Developments in Self-Extinguishing Plastics
SELF-EXTINGUISHING is a term applied to plastics and resins which will not support combustion in the absence of an outside flame. However, these products may be combustible as long as a flame is applied to them. Self-extinguishing resins contribute to safety against fire, the degree depending on other factors, notably on the thickness of the plastic sheet or laminate. A thin sheet of the material may provide much less fire protection than a thick layer of wood. However, at otherwise comparable conditions, selfextinguishing resins do indeed constitute an important element in fire prevention programs.
One of the earliest applications of these plastics was in the manufacture of air ducting in aircraft. It was felt the added fire-preventive features of using a self-extinguishing resin was justified in this case particularly because ducting reaches into inaccessible places. The object was to prevent the spread of fire which may be caused by a small spark and the like inside the aircraft. Noncombustibility was not a consideration in this instance since a major fire is presumed to be fatal.
This philosophy has since been applied to other applications of selfextinguishing plastics, notably railway rolling stock, commercial vehicles, and increasingly, in structural service. Protection must be provided against the spread of small fires which may be caused by electrical failure, by friction, or by dropping a match or cigarette, and in this function self-extinguishing plastics serve admirably.
Useful self-extinguishing formulations are available today in a very large line of commercial plastics including phenolics, ureas, melamines, nylons, alkyds, polyesters, polyvinyl chloride, polystyrene, polyolefins, polycarbonates, etc. Some of these are inherently self-extinguishing, even without further modification. This is particularly true of halogenated products. For example, polyvinyl chloride which contains 56 wt. per cent chlorine is self-extinguishing unless it is modified by a combustible plasticizer. As a rule of thumb, vinyl resins are self-extinguishing as long as total chlorine content (including chlorinated plasticizer) is above 30 per cent.
Other plastics which are inherently self-extinguishing include phenolics, nylons, polycarbonates, and chlorine plastics. However, their application in structural service is limited. In the case of cellulosic plastics, there is difficulty in formulating self-extinguishing resins because halogens attack the polymeric structure and may cause excessive stiffness in the molding process. Similar difficulties are encountered in rendering polystyrene self-extinguishing. Here the use of phosphate plasticizers has been recommended, but a high concentration is required and this leads to low heat distortion point and excessive softness. Chlorination of the styrene molecule does improve selfextinguishing characteristics, but the resulting polymer is expensive and differs significantly in its properties from polystyrene itself.
This difficulty of rendering polystyrene self-extinguishing has been the subject of considerable investigative work. Chlorinated waxes have been used, but the result has been loss of transparency and an excessive decrease in heat distortion point. Nor has the use of chlorinated rubbers in this service been successful except in certain high-impact styrenes.
Special steps must be taken to render most polyesters fire-retardant. Several lines of attack may be taken, namely: Physical admixture of fireretardant fillers or additives; chemical modification of acid components or of monomer; chemical combination of organometallic compounds with the resin. Usually, at least two modifications are jointly employed. Some typical approaches are as follows.
Change of Filler, Use of Additives: To retard burning rate, antimony oxide is the preferred filler. This material is generally incorporated jointly with highly chlorinated paraffin waxes or diphenyls, which themselves function as fire-retardant compounds. Unfortunately, these chlorinated organic additives have a tendency to act as plasticizers and they have generally unsatisfactory stability against migration.
According to Parkyns (British Plastics, January 1959), tricresyl phosphate added to chlorinated waxes in lieu of antimony improves fire-retardant characteristics significantly. Other possible additives are trichlorethyl phosphate, triethyl phosphate and diphenyl stibine.
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Chemical Modifications: Self-extinguishing characteristics may be raised by modifying either of the saturated acids incorporated into the resin. The usual approach is to employ chlorinated derivatives of the customarily selected acid. For example, tetra. chlorophthalic anhydride may be substituted in part for phthalic anhydride. Other possibilities are the use of chlorendic acid (a reaction adduct of maleic anhydride and hexachlorocyclopentadiene) and of chloromaleic acid.
