Frequently
Asked Questions About
Madison and our Products
FAQs Index:
-
What is a coating?
-
What is a
polyurethane coating?
-
How safe are
polyurethane coatings?
-
How
do polyurethane coatings cure to form a solid?
-
What is the difference between a urethane and a urea?
-
What is a V.O.C.?
-
Are all coatings designed to face similar exposure conditions?
-
Is special safety equipment necessary to apply a coating?
-
What does 100% solids mean, and why is this important in a
coating?
-
What
are plural-component polyurethane coatings?
-
How does a rigid polyurethane coating differ from an elastomeric?
-
What is the difference between aliphatic and aromatic polyurethane
coatings?
-
What are the advantages of polyurethane coatings over epoxy
coatings?
-
What are polyethylene coatings, and how do they compare with
polyurethane systems?
-
How do coatings
protect steel?
Q.
What is a coating?^Top
A. A coating is a barrier
used to separate two highly reactive materials. A coating must be a
completely continuous film in order to fulfill its function. Any
imperfection becomes a focal point for corrosion and the breakdown
of the structure.
Q.
What is a polyurethane
coating?^Top
A. A polyurethane coating
is an organic coating made from the reaction of isocyanate-rich and
polyol-rich compounds. Polymerization is made possible by using di-
and poly-functional isocyanates and polyols. There is no byproduct
from the reaction between isocyanates and alcohols. This is
important in terms of cost-effectiveness and health risk.
R-N=C=O + R’-OH --> R-NH-CO-O-R’
isocyanatealcoholurethane
Interest in polyurethane coatings is growing rapidly
because of their high chemical and abrasion resistance, excellent
electrical properties, and very low temperature cure due to the
exothermic nature of the polymerization. Polyurethane coatings are
capable of being made into soft, elastic materials, as well as into
very hard, tough, abrasion-resistant products.
Q.
How safe are
polyurethane coatings?^Top
A. Polyurethanes cure to a
safe, inert product. It has been estimated that the average family
owns approximately 25 to 100 pounds of polyurethane present in such
areas as insulation for refrigerators, foams for bedding and
cushions, and as dashboards and bumper covers in cars. The polyols
used for polyurethane formation are derived from natural products
such as sucrose, hence they are not classified as hazardous
substances. The main hazard in polyurethanes is the isocyanate.
However, isocyanates are
not carcinogenic as many people might believe. Allergic reactions
develop in a very small percentage (<1%) of the population but these
reactions desist once the over-exposure is stopped. Furthermore, the
isocyanates Madison uses are pre-polymers, which means that the
isocyanate monomer is reacted with a small amount of polyol to
produce a larger, less volatile and less hazardous polymer. Finally,
since polyurethanes are made by reaction between two low viscosity
liquids, there is little need for
solvents in the system. If there is a need to thin a coating,
reactive diluents are often used which provide the ability to reduce
viscosity, like solvents, while also undergoing reaction with the
polymer. Therefore, polyurethanes are generally not flammable and
they pose little risk in terms of health during application if the
proper precautions, such as a respirator and gloves, are taken.
Q.
How do
polyurethane coatings cure to form a solid?^Top
A. Polyurethane coatings
can cure in a number of ways depending on the design of the system.
Polyurethane coatings can be cured by: 1) reaction between the
isocyanate and the polyol; 2) evaporation of solvent (if present in
the formula); and 3) reaction between the isocyanate and moisture
from the air. Reaction between the isocyanate and water results in
the formation of
carbamic acid which decomposes to a yield carbon dioxide and amine.
R-N=C=O + H-O-H ----> [R-NH-CO-O-H] ----> R-NH2 +
CO2 (g)
watercarbamic
acidamine
Q.
What
is the difference between a urethane and a urea?^Top
A. Both a urea and a
urethane are made from an isocyanate. An isocyanate will react with
any compound containing a reactive hydrogen. However, whereas an
isocyanate reacts with an alcohol to form a urethane, a urea is made
from the reaction between an isocyanate and an amine.
R-N=C=O + R’-NH2 ---------> R-NH-CO-NH-R’
Q.
