WO2001036521A2

WO2001036521A2 - Thermoformed polyolefin foams and methods of their production

Info

Publication number: WO2001036521A2
Application number: PCT/US2000/041825
Authority: WO
Inventors: Kent G. Blizard
Original assignee: Trexel, Inc.
Priority date: 1999-11-05
Filing date: 2000-11-03
Publication date: 2001-05-25
Also published as: WO2001036521A3; AU2750201A

Abstract

The present invention provides thermoformed polyolefin foam articles and methods of their production. The thermoformed articles have a density of less than or equal to the density of the precursor foam sheets from which they are formed. The thermoformed articles may also have a thickness of greater than or equal to the thickness of the precursor foam sheets. The precursor foam sheets can be produced in an extrusion process and may be microcellular materials. A variety of thermoformed polyolefin foam articles, including deep drawn articles, can be produced.

Description

THERMOFORMED POLYOLEFIN FOAMS AND METHODS OF THEIR

PRODUCTION

Field of Invention

The present invention relates generally to polymeric foams, and more particularly to thermoformed polyolefin foams and methods of their production.

Background of the Invention Polymeric foams include a plurality of voids, also called cells, in a polymer matrix.

By replacing solid plastic with voids, polymeric foams use less raw material than solid plastics for a given volume. Thus, by using polymeric foams in certain applications instead of solid plastics, material costs may be reduced. It can be useful to characterize a foam by features of its cellular structure such as cell size, cell density, and the degree of cell interconnectivity. Microcellular foams (or microcellular materials) are a class of polymeric foams that have small cell sizes and high cell densities.

Polymeric foams can be produced using a number of known techniques. In an extrusion process, for example, foamed polymeric materials can be produced by introducing a physical blowing agent into a molten polymeric stream mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Alternatively, in the extrusion process, a chemical blowing agent can be added and caused to react in the molten polymeric stream, resulting in the generation of gas that forms cells in the polymer. The extrusion process can be used to produce polymeric material in sheet form.

In some cases, it is desirable to further process extruded foam sheets, for example, in a thermoforming process. The thermoforming process involves forming plastic sheets into parts through the application of heat and pressure. In the process, a heated sheet is typically forced against the contours of a mold by positive pressure or vacuum. In a technique known as "plug assist," a moveable plug may be used in conjunction with blown air or vacuum or both to force the sheet against the mold. Plug assist techniques may permit the formation of articles with deep draws.

Several patents describe aspects of thermoforming and, in particular, thermoforming of polypropylene foam sheets. U.S. Patent No. 5,149,579 (Park et al; September 22, 1992) discloses a thermoformable, rigid or semi-rigid polypropylene foam sheet. The sheet is produced by extruding high melt strength, high melt elasticity polypropylene, characterized by at least (a) either high M_z /M_w ratio, and (b) either high equilibrium compliance J_eo obtained from creep measurements or high recoverable strain per unit stress Sr/S obtained from steady shear measurements.

U.S. Patent No. 5,286,428 (Hayashi et al.; February 15, 1994) discloses a polypropylene resin foam sheet suitable for thermoforming. The sheet contains 10 to 50 percent by weight of an inorganic fine powder and has a density of 0.2 to 1.2 g/cm³. U.S. Patent No. 5,338,764 (Lesca et al.; August 16, 1994) discloses foamed polypropylene articles. The articles are prepared by subjecting pre-foamed beads to thermoforming by sintering. In one embodiment, a mold having the desired dimensions is filled with pre-formed beads and heated to a proper temperature to obtain finished articles with a homogenous structure, essentially without voids between the beads. The prior art thermoforming techniques of polyolefin foams, known to the Applicant, result in an increase in foam density and a decrease in foam thickness during thermoforming. That is, the density of the thermoformed article is greater than the density of the foam sheet from which its formed, and the thickness of the thermoformed article is less than the thickness of the sheet from which it is formed. This increase in density and decrease in thickness may arise from cell elongation or cell collapse, both of which may reduce mechanical properties of the thermoformed article. The increase in density also may increase the thermal conductivity of the article and, thus, reduce its ability to function as an insulator. Furthermore, the decrease in thickness during thermoforming, may reduce the rigidity of the resulting article because rigidity is proportional to thickness cubed. Accordingly, a need exists for a process for thermoforming polypropylene articles which does not increase the density and/or decrease the thickness of the material during the process.

Summary of Invention One aspect of the invention involves the Applicant's surprising discovery which provides a thermoformed polyolefin foam article having a density of less than or equal to the density of the precursor sheet from which it is formed. The thermoformed articles also may have a thickness of greater than or equal to the thickness of the precursor sheet. The precursor polyolefin sheet can be produced in an extrusion process, for example, and may be a microcellular material. A variety of thermoformed polyolefin foam articles, including deep-drawn articles, may be produced in accordance with the invention. One aspect of the invention provides a thermoformed polyolefin foam article formed from a precursor polyolefin foam sheet, wherein the thermoformed polyolefin foam article has a density of less than or equal to the density of the precursor polyolefin foam sheet. In another aspect, the invention provides a thermoformed polyolefin foam article formed from a precursor polyolefin foam sheet, wherein the thermoformed polyolefin foam article has a thickness of greater than or equal to the thickness of the precursor polyolefin foam sheet.

