U.S. patent application number 10/223451 was filed with the patent office on 2003-02-27 for method for producing semicrystalline polylactic acid articles.
Invention is credited to Bopp, Richard C., Whelan, Jason.
Application Number | 20030038405 10/223451 |
Document ID | / |
Family ID | 23216691 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030038405 |
Kind Code |
A1 |
Bopp, Richard C. ; et
al. |
February 27, 2003 |
Method for producing semicrystalline polylactic acid articles
Abstract
Amorphous sheets of PLA resins are thermoformed by heating the
sheets until they become semicrystalline, and then forming the
sheets on a relatively cold mold. Semicystalline formed articles
having improved heat resistance are made by the process.
Inventors: |
Bopp, Richard C.; (Golden
Valley, MN) ; Whelan, Jason; (New Hope, MN) |
Correspondence
Address: |
Gary C. Cohn PLLC
4010 Lake Washington Blvd., NE, Suite105
Kirkland
WA
98033
US
|
Family ID: |
23216691 |
Appl. No.: |
10/223451 |
Filed: |
August 19, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60313685 |
Aug 20, 2001 |
|
|
|
Current U.S.
Class: |
264/319 ;
528/503 |
Current CPC
Class: |
C08J 2367/04 20130101;
B29K 2995/004 20130101; B29K 2105/0008 20130101; C08J 5/18
20130101; B29K 2105/0026 20130101; B29K 2105/0032 20130101; B29C
51/002 20130101; B29K 2105/005 20130101; B29C 51/14 20130101; B29K
2105/0038 20130101; B29K 2995/0002 20130101 |
Class at
Publication: |
264/319 ;
528/503 |
International
Class: |
B28B 003/00; C08G
002/00 |
Claims
What is claimed is:
1. A method for making formed heat-resistant PLA articles,
comprising heating a sheet of an amorphous, crystallizable PLA
resin until it has obtained a surface temperature from about 80 to
about 155.degree. C., then thermoforming the heated sheet on a mold
that is at a temperature below 80.degree. C.
2. The method of claim 1, wherein the mold is at a temperature
below the T.sub.g of the PLA resin.
3. The method of claim 2, wherein the sheet is heated under
conditions such that the PLA resin attains a crystallinity of at
least 15 Joules/gram.
4. The method of claim 3, wherein the PLA resin is a homopolymer of
L-lactic acid or D-lactic acid, a random copolymer of L-lactic acid
and D-lactic acid, a block copolymer of L-lactic acid and D-lactic
acid or a mixture of two or more of these.
5. The method of claim 4 wherein the PLA resin contains a
nucleating agent, a plasticizer, or both.
6. The method of claim 5 wherein the PLA resin contains from about
1 to about 40 percent of a finely divided talc, based on the
combined weight of the PLA resin and the talc.
7. The method of claim 4 wherein the PLA resin has a
crystallization half-time at the temperature to which it is heated
during the heating step of less than 3 minutes.
8. The method of claim 7 wherein the sheet is heated under
conditions such that the PLA resin attains a crystallinity of at
least 20 Joules/gram.
9. The method of claim 8 wherein the sheet is heated under
conditions such that the PLA resin attains a crystallinity of at
least 24 Joules/gram.
10. The method of claim 1 wherein the PLA sheet contains one or
more additives selected from the group consisting of nucleants,
other inorganic fillers, plasticizers, reinforcing agents, slip
agents, lubricants, UV-stabilizers, thermal stabilizers, flame
retardants, foaming agents, antistatic agents, antioxidants and
colorants.
11. The method of claim 1 wherein the amorphous PLA sheet is
cellular.
12. The method of claim 1 wherein the amorphous PLA sheet contains
a foaming agent, and expands during the heating step to become
cellular.
13. The method of claim 1 wherein the PLA resin contains a
plasticizer.
14. The method of claim 1 wherein the sheet of amorphous PLA resin
forms a layer of a multilayer structure having at least one other
layer which is not a crystallizable PLA resin.
15. A thermoformed article made according to the process of claim
1.
