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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to a method for manufacturing biodegradable products,
to an apparatus which can be used therefor and to products obtainable
according to this method. The products to be manufactured according to the
invention have a foamy structure. More particularly, the foamy material
always comprises at least three parts: two relatively dense layers on the
outside, which as it were, form a skin, and between them a foam structure
as core. The dense layers are firm and strong and consist of substantially
closed, small cells. The foam structure of the core is generally open,
which means that the cells have burst to allow the gases evolving during
the manufacture, for instance water vapor or carbon dioxide gas, to
escape. The cells generally have a firm and solid cell wall due to the
relatively high pressure and temperature during the process.
In this description, "gelatinization" is understood to mean a change of a
natural polymer from a slightly or completely loose granular or comparable
granulate form into a cohesive form which may or may not be dry and/or
foamed, in which stretched polymers are present which are mutually bonded
to a limited extent, if at all. That is to say, a transition occurs from a
solid substance, a colloidal solution or suspension to a more homogeneous
fluid mass. Depending on the polymers used, "gelatinization" should
therefore be understood to include, for instance, gelling, gellating and
the like.
In foamed products where only gelatinization occurs, as a result of gas
evolution, bubbles are formed in the mass to be foamed, substantially
after gelatinization. This process occurs at relatively low temperatures
and pressures. Over the entire cross section, such products have
approximately the same structure of relatively small cells with walls of
substantially uncross-linked natural polymers.
In this description, "baking" is understood to mean a method in which both
gelatinization and cross-linking occur, at relatively high temperature
and/or pressure. As a result, the formation of gas arises relatively soon,
so that bubbles are already formed prior to or during gelatinization. As a
result of inter alia the high pressure adjacent strongly heated parts, the
polymers cross-link quickly when using a mold or like baking form with a
temperature at or above the baking temperature.
These baked products have a core with relatively large cells, enclosed
between skin parts with relatively small cells. The cell walls have a
relatively high density and the natural polymers included therein are
cross-linked to a high extent, which means that they have entered into
mutual chain bonds. Such a baked product therefore has a sandwich-like
structure.
International patent application 91/12186 discloses a method for
manufacturing biodegradable products by heating in a baking mold a batter
which comprises at least natural polymers in the form of starch or
derivatives thereof. The batter is introduced into an open platen set, for
instance, a wafer iron, whereafter the platen set is closed and the batter
is "baked". This results in a thin-walled product which is biodegradable
and yet firm and is relatively well resistant to moisture, at least as
long as the skin of the product is not damaged. The product is ready
immediately and so requires no post-treatment. Owing to the heating to
relatively high temperatures, a structure of blown cells and cross-linked
starch is created in the product. The products are relatively cheap to
manufacture, have good storing properties under different conditions, are
light and convenient in use and, owing to their biodegradability, are
environment-friendly.
A disadvantage using of platen sets is that the batter is introduced into
an open mold which is subsequently closed and, for instance, is passed
through a continuous oven, where it is heated, for instance by gas
burners. Energetically speaking, this is not very efficient and moreover
the temperature in the baking mold is not properly controllable and may
vary considerably during the baking process, which is disadvantageous to
the quality of the products. Moreover, the products which are obtained
according to this method are not particularly dimensionally stable and
allow no or only very slight differences in wall thickness, because
otherwise no homogeneous structure can be obtained. A further disadvantage
of this method is that the introduction of the batter and the removal of
the product is very laborious and will often lead to failure in the
production. Moreover, with this method no products can be manufactured
that are non-withdrawable, so that the molding freedom is limited.
European patent application 0 512 589 discloses a method for making
thin-walled biodegradable products, in which platen sets are likewise
used. In this known method, a starch-containing dough is introduced into
an open mold cavity in one of the platens, whereafter the platen set is
closed and is subsequently heated to a temperature at which only
gelatinization occurs but at which the dough is not "baked". The
temperature is therefore kept relatively low with respect to the
previously described method. With this method, products are obtained which
are directly ready for use, that is, they do not require any
post-treatment. It is true of the products obtained by this procedure too
that they have little dimensional stability and permit no, or only very
slight, differences in wall thickness in order to preserve a homogeneous
structure. Since the products are not "baked", they are less stiff and
exhibit relatively poor resistance to, for instance, water and varying
temperature conditions. Moreover, it is true of this method too that
filling the platen sets and removing the products is cumbersome and
time-consuming, that the products can easily be damaged when being removed
and cannot be non-withdrawable, so that the freedom in the molding design
is limited.
