Method of manufacturing a composite foamed polyolefin sheet

A method of manufacturing a foamed polyolefin sheet which comprises the steps of laminating a film of noncross-linked thermoplastic resin free from a cross-linking agent to at least one surface of a cross-linked polyolefin base sheet containing a thermally decomposable foaming agent or a polyolefin base sheet containing the above-mentioned foaming agent and a cross-linking agent, and carrying out the thermal foaming of said film-fitted sheet while placing it on a supporting member under atmospheric pressure with the film-laminated surface of the polyolefin sheet being in contact with said supporting member.

This invention relates to a method of manufacturing a foamed polyolefin 
sheet and more particularly to a method of manufacturing a cross-linked 
foamed polyolefin sheet, which prevents a foamed polyolefin base sheet 
from sticking to, for example, a supporting member carrying said base 
sheet, thereby providing a foamed polyolefin sheet having a smooth surface 
and displaying good thermal workability. 
Foamed plastics material generally has desirable properties including light 
weight, heat insulation, sound absorption, and shock absorption and is 
used in wide applications, for example, a structural material, packing 
material and floating material. Among others, a foamed polyolefin sheet 
has a proper degree of mechanical strength, excellent flexibility in 
particular and is little subject to permanent set, namely, is 
characterized by more desirable qualities than any other foamed plastics 
material, and consequently is favorably accepted in various fields of 
industry. A cross-linked foamed polyolefin sheet in particular is more 
improved than a noncross-linked foamed polyolefin sheet in respect of heat 
resistance which represents the common weak point of general foam, and 
consequently is characterized by good thermal workability. 
The above-mentioned cross-linked foamed polyolefin sheet should be 
manufactured by mixing a cross-linking agent and foaming agent with raw 
polyolefin, forming the mixed mass into a sheet in shape, and thermally 
foaming said sheet. However, this process has presented difficulties in 
producing the cross-linked foamed polyolefin sheet, because the foamable 
sheet, while heated for foaming, still remains in a molten state and often 
tends to stick to a supporting member carrying the sheet thereon, with the 
result that the foamed sheet, fails to be easily separated therefrom. 
In view of the above-mentioned drawbacks, the present inventors previously 
proposed a method (U.S. Pat. No. 3651183) of manufacturing a cross-linked 
foamed polyolefin sheet, which comprises mixing a cross-linking agent and 
a foaming agent having a higher decomposition point than said 
cross-linking agent with the raw polyolefin, forming the mixed mass into a 
sheet in shape, supporting the sheet on a supporting member of a wire net 
conveyor and heating the sheet by hot air in a furnace under atmospheric 
pressure to effect the foaming of said sheet. The above-mentioned proposed 
method further includes the process of supporting the foamable sheet on 
the wire net conveyor and applying strong heat only to the surface of the 
foamable sheet before carrying out its foaming in a furnace. According to 
the first mentioned method, reduction of a contact area between the 
foamable sheet and wire net supporting member, and hot air heating enable 
the foamed sheet to be easily separated from the wire net supporting 
member and moreover effect uniform heating. The latter process more 
facilitates the removal of the foamed sheet from the wire net supporting 
member. 
Where, however, the raw resin consists of polyolefin having a high melt 
index, or ethylene-vinyl acetate resin or said polyolefin or 
ethylene-vinyl acetate resin mixed with a flame retardant such as antimony 
oxide or halogen compounds, or with rubber or wax to improve softness, 
then a foamable sheet prepared from the above-mentioned resin or mixtures 
thereof presented high stickiness to the supporting member when melted for 
foaming. Therefore, any of the aforesaid proposed processes failed to 
attain the easy separation and in consequence the efficient manufacture of 
a foamed sheet. 
In view of the above-mentioned circumstances, the present inventors 
continued further studies and as a result, have found that when a film of 
noncross-linked thermoplastic resin free from a cross-linking agent is 
fitted to at least one surface of a cross-linked polyolefin base sheet 
containing a thermally decomposable foaming agent or a polyolefin base 
sheet containing a foaming agent and cross-linking agent and said 
film-laminated sheet is heated to foam under heat at atmospheric pressure 
with the film-laminated surface of said sheet being in contact with a 
supporting member, then said sheet can be efficiently foamed regardless of 
its composition without sticking to the support member, and that surface 
of the resultant foamed polyolefin sheet which faces the supporting member 
presents a smooth surface. 
