Patent Application
20160325472
The field of art to which this invention generally pertains is methods for making foam particles.
There are many processes that can be used and have been used over the years to produce foamed articles. While foamed articles have many advantages over their non-foam counterparts, such as using less material, lighter weight to transport, etc., they still use a significant amount of energy to produce, and can also raise environmental concerns for a variety of reasons.
The systems described herein meet the challenges described above, and can provide additional advantages, such as unique materials, and more efficient and effective manufacturing processes.
A method of forming polymer foam beads is described, including forming a plastic melt of the polymer, adding a blowing agent to the melt to form a polymer-blowing agent mixture, forcing the mixture through a capillary die, pelletizing the mixture as it exits the die, wherein the capillary die has a non-circular cross-section resulting in polymer foam beads with very thin walls and substantially uniform expansion properties.
Additional embodiments include: the method described above where the length of the capillary die is 50% or less than the length of a conventional circular cross-section die; the method described above where the pressure drop across the capillary die is substantially the same as a capillary die with a circular cross section; the method described above where the capillary die has tri-lobal cross section; the method described above where the capillary die has quadra-lobal cross section; the method described above where the capillary die has dendritic cross section; the method described above where the blowing agent is a physical or chemical blowing agent; the method described above where the blowing agent is nitrogen or carbon dioxide; the method described above where the polymer comprises homopolymers, graft polymers, or copolymers of polylactic acid, polystyrene, polyethylene terephthalate, thermoplastic polyurethane, polyvinyl chloride, polyethylene, polypropylene, or mixture thereof; and the method described above where the polymer comprises polyethylene and/or polypropylene copolymers.
An expanded polymer foam particle is also described having a thin skin and uniform expansion properties.
A molded article is also described made up of expanded polymer particles having a thin skin and uniform expansion properties having low density, improved mechanical properties, and cushioning properties suitable for protective packaging; an embodiment where the article is a foam cooler or shipping container is also described.
A method of making a molded article is also described including adding expanded polymer beads having thin skins and uniform expansion properties into a mold, pressurizing the beads, heating and cooling the beads, to produce a molded article in less time and utilizing less energy than conventional molding.
A capillary die specifically adapted for producing foam particles is also described where the capillary die has a non-circular cross-section and a length 50% or less than the length of a conventional circular cross-section die.
Additional embodiments include: the capillary die described above where the capillary die has a tri-lobal cross section; the capillary die described above where the capillary dies has a quadra-lobal cross section; the capillary die described above where the capillary die has dendritic cross section.
These, and additional embodiments, will be apparent from the following descriptions.
The particulars shown herein are by way of example and for purposes of illustrative discussion of the various embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.
The present invention will now be described by reference to more detailed embodiments. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for describing particular embodiments only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. All publications, patent applications, patents, and other references mentioned herein are expressly incorporated by reference in their entirety.
Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Every numerical range given throughout this specification will include every narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly written herein.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
In the extrusion foaming apparatus as shown, for example, schematically in
As mentioned above, the blowing agent can be any physical or chemical blowing agent typically used in foam processes, including CO2, butane, liquid butanol, nitrogen, etc.
As extrudate exits the die 24 and is foamed, rotating knives of the pelletizer 25 cut the bead at the face of the die. When cut, the foam is not completely finished. The foaming process continues to shape the structure of the bead after it has been cut. The blowing agent continues to evolve, typically expanding the particle 3 to 6 times the diameter of a round capillary. The outer skin of the particle remains rubbery while cut, allowing the surface of the foamed bead to flow and reform a smooth, solid surface. A multi-blade pelletizer 25 is typically mounted to the extruder 21 and die 24 assembly. The resulting foamed beads 26 are typically first cut at the face of the die 24 with the pelletizer 25 operating at, for example, 1500 to 2500 rpm. Conventional underwater pelletizers, available from Gala Industries, for example, can also be used to produce the beads as described herein. With water surrounding the beads as they are formed rather than air surrounding the beads, typically rounder beads can be generally formed.
