Polyamide 6 (PA6) NH (CH2)5CO Recycling

A polyamide is a polymer that contains amide monomers joined together by peptide bonds. They can occur both naturally and artificially; examples of these are proteins such as silk and wool, and can be artificially made through solid-phase synthesis and step-growth polymerization, examples being aramids, nylons and sodium polyasparatate.

Polyamides are used commonly in automotives, textiles, sportswear and carpets due to high strength and durability. PA 6 is also known as nylon 6. Paul Schlack at IG Farben developed this polymer, which is formed by ring-opening polymerization. It is a semicrystalline polyamide and was given the trademark Perlon in 1952.

Molecular formula - [-NH - (CH2) 5-CO] n
Density - 1.14 – 1.15 g/cm3
Melting point – 215 - 225°C
IUPAC ID - 7,14-diazacyclotetradecane-1,8-dione

Chemical and Physical Properties of PA6

The polyamide molecular structure includes CONH groups linked by methylene-group based linear aliphatic sections. The crystallinity, rigidity, thermal resistance and toughness of polyamide resins are because of the strong inter-chain attraction caused by the amide group polarity.

PA 6 has a lower melting point than PA 66 and a wider processing temperature range. It has a higher solvent resistance and impact strength when compared to PA 66; however, it has higher moisture absorption.

Moisture absorption impacts a number of properties that must be taken into account during resin design. A number of modifiers are added to enhance mechanical properties; glass is a commonly used filler. Impact resistance is improved by adding elastomers such as SBR or EPDM.

Manufacturing Process of PA 6

Polyamide 6 is manufactured preferably by a continuous process. For a more flexible production program and for small capacities alone, batch operation is recommended. In order to achieve flexible production of medium-to-high viscous polymer, presently caprolactam is polymerized with a catalyst in a dual-stage process.

Chip extraction, drying and production are similar for high and medium viscosity products. For chip production, different types of underwater granulators are preferably fused. Extracting the remaining oligomers and caprolactam is done in a multi or single tube wash column where the treatment of chips is done in a counter flow with hot water. The specific Zimmer design enables a water/chip ratio resulting in high-extract content in the washing water, hence saving energy in the caprolactam recovery plant.

The drying system comprises a closed loop for nitrogen recirculation and a separate gas distribution zones of advanced design including a purification system resulting in lesser consumption of fresh nitrogen. If needed, other additives and/or modifiers or titanium oxide proportionate to caprolactam quantities are fed as solutions or aqueous suspensions into the caprolactam before polymerization.

It is also possible to design the drying system for combined drying or solid-state polycondensation to enhance the relative viscosity of the chips over a broad range. There is a need to recover the oligomers and monomers dissolved in water during extraction as the polyamide 6 polymerization reaction is restricted to 90% conversion. The extraction water is reprocessed along with the solid waste from the overall production system, this being an economic method of recovering caprolactam.

The key process steps of caprolactam recovery are:

  • Multi-stage/single-stage evaporation
  • Oligomer separation
  • Depolymerization
  • Chemical treatment
  • Distillation

For certain applications, the extraction water is concentrated in a multi-stage evaporation system and directly fed back to the polymerization stage.

Applications of Virgin Material PA 6

The key applications of PA6 are:

  • Agricultural films
  • Food packaging
  • Medical films
  • Liquid packaging
  • Sports
  • Electrical and electronics
  • Furniture
  • Automotive
  • Protective packaging

Environmental Impacts of PA 6

During manufacture of PA6, a number of carcinogenic chemicals such as hydrogen cyanide gas and benzene are added. Nitrous oxide is produced during manufacture, which is a greenhouse gas 296 times more powerful than carbon dioxide for global warming.

Waste water generated during production may have the unreacted monomer caprolactam that has been proved to impact development, growth and mortality in crustaceans, fish, amphibians, insects, other organisms and mollusks. Hence it is important to remove this from waste streams. The hazard of storing PA 6 in landfills is that it easily catches fire and this results in hazardous smoke containing nitrous oxide, hydrogen cyanide and dioxins. It is very essential that the recycling process treats waste water, captures all emissions and recaptures the energy used.

Recycling Process

Recycling of polyamide 6 is very essential especially because its presence in landfills is very dangerous and may result in huge fires containing hazardous gases. The key use of polyamide 6 is in carpets and a recycling process for this was initially devised by DuPont in 1944 although recycling a dirty carpet is still a challenge. Statistics show that every 10 t of carpet waste produces 0.8 t of PA 6 and 1 t of PA 66. The Carpet Reclamation Program by DuPont processes more than 700 t of carpet each month, collecting carpets from more than 80 collection locations in the US. Waste material obtained from processing can be formed into fuel pellets for use in coal-fired power stations and cement kilns.

The different ways of recycling polyamide 6 are:

De-Polymerization or Chemical Recycling

This method disintegrates the long polymer chain into monomers that can be polymerized again converting the waste into products with the quality of the virgin polymer. It is possible to depolymerize polyamide 6 into its monomer caprolactam by acidolysis, aminolysis, hydrolysis or catalyzed depolymerization in vacuum. DuPont presently prefers aminolysis and catalyzed depolymerization is also gaining popularity.

Depolymerization is the main method used to recycle carpets and include the following steps:

Acidolysis – An acid catalyst is used to depolymerize PA 6 and the cut waste is melted in continuous reactor and subjected to steam treatment. Caprolactam is recovered for further usage.

Hydrolysis – Depolymerizing PA6 in a high-pressure steam reactor yields around 70% of caprolactam.

Aminolysis – Aminloysis has been accepted by DuPont as the ideal option for recycling carpet. Backing separation, dirt and contaminant removal is initially done after which chipping, shredding, passing through a hammer mill, screening then grounding to particles of 1.5 mm is done.

A slurry is formed by adding water and later separation by density is carried out forming a material having 98.5% purity.

Extracting Recycling or Recovery of Polymer Components

This method is used to recover individual components of the polymeric mixture without attaining the monomer level.

Mechanical Recycling or Re-melting

This technique is to perform melt-bending of the complete structure.

Thermal Recycling

This method involves only energy recovery during incineration of the polymer waste.

Applications of Recycled PA 6

Recycled PA 6 can be used for the following applications:

  • Carpet cushioning
  • Industrial block flooring material
  • Highway noise abatement panels
  • Automotive parts
  • Roofing shingles
  • Composite lumber

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