FOAMED ROTATIONALLY MOLDED ARTICLES

Abstract: Foamed rotationally molded articles and methods of manufacture are disclosed. The articles are prepared based upon inclusion of blowing agents in micro-pellets. The articles and the method for making them, display improved cell structure and uniformity leading to improved and generally uniform properties of the article, including for instance, insulation values and impact resistance.

Description: TITLE: FOAMED ROTATIONALLY MOLDED ARTICLES TECHNICAL FIELD This invention relates generally to improved rotationally molded articles and methods for producing foamed rotationally molded articles having improved cell structure. More specifically this invention relates to polymer micro-pellets that incorporate a blowing agent into the micro-pellets to facilitate the manufacture of rotationally molded articles having a foamed layer. BACKGROUND Rotationally molded articles containing a foamed layer are known. Typically such rotationally molded articles include at least one foam layer and at least one other thermoplastic layer. Generally, rotationally molded parts made utilizing an outer skin of a thermoplastic and at least one foamed layer, preferably on the interior of the first thermoplastic layer, have found commercial acceptance in such diverse articles as surf boards, wind surfing boards, insulated tanks for chemicals and potable water; children's toys, canoes, boats, material handling boxes (refrigerated and non refrigerated) and the like. Such articles generally employ at least one foamed layer to improve insulation, improve buoyancy, increase stiffness, or a combination of these properties, or for other purposes known to those of ordinary skill in the art. A number of techniques have been suggested to achieve such foamed rotationally molded articles. Among these suggested techniques are: utilizing a well stabilized outer-layer and a less or minimally stabilized inner-layer. Such a structure may provide a stabilized outer-layer to resist ultraviolet light degradation, heat degradation, oxygen degradation, and the like , and an inner-layer where the lower or minimum stabilized layer may be oxidized during heating (roto-molding) to assist the adhesion of a subsequently applied foam, for instance polyurethane or polystyrene. A method known in the art for achieving a similar structure is roto-molding a feed of a fine particle size and a coarse particle size where the fine particle size generally melts first and then the larger particle size melts second or next, providing again inner and outer layers of differently stabilized polyolefins . Another approach known in the art for producing rotationally molded articles having a foamed layer, includes using relatively small polymer particles containing no blowing agent and relatively large polymer particles containing a blowing agent. Another method known in the art, is to rotationally mold a first polyolefin charge and after it has been substantially melted or softened into the shape of the mold, a drop box (an internal box or boxes containing a second or sequential material which is inside the mold cavity and substantially insulated from the heat of the mold cavity) is used to drop a second charge. This second charge may, for instance, contain a polyolefin or another thermoplastic with a blowing agent, optionally a second drop box may be used for adding a third layer. Problems associated with foamed rotationally molded articles are found in substantially all of these suggested approaches. Such problems include, but are not limited to, first; delamination of a polyurethane, polystyrene, or thermoplastic foams from a rotationally molded article interior (e.g. the inside of the outer-layer of the part) causing either product failure or poor performance. Second; when such foamed structures contain two or more polymers, substantial limitations on recyclability may exist due to the dual polymer nature, especially where one of the polymers is, for instance, a thermoset. Third; the large particle, small particle combinations, as well as the drop box technology, present the rotational molder with complex and sometimes lengthy molding operations. Fourth; long known in the art, and a major industrial concern of those skilled in the art is that foams usually display generally uneven or wider margins of foam cell structure. Such varying cell structure, where for instance, there are areas of fine micro-cells, and areas of medium and/or large cells, and areas of large voids.can lead to poor performance. Such poor performance may be manifested in poor appearance or in poor end use performance, or both. For instance causing lower insulation in some areas than others, areas of poor or differing physical properties (such as impact), and the like. Further problems known in the art are evidenced with a thermoplastic/polyurethane or thermoplastic/polystyrene foamed rotationally molded article. These problems include environmental concerns with blowing agents frequently used in polyurethane foam. Additionally, polyurethane is often not a closed cell foam, once delamination occurs any holes in the skin layer can therefore render the structure generally non-functional, for instance, in applications utilizing buoyancy. In the two or three charge, drop-box technique, problems are based on, for instance, long cycle times required, because the