Shaped articles made from pre-processed starch.

The present invention refers to shaped articles made from pre-processed starch.

It is known that natural starch which is found in vegetable products and which contains a defined amount of water, can be treated at an elevated temperature and in a closed vessel, thereby at elevated pressure, to form a melt. The process is conveniently carried out in an injection molding machine or extruder. The starch is fed through the hopper onto a rotating, reciprocating screw. The feed material moves along the screw towards the tip. During this process, its temperature is increased by means of external heaters around the outside of the barrel and by the shearing action of the screw. Starting in the feed zone and continuing in the compression zone, the particulate feed becomes gradually molten. It is then conveyed through the metering zone, where homogenization of the melt occurs, to the end of the screw. The molten material at the tip can then be further treated by injection molding or extrusion or any other known technique to treat thermoplastic melts, to obtain shaped articles.

This treatment, which is described in the European Patent Application No. 84 300 940.8 (Publication No. 118 240) yields an essentially destructurized starch. The reason for this being that the starch is heated above the melting and glass transition temperatures of its components so that they undergo endothermic transitions. As a consequence a melting and disordering of the molecular structure of the starch granules takes place, so that an essentially destructurized starch is obtained. The expression "pre-processed starch" defines such essentially destructurized starch obtained by such thermoplastic melt formation.

Although articles obtained by injection molding of natural starch are useful, it has been found, that the shaped articles obtained therefrom show a relatively low physical strength. It has further been found that the process itself shows a relatively high instability due to the high dependency of the melt viscosity on the shear rate within the screw barrel which renders the processing for example by injection molding or extrusion more sensitive to conditions of screw speed, temperature, pressure and/or water content and reduces the average quality of the obtained articles. In this mentioned process of injection molding starch, there are two important steps, namely (A) the destructurizing step, i.e. to heat the starch granules above the melting points and the glass transition temperatures of their components to effect the high temperature transitions of the molecular structure and (B) the molding step, i.e. to form the shaped article e.g. by injection molding.

It has now been surprisingly found that the described difficulties are overcome if the mentioned two steps are separated from each other, i.e. the destructurized starch obtained in step (A) is solidified before heating it up again in a screw barrel to finally produce the shaped article. It has been found that by separating the destructurizing step (A) from the molding step (B) a shaped article with considerably improved physical properties is obtained and the molten material in the screw barrel, when carrying out step (B) shows a much reduced dependency of viscosity on the shear rate which again reflects itself in improved flow characteristics and an improved average quality of the produced shaped articles. The present invention refers to a process of forming shaped articles from starch, which process comprises: <ul id="ul0001" list-style="none"><li>(a) heating a composition comprising a pre-processed, and essentially destructurized starch/water material, at a water content in the range of from 10 to 20 % by weight based on the weight of the composition to a temperature sufficient to essentially destructure the starch to form a melt;</li><li>(b) transferring the melt to a mold while maintaining said water content and</li><li>(c) cooling the melt in the mold to a temperature below its glass transition temperature to form a solid shaped article.</li></ul>

The invention further refers to shaped articles obtained by said process.

Such pre-processed starch/water material is obtained by thermoplastic melt formation of starch as explained hereinabove.

It is interesting to note that the improvements obtained as described above are not a function of prolonged residence time of the starch within the screw barrel. The same residence time of the starch material in the molten state in the screw barrel will result in improved product quality if the steps (A) and (B) are carried out separatively.

The term "starch" is to be understood as chemically essentially non-modified starch. As such it includes for example generally carbohydrates of natural, vegetable origin, composed mainly of amylose and/or amylopectin. It may be extracted from various plants, examples being potatoes, rice, tapioca, corn, and cereals such as rye, oats and wheat. Preferred is potato starch and corn starch, especially potato starch. It further includes physically modified starch such as gelatinized or cooked starch, starch with a modified acid value (pH), e.g. where acid has been added to lower its acid value to a range of about 3 to 6. Further is included starch, e.g. potato starch, in which the types and concentrations of the cations associated with the phosphate groups of the starch have been modified to influence processing conditions e.g. temperature and pressure.

Such starch is suitably heated for destructurization in a screw barrel of an extruder above the melting points and the glass transitions point of its components for a time long enough to effect destructurization, which is generally between 3 and 10 minutes, depending on the process parameters. The temperature is preferably within the range of about 120°C to 190°C, preferably within the range of 130°C to 190°C depending on the type of starch used. For this destructurization , the starch material is heated preferably in a closed volume. A closed volume can be a closed vessel or the volume created by the sealing action of the unmolten feed material as happens in the screw of injection molding or extrusion equipment. In this sense the screw barrel of an injection molding machine or an extruder is to be understood as being a closed vessel. Pressures created in a closed volume correspond to the vapour pressure of water at the used temperature but of course pressure may be applied and/or generated as normally occurs in a screw barrel. The preferred applied and/or generated pressures are in the range of the pressures which occur in extrusion or injection molding processes and known per se, i.e. from zero to 150 x 10⁵ N/m² preferably from zero to 100 x 10⁵ N/m² and most particularly from zero to 80 x 10⁵ N/m².

