Flame Plasma Surface Treating System
By: Joseph D. DiGiacomo
Plastics and plastic-containing composites are currently utilized in a wide variety of packaging applications in the food, medical and pharmaceutical industries and in many other consumer products, from containers to automotive body components to geotextile materials.
In order to appeal to the sophisticated consumer of the 1990's, this packaging must be attractive and have eye catching appeal. This means that is needs to be decorated. To obtain barrier properties, in the case of thin films, the substrate is laminated to at least one, but usually more, other materials, such as aluminum or an other polyolefins.
The very properties that make polyolefins so appealing for use in packaging are detrimental to converting processes such as physical vapor deposition printing/decorating, coating and adhesion to other material. In order for polyolefins to be successfully converted, the reactivity of the polymer surface must be increases. This is especially true in the case of physical vapor deposition where, in order to increase adatom nucleation density, substrate surface reactivity must be increased.
As a general rule, acceptable adhesion is achieved when the surface tension of the substrate (measured in dynes/cm) is approximately 10 dynes/cm greater than the surface tension of the liquid. In this situation, the liquid is said to adequately "wet out" or adhere to the surface.
Surface tension which is a measurement of surface energy is the property, due to molecular forces, by which all liquids through contraction of the surface tend to bring the contained volume into a shape having the least area.
As long as solvent based inks and adhesives were used in converting operations, surface treatment, while important, was not critical, because the surface tension of these inks is low. This all began to change with the Clean Air Act of 1979 and subsequent amendments. In order to be in compliance with the Act, converters began to use water based inks, which while helping to reduce volatile organic compound (VOC) emissions created a criticality in surface preparation because water based inks have a much higher surface tension than solvent based systems (see table 2). Printing on films and non absorbative substrates is considerably more difficult with water based and coatings.
Polystyrene (low lonomer)
Acrylonltrille butadlene styrene
Polyvinyl acetate/polyethylene copolymer
Rigid Polyvinyl Chloride
Plasticized polyvinyl chloride
|Chlorotrifluoroethylene (Aclarfi) |
Fluorinated ethylene propylene
Polyvinylidone fluride 25 Polyvinyl fluoride (Tedlar ffi)
Fluorocarbon copolymer elastomer
N.D. - not determined
Table 2. Surface Tension For Liquids
The main objective of any surface treating method is to increase the substrate’s surface tension or reactivity of the surface. There are four methods commonly used to treat polyolefin surfaces. They are chemical etching, corona, electrical surface treatment and direct flame plasma treatment.
Chemical etching consists of a solvent rinse, then a chemical etch, a subsequent solvent rinse followed by a drying operation. This method is simple and cleans the surface at the same time. However, chemical etching has several significant disadvantages. The most significant problem is that the chemicals used are hazardous to workers and present both air pollution and waste disposal problems.
Coronal treatment basically consists of high voltage electrical discharge, a capacitor, in essence. Corona treatment produces an unwanted by-product, ozone, which must be eliminated by additional add-on equipment. Corona treatment does not easily conform to molded shapes and the resulting treatment is non-uniform. Corona treatment of web or sheet-fed polyolefins also produces undesirable results, such as pin-holing and back side treatment. In addition, secondary treatment is required when using water based inks.
Electrical surface treatment accelerates free electrons present in air by virtue of the high electric field imposed and ionizes the gas, which reacts with the surface of the polyolefin. This method operates at both high frequencies (up to 28 KHZ) and high voltages (up to 50kV). Disadvantages are obvious, namely operation at high voltage. In addition, the initial cost is quite high, as is operating cost. The system is practical for small objects.
Direct Flame Plasma Surface Treatment is the best method to obtain consistent, high dyne level treatment of polyolefin substrates. The system is easy to install, operate and maintain. This system can handle web widths from 2” (50.8mm) to 26 feet (8 meters). Most importantly, flame treatment does not produce ozone or other environmentally harmful products. In addition, nothing is used than can adversely affect operating or maintenance personnel.
Direct flame plasma treatment for web or sheet fed polymer materials offers significant advantages over corona treatment such as:
|...higher treatment levels|
...longer lasting treatment
...reliability of treatment
...uniformity of treatment
...positive control over treatment variables
In addition, unlike corona treatment, flame plasma treatment DOES NOT
In order for corona treaters to adequately increase the surface tension of polymer substrates for use with water based inks, secondary in-line treatment is required. The Flynn Series F3300 flame plasma treating system can achieve the substrate surface tensions required for use with water based inks without the need for secondary treatment. Consistent and superior surface treatment can be achieved by flame plasma treatment.
The exact mechanism that occurs on the surface of a chemically inert, non-polar polyolefin when direct flame is applied is unknown. There are several plausible theories advocated by knowledgeable people active in the field.