International patent application 93/08014 discloses a method for
manufacturing biodegradable products, in which the products are
manufactured by extrusion of a mixture comprising at least starch or
derivatives thereof. In this method, a dry, crude starch with less than
30% water is mixed with mild acid, which mixture is stirred with a
carbonate which, through reaction with the acid, can give rise to CO.sub.2
gas. This mixture is introduced into an extrusion tank and mixed with
water, while being pressurized and heated to such an extent as to give
rise to gelatinization of the starch. In the extrusion tank the acid is
reacted with the gelatinized starch, in such a manner that the average
molecular weight thereof decreases and the uniform bonds of the starch
chains are broken, while moreover, through reaction with the carbonate,
CO.sub.2 gas is produced for blowing up the modified starch. The thus
obtained mixture of blown starch with altered (micro) structure is
thereafter forced through an extrusion die, whereby under the influence of
the CO.sub.2 gas a closed-cell structure is obtained with a density of
less than 0.032 g per cubic centimeter. Owing to this structure, the thus
obtained product has elastic properties and permits of rapid biological
decomposition.
A disadvantage of this known method is that the raw materials are to be
supplied in relatively dry form and in the extrusion tank, are to be mixed
with water under simultaneous increase of the temperature in the tank,
whereby the desired gelatinization occurs. To that end, the mixture must
be heated, which is difficult to effect homogeneously in view of the
relatively large mass. As a consequence, the process is relatively poorly
controllable. A further disadvantage is that the products obtained in this
way have only limited durability and are not water-resistant and moreover
are not particularly dimensionally stable. As a result of the extrusion
process and the strong expansion occurring after the extrusion, the
freedom of design in this method is limited.
European patent application 0 118 240 discloses a method for manufacturing
biodegradable medicament capsules and like products by injection-molding
from a starch composite. To that end, a starch mixture with a low water
content is introduced into a closed space, in particular the hopper of an
injection-molding machine, where plasticization of the mixture is provided
for at a suitable specific temperature, pressure and humidity. The
temperature and pressure are increased to such an extent that the mixture
is adjusted to above the vitrification point. Thereafter the plasticized
mixture is forced into a cold mold and maintained under pressure, until
the, or each, product has cooled off sufficiently, whereafter the mold is
opened and emptied.
The advantage of this known method is that dimensionally stable
biodegradable products can be manufactured relatively fast. However, the
possible dimensions of products that can be manufactured with this method
are limited, owing to the flow path in the mold. In fact, the plasticized
mass forced into the mold is cooled directly, which gives rise to
solidification and prevents flow into the mass relatively soon after entry
of the mold. Moreover, no cross-linking of the starch in the mass occurs,
so that the products have relatively weak strength properties and exhibit
relatively poor resistance to water and moist conditions in general. In a
moist environment the products will take up a great deal of water and
thereby become slack; conversely, in a dry environment moisture will
evaporate from the products, so that they become hard and brittle. The
products obtained with this method have a high density and have no foamy
structure.
European patent application 0 407 350 proposes an improved composition of
starch composites for use in inter alia a method according to the
above-described European patent application 0 118 240, by which, for
instance through casting or extrusion, products can be manufactured with
better strength properties and better resistance to different conditions,
in particular as regards humidity and temperatures. To that end, to the
composite of starch, a thermoplastic plastic is added, whereafter the
mixture is transformed under very well regulated and controlled conditions
into a melt allowing subsequent casting or extrusion. By controlling the
conditions, it can be ensured that the thermoplastic plastic melts and
mixes with the starch without the starch disintegrating.
An advantage of this known method is that the products are dimensionally
stable, have good strength properties and exhibit relatively good
resistance to humidity and temperature fluctuations. One of the
disadvantages of this known method is that the thermoplastic plastic must
be added. This reduces the biodegradability and it is less attractive from
an environmental point of view. Further, these products too have a
relatively high density. Moreover, when using the composite for injection
molding, the above-mentioned drawbacks remain, such as, for instance, the
laborious and costly preparation of the composite, the limited
possibilities as regards dimensioning and the absence of cross-linking of
the natural polymers.
