Note: Descriptions are shown in the official language in which they were submitted.
METHOD OF ROTATIONALLY MOLDING PLASTIC ARTICLES
Background of the Invention
This invention relates to a process of rotationally
molding and ~e resulting product or article in which a blend ~ ~;
of high density polyethylene or ethylene-alkene copolymer and
an~:; ethylene-propylene copolymer or terpolymer with diene or a
mixture of these two with an EVA copolymer is used.
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The most pertinent prior art of which applicants are
aware are the following: -
U. S. patent 3,647,922 discloses polyethylene blended ~
with an ethylene-propylene copolymer. However, the blends of -
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this invention and the structures produced therefrom have
advantages that are not achieved with the blends of this ;~
patent. Thus in this patent the copolymers are block ;-~
copolymers while the on~s here~are
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essentially random as produced via transition metal catalysts.
Also the ethylene content of the reference coPolymer is low while
in the present invention it is high. Additionally, the blends
of the reference have low amounts of polyethylene while the blends
of this invention have high amounts of polyethylene.
U.S. Patent 3,261,889 discloses a blend of a low density
polyethylene and an ethylene-prooylene-diene terpolymer to improve
environmental stress crack resistance (ESCR). (A table of abbre-
viations is presented at the end of this s~ecification.) The
three component blends of this invention especially have the ESCR
improvement significantly above that achieved via a two component
blend due to the unexpected synergism of the blend components chosen.
In the conventional rotational molding methods using
resins and blends it is customary to use polyethylene of narrow
molecular weight distribution and with low melt flow to achieve
high impact stren~th. These prior res:ins are difficult to process
and it aPpears to be due to their high apparent melt viscosity
and they are generally not warp resistant. It has been known to
blend EVA (ethylene-vinyl acetate copolymer) with polyethylene
in oxder to im~rove the impact strength of the resin. However,
polyethylene-EVA copolymer blends are not nearly as effective -~
as ethylene-propylene copolymer or ethylene-propylene-diene
texpolymer in improving the impact resistance of rotomolded
structures. In addition, these prior and conventional rotational
molding blends containing Polyethylene do not have the above -
advantages of the blends of this invention.
SUMMARY OF THE INVENTION
The invention in its broader aspect as claimed pertains
to the method of rotationally molding articles of high impact
resistance, high stress crack resistance, with smooth surfaces
and an absence of excessive warpage. The method comprises
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placing in a rotational mold a blend comprising a three component
blend o~ (1) 60-80 wt.% of a member of a first class consisting
of high density polyethylene and high density ethylene-alkene
copolymers containing 4 to 6 carbon atoms in the alkene group
blended with (2l 5-20 wt.% of a member of a second class consist-
ing of ethylene-pro~ylene essentially random copolymer and an
essentially random terpolymer of ethylene-pro~ylene-monomer in
which the monomer is selected from the group consisting of 1,4-
he~adiene,5-ethylidine-2-norbornene and 5~methylene-2-norbornene
and (3) 5-30 wt~% of a copolymer of ethylene-vinyl acetate for a
total of 100%, biaxially rotating the mold about two perpendicular
axes while heating the mold and contents within the range of about
40~-9qaF., cooling said mold to about room tem~erature while
maintaining the coaxial rotation and removing the resulting
rotationally molded article.
In one aspect the component of the second class prefer- -
ably consists essentially of an ethylene-propylene copolymer -~
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containing 40-95 wt.% of ethylene and 5-60 wt.% of propylene
for a total of 100%, and the copolymer of ethylene-vinyl acetate ~ -
preferably contains 65-95 wt.% of ethylene and 5-35 wt.~ of ;, `
vinyl acetate for a total of 100%.
In another aspect the component of the second class
preferabl~ consists essentially of a ter~olymer containing 40-95
wt.% of ethylene, 5-60 wt.% of pro~ylene and 1-8 wt.% of the --~
group monomer for a total of 100%, and the ethylene-vinyl acetate
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copolymer contains 65-95 wt.% of ethylene and 5 35 wt.% of vinyl
acetate for a total of 100%.
