Thermoplastic elastomers having improved low temperature properties

Thermoplastic elastomers having improved low temperature properties are provided by incorporating suitable low molecular weight ester plasticizer into blends of crystalline polyolefin homopolymer or copolymer and olefinic rubber. The rubber component of the composition is at least partially cured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to thermoplastic elastomers having improved low 
temperature performance characteristics. 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 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 
rubber in various applications. 
2. Description of the Related Art 
Polymer blends which have a combination of both thermoplastic and elastic 
properties are generally obtained by combining a thermoplastic polyolefin 
with an elastomeric composition in a way such that the elastomer is 
intimately and uniformly dispersed as a discrete particulate phase within 
a continuous phase of the thermoplastic. Early work with vulcanized 
compositions is found in U.S. Pat. No. 3,037,954 which discloses static 
vulcanization as well as the technique of dynamic vulcanization wherein a 
vulcanizable elastomer is dispersed into a resinous thermoplastic polymer 
and the elastomer is cured while continuously mixing and shearing the 
polymer blend. The resulting composition is a microgel dispersion of cured 
elastomer, such as butyl rubber, chlorinated butyl rubber, polybutadiene 
or polyisobutene in an uncured matrix of thermoplastic polymer such as 
polypropylene. This patent describes the use of oil additives derived from 
coal tar, pine tar or petroleum in the composition. 
In U.S. Pat. No. Re. 32,028 polymer blends comprising an olefin 
thermoplastic resin and an olefin copolymer rubber are described, wherein 
the rubber is dynamically vulcanized to a state of partial cure. The 
resulting compositions are reprocessible. The addition of various 
lubricants to the cured blend at about one phr is taught to be useful for 
improving extrusion quality of the compositions. 
U.S. Pat. Nos. 4,130,534 and 4,130,535 disclose thermoplastic elastomer 
compositions comprising butyl rubber and polyolefin resin, and olefin 
rubber and polyolefin 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. The addition of 
plasticizers and aromatic, naphthenic and paraffinic extender oils to the 
blend is suggested. No details are given regarding the choice or 
suitability of any particular class or type of plasticizers. It is well 
known that different rubbers are compatible with certain types of 
plasticizers and that not all plasticizers are suitable with all rubbers. 
In U.S. Pat. No. 5,157,081 a dynamically vulcanized blend is described 
comprising a first butyl rubber based elastomer and a second 
ethylene-propylene polymer elastomer in a matrix of polyolefinic resin. 
Rubber process oils derived from petroleum fractions may be included, and 
a general suggestion is made that organic esters and other synthetic 
plasticizers can be used. 
SUMMARY OF THE INVENTION 
The present invention is based on the discovery that a thermoplastic 
elastomer composition having improved low temperature properties is 
provided by incorporating certain types of low molecular weight ester 
plasticizers into a blend of crystalline polyolefin homopolymer or 
copolymer and olefinic rubber. The rubber component of the composition is 
usually present as very small, i.e. micro-size, particles in the 
thermoplastic matrix, and it is preferably at least partially cured. 
Co-continuous morphologies are also possible. Unexpectedly, the inclusion 
of these organic esters in the composition provides a thermoplastic 
elastomer which has a significantly lowered glass transition temperature 
of both the rubber and polyolefin phases and improved impact strength at 
low temperatures, while maintaining the desirable properties of low 
compression set, high tear strength and good dynamic properties over a 
broad temperature range. The compositions have utility as constant 
velocity joint boots, rack and pinion boots, automotive elastoplastic 
components and mechanical rubber-plastic (thermoplastic elastomer) goods 
which need to be serviceable at low temperatures, e.g. -40.degree. C. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Polyolefin 
Polyolefins suitable for use in the compositions of the invention include 
thermoplastic, crystalline polyolefin homopolymers and copolymers. They 
are desirably prepared from monoolefin monomers having 3 to 6 carbon 
atoms, such as propylene, 1-butene, isobutylene, 1-pentene and the like, 
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 .alpha.-olefin comonomer of 4 to 16 carbon atoms, 
and mixtures thereof. The polypropylene can be highly crystalline 
isotactic or syndiotactic polypropylene, usually having a narrow range of 
glass transition temperature (T.sub.g). Commercially available polyolefins 
may be used in the practice of the invention. 
The amount of polyolefin found to provide useful compositions is generally 
from about 10 to about 90 weight percent, based on the weight of the 
rubber and polyolefin. Preferably, the polyolefin content will range from 
about 60 to about 90 percent by weight. 
