Patent Application
20110091688
The invention relates to a method of forming, preferably cold forming, a shaped foam article wherein the improvement is using a near net-shaped foam blank to produce the shaped foam article.
Various methods and techniques are currently known and employed in the industry for shaping articles from a thermoplastic foam material, such as extruded polystyrene (XPS) foams. For example, shapes such as toys and puzzles can be die-cut from foams that are formed by extruding a thermoplastic resin containing a blowing agent. There are also examples of foam sheet being shaped into articles such as dishes, cups, egg cartons, trays, and various types of food containers, such as fast food clam shells, take out/take home containers, and the like. More complex shaped foam articles can be made by thermoforming thermoplastic foam sheet. These methods lend themselves to the manufacture of relatively simple shaped articles from typically thin foams which are easily extracted from the molds used to produce them.
Recently, there have been significant advances in shaping more complex, and in particular, thicker thermoplastic foam (i.e., foams greater than 1 mm thick), shaped articles by pressing, or sometimes referred to as cold forming, unique foam compositions and/or structures, for example see USP Publication 2009-0062410. However, depending on the thickness and/or complexity of such foam shaped articles limitations may exist, for example in physical properties, such as cracking which may result in reduced part integrity; aesthetic properties such as the presence of read-through (visual appearance of features on the side opposite of forming); and/or the ability to meet critical and/or consistent dimensional tolerances, for example resulting from part warpage or part size which are too large/small. Furthermore, it is always desirable to improve environmental and manufacturing efficiencies by optimizing material consumption, scrap generation, and/or process economics. Thus, it would be desirable to have an improved method for producing a shaped foam article with one or more of improved physical properties, aesthetic properties, dimensional tolerances/reproducibility, lower environmental impact, and manufacturing efficiencies.
The present invention is such an improved process for forming a shaped foam article having one or more of reduced warpage, fewer cracks, less read-through while optimizing material utilization and lowering overall article material costs.
In one embodiment, the present invention is a method to manufacture one or more shaped foam article comprising the steps of: (i) extruding a thermoplastic polymer with a blowing agent to form a thermoplastic polymer foam plank, the plank having a thickness, a top surface, and a bottom surface in which said surfaces lie in the plane defined by the direction of extrusion and the width of the plank, wherein the foam plank has a vertical compressive balance equal to or greater than 0.4, (ii) cutting the foam plank to form a near net-shape foam blank with one or more pressing surface, (iii) shaping the one or more pressing surface of the foam blank into one or more shaped foam article and surrounding continuous unshaped foam blank by: (ii)(a) contacting the one or more pressing surface of the foam blank with a mold, said mold comprises one or a plurality of cavities each cavity having a perimeter defining the shape of the shaped foam article and a cavity surface and (ii)(b) pressing the foam blank with the mold at an applied strain whereby forming one or more shaped foam article.
In a preferred embodiment of the abovementioned method the foam has a cell gas pressure equal to or less than 1 atmosphere.
In another preferred embodiment of the abovementioned method the thermoplastic polymer is polyethylene, polypropylene, copolymer of polyethylene and polypropylene; polystyrene, high impact polystyrene; styrene and acrylonitrile copolymer, acrylonitrile, butadiene, and styrene terpolymer, polycarbonate; polyethylene terephthalate, polyvinyl chloride; polyphenylene oxide and polystyrene blend.
In another preferred embodiment of the abovementioned method the blowing agent is a chemical blowing agent, an inorganic gas, an organic blowing agent, carbon dioxide, water, or combinations thereof.
In another preferred embodiment of the abovementioned method the near net-shaped foam blank has a volume equal to or less than 1.7 times the volume of the formed shaped article.
In yet another preferred embodiment of the abovementioned method the maximum applied strain is equal to or less than 80 percent.
Another preferred embodiment of the present invention is a shaped foam article produced by the abovementioned method.
The present invention is an improved method for forming, preferably cold forming, a shaped foam article wherein the improvement comprises the use of a near net-shaped foam blank to produce the shaped foam article. The improvement of the present invention provides one or more of (1) improved raw material utilization, (2) improved manufacturing efficiency, (3) lower part cost, (4) lower part density/weight, (5) improved physical properties in the shaped foam article, (6) improved aesthetic properties in the shaped foam article, and (7) improved and/or more reproducible tolerance in the shaped foam article.
The foamed article of the present invention can be made from any foam composition. A foam composition comprises a continuous matrix material with cells defined therein. Cellular (foam) has the meaning commonly understood in the art in which a polymer has a substantially lowered apparent density comprised of cells that are closed or open. Closed cell means that the gas within that cell is isolated from another cell by the polymer walls forming the cell. Open cell means that the gas in that cell is not so restricted and is able to flow without passing through any polymer cell walls to the atmosphere. The foam article of the present invention can be open or closed celled. A closed cell foam has less than 30 percent, preferably 20 percent or less, more preferably 10 percent or less and still more preferably 5 percent or less and most preferably one percent or less open cell content. Conversely, an open cell foam has 30 percent or more, preferably 50 percent or more, still more preferably 70 percent or more, yet more preferably 90 percent or more open cell content. An open cell foam can have 95 percent or more open cell content. Unless otherwise noted, open cell content is determined according to American Society for Testing and Materials (ASTM) method D6226-05.
Desirably the foam article comprises polymeric foam, which is a foam composition with a polymeric continuous matrix material (polymer matrix material). Any polymeric foam is suitable including extruded polymeric foam, expanded polymeric foam and molded polymeric foam. The polymeric foam can comprise, and desirably comprises as a continuous phase, a thermoplastic or a thermoset polymer matrix material. Desirably, the polymer matrix material has a thermoplastic polymer continuous phase.
A polymeric foam article for use in the present invention can comprise or consist of one or more thermoset polymer, thermoplastic polymer, or combinations or blends thereof. Suitable thermoset polymers include thermoset epoxy foams, phenolic foams, urea-formaldehyde foams, polyurethane foams, polyisocyanurate foams, and the like.
