Definition of Aramid, Aramids Polymers and polyamide synthesis

The aromatic polyamide synthesis (aramids polymers) was started industrially in the early 1960s; in an industrial corporation (DuPont). Aramid fiber became a combination of science, research, and applications at an early stage. Due to a wide range of properties of Polyaramide and their structural variety they soon found their utility in diverse applications. A simple definition of aramid is a polymer having an amide group in the repeating unit.

We have series of topic on Aramids which you would like to go through;

If you like to study theoratical aspects of aramids, you can search wikipedia.

According to the U.S. Federal Trade Commission, an aramid is defined as the aromatic polyamide (aramids polymers) having at least 85% of the amide linkages which are attached directly to two aromatic rings.

Only four compounds have so far been commercially available corresponding to this definition:

  • polyaramide (m-phenylene isophthalamide) (MPDI)
  • poly (p-phenylene terephthalamide) (PPTA)
  • copoly (p-phenylene-3, 4-diphenyl ether terephthalamide) (ODAPPTA)
  • poly [5-amino-2(p-aminophenyl) benzimidazole terephthalamide] (SVM)

The commercialization process of aramid fibers and polyamide synthesis mostly involves a constant trade-off between properties, processability, and price.

Poly (p-phenylene terephthalamide) and poly (m-phenylene isophthalamide) are two well-known commercial aramids. Both of them have been utilized for various applications in different forms. They can be applied in the coating field and fillers in the aerospace industry, electrical insulations, bullet-proof armors, sports, and fabrics by transforming them into high-tensile strength and flame and cut-resistant synthetic Aramid fiber.

Definition of Aramid

Definition of Aramid is,

“Polyamide is polymers having the amide group in their repeating unit”.

The aromatic nature of the polymer backbone differentiates aromatic polyamides from aliphatic polyamides (Aramids polymers).

The aromatic structure of the main chain of the aramids plays an important role in providing certain special characteristics, which are oxidative stability, higher solvent resistance, as well as superior thermal and mechanical properties, hence they are also classified as high-performance polyaramide materials. 

Polyamide synthesis is of prime importance as Aramid fiber has high strength and modulus with low density and high elongation which imparts high impact resistance in the composites. Hence aramids polymers are also used as an important reinforcement material in aerospace applications where high tenacity is required.

One such application of polyaramide is a honeycomb structure that is utilized for structural applications in the aerospace industry due to reduced weight without compromising mechanical strength. Moreover, resin-impregnated Nomex® paper honeycomb structure is normally used as an airplane interior material due to its excellent flammability characteristics. This Nomex paper honeycomb structure is a real definition of Aramid in practical nature.

Polyamide synthesis (aramids polymers)

The most common polyamide synthesis methods for polyaramide or aramid fiber are polycondensation reaction of AB-type and AA with BB type monomers.  AB homopolymers are prepared by a head to tail reaction, in which a monomer having both amine and acid group undergoes polycondensation reaction. AA with BB type polymers is synthesized by polycondensation reaction between a diamine and diacid halide.

Reaction of AA with BB type Polymer
Reaction scheme of AB type Polymer

There are two main approaches used industrially for (aramids polymers or polyaramide) polyamide synthesis; solution polymerization and interfacial polymerization.

Solution Polymerization – (aramids polymers)

In solution polymerization, polyamide synthesis is carried out by providing a liquid medium for reaction of diacid halide with diamine monomers. The most commonly used solvents are dimethylacetamide (DMAc), N-methylpyrrolidone (NMP) and hexamethylphosphoramide.

Solvents for Solution Polymerization (aramids Polymers)

The reaction occurs at low temperatures to prevent side reactions which inhibit the formation of high molecular weight compounds. These reactions can be decarboxylation of the diacid and oxidation of the diamine.

Amide(polyaramide) solvent serves as an acid acceptor for the HCl that is formed as a byproduct, does not react with acid chlorides. Also, LiCl and CaCl2 promote solvation because of the coordination of metal ion with amide carbonyl, decreasing H-bonding between amide groups. This was shown by Morgan at DuPont, as well as workers in Japan and the Netherlands had shown an increase in the degree of polymerization as well as the inherent viscosity of the polymers formed (Aramid fiber).

Salts such as LiCl and CaCl2, or their mixture, are often used as solubility promoters as their cations interact with the amide groups, decreasing interchain hydrogen bonds’ strength.

The two important structural variants of aramids polymers (polyaramide), para-aramid and meta-aramid commercially known as Kevlar and Nomex (aramid fiber), respectively are also prepared by solution polycondensation.

Poly (p-phenylene terephthalamide) (Kevlar) is synthesized by polycondensation reaction of diamine and diacid halide in LiCl or CaCl2 solvents, Poly (m-phenylene isophthalamide) (Nomex) is synthesized by reacting diamine and diacid halide in amide solvent. Both of these processes were discovered and patented by DuPont in 1962.

Reaction scheme of para aramid
reaction scheme of meta aramid

Interfacial Polymerization – polyamide synthesis

In the interfacial method of polyamide synthesis (polyaramide), two fast-reacting intermediates are dissolved in a pair of immiscible liquids, one of the liquids is generally water.

In the water phase diamine and any added alkali are present, while diacid halide is present in organic liquid i.e. such as xylene, carbon tetrachloride, hexane, and dichloromethane etc.

The two solutions are agitated vigorously and brought together; the reaction takes place at or near the interface of the two phases. It is a low-temperature reaction method but it results in the synthesis of a broad distribution of molecular weights of aramids which are not suitable for producing aramid fiber.

For synthesis, you can follow science direct research papers.

Reaction Mechanism for Aramids polymers

The well-known reaction mechanism of polyamide synthesis (polyaramide) is Schotten-Bauman process. In this process, a fully aromatic monomer is reacted with a bifunctional monomer. The first step of the reaction is the attack of amine nitrogen on the carbonyl carbon of diacid halide. As shown in the scheme (Fig 2.5) by the elimination of HX an amide linkage is formed from the transition complex I.

Amine group (polyaramide) is transformed into a quaternary ammonium salt (II) by HX, excluding from the polymerization reaction. An acid acceptor is used to regenerate the amine ends to continue the polymerization process for polyamide synthesis. Solvents such as DMAc and NMP often function as an acid acceptor as well.

Reaction scheme of Schotten-Bauman process