Polyamides (or nylon) are considered high-performance plastics. They exhibit high temperature & electrical resistances. They find their use in the automotive, transportation, consumer goods, and E&E industry. There are serval types of polyamide classes available in the market today. But are you finding it hard to select the right one for your specific application?
Let us make your selection process easy by elaborating on the types of polyamides. Key features and processing conditions of each class have been explained here. Also, understand what makes them an ideal choice in high-end engineering applications.
What is Polyamide (Nylon)?
Polyamide or Nylon is abbreviated as PA. It is a major high-performance engineering thermoplastics class. It has a good balance of properties. They contain repeating amide linkages i.e. –CO-NH–. It is formed by condensing identical units, and copolymers with different units.
Nylon was discovered by Wallace Hume Carothers. A chemist hired in 1928 by DuPont de Nemours to lead an extensive research program on the design of original polymeric materials. In 1935, he developed the formula known as PA 66:
Polyamide 66
Polyamides exhibit high temperature and electrical resistance. Thanks to their crystalline structure, they also show excellent chemical resistance. They have very good mechanical and barrier properties. These materials can easily be flame retarded. Polyamides became the first truly synthetic fiber to be commercialized.
When reinforced with glass fibers, their stiffness can compete with metals. These glass fibers can be short or long. This is why Polyamides are often considered in metal replacement projects.
All polyamides tend to absorb moisture due to the amide chemical group. The moisture acts as a plasticizer. This thus reduces tensile modulus and increases impact resistance & flexibility. Moisture uptake also has a huge influence on dimensional variations. It must be taken into account when designing parts.
Polyamides find use in automotive, transportation, electrical & electronics, consumer goods, and many more.
How are polyamides made?
Polyamides are made by polycondensation of diacid with a diamine.
They can also be produced by ring-opening polymerization of lactams with 6, 11 or 12 carbon atoms.
- The monomers may be aliphatic, semi-aromatic or aromatic (aramids).
- They may be amorphous, semi-crystalline and of greater or lesser crystallinity.
Aromatic polyamides are also known as Aramids. They are obtained from the polycondensation of terephthalic acid with diamines. PA 6-3-T is one of the common examples of aromatic polyamide. It is amorphous and transparent in nature. Aramids can be processed at 280-300°C and are expensive. When compared to aliphatic polyamides, aromatic polyamides have:
- better dimensional stability,
- flame and heat resistance, and
- higher strength
What are the types of polyamide?
The monomers used in the manufacturing of different classes of polyamides is described below.
| Polyamide | Monomer(s) |
| Polyamide 6 | Caprolactum |
| Polyamide 12 | Laurolactam |
| Polyamide 66 | Hexamethylene Diamine/ Adipic Acid |
| Polyamide 69 | Hexamethylene Diamine/ Azelaic Acid |
| Polyamide 6-10 | Hexamethylene Diamine/ 1,12-Dodecanedioic Acid |
| Polyamide 6-12 | Hexamethylene Diamine/ Sebacic Acid |
| Polyamide 46 | 1,4-Diaminobutane/ Adipic Acid |
| Polyamide 1212 | 1,12-Dodecanediamine/ 1,12-Dodecanedioic Acid |
Aliphatic Polyamide Polymers and Their Monomers
Among this large polymer family, several types of polyamides are particularly suited for given applications. The best choice depends on the set of performances needed as well as the economical constraints.
- The two most widely used PAs are by far PA66 and PA6. They are often extruded to manufacture fibers (textile industry) or films (packaging), or injection molded.
- The polyamides with the highest performances are PPA and PA46. They are good candidates for metal replacement developments or very specific applications exposed to extreme conditions.
- Bio-based PA is also available. For instance, PA11 is based on castor-oil chemistry.
How to compare between polyamide 6 and polyamide 66?
Structure of Polyamide 6
Polyamide 6 (PA6) is also known as Nylon 6 or polycaprolactam. It is one of the most extensively used polyamides globally. It is synthesized by ring-opening polymerization of caprolactam. Melting point of polyamide 6 is 223°C.
