Coated paper is a common substrate in flexographic printing. To meet the specific printability requirements of the flexographic process, control measures must focus on three key aspects: ink absorbency, surface strength, and the elasticity/resilience of the coating layer.
Ink Absorption
Ink absorbency-defined as the paper's capacity to absorb ink or the ink's ability to penetrate the coating layer-is a combined manifestation of the inherent properties of both the paper and the ink under printing conditions; it is also the most critical factor in printability. Many printing defects arise from a mismatch between the coated paper's ink absorption characteristics and the specific printing conditions employed. Excessive ink absorption by the paper results in dull, matte prints or even "chalking" (powdering), whereas insufficient absorption slows down ink drying, leading to set-off (smudging on the back of the sheet).
The water-based inks used in flexography contain water-soluble resins with small particle sizes. Coated paper is a porous material; the voids formed by fibers and pigment particles constitute the basis for ink absorption, and the capacity of these voids to absorb ink serves as a key metric for evaluating the paper's absorbency. These voids determine the volume and rate of ink penetration once the ink is applied to the paper surface.
During printing, a portion of the ink is forced directly into the paper's voids by printing pressure. Once the remaining ink transfers to the paper surface, fiber interstices smaller than the ink pigment particles absorb the ink vehicle (binder) via capillary action, while the pigment particles remain on the surface to dry rapidly. Consequently, good surface microporosity is essential for effective ink absorption.
The composition of the coating layer significantly influences the surface microporosity of the paper. The coating typically consists of a pigment blend of kaolin clay and calcium carbonate. Research conducted by TCT Laboratory (Hagemeyer) on pigment blends of kaolin and calcium carbonate revealed that the Relative Sedimentation Volume (RSV) peaks at a 70:30 ratio; this ratio is considered optimal, as it yields a loose coating structure.
Coating weight also influences the microporosity of the coated paper surface. At higher coating weights, increased resistance within the filter cake layer slows the flow of liquid into the base paper, allowing more time for pigment particles to rearrange and pack efficiently; this results in a dense coating structure with reduced porosity, which is detrimental to ink absorption.
Binders are another factor affecting the microporosity of the coated paper surface. Natural binders tend to produce a more open coating surface and greater pore volume compared to synthetic binders, resulting in higher air permeability and strong ink absorbency. Conversely, synthetic binders-such as PVA and latex-exhibit excellent film-forming properties and interact strongly with pigment particles; they create a coating with low pore volume and a sealed surface, leading to reduced air permeability and weaker ink absorbency.