**5. Difference in and between Carbon and Silicate Oxide Coatings**

The current commercial carbon thin films have a slight, brownish to golden, tint [26]. Although this may restrict applicable product categories of carbon coated PET bottles, from the viewpoint of the visual quality of products, the degree of the tint appears to position within the scope of consumer acceptance, based on the commercial products of white wine (Figure 4) and edible oil categories in the Japanese market.

**Figure 4.** Example of DLC coated bottles for wine.

In case of beverage and liquor market in Japan, carbon coating is more often seen than silicate oxide coating in spite of the abovementioned disadvantage to carbon coating. The reason might be found in that carbon thin films are readily applicable to various product categories because carbon coating is inert to food and beverage solutions as long as the PET substrate is stable. On the other hand, some more remarkable limit in applicable product properties is known in typical silicate oxide coating. The gas barrier property of silicate oxide coatings may be decreased in contact with some solutions, for example, beverages of pH close to neutral [27].

C2H2 (acetylene) is the main material gas for carbon thin films for gas barrier-enhanced PET bottles. Derived from the hydrogen contained in acetylene molecules, the resultant carbon thin films contain hydrogen components up to 40% in atomic percentage. ERDA (Elastic Recoil Detection Analysis) analyses showed oxygen components up to 10% may be present in the carbon thin films [4,28], which is considered to be mainly derived from water vapor from PET substrates. The advantage of the use of acetylene lies in high deposition rates and economics, while CH4 (methane) is used in many studies [4,26]. At least carbon thin films derived from acetylene contain the carbon bonding of *sp*3, *sp*2, and *sp*1, based on XPS and FTIR studies [22]. In the Japanese market, carbon-coated PET bottles are derived from Kirin's DLC and Sidel's Actis™ technologies [14].

HMDSO (hexa-metyl-di-siloxan) and HMDSN (hexa-metyl-di-silazane) are the main material gases for silicate oxide thin films with aid of the controlled supply of oxygen. Based on the ratio in the mass flow of the material gas to oxygen and other conditions, the resultant thin films have different components consisting of silicate, carbon, oxygen, and hydrogen [29]. The components have impacts on gas barrier properties and stability in contact with beverage solutions, and sometimes also on tint. In the case with commercial gas barrier silicate oxide, thin films are totally colorless in visual observation. In the Japanese market, silicate oxide-coated PET bottles can be mainly seen in domestic edible oil and wine products, and rarely seen in imported carbonated water. Those bottles are derived from Toyo Seikan's Sibird™, Toppan's GL-C™, and KHS's Plasmax™ technologies [14].

From the viewpoint of the frequency of power used to cause the plasma states of material gas supplied, radio frequency (13.56 MHz) and microwave (2.45 MHz) are used in commercial technologies. The use of radio frequencies usually leads to a bi-electrode system, in other words, a type of capacitively-coupled plasma system, where sheath voltage and the resultant ion impact over the surface of the substrate can be controlled relatively precise manner [28]. It can be expressed that the use of these systems involves both merits and demerits to machine users. Examples of the merits are possible improvement in the performance of coating and stable application to relatively small or large containers, while those of the demerits are the possible increase of the change of mechanical parts for the application to containers of different shapes and sizes.

#### **6. Recent Advancement in Commercialized Technologies for Coating Plastic Containers**

In spite of the different nature of carbon and silicate oxide thin films as described above, it can be said that the difference between the two thin films is decreasing in the recent technical advancement.

It is obviously conceivable that the optimization of process conditions in parallel to the improvement in machinery has been performed in each technology, and as a result, deposition time has been shortened while the barrier properties of PET bottles coated are maintained or even improved. It is supposed that typical process conditions, including vacuum pressure, gas flow rate, and power application have been optimized. As a result, carbon coating has been less colored, and widened its applications (as shown in Figure 2). In the same way, silicate oxide coating has clarified and mitigated its limitation in applications, and widened its applications. In the case of the Japanese market, the use of coating technologies has been rapidly increased in recent years and, at present coating is the most abundant among technologies, compared to other gas barrier enhancement technologies applied to PET bottles [14].

An example of technological advancement has been found in the appropriate use of dielectric materials along electrodes in Kirin's DLC coating technology [30], and the modification of power frequency. Conventionally, this technology employed 13.56 MHz for power frequency and outer electrodes made of metal (conductive materials) parts only. Recently, power frequency was confirmed as one of the significant process parameters [28]. The use of 6 MHz for power frequency and outer electrodes fully or partially covered with dielectric material parts has been proposed in order to facilitate finding the appropriate process conditions for high gas barrier coatings (as shown in Figure 5) in addition to a decreased change of electrode parts for bottles of different shape and size. The results of the observation of coating thickness and plasma light emission indicate that the reason why 6 MHz power frequency showed the lowest gas barrier performance lies in the optimized spatial distribution of plasma. Compared to plasma produced with 13.56 MHz, where the plasma tends to concentrate around the neck part of the bottle, it seems that plasma with a lower frequency provides higher ion impacts to the PET substrate and the resultant secondary electrons modify the spatial distribution of the plasma to the direction of the bottom part of the bottle.

**Figure 5.** Example of the impact on power frequency to the performance of coated bottles. 500 mL PET bottles were coated with DLC using different power frequency ranging from 2.5 MHz to 13.56 MHz, and the oxygen transmission rate (OTR) of these bottles were measured. Optimized power frequency was found at 6.0 MHz in terms of OTR. The measurement of OTR was performed based on ASTM F1307 [31] method using an Oxran 2/21 device, Mocon Co., Ltd., (Brooklyn Park, MN, USA) under conditions of 23 ◦C and 90% relative humidity.

