WO1997006925A1

WO1997006925A1 - Filled biodegradable polymer material and media blast

Info

Publication number: WO1997006925A1
Application number: PCT/US1996/013110
Authority: WO
Inventors: Kent Blizard; Robert Kovar; Leslie Rubin; Justyna Teverovsky
Original assignee: Foster-Miller, Incorporated
Priority date: 1995-08-14
Filing date: 1996-08-14
Publication date: 1997-02-27
Also published as: AU1523097A; EP0844921A1

Abstract

A filled biodegradable polymer material, particularly adapted for use as a media blast, composed of a number of particles, each particle comprising at least one wholly or partially biodegradable organic, or naturally occurring inorganic filler material dispersed within a polymer carrier matrix comprised of at least one thermally processable biodegradable polymer, and a method of producing the same.

Description

FILLED BIODEGRADABLE POLYMER MATERIAL AND MEDIA BLAST

STATEMENT OF GOVERNMENT RIGHTS This invention was made with U.S. Government support under contract no. F33615-93-C-5350 and contract no. F33615-94-C-5609 both awarded by the U.S. Air Force. The U.S. Government has certain rights in the invention claimed herein.

FIELD OF THE INVENTION This invention relates to a filled, biodegradable polymer material, particularly adapted for a media blast, and the use thereof in the removal of paint and other coatings from metal, composite, and wood surfaces.

