DEGRADABLE COMPOSITE POLYMER AND METHOD OF MAKING SUCH COMPOSITE POLYMER

Abstract: A novel degradable composite polymer is comprised of a polymer comprised of lactic acid monomers, and a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer. This composite polymer is made by first combining the lactic acid polymer and the resin and subsequently extruding the combination through a heated extruder. These substances may be combined in various proportions to obtain materials of various flexibilities and hardnesses. This degradable composite polymer can be used in many applications, including use as a printable plastic material due to its unique ability to retain ink and as a plastic media blast material as it can be created in a variety of hardness levels.

Description: DEGRADABLE COMPOSITE POLYMERAND METHOD OF MAKING SUCH COMPOSITE POLYMER Statement Regarding Federally-Sponsored Research or Development Not applicable. Cross Reference To Related Applications This application claims priority from U.S. Provisional Application No. 60/040,779, filed March 14, 1997. Background of the Invention The present invention relates to composite polymers and methods for making such composite polymers. More specifically, the composite polymers of the present invention are degradable. Still further, the present invention includes applications for using composite polymers as printable plastic material or as plastic media blast material. Plastic materials are any synthetic materials comprised of polymers which can be shaped or molded. Printable plastic materials are plastic materials which are able to receive and retain ink. There are a number of disadvantages with currently available printable plastic materials. Many plastic materials used by the printing industry have a plasticizer incorporated into their formula which creates a residue on the surface of the finished plastic products. This residue prevents ink from sticking to the surface during printing, and surface treatment prior to printing often does not remove all of the plasticizer residue. Furthermore, many plastic materials currently used by the printing industry are not degradable. Polylactic acid resin, a degradable material, offers a quality printing surface since the biopolymer-based plasticizers used in the manufacture of this resin tend to stay within the material. However, polylactic acid resin does not thermoform well. Still further, because polylactic acid is extremely hard, it is very abrasive on cutting tools. Furthermore, when polylactic acid is extruded into sheets, it tends to curl and become quite brittle. For instance, if a polylactic acid resin is rolled, it tends to stay rolled forever. Also, because polylactic acid is a brittle material, it fractures, especially at corners, when cut, and when it is extruded into thicker sheets, it fractures even more. Other degradable plastics, such as the multipolymer resins sold by Novamont S.p.A., via G. Fauser, 8-28100 Novara, Italy, under the trademark Mater Bi™, have a great deal of flexibility, but are too ineffective to use in many sheet extrusion applications. These resins, which are comprised of a thermoplastic polymer, destructured starch, and a plasticizer, are very soft and rubbery, and ink cannot be printed on such resins. It is thus desirable to find a sheet extruded material with high elasticity and an exceptional ability to retain ink during printing. There are a number of disadvantages with current methods for removing coatings from various substrate materials. Commonly used methods for removing powder coatings include dipping the substrate material to be stripped into a pot of molten salt so as to turn the powder coating to ash and vapor. Another method involves putting the substrate material in a high temperature oven, approximately 600-900° F, to turn the coating to ash. The problem with these methods is that many substrates that are coated with powdered paint cannot survive the coating removal process, or if they do, their structural properties are significantly damaged due to the extreme heat used to remove the coating. Still another method involves using small glass beads in an abrasive process to remove the coatings. However, when used on relatively soft substrates, the impacting beads provide an unsatisfactory result by causing surface stress and possibly also changing the texture of the substrate. Thus, many coated materials must be scrapped since they cannot withstand the conditions of these conventional removal processes. Another conventional method for removing coatings involves using plastic media blast material. Plastic media blast is an effective way to surface clean various materials without damaging the surface of the material being blasted. Currently, certain petroleum-based plastics which are not too hard or too coarse are used as plastic media blast material. However, degradable plastics have been found to be too brittle or too weak to perform adequately as plastic media blast. In order to overcome the deficiencies found with conventional degradable polymers, a degradable composite polymer and a method for making such a composite polymer are needed for a variety of applications including those in which enhanced strength and flexibility are needed. Still further, such a degradable composite polymer should have improved printability characteristics for use as a printable plastic material and should be able to be produced in a variety of hardnesses for use as plastic media blast material. Summary of the Invention It is an object of the present invention to provide a degradable composite polymer and a method for producing the same wherein the composite polymer can be used as a plastic. A further object of this invention is to provide degradable composite polymers and methods of producing the same wherein the composite polymer can be produced in various textures and ranges of flexibilities. It is a further object of the present invention to provide a degradable composite polymer and