MXPA99011542A - Absorbent, extruded thermoplastic foams - Google Patents

Abstract

Disclosed is an absorbent, extruded, open cell thermoplastic foam. The foam has an open cell content of about 50 percent or more and an average cell size of up to about 1.5 millimeters. The foam is capable of absorbing a liquid at about 50 percent or more of its theoretical volume capacity when absorbing a liquid. The foam preferably has an average equivalent pore size of about 5 micrometers or more. The foam preferably has a structure substantially of cell walls and cell struts. Further disclosed is a method for absorbing a liquid employing the foam by elongation of the extrudate of the extrusion die. Further disclosed is a method of enhancing absorbency of an open cell foam by applying a surfactant to an exposed surface of the foam such that it remains at the surface and does not infiltrate a substantial distance into the foam. Further disclosed is a meat tray and a diaper containing the foam.

Description

EXTRUDE, ABSORBENT OPTIMAL FOAMS The prior art describes several foams that can be used in absorbency applications. Two varieties are high internal phase emulsion foams (HI PE) and extruded open cell thermoplastic foams. HIPE foams are seen, for example, in U.S. Patent Nos. 5,372,766 and 5,387,207 and extruded open-cell thermoplastic foams are seen, for example, in Canadian Patent Application 2, 129,278 and Japanese Application No. 2-120339. HIPE foams are formed by the interlaced polymerization of hydrophobic monomers as the continuous phase of a water-in-oil emulsion in which the water phase comprises at least 70 weight percent and typically more than 95 weight percent. The structure of the IPE foam depends on its composition and process to manufacture them, but the most desirable to absorb large amounts of fluid are substantially open cell with thin cell walls containing numerous pores in it in communication with neighboring cells. HI PE foams can be prepared to exhibit relatively high absorption regimes and have absorption capacities greater than 25 grams of water per gram of foam. Thus, HIPE foams are very useful for absorbing fluids. However, IPE H foams are expensive due to the large volumes of water used in their preparation.

Extruded open cell thermoplastic foams typically have more internal structure substantially than H IPE foams. They are typically formed of struts and walls interconnected with the open cell character that is derived from a relatively small number of small diameter pores within relatively thick cell walls. The struts are formed by the intersection of cell walls. The relatively substantial internal cell structure and the small pores in the cell walls induce viscous drag and resistance to flow into the foam. The relatively thick cell walls reduce the amount of fluid that can be absorbed within the foam. The relatively small number of small diameter pores can result in some of the portions of the foam not being accessible to fluid absorption. Thus, open-cell foams, extruded from the prior art even those of essentially 100 percent open cell content, typically exhibit both relatively low absorption capacity and a relatively low slow absorption rate. It would be desirable to have an extruded open cell thermoplastic foam exhibiting both, high absorption capacity and high absorption rate. It would also be desirable if the absorption regime could be increased in specific directions or dimensions within the foam.

BRIEF DESCRIPTION OF THE INVENTION In accordance with one aspect of the present invention, there is an extruded open cell thermoplastic foam. The foam has an open cell content of about 50 percent or more and an average cell size of up to about 1.5 millimeters. The foam is capable of absorbing a liquid up to about 50 percent or more of its theoretical volume capapity. The foam preferably has an average equivalent pore size of about 5 microns or more. The foam preferably has a structure substantially of cell walls and cell struts. According to another aspect of the present invention, there is a process for making an open-cell, extruded thermoplastic foam of about 50 percent or more of open cell content. The process comprises extruding and expanding an expandable thermoplastic gel comprising a mixture of a thermoplastic material and a blowing agent through an extrusion die to form an expandable extrudate which expands to form the foam. The extrudate is elongated as it exits the extrusion die and expands to a suffix extension to make the average cell size approximately 25 percent or more, larger in the dimension of elongation than the average cell size in either or both of the other dimensions. According to another aspect of the present invention, there is a method for increasing the absorbency of an open cell foam, comprising: a) providing the foam, b) applying a surfactant to an exposed surface of the foam such that the surfactant remains on the surface and does not infiltrate a substantial distance in the foam. Preferably, the surfactant is applied, in a solution form and subsequently allowed to dry to leave a residue on the exposed surface. The surfactant solution can be allowed to dry by evaporation or by application of heat. According to another aspect of the present invention, there is a method for absorbing a liquid wherein the present foam comes into contact with the liquid such that the liquid is absorbed. According to another aspect of the present invention, there is a meat tray capable of receiving and holding meat thereon, comprising: a tray and an insert, the insert is constituted by the open cell, extruded cell foam described above and is placed inside the tray. According to another aspect of the present invention, there is a diaper suitable for body use. The diaper comprises a foam sheet having an open cell content of about 50 percent or more and an average cell size of up to about 1.5 millimeters. The foam has a structure of substantially cell walls and struts and is capable of absorbing liquid up to about 50 percent or more of its theoretical volume capacity.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a photomicrograph of a cross section of an absorbent foam taken by scanning electron microscopy. The photomicrograph was taken at an amplification of 71.7. The foam has an average cell size of 200-300 micrometers. The foam is useful in the present invention. Figure 2 is a photomicrograph of a cross section of an absorbent foam taken by scanning electron microscopy. The photomicrograph was taken at an amplification of 1 13. The foam has an average cell size of 200-300 micrometers. The foam is useful in the present invention. Figure 3 is a photomicrograph of a cross section of an absorbent foam taken by scanning electron microscopy. The photomicrograph was taken at an amplification of 99.9. The foam has an average cell size of 200-300 micrometers. The foam is useful in the present invention. Figure 4 is a photomicrograph of a cross section of an absorbent foam taken by scanning electron microscopy. The photomicrograph was taken at an amplification of 44.4. The foam has an average cell size of 200-300 micrometers. The foam is useful in the present invention. Figure 5 is a photomicrograph of a cross section of an absorbent foam taken by scanning electron microscopy. The photomicrograph was taken at an amplification of 30.1. The foam has an average cell size of 200-300 micrometers. The foam is useful in the present invention. Figure 6 is a schematic side view of an extrusion process according to the present invention. Figure 7 is a schematic side view of another embodiment of an extrusion process according to the present invention. Figure 8 is a perspective view of an apparatus used to measure the equivalent average pore size. Figure 9 is a plot of pore volume distribution and cumulative volume absorbed versus pressure drop for a set of sample data as measured by the apparatus of Figure 3. Figure 10 is a perspective view of a tray for meat of the present invention wherein the meat tray has meat in it. Figure 11 is a cross section of the meat tray of Figure 4 along line 6-6.