Molecular Structures of Polyamide 6
Structure of Polyamide 66
Polyamide 66 (PA66) or Nylon 66 is one of the most popular engineering thermoplastics. It is majorly used as a replacement for metal in various applications. Nylon 66 is synthesized by polycondensation of hexamethylenediamine and adipic acid. These two monomers contain 6 carbon atoms each. Melting point of polyamide 66 is 255°C.
Molecular Structures of Polyamide 66 (bottom)
Main Properties of PA6 and PA 66
PA6 & PA66 are by far the most used polyamides globally. They are used in many applications due to their excellent performance/cost ratios. Their key properties include:
- High strength and stiffness at high temperature
- Good impact strength, even at low temperature
- Very good flow for easy processing
- Good abrasion and wear resistance
- Excellent fuel and oil resistance
- Good fatigue resistance
- Good electrical insulating properties
- High water absorption and water equilibrium content limits the usage
- Low dimensional stability
- Attacked by strong mineral acids and absorbs polar solvents
- Proper drying before processing is needed
Although, they exhibit similar properties, slight differences do remain.
| See detailed property comparison between PA6 and PA66 | |
| PA6 vs. PA66 | PA66 vs. PA6 |
| Slightly lower temperature resistanceSlightly less expansiveExcellent surface appearanceBetter processabilityBetter hydrolytic stabilityBetter long-term heat agingSimilar stiffness at temperature below 180°CLow cost and heat deflection temperature | Slightly less moisture absorption abilityHigher modulusBetter wear resistanceBetter short-term heat resistance |
Processing Conditions of PA6 and PA66
While processing PA6 and PA66 drying before is highly recommended. The moisture content should be a maximum of 0.2%. The maximum permissible drying temperatures lie in the range of about 80 to 110°C.
Polyamide 6 and Polyamide 66 are thermally stable up to 310°C. At temperatures above this lead to decomposition. The initial products formed are mainly carbon monoxide and ammonia, and caprolactam. While processing Polyamide 6 and 66 with injection molding and extrusion techniques, the following conditions are recommended.
Injection Molding
- L/D ratio of 18:22
- The melt temperature should be between 240-270°C (PA6) & 270-300°C (PA 66)
- The mold temperature should be in the range of 55-80°C
Extrusion
- Only highly viscous grades can be processed by extrusion
- A three-section screw with an L/D ratio of 20-30 is recommended
- The processing temperature during extrusion should lie between 240 and 270°C (PA6) & 270 to 290°C (PA 66)
What is polyamide 11 (PA11)?
Polyamide 11 (PA11) or Nylon 11 is a rare bio-based engineering plastic. Derived from renewable resources (castor plants). It is produced by the polymerization of 11-amino undecanoic acid.
Rilsan® is one of the first biosourced polyamide. Melting point of Polyamide 11 is 190°C.
Bio-based polyamide derived from renewable resources (castor plants)
Several properties of PA11 are similar to Polyamide 12 (PA12). PA11 comparatively offers superior thermal and UV resistance, low water absorption and lower environmental impact. It displays good impact strength and dimensional stability.
| Strengths | Limitations |
| The lowest water absorption of all commercially available polyamidesOutstanding impact strength, even at temperatures well below the freezing pointResistant to chemicals, particularly against greases, fuels, common solvents and salt solutionsOutstanding resistance to stress cracking, aging and abrasionsLow coefficient of frictionNoise and vibration damping propertiesFatigue resistant under high frequency cyclical loading conditionAbility to accept high loading of fillersHighly resistant to ionization radiation | High cost relative to other polyamidesLower stiffness and heat resistance than other polyamidesPoor resistance to boiling water and UVProper drying before processing is neededAttacked by strong mineral acids and acetic acid, and are dissolved by phenolsElectrical properties highly depend on moisture content |
What is polyamide 12 (PA12)?