Typically processes which can be used to generate the capillary dies described herein are those, for example, similar to those described in U.S. Pat. No. 3,868,870, the disclosure of which is herein incorporated by reference. In general (see
In
As mentioned above, the actual size of the die is not critical, but will depend on the size of the particle desired and the particular part to be molded. For some molding applications it may be desired to produce and mold larger rather than smaller beads.
As described herein, while benefits associated with the thinness of the walls of the foam beads produced contributes to the energy savings, shorter cycle time, improved properties etc. of the foam beads and articles produced, it is difficult to measure the exact thinness of such walls. Typically wall thickness is measured using electron microscope. However, in measuring such wall thickness of the particle, the particle is actually destroyed in the process. For example, the use of an electron high tension voltage of 8 to 15 kV will disintegrate the side face within a second while 25 to 30 kV disintegrates the thicker cylindrical wall in a second. Using this indirect measure, it is estimated that the thinner walls are less than half as thick as the walls in a conventionally formed bead. This results in a bead that expands significantly more with the same internal pressure, e.g., 50% to 80% higher. This is akin to a balloon with a thicker wall—with the same pressure exerted internally, it would not expand nearly as much.
The use of the die such as shown in
Utilizing the dies described herein, not only is a thinner skin generated on the beads or particles, but the thickness of the skin is more uniform around all sections of the particle, regardless of the ultimate bead shape produced. And in the particular embodiments shown in
Foam coolers and shipping containers (e.g., for personal and medical protective packaging, as well as for shipping fish and produce, computer related items, etc.) are some examples of articles that can be produced using the beads and processes described herein. In fact, using the beads described herein can result in thinner packaging with at least the same properties.
The ability to produce a thinner skin on the bead results in many advantages, especially in subsequent bead molding. For example, in a typical bead molding process, during processing, at least some of the CO2 escapes from the bead, leaving partial vacuum within the bead. Subsequent processing of the bead cannot happen until air diffuses in the bead to replace vacuum. Thinner wall beads allow shorter wait times before the next processing step. Also in a typical mold processing step, the bead-filled mold is filled with air under pressure, filling in these voids. With the beads as described herein, independent of the shape or size of the particle produced, the thinner wall allows all of this processing to happen much faster.
In a typical process as described herein, the beads exiting the die can have a size of about 1.5 mm, for example. Because, as a result of the processing described herein, they are still expanding, the beads will continue to expand, for example, to a size of about 3 to 6 times greater than the capillary diameter (depends on capillary shape) as they are formed and chopped by the spinning blade. Regardless of the shape described in
The process described herein produces a more uniform skin, and particles with a more uniform size distribution as well. All of these properties contribute to improved molding properties, such as improved and faster pressuring, cycle time, mechanical properties, etc. as well as improved properties in the ultimate molded foam article produced. For example, in the air pressuring step described above, it should take less energy to perform this step because the walls are thinner, the particles more uniform, it will take less time to pressurize, heat, and cool the beads, etc.
Pressure drop in non-circular pipes or capillaries can be approximated by using hydraulic diameter vs. diameter of a circular pipe in pressure drop calculation for a capillary. A hydraulic diameter is defined as 4 times the cross-section area of the capillary divided by the wetted perimeter. The concept behind the cross-section shown in
The dies typically are about one-half inch thick, typically made of metal or other material typically used to make capillary dies and spinnerets, with the sub-micron features, opening and side-walls, as described herein. They can be obtained from Nippon Nozzle, Kasen Nozzle, Ceccato Spinnerets, Invista Precision Concepts, Enka Technica etc., for example.
In addition to the shapes shown in
In addition to using conventional PLA polymer materials to make the improved foam beads, other conventional polymer materials can be used as well to produce foam beads as described herein, such as EPS (expandable polystyrene), EPP (expandable polypropylene), EPE (expandable polyethylene), EPET (expandable polyethylene terephthalate), EPVC (expandable polyvinyl chloride), and ETPU (expandable thermoplastic polyurethane), and mixtures, homopolymers, graft polymers and copolymers thereof, for example. And while polymers useful with the methods and systems described herein can have any glass transition or melt temperature, examples of polymers useful as described herein can have glass transition or melt temperatures between about 50° C. and about 95° C., or higher, for example, eTPU (expandable thermoplastic polyurethane) can have a melt temperature of about 170° C., and polypropylene a melt temperature of about 170° C. and a glass transition temperature (Tg) of less than about 10° C. See also, commonly assigned, copending U.S. Patent Application No. 62/084,839 entitled “Method of Making Foam Articles”, the disclosure of which is herein incorporated by reference.
