IE893340L

IE893340L - Thermoplastically workable starch and a method for the¹manufacture thereof

Info

Publication number: IE893340L
Application number: IE893340A
Authority: IE
Inventors: Ivan Tomka
Original assignee: Perfectly Possible Ltd
Priority date: 1988-11-03
Filing date: 1989-10-17
Publication date: 1990-05-03
Also published as: NO176401C; DK152290D0; CA2001587C; HU896292D0; EP0397819A1; MX169636B; KR900701901A; HU210587B; ATE121432T1; FI101229B; DE58909191D1; AU4427989A; JPH0764952B2; CA2001587A1; DK173567B1; HUT56122A; CN1042370A; WO1990005161A1; FI101229B1; BR8907146A

Abstract

In the manufacture of thermoplastically processable starch, an additive is essentially mixed with native or natural starch and the mixture is caused to melt by the application of heat and mechanical energy. The additive is a substance which lowers the melting point of the starch so that the melting point of the starch together with this additive lies below the decomposition temperature of the starch while, in addition, the additive has a solubility parameter of over 15 cal<1/2>cm<-3/2>. Once the starch-additive mixture has melted, the molten substance is mixed until it is at least virtually homogenous. In the melting range of the starch-additive mixture, the vapour pressure of the additive should be lower than 1 bar.

Description

CO 735 1 The invention relates to starch for thermoplastic processing, to a method of manufacturing starch for thermoplastic processing, to a method for manufacturing granules, flakes ©fc. and moulded bodies, extruclates, films ate. from starch, and to moulded bodies, fillers and carrier materials consisting 5 substantially of starch.

Natural or native starch, such as are obtained by washing and drying the raw starch of, for example, potatoes, cereals, maize ate., has a prominent macromolecular structure, in which the rnacromoteeutes do not or barely penetrate one another. This structure means that native starch is very 1 o inhomogeneous, and this inhomogeneity is normally at least partly retained even if the starch is melted down.

In the wake of increased use of hydrophilic polymers, which includes starch, as so-called natural plastics materials for a wide variety of applications, it has also been attempted to process starch by the known plastics processing techniques, 15 i.e. by injection moulding and extrusion for example.

Due to the properties of native starch outlined above, however, it has not been possible to manufacture a moulded body from a starch which has adequate mechanical properties, such as strength for example. According to the processing techniques currently; known for processing hydrophilic polymers, 20 such as gelatin or cellulose for example, it has been tried to homogenise starch with a specified water content, of the order of 10 % to 20 % relative to the total weight, in a worm extruder of an injection moulding machine, for example, and then to process the starch.

\ G3 1 014 801 discloses a method according to which a cooled mixture of native •» 25 starch 'containing 12 % to 15 % water, or having an insubstantially smaiGer water content, and a gelatinising promoter are processed to give a plastics-like product. According to the method described, the essential feature is that the 2 starch has a sufficient water content of at least 12 %.

In EP-A-0 118 240, a starch material and method of manufacturing it are described, the starch being suitable for manufacturing moulded bodies, such as in particular capsules for enclosing pharmaceutical products. In this method, 5 starch is plsstineci at high temperature arcl under pressure and with a water content of 5 to 30 % by weight, in order to bring the starch into the processable form required according to the invention.

FR -1 293 184 discloses the processing or plastification of starch derivatives, wherein it is proposed inter aiiato extrude "hydroxypropyl starch" with 10 % 1 o glycerol at 100°C and with a water content of 1-20 %, whereupon the plastified compound is conditioned for 24 hours at 25°C and 20 % relative humidity. The essential feature is that the plastified compound thus manufactured is water-soluble.

DE-OS-37 12 029 discloses a method of pressing moulded bodies from starch, 15 wherein a mixture of native starch containing inter alia 0.5 % lubricant and 10-22 % water is melted down in an injection moulding machine in a temperature range of 90°C to 240°C and is then processed. The essence of the invention is that the water content is retained throughout the process.

It has been found that this processing technique only produces an improvement 20 jm the mechanical properties of tie starch moulded bodies and therefore of the homogenising effect in the molten starch if the water in the intake zone of the worm piston is not driven out of the starch ami the' form of steam due to the effect of heat, but rather remains in the starch along the entire length of the piston. A further prerequisite is that extremely thorough mixing takes place, as is 25 achieved for example in a kneader or twin-shaft extruder with a correspondingly long piston. The shaft piston or kneader piston in this case forms a virtually sealed chamber, in which case the piston [length and temperature control along the piston are critical to the achievement of an adequate homogenising effect 3 This procedure for manufacturing homogeneous starch or rather thermoplastically processable starch is obviously complex and critical, but various processing parameters, such as the retention of an adequate water content, temperature, processing method, machine type, worm length etc.. are to 5 be maintained precisely. It is therefor© not surprising that in the known method according to EP-A-0 304 401 it is recommended to separate the homogenising stage or restructuring stage and the subsequent processing of the molten starch in order to obtain satisfactory properties an the starch moulded bodies. The essence of the process mentioned is that the first stage of the process is carried 1 o out in what is known as a sealed system, so that the water cannot escape in the form of steam, for example.

