Effect of a MFA on the processing and mechanical properties of a CB-filled NR compound.

Numerous process additives are used in rubber compounds, to reduce viscosity, accelerate mixing and promote release from metal surfaces. For effective selection and use of a process aid, it is necessary to understand how it functions and to be able to determine the appropriate amount to use. Here the effects of an experimental process additive on the mixing characteristics and properties of a carbon black filled natural rubber compound are explored in some detail. The additive is N-tallow 1,3 propanediamine distearate, described as a surface-active process aid (SAPA) or a multi-functional additive (MFA) in previous work (refs. 1 and 2).

The name MFA was chosen because the additive acts as a viscosity modifier, a cure accelerator and a mold release agent, in addition to its influence on vulcanizate properties. In this article, the optimum level of the additive will be determined and the changes caused by the use of the additive evaluated by comparison with an additive-free control compound mixed under similar conditions. The results will also be used to identify the main modes of action of the additive.

Experimental

Material

The material consited of: Natural rubber-SMR 20; MFA-EN 444; zinc oxide; stearic acid; poly-1, 2-dihydro-2, 24-trimethylquinoline; N-cyclohexylbenthiazyl sulfenamide; sulfur; carbon black N 330.

Equipment

The equipment used included: Automated, computer-controlled internal mixer; cure analyzer; capillary rheometer with conical die of 2 mm diam.; biconical rotor rheometer; servo hydraulic machine; scanning electron microscope; universal tensile tester; abrasion tester (BS 903: Part A9 Method A1)

Procedure

The modified ASTM 3191 formulation which is based on a conventional vulcanization system is given in table 1. An internal mixer(capacity 5.5 liters), with intermeshing rotors, was used for mixing the formulation. The mixer was warmed up before experimentation started. Using a fill-factor of 0.6 and a rotor speed of 35 rpm, mixing was carried out according to mixing cycle as given in table 2. Feeding was carried out automatically. Ram-down time is the time from the closing of the door after the input of materials into the mixer. The total time is the actual time taken by the mixer as shown in the power trace. The temperature of the rotors was set at 35[degrees]C, controlled by circulating water. The temperature of the discharged compound was determined by using a hand-held temperature probe. The mix was subsequently sheeted for thickness of 2-3 mm by a single pass through a two-roll mill 40 cm x 25 cm. Samples of the compound were taken for tests. 2g of rubber compound in round shape of 2.5 cm in diameter was used to determine the optimum cure times [t.sub.95] at 160[degrees]C using a cure analyser at frequency of 1.7 IIz., strain of 0.14 and pressure of 600 kPa. Small pieces of rubber (1 cm x 1 cm x 0.3 cm) were used to fill the barrel of the capillary rheometer for the approximate measurement of viscosity and melt fracture at 100[degrees]C using a 90[degrees] included angle conical die with a land region of 2 mm diameter and 1 mm in length. An uncorrected viscosity was calculated simply by dividing shear stress by shear rate. Melt fracture was determined visually as the departure from straightness or the onset of surface roughness. A rheometer with a smooth rotor of 3 cm diameter was used to measure the adhesion of cured rubber to metal rotor at 170[degrees]C using the optimum cure time.

Table 1 - formulation of the compounds

Ingredients                                          phr
SMR 20                                               100
Zinc oxide                                             5
Stearic acid                                           3
Flectol H                                              1
(Poly-1,2-dihydro-2,2,4-trimethylquinoline)
N330 carbon black                                     50
Sulfur                                               2.5
CBS                                                  0.5
(N-cyclohexylbenthiazyl sulfenamide)
MFA (levels investigated)                         0 0.3 1.0
                                                 2.0 3.0 5.0

Table 2 - mixing cycle of the ingredients

Label as     Sequence               Ram-down     Total
in fig. 1                            time(s)    time(s)
A            Rubber, Zno, St.
             acid and Flectol H           0       20
             MFA                       119
B            N330                      120       150
C            CBS                       180       240
D            Sulfur                    240       305
E            Discharge                 320       400

Dynamic properties were measured on bonded 'double shear' test pieces, using a servo-hydraulic mechanical tester running at 1 Hz. and room temperature (25[degrees]C). Transfer molding was used to produce the double shear test pieces, with rubber discs of 32 mm diameter and 8 mm thickness. Cyclic tests were performed on each specimen at double strain amplitudes (DSA) from 0.001 to 1. Tensile measurements as in BS 903:part A2 and tear strength as in BS 903:Part A3 were carried out using a universal tensile machine at room temperature. Abrasion resistance was measured using a DIN abrader (Method A1 as in BS 903:Part A9).

Results and discussions

Processing: Effect of MFA during mixing

The effect of MFA on mixing can be seen in the power trace as shown in figure 1. The major changes of power are due to ram-up and ram-down at the corresponding period of mixing cycle as in table 2. In the first ram-down (between A and B), the mixing power is used mainly to masticate rubber but also to incorporate zinc oxide, stearic acid and antioxidant. The next ram-down period (between B and C) follows the input of MFA and carbon black. The power increases as the carbon black is incorporated. The third ram-down period (between C and D) follows the input of accelerator at 240 s. It can be seen that the power is still increasing until it reaches a peak, which provides an indication of the carbon black incorporation time (BIT). The fourth ram-down (between D and E) follows the input of sulfur.

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The power trace illustrates the net effect of adding increasing levels of MFA to the rubber. It can be noticed that the control shows a lower level B-C and C-D, due to slower rate of CB incorporation and then it shows a high power consumption at the stable end of the mixing cycle when it is compared to the other mixes in presence of MFA. The MFA additions are speeding the incorporation stage and reducing the power needed at dispersion stage.

