When selecting accelerators from the aspects of process performance and product performance, single use of accelerators often fails to achieve the desired effect. For example, although the accelerator DPG can give the vulcanized rubber high tensile strength and elongation stress, the sulfur speed is slow, the flatness is poor, and the vulcanized rubber has poor heat aging resistance; the accelerator TMTD has a fast sulfur speed and high vulcanization degree, but The processing safety is poor, and it is easy to scorch and oversulfur; the vulcanizates of thiazole and sulfenamide have higher tensile strength, good wear resistance, heat resistance, aging resistance, and good vulcanization flatness. In order to further increase the vulcanization speed and achieve higher use value, 2-3 kinds of accelerators are often used in the design of the vulcanization system in the rubber formulation to achieve the effect of complementing each other or activating each other, so as to adapt to the processing technology and improve product quality. need.
When "accelerators" are used in combination, one type of accelerator is usually used as the main accelerator (called the primary accelerator or the first accelerator), and the other accelerator is used as the secondary accelerator (called the secondary accelerator or the second accelerator). The amount of the main accelerator is larger, and the amount of the secondary accelerator is smaller (generally about 10% to 40% of the amount of the main accelerator).
Combined use classification:
1. Accelerators are divided into three categories according to acidity and alkalinity. One is acid accelerator (indicated by letter A): including thiazole, thiuram, dithiocarbamate, xanthate; the other is alkaline accelerator (indicated by letter B): including guanidine Class and aldehyde amines; third is the neutral accelerator (represented by the letter N): including sulfenamides and thioureas. Accelerators are classified into AB, AA, BB, NA, NB, NN, etc. according to the combination of acid and alkali, but commonly used are acid-base (AB), acid-acid (AA), and neutral acid (NA).
2. Accelerators are divided into five categories according to the vulcanization speed. Internationally, it is customary to compare the vulcanization speed of accelerators based on the effect of accelerator M on natural rubber. Slow-speed grade: aldehyde amines, thioureas; medium-speed grade: guanidines; quasi-super-speed grade: thiazole, inosinamides; super-speed grade: thiuram; super-speed grade: dithiocarbamate, yellow Orthoate.
Generally, acid accelerator (A) type or neutral accelerator (N) type is selected as the main accelerator, namely thiazole and sulfenamide accelerators. As the main accelerator, thiurams are generally used in effective and semi-effective vulcanization systems. As the main accelerator, dithiocarbamate accelerators are often used in latex products and butyl rubber. As the secondary accelerator, alkaline accelerators such as DPG and H are generally used. Its role is to activate and promote each other. When sulfenamides are used as primary accelerators, secondary accelerators are generally not used. Because this accelerator can decompose accelerator M and amine compounds at the vulcanization temperature, M is an acid accelerator, and amine compounds are alkaline accelerators. It constitutes an acid-base (AB) combined system, but in order to speed up the vulcanization speed, a small amount of accelerator DPG or thiuram accelerator can be used together. The amount of accelerator DPG is generally 10% of the main accelerator. When the amount is too high, the vulcanization speed will tend to decrease.
use and match:
1. Acid-base (AB) combined use (mutual activation)
is a combined use method in which an acidic accelerator is the main accelerator and an alkaline accelerator is used as a secondary accelerator. The accelerator effect after the combined use is better than when A or B is used alone. Typically, thiazoles and guanidine accelerators are used in combination. For example, accelerator M is a quasi-super-speed grade, and accelerator DPG is a medium-speed grade, but the super-speed effect can be achieved after combined use, which is characterized by high vulcanization activity (fast starting point of vulcanization, fast vulcanization speed), tensile strength and fixed elongation of vulcanized rubber The stress, hardness and abrasion resistance are higher than that of a single accelerator.
Accelerator M and DPG are theoretically effective when used together in equimolar. At this time, the setting speed and vulcanization speed of the rubber compound are fast, and the mechanical properties are also high, but it is easy to scorch, the vulcanization flatness is poor, and the process safety is poor. Therefore, in order to ensure process safety in actual production, the combined dosage of accelerator M and DPG is generally controlled at M:DPG=5:2～4 (mass ratio).
