Anti-lumping compounds for use with expandable polystyrene...

Details Anti-lumping compounds for use with expandable polystyrene... Anti-lumping compounds for use with expandable polystyrene...

2002-09-18

2003-11-11

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C106S122000, C427S222000

Reexamination Certificate

active

06646018

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to expandable polystyrene beads and, more particularly, to an anti-lumping agent useful for the formation of expandable polystyrene beads and low density foams formed therefrom.
BACKGROUND OF THE INVENTION
Methods for the production of expandable polystyrene beads are well known. Such expandable beads generally comprise a mixture of an expandable styrene or styrene derivative polymer and a blowing agent. The expandable polystyrene beads are generally initially formed into beads of relatively small size having a diameter of from about 0.2 to 4 millimeters. The beads are generally formed in a suspension polymerization reaction. The blowing agent is introduced into the beads either during or after the polymerization reaction. The blowing agent is generally homogeneously dispersed within the polymer and the blowing agent may be, in general, a hydrocarbon which is gaseous or liquid under normal conditions and which does not dissolve the polymer. In addition, the boiling point of the blowing agent must be below that of the softening point of the polymer.
Typically, the expandable polystyrene beads go through at least a two-step process prior to molding. In the first step, the expandable polystyrene beads are expanded to form what are known as “prills” or as a “pre-puff”. This step comprises heating the beads to a temperature above their softening temperature and thereby above the boiling point of the blowing agent resulting in vaporization of the blowing agent and expansion of the beads to form individual particles of foam prills. These prills are quite fragile and prior to molding in the second step it is generally necessary to allow the prills to age for a period of time. To age the prills they are maintained under a normal atmosphere. During the aging process blowing agent within the prills diffuses out and air from outside diffuses into the prills. Once the prills have aged they can be placed in a mold and heated again such that the prills expand to fill the molding space and form a molded object. During this period the prills expand and fuse or adhere to each other to form the molded object. The molded objects can either be large blocks which are subsequently cut with a hot wire cutter into sheets or the prills can be molded directly into a particular shape.
Increasingly manufacturers of molded expanded polystyrene are attempting to shorten the expansion time frame and to mold increasingly lower density foams, thereby leading to an increase in profitability. One negative result of expanding foams to lower densities is that as the density of the foam decreases there is a significant increase in “lumping” of the prills. This problem is especially severe for expandable polystyrene beads that include low levels of blowing agents. Lumping is defined as the expandable polystyrene beads fusing together during the initial expansion step, which results in formation of large lumps of prills. These large lumps create at least two problems. First, the expansion process becomes difficult to control because of the formation of these large lumps. The second problem is that if the lumps are sufficiently large enough the material becomes unusable for the subsequent molding steps. In an attempt to control this lumping phenomenon, a number of external coatings have been utilized. Some examples of commonly used external coatings include zinc stearate, glycerol mono stearate, ethylene-bis stearamide (acrawax), and silica. Unfortunately, these commonly used lubricants are not successful in preventing lumping when the expandable polystyrene beads are expanded to very low densities.
Thus, it would be very advantageous to provide an anti-lumping compound that could be utilized with expandable polysytrene beads containing low levels of a blowing agent that would permit these beads to be expanded to very low densities previously not routinely obtainable.
SUMMARY OF THE INVENTION
In general terms, this invention provides expandable polystyrene beads having a reduced lumping tendency. In particular, this invention provides a class of anti-lumping agents for use with expandable polystyrene beads which significantly reduces lumping when expanding the polystyrene beads into low-density foams of from about 1.5 to 0.75 pounds per cubic foot (pcf), and more preferably from about 1.2 to 0.85 pcf. This invention also finds special utilization when preparing expandable polystyrene beads having low levels of blowing agents.
In a first embodiment, the present invention is an expandable polystyrene bead comprising an expandable styrene or styrene derivative polymer and from about 2 to about 5% by weight, based on the total weight of the bead, of a C
3
to C
6
hydrocarbon blowing agent, wherein the bead is coated with an anti-lumping agent comprising a block copolymer of propylene oxide and ethylene oxide blocks, said block copolymer comprising from about 80% to 65% ethylene oxide and from about 20% to 35% propylene oxide, and having a hydrophilic-lipophilic balance (HLB) value of from about 16 to 29.
In a second embodiment, the present invention is a method for the formation of an expandable polystyrene bead comprising the steps of: forming an expandable polystyrene bead containing from about 2% to about 5% by weight, based on the total weight of the bead, of a C
3
to C
6
hydrocarbon blowing agent; and coating the expandable polystyrene bead with an anti-lumping agent comprising a block copolymer comprising blocks of ethylene oxide and propylene oxide, wherein the block copolymer comprises from about 80% to 65% ethylene oxide and from about 20% to 35% propylene oxide, and has an HLB value of from about 16 to 29.
These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
In general terms, this invention provides expandable polystyrene beads having a reduced lumping tendency. In particular, this invention provides a class of anti-lumping agents for use with expandable polystyrene beads that significantly reduces lumping when expanding the polystyrene beads into low density foams of from about 1.5 to 0.75 pcf. This invention also finds special utilization when preparing expandable polymer beads having low levels of blowing agents.
The expandable polystyrene beads are generally formed via the well known aqueous suspension method wherein the monomers are polymerized and combined with a blowing agent and other additives to form the beads. The beads are then separated from the aqueous portion of the suspension, washed and dried. In at least a first expansion step the beads are heated, usually via steam, to a temperature above the boiling point of the blowing agent. This heating causes vaporization of the blowing agent and expansion of the beads into a form known as a prill or prepuff. The prepuff is then allowed to age for a sufficient period of time, usually suspended in mesh bags. During the aging process external air equilibrates across the prepuff and some residual blowing agent leaves the prepuffs. The prepuff is then placed into a closed mold and heated again to form a shaped object, be it a block or a more complex shape. In some procedures the prepuffs are expanded 2 to 4 times prior to their use in the closed mold.
In the aqueous suspension polymerization reaction the monomers used to form the polymer comprise styrene or styrene derivatives. The styrene derivatives include: alpha-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene, and ar-bromostyrene. The derivatives may include minor amounts of divinylbenzene, methylmethacrylate, or acrylonitrile. The preferred monomer is styrene. The monomer or mixture of monomers is suspended in an aqueous solution and polymerized. The blowing agent is generally added during the suspension polymerization, but can be added during later processing steps. The suspension polymerization also generally include

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