The Influence of the Multi-level Structure Under High Drawing on the Preparation of High Strength Lyocell Fiber

In order to research the multi-level structure of Lyocell fiber at different draw ratios and to reveal the limiting factors for preparing the high strength Lyocell fiber, the paper reports on the effect of draw ratio including low drawing (1–5), high drawing (6–11) and excessive drawing (12–20) on the multi-level structure and the mechanical properties of Lyocell fiber. The structure was determined by wide-angle X-ray diffraction, small-angle X-ray scattering and fibrillation test, and the result showed that, at low draw ratio stage, the breaking strength, yield strength and modulus of the fiber increased with the draw ratio owing to crystallinity as well as orientation increased while the micropore decreased, and there are almost no microfibrils on the fiber surface. At high draw ratio stage, the orientation of amorphous region increasing was the principal reason for the increase of fiber mechanical properties, and the micropores continued to decrease and a few short and thick microfibril was formed. At excessive draw ratio stage, the breaking strength remained constant mainly due to the basically unchanged crystallinity and orientation of the fibers, the yield strength and modulus decreased due to the slip of the highly crystallized and oriented elementary fibril. Meanwhile, the micropores still decreased and became much slenderer, the number of microfibrils increased and the microfibrils showed tenuous structure. It could be summarized that Lyocell fiber had the characteristics of multi-level structure, and the fundamental reason limiting the improvement of mechanical properties with draw ratio increase was the slip of elementary fibril.

A Review of the Mechanical and Physical Properties of Polyethylene Fibers

Since the 1970s and 1980s, a major effort has been made to study UHMWPE (Ultra-High Molecular Weight PolyEthylene) fibers with remarkable mechanical properties, based on a basic polymer such as PE (PolyEthylene). These performances are above all associated with a very strong alignment of the molecules and the microfibrillar structures formed using various processes. However, they vary greatly depending on many parameters, and particularly on the draw ratio. Thus, these characteristics have been extensively analyzed by dynamic, static tensile, and creep tests, and are predominantly viscoelastic. The behavior appears to be associated with physical considerations and with the characteristic orthorhombic-hexagonal solid phase transition. The presence of a hexagonal phase is detrimental to the behavior because the chains slide easily relative to each other. Shifting this transition to higher temperatures is a challenge and many factors influence it and the temperature at which it takes place, such as the application of stress or annealing. The objective here is to give an overview of what has been done so far to understand the behavior of UHMWPE yarns. This is important given future numerical modeling work on the dimensioning of structural parts in which these UHMWPE yarns will be reinforcements within composites.

Strain-sensing fiber with a core–sheath structure based on carbon black/polyurethane composites for smart textiles

Fiber-shaped sensors have great potential for real-time monitoring of human physiological signals thanks to the merging of electronic and textile technologies. This work reports on the fabrication of a core–sheath structured strain-sensing fiber based on the wet-spinning method. The sensing fiber is composed of a core of non-conducting polyurethane and a conducting sheath of carbon black in a polyurethane matrix. Microscopic observation reveals the irregular shape or scattered appearance of the core as well as the porous structure of the fiber, the diameter of which is in the range 200–500 μm. The electro-mechanical properties and their dependence on carbon black concentration in the sheath and draw ratio between the spinneret and first drafting roller are experimentally investigated. It has been found that the percolation threshold of the fiber is in the range 15–16 wt%. The resistance of the fiber rises stably as the fiber is stretched up to a strain of 120% and increases with the increase of draw ratio between the spinneret and first drafting roller. In the cyclic tensile tests, the resistance of the fiber exhibits good repeatability in subsequent loading–unloading cycles after pre-stretching, despite partial recovery of the resistance in the first few cycles. The integration of the strain-sensing fiber into textiles is demonstrated by the core-spun yarn fabricated based on a modified vortex spinning method. The results of this study indicate the fiber could be a promising candidate for a sensor for smart textiles.

Evaluation of the crimp formability of side-by-side PLA/PTT bicomponent fibers

To explore the feasibility of developing bio-based elastic fibers, bio-based polylactide (PLA) and polytrimethylene terephthalate (PTT) were selected for fabrication of side-by-side bicomponent fibers using bi-constituent melt-spinning technology. The structure development and performance of PLA/PTT bicomponent fibers was investigated using thermal mechanical analysis, differential scanning calorimetry, and wide-angle X-ray diffraction in order to evaluate the crimp formability of PLA/PTT bicomponent fibers. The PLA and PTT components could form regular boundary structure and exhibit excellent interface compatibility by regulation of the rheological behavior of the two melts. In the fiber forming process, the PTT component in PLA/PTT bicomponent fibers experienced higher tensile stress, and thereby enhanced the crystal and oriented structure development, while the structural evolution of the PLA component was inhibited. The difference in the structure of the two components causes the imbalance force existence in the PLA/PTT fibers, which is the main reason of fiber crimp. In addition, the crimp formability of the PLA/PTT fibers could be enhanced by expanding the shrinkage stress difference between the two components, which could be realized by increasing the PTT ratio in PLA/PTT bicomponent fiber or draw ratio. The maximum crimp extension that could be achieved was 85% for the bicomponent fibers with PLA30/PTT70 ratio at a draw ratio of 4.2.

Orientation of Poly(ε-caprolactone) in Its Poly(vinyl chloride) Blends Crystallized under Strain: The Role of Strain Rate

The blends of high and low molecular weights poly(ε-caprolactone) (PCL) with poly(vinyl chloride (PVC) were prepared. The samples before and after the crystallization of PCL were uniaxially stretched to different draw ratios. The orientation features of PCL in a stretched crystalline PCL/PVC blend and crystallized from the amorphous PCL/PVC blends under varied strains were studied by wide-angle X-ray diffraction (WAXD). It was found that a uniaxial stretching of crystalline PCL/PVC blend with high molecular weight PCL results in the c-axis orientation along the stretching direction, as is usually done for the PCL bulk sample. For the stretched amorphous PCL/PVC blend samples, the crystallization of high molecular weight PCL in the blends under a draw ratio of λ = 3 with a strain rate of 6 mm/min leads to a ring-fiber orientation. In the samples with draw ratios of λ = 4 and 5, the uniaxial orientation of a-, b-, and c-axes along the strain direction coexist after crystallization of high molecular weight PCL. With a draw ratio of λ = 6, mainly the b-axis orientation of high molecular weight PCL is identified. For the low molecular weight PCL, on the contrary, the ring-fiber and a-axis orientations coexist under a draw ratio of λ = 3. The a-axis orientation decreases with the increase of draw ratio. When the λ reaches 5, only a poorly oriented ring-fiber pattern has been recognized. These results are different from the similar samples stretched at a higher strain rate as reported in the literatures and demonstrate the important role of strain rate on the crystallization behavior of PCL in its blend with PVC under strain.