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Water-based
Polyurethane Dispersions
An
Introduction
Water-based Polyurethane dispersions (PUDs) are a
rapidly growing segment of the polyurethane coating industry due to
environmental legislations such as the clean air act and also due to
technological advances, that has made them an effective substitute for
the solvent-based analogs. They are versatile and environmentally
friendly coating materials that are available in a wide range of
hardness and solid content. Their zero to low volatile organic content
facilitate their formulation into a compliant coating for many different
substrates. Their versatility and wide range of superior properties,
such as abrasion resistance, impact strength and low temperature
flexibility are the driving forces to their continuously expanding usage
in many applications.
The
PUDs are not a direct plug-in for the solvent-based polyurethanes. The
film forming mechanism of the PUDs is very different from that of the
solvent-based resins. In order to realize their optimum properties, they
have to be skillfully formulated. The composition of the polymer
backbone as well as the formulating ingredients will have significant
influence on film formation properties.
Polyurerthane
dispersions are fully reacted polyurethane/ polyureas of small and
discrete polymer particles ranging in sixe from 0.01-5.0 microns in a
continuous water phase. They are free of isocyanate residues and have a
zero or a very low volatile organic content (VOC). They are safe to
handle and are a good choice for compliant coatings. PUDs can be made to
have 30-65% solid content and a wide range of film hardness.
One of the drawbacks of PUDs is their relatively
higher cost. To compensate for the higher cost and in some cases, to
improve adhesion to certain substrates, the PUDs are blended with other
low cost dispersions. Acrylic emulsions are the most commonly used
blending materials due to the familiarity of the formulators with their
properties.
Solvent-based
coatings are being replaced with eco-friendly water-based polyurethane
dispersions
Stringent
production standards required in the food, pharmaceutical and personal
hygiene industry, the world over, dictate that the actual formulating area
must be sterile and clean from bacteria and pathogens. These rooms are
generally air-conditioned, sealed environ- ments, under pressure of two
isobars. Inside, the floors, walls and ceilings are coated with a tough
resilient seamless coating to prevent biological contamination such as
germs or viral infestation which would find a hold in the cracks or joints
in the tiles or canters.
Epoxy coating
and solvent-based polyurethane coatings which are used in developing
countries such as India for clean rooms, are highly toxic in nature and
therefore fraught with health hazards. The solvent added to the above
coatings for better finish and to bring down the cost of these coatings,
is the proverbial nail in the coffin due to its negative long-term effects
on human health.
In the
developed world, especially Europe and the Americas, the use of
solvent-based coatings are being phased out. These are being replaced with
water-based Polyurethane dispersions (PUDs). PUDs are waterborne systems,
eco-friendly, having all the properties of the solvent-based systems,
minus the hazards. PUDs can be formulated to not only make floor coatings
but also for the walls which arte tough and resilient with scratch
resistance and stability
The PUDs have
a life of 10 years even in exposed areas as they have excellent UV resist-
ance against yellowing, cracking or peeling. They enjoy enhanced
penetration into the substrate and offer better bonding viz a viz water
based acrylics. They can also be formulated into coatings for glass, clear
glossy finishes for wood and veneers, anti- corrosive coatings for metals,
even in exposed areas with an enhanced life span. Technology is moving
rapidly as also awareness levels, with the advanced countries already
having phased out solvent-based systems in their work places and living
environments for health reasons.
Cleanrooms in
Indian pharmaceutical companies are not all designed with water-based PUDs.
Epoxy and solvent-based coatings are still used in clean rooms. Hardeners
used in solvent-based polyurethane coatings contain MDI, the health
hazards of which can result in skin rashes, cornea damage, asthma like
reactions, allergies. Isocyanides are also known to cause tumours in rats
and are therefore classified as human carcinogens.
Solvents are
also used in many items like Paints, varnishes, lacquers, paint removers,
etc. They include commonly used chemicals such as alcohol, mineral
spirits, petroleum distillates, turpentine, benzene, perchloroethylene,
toluene, xylene, trichloroethylene, gasoline, kerosene, butyl acetate,
acetone, metyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK). The list
goes on.
As per OSHA
(Occupational Safety and Health Administration, US Department of Labour),
health hazards associated with solvent exposure include cancer, skin
infections and reproductive damage. The smell of solvents hangs in the
air, days and weeks after the application of Epoxy solvent-based
polyurethane coatings.
Conclusion :
Solvent-based coatings are being phased out and replaced with water-based
PUDs, that are
eco-friendly with all the good properties of the solvent-based systems,
but, minus the hazards.
HYGIENE COATINGS :
Polyurethane
dispersions are used as the main or auxiliary binders in
"Hygiene" coatings, where the coatings main requirements of
toughness and reasonable gloss, single-pack, water based and low odour,
easy cleanability, rapid drying and fast property developments are some of
the more important properties. Food, pharmaceutical and personal hygiene
plants, hospitals and nursing homes, hotels and restaurants and anywhere
there exists a requirement for clean, bacteria resistant surface, these
coatings provide the answer. "Hygiene Coatings" may be used on
walls, ceilings and floors, all based on specific types of polyurethane
dispersions.
