Cellulose promises a low-carbon future for composites

Far from aiming for a ‘paperless’ society, Steve Eichhorn, Professor in Materials Science and Engineering at the University of Bristol, believes cellulose could hold the key to a sustainable, low-carbon future.

Composites are widely used in a diverse array of applications wherever a combination of mechanical strength and lightweight is needed. But composites typically rely on petrochemical-derived resins reinforced with carbon or other inorganic materials that are neither low-carbon nor sustainable to produce.

But, Eichhorn, who has dedicated his career to understanding cellulose, is convinced that composites based on natural, sustainable materials – particularly cellulose – can replace current petrochemical-derived materials without impacting on the environment.

His interest in cellulose began with an M.Sc. and Ph.D. in papermaking at Bangor University and UMIST in Manchester. He stayed in Manchester as a research scientist and lecturer, before moving to the University of Exeter in 2011, where he led the Engineering Department from 2014-2017. Now with a research group in Bristol, he is pursuing the development of renewable, sustainable composite materials based on cellulose.

Steve Eichhorn talked to Reinforced Plastics about his current research and future plans.

What are the major themes of your current research?

I have been working on cellulose, which is a plant material, for the last 20+ years. Most recently I’ve focused on what we call nanocellulose, which are fibers smaller than 100 nm extracted from plants chemically or mechanically, as reinforcement for composite materials. We are also trying to find ways to spin high-performance, novel cellulose fibers from ionic liquids [1].

We are also looking at gels, which are basically these fibers concentrated in water. Many of the creams and shampoos we use, as well as some of our foodstuffs like ketchups and mayonnaises, contain gels, known as ‘rheological modifiers, which give food texture and creams or shampoos the ability to spread on the skin or hair, hold fragrances, and improve the cleaning process. We would like to make these gels in different ways using less energy.

Recently, we’ve been moving more towards functional materials, where we use cellulose as the base of a material or device that has a particular function. For example, replacing the use of inks in plastics with films that are colored because of their structure [2]. We are also exploring ways of making materials magnetic by including magnetic nanoparticles in the structure, which could be useful as actuators [3]. We’ve also been looking at energy storage applications, where we convert the material to an all carbon-form that is conductive but also has a very high surface area, which could be suitable as electrodes in supercapacitors [4].

How and why did you come to work in these areas?

I studied physics as a first degree, but then did a Masters in paper science and forestry industries technology. I got really interested in cellulose at that point and did my PhD on the material. I realized that there are a whole range of things you can do with cellulose, which is the most widely used material in the world, and that there are many opportunities, particularly in sustainable applications. What really sparked my interest was the work of others and the possibilities of replacing oil-based polymers with cellulose. That captured my imagination.

What do you think has been your most influential work to date?

I think there are two. One is a piece of work we did a long time ago to work out the stiffness, or Young’s modulus, of tunicate cellulose nanocrystals, which are made by a type of marine organism [5]. We were the first group to establish the mechanical properties of this highly crystalline form of cellulose. From that initial work we were able to establish a fundamental understanding of the physical properties of nanocrystalline cellulose or nanocellulose. This work really started us off on the road we are on now, trying to make and understand nanocomposites.

The other piece of work that I am very proud of was done with a former colleague, Bill Sampson, at the University of Manchester [6]. We were both paper scientists at the time and we noticed that there were synergies between the structures of electrospun fibers, which were novel at the time, and paper fiber structures. We applied mathematical models, which Bill had developed, to electrospun fiber networks and were able to show quite clearly that rather than making smaller fibers for tissue engineering, thicker fibers were better for creating larger pores into which cells could infiltrate. This had, I think, a big impact on tissue engineering in the early days, particularly for producing artificial skin. It demonstrated how one area of specialism can have a major influence on another.

What is the relevance of your research to fiber-reinforced composites?

Cellulose is a lightweight material and has specific mechanical properties that are comparable to glass fibers. So using plant fibers to replace glass is quite an interesting area of research.

But if you reduce the length-scale, to nanocellulose, the number of defects within the material is reduced. This is a burgeoning area of research at the moment. In Japan, for example, the car manufacturer Toyota is seriously considering nanocellulose-based composite materials for car bodywork, while Ford is exploring similar plans in the US.

In our work, we look for ways to make new fibers and how those fibers disperse in resins to build up a picture of how these materials form, their structures, and what the barriers are to getting the best performance out of these new composite materials.

What have been the most major developments in this field over the past decade in your opinion?

The ability to process nanocellulose with thermoplastics has been a major development. Cellulose is hydrophilic, so it doesn’t like to interact with hydrophobic materials like thermoplastics. There have been huge inroads made in being able to injection mold, with thermoplastics, some of these materials on larger scales.

Where do you see this area of research going in the future?