a method for producing the same wherein the composite polymer is produced in a range of hardnesses so that it can be used as a plastic media blast material for a variety of surfaces. Still another object of this invention is to provide a degradable composite polymer and a method for producing the same wherein the composite polymer has the ability to retain ink during printing so that it can be used as a printable plastic material. It is a further object of this invention to provide a degradable composite polymer and a method for producing the same wherein the composite polymer is able to be cut easily without fracturing so that it may be cut into small plastic sheets. Another object of this invention is to provide a degradable composite polymer and a method of producing the same wherein the composite polymer has a high elasticity so that it is able to be extruded into sheets without curling or becoming brittle and is able to be injection or compression molded. Still another object of this invention is to provide a method for manufacturing a degradable composite polymer without the use of any corrosive or caustic chemicals in order to eliminate the hazards associated with using and disposing of such chemicals. Still another object of this invention is to provide a process for producing a degradable composite polymer which creates little waste. According to the present invention, the foregoing and other objects are achieved by a degradable composite polymer comprised of a polymer comprised of lactic acid monomers and a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer. This composite polymer is first created by combining the polymer and the resin, and subsequently, extruding them through a heated extruder. This degradable composite polymer may be used, among other things, as plastic media blast material or as a printable plastic material. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. Detailed Description of the Preferred Embodiment The novel degradable composite polymers of the present invention are comprised of a polymer and a resin. The polymer is comprised of lactic acid monomers, and the resin is comprised of a thermoplastic polymer, destructured starch, and a plasticizer. If the polymer comprised of lactic acid monomers is a copolymer, in addition to the lactic acid monomers, the other monomers in this polymer should be degradable monomers. Preferably, the polymer is polylactic acid (PL A). One method of producing PLA includes catalyzing crude lactic acid, which may contain impurities such as carbohydrates, proteins, amino acids, salts, metal ions and other organic acids, with 0.05-0.15 weight percent tin oxide (SnO). Other methods for making PLA also are conventionally available. Preferably, the polymer comprised of lactic acid monomers has a molecular weight of between 5,000 and 200,000, depending on the physical properties of the composite polymer which are desired in a specific application. For example, polymers having molecular weights at the higher end of this range produce composite polymers which are relatively stronger and harder. The lactic acid monomers in the polymer may be comprised of between about 0 and 90% D-lactide by weight. Preferably, between approximately 0 and 25% by weight of the lactic acid in the polymer is D-lactide. Most preferably, the lactic acid in the polymer is comprised of about 1% or less D-lactide by weight. The starch component of the resin may be any starch of natural or plant origin which is composed essentially of amylose and/or amylopectin. It can be extracted from various plants, such as potatoes, rice, tapioca, maize, as well as cereals, such as rye, oats, wheat and the like. Maize starch is preferred. Preferably, the starch component has an amylopectin content of more than 70% by weight. Chemically-modified starches and starches of different genotypes can also be used. Still further, ethoxy derivatives of starch, starch acetates, cationic starches, oxidized starches, cross-linked starches and the like may be used. Starch is provided without processing, such as drying, and without the addition of any water (the intrinsic bound water content of the commercial products is approximately 10-13% by weight). The starch is then destructured at temperatures above 90°C and preferably above 120°C. The term "destructured starch" means a starch which has been heat-treated above the glass transition temperatures and melting points of its components, so that the components are subjected to endothermic transitions to thereby produce a consequent disorder in the molecular structure of the starch granules. In other words, the crystallinity of the starch is destroyed. The plasticizer used in the resin is preferably a polyol, polyol derivative, polyol reaction product, polyol oxidation product or a mixture thereof. Preferably, the plasticizer has a boiling point of at least 150°C. Examples of plasticizers that can be used include, but are not limited to, glycerine, polyglycerol, glycerol, polyethylene glycol, ethylene glycol, propylene glycol, sorbitol, marmitol, and their acetate, ethoxylate, or propoxylate derivatives, and mixtures thereof. Specific plasticizers that can be used include, but are not limited to, ethylene or propylene diglycol, ethylene or propylene triglycol, polyethylene or polypropylene glycol, 1,2-propandiol, 1,3-propandiol, 1,2, 1,3, 1,4-butandiol, 1,5-pentandiol, 1,6-, 1,5-hexandiol, 1,2,6-, 1,3,5-hexantriol, neopentylglycol, trimethylolpropane, pentaerythritol, sorbitol acetate, sorbitol diacetate, sorbitol monoethoxylate, sorbitol dipropoxylate, sorbitol diethoxylate, sorbitol hexaethoxylate, aminosorbitol, trihydroxymethylaminomethane, glucose/PEG, the product of reaction of ethylene oxide with glucose, trimethylolpropane, monoethoxylate, mannitol monoacetate, mannitol monoethoxylate, butyl glucoside, glucose monoethoxylate, alpha-methyl glucoside, the sodium salt of carboxymethylsorbitol, polyglycerol monoethoxylate and mixtures thereof. The