The high water content of 10-20 % required in the process mentioned, as is generally known in plastics processing technology, is not necessarily advantageous either for processing nor for tie properties of the moulded bodies 15 to be manufactured. In particular, a water corriant of the order of 17 % or more for example prevents practical extrusion oflr 'the starch, e.g. for the manufacture of films, profiles or tubes. In general, it can be stated that an open method of processing, as is the case with extrusion for example, is rendered impossible due to the high vapour pressure of water of significantly over 1 bar. But if the 2 o water content is at least 25 % by weight, the mixture can be processed at 108°C in the "open" system, although the excruciate is sticky and loses its shape at. 20°C. it is therefore an object of the invention to create a thermoplastically prooessable s&arch and a method of manufacturing same without the above-25 mentioned disadvantages and in order to permit a simpler processing method and problem-free processing of the starch according to the known polymer processing techniques.

This ofDjeet ts achieved according to the invention by means of a method according to the wording of claim 1 and by thermoplastically prooessable starch 3 o according to the wording of claim 22. \ 4 A method is proposed for manufacturing themioplasticallv processable, homogeneous starch, which has a crystalline content in the starch of less than S %. By means of 'the crystaliinity, it can be established whether the starch is adequately homogenised and therefore whether the starch is thermoplastically 5 processable. Whereas native starch is highly crystalline, thermoplastically processable starch has virtually no crystalline content. According to the method, native or natural starch is mixed with at least 10 % by weight of at least one additive, relative to the total weight of the mixture of starch and additive, and is melted down by the application of heat, in which case the adciitive(s) has or 1 o have a solubility parameter of more than 30.7 (MPa)(- 15 (ealcm -3/2)) and upon mixing with the starch, reduces or reduce its melting point in such a manner that the melting point of the starch/addiitive mixture is below the decomposition temperature of the starch. The vapour pressure of the additive(s) in the mixture together with the starch is lass than 1 bar in the melting 15 temperature range of the homogeneous mixture. Finally, the water content of the starch at the time of mixing with the melt must be reduced to less than 5 %.

For this reason, sufficient additive must be present in the melt which displaces or replaces the water in order to reduce the water content of the melt to less than % relative to the starch. By adding additive or additives, in particular during 20 melting and mixing, the water content of approx. 17 % naturally occurring in starch is removed.

Preferably the solubility parameter of the additive in a temperature range of 100°C to 3G0°c is of the order of 30.7 to 51.0 (MPa) ^ (= 15 (eal 1'2cm -^)).

In particular, if the melting and mixing of the search with at least one additive is 25 carried out in an open environment, i.e. not under pressure, the vapour pressure of the additive in the melting range of additive and starch, in which mixing or processing of the thermoplasticaNy processable starch takes place, must be less than one atmosphere in order that the additive does not escape uncontrollably from the mixture with the starch.

It is further proposed that the at least one additive is so selected that the interfaciai energy between the additive and starch is not greater than 20 % oi the individual interracial energies relative to air. If the additive meets this 5 requirement, it is ensured that the interaction between the additive and starch is satisfactory.

The mixing of natural or native starch with the additive can be so effected that the mixture is fed to a plastics processing machine, such as a single or twin-shaft extruder or a kneader for example arid the mixture is mixed therein, for 1 o example in the shaft piston or kneader piston, to produce an at least almost homogeneous thermoplastic compound. Particularly rf the vapour pressure of the additive in the temperature range in which mixing is carried out is smaller than ore atmosphere, mixing can be carried out in any open or sealed vessel. Only if the vapour pressure of the additive in the temperature range in which 15 mixing is carried out is greater than one atmosphere do-as the vessel have to be sealed, so that the additive does not escape from the mixture. 8t is further proposed to use at least ore of the following substances as an additive: glycerol, formamide or N-methyl formamide.

Preferably, the 10 to 35 % by weight (relative to the total weight) additive(s) is 2 o added to the starch for mixing. .

It is further proposed to add to the starclh/&dditiv@ mixture a plastifier, in which case it te proposed in particular to add 0.5 to 15 % by weight (relative to the total weight of the starch/additive mixture) of one of the following substances, for example: sorbitol, glycerol, glycerol mono- or diaoetate, a polyalkyfbxide or a-25 citrate.

Preferably, mixing and homogenisaoon of the starch with the additive is carried out in a temperature range of 120 to 220°C. 6 According to a preferred modified embodiment, the starch is mixed with glycerol in a quantity of 10 to 35 % by weight of the total weight of the mixture and is melted down and then mixed with the rnelt until an ai least almost homogeneous mixture is obtained, in which after cooling the crystalline content 5 of the starch is less than 5 %.

Further preferred embodiments of the method according to the invention are characterised in the dependent claims 2 to 21.

