Hydrophilic fibres

Dichromate oxidation does tend to give a harsher handle and a less hydrophilic fibre, tending to cause handling problems in subsequent processes such as weaving, and is therefore less suitable for yarn dyeings. The technical merits of dichromate are overshadowed by ecological considerations, however, since chromium compounds in surface waters pose a direct threat to health and are increasingly restricted by water treatment... 

In the following review there are six technical sections, the first three covering dyes. The division has been made on the basis of the nature of the electronic character of the dye molecule. Anionic dyes are applied to hydrophilic fibres whilst cationic dyes are applied mainly to acrylic fibres. The neutral disperse dyes are used mainly on the hydrophobic polyester fabrics. Then follow sections covering pigments, a short summary of fluorescent brightening agents and, finally, an introduction to the subject of colour and chemical constitution. 

The largest proportion (80%) of FBA systems is based on stilbene (88) which itself absorbs at 324 nm. The parent molecule is usually substituted in the 4-position or disub-stituted in the 4,4 -positions. The introduction of sulfonic acid groups into the stilbene skeleton confers water solubility and hence affinity for hydrophilic fibres such as cotton. Products for use on hydrophobic substrates such as polyester require characteristics similar to those of disperse dyes. 

Fligherwet pickup with hydrophilic fibres Fligher wet pickup with low twist and/or open end yarns... 

The CAV for a particular process is dependent on fibre type and fabric construction and absorbency. A finish application below the CAV may result in a non-uniform speckled treatment, while an application above the CAV could lead to finish migration. Cellulosic fibres, becanse of their inherent hydrophilicity, have CAVs in the range of 35 0 % wet pickup. Hydrophobic fibres like polyester can have CAVs of less than 5 %, allowing much lower wet pickups than hydrophilic fibres. 

Schematic orientation of softeners on fibre surfaces, (a) Cationic softener and (b) anionic softener at fibre surface. Non-ionic softener at (c) hydro-phobic and (d) hydrophilic fibre surface. 

No thermal influence on the sample No swelling of hydrophilic fibres... 

Therefore, the first and the most important problem is the fibre-matrix adhesion. The role of the matrix in a fibre-reinforced composite is to transfer the load to the stiff fibres through shear stresses at the interface. This process requires a good bond between the polymeric matrix and the fibres. Poor adhesion at the interface means that the full capabilities of the composite cannot be exploited and leaves it vulnerable to environmental attacks that may weaken it, thus reducing its life span. Insufficient adhesion between hydrophobic polymers and hydrophilic fibres result in poor mechanical properties of the natural fibre-reinforced polymer composites. Pre-treatments of the natural fibre can clean the fibre surface. 

Inorganic fillers are widely used to reinforce polymers to improve their properties. Organic fibres such as cellulose-based fibres have started to replace synthetic minerals [27-35]. The main disadvantage of organic fibre is its enhanced moisture absorption and the poor adhesion between hydrophilic fibres and the hydrophobic polymer matrix. 

The pore model is mainly adopted for the diffusion of dyes into hydrophilic fibres from an aqueous solution in which fibres swell markedly. Therefore it is presumed that water-filled channels of the fibres provide a transfer route for dye molecules to reach their adsorption sites. However, Peters and Ingamells concluded that the pore model in its entirety cannot be accepted. The free volume mechanism mainly dominates when hydrophobic fibres are dyed. 

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