primary, secondary, and further layers must be molded substantially sequentially. Additional hardware is required for the rotational molder to make and use the drop box technology. The process is generally complex to operate, especially when one considers that a very low tolerance to leakage from the drop boxes is generally necessary due to the fact that such leakage may likely lead to discontinuities in one or more layers. The drop box process is difficult to optimize because the timing on opening of the drop box or boxes is critical. Poor uniformity of cell structure may also result. The third technique known in the art, wherein a powder or small particle size polymer with little or substantially no blowing agent, is charged to the mold at the same time that a larger particle size polymer with a chemical blowing agent melt compounded into the polymer. Such technology depends upon the polymer containing the blowing agent having characteristics which delay its melting or "laying down" during the process. Characteristics known to those of ordinary skill in the art which would permit this delay in melting or "laying down" are typically particle size and/or density. Difficulties with this old method again, are poor cell structure and/or uniformity and poor surface appearance characterized by the lack of skin continuity. There is a long felt commercial need, therefore, for a foamed thermoplastic rotationally molded part and a method for producing such a part which will have improved recyclability, improved process operability, lower levels of rotational molding operational complexity, relatively consistent uniform closed cell structure and size, and a minimum of delamination between a foam layer and other layers. SUMMARY What we have discovered is that compounding and/or mixing a thermoplastic with a blowing agent and forming micro-pellets of the combination, can provide advantages in the roto-molding process and articles made therefrom. The advantages include excellent flowability of the micro-pellets in a rotomold, a less complicated roto-molding process, and a molded part having at least one foamed layer that has a high percentage of relatively small, uniform, substantially closed cells. This last advantage leads to a generally lower range of variability of the physical properties of the molded part, such as insulation value, and impact strength. We have thus found that the above discussed disadvantages associated with prior solutions to the problem of obtaining a rotationally molded article having a foamed layer can be generally solved by the articles and methods of various embodiments of our invention. The term micro-pellets as used in the present specification and claims means a pellet or particle of a thermoplastic which may have any shape, discs and cylinders are preferred. Such discs or cylinders will have diameters generally in the range of 250 to 750 μm. The preferred shapes may also include for instance oblong, spheroidal, and the like. It will be understood by those of ordinary skill in the art that generally any shape will be acceptable with the understanding that the volume of such particle shapes be substantially similar to the volume of a sphere described by the above diameter ranges. Further, the shape and size of the pellet or particle should generally be such that its flowability in a rotational molding operation should be effective to permit flow into substantially all intricacies of a given mold to form a generally continuous rotomolded surface. The micro-pellets will optimally include a blowing agent, the blowing agent may be partially decomposed leading to some cell formation in the micro-pellets. The blowing agent will be present in the range of from 0.1 to 3 parts per hundred parts resin (thermoplastic). In variations of one of our embodiments, a thermoplastic may be melt mixed with the chemical blowing agent and then formed into micro-pellets. In another variation the micro-pellets may be utilized in a process for producing a molded part, preferably a rotationally molded part, where the process includes: a) charging a plurality of micro-pellets to a mold; b) rotating the mold on at least one axis; c) heating the micro-pellets to a temperature effective to produce a molded part, the molded part will include at least one layer of thermoplastic foam where the foam will have a density in the range of from 16 to 880 kg/m^. The foamed layer will advantageously have a cell size that will be generally small and the foam will have good cell uniformity. The cells will have an average size, depending upon density, in the range of from 10 to 1400 μm, and advantageously more than 70% of the cells will have cell sizes in the described range, further the foamed part will be substantially free from large cells, for example more than 50% larger than the average described cell size. For a 30 lb/ft • * (480 kg/rn^) density foam the range of cell sizes will be in the range of from 400 to 800 μm, with 70 % of the cells in the foamed part having a size in this range, preferably at least 75%, more preferably at least 80%, even more preferably at least 85 %, most preferably at least 95%. The thermoplastic may be selected from any suitable material, including but not limited to polymers of ethylene, propylene and other comonomers, including olefinic comonomers, as well as polyethylene terephthalate, polybutylene