WO1994025255A1 - Ethylene polymer foams blown with 1,1-difluoroethane and method of making same - Google Patents

Abstract

Disclosed are processes for making olefin polymer foam structures with 1,1-difluoroethane (HFC-152a). One foam structure is comprised of an ethylene polymer material, and is blown with a mixture of isobutane and 1,1-difluoroethane. Another foam structure has a cross section of 2 or more inches (5 or more centimeters) in one dimension and 18 or more inches (46 or more centimeters) in the other dimension, and is blown with 75 mole percent or more HFC-152a. Another foam structure is in coalesced strand form with a density of 16 to 48 kilograms per cubic meter, and is blown with a first blowing agent of 20 to 90 mole percent 1,1-difluoroethane and 80 to 10 mole percent of a second blowing agent selected from isobutane, n-butane, and propane.

Description

ETHYLENE POLYMER FOAMS BLOWN WITH 1 , 1- DIFLUOROETHANE AND METHOD OF MAKING SAME.

One aspect of this invention relates to an ethylene polymer foam structure having enhanced processability and physical properties and a process for making the foam structure Isobutane has become a preferred blowing agent for making ethylene polymer foam structures because of its zero ozone depletion potential and relatively high degree of processability and foamability, which can result in end products having generally desirable physical properties A drawback to using isobutane is that it is a volatile organic compound, which may cause environmental concern Another drawback to using isobutane is the sometimes poor compressive recovery encountered in end product foam structures at certain critical times in the product life cycle

A means of reducing the volatile organic content of a blowing agent containing isobutane is to replace part ot the isobutane with a hydrofluorocarbon, which may not cause the same measure of environmental concern A suitable hydrofluorocarbon is 1 , 1-dιfluoroethane (HFC- 152a)

Replacing isobutane with HFC- 152a can present processing and extrusion problems because of the relatively low solubility of HFC- 152a in melts of ethylene polymers The processing and extrusion problems would be expected to take the form of a relatively narrow range or "window" of foaming temperatures or less than desirable physical properties in the end product Undesirable physical properties can include poor skin quality, small cell size, high foam density, and small cross-section

Foam structures blown with a blowing agent comprised entirely of isobutane sometimes exhibit poor recovery after compression, which the foam structures are often subjected to during die cutting After compression and release from compression, foam structures blown entirely with isobutane recover a substantial proportion (that is, 88-95 percent by volume or thickness) of their initial volume prior to compression, but then shrink to some degree (that is, 3- 10 percent by volume or thickness) for an extended period of time before expanding and re-gaining a substantial proportion of the initial volume This temporary shrinkage is a considerable problem for cushion packaging end users or customers because the shrinkage typically occurs while the foam structure is being used as cushion packaging The shrinkage results in an undesirable loose fit between the packaging material and the article or articles being packaged

It would be desirable to have an ethylene polymer foam structure and process for making wherein the use of isobutane as a blowing agent is reduced It would further be desirable to make such a foam structure under a relatively wide range of processing conditions with desirable foam physical properties It would further be desirable to make such a foam structure without the compressive recovery problems sometimes encountered with foam structures blown solely with or with substantial proportions of isobutane

According to the present invention, there is an ethylene polymer foam structure comprising an ethylenic polymer material and a blowing agent The ethylenic polymer material comprises greater than 50 percent by weight ethylenic monomeric units The blowing agent comprises isobutane and 1 , 1 -difluoroethane (HFC- 152a) A surprising ai id unexpected feature of this invention was that a blowing agent of isobutane and HFC- 152a would acceptably or substantially maintain the processability of a blowing agent comprising solely or entirely isobutane Further surprising and unexpected was that desirable physical properties were likewise maintained to an acceptable or substantial degree Another surprising and unexpected feature was enhanced compressive recovery over a corresponding foam structure blown solely or entirely with isobutane

Further according to the present invention, there is a process for making an ethylenic polymer foam structure The process comprises a) heating an ethylenic polymer material to form a melt polymer material, b) incorporating into the melt polymer material at an elevated pressure a blowing agent to form a foamable gel, and c) expanding the foamable gel at a lower pressure to form a foam structure The ethylenic polymer material and the blowing agent are as described above

The novel features of the present invention and the context within which they are set will be better understood upon viewing the following specification together with the drawing

Figure 1 is a view of a representational comparative plot of the general compressive recovery behavior of ethylene polymer foam structures blown with blowing agents of isobutane and ιsobutane/HFC-152a Foam structures are generally prepared by heating an olefin polymer material to form a plasticized or melt polymer material, incorporating therein a blowing agent to form a - foamable gel, and extruding the gel through a die to form the foam product Prior to 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 may be incorporated or mixed into the melt polymer material by any means known in the art such as with an extruder, mixer, or blender The blowing agent is mixed with the melt polymer material at an elevated pressure sufficient to prevent substantial expansion of the melt polymer material and to generally disperse the blowing agent homogeneously therein Optionally, a nucleator may be blended in the polymer melt or dry blended with the polymer material prior to plasticizing or melting The foamable gel is typically cooled to a lower temperature to optimize physical characteristics of the foam structure The gel may be cooled in the extruder or other mixing device or in separate coolers The gel is then extruded or conveyed through a die of desired shape to a zone of reduced or lower pressure to form the foam structure Conventional dies include slit dies and multioπfice dies 1 he zone of lower pressure is at a pressure lower than that in which the foamable gel is maintained prior to extrusion through the die The lower pressure may be superatmospheπc or subatmospheπc (vacuum or evacuated), but is preferably at an atmospheric level

5 The blowing agent of foam structures of this aspect of the present invention comprises isobutane and HFC-152a The blowing agent preferably comprises 5 to 95, more preferably 1 5 to 85, and most preferably 25 to 75 weight percent isobutane based upon the total weight of the blowing agent The blowing agent further preferably comprises 95 to 5, more preferably 85 to 15, and most preferably 75 to 25 weight percent of HFC-152a A most

10 preferred blowing agent comprises entirely isobutane and HFC-152a

It was surprising that HFC- 152a could be used in a blowing agent with isobutane and maintain at least acceptable levels of certain desirable processing and physical properties in end product foam structures heretofore obtainable only with isobutane Preferably, desirable processing and physical properties are substantially maintained with the use of a HFC-

