دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و ششم شماره 3 (پیاپی 185، امرداد و شهریور 1402)

Due to environmental concerns and considering the petroleum resources, the bio-based materials have recently attracted more attention both in academia and industries. In addition to environmental sustainability and renewability, the bio-based adhesives have shown some competing advantages with respect to fossil-based products. This article covers the advantages and challenges of bio-based adhesives. The main advantages of bio-based adhesives include their less greenhouse gas emission, lower volatile organic compounds (VOC), lower environmental and human toxicity. The bio-based adhesives show higher tendency to biodegradability, which may be considered as their strong aspect compared to fossil-based resources. The bio-based materials may be incorporated in adhesive formulation through direct usage of biopolymer, in the form of building blocks and/or reactive monomer, and in the form of adhesive formulation additives. In bio-based adhesives, the pioneering technologies belong to fabrication of paper and different wooden products using starch and/or protein-based adhesives. The main goal was to remove as much as possible the harmful emitting formaldehyde gases during fabrication and product service life. The modification of bio-resource materials through introduction of new functionalities in the molecular architecture may bring some specific properties. There are many reports on the effect of various vegetable oils on the improved hydrophobicity in epoxy and polyurethane adhesives. The bio-based vegetable oils and cashew nut shell liquids now have acquired reliable position in adhesive and coating market. Recent investigations on some specific bio-resource structures such as lignin and tannin with complex structures have shown promising results for the improved thermal and antibacterial activities in adhesive formulation. In order to make a strong decision on the effectiveness of the bio-based adhesives, much investigations on different biological/economical aspects of bio-based materials are needed to achieve this goal.

Cellulose, as one of the most abundant natural polymers with a wide spectrum of applications, has drawn the attention of many researchers. In this regard, cellulose-base structures such as cellulose nanocrystals (CNCs) and nanocomposites are known as materials widely used in many fields. Therefore, development, production and use of cellulose-based structures by gaining the relevant insights are progressively increasing. In the present paper, the latest developments in the field of various extraction techniques of CNCs, their properties and dispersion methods in epoxy matrix to obtain desired mechanical properties in cellulose nano-crystals/epoxy nanocomposite structures are reviewed. First, cellulose nanocrystals are introduced as one of the most promising biodegradable and abundant nanomaterials widely used in various industries. The various production techniques of cellulose nanocrystals are briefly reviewed. Based on the various applications of cellulose nanocrystals in many fields, cellulose acid hydrolysis as one of the most practical and low-cost methods for cellulose nanocrystals preparation is described in details. In this regard, the source of cellulose, time and temperature of the acid hydrolysis, the concentration and type of acid as important factors of the hydrolysis process are investigated. Reinforcing epoxy-based nanocomposites using cellulose nanocrystals is the subject of another section. Dealing with problems associated with cellulose nanocrystals agglomeration phenomenon, the main challenge to achieve a homogeneous dispersion of cellulose nanocrystals within the epoxy matrix, is also thoroughly discussed. In this regard, various methods of chemical surface modification of cellulose nanocrystals are investigated. Finally, considering the investigated cases, the debate on the issue of appropriate methods for dispersing cellulose nanocrystals in the epoxy-based resin is comprehensively covered. It can be said that based on growing demands for high performance cellulose nanocrystals/epoxy nanocomposite structures, research on this field is ongoing. Therefore, in the present review, by providing a comprehensive discussion on the latest researche works conducted in this field, it has been attempted to present the new achievements of researchers and approaches to overcome the involved challenges. The most important achievement of researchers is the emphasis on selection of appropriate dispersion medium considering the cellulose nanocrystals surface energy to produce nanocomposites with suitable mechanical and thermomechanical properties.

Hypothesis: 

The skin is the largest organ and outer covering of the body, which acts as a barrier against microbial invasions as well as mechanical and chemical damage. But, sometimes the skin does not have the ability to regenerate the tissue on its own. In this context, tissue engineering (TE) is a promising and reconstructive solution for repairing serious skin tissue damage. This article refers to the importance of electerically conductive scaffolds based on tragacanth gum (TG) for skin TE owing to non-toxicity, metabolic compatibility, and the non-hazardous nature of its degradation products as well as the effect of electerical conductivity of the sacffold in performance of skin TE.

Electroconductive nanofibrous hydrogel scaffolds composed of tragacanth gam-polyaniline blend (TG-B-PANI) and poly(vinyl alcohol) (PVA) were fabricated with a weight ratios of 30:70 and 20:80 by electrospinning method. Their physicochemical and biological properties for skin TE application were studied by various experiments.  

