PU rubbers, vulcanizable via dimerized TDI, to produce wear-and-tear-resistant roll coverings.

Many applications, especially in the paper industry, require rolls in high hardness. Examples are: Forming rolls; suction rolls; breaker stack rolls; Yankee touch rolls; size press rolls; 3rd press rolls and calender gloss rolls.

All these rolls require elastomeric coverings in a hardness range of Shore D 40-60. Traditionally used synthetic rubbers like SBR and NBR do not fulfill today's requirements in terms of lifetime and overall productivity.

More recent developments like H-NBR modified with zinc oxide and methacrylic acid brought an improvement, but still have not met all expectations.

Therefore, much of this field has been left to cast polyurethane which, for itself, has its own problems due to the processing (special equipment necessary; each size requires a mold; core-bonding and de-lamination problems due to no-pressure curing, etc.).

Currently used sulfur or peroxide crosslinkable polyurethane rubbers have a great performance in the low and medium hardness range, but are, even methacrylate reinforced, not very suitable for high hardness applications.

An isocyanate crosslinkable high hardness polyurethane rubber first was developed by Bayer AG in Germany in the early 1960s, but processing and performance problems due to its simple diethylene glycol ester base did not make it very suitable for high performance roller applications.

New water-crosslinked, isocyanate vulcanizable polyurethane rubbers based on C4-ether (PTMEG) and specialty esters have been developed.

This article will outline the chemistry, the processing and the properties of these new high performance elastomers,

Chemistry of high hardness polyurethane rubbers

Polymer base

These newly developed polymers are based either on polyester glycol or ether. The chosen polyester glycols consist of adipic acid and a blend of diols; key to the required properties is the right blend of carefully chosen diols. These polyester glycols have a molecular weight of 800-4,000.

The ethers are based on polytetrahydrofurane; C2 or C3 ether don't give the required mechanical strength. These ethers have a molecular weight of 1,000-3,000.

These bases are then extended with aromatic isocyanates and water to a molecular weight of 10,000-15,000.

Vulcanization and development of hardness

Since free isocyanates are not suitable for ecological reasons and their too immediate reactivity, vulcanization of these new polymers takes place by crosslinking via hindered, blocked or dimerized isocyanates.

In theory, steric hindered 3-methyl-4,4'-diphenylmethane-diisocyanates or urea-diisocyanates can be used; in practice however, the only suitable product is a 1.3-bis(3-isocyanato-4-methylphenyl)-1.3-diazetidin-2.4-dione, made by dimerization of 2.4-toluene-diisocyanate. In short, we call this product dimerized TDI or TDI dimer.

The central uret-dion ring of TDI dimer does not split under typical vulcanization conditions; that means the TDI dimer reacts primarily bifunctional with a molecular weight of 348 (TDI has a molecular weight of 174) and therefore no free isocyanate develops under normal processing conditions.

[ILLUSTRATION OMITTED]

Eight parts per hundred rubber of this TDI dimer are stochiometrically necessary for the vulcanization of these polyurethane rubbers.

Vulcanization with TDI dimer crosslinks and chain-extends at the same time; this leads to a very unique possibility to increase the hardness of the vulcanizates by a chemical reaction. Using an excess of TDI dimer allows one to incorporate low-molecular components like hydroquinone-dihydroxy-ethylether (HQEE).

[ILLUSTRATION OMITTED]

Reaction of HQEE and excessive TDI dimer develops hard segments which are chemically embedded into the polymer matrix. This phenomenon is responsible for the unique characteristics of the vulcanizates like high resilience and very high elongation at break even at the highest hardness of up to Shore D 70.

HQEE and excessive TDI dimer naturally have to be used in molecular equivalence; since the molecular weight of HQEE is 198 and the molecular weight of TDI dimer is 348 the molecular ratio for HQEE is 0.57 for every part of excessive dimer.

