A Comparison of the Sealing, Forming and Moisture Vapour Transmission Properties of Polylactic Acid (PLA), Polyethene (PE) and Polyethylene Terephthalate (PET) Coated Boards for Packaging Applications

Due to the waste problems associated with plastic packaging and the desire to reduce fossil fuel-based packaging, many retailers and brand owners have set ambitious targets to reduce the use of non-renewable polymers. One aspect of this trend has been an increase in the use of bio-coated boards as an alternative to boards coated with traditional petroleum-based polymers such as PE and PET. This paper identifies the polymers used in a number of commercial boards coated with conventional or biopolymers and compares their performance in terms of the moisture vapour barrier, sealing behaviour, and their ability to be folded without cracking. It was found that PLA was the biopolymer used to coat the two bioboards studied, and while it compares favourably to PE and PET coated boards in terms of heat sealing capabilities, it has potential conversion issues due to it’s brittleness and has inferior moisture barrier properties compared to traditional petroleum-based coated boards.


Introduction
Plastic packaging has become increasingly prevalent due to its low cost, lightweight, high functionality, durability and processability. Most plastics are manufactured using fossil fuels [1] and the production of petroleum-based plastic has increased from 1.5 million tonnes in 1950 to ~ 400 million tonnes in 2018, and is expected to grow to 1800 million tonnes by 2050 [2]. The plastics industry currently consumes approximately 6% of the worlds yearly oil and gas, this too is expected to rise to 20% by 2050 [3].
Within the European Union, plastic packaging has a recycling rate of 41.9%, compared to 74.7% for glass, 84.6% for paper/cardboard, and 79.2% for metallic packaging [4]. A large proportion of plastic (approximately 45%) to be recycled is exported to countries with a lower gross domestic product and typically inadequate infrastructure [5]. Mismanaged waste can end up leaking into the natural environment, including oceans [6]. Plastic pollution at sea can cause damage to animals by reducing stomach capacity and causing internal injuries [7]. Plastics can also be broken down to microplastics (less than 5 mm length) and can be ingested by planktivorous fish and potentially make their way into human food, the consequences of which are still unknown [8][9][10].
Many brands and retailers aim to replace petrochemical based polymer packaging with alternatives which are either biodegradable or more easily recycled and have signed up to the UK Plastics Pact, aiming to eliminate problematic or unnecessary single use plastic packaging and ensure that 70% of plastic packaging is recyclable by 2025. Biobased 1 3 plastics in particular have gained increased attention. One form of bioplastic is commonly referred to as a "drop-in"; they have the same chemical structure and similar properties to conventional plastics but are derived from biomass (Bio-PE, Bio-PET etc.). Other types of bioplastics are derived from biomass, and unlike fossil-based plastics they are biodegradable. Biodegradability is the ability of a material to decompose under microbiological activity into naturally occurring substances such as carbon dioxide and water [11]. One such polymer is Polylactic acid (PLA) which is typically produced from corn [12]. Production of biobased plastics are expected to grow by 25% from 2018 to 2023 [13]. Although biodegradable, PLA discarded into the natural environment will degrade slowly as few PLA-degrading microorganisms exist below 30 °C [14]. Muniyasamy et al. identified PLA in soil at 23-25 °C was less than 5% biodegraded after 100 days [15]. Industrial composting facilities treat municipal and commercial biowaste. They operate between 50 and 60 °C for at least 21 days and for at least 7 days, the temperature must remain above 60 °C to eliminate pathogenic microorganisms [16]. Composting facilities are tightly controlled via standard PAS 100: Specification for composted materials [17]. Compostable packaging is governed by EN13432 [18] and carton board coated with a biodegradable bioplastic such as PLA can be certified to be either "Industrial Compostable or "Home Compostable" [19]. Bioplastic coated board typically has a coating of 10-25% weight, which is much less than the paper/board element and therefore composability is likely to proceed under the right conditions [20]. Although recycling of coated boards is preferred as per the waste hierarchy [21], the ability to compost coated boards gives brands the opportunity to have a responsible disposal method for their packaging, if it is contaminated with food/ liquid and cannot be recycled.