I think that one area where there will be major developments is in terms of large-scale, high-volume applications. But there are also an awful lot of low-volume, high-tech applications of nanocellulose on the horizon in areas such as medical diagnostics and energy storage materials. The idea of making a lab-on-a-sheet-of-paper that is able to diagnose common illnesses could have a major impact in the developing world.

In terms of other global-impacting technologies, I believe water purification systems will benefit greatly from nanocellulose-based filtration media that can capture viruses.

Although plants produce nanocellulose naturally, it is very difficult to make in the lab. In the future, it will become increasingly important to harness enzymes and bacteria that can produce cellulose as well as regular polysaccharide materials.

Nanocellulose and natural fibers are highly functionalizable in a myriad of ways. Because they possess a large number of hydroxyl groups, they have a very reactive surface to which we can tag or attach other molecules. This is not a feature readily available with many other polymer types.

And, of course, the main advantage of nanocellulose and natural fibers is that they are renewable! If we use more cellulose in the future, we will have to maintain and grow forests and plants, which are both sustainable and sequester carbon dioxide. The use of cellulose could be a significant contributor to excess carbon reduction by acting as a sink for carbon dioxide. We urgently need to move away from our reliance on oil-based materials towards more sustainable materials that can go back to the environment without causing harm. Cellulose is one such opportunity for us. We used to be very reliant on cellulose and I think we need to go back to that reliance on renewable materials.

Are there any developments that you would particularly like to see come to pass?

If we understood more about the plant cell wall, and the interactions that occur inside plant cells, we might be able to extract cellulose more cheaply and efficiently. I believe that would have a huge impact on our use of cellulose.

What factors do you believe will be key to the success of the field in the future?

Funding is key to keeping this field alive. I wish more people would get involved in cellulose research in the UK and elsewhere. We need industrial take up of materials for large-scale, large-volume applications and we need small-scale, high-tech end-users to come through to the market. Governments could also do more to push sustainable materials, to insist that we don’t use harmful persistent polymers in applications.

What specific questions or problems do you hope to tackle in the future?

I plan to go back to basics. I want to understand the interaction of cellulose with water: water plays a huge role in the interaction of sugars and influences molecular structure but very little is known about its influence on the molecular structure of cellulose.

But I am also going to start looking at how cellulose helps influence the properties of interfaces in composites and delamination toughness in larger structures.

My work in the future will range from fundamentals to applications.

What is the secret to successful research?

Diversity is really important. I’ve been very lucky to have a very diverse group over the years: everyone contributes and brings different strengths. As a group leader, I think it’s important to understand where people have come from, their background, and how best to help them develop, learn new skills, and contribute to knowledge. We all collaborate together, sharing techniques and expertise with materials. With diversity, come different ideas. If I surrounded myself with people who looked and thought like me, it really wouldn’t work!

Key publications?

1. C. Zhu, A.F. Koutsomitopoulou, S.J. Eichhorn, J.S. van Duijneveldt, R.M. Richardson, R. Nigmatullin, K.D. Potter. High Stiffness Cellulose Fibers from Low Molecular Weight Microcrystalline Cellulose Solutions Using DMSO as Co-solvent with Ionic Liquid. Macromolecular Materials and Engineering 303 (2018) 1800029. https://doi.org/10.1002/mame.201800029
2. P. Tzeng, D.J. Hewson, P. Vukusic, S.J. Eichhorn, J.C. Grunlan. Bio-Inspired Iridescent Layer-by-Layer-Assembled Cellulose Nanocrystal Bragg Stacks. Journal of Materials Chemistry C 3 (2015) 4260-4264. https://doi.org/10.1039/C5TC00590F
3. N. Sriplai, P. Sirima, D. Palaporn, W. Mongkolthanaruk, S.J. Eichhorn, S. Pinitsoontorn. White magnetic paper based on a bacterial cellulose nanocomposite. Journal of Materials Chemistry C 6 (2018) 11427-11435. https://doi.org/10.1039/C8TC04103B
4. A. Sturcova, G.R. Davies, S.J. Eichhorn. The Elastic Modulus and Stress-Transfer Properties of Tunicate Cellulose Whiskers. Biomacromolecules 6 (2005) 1055-1061. https://doi.org/10.1021/bm049291k
5. S.J. Eichhorn, and W.W. Sampson. Statistical Geometry and Spatial Statistics of Nanofibrous Porous Assemblies. Journal of the Royal Society Interface 2 (2005) 309-318. https://doi.org/10.1098/rsif.2005.0039
6. Q. Li, Y.Q. Zhu, S.J. Eichhorn. Structural Supercapacitors Using a Solid Resin Electrolyte with Carbonized Electrospun Cellulose/Carbon Nanotube Electrodes. Journal of Materials Science 53 (2018) 14598-14607. https://doi.org/10.1007/s10853-018-2665-x