Prepreg composites, which were once prevalent in the aerospace and performance sectors such as Formula 1, have become increasingly common across many sectors of the composites industry for their ease of use, consistent properties, high quality surface finish and the wide choice of types that are now available.
Here, Philippe Christou, Head of European Technical Support for Huntsman Advanced Materials, looks at the most recent developments and the key characteristics of the prepreg matrix in addressing how prepregs are evolving to offer new design freedoms and optimum performance.
Why prepreg?
A prepreg composite consists of a fibre reinforcement material, usually in fabric, roving or unidirectional form, pre-impregnated with a resin matrix in controlled quantities.
Prepreg manufacturers (prepreggers) partially advance the formulated resin system to a B-stage, providing fabricators with prepregs in a “semi-solid” which allows them to lay-up several plies, complete the cure with heat and pressure, and obtain the finished composite part.
The premixed fibre and resin is advantageous in enabling a very clean and controlled laminating process with ready-to-use materials for the production of composite parts.
The stringent control of resin content and dispersion eliminates fibre content variations and the build-up of resin intensive areas in the pregreg. This is essential for the delivery of very accurate and uniform weight ratios and ultimately, for achieving the desired properties of the final composite component.
Reduced labour costs and recent material developments have turned prepreg technologies into a commercially viable option.
Whilst the initial cost of prepreg materials is higher than materials for wet lay-up processing, it can be argued that the high level of control provided offsets this cost. With prepregs, the labour associated with manual wetting-out processes is removed and as the resin is pre-impregnated, the work involved in redistributing the resin is significantly less. Clean-up operations and the associated time and costs involved are also reduced as a result.
Typical prepreg applications
From the markets for large advanced composite parts, including the marine, wind and general industry (with applications such as wind blades, yacht masts, and pressure vessels and tanks) through to the mass automotive and sports and leisure industries (with products including car leaf springs, skis and bikes for example), prepregs are increasingly gaining wider acceptance and showing real performance benefits in a wide range of lightweight structures.
The role of the resin matrix
There are many different types of fibre reinforcements and advanced resin matrix systems that can be selected.
To establish the necessary controlled environment for the production of the prepreg, it’s important to consider the ‘component to composite’ value chain and the role that the matrix components, formulation and processing play in achieving the best performance.
The formulation for prepreg composites contain all or several of the following:
Resin(s) Main architecture
Hardener(s) Supporting architecture
Accelerator(s) Reactivity aid
Modifier(s) Performance modification
Additive(s) Processing aids
The majority of composite processes feature a combination of resin and hardener (or curing agent) with a reinforcing fibre, which may also include a solvent. Heat and pressure are typically used to shape and cure the prepreg into a finished part.
The resin holds and protects the reinforcement fibres, ensuring the transmission and distribution of the load to all filaments while also providing temperature and chemical resistance. The hardener acts as a polymerising agent for the resin by connecting the reactive groups of the resin and controlling their reaction rate. Thus, they play an important role in determining performance characteristics of the finished part.
Adding to this, the reinforcement imparts strength and other required properties. In some cases, accelerators, modifiers and additives are included to optimise matrix system performance. For example, accelerators help to modify curing times and modifiers increase resistance to crack propagation.
A formulation for success
The resin matrix is formulated by mixing the resin, hardener and any other material components either manually into a small mixing vessel, or for larger processes, the components are pumped into a mixing vessel. The prepregging process is then undertaken to impregnate the resin matrix into the reinforcement. Following this, the prepreg often needs to be placed in cold storage to prevent a chemical reaction occurring before the prepreg is used for manufacturing.
Performance optimisation
Reinforcement comes in various forms; components needing high strength and stiffness in one direction (i.e. yacht masts, skis or wind blades) will typically be made up of unidirectional strips. However, other structures, which require strength and stiffness in multiple directions, are more likely to employ reinforcement with an unlimited ply orientation, such as multi-axial non-crimp fibre.
It’s therefore important for the matrix architecture to be designed accurately to optimise weight distribution and flexibility in relation to single or multiple directions and the application of the selected reinforcement.
Thermosets rather than Thermoplastics are most commonly used in prepreg composite manufacturing with epoxy resin representing the primary chemistry of choice. Others include phenolic, bismaleimide and cyanate esters.
As the main ingredient, the resin needs to offer a low enough viscosity to enable formulation and development, facilitating air removal during processing and curing at specific temperatures/times. It should also provide excellent tack and drape characteristics, allowing the prepreg to mold properly whilst maintaining fibre orientation without the risk of moving during processing.
Addressing market demands
Temperature resistance has emerged as a key target for today’s prepregs. In the market for mass transportation for example, in order to introduce more advanced composite applications in close proximity to an engine, there is a requirement for improved heat resistance.
In order to match performance to requirement, the onus lies on keeping full control of the system’s curing kinetics without making any sacrifices to any other processing capabilities, such as long latency, or the prepreg’s final properties.
Maintaining and increasing thermal resistance whilst providing additional performance benefits has always been a main focus for Huntsman, from the scientists who discover new molecules to the production specialists who bring concepts to commercial molecules.
To illustrate this commitment, the following proven products can be highlighted:
· Tri-functional, TGHPM type epoxy resin Tactix® 742 provides a higher glass transition temperature resistance than any other resin. This solid resin is compatible with other epoxy resins, showing quite low viscosity when heated at medium temperatures and can be processed easily into prepreg formulations designed for high temperature applications
· Dyclopentadiene-based, novolac epoxy Tactix® 556 presents lower moisture absorption than many multifunctional epoxies commonly used in advanced composites – making it ideal for use where the retention of properties under hot and wet conditions is critical
· Solid latent hardeners and accelerators, Aradur® 976 for example, provide long latency in combination with high thermal and mechanical performances which support the development of high thermal resistance formulations
· Aradur® 3123 and Aradur® 1167 can act as hardeners or as accelerators with high latency and snap-cure behaviour above 110°C
In summary
Here we have looked at the elements which define the standard for prepreg matrices. As acceptance of prepregs continues to grow as a material of choice for mass production across a variety of commercial applications, demands for even greater efficiencies will be made and further investment in material innovation will be imperative.
Leveraging our core competencies in synthesis and formulation, this is a challenge that Huntsman intends to meet. Indeed, the molecules for tomorrow are already ‘cooking’ under the design phase in Huntsman’s R&T laboratories.
www.huntsman.com/advanced_materials
Patricia Albisser
Huntsman Advanced Materials (Switzerland) GmbH
K-401.5.77
Klybeckstrasse 200
CH-4057 Basel
Switzerland
Phone: +41-61-299 2664
Fax: +41-61-966 8130
Email: patricia_albisser@huntsman.com
