Shellac-based coating for sustainable packaging

Researchers at Mae Fah Luang University’s School of Science and Queen Mary University of London’s School of Engineering and Materials Science have developed a shellac-based coating that may improve the gas barrier properties of a recyclable, compostable and sustainably sourced packaging material to make it suitable for instant, dehydrated, frozen and chilled foods.

The most widely used sustainable packaging material, representing more than 30% of all paper-based packaging materials, is moulded pulp. Made from renewable materials such as eucalyptus wood or sugarcane bagasse, it is used to protect products in shipping and for food serving trays, containers and beverage carriers. Its feedstocks are renewable and it is suitable for recycling and composting.

The material, however, has a poor gas barrier and limited resistance to water and oil, making it unsuitable for maintaining the shelf life and quality of many products. Typically, this is resolved by laminating or coating the material with petroleum-based polymers such as polyethylene and a thin layer of metals, usually aluminium. However, this can make recycling or composting challenging.

The study, published in the SCI journal Polymer International, aimed to improve the barrier properties and service resistance of the moulded pulp while preserving its sustainability by developing a coating made from environmentally friendly, renewable and biodegradable materials.

Shellac has been produced for centuries for use in products ranging from nail varnishes to furniture lacquer and various dyes. It is derived from a resin secreted by lac bugs and its largest producers are India, Thailand and China, though it is also produced in Bangladesh, Myanmar, Laos, Vietnam and Mexico.

A biopolymer of the polyester group, shellac is widely used in the medicine and food industries due to its nontoxic nature, thermoplastic behaviour, oil resistance and good moisture barrier properties. It exhibits good adhesion finish and can be dissolved in low-toxicity solvents. Pure shellac is, however, not commonly used as a coating layer because of its high oxygen permeability and brittle nature.

In the research, a moulded pulp was coated with a nanocomposite layer consisting of nanofibrillated cellulose (NFC) and shellac to improve its barrier and surface resistance performance. The modified nanofibrillated cellulose (mNFC) was prepared via an esterification reaction to enhance the compatibility with the shellac phase and increase the water resistance of NFC. The researchers also prepared uncoated samples and samples coated with pure shellac for comparison.

The researchers examined the effects of the nanocomposite coating formulation as well as different thicknesses of the coating layer on the moulded pulp and on the morphology. They also examined barrier properties (water vapour transmission rate (WVTR), OTR), water and oil resistance, thermal stability and mechanical properties of the fabricated specimens.

They reported that water vapour and oxygen transmission rates were in the same range as those of conventional food packaging materials such as low-density polyethylene, oriented polypropylene and polyethylene terephthalate.

Testing of the water contact angle, oil contact angle and oil absorption rate also indicated that the nanocomposite coating layer provided superior water resistance and a promising greaseproof surface to the moulded pulp sheet. The coating layer also enhanced the tensile properties of the sheet samples, especially for the sample coated with shellac and mNFC. The samples achieved good thermal stability (ca 250°C) after introducing the shellac layer with lower thermal degradation temperature, confirming its practical uses for packaging applications.

Lead researcher Nattakan Soykeabkaew, School of Science, Mae Fah Luang University, said, “Our next aim is to develop a sustainable coating that is cheaper and scalable via materials selection and design as well as some process modifications.”

Soykeabkaew estimates that a barrier to the commercialisation of the product is its costs, which, at present, are three-to-ten times more than materials currently in use.

Image credit: Prof Nattakan Soykeabkaew