The oxidation–reduction of iron in the melt is affected by glass chemistry and oxygen partial pressure ([16, 17], Chapter 5.6)
(3)
and by the presence of other multivalent species either from additives or impurities,
(4)
In general, drawing processes for making finer or smaller diameter fibers (and typically smaller bushings also) require glasses with a relatively lower concentration of Fe2+ (hence, slower cooling rate) than coarse fibers made from larger bushings (hence, higher cooling rate). The control of the iron oxidation state (or ferrous ion concentration in glass) is directly related to the amount of fining agent in the batch and the total iron in the batch, both of which are adjustable variables to maintain process stability. The ferrous ion concentration in glass is routinely monitored in production with a colorimetric method through acid digestion of a glass powder sample or with a calibrated UV‐VIS spectroscopic method on an optically polished glass disk.
In commercial production, glass fibers are drawn at a temperature TF where melt viscosity is near 100 Pa∙s (Figure 4). Stable melt viscosity at the tip plate is critical for an efficient fiber‐forming process. If melt viscosity at the tip plate is too low (i.e. too high temperature at the tip plate), spreading of the glass melt across multiple tips on the tip plate results in flooding and disruption. If melt viscosity is too high (i.e. too low temperature at the tip plate), fiber drawing tension increases and the stable forming cone at the tip exit begins to fail, causing fiber breakage [3]. In addition, the actual forming temperature must be greater than the glass liquidus temperature, Tliq, by at least 50 oC to reduce the risk of devitrification. Microcrystals formed in colder spots along a primary canal or forehearth (Figure 2a), no matter how small, can lead to significant fiber breakage in the drawing process and hence, adversely impact productivity.
In making commercial glass fiber products for reinforcements, various processes are used to provide the desired end form. Direct draw or single end winding, direct chopped fibers, and numerous downstream secondary processes may be employed. Depending on the product needs, the types of bushings vary in dimension, number of tips, and tip diameters. In addition to the glass and process elements necessary to produce commercial glass fibers, the surface of the fiber must be treated to provide optimum compatibility of the inorganic glass fiber with the organic resins used in the reinforcements industry. This leads naturally into a discussion on the role of sizing chemistry.
3.4 The Role of Sizing/Binder in Glass Fiber Products
The long history of growth in both volume and breadth of glass‐fiber commercial successes has been driven by unique combinations of strength, stiffness, weight, and cost attributes that can be achieved through glass chemistry. However, to realize fully the value of the glass fiber in a reinforced composite, a means must be provided to facilitate the effective interactions between the inorganic fiber and the organic polymer that together make up a reinforced composite material. One maximizes this interaction by designing an interface that synergistically combines the material properties of each element. A design that provides for effective load transfer between the constituents transforms an inherently heterogeneous material into one that behaves as a homogeneous structure.
In fiber‐glass composites, the system that delivers the optimum interfacial properties is the sizing or binder that is applied to the surface of the fiberglass. The sizing is generally applied as a continuous coating just after the fibers are formed and before the individual glass filaments are gathered into a strand below the bushing. The most common sizing formulations comprise a water‐based mixture of molecular species such as adhesion promoters, film formers, lubricants, and other processing aids as summarized in Table 4. It is the role of the sizing chemist, working in conjunction with the glass scientists and the process engineers, to deliver a sizing formulation that appropriately enables efficient production rates of the fiberglass while also providing maximum compatibility with the targeted composite polymer matrix and the end‐use performance requirements.
Table 4 Classification and functionality of ingredients in fiber sizing/binder formulations.
| Classification | Functionality |
|---|---|
| Coupling agents | Adhesion promoters that bond or couple the glass surface with specific matrix resin systems; may also provide excellent filament protection and increased dry breaking strength. |
| Film formers | Hold filaments together and provide protection to the fiberglass strand. |
| Film modifiers | Modify the film formation to increase strength, flexibility, and tackiness. |
| External lubricants | Provide resistance to abrasion damage at external contact points such as strand guides in downstream processes. |
| Internal lubricants | Reduce the filament‐to‐filament abrasion within the fiberglass strand. |
| Emulsifiers/Surfactants | Form stable suspensions or emulsions of immiscible ingredients, generally in water‐based systems. |
| Other process aids | May be used as required to control foam, wetting, static behavior, biological activity, and any other special requirements by a particular end‐use or internal processing need. |
Good sizing design begins with a selection of the appropriate adhesion promoter that will provide good bonding between the inorganic glass surface and the reactive sites in the targeted polymer system. The most widely used class of chemicals for this function are organosilanes. These species can be designed to promote reaction between the silanol sites on the glass surface and on the organosilane molecule, leading to an Si–O–Si bond at the glass/polymer interface. The silanes as supplied are hydrolyzed in the final sizing formulation to provide Si–OH groups that can then condense with the Si–OH groups on the glass surface to provide strong interfacial bonds. The organic functionality of the silane is then chosen to maximize compatibility with the target polymer in the final composite. Common examples are shown in Table 5. Other classes of chemicals that have seen more limited utility as adhesion promoters include organotitanates and organo‐chromium complexes.
It is important to note that characterization of the quality of the sizing as well as the glass fiber in the finished composite is critical to confirming technical success of a fiberglass product. Good adhesion at the interface implies the formation of an interphase that makes the materials compatible through covalent bonding, and provides a discrete pathway for stress transfer between the materials. Many standardized tests related to interfacial adhesion, interfacial shear stress,