The Japenese Pat. No. 691,761 sets forth a process of producing a white 
foamed polyolefin sheet which consists of cross-linking a polyolefin base 
sheet containing a foaming agent, laminating a cross-linked film or a film 
containing a cross-linking agent to both surfaces of said cross-linked 
polyolefin base sheet and heating on a bath of molten metal salt the 
foamable laminate polyolefin sheet, thereby highly improving effective 
foaming ratio which is a ratio of an amount of gas by volume (cm.sup.3) 
contained in the foam thus obtained to that of total gas evolved from the 
foaming agent used. Where, however, the foamable polyolefin base sheet 
sandwiched between the above-mentioned films is actually foamed while 
being supported on, for example, a wire net, then said cross-linked films 
on the surfaces of said base sheet tends to stick to the wire net 
supporting member, fails to have a smooth surface and presents 
difficulties in being stretched with the resultant occurrence of cracks. 
Moreover, the polyolefin sheet thus foamed prominently shrunk when 
reheated for press molding or vacuum forming. Further, U.S. Pat. No. 
3,655,470 discloses a process of producing a foamed thermoplastic resin 
sheet by laminating a film free from a foaming agent to both surfaces of a 
base sheet of foamable thermoplastic resin and foaming said base sheet 
while supporting it on a wire net. This U.S. patent process, however, 
simply consists of foaming a base sheet of polyvinyl chloride resin 
containing a foaming agent after laminating a film of polyvinyl chloride 
resin free from a foaming agent to said base sheet. Unlike the present 
invention, the U.S. patent does not include the process of cross-linking a 
base sheet. As naturally expected, therefore, the foamed sheet obtained by 
said U.S. patent does not stick to a supporting member. Nor is roughened 
that surface of the resultant foamed sheet which faces the supporting 
member. Accordingly, said U.S. patent is quite different from this 
invention which is characterized by laminating a noncross-linked film to a 
foamable base sheet and cross-linking said base sheet. In short, said U.S. 
patent does not offer a process of eliminating the drawback. The reason is 
that cross-linked or cross-linkable thermoplastic resin itself, while 
being heated and placed on a supporting member, tends to stick to the 
supporting member, and is therefore difficult to take off therefrom. 
The object of this invention is to provide a method of manufacturing a 
foamed polyolefin sheet, which comprises the steps of laminating a 
noncross-linked thermoplastic resin film free from a cross-linking agent 
to at least one surface of a base sheet of cross-linked polyolefin 
containing a thermally decomposable foaming agent or a base sheet of 
polyolefin containing said foaming agent and a cross-linking agent and 
heating for foaming said film-laminated sheet under atmospheric pressure 
with the film-laminated surface of the sheet being in contact with a 
supporting member. 
The new methods of this invention may be more fully understood by reference 
to the appended flow sheet. Either a method employing an ionizing 
radiation or a method using a cross-linking agent can be used in this 
invention for obtaining the desired product of crosslinked polyolefin 
foam. As shown in the upper level of the appended flow sheet, the method 
of this invention for the case of utilizing ionizing radiation comprises: 
a first step of mixing a foaming agent into polyolefin, 
a second step of forming the mixture into a sheet, 
a third step of subjecting the sheet to ioning radiation for crosslinking 
the polyolefin, 
a fourth step of laminating a non-crosslinked resin film free from a 
crosslinking agent to the crosslinked sheet, and 
a fifth step of heating on a supporting member the film-laminated sheet, 
with the non-crosslinked resin film being in contact with the supporting 
member, at a temperature higher than the decomposition temperature of the 
foaming agent. 
The lower level of the appended flow sheet denotes the case of using a 
crosslinking agent. As shown in the drawing the method of this invention 
for this case comprises: 
a first step of mixing foaming and crosslinking agents into polyolefin, 
a second step of forming the mixture into a sheet, 
a third step of laminating a non-crosslinked resin film being in contact 
with the supporting member, at a temperature higher than the decomposition 
temperature of both the agents to form a foamed sheet. 
There will now be detailed the method of this invention for manufacturing a 
foamed polyolefin sheet. The method of the invention uses a base sheet of 
cross-linked polyolefin containing a thermally decomposable foaming agent 
or a base sheet of polyolefin containing said thermally decomposable 
foaming agent and a cross-linking agent. 
The former polyolefin base sheet is prepared by mixing a thermally 
decomposable foaming agent with raw polyolefin, forming the mixed mass 
into a sheet in shape by an ordinary process, and irradiating ionizing 
radiation on the sheet for cross-linking. The latter polyolefin base sheet 
is prepared by mixing the aforesaid foaming agent and a cross-linking 
agent with the raw polyolefin and forming the mixed mass into a sheet in 
shape by a customary process. 
