With the correct use of sizing agents, you can impart unique desirable qualities into your papers. The primary role of sizing agents is to enhance the resistance against the penetration of liquids such as water. That way, the paper becomes ready for writing, printing or any other purpose. To achieve this, manufacturers can use wet-end additives. Along with that, dried paper is coated on the surface with the best-suited agents. The use of sizing agents provides better control over the spreading of ink. Also, the paper accumulates a lesser amount of dust, becomes smoother and retains its fibers and fillers.
Sizing agents are divided into two major categories- Internal sizing agents and external sizing agents.
During the papermaking process, paper wet-end obtains all the desired traits with internal sizing agents. The chemicals used in internal sizing are called Alkyl Succinic Anhydride (ASA), Alkyl Ketene Dimer (AKD), and Rosin. As internal sizing agents, the mentioned chemicals showcase the following characteristics-
- High hydrophobicity
- Uniform distribution over the fiber surface
- Strong bonding with fibers
- Good fiber retention
The classification of internal sizing agents is also called acidic, neutral or basic type sizing. According to that, Rosin along with all its derivatives is called an acidic type agent. And, ASA and AKD are neutral or basic type agents.
Rosin as a sizing agent
Rosin, an acidic type agent used in paper sizing, is easy and cost-effective to produce. Raw materials of regenerative quality are used to produce various derivatives of Rosin. The cheap and easy production makes Rosin a sought-after agent for wet-end sizing.
The biggest advantage of Rosin is the ease of controlling the extent of sizing. Another benefit of this sizing chemical is its ability to provide fully cured sizing effects the moment your paper comes out of the paper machine.
In Rosin Sizing, the agent utilizes aluminum resinate as its active component. After attaining the reagent from rosin, the chemical poly aluminum chloride or aluminum sulfate is used. The overall sizing effect is obtained by creating electrostatic interactions on the cellulosic surface.
AKD as a sizing agent
Another common sizing agent is AKD or Alkyl Ketene Dimer that comes within the category of alkaline or neutral agents. The structure of AKD contains unsaturated lactones, which are obtained by conducting a synthesis process on fatty acids. When you touch it, the chemical feels like waxy and milky solid. It is mixed with water, which allowed the dispersion of tiny solid particles. The emulsification gets stabilized with the use of some type of cationic starch.
In the selection of filler material, calcium carbonate seems highly accurate due to its reasonable price and availability. However, this material acts in an unstable manner in any acidic medium. This is why the use of AKD as alkaline or neutral sizing agent has become popular.
AKD sizing improves the surface qualities of paper. It also reduces the chances of machine breakdown. As compared to an acidic agent, this one presents lesser difficulties in water recycling. However, the action of this sizing agent relies on a number of technological parameters like the drying method, time, temperature and raw material PH value.
ASA as a sizing agent
ASA or Alkenyl Succinic Anhydride also comes within the category of neutral or basic type of sizing agents. This feels like wax when you touch and provides effective results in internal sizing. The chemical structure of ASA includes a linear chain of unsaturated type along with an anhydride ring of five members. Maleic anhydride and Alfa-olefins are used to synthesize this agent. The anhydride rings contained in ASA take no time to act in a chemical reaction. It is widely used as an alkaline or neutral agent to stabilize the filler material calcium carbonate.
With ASA Sizing, the paper machine attains better operational capacity. It also preserves the dimensional stability of paper and increases the resistance against water penetration. The ASA agent also maintains paper’s surface conditions, reduces dust accumulation and protects the machine from breaking down.
In all types of sizing agents, different technological parameters act such as the storage temperature, exposure time, humidity and others. Hence, choosing the right manufacturer and following their instructions should be two major priorities for you.