Suitable thermoplastic polymers include any one or any combination of more than one thermoplastic polymer. Olefinic polymers, alkenyl-aromatic homopolymers and copolymers comprising both olefinic and alkenyl aromatic components are suitable. Examples of suitable olefinic polymers include homopolymers and copolymers of ethylene and propylene (e.g., polyethylene, polypropylene, and copolymers of polyethylene and polypropylene). Alkenyl-aromatic polymers such as polystyrene and polyphenylene oxide/polystyrene blends are particularly suitable polymers for of the foam article of the present invention. Other thermoplastic polymers useful for the foam used in the present invention can comprise high impact polystyrene; styrene and acrylonitrile copolymer; acrylonitrile, butadiene, and styrene terpolymer; polycarbonate; polyethylene terephthalate; polyvinyl chloride; and blends thereof.
Desirably, the foam article comprises a polymeric foam having a polymer matrix comprising or consisting of one or more than one alkenyl-aromatic polymer. An alkenyl-aromatic polymer is a polymer containing alkenyl aromatic monomers polymerized into the polymer structure. Alkenyl-aromatic polymer can be homopolymers, copolymers or blends of homopolymers and copolymers. Alkenyl-aromatic copolymers can be random copolymers, alternating copolymers, block copolymers, rubber modified, or any combination thereof and my be linear, branched or a mixture thereof.
Styrenic polymers are particularly desirably alkenyl-aromatic polymers. Styrenic polymers have styrene and/or substituted styrene monomer (e.g., alpha methyl styrene) polymerized in the polymer backbone and include both styrene homopolymer, copolymer and blends thereof. Polystyrene and high impact modified polystyrene are two preferred styrenic polymers.
Examples of styrenic copolymers suitable for the present invention include copolymers of styrene with one or more of the following: acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, itaconic acid, acrylonitrile, maleic anhydride, methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, methyl methacrylate, vinyl acetate and butadiene.
Polystyrene (PS) is a preferred styrenic polymer for use in the foam articles of the present invention because of its good balance between cost and property performance.
Styrene-acrylonitrile copolymer (SAN) is a particularly desirable alkenyl-aromatic polymer for use in the foam articles of the present invention because of its ease of manufacture and monomer availability. SAN copolymer can be a block copolymer or a random copolymer, and can be linear or branched. SAN provides a higher water solubility than polystyrene homopolymer, thereby facilitating use of an aqueous blowing agent. SAN also has higher heat distortion temperature than polystyrene homopolymer, which provides a foam having a higher use temperature than a polystyrene homopolymer foam. Desirable embodiments of the present process employ polymer compositions that comprise, even consist of SAN. The one or more alkenyl-aromatic polymer, even the polymer composition itself may comprise or consist of a polymer blend of SAN with another polymer such as polystyrene homopolymer.
Whether the polymer composition contains only SAN, or SAN with other polymers, the acrylonitrile (AN) component of the SAN is desirably present at a concentration of 1 weight percent or more, preferably 5 weight percent or more, more preferably 10 weight percent or more based on the weight of all polymers in the polymer composition. The AN component of the SAN is desirably present at a concentration of 50 weight percent or less, typically 30 weight percent or less based on the weight of all polymers in the polymer composition. When AN is present at a concentration of less than 1 weight percent, the water solubility improvement is minimal over polystyrene unless another hydrophilic component is present. When AN is present at a concentration greater than 50 weight percent, the polymer composition tends to suffer from thermal instability while in a melt phase in an extruder.
The styrenic polymer may be of any useful weight average molecular weight (MW). Illustratively, the molecular weight of a styrenic polymer or styrenic copolymer may be from 10,000 to 1,000,000. The molecular weight of a styrenic polymer is desirably less than about 200,000, which surprisingly aids in forming a shaped foam part retaining excellent surface finish and dimensional control. In ascending further preference, the molecular weight of a styrenic polymer or styrenic copolymer is less than about 190,000, 180,000, 175,000, 170,000, 165,000, 160,000, 155,000, 150,000, 145,000, 140,000, 135,000, 130,000, 125,000, 120,000, 115,000, 110,000, 105,000, 100,000, 95,000, and 90,000. For clarity, molecular weight herein is reported as weight average molecular weight unless explicitly stated otherwise. The molecular weight may be determined by any suitable method such as those known in the art.
Rubber modified homopolymers and copolymers of styrenic polymers are preferred styrenic polymers for use in the foam articles of the present invention, particularly when improved impact is desired. Such polymers include the rubber modified homopolymers and copolymers of styrene or alpha-methylstyrene with a copolymerizable comonomer. Preferred comonomers include acrylonitrile which may be employed alone or in combination with other comonomers particularly methylmethacrylate, methacrylonitrile, fumaronitrile and/or an N-arylmaleimide such as N-phenylmaleimide. Highly preferred copolymers contain from about 70 to about 80 percent styrene monomer and 30 to 20 percent acrylonitrile monomer.
Suitable rubbers include the well known homopolymers and copolymers of conjugated dienes, particularly butadiene, as well as other rubbery polymers such as olefin polymers, particularly copolymers of ethylene, propylene and optionally a nonconjugated diene, or acrylate rubbers, particularly homopolymers and copolymers of alkyl acrylates having from 4 to 6 carbons in the alkyl group. In addition, mixtures of the foregoing rubbery polymers may be employed if desired. Preferred rubbers are homopolymers of butadiene and copolymers thereof in an amount equal to or greater than about 5 weight percent, preferably equal to or greater than about 7 weight percent, more preferably equal to or greater than about 10 weight percent and even more preferably equal to or greater than 12 weight percent based on the total weight or the rubber modified styrenic polymer. Preferred rubbers present in an amount equal to or less than about 30 weight percent, preferably equal to or less than about 25 weight percent, more preferably equal to or less than about 20 weight percent and even more preferably equal to or less than 15 weight percent based on the total weight or the rubber modified styrenic polymer. Such rubber copolymers may be random or block copolymers and in addition may be hydrogenated to remove residual unsaturation.
