U.S. patent application number 11/372987 was filed with the patent office on 2006-10-19 for high modulus polymer composites and methods of making the same.
Invention is credited to Birgit Braun, John R. Dorgan.
Application Number | 20060235113 11/372987 |
Document ID | / |
Family ID | 37109363 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060235113 |
Kind Code |
A1 |
Dorgan; John R. ; et
al. |
October 19, 2006 |
High modulus polymer composites and methods of making the same
Abstract
The invention provides methods of producing composite polymers
by combining fillers with polymers in the presence of preformed
high molecular weight polymer. Monomer polymerization can be
initiated through the addition of initiators or by reactive
chemical groups on the surface of the fibers. The composite
materials formed possess superior mechanical properties compared to
similar polymer composites made by either purely mechanical mixing
or solely polymerization of monomers in the presence of the
fillers.
Inventors: |
Dorgan; John R.; (Golden,
CO) ; Braun; Birgit; (Golden, CO) |
Correspondence
Address: |
SHERIDAN ROSS PC
1560 BROADWAY
SUITE 1200
DENVER
CO
80202
US
|
Family ID: |
37109363 |
Appl. No.: |
11/372987 |
Filed: |
March 10, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60660972 |
Mar 11, 2005 |
|
|
|
Current U.S.
Class: |
524/13 |
Current CPC
Class: |
B29C 48/362 20190201;
B29K 2105/0002 20130101; B29K 2105/06 20130101; C08J 2367/04
20130101; Y10T 428/2998 20150115; Y10T 428/2991 20150115; B29C
48/022 20190201; C08J 5/045 20130101; B29C 48/04 20190201 |
Class at
Publication: |
524/013 |
International
Class: |
B29C 47/00 20060101
B29C047/00 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made with government support under
Environmental Protection Agency (EPA)-grant RD-83153001-1. The
government has certain rights in the invention.
Claims
1. A method comprising: mixing a filler simultaneously with a
monomer that can react to form a grafted polymer layer on the
surface of the filler and a pre-formed polymer; wherein an
interchange reaction takes place between a grafted layer on the
surface of the filler and the pre-formed polymer to form a
composite polymer.
2. The method of claim 1, wherein the filler is an organic
filler.
3. The method of claim 2, wherein the filler is selected from the
group consisting of wood fiber, wood flour, starch, straws,
bagasse, coconut hull/fiber, cork, corn cob, corn stover, cotton,
gilsonite, nutshell, nutshell-flour, rice hull, sisal, hemp, and
soybean.
4. The method of claim 1, wherein the filler is an inorganic
filler.
5. The method of claim 4, wherein the filler is selected from the
group consisting of a mineral, calcium carbonate, montmorilonite,
kaolin, titanium dioxide, alumina trihydrate, Wollastonite, talc,
silica, quartz, barium sulfate, antimony oxide, mica, magnesium
hydroxide, calcium sulfate, feldspar, nepheline syenite,
microspheres, carbon black, glass, glass fibers, carbon fibers,
metallic particles, magnetic particles, montmorillonite,
buckminsterfullerene, carbon nanotubes, carbon nanoparticles,
silicas, cellulosic nanofibers, synthetic silicates, and
synthetically prepared nanoparticles.
6. The method of claim 1, wherein the filler is cellulose that has
been pretreated with alkali prior to the mixing.
7. The method of claim 1, wherein the monomer is selected from the
group consisting of L-lactide, D-lactide, LD-lactide, caprolactone,
caprolactam, ring opening monomers, ethylene glycol-terphthalic
acid, sebacoyl chloride--1,6 hexadiamine, step reaction monomers,
ethylene, propylene, styrene, methyl methacrylate, and vinyl
monomers.
8. The method of claim 1, wherein the pre-formed polymer is
selected from the group consisting of poly-lactic acid (PLA),
polycaprolactone, poly(ethylene terephthalate), polyesters,
polycaprolactam (Nylon 6), poly(hexamethyl sebacamide) (Nylon
6,10), polyamides, polyurethanes, polycarbonates, polyolefins,
polyethylene, polybutadiene, polypropylene, polystyrene, and
polymethylmethacrylate.
9. The method of claim 1, wherein the mixing is conducted in the
presence of a catalyst that initiates or catalyzes the
polymerization reaction.
10. The method of claim 1, wherein the mixing is conducted in the
presence of a catalyst that catalyzes the interchange reaction
between the monomer and the pre-formed polymer on the surface of
the filler.
11. The method of claims 9 or 10, further comprising stopping a
polymer reaction by contacting the pre-formed polymer with a
compound that deactivates the catalyst.
