 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention:
This invention relates to improved foam materials, particularly for food
service and food packaging applications. These foams are made from
starshaped polymers with a plurality of poly(hydroxyacid) chains
(polylactide, polyglycolide, polycaprolactone, etc.) attached to a central
polyfunctional compound having a plurality of hydroxyl and/or amino
groups.
The materials of this invention are degradable, both biodegradable and
hydrolyzable. Furthermore, they are foamed with environmentally benign gas
and so contain no stratosphere ozone-depleting chemicals nor chemicals
that have significant low altitude smog forming photochemical ozone
producing reactivity. They have little global warming potential compared
to chlorofluorocarbon (CFC) blowing agents. Thus, this invention provides
readily degradable foam packaging materials with minimal environmental
impact.
2. Description of the Related Art:
Copending and commonly assigned U.S. patent application Ser. No.
07/826,357, allowed Mar. 3, 1992, relates to poly(hydroxy acid) foam
materials. The polymers for the foams of the present invention are
branched star polymers containing hydroxy acidic acid units polymerized
with other units that greatly enhance their foaming capabilities and the
properties of the resultant foams.
At the present time, the most commonly used food service and food packaging
materials are light weight closed cell foamed polystyrene, usually foamed
with chlorofluorocarbon (CFC) or hydrocarbon (HC) blowing agents. These
products pose serious environmental problems.
First, polystyrene is not degradable, either in landfills or as roadside
litter. Thus, unless collected and recycled, polystyrene foam products
have an unlimited litter life.
Second, CFC blowing agents trapped in the foam products, which make up a
high volume percentage of the products, eventually escape to the
stratosphere where the chlorine enters an ozone-depletion cycle.
Third, if HC blowing agents are used in place of CFCs, upon release from
the foam they are photochemically reactive and thus promote smog
formation. Also, they are flammable.
Thus, there is a need for light weight closed cell foam of a degradable
resin foamed by a blowing agent that does not enter into chemical
reactions that cause environmental damage.
Degradable polylactides are shown in Murdoch U.S. Pat. No. 4,766,182 (Aug.
23, 1988) and 4,719,246 (Jan. 12, 1988). These patents also disclose
forming open cell porous structures by the extraction of solvent from
polylactide gel. These open cell structures cannot be used for containers
for wet foods such as meats and drinks. Also, to the extent any of
Murdoch's blowing agents are present in the porous product, they are not
environmentally benign.
Battelle WO 90/01521 published Feb. 22, 1990 discloses degradable
polylactide resins, and their processing into solid films. The films may
be made into "foam" structures by dissolving therein a mixture of
petroleum ether and methylene chloride, and then placing the film into
boiling water to volatilize the liquid mixture. The Battelle "foam" films
are irregular and unsuitable. Also, upon degradation these products will
release a high volume percent of hydrocarbons that are deleterious to the
atmosphere.
Zhu et al "Super Microcapsules (SMC). I. Preparation and Characterization
of Star Polyethylene Oxide (PEO)-Polylactide (PLA) Copolymers.", Journal
of Polymer Science: Part A: Polymer Chemistry, Vol 27, pages 2151-2159
(1989) describe polymerization of lactide from 3-arm and 4-arm hydrophilic
poly (ethylene oxide) with hydroxylterminated arms.
Polylactic acid (PLA) polymers often show a very rapid drop in melt
viscosity with increasing processing temperature. At temperatures which
are high enough to allow processing of semicrystalline PLA polymers, the
melt viscosity drops very fast (partly because of thermal degradation). As
a result, a number of PLA melts have a low melt strength, which may affect
the foam-forming capacity of these polymers. An ideal foam-forming PLA
polymer should combine a lower processing temperature to limit thermal
degradation during processing with a higher melt strength to improve the
foam-forming capacity. Such combination of desirable features are
approached with branched (star) PLA polymers used in the foams of the
present invention.