terephthalate, nylon, polyvinyl chloride and the like. In still another variation the molded part can include at least one substantially non-foamed layer having a density in the range of from 900 to 1400 kg/m^. These and other aspects and advantages of certain embodiments of the present invention will become understood with reference to the following description and appended claims. Description Introduction This invention concerns certain classes of thermoplastic resins, thermoplastic micro-pellets made from these resins, and articles fabricated from these micro-pellets and processes for producing the articles from the thermoplastic micro-pellets. These thermoplastic micro-pellets have unique properties which make them particularly well suited for use in producing certain classes of fabricated thermoplastic articles. Rotationally molded articles made using the thermoplastic blowing agent combination of various embodiments of our invention, will have combinations of properties rendering them generally superior to articles previously available that used other techniques of producing foamed rotationally molded articles. Additionally, these thermoplastic micro-pellets show a surprising increase in their ability to be rotationally molded and to provide at least one foam layer in a rotationally molded article. Following is a detailed description of certain preferred thermoplastics, blowing agents, micro-pellets made from the thermoplastic/blowing agent combination, and methods for manufacturing rotationally molded articles based on these micro-pellets. Certain preferred applications of rotationally molded articles made according to the disclosure embodied herein are also included. Those skilled in the art will appreciate that numerous modifications to these preferred embodiments can be made without departing from the scope of embodiments of our invention. For example, while the properties of rotationally molded articles, thermoplastic/chemical blowing agent combinations from which they are made, and processes for using the thermoplastic micro-pellet combinations to produce foamed rotationally molded article are exemplified, those of ordinary skill in the art will appreciate that they have numerous other uses. To the extent that the description is specific, this is solely for the purpose of illustrating preferred embodiments of this invention and should not be taken as limiting this invention to these specific embodiments. The use of subheadings in the present application are intended to aid the reader, and are in no way intended to limit our invention. Various terms used in the specification and claims have been determined and are defined as follows: Impact Strength, measured by Association of Rotational Molders (ARM) test using a 15 lb. (6.8 Kg) weight dropped at various heights to give an impact energy in ft - lb. F or Joules. Test done at -40° C. Part Thickness known as the average part thickness, millimeters. The Flexural Modulus, at 1% secant, in KPSI (MPa) measured using ASTM D-790. Rotational Molding Cure Time (minutes): Using a clam shell rotational molding machine, Model FSP M-60 available from FSP Machinery Co.. The time necessary for a rotational molding formulation, typically in granular, micro-pellet, or powder form, to fuse into a part at a given temperature. Particle Size Distribution, measured by the amount retained on a screen, as defined by ASTM D-1921 using a Rototap Model B, 100 gm sample, 10 minute shake. Dry Flow of particles measured in seconds by a Funnel Flow Test, as defined by ASTM D- 1895, Method A on a 100 gm sample. Bulk Density in g/100 cc as defined by ASTM D-1895, Method A, using a minimum of a 200 gm sample. Melt Index is defined by ASTM D-1238 using 2160 grams at 190° C, units in gm/10 minutes, or decigrams minute, dg/min.). Foam Density: Molded part densities are measured in a densimeter. This method uses a water displacement technique in which the sample weight is measured in air, and then the volume is measured by displacement of water. Provision is made for air bubbles which may adhere to the surface of a part such that if any bubbles are observed they should be removed, if they cannot be removed the sample is discarded. Density is defined by ASTM D-1505. Cell Size: Cell sizes will be described by an average diameter, however it will be understood that such cells may be generally rounded or sphere-like, but shapes may vary substantially, by describing average cell size or diameter. It will be understood by those of ordinary skill in the art that we intend that this size be descriptive of a measurement made on a cross section of a cell or cells and the measurement will be of the widest point of the cells. Cell Uniformity: Cell uniformity may be determined by observing a cross- sectional area of the part containing a foamed layer. Using a magnifying glass or a microscope the area is viewed and cell size measurements made. Cell uniformity will be described by a percentage of the cells in a given cross-sectional area being within a certain size range. Heat Distortion Temperature: ASTM D 648-82 Aspect Ratio: ratio of pellet length to diameter The Thermoplastic Material The thermoplastic component may be a polyethylene, polypropylene, polyethylene terephhalate, polybutylene terephhalate, polyamide, polyvinylchloride, and the like. Preferred are polyethylene and