Patent Application
20080138598
This disclosure relates to a co-extrusion apparatus including a die feed block, a method of using the feed block to produce co-extruded articles having discontinuous phase inclusions, and co-extruded articles produced therewith.
Extruded polymers are used in many applications, including the production of filaments and fibers for use in fabrics; or thin films for use as tape backings, packaging materials, and the like. Exemplary polymeric materials suitable for extrusion include crystalline polyolefins, such as polyethylene, polypropylene, and polybutylene; polyamides such as nylon; polyesters such as polyethylene terephthalate (PET); and polyvinylidene fluoride. Although these polymeric materials and others are suitable for use in forming a polymeric fiber or web, they can have limiting characteristics that substantially narrow their suitable uses. For example, extruded polypropylene webs often have very good flexibility and tensile strength, but have less than desirable cross-web tear strength. On the other hand, PET exhibits good tear resistance, but a PET web may become brittle and may be not readily heat-sealable.
In order to improve the properties of extruded polymeric articles, several different polymeric materials are often co-extruded to form a multi-layer film or fiber. In general, each co-extruded layer forms a separate continuous phase within the article. An exemplary hybrid polymeric web combining two polymers is described in Krueger et al. (U.S. Pat. No. 5,429,856). Japanese Patent Document No. 55/28825 illustrates a multi-manifold die capable of producing a continuous core layer sandwiched within an upper layer and a lower layer. As exemplified by U.S. Pat. No. 3,397,428 to Donald, U.S. Pat. No. 3,479,425 to Lefevre et al, U.S. Pat. No. 3,860,372 to Newman, Jr., and U.S. Pat. No. 4,789,513 to Cloeren, encapsulation of a core stream in a surrounding multi-layer polymeric matrix is also known. U.S. Pat. No. 3,458,912 to Schrenk et al., U.S. Pat. No. 5,429,856 to Krueger et al., and U.S. Pat. No. 5,800,903 to Wood et al. describe various co-extrusion dies and co-extrusion methods for preparing composite articles having a discontinuous core layer sandwiched between two distinct skin layers formed of a polymeric matrix material.
Various methods have been described for producing co-extruded polymeric films incorporating a continuous polymeric phase as a core layer sandwiched between adjacent continuous polymeric phase layers. The art continually searches for new co-extrusion apparatuses and improved methods for preparing composite articles having unique phase configurations.
In general, the present disclosure relates to an improved co-extrusion apparatus and methods of preparing co-extruded articles. The improved apparatus includes a feed block having an internal die having a plurality of orifices used to form discontinuous phase inclusions in a continuous matrix material. The internal die allows formation of discontinuous phase inclusions of a first extrudable material embedded in a surrounding matrix of a second extrudable material, thereby forming a single-layer composite web. The feed block may be used in combination with an external die to form a single or multi-layer composite article in the form of a web, sheet, film, blown film, filament, fiber, tube, and the like.
In one aspect, the present disclosure provides a co-extrusion apparatus including a feed block having a first flow channel and a second flow channel, each of which has a transverse land channel in fluid communication with a first fluid delivery conduit. A die body is disposed between the first flow channel and the second flow channel within the feed block. The die body includes a transverse flow-providing passage in fluid communication with a transverse die exit channel. The transverse die exit channel includes a plurality of orifices formed on an external face of the internal die body in fluid communication with a second fluid delivery conduit. The feed block has a first internal wall which cooperates with a first face of the die body to form the transverse land channel of the first flow channel, and a second internal wall which cooperates with a second face of the die body to form a transverse land channel of the second flow channel. A feed block exit channel is formed in an external face of the feed block in fluid communication with the first flow channel, the second flow channel, and the transverse die exit channel.
In another aspect, the present disclosure provides a method of making a composite article having a discontinuous phase inclusion embedded in a continuous matrix material. The method includes introducing a first extrudable material into a first flow channel and a second flow channel formed within a feed block, introducing a second extrudable material into a plurality of orifices formed in a transverse exit channel on an external face of an internal die body disposed between the first flow channel and the second flow channel within the feed block, and combining the first extrudable material and the second extrudable material in a feed block exit channel to form a single-layer composite web. The first extrudable material forms a continuous matrix material surrounding a plurality of discontinuous included phases embedded in the continuous matrix material. The discontinuous included phases are separate from each other by being discontinuous in a cross-web direction, but are substantially continuous in the down-web direction.
