Viscosity is a fluid’s resistance to flow. The fluids that have less viscosity can flow easily (like water) while fluids with high viscosity can flow relatively more difficultly (like honey). The unit of viscosity is either “cp” or “mPa.s”. When defining the viscosity of a fluid, its temperature should always be mentioned because viscosity of fluids depends on the temperature. For example, the viscosity of a polyester resin at 20°C will always be higher than its viscosity at 25°C.
Thixotropy is the shear thinning property. Certain gels or fluids that are thick (viscous) under static conditions will flow (become thin, less viscous) easily when shaken, agitated, or otherwise stressed. For example, when a thixotropic polyester or gel coat is sprayed with spray gun, it is exposed to high pressure in the nozzle and its viscosity reduces, making it easy to spray. When the sprayed resin reaches the mold, it will stay on the mold and not run down because once it leaves the nozzle of the gun, the pressure is relieved and viscosity is recovered.
The period of time when the resin changes from a liquid to a non-flowing gel is called the “gel-time”, and is also the time available to work the resin once peroxide is added. Gel time varies with temperature as well as the amount of initiator and accelerator used.
Curing is a term in polymer chemistry that refers to the hardening of a polymer material by cross-linking of polymer chains after gel time.
Initiators are chemicals that cause the resin cure properly to achieve final properties. The choice of appropriate initiator is dependent on the application technique. Most commonly used initiator is MEK peroxide, which is added 10-25gr per 1kg of resin.
Accelerators are the chemicals that accelerate the decomposition of initiators thus helping the resin cure quicker. Generally metal salts are used as accelerators, Cobalt octoate being the most common one. It is mostly used in 6% solution with an addition ratio of 1.5-2gr per 1kg resin. IMPORTANT!!! Cobalt Octoate and MEK Peroxide must not be mixed or be in contact with each other. Mixing of these two materials causes violent reaction and possibly fire and explosion. Inhibitor: Inhibitors retard the gel and curing time of polyester resins. They are especially helpful to balance the gel time in warm environment.