Semi-finished Components, Materials, FRP

What are sandwich structures?

Sandwich structures consist of a lightweight core material covered by face sheets on both sides. Although these structures are lightweight, they have a high flexural stiffness and buckling strength. Hence, sandwich structures are an essential part of modern lightweight construction.

Core materials are:

• rigid foams
• honeycombs
• thermoplastic cores
• nap-cores

Resin impregnated woven fabrics or other carrier materials (so-called prepregs) can be used as face sheets. The basic properties of sandwiches, like flame retardance, mechanical properties and surface quality, are adjusted by selecting the resin, the woven fabrics and the core materials according to the desired application (e.g. aircraft interiors, facade facing).

Developments

Present developments include the development of new core materials as well as the development of prepreg resins for face sheets.

The novel prepreg resins are specifically adjusted reactive resins which meet specific requirements. These requirements are for example a plain surface where the structure of the subjacent core-material is not apparent (so-called telegraphing). Additionally, certain production parameters are required and the prepregs must have a sufficient shelf-life at room temperature. Many users of sandwich structures require a high flame retardance, which is influenced significantly by the face sheets because face sheets make-up a large part of the overall weight.

Core Materials are used in sandwich structures between two layers. They consist of lightweight materials that reinforce the top layers by absorbing the shear strengths.

Nap core

The structure of the nap core has several advantages over conventional core materials such as hard foams, honeycombs and themoplastic molds:

• good acoustics (sound damping)
• good top layer connection
• drapability
• cost-efficient production
• drainage
• wires can be integrated

Developments

The nap core as a core material has been known for years but the properties profile offered only few applications. New developments strongly improved the mechanical properties and paved the way for a wide spectrum of applications. By variating the basic materials and the production processes the nap core can be customized to different application purposes. The hitherto realized volume weight lies between 50 and 120 kg/m³. Higher or lower volume weights and the mechanical properties are adjustable according to requirements. Additionally to the use as core material the nap core can be customized as a crash absorber. Combined applications, such as sandwich structures with crash-absorbing function are also possible.

Foam on the basis of cyanates

The use of hard foams as core material is limited. Either hard foams are not flame-retardant enough (e.g. polyurethane resins) or too brittle (e.g. phenolic resins). Cyanate resins are intrinsically flame-retardant, heat resistant and the viscosity can be adjusted. They are recyclable and therefore sustainable. The same properties are expected for cyanate resin foams. Due to their chemical mechanism cyanate resins can be foamed in an environmentally-friendly way. We have produced the first cyanate resin foams and are currently working on the scale-up of these lab experiments.

Principle

Sandwich structures with a foam core often peel apart easily. As the commonly used rigid foams are brittle, use of optimized resins cannot prevent failure of the material, only shift it to the top layer. However, the peel resistance can be improved considerably by sewing the top layer and the core materials together.

Results

The peel resistance was increased up to 3-4 times depending on the sandwich configuration. Additionally the tendency to bend was decreased with sewn structures in comparison with an identical structure without sewing. Basic lamination and sewing technologies were used.

Application

Highly stressable lightweight sandwich constructions.

References

The project was carried out in co-operation with the BTU Cottbus and the RS-Technologie und Service GmbH. This work is promoted by the BMWi under the title „Entwicklung von hochfesten Leichtbaumaterialien in Sandwichbauweise“ (Indicator number: KF 0287501KMH1) (1.7.2001 - 30.6.2003).

Principle

Phase Change Materials (PCM) are materials that can accumulate and emit energy at room temperature through solidification and smelting.

Applications

There is increased interest in PCM in many areas such as decentral energy supply and devices for the building of constructions. The thermal capacity increases, the energy costs are reduced and a balanced room climate is established. The adjoining diagram shows the various effects on room temperature.

Science Activity

The Fraunhofer PYCO works in cooperation with the BTU Cottbus on PCM filling Sandwich Panels. These panels should be used as encasements for walls and slabs. Furthermore the PCM can be used in lightweight design.

References

This work is promoted by the BMWi with the title: "Energie-Optimiertes Bauen (EnOB): Thematischer Verbund LowEX – Entwicklung eines Messverfahrens zur Bestimmung des thermischen Beladungsgrades von PCM-Paraffin-Speichern" (Indicator number: 0327370F), (1.11.2004 - 31.10.2007).

