Fibre-reinforced composites
Fibre-reinforced composites combine low weight with excellent mechanical properties, and are therefore the material of choice for applications where lightweight construction concepts are required. The range of applications for fibre-reinforced plastics has expanded rapidly in recent years. Moreover, the fibre composite industry still holds great potential for growth looking forward.
WWe conduct research on fibre-reinforced composites and manufacturing technologies. Special attention is paid to the targeted modification of the matrix, for example to enhance the toughness, processability or flame-retardant properties of the fibre composite. We now also apply our knowledge of the targeted adaptation of resin systems in other areas of application such as medical technology.
Selected areas of application for our research
- Transport (Light-weight construction, hydrogen technology)
- Energy technology (Light-weight construction / wind energy, function integration)
- Medical technology (pure-resin formulations)
- Sports technology
The polymer matrix of fibre-reinforced plastics determines important properties of the composite in use, such as thermal resistance, damage tolerance, and fatigue behaviour. Yet even the manufacturing conditions of a composite are determined by the rheological properties of the matrix and its curing kinetics, and thus the viability of the whole manufacturing process.
One of our main research areas is the targeted modification of thermoset matrix systems with the aim of tailoring application and process-relevant properties. The work concentrates mainly on epoxy resins, but also includes research topics in the fields of polyester, polyurethane, acrylate, and benzoxazine resins.
In terms of process, the focus is on the common processing methods for producing fibre-reinforced plastics, such as prepreg impregnation, resin transfer moulding (RTM), vacuum infusion, and winding.
For the infusion process and prepreg semi-finished products, we have the entire process chain from resin formulation and processing to laminate characterisation under one roof. In particular, our equipment includes a 2-component RTM plant, a plant for impregnating thermoset prepregs, and a testing centre with extensive equipment for the mechanical characterisation of fibre composites. Resin developments can thus be validated on site.
The focus of resin modifications at Polymer Engineering is on:
- Optimisation of curing kinetics, e.g. the formulation of latent systems and rheological behaviour
- Increasing fracture toughness without sacrificing other material properties and process parameters
- Investigation of high and low temperature resistance and ageing behaviour
- Tailoring of electrical conductivity and thermal conductivity
- Development of individual flame protection concepts and investigation of fire behaviour
Compared to thermal curing systems, UV-curing resin systems have significantly faster reaction behaviour and higher curing speeds. These characteristics qualify UV-curing as a key technology for realising shorter process times and significant energy savings.
At present, the advantages described are already being used in the coatings and adhesives sector, as well as in additive manufacturing. In addition, this technology is currently being increasingly implemented in fibre composites, with glass fibre currently being the preferred material. Complete and rapid deep curing plays a central role here, which can be achieved with novel techniques such as “frontal polymerisation”.
One of our main areas of work is the targeted modification of UV-curing resin systems. The aim is to control the process- and application-relevant properties of resin systems in order to tailor the properties of various systems. In addition to cationic epoxy resin systems, which are being investigated in combination with different additives (accelerators, photosensitizers, reactive diluents, etc.), the focus is on radical acrylate resins.
From the point of view of process optimisation, it is primarily the viscosity profile and the curing kinetics that are adjusted. The investigation of application-relevant properties includes, for example, temperature resistance, toughness, thermal conductivity, as well as electrical insulation or conductivity, and fire behaviour. For the characterisation of UV-curing resin systems, we have a photo-DSC, photo-DEA, UV rheometer, and various light ovens at our disposal. In addition, Polymer Engineering’s own testing centre boasts an extensive range of machinery. It allows numerous mechanical parameters of both pure resins and composite materials to be determined on site.
“Prepregs” are semi-finished products for the manufacturing of fibre composite components consisting of continuous fibres impregnated with an uncured matrix polymer. The fibres can be arranged both as a unidirectional layer and as a fabric or scrim.
By using prepregs, it is possible to separate the impregnation process of the fibres from the shaping of the component. This makes it possible to realise more complex geometries and higher fibre volume contents. Prepregs can be processed by manual cutting and depositing, yet automated depositing processes are of particular importance for industrial applications. The reproducibility of these manufacturing processes qualifies prepreg technology for series production in high-performance, lightweight construction, for example, of structural components in aviation.
Prepreg materials are of particular importance for fundamental questions in the field of fibre-reinforced composites: Mechanical properties of composites can be determined on laminates built up from unidirectional prepregs with a defined resin content and with the lowest scatter compared to all other production technologies. This enables the systematic and precise elucidation of correlations between pure resin and composite properties.
We are also involved in the formulation of customised prepreg resin systems. Storage stability, viscosity, and curing kinetics of the matrix polymer have a crucial influence on processability. In addition, the surface tack of the prepregs must be optimally adjusted with regard to machine depositing processes. The application-oriented optimisation of the matrix systems goes hand-in-hand with research activities in the fields of halogen-free flame retardancy, toughening, and thermoset nanocomposites.
