Flame retarding materials.

There is currently a myriad of applications that are being produced that have flame retardant requirements. This article will center on the various polymers and on the additives available for them. This primer will dwell on the additive approach rather than the reactive one that deals with modification of the polymer backbone.

Table 1 lists the key polymers and several typical applications. The reason for the impetus in these types of materials today stems from their tremendous versatility, moldability and decreased weight that they possess over materials of the past such as wood, steel and metallic alloys. The problem that occurs is that the major constituents of all of these new materials are hydrocarbon or heteroatom based. These materials are inherently very flammable.

The purpose behind this primer is to discuss compounders' objectives and details of flame retardancy in which two basic mechanisms will be expanded upon including halogen-containing and halogen-free. We will then discuss ingredients that are typically used to retard the combustion reaction. These will be classified as to which phase they work in and how they relate to a combustion scenario. Afterwards, several scenarios and compounding hints, plus relationships will be presented.

There are several physical properties (polymer structural network characteristics) that can affect thermal stability. The change in state of cure (crosslink density), however, has very little effect on thermal stability (ref. 1). On the other hand, increase of the propylene content and quaternary carbons do decrease the thermal stability of a polymer (ref. 2). The increase of either of these should be avoided if the ultimate in flame retardant products is desired. Thermosets such as SBR, NR, PI, CR and EPDM and thermoplastics like PE, PP and PVC are relatively easy to flame retard and have mechanisms classified as normal. However, when IIR or PS and ABS are flame retarded they are somewhat more difficult and classified as abnormal due to the mechanism that is undergone during degradation and the dripping that is associated with it.

General mechanisms

If any polymer is exposed to heat and oxygen long enough, it will combust. It is our task as compounders to make changes to the ignition, flame spread and growth phases to retard combustion (ref 3). These changes can take place in a multitude of ways including the addition of flame retardant additives, fillers or polymer modification. This primer will deal with the additives approach which refers mainly to the first two areas.

Discussion of a typical scenario of rubber decomposition includes a general nature thermal decomposition and specific information on halogen-containing and halogen-free systems. The schematic shown in figure 1 will help to qualify this discussion.

Upon ignition, the first critical step in a fire situation is the rate of production of flammable gases with oxygen which determines the rate of burning. This fuel comes from the hydrocarbon moiety and the plasticizers present. This produces heat and flame which is available for further degradation. Without oxygen, combustion will not take place. Degradation takes place at relatively mild temperatures accompanied by evolution of volatiles (low molecular weight hydrocarbons which generate heat) (ref. 4). Besides flammable gases, there are also non-flammable gases that dilute the gaseous mixture and reduce the temperature of the flame. Char is also present in this scenario and reduces the rate of production of flammable gases. The char acts to hold the flammable gas fractions in the solid phase. Inert fillers and high carbon to hydrogen ratio polymers produces ash which can assist in char formation. Also, of great importance today, we have the smoke which is generated and is made up of particulates that are derived from the combustion reaction.

Halogen-containing system

We will now discuss the two major types of flame retardant systems that have recently become popular. The halogen-containing and halogen-free systems are the general categories that are used when dealing with additive flame retardancy. The halogen-based system usually contains antimony oxide along with the halogen in order to perform properly. It also can include zinc borate, alumina trihydrate (ATH) and phosphorous. In this case, there are a whole host of synergistic reactions that can take place which are very advantageous. Firstly, antimony oxide acts synergistically with zinc borate where both of them together can delay the onset of oxidation by up to 100 [degrees] C in PVC. (ref.5). The zinc borate reacts with the HCl present to give ZnCl which can cause crosslinking in PVC or polychloroprene resulting in added char. The zinc borate, at high temperatures, also forms a glassy layer which inhibits oxidation of the char and helps to maintain its integrity. The antimony oxide reaction with the hydrogen halide takes place in the condensed phase. The antimony oxyhalide or trihalide volatilizes to the gas phase and inhibits flaming oxidation.