Preferably, the natural or native starch together with the at least one additive and optionally further additives is fed to a plastics processing machine, such as 10 a single- or double-shaft extruder or a kneader, are melted down and are mixed into a homogeneous thermoplastic compound.

It is further proposed to add to the mixture composed c# starch ami additive at least one further additive, such as for example a filler, a lubricant, a plastifier, a softener, a pigment or other colourant and/or an agent to assist removal from the mould.

Hi® following materials are suitable as fillers: a polysaccharide, a cellulose derivative, a synthetic polymer which is at least almost soluble in an additive for starch, ami/or a gelatin phthalate.

It b proposed to add 0 to 50 % by weight filler, preferably 3 to 10 % by weight, 2 o relative to the total weight of the mixture of starch and additive.

It is further proposed that magnesium oxide is added to the starch/additive mixture in a concentration of 0.02 to 3 % by weight, preferably 0.02 to 1 %, of s tth© total mixture.

Suitable plastifying agents are in particular: a polyalkyl oxide, glycerol, glycerol 25 monoacetate or diaoetate, sorbitol or a citrate, which are added in a 7 concentration in the range from 0.5 to 15 % by weight, preferably 0.5 to 15 % by weight, of the total weight of the starch/additive mixture.

CO To colour the starch/additive mixture, in particular organic or inorganic pigments are suitable, with a concentration of the order of 0.001 to 10 % by weight, 5 preferably 0.5 to 3 % by weight.

To improve the flow properties, an animal or vegetable fat and/or lecithins, preferably in hydrogenated form, are used, these fats and other fatty acid derivatives preferably having a melting point of more than 50°C. in order to reduce the hydrophilic properties, and therefore the water 1 o permeability of the thermopSastically processable starch during and after its processing, it is further proposed to add to tie starch/additive mixture a cross-linking agent or an agent such as alkyl siloxane, for example, in order to modify the starch chemically As cross-linking agents, the following substances are particularly suitable: 15 divalent and polyvalent carbonic acids and/or their anhydrides, acid halsdes and/or acid amides of divalent or polyvalent inorganic acids, epoxy resins, formaldehyde and/or urea derivatives, diviny! sulphones, isocyanates, oxo-Gompoisntis, such as acetone formaldehyde or multivalent oxo-compourids and/or cyanamid©. If acid amides of carbonic acids are used, an acid group 20 should be available as a free esrboxyl group.

The processes proposed above for manufacturing thermoplastically processable starch are suitable in particular for manufacturing granules, flakes, pills, powder, tablets or fibres from thermoplastically processable starch.

\ The thermoplastically processable starch manufactured according to the - 25 process described previously can be further processed direct according to the conventional, known plastics processing methods. However, it should be ensured particularly in the extrusion of tubes, films and the like that the additive 8 lias a vapour pressure at the processing temperature of less than one atmosphere in order to prevent foaming of the starch, if the water content is too high, for example, the formation of bubble films, tubes etc. is not possible. By adding a sufficient quantity of additive, the absorption of water which ss a natural 5 property of starch is at teas! partly prevented.

Furthermore, thermoplastically processable homogeneous starch is proposed which has a crystalline consent in the starch of less than 5 % and which is obtainable by the homogeneous mixing of starch and/or a starch derivative with at least 10 % by weight, relative to the total weight, of at least one additive, 1 o wherein the additive(s), when mixed with the starch, reduces or reduce the melting point in such a manner that the melting temperature oi the starch/additive mixture Is below the decomposition temperature of the starch, and has or have, in the mixture with the starch in trie melting temperature range of the mixture, a vapour pressure which is less than 1 bar. During mixing in the 15 melt, the water content of the starch must to reduced to less than 5 %.

It is proposed that the molar mass of the starch used is in a range of greater than 1 rraPjon, preferably in a range of 3 million to 10 million.

Tii© solubility parameter *&, as is known, is composed substantially of the three components, in particular a polar component 8^, a component 6^ 20 corresponding to the hydrogen bridge-ring structures, and a dispersion component % or is a function of these three dimensions. According to the invention, it is proposed that the component ftp corresponding to polar interactions and the component corresponding to hydrogen bridge-ring structures are larger than the component % of the solubility parameter b of the 25 at least one additive corresponding to the dispersion forces. Preferably, and ff*- are twice as large as the dispersion component % Further embodiments according to the invention of thermoqlassfcaily 9 processable starch are characterised in the dependent claims 23 to 25.

The thermoplastically processable starch may take the form of granules, flakes, pills, tablets, powder or fibres.

The thermoplastically processable starch manufactured according to the 5 process described above is particularly suitable as a filler or as a formulation agent for adding to thermoplastic or duroplastic polymers. Its suitability as a filler or formulation agent arises particularly from the fact that the properties of the thermoplastically processable starch are controllable via the molar mass or molar mass distribution of the starch, which is relatively narrow. 1 o The thermoplastically processable starch is furthermore suitable as a carrier material, for example, for active ingredients, for example pharmaceutical active ingredients, and reagents, such as flaking agents for effluents. in the same way, thermoplastically processable starch is suitable for bonding water in a water-deprived environment and/or to a water-permeable substrate. 15 The starch is extruded, for example, to form large-area films or meshes and is Said on the substrate, consisting for example of sand or gravel. Due to hydrophilia, the starch bonds to water, whereby for example irrigation can be carried out more efficiently in desert-like regions.