15 152a/ιsobutane blowing agent compared to a blowing agent comprised entirely of isobutane on an equimolar basis (the total number of moles of isobutane and HFC-152a in the blowing agent being equal to the number of moles of isobutane in a corresponding blowing agent comprising entirely isobutane) Which processing and physical properties are desirable will vary according to the characteristics of a desired end product foam structure, and the HFC-

20 152a/ιsobutane blowing agent need not offer advantageous performance in every property Processing properties important in most conventional applications include foaming temperature range or window and cross-sectional size Physical properties important in most conventional applications include skin quality, cell size, density, open-cell content, dimensional stability, and compressive recovery

25 It was further surprising the use of HFC- 152a with isobutane in a blowing agent resulted in enhanced compressive recovery in end product foam structures compared to foam structures produced with a blowing agent comprised entirely of isobutane on an equimolar basis Foam structures blown with isobutane and HFC-152a were found to exhibit typically greater compressive recovery and more consistent compressive recovery Enhanced

30 compressive recovery provides better cushioning performance Specifically, the use of HFC- 152a with isobutane provides a foam structure with more consistent dimensional stability after compression than with isobutane alone

Figure 1 is a representation of the general compressive recovery behavior sometimes exhibited by foam structures blown entirely with isobutane as well as the general

35 compressive recovery behavior of foam structures of the present invention blown with isobutane and HFC-152a Figure 1 is representational, and does not necessarily correspond exactly to the examples below Figure 1 represents the inconsistent dimensional stability after compressive recovery sometimes exhibited by foam structures blown entirely with isobutane Dimensional stability is represented as a function of foam thickness as a percentage of the initial thickness versus time After such foam structures are compressed to a substantial degree, such as to 80 percent of initial volume, the compression is released to allow re-expansion of the foam structure to a temporary volume ratio peak at time Ti T→\ will vary according to foam structure composition and process conditions, but typically ranges from 1 hour to 3 days After T-→ for reasons unknown, the foam structures expanded with isobutane may continue to shrink by 3 to 10 percent in dimensions until a minimum is reached at T₂ T will vary according to foam composition and thickness, but typically ranges from 2 weeks to 5 weeks After T₂, foam thickness slowly recovers, but may take months to recover Such a slow, gradual change in foam dimension after compression renders the foam structure not suitable for commercial use since a fabricated foam structure is typically used as cushion packaging within a week after fabrication (die cutting) Figure 1 further represents the general compressi e recovery behavior of foam structures of the present invention blown with isobutane and HFC- 152a As shown in Figure 1 , recovery after release from compression generally follows that of corresponding structures blown solely with isobutane until about time T- Foam structures of the present invention typically exhibit about the same or greater peak recovery at T-- than corresponding foam structures blown solely with isobutane The present structure may not shrink at all or may shrink to a lesser degree, that is, 3 percent or less

The present foam structure may be cross nked or non-cross nked, but is preferably substantially non-crosshnked or substantially free of crosslinking Substantially non- crosshnked is inclusive however, of the slight degree of crosslinking which may occur naturally without the use of crosslinking agents or radiation

The present foam structure has a density of 200 or less, more preferably 100 or less, and most preferably 10 to 70 kilograms per cubic meter according to ASTM D- 1622-88 The foam has an average cell size of 0 1 to 5 0 preferably 0 5 to 3 0, and most preferably from 0 2 to 1 8 millimeters according to ASTM D3576-77 The foam component of the present foam structure may be closed cell or open cell Open-cell content may vary from O to 100 percent according to ASTM D2856-A Preferably, the present foam is 50 percent or less open-cell and most preferably 20 percent or less according to ASTM D2856-A

The present foam may take the form of sheet, rods, tubes, planks, or coalesced- strand planks

Another aspect of the present invention relates to a low density, large cross- section olefmic polymer foam structure blown with a blowing agent having a large proportion of 1 ,1 -dιfluoroethane Such a foam is useful in cushion packaging applications It has heretofore not been possible to make an olefin polymer foam structure in large cross-section and relatively low density with large proportions of HFC- 152a HFC 152a has presented unique processing and extrusion problems because of its relatively low solubility in melts of olefin polymers It would be desirable to have a process for making olefin polymer foam structures of large cross-section and low density using a large proportion of HFC- 152a

According to the present i ven ion, tι isre is an βΛ ruded, unitary, closed-cell olefin polymer foam structure in plank form The foam structure has a cross-section in one dimension of 2 or more inches (5 or more centimeters) and 18 or more inches (46 or more centimeters) in the other dimension The olefin polymer material comprises greater than 50 percent by weight of olefin monomeric units The olefin polymer material has a melt index of 3 5 grams/10 minutes or less The foam structure has a blowing agent comprising 75 mole percent or more 1 , 1-dιfluoroethane (HFC- 152a) based upon the total moles of the blowing agent It was found surprising that HFC- 152a, which has relatively low solubility in olefin polymers, could be used to make olefin polymer foam structures of large cross-section and low density

Further according to the present invention, there is a process for making an extruded, closed-cell olefin polymer foam structure in plank form The foam structure has a cross-section in one dimension of 2 or more inches and 18 or more inches in the other dimension The process comprises the steps a) heating an olefin polymer material to form a melt polymer material, b) incorporating into the melt polymer material at an elevated pressure a blowing agent to form a foamable gel, c) cooling the foamable gel to an optimum foaming temperature, and d) extruding the foamable gel through a die to form the foam structure Preferably, it is extruded at a shear rate of 900/seconds (seconds ') or more The olefin polymer material and the blowing agent are as described above -

A surprising aspect of the present invention was that it was possible to make an olefin polymer foam structure of large cross-section and low density It was not expected that HFC-152a, with its relatively low solubility and melts of olefin polymers, had adequate solubility to provide the measure of blowing or expansion necessary to make a foam structure of large cross-section and low density

It was found that desirable foam structures could be made by proper selection of an olefin polymer resin with desirable melt viscosity characteristics (melt index) in combination with proper selection of shear rate at the extrusion die

An important aspect of the present invention is to use an olefin resin or olefin resin blend of relatively high melt viscosity The larger the cross-section, the higher the desirable range or permissible minimum of melt viscosity Resin melt viscosity is most conveniently expressed in terms of melt index As melt viscosity increases, melt index decreases For the purposes of this invention, melt index is measured according to ASTM D- 1238 at 190°C/2 16 kg

