Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1994-04-28
2001-12-18
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S064000, C525S069000, C525S194000
Reexamination Certificate
active
06331595
ABSTRACT:
The present invention relates to the grafting of monomers onto polyolefins in the presence of organic peroxides, in which the organic peroxide is on a carrier polymer that, under the grafting conditions, undergoes chain scission in preference to cross-linking in the presence of organic peroxides.
Polymers of alpha-olefins in which the alpha-olefin is a hydrocarbon are well known. Such polymers, especially homopolymers of ethylene and copolymers of ethylene with the higher C
4
-C
10
alpha-olefins are used in large volumes for a variety of end-uses. These polymers are relatively non-polar, which is an important and beneficial characteristic for many end-uses. However, non-polar characteristics are also a disadvantage, for instance with respect to adhesion between polar materials and the polyolefins.
Properties of polyolefins may be modified by the grafting of polar monomers onto the polyolefin. Melt grafting processes are described in U.S. Pat. No.4,612,155 of R. A. Zelonka and C. S. Wong, which issued Sep. 16, 1986. In particular, that patent describes a grafting process in which polyolefin in particulate form is fed to an extruder together with grafting agent and organic peroxide, the latter being in the form of a composition with a second polymer of lower melting point and lower melt viscosity.
In melt grafting processes, it is necessary to obtain adequate mixing of the grafting monomer and organic peroxide in the polyolefin prior to significant formation of radicals upon decomposition of the organic peroxide. If the mixing is inadequate, there is a tendency for the polyolefin and/or grafting monomer to undergo cross-linking reactions which form localized cross-linked polymer that becomes apparent as gel, black contaminant particles or other specks in the grafted polymer in the extruder. Both gel and black contaminant particles or other specks are unacceptable in any significant level for many end-uses e.g. in films.
A grafting process that is less susceptible to gel formation and speck formation has now been found.
Accordingly, the present invention provides a method for the grafting of a monomer onto a polyolefin in the presence of an organic peroxide, said polyolefin being a polyolefin that, when molten, undergoes cross-linking in the presence of the organic peroxide, said method comprising:
(a) admixing in an extruder an admixture of (i) said polyolefin, (ii) 25 to 6000 ppm, based on the weight of the polyolefin, of an organic peroxide coated onto a carrier polymer, the amount of organic peroxide coated onto said carrier polymer being at least 0.2% by weight of the carrier polymer, and (iii) up to 5%, by weight of the polyolefin, of a grafting monomer capable of being grafted onto the polyolefin in the presence of the organic peroxide;
(b) heating the admixture to a temperature above the melting point of both the polyolefin and the carrier polymer under admixing conditions to effect grafting of said grafting monomer onto the polyolefin, said carrier polymer undergoing chain scission in preference to cross-linking in the presence of the organic peroxide at said temperature; and
(c) extruding grafted polyolefin from the extruder.
In a preferred embodiment of the method of the present invention, the grafted polyolefin so extruded has a lower level of gel and specks than if the carrier polymer had been a polyolefin that does not undergo chain scission in the presence of the organic peroxide. In particular, the grafted polyolefin has a lower level of gel and specks than obtained when the polyolefin of step (a)(i) is also used as the carrier polymer.
In another embodiment, the melting point of the carrier polymer is higher than the melting point of the polyolefin.
As used herein, it is understood that it may be determined whether a polymer preferentially undergoes chain scission or cross-linking in the presence of an organic peroxide at the temperature of grafting by extruding the polymer at the temperature of grafting in the presence of the organic peroxide and determining whether the molecular weight of the polymer increases or decreases. As disclosed above, the temperature of grafting is above the melting point of both the carrier polymer and the polyolefin that is to be grafted.
The process of the present invention involves feeding to an extruder an admixture of a polyolefin, organic peroxide coated onto a carrier polymer and a grafting monomer. The polyolefin may be a homopolymer of ethylene or copolymer of ethylene or propylene, including copolymers with the higher alpha-olefins e.g. C
4
-C
10
alpha-olefins, examples of which are butene-1, 4-methyl pentene-1, hexene-1 and octene-1. In addition, the polyolefin may be a copolymer of ethylene with one or more other ethylenically-unsaturated monomers that are polar in nature e.g. vinyl esters of carboxylic acids, vinyl halides and unsaturated carboxylic acids or esters thereof. Specific examples include copolymers of ethylene with at least one of acrylic acid, methacrylic acid, carbon monoxide, methyl acrylate, butyl acrylate, methyl hydrogen maleate and vinyl acetate. In addition, the polyolefin may be an ionomer e.g. a sodium, zinc or aluminum ionomer of an acid copolymer formed from ethylene and an ethylenically unsaturated carboxylic acid. Examples of the above polymers are available from Du Pont Canada Inc. or E.I. du Pont de Nemours and Company under the trade marks Sclair®, Elvax®, Nucrel® or Surlyn®, depending on the particular polymer.
The organic peroxide used in the process of the present invention has a half-life at 150° C. of from about one minute to about 120 minutes. The organic peroxide, which as used herein includes hydroperoxides, may for example be a peroxy ester, peroxy ketal, bis (tert.-alkyl peroxy alkyl) benzene, dicumyl peroxide or acetylenic diperoxy compound. Other organic peroxides are known to those skilled in the art, including t-butyl hydroperoxide and di-t-butyl peroxide. Preferred organic peroxides are 2,5-dimethyl-2,5-di(t-butyl peroxy) hexane and 2,5-dimethyl-2,5-di(t-butyl peroxy)hexyne-3 which are available under the trade marks Lupersol 101 and 130, respectively, from Elf Atochem. The organic peroxide is coated onto a polymer in an amount of at least 0.2% by weight, which may result in absorption into the polymer, such that the polymer acts as a carrier for the organic peroxide. The carrier polymer is a polymer that, under the grafting conditions, undergoes chain scission in preference to cross-linking in the presence of an organic peroxide. In preferred embodiments, the carrier polymer also has a melting point that is higher than the melting point of the polyolefin, although the melting point of the carrier polymer should not be unreasonably higher because both the polyolefin and carrier polymer need to become molten during the extrusion process, without excessive heating especially excessive heating of the polyolefin or carrier polymer above its melting point. Examples of the carrier polymer are polypropylene, copolymers of propylene with ethylene or other C
4
-C
10
alpha-olefin, poly-1-butene, copolymers of 1-butene with minor amounts of ethylene or other C
3
-C
10
alpha-olefin, polystyrene, and block copolymers of styrene with a C
4
-C
8
. diene.
The grafting monomer may be ethylenically unsaturated carboxylic acids and ethylenically unsaturated carboxylic acid anhydrides, including derivatives of such acids, and mixtures thereof, and vinyl trialkoxy silanes. Examples of the acids and anhydrides, which may be mono, di- or polycarboxylic acids, are acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, itaconic anhydride, maleic anhydride and substituted maleic anhydride e.g. dimethyl maleic anhydride or citraconic anhydride, nadic anhydride, nadic methyl anhydride and tetrahydro phthalic anhydride. Examples of derivatives of the unsaturated acids are salts, imides, amides and esters e.g. mono- and disodium maleate, acrylamide, maleimide, glycidyl methacrylate and diethyl fumarate. Examples of the vinyl trialkoxy silanes are vinyl trimethoxy silane
Kelly Peter Yates
Mitchell David John
Asinovsky Olga
Dupont Canada Inc.
Seidleck James J.
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