Mechanically Robust and Flame-Retardant Polylactide Composites Based on In Situ Formation of Crosslinked Network Structure by DCP and TAIC

The addition of intumescent flame retardant to PLA can greatly improve the flame retardancy of the material and inhibit the dripping, but the major drawback is the adverse impact of the mechanical properties of the material. In this study, we found that the flame retardant and mechanical properties of the materials can be improved simultaneously by constructing a cross-linked structure. Firstly, a cross-linking flame-retardant PLA structure was designed by adding 0.9 wt% DCP and 0.3 wt% TAIC. After that, different characterization methods including torque, melt flow rate, molecular weight and gel content were used to clarify the formation of crosslinking structures. Results showed that the torque of 0.9DCP/0.3TAIC/FRPLA increased by 307% and the melt flow rate decreased by 77.8%. The gel content of 0.9DCP/0.3TAIC/FRPLA was 30.8%, indicating the formation of cross-linked structures. Then, the mechanical properties and flame retardant performance were studied. Results showed that, compared with FRPLA, the tensile strength, elongation at break and impact strength of 0.9DCP/0.3TAIC/FRPLA increased by 34.8%, 82.6% and 42.9%, respectively. The flame retardancy test results showed that 0.9DCP/0.3TAIC/FRPLA had a very high LOI (the limiting oxygen index) value of 39.2% and passed the UL94 V-0 level without dripping. Finally, the crosslinking reaction mechanism, flame retardant mechanism and the reasons for the improvement of mechanical properties were studied and described.

Enhancing the physico-mechanical properties of ethylene propylene diene monomer rubber via ץ-radiation in the presence of bi-functional and tri-functional monomers

This study, uses two polyfunctional monomers (PFMs) namely ethylene glycol dimethacrylate (EGDMA) as a bifunctional monomer and trimethylolpropane triacrylate (TMPTA) as a trifunctional monomer were used as co-agents in irradiation crosslinking of Ethylene Propylene Diene Monomer Rubber (EPDM). The effect of concentration of each PFM and irradiation dose on the crosslinking density, gel content, swelling behavior in motor and brake oils, in addition to the mechanical and thermal stability properties of EPDM was investigated in detailed. The results showed a remarkable increase in the gel content, crosslinking density and mechanical properties as the concentration of PFMs increased from 1 to 5 phr (parts per hundred parts of rubber). The various blends of EPDM with the trifunctional monomer express the highest gel content and crosslinking density than those with the bifunctional monomer. The addition of 5 phr of TMPTA to EPDM causes a dramatic improvement in tensile strength (TS) of the prepared blend reached to 188% compared to neat EPDM at 50 kGy. At the same time, the maximum TS of the blend containing 5 phr of EGDMA achieved only 41% compared to neat EPDM at an irradiation dose of 100 kGy. The swelling of irradiated samples in brake oil revealed a stronger oil resistance than motor oil. For all irradiated samples, the oil uptake decreased with the irradiation dose up to 100 kGy. The EPDM samples containing 5 phr of TMPTA recorded the highest oil resistance at 100 kGy. The results also showed that the addition of PFMs and irradiation treatment of the various prepared blends improved the thermal stability of EPDM. Finally, neat EPDM and the blends containing 1 and 3 phr of EGDMA can be used as radiation dosimeters in the very high dose range (50–200 kGy).

Physically Crosslinked Chitosan/PVA Hydrogels Containing Honey and Allantoin with Long-Term Biocompatibility for Skin Wound Repair: An In Vitro and In Vivo Study

