Note: Descriptions are shown in the official language in which they were submitted.
1054Z98
This invention relates to a process for the production of
foam plastics with a fine cellular structure from polymers of
~ monoolefinically unsaturated monomers by cross-linking and
foaming the polymer in the presence of a highly disperse ampho-
teric oxide.
It is known to add organic peroxides and chemical blowing
agents to polyethylene and extrude the mixture to form a plate
under such conditions that neither cross-linking takes place to
any significant extent nor the blowing agent is decomposed. The
plate may then be heated in a hot air oven sufficiently to start
the cross-linking reaction and the temperatury may then be rai-
sed to decompose the blowing agent whereby the gas liberated
foams up the cross-linked polyethylene (see German Offenlegungs-
schrift No. 1,694,130 and US-Patent Specification No. 3,098,831).
The blowing agent preferably used for this process is azo
dicarbonamide because it has no deleterious effect on ths cross-
linking of polyethylene by organic peroxides.
But azodicarbonamide causes the formation of a foam with
relatively large cells. Therefore it was tried to lower the de-
composition point of azodicarbonamide by adding so-called "kik-
kers" and thus to influence the size of the cells of the fini-
shed foam product. Substances which have been used as kickers
includeJ eOgO compounds of magnesium, zinc, cadmium, barium or
leadO
These kickers, however, cannot be used in the process
for the production of polyethylene foam wherein the foam is
cross-linked by peroxides9 because the kicker destroy the func-
tion of the peroxide and hence partly or completely prevent
cross-linking and thus inhibit the subse~uent foaming process.
Accordingly foams with a density too high are produced. More-
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over, the cells are not smaller but, if anything, larger because decomposit-
ion of the blowing agent starts earlier, at a stage ~hen cross-linking has
not yet sufficiently progressed.
If was therefore an object of this invention to reduce or sub-
stantially to obviate the disadvantages of the known blowing agents and pro-
duce thermoplastic foams with particularly fine cells uniformly distributed.
It has surprisingly been found that this object could be achiev-
ed by adding a small quantity of an amphoteric oxide to the foamable mixture,
optionally together with blowing agents.
This invention relates to an improved process for the production
of a foam plastics with fine cells from a polymer based on an a,~-monoole-
f;n;cally unsaturated monomer by peroxidic cross-linking and foaming with a
chemical blowing agent selected from azodicarbonamide or dinitrosopentame-
thylene-tetramine, said polymer, the improvement comprises adding in addition
to of from 0.3 to 1.5% by weight, based on the total mixture of said perox-
ide and of from 2 to 30%, based on the total mixture of said blowing agent,
from 0.01 to 10% by weight, based on the blowing agent, of pyrogenic orpre-
cipitated silica to said polymer subjecting the mixture thus obtained to a
shaping process and cross-linking and foaming by heating it to a temperature
of at least 190& .
This invention also relates to foam plastic- material prepared in
accordance with the above process. The polymers based on a,~-monoolefini-
cally unsaturated monomers may be polyolefins with 2 to 4 carbon atoms, pre-
ferably polyethylene, including both low pressure polyethylene (d~ 0.94 -
0.97 g/cm3) and high pressure polyethylenes (d~ 0.01 to about 0.94 g/cm3) or
polypropylenes, polymers or optionally halogenated monovinyl monomers with 2
to 8 carbon atoms, e.g. polyvinyl chloride, polystyrene or ethylene vinyl
ester copolymers, preferably polyethylene vinyl acetate containing up to 30%
by weight of vinyl acetate.
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1054Z98
The peroxides used for cross-linking are organic peroxi-
des such as dicumyl peroxide, 2,5-dimethyl-2,5-di-(tert.-butyl-
p~roxy)-hexane, 2,5-dimethyl-2,5-di-(tert.-butylperoxy)-hexyne,
tert.-butylhydroperoxide, cumyl-tert.-butylperaxide, di-tert.-
butylperoxide or bis-(tert.-butyl-peroxi-isopropyl)-benzene, di-
cumyl peroxide being preferred.
The peroxides are used in quantities of 0.3 to 1.5 Yo by
weight, based on the total mixture. They have a decomposition
point or range of about 150 to 1~0C.
The chemical blowing agents with a deco~position point
or range of about 190C used are azodicarbonamide and dinitroso-
pentaméthylenetetramine, preferably azodicarbonamide in quanti-
ties of 2 to 30 %, based on the total mixture
-~ The highly disperse, amphoteric oxides are added to thepolymer to be foamed in quantities of 0.01 to 10 % by weight,
based on the blowing agent, preferably 0.01 to 2 % by weight.
The oxides preferably used are alumina, stannic oxide, zinc
oxide, pyrogenic or precipitated silica and the corresponding
commercial products such as Aerosilen@ or Vulkasilen~ (very
pure, highly disperse silica with amorphous structure). Silica
is particularly preferred.
The mixing of the polymers with the cross-linking agents,
blowing agents and the oxide may be carried out in any mixing -
apparatus, preferably in an extruder, followed by a shaping
step at temperatures below the decomposition point of the per-
oxide. The a~erage residence time Ln the extruder is about 5
minutes in order to pre~ent any substantially cross-linking or
foaming of the foamable mixture at this stage.
