PULPING: PLANTS INTO FIBERS

PLANTS AND PULPING

All paper is made from plant materials. More specifically, paper consists of a mat of fibers obtained from plant materials. These fibers are actually the conducting vessels or elongated cells which transport water from the roots of the plant into the leaves and distribute photosynthesis products throughout the plant.

Raw plants are generally not in a form ready for being made into paper. The plant fibers used to make paper are made up mainly of cellulose, a polymer of connected glucose molecules generated during photosynthesis. Cellulose has good tensile strength, but it is soft and flexible in nature. If a tree or plant consisted only of cellulose, it would not have enough stiffness or strength to stand upright. So, as the cellulosic conducting fibers are being manufactured, the plant generates another polymer called lignin and encrusts it inside of and in between the fibers. Lignin is much like an epoxy, with a huge molecular weight, random structure, and an enormous capacity to cross-link with other sites on the lignin molecule. The combination of cellulose and lignin makes wood a lightweight but extremely tough and durable material. For the papermaker, however, this structure presents a problem – how to get rid of the lignin and liberate the cellulosic fibers.

PULPING is the process of converting plant-based raw materials, such as wood or straw, into papermaking fibers. There are two basic methods of pulping:

· Mechanical Pulping – uses only mechanical force (and perhaps some heat energy) to pull the raw material apart into fibers. No lignin is removed. The resulting pulp is relatively crude, with a large content of damaged fibers.

· Chemical Pulping – uses heat and chemical energy to preferentially dissolve the lignin from the plant material, leaving only pure cellulosic fibers.

BLEACHING is a process used to do two things to the material produced from the pulping process:

· Remove the small amount of lignin remaining in the pulp.

· Chemically attack molecules in the pulp, which cause dark coloration.

In general, all of the lignin in a plant-based raw material cannot be removed by pulping. As the lignin content decreases, the pulping chemicals begin to attack the cellulose. Cellulose molecules are naturally white and do not need to be bleached. At some point, it is necessary to stop the pulping process and move on to the bleaching process. Bleaching chemicals tend to be more selective in their attack, preferring lignin. However, bleaching chemicals also are more expensive than pulping chemicals, so the determination of when to stop pulping and start bleaching is critical. It may seem strange, but after the pulping process the pulp is often much darker in color than the original raw material. This darkening is usually due to the fact that the chemical attack on the lignin causes some of the remaining lignin molecules to be modified in a way that absorbs light in the wavelengths visible to the human eye. These chromophores must be chemically attacked by bleaching chemicals in order to convert them back into colorless form.

REFINING

Even when liberated during pulping, plant fibers are not in a form most suitable for papermaking. The fibers are still hollow conducting tubes, which limits the surface area available for bonding to others fibers.

Illustration: Take a common plastic drinking straw, place it next to another. The point of contact is limited to a thin line between the two. Even when a number of straws are placed around the original straw, there is a significant amount of surface area on the straw that does not contact another straw.

If the straw could be collapsed into a flat ribbon, with no open area in the middle, the entire periphery of the fiber would be available for bonding.

Refining is an intense mechanical process carried out on pulp to ready the fibers for papermaking. Strong paper cannot be made without this treatment. Refining actually performs two key tasks to increase fiber-to-fiber bonding:

· Collapses the fibers into flat ribbons

· Shreds the outer layer of the fibers, called fibrillation, making it hairy-looking and producing more surface area for hydrogen-bonding

In the paper industry, refining is carried out by passing the pulp between grooved plates rotating at high speed. In the laboratory, a very similar action can be achieved using a common household blender.

Questions

1. What parts of plants are used to create the fibers to make paper?

2. What role does lignin play in plant structure?

3. Distinguish between mechanical and chemical pulping.

4. Why will chemical pulping be used in this activity to produce paper?

5. What is the purpose of bleaching pulp?

6. Why is it critical to determine when to stop the pulp process and begin the bleaching process?

7. What are chromophores? Describe how a chromophore’s properties are altered and the results of the alteration.

8. Using drinking straws, illustrate plant fibers before and after refining. This illustration must be permanently preserved in some manner. Provide a brief explanation to accompany for the illustration.

9. How will you refine plant fibers in the laboratory?