DETAILED DESCRIPTION The extruded open cell thermoplastic foams of the present invention exhibit excellent and unexpected absorption properties and characteristics. The present foams differ from the open cell foams, extruded from the prior art by their unique structure. The present foams have a substantially cell cell / cell wall structure which also exhibits a higher average effective pore size rate relative to the average cell size than the prior art foams. Extruded open cell foams of the prior art, even those with relatively high levels of open cell content, ie, 90-100 percent, have relatively small pores within their cell walls and limited pore incidence level in all the foam. The relatively small pores and the limited pore incidence level result in a relatively slow absorption rate and relatively low absorption capacity due to viscous drag and flow resistance. Although not limited by any particular theory, the largest effective average pore size regimen in relation to the average cell size can result from any or a combination of the following: cell walls that have larger pores in them, a higher proportion larger cell walls that have pores therein, a larger proportion of cell walls generally vertical and horizontal to the extrusion direction that have pores therein, and a smaller proportion of cell walls lost in the cell structure . Usually, the pore size and / or its level of incidence and / or the proportion of cell walls generally vertical and horizontal to the direction of extrusion that have pores therein and / or the proportion of cell walls lost in the cellular structure in the present foam it is higher than for the open cell, extruded foams of the prior art of substantially equivalent cell size and open cell content.

The lower viscous drag and resistance to liquid flow of the present foam enables its internal cell wall / substantial cell strut structure to be used as an advantage rather than a disadvantage. The substantial internal structure of extruded foams offers a relatively high internal surface area at the foam volume rate. The relatively high internal surface area at foam ratio volume of extruded foams offers the regime potential and high absorption capacity when there is relative compatibility between the material comprising the foam and the liquid to be absorbed. However, when the ratio of effective average pore size to average cell size is relatively small as in the open cell foams, extruded from the prior art, the viscous drag and flow resistance substantially strips or diminishes the potentially positive impact of the internal structure of cell wall / substantial cell strut. The present foam has an effective average pore size ratio at a sufficiently large average cell size to substantially reduce viscous drag and fluid flow resistance such that the potentially high absorption rate and capacity offered by the internal cell wall structure / Substantial cell strut can be performed. The potentially high absorption rate and capacity are realized with the present foam when there is relative compatibility, i.e., a contact angle of 90 degrees or less, between the thermoplastic material comprising the inner surfaces of the foam and the liquid to be absorbed.

The present foam has an open cell content of about 50 percent or more, preferably approximately 70 percent or more, more preferably about 90 percent or more, and most preferably about 95 percent or more according to ASTM D2856-A. The present foam preferably has an average cell size of about 1.5 millimeters or less and preferably about 0.01 to about 1.0 millimeters in accordance with ASTM D3576-77. A useful foam mode has an average cell size of from about 0.2 to about 0.7 millimeters in accordance with ASTM D3576-77. Another useful foam pattern has an average cell size of about 0.04 to about 0.06 millimeters in accordance with ASTM D3576-77. The present foam preferably also has an equivalent average pore size of about 5 microns or more, preferably about 10 microns or more, and most preferably about 15 microns or more. The average cell size and equivalent average pore size differ in that the average cell size refers to the average cell dimension in the foam and the equivalent average pore size refers to the average pore size within or through the cell walls of the foam cells. The equivalent average pore size is determined according to the method described above. The present foam has a density preferably from about 16 to about 250 kilograms per cubic meter (kg / m3) and more preferably from about 25 to about 100 kg / m3 in accordance with ASTM D-1622-88. The present foam is capable of absorbing about 50 percent or more, preferably about 70 percent or more, and most preferably about 90 percent or more of its theoretical volume capacity. The theoretical capacity of volume is the volume of liquid absorbed per unit weight of the foam and is commonly described in units of cubic centimeters of liquid per gram of foam. The theoretical volume capacity (TVC) is calculated according to the following: TVC = (1 / pf) x (1 -pf / pp) x (% o.c / 100) where: pf = foam density pp = polymer density% o.c. = percent of content of open cells, in accordance with ASTM D2856-A. The percent in absorbed volume is determined by immersing a foam of 5 millimeters thick under 2.5 centimeters of a liquid for 4 hours at atmospheric pressure. The skin layer of the foam is preferably removed prior to immersion of the foam. A liquid useful for measurement purposes will have a contact angle of 90 degrees or less with respect to the internal surfaces of the foam.

