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BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to a moldable hydrophilic polymer
composition, for use in a molding device preferably an injection molding
device.
When in the following description the term is used hydrophilic polymers are
polymers with molecular masses from approximately 10.sup.3 to 10.sup.7
Dalton carrying molecular groups in their backbone and/or in their side
chains and capable of forming and/or participating in hydrogen bridges.
Such hydrophilic polymers exhibit in their water adsorption isotherm (in
the temperature range between approximately 0.degree. to 200.degree. C.)
an inflection point close to the water activity point at 0.5. Hydrophilic
polymers are distinguished from the group called hydrocolloids by their
molecular dispersity. For the maintenance of the molecular dispersity of
said hydrophilic polymers, a fraction of water--according to the working
range of the present invention--of 5 to 25% by weight of said hydrophilic
polymers must be included; provided that the temperature of said
hydrophilic polymers is in the working range between 50.degree. C. and
190.degree. C. of the present invention.
There are other hydrocolloids, not hydrophilic polymers in the sense of
this definition, which contain more or less spherical or fibrous particles
whereby those particles are composed of several macromolecules of
hydrophilic polymer within the molecular mass range of 10.sup.3- 10.sup.7
Dalton giving rise to particle sizes between 0.01-10 microns which is the
typical range of colloidal particles. It is a primary object of the
present invention to utilize hydrophilic polymer compositions in the
production of capsules.
B. Description of the Prior Art
Capsule-making machines have been developed to utilize dip-molding
technology. Such technology involves the dipping of capsule-shaped pins
into a gelatin solution, removing the pins from the solution, drying of
the gelatin upon the pins, stripping off the gelatin capsule parts from
the pins, adjusting for length, cutting, joining and ejecting the
capsules. Prior art capsule-making machines have utilized the combination
of mechanical and pneumatic elements to perform these functions at speeds
up to about 1,200 size 0 capsules per minute. While the above described
apparatus is in general suitable for the intended purposes, it is
desirable to produce capsules at considerably high speed, over 15,000 size
0 capsules per minute, while at the same time precisely controlling the
properties of the gelatin in order to produce the capsules hygienically
and with minimum dimensional deviations so that the capsules can be filled
on high speed equipment.
A prerequisite for any material to be moldable by an injection process is
its ability to pass a glass transition point at a temperature compatible
with the thermal stability of the material and the technical possibilities
of an injection molding device.
Shirai et al. in U.S. Pat. No. 4,216,240 describes an injection molding
process to produce an oriented fibrous protein product. The fibrous
product as obtained by this process, differs fundamentally from the
transparent glass like material of the capsules obtained from the present
invention. Furthermore to obtain a flowable mass for the molding process,
the protein mixtures used by Shirai et al. have to be denatured and thus
lose their capacity to undergo dissolution.
Nakatsuka et al. in U.S. Pat. No. 4,076,846 uses binary mixtures of starch
with salts of protein materials to obtain an edible shaped article by an
injection molding process. With the present invention shaped articles from
protein material, preferably gelatin and other hydrophilic polymers can be
produced without the addition of starch.
Heusdens et al. in U.S. Pat. No. 3,911,159 discloses the formation of
filamentous protein structures to obtain edible products of improved
tenderness. With the present invention shaped articles are produced
without a filamentous protein structure.
The use of an injection molding device for producing capsules of gelatin
and other moldable hydrophilic polymers with similar properties is new and
has not been suggested in the technical literature.
The present invention distinguishes from the known as described above, by
the nature of the compositions and by the recognition that gelatin and
other hydrophilic polymers possess a dissolution point within a
temperature range usable for an injection molding process, provided the
water content of the gelatin and other hydrophilic polymers lies within a
characteristic range, giving allowance to avoid any essential drying or
humidification processes of the capsules.
