CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Application Serial No. 834,655, filed February 12, 1992, which is a continuation-in-part of U.S. Application Serial No. 685,217, filed April 12, 1991. FIELD OF THE INVENTION

This invention relates to biodegradable packaging materials that are derived from starches containing moderate amounts of amylose, and to methods for preparing such materials. BACKGROUND OF THE INVENTION

Starches are well known starting materials for a variety of shaped and formed products, as demonstrated by the following U. S. patents:

U. S. patent 3,116,351 refers to a process in which an aqueous, alkaline amylose solution is extruded into a coagulation medium to form an unsupported film.

U. S. patent 3,030,667 refers to the formation of tubing from an aqueous amylose solution.

U. S. patent 3,137,592 refers to the extrusion of starch into expanded, gelatinized products of various shapes.

U. S. patent 3,165,508 teaches the preparation of polyurethane foam in which a degraded starch glycoside polyoxyalk lene is reacted with a polyisocyanate.

U. S. patent 3,336,429 teaches the extrusion of an aqueous, caustic, amylose-containing solution to form clear, shaped structures.

U. S. patent 3,891,624 refers to the preparation of a dispersible hydrophobic, porous starch material by an extrusion process.

U. S. patents 4,863,655 and 5,035,930 refer to the production of a resilient packaging material by extruding a high amylose starch material through an extruder having oil heated barrels. The starch of the feed material is required to contain at least 45% by weight of the amylose starch fraction.

U.S. patent 4,673,438 refers to a method of producing a starch based injection molding composition in which the starch is said to form a "molecular dispersion" in the composition.

U.S. patent 5,095,054 refers to a thermoplastic polymer composition including a "destructurized" starch (as is referred to in U.S. patent 4,673,438) and a specified polymer.

Vegetable starches contain two starch components. One component, amylose, is characterized by a generally linear molecular structure, whereas the other component, amylopectin, has a relatively branched molecular structure. Starches from different sources, such as corn, rice and oat starches, may have widely varying proportions of these starch components.

For the most part, extruded starch products such as films have understandably employed, as starting materials, the linear, high amylose starches. Unfortunately, these starches are relatively expensive starting materials, particularly for such low cost products as biodegradable resilient packing materials as are referred to in U. S. patent 4,863,655.

Starch-based packing materials may be substituted functionally for the non-biodegradable but familiar polystyrene foam or expanded bead packing materials of which enormous amounts are used and discarded each year. The need thus exists for a manner of making a resilient, commercially acceptable biodegradable packing from less expensive starting materials. SUMMARY OF THE INVENTION

It has now been found that relatively inexpensive, low amylose starches that are components of, for example, vegetable flours, eg., cereal and legume flours, can be formed into highly acceptable resilient, water-dispersible packing materials.

In one embodiment, a composition comprising starch containing not more than about 42% of amylose (and preferably not more than 38% and most preferably not more than 28% of amylose) having a moisture content of about 5% to about 30% (preferably 18% to 22%) is partially dextrinized as by means of a chemical dextrinizing agent or by means of mechanical working in an extruder or both. The partially dextrinized starch composition is blended with a polymeric plasticizer that is preferably water soluble and that provides from 5% to 18% by weight of the blend, and the blend is extruded at a temperature greater than the boiling point of water into a lower pressure environment, causing the ropy extrudate to immediately expand. Preferably, the partial dextrinization occurs in one or more initial zones of an extruder to which the initial composition is fed, and the polymeric plasticizer is blended with the partially dextrinized composition and the blend is further extruded. In this embodiment, the mechanical energy input to the composition after blending of the polymeric plasticizer is not greater and most

preferably is substantially less than the mechanical energy input to the composition prior to such addition. I have found that if the polymeric plasticizer in added after partial dextrinization, it escapes the degradation that it may otherwise be subjected to if the polymeric plasticizer were combined in the initial starch feed.

