What are the influential characteristics of polymers in rotational moulding?

During rotational moulding, the intrinsic characteristics of the polymer play a decisive role in the success of the process, the final quality of the part and the stability of the thermal cycle. Rotational moulding is based on the progressive heating of a polymer powder in a biaxially rotating mould, without significant external pressure. In this context, the material must have a very specific combination of thermal, rheological and physico-chemical properties in order to melt progressively, spread out on the walls of the mould, coalesce correctly and solidify without defects. It should be emphasised that the behaviour of the polymer during the process is highly dependent on its molar mass, melt viscosity, thermal stability and crystallisation kinetics. Unlike processes such as injection moulding, where pressure is used to force the flow, rotational moulding depends essentially on gravity, surface tension and the low shear created by the rotation of the mould. As a result, the polymer must naturally have sufficient fluidity within a relatively narrow thermal window. This constraint explains why polyethylene largely dominates the rotational moulding market, accounting for around 90 % of the materials used, thanks to its low melting point, good thermal stability and suitable viscosity.

Molar mass is one of the most influential parameters in polymer behaviour in rotational moulding. It has a direct effect on the viscosity of the molten polymer and therefore on its ability to spread evenly over the internal surface of the mould. There is a critical molar mass corresponding to the entanglement threshold of macromolecular chains. If the molar mass of the polymer is too low, the macromolecular chains are relatively short and not very entangled. In this case, when the polymer melts during rotational moulding, its viscosity becomes very low and the material flows too easily. This excessive fluidity may seem favourable for the process, but it has a major disadvantage: after solidification, the short chains cannot create a network of entanglements sufficient to ensure good mechanical cohesion. The result is a more fragile part with low toughness and poor capacity to absorb deformation or impact. Conversely, when the molar mass becomes too high, viscosity increases sharply according to a power law, making flow and coalescence difficult. In rotational moulding, since shear rates are very low, Newtonian viscosity dominates rheological behaviour. The polymer must therefore find a compromise between fluidity and mechanical strength. Generally speaking, as the molar mass increases, so does the minimum temperature required to obtain an acceptable viscosity, which progressively increases the risk of polymer degradation.

Thermal stability of the polymer is also a fundamental property. The rotational moulding process is relatively slow compared to other polymer transformation processes, which implies long residence times at high temperatures. The material must therefore resist thermal and oxidative degradation throughout the heating cycle. It should be pointed out that increasing temperature improves fluidity but simultaneously increases the risk of chemical degradation. This contradiction is one of the major difficulties of the process. When degradation occurs, it generally leads to cuts in macromolecular chains, a reduction in molar mass and gradual embrittlement of the final part. Oxidation reactions are particularly problematic in the case of polyolefins heated for long periods in the presence of air. To limit these phenomena stabilising systems based on phenolic and phosphite antioxidants are often incorporated into the formulations. In some very demanding industrial cases, rotational moulding can even be carried out in a neutral atmosphere to limit oxidation. Thermal stability therefore has a direct influence on the permissible cycle time, the maximum processing temperature and the durability of the parts produced.

In the case of semi-crystalline polymers, crystallinity and crystallisation kinetics have a major influence on the final structure and mechanical properties of rotomoulded parts. For this type of polymer, the processing temperature must exceed the melting point. However, during cooling, the rate of crystallisation determines the final morphology of the material, residual stresses, volume shrinkage and dimensional stability. Slow-crystallising polymers, such as certain grades of PET or polyamide, are particularly sensitive to cooling conditions. Poorly controlled crystallisation can lead to deformation, local variations in density or brittle areas. Crystallinity also affects the rigidity, chemical resistance and heat resistance of the final part. In the case of polyethylene, the relatively favourable crystallisation rate and low melting point make the process much easier.

La powder granulometry polymer is another essential characteristic that directly influences the quality of rotomoulded parts. Rotational moulding uses polymers in the form of fine powders, and the size of the particles determines the fusion, coalescence and densification mechanisms. In rotational moulding, the grains must be fine enough to ensure homogeneous fusion, limit air entrapment and promote inter-diffusion of macromolecular chains between neighbouring particles. A powder that is too coarse encourages the presence of internal bubbles, porosities and mechanical defects. Recommended particle sizes are generally between 50 and 500 µm. Mixing different particle sizes also improves the compactness of the material by allowing the smaller particles to occupy the free spaces between the larger ones. During heating, the particles gradually melt and then coalesce under the effect of surface tension and molecular diffusion. The quality of this coalescence directly determines the final density, structural homogeneity and mechanical properties of the part.

Finally, the overall thermal properties of the polymer, in particular its heat capacity, his thermal conductivity and its thermal diffusivity, These factors have a major influence on heat transfer during the rotational moulding cycle. The polymer is heated primarily by conduction through the mould and then through the successive layers of molten material. A polymer with a low thermal conductivity will heat up more slowly, which will lengthen the cycle and increase the internal thermal gradients. Thermal diffusivity also influences the time required to achieve homogeneous melting throughout the thickness of the part.

The choice of polymer must therefore simultaneously take into account thermal properties, rheology, chemical stability and transformation kinetics in order to guarantee a sufficiently wide processing window. It is precisely this complex balance between fluidity, stability and crystallisation that determines the rotational mouldability of a polymer and the final quality of the industrial parts obtained.