16. A method for making formed heat-resistant PLA articles,
comprising heating a sheet of a semicrystalline PLA resin having a
crystallinity of at least 10 Joules/gram to a temperature at which
the sheet can be thermoformed but at which the crystallinity of the
sheet is not reduced below 10 Joules/gram, and then thermoforming
the sheet on a mold that is at a temperature below 80.degree. C. to
produce a formed article of PLA resin having a crystallinity of at
least 10 Joules/gram.
17. A method for making formed heat-resistant PLA articles,
comprising heating a sheet of an amorphous, crystallizable PLA
resin to a temperature at which the PLA resin forms crystallites
and for a time sufficient to impart a crystallinity of at least 10
Joules/gram to the PLA resin, and then thermoforming the heated
sheet on a mold that is at a temperature below 80.degree. C. to
produce a formed article of PLA resin with a crystalinity of at
least 10 Joules/gram.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to methods for forming
heat-resistant, semicrystalline articles from polylactic acid.
[0002] Polylactic acid (PLA) is useful for making films, fibers,
and various types of formed articles. One limitation on its use in
some food packaging and other applications is its tendency to
deform when heated. For example, many food packaging applications
require the resin to be subjected to the temperature of boiling
water without significant deformation. PLA articles often cannot
withstand such temperatures.
[0003] PLA tends to exist in an amorphous state when formed into
these kinds of articles. Experience with a more conventional resin,
poly(ethylene terephthalate) (PET) has shown that better resistance
to heat can be obtained if the polymer has a greater amount of
crystallinity. For PET, this has resulted in a specialized
thermoforming process for inducing crystallinity into PET articles.
In cPET thermoforming, a PET sheet is heated until soft enough to
be formed, then transferred to a hot mold and formed under vacuum
and pressure. The temperature and residence time in the mold are
such that crystallites form in the resin. Once the needed
crystallinity is obtained, the article is transferred into another
mold of identical dimensions. This second mold is held below the
glass transition temperature (T.sub.g) of the PET resin, usually
near room temperature. The colder temperatures "quench" the resin,
"locking in" the as-formed dimensions. If the resulting PET resin
is sufficiently crystalline, it may withstand use temperatures some
20-150 C. higher or more than amorphous PET.
[0004] The PET thermoforming process has the drawbacks of requiring
two molds, which increases capital investment and operating costs,
and long forming times in the mold to allow crystallization to be
completed, which reduces output per unit time and thus increases
costs.
[0005] As mentioned, the cPET thermoforming process is a
material-specific one that is designed around the particular
characteristics of PET resin.
[0006] It has been recognized that PLA, like PET, can be formed
into a more crystalline state by subjecting it to certain
temperatures. See Kolstad, "Crystallization Kinetics of
Poly(L-lactide-co-meso-lactide)", J. Applied Polymer Science 62,
1079-1091 (1996). As described by Kolstad, the rate of
crystallization is affected by various factors, including the
lactic acid enantiomer ratio, the use of nucleating agents, and the
thermal history of the polymer (i.e. time at crystallization
temperature and/or cooling rates).
[0007] Nonetheless, no cost-effective commercial process has been
developed for making formed crystalline PLA articles. It would be
desirable to provide such a process, as it would permit PLA to be
used in end-use applications that require improved heat
resistance.
SUMMARY OF THE INVENTION
[0008] In one aspect, this invention is a method for making formed
heat-resistant PLA articles, comprising heating a sheet of an
amorphous, crystallizable PLA resin until the sheet has obtained a
surface temperature from about 80 to about 155.degree. C., then
thermoforming the heated sheet on a mold that is at a temperature
below 80.degree. C.
[0009] In a second aspect, this invention is a method for making
formed heat-resistant PLA articles, comprising heating a sheet of a
semicrystalline PLA resin having a crystallinity of at least 10
Joules/gram to a temperature at which the sheet can be thermoformed
but at which the crystallinity of the sheet is not reduced below 10
Joules/gram, and then thermoforming the sheet on a mold that is at
a temperature below 80.degree. C. to produce a formed article of
PLA resin having a crystallinity of at least 10 Joules/gram.
[0010] In another aspect, this invention is a method for making
formed heat-resistant PLA articles, comprising heating a sheet of
an amorphous, crystallizable PLA resin to a temperature at which
the PLA resin forms crystallites and for a time sufficient to
impart a crystallinity of at least 10 Joules/gram to the PLA resin,
and then thermoforming the heated sheet on a mold that is at a
temperature below 80.degree. C. to produce a formed article of PLA
resin with a crystallinity of at least 10 Joules/gram.
