Patent Application
20140377523
The present invention generally relates to methods for producing layered or multi-layered cellular thermoplastic polymer structures, and more specifically to methods for producing lighter weight, selectively foamed plastic end products such as consumables and packaging.
For decades, plastics have been made in various forms and used in many different durable and non-durable applications. Plastics may generally be characterized as “thermoplastics” or “thermosetting plastics.” A product made from a thermoplastic becomes pliable or moldable above a specific temperature, and returns to a solid state upon cooling. Most thermoplastics have a high molecular weight with polymeric chains that associate through intermolecular forces, referred to as van der waal forces, thus thermoplastics may be remolded because they can be softened by heating and reformed into new or similar products upon cooling. Thermosetting plastics are formed with permanent or irreversible “cross-linking” bonds that are generated during the curing process. These cross-linked bonds break down or disassociate upon heating and do not reform upon cooling.
By way of example, non-durable plastics are commonly used in packaging and food service environments. In most cases, plastic packaging or plastic coverings that have been in contact with food are not meant to be recycled or reused, and therefore these products are typically disposed of after a single use. The disposal of non-durable plastics results in large amounts of waste. The recycling of non-durable plastic products requires that the product be cleaned. Cleaning, in turn, uses additional resources and energy. Nevertheless, cleaning processes may degrade the polymeric bonds of thermoplastics and may be insufficient for thermosetting plastics, and in either instance may result in recycling efforts that are less than optimal.
One type of plastic packaging in common use is referred to as solid or un-foamed plastic packaging, which is a highly dense form of plastic. Solid plastic packaging provides minimal, if any, cushioning for the packaged goods. In addition, solid plastic packaging generally has low thermal insulation properties, and thus does not adequately isolate hot and cold products from users or maintain product temperatures. The high density of solid plastic packaging results in an increased shipping weight, which in turn increases shipping and transportation costs.
The packaging industry, for example, has been switching to the use of foamed plastic to make packaging or other products. Foamed plastics are produced using a chemical blowing agent that decreases the density over a given volume. The foaming process adds some insulation and cushioning benefits as compared to solid plastics, but creates other problems.
Foamed plastics are more friable (e.g., brittle, easily crumbled or pulverized, frangible), which makes collecting and recycling more difficult. Styrofoam, a common type of foamed plastic, can be broken into small pellets that are nearly impossible to separate from a waste stream. The surface of foamed plastics is often cellular in nature, and unattractive to consumers. The chemical blowing agents used in producing many foamed plastics include fluorocarbons and/or chlorofluorocarbons, which may be environmentally restricted because of the contamination effects of these materials. The injection of fluorocarbons and/or chlorofluorocarbons into foamed plastics may render them non-recyclable from an environmental perspective.
Most foamed plastics are produced in the melt state of the polymer. Polymers such as polyethylene terephthalate (PET) and Polylactic acid (PLA) are not commercially feasible for producing foamed products using conventional means. Most foamed plastics are also difficult to thermoform, limiting their industry and commercial applications. Foamed plastics typically have poor barrier properties, so it is difficult to create foamed plastics with layered structures and hence bonded structures may be required, which increases the cost of the end product.
Microcellular foaming processes that utilize carbon dioxide or other high pressure gases as a foaming agent have been explored to resolve some of the drawbacks mentioned above. The main methods for commercial production of microcellular foams are extrusion foaming, semi-continuous production, and injection molding. Extrusion foaming, much like chemical blowing foaming, requires a high-quality input stream which may make it impossible to utilize recycled foams; the extruded result also may not be drawn deep into cup shapes. Microcellular injection molding cannot create thin walled shapes with a good surface finish. Semi-continuous production is a labor intensive process which consumes an interleaving layer and requires a large amount of plastic. Due to the fact that the plastic loses gas during processing, and that a large roll of plastic takes a long time to process, the properties at the start of a foam roll will differ from those of the end of a roll.
The problems concerning the utilization of plastics in packaging, in foam and solid form, extend to durable goods as well. A particular area of plastic usage for durable goods is composites, in which multiple phase separated materials are bonded together in order to create a single part with better properties. The center section of these parts is often foam. Most foam materials produced for these applications cannot be recycled after bonding, and are not biodegradable.
The life cycle of a conventional plastic product, such as a plastic cup, includes obtaining the raw material (e.g., petroleum, corn, plastic scrap, or combination thereof), refining the raw material, generating plastic pellets, processing the pellets into sheets or injection molding them into preforms, heating, shaping using mechanical means and/or air pressure, heating set sheet to increase crystallinity and service temperature, trimming and secondary operations to finalize product, distributing the product for customer consumption, disposing of the product as waste or as a recyclable material.
