INSULATING EXTRUDED FOAMS HAVING A MONOVINYL AROMATIC POLYMER WITH A BROAD MOLECULAR WEIGHT DISTRIBUTION

Abstract: This invention provides an insulating extruded thermoplastic polymer closed-cell foam having a thermal conductivity according to EN-13164 of about 35 mW/m. DEG K or less. The polymer is a monovinyl aromatic polymer with a broad molecular weight distribution. The foam contains blowing agent residuals from production of freshly formed cells of the foam using a blowing agent mixture that includes a primary blowing agent (one or more fluorine-containing carbon compounds and, optionally, carbon dioxide) and a secondary blowing agent (C1-4 alcohol, a C1-5 linear or cyclic hydrocarbon, an alkyl halide, water or a mixture thereof).

Description: INSULATING EXTRUDED FOAMS HAVING A MONOVINYL AROMATIC POLYMER WITH A BROAD MOLECULAR WEIGHT DISTRIBUTION This invention relates to an improved insulating extruded monovinyl aromatic polymer foam comprising a monovinyl aromatic polymer with a broad molecular weight distribution (MWD) of certain configuration, and to an improved process for making the foam. Historically, chlorofluorocarbons were employed as blowing agents in making monovinyl aromatic polymer foams for insulating applications. The foams provided excellent insulation performance of long duration. Due to concerns over potential ozone depletion, hydrochlorofluorocarbons (HCFCs), such as 1-chloro-1 ,1-difluoroethane (HCFC-142b), have largely replaced chlorofluorocarbons (CFCs) as a blowing agent in making monovinyl aromatic polymer foams for thermal insulation. Such foam applications provide excellent insulating performance while exhibiting substantially lower ozone depletion potential relative to foams made with CFCs. It is desirable to employ blowing agents with very limited, preferably zero, ozone depletion potential. Possible agents in the latter category include carbon dioxide (CO2) and 1 ,1 ,1 , 2-tetrafluoroethane (HFC-134a). A problem with using blowing agents such as CO2 or HFC-134a is that they exhibit high vapor pressure and low solubility, relative to CFCs, in melts of monovinyl aromatic polymers. As a result, extrusion foaming systems employing them can experience processing problems, such as high operating pressures and high die pressure, and product problems, such as poor skin quality, high density, and small foam cross-section resulting from very small cell sizes. U.S. Patent No. (USP) 5,011 ,866 discloses using HFC-134a or 1 ,1 ,1 trifluoroethane (HFC-143a), and optionally other blowing agents, to produce foam with good dimensional stability. USP 5,182,308 and EP-A 464,581 disclose a mixture of blowing agents containing CO2, ethanol, and an optional third component. USP 5,334,337 and EP A 543,242 disclose the use of a mixture of blowing agents including an alcohol or ketone, a fluorinated hydrocarbon and optional components. USP 5,650,106 discloses one approach to solving these problems by using a polystyrene with a broad MWD to produce foam at reduced operating pressures, increased cross-section and lower foam density relative to a conventional foam feedstock. EP 0 802 220 A2 discloses foamed plastic panels of a polystyrene (PS) thermoplastic matrix where the PS has a mean weight average molecular weight (M of from 175,000 and 500,000 and a MWD or M w /M n greater than (>) 2.6. Panel production uses a halogen-free blowing agent containing CO2, an ether and optional components. The aforementioned prior art processes generally produce foams with small cell size and poor insulating properties. It would be desirable to produce foam with good insulating qualities and dimensional stability with minimal environmental impact, while reducing processing problems associated with prior art attempts to achieve these goals. This invention provides an insulating, extruded, closed-cell, thermoplastic polymer foam having a thermal conductivity (TC) according to EN-13164 of about 35 mW/m. D K or less (<) comprising: (A) a polymer comprising, in polymerized form, about 50 percent or more by weight (wt%) monovinyl aromatic monomeric units, and having a mean weight average molecular weight (MJ of from 130,000 to 400,000, and a M w /M n of about 2.5 or greater (>); (B) blowing agent residuals from production of freshly formed cells of the foam by extruding a mixture of the polymer and a blowing agent mixture comprising: (i) a primary blowing agent comprising: (a) from about 30 to about 90 wt%, based on blowing agent mixture weight, of one or more fluorine-containing carbon compounds, and, optionally, (b) from > 0 to about 50 wt%, based on blowing agent mixture weight, (ii) from about 10 to about 70 wt%, based on blowing agent mixture weight, of a secondary blowing agent comprising an alcohol having from 1 to 4 carbon atoms(C-|_4), a linear or cyclic hydrocarbon having from 1 to 5 carbon atoms(C-|_5), alkyl halide, water or a mixture thereof. The CO2 and fluorine-containing carbon compounds are desirably present in a molar ratio of CO2 to such compounds < about 0.8. Further, this invention provides a process for producing such an extruded, polymer foam that comprises: a) producing a melt polymer material by heating a polymer comprising, in polymerized form, > about 50 wt%, based on weight of monovinyl aromatic monomeric units, and having a mean M w of from about 130,000 and 400,000, and a M w /M n > about 2.5 or greater; b) forming a foamable gel by incorporating into the melt polymer material at an elevated temperature a blowing agent mixture comprising: (i) a primary blowing agent comprising: (a) from about 30 to about 90 wt%, based on blowing agent mixture weight, of one or more fluorine-containing carbon compounds, and, optionally, (b) from > 0 to about 50 wt%, based on blowing agent mixture weight, co 2 ; (ii) from about 10 to about 70 wt%, based on blowing agent mixture weight, of a secondary blowing agent comprising a C-i .4 alcohol, a linear or cyclic C-| .5, alkyl halide, water or a mixture thereof; c) cooling the formable gel to a desirable foaming temperature; d) extruding the foamable gel through a die to form the foam. The scope of this invention also includes the foam produced by the aforementioned process. This invention involves using unique combinations of materials to produce extruded monovinyl aromatic polymer (preferably PS) foam structures, using a blowing agent mixture. The most preferred polymer, a broad MWD PS, and a preferred blowing agent mixture (comprising HFC-134a and/or another hydrogen containing fluorocarbons, CO2, an alcohol, and/or water, and/or a hydrocarbon) are mixed in an extruder or mixer at a temperature of 175-250° Celsius (°C). This mixture is then cooled in a controlled manner using a second extruder and/or heat exchangers before being allowed to expand in a controlled manner through a die at a temperature of 105-130°C. The PS desirably has a mean M w of from about 130,000 to about 400,000 and an M w /M n of > 2.5 (determined according to American Society for Testing and Materials (ASTM) 5296-92). A preferred PS has a mean M w of from about 130,000 to about 350,000. The use of these PS resins is desirable for the production of some materials, since it permits the production of desired foam cross-sections at low density and a high processing rate, even though the levels of a more soluble secondary blowing agent or processing agent are low. This PS is made with a non-symmetric distribution of molecular weights that provides a preferred melt strength for a very high melt flow PS. The PS has a melt flow rate (MFR) of 5-50 grams per 10 minutes (g/10 min) (ASTM-1238, Condition 200/5). The teachings of USP 5,650,106 are hereby incorporated by reference. Foams produced according to the present invention have low density and excellent long-term thermal conductivity, desirably < about 35 mW/m.°K (milli-watt per meter per degree Kelvin), preferably < about 30 mW/m.