Select Accelerators for Rubbers

Function of Accelerators

Accelerators are materials that are added in small amounts to speed up the curing of adhesives. The first accelerators were used in the 19th century. In that period, mostly oxides and hydroxides of inorganic compounds like lead, zinc, magnesium and calcium were brought to use. These days organic compounds are majorly used as accelerators.

Accelerators have made elastomers particularly latex systems, viable adhesives by reducing their cure time and temperature. Accelerators have also been found to be beneficial as a crosslinking agent in influencing final performance properties.

Definition of Accelerators

An accelerator is a material that, when mixed with a catalyst or a resin, speeds the chemical reaction between the catalyst and the resin. Therefore, accelerators are not usually employed alone, but they are used within a cure package.

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

Usually, the accelerator does not contribute directly to changes in the final physical or chemical properties of the formulation. However, it could indirectly affect properties by controlling the speed and order of side reactions in the polymerization process. Accelerators will also 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.

The term "accelerator" is very appropriate in that it succinctly describes the function of the additive. However, there is often confusion resulting from misuse or overuse of the term.

An accelerator has the following unique properties.

It is used along with a catalyst, curing agent or hardener to increase the rate of reaction, to lower the polymerization temperature, or to improve the efficiency of the reaction.
It does not become part of the final molecule and does not 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.
It is used in small concentrations, and the concentration of accelerator to other ingredients in the formula is usually of no consequence.
Accelerators can be completely consumed by the polymerization reaction or there may be an accelerator leftover depending on the formulation and polymerization conditions.

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

Term	Definition
Accelerator	An accelerator is a material that, when mixed with a catalyst and resin, speeds up the chemical reaction between the catalyst and the resin (usually in the polymerizing of resin or vulcanization of rubbers). Accelerators are also known as promoters when used with polyester resins and vulcanizing agents when used with rubbers.
Inhibitor, retarder	An inhibitor or retarder is sometimes incorporated into an adhesive formulation to de- accelerate the curing rate.
Activator	Activators (e.g., zinc oxide and stearic acid) are used to help initiate the cure.
Curing agent, hardener, vulcanizing agent	A curing agent is a substance added to an adhesive to promote the curing reaction by taking part in it. These affect cure by chemically combining with the base resin and becoming part of the final polymer molecule. They are specifically chosen to react with a certain resin. Curing agents will have a significant effect on the curing characteristics and on the ultimate properties of the adhesive system. Concentrations can be large and variations in concentration can be used to adjust properties.
Catalysts	Catalysts remain unchanged in the curing reaction, causing the primary resin to crosslink and solidify. Acids, bases, salts, sulfur compounds and peroxides are commonly used. Only small quantities are usually required to influence curing. Unlike hardeners, the amount of catalyst used is critical, and poor bond strengths can result when resins are over or under catalyzed.

Definitions of Some Additives that Control Curing Mechanism in Polymeric Systems

Classification of Accelerators for Rubbers

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 (normally stearic acid).

The most popular accelerators are delayed-action sulfenamides, thiazoles, thiuram sulfides, dithocarbamates and guanidines.

Part or all of the sulfur may be replaced by an accelerator that is also a sulfur donor such as a thiuram disulfide. The accelerator determines the rate of vulcanization, whereas the accelerator to sulfur ratio dictates the efficiency of vulcanization and, in turn, the thermal stability of the resulting vulcanizate.

Certain elastomers such as chloroprene can be vulcanized by the action of metal oxides such as zinc oxide as well as sulfur. As a result, several of the same accelerators that are used with sulfur vulcanization systems can be used with zinc oxide/neoprene systems. Because there are so many, accelerators are generally classified by chemical family.

Accelerators are also classified as being primary or secondary.

Primary accelerators are relatively slow and have a delayed onset of cure. These are used primarily to build physical properties.
Secondary accelerators are fast and mainly used to affect the cure rate.

The table below provides a summary of the major accelerator families and specific types within a family.