The process according to the invention will now be explained further by means 20 of a few main research results with reference to tables.

Table I shows the effect of additives on the melting temperature of native starch.

Table II shows the homogenisation process with different additive quantities and different homogenising conditions and their effects on the molar mass and erysialliniiy of the starch, Table III, selected mechanical values of homogenised and insufficiently homogenised starch, and Table IV, the dependence of the shear viscosity on the homogenisation temperature and composition of the starch. in order to examine the effect of additives on native starch, it is necessary to remove the water content of approx. 17 % naturally present in starch. This is carried out on the one hand by adding additives, in particular in the melting down operation and in mixing, and on the other hand by the conventional drying methods. According to the properties required in the moulded body fo be produced, such as sn particular thermal and mechanical properties, preferably approx. 10 to 25 % additive is mixed with the native starch, and thereby the water content of the starch is reduced by the addition of the additives. The melting temperature of the stanch can also be substantially influenced thereby, which has an effect on the processing of the starch on the one hand and on the hot-forming strength of the moulded bodies on the other hand.

This effect has been examined more closely by adding five different additives. First, the native starch was completely dried in order to exclude the effect of water. Then to the dried starch was added 10 % by weight in each case of addftive, and the mixture was slowly heated, the supply of heat being carried out accurately. Thus the thermal transformation could be precisely followed, and the temperature range at which the mixture is completely molten ootid be deduced. As additives, DMSQ, glycerol and ethylene glycol, and as a comparison test, propylene glycol and butylene glycol were used. The thermal transformation ranges measured, and therefore the effect of the additives on the melting temperature of the search are compiled in Table I. Finally, it should be noted in this case that the tower start of a peak is associated with brittle temperature, whereas melting takes place im the range at the upper end of the Off the additives used, ethylene glycol lowers the melting point of starch the most, whilst the use of butylene glycol produces a relatively high melting range 11 of approx. 200°C. Also investigated was the addition of propylene carbonate, in which case ths starch decomposed before melting. Lowering of the melting temperature is obviously associated with the effect of the molecular structure in the native starch, but this effect has not been fully investigated before.

If the mixture composed of starch and additive is processed further, in the homogenising operation, i.e. during mixing of the starch/additive melt, attention must be paid to the resulting melting temperature.

Thus for example mixtures of propylene glycol and native starch with a propylene glycol content of 10 to 20 % were fed to a kneader and then mixed at 1 o -j 750Q According to the quantify added, the dwell time of the mixture in the kneader was 40 to 100 sec., and by mixing in more additive, the processing temperature could be lowered in order to achieve sufficient homogeneity in the melt. In another example, glycerol was added, and accordingly it was possible to reduce the processing temperature in the kneader. From experience, it has 15 been found that the addition of 1 % additive produces a reduction in the melting range of approx. 10°C, or that at the same shear velocity, the same viscosity of the mixture at 1 % more additive is achieved even at a temperature which is approx. 10°C lower.

Toe average power in the kneader for the above-mentioned tests was approx. 20 10 kW per 100 kg mixture of March and additive. The homogeneity of the melt was tested by manufacturing test bodies and subjecting these to strainfetress tests. In the rang© where this measured mechanical properties, i.e. tensile strength or tear-resistance could not be further substantially improved, accordingly it could be deduced that the melt was sufficiently homogeneous. 2 5 On the basis of this strain/stress test, sufficient reference data can be processed to be abSe to make deductions concerning the appropriate dwell time in the extruder or kneader based on a specified composition of the starch/additive melt. 12 Table III shows the hornogenisation of starch, carried out under different homogenising conditions, and their effects on the homogeneity of the resulting thermoplastic starch.

Potato starch is used, in which case the potato starch P3 together with 15 % 5 additive covers a melting range of approx. 180°C, and potato starch P4 together with 15 % additive covers a melting range of approx. 195°C.

In column A the composition of the starch/additive mixture is indicated, the value A = grammes additive per (gramme starch 4- gramme additive). A value of A -0.15 therefore means 15 grammes additive to 100 grammes mixture consisting 10 of starch and additive. As an additive, a mixture of the previously mentioned, preferred additives is used, which contains a solubility parameter of eal. cm "^2 at -j 5G°c.

The temperatures T3 toT0 represent temperature reference values of the adjusted temperatures of the individual zones of the riornogenising apparatus. 15 As homogenising apparatus a kneader is used. is equal to the temperature of the compound upon discharge from the kneader.

B is equal to the speed of rotation of the kneader shaft in rpm.

C is equal to the power applied to the compound in the form of mechanical work 20 an (kW).

D is equal to the compound flow in the kneader, i.e. output of the melt through the kneader in (kg/h).