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Description of the Preferred Embodiments
The blends of this invention all of which contain poly-
30 ethylene blended with certain specified copolymers of either two
or three monomers have characteristics that make them highly
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useful for rotational molding to produce molded shaped structures.
' Thus the blends of this invention have good processability, high
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impact strength, produce smooth surfaces in the rotationally
molded structures with a virtual absence of warpage in the
structures, have easy flow during the rotational molding and
the structures produced have a high resistance to environmental
stress cracking.
The blends of this invention generally contain 50-95
wt.% of high density polyethylene (HDPE) with a preferred
composition range of 60-90 wt.%, and either ethylene-propylene
copolymer, a terpolymer of ethylene-propylene and a monomer
that is either 1,4-hexadiene, 5-ethylidine-2-norbornene or
5-methylene-2-norbornene or either of these copolymers further
blended with EVA.
To obtain maximum improvement in the impact strength
and ESCR of the blend, the preferred elastomer component, i.e.
ethylene-propylene copolymer and ethylene-propylene-diene ter-
polymer, should have a high molecular weight as measured by the -
Mooney viscosity and a narrow molecular weight distribution as
measured on the Gel Permeation Chromatograph. Tests have shown
that the blend properties improve with increasing Mooney -
viscosity and narrowing molecular weight distribution of the
elastomer component of this invention. Furthermore, the blend
properties also improve with increasing elastomer content up
to the concentration disclosed in this invention.
The following blends of polymers of this invention
; result in molding resins that have superior rotational molding
qualities as discussed above.
1. A blend of 70-95 wt.% of HDPE with 5-30 wt.% of
ethylene-propylene copolymer containing 40-95 wt.% of ethylene
and 5-60 wt.% of propylene in an essentially random copolymer
with a Mooney viscosity (ML1+4/250F.) of 30 to 100 or in some
instances even higher.
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2. A blend of 70-95 wt.% of HDPE with 5-3Q wt.% of
a terpolymer of 40-95 wt.% of ethylene, 5-60 wt.% of propylene
and 1-8 w-t.% of a termonomer that is either 1,4-hexadiene, 5-ethy-
lidine-2-norbornene or 5-methylene-2-norbornene or blends of
these monomers.
3~ Blend 1 above further blended with 5-40 wt.~ of a
copolymer of 65-95 wt.% of ethylene with 5-35 wt.% of vinyl
acetate and of an ~I range of 0.3 to 30.
4. Blend 2 above further blended with the ethylene-
vinyl acetate copolymer of blend 3 in an amount of 5-40 wt.% of
a copolymer of 65-95 wt.% of ethylene with 5-35 wt.% of vinyl
acetate and of an MI range of 0.3 to 30.
The blends of this invention may be prepared with any
desired mixer or blender so long as the resulting blends are
homogeneous and therefore uniform. Thus the mixers may be either
roll mills, banburies, kneaders or extruders or the like and
the mixing temperatures are prefera~ly above the softening point
of the ingredients and, for example, may be within the range of
175-500F. The blends of this invention are processed in the
normal manner in the thermoplastic range of the particular blend
to produce shaped structures that are themselves homogeneous and
that have desired improved characteristics including those
specifically noted above.
Certain values which are important for the proper pro-
cessing of the blends of this invention are standard procedures
that are customarily used in this art. Thus the test for en-
vironmental stress cracking of ethylene containing plastics is
the test designated D1693-70 of A.S.T.M. The test for impact
resistance of plastics is designated as ASTM Method A of D256-56.
The standard method used for testing tensile-impact energy to
break plastics is designated as ASTM D 1822-68 with the actual
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test specimen being that shown in Fi~ure 4~ of the paper en-
titled "Test for Tensile-Impact Energy to Break Plastics" at
page 590. Mooney viscosity data i5 obtained according to
ASTM-D1646, Viscosity and Curing Characteristics of Rubber By
The Shearing Disc Viscometer.
