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Epoxy and unsaturated polyester resins are the most used thermosetting polymers. They are commonly used in electronics, construction, marine, automotive and aircraft industries. Moreover, reinforcing both epoxy and unsaturated polyester resins with carbon or glass fibre in a fabric form has enabled them to be used in high-performance applications. However, their organic nature as any other polymeric materials made them highly flammable materials. Enhancing the flame retardancy performance of thermosetting polymers and their composites can be improved by the addition of flame-retardant materials, but this comes at the expense of their mechanical properties. In this regard, a comprehensive review on the recent research articles that studied the flame retardancy of epoxy resin, unsaturated polyester resin and their composites were covered. Flame retardancy performance of different flame retardant/polymer systems was evaluated in terms of Flame Retardancy index (FRI) that was calculated based on the data extracted from the cone calorimeter test. Furthermore, flame retardant selection charts that relate between the flame retardancy level with mechanical properties in the aspects of tensile and flexural strength were presented. This review paper is also dedicated to providing the reader with a brief overview on the combustion mechanism of polymeric materials, their flammability behaviour and the commonly used flammability testing techniques and the mechanism of action of flame retardants.

This study compares the degradation process of unsaturated polyester resin (UPR) and vinyl ester resin (VER) and their biocomposites with kraft lignin. In order to study their degradation, accelerated aging, immersion in different solvents, microwave radiation and high temperature were applied. The results show that, depending on the conditions, the degradation assumes a different course. The VER resin is more chemically resistant than the UPR resin. In the case of the composites immersed in an aggressive solvent (acetone), it can be observed that the polymer matrix is degraded, whereas in water only a small increase of weight takes place. Immersion in NaOH initiates the degradation process consisting in the hydrolysis of ester bonds, which are especially observed for pure resins. Under the influence of UV radiation and microwaves, the resins are additionally cross-linked. Thermogravimetric analysis shows that in the case of composites heated to 1000 °C, a residual mass remains, which is carbonized with lignin. In turn, composites treated with microwaves lost weight.

Unsaturated polyester resins, one of the most important thermosets, are invariably produced from oil-based monomers. Their application is limited in areas where high thermal stability is required due to their low Tg. Besides, these resins contain 30–40% hazardous styrene as a reactive solvent. Therefore, developing bio-based solventless unsaturated polyester resin with medium to high thermomechanical properties compared to petrochemical-based counterparts is important. In order to achieve this, a series of branched bio-based unsaturated polyester resins were synthesized using bulk polymerization method in two steps. In the first step, four different intermediates were prepared by reacting glycerol (as a core molecule) with either isosorbide (diol), 1,3-propanediol (diol), 2,5-furandicarboxylic acid (saturated diacid), or adipic acid (saturated diacid). In the second step, the branched intermediate was end capped with methacrylic anhydride to introduce reactive sites for cross-linking on the branch ends. The chemical structure of the resins was characterized by 13C-NMR. FT-IR confirmed the polycondensation reaction in the first step and the end functionalization of the resins with methacrylic anhydride in the second step. The effect of 2,5-furandicarboxylic acid and isosorbide on thermomechanical and thermal properties was investigated using dynamic mechanical analysis, differential scanning calorimetry, and thermo-gravimetric analysis. Results indicated that 2,5-furandicarboxylic acid based resins had superior thermomechanical properties compared to a commercial reference unsaturated polyester resin, making them promising resins for high-temperature composite applications. For example, the resin based on 2,5-furandicarboxylic acid and isosorbide and the resin based on 2,5-furandicarboxylic acid and 1,3-propanediol gave glass transition temperatures of 173 °C and 148 °C, respectively. Although the synthesized 2,5-furandicarboxylic acid based resins had higher viscosity (22.7 Pas) than conventional unsaturated polyester (0.4–0.5 Pas) at room temperature, preheated resins can be used for making high-temperature-tolerance fiber-reinforced composite.

We report a method of reinforcing and toughening unsaturated polyester resin (UPR) with a kind of hyperbranched polyester (HBP-1). Polyethylene glycol with different molecular weight was used as the core molecule of the preparation reaction, and the reaction product of phthalic anhydride and glycerol was used as the branching unit. The esterification reaction of polycondensation occurred, and then the hydroxyl-terminated hyperbranched polyester was prepared. The reaction product of maleic anhydride and isooctanol was added to the prepared hydroxyl-terminated hyperbranched polyester for esterification reaction. Both ends of the hyperbranched polyester had unsaturated double bond to obtain the hyperbranched polyester (HBP-1). The effects of this treatment on the morphology, mechanical properties and thermal properties of the composites were studied in detail. The HBP-1 was investigated by Fourier Transform Infrared Spectroscopy (FT-IR). The HBP-1/UPR composites were investigated by Thermogravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), mechanical properties analysis and Scanning Electron Microscope (SEM). The results showed that HBP-1 enhanced the thermostability and mechanical properties of UPR. However, DMA indicated that the addition of HBP-1 cannot effectively improve the thermodynamic properties of UPR due to the flexible chain in HBP-1 structure. The HBP-1 improves tensile strength, bending strength and impact strength compared to neat UPR.

A method for producing nanocomposites of unsaturated polyester resins (UPR) based on recycled polyethylene terephthalate (PET) as a matrix has been proposed. The upcycling method involves three successive stages: (1) oligoesters synthesis, (2) simultaneous glycolysis and interchain exchange of oligoesters with PET, (3) interaction of the obtained resins with glycol and maleic anhydride. UPRs were characterized by FTIR spectroscopy and gel permeation chromatography. The mechanical properties of nanocomposites obtained on the basis of these resins and titanium dioxide have been investigated. It has been shown that 1,2-propylene glycol units, despite their lower reactivity, significantly improve the properties of UPR. The most promising nanocomposite sample exhibited tensile strength 112.62 MPa, elongation at break 157.94%, and Young’s modulus 29.95 MPa. These results indicate that the proposed method made it possible to obtain nanocomposites with high mechanical properties based on recycled PET thus allowing one to create a valuable product from waste.

