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4.4 Putties
Putties are used to fill out damage to metal car body parts. They must be used if the part is
uneven and there is no possibility of replacing the metal part completely. Putties are paint-like
fillers which can be applied in layers up to several centimetres thick. The main requirements on
putties are optimum adhesion, the ready formation of smooth surfaces by manual application,
sandability, and recoatability within a short time. Such requirements are not easy to fulfil with
paint materials which contain solvents. Therefore, systems are chosen that contain resins and
reactive diluents. During film forming and initiator-induced crosslinking, the resin reacts with
the reactive diluent to form resistant film layers containing chemically bonded resin and solvent.
The resins employed here are unsaturated polyesters (UP resins). The reactive diluents are vinyl
compounds, mostly styrene so far.
Unsaturated polyesters
Unsaturated polyesters are members of the most important class of coating resins, polyesters. They
have a similar structure and are prepared similarly to the saturated polyester resins described in
Chapter 3.5.3.1. As their name suggests, they contain significant levels of unsaturated compounds,
molecules which contain olefin double bonds. By this is not meant the double bond systems of
monocarboxylic acids, namely fatty acids, which are suitable for preparing alkyd resins to yield
oxidatively drying films (see Chapter 3.6.3.1). Rather, the suitable unsaturated building blocks are
mainly polycarboxylic acids containing double bonds at the -position of carboxyl groups or their
derivatives. The most important compound is maleic acid (cis-butene diacid) or its anhydride. Other
unsaturated polycarboxylic acids – which are mostly mentioned in patents – are itaconic acid,
mesaconic acid and citraconic acid. There are also formulations based on fumaric acid, the
trans
isomer of maleic acid. However, fumaric acid has a very high melting temperature (287 °C under
pressure) and is difficult to esterify completely. It must also be borne in mind that large quantities
of the maleic acid are transformed into the isomer form (
trans
form) during the esterification proc-
ess, i.e. fumaric esters are formed. Unsaturated polyester resins are made by combining the maleic
anhydride with other dicarboxylic acids, for example phthalic anhydride, tetrahydrophthalic
anhydride, and – for very flexible formulations – adipic acid. The other components of unsaturated
polyesters are diols, e.g. ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol,
and neopentyl glycol. Triols, e.g. trimethylol propane, are used only if unsaturated polyesters are
modified with monocarboxylic acids. The average molecular weights of unsaturated polyesters
are relatively low. They are obtained not so much by selecting a molecular excess of polyols as by
limiting the degree of condensation; they therefore have high acid numbers. The reason for this is
that hydroxyl groups of polyols can react with the double bonds of unsaturated compounds to yield
branched molecules of high molecular weight and broad molecular weight distribution (Michael
addition). This side-reaction is avoided by making the unsaturated polyester resins at relatively
low temperatures, up to 180 °C, in contrast to the production conditions for saturated polyesters
and alkyd resins. After the end properties have been attained, the resin melt is cooled down and
Repair coatings
211
dissolved in the reactive diluent. To prevent premature copolymerisation reactions during dis-
solution, small quantities of inhibitors, e.g. hydroquinone, methyl hydroquinone, or 4-tert.-butyl
brenzcatechol, are added. The delivery forms of unsaturated polyesters are solutions in reactive
diluents, mainly in styrene, with a solids content of 50 to 70 % by weight.
The putties are prepared by combining the unsaturated polyester solutions with high levels of
pigments and extenders to yield
a pasty product. Colouring is performed with titanium dioxide
and small quantities of carbon black and iron oxides. Calcium carbonate, aluminium silicates,
especially talc and kaolin, serve as extenders. The pigment and extenders are dispersed in “dis-
solvers” or kneaders. The type and quantity of extender varies with the rheological behaviour
of the putty. The putties must be applied by knives, must spread easily and form relatively
smooth surfaces, and must not run.
Chemical curing of putties consists in free-radical-initiated copolymerisation of unsaturated
polyester together with reactive diluent (styrene). The initiators are peroxides, with benzoyl
peroxide preferred. When used in conjunction with a redox reagent, peroxide will generate
free-radicals at room temperature. Such redox reagents are mainly tertiary aromatic amines,
e.g. N,N-dimethyl aniline. The generation of free-radicals is shown in Figure 4.3.
The free-radicals initiate the copolymerisation process shown in Figure 4.4 (page 212).
There is very high probability that free-radicals initiate the process at the maleic ester in the
unsaturated polyester. The resultant free-radical absorbs a styrene molecule, and a new free-radi-
cal is formed. This free-radical starts a polymerisation chain by adding more styrene molecules.
However, chain propagation is fairly limited; polymerisation is terminated by recombination reac-
tions with other free-radicals. The outcome is a three dimensional molecular network containing
polyester chains which are interlinked several times by short polystyrene chains.
Crosslinked UP resins are very hard, and offer resistance to chemicals and excellent resistance to
solvents. As the reactive diluent is incorporated into the network structure by copolymerisation
and cannot evaporate, thick layers can be effectively through-cured. There are no blisters or similar
film flaws, which would occur when true solvents evaporated from film layers. The copolymerisation
reaction is exothermic. There are virtually no solvent emissions, but some styrene does evaporate.
Where large areas have to be smoothed and the requisite film do not have to be very thick, spray
putties (spray fillers) can be used. These contain the same ingredients as knifing putties. However,
to achieve the low viscosity needed for the spraying process, they may additionally contain some
real solvents. Of course, such solvents must evaporate efficiently during film forming, without
causing any problems. The EU has imposed an upper limit of 250 g solvent per litre paint on such
products since 1 January 2007.
Figure 4.3: Generation of free-radicals by reaction of benzoyl peroxide with N,N-dimethyl aniline
Putties
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