The raw polyolefin used in the method of this invention includes polymers 
of, for example, ethylene, propylene and butene, copolymers of said 
olefins and, for example, vinyl acetate or acrylic acid, or mixtures of 
said polymers and copolymers. If necessary, it is possible to mix the 
above-defined raw polyolefin with rubber or plastics material miscible 
therewith in a smaller amount than said raw polyolefin. 
The thermally decomposable foaming agent is of the type which thermally 
decomposes itself to evolve gas and includes, for example, 
p,p'-oxybis(benzene sulfonyl hydrazide), azodicarbonamide, 
dinitrosopentaethylenetetramine and p-toluene-sulfonyl semicarbazide. 
Where the foaming agent is applied jointly with a cross-linking agent, 
said foaming agent is preferred to have a higher decomposition temperature 
than that of the cross-linking agent. 
The cross-linking agent may be of ordinary type, such as of organic 
peroxides, for example, dicumyl peroxide; azides, for example, 
1,10-decane-bis-sulfonazide and m-phenylenediazide; silane compounds 
having a vinyl group, for example, vinyltriethoxysilane and 
vinyltrimethoxysilane; and silanol condensation catalyst, for example, 
dibutyltin dilaurate and dibutyltin diacetate. Sometimes, triallyl 
cyanurate and triallyl isocyanurate may be used as a cross-linking 
coagent. The amount of foaming agent and cross-linking agent mixed with a 
raw polyolefin should advisably be determined according to, for example, 
heating temperature for foaming and applications for which the product of 
foamed polyolefin sheet is intended. 
According to the method of this invention, a film of noncross-linked 
thermoplastic resin free from a cross-linking agent is laminated to one or 
both surfaces of a polyolefin base sheet. The film-laminated sheet is 
heated under atmospheric pressure on a movable supporting member with the 
film-laminated surface of the base sheet being in contact with the 
supporting member. It is preferred to place the film-laminated sheet on a 
wire net conveyor and heat the sheet by hot air in a furnace for the 
cross-linking and foaming of the polyolefin sheet. 
The thermoplastic resin film will well serve the purpose if it can be 
stretched while being closely attached to the polyolefin base sheet to the 
same extent as that to which said base sheet expands. Form this point of 
view, the thermoplastic resin film is usually preferred to be of the same 
type as the polyolefin base sheet. However, the film need not be limited 
to this type, but may be formed of others. A film made of polyethylene, or 
polypropylene is favorably accepted as the thermoplastic resin film used 
in this invention. It is generally desired to use a film formed of polymer 
having a small melt index or high melting point due to its low stickiness. 
The thermoplastic resin film laminated to a polyolefin base sheet is 
generally chosen to have a thickness of 0.05 to 1 mm, or preferably 0.2 to 
0.5 mm. When the polyolefin base sheet expands, the film is usually 
stretched to such an extent that its original thickness is reduced to 
one-tenth, constituting a skin of the foamed polyolefin sheet. Where the 
above-mentioned thermoplastic resin film is laminated only to one surface 
of a polyolefin base sheet and said sheet is heated for foaming while 
being placed on a supporting member such as a wire net conveyor, then it 
is necessary to mount the film-laminated sheet on the supporting member 
with the film-laminated surface of the polyolefin base sheet being in 
contact with said supporting member. The thermoplastic resin film is 
customarily laminated on a foamable polyolefin base sheet, after the sheet 
is prepared. However, it is possible to extrude a polyolefin base sheet 
and a thermoplastic resin film at the same time by two extruders 
respectively and cover the whole surface of the polyolefin base sheet with 
the thermoplastic resin film. Further, it is possible to apply plastics 
powder on the surface of a foamable polyolefin base sheet and melt said 
powder or coat a plastics solution on said surface followed by drying, 
instead of laminating a thermoplastic resin film thereto. 
As previously described, the method of this invention comprises fitting a 
film of noncross-linked thermoplastic resin free from a cross-linking 
agent to a cross-linked foamable polyolefin base sheet or a foamable 
polyolefin base sheet containing a cross-linking agent and thermally 
foaming said polyolefin film-fitted sheet while placing said sheet on a 
movable supporting member. Namely, said cross-linked polyolefin base sheet 
of polyolefin base sheet containing a cross-linking agent is thermally 
foamed without its directly touching the supporting member, but indirectly 
supported through the medium of the aforesaid thermoplastic resin film. 
Therefore, the foamed sheet does not stick to the supporting member, and 
can be easily taken off therefrom. 
