The present invention relates to a composition comprising crosslinkable polyethylene homopolymer, copolymer or mixture thereof. The composition includes a peroxide crosslinking agent, a crosslinking co-agent, and a metal compound having a cation selected from Group IIA and IIB of the Periodic Table of Elements. The composition is particularly useful for polymers which are formed using Ziegler-type catalysts and have complete terminal saturation. The present invention also includes a composition which has been found particularly suitable for use in critical molding processes such as rotational molding. This composition comprises a polyethylene homopolymer, copolymer, or mixture thereof. The present invention includes a method of rotational molding.

BACKGROUND OF THE INVENTION 
The present invention is in the field of crosslinkable polyethylenes; more 
particularly the present invention relates to a crosslinkable polyethylene 
composition and a method of rotational molding using the crosslinkable 
polyethylene composition. 
Rotational molding, also known as rotomolding, is used in the manufacture 
of hollow objects from thermoplastics. In the basic process of rotational 
molding, polymer is placed in a mold. The mold is first heated and then 
cooled while being rotated. The mold can be rotated uniaxially or 
biaxially and is usually rotated about two perpendicular axes 
simultaneously. The mold is heated externally and then cooled while being 
rotated. Included in polymers useful in rotational molding are a variety 
of polyolefins such as polyethylene including crosslinkable polyethylene. 
A general discussion of rotational molding is given in "Modern Plastics 
Encyclopedia 1979-1980", Vol. 56, No. 10A, beginning at page 381. 
Rotational molding has a feature such that it can result in hollow 
articles which are, as compared with those obtained by blow molding 
methods, complicated, large in size and uniform in wall thickness, and 
further the material loss is minor. 
Compositions of interest relating to crosslinking polyethylene are 
disclosed in U.S. Pat. Nos. 3,372,139; 3,806,555; 3,876,613; 3,974,132; 
4,018,852; 4,028,332; and 4,267,080. Crosslinkable polyethylene rotational 
molding compositions are disclosed in U.S. Pat. Nos. 3,876,613 and 
4,267,080. 
Polyethylene is commonly polymerized using two types of catalyst systems. 
The first is a chromium based system which results in a polymer having 
terminal unsaturation. The second type of catalyst is a Ziegler-type 
catalyst which results in the polymer having terminal groups which have 
substantially complete terminal saturation. Additionally, polymers formed 
with the Ziegler-type catalysts have been found to contain residue acidic 
compounds, most commonly chlorides. It has been found that crosslinkable 
polyethylene formed with use of the Ziegler-type catalysts cannot be used 
in critical molding operations, such as rotational molding. Articles 
rotationally molded from crosslinkable polyethylene made with a 
Ziegler-type catalyst exhibit satisfactory surface appearance, mold 
release, and impact properties when molded using methods disclosed in the 
art such as in U.S. Pat. Nos. 3,876,613 and 4,267,080. 
SUMMARY OF THE INVENTION 
The present invention relates to a composition comprising crosslinkable 
polyethylene homopolymer, copolymer or mixture thereof. The composition 
includes a peroxide crosslinking agent, a crosslinking co-agent, and a 
metal compound having a cation selected from Group IIA and IIB of the 
Periodic Table of Elements. The composition is particularly useful for 
polymers which are formed using Ziegler-type catalysts and have 
substantially complete terminal saturation. 
The present invention also includes a composition which has been found 
particularly suitable for use in critical molding processes such as 
rotational molding. This composition comprises a polyethylene homopolymer, 
copolymer, or mixture thereof. The polymers are formed using Ziegler-type 
catalysts and have substantially complete terminal saturation. The 
composition includes a peroxide crosslinking agent, and an allyl 
carboxylate crosslinking co-agent. 
The composition of the present invention has been found particularly useful 
in molding operations and most particularly in rotational molding. The 
present invention includes a method of rotational molding using the 
above-described compositions. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is a crosslinkable polymer composition where the 
polymer is selected from the group consisting of ethylene homopolymers and 
copolymers, and mixture thereof. The composition includes an organic 
peroxide crosslinking agent, a crosslinking co-agent, and preferably a 
metal cation selected from Group II of the Periodic Table of Elements. 
