# Optimization of Processing Parameters to Increase Thermal Conductivity of Rice Straw Fiber Film
## 3 Results and Discussion
### Response Surface Analysis
#### Interactive Effects on Wet Tensile Strength  
The combined effect of basis weight and wet strength agent content on wet tensile strength of the BMF with others staying constant (0.7% neutral sizing agent content, 10% charcoal addition ratio, and 45 \({}^{\circ}\)SR beating degree) is shown in Figure 3a. Both wet strength agent content and basis weight had significant effects on wet tensile strength. Wet tensile strength increased as both wet strength agent content and basis weight improved, especially at large wet strength content and high basis weight. This is mainly because the increase of the fiber number of the BMF enhanced the bonding ability between the fibers and resulted in increasing wet tensile strength. Meanwhile, the wet strength agent was acting as resin for enhancing the fiber networks through forming a strong crosslink with fibers, thus improving wet tensile strength. Figure 3a also shows the interaction of wet strength agent and basis weight had a significant effect on wet tensile strength. The largest wet tensile strength was attained at 70 g/m\({}^{2}\) basis weight and 1.6% wet strength agent content. The combined effect of basis weight and beating degree on wet tensile strength of the BMF with others staying constant (0.7% neutral sizing agent content, 10% charcoal addition ratio, and 1.2% wet strength agent content) is shown in Figure 3b. Wet tensile strength decreased slightly with the increase of beating degree when basis weight lower than 60 g/m\({}^{2}\). This is mainly because the overbeating degree shortened the fiber and weakened the strength of the single fiber, thus wet tensile strength declined. Wet tensile strength increased as the beating degree improved at 60 to 70 g/m\({}^{2}\) basis weight. The continuous increase of basis weight enhanced the number of fibers unit area and enhanced the binding force among the fibers, thus improving wet tensile strength [45; 46]. The highest wet tensile strength appeared at 70 g/m\({}^{2}\) basis weight and 55 \({}^{\circ}\)SR beating degree. Figure 3c shows the combined effect of neutral sizing agent content and beating degree on wet tensile strength of the BMF with other parameters remaining constant (60 g/m\({}^{2}\) basis weight, 10% charcoal addition ration, and 1.2% wet strength agent content). At low neutral sizing agent content and beating degree, there was a tiny improvement in wet tensile strength as beating degree and neutral sizing agent content improved. But at large neutral sizing agent content and beating degree, wet tensile strength decreased significantly as beating degree and neutral sizing agent content improved. This may be because of the following reasons. The neutral sizing agent readily reacted with the fibers to form ketene ester derivatives on the surface of the fiber. Meanwhile, the exposure of the internal hydrophobic group (long chain alkyl group) to the surface of fibers increased the hydrophobic properties of the fiber, thus enhancing the wet tensile strength at low neutral sizing agent content. When the neutral sizing agent increased to the high level, the emulsion particles became larger and the dispersion capability became weaker, thus the wet tensile strength decreased. The sizing effect reduced, thus decreasing wet tensile strength accordingly. According to Figure 3c, the effect of beating degree on wet tensile strength was a little stronger than that of neutral sizing agent content. The largest wet tensile strength appeared at 0.5% neutral sizing agent content and 55 \({}^{\circ}\)SR beating degree. Figure 3d shows the combined effect of charcoal addition ratio and wet strength agent content on wet tensile strength of the BMF with others staying constant (60 g/m\({}^{2}\) basis weight, 0.7% neutral sizing agent content, and 45 \({}^{\circ}\)SR beating degree). Wet tensile strength enhanced uncommonly as wet strength agent content improved at low charcoal addition ratio from 0% to 10%.