Polysand is a novel material made from waste plastic and sand. The production of cost effective polysand fenceposts facilitates environmental clear up and creates locally useful products.

Context

I focused my work in Gunjur, a village in The Gambia where WasteAid are working on a plastic recycling project. Here pollution has become a serious local issue Although the lack of waste management is a global problem.

My challenge was to find product applications for this material that are driven by local needs, are cost effective and take into account the properties of the material.

Fences and walls are ubiquitous throughout the world but hold a particular relevance for Sub-Saharan African countries where compound housing is incredibly popular especially in rural and low-income areas.

A case for Polysand

Polysand is novel material made from low density polyethylene (LDPE) and sand.

LDPE films are one of the most common and problematic waste streams. It is used widely for packaging and often cannot be recycled.

Typically, because of the cost of collecting and processing waste upcycled or recycled goods are artisanal and unscalable.

The addition of sand dramatically improves the material properties while dramatically lowering the cost of manufacturing compared to making pure plastic products.

Polysand is made from 75% sand which acts as aggregate bound together by the plastic. The material properties have been characterised by the researchers at Imperial College London (paper here) who liken the material to concrete.

Polysand is currently being used to make paving tiles in a WasteAid workshop in Gunjur in the Gambia.

Optimisation

Fenceposts are fairly low value, so to make a sustainable business case, the product had to be optimised for cost effectiveness and designed for low cost manufacturing techniques.

Different cross sectional geometries with the same baseline dimensions were compared in three categories: strength, mass and ease of manufacture. Strength was tested using FEA simulations of the fencepost under loading and the maximum stress was recorded. Ease of manufacture was approximated depending on the complexity of mould required to produce the shape. An overall score was calculated from these three metrics and the best cross section selected.

Although subjective, this method helped me select an appropriate design to then optimise numerically.

The following is the beam bending formulation that was used to numerically minimise the volume of the beam while satisfying that the bending stress is less than the strength of the material.

The MATLAB optimisation solution gave the dimensions for the fencepost design.

Prototype

I built a segment of fencepost with the correct cross sectional area as a proof of concept of the material capabilities and manufacturing techniques.

Working with a novel material meant that I had to learn the best way to manipulate it. I was also trying to emulate the low cost tools that would be available in the field.

After much experimentation, I built a robust three part mould that could be seperated easily to remove the end piece.

The material was heated in an oven and pressed into the mould.

Life Cycle Analysis (LCA)

The optimised solution was compared to current solutions using an open source LCA tool. A holistic analysis of materials, manufacturing, transport and end of life was considered.

Compared to concrete and pine wood solutions, Polysand had the lowest aggregrate score for Energy Consumption, Carbon Dioxide, and other Emvironmental Impact Factors.

Cost Analysis

An approximation of costs, using local values for wages and materials showed that the optimised deisgn was competitive with conventional materials.

Conclusion

Polysand as a material has a lot of potential as an alternative for fenceposts. However, the design needs to be tested in the field. I look forward to seeing the project evolve in The Gambia with WasteAid and beyond.