In screen printing, all choices depend, first and foremost, on the substrate. The substrate corresponds to the lower layer, which in a printing context refers to the surface to be printed, whether it is two-dimensional or three-dimensional. The chemical (composition) and physical (shape and roughness) characteristics of the substrate, as well as its intended purpose, define the type of ink to be used. Ambient temperature and humidity regulate any prior or additional procedures.
Inks can be classified according to several criteria, based on their visual, structural, or drying/curing characteristics. Visual characteristics are, naturally, related to color and opacity, reflected in the optical impression generated when looking at an ink. Structural characteristics are related to the viscosity, adhesion, and behavior of the ink in relation to a substrate, also reflected in the impression they cause when touching an ink. The drying and curing characteristics relate to the time and process of final formation of the ink layer on the substrate, also becoming one of the most used criteria for distinguishing printing inks. One of the major difficulties in the initial development of modern screen printing was precisely the drying of inks, due to the use of inadequate and not yet properly refined types.
Drying corresponds to a physical process of transition from a liquid state to a solid state by simple evaporation of solvents through the air. Curing translates into a chemical hardening process that can involve different agents, including air itself. Drying gives rise to a kind of coagulation, without molecular alteration of the resins contained, while curing originates a chain chemical reaction, which is called polymerization (union of monomers that give rise to a polymer). In general, the formation of the final ink film on the substrate involves both processes – a first evaporation phase, which gives it a dry appearance to the touch, and a second curing phase that corresponds to effective hardening. However, inks differ substantially in their primary hardening method, with exceptions for those whose process may be exclusively chemical or exclusively physical. Inks whose hardening is primarily a curing process are inks with improved or specific properties, given that polymerization also implies a transformation of the resins. These inks are naturally suited to less common substrates, those with difficult adhesion, for outdoor use, or those requiring intensive use, depending on the resins and type of curing.
Distinguishing screen printing inks based on their drying and/or curing processes, may be as follows: 1. Air-drying and curing inks – whose hardening occurs mainly through the intervention of air – by evaporation or oxidation 2. Absorption-drying inks – whose hardening occurs through the substrate 3. Thermal curing inks – whose hardening occurs through the intervention of high temperature 4. Chemical curing inks – whose hardening occurs through the action of a catalyst Induced curing inks – whose hardening occurs through the intervention of specific radiation
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