Prepreg

Prepreg is the English short form for preimpregnated fibers (American: preimpregnated fibers ), in German: "pre-impregnated fibers".

Prepregs are pre-impregnated textile fiber-matrix semi-finished products that are cured under temperature and pressure for the production of components. The reaction resins consist of a mostly highly viscous, but not yet polymerized, thermosetting plastic matrix, which v. a. is used in lightweight construction . The fibers contained can be present as a pure unidirectional layer , as a fabric or scrim . Prepreg, for example, is supplied in web form, wound on rolls. The term prepreg includes not only unidirectionally reinforced or flat semi-finished products, but also other preforms of basically any shape that, in the broadest sense, consist of an uncured thermoset matrix filled with fibers. The matrix is ​​in the partially cross-linked, so-called B-stage, and is pasty to solid, but can be liquefied again by heating.

Prepregs can be processed by machine and are therefore mainly used in automated processes. They result in a uniform and high quality. The advantages are their low undulation and high fiber volume . Curing at high temperatures enables short cycle times . The processing requires a high investment z. B. for autoclaves , laying robots, refrigerated storage.

Prepregs must not be confused with thermosetting fiber-matrix semi-finished products such as BMC ( Bulk Molding Compound ) or SMC ( Sheet Molding Compound ), which do not contain continuous fiber fabric, but rather shorter fiber snippets, usually <50 mm, as fiber content. In the literal translation of the word prepreg , it includes all thermoset fiber-matrix semi-finished products. In German-speaking countries, prepreg is mainly used for a continuous fiber-reinforced thermoset semi-finished product.

application areas

• In the aviation industry , prepreg is processed in large quantities. While fiberglass-reinforced plastic is mainly used for small aircraft or the main rotor of helicopters , the more expensive carbon fibers are increasingly being used in commercial aircraft and in the new generation of military technology . For example, the Airbus A380 is characterized by a mass fraction of around 20% and the Airbus A350XWB by a mass fraction of approx. 50% of carbon fiber prepregs. Carbon fiber prepregs have been used in the vertical stabilizers of the Airbus fleet for more than 20 years.
• In the automotive industry , proportionally little prepreg is processed. Due to the mostly high prices for prepregs, molding compounds such as BMC or SMC are used there. Make exceptions
• Motorsport
• Space travel
• Sports equipment
• Sailing yachts
• Orthopedic technology in both orthotics and prosthetics
• In electrical engineering as an "intermediate layer " in multilayer circuit boards and as an insulating material for electrical machines and transformers
• In wind turbines for rotor blade construction

Fiber types

All common reinforcing fibers can be used as fiber types. In practice, however, mainly

• Fiberglass
• Basalt fibers
• Carbon fiber
• Aramid fibers

processed into prepreg.

matrix

A distinction is made between the matrix systems according to their curing temperature and the type of resin. The hardening temperature has a major influence on the glass transition temperature and thus the operating temperature. 180 ° C systems are mainly used in military aircraft .

composition

The prepreg matrix consists of a mixture of resin and hardener, in some cases an accelerator. Freezing at −20 ° C prevents the resin from reacting with the hardener. If the cold chain is interrupted, the reaction starts and the prepreg becomes unusable. However, there are also high-temperature prepregs which can be stored at room temperature for a certain period of time. These prepregs can then only be cured in an autoclave at an elevated temperature.

The desired tack is achieved through special ripening processes and additives . This makes it possible to layer prepreg on convex shapes as well.

Resin types

Resins based on epoxy resin are predominantly used. Vinyl ester based prepregs are also available. Since vinyl ester resins have to be pre-accelerated with amine accelerators or cobalt, their processing time at room temperature is shorter than that of prepregs based on epoxy resin. Peroxides such as methyl ethyl ketone peroxide (MEKP), acetylacetone peroxide (AAP) or cyclohexanone peroxide (CHP) are used as catalysts (also called hardeners) . Vinyl ester resin is used where there is high impact stress.

processing

Prepregs are cured at an elevated temperature. They can be processed with the hot press technique or the autoclave technique. With both techniques, the pressure increases the fiber volume .

The best qualities can be produced with autoclave technology. The combination of pressure and negative pressure results in components with very little air inclusions.

The hardening process can be followed by a tempering process, which is used for complete crosslinking.

literature

• Hauke ​​Lengsfeld, Felipe Wolff-Fabris, Johannes Krämer, Javier Lacalle, Volker Altstädt: Fiber-reinforced plastics - prepregs and their processing . 1st edition. Carl Hanser Verlag , 2014, ISBN 978-3-446-43300-7 .
• Hexcel Composite, HexPly-Prepreg-Technology, 2013, [1]