Since 8 pphr of TDI dimer are necessary for vulcanization of the polyurethane rubbers, this leads to a very simple formula to calculate the amount of HQEE and TDI dimer in a compound formulation:

(amount of TDI dimer - 8) x 0.57 = amount of HQEE

In order to speed up the vulcanization, small amounts (0.2 pphr) of a lead dimethyldithiocarbamate can be used as an accelerator.

These combinations of dimerized TDI, HQEE and LD show very nicely balanced Rheo-Vulcameter characteristics: Good flow, fast vulcanization and a wide plateau with hardly any reversion.

Processing of high hardness polyurethane rubbers

Compound mixing

The described polyurethane rubbers have a low Mooney viscosity (ML 1+4/100 [degrees] C of 10-40, depending on the grade. Since these materials do not require any reinforcing fillers, compounding and mixing is very simple. Typical formulations are as follows:

Millathane 96 or Millathane E 49        100.0
Stearic acid                            0.5
Thanecure T9 (TDI dimer)                8.0 - 60.0
Thanecure HQ (HQEE)                     0.0 - 30.0
Thanecure LD (lead dithiocarbamate)     0.2

TDI dimer and HQEE don't increase the compound viscosity, therefore even compounds to produce extremely high hardness vulcanizates can be easily mixed without much heat build-up.

Compounds are preferably mixed on open mills, but mixing in internal mixers does not cause any problems as long as the temperature is kept below 70 [degrees] C. This is usually not a problem due to the low viscosities of the polymers and the compounds and the resulting low shear forces.

Forming

Molded parts can be produced via injection, transfer or compression molding. Extrudates sometimes show a certain die-swell which can be overcome by the addition of small amounts of fillers. Calendering is very easy and provides very smooth sheeting and rollers can be built in all common ways.

Since compounds only provide a good building tack when warm, the best way of producing rollers is by extruding directly on a rotating core. Calendered sheets should always be used pre-warmed.

Vulcanization

Press-curing does not cause any problems for all compounds based on the described polymers, especially since the required vulcanization temperature is only 130-135 [degrees] C. Even extremely thick parts can be press-cured without problems; the compounds hardly revert at all and therefore can't become over-cured.

Open steam-curing of rollers is very suitable for the ether-based polymer; the ester-based product is preferably vulcanized in a hot air autoclave. By curing an ester-based compound in open steam, a steam barrier (mylar-foil, aluminum-foil, etc.) should be used.

Both products require only very low vulcanization temperatures; ideal temperatures are between 125 [degrees] C and 140 [degrees] C.

By using small amounts of lead dithiocarbamate, no post curing is necessary; however, the final physical properties will be achieved a minimum of 24 hours after vulcanization.

Properties of high hardness polyurethane rubbers

Mechanical strength

Both the ester- and the ether-based new polyurethane rubbers show absolute outstanding mechanical strength in a hardness above 90 Shore A.

Table 1 shows the physical properties of three standard formulations each for the ester-and the ether-based new products.

Table 1 - properties of high hardness polyurethane rubbers

                                  Ester
Millathane 96             100.0   100.0   100.0
Millathane E 49
Stearic acid                0.5    0.5     0.5
Thanecure T9               20.0   30.0    40.0
Thanecure HQ                6.8   12.5    18.2
Thanecure LD                0.2    0.2     0.2

Hardness, Shore A            95     98      99
Tensile strength, MPa        38     39      38
Elongation at break, %      505    495     405
Tear die C, N/mm             97    120     138
Rebound resilience, %        39     42      46

                                  Ether
Millathane 96
Millathane E 49           100.0   100.0   100.0
Stearic acid                0.5     0.5     0.5
Thanecure T9               20.0    30.0    40.0
Thanecure HQ                6.8    12.5    18.2
Thanecure LD                0.2     0.2     0.2

Hardness, Shore A            90      95      97
Tensile strength, MPa        39      43      38
Elongation at break, %      550     455     500
Tear die C, N/mm            110      91     113
Rebound resilience, %        52      53      54

Table 2 compares the mechanical strength of these new polyurethane rubbers with the properties of HNBR and ZnO/MAA modified HNBR.

