Polyamides 101: How to Select the Right Grade?

Introduction to Polyamides

Definition

• Naturally occurring polyamides are proteins
• Synthetically occurring polyamides like nylons and aramids

History

Classification of Polyamides – When to Choose What?

1. Aliphatic polyamides offer the most versatile performance. They lack aromatic content allowing maximum flexibility, toughness, and ease of processing. Yet, it has reduced resistance to heat and chemicals compared to the other two classes.


2. Semi-aromatic grades enhance mechanical properties, moderate thermal performance, and chemical resistance. These properties are coupled with increased flexibility and impact resistance.


3. Aromatic polyamides contain a significant percentage of aromatic rings in their molecular structure. This gives them high thermal stability and chemical resistance. But it also makes them rigid and relatively brittle.

Aliphatic Polyamides – Structure, Properties & Processing

Structure of Polyamide 6 (Nylon 6)

Molecular Structures of Polyamide 6

Structure of Polyamide 6-6 (Nylon 6-6)

Molecular Structures of Polyamide 66

Main properties of PA 6 and PA 6-6

• 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 limit the usage
• Low dimensional stability
• Attacked by strong mineral acids and absorbs polar solvents
• Proper drying before processing is needed

Processing conditions of PA6 and PA66

Injection molding

• L/D ratio of 18:22
• The melt temperature should be between 240-270°C (PA 6) & 270-300°C (PA 6-6)
• 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 (PA 6) & 270 to 290°C (PA 6-6)

Polyamide 11 (Nylon 11)

Bio-based polyamide derived from renewable resources (castor plants)

Polyamide 12 (Nylon 12)

Molecular Structure of Polyamide 12

Key properties of PA 12

• It possesses lower impact resistance but shows good resistance to abrasions and UV.
• It has a lower water absorbency than PA 6, PA 6-6, and all other types of polyamides.
• The PA 12 grade displays good dimensional stability and reasonable electrical properties.
• PA 12 is ideal for applications where safety, durability, or reliability over time is critical.
• PA 12's transparent grades are also available, allowing high flexibility in terms of design and creation.

PA 11 and PA 12: Unique features you should know

• Excellent chemical resistance
• Flexibility
• Durability
• Cold impact resistance
• Thermal resistance

• 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...).
• PA 11 and PA 12 are particularly suitable when reliability over time is needed.

PA 11 & PA 12 processing conditions

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 the type of extrudate, thus a general recommendation can not be given.
• Temperature in the 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.

Polyamide 6-10 (Nylon 6-10)

Key features of PA 6-10

• Exhibits lower water absorption when compared to PA 6 or PA 6-6
• Has a lower brittle temperature than PA 6 or PA 6-6
• Has good abrasion resistance and chemical resistance
• Possesses lower strength and stiffness unlike PA 6-6
• 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)

Limitations of PA 6-10

• High mold shrinkage, and high cost compared to other low water absorption polyamides.
• It is attacked by strong mineral acids and absorbs polar solvents.

Polyamide 4-6 (Nylon 4-6)

Key properties of PA 4-6

Good Thermal Performances	Good Mechanical Properties Particularly at high temperatures	Excellent Wear Resistance	Excellent Chemical Resistance	Excellent Electrical Resistance

• PA 4-6 is often used to replace metal in demanding, high-temperature applications.
• Due to PA 4-6's excellent mar and wear resistance, it is used in gear applications. It offers a combination of mechanical and constant performances at high temperatures. It also offers excellent tribological behavior and high fatigue resistance in this industry.
• PA 4-6 can be metalized. It is also possible to color a part made of PA 4-6. However, the color resistance will depend on the behavior of the pigments at high temperatures.
• Due to its high fluidity, PA 4-6 is a good solution for complex shapes and parts with thin walls.

Polyamide 4-6 processing conditions

• Polyamide 4-6 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 4-6 does not stick to the mold surface and has good ejection properties.

Semi-aromatic Polyamides or Polyphthalamides

Key features of PPA

• Harsh chemical environments, and
• Extreme temperature conditions

• Improved dimensional stability
• Improved solvent and hydrolysis resistance
• Better high-temperature mechanical property retention

• Aliphatic polyamides such as PA 6-6
• Polyetheretherketone (PEEK) and some Liquid Crystal Polymers.

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.

Aromatic Polyamides or Aramids

• better dimensional stability,
• flame and heat resistance, and
• higher strength

Synthesis and Performance Profile

Reaction chemistry

Condensation polymerization

Condensation polymerization of polyamides

Ring-opening polymerization

Ring-opening polymerization of PA 6¹

Performance profile – Does your grade meet user requirement?

