Rubber accelerators had the output of 331thousand tons in 2016,accounting for 30% of the total output of rubber chemicals ,and were the second largest category of rubber chemicals,among them M influential dominance because M is the main material for many rubber chemicals.
1.Rubber accelerator concept
Vulcanizing accelerator is abbreviated as accelerator. The rubber is blended into a small amount of accelerator can greatly promote the reaction between rubber and vulcanizing agent (crosslinking agent).Vulcanization accelerator is a kind of complexing agent which can accelerate the rate of vulcanization reaction, shorten the curing time, reduce the sulfidation reaction temperature, reduce the dosage of vulcanizing agent, improve the physical and mechanical properties of sulfur adhesive.The same accelerator on the promotion of different rubber vulcanization is also different as the different types of rubber.So the user should choose reasonable vulcanization system.
1.Effect of rubber accelerator
①The accelerator can shorten the curing time, reduce the amount of sulfur, reduce the curing temperature, thereby improving the production efficiency and reducing the cost .
②Accelerators can improve scorch performance of rubber compound and play a decisive influence on the scorch time of rubber compounds. Accelerators with high curing speed and long scorch time are selected to ensure that the rubber compounds will not be prematurely vulcanized(Scorch) during normal processing (such as mixing, calendering and extruding) , to improve production efficiency and improve the safety of the processing.In the actual formulation of rubber products, it is seldom used alone as a accelerator, but in various combinations to control the rate of vulcanization and the scorch time.
③The accelerator makes the vulcanization of the rubber material smooth and not easy to over-sulfur, which is beneficial to improve the physical and mechanical properties of the rubber products especially the follow-up products.
④Improve the appearance quality of products.
On the one hand due to the reduced amount of sulfur will not produce sulfur spray phenomenon; the other hand, the curing temperature is reduced and the curing time is shorter, so the use of organic colorants make rubber products keep bright color.Choosing the accelerators must be non-toxic non-polluting.
⑤The accelerator should be well dispersed in the rubber：Attention should be paid to the liquid or low melting point accelerator in the mixer mixing process, which may cause the slip of the rubber, resulting in uneven dispersion or reduced production efficiency.
2.Accelerators by the rate of vulcanization classification
①Ultra speed accelerator：Dithiocarbamates、Xanthates Sulfur
②High speed accelerator：Thiurams
③Quasi high speed accelerator:Thiazoles、Sulfenamides
④Medium speed accelerator:Guanidine
⑤Slow speed accelerator:Aldehydes and amines、Thioureas
3.Accelerators by acid-base classification
①Acid accelerator :Thiazoles、Thiurams、Dithiocarbamates、Xanthates Sulfur
②Basic accelerator ：Guanidine、Aldehydes and amines
③Neutral accelerator ：Sulfenamides、Thioureas
4.Accelerators by chemicals structure classification
①Thiazoles:Acid accelerator.Thiazoles accelerator is characterized by fast curing; flat curing curve;good comprehensive properties of vulcanized rubber;good anti-aging properties;a wide range of application;suitable for natural rubber and synthetic rubber ;fit to match with acid carbon black ;attention should be paid to the danger of scorch when matching with furnace carbon black.;No pollution, can be used as light colored rubber products;a little bitter, should not be used in the food industry.The combination of thiazoles accelerators with Thiurams、Dithiocarbamates and Thioureas has good anti-coke function.Moreover it with basic accelerator constitute activation Thiazoles vulcanization system ,which shorten the curing time,improve the curing speed and the performance of rubber.The representative products are M、DM、MZ
②Sulfenamides：Sulfenamides have the same promoter base with thiazoles,but have one more anti-coke base and activation base than thiazoles. The promoter is acidic and the activating group is alkaline.Therefore the sulfenamides is a kind of accelerator for acid-base self combination.It has the advantages of thiazoles and overcomes the shortcoming of short scorch time. Its characteristics are as follows: (1) long induction period, fast curing, the curing curve is flat, good comprehensive properties of vulcanized rubber; (2) combined with furnace black have sufficient safety and sufficient liquidity for extrusion, rolling and molding compound;( 3)suitable for high temperature vulcanization and thick vulcanized products; (4)combined with acidic accelerator can form an activation sulfenamide vulcanization system.
In general,the duration of the Sulfenamides induction period is affected by the size of
the group attached to the amine group.The larger the group,the longer the induction period ,the better anti-scorching effect.
③Thiurams：It is also one of the widely used categories in the rubber industry.Commonly used varieties are as follows:TMTM,TMTM,TETD,TRA.It can be seen from the structural formulas that the common thiurams contains two active groups and two promoting groups.Therefore thiurams’ curing speed is fast,scorch time is short and it is a speeding acid promoter.Application should pay special attention to scorch tendency. When Z is larger than 2 in thiuram, it contains sulfide groups, sulfide precipitate active sulfur atoms, participate in the cross-linking reaction can make the rubber vulcanization. Therefore, TMTD, TETD and TRA can be used as vulcanizing agents for sulfur free vulcanization, preparation of heat-resistant adhesives or for high temperature vulcanization occasions.
④Dithiocarbamates：It can be seen from the structural formula that it is more active than Thiurams . Besides the active group and the same base, it contains a transition metal ion, which makes the unsaturated double bond of rubber more easy to polarize and faster than the latter. It is a kind of super high speed acid accelerator. It has a short induction period, is used for room temperature vulcanization and vulcanization of latex products, and is also used for vulcanization of low unsaturation rubber such as EPDM, which endows vulcanizate with excellent aging-resistant performance.
⑤Guanidine：It is medium slow alkaline accelerator，has the maximum dosage in the alkaline accelerator and Widely used in natural rubber and various synthetic adhesives. Commonly used are DPG, DOTG, its structure is characterized by active base, no promoter base and other functional groups, slow, safe operation, but slower curing rate. It is suitable for vulcanization of thick products such as cot, and improves the stretching stress of sponge. But guanidine accelerator is easy to make the product aging crack and discoloration pollution. It has the greatest advantage of activating the vulcanization system and overcoming its own shortcomings when combined with acid accelerators (e.g., thiazole) .
⑥Thioureas：The accelerator has low efficiency and poor scorch resistance. In addition to chloroprene rubber and chlorinated polyethylene for promoting and crosslinking, other diene rubber is rarely used. ETU and DETU are commonly used accelerators for chloroprene rubber .
⑦Aldehydes and amines：It is a condensation of aldehydes and ammonia, a weak basic accelerator with 4 per blocked active amine groups, a slow accelerator, free from the risk of scorch and generally used as a secondary accelerator combined with Other accelerators . In addition there are acetaldehyde amine which known as AA or AC is a slow accelerator.
⑧Xanthates Sulfur：It is a kind of acidic super-speed accelerator, its vulcanization speed is faster than Dithiocarbamates, it is mainly used in low-temperature mortar and latex industrial ,rarely applying on other areas.Its representative product is ZIX opropyl xanthate zinc.