had a molecular weight of approximately 300. The polymer blend had a tensile strength of 2,300 psi and an elongation at break of 543%. The polymer blend had a seal strength of 2.75 lbs/inch at a temperature of 235.degree. F. The sealing temperature results were essentially identical to those of Example 1.

In preparing plots of Seal Strength vs. Seal Temperature as previously described, it is noted that the curves for the polymer blends of the invention rise less steeply than do the curves for the ethylene polymers. As a consequence, effective heat seals can be obtained over a relatively wide range of sealing temperatures. This characteristic makes temperature control of the heat sealer less critical than is the case with other types of ethylene polymers.

In addition to being useful in the direct manufacture of film, the polymer blends of the invention can be used as one component in the manufacture of multilayered, coextruded film. Usually the polymer blend will be employed as a surface component to take advantage of its good heat sealing characteristics.