Mater. Proc. 2020 , 2 , 29
2 of 8
pharmaceutical coatings [1]: while providing excellent physical properties, such as high elasticity, high tensile strength and ease of film-forming, the films are soft and sticky [2]. The use of natural rubber as paper coatings is less developed, although the application on paperboard provides low water affinity and low absorption rates of the coated surfaces [3]. Although it has good potential to replace unrecyclable wax coating material on packaging papers, the blocking (sticking) tendency needs to be decreased with the content increase fillers—e.g., adding modified lignin can efficiently reduce the sticking problem of the coating [3]. In combination with cellulose fabrics, the natural rubber coatings were applied in a calendaring machine with good adhesion to the substrate, which is presumably due to the good mechanical and chemical compatibility of natural rubber and lignocellulose fibers [4]. In order to tune the composition and properties of the natural rubber coatings, additives are required to provide the requested surface properties, although compatibility with rheological properties and the molecular profile of the natural rubber should be investigated. In this work, typical coating fillers used in paper technology, including kaolinite, talc and organic styrene–maleimide nanoparticles, are used in combination with a natural rubber latex binder in order to investigate the effects on the processing and surface properties of the coating.
2. Materials and Methods
2.1. Materials Vytex Natural Rubber Latex (Vystar, Worcester, MA, USA) was used as a commercially available “ultra-low protein” natural rubber latex with an intrinsic solid content of 60% ( w / w ) and pH = 10.4. Three different types of fillers were used, including kaolinite (KAO) powder with particle diameter < 2 ΐ m and aspect ratio 20:1 (Imerys, Paris, France), talc powder, and styrene–maleimide (SMI) nanoparticles that were in-house synthesized according to a previous protocol [5]. The latex was used in non-diluted conditions for mixing with different filler types in concentrations of 5, 10, 20% (wt./wt.), using a three-blade propeller mixer under constant medium shear for about 1 h. The mixed latex suspensions were applied as a paper coating on a laboratory scale K303 Multi- coater (RK Print Coat Instruments Ltd., Litlington, Royston, Hertfordshire, UK), using a black metering bar number 4 (close wound wire diameter 0.51 mm) resulting in a wet film thickness of about 40 ΐ m. The coatings were dried for 2 min in a hot-air oven and further dried for one week under environmental lab conditions (23 °C; 50% RH). A reference paper grade was used for deposition of the films, including bleached Kraft pulp and internal sizing (350 ΐ m thickness). In parallel, free standing rubber films of the same composition were cast on a PTFE foil for following adhesion measurements (the free films were more flexible and used as counterpart for an adhesive loop test). 2.2. Characterization Rheological measurements on mixed latex suspensions were performed on Ares G2 equipment (TA Instruments) with a cylindrical bob-cup geometry operating at a gap distance of 2.10 mm. A suspension volume of 20.1 mL was added into the cup and first kept at rest for about 30 min before testing to relieve internal stresses. A continuous rotational shear test was performed under controlled shear rate between 0 and 1000 s ƺ 1 at a controlled temperature of 25 °C, while applying three subsequent sequences of ramp up (15 min)–rest at 1000 s ƺ 1 (5 min)–ramp down (15 min)–rest at 0 cm ƺ 1 (5 min)–ramp up (15 min), monitoring viscosity changes by eventual effects of hysteresis and/or internal history. The differential scanning calorimetry (DSC) measurements were done on a Q200 equipment (TA Instruments) on a sample mass of 8.5 mg in a heating range from ƺ 90 to 180 °C at 20 °C/min under continuous nitrogen flow. The results for T g (glass transition temperature) and NJ c p (heat capacity) are taken from the second heating step and averaged from two samples. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was done on separate natural rubber films (no paper coating) in order to focus on the effect of the fillers on the coating structure, without interfering spectral bands of the base paper. The measurements were done
Made with FlippingBook Ebook Creator