High performance millable urethanes. (Tech Service).

The first polyurethane rubbers in the 1950s were developed primarily for applications in the tire industry. However, the increasing dominance of the steel-belted radial tire has since shifted those elastomers to applications other than tires.

Typical applications of millable polyurethane high performance rubber now include gaskets and seals, membranes, hoses, rubber roll coverings and various kinds of belts (refs. 1 and 2).

During recent years, an increasing number of new specialty elastomers has been developed, however there is still a significant need for millable polyurethanes. Their unsurpassed mechanical properties, like abrasion resistance and tensile strength, are better than natural rubber. Their oil and fuel resistance is higher than NBR or HNBR, even for those types with higher acrylonitrile content. Gas permeation is as low as butyl rubber, and the low temperature properties of millable polyurethane were extended to -40[degrees]C through new developments to make it comparable with ECO and AEM with the added advantage of having a heavy metal free curing system and overall better mechanical properties.

Chemical structure and morphology

Conventional types of rubbers are based on unsaturated linear hydrocarbon chains, which are crosslinked by sulfur bridges. In some cases, these rubbers can also be crosslinked by peroxides that involve a different mode of action. In principle, both crosslinking systems can be used without any modifications of the elastomer structure.

However, in addition to crosslinking by peroxide and sulfur, millable urethanes can also be cured with a fully dimerized toluene diisocyanate. It should be noted that the various crosslinking systems require differing polyurethane compositions which are precisely adapted to the particular cross-linking chemistry:

* Peroxide crosslinking of polyurethane rubbers takes place through the incorporation of diphenyl methane diisocyanate (MDI) into the polymer backbone.

* Sulfur crosslinkable millable urethane incorporates glycerine allyl ether as the cure-site component.

* Isocyanate curable millable urethane requires urea segments within the polymer.

The chemical composition of the polyurethane rubber largely determines its range of properties. Based on the inventions developed by Bayer in the early 1930s, polyurethanes are produced by polyaddition of diisocyanates with microdiols (= chain extenders) and macrodiols (= polyols). This chemical reaction allows the production of an exceptionally wide range of structural variants which give rise to correspondingly varied material properties. Polyurethane elastomers should be viewed as segment copolymers which consist of a series of long-chain, highly flexible structural units, the so-called "soft" segments and rigid "hard" segment blocks inserted between the soft segments. The amorphous soft segments are represented by the macrodiol. The crystalline hard segments are formed by the reaction of isocyanate and microdiol (refs. 3-5).

Macrodiols

The nature and composition of the macrodiol define the characteristic vulcanizate properties such as elasticity, low temperature properties, mechanical strength and solvent resistance. The macrodiol is either based on polyester or polyether.

Ester based polyurethane rubbers (according to DIN/ISO 1629: AU) are primarily characterized by outstanding mechanical strength, as well as oil and solvent resistance. The outstanding mechanical strength is attributed to self-reinforcement effects due to the formation of hydrogen bridging bonds. Furthermore, the high polarity of the ester units results in outstanding oil and fuel resistance. However, the ester bonds can be attacked by some nucleophiles. In contrast. polyurethane, which derives from polyether macrodiols (according to DIN/ISO 1629: EU), exhibits excellent hydrolysis resistance and stability against acids and bases.

Microdiols

On one hand, the type and quantity of short-chained diols within the polymer enhances or reduces the thermoplastic nature of the rubber. Generally, a high content of microdiol results in increased glass transition temperatures and a more thermoplastic character. Therefore, the microdiol content is kept as low as possible in polyurethane rubbers. On the other hand, mechanical strength and temperature resistance is determined by the composition of the crystalline hard segments. These hard segments derive from the additional reaction between isocyanates and microdiols. Thus, the choice of microdiols has considerable influence on the overall properties of the rubber and the vulcanizates.

Another function attributed to some special microdiols is the incorporation of double bonds in the polymer backbone. This allows sulfur crosslinking of the rubber. Glycerine mono allylether is used for that purpose as a cure-site component. Of course this microdiol also allows the vulcanization by peroxides.

