EP1252224A1 - Mousse de polyolefine extrudee de qualite superieure en epaisseur et panneau mousse et procede associes - Google Patents

Mousse de polyolefine extrudee de qualite superieure en epaisseur et panneau mousse et procede associes

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Publication number
EP1252224A1
EP1252224A1 EP00911611A EP00911611A EP1252224A1 EP 1252224 A1 EP1252224 A1 EP 1252224A1 EP 00911611 A EP00911611 A EP 00911611A EP 00911611 A EP00911611 A EP 00911611A EP 1252224 A1 EP1252224 A1 EP 1252224A1
Authority
EP
European Patent Office
Prior art keywords
foam
blowing agent
percent
propane
isobutane
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP00911611A
Other languages
German (de)
English (en)
Inventor
Shau-Tarng Lee
Natarajan S. Ramesh
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sealed Air Corp
Original Assignee
Sealed Air Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sealed Air Corp filed Critical Sealed Air Corp
Publication of EP1252224A1 publication Critical patent/EP1252224A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/14—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent organic
    • C08J9/141—Hydrocarbons
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/02—Foams characterised by the foaming process characterised by mechanical pre- or post-treatments
    • C08J2201/03—Extrusion of the foamable blend
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment

Definitions

  • This invention relates to extruded polyolefin foams and to a process for producing an expanded polyolefin foam product of low density using an expandable polyolefin composition.
  • this invention relates to the use of blowing agents for incorporating into a plasticized polyolefin resin for foaming by extrusion.
  • Thermoplastic foam products can be produced by a wide variety of processes, of which extrusion is but one, that are in part responsible for the wide variety of foam products available today.
  • Foams range in consistency from rigid materials suitable for structural use to flexible substances for soft cushions and packaging materials. These foams range in cellular formation from open or interconnecting-cell foams to closed or unicell foams. The cell structure may range from large to fine. Electrical, thermal, mechanical, and chemical properties can be varied within wide limits depending on the thermoplastic resin composition and the method chosen to create the foam.
  • Foamed thermoplastics range in density anywhere from about 10 kg/m to over 1,000 kg/m 3 , although the latter perhaps more properly are called microcellular structures. True foams are considered to have a density of less than about 800 kg/m .
  • blowing agents work by expanding a thermoplastic resin to produce a cellular thermoplastic structure having far less density than the resin from which the foam is made.
  • Bubbles of gas form around "nucleation sites" and are expanded by heat or reduced pressure or by a process of chemical reaction in which a gas is evolved.
  • a nucleation site is a small particle or conglomerate of small particles that promotes the formation of a gas bubble in the resin.
  • Additives may be incorporated into the resin to promote nucleation for a particular blowing agent and, consequently, a more uniform pore distribution.
  • the foam is maintained by replacing the blowing agent in the cells with air. Diffusivity of the blowing agent out of the cells relative to air coming into the cells impacts the stability of the foam over time and whether the cells of the foam may collapse.
  • Additives may be incorporated into the resin and process conditions may be adjusted to assist in controlling the diffusivity of the blowing agent, to promote foam stability, and to limit collapse of the foam to acceptable limits.
  • Methods for producing a blowing agent in situ usually involve a chemical reaction that evolves gas.
  • Polyethylene, silicone, epoxy, and vinyl foams have all been produced by adding a substance that will produce a gaseous blowing agent chemically.
  • dinitroso compounds and hydrazides which evolve nitrogen gas on decomposition
  • bicarbonates which evolve carbon dioxide
  • Polystyrene foams often are produced by "bead molding,” in which partially expanded granules or beads are heated in a mold in the presence of a blowing agent to expand and fuse the particles into a rigid unicellular structure. A volatile organic compound or some other gaseous blowing agent is impregnated into the beads. Heat is applied and the pressure is released to cause the beads to expand and fuse.
  • blowing agent to a thermoplastic resin during processing to produce a foam.
  • Ureaformaldehyde and polyvinylformaldehyde foams have been produced by whipping air into a heated thermoplastic mass before it sets.
  • Polyolefinic foams have been produced by introducing air or some other gas or volatile solvent into a heated thermoplastic polyolefin mass and then heating the mass or reducing pressure to expand the gas and form pores of a desirable size.
  • One more specific method is to impregnate a thermoplastic resin with blowing agent under heat and pressure in a closed vessel. The pressure is released to expand the blowing agent to form "prefoamed,” or partially expanded, beads. Prefoamed beads usually are further expanded in an enclosed vessel such as a mold to produce a molded foam product, as is discussed above.