The rubber modified homopolymers or copolymers are preferably prepared by a graft generating process such as by a bulk or solution polymerization or an emulsion polymerization of the copolymer in the presence of the rubbery polymer. Depending on the desired properties of the foam article, the rubbers' particle size may be large (for example greater than 2 micron) or small (for example less than 2 micron) and may be a monomodal average size or multimodal, i.e., mixtures of different size rubber particle sizes, for instance a mixture of large and small rubber particles. In the rubber grafting process various amounts of an ungrafted matrix of the homopolymer or copolymer are also formed. In the solution or bulk polymerization of a rubber modified (co)polymer of a vinyl aromatic monomer, a matrix (co)polymer is formed. The matrix further contains rubber particles having (co)polymer grafted thereto and occluded therein.
High impact poly styrene (HIPS) is a particularly desirable rubber-modified alkenyl-aromatic homopolymer for use in the foam articles of the present invention because of its good blend of cost and performance properties, requiring improved impact strength.
Butadiene, acrylonitrile, and styrene (ABS) terpolymer is a particularly desirable rubber-modified alkenyl-aromatic copolymer for use in the foam articles of the present invention because of its good blend of cost and performance properties, requiring improved impact strength and improved thermal properties.
Foam articles for use in the present invention may be prepared by any conceivable method. Suitable methods for preparing polymeric foam articles include batch processes (such as expanded bead foam steam chest molding processes), semi-batch processes (such as accumulative extrusion processes) and continuous processes such as extrusion foam processes. Desirably, the process is a semi-batch or continuous extrusion process. Most preferably the process comprises an extrusion process, preferably by means of a single or twin screw extruder.
An expanded bead foam process is a batch process that requires the preparation of a foamable polymer composition by incorporating a blowing agent into granules of polymer composition (for example, imbibing granules of a thermoplastic polymer composition with a blowing agent under pressure). Each bead becomes a foamable polymer composition. Often, though not necessarily, the foamable beads undergo at least two expansion steps. An initial expansion occurs by heating the granules above their softening temperature and allowing the blowing agent to expand the beads. A second expansion is often done with multiple beads in a mold and then exposing the beads to steam to further expand them and fuse them together. A bonding agent is commonly coated on the beads before the second expansion to facilitate bonding of the beads together. The resulting expanded bead foam has a characteristic continuous network of polymer skins throughout the foam. The polymer skin network corresponds to the surface of each individual bead and encompasses groups of cells throughout the foam. The network is of higher density than the portion of foam containing groups of cells that the network encompasses.
Complex articles or blocks may be produced by steam chest molding. Blocks may be further shaped by cutting, for example by CNC hot wire, to a sheet of uniform thickness. A structural insulated panel (SIP) is an example of a steam chest molded block foam cut to a uniform thickness sheet and adhered to oriented strandboard OSB) or any other suitable facing.
The foamed article can also be made in a reactive foaming process, in which precursor materials react in the presence of a blowing agent to form a cellular polymer. Polymers of this type are most commonly polyurethane and polyepoxides, especially structural polyurethane foams as described, for example, in U.S. Pat. Nos. 5,234,965 and 6,423,755, both hereby incorporated by reference. Typically, anisotropic characteristics are imparted to such foams by constraining the expanding reaction mixture in at least one direction while allowing it to expand freely or nearly freely in at least one orthogonal direction.
An extrusion process prepares a foamable polymer composition of a thermoplastic polymer with a blowing agent in an extruder by heating a thermoplastic polymer composition to soften it, mixing a blowing agent composition together with the softened thermoplastic polymer composition at a mixing temperature and mixing pressure that precludes expansion of the blowing agent to any meaningful extent (preferably, that precludes any blowing agent expansion) and then extruding (expelling) the foamable polymer composition through a die into an environment having a temperature and pressure below the mixing temperature and pressure. Upon expelling the foamable polymer composition into the lower pressure the blowing agent expands the thermoplastic polymer into a thermoplastic polymer foam. Desirably, the foamable polymer composition is cooled after mixing and prior to expelling it through the die. In a continuous process, the foamable polymer composition is expelled at an essentially constant rate into the lower pressure to enable essentially continuous foaming. An extruded foam can be a continuous, seamless structure, such as a sheet or profile, as opposed to a bead foam structure or other composition comprising multiple individual foams that are assembled together in order to maximize structural integrity, thermal insulation and water absorption mitigation capability. An extruded foam sheet may have post-extrusion modifications performed to it as desired, for example edge treatments (e.g., tongue and groove), thickness tolerance control (e.g., via planning or skiving the surface), treatments to the top and/or bottom of the sheet, such as cutting grooves into the surface, laminating a monolithic or composite film and/or fabric, and the like.
Accumulative extrusion is a semi-continuous extrusion process that comprises: 1) mixing a thermoplastic material and a blowing agent composition to form a foamable polymer composition; 2) extruding the foamable polymer composition into a holding zone maintained at a temperature and pressure which does not allow the foamable polymer composition to foam; the holding zone having a die defining an orifice opening into a zone of lower pressure at which the foamable polymer composition foams and an openable gate closing the die orifice; 3) periodically opening the gate while substantially concurrently applying mechanical pressure by means of a movable ram on the foamable polymer composition to eject it from the holding zone through the die orifice into the zone of lower pressure, and 4) allowing the ejected foamable polymer composition to expand to form the foam. U.S. Pat. No. 4,323,528, hereby incorporated by reference, discloses such a process in a context of making polyolefin foams, yet which is readily adaptable to aromatic polymer foams. U.S. Pat. No. 3,268,636 discloses the process when it takes place in an injection molding machine and a thermoplastic with blowing agent is injected into a mold and allowed to foam, this process is sometimes called structural foam molding. Accumulative extrusion and extrusion processes produce foams that are free of such a polymer skin network.