Halogen-free system

The halogen-free system does not utilize antimony oxide or halogen and, therefore, does not exhibit the synergism that antimony oxide/zinc borate/halogen has in the first system. This systems relies on heavy loadings of two ingredients, ATH and zinc borate (ref. 6). Since it contains no halogens, there is no formation of harmful acids. The water of hydration of both of these materials provide the initial flame retardancy. The zinc borate also promotes a strong formation of char. The char is the polymer which has been diverted to carbon rather than fuel for gas phase combustion. The char also insulates the unburned polymer.

The zinc borate in combination with the ATH produces a porous hard residue which is the sintering due to high heat conditions and reaction of the zinc borate and ATH. This all takes place in the condensed or solid phase.

Building a flame retardant compound

The discussion and building of a flame retardant system will be approached as seen in table 2. Listed are several key categories that have the major additive flame retardants classified by function, chemical class and the phase in which the specific reaction (chemical/mechanical) takes place. Also find listed the typical usage level of each of these ingredients for a typical halogen-containing and a halogen-free system.

Tables 3-5 list several key points concerning the additives of interest in this discussion. The additives include antimony oxide, halogens, phosphorous, alumina trihydrate and various fillers.

Antimony oxide

Antimony oxide does not react with a halogen (Cl2 or Br2), but directly with HCl or HBr that is formed during combustion. It is mandatory that antimony oxide be accompanied by a halogen source in order for it to be effective. Antimony oxide in combination and reaction with the halogen source suppresses ignition but also may produce smoke. The reaction with the halogen source forms an antimony trihalide or oxyhalide and takes place in the molten polymer. This reaction promotes the formation of carbonaceous char, not volatile gases. Even though the initial reaction takes place in solid or condensed phase, the bulk of the reaction occurs in the vapor phase after volatilization takes place. These reactions inhibit flaming combustion in the gaseous phase.

Halogens

The halogens used suppress polymer fires in the listed order iodine > bromine > chlorine > fluorine. Chlorine and bromine are typically the halogens of choice. Also, aromatics are more chemically stable than the aliphatic type compounds. Concurrently as with the antimony, the halogen acts in a synergistic manner and along with the antimony oxide breaks the oxidative chain. It is also well known that chlorine slows the condensed phase reaction and when attached to a polymer such as CPE, CR, CSM or PVC it has better stability.

Phosphorous

The phosphorous containing materials promote dehydration by the formation of an acid. It functions in the condensed phase by forming a char with very low smoke evolution. The char form consists of polyphosphoric acid. The best sources of phosphorous are either halogenated or nonhalogenated compounds which contain at least 8-9% phosphorous. The three classes of major phosphate ester plasticizers of importance in the rubber industry include triaryl, alkyl diaryl and trialkyl. The triaryls are very good flame retardants, but poor plasticizers. The triaryls are poor flame retardants, but excellent plasticizers (ref. 7). It is, therefore, suggested that the alkyl diaryls be used for the best overall properties in flame retardant compounds.

Alumina trihydrate/magnesium hydroxide

The alumina trihydrate and magnesium hydroxide both act by releasing their water of hydration which cools the flame and acts as a heat sink to pull heat away from the flame front. ATH acts as a flame retardant and smoke suppressant because of its thermodynamic properties. When the ATH or magnesium hydroxide is exposed to heat it will absorb it prior to liberating water vapor. The endothermic decomposition of the 34.6% combined water takes place for ATH at 230 [degrees] C and 26% for magnesium hydroxide at 350 [degrees] C. This dehydration cools the polymer, acting like a heat sink, and dilutes with water vapor those gases that escape. This whole system can be considered to be mechanical in nature and occurs in the gas phase (ref. 8). It has also been found that ATH reacts synergistically with zinc borate.

Fillers

In all cases, fillers increase smoke density. Calcium carbonate and zinc oxide lower the LOI and clay and ATH increase the LOI. In most all cases, it is helpful to require more oxygen to support flaming combustion.