Diisocyanates

The choice of the isocyanate has a strong influence on mechanical strength and temperature resistance (see above). Moreover, MDI has proven to be a suitable reaction partner for the reactive intermediates formed during peroxide crosslinking. This is presumably due to the capability of diphenyl methane units to capture and thus to stabilize radicals derived from peroxide decomposition. In contrast, TDI (toluene diisocyanate) is not an application for the production of peroxide curable polyurethane rubbers.

Overview and classification of grades

Millable urethane rubbers are available as ether- or ester-based types in different hardnesses and, depending on the selected grade, they can be vulcanized by various crosslinking systems. The following criteria may be used to select the right grade for a particular application (table 1):

* Achievable hardness;

* mechanical properties;

* compression set;

* heat resistance;

* low temperature performance;

* impermeability to gases;

* oil resistance;

* resistance to fuels; and

* processing properties.

All grades of Bayer's millable urethane exhibit excellent resistance to oxygen and ozone. They contain no extractable constituents and their composition remains unchanged even after storage in organic solvents.

Product portfolio

Millable polyurethane elastomers can be classified by the hardness range achievable, the chemical base and the product's physical delivery form. U-600 vulcanizates, for instance, are available in a relatively high hardness range, whereas all other grades give a vulcanizate hardness in the low to medium durometer A hardness scale. Other differentiations of millable polyurethane grades lie in the chemical base of the polymers and their crosslinking system. In principle, the portfolio can be divided into three groups (table 2):

* Isocyanate crosslinkable ester-based material (AU-I);

* peroxide cured ester-based polymers (AU-P); and

* sulfur and peroxide cured polyether urethanes (EU S/P).

These different product families exhibit characteristic material properties, each of which will be discussed within the next sections.

Performance of vulcanizates

Mechanical properties

Finished rubber goods made from millable urethanes have unique properties including, e.g., excellent tensile strength, rebound resilience, modulus, elongation at break and abrasion resistance.

Even at extremely high hardness, U-600 vulcanizate, for instance, sustains an outstandingly high level in its mechanical properties.

Service temperature

Maximum thermal stability is of great importance, particularly for applications in the automotive industry. In order to reduce noise emissions, engines are frequently encapsulated. This can lead to such high temperatures in the engine compartment that the performance of the rubber vulcanizates can be critically impacted.

As with all elastomers, the maximum high-temperature resistance for polyurethane rubbers should be considered in the light of both the particular requirements of an application and the test methods applied. If hot air resistance is used as the criterion, peroxide crosslinkable millable urethanes exhibit a heat resistance up to 125[degrees]C; sulfur crosslinkable millable urethanes up to 100[degrees]C; and isocyanate crosslinked millable urethanes up to 70[degrees]C. Peroxide crosslinkable polyurethane rubber can even be exposed to peak temperatures of up to 150[degrees]C.

In regard to service temperature, the low temperature vulcanizate should also be taken into account. Like all rubbers, millable urethane hardens as the temperature falls. However, this tendency is clearly less pronounced compared with other rubbers. The glass transition temperature for millable urethane is approximately -30[degrees]C. However, it is advantageous that in certain applications, even below its glass transition temperature, this does not embrittle. Depending on filler load and grade, embrittlement can occur at temperatures as low as -70[degrees]C (ref. 6).

A new grade. U-643G, exhibits the best balance of the properties. According to our measurements, vulcanizates of U-643G have a glass transition temperature of -34[degrees]C and a brittleness point at -60[degrees]C. In addition, even when stored at 130[degrees]C for a longer period of time, U-643 G vulcanizate properties remain unchanged.

Gas permeability

These millable urethane vulcanizates are distinguished by very low permeability for gases, which is comparable with butyl rubber. In this respect, these grades crosslinked by peroxides exhibit particularly outstanding performance.