The melt of destructurized starch so obtained is extruded first (step A), cooled to solidify and cut into granules before it is further used in injection molding or pressure molding techniques (step B). The water content of the pre-processed and essentially destructurized starch/water material used according to the present invention (for step B) has a water content in the range of about 10 to 20 % by weight of the composition, preferably 12 % to 19 % and especially 14 % to 18 % by weight, calculated to the weight of the composition.

This destructurized starch/water material according to this invention is heated essentially above the melting points and glass transition temperatures of its components (step B). Such temperature is generally within the range of about 80 to 200°C, preferably within the range of about 120 to 190°C and especially at about 140 to 180°C. These temperatures when applied essentially destructure the starch to form a melt, i.e. a thermoplastic melt.

The minimum pressure (in step B) corresponds to the water vapour pressure produced at these temperatures. The process is carried out in a closed volume as explained above, i.e. in the range of the pressures which occur in extrusion and injection molding processes and known per se, e.g. from zero to 150 x 10⁵ N/m² preferably from zero to 100 x 10⁵ N/m² and most particularly from zero to 80 x 10⁵ N/m².

When forming a shaped article by extrusion the pressures are preferably as mentioned above. If the melt of the destructurized starch composition according to this invention is e.g. injection molded, the normal range of injection pressures used in injection molding is applied, namely from 300 x 10⁵ N/m² to 3.000 x 10⁵ N/m² preferably 700 x 10⁵ - 2200 10⁵ N/m².

The starch material of the present invention may contain or may be mixed with additives such as extenders, lubricants, plasticizers and/or coloring agents.

These additives may be added before the destructurizing step (step A) or after this step i.e. mixed with the solid granules of the destructurized starch. It mainly depends on the intended use of the destructurized starch.

Such additives are extenders of different kinds, e.g. gelatin, vegetable proteins such as sunflower protein, soybean proteins, cotton seed proteins, peanut proteins, rape seed proteins, blood proteins, egg proteins, acrylated proteins; water-soluble polysaccharides such as: alginates, carrageenans, guar gum, agar-agar, gum arabic and related gums (gum ghatti, gum karaya, gum tragacauth) pectin; water-soluble derivatives of cellulose: alkylcelluloses hydroxyalkylcelluloses and hydroxyalkylalkylcelluloses, such as: methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxpropylmethylcellulose, hydroxybutylmethylcellulose, celluloseesters and hydroxyalkylcelluloseesters such as: celluloseacetylphtalate (CAP), Hydroxypropylmethyl­cellulose (HPMCP); carboxyalkylcelluloses, carboxyalkyl­alkylcelluloses, carboxyalkylcelluloseesters such as: carboxymethylcellulose and their alkalimetal salts; water-soluble synthetic polymers such as: polyacrylic acids and polyacrylic acid esters, polymethacrylic acids and polymethacrylic acid esters, polyvinylacetates, polyvinylalcohols, polyvinylacetatephthalates (PVAP), polyvinylpyrrolidone, polycrotonic acids; suitable are also phtalated gelatin, gelatin succinate, crosslinked gelatin, shellac, water soluble chemical derivatives of starch, cationically modified acrylates and methacrylates possessing, for example, a tertiary or quaternary amino group, such as the diethylaminoethyl group, which may be quaternized if desired; and other similar polymers.

Such extenders may optionally be added in any desired amount preferably up to and including 50 %, preferably within the range of 3 % to 10 % based on the weight of all components.

Further additives are inorganic fillers, such as the oxides of magnesium, aluminum, silicon, titanium, etc. preferably in a concentration in the range of about 0.02 to 3 % by weight preferably 0.02 to 1 % based on the weight of all components.

Further examples of additives are plasticizers which include polyalkylene oxides, such as polyethylene glycols, polypropylene glycols, polyethylene-propylene glycols; organic plasticizers with low molecular weights, such as glycerol, glycerol monoacetate, diacetate or triacetate; propylene glycol, sorbitol, sodium diethylsulfosuccinate, triethyl citrate, tributyl citrate, etc., added in concentrations ranging from 0.5 to 15 %, preferably ranging from 0.5 to 5 % based on the weight of all the components.