When the TVC is tested on a polystyrene foam, a useful liquid is an aqueous solution (water) of detergent which exhibits the indicated contact angle range with respect to the internal surfaces of the foam. The foam exhibits superior liquid retention under load (under load of weight or other pressure induced externally). Preferably, the foam can withstand pressures of 210 kilopascals with loss of less than 10 percent of its retained liquid. The foam can take any physical configuration known in the art such as sheet or planks. Desirable foam sheets include those less than 0.95 cm thick in cross section. Desirable plank foams include those that have a cross sectional thickness of 0.95 cm or more. Useful sheet foams can be made by squeezing or sliding foams into planks in two or more sheets or by extrusion through an annular or slit die. Desirably, the closed cell skin of the foam formed by extrusion is cut, sliced, or discarded. It is possible to increase the absorption rate mechanically by perforating the foam with needles or other pointed, sharp or compressing objects. The excellent absorption behavior of both, relatively large average cell size and relatively large pore size can be obtained. The foam can be perforated or not perforated. Figures 1-5 are photomicrographs of cross sections of absorbent foams taken by scanning electron microscopy. The foams are useful in the present invention. Foam cells having pores within their cell walls and / or having a smaller proportion of missing cell walls are seen in the figures. In these figures where certain cell walls are missing, the foams retain a cell wall / cell strut structure substantially. The extruded thermoplastic foams are generally prepared by heating a thermoplastic material to form a plasticized material or molten polymer incorporating therein a blowing agent to form a foamable gei, and extruding the gel through a die to form the foam product. Before mixing with the blowing agent, the polymer material is heated to a temperature at or above its glass transition temperature or melting point. The blowing agent can be incorporated or mixed into the molten polymer material by any means known in the art such as an extruder, mixer, stirrer, or the like. The blowing agent is mixed with the molten polymer material at a high enough pressure to prevent substantial expansion of the molten polymer material and to generally disperse the blowing agent homogeneously therein. An optional nucleating agent can be mixed in the molten polymer or mixed dry with the polymer material before plasticizing or melting. The foamable gel is typically cooled to a lower temperature to optimize or obtain desired physical characteristics in the foam. The gel can be cooled in the extruder or other mixing device or in separate chillers. The gel is then extruded or transported through a die of desired shape to a zone of reduced or reduced pressure to form the foam. The lower pressure pad is at a lower pressure than that in which the foamable gel is maintained prior to extrusion through the die. The lower pressure can be superatmospheric or subatmospheric (evacuated), but preferably at an atmospheric level. As the die extrudate exits and expands, the foam is elongated by mechanical means to assist in the formation of pores and the formation of open cells. Elongation is discussed later. To assist in the extrusion of open cell thermoplastic foams, it may be advantageous to employ a polymer other than the predominant polymer employed in the thermoplastic material. Using a lower amount of a polymer different from the predominant polymer increases the open cell content development. For exampleBy making a polystyrene foam, smaller amounts of polyethylene or ethylene / vinyl acetate can be used. By making a polyethylene foam, smaller amounts of polystyrene can be used. The formation of extruded open cell thermoplastic foams of the desired high levels of average open cell contents and equivalent average pore size can be increased by lengthening the extrudate as it exits and expands from the extrusion die. The formation of foams by elongation is not required but is preferred. The elongation may increase the relative proportion of cell walls having pores in them and / or increase the average size of existing pores. The equivalent average pore size can be increased significantly. Thus, even extruded foams that exhibit very high open content, ie 95 percent or more, without elongation can have their absorption properties including packing regime and absorption capacity, significantly increased by elongation because the proportion of cell walls that they have pores in them and / or the average cell size of existing pores is increased. Elongation is best achieved by mechanically lengthening the extrudate as it exits and expands from the extrusion finger. Elongation can occur when a substantial portion of the thermoplastic material comprising the extrudate is at a temperature where it is soft or elastic. For a substantially amorphous thermoplastic material, this temperature will be in the vicinity of the range of glass transition temperatures. For a crystalline thermoplastic material substantially, this temperature will be in the vicinity of the crystalline melting point. The extrudate will cool as it expands and will eventually cool to a temperature at which it will not elongate any longer. The elongation of the extrudate gives more elongate foam cells dimensionally in the direction of elongation than they would be without elongation. The elongation also results in foam cells that are reduced in dimension in the two dimensions perpendicular to the elongation direction of what they would be without elongation. For example, elongation in the extrusion direction gives larger foam cells in dimension in the direction of extrusion but smaller in dimension in the vertical and horizontal directions than they would be without elongation. The larger the average foam cell size, the greater the extension extension possible because the cell walls will be thicker on average and will tend to cool more slowly than the thinner cell walls of foam cells of cell sizes lower average. In addition, to alter the dimensions of the foam cells, the elongation tends to make thinner cell walls in the direction of the elongation force, and thus, more likely to develop pores in those cell walls and / or make the cells larger. existing pores of what could be without lengthening. For example, elongation in the extrusion direction gives thinner cell walls in the horizontal (transverse) direction and vertical direction. Thus, the development of pores is more likely and / or they are larger in the horizontal and vertical directions than without elongation. Elongation in the horizontal (transverse) direction gives thinner cell walls in the direction of extrusion and vertical direction. Thus, the development of pores and / or which are larger in the extrusion and vertical directions than without elongation are more likely. The packing regime of a fluid in the foam is significantly improved by the presence of additional pores and / or larger pores. The elongation can be used to increase the packing rate of a liquid in the foam in a certain direction or directions. The vertical and horizontal packing regimes can be increased by lengthening in the extrusion direction. The packing rate in the extrusion direction can be increased by horizontal or transverse elongation. The extrudate can be lengthened to an extent necessary to result in a stable foam, expanded having an average cell size of approximately 25 percent or greater in any dimension compared to the average cell size in either or both of the other two dimensions. For example, the average cell size in the extrusion dimension may be approximately 25 percent or more, larger compared to the average cell size of either or both of the vertical dimension and the horizontal dimension. Similarly, the average cell size in the horizontal or transverse dimension may be approximately 25 percent or more, larger than the average cell size in the extrusion direction and / or the vertical dimension. The average cell size in any given dimension can be determined in accordance with ASTM D3576-77. The extrudate can be mechanically elongated to a point where the extrudate does not break, tear, or introduce substantial voids in the cell structure. The larger the cross section of the expanded extrudate, the greater the mechanical stress that must be applied to effect the desired extension of elongation. The elongation can be carried out by any of several means. For elongation in the extrusion direction, the extrudate can be stretched in the extrusion direction by a pair of opposed pinch rollers or bands placed downstream of an extrusion die. Such an elongation method is seen in an elongation apparatus 10 in Figure 6, which shows a pair of opposing rotating pinch rollers 20 pulling or stretching an extrudate 30, which is exiting an extrusion die 40. The elongation in both the direction of the extrusion and the transverse direction can be accomplished by using mechanical pressure in the extrudate by a pair of opposed forming plates placed just downstream of the extrusion die. The extrudate is elongated in the extrusion direction between the forming and elongation plates in the transverse direction around the sides or lateral to the forming plates. Figure 7 shows an elongation apparatus 60 with a pair of opposing forming plates 70 exerting pressure on opposite surfaces of an extrudate 80 (up and down) exiting an extrusion die 90. For horizontal or transverse elongation to the extrusion direction, a conventional stretching apparatus (not shown) downstream of the extrusion die can be used to stretch the extrudate in that direction. The elongation can be effective with both foams in foil and foams in plank, but is particularly effective with sheet foams. Although stretching is effective to produce absorbent foams of any thermoplastic material, it is particularly effective when foaming with relatively rigid thermoplastic materials such as alkenyl aromatic polymers. The foam can be formed of any thermoplastic or mixtures of thermoplastics that can be formed or blown in an open cell foam of the aspects described herein. Useful thermoplastics include natural and synthetic organic polymers.

