Prepreg

The term "prepreg" is actually an abbreviation for the expression "pre-impregnated." A prepreg is a fibrous polymer reinforcement that has been pre-impregnated with a resin. Prepregs are cured under specific pressure and temperature. As a result of curing, the impregnated resin hardens to form a composite structure that is lightweight, highly durable, and possesses high thermal and chemical resistance. Prepregs are divided into two main categories—thermoset and thermoplastic—and show significant technical differences between them.

Manufacturing stages with prepregs:

- Cutting prepreg layers from the roll with the aid of a mold, - Laying up prepregs at appropriate fiber angles, one on top of another, - Curing to allow the resin to reach its final form, - Preparation for assembly through cutting, drilling, and joining operations. As seen in Figure 1, there are two different types of prepreg manufacturing: unidirectional and woven. Comparisons between these two different structures are given in Table 1. With unidirectional prepregs, layers with all fiber orientations can be produced. In woven structures, only 0°/90° and ±ø (180°-ø) angles can be manufactured together. Additionally, woven prepreg offers process simplicity since it allows two layers to be laid up in a single operation.  

Thermoset Prepregs

Thermoset prepregs are more frequently used in prepreg composite manufacturing. The primary resin matrix used is epoxy. However, other thermoset resins, including BMI and phenolic resins, are converted into prepregs. With a thermoset prepreg, the thermoset resin begins as a liquid and completely impregnates the fiber reinforcement. Excess resin is completely removed from the reinforcement. Meanwhile, the epoxy resin undergoes partial curing, changing the resin state from liquid to solid. This is known as the "B-stage." At the B-stage, the resin is partially cured and is generally tacky. When the resin is brought to an elevated temperature, it remains in a liquid state for a brief period before hardening. Once cured, the thermoset resin at the B-stage is now fully cross-linked.

Thermoplastic Prepregs

Thermoplastic prepregs are composite reinforcements (fiberglass, carbon fiber, aramid, etc.) pre-impregnated with thermoplastic resin. Common resins for thermoplastic prepregs include PP, PET, PE, PPS, and PEEK. Thermoplastic prepregs can be supplied in unidirectional tape or in woven or stitched fabrics. The fundamental difference between thermoset and thermoplastic prepreg is that thermoplastic prepregs are stable at room temperature and generally have no shelf life. This is a direct consequence of the differences between thermoset and thermoplastic resins.

Advantages and Disadvantages of Prepregs

Prepreg manufacturing consists of coating continuous dry fibers on their front and back surfaces with resin-coated paper, and then bringing the resin to a semi-viscous state through light curing at a low temperature. Since the surfaces of prepreg sheets are tacky, they are always protected between protective papers. In this case, prepreg can be rolled and stored. If storage conditions are maintained, they can be preserved until the expiration date. A prepreg consists of fibers and resin that has been pre-cured and bonded with the fibers. Its most important advantage is that it serves as a raw material for manufacturing high fiber content composites. The thickness of the manufactured composite and the ratios of fiber and matrix materials are homogeneous. Its practicality during production additionally eliminates the need for any resin and hardener use, and compared to dry fiber that is not resin-impregnated, the laying-up process is easier due to adhesion to the mold surface and to each other, and fiber separation does not occur. Cure time is also shorter compared to hand lay-up, vacuum infusion, and vacuum bagging methods. Since the resin application has been completed beforehand, situations such as drying out or excessive resin absorption do not occur. This ensures that standard products are obtained without any loss of strength in the product. Depending on the type of prepreg used, the resin/fiber ratio varies between 35%-50%. While prepreg manufacturing has advantages, it also has some disadvantages; it and its manufacturing methods are quite expensive compared to others. High unit cost and the necessity of selecting fiber and matrix materials together are the most important disadvantages. In the autoclave method, temperature is set around 120-180°C according to the cure schedule and simultaneously performed under 3-7 MPa pressure. In oven manufacturing, the cure temperature must likewise be set to appropriate levels. Such autoclaves and ovens are quite expensive industrial products. Storage conditions for prepregs are also a major challenge for manufacturers. While some types can be stored at room temperature for 4-6 weeks, generally they require storage conditions in a freezer around -20°C. Additionally, they must be used after being removed from the freezer.

Applications of Prepreg Materials

In composite part production using prepregs, end product properties such as thickness, weight, and surface quality can be standardized and repeatable compared to other methods, enabling low scrap rates while ensuring high product quality. The adjusted resin ratio increases process efficiency by eliminating problems such as resin-rich areas, dry areas, and air voids. In the early 1990s, prepregs were considered an important material. They were used at about 5% in aircraft designs and in secondary structures. Today, they are a fundamental component of the aerospace sector and are present in more than 50% of the fuselages of Airbus A350 XWB and Boeing 787. Development was followed by space technology, wind energy, automotive, sporting goods, and other industrial equipment. In recent applications, prepregs are utilized in the oil and gas industry for pipeline and high-pressure tank manufacturing. Advanced prepregs and composite materials have enabled higher strength and unique designs to emerge. Prepared by: B. Serhat Cengiz

Source: Müge Armatlı Kayrak, Manufacturing Criteria Analysis of Aircraft Composite Components for Different Machine Systems, Anadolu University Faculty of Aeronautics and Astronautics M. Reyne, "Technologie des composites, Hermes", Paris, 1990, p. 165. M. Armatlı Kayrak, "Aviation Composites and Strength Cost Analyses", Anadolu University Publications, Eskişehir, 1999, p. 103. D. Gay, "Materiaux Composites", Hermes Paris, 1989, p. 506. İTÜ Solar Car Team, http://www.fibreglast.com/ product/about-prepregs/Learning_Center