The present invention involves the synthesis of a series of amylose (starch) derivatives with various degrees of substitution and amylose/amylopectin ratios. These chains are chemically crosslinked and then mechanically deformed (stretched) to produce a biodegradable and mechanically superior material. Specifically, the process consists of chemically modifying starch into starch derivatives such as starch ethers, starch esters and starch carbamates. The polymers have a percentage degree of substitution of from about 35% to about 95% (degree of substitution is from about 1.05 to about 2.85) and preferably have a percentage degree of substitution of from about 65% to about 90% (degree of substitution is from about 1.95 to about 2.70). The starch derivatives are crosslinked to obtain crosslinked chains and processed into sheets, films, fibers, threads or other articles as known in the art. After processing, the articles are swollen in a thermodynamically acceptable solvent or solvent mixture to a desired volume and deformed in a uniaxial or biaxial extension. The polymers materials are preferably stretched from about 1% to about 500% in the direction of stretching. Finally, the solvent is removed, yielding a homogeneous, highly-ordered material. The present invention improves the properties and the quality of sheets, films, fibers, threads or other articles with respect, for example, to mechanical strength. The materials are developed from starch, a natural renewable source which has low cost, high production levels and which replaces petroleum-based, synthetic polymers; the materials acquire high-strength, high-modulus, toughness and flexibility; and the materials exhibit structural and functional stability during processing, storage and use, yet are susceptible to biodegradation upon disposal.
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1. A method for synthesizing polymers comprising the steps of:
(a) chemically modifying starch into starch derivatives;
(b) crosslinking the starch derivatives to obtain lightly crosslinked chains by contacting said starch derivatives with a crosslinking agent;
(c) processing the lightly crosslinked polymers into sheets, films, fibers, threads or other articles as known in the art a desired physical article;
(d) swelling the articles article in a thermodynamically acceptable solvent or solvent mixture to a desired volume;
(e) deforming (stretching) the swollen articles, in uniaxial or biaxial extension the article; and
(f) removing the solvent, at constant strain or stress .
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This application claims the benefit of U.S. Provisional Application 60/113,550, filed Dec. 23, 1998.
A. Field of the Invention
The present invention relates to high-performance biodegradable materials made from substituted, lightly crosslinked starch polymers which have been stretched, a method of producing substituted, lightly crosslinked starch polymers, a method of producing sheets, films, fibers, threads or other articles that consist at least in part of substituted, lightly crosslinked starch polymers, and to sheets, films, fibers, threads or other articles consisting essentially of substituted, lightly crosslinked oriented starch polymers. The materials exhibit structural and functional stability during processing, storage and use, yet are susceptible to microbial and environmental degradation upon disposal.
B. Biodegradable Materials
Synthetic polymers were invented about 60 years ago. Since that time there has been enormous progress in extending their range of applications. These various chemicals have steadily been further developed to show more valuable properties. One characteristic of synthetic polymers is their durability which can also be a disadvantage. The persistence of plastics in the environment, the shortage of landfill space, concerns over emissions during incineration, and entrapment and ingestion hazards from these materials have spurred efforts to develop biodegradable plastics. The challenge in replacing conventional plastics with biodegradable materials is to design such materials that exhibit structural and functional stability during storage and use, yet are susceptible to microbial and environmental degradation upon disposal.
For several decades it has been a goal of industry to make plastic materials either biodegradable by microorganisms or environmentally degradable by sunlight, moisture, temperature and the like. It is a continuing goal to make materials as fully degradable or compostable as possible. A compostable material is one that under-goes chemical, physical, thermal and/or biological degradation such that it may be incorporated into and is physically indistinguishable from finished compost (humus) and which ultimately mineralizes (biodegrades to CO2, water and biomass) in the environment like other known compostable matter such as paper and yard waste. It would be highly desirable to provide a material that is compostable especially in a municipal solid waste composting facility where it may undergo biodegradation in the presence of heat, moisture and microorganisms.
In the search of suitable polymers to replace synthetic polymers, starch is of particular interest since this biopolymer should have no adverse impact on human or environmental health. Furthermore, the introduction of starch as a partial replacement for synthetic polymers will contribute to the preservation of nature resources such as petroleum since starch is a renewable resource.
C. Starch and Starch Related Materials
Starch, a natural polymer (C6H10O5)n derived from plant materials, is commonly found in the form of tiny microscopic granules (5-25 microns in diameter) comprised of stratified layers of starch molecules. Starch occurs naturally in the roots, seeds, and stems of numerous types of plants, including corn, wheat, rice, millet, barley, and potatoes and constitutes the main carbohydrate reserve of plants. Starch consists of two polysaccharides, amylose and amylopectin, which can be separated according to their differences in solubility.
