Paint
13.1
Why paint?
Without paint or similar products, we would have to live in an
environment filled with rusting iron, rotting wood, and eroded concrete and
with the consequences of all this deterioration. We would also miss out on a
great deal of colour and colour options, which make life a lot more pleasant
and enjoyable. Paint has three main functions:
protection
decoration
hygiene
13.2 Definition
of paint
Paint is a liquid, paste or powder product applied in thin or thicker
layers on a substrate, which is then converted during the drying process into a
solid film. In the first place, paint is intended to protect the substrate and
must therefore meet special requirements.
Over the past 20 years, the focus has also increasingly come to lie on decoration
and colouring (aesthetic aspects).
Paint consists of four major components, and each component helps
determine the performance characteristics of the paint:
binders
pigments and fillers
solvents and thinners
additives
13.3 Binders
Binders serve to bind the pigments and fillers in the paint layer and to
provide them with good adhesion to the substrate. The type of binder determines
to a large extent the various performance characteristics of the paint.
13.4 Pigments
and fillers
These are the filler materials which give paint its colour, strength and
covering power. The pigments also convert damaging shortwave UV radiation into
longer wavelengths that are less damaging. This helps slow down and prevent the
degradation and aging of the paint product by UV radiation.
13.5 Solvents and thinners
The function of these ingredients is to make the paint more workable and
easier to apply. Paint is actually a product composed primarily of solid
ingredients, some of which have been converted into liquids by the addition of
solvent. The solvent therefore functions as a transport agent but does not in
itself alter the characteristics of the paint film. Once the paint has been
applied, the solvent will disappear into the air via evaporation. Of course,
solvents are not present in solvent-free paint products, which are available in
liquid as well as powder form.
The most important types of solvent/thinners are:
aliphatic hydrocarbons such as turpentine, naphtha etc.
aromatic hydrocarbons such as xylene
alcohols such as ethanol, propanol, butanol etc.
esters such as ethyl acetate, butyl acetate etc.
ketones such as acetone, methyl ethyl ketone etc.
glycol ethers such as ethyl glycol, butyl glycol etc.
glycol esters such as ethyl glycol acetate etc.
13.6 Additives
These compounds fulfil various functions and therefore sometimes help
determine the desired characteristics of the paint:
Siccatives which cause drying of oxidative
binders (alkyds) and include calcium, cobalt, zirconium, etc.
Surface active agents which provide smoothness to
the surface
Anti-skinning agents which prevent paint from
forming a skin inside the packaging
Plasticizers which provide elasticity to the
paint film
Anti-settling agents which prevent heavy filler materials
from settling during storage
Wetting agents which ensure that the pigments are more easily dispersed throughout the paint
De-aerating agents which ensure that the air
that is mixed into the paint during production and application is released from
the paint film in a timely fashion after application and before drying
Anti-flooding agents which prevent the flotation
of pigments to the surface after application
Levelling agents which promote an optimal
flow after application and secure high quality finishing
Fungicides which ensure that no microorganisms can survive in the wet paint medium
(e.g. water-based wall paint) and/or ensure that no algal growth can take place
on the paint film surface (e.g. for anti-fouling coatings on ships)
13.7 Drying
The paint drying process can take place in various ways and depends upon
the type of binder used in the paint. The following types of drying process can
be distinguished:
physical drying
semi-physical drying
chemical drying
oxidative drying
two-component paints
catalytically mediated drying
13.7.1 Physical
drying
This drying process is caused by the evaporation of the solvent from the
paint film, causing the film to become increasingly dry and hard. After a paint
film has dried out, it can easily be dissolved again using the original solvent
present.
Examples of paints formulated in this fashion:
Nitrocellulose
paints (in varnishes for the paper and furniture industry)
Chlorinated
rubber paints (for ships and steel structures)
Polyvinyl
(for concrete and steel structures)
Bitumen
or bituminous paints (for offshore and steel structures)
13.7.2 Semi-physical
drying
This is characteristic for water-based acrylics, also known as
"dispersion paints." In these dispersions, the binder particles are
finely dispersed in water. After application of the paint, the water will first
start to evaporate. As evaporation progresses, the binder particles flow
together and are compressed to form a microscopically tight paint film.
Examples of such paints include water-based acrylic paints, which are
used for protecting and covering concrete, steel, wood and plasterwork.
13.7.3 Chemical
drying
This type of drying occurs because, during and after the evaporation of
any solvent present, a chemical reaction takes place. The following types of
chemical drying can be distinguished:
oxidative drying
two-component drying
catalytically mediated drying
13.7.4 Oxidative drying
Alkyd paints: Alkyd-based paints are some of the most popular paints in
the steel coatings market as well as the professional painting sector and do it
yourself hobby market. This is due to their good application properties, fast
drying, and relatively low price. Alkyd paints dry by absorbing oxygen from the
air. The higher the percentage of oil in the paint, the slower the drying
process is.
There are three types of alkyd paints, each with its own typical market
segment:
Short oil alkyd: this type contains the lowest percentage of oil, up to
circa 40%. This makes it the fastest drying type, and it is used primarily for
industrial applications such as steel coatings;
Medium oil alkyd: the percentage of oil in this type varies between ca.
40% and 60%. It dries relatively quickly and displays good outdoor durability
and performance. The largest markets for this group are the steel coatings
sector and the professional painting sector;
Long oil alkyd: the percentage of oil in this type is more than 60%. The
drying speed is therefore fairly standard. The largest markets for this group are
the professional painting sector and the do it yourself hobby sector. In the
steel coatings sector, it is often applied as a repair coating by brush in the
form of a conventional as well as High Solid product;
Baking finishes: the drying process for some of these types can be
accelerated by heating the object.
13.7.5 Two-component
paints
In these paints, the film formation process is based on a chemical
reaction between the curing agent and the primary component. the base
component. these products is created by a chemical reaction of the curing agent
with the base component. During this reaction, the chemical chains in the wet
paint film become tougher and harder. At the end of the chemical reaction the
paint film reaches its final hardness. The chemical reaction starts as soon as
both components are mixed together.
The two components are separately packed and supplied and need to be
thorough mixed just before application of the paint.
These paints include:
Epoxy
paints:
These
paints are very popular for providing protective coatings for steel, in
particular wherever good adhesion, mechanical durability, and chemical
resistance are desired. These epoxy coatings are often available in the
following types: Primers, zinc-rich primers, sealers, intermediate coatings
(with or without miox), coatings and finishes.
Polyurethane
paints:
These
paints have the same positive characteristics as the epoxy coatings, but loss
of gloss in the finish over longer periods of time is not an issue with
polyurethane paints. The curing process for these paints is also less
temperature sensitive than for epoxy paints.
Additional
advantages include: excellent mechanical strength, toughness and wear
resistance; excellent chemical resistance; reasonably heat resistant; good adhesion
after proper surface preparation; good performance; good gloss retention; good
long-term colour-fastness and retention.
13.7.6 Drying via catalytic
mediation
These coating types are also provided as two-component systems, but here
the curing agent is replaced by a catalyst. This catalyst functions as an
initiator or "starter." When it is mixed together with the primary
component, a strong molecular network is established consisting of the binder
(polyvinyl butyral), the pigment (zinc-tetra oxychromate) and the available
acid (phosphoric acid) in the catalyst. These primers are supplied in only a
few colours and are applied in relatively thin coating thicknesses.
The surface to be coated may be cold- or hot-rolled steel as long as
grease and oil have been effectively removed beforehand. Aluminium and other
metals are also suitable substrates, as this type of primer also etches itself
into the metal surface, thereby ensuring excellent adhesion. These primers are
often also referred to as “wash primers” and are sometimes supplied already
pre-mixed.
13.8 Corrosion
protection methods
There are three ways to slow down the process of corrosion:
Passive corrosion control (sealing)
Active corrosion control (via anti-corrosion pigments)
Cathodic protection
13.8.1 Passive corrosion control
This method is very popular in the "heavy duty" industry. The
working principle involved is sealing off the substrate from compounds that
cause corrosion such as oxygen, moisture and other chemicals present in the
environment. The level of protection is determined by the film thickness
applied and the selected coating system. The most popular systems are high
solids epoxy and polyurethane coatings, with or without iron oxides, aluminium
and micaceous iron oxides.
13.8.2 Active corrosion control
The addition of anti-corrosive pigments to the paint (e.g. zinc
phosphate) creates an active anti-corrosive product. The protection is based on
the fact that the zinc sacrifices itself for the steel substrate by corroding
first. This method is still one of the most popular methods in the protective
steel coatings sector, as protection can already be achieved at very thin
coating thicknesses. In addition, this method can be applied in practically all
qualities and types of coatings.
13.8.3 Cathodic protection
The working principle behind this method is that application of an
electrical potential prevents corrosion of the steel substrate.
Cathodic protection can be implemented in two ways:
Passive: by attaching sacrificing anodes (e.g. zinc) in a fixed fashion
to the steel substrate. These anodes will then corrode instead of the steel
substrate. This method is used for large constructions and ships.
Active: by applying a current to the construction (Impressed Current Cathodic Protection), thereby lowering the electric potential of the steel. This method is used for large bridges or similar traffic structures and underground steel pipes.
