Study On The Key Processing Technology And Properties Of Kapok Fiber Based On Its Microstructure

Kapok fiber is a non-cotton, natural cellulosic fiber which is harvested from the kapok fruit and has many unique properties relatively new to the textile industry. Kapok is a single-cell fiber with extremely high degree of hollowness (80%-90%) which is by far the largest in natural fibers. It has smooth surface without distinct convolution and extremely low specific gravity (0.29 g/cm3). Kapok appears in a cylindrical shape with round or oval cross-sections, and has a thicker wall in the middle and closed ends. Kapok is a natural fine fiber which has anti-bacterial, anti-mite and mildew-proof functions and can reproduce rapidly and degrade easily. Kapok tree can grow in the hilly area, and therefore can ease the tension in the land and the scarcity of resources. It has great potential to be a new type of alternative textile materials with many practical applications and high economic value under the goal of the sustainable development. However, hollow kapok fibers are easy to be squashed and broken during processing and use. This change would undermine the performance of the end-use products that take advantage of kapok’s hollowness. Therefore, the kapok fiber was limited as the buoyancy, filler and oil-absorbing materials for a long time.With the improvement of the kapok spinning technology, it was possible to produce the kapok blended yarns with the uniform evenness and the improved tensile strength after applying the freshly new pretreatment on the kapok fibers. However, the kapok fiber packing structure in the assembly and its hollowness can be changed after the repetitive pressure during manufacturing. Eventually it would lead to alter the structural uniformity of the sliver and the controllability of the fiber itself. Consequently, there were many significant questions emerging when using the kapok fibers under the repeated compressions, such as whether the piling structure of the kapok fiber in the assembly will change? Whether the cellulosic capsule of the single-cell fiber will break? Whether it is easy to revert the hollow structure of the squashed kapok fiber? Whether the end-use products could take the advantage of kapok’s hollowness? These questions needed to be answered in the application of kapok fiber and were also the purposes of this research.With these purposes, many research tasks including the microstructure of the kapok fiber, the compressibility and resilience of the kapok fibrous assembly, and the key processing technology and performance of the kapok blended yarns and fabrics were proposed and performed in this research. The main research methods and results were as follows:(1) The micro-structure of kapok fiber, including the fiber specific surface area, pore volume, structure, diameter size and distribution etc., were analyzed through the static nitrogen adsorption test. It was found that kapok possessed the nitrogen adsorption isotherm of type II which was defined by the International Union of Pure and Applied Chemistry (IUPAC) and reflected that the adsorption behavior of the kapok fiber was like the reversible adsorption on the surface of the non-porous solid. In the relative pressure area (0.85～1.0) the adsorption isotherm appears the hysteresis loop which could be recognized as H3 hysteresis behavior defined by IUPAC. Accordingly it can be concluded that the pore structure of the kapok fiber was slit pore structure. In addition, based on the adsorption experiment and BET multi-layer adsorption model, it was found that the specific surface area of the kapok fiber was evaluated as 2.99 m2/g in the relative pressure range between 0.05 to 0.35; the pore size distributed in 4～40 nm and mainly concentrated on 3～4 nm; the volume of the pore with the diameter of 2-40 nm accounted for about 80% of the total pore volume, and those with diameter over 40 nm accounted for about 20%. Compared with the macro-pore observed by the scanning electron microscope with the pore diameter around 300nm, it could deduced that there are smaller pores in the kapok fiber cell walls like meso-pore. By the Va～αs micro-pore evaluating method, it could be obtained that the micro-pore volume of the fiber is about 7.0399E-llml/g. These pore distribution characteristics of the kapok fiber plays a decisive role on the adsorption of the flame retardant finishing auxiliaries and the recovery rate of the hollowness.(2) Four different pretreatments on the kapok fiber assembly (KFA) were designed to study the dependence relationship between its compressibility and the pretreatment conditions including the relative humidity (RH) and external pressure. By Kawabata Evaluation System (KES)-FB3 the compressibility of the KFA under the low compressional load (0～50cN/cm2) was measured. The compression tests demonstrated that the wet treated kapok fibrous assembly possessed a much lower compressional resilience than the dry treated ones, and the bulkiness of the wet treated kapok fibrous assembly after repeated pressure treatment diminished. Though the bulkiness of the dry treated and the wet treated kapok fibrous assemblies both descended after the pressure treatment, the loss of interspaces among fibers and the hollow structure of kapok fiber in the dry-pressure assemblies was much less than that of the wet-pressure treated ones. The wet-pressure treated samples were easier to be squeezed. Both humidity and pressure can significantly affect the assembly’s fluffy structure and the kapok fibers’hollow structure. These results indicated that it should decrease RH or fiber moisture regain to retain the hollowness during processing and should increase RH or fiber moisture regain to squash the fiber in the packing or the pretreatment of spinning.