INSULATIVE PLASTIC COMPOSITE STRUCTURE

Abstract: Disclosed is a coextruded, insulative, composite structure of adjacent, alternating first and second layers wherein the first layer comprises a plastic foam and second layer of a plastic material containing a quantity of carbon black sufficient to reduce the thermal conductivity of and to enhance the insulating performance of the composite structure. Incorporation of the carbon black into the plastic material layers allows an insulative foam structure to be made while avoiding or reducing serious processing problems encountered when incorporating carbon black into plastic foams. Further disclosed is a process for making the above composite structure. Further disclosed is a composite plastic flow melt expandable to form the above composite structure.

Description: INSULATIVE PLASTIC COMPOSITE STRUCTURE The invention relates to a plastic foam composite structure having enhanced insulative performance. Many methods of enhancing the insulating performance of plastic foams have been taught in the literature. These methods include, among others, use of an insulating gaseous blowing agent, reduction in foam cell size, and incorporation of various additives, including carbon black, to enhance the infrared absorption or reflectance of the foam. One method of enhancing insulative performance of a foam, incorporation of carbon black into the foam matrix, is desirable because of the degree of increase in performance and the relatively low cost of the carbon black additive. The problem however, in incorporating carbon black into plastic foam is one of processing. Mixing and pressure parameters for plastic flow melt containing carbon black are more extreme than without, and the amount of process wastes from purge cycles is relatively higher. Thus, it would be desirable to have an insulative plastic foam utilizing carbon black yet not presenting unusual or undesirable processing problems. According to the present invention, there is a coextruded, insulative, composite structure comprising: a) a first layer of a plastic foam substantially free of carbon black; and b) a second layer of a plastic material having a quantity of carbon black sufficient to reduce the thermal conductivity of the composite structure. Incorporation of carbon black into the second layer imparts enhanced insulativeness to the composite structure while allowing the first plastic foam layer to be free or substantially free of carbon black. The first and second layers are adjacent and cohesive with respect to each other. The second layer is preferably substantially noncellular or unfoamed, and most preferably takes the form of a film. Preferably, the composite structure is comprised of a plurality of substantially alternating first and second layers. According to the present invention, there is a composite plastic flow melt suitable for expansion into a coextruded composite foam structure comprising a) a first stream layer of a flow melt of a plastic material and b) a second stream layer of a flow melt of a plastic -1- material. The first stream layer is substantially free of carbon black. The second stream layer has a quantity of carbon black sufficient to reduce the thermal conductivity of a composite structure made from the composite flow melt. The incorporation of the carbon black into a stream layer (the second stream layer) allows carbon black to be incorporated into the composite flow melt while reducing the processing problems associated with the use of carbon black. The first and second stream layers are adjacent each other. The composite flow melt preferably comprises a plurality of substantially alternating first and second stream layers. According to the present invention, there is a process for making a insulative composite structure comprising: (a) providing a first plastic melt stream having incorporated therein a blowing agent under an elevated pressure sufficient to prevent the expansion of the first stream, (b) providing a second plastic melt stream having incorporated therein carbon black, (c) integrating the first and second streams to form a composite melt stream, and (d) extruding the composite stream through a die to a zone of lower pressure to form a stable composite structure comprising a first layer of a plastic foam and a second layer of a plastic material. The present coextruded, insulative, composite structure is made up of at least one first layer of a plastic foam and at least one second layer of a plastic material, which may be foamed or unfoamed, and is preferably in film form. The second layer contains an amount of carbon black sufficient to enhance the insulativeness of and reduce the thermal conductivity of the composite structure. Incorporation of carbon black in the second layer or layers instead of throughout the entire composite structure allows for significantly reducing or eliminating processing difficulties associated with thermoplasticflow melts containing carbon black. Incorporation of carbon black into a film layer rather than a foam layer is advantageous because film-forming plastic melts typically require less processing than foam-forming plastic melts because they do not contain a blowing agent and are typically extruded closer to the die. Even if both the first and second layers are comprised of foam, processing problems are reduced because the first layers do not contain carbon black. The total number of first and second layers is at least 2, preferably from 3 to 1000, and most preferably from 3 to 20. The first and second layers are generally parallel and in stacked relationship with one another. The first and second layers preferably alternate consecutively along the stack of