CATEGORIES OF ORGANIC COATINGS SUBSTANCES
- Additives small quantity substances to improve properties such as
coalescence, flow, and other paint properties. An additive is typically
less than 2% of the formulation solids by weight.
- Binders (organic polymers) form the film
This includes the resin and the crosslinker. In your first exposure to
crosslinking theory, the crosslinker may be a small molecule, but a
crosslinker could also be a polymer. To make things more complicated,
a crosslinker may be called polymer (Polyisocyanate, for example) but
it is not a polymer. Sorry :(
- Carrier- either an organic solvent, liquid carbon dioxide, or water.
Technically any fluid can be called a solvent, but because the word
has such an unfavorable connotation, it is reserved for organic chemicals.
For powder coatings there is no carrier.
- Pigments (insoluble solids) provide the paint opacity that
obscures the surface. This is in contrast to a clearcoat in which you
want to see the surface.
PERFORMANCE OBJECTIVES OF A COATING
(This list is not comprehensive)
- A coating must flow over the surface of the object to form a
smooth surface, possibly a surface more smooth than surface of the
object prior to the application of the topcoat. The coating may be
expected to adhere to a vertical surface (vertical relative to gravity),
or even "upside down." Thus, the viscosity of the paint must be low enough
to permit flow, but high enough so that the paint doesn't sag. To
define sag, if you were to paint a wall using a paint with poor sag
characteristics, and you painted too much in one area, and the a drop of
paint runs down the side leaving a streak, then you're looking at a sag
problem. The sag problem would also occur if you painted a ceiling, and
the paint came to one spot and then dried like it was about to drip. Issues
such as leveling and sag, and others involving paint flow fall under
the category
rheology.
To some degree the coating may be expected to stand up to harsh
weather. Paints which survive contact with water are called exterior
paints. Paints which do not resist water, but which are used in
applications where substantial water contact is not expected, like inside
your house, are called interior paints. Paints may need to be
designed with corrosion resistance in mind. As an example, the
topcoat of a car that will be driven in a state where salt is put on the
roads to melt snow and ice in the winter will need more corrosion
resistance.
- The cure of the coating is important. How fast will the paint
media (solvent, water, etc.) dry, and if there is a crosslinking reaction
that forms the paint film, what is the required time frame for the
crosslinking reaction? Note that there are different classifications of
dryness:
- dry to the point where the film has the appearance it
will maintain for the life of the paint
-
dry to the point where you can
touch the film without leaving a fingerprint
-
dry to point where mechanical tests of the paint would show no further cure
As an example, a two component PUR crosslinked acrylic tested at UMR
showed evidence of additional cure of 10 day cure samples over 2 day cure
samples (i.e., peel strengths of 10 day samples were higher than peel strengths
of 2 day samples indicating further cure had taken place.)
On the other side of the picture, the paint should not cure in the
storage container. One way to prevent in package cure is to make a
two component system where the two species that react to make
the polymer binder are kept separate.
- The pot life of a paint is the duration of time you have to
work with a paint system once you prepare it for use. If you are working
with a two component system, i.e., a paint system where you mix two
systems together that will react to form the paint film, then once you
mix the two together, your paint begins the film forming process. Pot life
should be on the order of hours.
There are several tricks that can be tried to make a system that does
not begin to react until it is needed.
- make it a radiation crosslinked system. In storage there is
no radiation to make the system react.
- make it so that atmospheric components (water or oxygen) catalyze
the cure reaction.
- use a volatile inhibitor so that when the container is opened the
volatile inhibitor evaporates and the reaction proceeds.
- Use a reversible crosslinking reaction that gives off a volatile
by-product so that the reaction does not make any net progress along
the reaction coordinate until the container is opened.
REACTION RATES
a b
Reaction Rate = k [A] [B]
The reaction rate depends on the concentration of the reacting species,
A and B.