A number of blowing agents, including fluorohydrocarbons, are known in the
art. Spitzer U.S. Pat. No. 4,422,877 (Feb. 4, 1982) shows the use of a
number of blowing agents, including 1,1-difluoroethane, to form foams from
a number of resins. However, no PHA foam articles are disclosed.
Walter U.S. Pat. No. 4,988,740 (Jan. 29, 1991, filed Jun. 15, 1989)
discloses closed cell elastic foam material made from elastomeric polymer.
No rigid foam products are disclosed.
"The Elements of Expansion of Thermoplastics", Part I and Part II, James G.
Burt, Journal of CELLULAR PLASTICS, May/June 1979 (Part I) and
November/December 1978 (Part II) disclose in detail the mechanics of
meltfoaming thermoplastic resins. This article, while not disclosing PHA
foams, sets forth a large number of requirements that must be met by the
foaming ingredients to prepare an acceptable foam product, such as: amount
of plasticization of the molten resin by the blowing agent; volatility of
the blowing agent at foaming temperature; speed of solidification of the
blown resin on cooling; heat transfer necessary for solidification;
molecular migration of the blowing agent through the blown cell walls;
melt viscosity and melt strength of the resin during the
cooling/solidification; the rate of change of polymer viscosity with
temperature; and a number of other properties. One concludes that the
suitability of particular polymeric resins to be foamed can be determined
only by trial and error.
HFC manufacture is known in the art. See "Aliphatic Fluorine Compounds", A.
M. Lovelace et al. (1958), p. 55.
The above patents, patent application and literature references are
incorporated herein in total and made a part of this patent application.
SUMMARY OF THE INVENTION
It has been found that certain polymers containing hydroxy acid units form
excellent degradable environmentally benign light weight rigid closed cell
foam structures when foamed by inert environmentally benign blowing
agents. The products of the present invention are foam products,
particularly packaging and food container products, comprising one or more
specific thermoplastic polymers, the closed cells of which foam product
enclose one or more fluorocarbon blowing agent. The polymers useful in the
present invention foam products comprise star-shaped polymers having the
residue of a central polyfunctional compound having 3 to 100 and
preferably 3 to 20 hydroxyl or amino groups and 5 to 10,000 and preferably
5 to 100 carbon atoms with polymeric arms attached to the functional
groups. The polymeric arms are formed from lactide, glycolide or
caprolactone.
DESCRIPTION OF THE INVENTION
The foam products of the present invention comprise closed cell foam
degradable thermoplastic polymeric resins containing at least 50% by moles
of one or more star-shaped polymers wherein lactide is polymerized onto a
polyfunctional hydroxyl or amino containing compound.
The polymers used in the foams of the present invention, and their
preparation, are described in detail in copending and commonly owned U.S.
patent applications Ser. No. 07/922,269.
The polyfunctional macroinitiator compound generally contains from 3 to 100
and preferably from 3 to 20 hydroxyl and/or amino groups, and from 5 to
10,000 and preferably from 5 to 100 carbon atoms. While sugars, such as
monosaccharide or disaccharide hexoses or pentoses can be used, it is
preferred to use a polyfunctional compound in which all of the functional
groups are of the same reactivity, i.e., either all primary or all
secondary. Such a polyfunctional compound is inositol, with six secondary
OH groups. Furthermore, the high melting inositol can be reacted with a
mixture of L- and D-lactide to produce a macroinitiator containing six
short amorphous arms (about 800 g/mole) which are OH terminated. This
initiator makes possible homogeneous initiation of lactide polymerization
(the initiator is readily soluble in molten lactide and has readily
available OH groups remote from the central hub of the initiator
molecule).
Also suitable for use as polyfunctional compounds are oligomers or polymers
of unsaturated monomers containing one or two hydroxy or amino groups.
Suitable oligomeric or polymeric polyfunctional materials include poly
(vinyl alcohol), and hydroxyethylmethacrylate containing from 10 to 5,000
and preferably from 10 to 20 repeating vinyl alcohol or
hydroxyethylmethacrylate units.
