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providing torque in the optimal range. The viscosity of the final PLAX dispersion measured with the RV04 spindle at 100 rpm was 500 mPas. The detailed steps for PLA polymer synthesis and dispersion preparation are described by Mehtiö et al. (2017) [32]. After screening several thermally crosslinking bioORMOCER ® formulations and comparing application properties on the two papers, barrier properties and flexibility, one was selected that exhibits a very high degree of crosslinking. This is based on the high usage of aluminium alkoxide, which acts as a catalyst and a network former. The inorganic network is reinforced through the use of different silanes, including Tetraethylorthosilicate (TEOS) (ABCR; Karlsruhe, Germany). Hydroxypropylcellulose (HPC, Thermo Scientific) was incorporated as a biocomponent to enhance flexibility during the synthesis. The viscosity of the finished coating solution was measured at 39.7 mm/s 2 , with a solid content of 28%. To improve the pot life of the bioORMOCER ® formulation, a multicomponent variant was also developed, which could be mixed on site. Hereon, PLAX is referred to as ‘P’ and bioORMOCER ® as ‘O’. 2.2. Methods The coating layers were applied on both a semi-pilot and industrial scale. The first coating layer, P, was applied at an industrial scale production line using a rod coater (Walki Group Oy; Ylöjärvi, Finland). Dispersion P was diluted to 20% due to high viscosity. Rod V5 was used, and a coating layer was applied on the precoated side in the case of base paper A and on the smooth side of base paper S. The speed of the production line was 80 m/min, and the coatings were dried at 110 ◦ C along the whole drying section. De-curling was prevented using a steam moisturiser with a steam pressure of 0.45 bar. The latter coating layers of O and P were applied with the pilot coating line SUTCO (Surface Treatment and Coating line) at VTT Bioruukki (Espoo, Finland). Dispersions O and P were applied using the reverse gravure coating method. A gravure roll with a surface volume of 75 cc/m 2 (20 lines/cm) was used, with a coating speed of up to 20 m/min. The drying section consisted of three infrared (IR) and five air dryers. The first air dryer was set to200 ◦ C, and the temperature was gradually decreased to 170 ◦ C along the line. No IR dryers were used while drying O due to the ignition risk of ethanol. Corona treatment was used to improve the adhesion of the dispersions on coated substrates. A cooling roller with a surface temperature of 10 ◦ C was used before rewinding the coated papers. All coated samples were stored in standard laboratory conditions, 23 ◦ C and 50% relative humidity (RH), before characterisation. Table 1 lists the coating trial points produced in this work.
Table1. The list of samples produced in this work.
BasePaper
1st Layer
2ndLayer
3rdLayer
Abbreviation
Solide Lucent Solide Lucent Solide Lucent
PLAX
-
- -
S-P
PLAX bioORMOCER ® PLAX bioORMOCER ® PLAX bioORMOCER ® PLAX bioORMOCER ® PLAX -
S-PO
PLAX
S-POP
Asendo Asendo Asendo
- -
A-P
A-PO
PLAX
A-POP
2.3. Characterization 2.3.1. Coat Weight and Coating Thickness
The conditioned samples were cut with a circular sample cutter (50 cm 2 area, Thwing– Albert Instrument Company; West Berlin, NJ, USA) and weighed with an analytical balance (Precisa ES220A; Dietikon, Switzerland) to calculate the grammages, reported as g/m 2 as an average of five parallel samples. The coat weights were obtained by subtracting
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