Accelerators for rubbers: How to select the right grade?

Accelerators are chemical additives that, when mixed with a catalyst or a resin, speed the chemical reaction between the catalyst and the resin. This usually happens during the polymerization of resin or the vulcanization of rubber. Accelerators are also known as:

• Promoters, when used with polyester resins
• Vulcanizing agents, when used with rubbers

Therefore, accelerators are not usually employed alone, but are used within a cure package.

The accelerator itself does not usually become a part of the final molecular structure. However, it may cause physical or chemical changes in the reactant molecules that increase the speed at which they react. Its concentration may or may not be fully consumed by the reaction.

Accelerators are often confused with curing agents, hardeners, and catalysts.

Role of accelerators in the curing process

Properties of accelerators

An accelerator has the following unique properties.

• Function: It is used along with a catalyst, curing agent, or hardener to increase the rate of reaction, lower the polymerization temperature, or improve the efficiency of the reaction.
• Effect on final properties: It does not become a part of the final molecule. Neither does it directly affect the final chemical or physical properties of the formulation. However, it may indirectly affect these properties by controlling the rate or order of competing reaction that occurs within the polymerizing system.
• Concentration: It is used in small concentrations. The concentration of the accelerator relative to other ingredients in the formula is usually of no consequence.
• Consumption during curing: Accelerators can be completely consumed by the polymerization reaction, or there may be an accelerator leftover. This depends on the formulation and polymerization conditions.

Now that we know the fundamentals of accelerators, let's move on to their types to see how they are classified. 

Elemental sulfur is the predominant vulcanizing agent for general-purpose rubbers. It is used in combination with one or more accelerators and an activator system comprising zinc oxide and a fatty acid (usually stearic acid). Depending on the function in the cure system, we can broadly segregate accelerators into two types:

• Primary accelerators: They are the main curing agents in sulfur vulcanization systems, used to build physical properties. They are responsible for controlling the onset and progression of crosslinking. They are relatively slow and have a delayed onset of cure. Thus, allowing sufficient time for mixing and shaping before vulcanization begins. Examples of primary accelerators include sulfenamides and thiazoles.

   

• Secondary accelerators: They are used in combination with primary accelerators to enhance the rate and efficiency of vulcanization. They are generally faster-acting and help increase crosslink density, improve cure speed, and modify the final properties of the rubber compound. Examples include thiurams, dithiocarbamates, and guanidines.

The accelerator determines the rate of vulcanization, whereas the accelerator-to-sulfur ratio controls the efficiency of vulcanization. This, in turn, affects the thermal stability of the resulting vulcanizate. In some systems, part or all of the sulfur may be replaced by an accelerator that is also a sulfur donor, such as a thiuram disulfide.

Certain elastomers, such as chloroprene, can be vulcanized by the action of metal oxides such as zinc oxide and sulfur. As a result, several of the same accelerators that are used with sulfur vulcanization systems can be used with zinc oxide/neoprene systems.

Rubber accelerators are generally classified by their chemical families. Let's have a detailed look at each of these families in the section below.

Amines and aldehyde-amines

They are the condensation products of aldehydes and amines. Their accelerating effect is mainly decided by the aldehyde type, and the mole ratio of aldehyde and amine used in the reaction mixture.

They are the primary accelerators for natural and synthetic rubbers. They can also be used as secondary accelerators with dithiocarbamates and in combination with sulfenamides and thiazoles. Hexamethylene tetramine is an inexpensive secondary accelerator.

Cyclohexylethylamine has a strong secondary acceleration effect with dithiocarbamates for adhesive solutions. Zinc n-ethylphenyl dithiocarbamate is used in combination with cyclohexylethylamine. Some amines are used as activators for active silica fillers. 

Common examples: Cyclohexylethylamine, Hexamethylene tetramine (HMT), Ethylenediamine (EA), Butyraldehyde dianiline (BA) condensation product.

Guanidines

Guanidines are less important as individual accelerators due to their slower vulcanizing speed. They are important secondary accelerators for thiazoles and thiurams. They can be used as an activator of sulfenamides to increase the reaction rate.

