Abstract:
The invention refers to a process of forming shaped articles from starch, which process comprises: 
     (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. 
     (b) transferring the melt to a mold while maintaining said water content and 
     (c) cooling the melt in the mold to a temperature below its glass transition temperature to form a solid shaped article 
     and articles made by this process.

Description:
This application is a continuation of application Ser. No. 07/620,230, filed Nov. 29, 1990, now abandoned, which in turn is a continuation of application Ser. No. 07/209,402, filed Jun. 20, 1988, now abandoned. 
    
    
     The present invention is directed 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 &#34;pre-processed starch&#34; 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 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: 
     (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; 
     (b) transferring the melt to a mold while maintaining said water content and 
     (c) cooling the melt in the mold to a temperature below its glass transition temperature to form a solid shaped article. 
     The invention further refers to shaped articles obtained by this process. 
     Such a pre-processed starch/water material is obtained by thermoplastic melt formation of starch as described supra. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the stress/strain behavior of starch processed at a residence time of 450 seconds and a screw speed of 75 rpm, 
     FIG. 2 illustrates the stress/strain behavior of starch processed at a residence time of 450 seconds and a screw speed of 125 rpm. 
     FIG. 3 illustrates the stress/strain behavior of starch processed at a residence time of 750 seconds and a screw speed of 75 rpm. 
     FIG. 4 illustrates the stress/strain behavior of starch processed at a residence time of 750 seconds and a screw speed of 100 rpm. 
     FIG. 5 illustrates the stress/strain behavior of starch processed at a residence time of 450 seconds, screw speed of 75 rpm, temperature 165° C. and pressure 75×10 5  N/m 2 . 
     FIG. 6 illustrates the stress/strain behavior of starch processed at a residence time of 750 seconds, screw speed 75 rpm, temperature 165° C. and pressure 75×10 5  N/m 2 . 
     FIG. 7 illustrates melt viscosity versus shear rate for one-step and two-step processes. Lines (1), (2) and (3) represent a one-step process at residence times of 750, 600 and 450 seconds, respectively. Lines (4) and (5) represent a two-steo process at residence times of 750 and 450 seconds, respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is-interesting to note that the improvements obtained as so described 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 &#34;starch&#34; is to be understood as chemically essentially non-modified starch. As such it includes for example carbohydrates of natural, vegetable origin, composed mainly of amylose and/or amylopectin. It may be extracted from various plants, such as 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 150×10 5  N/m 2 , preferably from zero to 100×10 5  N/m 2  and most preferably from zero to 80×10 5  N/m 2 . 
     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 more preferably 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 with the range of about 140° to 180° C. These temperatures will 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 i.e. in the range of pressures which occur in extrusion and injection molding processes such as from zero to 150×10 5  N/m 2  preferably from zero to 100×10 5  N/m 2  and most preferably from zero to 80×10 5  N/m 2 . 
     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 injection molded, the range of pressures used is from 300×10 5  N/m 2  to 3000×10 5  N/m 2 , preferably 700×10 5  to 2200 10 5  N/m 2 . 
     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, depending on the intended use of the destructurized starch. 
     The extenders suitable for use herein include 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, such as alkylcelluloses hydroxyalkylcelluloses and hydroxyalkylalkylcelluloses, such as methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxpropylmethylcellulose, hydroxybutylmethylcellulose, cellulose esters and hydroxycellulose esters such as cellulose acetylphthalate (CAP), hydroxypropylmethcellulose (HPMCP); carboxyalkylcelluloses, carboxyalkylcelluloses, carboxyalkylcellulose esters such as carboxymethylcellulose and their alkali metal 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, and phtnulated 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 be added in any desired amount, preferably up to and including 50%, more preferably within the range of 3% to 10% based on the weight of all components. 
     Further additives include inorganic fillers, such as the oxides of magnesium, aluminum, silicon, titanium, preferably in a concentration in the range of about 0.02 to 3% by weight, more preferably about 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, added in the range of from about 0.5 to 15%, preferably ranging from about 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. 
     Additional compounds may further be added 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 a melting point of 50° C. or higher. Preferred are triglycerides with C 12  -, C 14  -, C 16  -, and C 18  -fatty acids. 
     These fats may be added without extenders or plasticizers. 
     Alternatively, these fats may advantageously be added 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 C 12  -, C 14  -, C 16  -, and C 18  -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 agents. 
     The materials described herein on heating and in a closed vessel, (i.e., under controlled water-content and pressure conditions) form a melt with thermoplastic properties. Such a melt may 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 and pharmaceutical capsules. 
     The following examples further explain the invention. 
     EXAMPLE 1 
     (a) Preparation of starch granules. 
     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 C 18  : C 16  : C 14  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 found to be hard, and have a white color with a fine foamed structure. The water content was 12%, as water was allowed to escape when the melt left the extruder nozzle. The granulated material so obtained 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 75×10 5  N/m 2 , the average residence time was 71/2 minutes (approx. 5 minutes heating time, approx. 21/2 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. 
     FIG. 1 shows the stress/strain diagram for a material produced according to Example 1(b) with a residence time of 450 seconds, a screw speed of 75 rpm, a break strain of 32.5±2.0%, a break stress of 40.0±1.0 MPa and a break energy per unit area of 450.0±30.1 KJ/m 2 . 
     FIG. 2 shows the stress/strain diagram for a further material obtained according to Example 1(b) with a residence time of 450 seconds, a screw speed of 125 rpm, a break strain of 29.4±2.0%, a break stress of 39.3±0.2 MPa and a break energy per unit area of 401±30.6 KJ/m 2 . 
     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 75×10 5  N/m 2 , the residence time was 121/2 minutes (approx. 8 minutes for heating, approx. 41/2 minutes in the molten state). The stress/strain behaviours are shown in the FIGS. 3 and 4. 
     FIG. 3 shows the stress/strain diagram for a material obtained according to Example 2, with a residence time of 750 seconds, a screw speed of 75 rpm, a break strain of 18.0±4.7%, a break stress of 33.8±7.7 MPa and a break energy per unit area of 241±68 KJ/m 2 . 
     FIG. 4 shows the stress/strain diagram for a further material obtained according to Example 2, with a residence time of 750 seconds, a screw speed of 100 rpm, a break strain of 8.8±3.1%, a break stress of 33.8±7.7 MPa and a break energy per unit area of 108±44 KJ/m 2 . 
     It can be seen from these results that the obtained extension values to break are relatively low, 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: 
     