Polyamide 12 (PA12) or Nylon 12 is a semi-crystalline thermoplastic. It has a similar performance to Polyamide 11. It can be derived from both petroleum and renewable sources. It is an expensive polymer compared to other polyamides.
Molecular Structure of Polyamide 12
Key Properties of PA12
- It possesses lower impact resistance but shows good resistance to abrasions and UV
- It has a low water absorbency than PA 6, PA66 and all other types of polyamides
- The PA12 grade displays good dimensional stability and reasonable electrical properties
- PA12 is ideal for applications where safety, durability or reliability over time is critical.
- PA12’s transparent grades are also available, allowing high flexibility in terms of design and creation
| Strengths | Limitations |
| Lowest water absorption of all commercially available polyamidesOutstanding impact strength, even at very low temperaturesGood chemical resistance, in particularly against greases, fuels, common solvents and salt solutionsOutstanding resistance to stress crackingExcellent abrasion resistanceLow coefficient of frictionNoise and vibration damping propertiesGood fatigue resistance under high frequency cyclical loading condition | Expensive than other polyamidesLower stiffness and heat resistance than other polyamidesLow UV resistanceProper drying before processing is neededElectrical properties highly depend on moisture content |
PA11 and bio-sourced PA12 demonstrate the following features:
- Excellent chemical resistance
- Flexibility
- Durability
- Cold impact resistance
- Thermal resistance
These properties give PA11 and PA12 an edge over traditional bio-based polymers.
- Even if they do not over-perform in terms of temperature resistance (HDT, peak temperature…), they exhibit outstanding retention of performance over time.
- Their remarkable long-lasting performance allows for their use in a wide range of conditions (temperature, pressure, chemical…).
- PA11 and PA12 are particularly suitable when reliability over time is needed.
PA 11 & PA 12 Processing Conditions
Drying before processing is highly recommended: 6-12 h at 80-90°C. Target moisture content should be a maximum of 0.1%.
Injection Molding
- For the plasticating unit, a three-zone screw with an L/D ratio between 18 and 22 is recommended.
- Melt temperature: 180 – 230°C
- Mold temperature: 30 – 100°C
- A decrease in mold temperature very often eases de-molding but a decrease in crystallinity then occurs.
Extrusion
- General temperature setting depends very much on the resins to be processed and type of extrudate, thus a general recommendation can not be given.
- Temperature in first heating zone: ~ 200°C
- Conventional three-zone screw with an L/D ratio of at least 24 is recommended.
- Mixing and shear elements may be useful to increase the melt homogeneity.
- Cooling of the feeding section is mostly required.
What is polyamide 6-10 (PA 6-10)?
Polyamide 6-10 (PA 6-10) is a semi-crystalline polyamide. It is produced by the polymerization of hexamethylene diamine with a dibasic acid i.e., sebacic acid. Melting point of polyamide 6-10 is 223°C.
Key features of PA 6-10 include:
- Exhibits lower water absorption when compared to PA6 or PA66
- Has a lower brittle temperature than PA6 or PA66
- Has good abrasion resistance and chemical resistance
- Possesses lower strength and stiffness unlike PA66
- Drying before processing PA 6-10 is highly recommended
- PA 6-10 is much stronger than PA 11, PA 12, or PA 6-12
- Low coefficient of friction
- Good electrical insulating properties
- High resistance against high energy radiation (gamma and X-rays)
Polyamide 6-10 is used to manufacture insulators for the electrical market. This is because of its good insulating properties, heat resistance, and flame retardancy.
Limitations of PA 6-10 include:
- High mold shrinkage, and high cost compared to other low water absorption polyamides.
- It is attacked by strong mineral acids and absorbs polar solvents.
What is polyamide 46 (PA46)?
Polyamide 46 (PA46) or Nylon 46 is manufactured by polycondensation of adipic acid and 1,4-diaminobutane. Diaminobutane is synthesized from acrylonitrile and HCN. Melting point of polyamide 46 is 295°C.
