Note: Descriptions are shown in the official language in which they were submitted.
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HYDROSILYLATION CURED THERMOPLASTIC ELASTOMERS
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to thermoplastic elastomer compositions prepared using
hydrosilyiation crosslinking of the elastomer component of the composition. A
thermoplastic
elastomer is generally defined as a polymer or blend of polymers that can be
processed and
recycled in the same way as a conventional thermoplastic material, yet has
properties and
functional performance similar to that of vulcanized rubber at service
temperatures. Blends or
alloys of plastic and elastomeric rubber have become increasingly important in
the production of
high performance thermoplastic elastomers, particularly for the replacement of
thermoset rubbers
in various applications. High performance thermoplastic elastomers in which a
highly
vulcanized rubbery polymer is intimately dispersed in a thermoplastic matrix
are generally
known as thermoplastic vulcanizates.
Description of the Related Art
Polymer blends which have a combination of both thermoplastic and elastic
properties are
generally obtained by combining a thermoplastic resin with an elastomeric
composition in a way
such that the elastomer component is intimately and uniformly dispersed as a
discrete particulate
phase within a continuous phase of the thermoplastic. Early work with
vulcanized rubber
components is found in U.S. Pat. No. 3,037.954 which discloses both static
vulcanization of the
2p rubber, as well as the technique of dynamic vulcanization wherein a
vulcanizable elastomer is
dispersed into a molten resinous thermoplastic polymer and the elastomer is
cured while
continuously mixing and shearing the blend. The resulting composition is a
micro-gel dispersion
of cured elastomer in an uncured matrix of thermoplastic polymer.
In U.S. Pat. No. Re. 32,028 polymer blends comprising an olefin thermoplastic
resin and
an olefin copolymer are described, wherein the rubber is dynamically
vulcanized to a state of
partial cure. The resulting compositions are reprocessible. U.S. Pat. Nos.
4,130,534 and
4,130,535 further disclose thermoplastic vulcanizates comprising butyl rubber
and polyolefin
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resin, and olefin rubber and polyoletin resin. respectively. The compositions
are prepared by
dynamic vulcanization and the rubber component is cured to the extent that it
is essentially
insoluble in conventional solvents. A range of crosslinkin~~. or curing,
agents for the
vulcanization of the rubber are described in the early art. including
peroxides, sulfurs. phenolic
resins, radiation, and the like.
U.S. Pat. No. 4,803.244 generally discusses the use of multifunctional
organosilicon
compounds in conjunction with a catalyst as an agent for crosslinking the
rubber component of a
thermoplastic elastomer by hydrosilylation. Hydrosilylation involves the
addition of a silicon
hydride across a multiple bond. often with a transition metal catalyst. This
patent describes a
rhodium catalyzed hydrosilylation of EPDM rubber in a blend with polypropylene
to produce
thermoplastic elastomers having a gel content of up to 34% (after conection
for the plastic
phase). This degree of vulcanization was achieved ow.;y with a high level of
catalyst.
A further modification of hydrosilylation crosslinking of the rubber in a
thermoplastic
elastomer composition is disclosed in European Patent Application No. 6~
1.009. A
compatibilizing agent containing in the same molecule a component having an
affinity for the
rubber and a component having an affinity for the thermoplastic resin is
incorporated into the
composition, and is said to improve adhesion between the rubber and resin in
order to prevent
agglomeration.
U.S. Pat. No. 5,672,660 discloses the preparation of thermoplastic elastomers
using
hYdrosilylation crosslinking of the rubber component. wherein very low amounts
of platinum
catalyst are used in conjunction with specific diene containing rubbers. and
further discloses the
desirability of conducting the reaction in a medium which is free of materials
with Lewis base
behavior.
International (PCT) Application WO 96/24632 describes the preparation of
thermoplastic
elastomers prepared using a phenolic curative. and stabilized with a
hydrolysis-insensitive HALS
compound.
SUMMARY OF TI-IE INVENTION
The present invention is based on the discovery that selected hindered amine
light
stabilizers (HALS) can be incorporated into a one pass dynamic vulcanization
process, using a
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platinum-catalyzed hydrosilylation cure. to prepare a thermoplastic elastomer
from a blend of
thermoplastic resin and unsaturated rubber. The resulting thermoplastic
elastomers can be fully
or partially cured, with desirable tensile and elasticity properties as well
as improved resistance
to degradation by ultraviolet (UV) light. The preferred structure of HALS
compounds for use in
the invention is one in which the sterically unhindered amine functionality is
minimized.