         ______________________________________natural potato starch: 80.0   partslubricant/release agent                  1.0    parts(hydrogenated fat):lecithin:              0.7    partstitanium dioxide:      0.3    partswater:                 17.0   parts                  100.0  parts______________________________________ 
    
     Analogous results as in Examples 1 and 2 were obtained as shown in the FIGS. 1, 2 (when processed analogously to Example 1) and FIGS. 3 and 4 (when processed analogously to Example 2). 
     EXAMPLE 4 
     Examples 1 and 2 were repeated with a composition containing polyvinylpyrrolidone, so that test pieces of following composition were obtained: 
     
         ______________________________________potato starch:    74.6%polyvinylpyrrolidone:             10.0%hydrogenated fat: 1.1%lecithin:         0.8%water:            13.5%             100.0%______________________________________ 
    
     The stress/strain behaviour was very similar to that shown in the FIGS. 1 and 2 when processed analoguously to Example 1 and to FIGS. 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, i.e., 9.5%, 10.8% and 13.5% water, and stress/strain curves determined. The results are shown in FIGS. 5 and 6. The results in FIG. 5 from pre-extruded starch are clearly superior. A homogenous material exists at all the water contents is used, with a residence time of 450 seconds, a screw speed of 75 rpm, a temperature of 165° C. and a pressure of 75×10 5  N/m 2 . The material in FIG. 6 shows inferior properties (less extension and energy to break) and a less reproducible behaviour at all water contents, with a residence time of 750 seconds, a screw speed of 75 rpm, a temperature of 165° C. and a pressure of 75×10 5  N/m 5 . Such behaviour is consistent with a less homogenous and less coherent material. 
     EXAMPLE 6 (PROCESSING STABILITY) 
     The viscosity 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 riscosity of a function of shear rate was calculated from the measurements using standard injection-molding theory together with measurements of refill times. FIG. 7 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 FIGS. 3, 4 and 6. 
     The melt viscosity as a function of shear rate in the two-step process according to Example 1, are similar to those of conventional thermoplastics, e.g. polyethylene, which are known to be processible by injection molding to give reproducible products. 
     In FIG. 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. Lines (1), (2) and (3) represent the one-step process according to Example 2, and have residence times of 750 seconds, 600 seconds and 450 seconds, respectively. Lines (4) and (5) represent the two-step process according to Example 1, and have residence times of 750 seconds and 450 seconds, respectively.