Papermaking

 

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Introduction

Paper is used for writing and printing, for wrapping and packaging, and for a variety of other applications ranging from kitchen towels to the manufacture of building materials. In modern times, paper has become a basic material, commonly found in almost all parts of the world. The development of machinery for its production in large quantities has been a significant factor in the increase in literacy and the raising of educational levels of people throughout the world.

The basic process of making paper has not changed in more than 2,000 years. It involves two stages: the breaking up of raw material (which contains cellulose* fibre) in water to form a pulp (i.e. a suspension of fibres*), and the formation of sheet paper by spreading this suspension on a porous surface, and drying, often under pressure.

Records suggest that paper was first made in China around AD 105. The technology was practised solely in China for the subsequent 500 years, and then spread to Japan in 610, and later into Central Asia. It appeared in Egypt about AD 800, but was not manufactured there for another 100 years. A variety of raw materials were used and these included mulberry bark, old rags and hemp.

Paper was introduced to Europe by the Moors on their invasion of the Iberian Peninsula, and the first mills were established in Spain in about 1150. The craft then spread into most of the rest of Europe during the next two centuries. The development of printing technology in the 15th century saw the start of the widespread publication of books and this greatly stimulated the paper-making industry. The first paper mill in England was established in 1495, and the first such mill in North America in 1690. A crisis arose in the early 19th century as raw material for paper production was in shortage. European papermakers had grown used to using rags for paper manufacture and the shortage forced manufacturers to seek alternative raw materials. This gave rise to the use of wood for paper manufacture. The solution to the problem of a cheap, readily available, raw material for paper making was achieved by the introduction of the groundwood* process of pulp making about 1840. At the same time, technological advances in paper making technology were taking place with the development of the first practical papermaking machine by Nicolas Louis Robert in 1798. Later, this machine was further improved, and put into manufacture, by the British brothers, Henry and Sealy Fourdrinier, their name still being closely associated with paper-making machinery today.

Today's paper-making plants are capable of producing 1,000 tonnes of pulp and paper per day, using sophisticated technology based on chemical as well as mechanical processes for reducing raw materials to pulp. Modern day raw materials are many and varied, depending on availability, cost, geographical location, etc. These are covered more fully in a later chapter.

Hand paper-making has enjoyed a major revival over the past 30 years, using new and innovative approaches to this ancient craft. Handmade paper has a unique texture and an individual quality that makes it not only a surface to write, paint, or print on, but an object of beauty in its own right. In addition, the versatility of paper in its wet form has led artists to experiment with paper-making as an art medium, creating two- and three-dimensional images of textural richness and diversity, some on a vast scale. This Technical Brief aims to cover only the area of small-scale papermaking technologies for application in developing countries.

For this purpose we will define scale in paper-making as shown in Table 1 below.

Typically, a hand-made paper producer will manufacture only a few tonnes of paper per year (depending on the number of employees) often for a highly specialised market. Mechanised plants, on the other hand, only become economically feasible when dealing with an output above several tonnes per day. In India, where paper making machinery is manufactured indigenously, and hence costs are kept lower, mechanised paper making on a small scale is very common, with plants operating at outputs of 5 tonnes per day and upwards.

Paper making in developing countries

Statistics show that there is a strong link between the per capita income of a country and the amount of paper consumed. Whilst in the industrialised countries of the world consumption can be as high as 300 kg per capita per year, in some of the world's poorest nations this figure can be as low as 1 kg, and rarely exceeds 15kg per capita per year in the developing world. Illiteracy is also closely associated with low levels of paper consumption, as few books or newspapers are available and schools lack basic resources. As per capital income grows, and society demands higher rates of literacy, so the demand for paper grows. Only with indigenous manufacturing capacity and locally sourced raw materials can this demand be met at a reasonable cost, avoiding import taxes, high purchase prices and loss of valuable foreign exchange.

Technically, there are several ways of meeting this demand. Large-scale paper making plants are one solution, but these larger plants often fail to meet the broader socio-economic requirements of developing nations. Smaller mills provide higher levels of employment, not only in the mill, but amongst associated industries, such as waste paper collection and machinery manufacture. Smaller mills are more flexible in their acceptance of raw materials. With the growing concerns over deforestation and natural resource depletion in many parts of Asia, Africa and South America, the use of agricultural residues such as baggase, wheat or rice straw for paper production, is often a necessity. The product range is also more flexible in small paper making plants, with the ability to cater for a variety of demands, albeit, sometimes, with a slightly lower quality than that of the larger dedicated plant.