BACKGROUND OF THE INVENTION Automobile panels, architectural structures, aircraft surfaces and other coated or painted substrates in many cases must be stripped of the coating(s). The routine paint stripping of military aircraft is needed to preserve structural integrity, control surface corrosion, insure safety and extend the life span of the aircraft. Care must be taken during this process that any delicate surfaces underlying the paint, such as fiberglass or other composite or anodized material, are not damaged. Previously, such paint removal was accomplished through chemical stripping. However, many of the chemical solvents used for paint removal have become the subject of environmental and safety regulations. An environmentally and occupationally safe alternative to the use of chemical solvents is the process of media blasting. Media blasting is a paint removal method whereby a plurality of dry, solid media or "grit" particles are entrained in a pneumatic stream. This stream of particles is then directed against the surface to be stripped and the paint and any other coatings are chipped or abraded off the substrate as the particles impact the substrate. Both the used media particles and any paint and coating debris slough off the aircraft surface as waste. While the used media particles are not usually classified as environmentally harmful, the paint and other coating debris, which typically comprise 5% to 10% of the waste volume, usually contain heavy metals such as chromium, cobalt, barium, and lead - all of which are considered hazardous under current government regulations. In many instances, the used media particles can not be easily separated from the paint and coating debris and the entire volume of the waste (approximately 20,000 lbs per fighter plane, for example) must be disposed of in hazardous waste landfill sites at a cost of as much as $800 per 55 gallon barrel, or incinerated at a cost of as much as $2000 per 55 gallon barrel. In view of escalating hazardous waste landfill tipping fees, media particles, that are biodegradable and therefore capable of being separated from the sloughed-off paint and coating debris through environmental degradation and bioremediation techniques would result in a substantial reduction in paint stripping waste disposal costs, and would also help to reduce the volume of waste disposed at hazardous waste sites. Bioremediation of the used media blast particles may be conducted through a number of existing bioremediation facilities such as facilities owned by DOT Technologies, Inc., (Delta, British Columbia, Canada) or ABB Environmental Services, Inc., (Wakefield, Massachusetts). In anaerobic bioremediation, an acclimated culture of anaerobic bacteria is added to a solubilized used biodegradable media blast solution in a reactor. After this anaerobic treatment, the reactor contents are transferred to another vessel equipped with an aeration device until remediation is complete. In aerobic bioremediation, the used biodegradable media blast is fed into a liquidification tank where water and enzymes are added. When this solubilization step is complete, the suspended paint solids are removed and filtrate is passed through ion exchange columns for removal of any dissolved heavy metals. The discharge from the ion exchange columns are then fed into a starch digester where the appropriate bacteria and nutrients are added to provide nitrogen. During digestion, the starch solution is constantly aerated from the bottom of the digester to provide the solution with oxygen until remediation is complete. Many materials have been employed for use as media blast particles in an attempt to find a suitable biodegradable media blast substitute for chemical paint removing. Organic materials that have been employed for use as media particles include vegetable materials, such as rice hulls, soybean hulls, and corncobs, as disclosed in U.S. Patent No. 3,424,616, lignocellulosic materials such as walnut shells and peach pits, grains such as cracked wheat and clover seed, as disclosed in U.S. Patent No. 2,622,047, and U.S. Patent No. 2,426,072, and sugar crystals. These organic materials were found to have certain performance limitations that make them imperfect paint removers, such as residue surface dusting. Ground walnuts shells, for example, disintegrate upon contact with the surface of the aircraft and produce a fine dust. Besides rendering the media blast substantially useless for more than one blasting application, the dust produces an unacceptable explosion hazard. This dust residue must be scrubbed off with hot soapy water before the aircraft structures and components can be further processed since complete removal of all surface residue is required to provide optimal substrate/coating adhesion during subsequent painting. Any dust residue remaining on the aircraft may cause an additional safety hazard since such residue may mask microcracks in the aircraft's exterior during visual inspections of the aircraft. Other organic materials, such as sugar crystals, were found to be very brittle and would shatter into particles too small to be reused by conventional blasting equipment, causing the media particles to be completely consumed after only one use. Still other organic materials, such as many lignocellulosic materials, are too hard and may cause damage to the delicate substrate surfaces by causing microcracks and scratches in the surface. This could eventually lead to structural damage of the aircraft Engineered plastic materials, such as disclosed in U.S. Patent No. 5,367,068 and U.S. Patent No. 5,095,054, have also been employed for use as biodegradable media blast particles. However, these plastic materials have also been found to have certain performance limitations. The plastic of U.S. Patent No. 5,367,068 has been found to have a low Moh hardness and, as a result, a low strip rate as measured in square inch of coating stripped from the substrate per minute. Because of the low strip rate, the substrate may require final hand sanding or the use of chemical solvents to completely strip coatings from the substrate surface. The plastic of U.S. Patent No. 5,367, 068 was also found to have a high rate of water absorption. At high humidity levels or if exposed to liquid water, this plastic media material becomes sticky and forms clumps that can clog the blasting nozzle. The plastic disclosed in U.S. Patent No. 5,095,054, unlike the above, has high strip rates, but is unable to remove more than one coating layer. For example, plastic media blast particles of this type are able to remove acrylic paint from an automobile auto body panel, but are unable to remove the aluminum etchant primer. Other engineered plastics, such as urea, acrylic, melamine and polyester, that are currently used as media blast particles are not biodegradable. The applications of an improved biodegradable media blast are not limited to paint removal on military aircraft, but extend to the non-military sector as well. Biodegradable media blast may be used to strip commercial aircraft, bridges, automobiles and other ground vehicles, but one of its most significant applications would be the removal of lead paint from the interiors and exteriors of residential and commercial buildings. The number of buildings containing lead paint in the interior alone is estimated to be over fifty-seven million. With removal costs averaging between $8,000 and $12,000 per residential building, many owners cannot afford to take the proper remedial measures at a high risk to the health of infants and young children. A continuing need therefore exists for an improved biodegradable media blast that achieves a strip rate comparable to existing media blast and similar or lower consumption rates, but which has a low rate of water absorption, does not result in a film or residue remaining on the substrate, is able to remove more than one coating layer, and provides an economical option to currently available media blast.