SUMMARY OF THE INVENTION
The present invention covers an improved hydrophilic polymer composition,
for use in an improved automatic injection molding device combined with a
microprocessor to control the optimum time, temperature, pressure and
water content of the composition informed shaped parts. The composition
has a molecular mass range of 10,000 to 2,000,000 Dalton or a molecular
mass range 10,000 to 2,000,000 and 10,000,000 to 20,000,000 Dalton.
The composition has a water content range of approximately 5 to 25% by
weight.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention both as to its organization and method of operation together
with the advantages thereof will best be understood by reference to the
following specifications and taken in conjunction with the accompanying
drawings.
FIG. 1 is a layout of the reciprocating screw injection molding device for
making capsule parts;
FIG. 2 is a schematic of the injection molding work cycle for making
capsule parts;
FIG. 3 is a schematic of the combined injection device microprocessor
apparatus for capsule parts;
FIG. 4 is an expanded schematic of the exit end of the injection molding
device;
FIG. 5 is the diagram of dependence of shear viscosity of gelatin within
the pertinent ranges of the shear rate in the present invention;
FIG. 6 is the diagram of molding area for gelatin within the ranges of
time, temperature, pressure and water content of the hydrophilic polymer
for the present invention.
FIG. 7 is the diagram of dependence of glass transition temperature range
and melting temperature range for the pertinent water content ranges of
gelatin a representative hydrophilic polymer, the subject of U.S. Ser. No.
698,264 the parent of this application;
FIG. 8 is the diagram of dependent of differential calorimeter scan in
which the heat consumption rate of the polymer covered by U.S. Ser. No.
698,264 application now U.S. Pat. No. 4,655,840 gelatin is plotted for the
pertinent temperature range of the present invention these values being
representative of all hydrophilic polymers;
FIG. 9 is a diagram of the logarithmic bulk elastic storage module of the
gelatin as representative of the temperature range of the present
invention;
FIG. 10 is a diagram of dependence of equilibrium water content of the
gelatin in the entire water activity range with is representative of the
hydrophilic polymers of the present invention; and
FIG. 11 is a diagram of dependence of differential heat of water adsorption
in the pertinent range of water content of the gelatin which is
representative of the polymers of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1 the injection molding device 27 generally consists
of three units: a hopper unit 5, an injection unit 1 and a molding unit 2.
The function of the hopper unit 5 is receiving, storing maintaining and
feeding the hydrophilic polymer 4 at a constant temperature and at a
constant water content. The hopper unit 5 comprises a vertical cylinder 30
having a closed top 31 which an inlet 32 therein to receive the
hydrophilic polymer 4. At the bottom of the vertical cylinder 30 is a
closed conical funnel 33 and a discharge outlet 34 to feed the hydrophilic
polymer 4 into an inlet 34 of the injection unit 1. There is an air duct
35 communicating between the closed top 31 and the conical funnel 33
wherein air is circulated by a blower 36, the air temperature is
maintained by a thyristor 37 and the air relative humidity is maintained
by a stream injector 38.
The function of the injection unit 1 is melting, dissolving in water, and
plasticizing in the extruder barrel 17 the gelatin 4 fed from the hopper
unit 5 into the extruder inlet 54 and injecting the plasticized
hydrophilic polymer 14 into the molding unit 2.
The function of the molding unit 2 is automatically holding, opening and
closing the mold 6 having capsule shaped cavities 19 therein, and ejecting
the capsule parts 7 therefrom.
Within the injection unit 1 the screw 8 both rotates and undergoes axial
reciprocal motion. When the screw 8 rotates, it performs the functions of
melting, dissolving water, and plasticizing the hydrophilic polymer 4.
When the screw 8 rotates, it performs the functions of melting, dissolving
in water, and plasticizing the hydrophilic polymer 4. When the screw 8
moves axially, it performs the function of injecting by transporting and
ramming the hydrophilic polymer 14 into the mold 6. The screw 8 is rotated
by a variable-speed hydraulic motor and drove 10, and its axial motion is
reciprocated by a duplex hydraulic cylinder 9.