°K, as determined with the slicing technique according to the test method EN-13164. The foams of this invention may be closed cell or open cell. Preferred foams of this invention that are suitable for use in some conventional insulation applications are primarily closed cell in nature. Conventional PS resins used in making foamed sheet and plank often have narrow MWDs and low melt flow rates (MFR<5 g/10 min). Such resins require high levels of blowing agents to give products with reasonable cross-sections. However, such blowing agent levels typically result in a high open cell content with poor insulating properties (relative to closed-cell foams). A low production rate is an additional penalty. However, if the foam is to be used in some other application, such as a vacuum insulation panel core, preferred foams are primarily open celled. Fluorinated hydrocarbons, particularly HFC-134a, HFC-134, HFC-32, HFC-245fa, HFC-365mfc, HFC-227, HFC-161 , HFC-152a, HFC-125 or HFC-143a and the non-fluorinated blowing agent, CO2, are known to have limited solubility in monovinyl aromatic polymers, particularly at the foaming temperature range, making their use with conventional PS resins limited. That is to say, foams can be made, but the cross-sectional area achieved is less than desirable and difficult to control due to very small cell size formation. Processing aids, such as alcohols, alkyl halides, hydrocarbons or mixtures thereof, can be added to the process as secondary blowing agents, and these help to improve resulting cross-sectional areas. However, high levels of, for example, alcohol, result in a product that requires long periods of curing before mechanical and thermal stability is achieved, and also results in undesirably high emission levels and flammability concerns during processing, storage, and foam use. By using a PS resin with a broad MWD or polydispersity and low levels of the secondary blowing agents, it is possible to produce products of high thickness range with a closed cell structure and acceptable density. Foam made with a PS resin that has a MWD > 2.5 will give improved physical properties over foam made with a PS resin with a polydispersity < 2.5. Desirably, the blowing agent mixture is used in a concentration of from about 0.08 gram-moles to about 0.2 gram-moles per 100 grams of polymer (gmph). The sum of gram-moles of fluorine-containing carbon compounds and CO2 is desirably < about 0.09 gmph, preferably < about 0.085 gmph. In one preferred embodiment of this invention, the foam comprises a linear, broad MWD monovinyl aromatic polymer having an M w of from about 130,000 to about 350,000. In meeting this MWD limitation, greater than (>) 5 wt%, preferably > 10 wt%, of the polymer's chains have a M w of from 500,000 to 1 ,000,000, based upon the total weight of monovinyl aromatic polymer chains. In addition, less than (<) 5 wt% of the polymer's chains have M w > 1 ,000,000, based upon the total weight of monovinyl or alkenyl aromatic polymer chains. For purposes of polymer chain wt% calculations, polymer chains are polymer molecules that have at least ten monovinyl aromatic monomeric units. Residual monomer and oligomer content are excluded (not included) for calculation purposes. The polymer preferably has a Z-average molecular weight (M z ) of from about 200,000 to about 900,000 and more preferably from about 250,000 to about 600,000. The polymer preferably has a M w /M n of from about 2.5 to about 10, more preferably from about 2.7 to about 7. The polymer preferably has a M z /M w ratio of from about 1.5 to about 5, more preferably form about 1.8 to about 4. The polymer preferably exhibits a MFR of from about 6 to about 99, more preferably from about 15 to about 60, g/10 min (ASTM 1238, Condition 200/5). M w and M z are both determined according to size exclusion chromatography (SEC) using ASTM 5296-92. The linear, broad MWD monovinyl aromatic polymer useful in foams of the present invention may be prepared by blending two or more linear monovinyl aromatic polymers of different MWDs or by in-situ polymerization. Advantageously, the broad MWD polymer may be formed by feeding a previously polymerized higher molecular weight polymer dissolved in monomer into a polymerization zone or reactor wherein an intermediate weight polymer is produced in the presence of the high molecular weight polymer. Such polymerizations are described in USP 4,585,825, the relevant teachings of which are incorporated herein by reference. By way of illustration, a broad MWD linear monovinyl aromatic polymer is shown in FIG. 3 of USP 4,585,825. It was found surprising that an extruded foam could be made at lower system pressure drop and die pressure with the broad MWD, linear monovinyl