Container Surface And Method Of Obtaining Same

Abstract

Vendability of nestable, resilient, plastic containers is improved by coating outside surface portions with a lubricant. These portions preferably are the nesting means and/or the lower sidewall area of the container. A process is provided for depositing the lubricant on the container which, in the case of a partially foamed container, may occur simultaneously with its exposure to a foaming agent.

Parent Case Text

This is a division of copending application Ser. No. 760,911, filed Sept. 19, 1968, and now abandoned.

Description

This invention relates to containers and more particularly to nestable containers for use in coin operated dispensing machines.

Thin wall containers made of non-foamed as well as of entirely and partially foamed plastic, such as those described in U.S. Pat. No. 3,374,922, are being increasingly used in vending machines for containing hot and cold beverages of all types. These containers usually have stacking or nesting means formed in a wall portion thereof which function to maintain adjacent walls spaced from each other when a number of such containers are nested together to form a stack. The stacking means usually comprises a series of cooperating shoulders or shelves having close dimensional tolerances, since these portions must be accurately and consistently reproduced with a minimum of variation or otherwise the supported surface of the stacking means of one container may proceed beyond the supporting surface of the stacking means of an immediately adjacent nested container during stacking or in subsequent handling of a plurality of such containers. This results in a wedging together of two or more containers, with the result that they do not consistently feed freely from a nested relationship within a vending machine when a coin is inserted therein by a customer. Such high tolerance requirements often result in numerous container rejects, thereby lowering the economics of a high speed, large volume container producing operation.

This problem of nestable container vendability is aggravated with the more recently developed type container having a foam band integrally along its sidewall for insulating purposes. During formation of this foam covering, the portion of the container which is not to be foamed is usually masked to prevent the foaming agent from contacting it. However, in high speed production operations, foam breakthrough often occurs in the container portion which is to be non-foamed. This foam has an unusually high coefficient of friction, and since portions of the non-foamed surface are usually those which contact the next adjacent container within the stack, as well as chute surfaces within the vending maching during movement of the container into its filling position, vendability of this type of container as limited by minute patches of foam breakthrough presents an unusual problem. In addition, in the process of producing these containers, masking is usually carried out by coating the surface to be non-foamed with a chemical composition, such as that set out in copending application Ser. No. 695,359, filed Jan. 3, 1968, now U.S. Pat. No. 3,565,660, and assigned to the assignee of the present invention. The non-volatile portion of this composition which remains on the surface of the container after foaming also has a coefficient of friction which is greater than that of the basic plastic substrate, and thus also tends to retard swift movement of the container along surfaces with which it comes into sliding contact.

Now there has been developed for use with either a non-foamed, partially foamed or entirely foamed nestable container of the type used in vending machines, means for improving behavior of such containers within a stack and for reducing frictional resistance of these containers when in sliding contact with the internal surfaces of automated vending machines.

Accordingly, it is a principle object of this invention to improve the vendability of nestable containers.

It is another object of this invention to provide a container having improved resistance against wedging within another container when a plurality of like containers are nested together within a stack.

It is an additional object of this invention to reduce the frictional effect between surfaces of a vending machine and a plastic container in contact with the surfaces.

It is a further object of this invention to provide a container exhibiting improved stacking performance yet permitting a relaxation of the tolerances of the stacking means by which the nesting is accomplished.

It is an additional object of this invention to provide an improved, partially foamed, nestable and vendable container as well as a method of producing such a container.

It is a still further object of this invention to decrease the frictional effects of a foamed surface on vendability of a partially or completely foamed, nestable container.

It is an additional object of this invention to decrease the effects of friction on vendability of a previously chemically masked portion of a partially foamed container.

It is yet a further object of this invention to provide a container and a method of forming such a container which overcomes the prior art difficulties discussed above.

Other objects of this invention will in part be obvious and will in part appear hereinafter.

These and other objects are accomplished by providing, in the broadest aspect of the invention, a nestable, thin wall, cup-like thermoplastic container having nesting means formed in its sidewall for maintaining the sidewall spaced from that of a similar container when nested there within, the sidewall having on at least a portion of its outer surface, a thin coating of lubricant to reduce frictional resistance to sliding between the coated surface and a supporting surface with which it comes into contact.

