Abstract:
The parts of a pallet or other article have different tailored thermoplastic composition to provide fire resistance according to the shape, location and function of the part in the pallet. In one embodiment, the high surface area to volume deck has high fire retardant content, whereas the heavier portions of the pallet, such as the base and columns have lower content. In another embodiment, the periphery of the deck that is prone to impact damage has low retardant content, compared to the rest of the deck and or the rest of the pallet.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Patent No. 60/657,678, filed Mar. 1, 2005, the entire contents of which are specifically incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     There are a variety of thermoplastic articles that benefit from fire-resisting properties. One method of imparting fire-resisting properties to a plastic is inclusion within the plastic of a fire-retardant component. However, inclusion of such fire retardants, even in minimally effective amounts, often carries adverse side effects. More specifically, such fire retardants can decrease mechanical properties and moldability, increase weight and cost, and be environmentally undesirable.  
         [0003]     Fire resistant coatings may also be used, however such coatings are subject to wear and degradation. Once the coating layer is penetrated, particularly with objects having large relative mass, there is little effect.  
         [0004]     Plastic pallets are an example of plastic articles, which might incorporate fire-retardant components therein. An important criterion for all pallet uses is that the pallet has adequate fire resisting properties. Indeed, all pallets must pass tests that determine how fast and with what heat evolution pallets burn in a simulated warehouse fire. These same pallets must also pass durability and impact tests.  
         [0005]     Plastic pallets have not gained wide acceptance due in large measure to the failure to satisfactorily meet fire resistance criteria, while additionally meeting the structural and durability standards of Grocery Manufacturers of America (GMA). Pallets should have fire resistance sufficient to not exceed the heat release set by Underwriters Lab Standard 3435. In a test under the standard, pallets are subjected to a fire in a test facility, to simulate a warehouse fire. Any fire resisting pallet also must be strong enough to carry specified loads, must not be too heavy, and must be durable in resisting damage during use, as measured by certain tests and field use.  
         [0006]     Because the heat and rate of combustion of the typical polyethylene or polypropylene pallet material are inherently high compared to wood and many other materials, it is, in one sense, desirable to include fire retardant components within the plastics of the pallet. However, while inclusion of fire retardants in plastic pallets may increase the chances of passing fire resistance tests, such inclusion at the same time generally decreases the chances of passing durability and impact tests.  
         [0007]     One partial solution is to construct the deck of a pallet can from metal, while constructing the rest of the pallet as plastic. That has the effect of reducing the amount of plastic in the pallet and the favorably improving the burn test characteristics, and reducing the gross amount of fire retardant in the pallet. However, it may not always be desirable to have a metal deck on a pallet.  
         [0008]     What is needed is a thermoplastic article that is satisfactorily fire resistant, and that is also satisfactorily impact resistant or durable.  
       SUMMARY  
       [0009]     The above described and other disadvantages of the prior art are overcome or alleviated by the presently disclosed thermoplastic article, comprising plastic parts that having differing fire resistances. In one embodiment, with regard to the rate of heat evolution is concerned compared to the total heat evolution, those portions of a plastic pallet having a higher surface area or those portions of the pallet that are more exposed have higher heat resistances relative to those portions having lower surface areas or those portions that are less exposed.  
         [0010]     In another exemplary embodiment, a pallet&#39;s thermoplastic material has different fire retardant content (composition) is tailored according to the portion of the pallet that the material forms. In one such embodiment, thinner and higher surface area to volume ratio sections have higher fire retardant content than do other sections. In another such embodiment, the edges of the top deck of the pallet, or like areas of some other article that are prone to impact damage, have lower fraction of retardant than those portions not prone to impact damage. In another embodiment, the deck of a pallet has higher fire retardant content than the columns upon which the deck is mounted. In another embodiment, part of, or all of, the periphery of the deck has less retardant or no retardant, compared to the interior of the deck which has fire retardant.  
         [0011]     In another exemplary embodiment, a pallet&#39;s differing components include a fire resistant coating or layer on portions having a higher surface area or on portions that are more exposed. In one such embodiment, the deck is formed of two or more layers of material that are joined together, wherein the deck comprises a top layer having less fire retardant than a bottom layer.  
         [0012]     The above-described and other features will be appreciated and understood by those skilled in the art from the following detailed description, drawings and appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Referring now to the accompanying FIGURES, which are meant to be exemplary and not limiting:  
         [0014]      FIG. 1  is a cross sectional view of an exemplary pallet;  
         [0015]      FIG. 2  is a cross sectional view of an exemplary pallet column;  
         [0016]      FIG. 3  is a cross sectional view of an exemplary pallet column and deck;  
         [0017]      FIG. 4  is a cross sectional view of an exemplary pallet corner;  
         [0018]      FIG. 5  is a cross sectional view of an exemplary pallet deck; and  
         [0019]      FIG. 6  is a cross sectional view of an exemplary pallet component mold.  
     
    
     DETAILED DESCRIPTION  
       [0020]     The present disclosure relates to articles comprising thermoplastics, such as high density polyethylene (HDPE) or polypropylene, among others. Components of such articles, e.g., pallets, may be constructed according to any convenient method, for example by injection molding.  
