How Much Peroxide Should I Use?
Generally, a small amount of peroxide (about 0.5 to 2 percent by weight) is sufficient to start a curing reaction. Peroxides should be selected that produce sufficient free radicals at the applied temperatures and result in complete or higher degree of cure. The rate of cure is also dependent on the presence of promoters/accelerators and inhibitors in the resins. Resins are often supplied with promoters or inhibitors premixed in the system, or end users can choose to add specific ones based on their cure requirements.
In pultrusion and filament winding applications for processing thin composite parts, a single peroxide like benzoyl peroxide (BPO) may be sufficient. A single fast-decomposing peroxide may not produce enough free radicals to complete the cure in some cases. For moderate to thick parts, multiple peroxide systems may be required. Also, highly exothermic reactions often cause problems with heterogeneous temperature distributions in the part that can lead to residual stress and cracking, especially in thick parts.
To combat such problems, one feasible approach is to use a combination of high-temperature slow-reactivity peroxides in addition to low-temperature fast-reactivity peroxides simultaneously in a resin/initiator system. In this way, line speeds and cure time can be coordinated and controlled. Figure 1 shows an example of cracks in the center of a part where these considerations may not have been properly controlled.
While hardly exhaustive, the above considerations touch on the main points an end user should consider when selecting a peroxide initiator. For illustrative purposes, we have attempted to separate the factors of process type, speed, part thickness, etc. In reality these factors are interrelated, and proper peroxide selection is not a trivial exercise. We hope this column provides food for thought to help an end user work with a peroxide supplier to choose the right one for their application.
Sandeep Vennam is a senior research and development engineer at PPG. Email comments to firstname.lastname@example.org. Ryan Emerson, Ph.D., is senior group leader for fiber glass applications development. Email comments to email@example.com.
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