Electric in-situ measurement of vulcanization.

Optimum vulcanization condition as cure temperature and cure time is essential for robber in order to obtain good vulcanizates properties. Torque measurement is the most extensively used technique for estimating the degree of vulcanization. We propose a new technique to monitor the progress of vulcanization based on the measurement of pressure and electrical properties (refs. 1 and 2). In rubber-carbon black compounds, rubber and carbon black form homogeneous and dispersed phases, respectively, and the electrical properties, therefore, are dependent on the degree of carbon black dispersion in rubber (refs. 3-6). Moreover, it has been known that the vulcanizates reinforced by carbon black are always lower in electrical resistivity than the uncured compounds (ref. 3), and this fact agrees with our results (refs. 1 and 2). In order to use electrical properties as a means for monitoring vulcanization, it is necessary to distinguish factors as concentration and dispersion of carbon black, degree of vulcanization, etc., that are affecting electrical properties of rubber under vulcanization. It is the purpose of the present work to extend the research of previous work to rubber compounds having more simple formulations to understand the conductivity variation of rubber compounds during vulcanization.

Experiments

Materials and vulcanization

The NBR used is JSRN230S. Two types of NBR compounds were used. The one was mixed by a rubber mixing company for practical use. The formulation of the rubber compound is given in table 1.

Table 1 - rubber compound formulation for practical use

Ingredients                          phr

Rubber JSR 230S                      100
Carbon black (Seast S)                65
Filler clay                           41
Ca[CO.sub.3]                          32
DOP                                   15
ZnO                                    5
Stearic acid                           1
Antioxidant            810NA           1
                       224             1
Accelerator            DM            0.3
                       CZ              1
                       TT            0.5
Sulfur                                 1
Total                              263.8

Another compound with a very simple formulation shown in table 2 was prepared in our laboratory using an internal mixer having a capacity of 738 ml as follows. After masticating the NBR robber for about 10 minutes at below 80 [degrees] C, carbon black and the other ingredients were added. The temperature of the rubber was adjusted below 80 [degrees] C by water-cooling to prevent the vulcanization. After desired mixing time, the compound was removed from the mixer and immediately put into a polyethylene bag followed by soaking in ice-cold water to avoid the vulcanization. The compound was sheeted off from a 100 mm x 240 mm roll mill to give a sample of about 3 mm thick. The uncured mix was cut into an approximate square (about 5 cm on a side). The square sheet was then vulcanized for 20 minutes at 130~170 [degrees] C in a press mold to give a sheet 113.5 mm by 118.5 mm by 2 mm thick.

Table 2 - rubber compound with simple formulation

Ingredients                           phr

JSR N230S                             100
Carbon black (Seast 3)                 40
ZnO                                     5
Stearic acid                            1
Accelerator              DM             1
                         TT             1
                         CZ             1
Sulfur                                  1
Total                                 150

Mold

The mold used for electrical measurement during vulcanization is shown in figure 1. The electrodes were made from stainless steel and were insulated from its surroundings by polytetrafluoroethylene. The upper and lower electrodes have diameters of 35 mm and 40 mm, respectively. The size of cavity is 118.5 mm by 113.5 mm by 2.0 mm thick.

[Figure 1 ILLUSTRATION OMITTED]

Electrical measurement

The dielectric loss tangent (tan [Delta]) was measured by a tan meter, applying 120 volts of alternating current. The electrical current was measured by an ammeter applying 40 volts of alternating current.

Measurement of crosslink density

The mold was opened at appropriate cure time and the rubber sheet was taken out, followed by immersing it into iced-water to stop vulcanization. The crosslink density (v) was determined by swelling the rubber strip in toluene for 72 hours. Flory-Rehner's equation was used to calculate crosslinking density.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [V.sub.0] is the molar volume of solvent, [V.sub.r] is the volume fraction of rubber in the swollen rubber, and [micro] is a parameter characteristic of interaction between rubber and solvent.

Results and discussion

Vulcanization of rubber compounds having a formulation for practical use

We have shown that a good correlation exists between generally accepted cure curves of torque-cure time curves and electrical current, tan [Delta]-cure time curves (ref. 2). Figures 2 and 3 show electrical current-cure time curves and tan 8-cure time curves for rubber compounds with and without sulfur and accelerators. The formulation of the rubber compound was shown in table 1.

[Figures 2-3 ILLUSTRATION OMITTED]

A rubber compound with sulfur and accelerators showed a region of cure time where electrical current and tan [Delta] increased rapidly, and then they showed a plateau region where the increase of electrical current and tan [Delta] became nil. On the other hand, NBR compounds without sulfur and accelerators showed a monotonous increase of electrical current and tan [Delta] with cure time. Therefore, the region where the rapid increase is observed may be interpreted as being that the vulcanization is occurring vigorously.

