Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to expanded foam articles from
synthetic thermoplastic materials and particularly to pre-expanding
beads of such thermoplastic ma-terials for use in subsec~uent molding
processes .
It is conventional in the art to initially pre-expand the thermo-
plastic beads to form so-called "pre-puffs. " These pre-puffs are
expanded to a-t leas-t fif-ty percent of volume of -the final molded
article. By preforming the beads, the rnold can be substantially
filled, thereby permitting a more uniform expansion of the beads,
the elimination of voids, lower molding pressures, higher production
cycles, and the formation of low density foam. The basic process
is described by G. R. Franson, Plastics Technology, July, 1956,
pp. 452-455.
In pre-expanding beads to form "pre-puffs" it is impor-tant to
produce uniform, dimensionally stable produc-ts at fast production
ra-tes. The type of pol~mer, the vola-tile content of -the bead before
and after pre-expansion, and the moisture in the environmen-t mus-t
all be consiùered. Finally, the equipment for pre-expandincJ and
molding the foam has -to be capable of high speed and trouble-free
operation.
The subject inven-tion relates to a me-thod and apparatus for
pre-expanding thermoplastic materials containing a blowing agent.
More specifically, the invention teaches a means of pre-expansion
which forms dimensionally stable "pre-puffs" which may be molded
directly to their final dimensions without fur-ther finishing. The
method and apparatus of the present invention produce a finished
product in substantially less time -than has heretofore been possible
and produce a produc t tha-t will be substan tially more uniform in
terms of its density and struc-tural integrity and which can be
manufactured at lower cost and with less capi-tal expenditure than
products produced with the methods and apparatus of the prior art.
Briefly, thermoplas-tic beads, e . g . polystyrene beads, contain-
ing a blowing agent, e . g . pentane, are pre-expanded by first
heating by conduction a measured amount of agitated beaùs to -their
softening point in a vessel maintained at substantially atmospheric
pressure. Heating is continued and controlled until the beads
expand to a predetermined degree . A gaseous medium, e . g . air,
may be swept -through the chamber to remove any blowing ayent
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which has been liberated from the bead to avoid any potential
safe~:y hazards. Thereafter, the vessel is closed and evacuated to
substantially reduce the blowin~ agent content of the beads. When
Lhe blowing agent is reduced l:o the appropriate amount, the vessel
is returned to atmospheric pressure.
The beads may then be transferred clirectly to a mold cavity.
Alternatively, they may be returned to a-tmospheric pressure in a
storage container which main tains the partially expanded beads at
an elevated temperature. By maintaining the pre-puffs at an ele-
va-ted temperature, it has been found -that the subsequent molding
process can be carried out much more rapidly and with a lower
overall energy consumption.
From the pre-expansion chamber, or hea ted s-torage container,
as the case may be, -the pre-puffs are inJected into a preheated
mold cavi-ty or cavities by conventional air-operated fill guns. A
number of fill guns are employed to carry out as rapidly as possible
the filling of the mold cavity. Also, the mold cavities may be
constantly subjected -to a vacuum from a suction pump to facilitate
the rapid filling oE the mold cavities.
When the tillin0 is completed, a vacuum suction is applied
throuyh one face of the mold cavity while a hea-ting 1uid such as
superhea ted steam or hot air is passed through the o-ther face of
: the mold cavity for a period of -time. Then, the direc-tion of flow of
the heating medium is reversed through the mold cavity to assure
uniform hea-ting and expansion of the pre-puffs to fuse the pro-
duct. The residual blowing agent in the pre-puff makes possible
the expansion during -the higher molding temperatures and is neces-
sary for complete fusion.
Upon completion of Lhe expansion and fusion of the pre-puffs,
the flow of the heating fluid is discontinued and, for a briel inter-
val, a negative pressure is maintained to remove substantially all oL
the moisture and blowing agent. This will also assis-t in forming a
skin on the molded article, which can also be accomplished by
cooling the mold surfaces. Thereafter, the interiors of the cavities
are re turned -to a-tmospheric pressure and the cavities opened to
effect automatic ejection of the finished article.
