Thesis winner of the Aticelca – Reno De Medici scholarship, packaging and innovative coatings

by Ayoub Fakhri – Proceedings of the “Aticelca Congress 2024”

Master degree course in: Technology and Production of Paper and Cardboard – Dipartimento di ingegneria Civile ed Industriale Università di Pisa Scuola di Ingegneria

This works aims to exploit the main novelty regarding the application, properties, and recycling strategies for innovative coating on cellulose-based substrates, in particular in the field of packaging. The thesis work has been divided into two main parts and the study, correlated to the European Project Preserve, aimed to find ecosustainable solutions for removing proteins, when used as oxygen barrier layer for polymeric substrates and Cardboard

The first part of this document – about application, properties, and recycling strategies for innovative coating on cellulose-based substrates – will deal with the importance of the production and characterization of sustainable coatings for cellulosic substrate, while the second part will be focused on recycling strategies to separate coatings and substrates, also based on plastic. The selection of materials, including cardboard, and the specific types of coatings were made to choose the best biobased formulation accounting properties such as:

– OxygenTransmissionRate (OTR) forgas barrier evaluation

– Water Vapour Transmission Rate (Wvtr) for moisture assessment

– Contact Angle measurements for liquid contact

– Cobb method (specifically Cobb 60) for water absorption

– Coating analysis through optical microscope.

The second part of the document, instead, deals with a study to remove the proteins that coat mono and multilayer polymer materials and cardboard to promote the recycle ability of these materials, that is facilitated by materials separation.

This study, correlated to the European Project Preserve, aimed to find ecosustainable solutions for removing proteins, when used as oxygen barrier layer for polymeric substrates and Cardboard.

The use of eco-sustainable solutions for protein removal was chosen for two main reasons:

– concern for the environmental impact of removal processes

– interest in more economically and sustainably viable solutions in the long term.

The ultimate goal of the project is to industrialize the protein removal process, making it possible to recycle the protein-coated polymer material and cardboard.

Introduction

The production of items derived from sustainable and renewable resources, not toxic for humans and the environment, is a pressing challenge facing our society [1]. In this context, the production of sustainable coatings with improved and multifunctional performances is necessary [2]. As such, the search for coatings that have to be bio-based, with good barrier, water resistance and antimicrobial features is underway [3].

Nowadays, extensively used materials, with excellent moisture barrier properties for the production of coatings, are fundamentally petro-based. This must be the barrier to break down in research in the coming years [4].

Among all the possible typologies of coatings, this study will focus on sustainable active coatings, which are applied to packaging materials to provide additional functionalities that enhance the properties of packaged foods. Active coatings play a crucial role in improving the safety, quality, and shelf life of both the packaged food and its packaging.

By extending the shelf life of food products, active coatings help reduce waste and enhance overall sustainability [5].

When applied as coatings in the packaging industry, proteins aim to enhance several properties, including gas and water vapor barrier capabilities, as well as seal-ability [6].

In particular, Whey protein isolate (WPI) coatings have demonstrated their efficacy as barriers when applied to Ldpe and PP [7, 8] effectively enhancing oil resistance and reducing water vapor permeability.

Furthermore, these coatings exhibit excellent visual appearance and favorable mechanical properties on the treated substrates.

Methods

Coating application

To conduct the coating process, firstly the substrate is fixed on the sample slide using a pneumatic and a manual clamp. The chosen wired rod is placed into the coating unit and pneumatically pushed onto the substrate with a set force. The coating solution is evenly spread in front of the wired rod using a pipette. Subsequently, the slide moves into the coating unit at the set speed. The coating solution is distributed on the surface of the substrate by the grooves of the wired rod. Inside the coating unit the coating solution is dried via a convective dryer.

Evaluation of oxygen barrier

The standard used was: “F1927 Standard Test Method for Determination of Oxygen Gas Transmission Rate, Permeability and Permeance at Controlled Relative Humidity Through Barrier Materials Using a Coulometric Detector”.

