David Herzog, director of research and development for Interplastic Corporation, created a list of critical parameters you need to consider when selecting a resin.
Resin Quantities: One of the first factors is the amount of resin you will purchase. Do you require one or several drums, truckloads or tankers per month? This will determine whether you need to purchase a resin stocked by a distributor in a regional warehouse or one made to order by a resin manufacturer. If you are purchasing a low volume you will likely be limited to resins that local distributors have in inventory for your process and application.
Manufacturing Process: Next, consider the manufacturing process. Will the product be manufactured by open molding spray-up, resin transfer molding, filament winding, infusion, pultrusion, centrifugal casting, open molding hand lay-up or another process? Each process has general properties like viscosity, gel and cure that resins are commonly set up to meet. If you do more than one process in your shop, select the dominant one or the most sensitive process.
Application and End Use: After process consideration, the application/end use is the next component to think about. A resin best suited for a heavy truck part will be different than one for a surfboard or shower stall. Suppliers have internal lists of key properties and requirements that resins for various applications are designed to meet, so knowing the end use will narrow the choices. Consider what attributes you might need, such as corrosion or fire resistance.
In addition, you may be guided by certification requirements from organizations such as UL, FM, Lloyds of London, ANSI Z124, CSA International and DNV. Required certifications will dramatically narrow the list of suitable resins further. For instance, having CSA approval on the resin so your product can be sold in Canada may be an important factor in resin selection.
Regulatory Standards: Some of the standards that affect resin selection may apply nationwide, such as the Environmental Protection Agency’s Title V operating permits and Maximum Achievable Control Technology (MACT) standards. Others may be regional, such as the South Coast Air Quality Management District’s Rules 1162 and 1132 in southern California. Be sure to know any applicable state and regional hazardous air pollutant (HAP) and volatile organic compound (VOC) requirements.
HAP and VOC regulatory requirements may dictate specified limits and carry financial penalties for amounts in excess of those limits. Other limits in HAP and VOC contents can arise as production increases, such as issues related to Title V operating permits for manufacturing sites. These can also limit HAPs and VOCs that can be processed at the site for continued production without obtaining a new operating permit.
Working Properties and Specifications: The next step in the resin selection process is to consider the working properties and specifications. Key working properties include gel time, viscosity, the ability to add various levels of fillers, the ability to make a thick part in a single step and mold/demold time. These affect how the resin works on the shop/production floor and productivity.
Selecting a resin that allows workers enough time to make the part with a small cushion to accommodate processing issues, but not too much extra time, will maximize productivity per shift. Being able to make a part in a single step rather than breaking it into several steps to control exotherm, cosmetics or other properties is another thing that factors into productivity. Some raw materials like peroxides and fillers have shipping limitations, so understanding what is available in the region is important.
Finished Properties: The final step in the resin selection process is determining which finished properties your application requires. Possible properties to consider include UV resistance, corrosion resistance, fatigue performance, fire resistance, surface cosmetics, stain resistance, stiffness, strength, toughness, blister resistance, thermo-shock and color/clarity. Some of these will be covered in the application/end use step, but there may be some property requirements outside of the basics ones. Let the resin supplier know you need a specific end property, such as a temperature capability that can be defined by the heat distortion or glass transition temperature or thermo-shock.
Overall, I recommend that you create your own set of requirements based on the above criteria and pass along that important information to the resin representative. Even though you have requirements, make sure you keep an open mind. There may be a better product available. New resins and additives are being developed and current products are being expanded. An alternate resin may offer you some processing and/or performance advantages even though they may have some different working properties and specifications.