amount of plasticizer in the resin is approximately 0.05-100% of the weight of the starch, and preferably about 20-100% of the weight of the starch. The thermoplastic polymer in the resin is a synthetic polymeric component which includes a polymer or copolymer of at least one ethylenically unsaturated monomer, the polymer or copolymer having repeating units provided with at least a polar group such as hydroxy, alkoxy, carboxy, carboxyalkyl, alkyl carboxy or acetal group. Preferred polymeric components included in the resin are polyethylene, polyvinyl alcohol, polyacrylonitrile, ethylene-vinyl alcohol copolymer, ethylene-acrylic acid copolymer and other copolymers of an olefin selected from ethylene, propylene, isobutene and styrene with acrylic acid, vinyl alcohol, and/or vinyl acetate and mixtures thereof. Most preferably, one of the polymers in the resin is an ethylene-acrylic acid copolymer with ethlylene contents of from about 10 to 44% by weight. The resin also may contain relatively low amounts, approximately 5% or less by weight of the overall composition, of hydrophobic polymers, such as polyethylene, polypropylene and polystyrene. Still further, other polymers such as polyamide, polyacrylic, polyester, and poly ether may be in the resin. The polymer and starch may be combined in a 1 : 19 to 19: 1 ratio by weight. Preferably, the polymer component of the resin has a higher molecular weight than the polymer comprised of lactic acid monomers, the other component used in forming the composite polymer of the present invention. Other components such as destructuring agents, cross-linking agents and neutralizing agents may, optionally, be added to the resin but are not essential components. Preferably, a destructuring agent is added while making the resin. The destructuring agent may be urea, alkaline and alkaline-earth hydroxides, and mixture thereof. Examples of alkaline and alkaline-earth hydroxides include but are not limited to sodium, potassium and calcium hydroxides. Most preferably, urea is added as the destr icturing agent. Urea improves the gelling of the starch with small amounts of water, and hence enables the production of a uniform film. Preferably, the amount by weight of destructuring agent added to the resin is 2-20% of the weight of the starch. However, if a destructuring agent is not added, it is still possible to destructure the starch through heat or pressure. The resin also may contain cross-linking agents such as aldehydes like formaldehyde, paraformaldehyde, and paraldehyde; keytones and glyoxals; epoxides like epichlorohydrin; process coadjuvants and release agents; and lubricants which are normally incorporated in compositions for molding or extrusion such as fatty acids, esters of fatty acids, higher alcohols, polythene waxes, and low density polyethylene (LDPE). The resin further may contain a neutralizing agent, such as ammonia or any amine, sufficient to neutralize some or all of the acid groups of the polymer if an acidic polymer such as ethylene-acrylic acid copolymer is used. Ammonia may be added to the resin in quantities up to about 7% of the weight of the dry starch. However, most of the ammonia should be removed before or during extrusion. Preferably, about 0.5% or less by weight of the ammonia remains in the final resin formulation. Urea, in addition to functioning as a destructuring agent, also may function as a neutralizing agent. Although optional, the use of boron containing compounds results in substantially better interpenetration between the hydrophilic starchy phase and the hydrophobic polymeric phase, with a resultant substantial improvement in mechanical properties, particularly tear strength and transparency of sheets and films obtained from various formulations of the resin. Boron, boric acid, borax, metaboric acid, or other boron derivatives may be used in the resin. Preferably, the boron containing compound, expressed as the boron content, is between about 0.002 and 0.4% and preferably between about 0.01 and 0.3% of the total weight of the resin. Other additives also may be mixed into the resin. For example, poly vinyl alcohol may be added to change the behavior of molded articles with water; UV stabilizers, such as, carbon black, may be added to improve the resistance of the articles to sunlight; and flame-proofing agents may be added if desired. The addition of inorganic salts of alkali or alkaline-earth metals, particularly lithium chloride and sodium chloride at concentrations between about 0.1 and 5% by weight of the resin, preferably between about 0.5 and 3% by weight, also was found advantageous. Other additives which may be in the resin include the conventional additives generally incorporated in starch-based molding compositions, such as fungicides, herbicides, antioxidants, fertilizers, opacifiers, stabilizers and plasticizers. All these additives may be used in conventional quantities as known to experts in the field or as easily determined by routine tests, and these additives may constitute up to about 20% by weight of the final composition. The resin is made by mixing the essential components, namely, the starch, plasticizer and thermoplastic polymer, and any other optionally included components, in a conventional device, such as a heated extruder, which ensures conditions of temperature and shearing stress suitable to render the starch and the polymer compatible from a rheological point of view. The starch's structure is interpenetrated or at least partially interpenetrated by the thermoplastic polymer so as to obtain a thermoplastic melt. The starch may be destructured before it is combined with the polymer, or as it is combined. A destructuring agent may be mixed with the starch and the plasticizer in a heated extruder to destructure it. Preferably, the mixture is extruded to form the resin at a temperature between about 100°C and 220 °C. Preferably, according to one formulation of the present invention, the resin is a film-grade