At the end of this module, you will be able to:
- Identify the three major classes of furnish components
- Define and describe a "colloid"
- Describe two ways to measure charge
- Describe the effect of furnish surface area on additive uptake
- Describe coagulation and flocculation
- Identify the sources of anionic trash and describe its effect on the system
- Explain why water plays a large role in papermaking
- Describe the pH Scale
- Describe the two classes of papermaking additives
- Identify common chemical additives and their purpose
- State the importance of wet end chemistry control
The following key questions are answered in this module:
What is "wet end chemistry"?
"Wet end chemistry" involves the chemistry of and chemical interactions between the components of a papermaking furnish. The furnish contains whole fibers, fiber fines, and chemical additives in a large amount of water.
What are "colloids"?
"Colloids" are particles that do not easily settle out of a solution under the influence of gravity. They have very small particle sizes and very large surface areas, and tend to carry electrical charge on their surfaces from the presence of anionic (negatively charged) and cationic (positively charged) groups.
What are "anionic trash" and "cationic demand"?
The furnish coming to a paper machine typically contains some negatively-charged materials that are collectively called "anionic trash" or "cationic demand." These colloidal and dissolved materials can build up in the recirculating water loops and preferentially adhere to cationically-charged additives, reducing the effectiveness of the additives. They can also play a role in deposit formation, spots and defects in the sheet, and reduced sheet strength properties.
Where does "anionic trash" come from?
Anionic trash can include wood components that are dissolved or dispersed during the pulping and bleaching processes, plus defoamer, talc, and other residual chemicals from these processes. It can also be generated from pulp fibers during refining and storage, or enter with the fresh water, chemical additives, broke fiber, or recycled fiber inputs.
Why is pH important?
Consistent pH is crucial to steady paper machine operation because pH indicates the level of ions, and ions can interfere with colloidal surface charge interactions. Among other things, pH affects chemical additive efficiencies, pulp refining, water removal on the former, and dryer coating characteristics.
Below is a transcript of the video sample provided for this module:
Wet end chemistry is dominated by colloidal chemistry because of the small size of the furnished components and because most chemical interactions occur at the component surfaces. Colloids are particles that do not easily settle out of a solution under the influence of gravity. They have very small particle sizes and very large surface areas. In fact, even the fibers can be considered to be colloids due to their small diameters and large surface areas.
Colloids often carry an electrical charge on their surfaces that results from the presence of anionic, or negatively charged, and cationic, or positively charged groups. Wood fibers and fiber finds normally have negatively charged surfaces, as do many other furnished components. Because most molecular interactions in paper machine stock systems are surface-charge interactions, chemical additives must be carefully selected. The selected additives, plus their addition points and rates, must take into account the surface charge characteristics of the additives and all of the other furnished components.
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