What are 100% Solids polyurethane coatings and how do they work?
Generally speaking, 100% Solids Polyurethane Coating are an organic coating made from the cross linking reaction of Isocyanate and Polyol compounds. The two compounds chemically react to form a Polyurethane Coating. There is no by-product, other than heat from the reaction between isocyanates and polyols.
R-N=C=O + R’-OH –> R-NH-CO-O-R’ + Heat
Isocyanate Polyol Polyurethane
100% Solids Polyurethane Coating can be applied at high film thicknesses (500-3000 Microns) with minimal sagging, in a single coat. They cure rapidly even at sub zero temperatures due to the exothermic reaction between the Isocyanate and the Polyol.
Due to their low permeability and high chemical resistance, 100% Solids Polyurethane Coatings act as a ‘barrier coating’ by blocking out moisture, oxygen and other chemicals from the surface. They have high electrical resistance to effectively block the corrosion cell on steel surfaces.
What does the term ‘100% Solids’ mean? What is a VOC?
The term 100% Solids is a little misleading implying a coating in solid form. Actually, it means that the coating contains no solvents or VOC’s. VOC stands for Volatile Organic Component. Zero VOC coatings have no fire hazard and low health risk while application. They are also very environment friendly as hazardous organic solvent vapour is not generated and released into the air. VOC’s contribute to ground level ozone and smog. There are increasing environmental restriction on using VOC’s in coatings.
Are 100% Solids Polyurethane Coatings conventional industrial coatings?
No! Conventional industrial coatings (100-400 Microns range) like Solvented Epoxy are “maintenance” coatings, requiring repainting after regular intervals (3-7 years).
100% Solids Polyurethane Coatings (500 – 3,000 Microns range) are “capital” coatings which are designed to last the lifetime of the utility and require no recoating. Service life in typical applications is 30-50 years or more ! They are in fact engineered polymers and not paint.
Are 100% Solids Polyurethane Coatings more expensive than conventional industrial coating?
It is now universally recognized that the true criteria for coating selection is not merely the initial cost per unit area but the cost per unit area per annum. It is important for the owner or agency to recognize that selection, placement, installation and inspection of a barrier system can be dominated by initial economic considerations to the detriment of performance potential.
Systems that costs less on an annual basis will most likely result in higher initial cost because the material and application labor used may be expensive, or a larger quantity of materials may be needed. This is true of 100% Solids Polyurethane Coatings as well.
Other factors to be considered for true determination of coating economics –
Materials cost inflation at re-coat.
Surface preparation cost escalation at re-coat due removal of prior coat. Blasting sometimes not feasible so succeeding coats less effective.
Manpower cost and time for tendering, supervision and billing.
Losses due to downtime
Is purchasing 100% Solids Coatings similar to buying any other industrial coating? Will it be available from our regular paint company?
No! Conventional industrial coatings require simple application equipment and there are a large number of painting contractors who can apply these products. There is usually very little involvement of the paint manufacturer in actual application – he simply sells the paint in cans. There are a large number of industrial coating manufacturers making similar products.
100% Solids Polyurethane Coatings are solvent free, generally very fast reacting and require specialized plural component application equipment. They are applied by the coating manufacturer (if he has coating application services) or through his ‘licensed applicator’ i.e painting contractors’ factory trained by the coating manufacturer. There is a great deal of technical support provided by the manufacturer to the application contractor including specifying correct equipment and application procedures. Over the years the coating company gains considerable technical skills which are shared with the coating contractor.
Due to the demands of the close interaction with applicators, these coatings are manufactured by specialised coating companies and not by large conventional paint manufacturers. Therefore there are limited companies with expertise in 100% Solids Polyurethane Coatings.
In recent years, due to the success of these coatings and increasing regulations for Volatile Organic Compounds (VOC’s) many new entrants have entered this market segment. It must be recognized however that these companies may not have the field proven technology and application expertise on par with the older, established companies.
What are the special application aspects of fast setting 100% Solids Polyurethane Coatings?
