Catalysts/Curing System
Polyester resin is converted from a low-viscosity liquid to a thermoset solid when mixed with an initiator (often called a catalyst), in conjunction with suitable promoters and accelerators. Commonly used catalysts for making corrosion-resistant materials are methyl ethyl ketone peroxide (MEKP), benzoyl peroxide (BPO) and cumene hydro-peroxide (CHP). Promoters are very selective in their work with peroxide catalysts and care must be taken to use the correct combinations.
Peroxide Initiators
The initiator is usually the final addition required to initiate the curing reaction. Promoters and/or inhibitors may be added to prepare it hours, days or weeks before it is to be used. The initiator is added immediately before use, and the amount employed is a factor that determines the working life of the resin. The most commonly used initiator systems for the resins include:
Methyl Ethyl Ketone Peroxide (MEKP):
Methyl ethyl ketone peroxide (MEKP) is used to cure unsaturated polyester resins in the presence of a cobalt accelerator at room and high temperatures. Curing System MEKP/Cobalt accelerator is particularly suitable for curing gelcoat resins, laminating resins, lacquers and castings; Additionally, the production of light resistant parts may be possible in contrast to the benzoyl peroxide/amine accelerator curing system. Many years of practical experience have proven that due to the low amount of water in MEKP and the absence of polar compounds, this peroxide is very suitable in GRP products e.g. Chemical application industrial tanks and pipes, marine application, etc. For room temperature use it is necessary to use MEKP with a cobalt accelerator
Cumene Hydroperoxide (CHP):
Cumin hydroperoxide (CHP) is a colorless to pale yellow liquid solution of cumyl hydroperoxide. It is an economical free-radical initiator for the styranation of unsaturated polyester resins, vinyl monomers, and oils and alkyds. CHP can be used alone or in combination with other peroxides in both ambient and elevated temperature processes. CHP is particularly suitable for curing unsaturated polyester resin systems at both ambient and elevated temperatures. The typical exotherm curve exhibited is usually mild and flat leading to a slow, but complete, cure. This characteristic can reduce crazing and cracking, especially in thick cross-sections. CHP is commonly used in pre-promoted resin systems that employ cobalt and/or manganese compounds as primary accelerators.
Benzoyl Peroxide (BPO):
Benzoyl peroxide is available in concentrated powder or liquid thin paste form. Benzoyl peroxide is used to harden the resin in hot molds. It is applied alone with acceleration at high temperatures and at room temperature. Generally, a lower critical temperature is used to adjust the gel time in the dye. A higher amount of BPO gives faster gelation at a relatively low temperature in the middle of the thick sections for uniform heat generation
Tert Butyl Perbenzoate (TBPB):
A yellow, mobile liquid with a faint aromatic odor, containing technically pure tert-butyl peroxy benzoate. This aromatic ester is used as a precursor (radical source) in the curing of unsaturated polyester resins. A major application of TBPB is hot press molding of SMC or BMC at 130–160°C.
Promoters, Inhibitors, and Accelerators
Initiators, catalysts, and inhibitors are used to initiate and control the cross-linking process that cures thermoset resins.
Initiators, commonly called catalysts, are used to produce the curing (molecular cross-linking) process with thermoset resins. Various formulations of methyl ethyl ketone peroxide (MEKP) and benzoyl peroxide (BPO) are used in a wide range of room temperature curing molding applications. Other initiator formulations are available that are heat activated or produce specific healing properties.
Promoters are added to the resin formulation to induce the curing reaction in the desired parameters and to control the curing time. Specific promoters are used to react with specific initiators and include cobalt naphthenate, cobalt octoate, dimethylaniline (DMA), or diethylaniline (DEA).
Inhibitors are added to polyester and vinyl ester resins by the manufacturer, but are occasionally modified by the user to produce specific gel times or cure characteristics. Hydroquinone and tertiary butyl catechol (TBC) are the most common inhibitors used in resin formulations.
Source : Content based on collections of information from public domain
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