High efficiency NR processing with good dynamic properties using a zinc free additive.

Viscosity reduction of natural rubber (NR) is a common problem in the entire rubber industry, and especially in tires where viscosity stabilized grades of NR are too expensive and not even available in sufficient quantities.

Mechanical mastication is rather inefficient, so chemical peptizers are widely used. Their effect in viscosity reduction is very good even in small dosages such as 0.1-0.2 phr, but they also produce a lot of short chain material which results in poorer dynamic properties and increased sticking on the production machines.

Moreover, due to changes in mixing procedures over the years, peptizers are not fully utilized any more. The strongest effect is achieved at temperatures above 120[degrees]C when oxidative chain scission becomes dominant, catalyzed by the so-called "boosters." Often the fillers have already been added at this time, thus the booster is deactivated.

To overcome this problem, a new concept of NR viscosity reduction has been introduced (ref. I). The new material, HT 105, avoids the drawbacks of chemical peptizers, but is still zinc based.

With view to the upcoming restrictions on zinc by the EU, a zinc free version of the new peptizer was developed, XP 1440.

Experimental

The new material, XP 1440, was compared to the zinc-containing HT 105 and a reference (control) with only mechanical mastication and without any additive. The formulations, based on a simple truck tire compound with a semi EV cure system, are shown in table 1.

Masterbatch mixing

All compounds were mixed for 3' 30" in a 1.5 liter lab internal mixer. Peptizing agents, if used, were added with the polymer, fillers after 1' 15". The mixing procedure is shown in table 2.

No significant differences in dump temperature (measured by needle pyrometer) or mixing energy were observed. The dynamic viscosities measured by RPA @ 100[degrees]C with 1 or 10 Hz showed a slightly higher reduction with the conventional HT 105 (figure 1) at equal dosage of 2 phr.

[FIGURE 1 OMITTED]

The mixing energy curves (figure 2) show the typical lubricant behavior of the additives in the beginning of the mixing cycle and also the principle drawback of a mixing cycle starving with a half empty machine. In the beginning, the energy input is sometimes, and unpredictably, low. Of course, the same can happen in full production scale, and depending on the equipment, ,night very often not even be detected.

[FIGURE 2 OMITTED]

The mixing temperature curves closely follow the pattern of the mixing energies as expected (figure 3). Towards the end of the masterbatch mixing cycle, differences are small.

[FIGURE 3 OMITTED]

Final mixing

Also, the final mixing was done in the lab mixer (table 3). As found in the masterbatches already, XP 1440 results in higher viscosities than HT 105. both in RPA or Mooney values.

Curing

Figure 4 shows the rheometer curves of the various compounds. It is obvious that the zinc containing HT 105 gives better reversion resistance than all the other compounds, a fact that is due to the unique influence of zinc ion oil the sulfur cure of diene rubbers.

[FIGURE 4 OMITTED]

More details of the cure characteristics are given in table 4. While there is little difference in tc 10% tc 90% and tc 100% are clearly shorter with the new additive. This effect can be used for higher productivity, and if not desired we have evidence (not shown here) that a reduction in zinc oxide can bring the values back to standard.

Vulcanizate properties

The physical properties are shown in table 5. The activating effect of XP 1440 is reflected in the higher modulus values. After heat aging (one week @ 70[degrees]C), no significant differences in hardness, tensile strength elongation or modulus were registered between the various compounds.

Dynamic data

The Goodrich Flexometer test showed no significant differences in heat build up or deflection. Under higher load (blow out conditions), the differences were also not significant.

Eplexor data

The equal HBU with 2 phr XP 1440 is supported by the Eplexor dynamic data (figure 5). Tan delta at 70[degrees]C. indicating the rolling resistance, is equal to HT 105 or mechanical break values are equal or higher with XP 1440.

[FIGURE 5 OMITTED]

Conclusion

XP 1440, the first zinc free peptizer/lubricant combination, gave good dynamic properties. The viscosity reduction is at least equivalent to HP 105. The new zinc free material is not able to provide any reversion resistance. This property seems to be unique to zinc containing materials.

Table 1--formulations

                       Control    HT 105        XP 1440

                                   2 phr    1.5 phr  2 phr
Natural rubber 5 WF    100         100      100      100

HT 105                               2
XP 1440                                       1.5
N-234 Statex            55          55       55       55
Sundex 790               5           5        5        5
Zinc Oxide               3.5         3.5      3.5      3.5
Stearic acid             1           1        1        1
Vulkanox 4020            2           2        2        2
Vulkanox HS/LG           1           1        1        1
Protektor G 35           1           1        1        1
Sulfur MS 80/90          1.5         1.5      1.5      1.5
Vulkacit NZ/EG           1.5         1.5      1.5      1.5
Total                  171.5       173.5    173.0    173.5

Table 2--mixing cycle

First pass--internal mixer GK 1.5 N. start 60[degrees]C, 65 rpm

           0'                    NR, peptizer
           1' 15"                1/2 black, ZnO, Protektor G 35, oil
           2' 30"                A.O., stearic acid, 1/2 black
           3' 00"                Sweep
           3' 30"                Dump

                                 Control    HT 105        XP 1440

                                             2 phr    1.5 phr    2 phr

Dump temperature ([degrees]C)        130       131        130      131
Energy input (kJ)                  1,891     1,976      1,939    3,014

Table 3--final mixing

                                 Control    HT 105         XP 1440

                                             2 phr    1.5 phr     2 phr

Second pass--internal mixer GK 1.5 N, start 60[degrees]C, 40 rpm
                                    0'    First pass batch, cures
                                    2'    Dump
Dump temperature ([degrees]C)        100       100        100       101
Energy input (kJ)                    888       896        904       894
RPA 2000
Frequency range, measurement [degrees]C/1 deg.
Visc. n* at 1Hz (Pa s)            59,116    53,070     52,698    51,090
Visc. n* at 10Hz (Pa s)            8,103     7,440      7,451     7,305
Mooney MS (1+4) 100[degrees]C         56        48         50        49
Mooney MS (1+4) 100[degrees]C        102        90         91        90

Table 4--rheometer data

                          Control    HT 105        XP 1440

                                      2 phr    1.5 phr    2 phr

Torque ML (dNm)              4.55      4.09       3.96     3.90
Torque MH (dNm)             22.0      21.5       21.9     21.4
tc 10% (min.)                2.67      2.48       2.54     2.57
tc 90% (min.)                4.58      4.73       4.47     4.54
tc 100% (min.)               7.30      8.13       7.26     7.44
Reversion t 95% (min.)      12.6      16.7       12.9     13.0

Table 5--physical properties

Vulcanizate properties

                             Control    HT 105         XP 1440

                                         2 phr    1.5 phr     2 phr

Cure time for 2 mm (min.)       8         8          8         8
Cure time for 6 mm (min.)       9         9          9         9
Density (g/ccm)                 1.12      1.12       1.12      1.12
Duro. hardness A (Sh.u)        68        68         69        68
Tensile strength (MPa)         28.2      29.0       29.0      27.5
Elongation at break (%)       398       511        509       493
Modulus 100% (MPa)              2.8       2.8        2.8       2.9
Modulus 300% (MPa)             14.5      14.8       15.0      14.9
Rebound (%)                    39        39         39        38
Abrasion loss (cmm)           114       118        113       117

Reference

(1.) K. Menting et. al., 5. DIK Kautschuk-Herbstkolloquium 2002, Hannover, October, 2002.