CA2233976A1

CA2233976A1 - Thermoplastic extrusion process

Info

Publication number: CA2233976A1
Application number: CA 2233976
Authority: CA
Inventors: Douglas Checknita; P. Scott Chisholm; Tony Tikuisis; Robert Edward Oswin
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-04-03
Filing date: 1998-04-03
Publication date: 1999-10-03

Abstract

Thermoplastic polyolefin compositions used to prepare extruded goods contain an additive package which includes a silicone process aid, at least 1000 parts per million of a particulate metal oxide and a polyethylene glycol. Exemplary compositions contain linear low density polyethylene as the predominant polyolefin and silica as the predominant particulate metal oxide. The performance of the silicone process aid in the presence of at least 1000 parts per million of silica is enhanced by the polyethylene glycol. Extruded goods such as profiles, films and tapes may be prepared according to this invention.

Description

FIELD OF THE INVENTION
This invention generally relates to the use of a silicone polymer as a process aid in thermoplastic extrusions. The invention further relates to an improved method for extruding thermoplastics which contain metal oxides using a silicone process aid.
BACKGROUND OF THE INVENTION
to 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 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, 3 o 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 \\NRTC-NT\MAW SONJ$\SCOTT1PSCSped9150can.doc those skilled in the art as "shark skin"; "melt fracture"; "orange peel"; and "snakeskin".
Polyolefins such as polypropylene and polyethylene are amongst the most common thermoplastics used in extrusion processes.
Accordingly, a great of effort has been directed towards the development of "process aids" to improve the extrusion of polyolefins, as briefly to described below.
Fluoropolymer process aids have been extensively investigated.
The use of elastomeric fluoropolymers to improve the extrusion of polyethylene is disclosed in United States Patent (USP) 3) 125,547 issued 17 March 1964 to E.I. DuPont de Nemours and Company ("DuPont"). The use of a thermoplastic vinylidene fluoride homopolymer is similarly disclosed in USP 4,753,995 (to Mobil). The use of either an elastomeric, or thermoplastic, fluoropolymer in combination with a polyalkylene polymer is disclosed in USP 5,015,693 (to Minnesota Mining and Manufacturing).
The use of a combination of an elastomeric fluoropolymer and a thermoplastic fluoropolymer is disclosed in USP 5,106,911 to DuPont.
The use of acrylic polymers as process aids for high density polyethylene ("HDPE") is disclosed in USP 4,963,622 to Union Carbide 3 o Corporation. Interestingly, this acrylic processing aid provides only a marginal improvement in the extrusion of linear low density polyethylene ("LLDPE") having a narrow molecular weight distribution.
The mechanism by which fluoropolymers function as process aids is still the subject of some debate. However, one generally accepted mechanism is that the fluoropolymers "phase separates" from the \\NRTC-NT\MAWSONJ$\SCOTT\PSCSpec\9150can.doc polyolefin melt and forms a coating on the metal die. This fluoropolymer coating improves extrusions by reducing the friction between the surface of the metal die and the polyolefin melt flowing across it.
This mechanism is consistent with an undesirable operating characteristic known as "die lip build up" or "plate out" (i.e. an accumulation of excess fluoropolymer on the die surface). This problem is 1o pa~icularly acute when the composition which is being extruded contains a finely divided particulate metal oxide, such as silica, having a median particle size of less than about five microns. Whilst not wishing to be bound by any particular theory, it is believed that this problem may result from an interaction between acidic groups in the fluoroelastomer (such as residual carboxylic acids from the emulsifier used in the synthesis of the fluoroelastomer) and basic groups in the particulate metal oxide additives (such as the hydroxyl groups contained in silica).
Thus, a need exists for process aids which function well in the presence of large amounts of metal oxide.
Silicone polymers have also been proposed for use as process aids. For example, USP 5,403,891, 5,356,585, and 5,708,085 to Dow Corning disclose the use of silicone polymers to improve the extrusion of 3 o polyethylene. These patents also disclose the use of an "absorbing filler"
(which may be silica). The '585 patent further teaches that the purpose of the silica is to minimize "slippage" of the extrusion screw. This is an interesting teaching because it suggests that silica may improve the effectiveness of silicone process aids in the extrusion of silica-filled M:\SCOTT\PSCSpec\9150can.doc 4 polyolefin compositions. However, our experiments show that high loadings of silica detract from the performance of silicone process aids.
Accordingly, there is still a need for a process aid which functions well in the presence of large amounts of finely divided metal oxide additives.
SUMMARY OF THE INVENTION
to In one embodiment the present invention provides a method for preparing extruded goods by the extrusion of a thermoplastic polyolefin composition which comprises a major portion of thermoplastic polyolefin) from 1000 to 7500 parts per million of a particulate metal oxide, from 100 to 2000 parts per million of a silicone process aid and from 400 to 2000 parts per million of polyethylene glycol, with the proviso that the weight of polyethylene glycol exceeds the weight of the silicone process aid.
In another embodiment the invention provides an additive package for a thermoplastic polyolefin, wherein the additive package contains a silicone process aid; polyethylene glycol and silica with the provisos that (a) the weight ratio of said silicone process aid to said polyethylene glycol is from 1 /1 to 1 /7; and (b) the combined weight of the silicone process aid and the polyethylene glycol is less than the weight of the silica.
