Plastic construction materials offer many advantages, making them attractive to builders and property owners worldwide. Yet many also present a serious disadvantage: extreme fire hazard. Combustible plastics have been the cause of numerous devastating fires, writes Clive Goodwin.

So you have the task of designing a building to meet both cost efficiency and hygiene needs of today's world? The cheapest and most cost effective way is to use products from the ever widening range of plastic building materials available throughout Europe. Cost and thermal efficiency advantages are well known and appreciated but what is the cost in terms of exposure to your business activities?

While many plastics can be produced in expanded or foamed form, polyurethane, the related polyisocyanurate and polystyrene have been most widely used as commercial products. Generally, they offer high insulating values and hence help control operating costs in facilities where maintaining a stable interior environment is important. Refrigerated food processing and storage is the most common occupancy that uses foamed plastic insulation.

Foamed plastics are economical, durable and relatively easy to fabricate and install. The material resists damage by corrosion, water and organisms. Products covered with plastic facings are easy to keep clean. Their light weight provides economy in shipping and handling. However, whether formulated as fire retardant or not, foamed polyurethane and extruded or expanded polystyrene will burn.

Polyurethane may have a low flame-spread rating by the American Society for Testing Materials (ASTM) E-84 Steiner Tunnel Test. But this test involves a low energy ignition source and tests products in the horizontal position only. Polyurethane will decompose at approximately 230°C and ignite somewhere in the range of 315°C to 370°C. Dense, acrid smoke is given off and flames may race across the material's surface. Large scale tests by Factory Mutual Research Corporation (FMRC) have shown that the addition of fire-retardant additives may not significantly change polyurethane's performance under realistic fire conditions.

Polystyrene melts at below 205°C. It has a low flame-spread rating according to the E-84 test method because of its tendency to shrink away from a heat source. In a more realistic, larger fire involving building contents the material will burn intensely, with rapid flame spread across its surface. Heat content has been found to approximate 42,000 kj/kg, which approaches the heat content of gasoline. Also, the material will melt and flow like a flammable liquid, with a flash point of 425°C. As it burns, it generates a dense, black smoke, leaving oily particles on exposed surfaces.

Smoke damage from burning foamed plastics can be specially severe in some settings, exceeding fire damage. Operations that are especially susceptible include food and pharmaceutical processing and storage, medical research and diagnostic laboratories, semiconductor fabrication and storage, and electronic equipment such as computers. Also, dense smoke during a fire will complicate manual firefighting efforts.

Loss history shows that exposed plastic foam insulation represents the highest fire challenge. Often this can cause fire to race ahead of sprinkler protection increasing significantly the resultant damage. Sprayed on polyurethane or exposed expanded/extruded polystyrene should be avoided to reduce the chance of a major fire event.

The food and beverage industry in particular typically looks to preformed insulated panels for hygienic construction. It is often considered that if the skins of pre-formed sandwich panels are steel and continuous the risk is negligible. However, this is not always the case. Insulated sandwich panels consist of a core of insulation "sandwiched" between two sheets or facings. The insulation core may be foamed polyurethane, foamed polyisocyanurate, glass fibre, mineral fibre, or honeycombed treated Kraft paper or aluminium. The facings may be steel, galvanised steel, painted steel, stainless steel, or aluminium. Panels may be designed as the exterior cover of a building, to be secured to the building frame. Or panels may be freestanding and not attached to the building frame, designed to interlock with each other and enclose a separate temperature-controlled structure within a building.

The facing material serves to exclude oxygen from the plastic core's surface, thereby controlling immediate flammability. Where sprinklers have been installed because of occupancy conditions, the facing must delay ignition of the plastic core for 10 to 15 minutes in order to allow the sprinklers to control the fire. It is our experience that with adequate skin thickness, metal to metal joint arrangements and adequate fixing to building steelwork this needed performance in a fire scenario can be achieved. However, the controlling factor is the sandwich core. Polyisocyanurate and polyurethane in certain formulations can achieve this. Polystyrene is the greatest concern. FMRC is undertaking a special research programme to establish methods of controlling this hazard. It appears that conventional sprinkler protection adequate for building contents may not control a fire involving these panels. The future appears to be the use of a thermal barrier which may cause problems in hygienic environments or additional sprinkler protection solely for the panels.

The question for the future is, of course, whether introducing such an exposure into your business even if well protected is not as cost effective as eliminating the hazard by the use of a mineral wool equivalent product. Cost/benefit will contrive to be the name of the game no doubt.

Building with rigid plastics

Rigid plastic panels are commonly used as exterior building panels as well as interior finish. For exterior use, panels are normally of corrugated fibreglass reinforced plastic (FRP), polyvinyl chloride (PVC) or polycarbonate, and are installed directly to the building frame without a backing. For interior use, flat FRP or PVC is commonly used, installed over a backing such as concrete block or gypsum board. Transparent or translucent acrylic, polycarbonate, or FRP is sometimes used in place of glass for glazing and skylights. Structural applications are appearing, especially at pulp and paper mills and chemical plants where economical use of more traditional building materials is difficult in view of the corrosive environments.

For many applications, the major advantage of plastics has been lower cost, ease of fabrication and installation, high strength-to-weight ratio, resistance to corrosion and moisture, and minimal maintenance. The light weight makes for easier handling and more economical shipping. Where colour is important, pigmentation is easily blended into the product during its manufacture; the application of a surface coating for colour, and its possible deterioration, is avoided. Translucent and clear plastics are also available in place of the more fragile glass. Resistance to solar ultraviolet radiation and electrical insulation properties are other advantages of some plastics.

However, combustibility of some plastics in construction has been a major disadvantage, although the degree of fire hazard can be reduced in some cases at the manufacturing stage or by the method of installation. Some plastics are easily ignited, some burn fast, and some give off dense smoke that will hinder firefighting efforts and increase damage at susceptible occupancies. In some cases, additives will reduce the fire hazard but result in a greater generation of smoke.

The heat of combustion, in kj/kg, is generally much higher than with a traditional product such as wood. Some plastics will melt, becoming dripping, burning pieces. Burning building components such as plastic ductwork and wall panels will allow the extension of fire from one area of a building to another. A fast-growing, hot fire will likely activate too many automatic sprinklers, robbing water from those sprinklers that are directly over the seat of the fire. Furthermore, plastics will shed water so that pre-wetting of unburned material, a key aspect of a standard sprinkler system's effectiveness, is minimised. A severe fire involving plastics is also a threat to critical steel building frames, possibly leading to weakening of joists and columns, collapse, and loss of sprinkler protection.

Full-scale testing by Factory Mutual Research Corporation (FMRC) has clearly shown that plastic panels are combustible, whether or not they are marketed as fire retardant treated. Fire-retarding additives may delay ignition and slow down initial fire spread. However, as the fire gets larger, fire spread will accelerate.

The message is clear, take great care when applying plastic construction materials. FMRC has a wide range of approved products that have been subjected to full scale fire tests and have been proved to perform to the designated standards. However, avoidance must be the best course of action.

Clive. Q. Goodwin is a chartered mechanical engineer specialising in loss prevention engineering related to the food, brewing and printing industries. On graduating from Manchester University (UMIST) in 1993 he first specialised in the mechanical design of precise mechanical control systems. Mr Goodwin is now based in Amsterdam with Protection International Limited following a career with Factory Mutual in London.