[0011] This invention provides a simple, efficient method for
making formed articles with improved heat resistance from
crystallizable PLA resin. Cycle times as short as about 3 seconds
or even less for the forming step can be achieved with this method
using commercial-scale equipment. Because only a single mold is
necessary, capital expense and operating expenses are minimized.
The crystallinity of the PLA resin can be controlled over a wide
range by adjusting time and temperature during the heating step,
and through control of the composition of the PLA resin itself, as
described more below. As certain properties of the PLA resin
(notably heat resistance) vary with the crystallinity, this process
provides a method whereby easy control over those properties can be
obtained. In particular, this invention provides a rapid and
inexpensive process for making formed PLA articles that can resist
exposure to temperatures of 100.degree. C. or more with minimal or
no distortion. This permits the articles to be used in a variety of
applications, in particular food packaging, in which the article
and its contents are to be heated.
DETAILED DESCRIPTION OF THE INVENTION
[0012] In this invention, a semicrystalline sheet of PLA resin is
thermoformed on a mold that is at a temperature of 80.degree. C. or
less. The semicrystalline sheet of PLA resin is obtained by heating
a sheet of amorphous PLA resin to a specific temperature range
until the desired crystallinity is obtained. The resulting
semicrystalline sheet can be immediately thermoformed, or else
cooled and later reheated to a thermoforming temperature under
conditions that the crystallinity is maintained and then
thermoformed.
[0013] In the preferred process, the heating step is performed as
part of the overall thermoforming process. In the first step of the
preferred process, a sheet of amorphous but crystallizable PLA
resin is heated. The function of the heating is two-fold--to soften
the sheet so that it can be formed in the subsequent step, and to
introduce crystallinity into the PLA resin. The conditions of the
heating are selected to achieve these purposes.
[0014] Softening is performed such that the sheet can be
thermoformed under commercially reasonable conditions, without
softening the sheet so much that it is too fluid to transfer to the
thermoforming mold and be formed into a part.
[0015] Crystallinity is introduced such that the sheet develops a
crystallinity during the heating step of at least 10 Joules/gram,
preferably at least 15 Joules/gram, more preferably at least 20
Joules/gram, even more preferably at least 24 Joules/gram, to about
55 Joules/gram, preferably to about 45 Joules/gram, even more
preferably to about 40 Joules/gram.
[0016] Crystallizable PLA resins are those that will form
crystallites rapidly when a sheet of the resin is heated to a sheet
surface temperature range which lies above the T.sub.g but below
its melting temperature (T.sub.m). The temperature range to which
the PLA resin must be heated will be a characteristic of the
particular PLA resin, and will depend largely on the lactic acid
enantiomer ratio and the presence of nucleating agents and/or
plasticizers, as described more fully below. In general, however,
the temperature at which the requisite crystallization will occur
within commercially reasonable time frames is from about 80.degree.
C. to about 160.degree. C. and is more typically from about
90.degree. C. to about 150.degree. C.
[0017] Thus, one method of controlling the process of the invention
is to monitor sheet surface temperature. Sheet temperature is
generally a suitable process control parameter for sheet
thicknesses that are typically used in thermoforming applications,
such as up to about 250 mil, preferably about 10-100 mil, even more
preferably about 15-50 mil, unless extremely rapid heating rates
are used. A thicker sheet, or one that is heated very rapidly, may
exhibit a significant temperature gradient between the surface and
the center. Under these conditions, therefore, surface temperature
measurements alone may be less suitable as a process control
parameter. For the best results, it is desirable to introduce
crystallinity throughout the thickness of the PLA resin, rather
than only near the surfaces of the sheet.
[0018] It is anticipated that in most cases, heating conditions
will be established empirically with respect to the particular
equipment and particular PLA sheet that is used. These empirically
derived heating conditions may be developed by establishing
suitable and/or optimal temperatures to which a particular PLA
resin sheet should be heated, and then relating those temperatures
to particular controllable processing conditions such as heating
time, line speeds, heater and/or oven temperatures, power to be
supplied to heating apparatus, and the like. Alternatively, heating
conditions can be empirically derived by measuring the
crystallinity of the heated PLA sheet while varying process
parameters.
