Recycling of Natural Fibre Composites

Table of Contents:

Introduction

The day-to-day process of environmental regulation involves many preventive measures. The environmental impact of the disposal of natural fibre-reinforced composite materials in landfills is an issue because of the industrial scale of commercialization. Landfill is a quite inexpensive disposal method but it is the least favoured waste management option.' Mechanical recycling, chemical recycling, and thermal recycling are used to separate the fibres and resins from the used composite material or the scrap produced during composite manufacturing. Industrial requests for products using recycled fibres and resins are still rare or partial because reclaimed or recycled fibre composites are considered to be of lower quality compare than virgin fibre-reinforced composites.2 Recycled fibres are not fully controlled in terms of length, fineness, length distribution, poor surface adhesion, and grade variations on batch production.2 The unsatisfactory quality factors can be eliminated by adopting appropriate technology for processing recycled fibres.

Recycling operations involve some conditions such as temperature, pressure, equipment volume, and toxicity of catalyst, and solvents may affect or determine the recycled fibre’s end use. A comprehensive assessment must be carried out in order to categorize these diverse technologies in terms of their efficiency, environmental impact, and commercial viability.1 Three types of recycling process are available, categorized as primary, secondary, and tertiary. In a primary recycling process, the recyclable fibre or resin is recovered from the composite and reused for the purpose of producing the parent or same composite product. In the secondary recycling process, the recyclable fibre or resin is reused in some other way, without reprocessing, to produce other products. In the tertiary recycling process, the composite material or product is chemically altered in order to make it reusable.4

Furthermore, in a universal and eco-friendly approach to the recycling process, one must examine and consider the end of life of reinforced composite products at an early stage in their development process.5 The recycling process for reinforced composite materials and products should include end-of-life strategies and logistics at the product development stage.6 Design and process engineers must balance energy efficiency, safety, cost, and need when thinking about how the composite product will be dealt with at the end of its useful life.7 The common characteristics of design reuse strategies are:8-10

  • • Recoverability: This measures whether the product can be recovered or not after use.
  • • Functionality: This is the average outstanding useful lifespan of a product at end-of-use compared to its premeditated lifetime.
  • Technological maturity: This is the degree of technical amendment adopted or the phases of the inclusion of new features in the product.
  • Components: This is the number of unique parts that can be made and adopted to various kinds of products.
  • Level of integration: This is the independence of product components or level of contribution needed to assure the functionality of a particular end product.

Chemical Recycling

Chemical recycling, also known as feedstock or tertiary recycling, uses a high-tem- perature process to break down the structural bonds in the natural fibre-reinforced polymer composite to be recycled." Chemical recycling encompasses diverse procedures such as pyrolysis, hydrocracking, hydrolysis, and gasification.12 Pyrolysis is carried out in an oxygen-lean atmosphere to break down the large molecules into smaller ones through the influence of heat. Pyrolysis is appropriate for use in dealing with mixed waste or residuals of automotive parts produced after shredding. The pyrolysis process converts plastics into gases, a varied liquid of hydrocarbons, and solid char.12

Some high molecular weight fibre-reinforced polymers, due to the absence of hydrogen and oxygen, are refined into high-value-added petrochemical feedstock as listed in Table 11.1.

Hydrocracking is the alteration of higher boiling point chemicals in crude oil to low boiling point, such as gasoline or jet fuel, using hydrogen under high pressure. Gasification involves the introduction of carbon, comprising material from coal and biomass, using a controlled amount of air or oxygen. The molecules are thereby broken down into hydrogen, carbon monoxide, and other gases.14 Hydrolysis process also helps to break down the large molecule compounds into smaller ones in the presence of water. An example of the hydrolysis process is the depolymerization of condensation polymers such as PU, PLA, and PET. The monomers that result can be reused in the synthesis of new polymers.15

Solvolysis is a process that involves chemical treatment by means of a solvent to degrade the resin or matrix of the reinforced composite. This technique is applied

TABLE 11.1

Recycling natural fibre-reinforced composites out of pyrolysis1213

Polymer

High-temperature products

Low-temperature products

PA-6

Aramid (aromatic polyamides)

Caprolactam

PE

Gases, light oils

waxes, paraffin, oils, a-olefins

PET

TFE

Benzoic acid, vinyl terephthalate

PTFE

TFE

Monomers

PMMA

Tetrafluoroethylene (TFE)

Methyl methacrylate (MMA)

PP

Gases, light oils

Vaseline, olefins

PS

Styrene

Styrene

PVC

Toluene

HC1, benzene

to unsaturated polyesters in sheet moulding composites. Hydrolysis takes place between 220°C and 275°C with or without added solvent/catalyst degrade into glycols, carboxylic acids and a styrene-fumaric acid as copolymer. Different conditions and solvents are used to recycle thermoplastic and thermoset fibre-reinforced composites.16 Alkaline catalysts include potassium hydroxide (KOH) or sodium hydroxide (NaOH). Acidic catalysts are purposely used to degrade high resistant resins like epoxy resins at low temperatures. Methanol, propanol, ethanol, and glycols are some other solvents that can be used with or without additives/catalysts.17

Low temperature and pressure (LTP) solvolysis is commonly carried out below 200°C at atmospheric pressure. Catalysts and additives are generally required to degrade the resin present in reinforced composites at low temperature along with a stirring action. Sulphuric acid, acetic acids, and nitric acid are used as the medium. In high temperature and pressure (HTP) solvolysis (at more than 200°C), alkaline conditions are used to recover the epoxy monomers. The solutions used in the LTP process cannot be used again and are difficult to recycle.18 Common recycling processes for fibre-reinforced polymer methods are depicted in Figure 11.1.

 
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