Resistance to oils, fuels and solvents

High perfomance elastomers such as millable urethanes are judged not only by their physical properties. Indeed, it is also their resistance to chemicals and environmental influences which make them of interest for many applications.

Ozone degradation of rubber is caused by attacking the double bonds in the polymeric backbone of the vulcanizate. Millable urethanes cured by peroxide and isocyanate have a saturated polymeric network. Thus, these grades are essentially impervious to damage by ozone. Even when sulfur cured, there is a high resistance to ozone because the double bonds are located in the polymer side chain.

Long term exposure to severe hydrolytic conditions can lead to damage to the ester-based grades. Its resistance is increased considerably by the addition of polymeric carbodiimides. Vulcanizates protected in this manner can be exposed to water for longer periods of time even at temperatures up to 70[degrees]C, depending on the grade selected. Due to its polyether structure, U-50 EL 06 G shows superior resistance in hydrolyzing media compared with ester-based grades.

One particularly important property of ester-based millable urethane is its excellent oil resistance. In that, our own measurements revealed no swelling or drop in physical properties in ASTM Oils No. 1 and No. 2. Even when stored close to the maximum service temperature in the strongly swelling ASTM-Oil No. 3, these vulcanizates remain essentially unchanged. However, since many additives are used in oils today, it is recommended that swelling tests be performed on the particular oil of choice.

Swelling in petrol, diesel or bio-diesel is not significant either. Finished articles have proven to be stable even in contact with aromatic and chlorinated solvents.

Vulcanizate properties by product

U-600

Even at high hardness, comparable to ebonite, articles made from U-600 have excellent mechanical strength, including high elongation at break and rebound resilience, outstanding tear resistance and very low abrasion. Moreover, due to their ester-base, molded parts of U-600 remain essentially unchanged after storage in oils and fuels.

U-640G, U-641G, U-643G

In general, peroxide curable polyester urethanes combine excellent mechanical strength with good compression set, oil and fuel resistance and a broad range of service temperatures. It also should be noted that compression set and hot air resistance data determine the service temperature. For instance, the compression set values measured at low temperatures reflect the low temperature performance of the vulcanizates (table 4). The data demonstrate that all peroxide curable ester-based millable polyurethanes have negligible or essentially no tendency to crystallization at the lowest temperatures. Thus, the articles made can be used in applications where flexibility is required at temperatures down to -40[degrees]C.

Heat resistance can be judged by means of both compression set measured at high temperature and hot air aging properties. The upper service temperature is approximately 125[degrees]C, although the molded parts can even withstand temperatures of 150[degrees]C for a short period of time.

The different AU-P grades have been developed in order to match specific requirements apart from the overall inherent performance mentioned above. For instance, U-640G exhibits the highest mechanical strength, lowest gas permeation and excellent oil resistance. U-641G offers the best hydrolysis stability and good heat resistance. U-643G combines the specific advantages of U-640G with the best low temperature performance. More details of the vulcanizate performance can be taken from table 1.

U-50 EL06G

In general, ether-based polyurethane rubbers exhibit a high stability against acids, bases and water. In addition, high mechanical strength and superior abrasion resistance represent valuable properties of this family of polyurethanes.

As shown in table 4, the ether-based U-50 EL06G was tested after sulfur and peroxide crosslinking. Although the ether-based grades differ only slightly in their chemical base, significant variations in the performance of those grades are found (table 4). For instance, typical advantages for U-50 EL06G molded articles are high modulus, excellent abrasion resistance, low gas permeation and outstanding hydrolysis resistance.

The service temperature of the polyether grade is 100[degrees]C maximum. U-50 EL06G is used in applications where excellent hydrolysis resistance is required. Owing to its ether base, these materials withstand even the most severe conditions in a humid environment at high temperatures.

Applications

Millable polyurethane elastomers are used in many sectors of the industry, including mechanical engineering and plant construction, the textile industry, the petroleum industry, transportation and vehicle construction. Typical applications are tile molds, hydraulic gaskets, diaphragms, seals, belts, roller coverings, o-rings, vibration insulators, bumpers, pump stators, cable jackets, sand-blasting hose and shoe soles. A few of these applications are described below.