Family	Common Types	General Characteristics
Amines and aldehyde-amines	Cyclohexylethylamine Hexamethylene tetramine (HMT) Ethyldiene aniline (EA) Butyraldehyde dianiline (BA) condensation product	They are the condensation products of aldehydes and amines. Their accelerating effect is majorly decided by the aldehyde type, and a mole ratio of aldehyde Á amine used in the reaction mixture. Hexamethylene tetramine is an inexpensive secondary accelerator. Primary accelerator for natural and synthetic rubbers. Used as secondary accelerators with dithiocarbamates, and in combination with sulfenamides and thiazoles. Cyclohexylethylamine has a strong secondary acceleration effect with dithiocarbamates for adhesives solutions and zinc n-ethylphenyl dithiocarbamate is used in combination with cyclohexylethylamine. Some amines are used as activators for active silica fillers.
Guanidines	Diphenyl guanidine DPG Di-o-tolyl guanidine (DOTG) DOTG Triphenyl guanidine (TPG)	Less important as individual accelerators because of lower vulcanizing speed. Important secondary accelerators for thiazoles and thiurams. Can be used as an activator of sulfenamides to increase the reaction rate. Slow development of crosslink density. Fatty acids have a retarding effect. Synergistic with thiazoles, sulfenamides, and thiurams. Presence of ZnO is necessary.
Thiurams	Tetramethyl thiuram disulfide (TMTD) Tetraethyl thiuram disulfide (TETD) Tetramethyl thiuram monosulfide (TMTM) Dipentamethylene thiuram tetrasulfate (DPTS) Dipentaethylene thiuram (DPTT)	Faster onset of cure and more complete cure than thiazoles. Especially good for isoprene and chloroprene. At high dosages (2-5%) can eliminate sulfur. Good corrosion resistance to metals. Good heat resistance in vulcanizate. Used as a secondary accelerator with dithiocarbamates, sulfenamides, or thiazole. Higher molecular weight have slower cure rates. Rate of cure: TMTD>TETD>DPTT>TMTM
Dithiocarbamates	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	Delayed action. Zinc compounds are used most frequently. Low-temperature vulcanization in the shortest time. Used in lattices and solution cements. Primary accelerator for natural and synthetic rubbers. Higher molecular weights have slower cure rates.
Thiazoles (mercapto compounds)	2-Mercaptobenzothiazole (MBT) 2,2’-Dithiobenzothiazole (MBTS) Sodium salt of MBT 2,4-dinitrophenyl mercaptobezothiazole (DMB) Zinc mercaptobenzothiazole (ZMBT) 2-morpholinochiobenzothiaxole (MBS)	Zinc salt (ZMBT) is used mainly with latex compounds. Relatively long activation time. Good resistance to aging. Used alone or in combination with others (premixed products are available). Very fast with basic accelerators such as HMT. All combinations are synergistic. In CR, MBT and MBTS act as retarders. Rate of cure: MBT>ZMBT >MBTS
Sulfenamides	N-cyclohexyl benzothiazole-2-sulfenamide (CBS) N-1-butylbenzothiazole-2-sulfenamide (TBBS) N-dicyclohexylbenzothiazole-2-sulfenamice (DCBS)	Largest class of accelerator in terms of quantity and value. Delayed onset of cure. Sulfenamides are generally used alone, but the rate can be increased by secondary accelerators (e.g., thiurams). Higher molecular rate generally gives slower cure rates. Provide good resistance to reversion. Limited shelf life (1 yr) under controlled conditions. Rate of cure: CBS>TBBS
Thioureas	Ethylenethiourea (ETU) Diethlyenethiourea (DETU) Diphenylthiourea (DPTU)	Mainly used during vulcanization of chloroprene rubbers. High crosslink density when used with zinc oxide. Used mainly for vulcanization of chloroprene and EPDM. Thiazoles can be used as retarders. Use is decreasing because of health concerns (dust is an issue). Primary accelerator with polychloroprene and secondary with NR, EPDM, and SBR. Cure rate: ETU>DETU>DPTU
Dithiophosphates	Zinc dithiophosphate	They are generally used with other accelerators. They show an optimum accelerating effect when used in combination with thiazoles. Fast cure rates. Higher molecular weight reduces the cure rate. Used mainly in crosslinking of EPDM. Used in combination with thiazoles, sulfenamides and dithiocarbamates.
Xanthates	Zinc isopropyl xanthate (ZIX) Sodium isopropyl xanthate (SIX) Zinc butyl xanthate (ZBX)	Similar to dithiocarbamates. Sometimes used for self-vulcanizing adhesives solutions and lattices.

