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Styrenic thermoplastic elastomers.

The properties of thermoplastic rubbers can best be appreciated by comparing them with other polymers, as shown in table 1. This classifies polymers using two criteria:

* Method of forming the final product, either thermosetting (chemical change) or thermoplastic (physical change).

* Properties of final product - either rigid, flexible or rubbery.
 Table 1 - classification of polymers
 Thermosetting Thermoplastic
Rigid Epoxies Polystyrene
 Phenol-formaldehyde Polypropylene
 Urea-formaldehyde Poly(vinyl chloride)
 High density polyethylene
Flexible Highly filled and/or Low density polyethylene
 highly vulcanized EVA
 rubbers Plasticized PVC
Rubbery Vulcanized rubbers Thermoplastic elastomers
 (NR, SBR, IR, etc.)


This gives a total of six classes. Five of these have been known for many years. Thermoplastic rubbers were introduced about 30 years ago and constitute a sixth class. Because changes involved in forming products from thermoplastic rubbers are physical, they are easily reversible. This gives thermoplastic rubbers several unique features. These are:

* They require no vulcanization and can be processed like thermoplastics.

* Scrap after molding is reusable.

* Many thermoplastic rubbers are soluble in common solvents and regain their properties when the solvent evaporates.

Structure and properties

Most thermoplastic rubbers have these properties because they are made up of distinct segments, that is they are block copolymers. An A-B-A triblock copolymer based on polystyrene (A segments) and a rubber (B segments can be written as:

AAAAAAA - BBBBBBBBBBBB - AAAAAAA

These polymers are very different from comparable random copolymers:

ABAABABBAABBBABABBABBBAABAABA or mixtures of two homopolymers:

AAAAAAAAAAAA + BBBBBBBBBBBBBB because in block copolymers the long sequences of A and B monomer units are joined by chemical bonds. In these A-B-A triblock copolymers each molecule has two segments of polystyrene separated by a rubber segment. The polystyrene and the rubber segments are incompatible and form a two-phase system which causes these polymers to show two distinct glass transition temperatures.

These transition temperatures are characteristic of the polystyrene and the rubber (in this case polybutadiene). In contrast, a random copolymer (SBR) shows only a single intermediate glass transition temperature, indicating a one phase system.

Many arrangements of two phase systems are possible. If the polystyrene is the minor constituent, it tends to disperse in the continuous rubber phase and form separate domains.

At room temperature these domains are hard and tie down the ends of the rubber chains o give an interconnected network. This termed phusical crosslinking. On heating, the domains soften and the block copolymer becomes fluid. When the heated polymer is cooled the domains become hard again and the network regains its strength. Similarly, both the polystyrene and the rubber segments will dissolve in some solvents to form low viscosity solutions. On evaporating the solvent, phases separate and the domains reform. This can be constrasted with the chemical cross-linking of rubber (vulcanization). In this process the links between the chains are chemical bonds and once formed, are permanent - thus the products are insoluble and infusible.

At room temperature, styrenic thermoplastic rubbers are elastic and resilient and so they resemble conventional vulcanizates. An example of this is shown in figure 1, where the elastic properties of a polystyrene-polybutadiene-polystyrene (S-B-S) block copolymer are compared to those of vulcanized natural rubber and SBR. One characteristic of an elastomer is that the extended polymer recovers when the stress is removed. In this respect, styrenic thermoplastic rubbers differ from flexible thermoplastics such as EVA. Figure 2 shows this difference in terms of the behavior of several polymers after being extended to 80% of their breaking strain, allowed to relax and then extended again. This description of polymer properties has been given in terms of an S-B-S block copolymer. It will also apply to block copolymers with multiple alternating blocks (A-B-A-B-A..) and to those with a branched structure such as (A-B)nX (where X represents a multi-functional junction point).

However, block copolymers such as A-B and B-A-B only have one hard segment per molecule and so cannot form a physically cross-linked network since only one end of each rubber chain is attached to the hard domains. Thus their properties are similar to those of conventional unvulcanized rubbers.

Synthesis

Typically, the poly(styrene-b-elastomer-b-styrene) materials are made by anionic polymerization using an alkyl-lithium initiator (R-[Li.sup.+]). This first reacts with styrene monomer.

[Mathematical Expression Omitted]

The product now acts as an initiator for further polymerization.

[Mathematical Expression Omitted]

This product (denoted as S-[Li.sup.+]) has been termed a living polymer because it can initiate further polymerization. If a second monomer, such as butadiene, is added:

[Mathematical Expression Omitted]

This reaction product, (denoted as S-B-[Li.sup.+]) can then initiate a further reaction with added styrene monomer to give S-B-S-[Li.sup.+]. The active [Li.sup.+] end group in turn can be reacted with an alcohol, R-OH, to give S-B-SH + LiOR.