The compositions of the present invention have utility as replacements for
thermoset
rubber compounds in a variety of applications. particularly where molding or
extrusion is
involved and the combination of thermoplastic and elastomeric properties. as
well as UV
stability, provide an advantage. Typical uses include molded articles for
automobile underhood
P~s~ engineering and construction materials. mechanical rubber goods,
industrial parts such as
hose, tubing and gaskets, electrical applications and household goods.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Thermoplastic elastomer compositions may generally be prepared by blending a
thermoplastic resin and a rubber. then melting the thermoplastic component and
mixing the melt
until the blend is homogeneous. If a composition of vulcanized rubber in a
thermoplastic matrix
is desired, crosslinking agents (also referred to as curatives or vulcanizing
agents) are added to
the blend and crosslinking occurs during the mixing. This latter process is
described as dynamic
vulcanization.
A wide range of thermoplastic resins and rubbers and/or their mixtures have
been used in
the preparation of thermoplastic elastomers. including polypropylene. HDPE.
LDPE.VLDPE,
LLDPE, cyclic olefin homopolymers or copolymers as well as olelinic block
copolymers,
polystyrene, polyphenylene suifide. polyphenylene oxide and ethylene propylene
copolymer
(EP) thermoplastics. with rubbers such as ethylene propylene dime (EPDM),
butyl, halobutyl,
acrylonitrile butadiene (NBR), styrene butadiene (SBR) and natural (NR) as the
elastomers.
Hydrosilylation Agents
Hydrosilylation has been disclosed as a crosslinking method. In this method a
silicon
hydride having at least two SiH groups in the molecule is reacted with the
carbon-carbon
multiple bonds of the unsaturated (i.e. containing at least one carbon-carbon
double bond) rubber
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component of the thermoplastic elastomer. in the presence of the thermoplastic
resin and a
hydrosilylation catalyst. Silicon hydrides useful in the process of the
invention include
methylhydrogen polysiloxanes, methylhydrogen dimethyl-siloxane copolymers.
alkylated methyl
hydrogen polysiloxanes. bis(dimethylsilyl)alkanes and bis(dimethylsilyl)
benzene.
The amount of silicon hydride compound useful in the process of the present
invention
can range from about 0.1 to about 10.0 mole equivalents of SiH per carbon-
carbon double bond
in the rubber, and preferably is in the range of about 0.~ to about 5.0 mole
equivalents of SiH per
carbon-carbon double bond in the rubber component of the thermoplastic
elastomer.
Thermoplastic Resins
Thermoplastic resins useful in the compositions produced by the invention
include
crystalline polyolefin homopolymers and copolymers. They are desirably
prepared from
monoolefin monomers having 2 to 20 carbon atoms. such as ethylene. propylene,
1-butene,
I-pentene and the like. as well as copolymers derived from linear and cyclic
olefins, with
propylene being preferred. As used in the specification and claims the term
polypropylene
includes homopolymers of propylene as well as reactor copolymers of
polypropylene which can
contain about 1 to about 20 wt% of ethylene or an olefin comonomer of 4 to 20
carbon atoms,
and mixtures thereof. The polypropylene can be cry~stalline~ isotactic or
syndiotactic. and may be
prepared by Ziegler-Natta or metallocene catalysis. Other thermoplastic resins
which are
substantially inert to the rubber, the silicon hydride and the hvdrosilylation
catalyst would also be
suitable. Blends of thermoplastic resins may also be used.
The amount of thermoplastic resin found to provide useful compositions is
generally
from about 5 to about 90 weight percent. based on the weight of the rubber and
resin. Preferably,
the thermoplastic resin content will range from about 20 to about 80 percent
by weight of the
total polymer.
Rubbers
Unsaturated rubbers useful to prepare thermoplastic elastomers according to
the invention
include monoolefin copolymer rubbers comprising non-polar. ruhbery copolymers
of two or
more -monoolefins, preferably copolymerized with at least one polyene. usually
a diene.
