Process for production of foamed shaped pieces with poreless surface made by the molding process

A process for producing foam-shaped pieces having a poreless and smooth surface from particulate expandable styrene polymer which comprises the steps of heating not prefoamed particles of expandable styrene polymer to a temperature between 100.degree. and 170.degree. C. under a pressure of between 3 and 30 bar to cause melting of the polymer cooling the melt polymer to temperatures of about 10.degree. to 40.degree. C. above the softening point of the styrene polymer, enlarging the capacity of the mold in which the particles are melted, expanding the resulting mold melt material without supplementary external delivery of heat and thereafter cooling the mold to about ambient temperature followed by removal of the shaped foamed piece.

The present invention relates to a process for production of foamed shaped 
pieces with a continuous poreless, surface, from particulate expandable 
styrene polymers, wherein the expandable particles are first heated in a 
non-gas-tight mold at elevated pressure to temperatures above the 
softening point of the styrene polymer, are then cooled in the mold, and 
are foamed within the mold without renewed heating. 
It is known that foamed shaped pieces can be prepared by the molding 
process if thermoplastic materials that contain a solid expanding agent 
are molded at temperatures above the softening point of the thermoplast 
and then foamed by renewed heating without pressure outside the mold. 
This process requires the use of a solid expanding agent, and it has the 
disadvantage that the foamable material has to be expanded outside the 
mold by renewed heating in a second work step. With this procedure we do 
not arrive at a form-stable foamed shaped piece. 
It is also known that thermoplasts containing a fluid expanding agent can 
be heated at high pressure to temperatures above the softening point of 
the thermoplast, cooled with maintenance of the pressure and then expanded 
by renewed heating outside the mold. Here also there are disadvantages 
similar to those of the process with solid expanding agents. 
It is also known that small-particle expandable, at least partially 
prefoamed polystyrene particles can be bonded in a closing form with 
variable, capacity, the mold not being gas-tight, and that then the mold 
capacity can be enlarged and the shaped pieces expanded in the mold by 
renewed heating. According to this work process, it has been found that 
only foamed materials that consist of relatively loosely bonded individual 
particles can be obtained. 
According to the state of the art, there are obtained foamed pieces with 
sufficient stability only by the mold process which, because of the high 
pressure that is applied, involves additional expense. However, foamed 
shaped pieces are not produced thereby in single work step: the pieces 
possibly receive a supplementary lamination because the expanding and the 
lamination that may be desired have to be undertaken in further work 
steps, conducted outside the mold. 
In the work process that starts with small expansible particles in molds 
with variable capacity, we only get shaped pieces with inadequate 
stability. 
The purpose of the invention is to develop a process that can be effected 
with relatively low pressures in a single mold with variable capacity, 
that allows production of foamed shaped pieces with sufficient stability 
and possibly also with simultaneous lamination. 
The solution of this problem is found if particles of expandable styrene 
polymer, that have not been prefoamed, are melted in a single closing mold 
with variable capacity, that is not gas-tight--at temperatures above the 
softening point, between 100.degree. and 170.degree. C, at pressures 
between 3 and 30 bar, with maintenance of the pressure at temperatures 
that are about 10.degree. to 40.degree. C above the softening point of the 
styrene polymer; then the polymer melt is cooled, the capacity of the mold 
then enlarged to the desired size of the shaped piece, whereby the 
operation proceeds without further external application of heat, and 
finally after cooling to about 30.degree. C the foamed shaped piece of 
polymer is removed. According to a special embodiment of the process of 
the invention, a single-sided or two-sided laminated foamed shaped piece 
is obtained in a single work step, if before charging the expandable 
polystyrene particles, a laminating sheet is laid in the mold, and with 
two-sided lamination, another sheet of the desired laminating material is 
laid on the charged expandable particles. In production of such laminated 
foamed shaped pieces, the work can advantageously be accomplished without 
additional adhesives. 
As particulate expandable styrene polymers, use may be made of styrene 
homopolymer, impact-resistant polystyrene and mixed polymers of styrene, 
e.g. with .alpha.-methyl styrene, acrylonitrile, methyl methacrylate, 
acrylic esters which comprise at least 50% by weight styrene monomer. As 
expanding agents, the styrene polymers receive--as is known--liquid or 
gaseous expanding agents that only swell and do not dissolve the polymer, 
and which boil below the softening point of the styrene polymer. Here it 
may be mentioned the known hydrocarbons such as pentane, isopentene, 
hexane, cyclohexane and halogen-containing hydrocarbons such as 
dichlorodifluoroethane. Obviously, there may also be mixtures of these 
expanding agents which generally are used in amounts of 2 to 15, 
especially between 5 and 7% by weight based on the weight of the polymer 
particles. Moreover, the expandable styrene polymers may contain fillers, 
flame-proofing agents and dyes in the usual way. 
The expandable particles are not prefoamed in the process of the invention, 
which saves another work step and intermediate holding. 
The expandable particles are charged into a closable heatable mold with 
variable capacity, the mold not being gas-tight. The mold may be already 
heated when the material is charged into it. After filling of the mold, 
the mold is closed and the closed mold is heated, advantageously with 
steam or oil, or electrically without locking pressure. 
The temperature at the surface of the movable upper mold is then between 
100.degree. and 170.degree. C, especially between 120.degree. and 
160.degree. C. The pressure, which may be substantially lower than in the 
usual molding process, is generally between 3 and 30 bar, advantageously 
between 10 and 20 bar. After the fusing of the particles, the upper and 
lower parts of the mold are cooled to temperatures that are about 
10.degree. to 40.degree. C., advantageously 20.degree. to 30.degree. C., 
above the softening point of the styrene polymer. This is generally in a 
temperature range between 100.degree. and 130.degree. C., advantageously 
110.degree. and 120.degree. C, with maintenance of the pressure. After the 
desired temperature is reached, the upper mold is moved, by a stroke that 
corresponds to the desired thickness of the finished foam. The available 
intrinsic heat within the thermoplast and the residual heat of the mold 
are sufficient for the necessary expansion. A further external application 
of heat is not necessary. Expansion time generally, depending upon the 
size of the mold, is 2 to 4 minutes. In the expansion there is 
considerable cooling. Finally the mold temperature is reduced to about 
30.degree. C. which can be effected by delivery of water or oil, and the 
finished piece is removed from the mold. 
Similarly, it is possible to prepare foamed layers or plates that are 
coated on one or both sides, whereby--as already 
indicated--advantageously, with no need for additional adhesive, the 
process can be effected in a single work step. If desired, of course, 
adhesives may be utilized. 
Suitable coating materials are foils, sheets, or plates of thermoplastic 
synthetic resins such as polystyrene, polyvinyl chloride, polyethylene, as 
well as asbestos cement, plasterboard or ceramic plates, wood plates or 
plywood.

