Commercial hardwood and softwood kraft and sulfite pulps

[17]. Fiber dimensions determined using Kajaani FS200.

Tab. 11.3 Fiber dimensions of fully bleached commercial paper

pulps, as determined with Kajaani FS200 [17].

Parameter Pine Spruce Eucalyptus Beech Spruce Spruce Beech

Kraft Kraft Sulfite Sulfite

ECF TCF ECF ECF TCF ECF TCF

Average fiber length

Arithmetic mm 0.90 1.04 0.55 n.d. 0.79 0.52 0.28

Length-weighted mm 2.16 2.36 0.72 n.d. 2.15 1.89 0.66

Weight-weighted mm 2.82 3.01 0.91 n.d. 2.93 2.91 0.86

Fines (P < 0.2 mm) wt% 3.5 2.7 2.0 n.d. 4.2 7.9 12.8

Coarseness mg m–1 0.213 0.215 0.074 n.d. 0.227 0.219 0.122

n.d. = not determined

Pulp Properties and Applications

11.3

Dissolving Grade Pulp

11.3.1

Introduction

Dissolving pulp refers to pulp of high cellulose content which is used to manufacture

Various cellulose-derived products such as regenerated fibers or films (e.g.,

Viscose, Lyocell), cellulose esters (acetates, propionates, butyrates, nitrates) and

Cellulose ethers (carboxymethyl-, ethyl-, methyl-celluloses). The wood-derived celluloses

which account for about 85–88% of the total dissolving pulp market are

Made by the prehydrolysis kraft and acid sulfite processes comprising additional

Purification stages such as hot and cold caustic extraction. The residual amount of

Dissolving pulps is based on cotton linters. The linters fibers, being attached to

The cotton seeds, are removed by the delintering process, producing fibers of different

Lengths. The shortest fibers or second-cut linters are used as chemical feedstock.

Purification is accomplished by a combination of mechanical and chemical

Steps comprising mild alkali treatment at elevated temperature to remove proteins,

Waxes, pectins and other polysaccharides and bleaching to achieve the required

Brightness level. Purified cotton linters represent the dissolving pulp of

Highest cellulose purity particularly used for manufacturing acetate plastics and

High-viscosity cellulose ethers.

When using softwoods and hardwoods as a raw material, more drastic conditions

In pulping and bleaching operations are required in order to obtain a highquality

Dissolving pulp. However, even least amounts of residual impurities such

As resins or inorganic compounds can adversely affect the filterability of viscose,

And some residual noncellulosic carbohydrates can promote yellowing of cellulose

Acetate spinning dope. Since the (almost) complete removal of noncellulosic impurities

Would be very expensive and,moreover environmentally harmful (high yield loss,

High chemical charges, additional equipment, etc.), the main emphasis is placed on

Adjusting the refining processes on the demands of each cellulose product.

Дата добавления: 2015-10-21