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Glossary of Coating PropertiesIndustrial Coating StandardsCoating Concrete – A Brief GuideCoating Steel – A Brief GuideOn Polyurethane Coatings

Glossary of Coating Properties

An Extensive Guide

We have compiled a list of various properties and terms related to polyurethane coatings or its applications along with a short description, some general information about them, and a brief explanation of each property is tested, how to interpret the results, and some information about the ranges where each property should acceptably fall. We encourage you to contact us if you require any more information on these properties or polyurethane coatings in general and we could also provide you other important information about our Purethane product family such as a technical datasheet.

Abrasion Resistance

Overview: Abrasion resistance is the ability of a coating to withstand mechanical action such as rubbing, scraping, or erosion that tends progressively to remove material from its surface. Such ability helps to maintain the coatings original appearance and structure and provide longer life, especially in applications where fine particle impingement type abrasion is present.

Testing Methodology: The abrasion resistance is usually measured as per ASTM D4060 – 10 Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser. In this test, a technician abrades a specimen using a 1000-gram load with a specific rotating grinding wheel (CS-17, H-10, H-18). Results report the weight loss in mg/1000 cycles. Lower the weight loss, higher is the abrasion resistance. In order to compare two coatings the type of wheel, weight and number of revolutions need to be identical.

Accelerated Weathering

Overview: The effects of outdoor weather sunlight (particularly ultraviolet (UV) radiation) , moisture & heat on a coating’s appearance and properties can range from a simple color shift to severe material embrittlement. After several years in direct sun, coatings can show reduced impact resistance (embrittlement), lower overall mechanical performance, cracking and chalking (breakdown of Polymer resulting loosening of pigment/ filler particles). The accelerated weathering tests attempts to replicate the effects in an accelerated manner. Although direct co-relation to real life longevity cannot be established, it is a useful method for comparative evaluation of the effects of weathering on different types of coatings.

Testing Methodology: Accelerated weathering is usually done as per ASTM G153 – 04(2010) ‘Standard Practice for Operating Enclosed Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials’. Coated specimens are scrutinized to see the effects of exposure after certain number of hours in the test chamber e.g. 500, 1000, 2000 hours. Higher the period the coating can withstand without defects, better is its weathering ability. A ‘Pass/Fail’ report including condition of coating after exposure is issued.

Cathodic Disbondment

Overview: Damage to pipe coating is almost unavoidable during transportation and construction. Breaks or holidays in pipe coatings may expose the pipe to possible corrosion since, after a pipe has been installed underground, the surrounding earth will be moisture-bearing and will constitute an effective electrolyte. Applied cathodic protection potentials may cause loosening of the coating, beginning at holiday edges. Spontaneous holidays may also be caused by such potentials. This test method provides accelerated conditions for cathodic disbondment to occur and provides a measure of resistance of coatings to this type of action.

Testing Methodology: There are several international test methods for Cathodic Disbondment such as: (1) ASTM G8-96(2010): Standard Test Methods for Cathodic Disbonding of Pipeline Coatings; (2) ASTM G42-11: Standard Test Method for Cathodic Disbonding of Pipeline Coatings Subjected to Elevated Temperatures; (3): ASTM G95-07 : Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings (Attached Cell Method).

Chloride Diffusion

Overview: In saline environments, the corrosion of steel reinforcing bars in concrete does not begin until the passive Iron Oxide film, normally present on the surface of steel , is made permeable by the action of Cl- ions. This happens due to Chloride ions penetrating from outside and coatings are used on the concrete surface to act as a impermeable barrier to chloride penetration. In order to determine the resistance of a coating to the penetration of Chloride into concrete, the chloride diffusion test is done.

Testing Methodology: This is a custom designed test – A coated slab is ponded with 1N Salt Solution and allowed to dry out naturally in the climatic room at 40C and 60% RH. At intervals distilled water is added to produce alternate wet / dry cycles. After a period of 50 days, the slab is cut and sections taken out at the depths of 0-5mm, 5-10mm, 10-15mm, 15-20 mm, 20-25mm, 25-30mm. Results are compared with an uncoated control. Results are reported in mg/m2/day for the coated concrete and the uncoated control.

Chemical Resistance

Overview: Coatings may be subjected to chemical exposure which could cause degradation if the coating is not resistant to the specific chemical. In order to determine the suitability of a coating for a particular chemical exposure application, chemical resistance tests are carried out. It must be noted that resistance of a coating is formulation specific and generalization based on generic type cannot be made. Also chemical resistance is reagent specific i.e. a coating may withstand acid attack very well but may be quickly degraded by solvent.