Another important aspect ot the present invention is having a shear rate at the extrusion die sufficiently high to form the desired large cross-section The larger the desired foam cross-section, the lower the minimum permissible shear rate Shear rate is a function of volumetric flow rate through the die and the geometry of the die ori fice For the purposes of this i nvention, shear rate = (6 X rale)/ (width X gap²), where " rate" is the volumetric flow rate, "width" refers to the width of the opening of the die orifice, and "gap" refers to the height of the opening of the die orifice A plank foam structure having a cross-section of 2 or more inches (5 or more centimeters) in one dimension and 18 or more inches (46 or more centimeters) in the other dimension can be made with an olefin polymer of a melt index of 3 5 grams/10 minutes or less at a shear rate of 900/seconds or more A foam structure having a cross-section of 2 or more inches (5 or more centimeters) in one dimension and 24 or more inches (61 or more centimeters) in the other dimension can be made with an olefin polymer of a melt index of 2 5 grams/10 minutes or less at a shear rate of 600/seconds or more A foam structure having a cross-section of 2 or more inches (5 or more centimeters) in one dimension and 32 or more inches (81 or more centimeters) in the other dimension can be made with an olefin polymer of a melt index of 0.8 grams/10 minutes or less at a shear rate of 500/seconds capable or more. A foam structure having a cross-section of 2 or more inches (5 or more centimeters) in one dimension and 48 or more inches ( 122 or more centimeters) in the other dimension can be made with an olefin polymer of a melt index of 0 6 grams/10 minutes or less at a shear rate of 400/seconds or more The shear rates above are preferred shear rates for a given cross-section In making the large cross-section foam structure, the secondary blowing agent comprises 25 mole percent or less of the total weight of the blowing agent Preferred secondary blowing agents include isobutane, n-butane, carbon dioxide, or mixtures of two or more of the foregoing A mixture of isobutane and carbon dioxide is especially preferred

The large cross-section foam structure has a density of 48 or less kilograms per cubic meter and most preferably from 24 to 44 kilograms per cubic meter according to ASTM D-3575 The foam has an average cell size of from 0 1 to 5 0 and preferably from 1 to 3 millimeters according to ASTM D3576-77

The large cross-section foam structure may be closed cell or open cell Preferably, the present foam is greater than 80 percent closed-cell according to ASTM D2856-A

Another aspect of the present invention relates to a process for making an extruded, coalesced strand olefin polymer foam structure of certain density with a blowing agent of 1 , -dιfluoroethane and any of isobutane, n-butane, and propane Such a foam structure is useful in cushion packaging applications It is difficult to make low density, coalesced strand olefin polymer foam structures, that is, 16 to 48 kilograms per cubic meter, with HFC- 152a because of foam collapse from excessive blowing agent loss HFC-152a has a relatively low solubility in melts of olefin polymers The low solubility renders it very susceptible to variation in foaming temperature High foaming temperatures will cause excessive blowing agent loss; thus, the foaming temperature must be carefully maintained to prevent excessive blowing agent loss

Maintaining low foaming temperatures is particularly difficult in making extruded, coalesced strand foam structures Making such foam structures involves extruding molten extrudate through a large number of relatively small holes or orifices in a multi-orifice die Much more shear heat is generated in such extrusion compared to extrusion through a conventional slit die The shear heat increases the temperature of the molten extrudate; thus, the foaming temperature is increased

One means of maintaining control of foaming temperature when making extruded, coalesced strand foam structures is to lower the temperature of the molten extrudate prior to entering the die However, this may result in "freezing" or plating out of the constituent polymer on the internal surfaces of processing and heat exchange equipment

It would be desirable to be able to make coalesced strand olefin polymer foam structures of low density with HFC-152a It would be further desirable to make such foam structures at low foaming temperatures According to the present invention, there is a process for making an olefin polymer foam structure of a plurality of coalesced, extruded strands of a foamed olefin polymer composition of a density of 16 to 48 kilograms per cubic meter The process comprises a) heating a olefin polymer material to form a melt polymer material; b) incorporating into the melt polymer material at an elevated pressure a blowing agent to form a foamable gel; c) cooling the foamable gel to an optimum foaming temperature; and d) extruding the foamable gel through a die having a plurality of orifices therein (multi-orifice die) to form a plurality of coalesced extruded strands or profiles of the foamed olefin polymer material forming the foam structure The blowing agent comprises a first blowing agent of 20 to 90 mole percent 1 , 1- dif luoroethane and 80 to 10 mole percent of a second blowing agent selected from the group consisting of isobutane, n-butane, and propane based upon the total moles of the blowing agents The foamable gel is extruded through the die at a rate sufficient to cause a temperature i ncrease of at least 1 °C i n the foamable gel as it proceeds from the entrance of the die to the exit of the multi-orifice die

Foam structures in coalesced strand form are formed by extrusion of the foamable gel through a multi-orif ice die The orifices are arranged so that contact between adjacent streams of the foamable gel occurs during the foaming process and the contacting surfaces adhere to one another with sufficient adhesion to result in a unitary foam structure The streams of foamable gel exiting the die take the form of strands or profiles, which desirably foam, coalesce, and adhere to one another to form the unitary structure Desirably, the coalesced individual strands or profiles of olefin polymer foam should remain adhered into the unitary structure to prevent strand delamination under stresses encountered in preparing, shaping, and using the foam Apparatuses and method for producing foam structures of coalesced strand form are seen in U S Patents 3,573, 152 and 4,824,720

Foam strands or profiles will vary in cross-sectional shape or geometry according to the shape or geometry of the orifices in the multioπf ice die The strands or profiles may be the same or different shape or geometry than the foam structure which they coalesce to form The orifices may take on a circular shape or a noncircular shape though circular is preferred o Suitable noncircular shapes include X-shaped, cross- or star-shaped, or polygonal-shaped The various orifices in the die may be spatially arranged in a desired configuration or array such as a sine wave, honeycomb, square saw tooth, or a triangular saw tooth wave pattern Preferably, the individual strands, have a major dimension in cross-section, diameter in the case of circular strands, of between 0 5 and 10 millimeters and most preferably between 1 0 and 5 0 5 millimeters