In direct flame treatment, the high temperature (adiabatic flame temperature is approximately 3300°F) combustion gases cause oxygen molecules to dissociate into free oxygen atoms, in addition, this high temperature gas, called plasma contains carbon, nitrogen, free electrons, positively charges oxygen and other ions and excited species. Ether (R-O-R) and ester (R-C=O-OR’) functional groups, carbonyl (-COOH), carboxyl and hydroxyl (-OH) groups are contained in the flame plasma.
In studies of bonding, it was found that surfaces with high oxygen content and/or with oxy-carbon functional groups exhibited favorable results.
The flame plasma has an electron density of approximately 108/cm and electron energies of o.5 eV (electron volts). While this is low compared to corona discharge, the mass flow rate of the flame plasma is much higher than corona discharge. The species contained in the flame plasma are highly reactive and affect the electron distribution and subsequently the electron density of the polyolefin molecule. The result is that the polymer surface from a mono-molecular layer to several nanometers 10-9 meters is polarized. By changing the polyolefin surface from a non-polar to polar surface, ink adhesion, coating, lamination and metallizing are enhanced. The surface is much more receptive to subsequent converting operations.
Recall that oxidation is the transfer of electrons and the species contained in the flame plasma are highly reactive. The electrons of the functional groups hold together atoms of different electronegativety and these factors affect the electron distribution and shape of the polyolefin molecule. The resulting reaction polarizes the surface of the polyolefin from a mono- molecular layer to several nanometers (10-9meters). By changing the polyolefin surface from a non-polar to a polar surface, ink adhesion, coating, lamination and metallizing are enhanced. The surface is much more receptive to subsequent converting operations.
The key in utilizing flame plasma surface treatment is control over key flame treating parameters such as:
-Energy output of the burner
-Boundary layer penetration
-Air/gas ratio control
In order to set and control these variables, we need to review some basic information relating to combustion. Combustion is defined as a rapid chemical reaction between a fuel and an oxidizing agent. In our case, the fuel is a gaseous hydrocarbon or mixture, and the oxidizing agent is the oxygen in the combustion air. The gas and combustion air must be supplied to the burner nozzle, to the reaction zone or actual flame area, at the required rate. There are two basic types or flames involved in combustion. The diffusion (laminar flow) flame and the premixed (turbulent) flame. The diffusion zone is characterized by a gas rich area into which oxygen must diffuse. The reaction kinetics of diffusion flames are slow compared to the premixed flames.
The premixed flame is characterized with the fuel and air in close molecular proximity available for the combustion process and consists of an inner or primary core, a secondary or outer core, and the oxidizing portion of the flame. The premixing is accomplished by use of the injection principle in the Flynn Unimixer and is the type of flame utilized in the Flynn F-3000 Flame Plasma treating system.
For natural gas, the following reaction describes the combustion reaction:
CH4 + 2 O2 + 8 N2 = CO2 = 2 H2O + 8 N2
This reaction is exothermic, i.e., produces heat. The ideal air/gas ratio is approximately 10 parts of air to 1 part fuel. This is stoichiometric air/gas ratio. A lower air/gas ratio is called sub-stoichiometric or “rich”. A mixture with a higher air/gas ratio is “lean” and contains excess air. To treat a polyolefin surface successfully the flame must be lean.
Flame Geometry is controlled in the Flynn F-3000 Flame Plasma treating system by the unique ribbon burner design which consists of a series of stainless steel “ribbons” whose crimp and angle are designed to provide an extremely uniform flame front. This provides an even treatment across the web width. By changing the ribbon stack width and depth an infinite variety of flame shapes can be achieved.
Energy output of the burner is controlled by the relationship between the air/gas mixer, combustion air pressure, burner discharge area and burner flame retention features. The Flynn ribbon burner is designed for a wide range of burner firing rates. In addition, the BTU output of the burner must be programmed to follow line speed. As line speed increases, BTU output must increase correspondingly. Treatment level is a function of line speed. In the control system of the F-3000 an algorithm is programmed relating burner output to line speed.
Boundary layer penetration is accomplished by producing a flame plasma with sufficient forward velocity to break through the boundary layer. A web which moves at typical operating speeds of 450-1500 FPM induces a layer of air parallel to itself. Unless the flame plasma penetrates the boundary layer effective surface treatment cannot occur. The ribbon burner design provides enough velocity to the air/gas mixture that the boundary layer is penetrated at line speeds of 2500 FPM.
Burner/Web Interface - The system for film requires a temperature controlled back-up roll and a water conditioning unit to provide water to the roll at approximately 45°C (105°F). This temperature assures that the film will be sufficiently flexible to have intimate contact with roll, but also eliminates any potential condensation. The water conditioning unit is also used for cooling of the burner face.