Compression of the plasticized hydrophilic polymer 14 in front of the
rotating screw 8 forces back the screw assembly 20 containing the screw 8,
the drive 10 and the cylinder 9. When the screw assembly 20 reaches a
pre-setback position a limit switch 12 is contacted. When a defined time
has elapsed during which the hydrophilic polymer 4 becomes fully
plasticized the hydraulic cylinder 11 brings the screw assembly 20 forward
and uses the screw 8 as a ram for the plasticized hydrophilic polymer 14
to be injected through a valve body assembly 50 (FIG. 4) including a
one-way valve 15, a needle valve 23, nozzle 22 and an outlet port 21 into
the molding unit 2. The one-way valve 15 prevents the plasticized
hydrophilic polymer 14 from going back over the helical flutes 16 of the
screw 8. The extruder barrel 17 has steam heating coils 18 to heat the
gelatin 4 while it is being compressed by the screw 8 into hydrophilic
polymer plasticized 14. It is desirable for the plasticized hydrophilic
polymer 14 to be heated at the lowest possible temperature and to be
transported with the lowest possible temperature and to be transported
with the lowest possible speed of the screw 8. The speed of the screw 8
and the heating of the plasticized hydrophilic polymer 14 within the
extruder barrel 17 by the steam heating coils 18 control the quality and
the output rate of the plasticized hydrophilic polymer 14 injected into
the molding unit 2. The molding unit 2 holds the mold 6 having capsule
shaped cavities 19 into which the plasticized polymer 14 is injected and
maintained under pressure. Refrigerant cooling conduits 24 encirle the
mold 6 so that when the plasticized gelatin 14 in the model 6 has cooled
and sufficiently solidified, the molding unit 2 opens, the mold 6
separates and the capsule parts 7 are ejected.
Referring now to FIG. 1 and also to FIG. 2 which depicts the injection
molding work cycle for the polymer 4 (containing approximately 17% water
by weight) plotted against time. In general the work cycle of polymer 4 is
as follows in the injection molding device 27 of the present invention:
a. Polymer 4 is fed into the hopper unit 5 where it is received, stored and
maintained under condition of temperature ranging from ambient to
100.degree. C., pressure ranging from 1-5.times.10.sup.5 Newtons per
square meter (N.times.m.sup.-2) and water content ranging from 5 to 25% by
weight of the of the polymer.
b. the stored polymer is melted under controlled condition of temperature
ranging from 50.degree. to 190.degree. C., water content ranging from 5 to
25% by weight of gelatin and pressure ranging from 600 to
3000.times.10.sup.5 N.times.m.sup.-2,
c. the molten polymer is dissolved in water under controlled conditions of
temperature ranging from 50.degree. to 190.degree. C. pressures ranging
from 600 to 3000.times.10.sup.5 N.times.m.sup.-2 and water content ranging
from 5 to 25% by weight of polymer.
d. the dissolved polymer is plasticized under controlled conditions of
temperature ranging from 50.degree. to 190.degree. C., pressure ranging
from 600 to 3000.times.10.sup.5 N.times.m.sup.-2 and water content ranging
from 5 to 25% by weight of gelatin.
e. the plasticized polymer is injected into the mold 6 under controlled
conditions of temperature below 50.degree. C., injection pressure ranging
from 600 to 3000.times.10.sup.5 N.times.m.sup.-2 and a clamping force of
the mold 6 below approximately 600,000 Newton, and
f. the capsule-shaped parts 7 are ejected from the molded polymer within
the mold 6.