polypropylene. Polypropylene Where the polyolefin is a polypropylene, various embodiments include, but are not limited to, homopolymer polypropylene and copolymer polypropylene. The copolymer polypropylenes may contain propylene and one or more monomer selected from the group consisting of ethylene, butene- 1 , 4-methyl- 1 -pentene, hexene- 1 , octene-1, and combinations thereof. Copolymers of polypropylene will generally contain in the range of from 0.2 to 10 mole percent comonomer or monomers, based on the total moles of copolymer. The polypropylenes may be produced with either conventional Ziegler-Natta catalysis or with metallocene catalysis. The density of such polymer of propylene will generally range from 0.89 to 0.910 g/cc. The melt flow of such propylene may range from 1 to 20 dg min. Polyethylene Where the thermoplastic is a polyethylene, the polyethylene may be a homopolymer or polymers of ethylene and one or more comonomers selected from the group consisting of propylene, butene- 1, 4-methyl- 1 -pentene, hexene- 1, octene-1, decene-1, and combinations thereof, preferred comonomers include butene- 1, hexene- 1, and octene-1. Comonomers may also include ethylinically unsaturated acrylic acid esters, acrylic acids, vinyl acetate and the like. Those of ordinary skill in the art will appreciate, that such copolymers of polyethylene will generally contain in the range of from 0.2 to 20 mole percent comonomer or comonomers, based on the total moles of copolymer. Such polyethylenes may include one or more of high density, medium density, low density or linear density polyethylenes, generally having densities in the range of from 0.915 g/cc to 0.970 g/cc, preferably in the range of from 0.915 to 0.950 g/cc, more preferably in the range of from 0.930 to 0.950 g/cc. Polyethylene homopolymers or copolymers suitable for use in embodiments of the present invention may be made utilizing conventional Ziegler-Natta catalyst systems and processes, so called Phillips catalyst systems and processes, or metallocene catalyzed polymers and processes. Melt indexes of polyethylene for use in rotational molding and generally for use in foamable rotationally molded articles can range from 1 to 20 dg/min., preferably in the range of from 2 to 10 dg/min.. The use of melt indexes higher than 20 dg/min. polyethylenes is also contemplated, especially when combined with cross-linking agents to improve the melt strength/cell structure balance in a foamed layer. Also contemplated are blends of thermoplastic materials such as polypropylene, low density polyethylene; high density polyethylene, linear low density polyethylene and other combinations of materials known to those of ordinary skill in the art to provide useful, functional, durable roto-molded objects. Such combinations can be for instance in the micro-pellets containing one or more thermoplastics and/or various thermoplastics, generally in a physical form such as micro-pellets and or ground powder to be conveniently roto-molded, can be used in conjunction with the foamable micro-pellets. Such additions (combinations) may be useful to incorporate different properties into or in addition to a foamable layer. Blowing Agent Blowing agents are known. The description which follows includes exothermic chemical blowing agents which are preferred, but is not limited thereto. Such exothermic chemical blowing agent's include but are not limited to azodicarbonamide, modified azodicarbonamides, p-toluene sulfonyl semi carbazide, p,p'-oxybis(benzene)- sulfonyl hydrazide, p-toluene sulfonyl hydrazide. Preferred is azodicarbonamide. Chemical blowing agents may employ activators . In this context activators will be understood by those of ordinary skill in the art as materials that can alter, for instance, raise or lower the decomposition or gas evolution temperature, temperature range and/or decomposition rate of the chemical blowing agents. Metal salts are known activators. Other examples of strong activators are; surface treated urea, zinc oxide, zinc stearate, dibasic lead phthalate, triethanolamine, and dibasic lead phosphite. Other activators include dibutyl tindilaurate, calcium stearate, citric acid, and barium/cadmium stearate combinations. Activators may be added, if used, at parts per hundred parts of thermoplastic levels similar to the chemical blowing agent itself. Additionally, chemical blowing agent decomposition temperatures, temperature ranges, or gas evolution rates may be affected due to the presence of other chemicals either in the polyolefin itself and/or additives, such as stabilizers, antioxidants, acids, metal catalyst residues and the like. Endothermic blowing agents may also be used in various embodiments of our invention. Endothermic agents maybe based on sodium bicarbonate/citric acid mixtures. Such endothermic blowing agents can be blended with exothermic blowing agents to provide a mixture of properties as is known in the art. Depending upon the amount of heat generated during compounding/melt mixing and micro-pelletization, (including one or more thermoplastics, other additives, a chemical blowing agent and optionally an activator) and/or rotational molding of the micro-pellet containing a chemical blowing agent, and the rate of heat generation, those of ordinary skill in the art will appreciate that adjustments may be made to the level of chemical blowing agent