4
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However, unsaturated monooletin rubber such as EPDM rubber is more suitable.
EPDM is a
polymer of ethylene, propylene and one or more non-conjugated diene or non-
conjugated dimes,
and the monomer components may be polymerized using Ziegler-Natta or
metallocene catalyzed
reactions, among others. Satisfactory non-conjugated dimes include 5-
ethylidene-2-norbornene
(ENB); 1,4-hexadiene (HD); ~-methylene-2-norbornene (MNB); 1,6-octadiene:
5-methyl-1,4-hexadiene; 3,7-dimethyl-1,6-octadiene; 1.3-cyclopentadiene; 1,4-
cyclohexadiene;
dicyclopentadiene (DCPD); 5-vinyl-2-norbornene ( VNB) and the like, or a
combination thereof.
Blends of any of the above rubbers may also be employed, rather than a single
olefinic
rubber.
In preparing the compositions of the invention, the amount of rubber generally
ranges
from about 95 to about 10 weight percent, based on the weight of the rubber
and thermoplastic
resin. Preferably, the rubber content will be in the range of from about 80 to
about 20 weight
percent of total polymer.
Hydrosilylation Catalysts
It has previously been understood that any catalyst, or catalyst precursor
capable of
generating a catalyst in situ, which will catayze the hydrosilylation reaction
with the
carbon-carbon bonds of the rubber can be used. Such catalysts have included
transition metals of
Group VIII such as palladium. rhodium, platinum and the like, including
complexes of these
metals. Chloroplatinic acid has been disclosed as a useful catalyst in U.S.
Pat. No. 4,803,244 and
European Application No. 651.009. which further disclose that the catalyst may
be used at
concentrations of S to 10,000 pans per million by weight and 100 to 200.000
parts per million by
weight based on the weight of rubber, respectively.
Platinum-containing catalysts which are useful in the process of the invention
are
described, for example, in U.S. Pat. No. 4.78.497: U.S. Pat. No. 3,220,972:
and U.S. Patent No.
2,823,218 all of which are incorporated herein by this reference. These
catalysts include
chloroplatinic acid, chloroplatinic acid hexahydrate. complexes of
chloroplatinic acid with
sym-divinyltetramethyldisiloxane. dichloro-bis(triphenylphosphine) platinum
(II),
cis-dichloro-bis(acetonitrile) platinum (II}, dicarbonyldichloroplatinum (II),
platinum chloride
and platinum oxide. Zero valent platinum metal complexes such as Karstedt's
catalyst are
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particularly preferred, as described in U.S. Pat. No. 3.775.=1~2: U.S. Pat.
No. 3,814.730: and U.S.
Pat. No. 4.288,345 all of which are incorporated herein by this reference.
Additives
The thermoplastic elastomer may contain conventional additives, which can be
introduced into the composition in the thermoplastic resin. the rubber. or in
the blend either
before, during or after the hydrosilylation and curing. Examples of such
additives are
antioxidants, processing aids, reinforcing and nonreinforcing fillers,
pigments, waxes, rubber
processing oil, extender oils. antiblocking agents. antistatic agents.
plasticizers (including
esters), foaming agents. flame retardants and other processing aids known to
the rubber
compounding art. Such additives may comprise from about 0.1 to about 300
percent by weight
based on the weight of the final thermoplastic elastomer product. Fillers and
extenders which
can be utilized include conventional inorganics such as calcium carbonate.
clays, silica, talc,
titanium dioxide, carbon black and the like. Additives. fillers or other
compounds which may
interfere with the hydrosilylation should be added after curing reaches the
desired level.
HALS
Hindered amine light stabilizers are regularly compounded into materials
requiring
improved UV resistance. LJV protection is provided by tile amine functionality
of the stabilizer,
which is easily oxidized to form nitroxyl amines. In the case of transition
metal catalyzed
hydrosilylation, such reactions are sensitive to the presence of Lewis bases.
It was thought that
these reaction systems should be essentially free of compounds such as amines.
sulfides and
phosphines. Interference with hvdrosiiylation reactions by these compounds is
believed to be
from bonds formed between the non-bonded pairs of electrons donated by the
Lewis base and the
transition metal center. Since this bond with the metal center is stronger
than those that
characterize the bonds of "good" ligands. the activity of the catalyst is
reduced.