The ethylene polymer used may be either a polyethylene such as high 
density, medium density or low density polyethylene, or an ethylene 
copolymer comprised of at least 85% by weight of ethylene and not more 
than 15% by weight of a C.sub.3 to C.sub.10 .alpha.-olefin. The ethylene 
polymers may be used alone or in combination. The ethylene polymers should 
be of a high fluidity, i.e., possess a melt index (as determined according 
to ASTM D-1238, condition E: hereinafter referred to as "MI" for brevity) 
of at least 2 g/10 min., preferably from 5 to 200 and more preferably from 
5 to 40 g/10 min. Preferably the ethylene polymers used in the invention 
have a melt index of at least about 10 g/10 min., and a density in the 
range of 0.920-0.970, preferably 0.940-0.965. 
The crosslinkable composition of the present invention is particularly 
useful when the ethylene polymer is produced by methods using a Ziegler 
catalyst such as those described in U.S. Pat. Nos. 3,070,549 and 3,901,744 
which result in a polymer having substantially colmplete terminal 
saturation. 
A review of the Ziegler catalyst and the mechanism in polymerization is 
found in Billmeyer, Textbook of Polymer Science, Second Edition, 
Wiley-Interscience, beginning a page 319 (1971). Here Ziegler-type 
catalysts are described as complexes formed by the interaction of alkyls 
of metals Group I-III in the Periodic Table with halides and other 
derivatives of transition metals of Group IV and Group VIII. A commonly 
used Ziegler-type catalyst is a complex between an aluminum alkyl and 
titanium halide where the halide is chloride. Postulated structures are 
shown in the Billmeyer test. When such a catalyst is used, typically 
chloride compounds remain in the ethylene polymer matrix after 
polymerization. It has been found that, as a result of this, residue 
resulting from Ziegler type catalysts, ethylene polymers which are 
crosslinked contain bubbles and non-homogenities. The gel content is not 
as high; and when molding, the crosslinkable ethylene polymers warp and do 
not have good mold release. 
There is preferably, a sufficient amount of a metal compound, containing a 
metal cation from Group IIA or IIB of the Periodic Table to substantially 
neutralize any acidic compound such as acidic catalyst residue or other 
acidic elements in the polymer matrix. Typically there are at least 0.005 
parts, more preferably 0.05 to 0.5 parts and more preferably 0.05 to 0.15 
parts of a metal compound which provides the metal cation. Adding the 
basic compound, preferably a metal compound containing a Group IIA or IIB 
metal cation, results in a composition which has good color stability upon 
crosslinking. The basic material is preferably a metal cation which is 
provided as a metal compound having a corresponding anion which can be 
selected stearates, oxides, hydroxides, hydrides, formates, acetates, 
alcoholates and glycolates. The most preferred metal compounds are zinc 
stearate, calcium stearate, magnesium oxide, and calcium hydroxide. The 
metal compound is useful in ethylene polymers to substantially neutralize 
acidic compounds in the polymer composition. Although the preferred 
ethylene polymers for use in the present invention have terminal 
saturation, the metal compound can be used in any ethylene polymer whether 
there is terminal unsaturation or not. 
The crosslinking agent can be any suitable crosslinking agent for 
crosslinking ethylene polymers. Preferably the crosslinking agent is an 
organic peroxide crosslinking agent. A preferred organic peroxide 
crosslinking agent has the formula 
##STR1## 
wherein R is a C.sub.2 to C.sub.12 divalent hydrocarbon radical, 
R' and R" are the same or different C.sub.1 to C.sub.12 alkyl groups, 
R'" and R.sup.iv and the same or different C.sub.1 to C.sub.12 monovalent 
hydrocarbon radicals, and 
n is 0 or 1. 
Crosslinking agents useful in the present invention are bis(t-alkylperoxy) 
alkanes, bis(-alkylperoxy) benzenes, and bis(t-alkylperoxy) acetylenes. 
Illustrations of the bis(t-alkylperoxy) alkanes are 
2,5-bis(t-amylperoxy)-2,5-dimethylhexane, 
2,5-bis(t-butylperoxy)-2,5-dimethylhexane, 
3,6-bis(t-butylperoxy)-3,6-dimethyloctane, 
2,7-bis(t-butylperoxy)-2,7-dimethyloctane and 
8,11-bis(t-butylperoxy)-8,11-dimethyloctadecane. Illustrations of the 
bis(t-alkylperoxy) benzenes are 
a,a.dbd.-bis(t-amylperoxy-isopropyl)-benzene. Of these crosslinking 
agents, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane and 
a,a'-bis(t-butylperoxy-isoproply)benzene are preferred 
Illustrative of acetylenic diperoxy compounds are those disclosed in U.S. 