1. High strength and toughness – Polyamides are chosen for their excellent mechanical properties. They can withstand high impact and stress. High strength and durability are common requirements. View PA grades with high toughness.


2. Thermal stability – Polyamides retain strength and stiffness at elevated temperatures. Hence, they are used in applications where thermal stability is required.


3. Chemical resistance – In applications like gears and bearings, resistance to wearing, grease, and oil is critical. So, specialized polyamides are developed to resist chemical damage.


4. Friction – A low coefficient of friction is required for bearing, gears, and other sliding parts.


5. Lightweight – In many applications, there is a desire for lightweighting so plastic formulators may specify density as a requirement.


6. Electrical insulation – The insulating properties make polyamides useful for electrical components. These applications include plugs, sockets, coil bobbins, and many more.


7. Moisture absorption – The moisture-absorbing properties of polyamides make them suitable for tubing and reservoirs.


8. Heat stability – Polyamides offer heat stability at temperatures ranging from 100°C exceeding 200°C. In some cases, the temperatures also exceed 500°C. Heat stabilizers can be added to improve the dimensional stability of polyamides.


9. Processability – Polyamide grades show good processability. They can be processed using injection molding, extrusion, 3D printing, and many other processing techniques.


10. Incorporating reinforcements like glass, carbon fiber, carbon black, aramid fiber, minerals, PTFE, and molybdenum sulfide may alter the mechanical properties of polyamides.
    
    
    • Glass fiber – When polyamides are added with glass fillers their stiffness can compete with metals. Hence, they are considered in metal replacement projects.
    
    
    • Carbon fiber – Adding carbon fiber may increase the strength, rigidity and dimensional stability of polyamides.
    
    
    • Carbon black – Carbon black fillers improve resistance to wear, UV, and electrical conductivity of polyamides.
    
    
    • Aramid fiber – Reinforcing with aramid fiber contributes to extreme tensile strength and heat resistance limiting creep.
    
    
    • Mineral fillers – Talc and calcium carbonate increase the stiffness and heat deflection temperatures of polyamides. This reduces the cost but tend to decrease strength and toughness.
    
    
    • PTFE – Provide solid lubricant properties and low coefficient of friction for bearings, gears, and other sliding applications while enhancing chemical resistance.
    
    
    • MOS₂ – Similar to PTFE, molybdenum disulfide powder provides the same performance. Additionally, they are self-lubricating in parts like bushings.

Physical Forms and General Processing Techniques

Forms of nylon grades

Pellets

Powder

Granules

Solid

Chips

Conversion modes

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

• 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

Sustainability Aspect of Polyamides

Can polyamides be recycled?

• Acidolysis
• Hydrolysis
• Aminolysis, or
• Catalyzed-de-polymerization in vacuum

• Multiple extraction and separation steps
• Mechanical recycling
• Thermal recycling, or
• Energy generator

(Source: AlliedSignal/NCSU)

Are polyamides biodegradable?

Introduction of hydrolytically unstable bonds

Blending with biodegradable fillers

Synthesis from bio-based monomers

Are polyamides good or bad for the environment?

Pros

• They are durable and long-lasting plastics. Hence, products made from nylons don’t need to be replaced often.
• They can often be recycled. Giving materials a second life by sending them to landfills.

Cons

• Nylons are made from petroleum. Its production requires extensive energy and generates a lot of nitrous oxide. This can lead to significant greenhouse emissions.
• Recycling rates for nylons are quite low globally. So, a lot of nylon products end up in landfills and oceans.

Are polyamides safe to use?

• Raw materials used in the preparation of nylons may be hazardous. These substances include strong acids and gases that are involved during the complete production process. But in finished consumer products, they are risk-free.
• Food-grade nylons comply with EU and FDA food contact regulations. They show no migration of harmful substances in food packaging.
• Medical grade nylons show excellent biocompatibility. Used in implantable devices that do not break down or leach harmful compounds into the body over time.
• Nylons can begin to decompose when exposed to very high temperatures (above 500°F).

Nylon vs Polyester - Key Differences You Should Know

1. Dubois, Philippe; Coulembier, Olivier; Raquez, Jean-Marie (2009). Handbook of Ring-Opening Polymerization, 166.
2. Kyulavska, M., Toncheva-Moncheva, N., & Rydz, J. (2019). Biobased Polyamide Ecomaterials and Their Susceptibility to Biodegradation. Handbook of Ecomaterials, 2925–2926.