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3. Possibilities of detection of colourless synthetic materials
3.1 Reflexion
Giving special attention to the qualities of synthetic materials it was noted that most of it has a glossy surface which should reflect more light compared to the raw cotton. Trials were conducted to see if this reflectance could be used. They did, however, not show a consistent result
3.2 Fluorescence
Fluorescence was also suggested as criterion. That means that each material emits some of the light absorbed. This property varies from material to material and has been used in textile industry for a long time. Material which shows an especially high fluorescence „normally called optical brightener“ is for example admixed to synthetic material. If a mixture of synthetic tufts treated in this way and normal cotton tufts is evaluated by means of an ultraviolet lamp you can easily recognize the higher fluorescence of the synthetic material treated in such a way. The striking fact is not the higher fluorescence of the synthetic material but that one of the optical brightener. Unfortunately it cannot be assumed that the impurities of cotton with synthetic material have been treated with optical brighteners. But this does not mean that cotton and synthetic materials have the same fluorescence. It, however, might be very difficult to use a possible difference to distinguish these materials. In addition, there is the problem that fluorescence can be recognized only in darkness. For this reason textiles treated with optical brighteners always are evaluated at a darkened place. With the instruments used for the detection of impurities the conditions are just opposite. The optical detection used in all instruments requires a brightly illuminated channel.
3.3 Spectral analysis
Investigating the light reflected from these materials is an additional possibility to distinguish materials. White light is composed of waves of different length. It is known that the composition of the light which is reflected by a certain material varies according to the respective material properties. Investigations have shown that the light which is reflected by synthetic material has an especially high proportion within the infrared range. The use of this criterion has already been tested in practice. Unfortunately, there were no useful results, at least not in respect to its use during the pneumatic transport of cotton with speeds up to 16 m/sec. The quantity of the required light energy for the detection of impurities seems in particular to be a problem.
3.4 Sound waves
The surface structure of synthetic materials offers another possibility to distinguish them from cotton. Conspicuous is the fact that cotton consists of soft tufts; impurities, however, mostly occur in the form of harder relatively flat formations. In these circumstances it was assumed that sound waves may be reflected differently by these materials. Later trials actually proved that sound waves are largely absorbed by fibrous material such as cotton but mostly reflected by synthetic contamination such as fabrics woven from tapes, individual tapes and foils.
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