CA2264463A1

CA2264463A1 - Fast clearing polyethylene

Info

Publication number: CA2264463A1
Application number: CA 2264463
Authority: CA
Inventors: P. Scott Chisholm
Original assignee: Nova Chemicals Corp
Current assignee: Nova Chemicals Corp
Priority date: 1999-03-05
Filing date: 1999-03-05
Publication date: 2000-09-05

Abstract

Linear low density polyethylene (LLDPE) which is produced by a gas phase polymerization process is difficult to extrude into plastic film.
This invention mitigates this problem by using a blend of a first LLDPE
(which is prepared in a gas phase polymerization process) with a small amount of a second LLDPE (which is prepared in a specific solution polymerization process) and a processing additive which comprises from 500 to 2000 parts per million of a poly(oxyalkylene) polymer, based on the combined weight of the two LLDPE resins.

Description

FIELD OF THE INVENTION
This invention relates to the extrusion of linear low density polyethylene.
BACKGROUND OF THE INVENTION
Polyethylene was first produced on a commercial basis using a free radical polymerization process that operates at high pressure. The resulting polyethylene (which is commonly referred to by those skilled in the art as "high pressure low density polyethylene", or "LDPE") is comparatively easy to "process" or "extrude". In particular, the preparation of plastic film using a blown film extrusion process is generally regarded as being much easier when using LDPE (than when using other polyethylenes which were developed at a later date).
In particular, the type of polyethylene known as linear low density 2o polyethylene (or "LLDPE") is generally regarded as being difficult to process or extrude.
LLDPE may be produced in a solution, slurry or gas phase polymerization process. The solution process to prepare LLDPE was developed more than 40 years ago. Successful efforts to improve the extrusion of LLDPE prepared by the solution process are reported in the literature. For example, Canadian patent 641,321 (Robinson, assigned to DuPont of Canada) discloses that a polyethylene composition containing a minor amount of very low melt index material (or alternatively stated, a minor amount of very high molecular weight material) improves the extrusion performance of the blend. Canadian patent 830,023 (Elston, assigned to DuPont of Canada) teaches that a solution polymerization M:lScott\PSCSpec\9184can.doc 2 process which uses a tubular reactor operated in a substantially adiabatic manner may be used to prepare the type of polyethylene compositions disclosed in the Robinson '321 patent.
However, most commercially available LLDPE produced by a gas phase polymerization process is difficult to extrude, as described below.
In a typical thermoplastic extrusion process, a molten thermoplastic composition is forced through an extrusion die so as to form an extruded product such as a profile, a pipe, a wire or cable covering, a film product or a plastic tape. In such a process, there is often an erratic "stick/slip"
movement of the polymer along the die surface and/or a velocity profile between the polymer which is at the die surface and the polymer in the bulk melt. In turn, the velocity profile causes shear stresses to occur in the polymer melt. These phenomena (i.e. "stick/slip", velocity profiles and 2o shear stresses) are affected by such factors as the inherent physical properties/rheology of the thermoplastic resin; the material of construction and the geometry of the extrusion die; the extrusion temperature and the extrusion rate (which is often expressed in terms of mass flow rate of polymer per unit surface area of the extrusion die). As might be expected, such stick/slip flow and/or shear stresses can cause imperfections in the extruded product, especially on the surface of the product. These imperfections may manifest themselves in the form of pinholes in plastic film, improperly shaped plastic profiles, extrusions which are too thick or too thin or, most commonly, surface irregularities which are referred to by those skilled in the art as "shark skin"; "melt fracture"; "orange peel"; and "snakeskin".
M:\Scott\PSCSpec\9184can.doc Accordingly, a great of effort has been directed towards the development of "process aids" to improve the extrusion of polyolefins, as briefly described below.
Fluoropolymer process aids have been extensively investigated.
The use of elastomeric fluoropolymers to improve the extrusion of polyethylene is disclosed in United States Patent (USP) 3,125,547 issued 17 March 1964 to E.I. DuPont de Nemours and Company ("DuPont"). The use of a thermoplastic vinylidene fluoride homopolymer is similarly disclosed in USP 4,753,995 (to Mobil). However, fluoropolymers are expensive so efforts have been made to minimize the use thereof.