Key Properties of PA46
 |  |  |  |  |
| Good Thermal Performances | Good Mechanical Properties Particularly at high temperatures | Excellent Wear Resistance | Excellent Chemical Resistance | Excellent Electrical Resistance |
PA46 is the polyamide exhibiting the highest temperature resistance. Its HDT at 1.8MPA is 160°C, and 285°C when filled with 30% of glass fibers. PA 46’s mechanical resistance is superior to PA66’s. Its fatigue resistance is 50 times that of PA66.
- PA46 is often used to replace metal in demanding, high temperature applications.
- Due to PA46’s excellent mar and wear resistance, it is used in gear applications. It offers a combination of mechanical and constant performances at high temperature. It also offers excellent tribological behavior and high fatigue resistance in this industry.
- PA46 can be metalized. It is also possible to color a part made of PA46. However, the color resistance will depend on the behavior of the pigments at high temperatures.
- Due to its high fluidity, PA46 is a good solution for complex shapes and parts with thin walls.
| Strengths | Limitations |
| Outstanding stiffness, fatigue and creep resistance, up to 220°CExcellent abrasion and friction behaviorVery good flow for easy processingVery low injection cycle time, due to its high crystallization rateExcellent fuel and oil resistanceGood impact strengthVery low flashGood electrical insulating propertiesHigh resistance against high energy radiation (gamma and X-rays) | High water absorption and water equilibrium contentHigh temperature processing, due to its high melting pointLow dimensional stabilityAttacked by strong mineral acids and absorbs polar solventsProper drying before processing is neededDarkens with exposure to high heat |
Polyamide 46 Processing Conditions
Polyamides are hygroscopic in nature and hence tend to absorb moisture when left in open. It is therefore highly recommended to dry Polyamide 46 for 2-8 h at 80°C before processing. This ensures that hydrolytic degradation does not occur. Target moisture content should be a maximum of 0.1%. For critical applications, the recommended moisture content is 0.05% or less. In this case, it is recommended to pre-dry pellets 24-100 h at 80-105°C.
- Polyamide 46 can be processed on standard reciprocating screw injection molding machines.
- An L/D ratio of at least 20 is recommended.
- Melt temperature should lie between 300-330°C
- Mold temperature should be in the range of 60-120°C.
- Polyamide 46 does not stick to the mold surface and has good ejection properties.
What is polyphthalamide (PPA)?
Polyphthalamides are formed by the reaction of aromatic acids with aliphatic diamines. They are produced using a combination of terephthalic acid and isophthalic acids. Polyphthalamide also known as PPA is a high heat resistance semi-aromatic polyamide.
Key Features of PPA
With its low moisture pick-up, PPA shows excellent retention of performances in:
- Harsh chemical environments, and
- Extreme temperature conditions
They also show excellent stiffness and creep resistance.
Polyphthalamides have an aromatic structure. Due to this structure, it offers several superior performances compared to other polyamides. They offer:
- Improved dimensional stability
- Improved solvent and hydrolysis resistance
- Better high-temperature mechanical property retention
Polyphthalamide resin features an excellent stiffness-to-cost ratio and a high strength-to-weight ratio. Both these properties are superior relative to PBT, PPS, PEI, PET, and PA 66.
Polyphthalamide is stronger and less moisture sensitive. They have better thermal properties compared to:
- Aliphatic polyamides such as PA66
- Polyetheretherketone (PEEK) and some Liquid Crystal Polymers.
Yet they are less ductile in comparison. Some impact grades are available.
| Strengths | Limitations |
| Very high stiffness and strength, compared to PA66Good heat, chemical and fatigue resistanceLow water absorptionVery low creep tendencyGood dimensional stability | Requires high processing temperatures (up to 350°C)Requires good drying equipmentNot inherently flame retardantAttacked by powerful oxidants, mineral acids, acetic acid and formic acid |
Polyphthalamide Injection Molding Processing Conditions
- Drying time and temperature: 2h at 120°C or at least 8h at 80°C
- Holding the melt at temperatures above 350°C may result in polymer degradation which should be avoided.