Suitable plastics include polyolefins, polyvinyl chloride, alkenyl aromatic polymers, cellulosic polymers, polycarbonates, starch-based polymers, polyetherimides, polyamides, polyesters, polyvinylidene chlorides, polymethyl methacrylates, copolymer / polymer blends, modified rubber polymers, and the similar ones. Aromatic alkenyl polymers include polystyrene and copolymers of styrene and other copolymerizable monomers. If desired, the foam can be blown from a thermoplastic material which is partially or substantially biodegradable. Useful polymers include cellulosic polymers and polymers based on starch. A useful thermoplastic foam comprises an aromatic alkeneium polymer material. Suitable alkenyl aromatic polymer materials include aromatic alkenyl homopolymers and copolymers of alkenyl aromatics and ethylenically unsaturated copolymerizable comonomers. The alkenyl aromatic polymer material may further include minor proportions of non-alkenyl aromatic polymers. The alkenyl aromatic polymer material may consist solely of one or more alkenyl aromatic homopolymers, one or more alkenyl aromatic copolymers, a mixture of one or more of each of the alkenyl homopolymers and aromatic copolymers, or mixtures of any of the foregoing with an aromatic non-alkenyl polymer. The alkenyl aromatic polymer material comprises more than 50 and preferably more than 70 weight percent aromatic monomeric alkenyl units. Most preferably, the alkenyl aromatic polymer material is composed entirely of aromatic alkenyl monomer units. Suitable alkenyl aromatic polymers include those derived from alkenyl aromatic compounds such as styrene, alphamethylstyrene, ethylstyrene, vinylbenzene, vinyl toluene, chlorostyrene, and bromostyrene. A preferred alkenyl aromatic polymer is polystyrene. Minor amounts of monoethylenically unsaturated compounds such as C2-β alkyl acids and esters, ionomeric derivatives, and C4.6 dienes can be copolymerized with alkenyl aromatics. Examples of copolymerizable compounds include acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, itaconic acid, acrylonitrile, maleic anhydride, methyl acrylate, ethyl acrylate, isobutyl acrylated, n-butyl acrylated, methyl methacrylated, vinyl acetate and butadiene. Useful alkenyl aromatic polymer foams may comprise substantially (i.e., more than 90 weight percent) or entirely polystyrene. Preferred aromatic alkenyl polymer foams comprise polystyrene of about 125,000 to about 300,000 average molecular weight, about 135,000 to about 200,000, about 165,000 to about 200,000 average molecular weight, and about 135,000 to about 165,000 molecular weight. Weighted average according to size exclusion chromatography. Polystyrene in these molecular weight ranges is particularly suitable for forming foams, particularly elongated foams, useful in the present invention. Useful extruded thermoplastic foams include extruded microcellular alkenyl polymeric high cell open cell foams and processes for making them are described in WO 96/34038, which is incorporated herein by reference. The foams described have an average cell size of about 70 microns or less and an open cell content of about 70 percent or more. In the process described in WO 96/34038, useful blowing agents include 1,1-difluoroethane (HFC-1 52a), 1,1,1-trifluoroethane (H FC-143a), 1, 1, 2- tetraf luoroethane (H FC-134a), chlorodifluoromethane (HCFC-22), carbon dioxide (CO2), and difluoromethane (HFC-32). Preferred blowing agents are H FC-152a, HFC-134a, and carbon dioxide. The above blowing agents will comprise 50 mole percent or more and preferably 70 percent or more of the total number of moles of blowing agent. The balance may be constituted by other blowing agents. The amount of blowing agent employed is from about 0.06 to about 0.17 gram-mol per 100 grams of polymer, preferably from about 0.08 to about 0.12 gram-mol per 100 grams of polymer, and most preferably from 0.09-0.10 gram. mol per 100 grams of polymer. The use of a relatively small amount of blowing agent allows the formation of a foam with a high content of open cells.