material comprised of about 10-90% by weight polymer or copolymer, about 10-90%) by weight destructured starch, about 2-40% by weight plasticizer, about 0-20% by weight destructuring agent, and about 0-6% by weight water. More preferably, the resin is comprised of about 20-70% by weight destructured starch, about 10-50% by weight polymer or copolymer, about 2-40% by weight plasticizer, about 0- 10% by weight destructuring agent, about 1-5% by weight water, and about 0.002-0.4%) by weight boron compounds. One of the most preferred formulations of the resin is 41% by weight ethylene-acrylic acid copolymer with 20% by weight acrylic acid, 12% by weight urea, 41% by weight destructured starch, 20% by weight plasticizer, and 6% by weight water. Most preferably, the resin which is used in making the degradable composite polymers of this invention is resin sold by Novamont, S.p.A., via G. Fauser, 8-28100 Novara, Italy, under the trademark Mater Bi™. The lactic acid polymer, which is preferably PLA, and the resin, which is preferably Mater Bi™, may be combined to form the degradable composite polymers of the present invention in a range of proportions depending upon the resultant hardness, flexibility, and texture which is desired. The addition of the resin to the lactic acid polymer reduces its brittleness and improves its flow and process ability. Composite polymers of this invention which contain more resin are softer and more flexible. Anywhere between about 10 and 90% by weight of the degradable composite polymer may be a lactic acid polymer and the remaining 10-90%) by weight of the composite polymer is the resin. This degradable composite polymer is made by first combining the polymer and the resin, and by subsequently extruding the combination through a heated extruder. The polymer and the resin may be combined in a container and then fed into the extruder, or the polymer and the resin may be combined in the extruder. Any conventional extruder capable of melting the mixture may be used. The extruder may have any of a variety of screw configurations, including single, twin, or mixing screws. A colorant also may optionally be added to the mixture. Preferably, the particles of resin and polymer added to the extruder are relatively close in size. The extruder should be sufficiently heated so that the composite polymer may form at any temperature between about 140 and 230°C. Preferably, it forms at a temperature of about 160-190°C if used as a printable plastic material, and at a temperature of about 140-230°C if used as plastic media blast material. If the mixture is heated above 230°C, volatile compounds in the resin begin to evaporate. The temperature at which the composite polymer forms affects its resultant hardness. Therefore, the extruder should be at higher temperatures to form harder composite polymers. Furthermore, the temperature chosen for the process should be influenced by the melting point of the polymer used. For instance, if a lactic acid polymer having a relatively higher melting point is used, the composite polymer should be formed at a higher temperature within the acceptable temperature range. Also, if the lactic acid polymer has a relatively higher molecular weight, then, preferably, the composite polymer is formed at higher temperatures within the disclosed temperature range. The degradable composite polymer forms in the heated extruder after the polymer and the resin are extruded for about 30-240 seconds. Preferably, the composite polymer forms after the polymer and the resin are extruded for about 50-90 seconds. When the mixture comes out of the extruder, it is a very soft, molten material. Because the crystallinity of the mixture is changed when it is extruded, the product cannot be reprocessed without re-annealing the mixture. Preferably, the resulting degradable composite polymer is cut with a specially hardened die so as not to dull the die. This is especially preferable for those formulations containing high percentages of a lactic acid polymer. Preferably, for cutting purposes, the polymer and the resin are combined in approximately a 1 : 1 ratio by weight so as to cause the least amount of die abrasion. The degradable composite polymer of the present invention has enhanced elongation viscosity, improved strength and flexibility, and improved printability characteristics when compared to other degradable plastics. It also is resistant to tearing and perforation, and it is a good oxygen and carbon dioxide barrier. Still further, it has a high melting point and the ability to thermoform. The degradable composite polymer of the present invention is an environmentally-friendly product because it is made from renewable resources and degrades in two years or less when composted. Degrades, in this context, means it is chemically decomposed. Certain formulations of the present invention that contain high percentages of the resin are even biodegradable, which means they are capable of being decomposed by naturally biological processes. The novel degradable composite polymer of the present invention is useful for a variety of applications. It may be used as a plastic substitute for a wide range of applications such as those currently reserved for petroleum-based plastics. Articles may be formed with this composite polymer by injection molding, thermoforming or blowing. Two specific applications particularly suited for this novel degradable composite polymer include use as a printable plastic material or as a plastic media blast material. If used as a printable plastic material, the composition should be comprised of approximately 10-90%) by weight of a polymer comprised of lactic acid monomers, and approximately 10-90% by weight of a resin comprised of a thermoplastic polymer, destructured starch, and a plasticizer. Preferably, for cutting purposes, it is comprised of approximately 30-70%) by weight of the polymer, and approximately 30- 70% by weight of the resin. Most preferably, for cutting