Fast setting 100% Solids Polyurethanes comprise of two components, the Resin (Polyol/ Polyamine/ Blend) and the Activator (Isocyanate). These are supplied is separate 200 Litre drums and are sprayed using plural component airless spray equipment. The spray system comprises Hydraulic pumps driven by air motors which lightly pressurise the two components and supply them to a proportioner. This unit proportions the two liquids in the designed ratio by volume (3:1 / 2:1/ 1:1) and develops a pressure of 2000-3000 Psi.
The two liquids under pressure are pumped to a mixer manifold / static mixer where they mix intimately and flow through a spray hose into a spray gun. The mixed material atomises on exiting the spray nozzle and is sprayed onto the surface. The mutual reaction of the two components is very fast causing an instant viscosity build up prior to spray and post spray fast cure into a solid. The whole system is dynamic with supply, mixing & spraying done continuously. There is no mixing of lots in containers and spraying. It is a high volume spray operation with typical spray tip spraying 120-200 Litres/ Hour. As soon as the spray is stopped, the section between the manifold and spray gun is flushed clean with a purge liquid to prevent the coating from blocking the system.
Using the plural component system needs factory trained operators.
Do all 100% Solids Polyurethane Coatings have the same properties? Which product is good for my application?
No ! Depending upon the requirements of the end use application 100% Solids Polyurethane Coatings are formulated with different physical and chemical properties. Hardness ranges from 45 Shore D to 80 Shore D. Elongation at break ranges from 10% to 400%. Chemical resistance ranges from mild to very high. Cathodic disbondment ranges from 0mm to > 20 mm. Some have very high impact and abrasion resistance whereas some have limited mechanical properties.
You need to select the right coating for the right application! Ask for specific recommendations. The ‘Product Matrix’ chart provides some guidelines for a correct selection.
Are all 100% Solids Coatings “meeting a generic specification” the same?
No ! Coating companies take different chemistry routes in developing proprietary formulations e.g Polyol may be Polyester / Polyether / Oligomer / Other with different functionality (di and poly functional) and molecular weights. Different chemistry results in different chemical resistance, corrosion resistance, affinity for water and longevity properties while meeting the listed mechanical properties. A wide range of additive and functional chemicals are added to control parameters like thixotropy, reactivity/ cure, adhesion etc. often with dramatic changes. These additives are based on different chemistries with large variation in costs.
Just like dishes created by a chef turn out differently, different coating formulations turn out differently, some excellent, some average and some simply not so good. The excellent ones perform well in service over the years; become ‘industry standards’ and ‘inspirations’ for copying attempts. User and specifying engineers convert the properties of well known and successful proprietary formulations into ‘generic specifications’ in the interest of ‘fair competition’. This approach is fraught with problems since the copycat products are often not as good the original and yet assumed to be the same quality because they claim to “meet the specification”!
What is the difference between Polyurethane and Polyurea? What are Hybrids? Are Polyureas better suited for damp/humid environments than Polyurethanes?
Isocyanates react with any compound containing Hydrogen. This can be a Polyol resulting in Polyurethane, a Polyamine resulting in a Polyurea or both resulting in a Polyurethane – Polyurea Hybrid.
R-N=C=O + R’-OH –> R-NH-CO-O-R’
isocyanate polyol polyurethane
R-N=C=O + R’-NH2 –> R-NH-CO-NH-R’
isocyanate polyamine polyurea
The reaction between Polyol and Isocyanates can be catalysed to any desired speed whereas the reaction of Polyamine and Isocyanate requires no catalyst (autocatalytic) and is very fast.
Standards such as AWWA C-222 allow usage of either or both as still classify the coating as ‘Polyurethane’. As an industry practice, products utilising > 80% Polyols are classified as Polyurethane, those with > 80% Polyamines as Polyurea and those in between as Hybrids.