With these raw materials and their characteristics in mind we will review several compounding hints that can be of help when producing a flame retardant article. Firstly do not use calcium carbonate if halogens or phosphorous compounds are present since it literally soaks up these materials and renders them inefficient. It also has been shown that antimony oxide and phosphorous compounds together are less effective than if used alone. There are also several conditions that can occur causing your compounded product to fail a specific flame test. They and the corrective action that should be taken to counteract the condition will be noted. If you encounter molten flow of your product leading to flame spread it is generally suggested to increase the filler level. If, however, you encounter breaking off of flaming solid particles and/or collapse it is suggested to increase the durometer by adding mineral fillers or by reducing the plasticizer level. Lastly, if you encounter excessive rate of flame travel it is suggested to decrease the level of conductive fillers (ref. 3). There are a whole host of additional situations that can be encountered but these are some of the main areas. We will also show for your review typical scenarios for thermosets (table 6) and thermoplastics (table7) of major interest in the rubber/plastics flame retardant market. In the tables we have listed the polymer, mechanism and various comments pertaining to each.

Failures in flame retardancy can take place due to poor dispersion. If the mixing can not be carried out effectively, it is suggested that large particle size materials be used. There are also many concentrates available in rubber/plastic binders. Pastes, emulsions and various other forms of dispersions are also very effective ways to carry liquid or solid flame retardants into polymer systems.

Future trends

There are several areas that are currently being investigated which will have an impact on methods of flame retardancy for the future (table 8). There have been studies with natural rubbers that are treated with polyhaloalkenes via radiation, the halogen then adds to the double bond and gives a flame retardant highly elastic compound (ref. 10).

The area of copolymerization includes products such as nylon (ref. 9) and polyurethanes which have a great deal of architectural synthesis capabilities (ref. 10). Other areas of great interest include halogen-free and reduced smoke systems; plus studies of the chemistry of combustion. Polymeric additives are also being evaluated as viable options since they emit less hazardous materials while improving physical properties. These areas of research are still in their infancy and are continuing to expand and capture interests of many companies.

Conclusions

As compounders, we have learned that there are basically two different types of polymer backbone - hydrocarbon and heteroatom. These in most cases, except when there are halogens available on the backbone, are very flammable. There are also many other materials such as plasticizers, oils and process aids that are detrimental to flame retarding of polymers. There are many inorganic/organic additives including fillers, that will act in one of the two major phases. The two major phases are the condensed or solid phase and the vapor or gas phase. There are two basic systems used in flame retarding technology, halogen containing and halogen-free. Within these systems, the various additive acts either chemically or mechanically to retard the fire. In some cases, as with zinc borate, antimony oxide and halogens there are synergistic reactions.

All polymers whether rubbers or plastics, degrade by different mechanisms and to a different degree and are dependent on their chemical structure Since these variations do indeed exist, many different materials are needed in order to produce flame retardant products.

References

[1.] S. Straus and S.L. Madorsky, Industrial Engineering Chemical, 48, 1212, (1956). [2.] L.A. Wall and S. Straus, J. Polymer Science, 44, 313, 1960). [3.] Culverhouse, D. Presented at Akron Rubber Group meeting on "Compounding rubber for fire resistance," Akron, OH, January, 1983. [4.] H.J. Fabris and J.G. Sommer, Rubber Chem. and Tech., pp. 523-564. Volume 50, 1977. [5.] Shen, K. presented at FRCA Annual Conference on "Zinc borate as a flame retardant, smoke suppressant, afterglow suppressant and its miscellaneous uses in the plastics and rubber industries," Callaway Gardens, GA, March, 1984. [6.] Shen, K. Technical Service Bulletin No. FRP-01, 1987, U.S. Borax Research Corp., Anaheim, CA. [7.] Hamilton, J.P., Plastics Compounding, 1978 "Flame retardants for thermoplastics." [8.] Hornsby, P.R. and Watson, C.L., "A study of the mechanism of flame retardancy and smoke suppression in polymers filled with magnesium hydroxide," Best PMAAD Paper of RETEC, 1989. [9.] E.G. Cockbain, T.D. Pendle, and D.T. Turner, Chemical Industries (London), 318 (1960). [10.] S.L. Madorsky, "Thermal degradation of organic polymers," Interscience, New York, 1964.