The orifices in the multioπfice die preferably will be of shape or geometry and be spatially arranged such that there will be sufficient channel volume or clearance between the streams of molten extrudate exiting from the same for them to foam to form the strands or profiles without substantial distortion of the resulting unitary foam structure relative to the 0 geometry of the overall arrangement of the orifices The streams of molten extrudate may foam to either partly or completely fill the open channel volume between the strands or profiles (open channel or closed channel)

Desirable olefin polymer materials may be employed to form foam structures of coalesced foam strands or profiles having a strand-to-strand tensile strength of at least 0 5 5 pounds force per inch length (lbf/ιn) (0 88 Newtons per centimeter (N/cm)) and preferably at least 2 0 Ibf/in (3 5 N/cm) The average strand-to-strand tensile strength is defined as the average tensile strength required to pull apart any two, given adjacent strands within the foam structure

In making the foam structure in coalesced strand form, the blowing agent 0 comprises a first blowing agent of 20 to 90 mole percent 1,1-dιfluoroethane and from 80 to 10 mole percent of a second blowing agent selected from the group consisting of isobutane, n- butane, and propane based upon the total moles of the blowing agent Preferably, the second blowing agent comprises 20 to 50 mole percent based upon the total moles of the blowing agent The second blowing agent functions to plasticize the foamable gel so that the gel may 5 be conveyed to the die at a lower temperature than possible without the second blowing agent The first blowing agent, HFC- 152a, has a relatively low level of solubility in the melt olefin polymer material; thus, foaming temperature must be carefully controlled to prevent excessive blowing agent loss upon extrusion through the die The foaming temperature is more difficult to control in a multi-orifice die compared to a conventional slit die because of the greater shear heating encountered in extruding the foamable gel through a large number of relatively small holes or orifices The shear heating results in foaming temperature increases of 1°C-3°C upon extrusion through the die Temperature increases of 1⁰C-3°C are very significant because of the crystalline nature of olefin polymers, which have a relatively narrow foaming temperature window Foaming temperature increases cause a problem when extruding foamable gel containing substantially or entirely HFC- 152a because of the gel's sensitivity to increased foaming temperatures

One means of addressing the problem of increased foaming temperatures is to o decrease the temperature of the foamable gel conveyed to the die Decreasing the temperature of the gel can, however, cause "freezing" in the gel " Freezing" causes polymer to plate out on heat removal equipment reducing its efficiency and resulting in an increase in polymer temperature rather than the desired decrease

Any of the second blowing agents, isobutane, n-butane, or propane, may 5 plasticize the melt to the olefin polymer material to a degree sufficient so that the temperature of the foamable gel conveyed to the multioπfice die can be lowered enough to prevent excessive loss of HFC- 152a upon foaming without "freezing" Also, the second blowing agent may increase operable foaming temperature range or "foaming window" as referred to in the art o Suitable foam structures in coalesced strand form have gross densities (that is bulk densities or densities of the closed-cell foam including interstitial channels or voids between strands or profiles), preferably varying from 0 2 to 3 pounds per cubic foot (pcf) (3 2 to 48 kilograms per cubic meter (kgm)) according to ASTM 1622-88 Most preferred foam structures have a density from 0 5 to 2 8 pcf (8 0 to 45 kgm) For specific uses in low weight cushioning 5 applications a preferable alternate embodiment comprises portions having densities less than 2 pcf (32 kgm) The individual strands of foam comprising the foam structure preferably possess a local or strand density from 1 0 to 3 0 pcf (8 0 to 96 kgm), and most preferably from 1 5 to 2 0 pcf ( 16 to 48 kgm)

Foam strands comprising coalesced strand structures having an average ceil size 0 of between 0 10 to 1 2 millimeters and preferably between 0 4 and 1 0 millimeters according to ASTM D3576-77

In the coalesced strand foam structure, preferably at least 70 percent of the total number of cells in the foam are closed-cell according to ASTM D-2856A not including interstitial channels or voids between the foam strands comprising the foam structure Suitable olefin polymer materials include olefinic homopol' ers and copolymers of olefinic compounds and copolymeπzable olefinically unsaturated comonomers The olefinic polymer material may further include minor proportions of non-olefinic polymers The olefinic polymer material may be comprised solely of one or more olefinic homopolymers, one or more olefinic copolymers, a blend of one or more of each of olefinic homopolymers and copolymers, or blends of any of the foregoing with a non-olefinic polymer Regardless of composition, the olefinic polymer material comprises greater than 50 and preferably greater than 70 weight percent of olefinic monomeric units Preferably, the olefinic polymer material comprises greater than 50 and more preferably greater than 70 weight percent of ethylene monomeric units (ethylene polymer material) Most preferably, the olefinic polymer material is comprised completely or entirely of olefinic monomeric units A most preferred olefinic polymer is a polyethylene homopolymer Polyethylenes may be of the high, medium, low, linear low, or ultra-low density type Low density polyethylene is most preferred The polyethylenes may be o linear, branched, or lightly cross-linked Polypropylene is another useful olefin polymer

Suitable olefinic copolymers may be comprised of olefinic monomeric units and minor amounts, preferably 20 percent or less by weight, of a monoethylenically unsaturated compounds copolymenzable therewith Suitable comonomers include C _6 alkyl acids and esters, lonomeπc derivatives, C₄ 6 dienes, and C₃ 9 olefins Examples of suitable comonomers 5 include acrylic acid, itaconic acid, maleic acid, methacryhc acid, ethacryhc acid, methyl acrylate, methyl methacrylate, ethyl acrylate, vinyl acetate, carbon monoxide, maleic anhydride, acrylonitπle, propylene, isobutylene, and butadiene