As stated above, not only can the beads produced by the process described herein mold much faster, at lower temperatures, with less energy, but they also can produce improved properties in the finished molded product, such as lower density, improved tensile strength and flexural strength in the molded part, etc.
As described above, the invention described herein can work with any shape of pellet. While the pellets are typically formed in rod or cylindrical shapes, they can be spherical, disk, or ellipse in shape. Because of the thinner skin produced by the process described herein, the beads as produced are typically formed in a more spherical than cylindrical shape.
Typically, the dies described herein will have a multiple number of holes, e.g., 10 to 72 holes, at the point where the blade is cutting the polymer to form the beads. Each die or spinneret hole is machined separately. While the hole in the die is quite small, because of the blowing agent, e.g., CO2 dissolved in the polymer, the bead expands very quickly to form a much larger bead, e.g., about 2 to about 5 mm, or more.
While the cross-section of the bead forming holes in the die historically have been circles, the holes in the dies as described herein are in a petals or flower shaped, for example, as shown in
With thinner walls, everything can happen very quickly. With less water and less air in the beads, for example, expansion happens much more quickly. For example, beads can be pressurized in ¼ to ⅛th the time and the articles can be molded with 20% or less of the energy typically required, by changing this cross-section profile of the spinneret hole.
As stated above, the beads expand with very little energy input because the walls are so thin, can expand a lot more and faster with less energy during processing. Because the bead walls are so thin, the beads can expand and contract very quickly. For example, when beads described herein are put in a tank, and the air pressure is increased until air diffuses into the beads. 30 psi for 12 hours is the normal pressure typically required to expand the beads, where here with the beads described herein, expansion can take place even at 20 psi for 1 hour.
As particles are formed coming out of the capillary die, as represented in
As can be imaged, it is of great cost, time, etc. advantage for pressurization time to be reduced. For example, pressurization tanks represent a significant cost associated with molding of the beads into usable parts. If pressurization time can be reduced, from 12 hours to 2 hours, for example (see Table above), capital cost for equipment could be potentially reduced by 40 to 45 percent. In addition, surface properties of the molded parts are improved as well. See, for example,
Using conventional underwater pellitization, can produce more spherical looking beads as the water allows end of the cylinders to bow further. But in any case, the beads actually still look more like strands with extrusion and blade cutting. What is actually being produced is stout cylinders with bulging ends or stout capsules. The wall thickness on face is much thinner than on the length, so on expansion, the cylinder expands even more. Scanning electron microscope electron high tension voltage is the only (relative) measure which has been used for detecting the difference in wall thickness. The act of measuring destroys the sample. But in any case, on expansion the cylinder elongates more than the diameter expands.
Because of the thinner walls, the beads expand more evenly, producing parts with better mechanical properties. The tensile strength is a lot higher, and the yield strength is a lot higher, so the molded part can't be pulled it apart so easily, because the beads interlock to a certain extent upon molding. See, for example,
The thicker walls along the sides vs. the face in the beads formed with conventional capillary dies is generally associated with residence time in the capillary. As shown in
So the longer the L (capillary length), the greater the pressure drop, which keeps the CO2 dissolved in the polymer longer. However, by changing the shape of the capillary as shown herein, the pressure drop can be increased without increasing the length of the capillary, and the more petals inserted, the greater the pressure drop per unit of length. So the desired pressure drop can be maintained, but the length of the capillary can be shorted. So the minimum pressure drop required to get a steady state flow can be maintained while a desirable shortening of the length of the capillary can be employed, resulting in thinner walls and the advantages described herein. The longer the length of the capillary, the thicker the walls produced. As described herein, the requisite back pressure to keep the CO2 in solution can be maintained with shorter length capillaries, resulting in thinner walls in the beads, i.e., wall thicknesses closer to face thicknesses.