In a preferred embodiment of the invention, the sidewall of the nestable container has a gripping portion of major extent having a foamed, slip resistant covering on its outer surface, a lower, non-foamed section intermediate the base and the gripping portion, and an upper, non-foamed section intermediate the gripping portion and the upper end, the upper and lower non-foamed sections having portions off-set laterally with respect to the gripping portion defining points of contact between the container and a planar surface when the non-foamed sections are placed against said surface with the foamed slip resistant covering located inwardly of this surface, nesting means formed in the sidewall for maintaining the foamed, gripping, slip resistant surface of the sidewall of the container spaced from the sidewall of a similar container when nested therewith, the outer surface of the lower section having a coating of lubricant thereon to enhance sliding contact of the container along the planar surface.

A method is provided for forming the previously mentioned partially foamed plastic container having a predetermined, lubricated, non-foamed external surface portion which comprises: masking a portion of the surface of a plastic container which is to be non-foamed; exposing at least part of the masked portion along with another surface portion of the container which is to be foamed to a medium which is absorbable by the plastic and convertible to a gaseous state below the melting point temperature of the plastic used to form the container; heating the plastic container to a temperature below the melting point temperature of the plastic but above the temperature at which the medium converts to the gaseous state to produce a foam covering having a boundary along the edge of one of said surface portions; and applying a thin coating of lubricant to at least a portion of the external surface of the container portion to be non-foamed.

In a preferred form of the method, the lubricant is dissolved in the absorbable medium and coated on the masked surface at the same time the portion to be foamed is exposed to the absorbable medium.

In describing the overall invention, reference will be made to preferred embodiments depicted in the accompanying drawings in which:

FIG. 1 is a perspective view of a container embodying the present invention;

FIG. II is a partially sectioned, elevational view taken along the axis of the container of FIG. I;

FIG. III is a partial, sectional view of two nested containers of the type shown in FIGS. I and II; and

FIG. IV is a diagrammatical view illustrating the operational steps for producing the container of FIGS. I and II.

With reference to the drawings, wherein identical numerals refer to identical parts, there is shown in FIGS. I-III a thin wall, nestable, cup-like thermoplastic container for beverages, foods and the like, which is of the throw-away type and is broadly identified as 10. Container 10 generally comprises base 12, sidewall 14 generally extending upwardly and outwardly from base 12 in a frustro-conical manner, forming open upper end 17 at the free end of sidewall 14. Sidewall 14 comprises middle finger gripping portion 16 having a relatively even outer surface of major extent, which diverges outwardly relative to the container axis at a substantially constant angle. The length of finger gripping surface 16 may range between about 40 to 90 percent of the sidewall axial length. Though not essential, surface 16 may have a partially foamed outer covering 18, obtained in a manner to be hereinafter described. Sidewall 14 further comprises lower section 20 intermediate base 12 and gripping surface 16, and upper section 22, intermediate gripping surface 16 and open upper end 17. Upper section 22 has portions offset from the angle of gripping surface 16 to define nesting means formed therein, which, together with a corner chime 40 at the peripheral joint between lower section 20 and base 12, cooperate with similar structure in a like container as illustrated in FIG. III, to maintain a radial clearance between the sidewalls of two adjacently nested containers.

Nesting means of upper section 22 of sidewall 14 comprises outwardly flaring arcuately spaced supporting shoulder 24, the upper edge of which integrally merges into an upwardly and inwardly inclined spacing skirt 26. Spacing skirt 26 is integrally formed with an upwardly and outwardly formed stacking shoulder 28 along its upper margin. As shown in FIG. III the point of integral adjoinment of stacking shoulder 28 and spacing skirt 26 forms an annular stacking edge 30. Stacking shoulder 28 integrally merges at its upper end into an upwardly extending sidewall portion 31. Rim 32 flares outwardly and consists of an upper rim forming wall 34 which merges into reversely bent flange 36, thereby defining an outer rim margin 38 as typically shown in FIGS. II and III.

Slide line 54 of container 10 as depicted in FIG. II, is defined by a plane tangent at one point to the outer surface of the container at the point of maximum diameter in an upper section above the midpoint of the sidewall, and at another point tangent to the outer surface of the radially outermost extremity of the section of the container below the sidewall midpoint. These points need not necessarily be at the upper sidewall extremity or at the lower joint between the sidewall and base though they are located at these points in the embodiment of FIGS. I-III. Corner chime as used herein is meant to include the radially outermost extremity of the lower end of the container. In FIG. II, slide line 54 is represented by a plane tangent to outer rim margin 38 of upper section 22, and to the outside surface of corner chime 40 of lower section 20.