International patent application 95/04104 discloses a method for
manufacturing foamed, biodegradable products from starch-containing raw
materials, in which an amount of starch is liquified in a pre-stage by
heating to a temperature far above the gelatinization temperature,
whereafter an amount of water-saturated ramie fibers is admixed. This
mixture is thereafter passed into or through a mold or converted to a dry
granulate. Upon heating of the mixture, the water is to escape from the
ramie fibers and to function as blowing agent. When using this known
method, a substantially dry granulate of starch is to be strongly heated
in the pre-stage, which granulate therefore cannot form a liquid batter.
Accordingly, this method suffers from the above-mentioned disadvantages of
the gelatinization of the mass prior to its introduction into the mold,
such as for instance the laborious and costly preparation of the
composite, the limited possibilities as regards to dimensioning.
International patent application 92/13004 discloses a method for
manufacturing solid and foamed, biodegradable products from
starch-containing raw materials. In this apparatus, an amount of moist
(20% water) starch is mixed with inter alia some water and mixed in a
heated vat in order to obtain gelatinization of the mass, whereafter it is
processed through an outlet opening into film or sheet. Thereupon, the
film can be deformed into, for instance, dish products. In this known
method too, the liquidity of the suspension to be processed is obtained by
heating the mass, to above the vitrification temperature prior to the
definitive processing. Moreover, to the heated mass an amount of steam or
alcohol vapor is added. The processing means (for instance rollers) are
maintained at a relatively low temperature (70.degree. C.). Further,
European patent application 0 634 261 discloses a method for manufacturing
biodegradable products utilizing a kind of injection-molding technique,
which starts from a mixture of a first and second biodegradable starting
material. The first has a melting temperature of above 100.degree. C., the
second of less than 100.degree. C. Either a substance which contains water
is added to the starting material, or water is incorporated in the
starting material, in such a manner that it can provide for the blowing of
the cells. In an extruder press, the mass is heated to above the
gelatinization temperature of at least the first starting material, mixed
and pressurized and subsequently sprayed into a mold cavity provided in a
pressurized space. After introduction of the mass, the pressure is
removed, so that the water in the mass expands, blows the cells and exits
through the permeable wall of the mold cavity. Such a method requires a
complicated composition of starting materials, which moreover are not
entirely biodegradable. Further, this known method has the above-mentioned
disadvantages resulting from the gelatinization of at least a part of the
mass prior to its introduction into the mold, such as for instance the
laborious and costly preparation of the composite, the limited
possibilities as regards to dimensioning. In particular, as a result of
inter alia the porous walls, the outer wall portions of the products
manufactured according to this method will not have a dense, compact wall
but a uniform distribution of cells of uniform size throughout the product
thickness.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method for manufacturing
biodegradable products, in which the supply of the starting material is
simple, in which the manufactured products are simple to remove from the
mold, which allows a relatively great freedom in design and whereby the
manufactured products have a good dimensional stability and exhibit
relatively good resistance to different conditions, including moist
environments and temperature fluctuations. In particular, the invention
relates to a method for manufacturing biodegradable products with a blown,
foamy structure, wherein a mass comprising at least natural polymers such
as starch is passed under pressure into or through a mold and the mass is
heated in the mold, in a manner such as to give rise to gelatinization and
crosslinking of the natural polymers, while the mass prior to the
introduction into the mold has a temperature which is below the
gelatinization temperature and in the mold is maintained at least for some
time at a temperature which is above the gelatinization temperature.
Owing to the supply of the mass from which the or each product is to be
formed at a temperature which is below the gelatinization temperature, the
supply of the mass can be realized in a simple manner, for instance via
pumps and pipes. Moreover, a stock of the mass can be previously prepared
and be fed to a processing apparatus directly from a storage tank. By
subsequently passing the mass under pressure into or through the mold and
only heating it in the mold, it is ensured that the mold is always filled
sufficiently. The flow path, that is, the, or each, path traversed by the
mass to and in the mold can then be long to very long with respect to the
cross sections of the passages. Only in the mold does the, first
gelatinization of the natural polymers occur, and then the cross-linking
of those polymers.