12. The method of claim 10, wherein the catalyst is selected from
the group consisting of titanium(IV) isopropoxide (TIP), dibutyl
tin oxide (DBTO), an alkyl tin(IV) compound, monobutyltin
trichloride (BuSnCl.sub.3), TBD
(1,5,7-triazabiscyclo(4.4.0)dec-5-ene), acid catalysts, sulfonic
and sulfuric acids, base catalysts, sodium methylate, sodium
methoxide, potassium methoxide, sodium hydroxide and potassium
hydroxide, organic bases, triethylamine, piperidine,
1,2,2,6,6-pentamethylpiperidine, pyridine,
2,6-di-tert-butylpiridine, 1,3-Disubstituted
tetrakis(fluoroalkyl)distannoxanes, 4-dimethyl-aminopyridine (DMAP)
and guanidine, alkaline metal alkoxides and hydroxides, basic
zeolites, cesium-exchanged NaX faujasites, mixed magnesium-aluminum
oxides, magenesium oxide and barium hydroxide,
4-(dimethylamino)pyridine (DMAP), 4-pyrolidinopyridine (PPY), salts
of amino acids, and enzyme transesterification catalysts.
13. The method of claim 1, wherein the monomer is lactide and the
pre-formed polymer is polylactide.
14. The method of claim 1, wherein the monomer is ethylene
glycol-terphthalic acid and the pre-formed polymer is poly(ethylene
terphthalate).
15. The method of claim 1, wherein the monomer is ethylene and the
pre-formed polymer is polyethylene.
16. The method of claim 1, wherein the monomer is an ester and the
pre-formed polymer is a poly(ester).
17. The method of claim 1, wherein the monomer is a polycarbonate
and the pre-formed polymer is a poly(ester).
18. The method of claim 1, wherein the monomer is a polyamide and
the pre-formed polymer is a poly(ester).
19. A method comprising: mixing cellulose fibers with lactide and
polylactic acid to form a premix contacting the premix with a
composition comprising stannous octoate, triphenylphospine, and
titanium isopropoxide, to produce a composite polymer contacting
the composite polymer with poly(acrylic acid) to form a stabilized
composite polymer.
20. A composite polymer made by mixing a filler simultaneously with
1) a monomer that can react to form a grafted polymer layer on the
surface of the filler and 2) a pre-formed polymer, wherein an
interchange reaction takes place between a grafted layer on the
surface of the filler and the pre-formed polymer to form a
composite polymer.
21. The polymer of claim 20, wherein the filler is cellulose, the
monomer is lactide and the pre-formed polymer is poly-lactic acid
(PLA).
22. The polymer of claim 21, wherein the mixing is conducted in the
presence of a catalyst selected from the group consisting of
titanium(IV) isopropoxide (TIP), stannous octoate Sn(Oct).sub.2,
triphenylphospine, and mixtures thereof.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119(e) from U.S. Provisional Patent Application No.
60/660,972 filed Mar. 11, 2005, which is incorporated herein in its
entirety by this reference.
FIELD OF THE INVENTION
[0003] The invention relates to polymer composites and more
particularly, the invention provides novel methods of making high
molecular weight filled polymeric matrices having desirable
physical characteristics.
BACKGROUND OF THE INVENTION
[0004] The introduction of fibers and other fillers into a
polymeric matrix is an established route to enhancing the physical
properties of a chosen polymer provided good dispersion and
intimate interfacial adhesion can be achieved. This method can also
be very cost effective providing the filling agents are obtainable
at low to moderate cost.
[0005] Previous studies have shown successful improvements in
mechanical properties of polymers by the formation of
microcomposites through purely physical mixing of fiber fillers
into the polymer. For example, Oksman et al. (Oksman, K., M.
Skrifvars, and J. F. Selin, Natural fibres as reinforcement in
polylactic acid (PLA) composites. Composites Science and
Technology, 63(9): 1317-1324, 2003) embedded flax fibers into a
polylactic acid (PLA) matrix and compared the resulting composite
properties to polypropylene (PP) filled with the same fibers. It
was found that the mechanical properties of the flax-PLA composites
are promising, since the composite strength was about 50% better
compared to similar flax-PP composites that are used in many
industrial applications. However, microscopy studies suggested a
lack of interfacial adhesion between the polymer matrix and the
fiber surface. Additionally, Huda et al. (Huda, M. S., et al.,
Effect of processing conditions on the physico-mechanical
properties of cellulose fiber reinforced poly(lactic acid). ANTEC
2004 Plastics: Annual Technical Conference, Volume 2: Materials,
2:1614-1618, 2004; Huda, M. S., et al. Physico-mechanical
properties of "Green" Composites from poly(lactic acid) and
cellulose fibers, at GPEC, Detroit, USA, 2004) showed improvement
of the tensile strength, tensile modulus and impact strength upon
reinforcing PLA with cellulose fibers. However, the introduction of
cellulose fibers did not affect the glass transition temperature
significantly as measured by DSC.
[0006] Thus, there is a desire for improved polymer composites but
there exits a need for an improved method of making these filled
polymers to achieve the desired polymer physical characteristics
rapidly and at an acceptable cost.
SUMMARY OF THE INVENTION
[0007] The technology disclosed herein is a method of making
polymer composites. These methods are rapid and economically
efficient ways to produce polymer composites. The polymer
composites produced by these methods have desirable physical
characteristics. The polymer composites produced by these methods
also have a homogenous or nearly homogenous distribution of filler
throughout.
[0008] The methods include the mixing of a monomer or oligomer with
a pre-formed polymer in the presence of a filler to initiate an
interchange reaction between a grafted layer of monomer or oligomer
on the surface of the filler and the pre-formed polymer, leading to
the formation of a composite polymer.
