MXPA97007361A - Rotationalally molded articles, foam - Google Patents

Abstract

Rotationally molded articles, foams, and methods for their manufacture are described. The articles are prepared based on the inclusion of blowing agents in micro-beads. The articles and the method of making them exhibit improved cellular structure and improved uniformity leading to improved and generally uniform properties of the article, including for example insulation values and impact resistance.

Description

ROTATIONALAMOLDED ITEMS. FOAM Technical Field This invention relates generato rotationamolded, improved articles, and methods for producing rotationamolded, foamed articles having an improved cell structure. More specifica this invention relates to polymeric micro-beads that incorporate an insufflation agent into the micro-beads to facilitate the manufacture of rotationamolded articles having a foamed layer. BACKGROUND Rotationamolded articles containing a foamed layer are known. Typica such rotationamolded articles include at least one layer of foam and at least one other thermoplastic layer. Genera rotationamolded parts, made using an outer epidermis of a thermoplastic and at least one foamed layer, preferably inside the first thermoplastic layer, have found commercial acceptance in various articles such as surf boards, boards for surfing with wind, isolated tanks for chemical products and drinking water; toys for children, canoes, boats, boxes for material handling (refrigerated and non-refrigerated) and similar. Such articles generaemploy 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 skilled in the art. Various techniques have been suggested to achieve such rotationamolded, foamed articles. Among these suggested techniques are: use a well stabilized outer layer and a less or minimastabilized inner layer. Such a structure can provide an outer layer stabilized to resist degradation by ultraviolet light, heat degradation, degradation by oxygen, and the like, and an inner layer where the lower or minimastabilized layer can be oxidized during heating (roto-molding) ) to assist the adhesion of a subsequently applied foam, for example polyurethane or polystyrene. A method known in the art to achieve a similar structure is to roto-mold a feed of a fine particle size and a coarse particle size, where the fine particle size generamelts first and then the larger particle size melts secondly or later, again providing internal and external layers of stabilized polyolefins differently. Another approach known in the art to produce roto-molded articles having a foamed layer includes using relatively small polymer particles without containing blowing agent and relatively large polymer particles containing an blowing agent. Another method known in the art is to rotationamold a first polyolefin charge and after it has been melted or softened substantiain the form of a mold, a drop box (an inner box or boxes containing a second material) is used. or sequential material that is inside the mold cavity and substantiaisolated from the heat of the mold cavity) to drop a second load. This second filler, for example, may contain a polyolefin or other thermoplastic with an insufflating agent, optionaa second dropping box may be used to add a third layer. The problems associated with rotationamolded, foamed articles are found in substantiaall of the suggested approaches. Such problems include, but are not limited to, first, the delamination of a polyurethane, polystyrene or thermoplastic foams from the interior of a rotationamolded article (eg, 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, there may be substantial limitations on the recyclability due to the dual polymer nature, especially in cases where one of the polymers is for example thermoformable. Third, the combinations of large particle, small particle, as well as drop box technology, present to those who perform rotational molding complex and sometimes prolonged molding operations. Fourth, it has been known for a long time in the art, and it is a major industrial concern for those skilled in the art that such foams generally exhibit generally uneven or wider margins of the foam cell structure. Such variable cellular structure, in cases where for example 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 can be manifested by poor appearance or poor end-use performance, or both. For example, causing less insulation in some areas over others, areas of poor or different physical properties (such as impact resistance), and the like. Additional problems known in the art are evidenced by a rotationally molded article, foamed thermoplastic / polyurethane or thermoplastic / polystyrene. 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 epidermal layer may therefore render the structure generally non-functional, for example in applications that use buoyance. In the drop box technique, of two or three loads, the problems are based, for example, on the long cycle times required, because the primary, secondary and additional layers must be molded substantially sequentially. Additional equipment is required to make and use drop box technology by those who rotationally mold. The process is generally complex to operate, especially when it is considered that a very low tolerance to leakage of the drop boxes is generally necessary due to the fact that such leaks can probably lead to discontinuities in one or more layers. The drop box process is difficult to optimize because timing is critical when opening the box or drop boxes. Poor uniformity of the cellular structure can also result. In the third technique known in the art, a polymer of small particle size