The pendant polylactide groups forming the arms of the star can be
copolymers of L-lactide and D-lactide or a polymer of greater than 98%
L-lactide or greater than 98% D-lactide. The copolymers of 10-90%
L-lactide and 90-10 D-lactide, and preferably 15-85% L-lactide and 85-15%
D-lactide, provide amorphous polymers. The most preferred copolymers have
an L/D ratio of at least 95/5. The polymers containing only one of greater
than 98% L-lactide or greater than 98% D-lactide provide polymers having a
crystallinity of about 37%. The all semicrystalline polylactide polymers
generally melt above 173.degree. C. and lactide begins to degrade
thermally at about 180.degree. C. For this reason, it is preferred to form
a portion of the arms with amorphous polylactide and a portion of the arms
with semicrystalline polylactide. This drops the melting point by
10.degree. to 20.degree. C., depending on the length of the
semicrystalline blocks, while still retaining the superior physical
characteristics of the semicrystalline polylactide. While either the
amorphous or semicrystalline block can form the inner arm segment and the
other type of block the outer arm segment, it is preferred that the inner
arm segments adjacent the polyfunctional compound be amorphous and that
the outer arm segments be semicrystalline. The individual inner arm
segments generally will have a molecular weight of 1,000 to 50,000 and the
outer arm segments a molecular weight of 1,000 to 50,000. The total
molecular weight of the individual arms generally will be from 2,000 to
100,000, and preferably above 40,000.
Polycaprolactone or polyglycolide, alone or a copolymer of L,D or both
lactides may be used to form the arms or preferably the inner arm
segments.
The polymerization can be done either in bulk or in solution. Preferred
solvents are toluene and the xylenes, for high temperature reactions and
methylene chloride or chloroforms for low temperature reactions. Suitable
reaction temperatures for solution polymerization range from -100.degree.
to 300.degree. C. with -40.degree. to 110.degree. C. being the preferred
range. Suitable reaction temperatures for bulk polymerization range from
100.degree. to 220.degree. C., with 160.degree. to 200.degree. C. being
the preferred range. Any catalyst for ring opening of lactide, glycolide
or caprolactone may b used for the polymerization step(s). Generally,
stannous 2-ethylhexanoate (tin octanoate) is used for the polymerization,
but other catalysts such as the yttrium or lanthanide series rare earth
metal catalysts disclosed in U.S. Pat. No. 5,028,667 issued Jul. 2, 1991
to Drysdale and McLain can be used for the polymerization step(s). The
polymerization reaction generally takes from 5 minutes to 72 hours
depending on the temperature used and the amount of catalyst present.
Generally, the ratio of monomer/catalyst is from 200/1 to 10,000/1. In the
preferred aspect of the invention the mixture of L- and D-lactide is
polymerized onto the polyfunctional compound until the monomer conversion
is greater than 95% and further addition of the L-lactide or D-lactide is
made, generally along with additional catalyst.
The polymer compositions will normally contain some unreacted monomers and
low molecular weight oligomers. To avoid extrusion and foaming problems,
it is desirable to keep the low molecular weight, under 450, units in the
polymer composition to less than about 7-1/2%.
These polymers are degradable and are excellent for forming superior closed
cell molded and moldable foam products.
Since the star polymers may have reactive groups at the ends of the arms,
they are amenable to crosslinking. This can be done during direct
thermoforming of the foam structure, giving a particularly rigid
structure.
As is well known in the art, such polymers may be modified with minor
amounts of various adjuvants such as stabilizers, fillers, plasticizers,
nucleating agents and the like.
It has been found that these polymers have a superior combination of
properties that make them outstanding for foaming, particularly adequate
melt strength over a wide enough temperature range and low diffusion rate
to contain the blowing agent. Particularly, they have low enough
crystallinity and rate of crystal formation to allow an adequate
temperature range of foamable melt viscosity of adequate strength to
maintain cell integrity without heating too severely causing polymer
degradation.
The resultant foamed products comprising resin and fluorocarbon are
excellent low density rigid products, that although non-elastomeric, can
be hot molded to shapes such as plates and cups that have good heat
insulating properties. The densities are not over 20 pounds per cubic foot
(pcf) and preferably under 10 pcf.