Beginning at point A of FIG. 2 the screw 8 moves forward and fills the mold
6 with plasticized polymer 14 until Point B and maintains the injected
plasticized polymer 14 under high pressure during what is called the hold
time from point B until Point C of FIG. 2. At Point A the one-way valve 15
at the end of the screw 8 prevents the plasticized polymer 14 from flowing
back from the nozzle 22 onto the screw 8. During hold time, additional
plasticized polymer 14 is injected, offsetting contraction due to cooling
and solidification of the plasticized polymer 14. Later, the outlet port
21, which is a narrow entrance to the molding unit 2 closes, thus
isolating the molding unit 2 from the injection unit 1. The plasticized
polymer 14 within the mold 6 is still at high pressure. As the plasticized
polymer 14 cools and solidifies, pressure drops to a level that is high
enough to ensure the absence of sink marks, but not so high that it
becomes difficult to remove the capsule parts 7 from the capsule-shaped
cavities 19 within the mold 6. After the outlet port 21 closes, at Point
C, screw 8 rotation commences. The plasticized polymer 14 is accommodate
in the increased cylindrical space in front of the screw 8 created by its
backward axial motion until Point d. The flow rate of the plasticized
polymer 14 is controlled by the speed of the screw 8 and the pressure is
controlled by the back pressure (i.e., the hydraulic pressure exerted on
the screw assembly 20) which in turn determines the pressure of the
plasticized gelatin 14 at the nozzle 22 in front of the screw 8. After
plasticized polymer 14 generation for the next shot into the mold 6, the
screw 8 rotation ceases at Point D. The polymer 4 on stationary screw 8
rotation ceases at Point D. The polymer 4 on the stationary scew 8
continues to melt from Points D to E by heat conduction from the steam
heating coils 18 on the extruder barrel 17. This period is called soak
time. Meanwhile, the solidified capsule parts 7 are ejected from the mold
6. Thereafter, the mold 6 closes to accept the next shot of polymer
gelatin 14. All of these operations are automated and controlled by a
microprocessor as hereinafter described.
Referring now to FIG. 2 and also to FIG. 3. The injection molding work
cycle of FIG. 2 is accomplished on the injection molding device 27 of FIG.
3 by hydraulic and electrical components and the corresponding circuits
controlled by the mircoprocessor 28 of FIG. 3.
Through the use of solid-state circuitry and speed, temperature, limit and
pressure switches fro the electric and hydraulic systems, the
microprocessor 28 of the present invention utilized command signals in its
memory 51 fro the parameters of time, temperature and pressure condition
of the Table 1 below for the injection molding work cycle of FIG. 2 to be
accomplished by the injection molding device 27 of FIG. 3 in producing
gelatin capsule parts 7.
TABLE 1
__________________________________________________________________________
Ranges of Time, Temperature and Pressure for the injection
Molding Work Cycle of FIG. 2:
POINTS
__________________________________________________________________________
Time 10.sup.-2 -1
10.sup.-2 -1
10.sup.-2 -1
10.sup.-2 -1
10.sup.-2 -1
(seconds)
Temperature ambient-100
50-190 50-190 50-190 50-190
(Celsius)
Pressure 1-5 600-3000
600-3000
0-3000 600-3000
((10.sup.5 .times. N .times. m.sup.-2)
(Newtons per square meter)
__________________________________________________________________________
Referring now to FIG. 3 illustrating the combined injection molding device
27 and microprocessor 28 utilizing the method of present invention.
The combined injection molding device 27 and microprocessor 28 comprises
six control circuits of which five are closed-loop, fully analog, and one
is on-off. Starting at molding cycle Point A in FIG. 2, the injection
molding work cycle operates as follows:
When sufficient plasticized gelatin 14 has accumulated in front of the
screw 8 (microprocessor time controlled) and also when the screw assembly
20 carrying the screw 8, drive 10 and hydraulic motor 9 has been pushed
far enough backwards against a constant back-pressure as controlled by
control circuit 3, limit switch 12 will be actuated by position sensing
circuit 14. Upon these two conditions control circuit 4 is actuated
causing the hydraulic fluid to flow into the forward portion of the
hydraulic cylinder 11. This rams the screw assembly 20 forward injecting
the plasticized polymer 14 into the mold 6 as molding cycle Point B of
FIG. 2 is reached, and, as controlled by the microprocessor 28, the screw
8 remains stationary in this forward position under high pressure for
certain period of time until Point C.
