This invention relates to a method of heat treating a foam cup, made from a foam resin sheet produced by the blown bubble technique to which a solid resin layer is applied to one or both sides of the foam sheet, to improve the sidewall stiffness.
Foam cups are widely used for individual servings of hot beverages, such as coffee and soup, and cold beverages such as soft drinks, with millions used every day. The cup must have sufficient strength to permit handling by the user, and at the same time, be made from a minimum amount of resin. Polystyrene and other resins can be used to produce the cup although the majority are now made from polystyrene.
U.S. Pat. No. 4,579,275 describes a method of treating a cup having a closed cell foam wall and a solid plastic film coating on at least the outer surface thereof by contacting the inner wall of the cup but not the bottom with a heated mandrel at a time and temperature sufficient to expand the wall.
U.S. Pat. No. 4,547,412 describes a method of improving stiffness of a polystyrene foam container having a sidewall and a bottom by heating the container in an unrestrained state sufficiently to increase the thickness of the sidewall at least 10%.
U.S. Pat. No. 3,344,222 describes treating foam cups produced by extrusion of foamable sheets of polystyrene with no solid layer present by heating the foam cup while holding the cup between restraining surfaces. This treatment is said to provide a density gradient with denser foam near the surface of the cup, thus increasing the strength of the cup. According to the patentees, the thickness of the wall or walls of the container does not apparently change. The patentee also states that "a wall gradient of two or three times the original thickness will severely impair the formation of the dense surface walls and the heat-insulating interior zones".
U.S. Pat. No. 4,359,160 describes a process wherein a sheet of polystyrene foam is produced by the extrusion of molten polystyrene containing a blowing agent and a nucleating agent from a slit-ring orifice. The polystyrene foam cup is thermoformed by a plug-system method which stretches the closed, generally spherical closed cells formed in the extruded sheet, and produces a product containing "pancake" shaped cells. Upon heating this thermoformed cup, as when hot beverage is added thereto, the cells tend to return to spherical shape.
Applicants have found, contrary to indications in the prior art, that stiffness as measured by the sidewall deflection of a polystyrene foam cup having a sidewall and bottom prepared from a laminate of polystyrene foam having generally closed cells and, on at least the outer surface thereof, a layer of solid polystyrene, is improved by placing the polystyrene foam cup around a heated mandrel, the heated mandrel having an outside diameter sufficiently less than the inside diameter of the foam cup so that sufficient space is present between the cup sidewall and the mandrel to allow the cup sidewall to expand toward the mandrel.
From the foregoing, it can be seen that the object of this invention is a method to provide a polystyrene foam cup with improved sidewall deflection strength. Other objects of this invention will become apparent to those skilled in the art upon reading this disclosure.