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Description  |
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FIELD OF THE INVENTION
This invention relates generally to expanded biodegradable starch containing products with improved resiliency and other properties appropriate for use as packaging materials. The invention includes an improved method for manufacture of such
products and improved expanded starch loose fill and sheets.
BACKGROUND OF THE INVENTION
Light-weight, resilient, compressible and biodegradable materials are presently in high demand for applications in packaging materials. Commonly used polystyrene (Styrofoam.TM.) e.g., ("peanuts"), polypropylene, polyethylene, and other
non-biodegradable plastic containing packaging materials are considered detrimental to the environment. The use of such non-biodegradable materials will decrease as government restrictions discourage their use in packaging applications. Biodegradable
materials having resiliency, compressibility and bulk density comparable to Styrofoam.TM. "peanuts" will become increasingly valuable in packaging applications. Biodegradable sheet materials having properties comparable to polypropylene and
polyethylene materials will be useful in laminating packaging materials. Starch-based products have been examined for such applications.
Attempts to prepare starch-based products with resiliency and other properties acceptable for packaging materials, particularly for loose fill, have generally focused on chemical or physico-chemical modification of starch or use of expensive high
amylose starch to generate expanded starch materials sufficiently resilient for packaging materials. U.S. Pat. Nos. 4,673,438, 4,133,784, 4,337,181, and 4,454,268 relate to starch-based products that have been destructurized or gelatinized by heating
under pressure in the presence of water. U.S. Pat. Nos. 4,863,655, 5,035,930 and 5,043,196 relate to expanded starch materials in which the starch has at least 45% by weight amylose (high amylose materials). Starches in these expanded high amylose
compositions have also been modified by reaction with oxiranes (e.g. ethylene oxide) and include the commercially available ECO-FOAM.TM. in which the starch is modified by reaction with propylene oxide. Expanded packaging materials made from high
amylose starch are too expensive for practical commercial use. U.S. Pat. No. 5,087,650 relates to addition of graft polymers to starch to make partially biodegradable products with acceptable elasticity and water stability.
In general, extrusion of non-modified starch without polymeric additives results in brittle (non-resilient) materials that have not been commercially useful for packaging applications.
More recently the following patents related to biodegradable starch products have issued:
U.S. Pat. No. 5,186,990 of Starcevich (Feb. 16, 1993) reports a lightweight biodegradable packaging material produced by extrusion of corn grit mixed with a binding agent (guar gum) and water. Corn grit is said to contain among other
components starch (76-80%), water (12.5-14%), protein (6.5-8%) and fat (0.5-1%).
U.S. Pat. No. 5,208,267 of Neumann et al. (May 4, 1993) reports biodegradable, compressible and resilient starch-based packaging fillers with high volumes and low weights. The products are formed by extrusion of a blend of non-modified starch
with polyalkylene glycol or certain derivatives thereof (up to 10% by weight) and a bubble-nucleating agent, such as silicon dioxide. The nucleating agent is said to enhance bulk resiliency and uniformity of cell structure of the extruded products. The
starch blend is "preconditioned" by steam injection prior to addition to the extruder. Bulk resiliency of the exemplified wheat starch products (shaped as small or large tubes) ranged from a low of 56.8% (0.5% silicon dioxide, small tube) to a high of
69.9% (6.0% PEGC [a polyethylene glycol derivative with molecular weight 20,000], 1.0% silicon dioxide, large tube). Bulk densities for these exemplary products was 1.69 lbs/ft.sup.3 and 1.22 lbs/ft.sup.3, respectively.
U.S. Pat. No. 5,252,271 of Hyrum (Oct. 12, 1993) reports a biodegradable closed cell light weight packaging material formed by extrusion of a modified starch. Non-modified starch is reacted in an extruder with certain mild acids in the
presence of water and a carbonate compound to generate CO.sub.2. Resiliency of the product is said to be 60% to 85%, with density less than 0.032 g/cm.sup.3.
U.S. Pat. No. 5,314,754 of Knight (May 24, 1994) reports the production of shaped articles from high amylose starch. A hot melt is formed under pressure at elevated temperature from a mixture of high amylosic starch with water and with
optional plasticizers and lubricants and extruded to form a shaped article. The hot melt is subjected to atmospheric or subatmospheric pressure just prior to extrusion or to subatmospheric pressure after extrusion to remove water from the hot melt.