Due to the cross-linking that occurs, a firm product is obtained. A natural
polymer provides for a relatively firm skeleton which extends around
preferably continuous cells that form in the mold due to moisture or other
blowing agents which, as a result of the heat in the mold and the pressure
therein, attempt to escape from the mass and thereby form bubbles. As a
result, the product obtained has a blown foamy structure. Since the
natural polymer provides for a relatively stiff jacket, the thus obtained
product is dimensionally stable upon exiting from the mold. Depending
inter alia on the extent of cross-linkage, the product obtained is more or
less flexible.
Since the mold is heated and not the mass prior to being forced into the
mold, the temperatures in the mold can be properly controlled, both for
the mold as a whole and for each separate portion thereof. As a result,
products can be manufactured with different and varying wall thicknesses
and with different mechanical properties. In fact, by heating more or less
and/or for a longer or shorter period and adjusting, for instance, the
pressure, for instance the extent of cross-linkage of the polymers can be
controlled locally, so that the mechanical and physical properties are
influenced. All this can be simply determined by those skilled in the art.
In an advantageous embodiment, a method according to the invention is
further characterized by the fact that the cells substantially have a wall
of natural polymers cross-linked during heating, while the cell size
across the wall thickness of each product is varied, such that it
decreases in outward dimension.
By controlling the process conditions, in particular the feed rate of the
mass, the temperature of the mold and the pressure in the mold, a product
is manufactured in which the cells are smaller adjacent the mold wall than
centrally between the walls of the mold. In other words, in the product,
the cell size increases from the inside to the outside. Thus, a relatively
closed, water-tight skin is obtained which properly protects the product
from premature decline, while the inside of the product comprises
relatively large cells which can keep the product light and flexible. A
further advantage of the skin with a relative large density is that, as a
result, a taut and smooth surface is obtained which affords the product an
agreeable appearance, has a pleasant feel, is simply removable from the
mold, is simply printable and moreover hygienic. Accordingly, in contrast
with the known methods, a cell structure is obtained which is
non-homogeneous, at least viewed across the wall thicknesses.
In a further advantageous embodiment, a method according to the invention
is characterized by products formed with a number of sheet, bar, or
dish-shaped parts, each having, in at least one direction, a dimension
that is relatively small with respect to the total dimensions of the
product.
By building up the products according to the invention from dish or sheet
parts, each having at least one slight thickness with regard to the other
dimensions, at least with regard to outside dimensions, voluminous
products can be manufactured which can yet be supplied at all points with
so much heat during the preparation that the desired extent of
cross-linkage occurs. Thus, dish-shaped products can be manufactured, that
is, also block-shaped products, with, for instance, a recess in which a
product to be packaged can be wholly or partly received, and filler blocks
for, for instance, packages, can be manufactured. Also, for instance
through extrusion, for instance hollow or finned profiles can be
manufactured. A further advantage of the relatively thin sheet parts is
that, as a result, relatively great flexibility is obtained while the
products maintain the desired strength properties and volumes.
In a first preferred embodiment, a method according to the invention is
characterized by the mass being formed by a liquid batter, comprising a
suspension or solution(s) of at least the natural polymers, such as
starch, in a liquid.
By using a batter which is liquid below the gelatinization temperature,
preferably at room temperature, the batter can be supplied in a simple
manner, for instance via pipes and using simple pumping means. Moreover, a
stock of the batter can be previously prepared and be fed to a processing
apparatus directly from a storage tank. In this connection, the liquidity
of the batter provides the advantage that the flow path in the mold is
particularly long. The water in the batter functions as blowing agent and
moreover, upon evaporating from the mold, provides space for the expansion
of the cells.
The batter preferably consists entirely of biodegradable constituents, in
particular, in the form of a suspension. As a result, good flow properties
of the batter are maintained and crude starting material such as starch
can be used, for instance potato starch or tapioca. Moreover, such a
suspension can be simply stored, at least better than a mixture already
gelatinized.