~3~96~3
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Thus, the present invention provides a method of pre-
expanding beads o~ synthet~c thermoplastic res~n material
containing a blowing agent, comprising the steps of~
: ~a3 heating the-beads in a chamber having an interior
surface heated to a~out the fusion temperature of
the beads and a~ove the boi:ling point of the blow-
ing agent;
(b~ cont~nuously, at substantially atmospheric pressure~
agitating the beads to prevent agglomeration while
the beads absorb heat from the interior surface of
the chamber, until said beaas reach t~eir softening
^ point and expand in volume;
~c) reducing the pressure in said chamber to extract a
substantial portion of the gaseous blowing agent
- 15 from the expanded beads so that the blowiny agent
content of the expanded beads is from about 0.75 to
2% by weight;
(d) permitting the expanded beads to return to sub-
stantially atmospheric pressure.
In a preferred embodiment, the invention provides a
method of pre-expanding beads of polystyrene containing an
n-pentane blowing agent comprising the steps of:
. (a) heating the beads in a chamber having an interior
surface which is heated to a temperature of from
. 25 180 to 250 F.;
(b) continuously, at substantially atmospheric pres-
sure, agitating the beads to prev~nt agglomeration
so that the beads absorb heat from the intexior
surface of the chamber, soften and expand in volume;
(c~ reducing the pressure in said chamber to from lO"
to 25" Hg to reduce the weight percent of the
n-pentane in the polystyrene to about 0.75% to 2%;
(d) permitting the expanded beads to return to sub-
stantially atmospheric pressure.
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In a preferred embodiment of the invention, the pre-puff is
further expanded in -the pre-expansion step by the injection of
steam into the vessel after the initial heating. This technique
serves to further soften the pre-puff and facilitaLe the migration of
5 the blo~ing agent out of the pre-puff. This embodiment shortens
the period of evacuation necessary to elirnina-te s~bstantially all of
the blowing agent and permits the -formation of' stable pre-puffs of
exceptionally low density.
The process of the present invention not only eliminates down
lû time and curing between the pre-expansion step and the molding
step, but also eliminates the requirement for pos-t-molding condilion-
ing of the finished article so that the product can be packa(led and
shipped to a customer directly from the molding apparatus.
Without being bound -to any particular theory, i t is believed
15 -that the uni~ue success of the inven-tion may be best understood by
considering the effect of each of the inventive process steps on the
l:hermoplastic particles. Initially, during the preheating stage, the
bead is tumbled with agitation against the hot. walls oli the pre-
expancler in a dry atmospher e . 'I'he bea~ is h~ated uniformly
20 throughoul and allowed to expand freely as the blowing agent
volatilizes. Because the pre-expander is not closed or later sub-
jected to superatmospheric pressure, -the expansion is not retarded.
During this stage, the density of the pre-pufE may be controlled by
varying the temperature of the bead. While in all cases the bead is
25 sof tened to some degree, at the higher range of temperatures it
becomes more fluid and greater expansion occurs. ~3ecause the
bead temperature (and therefore -the pre-puff densi-ty) can be easily
regulated for each batch loaded in-to the pre-expander merely by
con-trolling the jacket temperature and the heating periocl, a wide
~0 range of product.s can be ma~e.
'I'wo other advantages are obtained in the preheating step of
the invention. Firstly', because the beads are heated b-y conduction
with the walls of the pre-expander, the surface softens to a greater
extent than the interior, thereby forming, upon cooling, a dense
35 skin or crust around the particle. This lends dimensional stability
to the pre-puff. Steam does no-t have this effect because it per-
meates the pre-puff. Secondly, by pre-heating under a-tmospheric
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3~1968
-'I -
pressure, the escape of the volatilized blowing agen-t is not retard-
ed .
After the expansion is completed in the preheating stage, the
vacuum applied to the p-re-puff serves to reduce -the blowing agent
5 concentration to a minimum amount. Naturally, less time is required
for this step because a subs-tantial amount of -the blowing agent has
been eliminated during the preheating. The elimination of the
blowing agent is a key factor in ob taining a dimensionally stable
product. The pre-puff, however, must still contain a minimum
10 amount because it is needed to further expand the product during
the final molding.
