Abstract
Studies on composition optimisation showed that the mixing of nanoclays to whey protein-isolate (WPI)-based coating formulations offers an effective strategy to reduce the oxygen permeability of coated polymer films. The scaling up of the various processing stages of these formulations was undertaken to prove their industrial feasibility. The aim was to investigate the effect of various preparation methods at different production scales (pilot- and semi-industrial scale) on the barrier performance and morphological properties of the applied nanocomposites. A nano-enhanced composition was converted into a so-called “ready-to-use” formulation by means of a solid-state pre-dispersion process using ball-milling. The process yielded a nearly dust-free, free-flowing powder containing agglomerated particles, which can easily be mixed with water. The preparation of a coating formulation using the ready-to-use granules and its upscaling for roll-to-roll converting at pilot- and semi-industrial scale was also successfully implemented. The effects of both the production at various scales and ultrasound treatment on the morphology and barrier performance of the nanocomposites were characterized by transmission electron microscopy, scanning electron microscopy, as well as oxygen permeability measurements. Results have shown that the addition of nanoclays to WPI-based coating formulations ultimately led to significantly reduced oxygen permeabilities to 0.59 cm3, 100 µm·m−2·d−1·bar−1 (barrier improvement factor, BIF of 5.4) and 0.62 cm3, 100 µm·m−2·d−1·bar−1 (BIF of 5.1) in cases of pilot- and semi-industrial-processed coatings, respectively, compared to a reference without nanoclay. In both cases, a similar degree of nanoparticle orientation was achieved. It was concluded that the solid state pre-dispersion of the nanoplatelets during the production of the ready-to-use formulation is the predominant process determining the ultimate degree of nanoparticle orientation and dispersion state.
Highlights
The ability of protein-based films and coatings to act as a superior barrier against the permeation of oxygen in packaging materials has attracted a great deal of attention in recent years, as extensive research has demonstrated [1,2,3,4,5,6,7,8,9,10,11]
Transmission electron microscopy (TEM) was used to analyse the layer morphology of untreated coatings and coatings, which were treated with ultrasonication for 240 min (Figure 1)
For coatings that were treated by ultrasonication, it seems that fewer nanoparticle agglomerates were present in the dry coatings, and more individual nanoplatelets, instead of nanoplatelet-stacks, achieved an intercalated or exfoliated state (Figure 1b)
Summary
The ability of protein-based films and coatings to act as a superior barrier against the permeation of oxygen in packaging materials has attracted a great deal of attention in recent years, as extensive research has demonstrated [1,2,3,4,5,6,7,8,9,10,11]. While currently being currently routed to commercialisation, whey protein-based coatings offer several advantages compared to conventional petrochemically-sourced oxygen barrier materials, such as poly (ethylene–co–vinyl alcohol) (EVOH), as they are extracted from renewable resources and, despite usually being biodegradable, offer the opportunity to be recycled [14]. When applied on biodegradable polymeric substrates, e.g., poly (lactic acid) (PLA), the design and production of compostable packaging concepts is possible [16]. These aspects enable converters in the plastics packaging sector to cope with the EU action plan for a circular economy, which was adopted by the European commission in 2015 and supports the goal that all plastic packaging is recyclable by 2030 [17]
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