Process of making rigid polyurethane foam

Details Process of making rigid polyurethane foam Process of making rigid polyurethane foam

2002-03-22

2004-04-27

Cooney, Jr., John M. (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S137000, C521S155000, C521S170000, C521S174000

Reexamination Certificate

active

06727290

ABSTRACT:

This invention concerns a method of making a rigid polyurethane foam that is cost efficient and which yields a product that is relatively light in weight, yet has good crush resistance.
Polyurethane resins are made by reacting polyols with polyisocyanates. The reaction is exothermic. Cross-linking, or branching, of the polyurethane molecules can be achieved by including in the reaction mixture some polyol molecules and/or isocyanate molecules that have at least three functional groups, and by adjusting the ratio of reactants accordingly. With sufficient cross-linking, rigid, thermoset polymers are obtained.
To make rigid polyurethane foam, a mixture is made of a polyfunctional isocyanate, a polyol, a blowing agent, a catalyst, and, usually, a cell-size regulator (e.g., a surfactant). A urethane-forming reaction begins once the ingredients are combined, an exotherm forms, and the blowing agent or agents cause closed cells to form in the polymer as the mass expands and solidifies. The exotherm typically reaches a peak temperature of at least about 150° F. The isocyanate and polyol reactants include enough molecules with three or more functional groups that the degree of cross-linking or branching is sufficient to produce a rigid foam.
Aromatic polyisocyanates often are used when making rigid foam. Some examples are toluene diisocyanate (TDI) and polymeric isocyanate (PMDI), which is obtained by the condensation of aniline with formaldehyde.
Polyols that can be used include polyether polyols and polyester polyols. Propylene oxide adducts of polyfunctional hydroxy compounds or amines are one type of polyether polyols that can be used. Mixtures of polyester polyols and polyether polyols sometimes are employed.
Halogenated hydrocarbons, such as hydrochlorofluorocarbons and hydrofluorocarbons, can be used as blowing agents. Lower alkanes such as pentanes and cyclopentanes can be used as well. Water can also be used, as it will react with isocyanate to generate carbon dioxide in situ. Sometimes water or carbodiimide catalysts are used to generate carbon dioxide as a co-blowing agent. Often the blowing agent or agents are preblended with the polyol, together with the catalyst and the cell-size regulator, which usually is a surfactant.
All of this is well known to persons of ordinary skill in the art and is described, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, 4
th
Ed. (1997), vol. 24, pp. 695-715, which is incorporated herein by reference.
The cost of the reactants to make rigid polyurethane foam is relatively expensive. It has now been discovered that the cost per volume unit of the finished foam can be lowered by including in the polyurethane reaction mixture expandable polystyrene beads (EPS) that are made of a polystyrene that has a softening point that is equal to or below the maximum temperature reached by the exotherm during the urethane-forming reaction. The heat of the urethane-forming reaction causes the polystyrene beads to sinter, while trapped in the foaming matrix of polyurethane, creating pockets that are generally larger than the cells of the polyurethane. Since the polystyrene beads are less expensive, on a volume basis, than the foamed polyurethane, the materials cost of the polystyrene-containing foam is less than that of the same volume of straight polyurethane foam. The monetary savings can be substantial, as the cost ratio of polyurethane foam to fully expanded polystyrene, on a volumetric basis, is currently about 25/1 to 30/1.
A side benefit of including the polystyrene beads as a filler in the polyurethane is that the resultant foam is less dense than unfilled polyurethane. Yet another benefit is that the polystyrene-containing foam, despite the presence of the relatively low-cost filler (the expanded polystyrene), appears to have the same or better crush resistance as the unfilled polyurethane.
Expandable polystyrene beads are cellular pellets of expandable polystyrene that often are used to form lightweight molded objects. Created in a more or less granular form, and with an expanding agent in the cells, typically the beads are pre-foamed, or “pre-expanded,” by heating to a temperature above their softening point, which often will be in the range of about 165-185° F., until they foam to give a loose aggregate of the desired bulk density. The pre-foamed particles, which retain their cellular structure, may then be placed in a mold or other cavity and heated with live steam, causing them to sinter and fuse together to form a lightweight, cellular solid whose dimensions correspond to those of the mold cavity. When fully expanded, the beads often will have a diameter that is about 2 to 4 times that of the unexpanded, or “raw,” beads.
When present in the reaction mixture that forms a rigid polyurethane foam, the EPS beads, as mentioned above, sinter. In addition, however, at least a substantial portion of the beads lose their cellular structure, creating gas-filled pockets, of various sizes, in the foam, which are lined with the polystyrene of which the cellular structure was formed. It appears that isolated spherical beads generate relatively spherical pockets. In some sense, it might be said that there are thin-walled polystyrene globules dispersed throughout the polyurethane foam, as a result of the inclusion of the EPS beads in the reaction mixture.
Moreover, it appears that these polystyrene globules are coated with a layer of “skinned” polyurethane, i.e., a thin continuous layer of the polyurethane, much as is present on the outer surface of rigid polyurethane foam moldings. The presence of these double-walled hollow structures seems to enhance the foam's crush resistance.
Methods of making expandable polystyrene beads are well known. As disclosed in U.S. Pat. Nos. 3,991,020; 4,287,258; 4,369,227; 5,110,835; 5,115,066; and 5,985,943, for example, all of which are incorporated herein by reference, EPS beads may be made by polymerizing styrene in an aqueous suspension, in the presence of one or more expanding agents that are fed at the beginning, during, or at the end of polymerization. Alternatively, they may be made by adding an expanding agent to an aqueous suspension of finely subdivided particles of polystyrene.
The expanding agent, also called a “blowing agent,” is a gas or liquid that does not dissolve the styrene polymer and which boils below the softening point of the polymer. Examples of suitable blowing agents include lower alkanes and halogenated lower alkanes, e.g., propane, butane, pentane, cyclopentane, hexane, cyclohexane, dichlorodifluoromethane, and trifluorochloromethane. Often the beads contain about 3 to 15%, based on the weight of the polymer, of the blowing agent. Preferably, the blowing agent will be present at a level of about 3 to 7%.
By “beads” we here mean small particles of any geometry, e.g., spherical, cylindrical, or lumpy. By “polystyrene” is here meant a styrene homopolymer or copolymer containing 50 wt. % or more, preferably at least 80 wt. %, of styrene. Examples of suitable comonomers are &agr;-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes, acrylonitrile, esters of acrylic or methacrylic acid with alcohols having from 1 to 8 carbon atoms, N-vinylcarbazole, and maleic acid or anhydride. A minor amount of a copolymerized chain-branching agent may be included in the polymer as well. Suitable such agents are compounds containing at least two &agr;,&bgr;-ethylenically unsaturated groups, such as divinyl benzene, butadiene, and butanediol diacrylate. Branching agents are generally used in an amount of about 0.005 to 0.05 mol %, based on the styrene.
The polystyrene in the EPS beads usually has a weight average molecular weight in the range of about 130,000 to about 300,000.
Expandable polystyrene beads may contain other additives to impart specific properties either to the beads or to the expanded products. These include, for example, flameproofing agents, fireproofing agents, nucleating agents, decomposable organic dyes, lubricants, fillers, and anti-agglomerating additives. As di

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