The durability of water-borne acrylic coating systems.
The objectives of this research programme were to examine the influence of manufacturing
and processing parameters on the environmental stability of aqueous acrylic based latices and
to also extend the programme to an assessment of the behaviour of various stabiliser types
suitable to such materials through either end-group modification or direct in-situ addition.
During the research programme a number of physical and chemical techniques were assessed
to routinely characterise and monitor the photochemical and thermal degradation of acrylic
latices. These were based on emulsion polymerised formulations of methyl methacrylate and
butyl acrylate. In the first instance, acrylic latices based on different formulations were
chosen as homopolymers and copolymers. Here the nature of impurities and oxidation
products generated during various stages of their manufacture have to be characterised and
inter-related to their influence on subsequent environmental degradation. This involved the use
of reflectance FT-IR spectroscopy to show functional group changes together with
colourimetric U.V. analysis to determine photochemical generation of hydroperoxides. The
early chemical changes and their subsequent influence on the physical and chemical properties
of the latices during the later stages of environmental degradation were found to exhibit a
close inter-relationship. In this regard the formation and the manufacturing temperatures and
periods were crucial. Thermal methods of analysis were also used to characterise differences
in the properties of the latices before and during thermal and photochemical oxidation. The
nature of the residual persulphate initiator and the compositions of the latices in terms of end
group modification were important parameters. De-esterification and hydroperoxide formation
were found to be important processes during latex degradation while the use of low levels of
co-monomer addition. such as methacrylic acicL to the emulsion reaction gave latices with
improved light stability.
Subsequent aspects of the research programme involved a detailed investigation into the
behaviour and performance of various stabiliser types and formulations. Thus, while coreactive
hindered piperidine stabilisers was found to be effective, the incorporation of simple
terminal dialkyl acrylamide/methacrylaroide groups were also found to be effective. The
efficiency of dialkylamide groups operating through a sacrificial mechanism is discussed. The
influence of these processing operations on subsequent stabiliser activity provides valuable
information on the mode of action of these stabilisers in such complex media.
Further work involved a study into the effect of fluorinated methacrylates used in emulsion
polymerised formulations. Here, the presence of residual starting materials in the monomer
was found to have a detrimental effect in terms of photostability. Subsequent analysis of
acrylic resins, based on solution polymerised formulations of methyl methacrylate and
fluorinated methacrylates, was undertaken. The influence of reactive stabiliser types on these
resins was important and to improve stabiliser perfonnance, synergistic stabiliser formulations
This project will provide important fundamental information on the behaviour of active
chromophores produced during the manufacturing process and hopefully resuh in the
development of monitoring methodologies to establish paint quality.