What is a
V.O.C.?^Top
A. V.O.C. stands for
Volatile Organic Component, and it usually refers to solvents in the
system. However, not all solvents are V.O.C.s, which means that the
solids content of the coating can be less than the amount of V.O.C.s
in the system. The lower the amount of V.O.C.s in the coating, the
safer it is to use. V.O.C.s increase the degree of flammability of
the coating and also increase the health risk associated with
application of the coating. Therefore, there is a maximum amount of
V.O.C.s that can be present in a coating - lbs/gallon. All Madison
coatings contain much less than the maximum amount.
Q.
Are all coatings designed to face similar exposure conditions?^Top
A. No. There are three main
types of exposure to which coatings are subjected: atmospheric
exposure, immersion, and underground exposure. The main difference
between atmospheric exposure, immersion, and underground exposure is
weather resistance. A coating under atmospheric exposure must endure
a variety of conditions, including heating and cooling, oxidation
and
wetting and drying. Immersion coatings are primarily subjected to
water solutions ranging from pure water to high concentrations of
various chemicals. Coatings used in underground applications must be
resistant to ground water and soil forces, so they are generally
applied thicker than atmospheric or immersion coatings.
Q.
Is special safety equipment necessary to apply a coating?^Top
A. It depends on the type
of coating. Madison’s plural and single component products are
applied by spray. Therefore, it is advised that one wears a mask or
respirator and gloves when applying the product. For touch-up and
mix and apply coatings, no special equipment is required.
Q.
What does 100% solids mean, and why is this important in a coating?^Top
A.
100% solids refers to a lack of solvents: the thickness of a
100% solids coating remains the same –whether wet or dry – because
there are no solvents to evaporate. There are many advantages to
100% solids coatings. First, these coatings are more environmentally
friendly and generally safer to use due to their decreased levels of
flammability and health risk. Second, on a cost-per-mil of
thickness, they are very cost-effective. Finally, most 100% solids
coatings have the advantage of additional thickness that helps to
increase their physical properties and chemical resistance.
Q.
What are
plural-component polyurethane coatings?^Top
A. Plural-component
coatings are those in which the isocyanate part is packaged
separately from the polyol part. Just before the coating is applied
to a surface, the two parts are delivered through individual fluid
lines to a mixing device, which is located within the spray gun or
directly before the spray tip. It is very important that the right
mixing ratio is obtained, otherwise defects in the coating will
develop. Thus, both sides must have balanced viscosities in order to
be sprayed on ratio. It is possible to apply some two component
products using a brush or roller. Single-component products are
applied without any preparation, although catalysts may be added to
the product to speed up the cure time.
Q.
How does a rigid polyurethane coating differ from an elastomeric?^Top
A. The main difference
between these two types of coatings is the degree of cross-linking
density. Soft, elastomeric coatings are made from long chain polyols
that have little or no branching. Rigid coatings, on the other hand,
are made from short chain polyols that generally feature higher
degrees of branching, such as triols and quadrols. Water and
chemicals have easier passage through an elastomeric coating due to
the linear nature of the chemical bonds, whereas rigid coatings show
a high degree of chemical and moisture resistance due to their high
cross-linking density. Rigid coatings also demonstrate excellent
adhesion to substrates; this quality, together with strong chemical
resistance, position rigid coatings as the best choice for
protecting metal from corrosion. Although elastomeric coatings do
not perform as well in such areas, they are superior in terms of
abrasion and impact resistance. Therefore, flexible coatings are
more suited to the protection of substrates that demonstrate more
movement than steel, such as concrete.
Q.
What is the difference between aliphatic and aromatic polyurethane
coatings?^Top
A. Aliphatic and aromatic
coatings differ in the types of polyols and isocyanates used in the
formulation, hence their stability in atmospheric conditions differ
significantly. Because they are very stable when exposed to
ultraviolet light, weathering, and hydrolysis, aliphatic coatings
are the superior choice for exterior protection. Aromatic coatings
do not fair as well against atmospheric exposure since the UV light
causes yellowing and chalking. Aliphatic coatings are produced as a
result of the reaction between aliphatic isocyanates, such as HDI
and IPDI, and polyester or acrylic polyols. In contrast, aromatic
coatings are designed from aromatic polyisocyanates and polyether
polyols. The raw materials used in formulating aliphatic systems are
generally more expensive and have higher viscosities than their
aromatic counterparts. The need for solvents to reduce viscosity has
been an obstacle in trying to achieve 100% solids aliphatic
polyurethane coatings. New developments such as the use of lower
molecular weight resins, reactive diluents, and new polyurethane
prepolymers have resulted in higher solids systems.
