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A high-pressure differential scanning calorimeter chamber was used to investigate double crystal melting peak generation in polypropylene. The double crystal melting peak forms during the batch-based, expanded polypropylene (EPP) bead foaming process. This double peak structure is required for good sintering and the desired geometric shape of the final bead foam products. To form the double peak structure, isothermal treatment over a certain period of time is necessary at a high temperature and saturated pressure during the batch foaming process. This study investigated the influences of various saturation temperatures, time, and pressure on double crystal melting peak behavior. It was seen that the temperature was the most sensitive parameter on double peak generation. Also, the longer saturation time increased the amount of perfect crystals with a high melting temperature. Furthermore, as the saturation pressure increased, the required saturation temperature for generating the second peak with perfected crystals decreased through the plasticization effect of CO 2 .

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For sustainability considerations, the use of recycled aggregate in concrete has attracted many interests in the research community. One of the main concerns for using such concrete in buildings is its spalling in fire. This may be alleviated by adding steel fibers to form steel fiber reinforced recycled aggregate concrete (SFRAC). This paper presents an experimental investigation into the compressive properties of SFRAC cylinders after exposure to elevated temperatures, including the compressive strength, Young's modulus (stiffness), stress-strain curve and energy absorption capacity (toughness). The effects of two parameters, namely steel fiber volume content (0%，0.5%，1%，1.5%) and temperature (room temperature, 200℃， 400℃ and 600℃) on the compressive mechanical properties of concrete were investigated. The test results show that both compressive strength and stiffness of the concrete are significantly reduced after exposure to high temperatures. The addition of steel fibers is helpful in preventing spalling, and significantly improves the ductility and the cracking behavior of recycled aggregate concrete (RAC) after exposure to high temperatures, which is favorable for the application of RAC in building construction.

ABSTRACT:Poly(urethane-benzoxazine) films were prepared by blending a monofunctional benzoxazine monomer, 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (Pa), and polyurethane (PU) prepolymer that was synthesized from 2,4-tolylene diisocyanate and polyethylene adipate polyol (Mw ca. 1000) in 2:1 molar ratio, followed by casting as films and thermal curing of the films. The poly(urethane-benzoxazine) films obtained by the thermal cure at 190°C were yellow and transparent. All the films had only one glass transition temperature (Tgl from viscoelastic measurements, indicating good miscibility between PU and polybenzoxazine due to the in-situ copolymerization ofPa and PU prepolymer. The Tg shifted increasingly with the increase of Pa content. The films containing less than 50% of Pa had characteristics of elastomer, with elongation at break more than 200%. These elastic films exhibited good resilience with excellent reinstating behavior. The films containing more than 60% of Pa had characteristics of plastics. The poly(urethane-benzoxazine) films showed excellent resistance to the solvents such as tetrahydrofuran, N,N-dimethyl formamide, N-methyl-2-pyrrolidinone and DMSO.