Zinc soaps for improved vulcanizates.

The use of zinc carboxylates as a multifunctional additive for both processing and curing effects has been described (ref. 1). The structure of the organic acid and concentration of the zinc soap was shown to influence how the material performed. Of interest was the ability of certain structural classes to change vulcanization efficiencies. Because of the historical and commercial importance of natural rubber, it was of special interest to note that zinc soaps influenced both the cure and reversion (ref. 2). The most effective organic zinc structure studied for improving natural rubber vulcanizates was based on the use of an aromatic acid. This article will further explore this class of compounds and will examine some of the compounding and curing options that occur with the use of zinc containing fatty chemicals.

Experimental

The mixing was done in a laboratory size 1,600 cc internal mixer at fill factors of about 70% with commercial grade polymers and additives. The cure kinetics were obtained from an Alpha Technologies MDR 2000. The injection molding studies were run on a Desma 966 machine. The aryl zinc soap is available as Activator (or Aktivator) 73 from Struktol Company of America or Schill and Seilacher GmbH & Co., Germany.

Results and discussion

The curing of rubber is a complicated issue of both heat transfer and curing reactions. The parameters in heat transfer involve thermal conductivity, density, specific heats and the possibility of heats of reaction. The curing reaction kinetics and activation energies will determine the state of cure throughout the part. For curing, both the rate of formation of crosslinks and degradation of crosslinks (through reversion) are important. Reversion is marked by losses in physical properties on overcure. The cure curve for both reverting and nonreverting systems can be modeled based on rate constants for creation and destruction of crosslinks. The constants are determined from assuming first order kinetics for formation of strong crosslinks, formation of weak crosslinks and disruption of weak crosslinks and Arrhenius relationships were shown between 140-170 [degrees] C (ref. 3). The composition, shape and thickness of the part being cured will affect the efficiency of the curing process. For example, models using numerical algorithms have been described that simulate the trends of temperature and state of cure during curing a truck tire (ref. 4). For this type of profile, temperatures of 168 [degrees] C for the bladder, 140 [degrees] C for the mold and cure times of 50 minutes were assumed. For the standard type NR compound with a sulfonamide cure, these temperatures and times would probably result in an overcure situation for the material in contact with the bladder, and a possible overcure for the mold surface.

To understand the cure kinetics and reversion effects, cure temperatures ranging from 150 to 200 [degrees] C were studied. The test compound contained 2 phr stearic acid, and the zinc soap was added up to 10 pier. It was found that Arrhenius effects could be extended to include temperatures of up to 200 [degrees] C (ref. 3). The zinc soap gave linear first order curing and reversion kinetics (figures 1 and 2). The cure kinetics and activation energies were determined by the rheometer software. For reversion, they were manually plotted from the rheometer chart. The activation energies were determined to be affected by the concentration (table 1). The activation energy for the cure was similar to that for the weak crosslink formation, but for the reversion was slightly less (ref. 3). These values are most likely compound dependent. In normal measurement of reversion, the time to a certain loss in torque is normally reported. These results (table 2) show the effect of temperature on the time to reach a 3% torque drop. The zinc soap can significantly decrease the loss of torque due to overcure.

[Figures 1 and 2 ILLUSTRATION OMITTED]

Table 1 - values of activation energies

Phr     Cure [joules/mole]    Reversion [joules/mole]

 0           10.1 E 4                 12.1 E 4
 3           10.6 E 4                 13.0 E 4
10           11.7 E 4                 16.2 E 4
Table 2 - time in min. to three percent reversion

                   phr of aryl zinc soap
Temperature [C]      0       3       10

160                8.57    20.14     >30
180                2.16     3.57    7.43
200                0.57     0.87    1.57

Because the formation and destruction of crosslinks occurs at different rates and activation energies, the physical properties obtained were studied as a function of both cure time and temperature. The tan delta (figure 3) shows how the aryl zinc soap can produce a more efficient network, especially at higher temperatures. The cured stocks show how the modulus changes with cure time. The 300% modulus at 150 [degrees] C showed that the addition of aryl zinc soap gave a slightly higher state of cure. At 175 [degrees] C the differential between the modulus was even greater and the modulus for the control was decreasing at a fast rate (figure 4a). An injection molding process was used to study the short cure times above 180 [degrees] C. Although the modulus was shifted from the compression molding, the relative differences between the control and the aryl zinc soap efficiency was still present (figure 4b). The difference in hardness (figure 5) was not sensitive to molding methods, but showed significant losses for the control at high temperatures.

[Figures 3, 4 and 5 ILLUSTRATION OMITTED]

Curing thicker sections shows how properties of the rubber can be altered by the effect of overcure (table 3). Compression set decreased at each cure time. The aryl zinc soap gives lower sets than the control. A larger specimen showed both the effect of hardness loss for the control stock, and the effect of cure on heat generation from hystersis. The aryl zinc compound maintained its hardness when compared to the standard stress strain sheet hardness, and showed no change in heat generation at extensive overcure.

