Sustainable Surface Engineering of Lignocellulose and Cellulose by Synergistic Combination of Metal‐Free Catalysis and Polyelectrolyte Complexes

Abstract A sustainable strategy for synergistic surface engineering of lignocellulose and cellulose fibers derived from wood by synergistic combination of metal‐free catalysis and renewable polyelectrolyte (PE) complexes is disclosed. The strategy allows for improvement and introduction of important properties such as strength, water resistance, and fluorescence to the renewable fibers and cellulosic materials. For example, the “green” surface engineering significantly increases the strength properties (up to 100% in Z‐strength) of chemi‐thermomechanical pulp (CTMP) and bleached sulphite pulp (BSP)‐derived sheets. Next, performing an organocatalytic silylation with a nontoxic organic acid makes the corresponding lignocellulose and cellulose sheets hydrophobic. A selective color modification of polysaccharides is developed by combining metal‐free catalysis and thiol‐ene click chemistry. Next, fluorescent PE complexes based on cationic starch (CS) and carboxymethylcellulose (CMC) are prepared and used for modification of CTMP or BSP in the presence of a metal‐free catalyst. Laser‐scanning confocal microscopy reveals that the PE‐strength additive is evenly distributed on the CTMP and heterogeneously on the BSP. The fluorescent CS distribution on the CTMP follows the lignin distribution of the lignocellulosic fibers.


General
Chemicals and solvents were purchased from commercial suppliers. Commercial reagents were used as purchased without any further purification. Infrared spectrum was recorded with a Varian 610 IR microscope coupled to a Varian 670-IR spectrometer. Surface images were obtained using a field emission scanning electron microscope (MAIA3 model 2016, TESCAN).
Secondary electron images were generated using 3 kV accelerating voltage. Prior to imaging the samples, were coated with a 5 nm layer of Iridium.

Procedure for making handmade sheets of bleached sulphite pulp (BSP)
The various laboratory hand-sheets were made according to the the ISO 5269-2 method using a Rapid-Köthen sheet former ( Figure S1). The suspension of sulphite pulp (50 gram dry mass) dissolved in water (2 L) was stirred for 15 minutes and was next disintegrated with disintegration machine. After that, the mixture was diluted with water to 7.5 -8 kilogram total weight and stirred for 10 minutes. The resulting mixture was transferred into the Rapid Köthen sheet former S4 and hand-sheets were made. The sheets were dried at 95°C at an applied pressure of 96 kPa for 10 min; paper testing was performed in the standard testing climate described in ISO 187, i.e., 23 °C and 50% relative humidity.
A suspension of CTMP (50 gram of dry mass) was dissolved in hot water (2L) and mixed for 15 minutes. Next, hot disintegration was performed at 85 °C, which was followed by dilution of the mixture with water to obtain a total weight of 8 -8.5 Kg. After stirring for 10 minutes, the mixture was transferred into Rapid Kothen sheet former and hand-sheets were made. The sheets were dried at 95°C at an applied pressure of 96 kPa for 10 min; paper testing was performed in the standard testing climate described in ISO 187, i.e., 23 °C and 50% relative humidity.
Typical procedure for chemically modification of BSP or CTMP using a combination of PE-complex and organic acid catalyst: S5 reach 7.5-8 Kg in total weight followed by mixing for 5 minutes. Next, the solutions of cationic starch (50 g, 10g/1L cooked in water, 1%wt to pulp), CMC solution (0.5 g dissolved in 500 ml water, 1%wt to pulp) and citric acid (300 mg, 0.6%wt to pulp), which had been homogenized, was added to the pulp solution and the resulting mixture was mixed for 10 minutes. the mixture was transferred into Rapid Köthen sheet former and hand-sheets were made. The sheets were dried at 95°C at an applied pressure of 96 kPa for 10 min; paper testing was performed in the standard testing climate described in ISO 187, i.e., 23 °C and 50% relative humidity

Typical procedure for fluorescent marking of lignocellulosic hand-sheets of BSP or CTMP:
Sulphite pulp (25g) or CTMP (25 g based on calculated dry mass) was mixed in water (2L).
After mixing for 15 minutes, the pulp was disintegrated and the resulting mixture was diluted with water to reach a total weight of 3.5-4 Kg and then stirred for 5 minutes. Next, a solution of TAMRA-modified cationic starch (CS-TGA-TAMRA) (0.25 g cooked in 50 mL of water, 1%wt to pulp), CMC (0.25 g dissolved in 250 mL of water, 1%wt to pulp) and citric acid (150 mg, 0.6%wt to pulp) was homogenized and then added to the pulp solution and mixed for an additional 10 minutes. The mixture was thereafter transferred to the Rapid Köthen sheet former and hand-sheets were made. The sheets were next dried at 95°C at an applied pressure of 96 kPa for 10 min
After 24h, the solid starch material was washed with Soxhlet extraction using acetone for 24 h.