Task 3: Bleaching

Bleaching is necessary for several reasons:

To remove residual lignin in the pulp, which not only gives the pulp a dark color, but may also causes yellowing and degradation of the paper over time

To destroy color-causing chromophore molecules in the pulp, giving it a lighter color that is more suitable for printing and writing

To attack fiber bundles called shives and dirt in the pulp, improving its cleanliness

Most chemicals used for bleaching pulp are strong oxidizers, meaning that they conduct their bleaching action on the pulp by stealing electrons from molecules in the pulp. Bleaching chemicals fall into two categories:

Chlorine-Based: Chlorine, Hypochlorite and Chlorine Dioxide

These chemicals were used exclusively by the industry for many years. However, due to restrictions on the discharge of chlorinated organic byproducts into the environment, only chlorine dioxide is used today.

Oxygen-Based: Oxygen, Hydrogen Peroxide and Ozone

These chemicals produce no chlorinated byproducts and have therefore been used in increasing amounts.

Sodium or calcium hypochlorite solutions are more commonly known as household bleach or CloroxÒ . Even though hypochlorites are not used much in industry, it will be used for this activity because they work well under simple lab conditions.

Define (a) chromophore; (b) oxidizer; (c) shives

(a) Write the formulas for the chlorine-based bleaching chemicals.

(b) Write the formulas for the oxygen-based bleaching chemicals.

Which category of bleaching agents is best to use when mking paper? Explain

Calculate the proper bleaching chemical charge by completing the following steps:

Calculate the mass of OD fiber you will use based on the amount and type of wet fiber you were given.

OD fibers = (grams wet fiber) x (OD% consistency)

 

Chemical charge is the mass of chemical required to react with the pulp. It is usually specified in terms of the mass % of reacting chemical based on the mass of the over dried, OD fiber. The chemical charge of sodium hypochlorite is 8%. Calculate the grams of sodium hypochlorite you will need based on the grams of OD fiber determined in Step a.

 

Record the % sodium hypochlorite of the CloroxÒ you are using. This information should be on the label of the CloroxÒ bottle. The % sodium hypochlorite may vary from brand to brand.

Rewrite your % sodium hypochlorite as a conversion factor

grams sodium hypochlorite

100 g of CloroxÒ

Using the factor-label method and the % sodium hypochlorite, convert the grams of sodium hypochlorite determined in Step b to grams of Clorox you will have to mass out.

 

 

 

 

 

 

 

 

 

 

Put the wet pulp you were given into the blender container. Add 750 ml (750 grams) of warm water. Place the cover on container and energize blender at the lowest setting for 5 seconds to mix water and pulp.

Weigh out the grams of CloroxÒ you need based on the mass of wet fibers you were given. Pour the CloroxÒ into the blender with the fibers. Replace the cover and energize the blender for 5 seconds. Allow the mixture to sit for 30-60 minutes, turning on the blender ever 10 minutes or so for a few seconds to promote mixing.

Wash the pulp by placing 1 paper towel in the Buchner filtering funnel. Pre-wet the towel to make it conform to the shape of the funnel. Place the funnel into the filter flask, then connect the flask to the aspirator or other vacuum source. Turn on the vacuum. Slowly pour the contents of the blender container into the funnel. Liquid will start leaving the pulp immediately. When the liquid level in the filter flask reaches a point near the vacuum take-off, stop pouring into the funnel. Turn off the vacuum and remove the funnel, being careful not to dump out the contents. Pour the flask contents into the sink. Replace the funnel, turn the vacuum back on, and resume pouring in material from the blender container.

When all the material has been poured form the blender container, allow all the liquid to be pulled from the pulp, forming a thick mat. Fill the blender container with hot tap water to the 1-liter mark. Pour this water on top of the pulp mat in the funnel slowly, allowing the vacuum to pull it through. Turn off the vacuum and discard liquid as necessary.

Repeat Step 5 using another liter of hot tap water. The pulp should now be thoroughly washed.

Allow the pulp mat to drain until no liquid is noted coming out of the funnel. The mat will be slightly mushy to the touch. Turn off the vacuum and remove the funnel from the flask. Remove the paper towel from the funnel and lay flat. Separate the pulp mat from the paper towel, rolling the pulp ball across the towel to capture remaining fibers.

Place a sample of the bleached pulp next to a sample of the unbleached pulp to compare the brightness and color. Record your observations in the Observations Table.

Place the wet pulp in a sealed plastic bag to keep it from drying out. Label with the raw material type, pulping process used, and bleaching chemical type and charge (8% hypochlorite).