It is known that printing an acidic ink or fixer, typically a fluid containing an organic acid, together with a pigmented ink which is basic or neutral causes the pigments to destabilize or “crash” on the print media. This “crashing” effect causes improved optical density, strikethrough and edge acuity and decreased bleed.
A problem with these acidic inks and fixers is that the acid partitions or cuts into printhead plastics and adhesives over time. This problem has been mostly ignored until recently because the expected lifetime of a printhead has been relatively short, e.g., a few years. Furthermore, printheads have been relatively inexpensive to replace.
Ways to crash pigments without an acidic ink or fixer have also been found. Rather than employing acids, multivalent metal salts (e.g. calcium nitrate) have been employed in an ink or fixer fluid. Such solutions have been found to minimize adhesive degradation, but have lead to poor pen reliability because of the problems of decap and kogation caused by the salts.
As printheads have become larger and more expensive and have been designed to last longer, there has arisen a need to solve the problems caused by acidic inks and fixers partitioning into printhead adhesives.