Testing Methodology: Chemical resistance tests are usually carried out as per ASTM D543 – 06 ‘Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents’. Free films of coating are measured, weighed and immersed in the specified reagent for a 30 Day period at specified temperature (usually 25C). At the end of the period they are removed, rinsed, patted dry and allowed to stand for 24 hours before weighing and measuring. Results are reported in (%) change in weight and dimensions, lower the change better is the coatings resistance to the reagent. Coatings resistant to the reagent for immersion service will show less than 2% change (some standards allow up to 5% change, reflecting that the coating may not be immersed in the reagent but only exposed intermittently).

Dielectric Strength

Overview: A coatings dielectric strength, the best single indicator of a material’s insulating capability, measures the voltage the coating insulating material can withstand before electrical failure or breakdown occurs. Expressed as a voltage gradient, typically volts per mil of thickness, higher dielectric-strength values indicate better insulating characteristics. The dielectric strength of coating varies inversely with thickness: thinner specimens yield higher values. The values also tend to be higher at elevated temperatures. During Holiday Detection tests due care is taken not to exceed the coatings dielectric strength to prevent breakdown/degradation.

Testing Methodology: In the test for dielectric strength (ASTM D 149 or IEC 243), a flat sheet or plate is placed between cylindrical brass electrodes, which carry electrical current. Results are reported in Volts / Mil. (Conversion 1 Volt/Mil = 39.4 V / mm).

Elongation – Recoverable

Overview: Recoverable elongation provides data on the elasticity of coating and the relative ability to stretch without permanent deformation. Even plastic coatings will give a high elongation at break but will not recover and will undergo permanent deformation.

Testing Methodology: The standard tensile tests for rigid coatings (ASTM D 638 and ISO 527) or soft coatings and elastomeric materials (ASTM D 412) involves clamping a standard molded ‘Dumbbell Shaped’ specimen into the test device. The device’s “jaw” then moves at a constant rate of separation between the clamps and elongates the specimen close to break point at which point the load is released and specimen allowed to recover.

Flexibility

Overview: Coatings attached to substrates are elongated when the substrates are bent during the installation or in service. Flexibility tests are useful in rating attached coatings for their ability to resist cracking when elongated. They are also useful in evaluating the flexibility of coatings on flexible substrates.

Testing Methodology: ASTM D522 – 93a(2008) Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings. In this method coatings are applied on a flexible substrate like aluminium and bent 180 degree over a cylindrical or conical mandrel at 25C. Illuminated lens examination reveals formation of cracks in the coating. Results are reported as ‘Pass/ Fail’ over a specified mandrel size. Lower the mandrel diameter over which the coating of specified thickness bends without cracking, better is the flexibility. Standards tables & equations are also available to estimate the (%) elongation. Variable are coating thickness and mandrel size. To compare coatings, both parameters must be identical.

Flexibility – Low Temperature

Overview: The flexibility of coatings will drop significantly when the operating temperature is lowered. To assess the flexibility of a coating at lower temperature, the flexibility test needs to be done at the specified temperature.

Testing Methodology: Tests method is identical to the flexibility test, except that the specimen is cooled down to the specified temperature.

Glass Transition Temperature

Overview: Glass transition temperature provides data on the lowest temperature at which a coating can be used. It is the temperature under which the coating polymer undergoes a rather sudden transition from a flexible or elastomeric condition to a hard, glassy or brittle condition. The transition occurs when the coating polymer molecule chains which are free to rotate and slip past each other, become coiled, tangled and motionless at temperatures below the glass transition range.

Testing Methodology: Glass transition temperature is measured by Dynamic Mechanical Analysis (DMA), ASTM E1640 – 09 or Differential Scanning Calorimetry (DSC), ASTM E1356 – 08 instruments.

Hardness

Overview: Hardness gives a measure of the resistance of a coating material to compression, indentation and scratching. The hardness of a coating is measured by an indenting tool called Durometer (Shore Instruments). This test method is an empirical test intended primarily for control purposes.

Testing Methodology: ASTM D2240 – 05(2010) Standard Test Method for Rubber Property—Durometer Hardness. This test method is based on the penetration of a specific type of indentor when forced into the material under specified conditions. The indentation hardness is inversely related to the penetration.

