In the first part of the series on the processing of biobased plastics we gave a general introduction into the current market and the common perceptions in the plastics industry. To further our understanding of the processing of biobased plastics, we need to work with some general considerations that are applicable to plastic processing and the methods that are commonly applied.
One of the most important parameters that needs to be considered is the melting point (TM). At the melting point, chain mobility is observed, which reduces the mechanical properties significantly. Handling and shaping of the polymer is possible due to the fluid morphology of the material. The second important parameter is the processing temperature (TP). This temperature defines the temperature range that is necessary for the processing of the polymer and should be higher than the TM. Increasing the TP results in a reduction of viscosity and thus a lower flow resistance during processing1.
Extrusion is often the basis for most moulding processes and is vital to induce the necessary melt flow or for the formation of an intermediate product in other processes. During processing with extrusion it’s important to take into account the residence time of the melt when it flows through the extruder, as it’s also a determining factor for temperature degradation. Longer residence times2 basically imply a longer subjection to high temperatures, which increases the risk of degradation. The plasticator and die design and operation also affect degradation through for example, shear heating2. Shear heating in this case is the increase in surface temperature of the melt due to a process that is very similar to friction. However, this mechanism is instead based on internal (shear) stress profiles.
- Extrusion: a thermoplastic is changed into a melt by applying compressive and shear stress and heat using a plasticator and a heated barrel casing. This plasticator usually comes in the form of one or two screws that can apply pressure differentials by rotating. The melted thermoplastic is mixed and forced through the extruder by the pressure differential where it flows continuously through a shaping die at the output. This shaping die determines the form of the article, which is finalized after cooling3.
- Injection moulding: thermoplastic or thermosetting polymers are heated until a viscous melt is formed, by pushing the molten feedstock through an extruder. This viscous melt is shaped into the product article by forcing it into the form of a closed mould. The shape of this closed mould then determines the shape of the article. The end product is extracted from the mould after the melt has cooled and reverted to a solid4.
- Blow moulding: blow moulding is an extension of the previous two processes, but with the ability to produce hollow objects. The process first involves the formation of what is called a parison, which is essentially a hollow thermoplastic tube that is made malleable by heating it above the glass transition temperature. The hollow tube is usually produced by extrusion, but injection moulding is also possible. This parison is then subjected to air pressure from the inside and thereby expands into the form of mould that contains the object. The form of the mould then determines the shape of the article5.
- Compression moulding: thermosets, such as rubber, or thermoplastics can shaped into an article. The plastic is placed into a heated mould and then subjected to pressure by a hydraulic press. The plastic will take the form of mould and, if necessary, cure into a finalized state. It is also applicable for the creation of composites due to the minimal damage to the reinforcing fibres. Cold forming is also possible, but it prevents the possibility of curing the thermosets6.
- Transfer moulding: Transfer moulding is an improved form of the compression moulding that is not applicable for the fabrication of composites. A thermoset is added to a heated mould and pressurized, similar to compression moulding. This time the mould contains several small orifices from which the thermoset flows to multiple cavities. The advantage here is a more homogenous distribution with regard to temperature, which increases the speed of the curing reaction and the homogeneity of the end-product. Additionally, it allows for more complex shapes to be gives to articles7.
- Thermoforming: a malleable thermoplastic sheet is formed by extrusion and not yet cooled to room temperature. In this malleable state it is possible to shape the sheet into a desired form by applying pressure in a mould. This pressure can be achieved by utilizing a press, a vacuum process, mechanical bending and a variety of methods8.
- Additive Manufacturing (3D printing): is a process where design and fabrication occur in a parallel fashion. A three-dimensional design is translated from a computer model to a manufactured product by depositing a thermoplastic melt or thermoset in a layered process. Every layer is subsequently added to the previous layer and occupation is varied according to the model. Different materials can be integrated into the same article creating a heterogeneous product. In this way it’s possible to achieve a flexibility in material and form according to customer demand that is simply not possible with alternative processing methods9.
With this general overview of processing methods and dependent variables we can take a deep dive into specific plastic processing in the following parts of the series on the processing of biobased plastics.
Article by Wybren Kalsbeek
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- Ebnessajad, S. Melt Processable Fluoroplastics: 7 – Injection Moulding. (William Andrew Publishing, 2003).
- Belcher, S. L. Blow Moulding. in Applied Plastics Engineering Handbook (ed. Kutz, M.) 265–289 (William Andrew Publishing, 2017). doi:https://doi.org/10.1016/B978-0-323-39040-8.00013-4.
- A.B. Nair, R. J. Eco-friendly bio-composites using natural rubber (NR) matrices and natural fiber reinforcements. in Chemistry, Manufacture and Applications of Natural Rubber (ed. Shinzo Kohjiya, Y. I.) 249–283 (Woodhead Publishing, 2014). doi:https://doi.org/10.1533/9780857096913.2.249.
- Roy J. Crawford, P. J. M. NoProcessing of plastics. in Plastics Engineering (ed. Roy J. Crawford, P. J. M.) 279–409 (Butterworth-Heinemann, 2020). doi:https://doi.org/10.1016/B978-0-08-100709-9.00004-2.
- Throne, J. Thermoforming. in Applied Plastics Engineering Handbook (ed. Kutz, M.) 345–375 (William Andrew Publishing, 2017). doi:https://doi.org/10.1016/B978-0-323-39040-8.00016-X.
- Yang, J. et al. Mutimaterial 3D Printing technology. (Academic Press, 2021). doi:https://doi.org/10.1016/B978-0-08-102991-6.00014-8.