In still another aspect, the present disclosure provides a method of making a composite article having certain properties, for example a physical property gradient between the discontinuous phase and the surrounding matrix material.
In yet another aspect, the present disclosure provides a co-extruded composite article including a continuous layer of an extruded matrix material, and a multiplicity of included phases embedded in the continuous layer. The included phases are surrounded by the matrix material to form a single-layer. The included phases are separate from each other by being discontinuous in the cross-web direction, and the included phases are substantially continuous in the down-web direction. In some exemplary embodiments, the co-extruded composite article is in the form of a single-layer web, sheet, film, blown film, filament, fiber, and the like. In other exemplary embodiments, the co-extruded composite article is in the form of a multi-layer web, sheet, film, blown film, filament, fiber, and the like.
Incorporation of the discontinuous phase inclusions in the matrix material within the feed block may provide several advantages. Discontinuous phase inclusions with dimensions smaller in the web thickness direction than in the web width direction may be produced. Multiple layers may be extruded around the composite layer containing the discontinuous phase inclusions to form multi-layer composite films having an embedded composite layer having discontinuous phase inclusions within a matrix material.
The above summary is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The Figures and the Detailed Description that follow more particularly exemplify certain preferred embodiments using the principles disclosed herein.
Unless otherwise noted herein, the Drawings are for illustrative purposes only and are not drawn to scale.
With respect to the above discussion of composite co-extruded articles, Applicants have discovered that novel composite articles may be prepared using co-extrusion methods that make use of an improved co-extrusion apparatus. The improved apparatus includes a feed block having an internal die having a plurality of orifices. The plurality of orifices permits formation of discontinuous phase inclusions of a first extrudable material embedded in a surrounding matrix of a second extrudable material, thereby forming a single-layer composite web. By discontinuous, we mean that the phase inclusions are not continuous in extent in at least one direction (e.g. the cross-web direction). However, it will be understood that the discontinuous phase may be continuous in another direction within the web (e.g. the down-web direction) and may be formed in a time-wise continuous manner.
The single-layer composite web may be overlayed on one or both sides with one or more additional layers of additional extrudable material(s). The additional extrudable material(s) may be applied within the feed block and/or the die. The resulting single-layer or multi-layer composite articles (for example, a sheet, filament, fiber, tube, article, and the like.) may have a uniform surface without the surface waviness or “rippling” that accompanies formation of multi-layer composite articles by overlaying extrudable material on a discontinuous core material within an external die. The resulting composite articles may also have unique configurations in which the composite layer having the discontinuous phase inclusions embedded in a matrix material may be an outer layer (i.e. a top layer or a bottom layer) in a multi-layer stack. Such composite structures are unique to Applicant's disclosed method and apparatus for co-extrusion of articles having discontinuous phase inclusions.
The embodiments described herein may take on various modifications and alterations without departing from the spirit and scope of the disclosure. Accordingly, it is understood that the disclosure is not to be limited o the following described embodiments, but is to be controlled by the limitations set forth in the claims and any equivalents thereof. In particular, all numerical values and ranges recited herein are intended to be modified by the term “about,” unless stated otherwise. The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). Various embodiments of the disclosure will now be described with reference to the Figures.
Referring to
The feed block 18 includes an internal cavity 30 containing an internal die 31 having a plurality of orifices (not shown in
As illustrated by
In certain optional embodiments, one or more additional extruders, for example a third extruder 24 as shown in
In one such optional embodiment illustrated in
Referring to
Referring to
Although in some embodiments streams 11 and 13 are made up of the same material (e.g. second extrudable material 17), streams 11 and 13 may include different materials, provided that each transverse land channel 98 and 100 is in fluid communication with a separate fluid delivery conduit, each supplying a different extrudable material from a separate extruder (not shown in the Figures).
The apparatus 10 shown in
The orifices 44 can be made, for example, by electro-discharge machining (EDM) or other material removal means known in the art. Preferably, each orifice 44 is at least 1 mm from the nearest adjacent orifice in order to prevent merger of the discontinuous streams of the first extrudable material into a single continuous layer upon exiting the internal die body 31 but within the feed block 18.