aromatic polymer described above compared to conventional monovinyl aromatic polymers of equivalent M w typically employed commercially to produce foams. It was further found surprising that foams of lower density and larger cross-section could be produced compared to foams made with the conventional monovinyl aromatic polymers. Additionally, it was found surprising that specific relative amounts of the desired non-ozone depleting blowing agents (HFCs and CO2) are required to achieve desired foam closed cell content and insulating performance. The unique broad MWD of the linear monovinyl aromatic polymer may provide the surprising extrusion foaming performance. A majority distribution of polymer chains with a M w of from about 25,000 to about 100,000 provides a polymer in melt form that exhibits a low shear viscosity relative to conventional PS feedstocks for foam production. This shear viscosity affords a reduction in system pressure drop and die pressure, particularly when employing blowing agents of relatively high vapor pressure and low solubility in melts of monovinyl aromatic polymers as exemplified by CO2 and HFC-134a under typical process temperatures and pressures. The distribution of polymer chains with a M w > 500,000 provides greater (relative to the above lower Mw polymer chains and conventional PS feedstocks) polymer elongational viscosity in melt form, which affords relatively low density and large cross-section in the resulting foam product. The relatively high elongational viscosity of the melt polymer affords greater (relative to the above lower M w polymer chains and conventional PS feedstocks) melt strength and, subsequently, improved foam expansion of the melt out of the die prior to the cooling and cessation of expansion. The distribution of polymer chains with a M w > 1 ,000,000 is limited to minimize or prevent flow stabilization problems arising from excess elongational viscosity. Monovinylidene aromatic polymers are those comprising at least a major portion of an addition polymerized monomer of the formula: Table 1 ( * )=Comparative not an object of the present invention. The foams produced with conventional PS resin, narrow distribution and polydispersity of < 2.5 exhibited a very high amount of open cell, and having poorer thermal insulation performance, while the foams made with broader MWD polymer (M w /M n > 2.5) had a very low open cell, and excellent thermal conductivity. All foams were made with total moles of HFC-134a and CO2 < 0.085 gmph to show that the resin has an influence on the properties of the foam. Ex 2 There is a limit to the total moles of the HFC and CO2 blowing agent mixture that can be used to make a suitable foam such as that of Samps 3 and 4. This Ex shows that even with PS resin having a polydispersity of > 2.5, foam with suitable properties cannot be made if the total moles of HFC-134a and CO2 exceed 0.090 gmph. Samp 5, an example of the invention, was made with a PS resin of M w = 150,000 and M w /M n = 3.29. The physical property testing shows that very desirable foam properties can be produced with this blowing agent level. Samp 6, also an example of the invention, was made with a PS resin of M w = 200,000 and M w /M n = 2.75. Both examples used a broad MWD PS resin with total moles of HFC-134a + CO2 < 0.09 gmph and a mole ratio of CO2/HFC-134a < 0.8 to reach a desirable long term thermal conductivity. Comps 7 and 8 were also made with a broad MWD resin but with a sum of HFC-134a + CO2 moles > 0.090 gmph and a CO2 /HFC-134a mole ratio > 0.8. The data in Table 2 show that the foams of Samps 5 and 6 had better thermal conductivity results than the foams of Comps 7 and 8. See the sliced Lamda data to confirm this point Table 2 (*)=Comparative not an object of the present invention. Ex 3 In these experiments, PS having a M w of 147,000 and a M w /M n =3.3 was added to a 2-inch (50 mm) single-screw extruder without a fire retardant, but with other typical additives and melted prior to injection and mixing of the blowing agents. The resulting mixture was allowed to cool and converted to foam as in Ex 1. The mixture included isopropanol as part of the blowing agent mixture. The physical properties of the foam were