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End-Use Case Study

Food ContainerSatisfying each customer’s specific needs by developing “tailor-made” compounds from a variety of base resins and fillers is a specialty at RTP Company.

One of the more recent formulas developed by our engineers is for a thermoplastic microwave cookware, molded by Hartzell Manufacturers of St. Paul. The specially adapted compound uses mineral filled polymethylpentene (TPX) as its base resin. The unique qualities of this polymer produce top quality microwave cookware.

TPX has excellent resistance to most organic and inorganic chemicals which are present in grease and foods. This makes the cookware stain resistant, even to ketchup, mustard and grease stains which normally saturate other ordinary plastic surfaces. TPX also resists stress cracking which commonly occurs in other thermoplastic resins when they are exposed to some automatic dishwasher detergents.

The material is resistant to very high and very low temperatures. Its 464 degree Fahrenheit melting point allows the food to cook without melting the polymer container and makes the material dishwasher safe. Its low temperature ductibility gives the thermoplastic material its capacity for freezer storage of foods.

TPX has a very low dielectric constant which is extremely important in microwave cookware. Rather than absorbing microwaves which heats up the cookware, the material is microwave transparent, allowing the waves to pass through it and heat only the food.

In the custom formulas developed by RTP engineers for microwave cookware, the mineral filler greatly improves the structural and thermal properties of the base TPX.

By raining the thermal conductivity of the cookware, the compound’s mineral reinforcements and fillers promote uniform heating of foods. If there is a “hot spot” on the plastic plate, the minerals will distribute the heat uniformly over the food contact area. And because the mineral fillers increase the plastic’s heat distortion temperature, the cookware will not become deformed under high microwave temperatures.

The minerals also reduce the plastic’s flexibility, giving the plastic composite the rigidity it requires to withstand heat.

Of primary importance to molder, the mineral fill acts as a resin extended, reducing raw material costs by as much as 25 percent. This, coupled with the fact that the base TPX polymer is extremely lightweight (0.83 specific gravity), results in cost per part savings over many denser plastics.

Another benefit to molders is that the mineral fill improves the dimensional stability of resin, making it warp resistant during the molding process.

Competing with thermoplastic materials in the microwave cookware market are thermosetting plastics (polyesters), glass and ceramics. Thermoplastic is superior to glass and ceramics in that is weighs less and, in comparison to cookware of similar quality, costs less. Its advantage over all three is in processing and cookware fabrication. Thermoplastic composites are easier to injection mold than are thermosetting resins. And the fact that thermoplastic composites can be remelted allows processors to recycle rejects and virtually eliminate waste.

The polymethylpentene composites are manufactured under RTP’s 3000 product series nomenclature.