If desired, the starch composition may be subjected to exceptionally high shear forces during the extrusion operation. It is believed that the branched amylopectin is dextrinized, that is, partially depolymerized, during this process into shorter, more linear chain lengths which subsequently may recombine into longer, more linear chains. Thus, in another embodiment, the invention relates to a process for making an expanded, resilient, biodegradable, water-dispersible packing material, the process comprising extruding starch containing not more than about 42% of amylose (and preferably not more than 38% and most preferably not more than 28% of amylose) at a specific mechanical energy input of not less than about 0.23 hp/pound of product produced to thereby subject the material to high shear forces. The product is heated, primarily by application of said mechanical energy, to a temperature above 100 degrees C, and is expanded by extruding it into a region of lower pressure to enable moisture in the extrudate to rapidly evaporate.

The present invention in a further embodiment relates to a water-dispersible, biodegradable, expanded, starch-based product which has particular utility as an environmentally acceptable, resilient shock-absorbing packaging material, the product being derived from a starch-containing feed such as a cereal or legume flour, the starch component containing not

more than 42% by weight of amylose (and preferably not more than 38% and most preferably not more than 28% of amylose) . The product has a structure comprising a plurality of generally aligned, interwoven and adhered filaments, and is sufficiently flexible to deform substantially upon application of moderate finger pressure and to rebound readily when pressure is removed. The product may include air cells ranging in dimension from 3 to 100 microns in diameter. The product includes at least 5% by weight of a polymeric plasticizer as noted above, the presence of which contributes significant resiliency to the product. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is an enlarged photograph of typical packing material of the invention, showing its lofty, filamentary structure. DETAILED DESCRIPTION

In a preferred embodiment, the starch starting material, preferably a native starch or blend of starches, is first partially dextrinized, following which it is combined with at least 5% and preferably 5% to 18% by weight of a polymeric plasticizer. I have found that if the plasticizer in this amount is incorporated in a native starch composition and the resulting blend is fed through an extruder under conditions causing the the blend to be extruded at superatmospheric pressure and a temperature above the boiling point of water into a region of lower pressure, the extrusion process is difficult to control. It often produces an extruded product having properties that vary as the extrusion process proceeds, and this, in turn, leads to an extruded product that has less than the desired resiliency and also causes frequent costly shut-downs of the extrusion equipment for cleaning and maintenance. The

problem appears to be that the energetic extrusion working that is required degrades the plasticizer. By partially dextrinizing the starch composition prior to adding the polymeric plasticizer, degradation of the plasticizer can largely be avoided and the extrusion equipment can be run on a continuous basis under steady-state conditions with few shut-downs.

The initial dextrinization of the starch composition may be carried out in several ways. Preferably, however, the starch composition, prior to addition of the polymeric plasticizer, is subjected to extrusion working in an extruder, substantial mechanical energy being added to the starch to cause dextrinization. The extruder may include an atmospheric pressure zone following the zone or zones in which partial dextrinization occurs, and the polymeric plasticizer, desirably in dry, granular form, may be added to the extruder at this point. The partially dextrinized starch at this point desirably is a hot and crumbly but yet dry-to-the-touch particulate mass, and has a generally gritty character compared to the soft, silky feel associated with native starch.

Dextrinization may be accomplished by combining the native starch with a chemical dextrinization agent, such as acetic acid or hydrochloric acid, and in a preferred embodiment, a dilute solution of acetic acid is added to the native starch as the latter is fed to the initial stages of an extruder, as described above, to aid in the partial dextrinization process. Since the partial dextrinization of the starch feed most preferably occurs in the initial zone or zones of a multizone extruder, the preferred embodiments of the invention are described below in terms of performing the partial dextrinization in this manner.

- 1 -

Extruder Feed

The starch containing materials useful as extruder feeds in the process of the present invention can vary widely. The feed material may comprise a starch-containing cereal or legume flour such as corn, wheat, rice, peas, oat, potato and tapioca flour which contains at least about 80% by weight of starch, the starch component desirably containing itself less than 42% of amylose, preferably less than 38% amylose and most preferably not more than 28% amylose. Starches containing from 16% to 28% amylose are particularly preferred, and wheat, rice and potato starches may fall within this range. The extruder feed desirably is substantially entirely starch having an appropriate moisture content, but small amounts (up to about 20%) of non-starch materials, such as vegetable materials, water soluble polymeric binders such as polyvinyl alcohol, polyethylene glycol, and the like, may be tolerated. The natural moisture content of starches may be augmented by addition of further amounts of water, the water content of starch feeds ranging from 5% to 30% but preferably being in the range of 18% to 22%. The water content is adjusted to provide a starch feed that can be extruded, that is, that may be gelatinized within the extruder into a fluid mass.

