EP0912333A1

EP0912333A1 - Polystyrene foam sheet for forming deep drawn articles, and the deep drawn articles made therefrom

Info

Publication number: EP0912333A1
Application number: EP97925460A
Authority: EP
Inventors: James E. Plankar; Richard L. Mathis; Philip A. Wagner
Original assignee: Solo Cup Co
Current assignee: Solo Cup Co
Priority date: 1996-05-06
Filing date: 1997-05-06
Publication date: 1999-05-06
Also published as: CA2252759A1; EP0912333A4; WO1997042025A1; AU3059197A

Abstract

A polystyrene foam Sheet (A) for thermoforming deep drawn articles. The polystyrene foam sheet contains 0.5 to 15 wt.% of a rubber component, from 0.5 to 2.0 wt.% of a nucleating agent, has a uniform thickness of from 0.060 to 0.250 inches with a tolerance of ± 0.10 inches, a density in the range of 4 to 18 lb/ft3, and an average cell size of 0.12 to 0.34 mm. Optionally, one or more films (22) may be extrusion coated or laminated to the foam sheet prior to thermoforming. The resulting deep drawn articles have draw ratios greater than 1.0, have good heat insulation capacity with an attractive appearance.

Description

POLYSTYRENE FOAM SHEET FOR FORMING DEEP DRAWN ARΗCLES, AND THE DEEP DRAWN ARΗCLES MADE THEREFROM

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polystyrene foam sheet useful for thermoforming deep drawn articles, and the thermoformed deep drawn articles made from the polystyrene foam sheet. More particularly, the invention relates to polystyrene foam sheet having particular thickness, density and cell size characteristics that make the foam sheet suitable for thermoforming deep drawn articles either in a conventional two-stage foam extrusion and thermoforming operation or in a continuous operation without aging of the foam sheet. 2. Description of Prior Art Practices

Foamed polystyrene resins have long been used to fabricate packaging materials such as cups, tubs, bowls, trays and similar articles. One method of manufacturing such foamed polystyrene articles involves preforming a polystyrene foam sheet material utilizing well known thermoplastic extrusion techniques. The polystyrene foam sheet is subsequently preheated and placed between matched male and female mold halves, which, as they close, press and form the sheet into the desired product shape.

An alternate forming arrangement which may be employed to thermoform plastic sheet includes vacuum thermoforming. A vacuum is applied beneath the preheated sheet to be formed causing atmospheric pressure or introduced blown air to push the sheet down into contact with the mold. As the sheet contacts the mold it cools and sets in the desired configuration. Usually those areas of the sheet material which reach the vacuum mold member last are the thinnest, having been drawn to a greater extent than the remainder of the material being formed. Other prior art thermoforming techniques include a two-stage thermoforming technique whereby, utilizing a plug member, a preheated plastic sheet is only partially preformed into a desired configuration and, after the preforming step, the thermoforming step is completed whereby the matched mold members come together to form the desired finished article. U.S. Patent No. 3,825,166 discloses such a forming method.

Although these techniques are useful for forming polystyrene foam sheet into relatively shallow articles, such as bowls and trays, one limitation of the prior art practices has been the inability to easily form articles, such as cups, which have a depth to width ratio greater than 1.0. Draw ratios greater than 1.0 are known in the art as "deep draws".

One conventional approach for manufacturing deep drawn, cup-like articles has utilized beads of expandable thermoplastic resin. The thermoplastic beads are introduced into the mold cavity, and then are heated so that the beads expand and fuse together into a solid article. Because the mutual fusion of the beads is weak, however, the finished article has a reduced strength and reduced water-tightness, resulting in an article that can be easily crushed or may weep or leak.

Another approach for making deep drawn cups 5

is to separately form the sidewalls and the bottoms of the cups and then piece them together to form the article. Such articles, however, suffer from reduced water-tightness due to their pieced-together construction. U.S. Patent No. 3,969,173 illustrates such articles.

In view of these drawbacks with the prior art techniques, the art has been searching for a way to successfully manufacture deep drawn cup-like articles by thermoforming the article as a unitary piece from a single foam sheet.

