Common types of reinforcing fibers

In the past more than half a century, fiber-reinforced composite materials have been widely used due to their excellent properties, and reinforcing fibers play an important role in composite materials. Since the advent of composite materials, reinforcing fibers have undergone a transition from natural fibers to synthetic fibers.

At present, the most common reinforcing fibers include glass fibers, aramid fibers, carbon fibers, etc. This series of articles will introduce the types, properties and application methods of reinforcing fibers for composite materials in detail. This article will first briefly describe the common types of reinforcing fibers.

In composite materials, the primary role of the resin matrix is to bond the fibers together and transfer external loads from one fiber to the next. Most reinforcing fibers are bent and floppy, and if tension is applied to them, they will have sufficient tensile strength and stiffness.

Reinforcing fibers are usually bundled filaments, and the individual fibers tend to be very fine, such as glass fibers and carbon fibers with typical diameters ranging from 5 to 25 microns. For comparison, human hair is usually between 50 and 200 microns in diameter. All fiber-reinforced "structures" can be derived from a single fiber (filament), including tow, yarn, chopped fiber, milled fiber, etc., as shown in the figure below Show.

Currently common reinforcing fibers include: glass fiber

There are many different varieties of fiberglass, but for composites, two are the most common. E-glass fibers are the standard type for almost all glass fiber reinforced products, while S-glass fibers (also known as R-glass fibers or T-glass fibers) have significantly better tensile strength.

S-glass fibers are generally smaller than E-glass fibers, have better adhesion in the resin matrix, and have improved impact properties. However, it costs a lot more. S-2 glass fiber is a higher strength commercial S-glass fiber that has twice the tensile strength of typical E-glass fibers and is also about 10-20% stiffer. But for almost all applications, E-glass fibers are sufficient.

Glass fibers are produced by extruding molten (1700°C) mineral products (silica, aluminum, calcium oxide, etc.) through small diameter holes. Typically, E-glass fibers are about 10-25 microns in diameter, which is larger than carbon fibers.

Carbon Fiber

There are many types of carbon fibers with varying mechanical properties and cost. Carbon fibers are not extruded directly from molten material, but are made by heat-treating the precursor fibers, including pre-oxidation in air and carbonization in inert atmosphere. Under tension, the carbon structure within the fiber aligns, helping to maximize tensile strength and stiffness.

The most common precursor used for carbon fiber is polyacrylonitrile (PAN) fiber. Currently, the most common standard and medium modulus carbon fibers are produced based on PAN precursors; while carbon fibers prepared from pitch precursors tend to have higher modulus. Depending on the properties of the precursors, the fiber diameter, and the details of the heat treatment (oxidation, carbonization, graphitization) process, the mechanical properties of the final carbon fiber can range widely.

Usually the diameter of a single carbon fiber is smaller than that of glass fiber, only 5 microns. At present, the most common classification of carbon fiber is based on the mechanical properties of the fiber, especially the fiber modulus. It is mainly divided into standard modulus (Standard modulus), intermediate modulus (IM), high modulus (High modulus, HM) and ultra-high modulus The amount (Ultra-high modulus) carbon fiber, the representative products are shown in the table below.

Other commonly used reinforcing fibers

1. Kevlar Aramid Fiber: A synthetic aramid fiber developed by DuPont. Other commercial aramid fibers include Twaron, Technora and Nomex. As a reinforcing fiber for composite materials, aramid is mainly used in applications with high tensile strength and puncture resistance, abrasion resistance and crush resistance. Aramid fibers are often difficult to bond, cut and handle, and are often combined with carbon or glass fibers.

   
   

2. Basalt Fiber: Made using a melting and extrusion process similar to fiberglass. Its tensile strength and modulus are slightly higher than E-glass fiber, but not as good as carbon fiber. Density is similar to E-glass fiber. The price is between E-glass and carbon fiber. There is a limited supply of composite grade basalt, which is usually brown in color.

   
   

3. Ultra High Molecular Weight Polyethylene: Both Dyneema and Spectra are fibers made from extruded filaments of Ultra High Molecular Weight Polyethylene (UHMWPE) or "High Modulus Polyethylene" (HMPE). UHMWPE is used in tugboat cables, bowstrings, fishing lines and Vehicle armor, strong and durable. These fibers can be used in composite applications and are often blended with carbon fibers. Dyneema/carbon fiber hybrid reinforcements can increase the toughness of laminates and improve carbon fiber energy absorption and impact resistance. Spectra fabric can be used topically to increase resistance Abrasiveness.

   
   

4. High Molecular Weight Polypropylene: Innegra is a fiber made from High Molecular Weight Polypropylene (HMPP) by Innegra Technologies. While not as strong as Kevlar or Dyneema, Innegra is tough enough to resist shock and breakage at a lower cost. Often Innegra is used as a component of hybrid reinforcements, mixed with carbon or glass fibers to increase the toughness of the laminate.

   
   

5. Plant fibers: While glass fibers and carbon fibers are the most common reinforcing fibers, the oldest structural reinforcing fibers are wood and plant fibers. Over the past decade, there has been a resurgence of interest in laminated plant fibers, especially flax and jute, which provide useful mechanical properties and provide similar processing to standard fiber types. One challenge plant fibers face is that the range of mechanical properties is much wider than traditional engineering materials, and they are also not as strong as regular E-glass fibers. Moisture absorption is a problem with all bio-based composite reinforcements, which can cause problems for many composite processes.

   
   

6. Ceramic fibers: Ceramic matrix composites (CMC) have mechanical properties close to carbon fiber composites, but have extremely high temperature resistance. They are usually broken down by oxidative and non-oxidative fibers, depending on their chemical composition. On the non-oxide side, boron is one of the most well-known ceramic reinforcements, with incredible compressive strength. Silicon carbide (SiC) fibers have high strength and stiffness, and are very stiff. Oxide-based fibers have higher oxidation resistance but lower mechanical properties.