Blowing agents other than those described above may be employed in minor amounts Such blowing agents include inorganic agents, organic blowing agents and chemical 0 blowing agents Suitable inorganic blowing agents include carbon dioxide, nitrogen, argon, water, air, nitrogen, and helium Organic blowing agents include aliphatic hydrocarbons having 1-9 carbon atoms, aliphatic alcohols having 1-3 carbon atoms, and fully and partially halogenated aliphatic hydrocarbons having 1-4 carbon atoms Aliphatic hydrocarbons include methane, ethane, n-pentane, isopentane, and neopentane Aliphatic alcohols include 5 methanol, ethanol, n-propanol, and isopropanol Fully and partially halogenated aliphatic hydrocarbons include fluorocarbons, chlorocarbons, and chlorofluorocarbons Examples of fluorocarbons include methyl fluoride, perfluoromethane, ethyl fluoride, 1 , 1 , 1 -tπfluoroethane (HFC-143a), 1 , 1 , 1 ,2-tetrafluoro-ethane (HFC-134a), pentafluoroethane, difluoromethane, perfluoroethane, 2,2-dιfluoropropane, 1 , 1 , 1-trιfluoropropane, perfluoropropane, 0 dichloropropane, difluoropropane, perfluorobutane, perfluorocyclobutane Partially halogenated chlorocarbons and chlorofluorocarbons for use in this invention include methyl chloride, methylene chloride, ethyl chloride, 1 , 1 , 1 -trιchloroethane, 1 , 1 -dιchloro- 1 -fluoroethane (HCFC- 141 b), 1 -chloro- 1 , 1-dιfluoroethane (HCFC-142b), chlorodιfluoromethane (HCFC-22), 1 , 1-dιchloro-2,2,2-trιfluoroethane (HCFC-123) and 1 -chloro- 1 ,2,2,2-tetrafluoroethane (HCFC- 5 124) Fully halogenated chlorofluorocarbons include trιchloromonofluoromethane (CFC-1 1 ), dichlorodif luoromethane (CFC-12), trichlorotπf luoroethane (CFC- 1 13), 1 , 1 , 1 -tπf luoroethane, pentafluoroethane, dichlorotetrafluoroethane (CFC-1 14), chloroheptafluoropropane, and dichlorohexafluoropropane Chemical blowing agents include azodicarbonamide, azodnsobutyro-nitrile, benzenesulfonhydrazide, 4,4-oxybenzene sulfonyl-semicarbazide, p- toluene sulfonyl semi-carbazide, barium azodicarboxylate, N,N'-dιmethyl-N,N dinitrosoterephthalamide, and tπhydrazino tπazine

The amount of blowing agent incorporated into the polymer melt material to make a foam-forming polymer gel is from 0 2 to 5 0, preferably from 0 5 to 3 0, and most preferably from 1 0 to 2 50 moles per kilogram of polymer

Preferred foam structures exhibit excellent dimensional stability Preferred foams structures recover 80 or more percent of initial volume within a month with initial volume being measured within 30 seconds after extrusion Volume may be determined by the cubic displacement test in water

It is desirable to add a stability control agent to the foam structures of the present invention to enhance dimensional stabili ty Suitable agents include any of those known in the art Preferred agents include amides and esters of Cio ₂ fatty acids Most preferred agents include stearyl stearamide and glycerol monostearate Various additives may be incorporated in the present foam structure such as inorganic fillers, pigments, antioxidants, acid scavengers, stability control agents, ultraviolet absorbers, flame retardants, processing aids, and extrusion aids

In addition, a nucleating agent may be added in order to control the size of foam cells Preferred nucleating agents include inorganic substances such as calcium carbonate, talc, clay, titanium oxide, silica, barium sulfate, diatomaceous earth, and mixtures of citric acid and sodium bicarbonate The amount of nucleating agent employed may range from 0 01 to 5 parts by weignt per hundreα parts oy weignt of a polymer resin Tne preferreα range is from 0 1 to 3 parts by weight

The following are examples of the present invention, and are not to be construed as limiting Unless otherwise indicated, all percentages, parts, or proportions are by weight Example 1 and Control Example 1

An ethylene polymer foam structure of the present invention was prepared with a blowing agent of HFC- 152a and isobutane Control foam structures prepared with a blowing agent of either 1 , 1 -dif luoroethane (HFC- 152a) or isobutane (ιC₄) were also prepared The present foam structure and the control structures were compared for processability and physical properties

The apparatus used was a 25 mm (1 inch) screw type extruder having additional zones for mixing and cooling at the end of usual sequential zones for feeding, metering, and mixing An injection port for the blowing agent was provided between the metering and mixing zones A die having a rectangular orifice was attached at the end of the cooling zone The height of the orifice, hereinafter called the die gap, was adjustable while its width was fixed at 3 68 mm (0 145 inch) A granular low density polyethylene (LDPE) resin having a melt index (ASTM D- 1238 190°C/2 16 kgs) of 1 8 dg/min (decigrams/minute) and a density of 0 923 g/cmJ was pre- blended with a concentrate of glycerol monostearate (GMS) to yield an effective GMS level of 1 3 parts per hundred (pph) and a small amount (0 02 pph) of talcum powder to form a solid mixture The solid mixture was fed to the extruder, and extruded at a uniform rate of 2 3 kilograms per hour (kgs/hr) (5 pounds per hour (lbs/hr)) The temperatures maintained at the extruder zones were 100°C at the feeding zone, 160°C at the melting zone, 180°C at the metering zone, and 193°C at the mixing zone The blowing agent was uniformly injected at a rate so the blowing agent level was 1 3 gram-moles per kilogram of polymer (mpk) The temperature of the cooling zone was gradually reduced to cool the polymer/blowing agent mixture (foamable gel) to an optimum foaming temperature The die and the cooling zone were maintained at the same temperature

The present structure and the control structures were compared for foamabihty, an indicator of processability, by measuring the range or "window" of foaming temperature The foaming window was determined as follows Starting at the gel temperature where the foam rose and remained stabilized, a foam strand was saved at a critical die gap (a threshold die gap for pre-foaming) The gel temperature was then dropped one degree and another foam sample was taken The operation was repeated until there was an indication the cooling section was "froze off" The freeze-off (same as froze off) condition was indicated by deterioration of foam quality accompanied by a sharp increase of gel pressure entering into the cooling zone The foaming temperature window was defined as the range of foaming temperature providing an open cell content in the foam no greater than a specified value ( 10 percent open cell for the examples herein) The optimum foaming temperature wherein minimum foam density was achieved varied between 107°C- 109°C for the three tests The foam structures were aged for at least two weeks, and analyzed for cross- sectional size (area), cell size, density and open cell content Process and physical property data are set forth in Table 1

