Wet End Chemistry – Functional Papermaking Additives
Chemical additives are used in the papermaking process for many different reasons, and they are often divided into two groups. Functional chemicals directly improve properties of the sheet, while process chemicals affect operations on or near the paper machine. This module covers the first group – functional chemicals. It discusses the purpose of some of the more common functional additives used on paper and board machines, and describes how they work.
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Learning Objectives
• Describe the purpose and composition of papermaking furnishes • Differentiate between the two classes of papermaking additives • Identify strength properties of paper • Identify the variables that affect the dry strength characteristics of paper • Describe the purpose and function of common functional additives, including: dry strength additives, wet strength additives, internal sizing agents, fillers and dyes
Specs
| Course Level | Intermediate |
| Languages | English, Portuguese, French, Polish |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
Most chemical additives are delivered as “”aqueous”” solutions. What does that mean?
The chemical is dissolved or dispersed in water, and delivered as a liquid in a tote or tanker truck
What is the difference between the two main types of additives – functional and process?
Functional additives are added to impart or enhance a property of the sheet, while process additives are added to improve operations or the runability of the paper machine
What are the different ways that the dry strength of a sheet can be improved?
In general, dry strength can be increased by using longer fibers, less filler, more wet pressing (higher nip pressures or longer dwell times), or more refining. It can also be improved by the addition of dry strength additives.
How do wet strength additives work?
When paper gets wet, the fiber-to-fiber bonds are replaced by water-fiber bonds and the sheet falls apart. Wet strength additives work by creating additional water-resistant bonds between the fibers.
What kinds of additives are used to change the color of a sheet?
Acid dyes, direct dyes, basic dyes, and colored pigments can all be used to change the color of a sheet of paper or board
Sample Video Transcript
Dry strength additives increase sheet strength by increasing the strength and/or the number of fiber-to-fiber bonds in the sheet. Dry strength additives are natural or synthetic polymers that are added when refining cannot achieve the required dry strength levels. They also function as retention and drainage aids due to their bonding abilities with fibers and fiber fines. The most widely used dry strength additives are based on starch, mostly corn starch in North America and Western Europe. Starch is a carbohydrate, a polymer of glucose that is produced and used by all plants to store energy. The starch is often modified by heat or chemicals before it is delivered to the mill as a dry powder, granules, or flakes. At the mill, the dry starch is mixed with water and cooked with steam before it is diluted and added to the stock. The type of starch used and the addition point must take into account the other furnished components, including other additives. Synthetic polymers can also be used as dry strength additives. Anionic and cationic polyacrylamides are the most common, and they are sometimes used in combination with starch. Guar gum, carboxymethyl cellulose, and methyl cellulose can also be used, but they tend to be more expensive.
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