3o DETAILED DESCRIPTION
This invention is generally directed towards an improvement in the extrusion of thermoplastic polyolefin compounds which contain a high level of an inorganic metal oxide or hydroxide additive. The invention requires the use of a silicone process aid and polyethylene glycol.
\\NRTC-NT\MAWSONJ$\SCOT11PSCSpec\9150can.doc 5 The term "thermoplastic polyolefin" is meant to convey its broad, conventional meaning and to encompass a wide variety of plastics which are in general commercial use. The two most common thermoplastic polyolefins are polypropylene and polyethylene. Although this invention is suitable for use with all thermoplastic polyolefins, it is most suited for those polymers which are comparatively difficult to extrude - such as 1o thermoplastic copolymers of ethylene with at least one other alpha olefin.
The family of ethylene copolymers referred to as linear low density polyethylene (or LLDPE) is known to be difficult to process or extrude.
Commercially available LLDPE is a copolymer of ethylene with at least one alpha olefin selected from butene, pentene, hexene or octene and is characterized by having a density of less than 0.940 grams per cubic centimeter. LLDPEs which are often most difficult to process are those which have a narrow molecular weight distribution and a low melt index.
The molecular weight distribution of a polymer is calculated by dividing the weight average molecular weight ("Mw") by the number average molecular weight ("Mn"). As used herein, "narrow molecular weight distribution"
refers to a Mw/Mn value of less than 10. "Melt index" (also known as "12") is determined by ASTM standard test D1238 at 190° (using a 2.16 kg load). A "low melt index" polyolefin has an 12 of less than 4.
LLDPEs having a molecular weight distribution of less than 10 and/or an 12 of less than 4 are sufficiently difficult to process that they are often used in blends with other polymers which are easier to process [such as polyethylene produced by a free radical polymerization process, or the so-called "high pressure low density polyethylene"]. The use of M:\SCOT11PSCSpec\9150can.doc polyolefin blends, especially blends which are predominantly LLDPE with a minor amount of other easier-to-process polyolefins, is also part of this invention. A common blend is a combination of up to 30 weight % of high pressure low density polyethylene with LLDPE.
LLDPEs meeting the criteria of low melt index and narrow molecular weight distribution may be readily prepared using conventional to Ziegler-Natta catalysts or so-called metallocene catalysts in gas phase or solution polymerization processes. These catalysts and polymerization processes are widely reported in the open and patent literature and further description of them is not considered to be necessary to provide a full disclosure of this invention. However) it should be noted that the so-called "Phillips" polymerization process (which is reported to employ a supported chromium catalyst under slurry polymerization conditions) is generally regarded as not being suitable for the commercial production of LLDPE
having a narrow molecular weight distribution.
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 3 o 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 \\NRTC-NT UvIAW SONJ$\SCOT11PSCSpec\9150can.doc 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 (with associated shear stresses) in the melt - and, in turn, imperfections in so the extrudate. This problem may be accentuated by the presence of certain additives in the polymer melt - particularly high levels of metal oxides. Exemplary metal oxides include silica; talc; zinc oxide and titanium oxides. Silica and talc are used as fillers or antiblock agents in some extrusion compositions. Zinc oxide and titanium dioxide may be used as acid scavengers (i.e. to mitigate problems caused by the decomposition of catalyst residues from the polymerization process) or colorants for the compound. Small amounts of these metal oxides (i.e.
less than 200-300 ppm) usually do not cause significant difficulties with extrusion processes. However, the use of greater than 1000 ppm often does, particularly when small particle size silica (i.e. median particle size from 0.5 to 5 microns) or small particle size talc is used in amounts of from 1000 ppm to 5000 ppm as an anti-block. Typical compositions according 3o to this invention may contain more than one type of metal oxide -especially a mixture of zinc oxide and silica, with the silica being the "predominant" metal oxide (i.e. the major portion of the metal oxide is silica). The polyolefin compositions used in this invention may also include other conventional plastic additives. A non-limiting list includes antioxidants, stabilizers, pigments, anti-static agents and slip agents.
\\NRTC-NT\MAWSONJ$\SCOTT\PSCSpec\9150can.doc Commonly used antioxidants include the so-called hindered phenols, such as those sold under the trademark IRGANOX by Ciba and which are typically used in amounts between 100 and 2000 ppm. The term "stabilizers" includes organophosphorus compounds (including phosphites and phosphonites) which are typically used in amounts between 500 and 2000 ppm and the hindered amine light stabilizers (which are employed to for UV stability in goods intended for outdoor use) which are generally used in amounts of from 1000 to 3000 ppm. Both of these types of stabilizers are well known and widely available items of commerce.
The compositions may also include an optional anti-static agent such as glycerol monostearate (GMS) or glycerol mono-oleate (GMO).
The use of a small amount of these materials provides anti-static performance.
