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May 2006: Plastics

The 1946 film "It's A Wonderful Life" was one of the best movies ever made. It starred Jimmy Stewart as George Bailey. A friend of George's was Sam Wainwright played by Frank Albertson. Sam was a recent college graduate and he wanted George to work with him and become rich with this new product called "Plastics." Sam made a fortune with making plastic hoods for airplanes during WW II.

Plastics have evolved considerably since the 40's and their initial introduction to household and building materials. Now plastics can be found everywhere and in every home, office, warehouse or building. Today four kinds account for approximately 70% of all plastics production in the United States - polystyrene, polyethylene, polypropylene, and polyvinyl chloride.

Plastics are made from petroleum. Crude oil and natural gas are refined into ethane, propane and hundreds of other petrochemical products. Ethane and propane are "cracked" into ethylene and propylene, using high-temperature furnaces. A catalyst is combined with ethylene or propylene in a reactor, resulting in a powdered polymer. The polymer is combined with additives in a continuous blender, extruded and then melted. The plastic is then cooled and then fed to a pelletizer that cuts the product into small pellets. The pellets are then used in processes such as extrusion, injection molding and blow molding to make millions of plastic products.

Generally plastics are determined by heat of combustion using a one gram sample of a product burned in a calorimeter. Paper has a heat content of approximately 7,000 btu/lb, wood is about 10,000 btu/lb. Most plastics are in the 15,000 to 22,000 btu/lb heat content range. Gasoline has a 22,000 btu/lb heat content rate.

Plastics have two forms, unexpanded and expanded. Unexpanded plastics have a high density like a plastic tote and expanded plastics have a low density like foam packaging. The heat release rate for expanded plastics is generally greater due to the relatively low density and resulting high burning rate.

FM Global Data Sheet 8-1 Commodity Classification defines plastics into three Groups A, B and C.

Group A - High heat of combustion (btu/lb) and burning rate higher than Group B. Plastics such as Polystyrene and ABS are examples of Group A plastics.

Group B - Heat release rates that are higher than ordinary combustibles but less than Group A. The heat of combustion may be as high or sometimes higher than Group A but the burning rate is lower than a Group A plastic. Plastics such as Polyethylene, Polypropylene, Polycarbonate, Acrylics are examples of Group B Plastics.

Note: The heat release rate of Polystyrene is 440 lb/min and the heat release rate of Polypropylene is 233 lb/min.

Group C - Plastic products that incorporate heat of combustion and burning rates that are similar to Class 3 ordinary combustibles. Phenolics and Silicones are examples of Group C plastics.

How do you determine if the commodity or storage arrangement is a plastic or not:

- If more than 25% by volume is expanded plastic (ex. foam packaging), the commodity is Plastic.

- If greater than 5% and less than 25% by volume is expanded plastic, the commodity is Class 4.

- If more than 15% by weight is unexpanded plastic (ex. plastic tote) the commodity is Plastic.

- If between 5% and 15% by weight is unexpanded plastic the commodity is a Class IV.

- If less than 5% by weight or volume is unexpanded or expanded plastic the commodity is Class 1 - 3.

The National Fire Protection Association (NFPA) 13, Standard for the Installation of Sprinkler Systems - 2002 Edition has a slightly different definition for plastics than FM. NFPA 13 states in Chapter 5 the following:

- A Class IV commodity shall be defined as the following:

- a. A product that is partially or totally Group B plastics

- b. Consists of free flowing Group A plastic material

- c. Contains within itself or its packaging 5% to 15% by weight of 5% to 25% by volume of Group A plastics

- A Plastic commodity can be classified as Group A, B or C and have more than 15% by weight or 25% by volume of plastic

- NFPA Group A materials = FM Group A & B materials

- NFPA Group B materials = FM Group C materials

Plastics are much more hazardous than ordinary combustibles (Class 1 - 4 commodities) for three main reasons:

- A plastics fire develops extremely high temperatures up to 2000° F. (Some as high as three times more than a fire with paper or wood). At 1400° F. the physical property characteristics of steel start to fail. Building collapse is a concern with a plastics fire.

- A plastics fire exhibits a very high burning rate. The flammability of plastics is much higher than that of ordinary combustibles.

- A plastics fire gives off especially thick, dense smoke and toxic fumes. The smoke makes it extremely difficult for public fire personal to control the fire with hose streams.

Installing an automatic sprinkler system is the best form of protection to control a plastics fire. There are two factors that are especially important in designing protection. These are 1) the storage arrangement, and 2) the type of sprinkler protection provided.

Solid-pile and palletized storage with 12-in. flue space between stacks will burn more fiercely than a close-pile-array, with 6-in. spaces between stacks because air access in the open array promotes fire growth and heat radiation between stacks.

For rack storage, the abundance of flue spaces and the stability afforded by the racks to hold the commodity in place will result in a fire hazard more severe than solid-piled and palletized storage. Usually protection afforded by ceiling sprinklers needs to be supplemented by in-rack sprinklers in many cases to assure adequate fire control. Some suppression mode automatic sprinklers can offer adequate protection without in-rack sprinklers.

Plastics are used everywhere in our day-to-day life. Therefore most warehouse occupancies today are classified as plastics and need to be protected with a superior sprinkler system, and in most cases the public water supply will need to be supplemented with a booster pump to meet the increased automatic sprinkler demand for the plastic storage.

Providing adequate sprinkler protection for a plastic warehouse is a difficult task. There are so many variables that need to be taken into consideration to provide adequate protection. The best advice is to have a fire protection engineer be involved early in the design phase. We at Risk Logic are available to assist you in your sprinkler system design.

References:

The High Challenge Risk Stored Plastics published by Factory Mutual Global P7742

FM Global Data Sheet 8-1 Commodity Classification

The National Fire Protection Association (NFPA) 13, Standard for the Installation of Sprinkler Systems - 2002 Edition