[0019] The heating step can be carried out in any convenient
manner, such as convection heating, radiant heating (using devices
of various types such as visible light, infrared radiation,
microwave radiation, and the like), conductive heating (such as by
passing the sheet over a heated surface or between heated surfaces
such as heated rollers) and induction heating. In order to keep
cycle times short, heating is preferably done quickly and uniformly
without scorching the sheet or forming significant localized hot
spots. The sheet is conveniently held in a clamping frame or other
apparatus to give it physical support and to facilitate transfer in
and out of the heater and/or to the subsequent forming step.
[0020] The heating step may also be performed as part of the
process of extruding the sheet. In this variation, the PLA resin is
first extruded into sheet form. Because extrusion temperatures are
generally higher than the melting temperature (T.sub.m), it is
necessary to cool the sheet in order to reduce the temperature
below T.sub.m and into the temperature range at which
crystallization occurs. The heating step of this invention can be
accomplished by adjusting the temperature of the freshly extruded
sheet into the aforementioned temperature ranges, and holding the
temperature there until the requisite crystallinity has been
developed. The crystallized sheet can then be cooled below the
T.sub.g, in order to be thermoformed in a separate, later step. In
such a case, the sheet will be re-heated to the thermoforming
temperature (but not to so high a temperature that crystallinity is
destroyed) when the thermoforming process is performed.
Alternatively, the crystallized sheet may be fed directly into the
thermoforming step.
[0021] Following the heating step, the PLA sheet is transferred to
a mold and thermoformed. The transfer to the mold is performed so
that the PLA sheet remains at a temperature suitable for
thermoforming until the transfer is complete and the thermoforming
accomplished. The mold is at a temperature below 80.degree. C.,
preferably below the T.sub.g of the PLA sheet, more preferably no
greater than 50.degree. C., and especially no greater than
35.degree. C. Because of the relatively cold mold temperature, the
PLA sheet quickly hardens into the desired shape in the mold. Some
additional stress-induced crystallinity may be introduced due to
the orientation of the polymer during the thermoforming process,
but the amount is generally small.
[0022] Thermoforming is accomplished by positioning the softened
and crystallized sheet over a male or female mold, and drawing
and/or pressure forming the sheet on the mold to form a molded
part. The mold is most typically a female mold. Multiple formed
parts can be made simultaneously or sequentially from a single
sheet. Except for the mold temperature, which is held below the
temperatures discussed above, the process of this invention can be
conducted using conventional types of thermoforming apparatus,
which are adapted if necessary to provide a means to maintain the
mold at the requisite temperature. Examples of such apparatus and
general methods are described, for example, by Throne in
"Thermoforming Crystallizing Poly(ethylene Terephthalate) (CPET)",
Advances in Polymer Technology, Vol. 8, 131-146 (1988). Drawing is
performed using vacuum, and is the preferred method. The mold may
include a male half that is inserted into the female half during
the process to provide male mold forming. It may also be desirable
to prestretch the sheet; if so a pressure cap or other
prestretching device may be used and actuated prior to drawing the
sheet into the mold.
[0023] The thermoforming step is preferably operated such that
thermoforming cycle time (time to complete one thermoforming cycle
and get ready to perform the subsequent cycle) is minimized.
Thermoforming cycle times are advantageously less than 20 seconds,
preferably less than 10 seconds, more preferably no more than 5
seconds, and even more preferably no more than 3 seconds.
[0024] Once the part is formed and cooled below its T.sub.g, it is
demolded and separated from other parts and trimmed if necessary.
Various downstream operations, such as applying graphics or labels,
assembly to other parts, packaging and the like can be performed if
needed, depending on the type of part and its intended use.