Referring to
The internal die body 31 includes a transverse flow-providing passage 99 in fluid communication with a transverse internal die exit channel 102. The transverse internal die exit channel 102 includes a plurality of orifices (not shown in
Also positioned within feed block 18, in the embodiment of
Furthermore, in some embodiments, each layer-forming channel 46 and 48 may be positioned proximate a corresponding adjustable vane 84 and 86. Each adjustable vane 84 and 86 may be movably positioned to at least partially occlude the corresponding layer-forming channel 46 and 48, respectively. In some embodiments, at least one adjustable vane 84 or 86 may be positioned to fully occlude the corresponding layer-forming channel 46 and 48. Optional access port 58 and/or access port 60 may be installed in the feed block 18 to facilitate adjustment of the corresponding vane 84 and/or 86 from outside of the feed block 18.
Vanes 84 and 86 may, in some embodiments, be independently adjustable in at least one of two modes. Either one or both of vane 84 and/or vane 86 may be pivoted so the corresponding tip 55 or 57 can be moved closer to the exit of the corresponding layer-forming channel 46 or 48, thereby partially occluding the corresponding layer-forming channel 46 or 48, respectively, and causing a difference in gap for one or both of the layer-forming channels 46 or 48. This difference in gap can result in a different layer thickness for each additional layer made with an additional extrudable material (see e.g. additional extrudable material 25 in
Alternatively, in some embodiments, one or both vanes 84 and/or 86 may be positioned to completely occlude the corresponding layer-forming channel 46 and 48, respectively, thereby causing formation of a multi-layer web in which the co-extruded single-layer composite web is positioned as a layer adjacent to one major side surface of the multi-layer web (see e.g.
As described above, in certain embodiments, adjustable vane 84 and/or 86 may be adjusted by rotation around an axis through a pivotable fixture 87 or 88, respectively. Thus if one additional extrudable material 25 is less viscous than the other, it may be possible to narrow the gap through which the less viscous matrix material flows in order to maintain uniformity of layer thickness of each of the two matrix layers. The gaps can be altered during processing in order to account for variations in processing conditions, such as changes in the temperature, pressure, flow rate, or viscosity over time. Thus, if feed block 18 has a warmer upper portion than lower portion resulting in lower viscosity of materials flowing through the upper gap, then the gaps can be adjusted to account for this change in viscosity. In addition, the gaps can be altered to achieve a different thickness in each matrix layer. This may be particularly useful when each matrix layer may be of a different material, e.g., a thermoplastic elastomer and a pressure-sensitive adhesive, or where different properties are desired from each layer of the multi-layer web.
The manner in which the co-extruded single-layer composite web 2 (not shown) may be formed with the internal die body 31 within feed block 18 is shown with more particularity in
The die body 31 includes a transverse flow-providing passage 99 in fluid communication with a transverse internal die exit channel 102. The transverse internal die exit channel 102 exits the external face 104 of the internal die through a plurality of orifices 44 (see
The overall structure of the presently disclosed single-layer composite web may be formed by any convenient matrix forming process such as by pressing materials together, co-extruding or the like, but co-extrusion is the presently preferred process for forming a single-layer composite web with a discontinuous core embedded within a continuous polymeric matrix material. Co-extrusion per se is known and may be described, for example, in Chisholm et al U.S. Pat. No. 3,557,265, Leferre et al. U.S. Pat. No. 3,479,425, and Schrenk et al. U.S. Pat. No. 3,485,912. Tubular co-extrusion (i.e. to form a filament or fiber) or double bubble extrusion (i.e. to form a blown film) is also possible for certain applications. The discontinuous core and matrix material are typically co-extruded through a specialized die that will bring the diverse materials into contact to shape the composite material to the desired form.
Various conventional dies are known for forming a multi-layer co-extruded web or film having a continuous core layer sandwiched between additional layers. In such dies, the core stream exiting from the die is sandwiched within additional streams exiting from flow channels formed within the die body. Virtually any such conventional multi-layer extrusion die may be advantageously used in conjunction with Applicant's feed block configurations to form multilayer composite articles having discontinuous phase inclusions, as described below.
In addition, a number of co-extrusion dies are known for producing multi-layer composite articles, such as films, in which a discontinuous core is embedded between two additional film layers. Such dies may also be used advantageously with the feed block of Applicant's disclosure to produce multi-layer composite articles having an embedded layer of discontinuous phase inclusions. For example, Schrenk et al. employs a single main orifice and polymer passageway die. In the main passageway, which would carry the matrix material, may be a nested second housing having a second passageway. The second passageway would have one or more outlets, each defining an elastomeric core, which discharges matrix material flowstreams into the main passageway matrix flow region. This composite flow then exits the orifice of the main passageway, thereby forming a multilayer film having an embedded discontinuous core.