[0009] In one embodiment, a method of forming a composite polymer
includes mixing a filler simultaneously with a monomer that can
react to form a grafted polymer layer on the surface of the filler
and a pre-formed polymer. In this process, an interchange reaction
takes place between a grafted layer or monomer or oligomer on the
surface of the filler and the pre-formed polymer, to form a
composite polymer.
[0010] In this reaction, the filler may be an organic filler such
as wood fiber, wood flour, starch, straws, bagasse, coconut
hull/fiber, cork, corn cob, corn stover, cotton, gilsonite,
nutshell, nutshell-flour, rice hull, sisal, hemp or soybean.
[0011] Alternatively, the filler may be an inorganic filler such as
a mineral, calcium carbonate, montmorilonite, kaolin, titanium
dioxide, alumina trihydrate, Wollastonite, talc, silica, quartz,
barium sulfate, antimony oxide, mica, magnesium hydroxide, calcium
sulfate, feldspar, nepheline syenite, microspheres, carbon black,
glass, glass fibers, carbon fibers, metallic particles, magnetic
particles, montmorillonite, buckminsterfullerene, carbon nanotubes,
carbon nanoparticles, silicas, cellulosic nanofibers, synthetic
silicates or synthetically prepared nanoparticles. In a preferred
embodiment, the filler is cellulose that has been pre-treated with
alkali prior to the mixing.
[0012] The monomer(s) used in these methods may be any monomer or
oligomer capable of interacting with the filler, and particularly,
interacting with surface groups on the surface of the filler, and
also capable of participating in the interchange reaction with the
chosen pre-formed polymer. While these characteristics are
necessarily dependent upon the chosen filler and pre-formed polymer
to be used in these reactions, exemplary monomers include
L-lactide, D-lactide, LD-lactide, caprolactone, caprolactam, ring
opening monomers, ethylene glycol-terphthalic acid, sebacoyl
chloride--1,6 hexadiamine, step reaction monomers, ethylene,
propylene, styrene, methyl methacrylate, and vinyl monomers.
[0013] The pre-formed polymer(s) are similarly any polymer capable
of participating in the interchange reaction with the filler and
the monomer. Exemplary polymers for use as the pre-formed polymer
in these processes include poly-lactic acid (PLA),
polycaprolactone, poly(ethylene terephthalate), polyesters,
polycaprolactam (Nylon 6), poly(hexamethyl sebacamide) (Nylon
6,10), polyamides, polyurethanes, polycarbonates, polyolefins,
polyethylene, polybutadiene, polypropylene, polystyrene and
polymethylmethacrylate.
[0014] In certain embodiments, catalysts may be used to increase
the initiation rate or polymerization rate of the monomer(s) or to
increase the rate of the interchange reactions or both. The
polymerization and interchange reactions can be stopped through the
use of a compound that deactivates one or more catalysts used in
these processes.
[0015] Exemplary catalyst of the interchange reactions include
titanium(IV) isopropoxide (TIP), dibutyl tin oxide (DBTO), an alkyl
tin(IV) compound, monobutyltin trichloride (BuSnCl.sub.3), TBD
(1,5,7-triazabiscyclo(4.4.0)dec-5-ene), acid catalysts, sulfonic
and sulfuric acids, base catalysts, sodium methylate, sodium
methoxide, potassium methoxide, sodium hydroxide and potassium
hydroxide, organic bases, triethylamine, piperidine,
1,2,2,6,6-pentamethylpiperidine, pyridine,
2,6-di-tert-butylpiridine, 1,3-Disubstituted
tetrakis(fluoroalkyl)distannoxanes, 4-dimethyl-aminopyridine (DMAP)
and guanidine, alkaline metal alkoxides and hydroxides, basic
zeolites, cesium-exchanged NaX faujasites, mixed magnesium-aluminum
oxides, magenesium oxide and barium hydroxide,
4-(dimethylamino)pyridine (DMAP), 4-pyrolidinopyridine (PPY), salts
of amino acids, and enzyme transesterification catalysts.
[0016] Examples of particularly preferred combinations of monomers
and pre-formed polymers include (monomer/pre-formed polymer)
lactide/polylactide, ethylene glycol-terphthalic acid/poly(ethylene
terphthal ate), ethylene/polyethylene, an ester/poly(ester), a
polycarbonate/a poly(ester), and, a polyamide/a poly(ester).
[0017] In one embodiment, a method of mixing cellulose fibers with
lactide and polylactic acid to form a premix and then contacting
this premix with a composition including stannous octoate,
triphenylphospine, and titanium isopropoxide, to produce a
composite polymer and then contacting the composite polymer with
poly(acrylic acid) to form a stabilized composite polymer is
provided.
[0018] Additional embodiments include composite polymers made by
mixing a filler with a monomer that can react to form a grafted
polymer layer on the surface of the filler and a pre-formed polymer
to produce an interchange reaction between a grafted layer on the
surface of the filler and the pre-formed polymer to form a
composite polymer. In one embodiment, this composite polymer is
produced using a cellulose filler, lactide monomers and pre-formed
poly-lactic acid (PLA).
