Note: Descriptions are shown in the official language in which they were submitted.
103~
The present invention relates to a process for rotational molding
of foamed, chemically crosslinked polyethylene. The proc~ss enables
hollow objects to be achieved having a cellular wall structure rom
solid polyethylene.
Foamed polyethylene structure offers certain advantages over
solid polyethylene without excessive sacrifice o ~he excellent ~ -~
inherent properties of polyethylene such as chelmical and moisture
resistance, flexibility and toughness. The chief advantages arle
reduction of density, thus allowing a saving in weight of polyethylene, -
and improvement in the insulating and isolating properties.
A ew years ago, a U.K. Patent, 1,038.810, was issued on the ;~
rotational molding of polyolefin foams rom a finely divided powder
having a par~icle size of 800 microns using a blowing agent.
Optionally, a crosslinking agent selected from a polysuloneazide,
a polyazide or an azidoformate is suggested to be added in order to
obtain a "finer and more uniform cell structure". The disadvantage
of this method is perhaps the relatively high cost of the unco~mon
crosslinking agent claimed. The patent does not specify the extent
of crosslinking ob~ained in the polyolefin but it seems that no
specific requirement is imposed on this feature, since tbe presence
of the crosslinking agent is also only optional.
J The crosslinked polyethylene compounds have a number of
important advantages over the original ~hermoplastic olefin, these
advantages baing directly correlated ~o the extent o crosslinking.
Thus, for example, highly crosslinked polyethylenes are not
embrittled even when high loadings of fillers are used; they are
not subjected to environmental stress cracking, exhibit excellent ~ -
weathering properties and have ou~standing resistance to chemical ~ ~ -
solvents.
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It is the object of the present invention to provide a process
for rotational molding of foamed, chemically crosslinked polyethylene.
According to the invention there is provided a process for the
rotational molding of rela~ively high, chemically crosslinked,
foamed polyethylene in which solid polyethylene of the high, medium
o~ low density type having a melt flow index above 1.8 is blended
with a chemical blowing agent, a crosslinking agent and an
activator for regulating the decomposition temperature of the
blowing agent in a non-acidic medium and the blend is heated in
hollow mold at a temperature between 190C and 300C while
simultaneously rotating the mold, wherein the process is
characterized by simultaneous blowing and crosslinking of the
polyethylene to produce a foamed, crosslinked polyethylene having
a gel content o~ above 50%, the crosslinking agent being an organic
peroxide having a halflife in the range of 3 to 25 minutes and a
vapor pressure not higher than 5 mm Hg at 20C, said organic
peroxide being present in an amount of O.l - 2% by weig~1t of the ;
polyethylene.
The half lifetime figures of 3 to 25 minutes are measured
at 160C in benzene. The applicants have tried out a large
number of other crosslinking agents and even organic peroxides
which have a vapor pressure higher than 5 mm Hg at 20C but only
poor crosslinking, degree of expansion or appearance of the objects
obtained by the rotational mold m g of the solid polyethylene ;~
resulted. Examples of organic perosides which are suitable for
the present invention are 2,5-dimethyl-2,5-di(tert-butyl-
peroxy) hexane, 2,5-dimethyl-2,5-di(tert-butyl-peroxy~ hexyne-3,
characterized by their half life-time in the range of
3_25 minuSes (zzasur~d at 160CentigeJde in bznzene) and vzpor
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pressure not higher than 5 mm Hg at 2nc. Their use in the liquid
state is preferred.
After a thorough investigation of the prob]Lem in order to find
an explanation for the phenomenon, it was found that the two
parameters of the c~osslinking agents - half life-time and vapor
pressure - are very critical for the present invention. As known, ~;~
the ter~ 'half life' is commonly used to express the rate of
decomposition at a particular temperature, being defined as the time
required for one half of the organic peroxide originally present to `
decompose. The inventors have found that, in order to obtain products ~-
with relatively high crosslinking and good appearance, the foaming
and crosslinking processes should occur simul~aneously. This is
achieved when the crosslinking agents are characterized by the two
above-mentioned parameters. Poor results are obtained when the
crosslinking agents possess only one of the parameters. Thus, for
example, when di-tert-butyl peroxide which has a high life-time even ~ ;
higher than that of 2,5-dimethyl-2,5-di~tert-butyl-peroxy) hexyne-3
t20.4 minutes Yersus 18 minutes, both measured at 160C in benzene)
was used at the same concentration, poor crosslinking and foam
collapse in the products were observed. This is due to the
relatively high volatility - 19.5 mm Hg at 20C - o~ the di-ter-butyl
peroxide which causes poor crosslinking and instability of the fosm,
especially at the lower range of foam density. ;
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The same poor results were obtained when di-t-butyl peroxy
benzoate was used; this organic peroxide is a well-known catalyst
for high-temperature molding, being a non-vo~tile liquid, but its
half life-time is only 1.5 minutes ~measured at 160C in benzene).
