Raman microscopy and data acquisition

Raman microscopy has been used to study the curing of UF resin by Hill et al. (1984) who identified several spectral regions for distinguishing critical structural differences in cured and uncured UF resins (Tables A.3 & A.5). These peaks are used to identify UF resins in this Chapter.

Scheepers et al. (1993) characterized the curing of MF resins using Fourier transform (FT) Raman spectroscopy. They confirmed that MF resin formation consisted of two stages. Dur- ing the first stage water insoluble melamine dissolves in formalin under alkaline conditions undergoing a series of addition reactions (Pizzi, 1983a) to create various methylolmelamines. Condensation reactions then take place to create a low molecular weight resin. At this stage, the resin is both soluble and fusible. During the second stage, the resin cures with the applica- tion of heat or an acid catalyst. During curing, further chain extension and cross-linking takes place to form an insoluble, infusible three-dimensional network. Scheepers et al. (1993) were the first to assign bands and interpret Raman spectra of MF resins and their changes during cure. All spectra of melamine-containing compounds showed an intense band at 975 cm−1 which was attributed to the ring breathing vibration of the triazine ring. They also found that the band at 975 cm−1 was not influenced by degree of methylolation. Hence it can be used to identify and examine the distribution of MF resin in impregnated and pressed decor papers. Scheepers et al. (1993) found that “MF resins have significant absorptions in the Raman range, but the absorbtions at a specific wavelength are the same for various samples. Therefore, rel- ative band areas are not affected in a different way to the MF resins studied.” Scheepers et al. (1993) claim to be the first to identify this peak, however, their claim can be challenged as Dollish et al. (1973) identified 8 frequencies for s-triazine (1,3,5-triazine) and assigned the fre- quency 676 cm−1to “in-plane ring deformation”, and ring “breathing” at 1132 and 992 cm−1. They also found ring “breathing” vibrations at 1000-980 cm−1for various 2,4,6 s-triazines (Ta- bles A.4 & A.6). It was these peaks that were used to determine the level of MF resin migration and to answer the questions posed by Schnieder (1997b).

Scheepers et al. (1995) also identified the 676 cm−1 raman spectral band in melamine resins. The authors suggested that “it might be possible to use the intensity of the 676 cm−1 band for determination of the free melamine content of MF resins.” “The 676 cm−1ring mode vanishes upon methylolation, whereas the 975 cm−1 band retains constant intensity” (dur- ing and after methylolation). Both bands were assigned to deformations of the triazine ring (Scheepers et al., 1995). The authors concluded that “Raman spectroscopy can be used to determine the free melamine content in MF resins. Consequently confocal Raman microspec- troscopy is able to obtain spectral information on MF resin from a small volume element” (µm scale).

Figure 3.6: The Renishaw model 2000 Raman microscope

3.2.9.2 Experimental

A Renishaw model 2000 Raman microscope7using a near infrared laser (780 nm at 6.8 mW at 100% power output) was used to assess the distribution of melamine through treated pressed paper. Scanning was done using a 50 x objective with a scanning time of 50 s and an accumu- lation of 5 full scans to obtain the final spectrum (Figure 3.6).

In order to compare the amount of MF resin in the paper sample by position, paper type and UF resin treatment, the ratio of melamine to titanium dioxide within each paper was de- termined. This was quantified by measuring the area under the melamine peak at 975 cm−1 and expressing this as a ratio of the area under the two TiO2 peaks at 448 and 610 cm−1 (Otieno-Alego, 2000). A similar technique was used by Sun et al. (1997) to determine the lignin to cellulose ratio in pulp samples and predict pulp Kappa numbers. Peak areas here were obtained using Igor Pro.8 _{The extents of the two TiO}

2 peaks were from 395-490 cm−1 and 545-655 cm−1, and for melamine from 950-1000 cm−1. The baseline was interpolated between these points for all peaks. Initially peaks were analysed with and without baseline correction (Sun et al., 1997), however, subsequently base line correction was used to remove

7_{Renishaw Raman Imaging Microscope Windows - based Raman Environment Users Notes M-8012-1894 01}

Issue 1.0 Renishaw plc New Mills, Wotton-Under-Edge Gloucestershire, GL12 8JR

background fluorescence as it did not affect the melamine or TiO2 profiles for the corrected Raman spectra.

Two papers were not analysed. These were Black, because it had no detectable TiO2peak, and Kraft, because all samples fluoresced strongly and hence it was impossible to identify any peaks. The fluorescence in Kraft originated from the lignin because the pulp used in this paper was unbleached.

The purpose of experimentation was to determine whether there was a difference in the distribution of MF resins in papers treated with different levels of UF resin. Raman spectra were all obtained in the same plane, on the surface, half way to the middle of the paper and in the middle of the paper. The location of the line of scanning was random and orthogonal to the surface of the paper. Some dyes in the printed papers fluoresced making acquisition of data more difficult even using neutral density filters which reduced the intensity of the laser by 25%. Fluorescence even occurred when the back (non printed) side of such papers were scanned, showing that some of the ink dyes (or their solvents) had penetrated a substantial distance from the surface of the papers. It was still possible, however, to quantify the relevant melamine and titanium peaks from poorer quality spectra. The spectroscopic data (in ASCII format) were analysed using the wave analysis program Igor.

To determine whether UF resin flowed during LPM pressing, potentially resulting in inter- mixing with the curing MF resin, discs of paper treated only with UF resin were removed from the sample sheets and pressed. The distance that the resin flowed out from the discs was measured.