In Dutch patent application 9300102, incorporated herein by reference, a
number of examples of such batters are described. These batters comprise
500-1500 parts by weight of starch or starch derivatives, 0.5-50 parts by
weight of xanthan gum, 5-250 parts by weight of a reactive siloxane and
25-300 parts by weight of an inert filler in water. Additionally,
preferably 0.5-50 parts by weight of a salt are further included. However,
the embodiments described should not be construed as limiting in this
respect When other batter compositions are used, the processing
conditions, such as pressure, temperature and time, will often have to be
adjusted.
Mentioned as suitable natural polymers are native starch, for instance
potato starch, maize starch, wheat starch, waxy maize starch, tapioca
starch, pea starch, high-amylose starch or rice starch. Preferably,
however, potato starch is used, whose amolypectin content can vary between
75 and 100%. Starch derivatives can also be used, for instance, starch
which has been modified by etherification, ester-4 cation, acid
hydrolysis, oxidation, cross-linking -and/or the action of enzymes.
In an alternative embodiment, a method according to the invention is
characterized by introducing the mass into the mold in a relatively dry
condition, preferably in granulate form. Before being introduced into the
mold, the mass is slightly prefoamed without this giving rise to
gelatinization.
The use of relatively dry, optionally slightly prefoamed starting material
provides the advantage that relatively little water or other moisture
needs to evaporate in the mold, which has appreciable energetic
advantages, the more so since the mass only needs to be heated in the
mold, not in the pre-stage. The mass can for instance consist of granulate
material, in particular more or less spherical particles having small to
very small dimensions with respect to the passage openings to and in the
mold. This granulate material can contain a blowing agent, for instance in
the form of water or blowing agents simply released and/or evaporating
upon heating, such as bicarbonates, which provide for gas evolution
through decomposition at elevated temperature.
As starting materials, for instance, the natural polymers mentioned in
respect of the batter can be used.
In a further advantageous embodiment, a method according to the invention
is characterized in that as mold an injection mold is used.
By using an injection mold in a method according to the invention, products
can be manufactured with both regular and irregular shapes, which are
dimensionally stable and can have varying wall thicknesses. Products
manufactured in this manner can, for instance, be used as sheet and dish
parts, trays and boxes and like dish-shaped packages and as filler for,
for instance, packaging products in boxes and the like. One of the
important advantages that can be achieved with this method is that a
greater freedom in design is obtained than when platen sets are used. The
products can be manufactured in withdrawable as well as non-withdrawable
manner, since divisible cores and the like can be readily utilized. As a
result, for instance undercuts can be integrally molded. Moreover, greater
differences in height can be incorporated in the product in that the flow
path can be longer and gravity has no influence, at least no appreciable
influence, on the distribution of the mass.
In a further advantageous embodiment, a method according to the invention
is characterized in that an extrusion die is used. When using an extrusion
die in a method according to the invention, biodegradable sections and the
like can be manufactured in a simple manner with the above-mentioned
advantages of the cross-linked structure of the natural polymers. Owing to
the mass being supplied in cold, preferably liquid form, the preparation
thereof is particularly simple and products with the desired properties
can be manufactured in substantially one processing pass. In this manner,
for instance, sheets and sections can be extruded which are used in great
lengths or can be divided up and, for instance, be used as loose filler in
the packaging of products in boxes, crates, bags and the like. Extrusion
and the use of an extrusion die should herein be understood to mean, in
particular, forcing a molding mass under pressure through a relatively
small orifice, this orifice determining substantially at least one cross
section of the product. The delivery pressure can, for instance, be
generated with a pump or a plunger.
Products that are manufactured with a method according to the invention can
in a general sense be designed light with respect to the volume, have
sufficient strength and elasticity and are properly resistant to different
conditions, in particular when using a "skin" with a relatively high
density and a core with a relatively low density.
During the manufacture of the products according to the invention, gas
formation through evaporation of water or under the influence of blowing
agents occurs so fast that foaming occurs concurrently with, or preferably
before the gelatinization. At elevated pressure and/or temperature this
effect is achieved, while further more solid material is "compressed" as
cell wall. This not only yields a core made up of large cells with firm
cell walls but also skin layers with a higher degree of densification of
firm small cells.
In addition, there may be a number of other conditions that must be met to
obtain the desired result.
The colloidal particles and corresponding conditions must meet requirements
to provide for forming foam, which requires, among other things, a
particular load and particular surface tensions, in conjunction with an
internal and external pressure in the foam bubbles.