Chitosan/PVA hydrogel films crosslinked by the freeze–thaw method and containing honey and allantoin were prepared for application as wound dressing materials. The effects of the freeze–thaw process and the addition of honey and allantoin on the swelling, the gel content and the mechanical properties of the samples were evaluated. The physicochemical properties of the samples, with and without the freeze–thaw process, were compared using FTIR, DSC and XRD. The results showed that the freeze–thaw process can increase the crystallinity and thermal stability of chitosan/PVA films. The freeze–thaw process increased the gel content but did not have a significant effect on the tensile strength. The presence of honey reduced the swelling and the tensile strength of the hydrogels due to hydrogen bonding interactions with PVA and chitosan chains. Long-term cell culture experiments using normal human dermal fibroblast (NHDF) cells showed that the hydrogels maintained their biocompatibility, and the cells showed extended morphology on the surface of the hydrogels for more than 30 days. The presence of honey significantly increased the biocompatibility of the hydrogels. The release of allantoin from the hydrogel was studied and, according to the Korsmeyer–Peppas and Weibull models, the mechanism was mainly diffusional. The results for the antimicrobial activity against E. coli and S. aureus bacteria showed that the allantoin-containing samples had a more remarkable antibacterial activity against S. aureus. According to the wound healing experiments, 98% of the wound area treated by the chitosan/PVA/honey hydrogel was closed, compared to 89% for the control. The results of this study suggest that the freeze–thaw process is a non-toxic crosslinking method for the preparation of chitosan/PVA hydrogels with long term biocompatibility that can be applied for wound healing and skin tissue engineering.

Improving of thermal stability and water repellency of cotton fabrics via coating with silicone rubber under the effect of electron beam irradiation

Cotton fabrics for outdoor use were coated with room-temperature vulcanizing silicone rubber (RTVSR) and its catalyst to obtain improved thermal stability and water repellency properties. The coated fabrics were exposed to different doses of electron beam irradiation as an extra curing step for the coating. The effect of RTVSR coating and subsequent electron beam irradiation on the cotton fabrics was then studied. The thermal stability was investigated by thermogravimetric analysis, and surface morphologies were observed by scanning electron microscopy. The mechanical properties, crease recovery, gel content, swelling property, and water repellency of the fabrics were also investigated. The results showed that the thermal stability and the water repellency of the cotton fabrics were improved as a result of the coating process. Moreover, the crease recovery and swelling properties of the fabrics were enhanced, accompanied by decreases in their gel content, as a function of the increase in the electron beam irradiation dose. These findings recommend the produced coated fabrics under the optimum conditions for use in the industrial purposes as isolation sheets through the pipes connecting points, especially those that contain hot fluids.

Asphalt Incorporation with Ethylene Vinyl Acetate (EVA) Copolymer and Natural Rubber (NR) Thermoplastic Vulcanizates (TPVs): Effects of TPV Gel Content on Physical and Rheological Properties

Plastic waste has been incorporated with asphalt to improve the physical properties of asphalt and alleviate the increasing trend of plastic waste being introduced into the environment. However, plastic waste comes in different types such as thermoplastic or thermoset, which results in varied properties of polymer modified asphalt (PMA). In this work, four thermoplastic vulcanizates (TPVs) were prepared using different peroxide concentrations to produce four formulations of gel content (with varying extent of crosslinked part) in order to imitate the variation of plastic waste. All four TPVs were then mixed with asphalt at 5 wt% thus producing four formulations of PMA, which went through physical, rheological, and storage stability assessments. PMA with higher gel content possessed lower penetration and higher softening temperature, indicating physically harder appearance of PMA. Superpave parameters remained unchanged among different gel content PMA at temperatures of 64, 70, and 76 °C. PMA with any level of gel content had lower Brookfield viscosity than PMA without gel content at a temperature of 135 °C. Higher gel content resulted in shorter storage stability measured with greater different softening temperatures between top and bottom layers of PMA after 5 days of 163 °C storage. This study shows that asphalt with thermoset plastic waste is harder and easier to pave, thus making the non-recycling thermoset plastic waste more useful and friendly to the environment.