layers, but first layers may abut each other or second layers may abut each other occasionally through the stack. Preferably, the outer layers (the top and bottom of the stack) of the composite structure are second layers having carbon black incorporated therein to further enhance the insulativeness of and reduce the thermal conductivity of the present structure. Though the rectangular configuration will be the most typical shape of the present structure in both cross-section and on its surfaces, it is understood that structure of any shape such as cylindrical or circular may be produced. -2- The first and second layers typically are heat fused to each other, but also may be adhered to each other by intermediary layers made of tacky plastics such as ethylene-vinyl acetate copolymers, ethyl ene-acrylic acid copolymers, ethylene-propylene copolymers, urethanes, and the like. Preferably, the first and second layers will be heat fused to each other as contemplated in coextruded structures. When the first and second layers are referred to as being adjacent and cohesive with respect to each other, this is inclusive of the layers being fused together or being adhered by tacky intermediate layers. The second layers preferably take the form of a substantially noncellular or unfoamed film. The thickness of the film layers is not critical, but is preferably from 1 micron to 2 millimeters and more preferably from 3 microns to 1 micron. It is understood that the film layers may also take the form of thin sheets. Alternately, the second layers may take the form of a foam to reduce the density of such layers. The second layers preferably contain an amount of a plasticizing additive plasticizer and/or anti-static additive plasticizer sufficient to ensure generally uniform dispersion of the carbon black within the second layers. The plasticizer is a plasticizing substance different than the plastic material primarily comprising the second layers. The plasticizer is preferably present at from 0.01 to 1.0 and more preferably present at from 0.05 to 0.2 weight percent based upon the weight of the second layers. Any known plasticizer may be used; useful plasticizers include ethyl chloride, dibutyl phthalate, dioctyl phthalate and polyethylene glycol. Certain plasticizers, such as ethyl chloride, can act as blowing agents under certain conditions. If it is desired that the second layers be substantially unfoamed and be loaded with a plasticizer which could also function as a blowing agent, then the plastic material comprising same must be melt processed and extruded in a manner preventing foaming or the concentration of plasticizer must be limited to prevent foaming. The first and second layers may be comprised of the same or different plastic material. Both thermoplastic and thermoset materials are suitable. Suitable plastic materials may be selected from any of those which can be formed into film or blown into foam. Suitable plastics include polyolefins, polyvinylchloride, styrenic polymers, cellulosic polymers, polycarbonates, polyetherimides, polyamides, polyesters, polyvinylidene chloride, polymethylmethacrylate, polyurethanes, polyisocyanurates, phenolics, copolymers, terpolymers, polymer blends, rubber modified polymers, and the like. Suitable polyolefins include polyethylene r polypropylene and polybutylene. Suitable styrenics may include polystyrene and copolymers of styrene and other monomers, such as acrylic acid. The present invention is particularly useful with composite structures comprised of polystyrene, which may be formed into both a film and a foam, because polystyrene foams are commonly employed as an insulating plastic foam. The carbon black preferably comprises between 1.0 and 25 weight percent based upon the weight of the plastic material in the composite structure. The carbon black preferably -3- SUBSTITUTE SHEET has an average particle size between 10 and 100 nanometers. The carbon black preferably has an average surface area between 10 and 1500 square meters per gram of carbon. The present structure may contain additional additives such as pigments, fillers, antioxidants, extrusion aids, nucleating agents, stabilizing agents, antistatic agents, fire retardants, acid scavengers, or the like. The foam component of the present foam structure is closed-cell. Preferably, the present foam is greater than 90 percent closed-cell according to ASTM D2856-A. The foam component of the present composite structure preferably has density of 16 to 80 kilograms per cubic meter. The foam component further preferably has an average cell size of 0.05 to 2.4 mm. The process of the present invention for making the composite structure generally comprises providing a first plastic melt stream, providing a second plastic melt stream having incorporated therein carbon black, integrating the first and second streams to form a composite stream, extruding the composite stream through a die to a zone of lower pressure to form the composite structure. The first plastic melt stream is generally formed by melting a plastic material and incorporating therein a blowing agent. The blowing agent may be incorporated or blended into the plastic melt by any means known in the art such as with an extruder, mixer, blender, or the like. Prior to blending with the blowing agent, the plastic material is heated to a temperature at or above the glass transition temperature and preferably above the melting point of the plastic material. The blowing agent is blended with the plastic melt at an elevated pressure sufficient to prevent substantial expansion of the melt or loss of generally homogenous dispersion of the blowing agent within the melt. Such a pressure is typically at least above 