In the form of the invention depicted in FIGS. I-III, the annularly extending foam covering 18 of sidewall 14 extends from about three-eighths inch above corner chime 40, to about one inch below its uppermost peripheral margin. By further reference to FIG. II, and slide line 54, it can be seen that foamed section 18 of container 10 is radially inward of slide line 54, and is so located along the sidewall that it will never come in contact with a straight surface against which the container is placed. Consequently, a container 10 passing along the inclined dispensing tube of a vending machine will never contact the wall of the dispensing tube except tangentially at corner chime 40 and outer rim margin 38.

In accordance with the present invention, container 10 has on at least a portion of its outer surface, a thin adherent coating 19 of a lubricant such as silicone oil, which functions to reduce the frictional resistance between the coated surface and any other surface with which it comes into sliding contact, such as the inner surface(s) of an adjacently stacked container or any of the surface(s) of the cup dispensing portions of an automated vending machine. Lubricant 19 in the embodiment of FIGS. I - III is present on the outer surface of the nesting means in the upper portion 22 of the container sidewall between the top edge of the foamed gripping portion 18 and rim 32, on the lower portion of the sidewall 20 between the base 12 and the lower end of foam covering 18 including chime 40 and also on the outer surface of the margin of base 12.

In use, container 10 is readily adaptable to be nested or stacked together with a plurality of like containers in the manner depicted in FIG. III. When placed in nested position, it can be seen that the outer surface of supporting shoulder 24 is engaged by the annular stacking edge 30 of stacking shoulder 28.

In the event of the tendency of the inner non-foamed surface of an outer cup to move inward toward the foamed surface portion of the inner cup or vice versa, because for example, of a drag movement during transfer of nested stacks within the vending machine, non-foamed, outer anti-rub corner chime 40 of the inner cup will contact the non-foamed inner surface of the bottom section of the outer cup as depicted in FIG. III, to maintain the desired sidewall clearance of the foamed portion of the inner cup from the non-foamed portion of the outer cup. Contact between the chime and the inner surface of the next adjacent cup is in an edgewise manner rather than surface to surface, in order to minimize frictional drag between the non-foamed portions of the lower sections.

Lubricant film 19 is interposed (FIG. III) between the outer surface of supporting shoulder 24 of the inner container and the inner surface of stacking edge 30 of the outer container in the limited peripheral areas of contact in the upper sidewall portions of each. Lubricant 19 is likewise disposed between the outer and inner surfaces of immediately adjacent stacked containers at the lower contacting portions of the sidewalls. When it is desired to release the lowermost container (or to raise the uppermost container) from a stack of a plurality of similarly constructed units, the lubricant film acts in a sense like a ball bearing to insure uniform and rapid separation of the outer and inner containers around the peripheries of each at their upper ends, i.e., between edge 30 of the outer container and shoulder 24 of the inner container. Lubricant 19 also functions to minimize the frictional resistance against sliding of the inner surface of sidewall 18 of an outer cup along chime 40 at the lower end of the sidewall of the inner cup as the outer cup moves away from the stack. Since the lubricant utilized must in all cases bear approval for use with comestibles by the Food and Drug Administration, trace amounts of the lubricant appearing on the inner surfaces of a container, which may have been rubbed off of the outer surface of an adjacent stacked unit are not objectionable from the standpoint of food or beverage contamination.

Container 10 must subsequently fall by gravity along an inclined chute from a lower position within a nested stack in the upper section of an automated vending machine into place in the filling station below. Contact during travel along the chute, which may not be entirely smooth, is along slide line 54. In so doing, lubricant 19 on chime 40 in the lowermost portion of the sidewall again aids in reducing frictional resistance to sliding, thereby facilitating rapid and trouble free movement of the cup into its dispensing position within the machine.

Referring now to FIG. IV, an expeditious process is illustrated by which a partially foamed, plastic container having a predetermined, lubricated, non-foamed external surface portion is obtained.

