Fast-curing towpregs as part of a thermoset material toolbox
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In high-volume serial production of composite components today, cycle times must generally be less than 5 min. The SGL Group has developed a fast-curing epoxy resin system that permits curing times of 1.5 to 3 min. Fibre rovings or Towpregs pre-impregnated with this resin system offer an excellent material for efficiently constructing laminates using automated fibre placement or fibre winding.
In high-volume serial production of composite components today, cycle times must generally be less than 5 min and, better still, less than 3 min in order to achieve the required output quantities. If thermoset materials are used, not only must all the manipulations (positioning, shaping, impregnation, etc.) be carried out within the above-mentioned time span but curing of the matrix material must also be completed. This calls for resin systems with extremely short curing times, which are nevertheless able to meet the requirements of the intended application, e.g. in terms of mechanical and thermal properties.
To address these challenges, SGL Group has developed a fast-curing epoxy resin system (“E420“) that permits curing times of 1.5–3 min, depending on various factors. By using fibre rovings pre-impregnated with this resin system (so-called towpregs), additional efficiency benefits can be gained, e.g. by eliminating the impregnation step in component production and ensuring a homogeneous, reproducible starting material. Towpregs are therefore an excellent material for efficient build-up of laminates or stacks by automated fibre placement or fibre winding, followed by shaping and rapid curing.
An important difference from slit tapes is the fact that the rovings can be individually processed and no release paper is used in the winding operation. This gives rise to a cost saving and increased efficiency in terms of both the material and process but, of course, makes higher technological demands when it comes to unwinding the towpreg without any problems after lengthy storage. To achieve smooth unwinding, carefully balanced material parameters such as the tack of the resin system used, resin content, and degree of impregnation are crucial.
The advantage of using towpregs is that, in comparison with, for example, the wet winding process, all resin processing and impregnation operations are eliminated. As a result, processes can be run faster with greater consistency, the costs and labor for resin storage and mixing are saved, and it is possible to change over more quickly to other products. In the production of towpregs, the resin content can be adjusted and held very constant. In addition, a defined towpreg width can be guaranteed with minimal deviation. This permits overlapping and gapless placement of the towpregs in the plane and contributes to improved mechanical properties and appearance.
With suitable automated placement processes, towpregs offer the opportunity to produce near net shape stacks specially optimised for the particular component. The individual towpreg rovings can be placed in precisely the required number and orientation for the expected loads in the intended application. Towpregs also make it possible to produce optimised structures for a subsequent shaping operation, e.g. by leaving gaps or introducing specific cuts in the towpreg rovings during placement. So towpregs provide degrees of freedom and possibilities in component production that are not available with conventional, flat, impregnated semi-finished products.
The potential range of application for towpregs is increased throug h the separation of the impregnation process from the use of the impregnated fibres, permitting the production of towpregs with high-performance resins, of which E420 is a good example.
Material Toolbox based on Snap-cure Resin System E420
All products in the material toolbox offer diverse freedoms for application-specific modification, such as different types of reinforcing fibre (carbon, glass, etc.) and customised textile architecture, fibre areal weight, resin content, width, etc. In addition, the resin system can be formulated with an internal release agent, if required, to further increase efficiency in component production.
Carefully balanced material parameters such as the tack of the resin system used, resin content, and degree of impregnation are crucial.
Mechanical properties of a unidirectional laminate produced from Sigrapreg Towpreg based on SGL 50k carbon fibres and an E420 resin system (© SGL Group)
Typical mechanical properties at FVC 55 % (cure: 3 min at 150 °C)
E420 Towpreg based on carbon fibre C T50-4.4/255-E100
0° Tensile strength
0° Tensile modulus
0° Compressive strength
0° Compressive modulus
0° Flexural strength
0° Flexural modulus
Interlaminar shear strength
In hot presses, cycle times of 45 s are achievable under ideal conditions, if followed by a post-curing step, e.g. during the electrophoretic dip coating process. However, there is a certain limit to these fast press times since just heating the material takes a certain amount of time.
Design and Engineering
Typical one-stage curing cycles are 3 min at 150 °C or 2 min at 160 °C.
Conclusion and Outlook
By combining towpreg technology with fast-curing resin systems, it is possible with individual fibre placement methods to produce both stacks and then, in the subsequent shaping and curing process, also the components in rapid succession. Offcuts can be minimised by individual towpreg guidance and a near net shape placement pattern. With a product standardised in width, resin content, and length, further rationalisation effects can be obtained.
When correctly employed, towpregs are not only suitable for 2-D-stacks but could also be of interest in 3-D-placement processes for a very wide range of applications in all areas. With a simple towpreg guidance system, AFP machines can be converted from 2-D- to 3-D-placement at reasonable cost, which can represent a further optimisation step.
As regards resin systems, it is possible to achieve the complete property spectrum obtainable with conventional prepreg resins in towpregs as well, for example by using flame-retardant resins for aerospace applications. In this way, potential uses for towpregs could be opened up outside high-volume, automotive serial production, so expanding the range of application for towpregs.