In another aspect, the invention provides a method of forming a foam article. The method includes thermoforming a precursor polyolefin foam sheet to form a thermoformed polyolefin foam article having a density of less than or equal to the density of the precursor polyolefin foam sheet.

In another aspect, the invention provides a method of forming a foam article. The method includes thermoforming a precursor polyolefin foam sheet to form a thermoformed polyolefin foam article having a thickness of greater than or equal to the thickness of the precursor polyolefin foam sheet. Among other advantages, the invention can provide thermoformed articles without an increased density and/or reduced thickness as compared to the precursor foam sheets from which the articles are made. As a result, relatively thick, low-density thermoformed foam articles can readily be produced.

The thermoformed articles may have a uniform and uncollapsed cell structure throughout their thickness. In particular, cell collapse at the edges and corners of the thermoformed articles, which generally occurs during thermoforming, may be eliminated. Further, precursor sheets having a small cell size and/or a thin skin may be preferred for the production of the thermoformed articles of the present invention.

The thermoformed polyolefin foam articles typically exhibit excellent mechanical properties which may result from the uniform cell structure and/or article thickness. In particular, relatively thick, low-density thermoformed articles of the invention may have the same rigidity as conventional thermoformed articles which have higher densities, lower thicknesses, and greater weights. The thermoformed articles of the invention at lower weights, therefore, may replace conventional thermoformed articles in certain applications, which can result in significant material and cost savings.

The invention also may permit reducing precursor sheet sag during the thermoforming process by using lower weight precursor sheets than in conventional thermoforming processes. Sheets with lower weights sag less during thermoforming. Generally, the sag of the sheet limits the width of the sheet that can be used in a thermoformer. Thus, reducing the sag enables a wider sheet width to be used and, therefore, increases the dimensions of the thermoformed articles which can be made. The thermoformed articles of the invention also may have a higher quality surface finishes as compared to some conventionally thermoformed articles. In particular, the surface finish can enhance the printability of the thermoformed articles of the invention.

Other advantages, novel features, and aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying figures and from the claims. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

Brief Description of the Drawings

Fig. 1 schematically illustrates an extrusion system for producing a precursor polyolefin foam sheet.

Fig. 2 schematically illustrates a multi-hole blowing agent feed orifice arrangement and extrusion screw.

Figs. 3 and 3A schematically illustrate a thermoformer during the heating and the forming stages of the thermoforming cycle, respectively.

Detailed Description of the Invention The various embodiments and aspects of the invention will be better understood from the following definitions. Microcellular foam or microcellular material is defined as a foamed material having an average cell size of less than about 100 microns, or material of cell density of generally greater than at least about 10⁶ cells/cm³, or preferably both. Cell density is defined as the number of cells per cubic centimeter of unexpanded, solid plastic.

The present invention provides thermoformed polyolefin foam articles which have a density of less than or equal to the density of the precursor foam sheets from which they are formed. The thermoformed polyolefin foam articles according to the invention may also have a thickness of greater than or equal to the thickness of the precursor foam sheets from which they are formed. The precursor sheet has a cell structure, as described further below, which may promote density reduction and thickness enlargement during the thermoforming process. The cell walls of the precursor sheet generally are strong enough to resist rupture during the expansion of the gas in the cells when the sheet is heated. The cell walls are also generally strong enough to resist collapse as a result of forming pressures. As a result of the thermoforming process, the cells generally expand in size which leads to the thermoformed article having an equal or lower density, and typically an equal or greater thickness, than the precursor sheet. It is to be understood that though the present invention provides a thermoformed polyolefin foam article having a density of less than or equal to the density of the precursor sheet, in some embodiments certain portions of the thermoformed article may have a density of greater than the precursor sheet. Such portions, for example, may exist at areas which are particularly compressed or drawn during the thermoforming process. The present invention provides embodiments in which at least a portion of the article has a density of less than or equal to the density of the precursor sheet and embodiments in which the average density of the thermoformed article is less than or equal to the density of the precursor sheet. Similarly, the thermoformed articles in some embodiments may have portions which have a thickness that is less than the thickness of the precursor sheet. These portions also may exist in areas which are particularly compressed or drawn during the thermoforming process. The present invention provides embodiments in which at least a portion of the article has a thickness of greater than or equal to the thickness of the precursor sheet and embodiments in which the average thickness of the thermoformed article is less than or equal to the thickness of the precursor sheet. In preferred embodiments, the precursor foam sheet is a microcellular material. It is to be understood that the thermoformed article also may be preferably a microcellular material. In certain embodiments, the microcellular material has an average cell size of less than about 100 microns, in other embodiments less than about 75 microns, in other embodiments less than about 50 microns, and in other embodiments less than about 30 microns. The microcellular material, in certain cases, preferably has a maximum cell size of about 100 microns. In embodiments where particularly small cell sizes are desired for the precursor sheet, the maximum cell size can be about 50 microns, or preferably about 40 microns, and more preferably still about 30 microns. A set of embodiments includes all combinations of these noted average cell sizes and maximum cell sizes. For example, one embodiment in this set of embodiments includes microcellular material having an average cell size of less than about 30 microns with a maximum cell size of about 50 microns and as another example an average cell size of less than about 30 microns with a maximum cell size of about 35 microns, etc. That is, the precursor sheet can be designed for a variety of purposes and can be produced having a particular combination of average cell size and a maximum cell size preferable for that purpose. Additionally, greater density reduction may be observed with microcellular foam precursor sheets after thermoforming compared to non- microcellular foam precursor sheets.