In the preferred embodimen-t of the invention, to facilitate the
reduc-tion of blowing agent concentration and the molcling process, a
small amo-unt of high temperature steam is injected at the end of the
15 pre-heating period. This rapidly heats and further expands the
pre-puff. Not only does this reduce -the density of the pre-puff,
but it also increases the permeability of the s~lrface skin, thereby
reducing the time and energy required to extract the blowing agent
Erom the pre-puff d~lring the evacuation stage. Heating and the
20 evacuation o~ volatiles during thc molding step are also facilitated.
In the process of the invention, the mold is entirely filled with
the pre-puff. During -the molding step, therefore, there is li-ttle
change in the bulk densi-ty of the mass. In fac-t, however, the
individual pre-puffs do fur-ther expand -to fill the voids between the
25 par-ticles . I t is for this reason that a small amoun-t of blowing
agen-t must remain. Without it, no fwr-ther expansion would occur
under the molding conditions and fusion would only occ-ur at the
points where the spherical par-ticles touch. On the o-ther hand,
once molding is complete, substantially all of the blowing agent must
30 be removed. 'I'o do this and to fuse the particles, steam is passed
alternatively from one side of -the rnold while the mold is placed
under vacuum from the other side. The final evacua-tion step also
insures substantially complete removal of the hlowing agent and
mois ture . If both were not removed, the cooling time required
35 before the molded article could be removed from the mold would be
lengthened (because of the high specific heat of the water and the
pressure developed by the blowing agent) and dimensional stability
lost .
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In summary, it is apparent that the process of the invention is
effective because its unique combination of steps serves to quickly
remove the blowing agent and moisture, once their purposes have
been served, to produce an outstanding product hither:to unobtain-
able and especially at high production rates.
The FIGURE is a schematic illustration of the pre-expansion
and molding appara-tus and method of the present invention.
Referring to the Figure, there is illustrated a pre-expansion
apparatus, generally designated at 10, which includes a cylindrical
con-tainer 12, the axis of which extends generally horizontally. The
container 12 has in its end wall 1~ an opening for receiving unex-
panded synthetic, thermoplastic resin material in -the form of beads
16. I'he most widely used type of ma-terial is polys tyrene con-
taining n-pentane as the blowing agen-t. It has a density of about
0.6~ kg . /li-ter and the conditions descrihed hereinaEter are those
used for such ma-terial. The beads î6 are first deposited in a feed
hopper 1~. The desired quantil;y o~ beads 16 are then fed to the
interior of the container 12 by a chargin~ piston 20 which may be
air-actua ted .
The interior wall of the container 12 is heated to a temperature
of from about 107 to 121~ by a heating fluid which is circulated
through a heating jacket 22 which surrounds a major portion of the
exterior o-f the container 12. Hea ting fluid is introduced through
- the inlet 24 located adjacent the upper edge 23 of the heating
jacket, on one side of the container 12, and is removed through the
outle-t 26 located at -the bottom of the jacket from whence the fluid
is removed.
Means are providecl for agitatiny the beads in the container 12
in the form of a plurality of spokes 2~ mounted on a rotating rod
which is driven by a motor, not shown. The agitation prevents the
agglomeration of the beads and facilita-tes uniform heating.
Whereas the inlet adjacent the charging pis-ton 20 is located
above the central axis of the container 12, the outlet 30 is disposed
vertically below -the central axis so that, when the discharge piston
32 is actuated to open -the outlet 30, the expanded beads will flow
under -the influence of gravity to a storage container 3~. This
transfer may be accelera-ted by blowing the pre-puffs ou-t of the
con-tainer 12 with air from -the blower 36.
