Method of producing thermoformed articles from gas impregnated polymer

Abstract

A solid state process utilizes gas impregnation to enhance thermoforming of thermoplastic material. If the gas is plasticizing, the article is thereby plasticized for thermoforming. In some embodiments, the invention provides foaming the polymer prior to or during thermoforming by creating high levels of dissolved gas during gas exposure. Foaming may proceed spontaneously upon decompression from gas pressure, or foaming may be enhanced by heating the polymer sheet near to or above the polymer's glass transition temperature, thereby producing plasticized foamed polymer for thermoforming. When objects of unfoamed polymer are desired, foaming may be suppressed by thermoforming gas saturated articles under gas pressure. This process may be used to enhance the thermoforming performance of articles that have been previously foamed, including articles foamed by prior art processes. In some embodiments, polymer is sufficiently plasticized so that it may be thermoformed without heating. Plasticization of the polymer is reversible.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is a continuation of PCT Application No. PCT / US04 / 15246, filed May 14, 2004, which claims priority from U.S. provisional application No. 60 / 471,477, filed May 17, 2003, titled THERMOFORMED FOAMED THERMOPLASTIC PACKAGING.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention relates to a process of producing articles of thermoformed thermoplastic polymer. More specifically, this invention relates to improved thermoforming processes enabled by impregnating polymer with gas prior to thermoforming. [0004] 2. Description of the Related Art [0005] Thermoforming processes are used to produce a wide array of shaped articles in modern life. Products made from thermoformed sheets of thermoplastic include trays, bowls, beakers, signs, briefcase shells, refrigerator door liners, and packages. [0006] Thermoplastic materials used in thermoforming include acrylic, low density polyethylene (LDPE), high densit...

AI Technical Summary

Benefits of technology

[0032] The present invention utilizes a solid state process of gas impregnation to enhance the performance of thermoplastic material used in thermoforming. A roll of polymer sheet is provided with a gas channeling means interleaved between the layers of polymer. The roll is exposed to a non-reacting gas at elevated pressure for a period of time sufficient to achieve an elevated concentration of high-pressure gas within the polymer. If the gas is a plasticizing gas, exposure is for a period of time required to bring about a plasticizing effect of the polymer. The saturated polymer sheet is then separated from the gas channeling means and decompressed and subsequently thermoformed. In embodiments utilizing plasticizing gas, the glass transition temperature of the exposed polymer is reduced, and therefore thermoforming may take place at a lower temperature than used for thermoforming unexposed polymer.

[0033] In some applications, the invention provides foaming the polymer prior to thermoforming by creating high levels of dissolved gas during gas exposure. In some embodiments practicing foaming, bubble nucleation and growth proceeds spontaneously upon decompression, while in other foamed embodiments bubble nucleation and growth is initiated and enhanced by heating the polymer sheet near to or above the polymer's glass transition temperature, thereby producing foamed polymer for ready for immediate thermoforming. In embodiments practicing foaming, the processes of foaming and thermoforming may be continuous. In preferred embodiments practicing continuous foaming and thermoforming, foaming is performed by heating just prior to forming. It should be noted, in addition, that thermoforming of conventionally foamed polymers may be enhanced by solid state gas impregnation.

Problems solved by technology

Principal limitations of this thermoforming process are that, of necessity, edges and corners of objects are always rounded to an extent, and significant undercuts or reentries are not possible.

While not generally suited for very high speed production, crisp detail, 90 degree corners and complex features may be rendered in objects produced by this technique.

Higher temperatures required for low plastic viscosity require greater energy input and thus add to thermoforming costs.

Plastic viscosity that is inadequate for rendering of detail by less expensive and / or higher production processes requires employing more expensive and / or lower production thermoforming processes to achieve results.

As polymeric materials are subjected to higher temperatures, they are subject to thermo-oxidative degradation, wherein the polymer chains break and the polymer becomes more difficult to recycle, diminishing its value.

In addition, the viscosity of some polymeric material in the prior art simply cannot be lowered sufficiently by heating, no matter how high the temperature, for some thermoforming applications.

Prior art thermoforming of foamed polymeric materials presents additional problems.

Prior art foamed polymer is not suitable for thermoforming immediately after foaming extrusion, however, requiring a period of “curing”, in which the foamed polymer is exposed to the atmosphere for several days after extrusion.

Forming at this low absolute pressure may well cause buckling of cell walls and collapse of cells.

When prior art foamed polymer is exposed to the atmosphere for longer periods before thermoforming, however, the trapped foaming gas gradually dissipates from the cells in the polymer foam, and the pressure of the cells approaches atmospheric pressure.

Accordingly, on thermoforming polymer foam that has been stored beyond optimal periods for curing, a substantial percentage of the microcells in the foam collapse or distort during thermoforming, resulting in material that may tear or distort during forming or otherwise produce inferior thermoformed foamed products.

Since prior art foamed polymer is reused only with difficulty, repeating the process of foamed extrusion and curing of such exhausted foamed polymer is generally not an economically viable option.

Yet another limitation of prior art thermoforming is related to the rheological properties of heated prior art foamed materials, which limits the geometry of products that can be formed with such materials.

In the prior art, inadequate plasticity, strength and ducility at thermoforming temperatures limits both the steepness of object walls, whereby wall angles of less than 35 degrees from vertical are not possible, as well as the relative height of objects, whereby a depth to width ratio exceeding 1:1 cannot be achieved.

Yet another limitation of prior art foam thermoforming is the lack of a continuous smooth skin in untreated prior art foamed materials, resulting in thermoformed objects having poor appearance, low durability, lack of stain resistance, and other undesirable qualities.

Lack of skin may result in blistering or tearing of prior art foamed material during heating and thermoforming, limiting the suitability of such material for thermoforming.

In the prior art, a separate skin of unfoamed material may be laminated or otherwise attached to the foamed material in an effort to address these shortcomings, but at an economic and environmental cost, the latter because the attached skin may render the material and objects formed from it less suitable for recycling.

Attempts to address these shortcomings of prior art thermoformed foamed materials further include the use of closed or two sided molding, requiring considerably higher tooling expense and resulting in decreased production line efficiency, as is well understood by those of skill in the art.

First, because a curing period is necessary, the processes of foaming and thermoforming polymer are necessarily discontinuous, resulting in industrial inefficiency.

Second, foamed polymer that has been cured for overly protracted periods loses value for thermoforming, a loss for which the prior art provides no satisfactory remedy.

Third, the geometry of thermoformed foamed objects with prior art materials is considerably limited.

Fourth a lack of integral skin may necessitate adhesion of a layer of unfoamed material and / or using expensive tooling entailing decreased production line efficiency.

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