17-08-2012, 12:20 PM
Coatings
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The concept of the coatings
The concept of coatings most probably originated from the realm of
anatomy where it means the cover of animals’ bodies, isolating the organism
from the environment and, at the same time, providing contact between
the organism and that environment. In vertebrates the coating is
constituted by the skin. This concept has been imported into the realm of
technology.
The concept of coatings was given recognition in official documents
and specifications in the 1950s. It must, however, be admitted that to this date
there is no general terminological specification defining the coating and its
types. There are, on the other hand, specifications related to corrosion and
corrosion protection [1, 2], which though assume that the concept of “coating”
is self-evident and define the various types of coatings.
It seems appropriate to put forward a general definition of the concept in
the following form:
Structure of the coating
For obvious reasons, the coating has a laminar structure. Because of the
great variety of coatings, both from the point of view of material and
technology, which stems from different designated uses, it is difficult to
develop one universal model of coating structure. Specific models, pertaining
to the particular types of coatings, are given and discussed in literature
on the subject, usually with varying degrees of simplification.
Types of coatings
Coatings may be divided in different ways, depending on the criteria used.
The most significant division appears to be by material, by designation
and by method of manufacture of the coating.
Division of coatings by material
From the point of view of material, coatings may be divided in two groups:
metallic and non-metallic. Very often the name of the coating is derived
from the coating material used.
Metallic coatings
Such coatings are made from different metals, metal alloys and metal
composites, and deposited on substrates, most often themselves metallic,
by different methods. In principle, coatings may be manufactured from all
metals and composites. However, not all metals, alloys or composites are
applicable in practice on account of their properties and technological
difficulties in their manufacture. Most often used are:
Non-metallic coatings
These coatings, numerous and varied, are made from organic materials
(paint, rubber, plastic) and inorganic (enamels, ceramics), of natural and
synthetic origin and bearing many different trade and chemical names.
Most often used coating materials (or their basic constituents) are:
Paints - coating materials for covers, comprising suspensions of pigments
(organic and inorganic coloring substances) in a binder (filmogenic
substance) [9-15], manufactured in the form of liquids and powders. The
dispersed phase is the pigment, while the binder is the dispersing agent.
Typical inorganic pigments are zinc white, titanium white, chrome yellow,
minium, graphite, etc., while organic pigments are divided into natural and
synthetic, e.g., indigo, phtalocyanine blue [14, 15]. Depending on the binder
used, we distinguish the following types of paints: oil, cellulose [10], emulsion,
varnish [10], latex, limewash, silicate, thixotropic, ceramic (including
mirror and muffle), minium (rust-inhibitive), oil-resin (e.g., antifouling) [9],
fireproof, etc. [9-15]. They are most often deposited in the liquid form on a
painted substrate, on which, after drying (in the case of powder paints, after
remelting) they form the cover coating.