Water Vapor Transmission Rate

10 measurements were made following the “Astm D1434 – Standard Test Method for Determining Gas Permeability Characteristics of Plastic Films and Sheets”. Two measurements, per day, one in the morning and one in the afternoon, after preconditioning the samples in the conditioning chamber at 23 °C and 50% r.h. were carried out.

Contact angle measurements

The contact angle measurements have been conducted following the “UNI EN 828:2013”.

Cobb Method

This method ISO 17025 accredited for the determination of water absorption of paper and paperboard has been used for the determination of water absorption of paper, paperboard, corrugated board weighing more than 50 g/m2 or embossed paper. The values obtained show how much water 1 m2 of paper absorbed.

Analysis with the Optical Microscope

The samples were analyzed under a polarized light microscope.

Monolayers and multilayers separation trials

The methodology used for testing the layers separation was conducted as follows: 2.5 mm x 2.5 mm pieces of material had put them into a beaker, placed on a heated plate with magnetic stirrer and temperature control trying several approaches, using water, salt, liquid soap, different enzymatic detergents, a buffer solution or sodium hydroxide as previously described.

The working conditions were tuned changing the stirring and temperature.

– Stirring = 150, 300 and 500 rpm

– Temperature = 30 °C, 40 °C, 50 °C and 60 °C.

Fourier Transform Infrared Spectroscopy (Ftir)

Fourier Transform Infrared Spectroscopy (Ftir) is a form of electromagnetic radiation, that falls within the spectrum between visible light and microwaves.

In the case of the present work, we used Ftir to verify the presence or absence of proteins on the treated samples, by comparing the analysis of our sample with the reference with and without the coating. This allows us to obtain the resulting information.

Result and discussion

OTR

Summarizing, at measurement conditions T = 23 °C, RH = 50% the obtained transmission rates for each coating material are summarized as follows:

MelOx: the measured transmission rate was 31.44 cc/(m2 · day), indicating moderate barrier properties.

VBCoat: the transmission rate was significantly higher at 20798.4 cc/(m2 · day), suggesting relatively poor barrier performance.

VBCoat + MelOx: the combined coating showed improved barrier properties compared to VBCoat alone, with a transmission rate of 395.46 cc/(m2 · day).

Lactips (11%, 15%, 18% and 20% concentrations): all Lactips formulations exhibited failed the test, indicating poor barrier properties.

Chitosan (three different concentrations): similar to Lactips, Chitosan coatings demonstrated extremely low transmission rates.

Wvtr

The results obtained from the analysis of water vapor transmission rate data, with four replicates conducted for each type of coating (figure 1).

Upon examining the data, it becomes apparent that the coatings produced by Melodea, represented by the yellow bars in the graph, exhibit the best hydroscopicity when based on cellulose, as all values are below 20 g/m2 per day.

In contrast, the data for Lactips biopolymers, which are protein-based, show relatively high water vapor transmission rates. This suggests that a higher solid content in the coatings serves as a more effective barrier against the passage of water vapor.

Regarding the chitosan formulations, it is clear that incorporating polyvinyl alcohol (PVA) and reducing the aqueous solvent content contribute to an improvement in water vapor resistance.

Contact Angle: measurements

The first parameter that has been evaluated for all the coatings is the surface energy (figure 2). Surface energy and its dispersive and polar component were determined via contact angle with three different fluids.

From the results, it can be observed that Melox Coating exhibits the highest surface energy, indicating the presence of greater interfacial energy and thus better coatingsubstrate adhesion.

On the other hand, the cellulose-based coating, VBCoat, shows the worst result with a complete absence of dispersed surface energy, implying no Van der Waals interactions.

The Lactips-based coatings demonstrate a higher homogeneity between the polar and dispersed phases, with the total surface energy increasing as the amount of biopolymer increases.

Lastly, Chitosan exhibits a higher activity in the dispersed phase and an almost complete absence of the polar part, indicating a lack of dipole-dipole bonding formation.