At temperatures below Tg, the lack of free volume precludes the mobility
that would allow reaction between functional groups. At temperatures
just above Tg, the reaction rate depends on chain mobility, not the
reaction kinetics.
Solvent borne paints vs. waterborne paints
The use of organic solvents to dissolve the paint polymer molecules
increases the range of polymer systems that can be employed since organic
molecules are more likely to make
good solvents for polymer molecules. For a polymer to be water soluble
it must have substantial polar character. Some polymers can be dissolved
in water, an example of which is
poly(acrylamide), which is infinitely soluble in water.
If the waterborne system consists of a polymer that is not dissolved
in the water (it could be an emulsion dispersed using surfactants) then
there is no relation between
viscosity and
molecular weight.
PAINT APPEARANCE
A paint can be a clearcoat, or it can be a pigmented paint. The appearance
of a pigmented paint can be metallic or glossy. [I am not sure at this time
if these two terms are meant to be mutually exclusive.]
A clearcoat is a topcoat that has no pigment so that the substrate
surface
is visible through the polymer film. The watersealants advertized on
television to protect wooden decks from rain are an example of clearcoat
technology.
PIGMENTS AND HIDE
Pigments are added to paint system to:
- Hide the surface of the substrate
- Give color and possibly gloss to the topcoat
Pigment molecules hide the heterogeneous appearance of the steel of
a car frame. In the past, the paint gave off a color, but today the
paint also to some degree gives off "a reflection." Dr. Stoffer
reports seeing cars that reflect more than one color, depending on
the light. Perhaps this option is available only through custom
body shops.
Pigments can be organic or inorganic. Examples of inorganic pigments
include titanium dioxide (TiO2) which gives "T-shirt white", carbon
black, and aluminum trioxide (Al2O3, I think) which isn't as "white" as TiO2.
Pigments can be very expensive, and it may turn out that you need a
given volume of pigment for mechanical property reasons, but for just the
purpose of color,
you don't need that much. You can use a little of the expensive pigment
for color, and then you can use a second cheap no color extender
pigment for
the remainder of the needed pigment.
Pigments work because the refractive index of the pigment is much higher
than that of the pigment binders, so when light reaches the pigment/binder
interface, it is likely to be scattered back out, rather than to penetrate
through the surface of the painted object. If light reached the surface,
and then it went back out (as is the case for a clearcoat) then you could
see the surface of the painted object.
Examples of Pigments
Titanium Dioxide (TiO2)
Rutile titanium dioxide has a refractive index of 2.76, and anatase
titanium dioxide has a refractive index of 2.55. Titanium dioxide is
responsible for the white you see in bright white you see in briefs
(underwear.)
Zinc oxide is another white, and it is much cheaper than titanium dioxide,
but it has a refractive index of only 2.02, and so it gives less hide.
Mechanical Behavior of Pigments
I believe (but I want to confirm) that pigments such as mica, which are
hard, give a topcoat physical or mechanical advantages.
Chalking is when pigment particles rub off on contact. This is
usually undesirable, but in some instances it is desired, and the topcoat
is called "self cleaning." When the outermost pigment particles rub out,
they take with them the polymer binder molecules that hold the stain.
Metallic Appearance
The metallic appearance involves "color flop." Color flop indicates that
when you view the paint surface from a small viewing angle
that you see a dark color, or the flop color. If you view the paint
surface from a large viewing angle, you see a lighter color, the
face color.
Gloss
The gloss of a paint used for a ceiling should be low. A low gloss
ceiling paint does a better job of diffusing the light it reflects, and
this makes for better illumination of the room.
TYPES OF PAINT SYSTEMS
Both thermoplastic and thermoset polymers can be used for topcoats.
Thermoplastic polymers do not branch, but make use of high
molecular weight to guarantee good mechanical properties.
Thermoset polymers make use of branching to achieve good properties.
Thermoplastics polymers are like spaghetti, and thermoset polymers are
like a fisherman's net.