or powder is charged with little or substantially no blowing agent in the mold at the same time as a polymer with larger particle size with a chemical blowing agent. combined in the polymer. Such technology depends on the polymer containing the blowing agent having characteristics that delay its fusion or collocation during the process. Characteristics known to those skilled in the art that would allow this delay in melting or placement are typically particle size and / or density. Again, the difficulties with this old method are a poor cellular structure and / or uniformity and poor surface appearance, characterized by a lack of continuity in the epidermis. Therefore, there is a commercially long-felt need for a rotationally molded, thermoplastic, foamed part, and a method for producing such a part, which have improved recyclability, improved process operability, lower levels of rotational molding operational complexity, closed cellular structure relatively consistent and relatively consistent size, and a minimum of delamination between a layer of foam and other layers. SUMMARY OF THE INVENTION It has been discovered that the combination and / or mixing of a thermoplastic with an insufflating agent and the formation of micro-beads of the combination can provide advantages in the roto-molding process and articles made from the same. The advantages include excellent flow capacity of the micro-beads in a roto-mold, a less complicated roto-molding process, and a molded part having at least one foamed layer having a high percentage of substantially closed, uniform, relatively small cells. This latter advantage leads to a generally lower range of variability of the physical properties of the molded part, such as insulation value and impact resistance. It has been found in this manner that the above-discussed disadvantages associated with prior solutions to the problem of obtaining a rotationally molded article having a foamed layer can generally be solved by the articles and methods of the various embodiments of the present invention. The term "micro-beads", as used in the present description and in the claims, means a bead or particle of a thermoplastic which can have any shape, discs and cylinders being preferred. Such discs or cylinders will have diameters generally in the range of 250 to 750 μm. Preferred forms may include, for example, oblong, spheroidal and the like. It will be understood by those skilled in the art that generally any form will be acceptable, it being understood that the volume of such particle shapes will be substantially similar to the volume of a sphere described by the above diameter ranges. In addition, the shape and size of the bead or particle should generally be such that its flowability in a rotational molding operation is effective to allow flow to substantially all of the intricate locations of a given mold to form a generally broken-molded surface. keep going. The micro-beads will optimally include an insufflation agent, the insufflation agent may be partially decomposed, leading to some cell formation in the micro-beads. The blowing agent will be present in the range of 0.1 to 3 parts per 100 parts of resin (thermoplastic). In variations of one of the embodiments, a thermoplastic can be mixed in the melted state with the chemical blowing agent and then formed in the micro-beads. In another variation, the micro-beads can be used in a process to produce a molded part, preferably a rotationally molded part, where the process includes: a) charging a plurality of micro-beads to a mold; b) rotating the mold on at least one axis; c) heating the micro-beads to an effective temperature 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 16 to 880 kg / m3. The foamed layer will advantageously have a cell size that will be generally small and the foam will have a good cellular uniformity. The cells will have an average size, depending on the density, in the range of 10 to 1,400 μm, and advantageously more than 70% of the cells will have cell sizes in the described range; in addition, the foamed part will be substantially free of large cells, for example more than 50% larger than the average cell size described. For a foam with a density of 30 lb / ft3 (480 kg / m3), the range of cell sizes will be 400 to 800 μm, with 70% of the cells in the foamed part having a size in this range, preferably at less 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 co-monomers, including olefinic co-monomers, as well as polyethylene terephthalate, polybutylene terephthalate, nylon, polyvinyl chloride, and the like. . In yet another variation, the molded part may include at least one substantially unfoamed layer having a density in the range of 900 to 1,400 kg / m 3. These and other aspects and advantages of certain embodiments of the present invention will be understood with reference to the following description and the appended claims. Detailed Description Introduction This invention concerns certain kinds of thermoplastic resins, thermoplastic micro-beads made from these resins, and articles made from these micro-beads, and processes for producing the articles from the thermoplastic micro-beads. These thermoplastic micro-beads have unique properties that make them particularly suitable for use in the production of certain kinds of fabricated thermoplastic articles. Rotationally molded articles made using the combination of thermoplastic and blowing agent of various embodiments of the invention, will have combinations of properties that make them generally superior to previously available articles that used other production techniques of rotationally molded, foamed articles.

