Scorch control in the grafting of diacid anhydrides onto high density polyethylene

A composition comprising: PA1 (i) a homopolymer or copolymer of ethylene and one or more other alpha-olefins wherein the portion of the copolymer based on ethylene is at least about 60 percent by weight based on the weight of the copolymer, said polymers having a density of at least 0.940 gram per cubic centimeter; PA1 (ii) a homopolymer or copolymer of propylene and one or more other alpha-olefins wherein the portion of the copolymer based on propylene is at least about 60 percent by weight based on the weight of the copolymer; and PA1 (iii) an anhydride of an unsaturated aliphatic diacid.

TECHNICAL FIELD 
This invention relates to a composition which is useful in the grafting of 
diacid anhydrides onto high density polyethylene. 
BACKGROUND ART 
Some of the major technological applications for unsaturated aliphatic 
diacid anhydrides, particularly maleic anhydride, grafted polyolefins is 
to form adhesive olefin polymers, compatibilized olefin polymers, and 
reversible crosslinked base resins. Maleic anhydride grafted polyolefins 
also find utility as adhesion promoters because of their compatibility 
with polar copolymers such as ethylene/ethyl acrylate copolymers, 
ethylene/vinyl acetate copolymers, polyesters, and polyamides because of 
the nature of the anhydride. 
Because of its low melt/flow indices and other physical characteristics, 
maleic anhydride grafted high density polyethylene is considered to be 
particularly suited to rotomolding. Unfortunately, the technique for 
grafting maleic anhydride to the high density polyethylene is conducive to 
scorch, i.e., premature crosslinking, and the various methods, which have 
been developed to control scorch, show little benefit for high density 
polyethylene. Further, the scorch control additives tend to decrease the 
modulus of the grafted product while rotomolding applications require the 
retention of high modulus. 
DISCLOSURE OF THE INVENTION 
An object of this invention, therefore, is to provide a composition, which 
is suitable for use in the preparation of a maleic, or similar, anhydride 
grafted high density polyethylene essentially without the attendant 
scorch. 
Other objects and advantages will become apparent hereinafter. 
According to the present invention, the above object is met by a 
composition comprising: 
(i) a homopolymer or copolymer of ethylene and one or more other 
alpha-olefins wherein the portion of the copolymer based on ethylene is at 
least about 60 percent by weight based on the weight of the copolymer, 
said polymers having a density of at least 0.940 gram per cubic 
centimeter; 
(ii) a homopolymer or copolymer of propylene and one or more other 
alpha-olefins wherein the portion of the copolymer based on propylene is 
at least about 60 percent by weight based on the weight of the copolymer; 
and 
(iii) an anhydride of an unsaturated aliphatic diacid.