The use of either an elastomeric, or thermoplastic, fluoropolymer in combination with a poly(oxyalkylene) polymer (such as polyethylene glycol) is disclosed in USP 5,015,693 (to Minnesota Mining and 2o Manufacturing, or "3M"). USP 5,707,569 (to DuPont) teaches the use of an extrusion adjuvant (which may be an ethylene-(meth)-acrylic polymer or an "ionomer" prepared by partial neutralization thereof).
Copending and commonly assigned Canadian patent application 2,233,976 (Oswin et al) teaches the use of a silicone polymer as a process aid.
The use of a process aid does facilitate the extrusion of "gas phase LLDPE". However, it is not uncommon for the process aid to take up to one hour to completely eliminate extrusion defects. In other words, poor quality film may be produced for up to one hour before the process aid is fully effective.
M:\Scott\PSCSpec\9184can.doc 4 Accordingly, there is a commercial incentive to provide a "quick clearing" gas phase LLDPE (where the term "quick clearing" means that the film is substantially free of melt defects in less than 60 minutes).
SUMMARY OF THE INVENTION
The present invention provides a composition for the preparation of extruded polyethylene film, said composition comprising:
1 ) 100 parts by weight of a blend of linear low density polyethylene comprising:
a) 90 to 98 parts by weight of a first linear low density polyethylene which is characterized by being produced in a gas phase polymerization process and, to a total of 100 parts by weight;
b) from 10 to 2 parts by weight of a second linear low density polyethylene having a major component and a minor component wherein 2o i) said major component has a melt index, 12, which exceeds the melt index of said minor component by a factor of at least 103; and ii) said second linear low density polyethylene is produced in a solution polymerization process using a tubular reactor which is operated under substantially adiabatic conditions; and 2) a processing additive which comprises at least 500 parts by million of a poly(oxyalkylene) polymer, based on the weight of said blend.
DETAILED DESCRIPTION
Linear low density polyethylene (LLDPE) is a widely available item of commerce which is prepared by the copolymerization of ethylene with at M:\Scott\PSCSpec\9184can.doc least one other C3 to ~o alpha olefin. LLDPE may be prepared by gas phase, slurry or solution polymerization processes. The use of butene or hexene comonomer is typically preferred for gas phase processes. The use of butene or octene is generally preferred in known solution polymerization processes. The density of the resulting polymers may be from 0.905 to 0.940 grams per cubic centimeter, with a range of from 0.916 to 0.935 being typical.
The composition of this invention must contain:
1 ) a large amount of a first LLDPE produced in a gas phase polymerization process ("gas phase LLDPE");
2) a small amount of a second LLDPE produced in a solution polymerization process using a tubular reactor which is operated under substantially adiabatic conditions; and 20 3) a process aid which contains a poly(oxyalkylene) polymer-such as polyethylene glycol.
The use of the comparatively small amount of the second LLDPE, combined with the process aid, provides a "fast clearing" polyethylene composition which is predominantly gas phase LLDPE.
The compositions may further contain from 5 to 30 parts by weight of LDPE per 100 parts by weight of the LLDPE blend. This is a common practice in the preparation of film from LLDPE and is well known to those skilled in the art.
As used herein, the term "gas phase polymerization process" is meant to convey its conventional meaning - i.e. the well known polymerization process which may be used to prepare LLDPE. The M:\Scott\PSCSpec\9184can.doc process is widely reported in the literature (see for example, United States patents' 4,543,399 and 5,352,749, the disclosures of which are incorporated herein by reference).
The compositions must also contain a minor amount of a second LLDPE which has a minor amount ("minor component") of polyethylene having a melt index, IZ, which is at least 103 greater that the melt index, 12, of the major component of the second polyethylene. An alternative way of describing this second LLDPE is to say that it contains a minor amount of a very high molecular weight polyethylene. Such polymers are described in Canadian patent 641,321 (Robinson), the disclosure of which is incorporated herein by reference. Melt index, 12, is determined by ASTM D
1238 at 190°C using a 2.16 kg load.