Common Types & General Characteristics of Accelerator Families

Selecting the Right Vulcanizing Agents

The chemistry of rubber cure or rubber vulcanization is complex. There are several rubber vulcanization systems possible based on reactions with different chemicals. The selection of an accelerator will depend on the specific vulcanizing system.

Sulfur vulcanization processes are the most common, but peroxide and metal oxide systems are also used in the adhesives industry.

Check out the vulcanization systems generally used with common elastomers below.

Elastomer	Sulfur	Zinc oxide	Peroxide	Phenolic	Diamines	Diisocyanates
Polyurethane					✔	✔
Polychloroprene	✔	✔	✔
Natural rubber	✔		✔
Styrene butadiene copolymers	✔		✔
Silicone			✔
Ethylene propylene diene monomer	✔		✔
Acrylonitrile-butadiene copolymer	✔		✔
Butyl rubber	✔			✔
Isoprene	✔
Polysulfide					✔

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 vulcanization are based on sulfur. While sulfur alone will cure unsaturated rubbers on heating, the process is slow and inefficient. The mechanisms of sulfur curing are not well understood, but 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 usually complex organic compounds, often of proprietary composition. They include:

Sulfur-containing compounds such as thiocarbamates, thiazoles, sulfenamides, and thiuram sulfide
A few non-sulfur types 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.

Optimizing Properties for Rubber Vulcanization

It is the responsibility of the formulator to optimize the desired properties for the finished adhesive. This can be done by manipulating the levels of crosslinkers, activators, and accelerators. Typical vulcanization systems for several sulfur-cured elastomers are provided in the table below. Generally, the optimum components and concentrations are determined by trial and error.

In rubber, an accelerator to sulfur ratio typically of 1:5 is called 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.

	Natural rubber	Styrene butadiene copolymers	Acrylonitrile-butadiene copolymer	Butyl rubber	Ethylene propylene diene monomer
Sulfur	2.5	2.0	1.5	2.0	1.5
Zinc oxide	5.0	5.0	5.0	3.0	5.0
Stearic acid	2.0	2.0	1.0	2.0	1.0
Sulfenamide (CBS)	0.6	1.0
Thiazole (MBTS)			1.5	0.5
Thiazole (MBT)					1.5
Thiuram (TMTD)				1.0	0.5
Dithiocarbamate (ZDBC)					1.5

Vulcanization Systems for Several Sulfur-Cured Elastomers

Solvent-borne Vulcanizable Natural Rubber Adhesive Formulation

Vulcanized adhesives are usually supplied in two parts that are mixed together at the time of use. One part contains sulfur and no accelerator, and the other part contains accelerator and no sulfur. The mixed adhesives are stable for about 8 hrs 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.

Component	Ingredients	Parts by Weight
Part A	Natural rubber	10
	Zinc oxide	1
	Antioxidant	0.1
	Sulfur	0.1
	Solvent	80
Part B	Dithiocarbamate accelerator (10% solution)	4

Starting Formulation of a Solvent-Borne Vulcanizable Natural Rubber Adhesive

Checklist to Select an Accelerator

Selecting an accelerator system is one of the most difficult problems in compounding. A number of factors must be considered while selecting an accelerator. Selection of the proper accelerator and the amount to be used for any application depends on:

The resin
The temperature at which the resin is to be cured
Properties such as working life, shelf life, etc.

With the choices narrowed down to several products, questions of purely an economic or practical nature will probably determine the final choice of an accelerator. Sometimes no single accelerator is available that can meet all the requirements. Therefore, a combination of accelerators or catalysts and accelerators must be used to obtain the optimum results.

Since accelerators are used at a very small weight or volume percentage in a formulation, losses should be avoided in mixing. These losses could be due to dusting and escape of the accelerator if the accelerator is solid. For even dispersion in the formulation, the accelerator should be preferably in a powder or liquid form.

Accelerators also should be added last in the formulation. This helps to avoid the loss of the accelerator due to premature reactions with the system's chemistry. Important compounding or handling properties include the following:

Free-flowing for automatic weighing and bulk handling systems
No caking in transit and storage
Supply forms need to resist abrasion and break-up
Particle uniformity to prevent separation of blends in bulk storage
Dust-free to meet industrial hygiene standards
Improved processing and dispersion to shorten mixing cycles
High purity to target specific reaction sites
High activity to prevent undesirable side effects
Shelf-life stability

The list below offers a questionnaire that may be useful for selecting an accelerator or in discussing the formulator's needs with the accelerator supplier.