DETAILED DESCRIPTION 
The homopolymers and copolymers of ethylene having a density of at least 
0.940 gram per cubic centimeter can be prepared by conventional methods 
such as the low pressure process described in Introduction to Polymer 
Chemistry, Stille, published by John Wiley and Sons, New York, 1962, pages 
151 to 153. At least about 60 percent by weight of the copolymer is 
derived from ethylene, the balance being derived from one or more higher 
alpha-olefins, preferably having 3 to 12 carbon atoms. The melt index of 
the polyethylene can be in the range of about 0.1 to about 100 grams per 
10 minutes and is preferably in the range of about 1 to about 20 grams per 
10 minutes. 
The useful homopolymers and copolymers of propylene can also be prepared by 
conventional processes, e.g., see U.S. Pat. No. 4,414,132 issued on Nov. 
8, 1983. The copolymers include at least about 60 percent by weight 
propylene derived moieties with the remainder being derived from one or 
more other alpha-olefins, preferably having 2 or 4 to 12 carbon atoms. The 
density of the polypropylene can be in the range of about 0.890 to about 
0.910 gram per cubic centimeter and the melt index can be in the range of 
about 0.1 to about 100 grams per 10 minutes. 
The alpha-olefin comonomers in either the ethylene or propylene copolymers 
can be present in amounts of about 1 to about 40 percent by weight based 
on the weight of the copolymer, and are preferably present in amounts of 
about 2 to about 25 percent by weight. Useful alpha olefin comonomers are 
exemplified by ethylene, propylene, 1-butene, 1-hexene, 
4-methyl-1-pentene, and 1-octene. 
Anhydrides of unsaturated aliphatic diacids having 4 to 20 carbon atoms, 
and preferably 4 to 10 carbon atoms, are commonly grafted to various 
polyolefins. Examples of anhydrides, which are useful in this invention, 
are maleic anhydride, itaconic anhydride, and nadic anhydride. The 
preferred anhydride is maleic anhydride. Excess anhydride, if present 
after grafting, can be removed by devolatilization at temperatures in the 
range of about 200.degree. C. to about 250.degree. C. 
The grafting is accomplished by using an organic peroxide catalyst, i.e., a 
free radical generator, such as dicumyl peroxide; lauroyl peroxide; 
benzoyl peroxide; tertiary butyl perbenzoate; di(tertiary-butyl) peroxide; 
cumene hydroperoxide; 2,5-dimethyl-2,5-di(t-butyl-peroxy) hexyne-3; 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane; tertiary butyl hydroperoxide; 
isopropyl percarbonate; and 
alpha,alpha'-bis(tertiarybutylperoxy)diisopropylbenzene. The organic 
peroxide catalyst may be added together with the anhydride. 
The proportions of components in the composition based on 100 parts of 
component (i), i.e. the ethylene homopolymer or copolymer component, in 
parts by weight, are as follows: 
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Broad Preferred 
Component Range Range 
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(ii) propylene homopolymer 
about 1 to about 60 
about 5 to about 25 
or copolymer 
(iii) anhydride 
about 0.5 to about 10 
about 1 to about 5 
(iv) organic peroxide 
about 0.1 to about 5 
about 0.25 to about 1 
(v) antioxidant 
about 0.01 to about 1 
about 0.1 to about 0.5 
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Grafting temperatures can be in the range of about 100.degree. to about 
300.degree. C. and are preferably in the range of about 150.degree. to 
about 200.degree. C. 
A typical procedure for grafting maleic anhydride onto polyethylene is 
described in U.S. Pat. No. 4,506,056 issued on Mar. 19, 1985. 
Grafting can also be accomplished by adding a solution of anhydride, an 
organic peroxide catalyst, and an organic solvent to a mixture of 
polyethylene and polypropylene in particulate form. The organic peroxide 
catalyst is soluble in the organic solvent. Various organic solvents, 
which are inert to the reaction, can be used. Examples of useful organic 
solvents are acetone, methyl ethyl ketone, methyl propyl ketone, 
3-pentanone, and other ketones. Other carrier solvents which allow 
solubilization of peroxide and anhydride and which strip off well under 
appropriate devolatilization conditions may be used. Acetone is a 
preferred solvent because it acts as a stripping agent for residuals such 
as non-grafted anhydride or anhydride by-products. 
The anhydride solution can contain about 10 to about 50 percent by weight 
anhydride; about 0.05 to about 5 percent by weight organic peroxide 
catalyst; and about 50 to about 90 percent by weight organic solvent based 
on the total weight of the solution. A preferred solution contains about 
20 to about 40 percent anhydride; about 0.1 to about 2 percent peroxide; 
and about 60 to about 80 percent solvent. 
Conventional additives can also be added to the basic composition. The 
solution of anhydride, peroxide, and organic solvent may be added before 
or after all of the additives have been added. This will depend on the 
nature of the additives and their function. The amount of additive is 
usually in the range of about 0.01 to about 60 percent by weight based on 
the weight of the resin. Useful additives are antioxidants, ultraviolet 
absorbers, antistatic agents, pigments, dyes, fillers, slip agents, fire 
retardants, plasticizers, processing aids, lubricants, stabilizers, smoke 
inhibitors, viscosity control agents, vulcanizing agents, crosslinking 
agents, crosslinking catalysts, and crosslinking boosters. 
The maleic anhydride grafted resin is useful as a reversible crosslinkable 
base resin for rotomolding, pipe crosslink applications, and other molded 
article applications. 
A reversible crosslinkable resin is one which can be crosslinked, for 
example, with a diol at about 20.degree. C., and, at higher temperatures 
of about 150.degree. C., can be un-crosslinked, i.e., the resin is no 
longer crosslinked, becomes thermoplastic, is easily molded, and can be 
crosslinked, once again, at about 20.degree. C. The crosslinking agent can 
be an aliphatic diol, which are exemplified by ethylene glycol, propylene 
glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 
1,4-dimethanolcyclohexane, and 1,4 pentanediol. 
Patents and other publications mentioned in this specification are 
incorporated by reference herein. 
The invention can be illustrated by the following examples. 
EXAMPLE 1 TO 5 
A homopolymer of ethylene having a density of 0.942 grams per cubic 
centimeter and a melt index of 40 grams per 10 minutes and polypropylene 
having a density of 0.900 gram per cubic centimeter and a melt flow of 9.1 
grams per 10 minutes, and being produced from the comonomers propylene and 
1-hexene, are blended with maleic anhydride and antioxidant in a Brabender 
mixer fitted with a 40 gram head. The 1-hexene is present in the 
polypropylene in an amount of 3 percent by weight based on the weight of 
the copolymer. The blend is fluxed and mixed at 160.degree. C. before 
adding dicumyl peroxide. The temperature is then raised to 180.degree. C. 
for five minutes. Material is discharged, pressed, and used for rheometer, 
melt index, and flow index measurement, and then subsequently fluxed at 
150.degree. C. followed by the addition of 2.6 parts by weight 
cyclohexane-1,4-dimethanol (per 100 parts by weight total polymer). The 
product of reaction with the diol is measured by rheometer to determine 
the crosslink level, which is an indicator of the extent of incorporation 
of maleic anhydride in the graft. 
The proportions of components and the results are set forth in the Table. 
Notes to Table: 
1. The rheometer measures melt viscosity in pound-inches. The rheometer 
test is carried out according to the Monsanto Rheometer Test Procedure 
described in U.S. Pat. No. 4,018,852. 
2. Melt Index is determined under ASTM D-1238, Condition E. It is measured 
at 190.degree. C. and reported as grams per 10 minutes (g10 min). 
3. Flow Index is determined under ASTM D-1238, Condition F. It is measured 
at 10 times the weight used in the melt index test above. The value given 
is the average of two flow indices determined for the same composition. 
TABLE 
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(in parts by weight) 
Example 1 2 3 4 5 
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Component 
polyethylene 100 -- 95 85 75 
polypropylene 
-- 100 5 15 25 
maleic anhydride 
4.0 4.0 4.0 4.0 4.0 
antioxidant 0.1 0.1 0.1 0.1 0.1 
dicumyl peroxide 
0.5 0.5 0.5 0.5 0.5 
Results 
rheometer (no diol) 
12 &lt;1 6 4 1 
melt index (g/10 min) 
No Flow -- -- 0.50 0.32 
flow index (g/10 min) 
0.15 -- -- 32 103 
rheometer (plus diol) 
32 &lt;1 27 19 19 
rheometer (plus diol) 
20 0 21 15 18 
minus rheometer 
(no diol) 
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