Table 2 - comparison of high hardness compounds based on
polyurethane rubbers and HNBR/ZSC

          HNBR/ZSC                     Polyurethane rubber

ZSC 2295                100.0  50 Millathane 96      100
Zetpol 2020                    50 Millathane E 49            100
Crystalite USX                100 Thanecure T9        30      30
Peroximon F 40              8  10 Thanecure HQ      12.5    12.5
TMPT                           10 Thanecure LD       0.2     0.2

Hardness, Shore A          96  97                     98      96
Tensile strength, MPa      37  28                     39      43
Elongation, %              70  40                    495     455
Tear resistance, N/mm      75  56                     91     120
Rebound resilience, %      40  34                     42      53

Hardness in P&J points; P&J recovery

Both new products outperform all other natural or synthetic rubbers in terms of P&J hardness and P&J recovery.

Chemical resistance

Both products have a very good chemical resistance. The ester-based product has absolute outstanding oil-, fuel- and solvent-resistance but can be attacked by hydrolysis. The ether-based product is not attacked by hydrolysis and still offers a fuel-and oil-resistance comparable to medium ACN nitriles or HNBRS.

Table 3 shows the chemical resistance of the ester and the ether based product in a standard formulation.

Table 3 - chemical resistance and aging of high hardness
polyurethane rubbers vs. ZSC

Compound 1              Compound 2
(polyurethane rubber)   (ZSC/modified HNBR)

Millathane 96   100.0   ZSC - 2295      100.0
Thanecure T9     30.0   Peroximon F 40    5.0
Thanecure HQ     12.5   Antioxidant 445   1.5
Thanecure LD      0.2
Millstab P        3.0

                        Original     Aged in ASTM #3 oil @
                                       100 [degrees] C
                                      94 hrs.     504 hrs.

                        PUR   ZSC    PUR   ZSC    PUR   ZSC

Hardness Shore A         97    95     97    94     97    92
Tensile strength, MPa    39    42     41    35     32    21
Elongation, %           480   220    425   130    385    70
Volume swell, %                     +1.2   +13   +2.9   +27

                        Aged in hot air@ 100 [degrees] C
                         94 hrs.    504 hrs.

                        PUR   ZSC   PUR   ZSC

Hardness Shore A         98    96    97    96
Tensile strength, MPa    37    42    33    43
Elongation, %           378   175   348   130
Volume swell, %

Conclusion

Newly developed synthetic rubbers based on polyurethanes and crosslinked via dimerized TDI and HQEE, offer mechanical strength and hardness not achievable with any other rubbers.

Especially in the high hardness range - required by many roller applications in the steel and paper industry - the physical properties are significantly better than other rubbers:

* Higher tear and tensile strength;

* better wear and abrasion resistance,

* highest hardness of LIP to 70 Shore D achievable with no decrease in properties, and highest elasticity.

The properties are comparable or better than high performance cast polyurethanes, but the processing is a standard rubber roller building. In fact, it is a very easy one due to the simple Compounding. Advantages of these new polyurethane rubbers versus cast polyurethanes are:

* Higher production flexibility - no molds required;

* faster cure - no post cure required;

* better bonding to the metal core due to pressure and heat.

Due to the excellent properties, the easy processing and the (compared to HNBR) very reasonable prices, these new polyurethane rubbers have already found their way into the rubber roller industry and an exceptional growth in paper, steel and printing applications is expected.

References

[1.] "Applications in rubber roll coverings for the paper and steel industry," Zeon Chemicals Incorporated.

[2.] "Neue entwicklungen auf dem gebiet der chemie und technologie der walzbaren polyurethane," Dr. W. Kallert, Kautschuk und Gummi, Kunststoffe, 6/1966.

[3.] "Millathane 96 and Millathane E 49, new heavy duty polyurethane rubbers for the production of high hardness rubber rollers and other tear and wear resistant parts," TSE Industries Inc., 1996.