Coating formulation is a fundamental technology in the manufacture of coated paper and has a critical impact on the product's end-use. Consequently, coatings must be designed and manufactured to meet specific requirements regarding coating processability and printability; in other words, the design of the coating is directly linked to the type of coated paper being produced. In terms of volume, standard coated paper remains the dominant product, yet from a technological standpoint, there is a drive to master the technologies associated with high-end art paper (gloss-coated paper). Different products require specific coatings, coating methods, and coating machines. While all varieties must meet established quality standards, gaining customer approval is the ultimate priority.
There are many types of coatings used for paper, with water-based coatings being the most widely applied. The composition of water-based coatings can generally be categorized into four components: pigments, binders, additives, and water. The combination of these four components is closely linked to the quality of the finished coated paper. Table 1 below outlines the impact of coating components on paper quality.
Pigments
Pigments include clay (white clay), calcium carbonate, satin white, aluminum hydroxide, titanium dioxide, and plastic pigments.
1. Clay
Clays are classified by composition into categories such as kaolinite, pyrophyllite, and sericite. Kaolinite is further subdivided into varieties like standard kaolin and hydrated kaolin; all these clays are commercially produced. However, in the context of coated paper, the term "kaolin" is generally used to refer to the kaolinite group as a whole, effectively becoming synonymous with coating-grade clay.
Kaolin particles are hexagonal platelets with diameters ranging from 0.3 to 3 micrometers. The aspect ratio and particle size of these particles directly influence the quality of the pigment layer on the paper. Kaolin is currently the most widely used pigment for coated paper because it imparts high gloss to the unprinted sheet, requires a moderate amount of binder, and offers good viscosity and dispersion characteristics, allowing for higher solids content in the coating formulation.
2. Calcium Carbonate
Calcium carbonate is widely used in coated paper, ranking second only to clay in terms of usage volume. Calcium carbonate is particularly abundant in Taiwan; in recent years, it has increasingly replaced more expensive clay. A wide variety of grades have been developed to suit different end-uses and production methods for coated paper, making the optimal use of calcium carbonate a key focus in the formulation of paper coatings. Thanks to its high brightness and porosity, calcium carbonate pigment offers excellent ink absorbency, thereby improving printability-making it highly practical even for matte-finish coated papers.
3. Satin White
Satin white is a white pigment synthesized from slaked lime and barium sulfate; it has long been used as a pigment for art paper. It offers excellent brightness, gloss, ink absorbency, and surface smoothness. However, these qualities come with characteristics such as high coating viscosity and strong swelling properties, which result in drawbacks like poor runnability during the coating process and a high demand for binders. Historically, satin white was primarily used in air-knife coating formulations; it presents challenges when used in high-solids blade coating processes, so its application is generally limited to art-grade coated papers.
4. Aluminum Hydroxide
Offers good brightness, opacity, and ink absorbency.
5. Titanium Dioxide
Titanium dioxide possesses very high opacity, making it a common choice for light-weight coated (LWC) paper formulations. However, its use is restricted by its high cost and abrasive nature. It exists in two crystalline forms: rutile and anatase. Although the anatase form offers slightly lower opacity, it is widely used due to its lower cost and high brightness.
6. Plastic Pigments
Plastic pigments currently available generally fall into three categories: binder type, solid type, and hollow type, with the latter two being the most commonly used. During synthesis, variables such as monomer ratios, initiator concentrations, agitation speeds, and reaction temperatures can be manipulated to control particle size, uniformity, stability, and reaction rates.