Smoothness

 Smoothness is concerned with the surface contour of paper. It is the flatness of the surface under testing conditions which considers roughness, levelness, and compressibility. In most of the uses of paper, the character of the surface is of great importance. It is most important parameter for printer. It is common to say that paper has a "smooth" or a "rough" texture. The terms "finish" and "pattern" are frequently used in describing the contour or appearance of paper surfaces. Smoothness is important for writing, where it affects the ease of travel of the pen over the paper surface. Finish is important in bag paper as it is related to the tendency of the bag to slide when stacked. Smoothness of the paper will often determine whether or not it can be successfully printed. Smoothness also gives eye appeal as a rough paper is unattractive.

 

Smoothness (Bekk Method): This test is an indirect measure of paper smoothness when it is under moderate pressure( 100 kPa). The standards test procedure is described in TAPPI T 479.

 

Roughness (Sheffield Method): This test is an indirect measure of paper smoothness or roughness. It is a measurement of air flow between the specimen (backed by flat glass on the bottom side) and two pressurized, concentric annular lands that are impressed in to the sample from top. The standards test procedure is described in TAPPI T 538.

Roughness (Print-surf Method): Very similar to Sheffield methods. The standards test procedure is described in TAPPI T 555.

Typical Smoothness Values
Grade	Parker Print Surf (μm)	Bendtsen (mls/min)
Newsprint (40 - 49g/m2)	2.6-4.5	80-140
Stationery (45-135g/m2)	0.8-2.6	50-300
Business Papers (80g/m2)		100-300
Test Liner (186 g/m2)		1750

Temperature and Humidity: Conditioning of Paper

 

As explained above it is important to control the moisture content of paper and keep it stable during converting operation. To keep moisture content constant, it is important that paper is conditioned. Conditioning of paper is also of important in many printing and converting operations. In addition to the effect of moisture content on physical properties, it also determines the build up of static of the paper sheet subjected to pressure and to friction. The tendency for paper to develop static becomes greater with increasing dryness. Cellulose fibers are hygroscopic i.e. they are capable of absorbing water from the surrounding atmosphere. The amount of absorbed water depends on the humidity and the temperature of the air in contact with the paper. Hence, changes in temperature and humidity, even slight changes, can often affect the test results. 

So, it is necessary to maintain standard conditions of humidity and temperature for conditioning.