Roll-to-Roll Vacuum Deposition of Barrier Coatings

Charles A. Bishop

• Publisher: Wiley-Scrivener
• Year: 2015
• Print ISBN: 9781118946145
• eBook ISBN: 9781118946152
• Edition: 2
• Topic: Tecnologia e ingegneria
• Subtopic: Scienza dei materiali

It is intended that the book will be a practical guide to provide any reader with the basic information to help them understand what is necessary in order to produce a good barrier coated web or to improve the quality of any existing barrier product.

After providing an introduction, where the terminology is outlined and some of the science is given (keeping the mathematics to a minimum), including barrier testing methods, the vacuum deposition process will be described. In theory a thin layer of metal or glass-like material should be enough to convert any polymer film into a perfect barrier material. The reality is that all barrier coatings have their performance limited by the defects in the coating. This book looks at the whole process from the source materials through to the post deposition handling of the coated material. This holistic view of the vacuum coating process provides a description of the common sources of defects and includes the possible methods of limiting the defects. This enables readers to decide where their development efforts and money can best be used to improve the barrier performance of their own process or materials.

The 2 nd edition contains at least 20% new material including additional barrier testing techniques that have been developed and testing and cleaning equipment brought to market since the 1 st edition was published in 2010. The topic of adhesion is covered in more detail and there is a section on the Hanson Solubility Parameter which is a method of predicting the solubility of gases or liquids in materials.

Chapter 1

Introduction

To start let us explain what is meant when we refer to a barrier material and then go on to describe where these barrier materials might be used. A barrier provides a resistance against something. In the world of packaging, or encapsulation, barrier tends to mean a material that has a resistance to the ingress of something that might degrade the product being packaged or encapsulated. The barrier can also be to prevent the egress of attributes such as flavour or aroma.

Everyone has their own idea of what is meant by a barrier material depending on what is most detrimental to their product. For some people barrier only refers to protection against water vapour whereas for others it is more important to protect the goods against oxygen. So the word barrier is a generic term that needs to be elaborated on to provide more specific information. Being specific is important as a good barrier material against water vapour may have little barrier performance against oxygen and vice versa.

As will be shown there is a growing market for existing barrier materials of which food barrier is a large application. There is also an, as yet, unexploited market for barrier materials with a much better barrier performance. As the performance requirements increase the difficulty in producing the barrier materials also increases, as does the cost.

The barrier performance starts with the choice of materials and the whole manufacturing process. Glass bottles and tin cans are long established barrier materials for food packaging. Thin metal foil is used either directly or as a laminate for high barrier performance in some more critical applications such as pharmaceuticals.

It was believed that to improve the barrier performance of polymers was simply a matter of adding a thin enough glass or metal layer. It was expected that, if thin enough, this coating would not impair the flexibility of the polymer but would match the performance of the bottles, cans or foils. The standard method of applying a glass or metal coating to polymer substrates is by vacuum deposition. What was found was that any imperfections in the coating would result in a limitation to the barrier performance. If the polymer is coated with a coating with no imperfections the performance approaches that of a glass bottle or tin can.

In roll-to-roll vacuum deposition the quality of the supply rolls is a critical factor as too is any pre-treatment or cleaning process. Subsequent chapters will follow the process through from the polymer web production and any cleaning or pre-treatment through to the nucleation and growth of coatings deposited by various vacuum deposition techniques. In order to be able to compare the performance of different barrier coatings, it is necessary to be able to measure the performance and so there is also a chapter that describes the most common methods of measuring the barrier performance. In this way it is hoped that it can be shown how the ultimate performance of the barrier materials can be affected throughout the whole manufacturing process.

A barrier is anything that keeps things apart and we can see examples of barrier materials everyday in food packaging where food products are protected from a variety of different elements be they gases, liquids or solids. Depending on the food and the sensitivity of the foods to degradation they may need protecting from moisture, oxygen, light as well as bacteria, moulds, aromas and taint [1]. As might be expected different materials will perform differently as a barrier to liquids or gases and so there is not any one material used as a universal barrier. There are many possible solutions to providing a suitable barrier. In fact one of the problems we now have is the vast choice of materials that in combination could provide the necessary barrier performance.

