NL1006079C2 - Biodegradable foam moldings of thermoplastic starch. - Google Patents

Description

Biodegradable foam moldings of thermoplastic starch

The invention relates to foam moldings of a thermoplastic polymer, in particular a biopolymer such as starch, which can be used as a biodegradable packaging and filling material.

5 Foam materials for one-off packaging and filling purposes have the image of being environmentally harmful. Recycling of such materials is only very slow due to the low density (and therefore high transport costs) of these materials. The use of biodegradable foam materials in these applications is therefore a useful alternative.

WO 92/02559 describes a method for extruding and injection molding thermoplastic starch. The moldings thus obtained have a high density, i.e. more than 1000 kg / m 3, while for foam products for packaging o 10-100 kg / m is desired, and high production costs (long cycle times) for thick-walled products.

From WO 96/07693 a starch foam is known, which is obtained by extrusion of starch, after which the foamed semi-finished products obtained are glued into plates. This is a labor-intensive method, which also does not offer complete design freedom.

According to WO 95/15894, thin-walled moldings such as wafers, cups and the like are produced from thermoplastic starch by heating a mixture of 15-40% by weight of starch, 48-65% by weight of water and other components such as release agents and vegetable fibers. at 155-200 ° C. This method is not suitable for producing thick-walled moldings, such as desired for packaging and filling applications, for example.

According to EP-A-707034, foam moldings are obtained from a mixture of starch, rubber latex or polyvinyl alcohol, and water, which is first extruded, then granulated and finally brought into the desired shape in a mold under heating to 150-250 ° C. . These high temperatures are a disadvantage, especially for thick-walled products. In addition, the supply of thermal energy (heat) leads to non-homogeneous heating, when larger foamable particles (pellets or the like) are processed.

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There is therefore a need for foam moldings that are completely biodegradable and capable of being shaped into any desired shape under mild conditions, as well as a process for their manufacture.

A process has been found that meets this need, in which foam moldings of a biodegradable thermoplastic polymer are produced in a fast, semi-continuous process, by using the polymer in the presence of a substance with a permanent dipole (such as water or an alcohol). melting and elevating the elevated temperature in the form of thermoplastic granules or pellets, placing the granules or pellets in a mold and foaming them, the process being characterized in that the granules are foamed by application of electromagnetic radiation energy. This foaming method has been found to provide a rapid, homogeneous way of heating.

Particularly suitable biodegradable thermoplastic polymer is thermoplastic starch (TPS), i.e. fusible starch. The starch is, for example, from potatoes, peas, wheat, rice, corn or tapioca, and as such is used or mixed with other biodegradable polymers such as cellulose (derivatives), other polysaccharides such as guar gum, locust bean, dragant, pectin, Arab gum or other gums, natural rubber, polyesters such as polycaprolactone and polylactic acid, proteins such as gluten and casein. Proteins and starch can also be added together in the form of flour. The other polymers 20 can make up to 75% by weight of the mixture. The thermoplastic starch can be based on native starch or on physically or chemically modified starch. Chemically modified starch and starch derivatives include oxidized, carboxymethylcerd, hydroxyalkylated and otherwise derivatized starch.

Preferably a plasticizer such as a polyol (glycol, diethylene glycol or another alkylene glycol or polyalkylene glycol, glycerol, glycerol monoester, etc.), citric acid ester or urea is added to the starch, but water alone may also suffice. The amount of water is preferably 3-35% by weight relative to the total degradable polymer. The amount of additional plasticizer such as glycerol is preferably 0-25% by weight. Furthermore, an emulsifying agent such as lccithin or 3Q, a monoglyceride, a release agent such as an oil (e.g. castor oil), fatty acid or metal salt thereof (e.g. calcium stearate), a natural fiber such as flax or cellulose and / or a filler such as lime or chalk are added. Other possible additives are dyes, preservatives, and especially swelling or blowing agents such as sodium bicarbonate and nucleating agents such as talc.

The polymer can be granulated / pelletized according to known granulation or pelletization processes, for example by extrusion in a twin screw extruder at elevated temperature (60-180 ° C, in particular 100-150 ° C). The size of the grains is determined by the speed of granulation and any grinding steps after granulation. Depending on the presence of other plasticizers, the water content during extrusion can vary from 3 to 35% by weight. During this extrusion step, starch transitions from a highly ordered structure to a disordered, amorphous structure (destructuring).