The monomer itself, e.g., styrene, may be replaced by structures such as diallyl benzene phosphonate which yield a fairly good self-extinguishing polyester. According to Parkvns, polyesters based on diallyl benzene phosphonate are the best self-extinguishing plastic now available. Because of its high cost, however, other monomers are preferred for many applications. Here, too, chlorination will serve to decrease combustibility and dichloro styrene is being used to good advantage.
Finally, good performance has been reported for use of organometallic compounds in the resins, among them antimony alloxide. Obviously, the use of such modifying agents frequently changes the properties of the basic polyester resin. The change is most pronounced in the case of organic additives which tend to plasticize the resin. In the case of halogenated compounds, these materials may also cause corrosion of molds used in fabrication.
Self-Extinguishing Foams: The need for self-extinguishing plastics is particularly great in structural foams which are at the threshold of major expansion. One forecast expects United States requirements for plastic foam in roof construction to rise from the present 200 million board feet to 600 million board feet in the near future, Even more impressive is the anticipated growth of foamed panels and sidings from 30 million board feet today to 400 million board feet within a few years. The bulk of need is for polystyrene and polyurethane foams.
We have already noted the difficulties in formulating self-extinguishing polystyrene plastic for molding purposes. However, the structural requirements on polystyrene foam can be met more readily by fire-extinguishing formulations and such fire-retardant polystyrene foam is widely available today. As in other plastics, such formuiations are achieved by incorporation of phosphated or chlorinated plasticizers—use of chlorinated monomer is too expensive for most purposes.
Polyurethane foams are also rendered fire-retardant by incorporation of phosphated and halogenated plasticizers. However, a recently successful variant is the use of chlorendic acid as one component in the contained alkyd resin. In this method, chlorendic acid is actually incorporated into the molecular structure.
Urethane foam containing chlorendic acid is stated to exhibit excellent structural properties up to 250° F, low thermal conductivity, a high degree of adhesion to sandwich panels between which they foam and, above all, excellent self-extinguishing characteristics. Polyurethane foams incorporating chlorendic acid are already used commercially as insulation for refrigerated enclosures and as filling between sheet steel panels for building wall construction.
Advantage of formulation
Chief advantage of this formulation over urethane foams rendered selfextinguishing by use of additives, is reported to be better retention of high-temperature strength and improving aging characteristics. According to Hooker Chemical Corporation, these fire-retarding polyurethane foams require only a hydroxyl resin and the semi-prepolymer for their manufacture, using either continuous machine or batch methods.
Tests for fire-retardancy
The acceptability of self-extinguishing resins for various structural purposes is controlled by a multiplicity of building codes, standard industrial tests and military specifications. Typical of the approaches employed are the three key tests specified by the American Society of Testing Materials:
- ASTM D-635 which is a procedure for determining the relative flammability of rigid plastics in the form of sheet or molded bars over .050 inch in thickness.
- ASTM D-568, which determines flammability of sheets less than and films less than .050 inch in thickness.
- ASTM D-757 is a standard test which covers flammability of rigid plastics in the forms of sheets or plates 1/8 inch in thickness. This is particularly suitable for comparative tests in a series of plastics.
In Great Britain, the key applicable test is the British Standard 476. The second test in this series determines the surface spread of flame on building materials, including plastics. In the United States, ASTM tests are frequently supplemented by an Underwriters Laboratories test carried out on a ⅛-inch-thick work piece which is held in a flame for 10 seconds. Applicable military tests are M1L-M-14-F and M1L-M-14-E which are similar in concept to the methods recommended by ASTM and by Underwriters Laboratories.
ASTM D-568 determines whether flame applied to one end of the test piece will spread to a gage mark 3 inches from the end. while the gage mark in D-635 is 4 inches from the end. If the flame will not reach to this test mark, the plastic is said to be self-extinguishing by the applied tests.