Q.
What are the advantages of polyurethane coatings over epoxy
coatings?^Top
A. The advantages of
polyurethane coatings are:
- Polyurethane coatings can be made
to cure at virtually any given time by changing the amount of
catalyst or the type of polyol in the formulation. Thus,
fast-setting, one coat polyurethanes have a much faster turn
around time than epoxy systems. Epoxy coatings generally take
seven to ten days to fully cure and to allow the solvents to
evaporate. Some epoxy systems require force curing.
- One hundred percent polyurethane
coatings are solvent free and have lower toxicity levels than the
epoxies. Due to solvent content, epoxy coatings are extremely
flammable.
- Due to the exothermic nature of
the reaction between polyol and isocyanate, polyurethane coatings
can cure at almost any ambient temperature. This means that
polyurethane coatings can be applied even during the cold months
of the year. Epoxy coatings, on the other hand, usually require
temperatures above 50°F (10°C).
- One hundred percent polyurethane
coatings feature a unique “self-inspecting” property; they fail
almost immediately if they are incorrectly applied or if there is
a problem with the surface preparation or the mixing ratio. Thus,
polyurethane coatings can be inspected immediately after
application and any defects in the coating will be visible.
Q.
What are polyethylene coatings, and how do they compare with
polyurethane systems?^Top
A.
To produce a polyethylene coating, a pipe is filled with
beads of polyethylene and heated until the polyethylene melts and
adheres to the pipe interior. Polyethylene is a thermoplastic
material whereas polyurethane is a thermoset polymer. Polyurethane
coatings generally have greater adhesion, hardness, and chemical
resistance although polyethylene coatings perform well in impact and
abrasion resistance tests.
Q.
How do coatings protect
steel?^Top
A.
There are 3 recognized ways that coatings can protect steel:
barrier protection, inhibition, and sacrificial action. A coating
protects as a barrier by blocking moisture, oxygen, and other
chemicals from the steel substrate. All coatings are permeable to
some degree, but those coatings that protect through a barrier
mechanism have relatively low moisture permeability.
Coatings that protect by inhibition contain special pigments to
inhibit or interfere with the corrosion reactions on the steel
surface. As moisture passes through the coating film, the
anti-corrosive pigments slowly dissolve and aid in stopping
corrosion. Finally, sacrificial action is the method used by zinc-
and aluminum-rich coatings.
Plural-Component Polyurethanes
Plural-component polyurethanes are due to the "poly" - "urethane"
reaction between isocyanates and polyols:
R-NCO + R’-OH ® R-NH-COOR’
“Urethane", as used most often in the term "urethane
coating", is a misnomer for "polyurethane coating". "Urethane" is
used broadly and has 2 different meanings. One refers to ethyl
carbamate, which is a monourethane. Although it is a member of the
urethane family, it exists as a totally separate chemical that is
not used in polyurethane coatings. The other meaning of "urethane"
should more properly be stated as "polyurethane" and generally
refers to coatings or other materials made with isocyanate compounds
that contain multi-NCO groups.
A "urethane" linkage can be found by other ways
without involved isocyanates; however, the development has depended
largely on the chemistry of isocyanate. It is for this reason that
people are often mix up the two terms of "urethane" and
"polyurethane". The basic polyurethane reaction is an exothermic
(heat-generating) reaction. Therefore most polyurethane coatings,
especially those which are solvent-free, will have "cold-curing"
ability.
For solvent-free coating systems, the curing process
is mainly due to the exothermic reaction, rather than the
evaporation of solvents, or the oxidation of the resins. The heat
generated by the reaction will be enough to help the coating to cure
even at -40°C, particularly in those cases when:
