Alkaline surface degradation

It was also shown that treatment of PHB film with IM NaOH caused a decrease in pore size on film surface from 1-5 pm to around 1 pm, which indicates a partially surface degradation of PHB in alkaline media [40-41], At higher temperature, no weight loss of PHB films and threads was observed after ineubation of 98 and 182 days in phosphate buffer (pH=7.2) at 55°C and 70°C, respectively [22], 12 per eent and 39 per cent of PHB (450 and 150 kDa, respectively) films after 84 days incubation at 70°C [35, 40], 50 per eent and 25 per cent after 150 days incubation of microspheies (250-850 pm diameter) from PHB (50 kDa and 600 kDa, respectively) [42],... 

Shirahase et al. found that the addition of 30 wt% poly(methyl methactylate) (PMMA) enhanced the alkaline hydrolytic degradation of PLLA/PMMA blends, given that hydrotyrl ions could probably diffuse into the interface between the PLLA and PMMA-rich domains and catalyze the surface hydrolytic degradation of PLLA domains. 

Polymer composites in outdoor applications are susceptible to photoinitiated oxidation leading to surface degradation and are also sensitive to moisture-induced damage, alkaline... 

Note that PVDF may be adequate for high pH environments in applications where the surface degradation and fluoride leaching is not problematic, eg, alkaline waste streams (87)... 

On the other hand, pores enhance the alkaline and enzymatic surface hydrolytic degradation of aliphatic polyesters because of the increased surface area per unit mass of porous materials compared to that of nonporous materials [307]. Similar to this, the pores formed by the removal of water-soluble polymers such as poly(p-vinyl phenol) from PLLA/ water-soluble polymer blends accelerate the alkaline hydrolytic degradation of PLLA as traced by weight loss [249]. 

The efficiency of extraction was observed to be inversely proportional to the corn cob particle size. This was expected because the size reduction corresponds to an increase in total particle surface area. An increase in the time of the alkaline extraction and in the NaOH concentration also improves the efficiency of xylan extraction. This happened because when the NaOH concentration was lower, the xylan present in corn cobs could not be fully dissolved in the solution. Thus, it resulted in lower efficiency of xylan extraction. However, when the NaOH concentration was higher than 2 M, the yields decreased with continuously increasing of the NaOH concentration. This is probably due to the alkaline degradation of xylan chains, proceeding at the higher NaOH concentration, which indicated that the ideal NaOH concentration in the extraction was between 1.5 and 1.8 M (Unpublished data). 

Desizing by chemical decomposition is applicable to starch-based sizes. Since starch and its hydrophilic derivatives are soluble in water, it might be assumed that a simple alkaline rinse with surfactant would be sufficient to effect removal from the fibre. As is also the case with some other size polymers, however, once the starch solution has dried to a film on the fibre surface it is much more difficult to effect rehydration and dissolution. Thus controlled chemical degradation is required to disintegrate and solubilise the size film without damaging the cellulosic fibre. Enzymatic, oxidative and hydrolytic degradation methods can be used. 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

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