E is equal to the limit viscosity of the thermoplastic starch after leaving the kneader, measured in solution in 0.1 nKOH at 60°C after a dissolving time of 13 one hour at 110°C in the Ubelhc-der capillary viscometer, measured in (cm3 per gramme). E is a measure of the molar mass (M) of the thermoplastic starch in (g/Mole). The associated correlation is equal to: E = 0.2 x Mq0 4 (Mq = mean weight of molar mass).

E of native untreated starch is 260, which results in a value for Mq of native untreated starch of 8 x 10'.

F means the crystallite content as a percentage. From the crystallinity of the starch, it can be established whether sufficient hornogenisation has taken place and therefore whether the starch is thermoplastically processable. Native starch 1 o is highly crystalline, whilst thermoplastically processable starch has virtually no crystalline content left.

Measurement of crystalline content: Method of measurement: X-ray diffraction on powder; Unit Oi measurement: intensity of scattered radiation as a function of the scatter 15 angle, Native potato starch: sharp reflexes at the scatter angles (degrees) 6, 14,17, 20, 22. 24, 26, Measure of crystallinity: Fx = surface content of the scatter intensity - scatter angle function for the sharp 2 0 reflexes of the treated starch, Fn - surface content as above for native untreated starch.

F = Fx . 100 (%) 14 Fn The F-values of the treated starch in the homogeneous thermoplastic state are less than 5 %.

Commentary on Table II: Criteria for evaluating how well or badly hornogenisation of the starch has succeeded is represented by the value F for the crystalline content VaSuas of between 0 and 5 are optimal, whilst even values greater than 5 % indicate unsatisfactory homogeneity of the thermoplastic starch.

The values found for E, i.e. for the limit viscosity, are within a rational range, but 1 o even the lowest value found for E produces an average molar mass Mq of 1.5 million.

If the various values for A are compared, i.e. for the composition of the starch mixtures with Pg and P4, it becomes clear that the values in a range from 0.25 to 0.3 pass through an optimum. Thus for example mixtures from the starch P3 15 having A-vaSues of 0.35 and 0.4 after hornogenisation still yield a high crystalline content, whereas values for A of 0.3 and 0.25 have virtually no detectable crystalline content left The same picture as obtained furthermore for starch mixtures from P4 potato starch with additive. This statement obviously cannot be generalised, but the homogenising effect does also depend not 20 insignificantly on the composition or properties of the atiMitive(s).

An essential criterion for hornogenisation is the power applied, since it is dear from the experiments that the higher the power consumption of the kneader, the better the quality of hornogenisation of-the starch. This can be deduced, for example, from samples 3a. 3b and 3c, where hornogenisation takes place 25 virtually at the same temperature range and where the composition of the starch mixture is also equal, in particular with a value for A = 0.35. An identical effect can also be derived from a comparison of samples 9 and 10. where sample 10 produces adequate homogeneity with a higher power consumption of the kneader, whereas sample 9 has a crystalline content of 25 %.

In order to achieve adequate homogeneity in the thermoplastic starch, the choice of temperatures in the homogenising apparatus is also essential, as can 5 be seen for example from a comparison of the samples 5a and 5b. Thus, for example, sample 5b produces a better homogeneity, even though less power is supplied to the kneader. The improved homogeneity in this case results largely from the significantly higher temperatures selected in the kneader.

The readings cited in Table II show clearly, however, that no generalisation can 1 o be made about the composition, selected temperatures in the kneader and power supplied. It is therefore an object in any starch/additive mixture selected to optimise the execution of the process in order to obtain a thermoplastic starch. Finally, an essential criterion is whether it is desired to set a high or a tow molar mass. The answer to this question is ultimately determined by the 15 requirements as to whether the thermoplastic starch is to be used for injection moulding or for extrusion. As is known, the more viscous polymers are more suitable for extrusion, whereas the less viscous polymers are more suitable for injection moulding.

Returning to the subject of hornogenisation of the starch, it should also be 20 mentioned that obviously the dwell time of the material in the homogenising apparatus or kneader may affect the homogeneity of the material.

Commentary on Table III: Table III shows the homogeneity or crystallinity of the thermoplastic starch on mechanical properties with reference to the illustration given by way of example 2 5 of the effect on the stress and elongation at breaking point of the starch.

Sample I is a test body composed of thermoplastic starch material which is adequately or almost ideally homogenised, and which is accordingly 16 thermoplastically processable. Sample II is a starch sample which is unsatisfactorily homogenised and accordingly has too much crystallinity.

The column "module" shows the module of elasticity of the two materials, which is conveniently virtually the same for both materials. Column A shows the composition of the samples, column E, as in Table II, snows the limit viscosity of the samples, and column F the value for the crystalline content.

G signifies the relative elongation of the material at break in (%), and H stands for the energy input applied to the material until the breaking point of the material «as reached. Th© dimension of H is (kJ/rn2).