The granules / pellets obtained can be conditioned to an optimum moisture content for the process. Depending on the content of other plasticizers, the moisture content after conditioning varies between 4 and 30%, and in particular between 5 and 25%. The granules / pellets are then placed in a mold with the shape of the final foam product to be manufactured. An amount of adhesive (e.g. a mixture of a native starch and glycerol) can be added to the granules to promote adhesion between the granules / pellets during foaming.

The foaming takes place using electromagnetic radiation, in particular that in the microwave range (frequency between 20 MHz and 10 GHz and in particular between 50 MHz and 5 GHz). Use is made of the radiant energy absorption of the substance with the permanent dipole such as water or glycerol, whereby this substance warms up in a very short time and turns into vapor form. In the case of water, two processes take place here: 25 - The individual granules / pellets are foamed as a result of the evaporation of water or the other dipole-containing substance. The advantage here is that no thermal energy has to be supplied from the outside. The exact boiling temperature of the dipolar substance depends on the local air pressure. The foaming process can optionally be promoted by the presence of blowing agents.

The loose granules / pellets are "welded" together to form a three-dimensional foam molding. It is important for this that the outer side of the granules is fusible, for which an at least partial thermoplastic behavior of the polymer is required.

It has been found important that the foaming process takes place quickly, i.e. within seconds. This can be accomplished by using a high power microwave source (up to, for example, 50 kW) or by using a combination of microwave generator and die in which the pressure can be varied rapidly.

After the polymer is thus brought into the desired foam form, the mold is opened and the product is taken out.

The foam products obtained by the method according to the invention can have any desired shape. An important application is that as a packaging means, for example for fragile devices, glassware, postal items, etc. The foam product may also be in the form of spheres, discs, etc., which can be used as filler in packaging. The density of the product obtained is generally between 5 and 200 kg / m, in particular between 10 and 100 kg / m. Further advantages of this foamed material are the naturally anti-static behavior, the fact that water-soluble variants are possible and the biodegradability and compostability.

Example

A mixture of 100 parts dry corn starch (75% sodium starch, 25% modified corn starch), 3 parts lecithin and 12 parts lime is placed in a counter-rotating twin screw extruder at 140 ° C after adding water to a total moisture content of 30 wt.% extruded. The granules obtained (diameter 2 mm) are brought to a moisture content of 15-16% by weight in a conditioning step. The granules are placed in a mold in an amount of 10% by weight (relative to the granules) of a 50/50 mixture of sodium starch and glycerol. The mold is placed in a microwave oven with a frequency of 2.45 GHz. After 30 seconds of irradiation, a foam product corresponding to the mold is obtained, which is then taken out.

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Claims (9)

1. A process for producing foam moldings of a biodegradable thermoplastic polymer, wherein the polymer is melted at elevated temperature in the presence of a substance with a permanent dipole and brought in the form of thermoplastic granules / pellets, the granules / pellets mold and foam it, characterized in that the granules / pellets contain a plasticizer and / or an adhesive and the granules / pellets are foamed in one step into a three-dimensional product by application of electromagnetic radiant energy. 2. A method according to claim 1, wherein the thermoplastic polymer is starch, optionally combined with a natural rubber, polyester, other polysaccharide or protein. A method according to claim 1 or 2, wherein the electromagnetic radiation has a frequency between 20 MHz and 10 GHz. 4. Process according to any one of claims 1-3, wherein the polymer is brought into the form of the granules / pellets by extrusion. A method according to any one of claims 1-4, wherein the granules / pellets contain 5-25% by weight of water just before foaming. A method according to any one of claims 1 to 5, wherein the granules / pellets also contain a blowing agent such as a bicarbonate. A method according to any one of claims 1 to 6, wherein the granules / pellets also contain a plasticizer such as glycerol or urea. A method according to any one of claims 1 to 7, wherein the substance with a permanent dipole comprises water. Foamed thermoplastic polymer molding, obtainable by the process according to any one of the preceding claims. m 1 0 0 6 0 7 9