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Application of ATR-IR measurements to predict the deinking efficiency of UV-cured inks

In recent years, ultraviolet (UV)-curable ink has been developed and widely used in various printing applications. However, using UV-printed products (UV prints) in recovered paper recycling causes end-product dirt specks and quality issues. 

A new method was developed that can distinguish UV prints from other prints by means of attenuated total reflectance infrared (ATR-IR) spectroscopy. Application of this method could allow more efficient use of UV prints as raw materials for paper recycling. 

First, a mill trial was performed using UV prints alone as raw materials in a deinked pulp (DIP) process. Second, test prints were made with four types of UV inks: a conventional UV ink and three different highly-sensitive UV inks. Each print sample had four levels of four-color ink coverage patterns (100%, 75%, 50%, and 25%). Next, drinkability of all prints was evaluated by laboratory experiments. Finally, each print was measured using the ATR-IR method,
and the relationship between the IR spectra and deinkability was investigated. 

Mill trial results showed that UV prints caused more than 20 times as many dirt specks as those printed with conventional oil-based ink. There were variations in recycling performance among UV prints taken from bales used for the mill trial. Lab tests clearly revealed that not all UV-printed products lead to dirt specks. 

In order to clarify the factors that affected deinkability of UV prints, the print samples were investigated by lab
experiments. Key findings from lab experiments include:

• The number of dirt specks larger than 250 μm in diameter increased as the ink coverage increased.
• Higher ink coverage area showed stronger intensity of ATR-IR spectral bands associated with inks. These results
indicate that deinkability of UV prints could be predicted by analysis of ATR-IR spectra.
• Finally, the method was applied for assessment of recovered paper from commercial printing presses. It was confirmed that this method made it possible to distinguish easily deinkable UV prints from other UV prints.

Based on these findings, we concluded that the ATR-IR method is applicable for inspection of incoming recovered
paper.