purposes, it is comprised of approximately 50%> by weight of the polymer, and approximately 50% by weight of the resin. This printable plastic material is made by first combining a polymer and a resin and then extruding this mixture as discussed above . The extruded mixture can be formed into sheets by any method known to those skilled in the art. One method involves feeding the extruded mixture, which is a soft, hot, molten resin into a sheet die and forming the composite polymer into a continuous sheet. The composite polymer is fed through a series of cylindrical rollers, for example, three vertically-arranged rollers, which are evenly spaced apart so as to shape the composite polymer into a sheet having a uniform thickness. The spacing of the rollers determines the thickness of the sheets formed. Conventionally, steam is pumped into the rollers when forming sheets, but with the present invention, the rollers should be cooler than the temperature of the mixture leaving the extruder. Preferably, water having a temperature of about 10-50°C is pumped through the rollers. As the continuous sheet leaves the last roller, it is tensioned to keep it flat until it cools and hardens enough to be moved. Once the sheets are formed, they may be cut into large sheets or rolled for storage. It is preferable to store those composite polymer formulations containing relatively large percentages of lactic acid polymer in sheets rather than in rolls because if rolled they tend to remain curled. The stored plastic sheets are subsequently processed. They may be cut into smaller sheets so as to be used as plastic cards, such as phone cards or credit cards. If used as credit cards or phone cards, consumers prefer the feel of a material that is 10% by weight resin and 90% by weight polymer comprised of lactic acid monomers. This preference must be weighed against the desirability of a 50/50 mixture for cutting purposes. Ink is then printed onto the sheets, and this novel degradable composite polymer is able to act as a substrate and retain the ink. The degradable composite polymer of the present invention may be printed with conventional or ultraviolet (UV) inks. Preferably, for printing purposes, the polymer and resin are in about a 50/50 combination by weight so that the printing dies are not worn. If printed with conventional inks, the ink sets up at ambient temperature when exposed to light. If the material is printed with UV ink, then UV light must be used to cure or dry the ink. Preferably, if UV light is used, the UV light is flashed in intervals which last for a fraction of a second so as to reduce heat build-up on the composite polymer. Still further, preferably, one does not print on the same print line of both sides of the composite polymer substrate at the same time because this brings too much heat to the material. Instead of being extruded into sheets, the degradable composite polymer of the present invention may be injection or compression molded into shapes and forms while retaining its ability to be printed upon. This degradable composite polymer can be molded into stiff and thin wall articles. Preferably, the composite polymer, which is later molded, is formed at a temperature of about 160-190°C, if used as a printable plastic material. Conventional injection molding equipment, such as that used in the plastics industry, may be used. However, processing conditions should be adjusted to allow for the fact that the degradable composite polymer of the present invention has a slower cooling time than typical plastics. Subject to the size and complexity of the mold's shape, the mold release time, as compared to typical plastics, can be increased negligibly up to approximately 200%). The usual time increase appears to be about 30%. However, use of an air cooled or liquid cooled mold will allow cycle times which are comparable to those of conventional plastics. If the degradable composite polymer is injection molded, the injection temperature, preferably, is about 180 to 200 °C. The injection speed can be varied depending upon the size and shape of the mold and the number of cavities in the mold. Preferably, the nozzle pressure is about 1300 to 1400 bar. Preferably, the mold temperature is between approximately 10 and 65 °C. Either cold or hot runners may be used in the injection system, and the minimum gate size is about 1 mm full round. Preferably, the mold is made of an acid-resistant material such as an acid-resistant metal. The flowability of this degradable composite polymer into the mold cavities is enhanced when the molds are specially designed for a degradable material. One preferable design incorporates the use of rounded corners inside the mold, as opposed to 90 degree corners, because the degradable composite polymer does not like to flow into "sharp dead-ends." Most preferably, the mold design should take into account the following parameters or properties: 1) The rheological and other physical properties of the resin; 2) if cold runners systems are used, the sprue length should be as short as possible to avoid the composite polymer breaking inside the mold; 3) flow channels should have reduced cross-sectional area, free of stagnation points; and 4) gates should be fully rounded. Molded articles can be colored in numerous ways. The coloring of the degradable composite polymer can either be accomplished by compounding the PLA with the desired colorant or by substitution waxes as the matrix for the colorant. Still further, the materials used in making the degradable composite polymers of the present invention should be dried before being processed. Also, during the start-up and shutdown of the extrusion process, to reduce the material costs, the start¬ up can be performed using low density polyethylene (LDPE). Once the operating temperatures have been achieved, the components comprising the degradable composite polymer of the present invention can be substituted for the LDPE. At the end of the run, LDPE can