Latex
A latex is a high molecular weight polymer in water. The viscosity
is independent of the molecular weight of the polymer because the polymer
is not dissolved in the water. Because viscosity of the solution is not
molecular weight dependent, polymers can be synthesized with molecular
weights high enough so that crosslinking is not necesssary for good
mechanical properties.
Lacquer
A lacquer is a thermoplastic polymer dissolved in an organic
solvent. High molecular weight polymers are necessary, and since the
polymer molecules are dissolved, the amount of polymer dissolved in the
solvent is low. Lacquers could thus be called low solids
coatings.
Advantages of acrylic lacquers
- good for brilliant metallic colors
- excellent exterior durability
- gloss retention
- continued solubility (because they are thermoplastic rather than
thermoset) makes repair jobs easier.
Crosslinking
Most monomers for a polymer are bifunctional. That is, they contain
two functional groups for polymerization:
A - - - A
But if there are three functional groups on a monomer, making it
trifunctional, then at the point where that monomer becomes a
part of the chain, there will be a branch:
A - - - A
|
|
A
If you are looking at a quantitative expression for crosslink density,
be sure to pay close attention to the units. Crosslink density could
be expressed as number of crosslinks in a given volume of polymer, but
it could also be given as number of crosslinks in a given mass of
polymer (polymer volume corresponds to polymer mass) or in a given number
of polymer monomer. This last one, polymer monomer, may sound a little
confusing, but it may be the easiest. The statement 3 crosslinks per
100 monomer details the stoichiometry used to mix di and trifunctional
monomers.
Glass Transition Temperature
The glass transition temperature corresponds to the temperature
where when you heat to and above this temperature, the polymer goes
from a hard glassy material to a
rubbery material. On a plot of strength
vs. temperature, as you go through the glass transition temperature you
lose mechanical strength. Above the glass transition temperature,
a motion started at one point on a polymer chain will produce results,
say 50 atoms along the chain away from where the initial disturbance
occured.
The glass transition of a latex polymer is decreased if a plasticizer is
added.
Coalescence is the coming together of polymer particles to form
a film. The temperature of film formation must exceed the glass transition
temperature of the polymer molecules. The Minimum Film Formation
Temperature (MFT) thus relates to glass transition temperature.
Solvents
As you lengthen a polymer chain, you decrease the solubility. This makes
it increasingly difficult to find a solvent that will dissolve the
polymer. Most organic polymers will not dissolve in water. One
exception is polyacrylamide, which is insoluble in water to infinite
molecular weight.
A good solvent will cause a coiled solid polymer molecule (solid until
it was put in the solvent) to uncoil and dissolve.
Viscosity
Rheology is the science of flow. A coating must flow over the
surface and form a uniform topcoat with a smooth surface.
Shear Stress (force per unit area)
Viscosity = ------------ -----------------------------
Shear Rate (velocity per unit thickness)
The viscosity of a polymer in a good solvent increases with concentration.
Surface Tension- The liquid molecules experience the highest
level of attraction for other molecules of the same liquid so packing
takes place in a way that minimizes the contact with surface areas. For
the purposes here, surface areas include both the solid surface a drop of
the liquid is on, and the interface with the atmosphere.
To say it yet another way, molecules surrounded by like molecules are
more stabilized, so this could be thought of as maximizing the number of
molecules surrounded by like molecules.
Wetting involves the spread of a liquid over a surface.
A low energy droplet spreads over a higher energy surface to minimize
the surface free energy (for the high energy surface, coverage by a
liquid allows more "energy release" than contact with gas molecules.)
The liquid continues to spread as long as the additional benefit to
the high energy surface more than offsets the increase in energy to
the liquid (spreading exposes more liquid to the atmosphere interface and
this is the restricting factor to wetting.)