The product obtained by rotational molding had an extre~ely rough
internal surace and a density of 0.57 g/c~3, which indicatas t~
30 poor foam formation.
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10385t77 ~ ~
i Other reagents used in the rotational molding of the crosslinked ~
foamed polyethylene are temperature regulators and mold release ;
agents.
The temperature regulators are very import~t components of the ~ -
present invention dus to their role in decreasing the decomposition
temperature of the blowing agent. Examples of t:he temperature
~ re
regulators~various alkaline substances such as magnesium oxide,
calcium carbonate, zinc oxide or stearats, titanium oxide, etc.
These reagents are used in amounts in the range of 0.1 to 1 parts by
weight of the polyethylene.
Examples of the mold release agents found su.itable for the
rotationai molding of foamed, crosslinked polyethylene are: calcium
stearate, silicone spray, Te1On etc.
The foamed polyethylene products obtained according to the
process of the present invention are characterized by a relati~ely
high degree of cross-linking, i.e. j above 50%, which gives rise to ; ::
t h e :
improved properties of the products obtained.
It is known that organic peroxides which form free radicals
upon decomposition are considered chemically reactive and chemically
capable o forming crosslinking. However, rom the above-mentioned
U.K. Patent 1,038.810, it would appear that organic peroxides are
1 not suitable or the crosslinking o foamed polyethyljne in a
I rotational molding in view of the danger of an excessive degradation
of the polymer.
The amount o crosslinhing agent used has to be in the range
¦ of 0.1-2% by weight of the polyethylene and preferably between 0.3-1%.
, Amounts o crosslinking agent less 0.1% by weight of the polyethylene
i do not crosslink the polyethylene sufficiently to impart the necessary
` strength to withstand the force of the gas produced by the decomposed
blowing agent, especially in the lower range of foam density.
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~Q385'r77 :
On the other hand, amounts of crosslinking agents in excess of 2%
by weight may crosslink the polyethylene excessively and inhibit
foaming. The extent to which the polyethylene is crosslinked, i.e.
the extent to which its structure is transformed into a three-
dimensional structure, is determined by the gel content. Specifically, `
the crosslinked polyethylene, upon immersion in a boiling liquid
hydrocarbon, leaves an insoluble residue, known as gel, which is due
to the crosslinked structure. This amount of gel is indicative of ~
the degree to which the polyethylene is crosslinked. ~;
The gel was determined by refluxing a weighed sample of the
foamed polymer in boiling toluene; after drying, the insoluble
portion of the sample was weighed to calculate the percentage of gel
as follows:
gel = wt insoluble sam~le 100total wt of sample
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~This method is described in ~abelitem No. 128, Union Carbide ~1964) ;~
"Cure Testing of Vulcanized Polyethylene").
A non-acidic system is required to prevail since any acidic ~ -
component will inhibit the crosslinking reaction.
It was found that, to a certain extent, the presence o oxygen
disturbs the crosslinking of the polyethylene and that absorption of
oxygen from air by the polyethylene during the heating accompanying
the crosslinking react;on was responsible for the great variability
in cell size and ruptured cells. Thus, by carrying out the heating
in the substantial absence of oxygen, a foam is produced wherein the
cell size throughout the foam is reasonably uniformJ and a homogeneous ~,
crosslinking is achieved through all the thickness of the wall, as
expressed by the constant percentage of gel content. However, when
exygen was present, it was ~ound that a lower gel content exists in
the thin inner layer, the gel per cent beîng higher at the outer
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wall in contact with the mold than at the inner wall. Tharefore,
it is possible to obtain either a product with a constant, high
crosslinking content throughout the thickness of ~he wall when oxygen
; is absent in the system or a product with a lower gel content at the ; ~;~
inner surface, when oxygen is present in the system. ~A.'person skilled
in the art will select the proper conditions according to the specific
requir~ments of the desired products. `~
The process according to the presen~ invent:ion can be carried - ~ -
out by rotating the mold around two axes at a right angle to each other,
at a rotational speed in the range of 2-40 rpm. Another application of
, the process according to the present invention is the production of
i articles by the centrifugal molding, fo r example, in the production
of.cast tubes by a process which comprises rotating a cylindrical
mold around its longitudinal axis while said axis is horizontal.
The rota~ional speed or centrifugal molding may be varied over a
broad range, generally being between 200 and 2000 rpm.
