Correlation Between The Interface And Structure Characteristics Of Meta-aramid Fiber And The Properties For Sheetmaking

High performance aramid paper is generally acknowledged to be the superior sheet for electrical insulation and structural materials. In China, the great demand of aramid paper almost depends on import for a very long time; since the aramid paper made in China had not well meet the demand of the market and customs. There is a distinct differential between the properties of Chinese paper and Nomex in Du Pont. The properties of paper formation, thickness uniformity, and the physical strengths still need to be improved. The essential reason of the quality defect is due to the lacking of researches on basic theories and key techniques of the correlated cross-discipline.The key point of improving the paper properties is to determine the relationship between the fiber structures, which correlated to the fiber properties, and properties of the papermaking. To determine this, a series of questions will be dealt with from the perspective of material science and physical chemistry in this project.The adhesion properties of aramid fiber and fibrids will be revealed by study on their interfiber chemical forces, fiber surface energy, and electrochemical properties. The main factors of influence the aramid sheet formation, density and thermal resistance properties should be scientifically evaluated via investigating of the thermal resistance of aramid fiber and fibrids. In addition, study on the characteristic of fiber crystalline structure and dynamic viscoelastic properties were expected to support a theoretical analysis on the special difference of aramid sheet strength. With this foundation, more information of correlations between the surface properties, structure feature, and thermal properties with papermaking characteristics were studied. Thus, we can adjust the process of sheet forming and hot press, and further improve the techniques of fiber interfacial adhesion.The structure and surface properties of aramid fiber depends mainly on process it undergone in dry-spinning or wet spinning. Optical microscopy, scanning electric microscopy (SEM) and atomic force microscopy (AFM) were used for the microstructure analysis of aramid fibre/fibrids, and the cross sectional structure of aramid short fibre were observed by transmitting electric microscopy (TEM).The SEM observation showed the linear coverage of F1-fibres with discontinuous pleats, which are fairly uniformly distributed and run parallel to the fibre axis direction, no obvious skin-core structure. The surface pleats traces are fibrillar structure, which formed in the wet spinning process and caused by the steep change in the molecular structure. It is a precursor of a surface defects. Fibrids are soft like handkerchief with a few hundred microns or less in both length and width. They are non-rigid filmy, ribbon-like particles with irregular shape. Fibrids have an average length of 0.2 to 1 mm. These morphology characteristics of fibrids are beneficial to get well-dispersion and lap together in wet forming process.F2-fiber was brown, irregular shape with a thin groove in the middle of fibre surface, the cortex was in expanded state, which was also associated with spinning process. F3-fiber has a uniformity morphology and smooth surface. Cross-sectional TEM image of aramid fibers showed large differences in micro-fine degree between fibers of the various manufacturing processes. Nomex process F3-fiber had a very tiny micro-fiber and dense array.Heat preservation and cold impregnating process were conducted to the hot pressed aramid sheet, and the effects on sheet properties were studied. After the warm keeping time of 1- 3min, the tensile strength can be increased by 8%～10%, the paper elongation increased by 10%. After cold soaking the paper tensile strength index decreased by 4%, elongation increased by 10% or more, may due to water absorbing in the process of cold soaking. Tear index showed a fluctuate trend in both of the cold impregnating and heat preserving. Cold-impregnated samples were retreated by hot-pressing and warm keeping process. It was found that the strength parameters were significantly restored, which suggested significance of temperature for aramid fiber into paper. Therefore, a gradual cooling of sheet after hot pressing on the performance of aramid paper is a very important process.Sheet forming by laminating different layers with various furnish compounds was applied for better sheet structures. The sheet consists of three layers or more. In which, more fibrids were arranged in the surface and bottom layers of sheet, and more fibres in the middle layers than that of single layer sheet, so advantages of the two aramid fibre and fibrids were fully used, and a good formation and high strength aramid papers obtained.The hot-pressing process with warm keeping of aramid sheet was optimized by orthogonal experiment; the relevant factors affecting the performance of paper and level were analyzed. The results showed that the pressing temperature, warm keeping time and pressure were the main factors of F1-paper properties. F1-paper hot-pressing process was improved as follows: pressing temperature 240℃and holding time of 3min, thermal pressure 14MPa, pressure roller speed 1.5r/min,3-time pressing.main factors of F2-paper properties were hot-pressing temperature, heat stress and pressure roller, hot-pressing holding time was the secondary factors; F2-paper hot-pressing process were hot-pressing temperature of 240℃, holding time of 2min, thermal pressure 14MPa, roller speed 1.5r/min, pressing 4 times.Paper surface in XY and Z dimensions were investigated by image analysis techniques and SEM, TEM, AFM and confocal laser scanning microscope (CLSM). Internal structure and composition of sheet and its effect on sheet properties were also researched. The response of mechanical properties and the microstructure of an aramid sheet to the hot pressing process were present in this paper. It showed that hot pressing in aramid sheet manufacturing greatly improved the total properties of the sheets. Temperatures of calendering from 130-240℃were conducted in our experiment.Sheet density, tensile, and elongation were improved considerately by factors of 4,5 and 7 times, respectively in the hot calendering process at 240℃. Scanning electron microscopy (SEM) observation showed that the surface of the sheet becomes much smoother due to the melting of some of the fibrids in the sheet, which also contributed to a stronger adhesion between the fibres and fibrids. Fibres were deformed more than 20% by calendaring and the surfaces of fibres pulled out from the sheets were clean. Fibrids acted as binder and filler for the floc in the sheet structure. The increased tensile strength combined with the increased elongation suggested that the binding between the fibres and fibrids is mainly due to physical adhesion, rather than a chemical bonding, and this is verified to