A rubber that has good flexibility and is easy to shape is a winner, says Dr John Eustace, a professor of rubber and plastics engineering at the University of Exeter, UK.
“It doesn’t matter how much plastic or resin it has,” he says.
“You need a rubber that can hold a load for a long time and is very rigid.”
The rubber of choice for many industrial applications is polyurethane (PU) or polyethylene terephthalate (PET), which is a high-density, flexible polyureTHA (PUVA) plastic.
But polyureths are also being used in many other industries, including in food packaging, for the production of medical devices and the building of cars.
The researchers behind the study found that the best natural rubber was one that had good flexibility, stability and elasticity.
It was also the least costly and least energy-intensive.
“A rubber that is very soft, but flexible, and has good strength and elastic properties is a good choice,” says Eustaces co-author and co-director of the University’s Center for Materials Science and Technology, Mark Durnell.
“The rubber is soft and flexible, so it has good rigidity.
It also has a good surface area to water and moisture and it has a very high elasticity.”
Eustacises team tested about 100 rubber varieties, including a variety made from the softest of all rubber types, called kapton rubber, with various properties, including their ability to hold water and maintain its shape.
They then found that there was a clear winner among natural rubber products, which had good elasticity and stability, but were relatively cheap.
“If you are looking at rubber in terms of costs, it’s very similar to plastics, but its elasticity is much higher,” says Dr Durnells co-lead author and assistant professor of materials science and engineering at Harvard University, Daniel Wahl.
“So it is cheaper than plastic.”
Polyurethanes were chosen for their flexibility, but not in isolation, so the researchers also tested their strength, stability, strength and water resistance.
“When it comes down to it, we think there is no single rubber that performs well,” says Durnles co-leader, Dr John Haggerty, who also works for the Rubber Institute.
“There are many different types of natural rubber available, and there is a wide range of properties between the different kinds.
So it’s not surprising that different types have different properties.”
The researchers then used these properties to predict the properties of different types in a series of experiments.
“We found that polyuretha is not as good as other polymers,” says Haggerts co-led author and senior researcher on the research, Professor Andrew Tisdale.
“But there is one rubber that outperforms polyureta in all three categories.
It is called polyurethanol rubber.”
The research has just been published in Nature Materials, an online journal of the Royal Society of Chemistry.
It can be used in everything from medical implants to artificial skin to cars.
“One of the problems with rubber is it has poor strength,” says Tisdales co-head of research at the Rubber Research Centre, John Smith.
“Polyurethans are much stronger than other types of polymers.
And they are very soft.”
But it is still possible to design polyurethalates to be both flexible and stiff enough to make them good for rubber applications, says Tislales co to lead author, Professor Sarah Condon.
The next step, she says, is to make polyureThanes that can both be flexible and strong.
“For example, we are currently developing a new polyureylene that is much more flexible, but also has good stability and strength,” she says.
To see how the rubber of the future might look like, the team is also looking at how the properties are changing with time, says Hagan.
“Some of the properties will improve over time, but others will be less good over time,” she explains.
“And if we want to make it more flexible over time than the properties that are good now, we need to find better materials to make that flexible and flexible.”
The team is currently looking at what materials might be able to make these materials more flexible and more resilient.
The research is funded by the British Chemical Society and the Natural Environment Research Council.
The team will publish their findings in the Journal of Applied Polymer Science in May.