Correct material and design are critical in plastic tanks construction

Plastic tanks are becoming integral to many industrial processes and an understanding of the technology involved in advanced thermoplastic types will help optimise their performance. But correct material selections and design understanding are critical to successful tank construction - and increasingly so is chemical resistant thermoplastic sheet increasingly supplants steel, stainless steel, fibreglass or similar materials in the chemical vessel, process equipment and tank building industry. Thermoplastics choice as critical The main advantages of properly specified thermoplastics like polyethylene (HDPE), polypropylene (PP) and PVDF are: excellent chemical resistance; predictable service or 20 years or more; lightweight and eary to construct (butt or extrusion weldable); impact resistance; low maintenance; and safe. However, because plastics often look the same to the untrained eye, many people new to plastics technology presume that they all function in the same way and offer similar performance over time. This is a fundamental mistake ?and one that can not only lead to design failure, but, in the worst instances, profound safety issues. What needs to be understood at the outset is that the physical and chemical property of the plastic product is greatly dependent upon the widely varying polymer used. For example, with polyethylene the properties are primarily determined by density, molecular weight and molecular weight distribution. Higher density (higher crystallinity) increases tensile strength, rigidity, hardness, solvent resistance and permeation to gases. With this comes a lower impact strength, transparency and stress crack resistance. Where as increasing molecular weight increases impact and tensile strength, elongation at break and stress crack resistance. The variants in HDPE are immense, broad molecular weight distribution grades are easier to process and this is reflected in market price, but the properties required for long term serviceability are not there. Organisations such as Basell have developed processes that have given rise to so called third generation materials, the bimodal molecular weight distribution polymers. Special process design results in a blend of short and long chained molecules optimising the opposed properties of toughness and rigidity in the one polymer. HDPE grades such as Polystone 300, a PE80 style of polyethylene are products of this bimodal technology. Further enhancements to the polymer in recent years have given rise to the PE100 grade (Polystone PG100) that is remarkable in its excellent combination of toughness and rigidity. This results in additional advantages of high creep resistance, high resistance to slow and rapid crack propagation, good weldability and resistance to aggressive media. The situation is exactly the reverse in the case of LDPE as utilised in roto-moulding. This has lower crystallinity, leading to a lower elastic modulus and hardness along with ultimate strength. To pass temperature and corrosion tests, stabilisation systems are also added. Why does it matter? Because knowing the physical characteristics of the plastic involved and having a means of testing, allows you to match the characteristics of the thermoplastic with the task they are being asked to perform, especially the chemical, its concentration and temperature with which they will come into contact (be it an industrial process water, black water (sewage), grey water (recycled water) or potable water; and then build the tanks to a specification ?an internationally reliable standard from which you can calculate the ideal materials to use, eliminating otherwise unforeseen safety issues and confidently predict lifespan. You can thus arrive at a known performance outcome. Why is this important? Because it is obvious that, while there is wide general awareness of the benefits of plastics for tank coAir Max 2018 Dunk SB