It is not just food that requires barrier materials but anything that has some sensitivity to the ingress or egress of some other material, be it a gas or moisture, will require a barrier to protect it. Thermal insulation panels, used to improve the insulation performance of buildings, are designed to have a working lifetime of 50 - 100 years. Throughout this time these panels are expected to maintain their insulation performance which is, in part, dependent upon the evacuated panel remaining under vacuum and hence air and moisture have to be kept out for all this time. The reality is that this is not achieved by the barrier material performance alone but by a combination of the barrier material and scavenger materials incorporated into the product that getter what little amount of gas or moisture is passed through the barrier material. Once the scavenger material is saturated there will be a build up of gas or moisture and the performance of the insulation will then begin to decline.

In the area of electronics there are the organic light emitting devices (OLEDs) that are degraded by moisture ingress and are so sensitive to attack that the barrier requirements are six orders of magnitude higher than those used in most food packaging applications. These very high performance barrier materials are often referred to as ultra-barrier materials.

It is interesting to note that often the same materials are used for both the food packaging and for the ultra-barrier applications. There can be a huge performance difference for exactly the same materials that is dependent upon the quality of how the materials are supplied, handled and used to make the final barrier material. Polymer webs have a certain amount of barrier performance that is inherent, but it is often not enough to meet the customer specifications and so is coated with something to improve the barrier performance. The two materials that have been used for food packaging for decades are metal and glass and the expectation was that adding a very thin glass or metal layer would change the polymer barrier performance into the same perfect performance exhibited by the glass or metal. The metal and glass or glass-like very thin layers, sometimes as thin as a few nanometers, can be deposited using vacuum deposition techniques. The question that has taken time to answer is what happens to these materials when they are deposited as very thin coatings as they no longer perform as well as when they are in the more rigid thick form.

Vacuum deposition onto flexible webs is where a roll of material is loaded onto a winding mechanism that is enclosed in a vacuum vessel that can be pumped out to remove the air. Different materials can be evaporated, or deposited by a variety of means onto the web as it is wound between unwind and rewind rolls. The lack of air enables metals to be deposited with minimal oxidation or for controlled stoichiometry compounds to be deposited. Glass as used in packaging is very rigid, but if the glass is thinned down, it shows increasing flexibility. The very thin glass used for displays that is less than 500 microns thick can be flexed and bent without breaking. If this same glass is vacuum deposited onto a flexible polymer web at a thickness of less than 15nm, the glass becomes even more flexible making it suitable for use in flexible packaging applications. Similarly, metals are also much more flexible when vacuum deposited as thin films than when produced as a rolled thin foil. Aluminium foil has in some countries been banned from being used in packaging as it is deemed to have too high an environmental cost. As the vacuum deposited aluminium coatings are often around one hundredth of the thickness of the rolled foils these have been targeted at replacing many of the packaging foil products [1]. This foil replacement application is one of the highest growth markets.

When these coatings are examined in detail it becomes apparent that they are not perfect but contain a large number of defects. A detailed examination of the supply materials, previous processing and the vacuum deposition process, show that there are many factors that can affect the integrity of the coatings which in turn affect the resultant barrier performance.

Even with these less than perfect coatings the market for the vacuum deposited barrier films is huge with approximately 550,000 tonnes of vacuum coated products being sold into the packaging industry annually, and a predicted growth of ~5% per annum through to 2020. This represents the coating of approximately 22,000 million square metres of material. Of these packaging materials, metallised polypropylene takes the largest share at more than 50% with metallised PET being the second most widely used substrate. The market continues to grow partly encouraged by environmental pressures with metallised polymers being used to replace aluminium foil and also to replace tin cans. Within the area of vacuum deposited coatings there is a difference in market growth expectation for different materials. The deposition of metals, primarily aluminium, has existed for more than 50 years whereas the deposition of the transparent barrier materials is relatively new. It is only relatively recently that the costs have reduced enough, as well as the banning of a chlorine containing coating, to make the transparent barrier vacuum deposited coatings attractive to the packaging industry. This market sector of transparent barrier coatings has been growing in excess of 20% per annum albeit from such a small volume, the total volumes are small by comparison to the metallized films.

When we look at different barrier coatings we can group them into specific types such as packaging, intermediate and ultra-barrier coatings and then subdivide these into opaque or transparent barrier materials.

