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, Volume 10, Issue 5, pp 14–19 | Cite as

Fast-curing towpregs as part of a thermoset material toolbox

  • Markus Wezstein
  • Thomas Meinhardt
  • Andreas Erber
Materials Fast Curing Resin
  • 258 Downloads

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.

Introduction

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.

Towpregs

Towpregs are individually impregnated fibre rovings, which after impregnation are rewound onto a bobbin in a similar way to the starting fibres, Figure 1. All reinforcing fibres currently on the market, such as glass and carbon fibres, are suitable for this process, with heavy tow fibres being particularly suitable. The resin system (generally epoxy resins), resin content, and width are also material parameters that can be freely selected within certain limits.
Figure 1

Sigrapreg Towpreg bobbins with 50k and 24k carbon fibres (© SGL Group)

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

When employing different impregnated semi-finished products, it is desirable if a single resin system can be used to serve the widest possible variety of tasks and applications. This is why SGL Group has developed the snap-cure resin system E420, which can be used in different, complementary, impregnated semi-finished products, Figure 2. This minimises the time and money spent by the customer on qualifying resin systems for different applications and components, since in the ideal case, only one resin system needs to be qualified. Restriction to a single resin system not only reduces complexity but offers additional advantages in terms of freedom of component design, since the various materials in the thermoset toolbox can be freely combined with each other without risk of incompatibilities due to different matrix materials or curing properties. So, for example, prepregs based on an isotropic carbon fibre nonwoven (which therefore also have isotropic mechanical properties) can be selectively reinforced in load directions with UD layers or strands produced from prepregs or towpregs.
Figure 2

Thermoset material toolbox for high-volume, serial-production applications (© SGL Group)

Figure 3

Isothermal viscosity curve at different temperatures (© SGL Group)

Figure 4

Isothermal reaction curves based on kinetic modeling (© SGL Group)

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.

Resin System

The E420 resin system was developed for the fastest-possible curing. Typical one-stage curing cycles are 3 min at 150 °C or 2 min at 160 °C. Despite this high reactivity, the storage life of E420 prepregs and towpregs at room temperature is at least four weeks. The mechanical properties achieved with the above-mentioned curing cycles are comparable to those of standard conventionally cured epoxy resin prepregs, Table 1 and Figure 5. The high glass transition temperature of 140–150 °C and its rapid build-up also permit the component to be demolded while still hot without any risk of deformation. This further speeds up cycle times.
Figure 5

Comparison between relevant properties of UD prepregs based on the E420 snap-cure resin system and the standard E320 resin system (© SGL Group)

Table 1

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

2000 MPa

0° Tensile modulus

125 GPa

0° Compressive strength

1200 MPa

0° Compressive modulus

120 GPa

0° Flexural strength

1400 MPa

0° Flexural modulus

110 GPa

Interlaminar shear strength

80 MPa

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

In combination with suitable processing technology, towpregs offer the opportunity to produce components specifically designed for the load path. To achieve this, the boundary conditions for the production technology and material must be considered early on in the design and laminate planning stage. As shown in Figure 6, draft design of the component and laminate is carried out during the design and engineering phase of the product development process, taking due account of various factors such as the processing technology and curing process, material characteristics, and boundary parameters for the tooling.
Figure 6

Interaction within the product creation process (© SGL Group)

An ideal processing technology for towpregs is automated fibre placement (AFP). In AFP technology, it is possible to process, for example, 16 towpregs at the same time using a single placement head. Each individual tow can be separately controlled so that cutouts or component contours can be produced with minimal trim waste. Figure 7 shows a machine of this type manufactured by BA Composites and also demonstrator geometry that clearly indicates the material efficiency of the production process, Figure 8.
Figure 7

Automated fibre placement machine (© SGL Group)

Figure 8

Exemplary laying path of a structural geometry (© SGL Group)

Typical one-stage curing cycles are 3 min at 150 °C or 2 min at 160 °C.

To take into account the boundary conditions for the material and production technologies at an early stage of product development, design and engineering must be a fully parameterised and continuous process chain. As shown in Figure 9, this chain ideally extends from the concept phase through the design phase to virtual prototyping. With CAM connections, the results of the design and engineering process can then be transferred to production, for example using the AFP process.
Figure 9

Process chain (© SGL Group)

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.

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Markus Wezstein
    • 1
  • Thomas Meinhardt
    • 1
  • Andreas Erber
    • 1
  1. 1.SGL GroupMeitingenGermany

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