However, it has been discovered that careful selection of the HALS to be
employed
makes it possible to include such stabilizers in the dynamic vulcanization
reaction. even when
hydrosilylation is used as the crosslinking (curing) process. The preferred
HALS structures are
those substantially free of sterically unhindered amine functionality. HALS
compounds having
the following structures were tested.
6
SUBSTITUTE SHEET (RULE 26)
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N (CHZ~N,
N / N
HN N N
H3C-C-CH3 H H
CH2
H3C-C-CH3
CH3
O
O
(I
H-C N (CHZ)s N-C~H
N N
H H
OH
C18H37 N-C18H37
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N (CHZT~s--N-
/N
N
N
H H
~OJ
(IV)
O O
HN O-C (CH2)8 C-O NH
v ,
N)
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R R
R-N-'(CHz)a N (CHz)2 N (CHZ)3 N R
H H
1 4H9
N~ N \N-CH3
R_ ~ 1
N /N
C4H9 N N-CH3
(VI)
II II
O O
CaH.,~-'O.-N O-C- (CHZ)a-C-O N-O-CBH~~
(VIII
O
!I
HN O-C-R R = C~~ - CZo
(VIII)
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I Chimassorb~ 944
(Ciba)
II Uvinul~ 4050H
(BASF)
III FS042 (Ciba)
IV Cyasorb~ 3346
(Cytec)
V Tinuvin~ 770 (Ciba)
VI Chimassorb 119
(Ciba)
VII Tinuvin 123 (Ciba)
VIIICyasorb 3835 (Cytec)
Processing
The rubber component of the thermoplastic elastomer is generally present as
small, i.e.
micro-size, particles within a continuous thermoplastic resin matrix, although
a co-continuous
morphology or a phase inversion is also possibl _ depending upon the amount of
rubber relative
to plastic and the degree of cure of the rubber. The rubber is desirably at
least partially
crosslinked, and preferably is completely or fully crosslinked. It is
preferred that the rubber be
erosslinked by the process of dynamic vulcanization. As used in the
specification and claims, the
term "dynamic vulcanization" means a vulcanization or curing process for a
rubber blended with
a thermoplastic resin. wherein the rubber is vulcanized under conditions of
shear at a temperature
at which the mixture will flow. The rubber is thus simultaneously crossIinked
and dispersed as
fine particles within the thermoplastic resin matrix, although as noted above
other morphologies
may exist. Dynamic vulcanization is effected by mixing the thermoplastic
elastomer components
at elevated temperatures in conventional mixing equipment such as roll mills,
Banbury mixers,
Brabender mixers, continuous mixers. mixing extruders and the like. The unique
characteristic
of dynamically cured compositions is that, notwithstanding the fact that the
rubber component is
partially or fully cured, the compositions can be processed and reprocessed by
conventional
plastic processing techniques such as extrusion. injection molding and
compression molding.
Scrap or Clashing can be salvaged and reprocessed.
The terms "fully vulcanized" and "fully cured" or "fully crosslinked" as used
in the
specification and claims means that the rubber component to be vulcanized has
been cured or
crosslinked to a state in which the elastomeric properties of the crosslinked
rubber are similar to
those of the rubber in its conventional vulcanized state, apart from the
thermoplastic elastomer
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composition. The degree of cure can be described in terms of gel content. or
conversely,
extractable components. Gel content reported as percent gef (based on the
weight of
crosslinkable rubber) is determined by a procedure which comprises determining
the amount of
insoluble polymer by soaking the specimen for 48 hours in organic solvent at
room temperature,
weighing the dried residue and making suitable corrections based upon
knowledge of the
composition. Thus, corrected initial and final weights are obtained by
subtracting from the initial
weight the weight of soluble components, other than rubber to be vulcanized,
such as extender
oils, plasticizers and components of the composition soluble in organic
solvent, as well as that
rubber component of the product which is not intended to be cured. Any
insoluble polyolefins,
pigments, fillers, and the like are subtracted from both the initial and final
weights. The rubber
component can be described as fully cured when less than about 5%. and
preferably less than
3%, of the rubber which is capable of being cured by hydrosilylation is
extractable from the
thermoplastic elastomer product by a solvent for that rubber. Alternatively
the degree of cure
may be expressed in terms of crosslink density. All of these descriptions are
well known in the
art, for example in U.S. Pat. Nos. 4.93.062, 5.100.947 and x.157,081, all of
which are fully
incorporated herein by this reference.
