Light Composite Material for New Design Concepts
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Covestro has developed CFRTP tapes and sheets which, thanks to continuous fibers made of carbon or glass, are durable and can be processed in short cycle times. Due to its polycarbonate polymer matrix, the material can also be painted in Class A quality or even furnished with decorative films.
Composite for Mass Production
The increasing interest in ever more efficient lightweight materials for the automotive industry has led to the introduction of a whole range of extremely smart composite materials in recent years. When considering their specific stiffness, the list of possibilities is currently led by continuous fiber- reinforced composite materials in which high-performance carbon or economical glass fibers absorb and dissipate forces. Some of the parts made from these materials can even compete with metal construction materials in terms of load-bearing capacity.
Composites were originally developed and optimized especially for lightweight construction and high rigidity. For a long time they could only be used in manufactories and job lot production. Thermoplastics can now be integrated into mass production processes such as injection molding.
Designer and Engineering Material
So-called CFRTP tapes and sheets from Covestro can fill this gap in the current high-performance material portfolio. CFRTP stands for Continuous Fiber-Reinforced Thermoplastics. CFRTP technology is based on continuous fiber tapes, which — as one of many options — are made from several 10,000 parallel-oriented carbon or glass fiber filaments and impregnated with a thermoplastic resin, which is the matrix. These tapes can be up to 700 or more mm wide for thicknesses below 0.2 mm and are wound up on rolls. They are either supplied in rolls or processed into semi-finished products according to customer specifications.
Thermoplastics can be integrated into mass production processes.
Compared to conventional continuous fiber-reinforced composites, CFRTP tapes and sheets offer two main advantages: Firstly, thanks to their thermoplastic matrix, they are absolutely compatible with established and highly economical processing processes. Secondly, thanks to the thermoplastic matrix material, they are design-friendlier than almost any other composite material — with mechanical properties comparable to those of conventional continuous fiber-reinforced plastics. Some characteristics and properties of these new continuous fiber composites are presented in more detail below.
Processing Technology in Detail
Selected properties of a carbon fiber-reinforced CFRTP tape (© Covestro)
Typical characteristic value
Tensile modulus 0° (ISO 527-5, 1 mm/min, [GPa])
Tensile modulus 90° (ISO 527-5, 1 mm/min, [GPa])
Tensile strength 0° (ISO 527-5, 1 mm/min, [MPa])
Tensile strength 90° (ISO 527-5, 1 mm/min, [MPa])
Compression strength 0° (ISO 14126, [MPa])
Compression strength 90° (ISO 14126, [MPa])
Densit y (ISO 1183), [g/cm3]
Fiber content (internal) [Vol %]
For more complex tasks, however, the use of CFRTP semi-finished products, the so-called CFRTP sheets, which Covestro produces from the tapes and which are tailored very closely to the requirements of their future applications, is recommended. In these semi-finished products — with a variable polymer matrix adapted to the product — the tapes can also be arranged at any angle to each other: The user is only slightly limited to specified values.
In order to tailor the semi-finished product optimally to the requirements, the carbon fiber tapes in the sheets can also be arranged in up to twelve layers — this means that even the most complex load cases can be dealt with. The CFRTP sheets can currently measure up to 1200 × 730 × 3 mm.
Low Cycle Times
CFRTP sheets are generally processed in thermoforming processes or in a combination of injection molding and thermoforming (hybrid molding). After preheating, the pre-assembled plates are placed in a mold, formed when the mold is closed and backmolded in one step (one shot molding); this makes it easy to form further functional elements, for example ribs, grooves or clamps. The only difference compared to the classic injection molding technique with inserts is that the CFRTP semi-finished products must be heated by irradiation e. g. with an infrared lamp before being inserted into the mold. However, this can happen in series production parallel to the actual injection process, so that this step does not have a negative effect on productivity. Similar to injection molding, the typical cycle times for processing these semi-finished products are in the low single-digit minute range or even below — which represents a considerable added value for economical mass production.
In order to take advantage of CFRTP technology, existing plants can generally continue to be used and only need to be expanded; the necessary investment in the required expansions can therefore be moderate.
The mechanical properties of the composites produced in this manner exhibit a low density of approx. 1.5 g/cm3 and high stiffness. Thanks to their low density, both their specific strength and their stiffness in the direction of the fibers are significantly higher than those of steel and aluminum.
Comprehensive Design Options
Perhaps even more interesting from the point of product design are the possibilities that CFRTP technology offers the designer. One of the preferred matrix materials for CFRTP tapes and sheets is polycarbonate (PC), a thermoplastic material characterized for instance by its good properties in regard to paintability, shrinkage, surface finish, and adhesion to other materials. This makes the fiber-matrix combination significantly different from other composites widely used in automotive engineering.
Polycarbonate also offers a low moisture absorption (0.1 to 0.3 %), high glass transition temperature (145 °C), and a high impact strength.
Applying Films without Adhesive
Besides, polycarbonate can be dyed in almost any color. CFRTP tapes open up new creative opportunities through the combination of black fiber with colored matrix, which leads to new color and optical impressions. Polycarbonate is available in practically all colors, so that individual shades are possible.
Polycarbonate can be dyed in almost any color.
Application Examples in Automobiles
For example, in display housings, the number of which will certainly increase in the automotive cockpit in the future. As is well known, the magnesium alloys frequently used in this sector can only be painted in complex processes. Here CFRTP composites can help save assembly costs, and they also feature a lower weight compared to magnesium.
These vents will not stand isolated in the vehicle, but can be integrated, for example, into more global design concepts that restructure the cockpit using curved, organically designed decorative strips. These in turn will not only set optical accents, but also interact with the driver via integrated sensors or backlit PC foils — possibly with a holographic depth effect.
Seats are ideal for the use of current lightweight materials. Once vehicles drive autonomously, the interior of the vehicle will change into a passenger cabin in which space and freedom of movement are required. This will lead to a departure from the current, voluminous seat constructions; they will be replaced by more filigree, yet comfortable models that depend on extremely stable lightweight materials and at the same time must have aesthetically designed surfaces, as they will always be in the field of vision of the vehicle occupants. Here, lightweight and stable CFRTP components — also thanks to film technology — allow for completely new design approaches.
In addition to PC thermoplastic urethane is also a suitable matrix material.
So far, only CFRTP composites with carbon continuous fibers and polycarbonate as the matrix material have been discussed. However, the new technology is not limited to this combination. For tapes and sheets with continuous glass fiber reinforcement, the same applies as for the carbon fiber variant above: Here too, the fibers can be laid out at all conceivable angles to one another in order to meet any desired mechanical application profile.
The user is even more flexible when selecting the matrix material. In addition to PC, which is the polymer of choice in most cases due to its surface properties, thermoplastic urethane is also an option. Covestro has already had positive experiences with this. Furthermore, various types of polycarbonate are also available, such as flame-retardant variants. The bottom line is that CFRTP technology manifests itself as a composite construction kit that not only focuses on the mechanical properties of the compounds, for example in terms of maximum stiffness and strength with minimum weight, but also meets the needs of product designers with an aesthetic approach.