A third (unwanted) reaction is between the Isocyanate and water (humidity)
R-N=C=O + H-OH –> CO2 + Urea or Biuret
Isocyanate water Carbon Dioxide
The CO2 produced causes foaming and defects in the coating. Different methods are employed to prevent this from occurring:
Water absorbing Zeolites which trap and prevent the water from reacting with Isocyanates
Use of Hydrophobic Polyols which do not absorb atmospheric water during reaction
Use of selective catalysts which promote reaction of only Polyol – Isocyanate at the cost of the water – Isocyanate reaction.
Setting the mutual reaction speed so fast that the Water – Isocyanate reaction has no time to take occur. This can be done by high catalysis as well as by using Diamines and Polyamines
Polyurethanes have a wide range of ingredients to choose from in terms of chemical structure, molecular weight and functionalities. A wide range of products ranging from soft elastomers to rigid coatings can be produced for different applications. Speed of reaction can be set from long pot life to products which set in seconds. They utilize many methods to prevent unwanted reaction with water (a to d above) and can be formulated to have similar water insensitivity as Polyureas.
Polyureas have been promoted as products which are so fast setting (see d above) that they can be applied on damp surfaces or in humid environments. However the uncontrolled speed makes them prone to adhesion and surface finish problems. Primers are still necessary in humid environments due to dew condensation on the surface. Hybrids are preferred since they have controlled fast speeds, high adhesion and smooth finishes.
The Polyamines currently available produce Polyureas with linear molecular structures (not cross linked) which are not suited for coating steel since they have high permeability and low corrosion resistance. Also in areas such as concrete lining for wastewater service the high elongation (>300%) Polyureas will not work well since they have lower chemical resistance and high permeability.
What role do Primers play? In which products or situations is a primer necessary?
Direct to metal Polyurethane Coatings are highly cross linked products and have elongation usually less than 10%. These products are specially recommended for buried and cathodically protected surfaces.They do not require a primer and have a very strong adhesion directly to blast cleaned surface. Elimination of the primer step allows high production speeds for pipe mills. It also eliminates the chances of incorrect ratio primer application, overcoating without being fully cured etc.
Primers are essential for elastomeric 100% Solids Polyurethane Coatings with an elongation range of 30-400%. Great care must be taken in application – apply thin fog coat not thick coat, correct ratio of Resin to Activator and apply top coat only on the hard cured primer to avoid solvent entrapment.
Penetrating damp tolerant primers react with surface moisture to seal and toughen the surface. This prevents out gassing of air inside the concrete due to coating reaction exotherm minimising pin holing. It also allows application of 100% Solids Polyurethane Coatings on damp concrete.
The surface toughening results in longer service life of the coating. Strength of the concrete is very important to the success and performance of the coating system. Concrete with a weak surface layer will be incapable of supporting a coating system for long.
How safe are polyurethane coatings?
Cured Polyurethane coatings are highly inert. They are widely used as lining of Potable Water pipes and tanks where stringent testing is conducted to determine non-toxicity. Total Organic Carbon (TOC) extraction of Purethane Coatings into water has been tested to be as low as 2.05 Micrograms / Litre against US EPA standard of 2 Milligrams / Litre for Public Water Systems, i.e. 1/1000th of the maximum permitted.
The ingredients used in the Polyols used for Purethane Coatings are completely non-toxic and the coatings meet European Community Regulation on Chemicals (REACH) requirements which are very stringent.
Concerns on the safety of Polyurethanes revolve largely on the usage of Polymeric Isocyanates and Prepolymers. There are a lot of misconceptions on these products. Contrary to popular perception (perhaps originating from the misinformation spread by manufacturers of competing products), these are non toxic, non carcinogenic and cause no reproductive effects in animal and humans. In acute animal toxicity tests, Polymeric Di-isocyanates are low in oral and dermal toxicity, as demonstrated by following:
LD50 ( Oral ) : > 5,000 mg / Kg in Rats
LD50 (Dermal ) : > 5,000 mg / Kg in Rabbits
(LD50 – Dose that is lethal to 50% of exposed animals)
In comparison Polyesters and Vinyl Esters use hazardous ingredients like Styrene & Methyl Methcrylate Monomers which are toxic and inflammable. They are regulated for Sea and Air transport. In Epoxy Resins Bisphenol A and F are considered environmentally Hazardous Substances (UN number 3082, LD50 > 2,000 mg/Kg). Epoxy Activators contain toxic Polyamines (UN 2735, LD50 > 1,000 mg/Kg) , Nonyl Phenol, Benzyl Alcohol etc.