Principle

A prepreg (PREimPREGnation) is a matrix material that is already impregnated with resin and pre-hardened. These matrix materials can be fibres of different material (e.g. glass, carbon, aramide or natural fibers) and different processing (e.g. woven, non-woven and knitted fabric, papers, etc.). By impregnating the matrices with a reactive resin or a thermoplastic material, a semi-finished component, the so-called prepreg, is made, which can be processed in a later molding process (influence of pressure and temperature) and hardened.

Applications

Prepregs are used, amongst others, in the area of lightweight structures to produce a wide range of products. For example, they are used in so-called sandwich composites, which are used for many purposes in traffic engineering. In sandwich composites different materials (e.g. foams, honeycombs, wood,…) are bonded with prepreg sheets on the top and bottom, which creates extremely light but very stable constructions. Another big area of application are laminates, which are produced by molding of several layers of prepregs and can be used as conductor plates for electronics components.

Equipment

• Impregnation plant: For impregnation of flat materials (e.g. wovens non-wovens, papers) made from glass, carbon, natural and synthetic fibres with reactive resins (e.g. epoxide, phenol and cyanate resins) to produce prepregs. The impregnation plant can also be used for surface finishing and laminating of papers and films.

• UD-Impregnation plant: enables the production of prepregs with unidirectional oriented fibers.

There are many different ways of using prepregs and a variety of characterisation methods for the resulting components are available.

The Fraunhofer PYCO in Teltow took part in the subproject 14 "new construction materials for sandwich and adhesive applications" of BMWi’s aviation research project LUFO III-KATO. In this project prepreg resins based on cyanate resins and core materials made from nap combs. The project was planned from 1.7.2003 til 30.6.2007. An extension until 30.6.2007 was approved by the funding body.

Prepreg resins based on cyanate resins

The main challenge with prepreg resins based on cyanate resins is to harden them at relatively low temperatures without losing their very good characteristics. At the same time they have to be considerably stable at room temperature for storage. Compared to previously used phenol resins cyanate resins have a higher toughness and are recyclable. They have, similar to the phenol resins, an intrinsic flame resistance and comply with the strict requirements for fire resistance in aviation. As addition resins no low molecular compounds are produced during curing, which creates better surfaces. These and more properties are adjusted through targeted modification of the starting resin.

Core materials from nap combs

The challenge with nap cores is to reach the exceptional weight-stiffness ratio and the good flame resistance of the commonly used honeycombs. The nap core has some advantages over honeycombs, which make them a very interesting core material for aviation in spite of their slightly higher weight-stiffness ratio: The nap core is drainable, it offers good surface sheet binding, it is drapable, offers improved acoustic  dampening, production is simple and cost-effective, and ducts and wires can be integrated directly into the structure.

Aims and Results

Natural fibers such as hemp and flax are – due to their similar stiffness and modulus in relation to density - an ecological substitute for glass fibers in fiber composites. The aim of this project was to assess to what extent materials for railway vehicle manufacturing can be substituted with natural fiber composites.

Using flax non-woven mats with an average area mass of 234 ± 15 g/m² as the fiber component, the manufacture process and the properties of composites on basis of

• unsaturated polyester resin (UP-resin), cold curing
• epoxide resin (EP-resin), hot curing and
• phenol-formaldehyde resin (PF-resin), hot curing

were investigated. UP-resin composites were compacted in several layers in a vacuum bag and cured at room temperature. Non-wovens impregnated with EP- and PF-resins were dried to produce prepregs and hot-pressed into multilayer laminates with a pressure of up to 2 MPa. The processing technologies could be generally matched to those of conventional glass fiber composites.

The laminate materials reached a fiber content of circa 25 vol.% when using cold curing UP resins in the vacuum process. The prepreg processes with hot curing epoxide- and phenol-resins lead to laminated with circa 50 vol.% fiber content. Assessment of the mechanical properties gave parameters which largely fulfilled the requirements for epoxide resins (tensile strength > 60 MPa, elongation at break > 1% und flexural stress at break > 120 MPa). Mechanical properties were dependent on the direction of stress in relation to the position of the non-woven in the composite. Addition of flame retardants to the impregnating resin lead to a decrease in the mechanical properties of the laminates proportional to the amount of added flame retardant.

Applications

Lightweight components for e.g. vehicle and exhibition stand construction

References

The work was funded by the Bundesministerium für Bildung und Forschung within the network “RIO” of the InnoRegio program under funding reference 03I4609D

Cooperation

The work was carried out in close cooperation with AMIC GmbH, Berlin, Sächsischen Textilforschungsinstitut, Chemnitz, Hippe KG, Werk Spremberg and Dr. Hielscher GmbH, Teltow.

