Researchers create bio-based shellac coating for molded pulp materials

17 Feb 2023 --- Researchers from the School of Science, Mae Fah Luang University, Thailand and the School of Engineering and Materials Science, Queen Mary University of London, UK, have developed a shellac-based coating to improve the gas barrier properties of recyclable, compostable and sustainably sourced packaging materials. The coating helps liquid resistance while maintaining its environmentally sustainable properties.

Molded pulp materials such as eucalyptus wood or sugarcane made from renewables are widely used as sustainable packaging materials. Its production volume represents more than 30% of all paper-based packaging materials and is suitable for recycling and composting. 

However, the material has poor gas barrier properties and limited resistance to water and oil. This makes the material unsuitable for maintaining product shelf life and quality. The new coating is designed to be suitable for instant, dehydrated, frozen and chilled foods.

“Unlike non-recyclable and non-degradable laminated or metallized plastics, this coating makes molded pulps have good gas barriers and still be recyclable/compostable and is entirely based on renewable resources,” lead researcher Professor Nattakan Soykeabkaew, Mae Fah Luang University tells PackagingInsights. 

The researchers reported water vapor and oxygen transmission rates in the same range as those of conventional food packaging materials such as low-density PE, oriented PP and PE terephthalate. 

Researcher showcases the shellac-based coating made to decrease plastic pollution (Image credit: Nattakan Soykeabkaew). Biomaterial performance
To make these pulp materials higher functioning, companies apply a laminate or coating with petroleum-based polymers like PE and a thin layer of metals, usually aluminum, making recycling or composting more challenging. 

The researcher’s shellac coating is derived from a resin secreted by lac bugs. The largest producers of shellac are India, Thailand and China, but it is also produced in Bangladesh, Myanmar, Laos, Vietnam and Mexico.

Shellac is a biopolymer of the polyester group and is widely used in the medicine and food industries due to its nontoxic nature, thermoplastic behavior, oil resistance and good moisture barrier properties. However, a coating layer of pure shellac is not commonly used for industry applications due to its brittleness and has high oxygen permeability. 

“It can help with the plastic pollution, resource depletion and other consequential problems that we are facing,” says Soykeabkaew referring to how the coating can impact the environment. 

Putting it to the test
For the study, a molded pulp packaging was coated with a nanocomposite layer consisting of nanofibrillated cellulose (NFC) and shellac to improve its barrier and surface resistance performance.

To enhance the compatibility with the shellac phase and increase the water resistance of NFC, modified NFC was prepared via an esterification reaction. The researchers prepared uncoated samples and samples coated with pure shellac for comparison. 

“To have good barrier properties, the dispersion of nanocellulose inside the shellac-based coaBio-based coating created for pulp packagings makes them more grease resistant while maintaining recylability.ting layer that helps block transportation of gas molecules must be good. So, we chemically modified the nanocellulose slightly to assist them to have good dispersion in the coating layer,” continues Soykeabkaew. 

Water vapor and oxygen transmission rates were reported in the same range as those of conventional food packaging materials such as LDPE, oriented PP, 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 molded pulp sheet. 

Fighting cost 
The study was published in the SCI journal Polymer International. Its goal is to improve the barrier properties and surface resistance of the molded pulp while preserving its green profile and environmental sustainability by developing a new coating based on environmentally friendly, renewable and biodegradable materials.

Soykeabkaew says: “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.”

The lead researcher is referring to the current barrier to the commercialization of the shellac coating, its cost. Soykeabkaew estimates the price of the coating would be three-to-ten times more than the materials currently in use. 

“For large scale production, however, we still see the challenges about the materials cost and process upscaling and that is what we are now working on,” concludes Soykeabkaew.

By Sabine Waldeck