The second LLDPE is prepared using a tubular reactor that is operated under substantially adiabatic conditions (as described in Ganac~ian patent 830,023 (Elston), the disclosure of which is incorporated herein by reference).
The term "substantially adiabatic" is meant to convey its conventional meaning, namely that the enthalpy of polymerization is not deliberately removed from the polymerization medium. Thus, the polymerization temperature increases as the polymerization progresses. It is well known to those skilled in the art of solution polymerization that the polymerization temperature affects the molecular weight of the resulting polymer. Thus, the gradient in polymerization temperature produces a broad molecular weight distribution. The temperature gradient should be M:\Scott\PSCSpec\9184can.doc 7 at least 75°C (with an inlet temperature of from 100 to 110°C) to produce the required second LLDPE.
The compositions of this invention must contain at least 500 parts per million (ppm) of poly(oxyalkylene) polymer as described below.
The poly(oxyalkylene) polymers useful in this invention can include poly(oxyalkylene) polyols and their derivatives, and a useful class of poly(oxyalkylene) polymers can be represented by the general formula to A[(OR~)XOR2]y where A is an active hydrogen-free residue of a low molecular weight, initiator organic compound having a plurality of active hydrogen atoms (e.g. 2 or 3), such as a polyhydroxyalkane or polyether polyol, e.g.
ethylene, glycol, glycerol, 1,1,1,-trimethylol propane, and poly(oxypropylene) glycol; y is 2 or 3; the (OR~)x is a poly(oxyalkylene) 2o chain having a plurality of oxyalkylene groups, (OR'), wherein the R~
radicals can be the same or different, and are selected from the group consisting of C1 to C5 alkylene radicals and preferably C2 or C3 alkylene radicals; and x is the number of oxyalkylene units, Said poly(oxyalkylene) chain can be a homopolymer chain, e.g. poly(oxyethylene) or poly(oxypropylene), or can be a chain of randomly distributed (i.e. a heteric mixture) oxyalkylene groups, e.g. a copolymer of -OC2H4- and -OC3H6- units, or can be a chain having alternating blocks or backbone segments of repeating oxyalkylene groups e.g. a polymer comprising (-OC2H4-)a and (-OC3H6-)b blocks, where a + b = x, is about 5 to about 500 and preferably about 10 to 300. RZ is H or an organic radical such as alkyl, aryl or combination M:SScottIPSCSpec\9184can.doc thereof such as aralkyl or alkaryl, and may contain hetero atoms such as O or N. For example, RZ can be methyl, butyl, phenyl, benzyl, and acyl groups such as acetyl (CH3C0-), benzoyl (C6H5C0-) and stearoyl (C 17H35C~-) Representative poly(oxyalkylene) polymer derivatives can include poly(oxyalkylene) polyol derivatives wherein the terminal hydroxy groups have been partly or fully converted to ether derivatives, e.g. methoxy groups or ester derivatives, e.g. stearate groups, (C~7H35COO-). Other useful poly(oxyalkylene) derivatives are polyesters, e.g. prepared from dicarboxylic acids and poly(oxyalkylene) glycols. Preferably, the major proportion of the poly(oxyalkylene) polymer derivative by weight will be the repeating oxyalkylene groups, (OR). Said poly(oxyalkylene) polyols and their derivatives can be liquids or solids at room temperature and have a 2o molecular weight of a least 200 and preferably a molecular weight of about 400 to 20,000 or higher, e.g. 200,000 or more.
Poly(oxyalkylene) polyols useful in this invention include those sold under the trademark Carbowax, such as CarbowaxT~ 3350, H(OC2H4)"OH, where n is about 76, and those sold under the trademark Pluronic e.g.
PluronicTM F-77, H(OCZH4)d[OCH(CH3)CH2]e(OCZH4)f)H, where d + f is about 108, and a is about 35.
Preferred poly(oxyalkylene) polymers are poly(oxyethylene) glycols, often referred to as polyethylene glycols ("PEG"), having a molecular weight of about 1000 to 20,000.
Highly preferred types of PEG have a molecular weight of about 3000 to 8000. A minimum amount of 500 ppm of PEG is generally M:SScott\PSCSpec\9184can.doc required to improve the surface smoothness of the polyolefin extrusions (with from 600 to 1000 ppm being preferred). The use of greater than about 2000 ppm of PEG is not recommended as it may cause "surface bloom" on the polyolefin extrusions.
The processing additive may also optionally contain from 100 to 3000 parts per million of at least one other polymer selected from the group consisting of silicones, ethylene-(meth)-acrylic copolymers, ionomers and fluoropolymers (based on the weight of the two LLDPE
polymers).
