Method for producing foamed plastic sheet

A method for producing a foamed plastic sheet which comprises heating and kneading a thermoplastic synthetic resin composition containing a blowing agent in an extruder, extruding the resulting gel through a circular die provided at the end of the extruder, and pinching the tubular foamed extrudate with a pair of pinch rolls while it is still in the softened state to adhere the inside wall of the foamed tubing to itself, is characterized in that said pinching and adhesion are performed by using a pair of pinch rolls having a width at least 5% smaller than the width of the tubular foamed extrudate extruded in the flattened state so that non-adhered parts are formed at both side end portions of the pinched foamed sheet in the widthwise direction and the inside of the non-adhered parts communicates with the open air.

This invention relates to a method for producing a foamed plastic sheet. 
More specifically, it relates to an improvement in a method for producing 
a plate-like foamed plastic sheet which comprises extruding a gel of a 
foamable synthetic resin containing a blowing agent through a circular die 
provided at the end of an extruder, and pinching the tubular foamed 
extrudate with a pair of pinch rolls having a width at least 5% smaller 
than the tubular foamed extrudate in the flattened state while it is still 
in the softened state to adhere its inside surface to itself. 
A method is known to produce a foamed plastic sheet in flat form by 
extruding a gel of a foamable plastic composition comprising a 
thermoplastic synthetic resin such as polystyrene and a volatile blowing 
agent such as butane through a circular die provided at the end of an 
extruder, and pinching the tubular foamed extrudate with a pair of pinch 
rolls while it is still in the softened state to adhere its inside surface 
to itself (see, for example, U.S. Pat. No. 3,804,684). This method is 
useful for the production of a plate-like sheet having a thickness of 2 to 
20 mm which is difficult to make by a method which involves incising the 
tubular foamed extrudate obtained by using a circular die. To produce a 
plate-like foamed sheet stably for a long period of time by this method, 
technical problems inherent to it should be solved. 
The most frequently encountered drawbacks in the actual production of 
plate-like foamed sheets by this method are that the tubular foamed 
extrudate is liable to be distorted and cannot be obtained in a 
predetermined configuration and a uniform thickness, and that the 
melt-adhered surface of the inside surface of the tubular foamed extrudate 
is susceptible to delamination owing to incomplete adhesion. 
There are various causes for these troubles. One of them is the fluctuation 
of the pressure of the air forced into the inside of the tubular foamed 
extrudate. This is due to the partial dissipation of the gasified blowing 
agent during the foaming of the resin, which gas adds to the air already 
forced into the closed space within the foamed tubing. 
As a means for solving this problem, Japanese Laid-Open Patent Publication 
No. 1057/73 suggests a method for maintaining the pressure of the inside 
of the tubing constant by using a molding device equipped with a control 
means for the pressure of the inside of the tubular foamed extrudate. This 
method, however, has the defect that the apparatus is complicated. 
It is an object of this invention therefore to provide a method capable of 
controlling the fluctuation of the pressure of the air forced into the 
foamed tubing, and maintaining the forming and adhering of the tubular 
foamed extrudate in the best condition. 
The object of this invention is achieved by an improved method for 
producing a foamed plastic sheet which comprises heating and kneading a 
thermoplastic synthetic resin composition containing a blowing agent in an 
extruder, extruding the resulting melt through a circular die provided at 
the end of the extruder, and pinching the tubular foamed extrudate with a 
pair of pinch rolls while it is still in the softened state to adhere the 
inside surface of the foamed tubing to itself; characterized in that said 
pinching and adhesion are performed by using a pair of pinch rolls having 
a width at least 5% smaller than the width of the tubular foamed extrudate 
in the flattened state so that non-adhered parts are formed at both side 
end portions of the pinched foamed sheet in the widthwise direction and 
the inside of the non-adhered parts communicates with the open air. 
Since non-adhered parts are formed at both side portions of the flattened 
foamed sheet, the gas inside the tubular foamed extrudate is released into 
the open air through the non-adhered portions in the step of forming the 
plate-like foamed sheet from the tubular foamed extrudate, and therefore, 
the inner pressure of the tubular foamed extrudate can be controlled to 
bring about the advantages described hereinbelow. 
In the method of this invention, a mixture of 30 to 90 mole% of a volatile 
blowing agent having a solubility parameter (to be referred to as an SP 
value) of 7.0 to 8.0 and 70 to 10 mole% of a volatile blowing agent having 
an SP value of at least 6.0 but less than 7.0 can be conveniently used as 
the blowing agent. The use of such a mixed blowing agent makes it possible 
to effect fully satisfactory foaming. The foaming is uniform, and a 
tubular foamed extrudate having a predetermined configuration can be 
formed without distortion. Moreover, as a result of using the mixed 
blowing agent, when the tubular foamed extrudate is pinched with a pair of 
pinch rolls into a flat plate-like sheet, the inside surface of the tubing 
can be adhered to itself to provide a good and strong bonding. 