Column I shows a compulation of the values of various measurements for samples I, which have a aygtalSine content of the order of 0 to 5 % max.. These are therefore materials which are almost ideally homogenised and are accordingly perfect for thermoplastic processing.

Correspondingly, various measurements are compiled in column II for materials which are inadequately iiomogenised.

From column G, it can be clearly deduced that th© starch materials from .column Si are considerably more fragile than the materials in column I. The starch materials in column I flintier require a higher energy input according to column H in order to be elongated to breaking point.

On the basis of the strain/stress tests, which are illustrated numerically in Table IS!, the improvement in mechanical properties of thermoplastically processable starch can be dearly deduced. Due to the reduction in crystallinity of the materia! to lass than 5 %, starch materials are obtained having adequate to good mechanical properties. These mechanical properties can obviously be further improved by the quantity of the additive and an appropriate choice of additives. 17 Since, obviously, shearing forces in the molten starch/additive mixture are responsible for the homogenising effect and therefore lead to a material with lower crystallinity, the association between the shear viscosity of the melt and the shearing effect of the homogenising apparatus needs to be explored more 5 fully. To this end, the intrinsic viscosity of the starch melt, i.e. the dependence of the shear viscosity on the shear velocity, is examined in a capillary rheometer.

As a correlation between the two values, the following equation was found: rj = K . y In this case represents the shear viscosity of the melt in (Pa x sec) and y the 1 o shear velocity in (sec"1). K is a material constant, which is occasionally also called consistency. K is calculated from the following equation: K = exp [|a (1 - _D - a (A-A0)] R T To For the term ^A/R, the value of 5.52 x 104 (degrees Kelvin was found), where R = the gas constant; and EA = thermal activation energy of the place-exchanging process of the molecules as the melt flows.

T represents the temperature of the melt (in degrees Kelvin), and To a reference temperature of 458 degrees Kelvin. a also represents a constant having the value 2.78 x 102, and A, as is known, represents the composition of the starch/additive mixture. Ao represents the composition of a reference mixture having the value A = 0.1.

It then becomes clear that the term m is a function of the temperature of the melt 18 arid the composition of the starch/additive mixture.

Table IV shows values of m as a function of T (degrees Kelvin) and of A, these values being obtained by the corresponding adjustment of the shear viscosity in the measuring apparatus to the predetermined y, i.e. the shear velocity. For m, 5 therefore, a function having the general formula: m = (g(T) + f(A) + r (T, A)) is obtained.

The values from table IV underline the already suspected fact that at increased temperature of the melt the shear velocity can be reduced in order to obtain the same shear viscosity in the melt, and with increased additive content the 1 o temperature of the melt can be reduced in order to obtain the same shear viscosity in the melt.

The starch thus homogenised, or rather thermoplastic starch, can then be further processed direct according to the conventional plastics processing methods, such as injection moulding, extrusion, film-blowing extrusion, injection blow-15 moulding, deep-drawing etc.. In this case, however, particularly in extrusion, film-blowing extrusion, injection blow-moulding, etc., it should be ensured that the additive has a vapour pressure which is significantly below 1 bar at the temperature at which the melt leaves the extrusion nozzle. The same applies with reference to the water content in the melt, which must not be too high. It * ^ should therefore also be ensured that sufficient additive is present in the melt to displace or replace the water. If the water content is too high or an additive is used having a vapour pressure which is too high, the material foams as it leaves the nozzle.

By adding further additive, as described above, the properties of the moulded 25 bodies and extrudates can be further substantially affected. Thus, for example, by adding inorganic fillers, such as magnesium oxide, aluminium, silicon etc., the transparency can be reduced or eliminated altogether. Additives of plant or animal fats improve the flow properties of the melt and improve removal of the starch from moulds. However, modifying the properties is not the primary 19 subject of the invention, so that this matter is not described more fully here.

A further essential aspect is the adding of cross-linking agents to the starch, since moulded bodies and extrudates of pure starch are not water-resistant due to their hydrophilic property. By the addition of cross-linking agents and other 5 chemical modifying agents, parts composed of starch are rendered at least partially or almost fully water-resistant and can therefore be used in practice without problems. The choice and addition of one of the cross-linking agents mentioned above substantially depends on the additive and the quantity to be added to the native starch, in which case the additives may also play a role. 1 o Temperature and dwell time in the extruder, i.e. during melting, homogenising and processing, are therefore the essential criteria for the type of cross-linking agent to be selected. In principle, during the processing of the starch, cross-linking must not progress so far that the thermoplasticity of the starch is affected in such a manner as to make subsequent processing of the starch problematic; 15 but this situation is well known in practice from processing partially cross-linkable thermoplasts, from the manufacture of powder coatings etc., so that there is no need to go into further detail here.

The previously mentioned additives and processing conditions used by way of example were only intended to illustrate the invention more fully and can be 20 varied in any manner by the use of other materials and processing conditions according to requirements. The essential point is that by adding an additive to the native starch and mixing these two materials in the melt, a thermoplastically processable starch can be created. It is also essential that the additive has a cohesion energy (tensity which permits the additive to modify the molecular 25 structure of the native starch in such a manner as to render the starch thermoplastically processable. Finally, a further requirement is that the vapour pressure of the additives, at least in open processing, is less than 1 bar in the processing temperature range.

