The basic physical properties of paper include grammage, thickness, density and bulkiness. Density and thickness are two of the basic macroscopic properties of paper structure.

Since the basis weight affects all the basic properties directly, the weight per m2 is important.

Although the expression of grammage is used as “Basis weight” in the American measurement system and its unit is shown as lb (lb), the concepts call “basis weight” or “grammage” in the standard metric system basically mean the same thing and its unit is shown as g/m2.

The metric weight (Grams Square Meter) used in the metric system does not differ by paper type or usage. Instead, the weight reflects the actual weight of a sheet of paper cut into 1 x 1 square metre. Since the same size of paper is always weighed regardless of the paper’s properties, GSM (grams per square meter) is clear and easy to understand way of assessing paper weight and thickness (URL-2).

Density is expressed as the amount of mass per unit volume (kg/m3). It is as important as other parameters (fiber type and pigment combination, beating, wet pressing, and calendering) in terms of the predictability of many properties such as tensile strength, elasticity modulus, etc. (Isko et al., 1998).

Paper thickness is the distance between the bottom and top surface of a paper sheet, in mm or microns. All these features affect the bulkiness of the paper.

Bulkiness is defined as the amount of volume per unit weight. It is a property that varies according to the wood species, paper machine characteristics, and morphological characteristics of the additives used in papermaking.

For example, since refining and/or beating of pulp can change the mechanical properties of the paper, the bulkiness of the paper is affected (Sousa et al., 2011).

Bulkiness is expressed as the inverse of density and its unit is cm3/g.

The formula:

B: bulkiness and D: density.

Since the density value is found by the ratio of grammage to thickness and volume is the inverse of density, the equation can be showed as follows:

B: bulkiness, T: thickness, and G: weight.

Especially, when it is desired to obtain sheets with high bulkiness, hardwood fibers can be used as they have thick fiber walls. This provides a high degree of fiber stiffness and stiffness is more important than fiber length in situations where bulkiness is expected (Pettersson et al., 2015).

Since the fiber stiffness provides resistance against bending, the bulkiness of the paper increases.

Manufacturers can choose between woods with relatively long fibers such as spruce, fir, and pine (softwoods) that provide strength, rigidity, and structure, and trees with shorter fibers such as birch, eucalyptus, poplar (hardwoods). Fibers obtained from such woods provide high bulkiness (low density), tight texture, and surface smoothness (Chamberlain and Kirwan, 2013).

Another important factor affecting bulkiness is the pulp production technique. Papers obtained from mechanical pulp have higher bulkiness than those obtained from chemical pulp.

The degree of inter-fiber bonding obtained with such fibers is not high, so the sheets tend to be weak. Sheets from mechanically separated fibers have “high bulk” and low density; which means it has a relatively low weight per unit area for a given thickness (Chamberlain and Kirwan, 2013).

The bulkiness measurement provides information on understanding how compact the paper is. Glossy-coated papers are less bulky and thinner. However, uncoated, bulky papers feel thicker and stiffer (URL-1). Controlling the refining, drainage, and pressing processes can increase the bulkiness of the paper, but it should be taken into account that these approaches may also have an adverse effect on issues such as paper strength (Choi et al., 2018).

References:

  • ANONİM, 2004, ISO 534: Kâğıt ve Karton: Kalınlık, Yoğunluk ve Hacimlilik
  • Belirleme, Türk Standartları Enstitüsü, ICS 85.060
  • CHAMBERLAİN, D. ve KİRWAN, M.J., 2013, Handbook of Paper and Paperboard Packaging  Technology, Second Edition. Ed. Mark J. Kirwan, Published by John Wiley &Sons, Ltd.
  • CHOI, K.-H., KIM, A. R. & CHO, B.-U. 2018. Manufacture of high bulk paper using alkali swollen kraft pulp. Nordic Pulp & Paper Research Journal, 33, 503-511.
  • ISKO K., LAAMANEN J., ve KAINULAINEN M., 1998, Paper Bulk and Structure, Chapter 3,     Paper Physics, Ed. Kaarlo Niskanen, Finnish Pulp and Paper Research Institute, Printed    by Gummerus Printing, Jyvaskyla, Finland.
  • PETTERSSON, G., HÖGLUND, H., NORGREN, S., SJÖBERG, J., BERGSTRÖM, J., SOLBERG, D., PENG, F., HALLGREN, H., MOBERG, A. & LJUNGQVIST, C.-H. 2015. Strong and bulky paperboard sheets from surface modified CTMP, manufactured at low energy. Nordic Pulp & Paper Research Journal, 30, 319-325.
  • SOUSA, G. D. A., ABREU, C. T., AMARAL, J. L. & BRÁS, C. 2011. Office paper bulk optimization in a paper machine using multivariate techniques. O PAPEL, 72, 50-55.
  • URL-1 https://pinupdesign.co.uk/paper-weight-thickness/ Date accessed: 5/01/2023.
  • URL-2 https://www.ecoenclose.com/blog/points-pounds-and-gsm-whats-the-difference-and-what-do-you-need-in-your-paper-packaging/ Date accessed. 8/1/2023