[0025] By "PLA sheet", it is meant a sheet of a thermoplastic
poly(lactic acid) homopolymer or copolymer containing at least 50%
by weight (based on the PLA resin) of repeating units derived from
lactic acid. The PLA is more preferably a homopolymer of lactic
acid. The PLA resin may be blended with small (up to about 50% by
weight, based on the weight of the polymers) of another polymer
which is not a PLA, but is preferably not such a blend. The
preferred PLA resin is a homopolymer of either L-lactic acid or
D-lactic acid, a random copolymer of L-lactic acid and D-lactic
acid, a block copolymer of L-lactic acid and D-lactic acid, or a
mixture of two or more of these. As discussed below, the ratio of
the lactic acid enantiomers and the manner in which they are
copolymerized (i.e., randomly, block, multiblock, graft and like)
greatly influences the ability of the PLA sheet to crystallize in
the present process.
[0026] The PLA resin can be formed by polymerizing lactic acid or,
preferably, by polymerizing lactide. Thus, the term PLA resin is
used herein to include polymers made by polymerizing lactide.
Lactide is a dimeric form of lactic acid, in which two lactic acid
molecules are condensed to form a cyclic diester. Like lactic acid,
lactide similarly exists in a variety of enantiomeric forms, i.e.,
"L-lactide", which is a dimer of two L-lactic acid molecules,
"D-lactide", which is a dimer of two D-lactic acid molecules and
"meso-lactide", which is a dimer formed from one L-lactic acid
molecule and one D-lactic acid molecule. In addition, 50/50
mixtures of L-lactide and D-lactide that have a melting temperature
of about 126.degree. C. are often referred to as "D,L-lactide". Any
of these forms of lactide, or mixtures thereof, can be
copolymerized to form a PLA resin for use in this invention. The
L/D ratio in the PLA resin is controlled through the ratio of these
enantiomeric forms of lactide that are used in the polymerization.
In an especially preferred process mixtures of L-lactide and
meso-lactide are polymerized to form a polymer having a controlled
level of D-lactic acid enantiomeric units. Suitable processes for
polymerizing lactide to form PLA having controlled L/D ratios are
described, for example, in U.S. Pat. Nos. 5,142,023 and 5,247,059,
both incorporated herein by reference.
[0027] The PLA resin may be compounded with various additives and
processing aids such as nucleants, other inorganic fillers,
plasticizers, reinforcing agents, slip agents, lubricants,
UV-stabilizers, thermal stabilizers, flame retardants, foaming
agents, antistatic agents, antioxidants, colorants, and the like,
with finely divided inorganic solids being of particular importance
as discussed more below.
[0028] The PLA sheet is amorphous and capable of being crystallized
to the extent of 10 Joules/gram through heating at a temperature
between its T.sub.g and T.sub.m. Preferably, it is capable of being
crystallized to a crystallinity of at least 15 Joules/gram, more
preferably at least 20 Joules/gram, even more preferably at least
24 Joules/gram, especially at least 30 Joules/gram. For purposes of
this invention, a PLA sheet is considered "amorphous" if it
exhibits a crystallinity of less than 10 Joules/g when measured by
differential scanning calorimetry (DSC) as described more fully
below.
[0029] The composition of the PLA sheet is also preferably such
that the sheet can be crystallized to the desired extent in a short
period. Three important parameters affecting this are the ratio of
the lactic acid enantiomers in the PLA resin, the use of nucleating
agents and the use of plasticizers.
[0030] The ability of PLA to crystallize, all other things being
equal, is greatest when only one of the lactic acid enantiomeric
forms is present in polymerized form in the polymer. Thus,
homopolymers of L-lactic acid or D-lactic acid are the forms of PLA
that tend to crystallize most completely and rapidly. In random
copolymers of the L- and D-enantiomers, the ability to crystallize
falls off rapidly as more of the second enantiomer is present. In
this invention, it is preferred to use a PLA resin in which the
lactic acid enantiomer ratio is at least 90:10, preferably at least
95:5, more preferably at least 98:2, to about 99.9:0.1, more
preferably to about 99.5:0.5, even more preferably 99:1. It is
unimportant in terms of performance whether the predominant isomer
is the L- or D-form. PLA resins having predominantly the L-isomer
are more readily available commercially, and are preferred for that
reason.
