The quest for a material that is stronger, lighter and cheaper to make and maintain is the "Holy Grail" for the manufacturer of any product. Fire apparatus manufacturing is no different in that respect, especially when it comes to one of the components: the water tank.
Finding a material for the construction of the water tank on a pumper or tender that meets the stronger, lighter criteria provides the manufacturer several options they can then offer to their customer. Some of those options may include providing more water-carrying capacity using the same chassis, lowering the center of gravity on a tender to reduce the risk of rollover, or refurbishing a piece of apparatus to reduce its operating and maintenance costs.
Construction materials used in fire apparatus water tanks have constantly evolved to meet the wide variety of apparatus needs. Tanks constructed of steel beget tanks using aluminum, which were then followed by those using fiberglass. Each generation of material was touted as being stronger, lighter, and cheaper than its predecessors.
Today's "new kid on the block" is polymer materials, also incorrectly referred to as plastic.
If a polymer is not a plastic, what is it? In short, polymers are large molecules that are formed by chaining smaller monomers.
One of the most common polymers being used to construct fire apparatus water tanks is Polyprene, a material developed by Aristech Company in Pittsburgh. Polyprene is the outcome of chaining the ethylene and propylene monomers. Polypropylene is another commonly used polymer.
The resulting large molecules have incredible tensile strength that is comparable to that found in steel at a fraction of the weight. Tensile strength is not the only positive characteristic that this sheet stock material, and others made from comparable polymers, brings to the apparatus water tank construction process. The manufacturers of polymer water tanks for fire apparatus are able to offer their customers:
- A lower cost tank construction option. The cost of polymer tank is typically less than that of one made of fiberglass, though slightly higher than steel. The cost-benefit reality is that polymer tanks offer the buyer an option to obtain a tank with a price and weight that's comparable to aluminum, but with an impact strength on par with stainless steel.
- A lower operating cost over the life of the tank. Since polymer is a non-porous, Food and Drug Administration approved food grade material, and is not subject to microbial attack; the polymer tank lasts longer and is practically maintenance-free. Microbial induced corrosion (MIC) is capable of far more damage to a tank in far less time then typical electrolytic corrosion.
- A longer tank life than that of tanks constructed using metals or fiberglass. Even when electrodes are built into a metal-constructed tank, corrosion can occur over time as can MIC. As the glass molecules in a fiberglass-constructed tank crack and deteriorate over time, pits open up in the material and make it vulnerable to microbial attack. The microbes will use the resin in a fiberglass tank as a food source.
Apparatus water tanks constructed using polymers offer the buyer a tank that is lighter, cheaper and lasts longer than a comparable tank constructed of fiberglass. While it is true that the sub-frame to support a polymer tank is more expensive than that for a fiberglass tank, the increased cost for the sub-frame can be recouped over the life of the tank because the polymer tank will last longer.
A polymer tank is also resistant to MIC because the material is resistant to the pitting seen in metal-constructed tanks, and the fiberglass deterioration seen in fiberglass-constructed tanks, both of which can make the respective tank vulnerable to MIC
While metal-constructed tanks can be made with greater tensile strength than a polymer-constructed tank at a cheaper cost, the overall return on investment is greater for the polymer tank because it is virtually free of maintenance costs.
While the water tank on a piece of fire apparatus is only one component, albeit a significant one in the total system that is today's fire apparatus, polymer tanks are the latest development in the quest for fire apparatus that is more functional, more affordable and longer lasting. Polymer is today's response for manufacturers and buyers alike in their quest for lighter, stronger, cheaper fire apparatus construction materials.