be used to cleanse the equipment of the degradable composite polymer. A plastic media blast process is a process for the rapid, economic, and safe removal of coating from almost any product. It resembles sandblasting but does not use a hard abrasive, such as silica sand. A plastic media blast process is a dry stripping method by which small angular plastic particles are propelled against a covered surface lifting the covering off and leaving a clean, unmarred substrate. The process employs specially designed equipment which propels and recovers the sharp-edged non-toxic plastic granules. It is especially useful on surfaces which cannot tolerate damaging mechanical sanding or wet chemical stripping. The degradable composite polymer of the present invention is particularly useful as plastic media blast material. The plastic particles are pneumatically applied at pressures of about 10-40 psi. Plastic media blast material is made by first combining a polymer and a resin and subsequently extruding this mixture as discussed above. The extruded mixture can be formed into particles by any method known to those skilled in the art. One method involves feeding the extruded mixture, which is a soft, hot, molten resin, into a rod die and forming the composite polymer into a continuous rod. Preferably, the rod is cooled as it exits the rod die. This may be accomplished with a water bath, a fan or moving the rod through static air. Once the rod has cooled and hardened, it may be cut into pellets. The pellets are then further reduced in size by grinding so as to form composite polymer particles of a prescribed fineness. A hammer mill or any type of conventional grinder may be used to form the particulates. The hardness of the resulting composite polymer should affect the particular choice of grinders. For degradable composite polymers of this invention containing a large percentage of lactic acid polymer, an especially durable grinder must be used. Conventional steel blades may be destroyed through the grinding of the invented substance. Still further, the plastic media blast material of the present invention may consist entirely of a polymer comprised of lactic acid monomers. Polymers or copolymers made from lactic acid monomers make an effective plastic media blast which is relatively very hard. This is a novel use for such polymers or copolymers. However, plastic media blast material made entirely from lactic acid polymers cannot be made in a full range of hardnesses. Thus, in order to make softer plastic media blast material, the lactic acid polymer must be combined with a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer. The grit size of the plastic media blast particles can be varied. However, particles which are too large or too hard can damage the surface of a substrate. Preferably, the degradable composite polymer of the present invention is ground to a very fine powder so that it will not plug up the nozzles of the device which sprays the plastic media blast material. Preferably, the plastic media blast particles have equivalent diameters between about 0.2 and 2.5 mm. Most preferably, the plastic media blast particles have equivalent diameters of between about 0.4 and 1.2 mm. The plastic media blast material may be created in a variety of hardnesses to remove different substances from different surfaces. Specifically, it may be created so as to have a hardness between about 10 and 70 on the Rockwell scale. The plastic media blast material of the present invention can be used in a variety of processes including coating removal, substrate abrasion, industrial cleaning, particle removal, surface finishing, and deflashing molded items. Types of coatings that can be removed include paint, powder, primers, top coats, residues, contaminants, burrs, polymer coatings, and epoxy coatings. It may be used on any surface including flexible surfaces and sensitive substrates such as plastic, honeycomb-shaped structures, fiberglass, polymers, composite materials, metals, and woods, without harming these substrates. Applications where plastic media blast material can be used include, but are not limited to, the dry stripping or cleaning of an endless range of consumer household products, industrial machinery and equipment, vehicles, aerospace components, weapons systems and marine vessel hulls. The plastic media blast process is environmentally sound and is an effective replacement for wet chemical strippers, such as methylene chloride based chemical strippers which cause adverse environmental impacts. Using the degradable composite polymer of the present invention as a plastic media blast material avoids the use of methylene chloride, phenol, corrosives and caustics, methanol, toluene and acetone. Using proper techniques, the combination of low operating pressures, soft plastic particles and high flow rate permits rapid removal without warping panels or damaging surfaces. Furthermore, clad, anodized, galvanized and phosphate coatings may be left in tact. And, in many cases, paint can be removed layer by layer down to the base substrate or to the primer coating. The following are examples of various degradable composite polymers which are within the scope of this invention. These examples are not meant in any way to limit the scope of this invention. Example 1 A composition containing the following components was prepared: 47.5% by weight of semicrystalline polylactic acid (PLA) containing a trace amount of D-lactide with a molecular weight of 150,000, 47.5% by weight of ZF03U/A class Mater Bi™, and 5% by weight of titanium oxide as white colorant. All the components were pre-dried at 40 °C for 23 hours and premixed. The premixed components were then fed into an extruder attached with a flexible lip sheet die having an opening width of 724 mm (28.5 in). The screw diameter was 63.5 mm (2.5 in.), with a screw length/diameter ratio of 24: 1. The extruder barrel was divided into 4 zones. The temperatures employed were 180, 180, 