In one embodiment, the present invention generally relates to systems and methods for generating, processing, handling and forming plastic materials into an end product such as, but not limited to, consumables and packaging. The plastic materials are made into preforms or preform sheets, which in turn may be selectively masked to expose desired regions of the same for a saturation process, a heating process, or both. In addition, the various processes may be sufficiently controlled to obtain a desired micro-structure, and in turn obtain desired mechanical, structural, and aesthetic properties in the end product. By way of example, an embodiment of the present invention results in consumable, plastic cups that lighter weight and more structurally robust less than conventional plastic cups. Further to one or more embodiments of the present invention, various systems for material handling may be utilized to efficiently, timely and cost effectively produce the preforms.
In one aspect of the present invention, a method for making plastic end products includes the steps of (1) generating a plurality of plastic preforms, each preform having a inner region defining an area from which an end product will be formed, and each inner region bounded by a periphery region; (2) loading the plurality of preforms into a pressure vessel rack; (3) placing the pressure vessel rack into a pressure vessel; (4) applying a pressure to the pressure vessel rack; (5) while under pressure, saturating the plurality of preforms with a gas for a predetermined sorption time; (6) moving the plurality of preforms from the pressure vessel rack into a heating rack; (7) heating the plurality of preforms; and (8) forming each of the plurality of preforms into a configuration corresponding to a desired plastic end product.
In another aspect of the present invention, a cross-sectional micro-structure of a plastic apparatus includes an exterior surface; an interior surface; a first intermediate region located between the exterior and interior surface, the first intermediate region configured with a plurality of first foamed cells, wherein the first foamed cells include a plurality of first interstices disposed among a plurality of first plasticized portions; and a second intermediate region located between the exterior and interior surface, the second intermediate region configured with a plurality of second foamed cells, wherein the second foamed cells include a plurality of second interstices disposed among a plurality of second plasticized portions, and wherein the second interstices are larger than the first interstices.
In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings may not be necessarily drawn to scale. For example, the shapes of various elements and angles may not be drawn to scale, and some of these elements may be arbitrarily enlarged or positioned to improve drawing legibility. Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known systems and processes for producing plastic products may not necessarily be shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.
The present invention is generally directed to rapid foaming processes for creating polymer end products that have an integral solid skin. In one embodiment, the processes general include the steps of saturating a thermoplastic disc, sheet or preform; sending the saturated product through a heating cycle, and the forming the product into a final shape. The processes may be utilized with a variety of thermoplastic materials and the final product may include biodegradable fibers and adhesives to create a biodegradable composite product.
By way of example according to an embodiment of the present invention, plastic material may be obtained as a solid sheet. The sheet is placed into a pressure vessel at saturation pressures in a range of about 200 to about 2,500 pounds-per-square-inch (psi), with a preferred range of about 300 psi to about 1,000 psi, in which a gas used in the pressure vessel may be a gas such as, but not limited to, carbon dioxide. As the sheet absorbs the gas, a gas concentration in the sheet will be highest on an exterior surface of the sheet and the gas concentration will increase over time in the pressure vessel. In one embodiment, the sheet may be left in the pressure vessel to achieve an equilibrium gas concentration throughout its thickness, but preferably the sheet would be extracted from the pressure vessel prior to achieving equilibrium. As the gas concentration increases, crystallization of the sheet will begin to occur. Similar to the gas concentration, the crystallization may be initially higher on the exterior surface, but may reach an equilibrium state if the sheet is left in the pressure vessel long enough. For thin section of relatively fast-absorbing polymers such as, but not limited to, a 0.024 inch thick PLA, a saturation time in a range of about one (1) minute to about one (1) hour may be sufficient. For thicker sections and/or slower absorbing polymers such as, but not limited to, PET, a saturation time in a range of about thirty (30) minutes to about one-hundred (100) hours may be sufficient.
When the gas trapped in the sheet reaches a certain concentration, the pressure vessel is vented, opened, and the sheet is removed. After removal, the gas trapped in the sheet will begin to disperse into the atmosphere. Because the dispersion process will be fastest on the exterior surface of the sheet, the gas concentration will lower more quickly in the exterior surface as compared to an interior of the sheet. While the initial gas dispersion happens, the gas in the interior will continue to diffuse and make the gas concentration more even throughout the thickness of the sheet.
The sheet is then heated, which may be accomplished using an infrared heating source according to an embodiment of the invention. As the sheet heats up, the sheet softens and the gas trapped in the sheet increases in fugacity, which is defined as the desire of the gas to escape from a material. The softening (i.e., heating) process may also be referred to as a foaming process. In addition and as the sheet heats up, bubbles begin to nucleate at various sites in the softened sheet to create cells. In areas of higher crystallinity and higher gas concentration, there are more nucleation sites. In areas of lower crystallinity and lower gas concentration, there are fewer nucleation sites. If the exterior surface of the sheet has been allowed to lose a sufficient amount of gas then a skin may be created as the exterior surface. The skin may be solid, approximately solid, or at least have smaller cells as compared to other portions of the sheet that lost a lesser amount of gas during heating. High crystallinity portions of the sheet may require higher temperatures.