Examples of coloring agents include known azo dyes, organic or inorganic pigments, or coloring agents of natural origin. Inorganic pigments are preferred, such as the oxides of iron or titanium, these oxides, known per se, being added in concentrations ranging from 0.001 to 10 %, preferably 0.5 to 3 %, based on the weight of all the components.

The sum of the plasticizer and water contents should preferably not exceed 25 %, and should most preferably not exceed 20 %, based on the weight of all the components.

There may further be added compounds to improve the flow properties of the starch material such as animal or vegetable fats, preferably in their hydrogenated form, especially those which are solid at room temperature. These fats have preferably a melting point of 50°C or higher. Preferred are Triglycerides with C₁₂ -, C₁₄-, C₁₆-, and C₁₈- fatty acids.

These fats can be added alone without adding extenders or plasticizers.

These fats can advantageously be added alone or together with mono- and/or diglycerides or phosphatides, especially lecithin. The mono- and diglycerides are preferably derived from the types of fats described above, i.e. with C₁₂ -, C₁₄ -, C₁₆ -, and C₁₈ - fatty acids.

The total amounts used of the fats mono-, diglycerides and/or lecithins are up to 5 % and preferably within the range of about 0.5 to 2 % by weight of the total composition.

It is further recommended to add silicon dioxide or titanium dioxide in a concentration of about 0.02 to 1 % by weight of the total composition. These compounds act as texturizing agent.

The materials described herein above form on heating and in a closed vessel a melt with thermoplastic properties, i.e. under controlled water-content and pressure conditions. Such a melt can be used in various techniques just like thermoplastic materials. These techniques include injection molding, blow molding, extrusion and coextrusion (rod, pipe and film extrusion), compression molding, to produce known articles as produced with these techniques. These articles include bottles, sheets, films, packaging materials, pipes, rods, laminates, sacks, bags, pharmaceutical capsules.

The following examples further explain the invention.

Example 1

(a) Preparation of destructurized starch.

Natural potato starch, a lubricant/release agent (hydrogenated fat) and a melt flow accelerator (lecithin), are mixed together in the relative proportions in a powder mixer for 10 minutes so that a composition consisting of 81,3 parts of natural potato starch, one part of the hydrogenated triglyceride containing the fatty acids C₁₈: C₁₆: C₁₄ in a ratio of 65:31:4 weight percent, 0.7 parts lecithin, and 17 parts water in the form of a freely flowing powder is obtained. This material was then fed to the hopper of an extruder. In the screw barrel the powder was melted. The temperature within the barrel was measured to be 165°C, the average total residence time was 12 minutes (approx. 10 minutes heating time, approx. 2 minutes in molten state) and the pressure generated was equal to the vapour pressure of the moisture present in the volume of the extruder barrel. The melt was then extruded, and cut into granules of an average diameter of 2 to 3 mm. The material was hard, white with a fine foamed structure. The water content was 12 %, as water was allowed to escape when the melt left the extruder nozzle. The obtained granulated material was then conditioned to a water content of 17 %.

(b) Injection molding of the granules obtained under (a) above

The material obtained under (a) above was fed into the hopper of an injection molding machine. The material was formed into a melt within the screw barrel. The temperature there was kept at 165°C, the pressure at 75 x 10⁵ N/m²; the average residence time was 7 1/2 minutes (approx. 5 minutes heating time, approx. 2 1/2 minutes molten state). The melt was injected into a mold so that test pieces were produced suitable for testing their physical properties (stress/strain behaviour) on an INSTRON tensile listing apparatus. The samples were conditioned at 13.5 % water content and measured at room temperature using an extension rate of 10 mm per minute. <ul id="ul0002" list-style="none"><li><u style="single">Fig. 1</u> shows the stress/strain diagram for a material produced according to Example 1(b).</li><li><u style="single">Fig. 2</u> shows the stress/strain diagram for a further material obtained according to Example 1(b).</li></ul>

The test pieces were of standard DIN design (DIN No. 53455). Each group shows results from three samples injection molded under the same processing conditions as described above under (b) using the pre-processed (destructurized) starch as obtained under (a). It is immediately apparent that the test pieces are well reproducible in properties and the extension to break is about 30 %. This is consistently and remarkably higher than the results obtained in the comparative Example 2. Other processing conditions, e.g. injection molding residence time 600 sec., screw speed 75 rpm gave analogous results.