[0031] The proportion of "D" and "L" lactic acid repeating units in
enantiomeric polylactic acid (PLA) copolymers can be accomplished
by high performance liquid chromatography. A suitable chromatograph
is a Waters LC Module I HPLC with a Sumichiral OA6100 column set
and a Model 486 variable wavelength UV detector. Prior to analysis,
the PLA enantiomeric copolymer is fully hydrolyzed in a basic
aqueous solution into its constituent D- and L-lactic acid monomer
units. Enantiomer ratio is conveniently determined by neutralizing
the solution with 1N HCl and injecting it into the HPLC through a
0.45 micron filter. The concentration of the lactic acid
enantiomers is determined by comparison of the HPLC results to
standard curves generated using pure standards such as are supplied
by the Aldrich and Sigma Chemical Companies.
[0032] The PLA resin may be compounded with a nucleating agent in
order to improve its ability to crystallize quickly. Suitable
nucleating agents include finely divided solids that do not react
under the conditions of the heating step or the thermoforming
process. Particles having a median particle size of less than 5
.mu.m, preferably less than 1 .mu.m, are particularly suitable. The
most preferred nucleating agent is talc, as it often provides a
measure of reinforcement in addition to performing the nucleation
function well. Among the suitable commercially available talc
products are Ultratalc.TM. 609, available from Specialty Minerals,
Inc., and Zemex HTP Ultra SC.TM., available from Zemex Fabi Benwood
LLC. Nucleating agents are used in effective amounts, but if used
in too large quantities they can cause the physical properties of
the PLA resin to deteriorate. Preferred amounts for most nucleating
agents are from about 0.1, preferably from about 0.5 to about 10,
preferably to about 5, more preferably to about 2.5 percent of the
combined weight of the PLA resin and the nucleating agent. In the
case of talc, it may be desirable to employ larger quantities in
order to obtain a desirable reinforcement effect. Thus, preferred
usage levels for talc are from about 0.5, more preferably from
about 3, even more preferably from about 5 to about 40, more
preferably to about 30 and even more preferably to about 20 percent
of the combined weight of the PLA resin and the talc.
[0033] Nucleating agents are conveniently melt compounded into the
PLA resin using any suitable melt compounding equipment, such as
single and twin screw extruders, roll mills, Banbury mixers,
Farrell continuous mixers and the like. The nucleating agent can
also be added during the sheet extrusion process.
[0034] Plasticizers also tend to improve the rate at which PLA
crystallizes. In general, a suitable plasticizer is one that is
compatible with the PLA resin and stable under the conditions of
the heating and thermoforming steps. Suitable plasticizers include
phthalates (including dioctyl phthalate), citric acid esters,
lactic acid esters such as ethyl lactate, lactide esters, mineral
oil, triphenyl phosphate, glycerine, acetin and butyrin. Those that
are biodegradable are preferred. Suitable amounts of plasticizer
are from about 0.5 to about 30 percent, based on the combined
weight of the PLA resin plus the plasticizer. Plasticizers are
conveniently melt compounded into the PLA resin or added during the
sheet extrusion process as described before.
[0035] The ability of a PLA resin to crystallize at a given
temperature can be expressed in terms of a crystallization
half-time, as described more fully by Kolstad in "Crystallization
Kinetics of Poly(L-lactide-co-meso-lactide)", J. Appl. Poly. Sci.
62:1079-1091 (1986), incorporated herein by reference. In general,
the crystallization half-time is the time required for a PLA resin
to achieve one-half of its ultimate extent of crystallization under
particular heating conditions. Sheets of PLA resins exhibiting
crystallization half-times of less than 10 minutes, preferably less
than 3 minutes, and especially less than 1 minute at the
temperatures of the heating step are suitable. Sheets of PLA resins
that exhibit crystallization half-times of less than about 10
seconds may crystallize too rapidly to provide good process
control.
[0036] The sheet may be a multilayer type, in which at least one
layer is a crystallizable PLA resin as described. Other layers may
also be of crystallizable PLA resin, an amorphous PLA resin, or may
be composed of a different polymer (such as a barrier plastic),
provided that the sheet can be thermoformed under the conditions
described herein. Multilayer sheets may be formed by, for example,
coextrusion or lamination.
[0037] The sheet may also be cellular. Cellular sheet can be made
by incorporating a foaming agent into the PLA resin during the
sheet extrusion process, and extruding the sheet under conditions
such that the foam agent generates a gas and expands the sheet when
the sheet is extruded. Extrusion foaming processes are well known
and applicable here. Cellular sheet is preferably mainly closed
celled and has a density as low as about one lb/ft.sup.3.