  • halogenated hydrocarbons have been used for several years as blowing agents in various methods for producing extruded foams from thermoplastic resins.
  • the halogenated hydrocarbons include the chlorofluorocarbons (“CFCs”) and hydrochlorofluorocarbons (“HCFCs”).
  • CFCs and HCFCs are readily impregnable in thermoplastic resins and are readily expandable under relatively mild conditions.
  • CFCs and HCFCs generally produce foams of high quality with a minimum of processing difficulty.
  • the pore size is controllable, the foam has good stability with minimum tendency to collapse after a period of time, and the surface characteristics of the foam are smooth and desirable.
  • CFCs, HCFCs, and other halogenated hydrocarbons typically are either not flammable or are of low flammabihty, which greatly reduces the care with which they may be used. These compounds have the further advantage of low toxicity.
  • governmental regulation is phasing out use of halogenated hydrocarbons because the halogenated hydrocarbons may be responsible for damage to the earth's ozone layer. Producers of thermoplastic foam products have been seeking alternatives to
  • VOCs volatile organic compounds
  • the diffusivity of VOCs can be many times faster than that of the halogenated hydrocarbons and harder to control. Foam collapse and stability problems have been encountered, although high quality foams have been produced using, for example, butane.
  • VOCs typically are volatile and flammable, thus presenting handling problems and safety concerns.
  • Robin et al. U.S. Patent No. 5,314,926 describes a blowing agent comprising a mixture of one or more hydrocarbons or partially halogenated alkanes with a fluorinated propane, 1,1,1,2,3,3,3-heptafluoropropane.
  • the hydrocarbons are said to include propane, butane, isobutane, n-pentane, i-pentane, neopentane, n-hexane, 2 methylpentane, 3-methylpentane, and 2,2-dimethylbuntane.
  • the fluorinated propane is said to be useful even in small amounts in reducing the flammabihty of foamable plastics including polystyrene, poly vinyl chloride, polyethylene, and other non-polyisocyanate based foams.
  • foamable plastics including polystyrene, poly vinyl chloride, polyethylene, and other non-polyisocyanate based foams.
  • the behavior of VOCs in various thermoplastic resins and the foams prepared therefrom is somewhat unpredictable due to the differences in volatility of the various VOCs and VOC blends, the differences in the foaming behavior of different thermoplastic resins, the wide variation in kinds and amounts of processing additives that are added to the different thermoplastic resins, and a host of other factors too numerous to mention here.
  • Foaming agents are said to include nitrogen, carbon dioxide, lower molecular weight paraffins such as propane, butane, and methylchloride, lower molecular weight olefins such as ethylene, propylene, and butylene or mixtures of the above. No specific mixtures of foaming agents are disclosed.
  • a preferred thermoplastic composition for extrusion to form tongue depressors or ice cream sticks of 320 to 1000 kilograms per cubic meter is disclosed to include polystyrene beads having a pentane blowing agent integrated therewith.
  • Vesilyn U.S. Patent No. 3,287,477 discloses extrusion apparatus and methods for preparing polymer foams and is primarily directed to polystyrene foam. Vesilyn discloses that the extrusion apparatus can also be operated with other polymers including polymers derived from ethylene monomers to produce foam sheets. Blowing agents are said to include examples such as methane, ethane, propane, butane, n-pentane, isopentane, neopentane, hexanes, heptanes, and a variety of others.
  • Polystyrene typically is considered to have different foaming characteristics from the polyolefins.
  • Polystyrene is an amorphous material whereas the polyolefins are semi-crystalline.
  • Butane blowing agents normally are used for foam extrusion of low density polyolefin foams (over 30% expansion) in an amount of from about 15% to 16% by weight of the resin, typically in combination with active nucleating agents in amounts of about 0.5%) by weight of the resin. Butane is extremely flammable and at the levels used for blowing agent an aging period typically is provided to reduce residual butane blowing agents below explosive limits prior to shipment of the foam. Propane has successfully been used as a blowing agent for producing polyethylene thin foam sheet and foams of about 11 mm thickness or less, despite its volatility and flammability.
  • the invention claimed herein relates to the extrusion foaming of polyolefin resins into relatively low density thicker grade and plank foams having improved properties, particularly compressive strength, wherein the blowing agent is propane.
  • "Thicker grade" foams means foams of about 12 mm or more. Thin foam sheet is typically thought of as 1/4-in or less (6 mm or so). Plank is typically thought of as 2 inches or more in thickness (about 50 mm).
