Polymer chemistry has its own vocabulary, and navigating technical literature, coating specifications, and product data sheets requires familiarity with the terminology. This glossary defines the most commonly encountered terms in polymer science and coating technology, from fundamental chemistry concepts to application-specific jargon.
A–C
Adhesion — The strength of the bond between a coating and its substrate, measured in terms of the force per unit area required to separate the coating from the surface. Adhesion can be tested by pull-off methods (ASTM D4541) or cross-cut tape tests.
Aliphatic — Referring to organic compounds in which carbon atoms are arranged in open chains or non-aromatic rings. Aliphatic isocyanates (such as HDI and IPDI) produce coatings with superior UV stability compared to aromatic isocyanates, making them preferred for exterior applications where color retention matters.
Aromatic — Referring to organic compounds containing benzene rings or similar cyclic structures with delocalized electrons. Aromatic isocyanates (such as MDI and TDI) are more reactive and typically less expensive than aliphatic versions, but the resulting coatings yellow and chalk when exposed to ultraviolet radiation.
Catalyst — A substance that accelerates a chemical reaction without being consumed in the reaction itself. In polyurethane chemistry, organometallic catalysts (such as dibutyltin dilaurate) and tertiary amine catalysts are used to control gel time, cure rate, and the balance between blowing and gelling reactions in foam systems.
Chain extender — A low-molecular-weight, difunctional compound added to polyurea and polyurethane formulations to build molecular weight and modify hard segment content. Common chain extenders include diols and diamines. In polyurea coatings, amine chain extenders are standard components of the resin blend (Part B).
Cross-linking — The formation of chemical bonds between adjacent polymer chains, creating a three-dimensional network. Cross-linking improves hardness, chemical resistance, and thermal stability while reducing swelling and solubility. High cross-link density generally means a harder, less flexible material.
D–I
Delamination — The separation of coating layers from each other or from the substrate. Delamination is the most common form of protective coating failure and is typically traced to inadequate surface preparation, application errors, or coating-substrate incompatibility.
Dry film thickness (DFT) — The thickness of the coating film after all solvents have evaporated and the film has cured. DFT is a critical specification parameter because it directly affects barrier protection, and is measured using magnetic gauges (on steel) or ultrasonic gauges (on non-metallic substrates).
Elastomer — A polymer that exhibits large, reversible deformation under stress. Elastomers have high elongation (typically 100–600%) and recover their original shape when stress is removed. Polyurea coatings are classified as elastomers due to their high elongation and elastic recovery.
Elongation at break — The percentage increase in length of a material sample from its original dimension to the point of fracture under tensile stress. High elongation is a defining characteristic of elastomeric coatings. Measured per ASTM D412.
Glass transition temperature (Tg) — The temperature below which an amorphous polymer transitions from a rubbery, flexible state to a rigid, glassy state. Below Tg, segmental chain motion ceases, and the material becomes brittle. Tg is a critical parameter for coatings used in low-temperature environments.
Isocyanate — An organic functional group with the formula -N=C=O. The central reactive building block in polyurethane and polyurea chemistry. Isocyanates react with polyols to form polyurethane linkages and with amines to form polyurea linkages.
L–P
Mil — A unit of thickness equal to one-thousandth of an inch (0.001 inch = 25.4 microns). Coating thicknesses in protective coating specifications are often expressed in mils for the North American market.
Monomer — The small, repeating molecular unit that polymerizes to form a polymer chain. In polyurethane chemistry, the isocyanate and polyol are the functional building blocks from which the polymer chains are assembled.
Permeability — The ease with which a molecule (water, gas, or ion) can diffuse through a material. Lower permeability in a coating film means better barrier protection against corrosion-promoting species. Cross-link density, film thickness, and the chemical nature of the polymer backbone all influence permeability.
Polyol — An organic compound containing multiple hydroxyl (-OH) groups. Polyols are one of the two key reactive components in polyurethane chemistry. When a polyol reacts with an isocyanate, it forms urethane linkages. Polyols vary widely in molecular weight, functionality, and chemical nature, enabling a broad range of polyurethane properties.
Polyurea — A fast-curing elastomeric coating formed by the reaction of an isocyanate component with an amine-terminated resin component. Characterized by fast cure, moisture tolerance, high tensile strength, and high elongation. Used widely in waterproofing, containment, military protection, and industrial floor applications.
Polyurethane — A polymer formed by the reaction of an isocyanate with a polyol. Available in rigid and flexible configurations, including foams, coatings, elastomers, and adhesives. Distinguished from polyurea by the use of polyol rather than amine as the second reactive component.
S–Z
Shore hardness — A measure of a material’s resistance to permanent indentation. Shore A is used for soft elastomers; Shore D is used for harder materials. Higher numbers indicate greater hardness. Measured using a durometer per ASTM D2240.
Spallation — The fragmentation of a material on the reverse face when it is subjected to an impact or explosive event on the front face. Polyurea coatings are used as spall liners on armored vehicles and ballistic structures to capture and decelerate these secondary fragments.
Tensile strength — The maximum stress that a material can sustain under uniaxial tension before failure. Expressed in units of force per unit area (psi or MPa). High tensile strength in a coating contributes to tear resistance and resistance to mechanical damage. Measured per ASTM D412.
VOC (Volatile Organic Compound) — An organic compound that evaporates at room temperature and may contribute to air quality concerns. Regulatory limits on VOC content in coating formulations drive the development of waterborne, high-solids, and solvent-free coating systems.
Wet film thickness (WFT) — The thickness of a coating immediately after application, before solvent loss or chemical cure reduces the volume. WFT and the calculated volume solids percentage together predict the dry film thickness that will result, enabling real-time application monitoring.