[0038] A cellular sheet can be formed by foaming it during the
softening/crystallization step of this process. In this case, the
sheet will contain a foaming agent that generates a gas under the
conditions of the heating step. In this manner, the sheet can be
simultaneously heated and blown. Suitable blowing agents include
lower hydrocarbons, halogenated alkanes such as fluorinated alkanes
and perfluorinated alkanes, and water, as well as chemical blowing
agents such as citric acid/sodium bicarbonate mixtures, gasses such
as carbon dioxide and nitrogen, and the like.
[0039] Formed articles made according to the invention have
improved resistance to heat, compared to PLA articles that are
formed with lower crystallinity. Formed PLA articles having a
crystallinity of about 15 Joules/gram or more have been found to
withstand boiling water for several minutes without significant
dimensional changes or distortion. As such, the formed articles can
be used as containers for hot foods, as microwavable food trays, or
in other applications where they are exposed to moderately high
temperatures (up to about 100.degree. C. in many cases, and up to
the crystalline melting point of the article for more highly
crystalline articles). As PLA resin is hydrolyzable, the length of
time for which the article can withstand elevated temperatures will
depend on whether, and how much, water is present. In general, the
presence of liquid water or increasing humidity tends to shorten
this time, as the PLA tends to hydrolyze under those
conditions.
[0040] The formed articles tend to be opaque to somewhat
translucent in appearance, even when a nucleating agent is used.
The degree of opacity is generally related to the number and size
of the spherulitic crystallites developed in the crystallization
step. The opacity can be reduced through more effective crystal
nucleation. More effect nucleation is favored by using smaller
nucleating particles, in larger numbers. In some instances, the PLA
resin may become oriented during the forming process. This may
cause some reduction in opacity in the areas in which the
orientation occurs.
[0041] Among the articles that can be made according to the
invention are: beverage cups, other foodware such as trays and
plates, including those having attached lids; trays and plates for
non-food applications and separate lids or covers for any of the
foregoing.
[0042] The following examples are provided to illustrate the
invention, but are not intended to limit its scope. All parts and
percentages are by weight unless otherwise indicated.
EXAMPLE 1
[0043] A. Evaluation of Processing Window for Talc-Nucleated PLA
sheet
[0044] A single-stage, shuttle-type lab-scale thermoformer without
plug assist and a heating oven equipped with an IR radiant heater
positioned above and below the sheet is used to evaluate the
processing conditions for a nucleated PLA resin sheet. 20-mil
sheets made from an amorphous, random copolymer of 98.3% L-lactic
acid and 1.7% D-lactic acid and containing 10% by weight
Ultratalc.TM. 609 (trademark of Specialty Minerals, Inc.) talc are
placed for varying, predetermined times in the pre-heated
thermoformer oven with the heaters set at 80% power. After the
predetermined time in the oven, the sheets are racked out and their
surface temperature measured using a Raytek.TM. ST IR surface
thermometer. The sheets are then cooled by forcing cool air over
their surface. The crystallinity of the cooled sheets is measured
by DSC according to the procedure below. In this manner, induced
crystallinity is correlated both to oven residence time and to the
attained surface temperature, as follows:
1TABLE 1 Time in Oven (s) Attained Surface Temperature (.degree.
C.) Crystallinity (J/g) 16 111 22.1 20 128 30.1 25 143 31.7 30 151
36.4 40 162 5.8 55 182 6.5
[0045] The DSC measurements are made using a Mettler Toledo DSC
821e calorimeter running Star V. 6.0 software. Samples are 5-10
milligrams. Heating is performed from 25-225.degree. C. at
20.degree. C./minute, under air.
[0046] At a heating time of less than 16 seconds under these
conditions, the sheet is too rigid to form, even though the
measured crystallinity was less than 22 J/g. At 40-55 seconds
residence time, the PLA sheet contains less than 10 Joules/g
crystallinity, indicating that the sheet has reached too high a
temperature, thereby destroying the crystallinity. At more than 55
seconds residence time, the sheet is too fluid to form. Within the
range of 16 to 30 seconds heating time, the sheet becomes
thermoformable and attains the desired crystallinity.