According to the process of the invention there is simple production of 
insulating board, structural board, shaped pieces, elements with high 
strength and dense surfaces, in a single form without extra work steps. 
EXAMPLE 1 
In a mold (500 .times. 500 mm) that can be heated by oil, steam or 
electricity, 1400 grams of not prefoamed expandable polystyrene particles 
are charged. The used expandable polystyrene particles contain as 
expanding agent about 6% by weight of a mixture of about 15% (10-30%) by 
weight isopentane and about 85% (90-70%) by weight pentane. The mold is 
closed and heated to 150.degree. C. without locking pressure. After 
reaching 150.degree. C., there is plasticized (melted) within 3 minutes. 
Finally the molding pressure is immediately continuously raised to 20 to 
25 kp/cc. After the molding pressure is reached, the mold is cooled to the 
expansion temperature of 120.degree. C. under said pressure within 6 
minutes. Thereafter the upper mold part is lifted to the desired expansion 
height and fixed. The expansion time is 3 minutes, with no application of 
heat. After the expansion time has passed, there is cooling to the removal 
temperature of about 25.degree. C. 
EXAMPLE 2 
In a form (500.times.500 mm) that can be heated with oil, steam, or 
electricity, ceramic tiles are laid in the lower mold half, and over the 
tiles 1400 grams of not prefoamed expandable polystyrene particles 
corresponding to that of example 1 are distributed. The subsequent course 
of the process corresponds to that of example 1. The ceramic surface is 
bonded firmly to the obtained shaped piece, without use of an adhesive. 
The well-known expandable polystyrene particles which are to be used in 
the process according to the invention generally have a particle size 
between 0.4 and 3 mm diameter. Also those with a greater diameter may be 
used, said is to say the particles' size is not critical. 
The time till the particles have melted is only a few minutes and naturally 
depends upon the amount of the particles used, i.e., using 1.5 kg 
expandable polystyrene the time is 3 minutes, using 2.5 kg the time is 3.5 
minutes. Immediately the particles have been in the molten condition 
follows the cooling step to a temperature about 10.degree. to 40.degree. 
C. above the softening point of the styrene polymer. The melted particles 
therefore must not be maintained in the molten condition. The cooling time 
depends upon the amount of the expandable polystyrene, the material of the 
mold (heat conductivity) and the desired specific weight of the foamed 
shaped pieces; it differs only a few minutes. The cooling time lies 
generally between 3 and 7 minutes at a specific weight of the foamed 
shaped pieces from about 0.07 grs/cm.sup.3. The mold is filled in a height 
from about 8 to 10% of the height of the foamed shaped pieces. In other 
words the movable upper mold is lifted 10 to 12 fold the height of the 
original height of the unfoamed expandable polystyrene. 
Features of the invention will be additionally apparent in conjunction with 
the accompanizing drawings, wherein 
FIG. 1 shows the cross section of the unclosed mold 1 filled with non 
prefoamed expandable polystyrene 2. Cooling and heating means are not 
shown. 
FIG. 2 shows said cross section of the closed mold during the 
pressure-and-melting-step and the cooling step. 
FIG. 3 shows the closed mold during the expanding step. Cooling and heating 
means are not shown. 
As shown in FIG. 1 the preheated or not preheated mold 1 is filled with the 
non prefoamed expandable polystyrene particles 2. The amount of said 
polystyrene is at least that a non foamed (compact) shaped piece 8 
theoretically can be formed. After filing the mold 1 is closed and heated. 
As shown in FIG. 2 numbers 3, 4, 5 and 6 call the usual temperatures 
especially applied during the successive steps of the process; the 
corresponding lines call means by which heating or cooling take place. The 
polystyrene particles are plasticized between 150.degree. and 170.degree. 
C. (about 160.degree. C. [3]). The temperature of the molds' frame is 
about 90.degree. C. [4]. After the fusing of the particles is cooled to 
the expansion temperature of about 125.degree. C. [5]. The upper mold part 
as shown in FIG. 3 is lifted to the desired expansion height and fixed. 
After expansion there is cooled to removal temperature of about 30.degree. 
C. [6].