Another example has been found in the modification in the manner of material gas and oxygen supply during the coating process of KHS's Plasmax™. This technology is called Plasmax Plus™. Due to an extra carbon-rich layer formed on the conventional silicate oxide layer, the resultant coating can be stable in contact with solutions of pH close to neutral, which deteriorates the performance of coating based on the conventional process. Interestingly, the new coating manner requires no machinery modification [27].

#### **7. Possible Near Future Advancement in This Field**

The above description in this review mainly covered a brief history of gas barrier enhancement of PET bottles through plasma-assisted CVD techniques. On the other hand, a lot of effort has been made to other types of plastic containers and novel approaches to gas barrier enhancement.

Although the current era where PET bottles are the most abundant package format of rigid plastic containers is likely to last in this and the following decades because of their industrially-favorable balance between performance and economics, other plastic materials also have demands for functional thin film coating. Some polyolefins, such as PE (poly(ethylene)) and PP (poly(propylene)), are quite useful materials while the lack of oxygen and other barrier properties limits their benefits. For possible example, coated PP bottles or jars could keep the flavor quality of filled contents for certain extended periods of time in addition to high heat resistance, compared to PET containers, which are limited in applications below the boiling temperature of water.

The authors found a remarkable difference in the degree of oxygen gas barrier enhancement with DLC coating formed on various kinds of plastic film substrates, as shown in Table 5 and Figure 6. However, a positron annihilation [32] study by the authors indicates that, on these substrates, DLC coating can be formed homogeneously in terms of free volume, as shown in Figure 7. The positron annihilation method is based on a phenomenon where positrons implanted into a condensed matter annihilate with an electron and emits two 511-keV γ quanta. The spectra of γ energy, including the Doppler shift, are characterized by the S parameter, which mainly reflects changes due to the annihilation of positron-electron pairs with a low-momentum distribution. For amorphous materials, positronium (Ps: a hydrogen-like bound state between a positron and an electron) may form in open spaces (or free volumes). Figure 7 clearly shows that DLC films has a small S parameter, compared to polymer substrates, and that thin films of small free volume can function as barriers against gas

permeation. Empirically, packages made of PE and PP tends to have relatively rough surface, and rough surface is considered to lead to significant defects in coating. When the surface of them and PET bottles is observed using an atomic force microscopy, the *R*<sup>a</sup> of 1 μm square is usually 30–100 nm and less than 1 nm, respectively. A result of wet coating approach [33] supports this concept, where a specific type of organosilane materials placed between DLC thin films and PP substrates remarkably enhanced oxygen barrier property, even though the organosilane layer itself did not have a significant barrier property to the PP substrate. It should be noted that, when the surface of the organosilane layer is observed with an atomic force microscopy, the *R*<sup>a</sup> of 1 μm square is usually around 1 nm. In this case, the smoothed surface with an increased anti-crack property due to the organosilane layer caused the enhancement of the coating. These results suggest the interface between thin films and substrates plays a crucial role on the enhancement of gas barrier property with dense thin film coating, and technologies for surface conditions are considered to be a key for the future commercialization of coated containers made of various plastics such as PE and PP.

In the other way, a novel approach to gas barrier thin film coating has been proposed, where a hot wire or catalytic CVD technique is applied to bottle coating in an attempt to achieve decreased installing expenditure based on the simple configuration of coating machines compared to that of conventional plasma assisted CVD machines. Furthermore, the application of this technique can produce unique gas barrier coating to PET bottles, like an intermediate between carbon and silicate oxide thin films [34]. Since the machine installation cost seems to be the bottleneck to further distribution of thin film coating technologies, significantly low cost machinery may be a remarkable breakthrough in this field.


**Table 5.** List of plastic materials used for comparing the degree of gas barrier enhancement with DLC coating.

**Figure 6.** Comparison of (**a**) the oxygen transmission rate (OTR); and (**b**) BIF of DLC coating on different plastic films. Samples of 40 mm square size were placed on the center of body part of 500 mL PET bottles were coated and measured based on the ASTM D3985 method using Oxtran 2/21 devices, Mocon Co., Ltd. (Brooklyn Park, MN, USA), under conditions of 23 ◦C and 90% relative humidity, in the same manner in a previous study [28].

**Figure 7.** Depth profile of the positron annihilation of DLC coated samples. The *S* parameter of DLC coating (see the region of less than 1.5 keV) and plastic substrates (see the region of more than in 5.0 keV) was measured in the same manner in a previous study [28] for observing the relative free volume of DLC coating layers.

The above discussion on near future technologies indicates a high potential of further advancement in thin film coating technologies for hollow plastic containers in this field, including applications to food and beverage industry.

**Acknowledgments:** We would like to express our gratitude to Tetsuya Suzuki, and Taku Aoki, and Kazuhisa Tsuji, Keio University, Japan, for profound discussion. We are also thankful to all the staff related to this article in University of Tsukuba, Japan, and Kirin Co. Ltd., Tokyo, Japan.

**Author Contributions:** Masaki Nakaya conceived and designed the experiments on DLC coating to PET bottles in Figures 5 and 6, Table 5, and a part of Table 4. Masaki Nakaya and Akira Uedono conceived and designed the experiments in Figure 7. Masaki Nakaya and Atushi Hotta conceived and designed the experiments using organosilane. Masaki Nakaya performed the experiments in Figures 5 and 6, and a part of Table 4. Masaki Nakaya and Akira Uedono performed the experiments in Figure 7. Akira Uedono analyzed the data in Figure 7. Atsushi Hotta contributed PP substrate materials and analysis tools in the experiments using organosilane. Masaki Nakaya wrote the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