345 kϊlopascals gauge and more preferably above 6900 kilopascais gauge. The blowing agent is incorporated into the first plastic melt stream in a weight proportion of between 1 to 30 parts and preferably from 3 to 15 parts per hundred parts of the plastic material to be expanded. Blowing agents which optionally may be incorporated in the first plastic melt to make the first layer or layers of the present composite structure are described below. If it is desired that the second layer or layers of the present structure be cellular or foamed, the following blowing agents may likewise be incorporated into the second plastic melt. Useful blowing agents include gaseous materials, volatile liquids, and chemical agents which decompose into a gas or other byproducts. Suitable blowing agents include, but are not limited to nitrogen, carbon dioxide, air, argon, water, alkanes of 2 to 9 carbon atoms such as n- pentaneand isobutane, methyl chloride, ethyl chloride, butane, isobutane, pentane, isopentane, perfluoromethane, chlorotrifluoromethane, dichιorodifluoromethane, tri- chlorofluoromethane, perfluoroethane, 1-chloro-1,1-difluoroethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, 1,1,1-trifluoroethane, chlorotetrafluoroethane, 2-chloro-1,1,1,2- tetrafluoroethane, chloropentafluoroethane, dichlorotetrafluoroethane, trichlorotrifluoroethane, perfluoropropane, chloroheptafluoropropane, dichloropropane, dichlorohexafluoropropane, perfluorobutane, chlorononafluorobutane, per- fluorocyclobutane, azodicarbonamide, azodiisobutyronitrile, benzenesulfonhydrazide, 4,4-oxybenzene sulfonyl-semicarbazide, p-toluene sulfonyi semicarbazide, barium azodicarboxylate, N,N'-dimethyl-N,N'-dinitrosoterephthalamide, trihydrazinotriazine, and mixtures thereof. The blowing agent may comprise a mixture of two or more blowing agents. The formation of a second plastic melt stream suitable for extrusion into a film is carried out by melting a plastic material in the manner described above for the first stream and incorporating therein carbon black. The carbon black may be dry mixed with the plastic and subsequently melting and blended together, or may be mixed with the plastic melt. The plastic and the carbon black may be blended by any means known in the art such as with an extruder, mixer, blender, or the like. Following the incorporation of the blowing agent into the first melt stream and the carbon black into the second melt stream, the two streams are integrated to form a composite stream. The composite stream is then extruded through a die to a zone of lower pressure to form the stable composite structure of the present invention. The zone of lower pressure downstream of the die is a pressure lower than that which the first stream or the second stream if it contains a blowing agent were exposed to prevent such stream from expanding or losing their blowing agents. The stable composite structure is comprised of foamed plastic first layers and plastic foamed or unfoamed (film) second layers fused together or otherwise adhered together. The first and second streams are integrated together and extruded by means of coextrusion or simultaneous extrusion well-known in the art. Methods and apparatuses for coextrusion are disclosed in U.S. Patents 3,565,985, 3,557,265, and 3,884,606. Such methods and apparatuses may be used to create composite structures of up to several hundred layers. During coextrusion, the first and second melt streams are both divided or split into a plurality of first and second melt substreams. The plurality of substreams is then arranged and integrated to form a composite flow melt of a stack of alternating first and second substream layers. The composite flow melt is then extruded through a die to a region of lower pressure to form a composite structure of first sublayers of a foam of a plastic material and second sublayers of a plastic material wherein the sublayers are fused or otherwise adhered together. The second sublayers may or may not be foamed depending upon the presence of a blowing agent in the second melt substreams. For purposes of convenience, the first and second substream layers are referred to herein as first and second stream layers, and the first and second sublayers are referred to herein as first and second layers. Preferred methods and apparatuses for making the present composite structures are seen in U.S. Serial No. 577,723 filed September 5, 1990. -5- SUBSTITUTE SHEET There is a method for insulating a surface comprising affixing to the surface an insulating panel fashioned from the present composite structure. Such insulating panel is useful in known applications in buildings, furnace linings, packaging, cushioning, and the like. It is understood that a noncoextruded composite structure of the present invention may be made by conventional lamination or coating methods. For instance, a coextruded or molded bead plasticfoam substantiallyfree of carbon black could be laminated with another plastic foam, film, or material containing carbon black. Likewise, a plastic resin or melt containing carbon black could be coated, spread, or otherwise applied on a plastic foam substrate which is substantially free of carbon black. The following examples are presented to illustrate the invention, and are not to be construed as limiting. Film/foam composite structures of the present invention were prepared and tested forthermal conductivity. The structures had 7 or 12 percent by weight loadings of carbon black in the film layers based upon the weight of the plastic material in the entire structure. The loadings in the film layerswas 12 weight percent and more inthe film layers. The outer layers of each of the structures was a film