  • the benefits in compressive strength in thicker grade foams can usually be achieved even when the amount of butane used in the propane blowing agent is up to about 75%) by volume, depending on the polyolefin selected. Aging time to reach the blowing agent lowest explosive limit can be reduced as compared to butane, which is important from safety and flammability concerns. Extensional viscosity of the melt extruded through the plank die can be improved. The force and time needed to cut the foam is reduced and the degree of penetration into the foam of the cutting knife is reduced in comparison to conventional butane foams.
  • the foams of the invention have reduced concavity and better dimensional stability after cutting; the cut edges are straight and square.
  • the foams of the invention have improved flexural modulus, which is to say the foams are stiffer and resist bending more than conventional butane foams.
  • Foaming efficiency refers to the amount of blowing agent that is used to achieve a particular density. It is believed that more efficient blowing agents produce less of the blowing agent in cell walls of the foam and that the compressive strength of the foam is improved when less blowing agent is present in the cell walls of the foam.
  • Thicker grade polyolefin foams and plank made in accordance with the invention can have an apparent extensional viscosity at a die temperature of about 110 degrees Centigrade of from about 1.2 x 10 6 to 7 x 10 6 poise at a strain rate of from about 1.9 to 40 per second, respectively.
  • the resin typically enters the die at a much higher temperature than the die temperature.
  • Thicker grade polyolefin foams and plank made in accordance with the invention can have a compressive strength about one hour after extrusion of from about 8 to 45 psig at 25 percent compression by unit volume, depending on the density of the foam.
  • Foam plank made in accordance with the invention can have a compressive strength about two weeks after extrusion of from about 20 to 25 psig at 50 percent compression by unit volume and up to 55 or even 60 psig at 25 percent compression.
  • foam plank made in accordance with the invention can reach its lowest explosive limiting value within about two weeks after extrusion.
  • foam plank made in accordance with the invention has a cell count of from about 10 to 30 cells per inch and a density of from about 20 to 150, 160, 170, and up to 180 kg/m .
  • die cut performance for the foams of the invention is greatly improved.
  • the foam is remarkably easier to cut and has less concavity with sharp and square edges as compared to foam prepared with conventional butane blowing agent.
  • the foam of the invention ages more quickly, thereby reducing the time necessary for die cut of the foam. Less penetration of the die knife is required. Flexural stability is improved.
  • foam plank made in accordance with the invention is prepared from a low density polyethylene resin and a blowing agent selected from the group consisting of propane and a blend of propane with up to about 65% by weight of either normal butane or isobutane.
  • the polymer melt is extruded and expanded to a nominal thickness of at least about 12 mm or more to form a thicker grade foam or foam plank.
  • the polyethylene foam also comprises a processing additive, including, for example, glycerol monostearate (GMS), and a nucleating agent, including, for example, talc.
  • GMS glycerol monostearate
  • nucleating agent including, for example, talc.
  • thicker grade foam and foam plank made in accordance with the invention is prepared from a polypropylene resin and a blowing agent selected from the group consisting of propane and a blend of propane with up to about 75%> by weight of either normal butane or isobutane. While not wishing to be bound by theory, it is believed that the blowing agent efficiency of propane in polypropylene is greater than in polyethylene, so that less propane is required to achieve the benefits of the invention.
  • the invention provides, among other things, thicker grade foam and foam plank prepared from polypropylene and low density polyethylene resins that are expanded with a propane blowing agent. These foams have improved compressive strength and reduced aging time compared to plank made using a 100%) butane blowing agent and the resins have greater melt strength for extrusion through the plank die.
  • the benefits of the invention can be achieved in thicker grade polypropylene foams when the propane blowing agent includes up to about 75% by weight of isobutane or normal butane, and in thicker grade polyethylene foams with up to about 65% by weight of one or more butanes.
  • Figure 1 is a graph illustrating the relationship between apparent extensional viscosity and strain rate for freshly extruded plasticized polyolefin resin, including examples prepared with a blowing agent in accordance with the invention compared to an example prepared with a butane blowing agent and an example extruded in the absence of blowing agent.
  • An extruded rod of plasticized polyolefin resin is grabbed by a roller to accelerate the rod until it breaks.
  • the force required to drive the roller, the roller speed, and the die exit velocity are taken into account to calculate the extensional viscosity, which is considered to be an indication of the melt strength of the resin.
  • the data obtained is for a resin that is foaming and is in a transition state from the plasticized polyolefin resin in the extruder to the fully expanded cellular product.
