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Rigid building materials used to be the norm – engineers would value high-grade steel for its great strength, while groaning under the limitations of its weight. Doctors would use plaster for splinting broken bones, confident in its structural integrity but displeased by the amount of time and work which plaster casting entailed. Ever since the advent of fiber-reinforced plastics, these concerns have been a thing of the past. Thermoplastic tape has shown the benefits of its inherent combination of great in-situ flexibility combined with its immense strength once set.
The Structure and Function of Thermoplastic Tape
Thermoplastic tape is a composite combining the properties of fibers and plastics. Essentially, the product consists of a continuous roving pre-impregnated (prepreg) with polymer plastic. Such composites are highly flexible at average temperatures, but harden when the material is subjected to heat and cooled off again. This reaction is caused by the plastic impregnation of the thermoplastic tape melting and creating a homogenous liquid which, when cooled off, forms polymer links, providing a hardened composite element.
This provides the tape with a number of useful properties and abilities:
- Flexible placement before curing
- Great strength and rigidity after curing
- Easy to store and quick to apply
- High degree of recyclability
These properties make it an ideal product for organo sheet layups as well as in-situ applications. However, since the production of composite parts using non-crimp fabrics and organo sheets has been in decline ever since the proliferation of industrial direct fiber placement techniques, thermoplastic materials are today most frequently used by emergency services for splinting broken bones and similar applications.
The different Types of Fiber-Plastic Technology
There are essentially two different types of materials employing the principle of fiber-reinforced plastic composites: thermoplastic and duroplastic products. While both of these materials demonstrate similar properties, there are distinct differences in the details.
Most notably, thermoplastics feature resins which soften with the application of heat and harden at average temperatures. However, this reaction can also be performed in reverse, liquefying the resin and making the tape flexible again. Thus, these composites can be reused to some extent. Meanwhile, duroplastic composites feature resins which cross-link when heated, causing an irreversible chemical reaction.
This also means that thermoplastic tape is more susceptible to unwanted deformation at high temperatures, leading to the material chiefly being employed for in-situ nonpermanent applications such as splinting bones. Duroplastics, in contrast, are used to create more robust shapes meant to resist both deformation and impact even at a high temperature.
The production of thermoplastic tape is a relatively complex procedure, involving extensive matrix know-how.
In addition, thermoplastic tape requires more heat to set compared to duroplastic composite materials. It can also easily be stored at average temperatures. The plastic resins employed in duroplastics cause chemical reactions even at lower temperatures, which means that the latter material needs to be stored in cooler conditions than thermoplastics to prevent the resin from accidentally hardening irreversibly, making the tapes unusable.
Thermoplastic Tape versus direct Fiber Placement
Today, thermoplastics are still an upcoming solution for high volume applications. Easily shaped and re-shaped, the material provides immense strength, damage resistance and rigidity while being flexible and temperature-resistant. Combining tapes to injection molding processes is one very relevant path for CFRP in high volume application.
For the creation of sophisticated composite parts, Automated Fiber Placement techniques such as the patented Compositence system have the distinct advantage of being a cost-effective way to process thermoplastic tape into semi-finished products for injection molding or even final parts.