The polymer is converted to foam by nitrogen, carbon dioxide or a blowing
agent of the formula:
##STR1##
wherein all A's are independently hydrogen or fluorine, and n=1-4. The
nitrogen, carbon dioxide and HFC of formula (5) are chemically compatible
with and useable with conventional foam blowing equipment, and are
thermally stable and chemically non-reactive during the blowing and
subsequent thermoforming stages. A significant amount of blowing agent
remains in the closed cells of the foam product for an extended period of
time. Of course, air does diffuse into the individual cells.
The cells of these products are closed cells in a significant proportion
even after hot molding, usually above 90% of the cells being closed before
shaping and 50% after shaping. In its simplest concept, the present
invention is the use of nitrogen, carbon dioxide or HFCs of the formula
(5) as a foam blowing agent for specific resins to produce highly useful,
environmentally superior foam products.
The preferred products of the present invention are a degradable resin
foamed by and containing in closed cells an environmentally benign
hydrofluorocarbon (HFC) blowing agent.
By "foam" is meant a low density microcellular structure consisting
essentially of thin contiguous cell walls of 0.0003 to about 0.030 inches
thickness each side of which forms a wall of one or more closed cells
containing blowing agent. The cells have a maximum cell size of 0.03
inches.
The foams of the invention are rigid, in contrast to elastomeric. By
"rigidity" is meant that when compressed, the films of the invention will
be crushed and cell structure destroyed. They will not recover back to
their precompression shape upon release of compression. Thus, the present
products have little significant elastic recovery and cannot be severly
compressed without cell destruction.
While some cell walls may be broken, the majority of cells of the present
invention product are unbroken, giving the foam rigidity and structural
strength. After being made, foam may be shaped by molding, calendaring, or
cutting. The foams of the present invention, being essentially
non-elastic, retain their shape when thermo molded.
The term "degradable" as used here with respect to the polymers means that
the polymer is biodegradable and, more importantly, degradable by
hydrolysis. The degradation rate is consistent with its intended usage,
i.e., the product does not significantly degrade in normal storage and
usage, but will degrade significantly in a reasonable time after
discarding. For hydrolysis degradation, slightly acidic or basic
conditions may be used advantageously. By hydrolysis degradation, monomer
units can be recovered if desired for reconversion to useful polymers or
can be discarded as an environmentally benign waste material.
Acceptable blowing agents must have the following properties:
environmental acceptability
low toxicity
appropriate volatility
adequate solubility
low reactivity
acceptable diffusion rate
relatively low molecular weight.
Environmental acceptability means that the blowing agent, when released
from the foam product, will have no potential for stratospheric ozone
depletion. In this regard, blowing agents containing chlorine atoms are
unacceptable. Also, when released, the blowing agent must have a minimal
infrared energy absorbability and appropriate atmospheric lifetime so that
it will not have significant global warming potential, and also have
negligible photochemical reactivity so as not to promote smog formation.
Hydrocarbon blowing agents are unacceptable because they promote the
formation of low level ozone.
Low toxicity of the blowing agent is required to protect employees during
foam manufacture. Also, it must not be toxic in use of the foam products,
which is of particular importance in food service and food packaging
applications.
The blowing agent must also have appropriate volatility for use with the
thermoplastic resins of the present invention. Such blowing agents must
provide the solution pressure required to expand and foam the viscous
polymeric resin.
The blowing agent must have adequate solubility in the molten polymer,
which means that the blowing agent is readily contained in the molten
polymer when present in the concentration required for the needed degree
of foaming. If the blowing agent separates from the polymeric resin before
expansion of the blowing agent, gas pockets or non-uniform foam density
can be caused.
The blowing agent must have low reactivity, which means that it will not
react with the resin or decompose under the typical temperatures and
pressures conventionally used in thermoplastic resin foam production.
Similarly, the blowing agent must be nonreactive under normal use so as to
prevent product degradation.