Additionally, these thermoplastic micro-beads show a surprising increase in their ability to be rotationally molded and to provide at least one layer of foam in a rotationally molded article. The following is a detailed description of certain thermoplastics, blowing agents, preferred micro-beads, micro-beads made from the thermoplastic / blowing agent combination, and methods for making rotationally molded articles based on these micro-beads. . Certain preferred applications of rotationally molded articles made in accordance with the disclosure incorporated herein are also included. Those skilled in the art will appreciate that numerous modifications can be made to these preferred embodiments, without departing from the scope of the embodiments of the invention. For example, although the properties of the rotationally molded articles, combinations of thermoplastic / chemical blowing agent from which they are made, and processes for using combinations of thermoplastic micro-beads to produce rotationally molded, foamed articles, are exemplified. Technicians in the field 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 headings in the present application is intended to assist the reader, and is not intended in any way to limit the invention. Various terms used in the description and claims have been determined and are defined as follows: Resistance to impact, as measured by the test of the Association of Rotational Molders (ARM) using a 15 Ib (6.8 kg) weight dropped at various heights to give an impact energy in ft-lb or Joules. Test conducted at -40 ° C. Part thickness, known as average part thickness, millimeters. Bending modulus, at 1% secant, in KPSI (Mpa), measured using the ASTM D-790 method. Rotational molding curing time (minutes): using a clam shell rotational molding machine, model FSP M-60, available from FSP Machinery Co. The time required for a rotational molding formulation, typically in granular form, to micro-pearls or powder, is based on a part at a given temperature. Particle size distribution, measured by the amount retained in a sieve, as defined by the ASTM D-1921 method using a Rototap model B apparatus, a 100 g sample, agitation for 10 minutes. Dry particle flow, measured in seconds by pipette flow test, as defined by ASTM method D-1895, method A, in a 100 g sample. Bulk density in g / 100 ce, as defined by the method ASTM D-1895, method A, using a minimum of a 200 g sample. melt index, defined by the method ASTM D-1238 using 2,160 g at 190 ° C, units in g / 10 minutes or dg / minute. Foam density: the densities of molded parts are measured in a hydrometer. This method uses a water displacement technique in which the weight of the sample is measured in air, and then the volume is measured by water displacement. Provision is made for air bubbles that can adhere to the surface of a part such that if any bubbles are observed, they must be removed; if they can not be removed, the sample is discarded. Density, defined by the method ASTM D-1505. Cell size The 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 the shapes may vary substantially, describing the average cell size or diameter. Those skilled in the art will understand that this size is intended to be descriptive of a measurement made in a cross section of a cell or cells and the measurement will be from the widest point of the cells. Uniformity of cells: the uniformity of cells can 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 observed and cell size measurements are made. The uniformity of cells will be described by a percentage of the cells in a given cross-sectional area that is within a certain range of sizes. Heat distortion temperature: ASTM D 648-82. Aspect ratio: ratio of length to diameter of pearl. Thermoplastic material: the thermoplastic component can be a polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyvinylchloride, and the like. Polyethylene and polypropylene are preferred. Polypropylene Where the polyolefin is a polypropylene, various embodiments include, but are not limited to, polypropylene homopolymer and copolymer. The polypropylene copolymers may contain propylene and one or more monomers selected from the group consisting of ethylene, butene-1,4-methyl-1-pentene, hexene-1, octene-1, and combinations thereof. The polypropylene copolymers will generally contain from 0.2 to 10 mol% of co-monomer or monomers, based on the total moles of copolymer. Polypropylenes can be produced either by conventional Ziegler-Natta catalysis or by metallocene catalysis. The density of such propylene polymer will generally vary from 0.89 to 0.910 g / cc. The flow in the melted state of such propylene can vary from 1 to 20 degrees / min. Polyethylene Where the thermoplastic is a polyethylene, the polyethylene can be a homopolymer or polymer of ethylene and one or more co-monomers selected from the group consisting of propylene, butene-1,4-methyl-1-pentene, hexene-1, octene -1, decene-1, and their combinations; Preferred co-monomers include butene-1, hexene-1, and octene-1. The co-monomers may also include ethylenically unsaturated acrylic acid esters, acrylic acids, vinyl acetate and the like. Those skilled in the art will appreciate that such polyethylene copolymers will generally contain from 0.2 to 20 mole percent of co-monomer or co-monomers, based on the total moles of copolymer. Such polyethylene may include one or more polyethylenes of high density, medium density, low density or linear density, generally having densities in the range of 0.915 to 0.970 g / cc, preferably in the range of 0.915 to 0.950 g / cc, more preferably in the range of 0.930 to 0.950 g / cc. Suitable polyethylene homopolymers or copolymers for use in embodiments of the present invention can be made using conventional Ziegler-Natta catalyst systems and processes, so-called Phillips systems and processes, or polymers and metallocene catalyzed processes. The melt indexes of polyethylene for use in rotational molding and generally for use in rotationally moldable, foamable articles, can vary from 1 to 20 ° / min, preferably in the range from 2 to 10 ° / min. The use of polyethylenes having melt indices above 20 ° / min is also contemplated, especially when combined with crosslinking agents to improve the melt strength / cell structure balance in a foamed layer. Physical mixtures of thermoplastic materials such as polypropylene, low density polyethylene; high density polyethylene, linear low density polyethylene, and other combinations of materials known to those