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Description  |
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INTRODUCTION AND BACKGROUND
The present invention relates to water resistant, thermoplastic, starch
materials which can be processed into environmentally safe products,
particularly moldings and thermoplastically produced sheets as well as
cast sheets.
Starch esters, particularly acetates, have been known for a long period of
time. A distinction is made between highly substituted starch acetates
having a degree of substitution (DS) of 2-3 and low-substituted starch
acetates having a DS up to a maximum of 1. Whereas highly substituted
starch acetates have not yet gained any technical importance,
low-substituted starch acetates are well-established commercial products.
Known acetates having a high degree of substitution (DS 2.5-3) are
cellulose and amylose triacetates. As described in the literature, the
properties of films consisting of amylose triacetate are similar to those
made of cellulose triacetate. These films were made of chloroform.
The process used to produce starch acetates, according to methods and
common practices in the chemical industry, is also generally known and
described in numerous citations (e.g., by using acetic anhydride, acetic
anhydride pyridine, mixtures consisting of acetic anhydride and glacial
acetic acid, ketene, vinyl acetate, and acetic acid; potato and corn
starch being the predominantly used starches). Relatively long reaction
times, and drastic reaction conditions, had to be accepted to obtain
highly substituted derivatives.
Especially disadvantageous in connection with these processes is the strong
decomposition of the starch molecules into relatively short chains which
no longer have the film-forming properties typical of starch.
Little has been written about the production of high amylose starch acetate
having a high degree of substitution. A paper by Mark and Mehltretter is
found in U.S. Pat. No. 3,795,670 and in the corresponding publication
"Facile Preparation of Starch Triacetates" in the journal Starke (1972),
Issue No. 3, pages 73-100. A commercially available, high amylose corn
starch from National Starch, having an amylose content of about 70%, was
used as the starch. By avoiding the above conventional processes and
substance components used therein, the goal of complete acetylization was
obtained by the well-calculated selection of the catalyst, variation of
its quantity, and with a reaction time of approximately 5 hours. The
acetate which was obtained after 5 hours, having a degree of substitution
of 3, could be cast together with a dichloromethane solution into
flexible, transparent sheets. Although no other high amylose starches were
used, it was assumed that the general process, as indicated, would be
usable for high amylose starches with the same course and results, and
that the resulting starch triacetates could furthermore be converted into
fibers.
However, as expected, the triacetates obtained by this process, as well as
sheets produced therefrom, are not fully biodegradable and/or compostable
within acceptable periods of time; no advantage over cellulose acetates
has been observed in respect to this point.
SUMMARY OF THE INVENTION
The object of this invention is to provide starch materials which have the
following characteristics: distinguish themselves by a high technical
(utility) value; can be processed into cast sheets and also
thermoplastically into moldings and sheets which are environmentally safe;
and, when exposed to environmental forces, will completely decompose into
natural substances which do not create any lasting environmental hazard.
This problem is solved by a water-resistant, thermoplastic starch material
which can be processed into environmentally safe, thermoplastically
produced products and cast sheets. It is characterized in that it consists
of at least one starch acyl compound (sparingly soluble in water and made
of amylose-rich starches and the derivatives thereof) and at least one
plasticizer effecting gelation and which is biodegradable.
The invention further provides a process for the production of starch
material as outlined above, which is characterized in that the acyl
components (particularly the long-chain fatty acids) are simultaneously or
successively reacted with the starch in a one-pot reaction without
isolation of the intermediate products. A process is also provided for the
production of starch material as described above, which is characterized
in that the acyl components (particularly the long-chain fatty acids), are
simultaneously or successively reacted with the starch in a one-pot
process without isolation of the intermediate products, the starch acyl
compound is then obtained from the reaction product by means of solution
fractionation. Another alternative relates to a process for the production
of thermoplastic sheets and moldings by extrusion or injection molding in
a known manner utilizing the starch material as outlined above, wherein
the plasticizer(s) of the starch acyl compound are preferably not added
until the material is fed into the processing means. One embodiment
concerns a process for the production of cast sheets in a known manner
utilizing the starch material as described above, which is characterized
in that ethyl acetate, ethyl lactate, n-butyl acetate, chloroform,
methylene chloride, acetone, ethyl acetoacetate, acetylacetone, and the
mixtures thereof are used as the solvent.
