The importance of resin selection in Manufacturing Carbon fiber products

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Most carbon fiber products in the CFRP (Carbon Fiber Reinforced Polymer) industry are formed from a combination of fibers, sheets, strands, nanotubes and bound together by the medium that is resin. The resin not only binds the reinforcement matter together, but is also a determinator of the overall strength of the finished product. Some of the relevant examples are carbon fiber tubes and carbon fiber sheets.  The composition of the resin plays a very important role in the shaping of the targeted properties of the carbon fiber products. The performance and life of the carbon fiber product will greatly depend on the nature and characteristics of the resin. Choosing the right type of resin will determine if your product is going to work as designed or otherwise.

The choice of resin can be based on certain factors as follows:

• The application for end use
• The next operation or treatment intended (some further layering or bonding or treatment)
• The size of the part to be created. For instance, in case of marine and wind energy applications, the type of resins required for large parts will differ from small parts
• The shape of part to be created: e.g.: carbon fiber products like carbon fiber tubes and carbon fiber sheets
• The temperatures and environment the part is going to be subjected to. e.g., external, exposed to sunlight, or internal
• The intended service life
• Compatibility to a wet lay-up
• Suitability with vacuum bagging
• What is the intended use: Production or repair of an existing part. While repairing, compatibility with the existing resin is an important factor to be considered
• The technical parameters desired, like shear stress levels, modulus, impact resistance, etc.

Now let us look at the different types of resins.

Viscosity related Resins:

These can be classified as Newtonian or Non-Newtonian. This classification is based on maintainability of viscosity value. Non-Newtonian fluids or resins will have a variation in viscosity during the impregnation process. Long parts made using non-Newtonian resins may tend to sag when loaded in their end use. Newtonian fluids have better fluidity and therefore are useful in filling the cracks.

Epoxy based resins:

These are made in two parts, the resin, and the hardener. This type of resin is the most popular in the carbon composites and particularly the carbon fiber products industry, more particularly, the niche segments like defense and aerospace. These resins are also used extensively in carbon fiber tubes, regardless from the method of manufacturing, like filament winding or roll-wrapping, whether straight or tapered/conical.  Some special beneficial properties include Wood compatibility, elimination of bubbling, pigment compatibility, etc.

Polyester based resins:

These are the more economic type of resins that find application in the marine and shipbuilding industry, wherein, multiple-layering is the norm. Hence carbon fiber sheets find more conformity with the properties of polyester resins to some extent. Such resins consist of a monomer and a solution. Monomers of styrene category have a low viscosity and are easily polymerized from the liquid state. These resins lack the water proofing characteristics that are common with epoxy type resins.

Esters of Vinyl:

These type of resins consist of the best of both worlds, the epoxy, and the polyester. These have slightly better water proofing capacity. They can be deployd in areas needing protection against reaction as well as impact strength.

Besides carbon fiber products, these are compatible with other materials like Kevlar and glass-fiber.

All of the above-named resins can be broadly called “thermosetting resins” as these become processable at specific temperatures and are the most suited to work with carbon fiber products. Chemically speaking, molecular cross linking is the method of curing of such resins

PU (Polyurethane) based resins are suitable for pultruded carbon fiber tubes as they allow forming parts with thin walls that are not so prone to failure or tearing. Strength wise, these are located between the epoxy and the polyester resins. They are also ideal for making carbon fiber product fasteners.

Wrapping up:

The selection of resins is a highly skilled job as there is going to be high risk if the selection is erroneous, given the fact that carbon fiber products are not practically recyclable. Therefore, selection must be done with utmost care and planning.