1.1 Packaging

Packaging has to achieve a number of different functions. Ideally it provides containment to keep the product secure. It has to be convenient to use providing an opportunity for communication, have suitable aesthetics, be non-toxic, tamper-resistant (or tamper-evident), be functional in size & shape and compatible with the production process and the product it contains, low cost, recyclable, reusable or disposed of easily. In addition it has to preserve the product by providing protection against environmental (oxygen, water/moisture, light, chemical attack, contamination from micro organisms), physical attack (such as rodents, and insects), and mechanical hazards (handling damage) during storage and distribution. So when incorporating a barrier coating it needs to complimentary to the existing properties.

The largest volume of vacuum deposited packaging materials is used for the packaging of food. Often this market segment is driven by minimising cost, and as vacuum coating adds cost over the basic flexible webs, there has to be a cost benefit to justify using this coating process.

Extending the shelf-life of products is one of the most easily proven cost benefit that can be used to justify the addition of vacuum deposited coatings. If we take an example of potato crisps/chips where if we open the pack and the crisps/chips are left in the open, moisture will be absorbed and the crisps become soft and soggy. If the opened bag of crisps/chips are left out in the air and in daylight over a period of time, the taste of the crisps would decline to the point of inedibility, as the fats turn rancid because of degradation by oxidation or photo-oxidation by daylight. The same pack of crisps can be left unopened for weeks on the shelf, and then when opened will still be crisp with the same taste as when first made because the vacuum deposited coating has provided a barrier to the oxygen, moisture and light keeping the crisps dry and fresh. Providing this superior barrier performance means that the bags of crisps do not have to be sold within a few days of manufacture but can still be safely sold weeks later and so the waste and loss of profit is reduced.

The manufacturer of any food product will know what quantity of moisture, or oxygen or light will cause the product to degrade. The manufacturer will also choose a shelf-life that they wish to achieve and this information can be used to calculate how good the barrier performance of the packaging has to be to achieve these goals. An example of this type of calculation will be given in a chapter 5.

Most of the time we think of the barrier being to prevent things getting into the food but the reality is that it also prevents things escaping from the food too. If we think of water vapour it can turn food soggy but if lost from food it can allow the food to dry out too much. The drying out of food can be a problem for foods such as breads or cakes. A less obvious problem is in frozen food where the loss of moisture through the sublimation of ice can lead to freezer burn.

Oxygen from the air can oxidize some materials such as fats, turning them rancid, and also can oxidize vitamins such as vitamin C, causing a loss of potency. However, oxygen is not the only gas that can be controlled by barrier coatings. Controlling the permeation of a variety of different gases is used to advantage in controlled or modified atmosphere packaging (MAP). In modified atmosphere packaging the package is flushed out using a gas, such as dry nitrogen, and then the package is filled with a specific gas or mixture of gases. In this case the barrier is designed not only to keep the air out but also to keep the modified gas composition inside the package. The gas used to fill the package might be designed to slow down the ripening of fruit and so extending the shelf-life or it may be used to maintain the colour of the food which can be more about aesthetics than food safety.

Light can be quite detrimental to food with photocatalytic reactions causing the degradation of fats, flavours, vitamins, such as vitamins A, B12, D, E, K, etc and changes of colour. So one early choice is to decide if the product needs to be protected from light and so have an opaque metal coating deposited as the barrier. Where light is not a problem then it may be preferable for the foods to be visible to the customer and these would have a transparent barrier coating deposited.

The packaging also needs to be benign and not interact with the product. The packaging polymer may absorb aromas from the foodstuffs and this may reduce the aroma detected by the consumer. This process of aroma absorption is known as scalping. The packaging also should not taint the foodstuffs by losing anything from the polymer into the foodstuffs known as migration.

1.1.1 Opaque Barrier

Opaque light barrier vacuum deposited coatings are achieved primarily by aluminium metallization. The opacity of the very thin metal coating is usually quoted as the optical density of the coating. Opacity is a measure of light incident on the coating divided by the amount of light transmitted through the coating. The optical density (OD) of a coating is the opacity expressed as a logarithm to base ten. This measurement uses a white light source and detector. The transmitted light value of the substrate can be obtained before deposition starts each time to establish the 100% value, in this way the sub...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Biography
• Acknowledgements
• Preface
• Chapter 1: Introduction
• Chapter 2: Terminology
• Chapter 3: Measurements
• Chapter 4: Materials
• Chapter 5: Packaging Materials Calculations
• Chapter 6: Substrates, Surfaces, Quality and Defects
• Chapter 7: Vacuum Deposition Processing
• Chapter 8: Vacuum Deposition
• Chapter 9: Summary
• Index