Sisal fibres and their constitutent non-cellulosic polymers

To characterise the constitutive non-cellulosic components sisal fibres were sequentially extracted with CDTA, Na2CO3 and increasing concentrations of NaOH. Gel permeation chromatography (GPC) of the CDTA extract suggested that a pectic polysaccharide similar to rhamnogalacturonan II (RG II) and glucuronoxylan were present. Diffuse reflectance infrared (DRIFT) and solid state NMR spectroscopies indicated that these polysaccharides were attached to esterified cinnamic acids and/or oligo-lignins. Treatment with Na2CO3 also extracted a RG-I-like polysaccharide and glucuronoxylan and reduced the degree of esterification in the residue as confirmed by both DRIFT and NMR spectroscopies. Extraction with 0.1–4.0 M NaOH reduced lignin and non-cellulosic polysaccharide (NCP) contents with glucuronoxylan as the main extracted NCP. Small amounts of pectin and mannan-derived fragments were also identified in the 0.1 M and 1.0 M NaOH extracts. Treatment with 4 M mM H3BO3 dramatically altered the structure of the fibre, reducing the proportion of crystalline cellulose, as demonstrated by DRIFT and NMR spectroscopies. GPC of the extract revealed that xylan, amorphous cellulose and a glucomannan had been extracted. DRIFT spectroscopy also indicated that, despite extensive washing, the residue contained bound-borates. The NMR spectrum revealed that the lignin remaining after extraction with 4 M NaOH was almost exclusively guaiacyl.

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