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Quality rubber accelerators.

Rubber has become an indispensable part of the modem world. Whenever machines and engines require the use of bearings, whenever forces are transmitted and liquids are transferred, whenever rotating shafts and containers must be sealed, there is no getting around this material.

Generally, rubber can be cured with sulfur for manufacturing tires or general rubber goods. The chemical reaction between sulfur and the rubber chain is an extremely slow and inefficient process. It was estimated that this reaction takes around six hours at 140[degrees]C, which is uneconomical by any production standards. Rubber articles made through this process are extremely prone to oxidative degradation and do not possess adequate mechanical properties for practical rubber applications. Additionally, by adding a low content of sulfur, the rubber becomes soft; by adding a high amount of it, the rubber becomes hard. These limitations were overcome through the development of accelerators. They increase the speed of the chemical reaction so that a rubber article can be produced, e.g., in 10 minutes at 170[degrees]C.

The function of an accelerator is first to activate the sulfur, i.e., to open up the ring-shaped molecule (S8) and form precursors with the S-atoms. The precursor then transfers the sulfur, together with the attached accelerator residue, to the rubber molecule. The reaction of this pendent group with a further rubber molecule chain and the splitting-off of the accelerator residue cause the actual crosslinking.

Therefore, the interaction between sulfur and accelerator plays an important role in the rubber industry. This article investigates both a conventional high-sulfur cure natural rubber (NR) based system and a low-sulfur cure NR system (semi-efficient vulcanizing system), and includes an evaluation of the influence of the cure system on network properties.

Generally, the sulfenamide class accelerators are most popular in the rubber industry due to their delayed action, as well as faster cure rate offered by them during vulcanization of rubber compounds containing carbon black. For the investigation, two sulfenamide accelerators have been chosen, including N-cyclo-hexyl-2-benzothiazolsulfenamid (CBS from Lanxess called Vulkacit CZ), and N,N-dicyclohexyl-2-benzothiazolsulfenamid (DCBS from Lanxess called Vulkacit DZ). They are compared to a reference NR compound containing only sulfur. A general truck tire compound formulation has been used.

As shown in figure 1, the vulcanizate of NR with sulfur alone shows a very long cure time at 150[degrees]C. Similar behavior can be observed by using a lower level of sulfur, as shown in figure 2. Additionally, the vulcanization plateau is not reached after 40 minutes.

By adding CBS or DCBS to the NR compound, the scorch and cure time is reduced rapidly in both cure systems. The CBS curve shows a shorter scorch time compared to DCBS. An increase in dosage of CBS or DCBS shows improvement in scorch delay, cure rate and state of cure, as compared in figures 1 and 2.

Generally, both curves rise steeply until the onset point. After the maximum torque has been reached, the curves show a decrease (figure 1). This part of the curve is an indication of degradation of the compound resulting in breakdown of crosslinks (reversion). In figure 2, both curves show a good vulcanization plateau (stable curve after the maximum of torque), resulting in better resistance to aging and better compression set behavior.

The CBS compound shows a higher torque compared to DCBS. The torque value is an indication of the crosslink density. DCBS shows, therefore, a lower crosslink density (also seen in figure 4) which is due to the chemical structure of DCBS. Its reaction time is also longer compared to CBS.

It is known that sulfur vulcanization gives predominantly long polysulfidic crosslinks. It is obvious that long sulfur bridges tend to break, i.e., to re-crosslink and revert, and therefore change the physical properties of a vulcanizate.

A sulfur plus accelerators system has, therefore, two basic characteristics, including the kinetics of the network formation and the stability of the network produced

The chemistry of sulfur vulcanization and the changes in crosslinking structure of NR vulcanizates are described by Chapman and Porter (ref. 1). Short sulfur crosslinks provide high temperature stability (ref. 2), but insufficient tear and dynamic properties. As an example: In tire applications, polysulfidic crosslinks are most preferred due to their outstanding tear and dynamic properties.

The influence of the CBS/sulfur ratio has been analyzed. In figure 3, it can be seen that the ratio has a big influence on the crosslinks in the vulcanizate. Used at the same ratio of CBS/ sulfur, the vulcanizates show with increased dosage more monosulfidic crosslinks and less polysulfidic crosslinks (systems 1-3). With more free sulfur and less CBS (system 4), the vulcanizate has a high amount of polysulfidic crosslinks, as expected.

Additionally, a comparison of the development of crosslink density (XLD) and distribution of crosslink structure (XLS) with a CBS and DCBS based NR compound was analyzed. The data are measured at different points t75, t90, t100, t120 and t150. The 2.5 phr CBS and 3.3 phr DCBS have been taken to have an equal molar concentration.

The results are shown in figure 4. The distribution of the crosslink structure changes through time, resulting in the change of crosslinks from mainly polysulfidic crosslinks to more mono- and di-sulfidic. A CBS containing NR compound builds more mono- and disulfidic crosslinks through time compared to a DCBS based compound. As shown, the accelerated sulfur vulcanization produces different sulfur containing network structures. Additionally, the ratio of sulfur and accelerator has a huge influence on the properties of a vulcanizate.

CBS and DCBS are only two accelerators of the sulfenamide class. There are many other accelerators available for the vulcanization of rubber which show a wide variety of properties in vulcanization.

Lanxess is a global supplier of industrial chemicals and supports the rubber processing industries with a range of products and technical expertise. Accelerators, antidegradants and mastication agents are offered for rubber articles such as conveyor and transmission belts, seals, hoses and latex articles.

By Melanie Wiedemeier-Jarad and Hermann-Josef Weidenhaupt, Lanxess

References

(1.) A. V. Chapman and M. Porter, Natural Rubber Science and Technology, A.D. Roberts, Ed., Oxford University Press, Oxford, 1988, pp. 511-620.

(2.) W.F. Helt, B.H. To and W.W. Paris, Rubber World, 1991, 18.

Caption: Figure 1--vulcanization of NR at 150[degrees]C, 2.5 phr sulfur and 0.6 phr accelerator

Caption: Figure 2--vulcanization of NR at 150[degrees]C, 1.5 phr sulfur and 1.5 phr accelerator

Caption: Figure 3--influence of accelerator/sulfur ratio (NR compound at 150[degrees]C)

Caption: Figure 4--development of XLD and XLS with CBS and DCBS
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Author:Wiedemeier-Jarad, Melanie; Weidenhaupt, Hermann-Josef
Publication:Rubber World
Date:Aug 1, 2017
Words:1102
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