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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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

Glass Transition Temperature

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.

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

Hardness

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.

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

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.

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

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).

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

Tensile strength is used to compare the relative strength and stiffness of coatings.

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).