FILMS OF LINEAR LOW DENSITY POLYETHYLENE HAVING IMPROVED IMPACT STRENGTH

Abstract: Compression-rolled films of linear low density polyethylene have high impact strength and high tear strength in both the machine and transverse directions.

Description: FILMS OF LINEAR LOW DENSITY POLYETHYLENE HAVING IMPROVED IMPACT STRENGTH This invention relates to films of linear low density polyethylene (LLDPE) . LLDPE has a molecular structure which is characterized by the substantial absence of long 5. chain branching. In contrast, conventional low density polyethylene (LDPE) has substantial long chain branching. LLDPE also has a significantly higher melting point (typically 120-135°C) than LDPE (typically 105-115°). LDPE is sometimes re- 10. ferred to as high pressure polyethylene because it is produced at high pressures. LLDPE, on the other hand, is produced commercially at low pressures in a gas phase process. However, LLDPE may also be produced in a liquid phase solution process. 15. Various alpha-olefins are typically copolymerized with ethylene in producing LLDPE. The alpha-olefins , which have 3 to 15, preferably 4 to 8, carbon atoms, are present in the polymer in an amount of up to about ten weight percent. The 20. comonomer influences the density of the polymer, which is less than 0.94 grams per cubic centimeter, and preferably is from about 0.915 to 0.935 g/cc. This invention provides films of LLDPE having substantially improved impact strength compared to 25. conventional films of LLDPE. The films of the invention also have good tear strength in both the transverse and machine directions. The films of this invention are prepared by compression rolling films of LLDPE. The term 30. "compression rolling," as used herein and as understood in the art, refers to the process of passing a plastic film through the nip of a pair of rolls at a temperature below the softening point of the film at a pressure which is sufficient to 5. reduce the thickness of the film at least five percent, preferably at least fifty percent. The length of the film is increased by an amount corresponding to the reduction in thickness. For example, a film whose thickness has been reduced 10. fifty percent by compression rolling has a length which is about twice as great as its original length. Compression rolling is described in U.S. Patents 3,504,075 and 4,085,187, which are incorporated herein by reference. 15. These patents teach that virtually any polymer may be compression rolled, but compression rolling may adversely affect certain properties of a film to make it unacceptable. For example, as taught in an article by Williams and Morrison in the December 20. 1971 issue of SPE Journal (Vol. 27), pp. 42-48, compression rolling of high density polyethylene (HDPE) and polypropylene (PP) films decrease their tear stength in the machine direction. As shown in Table 2 of the article, rolled films of HDPE and PP 25. having a thickness of one mil had tear strengths in the machine direction of only 24 grams and 5 grams, respectively. Hence, compression rolling of films of HDPE and PP makes them "splitty" and reduces their impact strength, which severely limits their 3(3. commercial utility. An opposite phenomenon is observed with LDPE. As shown in Table 2 of the Williams article, rolling improves the tear strength of films of LDPE in the machine direction, but it reduces their tear OMP strength in the transverse direction. In fact, the tear stength of rolled LDPE film in the transverse direction is so low that LDPE film is difficult to roll at high speeds because it tends to tear in the 5. transverse direction as it is drawn from the rolling mill. This invention is based on the discovery that compression rolling of films of LLDPE substantially improves the impact strength of the films and 10. ensures that they have good tear strength in both the machine and transverse directions. This result is surprising and unexpected in view of the effect of compression rolling on the impact strength and tear strength of films of other polyolefins. For 15. example, the rolled films of LLDPE of this invention have notched Elmendorf tear strengths (ASTM D-1922) of at least 300 (in preferred embodiments at least 350) grams per mil in both the machine and transverse directions, which cannot be 20. said for other rolled polyolefin films. The films of this invention also have a falling dart impact strength (ASTM D-1709) of at least 500 grams per mil when the dart is dropped from a height of 8.75 inches, and at least 160, preferably at least 180, 25. grams per mil when the dart is dropped from a height of 26 inches. The impact strength of conventional LLDPE film is much lower. The compression rolling is preferably carried out with semiboundary lubrication, as taught in 30. U.S.P. 4,085,187, with the preferred lubricant being water. The film is preferably rolled at a temperature of about 15 to 80 °C, more preferably 20 to 55°C. The thickness of the film is preferably reduced between about 50 and 90 OM?I percent. The final thickness of the films of this invention is preferably between about 0.5 and 6 mils, more preferably between one and three mils. In addition to high impact and tear strength, the 5. films of this invention have other excellent properties compared to conventional films, in¬ cluding stiffness, tensile strength, and water vapor transmission rate. Other polymers can be blended or coextruded 10. with the LLDPE to provide compression-rolled films with tailored properties. Such other polymers include other polyolefins, such as HDPE (i.e. polyethylene having a density greater than 0.94 g/cc), PP and copolymers of ethylene or propylene 15. with minor amounts of other ethylenically unsaturated monomers. The proportion of LLDPE blended or coextruded with other polymers is preferably at least 25, more preferably at least 50, percent by weight. 20. In the following examples, which illustrate the invention, "MD" means machine direction; "TD" means transverse direction, "CNT" means cannot tear; "MI" means melt index; "NT" means not tested; "tear strength" means notched El endorf tear 25. strength (ASTM D-1922) measured in grams per mil; and "impact strength" is measured by the dart drop test of ASTM D-1709 in grams per mil, with the dart being dropped from a height of 8.75 or 26 inches. fTTRE- OMPI EXAMPLES 1 -4 AND COMPARATIVE EXAMPLES A-J In Examples 1-4 and F, H and J, cast films consisting of a single polymer, identified in the following Table, were compression rolled at ambient temperature. The ratio of the thickness of the 5. film before rolling to the thickness after rolling is shown in parentheses in the Table. The properties of the compression-rolled films are compared to representative cast and blown unrolled films (Examples A, B, C, D, E, G and I) in the 10. Table. OMPI JL TABLE * Impact Film Thickness Tear Strength Strength Polymer Type Mils MD TD 8.75" 26" Example LLDPE, MI=6, Cast 0.9 370 740 270 90 A density = 0.919 Rolled (4.9) 1.3 480 760 500 185 1 Blown 1.0 130 380 NT 130 B / Rolled (4.1) 1.5 490 CNT NT 160 2 LLDPE, MI=2.5, Cast 1.1 50 870 180 50 C density = 0.935 Rolled (5.0) 1.3 385 485 500 225 3 Blown 1.0 80 340 NT 65 D Rolled (4.3) 1.4 500 630 NT 230 4 HDPE, MI«6, Cast 1.1 16 50 130 NT E density - 0.960 Rolled (5.7) 1.1 40 270 85 NT F PP, MI=6.5, Cast 1.1 33 67 80 NT G density « 0.897 Rolled (3.0) 1.5 10 CNT 60 NT H These examples show that when films of LLDPE are compression rolled, the impact strength of the films is substantially improved, and the tear strengths of the films are over 350 grams per mil 5. in both the machine and transverse directions. In comparison, when films of HDPE and PP are compression roiled, the impact strength of the films is reduced, and their tear strength in the machine direction is very low. Hence, they tend to 10. split, which makes them unsuitable for many applications. When films of LDPE are compression rolled, the tear strength of the films in the transverse direction is reduced to less than about 250 grams per mil. Hence, they tend to tear when 15. they are drawn from the rolling mill. The rolled films are compared in the Table with unrolled films of comparable thickness, rather than with the films before rolling, because the thickness of the films before rolling is so much 20. greater that a comparison with them would not be as meaningful as a comparison with films of comparable thickness. Hence, while a comparison of Examples C and 3 might appear to suggest that rolling might reduce the tear strength of LLDPE in the transverse 25. direction, that is not believed to be the case. The very high tear strength in the transvere direction of the film of Example C is believed to be due to orientation of the film that inherently or inadvertently occurred when the film was 30. extruded. However, such orientation also appar¬ ently caused a sharp reduction in the tear strength of the film in the machine direction, which does not occur with compression rolling of LLDPE film.

