The Good, Better, Best Approach for RTM Processes

United Initiators

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 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:

  1. Two major considerations in closed mold are:
      a. Removing air, especially eliminating porosity caused by chemical reaction
      b. Reducing the “leading edge effect”
  2. Both of these considerations can be addressed by using products designed for gel coat¹. High dimer products such as 925-H reduce the thickening effect².
  3. 925-H, 9H, and MEC, in that order, are products that reduce porosity. These are considered “Better.” They also have the advantage of giving longer gel times to help fill the void, while having faster cure times, so often the “demold” time is actually faster.

For the “Best” we have to factor in other process considerations such as:

  1. Size and thickness of profile; thermal mass
  2. Dimensional stability or the ability to control the shrink and make the part the same each time
  3. Cosmetics, surface appearance
  4. How fast the need is to cycle the mold

To do this we need to determine where the heat is and control it. Do we want more or less?

  1. For more heat, we use the Azox family and possibly blends of Azox, MEKP, and TBPB
      a. SHP-90, SHP-40
      b. TLC-88 is a 925-H/TBPB blend that gives performance of 925-H with a kick
      c. AZOX/MEKP blends are point of use blends
  2. For less, we would use blends of CHP, MEKP and/or AZOX
      a. AZOX/CHP are Norox® 750 or 757
      b. MEKP/CHP has a whole range of the MCP products, HDP-75
  3. Modified CHP products such as CHM-50 are also useful for very thick parts, as is 100% CHP
  4. 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