They exhibit slow development of crosslink density. Fatty acids have a retarding effect. They have a synergistic relationship with thiazoles, sulfenamides, and thiurams. The presence of ZnO is necessary for guanidines.

Common examples: Diphenyl guanidine (DPG), Di-o-tolyl guanidine (DOTG), Triphenyl guanidine (TPG).

Thiurams

Thiurams exhibit a faster onset of cure and a more complete cure than thiazoles. They are especially good for isoprene and chloroprene. At high dosages (2-5%), thiurams can eliminate sulfur.

They offer good corrosion resistance to metals and good heat resistance in vulcanizate. They are used as a secondary accelerator with dithiocarbamates, sulfenamides, or thiazole. Higher molecular weight thiurams have slower cure rates. The cure rate in thiurams follows the order: 

TMTD > TETD > DPTT > TMTM

Common examples: Tetramethyl thiuram disulfide (TMTD), Tetraethyl thiuram disulfide (TETD), Tetramethyl thiuram monosulfide (TMTM), Dipentamethylene thiuram tetrasulfate (DPTS), Dipentaethylene thiuram (DPTT).

Dithiocarbamates

Dithiocarbamates show delayed action. Zinc compounds are used most frequently. They undergo low-temperature vulcanization in the shortest time. They are used in lattices and solution cements. Dithiocarbamates are primary accelerators for natural and synthetic rubbers. Higher molecular weights have slower cure rates.

Common examples: Zinc diethyl dithiocarbamate (ZDEC), N-dimethyl dithiocarbamate (ZDMC), Zinc n-dibutyl dithiocarbamate (ZDBC), Piperdiene pentamethylene dithiocarbamate (PPD), Sodium diethyl dithiocarbamate (SDC), Zinc ethyl phenyl dithiocarbamate.

Thiazoles (mercapto compounds)

Thiazoles have a relatively long activation time and good aging resistance. They are used alone or in combination with others (premixed products are available). Among them, zinc salt (ZMBT) is used mainly with latex compounds.

Thiazoles can be used alone or in combination with other accelerators. Their curing activity can be increased in the presence of basic accelerators such as HMT, with combinations often showing strong synergistic effects. However, in chloroprene rubber systems, MBT and MBTS act as retarders instead of accelerators, slowing down the cure process. The relative cure rate in thiazoles follows the order: 

MBT > ZMBT > MBTS

Common examples: 2-Mercaptobenzothiazole (MBT), 2,2'-dithiobenzothiazole (MBTS), Sodium salt of MBT, 2,4-dinitrophenyl mercaptobenzothiazole (DMB), Zinc mercaptobenzothiazole (ZMBT), 2-morpholinothiobenzothiazole (MBS).

Sulfenamides

Sulfenamides are the largest class of accelerators in terms of quantity and value, showing a delayed onset of cure. Sulfenamides are generally used alone, but the rate can be increased by secondary accelerators (e.g., thiurams).

In general, higher molecular rates of sulfenamides generally give slower cure rates. They provide good resistance to reversion and limited shelf life (1 year) under controlled conditions. The rate of cure follows the order: 

CBS > TBBS

Common examples: N-cyclohexyl benzothiazole-2-sulfenamide (CBS), N-1-butylbenzothiazole-2-sulfenamide (TBBS), N-dicyclohexylbenzothiazole-2-sulfenamide (DCBS).

Thioureas

Thioureas are primarily used in the vulcanization of chloroprene rubbers. When used in combination with zinc oxide, they enable the formation of high crosslink density. Though mainly used in chloroprene rubbers, they also find application in the EPDM and, to a lesser extent, in NR and SBR systems as secondary accelerators.

In chloroprene formulations, thioureas function as primary accelerators, whereas in other elastomers they serve a secondary role. Thiazoles may be incorporated as retarders to control the cure rate in such systems. However, the use of thioureas is gradually declining due to health and safety concerns, particularly related to dust exposure. The relative cure rate follows the order: 

ETU > DETU > DPTU

Common examples: Ethylenethiourea (ETU), Diethylenethiourea (DETU), Diphenylthiourea (DPTU).