Table I Thermal transformation (tarnp . in °C) peak from peak centre to "propylene glycol 78 142 175 ethylene glycol 40 80 120 glycerol 45 110 140 OMSO 65 80 150 *buiytene glycol 180 190 200 % by weigh! additive 0 % by weight water * control test TABLE II Starch Sample h Tl T« Tj. t6 Te B C D SS F r c) (°C) (°C) rc> <°C) (rpm) (kW) (kg/h) (g/Mol) (%) Pi 2a 0.4 130 120 110 no 105 150 8 81 193 50 P3 2b 0.4 110 98 36 no 108 150 90 165 - P, 3a 0.35 130 110 105 109 110 150 99 173 40 j 3b 0.35 130 110 105 105 106 150 8.5 90 188 50 Pg 3c 0.35 130 110 95 128 125 150 11 90 145 P, 4a 0.3 130 120 110 138 139 150 90 130 0 P3 4b 0.3 140 140 140 148 146 150 90 116 0 P*2 5a 0.25 90 88 85 143 125 150 22 90 66 5b 0.25 140 135 13S 155 150 150 18 90 89 0 6 0.4 105 95 85 109 108 150 11.5 90 239 40 P/ 7 0.35 105 95 119 114 150 14 90 153 P,' 8 0.3 140 140 130 149 145 150 16.5 90 126 0 P/ 9 0.25 140 140 135 157 150 150 90 73 P.' 0.25 90 85 85 144 125 150 22.5 90 68 K 11a 0.3 90 70 70 118 113 130 16.5 75 141 p« 12a 0.25 85 85 65 126 119 130 21 75 92 p« 12b 0.25 85 60 60 118 118 130 21.5 75 95 p^ 13 0.25 85 63 61 115 112 130 55 91 - p3 14 0.3 85 65 62 113 112 120 13 55 141 " 6 TABLE III Module (GPa) A <-) S (cm3/g) F (%) G (%) K (kJ/m2) 1 1. 6 — 2 . 2 0.15 60 - 150 0-5 4 0 - 55 500 - 650 11 1« 6 =* 2 . 2 0,15 150 =■ 200 -50 2-5 - 50 to NJ 23 TABLE IV HI T (°K) A 0.68 433 0.203 0.74 441 0.203 0.78 443 0.203 0.68 455 0„ 181 0.75 463 0.181 o 00 > 466 0.181 24

Claims

1. Claims 1. Process for manufacturing homogeneous starch having a crystalline content of less than 5 % and capable of being subjected to thermoplastic processing, characterised in that native or natural starch is mixed with at least 10 % by 5 weight of an additive, compared to the total weight of the starch/additive mixture, and is formed into a melt by the application of heat, wherein the additive has a solubility parameter of more than 30.7 (MPa) (= 15(cal. cm " 3^)) and when mixed with the starch lowers its melting point so that the melting temperature of the starch/additive mixture is below the decomposition 10 temperature of the starch, and the vapour pressure of the additive(s) in the mixture together with the starch is less than 1 bar in the melting temperature range of the homogeneous mixture, and in that the water content of the starch in the melt is reduced to less than 5 %.

2. Process according to claim 1, characterised in that the water content of 15 approx. 17 % naturally occurring in the starch is removed by the addition of an additive or additives during melting and mixing. 3. Process according to claim 1 or 2, characterised in that the starch is completely dried before mixing with at least one additive. 4. Process according to one of claims 1 to 3, characterised in thai at least one of 20 the following substances is used as an additive: glycerol, formamide or N- methyl formamide. 5. Process according to one of claims 1 to 4, characterised in that the starch is mixed with additive in a quantity of 10 to 35 % by weight of the total weight of the 25 mixture, is melted down and then mixed until the melt is homogeneous. 6. Process according to one of claims 1 to 5, characterised in thai mixing and hornogenisation of the starch melt with the additive is carried out in a temperature range of 120 to 220°C. 5 7. Process according to one of claims 1 to S, characterised in that the starch is mixed with glycerol in a quantity of 10 to 35 % by weight of the total weight of the mixture and is melted down.