This first extrudate can then be subject to a second extrusion to form a film or other shaped article. The film produced in the two-step process is said to be highly elastic with elongation before failure from about 80% to 240%.
U.S. Pat. No. 5,322,866 of Mayer et al. (Jun. 21, 194) reports the production of biodegradable blown films from a blend of unprocessed raw starch and the copolymers polyvinyl alcohol (PVA) or ethylene vinyl alcohol (EVA) and may also include
plasticizer and lubricant. For blown films useful ranges of PVA or EVA in the starch blend are said to be 20% to 80% by weight. Starting materials for the blown film are introduced into a twin-screw extruder fitted with a blown film die.
SUMMARY OF THE INVENTION
The present invention provides a method for producing an improved high resiliency expanded starch product which comprises extrusion of a starch-containing mixture through an expansion die followed by subjecting the extrudate to pressure
perpendicular to the direction of extrusion, for example by use of a rolling device. The rolled extruded strand is then cut to desired length to suit the planned application. Preferably the pressure applied to the hot extrudate strand is sufficient to
substantially crush the cells in the extrudate. Surprisingly, crushing the cells of the expanded product significantly improves the resiliency of the product. The preferred method of applying pressure to the extrudate to substantially crush the cells
is rolling the extrudate between opposed rollers. The rolled expanded starch product is biodegradable and has high resiliency and other properties attractive for use in packaging materials such as loose fill.
The invention provides improved expanded starch materials having high resiliency for use in packaging applications. These improved starch materials are produced by methods described herein by extrusion combined with a step of crushing
substantially all of the cells in the extrudate. The method of this invention can be employed essentially with any starch composition suitable for extrusion. In particular and preferably, the method can be used with starch compositions in which the
major ingredient is raw non-modified starch. The method of this invention allows the use of lower levels of plasticizers and humectant in base starch extruder feed than previously known to achieve desired high resiliency properties in the final product.
This invention provides a loose fill expanded starch product with resiliency higher than 50% useful for packaging applications. Loose fill products of this invention have compressibility, flexibility properties comparable to Styrofoam.TM. loose
fill. The methods of this invention are also applied to the manufacture of expanded starch sheet materials. Rolled expanded starch sheets are generally useful in manufacture of laminated materials for packaging applications, such as for manufacture of
mailing envelopes and related containers.
In one aspect, the invention provides new resilient expanded starch products. In a second aspect, the invention provides base starch mixtures, extrusion feed mixtures, for production of expanded starch products which contain lower levels of
humectants and plasticizers, yet which can be processed into high resiliency (low brittleness) expanded product. In another aspect, the invention provides a new method for manufacture of expanded starch products utilizing a conventional extruder
combined with a pressure rolling device. Expanded products can be shaped by the combined effect of extruder die shape and roller configuration. In yet another aspect, the invention provides a new extrusion device in which a conventional extruder is
combined with a rolling device which applies pressure to an extrudate perpendicular to the direction of extrusion. The pressure applied being sufficient to substantially crush the cells in the hot extrudate. Additional aspects and features of the
invention will become apparent in the following detailed description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of the extrusion/rolling/cutting device employed to make the biodegradable resilient starch products of this invention.
FIGS. 2A-2G are illustrations of various roller configurations that can be employed in the methods of this invention. Roller configurations having 2-4 flat or shaped rollers are illustrated. This figure also provides illustrations of several
possible shapes for pieces of loose-fill packaging material, including FIG. 8 and omega shapes among others.
DETAILED DESCRIPTION OF THE INVENTION
The improved process of preparing extruded starch products of this invention is described by reference to the schematic manufacturing extrusion apparatus of FIG. 1. Raw non-modified starch is mixed with any desired additives, for example in an
attached mixing chamber (1) and introduced into an extruder (2), preferably a twin screw extruder (shown schematically as 4). The mixture is further mixed and blended, subjected to shearing, increasing temperature and pressure in the extruder to form a
plasticized mass which is forced through an expanding die (3).
The shape of the die determines the shape of the extruded strand and die shape is typically chosen to prepare a desired shape of expanded product. The mixture is typically extruded out the die at a pressure of about 700 to 2000 psi and the
extrudate is at a temperature of about 120.degree. C. to about 200.degree. C. at extrusion. The extruder is typically heated and can have several zones along its length in which temperature is separately adjustable. A pressure drop occurs outside the
extruder and steam emitted from the mixture causes it to expand as it exits the die.