Pat. No. 3,214,422 thereby incorporated by reference. The compounds 
include hexynes, octynes, and octadiyenes. Examples of these include: 
2,7-Dimethyl-2,7-di(t-butylperoxy)octadiyne-3,5 
2,7-Dimethyl-2,7-di(peroxyethyl carbonate)octadiyne-3,5 
3,6-Dimethyl-3,6-di(peroxyethylcarbonate)octyne-4 
3,6-Dimethyl-3,6(t-butylperoxy)octyne-4 
2,5-Dimethyl-2,5-di(peroxy-n-propyl)carbonate)hexyne-3 
2,5-Dimethyl-2,5-di(peroxyisobutylcarbonate) hexyne-3 
2,5-Dimethyl-2,5-di(peroxyethylcarbonate)hexyne-3 
2,5-Dimethyl-2,5-di(alpha-cumylperoxy)hexyne-3 
2.5-Dimethyl-2,5-di(t-butylperoxy)hexyne-3. 
The amount of the crosslinking agent to be blended with the ethylene 
polymer may vary depending upon the desired degree of crosslinking, the 
activity of the crosslinking agent used, the crosslinking aid used and the 
crosslinking conditions. Usually, the amount of the crosslinking agent is 
in the range of from 0.1 to 3 parts by weight, preferably from 0.2 to 1.5 
parts by weight, and more preferably 0.5 to 0.8 parts by weight, based on 
100 parts by weight of the polymer. 
Preferred organic peroxide crosslinking agents have the formula: 
##STR2## 
wherein R is selected from the group of 
The crosslinkable ethylene polymer composition of 
##STR3## 
The crosslinkabe ethylene polymer composition of the present invention 
includes a crosslinking co-agent which helps to regulate the generation of 
free radicals and thus modifies the type of crosslink and improves the 
efficiency of the crosslinking reaction. Furthermore, the use of the 
crosslinking co-agent permits the use of less peroxide. A particularly 
preferred class of coagents for a composition particularly suited for 
rotational molding is allyl carboxylates. Crosslinking coagents include 
allyl compounds, diallyl compounds, triallyl compounds, dimethacrylate 
compounds, and 1,2-polybutadiene. Illustrative of allyl compounds is allyl 
methacrylate. Illustrative of the diallyl compounds are diallyl itaconate, 
and diallyl phthalate. Illustrative of compounds are triallyl 
trimellitate, triallyl trimethallyl trimellitate, triallyl cyanurate, and 
triallyl phosphate. Illustrative of dimethacrylate compounds are ethylene 
dimethacrylate, polyethylene glycol dimethacrylate, and trimethylol 
propane trimethacrylate. Other useful crosslinking co-agents include but 
are not limited to divinylbenzene, vinyltoluene, vinylpyridine, p-quinone 
dioxime, acrylic acid, cyclohexylmethacrylate. Other useful crosslinking 
co-agents are indicated as crosslinking aids in U.S. Pat. No. 4,267,080 
which is hereby incorporated by reference. 
Allyl carboxylates such as triallyl trimellitate (also known as 
1,2,4-benzenetricarboxylic acid tri-2-propenyl ester), trimethallyl 
trimellitate diallyl phthalate, allyl itaconate, and allyl methacrylate 
are useful for compositions to be used in rotational molding. These 
coagents help suppress bubble formation and enhance the release 
characteristics when used in combination with the crosslinking agent used 
in the invention. 
The amount of the crosslinking co-agent used is preferably from 0.5 to 5 
parts by weight, more preferably from 1.0 to 3.0 parts by weight based on 
100 parts of polymer. When the amount of the crosslinking coagent is less 
than the above-mentioned lower limit, bubbles are more likely to form due 
to decomposition of the crosslinking agent. When the amount of the 
crosslinking co-agent exceeds the above-mentioned upper limit, the 
ethylene polymer composition becomes poor in thermal resistance and thus, 
is colored to some extent due to the exposure to the rotational molding 
temperature. 