Starches purified to contain high levels of either amylose or amylopectin may be mixed as desired to produce various desired end results. The starch-containing extruder feeds employed in the invention also may be modified through esterification, esterification, oxidation, acid hydrolysis, enzyme conversion, etc., by methods known in the field of starch chemistry.

For purposes of this invention, many different flours derived from long and short grain rice, common

yellow and high amylose corn, acid and enzyme,modified corn starch, potato, wheat, pea and oat starch, and high and moderate amylose content corn starch can be employed by suitable blending to provide a low amylose starch product for use as an extruder feed. The extruder feeds employed in the instant invention may be considered to be starch-containing compositions such as cereal and legume flours that contain at least 80% of starch by weight, the starch component itself containing not greater than about 42%, preferably not greater than about 38% and most preferably not more than 28% by weight of amylose. Of the various starch products referred to above, wheat starch is preferred, and pea and potato starch have also given good results.

A wide variety of additives and plasticizers, such as mono- and di-glycerides, urea, ethylene glycol, glycerin, polyvinyl alcohol, polyethylene glycol, surfactants (e.g., fatty acid esters, triethylene glycols, chlorinated paraffins), polyurethane, and various cellulosics, may be employed in the extruder feed, commonly at not more than about 5% by weight of the feed. Surfactants may be used to promote bubble formation, and emulsifiers such as lecithin and its derivatives may be used to increase bubble size. Extrusion aids such as MgCl ₂ and glycerin may be incorporated in the extruder feed, typically at concentrations of 0.1% by weight or less.

Small amounts of bleaching agents (at less than about 0.1% by weight) such as sodium hypochlorite may be included in the feed to promote whiteness of the product, and the product may be colored as desired by incorporating in the extruder feed appropriate coloring materials such as dyes (particularly, food dyes) and pigments. Various fillers can also be used,

including fibers (e.g., powdered cellulosic fibers) and fumed silica.

Extruder Processing

To ensure appropriate plasticization during the extrusion process, the extruder feeds employed in the present invention should contain moisture in the range of 5% to 30% by weight, preferably not greater than about 22% but desirably at least about 18% by weight.

The extruder feeds referred to above are extruded through an appropriate extruder, such as a Buhler, Inc. 62 mm DNG twin screw extruder. Although the extruder feed may be preheated, it is desired that the extruder feed not enter the extruder at a temperature much above room temperature. The extruder barrel can be heated or cooled as desired by known means, but commonly it is found to be desirable to cool all sections of the extruder barrel from external sources, e.g., by use of a circulating water or oil external cooling system of known design.

As the extrusion process proceeds, the temperature of the extruder feed rises because of the shear forces that are imparted to it. In one embodiment, the specific mechanical energy ("SME") imparted to the extruder feed during the extrusion operation, per pound of product produced, desirably is at least 0.23 hp, preferably is in the range of 0.23 to about 0.34 hp, and most preferably is in the range of 0.24 to 0.27 hp. SME values, representing the mechanical energy imparted to the feed, can readily be measured by measuring the electrical energy input to the electric drive means powering the extruder screw or by other known means. The mechanical energy that is imparted to the extruder feed is substantially entirely in the shear mode rather that in the kneading mode, and the mechanical energy imparted to the feed

during extrusion is preferably the sole energy supplied to the feed to raise the temperature of the extrudate at the exit die to over 100°C. To avoid overheating of the feed, the feed desirably is charged to the extruder at a temperature of not great than about 25°C.

Desirably, an exit die is employed which includes an elongated portion of small cross section through which the physically worked extruder feed is extruded into the air. An elongated die opening of circular cross section having a diameter of 1/32 inches and a length of about 1.3 inches has given good results, although slotted or other die configurations may provide good results.