(3) The compressional resilience of the KFA under the compressional load (0～ 140N/cm2) was measured by Instron compression tester, and several indicators were used to describe its performance under the repetitive compression cycles. It was found that there were two yielding points and three main compressional stages in the single compressional stress-strain curve. Before the first yielding point, the KFA was in a low volume fraction, and had few contact fibers points and plenty of air among fibers. Thus the assembly deformed with an extremely low compressional modulus. At the first yielding point, the assembly was compacted to some compactness which made the fibers contact more tightly and ideally like fiber piling structure. With the compression continued, plenty of the fibers in the KFA started to be squashed which made the strain of the KFA increase rapidly until the second yielding point. At the second yielding point, the kapok fiber were squeezed as ribbon-like and even crushed and the KFA started to densify. After the second yielding point, the compressional strain of the KFA became very small. It was also found that the conditioning humidity made the wet KFAs deform easily and lowered its compressional strain drastically. However, both the dry and wet KFAs could revert the hollowness in the same level after unloading the compression which meant the reduction of the compressional resilience both on the dry and wet KFAs were similar. These results reflected that the wet pretreatment on the kapok fiber was important and the hollowness of the fiber wouldn’t lose easily.(4) A simple pipe-piling model was adopted to simulate the fiber arrangement in the kapok fibrous assembly. This model appears to be effective in estimating the changes of the assembly’s cross section area. A visco-elasto-plastic mechanical model based on the pipe-piling structure which was named Nishihara model was introduced. According to the compressional stress-strain relationship of the KFA in the three respective stages, three mechanical constitutive equations could be built from Nishihara model, and four mechanical parameters, including E1, E2, η1 and η2, could be worked out to characterize the compression behavior of the KFA quantitatively. It could be found that the KFA exhibited the elastic behavior before the first yielding point, performed the visco-plastic behavior between the two yielding points, and showed the transform from the elastic to visco-elastic behavior in the compressional stage after the second yielding point. These compressional characteristics could appeared repetitively in the compression cycles and the model parameters used to reflected it would change with the structure of the KFA. The conditioning humidity generally improved the consistence of the model parameters of the KFAs. It increased the strength of the Bingham model regarding the plasticity of the KFA, but lowered the viscosity of the Bingham model between the two yielding points. It also greatly increased the visco-elasticity of the KFAs after the second yielding point. The proposed visco-elasto-plastic model worked well in representing the properties of the KFA under repetitive compressions. These results demonstrated that the wet pretreatment was positive to remain the stable structure of the sliver and roving under the multiple pressure and draft by the roller and to improve their smoothness and evenness.(5) The new static pretreatment that supplements the humidification on the kapok fibers with a surface treatment for 24 hours was proposed to meet the demand of the batch processing in mechanization. The kapok fibers were sprayed by the treating fluid and kept in a close room for 24 hours which made the fluid be attached equably on the surface or be absorbed evenly in the internal pores of the kapok fiber by wicking, diffusion and evaporation. The pretreatment increased the weight of the fiber itself and the cohesive force which could resolve the difficulty on combing, meshing and rolling in the spinning. The pretreatment also can increase the flexibility of the fiber, reduce fiber damage and improve the quality of yarn. In addition,18 kapok blended yarns manufactured by the four different spinning technology, including embedding composite spinning, compact spinning, ring spinning and rotor spinning, were measured and evaluated. It could be found that the fineness evenness and the defect of the blended yarns made by the compact spinning and technology Ⅳ processing were better than that of the blended yarns made by the other three spinning and processing technologies. Additionally, the kapok blended yarns made by the ring spinning technology, the breaking tenacity of the yarns were higher, especially the yarns treated by the technology III which could supply the superior grade strength to yarn. Compared with the technology II, the technology III also did better on reducing the hairiness. The kapok blended yarns made by the rotor spinning have less defect but more hairiness, and they were worse than the yarns made by the other three spinning technologies in fineness evenness and breaking tenacity.(6) 6 kapok blended fabric and 3 comparative cotton fabric in different blending ratio, linear density and structure were manufactured to explore the processing technology and study the relevant characteristics. It can be found that the kapok blended yarn needed more consideration when manufacturing and the kapok blended yarn quality including its hairiness index and yarn strength required a stricter weaving system of the loom tension, back rest, air pressure, shedding mechanism, workshop temperature and humidity etc.. The structural and mechanical properties were studied comparatively between the kapok blended fabrics and the cotton fabrics. Some conclusions can be obtained as follows:the kapok fiber improved the compressibility and hand of its blended fabrics for the fine linear density and larger hollowness of the kapok fiber; it also increased the porosity and enhanced the anti-pilling property of the blended fabrics; the surface friction properties of the kapok blended fabric were similar with the cotton fabric, but the surface roughness of kapok blended fabric slightly less than the cotton fabric which meant the 20-30% kapok fiber in the weft yarn did not effectively improve the surface friction properties. Additionally, the napping finishing can promote the bulk of the blended fabrics without decreasing their compressional resilience. The thermal properties of the fabric were also studied through the air permeability, warm/cool feeling and thermal conductivity tests. It was found that the air permeability, qmax as the warm/cool feeling index and thermal conductivity of the fabrics generally reduced as the kapok fibers were blended which denoted that the kapok fiber could increase the windproof and thermal insulating properties and give a warm touch feeling of the fabric. The proposed napping finishing methods both of the peach skin and fleece finishing were positive on imparting a warm touch feeling and low thermal conductivity to the fabrics. The thickness was a dominating factor on the thermal resistance of the fabrics and the hairiness of kapok blended yarn, fabric weight and other factors also affected the thermal properties of the fabric.