Claims: CLAIMS 1. A compression-rolled film of linear low density polyethylene having substantially improved falling dart impact strength, substantially improved tear strength in the machine direction, and a tear strength in the transverse direction of at least 300 grams per mil. 2. The film of claim 1 having an impact strength of at least 180 grams per mil when the dart is dropped from a height of 26 inches. 3. The film of claim 1 having notched Elmendorf tear strength of at least 300 grams per mil in both the machine and transverse directions. 4. The film of claim 2 having notched Elmendorf tear strength of at least 350 grams per mil in both the machine and transverse directions. 5. The film of claim 3 having a thickness between about 0.5 and 6 mils. 6. The film of claim 3 having a thickness between about one and three mils. 7. The film of claim 5 having another polymer coex¬ truded with the polyethylene. 8. The film of claim 5 having another polymer blended with the polyethylene, with the proportion of the linear low density polyethylene being at least fifty percent by weight of the blend. OMPΓ 9. The film of claim 3 wherein the density of the polyethylene is less than about 0.94 g/cc. 10. The film of claim 9 wherein the polyethylene has copolymerized therewith up to ten weight percent of another alpha-olefin. 11. The film of claim 10 wherein the alpha-olefin has from four to eight carbon atoms. QMPI

Inventors: ZUSCIK EDWARD J, ZUSCIK, EDWARD, J.
Applicants: CROWN ZELLERBACH CORP, CROWN ZELLERBACH CORPORATION
Application year: 1984
Main classification: C08J5
IPC codes: B29C55, C08J5