Preferred foaming temperatures will vary from about 18 ° C to about 160 ° C. Most preferred foaming temperatures will vary from about 125 ° C to about 135 ° C. The amount of nucleating agent employed can range from about 0.01 to about 5 parts by weight per hundred parts by weight of a polymer resin. The preferred range is from 0.1 to about 3 parts by weight. To assist in the extrusion of open cell thermoplastic foams, it may be advantageous to employ a polymer other than the predominant polymer employed in the thermoplastic material. Using a smaller amount of a polymer other than the predominant polymer increases the development of open cell content. For example, by making a polystyrene foam, smaller amounts of polyethylene or ethylene / vinyl acetate copolymer can be employed. By making a polyethylene foam, smaller amounts of polystyrene can be employed. Useful teachings for preferred different polymers are found in Serial No. 08/880, 954 of E.U., which is incorporated herein by reference. Other aromatic extruded alkenyl foam of larger average cell size and processes for making it are seen in WO 96/00258, which is incorporated herein by reference. The content of open cells is approximately 30 percent or more according to ASTM D2856-87. The foams described have a density of about 24 kg / m3 to about 96 kg / m3 and preferably a density of about 32 kg / m3 to about 48 kg / m3 according to ASTM D-1622-88. The present foam it has an average cell size from about 0.08 millimeters (mm) to about 1.2 mm and preferably from about 0.10 mm to about 0.9 mm in accordance with ASTM D3576-77. In the process for making the foam in WO 96/00258, the foaming temperature, which is relatively higher than that for making closed-cell foams (less than 10 percent of closed cell according to ASTM D2856-87), can varying from about 18 ° C to about 145 ° C. The foaming temperature will vary according to the composition and concentration of the nucleating agent, composition and concentration of the blowing agent, characteristics of the polymeric material, and design of the extrusion die. The foaming temperature for the present open cell foam ranges from about 3 ° C to about 15 ° C and preferably from about 10 ° C to about 15 ° C higher than the highest foaming temperature for a corresponding closed cell foam (less than 10 percent closed cell according to ASTM D2856-87) of substantially equivalent density and cell size made with a substantially equivalent composition (including polymeric material, nucleating agent additives and blowing agent) in a substantially equivalent process . A preferred foaming temperature is about 33 ° C or more, greater than the glass transition temperature (according to ASTM D-3418) of the alkenyl aromatic polymer material. A most preferred foaming temperature is from 135 ° C to 140 ° C. The amount of blowing agent incorporated in the polymer melt to make a foaming gel is from about 0.2 to about 5.0 gram-mol per kilogram of polymer, preferably from about 0.5 to about 3.0 gram-mol per kilogram of polymer, and most preferably from about 0.7 to 2.0 gram-mol per kilogram of polymer. A nucleating agent such as those described above can be employed. To make foams of the physical properties described in WO 96/00258 having the pore size and the pore incidence level to be effective in the present invention, it may be necessary to incorporate different polymers in the alkenyl aromatic polymer material such as polyolefins of melting temperatures of 70 ° C or less, ethylene / styrene interpolymers, and styrene / butadiene copolymers or other rubber homopolymers or copolymers. Useful, extruded open-cell thermoplastic foams include those made of styrene / ethylene interpolymers and mixtures of such interpromomers with alkenyl aromatic polymers and ethylene polymers described in the U.S. Patent. , No. 5,460,818, WO 96/14233, and Serial No. 60/078091 of E. U., filed March 16, 1998, all of which are hereby incorporated by reference. Such interpolymers are particularly useful for making foams having an average cell size greater than 100 microns. The content of open cells and equivalent average pore size can be further increased by extruding a foam with a charge of a water soluble polymer into particles such as methyl cellulose. The particulate polymer can be subsequently washed from the foam matrix by exposure to water or steam. The spaces will remain in the foam matrix. The foam may be non-interlaced or lightly interlaced. Non-interlaced means that the foam is substantially free of crosslinked bonds or that it has the slight degree of interlocking bonds that can occur naturally without the use of crosslinking or radiation bonding agents. The non-interlaced foams contain no more than 5 percent gel according to ASTM D2765-84, Method A. The lightly entangled foams are those that have more than 5 percent gel but less than about 25 percent gel according to the same test. The present foams can be treated to give the inner cell surfaces of the foam more compatible with respect to a liquid to be absorbed. For example, internal cell surfaces can be made more hydrophilic to increase the absorption of aqueous liquids such as urine or blood. Similarly, the internal cell surfaces can be made more hydrophobic to increase the absorption of oily liquids or organic liquids. To increase the absorption of aqueous liquids, the internal surfaces of the foams can be sulfonated or surfaces treated with a surfactant. To give a more hydrophilic foam, the foams can be sulfonated by exposure to gases or sulfurous liquids such as sulfur dioxide, sulfur trioxide, or sulfuric acid. The foams are neutralized later. The surfactants can be applied by soaking and infiltrating a substantial portion of the entire foam with a solvent / surfactant solution such as an aqueous detergent or a soap solution followed by drying to remove the solvent (water in the case of an aqueous solution). When a solution is applied, the exposed surface is subsequently dried by evaporation under ambient conditions or post-extrusion normal processing conditions or by heating to leave a residue of the surfactant. The heating can be carried out by any conventional means such as by heated air, infrared heating, radio frequency heating, or induction heating. The surfactant remains as a residue on the internal surfaces of the foam. In the present invention, it was observed that the packing regimes are the fastest for foams of approximately 70 micrometers of average cell size and 15 micrometers of equivalent average pore size. In one aspect of the invention, it was surprisingly found that treating one or more exposed surfaces of the foam with a surfactant to alter the contact angle of the foam was substantially as effective as treating the entire foam to increase the absorbency of the foam if the Absorption occurs through a treated surface. The surfactant can be applied by any means known in the art such as brushing or spraying in the form of a solvent / surfactant solution on the exposed surface or the surfactant itself if it has a fluid consistency. When a water-soluble surfactant is applied, an aqueous solution is preferred. Although not preferred, it is also possible to apply a surfactant in a powder or solid form to the surface. The surfactant is applied such that a substantial distance does not infiltrate the foam and remains on the treated surface and portions of the foam contiguous to the treated surface. When a solution is applied, the exposed surface is subsequently dried by the means discussed above or the water or solvent is allowed to evaporate to leave a residue of the surfactant. During the absorption the liquid is removed or absorbed through the exposed exposed surface and the surfactant residue dissolves in the liquid making it more compatible with the thermoplastic material that constitutes the foam. The compatibilized liquid is then more easily absorbed and distributed within the portions of the foam where the surfactant residue was not present. This aspect of the invention for treating one or more exposed surfaces of a foam with a surfactant can also be employed in H I PE foams, such as those described in the U. , Nos. 5,372,766 and 5,387,207, which are incorporated herein by reference.

It is also possible to adjust the contact angle of the internal cell surfaces of a foam by incorporating a surfactant into the thermoplastic material constituting the foam as the foam is made. For extruded foams, the surfactant can be dry mixed with the thermoplastic or molten injected material in a melt of the thermoplastic material before extruding through the die. Surfactants and useful methods of incorporation are seen in Canadian Patent Application 2, 129,278, which is incorporated herein by reference. The term "surfactant" as used herein describes any substance that could be applied to the cell surfaces of the foam to make them more compatible (reduce the contact angle) with respect to a particular liquid or fluid to be absorbed. The surfactant could be used to make the thermoplastic material that makes the substrate more hydrophilic or, conversely, more hydrophobic. Useful surfactants include cationic, anionic, amphoteric, and nonionic surfactants. Useful anionic surfactants include the alkylsulfonates. The present foam is useful in a variety of absorbency applications such as in food or barrier packaging, capture and absorption of industrial hydraulic oil, cleaning, and baby and adult diapers for body use. The sheet foam is particularly adapted to be formed, cut, in diapers. The sheet foam is also particularly adaptable to be thermoformed or molded and otherwise formed into meat trays or other forms of food packaging. The foamed sheet is also particularly adaptable to be used as an insert or absorbent pad in a meat tray. The meat tray of the present invention is shown in Figures 10-1 1. The meat tray 210 comprises a tray 212 of closed cell plastic foam and an extruded, open cell foam insert 214 located therein. The meat 216 is located within the bottom of the tray 212 at the top of the insert 214. If desired, a tray bottom of a material other than a foam such as a paper-based material such as cover board or a non-foamed plastic material. If it is a foam as in the case of tray bottom 212, it typically has an open cell content much lower than the foam insert. The bottom tray and the insert are preferably manufactured separately with the insert that is placed in the receiving portion of the bottom tray. Optionally, an adhesive can be used to stick the insert to the bottom of the tray. You can pack any type of meat in trays with absorbent inserts. It is particularly advantageous to pack chicken meat in such trays since the chicken meat exudes relatively large amounts of liquid. In making extruded foams, other additives such as inorganic fillers, pigments, antioxidants, acid sequestrants, ultraviolet absorbers, flame retardants, processing aids, extrusion aids, and the like can be incorporated.