As the temperature of the sheet continues to increase, the sheet continues to soften and the fugacity continues to increase. Within the interior region of the sheet as the gas escapes, the cells may begin to grow, which may be referred to as foaming. Even after the plastic is removed from the heat source, additional foaming may occur as gas continues to escape and the sheet continues to stretch.
As the cells grow, they stretch local regions (e.g., boundary regions) of the sheet around the growing cells. The growing process may lead to local strain induced crystallization in the sheet, and the strain induces crystallization, which in turn may make the boundary regions more resistant to further growth. The amount of time and ramp up rate of the heating process should strike a balance between cell growth and a wall thickness of the boundary regions. For example, if the wall thickness becomes too thin then the cells may rupture. Strain induced crystallization may be viewed as a self-regulating process to help strike the aforementioned balance. From a micro-structural viewpoint, the heating process may generate additional foaming, increase cell sizes, and/or create oblong shaped cells at desired locations within the sheet.
Cell growth stops when the sheet reaches an equilibrium state for its temperature or when the sheet has cooled to where it no longer stretches due to internal forces from the gas within the cells. In a combined thermoforming and foaming process, the equilibrium state may be achieved in a mold or tool or even after the sheet has been removed from the mold or tool. Optionally, impacting the sheet with compressed air may also bring about the equilibrium state.
By way of example and according to an embodiment of the present invention, one process starts with a multi-layered structure, with each layer being either a different type of plastic, or a type of the same plastic with different crystallization characteristics such as, but not limited to, a polyactic acid (PLA) structure, which is non-crystallizable and foamable, an ethylene-vinyl alcohol (EVOH) copolymer structure, which is crystallizable and high barrier, and another PLA structure. The PLA portion may be rapidly foamed while the EVOH portion would provide superior barrier properties. Optionally, the multi-layered structure may include a first plastic not prone to crystallization on a periphery of a part, and a second plastic prone to crystallization at the center of the part. This type of multi-layered structure may yield a part whose internal structure remained rigid at high temperatures, while the non-crystalline periphery may allow a high depth of draw and provide toughness to the part.
The different layers may expand at different rates and/or by different amounts during heating. By way of example, an internal layer of EVOH or an internal layer of another highly crystalline material may not expand or may expand by only a small amount. Such minor or zero expansion will constrain the exterior surface layers (e.g., the PLA layers), so that a majority of any cell growth should occurs in a thickness direction to prevent waves and wrinkles from being formed in the sheet prior to a post-heat forming process.
In an embodiment of the present invention, a plastic end product is formed using a preform process. By way of example,
Supporting small unsupported sections of the preforms 200 during saturation may have process control advantage. As the preforms 200 soften when exposed to the gas, such as carbon dioxide, there may be saturation conditions in which it is desirable to heat the preforms during saturation.
The heating tray may have edges (not shown) that will assist in lip forming of end products such as, but not limited to cups or other end products. The periphery regions 202 trapped between the top and bottom trays, 224, 226 may have less gas trapped in those regions due to the trays acting as a gas and/or heat shield. Accordingly, the periphery regions 202 may be prevented from foaming.
During heating, the edges periphery regions 202 may be constrained to prevent warping. Warping prior to shaping is undesirable because it may cause non-uniform characteristics in stretched and formed end-products.
In another embodiment of the present invention and referring to
The conveyance system 400 includes an entrance chamber 402 that may be evacuated using a pump 404 that vents to an accumulator and/or carbon dioxide supply reservoir 406. At the entrance chamber 402, a first gate valve 408 is opened so the assembly may be inserted. The first gate valve 408 is then closed and the entrance chamber 402 is pressurized using the pump 404. Next, a second gate valve 410 is then opened and the assembly is moved into a main chamber 412.
Referring back to
Processing the assemblies directly in the pressure vessel may allow for the injection of carbon dioxide during injection molding or extrusion. In the aforementioned embodiment, the main chamber 412 may be employed to fully saturate parts, the assemblies. Alternatively, the assemblies may be injection molded or extruded with lower gas concentrations and then saturated or desorbed to achieve higher or lower gas concentrations through the surface.
The temperature in each chamber of the pressure vessel may be controlled independently to generate a desired layered structure for the end product. In one embodiment, the main chamber 412 may be segregated or controlled to be subjected to a low pressure to saturate the assemblies for a period time adequate to achieve a uniform low gas concentration. The low gas concentration would be able to create a large cellular structure to reduce the density of the end product. The assemblies may then be subjected to a higher pressure, as compared to the low pressure, for a shorter period of time to create an integral crystalline skin for the end product. Saturation pressures may be in a range of about 200 psi to about 2,500 psi, with a preferred range of about 300 psi to about 1,000 psi.