and activator, to optimize foaming, foam density and cell uniformity. It will be understood by those of ordinary skill in the art that the level of one or more blowing agents and optionally the level of an activator or activators will depend on many factors including but not limited to: level of other additives in the polymer, level of contained impurities in either a polymer or the aforementioned additives, the thermal history of the blowing agent and/or blowing agent thermoplastic combination, the rate and level of heating and temperature ranges used in the rotational molding process, and the like. Blowing agent decomposition temperature should be taken into account during melt mixing/compounding of the chemical blowing agent and thermoplastic, to minimize decomposition in the compounding and/or pelletizing step. Some decomposition of the blowing agent may take place during this step and is desirable, but generally it is preferred that the largest part of the decomposition, leading generally to gas evolution and foam cell formation, take place in the rotational molding process. Controlling such determinations will be the desired end product or foamed article. Levels of inclusion of chemical blowing agents into a micro-pellet may generally be in the range of from 0.1 to 3 parts per hundred parts of resin (thermoplastic). Preferably in the range of from 0.2 to 2 . More preferably in the range of from 0.5 to 1.5 parts per hundred parts of resin (thermoplastic). If blowing agent activators are included in the formulation, their presence will be at levels similar to but not necessarily the same as the levels for the chemical blowing agents. Compounding and/or Micropelletization of the Thermoplastic Compounding and/or pelletization of the thermoplastic chemical blowing agent combination or blend of various embodiments of our invention, can be carried out by any mixing/pelletizing scheme. Preferred are extruders commonly used to compound or mix ingredients and pelletize the resulting mixture or blend of thermoplastics, blowing agent, and a wide variety of possible additive components. In such extrusion operations, generally the thermoplastic or thermoplastics and additives including but not limited to antioxidants, anti-static agents, ultraviolet absorbers, ultra-violet blockers, colorants, acid neutralizers, blowing agents chemical blowing agent blowing agent activators, cross-linking agents and the like are blended with at least the thermoplastic in the melt phase, then extruded. Micro-pellets may be produced in a manner similar to "standard" sized pellets, in that the polymer (e.g. thermoplastic or polyolefin) is melted along with additives, in a mixing device, such as an extruder. The molten polymer is then extruded through die holes in the discharge end of an extruder and either "strand" cut, where strands exiting from die holes are solidified/cooled then "chopped", or the micro-pellets may be "under water cut". The "under water cut" generally allows a rapidly revolving blade to sweep or cut off the polymer extrudate as it comes through the die plate holes while the water covers and "freezes" the molten polymer cut off, forming a pellet or micro-pellet. Previous to our discovery, particles generally used in rotational molding were the result of "standard" sized pellets being ground into powder. By "standard" sized pellets, we intend that this terminology mean pellets that are commonly used in the thermoplastic industry for storage and handling. Whether strand cut or underwater cut, such pellets generally have a range of size averages often from 2,000 to 5,000 μm. These sizes offer several advantages which should fulfill a thermoplastic manufacturer's need to have a pellet size that may be pneumatically conveyed, reduce "bridging" in holding containers, and will have a bulk density that permits economic shipment of the thermoplastic. "Standard" pellets, as disclosed above of 2,000 to 5,000 μm are generally considered impractical for use in rotational molding, because such a relatively large particle size inhibits the particle's ability to easily reach and fill all of an intricate mold's features. Additionally sintering and fusing are made more difficult due to the relatively small surface to volume ratio (especially when compared to ground powders used commonly in roto-molding). Accordingly, thermoplastics intended for use in rotational molding are generally available in "standard" pellets. The "standard" pellets are ground, either cryogenically or at ambient temperature, to a 200 - 300 μm(average) particle size. It will be understood by those of ordinary skill in the art that such grinding processes result in a relatively wide particle size distribution, but a particle size and size distribution that has proven successful in flowing, sintering and fusing relatively well when used in a rotational molding operation. Attempts to compound a blowing agent into a "standard" sized pellet then grind it, might lead to premature foaming and/or some fugitive escape of decomposed chemical blowing agent (gas) leaving less gas/blowing agent available for foaming in the rotomolding process. However, such an approach is not foreclosed. Extruders used to produce micro-pellets can be any size, however the extruder generally should be capable of extruding a wide range of polymer viscosities through a die plate having numerous