3
17

BASIS WEIGHT OF PAPER
Introduction
Basis weight is the weight per standard area of paper, considering one side only. In the U. S. paper trade, one generally expresses basis weight in units of pounds per ream, where the ream usually consists of 500 sheets, occasionally of 480 sheets, of a given size. The sheet size employed in any particular case depends on the paper grade. For example, the basis weight of newsprint given in this system would be about 35 lb (24 x 36 - 500), with the sheet size of 24 by 36 inches and the number of sheets per ream, 500 in this case, given in parentheses in the manner shown. The ream designation does specify a sheet area, as this ream is equivalent to 24 x 36 x 500 in 2 = 432000 in 2 or
3000 ft 2 .
The metric system used in scientific work and in most foreign countries is often more convenient and expresses basis weight in units of grams per square meter. The basis weight given in these units is generally called the "grammage". In this system, the basis weight of newsprint considered above would be approximately 57 g /m
2
.
Paperboard is usually the heavier, thicker type of fibrous product characterized by its structural strength. Paperboard or board products are constructed for certain properties such as stiffness and ability to resist deformation. The basis weight of paperboards is usually given as the weight in pounds per 1000 square feet. A somewhat arbitrary, but useful definition of paperboard-as distinct from paper-is that it has a basis weight greater than 45 lb/(1000 ft
2
), which is roughly equivalent to 200 g/m
2
, and/or thickness greater than 0.012 in. However, corrugating media (usually
0.009 in), chipboard (as thin as 0.006 in), and linerboard are considered paperboard products.
Another common unit employed in the paper trade is the number of sheets of size 25 x 40 in
2 required to make a bundle of 50 lb. This is known as the "regular number".
In the following, two different methods of determining basis weight are described. For reasons that will become clear below, students will apply both methods to measure the basis weight of all their samples.
Significance
Most paper and paperboard transactions are based upon the weight of the product so basis
18

weight has great significance to both consumer and producer in determining price. Furthermore, many physical properties are evaluated in accordance with the particular basis weight involved.
Paper is ordered by the ream or roll, is bought and sold by the pound, and is used by the square foot.
Therefore, the producer prefers to make the paper heavy while the consumer wants the lightest weight suitable for his particular purpose. When the paper is heavier than specified, the user receives fewer sheets per pound. If the paper is lighter than specified, more sheets per pound would be received, but such paper might lack certain other requirements for the end use.
Basis Weight Variation Within a Paper Sheet
In the section entitled "Statistical Treatment of Data", we saw that all paper properties vary from place to place in a paper sheet. The same is true for basis weight. This variation of basis weight within a paper sheet is of particular concern to the papermaker, as can be readily appreciated from the adage that much of the art of making good paper consists of reducing these basis weight variations to a minimum. Thus, the papermaker is not only interested in determining and controlling the mean basis weight of his paper product, but he is also very much concerned with determining and controlling the variation or dispersion of basis weight around the mean. The tests outlined below permit both of these two aspects to be measured.
A few general remarks on basis weight variation within a sheet are in order. This variation can never be completely eliminated. Even if we had a perfect paper machine, with all imperfections in the approach flow system, the headbox, and the slice openings removed and with complete prevention of flocculation: the paper would still show basis weight variation, less than usual but still considerable. The reason is as follows. Under the ideal conditions just specified, the individual fiber is laid down in the plane of the sheet in a manner that is completely random as well as independent of all other fibers. The papermaker has no control over this process. Obviously, he cannot lay down fibers as orderly as the bricks in a masonry wall. It is this randomness in sheet formation which causes variation in basis weight even under ideal conditions. In simplest terms, the sheet is bound to have thick spots and thin spots.
On a real paper machine with all its imperfections, additional mechanisms act to further increase basis weight variation. For example, the basis weight profile in the cross direction often drops off toward the edges of the original wide roll. In many grades this is unacceptable. To be able
19

