Suitability of poly(butylene succinate) as a coating for paperboard convenience food packaging

ABSTRACT

Take out and convenience food packaging has seen rapid growth in recent years, particularly in the last few months due to restrictions of in-restaurant dining. Paper-based packaging is often thought to be a more sustainable option to plastics. However, paper-based food packaging materials generally require supplemental treatments, such as applications of perfluorinated chemicals and petroleum-based plastics to improve performance. These are effective but limit sustainable end-of-life options (e.g., recycling and composting). Landfill diversion strategies are needed to divert food packaging waste as consumer demand for ready-to-eat (RTE) convenience meal kits and single-use packaging continues to increase municipal solid waste accumulation. This investigation evaluated the suitability of poly(butylene succinate; PBS) as a direct-melt coating on recycled paperboard for grease resistance in microwave rapid reheat and heat and hold convenience food packaging applications. This research demonstrates PBS as a viable alternative to perfluorinated chemicals and petroleum polymers with a more sustainable end-of-life option due to its oil migration resistance at elevated temperatures and compostability properties.

KEYWORDS:

• Sustainable
• compostable
• microwave
• poly(butylene succinate)
• packaging

Introduction

As global demand for packaged food expands, the production of non-degradable and non-renewable sourced materials will escalate the strain on Earth’s environmental resources. Packaging comprises the largest market of plastics and is one of the major contributors to solid waste in landfills and marine environments in the United States [1]. In 2017, 80 million tons (29.9% of the total MSW) consisted of containers and packages [2]. Traditional petroleum-based convenience food packaging structures have very a short service lifetime and food contamination reduces recyclability which increases landfill accumulation potential. Therefore, utilizing sustainably sourced, degradable packaging, and diverting plastic waste from landfills and marine environments is crucial.

With the anticipated and continuous growth of the paper packaging sector at 4.82% CAGR ($321 billion by 2024) [3], the ability to compost post-consumer waste instead of landfilling is expected to increase environmental health by reducing accumulation potential. However, paper-based products generally do not have sufficient physical properties to function appropriately alone in all food packaging applications as listed in the Code of Federal Regulations (21 CFR 176.170) [4]. This is particularly true for high-temperature applications which generally requires supplemental treatments for performance such as toxic perfluorinated chemicals [5] and petroleum -based polymers [6]. While effective, these treatments limit recycling and composting potential leading to increased landfill accumulation.

Since the 1940s, perfluoroalkyl substances (PFAS) have been used in the food industry as barriers to moisture and oil penetration in paper packaging. These PFAS have been under intense scrutiny because of their connection to high toxicity which may induce cancer and other health concerns [7,8] which has led to regulations and bans in food packaging driving the increased demand for bio-based alternatives in paper-based packaging. These alternatives would be recyclable or compostable reducing waste in landfills thereby avoiding increased PFAS accumulation from treated paper products. Recently, both the European Union and the United States (H. R. 2827 and H.R. 5845) have proposed or passed PFAS and single-use packaging bans [9–14]. As a result, there is an urgent need for high performance bio-based alternatives to PFAS and petroleum-based feedstocks.

Potential alternatives for paper coatings are bio-based and degradable polymers such as poly(lactic acid) and poly(butylene succinate). For example, Rhim et al. coated paper with poly(lactide) coatings via the solvent cast method [15]. PLA was dissolved in chloroform at different concentrations and applied to paperboard via the drawdown method followed by water contact angle, water vapor permeability (WVP) and mechanical properties analysis. It was determined that water contact angle increased ~10 degrees indicating increased hydrophobicity but plateaued at 3 w/v % up to 5 w/v %. The WVP decreased dramatically from 4.08 to 0.16 (×10⁻⁹ g·m/m²·s·Pa) and plateaued again from 3–5 w/v %. The increase in contact angle and reduced water vapor permeability shows that coated paperboard, at 3 w/v %, is the most effective for water protection. Critically, these solutions were prepared using chloroform which presents significant problems in commercial production due to use of toxic solvents. Furthermore, the polymer morphology is anticipated to change when scaling from solvent casting to the more commercially relevant direct-melt method which may alter physical properties.

PBS is another degradable, biobased aliphatic polyester [16] with limited information for use as a paperboard coating for elevated temperature food packaging applications. Poly(butylene succinate) is produced through a poly condensation reaction of succinic acid and 1,4-butanediol (Figure 1). PBS biodegradation studies have determined gradual weight loss over a four-month period while buried in soil [16]. The degradation rate was determined to be nearly constant throughout the degradation study with an average of 13 mg/cm² of weight loss per month. Furthermore, PBS grades have been listed as certified compostable according to the Biodegradable Products Institute and are available in direct food contact grades [17]. The biodegradability of PBS is an attractive attribute for single-use food packaging since it is able to degrade at high rates over short periods of time [16]. This would decrease landfill accumulation through industrial and home compostable end-of-life options.

