The action of phloroglucinol in the presence of hydrochloric acid on photoaged, totally-chlorine-free, bleached chemical pulps

Abstract

UV-Vis diffuse reflectance spectrometry has been used to monitor the reaction of phloroglucinol (in the presence of hydrochloric acid) with totally-chlorine-free (TCF) bleached softwood bisulfite chemical pulps. Pulps (1–4) bleached via various sequences, which include ozone and hydrogen peroxide processes, were studied. The unirradiated pulps show a very small absorption band with a maximum at about 460 nm when the phloroglucinol test was performed. Pulp 3, which was bleached only with peroxide and contains the higher amount of lignin (Kappa number, 4), displays, after ageing with UV light and after the phloroglucinol test, a broad absorption band with a maximum at 560 nm, similar in behaviour to photodiscoloured lignin-rich pulps. Pulp 2, bleached by one ozone stage followed by a peroxide stage (Kappa number, 1.7) displays the same trend (broad band centred at 560 nm) although the lignin content was lower. This indicates that the phloroglucinol test is very sensitive to oxidized lignin. Pulps 1 and 4, which were fully bleached (Kappa number, 0.5 or less), still present a narrow absorption band around 555 nm after irradiation with UV light. The origin of this band remains uncertain. ortho-Quinone models adsorbed on cellulose paper appear to be insensitive to the phloroglucinol test, in contrast to 2-methoxy- para-quinone and coniferaldehyde. Such chromophores are probably formed by UV irradiation of the TCF pulps.