102
The free carboxylic groups are neutralised by adding inorganic or organic bases to generate
anions. Organic bases are preferred if they can evaporate. The neutralisation reaction is shown
in Figure 3.5.16.
On account of the low acidity of carboxylic acids, even partial neutralisation with amines yields
pH values above 7.5. This explains why – depending on the temperature – the ester groups of the
polyester molecules can be partly saponified (hydrolysed). This tendency to saponification is the
main reason that the use of polyester resins in water-borne systems is restricted. Saponification
may be the reason that acids forming the anionic carrier groups are cleaved and water-solubility
is lost. Or, if esters of the polyester main chains are saponified, the molecular weights decrease
and the acid values increase, which may lead to loss of physical properties of the coating films.
There are alternatives to polyesters that are much more resistant to saponification. However, so
as to retain the positive properties of polyesters for primer surfacers, several attempts have been
made to improve their saponification resistance. The results of such studies show: There are differ-
ences in the saponification stability of addition products of cyclic anhydrides due to the different
anhydrides. The following anhydrides are listed in order of increasing stability:
•
phthalic anhydride
•
trimellitic anhydride
•
maleic anhydride
•
tetrahydrophthalic anhydride
•
hexahydrophthalic anhydride
•
succinic anhydride
One way to improve the saponification resistance is to add trimellitic anhydride first and then to
statistically esterify one other carboxylic group of the addition product. The statistically remaining
free carboxylic groups are connected via two ester groups in the polyester molecules, which is a
precondition for improving resistance to cleavage reactions (see Figure 3.5.14). In addition, the
method
[94]
has the advantage that the carboxyl groups are better distributed over the polyester
molecules in comparison to the simple adduct of trimellitic anhydride, for the same acid values.
It is also possible to rank polycarboxylic acid contained in the polyester chains by their saponifica-
tion resistance. The following list shows the polycarboxylic acids or corresponding derivatives in
order of increasing saponification resistance:
•
phthalic anhydride
•
tetrahydro phthalic anhydride
•
adipic acid
•
hexahydro phthalic anhydride
•
isophthalic acid
•
terephthalic acid
•
fatty
acid dimers
A similar list could also be generated for diols as building blocks for polyesters. Diols containing
long chains, or, even better, aliphatic side chains, are more resistant to saponification than diols
containing short chains or diols containing ether groups (polyethylene glycols).
In addition, saponification resistance is influenced by the type and quantity of co-solvent. First,
co-solvents are used to improve the water solubility of polyesters. It is believed that they promote
un-coiling of molecules, thereby presenting the carrier groups to the surface of the colloidal coils
for neutralisation and forming solvates with water molecules. An equilibrium then exists between
co-solvent molecules in the colloidal coils and in the aqueous phase. If greater quantities of sol-
vent are in the molecular coils of the polyesters, it can protect the ester groups against hydrolysis
by water molecules, and the polyester molecules remain stable. From all these aspects, butyl
glycol (ethylene glycol monobutyl ether or butyl cellosolve) has proved to be the best co-solvent.
Automotive OEM coatings
103
It is also possible to employ small quantities of solvents which are incompatible with water. Such
solvents are placed in the polyester molecule coils and are also able to protect the ester groups
against saponification. The quantity of such solvents is restricted because of their lack of a media-
tor
role with the aqueous phase, which is important for optimum film forming.
Furthermore, the type and quantity of neutralisation agent influence the saponification resist-
ance. Suitable neutralisation agents are N,N-dimethyl ethanolamine (DMEA), diisopropanol
amine (DIPA), triethyl amine (TEA), aminomethyl propanol (AMP) and ammonia.
Water-borne polyester solutions often exhibit very strange and unusual viscosity behaviour when
thinned with water. During thinning, the viscosity rises significantly at first, unlike the case for
all organic solutions. After that, as thinning continues, the viscosity drops very rapidly to specific
values. This anomalous viscosity behaviour is sometimes termed a “water hill”. The process is
reversible, so it is not the effect of an inversion reaction (observed in emulsions). The water hill
may cause problems, first in the handling of water-borne polyesters in paint production, and
second during film forming, where such anomalies may interfere with levelling. The position and
height of the water hill are influenced by the polyester building blocks, the type of neutralisation
agent, the degree of neutralisation and type and quantity of co-solvent.
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