The ceramic industry uses tile molds produced from rubbers which combine elevated abrasion resistance, high hardness and excellent surface finish. U-600 has been used for decades as the material of choice.

Diaphragms for shock absorbers made from peroxide-curable grades are used in so-called self-regulating and hydropneumatic automotive suspension systems. The key properties required in this kind of application are, apart from excellent dynamic load bearing, outstanding oil resistance and low gas permeability.

In seals and gaskets, the required property is excellent resilience under pressure, and peroxide crosslinked grades are used for this purpose in a temperature range of -40[degrees]C to 125[degrees]C.

Rubber roller coverings made, for example fur the transport of paper in photocopy machines, must have a high coefficient of friction, low abrasion and excellent resistance against ozone.

Compounding

Compounding millable polyurethanes is not very different from compounding other special and/or standard elastomers. At the cure site, sulfur donors are not effective, so that for sulfur curing an increase in heat resistance is not possible.

To increase the hardness of the compound, filler dosage can be increased (this is valid for black and white fillers). Addition of a small amount of a fully dimerized toluene diisocyanate has shown a positive effect in increasing the hardness of the compound. On the other hand, in order to decrease the hardness, one can increase the dosage of plasticizer oil. However, at a certain point plasticizer oils, normally esters of adipic acid, are not only good plasticizers but also good solvents. The solution is the addition of factice that will act like a sponge or will permit a lower dosage of plasticizer oil. Factices suitable for sulfur and peroxide curing are available.

Certain blends with NR, EPDM and NBR have found commercial success in improving tensile strength, abrasion resistance and tear resistance; to improve the blending, a homogenizer resin is recommended.

In order to increase the tensile strength in compounds, Rhein Chemie has developed a special line of aramid fibers predispersed in rubber, which are totally compatible with the millable polyurethanes.

At dosages higher than 20 phr, the addition of a vinyl silane also has shown an increase in tensile strength.

In order to increase the processability of millable polyurethanes, the addition of processing promoters is recommended, which will reduce the sticking in the mixing chamber or on the mill. Other products, butadiene resins or very low Mooney polyurethanes, will increase the flow/throughput during extrusion or injection molding.

In the footwear industry, blends with SBR have shown that it is possible to increase wet traction.

The hydrolysis resistance of polyester polyurethanes can be improved by the addition of a polymeric carbodiimide or by blending with a polyether-type polyurethane (generally 90/10 or 80/20 ratio).

Conclusion

Excellent mechanical strength, outstanding impermeability to gases and resistance to oil constitute a unique combination of valuable properties in millable polyurethanes. In addition, due to continuing developments in chemistry and improved production and processing technology, these materials stand out as very high performance elastomers.

Table 1 - selecting the right grade
(-inadequate, 0 fair, + good, ++ excellent)

Grade                            640G   641G   643G   50EL06G   600
Type of polymer                  AU-P   AU-P   AU-P   EU-S/P    AU-
                                                                 I

Key properties
Physical/mechanical properties    ++     ++     ++       +
Compression set                   +      +      ++       0      ++
Heat resistance                   +      +      ++       0       0
Low temperature performance       +      +      ++       0       0
Impermeability to gases           ++     ++     +        +       +
Resistance to water               0      +      0       ++       +
Oil resistance                    ++     +      ++       0       0
Resistance to fuels               ++     ++     ++       0       +
                                                                 +
Processing properties
Compression molding               ++     ++     ++      ++
Transfer molding                  ++     ++     ++      ++      ++
Injection molding                 ++     ++     ++       +      ++
Extrusion                         ++     ++     ++       +       -
Steam vulcanization               -      -      -       ++       0
Hot air vulcanization             0      0      0       ++       -

Table 2 - three groups of polymers

Grade     Hardness range          Chemical   Delivery
                                  base (1)   from