be hydrogen bond by FTIR.The relationship between formation and grammages of aramid sheets were analyzed. Grammage also strongly influences the relation between formation and properties of aramid paper. The effects of formation components on properties of sheets with various grammages were discussed in detail. The results showed that the increase of grammage was benefit for improving the small scales of formation. The analysis of. components of formation theoretically demonstrated that the staple fibers mainly provide mechanical strength, while fibrids particles provide the dielectric strength for the aramid paper structure; furthermore, it can be inferred that improving of the full wave impulse are mainly depend on the furnish ratios of the two kinds of aramid fibers.Cross-sectional TEM observation of sheet showed that aramid microfibers were not melted together by hot-pressing process or a transition zone interface were formed between fiber and matrix. Different process raw materials present different interface bonding properties. Micro-fibers from F3-pulp-based matrix like fingerprints, showing a stripe and spiral-like structure. While the fingerprint features in bonding region of F1-and F2-pulp are not prominent; fiber surface of the groove marked with the pulp to form "pinning-type" interface structure; some regions of the fiber and pulp interface even form a "gap-type" interface with a great distance from each other, this phenomenon may be related to differences in heat shrinkable nature of fiber and pulp. Based on theory of droplet shape and the Young-Laplace equation, surface contact angle of a single aramid fiber were measured by using vector technology. Chemical reagents for aramid fiber surface treatment had significant impact on contact angle. DMAc, DMF, toluene, chloroform, dichloro-propane may make Kevlar fiber surface contact angle smaller, the minimum contact angle reduced to 44.9, which increase the fiber dispersion in water and improve the sheet formation.Surface energy, hot press bonding behavior and adhesion properties of aramid sheet materials were discussed. Interface microstructure characteristics and properties of aramid sheet were analyzed. Raw materials factor, manufacturing processes, thermal stresses generated by hot-pressing process of interfacial properties were comprehensive considered. The surface contact angles of several meta-aramid fibres and meta-aramid pulps as well as their sheets were measured, the surface energy of aramid fibre, aramid pulp and sheet were determined according to harmonic mean-based Wu equation. The relationship between surface energy and sheet properties in sheet processing were discussed according to surface energy as well as interfacial tension and work of adhesion of the aramid fibre and pulp. The results show that surface energy of aramid fibres is in the range of 35-45mJ/m2, the surface energy of fibres is slightly higher than pulps, polar component of surface energy is larger than dispersion one, wettability and work of adhesion are enhanced with increase of the surface energy of aramid fibres and pulp, when water and glycol are employed as the testing liquids,. The surface energy of aramid sheet decreases during hot-press process, and compared to Nomex sheet, the surface energy of self-made aramid sheet decreases sharply after hot-press process. The better matching of polar component and dispersion component, and the smaller the difference of surface energy and the smaller interfacial tension, the stronger adhesion between fibres and pulp with the improved strength of aramid sheet. Understanding the interactive force and bonding force between aramid fibres and fibrids is important in design wet-forming and hot pressing technologies for manufacturing aramid paper sheet and in producing the paper sheets of desirable properties. In this study, the morphology of both aramid fibres and fibrids were observed with SEM and AFM. The adhesion force between aramid fibres and fibrids were measured by AFM with a small piece of fibrid attached to a regular AFM tip. The results show that the adhesion force between aramid fibrid and fibrid is 9.20±0.86nN and that between fibrid and fibres is 1.71±0.42nN. This explains the importance of the fibrids in the aramid paper sheet in helping bonding between the aramid fibres and gives the resulting sheet a strong physical structure. The adhesion force measured by AFM was confirmed by a theoretical calculation with the Derjaguin-Muller-Toporov (DMT) theory.A relationship between fibre structure and thermal property has been found. Firstly, the structure of aramid fibre during the hot-calendering process have altered notably, the viscosity and viscosity-average molecular weight of aramid fibre decreades with increase of fibre ohesive energy density, crystallinity and glass transition temperature. Additionally, GPC-MALLS testing results show the average molecular weight and molecular weight distribution coefficient of aramid fibre and pulp are 100～150 thousand and 1～2 respectively. XRD testing results indicate that the crystallinity of aramid fibre is higher than that of aramid pulp with non-crystal structure thereof. And it is concluded from DSC-TG that the pyrolysis peaks can only be found within the aramid fibre, in contrast, the glass transition temperature (start at 270℃) and re-crystallinity (start at 300℃) happens in aramid pulp with increased temperature. The pyrolysis temperature of aramid fibres can heighten with increase of average molecular weight, decrease of molecular weight distribution coefficient and improved re-crystallinity, and re-crystallinity ability depends on primal structure of the aramid fibre. Pyrolysis causes increased low molecular weight and deepened color of aramid fibre, and the integral intensity of DSC pyrolysis peak and heat-resistant ability can be enhanced with increased crystallinity at the start of pyrolysis process.The relationship between surface energy and paper properties in paper processing were discussed according to the surface energy, interfacial tension and work of adhesion of the aramid fibre and pulp. The results show that surface energy of aramid fibres and pulps is in the range of 35～45mJ/m2, the surface energy of fibres is slightly higher than that of pulps, and the polar component of surface energy is larger than the dispersion one, the wettability and work of adhesion are enhanced with increase of the surface energy of aramid fibres and pulp, when water and glycol are employed as the testing liquids,. The surface energy of aramid paper decreases during hot-press process, and compared to Nomex paper, the surface energy of self-made aramid paper decreases sharply after hot-press process. The better matching of polar component and dispersion component, and the smaller the difference of surface energy and the smaller interfacial tension, the stronger adhesion between fibres and pulp with the improved strength of aramid paper but left unclear laws upon tear strength of aramid paper.