Amylose is a straight chain polymer of several hundred glucose units linked by a-1,4-glycosidic linkages. Amylose is mainly linear in structure, with molecular weight ranging from about 30,000 up to 1 million, although upper limits of 200,000 to 300,000 are more common.
Amylopectin is highly branched through carbon 6 and has a molecular weight of over 1 million. It is believed to consist of chains of 20 to 25 glucose units linked through carbons 1 and 4, as in amylose, but with the chains connected to each other through the 1,6 linkage. There is usually three times as much triethylsarch
Since there are only three free hydroxyl groups on each glucose unit of the starch backbone, the maximum DS is 3. The percentage degree of substitution is expressed as:
%DS=100×DS/3.
Typically, the substituted starch polymers of the present invention have a percentage degree of substitution of from about 35% to about 95% (DS is from about 1.05 to about 2.85). Preferably, the substitution percentage is from about 65% to about 90% (DS is from about 1.95 to about 2.70).
The presence of moisture in the substituted starch polymers may consume the crosslinking agent. In this connection it is advantageous if the starting material is dried or vented before crosslinking so as to reduce the moisture content to less than 5%. Preferably, the moisture content is less than 1%.
The next step consists of crosslinking the starch derivatives to obtain lightly crosslinked, substituted starch polymer chains, either by chemical reagents or high-energy radiation. This can be done either in the presence of solvent, or the solvent can be introduced subsequently, by swelling.
Other crosslinking methods are described in detail in the U.S. Pat. No. 5,102,597, Roe et al., issued Apr. 7, 1992, and in U.S. Pat. No. 4,076,663, the disclosures of which are incorporated herein by reference in their entirety. The substituted starch polymers are joined together by crosslinking agents, which are sufficient to react with the polymer material to form crosslink bonds between the starch polymer chains.
For example, for the preferred substituted starch polymer materials possessing hydroxyl or other crosslink sites, it is believed that the crosslinking agent reacts with the hydroxyl sites of the starch backbone or other crosslink sites of the starch substitution groups to form covalent chemical crosslink bonds between the chains of different precursor starch segments. These covalent chemical crosslink bonds generally arise as a result of the formation of bonds, such as ester, ether, carbamate, urethane, etc., by reaction of the functional groups of the crosslinking agents with the hydroxyl or other crosslink sites of the substituted starch polymer material.
Crosslinking agents useful in the present invention are those that react with the polymer material to form the crosslinked gels and can be any difunctional reagent which can provide crosslinked reaction products. Such crosslinking agents include compounds having at least two functional groups reactive with the starch derivative. Typically, the crosslinking agents are those compounds having at least two functional groups reactive with the starch derivative include including di- or poly-functional compounds which contain groups reactive with carboxyl, carboxylic acid anhydride, hydroxyl, amino or amide groups. Specific crosslinking agents useful in the present invention are described in more detail in U.S. Pat. No. 4,076,663.
The crosslinking agent is preferably one or more compounds selected from the group consisting of diisocyanates, aliphatic acid chlorides, hydridosilanes, and silanol-terminated oligosilanes. More preferably, the crosslinking agent is hexamethylene diisocyanate, silanol terminated polydimethylsiloxane or α,ω-dihydrido-oligo(dimethylsiloxane). Most preferably, the crosslinking agent is hexamethylene diisocyanate. One crosslinking agent or two or more substantially mutually nonreactive crosslinking agents selected from the group mentioned above may be used.
As used herein, the term “lightly crosslinked,” with reference to the various materials of the present invention refers to the density of crosslinking between polymer chains. The crosslinking density is determined by the number of crosslinks per starch derivative chain. In general, there is one crosslink for every 25 to 250 glucopyranose units. Preferably, there is one crosslink for every 100 to 150 glucopyranose units. However, the actual amount of a crosslinking to be used will vary depending upon the kind of substitution groups, the degree of substitution and the desired properties of the final article. In the present invention, other materials or agents can be used with the crosslinking agent(s) as an aid in producing the crosslinked polymer aggregates, or in promoting or assisting in the reaction of the crosslinking agent with the polymer material, or as solvents.
