Curing without Accelerators

Types of Latex Compounds. For comparison with dry-mbber compounds, some examples of various latex compounds and the physical properties of their vulcanizates are given in Table 23. Recipes of natural mbber latex compounds, including one without antioxidant, and data on tensile strength and elongation of sheets made from those, both before and after accelerated aging, are also Hsted. The effects of curing ingredients, accelerator, and antioxidant are also Hsted. Table 24 also includes similar data for an SBR latex compound. A phenoHc antioxidant was used in all cases. 

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

Alternatively, the same coatings can be cured by electrons from an electron accelerator without the use of photoinitiators. Electrons from a 150-600 kV accelerator are energetic enough to create free radicals on impact with the polymer molecules and curing ensues. Clear and pigmented coatings can be cured. Electron accelerators are extremely expensive, but are cheap to run. 

Phosphite functionality reduces epoxy viscosity without accelerating cure... 

Castings cured with the commercial aromatic amine derived from aniline-formaldehyde were prepared like the HHPA-cured castings, but without accelerator. The aromatic amine mixture was about 90% methylenedianiline (MDA) and tended to partially... 

One difference with acrylic adhesives is the fact that they do often contain large amounts of highly reactive substances (monomers) that are used to achieve some of the unique properties associated with this family of products. In some cases, certain monomers may have a tendency to autopolymerize (cure without the use of externally added accelerators or hardeners). Consequently, shelf stability with some systems ean be a problem at relatively low temperatures. Even at as low as 44°C some acrylic formulations will begin to cure and harden in as little as a few weeks if maintained at these temperatures. Large containers [above 5 gallons (ca. 19 L)] can worsen this problem. 

For most other surfaces, however, accelerators of various kinds similar to the activators of two-component acrylics are often required to achieve cure rates of practical lengths. Structural anaerobic adhesives cured without the use of pre-applied accelerators or primers frequently cure too slowly for rapid assembly techniques (3). ... 

CO and ECO are vulcanized without sulfur. Rather, they are generally cured by the action of thioureas or triazines in the presence of acid acceptors such as MgO or dibasic lead phosphite. The terpolymers can be cured by accelerated sulfur or peroxide curing systems as well as by the action of thioureas, and so on. 

Di(tert. butylperoxy) trimethyl cyclohexane liquid high activity 50% solution in aliphatics Versatile type for curing above 80°C Quickset in range 120-150 C for hot-press molding SMC or BMC can be accelerated by promoters. Special for SMC/BMC at 130-160°C without accelerator not sensitive to fillers pigments and promoters FGH gh... 

In general, reactivity of amines toward aromatic glycidyl ethers follows their nucleophilicity aliphatic amines > cycloaliphatic amines > aromatic amines. Aliphatic amines cure aromatic glycidyl ether resins at room temperature (RT) without accelerators, whereas aromatic amines require elevated temperatures. 

Polychloroprenes differ from other polydienes in that conventional sulphur vulcanization is not very effective. The double bonds are deactivated by the electronegative chlorine atoms and direct reaction with sulphur is limited. The vulcanization of polychloroprenes is normally achieved by heating at about 150°C with a mixture of zinc and magnesium oxides W type neoprenes also require an organic accelerator (commonly either a diamine or ethylene thiourea) but G types cure quite rapidly without acceleration. The mode of reaction has not been established with certainty, but it is generally supposed that cross-linking occurs at the tertiary allyUc chloride structures generated by 1,2-polymerization (see Section 18.8.3) and that a 1,3-allylic shift is the first step. The metal oxides may lead to ether cross-links as follows ... 

At RT or elevated temperature, which ever is the recommended procedure) (or post-cure needed.) At RT P/F and R/F Solvent bonding of certain plastics materials Nitrile phenolics Acrylics with polymer dissolved in monomer but without accelerator Casein and casein-latex Polyvinyl acetate... 

The disodium salt of hexamethylene bisthiosulphate (HTS) was evaluated as an accelerator in the zinc oxide mediated vulcanisation of tyre sidewall compounds containing a brominated isobutylene-p-methylstyrene copolymer. Rheometer profiles of blends with polybutadiene cured with sulphenamide, sulphur, zinc oxide and HTS showed a characteristic delayed action fast cure kinetic profile, while the same blends cured without HTS exhibited undesirable cure characteristics, with long early cure and fast late cure. Many vulcanisation characteristics and vulcanisate properties were largely dominated by a single cure component. State of cure was dominated by sulphur level, scorch delay and early cure time by HTS loading, and the time to reach t90 by the sulphenamide level. Time of flight secondary ion mass spectroscopy imaging showed that HTS favourably affected the distribution of curatives in blends with SBR. 11 refs. 

When the surface conditions are acidic or the ambient humidity is low enough to affect the cure significantly, a surface accelerator may be used to promote the reaction. Available from most manufacturers, these basic solutions may be dip, wipe, or spray appHed. Recentiy, new additive chemistry has been developed that accelerates the cure under adverse conditions without the need for a separate accelerator. 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

The resins are commonly cured by the use of peroxide with or without cobalt accelerators, depending on whether the hardening is to be carried out at room temperature or at some elevated temperature. Electron irradiation curing, which can be completed within a few seconds, has, however, been introduced for coatings on large flat surfaces such as plywood, chipboard and metal panels. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

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