Search

Possibility to control the adhesion properties is important in various industrial applications. Since the topmost surface layers have the greatest role in adhesion thus making surface-controlled atomic layer deposition (ALD) reactions appealing method for tailoring surface adhesion properties. In this presentation we show that ALD and molecular-layer-deposition (MLD) can be used to adjust wetting characteristics of polymers and metals. Recently we have been modifying surface properties for various applica-tions including ink-jet printed films, adhesion promoting layers for extrusion coatings and anti-ice coatings. In addition to conventional inorganic metal oxides, such as Al2O3, we have tested also inorganic-organic hybrid MLD materials. Various tools, such as XPS, FT-IR and TOF-ERDA, were used to characterize these films, and wetting behavior was determined using contact angles measurements with the sessile drop method

Atomic and molecular layer deposition techniques have been successfully combined to produce inorganic-organic hybrid films with properties derived from both the inorganic and organic components. There are few reports dealing with further converting of ALD/MLD thin films. Typically inorganic oxide layers on polymer films in most cases require an additional top layer to enable heat sealing and to protect the thin film from mechanical stresses and defects. Adhesion of low-density polyethylene, polyethylene terephthalate and polylactic acid on ALD Al2O3 have been tested previously, where good adhesion was achieved in most cases only with the low-density polyethylene. In this study we report deposition of a hybrid ALD/MLD layer, its properties, as well as its use as adhesion layer for polyactic acid (PLA) extrusion coatings. A mixture of both lactic acid optical isomers, D- and L-lactic acids, was used as MLD precursor. 25 and 45 nm thick hybrid films were analyzed. XPS analyses gave a quite similar composition for both of these two coatings. FTIR analyses also indicated presence and bonding of hybrid components. The oxygen to carbon and oxygen to aluminum ratios were in line with theoretical calculations for such a hybrid. Deposition mechanism is expected to involve both hydroxyl groups of the lactic acid. However, the results indicated also other types of bonding between lactic acid and TMA or between the lactic acid molecules. The thickness of the hybrid remained the same for at least a week after the deposition indicating certain level of stability. In next step cellophane film deposited with 25 nm Al2O3 and 13 nm TMA-lactic acid hybrid was used as a substrate for PLA extrusion at different melt temperatures. Uncoated cellophane and BOPLA were used as reference substrates. PLA adhered only to itself and to the ALD/MLD hybrid with peel strength values exceeding 100 N/m, while PLA did not adhere to plain cellophane. Thickness of the MLD/ALD hybrid on ALD Al2O3 layer had no obvious effect on the adhesion properties at sealing temperature of 170 °C.

Atomic layer deposition (ALD) is an efficient tool for improving the barrier properties of polymer films and polymer-coated papers/paperboards. However, adequate barrier properties are only one of the key requirements set for the packaging materials. Purpose of this paper is to present the safety aspects related to polymer films with a thin ALD grown Al2O3 layer. Several approaches were tested. Based on our analyses ALD grown Al2O3 thin layers do not fall under the European Commission's definition of nanomaterial, and thus the limitations set in the Commission Regulation (EU) No 10/2011 on plastic materials would not be valid in this case. Al2O3 is also accepted as an additive for plastics in contact with food. Abrasive testing of polymer films, although detrimental in several cases to the film itself, did not increase the number of airborne particles. Coated silicon wafer and oriented polymer films were also exposed to specific simulants and key material properties were followed. Finally, we also tested the films for migration, although overall migration from a similar thin film has been regarded extremely low. Our tests were based on the worst case scenario, where the thin inorganic layer is exposed directly to the simulant. The Al2O3 coated films were affected by the simulant exposure, but the degree of impact depended also on the base film. This could indicate different thin layer growth on different polymer films. As a conclusion, such ALD grown Al2O3 thin layers require additional protective layers acting also as a sealant layer.

Atomic and molecular layer deposition (ALD and MLD, respectively) techniques are examples of self-assembly based on repeated cycles of self-limiting gas-solid reactions. The precursors are pulsed into a reactor alternately, separated by inert gas pulses. During a single cycle, only one molecular layer is deposited on the surface, enabling tailored film composition in principle down to molecular resolution on ideal surfaces. These cycles are repeated until a layer with a specific thickness is achieved. Few materials are, however, ideal. During the early film growth precursors adsorb onto the surface and absorb into the near-surface regions depending both on the substrate and the deposition parameters. These precursors can react with the substrate affecting e.g. mechanical, barrier and surface properties. The deposited material starts then to form clusters leading to nonuniform early film growth. The purpose of this presentation is to demonstrate that thin and non-uniform layers can be used to tailor the surface characteristics of different substrates. For example, print quality (ink spreading and penetration) of inkjet printing on polymer films can be adjusted with metal oxide. Secondly, this can be used to control water penetration into porous nonwovens. Third example demonstrates how adhesion of extrusion coated biopolymer to inorganic oxides can be improved with a novel hybrid layer based on lactic acid. As the amount of material to be deposited is small, this will help to increase the speed in ALD/MLD processes designed for moving webs.

Atomic layer deposition (ALD) is an efficient tool for improving the barrier properties of polymer films and polymer-coated papers/paperboards. However, adequate barrier properties are only one of the requirements set for the packaging materials. The purpose here is to present the safety aspects related to polymer films with a thin ALD deposited oxide barrier layers, especially those with Al2O3. Several approaches for testing ALD material were utilized. Based on our analyses ALD grown uniform Al2O3 thin barrier layers do not fall under the European Commission's definition of nanomaterial and the related limitations in the Commission Regulation. Plastic food contact materials may also contain non-authorized substances provided that their migration to food do not exceed very low migration limit (0.01 mg/kg food) and they are not mutagenic, carcinogenic nor toxic for reproduction, or are in nanoform. However, functional barrier concept should not be used to separate food and nanomaterial. Meanwhile, U.S. Food and Drug Administration (FDA) has a more inclusive approach to identifying products of interest in context of nanotechnology including size and/or function. FDA does not categorically judge products containing nanomaterials to be inherently harmful. Al2O3 and organic compounds of interest, such as ethylene glycol and glycerol, are accepted in Europe as additives for plastics in contact with food. Likewise, in USA these are accepted as either as indirect additives used in food contact substances or generally regarded as safe. According to some producers AlOx coatings have also been approved in USA for indirect contact with food. In our experiments abrasive testing of polymer films, although detrimental in several cases to the film itself, did not increase the number of airborne particles. ALD Al2O3 deposited at 100 ºC onto silicon and commercial polymer films were also exposed to liquid food simulants while evaluating the change in surface and barrier properties. Finally, we also tested the films for migration, although overall migration from thin films has been previously regarded as extremely low. The Al2O3 coated films were affected by the simulant exposure, but the impact depended on the substrate material and the oxidizing precursor. This indicates different thin layer growth on different polymer films. As a conclusion, ALD grown Al2O3 thin layer requires an additional, for example, laminated or extrusion coated top layer to protect it from both mechanical and chemical stresses, to prevent migration to the food, and also to act as a sealant layer in food packaging.