☑ What are the base polymers, curing agents, catalysts and other components in the adhesive or sealant formulation?

☑ What commercial accelerators have been commonly used with the base polymer?

☑ What is the solubility of the accelerator in the adhesive or sealant system?

☑ What form is required for the accelerator (solid or liquid)? How is it to be incorporated into the formulation?

☑ What is the main reaction process (heat cure, moisture cure, UV cure, etc.) and what are the various processing stages?

☑ What are the curing conditions required (temperature, time, etc.) and what is the expected variance in production?

☑ Is the product to be a one-component or a two-component system?

☑ What is the expected shelf life of the final formulation?

☑ What is the expected working life (or pot life) of the final formulation?

☑ What is the known safety or health hazards for formulating the catalyst into an adhesive or sealant system and what are the known hazards to the end-user in applying the system?

☑ What are possible adverse effects on other properties of the adhesive or sealant formulation (uncured as well as cured)?

Selecting the Right Accelerator for Your System

Selection According to Your Elastomer

When a compounder sets out to develop a new curing system, he or she proceeds in two steps:

A base system is selected which will work with the base polymer chemistry and provide a level of performance requirements.
It will be necessary to refine that system to meet both the curing characteristics needed and the final physical properties.

The table below will help you select the accelerator according to the chosen elastomer.

Accelerator Family	Primary or Secondary Accelerator	Cure Rate	Onset of Cure	Crosslinking Density	Water Solubility	Form	Recommended Elastomer
Aldehyde- amines and amines	Secondary	Slow to medium	Short	Medium	Soluble	Liquid	Polyurethane Polychloroprene Polysulfide
Guanidines	Secondary	Slow to medium	Moderate	Small to medium	Slightly soluble	Solid	Polyurethane Polychloroprene SBC
Thiurams	Secondary	Very fast	Short	High	Generally insoluble	Solid	Polychloroprene EPDM ACN Isoprene
Dithiocarbamates	Both	Ultra-fast	Very short	High	Generally insoluble	Liquid and solid	Polychloroprene Natural Rubber SBC EPDM Isoprene
Thiazoles	Both	Moderate	Moderate	High	Generally insoluble	Solid	Natural Rubber SBC EPDM Isoprene
Sulfenamides	Primary	Fast	Long	Moderate to High	Generally Insoluble	Solid	Natural Rubber SBC EPDM Butyl Rubber
Thioureas	Secondary	Ultra-fast	Short	High	Soluble	Solid and Liquid	Polychloroprene Natural Rubber EPDM
Xanthates	Secondary	Slow to Medium	Short	Medium	Soluble	Liquid	Polychloroprene Natural Rubber SBC EPDM Isoprene

Accelerator Families & Properties Recommended for Certain Elastomers

Selection According to Curing Properties of your System

The selection of an accelerator family will also depend on:

The various cure properties (cure rate, speed of cure onset, crosslink density)
The convenience of compounding (solubility and form) that is provided by the accelerator.

These parameters are summarized for the main accelerator families in the table below.

Accelerator Family	Primary or Secondary Accelerator	Cure Rate	Onset of Cure	Crosslinking Density	Water Solubility	Form
Aldehyde-amines and amines	Secondary	Slow to medium	Short	Medium	Soluble	Liquid
Guanidines	Secondary	Slow to medium	Moderate	Small to medium	Slightly soluble	Solid
Thiurams	Secondary	Very fast	Short	High	Generally insoluble	Solid
Dithiocarbamates	Both	Ultra-fast	Very short	High	Generally insoluble	Solid and Liquid
Thiazoles	Both	Moderate	Moderate	High	Generally insoluble	Solid
Sulfenamides	Primary	Fast	Long	Moderate to High	Generally Insoluble	Solid
Thioureas	Secondary	Ultra-fast	Short	High	Soluble	Solid and Liquid
Dithiophosphates	Both	Fast	Short	High	Insoluble	Liquid
Xanthates	Secondary	Ultra-fast	Short	High	Soluble	Solid

Property Parameters for Accelerator Families

Find Suitable Accelerator Grade

View a wide range of accelerator grades available in the market today, analyze technical data of each product, get technical assistance or request samples.

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