The market for packaging and packaging solutions is expected to grow in the next four years due to factors like increased online shopping*. At the same time, the demand for sustainable packaging becomes more evident. In this article, we explore the compatibility of MFC with PLA and discuss what could be the benefits of such a mixture in various packaging products.
* Ref. article from interpac.com: Packing market continues to grow
Polylactic acid (PLA) is a biodegradable thermoplastic material produced from lactic acid, which is in turn commonly derived by fermentation of corn starch or sugarcane. PLA can be processed by conventional methods, such as molding and extrusion, which makes it a potential substitute for traditional oil-based polymers such as polypropylene (PP) and polyethylene (PE).
However, there are a few things which limit the use of PLA. Firstly, PLA is a brittle material with low impact strength. Secondly, PLA has a rather low softening temperature (55-60 °C) which limits its use in, for example, coffee cups or other products exposed to elevated temperatures. Despite these challenges, a variety of consumer products uses PLA, such as disposable plastic cups, plates, and food packaging.
The brittle nature of PLA limits its use in more demanding applications. That is why natural fillers, like hemp and kenaf, has been studied as strengthening aids. This is where MFC comes into the picture.
The properties of MFC, such as mechanical strength, high surface area, oxygen barrier and thermal stability, have inspired researchers all over the world to study the incorporation of MFC into PLA. The main challenge is that MFC usually comes in aqueous paste or suspension, which in turn creates problems for a homogenous dispersion of the hydrophobic polymer PLA. Different methods have been introduced to overcome the problems, and they all have their benefits and challenges.
Nakagaito et al., produced MFC reinforced PLA nanocomposites from aqueous suspension by a process which resembles a papermaking process. MFC was mixed with PLA fibers and, after dewatering and drying, compressed with a hot press (180 °C). They saw that the modulus doubled, and the strength tripled as the MFC content increased from 10 to 70 wt%. This type of processing method might be suitable for producing packaging materials of PLA and MFC.
Wang et al. recently used a similar kind of approach, but instead of PLA fibers, they used self-prepared PLA microparticles, and the composite was compression molded into the final form. To enhance the dispersion of PLA particles into water, they used polysorbate 80 as a surfactant. The compression molded nanocomposites showed increased modulus and strength, up to 58% and 210%, respectively. Compression molding fits well for producing objects, like car parts or cups.
SCA had a more radical idea for PLA/MFC composites: They wanted to make a foamed material for replacing expanded polystyrene (EPS) in, for example, as insulator. They demonstrated during the SustainComp project that PLA could be processed together with MFC into a homogeneous, foamed product. However, this processing method needs more development since the mechanical properties did not improve as expected.
To overcome the compatibility issues, hydrophobic surface modification of MFC either chemically, such as silylation or esterification, or by using emulsion aids, may be an option to help the incorporation of MFC into hydrophobic polymers. Naturally, when the stage of chemical modification increases, we might lose some of the sustainability benefits of the product.
MFC and PLA is a fascinating combination that, hopefully, will be explored further. When designing new formulations with MFC one should not just try to replace existing materials as one to one. Instead, the mindset should be on the compatibility and how to overcome the possible dispersion issues by different techniques or additives.