From molding cycle Point B of FIG. 2 onwards the plasticized polymer 14
cools down in the mold 6 and the port 21 closes at molding cycle Point C
of FIG. 2.
At molding cycle Point C of FIG. 2, the screw 8 starts to rotate again and
the hydraulic pressure reduced from the forward portion of the hydraulic
cylinder 9 to a pressure slightly less than the pressure set for the
backward portion of the hydraulic cylinder 9.
The barrel 17 is kept under constant pressure towards the mold 6 by the
pressure in the back position of the hydraulic cylinder 11. This is
achieved by means of the control circuit 2 where a proportional hydraulic
valve is controlled by a pressure sensor circuit I.sub.2.
As the screw 8 rotates a recharge of hydrophilic polymer 4 is made from the
hopper 5. During a certain time period and at a defined rotating speed of
the screw 8, controlled by control circuit 3, a precise amount of polymer
4 is fed into the extruder barrel 17. Control circuit 3 is actuated by
speed sensor circuit I.sub.3 measuring the rotating speed of the screw 8
and sensing back to a hydraulic proportional flow control valve O.sub.3
controlled by control circuit 3, thus assuring a constant rotating speed
of the hydraulic motor 10, irrespective of the changing torque resulting
from introduction of the polymer 4 recharge.
When the load time is completed, the screw 8 rotation is stopped and
molding cycle Point D of FIG. 2 is reached. The soak time from molding
cycle Points D to A of FIG. 2 allows for the polymer 14 to plasticize
completely under controlled temperature conditions as controlled by
control circuit 1.
A temperature sensor circuit I.sub.1 senses a thyristor heat regulator
O.sub.1 heating the extruder barrel 17 as directed by control circuit 1.
During the time interval from molding cycle Points B to E on FIG. 2, the
mold 6 has cooled down sufficiently so that the finished capsule parts 7
can be ejected from the mold 6.
After ejection of the capsule parts 7, the work cycle returns to Point A of
FIG. 2 where a certain volume of plasticized polymer 14 has accumulated in
front of the screw 8 (sensing circuit I.sub.4 is actuated and time has
elapsed) so that the work cycle of FIG. 2 can be repeated.
It is important to note the temperature and humidity control loops 5 and 6,
for the maintenance of precise water content of the polymer in the hopper
5, which is essential for proper operation at the desired speeds.
The microprocessor 28 includes a memory section 51 to store the desired
operating parameters; a sensing and signaling section 52 to receive the
sensing signals of actual operating conditions, to detect the deviation
between the desired and actual operating conditions, and to send signals
for adjustment through the actuating section 53 to the thyristor and
valves.
Referring now to FIG. 4 there is shown the valve assembly 50 including the
outlet port 21, the nozzle 22, the needle valve 23, and the one-way valve
15. These elements operate as follows:
At Point A in FIG. 2 the needle valve 23 is retracted for the outlet port
21 and the one-way valve 15 is retracted from the valve body 50 so as to
form an inlet opening 55 for plasticized gelatin 14 into the nozzle 22
which defines a charging chamber for plasticized polymer 14. The
plasticized polymer 14 is injected through nozzle 22 and into the mold 6
during the mold-filling time between Point A and B in FIG. 2. At Point C
in FIG. 2 the needle valve 23 is pushed forward so as to close the outlet
port 21 during which time between point C and E in FIG. 2, the mold 6 is
closed and the capsule part 7 in the mold 6 is cooling. The needle valve
23 remains closed between Point E and A in FIG. 2 during which time the
capsule part 7 is ejected from the mold 6. The total time period between
Point B and A in FIG. 2 must be less than 5 seconds in order that the
plasticized polymer 14 does not solidify in the nozzle 22. This is an
important aspect of the present invention because:
a. faster production times are made possible in order to achieve greater
output;
b. there is no loss of plasticized Polymer 14 in the production cycle due
to solidification in the nozzle 22 and the mold 6; and
c. there is a minimum risk of degradation of the plasticized polymer 14
because it remains in the production cycle for a short time and is only
utilized one in each production cycle because the plasticized polymer 14
is solidified in the capsule-shaped cavities 19 and not in the nozzle 22.