SUMMARY OF THE INVENTION In accordance with the above and other objects of the invention, the present invention provides a filled, biodegradable polymer material comprising at least one wholly or partially biodegradable organic, or naturally occurring inorganic filler material dispersed within a thermally processable (for example, thermoplastic) biodegradable polymer carrier matrix. The polymer carrier matrix alone, when used as a media blast, does not obtain truly effective stripping rates. In media blast produced from polymer carrier matrix alone and used in the removal of paint from an aluminum auto body panel prepared by cleaning and applying an aluminum etchant primer followed by a topcoat, the polymer carrier media blast did not remove the primer. It was found that when suitable fillers were added in accordance with the teachings of the present invention, both the primer and the topcoat were removed. Strip rates higher than or equal to and consumption rates lower than or equal to the polymer carrier matrix media blast particles alone were also obtained. The present invention employs a biodegradable polymer carrier matrix as a binder for abrasive filler material. Therefore, if the media blast particles of the present invention break apart, the binding effect of the polymer carrier matrix aids in preventing complete shattering of the media blast particles and loss of the filler material. This allows the particles to be reused and results in a low consumption rate of media blast particles. Dusting will also be controlled since the bound filler materials are less likely to break into particles fine enough to cause a powdering effect. Upon impact with the substrate surface, the media blast particles of the present invention fracture and additional sharp corners are produced due to the exposure of more filler material. Thus, the original abrasiveness of the media blast particles of the present invention will be maintained, allowing repeated reuse of the media blast particles. This further lowers the consumption rates of the media while providing the hardness necessary to abrade the substrate surface and to maintain a strip rate comparable to existing media blast. Consumption rates may be determined under MIL-P-85891A-93. The use of filler material with a binder is described in U.S. Patent No. 5,234,470. However, U.S. Patent No. 5,234,470 only describes the use of filler loading to control the composition of the resulting media blasts particles and does not disclose selection of a filler material and carrier matrix material based upon biodegradability, hardness of the material and particle size distribution to tailor the media blast toward a specific end use. For example, the hardness of the media blast particles of the present invention may be tailored to meet the requirements of specific applications through the selection of one or more filler materials with a hardness greater than the hardness of the paint or other coatings, according the description which follows. Additionally, the present invention lowers the basic material and manufacturing costs associated with biodegradable plastic media blast. Unlike other plastic media blasts, such as disclosed in U.S. Patent 5,234,470, which are thermoset plastic materials, the present invention is thermally processable and does not require specially designed machinery. The present invention may be extruded on standard extrusion equipment for reduced processing costs. Organic and naturally occurring inorganic filler materials are generally inexpensive may help to reduce material costs by as much as 48% compared to currently available media blast by reducing the amount of plastic material required. The foregoing and other objects, features and advantages of the invention will become better understood with reference to the following description and appended claims. DETAILED DESCRIPTION The biodegradable plastic of the present invention comprises a number of particles, wherein each particle comprises at least one wholly or partially biodegradable organic, or naturally occurring inorganic, filler material, dispersed into the volume of a polymer carrier matrix comprised of at least one thermally processable biodegradable polymer. In formulating the overall composition of the biodegradable plastic of the present invention as a media blast, a number of variables including the properties of the filler material, the properties of the polymer carrier matrix, and the properties of the surface to be abraded are taken into consideration. Using these variables, the biodegradable media blast is tailored and optimized in accordance with the present teachings to provide a biodegradable media blast to meet a particular end use specification (i.e., particular coating or coatings to be stripped from a particular underlying substrate). The biodegradable polymers of the present invention may be referred to as "biopolymers" and are defined as polymers that can undergo microbially induced chain scission leading to mineralization. Suitable thermally processable biopolymers for use as the carrier matrix in the present invention include but are not limited to starch, polylactic acid, cellulose acetate, polyhydroxyalkanoates, polyvinyl alcohol, and polycaprolactone. Preferred biopolymers include polylactic acid, available commercially from Cargill, Inc. of Minneapolis, Minnesota and cellulose acetate, which is produced by The Eastman Chemical Company of Kingsport, Tennessee. Particularly preferred thermally processable biopolymers for use as a carrier matrix in the present invention are starch-based thermoplastic polymers. Preferred starch based polymers are based on wheat starch, rice starch, potato starch, or corn starch, and are available commercially from a number of vendors. A particularly preferred starch based thermally processable biopolymer for use in the present invention is the corn starch based polymer NOVON™ available from Novon International, Inc., (Tonawanda, New York), a division of Ecostar. Other starch based polymers may be modified with any biodegradable hydrolyzable polymer with the requisite hardness, including polycaprolactone , polylactic acid , or polyhydroxybutyrate-valerate , to produce a thermally processable biopolymer for use in the present invention that may be processed on conventional extrusion equipment. One preferred embodiment of the present invention comprises a blend of at least two starch based thermally processable biodegradable polymers to lower water absoφtion and increased hardness in the resulting media blast particles. Water absorption, as measured by the equilibrium