3. Process according to one of claims 1 to 7, characterised in that the starch is mixed with at least one additive and melted down, the additive having a 1 o solubility parameter in a temperature range of 100 to 30G°C of 30.7 to 51.0 (MPa) (= 15-25(cal1 ^. cm ^)) and its interracial energy relative to the starch is not greater than 20 % of the individual interracial energies of the starch and additive relative to air. 9. Process according to one of claims 1 to 8, characterised in that the natural or 15 native starch and at least one additive are fed together into a plastics processing machine, such as a single- or twin-shaft extruder and a kneader, are melted down and are mixed into a homogeneous thermoplastic compound. 10. Process according to one of claims 1 to 9, characterised in that as a plasiifier, a polyalkvl oxide, glycerol, glycerol monoacetate or diacetate, sorbitol 20 or a citrate is used in a concentration in the range of 0.5 to 15 % by weight of the total weight of the mixture. 11. Process according to one of claims 1 to 10, characterised in that at least one of the following materials are added as a filler to the starch/additive mixture: 25 another polysaccharide, a cellulose derivative, 26 a synthetic polymer soluble in an additive for starch, a gelatin phthalate, more than 0 to 50 % by weight relative to the total mixture of a filler being added to the starch/additive mixture. 5 12. Process according to claim 11, characterised in that filler is added to the starch/additive mixture in a proportion of 3 to 10 % by weight of the total mixture. 13. Process according to one of claims 1 to 12, characterised in that at least one inorganic filler is added to the starch/additive mixture in a concentration of 0.02 to 3 % by weight of the total mixture. 10 1

4. Process according to one of claims 1 to 13, characterised in that magnesium oxide is added to the starch/additive mixture in a proportion of 0.02 to 3 % by weight of th® total mixture. 1

5. Process according to one of claims 1 to 14, characterised in that an organic or inorganic pigment is used in a concentration of the order of 0.001 to 10 % by 15 weight relative to the total weight. 18. Process according to one of claims 1 to 14, characterised in that an organic or inorganic pigment is used in a concentration of the order of 0.5 to 3 % by weight relative to the total weight. 17. Process according to one of claims 1 to 16, characterised in that, as a 20 means of improving the flow properties, an animal or vegetable fat, a lecithin, preferably in hydrogenated form, is used, these fats and other fatty acid derivatives having a melting point of more than 50°C. 18. Process according to one of claims 1 to 17 for the manufacture of at least partly cross-linked starch, characterised in that at least one cross-linking agent 25 is added to the starch/additive mixture. 27 \ 19. Process according to one of claims 1 to 18, characterised in that as a cross-linking agent at least one of the following substances is used: a divalent or polyvalent carbonic acid and/or an anhydride thereof, 5 - a halicte and/or an acid amide of a divalent or polyvalent carbonic acid, a derivative of a divalent or polyvalent inorganic acid, an epoxy resin, such as a multivalent glycidyl ether, formaldehyde and/or a urea derivative, 10 - an isocyanate, an oxo-compound, such as acetone formaldehyde or a multivalent oxo-com pound, cvanamide. 20. Process for manufacturing granules, flakes, pills, powder, tablets or fibres 15 from homogeneous starch capable of being subjected to thermoplastic processing, using the process according to one of claims 1 to 19. 21. Process for manufacturing moulded bodies, extrudates or films from homogeneous starch capable of being subjected to thermoplastic processing, using the process according to one of claims 1 to 20, characterised in that the 20 homogeneous starch/additive melt is processed direct 22. Homogeneous starch having a crystalline content of less than 5 % and 28 capable of being subjected to thermoplastic processing, obtainable by means of a process according to one of claims 1 to 21. 23. Starch capable of being subjected to thermoplastic processing according to claim 22, characterised in that the limit viscosity mean of the molecular weight of 5 the starch is in the range of 3 million to 10 million. \ 24. Starch capable of being subjected to thermoplastic processing according to claim 22 and 23, characterised in that it is present in the form of granules, flakes, pills and/or tablets, or as a powder, or in the form of fibres. 25. Starch capable of being subjected to thermoplastic processing according to 1 o one of claims 22 to 24, characierised in that the component 6p, corresponding to polar interactions, and the component 6 H, corresponding to the hydrogen bridge-ring structures, of the solubility parameter 6 of the at least one additive are larger than the component 6d of the solubility parameter 6 of the at least one additive. 15 2

6. Moulded body composed of homogeneous starch capable of being subjected to thermoplastic processing according to one of claims 22 to 25. 2

7. Use of the starch capable of being subjected to thermoplastic processing according to one of claims 22 to 25 as a filler for filling and/or formulating thermoplastic or duroplastic polymers. 20 2

8. Use of the homogeneous starch capable of being subjected to thermoplastic processing according to one of claims 22 to 25 as a support material for receiving additives and/or reagents. 2

9. Use of the homogeneous starch capable of being subjected to thermoplastic processing according to one of claims 23 to 25 for bonding water 25 in a water-depleted environment and/or to a water-permeable substrate, 29 wherein the starch is used in the form of a film, a mesh or other extrudate. 30. A process according to claim 1 for manufacturing homogeneous starch, substantially as hereinbefore described. 31. Homogeneous starch whenever manufactured by a process claimed in any one of claims 1-19 or 30. 32. A process according to claim 21 for manufacturing moulded bodies, extrudates or films, substantially as hereinbefore described. 33. A moulded body, extrudate or film, whenever manufactured by a process claimed in claim 21 or 32. 34. Use according to claim 28 or 29,, substantially as hereinbefore described. F. R. KELLY & CO., AGENTS FOE THE APPLICANTS.