skilled in the art to provide durable, functional, useful roto-molded objects. Such combinations, for example, in micro-beads containing one or more thermoplastics and / or various thermoplastics, generally in a physical form such as micro-beads or ground powder, to conveniently roto-mold, can be used in conjunction with the micro-beads. -foamy pearls. Such additions (combinations) can be useful to incorporate different properties in or in addition to a foamable layer. Insufflation Agent The blowing agents are known. The following description includes chemical exothermic blowing agents, which are preferred, but not limited thereto. Such exothermic, chemical blowing agents include but are not limited to modified azodicarbonamides, azodicarbonamide, p-toluene sulfonyl semi-carbazide, p, p'-oxybis (benzene) sulfonyl hydrazide, p-toluene sulfonyl hydrazide. Azodicarbonamide is preferred. The blowing agents can use activators. In this context, activators will be understood by those skilled in the art as materials that can alter, for example, elevate or reduce the temperature of decomposition or gas evolution, the temperature range and / or the decomposition rate of chemical agents. of insufflation. 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 tin dilaurate, calcium stearate, citric acid, and barium stearate / cadmium combinations. If used, the activators can be added at the rate of parts per 100 parts of thermoplastic levels similar to the chemical blowing agent itself. Additionally, decomposition temperatures, temperature ranges, or gas release rates of the chemical blowing agent may be affected due to the presence of other chemicals either in the polyolefin itself and / or additives, such as stabilizers, anti-oxidants, acids, metal catalyst residues, and the like. Exothermic blowing agents can also be used in various embodiments of the invention. The endothermic agents can be based on mixtures of sodium bicarbonate / citric acid. Such endothermic blowing agents can be physically mixed with exothermic blowing agents to provide a mixture of properties, as is known in the art. Depending on the amount of heat generated during the combination, mixed in the melted state and micro-pelletizing (including one or more thermoplastics, other additives, a chemical blowing agent and optionally an activator) and / or the rotational molding of the micro-pearl containing a chemical blowing agent, and the rate of heat generation, those skilled in the art will appreciate that adjustments can be made to the level of blowing and activating chemical agent, to optimize foaming, foam density and cell uniformity. Those skilled in the art will understand 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 impurities contained either in a polymer or in the aforementioned additives, the thermal history of the blowing agent and / or the blowing / thermoplastic agent combination, the rate and level of heating and the temperature ranges used in the rotational molding process, and Similar. The decomposition temperature of the blowing agent must be taken into account during the mixing in the melted state / combination of the chemical blowing agent and thermoplastic, in order to minimize the decomposition in the combination and / or pelletizing step. Certain decomposition of the blowing agent can take place during this step and is desirable, but it is generally preferred that the larger part of the decomposition, which generally leads to gas evolution and formation of foam cells, takes place in the molding process rotational. The control of such determinations will be the final product or desired foamed article. The inclusion levels of chemical blowing agents in a micro-bead can generally be in the range of 0.1 to 3 parts per 100 parts of resin (thermoplastic). Preferably, in the range of 0.2 to 2. More preferably, in the range of 0.5 to 1.5 parts per 100 parts of resin (thermoplastic). If blowing agent activators are included in the formulation, their presence will be at similar levels but not necessarily equal to the levels for the chemical blowing agents. Combination and / or Micro-Pelletization of the Thermoplastic The combination and / or pelletization of the combination or physical mixture of thermoplastic / chemical blowing agent of various embodiments of the invention can be carried out by any mixing / pelletizing scheme. The commonly used extruders are preferred for combining or mixing ingredients and pelletizing the resulting mixture or physical blend of thermoplastics, blowing agent, and a wide variety of possible additive components. In such extrusion operations, generally thermoplastic or thermoplastics and additives, including but not limited to anti-oxidants, anti-static agents, ultra-violet absorbers, ultra-violet blockers, dyes, acid neutralizers, chemical blowing agents, agents of blowing, blowing agent activators, cross-linking agents and the like, are physically mixed with at least the thermoplastic in the melt phase, then extruded. Micro-beads can be produced in a manner similar to standard size beads, in which the polymer (eg, thermoplastic or polyolefin) is melted together with additives, in a mixing device, such as an extruder. The melted polymer is then extruded through die holes at the discharge end of an extruder and each filament is cut, where the filaments emerging from die holes are solidified / cooled, then cut, or the micro-beads can be cut under water. Underwater cutting generally allows a fast revving knife to cut or cut the polymer extrudate as it passes through the holes in the die plate while the water covers and freezes the melted polymer cut, forming a pearl or micro-pearl. Prior to the present invention, the particles generally used in rotational molding were the result of standard size beads that were ground into a powder. By standard size beads, this terminology is intended to mean pearls that are commonly used in the thermoplastic industry for storage and handling. Whether they are cut into filaments or underwater, such pearls will generally have an average size range of often 2,000 to 5,000 μm. These sizes offer several advantages that must satisfy the needs of a thermoplastic manufacturer to have a pearl size that can be transported pneumatically, reduce bridging in containers, and have a bulk density that allows economical boarding of the thermoplastic. Standard pearls, as described, from more than 2,000 to ,000 μm, are generally considered impractical for use in rotational molding, because such a relatively large particle