percent water by weight at 100% relative humidity, of the media blast particles of the present invention must be low enough to prevent clumping of the media blast at high humidity levels or immediately upon contact with water, but high enough to allow water to be a solvent if the media blast particles are put into solution so as to enable bioremediation. Preferably, the biopolymer selected is less than about 10% water by weight at 100% relative humidity, and more preferably less than 10% water by weight at 100% relative humidity. The filler material selected for use in the present invention may be any one of a number of organic, or namrally occurring non-organic, materials which are capable of environmental remediation. One particularly preferred naturally occurring inorganic filler material is fluorspar, a mineral comprised of fluorite and varying amounts of silica. Particularly preferred is fluorspar consisting of about 7% silica. Other preferred naturally occurring inorganic fillers include but are not limited to the following minerals: calcium carbonate, pure calcium fluorite, and pure silica. Particularly preferred organic filler materials include walnut shell flour and granular cellulose fiber. Suitable filler materials for use in the present invention are selected based upon the qualities desired to impart to the finished product. For a media blast, the selection is based on the hardness and density of the filler material in comparison to the hardness of the substrate to be abraded. The exact hardness of the filler material selected for a particular embodiment of the media blast of the present invention is lower than the hardness of the substrate to be abraded, but higher than or equal to the coating being abraded. For example, a biodegradable filler having a hardness greater than polyurethane but not as great as aluminum is preferably selected if it is desired to produce a biodegradable media blast to remove polyurethane paint coating from an aluminum substrate. Preferred filler materials for most end use applications include any filler materials which have a hardness of about 2.0 Moh to about 4.0 Moh, and more particularly preferred are any filler materials which have a hardness of about 2.5 Moh to about 3.5 Moh. Preferably, the filler material of the present invention comprises a number of particles or granules with a particle size diameter distribution of about 200 microns to about 70 microns, and more preferably, of about 150 microns to about 70 microns. In addition to being a possible explosion hazard during extrusion, if the filler material particles or granules are finer than 70 microns, the filler material will clump and not disperse evenly throughout the biopolymer carrier matrix. Filler material particles or granules larger than 200 microns are too large to fit within the biopolymer carrier matrix and the resulting mixmre may contain loose filler material and unfilled carrier matrix material. The compressive strength of the material of the present invention is governed by the interfacial adhesion and cohesion of its components. Soft, compressible fillers such as some nutshell flours will reduce compressive strengths, whereas the reverse is true for hard solid fillers, such as fluorspar, provided that the adhesion between the components is at least as great as the cohesive strength of the matrix. Adhesion between the filler and the biopolymer may be increased through the use of surface agents added to the surface of the filler. Preferred surface agents include coupling agents, hydrophobic wetting agents, silanes, and stearic acid, which are preferably wholly or partially biodegradable. Media blast particles with a high density have increased momentum when directed against a substrate to be stripped. This increased momentum translates into an increased strip rate because more paint or coating will be chipped or abraded off upon impact by the force of the particle. By selecting a filler material with a high density, the density of the media blast particles of the present invention may be optimized. Preferably, the density of the filler material is greater than about 2 grams per cubic centimeter. Most preferred are filler materials with a density of about 3 grams per cubic centimeter. Again, the density chosen for the media blast particles is dependent upon the underlying substrate and the coating to be removed. For example, media blast particles with high densities may cause microcracks and abrasions in substrates with a low Moh hardness. Preferably , the media blast particles of the present invention contain less than 40 % filler by volume. Particularly preferred are media blast particles that consist of about 15% to 30% filler by volume. At a volume percent higher than 40%, the filler material will not disperse evenly into the biopolymer carrier matrix. Further, at filler loadings higher than 40% by volume, embrittlement of the media blast particle may occur. A media blast particle which is brittle will crumble apart more easily and generate more broken media blast particles at a higher rate. These broken media blast particles are too small to be reused in standard blasting equipment, resulting in more media blast particles being consumed. In the methods of the present invention, at least one wholly or partially biodegradable organic, or naturally occurring inorganic, filler material is compounded with at least one thermally processable biopolymer to achieve the desired results in sequential extrusion, drying, and grinding methods as described below. The following methods have been found to provide filled biodegradable polymer materials having improved properties. The methods include: (a) feeding at least one thermally processable biodegradable polymer and at least one wholly or partially biodegradable organic or naturally occurring inorganic filler material into an extruder; (b) heating the polymer(s) and filler(s) of step (a) under appropriate conditions in an extruder to form a melt; (c) exposing the melt of step (b) to a cooling means to form a solid product; and (d) using a grinding means to break the solid product of step (c) into particles. More than one biopolymer may be combined for use as the biopolymer carrier matrix for only one filler material or a mix of more than one filler material, or alternately, only one biopolymer may be used as the carrier matrix for only one or a mix of more than one filler material in accordance with the teaching of the present invention. The melting steps of the methods are accomplished through the use of conventional compounding and extrusion methods as described