An acceptable diffusion rate is necessary for the blowing agents in the
foams of the present invention. A very slow diffusion rate is preferred so
that the thermoforming of foamed sheet into shaped objects such as
hamburger containers, plates, etc. is easily achieved.
A relatively low molecular weight blowing agent is desirable. Molecular
weight determines the pounds of blowing agent needed to produce a given
volume of gas. Thus, use of a low molecular weight blowing agent minimizes
the blowing agent cost per unit of production.
It has been found that HFC blowing agents selected from those represented
by formula (5) are ideally suited, with respect to the aforementioned
properties, when used to make foam products with the polymeric resins of
the units for formulae (1)-(4). Suitable HFC blowing agents of formula (5)
used with a specific polymeric resin of the units of formulae (1)-(4) can
be readily determined by conventional techniques.
The following HFCs are suitable blowing agents for the foams of the present
invention:
______________________________________
HFC-32 CH2F2
HFC-125 CF3CF2H
HFC-134 HCF2CF2F
HFC-134a CF3CFH2
HFC-143a CF3CH3
HFC-152 CH2F--CH2F
HFC-152a CH3--CHF.sub.2
HFC-227 EA CF3CHF--CF3
HFC-356 MFF CF3--CH2--CH2--CF3
HCC-365 MFC CF3--CH2--CF2--CH3
______________________________________
The preferred HFCs are those containing one and two carbon atoms.
The foam products of the present invention have a unique combination of
properties making them environmentally acceptable both with respect to the
atmosphere and also for disposal. Concerning disposal, they can be
landfilled whereupon they will biodegrade and/or hydrolyze to harmless
degradation products, or they can be recycled by hydrolysis
depolymerization, preferably under slightly acidic or basic conditions, to
form monomeric units that can be used in the production of polymer.
The foam products of the present invention can be made by conventional
techniques. Thus, the polymeric resin with the blowing agent therein can
be pressure extruded at an appropriate temperature below the degradation
or reaction temperature of the resin and the blowing agent. These extruded
products can be hot formed into desired shapes. The foamed products can be
in the form of sheets or cast, molded or pressed shaped articles such as
hamburger containers, trays, plates, boxes and the like. These products
are particularly useful in food packaging and food service containers
because of the combined properties of low toxicity and environmental
acceptability.
The foam products can also be made directly into the desired shape by
introducing the material to be formed into a mold or press of the desired
shape which can be appropriately controlled in temperature and pressure to
develop the desired shape. Cross-linking of the star polymer can be
readily carried out in this type of direct product molding/thermoforming.
In addition to polymer and blowing agent, conventional adjuvants can be
included. Typical adjuvants are nucleants such as calcium silicate talc;
processing aids such as mineral oils; extrusion aids such as
dioctylphthalate (DOP); and color concentrates. The concentrations of the
additives are generally independent of the amount of blowing agent.
EXAMPLE 1
Preparation of macroinitiator
In a dry box, L-lactide (8.2 g, 56.94 mmol), D-lactide (1.40 g, 9.722
mmol), and inositol (0.40 g, 2.22 mmol) are weighed in an oven dried 100
ml onenecked round bottom flask equipped with a magnetic stirring bar.
After charging the reactants, the reaction flask is fitted with a rubber
septum secured in place with copper wire, and transferred into a hood. The
reaction flask, maintained under a positive pressure of nitrogen at all
times, is heated to 150.degree. C. and reacted, without catalyst, to
initiate ring opening of lactide by inositol OH groups. The initial
reaction mixture is heterogeneous, as inositol (mp=224.degree.-225.degree.
C.) is not readily soluble in molten lactide. However, after a few hours
at 150.degree. C., the reaction becomes homogenous and slightly viscous,
as lactide units are polymerized off the OH groups of inositol initiator.
The reaction is allowed to proceed for 12 hours before catalyst addition.
0.33 ml tin (SnOct) octanoate solution 0.1M in toluene [Monomer/Catalyst
(M/Cat.)=2000/1] is then added and the reaction is allowed to proceed for
additional 6 hours at 150.degree. C. to complete lactide polymerization.
