Plastics—Some Facts Fire Fighters Should Know
Plastics are everywhere, and in a fire, they can and do burn. Learning about plastics, their differences, their reactions in a fire, their chemical and physical properties, can go a long way in employing proper fire fighting techniques—as well as reducing the fear of an unknown material.
Structural fires today are different than those fought 20 years ago. The reason lies in the different materials being used in construction and in the contents of buildings. Many furnishings, decorations, even structural components of some buildings and rooms are made of plastic, where in the past they may have been wood, metal, cloth from natural fibers, or glass. These changes have been made for countless reasons, the more important of which are aesthetics, cost, design capabilities, lightness, strength and versatility. And with an increasing demand for these properties, the proliferation of conversions from the so-called “traditional” materials to plastics will escalate in the future. Indeed, the use of plastics is only limited by the imagination.
What are plastics really? How do they behave in fires? What combustion products are released if plastic burns or becomes subject to pyrolysis? How are burning plastics extinguished?
Plastics are a member of a family of materials known as polymers. A polymer is defined as a giant molecule made up of thousands of small molecules that have the unique chemical property of being able to react with themselves. These small molecules are units that repeat themselves as many as 100,000 times in a single, continuous chain. These small units are called monomers, and the process by which they are converted into polymers is called polymerization.
If all the industrial companies in the world that create polymers were to add their total production together, they couldn’t hold a candle to the greatest polymerizer of all—Mother Nature. The most common natural polymers are wood (or more correctly, the cellulose in wood), leather, silk, cotton (cellulose again) and wool. All cardboard and paper products fall into this category, as they are made from the cellulose in wood (and sometimes cloth).
As man found use for these natural polymers, as well as glass and metal, he encountered many problems, including excessive cost, processing problems, improper adaptation of materials (too heavy, too brittle, too bulky, etc.) and poor aesthetics. The answer to these problems came in the form of synthetic (i.e., made by man) polymers, developed one at a time to solve the dilemma.
These synthetic or man-made polymers include plastics, urethanes, and rubber (although the first rubber was natural, man had to polymerize it properly to make it useful).
There are two great divisions of those synthetic polymers we call plastics, depending on how they polymerize and how they are used. The two types are thermoplastics and thermosets. The difference is relatively simple, the thermosets may be processed (extruded, molded or otherwise formed) by heat only once, while the thermoplastic may be processed several times. That is, once a thermosetting plastic is formed by heat (and pressure), it may not be reformed using more heat. Any substantial heat applied to it will cause the material to degrade (decompose pyrolytically or burn). Common thermosets are phenolics, epoxies and urea-formaldehyde.
On the other hand, thermoplastics may be formed and reformed many times, depending upon the type plastic and the amount of heat applied to it during any particular processing cycle. Common thermoplastics are polyethylene, polystyrene, polyvinyl chloride, and ABS (acrylonitrile-butadiene-styrene).
Since urethane chemistry is so different from other types of polymer chemistry, many feel they make up a totally different group of plastics. Others feel urethanes are neither plastic nor rubber, rather they occupy a position somewhere between the two. Regardless of how chemists view urethanes, they must be discussed as a very important group of materials whenever plastics are discussed generally. Polyurethanes come as thermosets (foams) and as thermoplastics (film, sheet and coatings) and are very important to the consuming public.
The important thing to remember is that it is not correct to lump all plastics together when they are being discussed as a group of materials. The individual plastics are as different from one another as individual metals are different from one another.
These differences among plastics, their compounds and/or alloys go much beyond whether they are thermoplastic or thermosets. Plastics may be liquid, foam or solid. They may be very soft and pliable, or very hard and rigid. They may be electrically conductive or insulators. They may be transparent, translucent or opaque. They may be natural in color (milkywhite) or colored brightly. They may be merely decorative or vitally functional. They may be made to contain explosives, solvents, corrosives, oxidizers, poisons or foodstuffs; and they may be combustible or flame-retardant.
So you can see, the topic of plastics is not a simple one. However, our job here is not to become polymer chemists or plastics engineers. Rather it is to become more knowledgeable in the area of how plastics burn and what we can expect when this occurs.