  • thermoplastic resins solid pellets of thermoplastic resin are fed through a hopper to a melting zone in which the resin is melted, or plasticized, to form a flowable thermoplastic mass.
  • the plasticized thermoplastic mass generally is then metered to a mixing zone where the thermoplastic mass is thoroughly mixed with a blowing agent under pressure for subsequent cooling and expansion of the resin to form a foam.
  • Blowing agent typically is injected between the metering and the mixing zones.
  • the mixture of thermoplastic resin and blowing agent is then forced through a die, which imparts a shape to the thermoplastic mass, into a zone of lower pressure, such as atmospheric pressure.
  • the blowing agent expands to form the cells of the foam and the thermoplastic foam is cooled.
  • thermoplastic pellets are conveyed from a hopper through the melt zone, the mixing and cooling zones, and extruded through the die by a screw type apparatus.
  • Single screw extruders are common, although double screw extruders sometimes are used for greater mixing and tandem extruders can be used to provide greater cooling of the resin prior to foaming.
  • the blowing agent When a blowing agent is injected into the mixing zone of the screw extruder, the blowing agent initially forms a dispersion of insoluble bubbles within the plasticized thermoplastic mass. These bubbles eventually dissolve in the thermoplastic mass as the mixing continues and the pressure increases down the length of the extruder.
  • the extruder should have a length to diameter ratio of at least 30:1 and a sufficient length of mixing zone to ensure that proper mixing occurs.
  • Thermoplastic resins contemplated for use in the practice of the invention are the polyolefin resins, although not necessarily with equivalent results for different polyolefins.
  • Polyolefin resins may be defined as semicrystalline polymers derived from unsaturated hydrocarbons containing the ethylene or diene functional groups. Semicrystalline polyolefin polymers are to be distinguished from polystyrene polymers and the like, which typically are amorphous and can have fundamentally different foaming characteristics.
  • Polyolefin resins may include virtually all of the addition polymers, however, the term polyolefin typically is used for polymers of ethylene, the alkyl derivatives of ethylene (the alpha-olefins), and the dienes.
  • the more commercially important polyolefins are polyethylene, polypropylene, polybutene, and their copolymers.
  • Polyethylene is a whitish, translucent polymer of moderate strength and high toughness. Polyethylene is available in forms ranging in crystallinity from 25 to 95 percent. Polyethylene is available in low, medium, and high density polymer forms. For the low density material, the softening temperature is about 105°C to 115°C. The softening temperature for the high density material is some 25°C to 40°C higher, or from about 130°C to 140°C. Low, medium, and high density polyethylenes typically are suitable for extrusion foaming, including mixtures thereof, although not necessarily with equivalent results.
  • Polypropylene is also a whitish, translucent polymer and has a softening point of about 168 to 171°C, which is some 50 to 65°C higher than that for polyethylene. Polypropylene has high tensile strength and maintains its strength even after repeated flexing. However, polypropylene does not typically have the toughness of polyethylene.
  • the invention is useful for producing thicker grade polyolefin foams and polyolefin plank.
  • Thicker grades of polyolefin foams typically are considered to be about 12 mm or more thick and are often sold in thicknesses of about 1/2 inch, 1 inch, 1 1/2 inches, 2 inches or more.
  • Foam plank typically is considered to have a nominal thickness of about 50 mm (2 inches) or more.
  • the thicker grades are to be distinguished from polyolefin foam sheets and thin sheets, which are generally less than 12 mm, and even less than 6 mm.
  • Thicker grade foams can be used for a variety of purposes, including building materials, surfboards, rigid structural members, and insulation. Compressive strength is an important attribute of thicker grade foams and plank in many load bearing applications.
  • thermoplastic resin should be maintained at a temperature within a range above the melting point of the polymer that is sufficiently high so that the polymer has sufficient fluidity for mixing with blowing agent. This range normally will be from about 20°C to 100°C above the melting point of the resin.
  • the melting zone can be maintained at a somewhat lower temperature due to the heat that is generated by friction as the plasticized resin flows through the extruder.
  • the temperature of the mixture of resin and blowing agent should be lowered closer to the melting point of the mixture so that the polymer maintains its structure upon foaming, but not so much that complete expansion is hindered.
  • the blowing agent has a plasticizing effect on the resin reducing its viscosity, or resistance to flow, and so the melting point of the mixture of resin and blowing agent normally is below that of the resin alone.
  • the expansion temperature which is above the melting point of the mixture, is empirically determined and depends upon the composition of the resin, the length of the screw, whether single or double screws are used, on the specific resin, upon the amount of blowing agent, and the specific blowing agent. For a low density polyethylene, the expansion temperature will generally be in the range of from about 85°C to 120°C.