herein. To determine the extrusion conditions, the biopolymer may be characterized using differential scanning calorimetry (DSC) and torque rheometry. During the extrusion process, up to about 50 weight percent of at least one thermally processable biopolymer and up to about 50 weight percent at least one wholly or partially biodegradable organic or namrally occurring inorganic filler material is fed into an extruder where the mixmre is heated to form a melt. Because the most expensive component (and one of the softer components) of the present invention is typically the biopolymer, it is preferable to keep the polymer content of the melt as low as possible while maintaining the objectives of the present invention. The amount of filler material can be adjusted to provide increased hardness and density to the resulting product. Preferably, the biopolymer and the filler material are fed simultaneously in a metered fashion into the extruder, but those skilled in the art will recognize that the filler material may also be fed into the extruder downstream, after the biopolymer has melted. However, downstream addition can be problematic if steam losses are important to the final product. Preferably, a single screw or twin screw extruder is used during the extrusion process. Additional conveying and low compression screw elements may be needed to minimize shear and heat during the extrusion process to reduced biopolymer degradation and to provide a more uniform extrudate. After the biopolymer and filler material have been heated to form a melt, the melt is carried through the die slot. Upon exiting the extruder, the melt is cooled and dried by a cooling means and is then ground into particles by a grinding means. Optional components may be added to the melt of the present invention provided there is no interference with formation or with the desired final properties of the filled biodegradable polymer material. Such components or additives may include but are not limited to anti-static agents, such as zirconates and titinates, coupling agents, hydrophobic wetting agents, silanes, dispersants, pigments and so forth, and are well-known to those of ordinary skill in the art of polymer processing. Preferably, any optional components will be wholly or partially biodegradable. Specific processing considerations are important when extruding the biopolymer to better control water content of the resulting extruded product, particularly with starch- based biopolymers. Preferably, the vents of the extruder used in the present invention are plugged during the extrusion to prevent loss of water as steam during processing. More preferably, a non-vented extruder is used in the present invention. Or, water can be added downstream of the vents. Another preferred method of controlling the water content of the resulting media blast particles is to use air cooling as the cooling means since the filled biopolymer, as a melt, would take up water from the liquid cooling means traditionally used during extrusion, although liquid nitrogen may be used. Particularly preferred for air cooling is a tube containing small air holes which direct streams of cool, dry air onto the extrudate. Another preferred embodiment employs a dry ice trough of solid CO₂ as the cooling means. The cooled filled biopolymer melt is ground into particles to produce the media blast of the present invention. Grinding may be accomplished by any of a number of commercial grinding companies, such as AERO-BLAST™ of Sharonville, Ohio and ALL- GRIND PLASTICS, INC. of West Portal, New Jersey. Ground particles can be measured by a mesh size distribution which is based upon what percentage of the starting filled biopolymer melt will be retained on Tyler mesh of various sizes. For example, at the mesh size distribution of 20/30, the maximum amount of particles retained on a 16 Tyler mesh is 0.1 % of the total number of particles screened, the maximum amount of particles retained on 20 Tyler mesh is 15 % of the total number of particles screened, the m iimum amount of particles retained on a 30 Tyler Mesh is 80% of the total number of particles screened, the minimum amount of particles retained on a 40 Tyler Mesh is 95% of the total number of particles screened, and the minimum amount of particles retained on a 100 Tyler mesh is 98% of the total number of particles screened. The mesh size distribution needed is dependent upon the requirements of the blasting equipment to be used. In one generally preferred embodiment, the particles are ground to a mesh size distribution of about 12/30. This is a particle diameter size distribution of about 1.5 mm to about 0.5 mm. The ground media blast particles are then entrained in a pneumatic stream which is directed at the surface to be abraded. Devices employed for use in media blasting are well known to those skilled in the art and include jet blasters and centrifugal blasters. All of these devices utilize a wheel whose circumference is operated at a specific speed which mechanically accelerate the media blast and directs it against the substrate surface. There are a number of commercial blasting facilities at which the actual blasting may be done, such as MAXI-BLAST™ of South Bend, Indiana. To illustrate the improved properties of the biodegradable media blast of the present invention, various biodegradable media blast were prepared as taught herein. The ratios of filler and polymer in the total composition were adjusted to achieve biodegradable media blast having the desired properties as is shown in the embodiments that follow. EXAMPLE 1 In one particularly preferred embodiment of the present invention for removal of a polyamide epoxy primer and a polyurethane topcoat from a 0.032 inch thick aluminum panel, about ten pounds of NOVON™ Grade M1801 and about ten pounds of fluorspar (50 weight percent of fluorspar) were fed simultaneously in a metered fashion into a one inch Killion extruder with a 24: 1 L/d, 2.5:1 compression screw, with a breaker plate with seven holes, each approximately 6 millimeter in diameter, so as not to be plugged by the filler, only, and heated to approximately 132°C at a pressure of 6.2 MPa with a screw RPM of 77.5 to form a filled biopolymer melt. The filled biopolymer melt was extruded through a 3/8 inch rod die with a die temperature of 121 °C at a throughput of 9.0 lb/hr, cooled as it exited the die in an air-cool tube and pelletized in preparation for grinding by a commercial grinder into particles with a mesh size distribution of 16/46. This is a particle diameter size distribution of about 1 millimeter to about 350 microns.