Initially, a solution 60 is prepared (FIG. IVa) of a non-volatile lubricant 61 dissolved in a medium which is absorbable by the plastic and convertible to a gaseous state below the melting point of the plastic of the container being treated. The solution may be stirred by means of a suitable stirrer 64 in tank 65 to insure complete dissolution. A completely non-foamed thermoformed cup 62 of the type depicted in FIG. I is obtained, which it is desired to lubricate and to partially foam by providing previously described foam covering 18 along the major extent of its sidewall. To prevent absorption of the foaming agent into the plastic in the predetermined portions to be non-foamed, i.e., the upper and lower portions of the sidewall on either side of the foam band, these portions are coated with a chemical lacquer composition comprising a non-volatile resinous ester dissolved in an alcohol as described in copending application Ser. No. 695,359 filed Jan. 3, 1968, now U.S. Pat. No. 3,565,660, and assigned to the assignee of the present invention. This coating is accomplished by moving cup 62 into contact with a porous material 64 which is saturated with the chemical masking agent. The chemical masking agent is supplied to porous material 64 by means of a conduit connecting to a pressurized supply source of the composition. Porous applicator material 64 is supported in holder 66. Cup 62 is rotated as depicted at (b) while its upper and lower surfaces are in contact with porous material 64, thereby resulting in a coating of the exterior of the bottom chime and the nesting means with the chemical masking composition, as well as the areas immediately adjacent thereto. The chemical masking agent utilized is considered to a liquid which when coated on the surface of the plastic and dried thereon to evaporate the solvent when the masking agent is a lacquer, acts as a barrier against absorption into the plastic of the absorbable foaming agent. The cup, having preselected exterior portions thereof coated with the chemical masking agent, is then immersed in tank 65, which contains the solution of lubricant dissolved in the absorbable medium, as schematically depicted at (c) for a predetermined interval of time. At the end of this interval of time, cup 62 is withdrawn from tank 65 and subjected to a heat source as depicted at (d) which involves circulating a fluid, e.g., air or steam at elevated temperature through an inlet conduit 68 into the interior of cup 62. The heated fluid exits from the cup through outlet conduit 70 which has a diameter greater than that of inlet conduit 68, and which is equipped with a flange 72 adpated to rest against the upper surface of the rim of cup 62 while the gas is being circulated. The heat, which as shown is applied to the inner side of the cup, i.e., that which is opposite to the side which had been exposed to the absorbable medium, results in the formation of foam covering 18 along the area which was previously in contact with the medium and which had not initially been coated with the chemical masking agent. The cup 62 is shown at (e) with the foam covering thereon.

Also, as a result of exposure to the solution of lubricant and absorbable medium, a coating 19 of lubricant is left on top of the previously chemically masked upper and lower portions of the sidewall, on the nesting means and on the corner chime portions thereof. The lubricant is also present on the surface of the plastic portion which was subsequently foamed, and therefore results in the lubricant being interspersed throughout the foam cells after the foaming step. Lubricant 19 functions as previously described to enhance the stack release and vending properties of cup 62. Since the chemical masking agent is insoluble in the solution of lubricant and absorbable medium, the non-volatile portion of the masking agent is not disturbed from its place as a coating on the preselected portions of the container sidewall during the exposure step. Since the lubricant is relatively non-volatile, it remains on the cup surface and does not evaporate as does the absorbable medium during the heating step.

The above description and particularly the drawings are set forth for purposes of illustration only and should not be taken in a limited sense.

The following examples are given to illustrate the invention and are not intended as limitation thereof. Unless otherwise specified, quantities are mentioned on a weight basis.

EXAMPLE 1

A solution of lubricant dissolved in a liquid foaming agent was prepared by adding ten parts of liquid Dow Corning DC 200 silicone oil (polydiemethyl siloxane characterized by a viscosity of 500 centistokes, a specific gravity of 0.973 at 25.degree. C. and a surface tension of 21 dynes/cm..sup.2 at 25.degree. C.) to 1990 parts of Freon-11 (trichloromonofluoro methane). The mixture was stirred until a clear homogeneous solution was obtained.

A rubber-modified styrene polymer blend having a Staudinger molecular weight of approximately 50,000, i.e., a physical blend of 7.5 parts of styrene-butadiene graft copolymer with 92.5 parts of styrene polymer wherein the graft copolymer contains 28 percent by weight of styrene based on the weight of the copolymer, was thermoformed into a smooth, tapered cup approximately 4 inches in height and having an average inside diameter of 2 inches. The cup had nesting means formed in the upper sidewall portion thereof similar to that depicted in FIG. I.