Our in-house prepreg plant has been especially designed for the development and characterisation of new prepreg resin systems. The plant makes it possible to produce aerospace-quality prepregs in a reproducible manner and only small quantities of resin using the hotmelt process. The reinforcing fibres can be fed directly as rovings via an in-line spreading unit or unwound as fabric. A special feature of the system is the innovative and extremely flexible application unit, which combines a roller module, a commarakel system, and a resin bath. With this configuration, matrix systems can be processed over a viscosity range of approx. to 10 to 50,000 mPas, and, in addition, processability can be compared by means of different impregnation technologies without any need for conversion The B-staging zone comprises two calenders and allows temperatures above 200 °C, so that high-temperature systems such as benzoxazine or cyanate ester resins can be processed on the line as well.
Our equipment enables extensive thermoanalytical, rheological, and mechanical tests on prepreg resin systems, as well as the production of laminates from prepregs using the press method, and comprehensive thermal and mechanical (quasi-static and dynamic) characterisation.
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Resin injection processes are used in a variety of very different areas for the production of fibre composites and are increasingly replacing manual manufacturing processes. Resin transfer moulding (RTM) has gained particular importance with the introduction of continuous fibre-reinforced plastics into automotive series production. In addition to economic and material-specific advantages, RTM processes are essentially characterised by significantly improved surface finish of components, improved reproducibility, and the possibility of comparatively simple integration of cores and inserts.
Resin infusion processes are particularly suitable for large-area components, as only one moulding tool half is required here, and thus investment costs can be saved. As a consequence, infusion pressure is lower than that of the injection processes and corresponds at most to prevailing ambient pressure. This limitation of the process parameters must be taken into account in the design of the manufacturing process by using appropriate aids such as flow aids, membranes, special textile, and laminate architectures, flow channels in the component, multiple gating points, etc.
In the field of injection processes, we are working on the development of fast-curing systems for RTM processes. In addition to curing kinetics, which should enable low viscosity up to mould filling and then very fast curing, the focus is particularly on increasing temperature resistance and toughness, as well as improving the fire behaviour of the composites.
Developments in the field of infusion processes mainly address the production of rotor blades for wind turbines. For the resin systems to be used, a sufficiently long pot life is required in order to completely infiltrate the reinforcement textiles. At the same time, the matrix systems must have excellent fatigue tolerance to ensure reliability in application. In our dynamic testing laboratory, fatigue properties of the laminates produced can be characterised on site.
Only in a few cases are materials in fibre form purposefully usable as a component, despite their superior weight-related strength or stiffness. Fibre-reinforced plastics therefore consist of fibres and a matrix that serves as an embedding material. In addition to the matrix and fibre material, the fibre architecture, which enables application-specific properties, has a decisive influence on the properties of the composite.
In the course of manufacturing components by “Liquid Composite Moulding” (LCM), e.g. using the “Resin Transfer Moulding” (RTM) process relevant for the automotive industry, it is necessary to drape the fibres in dry form in the component contour and to fix the layer structure required for the component. Both sewing processes and binder-based processes can be used for this.
Our research work includes, on the one hand, investigations into the influence of the sewing process and yarn material on laminate properties. On the other hand, we are developing binder systems that lead to high preform stability without, at the same time, impairing laminate properties or the manufacturing process. Current work is also concerned with the development of functional binders that contribute to the targeted optimisation of the laminate’s properties (e.g. toughness increase).
Polymer foams as the core for sandwich structures are used in construction, logistics, shipbuilding, aircraft construction, the automotive industry, wind energy, and in many other areas. This class of materials has long been popular with engineers because they offer a perfect combination of strength and resilience for a given weight. Modern sandwich structures offer economic advantages in terms of cost, weight reduction, and reliability.
One major application of sandwich structures is, for example, in wind energy. The rotor blades of wind turbines specifically use sandwich composites to increase the stiffness and buckling resistance of their components under the enormous loads encountered in use. For example, polymer foams and even balsa wood are used as core materials for the sandwich structure. When using different core materials, weight, price, and reliability can be optimised.
We are particularly concerned with the manufacturing and mechanical analysis of sandwich composites. In order to represent the load cases as realistically as possible, we regularly develop new test methods for static and dynamic testing. Based on our extensive expertise in foams, we also develop novel core materials for various applications.
Although thermoset matrix systems dominated for a long time, the market for thermoplastic composites has been growing steadily in recent years. Shorter cycle times, higher damage tolerance, and the formability and recyclability of the materials are the drivers of this development. The expansion of the use of thermoplastic composites is often still held back by the comparatively high melt viscosity and the resulting challenges for complete fibre impregnation.
To address this challenge, we have developed a so far unique prototype for the impregnation of organo sheets. The process allows melt to be introduced directly from the extruder between two fibre layers, and distributed homogeneously in the fabric, with the help of a special impregnation unit. This renders the usual upstream process steps, such as the production of films or powders, superfluous. This direct impregnation leads to a significantly improved economic efficiency of production, and thus opens up new fields of application. In the past, this innovative technology has been used to successfully produce organo sheets with various matrix systems – ranging from the standard plastic polypropylene (PP) to the high-performance thermoplastic polyetherimide (PEI).