EXAMPLE 2 In another preferred embodiment of the present invention for removal of a polyamide epoxy primer and a polyurethane topcoat from a 0.032 inch thick aluminum panel, about ten pounds of cellulose acetate and about ten pounds of fluorspar (about 50 weight percent of fluorspar) were fed simultaneously in a metered fashion into a one inch Killion extruder with a 24:1 L/d, 2.5: 1 compression screw, with the breaker plate only, and heated to approximately 216°C at a pressure of 6.5 MPa with a screw RPM of 80.0 to form a filled biopolymer melt. The filled biopolymer melt was extruded through a 3/8 inch rod die with a die temperature of 174°C at a throughput of 11.8 lb/hr, cooled as it exited the die in an air-cool tube and pelletized in preparation for grinding by a commercial grinder into particles with a mesh size of 16/46.

EXAMPLE 3 In a further preferred embodiment of the present invention for removal of an aluminum etchant primer and an acrylic topcoat from an aluminum auto body panel, about fifty pounds of NOVON™ Grade Ml 801 and about fifty pounds of granular calcium carbonate of a mesh size distribution of 40/200 were fed simultaneously in a metered fashion into a one inch Killion extruder with a 24: 1 L/d, 2.5:1 compression screw, with the breaker plate only, and heated to approximately 245 °C at a pressure of 10 PSI with a screw RPM of 95 to form a filled biopolymer melt with about 40 volume percent of the calcium carbonate. The filled biopolymer melt was extruded through a 3/8 inch rod die with a die temperature of 245 °C, cooled as it exited the die in an air-cool tube and pelletized in preparation for grinding by a commercial grinder into particles with a mesh size distribution of 12/30.