A crosslinking solvent may also be used in conjunction with the crosslinking agent. A “crosslinking solvent” as used in this application, refers to the solvent used in the crosslinking step. These crosslinking solvents are used to promote uniform dispersion of the crosslinking agent. Crosslinking solvents useful in the present invention are typically organic solvents such as chloroform, toluene, benzene, dimethylsulfoxide, dimethylformamide, etc. However, other solvents capable of dissolving the starch derivatives, well known to those skilled in the art, can be used. Preferred crosslinking solvents are chloroform, toluene, benzene or mixtures thereof. Typically the crosslinking solvent comprises a mixture including an crosslinking agent such that the step of applying an crosslinking agent is carried out simultaneously with the step of applying a crosslinking solvent.
The actual amount of solvent to be used will vary depending upon the kind of substitution groups, the degree of substitution, and the degree of crosslinking of polymer material. The crosslinking agent may also be used in a mixture with one or more solvents. Other optional components may also be mixed with the solution containing the crosslinking agent.
The crosslinking reaction can be promoted by adding a catalyst to the crosslinking agent to reduce the time and/or the temperature and/or the amount of crosslinking agent required to join the precursor polymer chains together. The actual time and temperatures used will vary depending upon the specific polymer materials used for the precursor polymer chains, the specific crosslinking agents used, the presence or absence of an additive in the reaction step, and the thickness or diameter of the macrostructure.
The crosslinking solvent may be applied to the precursor chains by any of various techniques and apparatus used for applying solutions to materials including coating, pouring, spraying, atomizing, or immersing the solvent on the precursor chains. The crosslinking solvent can be mixed with the precursor polymer chains by any of a number of mixing techniques and mixing apparatus to insure that the precursor chains are thoroughly coated with the crosslinking solvent.
This crosslinking reaction can occur at ambient room temperatures. Crosslinking is typically carried out at a temperature of from about 18° C. to about 90° C. for from about 12 to about 48 hours, preferably from about 18° C. to about 50° C. for from about 24 to 48 hours. The crosslinking reaction can be carried out at elevated temperatures to speed up the reaction.
The reaction between the crosslinking agent and the polymer material may alternatively be activated, instead of a spontaneous reaction, to form the crosslink bonds between different substituted starch chains. The method for activating and completing the reaction depends on the type of precursor material used and the composition of the crosslinking agent and any optional components. In general, the reaction may be caused by irradiation (e.g., ultraviolet, gamma- or X-radiation), by a catalyst which functions as an initiator, by an activator, or by thermal activation (heating) using any of a number of different apparatus as are known including the various ovens or dryers as are known. When radiation is used to activate the crosslinking reaction, the starch derivative itself may contain a side group with a double bond capable of forming crosslinks with other starch chains. Radiation polymerization methods as are known in the art are suitable for use herein.
The method of producing the crosslinked polymer comprises the steps of providing precursor starch derivative chains of the type herein described, applying a crosslinking agent to a portion of the polymers, and reacting the crosslinking agent with the polymer material to form crosslink bonds between the polymer chains of different precursor polymer chains.
In an alternative embodiment of the present invention, after the crosslinking agent is applied onto the precursor substituted starch chains, the crosslinking agent is mixed with the precursor starch derivative by any of a number of mixing techniques to insure that the precursor substituted starch chains are thoroughly mixed with the crosslinking agent to thereby enhance the efficiency of the crosslink bonds between the precursor chains. The mixing can be accomplished using various techniques and apparatus, including various mixers or kneaders, as are known in the art.
In an alternative embodiment of the present invention, a mixture of starch derivative chains, a crosslinking agent, and a crosslinking solvent are mixed and extruded in a conventional extruder apparatus to feed a pair of driven compacting rolls having a fixed (but variable) gap between the rolls so as to compress the aggregate into the form of a film or fiber. The film or fiber is then processed to specific lengths to provide macrostructures that have a specifically designed size, shape, and/or density. Such a method is described in detail in the U.S. Pat. No. 5,102,597.
While the substituted starch polymer composition of the present invention is essentially composed of a substituted starch-based macromolecular substance, it may contain as necessary a variety of additives such as natural polymers other than those related to starch (polysaccharide type polymers, cellulose polymers, proteinaceous polymers, etc.), heat stabilizers, colorants, flame retardants, ultraviolet absorbers, fungicides, herbicides, antioxidants, etc.
The crosslinked, substituted starch polymers are then dried or vented before crosslinking so as to reduce the moisture content to less than 5%. Preferably, the moisture content is less than 1%.
In the process proposed by the invention, it is proposed to produce single- or multi-ply sheets, films, fibers, threads or other articles as known in the art with at least one ply containing cross-linked, starch derivative polymers. The dried, crosslinked materials formed are hard and brittle.
The biodegradable films of the present invention may be processed using many of the conventional procedures for producing films or fibers of polymers on film or fiber making equipment.