In yet another embodiment, the gate valves may take the form of dynamic seals that allow the entrance and removal of assemblies from pressure vessels without opening and closing the pressure vessel.
The conveyance system 600 in cooperation with the vertical conveyance system 612 allows for efficient and continuous material handling in moving the preforms through at least a saturation cycle or process while adequately supporting the preforms. Now referring briefly back to
As best illustrated in
In addition, the pressure vessel 702 may include one or more rollers 714 to guide the continuous sheet 712 through the pressure vessel 702. A cutter 716 operates to cut the continuous sheet 712 into segmented preforms or sheets 718. A seal or wall 720 located in the pressure vessel 702 separates a pressurizing chamber 722 from an exit chamber 724. In one embodiment, the wall 720 includes an internal guide 726 that is configured to direct the segmented preforms 718 into the exit chamber 724. Similarly, an exit guide 728 may operate to direct the segmented preforms 718 out of the exit chamber 724 of the pressure vessel 702. The exit chamber 724 may be vented to atmospheric pressure or to a gas reservoir (not shown).
The illustrated embodiment of
The micro-structure 900 may advantageously improve bending strength, prevent the spread of colorants during forming, and/or provide a smoother finish. Embodiments of the micro-structure 900 allow a manufacturer to customize the appearance and mechanical properties of the plastic apparatus.
Controlled heating of the preform or sheet may be used to induce specific properties during the saturation process. As discussed above, heating a single side of the preform or sheet may result in an asymmetric density and cellular distribution. By way of example, such an asymmetric density and cellular distribution may be manipulated to optimize mechanical properties, such as creating a plastic apparatus with an improved bending strength to resist external crushing forces while remaining sufficiently flexible to side loading so the apparatus may be easily removed from a stack. Optionally, one or more colorants or coloring dies may be selectively infused into the micro-structure 900 to create a colored, high density exterior while minimizing undesired density increases of one or both of the intermediate portions 906, 908. Controlling a size of the foamed cells 910, 916 may advantageously permit a desired amount of light diffusion or light refraction through or by the plastic apparatus. By way of example, foamed cells 910 having a measured length across a height or width of the foamed cell 910 within a range of about one (1) to about twenty (20) micrometers provides an aesthetically desired amount of light diffusion through the plastic apparatus. It is further appreciated that the size of the foamed cells 910 may operate to give the exterior surface 902 a rougher finish (e.g., large-sized foamed cells 910), a matte finish (e.g., medium-sized foamed cells 910), or a glossy finish (e.g., smaller-sized foamed cells 910).
In another embodiment, the micro-structure 900 includes an intermediate wall 922 located between the first intermediate region 906 and the second intermediate region 908. Preferably, the intermediate wall 922 may take the form, generally, of a solid wall having a wall density that is higher than both of the intermediate regions 906, 908. Inclusion of the intermediate wall 922 may advantageously permit a desired amount of resiliency for the plastic apparatus and thus allow it to spring back to a desired shape when loaded. The intermediate wall 922 may also provide a barrier for crack propagation, thus making the plastic apparatus more crack resistant. Alternatively or additionally, the intermediate wall 922 may make the plastic apparatus more resistant to in-plane or out-of-plane expansion or warping during one or more of the manufacturing processes.
In yet another embodiment of the present invention, it may be advantageous to distort or partially distort the preforms or sheets during the heating process. Referring back to
The systems and methods described herein may advantageously shield and constrain a portion of a preform during saturation and/or heating. The systems and methods may be used with a variety of thermoplastics. The saturation process preferably utilizes carbon dioxide gas, but other types of gas may be employed to optimize the mechanical or micro-structural properties of the end product. In one embodiment, the end product may be biodegradable or recyclable. In addition, the end product may have a smooth exterior surface that is aesthetically and structurally better than existing end products made with other methods. The systems and methods described herein may produce an end product that is less expensive than a similar product of solid plastic due to the lower amount of raw plastic material used and a more energy processes. The end product may be lighter in weight, which reduces downstream transportation and inventory costs. Moreover, material left over after processing the preforms or sheets, as described above, may be re-processed and/or re-used to make additional sheets or preforms.
The various embodiments described above can be combined to provide further embodiments. All of the above U.S. patents, patent applications and publications referred to in this specification are incorporated herein by reference. Aspects can be modified, if necessary, to employ devices, features, and concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all types of plastic end products (e.g., plastic cups, packaging, etc.) and processes for making the same. Embodiments of the processes and/or end products described herein may be utilized individually or in any combination. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.
The present application claims priority from U.S. Provisional Patent Application No. 61/837,110, filed on Jun. 19, 2013, and the subject matter of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61837110 | Jun 2013 | US |
