THE THEORY AND PRACTICE OF SCIENTIFIC PAINT MAKING
=Laws of Paint Making.= To secure a proper comprehension of the
composition of paints, and to be able to interpret the functions of
their various constituents, requires an understanding of the general
physical principles involved. The modern grinder has accepted the Law of
Minimum Voids, and upon this law he bases the design of paint formulae,
aiming toward the production of what have been properly termed
Scientifically Prepared Paints. Perry's formulation of the Law of
Minimum Voids in a paint coating, and the analogy which he has drawn
between a scientifically prepared paint and a well-proportioned
concrete, was the result of genuine scientific thought following
observation and experimentation. It must be admitted that analogies are
not always safe to draw conclusions from, but it surely is no fallacy in
reasoning to draw analogies between these two materials, when they
resemble each other in so many ways. To carry out processes of
reasoning, and to formulate laws from such close analogies, is certainly
a step in the right direction.
A graphic summary of the analogies between a properly proportioned
concrete and a paint, are shown on next page.
Although this table graphically summarizes the principles involved, the
matter is presented with greater clearness in the following:
Law No. 1--The law of minimum voids to be observed in constructing a
paint formula--this law having already been accepted as mathematically
correct and technically proved in the technology of concrete and cement.
Corollary--The requisite thickness of a paint film together with the
utmost attainable strength and impermeability can best be obtained by a
properly proportioned blend of pigments of three or more determinate
sizes.
AN EXHIBITION OF CERTAIN ANALOGIES GOVERNING THE MANUFACTURE OF CONCRETE
AND OF PAINT
1 Concrete aggregate = solids + vehicle|Paint aggregate = solids + vehicle
|
2 Solids = coarse + medium + fine |Solids = coarse + medium + fine
(stone) (gravel) (sand) | {pulverized }{precipi-}
|(pig- {cryst'lline}{tated }(fume)
|(ments {(etc.) }
|
3 Vehicle = |Vehicle =
= reactive binder + evapor'g thinner |= reactive binder+evaporating thinner
{ cement and com- } (excess water) | (linseed oil) (volatiles)
{ bining water } |
|
4 Solids + compacting = |Solids + compacting =
(tamping) | (brushing)
= elimination of accidental voids + | = elimination of accidental voids +
+ proper adhesive contact | + proper adhesive contact
|
5 Vehicle + reaction = hydrosilicates, |Vehicle + reaction = linoxyn
etc. |
(setting) | (drying)
|
6 Solids + vehicle + |Solids + vehicle +
+ lubrication + chemical reaction = | + lubrication + chemical reaction =
= final product { solidified binder+}| = final product {solidified binder+}
{ + solids }| {+ solids }
|
7 Final product = concrete |Final product = paint coating
{ shearing }| { strength }
(of max. strength { tensile }| (of maximum { impermeability }
{ crushing, etc. }| { durability }
* * * * *
If we assume for both paint and concrete
proper lubrication
proper proportion of vehicle and solids
Then the _essential difference_ between a thin film of
Concrete and Paint
is
Cement Binder Linoxyn Binder
_Disadvantages_
Non-elastic and hence an impracticable |Slowly perishable from oxidation by
binder for a film to protect non- |the air.
similar structural surfaces. |
_Advantages_
Durable and with the qualities of a |Semi-elastic and therefore a practic-
natural mineral. |able binder for a film to protect
|structural surfaces.
Postulate (def. Webster's Dictionary--A self-evident problem)
Postulate No. 1--The organic linoxyn or semi-elastic binder of the
paint vehicle (unlike the cement binder) is perishable and its purity,
strength and protection from attack means life to the paint coating,
and hence the _life_ of the oil is the _life_ of the paint.
Postulate No. 2--The inorganic or powdered mineral solids of a paint
coating will crumble unless held together by the binder, but the
imperishable pigments must be so ground and blended in the binder that
they will protect the binder and present the greatest possible solid
front to the atmospheric attack.
* * * * *
A paint, to secure the greatest protection and life for the linoxyn,
together with the durable qualities of cement,
_Therefore_
Should expose to air decay
within limits of physical strength |within limits required for elasticity,
The greatest amount of pigm't material |etc. The least amount of exposed
|linoxyn
(which is) | or
Durable and with the inert qualities of|Considering the linoxyn present be-
natural mineral |tween pigment particles as the void
|or point of attack,
| Then
|the minimum exposure of linoxyn
or minimum voids obtainable by proportioned pigments of different particle
sizes.
