Study On The Mechanism Of Improving The Structure And Properties Of P-Aramid Paper By Hot Calendering

P-aramid paper exhibits numerous excellent properties such as excellent mechanical properties, high temperature resistance and insulation with light-weight, and it is applied widely in many fields including aerospace, transportation and electricity, national defense and satellites communication. P-aramid fibrid is a kind of differentiated aramid fiber, which is developed in recent years, and p-aramid paper of industrialization are all taking the aramid pulp and chopped fiber as raw material. However, aramid fibrid has better wet processing performance and reinforcement effect than aramid pulp, what’s more, it can be papering directly and the p-aramid paper made of aramid fibrid and chopped fiber without hot calendaring has high strength, but the related research was few both at home and abroad. Hot calendaring process is essential to get final aramid paper product with higher performance, but the research was lacking on how the structure and performance of aramid paper was improving by hot calendaring.Firstly, through comparative analysis on the structure difference of aramid chopped fiber, aramid pulp and aramid fibrid, the subject studied the possibility of aramid fibrid used to make aramid paper without hot calendaring, which would provide the theoretical basis for the selection of raw materials of high performance p-aramid paper. The results showed that aramid fibrid had better flexibility, plasticity and dispersion stability in water relative to aramid pulp. The crystallinity degree of aramid fibrid was the lowest with 28.6%, but it had similar thermal stability with pulp and chopped fiber. Unlike aramid pulp, the fibrid can be separately made of paper, and the tensile index, tearing index and compressive strength of aramid fibrid paper without hot calendaring were 15.3N·m/g,8.4mN·m2/g,10.7kV/mm, respectively, while the tensile index, tearing index and compressive strength of aramid fibrid/chopped paper without hot calendaring were 34.1N·m/g、20.8mN·m2/g、1.9kV/mm, respectively. The mechanical strength of aramid fibrid/chopped paper without hot calendaring was close to the strength of that paper after hot calendaring.Secondly, the importance of hot calendaring process for obtaining high performance aramid paper was analyzed. The results showed that only the simultaneous action of force and heat during hot calendaring can improve the structure and performance of aramid paper. After hot calendaring, the paper structure became denser, and the pore is reduced. However, the chemical structure of aramid paper didn’t change, but the degree of intermolecular hydrogen bond association of aramid fibrid was increased. The crystallinity of aramid fibrid paper was improved by 27.9% from 34.5%, while the crystallinity of aramid fibrid/chopped paper was improved by 66.6% from 77.3%, but little changes in crystal form. In addition, the initial decomposition temperature and TG10% of both kinds of aramid paper had a slight increase, which meant their thermal stability was improved.The subject also studied the effect of hot calendaring on the surface morphology, surface element composition, mechanical properties and internal bond strength of aramid fiber, and the reasons for hot calendaring improving the structure and properties of aramid paper were analyzed from the mechanism of chopped fiber reinforced aramid paper. There were several reasons for hot calendaring increasing the strength of aramid paper. The chopped fiber distorted at high temperature and pressure and the specific surface area was increased, which increased the mechanical engagement force between the fibers and the diversionary force between the fibers because of surface fibrillation. Hot calendaring prompted chopped fiber and fibrid bonded together closely, improving the internal bond strength between the fibers. In addition, the content of polar group in the fiber surface increased, enhancing wettability of aramid fibers.Finally, the crystallization behavior of both aramid paper and aramid fiber during hot calendaring and the influence of crystallinity changes on their thermal properties were studied, and residual thermal stress of aramid fibers was analyzed as well, which further perfected the theoretical basis of hot calendaring improving the properties of aramid paper. The results showed that aramid chopped fiber after hot calendaring showed the phenomenon of light and dark under polarizing microscope due to the fiber deformation and crystallization change, and more chopped fiber appeared in the field of vision. The crystallinity of chopped fiber decreased slightly, but its crystal form was unchanged. The crystallinity, grain size and crystal plane of fibrid were all increased and its crystal form became perfect. The improvements of aramid paper crystallinity depend on fibrid crystallinity increasing and the volume fractions of chopped fiber increasing in the unit space. The initial decomposition temperature and glass transition temperature of aramid fibrids after hot calendaring have slightly elevated. The cold crystallization peak on loss tangent curve for aramid fibrids disappeared, which was attributed to its recrystallization during hot calendaring. However, the initial decomposition temperature of aramid chopped fiber decreased. Hot calendaring couldn’t lead to residual thermal stress on aramid fiber, which didn’t influence the combination of fibers.