Thin Al2O3 barrier coatings onto temperature-sensitive packaging materials by atomic layer deposition

https://doi.org/10.1016/j.surfcoat.2011.05.017Get rights and content

Abstract

Thin (25 nm) and highly uniform Al2O3 coatings have been deposited at relatively low temperature of 80 and 100 °C onto various bio-based polymeric materials employing the atomic layer deposition (ALD) technique. The work demonstrates that the ALD-grown Al2O3 coating significantly enhances the oxygen and water vapor barrier performance of these materials. Promising barrier properties were revealed for polylactide-coated board, hemicellulose-coated board as well as various biopolymer (polylactide, pectin and nano-fibrillated cellulose) films.

Highlights

► Al2O3 coatings (25 nm) were deposited at 80 and 100 °C. ► The atomic layer deposition (ALD) technique was used. ► The coating efficiently enhanced the barrier properties. ► Substrates were all bio-based. ► Coatings meet the requirements for food packages.

Introduction

Growing environmental concerns related to the use of synthetic polymers in the packaging industry have led to the need for new, especially bio-based, materials in such applications [1]. Currently synthetic polymers are widely used in packaging applications because of their relatively low cost and high performance. Bio-based packaging materials would have many advantages over their plastic competitors, such as sustainability and recyclability [2]. However, the sensitivity towards moisture restricts their extended use. One way to improve the water-sensitivity is to apply a surface coating.

“Barrier property” refers to a material's capability to resist the diffusion of a specific species (molecule, atom or ion) into and through the material. To be a good gas and vapor barrier, the material needs to be pore-free. When considering polymer-coated boards, the water vapor transmission rate (WVTR) is affected by e.g. the coating weight of the polymer as well as the temperature and humidity of the surroundings [3], [4]. The common polymers used in packages include low- and high-density polyethylene, polypropylene and polyethylene terephthalate [5]. Hygroscopic materials, such as many biopolymers, typically lose their barrier properties at high relative humidity due to water absorption [6]. There have been some efforts to improve the water vapor and oxygen barrier properties of polymer coatings with e.g. SiOx layers [7]. Based on our recent studies [8], [9], [10], [11] and studies by others [12], [13], [14], a thin Al2O3 coating layer grown by the atomic layer deposition (ALD) technique could work as a high-quality pore-free barrier film. The ALD technique is a surface-controlled layer-by-layer deposition process based on self-limiting gas-solid reactions [15]. It is well suited to produce inorganic gas barrier coatings on various materials.

Because of the covalent bonding, the adhesion of ALD-grown Al2O3 layer with the substrate is commonly excellent [16], [17]. Biopolymers typically have functional surface groups improving the bonding between the substrate and the Al2O3 layer. This makes biopolymeric materials, in our opinion, even more interesting substrates to create efficient gas and moisture barrier materials when combined with a thin Al2O3 coating than regular oil-based polymers, such as polyethylene, polypropylene or polyethylene terephthalate, for instance.

The ALD film growth characteristics on oil-based polymers have been previously studied by others [18], [19], [20], [21]. Metal oxide films were found to grow on the native substrate surface. The basis for the initial film growth and nucleation was the hydroxyl groups on the polymer [15], [22]. The Al2O3 growth mechanism on porous polymeric substrates was demonstrated to occur through the adsorption of the trimethylaluminum (TMA) precursor onto the surface or by absorption into the porous material leading to the formation of Al2O3 clusters and further on to the linear film growth rate after the nucleation period [19]. The same mechanism has been demonstrated for many polymers. However, the initiation period differs depending on the polymer [18].

Here we demonstrate that ALD is indeed a promising technique to fabricate thin Al2O3 barrier layers on bio-based temperature-sensitive packaging materials. We moreover show that the barrier properties can be further improved by coating the materials with a pre-barrier layer prior to the ALD-Al2O3 coating.

Section snippets

Material and methods

The packaging materials investigated were commercial boards (provided by Stora Enso Oyj) coated with bio-based polylactide (PLA). In addition, several different biopolymer films were investigated. The materials tested are presented in Table 1. From our previous thermogravimetric study performed for most of the present substrate materials [8], we may conclude that the materials do not degrade thermally at temperatures employed in our low-temperature ALD-Al2O3 process.

The ALD-Al2O3 depositions

Results and discussion

Our first task was to optimize the ALD-Al2O3 process for temperature-sensitive bio-based substrates. In these preliminary experiments two PLA-coated board samples, B1(PLA) and B2(PLA), were investigated and the deposition parameters considered were the deposition temperature (80 or 100 °C) and the choice of the oxygen source (H2O or O3). Interestingly, the growth per cycle (GPC) values for the H2O and O3 processes were found to be nearly identical, i.e. 0.1 nm/cycle (as measured for films grown

Conclusions

We have demonstrated that the oxygen and water vapor barrier properties of various bio-based boards and films are significantly enhanced by coating them with a 25-nm thick ALD-grown Al2O3 film. Through careful process optimization excellent barrier properties were reached for some of the bio-based materials investigated such that the materials satisfy the basic requirements set for commercial barrier materials for dry food or pharmaceutical packaging applications. Also shown was that there are

Acknowledgements

The authors thank VTT and the Academy of Finland (No: 126528) for funding. Stora Enso Oyj and polymer film suppliers are thanked for providing the substrates. The facilities of Nanomicroscopy Center, Aalto University were also used during this research.

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