Composite – Prepreg Basics

Take some composite reinforcement, such as carbon fiber mat, lay it out and saturate it with a thermoset resin. What you have then, in the uncured state, is prepreg. Sounds simple? Well, just think about the practical problems – for example shelf life. Resins cure once they have the catalyst or hardener has been added, so how is curing of a prepreg prevented?

surfacing-ply.jpg

Image Source : EASYCOMPOSITE

There are two approaches, both aspects of the same solution – thermal control.

Prepreg resins are specially formulated so that they cure very slowly or not at all at room temperature. If they are then stored in refrigerated conditions then curing can be halted completely.

The length of time the prepreg can spend at room temperature before partial curing prevents practical use is known as the material’s ‘out life’.

The freezer storage time of a prepreg without affecting practical use when thawed-out is known as its ‘freezer life’ or ‘shelf-life’.

Complex Curing

The second approach is to cure the shaped prepreg product (say a rowing scull) at a high temperature – in a pressurized oven. These special ovens are called autoclaves. Clearly there is a practical limit to the size of oven that can be economically built. The largest of autoclaves can cure massive sections of airplanes.

The curing process can be complex. Here are the instructions for curing a specialized commercial prepreg used for tooling:

30 minutes at 300°F/150°C, then 4 hours 350°F/177°C followed by 6 hours at 383°F/195°C

This kind of process is hardly practical for the average DIY person building a skateboard, but nevertheless prepregs can be used in the home workshop – and the kitchen too as a domestic cooking oven can be used for some home projects.

Voids

When multiple layers of prepreg are required in a structure, then there is a risk of voids being introduced between the layers. For example, aerospace requirements dictate less than 1% void content in composite structures and components.

Vacuum bagging is used during manufacture of the prepreg, but to minmize inter-layer voids then then lamination should also be vacuum bagged if void content is a concern.

Why Use Prepreg?

The convenience of using prepreg is considerable. Prepreg is easy to handle and depending on its ‘tackiness’ it is to place in a mold and sticks in place.

It can be bought ready to use, so there is no resin mixing and no ‘wetting-out’ to be done. This improves the quality and consistency of the finished product (saturating the mat with resin – ‘wetting-out’ – is usually done using a vacuum bag by the manufacturer). For example, aerospace requirements dictate less than 1% void content in composite structures and components. The result is that in the finished product surface blemishes are almost entirely eliminated and weak spots due to resin voids are likewise minimized.

Why People don’t use Prepreg

The key reason is practicality. This can be because the structure is too large for an autoclave, or simply than it is impractical to keep the material in a freezer and on-site mixing is necessary.

Also cost, prepregs generally cost more then the same dry fiber and a similar liquid resin.

In some applications – for example in satellites – any void at all can cause cavitation damage when the structure is itself used in a vacuum and the pressure in the void blows out into space.

The Future of Prepreg

The key problem with prepreg has been the need for the autoclave, making it uneconomic for large structures. However, research into Out-of-Autoclave (‘OOA’) techniques is now starting to change the picture. Using Vacuum-Bag-Only (‘VBO’) curing processes at near-ambient temperatures, we should soon have the technology to make prepreg economic and practical for building airplanes (as opposed to building components).

SOURCE : COMPOSITEABOUT.COM

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