Material selection

The final product needs to be lightweight, durable, stable, and relatively cheap to be affordable. Additionally, since the product will be used as a garden it has to be easy to clean and to be corrosion resistant. Another important aim is to be as sustainable as possible while also covering all requirements, this could be achieved either through recycling or using recycled materials. The material that best suits those requirements is plastic however plastics are not known for being the most sustainable materials that’s why we will aim to manufacture the product from recycled plastics and to increase the life span of the product as much as possible. We will aim to use as much as possible recycled plastic in the selected polymer without reducing the characteristics below the required.

The processes that use plastics are rotational moulding, injection blow moulding, injection moulding, extrusion blow-moulding and thermoforming. Most of those processes result in a hollow product with thin walls, our product will have both solid and hollow parts, it will have thicker walls and it will need to withstand a high load. The most suitable production option will be injection moulding however, the moulds could be expensive thus to be justified the product needs to be produced in high quantities.

The top most used plastics are Low-density Polyethylene (LDPE), High-density Polyethylene (HDPE), Polypropylene (PP), Polyvinyl chloride (PVC), Polystyrene (PS), and Polyethylene terephthalate (PET). However, we can not choose the material only based on only that. To make a more efficient choice for the material we are going to use GRANTA EduPack. The materials that we will look at are Polymers, they will be compared on their recyclability, Yield strength, and footprint for recycling. Out of all of the materials, only Polyester, Phenolics (PH), and Epoxies (EP) came up as not recyclable meaning they can be removed from the selection since they are not sustainable. Next, we will sort the material based on their Yield strength and recycling footprint. The material that came up with the lowest footprint and highest Yield strength is Polyhydroxyalkanoates (PHA, PHB) with a footprint of 0.291-0.321 kg/kg and a Yield strength of 35-40 MPa. To determine if the material can withstand the required weight multiple simulations and optimizations of the product need to be run, which is beyond the scope of our project in the current stage. For that reason, we will consider other polymers that have higher Yield strength, that could be used for future testing.

Some other polymers, which are stronger, that should be considered are Polylactide (PLA) and Polyoxymethylene (Acetal, POM), those two are the most efficient based on footprint and Yield strength. Polylactide (PLA) has Yield strength of 55-55 MPa and 0.9-0.995 kg/kg footprint, Polyoxymethylene (Acetal, POM) has 57.2-71.7 MPa Yield strength and 1.52-1.68 kg/kg footprint. However, those materials will also need to be tested with the product and it will be optimized based on their performance. Furthermore, some characteristics need to be check. First, it’s the Maximum service temperature, the three polymers can work perfectly at 40°C which should be more than enough, however, before exporting the product globally the maximum temperature that it is going to be exposed to needs further research (including an increase in temperature over the years) because Polylactide (PLA) has a Maximum service temperature of 44.9-54.9 °C the other two polymers have around 60°C and 80°C. Second, the durability in contact with water and soil needs to be looked, since the product will be in constant contact with them. All of the materials can work under constant contact with water on the other hand when it comes to soil, Polyhydroxyalkanoates (PHA, PHB) and Polylactide (PLA) can not be exposed constantly to soils. This depends on the type of soil that they will be exposed to depending on that the materials might still be suitable however, further research needs to be conducted to see if it will be possible, this does not fall into the scope of our project at this stage.

Reference

Yu, J., Sun, L., Ma, C., Qiao, Y., & Yao, H. (2016). Thermal degradation of PVC: A review. Waste Management, 48, 300–314. https://doi.org/10.1016/j.wasman.2015.11.041

A. Ibrahim, B., & M.Kadum, K. (2010). Influence of Polymer Blending on Mechanical and Thermal Properties. Modern Applied Science, 4(9). https://doi.org/10.5539/mas.v4n9p157

Shahab, H. (2021, June 11). Which Plastics Are Recyclable? 3devo. https://3devo.com/blog/recycling-plastics/