Curing system retarders

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

Curing Systems. Polychloroprene can be cured with many combiaations of metallic oxides, organic accelerators, and retarders (114). The G family of polymers, containing residual thiuram disulfide, can be cured with metallic oxides alone, although certain properties, for example compression set, can be enhanced by addition of an organic accelerator. The W, T, and xanthate modified families require addition of an organic accelerator, often ia combination with a cure retarder, for practical cures. 

Other flame retardants and/or smoke suppressants can also be used such as magnesium hydroxide, magnesium carbonate, magnesium-zinc complexes and some tin-zinc compositions. Zinc oxide is a common ingredient in many rubber base formulations used as part of the curing system. At the same time, the action of zinc oxide is similar to that of antimony trioxide, but less effective. 

Phenolic networks, 411 Phenolic-novolac-cured systems, 415 Phenolic novolac-epoxy networks, flame retardance of, 415 Phenolic oligomers, 375... 

Two genetic disorders of copper metabolism, Wilson s disease (see Section 62.2.3.3) and Menkes disease, are known. The latter involves impaired intestinal absorption of copper56,57 as well as probably subcellular metabolic defects which result in copper deficiency with respect to metal-loenzyme activity. The characteristic steely hair in Menkes disease results from free SH bonds in hair protein because of failure of lysyl oxidase to produce the disulfide links. Depigmentation of hair and skin, hypothermia, cerebral degeneration, central nervous system retardation, skeletal demineralization and arterial degeneration are all seen. Copper supplements may benefit hypothermia and increase pigmentation but the disease is not generally cured. 

Huang, W. K., Yeh, J. T., Chen, K. J., and Chen, K. N., Flame retardation improvement of aqueous-based polyurethane with aziridinyl phosphazene curing system, J. Appl. Polym. Sci., 2001, 79, 662-673. 

Use Elastomers, plasticizers, fire retardant chemicals, resins, and epoxy curing systems. 

Standard curing systems are based on two catalyst groups. Several different terms are used in the industry to cover these curing agents catalyst (not technically accurate, but widely used) and hardener or initiator. There are activators (also called promoters or accelerators) that are used to speed up and enhance the cure. Inhibitors (also known as retarders) perform the opposite function and are used to extend the curing time. 

Magnesium oxide can function as an activator, scorch retarder, and cure modifier. As an acid acceptor, magnesium oxide activates amine cure and improves heat resistance of thiuram-thiazole cures. It is effective as a scorch retarder with all cure systems (except amine systems). In carbon black filled compounds, magnesium oxide has a marked effect on scorch safety even at low level (0.25 phr), lengthening the available processability time. In mineral filled compounds, magnesium oxide can function in a role... 

The LP polymers are usually cured by oxidation using higher-valence-state metal oxides or peroxides. (Dichromate and manganese dioxide cure systems are used in aircraft systems.) Oxides such as lead dioxide react with the mercaptan groups to form higher polymers as well as crosslinks through disulfide bridges. Cure is accelerated by sulfur and water, and retarded by stearic acid. 

Stearic acid acts not only as an effective process aid, but also as a retarder with most cure systems used with ACM. Keep in mind that process aids may contain fatty acids, which will retard the cure rate hence an overall balance of stearic acid with certain process aid needs to obtain for minimum influence on cure rate. 

The Fast, Safe Cure System Consists of a Diamine, a Peroxide, a Coagent, an Accelerator and Optionally, a Scorch Retarder. 

DTDM (4,4 dithio-bis-morpholine or Sulfasan R) is the best retarder for the less desirable ETU cure system. If this mechanism is employed, a level of 0.5 pphr provides sufficient scorch safety for most processes. 

Corrqrlete curing system (activated lead oxides plus curing agent) in predispersed form Provides good processing safety and bin stability, moderate vulcanizate properties Acts as a retarder when scorchiness is caused by excess moisture... 

Sulphur systems. Conventional cure systems for most current, practical, tyre-related and mechanical goods formulations consist of zinc oxide, plus sulphur or sulphur-donors, accelerated with sulphenamides or benzothiazoles. These types of accelerator function initially as retarders but ultimately produce very high states of cure. MBT (mercaptobenzothiazole) is an exception in that it will scorch bromobutyl stocks, but it can be used... 

A study is made of the retarding and accelerating activity of Bayer s Vulkalent E, a sulphonamide derivative, in the vulcanisation of a number of rubber compounds, and of its effects on the mechanical properties of vulcanisates. It is shown that this additive acts as an effective retarder of scorch in several cure systems, and that in most cases it combines the function of scorch retarding at both processing and moulding temperatures with secondary... 

A study was made of the mechanisms by which tetraisobutylthiuram monosulphide (TiBTM) acts both as a retarder and a secondary accelerator in curing systems containing benzothiazole sulphenamides as primary accelerators. Model reactions between TiBTM andTBBS, CBS or MBTS were followed by HPLC to identify intermediates formed between TiBTM and its various analogues and the primary accelerators. Some of these intermediates were independently synthesised and evaluated as retarders and secondary accelerators in SBR/ polybutadiene compounds. The isobutyl groups were responsible for imparting scorch delay characteristics to one of these intermediates, while another intermediate was most likely responsible for the secondary accelerator effect. 14 refs. 

Optimised NR bushing compounds were developed using semi-efficient cure systems in which N-tert-butyl-2-benzothiazole was used as the primary accelerator, zinc dibutyldithiophosphate as the secondary accelerator, N-cyclohexyl thiophthalimide as retarder, andhexamethylene bisthiosulphate disodium dihydrate as a stabilising hybrid crosslinker. These cure systems gave compounds with improved stability of mechanical and dynamic mechanical properties and shortened mould cycle times. 

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