In U.S. Pat. No. 4,528,221 there is disclosed a polystyrene foam sheet suitable for thermoforming into containers, such as cups and trays. The foam sheet must have a polystyrene resin as the base resin, 1-30% (percent) of a rubber component and 1-20% of a filler component. In addition the foam sheet must have a bulk density of 0.13-0.7 g/cm³ (grams/centimeter cubed) (8.12-43.7 pounds per cubic feet) , a stretch ratio less than 1.25 and an amount of residual blowing agent less than 0.3 mole/kg (moles of blowing agent/kilogram) . According to the patent, the polystyrene foam sheet is suitable for thermoforming deep drawn articles, such as cups. However, none of the working examples in the patent employ a draw ratio of greater than 1.0. Rather, the articles made in the examples all have draw ratios of less than or equal to 1.0. Polystyrene foam sheet developed heretofore has not met the requirements for successful and efficient application to deeply drawn low-density foamed thermoplastic articles. SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a polystyrene foam sheet which can be formed efficiently into deeply drawn articles.

Another object of the present invention is to provide a polystyrene foam sheet which can be thermoformed into deeply drawn articles in a continuous extrusion and thermoforming operation without aging the foam sheet prior to thermoforming.

Another object of the present invention is to provide a thermoformed deeply drawn article made as a unitary article from the polystyrene foam sheet.

A further object of the present invention is to provide a thermoformed deeply drawn article having superior strength, crush resistance, and insulating properties. These and other objects of the present invention are achieved by a polystyrene foam sheet comprising a polystyrene resin which contains 0.5 to 15 wt% of a rubber component, based on the polystyrene resin, with the foam sheet having a thickness of from 0.060 to 0.250 inches with a deviation no greater than ±

0.010 inches, a density of from 4 to 18 lbs/ft³, and an average cell size of from 0.12 to 0.34 mm.

Also disclosed are the deep drawn articles made by thermoforming the polystyrene foam sheet. BRIEF DESCIPTION OF THE DRAWINGS

Figure 1 is a sectional view of a male/female mold pair used to form a deeply drawn thermoformed article, in this instance a cup.

Figure 2 is a deeply drawn article, a cup, formed using the mold pair of Figure 1.

Figure 3 is a cross-sectional view of the sidewall of Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The foamed polystyrene sheet of the present invention exhibits excellent and consistent formability when used for deep drawing. It is particularly suitable for producing deeply drawn cup-like formed parts having a desired strength and a draw ratio (b/a, where b is depth and a is the widest diameter) greater than 1.0. The critical characteristics of the polystyrene foam sheet are its uniform thickness, foam cell size, and foam density. It has been found that even minor variations throughout the foam sheet in any of these three characteristics can lead to difficulties in the thermoforming process. The deeper the draw, the greater the surface area that must be accommodated by the area of foam sheet seized between the plug and cavity in the thermoforming process. Accordingly, aberrations in the thickness, cell size, and density can result in sheet tears, poor formability in the former, and low production due to excess scrap.

The thickness of the foam sheet is from 0.060 to 0.250 inches. If the thickness is less than 0.060 inches, the foam sheet cannot be drawn deeply and the resulting formed part is insufficient in compression strength. If the thickness exceeds 0.250 inches, the formability and economics become poor; particularly it is difficult to balance the side wall thickness and the bottom wall thickness. The preferred thickness (including any non-foamed resin film) will be at least partially dependent on the thermoformed deep drawn article. The thickness per se of the foamed polystyrene sheet, while important, is not a critical characteristic because foam sheet of differing thickness have been used to produce useful deep drawn articles. Rather, it is the uniformity of the thickness that is a critical characteristic. Thus, the tolerance or deviation from the desired foam sheet thickness should be ± 0.010 inches. That is, no thickness of the foam sheet should be more than 0.010 inches greater or less than the desired foam sheet thickness. Thickness deviations in the foam sheet greater than the tolerance may result in a lack of uniformity in the sidewall thickness. Severe thickness deviations may result in an inability to thermoform the article due to a lack of material in the mold. Uniformity of the average foam cell size is a second critical characteristic of the foam sheet of the present invention. Average cell size must be in the range of 0.12 to 0.34 mm. Preferably, the average cell size is within the range of 0.16 to 0.30 mm, and most preferably within the range of 0.16 to 0.26 mm. Average cell size is determined by cutting a flat slice from two perpendicular planes of the foam specimen and counting the number of cell walls along a 1 mm grid line in both the horizontal and vertical directions. The number of cell walls counted in both directions are added, and the sum is divided by two to obtain the average cells counted. The average cell size in mm can then be determined from the average cells counted using a graph well known to those in the trade. Lack of uniformity in the average cell size can result in a lack of uniformity in the sidewall thickness of the thermoformed article. Lack of uniformity can also result in cell elongation, which can lead to cold tearing.