The precursor foam sheet may have a cell density of greater than about 10 cells/cm . In certain cases, the sheet includes a cell density of greater than about 10⁷ cells/cm\ in some cases greater than 10⁸ cells/cm³, and in other cases greater than about 10⁹ cells/cm³. Preferably, the cell density across the cross-section of the precursor foam sheet is substantially uniform. However, in other embodiments, the cell density may increase or decrease across the precursor foam sheet cross-section. Thus, in one case, the cell density may decrease from the center of the precursor foam sheet to its surface. In particular, the precursor foam sheet may have very low cell density or no cells near its surfaces.

In some embodiments, the cell size across the cross-section of the foam precursor sheet is substantially uniform. That is, the cell size across the cross-section of the precursor sheet generally varies by less than 20% of the average cell size. However, in other embodiments, the precursor foam sheet may have a cell size distribution or cell size gradient. In particular, the precursor foam sheet may have a gradient of cell sizes wherein it has larger cells near its center and smaller cells near its surface. Also, in some embodiments the foam sheet may have a closed cell structure. A closed cell structure has limited interconnection between adjacent cells and, as used herein, is meant to define a material that at a thickness of about 0.050 inches contains no connected cell pathway through the material. In other embodiments, the foam precursor sheet may have no skin or a thin skin. As used herein, skin thickness refers to the shortest distance from the surface to the closest cell averaged across the sheet surface. In other embodiments, the skin thickness is less than about 50 microns, in other embodiments less than about 25 microns, in other embodiments less than about 10 microns, in yet other embodiments less than 5 microns and still other embodiments less than about 1 micron. It may be preferable to utilize precursor sheets with no skin or thin skin to form thermoformed microcellular articles with greater density reduction from after thermoforming. In some cases, precursor sheets having thicker skin may not be preferable to form density reduced thermoformed articles even if the sheets have small cell sizes. The foam precursor sheet in the present invention can be produced over a broad density range as desired for the particular application. The density can be controlled by selecting appropriate processing parameters. In most cases, the precursor sheet has a density of between about 0.05 g/cm³ and 0.9 g/cm³. In some cases, the precursor sheet has a density between about 0.5 g/cm³ and 0.8 g/cm³, and, in some cases, between about 0.6 g/cm³ and 0.75 g/cm³.

The precursor polyolefin sheet also can be produced over a range of thicknesses. The sheet may have a thickness of between about 0.020 inches and about 0.080 inches. In certain embodiments, the precursor polyolefin foam sheet is between about 0.030 inches and about 0.040 inches. In other cases, particularly when deeper draws are desired, the sheet has a thickness between about 0.060 inches and about 0.080 inches.

The present invention encompasses precursor sheets, and thus thermoformed articles, including any polyolefin material or blends or copolymers thereof. In one set of embodiments, the precursor sheets and articles are made of polypropylene material. In some embodiments of this set, the material is a fractional melt flow polypropylene homopolymer. As known in the art, a fractional melt flow material has a melt flow index of less than 1. In other embodiments of this set, the material is a fractional melt flow polypropylene co- polymer. In another set of embodiments, the sheets and thermoformed articles are made of polyethylene material. In certain embodiments of this set, the material is a high-density polyethylene. In other embodiments of this set, the material is a fractional melt flow high- density polyethylene.

Though the precursor sheets and thermoformed articles of the invention include polyolefin material as the major component, the sheets and articles also may include a variety of other components. Such components can be other polymeric materials, fillers, nucleating agents, plasticizers, lubricants, colorants or any other additive or processing aid known in the art. In certain preferred cases, the precursor sheet and thermoformed articles include a percentage of talc and/or titanium dioxide in particulate form. Generally, talc and/or titanium dioxide is present in amounts between about 5% and about 30% by weight of the total polymer composition.

Where other polymers are blended with or copolymerized with polyolefins, olefin is present in an amount of at least about 50% by weight, based on the weight of the blend or copolymer, preferably at least about 70% by weight, more preferably at least about 80% by weight, more preferably at least about 90% by weight, and more preferably still at least about 95% by weight. Where olefin is a component of a copolymer, the copolymer can be a block copolymer, random copolymer (in which case "polyolefin" means olefin component of a copolymer), radial block copolymer, teleblock copolymer, etc. These systems are well- known in the art. Though the precursor sheet may be produced using a number of techniques, preferably, the sheet is extruded. One illustrative embodiment of an extrusion system 30 for the production of the precursor sheet is shown schematically in Fig. 1. The extrusion system includes a screw 38 that rotates within a barrel 32 to convey, in a downstream direction 33, polymeric material in a processing space 35 between the screw and the barrel. The polymeric material is extruded through a die 37 fiuidly connected to processing space 35 and fixed to a downstream end of barrel 32. Die 37 includes inner passageways (not illustrated) to shape an extrudate 39 which is used to form the precursor sheet.