Thermal and mechanical properties of PBS and PBS-corn starch blends have been previously reported [18]. Lai et al. determined that PBS melts at around 114°C with a glass transition temperature around −32.5°C. This work blended corn starch with PBS between 4 and 18.5 wt% and determined that these thermal properties did not practically vary (Tm: 113°C, Tg −35°C) suggesting that cellulosic fillers can be used to reduce cost without detrimentally altering important properties for convenience food packaging [18].

However, both the tensile and tear strength decreased when more starch was added indicating that property-cost optimization is required depending on the application. Thermogravimetric analysis identified increased mass loss at elevated temperatures which was attributed to excess water trapped in the starch blend. Since PBS is a polyester, this could present an issue at elevated temperatures due to hydrolysis reactions leading to additional performance degradation.

Figure 1. Molecular structure of poly(butylene succinate; PBS)

Our research reports the ability of poly(butylene succinate) to be used as a direct-melt coating for paperboard in two different ready-to-eat convenience food packaging scenarios: microwave rapid reheat and heat and hold (hot case convenience). Our goal was to evaluate the material’s performance capabilities in simulated retail environments. We assume that all federal safety considerations, such as compliance with the Code of Federal Regulations, are met for the specific conditions of use. These packaging scenarios correspond to how PFAS treatments and polymer coatings have been used to resist moisture and grease in convenience food packaging applications. The microwave rapid reheat procedure represents heating up food from room temperature in a commercial microwave. The heat and hold procedure simulates food being heated/cooked, placed in a package, then left at a warm temperature (e.g., under a heat lamp or in a hot display case) for an extended period of time. Both of these scenarios are often used in convenience ready-to-eat food packaging applications. The results presented herein will enable packaging manufactures and commercial retailers to source biobased food packaging with more sustainable end-of-life options without compromising performance. Replacement of traditional paper treatments with a compostable, degradable polymer for paper-based convenience food packaging will enable more sustainable structures which, when collected and composted, will reduce landfill accumulation and global contamination.

Methods and materials

Sample preparation

Poly(butylene succinate) (PBS; BioPBS FZ79AC, Mitsubishi Chemical, MI, USA) was melted directly onto kraft paperboard at 190.5°C at a speed of 380 cm/min (Figure S1) using a Cheminstruments laboratory melt coater model HLCL-1000 (OH, USA). Three unique coating trials using the same parameters were performed to determine the reproducibility of the coating technique. According to the manufacturer’s specifications, the BioPBS possesses a melt flow rate of 22 g/10 min at 190°C, a flexural modulus of 630 MPa, flexural strength of 40 MPa, and a yield stress of 40 MPa [19]. The coated paperboard was cut into 50 mm x 50 mm squares then attached to glass slides with double-sided tape. The edges were sealed with single-sided tape, and finally sealed with a 30-minute delayed setting two-part epoxy to prevent oil from saturating the paperboard via the edges. The utilized samples possessed a PBS coating thickness between 3.6 µm – 5.1 µm (1.4–2.0 mil).

Poly(butylene succinate) thermal characterization

Thermal transitions of poly(butylene succinate) were measured between −70°C and 200°C utilizing a modulated heating protocol (modulating ± 1.00°C every 60 s) at a rate of 3°C/minute with a TA Instruments Q2000 differential scanning calorimeter (New Castle, DE) in a nitrogen atmosphere. Specimens (3–6 mg) were singularly loaded into a hermetically sealed T-zero DSC pan and crimped prior to analysis. Total volatile content and thermal degradation properties were quantified via thermogravimetric analysis using a TA Instruments Q5000IR thermogravimetric analyzer (New Castle, DE) [20]. Samples (5–10 mg) were loaded into a platinum pan then heated at 10°C/min from room temperature to 700°C under a nitrogen atmosphere.

Surface morphology investigations

Scanning laser confocal microscopy was used to determine morphological changes in the poly(butylene succinate) direct-melt coatings before and after convenience food packaging simulations at 20 x magnification using a Keyence (IL, USA) VK-X1000 confocal laser scanning microscope equipped with a 661 nm laser.

Contact angle measurements

A Ramé-Hart 250 automatic goniometer (NJ, USA) was used to measure the contact angle of deionized water and avocado oil; these liquids were chosen to provide an understanding of water and oil resistance performance, respectively. The software triggered a video recording then a 20 µL drop of deionized water or avocado oil was manually delivered via 22-gauge or 30-gauge needle, respectively. The contact angle was measured using DROPImage Advanced software in accordance with ASTM D7334-08(2013) [21].