Uniformity in foam sheet density is a third characteristic of the foam sheet of the present invention. The foam sheet should have a bulk density of 4 to 18 lb/ft³, more preferably from 7 to 14 lbs/ft³, and most preferably from 8 to 12 lb/ft³. Higher densities require more resin and more heat is required for forming, resulting in increased expense. If the bulk density is lower than 4 lb/ft³, the foam sheet is insufficient in strength, resulting in an article that tends to lack dimensional accuracy.

The foamed polystyrene sheet of the present invention is composed chiefly of polystyrene resin, and also contains from 0.5 to 15 wt% (based on polystyrene weight) of a rubber component. Preferably the foam sheet contains from 0.5 to 10 wt% of a rubber component, most preferably the foam sheet contains 0.5 to 5 wt% of a rubber component. The polystyrene resin comprising the polystyrene foam sheet of this invention includes polymers made up of styrene-type vinyl monomers such as styrene, methylstyrene, and dimethylstyrene, and also includes copolymers made up of styrene-type vinyl monomers and other vinyl monomers such as acrylic acid, methacrylic acid or ester thereof, acrylonitrile, acrylamide, methacrylonitrile,and maleic anhydride. The polystyrene foam sheet of the invention can be prepared by extrusion-foaming the resin composition made up of a polystyrene resin and the specific required quantities of rubber component and, if required, a filler. The rubber component may include butadiene rubber, ethylene-propylene rubber, styrene-butadiene rubber, and polyethylene. The rubber component when used as a copolymer component includes such monomers as butadiene, isoprene, and chloroprene and oligomers thereof. They are copolymerized at a predetermined molar ratio with polystyrene resin. (In the case where a copolymer is used as the polystyrene resin, the copolymer containing the rubber component becomes a terpolymer.) Preferred for this invention are those high-impact polystyrenes that utilize a styrene/butadiene copolymer as the rubber component. The above rubber component may be added directly but is usually contained in a high-impact polystyrene which is then blended with a polystyrene homopolymer.

The impact polystyrene should have a weight percent rubber of between 1-15 weight percent, preferably 1-10 weight percent, rubber based on the rubber component, such as polybutadiene. The size of the rubber particle is approximately 0.2 microns. Preferably the weight percent rubber is between seven and ten. The weight average molecular weight li,, should be between 100,000 and 300,000, and preferably between 150,000 and 200,000. The molecular distribution, M/M,,, should be between 2.7 to 2.9. One preferred foam sheet is a blend of thirty percent of an impact polystyrene and seventy percent of a general purpose polystyrene homopolymer with a weight average molecular weight of about 325,000 and a melt flow rate of about 1.5 grams/10 minutes, such as for example, STYRON 685D, available from The Dow Chemical Company. More preferably the foam sheet has twenty percent of the impact polystyrene with the remainder being a general purpose polystyrene.

If the content of the rubber component is less than 0.5 percent (0.5 wt%) , the resulting foam sheet is not suitable for producing deeply drawn parts. Cups produced from such a sheet lack strength and tend to break at the lip. Moreover, such a sheet is insufficient in elongation and in productivity. On the other hand, if the content of the rubber component exceeds fifteen percent (15 wt%) , there is no additional benefit in thermoforming deeply drawn articles.