Carton board is a common fibre-based packaging material and is naturally biodegradable [22]. Carton board is lightweight, stiff and can be manufactured from sustainable renewable materials (wood from sustainably harvested forests). Due to the porosity of the paper fibre structure, carton board can absorb water which results in decreased performance characteristics. Plastics are widely used to coat carton board and enhance its properties [22,23]. Polymer coatings allow the board to be heat sealed, increase strength [22], reduce moisture/oxygen migration [23] and oil uptake. The global cartonboard market was approximately $110 billion in 2019 and with approximately 6% of this market being for coated products. The market for coated products is expected to increase 7.6% annually, between 2019 and 2024 [24]. Coated boards are used predominantly for chilled and frozen foods, juice bottles, washing powders, pet food and beverage cups. Typically, coatings have been conventional fossil-based plastics (mainly LDPE with a limited amount of PET) although there are an increasing range of products on offer with biopolymer coatings. Coatings can be either extruded or laminated onto boards [25]. This combination of biopolymer and boards allows for a piece of packaging to be made entirely from renewable resources [22].
To enable the recyclability of coated carton board, coatings ideally should be below 5% by weight of the board [26]. However, this depends upon the recycling process used and the end application. In order to achieve the required functionality, this aspirational target is currently tough to meet and the On Pack Recycling Label (OPRL) scheme in the United Kingdom, allows a coated pack to be labelled as "Recyclable" if the coating is below 15%, reducing to 10% by 2023 [27]. PLA is a contaminant of PET during conventional polymer recycling of products such as plastic bottles or food trays [28,29] however, during coated board recycling, both PLA and PET coatings are seen as equal contaminates to the paper/board recycling process and therefore the total covering should be kept to a minimum [26].
The addition of plastics to carton board increases the ability of the coated carton board to fulfil the "preserve" function of packaging [25]. Common plastics used in packaging have varying levels of resistance to water and oxygen transmission which helps ensure the packaged product remains fresh for the shelf life of the product. PLA has a CO 2 and O 2 transmission rates similar to PET [30] which is commonly used in the food packaging industry. PLA coated boards have the potential to replace a large amount of the petrochemical polymer coated boards currently used.
There is a lack of fundamental information on the chemical nature of the various coatings (many manufacturers simply state they are biopolymers) and on the coated board's ability to undergo traditional cartonboard conversion practices (folding/creasing) with the barrier intact. PLA is inherently a much more brittle polymer than conventional alternatives such as LDPE and PET and may lack the necessary ductility to allow the boards to be folded into cartons without failing. While there is some understanding of these issues within the industry, this knowledge is not widespread and there is a lack of published literature. Independent test data on the barrier properties of these boards compared to conventional alternatives is also lacking. In most applications the coated boards are either sealed to themselves or to a polymer film and the heat seal performance to commercially available films were also evaluated. To better understand the relative performance of biopolymer coated boards, 7 commercially available boards were tested, namely two biopolymer coated boards, 3 boards coated with conventional PE or PET and two uncoated boards. The coatings were analysed using thermal analysis (DSC), Fourier Transform Infrared Spectroscopy (FTIR), the moisture vapour transmission rate (MVTR) and sealing behaviour of the boards was also evaluated.

Materials
A range of commercially available board (both coated and uncoated) was sourced, see Table 1, and to ensure confidentiality is protected, the manufacturers of the boards are not disclosed. The sealing trails were conducted using a 45 µm PE/PP film with a PE sealing layer, a 28 µm PET sealing film and a 55 µm PLA sealing film (all supplied by Compac).