The ribbon burner used for flame plasma surface treating has a cast iron body with stainless steel ribbon ports designed to provide a very precise flame pattern. The flame is very smooth and continuous and will treat the surface evenly. The water cooling of the burner is provided by continuous tubes placed on both sides of the burner ribbon stacks, in addition to the back of the burner. Our latest design incorporates an electrical heater attached to the back of the burner which is used at initial start-up to prevent burner deflection due to thermal expansion caused b y temperature differences between the front and rear faces of the burner.
The burner/web gap should be increased with increased burner output so that the plasma portion of the flame, which is just beyond the unburnt cones of air/gas mixture, is just at the web surface. Burner should be designed so that little or no warping or bending due to heat or other factors occurs. This is achieved by the double water cooling feature and start-up heater.
Dwell time is an important factor in optimizing surface treatment. The moving web must be in contact with the flame plasma for sufficient time for the reaction kinetics to be maximized. This is accomplished by designing for multiple burner heads as a function of line speed.
Air/Gas Ratio control is the single most important variable in achieving effective flame surface treatment. There are many methods that can be used to control air/gas ratio such as volumetric/mass flow control, O2 analysis and flame temperature analysis. We investigated all of these techniques and found that none of them provided the close control required for effective flame treatment. We developed an air/gas ration controller called the Flame Plasma which controls air/fuel ratio very precisely. Our work has indicated an oxygen concentration in the range of 3-4% excess oxygen in the flue gases in the optimum for flame plasma treatment. A comparison of the scale for a typical O2analyzer and that for the Flynn Flame Plasma Analyzer illustrates the accuracy and precise control of our system.
Typical Oxygen Analyzer Scale (%O)
Surface treating involves a plasma at the surface to be treated. Our approach is to simulate the reactions taking place at the surface by taking a small continuous sample of air/gas mixture and producing and controlled flame in a closed chamber. The flame plasma produces an electrical signal which is processed to produce the plasma value. The plasma value is an accurate, reproducible measure of the treating ability of an air/gas mixture, and can be used in a control loop or displayed for information or manual control.
The Flynn Flame Plasma Analyzer works as follows:
A small sample of air/gas mixture is fed into the analyzer cell at a fixed rate. The cell and sensor are maintained at 812°C. The sample is ignited and the hot products of combustion flow past the ceramic/platinum sensor and produce the electrical signal which is amplified, linearized and processed to a voltage proportional to the percent excess oxygen. This signal is in turn fed to a comptroller that has the desired PID turning control loop constants necessary to operate the gas flow control valve. The Flynn Unimixer ensures a dynamically balanced mixture of air and gas.
A schematic representation of the Flynn Flame Plasma Analyzer
is represented in Figure 1
Safety And Other Considerations
When utilizing a gas fired system safety must be an important consideration. At Flynn, we design to I.R.I. standards which are the most stringent in the United States. Our systems are provided with the following safety interlocks/features:
|-High and low gas pressure switches|
-Low combustion air pressure switch
-Exhaust fan pressure switch
-Minimum speed switch
-Burner retraction mechanism
-Double block shut-off valves
-Electronic spark ignition
-Electronic flame monitoring
Other important variables, which we take into consideration are exhaust of the combustion products of carbon dioxide and water, flame ignition, sensing and shut-down, burner retraction, alarms, and automatic operation. We essentially utilize the same gas fired system for all surface treating applications. This consists of a cast iron ribbon burner, a mechanical control cabinet housing the combustion air blower, proportional mixer, motorized air and gas control valves, zero regulator, pressure transducers, pressure switches and fuel shut-off valves. An electronic/electrical control console containing a single board computer with operator interface, message display and keypad are provided in the electronic control console. Electrical relays, motor starters and flame safety relay are housed in the electrical cabinet.
In 1984 CAMVAC International opened a new metallizing facility in Morristown, TN.
In 1987, in order to increase their production of metallized paper to over 30 million Lb/year, CAMVAC installed a Magna-Graphics coater, designed to work in tandem with and existing Faustel Gravure coater. After metallization, the paper was run through the coater where a top coat was applied. The coater can handle webs up to 80" wide at speeds up to 200O FPM.
A corona treater was installed for treating the web to ensure proper print coat adhesion. However, due to the inability of the corona treater to provide a high enough dyne level, CAMVAC was unable to run the line at rated speed. This caused a bottleneck which adversely affected production. After running trials on our lab unwind/rewind station, CAMVAC replaced the coronal treater with a flame plasma surface treating system and was able to run at rated speed, while maintaining high quality.
With the successful development of the Flame Plasma Analyzer, effective flame plasma surface treatment is now available. A formerly elusive variable in flame treatment has been identified and can now be measured and controlled. Modern flame plasma surface treatment provides high treatment levels without the deleterious side effects of other treatments reliably, consistently and efficiently. The use of water based inks can now be actively pursued without concern about proper adhesion.