[0019] This preferred polymer may be mixed in the presence of a
catalyst such as titanium(IV) isopropoxide (TIP), stannous octoate
Sn(Oct).sub.2, triphenylphospine, and/or mixtures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a SEM micrograph showing untreated fibers (left)
and the fibers after alkali treatment (right).
[0021] FIG. 2 is a chemical scheme for one embodiment of the
present invention using a multifunctional initiator and the
resulting polymer structure.
[0022] FIG. 3 shows the molecular structure of cellulose,
exhibiting 6 hydroxyl groups available for initiating polylactic
acid (PLA) polymerization as described in the Examples appended
below.
[0023] FIG. 4 is a graph of the molecular weight development as a
function of reaction time with and without the addition of PAA.
(Experimental conditions: 180.degree. C., addition of PAA after 25
minutes reaction time).
[0024] FIG. 5 is a graph showing that the introduction of
pre-formed polymer increases the molecular weight (Experimental
conditions: 200.degree. C., 20 minutes reaction time).
[0025] FIG. 6 is a graph of T.sub.g versus molecular weight for
samples with various composition of L- and D-lactide.
[0026] FIG. 7 is a graph of T.sub.g versus fiber loading level for
samples containing only pre-formed PLA (100/0 P/L) and for samples
completely polymerized in the presence of cellulose fibers (0/100
P/L) as described in the appended Examples.
[0027] FIG. 8 is a graph showing the achievable crystallinity, as
measured by DSC for samples containing only pre-formed PLA (100/0
P/L) and samples completely polymerized in the presence of
cellulose fibers (0/100 P/L) as described in the appended
Examples.
[0028] FIG. 9 includes graphs of the storage modulus versus fiber
loading level for a) samples containing 100% pre-formed PLA, b)
samples containing 65% preformed PLA and 35% lactide at the
beginning of the reaction, c) samples containing 30% pre-formed PLA
and 70% lactide at the beginning of the reaction, d) samples
containing 100% lactide at the beginning of the reaction.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The quality of polymer composites are influenced by various
factors, including, principally 1) the aspect ratio of the filler
particle (fiber or other geometry) used, 2) the particle
orientation, 3) the volume fraction of filler, 4) the dispersion of
particles in the polymeric matrix, and 5) the interfacial adhesion
between the particle surface and the surrounding matrix.
[0030] The interaction between the filler surface and the polymer
matrix is an especially important factor for the reinforcement
potential of the filler. In general, macroscopic reinforcing
elements always contain imperfections. Structural perfection is
greater as the filler becomes smaller and smaller. However, the
competing effect of increasing specific surface area with
decreasing size has to be considered. Additionally, in the case of
a hydrophilic particle incorporated into a hydrophobic polymer, a
lack of dispersion and interfacial adhesion is observed. In order
to improve these factors, reactive compatibilization has been used
to mix the polymer monomers with the filler. Unfortunately, this
process has been hampered by the time needed to produce a composite
polymer with an average high molecular weight and insufficient
mixing of the filler within the formed polymer.
[0031] The methods described herein overcome these problems with
reactive compatibilization resulting in composites with superior
physical characteristics to those formed by conventional means. The
methods of the present invention include the mixing of a polymer
filler with both a low molecular weight monomer or oligomer and a
pre-formed high molecular weight polymer. The monomers polymerize
to become chemically grafted onto the filler and undergo an
interchange reaction with the pre-formed high molecular weight
polymers thereby greatly decreasing the time needed to form a
composite polymer. This reactive process also maintains a high
viscosity leading to greater stress within the mixture which aides
in dispersing the filler within the composite polymer.
[0032] The filler used in these processes can be any suitable
filler that can react with the monomer or oligomer. The polymer
system chosen may be any combination of polymers that are capable
of participating in interchange reactions. One or more initiators
or catalysts of the polymerization reaction can be added to the
reaction mixture. Additionally, one or more catalysts for the
interchange reaction can be added to the reaction mixture.
Preferably, the chosen filler has a reactive surface capable of
initiating the polymerization reaction of the monomers during
mixing. When surface groups are employed as initiators for the
polymerization, there are two major competing factors to be
considered: 1) in polymerization reactions, a high conversion rate
of monomer is desired and therefore a high number of initiating
groups is advantageous, 2) to obtain good mechanical properties, a
high molecular weight of the polymer is required. High molecular
weight implies a low number of polymer chains, corresponding to a
low number of initiating groups.
[0033] The simultaneous introduction of the monomer and high
molecular-weight, pre-formed polymer is able to address these
competing factors. The monomer polymerization is initiated from the
reactive surface groups on the fiber or particle surface. The
presence of pre-formed high molecular-weight polymer ensures a
higher average molecular weight of polymer chains grafted to the
fiber surface. The resulting material consists of high-molecular
weight polymer chains grafted to the particle surface in a polymer
matrix. The resulting composites show improved interfacial adhesion
and better dispersion properties. In addition, having high
molecular weight polymer present initially means the viscosity is
high so that more stress is transmitted during shearing of the
sample, this leads to more effective dispersive mixing.