Characterisation
To identify the coatings and confirm manufacturers' claims, Differential scanning calorimetry (DSC) and FTIR experiments were performed. DSC was performed using a DSC Q100 (TA Instruments). Approximately 5 mg of sample was scraped off the board using a scalpel and heated from − 70 to 250 °C at a rate of 10 °C/min in nitrogen.
Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (FTIR) was performed. The infrared spectra were recorded with a BIORAD FTS 3000MX Excalibur series spectrometer with an attenuated total reflection (ATR) accessory using a single reflection Germanium crystal. A total of 1000 scans for each were recorded between 800 and 4000 cm −1 . All spectra were baseline corrected using Orig-inPro 7 software.
For Scanning Electron Microscopy (SEM), the samples were splutter coated with gold and imaged at 10 kV using a Joel JSM6010Plus.

Sealing Performance
A Sencorp 12AS/1 laboratory sealing machine with a one inch wide sealing strip was used to seal the sealing film to the coated board and a minimum of 5 samples were produced and tested for each set of conditions, For each material type, samples were sealed at 130, 140, 150 and 160 °C at dwell times of 1.0 and 1.5 s. This was conducted to determine the optimal sealing conditions for each material combination and to compare the peel strength of the various materials. Peel tests were carried out at 180° peel angle according to EN1895 (Adhesives for paper and board, packaging, and disposable sanitary products). Maximum peel load and average peel strength were recorded using a Instron 3344 tensile testing machine fitted with a 50 N load cell at a crosshead speed of 300 mm/min.
Water vapour transmission rate (WVTR) testing was completed by Versaperm Limited according to ISO 15106-3 Plastics, Film and sheeting, Determination of water vapour transmission rate method. The measurements were taken at 23 °C at 75% RH using a Versaperm MKIV machine fitted with an electrolytic water vapour sensor. Moisture vapour barrier properties are important in many food packaging applications.

Coating Identification
In some cases, the manufacturer lists the coating used in their specification sheet e.g. PET, however, for the biobased coatings, the type of biopolymer used was not given.
To positively identify the coatings on the various boards DSC and FTIR analysis was performed and the results are shown in Figs. 1, 2, 3, 4, 5, 6.
Referring to Fig. 1, which illustrates DSC traces from the two varieties of PET coated board, it can be seen that the samples undergo melting at approximately 240-250 °C, have a glass transition temperature of approximately 71-75 °C and have a crystallization exotherm at 125-130 °C, all of which are characteristic of PET [31,32]. \* MERGEFORMAT Referring to Fig. 1, which illustrates DSC traces from the two varieties of PET coated  [31,32]. Figure 2 shows the FTIR spectra of the 350/40 Board/ PET and 260/40 Board/PET boards, which display several characteristic peaks associated with PET (see \* MERGEFORMAT Table 2). It can be concluded from the DSC and FTIR data for the 350/40 Board/PET and 260/40 Board/PET, that the samples are coated with PET as per the manufacturer's specification (Table 3).
Both 350/35 Board/Bio and 275/25 Board/Bio are coated with an unknown biopolymer. Referring to Fig. 3 above, the 350/35 Board/Bio has a glass transition at approximately 63 °C which is typical of PLA [36,37]. Two melting endotherms are visible for 350/35 Board/Bio at 150 °C and 170 °C and similar double endotherms were previously observed in PLA by Gracia-Fernández et al. [38]. Two separate theories have been used before to explain double melting endotherms: the melting recrystallisation model [38,39] and the lamellar thickness model [38,40]. For the recrystallisation model it is assumed the first endotherm is formed during initial lamellar melting which then recrystallises into a more crystalline lamellar and subsequently undergoes a secondary melting. The lamellar model proposes that there are two distinct lamellar layers of differing crystallite types which melt at differing temperatures. 275/25 Board/Bio does not have a pronounced glass transition endotherm. However, a slight dip can be seen at approximately 55-60 °C. A clear melting endotherm can be seen at approximately 168 °C which is consistent with results for PLA from literature [37].
Overall, from the DSC and FTIR analyses of boards 350/35 Board/Bio and 275/25 Board/Bio, it can be concluded that the manufacture specified "biopolymer" coating is PLA. Information on the type of polymer used in the coating was not provided by the manufacturers.
Referring to Fig. 5  not have the same pronounced peaks associated with LDPE, perhaps due to an issue with obtaining good quality spectra with the thin coating, however, small peaks can be seen at both 2845 cm −1 and 2910 cm −1 which are attributed to C-H stretching. The remaining peaks observed for 350/15 Board/ PE at 3300 cm −1 , 1305 cm −1 , 1157 cm −1 , 1051 cm −1 and 1027 cm −1 are typical peaks found in the spectra of cellulose [45][46][47]. It therefore appears that much of the spectra was from the underlying board rather than the LDPE coating. Overall, from the DSC and FTIR analyses of 350/15 Board/ PE, it can be concluded that the polymer coating was LDPE. Table 4 shows the maximum peel load and average peel strength for the PE and PLA coated boards.

