Composites and Fiberglass

Composite materials (or composites for short) are engineered materials made from two or more constituent materials that remain separate and distinct on a macroscopic level while forming a single component. There are two categories of constituent materials: matrix and reinforcement. At least one portion (fraction) of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical, electrical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from naturally occurring materials. Due to the wide variety of matrix and reinforcement materials available, the design potential is incredible.

The most primitive composite materials comprised straw and mud in the form of bricks for building construction. The most advanced examples perform routinely on spacecraft in demanding environments. The most visible applications pave our roadways in the form of either steel and aggregate reinforced portland cement or asphalt concrete. Those composites closest to our personal hygiene form our shower stalls and bath tubs made of fiberglass. Solid surface, imitation granite and cultured marble sinks and countertops are widely used to enhance our living experiences.

There are the so-called natural composites like bone and wood. Both of these are constructed by the processes of nature and beyond the scope of this text. Engineered composite materials must be formed to shape. This involves strategically placing the reinforcements while manipulating the matrix properties to achieve a melding event at or near the beginning of the component life cycle. A variety of methods are used according to the end item design requirements, and they are commonly named molding or casting processes, as appropriate. The principle factors impacting the methodology are the natures of the chosen matrix and reinforcement materials. Another important factor is the gross quantity of material to be produced. Large quantities can be used to justify high capital expenditures for rapid and automated manufacturing technology. Small production quantities are accommodated with lower capital expenditures but higher labor and tooling costs at a correspondingly slower rate.

Most commercially produced composites use a polymer matrix material often called a resin or resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common categories are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, and others. The reinforcement materials are often fibers but also commonly ground minerals. Fibers are often transormed into a textile material such as a felt, fabric, knit or stitched construction.

Advanced composite materials constitute a category comprising carbon fiber reinforcement and epoxy or polyimide matrix materials. These are the aerospace grade composites and typically involve laminate molding at high temperature and pressure to achieve high reinforcement volume fractions.

One component is often a strong fibre such as fiberglass, quartz, kevlar, Dyneema or carbon fibre that gives the material its tensile strength, while another component (called a matrix) is often a resin such as polyester, or epoxy that binds the fibres together, transferring load from broken fibers to unbroken ones and between fibers that are not oriented along lines of tension. Also, unless the matrix chosen is especially flexible, it prevents the fibers from buckling in compression. Some composites use an aggregate instead of, or in addition to, fibers.

In terms of stress, any fibers serve to resist tension, the matrix serves to resist shear, and all materials present serve to resist compression, including any aggregate.

Composite materials can be divided into two main categories normally referred to as short fiber reinforced materials and continous fiber reinforced materials. Continuous reinforced materials will often constitute a layered or laminated structure.

Shocks, impact, loadings or repeated cyclic stresses can cause the laminate to separate at the interface between two layers, a condition known as delamination. Individual fibers can separate from the matrix e.g. fiber pull-out.

Examples of composite materials:

• Fibre reinforced plastics:
  • Classified by type of fiber:
    • Wood (cellulose fibers in a lignin and hemicellulose matrix)
    • Carbon-fibre reinforced plastic or CRP
    • Glass-fibre reinforced plastic or GRP (informally, "fiberglass")
  • Classified by matrix:
    • Thermoplastic Composites
      • long fiber thermoplastics or long fiber reinforced thermoplastics
      • glass mat thermoplastics
    • Thermoset Composites

• Metal matrix composites or MMCs:
  • White cast iron
  • Hardmetal (carbide in metal matrix)
  • Metal-intermetallic laminate
• Ceramic matrix composites:
  • Cermet (ceramic and metal)
  • concrete
  • Reinforced carbon-carbon (carbon fibre in a graphite matrix)
  • Bone (hydroxyapatite reinforced with collagen fibers)
• Organic matrix/ceramic aggregate composites
  • Mother of Pearl
  • Syntactic foam
  • Asphalt concrete
• Chobham armour (see composite armour)

• Engineered wood
  • Plywood
  • Oriented strand board
  • Wood plastic composite (recycled wood fiber in polyethylene matrix)
  • Pycrete (sawdust in ice matrix)

Fiberglass or fibreglass is material made from extremely fine fibers of glass. It is used as a reinforcing agent for many plastic products; the resulting composite material, properly known as glass-reinforced plastic (GRP) or glass-fiber reinforced epoxy (GRE), is called "fiberglass" in popular usage.