A nucleating agent is effective in improving the appearance and the dimensional accuracy and stability of the formed part because it is effective in controlling the cell size. While not absolutely required, the use of a nucleating agent is preferred when making foam sheet. If the content of the nucleating agent is too little, it may be difficult to adequately control gas and cell characteristics, and consequently to control the thickness and consistency of the foam sheet and the thermoformed part. On the other hand, if the content of the nucleating agent is excessive, the resulting foam sheet is insufficient in elongation at the time of forming. The content of the nucleating agent in the present invention, if required, is 0.2 to 2.5 wt%, and preferably the content of the nucleating agent is 0.5 to 2.0 wt%. Most preferably the nucleating agent content is about 1.6 weight percent based on total resin weight.

Common examples of nucleating agent include talc, Hydrocerol, a citric acid and carbonate product obtained from Ingelheim (CF40 S) , calcium carbonate, volcanic ash, gypsum, carbon black, white carbon, magnesium carbonate, clay, natural silica, and other common inorganic fillers and metal powders. The foam sheet of this invention is produced by mixing the impact polystyrene, the general purpose polystyrene, and the nucleating agent and then extrusion-foaming the sheet. Preferably, mixing is carried out in a screw extruder to insure that the materials to be foamed are well mixed. Uniform mixing helps to insure uniformity in the average cell size. A volatile blowing agent in the range of about 0.5 to about 10 wt% based on the total weight of the composition, preferably about 1.8% by weight, is added to extrusion foam the sheet. The examples of the volatile blowing agent include hydrocarbons having a boiling point of - 40 degrees to 45 degrees C (centigrade) , such as propane, butane, isopentane and pentane; and polyfluorocarbon blowing agents, such as 1,1,-difluoroethane (HFC-152a) ; 1,2- difluoroethane (HFC-152) ; 1, 1, 1,2- tetrafluoroethane (HFC-134a) ; 1,1,2,2-

SUBSTΓΓUTE SHEET (RULE 26) tetrafluoroethane (HFC-134) ; l,l,l-trifluoroethane (HFC-143a) : and 1,1,2-trifluoroethane (HFC-143) ; pentafluoroethane (HFC-125) , (HFC-152a) and (HFC- 134a) , and (HFC-152a) ; chloroflurorcarbon and hydrochlorofluorocarbon blowing agents, such as chlorodifluoromethane (HCFC-22) , dichlorodifluoromenthe (CFC-12) and trichlorofluoromethane (CFC-11) . Of course, nitrogen, carbon dioxide, other inert gases, hydrocarbons and chemical blowing agents can be used in conjunction with the polyfluorocarbon blowing agents.

Butane, pentane and (HFC-152a) are preferred blowing agents. Butane is the most preferred blowing agent. If butane is used as the sole blowing agent, the most preferred blowing agent content is 1.8 parts per 100 parts of blended material. The blowing agent may be introduced into the extruder in any manner conventional in the art.

After forming the foam sheet, preferably the cells of the foam are substantially completely filled with air, making the foam sheet produced suitable for food contact applications. The foam sheet, after being initially extruded, is then taken up under tension, usually by being wound onto a roll. Biaxial orientation takes place in the case where a circular die is used. In such a case the foam sheet is usually slit and laid flat while still under tension before being wound onto a roll. Excessive imbalance of orientation between the machine direction and the transverse direction (that is, greater than 10%) should be avoided because it is detrimental to the strength of the final product. The polystyrene foam sheet thus prepared provides satisfactory formed parts, because the thickness, density and average cell size is uniform and controlled. The polystyrene foam sheet containing .5 to 15 wt% of rubber component is superior in elongation when heated for forming deep drawn articles. While not required, it is desirable to laminate or extrusion coat a non-foamed thermoplastic resin film onto at least one surface of the foam sheet in order to improve the compression strength, the printability and chemical gas barrier properties of the resulting thermoformed part. This non-foamed resin film is usually a 0.001 to 0.020 inches thick film of thermoplastic resin. This film may be laminated or extrusion coated onto one or both surfaces of the foam sheet in any conventional manner. The thermoplastic resin for the non-foamed film includes, for example, polystyrene, polyethylene, high-impact polystyrene which is a mixture or copolymer of polystyrene and rubber, polypropylene, and polyethylene terephthalate. Preferable among them from the standpoint of formability are high-impact polystyrene and high- density polyethylene; most preferable is high- impact polystyrene. A preferable film thickness is 0.003 to 0.015 inches.

