Study On Relationships Between Structure Of Aramid Fibers And Aramid Pulp And Properties Of Aramid Paper

Aramid papers are specialty papers made from aramid fibers and aramidpulp, which are well known for their excellent mechanical properties, hightemperature resistance and electrical insulating property, and are widely used inelectrical insulation, aviation, transportation and other key areas. Highperformance aramid paper is a high value-added electrical insulating paper andstructural material paper, is the strategic new material of our country, but theproduction technology of it is exclusively monopolized by the Dupont Company.Though aramid papers are achieved domestically currently, domestic aramidpapers are obviously weaker in formation, strength and color than Nomex papers.The reasons mainly lie in two aspects: firstly, demestic aramid fiber (especiallyaramid pulp) itself is not quite suitable for making aramid paper; secondly, theoptimum aramid paper microscopic structure and fiber interface bonding stateand mechanism are not very clear, which limits the production technologicalimprovement of high performance aramid paper. To break the technicalbottleneck of high performance aramid paper production, it is necessary toestablish a set of new synthetic fiber papermaking theory which is different fromthe traditional papermaking theory, to reveal the relationship between fiberstructure, thermal properties, interfacial bonding and paper structure, paperproperties. In this research, the relationship between the structure and thermalproperties of aramid pulp and the structure and properties of aramid papers hasbeen analyzed, and the influence of different surface modification methods onthe fiber surface structure and aramid paper structure has also been investigated.The aim is to analyze and characterize the optimum configuration of aramidpulp for making aramid papers, to evaluate the main factors influencing thestrength of aramid paper, to elaborate the relationship between the distribution of aramid fibers and aramid pulp and the structure and properties of aramid paper;to discuss fiber surface modification mechanism and its effect on the interfacialbonding properties, thereby provide necessary basic data and development goalsfor domestic aramid fiber production enterprises, and provide theoretical andtechnical support for improving fiber surface modification and interfaceenhancement technology, as well as for optimizing the forming and hotcalendaring technology of aramid papers to improve their quality.Bauer screen was used to screen the aramid pulp, and fiber quality analyzer,BET specific surface area analyzer, Schopper-Riegler beating degree tester,X-Ray Diffraction (XRD) and other methods were used to determine the averagefiber length, specific surface area, beating degree, crystallinity, and molecularweight of aramid pulp, and the relationship between the structure of aramid pulpand the properties of aramid papers was discussed. Results show aramid pulp isamorphous, and has lower crystallinity. Increasing the temperature and exertingpressure can improve the crystallinity of aramid pulp, and the molecular weightof aramid pulp has an effect on the crystallization rate. The crystallization rate ofaramid pulp with higher molecular weight is lower than that of aramid pulp withlower molecular weight. Aramid fibers and aramid pulp in uncalendered aramidpapers are bonded by simple physical blend, so the surface morphology ofaramid pulp is the fundamental determinant of the uncalendered aramid paperstrength, and the strength of uncalendered aramid papers improves with theincrease of specific surface area and beating degree of aramid pulp, however, themolecular weight and crystallinity of aramid pulp have little influence on theuncalendered aramid paper strength. After hot calendaring, the density andstrength of aramid papers are greatly increased, and the higher the crystallinityand density, the lower the porosity, the higher the paper strength. Aramid pulpwith medium molecular weight is suitable for making high strength aramidpaper.Differential Scanning Calorimetry (DSC) was used to determine the thermalperformance parameters of aramid fibers, aramid pulp with different molecularweight and aramid papers of different sources, and cold crystallization behavior,component compatibility and thermostability of aramid fibers, aramid pulp andaramid papers were analyzed. Results show that the glass transition temperature (Tg) of aramid fibers, aramid pulp and aramid papers are all higher than270℃,that is, they all have good heat resistance. However, they are different from eachother, and among them, the Tgof aramid fibers is higher that that of aramid pulp,and the Tgof aramid papers is in between. This is because that aramid fibers havehigh crystallinity after stretch orientation in the process of production, butaramid pulp has different process from aramid fibers, and it has low crystallinity.Aramid papers are made from the mixture of aramid fibers and aramid pulp, soits Tgis in between aramid fibers and aramid pulp. Furthermore, aramid papershave only one Tg, which indicates that the two components in aramid papershave good compatibility.Cold crystallization peak was also observed in the DSC of aramid fibers,aramid pulp and aramid papers when the temperature is higher than the Tg, andcompared with aramid fibers, aramid pulp has higher cold crystallizationtemperature and lower cold crystal enthalpy, and it indicates that aramid pulp hasgreater potential in increasing crystallinity through cold crystallization. Themelting temperature of aramid fibers, aramid pulp and aramid papers are in therange of430~445℃, and aramid fibers has the highest melting temperature andheat absorption capacity, but aramid pulp has the lowest melting temperature andheat absorption capacity. Because aramid paper is made from aramid fibers andaramid pulp, and it has wider molecular weight distribution, so its endothermicreaction is the slowest and its melting range is the longest. For aramid pulp andself-made aramid paper, there appears obvious pyrolysis peak when thetemperature is in the range of480~495℃.The glass transition temperature Tg, melting temperature and pyrolysistemperature of aramid pulp increase with the increase of its molecular weight,which indicates that the heat resistance of aramid pulp is closely related to itsmolecular weight and the uniformity of its molecular weight distribution. Thereare cold crystallization peaks in the temperature range of385~395℃for aramidpulp with different molecular weight.The melting temperature and heat absorption capacity of two domesticaramid papers (MetaStarTMand X-Fiper) are higher than that of Nomex papers,that is, the