size inhibits the ability of the particle to reach and easily fill the intricate characteristics of a mold. Additionally, sintering and melting become more difficult due to the relatively small surface to volume ratio (especially when compared to ground powders commonly used in roto-molding). Consequently, thermoplastics intended for use in rotational molding are generally available in standard beads. The standard beads are ground, either cryogenically or at room temperature, at a particle size (average) of 200 to 300 μm. It will be understood by those skilled in the art that such grinding processes result in a relatively broad particle size distribution, but a particle size and a size distribution that has proved successful in flowing, sintering and melting relatively well when used in a rotational molding operation. Attempts to combine an insufflation agent into a standard-sized bead, then grind it, can lead to premature foaming and / or some fugitive escape of chemical decomposition (gas) agent leaving less gas / blowing agent available for foaming in the roto-molding process. However, such an approach is not eliminated. The extruders used to produce micro-beads can be of any size; however, the extruder should generally be able to extrude a wide range of polymer viscosities through a die plate having numerous holes, at commercially viable rates. The number of holes will vary from 100 to 5,000 in relation to different capacities based on the melt index, the melt state viscosity, the extruder back pressure, the size of the extruder, and its die plate area. A die hole size generally of the size of the average diameter of the desired bead is used optimally. In the following examples, the 500 μm die hole size is used; however, a micro-bead, or a die hole from which it emanates, may vary from 250 to 1,500 μm. The size of the micro-beads contemplated in certain embodiments of the present invention may be an average size in the range of 250 to 1,500 μm, preferably in the range of 300 to 1,200 μm, more preferably in the range of 350. to 1,000 μm, even more preferably in the range of 400 to 800 μm, and most preferably in the range of 400 to 600 μm. The Rotationally Molded, Foamed Part The micro-beads, due to their improved flow capacity, may need a lower mold rotation speed to take advantage of their improved flow capacity. Parts made from the micro-beads described above (including a chemical blowing agent) exhibit a relatively smooth, smooth outer surface. This outer surface of the foamed parts made from micro-beads (the surface generally defined by the inside of the rotational mold) will generally have some surface roughness, in the absence of any material or technique to make the surface substantially smooth, but such a smoothness of surface is not prohibited. The inner surface of the foamed parts made from micro-beads will generally be smooth. Additionally, it is expected that a part or a cross section of part of a foamed part exhibits a density in the range of 1 to 55 lb / ft3 (16 to 880 kg / m3), preferably in the range of 2 to 35 lb / ft3 (32-560 kg / m3), more preferably in the range of 5 to 3 lb / ft3 (80-480 kg / m3), most preferably in the range of 5 to 25 lb / ft3 (80-400) kg / p?). However, the cross section may not exhibit a uniform density and / or uniformity of cells across the cross section (eg, from the outer to the inner surface); however, such uniformity or general lack of gradient is desirable. There has been some densification of the foam layer on these internal and external surfaces. Those skilled in the art will understand that this densification will depend on factors such as the transfer of heat to and from the mold itself, the amount of blowing agent escaping from the region of any of these surfaces, and similar mechanisms. In addition, methods are contemplated to obtain a smoother or smoother internal and / or external surface or to add an internal or external layer or layers. Such methods include, but are not limited to, spraying, dipping, painting, using a milled powder of smaller particle size in combination with the micro-beads in a rotational molding operation, and combinations thereof. The use of micro-beads containing chemical blowing agents in drop box techniques is also contemplated. The superior flowability of the micro-beads can lead to an improvement in the arrangement, especially in cases where micro-beads are used in a second load or subsequent load used in the roto-molding process. Rotationally molded, foamed parts made in accordance with preferred embodiments of the invention will show a surprisingly small cell size variation, and the cells will be substantially closed cells. An average cell diameter can be, for example, in the range of 50 to 1,300 μm, preferably in the range of 100 to 1,000 μm, more preferably in the range of 150 to 800 μm, with the greatest preference in the range from 400 to 800 μm. The variation in cell size can also be described as generally greater than 70% of the cells having a diameter in the above ranges, preferably 75%, more preferably 80%, even more preferably 85%, with the greatest preference more of 95%. Although the term "diameter" is used, it will be understood by those skilled 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. The cell size can also depend on the density of foam, the foams of lower density generally having larger cells. For example, the average cell size of a foam with a density of 30 lb / ft3 (480 kg / m3) will be 600 μm, with at least 70% of the cells being within ± 30% of the average size. For a foam with a density of 10 lb / ft3 (160 kg / m3), the average cell size will be 900 μm, with at least 70% of the cells being within + 30% of the average. Foamed articles that can be made by the various embodiments of the present invention include, but are not limited to surf boards, wind surfboards, insulated tanks for chemicals, drinking water and similar liquids, children's toys, boats, boxes for material handling (refrigerated and non-refrigerated), equipment for game sites, kayaks, sailboats, canoes, motor boats, boat seats, boat accessories, marine floats, buoys, marine flotation devices, marine platforms , coolers for field days, commercial display coolers, structural containers, recycle boxes, newspaper boxes, fish boxes, packaging, military packaging, and the like. The micro-beads of the various embodiments of the invention can be advantageously used in other processes of low shear stress, such as tube coating and other sintering processes.