The charging of the mold cavity must be complete within a very short time,
which entails requirements for the "flow" properties and the thrust:
during this short period the "flow" properties must remain sufficient to
ensure complete filling, while the driving force, the propellant or "foam"
gas, must remain present in a sufficient, amount to advance the mass
(which is increasingly hard to move). Flow should herein be understood to
include both the flow of a liquid, such as the liquid batter, and the flow
of a granulate-form, relatively dry substance such as small rolling and
sliding granules or powder, whether or not in slightly prefoamed form.
Accordingly, the length of the flow path is at least dependent on the
liquidity of the starting material and, given equal conditions, will be
greater for a liquid or suspension than for granulate material. Moreover,
the length of the flow path will be positively influenced by a greater
difference between the low supply temperature and the temperature of the
mold during the baking.
The invention further relates to apparatuses suitable for use in a method
according to the invention.
The invention moreover relates to biodegradable products manufactured with
a method and/or in an apparatus according to the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
To clarify the invention, exemplary embodiments will be described with
reference to the drawing.
FIG. 1 shows a biodegradable product, in particular a filler block,
manufactured by injection molding, in perspective view with a part broken
away.
FIG. 1a shows, on an enlarged scale, twice a cross section of a wall of a
product according to FIG. 1.
FIG. 2 schematically shows in cross-sectional view an injection-molding
apparatus according to the invention.
FIG. 2a shows on an enlarged scale a part of a mold, with mold cavity, in
cross-sectional view.
FIG. 3 shows a biodegradable product, in particular an insert tray for a
storage box, manufactured by injection molding, in cross section.
FIG. 4 shows a biodegradable product manufactured by extrusion, in
perspective view.
FIG. 5 schematically shows in cross-sectional view an extrusion apparatus
according to the invention.
DETAILED DESCRIPTION
The product shown in FIG. 1 is a filler block 1, for instance suitable for
locking a housing of a computer C in a box D with a proper fit. The
computer C and the box D are schematically represented in broken lines and
are mentioned only by way of example. The filler block 1 consists of a
body 2 and a number of first 3 and second ribs 4 extending downwards from
the body 2. The first 3 and second ribs 4 extend approximately at right
angles to each other. Enclosed between two first ribs 3, two second ribs 4
and the body 4 is a cavity 5 of approximately rectangular cross section.
The overall dimensions (length L, width B and height H) of the filler
block 1 are large with respect to the amount of material used, and hence
with respect to the weight, is light when compared with a comparable solid
block of the same material.
The first ribs 3 have a first part 6 which is relatively high with respect
to a second part 7 thereof. The second ribs 4 likewise have a first part 8
which is relatively high with respect to the second part 9 thereof. The
relatively high parts 6, 8 are arranged together, as are the relatively
low parts 7, 9. The low parts 7, 9 thus define an imaginary bottom surface
10. The approximately vertically extending transition parts 11 between the
high parts 6 and 8, respectively, and the low parts 7 and 9, respectively,
define two imaginary wall surfaces 12 which include an angle with each
other and with the bottom surface 10. The bottom surface 10 and the wall
surfaces 12 define an imaginary space in which, for instance, a corner of
the computer C can be received.
When the computer C is accommodated in a box D, the filler block 1 is
received between the computer C and three wall panels of the box D with a
proper fit. Preferably, such filler blocks 1 or comparable, suitably
shaped filler blocks 1 are fitted between the computer C and the box D at
several points, so that the computer C is prevented from shifting and
moreover a shock-absorbing capacity is obtained, so that damage is
avoided. It is noted that the ribs can be arranged in various orientations
and positions with respect to each other and the body 2, and more or fewer
(groups of) ribs can be used. Thus, for instance, the body can be arranged
on the side proximal to the product to be accommodated, so that a greater
contact surface between the product and the filler block is obtained. In
addition, cavities 5 can be open in different directions, or at least
locally all or some ribs can extend in one direction only and, for
instance, have a meandering, sinusoidal or otherwise bent shape. Further,
openings and recesses can be provided in the ribs and/or in the body.
These and many ocher modifications are considered to fall within the scope
of the invention.
The filler block according to FIG. 1 is pre | | |