The polyethylene suitable for the present invention includes low,
medium and high density homopolymers, provided that their melt flow
7 index is above 1.8. The use of a high quality polyethylane powder is
essential in rotational molding if good end products are to be
I obtained.
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Preferred types of polyethylene are the high density polyethylene,
I which has a density of about 0.96 and a melt flow index of about 5,
and the low density polyethylene which has a density of about 0.92 ;
! and a melt flow index of about 5.
The blowing agents to be used in the process according to the ~ ~
present invention are the common blowing agents known for this art, ~ ;
such as: 4,4' oxybis (benzene sulfonyl hydrazide3, dinitroso
pentamethylene tetraamine 80%, 4,4' oxybis (benzene sulfonyl
semicarbazide); 1-1 azobisformamide (azodicarbonamide). The :Last
one is the preferred blowing agent for the present invention.
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All these blowing agents act by producing N2, COJ C02, etc.
The gases expand the resin and control tha final density of the foam.
The amount of the blowing agent utilized in the preparation of
rotationally molded produc~s can be varied over a wide rango and will
obviously depend upon the degree of blowing desired. Usually, this
amount is in the range of 0.3-5~ and preferably in the range of
0.5-2%, based on the total weight of polyethylene. Amounts of
blowing agents outside this range generally do not produce desirable `;
foams. All the above-mentioned blowing agents have the property that
they decompose over a definite and short range of temperatures, the
gas being uniformly dispersed through the polymer. 'A~cording to the
present invention, it is possible ko obtain a process which is
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largely self-regulating by the proper selection of the amounts o~ ;
blowing and crosslinking agents. This comes out because the cell walls
become sufficiently strong to resist further expansion by the blowing ;~
agent. Accordingly, within reasonable limits, it is possible to
adjust the foam density of the finished products by adjusting the
mutual proportions of the crosslinking and blowing agents and other `
components of the system. A person skilled in the art will certainly
find the proper amounts within the range mentioned in the present `
specification in order to obtain products with the desired properties.
In accordance with another embodiment of the present invention, ;1
it is also possible to obtain composites of solid layers of a
thermoplastic resin and foamed layers of polyethylene. This is achieved
by a two-stage process: in the first one, a solid outer layer is
produced by rotational molding of a thermoplastic resin and~ in the
second stage, solid polyethylene, including the o~her ingredients
required according to the present invention, is added into the
rotating mold, thus obtaining an inner layer o foamed, crosslinked 1~
polyethylene. The outer layer may also be crosslinked, non-expanded .;
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polyethylene, in which case, the article produced by rotational
molding, although having two separate structural layers, will be
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based entirely on polyethylene. ~or certain purposes, this will be
of great advantage.
Fillers may also be used, either to reduce the cost of the
products or to impart specially desired properties to the foamed,
crosslinked polyethylene. The factors which have to be considered ;~
when adding fîllers are the viscosity of the mixture and the
required mechanical properties of the products.
It is absolutely requisite that a homogeneous mixture result$
from the mixing of the filler with the polyethylene, since otherwise -~ -
the products obtained will have a different appearance and feel on ~ ~-
their surfaces. Typical classes of fillers which can be used are
carbon black, cellulosic and other wood-derived fillers, inorganic
salts and oxides, inorganic flakes and fibers, etc. Examples of `~
the various fillers which can be used in the rotational molding of
crosslinked, foamed polyethylene are carbon black, calcium silicate,
aluminium silicate, silica gel, barium carbonate, glass beads, etc.
The proportion and particle size of the filler used depend upon ~;;
the rigidity and surface quality required in the product. Where the
proportion of filler is relatively low, the molding may have a
continuous surface skin of the polyethylene. High proportions of
filler, e.g. proportions of the order of 50% by weight, give products
having a rougher finished surface suitable for such applications as
insulating board or acoustic tiles.
The process according to this invention may further be used for
producing colored polyethylene molded items; to this end, plgmen$s
may be distribu~ed through the mixture entering into the rotational
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mold.
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The process can be employed to obtain unusual shapes of large
articles which would be difficult, if not impossible, to achieve by
conventional moldings. Shaped7 hollow articles o any size or shape
; can readily be prepared by rotational molding according to thepresent invention. For example, molds can be used for fabricating .
luggage, boat hulls, fuel tanks, aircraft stOTage tanks, etc.
The molds used are made of any s~eel or even aluminium. Aluminium
has been successfully used and found desirable from the standpoint
of temperature uniformity throughout the mold.