There has been a prior example in which two or more blowing agents are used 
to increase the expansion ratio of plastic foams. We, however, fail to 
find any prior example in which two or more blowing agents are used 
together in a method comprising forming a tubular foamed extrudate and 
pinching it with a pair of pinch rolls to form a sheet as in the method of 
this invention. In addition, the mixing of blowing agents of different SP 
values in specified proportions is a new combination in the production of 
a plate-like foamed sheet by flattening a tubular foamed extrudate as in 
the present invention.

In the method of this invention, the step of extruding a gel of a foamable 
synthetic resin through a circular die provided at the end of an extruder 
to form a tubular foamed extrudate is the same as a conventional method 
which comprises introducing a synthetic resin from a hopper 2 into an 
extruder 1 and a blowing agent from an injection port 3 and extruding the 
foamable resin through a circular die provided at the end of the extruder 
1. Accordingly, a detailed description of this step is omitted. 
In the method of this invention, the width W of each of a pair of pinch 
rolls 4 for pinching a tubular foamed extrudate 5 extruded from the 
circular die is made smaller than the width D of the pinched tubular 
foamed extrudate 51 so as to provide non-adhered parts 61 at both side end 
portions 6 of the pinched foamed sheet, as shown in FIG. 2. Since the 
non-adhered parts 61 are formed continuously in the longitudinal direction 
of the foamed sheet, it is necessary to allow sufficient width d for the 
formation of end portions which are not subject to pinching, according to 
the thickness of the sheet. To obtain sufficient width d, it is necessary 
to make the width W of the pinch roll at least 5% smaller than the width D 
of the tubular foamed extrudate 51. Otherwise, no special pinching 
conditions are required. 
The foamed sheet 51 delivered while having the non-adhered parts 61 at both 
side end portions is cut to a suitable length after leaving the pinch 
rolls. Thus, the tubular non-adhered parts communicate with the open air 
at the sites of cutting, and the gases inside the tubular foamed extrudate 
are gradually released into the open air through the non-adhered parts. 
The non-adhered parts which are not pinched are cut off later. 
To better the communication between the non-adhered parts and the open air, 
it is preferred to break the end portions 6 by any desired method such as 
those illustrated hereinbelow so that the inside of the tubular foamed 
extrudate 5 is communicated with the open air and the gases filling the 
tubing 5 are released little by little into the air through the 
non-adhered parts 61. Breaking of the non-adhered portions can be 
performed by various methods. Typical methods are described below with 
reference to the accompanying drawings. 
FIGS. 3 and 4 show a method comprising incising the non-adhered parts 61 
formed at both side end portions 6 to communicate the non-adhered parts 
with the open air, and then cutting off the non-adhered parts. FIG. 3 is a 
top plan view showing the related part, and FIG. 4 is a side elevation 
thereof. The both side end portions of the plate-like foamed sheet 51 are 
incised by an incising knife 7 to the non-adhered parts 61 in a horizontal 
direction, and then the end portions are cut off by a cutter, for example 
a knife 8. The removed end portions 6 are crushed by any desired method at 
any desired site, for example by means of a shredding crusher. 
Furthermore, the plate-like foamed sheet 51 is cut to a suitable length to 
form a final product. 
FIGS. 5 and 6 show a method which comprises incising both side end portions 
of the plate-like foamed sheet 51 to the non-adhered parts 61 by an 
incising knife 7 to communicate the non-adhered parts 61 with the open 
air. FIG. 5 is a front elevation, and FIG. 6 is a perspective view. The 
incised end portions are cut off in a subsequent step. 
FIG. 7 is a perspective view showing a method which comprises cutting off 
the both side end portions 6 with a knife 8 and subsequently cutting the 
end portions and the plate-like foamed sheet simultaneously to the desired 
lengths. According to this method, the non-adhered portions 61 communicate 
with the open air at the cut portions. The cut end portions 6 are directly 
crushed by a shredding crusher at a desired site without incising the 
non-adhered parts 61. On the other hand, the plate-like foamed sheet 51 
cut to a predetermined size is used as a final product. 
Communication of the non-adhered parts with the open air is carried out 
preferably as early as possible after flattening the tubular foamed 
extrudate by pinching it with a pair of pinch rolls. 
By releasing the gases inside the tubular foamed extrudate little by little 
into the open air, the rise of the pressure by the dissipation of the 
blowing agent can be prevented. The mechanism for the prevention of 
pressure rise is not entirely clear. It is theorized as follows, however. 
The non-adhered parts plays a role of an automatic flow control valve 
whereby the amount of the gases to be released increases with an increase 
in pressure and decreases with a decrease in pressure, and thus the 
pressure in the tubular foamed extrudate is maintained constant. As a 
result, the formation of the tubular foamed extrudate is carried out 
smoothly and stably. In addition, bubbles of gases are not stuffed into 
those parts of the tubular foamed extrudate which are to be adhered 
because no excessive rise in pressure occurs. Hence, no unevenness in 
adhesion occurs at the time of adhering the inside surface of the tubular 
foamed extrudate with a pair of pinch rolls. The quality of the tubular 
foamed extrudate is therefore stabilized. 
