Biodegradable polymeric mixtures based on thermoplastic starch

Details Biodegradable polymeric mixtures based on thermoplastic starch Biodegradable polymeric mixtures based on thermoplastic starch

1998-12-18

2001-05-22

Boykin, Terressa M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

Reexamination Certificate

active

06235815

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a biodegradable polymeric material essentially consisting of, or based on, thermoplastic starch, and to a polymer mixture comprising thermoplastic starch, a process for preparing a biodegradable material, a process for preparing a polymer mixture, and also to uses of the biodegradable material and of the polymer mixtures comprising thermoplastic starch.
Biopolymers based on renewable raw materials which are suitable for preparing biodegradable materials (BDM) are largely based on starch and comprise in particular thermoplastic starch, and also polymer mixtures made from thermoplastic starch and from other degradable polymeric components, such as polylactic acid, polyvinyl alcohol, polycaprolactone, tailored copolyesters made from aliphatic diols and from aliphatic or aromatic dicarboxylic acids, and also degradable polyester amides, which, with thermoplastic starch in an anhydrous melt via ester reactions and/or as polymer combinations form new degradable polymeric materials with a high proportion of renewable raw materials. There may be addition of other naturally occurring materials as additives and plasticizers such as glycerol and its derivatives, and hexahydric sugar alcohols such as sorbitol and derivatives of these.
EP 397 819 has for the first time specified a process for preparing TPS and also defined the new starch material known as thermoplastic starch (TPS) and specified the important differences, in particular in plastics processing technology, from the destructured starch which has been known for a relatively long time.
The thermoplastic starch is prepared with the aid of a swelling agent or plasticizer, not only without adding water but more particularly using dry or dried starch and/or starch which has been dried by devolatilization during the extrusion process while in the melt. Starches in the form of native starches commercially available comprise 14% of water, and potato starch as much as 18% of natural moisture at equilibrium.
If a starch with more than 5% of moisture is plasticized or pasted with exposure to temperature and pressure, this always gives a destructured starch. The preparation of the destructured starch is an endothermic procedure.
In contrast, the preparation of the thermoplastic starch is an exothermic procedure. In this case the essentially anhydrous (<5%) native starch is homogenized in an extrusion process with an additive or plasticizer (e.g. glycerol, glycerol acetate, sorbitol) which lowers the melting point of the starch, and is melted within a temperature range of from 120 to 220° C. by introducing mechanical energy and heat. The thermoplastic starch is free from crystalline fractions, or at least the crystalline fractions are less than 5% in the TPS, where the crystalline fractions remain unchanged and very small. The parameters of the process bring about a permanent rearrangement of the molecular structure to give thermoplastic starch, which now comprises practically no crystalline fractions and, contrasting with destructured starch, does not now recrystallize.
In destructured starch, the crystalline fractions immediately after preparation are likewise small, but these increase again when destructured starch is stored. This feature is also apparent in the glass transition temperature, which for thermoplastic starch remains at −40° C., whereas in destructured starch, in contrast, it rises again to above 0° C. For these reasons, destructured starch and materials or blends based on destructured starch gradually become relatively brittle on storage, and, depending on temperature and time elapsed, the stresses contained within the polymer cause creep and distortion of the material (memory effect).
A differentiation of destructured starch and thermoplastic starch is:
Preparation
Destructured
Thermoplastic
and properties
starches
starches
Water content
>5 to 50%
<5%, preferably
anhydrous in the melt
phase
Plasticizer
Water, glycerol,
Glycerol, sorbitol,
Additives
sorbitol,
glycerol acetate,
mixtures
essentially anhydrous
Crystalline
>>5% rising on
<<5%, no crystalline
fractions
storage
fractions, unchanged
on storage
Preparation
Endothermic
Exothermic
process
Glass
>0° C.
<−40° C.
transition
temperature
Storage
Increasing
Remains flexible
properties
embrittlement
Analytical
X-ray
X-ray diffraction of
differentiation
diffraction of
the crystalline
the crystalline
fractions
fractions
When polymer mixtures based on thermoplastic starch are prepared, compatibilizers are used to homogenize the hydrophilic and polar starch polymer phase and the hydrophobic and nonpolar other polymer phase, which are either added or preferably are produced in situ (e.g. by transesterification)ecified temperature and shear conditions, to give processable granules. The technology of preparing these thermoplastic blends involves coupling together the phase boundaries between the low-compatibility polymers in such a way as to achieve the distribution structure of the disperse phase during processing via the ideal range of processing conditions (temperature and shear conditions).
The twin-screw extruders which, for example, are used for the compounding are preferably corotating twin-screw extruders with tightly intermeshing screw profile and kneading zones which can be individually temperature-controlled. The twin-screw extruders used for the TPS compounding or preparation of TPS/polymer blends preferably have eight compartments or zones which where appropriate may be extended to ten zones and have, for example, the following construction:
Extruder design: Corotating twin-screw extruder, for example
Screw length-processing length =
32-40
L/D
Screw diameter D =
45
mm
Screw rotation rate =
230
rpm
Throughput =
50-65
kg/h
Die, diameter =
3
mm
Die, number =
4
Zone 1
Compressing with
Feed zone temp. 60° C.
devolatilization,
Pressure - bar
gradual melting of the
mixture (native and
glycerol)
Zone 2
as Zone 1
Mixing and plasticization
Temp. 140° C.
Pressure >1 bar
Water content 4-7%
Zone 3
as Zone 1
Plasticization
Temp. 180° C.
Pressure >1 bar
Water content 4-7%
Zone 4
as Zone 1
Plasticization
Temp. 185° C.
Pressure >1 bar
Water content 4-7%
Zone 5
Devolatilization, water
extraction
Temp. 160° C.
Pressure vacuum 0.7 bar
Water content <1%
Zone 6
(Side feeder, metering-
Metering-in of other
in of the additional
polymers
polymers, such as PCL)
Temp. 200° C.
Pressure >1 bar
Water content <1%
Zone 7
Transition zone,
Homogenization and where
compression zone,
appropriate transesteri-
reaction zone
fication
Temp. 200° C.
Pressure >1 bar
Water content <1%
Zone 8
Metering zone, where
Homogenization and where
appropriate evaporation
appropriate transesteri-
of water of reaction
fication
Temp. 205-210° C.
Pressure >1 bar
Water content <1%
Outside the extrusion plant: cooling and conditioning of the extrudates, where appropriate absorption of from 0.3 to 4% of water as plasticizer in a water bath, extrudate granulation, and bagging.
The extrusion conditions given above for preparing thermoplastic starch or mixtures based on thermoplastic starch are substantially directed toward the example of a TPS/PCL (polycaprolactone) polymer mixture. The processing or extrusion conditions change, of course, for polymer mixtures of other types. The example given above is intended merely to show how the prior art prepares polymer mixtures based on thermoplastic starch.
In connection with the original German Patent Application DE 19624641.5, in which the present invention was presented for purposes of a priority application, the relevant search by the German Patent Office mentioned the following publications: Kunststoffe 82 (11), pp. 1086-1089 (1992), WO95/33874, WO94/28029, WO94/03543, EP 580 032, EP 404 727, U.S. Pat. No. 5,453,144, U.S. Pat. No. 5,321,064 and U.S. Pat. No. 5,286,770. All of the publications mentioned, insofar as they refer to starch, relate to native starch or destructured starch, i.e. they have no connection w

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