Suitable blowing agents include one or any combination of more than one of the following: inorganic gases such as carbon dioxide, argon, nitrogen, and air; organic blowing agents such as water, aliphatic and cyclic hydrocarbons having from one to nine carbons including methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, cyclobutane, and cyclopentane; fully and partially halogenated alkanes and alkenes having from one to five carbons, preferably that are chlorine-free (e.g., difluoromethane (HFC-32), perfluoromethane, ethyl fluoride (HFC-161), 1,1,-difluoroethane (HFC-152a), 1,1,1-trifluoroethane (HFC-143a), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1,2 tetrafluoroethane (HFC-134a), pentafluoroethane (HFC-125), perfluoroethane, 2,2-difluoropropane (HFC-272fb), 1,1,1-trifluoropropane (HFC-263fb), 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), 1,1,1,3,3-pentafluoropropane (HFC-245fa), and 1,1,1,3,3-pentafluorobutane (HFC-365mfc)); fully and partially halogenated polymers and copolymers, desirably fluorinated polymers and copolymers, even more preferably chlorine-free fluorintated polymers and copolymers; aliphatic alcohols having from one to five carbons such as methanol, ethanol, n-propanol, and isopropanol; carbonyl containing compounds such as acetone, 2-butanone, and acetaldehyde; ether containing compounds such as dimethyl ether, diethyl ether, methyl ethyl ether; carboxylate compounds such as methyl formate, methyl acetate, ethyl acetate; carboxylic acid and chemical blowing agents such as azodicarbonamide, azodiisobutyronitrile, benzenesulfo-hydrazide, 4,4-oxybenzene sulfonyl semi-carbazide, p-toluene sulfonyl semi-carbazide, barium azodicarboxylate, N,N′-dimethyl-N,N′-dinitrosoterephthalamide, trihydrazino triazine and sodium bicarbonate.
Recent literature reveals that fluorinated olefins (fluoroalkenes) may be an attractive replacement for HFCs in many applications, including blowing agents, because they have a zero Ozone Depletion Potential (ODP), a lower Global Warming Potential (GWP) than HFCs, and high insulating capability (low thermal conductivity). See, for example United States patent application (USPA) 2004/0119047, 2004/0256594, 2007/0010592 and PCT publication WO 2005/108523. These references teach that fluoroalkenes can be suitable for blowing agents and are attractive because they have a GWP below 1000, preferably not greater than 75. USPA 2006/0142173 discloses fluoroalkenes that have a GWP of 150 or less and indicates a preference for a GWP of 50 or less. Particularly desirable fluorinated olefins include those described in WO 2008/118627.
The amount of blowing agent can be determined by one of ordinary skill in the art without undue experimentation for a given thermoplastic to be foamed based on the type of thermoplastic polymer, the type of blowing agent, the shape/configuration of the foam article, and the desired foam density. Generally, the foam article may have a density of from about 16 kilograms per cubic meter (kg/m3) to about 200 kg/m3 or more. The foam density, typically, is selected depending on the particular application. Preferably the foam density is equal to or less than about 160 kg/m3, more preferably equal to or less than about 120 kg/m3, and most preferably equal to or less than about 100 kg/m3.
The cells of the foam article may have an average size (largest dimension) of from about 0.05 to about 5.0 millimeter (mm), especially from about 0.1 to about 3.0 mm, as measured by ASTM D-3576-98. Foam articles having larger average cell sizes, of especially about 1.0 to about 3.0 mm or about 1.0 to about 2.0 mm in the largest dimension, are of particular use when the foam fails to have a compressive ratio of at least 0.4 as described in the following few paragraphs.
In one embodiment of the present invention, to facilitate the shape retention and appearance in the shaped foam article after pressing the shaped foam plank, particularly foams comprising closed cells, it is desirable that the average cell gas pressure is equal to or less than 1.4 atmospheres. In one embodiment, it is desirable that the cell gas pressure is equal to or less than atmospheric pressure to minimize the potential for spring back of the foam after pressing causing less than desirable shape retention. Preferably, the average pressure of the closed cells (i.e., average closed cell gas pressure) is equal to or less than 1 atmosphere (101.3 kilo Pascal (kPa) or 14.7 pounds per square inch (psi)), preferably equal to or less than 0.95 atmosphere, more preferably equal to or less than 0.90 atmosphere, even more preferably equal to or less than 0.85 atmosphere, and most preferably equal to or less than 0.80 atmosphere.
Cell gas pressures may be determined from standard cell pressure versus aging curves. Alternatively, cell gas pressure can be determined according to ASTM D7132-05 if the initial time the foam is made is known. If the initial time the foam is made is unknown, then the following alternative empirical method can used: The average internal gas pressure of the closed cells from three samples is determined on cubes of foam measuring approximately 50 mm. One cube is placed in a furnace set to 85° C. under vacuum of at least 1 Torr or less, a second cube is placed in a furnace set to 85° C. at 0.5 atm, and the third cube is placed in the furnace at 85° C. at atmospheric pressure. After 12 hours, each sample is allowed to cool to room temperature in the furnace without changing the pressure in the furnace. After the cube is cool, it is removed from the furnace and the maximum dimensional change in each orthogonal direction is determined. The maximum linear dimensional change is then determined from the measurements and plotted against the pressure and curve fit with a straight line using linear regression analysis with average internal cell pressure being the pressure where the fitted line has zero dimensional change.
The compressive strength of the foam article is established when the compressive strength of the foam is evaluated in three orthogonal directions, E, V and H, where E is the direction of extrusion, V is the direction of vertical expansion after it exits the extrusion die and H is the direction of horizontal expansion of the foam after it exits the extrusion die. These measured compressive strengths, CE, CV and CH, respectively, are related to the sum of these compressive strengths, CT, such that at least one of CE/CT, CV/CT and CH/CT, has a value of at least 0.40, preferably a value of at least 0.45 and most preferably a value of at least 0.50. When using such a foam, the pressing direction is desirably parallel to the maximum value in the foam.