One particular feed block useful in practicing a co-extrusion process according to the present disclosure is characterized by a removable die within a feed block, as described in U.S. Pat. No. 4,789,513. The die may be rigidly mounted between a first flow channel and a second flow channel, and extrudable material passed though the die to produce a continuous core of a first extrudable material surrounded on each side by a layer of a second extrudable material. Such known feed blocks are not capable of forming a discontinuous core layer, and the core layer must be sandwiched between two layers of the second extrudable material. This configuration precludes formation of a single-layer co-extruded article having an embedded discontinuous phase. This configuration also precludes formation of a multi-layer co-extruded film in which the single-layer composite web, having a discontinuous core layer of a first extrudable material surrounded by a matrix of a second extrudable material, is positioned as one of the outer layers of the multi-layer film.
However, by replacing the removable die within the feed block with a die configured with a plurality of cutouts or orifices as described below, the feed block may be used to form a single-layer or multi-layer film having a composite layer having a discontinuous core layer of a first extrudable material surrounded by a matrix of a second extrudable material. In addition, the composite layer may be positioned between additional layers of extrudable material, as an outer layer in a multi-layer film, or as a self-supporting single-layer film
Another advantageous co-extrusion process may be possible with a modified multi-layer, e.g. a three-layer, feed block or combining adapter such as made by Cloeren Co. (Orange, Tex.). Combining adapters are described in Cloeren U.S. Pat. No. 4,152,387 discussed above. The combining adapter may be used in conjunction with extruders, optionally in combination with multi-layer feed blocks, supplying the extrudable materials. Such an apparatus for producing multi-layer composite materials is shown schematically in
In another embodiment, the disclosure provides a method of making a co-extruded composite having discontinuous phase inclusions. The method includes introducing a first extrudable material into a first flow channel and a second flow channel formed within a feed block; introducing a second extrudable material into a plurality of orifices formed in a die body disposed between the first flow channel and the second flow channel within the feed block; and combining the first extrudable material and the second extrudable material in a feed block exit channel to form a single-layer composite web. The first extrudable material forms a continuous matrix material surrounding a plurality of discontinuous included phases embedded in the continuous matrix material.
The included phases may be separate from each other by being discontinuous in a cross-web direction as shown in
Incorporation of the plurality of discontinuous phase inclusions in the matrix material within the feed block may provide several advantages. Discontinuous phase inclusions with dimensions X in the cross-web direction greater than the composite web thickness Y may be produced. For example,
In some embodiments of Applicant's disclosure, the adjustable vanes within the feed block may be replaced with vanes that are fused in a fixed position. This has the effect of blocking subsequent layer flows while helping to form the discontinuous phase inclusions in the matrix material within the feed block. Multiple layers may be stacked upon this centralized layer or blocked off so that this discontinuous layer is offset from all other layers.
Fixing the position of the vanes in certain embodiments of Applicant's feed block configuration may allow fewer problems with leakage of the polymer melt stream as it passes through the shaping insert. In addition, since the core layer is formed discontinuously within the feed block, the composite web has additional time to undergo stress relaxation before entering the forming region with the die cavity. This additional relaxation time may act to reduce or eliminate expansion or contraction of the composite web upon exiting the die, thereby permitting more precise control of the width of the inclusions and thickness and uniformity of the composite web.
In some embodiments, this may permit formation of a composite web on or around which other extruded layers may be formed in the die to produce a multi-layer composite film without producing a non-uniform surface “ripple” pattern on the surface of the additional extruded layers due to swelling or contraction of the underlying discontinuous phase upon exiting the die. In certain embodiments, the thickness of the multi-layer composite film or web in a region overlaying a discontinuous phase varies by less than 5% from the thickness of the multi-layer composite film in a region not overlaying any discontinuous phase. In other embodiments, the thickness of the multi-layer composite film or web in a region overlaying a discontinuous phase varies by less than 1% from the thickness of the multi-layer composite web in a region not overlaying any discontinuous phase.