The non-foamed thermoplastic film can be laminated onto the foam sheet in various ways. For instance, the thermoplastic film may be laminated onto the foam sheet in a die by using a co-extrusion die (e.g., cross-head die). In another way, the foam sheet and the thermoplastic film extruded from the separate dies can be continuously laminated, or the previously extruded thermoplastic film can be laminated onto the foam sheet. The lamination may be achieved with an adhesive or by fusion-bonding. A variety of adhesives may be used for lamination, e.g., EVA copolymer and SBR in the form of solution, emulsion, or film.

The article which is specifically disclosed in this application is a deep drawn cup commonly utilized to contain hot fluids and to prevent irritation to the holder thereof. Of course, the article and process of the invention are equally applicable for use with cold fluids. Such cups can be made in standard sizes, such as 6, 8, 12 and 16 ounces, and even larger sizes. The foamed, cellular thermoplastic cup optionally can be provided with a non-porous densified skin layer on the inner surface, and optionally an outer densified, surface, and a low density cellular core. The lip may be rolled inwardly by suitable lip rolling equipment, such as helical screw lip rollers presently in common usage.

The conventional approach for making formed articles from foamed or cellular thermoplastics is a two-stage process. In the first stage, foam sheeting is extruded and collected on rolls. At this point, one may laminate one or more films onto the foam sheeting. The rolls are then stored until the second stage, which employs a conventional thermoforming machine for reheating the material on a progressive basis and forming it in molds through the use of differential air pressure, plungers, or both, whereupon the formed web is transported to a cutting machine for severing the formed articles from the web. The extrusion operation for producing the sheet material is thus, usually, an entirely separate operation (in relation to time and the utilization of heat energy) from the fabricating operation for forming and cutting the articles.

Although the two-stage process can be used with the polystyrene foam sheet of the present invention to make deep drawn articles, such a process has many limitations affecting cost, quality control, and operational control. Because of the separation of the extrusion and fabricating operations, quality control becomes more difficult and costly. Defects in the sheet which are not apparent until molding begins can not then be corrected, resulting in the rejection of large quantities of material. Since foam sheeting has excellent thermal insulating properties, it is difficult and costly to heat it properly during the fabrication step. To avoid these problems, it is desirable to have a continuous process in which the extrusion and fabrication steps follow without interruption. The polystyrene foam sheet of the present invention permits successful continuous extrusion and thermoforming, resulting in useful and attractive deep drawn articles.

It has been found that an important element for successful forming of the articles of the present invention is the specific design of the male and female mold members as well as its material of construction. The shape and material of the mold members may control the distribution of the foam sheet material along the side walls of the article being formed. Different materials of construction will result in marked differences in the distribution of material. Accordingly, mold member construction material must be individually selected depending upon the shape of the article being formed and the desired material distribution in the formed article. Suitable materials include steel, nylon, aluminum, polysulfane, and syntactic foam, for example. For this article, aluminum is the preferred mold member construction material. As can be seen in the sectional view of Figure 3, the male and female mold pair have been altered to provide a vacuum in both the male and female mold members to assist in forming the sheet into the articles of the present invention. It will be understood that both single and multiple cavity mold operations may be employed to make the articles of the present invention.