The term "ethylene- (meth)-acrylic acid copolymer" refers to polymers which may be produced by the copolymerization of ethylene with a carboxylic acid. So-called "ionomers" may be prepared by (at least partially) neutralizing the acid moiety with one or more cations such as lithium, sodium, potassium, magnesium, calcium, barium, zinc or aluminum. Such ionomers are well known articles of commerce. They are described, for example, in United States patent 3,262,272 (Rees) and are commercially available from DuPont under the trademark SURLYN.
Preferred ethylene-(meth)-acrylic acid copolymers include ethylene/acrylic acid (EA); ethylene/methacrylic acid (EMA) and ionomers prepared by (at least partially) neutralizing EA or EMA polymers. lonomers are highly preferred. Preferred ionomers have a melt index, 12, of from 0.5 to 20.
The silicone polymers which may be used are disclosed in commonly assigned Canadian patent application 2,233,976 (Oswin et al).
The polyofefin compositions used in this invention may also include other conventional plastic additives. A non-limiting list includes anti-block M:\Scott\PSCSpec\9184can.doc 1 agents (such as silica or talc), antioxidants, hindered amine light stabilizers, phosphorus-containing secondary stabilizers, pigments, anti-static agents and slip agents. Commonly used antioxidants include the so-called hindered phenols, such as those sold under the trademark IRGANOX by Ciba and which are typically used in amounts between 100 and 2000 ppm. The preferred compositions of this invention contain from 100 to 1000 ppm of a hindered phenol antioxidant and from 1000 to 2000 ppm of a phosphorus-containing secondary stabilizer. These stabilizers are organophosphorus compounds (such as phosphites and phosphonites) and are well known as polyolefin additives. Hindered amine light stabilizers (which are employed for UV stability in goods intended for outdoor use) which are generally used in amounts of from 1000 to 3000 ppm. These types of stabilizers are well known and widely available items 2 0 of commerce.
The compositions may also include an optional anti-static agent such as glycerol monostearate (GMS) or glycerol mono-oleate (GMO).
The use of a small amount of these materials provides anti-static performance. (Note: the use of at least 500 ppm of poly(oxyalkylene) polymer is essential to this invention. It is known to use a PEG having a molecular weight of less than about 5000 as an anti-static agent (and it may thus make the additional use of GMS or GMO redundant).
The inventive compositions may also include an optional slip agent.
As suggested by the name, "slip agents" are designed to facilitate the flow of the polyolefin melt along the extrusion die. Those skilled in the art do distinguish between the terms "process aid" and "slip agents" with the M:\Scott\PSCSpec19184can.doc 11 latter term being conventionally used to narrowly describe a family of fatty acid amides. The term "slip agent" as used herein is meant to convey the conventional, narrow meaning - i.e. a family of fatty acid amides such as those sold under the trademark KEMAMIDE by Witco.
The compositions of this invention are preferably mixed together by melt blending. This may be done in a single (large) screw extruder.
Alternatively, a "masterbatch" of a small portion of olefin and some of the additives could be premixed. The masterbatch is then fed to the extruder and blended with the remaining polyolefin and/or other additives.
Thermoplastic polyolefins are converted into finished goods using a larger number of fabrication processes - including injection molding, blow molding, rotational molding, compression molding and extrusion. This invention relates to the so-called extrusion process. In a typical extrusion 2o process, an "extruder" machine melts and mixes the polymer composition and forces the polymer melt through an extruder die. The most commonly used extruders are so-called "screw extruders" wherein the rotation of at least one flighted screw within a cylindrical barrel provides the energy to melt and mix the polymer. The extruder may be a "single screw" or twin screw extruder. A twin screw extruder may be operated in a co-rotating mode (i.e. both screws turning in the same direction) or a counter rotating mode (i.e. the screws turn in the opposite direction).
The polymer melt is then forced through a die to continue the extrusion process. As previously noted, the flow of the polymer melt across the die surface may lead to stick/slippage and/or velocity profiles M:\ScottIPSCSpec\9184can.doc 12 (with associated shear stresses) in the melt - and, in turn, imperfections in the extrudate. The problem is particularly acute with gas phase LLDPE.
In a blown film extrusion process, the polymer melt which exits the die is subjected to a flow of air, thereby producing a "bubble" of polyethylene film. The bubble is then slit to produce the film. This process is well known to those skilled in the art.

