Project
Plast-i-Com: Efficientreuse of contaminated polymers and polymer blends through compatibilisation andstabilisation (Plast-i-Com)
Project goals
One of the biggest challenges of the industry is to produce in a sustainable way. As a consequence, the reduction, re-use, and upgrade of waste is becoming increasingly important. Moreover, the raising prices of raw materials are pushing the industry to economize on raw materials and energy.
Some companies are already experienced in the re-use of intern polymer waste without a significant downgrading of mechanical properties. However, it becomes more difficult when companies have to recycle waste which might be contaminated by amounts of other polymers. In these cases, the recyclates often demonstrate poor mechanical properties, since most polymers are not compatible and therefore difficult to mix homogeneously and with a well-defined fine distribution in one another. In other cases, the presence of another polymer can lead to a severe degradation due to the instability of some materials at high temperatures or a catalytic effect of contaminants on further degradation. Detection of impurities, sorting and separation of different polymers makes the recycling process more expensive and rarely offers a 100% guarantee on the removal of all impurities.
Activitites and results
Because of the unsatisfactory quality of mixed recyclates, they are recycled into less valuable products (downgrading) or not recycled at all. In the latter case, waste is e.g. deposited into landfill or incinerated with or even without energy recovery.
A route to improve processability and to optimize the polymer quality is offered by the so-called “compatibilization”. Polymer blends can be compatibilized by creating an interphase interacting with both polymer phases. Mostly copolymeric compatibilizers or even reactive compatibilisers are proposed. Although these principles have already been known for decennia, compatibilisation is hardly applied on an industrial scale, and certainly not in the processing of recycled polymers.
Looking at plastic waste, many streams consist of polymers which are immiscible or so-called not-compatible. Within the project ‘plast-i-com’, it was evaluated whether specific additives, called compatibilizers, could be used to convert non-usable, ‘ready-to-incinerate’ streams into more valuable recyclate materials. This evaluation was approached from three different angles. First of all, a theoretical approach was applied, wherein chemical models were used to predict the compatibilizing efficiency of additives for the studied blends. To this end, solubility parameters were estimated for the polymers of interest, based on the Hoftyzer-Van Krevelen method. Secondly, experimental work was performed to evaluate the actual performance of the additives in different polymer blends. Processing techniques such as injection moulding and textile extrusion were used to produce the desired test samples, whereafter mechanical testing could be performed. Finally, in a third step, numerical simulations were performed of the polymer processing of (non-)compatibilized blends, either in injection moulding or extrusion, using flow and thermal behaviour properties as input data.
Based on the company’s interest, different market-relevant blends were selected for evaluation, including polyolefin (PO) blends, PET- and PA contaminated PO blends, PMMA-based blends, as well as other more niche blends.
For the polyolefin blends, it seemed that ethylene copolymers were most effective to increase the mechanical properties, whereas for the PET and PA-based blends reactive compatibilizers such as glycidyl methacrylate- (GMA) and maleic anhydride- (MAH) based compatibilizers proved to be very successful. For PE/PA blends for example, the PA contamination resulted in a 4 times lower impact strength compared to pure PE material. On the other hand, as also predicted by the chemical models, MAH-based compatibilizers could increase the impact strength with 200% compared to the non-compatibilized blends. Chemical modelling also showed that for the PP/PET blends the most suitable compatibilizer candidate is not available on the market yet. Nevertheless, existing grades are able to increase the mechanical properties, although to a lesser extent compared to the PE/PA blend. In general, it could be concluded that compatibilizers positively affect the elongation and impact properties of the blends, as well as the morphological distribution of the contaminant phase inside the polymer matrix. The material stiffness on the other hand was most of the times affected in a negative way. In an attempt to restore the stiffness losses, nucleating agent were evaluated, but they only seemed effective in case of virgin non-compatibilized blends.
It should also be emphasized that the material source (virgin, post-industrial vs. post-consumer grades) has a significant influence on the final compatibilization success. Next to the mechanical and morphological properties, compatibilizers could also contribute to the esthetical performance of the polymer blend.
Apart from evaluating the recycled blends on an end-product performance level, their stability on a processing level could also be considered. In the current project, a research strategy was developed for a company specific case to understand the acceptable variations in viscosity to still end up with the desired end-product quality after injection moulding.