Olefinic Rubber 
Suitable monoolefin copolymer rubbers comprise non-polar, essentially 
non-crystalline, rubbery copolymers of two or more .alpha.-monoolefins, 
preferably copolymerized with at least one polyene, usually a diene. 
Saturated monoolefin copolymer rubber, for example ethylene-propylene 
copolymer rubber (EPM) can be used. However, unsaturated monoolefin rubber 
such as EPDM rubber is more suitable. EPDM is a terpolymer of ethylene, 
propylene and a non-conjugated diene. Satisfactory non-conjugated dienes 
include 5-ethylidene-2-norbornene (ENB); 1,4-hexadiene; 
5-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); and the like. 
Butyl rubbers are also useful in the compositions of the invention. As used 
in the specification and claims, the term "butyl rubber" includes 
copolymers of an isoolefin and a conjugated monoolefin, terpolymers of an 
isoolefin, a conjugated monoolefin and divinyl aromatic monomers, and the 
halogenated derivatives of such copolymers and terpolymers. The useful 
butyl rubber copolymers comprise a major portion of isoolefin and a minor 
amount, usually less than 30 wt %, of a conjugated multiolefin. The 
preferred copolymers comprise about 85-99.5 wt % of a C.sub.4-7 isoolefin 
such as isobutylene and about 15-0.5 wt % of a multiolefin of 4-14 carbon 
atoms, such as isoprene, butadiene, dimethyl butadiene and piperylene. 
Commercial butyl rubber, useful in the invention, is a copolymer of 
isobutylene and minor amounts of isoprene. Other butyl co- and terpolymer 
rubbers are illustrated by the description in U.S. Pat. No. 4,916,180, 
which is fully incorporated herein by this reference. 
Another suitable copolymer within the scope of the olefinic rubber of the 
present invention is a copolymer of a C.sub.4-7 isomonoolefin and a 
para-alkylstyrene, and preferably a halogenated derivative thereof. The 
amount of halogen in the copolymer, predominantly in the 
para-alkylstyrene, is from about 0.1 to about 10 wt %. A preferred example 
is the brominated copolymer of isobutylene and para-methylstyrene. These 
copolymers are more fully described in U.S. Pat. No. 5,162,445, which is 
fully incorporated herein by this reference. 
A further olefinic rubber suitable in the invention is natural rubber. The 
main constituent of natural rubber is the linear polymer 
cis-1,4-polyisoprene. It is normally commercially available in the form of 
smoked sheets and crepe. Synthetic polyisoprene can also be used. 
Blends of any of the above olefinic rubbers can be employed, rather than a 
single olefinic rubber. 
In preparing the compositions of the invention, the amount of olefinic 
rubber generally ranges from about 90 to about 10 weight percent, based on 
the weight of the rubber and polyolefin. Preferably, the olefinic rubber 
content will be in the range of from about 40 to about 10 weight percent. 
Ester Plasticizer 
The addition of certain low to medium molecular weight (&lt;10,000) organic 
esters and alkyl ether esters to the compositions of the invention 
dramatically lowers the T.sub.g of the polyolefin and rubber components, 
and of the overall composition, and improves the low temperature 
properties, particularly flexibility and strength. It is believed that 
these effects are achieved by the partitioning of the ester into both the 
polyolefin and rubber components of the compositions. Particularly 
suitable esters include monomeric and oligomeric materials having an 
average molecular weight below about 2000, and preferably below about 600. 
It is important that the ester be compatible, or miscible, with both the 
polyolefin and rubber components of the compositions, i.e. that it mix 
with the other components to form a single phase. The esters found to be 
most suitable were either aliphatic mono- or diesters or alternatively 
oligomeric aliphatic esters or alkyl ether esters. Polymeric aliphatic 
esters and aromatic esters were found to be significantly less effective, 
and phosphate esters were for the most part ineffective. 
Esters may be screened for suitability by a simple test of their ability to 
swell a polyolefin such as polypropylene. For the purposes of this 
invention, polypropylene samples (2.0.times.20.times.50 mm) were immersed 
in various ester plasticizers or non-ester diluents such as mineral oils, 
and were swollen at 125.degree. C. to constant weight (normally about 24 
hours). If the total change in weight was greater than 40%, the diluent 
was considered significantly compatible with the polypropylene and 
therefore suitable for preparing compositions with enhanced low 
temperature performance. 