Permeability – Water Vapour Transmission

Overview: Corrosion of steel substrate depends upon the permeation of water vapour through the barrier coatings. All organic coatings are permeable to water vapour, lower the water vapour transmission, better is the barrier property of the coating.

Testing Methodology: Permeability is tested as per ASTM E96 / E96M – 10 Standard Test Methods for Water Vapor Transmission of Materials. Discs of coating are cut, the thickness measured and sealed to a glass dish filled with either desiccant (Method A) or deionized water (Method B). The dishes are then weighed, and placed into a temperature/humidity chamber maintained at specified temperature and relative humidity for a specified period. The dishes are weighed separately at various recorded intervals to record the weight gain (Method A – from chamber to dessicant) or loss (Method B – from water to chamber) and the results plotted on the graph. In method BW, the water filled cup is inverted.

Permeability – Oxygen Transmission

Overview: Corrosion of steel substrate depends upon the permeation of water AND oxygen through the barrier coating. Therefore in addition to water permeability, the Oxygen permeability should be tested (although industry mostly relies on the WVT data).

Testing Methodology: Gas chromatography as per ASTM D3985 – 05(2010)e1 Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor. The test is done at 1 Kg/cm2 feed pressure or at higher pressures such as 10 Kg/cm2. Results are reported as (cc cm) / (cm2 sec cm Hg)

Tensile Strength

Overview: Tensile strength expresses the maximum uniaxial stress a cured polyurethane film can resist before it ruptures, providing a direct indicator of the backbone integrity of the polymer network. It parallels overall film stiffness and load‑bearing capacity and is routinely used alongside elongation data to position formulations between flexible elastomers and rigid enamels. Excessively high tensile strength without adequate elongation often translates to brittle behavior, whereas too low a value signals under‑cure or an overly plasticized matrix. Temperature, humidity during cure, and post‑cure conditioning strongly influence the final tensile profile because they govern the extent of crosslinking and cure shrinkage. Designers use the property to model mechanical stresses in service such as pressurised tanks, bridge deck movements, or thermal cycling of concrete substrates.

Testing Methodology: The standard tensile tests for rigid coatings (ASTM D 638 and ISO 527) or soft coatings and elastomeric materials (ASTM D 412) involves clamping a standard molded ‘Dumbbell Shaped’ specimen into the test device. The device’s “jaw” then moves at a constant rate of separation between the clamps. Dividing the breaking load by the original minimum cross-sectional area gives tensile strength. Results are reported in N/mm2 or Psi. (Conversion 1 N/mm2 = 145 Psi).

Adhesion (Pull-off)

Overview: Adhesion characterises the molecular and mechanical forces that hold a polyurethane coating to its underlying substrate, making it the single most important predictor of in‑service durability. Pull‑off testing isolates a defined area and quantifies the tensile stress required to detach the film, thereby distinguishing adhesion failure at the interface from cohesive failure within the film. Surface preparation, profile amplitude, substrate cleanliness and primer chemistry can swing pull‑off values by an order of magnitude, so these parameters must be rigorously controlled both in the laboratory and on site. Loss of adhesion often precedes blistering, under‑film corrosion and wholesale delamination when structures are exposed to thermal swing or hydrostatic pressure. Immersion‑grade polyurethanes routinely deliver 4–7 MPa on blast‑cleaned steel and 2–4 MPa on prepared concrete, far exceeding the thresholds mandated by ISO 12944 or NORSOK M‑501.

Testing Methodology: Typically determined using ASTM D4541 or ISO 4624 pull‑off tests in which a metal dolly is bonded to the coating and the tensile force required to detach it is recorded; field cross‑cut tape checks may follow ASTM D3359.

Impact Resistance

Overview: Impact resistance gauges the coating’s capacity to absorb and dissipate sudden kinetic energy without cracking, chipping or disbonding from the substrate. Whereas tensile tests capture slow elastic deformation, impact loading simulates real‑world insults such as hail, gravel blast, tool drops or fork‑lift collisions that occur at high strain rates. Polyurethanes derive their high direct‑impact performance from a balanced hard‑segment/soft‑segment morphology and excellent adhesion; reverse‑impact resistance is more demanding because tensile stresses develop at the interface. Impact data guide specifiers when deciding on sacrificial guards, elastomeric interlayers or film‑thickness allowances in railcar linings and pipe bends. High‑build aromatic elastomers can survive 160 in‑lb (18 J) direct impact at 1 mm thickness, while thinner aliphatic decorative coats may trade impact strength for UV durability.

