How can fiberglass bond resin? It's all thanks to an invisible layer of glue.

I. Without Sizing Agent, Fibers and Resin "Don't Resist Each Other"

Fiberglass has a very smooth surface, like thin glass rods. Resin (epoxy, polyester, etc.) is a polymer liquid. If clean fiberglass are directly soaked in resin, they can "stick" together after curing, but the bonding strength is very weak. When pulled, the resin and fiber will "separate," and the fiber's strength cannot be fully utilized.

What's the problem? The surface of fiberglass is hydrophilic (containing a large number of -OH hydroxyl groups), while resin is hydrophobic. The two materials are "incompatible" and cannot bond firmly.

II. Sizing Agent: A Molecular-Level "Glue"

The sizing agent is a chemical treatment agent directly coated onto the fiber surface during the fiberglass drawing process. After the fiberglass is drawn from the stencil, it immediately passes through a coating device (oiler), where a thin layer of sizing agent-thinner than a hair and in very small quantity (usually only 0.5-2% of the fiber's weight)-is evenly coated on the surface. This sizing agent acts like "double-sided tape"-one end "grips" the fiberglass, and the other end "grips" the resin, establishing strong chemical bonds between the fiber and the resin.

III. What's in the Sizing Agent?

The sizing agent isn't a single component, but a "formulaic combination" of various chemical substances:

Coupling Agent (Core): The most crucial component, usually a silane coupling agent. It reacts with the hydroxyl groups on the fiberglass surface on one end and with the resin molecules on the other, building a "molecular bridge" between them.

Film-forming Agent: Forms a protective film on the fiber surface, preventing fiber abrasion and improving fiber cohesion (preventing the fibers from unraveling).

Lubricant: Reduces frictional damage to the fibers during spinning and weaving.

Antistatic Agent: Prevents the fibers from "frizzing" due to static electricity during processing.

IV. A Poor Sizing Agent Means Damaged Fibers

The quality of the sizing agent directly determines the upper limit of the fiberglass's strength. No matter how strong the fiber, if it doesn't "bond" firmly with the resin, the force cannot be transmitted, rendering the fiber useless. If the sizing agent is not applied properly, the interfacial bonding force between the fiber and the resin will decrease by 30-50%, and the bending strength and shear strength will also drop.