Note: Descriptions are shown in the official language in which they were submitted.
1046200
Expandable polystyrene particles are desirable
for molding many articles. Such molding is usually done
by the so-called steam-chest process wherein optionally
the particles are partially foamed or pre-expanded by
S exposure to steam at about atmospheric pressure. The
partially-expanded particles are optionally aged in air
and subsequently placed in a steam chest mold in a quantity
sufficient to fill the mold. The mold is closed and the
particles within the mold exposed to steam at about two
atmospheres for a sufficient length of time for the par-
ticles to expand and hopefully fuse together into a uniform
body having the desired configuration. When the steam has
been removed from the mold generally there is a period of
time in which the article must remain in the mold before
removal. Premature removal from the mold can result in
swelling or other distortion of the article. The minimum
time the article must remain in the mold before removal is
generally referred to as the cooling time. The cooling time
is a very important commercial property of expandable poly-
styrene particles. The shorter the cooling time, the more
rapidly the mold can be recycled, and the number of molded
articles produced per unit of time can increase signifl-
cantly as the cooling time is reduced, however, the cooling
time is one important factor. A second important factor is
the cell size of the resultant foam. If a molded article
is obtained with large size foam cells, for many purposes
it is unsatisfactory. Such articles tend to be brittle and
have an undesirable appearance. It is very desirable that
the cell siz~ of such articles or molding be small, that
is, not readily detected and distinguished by the naked eye.
18,177-F -1-
.
16)462(~0
Various additives have been incorporated in and on expand-
able polystyrene particles to improve their molding char~
acteristics, for example, British Patent 976,363 discloses
the use of solid water-soluble surface-active agents in
powdered form disposed on the surface of the expanded poly-
styrene materials, pre-expanded and subsequently washing
the surface-active agent therefrom. Fatty acid salts such
; as calcium stearate are indicated as useful.
U.S. 2,857,341 discloses the incorporation of
stearic acid in polystyrene and the subsequent steeping of
the polystyrene in normal pentane to provide an expandable
polystyrene particle. U.S. 3,461,088 discloses the use of
small quantities of a fatty acid to coat expandable poly-
styrene beads which have been pre-expanded to improve or
reduce the cooling time. U.S. 3,663,466 uses an alkali
metal salt such as calcium stearate in foamable polystyrene
polymers to improve fusion, cooling time and provide anti-
clumping properties. U.S. 3,736,273 discloses the preparation
of expandable polystyrene beads which are polymerized in the
presence of calcium stearate and subsequently impregnated
with blowing agent.
It would be desirable if there were available an
improved expandable polystyrene composition which on molding
exhibited a short cooling time.
It would also be desirable if there were avail-
able an improved foamable particulate polystyrene composi-
tion which exhibited a low cooling time and was moldable to
a product having fine generally uniform cell size.
18,177-F -2-
1046200
These benefits and other advantages in accordance
with the present invention are obtained in a mass of expand-
able polystyrene particles of a size suitable for molding,
the particles containing 3 to 10 parts per hundred parts of
polystyrene, a volatile fluid-foaming agent selected from
the group consisting of n-pentane, isopentane and mixtures
thereof and generally uniformly dispersed within the particles
from about 100 to 300 parts per million of stearic acid and
preferably 140 to 200 parts per million based on total weight
of the resinous composition and calcium stearate in a propor-
tion of from about 40 to 120 parts per million and preferably
60 to 100 parts per million based on the total weight of the
composition.
Expandable particles in accordance with the
present invention are readily prepared by methods. well known
to the art and as set forth in the hereinbeore cited U.S.
; patents, the teachings of which are hereby incorporated by
reference. Generally such particles are generally spherical .
: beads which vary in diameter from 0~1 millimeter to 5 milli-
metersO It is essential and critical to the composition of
the present invention that the stearic acid and calcium
stearate be uniformly dispersed within the composition, that
is, be present in each individual particle and not merely
deposited on the surface thereof. Such a composition can be
prepared in a malaxating apparatus, such as an extruder, by
admixing the calcium stearate and stearic acid with heat-
~plas~tified polystyrene either in the presence of a fluid-
-foaming agent or subse~uently adding fluid-foaming
agent thereto. The desired fluid-foaming agent may be
added in such an apparatus or beneficial.ly, if particularly
close process control is desired, the alkenyl
18,177~F -3-
1046Z~0
aromatic resinous material containing the organic acid
beneficially extruded and formed into unfoamed strands and
cut into pellets. The pellets are transferred to a pres-
surized reactor and the pellets suspended in water in the
presence of a fluid-foaming or blowing agent and heated for
a period of time sufficient for the blowing agent to permeate
the pellets. Advantageously, such heating is accomplished
at temperatures from about 90 to 150C under superatmos-
pheric pressure and generally spherical particles are obtained.
