Paper and cardboard are very common materials that are all around us all the time, and are used in a multitude of objects. They are also natural and easily recyclable.
Paper and its derivatives is a natural material that consists of a thin sheet made from cellulose made from a pulp of vegetable wood fibres. It is called virgin fibre when used for the first time and when used after recycling it is called recycled fibre. It is the same fibre in different stages. These cellulose fibres are then ground suspended in bleached water and then dried and hardened. Substances such as polyethylene or polypropylene are usually added to give it special characteristics. The mixture is then placed on a long belt that runs on rollers. The water is then removed using different processes: gravity, vacuum, pressure and drying. The result is a large sheet of paper wound on a reel.
Cardboard is a material produced by bonding several layers of paper pulp together. These layers are glued together with moisture and then compressed and dried by evaporation. As it dries, the paperboard becomes consistent. Because it has many layers, it is stronger, thicker and harder than paper. For this reason, this material is used in the manufacture of boxes, packaging, containers... and is also used to protect certain products. The cardboard used for writing or printing is a thinner cardboard. This material is relatively inexpensive to manufacture and therefore its use is very common, especially for packaging and transporting products, although it is beginning to be used for other purposes.
Characteristics of paper and board
The properties defined below are basic properties of paper and paperboard.
Calibre or thickness
Thickness determines how bulky or dense the paper is. It is measured with a micrometer as the perpendicular distance between two sheets of paper under a pressure of 1 kg/cm2. Variations in gauge can affect several basic properties, such as strength, optics and roll quality.
Basis weight or grammage
This is the fundamental property of paper and board. The basis weight of paper is the weight per unit area. It is usually expressed as the weight in grams per square metre.
The main component of paper, cellulose fibres, increase in diameter by 15-20% from their dry condition to the point of moisture saturation. Moisture absorption can cause changes in size that seriously affect printing processes.
Friction is the resistance force that occurs between two paper or board surfaces in contact. This property is measured as a coefficient, which is the ratio of the frictional force to the force acting perpendicular to the two surfaces.
Almost all types of paper have some percentage of moisture. In this material, the moisture content varies between 2 and 12%, depending on the relative humidity, the pulp used, the degree of refining and the chemical used.
Formation is an indicator of how evenly the fibres and fillers are distributed in the sheet. This characteristic plays an important role as most of the paper properties depend on it.
Other properties can be analysed, e.g. optical properties, where colour, gloss, finish, fluorescence or opacity come in. We can also measure some of the strength properties of paper or board, such as tear strength or compressibility.
Paper and cardboard recycling
Both paper and cardboard waste are recyclable, but the number of times they can be recycled is limited as in each cycle there is a portion of the fibre that becomes unusable. Once used, they are collected for recycling by municipal separate collection or by private operators in large supermarkets. These materials go to the recycling sector warehouses and there they receive the necessary treatment where they are sorted, conditioned and baled. The last step would be the paper mills. They buy these materials and recycle them to use them as raw material to make recycled paper and cardboard. In the case of paper, it is mixed with water in the pulper and shaken. The paper fibres are then separated from plastics and other possible materials and are fed into the paper machine to become paper again.
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