The arm-length of macroinitiator is determined by the molar ratio of
monomer/OH groups and it is approx 720 g/mole (or 5 lactide units) for the
example described above. The final reaction mixture is dissolved in
CH2Cl2, precipitated from hexane/methanol, 50/50 v/v, and dried in a
vacuum oven at room temperature for 72 hours prior to use.
Characterization
Theoretical number average molecular weight (Mn)=4,320 (calculated from the
molar ratio of Lactide/OH groups); Experimental Mn=5980 g/mole (titration,
OH#); Mn=7,350 g/mole; Weight average molecular weight (Mw)=8890 g/mole;
P/D=1.21; from Gel permeation chromatography (GPC), linear polystylene
standard (PS STD); Glass Transition Temperature (Tg)=36.degree. C. as
determined by differential scanning calorimetry (DSC); No Tm (amorphous
material).
EXAMPLE 2
Synthesis of amorphous 6-arm polylactide star
In a dry box, 15.4 g (106.94 m moles) L-lactide, 3.8 g (26.389 m moles)
D-lactide, and 0.30 g (0.050 m moles) hexafunctional hydroxyl containing
macroinitiator from Example 1 Mn 5,980 are weighed into an oven dried 100
ml. 3-necked round bottom flask equipped with overhead stirrer. The
reaction flask is then transferred in a hood, placed under an inert
atmosphere, and heated to 150.degree. C., until the reaction mixture forms
a homogeneous melt. 0.7 ml SnOct solution 0.1M in toluene is added
(M/Cat.)=2000/1 molar ratio) and the reaction is allowed to proceed for 1
hour at 150.degree. C. A viscous homogeneous melt is formed shortly after
catalyst addition, and the viscosity increases with reaction time;
however, stirring is possible throughout the reaction. The final reaction
mixture is cooled to room temperature and dissolved in 200 ml CH2Cl2. The
polymer is isolated by precipitation from hexane, and dried in a vacuum
oven at room temperature for 24 hours.
Characterization
Mp=223,000 (GPC, linear PS STD); Tg=56.degree. C. (DSC); no Tm (amorphous
polymer)
EXAMPLE 3
Synthesis of polylactide 6-arm star with amorphous/semicrystalline block
structure
In a dry box, 4.8 g (33.333 m moles) L-lactide, 1.6 g (11.111 m moles)
D-lactide, and 0.30 g (0.050 m moles) hexafunctional hydroxyl containing
macroinitiator from Example 1 (Mn 5,980) are weighed into an oven dried
100 ml. 3-necked round bottom flask equipped with overhead stirrer. The
reaction flask is then transferred in a hood, placed under an inert
atmosphere, and then heated to 150.degree. C. without catalyst until the
reaction mixture forms a homogeneous melt. When the reaction mixture
becomes homogeneous, 0.25 ml SnOct solution 0.1M in toluene is added
(M/Cat.=2000/1 molar ratio) and the reaction is allowed to proceed for 1
hour at 150.degree. C., at which time the conversion of lactide is greater
than 95%. A viscous homogeneous melt is formed shortly after catalyst
addition, and the viscosity increases with reaction time. After 1 hour, a
second monomer portion consisting of 12.8 g (88.888 m moles) L-lactide is
added and allowed to dissolve in the previous reaction mixture, under
inert atmosphere. When the reaction mixture becomes homogeneous again, a
second catalyst portion (0.5 ml SnOct 0.1M in toluene) is added, and the
polymerization is allowed to proceed for 30 more minutes. Shortly after
second catalyst addition the reaction viscosity increases considerably and
the stirring becomes difficult. The final reaction mixture is cooled to
room temperature and dissolved in 150-200 ml CH2Cl2. The polymer is
isolated by precipitation from hexane, and dried in a vacuum oven at room
temperature for 24 hours.
Characterization
Mp=218,000 (GPC, linear PS STD); Tg=56.degree. C. (DSC); Tm=165.degree. C.
