Poly(vinyl alcohol) (PVA) thin films are widely used for humidity sensing due to their high sensitivity towards water but have some drawbacks such as instability at high levels of humidity, nonlinearity and presence of hysteresis. In this study, in order to improve PVA sensing properties hybrids comprising hydrophobically modified PVA copolymers and silica particles were utilized as humidity sensing media.

Series of poly(vinyl alcohol-co-vinyl acetal) copolymers (PVAac) with acetal content in the range 18-28 % were synthesized by partial acetalization of hydroxyl groups of PVA with acetaldehyde. Copolymer solutions of concentration 1 wt % in mixed water-methanol solvent (20:80 volume ratio) were used for thin film deposition via spin-coating method. To obtain hybrid polymer-silica thin films, SiO₂ particles were in situ generated in copolymer solutions via the sol-gel method. Properties of PVAac and PVAac-SiO₂ films were compared in terms of optical constants, sensing behaviour toward different levels of humidity and sensor element characteristics like sensitivity, hysteresis and reflectance change (ΔR or color change). It was shown that acetal modified PVA films doped with appropriate amount of SiO₂ particles offers linearity in the entire humidity range. The feasibility of hydrophobically modified PVA thin films for optical sensing of humidity is demonstrated and discussed.

Acknowledgments: S. Bozhilova acknowledge the National Scientific Program for young scientists and postdoctoral fellows, funded by Bulgarian Ministry of Education and Science with PMC № 271/2019. This work was partially supported by the European Regional Development Fund within the Operational Programme “Science and Education for Smart Growth 2014 - 2020” under the Project CoE “National center of mechatronics and clean technologies“ BG05M2OP001-1.001-0008-С01.

In this paper, we report the fabrication of polyvinyl alcohol (PVA) and polyethylene oxide (PEO) blended polymer nanocomposite (PNC) films loaded with different percentages of carbon black (CB) using the stencil printing method. The effect of the PEO-PVA blend weight ratio on the existence of the dual-phase was studied. The fabricated films were characterized using the high-resolution scanning electron microscope (HRSEM), the surface profilometer, and the four-point probe resistive measurements. HRSEM analysis showed the homogenous dispersion of carbon black fillers in the polymer blend matrix. It has also revealed the formation of carbon black agglomerations in the polymer matrix. The topographical properties of the films such as the surface roughness and thickness were obtained. The CB concentration in the polymer blend had been varied from 0 to 14wt%. A typical PNC film loaded with 14wt% CB had a thickness of 120 ± 0.24 µm. The I-V characteristics of the PNC films were obtained to investigate the effect of thickness and CB content variation. An electrical conductivity of 0.417 S/m was achieved with 14wt% CB loading. The percolation threshold and critical exponent of the PNC films was estimated to be 0.2 vol% and 1, respectively. The latter indicated the presence of a two-dimensional conductive network. In general, the CB-polymer composite films with improved structural and electrical properties were fabricated and characterized for potential sensor applications.

The microencapsulation method is an exhaustively used technique for phase change material (PCM) shape-stabilization. This technique has been used in a broad spectrum of applications such as building, medical, electronics, food, etc. Polymeric encapsulation is characterized with high toughness and good heat transfer property due to large surface area of capsules. In this paper, phenol-formaldehyde shelled PCM microcapsules (MPCM) were fabricated and their processing parameters were analysed with Taguchi method. Core to shell ratio, surfactant concentration and speed of mixing are the parameters which were optimized in five levels. The optimized values for surfactant concentration, core to shell ratio and agitation speed were 3%, 1:1 and 800 rpm. The obtained microcapsules were spherical in shape. The melting enthalpy of MPCM synthesized with optimized processing parameters was 148.93 J/g in the range of 35-62 ⁰C. The obtained temperature range of phase transition temperature can be used for storing different food articles such as chocolate and hot served foods.

Hot served food items are maintained around 60⁰C temperature. The food delivery system should be quick to transport such items. If these food items once prepared and stored in container equipped with phase change material (PCM) slabs with phase change temperature around serving temperature, it can maintain temperature of food item for longer time in food delivery process while utilizing lesser amount of energy. Beeswax is a bioderived PCM with phase change temperature around 60⁰C. Beeswax should be shape stabilized to serve thermoregulation purpose. This report has used recycled paperboard as matrix for shape stabilizing beeswax. Optimization study revealed that 45% is the maximum amount of beeswax that can be incorporated in paper composite. Beeswax showed melting enthalpy of 216.09 J/g and melting enthalpy of composite with beeswax content 45% was 102.51 J/g. The enthalpy of melting is directly proportional to amount of beeswax in composite. Thermal conductivity of beeswax and composite with 45% beeswax calculated with T-history method as 0.285 and 0.157 respectively. The carton containing PCM sheets maintain temperature at higher level for longer time than for carton without PCM. To address concerns such as toxicity, environment friendly nature and recycling, beeswax-recycled paperboard composite should be considered as promising candidate.

Plastics are very useful for a wide range of applications, given the mechanical and chemical properties, and ease of manipulation. Unfortunately, because of the issue of non-degradability, plastic waste pollution poses significant threat to the ecological environment. This aspect has become particularly relevant in the sustainable development of industrial production. Nonetheless, additive manufacturing (AM), well-known as 3D printing, is emerging as a crucial industrial technology for rapid prototyping, to convert a numerical model into material deposition and 3D printed parts. Bio based polymers and recycling operation through circular use are representing alternative solutions to reduce plastic waste and limit the environmental impact of AM process. In this framework, this study investigates the possibility to adopt recycled polymers in the AM technology by replacing virgin matrices. At regards, two commercial filaments, made from polylactide acid (PLA), -the second (recycled) obtained from the production waste of the first one (virgin)-, were initially characterized using infrared (IR) spectroscopy, thermogravimetric analysis (TGA) and dynamic rheology. Then, the filaments were extruded in a 3D printer and characterized by dynamic mechanical analysis (DMA). Despite of a small reduction of the intensity in correspondence of typical absorption bands of PLA polymer in the case of recycled material compared to virgin one (as attested by IR spectra), the thermal-mechanical results allowed to attest very similar characteristics of recycled and neat filaments. The onset of the thermal degradation was found around 315°C in both systems. Both materials exhibited the same time-dependent trend of complex viscosity, with a reduction of approximately 50% after 900 seconds of testing. When the samples were dried at 80°C under vacuum for 10 hours, the stabilization of the rheological features against time was improved. There is no significant difference in the storage modulus (E') of 3D printed parts made with different types of PLA-based filaments.