The cup formed by the above described operation was coated by contacting 1 inch of the lower outer portion of the cup sidewall as well as the outer surface of the nesting means in the upper portion of the cup sidewall with a porous pad saturated with a resinous ester dissolved in alcohol. This latter composition was prepared by adding 35 parts by weight of "Amberol 750" (a resinous ester formed by esterifying the reaction product of maleic anhydride and rosin acid with glycerol) to 60 parts of methanol. After a 3 second interval the cup was separated from contact with the pad and allowed to dry for 45 seconds.

The cup, chemically masked in this manner, was then immersed in the previously prepared solution of polydimethyl siloxane and Freon-11 up to within about one-half inch of the top of the sidewall. The cup was maintained in this immersed condition for a period of 10 seconds after which it was withdrawn from the solution. After a 20 second interval at dry room temperature conditions, hot air at a temperature of 250.degree. F. was circulated through the interior of the cup for a period of 10 seconds.

The resulting cup had a foamed outer surface throughout the area of immersion in the solution of foaming agent and lubricant, extending into the body of the cup for approximately one-third of its overall thickness, except for those portions of the cup which had been previously coated with the resinous ester masking composition. These latter preselected portions were non-foamed and had a thin coating of the polydimethyl siloxane lubricant deposited thereon. Examination of the cup disclosed that its overall structural properties were not seriously altered by the steeping operation while at the same time the insulating properties of the cup were increased several fold. The foam was of excellent quality, the cells being fine in size and uniform in appearance.

Additional cups were thermoformed as set out above for test purposes to be hereafter described. Certain of these cups were chemically masked and foamed as set out above, except that the lubricant was not added to the Freon-11 foaming agent. The entirely non-foamed cups, the partially foamed, non-lubricated cups and additionally prepared cups partially foamed and lubricated in the manner set out above, were passed down an inclined hollow cylindrical chute of the type utilized in automated vending machines, which made an angle with the horizontal of about 60.degree., and observed for speed relative to each other. The following results were obtained. Numbers refer to the quantity of cups falling into the categories identified by the key below. ##SPC1##

As can be seen from the above, 100 percent of the chemically masked, lubricated, partially foamed cups tested were considered to have adequate or better speed while sliding down the vending tube, with about 77 percent of this category considered to have fast speed, thereby producing excellent vendability. Results were slightly better with the unmasked and non-foamed control cups and substantially poorer with the chemically masked and partially foamed cups.

EXAMPLE II

A solution of lubricant dissolved in a liquid foaming agent is prepared by adding thirty parts of liquid Dow Corning DC 200 silicone oil to 1970 parts of Freon-11. The mixture was stirred until a clear homogeneous solution was obtained. Chemical masking, exposing, foaming and testing are carried out in the same manner as in Example I utilizing similarly shaped initially thermoformed cups. Substantially equivalent results are obtained.

In general, the plastics used in the practice of this invention are substantially thermoplastic materials which have been fabricated into containers having nesting means simultaneously formed therein, and include such materials as polystyrene, styrene-buta-diene-acrylonitrile graft and/or mechanical polyblends, other rubber-modified graft and mechanically blended styrene polymers, vinyl chloride, vinyl acetate copolymers, polyethylene, chlorosulfonated polyethylene, chlorinated polyethylene and ethylene-vinyl acetate copolymers which are graft and/or mechanically blended with polyvinyl chloride homopolymers and copolymers, polyvinyl halide homopolymers and copolymers including interpolymers, mixtures and plasticized versions of the same.

The plastic solvent foaming agent may be any material which is (1) at least a partial solvent for the plastic being processed, and (2) convertible to a gaseous state at a temperature below the melting point temperature of the plastic. Freon-11 (trichlorofluoromethane) is generally preferred for polystyrene homopolymers and rubber-modified interpolymers, with methylene chloride, acetone, dichloro-ethylene, xylene, carbon tetrachloride, methyl ethyl ketone, benzol, toluol, chloroform and the like being likewise effective depending on the particular plastic being used.

