Luminance is the ratio of the reflectance of an opaque […]
Luminance is the ratio of the reflectance of an opaque paper to near blue light (wavelength 457 nm) and the reflectance of a magnesium oxide plate of known reflectance under the same illumination, expressed as a percentage. (Brightness and whiteness are two different concepts, but paper workers in China are accustomed to refer to brightness as whiteness, and sometimes whiteness terminology is actually brightness). There are many factors affecting whiteness, and it is not just a problem of insufficient whiteness and bleaching.
The factors that affect the whiteness of paper are:
1) Whiteness of pulp
Pulp whiteness is the basic whiteness that determines the whiteness of the paper. The whiteness of the pulp is high and the whiteness of the produced paper is also high, and vice versa.
2) Addition amount of filler
Generally, the whiteness of ordinary fillers (such as clay, talc, calcium carbonate, titanium dioxide, etc.) is high, so adding filler to the pulp can increase the whiteness of the paper. In production, due to the different filler content on both sides of the paper, the whiteness of the two sides of the paper differs by 0.5 to 3.0 percentage points. When the surface coating is carried out with a white pigment, the whiteness is remarkably improved.
3) Effect of sizing
Applying rosin gum can reduce the whiteness of the paper by 3~5 percentage points. With AKD sizing, the whiteness of the paper is higher than that of rosin. For example, when producing coated paper, AKD is used to sizing, and the whiteness of the paper is 5-6 percentage points higher than that of rosin.
4) Effect of pulp lignin content
High lignin content in the pulp or excessive bleaching will cause the pulp to age and yellow, reducing the whiteness of the pulp. This is most prominent in the newsprint that is ground with wood pulp.
5) The effect of drying
Excessive paper drying, high paper roll temperature and long storage time; paper after calendering will also reduce whiteness.
6) Turbidity of production water has an effect
The paper whiteness produced by water with clean water and low turbidity is higher than that of water produced by water quality.