Testing Methodology: Measured with ASTM D2794 falling‑weight (Gardner) impact test for both direct and reverse impact; results reported as inch‑pounds (or joules) the film withstands without damage.

Gloss and Color Retention

Overview: Gloss and color retention describe a coating’s ability to maintain its original appearance, reflectivity, and chromatic fidelity after prolonged exposure to sunlight, moisture, and environmental contaminants. Polyurethane topcoats are prized for their exceptional resistance to UV-induced chalking and fading, making them a preferred choice in architectural, automotive, and marine applications. Loss of gloss typically signals polymer degradation or surface microcracking, while color shift (ΔE) is a quantitative marker of pigment stability and binder resilience. High gloss and color retention reduce maintenance cycles, preserve asset aesthetics, and are often prerequisites in warranty specifications. Accelerated weathering tests and Florida panel exposures are standard means to benchmark these properties and compare different resin chemistries or pigment packages.

Testing Methodology: Initial gloss per ASTM D523 (20°, 60° or 85°) and color ΔE*ab per ASTM D2244 are compared with measurements after QUV exposure (ASTM G154) or natural weathering (ISO 2810 Florida panels).

Pot Life / Working Time

Overview: Pot life, also referred to as working time, is the time window after mixing two-component polyurethane systems during which the material remains sufficiently fluid for application and yields a defect-free film. It is governed by the onset of the curing reaction—often catalyzed by temperature, humidity, and batch size—which causes viscosity to rise until the product becomes unworkable. Short pot life can lead to wasted material and hurried application, while excessive pot life may signal insufficient reactivity or risk dust contamination. Pot life is crucial for planning large-scale projects, multi-coat systems, or complex geometries where continuous application is needed. Manufacturers typically specify pot life at a standard temperature (e.g., 25°C), and field conditions may require adjustments to crew size or batch size accordingly.

Testing Methodology: Determined via ASTM D4212 or ASTM D1640 by measuring the viscosity rise at a fixed temperature until it doubles, or via a gel‑time cup; report minutes at the stated °C.

VOC and % Solids by Volume

Overview: VOC (Volatile Organic Content) quantifies the amount of solvent released to the atmosphere during application and cure, impacting both environmental compliance and worker safety. Regulations in many regions cap allowable VOC levels for industrial and architectural coatings, making this property a key consideration in product selection. Percent solids by volume determines how much of the applied coating remains as a protective film after curing, directly affecting coverage rates and the calculation of required wet-film thickness to achieve target DFT (dry film thickness). High-solids and zero-VOC polyurethanes are increasingly favored for their sustainability and cost efficiency. Accurate knowledge of both parameters ensures correct specification, minimizes rework, and supports LEED or green-building certifications. Contractors rely on these values to optimize material usage and meet project environmental requirements.

Testing Methodology: VOC content per ASTM D3960; % solids by volume per ASTM D2697 (pycnometer) or ASTM D1644 (solvent evaporation); values reported in g L⁻¹ and %, respectively.

Dry‑to‑Touch / Through‑Cure Time

Overview: Dry‑to‑touch time measures how quickly a polyurethane coating loses surface tackiness, allowing for safe handling or overcoating without marring. Through‑cure time, in contrast, indicates when the film has reached its full mechanical and chemical resistance, permitting exposure to traffic, immersion, or aggressive cleaning. Both times are sensitive to temperature, humidity, and film thickness, and deviations can signal problems with mixing, application, or ambient conditions. Fast dry‑to‑touch is desirable for productivity but must be balanced with sufficient open time for leveling and flow. Through‑cure data help maintenance planners minimize downtime and coordinate follow-on trades. Published values are typically based on controlled laboratory conditions and may require adjustment for field environments.

Testing Methodology: ASTM D1640 Method B (dry‑to‑touch) and Method D (thumb‑twist dry‑through) at the specified temperature and relative humidity.

Recoat Window

Overview: The recoat window defines the optimal period after initial application during which a subsequent layer can be applied without sanding or abrading, ensuring chemical bonding and reliable intercoat adhesion. Applying a topcoat too early may trap solvents and cause blistering, while waiting too long can lead to poor adhesion due to surface curing or contamination. Polyurethane systems often have narrower recoat windows than epoxies, especially in humid or cold weather. Manufacturers specify both minimum and maximum recoat intervals, which are critical for scheduling multi-layer builds or field repairs. Exceeding the recoat window typically requires mechanical preparation (e.g., sanding or sweep blasting) to restore surface reactivity and guarantee system integrity.