Heat Sealing
Referring to Table 4, both Bio coated boards showed a significantly higher seal strength than the PE coated board. The PLA film failed before the bond for the 275/25 Board/ Bio board sealed at 150 °C and 160 °C, this is likely due to a weld seal, which occur when the seal is performed at temperatures above the fusion temperature [50], causing the two polymers to mix and fuse. Similar behaviour was observed while sealing PET film to the two PET coated boards, 350/40 Board/PET and 260/40 Board/PET, at 210 °C to 240°, where film failures were observed before seal failures. The data for the PET boards were therefore not included, as it only relates to the strength of the film. One must determine whether it is desirable to produce a welded permanent seal or one which is "peelable", which in-turn aids recycling by allowing a sealed film to be removed by a consumer.
For both the Bio and PE coated boards, dwell time did not significantly increase either seal strength which confirms the findings of Meka and Stehling [51], that dwell time has less influence than temperature on the peel performance. As the dwell times chosen were kept to 1.0 s and 1.5 s to mimic a manufacturing environment where dwell time is kept to a minimum, further studies at higher dwell times would be required to fully investigate this.  Carboxylic acid group C = O stretching [33,34]  C=O stretching [41,42] To fully assess the optimum sealing range for a new packaging film/board, a trial over a wider range of temperatures/dwell times would be required along with varying the applied pressure. This type of trial can be time consuming and companies often do not have sufficient time to find the optimum settings and therefore use trial and error to obtain a set of conditions that work for their product. To reduce testing time, a design of experiment approach could be taken as shown by Dixon et al. [52]. Figure 7 display SEM micrographs of the various boards bent through 180° which replicates some of the board movements that occur during cartonboard converting practices. For critical applications barrier boards require the ability to undergo converting without damaging the barrier properties of the boards. Figure 7 shows that no obvious cracks were present when the PE and PET coated boards were folded through 180°. However, cracking did occur with the Bioboards, 350/35 Board/ Bio and 275/25 Board/Bio. The cracks were less visible for the 350/35 Board/Bio as compared to the 275/25 Board/Bio, but are faintly visible at a magnification of × 250. Higher magnification images of the PE and PET coated boards did not show evidence of any cracking. Such cracking during the converting of PLA coated boards may result in the board barrier being compromised. Depending on where cracks/pinholes form, the board may uptake moisture if the board was being used to pack a moist/greasy product, this would therefore potentially result in a saturated board which would be unsightly to the consumer and could result in wastage of the product packaged.   [53]. The results signify that while bioboards may be suitable for some products, they offer a less effective barrier against moisture than conventionally coated boards. It is also worth noting that PLA is compostable and degrades into harmless products over a period of several months when in contact with water. This will render PLA coated boards unsuitable for packaging moist or wet foods.

Conclusion
There is a major move away from petrochemical based polymers in food packaging applications such as coated fibre boards and the use of biopolymers including PLA are an attractive alternative to conventional polymers such as PE and PET. This paper has shown that PLA coated boards seal well to PLA films. The inferior barrier performance of PLA coatings compared with PE or PET, combined with the incompatibility of PLA with wet foods, limits the applications of PLA boards. However, PLA coatings may be suitable for applications such as beverage cups which are only subjected to short term exposure to liquids or for use with "dry" foods such as breaded chicken or fruit.
There are also question marks over the ability of PLA coated boards to be folded in cartons without cracking.