In order further to illustrate the nature of this invention and
the manner of practicing the same fully, the following examples are
presented for clearness of understanding therefrom, without being
limited thereto, as modifications will be obvious to those skilled
in the art. The physical properties of the foamed products given
in the examples were determined according to the test procedure
described in ASTM D-638.
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~ Example 1
¦ 100 parts of high density polyethylene ~melt ~low index 5.0,
density 0.95) were mixed with 0.7 parts of azodicarbonamide, 1 part
zinc oxide, 0.2 parts calcium stearate and 0.6 parts liquid 2,5-
dimethyl-2,5-di ~tert-butyl~peroxy) hexyne-3. All the ingredients
were homogenized for 2 minutes, introduced into a rotating cubic
mold of aluminium ~100 x 100 x 100 mm) and inserted into a cold
oven; the mold was then rotated at a speed of 20 rpm. The oYen
, was heated to 275C for 25 minutes while the mold continued to -`
i rotate. The oven was then opened and the rotating mold cooled by air ;~
i followed by water spray. A perfect cube of foamed, crosslinked`,~ polyethylene was obtained, having an average gel content of 70%,
density of 0.45 g/cm , tensile strength of 908 lb/sq in, and 40 per -;
' 30 cen~ elongation at failure.
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1~3~35~77 ` ~ ~ ~
Example 2
:
100 parts of low density polyethylene (melt flow index 6.5,
density 0.918) were mixed with 0.5 parts of azodicarbonamide, 0.25 -~
parts zinc~ oxide, 0.1 parts calcium stearate and 0.75 parts liquid
2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexyne-3. All the
ingredients were homogenized for 2 minutes, introduced into a `
rotating cubic mold of aluminium (100 x 100 x 100 mm) and inserted
into a cold oven; the mold was rotated at a speed of 20 rpm.
The oven was heated to 250C for 20 minutes while the mold continued
to rotate. The oven was then opened and the rotating mold cooled by
air, followed by water spray. A perfect cube of foamed, crosslinked ~` !
polyethylene was obtained, having an average gel content of 70%,
density 0.45 g/cm3, tensile strength 250 lbtsq in, and 95 per cent
elongation at failure. ``
Example 3
The same experiment as in the previous example ~2) was repeated
except that 0.5 parts of NaHC03 as blowing agent wereused instead of ; ;
azodicarbonamide and 1 part zinc stearate instead of zinc oxide.
The other reagents used and amounts were the same as in the previous
example. The cube o foamed, crosslinked polyethylene had a
gelcontent of 65% and a density of 0.395 g/cm3.
Example 4
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The same procedure as that in experiment 1 was used in this
example, with the same high density polyethylene and other ingredients -~
as mentioned there, except that, in this experiment, 2,5 parts of
powdered 2,5-dimethyl-2,5-di (tsrt-butyl-peroxy) hexane (50% acti~e)
were used. The cube of foamed, crosslinked polyethylene had a gel
content of 52%, density 0.42, tensile strength 725 lb/sq in, 34.4
per cent elongation at failure and 26% elongation at yield.
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Example 5
The same procedure at that in experiment 1 was carried out in
this example, using high density polyethylene (melt flow index 4.5
and density 0.945). The amounts of the ingredients were the same
as in experiment 1, the only difference being the use of 0.7 parts ~ ;
of tryhydrozinotria~ine (Trademark Genitron ~HT~ as blowing agent
instead of azodicarbonamide. In this experiment, the oven was `
heated to 290C. A perfect cube of a foamed, crosslinked polyethylene
was obtained which ha~ a gel con~ent of about 70% and density of
0.39 g~cm3.
i Example 6
50 parts of high density polyethylene (melt flow index 5.0,
density 0.95) were introduced lnto a rotating cubic mold o~ aluminium
(100 x 100 x 100 mm), the mold put into a cold oven and heated to
275C for 20 minutes while the mold continued to rotate. l`he oven was
then opened and the rotating mold cooled by air followed by water -~
spray. The mold was opened, the cube taken out and a hole of 1 cm ``
diameter drilled, through which a homogeneous mixture consisting of
the following ingredients was introduced: 100 parts of high density
polyethylene (melt flow index 5.0, densit~ 0.95), 0.7 parts ;
, azodicarbonamide, 1 part zinc oxide and 0.6 parts liquid
2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexyne-3. The cube was
again inserted into the mold and rotated at a speed of 20 rpm for 25
minutes at a temperature of 275C. The oven was then opened and
the rotating mold cooled by air followed by water spray. A perfect
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cube of a double layer of polyethylene was obtained, the outer layer
being solid high density polyethylene and the inner one being foamed,
crosslinked polyethylene of 68% gel content and 0.43 g/cm3 density.
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