A composite material or composite material (short composite , English composite [material] ) is a material of two or more related materials, which other material properties has as its individual components. The material properties and geometry of the components are important for the properties of the composite materials. In particular, size effects often play a role. The connection is made by material or form fit or a combination of both. In the case of packaging , the term composite is also used for materials manufactured for this purpose. Occasionally the term compound is used for composite materials with a plastic component.
Composite materials are mixtures of single-origin raw materials. A solution of the individual raw materials with one another does not take place or takes place only superficially. The compounding thus combines at least two substances. The aim of compounding is to obtain a material that combines particularly favorable properties of the components. It is usually important to be able to ensure an intimate connection between the phases over the long term and under stress.
The goals of compounding are varied and depend on the desired properties of the later component. Here, in the sum of the requirements, compromises often have to be made, as certain properties can negatively influence one another.
Typical compounding goals are:
- Change in the mechanical properties of a binder (base polymer). In this case, mechanical properties such as tensile strength, elongation at break (see also tensile test ) and impact strength are set by adding reinforcing materials and fillers as well as modifying impact strength .
- Color settings. The desired color is set by adding pigments or so-called masterbatches or liquid colors . However, some additives can have a very significant effect on the mechanical properties.
- Flame retardant. The addition of flame retardants can prevent flammable binders from igniting or from continuing to burn after the ignition source has been removed.
- Addition of stabilizers and stabilizer systems. The main reasons for stabilization are:
- Temperature-initiated chain breakdown of polymers during processing. This can result from excessive shear of the material or from too long dwell times in the processing machines. It is prevented by a simple stabilization designed for a short-term load.
- Temperature-initiated chain degradation in the application. Plastic parts that are exposed to high temperatures during use, e.g. B. in the engine compartment of a motor vehicle, must be stabilized to this load.
- Improvement of the weather resistance : Plastic parts in outdoor areas are exposed to severe damage from oxidation and hydrolysis . These can be compensated to a certain extent by using special stabilizers. Depending on the base polymer and stabilization, these effects can be delayed for different lengths of time.
- Addition of processing aids. This group of substances essentially improves the processing of the polymers. In this way, z. B. easier demolding in the injection molding process by mold release agents . This group of additives is less relevant for the end application.
A distinction is made according to the geometry of the composite:
- Particle composite materials , also known as particle composite or dispersion materials,
- Fiber composite materials ,
- Layered composite materials , also known as laminates ,
- Penetration composites,
- Structural composites.
The components of a composite material can themselves be composite materials.
In the case of particle and fiber composite materials, particles or fibers are embedded in another component of the composite material, the so-called matrix . In fiber composite materials, the fibers can run in one or more specific directions or have preferred directions.
Fiber composites can be produced in layers, but are not yet layer composites if the successive layers are similar. However, the term laminate is also used here. Layered composite materials consist of different numbers of layers on top of one another.
The special case of three layers, two of which are identical outer layers, is also known as a sandwich composite. A sandwich composite material often consists of hard, resilient outer layers and a light middle layer that serves as a spacer and shear bond .
Penetration composite materials consist of an open-pored carrier material, which is filled with the matrix-forming binder . They are produced, for example, by impregnating an open-pored sintered material (such as a foam ceramic) with a melted second substance.
The basic combination options for composite materials result from the material classification of the materials into polymers ( plastics ), metallic , ceramic and organic materials . Thereby, application-specific attempts are made to combine the different advantages of the individual materials in the end material and to exclude the disadvantages.
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Examples of fiber composites:
- glass fiber reinforced glass,
- Metal matrix composite , e.g. B. boron fiber reinforced aluminum
- Fiber cement (e.g. "Eternit"),
- Ceramic fiber composite (e.g. carbon fiber reinforced silicon carbide in high-performance brake discs)
- self-reinforced thermoplastics (plastic fibers in a plastic matrix of the same composition)
- Reinforced concrete ,
- Fiber concrete , steel fiber concrete ,
- Fiber-plastic composites
Examples of composite materials:
- Composite panels (e.g. plywood)
- Composite pipes
- TiGr-Composit: A material made of titanium , carbon fibers and epoxy resin
- Glass fiber reinforced aluminum : A material made of glass fiber reinforced plastic and aluminum
- Sandwich construction
- Hylite, a sandwich structure made of a plastic plate that is embedded between two aluminum plates / foils.