Extrusion screw 38 is operably connected at its upstream end, to a drive motor 40 which rotates the screw. Although not shown in detail, extrusion screw 38 may include feed, transition, gas injection, mixing, metering and cooling sections. Positioned along extrusion barrel 32, optionally are temperature control units 42. In some embodiments, the temperature control units 42 are also positioned on die 37. Control units 42 can be electrical heaters, can include passageways for temperature control fluid, or the like as is known in the art.

Units 42 can be used to heat a stream of pelletized or fluid polymeric material within the extrusion barrel to facilitate melting, and/or to cool the stream to control viscosity, skin formation and, in some cases blowing agents solubility. The temperature controlling units can operate differently at different locations along the barrel, that is, to heat at one or more locations, and to cool at one or more different locations. Any number of temperature control units can be provided.

Extrusion barrel 32 is constructed and arranged to receive a precursor of a fluid polymeric material. The polymeric material, as described above, is a polyolefin material. Typically, the system includes a standard hopper 44 for containing pelletized polymeric material to be fed into the extruder barrel through an orifice 46, although a precursor can be a fluid prepolymeric material injected through an orifice and polymerized within the barrel via, for example, auxiliary polymerizing agents. The pellets can be compounded to include additives such as talc or titanium oxide, or in other cases the additives may be added to the hopper separately.

Pellets, generally, are received into the feed section of the screw and conveyed in a downstream direction as the screw rotates. Heat from extrusion barrel 32 and shear forces arising from the rotating screw, act to soften the pellets within the transition section.

The blowing agent is introduced into the polymer stream through a port 54 in fluid communication with a source 56 of a physical blowing agent. The port can be positioned to introduce the blowing agent at any of a variety of locations along the extrusion barrel 32. Preferably, as discussed further below, the port introduces blowing agent at the gas injection section of the screw, where the screw includes multiple flights.

Any of a wide variety of blowing agents known to those of ordinary skill in the art such as hydrocarbons, chlorofluorocarbons, nitrogen, carbon dioxide, and the like can be used in connection with this embodiment of the invention and, according to a preferred embodiment, source 56 provides carbon dioxide as a blowing agent. In another preferred embodiment, source 56 provides nitrogen as a blowing agent. In particularly preferred embodiments, solely carbon dioxide or nitrogen is respectively used. A pressure and metering device 58 typically is provided between blowing agent source 56 and port 54.

Blowing agents that are in the supercritical fluid state in the extruder are especially preferred, in particular supercritical carbon dioxide and supercritical nitrogen.

Device 58 can be used to meter the blowing agent so as to control the amount of the blowing agent in the polymeric stream within the extruder to maintain a level of blowing agent. In a preferred embodiment, device 58 meters the mass flow rate of the blowing agent. Though the amount of blowing agent depends upon the particular process, the blowing agent is generally less than about 15% by weight of polymeric stream and blowing agent. In some embodiments, blowing agent levels of less than 10%, or less than 5%, by weight of polymeric stream and blowing agent are used. In other embodiments, lower levels of blowing agent may be used such as less than 2%, or even less than 1%, by weight of the polymeric stream and blowing agent. In most embodiments, when nitrogen is used the blowing agent levels are lower than when carbon dioxide is used. In some embodiments, talc may be used as a nucleating agent which permits using lower levels of blowing agent.

The pressure and metering device can be connected to a controller (not shown) that also is connected to drive motor 40 and/or a drive mechanism of a gear pump (not shown) to control metering of blowing agent in relationship to flow of polymeric material to very precisely control the weight percent blowing agent in the fluid polymeric mixture.

