Utilization of powdered natural rubber vulcanizate as a filler.

Waste rubber disposal remains a threat to society although several approaches to recycling have been practiced from very early days (refs. 1 and 2). Utilization of powdered waste rubber in fresh polymer compounds seems to be an effective approach since it is environmentaly friendly (refs. 3-5). But the commonly used powdering methods like ambient grinding and cryogenic grinding provide coarser rubber particles which raise stress concentrations in the polymer matrix (refs. 6 and 7). Furthermore, the smooth particle surface obtained by the cryogenic grinding results in poor adhesion between the matrix and the rubber particles, which in turn necessitates expensive modification steps such as surface functionalization and devulcanization (refs. 8-10).

It has been reported by Jacob et al that EPDM vulcanizate powder obtained by an abrasion technique could be used as a low reinforcing filler in EPDM compounds (refs. 11 and 12) and also in NR and SBR matrices (ref. 13). The mechanism of abrasion has been studied by several workers (refs. 14-16). Since the abrasion technique is a single step process and the powder (EPDM) obtained by this technique is ultra fine (2-100 [micro]m), having the desirable rough and convoluted surface, the economic incentive is expected to be favorable. But it is reported that the chances of degradation, especially due to the heat build-up during wear is maximum for natural rubber (ref. 14). In this article, we report the results of studies on the effect of NR vulcanizate powder, prepared by the abrasion technique, on the properties of a typical NR compound. Here the vulcanizate powder is chosen as the model waste rubber.

Experimental

Materials used

The NR used for this study was ISNR-5 obtained from the Rubber Research Institute of India. It has the following characteristics: Dirt content, 0.05%; volatile matter, 1.0%; ash content, 0.6%; and a plasticity retention index (PRI) of 60. Other ingredients are rubber grade chemicals purchased from local market.

Preparation of NR vulcanizate powder

The formulation used for preparing the model waste vulcanizate is given in table 1. It is based on a typical NR truck tire tread compound. Mixing was done on a two-roll mill as per the standard procedure.

Curing characteristics of the compound were determined using a moving die rheometer. Thick sheets (~10 mm) were compression molded according to the optimum cure time obtained from the rheograph, at 150[degrees]C under 5 MPa pressure. Extra curing time of five minutes was provided due to the thickness of the specimen. The samples were aged at 70[degrees]C for 24 hours in order to simulate the long storage conditions of factory scraps or the service conditions of the post consumer products. The aged samples were then subjected to abrasion against an abrasive wheel of silicon carbide rotating at 2,950 rpm. The powder was collected in a holder placed below the wheel.

Thin sheets (formulation same as in table 1) were compression molded according to the optimum cure time obtained from the rheograph. Stress-strain properties of the precursor vulcanizate used for powder preparation, determined as per the ASTM standards, are as follows: Tensile strength, 22.0 MPa; elongation at break, 593%; modulus at 100% elongation, 2.60 MPa: and tear strength, 60.2 kN/m.

Particle size of the powdered rubber

The particles obtained by the grinding method are highly aggregated, and hence separation by sieving was not successful. In order to study the particle size and shape, the powder was spread over a mount and sputter coated with gold and examined under a scanning electron microscope.

The particle size range and distribution were determined by a light optical microscopy (LOM) ultrasonic technique, at a magnification of 200x. Particles were suspended in hexane, subjected to ultrasonic dispersion. Images of representative areas were transmitted to an on-line automated image analysis system. The individual particles were identified and the sizes measured. The data were subsequently transferred to an Excel spreadsheet, and the respective histograms were obtained.

XPS analyses of the precursor thick sheet and the powder (W-NR)

XPS analyses were done using a VG Escalab 220iXL. This system has a monochromatic Al K [alpha] x-ray source (1,486.6 eV) that can be focused to a spot size of 0.6 mmx 0.3 mm. Operating pressure was 2 x [10.sup.-9] mbar. Experiments were done for representative powdered samples, as well as for the corresponding precursor vulcanizates. All spectra were referenced to the Cls peak for carbon, which was assigned a value of 285.0 eV. Peaks in the spectra were deconvoluted using a standard Guassian multiple peak analyzer program and the concentration of the elements was calculated from their peak areas and standard photo-ionization cross section.

Preparation of powdered rubber filled NR compounds

The formulations used are given in table 2. Mixing was done on a two-roll mill as per the standard procedure.

Determination of Mooney viscosity and scorch time

Mooney viscosity of the compounds ML (1+4) was determined by using a Mooney viscometer at 100[degrees]C as per ASTM D1646-96a. The time for five units rise over the minimum value was taken as the Mooney scorch time.

Determination of cure characteristics

Cure characteristics of the compounds were determined using a moving die rheometer at 150[degrees]C.

Molding and determination of mechanical properties

Molding of the compounds was done using a hydraulic press at 150[degrees]C under 5 MPa pressure according to the cure time obtained from the rheograph.

Dumbbells and un-nicked angle specimens were punched out from the molded sheets. Tensile testing was done as per ASTM D412-98a at a crosshead speed of 500 mm/min. Tear strength was determined on angle specimens as per ASTM D624-98. Tension set at 100% elongation was determined on the dumbbell specimen as per ASTM D412-98a. Hardness was determined using durometer A as per ASTM D2240-97.