Four groups of polymers
Since only carbon and hydrogen make up polyethylene, polypropylene and polystyrene, you would expect them to behave in many ways like other hydrocarbons; and chemically they do. The fact that they are colorable, formable, reusable and might have engineering applications separates them from all other hydrocarbons, whether gas, liquid or solid.
The presence of oxygen in addition to the carbon and hydrogen in polymers like acrylics, polyesters and cellulosics makes these materials similar to wood and other cellulose-containing natural polymers. Again, chemically, they may react as wood does, but physically they are entirely different due to their plastic properties.
Polyvinyl chloride is different from the above-mentioned polymers in that its molecule contains chlorine in addition to carbon and hydrogen. PVC by itself, as mentioned earlier, is useless until certain other materials are added to it to make it useful. These range from minerals like calcium carbonate (ground limestone) and mineral pigments (like iron oxide) to organic esters and organic pigments. The list of possible additives for PVC is almost endless, so the resulting compound may have any number of different chemical and physical properties.
Other polymers, like ABS, nylon and the urethanes contain nitrogen in addition to carbon and hydrogen in their molecules, much like natural polymers like wool and leather. These polymers are once again set apart from the natural polymers by their plastic properties. Additives may be used with these polymers to radically change their physical and chemical properties.
This very rough grouping of four types of polymers according to their chemical composition is a crude attempt to point out the differences in plastics, so that when one mentions “plastic,” it is important to identify which polymer is under discussion.
How plastics are used
As fire fighters, you are very concerned as to where you will encounter these materials. That can be answered in a word: Everywhere!
However, even though you will find plastics everywhere, there are many situations where their use is so small as not to warrant your attention. We, of course, will be discussing those situations where you will be likely to meet many different plastics, sometimes in tremendous quantities.
The home: In the interior of a house or apartment, the use of plastics abounds. PVC may be used as floor tile, wall covering and trim and moldings. It will appear as furniture upholstery, shower curtains, bottles and containers, and wire insulation in the telephone and electrical circuits behind the walls. It may even be used as a wall surround or shower stall in the bathroom. Polyethylene may be found in bottles and containers, housewares, wire insulation, pipe, furniture and toys. Polypropylene may be found as containers, carpets, upholstery and housewares.
Polystyrene may be found in packages, lighting fixtures, appliances, housewares and furniture. ABS can be found as pipe, luggage, appliances, power tools, toys and telephones. Acrylics may be used for lighting fixtures, glazing and sanitary ware. Cellulosics are found as glazing, toys, appliances and electrical insulation. Nylon may be found as clothing, upholstery, carpeting, furniture and power tool housings.
Polyesters will be found as bottles, clothing, carpeting and upholstery. Polyurethanes will be found as furniture padding, carpet padding and backing and insulation.
On the exterior of the private dwelling, you may find PVC as siding, window and door frames, and gutters and downspouts. Modified styrene-acrylonitrile plastics may be found as thin layers covering the PVC in these applications.
Polystyrene may be used in shutters that are decorative rather than functional. It may also be used as insulation panels behind siding, or as an ingredient in roofing shingles. Clear polystyrene and/or acrylics may be found as panels over a frame covering a patio.
Commercial uses: The same uses for plastics may be found inside and outside commercial buildings, used in the same manner as they are in the home. You must remember that these uses are preferred in hotels, motels and nursing homes just as they are in your own home.
Anywhere that people congregate for any purpose, whether work or play, plastics will be found. Motorcycles, snowmobiles, campers, mobile and motor homes, athletic equipment, appliances and safety equipment all utilize plastics in one form or another, and in some cases, plastics represent the largest part of the material used in the product. Retail establishments will have concentrated groupings of products made or utilizing great amounts of plastics. It is literally impossible to list all the uses and locations of plastics, not to mention industrial operations where plastics are made (polymerized), processed or fabricated.
How plastics ignite in a fire
If you have a fire involving plastics, what does it take to ignite a plastic? Once it ignites, will it burn freely or will it go out by itself once the ignition source is removed? Do plastics undergo pyrolysis? Is flame spread as bad as with wood and paper products?
Contrary to popular opinion, there are no easy or simple answers to any of these questions. For some reason, fire fighters and others want to treat plastics as one large group of materials, with every plastic of every shape and use reacting alike—”Plastics are liquid petroleum and they burn like hell!”
