Autoclave Curing Guide for Uncured FKM/Viton Elastomeric Sheet Materials
PD3 offers uncured “b-stage” FKM sheet materials that can be cured at high temperature. Note that Viton is a Chemours brand of FKM. Unless specifically noted on the Technical Data Sheet, the following information pertains to every PD3 uncured FKM/Viton material.
FKM rubber is commonly used in industrial applications where its high temperature resistance or fluid resistance to common hydrocarbon fuels and fluids are required. It is a relatively high-cost elastomer with lower room-temperature mechanical properties than other less expensive elastomers. It is sometimes referred to by the brand name Viton, especially in the USA, although there are a number of different manufacturers that synthesize this elastomer. FKM and Silicone are the two most common elastomer types that can be used in applications requiring 350F and higher temperature resistance. There are other specialty types like fluorosilicone, FFKM, and TFE/P that also can be used at high temperatures, but these are even more costly and are only used when specific types of chemical compatibility are required.
Applications in the Composites Industry:
FKM is used in the composites manufacturing industry in sealing applications, like o-rings on vacuum quick disconnects, which are similar to conventional applications in other industries. These parts are supplied by typical rubber molders.
FKM is also used to fabricate “caul plates” or for elastomeric tooling that is used in the fabrication process of advanced composites made by autoclave/vacuum bag molding. Caul plates, or “cauls”, are used between the vacuum bag membrane and the composite laminate, and are designed to improve part quality, consistency, and surface finish. Rigid caul plates are made from composite or metal. Elastomeric cauls or “soft cauls” are made high temperature rubber materials like silicone or FKM. In many cases they are reinforced with several layers of carbon composite prepreg. This is very important on larger caul plates, since the dimensional stability of rubber cauls with no rigid reinforcement is poor. This document does not address the design or use of caul plates.
FKM elastomer is also occasionally used in manufacturing inflatable mandrels or solid mandrels. These are more difficult to manufacture and are typically made by specialists rather than the end-user of the tooling.
Storage:
All of the uncured FKM materials that PD3 currently sells (as of March, 2025) use a peroxide curing agent. These materials have the advantage of having a relatively long shelf life, and can be kept at room temperature for one year and then cured to full properties. They have an even longer shelf life in a refrigerator or freezer.
Curing:
All uncured FKM sheet products from PD3 use an aliphatic dialkyl peroxide curing agent: 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. This curing agent is commonly called DBPH or DBPH-50 when it is provided in a powder carrier. It is heat activated, and FKM rubber using this curing agent typically require a minimum cure temperature of about 290F. The cure time declines by about 50% every additional 15 degrees F of temperature exposure. At 350F the rubber material will typically fully cure in a few minutes, with the precise number depending on the formulation.
There are a number of methods that may be used to cure the FKM. For example, it can be cured on a heated press in a compression mold or “transfer mold”. Most composites fabricators will cure under vacuum in an autoclave, and that process is described in this document. The key features of a successful cure are that the FKM is heated to at least 300f, in an environment with little to no oxygen present, at pressure of at least 50 PSI.
Figure 1. Exploded view of layup for curing FKM.
The material properties are not highly dependent on cure schedule, and there is no need for a slow ramp up. However, the autoclave should be ramped to full pressure prior to reaching 200 F. The autoclave is typically ramped up to full pressure first, and then the temperature is adjusted to 350F. The cure is "air inhibited"- because oxygen will inhibit the cure reaction. Trace amounts of oxygen are not a problem.
Various types of release materials may be applied to the tool surface but Frecote 700 NC and other typical release chemicals have been found to work. Many composite release coatings are silicone based polymers dispersed in a hydrocarbon solvent, and these have generally been found to provide a good release. Dry PTFE based release, which have been used in the rubber industry for many decades, also can provide an effective release. The FKM may be applied to the mold as a single full-thickness sheet, or it can be layered and overlapped. It should be covered with a thin non-perforated release, such as FEP or ETFE, and then breather, and vacuum bag. This is illustrated in figure 1 above. Autoclave pressure must be at least 40 PSI, or there may be voids in the laminate, especially in corner radius areas or where FKM layers overlap. Typically between 50 PSI and 100 PSI autoclave pressure is used.
The material will fully cure in about 30 minutes at 325 f, but it is recommended set the oven to 350f and cure for 2 hrs to ensure that the tool is up to temperature. A very heavy tool could take longer due to thermal mass. It may be helpful to use thermocouples on the tool to ensure it has reached a high enough temperature in the case of larger molds. There is no issue with leaving the material in the autoclave too long. For example, an 8 hour cure at 375F would not cause any problem.
Note that certain types of peroxide-cure FKM can be “co-cured” with peroxide cure silicone rubber. This can be desirable in a number of different applications. Please consult PD3 for additional information if this is of interest.
Optional Postcure:
After cure, the vacuum bag can be removed and a "freestanding post cure" of 4 hrs at 350 can be conducted. This is not strictly required, but it may remove trace amounts of unused curing agent and other bi-products. This may be helpful if the FKM is required to pass a stringent outgassing test, or undergo other evaluations.
Note that during the cure and especially free-standing postcure, a circulating oven should be use that vents outside, to allow the undesirable smell of the cure reaction to be removed.
Heating FKM above 395F may release trace amounts of fluorine gas. When heating FKM to temperatures above 395F under vacuum bag, the vacuum pump should be vented to the outdoors. When heating FKM to temperatures above 395F in an open oven, the oven should vent to the outdoors. Human exposure to fluorine gas in concentrations above 1.7 ppm can be an irritant, and higher concentrations can be dangerous. Please consult the product MSDS for other safety details.
Repair and defects:
It is not possible to fully repair FKM that has torn or ripped. There are products available that market themselves as FKM adhesives, but they have not been found to produce good adhesion to cured FKM rubber.
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