EXAMPLE 4 In still another embodiment of the present invention for removal of a polyamide epoxy primer and a polyurethane topcoat from a 0.032 inch thick aluminum panel, about ten pounds of polylactic acid and about ten pounds of fluorspar were fed simultaneously in a metered fashion into a one inch Killion extruder with a 24: 1 L/d, 2.5:1 compression screw, with the breaker plate only, and heated to approximately 177 °C at a pressure of 4.4 MPa with a screw RPM of 52.0 to form a filled polymer melt with about 25 weight percent fluorspar. The filled polymer melt was extruded through a 3/8 inch rod die with a die temperature of 152 °C at a throughput of 6.1 lb/hr, cooled as it exited the die in an air-cool tube and pelletized in preparation for grinding by a commercial grinder into particles with a mesh size of 16/46.

EXAMPLE 5 In a further preferred embodiment of the present invention for removal of an aluminum etchant primer and an acrylic topcoat from an aluminum auto body panel, about fifty pounds of NOVON™ Grade M4900 and about fifty pounds of granular calcium carbonate of a mesh size distribution of 40/200 were fed simultaneously in a metered fashion into a one inch Killion extruder with a 24: 1 L/d, 2.5:1 compression screw, with the breaker plate only, and heated to approximately 290°C at a pressure of 10 PSI with a screw RPM of 94 to form a filled biopolymer melt. The filled biopolymer melt was extruded through a 3/8 inch rod die with a die temperamre of 300°C, cooled as it exited the die in a water bath and pelletized in preparation for grinding by a commercial grinder into particles with a mesh size distribution of 12/30. In the testing of the material of this invention as a media blast, the organic filler materials tested included walnut shell flour, pecan shell flour, macadamia shell flour, and cellulose fibers. The namrally occurring inorganic filler materials tested included calcium carbonate and fluorspar. For stripping coatings, the inorganic fillers accomplished better results, likely due to the hardness and density of those fillers.

Three separate plastic formulations according to this invention were compounded, grounded to a media blast, and tested. The results are shown below in Table 1. In all cases, the polymer was NOVON grade M1801. Fill "A" was formulated at about 30 weight percent granular calcium carbonate filler of a mesh size distribution of 40/200. Fill "B" was filled with about 30 weight percent fluorspar of a mesh size distribution of 30/200. The substrate was an aluminum alloy 2024-T3, MIL-C-81706 chromate conversion coated, MIL-P-23377 epoxy primer, and MIL-C-83286 polyurethane topcoat, all applied to comply with Air Force TO 1-1-8. The nozzle used was a Vi inch diameter round, long venturi type.

TABLE 1

Strip Rate (ftVmin)

Run # Mass Flow Nozzle Distance Angle Unfilled Fill "A" Fill "B" Rate (lb/hr) Pressure (PSI) (inches) (degrees) Polymer

1 Below 140 25 4 20 .004 .015 .006

2 Below 140 35 7 45 .025 .024 .021

3 Below 140 45 10 80 .039 .033 .017

4 140-170 25 7 80 .027 .010 .006

5 140-170 35 10 20 .001 .001 .001

6 140-170 45 4 45 .045 .095 .060

7 Above 170 25 10 45 .007 .006 .003

8 Above 170 35 4 80 .063 .054 .034

9 Above 170 45 7 20 .026 .027 .014

Additives to increase the adhesion between the filler particles and the polymer have also been explored. Improved adhesion leads to less fracturing of the media blast particles, which allows the media blast to be reused one or more times. Such recycling of the media blast decreases the cost per use. In addition, the preferred polymer of this invention is thermoplastic. This allows the material to be melted once the media blast is spent, and reformulated again into a media blast. If contamination of the media blast can be handled or dealt with in a manner sufficient for the application of the media blast, theoretically this would allow the material of this invention to be reformulated as a media blast over and over indefinitely. Coupled with the reuse of the media blast, which is partially due to improved adhesion, the economics of the material of this invention as a media blast are greatly improved. One method of improving adhesion of mineral particles to NOVON is to add compatibilizers and wetting agents during compounding. Formulations of the material of this invention have been tested with stearic acid, a titanate compound, and a hydrophobic wetting agent. The effect on the material of adhesion enhancers, and mineral particle size, has been determined using material impact strength for purposes of evaluation. Table 2 below provides results of testing of NOVON grade 1801 with 30% by weight loading of 74 micron calcium carbonate filler material, with a hydrophobic wetting agent (Ken-React from Kenrich Company), a titanate compound (KenStat from Kenrich Company) and stearic acid (Hystrene from Witco Corporation) at three additive levels. The tests were performed by compression molding samples of extruded material into 3.5 x 3.5 inch squares, and testing per ASTM D 3763-93 (12.7 mm nose at 10.94 ft/sec on a Dynatap Model 8200 test apparams with a 3.0 inch base plate. The total energy was divided by sample thickness to normalize the results, which were averaged over four specimens. All of the additives increased the impact strength over the same filled polymer without additives.