The high density polyethylene used in this invention
incl~des ethylene homopolymers as well as ethylene butene-l
and ethylene hexene-l copolymers preferably with an annealed
density range of 0.940 to 0.970. These high density poly-
ethylene resins can be produced by Ziegler or modified Zieglercatalysts~ chromium oxide catalysts, molybdenum oxide catalysts
or any of the available processes for producing essentially
linear crystalline polyethylene. The EVA copolymers used in
this invention can be produced by free radical catalysts at
high pressures. The vinyl acetate content in these copolymers
can range from 5% to 35%. The ethylene-propylene copolymers
and ethylene-propylene-diene terpolymers used in this invention
are essentially random copolymers that can be produced via the
transition metal catalyst. The preferred compound should have
~0 an ethylene content above 40~ with high molecular weight and
narrow molecular weight distribution. Where an ethylene-alkene
copolymer is used in the blend the alkene content may be up to
10 wt.% of the ethylene-alkene copolymer with the ethylene being
from 90-100 wt.%. The preferred alkene groups are those of 3-8
carbon atoms. Thus the individual polymers of the blends of
this invention are well known materials.
The three component blends of this invention produce
a synergistic eEfect with relation to the impact strength and
ESCR properties of the resin in that the overall effect is
greater than that which would be expected by the addltive
properties of the individual components.
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The preferred ethylene-propylene copolymer and ethylene- -
propylene-diene terpolymer for the three component blend are the
same as descri~ed above ~or the two component blend. The
preferred ethylene-vinyl acetate copolymer should contain at least ;
5% vinyl acetate with an MI of 0.1 or above. ~he blend properties
are shown to improve with increasïng vinyl acetate concentration
up to the preferred range disclosed herein. The processability
of the blend on rotational molding machines improves when the
polyethylene and the EVA copolymer components have a good match
of melt viscosities.
Specific Examples
:
Example 1 -
A blend of 15% ethylene-propylene-diene terpolymer
having a Mooney viscosity of about 50 (ML1+4/250F.) and 85%
of ethylene-hexene-l copolymer of 0.955 density and 18 MI was
produced on a Banbury mixer. The mixing times was 3-1/2 minutes, ;
achieving a drop temperature of approximately 320F. The mixture
was dropped into an extruder and pelletized. The resulting blend
has a nominal MI of 6.5 and 0.942 dens:ity. A comparison of parts
rotomolded from this blend and other commercial rotomolding resins
under the same molding conditions show that the blend of this
example is characterized by excellent impact strength and dimen-
sional stability (very low warpage) plus smooth glass-like
interior surfaces and an unusually wide range of molding tempera-
ture and cycle times.
Example 2
A nominal 0.96+ density and 6.0 MI blend of high density
polyethylene was made under similar conditions as in Example 1.
The rotomolded parts from this resin show very poor dimensional
stability (warpage was so bad that the part was useless), very
rough interior surface, and very low resistance to tear.
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Example 3
A comparison between the phy-sïcal properties of roto-
molded parts from the blend ïn Example 1 and a direct synthesis
copolymer rotomolding resin is shown in Chart I.
CHART I
Physical Propertie_ of Rotomolded Parts
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Blend in HDPE Copolymer
Ex. 1 Rotomolding Resin
MI (g/10 min) 7.1 4.5
Density (g/cc) 0.936 0.935
Tensile Impa~t
(ft-lbs/in') 64 21
Notched Izod
(ft-lbs/in wid h) 4.6(P~ 0.81 (C)
Bell ESCR F50 (hrs) 4-3 break on bending
The above results indicate that the blend of this invention has
significantly better properties on rotomolded structures than
that of HDPE copolymer resins with similar MI and density
properties but produced by direct synthesis.