1~L3~68
With the char ging piston 2() loaded wil:h the appropriate quan-
tity ol beads 1~, the wa~ls of the container 12 are heated to the
appropriate expansion temperatLlre. 'T`his is a temperature sufficient
to soi`ten -the surface of the beads and to volatilize the blowing
5 agent, but not sufEicienl to cause t`usion between the oeads during
agitation. The degree of softening of the bead and the period of
preheating are dependent on the densi-ty of the pre-puff and fin-
ished product desired. As is well known to -those skilled in the
art, the more the bead is sof-tened, the greater the expansion and
10 the less the density of the pre-puff. For example, using a feed of
20 kilograms of polys-tyrene beads containing 7~O of pentane as tlle
blowing agent, if one is seeking to produce a finished product
having a densi-ty of 0.096 kg/liter, the steam entering the heating
jacket should be set for 107C and the pre-heat period be 70
15 seconds. On -the other hand, if a 0.026 kg/liter product is de-
s;red, the temperature of the steam to the hcatiny jacket should be
set at 121C with a 70 second pre-heat time period. By doubling
the pre-heat time to 1~0 seconds, at 121C steam temperature, the
density can be reducecl to 0.020 kg/liter. One skilled in the art
20 may readily determine the appropria-te combination of -temperature
and time for the pre-heat step, by considering the type of thermo-
plastic bead, the weight of the charge, the equipment and the
desired density. The charging cylinder 20 is operated to load the
beads into the interior of the container 12 while the agitator bars
25 28 are rotated to continually bring the beads into contact with the
heated surface of -the con-tainer ~2
Hot air from a source 36 may be introduced preferably at a
temperature above the softening point of the beads through conduit
38 and wall l~ into the inter ior ot the con-Lainer 12 . A strong
30 current of hot air is maintained in the chamber 12 by drawing out
the atmosphere in the container -through a conduit 40 which is
connected to a vacuum pump. Thus, the in-terior of -the con-tainer
will be maintained a-t or very near atmospheric pressure, and any
vapor or gases that are evolved during the expansion of -the beads
35 will be quickly removed. The lat-ter is important since it aids in
the removal of the pen-tane from the bead and serves as a safety
consideration since -the pentane gas evolved and released into the
. .
3L~3¢3~
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atmosphere of t:he con~ainer is highly e/Yplosive when mi~ed with
oxygen. Moreover, where the incoming beads carry mois-ture as a
result of lony periods of storage or ~Iny pre processing cleaning
steps, it is impor-tant that the moisture be removed so that the
expansion of all of the beads will proceed at a fairly uniform rate.
In -those instances where a particularly low density foam or
higher production rates are desired, a rrleasured a~ount of high
temperature steam may be introduced through a plurality of inlet
lines (not shown) in the bottom of the container 12 after -the initial
expansion of -the beads wi-th -the dry air. This steam is at a tem-
perature greater than tha-t of the pre-puf-f, e.g. from 100 to 177C
and releases latent heat oE condensa-tion on the polys-tyrene par-ti-
cles. This serves -to rapidly increase the temperature of the parti-
cles and provide additional expansion. This s-tep, too, is performed
at or near atmospheric pressure. Only a limited amount of conden-
sate is formed during this step becawse the b~llk ot the heat provid-
ed to the beads is added by convection cluring the preheating step.
-I'his condensation is completely removed thro~lyh a plllrality of
outlet ports (not shown) on the top ancl side of the container 12
during the evacuation step described below. The use o:l~ a plurality
of s-team inlet and outlet ports assures even distribution of the
steam throu~h the agitating beads.
When the expanded beads or pre-puffs reach -the desired
density, the pressure in chamber 12 is lowered to subatmospheric
pressure (from 5.l cm to 6~ cm Hg vacuum, preferably 25 cm to 51
cm ~Ig) by actuation of the vacuum pump through line 40. By
lowering the pressure at -this stage the residual gas in the pre-puffs
can be reduced to give -the pre-puffs excellent slahility. This
vacuum step removes the majority of the blowing agent (and concten-
sate, where steam addition is used), while leaving an amount just
sufficient to permit Eur-ther expansion of -the beads during the
molding s-tep. Preferably, where the polys-tyrene beads originally
have about 5% to 7% by weight of the blowing agent, -the vacuum
step r educes the level from û ~ 75% to 2% by weight, preferably to
about 1%, of gas remaining. By using the preferred embodiment of
-the invention, i . e . steam addition after preheating, -the evacuation
of the blowing agent is facilitated. It is believed that this is
hecause the skin becomes more permeable to the gases.