layer. For structures having 2 film layers, the middle layer was a foam layer. For structures having 3 film layers, one film layer is sandwiched between 2foam layers, which are in turn sandwiched by outer film layers. The configuration and loading of each structure is seen in Tables 1-3. A control structure (sample 3089-09-22) was prepared as for the examples above except that no carbon black was loaded into the structure. Process conditions for making the present structure and the control structure are seen in Table 4. The indicated solids for the film and foam layer preparation were mixed together in separate 1 4-inch (3.8 cm) extruders. The blowing agent was added to the foamable meltin a mixer downstream of the foam extruder and before the film extruder. Both film- forming and foamable flow melts were passed through a cooler, and then passed through the feedblockto form the coextruded composite structure. As seen in Table 2, the structures of the present invention exhibited lower measured and corrected thermal conductivity than the control structure. The corrected thermal conductivity value was obtained by mathematically compensating for the variation in cell size. Thermal conductivity was assumed to be 0.008 Btu in/(hr. ft* F°) (1.15 x 103 W/(m.K°)) lower for every 0.1 millimeter (mm) smaller in cell size, and 0.35 mm was used as a reference size. -6- Table 1 Properties of Film/Foam Structures ''not an example of the present invention *not an example of the present invention -7- SUBSTITUTE SHEET Table 3 Carbon Black Concentrations of Film/Foam Structures Carbon Black Concentration *not an example of the present invention -8- Table 4 Run Condi tions/Formulations for Coextruded Film/Foam Compos i tes Foam Layers Solids polystyrene talc (pph) HBCD (pph) MgO (pph) CaSt. (pph) Rate (Kg/hr) Blowing Agent HCFC-142b (pph) EtCl (pph) Cθ2 (pph) Temperature (°C) Into Feedblock Pressure (kPa)) Into Mixer D Film Layers Solids polystyrene blue pigment(pph) i I carbon black HBCD (pph) MgO (pph) CaSt (pph) Talc (pph) Rate (kg/hr) Processing Aid EtCl (pph) Temperature (°C) Into Feedblock Pressure (kPa) Into Temp. Control Zone 21,370 30,000 21,030 HOTE: pph (parts per hundred) by weight based upon polystyrene and carbon black (where present) and based upon total solids weight in instance of blowing agent;CaSt - calcium stearate; MgO - magnesium oxide; EtCl - ethyl chloride; HBCD - hexabromocyclododecane; CO2 - carbon dioxide; HCFC - 142b - l-chloro-1,1 difluoroethane While embodiments of the composite structure of the present invention have been shown with regard to specific details, it will be appreciated that 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. -10-

Claims: Claims : 1. A process for making a coextruded insulative composite structure, characterized by the following steps: providing a first plastic melt stream having incorporated therein a blowing agent under an elevated pressure sufficient to prevent expansion of the first stream and being free of carbon black; providing a second plastic melt stream having incorporated therein an amount of carbon black sufficient to reduce the thermal conductivity of the composite structure relative to a corresponding composite structure free of carbon black; and integrating the first and second streams to form a composite melt stream; extruding the composite stream through a die to a zone of lower pressure to form the composite structure comprising a first layer of a plastic foam and a second layer of a plastic material. 2. The process of Claim 1 , wherein the first melt stream and the second melt stream are each split into a plurality of first melt streams and second melt substreams and integrated in an alternating sequence to form the composite melt stream, and wherein the composite melt stream is extruded through the die to form the composite structure of a plurality of alternating first and second layers. 3. The process of Claims 1 or 2, wherein the composite structure has between 1.0 and 25 weight percent carbon black based upon the weight of the plastic material in the composite structure. 4. The process of Claims 1 or 2, wherein the plastic material is a styrenic polymer. 5. The process of Claims 1 or 2, wherein the second plastic melt stream is further provided with a plasticizing additive therein. 6. The process of Claims 1 or 2, wherein the second plastic flow melt stream is further provided free of the same or a different blowing agent. 7. The process of Claim 1 , wherein the first stream is provided, integrated, and extruded under conditions sufficient to result in formation of a first layer of a closed-cell plastic foam. -1 1- SUBSTITUTE SHEET 8. The process of Claim 2, wherein the first substream is provided, integrated, and extruded in a manner resulting in formation of a first sublayer of a closed-cell plastic foam. 9. A coextruded insulative composite structure made by the process of Claims 1 or2. 10. A process for making an insulative plastic composite structure having a first layer of a closed-cell foam, characterized by the following steps: providing a first layer of a foam of a plastic material free of carbon black; providing a second layer of a plastic material having a quantity of carbon black therein sufficient to reduce the thermal conductivity of the composite structure relative to a corresponding structure not having carbon black; apply the second layer to the first layer such that the layers are adjacent each other. -12-

Inventors: GRIFFIN WARREN HERBERT, PAQUET ANDREW NORMAN, SUH KYUNG WON, GRIFFIN, WARREN, HERBERT, PAQUET, ANDREW, NORMAN, SUH, KYUNG, WON
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: B32B3
IPC codes: B29C44, B29C47, B29K105, B29L9, B32B5, B32B27