Kevlar is a heavy-duty polymer which thanks to its orderly crystalline structure has a resistance and toughness even greater than that of steel itself, so it has numerous uses including various critical applications, such as the manufacture of helmets, safety gloves and clothing, bullet-proof vests and puncture-resistant tyres. However, most heavy-duty polymers are sensitive to ultraviolet light and high temperatures, and given that their applications are for outdoor use, they need to be protected against sunlight. The most widespread strategy tends to be to coat them in resin or metal oxides, but this increases the weight of the polymer and diminishes its elasticity.

In order to seek another kind of solution for this problem, the chemist Itxasne Azpitarte-Irakulis resorted to hybridizing Kevlar with another material in a piece of research set up with the UPV/EHU’s LABQUIMAC laboratory. “In nature there are biomaterials with this structure, with inorganic fractions interspersed in organic matrices.” Following this path, she took Kevlar fibres as the organic matrix and hybridized them with zinc oxide. The hybridization produced "an intermediate phase between the two materials in which both appear blended, and what is more, we created a zinc oxide coating a few nanometres thick around the Kevlar fibre. We saw that the resulting hybrid Kevlar fibres displayed greater stability in the presence of ultraviolet light and temperature, and given that the coating is on a nanometric scale, the polymer does not acquire any additional weight,” explained the researcher.

To carry out the hybridization they also followed a somewhat unusual technique: “We had the chemical compounds we wanted to intersperse in the polymer in the gas phase so they were inserted into the pores naturally present in the polymer, where they created chemical bonds with the inside molecules; in other words, an infiltration took place. And around it, as we said, we formed a coating by means of various layers of zinc oxide,” said Dr Azpitarte.

Hybridization using two metal oxides in a second phase

In the second phase of the research the aim was to improve the results obtained in the previous one. “Although we improved the thermal properties and sensitivity to ultraviolet light, the polymer was found to be slightly debilitated because zinc oxide reacts with ultraviolet light,” said the researcher. So they continued to use zinc oxide for the infiltration but for the coating they resorted to another metal oxide: aluminium oxide. “This combination was the one that enabled us to fully overcome ultraviolet light sensitivity, to improve temperature sensitivity, and what is more, not to compromise mechanical properties,” she went on.

After these improvements had been achieved, they explored whether the hybrid Kevlar could have new functionalities. “We went for electrical conductivity and photocatalytic properties. Zinc oxide is an electrical conductor by nature, but we saw that the Kevlar fibres hybridized with this oxide displayed much better conductivity than zinc oxide on its own.  When studying them in greater detail we saw that it was the part where the zinc oxide is infiltrated into the Kevlar that provides the hybrid fibre with that little bit of extra conductivity,” said Azpitarte. At the same time, given that these fibres have photocatalytic properties, they have the capacity to decompose organic matter when exposed to visible light.

This will mean that Kevlar may have applications that it could not have until now. “Thanks to the photocatalytic properties, smart fabrics that wash themselves, for example, could be made as they would decompose the dirt when exposed to sunlight. Electrical conductivity could be used to develop flexible devices, or be incorporated into garments.” At the end of the day, uniting the useful properties of the two materials, such as the lightweight feature and elasticity of the polymer and the resistance and thermal and chemical stability of the metal oxide, “it will be possible to create endless applications for these hybrid materials,” concluded Azpitarte.

Additional information

The researcher Itxasne Azpitarte-Irakulis (Zornotza, Basque Country, 1991) defended her PhD thesis entitled ‘Vapor Phase Modification of Kevlar Fibres’ in the Department of Materials Physics at the UPV/EHU’s Faculty of Chemistry. She conducted her research work at the CIC nanoGUNE research centre, under the supervision of the head of the Namoaterials research group Mato Knez. She also had the collaboration of the LABQUIMAC laboratory of the UPV/EHU and the Irish Tyndall Institute. This institute conducted the theoretical part of the experimental work done by Azpitarte, which enabled her to understand the chemical interactions taking place in what she observed in her experiments.