Agricultural waste from prickly pear cactus could provide a renewable source of reinforcement for biobased composite materials, according to researchers at the University of Bath. By extracting the naturally occurring honeycomb fibre network from discarded cactus pads, the team is developing lightweight composites that could offer a lower-carbon alternative to conventional fibre-reinforced materials for construction and product design.

By replacing synthetic reinforcing fibres with plant-derived alternatives sourced from agricultural waste, the approach aims to reduce both embodied carbon and waste while creating new applications for an abundant by-product.

Using Agricultural Waste Instead of Synthetic Fibres

Composite materials typically combine reinforcing fibres with a lightweight matrix to create materials with improved strength and stiffness. Conventional composites rely on synthetic fibres such as carbon fibre, fibreglass or Kevlar, which require energy-intensive production and remain difficult to recycle at the end of their service life.

Natural fibres such as flax and hemp have already been investigated as alternatives, but cultivating dedicated fibre crops requires agricultural land, irrigation, fertilisers and pesticides. The researchers instead focus on agricultural waste from Opuntia ficus-indica, commonly known as prickly pear cactus. The fast-growing species thrives in hot, dry climates and generates substantial quantities of discarded biomass through food production and routine pruning.

Inside each cactus pad is a woody honeycomb-like fibre network that provides structural support while allowing the plant to withstand bending forces. Rather than extracting individual fibres, the research aims to preserve this natural architecture and use it to reinforce bio-based composite materials.

Comparing Fibre Extraction Methods

To evaluate the most suitable production route, the team compared two extraction methods. The first was traditional water retting, in which cactus pads are soaked for several weeks until the soft tissue decomposes and the fibre network can be separated. The second method used alternating water pressures to remove the plant tissue, reducing processing time by approximately 90%.

Although the pressure-based method proved considerably faster, water retting produced cleaner and stronger fibre networks with fewer residual materials that could affect composite performance. The researchers also found that fibres taken from older cactus pads were stronger and easier to extract than those from younger plants.

Lightweight Composites for Low-Carbon Applications

When incorporated into plastic matrices, the cactus fibres increased both stiffness and strength compared with the plastic alone, particularly under bending and low-impact conditions. While the resulting composites do not match the performance of carbon fibre and are unsuitable for demanding structural or high-temperature applications, they compare favourably with other plant-based composite materials.

Potential applications include non-load-bearing wall panels, lightweight façade cladding, automotive interior components and product design applications such as surfboard cores. In addition to their mechanical properties, the composites retain the cactus’s distinctive honeycomb structure, creating a natural surface aesthetic that may appeal to designers.

The research team will now investigate how the fibres bond with a wider range of biobased resins and construction polymers while further evaluating their mechanical performance. The project forms part of a broader programme focused on scalable, fully bio-based composite materials for lower-carbon construction and manufacturing.

Source: University of Bath
Photo: Angeleses