M:\Scott\PSCSpec\9184can.doc 13

Claims

1. A composition for the preparation of extruded polyethylene film, said composition comprising:
1) 100 parts by weight of a blend of linear low density polyethylene comprising:
a) 90 to 98 parts by weight of a first linear low density polyethylene which is characterized by being produced in a gas phase polymerization process and, to a total of 100 parts by weight, b) from 10 to 2 parts by weight of a second linear low density polyethylene having a major component and a minor component wherein i) said major component has a melt index, 12, which exceeds the melt index of said minor component by a factor of at least 10 3; and ii) said second linear low density polyethylene is produced in a solution polymerization process using a tubular reactor which is operated under substantially adiabatic conditions; and

2) a processing additive which comprises from 500 to 2000 parts per million of a poly(oxyalkylene) polymer, based on the weight of said blend.

2. The composition of claim 1 wherein said first linear low density polyethylene has a density of from 0.916 to 0.935 grams per cubic centimeter.

3. The composition of claim 2 wherein said first linear low density polyethylene is a polymer selected from the group consisting of ethylene-butene copolymers and ethylene-hexene copolymers.

4. The composition of claim 3 wherein said second linear low density polyethylene is selected from the group consisting of ethylene-butene copolymers and ethylene-octene copolymers.

5. The composition of claim 1 which further contains from 5 to 30 parts by weight of high pressure low density polyethylene per 100 parts by weight of said blend.

6. The composition of claim 1 wherein said processing additive contains, in addition to said poly(oxyalkylene) polymer, from 100 to 3000 parts per million of at least one further polymer selected from the group consisting of silicones, ethylene-(meth)-acrylic copolymers, ionomers and fluoropolymers.

7. The composition of claim 6 wherein said fluoropolymers are elastomeric copolymers of vinylidene fluoride and hexafluoropropylene.

8. The composition of claim 1 wherein said poly(oxyalkylene) polymer is polyethylene glycol having a molecular weight of from 3000 to 8000.

9. A process for the preparation of polyethylene film, said process consisting of the extrusion of a composition for the preparation of extruded polyethylene film, said composition comprising:
1) 100 parts by weight of a blend of linear low density polyethylene comprising:
a) 90 to 98 parts by weight of a first linear low density polyethylene which is characterized by being produced in a gas phase polymerization process and, to a total of 100 parts by weight;
b) from 10 to 2 parts by weight of a second linear low density polyethylene having a major component and a minor component wherein i) said major component has a melt index, I2, which exceeds the melt index of said minor component by a factor of at least 10 3; and ii) said second linear low density polyethylene is produced in a solution polymerization process using a tubular reactor which is operated under substantially adiabatic conditions; and 2) a processing additive which comprises from 500 to 2000 parts per million of a poly(oxyalkylene) polymer, based on the weight of said blend.

10. The process of claim 9 wherein said extrusion is undertaken on a blown film extrusion line.