Examples of esters which have been found satisfactory for use in the 
present invention include isooctyltallate, isooctyloleate, n-butyltallate, 
n-butyloleate, butoxyethyloleate, dioctylsebacate, di 
2-ethylhexylsebacate, dioctylazelate, diisooctyldodecanedioate, 
alkylalkylether diester glutarate and oligomers thereof. Other analogues 
expected to be useful in the present invention include alkyl alkylether 
mono- and di-adipates, mono- and dialkyl adipates, glutarates, sebacates, 
azelates, ester derivatives of castor oil or tall oil and oligomeric mono- 
and diesters or mono- and dialkyl ether esters therefrom. Isooctyltallate 
and n-butyltallate are particularly preferred. These esters may be used 
alone in the compositions, or as mixtures of different esters, or they may 
be used in combination with conventional hydrocarbon oil diluents or 
process oils, e.g. paraffin oil. The amount of ester plasticizer in the 
composition will generally be less than about 250 phr, and preferably less 
than about 175 phr. 
Additives 
In addition to the polyolefin, rubber and ester components, the 
compositions of the invention include curatives and may also include 
reinforcing and non-reinforcing fillers, antioxidants, stabilizers, rubber 
processing oil, extender oils, lubricants, antiblocking agents, antistatic 
agents, waxes, foaming agents, pigments, flame retardants and other 
processing aids known in the rubber compounding art. Such additives can 
comprise up to about 50 wt % of the total composition. Fillers and 
extenders which can be utilized include conventional inorganics such as 
calcium carbonate, clays, silica, talc, titanium dioxide, carbon black and 
the like. The rubber processing oils generally are paraffinic, naphthenic 
or aromatic oils derived from petroleum fractions. The type will be that 
ordinarily used in conjunction with the specific rubber or rubbers present 
in the composition, and the quantity based on the total rubber content may 
range from zero to a few hundred phr. However, it is an important aspect 
of the present invention that processing oil need not be present, and in 
fact it may be totally replaced by the ester plasticizer component of the 
composition. In other word, depending upon the properties desired in the 
thermoplastic elastomers of the invention, the composition may be free of 
processing oil or it may contain a combination of processing oil and 
ester. 
Processing 
The olefin rubber component of the thermoplastic elastomer is generally 
present as small, i.e. micro-size, particles within a continuous 
polyolefin matrix, although a co-continuous morphology or a phase 
inversion is also possible depending on the amount of rubber relative to 
plastic, and the cure system or degree of cure of the rubber. The rubber 
is desirably at least partially crosslinked, and preferably is completely 
or fully crosslinked. The partial or complete crosslinking can be achieved 
by adding an appropriate rubber curative to the blend of polyolefin and 
rubber and vulcanizing the rubber to the desired degree under conventional 
vulcanizing conditions. However, it is preferred that the rubber be 
crosslinked 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 contained in a thermoplastic 
elastomer composition, wherein the rubber is vulcanized under conditions 
of high shear at a temperature above the melting point of the polyolefin 
component. The rubber is thus simultaneously crosslinked and dispersed as 
fine particles within the polyolefin matrix, although as noted above other 
morphologies may also exist. Dynamic vulcanization is effected by mixing 
the thermoplastic elastomer components at elevated temperature 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 flashing can be salvaged and reprocessed. 
Those ordinarily skilled in the art will appreciate the appropriate 
quantities, types of cure systems and vulcanization conditions required to 
carry out the vulcanization of the rubber. The rubber can be vulcanized 
using varying amounts of curative, varying temperatures and varying time 
of cure in order to obtain the optimum crosslinking desired. Any known 
cure system for the rubber can be used, so long as it is suitable under 
the vulcanization conditions with the specific olefinic rubber or 
combination of rubbers being used and with the polyolefin. These curatives 
include sulfur, sulfur donors, metal oxides, resin systems, peroxide-based 
systems and the like, both with and without accelerators and co-agents. 
Such cure systems are well known in the art and literature of 
vulcanization of elastomers. 
The terms "fully vulcanized" and "completely vulcanized" as used in the 
specification and claims means that the rubber component to be vulcanized 
has been cured 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 composition. The 
degree of cure can be described in terms of gel content or, conversely, 
extractable components. 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. 5,100,947 and 5,157,081, both of 
which are fully incorporated herein by this reference. 
The following general procedure was used in the preparation of 
thermoplastic elastomers of the invention as set forth in the examples. 
The polyolefin and rubber were placed in a heated internal mixer, with an 
appropriate portion of the ester and other desired additives. The mixture 
was heated to a temperature sufficient to melt the polyolefin component, 
the mixture was masticated and curative was added while mastication 
continued. After a maximum of mixing torque indicated that vulcanization 
had occured, additional ester was added as indicated, and mixing was 
continued until the desired degree of vulcanization was achieved. The 
order of addition of the various components may vary. The compositions 
were then removed from the mixer, molded and tested for their physical 
properties.