Simply put, the manufacture of coated paper involves applying a coating to a base paper, drying it, and then smoothing the surface with a calender; consequently, both the base paper and the coating are critical factors determining the quality of the final product.
Consider a coating layer with a coat weight of 20 g/m²; assuming the dry specific gravity of the coating is 1.5, the thickness of the layer is merely about 0.0013 cm. Given such minimal thickness, the quality of the base paper plays a pivotal role.
At slow coating speeds, the base paper-having absorbed water-must retain sufficient strength to avoid failure upon reaching the drying section; therefore, appropriate sizing and wet-strength retention are essential. Conversely, at high coating speeds, the paper must possess the strength to withstand the mechanical forces involved, while uniform water absorbency is also crucial.
In addition, base paper intended for coating must exhibit the following properties:
Good dimensional stability (minimal change due to moisture fluctuations).
Strong affinity for the coating material.
Resistance to curling or warping, which could otherwise cause coating streaks and lead to paper breaks.
Freedom from defects such as wrinkles or holes.
Ability to be wound tightly into rolls without breaking during the coating process.
Key base paper properties affecting the coating process include uniformity, smoothness, surface absorbency, surface strength, surface condition, chemical compatibility, two-sidedness, and optical properties; ultimately, the quality of the finished paper depends heavily on the quality of the base paper itself.