Glassmakers throughout history have experimented with glass fibers, but mass manufacture of fiberglass was only made possible with the advent of finer machine-tooling. In 1893, Edward Drummond Libbey exhibited a dress at the World's Columbian Exposition incorporating glass fibers with the diameter and texture of silk fibers. What is commonly known as "fiberglass" today, however, was invented in 1938 by Russell Games Slayter of Owens-Corning as a material to be used as insulation. It is marketed under the trade name Fiberglas (sic), see also genericized trademark.

Formation

Glass fiber is formed when thin strands of silica based or other formulation glass is extruded into many fibers with small diameters suitable for textile processing. Glass is unlike other polymers in that, even as a fiber, it has little crystalline structure (see amorphous solid). The properties of the structure of glass in its softened stage are very much like its properties when spun into fiber. One definition of glass is "an inorganic substance in a condition which is continuous with, and analogous to the liquid state of that substance, but which, as a result of a reversible change in viscosity during cooling, has attained so high a degree of viscosity as to be for all practical purposes rigid." (Loewenstein, 4)

The technique of heating and drawing glass into fine fibers has been known to exist for thousands of years; however, the concept of using these fibers for textile applications is more recent. The first commercial production of fiberglass was in 1936. In 1938, Owens-Illinois Glass Company and Corning Glass Works joined to form Owens-Corning Fiberglas Corporation. Until this time all fiberglass had been manufactured as staple. When the two companies joined together to produce and promote fiberglass, they introduced continuous filament glass fibers (Loewenstein, 2). Owens-Corning is still the major fiberglass producer in the market today.

Chemistry

The basis of textile grade glass fibers is silica, SiO₂. In its pure form it exists as a polymer, (SiO₂)_n. It has no true melting point but softens up to 2000°C, where it starts to degrade. At 1713°C, most of the molecules can move about freely. If the glass is then cooled quickly, they will be unable to form an ordered structure (Gupta, 544). In the polymer it forms SiO₄ 4- groups which are arranged as a tetrahedron with the silicon atom at the center and four oxygen atoms at the corners. These atoms then form a network bonded at the corners by sharing the oxygen atoms.

The vitreous and crystalline states of silica (glass and quartz) have similar energy levels on a molecular basis, also implying that the glassy form is extremely stable. In order to induce crystallization, it must be heated to temperatures above 1200°C for long periods of time (Loewenstein, 6).

Molecular Structure of Glass

Although pure silica is a perfectly viable glass and glass fiber, it must be worked with at very high temperatures which is a drawback unless its specific properties are needed. It is usual to introduce impurities in the form of other materials into the glass to lower its working temperature. These other materials also impart various other properties to the glass which may be beneficial in different applications. The first type of glass used was soda-lime glass or A glass. It was not very resistant to alkali. A new type, E-glass was formed that is alkali free (< 2%) and an alumino-borosilicate glass (Volf, 338). This was the first glass produced for continuous filament formation. E-glass still makes up most of the fiberglass production in the world. Its particular components may differ slightly in percentage, but must fall within a specific range. The letter E is used because it was originally for electrical applications. S-glass is a high strength formulation when tensile strength is the most important property. C-glass was developed to resist attack from chemicals, mostly acids which destroy E-glass (Volf, 340).

Since E-glass does not really melt but soften, the softening point is defined as, “the temperature at which a 0.55 – 0.77 mm diameter fiber 9.25 inches long, elongates under its own weight at 1 mm/min when suspended vertically and heated at the rate of 5°C per minute” (Lubin, 152). The strain point is where the glass has a viscosity of 10 14.5 poise. The annealing point, which is the temperature where the internal stresses are reduced to an acceptable commercial limit in 15 minutes. The viscosity at this point should be 10 13 poise (Lubin, 152).

Properties

Glass fibers are useful because of their high ratio of surface area to weight. However, the increased surface makes them much more susceptible to chemical attack.

Glass strengths are usually tested and reported of "virgin" fibers which have just been manufactured. The freshest, thinnest fibers are the strongest and this is thought to be due to the fact that it is easier for thinner fibers to bend. The more the surface is scratched, the less the tenacity is (Volf, 351). Because glass has an amorphous structure, its properties are the same along the fiber and across the fiber (Gupta, 546). Humidity is an important factor in the tensile strength. Moisture is easily adsorbed, and can worsen microscopic cracks and surface defects and lessen tenacity.