holes, at commercially viable rates. The number of holes will range from 100 to 5000 relating to different capacities based on the melt index, melt viscosity, extruder back-pressure, size of the extruder, and its die plate area. A die hole size generally the size of the average diameter of the pellet desired is optimally utilized. In the examples which follow the die hole size of 500μm is used however, a micro-pellet or the die hole from which they emanate may range from 250μm to 1500μm. The size of the micro-pellets contemplated in certain embodiments of our invention can have an average size in the range of 250 to 1500 μm, preferably in the range of from 300 to 1200 μm, more preferably in the range of from 350 to 1000 μm, even more preferably in the range of from 400 800 μm, and most preferably in the range of 400 to 600μm. The Foamed Rotationally Molded Part Micro-pellets, due to their improved flowability may necessitate a slower mold rotation speed to take advantage of their improved flowability. Parts made from the micro-pellets described above, (including a chemical blowing agent) display a relatively smooth outer surface. This outer part surface of foamed parts made from micro-pellets (the surface generally defined by the inside of the roto-mold) will generally have some surface roughness, absent any material or technique to render the surface substantially smooth, but such surface smoothness is not precluded. The inner surface of foamed parts made from the micro-pellets will generally be smooth. Additionally it is expected that a part or a part cross section of a foamed part will display a density in the range of from 1 to 55 lb/ft 3 (16 to 880 kg/m 3 ), preferably in the range of from 2 to 35 lb/ft-* (32-560 kg/m 3 ), more preferably in the range of from 5 to 30 lb/ft 3 (80-480 kg/m 3 ), most preferably in the range of from 5 to 25 lb/ft 3 (80-400 kg/m 3 ). However, the cross section may not display a uniform density and/or cell uniformity across the cross section (e.g. from outside to inside surface) however, such uniformity or general lack of gradient is desirable. There will be some densification of the foam layer at these inside and outside surfaces. Those of ordinary skill in the art will understand that this densification will depend on factors such as heat transfer to and from the mold itself, the amount of blowing agent that escapes from the region of either of these surfaces, and similar mechanisms. Further, the methods for obtaining a smooth or smoother inner and/or outer surface or adding an inner or outer layer or layers are contemplated. Such methods include, but are not limited to, spraying, dipping, painting, using a ground powder of smaller particle size in combination with the micro-pellets in a rotational molding operation, and combinations thereof. Use of micro-pellets containing chemical blowing agents in drop box techniques is also contemplated. The superior flowability of micro-pellets may lead to an improvement in lay down, especially where micro- pellets are employed in a second or subsequent charge used in the roto-molding process. Foamed rotationally molded parts made according to preferred embodiments of our invention will show a surprisingly small cell size variation, and the cells will be substantially closed cells. An average cell diameter may be for instance in the range of from 50 to 1300 μm, preferably in the range of from 100 to 1000 μm, more preferably in the range of from 150 to 800 μm, most preferably in the range of from 400 to 800 μ m. The cell size variation may also be described as generally greater that 70% of the cells have a diameter in the above ranges, preferably 75%, more preferably 80%, even more preferably 85%, most preferably greater than 95%. While the term diameter is used, it will be understood by those of ordinary skill in the art that the cells will generally have a rounded, but not necessarily spherical shape. The measurements discussed above can be applied to the widest and or deepest dimension of a foam cell. Cell size may also be dependent upon foam density, lower density foams generally having larger cells. For example, the average cell size of a 30 lb/ft 3 (480 kg/m 3 ) density foam will be 600 μm, with at least 70% of the cells being within ± 30% of the average size. For a 10 lb/ft 3 (160kg/m 3 ) density foam the average cell size will be 900 μm, with at least 70% of the cells being within ± 30% of the average. Foamed articles that may be made by various embodiments of our invention include, but are not limited to surfboards, wind surfboards, insulated tanks for chemicals, potable water and similar liquids, children's toys, boats, material handling boxes (refrigerated and non-refrigerated), playground equipment, kayaks, sailboats, canoes, power boats, boat seats, boat accessories, marine floats, buoys, marine floatation devices, marine decking, picnic coolers, commercial display coolers, structural containers, recycle boxes, newspaper boxes, fish boxes, packaging, military packaging, and the like. The micro-pellets of various embodiments of our invention may be advantageously used in other low shear processes, such as pipe coating and other sintering processes. Examples Example 1 Resins were chosen for micro-pellet screening analysis. All polyethylene resins are available from Exxon Chemical Company. Escorene® LL-8460.27, a 3.3 