**1. Background of Thin Film Coatings on Plastic Containers**

Polymer materials have unique properties, such as being easy to shape, and are elastic to physical impacts compared to other types of materials, like metal, glass, and ceramics, and nowadays quite a wide variety of plastic containers are seen in the food and beverage industry. For example, PET (poly(ethylene terephthalate)) bottles are the most widely used package format in the soft drink segment and further use of PET bottles is expected both inside and beyond the soft drink segment [1,2].

From the view of package performance, light-weight, unbreakable, and transparent properties are favorable advantages of common plastic containers. To the contrary to these consumer benefits, gas permeability is a remarkable disadvantage of plastic containers compared to metal and glass containers [3], which virtually eliminate gas permeation, except sealing parts where polymer materials are usually used.

Especially, the permeation of oxygen and carbon dioxide molecules often limits the shelf-life of sensitive products. One of the most sensitive products to gas permeation is beer. Beer is quite sensitive to oxidation, and also sensitive to carbon dioxide release. From the view point of shelf-life extension, the degree of gas barrier improvement is often expressed by BIF (barrier improvement factor) [4]. The value of BIF can be calculated based on the gas transmission rate of normal container(s) divided by that of barrier improved container(s). PET bottles of single serve size require 10 or more times the oxygen barrier in BIF in order to achieve a realistic shelf-life of beer. Furthermore, they require seven or more times the carbon dioxide barrier in BIF if the equivalent shelf-life in glass bottles is demanded.

Since these sensitive products are seen quite often in our daily diet, like in juice, teas, seasoning, edible oil, and wine, as well as beer, significant effort has been made for improving the gas barrier

performance of plastic containers. Among rigid containers used in the food and beverage industry, PET bottles are the most intensive category of plastic containers for gas barrier enhancement study because of their industrial scale of use. It should be stressed that the demand for high gas barrier PET bottles has been increasing because of the global trend in weight reduction, where thinner bottle walls show less gas barrier performance [5], and of a gradual increase of the applications of PET bottle formats.

Based on these backgrounds, this paper reviews the past and recent progress of gas barrier enhanced PET bottles, especially gas barrier thin film coated bottles.

#### **2. Approach to the Gas Barrier Enhancement of PET Bottles Other Than Thin Film Coating**

Major technologies to enhance the gas barrier property of PET bottles used in today's industry can be roughly classified into four categories, that is, (i) coating; (ii) multi-layer; (iii) blending; and (iv) oxygen scavengers, as illustrated in Table 1. It should be noted that different approaches can be combined together. For example, the core layer explained below in the multi-layer approach may include oxygen scavengers, or the blending additives explained below are added in PET layers of multi-layer walls.


**Table 1.** Rough classification of the current major gas barrier technologies for PET bottles [6,7].

The multi-layer approach employs at least one core layer with higher gas barrier properties placed between PET layers. The core layer(s) provides the majority of the gas barrier property of the whole bottle. Some specific grades of polyamides are often used for core layer materials, even though other materials had been attempted [3,8]. While the multi-layer approach is widely used in many industrial fields and its process control has been well established, economics due to the use of specific injection machines for multilayer preforms and of relatively expensive core-layer materials are the barrier to further distribution in the PET bottle industry. From a technical standpoint of view, the core layers are usually adjusted to occupy several percentages of the whole bottle weight to shape the bottle properly, and the core layers of a bottle usually do not exist near the mouth part and the center of the bottom part. These factors limit the maximum oxygen barrier property of multi-layered bottles compared to oxygen scavengers and coating approaches. In Japan, the market share of barrier PET bottles based on this approach has been decreasing.

In the blending approach, higher gas barrier materials are added into melted PET resin before the shaping process. The additives increase the gas barrier property of the whole bottle depending on the concentration in the PET matrix. Some specific grades of polyamides are often used for additive materials [9], even though other materials had been attempted [3,10]. Due to the cost of additives and limited barrier performance compared to other approaches, the use of this approach is limited in these days in Japan. Additionally, in some countries such as Japan, possible mass use of polyamide additives is a concern to their recycling systems.

Oxygen scavengers are a type of additive which reacts with the oxygen permeant and results in restricting the passage of oxygen molecules through the bottle wall. The addition of a certain polyamide and transition metal complex into the PET matrix is an example of this approach [11]. In ideal conditions, this approach can inhibit the increase of dissolved oxygen in the liquid content of the bottle. However, it makes it difficult for bottle manufactures to control the quality of their products as additive concentration and shaping conditions of bottles affects to each other. Some application may not accept the tint and haze due to typical types of scavenger additives.
