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This application claims benefit of U.S. Provisional patent application Ser. No. 60/657,488, filed Mar. 1, 2005 and is a continuation-in part of U.S. patent application Ser. No. 10/729,615, filed Dec. 5, 2003, which is a divisional of U.S. patent application Ser. No. 09/938,954, filed Aug. 24, 2001, now U.S. Pat. No. 6,705,237, which claimed priority to U.S. Provisional patent application Ser. No. 60/227,537, filed Aug. 24, 2000, the entire contents of each of which are specifically incorporated herein by reference.
This disclosure relates to a device for the transportation of packaged goods, and, more particularly, to a pallet that meets certain standards set by the Grocery Manufacturers Association (GMA) and others for weight, durability, and strength.
One criterion which is used to evaluate plastic pallets is to compare their performance to wood pallets, to find if they perform equal or better than wood pallets under fire conditions. In particular, Underwriters Laboratories (Northbrook, Ill., U.S.) has developed a Standard, UL 2335, “Classification Flammability of Plastic Pallets”. The Standard is consistent with U.S. National Fire Protection Association (NFPA) Standards 231 and 231 C, which relate to warehouses and rack (pallet) storage of materials in warehouses. Under the UL Standard, measurements are made of the rate of spread of a fire within a stack of pallets, heat release, and structural stability (resistance to collapse).
Most common thermoplastic materials, which may be used in substitution of wood, have undesirable fire-related characteristics compared to wood. On a typical equivalent article basis, the heat release rate of the thermoplastic material is substantially greater than wood, and the loss of strength and resultant structural failure more rapid. Furthermore a plastic article can tend to melt and form into a burning pool of liquid. Thus, it is found that the existing conventional warehouse fire suppression systems (which are generally designed for wooden pallets) can be inadequate and unsafe with regard to plastic pallets.
What is needed in the art is a plastic pallet construction having good fire resistance (which may approximate the fire resistance of wooden pallets) that is also not too heavy, too costly, and that does not lack the combination of mechanical properties such as flexural strength and impact strength which a successful pallet must have.
The above described and other disadvantages of the prior art are overcome and alleviated by the present thermoplastic pallet comprising a plastic structural member including a foam thereon. The thermoset foam may be provided on the exterior, on the interior, or on both (e.g., when the structural member is hollow) of plastic structural members.
In one exemplary embodiment, structural thermoplastic members (e.g., polypropylene (PP) or high density polyethylene (HDPE)), such as columns, have hollows which are filled with thermoset foam. In another exemplary embodiment, the foam is a thermoset plastic, such as polyurethane, which provides impact strength to the hollow structural section. Other foams may also be used. In another exemplary embodiment, the foam further includes a fire retardant.
In another exemplary embodiment, thermoset foam is attached to the exterior of a structural member. Both the interior foam filling and exterior foam embodiments may be used together.
The foregoing and other objects, features and advantages of the present pallet will become more apparent from the following description of exemplary embodiments and accompanying drawings.
Referring now to the accompanying FIGURES, which are meant to be exemplary and not limiting:
FIG. 1 is a partial cross sectional view of an exemplary pallet column incorporating a foam therein;
FIG. 2 is a partial cross sectional view of exemplary foam filled pallet deck and columns; and
FIG. 3 is a cross sectional view of an exemplary pallet I-beam including a foam.
As is noted above, the presently described thermoplastic pallet includes a foam material, which material by composition and placement, impart fire-resistant to the thermoplastic pallet. By “fire-resistant”, as described herein, it is meant that the pallet has a desirable combination of properties, including ignition, burning rate, heat release rate, and retention of structural strength, as such are observed when the pallet is subjected to standard and non-standard fire test conditions. For example, the presently described thermoplastic pallet will generate energy at a lower rate than such pallet would otherwise generate.
A unique plastic pallet design comprising metal reinforcing bars and other special features, to provide efficient design, and to give strength to a pallet under normal and elevated temperature conditions is described in copending U.S. patent application Ser. No. 10/729,615. The pallet comprises several structural polymer or metal elements that are assembled and then joined together. In an exemplary embodiment, the pallet structure is made of polypropylene (PP) or high density polyethylene (HDPE). The pallet may have metal pieces as strengthening beams and a corrugated aluminum deck. The assembly nature of pallet design makes it convenient to have portions with differing material properties.
However, it is noted that in tests, the polyolefin thermoplastics, and in particular common HDPE and PP have poor fire resisting properties. By their nature they lose rigidity and structural strength, and they melt and can form puddles of burning plastic, when subjected to fire. The metal deck helps in providing strength during burning, and the metal deck, compared to a plastic deck, reduces the rate of heat release.
When there are structural sections of plastic, they may be in cross sections which approximate the cross section shape of I, H, C, O, etc., as appropriate, rather than being made solid, in accord with good design to lighten weight and keep down the cost of polymer and the pallet. Thus, a monolithic polymer beam may be made in the above-described cross section shape. Lightening holes or analogous filigree structure may be used in selected areas, such as in the web of an I beam.
The pallet component shapes may be complex, but approximations of those various features are used. However, insofar as fire resistance is concerned, these approaches also have the undesirable effect of providing a greater surface area of the plastic member when it partially burns through, and that enhances the propensity for burning in an oxidizing environment. The present application provides a good solution to those undesirable effects by selective application of foam to these structural components.