Law No. 2--The law of the flat arch in paint coatings--i.e., the fact
that in studying the fundamental physical principles governing the
strength and durability of a paint coating it is necessary to regard the
coating as consisting of a series of flat arches, in which the pigment
particles of largest characteristic size serve as the piers or supports
for the flat arches of which the continuous film is composed.
Corollary A--The strength and durability of a paint coating is
determined by the strength and durability of the piers or supports
(which consist of the characteristic pigment particles of the largest
size).
Corollary B--Owing to their inherent strength and durability the pigment
particles of largest characteristic size which serve as supports for the
paint coating should consist, in part at least, of chemically inert
pigments, such as natural crystalline barium sulphate, calcium
carbonate, magnesium silicate, etc.
Corollary C--It follows directly that the thickness of a paint coating
is determined by the particles of pigments having the largest
characteristic size, even if that pigment be present only in moderate
percentage. Upon this principle depends the comparatively great
thickness of film and moderate spreading rate of paints composed of such
pigments as basic carbonate--white lead, red lead, barytes, etc., and
the strongly contrasted thinness of film and high spreading rate of
paints composed of the sublimated pigments such as lamp black, zinc
oxide, basic sulphate--white lead, zinc-lead white, leaded zinc, etc.
In commenting upon the announced laws set forth above, Heckel says: "The
recognition of these laws was an exercise of pure deduction. Paint
manufacturers before Mr. Perry's announcement were producing paints
containing three or more pigments with particles of varying
characteristic sizes; but their procedure was based largely on empirical
knowledge, the result of accumulated experience, due to a conscientious
endeavor to produce the highest type of paints for economic service. In
the absence of any law to govern or to limit the use of the reinforcing
pigments, inexperienced manufacturers had brought upon the market paints
which were badly proportioned as to the several pigments, or burdened
beyond the limits of effectiveness with reinforcing pigments. To all
paint manufacturers Perry rendered a substantial service in deducing for
them the laws set forth in his address. In the results following a
recognition of these laws there was nothing new or startling, but Perry
was the first to give the principles from which it can be determined in
advance whether a paint formula will prove to be physically good or bad
in practice.
[Illustration: Series of Paint Chasers, Mixers, and Grinders]
[Illustration: Overhead Churn Mixer]
[Illustration: Battery of Paint Mixers and Grinders of Modern
Underdriven Type]
[Illustration: _Photographs courtesy of Ernest Heath_
View showing Shrinkage in Bulk of Paint Pigment after being ground in
Oil. Filled Barrel on Right with the Oil forms one-third Barrel Paste as
shown in Barrel on Left]
[Illustration: View showing careful Dressing of Bull Stone Mill from
Grinder]
"As has been before stated, he was not the first to recognize the law
governing minimum voids, but by that scientific use of the imagination
which Tyndall so highly commends, he recognized, as by inspiration, the
fundamental similarity existing between a film composed of solid
particles cemented together by a semi-solid homogeneous menstruum and a
layer of concrete composed of solid particles cemented together by a
solid homogeneous medium. His application of the law permits the paint
manufacturers to design a paint formula with full knowledge of the
controlling conditions, so that it shall produce a coating neither too
thick, and therefore uneconomical and subject to excessive internal
strains, nor too thin, and thus weak and inefficient for protection.
That Mr. Perry's contention was well-founded, other paint technologists
have since demonstrated; notably Mr. Wirt Tassin, in his microscopic
studies of paint films in situ, and Prof. G. W. Thompson who, in his
address to the Penna. Association of Master Painters at Reading,
said:--"I want to agree with Mr. Perry * * * where he says that a
pigment should be made up of particles of different sizes. Mr. Perry
also draws a further parallel between paint and concrete where he refers
to the form of the reinforcing pigment particles and suggests that in
paint coatings as in concrete a field can be found for the chemically
inert pigments with rod-like or hair-like structure, to strengthen the
film, just as the steel rods and iron mesh are used to reinforce
concrete in structural work--a suggestion which, since the first
publication of the address, has been widely accepted as a practical aid
in the manufacture of good paints.""
=Use of Inert Pigments.= There seems to be no reasonable doubt as to the
efficiency of a small amount of inert pigments in paint, and the writer
has often compared the manufacture of paint of the above type to the
making of various alloys wherein zinc, copper, and other metals are
added to gold in order to make a product possessed of greater
durability, etc.
[Illustration: Batteries of Color Grinding Mills]
There has been considerable inquiry as to just what is meant by the
statement that "a moderate percentage of inert pigments, combined with
properly adjusted mixtures of white lead and zinc oxide, have given
wonderful service in all the tests." The writer has been asked to define
what "moderate" means. A "moderate percentage of inert pigments" should
be defined as that amount of natural crystalline pigments that will,
when mixed with white lead and zinc oxide, not materially detract from
the hiding power of white lead and zinc oxide. It is possible to mix a
certain percentage of these crystalline pigments with white lead and
zinc oxide, and, by thorough grinding, incorporate them in such a manner
that the mixture will show nearly as good a hiding power as the straight
white lead and zinc oxide. When certain limits have been reached,
however, and these limits must be determined by the manufacturer and
painter in making practical tests, the further addition of inert
pigments lowers the hiding power of the paint and therefore lowers the
value of the paint. These remarks do not apply to artificial crystalline
pigments, such as precipitated whiting, which possess greater hiding
values than the natural pigments.
=Perry's Principles of Paint Making.= Parts of the original paper[18] in
which Perry so clearly set forth the principles from which the preceding
laws were formed, follow:
[18] Physical Characteristics of a Paint Coating. R. S. Perry.
Michigan Chapter, Amer. Institute of Architects, 1907.
=Sealing Quality or Imperviousness of the Coating.= "It has been
emphasized that for durability and protection, the strength and
imperviousness of a paint coating are vital factors. The protective
value of the paint coating of course ceases with its chalking or
disintegration, but, while it is true that the protecting or final life
of the coating ceases with this disintegration, it is also true that a
paint coating has always during its true life more or less porosity from
the nature of the linoxin or oxidized linseed oil. Therefore during its
protecting life the degree of its imperviousness influences its
resistance to attack upon its own life and its protection of the
underlying materials. The more impervious the paint coating without loss
of strength, the slower the oxidation or disintegration of the paint
coating itself and the greater protection to the underlying material.
"A coating of linseed oil alone is not only weak, but the simplest and
crudest experiments will show its porosity and this porosity increases
rapidly with progressive oxidation, the porosity of course definitely
hastening the over-oxidation or chalking. In proportion, therefore, to
our success in filling the voids in the linseed oil film with proper
pigment materials, we will in that degree succeed in excluding agencies
of decay, not only from the mass of the paint coating itself, but also
from the surface to be protected. These conditions are exactly parallel
in the requirements and performance of the best-made concrete, and
Taylor & Thompson in their work on concrete have clearly stated that to
obtain imperviousness there must be freedom from voids, and that to
obtain these conditions, the materials used must have at least three
determining sizes.
[Illustration: Equal Volume (One Cubic Centimetre) of Each Size of Shot
Taken. Note that the Smaller Shot Cover more than Half as much again as
the Larger Shot and the Voids are Smaller.]
[Illustration: Diagram Illustrating Two Determining Sizes of Solid
Particles in Concrete]
[Illustration: Diagram Illustrating Three Determining Sizes of Solid
Particles in Concrete]
"'It is a fact that with particles of different sizes as against uniform
size the densest mixture can be obtained. This is so evident as to
require no proof.' It follows that the least density and hence the
largest percentage of voids occur when the grains are all of the same
size, and it is shown that the most voids occur in a mass of large
particles. The least voids occur when the voids between the large
particles are filled with smaller particles and when these smaller voids
between the smaller particles are in turn filled with still finer
particles. In other words--particles with three determining sizes will
fill up a given space more completely than particles of two determining
sizes and very much more completely than particles of one size.
=Elasticity and Strength.= "The paint coating here again is governed by
many of the laws which govern the similar material, i.e., concrete. We
find, by again referring to Taylor & Thompson, on Concrete, page 275,
that tests at the Watertown Arsenal on concrete convinced the
investigators that the ultimate strength of a concrete is identical with
the shearing strength of particles of stone making up the aggregate.
"This means that in its ultimate form the good concrete will crack or
shear through the broken rock contained therein, and resistance to
shearing is directly proportionate to the strength of the broken rock
chosen for the mixture. The film of semi-liquid linseed oil when fresh
is extremely weak, but as it hardens, its characteristics and physical
properties will obviously be those qualities which are a composite of
the qualities of the solid particles and of the semi-solid linolein
incorporated together in the paint coating. These physical properties of
the suspended and incorporated pigments profoundly modify the film in
this respect.
"The dried vehicle, linoxin, is notable for its elasticity, and it is
weak in crushing and tensile strength, and in hardness or resistance to
surface wear. The fact that it is a semi-solid furnishes an opportunity
to modify and improve those characteristics of a solid in which it is
deficient. The semi-solid, rubber-like linoxin between the coarser
particles of the pigment obviously uses these coarser particles as
supporting points. The medium sized particles of the second group of
alteration products serve the same purpose as the broken rock in
concrete. The coarser particles absolutely do not, and can not, serve
the purpose of stiffening or of reinforcing or modifying the consistency
and qualities of the semi-solid linoxin, for a number of reasons, one
of which may be mentioned, namely, that particles of the first, or
coarse, class have a determining size which is a large fraction--a heavy
percentage--of the total thickness of coating, and are in some instances
thicker in diameter than the thickness of an oil coating not reinforced
with the fine or fire group.
"We must think of the coarser particles as piers. The mixture of linoxin
with the other two groups of particles in the spaces between these
coarser particles, or piers, is the true paint body and consists of flat
reinforced arches which have the extra support of falsework, in the
shape of the structural material on which the coating rests. Asbestine
pulp, a natural product and one of our most important natural
reinforcing pigments, serves not only in the coarse group as supporting
particles for the linoxin arch, but also because of its peculiar
properties serves the more important purposes of reinforcement. It
retains, no matter how finely ground, its peculiar needle-like, or
rod-like, form of particles, and obviously serves the purpose of
reinforcing the flat arch of linoxin, exactly as iron bars or iron
netting serve in reinforced concrete arches. The medium sized particles
of the second group of pigments produced by chemical alteration or
precipitation, serve the purpose of the broken rock in concrete, and
together with the coarser supporting particles and the finest
reinforcing particles, give minimum voids and a maximum imperviousness
to agencies of internal decay.
"It goes without saying that the pigments of any one group contain
particles of dimensions which fall into the other two groups, but no one
pigment supplies the correct proportion of each of the three required
dimensions, and each pigment has so large a percentage of approximate
dimensions as to bar it from exclusive use in the other two groups.
Given similar homogeneous coatings under identical conditions, we
recognize the law that elasticity will vary directly with thickness.
Direct deduction from this law teaches us that of two paint coatings
equal in wear, in strength, opaqueness, and in all other qualities
except thickness, we should choose the thinner coating. Therefore if we
have two paint coatings fulfilling every requirement, the first
compounded with pigments giving a thicker coating and the second with
pigments yielding a thinner coating, we must choose the second formula
and obtain the thinner coating.
=Adhesive Power.= "The adhesion of the linoxin to the coarse group of
particles and to the underlying material is vital to the life of the
paint coating. If the coating parts from the surface beneath, we have
scaling or peeling. It is universally admitted that this will result
from use of zinc oxide as the sole pigment. We have only to conceive of
our flat arch of reinforced linoxin and leave out our points of support,
to realize that this is the inevitable result if the coating be subject
to extreme exposure, although good results may be obtained from zinc
oxide used alone, as, for instance, in interior house painting where
extreme changes of temperature and exposure are avoided.
"Three major lines of force hold our linoxin in place--adhesion toward
the underneath surface, adhesion to the coarse particles, and cohesion
within the linoxin itself. These lines must be represented by a flat
arch of linoxin with a downward pointing magnet therefrom, to represent
adhesion to the surface. Magnets on each side of the arch pointing
toward the supporting coarse particles, and two magnets within the arch
and pointing toward each other, or to the centre of the arch, these
latter to represent the force of cohesion."