The one-way valve 15 and the needle valve 23 are actuated by a
spring-tensioned lever 25 which normally closes both the outlet port 21
and the nozzle 22 until the lever 25 is cam-actuated pursuant to signals
from the microprocessor 28.
The thermomechanical properties of gelatin, i.e. storage and loss shear
modules at different temperatures, are strongly dependent on its water
content. The capsule molding process of the present invention can be used
for the polymer with a water content preferably within a range of 5 to
25%. The lower limit is defined by the maximum processing temperature of
190.degree. C. which in turn cannot be exceeded in order to avoid
degradation. The upper limit is determined by the stickiness of the
finished capsules. The abbreviations in Table 2 below will be used
hereinafter in this application:
TABLE 2
__________________________________________________________________________
Abbreviations of Used Physical Parameters
ABBRE-
VIATION UNIT DESCRIPTION
__________________________________________________________________________
T.sub.a,p.sub.a
Degree C., N .times. m.sup.-2
Ambient temperature
and pressure.
H(T,P) KJoule .times. Kg.sup.-1
Enthalpy of the
hydrophilic polymer-
water system at a
given pressure and
temperature.
(T,P) N.sup.-1 .times. m.sup.2
Compressibility of
the hydrophilic
polymer at a given
temperature and
pressure. Its
numerical value is
the relative volume
change due to change
of pressure by a unit
amount.
(T,P) (Degree C.).sup.-1
Volumetric thermal
expansion coefficient
of the hydrophilic
polymer at a given
temperature and
pressure. Its
numerical value is
the relative volume
change due to change
of temperature by a
unit amount.
V(Q,T,P)
Kg .times. sec.sup.-1
Flow rate of the
hydrophilic polymer
at a given
temperature and shear
deformation rate and
pressure. Its
numerical value is
the volume of a melt
leaving the exit
crosssectional area
of an injection
molding device in
unit time due to the
deformation rate.
T.sub.G1; TG2 (X)
Deg C. The temperature range
of the glass-
transition of the
hydrophilic polymer.
T.sub.M1;R TM2 (X)
Deg C. The temperature range
of the melting of the
partially crystalline
hydrophilic polymer.
T.sub.E (t)
Deg C. The temperature of
the hydrophilic
polymer in the nozzle
area of the injection
unit.
T.sub.M (t)
Deg C. The temperature of
the hydrophilic
polymer in the mold.
P.sub.E N .times. m.sup.-2
The pressure in the
nozzle area of the
hydrophilic polymer,
expressed as the
weight fraction of
the water hydrophilic
polymer system.
__________________________________________________________________________
For the control and regulation of the injection molding process (IMP) we
need the knowledge of the
(1) heat consumption of the melting process:
H(T.sub.E, P.sub.E)-H(Ta; Pa)
(2) the heating rates of the hydrophilic polymers in the injection molding
device. To calculate this we need the heat conduction number of the
hydrophilic polymer ant the heat transfer number of the hydrophilic
polymer ant the heat transfer number of the hydrophilic polymer and the
specific material of construction of the barrel which is in contact with
the hydrophilic polymer. The heating rate and the heat consumption of the
hydrophilic polymer give the minimum time interval necessary to make the
hydrophilic polymer ready to inject and the necessary heating power of the
injection molding device.
(3) the T.sub.E depends on X of the hydrophilic polymers. If the water
content of the hydrophilic polymer in the mold is too low the resulting
T.sub.E will be too high and cause degradation. A minimum water content of
5% by weight is required to keep T.sub.E below 190.degree. C.