Table 2

Material Impact Strength (ft-lb/in)

Unfilled NOVON 11.5

Filled NOVON 4.5

Filled NOVON and 1 % KenReact 7.5 2% KenReact 13.5 3% KenReact 11.0

Filed NOVON and 1 % KenStat 10.5 2% KenStat 11.0 3% KenStat 14.5

Filled NOVON and 1 % Hysterene 13.5 2% Hysterene 7.5 3% Hysterene 10.5

The effect of filler particle size was tested by varying the particle size of calcium carbonate filled at 30 weight percent, with 1 % stearic acid to promote adhesion. 74 micron calcium carbonate without the adhesion promotor as a standard had an impact strength of 4.5 ft-lb/in. With 1 % stearic acid the impact strength went to 13.5. A two micron filler had an impact strength of 14.5, and the 3 micron had an impact strength of 17. The results indicate that the additive is effective at increasing impact strength, and that the filler particle size also appears to have an effect on impact strength.

Three of the runs from Table 1 were tested for Almen arc height according to MIL-P-85891A-93. Run number 3, unfilled polymer, provided an arc height of about 5.3 mils; run number 6 for fill A provided an arc height of about 13 mils; and run number 6 for fill B provided an arc height of about 7.3 mils. Type V MILSPEC material provided an arc height range of 8 to 15.5 mils. The invention has been described in detail with particular reference to the preferred embodiments thereof. However, it will be appreciated that modifications and improvements within the spirit and teachings of this invention may be made by those skilled in the art upon considering the present disclosure. The invention should therefore be limited only by the scope of the following claims.

Claims

CLAIMS 1. A biodegradable media blast particle comprising at least one wholly or partially biodegradable organic or naturally occurring inorganic filler material dispersed into a polymer carrier matrix comprised of at least one thermally processable biodegradable polymer.

2. A biodegradable media blast particle according to claim 1 wherein the filler material has a hardness of about 2.0 Moh to about 4.0 Moh.

3. A biodegradable media blast particle according to claim 1 wherein the filler material is a mineral.

4. A biodegradable media blast particle according to claim 1 wherein the filler material is Flourspar.

5. A biodegradable media blast particle according to claim 1 wherein the filler material is calcium carbonate.

6. A biodegradable media blast particle according to claim 1 wherein at least one of the thermally processable biodegradable polymers is cellulose acetate.

7. A biodegradable media blast particle according to claim 1 wherein at least one of the thermally processable biodegradable polymers is polylactic acid.

8. A biodegradable media blast particle according to claim 1 wherein at least one of the thermally processable biodegradable polymers is starch based.

9. A biodegradable media blast particle according to claim 1 wherein at least one of the thermally processable biodegradable polymers is cornstarch based.

10. A biodegradable media blast particle according to claim 1 wherein at least one of the thermally processable biodegradable polymers is wheatstarch based.

11. A biodegradable media blast particle according to claim 1 wherein the filler material has a density greater than about 2 grams per cubic centimeter.

12. A biodegradable media blast particle according to claim 1 wherein the particle contains up to about 40 volume percent of the filler material.

13. A biodegradable media blast particle according to claim 1 further including at least one agent for binding to the filler material and the polymer.

14. A biodegradable media blast particle according to claim 1 wherein the filler material has a hardness less than the substrate to be abraded and greater than the coating to be removed from the substrate by the media blast particle.

15. A method of producing a biodegradable media blast particles, each particle comprising a carrier matrix of at least one thermally processable biodegradable polymer and at least one wholly or partially biodegradable organic or naturally occurring inorganic filler material within the volume of the carrier matrix, wherein the method comprises: a) feeding the biodegradable polymer and the filler material into an extruder; b) heating the biodegradable polymer and the filler material of step (a) under appropriate conditions in an extruder to form a melt; c) exposing the melt of step (b) to a cooling means to form a solid product; and d) using a grinding means to break the solid product of step (c) into particles.

16. A filled biodegradable polymer material comprising at least one wholly or partially biodegradable organic or naturally occurring inorganic filler material dispersed into a polymer carrier matrix comprised of at least one thermally processable biodegradable polymer.

17. The filled biodegradable polymer material of claim 16 in which the filler is a mineral.

18. The filled biodegradable polymer material of claim 17 in which the polymer is starch-based.

19. The filled biodegradable polymer material of claim 16 further including at least one agent for binding the filler material and the polymer.