With many governments now opting for rural and regional development as a model for their country's growth, it is becoming more popular to assist in the development of small-scale industries in the regions. Paper making is an ideal example of how small industry can be developed to make of use of local resources, both in terms raw materials and energy, while cutting transport costs and catering for a slowly growing local market.

The initial capital investment requirement for small-scale papermaking plant is also lower and therefore more attractive to prospective small business people with limited capital to hand. This is especially so in countries like India where machinery and equipment for manufacture is produced in-country. Government measures are often needed to support such initiatives, and where such measures are put in place small-scale industry can flourish.

Types, characteristics and physical properties of paper

Paper comes in an enormous variety of shapes, sizes, qualities, grades, colours and finishes. There are as many types of paper as there are uses for paper. Some common types of paper in production include the following:

• Printings and writings (stationery paper)
• Currency paper
• Newsprint
• Hygienic tissue paper
• Corrugated case material § Photographic paper § Light-weight coated paper for magazines
• Wrapping and packaging paper
• Paper card
• Solid board for boxes

• Weight in grams per square metre (referred to as gsm or grammage)
• Brightness / shade
• Porosity
• Thickness or calliper (measured in microns) § Smoothness / gloss § Density or bulk (a function of the previous two qualities)
• Oil-resistance
• Moisture absorption
• Tensile strength § Moisture content
• Burst
• Folding
• Optical properties

The quality of paper is often controlled by the National Standards organisation in the country concerned. It is always worth consulting these Standards well in advance if contemplating setting up a paper manufacturing facility.

Raw Materials and Additives

Raw Materials

For economic production of paper there must be a secure supply of suitable raw material at a reasonable price. Fortunately, there are many fibres which are well suited to paper making. In tropical developing countries, where wood is often in short supply there are a number of other sources of fibre, often by-products of the agriculture or textile industries. Below are some examples.

Waste paper

Compared with producing a tonne of paper from virgin wood pulp, the production of one tonne of paper from discarded paper may use half as much energy and water. It results in 74% less air pollution, saves 17 pulp trees, reduces solid waste going into landfill sites and creates 5 times more jobs (Earth Care, 1988).

Thirty four per cent of the worlds pulp is derived from reclaimed paper (WRF, 1997), and it is estimated that it could contribute 30% of the needs of developing countries.

Box 1

Additives

The paper making process

The process of making paper is based on the fact that wet cellulose fibres bind together when dried under restraint. The processing of paper usually involves the initial separation of the cellulose fibres to form a wet pulp, some form of treatment, such as beating and refining, while in the pulped state, to enhance the quality of the final product, then forming of the sheet paper by hand moulding or by paper making machine, and drying. Some further processing is often carried out before or during drying to acquire the desired finish.

It is worth remembering that paper production and the related technologies are often complex and sophisticated and only a brief overview with a few examples can be given in this document. In this section we will look at the stages involved in transforming raw materials into paper in a small-scale mill. The process is similar, whatever the raw material (or mixture of raw materials), and at whatever scale of paper production, but the complexity of the technology involved may vary considerably.

Delivery and Preparation

Depending on the size of the plant and the arrangements for procuring raw materials, deliveries will be made either by truck, or by collectors who deliver small quantities of recycled material. It is important to ensure that there is sufficient storage capacity for the raw material. This is particularly important where seasonally available raw materials, such as straw or bagasse*, are used and a large supply will have to be stored for later use. The storage requirement can be considerable. For example, a mill producing 10 tonnes of paper per day with a mixture of 70% straw pulp and 30% imported pulp, with 3 months storage capacity, will require an area of 2,000m3 and 16m high. (Small-scale paper-making, Practical Action Publishing)

Straw preparation, for example, requires that the straw be cleaned to remove dust and cut into short lengths. Bagasse, on the other hand, will have been reduced to a suitable size at the sugar mill, but the pith will need to be removed. Wood will be chipped to an appropriate size. Specialised equipment is required for this kind of preparation. The material will then usually be transported to the pulping area on a conveyor belt or by hand.

On a global scale there is a large market in pulp. The world trade in 'market pulp' is enormous and large-scale plants will often use a mixture of market pulp and pulp from locally sourced raw materials.

The pulping process

Digestion, the first stage of the pulping process, is the process of removing lignin and other components of the wood from the cellulose fibres which will be used to make paper. Lignin is the "glue" which holds the wood together; it rapidly decomposes and discolours paper if it is left in the pulp (as in newsprint, which is usually made from groundwood* pulp with little or no chemical treatment).