Referring now to Fig. 2, a preferred embodiment of the blowing agent port is illustrated in greater detail and, in addition, two ports on opposing top and bottom sides of the barrel are shown. In this preferred embodiment, port 54 is located in the gas injection section of the screw at a region upstream from a mixing section 60 of screw 38 (including highly- broken flights) at a distance upstream of the mixing section of no more than about 4 full flights, preferably no more than about 2 full flights, or no more than 1 full flight. Positioned as such, injected blowing agent is very rapidly and evenly mixed into a fluid polymeric stream to promote production of a single-phase solution of the polymeric material and the blowing agent. Port 54, in the preferred embodiment illustrated, is a multi-hole port including a plurality of orifices 64 connecting the blowing agent source with the extruder barrel. In other embodiments, a single blowing agent port may be used including a single orifice or a plurality of orifices. As shown, in preferred embodiments a plurality of ports 54 are provided about the extruder barrel at various positions radially and can be in alignment longitudinally with each other. For example, a plurality of ports 54 can be placed at the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions about the extruder barrel, each including multiple orifices 64. In this manner, where each orifice 64 is considered a blowing agent orifice, the extrusion system includes at least about 2, in others at least about 4, in others at least about 10, in others at least about 40, in others at least about 100, in others at least about 300, in others at least about 500, and in others still at least about 700 blowing agent orifices in fluid communication with the extruder barrel, fiuidly connecting the barrel with a source of blowing agent. Also in preferred embodiments is an arrangement (as shown in Fig. 2) in which the blowing agent orifice or orifices are positioned along the extruder barrel at a location where, when a preferred screw is mounted in the barrel, the orifice or orifices are adjacent full, unbroken flights 65. In this manner, as the screw rotates, each flight passes or "wipes" each orifice periodically. This wiping increases rapid mixing of blowing agent and fluid foamed material precursor by, in one embodiment, essentially rapidly opening and closing each orifice by periodically blocking each orifice, when the flight is large enough relative to the orifice to completely block the orifice when in alignment therewith. The result is a distribution of relatively finely-divided, isolated regions of blowing agent in the fluid polymeric material immediately upon injection and prior to any mixing. In this arrangement, at a standard screw revolution speed of about 30 rpm, each orifice is passed by a flight at a rate of at least about 0.5 passes per second, more preferably at least about 1 pass per second, more preferably at least about 1.5 passes per second, and more preferably still at least about 2 passes per second. In preferred embodiments, port 54 is positioned at a distance of from about 15 to about 30 barrel diameters from the beginning of the screw.

Referring again to Fig. 1, screw 38, following the blowing agent injection port preferably, includes a mixing section having a series of unbroken flights which break up the stream to encourage mixing of the blowing agent and polymer stream. Preferably, a single- phase solution of the blowing agent and polymer stream is formed prior to die 37. Die 37 includes internal passageways (not shown) fiuidly connected to polymer processing space 35. The shape and dimensions of the internal passageways can be configured, as known in the art, to control the shape of extrudate 39. For example, when producing microcellular material, as known in the art, the dies are designed with specific dimensions that result in sufficient pressure drops, pressure drop rates, and other factors. Preferred dies used in microcellular processing are described in international patent publication no. WO 98/08667, filed August 26, 1997, by Burnham et al., which is incorporated herein by reference.

The die, in some embodiments, can be a sheet die which extrudes the material in sheet form. In other embodiments, the die can be an annular die which extrudes the material in annular form that can be cut, as known in the art, to produce sheets.

In certain embodiments, a thermoformer is installed downstream of the extruder to provide in-line thermoforming of the extruded sheet. More typically, the extruded sheet is collected and stored for a time period prior to thermoforming. The time period is preferably long enough to permit diffusion of the blowing agents out of the cells and/or the diffusion of air into the cells of the precursor sheet. In these embodiments, the thermoforming process is conducted separately from the extrusion process. When CO₂ or N₂ gas is used in the production of the precursor sheet, the time period between extrusion and thermoforming may advantageously be significantly less than the time period required in conventional processes for forming precursor sheet that use high molecular weight blowing agents, such as HCFC^'s. High molecular weight blowing agents diffuse much slower than C0₂ or N and, thus, sheets produced using high molecular weight blowing agents may require longer times between extrusion and thermoforming.

Referring to Figs. 3 and 3 A, one illustrative embodiment of a thermoformer 70 is schematically shown. The thermoformer includes an oven 72 having a heating element 74 for heating a precursor foam sheet 76. The precursor sheet may be secured to a female mold half 78 with a clamping element 80. The female mold half has a surface 82 contoured to produce the thermoformed article in the desired shape. Vacuum holes 84 in the female mold half connect a space 86 below the precursor sheet to a vacuum pump (not illustrated).

It is to be understood that the thermoformer 70 may have a variety of configurations. In some configurations, the female mold is provided on a sliding assembly which can move into and out of the oven at appropriate times during the thermoforming cycle. In other configurations, the oven may be moveable relative to the female mold half. In other configurations, a heating element may be used without the surrounding oven. Other configurations may employ the use of a plug which moves relative to the female mold half during the thermoforming cycle. Any thermoforming configuration known in the art is suitable for use in the invention. During use, the precursor foam sheet is transferred to the oven and heated to an appropriate temperature for a selected time period (Fig. 3). After the selected time, the pump is activated to create a vacuum in space 86. The vacuum forces the precursor sheet against the contours of the mold (Fig. 3 A). The female mold half is then typically removed from the oven to permit cooling of the mold and article. After an appropriate cooling time, the thermoformed article may be removed from the female mold half by an appropriate ejection mechanism (not shown). The processing parameters such as heating temperatures, heating times, and cooling times may be selected, as known in the art, according to sheet thickness, sheet density, material type, the shape of the desired product, and other factors.