The mold pair is made of several pieces. The male mold member 10 is a single piece. The female mold member 50 has four pieces, the top sidewall piece 70, the top sidewall ring 100, the bottom sidewall piece 80 and the convex bottom piece 60. The top sidewall piece 70, the bottom sidewall piece 80 and the convex bottom piece 60 are held together by four bolts 90. The top sidewall ring is bolted into the top sidewall piece 70 with three equally spaced bolts 110. Male mold member 10 has four equally spaced vacuum holes 12, whose diameters are 0.020 inches (0.051 millimeters), in the concave bottom of the piece, with the four holes forming a square around the central end point of a central vacuum channel 14 in the male mold member at the point of greatest extension into the female mold member 50. Thirty two additional vacuum holes 16 are located at the topmost area 17 in the male mold member of the article to be thermoformed, in this case a cup rim. These two vacuum holes 16 communicate with vacuum channel 18 which is also in communication with vacuum channel 14, while the other thirty vacuum holes have a channel about 0.12 inch in diameter, just deep enough to communicate with the holes as opposed to extending completely through the male mold member, as does vacuum channel 18.

Female mold piece 60 has three vacuum holes of the same diameter (0.020 inches) located in the center 61 of the convex bottom 60 of the female mold with one hole 62 located at the highest convex point and the other two holes placed linearly left and right of the center hole spaced a small distance apart. All three holes communicate with vacuum channel 63. Female mold piece 70 also has eighteen equally spaced holes 77 which each communicate with vacuum channel 78 about 0.12 inch in diameter. There are also eighteen additional vacuum holes 79 which communicate between the interior and the exterior of the female mold piece 70. The top sidewall ring 100 has been slightly oversized so as to produce enough of a gap between the top sidewall piece 70 and the top sidewall ring 100 so that the vacuum channels 78 and the vacuum holes 79 are accessible when reducing pressure. In the bottom sidewall piece 80 of the female mold 50, there are thirty two equally spaced vacuum holes 84 of 0.020 inch diameter which communicate with an annular groove 86 which is part of vacuum channel 85. An annular ring 67, with an opening of about 0.025 inch, communicates with an annular vacuum channel 68 in the convex bottom piece 60. The annular vacuum channel 68 and vacuum channel 63 are also in communication with the four equally spaced vacuum channels 85. The annular ring provides a full annular ring vacuum when thermoforming as opposed to separate and non-interconnected vacuum holes in a ring formation.

The gap between the male mold member 10 and the female mold member may range between about 0.01 and about 0.15 inch and is most preferably set at a gap of about 0.06 inch.

This mold pair is then completely placed in a unit which can be used to reduce air pressure and obtain a partial vacuum.

Surprisingly, in the mold members of the present invention, vacuum is present both above (in the male mold member) and below (in the female mold member) the foam sheet which is being thermoformed, as opposed to just using the vacuum assist to pull the sheet into the female mold member.

The following examples illustrate the polystyrene foam sheet and thermoformed articles of the present invention. EXAMPLE 1

Polystyrene foam sheet was produced by extrusion from the following starting materials:

(1) 73.0 wt. % crystal polystyrene obtained from Dow Chemical Company as product number 685D;

(2) 16.5 wt. % high impact polystyrene (having 12% rubber with particle size of about .2 microns) obtained from the Dow Chemical Company as product number XU70025; (3) .8 wt% 40% active Hydrocerol available from Boehringer Ingelheim (CF40 S) ;

(4) 3.2 wt.% talc (nucleating agent) (containing 40 wt.% talc blended with 60 wt.% crystal polystyrene). The foam sheet extruded from the extruder contained approximately 1.9 - 2.0 wt % of rubber having particle size of about 0.2 microns. The blowing agent used in the foam sheet of this example 1 was butane. The foam sheet was produced by extrusion with a thickness of about 0.120 inch with no portion of sheet having a thickness less than 0.110 inch or greater than 0.130. Thus, the thickness tolerance was 0.010 inches.

The cell size of the foam sheet was .24 mm. The extruder used to extrude the foam sheet was a 2.5 inch (primary extruder) - 3.5 inch (secondary extruder) foam extrusion line extruder available from Cincinnati Milicron. The density of the resulting foam sheet was about 11 lbs/ft³. After extrusion the foam sheet was laminated on one major surface by extruding a molten high impact polystyrene (Dow product number 482) onto the foam sheet to form a film. The film had a thickness of 0.0075 inches (0.19 mm). Film thicknesses of between 0.001 and 0.020 inches are acceptable, with thickness of 0.003 to 0.015 inches being preferable. This sheet was then rolled and allowed to age at least 120 hours. This aging permits equalization of cell pressure. During thermoforming, the foam sheet was passed through the thermoformer so that this first film constituted the inner surface of the final thermoformed article.

