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UV curable powder coatings is perhaps one of the most challenging developments in radiation
curing today. The desire to be able to apply powder coatings on heat sensitive substrates has
triggered extensive research during the last decade.
One of the main advantages with powder coatings is that they are zero-VOC systems, i.e. they
contain no solvents, which is advantageous both from environmental as well as economical
aspects. The use of powder coatings has until now mainly been directed towards the coating of
metal substrates since the high curing temperatures limit their use on heat sensitive substrates
such as wood and plastics. The lowest curing temperature at this point is around 120 °C for the
conventional systems on the market. This limit is set by the combination of two factors, storage
stability and the film fusion process. (1).
One way to reduce the melt viscosity is to introduce crystallinity in the resin system (2,3,4) since
the viscosity drops more rapidly above the melting transition, compared the decrease in viscosity
above Tg for an amorphous polymer. This allow film formation at lower temperatures while still
retaining good storage stability at room temperature. The fast drop in viscosity may, however,
cause sagging problems. The low viscosity may also cause unwanted penetration into the
substrate if this is porous, e.g. wood, and the molar mass of the crystalline component is to low.
Non-crystallized low molecular weight components also tend to act as softeners for the system.
Hence the optimum rheological performance is a rapid drop in viscosity above the melting point but only down to a specified level. A low curing temperature also requires an initiating system which is activated at low temperatures but stable at room temperature. This is difficult to achieve with conventional thermal initiators why the use of UV-initiation has been suggested (3,5). Thermally stable UV-initiators will ensure chemical stability at ambient temperature and the crystalline component induce good flow properties.
Another research area in the field of macromolecules is related to improvement of material
properties by changes in the macromolecular architecture (6). By the end of World War II synthetic
polymers started to be utilized for commercial products. Ever since, material engineers have been
trying to improve polymer properties with an increasing ways of technologies and ingenuity.
Polymers have been modified in numerous different ways in order to alter their properties. The
most utilized ways to alter properties have either been to simply develop a new monomer and
synthesize a new polymer or to modify an existing polymer by some chemical route. Modification
normally consist of changing a catalyst or using different co-monomers. When Paul Flory in 1952
wrote his famous book Principles of Polymer Chemistry" he indicated an alternative scheme for
polymer synthesis. He theorized about synthesizing condensation polymers from multifunctional
monomers. The polymers were predicted to have a broad molar mass distribution and to be nonentangled and non-crystalline due to their highly branched structure. Long chain branching has been utilized a lot for modifying properties such as crystallinity and viscosity. Various grades of branched polyethylenes play an important roll in engineering polymers today.
A little more than 30 years later the first papers on synthesis of dendritic polymers emerged
(dendritic, greek for treelike) and revealed properties nobody could have foreseen. Dendritic
polymers synthesized from comprise monodisperse dendrimers, with exact branching, and irregularly branched, polydisperse hyperbranched polymers. The dendritic polymers turned out to have a number of very unique and different properties compared to their linear 634 analogs, for instance at high enough molar mass they were found to be globular. Dendritic polymers have been demonstrated to possess low melt viscosity as a function of molar mass and high solubility in various solvents compared to their linear analogs. It has been demonstrated that yperbranched polymers can act as scaffolds for functional groups (resin structures), that crystallinity can be introduced by attachment of crystalline end-groups on the polymers (7). This paper describes the synthesis of solid semi-crystalline resins based on hyperbranched polymers onto which crystalline segments are grafted and finally end-capped with crosslinkable methacrylate groups. The chemical, rheological, and final network properties of these polymers are described. It is demonstrated how these properties fulfil some of the demands for low temperature UV curable powder coatings.