Alternatively the S-B-[Li.sup.+] may be reacted with a coupling agent such as an organohalogen.

[Mathematical Expression Omitted]

These polymerizations proceed only in the absence of terminating agents such as oxygen, C[O.sub.2], or water; therefore, polymerization is usually carried out in an inert hydrocarbon solvent and under a nitrogen blanket. These conditions produce polymers with narrow molecular weight distributions and precise molecular weights.

There are only three common monomers - styrene, butadiene and isoprene - that are easy to polymerize using this process and so only two poly(styrene-b-elastomer-b-styrene) block copolymers are directly produced on a commercial scale. These are poly(styrene-b-butadiene-b-styrene) (S-B-S) and poly(styrene-b-isoprene-b-styrene) (S-1-S). In both cases the elastomer segments contain one double bond per molecule of otiginal monomer. These bonds are quite reactive and limit the stability of the product. To improve stability, microstructure modifiers are added and as a result the polybutadiene niid-segment is produced as a random niixture of two structural forms, the 1,4 and 1,2 isomers. On hydrogenation these isomers give a polymer that is essentially a copolymer of ethylene and butylene (EB).

[Mathematical Expression Omitted]

Structural variations

In the typical A-B-A block copolymers, several structural variations are possible:

Molecular weight

Compared to homopolymers of similar molecular weight, the melt viscosities of styrenic block copolymers are very high. They also are unusually sensitive to the molecular weight of the polymer.

Both these effects are caused by the persistence of the two-phase domain structure in the melt and the extra energy required to disrupt it during flow.

In contrast, if the styrene content is held constant, the total molecular weight has little or no effect on the modulus of the material at ambient temperatures. This is because the modulus of the elastomer phase is inversely proportional to the molecular weight between entanglements in the elastomer chains. This quantity depends on the nature of the elastomer chains but not on their total molecular weight.

Hard/soft segment ratio

The ratio of the hard polystyrene (A) segments to the elastomeric (B) segments can be varied within quite wide limits. As would be expected, as the amount of polystyrene is increased the polymer gets harder and stiffer until eventually it becomes a clear flexible thermoplastic (e.g., Phillips Kresin). As the volume ratio from the A to B segments in an A-B-A block copolymer is increased, the phase morphology changes from a dispersion of spheres of A in a continuous phase of B to a dispersion of rods of A in a continuous phase of B and then to a lamellar or sandwich structure in which both A and B are continuous. If the proportion of B is increased still further, the effect is reversed in that A now becomes disperse and B continuous.

Diblock content

Many of these A-B-A polymers contain significant amounts of A-B diblock. This is usually the result of incomplete coupling during production. The diblock makes the product softer, weaker and less viscous. For some purposes (mostly adhesives and sealants) this diblock content is desirable and polymers with up

to 80% diblock are produced commercially.

Elastomer segments

Analogous S-B-S, S-I-S and S-EB-S polymers have somewhat different properties (table 2).
 Table 2 - comparison of S-B-S, S-I-S and
 S-EB-S block copolymers
 Relative Relative Stability Degradation
 stiffness cost product
S-B-S 1.0 1.0 Moderate Cross-linking
S-1-S 0.5 1.3 Moderate Chain scission
S-EB-S 2.0 2.0 Excellent Chain scission


The differences in the relative stiffness of these polymers is due to the difference in the degree of entanglements in the three types of elastomer segment. Poly(ethylene-butylene) is the most highly entangled and so has the most effective cross-links per unit volume of polymer, thus giving S-EB-S block copolymers the highest modulus. In contrast, polyisoprene is the least entangled and so S-I-S block copolymers are the softest of the three types. All these differences are reflected in the end uses. S-B-S polymers are often used to make lower cost products where stability is not critical (e.g., footwear). S-I-S analogs are softer and stickier and are mostly used in adhesives. S-EB-S polymers are the hardest of the three and also the most resistant to degradation. Thus they are used where high stability is required (e.g. automotive parts and wire insulation).

Applications

Unlike most other thermoplastics, the styrenic thermoplastic elastomers have virtually no end uses as pure materials. In almost all cases the final products contain less than 50% of the block copolymer. Thus a study of their end uses is in effect a study of how they are blended to achieve the properties needed for the particular application. These block copolymers can be blended with a wide range of resins, oils, other polymers, fillers, etc., to give products optimized for the various end uses.

Before discussing the end uses in detail, it is important to consider how the various possible added materials are distributed with respect to the two phases in the A-B-A block copolymer. For any additive there are four possibilities.

* It can go into the polystyrene phase. In this case the additive increases the relative volume of the polystyrene phase and so makes the product harder. The glass transition temperature of the additive should be similar to or greater than that of polystyrene (100 [degrees] C), otherwise it will reduce the high temperature performance of the final product.