The equivalent average pore size is determined by a liquid intrusion technique. The technique measures the liquid that is collected by the foam through an applied pressure gradient. The information is analyzed according to the Laplace relationship between the pressure drop and the pore radius: ? P = 2? Cos? / R where? P is the pressure gradient required to introduce a liquid with a surface tension? in a pore of radius R (micrometers) where the contact angle between the liquid and the foam is? Figure 8 shows an apparatus for measuring the equivalent average pore size. A sample 100 of foam is placed on the bottom of a desiccator 1 10 below a desiccator plate 120. Plastic tubing 130 is used to connect the desiccator 1 10 to a first filter flask 140, which functions as a liquid reservoir.

Plastic tubing 150 is used to connect the first filter flask 140 with a second filter flask 160, which functions as a trap for liquids. Plastic tubing 150 is used to connect the second filter flask 160 with a vacuum pump 180, which is used to create a pressure gradient across the system or remnant of the apparatus. The vacuum pump 180 is graduated to a desired vacuum pressure level and the pressure is allowed to stabilize within the system for a time, approximately 10 minutes. Once the system pressure is stable, the end of the plastic tube 130 that enters the bottle 140 is inserted into the liquid retained in that bottle. The vacuum pump 180 is then turned off, which re-pressurizes the system and pushes the liquid from the bottle 140 to the desiccator 1 10. There must be sufficient liquid in the bottle 140 to cover the plate 120 of the desiccator. After approximately 15 minutes, the foam sample 100 is removed from the liquid and dried with a paper towel or other absorbent means to remove any excess water on its surface. The foam sample 100 is weighed to determine the amount of liquid absorbed. This is repeated during a series of different pressure levels, including essentially complete vacuum, by recording the amount of liquid collected at each point. The increasing volume absorbed with each change in pressure level (pressure drop) is related to the pore size distribution. After collecting the information for the amount of liquid absorbed vs. ? P (pressure drop), the pore size distribution can be determined. The pore radius (pore size) corresponding to each? P can be calculated from the Laplace ratio described above. Figure 9 illustrates a data packet demonstrates for the amount of liquid absorbed vs. ? P. The first derivative of this curve with respect to the pore volume (or? P) is the pore volume distribution. If desired, the equivalent average pore size can also be determined using an automated porometer, such as the Perm Porometer 200 PSI from PMI (Porous Materials, Inc.).

The following are examples of the present invention, and should not be construed as limiting. Unless otherwise indicated, all percentages, parts, or proportions are by weight.

EXAMPLES Example 1 Extruded open cell polystyrene foams were subsequently sulfonated and tested for absorbency. The foams were made with a foaming apparatus comprising an extruder, a mixer, a cooler, a die, and sequential forming plates. The polystyrene resin of 200,000 weight average molecular weight according to size exclusion chromatography (SEM) was fed to the extruder and mixed with talc, graphite, and calcium stearate to form a polymer melt. The molten polymer was fed to the mixer and mixed with a blowing agent mixture of 1,1,1,2-tetrafluoroethane, ethyl chloride, and carbon dioxide to form a polymer gel. The polymer gel was cooled to a desirable foaming temperature in the cooler and subsequently transported through the die to a region of lower pressure to effect expansion of the extrudate to a foam product. During the expansion, the extrudate was extended downstream of the die or by placing forming plates that contact the extrudate from above and below to reduce the expansion of the foam in the vertical direction and increase the expansion of the foam in the extrusion directions and horizontal.

The foams had an average cell size of 50 microns, an average pore size of 15 microns, and an average open cell content of essentially 100 percent. The foams had a thickness of 5.1 centimeters (cm). The foam was sulphonated by i) exposing it to sulfur trioxide gas purging for one minute followed by a reaction time of ten minutes, ii) neutralize it with aqueous ammonium hydroxide for 1-3 minutes, iii) rinse it with water, iv) and dry it at an elevated temperature to remove the water. Two different levels of sulfonation were employed. Two foam samples were made at each level of sulfonation. One pack (Foam # 1) of foam samples had an average of 2.3 weight percent sulfur and the other pack (Foam # 2) had an average of 2.0 weight percent sulfur based on the weight of the foam ? The sulfur concentration was determined by energy analysis by neutron activation. The foams were tested for vertical packing to determine both the amount of liquid absorbed (collected) and the absorption rate. A foam sample of 15.2 cm in length, 2.5 cm in width and 0.32 cm in thickness was cut from the middle part of the foam in the extrusion direction and was erected vertically thereafter. The sample was submerged to a depth of 12 cm in liquid. Packing height as a function of time was established. The absorbed liquid was a synthetic urine composition similar to the JAYCO synthetic urine described in U.S. Patent No. 5,260,345. The composition is made by mixing 1.0 grams of KCl; 1.0 grams of Na2SO4; 0.42 grams of NH4H2PO4; 0.07 grams of (N H4) HPO4; 0.12 grams of CaCl2-2H2O; 0.25 grams of MgCl2-6H2O; and 497.14 grams of distilled water. The synthetic urine composition had a surface tension of approximately 72 dynes / centimeter. The weight of synthetic urine absorbed by the foam (in grams of urine per gram of foam) was 20.7 for each of the two samples of Foam # 1 and 23.2 for each of the two samples of Foam # 2. The values of Theoretical collected for these foams were 21.8 and 23.2 grams of urine per gram of foam, respectively, as calculated by theoretical volume available based on the content of open cells. Thus, both foams absorbed substantially up to their theoretical volumetric limits of synthetic urine in the vertical packing test. The time for packing vertically for a height of 6 centimeters was 33 and 28 seconds for the two samples of Foam # 1 and 35 and 40 seconds for the two samples of Foam # 2. Percent or urine absorbed based on the collected The theoretical for Foams # 1 and # 2 was 95 percent and 100 percent, respectively. These absorption levels far exceeded those of the extruded open cell foams of the prior art, which typically exhibit absorbency based on the theoretical collection of only about 15 percent or less.

