EP0131617A1 - Films of linear low density polyethylene having improved impact strength - Google Patents

Films of linear low density polyethylene having improved impact strength

Info

Publication number
EP0131617A1
EP0131617A1 EP19840900656 EP84900656A EP0131617A1 EP 0131617 A1 EP0131617 A1 EP 0131617A1 EP 19840900656 EP19840900656 EP 19840900656 EP 84900656 A EP84900656 A EP 84900656A EP 0131617 A1 EP0131617 A1 EP 0131617A1
Authority
EP
European Patent Office
Prior art keywords
film
films
polyethylene
impact strength
grams per
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19840900656
Other languages
German (de)
French (fr)
Inventor
Edward J. Zuscik
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
James River Corp of Nevada
Original Assignee
Crown Zellerbach Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Crown Zellerbach Corp filed Critical Crown Zellerbach Corp
Publication of EP0131617A1 publication Critical patent/EP0131617A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/18—Manufacture of films or sheets
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor
    • B29C55/02—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets
    • B29C55/18—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets by squeezing between surfaces, e.g. rollers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C08J2323/06—Polyethene

Definitions

  • 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 chain branching.
  • 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 referred to as high pressure polyethylene because it is produced at high pressures.
  • LLDPE is produced commercially at low pressures in a gas phase process. However, LLDPE may also be produced in a liquid phase solution process.
  • 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 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 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.
  • compression rolling 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 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 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.
  • This invention is based on the discovery that compression rolling of films of LLDPE substantially improves the impact strength of the films and 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.
  • 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 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, grams per mil when the dart is dropped from a height of 26 inches.
  • ASTM D-1709 falling dart impact strength
  • the impact strength of conventional LLDPE film is much lower.
  • the compression rolling is preferably carried out with semiboundary lubrication, as taught in 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 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.
  • the films of this invention have other excellent properties compared to conventional films, including stiffness, tensile strength, and water vapor transmission rate.
  • LLDPE low density polyethylene
  • PP polypropylene
  • copolymers of ethylene or propylene with minor amounts of other ethylenically unsaturated monomers are also useful.
  • the proportion of LLDPE blended or coextruded with other polymers is preferably at least 25, more preferably at least 50, percent by weight.
  • 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 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 Table.
  • 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 greater that a comparison with them would not be as meaningful as a comparison with films of comparable thickness.
  • rolling might reduce the tear strength of LLDPE in the transverse 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 extruded. However, such orientation also apparently 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.

Landscapes

  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Manufacture Of Macromolecular Shaped Articles (AREA)

Abstract

Films en polyéthylène à basse densité linéaire laminés par compression possèdant une résistance élevée à l'impact et une résistance élevée au déchirement aussi bien dans le sens de la machine que dans le sens transversal.Linear low density polyethylene films laminated by compression with high impact resistance and high tear resistance both in the machine direction and in the transverse direction.

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 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 referred 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. 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 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 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 "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 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 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. 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 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 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 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 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 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 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 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 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, 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 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 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 films of this invention have other excellent properties compared to conventional films, including stiffness, tensile strength, and water vapor transmission rate.
Other polymers can be blended or coextruded 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 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. 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 Elmendorf tear 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. 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 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 Table.
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 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 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 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 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 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 extruded. However, such orientation also apparently 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

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 coextruded 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.
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.
EP19840900656 1983-01-10 1984-01-03 Films of linear low density polyethylene having improved impact strength Withdrawn EP0131617A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US45667883A 1983-01-10 1983-01-10
US456678 1983-01-10

Publications (1)

Publication Number Publication Date
EP0131617A1 true EP0131617A1 (en) 1985-01-23

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ID=23813707

Family Applications (1)

Application Number Title Priority Date Filing Date
EP19840900656 Withdrawn EP0131617A1 (en) 1983-01-10 1984-01-03 Films of linear low density polyethylene having improved impact strength

Country Status (2)

Country Link
EP (1) EP0131617A1 (en)
WO (1) WO1984002707A1 (en)

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB8914703D0 (en) * 1989-06-27 1989-08-16 Dow Europ Sa Bioriented film
AU6429599A (en) * 1998-10-16 2000-05-08 Exxon Chemical Patents Inc. Polyolefin microporous breathable film having improved tear, impact strength, and softness and method of making same
EP1927617A1 (en) * 2001-07-19 2008-06-04 Univation Technologies, LLC Polyethylene films with improved physical properties.
US6936675B2 (en) 2001-07-19 2005-08-30 Univation Technologies, Llc High tear films from hafnocene catalyzed polyethylenes
JP4221363B2 (en) * 2001-07-19 2009-02-12 ユニベーション・テクノロジーズ・エルエルシー Polyethylene film with improved physical properties

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1134826B (en) * 1959-06-25 1962-08-16 Huels Chemische Werke Ag Manufacture of polyethylene films
JPS532668B2 (en) * 1974-04-30 1978-01-30
US4085187A (en) * 1975-03-27 1978-04-18 Revere Copper And Brass, Inc. Process for compression rolling of polymeric films
JPS6011925B2 (en) * 1978-10-18 1985-03-29 三井化学株式会社 Method for producing ethylene copolymer
US4363904A (en) * 1979-06-18 1982-12-14 Union Carbide Corporation High tear strength polymers
SE422070B (en) * 1980-09-04 1982-02-15 Unifos Kemi Ab DRAWN POLYETENE BASED THERMOPLASTIC MATERIAL AND PROCEDURE FOR ITS MANUFACTURING

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8402707A1 *

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Publication number Publication date
WO1984002707A1 (en) 1984-07-19

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Inventor name: ZUSCIK, EDWARD, J.