In additional embodiments, a physical property gradient (e.g. a refractive index, light transmission, density, compositional, color, or physical property gradient) between the discontinuous phase inclusions and the surrounding matrix may be created by controlling the design of the feed block die insert and the pressure and mass flowrate of the extrudable materials within the feed block. Such physical property gradients may be useful in preparing films for use in identification cards, document security, anti-counterfeiting applications, and the like. Another variation of a physical property variation is to use a lower molecular weight polymer as the discontinuous phase inclusions and a higher molecular weight polymer as the continuous matrix material. The resulting rheological properties of the polymers can be used to cause the discontinuous phase inclusions to spread within the die to make a continuous layer.
It will be understood by one skilled in the art that although the foregoing discussion refers to formation of a web or film including a composite layer having discontinuous phase inclusions, other co-extruded articles, for example ropes, fibers, melt-blown articles, and the like, may also be advantageously produced using the apparatus and methods described in this disclosure. The inventive composite web material has an unlimited range of potential widths (or diameters if formed into a filament or fiber), the width limited solely by the fabricating machinery width limitations.
The precise extruders employed in the inventive process are not critical as any device able to convey melt streams to a die of the invention may be satisfactory. However, it may be understood that the design of the extruder screw will influence the capacity of the extruder to provide good polymer melt quality, temperature uniformity, and throughput. A number of useful extruders are known and include single and twin screw extruders. These extruders are available from a variety of vendors including Davis-Standard Extruders, Inc. (Pawcatuck, Conn.), Black Clawson Co. (Fulton, N.Y.), Berstorff Corp (N.C.), Farrel Corp. (Connecticut), and Moriyama Mfg. Works, Ltd. (Osaka, Japan). Other apparatus capable of pumping organic melts may be employed instead of extruders to deliver the molten streams to the forming die of the invention. They include drum unloaders, bulk melters and gear pumps. These are available from a variety of vendors, including Giraco LTI (Monterey, Calif.), Nordson (Westlake, Calif.), Industrial Machine Manufacturing (Richmond, Va.), Zenith Pumps Div., and Parker Hannifin Corp. (N.C.).
Once the molten streams have exited the pump, they are typically transported to the die through transfer tubing and/or hoses. It may be preferable to minimize the residence time in the tubing to avoid problems of, for example, melt temperature variation. This can be accomplished by a variety of techniques, including minimizing the length of the tubing. Alternatively, melt temperature variation in the tubing can be minimized by providing appropriate temperature control of the tubing, or utilizing static mixers in the tubing. Patterned tools which contact the web can provide surface texture or structure to improve the ability to tear the web in the cross web or transverse direction without affecting the overall tensile strength or other physical properties of the product.
By using the internal die body 31 within the feed block 18, extrudable materials 15 and 17 may be co-extruded in a controlled manner. The materials may be brought together in the melt state, thereby allowing for improved adhesion to one another. In addition, even when the materials are not normally compatible, they may still be co-extruded in order to produce a web retaining the properties of each of the materials. The die and feed block used are typically heated to facilitate polymer flow and layer adhesion. The temperature of the die depends upon the polymers employed and the subsequent treatment steps, if any. Generally the temperature of the die may be not critical but temperatures are generally in the range of 350° F. to 550° F. (176.7° C. to 287.8° C.) with the polymers exemplified. Accordingly, the composite web, whether in the form of a single-layer or multi-layer web, is preferably cooled upon exiting the external die.
A number of additional steps can optionally be performed after extrusion. For example, the web may be uniaxially (i.e. lengthwise or width-wise) or biaxially (i.e. both length-wise and width-wise) oriented, either sequentially or simultaneously, can be cured (such as through heat, electromagnetic radiation, etc.), or can be dusted with various tack-reducing agents.
Another way of modifying the properties of the co-extruded webs of the invention may be to use specific materials having desired properties for the layers of the matrix and the embedded discontinuous phases. Suitable polymeric materials for forming the matrix layers and embedded phases of the inventive co-extruded web are any that can be thermally processed and include pressure sensitive adhesives, thermoplastic materials, elastomeric materials, polymer foams, high viscosity liquids, etc. Suitable materials useful in practicing the various embodiments of the present disclosure are known to those skilled in the art. Exemplary materials are described in U.S. Pat. No. 6,447,875 to Norquist et al.
Gases or supercritical fluids may also be incorporated as the discontinuous included phase to form a foam. Foams are those materials made by combining the above polymeric materials with blowing agents. Physical foaming agents, for example gases like carbon dioxide or nitrogen, ethane, butane, isobutane and the like; heated water or steam may be incorporated in the discontinuous included phase to form a foam, a void, a channel or a tube.
