Nylon structure

- May 22, 2019-

Nylon structure

Polyamide, commonly known as nylon (Nylon), English name Polyamide (PA), density 1.15g/cm3, is a generic term for thermoplastic resins containing repeating amide groups - [NHCO] in the main chain of the molecule, including aliphatic PA, fat-fragrance Family PA and aromatic PA. Among them, aliphatic PA has many varieties, large yield and wide application, and its name is determined by the specific carbon number of the synthetic monomer. Invented by the famous American chemist Carothers and his research team.

Nylon is a term of polyamide fiber (nylon) that can be made into long or short staple fibers. Nylon is the trade name of polyamide fiber, also known as Nylon. The English name Polyamide (abbreviated as PA), whose basic constituent substance is an aliphatic polyamide linked by an amide bond - [NHCO].

Molecular Structure

Commonly used nylon fibers can be divided into two categories.

One type is polyhexamethylene adipate which is obtained by polycondensation of a diamine and a diacid. The chemical structure of the long-chain molecule is:

H[HN(CH2)XNHCO(CH2)YCO]OH

The relative molecular weight of such nylons is generally from 17,000 to 23,000. Depending on the number of carbon atoms of the diamine and dibasic acid used, different nylon products can be obtained, and the difference can be obtained by adding the number after the nylon, wherein the former number is The number of carbon atoms of the diamine, the latter number being the number of carbon atoms of the dibasic acid. For example, nylon 66, which is prepared by polycondensation of hexamethylenediamine and adipic acid; nylon 610, which is derived from hexamethylenediamine and sebacic acid.

The other type is obtained by polycondensation or ring-opening polymerization of caprolactam, and the chemical structure of the long-chain molecule is:

H[NH(CH2)XCO]OH

According to the number of carbon atoms contained in the unit structure, the naming of different varieties can be obtained. For example, nylon 6, indicating that it is obtained by ring-opening polymerization of caprolactam having 6 carbon atoms.

Nylon 6, nylon 66 and other aliphatic nylons are composed of linear macromolecules with an amide bond (-NHCO-). The nylon molecules have -CO-, -NH- groups, which can form hydrogen bonds in the intermolecular or intramolecular groups, and can also be combined with other molecules, so the nylon has better moisture absorption capacity and can form a better crystal structure.

The -CH2-(methylene) in the nylon molecule can only produce a weak van der Waals force, so the molecular chain of the -CH2-segment portion has a large degree of curl. The variety of various nylons, CH2, is different, so that the binding forms of intermolecular hydrogen bonds are not exactly the same, and the probability of molecular curling is also different. In addition, some nylon molecules are also directional. The directionality of the molecules is different, and the structural properties of the fibers are not identical.

Morphology

The morphological structure observed in the microscope by the melt spinning method has a circular cross section and no special longitudinal structure. Filamentous fibril tissue was observed under an electron microscope, and the fibril of nylon 66 was about 10-15 nm wide. If a special-shaped spinneret is used, nylons of various special cross-section shapes, such as polygonal, multi-lobed, hollow, and the like, can be made. Its focal state structure is closely related to the stretching and heat treatment of the spinning process. The macromolecular backbones of different nylons are composed of carbon atoms and nitrogen atoms.

Shaped fibers can change the elasticity of the fiber, giving the fiber a special luster and bulkiness, and improving the fiber's cohesive and covering ability as well as anti-pilling and static reduction. For example, the triangular fiber has a glittering effect; the five-lobed fiber has a lustrous luster, has a good hand feeling, and is resistant to pilling; the hollow fiber has a small density and good warmth due to a cavity inside.

Polyamide (PA, commonly known as nylon) is the first resin developed by DuPont in the United States for fiber, which was industrialized in 1939. In the 1950s, the development and production of injection molded products began to replace metals to meet the requirements of lighter weight and lower cost of downstream industrial products. The polyamide backbone contains many repeating amide groups. When used as a plastic, it is called nylon. When used as a synthetic fiber, we call it nylon. Polyamide can be made from diamines and dibasic acids. It can also use ω-amino acids or rings. The lactam is synthesized. Depending on the number of carbon atoms in the diamine and the dibasic acid or amino acid, a wide variety of polyamides can be made. There are dozens of polyamides, including polyamide-6, polyamide-66 and poly. Amide-610 is the most widely used.

The chain structure of polyamide-6, polyamide-66 and polyamide-610 are [NH(CH2)5CO], [NH(CH2)6NHCO(CH2)4CO] and [NH(CH2)6NHCO(CH2)8CO, respectively. ]. Polyamide-6 and polyamide-66 are mainly used for spinning 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-lubricating properties, low friction coefficient, certain flame retardancy, easy processing, suitable for fiberglass and Other fillers are filled with enhanced modification to improve performance and extend the range of applications.

There are many varieties of PA, such as PA6, PA66, PAll, PAl2, PA46, PA610, PA612, PAl010, etc., as well as many new varieties of semi-aromatic nylon PA6T and special nylon developed in recent years. Nylon-6 plastic products can be made by using sodium metal, sodium hydroxide and the like as the main catalyst, and N-acetyl caprolactam as the cocatalyst, so that δ-caprolactam can be directly produced by negative ion ring-opening polymerization in the model, which is called casting nylon. This method is convenient for manufacturing large plastic parts.