Prepreg - A171

Prepreg
Document Type	Article
Document Identifier	171
Relevant Class	Material
Tags	Prepreg Thermoplastic polymers Thermoset polymers

Introduction

Prepreg is composed of fibre that is pre-impregnated with a thermoset resin that is later brought to a 'B-stage' cure. The resin is typically a high performance epoxy. The fibre reinforcement in the prepreg is commonly unidirectional or woven fabric. Graphite/epoxy prepreg is the most common prepreg material. Glass and aramid fibers can be also be made into prepregs, they are less common because prepreg are typically used for light weight, high performance applications. Other resin systems such as polyimides, polycyanate, and BMI can also be used in prepreg. Most prepreg material is cured in an autoclave or oven. Prepreg is known for its repeatable properties, high fiber volume fraction and excellent performance.

Scope

This page will discuss prepreg material forms. Manufacturing processes that use prepreg will be briefly presented, as it is a necessary consideration when selecting prepreg materials. Material suppliers of prepreg materials will also be provided.

Significance

The majority of high performance composite components are made from prepreg material because it provides excellent control of the reinforcement and matrix content. With deposition methods such as automated tape laying (ATL), filament winding (towpreg winding) and automated fiber placement (AFP), the fiber orientation can be precisely controlled, giving designers and engineers a high degree of freedom to place material exactly at where it is needed. Combining with autoclave or oven curing, components can be made with high quality, reliability, and performance.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Thermoset polymers - A105
• Thermoplastic polymers - A161
• A144
• A145
• A146

Prepreg manufacturing processes

Storage & Handling

Prepreg is shipped and stored under refrigerated conditions. If prepreg must be transported under ambient temperature then dry ice can be used to keep the material frozen. The time spent under ambient temperature (out time) should be documented and minimized. Before preparing for deposition, prepreg is slowly thawed to room temperature in sealed packaging to avoid condensation.

In the case of hand layup, the room temperature prepreg is cut into the desired size and shape before layup. The cutting of the prepreg should be performed in a clean room where temperature, humidity, and UV light are controlled. Ideally the clean room should have slight positive air pressure to keep the dust particles out. Inlet air should be also be filtered to minimize dust particles. Similar standards should be applied to the prepreg deposition environment to keep contamination to a minimum.

Deposition and consolidation

Prepreg can be deposited by hand, automated tape laying (ATL), filament winding (towpreg winding) or automated fiber placement (AFP). Hand layup is typically the most labor intensive deposition method but may be more advantageous for small and complex parts with low production volume. Automated tape laying is more suitable for laying up large, flat geometry that has mild curvatures, such as a wing-skin. ATL can also be used to layup flat charges for forming operations. Filament winding can achieve high material deposition rate and is used for positively curved parts with no re-entrant geometries such as pressure vessels or fuselages. Parts made with filament winding can have extremely high (up to 70%) fiber volume fraction. AFP has characteristics of both ATL and filament winding. AFP is capable of laying up large complex geometries that can not be achieved by ATL or filament winding.

Gaps between plies should be specified on engineering specifications. For unidirectional materials, gaps are typically less than 0.762 mm (or 0.030 inch) wide and overlaps or butt splices are not allowed. For woven prepreg materials, butt splices are allowed with overlap of 12.7 mm to 25.4 mm (0.5 inch to 1.0 inch). The figure below describes this. The engineering specifications should specify ply location, location accuracy and ply drop-offs at all locations of a part . The gap and overlap distance can be precisely controlled in ATL or AFP processes, less so in hand layup. Defects associated with gaps and overlaps can affect performance/failure of the composite part negatively .

Debulking

During and after the deposition, prepreg is typically de-bulked (consolidated) with vacuum to remove entrapped air within the laminate stack. Hand layup parts are typically de-bulked every three to five layers depending on the shape complexity. Debulking can also be perform at elevated temperatures (65°C to 93 °C or 150 °F to 200 °F) to soften the resin for better consolidation. In ATL or AFP, the shoe or roller that deposits the prepreg can provide in-situ consolidation to some extent. Vacuum is still required for de-bulking and subsequent curing. Shrink tape can be wrapped around filament wound parts to provide compaction.