The masking step of the above-described process of the present invention may be achieved by either coating with a chemical composition as set forth in the preferred embodiment or by mechanical means. Typical of the latter approach is the use of a sealed chamber imposed around the cup surface(s) to be non-foamed to shield these portions during exposure of the cup to the absorbable medium. Also useful for effective masking are the utilization of cooled surfaces in contact with either side of the surface to be non-foamed, while the surface to be foamed is heated. This may be accomplished by means of cooling rings in contact with the outside surface portion of the cup which is to be non-foamed or by means of a composite mandrel placed against the wall of the interior of the cup prior to exposure to the absorbable medium, the mandrel having both heated portions for creating foam and cooled portions for retarding foaming.

In the preferred chemical masking approach the coating materials employed may be any fluid substances which will serve as a barrier against absorption of the medium by the plastic. The choice of the fluid substance, utilized will, of course, depend on the medium employed in the process. In some instances, it may be desirable to employ fluid substances which are considered insoluble in the particular medium employed. Preferentially, the coating composition is a lacquer comprising a resinous ester which is a derivative of rosin or rosin acids, a modified rosin or a modified tarpene hydrocarbon, which is soluble in alcohol and generally insoluble in aliphatic and petroleum hydrocarbons. This composition has been found to effectively shield the portions to be non-foamed from the absorbable medium. However, the non-volatile resinous ester portion of the composition, after evaporation of the solvent, is rather rough and slip resistant which, as mentioned, causes vending and stack separation problems with the cups. The problem is unusually overcome by the present invention wherein these slip resistant surfaces are subsequently coated with a slip promoting type of release agent during the exposure step.

Regardless of the manner in which the masking is achieved in the present invention, patches of almost imperceptible foam breakthrough often appear in the area of the cup which was meant to be non-foamed. This is especially true in high speed production operations because of incomplete masking, whether it be a result of a slight discontinuity in the coating of chemical masking agent or incomplete shielding or surface contact if one of the other methods of masking is used. This foam, as mentioned, has a rather high coefficient of friction which may cause cup sliding problems. However, the effect of this deficiency is diminished by the manner of coating of the preferred embodiment wherein a film of lubricant has already been deposited on the surface prior to the foaming step when the patches are generated. Thus the present invention can reduce the criticality of the masking step since foam patch breakthrough is no longer as much of a problem.

In general, the plastic article is permitted to dry for a short interval of time after exposure to the foaming agent -- lubricant solution and prior to heating. This drying interval allows deeper diffusion and serves to harden the outer surface of the cup somewhat, forming a skin which aids considerably in the formation of the closed cell structures which are necessary for good insulating properties. If desired, the drying, interval may be accelerated by forced air blasts, moderate heating conditions, etc.

After the drying interval, the plastic article is heated to foam the outermost portions of the plastic article. The heating means utilized may vary, the final results being affected by the uniformity of heating, rate of heating and temperature level at which it is conducted. Heating may be on either the same or the opposite side of the cup surface which had been exposed to the absorbable medium. An especially uniform foam has been obtained by exposing the side opposite to that which had been exposed to the foaming agent.

Though it is preferred for operational simplicity to coat the container portion to be non-foamed with the lubricant simultaneously with immersion in the medium, any suitable equipment may be employed to coat the portion of the plastic container to be non-foamed as a separate step either before or after exposure to the foaming agent. For example, this could even occur by utilizing a brush, roll or similar device either with or without a partial foaming step.

The quantity of coating applied should be such that the dry coating weight is from about 0.1 to 1.5 parts of lubricant per 98.5 to 99.9 parts of liquid foaming agent, and preferably 0.1 to 0.5 parts of lubricant per 99.5 to 99.9 parts of foaming agent.

The lubricant utilized on the container surfaces of the present invention must have clearance of the Food and Drug Administration for use in food packaging applications, since this is the type of container contemplated by the invention. Consequently, it should be inert, clear and non-toxic, but may be either a solid or liquid at room temperature conditions prior to dissolution. It must have the ability to reduce friction between the plastic container and another surface along which the container is sliding. When utilized in conjunction with the partial foaming process wherein coating takes place during exposure to the absorbable medium as in the preferred embodiment, the lubricant should be miscible in the absorbable medium. It must be highly efficient at low concentrations, be soluble in water or organic solvents and have no effect on the quality of the foam produced during the heating step. It should be relatively non-volatile at temperatures up to about 200.degree. C, in order to avoid evaporating during the foaming step. Effective lubricants include fatty materials such as fatty acids having from 8 to 24 C atoms, ethers, alcohols and amides such as oleamide, palmitamide, linoleamide and stearamide; polyethylene glycols having average molecular weights in the range of between about 400 to 4,000; fully refined paraffin waxes melting at between about 145.degree. to 180.degree. F.; petrolatum, glycerine, castor oil, mineral oil, fluorocarbon polymers and fluorocarbon surfactants. Particularly preferred when the lubricant is applied during the partial foaming process are polymeric silicone materials characterized by a repeating siloxane linkage, i.e., ##SPC2##

wherein R.sub.1 and R.sub.2 are either C.sub.1 or C.sub.2 alkyl groups.