Example 1 The resins were selected for micro-bead screening analysis. All polyethylene resins are available from Exxon Chemical Company. Escorene® LL-8460.27, a material with nominal density of 0.938 g / cc, nominal melt index of 3.3 ° / min, Escorene® HD-8660.26, a material with density of 0.942 g / cc, melt index of 2.2 ° / min. These materials were micro-pelleted in a 2.5-inch (6.35 cm) Davis Standard single-screw pelletizing extruder. aj2la_ * Pearl length, 740 microns; pearl diameter, 500 microns; aspect ratio, 1.45. ** Pearl length, 820 microns; pearl diameter, 630 microns; aspect ratio, 1.30. *** Pearl length, 630 microns; pearl diameter, 680 microns; aspect ratio, 0.92. All micro-beads were extruded from die holes nominally 0.020 (500 μm) in diameter.

Variations in the size and shape of the beads will be seen due not only to the speed of the bead cutter, but also to the fact that the speed of the bead cutter may not be constant across the die cross section, ie for all holes. The results are shown in Table 1. Screen analysis and dry flow results for the bead shapes (described above) compared to commercially available ground powders that are commercially acceptable usually for roto-molding operations are shown below, in Table 2. The beads produced had the following properties, as compared to a ground powder (Escorene® LL-8461.27, a polyethylene with nominal average particle size of 300 μm having substantially the same melt index and density as the previously described LL-8460.27): It can be seen that the micro-beads have excellent smooth flow characteristics. Dry flow values of 15 seconds are obtained compared to 26 seconds for ground powders, generally indicating improved flowability and the consequent improved mold filling capabilities.