Example 2

(comparative test to Example 1)

The same starting composition as described in Example 1 (a) was fed into the hopper of an injection molding machine and test pieces as obtained under Example 1 (b) were directly produced in a single step process. The temperature in the screw barrel was 165°C, the pressure 75 x 10⁵ N/m², the residence time was 12 1/2 minutes (approx. 8 minutes for heating, approx. 4 1/2 minutes in the molten state). The stress/strain behaviours are shown in the Figures 3 and 4. <ul id="ul0003" list-style="none"><li><u style="single">Fig. 3</u> shows the stress/strain diagram for a material obtained according to Example 2.</li><li><u style="single">Fig. 4</u> shows the stress/strain diagram for a further material obtained according to Example 2.</li></ul>

It can be seen from these results that the obtained extension values to break are relatively low and inconsistent and remarkably inferior to those obtained according to Example 1.

Example 3

The Examples 1 and 2 were repeated but the starting composition in Example 1(a) was replaced by the following components: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">- natural potato starch:</entry><entry namest="col2" nameend="col2" align="char" char=".">80.0 parts</entry></row><row><entry namest="col1" nameend="col1" align="left">- lubricant/release agent (hydrogenated fat):</entry><entry namest="col2" nameend="col2" align="char" char=".">1.0 parts</entry></row><row><entry namest="col1" nameend="col1" align="left">- lecithin:</entry><entry namest="col2" nameend="col2" align="char" char=".">0.7 parts</entry></row><row><entry namest="col1" nameend="col1" align="left">- titanium dioxide:</entry><entry namest="col2" nameend="col2" align="char" char=".">0.3 parts</entry></row><row><entry namest="col1" nameend="col1" align="left">- water:</entry><entry namest="col2" nameend="col2" align="char" char="."><u style="single">17.0 parts</u></entry></row><row rowsep="1"><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="char" char=".">100.0 parts</entry></row></tbody></tgroup></table></tables> Analogous results as in Examples 1 and 2 were obtained as shown in the Figures 1, 2 (when processed analoguously to Example 1) and Figures 3 and 4 (when processed analoguously to Example 2).

Example 4

The Examples 1 and 2 were repeated with a composition containing polyvinylpyrrolidone, so that test pieces of the following composition were obtained: <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">potato starch:</entry><entry namest="col2" nameend="col2" align="char" char=".">74.6 %</entry></row><row><entry namest="col1" nameend="col1" align="left">polyvinylpyrrolidone:</entry><entry namest="col2" nameend="col2" align="char" char=".">10.0 %</entry></row><row><entry namest="col1" nameend="col1" align="left">hydrogenated fat:</entry><entry namest="col2" nameend="col2" align="char" char=".">1.1 %</entry></row><row><entry namest="col1" nameend="col1" align="left">lecithin:</entry><entry namest="col2" nameend="col2" align="char" char=".">0.8 %</entry></row><row><entry namest="col1" nameend="col1" align="left">water:</entry><entry namest="col2" nameend="col2" align="char" char="."><u style="single">13.5 %</u></entry></row><row rowsep="1"><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="char" char=".">100.0 %</entry></row></tbody></tgroup></table></tables> The stress/strain behaviour was very similar to that shown in the Figures 1, 2 when processed analoguously to Example 1 and to Figures 3 and 4 when processed analoguously to Example 2.

Example 5

Further test pieces were molded from destructurized starch as in Example 1(b) and from native starch as in Example 2 using the same processing conditions.

The molded pieces were conditioned to various moisture contents, 9.5 %, 10.8 % and 13.5 % water, and stress/strain curves determined. The results are shown in Figures 5 and 6. The results in Figure 5 from pre-extruded starch are obviously superior. A homogenous material exists at all the water contents used. The material in Figure 6 shows inferior properties (less extension and energy to break) and a less reproducible behaviour at all water contents. Such behaviour is symptomatic of a less homogenous and less coherent material.

Example 6

(processing stability)

The viscosity within the screw barrel in the molten state of the composition as described in Example 1(a) was measured as a function of the shear rate when treated (1.) as in Example 1(b) and (2.) as in Example 2. The results were obtained under well adjusted machine conditions (Netstal machine type 235/90). The melt viscosities in dependence of shear rate were calculated from the measurements using standard injection-molding theory together with measurements of refill times. <u style="single">Figure 7</u> shows the results from the two-step process according to Example 1 as well as the results from the one-step process according to Example 2. The materials processed according to Example 2 (one-step process) show higher melt viscosities with greater sensitivities to residence times and to shear rate. These higher values and sensitivities give a lower processing stability and a lower product reproducibility, which is also evident from Figure 3,4 and 6.

The shear rate dependences of the melt viscosities in the two-step process according to Example 1, are similar to those of conventional thermoplastics, e.g. polyethylene, which are known to process stably by injection molding to give reproducible products. In Figure 7 log (η/Pa s) means the logarithm to the base 10 of the value of the melt viscosity (η ) in units of Pa s; log (γ/s⁻¹) means the logarithm to the base 10 of the value of the shear rate in units of reciprocal seconds.