Polydimethyl siloxane is particularly preferred since it has been found to have no effect on the quality of foam produced on the finished container, with the resulting cell size being equivalent to that obtained without use of the lubricant. Also, off grade scrap cups having this type of lubricant coated on their surfaces showed no signs of incompatibility or other adverse processing behavior when reground and re-extruded.

It should be noted that the present invention is applicable to deposition of lubricant on any exterior surface portion of a nestable container of a type wherein it is desirable to increase its sliding properties. It is preferred however that the coating be applied on the lower portion of the sidewall in the area of its joint with the base and on the surfaces of the nesting means. Satisfactory results are obtained by coating at either or both of these sections of the cup.

As an advantageous side effect of the lubricated exterior surface portions of the containers of the present invention, it has been noted that minor amounts of the lubricant are rubbed off the surfaces of the cups onto various cup supporting surfaces of the vending machines over a period of time. This also decreases the effect on cup dispensing speed of sticky internal surfaces of these machines which may be caused by vapors condensing from hot liquids within the machine which are present as parts of the constituents of the beverage being dispensed.

The nestable containers of the present invention are formed from resilient thermoplastic materials. The structures are used in coin operated beverage dispensing machines and broadly where space requirements require that a plurality of containers be nested together within a stack. The lubricated surfaces of the invention are especially applicable in insulated containers for holding hot or cold substances having integral non-foamed wall portions which provide improved structural properties. The invention is likewise applicable, however, to entirely foamed and entirely non-foamed containers.

It is obvious that many variations may be made in the products and processes set forth above without departing from the spirit and scope of the invention as hereinafter claimed.

Claims

I claim:

1. A method of forming a partially foamed, plastic container having a predetermined, lubricated, non-foamed external surface portion which comprises:

a. dissolving a lubricant in a medium which is absorbable by the plastic and convertible to a gaseous state below the melting point temperature of the plastic used to form the container, to form a solution of lubricant in the absorbable medium;

b. masking a portion of said foamable plastic container which is to be non-foamed;

c. exposing at least part of said masked portion along with another portion of said container which is to be foamed to said solution to simultaneously coat said exposed masked portion with said lubricant and permit said medium to be absorbed into the portion of the plastic to be foamed; and

d. heating the plastic container to a temperature below the melting point temperature of the plastic but above the temperature at which the medium converts to the gaseous state to produce a foam covering having a boundary along the edge of one of said container portions.

2. The method of claim 1 wherein masking is accomplished by coating with a liquid chemical composition which is substantially insoluble in the absorbable medium.

3. A method of forming a partially foamed, plastic container having a predetermined, lubricated, non-foamed external surface portion which comprises:

a. dissolving a lubricant in a medium which is absorbable by the plastic and convertible to a gaseous state below the melting point temperature of the plastic used to form the container, to form a solution of lubricant in the absorbable medium;

b. depositing a chemical masking composition which is substantially insoluble in said absorbable medium on an outer surface portion of a container which is to be non-foamed;

c. entirely exposing said masked outer surface portion of said container along with another outer surface portion of said container to be foamed to said solution of lubricant in absorbable medium, to coat said previously chemically masked surface portion with said lubricant to form said predetermined, lubricated non-foamed external surface portion without the absorbable medium portion of the solution dissolving said chemical masking composition, while concurrently permitting said absorbable medium to be absorbed into the surface portion of the plastic to be foamed; and

d. applying heat to the inside of the container to cause said outer surface portion of the container which had absorbed said medium to foam.

4. The method of claim 3 wherein:

a. the masking composition is a resinous ester dissolved in alcohol;

b. the lubricant is silicone oil; and

c. the absorbable medium is trichlorofluoromethane.