The present foam structure blown with the HFC- 152a/ιsobutane blowing agent (Test 1 1) was of good quality at a wide range of foaming temperatures Its foaming temperature window ( 13°C) was shown to be one degree wider than the control structure blown with isobutane alone (Test 1 2) and significantly greater than the control structure blown with HFC- 152a alone (Test 1 3) The present foam structure was very comparable with the control structure blown with isobutane alone in foam cross-section, density, cell size, and skin quality, and clearly superior in those physical properties to those control structures blown with HFC-152a alone It was very surprising the desirable processing and physical properties of the foam structure blown with isobutane alone could be substantially maintained in the foam structure blown with one-half by mole isobutane and one-half by mole HFC- 152a in view of the much less desirable processing and physical properties of the foam structure blown with HFC-

152a alone

Example 2 and Control Example 2

An ethylene polymer foam structure of the present invention was prepared with a blowing agent of HFC- 152a and isobutane A control structure blown with HFC-152a alone was also prepared The foam structures were compared for processability and physical properties

The polymers used were a 74/26 by weight blend of a LDPE resin having 0 919 g/crr density and 0 22 dg/min melt index and another LDPE having 0 919 g/cm^ density and 7 o dg/min melt index The polyethylenes were pre-blended with a concentrate of stearyl stearamide to yield an effective stearamide level of 1 3 pph No nucleating agent was employed The blowing agent was employed at a level of 1 3 mpk

The foam structure blown with a 75/25 by mole HFC- 1 52a/ιsobutane was compared with a control foam structure blown entirely with HFC-152a Properties were 5 determined substantially as in Example 1 Process and foam structure property data are set forth in Table 2

The present foam structure blown with HFC-152a/ιsobutane had significantly better processing and physical properties than the control structure blown with only HFC- 152a

The control structure had an undesirably small cell size even though no nucleating agent was 0 used The control structure had an undesirably small cross-sectional size, relatively high open cell content (greater than 10 percent), and virtually no foaming window (zero) The present structure exhibited substantially better performance in most of these properties

Example 3 and Control Example 3

Ethylene polymer foam structures of the present invention were prepared with a 5 blowing agent of varying proportions of HFC- 152a and isobutane Control foam structures were prepared with blowing agents of either HFC-152a or isobutane alone The

HFC- 152a/ιsobutane mixed blowing agent provided the present foam structures with better compressive recovery than one blown with isobutane alone

The apparatus used was a 3 50 inch (8 9 cm) screw-type extruder having 0 substantially the same configuration as the extruder of Example 1 The apparatus was equipped with a gap-adjustable slit die with 2 25 inch (5 72 cm) width at the end of the cooling zone

The polymer used was a 75/25 blend by weight of a granular LDPE resin having

0 22 dg/min melt index and 0 919 g/cm^J density and another granular LDPE resin having 8 5 dg/min melt index and 0 919 g/cm^ density The polyethylene blend was fed into the extruder together with 1 pph stearyl stearamide (in concentrate form), 0 13 pph Hydrocerol CF-5 brand nucleating agent (Boehπnger Ingelheim K G , Germany) and 0 03 pph Irganox 1010 brand antioxidant (Ciba-Geigy Corp ) at a uniform rate of 400 Ibs/hr ( 182 kgs/hr) The temperatures maintained at the extruder zones were 100°C at the feeding zone, 120 'C and 140"C at the transition zones, 165°C and 180"C at the melting zones, 220°C at the metering zone, and 200^UC at the mixing zone The blowing agents were injected at rates such that total blowing agent content was 6 8 pph Cooling zone temperatures were adjusted to maintain optimum foaming temperatures of 1 12°C for Tests 3 1 through 3 4 and 1 13°C for Test 3 5 Physical property data is seen in Table 3

Foam structures of 39-42 mm thickness and 203-213 mm width formed at a fixed die gap of 0 13 inch were recovered Two specimens of 254-280 mm length from each test were examined for dimensional stability by periodically measuring the dimensions of the foams o Compressi e recovery tests were conducted upon foam structures aged at ambient temperature (72°F) for specific periods of time Foam structures were compressed to 80 percent of original thickness at 0 5 inches/minute rate, and allowed to recover During recovery, thickness was monitored periodically

Surprisingly, the present foam structures exhibited greater compressive recovery 5 than the control structure blown with isobutane alone The greater compressive recovery allows retention of foam structure dimensions after die cutting Example 4

Olefin polymer foam structures of large dimensions in plank form were blown with HFC- 152a according to the process of the present invention The olefin resins employed 0 were polyethylene homopolymers

Various polyethylene resins of various melt indexes were melted and mixed with a blowing agent to form a foamable gel, which was extruded through dies of various sizes to form olefin polymer foam structures The blowing agent comprised HFC- 152a Additives were 1 3 pph stearyl stearamide as a stabilty control agent and Hydrocerol CF-20 (Boerhinger 5 Ingelheim) as a nucleating agent, 0 03 pph of Irganox 1010 (Ciba-Geigy) as an anti-oxidant

The foamable gel of the polymer melt and the blowing agent was extruded through the dies at various shear rates Shear rate varied by adjusting the size of the dies and extrusion throughput rates The size of the dies was varied to form foams of various cross-sections The size (area) of the die was varied by adjusting the die gap 0 The surface (skin) quality of the foam structure was examined for quality Good foam structures exhibited relatively smooth, matte-like surfaces Fair foam structures exhibited matte-like surfaces with some bumps and surface irregularities Poor foam structures exhibited rough surfaces, some grooving or crevices, and larger irregularities Acceptable foam structures had fair or good surface quality Results are seen in Table 4 5 Example 5

An ethylene polymer foam structure in coalesced strand form was made according to the process of the present invention with a blowing agent of 75 pph HFC-152a and 25 pph isobutane The apparatus comprised an extruder and a multi-orifice die in series The die had 224 0 064 inch ( 1 62 millimeters (mm)) orifices therein

The polymer used was a blend of low density polyethylenes having an average melt index of 15 according to ASTM D 1238 ( 190^oC 2 16 kg) Additives used were glycerol monostearate (GMS) at 1 pph as a stability control agent and Hydrocerol CF-70 (Boehringer Ingelheim) at 0 3 pph as a nucleating agent The polymer and the addi tives were dry-blended, and fed to the extruder The polymer and additives were melted and blended in the extruder to form a molten, flowable melt

The blowing agent was injected into and homogeneously dispersed within the o flowable melt at an elevated temperature to form a foamable gel The foamable gel was cooled to an optimum foaming temperature, and conveyed through the die to form the foam structure The foaming temperature was 108 5^ϋC

