The body engineer has a choice of materials, ranging
from carpeting of the Wilton type for prestige
vehicles, through polyamide and polypropylene
moulded needle felts, to rubber flooring for economy
versions. The main requirements of flooring are
wear resistance, colour fastness to light and water,
and adequate strength to enable the customer to
remove the flooring from the car without it suffering
damage. The method of manufacture of pile carpets
varies: in some cases the pile is bonded to hessian
backing; in other cases it is woven simultaneously
Table 4.12 Undersealing and protection materials used in vehicle body repair work
Type Base material Application
Coatings for underbodies, Bitumen/rubber Coating for underbodies, wheel arches, new and repaired
spray type parts. Also a corrosion protection for vehicle underbodies
against elements such as moisture, road salt, road stone
chippings. Good adhesion, and durable at extreme
temperatures. Applied with a spray pistol
Rubber/resin Suitable for underbodies, wheel arches, front and rear
aprons, sills, new parts, repair sheets. Can be painted over
and has high abrasion resistance. Applied with a spray pistol
Wax Suitable for underbodies, touching up and subsequent
treatment of all protective coatings
Polymer wax Long-term corrosion protection even when thinly applied.
Has good flowing properties
Small-scale repair Rubber/bitumen Brushable coating, suitable for underbody and wheel arches.
application (brushable) High abrasion resistance and good sound deadening
properties
Road stone chip repair Synthetic/dispersion Good protection against stone chips and corrosion.
material Particularly suitable for front and rear aprons, sills, spoilers,
wheel arches
Metals and non-metals used in vehicle bodies 147
into the backing and then anchored in position with
either a rubber coating on the back surface or a vinyl
coating. Quality is normally controlled by characteristics
such as number of rows of tufts of pile per unit
length, height and weight of the free pile, overall
weight of the carpet, strength as determined by a
tensile test in both the warp and weft directions,
together with adhesion of pile if applicable. Rubber
flooring generally has a vinyl coating to provide
colour.
Leather (hide)
Large numbers of motor vehicle users all over the
world continue to specify hide upholstery when the
option is available, and will gladly pay the extra
cost involved for a material which defies complete
simulation. Great advances have been made in the
development of suitable substitutes, and the best of
the plasticized materials are to many people quite
undistinguishable by eye from leather. The unique
character of leather lies in its microstructure, the
like of which is not obtained in any manmade material.
Under a microscope leather can be seen to consist
of the hairy epidermis and under that the
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corium, or bulk of the hide, this being the basis of
the leather as we know it. By virtue of the millions
of minute air spaces between the fibres and bundles
of fibres, leather is able to breathe. To the motorist
this means that leather does not get hot and uncomfortable
in warm weather or cold and inflexible in
winter, and although permeable to water vapour it
offers sufficient resistance if it is exposed to normal
liquids. It is also strongly resistant to soiling, and
when it does get dirty the dirt can usually be
removed fairly easily without special materials.
Unlike some plasticized materials, leather does not
appear to attract dirt and dust as a result of static
electricity. Many people, moreover, regard the distinctive
smell of leather as an asset, and this defies
imitation by manufacturers of substitute materials.
However, natural hide has to go through many complex
processes before it attains the form familiar to
the upholsterer trimmer or motorist.
Fabrics for interior trim
Vinyl coated fabrics are now well established as
trim material. Their vast superiority over the linseed
oil coatings, and later the nitrocellulose coatings, of
yesteryear are almost forgotten in the march of
progress. Vinyl coatings are now sufficiently familiar
for their merits to be taken for granted; nevertheless
they continue to provide a material which
for durability, uniformity and appearance at a reasonable
cost so far remains unsurpassed. The resin
polyvinyl chloride, the main ingredient of the coating,
became available in commercial quantities in
the early 1930s, and now a coating based on
polyvinyl chloride (PVC) is used for seating material
in this country. Although by tradition PVC is
produced with a simulated leather appearance, on
the Continent and particularly in the USA it is
widely used with fancy embosses, and patterns. An
extremely wide range of qualities is available, and
in recent years there has been an effort to achieve
some degree of rationalization. Additional qualities
are necessary for tilt covers, headlinings and hoods
for convertibles; the material for hooding convertibles
must be resistant to mildew, to shrinkage and
to wicking, this last term relating to the absorption
of water on the inside surface of the cloth from the
bottom edge of the hood. The characteristics necessary
to provide serviceability over the life of the
vehicle are the strength of the material under tension
and under tearing conditions; adhesion of the
coating to the backing cloth; resistance to flexing,
and resistance to cracking at low temperatures; low
friction, to enable the owner to slide on the seat;
colour fastness, soiling resistance and, of course,
wear resistance. In the case of the breathable
leathercloth, the air permeability of the fabric has to
be controlled.
Modern trends
The interior furnishing of a car is gaining in importance
within the automotive trade. In response stylists
are endeavouring to upgrade and soften the
interior, using fabrics with the appearance and feel
of textiles to appeal visually and functionally. This
has manifested itself in all areas of the car, including
the boot, seating, carpets, door trims and headliner
cover fabrics. In all four areas of fabric use in
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car interiors (seating, door and side panels, bolsters
and headliners) fabrics are gaining ground against
exposed plastics. Both polyamide and polyester are
giving designers new scope for attractive colours
and variation in seating upholstery and in panels,
while fully meeting light fastness and other performance
standards.
148 Repair of Vehicle Bodies
4.12 Plastics
Development
Celluloid might well qualify for the honour of being
the first plastic, though its inventor, Alexander
Parkes, was certainly not aware of that fact. He
made it around 1860 and patented his method for
making it in 1865. An American, John Hyatt, found
a way of solving the technical problems which
plagued Parkes, and he set up business in 1870 to
sell the same sort of material. He called it Celluloid
to indicate its raw material, cellulose.
In 1920 a German chemist, Hermann Staudinger,
put forward a theory about the chemical nature of a
whole group of substances, natural and synthetic.
He called them macromolecules; today we call
them polymers. His theory not only explained the
nature of plastics, but also indicated the ways in
which they could be made. It provided the foundation
for the world of plastics as we know it.
Polymerization
The raw materials for plastics production are natural
products such as cellulose, coal, oil, natural gas and
salt. In every case they are compounds of carbon (C)
and hydrogen (H). Oxygen (O), nitrogen (N),
chlorine (Cl) and sulphur (S) may also be present.
Oil, together with natural gas, is the most important
raw material for plastics production.
The term plastics in the broadest sense encompasses
(a) organic materials which are based on
(b) polymers which are produced by (c) the conversion
of natural products or by synthesis from primary
chemicals coming from oil, natural gas or coal.
The basic building blocks of plastics are monomers.
These are simple chemicals that can link together to
form long chains or polymers. The type of monomer
used and the way it polymerizes, or links together,
give a plastic its individual characteristics. Some
monomers form simple linear chains. In polyethylene,
for example, a typical chain of 50 000 ethylene
links is only about 0.02 mm long. Other monomers
form chains with side branches. Under certain circumstances,
the individual chains can link up with
each other to form a three-dimensional or crosslinked
structure with even greater strength and stability.
Cross-linking can be caused either chemically or
by irradiating the polymer.
To get the advantages of two different plastics,
two different monomers can be combined in a
copolymer. By combining the monomers in different
proportions and by different methods, a vast range
of different properties can be achieved (see Tables
4.13 and 4.14). The properties of plastics can also be
enhanced by mixing in other materials, such as
graphite or molybdenum disulphide (for lubrication),
glass fibre or carbon fibre (for stiffness), plasticizers
(to increase flexibility) and a range of other
additives (to make them resistant to heat and light).
