Bull. Inst. Chem. Res., Kyoto Univ., Vol. 70, No. 2, 1992
Study on Photo-induced Colour Reversion
Wood Pulp by FT-IR
and Derek G. GRAY* Fumiko KIMURA * , Tsunehisa KIMURA*
Received June 10, 1992
The photo-induced colour reversion of handsheets of unbleached and bleached stoneground wood
pulp was studied by FT-IR transmission spectroscopy. Change in the chemical structure of thin hand-
sheets were monitored quantitatively as a function of repeated exposures to UV irradiation at 320 nm
followed by storage in the dark. A slight loss of carbohydrate structures was observed throughout the
cycles, while a pronounced loss in lignin aromaticity was produced by the UV irradiation. For the un-
bleached sample, irradiation initially caused the preferential destruction of non-etherified (free) phenol-
ic groups. The bleached samples, with a higher initial content of non-etherified groups, showed pref-
erential destruction of etherified groups throughout the irradiation cycles. A rapid initial decrease in
a-carbonyl group content occurred for both bleached and unbleached samples; non-conjugated car-
bonyl group content increased with irradiation.
KEYWORDS : Fourier Analysis / Infrared Spectroscopy / Transmission / Colour Rever-
sion / Stoneground Wood / Ultraviolet Radiation / Radiation Effects /
Pulps / Lignins / Cellulose
The use of mechanical wood pulps in a number of applications is limited because of
their rapid loss of brightness. Some understanding of the chemical changes responsible for
brightness loss is required if this colour reversion is to be minimized. The chemical
changes responsible for colour reversion are difficult to determine in situ, in part because of
the relatively small chemical changes that are occurring in the heterogeneous and complex
fibre surface matrix. A variety of infrared spectroscopic techniques1-5) have been applied
to the problem. The difficulty of detecting the small IR spectral change during reversion is
compounded by the variability in composition from point to point on the paper surface.
In this study, the effects of non-uniformity of the sample were minimized by using the
same spot on the sample for both the irradiation and IR measurements. This facilitated a
quantitative analysis of the chemical alteration of the pulps during irradiation and storage.
Furthermore, the sheet was irradiated with a narrow band of light centred at 320 nm, in
order to simplify the initial photochemical processes. The maximum in the action spectrum
for brightness loss occurs close to this wavelength.° The spectra were measured in trans-
* **T t
3420 University Street, Montreal, Quebec, Canada H3A 2A7
, ** ha A,
G. `/ L -r : Pulp and Paper Research Centre, McGill University,
F. KIMURA, T. KIMURA and Derek G. GRAY
mission mode on very thin sheets of bleached and unbleached stoneground wood pulp.
The samples were very thin handsheets (--10 g/m2) of unbleached and commercial
alkaline peroxide bleached stoneground wood (SGW) from a Norwegian spruce furnish.
Each sample sheet was pressed by a pellet maker die with a pressure of 10 tons to obtain a
transparent sample, which was then fixed in an IR sample holder.
IR measurement, W irradiation and storage cycles:
Figure 1 illustrates a series of cycles: each cycle consists of an IR measurement, UV
irradiation for 30 min, and one day storage, except the 5th cycle where an 8-day storage
was employed without irradiation. After the final cycle, an additional IR measurement
The IR spectra were obtained with a Nicolet 7199 spectrometer equipped with a liquid-
nitrogen cooled MCT detector. One thousand scans were acquired and the resolution was
2 cm-1. UV irradiation was performed with a 450-W Xenon lamp, and the monochroma-
tor of a SPEX spectrofluorometer equipped at a wavelength of 320 nm. During the irra-
diation, the sample was kept fixed in the IR sample holder in order to ensure that exactly
Cycle I Cycle 2 Cycle 3
(8 days)UasVwJ Store
Cycle 5 Cycle 6
• Cycle 7 Cycle 8Cycle
U and B stand for hand-Figure 1. Sequence of UV irradiation and IR measurements.
sheets of unbleached and bleached SGW. UV and IR signify UV irradiation and
IR measurement. The samples were irradiated for 30 min, then stored for 1 day,.
except in Cycle 5, where the samples were stored for 8 days without irradiation.
FT-IR Study on Colour Reversion of Pulp
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