The use of at least 400 ppm of polyethylene glycol (PEG) is essential to this invention. PEG is a commercially available polymer that is sold, for example, under the trademark CARBOWAX by Union Carbide Corporation. PEG is available is molecular weights of from about 400 to more than 20,000. Preferred PEGs have a molecular weight of from about 3000 to about 8000. It is known to use a PEG having a molecular 3 o weight of less than about 5000 as an anti-static agent (and it may thus make the additional use of GMS or GMO redundant). The use of less than 400 ppm of PEG may not be effective to completely mitigate the deleterious effect of the silica. The use of greater than about 2000 ppm of PEG is not recommended because difficulties in mixing the polar PEG
with the non-polar polyolefin may arise.
\\NFTC-NT\MAW SONJ$\SCOTT\PSCSpec\9150can.doc 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 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 1o 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 must include a silicone process aid. A "silicone" is a polymer which contains repeating Si - oxygen bonds in the backbone with substituents (usually organic substituents) on the Si atoms.
Silicones are well known articles of commerce and are extensively described in the open literature (see, for example) Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 22, pp. 82-141 ). The most common silicones are polydimethyl silicones ("PDMS").
The term PDMS-type silicones includes polymers which may be differentiated on the basis of molecular weight and/or the presence of minor amounts of substituents. PDMS polymers having ultra high 3 o molecular weight ("UHMW") have recently been described as additives for a variety of plastics ("UHMW" silicone is defined as a silicone having a viscosity of from 10 to 50 million mm2/s (centistokes)). Lower viscosity PDMS - especially those having a viscosity of from 10,000 mm2/s to 60,000 mm2/s - have been used as mold release agents for more than 25 years. Trade literature from Dow Corning (a manufacturer of PDMS) \\NRTC-NT\MAWSONJ$\SCOTTPSCSpec\9150can.doc 1 shows that UHMW PDMS can be mixed ("masterbatched)" with polyolefins to facilitate the addition of silicones to polyolefins.
A silicone process aid improves the extrusion of polyolefins.
Specific examples of known silicone process aids are disclosed in USP
5,356,585 and USP 5,403,891 (both to Dow Corning Corporation). The '585 patent also teaches that the use of a minor amount of an "absorbing to filler" (which may be silica) can improve the performance of the silicone process aid. However our data illustrate the deleterious effect of a large amount of silica on the effectiveness of the process aid.
The silicone process aid is used in an amount between 100 and 2000 ppm. As silicones are relatively expensive, there is an economic incentive to minimize the usage thereof whilst maintaining satisfactory extrudate appearance. This will generally require some routine, non-inventive optimization work. A starting point of 500 ppm silicone process aid is suggested.
The experimental data in the accompanying examples show that 1000 ppm of silicone is a very effective process aid for polyethylene extrusions which do not contain large amounts of metal oxide additives.
The data further show that the additional use of polyethylene glycol in 3 o such extrusions did not significantly improve the effectiveness of the process aid. The addition of 3000 ppm of silica destroyed the effectiveness of the silicone process aid (and 750 ppm of polyethylene glycol made no discernable improvement). However, a very good result was obtained when the amount of silicone process aid was reduced to 500 ppm. This is an interesting and unusual result (i.e. 500 parts per million of \\NRTC-NT\MAWSONJ$\SCOTT\PSCSpec\9150can.doc 11 silicone was effective but 1000 ppm was ineffective). Whilst not wishing to be bound by any theory, it is possible to postulate that the PEG facilitates the transport of the silicone through the bulk polyethylene melt to the extrusion die surface (and that an excess of PEG over silicone is required for this transport mechanism to occur).
The compositions of this invention are preferably mixed together by to 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.
Further details are illustrated in the following non-limiting examples.
EXAMPLES
Polyethylene compounds having the formulations shown in Table 1 were prepared in a laboratory scale twin screw extruder operating in counter-rotating mode at 175°C and 100 revolutions per minute. Brief descriptions of each of the additives are provided as footnotes to the Table. The polyethylene resin used was an LLDPE having a density of 0.917 g/cc and a melt index ("12") of 0.9.
The compounds were then extruded at 200°C through a capillary 3 o rheometry (brand name "Kayness Galaxy V") having a die diameter of 0.030" and a length/diameter ratio of 20/1. The shear rate through the die was increased until either a shear rate of at least 500 S-' or until surface roughness was observed on the monofilament extrudate. The shear rate (reported in reciprocal seconds) at which roughness is first observed is called the onset of melt fracture (or "OSMF") M:\SCOTI1PSCSpec\9150can.doc 12 Table 1 ExperimentA/O-1'A/O-22SilicaSIip3 SPA-14SPA-25 PEGS OSMF
~Ppm)~PPm) ~pPm)~PPm) ~PPm) ~PPm) ~PPm)CS
~) 6 500 1500 3150 100 500 0 750 >500 Footnotes:
1. A hindered phenol sold under the trademark IRGANOX 1076 by Ciba.
2. Tris-nonyphenyl phosphate.
3. A fatty acid amide sold under the trademark KEMAMIDE E-Ultra.
4. Ultra high molecular weight silicone process aid (PDMS, viscosity of from 10 to 50 million mm2/s) sold by Dow Corning.
5. Silicone process aid sold by Dow Corning (PDMS, viscosity of less than 100,000 mm2/s).
6. Polyethylene glycol (sold under the trademark CARBOWAX 3350 3 o by Union Carbide).
M:\SCOTT\PSCSpec\91 SOcan.doc 13

