Paper fibre chemically consists of cellulose and hemicellulose, which contain hydroxyl groups. These hydroxyl groups have a strong and natural tendency to interact with water. Therefore some chemical treatment is required for papers that inhibit the absorption of liquid and the process is known as sizing. Therefore sizing may be defined as a process of making fibres and fillers hydrophobic with the aim to control liquid-paper interactions in order to introduce liquid barriers, enhance wet strength properties or to improve paper printability by controlling water or other liquid spreading and absorption. The word "sizing" as it applies to paper products originates from the French word sise, which literally means to "set on" or "fix". Chemicals that are used in the process of sizing are known as sizing agents. The process of sizing can be accomplished in two principal ways:

• Internal sizing;
• Surface sizing.

Internal sizing agents have hydrophobic groups, which repel the water molecule to be absorbed. These agents retard the rate of penetration of a fluid, usually water, through capillaries formed both within and between fibres. Retardation is thus brought about by the creation of a low energy hydrophobic surface at the fibre-water interface which increases the contact angle formed between a drop of liquid and the surface and thus decreases the wettability. So, in the papermaking process internal sizing plays an important role in controlling the absorption and penetration of liquid such as water and ink etc. into the paper, paper board and sheet material. The purpose of internal sizing is to inhibit penetration of liquids into the internal structure of paper. If the liquid is polar, then the sizing is accomplished by converting the hydrophilic fibre surface to a non-polar, hydrophobic surface by covering it in a controlled manner with a water repellent material. The introduction of hydrophobic groups onto the fibre surface has to be done in a way which will allow a good distribution of the sizing molecule through the body of the sheet but which will not interfere with interfibre bonding. Thus internal sizes are introduced at the wet end of the papermaking system, usually as colloidal suspensions, which are retained in the fibre network during sheet formation. Adequate sizing maintains consistent runnability through the size press and helps to provide uniform printability in uncoated papers. For coated paper basestock, adequate sizing helps to control binder migration. Many factors influence the degree to which paper will resist penetration by liquids. These may include such paper properties as porosity and hydrophobicity.

Internal sizing agents have very little effect on the porosity of paper, therefore their contribution to the pore radius is negligible. Their major effect is on the contact angle at the liquid/paper interface. Pulp fibres are extremely hydrophilic and unsized paper has a contact angle with water approaching zero. Therefore, penetration of water into unsized paper occurs rapidly. With the use of sizing agents, their hydrophobic characteristic increases the contact angle and the rate of water penetration is decreased. At a contact angle of 90° or greater, no penetration occurs unless external pressure is applied to force the liquid into the pores. At a given contact angle, the pore size of the paper becomes an important factor. Decreasing the pore size to give a less porous, closed sheet tends to increase sizing by hindering the rate of liquid transport.

Davison has compiled a list of practical requirements for a useful sizing agent. Further, Strazdins and Dumas reported that for an internal sizing system to be effective, it must meet all of the following requirements:

• The sizing agent must be highly hydrophobic i.e. the sizing agent must produce hydrophobic fibre surfaces having low wettability by aqueous liquids; thus, the initial contact angle between a liquid and the treated paper must be greater than 90° and, preferably, remain constant.
• A practical method for putting the agent on fibre surfaces must be available: for internal sizing, good adsorption of the size onto pulp surfaces in the slurry and its subsequent retention in the formed sheet are required. The size must be well-distributed over all the fibre surfaces, to ensure adequate water repellency throughout the paper structure.
• The sizing agent and its anchoring system must be retained to the fibre surface of the paper or become permanently immobilised in some other way. If amphipathic, the size molecules must become favourably oriented with their polar groups pointing inward toward the cellulose substrate. Therefore the sizing agent must have good coverage on the fibre surface.
• The sizing agent and its anchor must be chemically inert to the liquid.
• The sized surfaces must have a high degree of chemical inertness toward the impinging aqueous liquid. Ideally, the sizing agent should have no adverse effects on either the papermaking process or paper product properties.