In accordance with th e the present invention, a layer composed of a substituted starch polymer substance can be laminated with a layer composed of a different substrate material. The different material may, for example, be paper, woven cloth, nonwoven cloth or wood board.
For laminating a layer comprised of the above composition with a layer comprised of said different material, any of the known methods such as dry lamination, extrusion, co-injection, multi-layer extrusion, etc. can be variously employed. In manufacturing such a laminate, an adhesive can be employed with advantage.
Before the polymers can be oriented, the articles are swollen in a thermodynamically acceptable swelling solvent or solvent mixture to a desired swollen volume. A “swelling solvent” as used in this application, refers to the solvent used in the swelling step. These swelling solvents are used to prepare a gel state in the articles which is capable of undergoing large mechanical deformation. Swelling solvents useful in the present invention are typically organic solvents such as chloroform, toluene, benzene, etc. However, other solvents capable of swelling the starch derivative polymers can be used. Preferred swelling solvents are chloroform, toluene, benzene or mixtures thereof. The actual amount of swelling solvent to be used will vary depending upon the kind of polymer material.
The swollen networks are then stretched, in uniaxial or biaxial extension, to induce segmental orientation. Orientation refers to stretching a film or fiber in at least one direction which allows for alignment and ordering of the polymer molecules along the direction of stretching.
Orienting can be uniaxial, which is typically in the direction the film or fiber travels as it is processed. Alternately, orienting can be biaxial which is typically in the direction the film or fiber travels as it is processed and in a second direction transverse to the first.
The films are preferably oriented at a polymer concentration from about 10 volume percent to about 70 volume percent. The films are more preferably oriented at a polymer concentration from about 15 volume percent to about 30 volume percent. The films are most preferably oriented at a polymer concentration from about 20 volume percent to about 25 volume percent.
Orientation is generally performed within a temperature range of from about 18° C. to about 90° C. Preferably, the stretching is performed within a temperature range of from about 20° C. to about 50° C.
The extension ratio during drying can be from about 1% to about 500% in the direction of stretching. The preferred stretching amount is in the range from about 10% to about 250% in the direction of stretching. The most preferred amount being in the range of from about 25% to about 150% in the direction of stretching.
Finally, the swelling solvent is removed at essentially the same, substantially deformed state, yielding a homogeneous and highly-ordered material. The finished starch derivative product is preferably dried or vented during production so as to reduce the final moisture content to less than 5%. Preferably, the final moisture content is less than 1%. Most preferably, the final polymers should be dried to the point where they are almost solvent or moisture-free.
The oriented films or fibers may be dried by allowing the solvent to evaporate under ambient conditions. The rate of the evaporation should be kept low to prevent inhomogeneous shrinkage in the direction perpendicular to the stretching. The orientation direction of the film or fiber is kept at constant length. Usually several days are required to obtain a dried film with homogeneous density.
Alternatively, the films or fibers may be dried using a coagulation process. Such coagulation processes are widely used in industry to obtain films or fibers for the polymer solutions, for example in “wet spinning”. Gel formation by coagulation depends upon the differential solubility of polymers in a blend of solvents and nonsolvent, and the solubility changes resulting from concentration changes of added chemicals. For example, solvents A and B may be a good solvent and a poor solvent (or nonsolvent), respectively, for polymer Y. A nonsolvent is a solvent in which the polymer neither dissolves nor swells. A polymer solution of Y is prepared in solvent A and extruded into a bath of solvent mixture of A and B. The concentration difference causes diffusion of A into the bath and the B into the polymer solution. As the concentration of B increases and that of A decreases, the polymer becomes less soluble and yields eventually the desired unswollen film. Changes in pH or supersaturated solutions may also be sufficient to reverse the solubility or change the chemical nature of a polymer in solution, transforming it into an insoluble, stable gel.
For example, for films swollen in chloroform, a nonsolvent with good miscibility with chloroform is required. Petroleum ether can then be employed because of its low cost, low boiling point, and bulk availability, its miscibility with chloroform and because it is a thermodynamically poor solvent for triethylamylose. Other methods of drying the films or fibers may be employed as known to those skilled in the art.
The following examples illustrate and explain the present invention but are not to be taken as limiting the present invention in any regard.
Methyisulfinylmethylcarbanion Methylsulfinylmethylcarbanion (MSMC) is prepared using 6.67 g NaH (as 60% suspension in mineral oil) and washed four times, with 100 mL dry pentane each time. The pentane is decanted and the residue drawn off by vacuum. To the dry, powdery NaH, 75 mL of DMSO is added under argon and the mixture is stirred at 65-70° C. for two hours or until the hydrogen evolution ceases. The reddish black methylsulfinylmethylcarbanion is cooled to room temperature and is used for alkylation.