Example 2: Extruded, open-cell foam samples similar to those of Example 1 were contacted with an aqueous detergent solution, dried, and subsequently tested for synthetic urine absorbency. Four samples of the foam were saturated by vacuum saturation with an aqueous detergent solution of 0.5 weight percent freight washing liquid JOY brand (Proctor and Gamble) based on the total weight of the aqueous detergent solution (real solids in the detergent solution was 0.13 weight percent based on the weight of the aqueous solution) and then dried by heating at 80 ° C in a forced air oven. The increase in weight of the foams ranged from 0.036 to 0.041 grams with an average of 0.038 grams. This corresponded to the amount of surfactant residue remaining on the surfaces of the foam after drying the detergent solution. This also corresponded to 3.59 percent to 4.05 percent with an average of 3.76 percent surfactant residue based on the weight of the foam. The foams were subjected to the vertical packing test as in Example 1. The weight of synthetic urine (in grams) absorbed by the foams (in grams) in a vertical packing test ranged from 21.8 to 22.4 for an average of 22.00. This compares favorably with an average of 24.4 grams of aqueous solution of absorbed detergent per gram of foam during vacuum saturation during the initial preparation of the foam samples. The packing time (regime) vertically to a height of 6 cm for the four foams ranged from 12 to 160 seconds. The absorption performance was excellent. The percent of urine absorbed based on the theoretical collection for Foams # 1 and # 2 was 90 percent and 92 percent, respectively.

Example 3: Extruded open-cell polystyrene foams were prepared and tested for absorbency of a detergent solution. The foams were prepared with the apparatus described in Example 1.

The process conditions and the physical properties of the foam are described in Tables 1 and 2. The polystyrene resin (PS) used was 135,000 weight average molecular weight according to size exclusion chromatography. The Kraton G 1657 resin was a SEBS copolymer (styrene / ethylbenzene / styrene) having 13 percent monomeric styrene content by weight and having a structure which is 65 percent linear and 35 percent diblock by weight. The ethylene polymer H F 1030 was an ethylene / octene copolymer sold under the trademark I NSITE by The Dow Chemical Company. The H F 1030 had a density of 0.935 grams / cubic centimeter, a melt index of 2.5, and a melting temperature of 125 ° C. The absorbed liquid was an aqueous detergent solution of 1.5 percent by weight of JOY brand dishwashing liquid (Proctor and Gamble) based on weight total of the aqueous detergent solution (real solids in the aqueous solution was 0.75 weight percent based on 3.6 the weight of the aqueous solution). The foams were subjected to the vertical packing test described in Example 1.

Table 1 CO2 - Carbon dioxide EtCl - Ethyl chloride 134a - 1, 1, 1, 2-tetraf luoroethane pph - Parts per hundred parts of polymer by weight Tf - Foaming temperature Table 2 Content of C.A. - Open cell content E.A. . - Average Equivalent Pore Size V.W. H. - Vertical Packing Height Kg / m3 - Kilograms per Cubic Meter As seen from Table 2, the absorption performance was good even with foams with relatively large cell sizes.

Although the embodiments of the foam and the methods of the present invention have been shown with respect to specific details, it will be appreciated that depending on the manufacturing process and the manufacturer's wishes, the present invention may be modified by various changes while still within of the scope of the teachings and novel principles in the present settled.

Claims (46)