The following general procedure was used in the preparation of thermoplastic
elastomers
by the process of the invention, as set forth in the examples. The
thermoplastic resin and oil
extended rubber were placed in a heated internal mixer, with the
hydrosilylation agent,
hYdrosilylation catalyst and HALS compound. The hydrosilylation agent and
catalyst can be
incorporated into the composition by any suitable technique. for example by
injection as
solutions in oil or as neat components, although a dilute catalyst solution is
preferred. Additives
such as antioxidants, ultraviolet stabilizers and fillers may also be added as
a slurry in oil.
Masterbatches of the components may also be prepared to facilitate the
blending process. The
mixture was heated to a temperature sufficient to melt the thermoplastic
component, and the
mixture was masticated, with added processing oil if desired, until a maximum
of mixing torque
indicated that vulcanization had occurred. Mixing was continued until the
desired degree of
vulcanization was achieved.
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The invention will be better understood by reference to the following examples
which
serve to illustrate but not limit the present process. In the examples. the
following test methods
were used to determine the properties of the thermoplastic elastomer products.
Hardness (Shore A/D) - ASTM D 2240
Ultimate tensile strength (UTS - psi) - ASTM D 412
Ultimate elongation (UE - %) - ASTM D 412
Modulus at i 00/300% elongation
(M 1 or M3 - psi) - ASTM D412
Tension set (TS - %) - ASTM D 412
Oil swell (OS - %) - ASTM D 471
EXAMPLES
Compositions were prepared by the method of the invention as generally
described
above, using polypropylene resin and EPDM rubber containing ~-vinyl, 2-
norbornene as the
diene component. The thermoplastic (41 parts) and rubber (100 parts) were melt
mixed in a
Brabender mixer at 180°C until the polypropylene was melted. Silicone
hydride (alkylated
methyl hydrogen polysiloxane) [2 phr] was added dropwise to the melt mix,
followed by addition
of an oil solution containing 0.75 ppm platinum [platinate (II) hexachloro,
dihydrogen reaction
product with 2,4,6,8-tetraethenyl-2,4.6,8-tetramethyl cyclotetrasiloxane]. The
HALS compound
was added to the blend neat after silicone hydride addition, in a ratio of 1.5
gram of HALS to 60
2p grams of plastic/rubber blend. The rubber was dynamically vulcanized by
mixing the blend until
the maximum torque was reached. The product was removed from the mixer, then
returned to
the mixer and masticated at I 80°C for an additional minute. Plaques
were prepared by
compression molding the products of the dynamic vulcanization at 200°C
to a thickness of 60 miI
and cooling under pressure, and the physical properties were determined using
these plaques. All
of the products were elastomeric, as defined by ASTM D 1566, i.e. all had
tension set values of
less than 50%. The compositions and their properties are set forth in Table I
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TABLE
I
Blend HALS Hardness UTS (psi)M 1 IvI3 UE (%) OS (%)z
(psi) (psi)
1 None ~8 822 450 232 96
2 I ~4 653 264 566 394 168
3 II ~1 X00 250 410 535 240
- 4 III+VI 50 430 215 400 400 220
IV 57 916 352 770 396 113
6 V 57 1031 428 911 368 100
7 VI 55 940 340 710 460 116
8 VII 59 650 648 168 82
9 VIII 61 822 478 230 96
' Shore A hardness
'' 125 C for
24 hours
It can be seen from the data set forth in Table I that a HALS having no
sterically
unhindered amine functionality (e.g. structures V and VIII) gives
thermoplastic elastomer
products with properties essentially the same as the control blend 1 (i.e.
without HALS).
Compositions prepared using HALS having amine functionality capable of
reacting with
platinum (e.g. structures I, II and III) have poor properties compared to the
control.
While the best mode and preferred embodiment of the invention have been set
forth in
accord with the Patent Statutes, the scope of the invention is not limited
thereto, but rather is
defined by the attached claims.
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