A further process relates to the use of the above-described starch material
for the production of thermoplastic sheets and moldings by extrusion,
injection molding and calendering. Finally, this invention concerns the
use of the starch material as outlined above for the production of cast
sheets.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have found that in order to be able to produce transparent,
flexible cast sheets, the former demand for complete acetate formation
(and thus a necessary starch acetate substitution degree of 3) could be
dropped when using plasticizers and special corn starches as well as the
now available cultivated high amylose corn and pea starches. This is true
even though the latter do not differ from the corn starch type used in the
above process according to U.S. Pat. No. 3,795,670 as regards their
amylose content.
It is also surprizing that uniform, thermoplastically processable materials
can be obtained at all when such starches (comprised preferably of 70%
amylose and 30% amylopectin) are used as a result of the corresponding
amylose products.
In the direct acetate formation of high amylose corn starch (i.e., without
subsequent DS adjustment as is necessary in the case of cellulose
acetates), it is possible to produce materials which not only form clear
solutions in organic solvents from which clean, transparent sheets can be
made, but which also supply clear materials when processed
thermoplastically.
When the degree of substitution of the acetates (which are obtained from
the employed corn starches and have these properties) was determined,
values around 2.5 and down to 2.2 were established and which are markedly
below a DS value of 3. This is all the more surprising because these
values are decidedly lower than those of the pure amylose and cellulose
acetates; it having been assumed, according to the state of the art, that
these values are absolutely necessary for the sheet production and
quality. Corresponding to the cited process, until now the person skilled
in the art also assumed that acetate derivatives having a degree of
substitution of less than 3, and obtained by direct acetate formation
without new saponification, do not form clear solutions as is also the
case for many cellulose acetates. However, according to the present
invention, when the plasticizers are used it is possible to directly
produce the desired products from the corn starch acetate (plus
plasticizer) with such low DS values.
According to the invention, surprisingly and contrary to the assumptions
made in the cited process, when another high amylose starch (e.g., high
amylose pea starch) is used, a high amylose pea starch acetate, having
properties markedly different from those of the corn starch, is obtained
with equal reaction conditions.
In this connection, the high amylose pea starch proved to be a much more
favorable raw material. Substitution degrees of approximately 2.5 are
obtained after a reaction time of only 2 hours, which together with the
plasticizer properties result in a material of the desired type. In this
way, the substitution degree can be reduced even below 2.5 without
deterioration of the properties of the resulting materials. By means of
prolonged reaction times, it is possible to increase the DS value up to 3,
and correspondingly reduce the plasticizer addition if desired, owing to
certain demands made as to the end products.
By the addition of the plasticizer, effects were obtained which could not
be foreseen. For example, the brittleness of the starch acetates is
reduced due to the plasticizer(s) effecting reversible gelation, without
sustained influence and/or reduction in the strength thereof, as is
usually the case. Accordingly, only the substances previously mentioned
have been established as plasticizers. The plasticizers under
consideration are starch acetate-dissolving liquids, preferably having low
vapor pressure; they have the necessary gelation capacity and/or a low
strength-reducing effect, as well as a dissolving property, which results
in a brilliant transparency and, as can be proved, are at the same time
fully biodegradable.
The following is stated concerning the above-mentioned reversible gelation.
According to the invention, increasing the amount of plasticizers in
combination with the solid starch acetates lead, in the case of mechanical
thorough mixing (kneading, shaking, stirring), initially to a strong
swelling of the solid starch acetates, then to extremely tough, sticky
compositions, and finally to transparent viscous quasi-solutions which
form gels when allowed to stand. These gels are again converted into
quasi-homogeneous solutions by renewed mechanical and additional heating
(optional). This applies to mixtures of starch acetate plus plasticizer(s)
as well as the addition of suitable, highly volatile solvents for the cast
sheet production. It has been found that the temporal stability of the
solutions is sufficient for the desired processes. For example, even those
solutions can be drawn out homogeneously as films at room temperature;
which, when allowed to stand at room temperature, would form gels that are
no longer castable and capable of being drawn-out homogeneously. The
drawn-out solution films dry to form brilliantly transparent sheets, the
gel formation also being the decisive factor for the brilliance of the
sheets.