dg/min. nominal melt index 0.938 g/cc nominal density material, Escorene HD-8660.26, a 2.2 dg/min melt index dg/min., density 0.942 g/cc. These materials were micro-pelletized in a 2.5 inch (6.35 cm) Davis Standard, single screw pelletizing extruder. TABLE 1 Cutter Speed Resin Used LL8460.27 HD8660.26 2600 rpm Longer cylinders* Longer cylinders 3300 rpm Shorter cylinders** Shorter cylinders 4000 rpm Discs*** Discs * Pellet length 740 microns, pellet diameter 500 microns, Aspect Ratio 1.45 ** Pellet length 820 microns, pellet diameter 630 microns, Aspect Ratio 1.30 *** Pellet length 630 microns, pellet diameter 680 microns, Aspect Ratio 0.92 All the micro-pellets were extruded from die holes nominally 0.020" (500μm) in diameter. Variations will be seen in the size and shape of the pellets due not only to the speed of the pellet cutter, but also to the fact that the pellet cutter speed may not be constant across the cross section of the die, i.e. for all holes. Results are shown in Table 1 The sieve analysis and dry flow results for the pellet shapes (described above) in comparison to commercially available ground powders that are generally commercially acceptable for roto-molding operations is shown below in Table 2. The pellets produced had the following properties compared to a ground powder (Escorene LL-8461.27, a nominal 300 μm average particle size polyethylene having substantially the same melt index and density as the above described LL- 8460.27): TABLE 2 The micro-pellets can be seen to have excellent, smooth flow characteristics. Dry flow values of 15 seconds are obtained compared to 26 seconds for ground powders, generally indicating an improved flowability and attendant improved mold filling capabilities. Table 3 summarizes the physical properties of rotational molded parts using ground powders and micro-pellets. As can be seen there is generally little difference in the physical properties measured. Processing cycle time for a typically rotationally molded object would also be substantially the same for both micro-pellets and ground powders. Processing characteristics were demonstrated on a small scale roto-molder. Other tests were run to determine processing characteristics on a large scale roto- molder (Model FSP-60). Table 4 summarizes range of angle of repose measurements for a ground power and the micro-pellets. The micro-pellets generally have a lower angle of repose than the typical ground powders usually indicating improved flowability. As can be seen from Table 4 the angle of repose of micro-pellets is in the range of 15 to 25% lower than ground powder made from the same polymer. Example 2 The polyethylene of Example 1 (Escorene® LL-8460.27 available from Exxon Chemical Company) was compounded with 0.5 parts per hundred parts of resin of azodicarbonamide (with a 2 micron particle size (Celogen® AZ 2990 available from Uniroyal Chemical)). The combination was micro-pelletized in the extruder of Example 1 at a melt temperature of 375°F (190.5°C) and a die plate temperature of 500°F (260°C). The pellet cutter speed was 3550 rpm, pellet cutter water was 180°F (82.2°C), and the die hole diameter was 0.020" (500 μm). The pellets produced had the following properties. 35 mesh retention 99.4% 50 mesh retention 0.6% bulk density 0.42 g/cc dry flow 16 sec The micro-pellets were placed in a rotation mold an FSP model 60 clam shell rotational molding machine using a hexagonal shaped mold and cured at an oven set point of 600°F (315.5°C) for 25 minutes. The molded polymer is allowed to cool for 5 minutes with the top of the oven closed and then 5 minutes with the top of the oven open with ambient air circulated by a fan, followed by 11 minutes of water spray onto the mold and part, then a 3 minute period of drying. The part thickness made was 1200 μm Association of Roto-Molders Impact at -40°C was 42 ft.-lb (57 Joules). Examples 3. and 4 Three formulations (Example 2, 3, and 4) were roto-molded in the model FSP- 60 roto-molder. Example 2 utilizes micro-pellets and the chemical blowing agent. Pellet size is as described above. Example 3 is a dry blend of ground powder LL-8461 (described above) and Celogen® AZ 2990 at a nominal 2 μm particle size. Example 4 is a physical blend of 20% of LL-8461, a commercial ground powder and 80% of a pellet formed by melt mixing HD-6705 a 19 dg/min, 0.952 g/cc polyethylene (available from Exxon Chemical Company) and 0.5 parts by weight Celogen AZ 2990 and pelletizing to a "standard" nominal 3000 μm pellet. As can be seen from the results in Table 5, the part thicknesses of Examples 2, 3 and 4 are 1.27 cm, 1.27 cm and 1 cm, respectively. The Association ofRoto- molders impact ( 42 ft./lb. (57 joules) @ -40° C ) of Example 2 exceed those of dry blended ground PE and blowing agent (Example 3 29 ft/lb (39.3 joules)) by over 40%, while the Association of Roto-molders impact of Example 2 exceeds the Association of Roto-molders impact of Example 4 (12 ft/lb or 16.2 joules) by 350%. Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, other chemical blowing agent's, other micro-pellet sizes, and additional layer formation are contemplated. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein. TABLE 3 SUMMARY OF PHYSICAL PROPERTIES MICROPELLETS VS. GROUND POWDER GP=Ground Powder MP=Micro-pellets l=Escorene 8360, 5 dg/min, 0.932 g cc 2=Escorene 8460, 3.3 dg min, 0.938 g/cc 3=Escorene 8555, 6.7 dg/min, 0.936 g/cc 4=Escorene 8660, 2.2 dg min, 0.942 g/cc 5=Escorene 8760, 5 dg min, 0.948 g/cc All available from Exxon Chemical Co. TABLE 4 ANGLE OF REPOSE MEASUREMENTS TABLE 5 Physical Properties Example Appearance Thickness ARM Impact (cm) (joules for -40° C) 2 - uniform in cell structure 1.27 57 -absence of large voids smooth surface 3 - broad cell size 1.27 39.3 4 - mottled surface 1.0 16.2 (large voids)