The paper investigates the synthesis of eco-friendly composites and their properties before and after immersion in solvents of different chemical natures. For their preparation, unsaturated polyester resin (UPR) based on recycled poly (ethylene terephthalate) (PET) and peanut shell powder (PSP) were used. Polymerization was carried out in the presence of environmentally friendly polymeric cobalt. Distilled water, acetone, 10% hydrochloric acid, 40% sodium hydroxide, toluene, and 2% sodium carbonate were used as solvents in the chemical resistance test. Changes in the structure, properties, and appearance (morphology) of composites after 140 days of immersion in solvents were investigated. The results show that both the resin and its composites show resistance towards 10% HCl and toluene. The immersion in water has no significant effect on the resin, but for PSP composites, the plasticizing effect of water was observed. In acetone, after only one day, the resin and its composite with 10% PSP shrink and fall into pieces. However, the most destructive is an alkaline environment. After the immersion test, a huge increase in mass and a deterioration of gloss and thermomechanical properties were observed. The destructive influence of the 40% NaOH environment mainly concerned the resin.

Unsaturated polyester resins (UPR) are broadly applied in chemical, construction, transportation and electrical fields, etc. However, UPR materials are extremely flammable. Through radiation, conduction and convection, the released heat brings serious thermal hazards to surrounding life and the environment. Meanwhile, the combustion of UPR also releases a vast of toxic pyrolysis gases and smoke, causing extremely serious non-thermal hazards, endangering life and health, reducing fire visibility and hindering escape and rescue efforts. Therefore, the flame retardant modification of UPR materials is an inevitable measure to reduce its flammability and broaden its application breadth. In this review, the flammability of UPR materials is firstly analyzed from the perspective of their compositions and structures. Meanwhile, based on the differences in preparation methods of flame retardant UPR (FR-UPR) in literature research, it is divided into additive flame retardant, intrinsic flame retardant and interfacial flame retardant methods. The relationship between the highly efficient flame retardant structures and the properties, and the gaseous-phase flame retardant and condensed-phase mechanisms in FR-UPR composites are analyzed. Compared with traditional additive flame retardants, polymeric flame retardants, nanosheet flame retardants and intrinsic flame retardant method are important directions for the future development of FR-UPR composites. In addition, the interfacial flame retardant method can effectively improve the interfacial adhesion and flame retardant properties of fiber-reinforced UPR composites, and also has application prospects. This review analyzes the current challenges faced by FR-UPR, and looks forward to its possible future development directions to guide the preparation and application of high-performance FR-UPR.

In this study, a bio-based resin containing glycerol, isosorbide, and 2,5-furan dicarboxylic acid was used to produce a glass fiber reinforced composite. The thermomechanical properties of the resin were examined through dynamic mechanical analysis, thermogravimetric analysis, and differential scanning calorimetry, and were compared with those of commercially available unsaturated polyester resin and epoxy resin. Glass fiber composites were prepared using the synthesized bio-based resin, commercial unsaturated polyester resin, and commercial epoxy resin. Tensile tests, flexural tests, and aging tests were performed on all three types of composites and the results were compared. The findings suggest that the bio-based resin exhibits superior thermomechanical properties compared to the commercial resins. Bio-based resin demonstrates a high storage modulus of 4807 MPa and a loss modulus of 72 MPa at 25 ℃, along with a high glass transition temperature of 173 ℃. The flexural and tensile properties of the bio-based resin were better than that of the commercial resins. The composite produced from bio-based resin shows a flexural strength of 334 MPa and a tensile strength of 256 MPa. Aging results indicate that the synthesized bio-based resin was fairly stable at elevated temperatures. The outcome of this work shows that the bio-based glass fiber reinforced composite is a promising composite for high temperature applications.

The paper investigates the properties of unsaturated polyester resins and microcrystalline cellulose (MCC) composites. The influence of MCC modification on mechanical, thermomechanical, and thermal properties of obtained materials was discussed. In order to reduce the hydrophilic character of the MCC surface, it was subjected to esterification with the methacrylic anhydride. This resulted in hydroxyl groups blocking and, additionally, the introduction of unsaturated bonds into its structure, which could participate in copolymerization with the curing resin. Composites of varying amounts of cellulose as a filler were obtained from modified MCC and unmodified (comparative) MCC. The modification of MCC resulted in obtaining composites characterized by greater flexural strength and strain at break compared with the analogous composites based on the unmodified MCC.

Nowadays, unsaturated polyester resins (UPR) are mainly obtained from non-renewable resources. The ever-increasing regulations and the continuous demand for more sustainability have led to extensive research towards more environmentally suitable alternatives to petroleum-based materials. However, one of the main disadvantages of bio-based UPR is their relatively high viscosity compared to petrochemical ones. In order to overcome this drawback, in this work, we investigated the possibility to lower the resin viscosity utilizing a mixture of dimethyl itaconate (DMI) and methyl methacrylate (MMA) as a reactive diluent. The effect of the DMI and MMA ratio on resin rheological properties was investigated. The optimal curing parameters were determined and all UPRs had a high gel content, which was shown to be dependent on the DMI and MMA ratio in the formulation. Furthermore, thermomechanical and mechanical properties of the resulting network were also found to be affected by the used reactive diluent mixture. A small substitution of DMI by MMA proved to be advantageous since it offers lower resin viscosity and improved mechanical properties.