Polyamides, PA for short, have become an integral part of the world of plastics. They are appreciated for their high strength, rigidity, excellent impact resistance and good abrasion and wear resistance. In addition, organic solvents such as fuel, lubricants, alcohol or acetone cannot harm them. In the plastics pyramid, they therefore hold their own among the engineering thermoplastics.
Chart 1: Plastic pyramid: the performance of the materials increases from the bottom to the top of the y‑axis, while the production volume decreases.[1]
According to the journal Kunststoffe, global consumption of PA 6 and PA 66 reached a volume of 6.7 million tonnes in 2011 compared to 6.6 million tonnes in 2010 (source: PCI Yellow Book 2012). The largest purchasers with a share of around 53% were producers of fibres and filaments. Around 41% (2.7 million tonnes) were processed into technical compounds. The manufacture of films accounted for around 6 %.
Use of polyamide by industry
Chart 2: use of polyamide by industry.[2]
Where is polyamide used?
Polyamide has become a staple in countless industries. Here are some examples:
Textile industry
Polyamide fibres, i.e. the nylon or Perlon that we are all familiar with, can be found in stockings, sportswear, underwear and many other items of clothing. The fibres are also used as technical fabrics, as fishing lines or to tension tennis rackets.
Automotive industry
In addition to being used as fibres in the textile industry, polyamides are firmly established as engineering thermoplastics in the automotive industry – in fittings, mudguards or under the bonnet for tanks or fuel lines, brakes, transmission components and in e‑mobility.
Electrical and electronics industry
Polyamides can also be found in housings, printed circuit boards, insulation materials, household electrical appliances or pumps in the electrical and electronics industry.
Construction industry
Polyamide is used in construction as a thermal break in profiles for windows and façade constructions.
Packaging industry
Polyamide plays a particularly important role in flexible packaging. It is found in the vacuum packaging of foodstuffs such as cheese, sausage and frozen goods.
Chart 3 shows that the share of polyamide in both the automotive and packaging industries is steadily increasing.
Value of Polyamide (PA) 6 Consumed by end user industry
USD, Global, 2017 — 2029
Chart 3: industry shares of polyamide as technical compounds.[3]
A closer look at polyamide
But what exactly does polyamide look like and how is it produced? This requires a brief excursion into organic chemistry. Polyamides are linear polymers. What makes them stand out is their regularly repeating amide bonds along the main chain. Almost all important polyamides are derived from primary amines. What does that mean? The functional group ‑CO-NH- occurs in its repeating units along the polymer chain (see structural formula).
Structural formula of polyamide: the blue amide group is repeated along the polymer chain. R stands for the remainder of the compound used for synthesis.
The amide group is formed during the production of polyamide from a carboxylic acid and an amine. The resulting amide bond is hydrolytically cleavable – it can therefore be broken again by a reaction with water. For example, the industry uses aminocarboxylic acids, lactams, diamines and dicarboxylic acids as monomers for polyamides.
Production of PA6 and PA66
PA66 and PA6 are often used as technical compounds. Although these two polyamides are chemically and physically similar, their production process is fundamentally different and is based on different raw materials. Polyamide 66 – generally known as nylon – can be produced from the diamine hexamethylenediamine (HMD) and adipic acid, a dicarboxylic acid. The carboxyl group (-COOH) of adipic acid reacts with the amino group of diamine (-NH2). This creates the peptide bond (-NH-CO-). It is the basic building block of all polyamides. Polyamide 6, on the other hand – known as Perlon – is produced by ring-opening polymerisation from ε‑caprolactam with water as a starter. At elevated temperatures, ω‑aminocaproic acid (chemically: ω‑amino hexanoic acid) is formed from the ring-shaped ε‑caprolactam. It has a carboxyl group at one end and an amine group at the other. This property is referred to as bifunctional. It is possible for ω‑aminocaproic acid to react with itself to form long-chain polyamides.
DIN ISO 1043–1 defines the nomenclature of polyamides: PA is the abbreviation for all polyamides. The abbreviation is followed by the number of carbon atoms present in the monomers. For example, if it is Perlon, made from the bifunctional ω‑amino hexanoic acid with six carbon atoms, the abbreviation is PA6. If the polyamide is formed from a dicarboxylic acid with a diamine, as is the case with nylon, the carbon atoms of the two monomers are relevant, counted and listed one after the other. This is why nylon is called PA66. [4]
Properties of PA6 versus PA66 — similar and yet different
Both PA6 and PA66 have good sliding and damping properties, are highly abrasion-resistant, impact-resistant and resistant to weak alkalis, lubricants, oils and greases. Due to the different arrangement of the polymer chains, PA66 has slight advantages, as the table shows.