heat resistance of domestic aramid papers are superior to that ofNomex papers. However, Nomex papers have narrower endothermic peak than the two domestic aramid papers, which indicates that Nomex papers have moreuniform molecular weight distribution, and this may be one of the reasons fortheir superior physical strength.Dynamic Mechanical Analyzer (DMA) was used to discuss the storagemodulus, loss modulus and loss factor of self-made aramid papers, Nomexpapers and pure aramid pulp paper before and after hot pressing, and to find howthese parameters change with temperature. Combined with the changes incrystallinity, the glass transition, relaxation of the chain segments and otherstructure differences and change process of different aramid papers werediscussed, thereby analyzing the influence of fiber structure and hot pressing onthe physical properties of aramid papers. The results show that the Tgofself-made aramid before and after hot pressing are a little higher than that ofNomex papers, self-made aramid papers have better heat resistance, and thisresults from the higher molecular weight of aramid fibers and aramid pulps.However, the storage modulus of self-made aramid papers before hot pressingare far less than that of Nomex411papers. On the one hand, it is related to thecrystallinity of aramid raw materials, on the other hand, it is related to theinterfacial bonding between aramid fibers and aramid pulp. The lower initialcrystallinity and weaker interfacial bonding make the modulus and strength ofself-made uncalendered aramid papers far lower than that of Nomex papers. Hotpressing significantly improves the modulus of self-made aramid papers, Nomex410papers and pure pulp aramid papers. Heating makes aramid pulp partlymolten, and the specific surface area of aramid pulp increases under the pressure,which increases the interfacial bonding between aramid pulp and fibers.Meanwhile, aramid molecular chains rearrange and become more regular duringhot pressing, which improves the orientation and crystallinity of molecularchains, and increases the molecular interaction, thus improving the modulus andphysical strength of aramid papers. Because fiber distribution in self-madeuncalendered aramid papers is disorderly and irregualarly, after hot pressing,there are still some voids in self-made aramid papers, and the interfacial bondingbetween aramid fibers and pulp in self-made aramid papers is less than that inNomex410papers, which leading to a lower modulus and strength of self-madearamid papers than that of Nomex410papers. Various kinds of characterization methods were used to analyze the effectsof paper formation, fiber orientation distribution on paper properties. Resultsshow that β Formation Tester and Micro-scanner Formation Analyzer aresuitable for uncalendered aramid papers, and the paper formation characterizedby these two methods have good relation with the physical strength of aramidpapers. The bigger the basis weight standard deviation of uncalendered aramidsheets, the less the tensile strength and tear strength of them, and the higher theformation index by Micro-scanner formation analyzer, the higher physicalstrength of uncalendered aramid papers. After hot pressing, the tensile strengthof aramid papers have good relations with their basis weight standard deviationand Paper Perfect Formation (PPF) values, but there is no clear correlationbetween the tear strength of aramid papers and their formation determinationresults.Small size clouds on the surface of aramid papers is unfavorable to thestrength of aramid papers, while large size clouds on paper surface is beneficialto the strength of aramid papers. Changing papermaking technology andallowing more aramid pulp deposit onto the surface of paper will improve thesurface smoothness and texture of aramid papers, thus improves their physicalstrength.Tensile stiffness orientation angle (TSOAngle) of calendered aramid papers isfar larger than that of the common writing paper, printing paper and packagingpaper. The tensile stiffness index in machine direction (TSIMD), tensile stiffnessindex in cross machine direction (TSICD) and TSIMD/CDof two domestic aramidpapers and self-made aramid papers are all lower than that of Nomex410papers.The lower tensile stiffness of domestic aramid papers mainly due to their lowermodulus, and the lower TSIMD/CDof domestic aramid papers relates to theirforming and hot-pressing technology. Changing the papermaking technology andhot-pressing technology to improve the fiber orientation as far as possible andavoiding overcompaction during hot pressing are useful for improving the TSIMDand TSIMD/CDof aramid papers.Air permeability of self-made aramid papers, especially air permeability oftwo domestic aramid papers are higher than that of Nomex410papers with thesame basis weight, but the density of Nomex410papers are lower than that of self-made and two domestic aramid papers. Multi-ply forming can let morearamid pulp cover on the surface of aramid paper like a thin film, therebyimproves the surface density of aramid paper and decreases its air permeability,which is beneficial to improve the smoothness, strength property and electricalperformance of aramid papers.Hot pressing decreases the whiteness of aramid papers, and surfacefunctional groups change of aramid pulp during hot pressing is perhaps the majorreason. Addition of bismuth oxide or bismuth nitrate into aramid paper caninhibit oxidation and reduce whiteness reversion of aramid paper during hotpressing, however, the tensile strength of aramid paper has no obviously change.In this work, surface modification of aramid fibers were performed by usingdifferent consistency of phosphoric acid solution (PA), silane coupling agent(KH-550), nitration/reduction agent and sodium hydroxide solution (NaOH),respectively, and modified aramid fibers were used to make aramid papers witharamid pulp, then the optimal modifying process was studied. Results show thattreatment with20%phosphoric acid solution roughs the surface of aramid fibersand increases the oxygen content of fiber surface, thus improves the interfacialbonding between aramid fibers and aramid pulp and increases the tensile strengthand tear strength of aramid papers. Surface modification of aramid fibers bysilane coupling agent, nitration/reduction agents and NaOH can all increase thebonding between aramid fibers and aramid pulp at a certain extent, and improvethe tensile strength and tear strength of aramid papers. In contrast, surfacemodification with20%phosphoric acid has the best effect on improving thestrength of aramid papers, silane coupling agent and NaOH modification areworse than20%phosphoric acid, but better than nitration/reductionmodification.