The paper density of a type of paper or cardboard is the mass of the product per unit of area. The term density here is not used in its traditional sense of mass per unit volume. "Paper density", rather, is a measure of the area density. Paper products that let little or no light pass through (e.g. poster board) are considered dense or heavy. Paper products that allow some light to pass through (e.g. tissue paper) are considered lightweight.
Two ways of expressing paper density are commonly used:
- Expressed in grams per square metre (g/m²), paper density is also known as grammage. This is the measure used in most parts of the world.
- Expressed in terms of the mass (expressed as weight) per number of sheets, it is known as basis weight. The convention used in the United States and a few other countries using US paper sizes is pounds of a ream of 500 (or in some cases 1000) sheets of a given (raw, still uncut) basis size. Japanese paper is expressed as the weight in kg of 1000 sheets.
Coated paper is paper which has been coated by a compound to impart certain qualities to the paper, including weight, surface gloss, smoothness or reduced ink absorbency. Kaolinite or calcium carbonate are used to coat paper for high quality printing used in packaging industry and in magazines. The chalk or china clay is bound to the paper with synthetic viscofiers, such as styrene-butadiene latexes and natural organic binders such as starch. The coating formulation may also contain chemical additives as dispersants, resins, PE: to give water resistance and wet strength to the paper, or to protect against ultraviolet radiation.
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