  • the blowing agent contemplated for use in practicing the invention claimed herein comprises propane. Many of the benefits of the invention normally can still be achieved when butane is incorporated into the propane blowing agent in amounts of from 10, 20, 30, or 40% or more by volume up to a maximum of about 75%>, depending on the polyolefin selected. Unlike butane foams of the prior art, foam produced by the process of the invention with either pure propane or with mixtures of propane and either normal butane and isobutane typically can be shipped in a relatively short period. With needle punching, the residual levels of blowing agent are below the least explosive limit within about one week after the foam is produced, as explained below.
  • Table 1 below shows the upper and lower flammability limits in air for normal butane, isobutane, propane, and ethane based on the percent by volume of the hydrocarbon in air.
  • the lower explosive limit for propane is 2.1 percent by volume in air compared to 1.8 percent for the butanes.
  • 100 percent propane presents a concentration in fresh foam that rapidly declines below explosive limits, whereas butane often remains above the lowest explosive limit for a period of time.
  • Propane has a higher foaming efficiency than the butanes. For a given weight, propane generates more gas volume than butane. Less propane typically is required than is required of the butanes to achieve a comparable foaming efficiency. Only about 12 percent by weight of the resin of propane is used as a blowing agent when pure propane is used, compared to about 16 percent for pure butane. Therefore, the plasticizing effect of propane as a blowing agent is much less than in the case of butane. The temperature of the resin in the extruder and die tends to increase and nucleation is more difficult to control with the typical active nucleators. Propane blowing agent used in the same amounts and with the same nucleators as butane produces small bubbles and a high density foam that is subject to collapse. Prefoaming in the extruder and corrugation of the foam can occur.
  • process conditions and nucleation are carefully controlled to produce low density foams from a propane or propane and butane blend blowing agent that are dimensionally stable.
  • a minimum die opening sufficient for preparing plank can be used to maintain sufficient pressure to prevent prefoaming.
  • the minimum die opening increases the shear and heat and sensitivity of the system to nucleation.
  • Very low levels of relatively inactive metal oxide nucleators from 0 to 0.8 percent by weight of the resin, such as zinc oxide and zirconium oxide, have been used in combination with propane blowing agent in accordance with the invention for foam extrusion of polyethylene foam.
  • Another nucleator comprising sodium bicarbonate and citric acid is useful for producing fine cells in foams.
  • the sodium bicarbonate and citric acid nucleator blend is available from BI Chemicals in Winchester, Virginia under the trade name Hydrocerol.
  • the foam can be made with a single screw extruder, it is helpful in the process of the invention to use tandem extruders where the first extruder is used for mixing and the second extruder is used to maximize cooling of the resin prior to foaming.
  • the butane can be present in a ratio of from about 0:1 to 3 : 1 by weight for producing polypropylene foam, which is from 0 to about 75 percent by weight of butane.
  • the butane can be present in a ratio of from about 0:1 to 1.9:1 by weight for producing polyethylene foam, which is from 0 to about 65 percent by weight of butane.
  • the blowing agent is mixed into the plasticized polyethylene polymer resin in proportions to achieve the desired degree of expansion in the resulting foamed cellular product.
  • Stable foam densities from 40 kg/m down to as low as 20 kg/m may be made by practice of the invention.
  • Stable foams of higher density, up to about 100 to 180 kg/m 3 can also be produced, if desired.
  • the blowing agent generally is mixed with the resin in a ratio of about one part blowing agent to ten parts resin.
  • the maximum useful proportion of blowing agent in the plasticized resin is density related and is related to the pressure that is maintained on the resin in the extrusion die passage, as is believed to be well known to the skilled artisan.
  • blowing agent in accordance with the invention claimed herein may be enhanced in preparing polyethylene foams by using a combination of a nucleation agent and an aging modifier to control cell size and development and to control the replacement of blowing agent with air in the cells of the foam, respectively.
  • a combination of low levels of relatively low activity metal oxide nucleation agents such as zinc oxide, zirconium oxide, talc, and others, in combination with an aging modifier, including, for example, glycerol monostearate (GMS)
  • GMS glycerol monostearate
  • the aging modifier is mixed with the polyethylene resin prior to melting in an amount sufficient to produce a desirable rate of exchange of air with blowing agent in the cells of the foam. More specifically, GMS is mixed with the polyethylene resin prior to melting in an amount from about 0.5 to 5 kg per 100 kg of polyethylene resin. Still more specifically, GMS is added in an amount of 1 kg per 100 kg of polyethylene resin.