Aliphatic groups form materials with the lowest surface energy. Methyl
groups give lower surface tensions than methylene groups. From this it
stands to reason that perhaps polypropylene will have a lower surface
energy than polyethylene.
Water is the media with the highest surface energy.
Surface tension decreases with increasing surface energy.
The surface tension of a resin in solution increases as the solution
concentration increases.
Volatile Organic Chemicals
There is an ongoing industry wide effort to develop coatings with reduced
VOCs. When volatile organic chemicals get into the atmosphere they
create smog.
Efforts to reduce VOCs
- Waterborne topcoats
- Higher Solids formulations
- Powder Coatings
The
Polymer Science Department of the University of Southern Mississippi and
the Southern Coatings Society sponsor an annual Symposium in New Orleans
that covers the three subjects listed above.
Application of Coatings
POLYMERIZATIONS
Solution polymerization involves a polymerization where the monomer dissolves
in the media (either organic solvent or water--we can't call water a solvent)
and the polymer is also soluble in the media, up to a certain molecular
weight. Beyond this molecular weight the polymer molecule precipitates out
of the solution, and the polymer will grow no further.
But polymerizations are possible in which the monomer is insoluble, or only
partially soluble in the media.
Suspension polymerization involves an initiator that dissolves in the
monomer.
Emulsion polymerization involves using a water soluble initiator,
such as ammonium peroxydisulfate. If the reaction is carried out at a
lower temperature at which the speed of radical production is insufficient,
then catalysts such as mixtures of ferrous, thiosulfate and persulfate
salts can be used to accelerate the formation of radicals. Room temperature
can then be used. Surfactant molecules stabilize the forming polymer
molecule. The presence of surfactants makes the formed polymer more
water sensitive.
Batch Process vs. Continuous Process
For addition polymerization, a batch process (i.e., a process where
everything is added together and then you turn the heat on and let
it react to completion) is not a good idea. Because addition reactions
are exothermic, the heat produced by the reaction would probably cause
the reaction to run out of control, with property damage and possible
human casualty. If runaway reaction does not occur, there would still
be a problem with a high polydispersity.
EXAMPLES OF POLYMERS USED FOR COATINGS
Poly(tetrafluoroethylene) (PTFE) Aqueous dispersions of PTFE are
used to coat chemical process equipment and cooking utensils. Sintering
temperatures of 425 C are used.
Poly(vinyl chloride) (PVC) has a glass transition temperature (Tg)
that is too high (81 C), it shows poor adhesion characteristics, and the
list of solvents that will dissolve poly(vinyl chloride) is rather small.
A terpolymer has been tested which alleviates the above concerns, which
consists of:
- vinyl chloride
- vinyl acetate
- maleic acid
weight ratio- vc : va : ma = 86 : 13 : 1
volume ratio- vc : va : ma = 81 : 17 : 1
The vinyl acetate reduces the Tg and increases the solubility, and the
maleic acid increases the adhesion.
PVC requires stabilizers because it has an inherent tendency to undergo
thermal and photochemical degradation via dehydrochlorination. Think
back to early in your semester of Organic I where the lecture discussed
dehydrohalogenation, where a hydrogen and an adjacent halogen would
come off when the system was heated, to form a double bond. The formation
of double bonds discolors the resin. The system can then crosslink and
become highly brittle. Stabilizers include dibutyl tin dilaurate,
and maleates.
Miscellaneous
Flocculation occurs when the particles which are supposed to be
separated come together and are held together by van der waal forces.
A plastisol is where a polymer with a Tg above room temperature is
dispersed in a plasticizer. The polymer (by definition) does not dissolve
in the plasticizer at room temperature. When the polymer is heated above
both the glass transition temperature of the polymer and the melt
temperature of the crystal domains of the polymer, the polymer dissolves
into the plasticizer to form a homogenous system.
If an organic solvent is added to a plastisol, it is then called an
organosol.
Gel Point
Powder Coatings
Infoseek
Last Update- April 29, 1995- wld