An introduction to the chemistry of polyurethane rubbers.

Professor Dr. Otto Bayer in Leverkusen, Germany invented the first polyurethane elastomer in 1937. In the 1950s the rubber, and particularly the tire industry, encouraged their colleagues in the chemical plants to make polyurethane elastomers feasible for rubber applications. U.S. Rubber and Bayer started joint developments for treads by using an ether-based peroxide crosslinked polyurethane rubber. DuPont started similar developments on ether-based sulfur crosslinked polyurethane elastomers.

At the end of that decade, U.S. Rubber (later to become Uniroyal) started the production of ITVE Vibrathane, which later became Vibrathane 5003, 5004. DuPont started Adiprene C and Bayer Desmophen A, an ester-based, isocyanate crosslinked polyurethane rubber, and Urepan E, an ester-based, peroxide crosslinkable polyurethane rubber.

In the 60s, an inflation of millable polyurethane was clearly visible. General Tire started the production of their ester peroxide grades under the name of Genthane. Goodyear and Michelin introduced their ester sulfur grades under the names of Chemigum and Gurane. American Cyanamid started to produce their ester sulfur grade Cynaprene VG. Thiokol introduced their ester and caprolactone-based, sulfur crosslinkable grades under the name Elastothane, while Witco developed Fomrez MG - a sulfur curable TDI/polyester. Bayer established a range of four Urepan grades, all based on esters, two of which were isocyanate crosslinkable and the others were cured with peroxide.

All these products faced processing problems, and very often the overall properties were not as expected.

The production process was also very difficult, which resulted in a very inconsistent material, and because of this Michelin and General Tire halted the production of their millable polyurethanes in the 70s. They were to be followed by American Cyanamid, Witco and Thiokol.

In this decade, TSE Industries started the production of their millable polyurethanes under the name Millathane. DuPont sold their Adiprene business around 1986 to Uniroyal. There are only three producers of millable polyurethane rubber who still produce in the western world, Uniroyal, Bayer and TSE Industries. TSE is the world's largest producer of polyurethane rubber, followed by Bayer and Uniroyal.

Does a real requirement for polyurethane rubber exist?

During recent years, we have witnessed a vast development of new special polymers, i.e., hydrogenated acrylonitrile butadiene rubber (HNBR), epichlorhydrin rubber (ECO), new peroxide crosslinkable fluoro-rubbers and others. By having a choice of these new polymers, in addition to all the old well-known synthetic rubbers, we have to ask ourselves whether a real requirement for polyurethane rubber exists? Today, more than ever before, the answer is a clear yes.

Not only are specific requirements becoming increasingly tough (i.e., oil and fuel resistance, heat stability, low temperature flexibility, high hardness, etc.), but there is a growing demand for technical parts with an outstanding balance of overall properties.

By using the full range of polyurethane rubber, it is possible to achieve:

* Mechanical properties such as abrasion resistance and tensile strength far superior to all other known natural or synthetic rubbers;

* hardness from 25 A to 60 D;

* oil and fuel resistance better than NBR or HNBR;

* gas permeation as low as butyl rubber;

* ozone resistance as good as EPDM;

* heat resistance up to 110 [degrees] C; 150 [degrees] C intermittent; and

* low temperature flexibility down to -55 [degrees] C.

Difference of polyurethane to other PU elastomers As with all polyurethane elastomers, polyurethane rubber is based on the three main ingredients: polyol, isocyanate and chain extenders. Polyols are either polytetramethylene ether glycols or polyester adipates. On the isocyanate side, a wide variety of aromatic or aliphatic isocyanates is suitable, and chain extenders can be ethyleneglycol, butanediol, glycerol-monoallylether, trimethylolpropane-monoallylether or even water (ref. 1). Usually, polyurethane elastomers are produced with a final stoichiometric equivalence of NCO to NCO-reactive (OH) groups (ref. 2).

Cast polyurethane pre-polymer systems are made by reacting polyol with a surplus of isocyanate in order to be liquid during processing. Then, during final processing, the material is mixed with chain extenders to reach stoichiometric equivalence.

Thermoplastic polyurethanes are mostly produced in one step with a slight excess of isocyanate (NCO) versus the combined OH number of polyol and chain extender.

Polyurethane rubber is produced with a final stoichiometric deficiency of isocyanates in order to obtain the necessary millable state. Polyurethane rubber is therefore in need of further crosslinking or vulcanization (ref. 3).

Diversification of polyurethane rubber

Polyurethane rubbers can be classified in accordance to either the chemical base or the type of vulcanization.

As polyols, either polytetramethylene ether glycol ([C.sub.4] ethers based on polytetrahydrofurane) or polyester adipates (based on adipic acid and diols like ethandiol, butanediol, methylpropanediol, hexanediol, neopentylglycol, cyclohexanedimethanol, etc.) can be used (ref. 4).