In contrast to carbon fibers, glass can undergo more elongation before it breaks (Gupta, 546).

The viscosity of the molten glass is very important. During drawing the viscosity is relatively low. If it is too high the fiber will break during drawing. If it is too low the glass will form droplets rather than drawing.

Manufacturing Processes

There are two main types of glass fiber manufacture and two main types of glass fiber product. First, fiber is made either from a direct melt process or a marble remelt process. Both start with the raw materials in solid form. They are mixed together and melted in a furnace. Then, for the marble process, the molten material is sheared and rolled into marbles which are cooled and packaged. The marbles are then taken to the manufacturing facility where they are inserted into a can and remelted. Then the molten glass goes to the bushing to be formed into fiber. In the direct melt process, the molten glass in the furnace goes right to the bushing for formation. (Lubin, 149)

The bushing is the most important part of the machinery. This is a small metal furnace containing nozzles for the fiber to be formed through. It is almost always made of platinum alloyed with rhodium for durability. Platinum is used because the glass melt has a natural affinity for wetting it. When bushings were first used they were 100% platinum and the glass wetted the bushing so easily it ran under after exiting the nozzle and accumulated on the underside. Also, due to its cost and the tendency to wear, it was alloyed with rhodium. In the direct melt process, the bushing serves as a collector for the molten glass. It is heated slightly to keep the glass at the correct temperature for fiber formation. In the marble melt process, the bushing acts more like a furnace as it melts more of the material. (Loewenstein, 91)

The bushings are what make the capital investment in fiber glass production expensive. The nozzle design is important also. The number of nozzles ranges form 200 to 1600 in multiples of 200. The important part of the nozzle in continuous filament manufacture is the thickness of its walls in the exit region. It was found that inserting a counterbore here reduced wetting. Today, the nozzles are designed to have a minimum thickness at the exit. The reason for this is that as glass flows through the nozzle it forms a drop which is suspended from the end. As it falls, it leaves a thread attached by the meniscus to the nozzle as long as the viscosity is in the range for fiber formation. The smaller the annular ring of the nozzle or the thinner the wall at exit, the faster the drop will form and fall away and the lower its tendency to wet the vertical part of the nozzle (Loewenstein, 94). The surface tension of the glass is what influences the formation of the meniscus. For E-glass it should be around 400 mN per m (Volf, 360).

The attenuation speed is important in the nozzle design. Although slowing this speed down can make coarser fiber, it is uneconomic to run at speeds for which the nozzles were not designed (Loewenstein, 94).

In the continuous filament process, after the fiber is drawn, a size is applied. This size helps protect the fiber as is wound onto a bobbin. The particular size applied relates to end-use. While some sizes are processing aids, others make the fiber have an affinity for a certain resin, if the fiber is to be used in a composite (Lubin, 100). Size is usually added at 0.5–2.0% by weight. Winding then takes place at around 1000 m per min (Gupta, 544).

In staple fiber production, there are a number of ways to manufacture the fiber. The glass can be blown or blasted with heat or steam. Usually there fibers are made into some sort of mat. The most common process used is the rotary process. The glass enters the rotating spinner and due to centrifugal force are thrown out horizontally. The air jets push them down vertically and binder is applied. Then the mat is vacuumed to a screen and the binder is cured in the oven (Mohr, 13).

End uses for regular fiber glass are mats, insulation, reinforcement, heat resistant fabrics, corrosion resistant fabrics and high strength fabrics.

See also

• Optical fiber
• Glass microsphere
• Carbon fiber
• Basalt fiber
• Fiberglass Molding

Bibliography

• Gupta, V.B. and V.K. Kothari; Manufactured Fibre Technology. Chapman and Hall. London. 1997.
• Loewenstein, K.L.; The Manufacturing Technology of Continuous Glass Fibers. Elsevier Scientific. New York. 1973.
• Lubin, George; Handbook of Fiberglass and Advanced Plastic Composites. Robert E. Krieger. Huntingdon NY. 1975.
• Mohr, J. G. and W. P. Rowe; Fiberglass. Van Nostrand Reindhold. Atlanta. 1978.
• Volf, Milos B.; Technical Approach to Glass. Elsevier. New York. 1990.