Softwood fibres

A fibre bin or bunker, sometimes ineorporated in the mat former, provides a buffer between the refiner and the press. Bin eapacity is quite limited as a tonne of fibre occupies 33 m, corresponding to a bulk density for the fibre of around 30 kg/m for a softwood fibre derived from chip. With press lines requiring 10-50 tonnes/h of fibre, the provision of even a few minutes capacity between fibre production and panel manufaeture involves a substantial bin volume. There is logic in making this bin as wide as the press line so that the process of distributing the fibre uniformly aeross the width of the press line can be partially accomplished in the fibre bin. 

Figure 5.8 Softwood fibres in various amounts by optical microscopic pictures. C Fraunhofer ICT, 2013.)...

Figure 7.1. Structure of softwood fibre showing the architecture of the fibre wall with a lignin-rich middle lamella, the primary wall enforced by a network of cellulose fibrils, and the secondary wall built up of three layers with different fibrilar orientations. (Reproduced from Mr. Rundlbf, Licentiate thesis, Royal Institute of Technology, Stockholm 1996, with permission from the author)...

Figure 19.7 Scanning electron micrographs of a freshly fractured surface of a PP film filled with 20 wt% of raw untreated (a) and (b) MAPP coated softwood fibres. Reproduced with permission from Reference [24].

Dried softwood, made from 60% pine and 40% spruce, was refined to 23°SR and fractionated using a 30-mesh screen to obtain fractionated softwood fibres, used as long fibres in this study. Unrefined regenerated cellulose and imrefined eucalyptus fibres were also used as reinforcement fibres. 

Conventional laboratory reference handsheets were formed from the above-mentioned nonfractionated softwood fibres and dried birch hardwood pulp refined to 28°SR. A 70 30 mixture of hardwood and softwood pulp was used as the base furnish. Standard commercial copy paper, composed of 70% birch and 30% mixed softwood of pine and spruce, was used as another reference. 

The grammage, PCC content and thickness of the sheets formed are shown in Table 5.6. The thicknesses of the handsheets formed in the laboratory were the same for both the high-filler-content and reference samples. Commercial copy paper has a lower thickness than the laboratory-made paper. Among the laboratory handsheets, softwood fibre samples exhibit higher thickness. 

The bulk and bending stiffness of the samples at various PCC contents are compared in Fig. 5.13. Handsheets formed from long softwood fibres and 23% filler showed the highest bulk due to the open structure of the network. As expected, 30% addition of fines to this furnish significantly densities the stracture of paper (Retulainen 1997). Increasing filler addition to this network to form a fines-filler... 

Fig. 5.13 Bulk and bending stiffness of the new composite and reference handsheets. Ctnnmercial copy - bending stiffness is a geometric mean value. Abbreviations C new composite handsheets. Fibre ccnnponents euca eucalyptus imrefined, sw softwood fibres...

The formation of the new composite and reference samples is illustrated in Fig. 5.21. It can be concluded that the formation improves with increasing amounts of nanofibrillar cellulose and PCC in the paper. Addition of 10% regenerated cellulose long fibres impairs the formation, while addition of softwood fibres results in a formation comparable to that of the reference handsheets. The handsheets were prepared from a suspension with a consistency of 4.3%. This combination of raw materials might also be suitable for higher consistencies, so the formation of handsheets at higher consistencies needs to be evaluated in detail. 

Cellulose fibres are obtained from softwood and hardwood. Softwood fibres are 30-45 pm in diameter and 3-7 mm long. Hardwood fibres are thinner (10-20 pm) and shorter (1-2 mm). Their tensile strength is by an order greater than that of the wood for example, lumber wood without macro-defects may have tensile strength equal to 70 MPa and a single fibre, 700 MPa. The fibres are extracted from wooden pulp by different chemical and mechanical processes. 

Today s interest in kenaf focuses on it as an alternative source of paper pulp although still on a very limited scale. Ultimately refined bast fibres measure on average 2.6 mm in length and resemble the best softwood fibres while core fibres are only about 0.6 mm long and are similar to hardwood... 

Paavllalnen, L. (1991). Influence of morphological properties of softwood fibres on sulphate pulp fibre and paper properties, in Proceedings of the International Paper Physics Conference. Atlanta, GA TAPPI Press, pp. 383-95. 

The comparisons between different types of fibres that were treated similarly indicate that the composites produced may have a similar flexural strength, but may be quite different in toughness (Table 11.3) [26,29]. These differences cannot be explained on the basis of density (which is similar in all the specimens), or the geometry (aspect ratio) of the fibre, which is highest for the fibre which led to the composite of lowest toughness (Table 11.3). Similar trends were reported by Coutts [30] for softwood and hardwood fibres prepared with room temperature cured matrix. There, the better performance of the softwood fibres was attributed to the higher fibre aspect ratio. 

In softwoods, the main cell type is the tracheid, which is often mistakenly referred to as a fibre. Tracheids constitute over 90% of the volume of most softwoods, and are the principal paper-making cells of softwoods. Their average length is usually between 2 and 4 mm, with a lengthrwidth ratio (aspect ratio) often in excess of 100 to 1, but there is a wide distribution of tracheid lengths, and it is possible for some to be as short as 1 mm and for others to be as long as 5 mm (Table 2.1). The lumen, or central cavity, is several times wider than the cell wall thickness. There is also a difference between spring wood (i.e. cells synthesised in the early part of the annual... 

Figure 2.2 Light photomicrographs of fibre preparations illustrating the morphological differences between softwood and hardwood commercial pulps (a) bleached sulfate pine (softwood), (b) bleached sulfate eucalyptus (hardwood). Scale bar = 200 /an.

Figure 4.11 Flocculation and formation effects in a chemically pulped bleached softwood pulp (slightly refined). Fibre suspensions settled for 40 min. Sheets (60 gm 2) photographed in transmitted light, (a) no additives, (b) polyelectrolyte added to induce flocculation. Scale bar = 2 cm.

Figure 5.2 Environmental scanning electron photomicrographs of fibres of a chemically pulped (sulfite process) softwood (a) before refining and (b) after refining.

Fibres are usually classified into fibre tracheids, libriform fibres, and septate fibres. Libriform fibres (Figures 1.27 and 1.28) are longer then fibres tracheids and have moderate to very thick walls and simple pits. Their function is one of support. The shorter fibre tracheids have moderately thick walls and bordered pits. They function in both conduction and support although their occurrence in vesselled woods suggests that their function is primarily one of support. It is likely that they represent an intermediate evolutionary form between the softwood tracheid and the true libriform fibre. The fibres in some woods have their fibre lumens divided into chambers by septa. Such fibres are known as septate fibres. The septa only cross the fibre lumen and do not connect to the primary wall. They are produced by a late sequence of division in the fibre prior to death of the cytoplasm. Septate fibres resemble axial parenchyma in some woods and are most abundant in woods where the latter are poorly represented. This has led to the general belief that septate fibres have evolved as an alternative site for the storage of starches, oils and resins. 

Bark has very little meehanieal strength on aeeount of its lack of fibres. The bark of softwoods has no thiekened axially-elongated cells, but the bark of some hardwoods do sometimes eontain small numbers of short fibres similar to wood fibres. 

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