Reinforcement materials are inorganic or organic additives used in plastics that reinforce the plastic matrix. Reinforcement is understood to mean the improvement of mechanical and physical properties, such as elasticity , flexural strength, creep mechanics and heat resistance. Reinforcing materials are used specifically to improve these material properties.
Classification of reinforcement materials
Takes place on the one hand according to the chemical composition, on the other hand also according to the physical shape of the substance. There are flat reinforcing materials in the form of woven fabrics, scrims, knitted fabrics, and knitted fabrics.
The starting materials for these flat reinforcing materials are fibrous reinforcing materials , whereby the fibers are usually made of glass , carbon , aramid , polyester or natural products such as. B. flax , jute . are formed.
The properties of reinforced thermoplastics are primarily influenced by the volume fraction of the reinforcing material, its shape (form factor, length / diameter, L / D or aspect ratio) and the interaction at the boundary with the matrix.
Additives in a wide variety of forms are used to fill and reinforce thermoplastics. The form factor, which is important for the mechanical properties of the composite, is defined as the ratio of its length to its thickness (L / D ratio).
- Spherical and cubic particles have a form factor of 1. Examples are glass spheres or calcium carbonate.
- Fibers or other anisotropic platelet-shaped fillers can have very high form factors and this is usually well above 100.
- Platelet-shaped reinforcing materials, which include sheet silicates such as talc and mica, are usually between 5 and 50.
Reinforcing materials with a high L / D ratio generally stiffen polymer matrices more than fillers with a lower aspect ratio.
The reinforcement effect is based on the fact that an applied mechanical stress is absorbed by the polymer matrix and transferred to the reinforcement material. The greater the aspect ratio of the reinforcing material, the better the energy caused by the stress can be dissipated in the material. Coating the additives with coupling reagents (so-called couplers) can also significantly improve the compatibility with the matrix and thus the processability and also the resulting mechanical properties. Innovative compounding companies succeed in producing high-quality compounds through optimal formulation of the recipe and the use of suitable coupling systems.
Platelet-shaped reinforcing materials usually have a lower modulus of elasticity than fibrous reinforcing materials, but nevertheless increase the modulus of elasticity considerably. A major advantage of the particulate reinforcement materials is that the final properties of the plastic component are almost isotropic, i.e. independent of direction, due to the particle shape. Due to the aspect ratio between 5 and 50, flake-form reinforcing materials such as talc are a very good solution for reinforcing plastics, but at the same time not influencing the directional dependence of the properties too negatively.
The improvement of properties through reinforcing materials concerns, for example:
- the increase in the modulus of elasticity
- the increase in flexural strength
- the positive influence on the shrinkage behavior
- the improvement of creep behavior
- or increasing the heat resistance.
Talc is used as a reinforcing material, for example, to reinforce polyolefins such as HDPE or PP, for a wide range of uses in the automotive and construction industries. Reinforced polypropylene compounds have been on the market for around 30 years. At the end of the 1960s, talc (TV) and glass fiber reinforced (GFV) products based on PP were offered for the first time.
- Glass fiber : Short ("KGF") or long glass fiber ("LGF") are the most frequently added reinforcing materials. They are significantly cheaper than carbon fibers, for example.
- Carbon fibers : The lightest, but also the most expensive fiber for reinforcements.
- Wollastonite : Wollastonite is a borderline case between reinforcement and filling. Because of its rod-shaped crystal structure, however, a reinforcing effect can be achieved by adding it.
- all TPE (thermoplastic elastomers)
- colored materials
- PP with 40% chalk
- PP with 30% glass fiber (KGF or LGF)
- PA 6 or 66 with 30% glass fiber (KGF or LGF)
- ABS with 16% glass fiber (KGF)
- PC with 20% glass fiber
- ABS, PC, PP flame retardant
- Walter Krenkel: Composites. John Wiley & Sons, 2009, ISBN 978-3-527-62712-7 .
- Manfred Neitzel: Handbook composite materials. Carl Hanser Verlag, 2014, ISBN 978-3-446-43697-8 .
- Wolf-Ekkehard Traebert: Composite materials, attempt of a new system. Beuth-Vertrieb GmbH, Berlin, Cologne, Frankfurt (M), 1967.