By decreasing the capillary length, thinner bead walls are produced. But decreasing the length, decreases the pressure drop, which can cause foaming within the counterbore, which causes problems in flow, etc. Uneven flow changes polydispersity, which can cause polymer sputter and spit out, process flow harder to control, etc. But by changing the cross section shape of the capillary, the same pressure drop with a shorter length or height capillary can be attained.
This technology is extremely useful for EPP and ETPU. These materials are particularly useful in the automotive industry, where lighter materials are desired, e.g., bumpers, seats, headrests, etc. But processing times for ETPU and EPP molded parts can be relatively long, e.g., 3 to 10 minutes. Processing with the beads described herein, can cut down processing time for these materials significantly, including a significant change in temperature of steaming. See the Table for example. Processing time of ETPU for use in shoes, for example, can also be significantly reduced, making this product less expensive to produce. In addition to processing advantages, a thinner bead wall is produced, which produces better mechanical properties in the finished product, with smoother surface properties as well.
As described, the pressure drop in the capillary is the same as with conventional circular cross section capillaries. A cross section is selected which results in the same pressure drop, and causes foaming to begin at the same spot, while decreasing the capillary length. Decreasing the capillary length, while maintaining the pressure drop, allows nucleation and foaming to happen at the same location in the bead forming capillary system, but with thinner walls produced in the beads. A uniform, homogeneous molten polymer with dissolved gases undergoes heterogeneous nucleation. A nucleating agent such as talc is generally added. As soon as the pressure drops below a threshold pressure, 600 psi to 1000 psi for example, start to see CO2 phase separation from liquid to gas. Bubbles of CO2 begin to nucleate from liquid to gas in the polymer melt in the capillary. The capillary design provides a gradual pressure change allowing foam development in a controlled fashion.
Normally nucleation and foam formation begins in the capillary, but if the length of the capillary gets too short, process control and uniformity of the beads becomes a problem. Decreasing the capillary length but maintaining the pressure drop, is accomplished by choosing a particular cross section as described herein, producing polymer foam beads with very thin walls. While any cross sectional shape may be selected which accomplishes this objective, the tri-lobal shape (
Capillary length as described herein can be reduced by 50% or more while still maintaining the requisite pressure drop to form the foam beads, while attaining the thinner walls desired. In order to get the foaming in the capillary as opposed to up in the counterbore, the length can be reduced by more than half with enough petals in the capillary—as long as the requisite pressure drop is maintained. For example, for a given polymer at a defined temperature, with a capillary length of 3 mils (0.076 mm), pressure drop for a given diameter could be 200 psi with circular capillary cross section, but with tri-lobal, for the same length, pressure drop could be 800 psi. Accordingly, the length could be decreased from 3 mils (0.076 mm), to 1.5 mil (0.038 mm), to 1 mil (0.025 mm), or shorter in length with the tri-lobal cross section and still maintain a satisfactory pressure drop by changing the cross-section shape of the capillary. In addition, the change would result in thinner walls in the beads produced, with the benefits described herein. Modifying or adjusting the capillary cross-section can allow reduction of capillary length to ⅓ the length or more, a ⅔ reduction, while maintaining the desired pressure drop.
Pressure drop which is desired in the capillary will depend on polymer being processed, what the dissolved gases are, the density trying to be achieved, etc. For a particular die, there may be 60 of these capillaries, more, or less. Since measuring the pressure drop changes the flow, it is typically measured relatively far above where the flow is, and is then typically computed. Representative pressure drops in a typical bead forming process can be as low as 150 psi, and as high as 400 psi or more—800 psi, for example.
It should be noted that as a result of the thinner walled beads, conventional molding machines can be utilized, utilizing less energy, and taking less time to mold a particular article. In some cases, lower steam pressure machines like conventional EPS machines may be suitable replacement for expensive higher pressure EPP machines.
Because of the thin skin as described herein, the beads interlock better during molding. This provides for more uniformity, not only within the molded article, but the surface appearance as well. The beads expand more uniformly to fill the gaps. The conventional cylinder shape is also not visible with the more uniform expansion produced with the beads described herein. For example, with the tri-lobe formed beads, once it expands there is little or no evidence that the bead has any resemblance to a cylinder. Utilizing the conventional circular cross section capillary, the cylindrical appearance is detectable even after molding.