         [0021]     Solid particulate fire retardants may also be included in the thermoplastic compositions, for example Grafguard graphite intumescent material, aluminum trihydrate, magnesium hydroxide or antimony trioxide (often used with bromine compounds), among others. The solid fire retardants may be used in combination with other types of fire retardants, for instance, brominated hydrocarbons. It will be appreciated that the invention may be applied to the inclusion of other ingredients, solid particulate or not, in a pallet and to other articles.  
         [0022]     The present invention recognizes that addition of fire retardants in quantities sufficient to impart minimal to good fire resistance, which in one embodiment, and depending on the material, ranges from 10 to 30 weight percent retardant, correspondingly decreases the fracture toughness of that article. In the example of plastic pallets, the pallet becomes too prone to breakage, particularly around the edges of the pallet where the shanks of the tines of a forklift truck may impact the pallet. Accordingly, an improved article is described, wherein fire resistance of the article is selectively tailored with regard to the geometry and/or position of a component of the article.  
         [0023]     Referring now to  FIGS. 1-4 , partial vertical cross sections of portions of exemplary thermoplastic pallets are illustrated. Referring to  FIG. 1 , exemplary pallet  20  comprises a deck  22 , which is welded to the columns  26  of base  24 . In this exemplary embodiment, the deck includes a plurality of holes (the deck may take other configurations, e.g., solid or grid-like, among others). The columns  26 , which have hollows at their top ends, are interconnected by rails  30 . Hollow square cross section metal beams  28  are within the rails. In one exemplary embodiment, deck  22  of pallet  20  has a higher concentration of retardant relative to the frame  24 . As an example, HDPE deck has 10% intumescent composition, and the frame has 5%. (All concentrations are by weight unless otherwise indicated.)  
         [0024]     Referring now to  FIG. 2 , a portion of exemplary pallet  22 A is illustrated, wherein beam  28  is covered by a floor plate  29 . In an exemplary embodiment, the plate  29  is configured to incorporate of the beam  28  within the base and comprises no or low fire retardant relative to other portions of the pallet. In such embodiment, the beam  28  is strategically engineered to fail in the event of a fire. This is due to the lack of significant fire retardant in the plate  29 . At an early stage in a fire, the beam  28  will be subjected to heat and will fail according to engineered design.  
         [0025]     Referring now to  FIG. 3 , an exemplary pallet  20 B is illustrated, wherein such pallet lacks base rails. An exemplary deck is a two-layer composite structure, which may be made for example by co-extrusion, by joining one sheet to another, or by injection molding, among other methods. In one embodiment, the underside layer  27  comprises a first composition with a large amount of fire retardant relative to the top layer  25 . Such exemplary configuration provides durability for the top of the deck, but at the same time provides fire protection to the pallet (oftentimes flames will rise up from below, and the lower deck layer provides a lower barrier relative to heat sources from above as well). Other layers may also be interposed between the top and bottom layers, for other properties or fire resistance. The layers may also be contoured to create cavities therebetween, and the cavities may be filled with foam, such as urethane.  
         [0026]     Referring now to  FIG. 4 , opposing edges of an exemplary pallet  20 D is illustrated, wherein deck  22 D has an inner portion comprising a first material having fire retardant (and thus diminished impact or other properties) and an integral edge portion  42  with less fire retardant and better impact properties. In another exemplary embodiment, the edge portion that has minimal or no fire retardant at least spans the openings that are between columns  26 D, through which forks enter the space under the pallet for transport.  
         [0027]     Referring not to  FIG. 5 , a top view of exemplary pallet  20 C is illustrated, wherein deck  22 C has a central area  34  with higher retardant content than a periphery portion  32 . The dashed boundary line  33  is one exemplary indication of where the composition changes. Depending on the manufacturing technique that is used, and the objective, the demarcation of composition change may be definite or gradual. While the periphery of the deck may be thin and thus should have fire retardant in accord with another teaching herein, the volume of plastic, which has the inferior fire retardant, is a small fraction of the total pallet. Thus, while burn test performance might be somewhat reduced the performance can still be acceptable, and the “give up” is well traded against durability and strength, in considering the total pallet design.  
         [0028]     Referring now to  FIG. 6 , a simplified cross section view of an exemplary mold for a pallet other dual property object is illustrated. The exemplary cavity parts  26 CC,  32 CC and  34 CC are illustrated as corresponding to numeral parts of exemplary pallet  20 C. The mold comprises two mating parts  36 ,  38 . When installed in a molding machine, injection mold nozzles feed molten plastic through ports  54 ,  56 . Two different material compositions, one with high retardant content, the other with low or no retardant content are provided by two different sets of nozzles, fed by appropriate injection extruders and supplies. The low content material is injected in the ports  54  while the high content material is injected in the ports  56 . In another exemplary manufacturing alternative, with reference to  FIG. 5 , the deck parts  34 ,  32  may be separately fabricated and then joined together, as by welding.  
         [0029]     While exemplary embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. It is to be understood that the present invention has been described by way of illustration and not limitation.