UV Curing? No, Photopolymerization! – Uncovering the Scientific Pitfalls Behind Industry Terminology

In the field of UV curing technology, have you ever been confused by terms like "oligomer," "resin," and "photoinitiator"? The Professor’s Corner column sharply points out: the professional vocabulary we take for granted may be obscuring the essence of the technology and even causing misunderstandings!

“Curing” or “Polymerization”? A Crucial Fundamental Difference

Traditionally, “curing” refers to the process where linear or branched polymers are crosslinked via covalent bonds (e.g., thermal curing). However, UV/EB technology is fundamentally different:

Core ingredients are NOT polymers: The main components of a formulation are relatively low-molecular-weight non-polymeric materials – oligomers and monomers.

Dual Process: When exposed to UV or EB energy, these materials undergo simultaneous polymerization (forming macromolecular chains) and crosslinking (curing) directly on the substrate.

Therefore, more precise terms should be "UV/EB polymerization" or "energy polymerization", and the raw materials should be called "UV/EB polymerizable" materials. This isn't mere semantics; it accurately describes the unique chemical nature of the process.

The Oligomer Quandary: Monomer or Resin?

By definition, an oligomer refers to a molecule formed by "a few" linked monomers. But the industry reality is full of contradictions:

Misnamed "Monomers": Many key "oligomers" (like the common epoxy acrylate, diglycidyl ether of bisphenol-A diacrylate / DGEBA-DA) contain only one reactive group (one "mer") in their molecular structure, making them, strictly speaking, monomers! They are classified as oligomers primarily due to their extremely high viscosity (up to millions of cps, far exceeding typical monomers).

The "Resin" Temptation: Due to their high viscosity and sticky properties resembling natural resins (e.g., tree sap), they are often loosely called "resins." But "resin":

Originates from natural materials or broadly refers to synthetic polymers.

Often denotes a fully formulated end product, not a single component.

Is an archaic term predating photopolymerization technology.

Despite the "oligomer" label being chemically imprecise for some structures, its continued use for these core formulation components is more appropriate and specific. This is due to their critical role (determining final properties) and distinct physical characteristics (high viscosity vs. monomers), better reflecting the technology's sophistication.

Photoinitiators: A Tangled Web of Terminology

Terminology confusion is even worse in the realm of photoinitiators. Terms like "photoinitiator," "photosensitizer," and "coinitiator" are often used interchangeably, but they are NOT synonyms! The distinction lies in their mechanism of action:

Norrish Type I (Photocleavage):

A single molecule both absorbs light energy and generates active species.

After absorbing light, it undergoes direct homolytic cleavage, producing two free radicals.

Examples: Irgacure®184, Darocur®1173.

The term "Photoinitiator" is accurate here.

Norrish Type II (Hydrogen-Abstraction):

Requires two molecules working together:

Photosensitizer: Absorbs light energy, reaching an excited state.

Coinitiator: Donates a labile atom (usually hydrogen) to the excited photosensitizer, itself forming a radical that initiates polymerization.

May produce byproducts but can be less expensive and helps overcome oxygen inhibition.

Example: Benzophenone (photosensitizer) + an amine with labile hydrogens (e.g., DMEA, MDEA - coinitiator).

Distinguishing between "Photosensitizer" and "Coinitiator" is crucial here.

Why is Terminological Precision So Important?

Knowledge Transfer: Newcomers facing chaotic terminology become easily confused, hindering learning efficiency and depth of technical understanding.

Clear Communication: Precise terms are fundamental for avoiding ambiguity and improving efficiency in R&D, production, and customer interactions.

Technological Innovation: Accurately describing chemical processes (e.g., distinguishing "polymerization" from "curing") helps grasp the core technology and push its boundaries.

Embracing Precision: Technological Evolution Starts with Language

UV/EB technology is an advanced, efficient, and environmentally friendly manufacturing process. Its terminology should also evolve, shedding historical baggage and vague expressions. Discarding the vague label "resin," affirming the "oligomer's" status as the formulation cornerstone, strictly differentiating types of photoinitiators, and promoting the more fundamental term "photopolymerization" – this is not just respect for science, but a necessary step for the industry towards maturity and standardization. Only when we can name the technology more accurately can we see its future more clearly.

Reference Source: Christmas, B.K. (2023). Professor’s Corner: Back to the Basics of UV/EB. UV+EB Technology, Quarter 2, 2023, pp.18-19.