Table 3 summarizes the physical properties of rotationally molded parts using ground powders and micro-beads. As can be seen, there is usually little difference in the measured physical properties. The processing cycle time for a rotationally shaped object typically would be substantially the same for both micro-beads and ground powders. The processing characteristics were demonstrated in a small-scale roto-molding apparatus. Other tests were run to determine the processing characteristics in a large-scale roto-molding apparatus (model FSP-60). Table 4 summarizes the resting angle measurements for a ground powder and the micro-beads. Micro-beads generally have a lower resting angle than typical ground powders, usually indicating improved flowability. As can be seen from Table 4, the resting angle of the micro-beads is in the range of 15 to 25% less than for ground powder made from the same polymer. Example 2 The polyethylene of Example 1 (Escorene® LL-8460.27, available from Exxon Chemical Company) was combined with 0.5 parts per 100 parts of azodicarbonamide resin (with a particle size of 2 microns (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 speed of the bead cutter was 3.550 rpm, the water of the bead cutter was at 180 ° F (82.2 ° C), and the hole diameter of the die was 0.020 inch (500 μm). The pearls produced had the following properties: Retention 35 mesh 99.4% Retention 50 mesh 0.6% Bulk density 0.42 g / cc Dry flow 16 sec. The micro-beads were placed in a rotational mold in a FSP 60 model shell rotational molding machine using a hexagonal shaped mold and cured to a fixed oven at 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 means of a fan, followed by 11 minutes of water spray on the mold and part, then a period of 3 minutes of drying. The thickness of part manufactured was 1,200 μm. The impact according to the ARM at -40 ° C was 42 ft-lb (57 Joules). Examples 3 and 4 Three formulations (Examples 2, 3 and 4) were roto-molded in the roto-molding apparatus model FSP 60. Example 2 uses micro-beads and the chemical blowing agent. The pearl size is as described above. Example 3 is a dry physical mixture of ground powder LL-8461 (described above) and Celogen® AZ 2990 at a nominal particle size of 2 μm. Example 4 is a physical blend of 20% LL-8461, a commercial milled powder, and 80% of a bead formed by melt blending of HD-6705, a polyethylene of 19 ° / min, 0.952 g / cc ( available from Exxon Chemical Company), and 0.5 parts by weight of Celogen AZ 2990 and pelletizing to a standard 3,000 μm nominal bead. As can be seen from the results of Table 5, the thicknesses of the parts of Examples 2, 3 and 4 are 1.27, 1.27 and 1 cm, respectively. The ARM impact value (42 ft-lb (57 Joules) -40 ° C) of Example 2 exceeded that of PE ground, dry blended and blowing agent (Example 3, 29 ft-lb (39.3 Joules)) in more than 40%, while the ARM impact value of Example 2 exceeds the value 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 agents, other micro-bead sizes, and additional layering 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 Compendium of Physical Properties Micro-Pearls Vs Ground Powder GP = ground powder MP = micro-beads l = Escorene 8360, 5 ° / min, 0.932 g / cc 2 = Escorene 8460, 3 5 ° / min, 0.938 g / cc 3 = Escorene 8555, 6 7 ° / min, 0.936 g / cc 4 = Escorene 8660, 2 2 / min, 0.942 g / cc 5 = Escorene 8760, 5 ° / min, 0.948 g / cc All are available from Exxon Chemical Company.

