HexPly M21 toughened epoxy prepreg matrix

HexPly M21 is a toughened epoxy prepreg matrix from Hexcel, a 180 degree Celsius curing resin system for primary aerospace composite structures. It is supplied as prepreg with unidirectional or woven carbon or glass fibres, and it is used where laminates need high damage tolerance in structures such as airframe skins, spars and control surfaces.

The maker describes the matrix as very tough, with particular strength against high-energy impact, and with high residual compression strength after impact. For a structural engineer this means that a laminate with impact damage retains more of its compressive capability, which supports damage-tolerant design allowables on aircraft parts exposed to tool drops, runway debris or handling damage. The system also translates fibre properties effectively, especially with intermediate-modulus carbon fibre, so the stiffness and strength of the reinforcement are used efficiently in the cured laminate.

Temperature capability is stated in two ways. M21 was developed as a controlled-flow system for operation in environments up to 121 degrees Celsius, and it shows good hot-wet properties up to 150 degrees Celsius. Its low exotherm allows simple cures of thick structures up to 48 mm. The prepreg also has a good tack life, which helps during manual or automated lay-up of large parts.

Processing is by press or autoclave cure to obtain the best mechanical performance. The data sheet gives typical autoclave cycles. A monolithic part under 15 mm thick is cured under 7 bar gauge pressure, heated at 1 to 2 degrees Celsius per minute to 180 degrees plus or minus 5, and held for 120 minutes. A monolithic part between 15 and 48 mm uses a hold at 150 degrees for 180 minutes before the final 180 degree hold. Sandwich structures with honeycomb or foam core use a vacuum-only stage with a hold at 135 degrees for 60 minutes, then 2 to 3 bar gauge pressure and a 180 degree hold. Cooling is at 2 to 5 degrees Celsius per minute and the autoclave is vented at 60 degrees or below.

For lay-up the first ply should be vacuum debulked to the mould at room temperature, and further debulking of subsequent plies may be needed to remove trapped air, depending on part size and complexity.

Typical uses are primary and secondary load-bearing structures of large uncrewed aircraft, eVTOL airframes and helicopter and fixed-wing components, in both solid laminates and sandwich panels.