(4) the flow rate V(Q,T.P) is as well strongly dependent on the water
content of the hydrophilic polymer. To speed up the IMP we need a high
flow rate V(Q,T,P) which can be achieved by a high water content.
The upper limit of the water content is defined by the stickiness and
mechanical failure of the capsules; a water content of 25% (0.25) by
weight cannot be generally exceeded. The range within which capsules can
be molded by the method of the present invention if therefore within 0.05
to 0.25 of water content. Better capsules are made with a water content in
the range between 0.10 and 0.20; the best capsules were made with the
water content in the range between 0.12 and 0.18.
The hydrophilic polymer in the mold will reduce its volume due to the
temperature change T.sub.M -T.sub.a. This would result in voids and
diminution of size of the capsule, which therefor would be of unacceptable
quality. It is an absolute requirement in capsule making that the
dimensional absolute requirement in capsule making that the dimensional
deviations are less than 1%. To compensate for shrinking by the
temperature change the mold must be filled at a distance pressure P.sub.M.
This filling pressure is determined by the quantities (T,P) and (T,P). The
injection pressure (P.sub.E) depends again on T.sub.E, which as was shown
already is in turn strongly dependent on X.
Referring now to FIG. 5, the shear rate dependent shear viscosity of
gelatin at 90.degree. C. is show for gelatin with a water content X of
0.17. The capillary has a diameter of d=1.05 mm, and a length of 5.0 mm;
the ration of length to diameter is therefor L/d=4.75.
Referring now to FIG. 6, the molding are diagram for gelatin with water
content of 0.17. During injection molding the plasticized gelatin is
discontinuously extruded and immediately cooled in a mold of the desired
shape of the capsule part. Moldability depends on the polymer properties
and the process conditions, of which the thermomechanical properties of
the gelatin as well as the geometry and the temperature and pressure
conditions of the mold are the most important. In the molding area diagram
of FIG. 6 the limits of pressure and temperature are indicated for the
processing of gelatin in the combined injection molder-microprocessor of
the present invention. The maximum temperature of 190.degree. C. is
determined by visible degradation of the polymer above that limit. The
lower temperature limit of 50.degree. C. was determined by the development
of too high viscosity and melt elasticity in the recommended water content
range X: 0.05 to 0.25. The higher pressure limits of 3.times.10.sup.8
.times.m.sup.-2 are given by the start of flashing when the melted polymer
flows in a gap between the various metal dies which make up the molds,
thus creating thin webs attached to the molded capsule parts at the
separating lines. The lower pressure limits of about 6.times.10.sup.7
N.times.m.sup.-2 are determined by short shots, when the mold cannot be
completely filled by the polymer.
______________________________________
WORKING PARAMETERS FOR
INJECTION MOLDING PROCESS
______________________________________
Density 1.3-1.2 .times. 10.sup.3 kg .times. m.sup.-3
Crystallinity 25%
H(T.sub.E,P.sub.E)-H(Ta,P.sub.a)
0.32 KJoule .times. kg.sup.-1
Net heating performance
3.5 .times. 10.sup.5 KJoule
for 10 kgs. melt/h
(corresponding to 10.sup.6
capsules/h)
Heat conduction number
1.0 KJoule .times. m.sup.-1 .times. h.sup.-1 .times.
Degree.sup.-1
(20.degree. C.) for gelatin
Compressibility (T.sub.E,P.sub.E)
5 .times. 10.sup.-10 N.sup.-1 .times. m.sup.2
(Ta,Pa) 8 .times. 10.sup.-5 (Degree C.).sup.-1
Contraction due to
negligible
crystallization
Critical shear
10.sup.4 -10.sup.5 sec.sup.-1
deformation rate
______________________________________
The hydrophilic polymers are extruded and injected under the following
conditions:
Referring now to FIG. 7 the glass transition range and the melting
temperature range as a function of the composition of the polymer-water
system is shown. Gelatin is representative of similar polymer systems
within this invention although specifically excluded because it is the
subject of the parent application. At temperatures below the glass
transition range ordinary gelatin, as available commercially, is a
partially crystalline hydrophilic polymer containing approximately 70%
amorphous and approximately 30% crystalline parts by volume (Area I in
FIG. 7). Such gelatin preparations are commonly called cold dryed
gelatins. By raising the temperature of said gelatin preparation at a
distinct water content the gelatin passes through the glass transition
range.