Water can be added to the system during the residence of the base starch mixture in the extruder to adjust the extent of expansion of the product. Lubricant levels in the extrudate can also be adjusted to improve flow through the extruder.
Glycerin levels in the extruder can also be adjusted to improve flow or vary properties of the expanded product. There is no requirement to remove water from the base mixture in the extruder prior to extrusion. Specifically there is no requirement to
apply subatmospheric pressure to the extrudate or extruded strand to remove water.
The extrudate strand (9) is fed through a rolling device (10) having two or more rollers (11) which apply pressure to the strand substantially perpendicular to the direction of extrusion. The rollers form an opening corresponding to the shape of
the desired product. The rollers exert sufficient pressure upon the strand to substantially crush the cells in the strand. The rollers also finalize shaping of the expanded product.
The process step of rolling and application of pressure to the extrudate strand perpendicular to the direction of extrusion is an important improvement in the manufacturing process of this invention which results in significantly improved
resiliency (decreased brittleness) of extruded starch products. Rolling also reduces dustiness and chipping of extruded products and increased bulk density. The roller pressure required to achieve substantial cell crushing leading to such improved
properties depends on the shape of the extrudate, cell size and bulk density of the extrudate which in turn depends on the composition of the extrudate including the type of starch and water content. The required roller pressure needed to achieve the
desired cell crushing is readily determined for a particular starch composition and product shape empirically without expense of undue experimentation. In general, roller pressure for making sheet products will be higher than in making shaped loose
fill.
The substantially crushed extrudate is cut into desired lengths by a cutting device, for example with a rotating knife (12) positioned after the rollers. The resulting crushed and cut strands (15) are preferably cured under controlled
temperature and humidity conditions. Curing is preferably done between 18.degree.-30.degree. C., preferably 21.degree. C. at humidity ranging from about 40%-80%, preferably 50% for up to 1 or 2 days.
The manufacture of expanded starch loose fill and sheets is typically done in a continuous mode by continuous feed into the extruder, extrusion, rolling and cutting.
A conventional twin screw extruder having feed screws, single lead screws, shear paddles (preferably 3 or more, 3-10 being typical) and mixing paddles (typically 1-5) can be employed in this process. A typical expanding type screw configuration
can be used in the twin-screw extruder. Typical residence time of the base mixture (i.e. the starch with any additives) in the extruder is from about 9 to 20 seconds and extrusion occurs at a rate from about 100 to 200 lb/h per die insert.
The size and configuration of the die opening determines the cross-sectional shape of the extrudate strand. A variety of shapes and sizes of final loose fill products can be made. The starch can also be extruded as an expanded sheet to make
sheets of cushioning material. Typically product sheets of thickness about 1/10 to 1/4 inch can be made by the extrusion/rolling method. Extruded starch products for loose fill packaging material, for example, can have several cross-sectional shapes,
including among others, capital omicron, FIG. 8, short rods, S-shaped, C-shaped, oval, and an omega. The product shape does not significantly effect resiliency of loose fill packing material. Bulk density of loose fill is however dependent upon shape.
The rolling device applies pressure to the extrudate strand perpendicular to the direction of extrusion. The hot extrudate strand can simply be passed through opposed flat rollers (which may distort the shape imposed by the die). Preferably the
roller or rollers apply pressure symmetrically to the hot extrudate conforming to the desired product shape. Rollers are made of stainless steel, teflon or a related material that is inert to the extrudate. Rollers are preferably positioned with
respect to the extruder die such that the hot extrudate strand can be directly fed into the rollers while the extrudate strand is still hot.
FIGS. 2A-2G illustrate exemplary roller configurations of 1-4 roller combinations. FIG. 2A illustrates the use of two flat rollers (20a and 20b) in combination with a slot-shaped extruder die (3a) for forming the extrudate strand (9a) into
expanded starch sheets. FIG. 2B illustrates the use of two opposed rollers (21a and 21b) in combination with die 3b to form FIG. 8 shaped products. The shaped roller configuration 21a and 21b is shown separately FIG. 2C. FIG. 2D illustrates a
three-roller configuration (22a, 22b and 22c) to form a triangular-cross section product. The direction of extrusion through the rollers is indicated by the arrow 9c. FIG. 2E illustrates a second three-roller configuration (23a and 23b and 24) to form
an omega-shaped product. FIGS. 2F and 2G illustrate two different four roller configurations (25a-d and 26a-d, respectively) that result in products with concave or convex sides. Many other roller configurations containing flat, curved or otherwise
shaped rollers can be made to achieved desired product shapes. The directions of extrusion are indicated in FIGS. 2E-2G by arrows labelled 9d-9f. In each of the drawings of FIG. 2 roller spindles and the direction of rotation are indicated.