As discussed above, the thermoformed polyolefin foam articles have a density of less than or equal to the density of the precursor foam sheet. The thermoformed articles also may have a thickness of greater than or equal to the thickness of the precursor sheet. The actual decrease in density and/or increase in thickness depends upon processing parameters in the thermoforming process such as thermoforming temperature and time. Generally, the density is significantly lower and the thickness is significantly greater of the thermoformed part as compared to the precursor sheet at shorter oven times. At longer oven times, the density and thickness may be approximately equal to the density and thickness of the precursor sheet. In some embodiments, the percentage of density reduction is greater than about 5%, in other embodiments greater than about 10%, and in still other embodiments greater than about 20%. In some embodiments, the percentage of thickness increase is greater than about 5%, in other embodiments greater than about 10%, and in still other embodiments greater than about 20%. Depending on the density of the precursor sheet and the degree of density reduction in the thermoforming process, thermoformed articles can be produced over a wide range of density. In some embodiments, the thermoformed polyolefin foam articles have a density between about 0.1 g/cm and about 0.9 g/cm\ In other embodiments, thermoformed articles have a density between about 0.5 g/cm³ and 0.8 g/cm³, and in other embodiments between about 0.6 g/cm³ and 0.75 g/cm³.

The thermoformed foam articles according to the invention can be produced in a number of shapes and sizes as can typically be done in thermoforming. Cups, plates, bowls and similar articles can be readily formed. In one set of embodiments, the thermoformed articles have large draw ratios. A draw ratio is the ratio of the depth of the article to its opening dimension at the top. For example, foam articles according to this set of embodiments may have draw ratios of greater than 0.4, greater than 0.7, greater than 1.1, and in some cases greater than 1.5. The cell structure of the foam precursor sheets generally permits deeper draws than in conventional foams. In particular, it is believed that the more uniform and smaller cells in the precursor sheet enables deeper draws than conventional foams which tend to rupture at weak points which may arise from, for example, large or interconnected cells. The function and advantage of these and other embodiments of the present invention will be more fully understood from the examples below. The following examples are intended to illustrate the benefits of the present invention, but do not exemplify the full scope of the invention.

Example 1. Extrusion of a Polypropylene Foam Precursor Sheet

A tandem extrusion line including a 2 ' _ in, 32:1 L/D single screw primary extruder (Akron Extruders, Canal Fulton, OH) and a 3 in, 36:1 L/D single screw secondary extruder (Akron Extruders, Canal Fulton, OH) was arranged in a parallel configuration. An injection system for the injection of CO into the secondary was placed at approximately 8 diameters from the inlet to the secondary. The injection system included 4 equally spaced circumferential, radially-positioned ports, each port including 176 orifices, each orifice of 0.02 inch diameter, for a total of 704 orifices. The injection system included an air actuated control valve to precisely meter a mass flow rate of blowing agent at rates from 0.2 to 12 lbs/hr at pressures up to 5500 psi.

The screw of the primary extruder was specially designed to provide feeding, melting, and metering of the solid plastic pellets. The screw also included a mixing section for the dispersion of blowing agent in the polymer. The outlet of this primary extruder was connected to the inlet of the secondary extruder using a transfer pipe of about 24 inches in length.

The secondary extruder was equipped with specially designed deep channel, multi- flighted screw design to cool the polymer and maintain the pressure profile of the microcellular material precursor, between injection of blowing agent and the die.

Two types of polypropylene pellets were blended in the hopper and then gravity fed into the extruder. The first type was a standard homopolymer polypropylene resin having a nominal melt flow index of 0.5 g/10 min (Montell 6823). The second type was a talc/PP concentrate containing 40% by weight of talc (Spartech Polycom EP5140 Al). The talc/PP concentrate pellets were added using a side auger feeder into the hopper. The side auger screw speed was adjusted to feed polymeric material having 5% by weight talc into the extruder.

The primary screw speed was set to 48 RPM, giving a total output of approximately 100 lbs/hr of material. The secondary screw speed was set to 16 RPM. The barrel temperatures of the secondary extruder were set to maintain a melt temperature of 424°F measured at the end of the secondary extruder. CO₂ blowing agent was injected at a rate of 0.5 lbs/hr resulting in 0.5% blowing agent by weight of polymeric material. A die adapter at the discharge of the secondary extruder was connected to a flat sheet T-type die having a die exit of 11 inches width and a gap of 0.030 inch. A parallel land of 0.5 inch length immediately preceded the exit of the die. The die had both melt and pressure indicators. The pressure profile between the injection ports and the inlet of the die was maintained between 1580 and 1960 psi.

The extruded sheet was taken up using a three-roll stack. The material was nipped between the top and middle rolls, with the application of nominal pressure to obtain a good surface finish. The temperature of all three rolls was maintained at 100 °F by use of recirculating oil. The rolls were driven at 11.4 feet/minute, resulting in a final sheet thickness of about 0.036 inches and a final sheet width of about 10 inches after trimming of the edges. Scanning electron microscopy was performed on the cross-section of the extruded precursor sheet by fracturing the sheet in the transverse direction. The microscopy analysis determined that the precursor sheet was microcellular material. A uniform dispersion of cells having an average diameter of about 35 - 50 microns was observed. The density of the foam precursor sheet was measured by a densimeter (Mettler Toledo AG104) to be 0.71 g/cm .

