Recovered Carbon black as a more sustainable feedstock

What is virgin carbon black?

Virgin carbon black is a fine black powder composed of elemental carbon. It has remarkable properties owing to its small particle size, aggregate structure & surface chemistry. Nowadays, virgin carbon black can be used in a multitude of applications due to its excellent properties. Generally, the particles are spherical and small in size, creating a large surface area, making virgin carbon black a fantastic pigment in paints, printing inks and coatings.

Figure 1: Virgin carbon black is widely used to give the black colour seen in many applications including rubber as in tyres seen here.

What is virgin carbon black used for?

Plastics such as refuse sacks, industrial bags, and household containers all contain carbon black. Conventionally, carbon black is added via masterbatch due to its excellent dispersibility and tinting properties. It also offers UV protection by absorbing UV light and converting it into heat, resulting in the polymer being more resistant to thermal degradation whilst providing colour.

High-performance coatings in wood, marine, aerospace, decorative and industrial applications also implement the use of carbon black for pigmentation & UV- protection. Similarly, it is used in toner for laser printers and screen ink electronics due to its superior efficiency, and fastness against light.

Aside from pigmentation and UV protection, the structure of carbon black makes it a brilliant reinforcing filler. Consequently, many rubber products such as hoses, belts and gaskets use carbon black.

Environmental impact of carbon black

In April 2022 a plastic packaging tax of £200 per tonne was introduced by the UK government to reduce the amount of plastic waste that ends up in landfill. It applies to producers and importers of plastic packaging with less than 30% v/v of recycled plastic. Virgin carbon black is widely used in plastics, however, is not produced from recycled feedstock. Instead, it is synthesised during the incomplete combustion of petroleum products such as oil and natural gas. As the demand for carbon black surges, significant environmental and health concerns are associated with its use. Typically, the production of carbon black uses a furnace process at elevated temperatures of 1200-1900°C. This uses a substantial amount of energy and creates sulphur and nitrogen oxide emissions, making the entire process undesirable from a green perspective. Producing 1 tonne of virgin carbon black generates 2.5-3 tonnes of CO₂, this is problematic and raises the question of how we can make virgin carbon black a more sustainable material.

So, how can we make the production of carbon black more sustainable?

Recovered carbon black (rCB) is a substitute for virgin carbon black, manufactured using sustainable material from end-of-life tyres. End-of-life tyres are a plentiful feedstock. Each year, 1.5 billion tyres enter the waste stream raising even more waste and environmental concerns. The circular economy approach used with rCB drastically reduces the CO₂ emissions that would otherwise be used in virgin carbon black production, whilst contributing towards the 30% of recycled material needed to avoid the UK plastic tax.

Figure 2: The mechanism behind recovered carbon black involves pyrolysis, devolatisation, milling and sometimes pelleting. This method is a novel method in the UK performed by Waverly Carbon.

Production of recovered carbon black

Production of rCB involves pyrolysis which uses lower temperatures than the furnace process of virgin carbon black. During pyrolysis, the thermal components of the tyre are broken down, which results in a condensed gas called tyre pyrolysis oil (TPO) being produced. Usually, this oil is sold and can be used for energy production in industrial settings or can be processed further in refineries. At the end of the process, a carbon-containing residue called char remains. Char resembles carbon black but lacks the consistency and quality to be used directly as a carbon black substitute. This is attributable to the metal contaminants present and the variable particle size of char. Therefore, it is essential to refine the char to produce high-quality rCB.

Waverly carbon is the UK’s only manufacturer of recovered Carbon Black. They source high-quality tyre char from various pyrolysis plants and focus on refinement, which includes steps such as devolatisation, milling, and optional pelleting. Char quality can vary depending on the type of tyre used and the temperature used in pyrolysis. ‘Dirty’ char can be produced if the pyrolyser is not run at a high enough temperature resulting in a strong odour and weak reinforcement properties. Waverly carbon put the spotlight on refining rather than producing char, this means the char is tested before refinement, to ensure it has exceptional quality and is ‘clean’.

How does rCB compare to virgin carbon black?

The carbon content of traditional virgin carbon black is close to 100%, as it’s sourced from heavy petroleum products such as coal. On the other hand, rCB is acquired from end-of-life tyres so contains 80-85% carbon black and 15-20% inert ash, predominantly consisting of silica & zinc.

Jetness & reinforcing properties

Waverly carbon offers rCB under the trade names Emerald 210, Emerald 215, Emerald 220 and Emerald 225.  In the polymer industry, one of the most important properties of virgin carbon black is the deep jet-black colour which arises from the large surface area of the small particles. During production, Waverly carbon refines rCB into a small uniform particle size in the milling process, creating a large surface area for good jetness and opacity. This is especially useful for coatings, plastics and rubber. However, the major difference between using rCB over virgin carbon black is the ash content, which leads to a slight reduction in the overall jetness.

To counter this Emerald 220 & 225 use different feedstock. By sourcing truck and passenger tyres as opposed to passenger tyres alone, lower levels of ash content can be attained, resulting in a higher degree of jetness.

In rubber, various grades of virgin carbon black exist. N-550 is a grade of virgin carbon black commonly used in the cord layer of tyres and calendared sheets for the roofing industry due to its reinforcing properties. In addition to providing reinforcement, virgin carbon black can also be used in rubber due to its damping properties. rCB by Waverly exhibits similar reinforcement properties to grades N-550 and above which are considered semi-reinforcing grades, whilst improving the damping properties of rubber. So in conclusion, rCB should be considered a sustainable replacement for virgin carbon black for most rubbers containing semi-reinforcing carbon blacks and for pigmenting purposes.

Figure 3: Recovered carbon black comes in pelletised or powder form both of which have small uniform particle size as can be seen above.

PAH levels

Polycyclic aromatic hydrocarbons (PAH’s) are found in virgin carbon black, which are renowned for being hazardous to human health and the environment. PAHs have been discovered in many consumer products and have been restricted by the EU and businesses due to health concerns. Therefore, it is more vital than ever to minimize the levels of PAH’s.

Benzo-alpha-pyrene is the common benchmark used to determine the concentration of PAH. During testing, benzo-alpha-pyrene could not be detected in rCB by Waverly carbon as these are destroyed during the pyrolysis process. For that reason, Waverly rCB can be considered a safer, more sustainable alternative to virgin carbon black.

Formulating with carbon black

Virgin carbon black can be produced in powder and pellet form. Powder form carbon black is semi-free flowing so is usually employed in pigments. Although, this can be challenging to handle as it tends to cake, pack, dust and deposit onto equipment. From a health and safety point of view, this is troubling as carbon black is a potential carcinogen. For this reason, Waverly carbon has developed rCB in pelleted form which closely matches the characteristics of pelleted virgin carbon black. Pelletised carbon black is made from larger particles, so the bulk density is higher resulting in easy shipping, storage, handling and most importantly less dusting.

Summary

rCB produced by Waverly is a more sustainable alternative to virgin carbon black. Whilst marginally higher concentrations of rCB may be required to achieve the same level of opacity, this can be offset by the environmental benefit of rCB. rCB plays a critical role in the drive to use more sustainable materials.

If you would like more information about recovered Carbon Black or you would like to discuss how it could work in your masterbatch, get in contact on 01827 314151 or via the website enquiry form.