[0034] Examples of fillers useful in the methods of the present
invention include, but are not limited to, organic based fillers
and combinations thereof like, wood fiber, wood flour, starch,
straws, bagasse, coconut hull/fiber, cork, corn cob and corn
stover, cotton based fillers, gilsonite based fillers, nutshell and
nutshell-flour based fillers, rice hull based fillers, sisal, hemp,
and soybean based fillers. In addition, mineral fillers and
combinations thereof may be employed including, but not limited to,
calcium carbonate, montmorilonite, kaolin and other clay-based
minerals, titanium dioxide, alumina trihydrate, Wollastonite, talc,
silica, quartz, barium sulfate, antimony oxide, mica, magnesium
hydroxide, calcium sulfate, feldspar and nepheline syenite, various
types of microspheres (solid or hollow), carbon black, glass, glass
fibers, carbon fibers, and various metallic or magnetic particles.
This also includes any nano-sized fillers and combinations thereof
including, but not limited to, montmorillonite and other clay based
particles, buckminsterfullerene, carbon nanotubes, other carbon
based nanoparticles, silicas, glasses, cellulosic nanofibers,
synthetic silicates or any type of synthetically prepared
nanoparticles. In one embodiment in which an initiator other than
the filler is added to the reaction, multifunctional initiators are
preferred to further increase the speed of the polymerization
reaction and to encourage the formation of more highly-branched
polymers.
[0035] The filler particles employed may be functionalized to
enable reaction using a variety of coupling agents. Such coupling
agents include, but are not limited to, silane coupling agents,
titanates, zirconates, aluminates, teolomers, and others.
[0036] The monomer and polymer molecules can be chosen from a wide
variety of known polymers. Preferably, the polymerization of the
monomers is initiated in the presence of reactive groups on the
surface of the filler. Exemplary monomer/polymer combinations
include, but are not limited to, ring opening polymerization
systems like lactide and polylactide, caprolactone and
polycaprolactone, caprolactam/polycaprolactam (Nylon 6), and
numerous others. Also, condensation polymer systems can be utilized
like ethylene glycol-terphthalic acid/poly(ethylene terphthalate),
sebacoyl chloride--1,6 hexadiamine/poly(hexamethyl sebacamide)
(Nylon 6,10), and numerous others. Also vinyl polymer systems can
be used including, but not limited to, ethylene/polyethylene,
propylene/polypropylene, styrene/polystyrene, methyl
methacrylate/polymethylmethacrylate, and many others. When
polyester systems are utilized, the most important transreactions
are alcoholysis, acidolysis, and transesterification.
[0037] Polycarbonates (PC) can also be reactively extruded with
polyesters; the main reaction is the direct ester-carbonate
exchange reaction. Polyamides (Nylons) can also be transreacted
with polyesters or with themselves; the relevant reactions include
acidolysis, aminolysis, and amidolysis (Porter, R. S. and Wang, L.
H., Compatibility and transesterification in binary polymer blends,
Polymer, 33:2019-2030; Groeninckx, G., Sarkissova, M. & Thomas,
S. in Polymer Blends, Volume 1: Formulation (eds. Paul, D. R. &
Bucknall, C. B.) 417-459 (John Wiley & Sons, New York, 2000).
Other transreactions practicable under the present art include, but
are not limited to, the transreaction of chemical functionalities
between amides-ethers, esters-lactams, esters-olefins,
amide-olefins, and olefins-olefins.
[0038] As noted above, the reactions may optionally be conducted in
the presence of a catalyst that catalyzes the interchange reaction
between the growing grafted polymer chain on the surface of the
filler and the pre-formed polymer. In the case of grafted
polyesters, a transesterification catalyst can be used to further
decrease the time of formation of a high average molecular weight
composite polymer. Suitable catalysts of the interchange reactions
include titanium(IV) isopropoxide (TIP), dibutyl tin oxide (DBTO),
Mono-, di-, and tetraalkyl tin(IV) compounds, monobutyltin
trichloride (BuSnCl.sub.3), TBD
(1,5,7-triazabiscyclo(4.4.0)dec-5-ene), acid catalysts like
sulfonic and sulfuric acids, base catalysts like sodium methylate,
sodium methoxide, potassium methoxide, sodium hydroxide and
potassium hydroxide, organic bases like triethylamine, piperidine,
1,2,2,6,6-pentamethylpiperidine, pyridine,
2,6-di-tert-butylpiridine, 1,3-disubstituted
tetrakis(fluoroalkyl)distannoxanes, 4-dimethyl-aminopyridine (DMAP)
and guanidine, alkaline metal alkoxides and hydroxides, basic
zeolites and related solid compounds such as cesium -exchanged NaX
faujasites, mixed magnesium-aluminum oxides, magenesium oxide and
barium hydroxide, 4-(dimethylamino)pyridine (DMAP),
4-pyrolidinopyridine (PPY), salts of amino acids, and enzyme
transesterification catalysts.
[0039] Post-reaction deactivation of the catalyst may be used to
ensure a controlled termination of the reaction and avoid further
molecular weight changes during any subsequent molding and/or
characterization steps.