Though the reason for such favorable effect has not yet been clearly 
defined, it is assumed that a thermoplastic resin film free from a 
cross-linking agent is little oxidized and has a lower stickiness to the 
supporting member than a cross-linked polyolefin base sheet. Further, the 
method of this invention allows a foamable base sheet made of one kind of 
material to be laminated with a film formed of another kind of material. 
Therefore, it has become possible to foam continuously and easily even a 
polyolefin sheet consisting of an ethylene-vinyl acetate copolymer 
containing a large amount of vinyl acetate group or mixed with a flame 
retardant such as antimony oxide or chlorinated paraffin which has 
hitherto been regarded as unusable because, when foamed, it is highly 
liable to stick to a supporting member, by laminating a film made of, for 
example, polyethylene to said polyolefin sheet. 
Another reason why a polyolefin sheet foamed with the laminated film being 
in contact with a supporting member does not stick to the supporting 
member is presumably that a cross-linked polyolefin base sheet has a 
property of uniformly expanding in three dimensional directions; release 
of gas through the film fitted to the polyolefin base sheet is more 
restricted than that of gas from the upper surface of a polyolefin base 
sheet which has no film thereon, leading to the larger expansion of the 
underside of the polyolefin base sheet, and consequently causing the both 
longitudinal ends of the foamed polyolefin sheet to be curved upward, that 
is, in the form of a gutter laid horizontally relative to the surface of a 
supporting member; and the film-fitted foamed polyolefin sheet is carried 
forward under this condition while being supported on the supporting 
member. Where, therefore, a film is laminated also to the upper side of a 
polyolefin base sheet, said upper side film should preferably have a 
higher gas permeability than the lower side film. 
Where, according to this invention, a polyolefin base sheet on which the 
aforesaid thermoplastic resin film is laminated is thermally foamed by 
hot-air heating in a furnace while being supported on a wire net conveyor 
received therein, then said film-laminated sheet thus foamed is more 
easily separated from the wire net supporting member. A wire net used as 
said supporting member is generally made of stainless steel and has 10 to 
100 meshes or preferably 30 to 50 meshes. Where a polyolefin base sheet 
laminated with a film of noncross-linked thermoplastic resin free from a 
cross-linking agent is foamed while being supported on the supporting 
member, then that surface of the resultant foamed sheet which faces the 
supporting member presents an attractive, smooth surface. This is supposed 
to be because, when heated on a supporting member, a cross-linked 
polyolefin base sheet or a polyolefin base sheet containing a 
cross-linking agent is thermally deteriorated due to overheating, unless 
the base sheet has a noncross-linked thermoplastic film free from a 
cross-linking agent thereon, whereas a noncross-linked thermoplastic film 
free from a cross-linking agent is little liable to thermal deterioration 
due to overheating and presents a smooth surface. Laminating of a 
thermoplastic resin film to a polyolefin base sheet has the advantage that 
when the base sheet is heated for foaming, gases evolving from a foaming 
agent or cross-linking agent contained in the base sheet are shut up in 
the base sheet itself by the laminated film and prevented from escaping to 
the outside, thereby suppressing air pollution by gases, and moreover when 
the base sheet is thermally foamed, the laminated film checks the 
occurrence of cracks on the surface of the foamed base sheet which might 
otherwise result from the oxidation of said surface, thus enabling a 
foamed polyolefin sheet to be manufactured with a larger thickness than 
has been possible in the past. 
According to this invention, a foamed polyolefin base sheet of high heat 
resistance covered with a noncross-linked film skin can be heat laminated 
with other materials at a relatively low temperature and under a stable 
condition. The film skin has an extremely uniform thickness and is little 
subject to fructure, presenting prominent water-proofness.

This invention will be more fully understood with reference to the 
following examples, in which parts are expressed by weight. 
EXAMPLE 1 
100 parts of low density polyethylene having a melt index of 1 was 
homogeneously mixed with 15 parts of azodicarbonamide, and 1 part of 
dicumyl peroxide. The mixed mass was extruded into a base sheet 350 mm 
wide and 1.7 mm thick. A 0.3 mm thick film formed of similar low density 
polyethylene was laminated to the underside of the above-mentioned base 
sheet. The laminated sheet was heated for 5 minutes by hot air at 
220.degree. C in a furnace while being supported on a wire net conveyor to 
foam the aforesaid sheet. The foamed polyethylene sheet thus obtained had 
a width of 1000 mm, a thickness of 6 mm and a density of 0.035 g/cc, and 
had its underside covered with a film skin of uniform thickness. The 
underside of said foamed sheet presented a smooth surface. A foamed 
polyethylene sheet was produced continuously for 72 hours. However, the 
foamed polyethylene sheet did not stick to the wire net conveyor at all. 
