What Are the Next Best Materials for Industrial Use?
For any mechanical or civil engineering project, material selection has always involved trade-offs. With so many variables — dozens of physical properties, cost, sustainability — there can never really be a perfect material for any job. Still, engineers and scientists keep on searching.
Finding the perfect material for a specific industrial or construction application can be a complex process. Whether you rely on so-called Ashby charts, multi-criteria analyses or even artificial intelligence, the common thread in the decision-making processes is balancing objectives and constraints.
Recent decades have seen an explosion in the variety of materials available to engineers. It hasn’t changed the fundamental need to make trade-offs, but it has perhaps led to a subtler change in this balance, with objectives becoming more important than constraints.
Or, to put it another way, you’re more likely to choose materials according to what you want — rather than what you’re prepared to give up.
Below, we examine how innovations in material design continue to expand the options and possibilities open to engineers.
When thinking about new materials in almost any industry – aviation, transportation, energy, civil engineering, machine-building and many more – you inevitably start with composites. It’s more of a category than a material because a composite can be any combination of two or more materials that results in different properties to those of its components. But in general, in most industries, ‘composite’ refers to a combination of polymers and reinforcing materials.
As a concept, this type of composite isn’t new. Fiber-reinforced composites — with glass fibers used to strengthen unsaturated polyester resins — were invented in the 1930s. Over the decades that followed, innovations like carbon fibers and the use of epoxy resins led to this technology being used in military and marine applications. But it was the 1970s when it made a genuinely transformative impact.
With rising oil prices, the high strength-to-weight properties of carbon fiber reinforced polymers (CFRPs) became extremely attractive to the aviation industry. Reducing the weight of aircraft becoming economically compelling, which pushed forward the development and commercialization of CFRPs.
Many advantageous properties
The high strength-to-weight ratio arguably remains the most outstanding quality of these composites, but they can have many more valuable properties. These vary according to which polymers are used, but as a rule, CFRPs have high thermal and electrical conductivity, corrosion resistance, tensile strength, and stiffness. Using different reinforcement materials alters these qualities dramatically. For example, if an aramid (a strong synthetic fiber) is used instead of carbon, then the resulting composite will be more flexible, durable and non-conductive.
This diversity of properties helps explain why composites continue to be used in so many industries and applications. Recent innovations have led to CFRPs being used as cables on cable-stayed bridges and, with their damping properties, for fast-moving components in industrial machines.
The primary barrier to using composites even more broadly has been production cost. Besides, using multiple materials and arranging reinforcement fibers in various matrices increases structural complexity and can make it more challenging to predict mechanical behavior and wear. Devising safe and robust joints has also been a challenge in many industries, leading to the development of advanced bolting technologies like Nord-Lock X-Series washers. They employ a spring mechanism to compensate for the slackening that can occur when bolting together two polymers.
Bio-based polymers and composites are promising
Most of the polymers used in industrial applications are still derived from fossil fuels, which raises sustainability issues. In recent years, interest in bio-based polymers, which use renewable resources as a feedstock, has grown rapidly.
Peter Mannberg, a unit manager at independent and state-owned RISE — Research Institutes of Sweden — works in research that tackles polymers and composites’ environmental impact.
“Our goal is to find sustainable solutions for lightweight applications,” he says. “The most-used composite materials have their origins in fossil oil, both carbon fibers and plastics. We want to replace them with renewable resources. That means using the feedstocks that we have — the available building blocks — to build new materials to replace the ones affecting the environment.”
Mannberg’s team have looked at forestry and agricultural residues for source materials, but one feedstock in particular seems to have captured his interest. “Reed canary grass grows on marshlands,” he says, “so it can be cultivated without using land that would otherwise be used for growing food. That’s important. We can use this grass in several different ways to create composites.”
The simplest is to use the stems and wood-like material as reinforcement fiber. The resulting composites, though, have relatively limited applications and are robust enough only for indoor use. A more ambitious method involves using the grass to create carbon fibers.
“For many years at RISE, we’ve been looking at using lignin to create a fiber, which we then carbonize,” Mannberg explains. “You can also do this with cellulose and hemicellulose — the other two basic components in biomass. The lignin from the grass is used to create fibers, which are then carbonized in quite a complicated process.”
The result is carbon fibers, which are the strongest fibers we have at the moment and that can be used for composites in high-level applications.
Replacing fossil-based materials
Of course, this only accounts for one of the components in a carbon fiber composite. Mannberg is optimistic, though, that reed canary grass can also be used for producing polymers.
Low-quality plastics created from bio-materials are already available on the market, in plastic bags, for instance,” Mannberg says. “We’re looking to find ways of creating bio-based plastics that can be used in automotive and aeronautic applications, replacing the epoxies and thermosets that are used there. It involves breaking lignin down to a molecular level and building it up to create something that is identical to the materials that are currently derived from oil.”
Although some companies are experimenting with using lignin to create carbon fibers, much of the work that Mannberg describes is still at a research stage.
“These are all things that we can do at a lab level,” he explains. “At the moment, it’s a more expensive process to derive the molecules and create the plastics and fibers than it is to make them from oil. So, it would require a combination of legislation and drive from consumers to get to the point where these products are used commercially.”
As an institute that focuses on applied research, RISE is also involved in projects to make it more feasible to work with materials that have for many years been assumed to be the future of engineering materials — nanocomposites.
Nanocomposite is another term that can encompass a wide range of materials. It can describe any composite material where nanoparticles enhance a component part. These are particles that have at least one dimension smaller than 100 nanometers (nm). Incorporating particles of this size can radically alter the physical properties of a material.
Guan Gong is a senior scientist at RISE whose work includes using nanomaterials to modify certain properties of composite materials to suit specific industrial requirements.
We are interested in using nanomaterials to enhance or modify different properties, according to what the end-users want,” she explains.
“For instance, customers could come to us and say, ‘I want improved electrical and thermal conductivity, or I just want much better thermal conductivity.’ Or, ‘I need the composite component to have good barrier properties against oxygen or many other things.’ Based on those requirements, we screen nanomaterials to find ones that have those outstanding qualities, then devise and verify a solution. Our general methodology is to first ask, what is required? What is the most critical quality the customer is looking for?”
A demanding and challenging process
Unsurprisingly, it’s not quite so simple as looking up a few tables. With the vast range of physical attributes, plus factors like cost, energy efficiency, and ease of production, finding the right combination of nanomaterials, composites and processes is always complicated. Gong explains that this isn’t the only barrier to nano-modified composites becoming commonplace:
“The main technical barrier is about dispersion. To convert the outstanding properties of nanomaterials into composite materials, you need to disperse the particles in the composite successfully,” Gong says. “You can use different techniques, but it’s still very difficult to get the dispersion status that you want, especially when fiber reinforcement is present. Industrial implementation of nano-modified composites is not yet robust.
“Most of the nanomaterials, like carbon nanotubes and graphene, are expensive. The way to get around this is to use very small amounts of nanomaterials, but because we can’t reach a good dispersion, you have to use more than is strictly necessary.”
Also, following strict safety rules are vital when creating or handling nanomaterial. Otherwise, there may be a threat to human health and the environment.
Nevertheless, Gong’s unit has successfully collaborated in this area with many private sector partners, including companies from the aeronautics, marine, automotive, forestry and energy industries.