Well, most plastics are petroleumbased, and since they are organic (carbon-based chemistry), they will burn like any other organic material, subject to its own chemistry, built-in safeguards, shapes and uses.
A plastic coat covering a piece of metal would not be expected to ignite or burn as easily as the plastic might if it were free-standing and subjected to the same ignition source. Also, some powdered plastics are as dangerous in the air as other dusts are, just as fine sawdust is explosive when mixed in air. The same plastic in large pieces might be very difficult to ignite, just as it takes a great deal of coaxing to raise a log to its ignition temperature in a fireplace.
Those plastics that contain only carbon and hydrogen (polyethylene [PE], polvbutvlene [PB], polypropylene [PP] and polystyrene [PS]) may be expected to behave in a fire situation like other solid hydrocarbons, but they don’t. While solid hydrocarbons like paraffin and asphalt may melt rapidly and produce flammable vapors, plastics will sag and may melt more slowly. While plastics do pyrolyze, the action is nowhere near as rapid as in natural hydrocarbons. Of course, when the heat is extreme, the hydrocarbon plastics w’ill melt and flow like other similar materials, burning as it flows.
Those plastics that contain carbon, hydrogen and oxygen (acrylics, polyesters and cellulosics) would be expected to act like wood in a fire, but again the reaction is different. These materials will sag and melt, very often flowing away from the heat source, thereby preventing ignition. Pyrolysis is again possible, but relatively slow when compared to wood. These plastics are more difficult to ignite than polyethylene, polypropylene and styrene (all things being equal), and will not burn as hot.
Those plastics that contain nitrogen in addition to carbon and hydrogen (nylons, ABS and polyurethanes) will burn, but how they act depends very much on their chemistry. Nylon very often will melt and flow away from heat sources preventing ignition, while ABS plastics, which are made up of two hydrocarbon polymers (styrene and butadiene) and acrylonitrile, which contains nitrogen, have a higher melting point and therefore sag very slowly when heated. ABS, therefore, has a higher probability of igniting, since it would move away from a heat source more slowly. Urethanes, on the other hand, had a serious problem in their earlier days with combustibility, and today the vast majority of urethane foam is compounded with flame-retardants to prevent it from burning.
Polyvinyl chloride (PVC or vinyl) is totally different from any of the above plastics. The presence of chlorine in the molecule makes ignition difficult, and when the ignition source is removed, the fire tends to go out by the liberation of chlorine from the molecule. A somewhat related group of plastics, the fluoroplastics, are almost never a fire problem. These materials, one of which is best known as a nonstick cooking surface for pots and pans, contain fluorine in the molecule. It is considerably more difficult to ignite than almost all plastics, and is considered to be a nonburning plastic.
There are many other types of plastics, some having radically different chemistry than the above, but you are not likely to encounter them due to their specialized nature.
With all due respect to the chemistry of the above polymers, their reactions in a fire situation can be modified by the addition of certain materials that will make them more or less combustible. Plasticizers, when added to PVC, will increase the probability of combustion, while antimony oxide decreases it.
A variable that must be considered in any discussion of the combustibility of plastics is that of the ignition source, which might be a rapidly approaching fire involving materials other than plastics. Notwithstanding chemistry and compounding additives, if an already large fire involves any of the above plastics, they will burn.
Size and shape of the plastic part is critical, as it is in considering any combustible material. Plastic film and sheet tend to distort and “back away” from a heat source. Large chunks of plastics are difficult to ignite, while plastic dusts are explosive, as any finely divided organic materials are. Whether the plastic is free-standing, a coating, laminated over, or otherwise in intimate contact with another material will determine its irritability in a fire. Each plastic has, in its pure state and according to shape, a particular ignition temperature, which again can be altered by compounding ingredients.
Therefore, given the base polymer, the additives compounded into it, its use, its shape and its proximity to other materials and its resulting ignition temperature, you can then determine whether the plastic part will sag, melt, flow, pyrolyze, ignite, spread flame rapidly, or simply do nothing.
Probably more than any other hotly debated topic concerning plastics and the fire fighter, the topic of combustion products of plastics is the most questioned. While many know that particular combustion products occur with plastics, the hazard produced by the amount of these combustion products is greatly exaggerated.
