(Polylactams, Polyphthalamides and Polyaramids)
Polyamides (PAs) are an important class of high performance engineering thermoplastics produced either by the reaction of a diacid with a diamine or by ring-opening polymerization of lactams. They can be aliphatic, semi-aromatic or fully aromatic themroplastics. The aromatic polyamides, called aramids, have higher strength, better solvent, flame and heat resistance and greater dimensional stability than the all aliphatic amides (Nylon) but are much more expensive and more difficult to produce.
The two most important aromatic amides are poly(p-phenylene terephthalamide), also known as Kevlar, and poly(m-phenylene isophthalamide). The fully aromatic structure and the strong hydrogon bonds between the aramid chains result in high melting points (generally above their decomposition temperature > 750 K), ultra high tensile strength at low weight, and excellent flame and heat resistance as well as good dimensional stability and solvent resistance at room and elevated temperature.
The aliphatic polyamides are produced on a much larger scale than the fully aromatic polyamides and are the most important class of engineering thermoplastics. They are amorphous or only moderately crystalline when injection molded, but the degree of crystallinity can be much increased for fiber and film applications by orientation via mechanical stretching. The two most important polyamides are poly(hexamethylene adipamide) (Nylon 6,6) and polycaprolactam (Nylon 6). Both have excellent mechanical properties including high tensile strength, high flexibility, good resilience, low creep and high impact strength (toughness). They are easy to dye and exhibit excellent resistance to wear due to a low coefficient of friction (self-lubricating). Both amides have a high melting temperature (500 - 540 K) and glass transition temperature resulting in good mechanical properties at elevated temperatures. For example, the heat deflection temperature (HDT) of PA-6,6 is typically between 180 and 240°C which exceeds those of polycarbonate and polyester. They also have good resistance to oils, bases, fungi, and many solvents. The main limitation is the strong moisture sensitivity (water acts as a plasticizer) and the resulting changes in mechanical properties. For example, the tensile strength of moist polyamide can be more than 50 percent lower than that of dry polyamide. Another important polyamide is Nylon 6,12. It is less hydrophilic than Nylons 6,6 and 6 due to the larger number of methylene groups in the polymer backbone. For this reason, it has better moisture resistance, dimensional stability, and electrical properties, but the degree of crystallinity, the melting point and the mechanical properties are lower. Other commercially available polyamides include Nylon 4,6, Nylon 6,10 and Nylon 11.
Another important class of polyamides are semi-aromatic polyamides, also knowns as polyphthalamides (PPA).1 They are melt-processible, semi-crystalline thermoplastic resins made from the condensation of an aliphatic diamine such as hexametylene diamine with terephthalic acid and/or isophthalic acid. The aromatic portion typically comprises at least 55 molar percent of the repeat units in the polymer chain. The combination of aromatic and aliphatic groups greatly reduces moisture absorption which results in little dimensional changes and much more stable properties. Thus, PPAs fill the performance gap between aliphatic nylons such as PA6,6 and PA6, and the much more expensive polyaramids. They are mostly crystalline and offer high strength and stiffness at elevated temperatures. However, these resins are more expensive than aliphatic amides and are more difficult to process due to their higher melting point. To improve processability and to lower cost, they are sometimes blended with aliphatic polyamides such as Nylon 66.
The two most common semi-aromaitc amides are poly(hexamethylene teraphthalamide) (PA 6T) and poly(hexamethylene isophthalamide) (6I). These resins have a very high melting point (6T: Tm ≈ 595 K) and glass transition temperature (6T: Tg ≈ 410 k). They are known for their excellent dimensional stability, low creep at elevated temperatures and good chemical resistance comparable to many high performance engineering plastics.
Polyamides have several advantages over other classes of engineering polymers. For example, they are more resistant to alkaline hydrolysis than polyesters but not as resistant to acid hydrolysis. They also have better solvent resistance to many organic liquids when compared with PET and PC.
Major manufacturers of unfilled and filled aliphatic polyamides (Nylon) areSpectra®), Teijin ( Teijin, Teijinconex) and DuPont ( Kevlar®, Nomex®).
|Polyamide||Structure of Repeat Unit||Trade Name|
|Aliphatic Polyamide||Aegis®, Ultramid®, Nypel®|
|Aromatic Polyamide||Kevlar®, Technora®, Teijin®|
|Semi-Aromatic Polyamide||Arlen®, Ultramid T KR®|
Aliphatic polyamides such as Nylon 66 and Nylon 6 are widely used for engineering and industrial applications. The unreinforced grades have an upper temperature for continuous use of around 340 - 350 K, and the glass and mineral reinforced grades of about 370 - 390 K. Applications include almost every industry and market. For example, in the automotive industry, nylons are used for wire and cable jacketing, cooling fans, air intake, turbo air ducts, valve and engine covers, brake and power steering reservoirs, gears for windshield wipers and speedometers. Engineering polyamides are also used for power tool housings, valves and vending for various machines and pumps and for many electrical/electronical parts including switches, sockets, plugs and antenna-mounting devices.
More than 60 percent of the aliphatic polyamides produced are used in commercial fiber applications. This includes carpets, garments, seatbelts, upholstery, ropes and tire reinforcements. However, fabrics made from Nylon have less wrinkle resistance than cloth made from polyesters are more expensive. For these reasons, Nylon fibers have lost some market share to PET fibers over the years.
The aromatic polyamides are much more expensive but have excellent mechanical properties. They are used for very demanding applications in many industries. Examples include ropes and cables, bulletproof vests, tennis strings, hockey sticks (as a composite), snowboards, jet engine enclosures, brake and transmission friction parts, and gaskets.
Semi-aromatic polyamides are often a cost effective alternative to the more expensive fully aromatic aramids. They fill the performance gap between aliphatic nylons and the much more expensive polyaramids. They are often a good choice when the products have to withstand prolonged exposure to harsher chemicals and/or higher temperatures. Common applications include motor parts, fuel line connectors, coolant pumps, bushings, bearing pads in aircraft engines, charge air coolers, resonators, engine cover components and heat shields, fuel cutoff and water heater manifold valves, connectors, high voltage bushings, motor housings, and head light components.
- Nylon 3 - Poly(propiolactam)
- Nylon 6 - Poly(caprolactam)
- Nylon 8 - Polycapryllactam
- Nylon 10 - Poly(decano-10-lactam)
- Nylon 11 - Poly(ω-undecanamide)
- Nylon 12 - Poly(ω-dodecanamide)
- Nylon 6,6 - Poly(hexamethylene adipamide)
- Nylon 6,10 - Poly(hexamethylene sebacamide)
- Nylon 6,12 - Poly(hexamethylene dodecanediamide)
- Kevlar - Polyaramide
- Nomex - Polyaramide
1Polyphthalamide are defined as semi-aromatic polyamides in which the residues of terephthalic acid and/or isophthalic acid comprise at least 55 molar percent of the repeat units (ASTM D5336). Thus, resins with less than 55 percent aromatic di-acid content are not PPA resins.