The polymer used to make the foam article of the present invention may contain additives, typically dispersed within the continuous matrix material. Common additives include any one or combination of more than one of the following: infrared attenuating agents (for example, carbon black, graphite, metal flake, titanium dioxide); clays such as natural absorbent clays (for example, kaolinite and montmorillonite) and synthetic clays; nucleating agents (for example, talc and magnesium silicate); fillers such as glass or polymeric fibers or glass or polymeric beads; flame retardants (for example, brominated flame retardants such as brominated polymers, hexabromocyclododecane, phosphorous flame retardants such as triphenylphosphate, and flame retardant packages that may including synergists such as, or example, dicumyl and polycumyl); lubricants (for example, calcium stearate and barium stearate); acid scavengers (for example, magnesium oxide and tetrasodium pyrophosphate); UV light stabilizers; thermal stabilizers; and colorants such as dyes and/or pigments.
A most preferred foam article is a shaped foam article which may be prepared from a foamed polymer as described herein above in the form of a near net-shape foam blank 26 cut from a foam plank 20 and further shaped to give a shaped foam article (
When a foam plank 20 is cut 25, the resulting cut foam structure is referred to as a ‘foam blank’. The foam blank is removed from and/or separated from the foam plank prior to shaping. One or more cuts may be necessary to prepare the shape of the near net-shaped foam blank. At least one of the cut surfaces of the foam blank becomes the first pressing surface 30. This terminology applies whether the foam plank is cut in half (providing two foam blanks 26 and 27, each with a first pressing surface) or only a few millimeters is cut or removed from the surface of the foam plank (providing a single foam blank with a single pressing surface). Multiple (e.g., 2, 3, 4, 5, or more) foam blanks may be cut from a foam plank. The conventional foam blank is rectangular and results from a cut through, and parallel to, the top and bottom surfaces of the foam plank. When a foam plank is cut to provide at least two near net-shaped foam blanks 26 and 27, the two near net-shaped foam blanks may be identically shaped with the same dimensions, i.e, 26=27 where 15=29 and 18=28 or they may be similarly shaped with different dimensions, i.e., 26≠27 where 15≠29 and/or 18≠28.
The improvement in the process of the present invention is the use of a ‘near net-shaped foam blank’. The term ‘near net-shape foam blank’ is used to describe a foam plank 20 wherein a first cut 25 provides shape to the blank as well as a pressing surface. In other words, the cut provides a two dimensional shape to the foam blank which approximates (is ‘near’ to) the shape or contour of the final ('net-') shaped foam article 26. The cut surface becomes the first pressing surface 30, if the opposite surface of the blank is also cut or removed the resulting surface becomes the second pressing surface. In comparison to conventional blank preparation, rectangular foam blanks required a cut to prepare a pressing surface so the cut in the near net-shaped foam blank of the present invention does not necessitate an additional step.
The volume of the near net-shaped foam blank of the present invention is dependent on the final shape of the shaped foam article. However, generally the near net-shaped foam blank has a volume equal to or less than 2 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.9 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.8 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.7 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.5 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.4 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.3 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.2 times the volume of the resulting foam shaped article made therefrom, more preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.1 times the volume of the resulting foam shaped article made therefrom, and most preferably the near net-shaped foam blank of the present invention has a volume equal to or less than 1.05 times the volume of the resulting foam shaped article made therefrom.
In a common embodiment of the present invention, the near net-shaped foam blank is cut from a foam plank wherein the cut is not parallel to the top or bottom surface of the foam plank. For a cut defined as a non-parallel plane through the foam plank, two near net-shaped foam blanks having a tapered shape are produced. Depending on how the cut is applied (specifically the angle and depth where the cut starts and stops through the plank), the resulting two tapered near net-shaped foam blanks may have the same dimensions or different dimensions. A tapered near net-shaped foam blank used in the process of the present invention improves raw material utilization and reduces raw material costs as compared to a conventional rectangular foam blank. For example, if a depth, db is required in a foam blank to produce a foam article two conventional rectangular foam blanks would require a foam plank having a depth of df equal to or greater than 2db, in other words, at least twice as much material. However, since near net-shaped foam blanks can nest, or be complementary in shape, two near net-shape foam blanks can be cut from a foam plank of depth less than 2db. Further, a foam article shaped from a conventional rectangular foam blank will have a density (weight) greater than that of a shaped foam article made from a near net-shaped foam blank.
In another embodiment, the near net-shaped foam blank for such an article cut from the foam plank is a sinusoidal shaped blank (for example used to produce a shaped foam article in the shape of Spanish roofing tiles). Like the example of the tapered near net-shaped foam blank above, a sinusoidal cut may provide two identical near net-shaped foam blanks from a single foam plank. For this kind of shape, the two cut near net-shaped foam blanks effectively ‘nest’ with each other and can result in improved raw material utilization as much as 100 percent while cutting the raw material costs by as much a half.
The following shapes are representative, but this list is neither limiting nor inclusive, as to the shapes a near net-shaped foam blank may comprise: tapered, sinusoidal, triangular, stepped, zig-zag, concave, convex, and the like. The shape of the near net-shaped foam is determined by the shape of the shaped foam article and is not limited to the shapes listed hereinabove.
As defined herein, shaped means the foamed article typically has one or more contour that creates a step change (impression) in height 32 of at least 1 millimeter or more in the shaped foam article 10 having a maximum thickness 17 as shown in
The forming of the shaped foam articles is surprisingly enhanced by using a near net-shaped foam blank cut 26 from a foam plank 2 that has at least one direction where at least one of CE/CT, CV/CT and CH/CT is at least 0.4 said one of CE/CT, CV/CT and CH/CT (compressive balance), CE, CV and CH being the compressive strength of the cellular polymer in each of three orthogonal directions E, V and H where one of these directions is the direction of maximum compressive strength in the foam and CT equals the sum of CE, CV and CH.