First, all plastics are organic materials (except silicones) and when carbonbased organic materials burn, toxic combustion products are formed— principally carbon monoxide. While products other than carbon monoxide are formed having different degrees of toxicity, these products including formaldehyde and acrolein, will not be formed as easily as when wood or other natural products burn. This is because pyrolysis does not occur as readily in plastics as in wood. Depending on the chemistry again, each plastic has its own list of combustion products, and carbon monoxide is present in every one.
Where carbon and hydrogen, or carbon, hydrogen and oxygen are present in the molecule, the only combustion products possible are water vapor, carbon dioxide, and carbon monoxide, with trace amounts of various aldehydes and short-chain hydrocarbons (monomer).
In PVC, a plastic in which chlorine replaces some of the hydrogen atoms, its decomposition from fire produces white smoke and acrid fumes which may be corrosive. The chlorine is mostly released as hydrogen chloride. If dissolved in water, it forms hydrochloric acid. PVC, however, also produces carbon dioxide, carbon monoxide, and traces of aldehydes as combustion products.
Where nitrogen is present, combustion products may include water vapor, carbon dioxide, carbon monoxide, oxides of nitrogen (irritants) and hydrogen cyanide gas. It is this evolution of hydrogen cyanide (HCN) and hydrogen chloride (HC1) from PVC that contributes to the controversy concerning burning plastics.
The point of all this is: plastics do produce toxic combustion products when they burn (as do all other carbon-containing materials). Those plastics that contain nitrogen may evolve hydrogen cyanide as a combustion product (as do all natural materials that contain nitrogen); carbon monoxide is the principal killer in toxic poisoning in these fires.
Many fires involving plastics (as well as other hydrocarbon-based materials) are very smoky. Work is being done in the plastics industry to reduce the amount of smoke generated and the combustibility of burning plastics.
The composition of this smoke is very similar to other hydrocarbon-based materials, with the major product being black, unburned carbon. The presence of so large an amount of unburned carbon is the telltale sign that large amounts of carbon monoxide are being generated.
Besides carbon, the smoke will consist of gaseous and vaporous combustion products, including carbon dioxide, carbon monoxide, water, acrolein, formaldehyde, acetaldehyde, propionaldehyde and butyraldehyde. In addition to these products, which are the only ones produced by those polymers containing carbon, hydrogen and oxygen (or just carbon and hydrogen), hydrogen chloride will be present in the smoke of burning PVC, while hydrogen cyanide and/or various oxides of nitrogen may be present in the smoke of burning ABS, nylons, urethanes and acrylonitrile-based plastics.
Needless to say, as in all fire situations, the fire fighter must wear SCBA throughout the incident, including overhaul.
Plastics are Class A materials, and fire involving plastics may be extinguished by using Class A fire extinguishers, Class A extinguishing agents and Class A fire extinguishing techniques. While it is true that most plastics will sag and begin to flow, very seldom do they ever reach the low viscosity flowing characteristics of flammable or combustible liquids. Water sprays and fogs will not only extinguish the fires, but they will cause the solidification of the softened plastic through the rapid cooling effect of water.
As mentioned earlier, fire fighters should become familiar with the melting temperature, minimum service temperature and the ignition temperature of each of the most common plastics and compounds (see table 1). These properties will provide clues to when plastic parts may fail in use and how to extinguish them if indeed they do become involved in the fire.
It is not an impossible job to understand all the different plastics and their behavior in a fire, not when you look back and count all the flammable and combustible materials with which you have become familiar and can handle with relative confidence. Since plastics are not going to go away, the most sensible thing to do (not to mention the safest) is to add plastics to your store of knowledge and experience, and approach them as you would any other material.
Learn their minimum ignition temperatures and burning characteristics (polyethylene and polypropylene burn with a dripping flame and smell like burning candle wax while polystyrene generates huge amounts of large particles of soot and has a faint odor of natural gas). Learn the minimum service temperatures so when the plastic part fails in a fire you won’t be surprised (polycarbonate and/or acrylic glazing may sag and give you unexpected and untimely ventilation). Learn where you are most likely to meet these materials, and be aware of how much of each may be used or stored in the building containing the emergency to which you are responding.
Unfamiliarity breeds fear and mistaken and misguided emergency procedures. Approach plastics as you would any other group of materials and your problems will probably diminish substantially.