Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C41/00—Shaping by coating a mould, core or other substrate, i.e. by depositing material and stripping-off the shaped article; Apparatus therefor
  • B29C41/02—Shaping by coating a mould, core or other substrate, i.e. by depositing material and stripping-off the shaped article; Apparatus therefor for making articles of definite length, i.e. discrete articles
  • B29C41/22—Making multilayered or multicoloured articles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
  • B29B9/00—Making granules
  • B29B9/12—Making granules characterised by structure or composition
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C41/00—Shaping by coating a mould, core or other substrate, i.e. by depositing material and stripping-off the shaped article; Apparatus therefor
  • B29C41/003—Shaping by coating a mould, core or other substrate, i.e. by depositing material and stripping-off the shaped article; Apparatus therefor characterised by the choice of material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C41/00—Shaping by coating a mould, core or other substrate, i.e. by depositing material and stripping-off the shaped article; Apparatus therefor
  • B29C41/02—Shaping by coating a mould, core or other substrate, i.e. by depositing material and stripping-off the shaped article; Apparatus therefor for making articles of definite length, i.e. discrete articles
  • B29C41/04—Rotational or centrifugal casting, i.e. coating the inside of a mould by rotating the mould
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
  • B29C44/02—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles for articles of definite length, i.e. discrete articles
  • B29C44/04—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles for articles of definite length, i.e. discrete articles consisting of at least two parts of chemically or physically different materials, e.g. having different densities
  • B29C44/0423—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles for articles of definite length, i.e. discrete articles consisting of at least two parts of chemically or physically different materials, e.g. having different densities by density separation
  • B29C44/043—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles for articles of definite length, i.e. discrete articles consisting of at least two parts of chemically or physically different materials, e.g. having different densities by density separation using a rotating mould
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
  • B29C44/34—Auxiliary operations
  • B29C44/36—Feeding the material to be shaped
  • B29C44/38—Feeding the material to be shaped into a closed space, i.e. to make articles of definite length
  • B29C44/44—Feeding the material to be shaped into a closed space, i.e. to make articles of definite length in solid form
  • B29C44/445—Feeding the material to be shaped into a closed space, i.e. to make articles of definite length in solid form in the form of expandable granules, particles or beads
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
  • B29B9/00—Making granules
  • B29B9/12—Making granules characterised by structure or composition
  • B29B2009/125—Micropellets, microgranules, microparticles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
  • B29K2023/04—Polymers of ethylene
  • B29K2023/06—PE, i.e. polyethylene
  • B29K2023/0608—PE, i.e. polyethylene characterised by its density
  • B29K2023/0641—MDPE, i.e. medium density polyethylene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2067/00—Use of polyesters or derivatives thereof, as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2077/00—Use of PA, i.e. polyamides, e.g. polyesteramides or derivatives thereof, as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
  • B29K2105/04—Condition, form or state of moulded material or of the material to be shaped cellular or porous
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
  • B29K2105/25—Solid
  • B29K2105/251—Particles, powder or granules

Definitions

• 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.
• Rotationally molded articles containing a foamed layer are known.
• Such rotationally molded articles include at least one foam layer and at least one other thermoplastic layer.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m.
• 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).
• a thermoplastic may be melt mixed with the chemical blowing agent and then formed into micro-pellets.
• 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 ⁇ m, 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.
• 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 ⁇ .
• thermoplastic resins thermoplastic micro-pellets made from these resins
• articles fabricated from these micro-pellets and processes for producing the articles from the thermoplastic micro-pellets.
• thermoplastic micro-pellets have unique properties which make them particularly well suited for use in producing certain classes of fabricated thermoplastic articles.
• 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.
• 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.
• 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.
• 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 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.
• the thermoplastic component may be a polyethylene, polypropylene, polyethylene terephhalate, polybutylene terephhalate, polyamide, polyvinylchloride, and the like. Preferred are polyethylene and polypropylene.
• polystyrene 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.
• 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.
• 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..
• 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.
• 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 agents are known.
• the description which follows includes exothermic chemical blowing agents which are preferred, but is not limited thereto.
• 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 .
• 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.
• 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.
• 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.
• 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.
• 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.
• Standard pellets as disclosed above of 2,000 to 5,000 ⁇ m 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).
• 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 ⁇ m(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.
• 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 500 ⁇ m is used however, a micro-pellet or the die hole from which they emanate may range from 250 ⁇ m to 1500 ⁇ m.
• 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 ⁇ m, preferably in the range of from 300 to 1200 ⁇ m, more preferably in the range of from 350 to 1000 ⁇ m, even more preferably in the range of from 400 800 ⁇ m, and most preferably in the range of 400 to 600 ⁇ m.
• 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, 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.
• 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 ).
• 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.
• 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 ⁇ m, preferably in the range of from 100 to 1000 ⁇ m, more preferably in the range of from 150 to 800 ⁇ m, most preferably in the range of from 400 to 800 ⁇ 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%.
• 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.
• the average cell size of a 30 lb/ft 3 (480 kg/m 3 ) density foam will be 600 ⁇ m, with at least 70% of the cells being within ⁇ 30% of the average size.
• the average cell size will be 900 ⁇ m, 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.
• 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
• 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.
• the pellets produced had the following properties compared to a ground powder (Escorene LL-8461.27, a nominal 300 ⁇ m average particle size polyethylene having substantially the same melt index and density as the above described LL- 8460.27): TABLE 2
• 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 1 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 ⁇ m). The pellets produced had the following properties.
• micro-pellets 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 ⁇ m Association of Roto-Molders Impact at -40°C was 42 ft.-lb (57 Joules).
• Example 2 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 ⁇ m 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 ⁇ m pellet.
• 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%.

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• Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)
• Processing And Handling Of Plastics And Other Materials For Molding In General (AREA)

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• 1996
  • 1996-03-15 WO PCT/US1996/003478 patent/WO1996030180A1/en not_active Application Discontinuation
  • 1996-03-15 CA CA002215253A patent/CA2215253A1/en not_active Abandoned
  • 1996-03-15 EP EP96911303A patent/EP0817711A1/de not_active Withdrawn

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