Mechanical pulping. Approximately 15% of the world's pulp is formed using a mechanical process (WRF 1997) whereby the raw material is broken down by attrition into its individual fibres by grinding. This process is not wholly satisfactory, as the fibre are broken into smaller pieces and relatively little lignin is released, resulting in a poor quality, 'woody' paper. The mechanical process is also energy intensive. Mechanical pulp is used for newsprint as the paper is highly absorbent and therefore soaks up ink and dries quickly.

Chemical pulping. Forty two percent of all the world's pulping capacity uses a variety chemicals as part of a high temperature cooking, or digestion, process (WRF, 1997) which breaks down the lignin, freeing the cellulose fibres. This process produces a high quality product, although the type of chemical used will determine the properties of the final product:

• Caustic soda or sodium sulphate will produce a pulp with coarse, strong fibres (known as Kraft) suitable for strong boxes.
• Ammonia or calcium sulphate will produce a finer fibre suitable for high quality printing and writing paper.

For smaller mills the chemical process is usually used, as purchase of dedicated equipment for wood grinding is available only for larger scale operations and is expensive to purchase. Furthermore, suitable types of wood for paper production are seldom available in developing countries.

The prepared stock is fed into the top of a digester and mixed with the cooking chemicals, which are called "white liquor" at this point. Digestion may be carried out on a batch or a continuous basis. For small-scale mills of up to, say, 30 t.p.d., batch cooking is preferred. Batch digesters are able to cope with a variety of stock feeds, for example straw, baggase, cotton and wood, in the same mill. As the stock and liquor move down through the digester, the lignin and other components are dissolved, and the cellulose fibres are released as pulp. After leaving the digester, the pulp is rinsed, and the spent chemicals (now known as "black liquor") are separated and recycled (see later). In a typical rotating spherical batch digester capable of handling 30 t.p.d., the complete process from filling to emptying takes approximately 5-7 hrs. (Small-scale Paper Making', ILO, 1985).

Bleaching and Refining

At this point, the "brownstock" pulp is free of lignin, but is too dark to use for most grades of paper. The next step is therefore to bleach the pulp by treating it with chlorine, chlorine dioxide, ozone, peroxide, or any of several other treatments. A typical mill uses multiple stages of bleaching, often with different treatments in each step, to produce a bright white pulp. Chlorine bleaching generally provides the best performance with the least damage to the fibres, but concerns about dioxins and other by-products have led the industry to move towards more environmentally friendly alternatives.

At this point, the individual cellulose fibres are still fairly hollow and stiff, so they must be broken down somewhat to help them stick to one another in the paper web. This is accomplished by "beating" the pulp in the refiners, vessels with a series of rotating serrated metal disks. The pulp will be beaten for various lengths of time depending on its origin and the type of paper product that will be made from it. At the end of the process, the fibres will be flattened and frayed, ready to bond together in a sheet of paper.

Forming the Sheet

Once the pulp has been bleached and refined, it is rinsed and diluted with water, and fillers such as clay or chalk may be added. In the mechanised process, this "furnish*", containing 99% water or more, is pumped into the flowbox of the paper machine. From the headbox, the furnish is dispensed through a long, narrow "slice*" onto the "wire*", a moving continuous belt of wire or plastic mesh. As it travels down the wire, much of the water drains away or is pulled away by suction from underneath. The cellulose fibres, trapped on the wire as the water drains away, adhere to one another to form the paper web. From the wire, the newly formed sheet of paper is transferred onto a cloth belt (or "felt") in the press section, where rollers squeeze out much of the remaining water. In smaller paper mills the newly formed sheet may be handled manually and stacked one layer on top of another and pressed using a hydraulic press to remove the excess moisture.

Coating, Drying, and Calendering

After leaving the press section, the sheet encounters the drying cylinders. These are large hollow metal cylinders, heated internally with steam, which dry the paper as it passes over them. The sheet will be wound up and down over many cylinders in the drying process. Between dryer sections, the paper may be coated with starch to improve the printing and strength characteristics. After another round of drying, the paper sheet is passed through a series of polished, close-stacked metal rollers known as a "calender" where it is pressed smooth. Finally, the sheet is collected on a take-up roll and removed from the paper machine.

Cutting and Packaging

In many cases, the new paper roll is simply rewound on a new core, inspected, and shipped directly to the customer. Other paper grades, however, may be further smoothed by passing them through a "supercalender" where the sheet is polished by passing between steel and hard cotton rollers (much like ironing fabric), or they may be embossed with a decorative pattern. The paper may also be cut into sheets at the mill, often by automatic equipment which accepts a roll of paper at one end and delivers packages of cut sheets at the other, already boxed and wrapped for shipping.