The foam structure stabilized to a desirable density of 1 95 pcf (31 2 kgm) The structure formed without collapsing The foaming temperature was sufficiently low to prevent 5 excessive loss of blowing agent, and, thus, collapse Comparative Example 5

An ethylene polymer foam structure in coalesced strand form was made with substantially the same apparatus and procedure of Example 5 except the blowing agent was entirely HFC- 152a ( 10 pph) 0 The foam structure rapidly collapsed upon exiting the c ϊ to a stabilized density of 3 52 pcf (56 3 kgm), an undesirably high level Foaming temperatures from 106°C-1 10°C were attempted at 0 5^CC intervals, but all resulted in rapid loss of blowing agent and collapse Example 6

An ethylene polymer foam structure in coalesced strand form was made 5 according to the present invention with a blowing agent of HFC-152a and propane

A smaller-scale version of the apparatus of Example 5 was used except the multi- oπfice die had 13 0 060 inch ( 1 52 mm) holes therein The procedure used was substantially the same as in Example 5 except the polymer and additives were fed at a rate of 20 Ibs/hr (9 1 kg/hr), and the blowing agent was composed of 4 pph HFC-152a and 4 pph propane 0 The foam structure stabilized to a desirable density of 2 00 pcf (32 0 kgm) The structure formed without collapsing The foaming temperature was sufficiently low to prevent excessive loss of blowing agent, and, thus, collapse

While embodiments of the foam structure and the process for making it of the present invention have been shown with regard to specific details, it will be appreciated that 5 depending upon the manufacturing process and the manufacturer's desires, the present invention may be modified by various changes while still being fairly within the scope of the novel teachings and principles herein set forth Table 1 Processing and Physical Properties of the Foam Structure of Example 1 and Control Examples 1

Foam Cross-

Blowing Blowing Foam Minimum Sectional Cell Size^ Foaming

Test No Agent Agent Ratio¹ Density- Open Cell Skin Quality⁷ Area- (mm) Window' (°C)

Type (molar) (kgm) (percent) Urn²)

1.1 152a/iC₄ 50/50 1.2 38 1.6 0 13 G

1.2* 152a 100 0.8 42 0.6 9 1 A

1.3* iC₄ 100 1.3 39 2.0 0 12 G

10

* Not an example of this invention i Mixture ratio of blowing agent by mole (molar)

² Cross-sectional area of foam body in square centimeters (cm²) σ ³ Density of aged foam body in kilograms per cubic meters (kgm)

⁴ Cell size in millimeters (mm) determined per ASTM D3576-77

5 Minimum open cell content of the foam made during the temperature scan in percentage (%) determined per ASTM 2856-A

15 6 Window of foaming temperatures in degrees Celsius (°C) providing an open cell content no greater than 10 percent

⁷ Quality of foam body: G = good foam; A = acceptable

20

25

Table 2 Processing and Physical Properties of the Foam Structure of Example 2 and Control Example 2

Foam Cross-

Blowing Foam Minimum Foaming

Blowing Sectional Cell Size⁴

Test No. Agent Ratio¹ Density-' Open Cell^" Window Skin Quality^" Agent Type Area? (mm)

(molar) (kgm) (percent) (°C) (cm²)

2.1 152a/iC₄ 75/25 1.3 36 1.1 3 6 G

2.2* 152a 100 0.5 37 0.3 12 0 G

* Not an example of this invention

10 i Mixture ratio of blowing agent by mole (molar)

² Cross-sectional area of foam body in square centimeters (cm²)

3 Density of aged foam body in kilograms per cubic meters (kgm)

⁴ Cell size in millimeters (mm) determined per ASTM D3576-77

5 Minimum open cell content of the foam made during the temperature scan in percentage (%) determined per ASTM 2856-A

6 Window of foaming temperatures in degrees Celsius (°C) providing an open cell content no greater than 10 percent

7 Quality of foam body: G = good foam

15

20

25

Table 3 Processing and Physical Properties of the Foam Structures of Example 3 and Control Examples 3

Recovery after Compression

Dimensional iC/ι/152a Ratio^; Foam Density- Cell Size:-

Test No. Stability⁴ (molar) (kgm) (mm) (percent) 1 week^" 2 week? (percent) (percent)

3.1 * 100/0 40 1.7 100 86 89

3.2 75/25 40 1.8 99 90 91

3.3 50/50 40 1.5 100 93 94

10

3.4 25/75 41 1.4 100 95 96

3.5* 0/100 43 1.5 100 96 97

* Not an example of this invention

00 i Molar ratio of isobutane and HFC-152a blowing agents

² Density of aged foam body in kilograms per cubic meters (kgm)

15 3 Cell size in millimeters (mm) determined per ASTM D3576-77

⁴ Minimum volume of the foam body as a percentage (%) of the initial volume measured one hour after extrusion during aging at 72°F

6 Thickness of foam body as a percentage (%) of the original thickness one week after a one week-old foam was compressed to 80 percent of its thickness

6 Thickness of foam body as a percentage (%) of the original thickness one week after a two week-old foam was compressed to 80 percent of its thickness

20

25

TABLE 4 Surface (Skin) Quality of Foam Structures

Melt

Cross-Section Shear

Run Resin Blend Index Actual Surface

(inches) Rate No (percent) of Quality

[cm] (1/sec) Blend

1 (2x24) 742 Resin B 18 Fair-Good [51x61]

2 (2x24) 742 Resin B 18 Fair [51x61]

3 (2x24) 742 25% Resin C 05 Fair-Good [51x61] 75% Resin A

4 (2x24) 689 25% Resin C 05 Good [51x61] 75% Resin A

5 (2x24) 618 80% Resin B 12 Fair [51x61] 20% Resin A

6 (2x24) 618 25% Resin C 05 Fair [51x61] 75% Resin A

7 (2x24) 618 35% Resin B 05 Fair-Good [51x61] 65% Resin A

8 (2x325) 529 90% Resin B 15 Poor [51x83] 10% Resin A

9 (2x325) 529 50% Resin B 06 Fair-Good [51x83] 50% Resin A

10* (2x48) 487 Resin B 18 Poor [51x120]

11* (2x44) 458 Resin B 18 Poor [51x110]