3~9~8
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A~ter the vacuum step, Lhe discharge pis-ton is actuated to
rapidly eLfect dumping of the pre-puffs into the insulated storage
container 3~1, maintained at atmospheric pressure and a temperature
of from ~19 to 77C. From the storage container 34, the partially
5 expanded beads are pneumati(ally fed through conduits 46 and ~8 to
a molding apparatus generally designated at S0.
Al-ternately, the outlet ~2 may be directly coupled to a tubular
conduit to effect rapid discharge of the container 12 and immedia-te
loading of a mold cavi-ty, as described helow In such event, the
10 pre-puffs must be res-tored to atmospheric pressure prior to loading
the mold cavi-ty.
As described in more detail below, pressurized air actuated fill
guns are employed -to inject the pre-puff in-to a pair of identical
mold cavi-ties 52 and 5~ which are schematically illus-trated in the
Figure.
The molding apparatus 50 includes mold platens 56 and 58
which are Lixedly mounted on frame means generally desiclnated at
60. In addition, a mol(i plal:en 62 is movably mounted ()n frame
means 60 opposite mold platen 56 and identical mold pla-ten 64 is
movably mounted on frame means 60 opposi-te mold platen 58. As
described in detail below, each of the mold platens 56, 58, 62 and
64 includes a number of perforations -there-through, some of which
- are for the purpose of admitting pre-puff from the fill gun 66 andothers of which are for admi-tting a heating fluid and for evacuation
of the interior of each of the mold cavities.
The mold platens 62 and 6~1 are movable -towards and away from
their facing coun-terparts. With this arrangement, molded articles
such as insulation board can be produced with a wide range of
thicknesses and shapes simply hy varyin~ the size of the mold
30 cavities 52 and 5~ as well as the configuration of the mold faces
themselves .
The fluid handling system of -the molding apparatus 50 will now
be described in conjunc-tion with the Figure.
In the preferred method, a hea-ted fluid medium such as hot
35 air is supplied to two separate plenum chambers 68 and 70 which
are appropriately located close to -the molding appara-tus 50. While
. hot air may be used as the hea-ted fluid medium, superhea-ted steam
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11 30968
at a lemperature of from 93" to L35(:~ is prefe~ l~ed. ~rom ~lenum
chamher 68, the heated medium is supplied through one or more
condui-ts 72 to a plenum chamber (not shown) located behind the
face ot` mold platen 62. A magnet;cally operated valve, movable
between an open and closed position, controls the flow of the fluid
medium through conduit 72. Sirnilarly, one or more conduits 76
controlled by a similar valve 78 supplies the fluid medium -to the
plenum chamber behind the tace of mold platen 64. Since the mold
platens 62 and 64 are movable on the frame rneans 60, i t is desir-
able -that the conduits 72 and 76 be flexible to compensate for this
movemen t
From plenum chamber 7û, the heating fluid medium is fed by
one or more conduits 80 and 84 to mold platens 56 and 68, respec-
tively Magnetically operated valves 82 and 86 likewise control flow
through conduits 80 and 8'1, respectively. As described below, a
drain tube 88 is preferahly located vertically below the mold appa-
ratus 50 and i9 connected by a condui-t: 90, throuyh valve 92 to the
plenum chamber hehind the Eace Or molcl pklten 62. Similarly,
conduit 94, through valve 96, is connected at a number of points to
the plenum chamber behind the face of mold platen 56 and to -the
drain tube 88. Conduits 98 and 100, through valves 102 and 104,
respectively, connect the plenum chambers behind -the faces of n-old
platens 58 and 64, respectively, with the drain tube 88. The drain
tube 88 is connected through a valve 106 to a large capacity vacuum
purnp schematically indicated a-t 108. A drain to atmosphere is
provided for drain tube 88 through valve 110. Preferably, all of
the valves used in the Eluid handling system are magnetically oper-
ated to permit remote control operation thereof, preferably by
means of a program. Also, all o~ the valves are of the on-off type
so that they operate to be either fully open or fully closed.