The composition of the present invention preferably includes a peroxide 
scavenger, preferably a thio compound. pound. There is 0.01 to 0.1, and 
preferably 0.05 to 0.08 parts of the peroxide scavenger. The peroxide 
scavengers prevent premature crosslinking during compounding and flow out 
stages of molding. This results in molded articles with good impact 
resistant properties. Peroxide scavengers known in the art can be used. 
Useful peroxide scavengers include organic phosphonates, organic 
phosphonites, organic phosphates, with thio compounds being most 
preferred. 
Illustrative of the peroxide scavengers which can be used include 
di(stearyl)pentaerythritol diphosphite, 
tetrakis(2,4-di-t-butylphenyl)4,4'biphenylyene-diphosphonite, 
pentaerythrityl hexylthiopropionate, and thiodipropionate polyester. The 
thio compounds also include those disclosed in Japanese Kokai JP No. 
57,126,833. The preferred peroxide scavengers are esters of 
thiodipropionic acids of the formula: 
EQU R.sub.1 --OOC--CH.sub.2 --CH.sub.2 --S--CH.sub.2 --CH.sub.2 --COO --R.sub.2 
wherein R.sub.1 and R.sub.2 are selected from the group consisting of 
hydrogen and alkyl, alkenyl, aryl, and cycloalkyl hydrocarbon radicals and 
combinations thereof such as alkaryl, aralkyl and alkylcycloalkyl, having 
up to 22 carbon atoms and wherein at least one R has at least 10 carbon 
atoms per molecule. These esters result in a composition having good low 
temperature impact strength. 
Some suitable R radicals include for example, methyl, ethyl, propyl, 
isopropyl, butyl, isobutyl, t-butyl, amyl, isoamyl, n-octyl, isooctyl, 
2-ethyl hexyl, t-octyl, decly, dodecyl, octadecyl, allyl, hexenyl, 
linoleyl, ricinoleyl, lauryl, stearyl, myristyl, oleyl, phenyl, xylyl, 
tolyl, ethylphenyl, naphthyl, cyclohexyl, benzyl, cyclopentyl, 
methylcyclohexyl, ethylcyclohexyl and naphthenyl. 
Examples of suitable thiodipropionic acid esters include, monolauryl 
thiodipropionic acid, butyl stearly thiodipropionate, 2-ethylhexyl lauryl 
thiodipropionate, dissodecyl thiodipropionate, isodecyl phenyl 
thiodipropionate, benzyl lauryl thiodipropionate, the diester of mixed 
coconut fatty alcohols nd thiodipropionic acid, the diester of mixed 
tallow fatty alcohols and thiodipropionic acid, the acid ester of mixed 
cottonseed oil fatty alcohols and thiodipropionic acid, the acid ester of 
mixed soybean oil fatty alcohols and thiodipropionic acid. 
thiodipropionate polyesters can also be used. 
A presently preferred group of esters of thiodipropionic acid in which both 
R radicals have 12-20 carbon atoms, more preferably esters in which both R 
groups are the same, including the dilauryl, distearyl, dimyristyl, 
dioleyl and diricinoleyl esters. 
In addition to the ethylene polymer, the crosslinking agent and co-agent, 
the compositions of the present invention also advantageously include 
about 0.01 to 0.1 and, preferably 0.02 to 0.05, parts by weight of one or 
more suitable high temperature antioxidants for the ethylene polymers, per 
100 parts by weight of polymer in such compositions. 
The preferred antioxidant is a hindered phenol type of antioxident. Such 
compounds include 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxy 
benzyl) benzene; 1,3,5-tris(3,5-di-t-butyl-4-hydroxy 
benzyl)-5-triazine-2,4,6-(1H,3H,5H) trione; tetrakis- [methylene-3-(3', 
5-di-t-butyl-4'-hydroxy phenyl)-propionate] methane; and 
di(2-methyl-4-hydroxy-5-t-buty phenyl)sulfide. Polymerized 2,2,4-trimethyl 
dihydroquinoline, 2,6-di-t-butyl-4-methylphenol, tetrakis [methylene 
3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane can also be 
used. 
A preferred hindered phenol is tetrakis [methylene 
3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane and the 
preferred thio compound is distearyl thiodipropionate. 