Chemical blowing agents may also be used to generate foams, voids, channels or tubes in melt processable materials. Suitable blowing agents are known in the art and include, for example, SAFOAM™ RIC-50, a citric acid sodium bicarbonate-based chemical blowing agent, or certain isocyanates that may be reacted in situ with water to release carbon dioxide gas. The resulting mixtures may then be subjected to various conditions known in the art to activate the blowing agent used to form a multiplicity of cells within the polymer. Additional cross-linking may occur to cause the resulting foams to be more stable.
Thermoexpandable microcapsules or beads of encapsulated blowing agents (e.g. hydrocarbons, such as butane or isobutene) covered with a thermoplastic resin shell material may also be used. Heating the microcapsules causes softening of the shell material and vaporization of the blowing agent leading to increased internal pressure and rapidly increase volume by 100 times or more. Suitable thermoexpandable microcapsules are the CLOCELL® materials (available from PolyChemAlloy, Granite Falls, N.C.).
High viscosity liquids are suitable as embedded discontinuous included phase materials. They are any liquids that do not diffuse through the matrix material and prematurely escape the article of the invention. These include, for example, various silicone oils, mineral oils and specialty materials having a sharp melting temperature around or below room temperature.
Viscosity reducing polymers and plasticizers can also be blended with the elastomers. These viscosity reducing polymers include thermoplastic synthetic resins such as polystyrene, low molecular weight polyethylene and polypropylene polymers and copolymers or tackifying resins such as Wingtack™ resin (from Goodyear Tire & Rubber Company, Akron, Ohio). Examples of tackifiers include aliphatic or aromatic liquid tackifiers, aliphatic hydrocarbon resins, polyterpene resin tackifiers, and hydrogenated tackifying resins.
Various additives may be incorporated into the phase(s) and/or the matrix to modify the properties of the finished web. For example, additives may be incorporated to improve the adhesion of the discontinuous phases and the matrix to one another. Additives such as dyes, pigments, antioxidants, antistatic agents, bonding aids, antiblocking agents, slip agents, heat stabilizers, photostabilizers, foaming agents, glass bubbles, starch and metal salts for degradability or microfibers can also be used in the elastomeric phase. Suitable antistatic aids include ethoxylated amines or quaternary amines such as those described, for example, in U.S. Pat. No. 4,386,125 (Shiraki), which also describes suitable antiblocking agents, slip agents and lubricants. Softening agents, tackifiers or lubricants are described, for example, in U.S. Pat. No. 4,813,947 (Korpman) and include coumarone-indene resins, terpene resins, hydrocarbon resins and the like. These agents can also function as viscosity reducing aids. Conventional heat stabilizers include organic phosphates, trihydroxy butyrophenone or zinc salts of alkyl dithiocarbonate.
The co-extruded web may also be laminated to a fibrous web. Preferably, the fibrous web may be a nonwoven web such as a consolidated or bonded carded web, a melt-blown web, a spunbond web, or the like. The fibrous web alternatively may be bonded or laminated to the co-extruded web by adhesives, thermal bonding, extrusion, ultrasonic welding or the like. Preferably, the co-extruded web can be directly extruded onto one or more fibrous webs. Short fibers or microfibers can also be used to reinforce the embedded phase(s) or matrix layers for certain applications. These fibers include polymeric fibers, mineral wool, (glass fibers, carbon fibers, silicate fibers and the like.
Glass bubbles or foaming agents may be used to lower the density of the matrix layer or embedded phases and can be used to reduce cost by decreasing the content of an expensive material or the overall weight of a specific article. Suitable glass bubbles are described in U.S. Pat. Nos. 4,767,726 and 3,365,315. Furthermore, certain filler particles can be used, including, carbon and pigments. Fillers can also be used to some extent to reduce costs. Exemplary fillers, which can also function as anti-blocking agents, include titanium dioxide and calcium carbonate.
Additional embodiments and advantages are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.
It is apparent to those skilled in the art from the above description that various modifications can be made without departing from the scope and principles of this disclosure, and it should be understood that this disclosure may be not to be unduly limited to the illustrative embodiments set forth hereinabove. All publications and patents are herein incorporated by reference to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference. Various embodiments of the disclosure have been described. These and other embodiments are within the scope of the following claims.
