**3. Thin Film Coating for the Gas Barrier Enhancement of PET Bottles**

Coating forms thin films over the surface of PET bottles. Dense structures of the thin films, typically several tens of nanometers in thickness, behave like glass or ceramics, and block the passage of gas permeants. The current approach generally uses two types of thin film species, that is, (A) carbon thin films, often described as diamond-like carbon (DLC) or a-C:H [3,4], or (B) silicate oxide thin films, often described as SiO*x*, where *x* is a number and often somewhere between 1.5 to 1.8 [3,12].

While each approach described in the previous chapter has its own advantages and disadvantages, the use of coating is an expanding trend, or is expected to expand [13]. At least in the Japanese market, the trend is remarkable in recent years [14]. One of the advantages in the coating approach is that relatively high gas barrier enhancement is possible to various gas components including oxygen, carbon dioxide, water vapor, and flavors. This favors the quality retention of beverages where quite complex combinations of flavors contribute to unique taste and mouth-feeling, for example, seen in wine and beer [15–17]. Another advantage lies in high recyclability. While other categories of the gas barrier enhancement approach of PET bottles usually require several percentages of foreign materials in the PET matrix in terms of weight, the foreign materials derived from coating amount to be, at most, several parts per million in terms of weight. As a result, coated bottles are usually no problem in recycling of normal PET bottles even in the case of mass use. From an economic point of view, relatively high capital cost to install coating machines is disadvantageous to coating, and this can explain the cause of the relatively slow increase of the use of coated bottles. On the other hand, relatively low operation cost is advantageous, and, in the case with high operational efficiency, coating is expected to require the lowest operation cost [6–8]. In brief, in the case where a remarkable increase of barrier PET bottles happens, especially involved with the mass use in beer and carbonated soft drinks, coating approaches are most likely to be accepted from the viewpoint of bottle performance, social systems, and economics. In other words, at present, coating can be considered to have the largest growth potential among the barrier enhancement technologies of PET bottles.

#### **4. Current Methodology to Thin Film Formation onto PET Bottle Surface**

While various techniques are known to form thin films on substrates, plasma assisted chemical vapor deposition (CVD) techniques are currently available for mass production machinery for gas barrier thin film coating of PET bottles. These techniques meet the requirements for food and beverage containers. At least several requirements are essential, as summarized in Table 2.


**Table 2.** Basic requirements for thin film coating to PET bottles.

One of major conceivable reasons of the use of plasma-assisted CVD lies in low heat load to the substrate. The deformation of the containers is likely to occur when the temperature of the substrate increases above its glass transition temperature which, in the case of polyester-based plastic containers like PET and PLA bottles is 70–80 ◦C, and 60–70 ◦C, respectively [18].

A second conceivable reason is that plasma can relatively readily occur inside a bottle. While coating may be applied to the outer surface of a bottle, these types of technologies involve some difficulty to protect the physical damage to the coating during production in filling lines and transportation to retailers, and also to control coating conditions along with accumulating coating dusts inside vacuum chambers. On the other hand, in the case of coating on the inner surface of a bottle, the thin film is protected with the bottle wall from physical impacts from the outside of the bottle, and most coating dusts can be deposited inside the bottle and removed from the vacuum chamber. Physical impacts may be a concern even with the internal coating due to known "abuse", while typical production and transportation processes seem harmless to the barrier performance of the coating inside the bottle, as far as coated bottles were observed in Japanese market. Additionally, it should be noted that dust control is significantly important for continuous production which might last 20 h or longer. In the case of coatings over the inner surface of containers, thin films tend to come in contact with food and beverages, and are required to have physio-chemical stability which secures the safety to human diet.

The third reason is the relatively short process time for thin film formation. Usually, thin films of 10–100 nm in thickness are used in current technologies. Coating thickness is determined, depending on thin film species, based on economics and the optimal thickness for gas barrier properties [2,12]. It should be noted that an excessively thin film lacks in barrier property, and an excessively thick film decreases in visual and barrier quality due to the occurrence of cracks [2,19].

