Methacrylate homo and copolymers with controlled architecture as adhesive viscosity modifiers in UV curable ink-jet inks

Reversible addition-fragmentation chain transfer (RAFT) controlled polymerisation was used to prepare a range of methacrylate polymers;- poly(methyl methacylate) (PMMA), poly(n-butyl methacrylate) (PnBMA), poly(hexyl methacrylate) (PHMA) and poly(methyl methacrylate)-co-poly(n-butyl methacrylate) (PMMA-co-PnBMA) with a range of molecular weights. The resulting polymers were characterised by 1H and 13C NMR spectroscopy and the molecular weights and distributions determined by gel permeation chromatography (GPC).

The viscosity modification ability and the effects on adhesion and wettability were evaluated. Polymers were dissolved into a UV-curable ink formulation composed of a pthalocyanine blue pigment dispersion, monomers n-vinyl caprolactam, phenoxy ethylacrylate and photo-sensitive initiators;- 1-hydroxy cyclohexyl phenyl ketone, acyl phosphine oxide and benzophenone. An evaluation of the surface energies and polar and dispersive components of a range of thermoplastic printing media was undertaken using static contact angle analysis. Dynamic contact angle and adhesion testing was performed on typical printing media of high and low surface energy (dynes cm-1).

The addition of the PMMA, PnBMA and PMMA-co-PnBMA copolymer enabled control over viscosity both by concentration and molecular weight. PHMA provided minimal control over viscosity at 3 wt% addition with varying molecular weight.

Improved adhesion to particular grades of polypropylene was achieved with the addition of PnBMA, however a reduction in adhesion to poly(vinyl chloride) was also observed. The low molecular weight PMMA-co-PnBMA copolymer at 3 wt% achieved marginal improvement in adhesion to the printing grades PP without detrimental effects to adhesion of the PVC grades.

The effect on polymer architecture therefore, appears to have a significant effect on the level of adhesion achieved under the test conditions outlined within the project. Although a significant correlation between molecular weight and viscosity was observed no significant difference was detected between adhesion and molecular weight.

Measured surface free energy (ys) using static contact angle is an effective method of determining the true surface characteristics of printing substrates; however this study determined that a correlation to high and low surface energy and good and poor adhesion is not easily identifiable.