In order to improve operational safety, the combination of accelerators DM and DPG is currently more commonly used, which is similar to the combined use of M and DPG, except that the start of vulcanization and the vulcanization speed are slightly slower. In fact, the combined use of thiazoles and hexamethylenetetramine is also effective.
The combined use of accelerators M, DM and DPG is the development of the combined use of M and DPG. Among them, M and DM should be regarded as the first accelerator together, and DPG as the second accelerator. This combined method is suitable for use when the total amount of accelerator is large and the process operation requires good stability, and the increase or decrease of DM can be used to adjust the starting point of vulcanization.
The AB type combined system has the following advantages: (a) It can shorten the vulcanization time or reduce the vulcanization temperature, and can reduce the amount of accelerators, so it can improve production efficiency and reduce production costs; (b) The vulcanization start point is fast, and it is suitable for non-model vulcanization requirements. Where the rubber material has a fast setting speed; (c) The tensile strength, fixed elongation stress and abrasion resistance of the vulcanized rubber are improved; (d) It makes up for the shortcomings of poor aging resistance when the accelerator DPG is used alone.
2. Acid and acid (AA) combined use (mutual resistance)
This is the combined use of two different types of acid accelerators. After their combined use, the activity of the system is often suppressed at a lower temperature (referring to the operating temperature), thereby improving the scorch performance, but at the vulcanization temperature, it can still exhibit rapid vulcanization. effect. For example, when TMTD and M are used together, the conversion of part of TMTD into DM inhibits the activation of TMTD and improves scorch. Zinc dithiocarbamate is formed in the system, which plays a role of rapid vulcanization after the start of vulcanization.
There are two types of AA commonly used in combination. One is that the primary accelerator is an overspeed or superspeed grade (such as TMTD or ZDC), and the secondary accelerator is a quasi-overspeed grade (such as M or DM). For example, in natural rubber, when one part of accelerator ZDC is used alone, the scorch time is 3.5 minutes. If the 10% amount is replaced with accelerator M, the scorch time can be extended to 8.5 minutes, while the vulcanization speed remains unchanged. , And make the tensile strength of the vulcanized rubber have a certain increase. The other is the accelerator M (or DM) as the main accelerator, TMTD as the secondary accelerator, and similar effects can be obtained after combined use. For example, the fiber carcass compound formula of bias tires can adopt this combined method, which has a slow start of vulcanization and a fast vulcanization speed. The starting point of vulcanization is slow, and the rubber compound has good fluidity before the initial vulcanization, which helps to improve the adhesion between the rubber compound and the fiber cord; the fast vulcanization speed can ensure that the inner layer of the rubber is heated later. It can be vulcanized synchronously with tread rubber. When the accelerator M (or DM) is used in combination with TMTD, if the amount of M (or DM) is about 1 part, the amount of TMTD is generally 0.05-0.1 part (up to 0.2 part or more in synthetic rubber). If the amount of TMTD is too high, the flatness of the vulcanization will decrease, and the fatigue crack resistance of the vulcanized rubber will decrease.
This type of combined use is suitable for model products. After combined use, it does not increase the tensile stress like AB combined use, but it has a higher elongation and the product is softer.
3. Neutral acid (NA) combined use
This kind of combined use can activate the neutral accelerator, thus speeding up the vulcanization speed of the system, but shortening the scorch time of sulfenamide to a certain extent. There are two typical NA-type combined systems. One is the combination of CZ (or NOBS) and M (or DM), which is more common in tire tread rubber and conveyor belt covering rubber formulations that use natural rubber and synthetic rubber. It has the characteristics of low scorch tendency, fast sulfur speed, good comprehensive performance of vulcanized rubber, and good aging resistance.
4. In combination with alkali (NB):
CZ (or NOBS) and TMTD (or ZDC) are used together. This combined method can replace the combined use of thiazole and guanidine. Compared with the combined use of thiazole/guanidine, it has less total amount of accelerator, faster sulfur speed, and scorch time. It is longer, the degree of cross-linking is increased, and the compression set is smaller. The disadvantage of this system is that the flatness is slightly worse.