Diamond carbon film coating: high-pitched "underwear" for PET bottles

PET is the best material for soft drink packaging compared to other plastic packaging materials. This is due to the high melting temperature, strength and transparency of PET, excellent gas barrier properties, flavor barrier properties, low cost, ease of molding and recyclability, and the like.


However, PET bottles do not meet the special requirements of some beverages, such as hot drinks and some soft drinks. PET is used for the packaging of beverages such as beer and juice. The barrier properties against oxygen, carbon dioxide, water vapor and flavor cannot meet the packaging requirements, which affects the quality of these products. When the oxygen in the air enters the plastic bottle or the carbon dioxide in the carbonated beverage in the bottle is released, it will accelerate the deterioration of the beverage. While other high polymers (such as PEN) can solve the barrier problem, they are too costly and difficult to recycle. Improving the barrier properties of PET to oxygen and carbon dioxide has attracted wide attention. At present, some technologies combine PET and some materials with barrier properties to improve their performance.

PET bottle coating treatment
In order to ensure the quality of soft drinks, it is necessary to prevent the gas from penetrating on the walls of the plastic bottles. This requires the use of packaging materials with low gas permeability, which are generally referred to as high barrier packaging materials. There are four main ways to improve the gas barrier properties of PET bottles: multi-layer coextrusion technology, which has a multi-layer composite structure; surface coating treatment; adding some gas-trapping agents; blending with other materials with barrier properties . Among them, the multi-layer coextrusion technology is a method that has been successfully applied at present, but the improvement of the barrier performance is small, and it is difficult to recycle, and the cost is high. The surface coating is an emerging technology for improving the gas barrier properties of PET bottles. It can obtain a very thin gas barrier film, which can effectively prevent gas molecules from passing through the PET bottle wall, and can solve the problem of recycling.

Some of the coating materials currently in use have been tested in practice, with diamond carbon film (DLC) and silicon oxide materials being industrially very popular materials. The silica coating is used to improve the gas barrier properties of PET films earlier than diamond carbon films. In the early 1980s, silica coatings were obtained by physical vapor deposition and chemical vapor deposition. However, the silicon oxide film is brittle, and in order to obtain a long-lasting mechanical strength on the PET surface, a relatively complicated process is required. In recent years, some products of diamond carbon film (amorphous hydrocarbon) coating on the surface of PET have appeared on the market, and the gas barrier properties of PET bottles have been greatly improved.

Diamond carbon film coating process
The diamond carbon film coating is generally subjected to plasma chemical vapor deposition (PCVD), and a very thin DLC coating is obtained on the inner surface of the PET bottle by plasma vapor deposition under vacuum. The preparation process of the coating is as follows: first, the bottle is placed in a vacuum chamber for vacuum treatment, and then C2H2 gas is injected to generate a plasma of hydrocarbon under the action of frequency conversion high pressure, and finally ions and free radicals are deposited on the inner surface of the PET bottle. , get DLC coating.

Characteristics of diamond carbon film coating
DLC membranes contain a large amount of hydrogen, so they have very good cohesiveness and fission resistance. At the same time, PET bottles have the following advantages: high gas barrier properties, blocking oxygen and carbon dioxide, ensuring the quality of beverages; flavor barrier properties, preventing bottles and The migration and adsorption process between the packages allows the soft drinks to maintain the original flavor; the UV barrier property has the function of absorbing ultraviolet rays, avoiding the soft drinks being affected by ultraviolet rays; being chemically inert, not reacting with other substances, and having chemical stability; Conducive to recycling while maintaining the existing advantages of PET bottles.

The DLC coating greatly improves the barrier properties of PET bottles, but the barrier properties of PET bottles have a great relationship with temperature changes. When the temperature is raised, the barrier properties of the coated and uncoated PET bottles are reduced, indicating that although the coating is a thermally stable material, the thermal motion of the molecules still affects the gas barrier properties of the PET bottle.

Development of DLC coated PET bottles
The future of DLC coated PET bottles is mainly based on market demand, focusing on the development and improvement of technology. The application prospect is very promising, the market potential is very large, it is estimated to occupy 10% of the market share of PET, widely used in the packaging of soft drinks, including tea, juice, carbonated drinks and so on. At present, packaging for tea beverages in Japan has already appeared on the market, and in 2005, 50 million have entered the market. With the gradual expansion of the soft drink market, DLC coated PET bottles will be further promoted and applied.