CONTROL 1 
A high pressure polyethylene base sheet consisting of the same material as 
in Example 1 was foamed in the same manner as in Example 1, with the 
polyethylene film laminated to the upper surface on the base sheet. The 
foamed polyethylene sheet thus obtained had a tendency to become sticky 
and indicated traces of adhesion to the wire net in several places. 
Consequently, that side of the foamed polyethylene sheet which faced that 
wire net did not present a smooth surface. 
EXAMPLE 2 
The low density polyethylene film laminated to the low density polyethylene 
base sheet of Example 1 containing a foaming agent and cross-linking agent 
was replaced by a polypropylene film having the same thickness as said low 
density polyethylene film. Foaming was carried out in the same manner as 
in Example 1. The foamed polyethylene sheet thus obtained had a good 
surface appearance like the product of Example 1 and did not stick to the 
wire net conveyor. That surface of the foamed polyethylene sheet on which 
the polypropylene film was laminated was very smooth. 
EXAMPLE 3 foamed 
100 parts of ethylene-vinyl acetate copolymer (having a melt index of 2 and 
containing 25% of vinyl acetate) was mixed with 15 parts of 
azodicarbonamide, and 0.5 part of dicumyl peroxide. The mixed mass was 
extruded into a base sheet 350 mm wide and 1.4 mm thick. A 0.1 mm thick 
high density polyethylene film (having a density of 0.96 and a melt index 
of 2) was laminated to the upper surface of the sheet and a 0.3 mm thick 
film of the same material was laminated to the underside of said base 
sheet. The laminated sheet was heated to 230.degree. C while being 
supported on a stainless steel net conveyor received in a furnace to foam 
the laminated sheet. The foamed sheet did not stick to the stainless steel 
net at all. The foame sheet thus obtained had a density of 0.035 g/cc and 
displayed a smooth surface. 
In contrast, a base sheet consisting of the same material as described 
above, but lacking the high density polyethylene film stuck to the 
stainless steel net when foamed under the same conditions, and failed to 
present a smooth surface. 
EXAMPLE 4 
100 parts of low density polyethylene having a melt index of 2 was first 
mixed with 15 parts of chlorinated paraffin, 15 parts of antimony trioxide 
and 1 part of basic magnesium carbonate and then 10 parts of 
azodicarbonamide and 1 part of dicumyl peroxide. The mixed mass was 
extruded into a base sheet in the same manner as in Example 3 and the same 
kind of film as used in Example 3 was laminated to both surfaces of the 
base sheet. The laminated sheet was heated to foam the base sheet. The 
foamed sheet thus obtained had a density of 0.06 g/cc, showed a smooth 
surface and proved flame retardant. 
In contrast, where the high density polyethylene film was omitted, the 
foamed sheet struck to the supporting member when thermally foamed in the 
same manner as in Example 3, and failed to have a smooth surface. 
CONTROL 2 
The low density polyethylene film fitted to the foamable base sheet used in 
Example 1 was replaced with a low density polyethylene film cross-linked 
by irradiation of electron ray of 10 mega rad units. The sheet was foamed 
with said cross-linked film being in contact with the supporting member. 
The foamed sheet thus obtained tended to stick to the supporting member. 
The cross-linked film which had a low elongation was subject to cracking, 
preventing the foamed sheet from presenting a smooth surface. Further, the 
foamed sheet noticeably shrunk upon reheating and had low thermal 
workability. 
EXAMPLE 5 
A foamed sheet obtained in Example 1 was laminated under pressure with an 
iron plate covered with an embossed soft vinyl chloride resin sheet on one 
side, which plate was heated to 120.degree. C, while the film-laminated 
surface of the foamed sheet was brought into contact with the iron plate 
on the opposite side. Said lamination was effectively carried out because 
the foamed sheet was provided with a noncross-linked film skin. In 
contrast, a foamed sheet obtained without laminating a film thereto failed 
to be satisfactorily laminated with the above-mentioned iron plate. To 
effect lamination between said iron plate and a foamed sheet lacking a 
noncross-linked film required said iron plate to be heated to a higher 
temperature than 160.degree. C. However, such high temperature deformed 
the embossed surface of the soft vinyl chloride resin sheet coated on the 
iron plate. 
As mentioned above, the method of this invention provides the foamed 
polyolefin sheet which does not stick to the supporting member and 
presents a very smooth surface and displays high thermal workability.