Other additives which may be employed in the compositions of the present 
invention include additives commonly employed in crosslinkable ethylene 
polymer based compositions including fillers, such as carbon black, clay, 
talc and calcium carbonate; blowing agents; nucleating agents for blown 
systems; lubricants; UV stabilizers; dyes and colorants; metal 
deactivators and coupling agents. The additional ingredients should not 
adversely effect crosslinking or rotational molding when the composition 
is used in a process to rotationally mold. 
The compositions of this invention are prepared by mixing the ethylene 
polymer, crosslinking agent, the crosslinking co-agent, the stabilizer 
system using convention techniques employed in the polymer field, such as 
passage through mixing rolls or dispersion using conventional type mixers. 
The mixture of the crosslinking compound, the polymer and the stabilizing 
system can be performed at about room temperature or a temperature 
different from room temperature but below that at which the crosslinking 
is effected. After uniform distribution of the crosslinking compound and 
the stabilizing system in the polymer, the mixture can be formed and 
shaped by conventional procedures. The composition of the present 
invention is particularly suitable for rotational molding. 
The composition of the present invention results in a crosslinked 
polyethylene or polyethylene copolymer which is bubble free, and has a 
high gel content, preferably greater than 75 percent and more preferably 
greater than 80 percent when measured according to ASTM Test D2765. The 
composition can be molded to form articles having low temperature, high 
impact strength, preferably greater than 50 ft-lbs and more preferably, 
greater than 60 ft-lbs at -40.degree. F. when measured according to the 
Association of Rotomolder (ARM) Dart Impact Test Procedure. The 
composition results in crosslinked material having good stress crack 
resistance. 
The present invention includes a method of rotational molding using the 
composition described above. the composition can be rotationally molded by 
conventional methods of rotational molding. 
Typically the rotational molding process with the above-described 
composition comprises the steps of preparing the composition. The 
composition can be in powder or pellet form. Powders are preferably used 
with the powders preferably 35 mesh or smaller. The composition is heated 
within the mold as the mold is rotated. Typically, the mold rotates 
simultaneously about two perpendicular axes. The mold is heated until the 
polymer within the mold melt flows together and begins to crosslink on the 
inside surface of the mold. Generally, the mold rotates with a forced air 
circulating oven. The mold is then cooled and the molded article is 
removed. 
The composition of the present invention can be processed in most 
commercial rotational molding machines. The oven temperature range during 
the heating step is from 400.degree. F. to 700.degree. F., preferably 
about 500.degree. F. to about 650.degree. F., and more preferably from 
about 575.degree. F. to about 625.degree. F. If the temperature is too 
high during rotational molding the molding, the crosslinking begins 
prematurely and will not be uniform. 
After the heating step the mold is cooled. The part must be cool enough to 
be easily removed from the mold and retain its shape. Preferably the mold 
is removed from the oven while continuing to rotate. Cool air is first 
blown on the mold. The air can be an ambient temperature. After the air 
has started to cool the mold for a controlled time period, a water spray 
can be used. The water cools the mold more rapidly. The water used can be 
at cold tap water temperature, usually from about 4' C. (40.degree. F.) to 
about 16.degree. C. (60.degree. F.). After the water cooling step, another 
air cooling step may optionally be used. This is usually a short step 
during which the equipment dries with heat removed during the evaporation 
of the water. 
The heating and cooling cycle times will depend on the equipment used and 
the article molded. Specific factors include the part thickness in the 
mold material. Typical conditions for a 1/8 inch thick part in a steel 
mold are to heat the mold in the oven with air at about 316.degree. C. 
(600.degree. F.) for about 15 minutes. The part is then cooled in ambient 
temperature forced air for about 8 minutes and then a tap water spray at 
about 10.degree. C. (50.degree. F.) for about 5 minutes. Optionally, the 
part is cooled in ambient temperature forced air for an additional 2 
minutes. 
During the heating and cooling steps the mold containing the molded article 
is continually rotated. Typically this along two perpendicular axes. The 
rate of rotation of the mold about each axis is limited by machine 
capability and the shape of the article being molded. A typical range of 
operation which can be used with the present invention is to have the 
ratio of rotation of the major axis to the minor axis of about 1:8 to 10:1 
with a range of from 1:2 to 8:1 being preferred. 
