Besides the quality of the resin, there are two decisive factors for obtaining an excellent final result:
- the types of the glass employed;
- and using the different types of glass in the most appropriate way possible to exploit their structural properties to the fullest.
Glass rovings are being fed to the E-CFW winding machine. Glass fibers are chopped and drop on the mandrel.
Glass fibers are obtained with the silica glass-based melting mixtures of inorganic materials at temperatures varying between 1300°C and 1600°C. Under these conditions, the cross-linked structure of the silica is destroyed, its continuity is interrupted and its structure is modified by the introduction of other oxides.
The fibers are formed by drawing and rapidly cooling the molten material which is coming out from a large number of very accurately dimensioned holes in a heated platinum alloy die in the form of threads. A constant head of glass is maintained under accurate temperature control, to produce the required diameter strand to tight tolerances when the fibers are wound onto a high-speed mandrel or collect running at several thousand meters per minute.
Glass Fiber Coil
From this basic thread, by direct or indirect operations, various commercial forms of reinforcements are obtained and appropriately designed to meet all the technical and technological requirements.
Single End Rovings (Hoop Winding)
Single End Rovings are coils made of continuous E-glass winding without twisting. This reinforcement is used in the winding operations due to its high glass content by providing high tensile strength.
Multi End Rovings (Chopped)
Multi-End Rovings are continuous glass fibers bonded together with high-performance sizings and gathered together into strands without a twist. This reinforcing material is obtained by cutting the continuous “E” or “ECR” glass rovings to build pipe wall structure. The recommended binder is based on the silane.
The reinforcements used in the manufacturing of industrial products, such as vessels, silos and pipes, are made from three different types of glass compositions:
- “C” glass, which displays very good properties of the chemical inertness to the corrosive environment;
- “E” glass, which is not so resistant to corrosion but displays a very high mechanical strength;
- “ECR” glass, which is similar to the E-glass, but it doesn’t contain boron and fluorine. Due to the removal of these components, the chemical resistance (including water resistance, acid resistance and alkali resistance) is greatly improved. When compared to the E-glass fibres, the ECR-glass shows higher temperature resistance, better dielectric strength, lower electrical leakage, and higher surface resistance.
Typical Chemical Compositions
The following table shows the typical compositions of the "C" glass and the "E" glass (the values are in % per weight):
| Component | "C" glass | "E" glass |
| SiO2 | 64.8 | 54.5 |
| B2O3 | 4.9 | 7.9 |
| CaO | 14.1 | 17.7 |
| MgO | 3.2 | 4.1 |
| F2 | / | 0.2 |
| K2O | 0.5 | 0.1 |
| Na2O | 8.2 | 0.5 |
| Al2O3 | 4 | 14.9 |
| Fe2O3 | 0.3 | 0.1 |
Physical and Mechanical Properties
The physical and mechanical properties of the "C" and the "E" glass are shown in the following table:
| Unit | C glass | E glass | |
| Tensile strength at 22°C | N/mm2 | / | 2,400 |
| Young's modulus | N/mm2 | / | 73,000 |
| Elongation at break | % | / | 4.5 |
| Specific gravity | g/cm3 | 2.53 | 2.60 |
| Creep | - | Absent | Absent |
| Hysteresis | - | Absent | Absent |
| Filament diameter | mm-3 | 10 | 10-24 |
| Coeff. of thermal expansion | m/m° C | 7.1*10-6 | 5*10-6 |
Surface treatments
The main function of the surface treatment (also called “size” or “sizing”) is to protect the glass filaments during the fibers manufacturing and during the transformation into the reinforced polymer. The functions of the main components of the size formulations are:
- Film former: provides the adhesion between the filaments forming the strands and prevents the adhesion between the strands during the winding operations;
- Lubricants: they protect the filaments from abrasion;
- Antistatic agents: they help avoid the formation of electrostatic charges during the manufacturing and the use of the fibers;
- Coupling agent: provides the adhesion between the fibres and the polymeric matrix.
The mix of these components is used in the water solution in a concentration of 6 ÷ 10% with well-defined ratios regarding the type of glass and the type of matrix.
Coupling Agents
The coupling agent enhances the adhesion between the reinforcement and the matrix and improves the wettability of the reinforcement, thus making the impregnation operations easier. The selection of the best coupling agent is fundamental to guarantee good stress transmission from the matrix to the reinforcements and the optimum mechanical behavior of the composite.
Composite Material
The employed coupling agent depends on the composition of the reinforcement and the resin itself. The most widely used coupling agents are silanes.
The ideal coupling agent is a molecule containing two kinds of chemical groups, one of which reacts to adhere to the reinforcement, while the other binds to the resin, as shown in the following figure.
The sketch of the reaction involved between the coupling agent, the reinforcement and the resin.
The reinforcements that are available on the market are already silane-coated, so they can be used immediately, without further processing.
Please note: before the purchase of the reinforcements it is necessary to preventively ask the supplier about the superficial treatment that makes the fibers suitable to one matrix typology instead of another one. On the market, we can find reinforcements with good wettability for phenolic, polyester, vinylester and epoxy resins.