After a foam plank is formed, shape and a first pressing surface 30 of a near net-shaped foam blank 26 is created by cutting the foam plank between the top and bottom surface of the plank. A second pressing surface can be created on the resulting near net-shaped foam blank by removing a layer from the surface opposite the cut surface (originally the plank top or bottom surface). Alternatively, a second pressing surface may be created by taking the abovementioned near net-shaped foam blank and cutting it a second time thus providing a first near net-shaped foam blank with two pressing surfaces and a second foam blank with one pressing surface that may or may not be shaped. Suitable equipment useful for cutting the foam plank and/or blank and preparing a pressing surface are band saws, computer numeric controlled (CNC) abrasive wire cutting machines, CNC hot wire cutting equipment, foam “skiving” equipment to split the foam via use of a wedge block that effectively splits the foam with a stationary wedge and moving plank, and the like. When removing a layer, the same cutting methods just described may be used and other methods such as planing, grinding or sanding may be used.
Typically, after removing a layer from the top and/or bottom surface of the foam plank and/or cutting the plank, the resulting blank with pressing surface is at least about several millimeters thick to at most about 60 centimeters thick. Generally, when removing a layer from the top or bottom surface of the plank, the amount of material is at least about a millimeter and may be any amount useful to perform the method such as 1.2, 1.4, 1.6, 1.8, 2, 2.5, 3, 3.5, 4, 5 millimeters or any subsequent amount determined to be useful such as an amount to remove any skin that is formed as a result of extruding the thermoplastic foam, but is typically no more than 10 millimeters. In another embodiment, the foam plank is cut and a layer is removed from the top or bottom surface opposite the cut surface to form a near net-shaped foam blank with two pressing surfaces.
In a particular embodiment, the near net-shaped foam blank 26 having a pressing surface 30, has a density gradient from the pressing surface 30 to the opposite surface of the near net-shaped foam blank 22. Generally, it is desirable to have a density gradient of at least 5 percent, 10 percent, 15 percent, 25 percent, 30 percent or even 35 percent from the pressing surface to the opposing surface of the foam plank. To illustrate the density gradient, if the density of the foam at the surface (i.e., within a millimeter or two of the surface) is 3.0 pounds per cubic foot (pcf), the density would be for a 10 percent gradient either 2.7 or 3.3 pcf at the center of the foam. Preferably, the local density at the pressing surface is lower than the local density at the opposite surface (non-pressing surface) of the foam blank respectively. Thus, when the non-pressing surface has a density of 3 pcf, it is desired for the pressing surface to be 2.7 pcf.
In one embodiment of the present invention, the shaped foam article 10 may be formed in a near net-shape foam blank 26 and in a subsequent and separate step, the shaped foam article is separated, or trimmed from the continuous unshaped foam blank 16. In another embodiment, the near net-shape foam blank 26 may be cut to fit into a forming tool prior to contact with the tool. In another embodiment, the final shape may be cut from the pressed plank, for example, the foam near net-shape foam blank 26 may be pressed to form a shape into the pressing surface and the shaped foam article subsequently cut from the pressed foam near net-shape foam blank. When cutting the foam, any suitable method may be used, such as those known in the art and those described previously for cutting the foam to form the pressing surfaces. In yet another, preferred embodiment, the shaped foam article is trimmed from the continuous unshaped near net-shape foam blank by a trimming rib 51 simultaneously as the shaped foam article is formed. In addition, methods that involve heat may also be used to cut the foam since the pressed shape has already been formed in the pressing surface.
The method of the present invention may use a molding machine, sometimes referred to as a press, to form the shaped foam article of the present invention. This process is often referred to as discontinuous as it consists of a cycle where a near net-shape foam blank is placed in an open mold, the mold closes to form an article, then after the article is formed the mold opens. The shaped foam article is removed from the mold, a new near net-shape foam blank is inserted into the mold and the process repeated.
Typically, a press has a stationary platen and a movable platen to which a forming tool may be affixed. The pressing surface(s) of the near net-shape foam blank is contacted with a forming tool such as a die face or mold. Herein die face and/or mold means any tool having an impressed shape and/or cavity that when pressed into the near net-shape foam blank will cause the foam to take the shape of the die face. That is, the material making up the forming tool is such that it does not deform when pressed against the near net-shape foam blank, but the near net-shape foam blank deforms to form and retain the desired shape of the forming tool, die face, and/or mold cavity. Typically, a mold comprises a cavity portion, or cavity half and a core portion, or core half. The cavity half of the mold may be affixed to the stationary platen, but more often is affixed to the movable platen. Hereinafter, when the mold half with a cavity is affixed to the movable platen is referred to as the movable forming surface and the stationary platen is referred to as the stationary forming surface. The stationary platen may or may not have a mold half with a core affixed to it.
Both sides of the near net-shape foam blank may be shaped. In this embodiment both the mold half with the cavity and the mold half with the core impart shape to the shaped foam article. In another embodiment, only one surface of the near net-shape foam blank is shaped. In this embodiment, the foam article is shaped only on one surface pressed by the platen having the half of the mold with the cavity. In this embodiment the near net-shape foam blank may be pressed directly against the other platen or against a mold half with a core affixed to the other platen.
Typically when pressing, at least a portion of the foam is pressed such that the foam is compressed to a thickness of 95 percent or less of the to-be-pressed foam thickness 17 as shown in
The forming tool, because a shape is most often desired, typically has contours that create an impression (step change) in height 32 of at least a millimeter in the shaped foam article 10 having thickness 17 as shown in
The step change, surprisingly, may be formed where the foam undergoes shear. For example, the foam may have a shear or draft angle 33 (θ) of about 45° to about 90° from the press surface 30 of the shaped foam article 10 in a step change of height 33. It is understood that the shear angle θ may not be linear, but may have some curvature, with the angle in these cases being an average over the curvature. The angle surprisingly may be greater than 60°, 75° or even by 90° while still maintaining an excellent finish and appearance. The draft angle at any point along the mold surface is defined as the tangent of the angle taken at that location of the mold.
