What are Prepregs?

Prepregs are reinforcement fibres (organic, inorganic, thermoplastic or even natural) that are generally preimpregnated with thermosetting resins and in most cases epoxy; although we can find special applications in which other resins such as phenolic, cyanoester, bismaleimide, etc. are used.

From prepregs, we can build complex components with high mechanical and thermal strength and also light weight. Depending on the area of application, different fibre-matrix combinations with different fibre types can be used. These materials are today an essential part of the manufacture of high-performance components in the following industries: aerospace and aviation, railway, construction, sports boats, naval (military applications), automotive, sports applications, wind energy, among many others.

The main reason for the widespread use of these materials lies in their excellent mechanical, physico-chemical and thermal properties.

Conventional techniques for manufacturing composite laminates with dry fibres and room temperature curing resins have been used for several decades. These techniques have an important limitation, which is determined by the time that elapses when the resin is catalysed to when what is known as polymer gelation, also known as "working time", occurs. Before the resin reaches its gel state, it remains in a low viscosity liquid state, allowing the user to easily perform tissue impregnation. The operator is limited in the time of use of the resin depending on the gel time, which in some cases is reduced, preventing large applications.

In most of cases we are describing, resin distribution is carried out manually and even if this is done by expert hands, there is a high risk of making mistakes during mixing (resin/catalyst/hardener) and/or during the reinforcements impregnation.

Pre-preg technology has come to solve this problem.

The fibres that make up these materials are industrially impregnated by a process that uses the optimum amount of resin, and at the same time distributes it completely evenly. These resins already incorporate their hardeners in the stoichiometric mixing ratio, which means in the exact proportions necessary for a complete and efficient chemical reaction.

The resins and catalytic systems used for the production of prepregs are low-reactivity polymers, which need the input of a heat source to complete their curing process. This is why an oven or autoclave is absolutely necessary for the manufacture of composite parts. In addition, the oven or autoclave curing processes for these types of materials are also carried out in a vacuum environment: the parts are laminated in moulds and subjected to the action of a compacting process, making use of a vacuum film that is hermetically sealed to the contour of the mould. In other cases, the mould is placed in a tubular vacuum bag in which both the laminate and the mould itself are compacted.

Los moldes deben resistir la presión positiva ejercida por la autoclave (generalmente entre +1 y +12 bar), la compactación inducida por el vacío (aproximadamente -0,95 bar) y el ciclo térmico que varía entre rangos de temperatura de aproximadamente 65 y 250 oC

The main advantages of using prepregs are the following:

Absolute control of the amount of resin per m2 of reinforcement: this allows us to design prepregs that will later be used in cosmetic applications (exposed carbon, laminates with total absence of microporosity, etc.), structural applications with high mechanical performance, ballistic applications, etc. The amount of resin determines, for example, the greater or lesser flexibility of a laminate, as well as its weight, shrinkage or aesthetics. The elimination of excess resins makes it possible to have a much lighter product, impregnated with only the material necessary to ensure cohesion between the reinforcing materials.

Another major advantage of prepregs is a higher fibre volume content compared to resin, in contrast to traditional processes. This is about 60%, while for example in manual wet lamination it is about 40%. The high fibre volume content and the optimised fibre impregnation allow the production of extremely high-performance, lightweight components. This impregnation process is extremely precise and the resin/fibre contents are repeated very precisely in each production run. This allows us to produce theoretically identical parts in series production. In manual laminates, the dispersion is enormous and it is very difficult to control resin consumption.

With prepregs the applicator does not run the risk of making resin/hardener mixing mistakes that ruin the application.

The health hazards for the workers are minimised by not working with chemicals in a liquid state. The polymer is in a pre-polymerised state in prepregs, also known as B-stage. Working environments are much safer and cleaner, as there is less exposure to potential volatiles emitted by resins or to suspended reinforcement fibre particles as dangerous as those from carbon or glass fibre.

In this state the resin system needs to be stored refrigerated at -18°C to be stable for generally 12 months. At the time of use, the prepreg is removed from cold storage and thawed at room temperature in a slow (unforced) process.. At room temperature of 21°C the prepregs remain stable for between 3 and 21 days. This means that the operator, whatever the prepreg, has a long time to work with these materials in the construction of his parts or even moulds.

Cutting and handling are significantly simplified. When it´s applied, the B-stage is characterised by a slight tackiness of the resin, which significantly improves the reinforcements adhesion to the moulds surface.

With the use of prepregs, material waste is significantly minimised. Are you sure you have never wasted a resin in a container because you didn't have enough time to laminate and impregnate the reinforcements? 😊 Don't fool yourself... With prepregs this will never happen to you.

Prepregs are ideal for the manufacture of sandwich structures, especially if honeycomb cores are used, but due to their low resin content, the surfaces in contact with the core must be enriched by the use of special adhesive films such as epoxy adhesive film MTFA500, because they have higher adhesion properties than the resin itself and have the advantage of being placed only in the areas where they are needed, which helps to limit the final weight increase of the part.