The scorch safety period of a rubber compound is the period of time required before the rubber compound begins to crosslink at a certain temperature. If the scorch safety period is too short, the rubber compound may scorch at a certain processing stage before entering the vulcanization process, resulting in waste. Therefore, it is necessary to find some way to increase the scorch safety period of the rubber compound. The following experimental scheme may extend the scorch safety period of the rubber compound. The reader should study and read the relevant literature sources in this book, including the literature cited later. Note: These general experimental schemes may not be applicable to every specific situation. Any variable that can extend the scorch safety period will definitely affect other properties, either for good or bad, but this book does not explain the changes in other properties. This book also does not explain safety and health issues.
1.SBR
The scorch safety period of SBR extruded rubber is longer than that of BR.
2.NBR
NBR with low ACN content or low Mooney viscosity has a longer scorch safety period.
3.Carbon black
Rubbers filled with low structure or low specific surface area carbon black can obtain a longer scorch safety period. When the carbon black filling amount of the rubber is reduced, its scorch safety period is extended.
4. White carbon black
Precipitated white carbon black with high specific surface area can extend the scorch safety period of halogen-free rubber, while also reducing the vulcanization rate and incision growth.
5. Increase the amount of zinc oxide
When the vulcanization system containing sulfonamide vulcanizes diene rubber, if the amount of zinc oxide is too low, it will cause scorch problems, so the amount of zinc oxide is often appropriately increased.
6. Choice of accelerator
Consider the following accelerators:
1) Sulfonamide: Longer scorch safety period.
2) Sulfonimide: Longer scorch safety period.
3) Thiazoles: The scorch safety period is acceptable.
4) Thiuram: The scorch safety period is short.
5) Dithiocarbamates: The scorch safety period is short.
7. Sulfonamide vulcanization system
MBS accelerator can provide a longer scorch safety period for rubber than TBBS, but MBS can release nitrosamines, which are harmful to personal safety.
8. Sulfonamide accelerator
In the sulfonamide vulcanization system, adding TATD instead of low molecular weight thiuram as an accelerator can prevent the scorch safety period from being too short, but at the same time increase the vulcanization speed.
9. Ethylac accelerator
The accelerator is used together with the main accelerator to increase the vulcanization speed and shorten the scorch time. Studies have found that compared with other accelerators, Ethylac has less effect on the scorch time as an accelerator.
10. Decelerators
When using thiazole accelerator vulcanization system to vulcanize diene rubber, salicylic acid, benzoic acid or phthalic anhydride can be used as a decelerator to extend the scorch safety period.
11. Inhibitors
For sulfonamide vulcanization systems, cyclohexylthiophthalimide (CTP) should be used with caution as a pre-vulcanization inhibitor, which can effectively adjust the scorch safety period.
12. DTDM-Sulfur donor for effective vulcanization systems
For effective vulcanization systems with low sulfur or sulfur-free vulcanization, DTDM rather than TMTD should be used as the sulfur donor, which can give the rubber a longer scorch safety period.
13. ISB accelerators
A laboratory-grade accelerator, 2-isopropylsulfinylbenzothiazole, can be used in combination with co-accelerators such as tetraethylthiuram disulfide (TETD) or the less environmentally friendly tetramethylthiuram sulfide to obtain a safe scorch period and a fast vulcanization rate.
14. Sulfonamide accelerators should avoid moisture
For sulfonamide accelerator vulcanization systems (such as MBS, etc.), in order to have a sufficient scorch safety period, it is necessary to avoid moisture in the rubber compound, otherwise the accelerator will hydrolyze, which will lead to a decrease in the scorch safety period.