The method of this invention, therefore, makes it easy to produce a 
plate-like foamed sheet of excellent quality which has uniformity in 
regard to thickness, expansion ratio, etc., and good dimensional stability 
and does not develop delamination of the adhered parts. 
As described hereinabove, it is preferred to use a mixture of mixed blowing 
agent composed of a volatile blowing agent having an SP value of 7.0 to 
8.0 and a volatile blowing agent having an SP value of at least 6.0 but 
less than 7.0. 
Examples of suitable volatile blowing agents having an SP value of 7.0 to 
8.0 are shown below. 
______________________________________ 
SP value 
______________________________________ 
n-Pentane 7.0 
Trichlorotrifluoroethane 
7.2 
Trichlorofluoromethane 
7.5 
Dichloromonofluoromethane 
8.0 
______________________________________ 
Examples of blowing agents having an SP value of at least 6.0 but less than 
7.0 are as follows: 
______________________________________ 
SP value 
______________________________________ 
Dichlorodifluoromethane 
6.1 
Dichlorotetrafluoroethane 
6.2 
Propane 6.2 
n-Butane 6.6 
______________________________________ 
In the method of this invention, the mixing ratio of these blowing agents 
must be within the specified range given hereinabove. If the proportion of 
the blowing agent having a lower SP value exceeds 70 mole%, distortion and 
poor adhesion of the tubular foamed extrudate tend to occur. If, on the 
other hand, the proportion of this component is less than 10 mole%, the 
expansion ratio is low and the product obtained is soft and has low 
dimensional stability. 
The amount of the mixed blowing agent is 0.015 to 0.200 mole per 100 g of 
the resin. 
Specific examples of the thermoplastic synthetic resin that can be used in 
this invention include polystyrene, high impact polystyrene, and 
copolymers of styrene and other copolymerizable vinyl monomers such as a 
styrene/maleic anhydride copolymer of a styrene/acrylate copolymer. 
Addition of the blowing agent to the thermoplastic synthetic resin and the 
extrusion of the resin composition can be effected by known conventional 
methods, and no special conditions are required. In the method of this 
invention, extrusion needs not to be carried out at such a high 
temperature as to cause rupture of cells within the tubular foamed 
extrudate as is the case with a conventional method, because the selection 
of the specified combination of blowing agents renders the inside surface 
of the tubular foamed extrudate essentially easy to adhere. 
According to the method of this invention described hereinabove, there can 
be easily produced a plate-like foamed sheet of excellent quality which 
has uniformity in regard to thickness, expansion ratio, etc., and good 
dimensional stability and does not develop delamination of the adhered 
portions. 
As required, a sheet of paper, etc. may be laminated to one or both 
surfaces of the plate-like foamed sheet obtained by the method of this 
invention. 
The following examples illustrate the present invention. 
EXAMPLES 1 TO 3 AND COMATIVE EXAMPLES 1 AND 2 
Polystyrene was fed into an extruder having a diameter of 90 mm through a 
material feeding hopper. Each of the mixed blowing agents shown in Table 1 
was fed into the extruder through a blowing agent injecting port. They 
were heated and mixed to form a foamable polystyrene gel containing the 
blowing agent. The gel was extruded at 145.degree. C. through a circular 
die having a slit with a diameter of 225 mm and a thickness of 1 mm. The 
resulting tubular foamed extrudate was pinched with a pair of pinch rolls 
having a width of 915 mm and disposed at a position 1000 mm away from the 
end of the circular die to form a plate-like foamed sheet having a width 
of 970 mm and including non-adhered parts at both side end portions. The 
non-adhered parts were then cut off at a point 27.5 mm from the side edge, 
and crushed. 
The resulting plate-like foamed sheet had a thickness of 5 mm, an expansion 
ratio of about 12, and the characteristics shown in Table 1. 
For comparison, the above procedure was repeated except that a pair of 
pinch rolls having a width of 1000 mm were used instead of the pinch rolls 
used in the above procedure, and the tubular foamed extrudate was pinched 
along its entire width without forming non-adhered parts at its end 
portions. The results are also shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Mixed blowing agent Plate-like foamed sheet 
Amount 
Mixing 
Total 
Adhesion of Compression 
(moles) 
ratio 
amount 
the inside strength 
Constituents 
(*) (mole %) 
(moles) 
surface 
Appearance 
(kg/cm.sup.2) 
__________________________________________________________________________ 
Example 
1 Trichlorofluoromethane 
0.030 
50 
0.060 
Excellent 
Excellent 
3.8 
Butane 0.030 
50 
2 Trichlorofluoromethane 
0.041 
67 
0.061 
Excellent 
Excellent 
3.2 
Propane 0.020 
33 
3 Trichlorofluoromethane 
0.060 
92 
0.065 
Good Good 2.0 
Butane 0.005 
8 
Comparative 
Example 
1 Trichlorofluoromethane 
0.062 
95 
0.065 
Good Shrinkage 
1.0 
Butane 0.003 
5 
2 Trichlorofluoromethane 
0.012 
20 
0.068 
Delamination 
Shrinkage 
3.7 
Propane 0.048 
80 
__________________________________________________________________________ 
(*) Per 100 g of polystyrene.