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.

short note on corona discharge surface treatment on plastics

Surface treatment with high voltage discharge modifies only the surface characteristics without affecting material bulk properties.

Materials

Many plastics, such as polyethylene and polypropylene, have chemically inert and nonporous surfaces with low surface tensions. hence they are non-receptive to bonding with printing inks, coatings, and adhesives. Although results are invisible to the naked eye, surface treating modifies surfaces to improve adhesion.

Polyethylene, polypropylene, nylon, vinyl, PVC, PET, metalized surfaces, foils, paper, and paperboard stocks are commonly treated by this method. It is safe, economical, and delivers high line speed throughput. Corona treatment is also suitable for the treatment of injection and blow moulded parts, and is capable of treating multiple surfaces and difficult parts with a single pass.

Equipment

Corona discharge equipment consists of a high-frequency power generator, a high-voltage transformer, a stationary electrode, and a treater ground roll. Standard utility electrical power is converted into higher frequency power which is then supplied to the treater station. The treater station applies this power through ceramic or metal electrodes over an air gap onto the material’s surface.

Three basic corona treater stations are used in coating applications. Bare Roll and Covered Roll and Universal roll. On a Bare Roll treater station the dielectric encapsulates the electrode. On a Covered Roll station it encapsulates the treater base roll. The treater consists of an electrode and a base roll in both stations. In theory a Covered Roll treater is generally used to treat non-conductive webs, and a Bare Roll treater is used to treat conductive webs. However, manufacturers who treat a variety of substrates on the same production line may choose to use a Bare Roll treater.

In a covered roll station the roll is insulated with some type of dielectric material and the electrode is bare metal. In a bare roll system the roll has no insulation, but the electrode is insulated. In a universal roll station both the ground roll and the electrode are insulated.

Covered roll system can only treat non-conductive substrates. Bare roll or universal roll systems can treat both conductive and non conductive substrates.

Shelf life of treated surfaces

The shelf life of pre-treated materials ranges from hours to years, depending on the plastic, its formulation, how it was treated and its exposure to elevated temperature after treatment. Material purity is the most important factor. Shelf life is limited by the presence of low molecular weight components such as antiblock agents, mould release, antistatics, etc. Eventually, these components migrate to the surface of clean polymers. It is therefore recommended to print or bond to the material soon after treatment. However, once the treated surface has been interfaced with a coating, ink, adhesive, or another material, the bond becomes permanent.

Applications:

·         Treatment of surfaces of bio-medical testing devices to improve wettability of surfaces for confluent liquid flow.

·         Treatment of syringe barrels prior to printing.

·         Treatment of the inner surface of needle hubs prior to bonding a stainless steel needle.

·         Treatment of electronic cable insulation to improve adhesion of inks and coatings.

·         Treatment of lids and covers of chemical containers prior to gasket material application or printing.

·         Treatment of plastic bottles prior to application of adhesive labels.

Treatment of automotive profiles made of EPDM rubber prior to application of an adhesive for retaining flocking bristles or decorating fabric.

Thoery: corona discharge method for surface treatment of plastic

Corona discharge: (air plasma)

Corona:

It is ionized air created by discharging high frequency high voltage energy across a metal or insulated electrode. This electrode is positioned over a grounded roll. The space between the electrode and the roll is typically .060". It is in this air gap that corona is generated.

It is a surface modification technique that uses a low temperature corona discharge plasma to impart changes in the properties of a surface. The corona plasma is generated by the application of high voltage to an electrode that has a sharp tip. The plasma forms at the tip. A linear array of electrodes is often used to create a curtain of corona plasma. Materials such as plastics, cloth, or paper may be passed through the corona plasma curtain in order to change the surface energy of the material. All materials have an inherent surface energy. Surface treatment systems are available for virtually any surface format including dimensional objects, sheets and roll goods that are handled in a web format. Corona treating forms low-molecular-weight (LMWOM) on film surface; oxidizes film surface; and forms positive and negative sites by adding and deleting electrons.

Basics of high voltage discharge in air and its application to surface treatment.

