2K Catalyzed Polyurethane FinishesPaul S
Catalyzed polyurethane clear coats and paints are prized for their durability and versatility around the world. They are used on everything from wood cabinetry, mill work and furniture to planes, trains, and automobiles. Two component catalyzed polyurethanes have several acronyms including 2-part, 2-component, 2-pack, 2-package, 2K urethane, and 2K-PU. To avoid confusion with other 2 part coatings like epoxy, it’s wise to include the “PU” when using the acronym.
The properties that make 2K-PU a top choice include the following;
- Excellent clarity.
- Resilient and durable.
- Great balance of hardness and flexibility.
- Excellent chemical and mechanical resistance.
- Sheen ranges from dead flat to high gloss wet look with exceptional DOI.
- Excellent pore formation when an open pore look is required or multiple coats can be applied to achieve a full-filled, close pore finish.
- Fast drying at room temperature. Can be oven dried for greater speed.
- Can be formulated to be non-yellowing and colorfast with superior performance in an outdoor environment.
What is Polyurethane?
In simplest terms, polyurethanes are polymers containing urethane (aka, carbamate) linkages formed by the reaction of an alcohol with an isocyanate. More specifically, polyurethane is formed when a compound containing two isocyanate groups (called a diiscyanate) reacts with a compound containing more than one hydroxyl group (called a polyol). This reaction is usually aided by a chemical catalyst that does not become part of the resulting polymer. Common PU catalysts include DABCO (diazabicyclo octane) and DBTL (dibutylin dilaurate).
There are many types of polyurethanes and they are used to make a wide range of products that include paints, expanding spray foams, flexible foams, insulation materials, surface coatings, car seats, furniture, foam mattresses, under-carpet padding, packaging materials, shoes, laminated fabrics, polyurethane rubber, and adhesives. For wood finishing, we’re only concerned with the PU that’s used to make clear and pigmented film forming finishes.
The Two Components
The two primary components, alcohol (polyol) and isocyanate, are stored in separate containers and mixed just before use. The first can (labeled Part A) contains the polyol co-reactant (typically acrylics or polyester), pigments (if it’s a paint), catalyst (e.g., DABCO or DBTL) and other additives. The second can (labeled Part B) contains the polyisocyanate (also known as the activator or hardener). Polyisocyanates are moisture-sensitive and should be resealed quickly after dispensing and stored with an inert gas blanket covering the surface (e.g., nitrogen, Bloxygen, etc.). Additionally, the thinners and solvents used with 2K PU have to be water and alcohol free to prevent problems during storage and with the final finish.
NOTE: manufacturers may switch which component is in which can (Part A and Part B). Be sure to follow the data sheet for measuring and mixing.
The composition of the co-reactant polyol determines the final properties of a polyurethane coating. The resin package is commonly referred to as “the backbone” of the coating. The most common polyols used in 2K PU are polyester, acrylics, and polyester/acrylic blends (though polyaspartic, polyacrylate, polyether, vinyl, fluoro, or epoxy may be used in some proportion to take advantage of their unique properties). In broad terms, polyester resins allow for a higher solids finish with excellent solvent resistance and adhesion to metals while the acrylics provide faster dry, lower cost, and excellent exterior performance.
Isocyanates – The 2 Types of Polyurethane
There are two types of isocyanates used in 2K-PU – aromatic and aliphatic. Polyurethane based on aromatic polyisocyanates cost less but will turn yellow with exposure to sunlight. Aromatics are most often teamed with polyester resins and the resulting PU is a popular choice for interior wood furniture and cabinetry. On the other hand, aliphatic isocyanates are more expensive but provide a non-yellowing film. They are usually paired with acrylic resins to create a PU with excellent outdoor weathering characteristics to include colorfastness, gloss retention, and lack of damage from UV radiation and water. Adding hindered amine light stabilizers (HALS) to the acrylic polyurethane double or triples its service life. Aliphatic polyurethane is the go-to coating for outdoor signs, cars, motorcycles, trains, and planes.
“There are two types of catalyzed polyurethane based on the isocyanate used – aromatic and aliphatic.”
If you’re not sure which type of polyurethane you’re working with or considering buying, take a look at the material safety data sheet (MSDS) to see which isocyanate is listed. Aromatic polyisocyanates include TDI (toluene diisocyanate) and MDI (methylene diphenyl diisocyanate). Aliphatic polyisocyanates include HDI (hexamethylene diisocyanate), IPDI (isophorone diisocyanate), and H12MDI (methylene dicyclohexyl diisocyanate), XDI (xylylenediisocyante), TMXDI (tetramethyl-m-xylidene diisocyante), TMI (m-isopropenyl-dimethylbenzylisocyanate), and TMHDI (trimethylhexane diisocyanate).