EXAMPLE 4
Synthesis of fully semicrystalline 6-arm L-Polylactide Star
In a dry box, 19.2 (133.33 m moles) L-lactide, and 0.30 g (0.50 m moles)
hexafunctional hydroxyl containing macroinitiator from Example 1 (Mn
5,980) are weighed into an oven dried 100 ml. 3-necked round bottom flask
equipped with overhead stirrer. The reaction flask is then transferred in
a hood, placed under an inert atmosphere, and heated to 150.degree. C.,
until the reaction mixture forms a homogeneous melt. 0.7 m SnOct solution
0.1M in toluene is added (M/Cat.=2000/1 molar ratio) and the reaction is
allowed to proceed at 150.degree. C. A viscous homogeneous melt is formed
shortly after catalyst addition, and the viscosity increases with reaction
time. Approximately 10 minutes after catalyst addition the reaction can no
longer be stirred and the polymerization is allowed to continue in the
"solid state", without stirring, for 20 more minutes. The final reaction
mixture is cooled to room temperature and dissolved in 200 ml CH2Cl2 using
heat to ensure complete solution. The polymer is isolated by precipitation
from hexane, and dried in a vacuum oven at room temperature for 24 hours.
Polymer properties
Mp=278,000 (GPC, linear PS STD); Tg=56.degree. C. (DSC); Tm=173.degree. C.
EXAMPLE 5
Synthesis of amorphous/semicrystalline 5-arm L/D polylactide star
In a dry box L-lactide (6.20 g, 0.0430 mole, D-lactide (2.10 g, 0.0140
mole), and alpha-D-glucose (0.30 g, 0.000166 mole) are weighed into an
oven dried 100 ml, 3-necked round bottom flask equipped with overhead
stirrer. The reaction flask is then transferred in a hood, placed under an
inert atmosphere and heated to 150.degree. C. without catalyst until the
reaction mixture forms a homogeneous melt. When the reaction mixture
becomes homogeneous 0.29 ml of 0.1M tin octoate is added (M/cat.-2000/1
molar ratio) and the reaction is allowed to proceed for 1 hour at
150.degree. C. A viscous homogeneous melt is formed shortly after catalyst
addition, and the viscosity increases with reaction time. After 1 hour, a
second monomer consisting of 25 g (0.1736 mole) L-lactide is added and
allowed to dissolve in the previous reaction mixture, under inert
atmosphere. After all additional monomer is dissolved and the reaction
mixture becomes homogeneous again, a second catalyst portion (0.9 ml tin
octanoate 0.1M in toluene) is added, and the polymerization is allowed to
proceed for 30 more minutes. Approximately 10 minutes after catalyst
addition the reaction mixture can no longer be stirred and the
polymerization is allowed to continue in the "solid state", without
stirring, for 20 more minutes. The final reaction mixture is cooled to
room temperature and dissolved in 200 ml CH2Cl2 using heat to ensure
complete solution. The polymer is isolated by precipitation from hexane,
and dried in a vacuum at room temperature for 24 hours. The overall
L-lactide D-lactide ratio in the polymer is 93.7/6.3. the overall
amorphous/semicrystalline content of the polymer is 25%/75%. The
theoretical number average molecular weight of the amorphous segment of
each arm is 10,000. The theoretical number average molecular weight of the
semicrystalline segment of each arm is 30,000. The theoretical number
average molecular weight of each arm is 40,000 and the total number
average theoretical weight of the 5 arm polymer is 200,000. The polymer
has a glass transition temperature of 54.degree. C. and a melting point of
162.degree. C.
EXAMPLE 6
Example 5 is repeated but varying the ratio of amorphous to semicrystalline
segments in the five arms of the polymer.
(a) A polymer having an amorphous to semicrystalline ratio in each arm of
33.3%/66.6% wherein the amorphous segment has a theoretical number average
molecular weight of 15,000, a theoretical total arm number average
molecular weight of 45,000 and a total theoretical polymer number average
molecular weight of 225,000 is found experimentally to have a Mp of
273,000, a glass transition temperature of 56.degree. C. and a melting
point of 160.degree. C.