A rolling step has not previously been applied to the production of expanded starch products. Preferred rollers are spring-loaded and the pressure applied is adjustable. Rolling processes and equipment that is standard in food processing, e.g.
processing of doughs, can be readily adapted to use in the manufacturing process of this invention. While rolling is the preferred method for applying pressure to the extruded strand to achieve substantial cell crushing, those of ordinary skill in the
art will appreciate that other methods may be applied to achieve the desired result.
Rollers are positioned sufficiently close to the extruder die such that the extrudate remains hot during rolling. For ease of illustration, the roller devices of FIGS. 2A-2G show one set of rollers. Two or more sets of rollers in sequence can
be employed in the processing method of this invention. The use of more than one set of rollers may improve efficiency of crushing of cells. For production of starch sheets, in particular, the use of two or more rollers may be required to achieve
desired crushing.
The shape of loose fill packaging material affects the interlocking capability of the material. Loose fill shaped so that there is some interlocking between pieces is less readily displaced by jostling and better protects packaged items from
impact damage. Loose fill shapes preferred for interlocking capability are those with protrusions or indentations which allow pieces to interlock.
The extrudate of this invention contains raw non-modified starch as the major ingredient (about 70% to about 95% by weight) with added water up to about 30%, minor amounts of additives including lubricant, plasticizer and humectant, nucleating
agent and optionally another blowing agent (in addition to water) wherein the additives comprise up to a maximum of 25% by weight of the total weight of the extrudate. Preferred extrudate contains a minimum amount of plasticizers, including polyvinyl
alcohol or ethylene vinyl alcohol, to obtain desired resiliency when processed by the improved extrusion/rolling method described herein. In particular, preferred extrudate contains a maximum of 5% by weight of polyvinyl alcohol or ethylene vinyl
alcohol.
In preferred embodiments, this invention uses raw, non-modified, non-derivatized starch which is essentially starch as it occurs in nature other than having been physically separated from other plant components. The starch is typically a powder
(fine or coarse) or granular. Cornstarch is the preferred starch in this invention, however any type of non-modified starch from wheat, rice, potatoes, tapioca or the like can also be used. The invention does not require the use of high amylose starch
to achieve desired high resiliency. The invention does not require the use of derivatized starch, such as starch which has been chemically reacted at the hydroxy groups (e.g. esterified, etherified or phosphorylated). The invention does not require
preconditioning of starch by treatment with heat or pressure, gelatinization or destructurization of starch before to addition to the extruder. Nevertheless starch composition with high amylose starch, modified or derivatized starch or starch
compositions that are preconditioned, gelatinized or destructurized can be processed by the improved extrusion/rolling process of this invention to provide extruded materials with improved properties for use as packaging materials.
High resiliency expanded starch products of this invention can be prepared with lower levels of plasticizers, particularly polyvinyl alcohol, ethylene vinyl alcohol and related materials than taught in the prior art.
Water in the base starch mixture functions as a blowing or expanding agent. The amount of water in the extrudate is adjusted to maximize expansion of the extruded product while avoiding a soft or runny extrudate. Water can be premixed with or
added separately from other base starch mixture ingredients. For example, water can be added and its addition adjusted and control led through inlet ports into the extruder. The amount of water that must be added depends on the moisture content of the
raw starch. The total water content of the base starch mixture prior to extrusion is typically between about 6% and 18%.
The pre-mixed base starch mixture (including water, lubricant and any glycerin) can be directly introduced into the extruder or the individual components can be added in any order into the extruder for mixing therein. In a preferred method of
addition of the components, water, lubricant and glycerin are added to the remaining ingredient in the extruder through independently adjustable inlets into the extruder.
The base starch mixture of this invention contains minor amounts of humectants, plasticizers, lubricants, nucleating agents and optional blowing agents and agents which function to reduce cell size. Glycerin, related polyols including
pentaerythritol, and vegetable oil among others can function in starch extrusion as humectants and lubricants to improve the flow properties of the mixture and provide smooth surfaces for extrusion. Polyvinyl alcohol, ethylene vinyl alcohol and related
polymeric polyols can function as plasticizers. Glycerin can also function as a plasticizer. Talc, protein as grain meal, in egg albumin or blood meal or other protein sources, and materials like Hydrocerol.TM. can function as nucleating agent.