Testing Methodology: Typically established by performing pull‑off adhesion (ASTM D4541) or cross‑cut adhesion (ASTM D3359) on panels recoated at various intervals; manufacturers publish acceptable ranges.

Sag Resistance

Overview: Sag resistance is the coating’s ability to remain in place on vertical or overhead surfaces without running or forming drips during application. It is especially important for high-build or thick-film polyurethane systems, where excessive sag can result in uneven coverage, poor appearance, or insufficient protection. Sag resistance is influenced by formulation rheology, solvent content, and environmental conditions such as temperature and airflow. Additives like thixotropes or fumed silica are often used to enhance sag control. Applicators rely on this property to maximize productivity—applying thicker coats in a single pass—while avoiding costly rework. Manufacturers publish sag resistance values to guide field crews on safe maximum film builds per coat.

Testing Methodology: Evaluated with ASTM D4400 using a Leneta Anti‑Sag Meter; results expressed as Sag Index or maximum mils/mm before sag occurs.

Film Thickness

Overview: Film thickness, measured as both wet‑film thickness (WFT) during application and dry‑film thickness (DFT) after cure, is a critical determinant of coating performance and durability. Too thin a film may result in premature failure, pinholing, or insufficient barrier protection, while excessive thickness can cause solvent entrapment, cracking, or sagging. Accurate control of film thickness is essential to meet warranty requirements, pass third-party inspections, and ensure long-term protection against corrosion or chemical attack. Application method, spray equipment, and operator skill all impact the consistency of film build. Regular monitoring with calibrated gauges allows for real-time adjustments and quality assurance throughout the project.

Testing Methodology: WFT measured with notch/comb gauges per ASTM D4414; DFT with magnetic or eddy‑current gauges per SSPC‑PA 2 or ISO 19840.

Thermal Shock Resistance

Overview: Thermal shock resistance measures a coating’s ability to survive rapid or repeated temperature changes without cracking, delaminating, or losing adhesion. Polyurethane coatings are often used on substrates—such as concrete or steel—that experience thermal cycling from operational processes, weather, or cleaning with hot water or steam. Failure to withstand thermal shock can manifest as microcracking, loss of gloss, or catastrophic disbondment, especially at joints and edges. The property is influenced by the coating’s glass transition temperature, flexibility, adhesion, and coefficient of thermal expansion relative to the substrate. Designers use thermal shock resistance data to select systems for freezers, hot water tanks, or exterior decks in variable climates. Field testing often supplements laboratory cycling to validate real-world performance.

Testing Methodology: ASTM D7843 or internally specified cycling between defined temperature extremes with inspection every cycle; pass/fail based on visual defects and adhesion.

Slip Resistance (Coefficient of Friction)

Overview: Slip resistance, expressed as the coefficient of friction (COF), quantifies how much traction a polyurethane-coated surface provides under foot or wheel, especially when wet or contaminated. This property is crucial for safety in walkways, ramps, pool decks, parking garages, and industrial floors where slips and falls are a major liability. The COF depends on surface texture, aggregate loading, and the inherent properties of the cured film. Regulatory standards and building codes often specify minimum COF values for public spaces, and coatings can be tailored with anti-slip additives to meet these requirements. Ongoing maintenance, cleaning practices, and environmental exposure can change slip resistance over time, so periodic testing is advised. Proper specification and verification of slip resistance help prevent accidents and ensure compliance with occupational safety guidelines.

Testing Methodology: Measured by ASTM E303 pendulum or ANSI A326.3 dynamic coefficient of friction; many specifications call for ≥ 0.42 (wet) on walkways.

Pencil Hardness

Overview: Pencil hardness is a simple, standardized method for evaluating the scratch and mar resistance of a polyurethane coating’s surface. The test uses pencils of varying hardness (from soft 6B to hard 9H) to determine the hardest grade that will not cut or gouge the cured film. It serves as a practical, rapid screening tool for surface durability in environments prone to abrasion, cleaning, or incidental contact. Higher pencil hardness values indicate a tougher, more scratch-resistant surface, but may also correlate with increased brittleness or reduced flexibility. Pencil hardness complements other hardness measures like Shore or König, providing insight into the balance between surface protection and film toughness. It is widely used in quality control for flooring, cabinetry, automotive, and decorative finishes.

Testing Methodology: ASTM D3363 Wolff‑Wilborn pencil test using the 6B–9H scale; report the hardest pencil that does not cut or gouge the film.