(b) A polymer having an amorphous to semicrystalline ratio in each arm of
37.56/62.5% wherein the amorphous segment has a theoretical number average
molecular weight of 15,000, the semicrystalline segment has a theoretical
number average molecular weight of 25,000 to provide a theoretical total
arm number average molecular weight of 40,000 and a total theoretical
polymer number average molecular weight of 200,000 is found experimentally
to have Mp of 201,000, a glass transition temperature of 56.degree. C. and
a melting point of 160.degree. C.
(c) A polymer with the arms formed of randomly polymerized L-lactide and
D-lactide and a theoretical total polymer molecular weight of 2000,000 has
an experimental Mp of 129,000, a glass transition temperature of
52.degree. C. and a melting point of 146.degree. C.
EXAMPLE 7
Continuous polymerization of a 3-arm polylactidestar polymer
To a 30 mm twin screw extruder is continuously fed 18 pounds per hour of
L-lactide, 2 pounds per hour of D-lactide, 63 mL of a 0.5 molar toluene
solution of tin octoate (for a monomer/catalyst ratio=2000/1), and 19 ml
per hour of "Tone" 301 (trimethylol propane caprolactone oligomer sold by
Union Carbide Co.). The product is quenched in water, pelletized and
dried. Conversion via thermogravimetric analysis is 99%. Mn is 134,000,
Mw/Mn is 1.5. Melt strength as measured according to Busse, J. Poly. Sci.,
Part A-2, Vol. 5, p. 1249 (1967) is 11.1 centiNewtons as compared to nil
for a linear p-lactide prepared under similar conditions.
EXAMPLE 8
Foam Production
Following the procedure of Example 7, adequate quantities of the 3-arm star
polymer are prepared for making foam. The foam is made by feeding to a
41/2"-6" tandem extrusion system with annular die and mandrel a mixture of
21/2 lbs. per hour of calcium silicate talc nucleating agent and 472.5
lbs./hr. of molten polymer. Into the molten polymer in the extruder is
introduced 25 lbs. per hr. of CF3-CFH2 (HFC-134a). The temperature of the
material in the 41/2" extruder is maintained at 400.degree. F., and the
pressure at 2500 lbs/sq. in. gauge. Prior to exiting the 6" extruder the
temperature is dropped low enough to prevent cell collapse upon discharge
from the extruder. The HFC comes out of solution in the polymer forming an
oriented cell wall degradable thermoplastic closed cell foam product. This
product is a rigid foam sheet 44 inches wide and 100 mils thick, having a
density of 6 lbs. per cubic foot, with the individual cells containing
HFC-132a.
The sheet foam is then heated, softened and stamped into 12" circular
plates using a thermoformer. The plates are closed cell foams containing
blowing agent.
Following the same procedures, sheets ranging from 50-100 mils thick are
prepared and thermoformed into hinged food service containers.
EXAMPLE 9
Following the continuous polymerization technique of Example 7 and the
foaming technique of Example 8, sheets of closed cell foams are formed
from the polymers of Examples 2-6 and the ingredients listed below:
______________________________________
Poly- Poly- Calcium
mer of mer Silicate
Ex- Blowing #/ Talc
ample Agent M.W. #/hr. hr. Nucleator
______________________________________
2 CH2F2 52 2.55 494.95
2.5
(HFC-32)
3 CH3CHF2 66 3.24 494.26
2.5
(HFC-152a)
4 CF3--CH2F 102 5 492.5 2.5
(HFC-134a)
5 CF3CHFCF3 170 8.33 489.17
2.5
(HFC-227 EA)
6(a) CF3CH2CF2CH3 166 8.14 489.36
2.5
(HFC-365 MFC)
6(b) N2 28 1.37 496.13
2.5
6(c) CO2 44 2.16 495.34
2.5
______________________________________
* * * * *
|
|
 |
|
 |
Description |
|
