1. Field of Invention
This invention relates to vehicle tires and especially to polypropylene foam tire cores and a method whereby such tire cores are molded from polypropylene foam beads under steam pressure and are inserted in a pneumatic tire casing.
2. Prior Art
Conventional pneumatic vehicle tires consist of an outer casing which is given the desired load-bearing capacity and elasticity by means of air pumped into the casing or into an inner tube fitted within the casing. Unfortunately, such pneumatic tires are subject to explosive decompression, when punctured, which may create serious hazards for the occupants of the vehicle or of nearby vehicles, especially if the puncture occurs while the vehicle is travelling at high speed or on a crowded road, such as a freeway. Numerous attempts have been made heretofore to overcome these disadvantages by filling the tire casing with other materials. A search in the United States Patent Office has revealed the following:
______________________________________ PATENT NO. INVENTOR ISSUED ______________________________________ 1,415,140 Beckman 1922 1,470,048 Barker 1923 1,488,998 Marshall 1924 2,166,511 Witzenmann 1939 3,022,810 Lambe 1962 3,866,652 Ahmad 1975 4,033,395 Berg et al 1977 4,058,152 Beck et al 1977 4,094,353 Ford 1978 4,125,660 White et al 1978 4,698,191 Endo et al 1987 4,720,509 Nakamura et al 1988 4,777,000 Hideki et al 1988 Brit. 288,040 Senitha 1928 ______________________________________
The Beckman patent proposed the use of a "liner" between the inner tube and the tier casing consisting of a leather or fabric bag filled with hollow rubber balls, while the patents to Marshall and Witzenmann proposed filling part of the inner tube with sponge rubber and inflating the rest. Barker suggested filling a tire with a rubber foam containing closed sells in a matrix of rubber-like material; with the density of the outer portions being high, while the density of the inner portions was relatively low. Senitha suggested a similar idea. Unfortunately, none of these concepts has been found to be commercially successful. Solid tires and tires filled with polyurethane foam have also been known. Thus, Lambe suggested filling a pneumatic tire partly, or entirely, with intrinsically compressed polyurethane or polyester foam produced directly within the tire at the desired pressure. Ford proposed a puncture-proof tire using a mixture of polyoxypropylene, polyether, polyol and diphenylmethane disocyanate injected into a tire casing to form a solid polyurethane tire filling material. Berq et all suggested an extruded tire with the inner space filled with a foam plastic made by injecting a plastic material, such as polyurethane, into the tire for foaming to fill the space. White et al suggested a zero pressure device composed of either microcellular or homogeneous polyurethane made by reacting an organic polyisocyanate, a polyol, a polyol ester and a polyether polyol to produce a device having an average density of 60-65 pounds per cubic foot, as a wheel assembly, and 30-42 pounds per cubic foot as a tire. Ahmad proposed a resilient tire and wheel assembly in which the cavity of a pneumatic tire is filled with a solid, resilient elastomeric polyurethane mixed with hollow glass or ceramic spheres.
However, polyurethane foam filled tires have been found to have low resiliency and poor hysteresis, which limits their usefulness. Moreover, since the polyurethane material is foamed within the tire under pressure, the manufacture of such tires is very complicated and the distribution of the foam within the tire is usually not uniform. Furthermore, polyurethane foam is susceptible to damage by oil and gasoline and the reactive materials and gas which are formed during foaming are quite toxic. Also, because low density polyurethane foam created under pressure within the tire relies on the tire casing for containment of pressure exerted by gas in the cells, such foam adds little, if any, internal support to the tire.
Beck et al proposed tubeless tires filled with polyolefin foam, in which the foam used was closed-cell 4-5 mm. foamed particles of a partially crystalline olefin polymer. The foam particles were inserted into the tire through a sealable opening in the rim of the wheel after first being "pre-shrunk" in sub-atmospheric pressure. The particles subsequently expanded against one another to fill the tire. When such a tire is punctured, the particles expand further to seal up the puncture.