Such procedures are well known in the art and are set forth
in U.S. Letters Patents 2,950,Z61 and 3,086,885.
Conventional suspension polymerization techniques
~; ~ay be employed to prepare polystyrene particles in accor-
dance with the present invention. Generally it is desirable
to disperse the stearic acid and calcium stearate within the
styrene monomer prior to dispersing the styrene monomer
within the suspending water phase. Generally pol~merization
is accomplished at a temperature of from about 80 to 100C
and subsequently the volatile fluid-foaming agent is added
when polymerizing is for practical purposes complete. The -
temperature of the reaction mixture is raised and the volatile
; fluid-foaming agent is added to the reaction vessel until the
volatile fluid-foaming agent has permeated the polymer par-
ticles. The temperature of the reaction mixture is lowered,
usually to a temperature below 50C and the expandable
polystyrene particles are recovered by filtration. The
parti~les are water-washed and dried. Foamable particles
beneficially are prepared by incorporating a minor propor-
~ion, sufficient to cause foaming, n-pentane, isopentane,
and mixtures thereof.
1~,177-F -4-
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1046ZQ0
Advantag-ou~ly, the stearic acid and calcium
stearate utilized in the practice of the present invention
may be of commercial purity such as technical grades. The
proportion o~ calcium stearate to stearic acid is most
easily regulated and controlled when particles ln accordance
with the present invention are prepared by admixing the
components in a heat-plastified melt. When using the sus-
pension polymerization technique when preparing particles
in accordance with the present invention, each polymerization
1~ ~y~t~m, that is, suspending agent, catalyst, water-phase
inhibitor if employed must be standardized for conversion
of calcium stearate to stearic acid during the polymeriza-
tion. It is undesirable to carry out such polymerizations
at a pH less than about 3 or greater than about 6. Con-
veniently, such pol~ erizations are done in a suspending
medium having a pH of about 5.2 + O.5 pH units. In the
preparation of particles in accordance with the present inven-
~ tion ~y the suspension system, one can add calcium stearate -
- or stearic acid and convert a portion of the calcium stearate
t~ stearic acid by operating in the lower pH region or
alternately by operating in the higher pH region one may add
stearic acid in the presence of calcium ions and form calcium
stearate in situ. Alternately, the careful control of the
pH and knowing the distribution coefficients for stearic acid
and calcium stearate between the water phase and the styrene
monomer, an approximation may be obtained for the appropriate
amounts of stearic acid and/or calcium stearate to be added
to the reactor.
: ~
18,1/l-F -5-
10462~0
By way of further illustration, a 100 gallon
agitated jacketed polymerization reactor was charged with
' 352 pounds deionized water, 210 grams of sodium dichromate
(water-phase polymerization inhibitor), 116 grams of car-
box~methyl methylcellulose. The mixture was agitated until
all components had dissolved. In a second agitated vessel,
an oil phase mixture was prepared which consisted of 171
~rams of azobisisobutyl nitrile, 348 grams of benzoyl per-
oxide, 136 grams of tertiarybutyl butylperoxybenzoate,
80 grams of calcium stearate, 440 pounds of styrene. The oil
phase mixture was added to the polymerization vessel and the
agitator rotated at 36 revolutions per minute. The pH of
the aqueous phase was about 5.2. The temperature of the
contents of the polymerization reactor was ~aised to 70C
over a period of about one hour and in the following six
~i hours the temperature was raised at a generally uniform rate
at a temperature of 120C. About 30 minutes after reaching
120C, 132 pounds of n-pentane was slowly added to the
reactor and the temperature maintained at 120C for a further
- 20 3 1/2 hour period to insure permeation of the n-pentaneinto the polystyrene beads. The reactor contents were then
cooled to 90C and a slurry of 8 pounds, 6 ounces of penta-
bromomonochlorocyclohexane, 29 grams carboxymethyl methyl
cellulose in 16 pounds of water were added and the temperature
maintained at 90 for four hours. The reactor contents were
then cooled to about 35CC. The resultant expandable poly-
-` styrene particles were separated from the aqueous phase,
water-washed and dried, and are arbitrarily designated as
Sample ta). A portion of the resultant polymer beads were
analyzed and found to contain 76 parts per million calcium
18,177-~ -6-
104~;200
stearate, 140 parts per million stearic acid. Although
400 parts per million of calcium stearate had been added
initially to the reactor, a portion of this had been lost
in the aqueous phase.