Vacuum bagging

Prepreg can be designed to bleed a certain amount of resin or as a net resin system that should not bleed excess resin. When bleeding is required, the raw uncured prepreg contains more resin than the final part. During early stage of the cure, before the resin gels, the excess resin is squeezed out and absorbed by the bleeder to increase the fiber volume fraction. Whereas a net resin system has a pre-determined resin content that does not change appreciably during cure, allowing bleeders and barriers to be omitted. Depending on whether the prepreg system requires bleeding or is a net resin system, the vacuum bagging setup is illustrated below. The purpose of each consumable is explained here below. They are typically applied in the order presented, starting from the tool/mould surface:

• Non-porous release film: assist releasing the part from the mold. In complex geometries, release agents are used instead
• Peel ply (optional): provide texture if either the mold side or the bag side of the part will be adhesively bonded after cure
• Porous release film: prevent breather from bonding to the peel ply and the laminate while allowing gases to escape and/or resin to flow into the breather
• Bleeder: absorb and retain excess resin if the resin is designed to bleed
• Inner bag: separates the bleeder from the breather
• Breather: provide air and volatile passage to the entire part surface, allowing for uniform vacuum pressure. Breather can also be used to pad sharp corners or thermal couples to prevent puncturing the vacuum bag
• Dam: provide the laminate with a straight vertical edge. If not used, the edge can taper from vacuum bagging. Dams are not necessary for very thin laminates
• Vacuum bag sealant tape: seal the vacuum bag onto the tool, creating a pocket for the application of vacuum or external pressure. Sealant tape is visco-elastic and can soften at elevated temperature. Operator should ensure there is no high tension in the vacuum bag which can pull and deform the sealant tape, causing vacuum leaks when the part is heated to cure
• Vacuum bag: seal the consumables and laminate allowing vacuum or external pressure to consolidate the layup

Material bridging

A key consideration is to avoid bridging in concave/corner areas of the part. Bridging is when the material takes a shorter path across the inside of an angle and does not conform to the mould/tool surface. During prepreg deposition, prepreg should be compacted firmly against the tool. Consumables such as release films, peel ply, bleeder and breather cloth should also have enough slack to conform to the concave/corner features when being consolidated. Pleats should be made to allow excess vacuum bagging material to conform to the concave/corner. Failing to do the above mentioned can cause bridging of the prepreg, consumables or vacuum bag. The bridged area creates a low pressure region where resin can flow into during cure. This can lead to resin rich areas, porosity, and residual stress in the laminate around bridged region.

Curing

Most prepreg are cured at elevated temperature and pressure compared to room temperature and atmosphere pressure. Prepreg are commonly cured with an autoclave, oven, heat blanket or internally heated tool. The use of an oven or autoclave is more common.

Autoclave

Autoclaves are used to cure prepreg for aerospace and other high performance applications. With the application of high pressure, an autoclave can provide excellent consolidation and produce parts with low porosity. Typically vacuum is applied along with autoclave pressure to provide further consolidation and help remove volatiles. The temperature, pressure and vacuum schedules should be tailored to the specific combinations of material(s), shape(s), tool(s) and the autoclave itself. The Thermal Management section covers this in detail.

Oven

Ovens can be used to cure out-of-autoclave (OoA) prepreg material. This form of prepreg was developed to provide a more economical alternative compared to material that requires an autoclave to cure. Both the equipment and tooling cost can be significantly lower compared to autoclave cure. However, because the consolidation pressure comes from just the vacuum bag, oven cured parts are generally more susceptible to defects including higher porosity (on the order of 5-10%). Surface finish and dimensional control of oven cured parts may also be less satisfactory due to the lack of external pressure.

Suppliers

Product suppliers

Common providers of this material include:

• Toray
• Hexcel
• Solvay
• Mitsubishi Chemical Carbon Fiber and Composites

Expert support providers

A selection of people and companies capable of providing support for using this material to manufacture composite parts include:

• Composites Research Network
• Composites Knowledge Network
• Polyvations

References

About	Help

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

A single unit of fibre reinforcement that cannot be separated further without breaking it. It often has a circular cross section that is formed by the dies used to create it or its precursor material. Many filaments are manufactured in parallel to produce a strand or tow. The number of filaments in a strand or tow is often references by how many thousands of filaments comprise the tow. e.g. an 18k tow is made up of ~18000 filaments.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).