measured and reported in Table 3. Table 3 ( ) = Comparative, not part of the present invention. The data in Table 3 illustrate several points. First, the data show that isopropanol, either alone or in admixture with ethanol, effectively substitutes for ethanol. Second, the Comps show that use of a HFC/CO2 combination > 0.090 gmph and a molar ratio of CO2 to HFC > 0.8 yields a foam with an unacceptable thermal conductivity. Third, even in the absence of CO2, an HFC content > 4.8 pph (7.5 pph in Samps 10-12) yields a foam with an acceptable thermal conductivity. Ex 4 A series of styrenic polymer foams was prepared using the apparatus of Ex 3 and the compositions and process parameters shown in Table 4. Table 4 also summarizes foam property data for such foams. A primary feedstock consisting of a PS was fed into the extruder, along with other formulation additives including 2.7 pph (part per hundred part of resin) of fire retardant hexabromocyclododecane (HBCD) and 0.9 pph of additives, composing of extrusion aid, dye and acid scavenger. Comp 13 ' was made with a narrow MWD PS, having M w =136,000 and M w /M n =2.3. The blowing agent mixture contained HFC-134a (67 %), HFC-32 (11 %) and pentane isomers (22 %). The total HFC content was 0.078 gmph . Comp 14 ' was made with a broad MWD PS, having M w =147,000 and M w /M n =3.3, but the blowing agent mixture was a mixture of HFC-134a (67 %), HFC-32 (22 %) and pentane isomers (11 %). The total HFC content was 0.097 gmph. Comp 15 ' was made with a broad MWD PS, having M w =147,000 and M w /M n =3.3, but the blowing agent mixture contained HFC-134a (76 %) and HFC-32 (24 %). The total HFC content was 0.102 gmph. Samp 16 was made with the blowing agent composition of Comp 13 and the PS of Comp 16 ' , that is, with a broad MWD. Samp 17 was made with the broad MWD PS, using a mixture of HFC-134a/C5 : 65/35 percent by weight. Samp 18 was made with the broad MWD PS, using a mixture of HFC-134a (74 %), pentane isomers (16 %) and ethanol (10 %). The total HFC-134a content was 0.069 gmph. Samp 19 was made with the broad MWD PS, using a mixture of HFC-134a (70 %), pentane (12 %) and propane isomers (18 %). The total HFC-134a content was 0.059 gmph. Samp 20 was made with the broad MWD PS, using a mixture of HFC-134a (74 %), CO2 (5 %) and ethanol (21 %). The total HFC-134a content was 0.080 gmph. As can be seen, the Comps produced with a conventional PS (narrow MWD) or with a high level of HFCs+ CO2 (>0.09 mph), exhibited a very high open cell content, and poor thermal conductivity despite smaller cell size. The foams made with broad MWD PS and with a lower level of HFCs + CO2 moles (< 0.090 gmph) presented excellent thermal conductivity and very low open cell content. The data for such foams also show very good mechanical properties and excellent thermal dimensional stability. The foams are suitable for use as insulating material. Note that all Samps used a mole ratio of CO2 /HFC-134a of < 0.8. Table 4

Claims: 1 . An insulating, closed-cell, extruded thermoplastic polymer foam having a thermal conductivity according to EN-13164 of about 35 mW/m.°K or less comprising: (A) a polymer comprising, in polymerized form, about 50 percent or more by weight monovinyl aromatic monomeric units, and having a mean weight average molecular weight of from about 130,000 and 400,000, and an Mw/Mn of about 2.5 or greater; (B) blowing agent residuals from production of freshly formed cells of the foam by extruding of a mixture of the polymer and a blowing agent mixture comprising: (i) a primary blowing agent comprising: (a) from about 30 to about 90 percent by weight, based on blowing agent mixture weight, of one or more fluorine-containing carbon compounds, and, optionally, (b) from greater than 0 to about 50 percent by weight, based on blowing agent mixture weight, carbon dioxide; (ii) from about 10 to about 70 percent by weight, based on blowing agent mixture weight, of a secondary blowing agent comprising an alcohol having from 1 to 4 carbon atoms, a linear or cyclic hydrocarbon having from 1 to 5 carbon atoms, alkyl halide, water or a mixture thereof. 2. The foam of Claim 1 , wherein the blowing agent mixture is used in a concentration of from about 0.08 gram-moles to about 0.2 gram-moles per 100 grams of polymer. 