Angle Measurements of Rest Table 5 Physical Properties

Claims (8)

1. A molded article comprising at least one foamed layer, said foamed layer having a density in the range of 1 to 55 lb / ft3 (16 to 880 kg / m3); wherein said foamed layer includes a thermoplastic selected from the group consisting of polyethylene, polypropylene, polyamide, polyethylene terephthalate, polybutrylene terephthalate, and combinations thereof, preferably of the group consisting of low density polyethylene, linear low density polyethylenes. , medium density polyethylene, high density polyethylene, and combinations thereof, wherein said foamed layer has a cellular structure where at least 70% of said cells have a diameter in the range of 400 to 800 μm, and where said foamed layer includes cells where a majority of said cells are closed cells.

2. The molded article of claim 1, including at least one second layer, said second layer having a density in the range of 850 to 1400 kg / m3; 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 as or different than said thermoplastic in said second layer.

3. A molded article, comprising: a) at least one first layer, said first layer including a foamed thermoplastic, said foamed thermoplastic having a density in the range of 16 to 880 kg / m3, wherein said foamed thermoplastic includes cells wherein a Most of said cells are closed cells; and b) at least a second layer, said second layer including a thermoplastic, said thermoplastic having a density in the range of 850 to 970 kg / m3.

4. In a process for producing a molded part, comprising: a) loading a plurality of micro-beads into a mold; b) rotating said mold on at least one axis; c) heating said micro-beads to an effective temperature to produce a molded object, characterized in that said molded object includes at least one layer of a thermoplastic foam having a majority of closed cells, said thermoplastic foam having a density in the range of 16 at 640 kg / m3; wherein said micro-beads have a volume equal to the volume of a sphere having a diameter in the range of 250 to 750 μm; wherein said thermoplastic foam has cells where at least 70% of the cells have an average size in the range of 400 to 800 μm; and wherein said micro-beads include at least one thermoplastic resin and a chemical blowing agent, preferably wherein: i) said thermoplastic is selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyvinyl, polyamide, and combinations thereof; and ii) wherein said molded part includes a second layer, wherein said second layer has a density in the range of 850 to 1,400 kg / m 3, said second layer including a thermoplastic selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyvinyl chloride, polyamide, and combinations thereof, preferably where said process is a roto-molding process.

The process of claim 4, further comprising: loading a thermoplastic powder into said mold, wherein said powder has an average particle size in the range of 200 to 300 μm.

The process of claim 4, wherein the thermoplastic of said thermoplastic powder and the thermoplastic of said thermoplastic foam can be the same or different, and the thermoplastic of said thermoplastic powder or thermoplastic of said thermoplastic foam is selected from the group that It consists of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyamide, and combinations thereof.

7. A thermoplastic bead, comprising: a) a thermoplastic, preferably where said thermoplastic is selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polybutrylene terephthalate, polyamide, and combinations thereof; and b) an insufflation agent; characterized in that said beads have a particle size equivalent to the volume of a sphere having a diameter in the range of 25 to 1,500 μm, preferably where said particle size is in the range of 300 to 1,200 μm; and wherein said blowing agent is a chemical insufflation agent, said blowing agent being present in said bead in the range of 0.1 to 3 parts per 100 parts of said thermoplastic, preferably where said blowing agent is selected from the group consisting of in azodicarbonamide, modified azodi-carbonamides, p-toluene sulfonyl semi-carbazide, p, p'-oxybis (benzene) sulfonyl hydrazide, p-toluene sulfonyl hydrazide.

8. The use of a plurality of thermoplastic beads according to claim 7 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 16 to 855 kg / m3, wherein said foamed layer has at least 70% cells of said foam with an average size described by a diameter in the range of 50 to 1,300 μm, and where a majority of said cells are closed cells.