[0040] The temperature at which the reaction is carried out is
chosen to aid in controlling the rate of reaction in order to
minimize the time of formation for the desired composite polymer
while allowing controlled polymerization and interchange of the
monomer and polymer molecules to allow for uniform distribution of
the filler. Typically, the temperature of the reaction is between
about 20.degree. C. and about 500.degree. C. Preferably, the
reaction temperature is between about 25.degree. C. and about
400.degree. C.
[0041] Mixing of the reagents during the polymerization and
interchange reactions is typically applied through a controlled
mechanical means such as a commercial reactor, mixing device, or
extruder, the extruder may have a static mixer attached.
[0042] The composite polymers formed by the methods of the present
invention are preferably processed into a commercially desirable
form such as pellets through cutting or grinding procedures after
the polymerization and interchange reactions have been
terminated.
[0043] The novel methods of forming polymer composites of the
present invention effectively increases the reinforcement of the
material compared to purely mechanical mixing for the same mixing
conditions. The properties of the polymer materials produced by the
methods of the present invention become limited by a decrease in
molecular weight as the fiber loading level increases due to an
excess of initiating groups. Therefore, both the fiber loading
level and the ratio of pre-formed material to lactide at the
beginning of the reaction are important factors in designing a
commercial process based on this new approach.
[0044] Additional objects, advantages, and novel features of this
invention will become apparent to those skilled in the art upon
examination of the following examples. Those of ordinary skill in
the art will readily understand that these examples are not
limiting on the invention as defined in the claims which
follow.
EXAMPLES
[0045] The methods of the present invention are exemplified here by
the formation of a polylactic acid (PLA)-cellulose composite
polymer formed by an interchange reaction including
transesterification of the lactide by the PLA in the presence of a
transesterification catalyst.
[0046] L-lactide and pre-formed polylactide (PLA) were purchased
from Cargill Dow Polymers, now NatureWorks, (Minnetonka, Minn.) and
used without further purification. Lactide was dried under vacuum
(22 inch Hg) at 50.degree. C. for at least 8 hours prior to use.
PLA was dried under vacuum (25 inch Hg) at 80.degree. C. for at
least 14 hours before being processed.
[0047] The cellulose fibers used in this study (CreaTech TC 2500)
were supplied by CreaFill Fibers Corp (Chestertown, Md.). These
fibers have an average length of 900 .mu.m and an average width of
20 .mu.m (L:D ratio=45). In order to remove surface impurities and
increase surface reproducibility, the fibers were pretreated
according to the following procedure: 1) 10 g fibers were suspended
in 500 ml aqueous solution of 8 wt % sodium hydroxide (NaOH), 2)
the solution was placed in a sonicator for 6 hours at slightly
elevated temperature (about 37.degree. C.), 3) the fibers were then
filtered and washed with distilled water until neutrality, 4) the
washed and filtered fibers were dried in a convection oven at
70.degree. C. for 24 hours, 5) before being used in the preparation
of composites, the fibers were again dried under vacuum (25 inch
Hg) at 80.degree. C. for about 8 hours. Repeated washing with
distilled water was found to be necessary in order to remove any
degradation products of cellulose during the base treatment, since
these by-products would cause the treated fibers to stick together
and yellow upon drying. FIG. 1 shows the fibers before and after
the pretreatment.
[0048] A special catalyst package was employed in the
polymerization reaction. Commercially, PLA is polymerized from
lactide that is produced from corn sugars by fermentation followed
by reactive distillation. Various catalysts can be employed in the
polymerization reaction. However, the most common catalyst used is
the tin-compound stannous octoate Sn(Oct).sub.2. This catalyst
requires hydroxyl groups as initiators, however, there is always a
competition between natural initiators (residual water, alcohols)
present in the starting materials and added initiators (compounds
added to the system containing hydroxyl groups in order to initiate
the reaction). Stannous octoate, Sn(Oct).sub.2, used in these
reactions was obtained from Sigma Aldrich and used as received. The
molar ratio, R, of lactide to stannous octoate for all reactions
was R=2500. The co-catalyst triphenylphospine, P(.phi.).sub.3, has
a beneficial effect on the polymerization kinetics of L-lactide in
reactions with Sn(Oct).sub.2 as catalyst. P(.phi.).sub.3 was
purchased from Sigma Aldrich and added without further purification
in an equimolar amount to Sn(Oct).sub.2. Titanium(IV) isopropoxide
(TIP) was used as transesterificiation agent in samples containing
pre-formed polymer (at a level of 0.1 wt % of PLA). For handling
purposes, solutions of the catalyst and co-catalyst as well as the
transesterificiation agent were prepared in dry, distilled
toluene.
[0049] Samples were mixed and polymerized in a Haake Rheomix 3000.
Polymerizations were performed at 200.degree. C. for 20 minutes at
100 rpm. After melting and premixing lactide with the pre-formed
PLA, the fibers were fed into the mixer. Once a reaction
temperature of 200.degree. C. was reached, the required amount of
solution containing the catalyst and co-catalyst was added. At the
end of the reaction time, the required amount of PAA solution was
added and mixed with the material for at least a minute prior to
extraction from the mixer. The catalyst was deactivated using
poly(acrylic acid), PAA of a molecular weight of 2000 g/mol, (at a
level of 0.25 wt % of lactide) purchased from Sigma Aldrich and
dissolved in dioxane for transfer purposes.