In general, a velocity of product through the die in the range of 30-60 ft/min. is desired, and the transit time of material through the length of the extruder preferably is not greater than about 45 seconds. Moreover, the die opening should be closely adjacent to the end of the extruder barrel so that the feed material continues to be subjected to shear forces in its passage through the extruder until very shortly before the feed issues from the die opening. The time interval between the moment that feed exits the screw within the barrel and the moment that the feed exits the exit die should not exceed 3 seconds and preferably is less than 1 second. The use of an extrusion die providing a lengthy (in time) passage from the extruder barrel to the die opening is desirably avoided, since this may lead to premature recrystallization of the extrudate with consequent plugging of the die.

As desired, the extrusion die may lend itself to formation of an extrudate with a cross section resembling a letter, number or simple design, and a

chopping or shearing of the extrudate may thus produce a particulate packing material, the particles of which have a desired letter, number or design shape.

The polymeric plasticizers that are incorporated into products of the invention are thermoplastic and preferably are water soluble, that is, they absorb substantial amounts of water, preferably at least equal to their own weight. Polyvinyl alcohol preferably provides the major ingredient of the plasticizer and most preferably is the sole plasticizer. Other water soluble polymers include polyvinylpyrrolidone and polyacrylamide. The polymeric plasticizer may include blends of different polymers, one such blend being polyvinyl alcohol and polypropylene glycol, in approximately equal amounts, the plasticizer also may include various polymers such as rubbers (e.g., SBR rubbers) chosen to impart specific qualities to the final expanded product. Preferably, however, the polymeric plasticizer consists of polyvinyl alcohol in the form of granules that are added to the extruder barrel at a point following the initial partial dextrinization of the starch composition. Polymers such as polyvinyl alcohol appear to become incorporated in the bubble walls of the product and appear to modify the water sensitivity and resiliency of the product, since the polymer fraction commonly dissolves less quickly in water than does the starch fraction of the product.

In the embodiment employing such a water soluble thermoplastic plasticizer, the SME employed may be less than that stated above. In particular, the SME may be as low as about 0.08 hp/pound of product and may be as high as about 0.34 hp/pound. Preferably, the SME is between about 0.08 and about 0.12 hp/pound, with an SME value of about 0.086 hp/pound being particularly preferred.

The SME employed in one embodiment of the process of the invention with external cooling or chilling of the product is significantly greater than SMEs encountered when substantial external heat energy is added to starch-containing extruder feeds. The substantial amount of mechanical energy that is imparted to the extruder feed is believed to cause at least partial saccharification of at least the amylopectin molecules, that is, the breakdown of these molecules into short starch chains. When the extruder feed is thus thoroughly mechanically worked and passed through the die, it is subjected to very high shear rates, and the dextrinized starch molecules are believed to recombine and polymerize or otherwise unite into longer chain molecules, the starch molecules becoming generally aligned in the direction of extrusion such that the product issues from the extruder die as an expanded, fibrous product. The pressure drop across the length of the exit die may range from about 300 psi to about 500 psi.

As the hot (above 100 degrees C) product issues from the end of the exit die into, desirably, an atmosphere at room temperature and one atmosphere, the rope-like extrudate desirably immediately expands as water is flashed off. The extrudate retains an elongated, filamentous structure. The rope-like extrudate can then be chopped into short lengths, e.g., 1-2 inches in length or thereabouts. The fibrous, cellular product is found to have substantial flexibility and resilience, an unexpected property considering that the starch feed contains only a low concentration of amylose - less than 42% and preferably less than 38% and most preferably not greater than about 28% of amylose.

The degree to which the product expands as it issues from the exit die depends upon a variety of controllable factors, including the pressure drop across the exit die and the temperature of the extrudate. The expansion desirably is controlled so that the density of the product is in the range of about 0.008 to 0.015 kg/1.

Resilience of the product may be assessed by comparing it with polystyrene packing material of the type commonly employed in a container within which is placed an article to be protected. Polystyrene pieces of the type described can be readily compressed between thumb and forefinger, and rebound slowly when finger pressure is removed to regain a portion of their original expanded dimensions. The expanded product of the present invention, when demonstrating a resilience approximately comparable to that of polystyrene foam packing materials, is rated with a "very high" resiliency value. Product which may be compressed by hand only with some effort and which retains a substantial permanent set is referred to as having a "medium" resiliency, and product which can be compressed only slightly by finger pressure or which crumbles easily when compressed is said to have a "low" resiliency value. The resiliency of the product of the invention immediately after it has expanded upon leaving the extruder often is fairly low, but may be increased by simply storing the product for a short time in a humid atmosphere or for a longer time, eg., 24 hours, in a less humid atmosphere.