Principle

Fiber composite materials are used for lightweight structures because of their low density and good material properties. For common construction applications glass fibers are mainly used as reinforcement. However, natural fibers like hemp, flax, jute and sisal can be used as a green substitution for glass fibers in many composites due to their strength and modulus levels.

Applications

Apart from already existing areas of application for natural fiber-plastic-composites such as interior paneling or coverings in cars, there is also interest in structural components for vehicles (Rails and Street) on the basis of reactive resins as well as in molded components made from short-fiber reinforced thermoplasts, for use where the natural fibers’ low density enables a reduction in weight compared to glass fiber reinforced materials.

Scientific Achievement

There are a few additional problems with the use of natural fibers instead of glass fibres in vehicles:

Flammability of the fiber and therefore additional need to develop flame protection

Continuous glass fibers are monofil. Natural fibers are spun threads from single fibers, which already have a structure. Due to the resulting different conditions of composite formation between polymer matrix and fiber the processes need to be adjusted.

The hydrophilicity of natural fibers needs to be under control in structural components to prevent damaging water absorption.

References

The work was funded by the Bundesministerium für Ernährung, Landwirtschaft und Forsten with  the Agency for renewable feedstocks  as the executing body under the funding reference 97NR066.

Involved in the development : Landgraf Kunststoffe, Fürstenwalde; Dr. Hielscher GmbH, Teltow, Brandenburgische Technische Universität Cottbus, Lehrstuhl Polymermaterialien

Principle

The thermal expansion (CTE) of polymer materials can be reduced by adding fillers. Within this project ceramic materials are synthesized, which have a negative thermal expansion ((β-eukryptite), to achieve very low CTEs. High fill levels are also necessary. To achieve this, filler blends with defined particle size distribution were made on the basis of theoretical sphere packing, to combine high packing density and good processability of the composites.

Applications

The motivation for this project is the often not tolerable thermal expansion of polymer materials, e.g. for applications in microelectronics. If materials with different coefficients of thermal expansion are combined, it can lead to system failure due to this thermal mismatch. Often silicon chips (CTE = 3·10-6 K-1) are attached to polymer conductor plates (CTE = 70·10-6 K-1). There will be shear forces at the soldered connections in a mismatch.

Scientific Achievement

For the same amount of filler the thermal expansion of the β-eukryptite composites is lower than that of composites with commercially available quarz sand and is within the area of stretching of silicon composites. The work continues.

References

This work is supported by the Brandenburgische Technische Universität Cottbus within the HWP research project funding 2004-2006 of the MWFK of Brandenburg.

Publications

M. Uhlmann, C. Olschewski, M. Bauer, S. Landeck, S. Vieth: Rheologische Eigenschaften von Suspensionen aus silanisierten Keramikpulvern, in: Keramische Zeitschrift (eingereicht März 2006).

S. Vieth, M. Uhlmann, B. Beckers, F.-D. Börner: Non-volatile organic liquids for electrostatically, electrosterically and sterically stabilised suspensions of ceramic powders, in: Cfi/Ber. DKG (eingereicht März 2006).

S. Landeck, M. Uhlmann, M. Bauer, C. Uhlig und S. Vieth: Polymer-matrix composite incorporating inorganic filler with negative thermal expansion, in: Composites: Part A (eingereicht März 2006).

Principle

Sandwich constructions with a foam core often have a low peel resistance. As the used hard foams are brittle, using optimized adhesives only moves the sandwich’s point of failure under peel stress to the outer layer of the foam core.

The peel resistance can be improved significantly by sewing  the top sheets to the core.

Results

The peel resistance was improved by a factor of 3 resp. 4 according to the structure of the sandwich. In addition to the peel resistances other properties of the sandwiches were improved by the sewing. For example the bend resistance of sewn structures was increased by up to 140% compared to identical structures that had not been sewn.  Broadly introduced technological solutions could be used for the production of the composite materials with reference to sewing and laminating technology.

Applications

Heavy duty lightweight sandwich structures

References

The project was carried out in cooperation with the Chair in Polymer materials of BTU Cottbus and the RS- Technologie und Service GmbH. We thank the BMWi for funding under the title „Entwicklung von hochfesten Leichtbaumaterialien in Sandwichbauweise“ (Förderkennzeichen: KF 0287501KMH1).
(i1.7.2001 - 30.6.2003)