It should be noted that due to the low toxicity, Polymeric Di-isocyanates are not regulated by transport agencies like the U.S DOT (Department of Transportation), International Civil Aviation Authorities etc. and the cargo is not classified as dangerous. > 2,000,000 MT annually of Polymeric MDI is produced and used worldwide in diverse end use applications such as Polyurethane insulation foam, soft mattresses, footwear etc.
During spray of high performance coatings personal protective equipment such as full goggles, gloves, disposable face masks is mandatory. This is true of 100% Solids Polyurethane Coatings as well. Threshold Limit Values ( TLV ) for Polymeric Di-isocyanates has been set at 0.02 ppm by The American Conference of Governmental Industrial Hygienists ( ACGIH ).
Prolonged or repeated exposure to the aerosols well in excess of the TLV can produce irritation of the mucous membrane in the respiratory tract, productive cough, breathlessness, chest discomfort and asthma type symptoms. These effects are reversible. This “early warning signal” allows corrective action to be taken. The effect is usually only manifested in people with pre-existing respiratory problems like asthma, bronchitis or allergies. Such people should not be allowed near the aerosol generation.
As of the date of writing, we have sprayed millions of over litres of Purethane Coatings with no documented case of allergic reaction.
How important is the application? Do I need to look into this aspect in detail?
Proper application is critical to the success of 100% Solids Polyurethane Coatings. High technology coating products are very unforgiving for shortcuts in application. The most common outcome of incorrect application is poor adhesion and disbondment. Other defects resulting from incorrect application are mis-metering (soft & hard areas), blistering, foaming, pin holing etc.
Verify the application capability of the coating manufacturer or his applicator.
Call and check on his listed work.
Spray equipment should be from Graco Inc., USA. Do not accept cheap makes or reconditioned shipyard equipment. Cheap, non-standard equipment will lead to mis-metering (off-ratio), inadequate pressure (low atomization with dry fall and holidays), improper fan pattern (thickness variations) high material wastage and low production rates.
Ask them to submit a minimum list of equipment (with makes & capacity) he will deploy for the project. Verify the capacity of the blast cleaning equipment and air supply as surface preparation is the slowest process! Blast cleaning requires large volumes of compressed air.
Ensure that he will list and deploy all the test equipment required at site :
Blast Profile Gauge (Elcometer 123 / Equal ) or Testex Tape
Dry Film Thickness (Elektrophysic eXacto F or Equal)
High Voltage Holiday Detector (Tinker Rasor AP/W or Equal)
Hardness Durometer (Shore D 0-100)
Portable Hydraulic Adhesion Tester (Positest AT Digital or Equal)
Concrete will require a Delmhorst BD-10 moisture detector.
How do I know if the vendor is estimating materials properly? Is it of any concern to me since he has to meet the specified thickness?
100% Solids Polyurethane Coatings are solvent free. Therefore the wet film thickness (WFT) is the same as the dry film thickness (DFT). The applied coating can be imagined as a 3 dimensional sheet with volume calculated as L X W X Thickness. Therefore 1 Sq. M @ 1.00 mm thickness required 1,000 cc or 1 Litre of coating. This figure does not change between manufacturers.