While the hydrophobicity and the distribution of size on fibres are more related to the inherent properties of the sizing agent, mordants are usually required to retain and anchor the size molecules onto fibre surfaces. Energetic analysis done by Zhuang and Biermann of the rosin sizing process leads one to believe that rosin soap size must be anchored to the fibre surfaces through primary chemical forces such as covalent bonds (dative bonds, which are also called co-ordinate covalent bonds, are a specific type of covalent bonds) to effect sizing; the stronger the bonding between rosin size and fibres, the better the sizing can be. Secondary chemical forces such as hydrogen bonding or Vander Waals forces are not strong enough to resist the attack of polar water molecules, so are not effective bonding types. The electrostatic characteristics of alum under various conditions may be important for the retention of rosin size, but should have little effect on the mechanism of sizing development.

Most of the papers are manufactured with internal sizing agents added to the wet end so that the paper resists water absorption. Internal sizing agents modify the surface energy of the paper as opposed to agents that work by plugging the voids in paper (for example, with starch in a size press). Internal sizing agents are hydrophobic substances, which must attach to the paper (through the use of an appropriate functional group) to present a hydrophobic surface. Based upon the pH, the internal sizing can be categorised into three types:

• Acid sizing;
• Neutral sizing;
• Alkaline sizing.

As the name indicates, acid sizing is done at acidic pH while neutral/alkaline sizing is done at neutral/alkaline pH.

At different pH, different internal sizing agents are used. The three most common internal sizing agents currently being used are rosin systems, alkene ketene dimers (AKD), alkenyl succinic anhydrides (ASA). Table 1 presents the global market of sizing agent (BSF) and Table 2 presents the European market (BSF).

A papermaker chooses the appropriate sizing agent by observing the processing conditions in the wet end of the paper machine and the particular grade being produced. For paper being made with acidic conditions in the wet end, a papermaker would choose to use a rosin size as an internal sizing agent. In the past, mills used to prepare their own rosin size. But now days, ready- made fortified rosin, either in powder or liquid form, is available for use. In acid sizing, the alum chemistry plays a very important and critical role in imparting sizing property to the paper/board. At low pH (< 4.0) the aluminum exists as a soluble ionic species, but as the pH is raised, a colloidal hydroxylated alum floc forms. At higher pH values (> 7.0), the alum flocs tend to redissolve and soluble aluminate is formed. As alum floc is a cationic material, it has affinity for anions such as anionic rosin particles, and fibre, fines and fillers. Thus, alum serves as a precipitating and anchoring agent for the size and also serves as a retention aid for fines fillers, and dyes, improves drainage etc. Extensive studies have been carried out on rosin sizing at neutral/alkaline pH, which include substitution of alum by other chemicals or with other retention aids. For effective sizing, alum may be replaced with PAC which increases the efficiency of rosin sizing significantly at pH greater than 5.5. Some polyaluminium phosphate sulphate compounds can get good sizing results using rosin size at neutral/alkaline pH. Similarly polyamines are known to improve rosin-sizing efficiency at neutral/alkaline pH because of their large charge densities in the neutral region and high molecular weight.

For neutral/alkaline paper machines, a papermaker would prefer to use a synthetic sizing agents like AKD and ASA. In contrast with the rosin-alum complex, which agglomerates on the surface of the fibre, these compounds actually react with the exposed hydroxyl group present in the fibre via a chemical bond and give rise to a very hard and permanent sizing. AKD is used as sizing agent in pH range of 6-9 i.e. both in neutral and alkaline media. Internal sizing at neutral/alkaline pH gives the possibility to use calcium carbonate as a filler. Another benefit that neutral/alkaline sizing gives is that the paper gains an improved storage durability compared to rosin-sized paper e.g. archive papers. ASA is another type of cellulose-reactive sizing agent that has found wide domestic acceptance in recent years due to its advantages. ASA is preferred for many grades because of its on-machine sizing and size press hold-out. Sizing with ASA is typically 80-100 per cent developed on the machine, and paper can immediately be converted and shipped. ASA is found to be more reactive than AKD. ASA is typically used as an internal sizing agent for alkaline papermaking in the pH range of 7-9 with good sizing efficiency. The ASA are quite reactive molecules and can promote sizing without heating. Therefore, alkaline papermaking with an ASA size offers the papermaker an opportunity for enormous saving in raw materials and production cost. AKD use is more common in Europe while in the US the application of ASA is growing and today surpasses that of AKD.

While alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) have dominated the sizing of commercial papers under alkaline conditions for many years, they are by no means the only chemical compounds capable of sizing paper in this pH range. Several other compounds have been investigated for alkaline paper sizing and found to be successful at least on a laboratory scale. Some of these alternative chemistries include various forms of acid chlorides, acid anhydrides, enol esters, alkyl isocyanates and rosin anhydrides. Polymers and copolymers of styrene and acrylates exhibit great promise as both an internal and surface sizing agent and offer some interesting effects from economic and technical aspects. The use of these products makes it possible to create a hydrophilic/hydrophobic matrix on the paper surface that improves copy, inkjet, laser jet and offset printing results.

The use of stearic acid as an internal sizing agent is limited to specialty papers like photographic papers. The method of size preparation and application is similar to rosin sizing. Stearic acid provides excellent sizing resistance against liquids and, in contrast to rosin, sizing it provides excellent brightness and whiteness stability. The use of stearic acid is restricted due to its relatively higher cost.

Fluorinated hydrocarbon sizes are another specialty group of products; they are used to provide certain paper products (e.g. paper plates) with oil and grease resistance, as well as water repellency. These fluorochemicals provide an exceedingly low energy surface, one that is wetted only to a limited extent by hydrophobic oily liquids. The size is added internally at the 0.2-0.3 per cent addition level, together with a suitable retention aid.

Many other materials have been also proposed as sizing agents for paper. A number of them have been patented, and some are now commercial products. Asphalt is the most commonly used of these materials. Asphalt particle suspensions, stabilised by fatty acid or resin acid soaps, have been used for many years to size structural paper-board products. Here, colour is not of concern, and large amounts (5-10 per cent) of asphalt-based size are added internally, together with alum as the retention aid. Fatty acid soap sizes are used only for special applications in the paper industry, such as sizing photographic papers against wetting and penetration by alcohol-water mixtures. Bis-stearamide sizes are the product obtained by reacting 2 mol of a fatty acid with 1 mol of a polyamine, such as diethylenetriamine. The water-insoluble product is available as an aqueous suspension of particles. This type of size can either be used alone or in conjunction with AKD size.

Because of the very hydrophilic nature of cellulose fibre surfaces, a reasonable amount of water repellency can be developed by sizing paper with a variety of relatively hydrophobic materials. Thus, various hydrocarbon resins have been evaluated, such as terpene resins, resinous polymers obtained from hydrocarbon cracking, and naphthenic derivatives, hydrophobic poly-functional amines, and copolymers of various hydrophobic and hydrophilic monomers.

There seems to be some advantage to be gained in some grades of paper by using mixed sizing systems. Liquid packaging board for aseptic packaging systems have to meet a rigorous high-temperature hydrogen peroxide edgewick specification and must also be resistant to lactic acid and other aggressive low surface energy penetrants. A combined sizing system of AKD and a cationic rosin size used in conjunction with alum gives a synergistic improvement when used in these types of board. Combinations of AKD and ASA when used in full, or pilot scale production of fine paper containing precipitated calcium carbonate, have also been shown to give a better sizing response, less size reversion, and allowed the use of higher filler levels with higher specific surface area. There was also less migration, higher frictional resistance and better inkjet printability.

Surface sizing is an ancient operation as old as papermaking itself. Surface sizing includes the application of some hydrophobic chemicals to the surface of already formed paper or board at the appropriate location at the dry end of the paper machine. The principal surface-sizing agent used commercially is starch. Others may include methylcellulose, polyvinyl alcohol, animal glue, carboxymethyl cellulose, wax chemicals and certain resin polymers that are sometimes used to obtain special effects. In surface sizing it is desirable to keep the size on the surface of the paper as much as possible. To meet certain specifications the polymer dispersion may be used as surface sizing agents with small amounts of internal sizing agent. These are used for controlling surface porosity for printing, decreased surface fuzz, improving surface finish, increasing certain strength properties such as tensile strength, folding endurance and erasure resistance and in improving surface picking resistance.