A novel route in preparing silica reinforced NR composite.

Rubber products are used in almost all areas of life due to their special properties. The high and reversible deformability of natural rubber is of great industrial importance. But since the initial modulus and durability of this polymer is very low, an additional reinforcement is required for practical uses (ref. 1). Physical properties of rubber vulcanizates are improved by reinforcing with fillers such as carbon black, clay, silica, etc. (refs. 2-6). For better reinforcement, there must be strong interaction between the polymer matrix and the filler, which depends on filler surface area, morphology and surface activity.

As carbon black can only be used in black articles, the use of silica has attracted a great deal of interest. However various contrivances are necessary for silica to combine with the rubber without hurting the performance and function. Silica has a number of hydroxyl groups on the surface, which result in strong filler-filler interaction and adsorption of polar materials by the hydrogen bonds (refs. 5 and 7). This property can result in poor dispersion of silica in rubber compounds. The adsorption of curatives by silica results in delay in curing of the silica filled rubber compound. The well known methods for improvement of the reinforcement effect are the treatment of the silica surface with suitable agents, such as a silane coupling agent like bis(3-tri ethoxy silyl)-propyl)-tetra sulfide (TESPT) (refs. 8-11), sol-gel method and in-situ precipitation.

We propose a novel route for precipitating silica in-situ in concentrated natural rubber latex followed by coagulation of the latex. The silica filled latex coagulate is mixed with block rubber by mill mixing so as to attain the desired concentration of silica in NR matrix. The mechanical properties of the vulcanizates prepared by this novel technique are compared with those prepared from commercially available nano silica (VN3) and conventional silica.

Experimental

Materials

Natural rubber latex of 60% dry rubber content was supplied by Njavallil Latex, Kochi, India. Natural rubber of grade ISNR-5 was obtained from the Rubber Research Institute of India, Kottayam, India, and other chemicals used were of commercial grade.

Precipitation of silica

Natural rubber latex was filtered and stabilized with a nonionic stabilizer, Vulcastab VL, mixed with required the amount of sodium silicate (60%), stirred for one hour and then kept overnight. A saturated solution of ammonium chloride was added dropwise with constant stirring to precipitate the silica. Latex was coagulated as crumbs by the addition of 2% acetic acid. The crumbs were washed with water until the washings were neutral, pressed to remove water and dried in an air oven at 70[degrees]C for 48 hours.

Preparation of the composite

The rubber with in-situ precipitated silica was kept as the master batch and was incorporated into ISNR-5 by mill mixing. Different quantities were added to get the desired concentrations of silica in NR. Mixing was done in a laboratory size (16 x 33 cm) two-roll mill at a friction ratio of 1:1.25 as per ASTM D 3184-80 and ASTM D 3182-82. The compounding formulation used in the present study is given in table 1. After complete mixing, the stock was passed six times through a tight nip and finally sheeted out at a fixed nip gap. The samples were kept overnight for maturation. Optimum cure time was determined using a rubber process analyzer. Blanks cut from unvulcanized sheets were vulcanized at a temperature of 150 [+ or -] 2[degrees]C and at a pressure of 200 kg/[cm.sup.2] in an electrically heated hydraulic press, to their respective optimum cure times.

Dumbbell shaped tensile and angle tear test specimens were punched out from the vulcanized sheets. The measurements were carried out at a crosshead speed of 500 mm per minute on a universal testing machine according to ASTM standards D 412-68 and D 624-54 respectively.

Silica filled rubber samples were kept at 550[degrees]C for two hours in a muffle furnace for burning off the rubber. The photographs of the ash were taken using a digital camera.

Bound rubber content and rubber filled interaction

Rubber compounds containing the same concentration of filler (latex precipitated silica, VN3 and conventional silica) were immersed in toluene for 24 hours and photographs were taken. The rubber compounds were also immersed in toluene for 24 hours for determining the bound rubber content. The bound rubber fraction was calculated from the insoluble rubber fraction using the equation (ref. 12):

Bound rubber fraction (wt. %) =

insoluble rubber fraction - silica in the sample/rubber quantity in the sample x 100

The strain sweep measurements on unvulcanized samples were conducted to study the rubber-filler interaction. These were done using the rubber process analyzer at 100[degrees]C and 0.1 Hz frequency by varying the strain from 0.5 to 100%.

Results and discussion

Figure 1 shows the photographs of a rubber sample containing in-situ precipitated silica before and after ignition in a muffle furnace. During ignition, the rubber particles were burned off leaving only silica. The pictures show that there exists a uniform distribution of filler network in the in-situ precipitated silica-containing sample. In the case of conventional silica and VN3, the samples crumpled on ignition, indicating that such a uniform distribution did not exist in the samples containing VN3 and conventional silica.

[FIGURE 1 OMITTED]

Figure 2 shows that the samples containing conventional silica and VN3 were dissolved completely when they were immersed in toluene. But the sample containing in-situ precipitated silica became a swollen mass. This shows that the filler-polymer interaction is present to a greater extent in the sample containing in-situ precipitated silica.

[FIGURE 2 OMITTED]

Table 2 shows that the bound rubber content increases with increase in concentration of in-situ silica, and approaches a maximum at a concentration of 20 phr silica. So the sample containing this concentration is considered a masterbatch.