[0050] After preparation, the material was stored in a freezer for
at least 3 days prior to grinding in a Foremost 2A-4 grinder to a
maximum particle size of about 5mm with the vast majority of the
pellets being about 2-3mm in diameter. Samples for further testing
were prepared by a combination of vacuum- and compression-molding.
The material was first melted under vacuum (about 25 inch Hg) at
190.degree. C. until the amount of gas released decreased
significantly. Afterwards the material was compression molded at
180.degree. C. for about 5 minutes under a load of 5000 psi and
then quenched between water-cooled plates.
[0051] Samples were analyzed for glass transition temperature
(T.sub.g) and amount of crystallinity in a Perkin Elmer DSC-7. The
machine was calibrated against an indium standard twice, and a
baseline established on a daily basis. The DSC testing protocol was
as follows: 1) heat from 5.degree. C. to 200.degree. C. at
10.degree. C./min, 2) hold at 200.degree. C. for 5 minutes, 3) cool
from 200.degree. C. 4) heat from 5.degree. C. to 200.degree. C. at
10.degree. C./min. T.sub.g and amount of crystallinity were
determined from data obtained on the second heating cycle by
inflection point method.
[0052] The glass transition temperature (T.sub.g) of samples
containing no cellulose fibers and polymerized from L-lactide
corresponds to previous studies that evaluated the influence of
proportions of L- and D-lactide on the T.sub.g. The Fox-Flory plot
for PLA, shown in FIG. 6, demonstrates that the glass transition
temperature increases with increasing molecular weight before
reaching a plateau at about 59.degree. C. at a molecular weight of
about 50000 g/mol. The materials prepared in this study reach a
molecular weight within the range of constant T.sub.g for 100%
L-lactide.
[0053] The introduction of micro-sized cellulose fibers into a
matrix of pre-formed PLA does not significantly affect the glass
transition temperature as measured by DSC as shown in FIG. 7. This
observation is consistent with previous studies. If no pre-formed
PLA is present and the material is completely polymerized in the
presence of cellulose fibers, then the glass transition temperature
decreases with increasing fiber loading level, as shown in FIG. 7.
According to the Fox-Flory equation, a decrease in the glass
transition temperature is an indication of lower molecular weight:
T g .function. ( M _ n ) = A - B .times. M _ n ##EQU1## A and B in
the above equation are constants, with A = lim .times. M _ n
.fwdarw. .infin. .times. T g .function. ( M _ n ) ##EQU2##
[0054] For 100% L-lactide PLA, values were determined to be:
A=60.2.degree. C., B=-71.1.degree. C. g/mol. Therefore, with
increasing fiber loading level, lower molecular weight PLA is
produced due to an excess of initiating groups.
[0055] Achievable crystallinity was also determined by DSC and is
shown versus the fiber loading level in FIG. 8. For samples
containing only pre-formed PLA (100/0 P/L) no variation of the
achievable crystallinity was observed, although an increase would
be expected if the fibers serve as nucleation agent. The reason for
this observation is the presence of talc in the pre-formed
commercial material that already serves as nucleation agent.
Samples completely polymerized from lactide (0/100 P/L) show a
significantly lower percentage of achievable crystallinity for
unfilled systems. As the fiber loading level increases, the
achievable crystallinity increases for 0/100 P/L samples because
the fibers can serve as nucleating agent.
[0056] Mechanical properties were determined by Dynamic Mechanical
Thermal Analysis (DTMA) using an ARES Rheometer (TA Instruments,
Piscataway N.J.) with torsional rectangular fixtures. Before
testing, the machine was calibrated for normal force and torque.
Test conditions were 0.11% strain and 1 Hz. The thermal scanning
was performed as follows: 1) heat from 30.degree. C. to 110.degree.
C. at 20.degree. C./min, 2) hold for 5 minutes at 110.degree. C.,
3) cool from 110.degree. C. to 30.degree. C. at 20.degree. C./min,
4) heat from 30.degree. C. to 110.degree. C. at 10.degree. C./min
(end condition: 30.degree. C.). Moduli were determined from data on
the first heating run.
[0057] The measured values of the storage moduli G' are summarized
in Table 1, and plots of the results are shown in FIG. 9. The
modulus increases when pre-formed PLA is physically mixed with
cellulose fibers (FIG. 9a); these results are consistent with
previous studies. In the system containing 35% lactide, at the
beginning of the reaction the modulus increases with the fiber
loading level up to 25% by weight of the polymer mass; this is
followed by a decrease in modulus for a further increase in fiber
loading to 35% (FIG. 9b). This indicates a drop in molecular weight
at the highest fiber loading level due to excess of initiating
groups. However, the highest achievable modulus in this system
exceeds G' achievable by purely mechanical mixing--there is a 43%
increase with 35% fibers by physical mixing but a 50% increase with
25% fibers by reactive compatibilization. For systems containing
70% lactide at the beginning of the reaction (FIG. 9c), a similar
trend was observed; an initial increase in G' with fiber loading
level was followed by a decrease beyond 25% fibers due to an excess
amount of initiating groups resulting in low molecular weight PLA.