Hydrocerol.TM. is a combination of encapsulated sodium bicarbonate and citric acid which generates CO.sub.2 and sodium citrate. Glycerol monostearate and other glycerol monoesters of fatty acids can function to aid in formation of uniform small
cells in the extruded material. Superheated water in the extrudate acts as a blowing agent. The base mixture of this invention can also include additional minor amounts of blowing agents including among others carbonate salts including sodium,
potassium and ammonium salts. Any vegetable oils or polyols employed in the base mixture are preferably pumpable. In general the base starch mixture of this invention can contain combinations of one or more humectants, plasticizers, lubricants,
nucleating agents and/or blowing agents. The maximum amount of these additives is 25% by weight of the total base starch mixture (including added water). The base starch mixture preferably contains a minimum of these additives needed to attain desired
properties of extruded product. The base mixture contains a minimum amount of plasticizer to decrease its cost. The rolling step of this invention allows the use of much lower amounts of plasticizers than are required in prior art processes. The
preferred base starch mixture of this invention contains a maximum of 5% by weight of polyvinyl alcohol, polyethylene alcohol or related polymeric vinyl alcohols (or mixtures thereof). Preferred base starch mixtures of this invention include those in
which the amount of polymeric vinyl alcohol is less than 5% by weight and, specifically, those having less than 2.5% by weight as well as those having less than 1% by weight of polymeric vinyl alcohol. Mixtures of this invention also contain relatively
low levels of glycerin or other humectant. Mixtures include those in which glycerin is less than 1.5% and those in which glycerin is less than 0.5%.
For example, the base starch mixture can contain the following ingredients expressed in weight percent: raw unmodified starch (about 70% to about 95%); added water (up to about 30%); vegetable oil (about 0% to about 6%); glycerin (about 0% to
about 3%); polyvinyl alcohol (about 0% to a maximum of 5%); proteinaceous grain meal (about 0% to about 5%); glycerol monostearate (about 0% to about 0.5%); additional blowing agent (about 0% to about 0.8%); and talc (about 0% to about 5%) with the
provision that the mixture must contain a lubricant, a nucleating agent and some combination of humectant and plasticizer.
The base starch composition of this invention can optionally contain coloring agents, fragrances, bactericides and mold-inhibiting agents.
Resiliency (also called bulk resiliency) is the capability of a material to recover its size and shape, i.e. to recover its original volume, after deformation by a compressive force. Bulk resiliency of a material is assessed as percent recovery
of volume by measuring the original volume (as height of a given amount of material in a container) and the volume after about a 30-second recovery from the application of a compressive force. The loose fill products made as described herein will
generally have resiliency of 50% or more. Preferred loose fill products will have resiliency over 60%, and more preferred loose fill products will have resiliency of 75% or more. Most preferred loose fill product will have resiliency between about 85%
to 95%. In the present invention high resiliency is substantially imparted to shaped expanded products by the physical processing step of rolling the hot extruded product to substantially crush the cells therein. The minor amounts of humectants and
plasticizers that are included in the base starch mixture do not in themselves account for the significant improvements in resiliency.
Packaging materials can also be compared and assessed based on their bulk density (weight/unit volume) and bulk compressibility (i.e., the maximum force needed to compress the sample to a preselected % of its original volume, typically to 2/3 of
its original volume). Flexibility, the capability to bend without breaking) of loose-fill can be assessed qualitatively by applying hand pressure (squeezing) to the final extruded products to assess how much force can be applied before the extruded
product breaks.
Crushed extrudates of this invention have bulk density less than about 2.0 lbs/ft.sup.3, but greater than 0.25 lbs/ft.sup.3. Typical bulk density for corn starch products of this invention is about 0.5 to 0.8 lbs/ft.sup.3. The material as
extruded is much lighter than the final product since crushing the cells with the rolling procedure adds bulk density. The expanded starch products have moisture content generally lower than the starch base mixture (e.g., feed material). Typically, the
rolled expanded products contain from about 5% to about 9% by weight moisture and more typically from 6% to 7%.
The high resiliency extruded expanded starch products of this invention are useful in applications to shaped articles for packaging materials. In addition to loose fill packaging material, starch sheets for wrapping or other applications can be
made by the extrusion/rolling method of this invention. In particular, starch sheets produced by methods herein can be combined with paper and other biodegradable materials to make biodegradable laminate | | |