Vulcanization of compounds having simple formulation

The conduction mechanism of electricity through rubber compounds during vulcanization is complicated because they contain many kinds of chemical reagents. The authors focused our attention on vulcanization reaction, and therefore, the electrical measurements were made for rubber compounds having only reagents required for vulcanization reaction to occur. The compound formulation is shown in table 2. Figures 4 and 5 show the tan [Delta]-cure time and electrical current-cure time curves, respectively. The shapes of the curves are similar to the torque-cure time curves generally used as cure curves. An increase in curing temperature caused (1) a decrease in the time and an increase in the slope of the curve at which rapid increase in tan [Delta] and electrical current occurs; (2) a decrease in the time at which tan [Delta] and electrical current level off; and (3) an increase in the values of tan [Delta] and electrical current. Generally, a moderate decrease in vulcanizate resistivity with rising temperature was found (ref. 6). Figures 6 and 7 show tan [Delta] and electrical current-cure time curves for the same compounds shown in table 2, except containing no carbon black.

[Figures 4-7 ILLUSTRATION OMITTED]

Compared to the curves of carbon black containing compounds at the same temperature, the level of the electrical current was lower. This is quite natural as carbon black is a semi-conductor. The time at which rapid increase in tan * or electrical current occurs was shifted to shorter time. The most significant point is that the distinct increase can be seen even in the curves of rubber compounds containing no carbon black. Of course, the electrical current of rubber compounds without vulcanization reagents as sulfur and accelerators is very low.

Effective crosslinking chain concentration

A more definitive way to follow the progress of vulcanization is to measure the variation of effective crosslink concentration (v). Figures 8 and 9 represent the cure time dependence of v and electrical current of NBR compounds with and without carbon black, respectively.

Two curves in each graph gave a good correlation. That is, the time region at which rapid increase in v is occurring or vigorous vulcanization is occurring, coincided with that for the electrical current curve. The time at which v levels off is considered that the vulcanization was almost completed, and this time also coincided with that for the electrical current curve. These suggest that the tan [Delta] and electrical current curves are very useful as a cure curve.

Mechanism of electric current conduction

In carbon black reinforced vulcanizates, semi-conductive carbon blacks were dispersed in insulator NBR and so the conductivity of rubber strongly depends on the dispersion state of carbon black. However, it is evident from figures 8 and 9 that electric current flows to some extent even through vulcanizates containing no carbon black, especially when the vulcanization is occurring. This phenomenon suggests that sulfur vulcanization itself may form a more conductive network, or ions or radicals formed from reactions between ZnO, accelerators, sulfur, polymer, etc., may play a role as electron carriers. If the intermediates are acting like this, electric current should decrease when the vulcanization is over because of the disappearance of the intermediates. This was observed in the case of NR and EPR rubber, but not observed in the case of NBR.

[Figures 8-9 ILLUSTRATION OMITTED]

Equivalent parallel circuit

When the alternating voltage is applied between the rubber compound, the dielectric loss will happen. The rubber compound may be expressed by a parallel circuit of resistance R and capacitance C. When alternating voltage of [E.sub.0]sin [Omega] t is applied, the electrical currents which flow throughout the C and R are [Omega] [CE.sub.0]sin([Omega] t + [Pi]/2) and [E0]sin [Omega] t/R, respectively. Therefore, the total current which flows through the circuit is expressed as follows.

(1) |I| = [E.sub.0][([[Omega].sup.2][C.sup.2] + [1/R.sup.2]).sup.1/2]

(2) tan [Delta] = 1/R [Omega] C

Tan [Delta] and [Delta] are called dielectric loss tangent and loss angle, respectively. In this study, tan [Delta] and electrical current were measured on 120 and 200 volts, respectively. If it is assumed that the tan [Delta] value does not change by this voltage difference, C and R are calculated by equations 1 and 2. Figure 10 shows the variation of C and R with cure time. With the increase of the cure time, C increased and R decreased. Both C and R became almost constant after a certain cure time. The mode of the variation of C and R corresponds to that of tan [Delta], electrical current or torque. When C and R are calculated from an equivalent series circuit of C and R, the value of R is almost the same obtained by parallel circuit, but the value of R is around [10.sup.-6]F (Farads), which is far different from the observed value. Therefore, the parallel circuit is adequate.

[Figure 10 ILLUSTRATION OMITTED]

References

(1.) Gondoh, T., Mori, T., Toh, T., Fujie, S. and Okai, D. (1996), Nippon Gomu Kyokaishi, 59 (10), 708.

(2.) Gondoh, T., Eto, S., Matsuoka, Y., Toh, M., Mori, T. and Okai, D. (1997), Kobunshi Ronbunshu, 54 (5), 359.

(3.) Boonstra, B.B. (1977), Rubber Chem. Technol., 50, 194.

(4.) Medalia, A.V. (1986), Rubber Chem. Technol, 59, 432.

(5.) Thompson, C.M. and Allen, J.S. (1994), Rubber Chem. Technol, 67, 107.

(6.) Polley, M.H. and Boonstra, B.B.T., (1961), Rubber Chem. Technol., 34, 119.