FAQ on 100% Solids Polyurethane Coatings.

Property / Parameter - In Alphabetical Order

▼  Abrasion Resistance

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DescriptionTest Method

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

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DescriptionTest Method

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.

▼  Adhesion

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DescriptionTest Method

Adhesion tests measure how firmly the coating is bonded to the surface. The pull-off strength of a coating is an important performance property and is usually considered to be a good indicator of the longevity of the coating. High adhesion helps prevents mechanical damage during handling/ service as well as disbondment in service by factors such as soil stress, cathodic protection currents etc. Loss of adhesion usually precedes commencement of corrosion underneath the coating.

Adhesion tests are usually done using portable 'Pull Off' testers as per ASTM D4541 - 09e1 'Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers'. The test determines the greatest perpendicular force (in tension) that a surface area can bear before a plug of material is detached and is reported in N/mm2 or Psi. (Conversion 1 N/mm2 = 145 Psi).

▼  Cathodic Disbondment

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DescriptionTest Method

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,
ASTM G8-96(2010): Standard Test Methods for Cathodic Disbonding of Pipeline Coatings
ASTM G42-11 : Standard Test Method for Cathodic Disbonding of Pipeline Coatings Subjected to Elevated Temperatures
ASTM G95-07 : Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings (Attached Cell Method).

A coated pipe/ coupon with a specified coating holiday (initial hole in the coating) is immersed in an electrolyte and DC current supplied. At the end of the test period, radial cuts from the initial hole are made and the coating wedges peeled off to see the extent of disbondment. Results are reported in mm radius of disbondment from edge of initial hole. Lower the disbondment, better is the resistance of the coating.

Variables include Test Voltage, Coating Thickness, Initial Hole Diameter, Electrolyte Composition, Test Temperature & Duration. Comparision between coatings can be done only when all the parameters are identical.

▼  Chloride Diffusion

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DescriptionTest Method

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

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DescriptionTest Method

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 e.g 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).

Variables in chemical resistance are coating thickness, chemical concentration, temperature and duration. When comparing chemical resistance of two coatings, these parameters must be identical to get an accurate result.

▼  Crack Spanning Capacity

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DescriptionTest Method

In high quality concrete the transport of gases and ions is a slow process, however it is much faster through an interconnected network of microcracks and capillary voids by capillary action. Such a network of microcracks already exist in reinforced concrete and the widening of cracks in service (by thermal and humidity changes) precedes the corrosion process. As soon as the Chloride ions reach the rebar, one has the powerful anodic-cathodic potential , and with free moisture abundantly available corrosion will proceed.

Any coating installed on the surface of concrete to act as barrier to ingress of Chloride ions must be able to span (bridge) existing as well as new cracks opening up under the coating without cracking. To test this ability, the crack spanning capacity is tested.

This is a custom test method. The coated beams are subjected to a four point load to measure the crack size at which rupture through the coating occurs. The test is also repeated after subjecting the coated specimens to accelerated weathering (Ultraviolet Radiation and Temperature Cycles) in order to assess the loss in crack spanning capacity after years of service. A similiar international standard is ASTM C 1522 'Standard Test Method for Extensibility After Heat Aging of Cold Liquid-Applied Elastomeric Waterproofing Membranes' - this is applicable for high elongation waterproofing materials.

This is a particularly severe test where cracks are initiated and opened up under the coating. Coating onto concrete already cracked is likely to be a less onerous condition.

▼  Dielectric Strength

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DescriptionTest Method

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 dur 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 @ Break

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DescriptionTest Method

Elongation at break provides data on the ability of coatings to stretch without early rupture. Elongation is recorded at the moment of rupture of the specimen, expressed as a percentage of the original length. Specimens and instrument is the same as tensile strength test. Brittle coatings break at low strain levels; ductile and elastic materials attain high strain levels before breaking.

Listed in place of elongation at break, ultimate elongation is often shown for highly elastic coatings, such as elastomeric polyurethanes, some of which can stretch over 400% before failing. The test for ultimate elongation uses narrower test bars and faster deflection speeds than the elongation-at-break test.

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 'Dumbell 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 till break point. Elongation @ break is calculated as : (The increase in length between gauge marks) / (Original length between gauge marks) X 100

▼  Elongation - Recoverable

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DescriptionTest Method

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 'Dumbell 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.