Convenience food packaging simulations

In the microwave rapid reheat procedure, a fiber-optic probe attached to Optocon FOTEMP1-4, Fiber optic temperature monitoring system was taped atop each sample along with an ambient-temperature avocado oil-soaked sponge (see Figure S2). The sample was placed in a Sharp (1000 W/R-21LT; Thailand) 1000 W commercial microwave oven and heated from ambient temperature to 100°C, after which the oven was turned off. The sample was maintained in the unopened microwave for 2 min then removed and cooled to room temperature, after which the surface morphology was investigated via scanning laser confocal microscopy. A typical heat profile can be found in Figure 2.

Figure 2. Representative temperature profile at the food simulant/packaging surface interface in the microwave rapid reheat procedure

In the heat and hold procedure, the sample and optical temperature probe were arranged in the same manner as the microwave rapid reheat procedure. An avocado oil-soaked sponge was placed in an oven and heated to 94°C (201°F). Each sample was placed in a Fisherbrand Isotemp oven at 60°C (140°F) and the heated sponge placed on top. These temperatures were selected as they were above the Food and Drug Administration Food Code (Chapter 3) for reheat temperature and equivalent to the minimum storage temperature for serving hot food [22,23], similar to what would be expected in convenience food bars. The samples were heated for a total of 4 hours, after which the oven was turned off and the samples slow cooled to room temperature in the oven. After cooling to room temperature, the sample surface morphology was investigated via scanning laser confocal microscopy. A typical heat profile can be found in Figure 3.

Figure 3. Representative temperature profile at the food simulant/packaging surface interface of the heat and hold procedure

Results and discussion

The thermal properties of poly(butylene succinate) were determined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). These were used to evaluate potential upper limits of temperature abuse for PBS coated paperboard convenience food packaging. The DSC data in Figure 4 show that PBS cold crystallizes at 93.5°C, with a standard deviation of 0.5°C, indicated by an exothermic peak. The endothermic event at 114.7°C, with a standard deviation of 0.8°C, shows the melting temperature of PBS. The onset of melting commences around 100°C and the glass transition temperature was measured to be −33.1°C, with a standard deviation of 2.2°C. Indeed, with an onset of melting ~100°C, the coating system likely cannot be used for applications requiring higher temperatures for extended periods of time. Furthermore, cold crystallization temperatures slightly above 90°C are expected to exhibit whitening which may detrimentally influence consumer perception that the packaging is “bad” or “over cooked” from heating.

There was no measurable volatile content from the PBS pellet up to the onset of degradation temperature indicating no likely adulteration of food from the evolution of small molecules from the polymer. However, a 2.5% additional mass loss was determined at 100°C for the coated paper sample compared to the pellet which is anticipated to be absorbed water in the paper structures (Figure 4 and S3). Values of melting and glass transition differ slightly from that of the PBS pellet. Interestingly, the cold crystallization temperature and melting temperature decreased ~6°C after application to the paper. This may be attributed to water absorption of the polymer during application which would be mitigated during commercial manufacturing with a nitrogen blanket and closed hopper system. These results were consistent across all three lots (see Figure S4). A second endothermic event was detected above the melting point of the PBS pellet ~147°C. This is likely attributed to melting temperature of the paperboard’s binders. This hypothesis is supported in Figure S5 where the paperboard alone possessed two endothermic events near 134°C and 183°C.

Figure 4. Representative thermogravimetric analysis thermograms (left) and differential scanning calorimetry thermograms (right) of food-grade poly(butylene succinate) in pellet and coated paper forms

Convenience single-use packaging is often used for fatty or oily foods at elevated temperatures. A paperboard coating is therefore required to resist oil penetration (i.e., “bleeding”) to avoid oily contents from reaching consumers and to maintain structural integrity. The contact angle of water and avocado oil was determined to be 85 degrees and 80 degrees with standard deviations of 7.7 and 12.3 degrees, respectively, indicating a slight wetting of the surface for both liquids. This suggests a relatively balanced surface for the protection of both oil and water liquids. The effect of oil on coated and uncoated paperboard at ambient temperature is shown in Figure S6. On uncoated paperboard, the avocado oil bleeds through immediately which presents as a rapid color change of the paperboard. This provides a rapid and visual inspection method for determining oil bleed-through of polymer coatings for the convenience food packaging simulations.

