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Retarders

Vulcanization retarders comprise the final class of chemicals involved in rubber vulcanization. The purpose of these materials is to delay the initial onset of cure in order to guarantee sufficient time to process the unvulcanized rubber.

Three main classes of materials are used commercially. These include organic acids and anhydrides, cyclohexylthiophthalimide (Santogard PVI or CTP) and a proprietary sulfonamide material (Vulkalent E).

Examples of organic acid retarders include phthalic anhydride and benzoic and salicyclic acids. These materials are thought to function by reaction with basic components present as accelerator fragments, from other basic compounding ingredients and from impurities. These basic moieties, which would normally serve to accelerate vulcanization and produce a higher state of cure, are neutralized by the acid retarders. Therefore, they are effective in delaying the initial onset of cure. However, they also retard cure rate and in many cases, detract from the final performance properties. This retardation of cure rate and loss in performance is a high price to pay for improved scorch safety (fig. 8, ref. 16).

The thiophthalimide (CTP) and sulfonamide classes of retarders differ from the organic acid types by their ability to retard scorch, or onset of vulcanization, without significantly affecting cure rate or performance properties.

Much has been published on the mechanism of CTP retardation. It functions particularly well with sulfenamide accelerated diene polymers - typically those used in the tire industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with more sulfenamide to complete the vulcanization process. If the MBT initially formed could be removed as soon as it forms, vulcanization will not occur. It is the role of CTP to remove MBT as is forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior (fig. 9).

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfonamide.

Like CTP, this product is most effective in sulfenamide cure systems, but it also works well in thiazole systems. Performance properties are generally not affected except for a slight modulus increase. In some cases this feature allows for the use of lower levels of accelerator to achieve the desired modulus with the added potential benefits of further scorch delay and lower cost cure system. This is illustrated in the rheograph below for a thiazole (MBTS) accelerated nitrile rubber compound (fig. 10, ref. 18).

Note that both sulfur and MBTS levels can be reduced when 0.5 phr sulfonamide are added without any sacrifice in modulus level.

Crosslinking mechanisms

Accelerated sulfur cure systems

Accelerated sulfur curing is the most common technique for crosslinking elastomers. it is applicable for all diene containing elastomers including the common tire polymers: natural rubber. SBR and polybutadiene as well as for high volume non-tire elastomers such as nitrile rubber and EPDM. Cure systems for these elastomers typically contain examples of each of the vulcanization chemicals previously discussed:

  Vulcanizing agent} sulfur   0.25 - 5.0 phr
  One or more accelerators    0.2 - 5.0+ phr
  Activators} stearic acid    0.5 - 3.0 phr
                zinc oxide    1.0 - 10.0 phr

Retarders (as required for adequate scorch safety)

One mechanism proposed for these materials to effect vulcanization follows the scheme mentioned in the discussion on retarders; i.e. sulfenamide decomposition to form MBT and amine. Following are the reactions thought to be involved in this scheme. Here, BT refers to the benzothiazole structure.

(1) BT - S - NHR2 [right arrow] BTSH + HNR2

(sulfenamide) (MBT) (amine) (2) BT-S-NHR2 + BTSH [right arrow] BTS-SBT + HNR2

(sulfenamide) (MBT) (MBTS) (amine) (3) BTS-SBT + S8 [right arrow] BTS-Sx-SBT (4) BTS-Sx-SBT + - [CH = CH-CH2] - [right arrow] - [CH=CH-Sx-SBT) + BTSH

(diene rubber) (MBT) (5) BTS-Sx-C- + CH [right arrow] C - Sx-CH + BTSH

                                             (MBT)
           CH    CH              CH     CH
           CH    CH              CH      CH
            (rubber)             sulfur crosslinked rubber

The length of the sulfur crosslink (Sx) can be varied from 1 to > 20 depending upon the specific cure system chosen and the curing conditions (time/temperature) employed. Another schematic summarizing natural rubber vulcanization is shown in figure 11. This diagram clearly shows the possible crosslink types that can form (ref. 14).

Control of both type and amount of sulfur crosslinks significantly influences performance properties. How this is done will be discussed in the section on cure system design.

Peroxide curing

Unlike sulfur vulcanization which works only with a diene or double bond containing polymer, peroxide curing is effective with most (but not all) elastomers. Peroxide curing provides thermally stable carbon-carbon crosslinks, and these impart superior heat resistance, resistance to permanent set and better creep and stress relaxation properties. However, the short C-C crosslinks are not as "flexible" as the labile sulfur crosslinks and this results in poorer fatigue properties as well as poor hot tear and tensile strength.

Peroxides suitable for rubber crosslinking must be active enough to provide practical cure cycles but not so active or volatile as to be difficult to control or unsafe to handle. Typical examples of commercial peroxides are shown in table 6 (ref. 19).

[TABULAR DATA OMITTED]

The mechanism proposed for peroxide curing is a three step process involving thermal decomposition. The general scheme for this mechanism follows:

Initiation: R-O-O-R [right arrow] 2R-O.

(Peroxide) (Free radical)

Propagation: R-O. + -CH-C-C- [right arrow] ROH + -C-C-C-

                (Elastomer chain)
Termination:  -C-C-C- + -C-C-C   -C-C-C-
                                 -C-C-C-
                        (Crosslinked elastomer)

Vulcanization rate is controlled by temperature and choice of peroxide used. Peroxide activity is measured in terms of decomposition half life as function of temperature, and these data are available from peroxide suppliers. Typical half life curves for two commercial peroxides are shown in figure 12 (ref. 20).

Clearly, peroxide "A" will crosslink rubber faster than "B" because "A's" reaction rate is faster at any given cure temperature.

Caution must be taken to avoid compounding ingredients which can reduce peroxide efficiency by reacting with the free radicals formed before they can form crosslinks. Examples include acidic materials such as certain nonblack fillers and fatty acids and antioxidants which function as free radical scavengers, thereby reducing peroxide cure efficiency.

Polymers which contain a tertiary carbon structure in the backbone should also be avoided because peroxides tend to effect chain scission rather than crosslinking. Examples are butyl rubber and epichlorohydrin polymers.

Resin cure mechanism

Certain difunctional phenolic resins are capable of forming thermally stable bridges between polymer chain segments thereby serving as crosslinkers (formula 1, ref. 21 ).

The first step is an acid catalyzed condensation to liberate water and form an active =CH2 group which is capable of further reacting with the allylic position of the polymer backbone to form a rubber-resin intermediate. The reaction repeats in a similar manner to form the final crosslink.

Metal oxide crosslinking

This technique is commonly used for the commercially important polychloroprene (neoprene) class of elastomer. Zinc oxide is commonly used as the crosslinking agent in this scheme. The reaction involves the chlorine at the allylic position according to formula 2 (ref. 22).

Sulfur containing materials such as ethylene thiourea (ETU) can also be used effectively to increase cure rate and improve physical properties by the mechanism shown in formula 3 (ref. 22).

Caution should be exercised in using ETU, however, because of suspected toxicological concerns.

Michael A. Fath is president and founder of Elastomer Technology, Inc., a consulting company specializing in commercial development and marketing. Prior to forming ETI, he worked in the rubber industry for 29 years with BFGoodrich, Goodyear, Monsanto and Polysar.