Plastic materials selection and their additives according to their recyclability and their energy recoverability
Plastics are recyclable materials whose recyclability can vary, either by the intrinsic nature of the material (the material can not be recycled or recycling would lead to a high deterioration in its initial properties) or by the lack of industrial and organized recycling system.
The possibility of energy recovery from plastic may also vary depending on the composition of materials. Some plastics have calorific values ​​comparable to those of oil (polypropylene, polyethylene, …). Other, in contrary, have calorific values ​​three to four times less than oil and are more similar to that of household waste (Composites with high rate of glass fibre or mineral fillers). Moreover, the presence of halogens (chlorine, fluorine, ..) in some plastics (PVC, PTFE, …) require incinerators equipped with flue gas treatment systems which comply with standards and which generate large quantities of ultimate waste that must be stored in special technical landfill.
Some fillers included in the materials can also cause problems for recycling or energy recovery (e.g. the brominated flame retardants).
Recyclability rating of plastics.
| Commonly recycled | Recycled | Few or no recycled |
| PP, PEhd, PET, PVC (rigid) | ABS , HIPS, PA6, PA66, PBT, PC, PC+ABS, PVC (flexible), | POM, BMC, SMC, EPOXY, PBT, PVC complex with a rate <50% of PVC, alloys, PUR, PLA, PBT, EVA, PTFE, bakelite |
The fillers and additives can have negative effects on recycling
| Is well tolerated for recycling | Is a little tolerated for recycling | Is not tolerated for recycling |
| Talc, calcium carbonate, barytes, TiO2 | Small fibre concentrations (<5%) | Flame retardants, high concentrations of glass fibres (> 10%), vegetable fibres, nano-particles, pigments with heavy metals |
Fillers and additives are not allowed for recycling for several reasons:
– Lack of information on additives in plastics for the recycler make sorting of plastic involved impossible.
– Impossible (technical / economic) to separate the fibres (glass or vegetable) from the polymer.
– Health hazard of heavy metals that may be present in the pigments.
– Health hazard, incompatibility of different types of flame retardant.
Energy recoverable plastics
| Energy recoverable | Moderately energy recoverable | Not energy recoverable |
| PP, PEhd, PET, ABS, HIPS, PBT, PC, PC+ABS , PLA,EVA | BMC, SMC, EPOXY | Bakelite, Plastics with halogens: PVC, PTFE, … |
Charges that could have a negative effect for energy recovery from plastic
| Well tolerated for energy recovery | Tolerated for energy recovery | Not allowed for energy recovery |
| Vegetal fibre | Glass fibre, talc, calcium carbonate, TIO2, baryte | Flame retardants, pigments with heavy metals, halogens (Br, Cl, F) |
Association rules of plastic materials
The plastic materials are often incompatible. This means that the recycling of incompatible materials leads to a mixed recycled material with degraded properties, which will not respond to noble applications with high added value.
It is recommended within the same part to use plastic materials compatible with each other, because the properties of recycled plastics fall rapidly if they are contaminated with small amounts of incompatible plastics (tolerance rarely exceeds 2% of contaminants in a mixture).
The following table shows the levels of compatibility of materials:
Figure 1: Recycling and Design Recommendations for Design and Production – BAYER AG (95)
In the case of a major obligation of using incompatible plastic within the same part, we must ensure that these materials are easily separable for a disassembly phase, or sorting techniques become more efficient, can be separated by physical-mechanical sorting after the grinding step of the part.
It is therefore, for the incompatible plastics, to respect rules of automatic sorting after grinding of the product or part, or default to facilitate easy access and removal of incompatible plastic parts.
Note: The surface coating of plastic
For aesthetic reasons, designers often have to advocate films of paint or coating type textile or leather (PVC) on plastics. These coatings should be considered as material inputs which must follow the rules of compatibility or shall be separable after grinding (especially by air classification, for textiles and paints). It is therefore necessary to ensure the compatibility of materials and their separability after grinding.
The recommendation is therefore to avoid these coatings and to provide dyed materials rather than painted for coloured plastics.
Sorting rules for incompatible plastic materials
Sorting methods by near infrared spectroscopy have grown strongly. They allow efficient way to sort mixtures of clear plastic by type of polymers matrix. They are now able to sort plastic mixtures of incompatible plastics with efficiencies above 95%, which is close to optimal recycling.
In contrary, for dark materials, such techniques by near infrared are now ineffective. During recycling, separation techniques are most commonly based on the density of plastic. So, during design of multi-material part, the dark plastics should have densities different with at least a gap of 0.15. cf. density table of some polymers
Note: Sorting techniques by near infrared do not recognize fillers, in particular brominated, vegetable fibres, that are incompatible for recycling. This means that the use of clear plastic with incompatible fillers (Br, plant, …) lead to mixtures difficult to recycle. The recycler must include an other phase of density sorting after infrared sorting to remove filled materials. Which is not always the case.
The rules for easy sorting are as follows:
– Reduce the percentage of incompatible plastics in a part (best part = single-material)
– Use clear plastic material, or without pigment (economic gain, freedom for colouring the recycled materials)
– Avoid incompatible fillers, even in clear plastic materials
– When mandatory use of dark materials, ensure that there is a density range of 0.15 to facilitate their separation.
Table density separation
Consider limitations of current recycling processes
Some design habits of plastic parts or assembly can lead to breakage problems or maintenance of recycling facilities, and induce additional sorting steps.
Design rules:
– Cases of metal screws and metal inserts overmolded
Will be encouraged not to exceed diameters of 3mm steel screws and inserts large metal thickness greater than 1 mm to prevent breakage or premature wear of the grinding of plastic parts. The aluminium screws can go up to 5mm.
– The case of very thick pieces of plastic or composite
The large composite parts filled with glass fibres of a thickness greater than 5mm should be avoided.
Plastic parts with low filler percentage and thickness less than 10 mm, should also be avoided.
Synthesis
We may summarize plastic parts design rules with the following recommendations (not exhaustive):
– Choose the polymer families most easily recyclable.
– Optimize the number of different plastics present within the same product.
– In case several types of plastic are needed, make sure they are not incompatible.
– Facilitate the separation of different plastics, this will facilitate sorting.
– Reduce the use of highly filled plastics containing brominated flame retardants.
– Avoid the presence of paint, film, tissue, metal insert / screw on or in a plastic part.
– Facilitate the identification of plastics, avoid dark plastic harder to sort.