Claims

1. A method for preparing extruded goods by the extrusion of a thermoplastic polyolefin composition which comprises a major portion of thermoplastic polyolefin, from 1000 to 7500 parts per million of a particulate metal oxide, from 100 to 2000 parts per million of a silicone process aid and from 400 to 2000 parts per million of polyethylene glycol, with the proviso that the weight of said polyethylene glycol exceeds the weight of said silicone process aid.

2. The method of claim 1 wherein said particulate metal oxide is predominantly silica.

3. The method of claim 2 wherein said silica has a median particle size of from 0.5 to 5 microns.

4. The method of claim 1 wherein said polyethylene glycol has a molecular weight of from 3000 to 10,000.

5. The method of claim 1 wherein thermoplastic polyolefin is predominantly linear low density polyethylene.

6. The method of claim 5 wherein said linear low density polyethylene is a copolymer of ethylene and at least one alpha olefin selected from butene, hexene and octene.

7. The method of claim 6 wherein said linear low density polyethylene has a melt index of less than or equal to 4.

8. The method of claim 6 wherein said composition further contains an antioxidant and a secondary stabilizer.

9. The method of claim 6 wherein said antioxidant consists of from 100 to 500 parts per million of a hindered phenol antioxidant and said secondary stabilizer consists of from 500 to 2000 parts per million of an organophosphorus compound selected from a phosphite and a phosphonite.

10. The method of claim 1 wherein said silicone process aid has a viscosity of from 10 to 50 million mm2/s.

11. An additive package for an extrudable polyolefin composition comprising (1) a silicone process aid; (2) polyethylene glycol; and (3) at least 1000 parts per million of a particulate metal oxide characterized in that:
(A) the weight ratio of said silicone process aid to said polyethylene glycol is from 1/1 to 1/7; and (B) the combined weight of said silicone process aid plus said polyethylene glycol is less than the weight of said antiblock agent.

12. The additive package of claim 11 wherein said particulate metal oxide is silica.

13. The additive package of claim 12 wherein said silica has a median particle size of from 0.5 to 3 microns.

14. The additive package of claim 13 wherein said polyolefin is linear low density polyethylene.

15. The additive package of claim 11 wherein said silicone process aid has a viscosity of from 10 to 50 million mm2/s.

16. A method for preparing thermoplastic extrusions by the extrusion of a polyethylene composition comprising a majority of linear low density polyethylene from 100 to 2,000 parts per million of a silicone process aid, from 300 to 2,000 parts of polyethylene glycol having a weight average molecular weight of from 500 to 30,000 and from 1000 to 7500 parts per million of silica, with the further provisos that:
(A) the weight ratio of said silicone process aid to said polyethylene glycol is from 1/1 to 1/7; and (B) the combined weight of said silicone process aid plus said polyethylene glycol is less than the weight of said silica.

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