short note on Plasma treatment (Gas plasma surface treatment)

Plasma is a fairly simple concept which refers to the fourth chemical state of matter. When enough energy is added to each state it changes in sequence from solid to liquid and from liquid to gas. Once in the gas phase if additional energy is forced into the system, then the gas becomes ionized and reaches the plasma state. When the plasma comes into contact with the material surface it transfers the additional energy from the plasma to allow for subsequent reactions to take place on the material surface. This treatment either cross links the surface molecule to create a tight, coherent skin permitting stronger adhesive bonds or forms free radical on the polymer surface providing strong chemical bonds to coatings.

Pretreatment

The pretreatment process consists of three segments: cleaning, activation, and surface bonding. These steps are an extremely important part of plasma treatment because it directly affects the quality of the surface in which ink or adhesive will later be applied. Cleaning the substrate surface can be done using a plasma system to remove even the finest particles of dust. The plasma actually consumes many of the particles through the surface reaction and completely removes them from the environment. No mechanical damage is done to the surface when this process is done with a dry plasma treatment, which also emits no harmful waste and is therefore an environmentally friendly process.

Plastics are made of long polymer chains which have a non-polar surface and are fairly difficult to bond and coat. When the surface is activated the plasma actually changes the chemical structure and polarity of the substrate. When properly activated with plasma plastics as well as many other materials have improved adhesive properties, increased surface wettability, and in many cases improved durability of adhesive joints.

The bond strength is in many cases the final step through the plasma treatment process. It is very important when applying adhesive that the surface has been thoroughly cleaned and activated so the adhesive can perform at its optimum level. Once the bond is made it is permanent and does not degrade over time.

Machinery

The most important components of the plasma system are the plasma jets and generators. Plasma is generated through a high-voltage discharge from within the jet. In blown ion systems, oxygen and sometimes other gases such as nitrogen are typically directed through the discharge which detaches part of the plasma and forces it through the diaphragm to the material being treated6). The diaphragm also limits what is released; mainly any part of the plasma stream containing a charge, which is vital to the process of properly treating a surface.

Depending on the surface being treated, a normal/small plasma beam has a typical treatment width around 25mm and can treat at speeds of 6-600 mm/minute. There are also rotary systems available which consist of numerous plasma jets rotating at extremely high speeds to treat large areas. These systems if properly tuned can treat surfaces up to 2,000mm wide in a single pass. Some of the basic forms of plasma treatment systems include: atmospheric/air plasma (dry), flame plasma, and atmospheric chemical plasma. Air plasma systems utilize either blown ion or blown arc technology. The main difference between these two is that the blown ion method is effective with both conductive and non-conductive surfaces. The chemical interaction of oxygen based plasma systems also create strong covalent carbon-oxygen bonds which are of greater polarity than the initial carbon-hydrogen bonds. Blown ion systems are one of the most commonly used plasma treatment methods. The discharge occurs inside of the chamber making this method different from most other forms of plasma treatment. Pressurized air is forced past a single electrode inside of the chamber; electrons then become excited and create positively charged ions. The pressurized air forces these positively charged ions out of the tip and onto the surface of the substrate. The new positively charged surface is now much more receptive to inks and adhesives similar to the blown ion, the blown arc system forces air past two high-voltage electrodes positively charging ion particles10). This has a much larger surface area than the blown ion method, but operates using the same basic concept. It is claimed that it is capable of treating polyethylene, polypropylene, polyethylene terephthalate, nylon, vinyl, polystyrene, polycarbonate, polyvinyl chloride, and all other types of thermoformed plastics. Flame plasma systems combine compressed air and a flammable gas which is combusted to create a large blue flame. The material surface only has to be exposed to the flame for a brief period of time to become polarized through oxidation. This process also leaves behind other chemicals on the surface that allow for a greater surface adhesion in comparison to an air plasma treatment. The only setback is the heat level required for this treatment. It is typically used to treat injection and blow-molded products because in most cases their thickness can withstand the heat. Atmospheric chemical plasma treatment systems are able to treat materials which would previously be deemed untreatable. This method is considered a phenomenal breakthrough simply because it is ideal for virtually any surface despite how rough or delicate it may be. The plasma used is formed similarly to that of both the air plasma and flame plasma treatment methods, but at low temperatures. Oxygen and acetylene reactive gases are introduced to an electrically charged atmosphere with a proprietary electrode, which produces a high density glow discharge. The resulting plasma sends a bombardment of ions and electrons to the surface of the material. Low molecular weight materials such as water vapor, carbon dioxide, and other non-toxic gases are removed to expose a fresh new surface. While these contaminants are being removed a fraction of the reactive components in the plasma and chemicals create a chemically altered surface by depositing polar functional groups ready for adhesion. Utilization of an atmospheric chemical plasma treatment creates a better surface tension than air plasma systems, but similar in quality to that of a flame plasma treatment. High temperatures; however it cannot treat as wide a variety of materials for the same reason. Atmospheric chemical plasma is an incredible break-through because it removes any previous limitations created by previous methods of surface treatment.

Within the packaging industry treatments have to be homogenous and consistent in every way imaginable. Plasma facilitates a method of surface treatment so advanced that it does not matter whether the process is to print on a glass bottle, a plastic grocery bag, or the side of an airplane. Plasma technology has enabled us to create an optimum surface anywhere.

Types of Plasma treatments:

1.      In-line atmospheric (air) plasma

2.      Flame plasma

3.      Chemical plasma systems.

Atmospheric plasma treatment

Atmospheric-pressure plasma treatment is very similar to corona treatment but there are a few differences between them. Both treatments may use one or more high voltage electrodes which charge the surrounding blown gas molecules and ionizes them. However in atmospheric plasma systems, the overall plasma density is much greater which enhances the rate and degree to which the ionized molecules are incorporated onto a materials' surface. An increased rate of ion bombardment occurs which may result in stronger material bonding traits depending on the gas molecules used in the process. Atmospheric plasma treatment technology also eliminates a possibility of treatment on a material's non-treated side; also known as backside treatment.

Flame plasma

Flame plasma treaters generate more heat than other treating processes, but materials treated through this method tend to have a longer shelf-life. These plasma systems are different from air plasma systems because flame plasma occurs when flammable gas and surrounding air are combusted into an intense blue flame. Objects’ surfaces are polarized from the flame plasma affecting the distribution of the surface’s electrons in an oxidation form. This treatment requires higher temperatures so many of the materials that are treated with a flame plasma can be damaged.

Chemical plasma

Chemical plasma is based on the combination of air plasma and flame plasma. Much like air plasma, chemical plasma fields are generated from electrically charged air. But, instead of air, chemical plasma relies on a mixture of other gases depositing various chemical groups onto the treated surface.