The required low vapor pressure is, of course, not based on the vapor
pressure of the individual components but corresponds to the effective
vapor pressure of the mixture.
In addition, it has been found that compositions which are completely
decomposable (e.g., by means of the ethyl esters of citric acid, lactic
acid and the glycerol acetates as well as the phosphoric esters employed
as plasticizers) can be produced and processed into clear moldings by
means of common plastic processing machines such as extruders, calenders
and injection molders.
A biodegradation test carried out in compost soil with the derivative per
se, and the sheets and moldings produced therefrom, resulted in a rapid
attack and decay of the material so that compostability can be assumed. In
this regard, the plasticizers which were added for processing proved to be
of decisive importance for creating attacking components and
decay-promoting conditions which determine the degradation behavior.
It is also possible to produce clear, transparent, very flexible, and fully
biodegradable sheets from organic solvents such as ethyl acetate. It is
preferred to use 5 to 20% solutions, based on the sum of plasticizer and
starch acetate, the ratio of plasticizer to starch acetate preferably
ranging from 0.5 to 50%. Surprisingly, the mixtures without additional
solvent can be calendered successfully, with temperatures far below the
extrusion temperatures being more than sufficient to give brilliantly
transparent sheets. Thus calendering, extruding and also injection molding
are possible by use of the plasticizer and starch acetate alone.
According to the invention, novel starch materials and products having
different properties are obtained by minor additions of relatively
long-chain fatty acids (e.g., palmitic or stearic acid in the form of the
free acids and/or as fatty acid chlorides) during the acetate formation by
means of the starch fat acyl compound (e.g., acetic anhydride or the
corresponding mixed anhydrides). For example, sheets and moldings can be
made from these derivatives, which, as compared to those made from pure
high amylose corn starch acetates and pure high amylose pea starch
acetates, have a markedly greater flexibility, dimensional stability,
pliability, and wear resistance.
Along with the advantage resulting from the further improvement of the
starch acetate/plasticizer material by introducing the relatively
long-chain fatty acids (particularly the natural fatty acids having 12-22
carbon atoms or the derivatives thereof), there is also the technical
advantage that this process does not render the simple production of the
material more labor intensive, time-consuming or costly. Since it had to
be initially assumed that the introduction of the fatty acids into the
reaction product would not readily be possible, a one-pot reaction is also
possible in the matter of this especially advantageous modification of the
material (i.e., simultaneously or successively reacting the components,
particularly the long-chain fatty acids, with the starch in a one-pot
reaction without isolation of the intermediate products).
By fractionation of the starch acyl compound solution, it is also possible
to produce specific materials, having differing properties, which can be
processed thermoplastically into sheets and moldings.
In addition to said starches, particularly those having an amylose content
of over 70%, the resultant derivatives, such as hydroxypropyl and
hydroxyethyl starches having a low DS value of 0.1 to 0.2, are also usable
according to the invention as a raw material for acetate formation.
The below examples clarify the simple production of the materials according
to the invention, as well as their excellent suitability as starting
materials for cast sheets and thermoplastic moldings having unlimited
biodegradability. Solvents which are used for the cast sheet production
are selected from ethyl acetate, ethyl lactate, n-butyl acetate,
chloroform, methylene chloride, acetone, ethyl acetoacetate,
acetylacetone, and mixtures thereof.
EXAMPLES
Example 1
Production of high amylose corn acetate
460 g of Hylon VII are supplied to a 10 liter, four-neck flask with a
reflux condenser, drip funnel, and thermometer, and is blended by stirring
with 1500 ml of acetic anhydride. After about 5 minutes of thorough
mixing, 88 g of 50% NaOH are added dropwise. Then the mixture is heated
until a constant reflux occurs. The boiling temperature is approximately
125.degree. C.; in this regard, overheating (which may lead to burning of
the starch to the flask rim) is to be avoided.
