At present, the microfiber has no accurate definition in the world. The definitions given by every country are slightly different, but they all take the line density as standard. If the line density of PET is lower 1.2 dtex and PA’s is lower than 1.0 dtex, this kind of single fiber is called microfiber by German Textile Association. American PET commission think that the fiber with size of 0.3-1.0 dtex is microfiber. Italy takes the fiber with less than 0.5 dtex as microfiber. Japanese chemical fiber industry generally calls the fiber which is less than 0.3 dtex as microfiber. However, at present, the industry takes the fiber with less than 1.1 dtex as microfiber.
Microfiber is a typical representative of synthetic fibers which develop towards high-tech and high simulation. The superfining of synthetic fiber originates from the constant pursuit of people to the functionalization of fiber. Its development history is to continuously simulate and exceed natural fiber, which basically can be divided into three stages.
The first stage is the simulation to silk. In 1960s, Japanese companies developed special spinning method and stripping method according to different methods, so that creating distinctive microfiber is produced, such as multicore type, radial type, etc.
The second stage is the imitation to the performance of natural fiber in 1970s, which focuses on the modification of synthetic fiber. In 1980s, on one hand, it pays more attention to natural of synthetic fiber, and on the other hand, it devotes to discover special functionality of chemical fibers. Microfiber is a representative of new synthetic fiber.
The third stage is to research and explore the function of microfiber since 1990s. Its characteristics are explored and application area is broadened. Many new kinds of microfiber are developed, such as superfine fiber, functional microfiber and so on.
1 The manufacturing method of microfiber
According to different technologies, the microfiber with different size, types and application can be produced. The manufacturing methods can be divided into direct spinning, compound segmentation and sea-island method.
1.1 Direct spinning method
Traditional melt spinning technology is used to manufacture filament type microfiber. The main advantage is that the microfiber containing single component can be directly obtained. In the process, peeling or dissolution of two components is not needed, so the cost is less. The disadvantage is strict process conditions. Because the linear density of monofilament is small, it tends to appear end breaking, broken filament, uneven linear density and the change of sectional shape, as well as it is difficult to obtain fiber with high quality. Currently the most thin microfiber produced by direct spinning is “Besaylon” of Asahi Kasei Corporation, the linear density of monofilament is 0.165dtex.
1.2 Compound segmentation method
Several kinds of polymer components, which are thermodynamically in compatible but have similar viscosity, come together along the channel in spinning pack and form preseted crosssectional shape, such as cross shape and stratiform. The key of segmentation method is to improve the amount of segmentation of two components, otherwise it is impossible to achieve superfining. In opening process mechanical or chemical method usually is used to separate every component, so that the mixed microfiber with abnormal shape is obtained. There is no need for specific components to be dissolved out, so the polymer has no loss. The commonly used is split fiber of PA6/PET.
1.3 Sea-island method
Two（or more）kinds of polymers which are thermodynamically incompatible are used to manufacture special fiber by blend spinning or composite spinning, which is called sea-island fiber. One of the polymers(dispersed phase)is dispersed in another polymer(continuous phase), if the continuous phase is removed, the microfiber formed by dispersed phase can be obtained. If the dispersed phase is removed, the hollow and porous fibers can be got. At present, the components used as island are PET, PA6, PVN, etc. These used for sea include PS and PE which is soluble in solvent, COPET can be soluble in hot alkali solution and PVA soluble in hot water. The microfiber which can be used in synthetic leather can be classified into figured and unfigured sea-island fiber according to spinning technology.
1.3.1 Figured sea-island fiber
The molten ”sea” and “island” components are fed by respective screw extruder, and then they mixed together in spinning pack. The distribution of island component is continuous and uniform in the direction of fiber length, as well as the number of island is fixed and their size is consistent. After composite spinning they exist in normal size, only when the “sea” component is dissolved away, real microfiber can be got. The size of monofilament is about 0.05-0.08dtex, which can be as staple and filament. For figured sea-island fiber, PA or PET usually is as island component an COPET as sea components.
1.3.2 The unfigured sea-island fiber
The sea and island components are blended in the same screw extruder, and then through melt spinning the unfigured sea-island fiber is made. After spinning the fibers exist in normal size. The size, number, distribution and length of island in unfigured sea-island fiber are all random within certain range. The number of island is more, so the average linear density is smaller. After the sea component is extracted by solvent, the fibers are sarcniform and their size usually is 0.01-0.001dtex, even it can reach 0.0001dtex, which is more similar to collagen fiber. Because microfiber is formed through stretching of particles, it only can produce staple fiber but not filament. The commonly used is PA6/PE, PS/PA6, PET/PE, etc.
In the manufacturing process of microfiber, different spinning methods often are adopted according to the desired fineness. When the fineness is more than 0.55dtex, direct spinning method generally is used, for 0.33-0.55dtex segmentation method usually is used. For 0.11-0.33dtex, segmentation method or sea-island generally is used and for 0.0011-0.11dtex, sea-island method can be used.