| PA6 | PA66 | |
|---|---|---|
| Melting point in °C | 220 | 260 |
| Glass temperature in °C (dry) | 50–60 | 50–60 |
| Density | 1.15 g/ml | 1.2 g/ml |
| Moisture absorption in % (23 °C, 50 % humidity) | 2,6–3,4 | 2,5–3,1 |
| Machinability — low tool wear and surface quality | Good | Better |
| Shrinkage | Low | Greater |
| Water absorption | Higher | Lower |
| Tensile modulus in MPa — dry | 2700–3500 | 2700–3500 |
| Tensile modulus in MPa — humid | 900‑1200 | 1000–1600 |
The different arrangement of the molecules results in slight mechanical advantages for PA66. PA66 is also more temperature-resistant and absorbs less moisture, making it more dimensionally stable. But PA6 also has some plus points: the surface finish is better, especially with the glass fibre-reinforced types, as the melt solidifies more slowly. In addition, the lower processing temperatures of PA6 lead to lower energy consumption, resulting in lower production costs compared to PA66. Due to its higher moisture absorption, PA6 is more impact-resistant than PA66.
The mechanical and thermal properties of polyamide can be significantly improved by adding additives, glass or carbon fibres. This makes it possible to produce highly impact-resistant, heat- and hydrolysis-stabilised polyamides. PA6 and PA66 are suitable for processing in injection moulding, extrusion and blow moulding. For the fibre industry, the fibres are spun directly from the melt.
Advantages and disadvantages of polyamide
The advantages
The advantages of polyamide as a fibre are obvious: the material scores highly in terms of strength, durability, elasticity and dimensional stability. It is light, quick-drying, resistant to moisture and less prone to creasing.
Polyamides have gained a steadily growing range of uses in technical applications. Due to its high mechanical strength and chemical resistance, engineering plastic has now replaced many metal parts in vehicle construction. Highly stressed components with complex geometries can be manufactured comparatively easily and cost-effectively in large quantities. Replacing metal with polyamide reduces the weight of the vehicle and thus its fuel consumption.
Slide bearings, drive belts, gear wheels or rollers made of polyamide have proven their worth because the material scores highly in terms of wear resistance and good sliding properties. Carbon fibres improve the already good gliding properties even further.
The electrical and electronics industry is the second largest purchaser of technical polyamides after the automotive industry. They are used in housings and for insulating electronic components, as the material has high electrical insulation and tracking resistance. If required, the material can also be made electrically conductive by adding metal and graphite.
The disadvantages
But where there is light, there is also shadow. Despite the numerous advantages of polyamide, there are some disadvantages. The material has poor chemical resistance to acids and bases,
and there are some drawbacks in terms of manufacture, use and disposal. Crude oil is needed for the production of virgin polyamide. Production is very energy-intensive, as is the processing of polyamide. The associated climate and environmental impacts are obvious.
Further environmental damage is caused when polyamides enter bodies of water and the environment – for example via items of clothing: tiny fibre particles are rubbed off in the washing machine. These particles, known in the media as microplastics, end up in wastewater. With the technologies available today, it is not possible for sewage treatment plants to completely filter out microplastics. Around half a million microfibres can end up in the wastewater in a single wash of nylon textiles. [7]
Recycling polyamide: opportunities and challenges
Like many plastics, polyamide is also being discussed in terms of sustainability and environmental protection. A major topic here is recycling. This is the only way to minimise polyamide waste and return it to the material cycle. The advantages: no crude oil is required for the recyclates; the energy requirement per kilogramme of recycled polyamide is far lower than that of virgin material; and the material does not end up in landfill sites, incineration plants or in nature. This benefits both the climate and the environment. Polyamides are easy to recycle mechanically. However, comprehensive expertise is essential due to the chemical structure and the technical requirements for processing. The challenge: the recycled material must achieve a consistent quality with properties as similar as possible to virgin material. This is the only way plastics processors can manufacture high-quality products that perform optimally during their use in vehicles, aeroplanes or the electronics industry.
To achieve this goal, recyclers must be able to procure polyamide waste of consistent quality and in sufficient quantity to process it carefully. It is precisely this ability that characterises ENNEATECH.
ENNEATECH: Recycled polyamide reducesCO2 footprint by 90%
The reason for the positive CO2 balance of Enneatech’s products is the raw materials: polyamide by-products from the high-end textile industry form the centrepiece of impact-resistant recycled polyamides. The company uses these to produce a homogeneous starting material. This is where Enneatech benefits from its many years of expertise in the textile industry and equally longstanding customer-supplier relationships. As the company is one of the pioneers in the market, it has a large supplier portfolio and can deliver reliably even in difficult market situations. By recycling high-quality textile waste, Enneatech produces polyamides with a traceable and documentable CO2 footprint.
With decades of experience in polyamide recycling, Enneatech has succeeded in offering an extremely impact-resistant, recycled filled PA6 and PA6.6 compound with the Entron Eco product line: Entron Eco Compounds. With its own compounding line, the company can fulfil individual customer specifications on request. The compounds not only have outstanding mechanical properties, but also a CO2 footprint that is up to 90% lower than that of virgin material. In addition to the filled compounds, Entron Eco is also available as granules. It is produced without using foreign polymers and additives and is the basis of the filled compounds. Of course, the same applies here: the CO2 footprint is always 90% lower than that of new goods.