The composition of the present invention is a crosslinkable polyethylene 
composition based on a Ziegler-type catalystized polymerication or 
polyethylene having substantial terminal saturation, or polyethylene with 
residual acidic groups in the polymer matrix. The composition neutralized 
is useful in critical molding operation such as rotational molding where 
previously premature crosslinking and bubble formation had been a 
persistent problem. 
Rotational molded articles of the present invention can be used where 
durability is essential in the sense that there is crack and puncture 
resistance. Examples of articles which can be made include gasoline tanks, 
large trash containers, and large bins or silos for fertilizer, etc. 
Several examples are set forth below to illustrate the nature of the 
invention and the manner of carrying it out. However, the invention should 
not be considered as being limited to the details thereof. All parts are 
based on a composition containing 100 parts by weight of an ethylene 
polymer.

EXAMPLES 1-6 
In Examples 1-6 crosslinkable high density polyethylene compositions were 
prepared and rotationally molded. The high density polyethylene had a 
density as measured by ASTM D-1505-63T in grams per cubic centimeter of 
0.955. The polyethylene was made using a Ziegler type catalyst and had 
substantially complete saturation of terminal groups. The crosslinking 
agent was 2,5-dimethyl-2,5-[di(t-butylperoxy)hexane] sold as Lupersol 101 
by Pennwalt Corp. The crosslinking coagent was triallyl trimellitate. The 
antioxidant was tetrakis[methylene 
3-(3',5'-di-t-butyl-4'-hydroxyphenyl)proprionate]methane sold as Irganox 
1010. The thio compound was distearyl thiodipropionate. The metal compound 
was zinc stearate. Additionally, a UV stabilizer, 
2-hydroxy-4-n-octoxybenzophenone sold as Cyasorb 531 was used. 
The compositions were prepared by first mixing the liquid components. The 
liquid was slowly added and continuously mixed with the other components 
including polyethylene which were in powder form. The physical blend was 
extruded through a 1 inch diameter single screw extruder having a L to D 
ratio of 24 to 1. The extruder was operated at the following temperatures: 
Zone 1-200.degree. F., Zone 2-260.degree. F., Zone 3-260.degree. F., Zone 
4"26020 F., Die-260.degree. F. The polymer was extruded as strands which 
were chopped to form pellets and ground on a Wedco SE-12 lab mill to a 
nominal size of 35 mesh or less. 
The powder was used to rotationally mold a rectangular box 
51/2.times.51/2.times.11 inches. The box had a wall thickness of about 125 
mils. The mold was rotated uniaxially about its longest axis at about 5 
rpm. The polymer was fed to the mold and the mold started to rotate. The 
rotating mold was heated for 35 minutes in an oven at 590.degree. F. After 
the heating step the mold was cooled in forced air at about 72.degree. F. 
for 7 minutes followed by tap water cooling for 8 minutes. 
The molded boxes were tested for percent gel according to ASTM Test No. 
2765 which measures the percentage of polymer insoluble in boiling xylene. 
The molded boxes were tested for cold temperature impact resistance 
according to the Association of Rotational Molders Dart Impact Testing 
Procedure using a 15 pound dart having a 1 inch diameter stem tip with a 
1/2 inch diameter hemisphere dropped into a 125 mil sample. Samples were 
tested for stress crack resistance in accordance with ASTM Test No. 1693. 
All Example samples tested to greater than 1000 hours without stress 
cracking, while the Comparative cracked at less than 1000 hours. 
The compositions and test results are summarized on the Table below. In 
Examples 5 and 6 the triallyl trimellitate was changed to triallyl 
isocyanurate and diallyl itaconate, respectively. Comparative 1 
illustrates a composition without a co-agent. Compositions and results are 
summarized on Table 1 below: 
TABLE 1 
______________________________________ 
Ex 1 Ex 2 Comp 1 Ex 3 Ex 4 Ex 5 Ex 6 
______________________________________ 
HDPE 100 100 100 100 100 100 100 
MI g/10 min. 
6 20 20 30 40 20 20 
Lupersol 101 
.3 .5 .5 .5 .8 .5 .5 
Irganox 1010 
.02 .02 .02 .02 .02 .02 .02 
DSTDP .05 .05 .05 .05 .05 .05 .05 
Cyasorb 531 
.2 .2 .2 .2 .2 .2 .2 
Zn St.sub.2 
.05 0.05 .05 .05 .05 0.05 .05 
triallyl trimell. 