In another aspect of the invention, a foam having a higher concentration of open cells at a surface of the foam than the concentration of open cells within the foam is contacted and pressed to form the shape. In this aspect of the invention the foam may be any foam, preferably a styrenic foam such as the extruded styrenic polymer foam described above. It may also be any other styrenic polymeric foam such as those known in the art including, for example, where the blowing agent is added to polymer beads, typically under pressure, as described by U.S. Pat. No. 4,485,193 and each of the U.S. patents cited hereinabove.
With respect to this open cell gradient, the gradient is as described above for the density gradient where the concentration of open cells if determined microscopically and is the number of open cells per total cells at the surface.
Generally, the amount of open cells in this aspect of the invention at the surface is at least 5 percent to completely open cell. Desirably, the open cells at the surface is at least in ascending order of 6 percent, 7 percent, 8 percent, 10 percent, 20 percent, 30 percent, 40 percent, 50 percent, 60 percent, 70 percent, 80 percent, 90 percent and completely open cell at the surface.
The foam may have the open cells formed at the surface by mechanical means such as those described above (e.g., planing/machining or cutting) or may be induced chemically, for example, by use of suitable surfactants to burst closed cells at the surface.
The foam surface with the higher concentration of open cells is contacted with a forming tool and pressed as described above. In a preferred embodiment for such foams, one or both sides of the forming tool, e.g., both sides of the die face and/or mold are heated, but the “bulk” foam (i.e., greater than 50 percent) is not (ambient 15-30° C.) and the foam is pressed. Surprisingly, heating the die faces with the foams having open cells at the surface results in superior surface contour and appearance as compared to doing the same with a foam without such open cells at the surface, in this case, the appearance of the foam is degraded.
Applied strain is a function of the initial thickness of the foam blank and the degree of compression or deformation the foam experiences while being compressed by the tool. We have discovered an important correlation between lower applied strain and improved shaped foam article quality, appearance, properties, and performance. A typical plot of stress versus applied strain is represented graphically in
In another embodiment of the present invention, the shaped foam article may be perforated. Such an article may have a plurality of perforations. Perforation is defined herein to mean one or more hole which passes through the near net-shape foam blank/shaped article one surface to another, i.e., from the top surface to the bottom surface. Perforation may occur at any time, in other words, it may be done to the near net-shape foam blank prior to shaping, to the shaped foam article, or a combination of the two. The perforations extend through the shaped foam article, for instance for a shaped foam article made from a near net-shape foam blank, through the depth of the near net-shape foam blank. The foam may be perforated by any acceptable means. Perforating the foam article may comprise puncturing the foam article with a one or more of pointed, sharp objects in the nature of a needle, pin, spike, nail, or the like. However, perforating may be accomplished by other means than sharp, pointed objects such as drilling, laser cutting, high-pressure fluid cutting, air guns, projectiles, or the like. The perforations may be made in like manner as disclosed in U.S. Pat. No. 5,424,016, which is hereby incorporated by reference.
When pressing with a heated forming tool, the contact time with the foam is typically from about 0.1 second to about 60 seconds. Preferably, the dwell time is at least about 1 second to at most about 45 seconds. Dwell time is defined as the duration at which the forming tool remains stationary with the foam subjected to maximum applied strain.
When pressing with a heated forming tool, the temperature of the forming tool is not so hot or held for too long a time such that the foam is degraded. Typically, the temperature of the forming tool is about 50° C. to about 200° C. Preferably, the temperature is at least about 60°, more preferably at least about 70° C., even more preferably at least about 80° C. and most preferably at least about 90° C. to preferably at most about 190°, more preferably at most about 180°, even more preferably at most about 170° C. and most preferably at most about 160° C.
The forming tool provides the shape to the shaped foam article. The forming tool comprises the forming cavity (shape) and all the necessary equipment for temperature control, trimming, ejection, etc. The most frequent case, the forming tool, such as a mold, comprises two halves, one which may be the stationary platen 60 or which is mounted to a stationary platen (sometimes referred to as the core side or stationary forming surface), the other mold half 50 to a moveable platen 70 (sometimes referred to as the cavity side or movable forming surface) and moving with it. The shape of the article will dictate the design and complexity of the forming tool. In the simplest case, the mold half with the cavity is affixed to the movable platen and the stationary forming surface is the stationary platen itself 60
In one embodiment of the present invention the shaping/trimming step of the present invention, the surface of the near net-shape foam blank 22 opposite the pressing surface(s) 30 of the near net-shape foam blank is placed on a stationary forming surface, such as a stationary platen 60. A movable platen 70 which can move toward or away from the stationary platen on which the near net-shape foam blank is placed comprises a movable forming surface of the forming tool 50 for example, a single cavity mold or optionally a multiple cavity mold. To shape the foam, the movable platen moves towards the stationary platen such that the pressing surface(s) 30 of the near net-shape foam blank is contacted and pressed with the movable forming surface of the forming tool 50. For a multi-cavity mold, each cavity may be identical in shape or there may be as many different shapes as cavities or there may be a combination of multiple cavities with the same first shape in combination with multiple cavities with one or more shapes different than the first shape. The layout of cavities in a multi-cavity mold may be side by side, in tandem, or any other desirable configuration. A multi-cavity mold produces more than one shaped article in a plank per molding cycle.