Contact Angle: evaluation

Water repellence was evaluated by measuring the contact angle of drops on coated paper samples according to EN ISO 19403-2:2017 using three liquids (figure 3) in which the contact angle data obtained with three different liquids were summarised, it is immediately apparent that VBcoat, which had given the worst results with regard to the oxygen barrier, instead gives the most positive results with regard to hydrophobicity with contact angles practically always above 90 °C.

This is followed closely by the chitosan-based coatings, although they have values below the hydrophobicity threshold (90°) and are strongly hydrophilic if the liquid used is diode-methane.

Rather low values for Melox and Lactips based coatings, showing once again the antithetical behaviour between oxygen barrier and obicity.

Cobb tests

The Cobb test reveals a particular result, namely that all coatings enhance the water barrier.

Furthermore, they confirm the contact angle result, meaning VBcoat is the most hydrophobic coating, while chitosan and lactips-based coatings, despite having a significant margin of error, do not have hydrophobicity as their main property (figure 4).

Optical analysis

One possible explanation for why VBCoat performed well as a barrier is that it had perfect adhesion to the surface, ensuring a consistent and uninterrupted coating, thereby minimizing defects that could compromise its barrier performance (figure 5).

Screening phase with WPI coated PET films

We have tried all the solvents with different operative parameters on the material: WPI coated PET film with the aim to select the best combination solvent/conditions and then transfer this info to the other coated samples.

To understand if the solvent worked well, we used Ftir to evaluate the presence of the absence of the WPI coated layer. We selected two solvents to test on the rest of the materials:

1. extrazim Plus at the optimum working conditions, i.e. the optimum concentration and the shortest possible time and at the temperature as close as possible to the ambient temperature : concentration of 0.25% and 0.5% with temperature T = 30 °C, and with times t = 0.5 h and t = 1 h (figure 6).

2. sodium hydroxide under the following conditions: 0.1 molar, temperature T = 30 °C and with times t = 0.5 h (figure 7).

In all the graphs present in the following paragraphs:

– the red curve is the substrate without coating

– the blue curve is the coated sample

– the green curve is the spectrum after the treatment.

Conclusion

After conducting a study in collaboration with different companies for the selection of materials for coating, with their application at various concentrations, our goal was to improve barrier properties for future use in food packaging. 

In terms of future applications, WPI appears to be a promising coating for OTR. Additionally, Melodea nanocellulose-based products have shown good results, particularly in Wvtr and water absorption. We envisage that the present study will serve as a solid starting point to address this innovative and sustainable topic, which also presents significant challenges to overcome.

The coating process was optimized in the laboratory to ensure homogeneity and minimize drying time. The PE, PET, and PLA films obtained were characterized by their oxygen and water vapor barrier properties. The results align with measurements obtained from melt films, specifically in terms of the applications of whey protein isolates. Additionally, efforts were made to remove the coating to enable recyclability of the material after use. The most effective solvents for coating removal were Extrazim Plus and sodium hydroxide (NaOH). These experiments were conducted at a laboratory scale with consideration for future industrial-scale implementation aimed at recycling food packaging materials. Once the coating is removed, plastic materials are intended to be recycled. The optimization process involved finding the optimal temperature, which was determined to be 30 °C to minimize energy consumption. Once the temperature was fixed, other variables such as solute concentration were optimized to reduce costs and environmental impact. Furthermore, the working time was optimized, and the maximum achieved was half an hour.

References:

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[6] Tyagi P, Salem KS, Hubbe MA, Pal L. Advances in barrier coatings and film technologies for achieving sustainable packaging of food products – A review. Trends Food Sci Technol 2021;115:461–85. https://doi.org/10.1016/j.tifs.2021.06.036.

[7] Hong S-I, Krochta JM. Whey protein isolate coating on LDPE film as a novel oxygen barrier in the composite structure. Packag Technol Sci 2004;17:13–21. https://doi.org/https://doi.org/10.1002/pts.634.

[8] Yousefi N, Abbasi S. Food proteins: Solubility & thermal stability improvement techniques. Food Chem Adv 2022;1:100090. https://doi.org/10.1016/j.focha.2022.100090.