With the mold apparatus as thus far described, the processing
steps of the presen-t invention will now be set forth.
Wi-th the mold platens 62 and 64 moved to their closed positions
relative -to their respec-tive counterparts 56 and 58, the temperature
of the mold cavities 52 and 54 is rnain-tained at 93 to 135"C, if
necessary by passing the fluid rnedium from one or the other of the
plenum chambers 68 and 70 in to the mold cavi-ties . Wi th the mold
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pl o~ r ly lo~ k~ t~ , vi~ lv~
are close(i, while valves ~2, 9~j, l()2, ll)~ and l()~j are o,ocn an(l the
vacuum pump 103 is turned on to lower lhe pressure to subatmos-
pheric, preferably to 20 cm to 23 cm Hg vacuum in the cavities 52
5 and 5~1. Shortly thereafter, the fill yuns 66 are actuated to inject
the partially expanded hot beads into the mold cavities 52 and 54 to
fill the cavi ties . When the cavi-ties have been filled, -the Lill guns
66 are -turned off ancl valves 82, 86, 92, 104 and 106 are in their
open positions while valves 96, 102, 74, 78 and 110 are placed in
10 -their closed positions. Thus, at this stage, the fluid medium from
plenum chamber 70 will flow through conduits 80 and 84, through
-the faces of the respective mold pla-tens 56 and 58 across -the res-
pective mold cavities 52 and 5~1 and out through the faces Or Lhe
mold platens 6~ and 6~1 through the conduits 90 and 1()0 to the
15 vacuum pump 108. After a period of time, I:he tlow direction of the
heated fluid mediurn may be reversed.
rE'o ei~ect reversal when used, valves 80 and 89 are closed
while valves 7~1 and 78 are opened and valves 92 ancl 1.0~ are closecl
while valves 96 and 102 are opened whereby the fluid medium will
20 flow from plenum chamber 68 through the faces of mold platens 62
and 64 across -the cavities in the opposite direction and out through
the orifices in the faces of the mold pla tens 56 and 58, through
conduits 94 and 98 to the vacuum pump 108. Wi th this arrange-
men-t, the possibility of crea-ting a density gradient across the
25 thickness of the molded article is substantially reduced so that
substantially more uni~orm and salable product will be obtained In
addition, as is well known, the partially expanded beads function as
very efficient insulatiorl barriers so that where t:he heating medium
is only deliverecl from one si(le Or a mold cavity and a very thick
30 mold board is being made, substantially greater molding time would
have to be employed to assure complete fusion of the beads
throughout the mold cavity. However, with the method and appara-
tus of the present invention, the molding time is substantially
reduced since the fluid medium saturates the partially expanded
35 beads from opposite sides, -thus assuring intimate and thorough
hea-ting of each of -the beads.
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Cooling of the proàuct in the mold cavities is then accom-
plished by closing all of the valves from the plenum chambers 68
and 70 and opening valves 92, 96, 102, :lO~ and 106 ~ith the vacuum
pump on and valve 110 still closed to reduce the pressure to subat-
5 mospheric, preferably to about 25 cm Hg vacuum. The moistureand any gases in the mold cavities 52 and 54 will then be evacuated
` and the temperature in the cavities will also drop as the pressure
drops. In this step the blowing agent concentration is reduced to
less than 0.5 wt. %, preferably below 0.3 wt. %. Subsequently,
then, valve 106 is closed and valve 110 opened to bring the mold
cavities back to atmospheric pressure whereupon the mold platens
are unlocked and opened, and the finished mold produc-t is au-to-
matically ejected from each of the mold cavi-ties.
