Wood pulp is the most widely used paper pulp, it can be divided into hardwood pulp and softwood pulp. The hardwood takes about 65% of forest growing stock, and its proportion gradually increase. The hardwood pulp is competitive among all kind of paper pulp. Compared to the softwood, the hardwood is dense, so the large increase the charge of pulp digester, which improves the production capacity of paper and pulp mill. The hardwood contains less lignin compared with softwood, easy to be dissolved in the cooking liquid. The hemicellulose content of hardwood is higher, and pulp yield is relatively high, so the hardwood pulping has better economic benefit. Currently, hardwood pulp has a higher growth rate than softwood pulp and non-wood pulp. In a word, hardwood pulp has a bright future in the paper and pulp industry.
As for the uses of hardwood pulp, it can be applied to various kinds of papermaking, such as printing paper, writing paper, tissue paper, etc. Due to the hardwood pulp fiber is thin and short, so it can improve the paper property like evenness, opacity and surface property. As we all know, the short fiber can obtain even separation without long-period beating, save the energy consumption. Tissue paper is required to have a higher thickness, better softness and absorptivity, hardwood pulp not only achieve this goal but make the good stability of tissue paper. Although hardwood pulp has worse pulp strength than softwood pulp, above advantages is enough to make it become the necessary material for papermaking.
Fiber morphology of hardwood
As the data are shown in the table1, the density of pulpwood will have an effect on the elasticity of fiber. Birch and poplar contain less parenchymal cell, Eucalyptus contains fewer vessels, but the size of the vessel is big, Beech contains many vessels with small size. We can also find that beech has lowest cellulose content among four types of hardwood, poplar has the lowest cell wall ratio. Beech fiber is hardest, poplar fiber is softest, and eucalyptus fiber is shortest.
| Pulp wood | density | parenchymal cell content | Vessel content | Vessel length | Vessel number | Cellulose content | Cellulose diameter | Cellulose length | wall cavity ratio | Cellulose number |
| Beech | 630 | 20 | 40 | 0.05 | 127 | 40 | 0.018 | 1.3 | 1.00 | 6 |
| Birch | 610 | 12 | 25 | 0.08 | 45 | 63 | 0.019 | 1.3 | 0.61 | 10 |
| Poplar | 450 | 13 | 26 | 0.05 | 47 | 61 | 0.021 | 1.3 | 0.30 | 13 |
| Eucalyptus | 520 | 15-30 | 8-20 | 0.15 | 8 | 60 | 0.018 | 1.0 | 0.56 | 17 |
Table1
Chemical composition of hardwood
The chemical composition of pulpwood can decide the adaptability of pulp. Cellulose and lignin content of pulpwood can affect pulp yield, if the pulpwood contains higher content of cellulose, the pulp yield is also high, inversely, low lignin content of pulpwood will bring low pulp yield. Besides, hemicellulose content also affects the pulp properties, for example, high hemicellulose content can improve the beating and pulp tensile strength, but reduce the opacity of pulp.
| Pulpwood | lignin | Cellulose | Pentosan | Glucomanna | Extractives |
| Beech | 24.8 | 39.4 | 27.8 | 1.3 | 0.8 |
| Birch | 22.0 | 41.0 | 27.5 | 2.3 | 3.0 |
| Poplar | 20.8 | 49.2 | 21.4 | 3.1 | 2.9 |
| Eucalyptus | 21.9 | 51.5 | 19.9 | 1.4 | 2.5 |
Table2
Paper pulping process of hardwood
KP and ASA are two common kinds of pulping process for hardwood. Generally, the xylan of hardwood has good stability under the alkaline condition, so pulp yield of hardwood is high. ASA pulp yield will be higher than KP pulp yield.
Conclusion
All in all, pulp property can be affected by pulpwood, fiber morphology, chemical composition or paper pulping process. The fiber morphology and paper pulping process have a bigger influence than other two elements.
- ASA pulping have higher pulp yield than KP pulping, because the ASA use low alkalinity in the pulp cooking process, protect the carbohydrate, especially the hemicellulose. The different pulping process also affect the pulp beating, ASA pulping has fast beating process, high hemicellulose content of pulp improve the tensile strength of paper, but have a bad influence on the tearing strength.
- Fiber morphology of difference pulpwood is beneficial for tensile strength and tearing strength of pulp. Speak to the tensile strength, birch pulp is highest and eucalyptus pulp is lowest among 4 kinds of hardwood. On the contrary eucalyptus pulp has heist tearing strength and birch pulp has lowest tearing strength.
- Light scattering and volume of paper pulp are also two indexes. Eucalyptus pulp is best, beech pulp is next, birch pulp and poplar pulp is worse.
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