Claims: CLAIMS: We Claim: 1. A molded article comprising at least one foamed layer, said foamed layer having a density in the range of from 1 to 55 lb/ft 3 (16 to 880 kg/m 3 ); wherein said foamed layer includes a thermoplastic selected from the group consisting of polyethylene, polypropylene, polyamide, polyethylene terephthalate, polybutylene terephthalate, and combinations thereof, preferably from the group consisting of low density polyethyelene, linear low density polyethylenes, medium density polyethylene, high density polyethylene and combinations thereof and wherein said foamed layer has a cell structure wherein at least 70% of said cells have a diameter in the range of from 400 to 800 μm. 2. The molded article of claim 1 including at least a second layer, said second layer having a density in the range of from 850 to 1400 kg/m 3 ; wherein said second layer includes a thermoplastic selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyvinyl chloride and combinations thereof, wherein said thermoplastic in said foamed layer may be the same or different than said thermoplastic in said second layer. 3. The molded article of claim 1 wherein said foamed layer includes cells wherein a majority of said cells are closed cells. 4. A molded article comprising: a) at least a first layer, said first layer including a foamed thermoplastic, said foamed thermoplastic having a density in the range of from 16 to 880 kg/m 3 ; and b) at least a second layer, said second layer including a thermoplastic, said thermoplastic having a density in the range of from 850 to 970 kg/m 3 . 5. In a process for producing a molded part comprising. a) charging a plurality of micro-pellets into a mold; b) rotating said mold on at least one axis; c) heating said micro-pellets to a temperature effective to produce a molded object characterized in that said molded object includes at least one layer of a thermoplastic foam, said thermoplastic foam having a density in the range of from 16 to 640 kg/m 3 ; wherein said micro-pellets have a volume equal to the volume of a sphere having a diameter in the range of from 250μm to 750μm; wherein said thermoplastic foam have cells wherein at least 70% of cells have an average size in the range of from 400 to 800 μm; and wherein said micro-pellets include at least one thermoplastic resin and a chemical blowing agent, preferably wherein: a) said thermoplastic is selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyvinyl chloride, polyamide and combinations thereof; and b) wherein said molded part includes a second layer, wherein said second layer has a density in the range of from 850 to 1400 kg/m 3 , said second layer including a thermoplastic selected from the group consisting of polyethyene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyvinyl chloride, polyamide, and combinations thereof, preferably wherein said process is a roto-molding process. 6. The process of claim 5 further comprising: charging a thermoplastic powder to said mold, wherein said powder has an average particle size in the range of from 200 μm to 300 μm. 7. The process of claim 5 wherein the thermoplastic of said thermoplastic powder and the thermoplastic of said thermoplastic foam may be the same or different, and thermoplastic of said thermoplastic powder or the thermoplastic of said thermoplastic foam is selected from the group consisting of polyethyene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyamide, and combinations thereof. 8. A thermoplastic pellet comprising: a) a thermoplastic, preferably wherein said thermoplastic is selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthlate, polyamide, and combinations thereof; and b) a blowing agent; characterized in that said pellets have a particle size equivalent to the volume of a sphere having a diameter in the range of from 25 μm to 1500 μm, preferably wherein said particle size is in the range of from 300 μm to 1200 μm; and wherein said blowing agent is a chemical blowing agent said blowing agent being present in said pellet in the range of from 0.1 to 3 parts per hundred parts of said thermoplastic, preferably wherein said blowing agent is selected from the group consisting of: azodicarbonamide, modified azodicarbonamides, p-toluene sulfonyl semi carbazide, p,p'-oxybis(benzene)-sulfonyl hydrazide, p-toluene sulfonyl hydrazide. 9. Use of a plurality of thermoplastic pellets according to claim 8 to form a molded article.preferably a rotationally molded article, wherein said molded article contains at least one foamed layer having a density in the range of from 16 to 855 kg/m 3 , wherein said foamed layer has at least 70% of cells of said foam have an average size described by a diameter in the range of from 50 to 1300 μm.

Inventors: ANDERSON MICHEL, SCHRAM CALVIN K, ANDERSON, MICHEL, SCHRAM, CALVIN, K.
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1996
Main classification: B29B9
IPC codes: B29B9, B29C41, B29C44