NOTE: TDI is also used as a modifier in alkyd resins to produce uralkyds (aka, oil modified urethanes [OMUs] or oil-base polyurethane). These polyurethanes are “1K” – single component finishes.
Mixing the Components
Parts A and B are mixed together in precisely measured amounts (by weight or volume) just before use. The chemical reaction between the two components starts immediately at room temperature (above 65 degrees Fahrenheit) so you have to use it quickly (e.g., 2-8 hours) once it’s been mixed – make sure you check the data sheet for the specific products you use. The time period the mixed finish remains usable is called the “pot life.” The higher the temperature you’re working in, the faster the reaction takes place – adjust your pot time as needed. Once the time is up, discard any surplus. Though you will still be able to spray it, the finish quickly loses long term durability properties as the reaction continues. Left in a cup or spray gun, the finish will gel and harden in a few days or weeks.
NOTE: for larger scale spray operations the spray equipment mixes the two components at the nozzle of the spray gun as you spray. This avoids the limitation of the pot life and potential waste of unused product. This is called plural component spray equipment.
2K Polyurethane Health and Safety
This section contains an introduction to the heath related safety issues and protective measures associated with catalyzed polyurethane wood finishes. It’s not meant to be comprehensive, so be sure to learn and follow all the appropriate health and safety requirements specified in OSHA Standards, product material safety data sheets (MSDS), and all other local, state, and federal regulations and guidelines. Also be sure to use all of your personal protective equipment (PPE) in accordance with the manufacturer’s guidelines.
Health Effects of Exposure to Isocyanates
The health effects of isocyanate exposure include irritation of skin and mucous membranes, chest tightness, and difficult breathing. Overexposure from isocyanates is most likely to occur from inadvertent inhalation during spray application of 2K PU. The main effects of hazardous exposures are occupational asthma and other lung problems, as well as irritation of the eyes, nose, throat and skin.
Isocyanates are powerful irritants to the mucous membranes of the eyes and gastrointestinal and respiratory tracts. Direct skin contact rarely causes any damage but may produce irritating redness and inflammation.
Acute exposure to isocyanates through inhalation can lead to occupational asthma. Chronic exposure through inhalation of diisocyanates can lead to hypersensitivity. Death from severe asthma in some sensitized subjects has been reported. Workers potentially exposed to isocyanates who experience persistent or recurring eye irritation, nasal congestion, dry or sore throat, cold-like symptoms, cough, shortness of breath, wheezing, or chest tightness should see a physician knowledgeable in work-related health problems.
It’s possible to develop sensitization to isocyanates from a single, large overexposure—for example, from a spill or accident—or from repeated overexposure at lower levels. Once an individual is sensitized, the asthma attacks can be induced by isocyanate levels even below the occupational exposure limits. Avoiding exposure completely is the safest course of action.
As with all hazardous chemicals, you must select and use the appropriate respiratory protection and personal protective equipment (PPE) and follow safe handling procedures to ensure your well being.
The general requirements for personal protective equipment are found in 29 CFR 1910.132 and the Respiratory Protection Standard is 29 CFR 1910.134.
When choosing a respirator, keep in mind that most do not have an end-of-service life indicator (ESLI) and there are none available for isocyanates. Also, if you can smell the isocyanate, that means you are already above the permitted exposure limits (the concentration required to produce a smell is higher than the allowable exposure limits). Because cartridge respirators can’t warn you to exposure, a positive pressure fresh air respirator with a full face piece is recommended.
Isocyanates also pose a health threat resulting from contact with the eyes or skin. To protect yourself, wear a spray suit, gloves, head covering (may be part of the fresh air respirator or spray suit) and gloves. When mixing the liquids prior to spraying, use splash proof goggles.
How to Paint a Train
For those of you that do your spray finishing in an open face paint booth or an enclosed automotive style booth, you might like this video. I came across it a while back and found it interesting due to the scale of the job. The video is about 6 minutes long, but the end result is worth the wait. Note the system they use to reach the high sections and extend the air hoses….
In this video Norfolk Southern is placing a one-of-a-kind “Veterans Locomotive” into freight service on its system to honor people who have served in the military and reserves, especially those employed by the railroad. Painting the Veterans Locomotive involved 26 NS employees, two of whom are veterans, using 66 gallons of primer and paint over a 112-hour period.