600       70 Dur. A - 50 Dur. D   AU-1       Bales
640G      40 Dur. A - 90 Dur. A   AU-P       Granules
641G      40 Dur. A - 90 Dur. A   AU-P       Granules
643G      40 Dur. A - 90 Dur. A   AU-P       Granules
50EL06G   40 Dur. A - 90 Dur. A   EU-S/P     Granules

AU = polyester rubber; EU = polyether urethane rubber;
P = peroxide crosslinking; S/P = sulfur and peroxide crosslinking

Table 3 - vulcanizate properties of AU-1

Formulations                              A      B      C

U-600                                   100    100    100
Stearic acid                            0.5    0.5    0.5
Crosslinker 30/10                        --      7     10
Thanecure T9                              8     20     27
Rhenogran AUI-70                        0.5    0.5    0.5
Physical properties                       1      2      3
Hardness (durometer A)                   70     90     95
Tensile strength (MPa)                   24     28     33
Elongation at break (%)                 680    600    560
Modulus 100% (MPa)                      2.8    6.3    7.5
Modulus 100% (MPa)                      7.6   10.1   11.2
Rebound resilience (%)                   54     44     42
Tear resistance, graves (N/mm)           30     47     53
Abrasion loss (qmm)                      20     30     32
Compression set, 24 h., 70[degrees]C     35     45     65
Modulus 300% (MPa)                      9.9   14.9   11.2
Elongation at break (%)                 380    390    390
Weight increase (%)                     8.6    3.5    8.2
Volume increase (%)                    11.1    4.3   10.4

Table 4 - vulcanizate properties of EU-S/P

Formulations                                          G      H

50EL06G                                              100    100
Zinc stearate                                        0.5     --
Stearic acid                                          --    0.5
Carbon black N330                                     30     --
Carbon black N550                                     --     30
Aflux 54                                               2      2
Rhenogran MBTS-80                                    3.8     --
Rhenogran MBT-80                                     3.8     --
Rhenogran S-80                                       1.9     --
Rhenocure AUR                                          1     --
Poly-Dispersion E(VC)D-40                             --      2
Rhenofit TAC/S                                        --    0.5
Physical properties
Hardness (durometer A)                                72     63
Rebound resilience (%)                                53     57
Tensile strength (MPa)                                29     19
Modulus 100% (MPa)                                   5.2    2.1
Modulus 300% (MPa)                                  17.7    7.3
Modulus 500% (MPa)                                    --   12.8
Elongation at break (%)                              450    690
Tear resistance, graves (N/mm)                        19     30
Abrasion loss (qmm)                                   60     78
Compression set, 72 h.
  25[degrees]C                                        11     37
  70[degrees]C                                        54     54
Q, N2, 80[degrees]C (cm3xmm/m2xdxbar)                578    798
Hydrolysis resistance (21 d, 80[degrees]C, water)
  Hardness (durometer A)                              64     40
  Tensile strength (MPa)                              17     11
  Modulus 100% (MPa)                                 3.4    1.1
  Modulus 300% (MPa)                                13.5    4.6
  Elongation at break (%)                            360    630
  Weight increase (%)                                 12     16
                                                      13     21

References

(1.) U. Hoffmann, KU-Kunststoffe, 110, Vol. 89 (1999) 5.

(2.) U. Hoffmann, KU-Kunststoffe Plast Europe, 28, Vol. 89 (1999) 5.

(3.) Th. Thimm, Kautschuk-Gummi-Kunststoffe, 568, Vol. 35 (1982).

(4.) Th. Thimm, Kautschuk-Gummi-Kunststoffe, 257, Vol. 36 (1983).

(5.) Th. Thimm, Kautschuk-Gummi-Kunststoffe, 933, Vol 37 (1984).

(6.) Bayer--Manual for the rubber industry.

Giorgio Mueller has been with Rhein Chemie since 1998. Since 2000 he has been the chief chemist at their Ohio location. Uwe Hoffmann has been with Rhein Chemie Germany the past seven years in Application Development/Technical Customer Service for specialty elastomers.