Effect of benzoxazine resin on mechanical properties of nitrile rubber composites: Experiment and molecular simulation

Abstract

"Benzoxazine resin, being a new type of phenolic resin deve-loped to overcome the shortcomings of traditional phenolic resins, has been synthesized from phenol, formaldehyde and amine, and does not require solvent elimination or monomer purification to obtain a relatively clean precursor. It has potential application in the field of aerospace due to its low expansion coefficient, high weather resistance, high carbon yield, good mechanical strength, and excellent ablation resistance［1］. It can be mixed with various other resins or polymers to produce new resins with a broad range of applications［2-3］. In this work, the mechanism of enhanced mechanical properties and dynamic mechanical properties of nitrile rubber(NBR)/polybenzoxaine (PBR) composites was investigated by combination of molecular dynamic (MD) simulation and experimental methods［4］. The composition of rubber compounds was NBR 100 phr (in mass, the same below), 4010 NA 2.0 phr, zinc oxide 2.0 phr, SA 2.4 phr, CZ 2.2 phr, sulfur 1 phr and PBR varies. These samples were named as NBR/PBR x, where x stood for the amount of PBR. The MD simulation results showed binding energy (Ebinding), hydrogen bond (H-bond), and fractional free volume (FFV) of NBR/PBR composites, as shown in Table 1. When the PBR amount increased to 20 phr, the number of H-bond was the largest, Ebinding was the highest, and FFV was the smallest, indicating that the intermolecular interaction between PBR and NBR was the strongest, so that the molecular chains were closely packed and FFV were decreased. When the PBR amount exceeded 20 phr, the number of H-bond and Ebinding decreased and FFV increased, which was ascribed to the aggregates formed by PBR molecules. As shown in Table 2, with the addition of PBR, tensile strength increased until it reached its highest value at PBR of 20 phr, and then decreased, but elongation at break gradually decreased. It might be attributed to the limited uniform dispersion of PBR in NBR, and PBR particles begin to aggregate exceeding a certain concentration. It might be also reasonable to assume that at the higher amount of PBR, the interphase surrounding the particles was beginning to overlap. Under higher stresses, elasticity was lost, and the material became stiffer, and most materials underwent plastic deformation process to alter their shape in a permanent way［5］. Additionly, dynamic mechanical properties of NBR and NBR/PBR composites revealed that glass transition temperature (Tg) increased with the increase of PBR amount. The reason was that increase in interaction between NBR and PBR reduced the movement of molecular chain. However, when the PBR amount exceeded 20 phr, Tg of NBR/PBR composites started to decrease, which was due to poor dispersion of PBR in NBR, resulting in a loose structure and a decrease in Tg."

Keywords

• benzoxazine resin，nitrile rubber， enhance， mechanical properties，hydrogen bonding，combining energy， free volume fraction，