Although an improvement over polyurethane foam filled tires, tires filled with small particles of polyolefin foam also have many disadvantages. First, the polyolefin foam particle filled tires rely on the tire casing for containment of the foam particles and, therefore, add no internal support to the tire, other than that of the inflated particles pressing against each other and against the tire casing. Furthermore, while a small puncture may be partly sealed by the expanding foam particles, a larger puncture may permit escape of the particles and subsequent deflation of the tire. Moreover, since the separate polyolefin particles offer no cohesive internal structure to the tire casing, they offer no protection against rupture following large punctures. In addition, although deflation is delayed, following small punctures, it eventually does occur due to diffusion of air and loss of pressure within the inflated foam particles. In addition, containment of the particles, when the tire is removed for repair or replacement, is next to impossible due to the electrostatic surface properties of polyolefin foam. Moreover, polyolefin foam particles are not biodegradable and create severe environmental hazards, since the small white particles are often mistaken for food by birds and other animals and have been shown to be extremely lethal to many species. Finally, movement of the particles against one another and against the tire casing, during use, develops internal frictional heat which cannot be avoided. This requires extremely complicated methods of conducting and dissipating heat buildup through the tire casing, which is obviously harmful to the longevity and safety of the tire casing.
These disadvantages do not apply to foam tire cores molded of fused polypropylene foam beads. Polypropylene foam beads are structural units consisting of ovoid particles of microcellular, closed cell polypropylene foam completely surrounded by a skin of polypropylene film. In contrast, the polyolefin particles of Beck et al consist of uniform pieces of polyolefin foam, preferably polyethylene, with no structural surrounding skin. Polypropylene is the lightest of the major plastics, with a specific gravity of 0.90 to 0.91. Moreover, expanded polypropylene foam can be produced with densities ranging from 0.5 pounds per cubic foot (PCF) to 18 PCF. Expanded polypropylene foam articles of 3.75 PCF have a strength to weight ratio twice that of polyurethane and 14 times greater than steel. The high strength of polypropylene, relative to other plastics, is not fully understood. However, the branched molecular arrangement appears to provide stereometric structural cohesion between molecules and there is some evidence of intrahydrogen bonding. Unlike most plastics, polypropylene seems to behave, physically, as if it were a single molecular unit, resulting in great strength and particular resistance to stretching. For this reason, thin polypropylene films have found varying applications ranging from food packaging to high altitude balloons.
It has also bee found that the expanded polypropylene foam products have outstanding shock absorbing properties. In fact, since 1985, expanded polypropylene foam has found commercial application in automobile bumper cores, reusable containers and cushion packaging. A technical field for molding polyolefin foam beads has been established, directed to steam chest molding of polypropylene foam beads, and is well known in the art. For example, Endo et al suggested a method of producing a polypropylene resin molded product from foamed polypropylene beads which comprised of introducing a pressurizing gas into foamed polypropylene foam beads having closed cells until the volume of the beads is reduced 50-99%, charging the beads in a mold cavity and introducing steam into the cavity to cause adhesion of the compressed foam beads.
Polypropylene beads may also be molded without pre-treatment to build up internal pressure within the foam beads before molding. For example, Hideki teaches a method for production of an expansion-molded article of polypropylene resin which comprised filling pre-foamed polypropylene resin beads in a mold and then heating the beads to cause the beads to expand and fuse together to form the expansion molded article conforming to the mold. In 1988, Nakamura proposed a process for preparing a foamed article which comprised charging pre-expanded polypropylene beads, having two melt temperatures, into a mold which is able to be closed, but unable to be sealed, without a procedure for giving an internal pressure to the pre-expanded beads, and heating the pre-expanded beads with steam.
Thus, it has been shown that products formed of expanded polypropylene foam are found to be strong, flexible, resistant to fatigue and chemical shock and are durable. Such qualities make this material ideally suited for forming molded tire cores. Nevertheless, none of the prior art patents have suggested this use and no method for molding such tire cores has been proposed heretofore. Thus, none of the prior art techniques have been entirely satisfactory.