Figure 3 shows the SEM pictures and figure 4 optical microscopic photographs of the precipitated silica and commercially available nano silica VN3. These pictures show that precipitated silica has a structure more viable to interactions with the polymer. The SEM pictures show that there is a reduction in particle size for precipitated silica compared to commercially available silica. Small particle size provides large surface area, which results in an improvement in the mechanical properties. Table 3 shows the mechanical properties of the latex precipitated silica reinforced natural rubber composite. The tensile strength of the composite is comparable to those reinforced with commercially available silica (VN3 and conventional silica). Tear strength measurements, as well as modulus values, also show the same behavior.

[FIGURES 3-4 OMITTED]

In the tensile fractured surfaces of rubber vulcanizates (figure 5), free silica particles are not seen in the compound containing in-situ precipitated silica, which indicates that filler to polymer interaction is present in this sample to a greater extent.

[FIGURE 5 OMITTED]

Figure 6 shows the variation of storage modulus on strain sweep for unvulcanized samples of precipitated silica reinforced NR composites. The contribution to G' in unvulcanized filled compounds arises from the gum polymer, a hydrodynamic effect, filler networking and polymer-filler linkages. The difference in G' values provides a means to exclude the contribution of the gum polymer and the hydrodynamic effect so that it gives a measure of the rubber-filler interaction. It is obvious from figures 7 and 8 for the VN3 grade silica and commercial silica that, in these cases, the interaction between the rubber and the filler is lower as compared to the latex precipitated silica. It is observed that the novel composite shows superior modulus and delta G values, indicating higher load carrying capacity and filler-rubber interaction.

[FIGURE 6 OMITTED]

Conclusions

The study shows that silica can be incorporated in NR latex by a precipitation procedure. The vulcanizates prepared by this novel technique show superior mechanical properties compared to those prepared from commercial silicas. The latex precipitated silica is found to have better rubber- filler interaction compared to commercial silicas.

References

(1.) Composites Science and Technology, 63, (2003), 1,647-1,654 (and references therein).

(2.) Sung-Seen Choi, et al, Polym. Adv. Technol., 15(3), pp. 122-127, 2004.

(3.) G.R. Cotten, Rubber Chem. Technol., 57-1, p. 118, 1984.

(4.) T.C. Gruber, Rubber Chem. Technol., 70-5, p. 727, 1997.

(5.) J.T. Byers, Rubber World, 218-6, p. 38, Sept. 1998.

(6.) S. Wolff, M-J. Wang, Rubber Chem. Technol., 65-2, p. 329, 1992.

(7.) Y-C. Ou, et al, Rubber Chem. Technol., 67-5, p. 834, 1994.

(8.) U. Gorl, paper no. 76 presented at the 151st meeting of the Rubber Division, ACS, (1996).

(9.) A.S. Hashim, et al, Rubber Chem. Technol., 71-2, p. 289, 1998.

(10.) E.P. Plueddemann, Silane Coupling Agents, Plenum, New York, 1982.

(11.) R.J. Pickwell, Rubber Chem. Technol., 56-1, p. 94, 1983.

(12.) K. Yoshikai, et al, J. Applied Poly. Sci., 85 (10), pp. 2,053-2,063, 2002.

K.S. Maya, Cochin University of Science and Technology, and Rani Joseph, K.K.T.M. Govt. College, Kerala, India

Table 1--compound
formulation

Ingredients        Amount
                    (phr)

Natural rubber        100
ZnO                     5
Stearic acid            2
Ethylene glycol         1
CBS                   0.6
TMTD                  0.2
Sulfur                2.5

Table 2--the bound
rubber content of
rubber samples

Concentration      Bound rubber
of silica (phr)    content (%)

       1               0.0
       2               0.0
       3               0.15
       5               9.86
      10              39.24
      15              63.45
      20              99.44

Table 3--results of mechanical analysis (reported
values are the average of at least six
measurements)

Sample          Amount of       Tensile             Tear          EB
                 silica         strength          strength        (%)
                  (phr)      (N/[mm.sup.2])    (N/[mm.sup.2])

Latex               1             30.5              40.2         545.5
precipitated        2             32.6              43.2         575.8
silica              3             33.4              46.1         600.4
                    5             35.8              48.5         631.4
                   10             38.6              54.2         754.5
                   15             41.1              64.5         820.1
VN3                 1             30.1              38.8         538.9
                    2             32.9              44.2         579.6
                    3             33.8              45.2         611.2
                    5             36.1              48.5         628.5
                   10             39.2              51.3         761.2
                   15             41.2              55.7         814.5
Commercial          1             29.7              40.1         512.3
                    2             30.1              44.3         548.8
                    3             31.1              44.9         622.1
                    5             33.5              49.6         630.1
                   10             36.6              52.2         715.3
                   15             40.2              55.9         809.5

Sample          Amount of       Modulus
                 silica      (N/[mm.sup.2])
                  (phr)

Latex               1             2.01
precipitated        2             2.19
silica              3             2.21
                    5             2.57
                   10             3.02
                   15             3.89
VN3                 1             1.95
                    2             2.19
                    3             2.43
                    5             2.66
                   10             2.88
                   15             2.93
Commercial          1             1.53
                    2             1.70
                    3             1.95
                    5             2.02
                   10             2.11
                   15             2.96