15. Peroxide vulcanization
For peroxide vulcanization systems, in order to have a longer scorch safety period, it is necessary to consider using high-performance HP peroxide formulas. By replacing DCP with a smaller amount of BBPIB, the scorch safety period of peroxide-cured rubber can be extended. In addition, peroxides with higher half-life temperatures can be selected. The peroxide 2,5-dimethyl-2,5-di(tert-butylperoxy)3-hexyne (DMBPHy, trade name Laperox 130) has a higher stability temperature than DCP and can therefore have a safer scorch period. In order to effectively increase the scorch safety period of peroxide vulcanization, a vulcanization inhibitor such as butylated hydroxytoluene (BHT) can be used. The amount of BHT added can be obtained by experimental methods, that is, the maximum torque value of the vulcanizer is reduced by 25%-50%. Studies have found that the amount of BHT used is generally 0.1-1 parts (mass parts), and then the vulcanizing agent is added to restore the maximum torque value. This can ensure that the scorch safety period is increased and the vulcanization speed is fast.
16. Peroxide vulcanizing agent
The vulcanizing agent in the peroxide vulcanization system can generally shorten the scorch safety period. Compared with other vulcanizing agents, the use of low molecular weight liquid high vinyl 1,2 polybutadiene resin (such as Ricon) as a vulcanizing agent for peroxide vulcanized EPDM rubber can give the rubber the best scorch safety period.
In peroxide curing systems, try to avoid using trimethylolethane (TMPTA), trimethylolpropane triacrylate (TMPTMA) and N,N'm-phenylenedimaleimide (HVA-2),
because they shorten the scorch safety period. 1,2-polybutadiene, dibutyl phthalate (DAP), triallyl isocyanurate (TAIC) and triallyl cyanurate (TAC) have little effect on the scorch safety period.
17. Avoid urethane curing
It is reported that urethane can make natural rubber compounds very easy to scorch.
18. Antioxidants
When choosing PPD antiozonants, avoid using dialkyl PPD, because it is the most likely to cause the rubber compound to scorch among the three types of PPD. In the sidewall rubber compound, the use of 6PPD alone may shorten the scorch safety period, but if 6PPD and TAPDT (a high molecular weight antiozonant) are used together, it can have a certain scorch safety period and ensure that the rubber compound has good fatigue resistance and anti-ozone cracking under dynamic conditions.
19. Mixing
The discharge temperature of the internal mixer should be low, otherwise it will increase the thermal history of the rubber compound and shorten the scorch safety period. The selected internal mixer should be equipped with a cooling system so that the temperature of the rubber compound does not rise too quickly and shorten the scorch safety period. In addition, if the temperature of the rubber compound rises too quickly, the viscosity drops rapidly, resulting in too low shear force and not conducive to uniform dispersion of the filler. Assuming that the optimal mixing amount has been determined once and there is a cooling system, if you still want to adjust the mixing temperature of the rubber compound, you can do so by adjusting the upper bolt pressure or the rotor speed. In short, the thermal history of the rubber compound during the entire mixing process should be reduced as much as possible to improve the scorch safety period.
20. Cooling after mixing
In order to obtain a longer scorch safety period, the rubber should be properly cooled after being discharged from the internal mixer.
21. Low viscosity rubber and viscous heat generation
Low structure carbon black filled rubbers often have fewer scorch problems. This is because the viscosity of such rubbers is lower, and the viscous heat generated by shear during mixing is less, so they are much less likely to cause premature scorch than high structure carbon black filled rubbers.
22. Halogenated butyl rubber (HIIR)
For halogenated butyl rubber, acid components usually shorten the scorch time, but alkaline components tend to increase the scorch time, which is exactly the opposite of non-halogenated butyl rubber. Therefore, the curing components of halogenated butyl rubber must be carefully considered and selected to ensure sufficient scorch safety period without compromising other performance aspects.
