Recycling Of Composites Materials

Question 1

What are the objectives of composites recycling?

Answer 1

Composite materials are used in a wide range of applications, such as automotive, aerospace and renewable energy industries.

Question 2

What are the materials in composites?

Answer 2

1. Fibres (or particles) for reinforcement
   
   • Glass fibres
     - E-Glass
     - S-Glass
     - Quartz
   • Carbon and graphite fibres
   • Metal fibres: for ceramic matrix composites
   • Ceramic fibres: for metal or ceramic composites
2. Matrix for holding fibres together
   
   • Polymers
     
     • Thermosets (70 %, very difficult to recycle!)
     • Thermoplastics (30 %)
   • Metals (Aluminium)
   • Ceramics
3. Other materials
   
   • Coatings
     
     • Improve bonding and load transfer at the fibre-matrix interface
   • Fillers
     
     • Added to polymers to reduce cost and improve dimensional stability

Question 3

What are the composite recycling options?

Answer 3

1. Primary recycling: Recycling back to “new” composite materials
   
   • Very difficult
   • Special manufacturing process
2. Secondary recycling: Recovery of reinforcement fibres
   
   • Main focus, and economic driver
   • Matrix (polymer type) has much less value, and is difficult to recover
3. Tertiary recycling: Recovery of both reinforcement fibres and matrix
   
   • Chemical and thermal processing
   • Matrix resins: as new resin or feedstock
4. Quaternary recycling: Energy recovery
   
   • No materials recycling (loss of fibre)
   • Incineration/combustion (resin)

Question 4

What are the methods of recycling composite materials?

Answer 4

• Mechanical recycling
• Thermal recycling
• Chemical recycling

Question 5

Mechanical recycling of composite materials

Answer 5

• Shredding and grinding, followed by screening to separate fibre-rich and resin-rich fractions for reuse
• Very energy intensive
• The recyclates have relatively low quality

Question 6

Thermal recycling of thermal recycling

Answer 6

• Using high temperature to decompose the resin (b/w 300 and 1000 °C)
• Separation of the reinforcement fibres and fillers from organic resins
• Clean fibres or inorganic fillers are regenerated
• Secondary fuel or thermal energy can be produced
  
  • Pyrolysis: heating in the absence of O2
  • Gasification or combustion (with deficient O2)
• However, the quality of the recovered fibres or filler materials degrades to a varying extent during thermal processing

Question 7

Chemical recycling of composite materials

Answer 7

• Chemical depolymerisation or removal of the matrix, and liberation of fibres for further recycling
• Using organic or inorganic solvent, e.g. supercritical water
• High temperature (200 - 450 °C), high pressure (5 - 30 MPa)
• Tertiary recycling
• Recovered fibre: no degradation
• Recovered resins: reusable
• Problems:
  
  • Expensive
  • Generation of waste acid or chemicals (toxic: using alkaline catalyst, such as NaOH and KOH)
  • Only in the research state

Question 8

Recycling of composite materials: ReFiber’s recycling concept

Answer 8

• On site cutting to ‘container’ size pieces w/ hydraulic shear or similar tools (mechanical)
• At the plants, the parts are shredded to hand-sized chunks (mechanical)
• The materials is fed continuously into an oxygen-free rotating oven w/ a temp. of 500 °C => the polymer is now pyrolysed to a synthetic gas (thermal)
• The gas is used for electricity production and for heating the rotation ovens
• In a second rotating oven, the glass fibre material is ‘cleaned’ in the presence of atmospheric air
• Metals (steel) are removed by magnets for recycling
• The dust is removed from the cleaned glass fibres
• The glass fibres are mixed w/ a small amount of polypropylene fibres and pass through an oven where the PP fibres melt and connect to the glass fibres, creating stable insulation slab

Question 9

What types of composite materials or their components are recyclable?

Answer 9

From the composites, only the carbon or glass fibres are being recycled, not the matrix (for economic reasons).

Question 10

What are the current challenges and problems for recycling composite materials?

Answer 10

The main obstacles are:

• Technological immaturity
• Lower quality of the recovered materials (degraded)
• Too high cost of recycling process
• Lack of market of the recovered materials (fibres)

Question 11

How to improve recyclability of composite materials?

Answer 11

• Material development for new and easily recyclable composite materials
• Material recyclers for more efficient and intensified separation and purification technologies
• Production mechanisms that can use more recycled fibres, instead of only new fibres
• Incentives for recycling: governmental role, legislation, ban for landfill, materials recovery quota