Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-10-27
2001-05-22
Szekely, Peter A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S128000, C524S386000, C524S387000, C524S599000, C524S601000, C524S845000, C524S539000, C525S054240
Reexamination Certificate
active
06235816
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to a biologically degradable polymer mixture, to a process for its preparation and to a method of processing the polymer mixture according to the invention. More particularly, the invention relates to the manufacture of biodegradable polymer blends that include thermoplastic starch and polymers that are at least partially biodegradable.
2. The Relevant Technology
Proposals for the preparation of biologically degradable polymer mixtures are known from a large number of patent documents and articles. The great problem in the case of polymer mixtures lies, as a rule, in that those mixtures which have an excellent, biological degradability have only limited possibilities for use in the area of engineering plastics, thereby explaining the relatively modest success to date. Polymer mixtures having improved properties are either biologically inadequate or degradable with increased effort or else are too expensive.
From EP 535 994 a polymer mixture is known consisting essentially of starch and an aliphatic polyester, for example polycaprolactone, where the starch is preferably destructurized with water.
Moreover, in an article in the journal “Starke”, Volume 45, No. 9, Sep. 1, 1993, Weinheim, pages 314 to 322, the use of starch in the modification of synthetic plastics is described. Here it is proposed first of all to gelatinize the starch by means of water, in connection with which it is mentioned that this gelatinization takes place by means of an endothermic process. In view of this, one may assume that the “gelatinized starch” mentioned therein is most likely destructurized starch, which is mixed with a range of synthetic plastics in order to develop new composite materials.
In contrast, and in knowledge of the inadequacies of starch destructurized solely with water, it is proposed in DE 42 37 535 for a biologically degradable polymer mixture to incorporate therein thermoplastic starch that is prepared using a suitable plasticizing agent in addition to processing to remove water, either before or during the process of mixing starch with the plasticizing agent. The proposed polymer mixtures contain thermoplastic starch, a hydrophobic polymer and a phase mediator, with a mixture of thermoplastic starch and polycaprolactone being proposed as the preferred mixture.
Aliphatic polyesters are, per se, suitable mixing components for the preparation of biologically degradable polymer mixtures, since they have a good biological degradability. However, aliphatic polyesters have only moderately appropriate material properties, for example, with regard to melting point, tensile strength, and the like, which is why even corresponding mixtures using a polymer prepared on the basis of renewable raw materials, for example thermoplastic starch, have only moderately appropriate properties, thereby again placing in question the possibility of using such blends in the field of engineering plastics.
SUMMARY AND OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a biologically degradable polymer mixture which both is flawlessly degradable biologically and in addition has good mechanical and thermal properties, so that use as an engineering plastic or as a polymer material is appropriate. A further prerequisite for suitability as a polymer material also lies in the price for the polymer mixture proposed having an acceptable magnitude.
The present invention is directed to compositions that include blends of thermoplastic starch and one or more additional polymers. The thermoplastic starch portion imparts good biodegradability and low cost while the additional polymers impart improved chemical and/or physical properties to the blends.
The term “thermoplastic starch” describes starch-based compositions made by mixing starch and with an appropriate plasticizing agent such as glycerol or sorbitol under conditions that yield a thermoplastic starch melt. The water is reduced to below about 5% by weight of the thermoplastic starch composition either before or during the mixing process. For example, the starch can be predried before mixing with the plasticizing agent. Alternatively, native starch that includes its natural water content can be mixed with the plasticizing agent in a heated mixing apparatus that allows for venting of the excess moisture in the form of water vapor.
Suitable plasticizing agents include glycerol, sorbitol, sugar alcohols, hydroxy acids, salts of hydroxy acids, hydroxy acid oligomers, salts of hydroxy acid oligomers, low molecular weight polyesteramides, polyvinyl alcohol, and the like.
The one or more additional polymers mixed with the thermoplastic starch to form the inventive blends include aromatic polyesters, polyester copolymers having both aliphatic and aromatic blocks (i.e. the so-called “aliphatic-aromatic copolyesters”), polyester amides, polyethylene oxide polymers, polyglycols, and polyester urethanes.
Additional components may be added to the thermoplastic starch blends according to the invention, including additional plasticizers, stabilizers, antiflaming agents, additional biologically degradable biopolymers, plant fibers, organic fillers, cellulose esters, polyvinyl alcohol, polyvinyl acetate, polyacrylates, hydrophobic proteins, gelatin, zein, polysaccharides, polylactides, fatty acid derivatives, lecithin, chitosan, shellac, and cross-linking agents is further added to the thermoplastic starch/polymer blend. Suitable aliphatic polyesters may be added, including polylactic acid, polyhydroxybutyric acid, polyhydroxybutyric acid-hydroxyvaleric acid copolymer, and polycaprolactone.
In a preferred process the thermoplastic starch melt is blended with the one or more additional polymers under conditions that yield a melt having a water content of less than 1% by weight of the composition. By removing the water either before or during processing, it is possible in some cases to form a condensation reaction product between the starch and the additional polymer. This condensation reaction product acts as a phase mediator to yield a more homogenous mixture of the more hydrophilic starch and the generally more hydrophobic polymer. Alternatively, or in addition, a phase mediator comprising a block copolymer of starch and one or more polymers such as aliphatic polyesters, aromatic polyesters, aromatic-aliphatic copolyesters, polyesteramides, or polyesterurethanes may be added to the thermoplastic starch/polymer blend.
After forming a thermoplastic melt from thermoplastic starch and the at least one polymer, the blend is preferably cooled with water and formed into granulates. This causes the blend to become substantially solidified. It also allows the blend to reabsorb moisture, typically to a final water content of about 1% to about 6% by weight. Water-conditioned blends are more easily processed.
The starch/polymer blends are typically processed at temperatures in a range from about 120° to about 260° C., more preferably in a range from about 140° to about 160° C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The multitude of biopolymers or biologically degradable polymer mixtures proposed in the prior art are to a great extent constructed on the basis of starch, although native starch is hardly suitable as a technically usable polymer on its own. Starch is proposed because it is readily degradable biologically, has a favorable price, and is free of petroleum products because it is based on a renewable raw material. Because of the poor suitability of native starch as an “engineering plastic” it is proposed according to the invention to use so-called thermoplastic starch, as is proposed, for example, in PCT/W090/05161. Thermoplastic starch as described therein is obtained by processing native starch in the melt, by means of a plasticizing or swelling agent, to a homogeneous mass, where the proportion of swelling or plasticizing agent is preferably in a range from about 10% to about 40% by weight of the overall mixture. Suitable swelling or plasticizing a
Lorcks Jurgen
Pommeranz Winfried
Schmidt Harald
Biotec Biologische Naturverpackungen GmbH
Szekely Peter A.
Workman & Nydegger & Seeley
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