Dithiophosphates

Dithiophosphates are generally used with other accelerators. They show an optimum accelerating effect when used in combination with thiazoles. They provide fast cure rates and improved crosslinking efficiency. However, higher molecular weight dithiophosphates tend to reduce the cure rate.

These accelerators are primarily employed in the crosslinking of EPDM rubber. They are commonly used alongside thiazoles, sulfenamides, and dithiocarbamates to create synergistic effects and improve overall performance in complex rubber formulations.

Common examples: Zinc dithiophosphate.

Xanthates

Xanthates are accelerators chemically similar to dithiocarbamates, offering fast curing in rubber formulations. They are sometimes used for self-vulcanizing adhesive solutions and lattices.

Common examples: Zinc isopropyl xanthate (ZIX), Sodium isopropyl xanthate (SIX), Zinc butyl xanthate (ZBX).

Not sure which accelerator chemistry fits your formulation? Use the “chemical family” filter on our platform to quickly narrow down your options.

With a clear understanding of accelerator families and their characteristics, let's look at how they drive the vulcanization process at a chemical level.

Natural rubber and many synthetic rubbers contain unsaturated molecules (i.e., molecules that contain double bonds, providing sites for the vulcanization or crosslinking reaction). It is through these double bonds that vulcanization occurs.

The most common curing systems for rubber adhesive vulcanization are based on sulfur. However, peroxide and metal oxide systems are also used in the adhesives industry. While sulfur alone will cure unsaturated rubbers on heating, the process is slow and inefficient.

The mechanisms of sulfur curing are not well understood. However, it is thought to include, among other things, the formation of sulfide or disulfide links between chains and the abstraction of protons from adjacent chains, with the chains crosslinking at the remaining unshared electrons.

To speed the vulcanization process, accelerators are used. These are typically complex, often proprietary organic compounds. They include:

• Sulfur-containing compounds such as thiocarbamates, thiazoles, sulfenamides, and thiuram sulfide
• Non-sulfur compounds such as phenols, guanidines, and amines

Except for the fact that accelerators contribute to vulcanization, little is known about their specific action in speeding up vulcanization. Check out the vulcanization systems generally used with common elastomers below.

Vulcanization systems used with common elastomers

Formulation for sulfur-cured elastomers

Formulators optimize the desired properties of the finished adhesive. This can be done by manipulating the levels of crosslinkers, activators, and accelerators. Generally, the optimum components and concentrations are determined by trial and error.

In natural rubbers, an accelerator-to-sulfur ratio of 1:5 is referred to as a conventional vulcanizing system. It gives a crosslinked network. The same principles apply to synthetic rubbers, although the optimum accelerator-to-sulfur ratio may not be the same as in natural rubber.

Typical vulcanization systems for several sulfur-cured elastomers are provided in the table below.

Formulation for solvent-borne natural rubbers

Vulcanized adhesives are usually supplied in two parts that are mixed at the time of use.

• One part contains sulfur and no accelerator
• The other part contains an accelerator and no sulfur

The mixed adhesives are stable for about 8 hours after mixing and cure completely in about 2 weeks. The table below provides an example of a starting formulation for a solvent-borne vulcanizable natural rubber adhesive using dithiocarbamate as an accelerator. It is used for bonding leather, fabric, paper, and elastomers.
 

Advantages

• Accelerators make elastomers, mainly latex systems, viable adhesives by reducing their cure time and temperature.
• They are beneficial as a crosslinking agent in influencing final performance properties.
• They affect practical, rate-related properties such as storage life, working life, gel time, set time, and so forth. Therefore, accelerators should be used in adhesive or sealant formulation only when they are needed to control the cure rate and after their full effect on the final properties is determined.

After looking at the role of accelerators in the vulcanization process, let's get to the final section, which outlines the considerations and guidelines used to select the right accelerator for a given formulation and application.