The apparatus of the present inven-tion, as described above,
15 will rapidly perform a molding sequence chiefly by virtwe of the
fact that the molds can be rapidly filled with the partially expanded
beads and then the beads can be uni~ormly heated to their fusion
temperature to consolit:late the beads in the finished artic~e ~orm~
The molded product is then ejected from the mold cavities and is
20 immediately ready for packaying and shipmen-t to a customer.
While the invention has been specifically described using
polystyrene beads containing n-pentane blowing agent, -these are
bu-t preferred materials. O-ther foamable particulate polymer mater-
ial which may be used includes other homopolymers and copolymers
25 derived from vinyl monomer s such as vinyl chloride, divinyl ben-
zene, alpha-me-thylstyrene, nuclear dime-thylstyrene, and vinyl
naphthylene. ~n addition to the polystyrene homopolymers, copoly-
mers of polystyrene with alpha-methylstyrene, divinyl benzene,
butadiene, isobutylene and acrylonitrile having about 5()% or more
30 styrene are especially suitable~ Useful blowing agen-ts include other
volatile alipha-tic or cyclo-aliphatic hydrocarbons, generally having
from 1 to 7 carbon atoms per molecule~ rrhese include methane,
ethane, propane, butane, hexane, petroleum ethers, cyclopentane,
cyclohexane, cyclopentadiene, and halogenated deriva tives which
35 have boiling points below the softening poin-t of the polymers~
Others include dichloroethylene, dichlorodifluoromethane, acetone,
methanol, methyl acetate, ethyl acetate, methyl ~ormate, ethyl
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134V96~3
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formate, propionaldehyde, dipropyl e-ther The expanding agent is
generally present in amounts of from 3% to 15% by weight of poly-
mers; from S to 8% is preferr~d.
To ill~lstrate more fully the instant invention, attention is
directed to the ~ollowing examples:
Example 1
Using the apparatus described above, 16 kg of polystyre-rle
beads containing 7% of n-pentane blowing agent are charged to the
container 12. The jacket tempera-ture is set for l07C and the
beads preheated for 100 seconds, while air fiows through the con-
tainer at a rate of 2832 liters per minute. During the preheatiny
period, the pressure in the con-tainer is approxima tely a-tmospheric
and -the beads expand. Thereafter, the container is sealed and a
vacuum of 58 cm ~Ig applied for 3 minutes. This reduces the
n-pentane concentration to about 1~ by weight. The pre-puffs
obtained have a densily o~ 0.026 kg/liter. 'rhe pre-puff is subse-
quently molded into a board on a clual-platen boarcl machine which
is equipped with a platen si~:e oE 1.2 meters x 2.4 meters and has a
thickness of 51 mm. 'I'he molding takes place at a steam pressure
of 0.0070 kg/mm2. Excellent Eusion of -the particles is obtained.
The product is cooled under vacuum for 3 minutes. It has good
dimensional s-tability and a blowing agent concentration of about
0 . 25% .
Example 2
The process of Example 1 is repeated, excep t -tha t after the
prehea ting period, s-team a t a pressure of 0 . 070 kg/mm2 is in tro-
duced in-to -the container for a period oF 6 seconds, during which
the container is maintained at atmospheric pressure. The container
is then placed under vacuum, exhausted for 30 seconds and the
vacuum maintained for 1.5 minutes. This is sufficient -to reduce the
pentane content to about 1% by weight. The resulting product has
an extremely low density of 0 . 012 kg/li-ter and is dry . This pro-
duct is also molded as described in Example 1, except that steam
pressure of only 0. 0056 kg/rmn2 is required to permeate the pre-
puffs. The cooling time is reduced -to 20 seconds. Excellent fusion
of the beads is ob-tained and -the product has ou-tstanding dimen-
sional stability. The reduced cooling time shows that the mold
produc-t is free of all moisture and blowing agent.
- ~ ~36Q96~
II(Ivin~J d(`~ il)c~(J Ihe invelllion, il will 1~(` a~ l lo Iho~;e~
skilled in the art that ~,~arious modilicatiolls niay b m.l~ie Ihe1~elo
wilhout departing from the spirit and scope of the invention as
defined in the appended claims.
