In December 1998, a large multitenant warehouse facility in Mt. Vernon, Indiana, went up in flames and burned for more than three days before the fire was finally extinguished. More than 28,000 tons of various plastic products were stored in this facility. The products not only burned but also created a serious environmental problem that included hot liquid plastic runoff and acrid smoke.

Firefighting actuaries involved about 40 fire departments responding from Indiana, Kentucky, and Illinois. The Mt. Vernon fire and rescue chief served as incident commander. The U.S. Environmental Protection Agency responded to the environmental problems that resulted from the burning and molten plastic material. Outside contractors were called in to monitor the atmosphere and construct berms and dikes to stem the flow of hot plastic to waterways and sewers.

This article covers one aspect of this tremendous loss-the source of the fire, its probable cause, and corrective procedures that could be employed to minimize, or even avoid, the conditions that could lead to another such event.


The fire began in the plastics-fabrication and storage area. This plant was extruding foamed polyethylene sheeting to be used as building insulation. Butane, which dissolves in the polystyrene under elevated temperature and pressure, was employed as the foaming agent. This hot, liquid material is forced through a circular die with a small gap and, as the plastic/butane matrix cools, the blowing agent gasifies under the reduced pressure and expands, creating a flexible foamed cylinder. Cold air is blown onto the foamed plastic to toughen the skin, after which the cylinder of plastic is pulled and sliced to form two flat blankets of foamed plastic insulation. As the material cools, some of the butane is lost in the atmosphere while about 70 percent remains in the cellular structure of the insulation. The remaining butane slowly diffuses from the foam, and air replaces the lost gas until equilibrium is reached and a small amount of butane is left.

The finished product off the extrusion line is a flexible sheet of foamed polystyrene, about 50 feet long, 4 feet wide, and 14 inch thick, folded fan-wise across a crimped line every two feet, yielding a packet some seven inches in thickness and weighing 10 to 15 pounds. About 50 of these packets are placed on a pallet, covered on top, and then shrink-wrapped. A lift truck then carries the finished load to the storage area, which is in the same large room as the extrusion line. The completed pallet must remain in the storage area for two to three weeks to allow the blowing agent (butane) to dissipate. It should be noted that the finished insulation has an Underwriters Laboratories Inc. (UL) flame spread rating of less than 40, which means that no significant amount of butane should be allowed to remain in the cellular structure at the time of installation.


The insulation extrusion line had been in production for about one month when the fire broke out. On that cold day in December, a lift truck operator was placing a pallet load in the storage area where two high pallets of previously manufactured material were stored (the propane-fired lift truck was not of explosion-rated construction). He was driving his truck to pick up a fresh load from the wrapping area when a loud boom was heard in the plant. Witnesses initially saw flame two to three feet high above the most recently stored pallets, followed by heavy black smoke. The alarm was sounded, at which time the butane supply was shut down and all personnel were safely evacuated. Although a wet pipe sprinkler system was installed, it did not activate during the fire. Employees testified that they believed the system was not adequately flushed after installation and dirt may have blocked the flow. (The factory was only several weeks old.)

The fabricating area was equipped with roof fans, which blew in fresh air in a downward direction, and exhaust fans on one side wall, which pulled air out of the plant. The fans, which reportedly were left on during the early stages of the fire, were not controlled by heat/smoke sensors.


The manufacture and handling of plastic films create static electricity on the exposed surfaces. This static charge can reach thousands of volts before it discharges, resulting in sparks capable of igniting flammable gases and combustible dusts. The extrusion process, which includes the addition of butane to the melted polyethylene, creates a high probability for local fires in the vicinity of the extrusion die and downstream as the formed sheets are cooled, cut, and folded. Indeed, several firefighting means, including the following, were installed for the line:
• fugitive gas detectors,

• electrical grounding of all associated equipment,
• static electricity neutralizers (ionized air blowers, for example),
• portable fire extinguishers nearby, and
• an automatic fire extinguishing system (hopefully adequate).

However, the fire did not start in the manufacturing area; it began in the post-production storage area, where pallets of finished insulation were placed to be “aged.”

The problems of fire in the storage area are not exactly the same as those on the extrusion line, but there are similar issues, which are addressed below.

• Static charge on the shrink-wrapped pallets remains for some time after being placed in storage. One employee reports “his hair standing on end” as he passed the newly placed loads.

• Butane gas continually exudes from the insulation and is trapped in the shrink-wrapped pallet. Since butane is heavier than air, the gas tends to sink to the floor through the pallet openings.

• The polyethylene insulation (with or without butane) is a significant fire load with a rapid burning rate and produces a great deal of dense, black smoke when burning. An adequate automatic fire extinguishing system must be employed in this area.

• The lift truck employed to move the pallet loads from the shrinkwrap area to the storage area was not of explosion-rated design, nor did it have electrical grounding capability.

• The floor in the storage area was ordinary finished poured concrete, essentially nonconductive. Any moving object (person, lift truck) can carry static charges to the storage area.

• Ventilation between the stacks of the stored insulation was poor, allowing butane to stagnate at floor level, under and between loads.

It is difficult to exactly pinpoint the cause of the initial ignition of the fire; however, it is reasonable to conclude that the lift truck activity in the area just prior to the explosion and fire was casually related. Butane gas on the floor (lower explosive level 1.9 percent by volume) was ignited by a static discharge from the load just placed or a back-fire from the lift truck exhaust or sparks from electrical components. Nevertheless, whatever the ignition source, it is clear that low-lying butane gas was present and created the initial explosion followed by the ignition and rapid burning of the shrink-wrap and the butane-laden insulation in the storage area.

The fire quickly spread through the plastics storage and manufacturing area, and it broke through the walls (nonfire rated) to the adjacent warehouse properties before the local fire departments could reach the site and prevent total destruction to building and property.


Lawsuits were filed, with many cross-claims, among the new owner of the building; the original owner and builder; the warehouse tenants; and, of course, the plastics manufacturer. Experts were retained, reports were filed, and settlements of all claims were reached prior to trial. The plant was not rebuilt. Significantly, the foamed plastics manufacturer corrected many of the shortcomings described above by instituting several changes in storage areas of other plants producing butane-foamed plastics. These changes include better cross-ventilation, monitoring of flammable gases at floor level, using only explosion-rated lift trucks with grounding straps, and installing conductive flooring and selecting appropriate clothing for personnel to minimize static charge buildup. Fire protection from fixed automatic extinguishing systems was to be upgraded where necessary. Fire shutters and shutdown controls, which could be employed in a fire, were added to ventilating systems.

Despite the fact that the technology for all the preventative procedures existed prior to the construction of the plant and the manufacturing process, the plastics manufacturing facility failed to adopt these techniques beforehand. The manufacturer learned a bitter lesson in the loss of its own as well as neighboring property.

MARVIN A. SALZENSTEIN, PE, is chairman and principal consulting engineer at Polytechnic Inc., Lincolnwood, Illinois. Polytechnic, Inc. provides safety, testing, and forensic engineering services. Salzenstein has a B.S. in mechanical engineering from the Illinois Institute of Technology and did postgraduate work in technical optics at the University of London.

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