Technique Takes Deeper Look at Paper

Aaron J. Hand

When papermakers cook up a new recipe for paper, they want to be sure it's going to work right before making reams and reams of it. Chemists at the University of Jyväskylä have developed spectroscopic techniques to get a three-dimensional analysis of paper, helping the manufacturers monitor the factors that affect print quality.

Researchers used laser-induced plasma spectroscopy to create distribution maps from consecutive layers of a paper's coating, showing variations in pigment content. Red areas indicate high pigment content; the dominance of blue in the bottom map reflects low pigment content of the base paper below the coating.

Paper coatings -- which contain pigments glued together by binders -- fill the voids of the base paper, providing a smooth surface for printing. Using laser-induced plasma spectroscopy, the researchers can get a snapshot of the variation of the coating weight or the binder within the coating, which are important factors in predicting printability, said Jouko E.I. Korppi-Tommola, a professor of physical chemistry at the university.

3-D analysis

Korppi-Tommola and Heikki J. Häkkänen reported their latest research results in the Nov. 15, 1998, issue of Analytical Chemistry. With laser-induced plasma spectroscopy, they can get a 3-D picture of pigment and binder distributions of a paper's coating layer at a lateral resolution of at least 100 µm and depth resolution of 1 µm.

"In a typical analysis, we can take 20,000 samples from an area of a few square centimeters and 50 µm in depth and record the element information at each location in about 15 minutes," Korppi-Tommola said. "This is quite a bit of information from a piece of paper. No other method can offer this information at this speed and cost."

Most competing methods -- which include a range of microscopy and spectroscopy techniques -- are restricted to two-dimensional analysis, he said. "Vacuum methods only see the topmost layer, typically 5 nm thick," he said. Most of the coating material resides below this depth, however, so studying depth profiles of multiple coatings is out of reach of these methods.

The Finnish group's system uses a 308-nm excimer laser emitting 60-mJ pulses from which 1 to 2 mJ are focused onto the paper's surface. But the group would like to replace that excitation source with another UV light source, Korppi-Tommola said, preferably a small diode-pumped Q-switched frequency-quadrupled Nd:YAG delivering 10-mJ pulses at a 20-Hz repetition rate.

The chemists have been working closely with paper and paper-machine companies in Finland, Austria and Sweden. With financial help from the Technological Development Center of Finland, they were able to ease the analysis process, Korppi-Tommola said, by replacing a boxcar and two monochromators with a gated charge-coupled device detector to record the spectrally resolved light.