The fabricated scaffolds were tested using FTIR, SEM, TGA, UV-Vis, and cyclic voltammetry (CV). SEM images indicated the achievement of uniform fibers within their nano-scale domain. The cytocompatibility and cells proliferation characteristics of the scaffolds were approved by MTT assay using L929 mouse fibroblast cells. The fabricated scaffolds exhibited excellent hemocompatibility and human serum albomin adsorption capacity. The fabricated scaffolds showed proper physicochemical and biological properties for skin TE. The scaffold made with 20% (wt) of TG-B-PANI showed higher potential in adhesion and proliferation of L929 mouse fibroblast cells than those of scaffold with 30% (wt) of the above polymeric blend.

Hypothesis:

 The curing process for phenolic resin composites is always carried out under press or autoclave pressure, as water by-products are released during curing process. As a result, when phenolic composite parts are formed using the vacuum bag-only method, the costs of the mold and challenges of the pressure vessels would be eliminated, and a significant step is taken in easier manufacture of these parts.

For the purpose of comparing the two methods, phenolic laminates were prepared using 3 bar pressure autoclave and vacuum bag-only methods. In order to investigate the effects of thickness on different properties, the samples were subjected to bending tests, short beam strength tests, void percentage tests, and fractured surface morphology tests.

As the sample’s thickness increases, the flexural modulus increases while the flexural strength and short beam strength decrease. Furthermore, the modulus, bending strength, and strength of the short beam in the autoclave sample have increased by 27%, 17%, and 17%, respectively, compared to the vacuum bag-only sample. Morphological studies also showed that more void content was formed in the vacuum bag samples and the resin-fiber interaction was reduced compared to the autoclaved samples. A decrease in the bonding between resin and fibers and in the penetration of resin between fiber strands has also been observed with increasing thickness. Samples with a thickness of 1 mm had a void content of 3.5 ± 1% and in samples with a thickness of 9 mm, it was 15% ± 1%.

Hypothesis: 

Today, due to the widespread use of coarse particle suspensions in chemical industry, the tendency to study the rheological behaviour of suspensions has increased significantly. One of the most important research fields is the study on friction of particles between them and the shear thickening behavior of their suspensions. In high-filled suspensions, the viscosity and the thickness of shear are proportional to the thickness of the suspended particles. 

This research is based on the syntheses of smooth particles (styrene/acrylic acid copolymers) and rough particles (silica-coated styrene/acrylic acid copolymers), with a volume proportion of 20%, 34% and 49% in the ethanol/water solution. The initial critical shear rate of suspension thickening regions was obtained in different ratios of rough and smooth particles. In addition, experimental and semi-empiric models such as Herschel Barclay and Gopalakrishnan have been used to describe the relationship between suspension rheology and microstructure. 

 It was observed that with the increase in roughness of the composition of the same percentage of particles, a more severe shear thickening behavior occurs in smaller amounts of rough particles. It was also observed that hydro-clusters were formed in samples that contain the highest proportion of suspension composition and consist of 100% coarse particles with the lowest amount of Peclet.  An increase in the amount of rough particles leads the system to an increase in viscosity at lower shear rates. Furthermore, the adaptation of the Gopalakrishnan model to experimental data clearly shows that an increase in roughness leads to a reduction in the critical value of Pe at the beginning of the shear thickening zone and a stronger shear thickness behavior in the system.

Hypothesis:

 Solid/liquid phase change materials (PCMs) are among the materials used to store thermal energy. By nanoencapsulating these materials, the problem of leakage during melting can be solved and the thermal efficiency of the system can be increased. In previous studies, researchers have used complex and expensive methods to prepare nanocapsules of PCM. In this work, in order to simplify and reduce the time and costs of the synthesis process, the difference in the solubility parameter of the core and shell materials at different temperatures and the sequence in phase separation have been used for the synthesis of PCM nanocapsules.

In order to make the desired PCM nanocapsule, the difference in solubility parameters of polyethylene glycol, polystyrene and toluene at temperatures of 5, 25 and 80ºC has been used. In fact, this difference in the solubility parameter creates a homogeneous solution of these three substances at a temperature of 80ºC. By decreasing the temperature to 25°C, primary cores are formed and solid polyethylene glycol nanoparticles are completely separated. Further, by decreasing the temperature to 5ºC, polystyrene is separated from the solution and completely covers the nanoparticles of polyethylene glycol. In this way, using the sedimentation with temperature gradient method, polyethylene glycol nanoparticles were first synthesized, and polyethylene glycol nanoparticles were coated with polystyrene deposition. Also, to increase the thermal conductivity of the shell and the thermal efficiency of the PCM system, carbon nanoparticles have been used in polystyrene shells. 

Examining and evaluating the morphology of the synthesis PCM system in this work confirmed the creation of a polyethylene glycol/polystyrene core/shell nanocapsule. The thermal energy absorption efficiency of the heat transfer fluid prepared from these nanoparticles at the applied temperature of 55ºC is about 17% more than that of water.