Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
1997-12-09
2001-06-26
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S053000, C425S00400R, C425S00400R, C425S461000, C425S463000, C425S549000
Reexamination Certificate
active
06251318
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for manufacturing biodegradable products, to an apparatus which can be used therefor and to products obtainable according to this method. The products to be manufactured according to the invention have a foamy structure. More particularly, the foamy material always comprises at least three parts: two relatively dense layers on the outside, which as it were, form a skin, and between them a foam structure as core. The dense layers are firm and strong and consist of substantially closed, small cells. The foam structure of the core is generally open, which means that the cells have burst to allow the gases evolving during the manufacture, for instance water vapor or carbon dioxide gas, to escape. The cells generally have a firm and solid cell wall due to the relatively high pressure and temperature during the process.
In this description, “gelatinization” is understood to mean a change of a natural polymer from a slightly or completely loose granular or comparable granulate form into a cohesive form which may or may not be dry and/or foamed, in which stretched polymers are present which are mutually bonded to a limited extent, if at all. That is to say, a transition occurs from a solid substance, a colloidal solution or suspension to a more homogeneous fluid mass. Depending on the polymers used, “gelatinization” should therefore be understood to include, for instance, gelling, gellating and the like.
In foamed products where only gelatinization occurs, as a result of gas evolution, bubbles are formed in the mass to be foamed, substantially after gelatinization. This process occurs at relatively low temperatures and pressures. Over the entire cross section, such products have approximately the same structure of relatively small cells with walls of substantially uncross-linked natural polymers.
In this description, “baking” is understood to mean a method in which both gelatinization and cross-linking occur, at relatively high temperature and/or pressure. As a result, the formation of gas arises relatively soon, so that bubbles are already formed prior to or during gelatinization. As a result of inter alia the high pressure adjacent strongly heated parts, the polymers cross-link quickly when using a mold or like baking form with a temperature at or above the baking temperature.
These baked products have a core with relatively large cells, enclosed between skin parts with relatively small cells. The cell walls have a relatively high density and the natural polymers included therein are cross-linked to a high extent, which means that they have entered into mutual chain bonds. Such a baked product therefore has a sandwich-like structure.
International patent application 91/12186 discloses a method for manufacturing biodegradable products by heating in a baking mold a batter which comprises at least natural polymers in the form of starch or derivatives thereof. The batter is introduced into an open platen set, for instance, a wafer iron, whereafter the platen set is closed and the batter is “baked”. This results in a thin-walled product which is biodegradable and yet firm and is relatively well resistant to moisture, at least as long as the skin of the product is not damaged. The product is ready immediately and so requires no post-treatment. Owing to the heating to relatively high temperatures, a structure of blown cells and cross-linked starch is created in the product. The products are relatively cheap to manufacture, have good storing properties under different conditions, are light and convenient in use and, owing to their biodegradability, are environment-friendly.
A disadvantage using of platen sets is that the batter is introduced into an open mold which is subsequently closed and, for instance, is passed through a continuous oven, where it is heated, for instance by gas burners. Energetically speaking, this is not very efficient and moreover the temperature in the baking mold is not properly controllable and may vary considerably during the baking process, which is disadvantageous to the quality of the products. Moreover, the products which are obtained according to this method are not particularly dimensionally stable and allow no or only very slight differences in wall thickness, because otherwise no homogeneous structure can be obtained. A further disadvantage of this method is that the introduction of the batter and the removal of the product is very laborious and will often lead to failure in the production. Moreover, with this method no products can be manufactured that are non-withdrawable, so that the molding freedom is limited.
European patent application 0 512 589 discloses a method for making thin-walled biodegradable products, in which platen sets are likewise used. In this known method, a starch-containing dough is introduced into an open mold cavity in one of the platens, whereafter the platen set is closed and is subsequently heated to a temperature at which only gelatinization occurs but at which the dough is not “baked”. The temperature is therefore kept relatively low with respect to the previously described method. With this method, products are obtained which are directly ready for use, that is, they do not require any post-treatment. It is true of the products obtained by this procedure too that they have little dimensional stability and permit no, or only very slight, differences in wall thickness in order to preserve a homogeneous structure. Since the products are not “baked”, they are less stiff and exhibit relatively poor resistance to, for instance, water and varying temperature conditions. Moreover, it is true of this method too that filling the platen sets and removing the products is cumbersome and time-consuming, that the products can easily be damaged when being removed and cannot be non-withdrawable, so that the freedom in the molding design is limited.
International patent application 93/08014 discloses a method for manufacturing biodegradable products, in which the products are manufactured by extrusion of a mixture comprising at least starch or derivatives thereof. In this method, a dry, crude starch with less than 30% water is mixed with mild acid, which mixture is stirred with a carbonate which, through reaction with the acid, can give rise to CO
2
gas. This mixture is introduced into an extrusion tank and mixed with water, while being pressurized and heated to such an extent as to give rise to gelatinization of the starch. In the extrusion tank the acid is reacted with the gelatinized starch, in such a manner that the average molecular weight thereof decreases and the uniform bonds of the starch chains are broken, while moreover, through reaction with the carbonate, CO
2
gas is produced for blowing up the modified starch. The thus obtained mixture of blown starch with altered (micro) structure is thereafter forced through an extrusion die, whereby under the influence of the CO
2
gas a closed-cell structure is obtained with a density of less than 0.032 g per cubic centimeter. Owing to this structure, the thus obtained product has elastic properties and permits of rapid biological decomposition.
A disadvantage of this known method is that the raw materials are to be supplied in relatively dry form and in the extrusion tank, are to be mixed with water under simultaneous increase of the temperature in the tank, whereby the desired gelatinization occurs. To that end, the mixture must be heated, which is difficult to effect homogeneously in view of the relatively large mass. As a consequence, the process is relatively poorly controllable. A further disadvantage is that the products obtained in this way have only limited durability and are not water-resistant and moreover are not particularly dimensionally stable. As a result of the extrusion process and the strong expansion occurring after the extrusion, the freedom of design in this method is limited.
European patent application 0 118 240 discloses a method for manufacturing biodegradable medicament capsules and like products by in
Arentsen Johan Hendrik
Huisman Jan Wietze
Kuhns Allan R.
Michaelson Peter L.
Michaelson & Wallace
Vertis B.V.
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