2.0 2.0 -- 2.0 3.0 -- -- 
triallyl isocy. 
-- -- -- -- -- 2.0 -- 
diallyl itaconate 
-- -- -- -- -- 2.0 2.0 
% gel 83.2 87.1 83.2 85.8 84.8 83.2 81.0 
impact ft-lb 
@ .sup.- 40.degree. F. 
75 60 Shatter 
60 60 60 60 
______________________________________ 
The molded boxes of Examples 1-6 had good surface appearance, mold release 
and no warpage. The polymer was homogeneous throughout. In Comparative 1 
on Table 1 below, the polymer had bubbles and shattered upon impact. 
EXAMPLES 7-8 
These examples illustrate the use of peroxide scavengers in situations 
where it is desirable to inhibit the crosslinking reaction during 
preliminary processing. None of the compositions contained peroxide 
scavengers. The compositions of Comparative 2 and Examples 7 and 8 were 
prepared, processed, molded, and tested using the same procedures as 
Examples 1-6. The compositions and test results are summarized in Table 2 
below. Environmental stress crack resistance was measured in hours to 
stress crack. 
TABLE 2 
______________________________________ 
Ex. 7 Ex. 8 Comp 2 
______________________________________ 
HDPE 100 100 100 
MI g/10 min. 20 40 6 
Lupersol 101 0.5 0.8 0.3 
Irganox 1010 0.02 0.02 0.02 
DSTDP -- -- -- 
Cyasorb 531 0.20 0.20 0.20 
Zn St.sub.2 0.05 0.05 0.05 
triallyl trimell. 
2 2 2 
% gel 88.0 85.4 85 
impact @ -40.degree. F. (ft-lb) 
60 60 45 
hours to stress crack 
&gt;1000 &gt;1000 90 
______________________________________ 
Examples 1, 7 and 8, and Comparative 2 show that for lower melt index 
polymer such as Example 1 having a MI of 6 g/10 min it is preferred to use 
a peroxide scavenger such as DSTDP when the composition is exposed to 
conditions which could cause premature crosslinking. Comparative 2 had a 
lumpy inner surface compared with the smooth inner surfaces of the 
rotationally molded boxes in Examples 1, 7 and 8. 
COMATIVE 3 
A composition was prepared using polyethylene made using chromium on 
SiO.sub.2 catalyst in a solution polymerization conducted in cyclohexane. 
The polyethylene so produced had terminal group unsaturation 
--CH=CH.sub.2. The solution method results in a polymer having a minimum 
of catalyst residue or acidic residue. The polyethylene had a Melt Index 
of 25 g/10 min. The composition contained 100 parts of polyethlene, 0.8 
parts of 
##STR4## 
and 0.02 parts of Irganox 1010. The composition was prepared, blended, 
molded and tested in accordance with the same procedures as used in 
Examples 1-6. The molded article had a drop weight impact at -40.degree. 
F. of 60 ft-lbs, no stress cracking after 1000 hours and good molding 
properties and appearance. 
COMATIVE 4 
Phillips Chemical Co. rotational molding grade polyethylene CL-100 was 
evaluated using the molding procedure described in Examples 1-6. The 
polyethylene is a solution polymerized material using a chromium type 
catalyst and has terminal unsaturation. It has a melt index of about 
25-35. The composition contains about 0.80 parts by weight of the peroxide 
of Comp. 3 per 100 parts of polymer. The polymer has no significant 
catalyst residue. The composition does not contain a crosslinking 
co-agent. The mold composition had a drop weight impact value at 
-40.degree. F. of 60 ft-lbs, no stress cracking after 100 hours and good 
molding properties and appearance. 
Comparative 1 and the Examples 1-8 indicate that the polyethylene having 
substantial complete terminal saturation requires a crosslinking co-agent 
to produce a satisfactory rotationally molded article, while Comparatives 
3 and 4 indicate that polyethylene having terminal unsaturation require no 
such crosslinking co-agent to produce a satisfactory rotationally molded 
article. It is noted that Comparatives 3 and 4 had satisfactory color 
stability although a metal compound was not added. 
While exemplary embodiments of the invention have been described, the true 
scope of the invention is to be determined from the following claims.