What is paper bulk?

The volume of an object

A paper with higher density has lower bulk, meaning it is more compact. And of course, the opposite is true for a paper with low density. It has more volume and contains more air.

The term bulk is used to describe the volume of an object, and paper bulk is measured in cm3 per gram. If 1 gram of paper has a volume of 1 cm3, it has a bulk value of 1.0. So a low bulk value means that the paper is compact, with little air between the fibres. A paper with high bulk value instead has a lot of air between the fibres, making it thick, but without adding weight.

Paper thickness

Imagine that you have a sheet of paper that weighs 60 gram per square metre. This paper has a bulk of 1.5 and a caliper thickness of 90 µm. If you compare this paper with another paper of the same grammage, 60 gsm, with a bulk of 2.0 the caliper thickness will increase to 120 µm. This means that the paper weighs the same, but the sheet is actually thicker because it has lower density and contains more air.

What does that mean when buying paper?

Well, you need to stop thinking about your ideal paper as defined by its grammage or weight per m2, and instead focus on the thickness of the paper sheet. If you do that, you will see that a high bulk paper will be able to give you the same thickness in µm, as your old paper, but it will weigh less because it contains less paper fibres and more air.

Why is that a good thing?

You buy paper in tonnes, right? But what you really want to get out of that massive, potentially toe crushing reel is square metres of printing surface. If the paper weighs less per m2, you get more printing surface for each tonne that you buy. In some cases, the difference in weight can be as much as 16%.

High bulk paper benefits

There are a number of benefits with using a high bulk paper. Now that you no longer define your paper by just looking at the grammage, have a look at some other parameters that play an important part when choosing paper:

What does higher opacity mean for paper?

Since the paper contains more air and is thicker, your get better opacity compared to a paper of the same grammage with lower bulk. The opacity is a measurement that tells you the amount of light which can be transmitted through the paper sheet.

A paper with high opacity lets very little light pass through it, whereas a paper with low opacity allows a higher amount of light to pass through the sheet, it is more translucent. Opacity is an important factor in preventing your print from showing through from the other side of the paper.

Why does a high bulk paper maintain its stiffness?

A high bulk paper made from fresh wood fibre in a thermomechanical pulping process is stiff, even in lower grammages.

Paper stiffness is a property that measures the ability of the sheet to withstand a bending force. The good thing about a stiff paper is that you can use a lower grammage, without getting a floppy end product. Since the high bulk paper has a large volume, it maintains its stiffness even at lower grammages. Low bulk papers have a tendency to become soft and limp in lower grammages.

Why use a high-bulk paper in a lower weight?

A high bulk paper is made to retain a thickness that cannot be found in other papers of the same basis weight. To get the same paper thickness that you are used to, you can switch to a paper that weighs less.

And using a paper that weighs less allows you to lower your distribution costs. Every gram you save, on each unit, can make a huge difference when you add them all up. It can also help you to reduce your climate impact, since you will get more printing surface per tonne, and thereby you will buy less paper.