As a result, based on the deposition rates of roughly 2–60 nm per seconds, 1–5 s are taken for thin film deposition under vacuum conditions, such as 1–20 Pa before coating and 5–30 Pa during coating. The whole process time ranges from 6–30 s per one bottle coating, depending on coating conditions and machine configurations. These process conditions are summarized in Table 3.


**Table 3.** Summary of plasma assisted CVD techniques used for PET bottle coating.

As a result, high throughput machines with a capacity of up to 40,000 bottles per hour have been in operation in soft drink and beer segments based on industriall-realistic economics. Figure 1 and Table 4 show an example of high throughput machine and details on coating process and performance, respectively, based on Kirin's DLC coating method [20].

**Figure 1.** Example of high throughput rotary coating machine for PET bottles (photo provided by courtesy of Mitsubishi Heavy Industry Food and Package Co., Ltd., Nagoya, Japan).


**Table 4.** Typical process conditions for DLC coating to PET bottles [20].

Although differences in processes for coating bottles can be found among the current plasma-assisted CVD technologies, they have the basic process concept in common, that is, (i) to place a bottle into a vacuum chamber, and to vacuum the chamber; (ii) to supply material gas into the bottle; (iii) to apply electromagnetic wave to the inside of the bottle so that the material gas is decomposed into a plasma state; (iv) to allow the plasma to form a thin film on the inner surface of the bottle; and (v) to release the chamber to the atmospheric pressure, and to remove the coated bottle (as summarized in Figure 2). Obviously, these processes can be repeated continuously.

Figures 2 and 3 show an example of the coating processes of Kirin's DLC coating method and the coating system, respectively. In this system, an outer electrode functions as a part of vacuum chamber. Moreover, its internal shape similar to the bottle shape enables evenly distributed coating over the entire part of the bottle, based on that distance between the inner surface of the outer electrode and the bottle can control the voltage of the bottle surface and the resultant plasma distribution.

**Figure 2.** Schematic plasma CVD process for coating plastic bottles in case of Kirin's DLC coating. (**a**) Bottle placement into the coating chamber and vacuuming; (**b**) material gas supply; (**c**) power application to the coating chamber; (**d**) thin film deposition; and (**e**) pressure release and bottle removal from the coating chamber.

**Figure 3.** Example of the components of coating system for PET bottles: (**a**) schematic model; and (**b**) the corresponding part of the production machines (photo provided by courtesy of Mitsubishi Heavy Industry Food and Package Co., Ltd., Nagoya, Japan).

This basic process concept for hollow containers was seen at least as early as the 1980s, and some coating machines intended for commercial use were introduced early in 1990s [21,22], and various process conditions, including different material gas species, have been tried. As a result, the main difference of the processes among the current coating technologies for PET bottles, in general, lies in the material gas species and the frequency of power used to create plasma states.

Nowadays, types of metal oxides and nitrides, as well as carbons, are known to be possible to function as gas barrier thin films [23]. Carbon and silicate oxide thin films are, however, only two thin film materials available for mass production technologies for gas barrier enhanced PET bottles. The major reasons for the use of carbon and silicate oxide thin films for PET bottle applications lie in safety in food contact, the availability and relatively easy handling of material gas, and the economics to achieve sufficient gas barrier performance. Although aluminum and aluminum oxide thin films have a long history of use for the gas barrier enhancement of film and sheet applications [24], appropriate material gas species and coating processes for container applications have not yet been found.

In addition, the current plasma assisted CVD processes which are practical in the mass production can be found in vacuum conditions. Although it has been proved that certain atmospheric plasma-assisted CVD techniques can form gas barrier carbon and silicate oxide thin films based on dielectric barrier discharge techniques [25], their technical problems, such as dimensional limits, remain yet unsolved for the application of three-dimensional objects like PET bottles.