* It can go into the elastomer phase. Conversely, in this case the additive decreases the relative volume of the polystyrene phase and so makes the product softer. The glass transition temperature of the additive will modify the glass transition temperature of the elastomer phase. This in turn affects such end use properties as tack and low temperature flexibility.

* It can form a separate phase. Unless the molecular weight of the additive is substantially less than that of either segment in the A-B-A block copolymer, this is the most likely outcome. Thus only low molecular weight resins and oils are compatible with either of the existing two phases - polymeric materials tend to form a separate third phase. This polymeric third phase is usually co-continuous with the block copolymer and so confers some of its own characteristic properties on those of the final blend.

* It can go into both phases. This is usually avoided because such an additive will lessen the degree of separation of the two phases and so weaken the product. This ability to be blended with so many different materials gives these polymers an exceptionally wide range of end uses. Some examples are:

Mechanical goods

In this application, styrenic thermoplastic elastomers can be formed into useful items by extrusion, blow molding, injection molding, etc. In other words, the fabricator can use plastics molding machinery to make rubber articles. Vulcanizing agents or cure cycles are not required. There are no residues from vulcanization and scrap is reusable. Typical applications include footwear, wire and cable insulation, automotive and pharmaceutical items. Auto parts can be painted to match sheet metal. Depending on the degree of stability required in the final product, compounds based on either S-B-S or S-EB-S can be used in these applications and can have hardnesses as low as 20 Shore A.

In all these cases, the S-B-S and S-EB-S polymers are blended with substantial amounts of other materials such as oil, other polymers and fillers. Some guidelines for these blends are given in table 3.

[TABULAR DATA OMITTED]

Polystyrene improves the processability of S-B-S based compounds and makes them harder. Oils also help processability but lower the hardness. Oils with low aromatic content must be used since other oils plasticize the polystyrene domains and weaken the product. Large amounts of oil and polystyrene can be added to these block copolymers, up to the weight of the polymer in some cases. Instead of polystyrene, semi-crystalline polymers such as polypropylene, polyethylene or ethylene-vinyl acetate copolymers can be blended with S-B-S and S-EB-S block copolymers. Blends of semi-crystalline polymers with S-B-S block copolymers usually show greatly improved ozone resistance (S-EB-S already has excellent ozone resistance). In addition, these blends have some solvent resistance. In compounds based on S-EB-S polymers, polypropylene is a particularly valuable additive. As well as improving solvent resistance, it makes the blends more processable and increases the upper service temperature. Presumably, at least some of the increase can be attributed to the high crystal melting temperature (165 [DEGREES] C) of the polypropylene network. Blends of S-EB-S block copolymer with paraffinic or naphthenic oils and polypropylene are transparent. This is probably due to the fact that the refractive index of an S-EB-S polymer/oil blend almost exactly matches that of crystalline polypropylene. Most of these changes are produced by a continuous network of the added polymer, which is developed when the polymer mixture is sheared and then quickly cooled. For this reason, compression molded samples do not show the same improvements as injection molded or extruded ones.

Inert fillers such as whiting, talc and clays can be used in these compounds. Frequently, up to 200 parts are used in compounds intended for footwear applications. Reinforcing fillers such as carbon black are not required and, in fact, large quantities of such fillers make the final product stiff and difficult to process.

Another advantage of S-EB-S polymers is that because of their lower midsegment solubility parameter, they are very compatible with paraffinic or naphthenic oils. Large amounts of these oils can be added without bleedout. In addition, the resistance of pigmented stocks to outdoor exposure is very good.

Surprisingly, even oils which themselves are stable to UV radiation reduce the stability of the blends, but the effects can be minimized by the use of UV stabilizers and absorptive or reflective pigments (e.g., carbon black or titanium dioxide).

Adhesives, sealants, coatings, etc.

