Pulp Properties and Applications

Bution. The decrease in molar mass of the wood polymers during pulping and

Bleaching procedures is normally estimated by measuring the intrinsic viscosity

Of the pulp fibers. Since celluloses from natural sources and after chemical treatment

Are always polydisperse, the determination of intrinsic viscosity is insufficient

To predict specific fiber properties. Additional information is provided by

Measuring the molecular weight distribution of paper pulps. The results of GPC

measurements of cellulose solutions in LiCl/DMAc with MALLS/RI detection

according to Schelosky et al. [23] are provided in Tab. 11.2 and Fig. 11.3.

3 4 5 6 7

HW-SULFITE

SW-SULFITE

HW-KRAFT

SW-KRAFT

Log Molecular Weight

dW/d(log M)

Fig. 11.3 Molar mass distribution of a representative selection

of fully bleached commercial paper pulps [17].

The typical pattern of molecular weight distribution of the four different categories

Of chemical paper grade pulps is clearly demonstrated in Fig 3. The kraft

Pulps can easily be distinguished from the sulfite pulps by a hemicellulose distribution

Centered at a rather high molecular weight. The bimodal character of the

Molecular weight distribution is particularly pronounced for the hardwood pulps,

Comprising a clear minimum between the cellulose and hemicellulose distributions.

The polydispersity index (PDI) calculated from the ratio of mass-weighted (Mw)

and number-weighted (Mn) molar masses (PDI = Mw/Mn), as shown in Tab. 11.2.

Is significantly higher for the sulfite as compared to the kraft pulps, and indicates

Paper-Grade Pulp

Tab. 11.2 Numerical evaluation of molecular weight distribution

of fully bleached commercial paper pulps [17].

Parameter Pine Spruce Eucalyptus Beech Spruce Spruce Beech

Kraft Kraft Sulfite Sulfite

ECF TCF ECF ECF TCF ECF TCF

DPv (Cuen) 2207 1648 2355 2240 3074 2486 3489

DPw 2827 2251 2847 2636 3648 3144 4050

DPn 659 650 612 590 572 504 517

PDI 4.3 3.5 4.6 4.5 6.4 6.2 7.8

DP < 120 wt% 3.5 3.1 3.0 3.5 5.4 6.3 7.4

DP < 200 wt% 8.2 7.9 8.2 10.7 8.8 8.7 12.8

DP > 2000 wt% 38.7 29.7 40.5 39.5 49.8 44.0 53.4

Zero-span tensile strength

dry Nm·g–1 130.8 133.7 147.1 n.d. 113.1 103.6 98.8

wet Nm·g–1 116.2 109.4 n.d. n.d. 96.5 82.9 84.4

A broader distribution of the former. The high PDI of the sulfite pulps originates

From rather high molecular-weight cellulose and very low molecular-weight hemicellulose

Fractions owing to the heterogeneous degradation of the wood pulp constituents

Across the cell wall layers. Intrinsic strength properties from a polymeric

Material are always related to the weakest part in the polymer, which is represented

By the short-chain molecules. A detailed evaluation of the numerical data from

GPC measurement revealed that zero-span tensile strength correlates well with

the weight percent of the DP <120 fraction, as illustrated in Fig. 11.4.

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