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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, tensile stress, modulus, and hydrolytic stability have been developed over the years and are widely available in international standard testing organizations such as ASTM and ISO. Many academic laboratories have also developed more sophisticated tests. One example is the measurement of composite critical fiber length [18]. As defined by Eq. (5), the critical fiber length (l_c) is the minimum fiber length at which fibers with given diameter d reach their ultimate tensile strength σ_fu and the matrix/fiber interface experiences a maximum interfacial shear stress τ_y [18]. When l > l_c, an increase in interfacial adhesion will result in a decrease in critical fiber length and may lead to improvements in other properties such as impact strength in certain resin systems. However, the sizing may also act to protect the fiber from surface damage and defects in addition to providing improved adhesion. In that case, reduction of fiber surface damage would lead to an increase in the critical fiber length by increasing σ_fu. Interestingly enough when l > l_c, and the composite is stressed to failure, then the fiber will break at regular intervals of l_c within the composite. This result can be used to estimate the interfacial shear stress:

(5)

This example also serves to illustrate that broader conclusions from micromechanical experiments should consider that different outcomes may result depending upon material selection. The most common material options include chopped or short fibers vs. continuous fibers and thermoset vs. thermoplastic resin systems, which of course complicates the task of the sizing chemist in product development.

It remains for the sizing chemist to complete the establishment of the final formulation from a virtually limitless palette of film formers, process aids, and modifiers to deliver the desired performance of the fiberglass as a commercial product. Commercially, the resultant amount of sizing on the fiberglass surface is typically in the range of 0.3–1.5 wt % of the dry fiberglass.

An excellent in‐depth review of the complexity of this field from the sizing chemists' point of view can be found in Thomason's book, Glass Fibre Sizings: A Review of the Scientific Literature [19].

Table 5 Common organosilanes.

Chemical name	Structure	Class
γ‐Aminopropyl trialkoxy silane	(RO)3Si–(CH2)3–NH2	Amine
γ‐Methacryloxypropyl trialkoxy silane	(RO)3Si–(CH2)3–OOC(CH3)C=CH2	Methacryl
γ‐Glycidyloxypropyl trialkoxy silane	(RO)3Si–(CH2)3–O–CH2CHCH2O	Epoxy
Vinyl trialkoxy silane	(RO)3Si–CH=CH2	Vinyl
	R = CH3CH2– or CH3–