190 and 190°C, respectively, for the first, second, third and fourth zones. The operating conditions were as follows: The rotating speed of the screw was 75 rev/min. The feed flow rate was 85.4 kg/hr (188 lb/hr). The sheeting rolls consisted of three vertically arranged cylindrical rolls measuring 305 x 1016 mm. The temperatures for the three rolls were 48, 60, and 43 °C, respectively for the top, middle and bottom rolls. The extruded sheets were cooled and cut into 610 mm by 610 mm (24 in by 24 in) squares and packed. Example 2 A composition containing the following components was prepared: 90% by weight of amorphous polylactic acid (PLA) containing 18%> D-lactide by weight with a molecular weight of 80,000, and 10% by weight of ZF03U/A class Mater Bi™. All the components were pre-dried at 40 °C for 24 hours and premixed. The premixed components were then fed into a twin screw extruder. The screw diameter was 20 mm, and the screw length/diameter ratio was 40. The diameter of the nozzle opening was 3 mm. The extruder barrel was divided into 5 zones. The temperatures employed were 90, 150, 150, 145, and 145°C, respectively for the first, second, third, fourth and fifth zones. The operating conditions were as follows: The rotating speed of the screw as lOO rev/min. The extrudates were air cooled and then granulated using a C.W. Brabender laboratory pelletizer. The obtained resins were then ground to particle sizes having equivalent diameters of about 1.5 mm using a C.W. Brabender Granu-grinder rotating at 3500 rev/min. Example 3 A composition containing the following components was prepared: 70% by weight of semicrystalline polylactic acid (PLA) containing a trace amount of D- lactide with a molecular weight of 150,000, and 30% by weight of ZF03U/A class Mater Bi™. All the components were then fed into a twin screw extruder. The screw diameter was 20 mm, and screw length diameter ratio was 40. The diameter of the nozzle opening was 3 mm. The extruder barrel was divided into 5 zones. The temperatures employed were 140, 190, 190, 185, and 185°C, respectively for the first, second, third, fourth and fifth zones. The operating conditions were as follows: The rotating speed of the screw as 100 rev/min. The extrudates were air cooled and then granulated using a C.W. Brabender laboratory pelletizer. The obtained resins were then ground to particle sizes having equivalent diameters of about 1.0 mm using a C.W. Brabender Granu-grinder rotating at 3500 rev/min. Example 4 A composition containing the following components was prepared: 47.5%o by weight of amorphous polylactic acid (PLA) containing 18% D-lactide by weight with a molecular weight of 80,000, 47.5% by weight of ZF03U/A class Mater Bi™, and 5% by weight of titanium oxide as a white colorant. All the components were pre-dried at 40°C for 24 hours and premixed. The premixed components were then fed into an extruder attached with a flexible lip sheet die having an opening width of 724 mm (28.5 in). The screw diameter was 63.5 mm (2.5 in.), and screw length/diameter ratio was 24: 1. The extruder barrel was divided into 4 zones. The temperatures employed were 152, 149, 160 and 160°C, respectively for the first, second, third and fourth zones. The operating conditions were as follows: The rotating speed of the screw was 75 rev/min. The feed flow rate was 85.4 kg/hr (188 lb/hr). The sheeting rolls consisted of three cylindrical rolls arranged vertically. The temperatures for the three rolls were 48, 60, and 43 °C, respectively for the top, middle and bottom rolls. The extruded sheets were cooled and cut into 610 mm by 610 mm (24 in by 24 in) squares and packed. Example 5 A composition containing the following components was prepared: 67.5%) by weight of semicrystalline polylactic acid (PLA) containing less than 3% D- lactide by weight with a molecular weight of 80,000, 27.5% by weight of ZF03U/A class Mater Bi™, and 5% by weight of titanium oxide as white colorant. All the components were pre-dried at 40°C for 24 hours and premixed. The premixed components were then fed into an extruder attached with a flexible lip sheet die having an opening width of 724 mm (28.5 in). The screw diameter was 63.5 mm (2.5 in.), and screw length/diameter ratio was 24: 1. The extruder barrel was divided into 4 zones. The temperatures employed were 152, 149, 160 and 160°C, respectively for the first, second, third and fourth zones. The operating conditions were as follows; The rotating speed of the screw was 75 rev/min. The feed flow rate was 85.4 kg/hr (188 lb/hr). The sheeting rolls consisted of three cylindrical rolls arranged vertically. The temperatures for the three rolls were 48, 60 and 43 °C, respectively for the top, middle and bottom rolls. The extruded sheets were cooled and cut into 610 mm by 610 mm (24 in by 24 in) squares and packed. The degradable composite polymers of the present invention can be used as a plastic. It can be produced in various textures and ranges of flexibilities. It also can be produced in a range of hardnesses so that it can be used as a plastic media blast material for a variety of surfaces. The composite polymer also has the ability to retain ink during printing so that it can be used as a printable plastic material. The composite polymer is able to be cut easily without fracturing so that it may be cut into small plastic sheets. It also has a high elasticity so that it is able to be extruded into sheets without curling and becoming brittle. Still further, it degrades by composting in two years or less and is made from renewable resources, thus making it an environmentally-friendly product. From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects hereinabove set forth together with other advantages which are obvious and inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Claims: We claim: 1. A degradable composite polymer, comprised of the following: a first polymer comprised of lactic acid monomers; and a resin comprised of a thermoplastic polymer, destructured starch and plasticizer. 