3. The foam of Claim 1 , wherein the sum of gram-moles of fluorine- containing carbon compounds and carbon dioxide is equal to or less than about 0.09 gram- moles per 100 grams of polymer. 4. The foam of Claim 1 , wherein a molar ratio of moles of carbon dioxide to moles of fluorine-containing carbon compound is about 0.8 or less. 5. The foam of Claim 1 , wherein fluorine-containing carbon compounds are present in the blowing agent mixture in an amount of 4.8 parts or more per hundred parts of polymer. 6. The foam of Claim 1 , wherein carbon dioxide is present in an amount of from 0.1 to about 20 percent by weight of the blowing agent mixture. 7. The foam of Claim 1 , wherein the fluorine-containing carbon compound is selected from methyl fluoride, difluoromethane (HFC-32), perfluoromethane , ethyl fluoride (HFC-161 ), 1 ,1 -difluoroethane (HFC-152a), 1 ,1 ,1 -trifiuoroethane (HFC-143a), 1 ,1 ,1 ,2-tetrafluoroethane (HFC-134a), 1 ,1 ,2,2-tetrafluoromethane (HFC-134), pentafluoroethane (HFC-125), perfluoroethane, 2,2-difluoropropane (HFC-272fb), 1 ,1 ,1-trifluoropropane (HFC-263fb), 1 ,1 ,1 ,3,3-pentafluoropropane (HFC-245fa), heptafluoropropane (HFC-227ca and HFC-227ea), 1 ,1 ,1 ,3,3-pentafluorobutane (HFC-365mfc), perfluoropropane, perfluorobutane or perfluorocyclobutane. 8. The foam of Claim 1 , wherein the fluorine-containing carbon compound is 1 ,1 ,1 ,2-tetrafluoroethane (HFC-134a). 9. The foam of Claim 1 , wherein the secondary blowing agent is methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, cyclobutane, cyclopentane, methanol, ethanol, n-propanol and isopropanol, or a mixture thereof. 10. The foam of Claim 1 , wherein the foam has a closed cell content of about 90 percent or greater as measured by ASTM D-2856. 1 1 The foam of Claim 1 , wherein the foam has a cross sectional area in the direction of extrusion of about 50 crτι2 or greater. 12. The foam of Claim 1 , wherein the foam has a thickness of from about 5 mm to about 250 mm. 13. The foam of Claim 1 , wherein the foam has cells with an average cell size of from about 0.07 to about 1.4 mm. 14. The foam of Claim 1 , wherein the foam further comprises an infrared attenuator. 15. The foam of Claim 14, wherein the infrared attenuator is selected from carbon black, graphite, aluminum flake or titanium dioxide. 16. The foam of Claim 1 , wherein the foam has a density of from about 10 to about 150 kg/m 3 (kilograms per cubic meter) according to ASTM D-1622-88. 17. A process for producing the extruded, monovinyl aromatic polymer foam of Claim 1 comprising: a) producing a melt polymer material by heating a polymer comprising, in polymerized form, about 50 percent or more by weight monovinyl aromatic monomeric units, and having a mean weight average molecular weight of between about 130,000 and 400,000, and an Mw/Mn of about 2.5 or greater; b) forming a foamable gel by incorporating into the melt polymer material at an elevated temperature a blowing agent mixture comprising: (i) a primary blowing agent comprising: (a) from about 30 to about 90 percent by weight, based on blowing agent mixture weight, of one or more fluorine-containing carbon compounds and, optionally, (b) from greater than 0 to about 50 percent by weight, based on blowing agent mixture weight, carbon dioxide; (ii) from about 10 to about 70 percent by weight, based on blowing agent mixture weight, of a secondary blowing agent comprising an alcohol having from 1 to 4 carbon atoms, a linear or cyclic hydrocarbon having from 1 to 5 carbon atoms, alkyl halide, water or a mixture thereof; c) cooling the formable gel to a desirable foaming temperature; d) extruding the foamable gel through a die to form the foam. 18. A foam produced by the process of Claim 17.

Inventors: DUFFY JOHN D, MASON JEFFREY J, PAQUET ANDREW N, VO CHAU V, DUFFY, JOHN, D., MASON, JEFFREY, J., PAQUET, ANDREW, N., VO, CHAU, V.
Applicants: DOW CHEMICAL CO, DUFFY JOHN D, MASON JEFFREY J, PAQUET ANDREW N, VO CHAU V, DUFFY, JOHN, D., MASON, JEFFREY, J., PAQUET, ANDREW, N., THE DOW CHEMICAL COMPANY, VO, CHAU, V.
Application year: 2000
Main classification: C08J9
IPC codes: B29C47, B29K25, B29K105, C08J9