Theoretical consumption will therefore be as under:
1 Sq.M @ 1.00 mm = 1.00 Litres
1 Sq.M @ 1.50 mm = 1.50 Litres
1 Sq.M @ 2.00 mm = 2.00 Litres
In addition, there is overspray/ wastage depending upon substrate, measurement technique and geometry of surface: Large Flat Steel Surface – SSPC PA 2 (Average Thickness) : 25-30%
Large Flat Steel Surface – User Spec (Minimum Thickness) : 35-40%
Pipeline – Automated – SSPC PA 2 (Average Thickness) : 10-15%
Pipeline – Automated – User Spec (Minimum Thickness) : 20-25%
Large Flat Concrete – (Average Thickness) : 15-20%
If a vendor offers a very low material quantum estimate, you can be sure he will provide less than specified thickness in application and his bid is not on par with others. He will fudge on DFT tests since you cannot change the laws of Physics at site!
How can I ensure that the site work is carried out properly?
The best drawn specifications are of no use unless they are actually implemented during coating! Amchem has designed and follows a quality methodology and documentation QA/QC procedure and Field Quality Plan (FQP) to ensure proper onsite application. Please ask for a copy for specific projects.
This plan should be included in the contract documents to ensure that everyone on site – owner’s engineers, applicators, third party inspection personnel are on the same page as to how the coating has to be applied and quality tests conducted.
It is also important that coating supply and application not be bifurcated to different entities. Deal only with the coating manufacturer (if he has coating application services) or through his licensed applicator on ‘supply & apply’ basis. This will ensure that the vendor has full liability of the installed coating and will not take any short cuts in application! Also he will ensure materials overspray/ wastage is restricted to pre-determined levels.
Do not buy materials from a manufacturer and provide on free issue to a coating contractor. In this scenario, in the event of a problem both will blame each other and the owner will suffer. Also the overspray / wastage will be very high.
How are weld joints coated?
After welding, the joint shall be cleaned so as to be free from mud, oil, water, grease, welding flux, weld spatter, dust and loose residue. The entire area to be coated shall be clean, dry and uncontaminated.
Wipe clean the weld area, plus an area of 150 mm on shop coating, using solvent and a clean cloth. Allow to dry. If contaminants, Oil & Grease are not removed, they can be pushed into the blast cleaning profile. If the pipe surface does not have any Oil & Grease, the solvent can be eliminated and area cleaned with a clean cloth.
Abrade and feather the shop coating edges, using power tool, 25 mm from the exposed metal. Blast clean, using permitted disposable abrasive and compressed air, the exposed steel to SSPC SP-10 with a 75+ Microns Anchor Profile. During this process, adjacent coating also gets roughened. If environmental regulations/ logistics do not permit open blast cleaning, use Rotary Flap Impact Tools or Needle Gun as per SSPC SP-11 (3.4.1 and 3.4.2). However, the same surface finish and Anchor Profile (>75 Microns) should be obtained. Also all parties must recognize that performance may not be on par with blast cleaned surface. If any flash rusting takes place, reblast.
Prime the exposed steel substrate and roughened shop coating. Allow to be tack free. Apply primer within 4 hours of blast cleaning / power tool cleaning. Primer must be thinly applied (25-40 Microns), as evenly as possible. DO NOT RE-COAT TILL TOUCH DRY. Do not prime the non-roughened area of shop coating – it will not bond and coating applicator may inadvertently apply coating over the visible primer.
For direct to metal products, omit the priming step.
Spray apply 100% solids Polyurethane using specified equipment. Apply the coating preferably within 4 hours of primer becoming touch dry and not later than 24 hours. Do not spray on shop coating which has not been abraded. Use masking tape to ensure overspray does not fall on the non-roughened shop coating.
Where spray coating is not possible, use weld joint hand mix kit and apply using brush or squeegee to the primed surface, taking care to coat within the abraded and primed area only. Ensure that the coating is reasonably level with no depressions in the middle. DO NOT use any solvent or extenders in the hand mix coating. Do not apply coating on shop coating which has not been abraded – it will peel. Use masking tape to ensure coating does not extend on the non-roughened shop coating. The mix must be homogenous before application and any cured coated areas showing soft and hard patches must be removed and redone.
Follow dew point criteria when priming and coating (surface to be 3C over the dew point).
Test the weld joint coating for DFT, Holiday & Adhesion.