Referring to FIG. 1 said heating process of the polymer will take place
within the extruder barrel 17. Referring to FIG. 2 said heating process of
the polymer will take place during the entire injection molding work
cycle. The area in FIG. 7 between the glass transition range and the
melting polymer and a polymer melt. The glass-transition is not
thermodynamic transition range of any order but is characterized by a
change of the molecular movement of the polymer molecules and by a change
of the bulk storage module of the amorphous gelatin by several orders of
magnitude. By passing from area II to are I in FIG. 7 the translational
movements of the molecules or those of large parts of said molecules will
be frozen in the glass transition temperature range and this is reflected
by a change in the specific heat (C.sub.p) and the volumetric thermal
expansion coefficient (a) in said temperature range. By passing from area
II to area III due to crossing the melting range of the crystalline
gelatin the helically ordered part of the gelatin will melt.
The heating process of the gelatin or similar polymer will take place
within the extruder barrel 17. Referring to FIG. 2 the heating process of
the polymer will take place during the entire injection molding work
cycle. Said helix-coil transition is a true thermodynamic transition of
the first order and is an endothermic process. These transitions can be
detected by scanning calorimeter or by measurement of the change of the
linear viscoelastic bulk storage module due to change of the temperature.
A typical plot of temperature scan with a differential calorimeter is
shown in FIG. 8. On the ordinate is plotted the velocity of the heat
consumed by the sample relative to a reference (empty sample holder). The
velocity of heat consumption of the sample is due to the change of the
temperature of the sample, and said temperature is plotted on the abscissa
as degrees Kelvin. The base line shift on said plot is corresponding to
the glass transition and the peak to the melting or to the helix-coil
transition. The linear viscoelastic bulk storage module E can be measured
at small sinusoidal shear deformations of the polymer sample. The change
of said module of a typical gelatin sample at water content X=0.13 is
plotted as a function of the sample temperature in FIG. 9. This value is
similar to the hydrophilic polymers of this invention having both
crystalline and amorphous structure at the glass transition temperature
and at the melting or helix-coil transition temperature said module
changes several orders of magnitude. As is shown in FIG. 9 there exist a
further transition temperature above the melting range, and said
transition is characterized by a further drop in said module e. We will
call the temperature of said transition the solution temperature. In the
temperature rant T.sub.g to T.sub.M the gelatin is in the rubber elastic
state, and the crystalline ranges or fibrils represent the elastically
active elements of the network.
Similar networks exist for example in the plasticized microcrystalline
polyvinylchloride (PVC). The crystalline regions give rise to diffraction
patterns of x-rays in said PVC but not in the gelatin [I. Tomka, Chimia
30, 534-540 (1976); I. Tomka et al. Phot. Sci. 23, 97 (1975)]. In the
temperature rant T.sub.M to T.sub.S the gelatin is in the viscoelastic
rubber-elastic state. The elastically active network in said state of the
gelatin is like in most polymer melts a temporary network. Said temporary
network is due to entanglements of the polymer molecules. Specifically in
the gelatin the strong interactions between the macromolecules
(hydrogen-bridges, dipol-dipol interactions) contribute an important part
to the elastically active temporary network. At the solution temperature
said temporary network disrupts and the gelatin molecules, specifically
due to the presence of water, dissolve. At a temperature higher than
T.sub.S the storage modul | | |