Referring now to FIG. 1, a vertical cross section through a plane, just off the center of an exemplary pallet is illustrated. As illustrated, the exemplary pallet 20 has a base 36 comprising of cross rails having internal metal beams 26A, 26B, and columns 34 running up from the base (exemplary column 34 is illustrated as positioned at an outer edge 24 of pallet 20), which support a rectangular frame 38, having internal metal beam 26T. A corrugated metal deck 22 is mounted in the frame 38. It should be noted that exemplary components are shown in simplified and conceptual manner. The beams 26A, 26B, 26T are contained within solid plastic sections. In an exemplary embodiment, the structural components also include hollows. More particularly, the outer portion of the frame 38 and the columns (of which in one exemplary embodiment, there are eight around the periphery and one in the center) may be hollow. The hollows 30 of the columns are illustrated as partially or fully filled with foam, which in one embodiment may be a semi-rigid closed cell polyurethane foam. Depending on the nature of the hollows in the frame (with regard, e.g., to design for impact resistance, etc.), and manufacturing limitations, hollows 32 in the frame may or may not be filled with foam.
Referring now to FIG. 2, another simplified and exemplary configuration of pallet 20A is illustrated, wherein the deck 22A and columns (or feet) 34 are comprise hollow structural plastic. A foam is provided within the hollow portions of the deck 22A and columns 34. It should be noted that filling the hollow portions of the pallet with semi-rigid foam may provide increase in bending or column strength (particularly with regard to the present embodiment). Also, a significant increase in impact strength may be achieved.
Any of the foam described herein may optionally include fire retardants (it being understood that the presently described structural components including foam are beneficial without flame retardants). That result can be attributable to the fact that when the foam is confined within the structural plastic which defines the portion of the pallet, the portion acts largely as it would if it were solid. The foam tends not to melt when burning to the extent that the structural plastic would. And of course, being largely void, it has less mass. Furthermore it is insulative. Second, if there is bum-through of the structural exterior, the fire spreads less slowly in the interior because the foam inhibits oxygen and heat from reaching the other interior surfaces of the structure. Thus, the rate of burning and heat evolution of a hollow structural element is thus inhibited by putting foam inside of a hollow article. The amount of foam can also be varied along the length of a member, so that a controlled collapse of the pallet can be achieved, where it may be desirable to collapse part or all of a burning pallet to reduce exposed surface area and consequently reduce the amount of pallet burn.
In one exemplary embodiment of the invention, a hollow structure of HDPE or PE (for example the column 34 of FIG. 1), having a wall thickness of 0.050 to 0.200 inch, is filled Class I polyurethane foam having a density of 2 to 4 lbs per cubic foot (for example, such as is supplied by BASF, Livonia, Mich.). The foam may completely or partially fill the hollow portions and/or may cover the exterior.
If a fire retarding additive is used in the foam, an exemplary material is Grafguard intumescent graphite material. The fire retarding additive may be mixed into the the polymer material, e.g., polyurethane, which will comprise the foam before it is “shot” or injected into the hollow spaces. In one embodiment, the intumescent additive content is 5-20 weight percent. Analogously with amount of foam, the amount of fire retardant can be controlled according to the burning behavior which is sought. Other substances may also be incorporated into the foam, as desired.
The foam which is used will provide a thermoplastic section of the pallet with improved burn test heat release characteristic; and preferably improved impact strength. When a structural element is exposed to fire generally, or ignited at one end, the rate of burning is decreased compared to when foam is not used. Where there is foam, it is first of all insulative. Second, its character upon burning, that it remains substantially in place inhibits oxygen and heat from reaching the surface of the HDPE structural element. Thus, when there is a hollow element filled with foam, even though part of the skin burns away, say at the first end which is ignited, the flame will only progress according to the oxygen which reaches the element on its unprotected exterior, since the interior foam greatly inhibits such on the interior. Thus, the element, and the article as a whole, is found to burn more slowly, which means its rate of heat release is desirably less, in fulfillment of meeting the aims of the UL Standard. Likewise, a structure which heats and burns more slowly will retain its structural strength for a longer time. Other foam materials, and other porous materials, known in the art which behave as described may be alternatively used in substitution of a thermoset foam or in combination with it. For example, ceramic or glass or expanded mineral foams may be used as fillers within a thermoset or other foam, or by themselves.
In another exemplary embodiment, a molded column 34 of a pallet, having exemplary dimensions of 8×5×6 inches, is filled with foam. The foam may be placed within the hollow sections by injection after the pallet is formed, including by use of the techniques where nubs or feet seal the holes of injection at the bottom of a pallet column.
Referring now to FIG. 3, another exemplary embodiment illustrates a thermoplastic structural element comprising an I-beam section 30 that is partially or fully covered with foam 42. In one exemplary embodiment, the foam is a low density polyurethane foam (or other composition foam exhibiting comparable properties) adhered to the surface of the structural element. The foam on the exterior may be somewhat less effective than filled interior embodiments, and the foam may be susceptible to mechanical damage, but a significant benefit in burning characteristic is still realized. As noted above, foam may also be placed on the interior and exterior of a hollow member.
It should be apparent that to the extent inclusion of a fire retardant chemical is economic, does not significantly raise weight, and is otherwise acceptable, it may be included in the foam and will likely enhance performance of the foam.
Although this invention has been shown and described with respect to exemplary embodiments, it will be understood by those skilled in this art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.