- A melting temperature of 320-345°C.
- A mold temperature of 80-140°C.
- Use a screw with an L/D ratio of 18-22 during the plasticizing phase.
What are the main differences between nylon and polyester?
Both nylon and polyester are thermoplastic materials. But polyester compounds can be thermosets as well. They both are majorly synthetic in nature. Their main differences are listed in the table below.
| Nylon | Polyester | |
| Type | Thermoplastic Polymers commonly known as Polyamides | Thermoplastic or Thermoset |
| History | First Nylon was produced by Wallace Carothers in 1935 | First polyester fiber called Terylene created in 1941 |
| Production | Nylon is formed by the condensation of copolymers. Equal parts of dicarboxylic acid and diamine are used for the process. There are peptide bonds on the ends of the monomers | Synthetic polyesters are made up of dimethyl ester dimethyl terephthalate (DMT) or the purified terephthalic acid (PTA). |
| Uses | Used in apparel, flooring, molded parts for cars, electrical equipment, etc., packaging films | Used to manufacture a variety of products, including textiles, belts, furniture, insulation, padding, tarps and glossy finishes for hardwoods |
| Touch | A silky, smooth touch | Fiber feeling |
| Durability | Exceptionally strong, abrasion resistant, resistant to damage from oil and many chemicals | Strong, resistant to stretching and shrinking, resistant to most chemicals, crisp and resilient wet or dry, abrasion resistant |
| Stretchability | Low moisture absorbency allows fabric to stretch | No water absorbance, faster drying, wrinkle resistant |
How to process polyamide?
Polyamides can be processed by all common melt processing techniques. Though low melt viscosity polyamides need particular attention. Due to their semi-crystalline nature, one must control the processing of polyamides. This is done to optimize the physical properties of the end component.
Thanks to their crystalline structure Polyamides are easy to inject, showing high fluidity. This is particularly appreciated when injecting thin-walled parts.
Due to their moisture sensitivity, Polyamides need an efficient drying process. Insufficient drying will lead to splays and unaesthetic marks on part surfaces. They lower the mechanical properties due to material degradation. This degradation by heat and water leads to oxidation.
Injection Molding
All polyamide materials can be processed by injection molding.
- If the moisture content is >0.2%, drying in a hot air oven at 80°C (176°F) for 16 hours is recommended. If the material has been exposed to air for more than 8 hours, vacuum drying at 105°C (221°F) for more than 8 hours is recommended.
- Mold Temperature: 60-80°C
- Melt Temperature: 230 – 280°C; 250 – 300°C for reinforced grades
- Material Injection Pressure: 75 – 125 MPa (depends on material and product design)
Extrusion
Polyamides can be processed by extrusion.
- Maximum allowable moisture content 0.1%
- Melt Temperature: 230-290°C
- The compression ratio: <4.0
- The L/D Ratio: 25-30 (Barrier Screw or Polyolefin Screw with equal feed, transition and metering section)
3D Printing
Polyamides are also widely used to produce 3D parts printed by selective laser sintering (SLS). 3D printing technique used to produce plastic prototypes offer several benefits such as production of complex parts, individual designs, cost-effective in small scale production.
Can polyamides be recycled?
The key use of polyamide 6 is in carpets. A recycling process for this was initially devised by DuPont in 1944. Although recycling a dirty carpet is still a challenge.
Polyamide polymer can be chemically recycled or de-polymerized
De-polymerization method involves breaking down the long polymer chains into monomers. These monomers can be then re-polymerized. This converts the waste into products having a quality equal to the “virgin” polymer.
For example, Polyamide 6 can be depolymerized to its monomer – caprolactam by:
- Acidolysis,
- Hydrolysis,
- Aminolysis, or
- Catalyzed-de-polymerization in vacuum.
Other methods include the recovery of polymer components without reaching the monomer level. These recycling methods include:
- Multiple extraction and separation steps,
- Mechanical recycling
- Thermal recycling or
- Energy generator.
(Source: AlliedSignal/NCSU)