Pretreatment Methods and Mechanisms

Apart from the factors mentioned above formation of inhibitors during pretreatment, for example, furfural and 5-hydroxymethylfurfural (HMF), carboxylic acids, ketones, phenolics, and aldehydes, affects the subsequent processes. The chemistry and the effect of these inhibitors are well studied (Taylor et al 2012). Effective pretreatment methods to reduce the inhibitor concentration, and to improve the sugar yields, are presently being researched. 

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SURFACE PRETREATMENT METHODS AND NUCLEATION ENHANCEMENT MECHANISMS... 

The hydrolysis of polysaccharides into monomeric sugars is a well-studied process its mechanism is still believed to be accurately described by the work of Saeman in 1945.432 Its significance is linked to the production of fermentable sugars for ethanol manufacture. Many of these processes are based on acid catalysis and overlap closely the pretreatment methods described earlier.363,364,373 Treatments with both concentrated and dilute acids are well known as methods for the hydrolysis of cellulose and hemicellulose, as is the use of organic dicarboxylic acids as alternative catalysts to mineral acids.433... 

For designing and developing catalysts, the focal point of all research, the pay-off, is correlation between catalyst properties on the one hand and mechanisms on the other. Choice of the active components, type of support and promotion, method of preparation, pretreatment, and formulation must be related to mechanism, kinetics, adsorption, or mass transfer. Often data are incomplete or speculative. Correlations may be empirical but prove... 

Finally, an important difference between mechanical and chemical surface pretreatment methods should be pointed. Concerning their effects, the former are only limited to the adhesive surface area, that is, they are not able to protect the neighboring areas of the adhesive surface against possible stress from the ambience. 

Owing to the deformability of plastics - in particular of thermoplastics - mechanical pretreatment methods are applicable only to a very limited extent. So for example, if jet pressure is too high the blasting abrasive can be shot into the surface. For polyethylene and polypropylene for instance, the SACO-mefhod described in Section 7.1.2.1 has proven its worth. It develops a surface suitable for the formation of adhesive forces by means of chemically modified blasting abrasives (silication). 

ES-EPA (Expert System for Environmental Pollutant Analysis) is an expert system for producing laboratory test plans, including the appropriate sampling methods, pretreatments, test methods and their order. ES-EPA generates test plans in a stepwise manner from abstract plans called "templates" to detailed plans, using a hierarchical planning mechanism. The knowledge base contains information on analysis items, test methods, test equipment, pretreatments and other necessary information. The prototype system has been successfully tested for various cases in the domain. The development of a delivery version has been completed and it will soon be used in the field on a daily basis to further verify its feasibility. 

The first method, although effective, was soon abandoned for safety reasons, while the use of chromic acid has been largely discontinued. The other two have remained the most widely used pre-treatments, not only for LDPE but also for high-density polyethylene (HDPE) and polypropylene (PP). Other methods have been found to be effective, but for reasons of cost, safety or convenience, they have not been widely used. The pre-treatments include fuming nitric acid, potassium permanganate, ammonium peroxydisulphate, ozone, fluorine, peroxides, UV radiation, grafting of polar monomers, plasmas (see Plasma pretreatment), electrochemical oxidation (see Electrochemical pre-treatment of polymers) and the use of solvent vapours. The corona, flame and plasma methods and the use of trichloroethylene are now discussed briefly the latter is included because it involves a different mechanism. 

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