Chlorobutyl rubber has a longer scorch safety period than bromobutyl rubber. Basic fillers (such as calcium stearate) will increase the scorch safety period of halobutyl. Acid clay in halobutyl will increase the cure rate, but it needs to be used in conjunction with a scorch inhibitor (such as magnesium oxide) to ensure an adequate scorch safety period. As a rule of thumb, phenolic resins and other tackifying resins will shorten the scorch safety period for halobutyl compounds. Magnesium oxide is a scorch inhibitor in halobutyl, except in amine cure systems. In halobutyl compounds, be cautious when using phenolic resins, aromatic oils, wood rosin, etc., as these components will directly or indirectly shorten the scorch safety period. In halobutyl compounds, be careful with some amine antioxidants or antiozonants as they can shorten the scorch safety period. Be careful when choosing a cure system for halobutyl, as some are extremely prone to scorch, such as the Vultac alkylphenol disulfide series. In halogenated butyl rubber, zinc stearate is prohibited because zinc is a vulcanizing agent that can cause scorch problems.
23. Australian isobutylene-p-methylstyrene rubber (BIMS)
Compared with general halogenated butyl rubber, BIMS has a safer scorch period.
24. Vulcanization of chloroprene rubber
In chloroprene rubber, W type has a safer scorch period.
For chloroprene rubber, replacing the traditional ethylene thiourea (ETU) vulcanization system with TMTM/DOTG/sulfur can make the rubber have a longer scorch safety period.
Increasing the amount of magnesium oxide or the specific surface area can extend the scorch safety period of chloroprene rubber. Highly active magnesium oxide (specific surface area greater than 100m²/g) is usually used to vulcanize oxyprene rubber, and the general amount is 4 parts by mass.
In chloroprene rubber, high-activity magnesium oxide in the form of "coating" can be considered. Because generally, powdered high-activity magnesium oxide will quickly react with moisture and lose its activity within 30 minutes when exposed to a hot and humid environment.
In the chloroprene rubber vulcanization system, stearic acid often acts as a vulcanization inhibitor, which will extend the scorch safety period and reduce the vulcanization speed.
When vulcanizing chloroprene rubber, consider using a larger amount of MBTS and ETU together, which can give the rubber a longer scorch safety period and a faster vulcanization speed.
Chloroprene rubber containing ZMTI is easy to scorch, and one way to extend the scorch safety period is to add CBS accelerator.
In traditional ETU-vulcanized chloroprene rubber, N-cyclohexylthiophthalimide can act as a vulcanization inhibitor and effectively extend the scorch safety period. When vulcanizing W-type chloroprene rubber, using symmetrical diphenylthiourea (A-1) as an accelerator, cyclohexylthiophthalimide (CTP) can be a good vulcanization inhibitor, which can effectively extend the scorch safety period.
25. Thiadiazoles extend the safe scorch period of CR and HIIR
Vanderbilt's thiadiazole, Vanax 189, is an accelerator for the vulcanization of CR or halogenated butyl rubber, which can give the rubber a safer scorch period than many other accelerators.
26. Fluororubber
The vulcanization system that can give fluororubber rubber a longer safe scorch period is bisphenol rather than diamine.
27. EPDM
EPDM rubber with a wide molecular weight distribution has a safer scorch period.
28. "Black" scorch of EPDM
The "black" scorch of EPDM rubber has nothing to do with the vulcanizer and can occur in EPDM compounds without vulcanizer. It is called "black" scorch because it often occurs in EPDM rubber filled with carbon black. The scorch safety period can be extended by adding a small amount of ethylidene norbornene ENB. "Black" scorch in EPDM can also be reduced by using low-structure carbon black. Interestingly, "black" scorch in EPDM can be delayed by adding small amounts of sulfur early on; small amounts of sulfur-containing accelerators have also been reported to improve "black" scorch. However, some sulfur-containing accelerators can make EPDM compounds more susceptible to "black" scorch, which needs to be determined through testing. "Black" scorch in EPDM compounds can be improved by re-refining the compound.
"Black" scorch in EPDM compounds can be improved by filling with carbon black with low surface polarity. The more polar the carbon black surface, the less compatible it is with the non-polar EPDM and the more "black" scorch it will cause. The polarity of the carbon black surface will vary from manufacturer to manufacturer.
