Figure 1. Economic drivers for the suitability of recycling materials.
Figure 1. Economic drivers for the suitability of recycling materials.
Figure 2. Hierarchical options for waste handling of production scraps: a) The traditional linear organisation for disposal options, and b) The less well used cyclical hierarchy. Other hierarchy formats that could be employed are also available.
Figure 2. Hierarchical options for waste handling of production scraps: a) The traditional linear organisation for disposal options, and b) The less well used cyclical hierarchy. Other hierarchy formats that could be employed are also available.

Although advanced fibre reinforced plastics (FRP) have been utilised for over half a century, it is only in recent years they have started to be used in mass market products, such as the high volume automotive sector. Previously the use of advanced composites was limited to high performance markets such as aerospace, defence, high performance automobiles (at low volumes), and racing. This perhaps contributed to a relatively low effort in researching the recycling of composites, especially as R&D mostly sought structural performance gains, but also many of the materials are heterogeneous and so difficult to recycle. In the modern era, with the introduction of legislation which limits the amount of non-recyclable material allowed on new automotive vehicles, levies applied to the disposal of waste by landfill or incineration, combined with the increasing volume of material used; research in this area has begun to become a focal point.

Generally it is up to manufacturers to design their vehicles in such a way that recovery of all materials is possible, i.e. design for recycling. It is likely that if recycling solutions are not made commercially viable in the near future, as composites become more affordable for mass market items, vehicle manufacturers will be forced to use more easily recyclable materials to satisfy their obligations (or invest in highly bespoke and likely costly solutions). This seems illogical since the use of lightweight materials can aid fuel efficiency, which in turn benefits the environment and slows the pollution rate. The point is especially pertinent with the present trend towards hybrid and electric powered vehicles. These vehicles require significant structural weight reductions due to the added mass of the storage cells (in order to have performance similar to traditionally powered vehicles), and composites could provide that solution. Thus a well incorporated recycling process for these materials must be included. Ultimately, the situation is that the introduction of legislation is just a small part of the wider financial driver for composite recycling. But the risk is that needs in terms of End of Life dominate recycling thinking, whereas options for production wastes are also required.

Figure 1 shows the major drivers for economic suitability and how they are interlinked. It also highlights that waste material is produced by the user both during production and at the end of the component's life. In terms of a global outlook on advanced composites waste generation, no reliable hard numbers are known to be openly available; although in 1997 it was suggested that around 853 tonnes of scrap carbon fibre prepreg were produced in industry. It has been suggested that the aerospace industry suffers a historic buy-to-fly ratio of up to 1.7: 1 (i.e. the amount of material purchased versus the material used on the final shipped product), although this may not be representative of modern builds and all sectors. Nevertheless, this ratio highlights scrap as a priority for composites production. Other views have estimated manufacturing waste to be in the region of 10% to 30%, depending on component complexity, etc. These estimates appear reasonable considering other studies.

The advantages of production waste streams over End of Life structures are that some forms of it should be easily reused within an ongoing production process, the history of the material can be easily traced, it is local, and there should be a low level of contamination. It presents a good opportunity to reduce the amount of useable waste going to landfill, and for those appropriate forms, reuse in high value applications. Alongside a brief review of current recycling options for thermoset composite waste, this article summarises opportunities for reuse of production waste; including some research from the University of Bristol, UK.

Options for waste disposal in composites manufacture

Despite quantities being difficult to understand, it is important to develop an understanding of the scrap types possible. Advanced composite waste may be generally divided into five groups of (1) uncured or dry material/liquid resin rolls and scrap, (2) uncured trim, (3) cured trim, (4) cured scrap, and (5) End of Life. From this, it can be found that manufacturing scrap is broadly defined as (1–4). These categories are perhaps an oversimplification, and do not consider consumables or inserts/core materials etc. They also do not consider defunct materials, though the same classifications could be employed to the material producers. Unfortunately such simplifications also fail to identify the diversity of material types available. For example, there are a large volume of types of carbon fibre, ranging in type, weight, tow count etc.; and identification without any form of labelling is very difficult. Similar difficulty is found with the resins (in type and class); although some resins cannot be mixed and so have a further waste reuse aspect.

1. Manufacturing waste

Previously, reviews were undertaken for US users of advanced composites regarding the type, quantity, and disposal methods of the waste streams. It was found that the dominating scrap material was uncured prepreg with approximately 66% of the total scrap generated. Some variation in this figure can also be found by simply reviewing a specific geographical area. It was suggested that up to 68.5% of waste composite material was available for capture or could be saved through source reduction, and the largest contributing factor to waste was seen to be the manufacturing technique employed, with hand lay-up the worst process for waste generation.

Manufacturing scrap is an immediate problem as it requires disposal/treatment at the point of creation, and requires immediate handling. It is often quoted that manual procedures only utilise 40% of the available material. In a given component, the proportion of material wasted post-processing in trimming and machining etc. ranges between 2% and 40%, with around 16.2% on average. But the largest area for concern appears to be in the preparation of plies from the feedstock rolls, as this phase of manufacture is often reported to create scrap anywhere between 25% and 50% of the input material.

2. End of life waste

The end-of-life scenario is complicated by the length of time a component can remain in service. It may only become waste when maintenance costs become excessive, technology upgrades are required, and/or part replacements become scarce. Essentially, they only become waste when it is no longer economical to use them. It is difficult to predict when and more importantly where End of Life components will become available (other than those that are in catastrophic crash events). This is perhaps more true for aerospace than automotive, but the lack of a reliable feedstock is an issue that needs addressing while composites are penetrating new markets; while for those established markets the distribution of product may make component collection economically challenging.

3. Manufacturing scrap material: reduce, reuse, recycle potential

When reviewing waste management strategies the waste hierarchy principle or cyclic hierarchy of materials use, as illustrated in Figure 2, should be considered. Whilst the waste hierarchy principle can validly be applied to composites, it is suggested that the cyclic hierarchy is more appropriate, particularly when considering manufacturing waste. Disposal as landfill or energy recovery have been omitted, but it is recognised these can still offer waste management options.