Fully Bio-Based Thermosetting Polyurethanes from Bio-Based Polyols and Isocyanates

The trend towards the utilization of bioresources for the manufacturing of polymers has led industry players to bring to the market new monomers. In this work, we studied 3 polyisocyanates and 2 polyols with high renewable carbon contents, namely L-lysine ethyl ester diisocyanate (LDI), pentamethylene-diisocyanate (PDI) isocyanurate trimer, and hexamethylene-diisocyanate (HDI) allophanate as the isocyanates, as well as castor oil and polypropanediol as the polyols. These monomers are commercially available at a large scale and were used in direct formulations or used as prepolymers. Thermosetting polymers with Tg values ranging from −41 to +21 °C and thermal stabilities of up to 300 °C were obtained, and the polymerization was studied using NMR, DSC, and rheology. Cured materials were also characterized using FTIR, DMA, gel content, and swelling index determinations. These high bio-based content materials can successfully be obtained and could be used as alternatives to petro-based materials.

The Effects of Stearic Acid (SA) on the Thermal and Physical Properties of LLDPE/DS Composites

In this study, polymer composites were synthesized from date seeds (DS) powder as natural filler and low-density polyethylene (LLDPE) as polymer matrix. This composite was exposed into chemical modification using different content of Stearic Acid (SA) (3, 6 and 9 wt.%). The composite was fabricated by using the process of extrusion and injection molding respectively. The prepared samples were examined using TGA, DSC, FTIR, and Gel Content test. It can be clearly observed that all the treated samples have presented three-steps of decomposition as shown in TGA curves. It is also observed that the Tm, Tc, and degree of crystallinity of the modified LLDPE/DS biocomposites increased as SA increase. Thr FTIR spectra have shown different type of stretching bands, the band at 3346 - 3347 cm-1 appeared because of hydroxyle (OH) groups that is described as a hydrophilicity measure. Beside, there was a minor decrease on the peak between 3346 and 3347 cm-1 of modified composite, whichwas attributed to the hemicellulose removal from the modified composite. The gel content of the treated mples increased due to the increase of crosslinking between DS and LLDPE in existence of SA.

STUDY OF THE INFLUENCE OF THE CROSSLINKING METHODS ON THE PROPERTIES OF POLYETHYLENE

Polyethylene (PE) is considered one of the most versatile thermoplastics available today. However, it exhibits several limitations related to its low melting point, low heat resistance, tendency to crack propagation, and its low resistance to rupture under stress. In order to overcome these deficiencies, several processes for crosslinking PE chains were developed, which makes this material more stable to temperature changes. In this work, methods based on peroxide (PEX-A) and silane (PEX-B) as chemical modification agents for PE chain crosslinking were analyzed, aiming to apply PE in pipes for conduction water. The materials were characterized by infrared spectroscopy (FTIR), gel content, and thermal analysis (TGA and DSC) to achieve those objectives. Also, some mechanical properties such as tensile strength and hardness and the determination of the heat deflection temperature (HDT) were evaluated. Spectra demonstrated the presence of the functional groups characteristic of PE and the incorporation of siloxane groups in PEX-B. Gel content values obtained were above 60% and indicated the formation of crosslinked chains between the molecules of the polymers. The thermal analysis suggests a greater efficiency in forming a chain network by the silane crosslinking process compared to the peroxide process. As for the mechanical tests, they also showed improvement in the mechanical properties of the crosslinked polymer when compared to the respective original PE. Thus, the silane method provided sufficient results to conclude that the properties evaluated are superior compared to the peroxide crosslinking method.

Physico-mechanical properties and cell microstructure of cross-linked PVC/organoclay nanocomposite foams prepared at various processing conditions

Cross-linked polyvinyl chloride (C-PVC) foams and their nanocomposite foams, containing Cloisite 30B nanoclays were prepared. The effects of compression molding pressure and time on the morphology and mechanical properties of different foams were studied. Increment of molding pressure led to higher apparent density, gel content, cell density, and expansion ratio, and wider cell size distribution, which improved the mechanical properties of the foams. Additionally, with the increasing of molding time, lower cell density and final expansion ratio, narrower cell size distribution, and higher gel content and mechanical properties could be obtained. Moreover, incorporation of Cloisite 30B nanoclay in a PVC matrix not only improved cellular microstructure and mechanical properties but also reduced water uptake ratios of nanocomposite foams.