Examples 4-8
The effect of EPDM rubber concentration on the physical
properties of the plaques compression molded from these blends is
shown in Chart II. The materials and mixing conditions are the
same as in Example 1.
CHART II
Effect of EPDM Concentration
Tensile Notched
Impact Izod
EPDM MI Density ft-lbs? ~ft-lbs
Ex. Conc. (g/10 min) (g/cc) in2 ~in width
4 0 18 0.955 35 0.8(C)
5% 14 0.951 37 0.83(C)
6 10~ 9.6 0.948 47 1.2(C)
7 12.5% 9.0 0O944 46 2.0(H)
8 15% 5.6-6.7 0.941-3 61-80 4.4-NB
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The above results indicated improved impact properties as the
EPD~ content of the blend was increased.
Example 9
A blend of 10% ethylene-propylene-diene terpolymer
with 20~ 0.950 density, 2.5 MI, 27.6% vinyl acetate content EVA
copolymer plus 70% of a 0.964 densïty, 12 MI homopolymer poly-
ethylene was produced on a sanbury mixer and pelletized as in
Example 1. Rotomolded parts from this nominal 5.5 MI, 0.953
density blend were characterized by excellent impact resistance
and superior ESCR (about 60 hours). The impact strength and
especially the ESCR are the result of a synergistic effect be-
tween the EPDM and the EVA copolymer. Chart III shows this
effect for compression molded samples.
Example 10
A blend of 30% EPDM and 70~ homopolymer is prepared
using the same materials and procedures as in Example 9.
Physical properties of compression mo]ded plaques are shown
in Chart III.
Example 11
A blend of 30~ EVA copolymer and 70~ homopolymer
polyethylene is prepared using the same materials and procedures
as in Example 9. Physical properties of compression molded
plaques are shown in Chart III hereinafter.
Examples 12-20
The effect of EPDM and EVA concentration on the physical
properties of compression molded plaques from the resulting
blends is shown in Chart IV hereinafter. The materials and
mixing conditions are the same as in Example 9 except that the
hexene copolymer used in Example 1 was used in place of the
homopolymer used in Example 9. This data also shows the
synergistic effect of the E~A-EPDM combination.
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Exam~le 21
66.5% of a 0.964, 12 MI homopolymer polyekhylene is
blended with 28~5~ of a 1 MI, 0.930 density, 9~ vinyl acetate
content EVA copolymer and 5% of an ethylene/propylene/1,4-hexa-
diene monomer rubber using the same conditions as in Example 1.
Rotomolded parts from this resin show improved impact strength
over rotomolded parts from the blend in Example 1.
In the rotational molding process of this inventioII
the mold may be rotated at from about 2-12 rpm about the malor
axis and about 5-40 rpm about the minor axis although
the conditions for rotomolding any blend of polymers within
the method of this invention can be easily determined by those
skilled in the art of rotational molding. Thus, for example,
in the rotomolding of the resins of Chart.I above the mold was
rotated for 13 minutes at 10 rpm about the major axis and 11.4
rpm about the minor axis while the mold was heated at about
650P. At the end of the 13 minutes the mold and contents was
transferred to a cooling chamber that was cooled by water and
air for 7 minutes to approximately room temperature and the
20 resulting molded article was then removed from the mold. As ~ '
a general statement it can be noted that a practical range of ,,
temperatures may be from 400-900F.
All parts and percentages herein are by weight.
Abbreviations
EPDM - ethylene propylene diene monomer
ESCR - environmental stress crack resistance '; ,
EVA - ethylene-vinyl acetate
HDPE - high density polyethylene
MI - melt index
MWD - molecular weight distribution
Having described our invention as related to the embodi-
, ments,set out herein, it is,our intention that the invention be
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not limited hy any of the details of description, unless
otherwise specified, but rather be construed broadly within its .
spirit and scope as set out in the appended claims.
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