The recoverable elongation is calculated as : (Original Gauge Length - Non Recovered Increase) / Original Gauge Length X 100. Higher the Recoverable Elongation (%), more elastic is the coating.

▼  Flexibility

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DescriptionTest Method

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

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DescriptionTest Method

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.

▼  Friction - Coefficient Of

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DescriptionTest Method

The coefficient of friction is the ratio of friction force, the force needed to initiate sliding, to normal force, the force perpendicular to the contact surfaces.

In many applications, such as internal lining of pipes, a smooth coating with a low coefficient of friction will reduce the resistance to flow and reduce pumping horsepower leading to cost savings.

Friction largely depends upon the relative roughess of the surface, tested by a Scanning Tunneling Microscope. Well formulated 100% Solids Polyurethanes have a relative roughness of 25 Microinches or less. For pipe lining, friction factor depend upon the relative roughness (surface roughness / pipe diameter) AND the type of flow (laminar/ turbulent) in the pipes. Mannings 'n' is normally between 0.009 - 0.01 depending upon flow type. Hazen Williams 'C' is approx. 149.

▼  Glass Transition Temperature

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DescriptionTest Method

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

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DescriptionTest Method
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.

Scales on the Durometer read from 0 to 100, with higher number denoting greater hardness. Two scales are available, Shore A for softer coating and Shore D for harder coatings. The scales overlap, with 100 Shore A being approximately equal to 60 Shore D.

▼  Impact Resistance

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DescriptionTest Method

"Impact strength, is the coating's ability to withstand damage by impacting objects. Coating which have the ability to dissipate energy (such as Elastomeric Polyurethanes) have higher impact strength than coatings without this ability (Rigids). The Impact Strength also varies with substrate shape, coating thickness and temperature.

If the impact is able to cause a discontinuity and unprotected path to the substrate, localised corrosion will take place. In pipelines, impact discontinutity will increase the non-insulated area and cause increase in cathodic protection currents.

For pipelines, the falling dart impact test (ASTM G14) is used, in which a weighted puncturing device with a standard tip diameter — usually 5/8 inch — drops onto a supported sample pipe from increasing heights until the impact causes a rupture or cracking.

The falling dart impact strength is the drop energy of the average height causing rupture and is typically reported in Inch - pounds (In-Lbs) or Newton Meter (Nm) or Joule. Higher the value, better is the impact resistance of the coating. (Conversion 1 Nm or J = 8.85 In-Lbs).

▼  Permeability - Water Vapour Transmission

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DescriptionTest Method
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 dessicant (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. The rate of transmission (from linear region of the curve) is reported as Water Vapour Transmission in gms / Sq.M / 24 hours. Lower WVT, better is the coating.

Variables are the method used A/B/BW, temperature, humidity conditions and coating thickness. For comparing coatings, all variables must be the same.

Water Vapour Permeability is the result reported for unit thickness.

▼  Permeability - Oxygen Transmission

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DescriptionTest Method

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)

▼  Salt Spray Resistance

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DescriptionTest Method

Salt spray test is an accelerated corrosion test that produces a corrosive attack to the coated samples in order to predict its long term ability to protect steel substrates. Test duration depends on the corrosion resistance of the coating; the more corrosion resistant the coating is, the longer the period in testing without showing signs of corrosion.

It must be noted that the result of the salt spray test cannot be used to estimate the actual service life of the coating. It however provides good data on the effectiveness of a coating system in corrosion protection and relative ability of different coating systems.

Test method is as per ASTM B117 - 11 'Standard Practice for Operating Salt Spray (Fog) Apparatus'. The apparatus consists of a closed testing chamber, where a salted solution (mainly, a solution of 5%sodium chloride) is atomized by means of a nozzle. This produces a corrosive environment of dense saline fog in the chamber so that coated coupons exposed in it are subjected to severely corrosive conditions. The standard describes the necessary information to carry out this test; testing parameters such as temperature, air pressure of the sprayed solution, preparation of the spraying solution, concentration, pH, etc.

Results are reported in hours (such as 500, 2000, 10000) the coating has survived without showing corrosion or other deleterious effects. Higher the number of hours, better is the coating's ability to protect the steel from corrosion.

▼  Tensile Strength

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DescriptionTest Method

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 'Dumbell 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).