The terms “resin” and “polymer” are often confused, especially when discussing materials used in adhesives, plastics, and coatings. They describe fundamentally different concepts in chemistry and materials science. A polymer is defined by its molecular structure as a large chain of repeating units. In contrast, a resin describes a material’s physical state, typically a viscous liquid or solid precursor. Clarifying the relationship between these two requires understanding their specific chemical definitions.
What Defines a Polymer
A polymer is a large molecule, or macromolecule, built from many smaller, identical units called monomers. This structure is the defining characteristic of a polymer, where monomer units are linked together by covalent bonds in a long, repeating chain. The process of forming this chain is known as polymerization, which increases the substance’s molecular weight.
Monomers are the simple building blocks, such as ethylene or styrene, that possess functional groups allowing them to react and join. The resulting polymer chain can be linear, branched, or form a complex three-dimensional network. This molecular structure determines the final properties of the material, such as whether it is a flexible plastic or a rigid solid.
Understanding Resins
In contrast to a polymer, the term “resin” describes a material’s physical state and function. It is typically a viscous, sticky substance that can be converted into a hardened product. Historically, resins were natural exudates from plants, such as tree sap, valued for their ability to dry or harden into a solid.
Modern synthetic resins, such as epoxies, polyurethanes, and polyesters, mimic this characteristic of being a liquid or semi-solid precursor material. These synthetic resins are formulated specifically as starting materials that undergo a chemical transformation. They are designed to be easily handled in their liquid state before being converted into a durable, final form.
The Polymerization of Resins
The question of whether a resin is a polymer is nuanced, depending on the type of resin and its stage of transformation. A synthetic resin, in its un-cured liquid form, is often not a high-molecular-weight polymer, but rather a collection of reactive monomers or short-chain molecules called oligomers. The resin component is the reactive part of a system, ready to undergo the chemical process that defines polymerization.
This chemical change is known as curing or cross-linking, which is the mechanism by which the liquid resin components form a solid polymer network. For example, in a two-part epoxy resin, the resin (Part A) is mixed with a hardener or curing agent (Part B), initiating a reaction. The monomers or oligomers in the resin begin to link together, forming long polymer chains and cross-links, which creates a rigid, three-dimensional structure.
This irreversible process transforms the low-viscosity liquid into a durable, solid, high-molecular-weight polymer. The liquid “resin” is best understood as the precursor material, which becomes the polymer after curing. The vast majority of synthetic resins are specifically designed to be converted into a solid polymer through a controlled chemical reaction.