SPECTRA fibres

The dependency of critical fibre volume on fibre-matrix bond and the composite crack-tip toughness is demonstrated in Figure 12.4, for ultra-high density polyethylene (Spectra) fibre, with modulus of 117 GPa and 38 fim diameter. It clearly shows that to maintain a fibre content below 2% there is a need for a bond strength of about 1 MPa. I n order to accommodate fibres with bond of about 0.5 MPA, which is typical to many polymeric fibres, a fracture toughness below... 

Today a very wide range of acrylic materials is available with a broad property spectrum. The word acrylic, often used as a noun as well as an adjective in everyday use, can mean quite different things to different people. In the plastics industry it is commonly taken to mean poly(methyl methacrylate) plastics, but the word has different meanings, to the fibre chemist and to those working in the paint and adhesives industries. Unless care is taken this may be a source of some confusion. 

Caution is needed in applying the results of general chemical tests for glass durability across a wide spectrum of glass properties. Glass fibre strengths, for example, are sensitive to the physical, as well as the chemical, nature of the environment and should only be assessed by the direct strength measurements in conditions which closely approximate the final application situation . 

The testing does not yet allow for setting specific input criteria. However, the testing made clear that the process probably can handle a broad spectrum of materials, such as wood, biomass, mixed plastic and pure PVC waste. For instance tests have been done on PVC waste but also with a mixture of PVC, PE, other polymers, Cu, Al, chalk, cement and fibres. 

We will confine ourselves to those applications concerned with chemical analysis, although the Raman microprobe also enables the stress and strain imposed in a sample to be examined. Externally applied stress-induced changes in intramolecular distances of the lattice structures are reflected in changes in the Raman spectrum, so that the technique may be used, for example, to study the local stresses and strains in polymer fibre and ceramic fibre composite materials. 

The selection of a light source, a photodetector and an optical fibre should be compatible with spectral properties of the indicator used. A typical spectrum of a pH indicator is presented in Figure 7-left. 

The application of near-IR spectroscopy for real-time monitoring of glucose, lactic acid, acetic acid and biomass in liquid cultures of microorganisms of the genera Lactobacillus and Staphylococcus has been recently published [76]. The NIR spectrum acquired by the optical-fibre probe immersed in the culture is exploited using a partial least squares (PLS) calibration step, a classical method for IR techniques. 

Recent developments in fibre optics have made it possible to measure the spectrum in any kind of vessel, including any which form part of a vacuum apparatus, so that the problem of fitting a cell which is part of a vacuum rig into a spectrophotometer is now effectively obsolete. However, for single or occasional measurements the devices discussed here are simpler and very much cheaper than fibre-optic gadgetry. 

Figure 7.15 Schematic loss spectrum of an optical fibre.

Fluorescence is not useful simply for chemical analysis. For example, a fluorescent additive that sticks to textile fibres is added to laundry soap. This compound absorbs solar radiation in the non-visible part of the spectrum and re-emits at longer wavelengths in the blue spectral region, which makes clothing appear whiter. Another application of fluorescence encountered daily is cathode tube lighting. The internal walls of these tubes are covered with mineral salts (luminophores) that emit in the visible region due to excitation by electrons. 

Table 2. Laser-Raman spectrum of oriented polyethylene fibres...

Polyethylene. The Raman spectrum of polyethylene has been known for some years and all the bands have been identified seemingly unambiguously. Recently the spectrum has been recorded using stretched large-diameter fibres as a sample. The spectra were recorded with the fibre bundled vertically, horizontally and end-on in a Cary 81 spectrometer. The arrangements are illustrated in Fig. 3. The electric vector of the helium-neon laser is set vertically and therefore in the first orientation (a), the fibre axis and electric vector are parallel. 

Using this approach, dichroism is found in the spectra which enable one to distinguish modes where the vectors are parallel or perpendicular to the fibre axes. The type of dichroism typical in these experiments is illustrated in Fig. 4. If one accepts the principle that the Raman band will be maximized in intensity when the electric vector of the source and the movement vectors of the atoms in the normal mode are parallel, it is possible to assign the Raman spectrum of polyethylene to fundamental... 

The first example to appear on dichroism in the Raman spectrum of oriented polymers concerned fibres and films of isotactic polypropylene (17). Unfortunately it was not possible to interpret the results because... 

A dye is a compound which is added to give colour to a normally colourless material, for instance natural or artificial fibres. By their very nature dyes must absorb light in the VIS region of the spectrum, and this can lead to two separate problems. 

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