Advanced Composites Group

Advanced Composites Group (ACG), which has manufactured and supplied advanced prepreg systems for over 20 years, offers a range of standard prepregs moulded at high temperature and pressure (typically 120°C/250°F and 3 to 6bar/45 to 90psi in a high pressure autoclave). The Group also offers pioneering low temperature moulding with the LTM® low temperature prepregs.

Building on this experience, ACG now provides a unique and extensive range of products that are mouldable at low temperature and pressure. In particular, VTM®, ZPREG® and VTF surfacing technologies, which allow the moulder to achieve high mechanical properties and superb surface finishes via low cost processing routes - using only vacuum bag and oven curing, make prepreg technology a viable and cost effective composite processing option for all high performance components.

This general guide offers the prospective moulder an insight into the extensive range of ACG prepregs, while also acting as a brief guide to the material and process route options available.

Please refer to the ACG data catalogue for the complete range of products.

The ACG guide, An Introduction to Advanced Composites and Prepreg Technology, which provides a general round-up of advanced composites and an introduction to prepreg technology, compliments this selector guide and is provided for familiarisation purposes.

The Composite Advantage

Advanced composites offer engineers a new freedom to design functional, aesthetically pleasing structures of optimum performance not previously possible with more traditional materials. The performance of these unique and exciting materials is now being realized in many everyday applications.

Composites offer:

• Low weight.
• High specific stiffness and strength.
• Fibre orientation with the directions of principle stresses.
• Excellent environmental and corrosion resistance.
• Very low coefficient of thermal expansion.
• Improved vibration damping properties.
• Good fatigue resistance.
• Relatively simple repair of damaged structures.
• Ease of manufacture of complex shapes. 
• Low cost tooling options for one-offs, etc.

The Prepreg Advantage

A prepreg consists of a reinforcement material (carbon, glass, etc.) in unidirectional or fabric form pre-impregnated at a pre-determined and controlled level with a resin matrix. Prepreg is a fully compounded, reactive system (no handling or mixing of potentially dangerous chemicals). Simply take it from the roll and laminate.

All prepregs have a clearly defined working life (or out life) at room temperature. Material is stored frozen to maximise its life. The reactivity of a particular prepreg system is predetermined and this, in turn, dictates the minimum temperature at which a particular resin system will cure.

The out life and the minimum cure temperature are key considerations when optimising material selection, balancing lay-up time (large structures) and selection of the mould material (low temperature curing - low cost tooling).

Prepregs are supplied with a controlled level of tack. Plies can be easily and accurately positioned on the mould to give a high level of fibre orientation control without the risk of movement, fibre wash out or the development of resin rich areas during subsequent processing, all of which will lead to loss of performance.

Cored structures (foam, honeycomb, etc.) are also easily produced using complementary, formulated film adhesive systems which offer high strength bonding and controlled and consistent bond line thickness, rather than just depending on the parent resin matrix.

Prepregs offer:

• Simplified storage and reduced health and safety issues.
• Low cost tooling options.
• Reduced part labour content.
• Rapid and accurately controlled lay-up, optimising and maximizing final component properties.
• Weight saving.
• Reduced processing risks compared to infusion processes.

Prepreg vs Wet Lay-up vs Infusion
(all composite processes fall into three basic categories)

Glossary of Prepreg Technology

· Debulking:
Application of vacuum pressure periodically throughout a lay-up sequence to insure all plies are consolidated, improving final laminate performance.

· Fibre Volume Fraction (%Vf):
The proportion of fibre in the prepreg (by volume).This is a key determinant of laminate performance.

· Glass Transition Temperature (Tg):
The temperature above which a resin will begin to soften, loosing stiffness and strength. Generally, the maximum end use temperature for a resin should be at least 20°C below the Tg.

· Out Life:
Period of time a prepreg remains usable at room temperature after removal for storage at at -18°C/0°F. Note. The out life is cumulative each time the material is removed from storage.

· Partially Impregnated Prepreg Systems:
Resin & fabric combined but not fully impregnated.

These systems offer a number of advantages, most particularly:

• Simple vacuum bag processing.
• Multiple or mixed ply construction for rapid lay up.
• Partial impregnation retaining fabric drape and hand ability.
• Inbuilt air paths to optimising air removal and reducing void content.

· Resin Weight (%RW):
The proportion of resin in the prepreg (by weight) quoted as a percentage of the TOTAL weight of the prepreg.

· Shelf Life:
The length of time the prepreg can be stored under specified conditions (usually -18°C) and remain suitable for its intended function.

· Tack:
Measurement of the capability of an uncured prepreg to adhere to itself and to mould surfaces.

· Tack Life:
The length of time the prepreg can be stored at room temperature (20°C/68°F) and continue to have sufficient tack.

The ACG Advantage

ACG leads the way in the development and supply of prepreg materials into high performance markets such as aerospace, motorsport, marine and sports equipment, and in the provision of innovative new material formats and low cost processing options.
ACG’s Low Temperature Moulding (LTM®) prepregs revolutionised moulding and contributed to the development of the Group’s Variable

Temperature Moulding (VTM®) flexible moulding system. The VTM® system offers optimised low temperature, low pressure processing, and flexible cure temperatures between a modest 65°C and 120°C, eliminating the necessity for autoclave curing. In addition, the 30 days out life presents a wider process window in which to complete the laminating process.

Complemented by an extensive portfolio of Medium Temperature Moulding (MTM®) (120°C/248°F) and High Temperature Moulding (HTM®) systems (180°C/356°F), ACG offers an unsurpassed range of processing and performance options to the composite industry.

Building on its Out-Of-Autoclave (OOA) processing experience, ACG is now realising the next generation of prepreg and film infusion systems for high performance applications. MTM® OOA processing systems now meet the levels of performance, laminate quality and damage tolerance demanded by the aerospace industry, offering simplified processing, reduced costs and increased production rates necessary to meet the new challenges of this rapidly advancing market sector.

Resins

ACG prepreg resin systems are classified in terms of the temperature at which they are cured.

Reinforcements

ACG prepregs are produced using a wide range of fibre reinforcements:

· Carbon:
Fibres are available in high strength, intermediate modulus, high modulus, and ultra high modulus grades. These fibres are mostly used in high strength, high stiffness applications, where benefit from weight saving outweighs the additional material cost.

· Glass:
Most commonly used and lowest cost reinforcement is E-Glass. S-Glass is a higher strength alternative. Glass is much denser than carbon and has lower strength and stiffness values.

· Aramid:
More commonly known as Twaron® or Kevlar®. Aramid is a low density fibre normally applied to areas or in components where there is a likelihood of impact. Aramid has the ability to absorb and dissipate energy, while also exhibiting excellent abrasion resistance properties, but is poor in compression.

· Polyethylene:
Better known as Dyneema® or Spectra®, Polyethylene has similar energy absorption characteristics to Aramid fibres, but is a much lighter, more expensive fibre. Polyethylene has little or no compressive strength and begins to shrink and lose its properties at a relatively low temperature (100°C/212°F).

· Hybrids:
All the above fibres can be mixed or hybridised, yielding property combinations not normally available with the individual fibres.

Relative Fibre Stiffness

Formats

Storage

Prepregs should be stored, wrapped and sealed in polythene, at -18°C (0°F) for maximum shelf life. The material must be fully thawed before breaking the polythene seal in order to avoid moisture contamination.

Health and Safety

Prepregs are low-risk in terms of handling hazards; however, the usual precautions should be applied. Gloves and protective clothing must be worn and mechanical exhaust ventilation used when heat curing prepreg systems.