Pulping and Bleaching PSE 476: Lecture 171 Pulping and Bleaching PSE 476/Chem E 471 Lecture #17 Introduction to Bleaching Lecture #17 Introduction to Bleaching

Pulping and Bleaching PSE 476: Lecture 172 Agenda Brightness General Bleaching Principles »Chemistry »Process »Chemicals -Description -Advantages/Disadvantages Brightness General Bleaching Principles »Chemistry »Process »Chemicals -Description -Advantages/Disadvantages

Pulping and Bleaching PSE 476: Lecture 173 Why Bleach? Improve brightness. Improve brightness stability. Clean up pulp (impurities). »Wood based (bark, resins, sand, shives). »Process based (carbon specs, rust, rubber). »External sources based (plastics, grease, ash). Increase capacity of paper to accept printing. Remove impurities from pure cellulose (rayon). Improve brightness. Improve brightness stability. Clean up pulp (impurities). »Wood based (bark, resins, sand, shives). »Process based (carbon specs, rust, rubber). »External sources based (plastics, grease, ash). Increase capacity of paper to accept printing. Remove impurities from pure cellulose (rayon).

Pulping and Bleaching PSE 476: Lecture 174 Brightness Determination (1) Light is scattered by fibers at air/fiber interfaces Light is adsorbed by certain chemicals in the fibers (lignin) Light Transmitted Reflected Absorbed Light shinning on a sheet of paper is either transmitted, adsorbed, or reflected.

Pulping and Bleaching PSE 476: Lecture 175 Brightness Determination (2) Brightness is measurement of how much light is reflected from a sheet of paper. Whiteness does not mean brightness. »Whiteness is a physical phenomena related to how the eye views the paper. »A very white looking piece of paper may not have high brightness. »Example: blue dye added to a yellow tinged sheet of paper will give a white sheet of paper with low brightness. Brightness is measurement of how much light is reflected from a sheet of paper. Whiteness does not mean brightness. »Whiteness is a physical phenomena related to how the eye views the paper. »A very white looking piece of paper may not have high brightness. »Example: blue dye added to a yellow tinged sheet of paper will give a white sheet of paper with low brightness.

Pulping and Bleaching PSE 476: Lecture 176 Brightness Determination (3) Brightness determination method: »Light reflectance measured and compared to light reflectance from MgO. »MgO assumed to reflect 100% light. »Brightness is reported as % of MgO reflectance (85 brightness is equivalent to 85% of MgO). Variables: »Angle of light: Light is applied to sheet at 45° angle. »Wavelength: 457 nm (blue light most sensitive). Brightness determination method: »Light reflectance measured and compared to light reflectance from MgO. »MgO assumed to reflect 100% light. »Brightness is reported as % of MgO reflectance (85 brightness is equivalent to 85% of MgO). Variables: »Angle of light: Light is applied to sheet at 45° angle. »Wavelength: 457 nm (blue light most sensitive).

Pulping and Bleaching PSE 476: Lecture 177 General Principles Two types of bleaching: »Lignin removing: chemical pulps. »Lignin retaining: mechanical pulps. Bleaching is used because at a certain point in the pulping process, carbohydrate degradation becoming greater than lignin removal. »Bleaching chemicals are more selective for lignin. »Bleaching chemicals much more expensive than pulping chemicals so they are not used in pulping. Two types of bleaching: »Lignin removing: chemical pulps. »Lignin retaining: mechanical pulps. Bleaching is used because at a certain point in the pulping process, carbohydrate degradation becoming greater than lignin removal. »Bleaching chemicals are more selective for lignin. »Bleaching chemicals much more expensive than pulping chemicals so they are not used in pulping.

Pulping and Bleaching PSE 476: Lecture 178 General Principles: Chemistry Pulping »Pulping typically involves cleavage of ether linkages and some substitution (sulfonation). Bleaching »Bleaching involves attacks on aromatic rings, olefinic structures, and carbonyl groups. »Substitution reactions play a big role. Pulping »Pulping typically involves cleavage of ether linkages and some substitution (sulfonation). Bleaching »Bleaching involves attacks on aromatic rings, olefinic structures, and carbonyl groups. »Substitution reactions play a big role.

Pulping and Bleaching PSE 476: Lecture 179 General Principles: Process Bleaching uses a combination of chemicals in series. »One chemical alone will not remove residual lignin. »Each step reacts with material modified in previous step. Bleaching uses a combination of chemicals in series. »One chemical alone will not remove residual lignin. »Each step reacts with material modified in previous step. Unbleached Pulp Bleached Pulp OD EO D NaOH O2O2 ClO 2 NaOH O2O2 ClO 2

Pulping and Bleaching PSE 476: Lecture 1710 General Principles: Chemicals (1)

Pulping and Bleaching PSE 476: Lecture 1711 General Principles: Chemicals (2)

Pulping and Bleaching PSE 476: Lecture 1712 General Principles: Chemicals (3a)

Pulping and Bleaching PSE 476: Lecture 1713 General Principles: Chemicals (3b)

Pulping and Bleaching PSE 476: Lecture 1714 General Principles: Chemicals (3c)

Pulping and Bleaching PSE 476: Lecture 1715 Bleaching Generalities It is important to note that when bleaching with a specific reagent, it will be converted into a number of different reactive species which will react with lignin and carbohydrates differently. A simple example is when chlorine gas is added to water; both hypochlorous acid and/or hypochorite is formed depending on the pH.

Pulping and Bleaching PSE 476: Lecture 1716 Bleaching Generalities (2) Often, radical species are generated from the bleaching chemicals. Let us use oxygen for an example. Molecular O 2 reacts with ionized free phenolic groups generating a phenoxy radical and a superoxide radical (under alkaline conditions). The species is not terribly reactive with lignin but under a variety of other reactions can be reduced to hydroxide radical which reacts very very rapidly with lignin and carbohydrates.