The foregoing procedure was repeated with the
exception that calcium stearate was omitted. The expandable
beads are designated as Sample (b).
The procedure of the first illustration was
repeated with the exception that calcium stearate was
omitted and replaced by 300 parts per million based on the
; styrene, of stearic acid and the product designated Sample
` (c). Beads of Samples (a), (b) and (c) were pre-expanded
; in a Rodman type bead expander and aged at ambient tempera-
ture and pressure maintained for a period of about sixteen
hours. The aged beads were molded in a steam-chest mold
measuring in feet, 4 x 2 x 1, employing steam as a heat-
~ransfer agent under a pressure of ten pounds per square
inch guage and the cooling time determined. The cooling
time for Sample (a) was eight minutes; for Sample (b) was
46.6 minutes; and for Sample (c) 5.5 minutes. Sample (c)
showing the shortest cooling time was of poor quality due
to large cells appearing in the foam beads. Sample (a) had
a fine uniform cell structure.
~ Employing the hereinbefore described polymerization
~ 25 procedure in a 100 gallon reactor, a number of other samples
of beads were prepared which differed in composition only in
respect to the content of stearic acid and calcium stearate.
Foam blocks were prepared using the product of such poly-
merizations and the cooling time determined. Portions of the
... .
. :
18,~77-F -7-
.,
: : :: . ' . .' . :': : -
1046Z~0
~ various polymerization runs were analyzed to determine the
; calcium stearate and stearic acid content. The results are
set forth in the following Table.
TABLE I
COMPOSITIONS AND COOLING TIMES OF F.I.P. BEADS
COOLING TIME
STEARIC ACIDCALCIUM STEARATE (minutes)
160 60 6.83
170 75 10.17
225 115 3.~2
280 108 2.0
300 105 2.32
140 76 7017
- 120 67 20.0
140 75 12.5
140 72 16.3
110 65 25
47 62.
110 42 62
38.75
~40 117 19.5
120 55 11.83
160 56 11.25
130 44 9.5
In the columns headed "Stearic Acidn and "Calcium
Stearate", the numbers indicate parts per million parts of
product present in the expandable beads. The results of the
experiments were statistically analyzed by linear regression
18,177-F -8-
10462~0
analysis as set forth in Statistical Analysis in Chemistry
and the Chemical Industry, C. A. Bennett and N. L. Franklin,
John ~iley & Sons, Inc., New York, 1954, pp. 222-245, 273-283.
The dependance of cooling time upon stearic acid,
; 5 or calcium stearate content was best represented by the
equation
T = a xb
where T is the cooling time in minutes, x the concentration
of stearic acid in ppm and a and b are constants. The values
of a and b found, together with the correlation coefficients
are
a b Correlation
Coefficient
Stearic Acid 1.115 x 107-2.75 0.92
Calcium
Stearate 2.994 x 104-1.83 0.59
15 - The values of the correlation coefficient demonstrate that
there is a good correlation between cooling time and stearic
acid content, but not between cooling time and calcium
stearate content. The calcium stearate, however, is necessary
to obtain good cell size and block appedrance~ as demonstrated
above.
Similar beneficial results are obtained when iso-
pentane alone or mixtures of n-pentane and isopentane are
employed in place of n-pentane.
As is apparent from the foregoing specification,
- ~ 25 the pxesent invention is susceptible of being embodied with
various alterations and modifications which may differ par-
ticularly from those that have been described in the preceding
18,1~ F ~9~
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, : - . : .
1~46ZOO
specification and description. For this reason, it is to
be fully understood that all of the foregoing is intended
to be merely illustrative and is not to be construe~ or
: interpreted as being restrictive or otherwise limiting of
the present invention, excepting as it is set forth and
defined in the hereto appended claims.
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18,177-F