to take appropriate corrective action, the papermaker will monitor the basis weight distribution in cross direction by, for instance, taking square-shaped samples of size 10 cm by 10 cm all along the cross direction and measuring their basis weight. Another example are the periodic changes in basis weight along the machine direction called wet-end barring. Finally, a very important contributor to basis weight variation in paper is flocculation; in the papermaking slurry (furnish), the fibers have a pronounced tendency to clump together and to form flocs. A paper sheet formed by the deposition of flocs will, of course exhibit much greater basis weight variation than a sheet formed by the deposition of individual fibers.
The variation of basis weight caused by these different mechanisms occurs over very different characteristic distances; one often speaks of the scale of the variation. In an ideally random sheet, most of the basis weight variation occurs over distances of about 5 mm; the basis weight drop toward the edges of the paper machine occurs over a distance of 1 ft or so; the periodicity of basis weight variation in wet-end barring depends on many factors and can be of the order of several ft; and the scale of basis weight variation due to flocculation depends primarily on the floc size, which, depending on conditions, is at least of the order of a fiber length and may reach dimensions of up to several inches.
Given these fairly large differences in scale, the papermaker must choose his measuring technique according to the particular variation he is concerned about. For illustration, consider the second test method described below, which requires that the student cut 5 specimens of size 10 cm by 10 cm from a paper sample he is given for analysis. By weighing the specimens and relating their weight to their known area of 100 cm
2
, he determines the 5 corresponding basis weights, the mean basis weight, and the standard deviation s, which serves as a measure of the variation in basis weight.
The important point to appreciate is that the random-sheet-variation of scale 5 mm will not contribute much to s because this variation will be effectively averaged out in specimen squares of side 10 cm. Only those basis weight variations that have scale comparable to or greater than 10 cm will contribute significantly to s as determined in this test.
Tests
First Method: Use of the Basis Weight Balance (Toledo Scale Corporation)
This method will give an accurate mean value of basis weight, and one determination is sufficient. This implies that no standard deviation can be obtained. For a quick zero check, take a
20

sheet out of a ream with known basis weight and read the scale for ream size. If the numbers do not match, consult the equipment information part of this piece. Place one sheet or several sheets of a paper sample in the sample basket of the balance. Use the largest sheet size possible. The weight can be read from any one of the three scales; that is, either in lb/(1,000 sheets), lb/(500 sheets), or lb/(480 sheets). Determine the width and length of the sheet. For the sake of illustration, suppose the width is 8 1/2 in and the length 11 in. Suppose further that the weight is read off the scale corresponding to
1,000 sheets and happens to be 11.0 lb/(1,000 sheets). If the basis weight W is required in units of lb
(24 x 36 - 500), the calculation is as follows:
W = 11.0 lb (8½ x 11–1000)
= 11.0 lb x ream (8½ x 11–1000) x 24 x 36 x 500 in 2
ream (8½ x 11–1000) 8.5 x 11 x 1000 in 2 ream (24 x 36-500)
= 50.8 lb (24 x 36-500)
Notice that this method works with paper sheets of any size. The above example does not imply that sheets should be cut to size 8 1/2 x 11 in 2 for basis weight determination. If, in the above example, the basis weight is required in units of g/m 2 , one proceeds in a similar manner:
W
= 11.0 lb (8½ x 11-1000)
= 11.0 lb x 453.6 g x ream (8½ x 11-1000)
ream (8½ x 11-1000) 1 lb 8.5 x 11 x 1000 in
2 x 1 in
2
(2.54 x 10
-2
)
2
m
2
= 82.7 g/m 2
If the weight of a single sheet is very low, precision of the basis weight determination may be insufficient because too few significant figures can be read off the scale. Precision and accuracy can be improved by weighing several sheets at a time. Their combined weight, indicated on the scales, must of course be divided by the number of sheets weighed. Clearly this method enables one to obtain the basis weight to more significant figures.
21

A special feature of the "Toledo" basis weight balance is the poise that rides on the beam of the balance and has two resting positions. The left position near the sample basket is marked "0" and is the standard position that is used most of the time. The right hand position far from the basket is marked
200 lb
100 lb
1000 sheets
500 sheets
96 lb 480 sheets and can be used to double the range of the balance. This may be useful in case the weight of the sheet exceeds the scale range when the poise is in "0" position. If the poise is moved to the right hand position, the zero of the three scales now corresponds to 200 lb, 100 lb, and 96 lb for the top, center, and bottom scale, respectively. The appropriate zero-level weight must of course be added to the basis weight read off one of the three scales.
Second Method: Use of an Analytical Balance
Use one of the square shaped metal templates provided to cut 5 specimens with a razor blade to the precise size of 10 cm by 10 cm. (Please be careful when using the razor blade and any of the paper cutters.) Tare the scale to assure a proper zero. Weigh each of these specimens on the analytical balance to as many significant figures as the balance is capable of providing. Calculate the basis weight for each of the 5 specimens, determine the mean of these 5 values, and calculate the standard deviation.
Report
Express the mean basis weights obtained by the above two methods in units of g/m 2 as well as lb (24x36 - 500). For the results of the second method, report the standard deviation, which is a measure of the variation of basis weight in the paper samples. Also report 95 % confidence limits for the mean.
22

Equipment Information
Toledo Scale – Model 9210
WB Porter & Co. - Rick Porter
1721 Lake Wheeler Road
Raleigh, NC 27603-2329
(919) 828-1750
References
ASTM D 646
TAPPI T 410
Analytical Balance – Model PM 200
Testing Machines, Inc. - Margaret Sexton
2910 Expressway Drive South
Islandia, New York 11749
(631) 439-5400
23