With bead foam, the smaller the capillary hole, the finer the particle, the stiffer the article, modes of energy dissipation increase, and every bead is much smaller, including a significant change in mechanical properties. Up to now there has been a limit as to how small capillaries can be made (and to some extent how fast the blade can spin)—but with micromachining this is changing rapidly. And with the ability of using capillaries of a shorter length (e.g., less than 1 mil (0.025 mm)), molded articles which are lighter, stiffer, having increased tensile and yield strength over the traditional round cross section capillaries are possible.
A dry mix blend of plastics is produced consisting of approximately 75% by weight of NatureWorks INGEO 80521) polylactic acid (PLA), approximately 15% by weight of Nature Works INGEO 4032D, 7% by weight of GreenDot GDH 919 modified starch elastomer, 2% by weight of Clariant CESA-extend OMAN698498 styrene-acrylic multifunctional oligomeric reactant, and approximately 1% by weight of conventional talc masterbatch (Cereplast ECA-023, for example). The dry mix of pellets is fed gravimetrically into the feed throat section of a twin-screw extruder. The feed rate for the solids is set to 18.1 kg/hr (40 lbs/hr), and the screws rotate at 40 rpm. Supercritical carbon dioxide (CO2) is injected into the plastic melt at 16 g/min (grams/minute) at a pressure of about 17.2 MPa (2500 psi). An 54-hole die having a cross-section described in
The temperature profile of the barrel sections from feed to exit in the mixer/extruder is systematically adjusted to achieve 190° C., 190° C., 190° C., 175° C., 130° C., 111° C., 111° C., 111° C., 111° C., and 128° C. across the extruder and die. At these conditions, the melt pressure at the die is 14.5 MPa (2100 psi). The extrudate is foamed to a density less than 0.034 g/cm.3 (2.1 lb/ft.3) with a closed cell structure. The surface temperature of the strand extrudate is less than 40° C.
An on-axis, two-blade cutting system (pelletizer), operating at 2600 rpm. is mounted to the extruder and die assembly. Foamed beads are cut at the face of the die. The foamed beads are free flowing and do not stick together. The surface of the foamed beads is complete and does not exhibit open or broken cells. The density of the foamed beads is less than 0.034 g/cm3 (2.1 lb/ft3), and the bead diameter is approximately 3 mm. with a closed cell structure with cell size in the range of 50μ to 100μ.
The foamed beads from Example 1 are pressurized in a sealed vessel using the following pressurization scheme: pressure of 0.07 MPa (10 psi) is applied for 20 minutes, pressure is increased to 0.14 MPa (20 psi) and maintained for 20 minutes, and finally increased to 0.18 MPa (25 psi) and maintained for about 80 minutes. A rapid depressurization of the vessel is performed when foamed particles move to molding machine supply hopper. The beads are conveyed into the cavity of a conventional EPS molding press (Hirsch HS 1400 D) within 15 minutes of removal from the pressure vessel. The machine supply hopper is filled from the pressurization tank every 5-8 shots to minimize variation in foam bead expansion capability during molding. A conventional aluminum mold for expandable polystyrene (EPS) is used in the shape of a box. The following process is used for molding a final product. The mold is filled using conventional fill guns. The first cross steam process is set for 3.1 seconds at 0.65 bar steam pressure and a 90% valve opening. A second cross steam process, reversing the direction of steam flow, is used for 2.9 seconds at a steam pressure of 0.65 bar and a 90% valve opening. The drain is opened to eliminate any potential condensate from mold. 31.0 seconds of vacuum is applied. Vacuum is replaced with air by opening the drain. The molded box is ejected from the press. The shapes of the beads after molding clearly demonstrate secondary expansion of the foamed beads within the mold. Surface depressions and textures from the mold cavity are replicated into the surface of the article. Based on weight and geometry of the box, the density of the molded article is typically about 1.8 lb/ft3.
The processes and materials described herein allow for the conversion of existing EPS manufacturing plants to produce a foamed article based on a compostable or biobased polymer.
Thus, the scope of the invention shall include all modifications and variations that may fall within the scope of the attached claims. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