Table 3 - thick section curing - vibration isolator

                   Originals - cured three minutes at
                           175 [degrees] C
                   [T.sub.90]     Hardness       Modulus
Control               2.2           64             10.2
4 phr aryl Zn         2.5           67             12.1
soap

                   Compression set - 22 hrs. at 70 [degrees] C
                           (12. 7mm disc)
                Control   4 phr aryl   Control   4 phr aryl
                           Zn soap                Zn soap
(*)Cure time        4          4           6          6
   (min.)
Compression      34.4         32        25.1       19.6
   set

                   Compression set - 22 hrs. at 70 [degrees] C
                           (12. 7mm disc)
                Control   4 phr aryl
                           Zn soap
(*)Cure time        8          8
   (min.)
Compression      23.2       16.8
   set

                   Firestone Flexometer (2.5cm x 3. 75cm x 5cm)
                Control   4 phr aryl   Control   4 phraryl
                           Zn soap                Zn soap
(*)Cure time       15         15          20         20
  (min.)
Hardness           58         65          58         65
Heat buildup      127        126         138        129
  (C)

                   Firestone Flexometer (2.5cm x 3. 75cm x 5cm)
                Control 4 phr aryl
                         Zn soap
(*)Cure time       25       25
  (min.)
Hardness           58       65
Heat buildup      142      127
  (C)

(*) Cure temp - 175 [degrees] C

Specialty cure systems have been developed to influence the thermal stability of the crosslink. These systems have used different materials to influence the kinetics of the vulcanization process to compensate for the loss of network from reversion. With the equilibrium cure system, the rate of crosslinking of a tetra-sulfur silane, Si69 (Degussa) occurs at about the same rate as the conventional reversion rate (ref. 5). A controlled olefin addition reaction with formation of carbon-carbon bonds occurs with Perkalink 900 (ref. 6) (Flexsys). The crosslink compensation occurs at high temperatures where the rate of reversion is high. Duralink HTS (Flexsys) gives postvulcanization stabilization by a hybrid crosslinking mechanism which produces shorter sulfur attachments with a flexible chain within the crosslink (ref. 7). Liquid polysulfide polymers, such as the Thiokol LP series (Morton International), are also able to form reactive crosslinking short chains through accelerator peptization of the sulfurs in the polymer chain. These reactive intermediates can then form modified crosslinks (ref. 8).

The aryl zinc soap is thought to improve the vulcanization by making the conventional vulcanization pathway more efficient (ref. 1). The aryl zinc soap combined with the specialty cure systems should show greatly enhanced thermal stability because of the combinations of mechanisms occurring. For the equilibrium cure, enhanced cure properties were obtained at lower levels of coupling agent (table 4). The postvulcanization stabilization package is enhanced with improvements in tear and dynamic properties at reduced accelerator levels (table 5). Where crosslink compensation is being used, balanced or enhanced reversion, set and severe hystersis properties can be achieved. With these specialty cure systems, concentration studies would be recommended to obtain the best balance of cost and performance.

Table 4 - equilibrium cure comparing Si69 and aryl zinc soap in a solid tire (NR, 50 phr mineral filler, sulfenamide - DPG cure)

                                         A          B

(*) Si 69 (pier)                        4.75        3
Aryl zinc soap (pier)                   0           3.5
[T.sub.90] (150 [degrees] C)           22.8       20.2
Cure time @ 150 [degrees] C (min.)    40   120   40   120
Elongation (%)                       630   630  640   640
300% modulus (mpa)                   7.2   6.9  7.4   7.1

    Firestone Flexometer (cured 120 min. @ 150 [degrees] C)
Heat build-up (C)                       138         132

(*) Degussa

Table 5 - post vulcanization stabilization of a
shock absorber (NR/BR blend)

(*) Duralink HTS           1.5          0        1.5
Aryl zinc soap             0            3        3
Accelerator                0.8          0.6      0.6
Reversion                 78           73      108
(see, [T.sub.max]-5%
 200 [degrees] C)

                          Tear (N/mm)
T90 @ 150 [degrees]
  C (min.)                39           33       46
T90 @ 200 [degrees]
  C (min.)                16.7         29.3     26.1
5 x [T.sub.90] (min.)      8.8          9.4     11.3

                          Goodrich, Set

(2MPA, 75' @ 100
  [degrees] C,)           Blow out     34%      39%

(*) Flexsys

The ability of the aryl zinc soap to produce a nonreverting system, and improved retention of physical properties over a range of cure temperatures has been shown to be a viable route to increase productivity and performance. For example, factory experience with a heavy duty pneumatic tire confirmed that higher cure temperatures could be utilized to decrease mold time and increase output without sacrifice in physical properties. Other applications that have shown field acceptance include mechanical goods, such as pads and bearings, fenders, solid tires and mineral filled compounds. In addition to the cure phenomena, the processibility of the stocks is improved with Mooney viscosity decreases of up to 10 points at 3 phr of aryl zinc soap being common.

Because the zinc soaps affect so many properties, a concentration profile should be run to determine what level is required. For the standard cure systems, most responses seen are linear to concentration. However, because the cure efficiency is improved, adjustments to the cure package might have to be made. In some cases, the zinc soap has been used to replace part or all of the stearic acid. Normal usage levels of the zinc soaps are generally between 2 and 5 pier.

Conclusion

The use of an aryl zinc soap with conventional and modified sulfur cures in NR is an easy way to improve curing and reversion. The use of standard first order kinetics has been shown to extend to elevated cure temperatures and overcure times. The sulfur network can be made thermally more stable by the use of the aryl zinc soap. This means that many of the issues associated with the reversion experienced with conventional NR systems can be addressed. This allows for the use of higher mold temperatures to decrease the cure time and extensive overcures without the loss of physical properties. This results in improved output and performance.

References

(1.) J. Vander Kooi, Rubber and Plastics News, p. 17, May 23, 1994.

(2.) A.D. Roberts, ed., Natural Rubber Science and Technology, Oxford Science Publications, NY, 1988.

(3.) G. Rimondi, et al., Tire Science and Technology, 24, 77-91, 1996.

(4.) I.S. Han, et al., Tire Science and Technology, 24, 50-76, 1996.

(5.) Degussa literature, Si 69, reinforcing agent, DEG0096.

(6.) Flexsys literature, Perkalink 900, reversion resistance by crosslink compensation.

(7.) Flexsys literature, W.F. Helt et. al., Post vulcanization stabilization for natural rubber, presented at NEO, April 16, 1991.