1. REJVIN DICATIONS 1. An absorption method, which comprises contacting a liquid and an extruded open-cell thermoplastic foam, the foam having a structure substantially of cell walls and cell struts, the foam having an overall content of open cells of about 50 percent or more, foam having an average cell size of up to about 1.5 millimeters, foam that is capable of absorbing liquid at approximately 50 percent or more of its theoretical volume capacity.
2. 2. The method of claim 1, wherein the foam has an equivalent average pore size of about 5 microns or more.
3. 3. The method of claim 1, wherein the foam has an equivalent average pore size of about 10 microns or more.
4. 4. The method of claim 1, wherein the foam is capable of absorbing approximately 70 percent or more of its theoretical volume capacity.
5. The method of claim 1, wherein the foam is capable of absorbing approximately 90 percent or more of its volume capacity.
6. The method of claim 1, wherein the thermoplastic material involves more than 50 percent or more by weight of alkenyl aromatic monomer units.
7. 7. The method of claim 1, wherein the foam loses 10 percent or less of its retained liquid when exposed to a pressure of 210 kilopascals.
8. The method of claim 2, wherein the thermoplastic foam is a polystyrene foam, the polystyrene being of a weight average molecular weight of about 125,000 to about 300,000.
9. The method of claim 2, wherein the thermoplastic foam is a polystyrene foam, the polystyrene being of a weight average molecular weight of from about 165,000 to about 200,000.
10. The method of claim 1, wherein the overall content of open cells is about 90 percent or more. eleven .
11. The method of claim 1, wherein the overall content of open cells is about 95 percent or more.
12. 12. The method of claim 1, the foam having an equivalent average pore size of about 15 microns or more.
13. The method of claim 1, wherein the foam has an average cell size of greater than about 0.01 to about 1.0 millimeters.
14. The method of claim 1, wherein the foam has an average cell size of greater than about 0.01 to about 0.07 microns.
15. 15. The method of claim 1, wherein a portion or a substantial portion of the inner cell surfaces have a surfactant deposited therein.
16. 16. The method of claim 1, wherein a portion or a substantial portion of the internal cell surfaces are sulfonated.
17. 17. The method of claim 1, wherein the foam is a sheet foam less than 0.95 cm thick.
18. 18. The method of claim 1, wherein the foam is a plank foam having a thickness of 0.95 cm or more.
19. 19. The method of claim 1, wherein the density of the foam is from about 16 to about 250 kg / cubic meter.
20. The method of claim 1, wherein the density of the foam is from about 25 to about 100 kg / cubic meter. twenty-one .
21. The method of claim 1, wherein the foam has an average cell size in one dimension that is about 25 percent or more, greater than the average cell size in either or both of the other two dimensions.
22. The method of claim 1, wherein the foam has an average cell size in one dimension that is approximately 50 percent or more, greater than the average cell size in either or both of the other two dimensions.
23. The method of claim 1, wherein the foam has an equivalent average pore size of about 15 micrometers more, the foam being able to absorb approximately 70 percent or more of its theoretical volume capacity, the foam having an overall open cell content is about 90 percent or more, the foam having a density from about 16 to about 250 kg / cubic meter, the thermoplastic material comprising more than 50 percent or more by weight of alkenyl aromatic monomer units, the foam having an average cell size of up to about 0.01 to about 1.0 millimeters.
24. The method of claim 23, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of from about 125,000 to about 300,000.
25. 25. The method of claim 23, wherein the thermoplastic foam is a polystyrene foam, polystyrene having a weight average molecular weight of from about 165,000 to about 200,000.
26. The method of claim 1, wherein the foam has an equivalent average pore size of approximately 10 micrometers more, the foam being able to absorb approximately 90 percent or more of its theoretical volume capacity, the foam having an overall open cell content is about 90 percent or more, the foam having a density from about 25 to about 100 kg / cubic meter, the thermoplastic material comprising more than 50 percent or more by weight of aromatic monomeric units of alkenyl, the foam having an average cell size of up to about 0.01 to about 0.07 millimeters.
27. The method of claim 26, wherein the foam of the method of claim 2, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of from about 125,000 to about 300,000.
28. The method of claim 26, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of from about 165,000 to about 200,000.
29. 29. A process for making an extruded open cell thermoplastic foam of about 50 percent or more of open cell content, the process comprising extruding and expanding an expandable thermoplastic gel comprising a mixture of a thermoplastic material and a blowing agent. of an extrusion die to form an expanding extrudate which expands to form the foam, the improvement which is to lengthen the extrudate as it leaves the extrusion die and expands to a sufficient point to make the average cell size of approximately 25 percent or more, larger in the dimension of elongation than the average cell size in either or both of the other dimensions.
30. 30. The process of claim 29, wherein the extrudate is elongated by stretching in the extrusion direction.
31. 31 The process of claim 29, wherein the extrudate is elongated by stretching in the transverse direction.
32. 32. The process of claim 29, wherein the extrudate is elongated in the extrusion direction by pressing the forming plates that contact the opposite surfaces of the extrudate downstream of the die.
33. 33. The process of claim 29, wherein the extrudate is elongated in the extrusion direction by opposing gripping rollers downstream of the extrusion die.
34. The process of claim 29, wherein the extrudate is elongated to a point sufficient to make the average cell size approximately 50 percent or more, larger in the elongation dimension than the average cell size in either or both of the other dimensions.
35. 35. The method for increasing the absorbency of an open cell thermoplastic foam, comprising: a) providing the foam, b) applying a surfactant to an exposed surface of the foam such that the surfactant remains on the surface and does not infiltrate substantial distance in the foam.
36. 36. The method of claim 29, wherein the surfactant is applied in a solution form and allowed to dry subsequently to leave a residue on the exposed surface.
37. 37. The method of claim 29, wherein the foam is dried by exposure to heat.
38. 38. The method of claim 29, wherein the foam is an extruded thermoplastic foam.
39. 39. A meat tray capable of receiving and retaining meat therein, the meat tray comprising a tray and an insert, the insert comprising a tray and open cell, extruded thermoplastic foam placed inside the tray, the foam which has an open cell content of about 50 percent or more, the foam which is of an average cell size of up to about 1.5 millimeters, the foam which is of a structure of walls and struts of cells substantially, the foam which is capable of absorbing, if it absorbs liquid, up to about 50 percent or more of its theoretical volume capacity, the foam has a thickness of less than about 0.95 cm.
40. 40. The meat tray of claim 39, wherein the foam has an equivalent average pore size of about 5 microns or more, the foam is capable of absorbing approximately 70 percent or more of its theoretical volume capacity, the foam having a total content of open cells is about 90 percent or more, the foam having a density from about 16 to about 250 kg / cubic meter, the thermoplastic material comprising more than 50 percent or more by weight of monomer units alkenyl aromatics, the foam having an average cell size of up to about 0.01 to about 1.0 millimeters.
41. 41 The meat tray of claim 39, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of about 125,000 to about 300,000.
42. 42. The meat tray of claim 40, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of from about 135,000 to about 200,000.
43. 43. The meat tray of claim 40, wherein the foam has an equivalent average pore size of about 5 microns or more, the foam being capable of absorbing approximately 90 percent or more of its theoretical volume capacity, foam having an overall open cell content is about 90 percent or more, the foam having a density from about 25 to about 100 kg / cubic meter, the thermoplastic material comprising more than 50 percent or more by weight units aromatic alkenyl monomers, the foam having an average cell size of up to about 0.01 to about 0.07 millimeters.
44. 44. The meat tray of claim 43, wherein the foam, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of about 125,000 to about 300,000.
45. 45. The meat tray of claim 43, wherein the thermoplastic foam is a polystyrene foam, the polystyrene having a weight average molecular weight of from about 135,000 to about 200,000.
46. 46. A diaper suitable for body use, the diaper comprising a flexible sheet foam, the foam having an open cell content of about 50 percent or more, the foam which is of an average cell size of up to about 1 .5 millimeters, the foam which is of a structure of walls and cell struts substantially, the foam which is capable of absorbing liquid in approximately 50 percent or more of its theoretical volume capacity.