After 1 to 2 hours, the viscosity increases until, after 3 to 4 hours, a
tough, brownish, clear composition has formed. After a period of about 5
hours (i.e., the required reaction time), 50 to 100 ml of acetic acid are
distilled off at 118.degree. C. and thereafter 200 ml of ethanol are added
dropwise. Then stirring takes place for another 30 minutes with slightly
reduced heating, and thereafter about 100ml of the resulting solvent
mixture (consisting of acetic ester and acetic acid forming when ethanol
is reacted with acetic anhydride) are distilled off at 102.degree. to
105.degree. C. Then heating is discontinued and the composition is allowed
to cool for 0.5 to 1 hour. Following this, 200 ml of ethanol are again
added dropwise. Thereafter, precipitation is slowly carried out with about
2 liters of methanol. The product is washed several times with alcohol,
the alcohol is then withdrawn, and the product dried in air. For further
processing, the product is finely ground and screened.
The yield was repeatedly from 670 to 680 g. The determined DS value was
approximately 2.7 (2.5-2.9). The desired materials used for cast sheets
and thermoplastic moldings can be provided by adding the indicated
plasticizers before or during further processing.
The process, in which relatively large amounts have already been used in
laboratory tests, can readily be applied to normal outputs in the kilogram
range by distilling off the solvent mixture prior to the alcohol
precipitation.
EXAMPLE 2
Production of pea starch acetate
As in Example 1, 150 g of pea starch are suspended in 600g of acetic
anhydride, and 33g of 50% sodium hydroxide solution is slowly dropwise.
The reaction time required to obtain an acceptable starch acetate, having
a DS value of 1.86, is only 0.5 hour; it is well usable for the material.
A well usable starch acetate having a DS value of 2.61 is obtained after 2
hours.
By means of this formula, a cast sheet obtained by using 30% of citric acid
ethyl ester (Citroflex 2) as the plasticizer, is transparent and pliable.
EXAMPLE 3
Starch acetate material on the basis of Hylon VII with palmitic acid
chloride and glycerol acetate
23 g of Hylon VII are supplied to a four-neck flask and blended by stirring
with 80 g of acetic anhydride. Then 4.4 g of 50% NaOH solution are added
dropwise. After a reaction time of 2 hours, 3.44 g of palmitic acid
chloride are slowly added dropwise and the mixture is stirred for another
3 hours. Isolation is carried out as described in Example 1. The
determined DS value is approximately 2.5.
7 g of this starch acetate were then blended with 3 g of glycerol acetate
and pressed at 180.degree. C. for 2 minutes without pressure and 2 minutes
with pressure (5 tons) to give a clear, elastic sheet that could be
deep-drawn.
EXAMPLE 4
Starch acetate material on the basis of Hylon VII with palmitic acid and
glycerol acetate
23 g of Hylon VII are supplied to a four-neck flask and blended by stirring
with 80 g of acetic anhydride and 8 g of palmitic acid. After a reaction
time of 5 hours, the product is isolated as described in Example 1. The DS
value is around 2.5.
7 g of this starch acetate were then blended with 3 g of glycerol acetate
and pressed at 180.degree. C. for 2 minutes without pressure and 2 minutes
with pressure (5 tons) to give a clear, elastic sheet that could be
deep-drawn.
On the one hand, Examples 3 and 4 show that the substitution of the acetate
by fatty acid is not restricted to certain portions. On the other hand, it
becomes evident that in the one-pot reaction the fatty acid component can
be used both from the very beginning and later on in the process.
Numerous other relatively long-chain fatty acids such as C.sub.6 -C.sub.24,
saturated, monounsaturated and polyunsaturated (as occurring in natural
oils and fats), can be substituted in place of palmitic acid and its
chloride.
EXAMPLE 5
Production of a cast sheet
80 g of solvent mixture consisting of 80% by volume of acetone and 20% by
volume of ethyl lactate are supplied to a 250 ml, two-necked flask having
a reflux condenser. 6 g of citric acid ethyl ester or 6 g of glycerol
acetate are added and the mixture is thoroughly stirred. Then 14 g of
starch acetate, produced according to one of the preceding examples, are
weighed and added, and the mixture is heated to approximately 80.degree.
