Polyamide, commonly known as nylon (Nylon), English name Polyamide (PA), is a general term for thermoplastic resins containing repeating amide groups—[NHCO]— on the main chain of the molecule , including aliphatic PA, aliphatic-aromatic PA and aromatic PA . Among them, aliphatic PA has many varieties, large output and wide application, and its name is determined by the specific carbon number of the synthetic monomer. It was invented by the famous American chemist Carothers and his scientific research team.
Molecular Structure
Commonly used nylon fibers can be divided into two categories.
One type is polydiacid diamine obtained by polycondensation of diamine and diacid. The chemical structure of its long-chain molecule is:
H-[HN(CH2)xNHCO(CH2)yCO]-OH
The relative molecular weight of this type of nylon is generally 17000-23000. Depending on the number of carbon atoms of the diamine and the dibasic acid used, different nylon products can be obtained, which can be distinguished by the number added to the nylon. The first number is the number of carbon atoms of the diamine, and the latter number is The number of carbon atoms of the dibasic acid. For example, nylon 66 indicates that it is made by polycondensation of hexamethylene diamine and adipic acid ; nylon 610 indicates that it is made of hexamethylene diamine and sebacic acid .
The other type is obtained by lactam condensation polymerization or ring-opening polymerization, and the chemical structure of its long-chain molecule is:
H-[NH(CH2)xCO]-OH
According to the number of carbon atoms contained in its unit structure, different varieties can be named. For example, nylon 6 shows that it is obtained by ring-opening polymerization of caprolactam containing 6 carbon atoms.
Nylon 6, nylon 66 and other aliphatic nylons are all composed of linear macromolecules with amide bonds (-NHCO-). Nylon molecules have -CO- and -NH- groups, which can form hydrogen bonds between or within molecules , and can also be combined with other molecules, so nylon has good moisture absorption capacity and can form a better crystalline structure.
Since the -CH 2- (methylene) in the nylon molecule can only generate weak van der Waals force , the molecular chain of the -CH 2 -segment has a higher degree of curling. Because of the different number of -CH 2 -of various nylons , the intermolecular hydrogen bonds are not completely the same, and the probability of molecular curling is also different. In addition, some nylon molecules have directionality. The directionality of the molecules is different, and the structural properties of the fibers are not exactly the same.
Morphology
The morphological structure of nylon made by melt spinning method observed in the microscope has a circular cross section and no special longitudinal structure. The filamentous fibril structure can be observed under the electron microscope , and the fibril width of nylon 66 is about 10-15nm. If the special-shaped spinneret is used, it can be made into nylon with various special cross-sectional shapes, such as polygonal, multi-leaf, hollow and other special-shaped cross-sections. Its focused state structure is closely related to the stretching and heat treatment of the spinning process. The main chains of macromolecules of different nylons are all connected by carbon atoms and nitrogen atoms.
Shaped fiber can change the elasticity of the fiber, make the fiber have special luster and bulkiness, and improve the cohesive performance and covering ability of the fiber, as well as anti-pilling, reducing static electricity and other properties. For example, the triangular fiber has a flashing effect; the five-lobed fiber has a plump luster, has a good hand feel, and resists pilling; the hollow fiber has a cavity inside, which has a low density and good warmth retention.
Polyamide (PA, commonly known as nylon) is the first resin developed by DuPont in the United States for fiber, and it was industrialized in 1939. In the 1950s, it began to develop and produce injection molded products to replace metal to meet the requirements of downstream industrial products to reduce weight and reduce costs. The main chain of polyamide contains many repeating amide groups. It is called nylon when used as plastics, and we called nylon when used as synthetic fibers. Polyamides can be made from diamines and dibasic acids, or omega-amino acids or rings. Lactam to synthesize. According to the number of carbon atoms contained in diamines and diacids or amino acids, a variety of different polyamides can be prepared. There are as many as dozens of polyamides, including polyamide-6, polyamide-66 and polyamide Amide-610 is the most widely used.
The link structures of polyamide-6, polyamide-66 and polyamide-610 are [NH(CH 2 ) 5 CO], [NH(CH 2 ) 6 NHCO(CH 2 ) 4 CO] and [NH(CH 2 ) 4 CO], respectively 2 ) 6 NHCO(CH 2 ) 8 CO]. Polyamide-6 and polyamide-66 are mainly used to spin synthetic fibers, called nylon-6 and nylon-66. Nylon-610 is a thermoplastic engineering plastic with excellent mechanical properties .
PA has good comprehensive properties, including mechanical properties, heat resistance, abrasion resistance, chemical resistance and self-lubrication, and has low friction coefficient, certain flame retardancy, easy processing, suitable for glass fiber and Other fillers are filled to enhance modification, improve performance and expand the scope of application.
There are many varieties of PA, including PA6, PA66, PAll, PA12, PA46, PA610, PA612, PA1010, etc., as well as many new varieties of semi-aromatic nylon PA6T and special nylon developed in recent years. Nylon-6 plastic products can use sodium metal, sodium hydroxide, etc. as the main catalyst, N-acetyl caprolactam as the co-catalyst , so that δ-caprolactam can be directly prepared by anion ring-opening polymerization in the model, which is called cast nylon. This method facilitates the manufacture of large plastic parts.
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