The careful selection of diol/glycol and the molar ratio of glycol-blends influence the final properties of the polyurethane rubber vulcanizate dramatically. Some examples of this include: ethylene glycol gives excellent oil and fuel resistance, but a poor hydrolysis resistance; butanediol or even better methylpropanediol give outstanding low temperature properties; hexanediol leads to a good hydrolysis and heat resistance; cyclohexanedimethanol brings outstanding gas impermeability (ref. 6), etc.

Also, the right molar ratio of glycol blends is extremely important. As an example, a blend of ethylene and butylene can make a polyurethane rubber with a nicely balanced chemical resistance and low temperature behavior. But be careful. At a molar ratio of approximately 70:30 or higher for the ethylene part, the soft segment will crystallize at even moderate low temperatures and is not usable for many applications (ref. 7).

The diisocyanate component is either aromatic diisocyanates like methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) or aliphatic diisocyanate like dicyclohexylmethane diisocyanates ([H.sub.12]MDI) or TMXDI (tetramethylxylene diisocyanate) which is a light stable isocyanate where the isocyanate groups are separated from the aromatic ring by methylene groups.

Aromatic diisocyanates provide the better mechanical strength, whereas aliphatic diisocyanates give better heat and hydrolysis resistance.

Aliphatic diisocyanates are also necessary if a light and color stable vulcanizate is intended to be produced (i.e., transparent outsoles for athletic footwear (ref. 8).

Chain extenders are of low molecular weight like ethylene glycol, 1,4-butanediol, hydroquinone bis(2-hydroxy-ethyl)ether, glycerolmonoallylether, trimethylolpropane-monoallylether or water. Again, the amount and type of chain extender influences the properties and the processing behavior of the polyurethane rubber substantially. Water, for an example, not just chain extends, but also incorporates urea groups which give a good solvent resistance.

Vulcanization of polyurethane rubber

The vulcanization of polyurethane rubber leads to crosslinking between molecular chains, which results in a network structure. This resembles the concept of other vulcanized rubbers, but compared to other polyurethane elastomers, to a smaller number of urethane groups. These urethane groups form hydrogen bonds and contribute substantially to improved mechanical strength. For this reason, most polyurethane rubbers require the addition of active fillers like carbon blacks or silicas, which reinforce polyurethane rubbers in the same manner as with other rubbers.

Sulfur vulcanization requires unsaturated components to be built into the structure of polyurethane rubbers. This is done by using OH functional compounds with a double bond as chain extenders, i.e., glycerolmonoallylether (GAE) or trimethylpropane-monoallylether (TMPMAE).

Of all isocyanates, only MDI as a constituent of the macromolecule is a suitable co-reactant for peroxide vulcanization; here we get radical formations through the central methylene group.

Polyurethane rubbers based on other isocyanates, i.e., aliphatic isocyanates, require unsaturation for peroxide vulcanization. Opposite to sulfur vulcanization, only small amounts of unsaturation are sufficient. As with all synthetic rubbers, the higher the polymeric backbone's saturation, the higher the heat and oxidation resistance will be.

A different approach is the isocyanate vulcanization of polyurethane rubbers. Here we see a chain extension through the hydroxyl end groups, plus a crosslinking through formation of allophanate or biuret structures. This provides outstanding mechanical strengths and very high elasticity, even at very high hardness (refs. 5 and 9).

Properties of polyurethane rubbers

All polyurethane rubbers provide outstanding mechanical strengths and a high chemical resistance, but depending on the requirements of the final vulcanized parts and the processing equipment, the appropriate polyurethane rubber grade has to be chosen carefully. Generally, ether-based polyurethane rubber provides an excellent hydrolysis resistance, but poorer heat resistance, while ester based polyurethanes are typical for their outstanding oil and fuel resistance.

Peroxide vulcanization gives the best heat resistance and the lowest compression set. Sulfur vulcanization allows a wide processing flexibility and the isocyanate vulcanization is used for the production of higher hardness vulcanizates.

Table 1 gives an overview of some commercially available polyurethane rubber grades, their chemical basis and type of crosslink. Table 2 shows their key properties, and table 3 their main applications.

Table 1 - commercial grades of PU rubber-chemical base

Type                      Chemical base            Vulcanization

Polyester/MDI             Ethylene                 Peroxide
                          glycol/butanediol
                          adipate and methylene
                          diphenyl diisocyanate

Polyester/TDI             Ethylene/propylene       Sulfur,
                          glycol adipate and       peroxide
                          toluene diisocyanate

Polyester/[H.sub.12]MDI   Hexanediol/neopentyl     Sulfur,
                          glycol adipate and       peroxide
                          dicyclohexylmethane
                          diisocyanate

Polyether/TDI             Polytetramethylene       Sulfur,
                          glycol and toluene       peroxide
                          diisocyanate

Polyether/[H.sub.12]MDI   Polytetramethylene       Peroxide
                          glycol and
                          dicyclohexylmethane
                          diisocyanate

Polyester/ISO             Diethylene glycol        Isocyanate
                          adipate and toluene
                          diisocyanate and water

Polyether/ISO             Polytetramethylene       Isocyanate
                          glycol and toluene
                          diisocyanate and
                          water
Table 2 - product type - key properties

Grade               Hardness                Service temp.