  • the blowing agent efficiency of propane is relatively high and various polyolefin resins can have different foaming characteristics.
  • polypropylene has a softening temperature that is about 50 to 65 °C higher than that for polyethylene. Less blowing agent is required to prepare polypropylene foams than is required to prepare polyethylene foams.
  • Aging modifiers, including GMS are not typically needed to prepare polypropylene foams in accordance with the invention, although nucleation agents are used. GMS and similar compounds can be used to reduce friction induced static in polypropylene resins and foams.
  • Nucleation agent is mixed with polyolefin resin in an amount sufficient to promote nucleation and to develop a pore structure of the desired size. More specifically, nucleation agent is mixed with the resin in an amount of from about 0.05 to 0.5 kg per 100 kg of polyolefin resin.
  • low activity metal oxide nucleators have proved useful for propane blown foams and the propane and butane blends described herein. As the percentage of CO 2 increases, it is helpful to use a low activity nucleator. Similar nucleation agents in similar amounts are beneficial in producing both polypropylene and polyethylene foams in accordance with the invention.
  • Example 1 Low density polyethylene resin having a melt index of 2 (Novacor 219) was prepared by adding 1 pph of talc and 1.2 pph of glycerol monostearate (GMS) to the plasticized resin and mixing thoroughly. Selected blowing agents were thereafter injected through a single port in the primary extruder of a tandem screw extruder. In one case, 7.5 parts per hour isobutane was injected as a blowing agent. In a second case, 6.4 parts per hour propane was injected as a blowing agent. In each case, the resin was then cooled in the secondary extruder of the tandem system and dispersed in a slab die for even expansion to form foam plank that was 54 millimeters thick (2.1 inches) and 64.5 centimeters wide.
  • GMS glycerol monostearate
  • foams whether produced with isobutane or propane blowing agent had an initial density of about 38.5 kilograms per cubic meter.
  • the hot foam plank was allowed to reach room temperature naturally. Typically, a few hours is required for the plank core to reach room temperature. At one hour, 25 percent compression was applied to each foam sample to check the center strength.
  • Plank made with isobutane had a compressive strength of 7.5 to 8.5 psig at 25 percent compression.
  • the propane plank sample had a compressive strength of 9.5 to 11.0 psig at 25 percent compression.
  • the isobutane plank had a compressive strength of 19 to 21 psig at 50 percent compression.
  • the propane plank had a compressive strength of 21 to 23 psig at 50 percent compression, which shows an improvement in compressive strength.
  • Example 1 The propane and isobutane foams of Example 1 were needle punched in accordance with the procedures set forth in U.S. Patent Nos. 5,424,016 and 5,585,058.
  • Example 3 The same low density polyethylene resin prepared as in Example 1 was mixed with a blowing agent comprising 35 percent by volume of propane and 65 percent by volume of isobutane at a concentrations shown in Table 2. The resin was extruded on a 150 millimeter twin screw extruder. The results are shown in Table 2 below.
  • a foam prepared in accordance with the invention has improved compressive strength at 25% compression 1 hour after extrusion compared
  • Example 4 A polypropylene resin was prepared in the absence of talc or glycerol monostearate or other aging modifiers and nucleating agents. The resin was mixed with either isobutane, propane, or a blend of isobutane and propane blowing agents
  • Cogswell developed the extensional viscosity equation for the orifice die.
  • the Cogswell formula is a useful tool for evaluating resin foaming characteristics and has been used to generate yield 15 force data for calculating extensional viscosity.
  • extensional viscosity of a resin melt in an extruder When the extensional viscosity of a resin melt in an extruder is measured, the resin melt undergoes acceleration in the direction of flow when the cross-sectional flow area is decreased, such as when the polymer melt passes through a constriction such as a slab die. The polymer tends to react and the resistance toward the stretching 20 that is induced by acceleration of the polymer in the flow direction is evaluated as "extensional viscosity.” For semi-crystalline, low density polyethylene, a higher extensional viscosity correlates with a higher degree of branching then enhances melt strength for better forming.
  • a rod of plasticized polyolefin resin is extruded through a capillary die and then grabbed by a roller outside the capillary die to accelerate the rod until it breaks. Foam is starting to appear at this stage.
  • the extruded rod is in a transition state between the plasticized polyolefin in the extruder and an expanded cellular product.