Curing tests at 160[degrees]C

The effect of MFA on curing characteristics is shown in table 3. Compounds with MFA illustrate a reduction of scorch time and curing time with addition of MFA. It should be realized that these tests are all carried out with the full normal amounts of curatives present. The control sample shows a longer time at 5% and 95% cure. The time for 5% and 95% curing decreases with the increasing addition of MFA. The difference between maximum and minimum torque (an indication of crosslink density) appears to show a small change which may account for the effect of the MFA on modulus. Thus, MFA is observed to enhance the curing of the compound, allowing scope for a considerable saving in the amount of accelerators needed.

Table 3 - curing characteristics of the compounds at 160[degrees]C

Properties                                  Mix
MFA content (phr)             0      0.3    1      2      3      5
Max. torque (Nm)           2.41   2.51   2.55   2.58   2.51   2.41
Min. torque (Nm)           0.64   0.69   0.66   0.65   0.61   0.53
Difference torque (Nm)     1.77   1.82   1.89   1.93   1.90   1.88
Time @ 5% cure(s)           40      34     29     27     26     19
Time @ 95% cure(s)          375    356    330    291    287    263

Capillary extrusion rheometer at 100[degrees]C

The effects of MFA on capillary flow properties are indicated in figures 2 and 3. The effect of MFA on apparent viscosity and melt fracture is substantial. The apparent viscosity falls rapidly with addition of MFA, especially at low shear rates. The plots of apparent viscosity tend to plateau at 2 phr of MFA, which approxmate calculations show would coincide with the presence of enough MFA to give a complete monolayer over the surface of the carbon black. The critical shear rate at which melt fracture of rubber compounds occurs increases with the addition of MFA. These effects will all be beneficial for production of extrusions.

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Effect of MFA on adhesion tests

A typical graph of stress against strain at 170[degrees]C is shown in figure 4. When a shear rate of 0.1 [s.sup.-1] is imposed upon the cured rubber compound, the stress does not immediately assume a constant value. at first there is a considerable buildup of stress, reaching a peak value and then subsiding to a plateau. This may be explained in terms of a build-up of elastic stress while the cured rubber adheres to the rotor, followed by a drop to a constant value after total debonding of the rubber. In figure 5 a plot of peak stress against MFA addition is shown. It can be observed that MFA acts as a mold release, with the improvements becoming marked for additions of 2 phr and more.

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Vulcanizate properties in presence of MFA

Mechanical properties of vulcanizates in presence of MFA

Tensile strength, tear strength, tensile modulus and abrasion resistance are all greater than the control with addition of MFA until an optimum level of 2 phr is reached when a further increase in MFA brings about a decrease in these properties. Enhancement of these properties are attributed to improvement in filler dispersion and an increase in crosslink density. It can be postulated that MFA functions as a filler-rubber wetting agent which gives a more efficient disagglomeration of carbon black during mixing and hence an improved dispersion. Above 2 phr of MFA, such as at 3 and 5 phr of MFA, the decrease in these physical properties is assigned tentatively to a weak interface between adjacent layers of MFA molecules. The formation of a level of 2 phr is identified as the critical amount to coat the carbon black with a monolayer of MFA.

The same trend is observed with the dynamic modulus as with the other mechanical properties. However, the energy loss or dynamic testing tends to show a minimum at about 1 phr with a subsequent rise to a high plateau value at about 3 phr. This result is independent of double strain amplitude. It has the useful consequence of giving low hysteresis which is still low at MFA levels where other mechanical properties are peaking.

Crosslink density by swelling measurement

As shown in figure 6, the plot of weight of uptake toluence per gram of rubber vulcanizate against the level of MFA, the values of toluene uptake do not change significantly with the increasing level of MFA. The difference between maximum and minimum value of toluence uptake after equilibrium is about 6% at constant swollen weight. Hence, the inclusion of MFA has no strong effect on crosslink density, so this is not a primary mode of action of MFA for causing property changes.

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Carbon black dispersion using SEM

Figure 7 belongs to a control sample and figure 8 belongs to a 2 phr of MFA. It shows that control mix displays many agglomerates of undispersed carbon black aggregates while an almost homogeneous microdispersion of carbon black aggregates can be seen on the surface of 2 phr of MFA. These micrographs confirmed that inclusion of MFA has improved carbon black dispersion.

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Conclusions

In the case of processability, the MFA tested has improved the mixing of natural rubber with carbon black by speeding the incorporation stage and reducing the power at the dispersion stage. In curing tests, it reduces the scorch and cure times of the rubber and causes a small increase in crosslink density, as measured by a curemeter. The use of MFA reduces the viscosity of the compounds and improves extrusion by increasing the shear rate prior to the onset of melt fracture. It can also give improvements in mold release.

In vulcanizates, the enhancement occurs for tensile strength, tear strength, tensile modulus, abrasion resistance and dynamic modulus while hysteresis shows a low value.

The improvements produced by the MFA are thought to be mainly due to improvements in carbon black dispersion with minor effects coming from crosslink density. The optimum mechanical properties occur at the MFA level which has been estimated to give a monolayer coverage of the filler surface.

References

(1.)Hepburn, C. and Mahdi, M.S. Plastics and Rubber Processing and Applications 6 (1986) 247-256, 257-265 and 267-271.

(2.)European Patent 0146579 B1 "A Process for Treating Rubber" 07.08.1991.