The reinforcement achievable in this system is better than in
systems only relying on physical mixing or systems containing 35%
lactide at the start of the reaction.
[0058] If the polymer is completely polymerized in the presence of
the fibers, the modulus slightly increases for a fiber loading
level of 15% followed by a sharp decrease at higher loadings (FIG.
9d). The material obtained with 35% fibers did not allow mechanical
testing due to very low molecular weight. Overall, comparison of
the moduli measured at 40.degree. C. shows that the best
reinforcement for this lactide-PLA-cellulose composite polymer is
achieved at 25% fiber loading level in the presence of 30%
pre-formed PLA. TABLE-US-00001 TABLE 1 Summary of storage modulus
G' for materials tested. Fiber loading level Modulus G'
Reinforcement [wt %] [GPa] [%] 100% preformed PLA, 0% lactide
(100/0 P/L) 0 1.4 -- 15 1.9 36 25 1.9 36 35 2.0 43 65% preformed
PLA, 35% lactide (65/35 P/L) 0 1.4 -- 15 1.7 21 25 2.1 50 35 1.5 7
30% preformed PLA, 70% lactide (30/70 P/L) 0 1.5 -- 15 1.8 20 25
2.3 53 35 2.0 33 0% preformed PLA, 100% lactide (0/100 P/L) 0 1.6
-- 15 1.9 19 25 1.4 -22 35 -- --
[0059] The molecular weight of materials without fillers was
determined by intrinsic viscosity measurements. Solutions of the
material were prepared in tetrahydrofuran (THF) at concentrations
of 0.005 g/ml and the viscosity of the polymer solution measured in
an Ubbelohde viscometer at 30.degree. C. The Schultz-Blaschke
relationship allows a single-point determination of the intrinsic
viscosity [.eta.] from viscosity measurements of the pure solvent
and the sample solution, given the Schultz-Blaschke coefficient
k.sub.SB is known for the given polymer-solvent system at a
constant temperature. [ .eta. ] = .eta. sp c * ( 1 + k SB * .eta.
sp ) ##EQU3##
[0060] In the Schultz-Blaschke equation, where c is the solution
concentration in g/ml, k.sub.SB the Schultz-Blaschke coefficient,
and .eta..sub.sp is the specific viscosity defined by .eta. sp =
.eta. .eta. 0 - 1 = t t 0 - 1 ##EQU4## where t and t.sub.0 is the
time necessary for the solution and the solvent, respectively, to
pass through a defined part of the viscometer. The Schultz-Blaschke
coefficient for PLA in THF at 30.degree. C. was previously
determined to be 0.298.+-.0.005.
[0061] Knowing the intrinsic viscosity, the viscosity average
molecular weight can be calculated by rearranging the Mark-Houwink
equation M V = ( [ .eta. ] K ) 1 a ##EQU5##
[0062] The parameters in this equation, K and a, are also specific
for the polymer-solvent system at a given temperature.
[0063] The Stockmayer-Fixman equation allows the calculation of the
weight-average molecular weight and appears as [ .eta. ] .times. M
_ W 1 / 2 = K .THETA. + b * M _ W 1 / 2 ##EQU6##
[0064] The factor K.sub..THETA. (Mark-Houwink constant for
theta-conditions) is an experimental quantity often used to
quantify unperturbed polymer chain dimensions in dilute solutions.
Values reported in the literature show a large variability.
However, careful experiments have determined this constant
K.sub..THETA. to be 0.107.+-.0.022 mL/g.
[0065] Post-reaction deactivation of the catalyst produced a
controlled termination of the reaction while avoiding further
molecular weight changes during molding and characterization
measurements. Samples were collected during the polymerization of
PLA and the molecular weight determined as a function of time as a
means of assessing the effectiveness of polyacrylic acid (PAA) as a
reaction-stopping reagent. As shown in FIG. 4, PAA is very
effective in deactivating Sn(Oct).sub.2 compared to a control
sample. There is no further increase in molecular weight after the
addition of PAA to the system, while the final molecular weight
without PAA addition is not reached within an hour under the
present experimental conditions.
[0066] The average molecular weight of samples that are completely
polymerized in the Haake reaches about 77600 g/mol, whereas the
pre-formed material is of a molecular weight of 108100 g/mol after
processing for 20 minutes at 200.degree. C. in the presence of TIP.
As expected, the introduction of pre-formed polymer to the PLA
synthesis leads to an increase in average molecular weight as
depicted in FIG. 5.
[0067] The foregoing description of the present invention has been
presented for purposes of illustration and description.
Furthermore, the description is not intended to limit the invention
to the form disclosed herein. Consequently, variations and
modifications commensurate with the above teachings, and the skill
or knowledge of the relevant art, are within the scope of the
present invention. The embodiment described hereinabove is further
intended to explain the best mode known. for practicing the
invention and to enable others skilled in the art to utilize the
invention in such, or other, embodiments and with various
modifications required by the particular applications or uses of
the present invention. It is intended that the appended claims be
construed to include alternative embodiments to the extent
permitted by the prior art.
* * * * *