  • the force required to drive the roller, the roller speed, and the die exit velocity are taken into account to calculate the extensional viscosity, which is considered to be an indication of the melt strength of the resin in the machine direction. It is believed that this property is similar in the transition state, it which some foaming has occurred, to the same property in the fully expanded cellular state. It is also believed that the properties should be somewhat similar in the machine direction, the cross direction, and the thickness direction.
  • the extensional viscosity of plasticized polyolefin resin having a propane blowing agent mixed therewith is close to the extensional viscosity for the plasticized resin in the absence of blowing agent, indicating that the melt strength of the resin is not significantly reduced by the presence of propane blowing agent.
  • Figure 1 shows that resin incorporating a propane blowing agent has a much better melt strength than resin incorporating an isobutane blowing agent.
  • Resin with isobutane was shown to have an apparent extensional viscosity of at least about 4.5 x 10 5 to 4.6 x 10 6 poise at a strain rate of from at least about 1.9 to 40 per second.
  • Resin with no blowing agent was shown to have an apparent extensional viscosity of at least about 1.8 x 10 6 to 6.1 x 10 6 poise at a strain rate of from at least about 2.1 to 28 per second.
  • Resin with propane which is a blowing agent for use according to the invention, is shown to have an extensional viscosity of from at least about 1.2 x 10 to 7 x 10 poise at a strain rate of from at least about 1.9 to 40 per second.
  • Example 6 Foams prepared from propane and from conventional butane blowing agents were prepared and compared. Plank foams having a thickness of 57 mm (2.25 inches) were produced at a cell count of 33 cells per inch.
  • the resin was a low density polyethylene resin having a melt index of 3.5.
  • the resin was mixed with Hydrocerol CF-20 nucleating agent, which is available from B.I. Chemicals, Inc., and a glycerol monostearate aging modifier.
  • the foams were produced on a tandem extruder having
  • y rocero - was a e a . o e res n ra e an ycero monostearate was added at 1.05% of resin rate
  • Table 4 shows that plank foams made in accordance with the invention as compared to conventional butane foams can have a higher compressive strength about 1 hour after extrusion of up to about 41 psig at 25 percent compression by unit volume for a foam having a density of about 148 kg/cubic meter.
  • Table 4 shows that plank foams made in accordance with the invention, in which propane blowing agent was added to the resin at 1.16 percent of the resin flow rate, by weight, exhibit about 14 percent higher compressive strength than planks made with conventional butane blowing agent, even where the propane foams of the invention were produced at 2 percent lower foam density about 1 hour after extrusion.
  • Foam plank made in accordance with the invention can have a compressive strength after 10 days of up to about 55 or 60 psig at 25 percent compression. Compressive strength is improved as compared to foams prepared from isobutane at comparable density. The data on compressive strength shown in Table 4 was calculated by dividing the maximum compressive load in pounds force carried by the sample during the test by the original minimum cross-sectional area of the sample, which in all cases was 16 square inches for a 4 x 4-inch sample. Compressive strength is a function of several factors, and is directly related to foam density for foams made with the same blowing agent. For a given blowing agent, compressive strength is reduced as foam density decreases because there is less polymer for the same volume to carry the compressive load.
  • Ef compressive strength in kPa
  • K is an empirical constant, typically about 0.089 for many LDPE foams
  • p foam density, normally in kg/cubic meter
  • n is 1.637.
  • compressive strength 1 hour after extrusion typically ranges from about 8 psig for a LDPE foam of density about 20 kg/cubic meter, to about 20 psig at 65 kg/cubic meter, about 41 to 43 psig at 148 to 151 kg/cubic meter, and about 43 to 45 psig at about 180 kg/cubic meter.
  • Example 7 Die cut properties were evaluated for low density polyethylene foams prepared in accordance with the invention. Foams were prepared with propane and with conventional butane blowing agent on a tandem line extruder at approximately 454.5 kg/hr flow rate. Plank foams of 57 mm thickness were produced at a density of approximately 39 kg/m .
  • the density of the planks after 45 days of curing was 38.4 kg/cubic meter.
  • a comparison of die cut force for propane foams is compared to that for conventional butane foams as shown in Table 5.
  • the lower die cut force that is demonstrated for the foams of the invention is associated with a foam product having better cut edges with less concavity.
  • the die cut force is the force required to cut the foam using a typical die fixture operated at 20 inches per minute crosshead speed using the Instron 4202 test frame.
  • the higher die cut speed was used to simulate real life situations experienced by foam fabricators.