In the presence of a high voltage discharge in an air gap, free electrons, which are always present in the air, accelerate and ionise the gas. When the electric discharge is very strong, collisions of high velocity electrons with molecules of gas result in no loss in momentum, and electron avalanching occurs. When a plastic part is placed in the discharge path, the electrons generated in the discharge impact the surface with energies 2 to 3 times that necessary to break the molecular bonds on the surface of most substrates. This creates very reactive free radicals. These free radicals in the presence of oxygen can react rapidly to form various chemical functional groups on the substrate surface. Functional groups resulting from this oxidation reaction are the most effective at increasing surface energy and enhancing chemical bonding to the resin matrix. These include carbonyl (-C=O-). carboxyl (HOOC-), hydroperoxide (HOO-) and hydroxyl (HO-) groups.

Types of surface treatment

There are many methods used for increasing the polarity and surface energy of plastics. Treatments that involve high temperature, wet chemicals, etc. are not environment friendly. Methods treating the plastic surface to a high voltage corona and plasma surface activation is friendlier towards the environment than wet chemicals and high flames.

1.      Washing and cleaning

2.      Mechanical abrasion

3.      Chemical etching

4.      Solvent treatment

5.      Priming

6.      Flame treatment or heat treatment

7.      Corona discharge

8.      Plasma treatment

Washing and cleaning:

It is required to remove any mold release, dust or debris on the product surface. Simple detergent or soap wash is employed.

Mechanical abrasion:

Abrasive belts or grit roughens the surface to improve adhesion and to give it “Tooth”.

Chemical etching:

Strong oxidizing agents etch the plastic surface to improve adhesion.

Solvent treatment (hot or cold):

Solvents soften the substrate without causing surface deformation, crazing or cracking.

Priming:

Prime coat or the base coatings are formulated to adhere to the plastic substrate and to provide a good bond for the top coat.

Flame treatment or heat treatment:

It provides and oxidized surface without using any liquid agents.

Theory of surface treatment of plastics

Significance:

It is required for improved wet ability for proper adhesion of paints, inks, coats, glues, sealants etc. poor adhesion result in to rubbing off, coatings or paint not sticking on the surface, weak sealing and failed gluing.

Some Plastic material that requires surface treatment:

·         Polyethylene (PE)

·         Plexiglas (PMMA)

·         Polypropylene (PP)

·         Teflon (PTFE)

·         Polystyrene (PS)

·         Polycarbonate (PC)

·         EPDM-rubber

·         Polyurethane (PUR)

·         ABS etc.

Technical reasons for poor adhesion:

The materials that possess low surface energies (between 29-36 dyne/cm) or non- polar surface, exhibit adhesion problem. (e.g. HDPE, PP, EDPM, PE, Polyolefins etc.). Due to their slickness they are unresponsive to printing, bonding, coating, or painting. Surface tension and the comparative surface energy of a material determine the potency of a bond existing between the coating and the material itself. If a solid possesses high levels of surface energy as compared to the surface tension of a liquid, there will be increased molecular attraction drawing closer together the adhesive and the ink or paint etc resulting in superior bond strength. Similarly, if the solid’s surface tension is lower than that of the liquid, then the attractive forces will weaken thus, resulting in repelling of the coat. During the surface treatment of plastic, the surface’s energy level is made greater than the surface tension of the coating, printing ink, and paint or adhesive, to increase the chemical attraction. This results in proper adhesion due to improved wettability of the surface. Generally, a substrate’s surface energy should range at least 5 mN/m (dyn/cm) above the surface tension of the adhesive, paint, coating or ink to be used on the surface.

A surface to be bonded should be dust free, clean, smooth, dry, non-porous and wettable i.e. having high surface energy and increased polarity.

Wettability of surface depends on the surface energy (surface tension) of the surface. It is measured in mN/m. it measured in terms of contact angle.

Contact angle:

 

It is the angle between the tangent line at the contact point and the horizontal line of the solid surface. When a liquid droplet is set on a smooth solid horizontal surface, it may spread out over substrate and the contact angle will approach zero if complete wetting takes place. If the wetting is partial the angle will be between 0 – 180 and vice versa. If the surface energy of the solid substrate is higher than the surface tension of the liquid, the wettability is better; it is verified by the smaller contact angle. In order for a proper bond to exist between a liquid and a substrate surface, the substrate’s surface energy should be at least 2-10 mN/m higher than that of the liquid’s tension.