Polyester/MDI      40 to 85 A   -40 [degrees] C to +140 [degrees] C
Polyester/TDI      25 to 85 A   -30 [degrees] C to +100 [degrees] C
Polyester/
 [H.sub.12]MDI     40 to 85 A   -35 [degrees] C to +150 [degrees] C
Polyester/ISO    70 A to 60 D   -35 [degrees] C to +100 [degrees] C
Polyether/TDI      35 to 85 A    -40 [degrees] C to +90 [degrees] C
Polyether/
 [H.sub.12]MDI     50 to 85 A    -40 [degrees] C to +90 [degrees] C
Polyether/ISO    70 A to 60 D   -40 [degrees] C to +100 [degrees] C

                 Oil/fuel   Hydrolysis     Mech.
Grade              res.        res.      strength

Polyester/MDI        +++            +        +++
Polyester/TDI        +++         -(*)        +++
Polyester/
 [H.sub.12]MDI       +++        +(**)         ++
Polyester/ISO        +++         -(*)        +++
Polyether/TDI          +          +++        +++
Polyether/
 [H.sub.12]MDI         +          +++         ++
Polyether/ISO         ++          +++        +++

(*) with addition of polycarbodiimide

(**) with addition of polycarbodiimide

- poor

+ fair

++ good

+++ excellent

Table 3 - application examples

Grade           Application examples

Polyester/MDI   Oil-, fuel- and heat-resistant molded
                parts like seals, gaskets, o-rings, membranes,
                dust covers, mounts, bearings
                and belts for the automotive industry
                and other hyrdaulic or pneumatic applications
                and paper handling rolls, flippers
                and belts for office machines.

Polyester/TDI   Oil-, fuel- and solvent-resistant molded
                parts and rollers with exceptional
                mechanical strength even at very low
                hardness, i.e., printing rollers, bottle
                grippers, can tester pads, etc.

Polyester/      Oil- and fuel-resistant seals, rollers and
[H.sub.12]MDI   belts to perform in hot and humid environments,
                i.e. poly V-belts, timing belts.

Polyester/ISO   High hardness molded parts and rollers
                with exceptional oil-, fuel- and
                solvent-resistance and high mechanical
                strength, i.e., oil seals, bearings,
                bushings, ceramic tile dies, etc.

Polyether/TDI   Hydrolysis resistant molded parts and
                rollers with high mechanical strength
                and abrasion resistance, i.e., rollers,
                wear- and tear-protective parts and
                conveyor belts for many industrial
                applications.

Polyether/      Glass clear and light stable elastomeric
[H.sub.12]MDI   parts with excellent abrasion resistance
                and outstanding wet and dry traction,
                i.e., skate rollers and athletic shoe soles.

Polyether/ISO   Rubber roll coverings and molded parts
                in high hardness with exceptional
                mechanical strength and hydrolysis
                resistance, highest tear- and cut-resistance
                of all known synthetic or natural
                rubbers, rollers for paper-, steel- and
                wood-processing industry, rice de-husking
                rollers, sheets and screens for mining.

Conclusion

Rarely can an elastomer be found that can offer a similar balance of achievable properties as polyurethane rubber. However, due to its wide spectrum of chemical differences and properties, the right grade for the application in mind has to be chosen very carefully.

Lack of knowledge, poor technical advice, and consequently the wrong choice of grade or inadequate processing are the main causes for the comparatively low usage of polyurethane rubber in the worldwide rubber industry.

But in many extremely demanding applications, i.e., membranes for active suspension systems and load-leveling shock absorbers, polyurethane rubber has outperformed all others - even much more expensive synthetic rubber; and recently, new grades have been developed and been introduced into the rubber industry.

References

(1.) "Neue Entwicklungen auf dem Gebiet der Chemie und Technologie der walzbarenpolyurethane," published in Kautschuk und Gummi, Kunststoffe (1966); Dr. W. Kallert.

(2.) Polyurethane Handbook, Dr. Oertel.

(3.) "Urepan - a new range of polyurethane rubber, "published in Kautschuk Gummi Kunststoff (1/1995); A. Schroeter.

(4.) "Development of a new polyurethane rubber grade, suitable for extreme low temperature applications," Rubbercon '95, Gothenborg, Sweden 1995; A. Schroeter.

(5.) "Developing polyurethane rubbers for very high hardness roll coverings," published in Rubber Technology International (1997); A. Schroeter.

(6.) "Cyclohexyldimethanol/methylpropanediol based polyurethane rubber for improved low temperature properties and gas impermeability," published in Rubber Science and Technology (1997); A. Schroeter.

(7.) U.S. Patent 5,760,158 Polyurethane rubber vulcanizable by peroxides or sulfur with improved low temperature and gas impermeability properties (granted June 1998); A. Schroeter.

(8.) "PU-rubber outsoles for athletic footwear," published in Rubber World (12/1998); Jim Ahnemiller.

(9.) "Wear, tear resistant roll coverings, "published in Rubber World (4/1998); A. Schroeter.3