Vacuum Deposition onto Webs, Films and Foils

Charles Bishop

• Editorial: William Andrew
• Año: 2011
• ISBN:
  9781437778687
• Edición: 2
• Categoría: Technology & Engineering
• Categoría: Materials Science

Roll-to-roll vacuum deposition is the technology that applies an even coating to a flexible material that can be held on a roll and provides a much faster and cheaper method of bulk coating than deposition onto single pieces or non-flexible surfaces, such as glass.

This technology has been used in industrial-scale applications for some time, including a wide range of metalized packaging (e.g. snack packets). Its potential as a high-speed, scalable process has seen an increasing range of new products emerging that employ this cost-effective technology:

• solar energy products are moving from rigid panels onto flexible substrates, which are cheaper and more versatile
• in a similar way, electronic circuit 'boards' can be produced on a flexible polymer, creating a new range of 'flexible electronics' products
• flexible displays are another area of new technology in vacuum coating, with flexible display panels and light sources emerging

Charles Bishop has written this book to meet the need he identified, as a trainer and consultant to the vacuum coatings industry, for a non-mathematical guide to the technologies, equipment, processes and applications of vacuum deposition. His book is aimed at a wide audience of engineers, technicians and production management. It also provides a guide to the subject for sectors in which vacuum deposition is a novel technology, such as solar energy and flexible electronics.

• Bishop's non-mathematical explanation of vacuum deposition technologies will empower a wide range of technicians, production managers and engineers in related disciplines to improve performance and maximize productivity from vacuum coating systems
• Provides the knowledge and understanding needed to specify systems more effectively and enhance the dialogue between non-specialists and suppliers / engineers
• Provides those in the rapidly expanding fields of solar energy, display panels and flexible electronics with the know-how to unlock the potential of vacuum coating to transform their processes and products

1. What Is a Vacuum?

Here is a brief description of what a vacuum is, what units it is measured in, and how it relates to pressure. Having defined a vacuum, the reasons why it is important in the deposition of coatings is explained. This includes the descriptive measure of the distance atoms travel between collisions, known as the mean free path.

Key Words

mean free path, partial pressure, saturated vapor pressure, vacuum, vapor pressure.

1.1. What Is a Vacuum?

The word vacuum is derived from the Latin word “vacua” meaning empty. If we empty the chamber of gas, we produce a vacuum.

A vacuum could be described as where within an enclosed volume there is less gas per unit volume than is present in a similar volume in the atmosphere surrounding the enclosed volume. This is something that can be used to our advantage.

Those of us who drink tea have all heard tales of not being able to brew a good cup of tea when on the higher slopes of Mount Everest because of the lower pressure and the problem of boiling water at a lower temperature. This effect of reducing the boiling point of materials when under vacuum is one advantage that can be used to good effect.

Many materials, particularly when raised in temperature to the boiling point, are prone to oxidation. Thus, another advantage of operating in a vacuum is that materials that would normally be excessively affected by oxidation can have oxygen and water vapor kept away. This is achieved by being within a volume where there are few gas molecules, that is, a vacuum.

1.2. What Is a Gas?

If we look at materials in general, they can be in the form of a solid, liquid, or gas. The structure changes with each form. Solids have atoms closely spaced and in fixed positions. On heating the material, the form changes to a liquid where the atoms are disordered and the distance between atoms is greater. On further heating, the disorder is still greater and the spacing is also much greater. The speed of motion of the atoms also increases with temperature. So let us look at a few facts and figures about gases.

A gas is where atoms or molecules are free to move in any direction and are in constant motion. Typically, these particles are traveling at speeds of approximately 1650kph (1000mph).

In air, gas molecules occupy approximately 0.01% of the space as compared to a solid where the molecules occupy approximately 74% of the space. The particles collide with each other or surfaces at a rate of 10,000,000,000 per second. These collisions mean that the gas particles have random motion and will rapidly expand to fill the whole volume. If the number of collisions seems to be large, bear in mind that there will be around 20,000,000,000,000,000,000 particles per cubic centimeter and the mean free path (the average distance a particle has to travel before it hits another particle) is only 100nm.

1.3. Pressure

All atoms or molecules have mass, and when they hit and bounce off a surface they exert a force on that surface. This force per unit area is known as pressure.

All atoms or molecules in the atmosphere with their mass are subjected to the gravitational pull of the earth and are attracted to the earth. Thus, at high altitudes the pressure is low because the density of gas is low.

At the top of Everest, the pressure is less than one third of that found at sea level. A simple rule of thumb is that the pressure is halved every 5km away from the earth's surface (sea level).

Atmospheric pressure taken at sea level and 45°N latitude is 14.69 pounds per square inch (psi) or 1kilogram force per square centimeter (kgf/cm²). If the pressure is taken at a temperature of 0°C, the pressure is said to be 1 standard atmosphere (1 std atm).

Atmospheric pressure as a value is often rounded up for convenience:

There is an issue regarding the units used to designate pressure that can be problematic. As long ago as 1978, pascal replaced torr as the acceptable measure of pressure. However, many vacuum systems that were built before this time are still in regular use, and also old habits die slowly, and so it is still common to find systems using torr as the measure of pressure.

The Italian, Torricelli, in 1644 made a measurement of pressure using a mercury manometer. His measurement of 760mmHg for atmospheric pressure is the basis of the torr scale used today:

The European preferred unit of pressure is the pascal or the newton per square meter. As ever, the Europeans have adopted a unit of measurement of pressure that does not quite conform because of it not being related to the SI unit by a facto...