Example 2. Thermoforming a foamed polypropylene precursor sheet into a shallow rectangular bowl

The polypropylene foam precursor sheet produced in Example 1 was used. A laboratory thermoformer was used to thermoform the sheet. The thermoformer had a frame of approximately 9 x 13 inches for clamping the sheet and included sliding rails leading into an oven. The oven included upper and lower banks of ceramic heaters. A potentiastat was used to control the equilibrium temperature of the heaters. The forming station of the thermoformer included a female mold having a rectangular cavity with an insert that defined its depth. The dimensions of the mold cavity were nominally 3 inches square by 1.375 inches deep. Holes were provided in the surface of the female mold which connected to a vacuum pump. The forming station also included a plug capable of moving in a downward direction toward the surface of the female mold. The sheet was clamped to the frame and inserted into the oven. Heater temperature was maintained at approximately 900 °F which resulted in a sheet temperature of less than 330 °F. The sheet was withdrawn from the oven after a dwell time of about 9 seconds and inserted in the forming station. The female mold was then raised to the bottom surface of the sheet, while the plug moved downward and the vacuum pump was activated to force the sheet against the female mold surface. After cooling period of approximately 20 seconds, the mold was opened and the thermoformed bowl was withdrawn by hand.

The density and thickness of the resulting bowl was measured at different locations on the bowl. The bottom of the bowl had an average density of 0.68 g/cm³ and the sides an average density of 0.62 g/cm³. This represented a density reduction of 27% (bottom) to 33 % (sides) from the initial solid sheet density of 0.92 g/cm³ and a density reduction of 4% (bottom) to 13%) (sides) from the precursor polypropylene foam sheet. The foamed bowl had an average side wall thickness of 0.013 inches and an average bottom thickness of 0.029 inches. Based on final product dimensions, this represented a draw ratio of 0.46: 1 (depth to side wall length).

Example 3. Thermoforming a foamed polypropylene precursor sheet into a rectangular bowl

The polypropylene foam sheet produced in Example 1 was used.

The laboratory thermoformer described in Example 2 was modified to include a different insert that provided a mold depth of 2.25 inches and was used to thermoform the sheet.

The sheet was cut to the correct size, clamped into the thermoformer frame, and transferred into the oven for 8.5 seconds at the same heater setting as in Example 2. The sheet was then withdrawn to the forming station to form the bowl as described above. After a cooling period of approximately 20 seconds, the mold was opened and the thermoformed bowl withdrawn by hand.

The density and thickness of the resulting bowl were then measured. The bottom and side walls of the bowl had an average density of 0.63 g/cm³. This represented a density reduction of 31.5% from the initial solid sheet density of 0.92 g/cm³ and an 1 1% additional density reduction from the precursor polypropylene foam sheet. The foamed bowl had an average side wall thickness of 0.012 inches and an average bottom thickness of 0.017 inches. Based on final product dimensions, this represented a draw ratio of 0.75: 1 (depth to side wall length).

Example 4. Thermoforming a foamed polypropylene precursor sheet into a circular bowl A foamed polypropylene precursor sheet having a density of 0.74 g/cm³ and a thickness of 0.034 inches was used. A laboratory thermoformer was used to thermoform the sheet.

The thermoformer had a frame of approximately 9 x 13 inches for clamping the sheet and sliding rails which permitted automatic insertion into an oven. The oven included upper and lower banks of ceramic heaters. A temperature controller was used to set the temperature of the heaters. The thermoformer included a timer which triggered automatic removal of the sheet after a selected time period.

The forming station included a female mold having a circular cross section cavity with a depth of 2.75 inches and an opening of approximately 4 inches at the top tapering to 3 inches at the bottom. Holes were provided in the surface of the female mold which connected to a vacuum pump. The forming station also included a plug capable of moving in an downward direction toward the surface of the female mold.

The precursor sheet was clamped and automatically inserted into the oven using the sliding rails. The equilibrium temperature of the heaters was set at 560°C. The dwell time of the sheet in the oven was set at 29 seconds, after which the sheet was automatically removed and transferred to the forming station. The female mold was then raised to the level of the sheet, while the plug moved downward and the vacuum pump was activated to force the sheet against the female mold surface. After forming, the mold was cooled for a period of approximately 20 seconds and then opened. The thermoformed bowl was withdrawn by hand.

The density and thickness of the resulting bowl were then measured. The bottom of the bowl had an average density of 0.70 g/cm and the sides an average density of 0.63 g/cm .

This represented a density reduction of 24% (bottom) to 31.5% (sides) from the initial solid sheet density of 0.92 g/cm³, and a 5% (bottom) to 15% (sides) additional density reduction from the precursor foam sheet. The foamed bowl had an average side wall thickness of 0.011 inches and an average bottom thickness of 0.028 inches. Based on diameter of the bowl at the top opening, the thermoformed bowl had a draw ratio of 0.69: 1. Having described several embodiments and examples of the invention in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and the scope of the invention. Furthermore, those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the system of the present invention is used. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined by the following claims and their equivalents.

What is claimed is:

Claims

1. An article comprising: a thermoformed polyolefin foam article formed from a precursor polyolefin foam sheet, wherein the thermoformed polyolefin foam article has a density of less than or equal to the density of the precursor polyolefin foam sheet.

2. An article comprising: a thermoformed polyolefin foam article formed from a precursor polyolefin foam sheet, wherein the thermoformed polyolefin foam article has a thickness of greater than or equal to the thickness of the precursor polyolefin foam sheet.

3. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises microcellular material.

4. The article of claims 1 or 2, wherein the precursor polyolefin foam sheet comprises microcellular material.

5. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a density between about 0.5 g/cm³ and about 0.8 g/cm³.

6. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a density between about 0.60 g/cm and about 0.75 g/cm^J.

7. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a density of less than the density of the precursor polyolefin foam sheet.

8. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a density of at least 5% less than the density of the precursor polyolefin foam sheet.

9. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a density of at least 10% less than the density of the precursor polyolefin foam sheet.

10. The method of claims 1 or 2, wherein the thermoformed polyolefin foam article has a density of at least 20% less than the density of the precursor polyolefin foam sheet.

1 1. The article of claims 1 or 2, wherein the precursor polyolefin foam sheet has a thickness between about 0.020 in and about 0.080 in.

12. The article of claims 1 or 2, wherein the precursor polyolefin foam sheet has a thickness between about 0.030 in and about 0.040 in.

13. The article of claims 1 or 2, wherein the precursor polyolefin foam sheet has a thickness between about 0.060 in and about 0.080 in.

14. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article further comprises talc.

15. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article further comprises titanium dioxide.

16. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article further comprises talc and titanium dioxide.

17. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises polypropylene.

18. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises a fractional melt flow polypropylene homopolymer.

19. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises a fractional melt flow polypropylene copolymer.

20. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises polyethylene.

21. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises high-density polyethylene.

22. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article comprises a fractional melt flow high-density polyethylene.

23. The article of claims 1 or 2, wherein the article has a draw ratio of greater than

0.4.

24. The article of claims 1 or 2, wherein the article has a draw ratio of greater than 0.7.

25. The article of claims 1 or 2, wherein the article has a draw ratio of greater than 1.1.

26. The article of claims 1 or 2, wherein the article has a draw ratio of greater than 1.5.

27. The article of claims 1 or 2, wherein the article has a closed cell structure.

28. The article of claim 1, wherein the thermoformed polyolefin foam article has a thickness of greater than or equal to the thickness of the precursor polyolefin foam sheet.

29. The article of claims 1 or 2, wherein the precursor polyolefin foam sheet has a skin thickness of less than about 50 microns.

30. The article of claims 1 or 2, wherein the precursor polyolefin foam sheet has an average cell size of less than about 100 microns.

31. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a thickness of greater than the thickness of the precursor polyolefin foam sheet.

32. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a thickness of at least 5% greater than the thickness of the precursor polyolefin foam sheet.

33. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a thickness of at least 10% greater than the thickness of the precursor polyolefin foam sheet.

34. The article of claims 1 or 2, wherein the thermoformed polyolefin foam article has a thickness of at least 20% greater than the thickness of the precursor polyolefin foam sheet.

35. The article of claims 1 or 2, wherein the entire thickness of the thermoformed polyolefin foam article is greater than or equal to the thickness of the precursor polyolefin foam sheet.

36. A method of forming a foam article comprising: thermoforming a precursor polyolefin foam sheet to form a thermoformed polyolefin foam article having a density of less than or equal to the density of the precursor polyolefin foam sheet.

37. A method of forming a foam article comprising: thermoforming a precursor polyolefin foam sheet to form a thermoformed polyolefin foam article having a thickness of greater than or equal to the thickness of the precursor polyolefin foam sheet.

38. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article comprises microcellular material.

39. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a density of less than the density of the precursor polyolefin foam sheet.

40. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a density of at least 5% less than the density of the precursor polyolefin foam sheet.

41. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a density of at least 10% less than the density of the precursor polyolefin foam sheet.

42. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a density of at least 20% less than the density of the precursor polyolefin foam sheet.

43. The method of claims 36 or 37, wherein the precursor polyolefin foam sheet comprises microcellular material.

44. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article comprises polypropylene.

45. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article comprises a fractional melt flow polypropylene homopolymer.

46. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article comprises a fractional melt flow polypropylene copolymer.

47. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article comprises high-density polyethylene.

48. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article comprises a fractional melt flow high-density polyethylene.

49. The method of claims 36 or 37 further comprising drawing the precursor polyolefin foam sheet to a draw ratio of greater than 0.4.

50. The method of claims 36 or 37 further comprising drawing the precursor polyolefin foam sheet to a draw ratio of greater than 0.7.

51. The method of claims 36 or 37 further comprising drawing the precursor polyolefin foam sheet to a draw ratio of greater than 1.1.

52. The method of claims 36 or 37 further comprising drawing the precursor polyolefin foam sheet to a draw ratio of greater than 1.5.

53. The method of claims 36 or 37, wherein the precursor polyolefin foam sheet comprises microcellular material.

54. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a thickness of greater than the thickness of the precursor polyolefin foam sheet.

55. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a thickness of at least 5% greater than the thickness of the precursor polyolefin foam sheet.

56. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a thickness of at least 10% greater than the thickness of the precursor polyolefin foam sheet.

57. The method of claims 36 or 37, wherein the thermoformed polyolefin foam article has a thickness of at least 20%) greater than the thickness of the precursor polyolefin foam sheet.