Paper

0.5-1.5 usd/kg
Circularity potential
Medium
Strength
Medium
Production energy
Medium
Stiffness
Low
Embodied CO2
Medium
Density
Low

Ranging from webs designed for printing at incredible speeds with precise surface quality to soft tissue micro-embossed to provide superior water absorption, a range of paper formats have evolved for many everyday applications. Paper is a fibrous material with anisotropic properties: it is stronger and stiffer in-line with the direction it comes off the forming machine. This is because the fibres are inherently more aligned with machine direction (MD) than cross direction (CD). In thicker materials the properties through thickness (ZD) may also become relevant, such as for packaging frozen foods.

Paper is often coated to improve surface quality. Plastic laminates, such as polypropylene (PP) and polyethylene (PE), are also used to enhance strength and create an impermeable barrier. These additions can make it difficult, if not impossible, to recycle those kinds of paper. Some paper mills can handle paper with plastic coating on only one side. The paper is re-pulped and plastic filtered out and burnt for fuel. If there is plastic on both sides, then this is not possible and the material is destined for landfill.

On top of this, additives are used to enhance the processing and performance of paper. While many of these are safe, the most concerning are fluorocarbon additions such as polyfluoroalkyl substances (PFAS), which are widely used with pulp to provide a cheap solution to achieve oil and water barrier properties, such as is useful for food packaging and beverage cups. Also known as ‘forever chemicals’, these hazardous ingredients are found in many industrial materials and applications, and do not degrade or break down – they are shown to be extremely persistent in both the environment and in the human body. In fact, it can already be found in the blood of people and animals all over the world, and small amounts in some food products. There are many different types of PFAS and the most commonly studied are perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) – also known as C8, for their eight-carbon chain structure. These two in particular, with their long chains, are considered so dangerous that production has now been largely phased out in the US and there is a trend among global manufacturers to replace them with shorter chain PFAS or non-PFAS products. The eye-opening documentary, The Devil We Know, looks at allegations of health hazards from these chemicals, in particular around the Dupont Teflon manufacturing facility in West Virginia, US.

While paper is renewable, recyclable and compostable, a great deal of energy and water is required to extract it from wood and plants and convert it into high quality sheets and webs (rolls). There are three main types of pulping process: thermo-mechanical (TMP), chemi-thermomechanical (CTMP) and chemical. The aim of the process is to liberate the cellulose fibres from the woody matrix (lignin). This breaks down the outer walls of the fibres and so reduces their strength and stiffness. Once removed, the lignin is typically used as a source of energy, to power the process. Alternatively, it may be converted into a type of bioplastic, such as injection mouldable Arboform.

Thermo-mechanical pulp (TMP) uses force and heat to breakdown the wood and obtain pulp. Heating up the wet pulp helps soften the lignin. It is the most efficient and cost-effective way to produce pulp, with relatively high yield. The lignin is not removed fully, which means the paper will become brittle and turn yellow over time. Therefore, it is only suitable for non-archival applications like newsprint and engineered wood.

Chemi-thermomechanical pulp (CTMP), also called semi-chemical, consists of a two-stage process. The lignin is first softened with chemicals (sulphite or cold soda) and then the wood is ground by mechanical means. It yields a high quality pulp suitable for tissue, for example, but the yield is a little lower than for the thermo-mechanical method. The pulp is suitable for applications like corrugated card.

Chemical pulping includes sulphate (kraft), sulfite, and soda methods. It yields the strongest fibre with the least lignin. The most common is sulphate, so called because the wood chips are cooked in sodium sulphite (Na2S) and sodium hydroxide (NaOH) solution (white liquor). This dissolves the lignin, allowing it to be entirely removed. As a result, the yield is only around 50%, but the removal of lignin means it is suitable for archival applications and the highest quality print and packaging.

Before being formed into paper the pulp may pass through several stages of chemical and mechanical refinement, such as to produce brighter, whiter pulp. Applications of pulp outside paper making include moulded pulp, insulation, engineered wood (MDF and hardboard), plastic reinforcement and starch composite packaging.

Many paper products contain a mix of recycled and virgin fibre, with some types 100% recycled as standard, such as greyboard. The amount of recycled fibre will affect environmental impact, with predominantly recycled paper and board averaging 40% less carbon footprint (kgCO2e/kg) and 35% embodied energy (MJ/kg).

Virgin fibre comes predominantly from softwood trees, such as spruce. Depending on the location, many other cellulose fibre yielding plants are used including hardwood, eucalyptus, flax (linen), cotton, jute and kenaf. These may also be recycled, such as from old clothes.

Print
PAP
Printing and book paper, 50-120 g/m2

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Sustainability concerns
Raw material generates polluting by-products


Printing paper specification depends on the process. They may be uncoated and referred to as machine finished or supercalendered depending, on the finish applied during production. Uncoated paper is popular for pages with a lot of text, because it yields a natural matte finish that is easier to read. It is more porous than coated paper and allows for better ink absorption. Whereas a novel may be printed on 80-100 g/m2 stock, children’s books tend to be printed on heavier stock, around 120-170 g/m2.

Paper coated with mineral, such a China clay (kaolin), known as machine-finished coated (MFC), has improved smoothness, gloss and print quality. This is particularly useful for enhancing image quality.

Lightweight coated paper (LWC), typically 40-75 g/m2, also known as bible paper, is thin high quality print paper used for books, magazines and brochures, for example. Medium- and heavyweight (MWC and HWC) papers provide a more substantial substrate for colour printing and magazines. Art paper, 120-150 g/m2 is a high-quality paper used for art books, text books and cookery books, for example.

Bank paper and bond paper, also called copy paper, are high quality print and letterhead papers, such as used for regular A4 colour printing in the home and office. They have good stiffness and strength, bright white and clean appearance. Bank paper is 50 g/m2, or less, and bond paper is >50 g/m2. It is made with chemically refined pulp to produce an acid-free stock, and sometimes mixed with cotton for improved strength and smoothness. It is available in a range of colours.


Design properties
Cost usd/kg
0.6-1
Embodied energy MJ/kg
13-61
Carbon footprint kgCO2e/kg
0.4-4
Density kg/m3
720-750
Tensile modulus GPa
1-5
Tensile strength MPa
15-30
Hardness Mohs
1
Thermal expansion (µm/m)/ºC
10-18
Thermal conductivity W/mK
0.05
Temperature min-max °C
-40 to 150
Thermal
insulator
Electrical
insulator