Another major application of styrenic thermoplastic rubbers is in adhesives, sealants and coatings. Tackifying and reinforcing resins are added to achieve a desirable balance of properties, as are oils and fillers. Oils and resins which associate with the center rubber segments give softer, stickier products, while resins which associate with the end polystyrene segments increase hardness and strength. Some examples are shown in table 4.
Table 4 - compatibility of resins and oils with
 block copolymers
Type of resin or oil Segment compatibility(*)
Polymerized [C.sub.5] resins I
(synthetic polyterpenes)
Hydrogenated rosin esters B
Saturated hydrocarbon resins EB
Naphthenic oils I,B
Paraffinic oils EB
Aromatic resins S
*I - compatible with polyisoprene segments
B - compatible with polybutadiene segments
EB - compatible with poly(ethylene-butylene) segments
S - compatible with polystyrene segments
 These adhesives and sealants can be applied either from
solvents or as hot melts. The relatively low molecular weight
of the polymers means that concentrated solutions can be
processed, which reduces the amount of solvent required. In
recent years, hot melt applications have gained more attention.
In this case the oils and resins serve a double purpose.
Firstly they modify the properties of the block copolymer to
give it the necessary adhesive properties. Secondly they serve
in place of the solvent and allow the block copolymer to be
processed as a low viscosity melt. Generally, oil and soft
resins that associate with the polystyrene segments are to be
avoided, since they plasticize the domains and allow the
polymer to flow under stress. An exception to this might be
in sealants where a controlled amount of stress relaxation or
plastic flow could be desirable.
 Some applications of this type involve the use of very
large amounts of oils or resins. In an extreme case, more than
900 parts of oil were used with 100 parts of an S-EB/S-EB-S
blend to form a cable filling compound. This is used to fill
the interstices in underground multi-wire phone cables. It is a
hydrophobic gel and its function is to keep water out of the
cable.
Blends with thermoplastics or other polymeric materials
 Styrenic block copolymers are technologically compatible
with a surprisingly wide range of materials and can be blended
to give useful products. Blending can often be carried out
on the equipment producing the final article. Blends of S-B-S
with polystyrene,. polyethylene or polypropylene show
improved impact and tear resistance, as do three component
blends of polystyrene, poly(phenylene oxide) and either S-B-S
or S-EB-S. Similarly S-EB-S can be blended with the less
polar engineering thermoplastics such as poly(phenylene
oxide) and polycarbonate. An unusual feature of these block
copolymers is their ability to enable useful blends to be made
from incompatible polymers, e.g., polystyrene or poly(butylene
terephthalate) with polyethylene. A new development is
the use of functionalized S-EB-S block copolymers as impact
modifiers for more polar enginecring thermoplastics such as
polyesters and polyamides. The functionality is given by
maleic acid/anhydride groups grafted to the S-EB-S polymer
chain. These functionalized S-EB-S block copolymers have
also been found useful in the compatibilization of polyolefins
with polyamides and polyphenylene ether.
 Special grades of styrenic block copolymers are useful
modifiers for sheet molding compounds (SMC) based on
thermoset polyesters. They improve surface appearance,
impact resistance and hot strength.
 Blends with styrenic block copolymers improve the flexibility
of bitumens and asphalts. The block copolymer content
of these blends is usually less than 20%, even as little as 3%
can make significant differences to the properties of asphalts.
The block copolymers make the products more flexible
(especially at low temperatures) and increase their softening
point. They generally decrease the penetration and reduce the
tendency to flow at high service temperatures; and they also
increase the stiffness, tensile strength, ductility and elastic
recovery of the final products. Melt viscosities at processing
temperatures remain relatively low and so the materials are
still easy to apply. As the polymer concentration is increased
to about 5%, an interconnected polymer network is formed.
At this point the nature of the mixture changes from an
asphalt modified by a polymer to a polymer extended with an
asphalt.
 Applications include road surface dressings such as chip
seals (these are applied to hold the aggregate in place when a
road is resurfaced), slurry seals, hot mix asphalt concrete
(this is a mixture of asphalt and aggregate used in road surfaces),
road crack sealants, roofing and other waterproofing
and adhesive applications. Because of their lower cost, S-B-S
block copolymers are usually chosen for this application, but
in roofing and paving applications the S-EB-S block copolymers
are also used because of better long term aging resistance.
 References
[1.] G. Holden, E.T. Bishop and N.R. Legge, J. Poly. Sci. C.
26, 37 (1969).
[2.] S C Wells, J. Elastoplastics 5, 102 (1973).
[3.] O.L. Marrs and L.O. Edmonds, Adhesives Age 14 (12), 15
(1971).
[4.] Applied polymer science, 2nd Ed. (R. W. Tess and G. W.
Poehlein, Eds.), ACS, Organic Coatings and Plastics
Chemistry Division, Washington, 1985, Ch. 9.
[5.] Block and graft copolymerization (R.J. Ceresa, Ed) Vol. 1,
John Wiley and Sons - New York, 1973.
[6.] Handbook of thermoplastic elastomers, 2nd Ed. (B.M.
Walker and C.P. Rader, Eds.), Van Nostrand Reinhold, New
York, 1988.
[7.] Thermoplastic elastomers - a comprehensive review (N.R.
Legge, G. Holden and H.E. Schroeder, Eds.), Hanser &
Oxford Univ. Press - Munich/New York (1987).
[8.] Anionic polymerization: principles and practice, (M.
Morton), Academic Press New York, NY (1983).
[9.] J.P. Kirkpatrick and D.T. Preston, Elastomerics 120 (10)
30 (1988).
COPYRIGHT 1993 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Holden, Geoffrey
Publication:Rubber World
Date:May 1, 1993
Words:3330
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