Results and discussion

Characterization of the powdered rubber

An SEM photomicrograph of a single particle of W-NR is given in figure 1. The particle is irregular in shape.

[FIGURE 1 OMITTED]

The particle size range obtained by the LOM-ultrasonic technique is 2.0 to 153 [micro]m, the average particle size being 56 [micro]m. Particle size distribution is as given in figure 2.

[FIGURE 2 OMITTED]

XPS analysis of the precursor sheet (used for making W-NR) shows a peak for Cls at 285.0 eV (atomic %, 99.8) which corresponds to the polymer backbone (figure 3). The Ols peak at 532.0 eV (atomic %, 0.184) may be contributed by the compounding ingredients present in the compound. In addition to the peaks in the spectra of the precursor sheet, extra peaks are observed in the spectrum of W-NR at 288.5 eV (atomic %, 0.414) and 533.4 eV (atomic %, 0.255) for Cls and Ols respectively. The peaks correspond to that of C=O groups. This indicates that mild oxidation of the particle surface occurs during the grinding or abrasion of the precursor sheet, which in turn may enhance the aggregate formation.

[FIGURE 3 OMITTED]

Mooney viscosity and curing characteristics of the powdered rubber-filled compounds

The variation in Mooney viscosity by the incorporation of the W-NR is negligible (table 3). But the decrease in scorch time is drastic by the addition of even 20 phr of the W-NR. This is attributed to the migration of accelerators from W-NR to the matrix rubber as is observed in the case of EPDM compounds (ref. 12) and other rubbers (refs. 17 and 18). The curing characteristics are summarized in table 3. The [M.sub.L] value (the minimum torque in the rheograph) shows negligible changes as is observed in the case of Mooney viscosity. But the [M.sub.H] value (maximum torque) decreases slightly with increase in W-NR loading. This can be ascribed to the migration of sulfur from the matrix rubber to W-NR owing to the concentration gradient of free sulfur in the two phases (refs. 19-21). The optimum cure time decreases, and the cure rate increases with the incorporation of W-NR, which is believed to be due to the accelerator migration, as discussed earlier.

Stress-strain properties

The stress-strain properties of W-NR filled vulcanizates are given in table 4. Tensile strength and low strain modulus remain constant even at a loading of 50 phr of the powdered rubber, while there is only a marginal decrease in elongation at break. There is a marginal increase in high strain modulus, hardness and tear strength on incorporation of the powdered rubber, while the tension set is negligible in all compositions.

The results obtained reveal the potential of using powdered rubber waste as a filler in virgin rubber compounds without adversely affecting the properties of the final compound. Previously, it has been reported that there is considerable deterioration in properties of the rubber compound containing ground rubber particles, obtained either by the cryo-grinding or ambient grinding techniques (refs. 22-24), which are believed to provide coarser particles (>150 [micro]m). Contrary to the observations made earlier, improvements in properties observed in the present study can be attributed to the ultrafine particle size (<100 [micro]m).

Conclusions

The abrasion technique can be used for obtaining ultra fine powder (~50 [micro]m).

The vulcanizate powder can be used as a cheap filler in virgin rubber compounds without adversely affecting the final vulcanizate properties.

The process has potential in utilization of waste rubber products.

Table 1 - formulation used

Ingredients           Phr

NR                    100
ISAF carbon black      50
Aromatic oil          4.0
Antioxidant TQ        1.0
ZnO                   4.0
Stearic acid          2.0
CBS                   1.0
Sulfur                2.0

Table 2 - formulation used for the study

                        Mix designation

Ingredients    [N.sub.0]   [N.sub.20]   [N.sub.50]

NR             100         100          100
W-NR *         0           20           50
ZnO            4.0         4.0          4.0
Stearic acid   2.0         2.0          2.0
CBS            1.75        1.75         1.75
Sulfur         1.75        1.75         1.75

* Powdered NR vulcanizate

Table 3 - Mooney viscosity and curing
characteristics of the compounds

                                              Mix designation

Property                           [N.sub.0]   [N.sub.20]   [N.sub.50]

ML(1+4)100[degrees]C                      10           10           12
Mooney scorch time, [t.sub.5],
  min.                                   103           49        27.5
[M.sub.L], dNm                          0.18         0.16         0.23
[M.sub.H], dNm                          6.82         6.70         6.33
[M.sub.H] - [M.sub.L], dNm              6.64         6.54         6.10
Optimum cure time, t90, min.             8.6          4.2          3.1
Cure rate index, % [min..sup.-1]        34.3         71.0         80.0

Table 4 - mechanical properties of the
vulcanizates

                                           Mix designation

Property                          [N.sub.0]   [N.sub.20]   [N.sub.50]

Tensile strength, MPa                  20.8         20.1         20.1
Elongation at break, %                  988          926          868
Modulus at 100% elongation, MPa        0.85         0.80         0.81
Modulus at 300% elongation, MPa        2.09         2.22         2.46
Hardness, durometer A                    35           36           37
Tear strength, kN/m                    29.8         30.9         31.7
Tension set, %                            0            0            0

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Ceni Jacob, P.P. De and S.K. De, Indian Institute of Technology