Cross-linking takes place at temperatures upwards of
160C and foaming at temperatures of 190 to 250C. According
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to a preferred embodiment, the highly disperse oxide in the ~orm
of a solid, solution or gel is first mixed with the blowing
agent, e.g. by grinding in a ball mill, and then added to the
peroxide-containing polymer. Alternatively, the oxide may be
added to the blowing agent in a preiiminary stage durLng the
preparation of the blowing agent.
The foam products produced by the process according to
the invention are distinguished by their uniform, fine cellu-
lar pore structure and soft, velvety surface and may be used
in the building and packaging industry as insulating material.
The densities of the foams produced according to the invention
lîes between 20 and 200 kglm3. The foams preferably have 18 or
more cells per cm, which corresponds to a pore number of
194,400 per g in a foam which has a density of 30 kg/m3.
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Comparison Example 1
A paste consisting of
50 parts of polyvinylchloride paste, K-value 70,
37.5 parts of dioctylphthalate,
12.5 parts of ben yl butyl phthalate,
1.0 part of azodicarbonamide,
1.5 parts of dibasic lead phthalate
is painted on a siliconised cotton fabric and foamed up in
a gelling channel at a temperature of 190C for 3.5 minutes.
The pure white foam obtained, which has a thickness of
1.1 mm and a density of 0.35 g/cm3, has a soft, smooth
surface with a velvety handle and uniform cell structure
with an average pore diameter of 0.25 mm.
Example 1
990 g of azodicarbonamide and
10 g of disperse silicic acid are homogenised in a ball
mill for 5 hou~s.
A polyvinylchloride paste of the same composition as in
comparison Example 1, except that silica has been added to the
azodicarbonamide, is painted on a siliconised cotton fabrlc
and foamed in a gelling channel at 190C for 3.5 minutes.
The resulting foam has a thickness of 1.19 mm and a
density of 0.31 g/cm3 and a completely flat, matt surface,
soft handle and absolutely uniform cell structure. The
cell diameter is 0.18 mm.
Comparison ExamPle 2
A mixture of 70 part~ of a high pressure polyethylene
having a density of 0.921 g/cm3 and a melt index of 3.5 g
per 10' at 190C and 2.16 kp loading and 30 parts of a
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commercial azodicarbonamide is homogenised and granulated
in a double screw extruder of the kind conventionally u~ed
for producing thermoplast granulates.
The resulting granulate, mixed in proportions of
1 : 1 with a granulate which has been obtained in the same
manner from 98 parts of the polyethylene described and 2
parts of dicumyl peroxide, is used to produce a band 30 mm
in width, ~.5 mm in thickness and any desired length. This
band is cross-linked and ~oamed up by passing it through a
continuous heatLng furnace in which it is heated to 215C.
A cross-linked polyethylene foam 11 mm in thickness,
850 mm in width and weighing 33 g/litre is obtained. ~he
interior of the foam was pure white but the surface was
yellowish and slightly uneven. It had a uniform cell
structure and an average pore diameter of 1 mm.
Example 2
~he azodicarbonamide concentrate described in
; comparison Example 2 is prepared not from commercial azo-
dicarbonamide but from the mixture of azodicarbonamide and
silica homogenised in a ball mill as described in Example 2.
This concentrate is processed to produce a cross-
linked polyethylene foam as described in comparison
Example 2.
The resulting foam is 910 mm in width, 13 mm in
thickness and has a density of 28.5 g/litre. It has
almost pure white, even surfaces and a very uniform cell
structure with an average pore diameter of 0.35 mm~
Example 3
50 kg of azodicarbonamide, either moist or dry, are
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suspended in 50 1 of water. 4 kg of sodium silicate
(d24 : 1.35, approximately 25~ by weight silica) are
added to the stirred dispersion which is then adjusted
to pH 1-2 with approximately 2.5 l of hydrochloric acid
(dilution 1 : 1). ~he reaction mixture is stirred for
30 minutes, filtered and washed with water until neutral
and the moist blowing agent is dried under vacuum. When
the agglomerates have been broken up mechanically, a free
flowing powder is obtained which is used for foaming high
pressure polyethylene as described in comparison ~xample 2.
A foam 12 mm in thickness and 900 mm wide and weighing
32 g/l is obtained. It has a pure white, uniform cell
structure with an average pore diameter of 0.45 mm.
Example 4
A12 (S04)3 . 18 H20 is dissolved in water, precipitated
with sodium hydroxide solution and redissolved in an excess
of water and added to an aqueous suspenslon of azodicarbon-
amide in a quantity of 2 % calculated as the hydroxide and
based on the quantity of azodicarbonamide. The suspension
is stirred for 30 minutes and then adausted to pH 1-2 with
half concentrated hydrochloric acid. After stirring for
30 minutes, the reaction mixture is filtered and washed with
water until neutral and the moist blowing agent obtained is
dried under vacuum. After the agglomerate has been broken
up mechanically, a free flowing powder is obtained which is
used for foaming high pressure polyethylene as described in
comparison Example 2.
A foam which is only slightly yellow on the surface is
obtained. It has a thickness of 11 mm, a width of 890 mm, a
density of 35 g/l and a pore diameter of 0.45 mm,
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