Prior to thermoforming, a second 0.0075 (0.19 mm) skin layer of the same high impact polystyrene (Dow product no. 482) was laminated to the other major foam sheet surface that had not been extrusion coated. This film constituted the outer surface of the final thermoformed article. Again, skin layer thicknesses of 0.003 to 0.015 inches are preferable.

The laminated foam sheet was then fed into an oven for preheating and to increase its temperature for the thermoforming step. The oven and thermoformer was a conventional oven available from Brown Machine, Model No. CS 2100. This oven permitted the preheating of the foam sheet to a temperature necessary for thermoforming. The goal of the preheating is to bring the temperature of the center of the foam sheet above the vicat temperature of the heat crystal polystyrene (Dow 685D) to a temperature where the foam is sufficiently softened to enable the male and female mold members of the thermoforming step to deform the foam sheet. The vicat temperature for this material (Dow 685D) is approximately 227° F. The temperature of the center of the foam sheet must be greater than 227° F. In order to achieve this center temperature the top surface is preferably in the range of about 285° F to about 305° F, more preferably between 295° F to about 300° F, and most preferably about 300° F. Although the bottom surface temperature was not measured, it is believed and preferred that the temperature of the bottom surface duplicates the temperature of the top surface.

The heating area is preferably designed to minimize the temperature gradient between the outer surfaces and the center of the foam sheet to be thermoformed. Accordingly, a gradual heating that results in a surface temperature equal to that of the center temperature would provide the optimal temperature gradient in the foam. However, this gradual heating is impractical because it can be both very expensive and time- consuming.

At the other heating extreme, the outer surfaces cannot be made to reach a temperature greater than 680° F because 680° F is the flash point and 925° F is the auto ignition point of the laminated skin layer. Accordingly, the outer surface will never be exposed to a heat of 680° F. The goal in heating the foam sheet is to bring the temperature of its center within the preferred range in the shortest amount of time without damaging the foam or skin layers.

During this preheating stage, the foam sheet also undergoes post-expansion. The amount of post-expansion depends on the composition of the gas resident in the foam cells at the time of preheating in the oven. The amount of post- expansion desired will depend on the gap between the plug and the mold of the thermoforming step. In the present example, the plug and mold gap was 0.060 inches. The desired foam thickness for this 0.060 inches gap is about .150 inches. This thickness includes the foam sheet and any films laminated to the sheet. The material of this example had a thickness of about .115 inches prior to the preheating stage. Thus, the post-expansion accounted for about 0.035 inches of expansion or about 30.4% expansion. Given this mold gap and thickness of the foam, a minimum of 20% post expansion is required and 30% is preferred. Post- expansion of between 0% and 60% is possible depending on the characteristics of the foam sheet and the gas resident in the foam cells. Green foam (i.e., foam sheet that has not been aged) that has been foamed using butane as the blowing agent undergoes a post-expansion of about 52%.

Cups were then thermoformed using the conventional continuous feed thermoformer of Figure 1. The vacuum capacity of the female mold of the thermoformer was used in this thermoforming process. A cooling mechanism was used in conjunction with the male and female mold pair. The male mold member 10 may be cooled to a temperature of between 70° F and 200° F and the female mold piece 70 is cooled to a temperature of between 70° F and 200° F. Typically, the temperature of the female mold piece 70 is cooled to a temperature lower than that for the male mold member 10 and preferably to a temperature about 50° F cooler. In the thermoforming of the foam sheet of this example 1, the temperature of the male plug was 150° F and that of the female cavity was 100° F. The mold and plug were made of aluminum.

The male and female mold members were then moved together into the final forming position to stretch the foam sheet around the male mold member and into the female cavity. Vacuum was applied to the female mold member.

Then the final shape of the foam sheet is set by chilling. The chilling is accomplished by allowing the mold members whose temperature is described above to remain in the final position long enough to reduce the foam sheet temperature below the vicat temperature or softening point.