The extrudates obtained through use of the present invention may be modified as desired to produce textures which are similar to popcorn with similar resiliency, and also can be formed into specific shapes or designs and coated with caramel or

other sweeteners as well as cheese or other salty condiments, and can be marketed and sold in the snack industry. Such textures in food-based products may be desirable for snacks, confectionery and the like.

The internal structures of products of the invention can vary from having very small air cells such as is found in popcorn (3-10 microns in diameter) to air cells of the type found in snack foods, having air cells in the range of 10-100 microns in diameter. The length of individual filaments in the expanded product can vary widely; many are in the range of 1-10 cm in length. Figure 1 depicts a typical product in which filaments (10) are shown as being generally parallel and aligned with each other. The filaments may be branched or adhered together; they can easily be pulled apart.

In the following example, use was made of a Buhler, Inc. 62 mm DNDG twin screw extruder, in which the extruder feed was initially preheated to a temperature of 79°C. An exit die was employed having an elongated opening of circular cross section, the diameter of the opening being approximately 1/32-inch. The length of the die, in the direction of extrusion, was about 1.3 inches. Except for the mild preheating of the extruder feed, no direct heat energy was added to the extruder; rather, the rise in temperature of the material being extruded was caused entirely by the input of mechanical shear forces to the material. The specific mechanical energy that was added to the feed material was measured in each case, and the resulting extrudate issuing from the die was allowed to cool to room temperature and then was tested using the finger squeeze test referred to above.

Example 1

A variety of starches from various sources and containing varying concentrations of amylose are

prepared having a moisture content of 20% and are utilized as extruder feed materials. The thus-described materials are extruded as indicated above, with the temperature of the material exiting from the die being well above 100° C. The measured specific mechanical energy imparted to the extruder feed is reported in the following table in horsepower per pound of product. Upon exiting from the extruder orifice, the ropy extrudate is cut into short lengths. After storage for several hours, the products are evaluated for resiliency as set out above.

Table 1

A blend of wheat starch and corn starch having an amylose content of less than 38% and a moisture content of 21.5% was combined with a minor amount of a dilute, aqueous acetic acid solution and was fed to the extruder described in Example 1. The extruder was provided with an initial "hard screw" zone in which the starch feed was subjected to intense extruder working. In the next extruder zone, the pressure of the now partially dextrinized, hot and grainy starch composition was reduced to atmospheric pressure and a small sample, removed for examination, exhibited a

granular, rough and course texture. Through a port in the extruder barrel at this point was added granular polyvinyl alcohol in an amount providing about 15% by weight of the resulting blend. Extrusion was continued in a subsequent downstream extruder zone in which less mechanical energy was added to the blend than was added to the feed in the initial zone. The temperature just prior to the exit die was measured as 350 degrees F. Immediately upon issuing from the die, the extrudate expanded and was mechanically chopped into short lengths.

The resulting product was soft to the touch, substantially odor free and very resilient; it could be crumbled by the fingers only with difficulty. The addition of the polyvinyl alcohol plasticizer was temporarily stopped, and the resulting product exhibited a smaller degree of expansion and was noticeably harder and less resilient. The desired high expansion and resilient nature of the product were rapidly regained when addition of the plasticizer was resumed. In earlier trials in which the same polyvinyl alcohol plasticizer was added in about the same concentration to the initial starch feed rather after partial dextrinization of the feed, the resulting product was less resilient, slightly darker in color, and exhibited a slight odor which was attributed to degradation of the plasticizer.

The expanded, biodegradable products of the invention are characterized as having uniform, lofty, fibrous structures with substantial resiliency and water solubility. They can be formed into a variety of shapes through extrusion and cutting or by other shaping methods. The product may be colored as desired, as by the addition of suitable dyes or pigments or other coloring additives to the extruder

feed. The feed may incorporate such antibacterial, antif ngal, or rodent or insect repelling or killing materials as desired to meet product needs.

While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.