The Good, Better, Best Approach to Picking Catalysts for RTM Processes
The Good, Better, Best approach to picking the proper peroxide for RTM applications is a matter of matching up basic chemical properties with the size and thickness of the profile to be made. Many fabricators are able to use standard products like Norox® MEKP-9, 900, and 925 with satisfactory results, as such are considered “Good.” However, as we want to go faster, improve cosmetics, or ensure each part is the same finished size, we have to make some choices. A “process flow chart” may go as follows:
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Two major considerations in closed mold are:
a. Removing air, especially eliminating porosity caused by chemical reaction
b. Reducing the “leading edge effect” -
Both of these considerations can be addressed by using products designed for gel coat¹. High dimer, low Hydrogen Peroxide MEKP, Norox® 925-H, reduces the thickening effect².
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Norox® 925-H, 9H, and MEC, in that order are products that can reduce chemical porosity. These are considered “Better.” They also have the advantage of each giving a specific gel time as needed for desired fill time, while having faster cure times than their “Good” counterparts, so often the “demold” time is actually faster. Norox® MEC also has the advantage of being less susceptible to variations in ambient temperature and humidity, resulting in more consistency under varying conditions.
For the “Best” we have to factor in other process considerations such as:
- Size and thickness of profile; thermal mass
- Dimensional stability, or the ability to control shrinkage and make repeatable parts
- Cosmetics, surface appearance
- Desired cycle time for the part/mold
To do this we need to determine where the heat is and control it. Do we want more or less?
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For more heat, we use the Norox® Azox (AAP) family and possibly blends of AAP, MEKP, and TBPB.
a. AAP/TBPB blends Norox® SHP-90, SHP-40 give fast processing and very good cure
b. 925-H/TBPB blend Norox® TLC-88 gives the performance of 925-H with a kick
c. AAP/MEKP blends Norox® AMP-20 and AMP-50 enhance cure rate and final cure -
For less heat, we would use blends of CHP, MEKP and/or AZOX.
a. AZOX/CHP such as Norox® 750 or 757 for very good process and shrinkage control
b. MEKP/CHP a whole range of the Norox® MCP products, as well as HDP and HDP-75 -
Straight or Modified CHP products such as Norox® CHP and CHM-50 are also useful for very thick parts. One may also want to use specialty products like L40-LV or MIBKP 9 in certain very large profiles. Both can give very long gel times and relatively fast cure and higher peaks for thin cross sections, such as deck hulls.
An emerging technique is to heat the resin to 100°F or higher. This allows the resin viscosity to be thinner, aiding wet out and air release, resulting in reduced infusion time, better cure, ultimately less time under vacuum. The peroxides that work well in this range are the Azox blends, CHP blends, L40-LV and MIBKP.
Consult your Norox® representative to help with a process audit to determine the best way to optimize your process.
¹Multi-Component Peroxides for Improved Cure by Ken Weber, Frank Long, Dennis Fink
²Optimizing Peroxide Performance for Vacuum Infusion by Dennis Fink