The density profile through the thickness of each foam blank was tested using a QMS Density Profiler, model QDP-01X, from Quintek Measurement Systems, Inc. Knoxville, Tenn. The High Voltage kV Control was set to 90 percent, the High Voltage Current Control was set to 23 percent and the Detector Voltage was approximately 8v. Data points were collected every 0.06 mm throughout the thickness of the foam. Approximate thickness of the foam samples in the plane of the x-ray path was 2 inches. Mass absorption coefficients were calculated for each sample individually, based on the measured linear density of the foam part being tested. The skin density, ρskin, was reported as a maximum value whereas the core density, ρcore, was averaged within an approximate 5 mm range. The density gradient, in units of percentage, was then computed in accordance with the following equation:
The compressive response of each material was measured using a Materials Test System equipped with a 5.0 displacement card and a 4,000 Ibf load card. Cubical samples measuring the approximate thickness of each plank were compressed at a compressive strain rate of 0.065 s−1. Thus, the crosshead velocity of the MTS, in units of inches per minute, was programmed in accordance with the following equation:
Crosshead Velocity=Strain Rate*Thickness*60
where the thickness of the foam specimen is measured in units of inches. The compressive strength of each foam specimen is calculated in accordance with ASTM D1621 while the total compressive strength, CST, is computed as follows:
C
ST
=C
SV
+C
SE
+C
SH
where CSV, CSE and CSH correspond to the compressive strength in the vertical, extrusion and horizontal direction respectively. Thus, the compressive balance, R, in each direction can be computed as shown below:
R
V
=C
SV
/C
ST
R
E
=C
SE
/C
ST
R
H
=C
SH
/C
ST
Open cell content was measured by using an Archimedes method on 25 mm×25 mm×50 mm samples.
While certain embodiments of the present invention are described in the following example, it will be apparent that considerable variations and modifications of these specific embodiments can be made without departing from the scope of the present invention as defined by a proper interpretation of the following claims.
Percent crack reduction Cr can be determined from the ratio of the rough crack value Rcv to the smooth crack value Scv by the following formula:
C
r=(1−Rcv/Scv)*100
wherein crack values are manually calculated for a shaped foam article pressed by a mold with a smooth cavity surface Scv by first measuring the length of each crack in the shaped foam article (or a specified portion thereof) made from a mold with a smooth cavity surface and then adding each of the individual crack lengths together to get an overall smooth crack value Scv in units of length. Crack values are manually calculated for a shaped foam article pressed by a mold with a reduced-slip cavity surface Rcv by first measuring the length of each crack, if any, in the shaped foam article (or the same specified portion as used in the shaped foam article pressed from the mold with a smooth cavity surface) made from a mold with a reduced-slip cavity surface and then adding each of the individual crack lengths together to get an overall reduced-slip crack value Rcv in units of length.
Induced strain is a function of the initial thickness of the foam blank and the final part thickness and is calculated as follows:
wherein to is original thickness of the foam blank and tf is the final thickness of the pressed shaped foam article, both measurements are measured and recorded using a digital linear gage.
Applied strain is a function of the initial thickness of the foam blank and the degree of tool compression and is calculated as follows:
wherein to is original thickness of the foam blank and dt is the distance the tool is pressed into the foam blank.
For Comparative Example A and Examples 1 blanks are prepared from six inch thick ROOFMATE™ SL-A Foam Plank available from The Dow Chemical Co., Midland, The ROOFMATE SL-A Foam Plank is an extruded polystyrene foam. The ROOFMATE SL-A Foam Plank has a density gradient of about 2.67 percent, an open cell content of about 5.5, a compressive balance of 0.4, and a cell gas pressure of about 0.6 atmosphere (atm).
Comparative Example A is a conventional straight foam blank cut with a Baumer abrasive wire saw from the ROOFMATE SL-A Foam Plank. Comparative Example A measures approximately 997 mm by 350 mm by 80 mm, in the length, width and thickness directions respectively. Example 1 is a tapered-shaped foam blank cut with an abrasive wire saw from the ROOFMATE SL-A Foam Plank and has a volume/weight of 27,440 cubic centimeters (cc)/1.043 kilograms (kg). Example 1 measures approximately 997 mm by 350 mm, in the length and width having a thickness of 80 mm at one end tapering to a thickness of 64 mm at the other end and has a volume/weight of 24,696 cc/0.938 kg. Example 2 is a sinusoidal-shaped foam blank cut with an abrasive wire saw from the ROOFMATE SL-A Foam Plank. Example 2 measures approximately 997 mm by 350 mm, in the length and width having a repeating sinusoidal pattern with a wavelength of about 197 mm with a maximum thickness of 82 mm and a minimum thickness of 32 mm and has a volume/weight of 20,666 cc/0.785 kg. The shapes for foam blanks Comparative Example A and Examples 1 and 2 are graphically represented in
The cut, or core, surface of the foam blank is then compressed against the movable forming surface comprising a mold cavity in the shape of Spanish roofing tiles (
The foam blank is pressed by an aluminum compression fixture (mold) with a pressing surface milled in the shape of Spanish roofing tiles. The resulting shaped foam article is a panel with the appearance of Spanish roofing tiles measuring 997 mm×600 mm×78 mm having a volume of about 16,863 cc. The periphery of the mold cavity/panel is defined by a trimming rib measuring about 0.38 inch (in.) wide and about 1,125 in. long. The fixture is mounted to the movable platen of a MTS Millutensil Spotting Press. The Millutensil is programmed for a crosshead velocity of 12 inch per minute (in./min.) and the foam sample is compressed 2.25 in. (i.e., the movable platen is 0.75 in. from the stationary platen). The pressing surface of the mold cavity is machined from a solid billet of aluminum then textured with a Wheelabrator 48 inch Spin Blast media blaster loaded with SN-460 Steel Nugget media.
The maximum applied strain is calculated for each sample at the bottom of the middle valley of the shaped foam article, these results are summarized in Table 1:
Furthermore, the near net-shaped blank imparts the requisite shape to the article with minimal waste or scrap depicted by additional part weight. Waste handling costs are also improved or eliminated due to compressing the final part shape into the final formed foam article. Tooling costs are reduced due to less sophisticated ejection systems for decreasing applied strain levels. Shipping costs are reduced due to proper nesting of contoured foam blanks which all serve to make the technology viable for a host of applications.
This application claims benefit of U.S. Provisional Application Ser. No. 61/252,405, filed Oct. 16, 2009, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61252405 | Oct 2009 | US |
