Synthesis and Swelling Behavior of Carrageenans-Graft-Poly(Sodium Acrylate)/Kaolin Superabsorbent Hydrogel Composites
In the present paper, a novel superabsorbent hydrogel composite based on carrageenan have been prepared via graft copolymerization of acrylic acid (AA) in the presence of kaolin powder using methylenebisacrylamide (MBA) as a crosslinking agent and ammonium persulfate (APS) as an initiator. The certain variables of the graft copolymerization (i.e. the monomer, the initiator and the crosslinker concentration as well as kaolin/carrageenan weight ratio) affected on the ultimate water-swelling capacity were optimized. The composite structure was confirmed using FTIR spectroscopy. A new absorption band at 1707 cm in the 1 composite spectrum confirmed kaolin-organic polymer linkage. The effect of kaolin amount and MBA concentration showed that with increasing of these parameters, the water absorbency of the superabsorbent composite was decreased. Because of their exceptional properties, i.e. anhydrogalactose units (13). The types of carrageenans biocompatibility, biodegradability, renewability and differ only in the position and number of ester sulfate non-toxicity, polysaccharides and proteins are the groups. Following a continuous research on modification main part of the natural-based superabsorbent of carrageenans (14), in this work, we attempt to hydrogels. The higher production cost and low gel synthesize and characterize new superabsorbent strength of these superabsorbents, however, restrict composites based on carrageenan in the presence of their application widely. To improve these limitations, kaolin particles. The preparation of the biopolymer-based inorganic compounds with low cost can be used. The superabsorbent composites can also improve the introduction of inorganic fillers to a polymer matrix mechanical properties of materials and can lower the cost increases its strength and stiffness properties. of the finished product compared with the synthetic Among inorganic compounds, special attention has counterparts as well as providing biodegradable been paid to clay minerals in the field of nanocomposites characteristics.
Free radical degradation of chitosan with potassium persulfate
A thermal dissociation initiator, potassium persulfate (KPS), is added to the chitosan solution at 70 � C; immediately, the solution viscosity and the molecular weight of chitosan decrease in a very short time. Size exclusion chromatography, nuclear magnetic resonance and electron spin resonance were used to study the degradation mechanism. A free radical degradation mechanism of chitosan by KPS is then proposed. When KPS is thermally dissociated into anionic radicals, they are attracted to the cationic amino group in the chitosan ring. Subsequently, the anionic radical attacks the C-4 carbon and transfers the radical to the C-4 carbon by subtracting the hydrogen from it. The presence of free radical at C-4 carbon eventually results in the breakage of the glycosidic C– O–C bond in the chitosan main chain. According to this mechanism, the concentrations of KPS, total free radicals and the degraded chitosan chain at different degradation times are all calculated by solving the rate equations. Finally, the calculated average molecular weights of the degraded chitosan chains at different reaction times agree with the experimental values. # 2001 Elsevier Science Ltd. All rights reserved.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.