2. A composite polymer as in claim 1 wherein said first polymer has a molecular weight between approximately 5,000 and 200,000. 3. A composite polymer as in claim 1 wherein said first polymer is polylactic acid. 4. A composite polymer as in claim 1 wherein said lactic acid monomers are comprised of about 1% or less D-lactide by weight. 5. A composite polymer as in claim 1 wherein said resin is Mater Bi™ resin. 6. A composite polymer as in claim 1 wherein said resin is comprised of about 10-90% by weight polymer or copolymer, about 10-90% by weight destructured starch, about 0-20% by weight destructuring agent, and about 0-6%> by weight water. 7. A composite polymer as in claim 1 wherein said resin is comprised of about 20-70%> by weight destructured starch, about 10-50% by weight ethylene-acrylic acid copolymer, about 2-40%> by weight plasticizer, about 0-10% by weight urea, about 1-5% by weight water and about 0.002-0.4%) by weight boron compounds. 8. A composite polymer as in claim 1, comprised of the following:approximately 10-90%) by weight of said first polymer; and approximately 10- 90% by weight of said resin. 9. A plastic media blast material, comprising: particulates having equivalent diameters between about 0.2 and 2.5 mm formed from a mixture of approximately 10-100% by weight of a first polymer comprised of lactic acid monomers, and approximately 0-90%o by weight of a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer. 10. A plastic media blast material as in claim 9 wherein said particulates are formed from a mixture of approximately 30-70% by weight of said first polymer, and approximately 30-70%) by weight of said resin. 11. A plastic media blast material as in claim 9 wherein said particulates are formed from a mixture of approximately 50%> by weight of said first polymer, and approximately 50%> by weight of said resin. 12. A printable plastic material, comprising: a sheet formed from approximately 10-90% by weight of a first polymer comprised of lactic acid monomers, and approximately 10-90% by weight of a resin comprised of a thermoplastic polymer, destructured starch, and a plasticizer. 13. A printable plastic material as in claim 12 wherein said sheet is formed from approximately 30-70%> by weight of said first polymer, and approximately 30-70% by weight of said resin. 14. A printable plastic material as in claim 12, wherein said sheet is formed from approximately 50% by weight of said first polymer, and approximately 50% by weight of said resin. 15. A printable plastic material, comprising: a molded article formed from approximately 10-90%) by weight of a first polymer comprised of lactic acid monomers, and approximately 10-90%> by weight of a resin comprised of a thermoplastic polymer, destructured starch, and a plasticizer. 16. A method for making a degradable composite polymer, comprising the following steps: combining a first polymer comprised of lactic acid monomers with a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer; and extruding said first polymer and said resin through a heated extruder. 17. A method as in claim 16 wherein said composite polymer forms at a temperature between about 140°C and 230°C. 18. A method for making printable plastic material, comprising the following steps: combining a first polymer comprised of lactic acid monomers with a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer; and extruding said first polymer and said resin through a heated extruder into a sheet die to form a sheet of printable plastic material. 19. A method as in claim 18, further comprising: cutting said sheet of printable plastic material into pieces of a desired size. 20. A method as in claim 18, further comprising: printing ink on said sheet of printable plastic material. 21. A method for making printable plastic material, comprising the following steps: combining a first polymer comprised of lactic acid monomers with a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer; extruding said first polymer and said resin mixture through a heated extruder to form a printable plastic material; and molding said printable plastic material into shaped articles. 22. A method as in claim 21 wherein said printable plastic material is molded by injection molding or compression molding. 23. A method for making plastic media blast material, comprising the following steps: combining a first polymer comprised of lactic acid monomers with a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer; and extruding said first polymer and said resin through a heating extruder into a rod die to form rods which are then processed into plastic media blast material. 24. A method as in claim 23, further comprising: cooling said rods which exit from said rod die. 25. A method as in claim 24, wherein said extruded rods are cooled in a water bath. 26. A method as in claim 23, further comprising: cutting said rods which exit from said rod die into pellets. 27. A method as in claim 26, further comprising: grinding said pellets into particles having a prescribed fineness. 28. A method as in claim 23 wherein said rods are ground into particles having equivalent diameters between 0.2 and 2.5 mm. 29. A method as in claim 23 wherein said plastic media blast material has a hardness between approximately 10 and 70 on the Rockwell scale. 30. A method for removing material from a substrate, comprising: projecting particulates formed from a mixture of approximately 10-100% by weight of a first polymer comprised of lactic acid monomers, and approximately 0-90% by weight of a resin comprised of a thermoplastic polymer, destructured starch and a plasticizer against a substrate; and dislodging material from said substrate.

Inventors: BIBY GERALD, FANG QI, HANNA MILFORD, BIBY, GERALD, FANG, QI, HANNA, MILFORD
Applicants: UNIV NEBRASKA AT LINCOLN, UNIVERSITY OF NEBRASKA AT LINCOLN
Application year: 1998
Main classification: C08L3
IPC codes: B24C11, C08L3, C08L23, C08L67