One Article to Understand Accelerators,M Influential Dominance

One Article to Understand Accelerators,M Influential Dominance

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. 

6.Cautions for accelerators selection and use 
①Rubber types：Different rubbers adopt different curing systems, and different vulcanization systems naturally include different types of accelerators to ensure the matching. 
②Scorch performance：The accelerator determines the scorch time of the compound (the time before the heat vulcanization begins). Therefore, the accelerator should make the compound have a considerable scorch time, which plays an important role in the operation safety, the fluidity of the initial rubber and the adhesive force of the fabric in the process of mixing, calendering, extrusion or injection. Especially in recent years, in order to improve production efficiency, processing conditions become much stricter, the anti-scorch performance requirements of the rubber compound is also higher and higher. But the excessive long burning time will lead to the increase of the total vulcanization time, which is not only detrimental to the improvement of labor efficiency, but also to the deformation of non mold curing products in the process of vulcanization will also have adverse effects. The scorch resistance of the accelerator is directly related to its critical temperature ( the temperature at which the accelerator acts on the curing process). Under this temperature, the accelerator activity is not significant; above this temperature, the accelerator is activated and its sulfidation is fully brought into play. Unfortunately, most accelerators have not yet measured this temperature.When a plurality of accelerators are used in combination, the activity of some accelerators is inhibited, and some accelerators exhibit greater activity even below their critical temperature. 
③ Vulcanization flatness：Accelerators should allow the rubber to have a broad flat curve during the optimum curing process. In other words, the properties of the vulcanizates should not change significantly during this period, which is particularly important for rubber thick and solid products.Rubber is a poor conductor of heat, the surface and internal heating of vulcanized rubber is not consistent, wide vulcanization flatness is to avoid over-sulfur and ensure that the product parts of vulcanization uniform. 
④Effect on properties of rubber compound and vulcanizate：The accelerator participates in the formation of rubber net structure during vulcanization, which is directly related to the structure of vulcanized rubber, that is to say, its physical and mechanical properties and aging properties. Some accelerators soften the rubber compound, increase the plasticity of the rubber compound, and change the process ability of the rubber compound. Therefore, it is necessary to choose accelerators to understand these properties. 
⑤Dispersivity：The accelerator with poor dispersing property in the rubber compound not only needs longer mixing time, increases the power consumption of the equipment, but also tends to cause uneven vulcanization due to uneven mixing. For some difficult dispersing accelerator, it can be used as rubber compound.
⑥Pollution and coloring：Some accelerators make rubber products colored because of their different colors.And some are discolored by chemical or sunlight exposure which can not be used as white or bright colored products. In addition, some promoters can contaminate other substances by contact migration.  
⑦Water solubility：For the production of latex products, the use of water-soluble accelerator is more convenient. 
⑧Toxicity：Most of the accelerators currently used are non-toxic.But some promoters have a bad smell and individually some are toxic,which cannot be used in medical products and food-contact rubber products.The use of toxic accelerators should be protected during operation.
⑨The combination of accelerators：Each accelerator has its own characteristics. In order to meet the good performance of the rubber in the process of operation and to make the vulcanizate have excellent physical and chemical properties, several accelerators can be used together to complement each other. 
⑩Factors such as interaction with other ingredients and cost also need to be considered.