[0047] Similar evaluations of 20 mil sheets of other PLA resins
demonstrate how the process window can vary with the composition of
the PLA resin. When a PLA resin containing 1.7% D-isomer and 2.5%
Ultratalc 609 is tested, the optimum oven residence time under
these conditions is about 18-33 seconds. The attained surface
temperature that correlates to 15 J/g or more crystallinity is
about 115-155.degree. C. For a 1.2% D-isomer PLA containing 5%
UltraTalc 609, the optimum oven residence time under these
conditions is about 10-23 seconds and the surface temperature
correlating to 15 J/g or more crystallinity is 105-155.degree.
C.
[0048] B. Thermoforming of PLA Resin Sheet on Cold Mold
[0049] Sheets of amorphous PLA resin as described in A above are
thermoformed by heating them in the pre-heated thermoformer oven
for 15 seconds at 90% power, and then immediately vacuum forming
the heated sheets onto a 23.degree. C., single compartment tray
mold having dimensions of 31/8.times.35/8.times.1.5 inches. The
time on the mold is varied in order to determine the minimum cycle
time and to assess the effect of in-mold residence time on
crystallinity. All parts come off the mold rigid, regardless of
in-mold residence time. The results are summarized in Table 2
below.
2TABLE 2 Time on Mold (s) Overall Cycle Time (s)* Crystallinity
(J/g) 4 19 16.7 3 18 16.1 2 17 16.4 *Includes 15 seconds residence
time in thermoformer oven.
[0050] The data in table 2 demonstrate that thermoformed articles
of semi-crystalline PLA are easily prepared via the process of the
invention. Note that under these conditions, the in-mold residence
time has no effect on crystallinity, indicating that all
crystallinity is developed in the thermoformer oven during the
heating step. This is confirmed by DSC evaluation of a sheet that
is taken from the thermoformer oven and rapidly cooled without
undergoing the forming step.
[0051] C. Heat Stability of Formed Parts
[0052] The heat resistance of the parts made in B above is
evaluated by filling the part with water and microwaving at high
power for five minutes, or longer if required to bring the water to
boiling. All parts retain their shape under this test. By contrast,
a similar part having a crystallinity of less than 10 J/g nearly
completely loses its shape under this test, flattening almost
entirely so that the water it contains at the start of the test is
completely spilled.
EXAMPLE 2
[0053] A 98.8% L/1.2% D PLA resin containing 5% Ultratalc 609 is
used in this Example. It is extruded into 16" wide, 20 mil sheet
using a single screw extruder equipped with a general purpose
screw, a Maddock mixing head and a 28" horizontal cast sheet die.
The freshly extruded, substantially amorphous sheet is run through
a heated three roll stack, trimmed and wound into rolls.
[0054] This sheet is thermoformed on an Irwin Mini Mag 28 in-line
thermoformer equipped with a 6" soup bowl mold. The machine is
operated at 14" and 24" index lengths. Index length affects oven
residence time, with larger indices reducing residence time
proportionately, for a given cycle time. The oven is operated at a
top oven temperature of between 480 and 675.degree. F. and a bottom
oven temperature of between 430 and 635.degree. F., as indicated in
Table 3 below. The temperature of the sheet is measured as it
leaves the oven just prior to forming, using an IR thermometer. The
mold temperature is about 23.degree. C. In-mold residence time is
less than 3 seconds. Samples of the sheet trim and the molded bowls
are taken in order to measure crystallinity. It is found that the
crystallinity of the sheet trim that is not quenched by the mold is
not significantly different from that of the corresponding bowls.
This confirms that crystallization has occurred in the heating step
prior to forming. Results are summarized in Table 3.
3TABLE 3 Index Crystallinity Length Top Oven Bottom Oven Attained
Sheet (Molded Parts) (in.) Temp. (.degree. C.) Temp. (.degree. C.)
Temp. (.degree. C.) (J/g) 14 480 430 120 16.3 14 490 445 133 16.5
14 500 450 ND 21.6 14 505 460 140 25.7 14 510 465 143 26.5 14 520
475 143 25.1 14 530 485 149 25.9 24 635 585 137 16.6 24 650 610 146
18.3 24 675 635 155 14.0
* * * * *