12* (2x48) 439 70% Resin B 10 Poor [51x120] 30% Resin A

* Not an example of the present invention

-Melt Index in grams/10 minutes according to ASTM D-1238 (190^ϋC/216 kg) -Resin blends are percentages by weight based upon total weight of the polymer -Melt index is calculated by the following formula Log (melt index of blend) = Weight Fraction! X Log(Melt Index^ + Weight Fractιon₂ X Log(Melt lndex₂) -Resins A, B, and C are polyethylene homopolymers having melt indexes of 022, 18, and 60 respectively as determined by ASTM D-1238 (190°C/216 kg) -Resins A,B, and C have densities of 0921, 0923, 0924 grams/cubic centimeters, respectively

Claims

CLAIMS :

1 A process for making an ethylene polymer foam structure, comprising: a) heating an ethylenic polymer material comprising greater than 50 percent by weight ethylenic monomeric units to form melt polymer material ,

5 b) incorporating into the mel t polymer material at an elevated pressure a blowing agent to form a foamable gel ; and c) expanding the foamable gel at a lower pressure to form the foam structure, the process being characterized in that the blowing agent contains isobutane and

1 , 1 -difluoroethane. ^■ _| 0

2. The process of Claim 1 , wherein the foamable gel is cooled to an optimum foaming temperature and then expanded by extruding it through a die into a region of lower pressure to form the foam structure

3 The foam structure of Claim 1 or 2, wherein the blowing agent comprises 5 to

95 weight percent isobutane and 95 to 5 weight percent 1 , 1 -dif luoroethane, the blowing 15 agent weight percentages being based upon the total weight of the blowing agent.

4. The foam structure of Claim 1 or 2, wherein the blowing agent comprises 25 to 75 weight percent isobutane and 75 to 25 weight percent 1 , 1 -difluoroethane, the blowing agent weight percentages being based upon the total weight of the blowing agent.

5. The foam structure of Claim 1 or 2, wherein the foam structure is 20 percent or 20 less open-cell.

6. The foam structure of Claim 1 or 2, wherein the foam structure is 10 percent or less open-cell

7. The foam structure of Claim 1 or 2, wherein the blowing agent contains entirely isobutane and 1 , 1-difluoroethane

25 8 A foam structure obtainable according to the process of Claim 1 or 2

9 A process for making an extruded, closed-cell olefin polymer foam structure in plank form having a cross-section in one dimension of 5 or more centimeters and 46 or more centimeters in the other dimension and a density of 48 kilograms per cubic meter or less, comprising. 30 a) heating an olefin polymer material comprising greater than 50 percent by weight olefin monomeric units to form a melt polymer material ; b) incorporating into the melt polymer material at an elevated pressure a blowing agent to form a foamable gel , c) cooling the foamaDle gel to an optimum foaming temperature; and

35 d) extruding the foamable gel through a die to form the foam structure, the process being characterized in that the olefin polymer material has a melt index of 3.5 grams/10 minutes or less; the process being further characterized in that the blowing agent contains 75 percent or more by weight 1 , 1 -dιfluoroethane based upon the total moles of the blowing agent

10 The process of Claim 9, wherein the foamable gel is extruded through the die at a shear rate of 900/seconds or more 1 1 The process of Claim 9 or 10, wherein the foam structure has a cross-section in one dimension of 5 or more centimeters and 61 or more centimeters in the other dimension, and the olefin polymer material having a melt index of 2 5 grams/10 minutes or less

12 The process of Claim 1 1 , wherein the foamable gel is extruded through the die at a shear rate of 600/seconds or more 13 The process of Claim 9 or 10, wherein the foam structure has a cross-section in one dimension of 5 or more centimeters and 81 or more centimeters in the other dimension, and the olefin polymer material having a melt index of 0 8 grams/10 minutes or less

14 The process of Claim 13 wherein the foamable gel is extruded through the die at a shear rate of 500/seconds or more 15 The process of Claim 9 or 10, wherein the foam structure has a cross-section in one dimension of 5 or more centimeters and 122 or more centimeters in the other dimension, and the olefin polymer material having a melt index of 0 6 grams/10 minutes or less

16 The process of Claim 15, wherein the foamable gel is extruded through the die at a shear rate of 400/seconds or more ¹7 The process of Claim 9, wherein the blowing agent contains 90 mole percent or more of 1 , 1 -dif luoroethane based upon total moles of the blowing agent

18 The process of Claim 9 or 10, wherein the blowing agent contains a secondary blowing agent selected from the group consisting of isobutane, n-butane, carbon dioxide, or mixtures of any two of the foregoing 19 The process of Claim 9 or 10, wherein the foam structure has a density of 24 to

44 kilograms per cubic meter

20 The foam structure obtainable according to the process of Claim 9 or 10

21 A process for making an olefin polymer foam structure of a plurality of coalesced extruded strands of a foamed olefin polymer material of a density of 16 to 48 kilograms per cubic meter, comprising a) heating a olefin polymer material comprising more than 50 percent by weight olefinic monomeric units to form a melt polymer material, b) incorporating into the melt polymer material at an elevated pressure a first blowing agent to form a foamable gel, c) cooling the foamable gel to an optimum foaming temperature; and d) extruding the foamable gel through a die having a plurality of orifices therein to form a plurality of coalesced extruded strands or profiles of the foamed olefin polymer material forming the foam structure, the process being characterized in that the blowing agent contains from 20 to 90 mole percent 1 , 1-dιf luoroethane and from 80 to 10 mole percent of a second blowing agent selected from the group consisting of isobutane, n-butane, and propane based upon the total moles of the blowing agents

22 The process of Claim 21 , wnerein the foamable gel is extruded through the die at a rate sufficient to cause a temperature increase of at least 1°C in the foamable gel as it proceeds trom tne entrance oi tne die to tne exit of the die

23 The process of Claim 21 or 22, wherein a nucleating agent is incorporated into the melt polymer material

24 The process of Claim 21 or 22, wherein the blowing agent is incorporated into the melt polymer material at a concentration of from 0 2 to 5 0 moles per kilogram of melt polymer material

25 The process of Claim 21 or 22, wherein the second blowing agent comprises 20 to 50 mole percent based upon the total moles of the blowing agent

26 The process of Claim 21 or 22, wherein the second blowing agent is isobutane 27 The process of Claim 21 or 22, wherein the second blowing agent is n-butane

28 The process of Claim 21 or 22, wherein the second blowing agent is propane