The thermoformed cups produced in Example 1 had a draw ratio on the order of about 1.1:1. The articles resulting from example 1 yielded excellent cups with good heat insulation capacity and attractive appearance.

EXAMPLE 2 Example 1 is repeated except that, instead of aging the foam sheet, the foam sheet is laminated with a second 0.0075 (0.19 mm) skin layer of high impact polystyrene (Dow product no. 482) and is passed directly to the thermoformer such that this second skin constitutes the outer surface of the final thermoformed article. The laminated foam sheet is preheated in an oven in a manner similar to Example 1. The sheet is then thermoformed into cups in accordance with the thermoforming operation described in Example 1, using the thermoformer of Figure 1. EXAMPLE 3 Polystyrene foam sheet was produced by extrusion from the following starting materials:

(1) 73 wt% crystal polystyrene obtained from Dow Chemical Company as product number 685D;

(2) 23 wt% high impact polystyrene (having 7.5% by weight rubber) obtained from the Dow

Chemical Company as product number 482;

(3) .8 wt% 40% active Hydrocerol, as a nucleating agent;

(4) 3.2 wt% talc (filler) (40 wt% talc blended with 60 wt% crystal polystyrene) .

The foam sheet produced by extrusion had a thickness of about 0.115 inches, with a thickness tolerance of 0.010 inches. The cell size of the foam sheet was .24 mm. The density of the resulting foam sheet was about 11 lbs/ft³.

After extrusion the foam sheet was laminated on one major surface by extruding a molten high impact polystyrene (Dow product number 482) onto the foam sheet to form a film. The film had a thickness of 0.0075 inches (0.19 mm).

Prior to thermoforming, a second 0.0075 inch (0.19 mm) skin layer of the same high impact polystyrene (Dow product number 482) was laminated to the other major foam sheet surface that had not been extrusion coated. This second film constituted the outer surface of the final thermoformed article. The sheet was then rolled and allowed to age at least 120 hours. The foam sheet was then preheated and thermoformed as described in Example 1 using the thermoformer of Figure 1. The thermoformed cups of this Example 3 had a draw ratio of 1.12:1, a density of about 11 lbs/ft³, and had good heat insulation capacity and an attractive appearance. Example 4

Example 3 is repeated, except that, instead of aging the foam sheet, the foam sheet is passed directly to the oven for preheating in accordance with the procedure described in Example l. The preheated sheet is then thermoformed into cups in accordance with the thermoforming operation described in Example 1, using the thermoformer of Figure 1.

It is to be understood that the foregoing description is merely illustrative of preferred embodiments of the invention, of which many variations may be made by those skilled in the art within the scope of the following claims without departing from the spirit thereof.

Claims

WHAT IS CLAIMED IS:

1. A polystyrene foam sheet for thermoforming deep drawn articles comprising a polystyrene resin which contains .5 to 15 wt% of a rubber component, based on the polystyrene resin, with the foam sheet having a thickness of from 0.060 to .250 inches, with a deviation no greater than ± 0.010 inches, a density of from 4 to 18 lb/ft³, and an average cell size of 0.12 to 0.34 mm.

2. A polystyrene foam sheet of claim 1, having a density of 7 to 14 lb/ft³.

3. A polystyrene foam sheet of claim 1, having a density of 11 to 12 lb/ft³.

4. A polystyrene foam sheet of claim 1, having an average cell size of 0.16 to 0.30 mm.

5. A polystyrene foam sheet of claim 1, having an average cell size of 0.16 to 0.26 mm.

6. A polystyrene foam sheet of claim 1, which also has from 0.5 to 2.0 wt% of a nucleating agent.

7. A polystyrene foam sheet of claim 1, having a non-foamed film on at least one major surface.

8. A polystyrene foam sheet of claim 1, having a non-foamed film on both major surfaces.

9. A deep drawn article comprising the polystyrene foam sheet of claim 1, the deep drawn article having a draw ratio greater than 1.0.

10. The deep drawn article of claim 9 having a non-foamed film on at least one major surface thereof.