THERMAL REFLECTIVE
COATINGS :
"Thermal Reflective - Insulating Coatings", is another entrant
in the construction exterior coatings market. These functional
polyurethane coatings are specially formulated to reflect, refract and
dissipate the radiant heat of the Sun. Use of these polyurethane coatings
on exterior of Roofs, Building Side Walls, Liquid and Gas Storage Tanks,
Boilers, Air Condition Ducts, and many more areas keeps the interiors
cool, thus saving evaporation losses and reducing energy requirements. The
"Thermal Reflective - Insulating Coatings" can, additionally be
provided with waterproofing properties, especially when used on building
roofs.
SEALERS FOR CONCRETE :
Waterborne
polyurethane dispersion based Concrete Sealers can be designed to
penetrate deeply into the pores of masonry surfaces to "Seal"
the concrete, from ingress of sulphates & chlorides and thereby
provide corrosion protection to the steel reinforcements. Floor Sealers
can also be formulated to stop the concrete floors from dusting and to
form a foundation to support different types of polyurethane based
coatings. The sealants may be formulated to provide excellent chalk
binding & alkali resistance properties. These concrete sealers are
unpigmented, resin-rich compositions, but may also be pigmented to fill
concrete blocks in one coat.
COMMERCIAL FLLOR
COATINGS ONE-COMPONENET SYSTEMS
If the floor is dusty
or if the floor in a supply room, warehouse, restaurant, shopping area and
requires to be made resistant to pedestrian traffic, or if the look and
aesthetics of a floor has to be improved upon, a single-component
water-based polyurethane floor coating system should be suitable. These
coatings, in addition to being water-based have low VOC and practically no
odour. They dry quickly and can be recoated within 2 to 4 hours depending
on the prevailing ambient temperature and humidity and are ready to take
traffic within 24 hours or earlier. The water-based polyurethane coatings
can be applied directly to cleaned concrete floors as also on previously
painted surfaces provided the existing paints are still in good condition.
Porous floors however need a penetrative & impregnating sealant as the
first coat. Maximum dry film thickness recommended for such floors is 60
microns. They are one-component coatings hence there is no on-site mixing
required nor is a limit on their pot life placed and they can be applied
quickly and easily by brush and roller. The coated floors become less
susceptible to pick up dirt, and are fast and easy to clean. These
coatings possess adequate wear and chemical resistance for the purpose
they are designed for.
While many colours can be formulated using the water-based polyurethane
dispersions, only a limited range of colours provides the decorative
effect for commercial floors. The decorative effect can be further
enhanced using multi-coloured pigment flakes, quartz sand, powdered hard
minerals, etc. Using an appropriate primer, these coatings can also be
painted over linoleum, vinyl tiles and sheets, over decorative cement
tiles, slabs, etc.
TWO-COMPONENT SYSTEMS
In the case of floors
that are subject to significant wear and tear due to heavy vehicular
traffic, protection and ongoing maintenance is essential. Floor coatings
that would stand up over a long period of time to heavy traffic loads,
such as forklifts and vehicles that carry heavy trailers and large parts
and in addition have high abrasion resistance can be formulated using
waterborne polyurethane dispersions crosslinked with polyisocyanates.
These coatings dry rapidly and can be walked on in less than 3 to 4 hours
after application and they are odor-free.
The coating system includes a water-based epoxy primer or 100% solids
epoxy primer, a 100% solids epoxy intermediate coat and a pigmented
two-component water-based polyurethane topcoat. The primer provides good
adhesion and moisture resistance and is applied at 1.2 to 1.5 mils dry.
The intermediate coat may be applied between 5 and 10 mils dry depending
on the load behavior required of the floor. The polyurethane topcoats
offer excellent gloss retention, long-term colour retention and high wear
resistance when applied at 1.9 to 2.4 mils dry film thickness.
SUMMARY
The need to protect
concrete surfaces from early deterioration has been an issue that has been
debated for many years. With the rapid industrialization that is taking
place in the country, pollution is likely to be one of the causes for
shortening the life of concrete structures. Unless protected, the
replacement cost of these structures can become phenomenal. Water-based
PUDs, can be tailored to suit every single application. They can be made
rigid and hard and they can be designed to be flexible and soft, and
anything in between. PUDs are resistant to UV light and are known to
retain gloss levels for long periods. They are compatible with many
acrylic and other dispersed resins, and can be easily pigmented. PUDs are
available as thermoplastic and thermoset resins and can be readily
cross-linked with a variety of commercially available cross-linkers to
further enhance their properties and in designing high-quality and durable
coatings.
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