  • the die cut fixture that was used to obtain the data was a straight knife, 2 inches long and 1 inch deep. This knife was attached to the top-loading nose of the Instron machine and a foam speciman of density 38.4 kg/cubic meter was placed on the bottom circular plate. The maximum force needed to cut into the foam and the distance and time required to initiate cutting were recorded as the top-loading nose traveled in the downwards direction at a speed of 20 inches/minute.
  • Table 5 shows that the propane foams of the invention exhibited stabilized die cut time only 9 days after extrusion. In comparision, conventional butane foams take about 30 days to achieve die cut stability.
  • Table 6 shows that it takes less time to cut through the foams of the invention as compared to typical foams prepared from conventional butane blowing agents and that the foam ages faster with the propane blowing agent of the invention. Table 6 compares die cut time for the propane and isobutane foams.
  • Die cut time is defined as the time required to cut the foam using a typical die fixture, as described in connection with Table 5, at 20 inches per minute crosshead speed using the Instron 4202 test frame.
  • the die cut time leveled out after 9 days for the propand blowing agent.
  • the die cut time stabilized after only 30 days for the conventional butane blowing agent.
  • Tables 5 and 6 show that for the foams of the invention, the die cut force and die cut time is less than for foams prepared with conventional butane blowing agents and that there is less concavity produced after die cutting.
  • Table 7 below shows that the maximum distance traveled by the die cut knife to obtain a die cut is less for the foams of the invention than for those prepared from conventional butane blowing agent. Table 7 shows that the die knife travels a longer distance into the foams prepared from butane blowing agent before the foam is cut, which creates a higher concavity in the conventional foams and leaves a dip impression. Die cut distance for the propane and butane foams is compared in Table 7.
  • Example 8 A test was performed to compare the flexural stiffness of foams prepared from conventional butane blowing agents with that of foams prepared from propane blowing agents of the invention. Bending force was measured at 1 inch deflection after 14 days of aging for propane and butane foam samples of density 38.4 kg/m and measuring 2 inches in width by 7 inches in length and 57 mm thick. The foams of the invention were 11.1% stiffer than conventional butane foams as shown in Table 8, demonstrating a higher flexural stiffness for the foams of the invention.
  • the measurements were taken in accordance with ASTM D790 modified for applicability to foams.
  • a foam of rectangular cross section rests on two supports and is loaded by a loading nose midway between the supports.
  • the apparatus was the Instron 4202 bending apparatus as supplied by Instron with a standard 3 point bending fixture and operated at 20 inches per minute crosshead speed deflection.
  • the rectangular foam samples were placed on the two supports separated by a span length of 5 inches.
  • the loading nose had curved radii with a smooth surface to avoid compression and excessive indentation during bending and to accurately reflect plank stiffness.
  • the bending nose deflected the foam samples at the center point of the sample and measurements were taken of the average maximum force needed to bend the foam sample to 1 inch deflection after 14 days of aging. Three specimens of each of the samples were examined and the data averaged.

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Abstract

L'invention se rapporte à une mousse de polyoléfine extrudée se présentant en épaisseurs d'au moins 12 mm environ, qui est dotée d'une résistance à la compression et d'une viscosité d'extension améliorées, et d'une durée de vieillissement réduite. L'invention se rapporte également à un procédé de fabrication de ladite mousse. Cette mousse est préparée à l'aide d'un agent d'expansion à base de propane. Les caractéristiques avantageuses conférées aux mousses et aux panneaux mousse de qualité supérieure en épaisseur par un agent d'expansion à base de propane peuvent être obtenues avec des quantités atteignant 75 % en volume de butane normal ou d'isopropane, ou de ces deux composés, dans l'agent d'expansion, suivant la polyoléfine sélectionnée.
EP00911611A 2000-01-21 2000-01-21 Mousse de polyolefine extrudee de qualite superieure en epaisseur et panneau mousse et procede associes Withdrawn EP1252224A1 (fr)

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CN114940738B (zh) * 2022-06-02 2024-02-27 万华化学（宁波）容威聚氨酯有限公司 一种聚氨酯组合物、聚氨酯泡沫及其制备方法与应用

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DE4201763C2 (de) * 1991-01-24 2002-07-18 Astro Valcour Inc Polyolefin-Schaum, diesem zugrundeliegende Polymermasse sowie Verfahren zur Herstellung des Schaums
DK0602262T3 (da) * 1992-12-15 1997-09-01 Scriptoria Nv Baneformede skumplastplader af polyolefiner
US5667728A (en) * 1996-10-29 1997-09-16 Sealed Air Corporation Blowing agent, expandable composition, and process for extruded thermoplastic foams

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