All samples tested exhibited similar behavior under all convenience packaging simulations. Dimpling on the coating surfaces of 2–14 microns were observed after microwave treatments via scanning laser confocal microscopy (Figures 5 and 6). Multiple coating trials were completed and similar dimpling was observed for each trial and can be found in Figures S7 and S8. This is consistent with the localized temperatures exceeding the onset of PBS melting (~100°C) from concentrated microwave energy absorption by oil droplets at the coating surface. Therefore, we attribute the dimpling to localized heating of the oil above the melting temperature of PBS (Figures 5 and 6) that was not captured by the temperature probe. The dimpling did not result in oil bleed through for all samples tested except for one where confocal microscopy identified defects in the coating prior to testing (Figure S9) resulting in inconsistent coverage of the paper substrate. This oil bleed through was not seen on other coatings since, through confocal images, defects were not observed to be deep enough to provide insufficient coverage of paperboard. Presumably, these defects would be minimized during commercial manufacturing. The dimpling did not significantly affect the coating performance against the oil penetration but after the oil-soaked sponges were removed, surface whitening was observed (Figure 5, 6, S7, and S8). This can be attributed to the surface reaching the cold crystallization temperature of PBS. Although dimpling was observed in all microwaved samples, their depth was insufficient to generate direct access of the oil to the paper substrates and still provided protection from oil bleed through. These results are expected to translate to an industry targeted thickness of 25 microns. It is anticipated that the localized rise in temperature-induced dimpling could result in oil bleed through if temperatures increased or were sustained for longer periods of time.

Figure 5. Representative confocal images of the first trial poly(butylene succinate) coated paperboard surfaces after microwave rapid reheat convenience food packaging simulation (upper left) and line scan of observed dimpling (upper right). Optical images front (lower left) and back (lower right) after the microwave rapid reheat procedure

Figure 6. Representative confocal images of the second trial of poly(butylene succinate) coated

Paperboard surfaces after microwave rapid reheat convenience food packaging simulation (upper left) and line scan of observed dimpling (upper right). Optical images before (lower left) and after (lower right) the microwave rapid reheat procedure.

Unlike the microwave rapid reheat test, there was no change in the surface morphology according to the confocal imaging before and after the heat and hold simulation procedure (Figure 7). No dimpling was observed since the temperature did not reach the melting temperature of PBS. This also explains the absence of surface whitening observed in the microwave rapid reheat simulation. None of the PBS coated paperboard samples exhibited oil bleed through the coating because no melting in the PBS occurred and no major surface defects in the paperboard were observed.

Figure 7. Representative confocal images of poly(butylene succinate) coated paperboard surfaces after heat and hold convenience food packaging simulation (upper left) and line scan (upper right). Optical images of front and back images after the heat and hold procedure

Conclusions

Poly(butylene succinate) is a degradable-biobased thermoplastic polyester with grades available for direct food contact. With more scrutiny over end-of-life considerations and impending legislation for more sustainable single-use packaging, the identification and availability of compliant alternatives are critical. We coated paperboard with PBS via direct-melt application and subjected samples to simulated convenience packaging scenarios. Some surface dimpling and whitening were observed under microwave rapid reheat conditions which may detrimentally affect consumer perception although this was not observed for the heat and hold scenario. This study demonstrated that poly(butylene succinate) is a suitable replacement for perfluoroalkyl substances and petroleum-based chemicals in convenience ready-to-eat food packaging using the simulation parameters employed here. Replacement of PFAS and petroleum polymer coatings with a compostable, biobased polymer for paper-based convenience food packaging will enable the manufacturing of more sustainable and fully compostable structures which, when collected and composted, will reduce landfill accumulation and global contamination. Future work includes determination of compliance for direct food contact in the convenience food packaging conditions of use and investigation of performance in similar scenarios with more complex chemistries, such as blends of oil, acetic acid, and water.

Acknowledgments

The authors would like to thank Alexandra Ivey and Dr. William Colona of Iowa State University for their valuable input into this work and guidance for testing.

Disclosure statement

The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Data availability statement

Data will be made available upon request to the corresponding author at https://[email protected].

Additional information

Funding

This study was supported by the Polymer and Food Protection Consortium at Iowa State University and the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa and Project No. IOW04202 Accession # 1021300 by the Hatch Act and State of Iowa. The authors would like to thank InnoPak, LLC., Thermo Fisher Scientific, GC Innovations America, and Mitsubishi Chemical for their generous support;U.S. Department of Agriculture [IOW04202]

Notes on contributors

Henry Thurber

Henry Thurber is an undergraduate student studying Materials Science and Engineering at Iowa State University. He is an undergraduate research assistant in the Polymer and Food Protection Consortium at Iowa State University.

Greg W. Curtzwiler

Dr. Greg W. Curtzwiler is an Assistant Professor in the Polymer and Food Protection Consortium at Iowa State University. His research is focused on commercially viable sustainable materials for adhesives and coatings in the packaging and automotive industries including bio-based, compostable, and recycled polymers. He is currently working on understanding the structure–property relationships between renewably sourced biobased polymers, hydroplasticization, and compatibilization of biobased waste diverted fillers for adhesives and coatings.