Process for Producing Thermoplastic Starch

The invention relates to a process for producing biopolymeric, thermoplastic starch on the basis of renewable raw materials by extrusion in the form of a strip, threads or in other geometrical shapes. This extrudate can be further processed into a granulate.

In the prior art granulates from thermoplastic starches and other modified starches are obtained by extrusion in such a manner that directly at the outlet of the extruder the hot modified starch is cut to granular pieces by a rotating blade. During the extrusion step the starch is disintegrated under pressure and temperature. Normally the natural water content of 10 to 20% is sufficient and optionally solvents, additives and plasticizers can be added. The plastic hot starch mass is subject to mechanical and/or physico-chemical damage by the reaction conditions in the extruder and the high shearing stress, i.e. as undesired side-effect an unavoidable molecular decomposition of the starch polymer is accepted. As a result, the binding strength and film strength of the extruded starch deteriorate as the film strength of the starch is reduced in dependence of the molecular weight. The twin-screw extruder experiences an above-average wear due to the high viscosity and stickiness of the destructurized starch. Therefore, either very expensive, wear-resisting machine parts must be used for the extruder casing and the screw material or the respective components are subject to an over-proportional wear entailing both high costs and non-constant quality of the produced thermoplastic starch. Due to the wear and tear problems of the extruder and the quality problems of the starch thermoplastics this process, which is for instance disclosed in GB-A-2 208 652, has so far not been widely accepted.

It is the object of the present invention to provide an improved process for producing a starch extrudate, preferably in granulate or fibrous form, such that the quality of the product is improved, the mechanical wear of the extruder equipment is reduced while increasing its capacity.

This object is achieved with the features of the patent claims.

The present invention is based on the following concept:

Thermoplastic starch is disintegrated with low viscosity and low shearing strength and is chemico-physically modified in the extrusion process. The highly viscous liquid thermoplastic starches leave the extruder in the form of a strip, filaments or in other geometrical shapes and are then conditioned to compact, stabilize and carefully after-treat them. Subsequently, the solidified starch may be granulated or further processed into fibers and adjusted in a further conditioning step to the desired residual moisture and packaged.

The invention exhibits the following advantages: protection of the thermoplastic starch, no molecular decomposition, considerably improved quality, protection of the extruder equipment, no wear as the inner friction is considerably minimized, and in particular higher yields at lower torques and energy input. For example, the process according to the present invention yields thrice the amount of the nominal output of the twin-screw extruder than currently known processes.

The invention is illustrated by way of a working example.

The pertaining drawing shows in

FIG. 1 the inventive principle of the process for producing granulated thermoplastic starch.

As shown in FIG. 1, according to the process for producing thermoplastic starch the starting materials are mixed in a mixer 1. The mixture of raw materials from renewable raw materials such as polymeric carbohydrates from corn, wheat, potatoes, tapioca, hybrid corn or other cereal or starch containing roots or tubers is used and additional suitable emulsifiers such as glycerin monostearate are added to the mixture. After mixing the homogenous mass is added to a buffer container 2 which always carries a sufficient amount of mixture for a continuous process. From said container a feed screw 3 transports the mixture to a dosage device 4 which provides a twin-screw extruder 6 with a predetermined amount of mixture via a first inlet 5. Simultaneously with the addition of the mixture further additives are fed to the twin-screw extruder 6 via a second inlet 7, in particular plastifying, solubilizing, hydrophobizing, dyeing, film-forming, film-formation enhancing and/or fibrous adjuvants. In addition, inter alia glycerin to enhance plastification and/or textile fibers to form a supporting texture can be used. Altogether, these further additives serve to further reduce the melt's viscosity and thus contribute to maintaining the molecular structure of the starch polymer by resulting in a lower shearing stress on the starch during extrusion. Also, the mechanical stress of the machine parts of the extruder is considerably reduced and premature signs of wear in the extruder casing or the extruder screw can be prevented. The percentage of renewable raw materials in the mixture is generally 5 to 95% preferably is 50 to 90% particularly 60 to 85%, but it can have a higher or lower percentage depending on the requirements on the properties of the starch product to be produced. During the reaction the mixture of raw materials is destructurized in the twin-screw extruder 6 under pressure and temperature (e.g. heating by heating devices 8) and chemico-physically modified. Appropriate temperature control devices serve to adjust the temperature profile as regards time and place in the extruder 6 to optimize the treatment, e.g. according to the adjustments in German patent application P 42 28 016.0. Immediately before leaving the twin-screw extruder 6 the viscosity of the melt is increased by removing humidity while at the same time its temperature is reduced and the starch is discharged in the form of a strip, threads or in other geometrical shapes under a pressure of e.g. 25 to 150 bar and at a temperature of e.g. 30 to 220.degree. C., preferably 30 to 99.degree. C., and optionally spun into continuous filaments.

On a treatment line 9 subsequent to the twin-screw extruder 6, e.g. in form of a revolving conveyor belt, the extrudate may be chemico-physically aftertreated by way of an airation device 10, in particular it may be conditioned to solidify and stabilize the texture.

When a granulate is to be produced, the solidified starch is crushed in a mill and granulated and put into a dryer 12 to remove liquid, volatile plasticizers and adjuvants. The drying step comprises the continuous revolution of the granules in the dryer 12, ventilation of the dryer 12, e.g. via fans 13 and exhauster devices 14 as well as by way of a heating device 15. Finally, the granules are fed to a sieve 17 via a conveyer 16 to be sorted, then they are weighed and packaged in a filling station 18. For a control of the amount of granules fed to the sieve 17 the latter is provided with an overflow container 19.

The following example is intended to illustrate the composition of the mixture and the process conditions:

In an intimate mixer 1 a mixture of 100 kg corn starch and 1 kg glycerin monostearate is produced. This mixture is continuously fed to a twin-screw extruder 6 "Continua 37" of the company Werner & Pfleiderer AG, Germany, having a screw diameter of approx. 40 mm at a flow rate of 45 kg/hr. At the same time glycerin is added at a flow rate of 15 kg/hr. By cooling the last two sections the outlet temperature at the die is adjusted to 90.degree. C. At a screw rate of 210 rpm a mass pressure of 62 bar occurs. The released filaments are aftertreated on a conveyor belt 9 and fed to a mill 11. The crushed material is continuously dried in a paddle dryer 12 to a moisture of 2%.