A solution of 5 g amylomaize VII (70% amylose/30% amylopectin) in 250 mL DMSO is mixed with MSMC under argon. The mixture solidifies to a greenish gray gel instantly and it becomes liquid again after being stirred vigorously for 5-10 minutes. After continuous stirring for two more hours, 15 mL of iodomethane is added at 20° C. in two hours. The solution first becomes greenish and then it turns to dark yellow and after complete addition of iodomethane, it then turns to a light yellow. The mixture is stirred continually for 16-18 more hours before being poured into 0.5 L of ice water. The triethylamylose is separated out as a white or pale yellow precipitate. The crude product is then washed several times with a sodium bisulfide solution to remove trace amounts of iodoethane. Purification is carried out by precipitating the triethylamylose toluene solution from petroleum ether. The triethylamylose prepared from amylomaize VII has a degree of substitution ranging from 65 to 90%.
The highly-substituted triethylamylose is then partially crosslinked with hexamethylene diisocyanate. Two grams of the highly-substituted triethylamylose is dissolved in 10 mL of dry toluene. An appropriate amount of hexamethylene diisocyanate is added via a syringe, followed by a catalytic amount of triethylamine. The mixture is stirred for about 3-4 hours before being transferred into a round TEFLON® pan (diameter of 2 inches). The gelation is completed in the pan overnight at 40-50° C., and solvent is almost gone by then. A brittle film is obtained after the solvent is evaporated, and it does not dissolve in common organic solvents.
The cross-linked amylose (starch) ether films are swollen to about 4-5 times their original volume in chloroform. The swollen films are then uniaxially deformed to a desired extension by moving two clamps at both ends. The specimens are stretched to about 150% of their original lengths. The stretched films are dried at constant length first under ambient conditions for 1-2 days followed by further drying in vacuo at 50° C. for an additional day. The resulting films are stored in a desiccator.
After orientation the triethyl amylose films become tougher. The tensile strength and modulus of dried samples having dimensions of 50 mm3×5 mm3×0.5 mm3 are measured using an Instron mechanical tester (Model 1122). The cross-head speed is 0.1 in/min with the initial gauge length of 0.7 inch. The force and deformation are recorded with an x-y recorder.
The tensile strengths and moduli increase monotonically with an increase of extension ratio during drying. A 3.5-fold increase in tensile strength, and a 30% increase in tensile modulus (in comparison to the cross-linked unoriented films) is observed at an extension ratio of 135%. The oriented films show a 20-fold increase in elongation at break and a 100-fold increase in toughness with an extension ratio of 135%.
Similar to Example 1 except that the film is dried using a coagulation process. A polymer solution of crosslinked amylose (starch) ether is prepared in chloroform and extruded into a bath of solvent mixture of chloroform and petroleum ether. The concentration difference causes diffusion of chloroform into the bath and the petroleum ether into the polymer solution. As the concentration of petroleum ether increases and that of chloroform decreases, the polymer becomes less soluble and yields eventually the unswollen, crosslinked amylose (starch) ether film.
Similar to Example 1 except that the crosslinked amylose (starch) ether polymer is processed to make a fiber.
Similar to Example 1 except that tri(ethyl-allyl)amylose is used as the starch derivative.
Similar to Example 1 except that the crosslinking agent used is a hydridosilane. The desired amount of α,ω-dihydrido-oligo(dimethylsiloxane) with hydride terminal groups (M.W. 400) is added to the starch derivative dissolved in toluene. Then 1 wt. % dibutyltin dilaurate is added.
Similar to Example 1 except that biaxial orientation is performed by stretching the swollen polymer film simultaneously in two directions with the same extension ratio to about 50% of their original lengths. The stretched films are dried at constant length first under ambient conditions for 1-2 days followed by further drying in vacuo at 50° C. for an additional day. The resulting films are stored in a desiccator.
The present invention improves the properties and the quality of sheets, films, fibers, threads or other articles that consist at least in part of lightly crosslinked, substituted starch polymers. The materials are developed from starch, a natural renewable source, the materials acquire high-strength, high-modulus, toughness and flexibility and the materials exhibit structural and functional stability during processing, storage and use, yet are susceptible to biodegradation upon disposal.
Mark, James E., Peterson, Brooke Zhao, Eman, Burak, Bahar, Ivet, Kloczkowski, Andrzej
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