C. while being vigorously stirred. When dissolution is completed, after 15
minutes to approximately 1 hour, pressure filtration may be used for
purification. Thereafter, the product, in a thickness of about 700 .mu.m,
is spread onto a glass plate by means of a coating knife. The dried sheet
has a thickness of about 50 .mu.m and is hung up for approximately 2 hours
after drying for the purpose of homogenization.
Acetic ester can also be used instead of the solvent mixture, and in the
laboratory it is also possible to use chloroform. Sheets obtained in this
manner exhibit an even better brilliance. Additions of triphenylphosphate,
triallylphosphate, tricresylphosphate, ethyl acetoacetate and
acetylacetone have a positive effect on the material properties of the
resulting sheets.
When a solvent mixture consisting of 70% by volume of acetone, 20% by
volume of ethyl lactate and 10% by volume of butylacetate is used, sheets
are obtained having a leathery surface structure.
EXAMPLE 6
Production of a pressed sheet
7 g of starch acetate are blended vigorously with 3 g of citric acid ethyl
ester and then placed between two teflon disks at 200.degree. C. on a
thermopress, initially for 2 minutes without pressure and then for 2
minutes with a pressure of 5 tons. The sheets thus produced can be
deep-drawn.
EXAMPLE 7
Processing of the material in an extruder
Citric acid ethyl ester, the compatible plasticizer, is not premixed with
the starch acetate (e.g., according to the above examples), but is
directly supplied in doses to the extruder during processing. The
temperature setting of the extruder was adjusted as follows: Zone I cold,
Zones II and III 150.degree. C., Zones IV and V 100.degree. C., and Zone
VI 150.degree. C. The temperature of the mixture was 149.degree. C. and,
with a speed of 151 rpm, the pressure was 10 bar. The nozzle diameter was
3 mm.
The material is easily processed. It was possible to cool the extruded
strand in a water bath. Cutting it into pellets did not pose any problems.
Modifications of the processing pressure (e.g., by nozzle diameter
variations) showed that the machine settings are widely variable, so that
the person skilled in the art can determine the optimum operating
parameters for the special material composition consisting of starch
acetate and plasticizer, depending on the amounts used in each case.
The above examples describe only a small portion of the total number of
usable substances and their quantity ratios. For example, NaOH was always
used as the catalyst. However, it must be assumed that another alkaline
solution can be employed as well. The DS value depends on the reaction
time and also on the solution fractionation. Although not necessary for
this material, this value can be raised to over 2.7 in the case of pea
starch if this is favorable to the products to be manufactured (less
plasticizer). Nevertheless, these few examples show that the full acetate
formation is no longer necessary. In particular, the time required for the
production of the starch acetate component of the material can be
advantageously reduced and simplified. Furthermore, the DS value is also
influenced by the substitution of fatty acyl component portions by
relatively long-chain fatty acids. Thus, it is possible to further lower
the DS value without lessening the quality of the product.
According to the invention, further examples of calendering the material
are described below:
EXAMPLE 8
35 g of starch acetate 15 g of ethyl citrate are thoroughly mixed and
processed into sheets on a rolling mill or calender which has been
adjusted to 105.degree. C.
EXAMPLE 9
35 g of starch acetate 15 g of glycerol triacetate are thoroughly mixed and
processed into sheets on a rolling mill or calender (100.degree. C.).
EXAMPLE 10
35 g of starch acetate 15 g of glycerol triacetate 1.5 g of adipic acid
dimethyl ester are thoroughly mixed and processed into sheets on a rolling
mill or calender (105.degree. C.).
EXAMPLE 11
35 g of starch acetate 10 g of glycerol triacetate 5 g of benzyl butyl
phthalate are thoroughly mixed and processed into sheets on a rolling mill
or calender (140.degree. C.).
The starch acetate referred to is a high amylose corn starch with the trade
name being Hylon VII. As described above, this starch is derivatized to
form the corresponding acetate.
The plasticizers indicated in the examples can be mixed with one another in
varying combinations (e.g., the combination of benzyl butyl phthalate and
ethyl citrate is also possible even though it is not specifically
mentioned); the plasticizer amounts may also vary. An amount of up to
about 50% of plasticizer can be blended with the starch acetate with the
composition becoming too soft only when this value is exceeded.
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