Flame Plasma Treatment for Indian Manufacturers
Flame treatment is a gas-phase pretreatment method used by plastics manufacturers in India and worldwide, in which the combustion of a hydrocarbon fuel, under controlled conditions, generates the flame plasma that modifies the substrate surface without affecting the bulk properties of the polymer. The oxidation of the polymer surface as a result of flame treatment occurs through multiple physical processes, including homogeneous combustion of the premixed reactants, transport of the products of combustion to the polymer surface and heterogeneous reaction of flame products with the polymer. The adiabatic flame temperature is approximately 3,300°F (1,816°C). The mechanism of flame treatment is the controlled combustion of a hydrocarbon fuel to generate the flame plasma. Premixed laminar flames produce an exothermic reaction. The exact ratio of oxidiser to fuel needed for complete combustion is known as the stoichiometric ratio. At stoichiometric combustion there is no excess oxygen or fuel. The stoichiometric ratio is approximately 10:1 for natural gas and 24:1 for propane.
The flame is a mixture of a fuel and an oxidiser thoroughly mixed before combustion. The molar ratio of the fuel to the oxidiser is probably the most important parameter within the flame treatment process. For natural gas, the following equation describes the combustion reaction:
CH4 + 2O2 + 8N2 ? CO2 + 2H20 + 8N2 + flame plasma
The three main process control variables are flame chemistry, distance of the substrate from the flame and dwell time of treatment. In a combustion system the flame is a subsonic wave characterised by a velocity called laminar flame speed, which is defined as the velocity at which unburned gases move throughout the combustion wave in the direction normal to the wave surface. A laminar flame profile consists of three zones – reducing, luminous and post-combustion. Each zone has different temperature gradients and chemical reactive species. Placement of the part moving through the flame is critical and must be precise for maximum surface wetting and functionalisation.
Optimal treatment (oxidation) occurs in the main reaction luminous zone or oxidising. This is the hottest flame region, where temperature of the combustion system reaches approximately 1,700°C for natural gas and 1,900 to 2,000°C for propane-based mixtures. In this zone, free radical reactive species content (including hydroxyl, carbonyl, carboxyl, ether and ester) increases dramatically to the detriment of the reactant concentration. The high concentration of radical species makes this region strongly oxidising, in contrast to the reducing zone.
The colour of this zone depends on the fuel/air ratio. A deep bluish violet radiation, with the flame becoming almost transparent if the quantity of gas is increasingly reduced, is produced when the mixture is gas-lean (due to excited CH radicals). A green radiation appears when the mixture is gas-rich (due to excited C2 molecules). When the gas in the mixture increases still further, the radiation turns yellowish because of the carbon particles formed. Ideal flame chemistry is that which provides for an oxygen concentration in the flame plasma that is, after the combustion reaction, of 0.1% to 0.5%.
The optimal treatment distance (position) of the substrate from the flame (just above the pre-combustion reducing zone) is typically between 3/8″ and ½” (9.5 mm to 12.7 mm). The actual treating portion of the flame extends approximately 1½” (38.1 mm) beyond the flame tip, with about ½” (12.7 mm) producing the highest level of treatment. Reference the yellow dotted line in Diagram 1. The surface to be treated should never contact the reducing zone (sub-stoichiometric) of the flame.
The pre-combustion or reducing zone is the coldest region in the flame, where the premixed unburned gases have not reached the optimal oxidising condition. Characterised by its bright steel-blue colour, this region is ineffective for surface activation and offers no oxidation benefits. The post-combustion zone is the largest of the three regions in a laminar flame profile. The temperature remains high due to the exothermic oxidation reaction of CO into CO2, with a release of heat.
This region is characterised as intermediate, between reducing and luminous zones, in terms of temperature and oxygen radicals concentration. The high temperature burned gases generally present a reddish colour, given by water vapour and carbon dioxide (combustion products with hydrocarbon combustibles) radiation and some dirt particles.
The gap (distance) between the luminous flame and substrate surface is a critical factor in determining the extent of activation accomplished by the treatment. Generally, when the substrate passes through the flame, a rapid depletion in the wettability of the treated surface occurs as the distance increases between the cones of the flame and substrate. However, a beneficial effect arising from the treatment is still appreciable several millimetres beyond the post-combustion zone. To maximise the benefit from the treatment, the flame should work in tandem with its luminous zone, which is the richest in active oxidising species (OH radicals and O atoms) and the one at the highest temperature within the whole combustion system.
Zero gas pressure regulators should be installed with premix burner systems, using venturi air/fuel mixers to maintain a constant air/fuel ratio regardless of the burner firing rate. The venturi mixer is a proportional mixing device that has inlets for both the fuel gas (natural or propane) and air/oxygen, and inside the body mixes these gases properly and distributes them through the outlet. Functionally, as combustion air flows through the orifice in the air/fuel mixer, it causes a pressure drop that is sensed internally as a negative pressure through a downstream sensing tube in the zero gas pressure regulator – to maintain zero pressure relative to atmosphere.
Proper selection of burner type is critical to the uniformity and totality of treatment. Product design, geometry and polymer substrate are important factors. A well-designed burner will have a high level of stability, reducing the occurrence of flashback and flame lift, which are two types of instabilities that occur due to an imbalance between the fuel/oxidiser mixture flow velocity and the burn velocity of the combustible mixture. Flashback can develop in a premixed burner if the burning velocity of the flame exceeds the stream velocity flowing out of the port. Under these conditions, the maximum port diameter ensuring necessary thermal losses has been exceeded, thus allowing the flame to propagate back into the mixing chamber.
Two types of burners are used for flame treating: ribbon and drilled port. Ribbon burners consist of a series of crimped stainless steel ribbons inserted into brass, stainless steel, mild steel and cast iron. The number of ribbons, the crimp pattern and number of burner ports and other ribbon pattern design features depend on the application. Ribbon burners can offer advantages because of their ability to provide large flame surface and flame stabilisation. The second type of burner is the drilled port. Drilled port burners are normally manufactured in stainless steel and aluminium.
Ribbon burners are the latest generation of burners and most widely adopted solution at an industrial level because the flame patterns are customisable by adjusting the width of the slot and configuring the ribbons3. Ribbon burners are the optimal solution for the surface modification or flame treatment of polymer films.
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Frequently Asked Questions
How does flame treatment change a plastic surface?
Controlled combustion of a hydrocarbon fuel generates the flame plasma, which oxidises the polymer surface without affecting the bulk properties of the part. The oxidation happens through several processes at once: homogeneous combustion of the premixed reactants, transport of the combustion products to the surface, and heterogeneous reaction of those products with the polymer. The adiabatic flame temperature is around 3,300F (1,816C).
Where in the flame should the part sit?
In the luminous, oxidising zone -- the hottest region, where free-radical species including hydroxyl, carbonyl, carboxyl, ether and ester rise sharply. The optimal distance is typically 3/8 to 1/2 inch (9.5 to 12.7 mm) above the reducing zone. The treating portion of the flame extends about 1-1/2 inches (38.1 mm) beyond the flame tip, with roughly the first 1/2 inch giving the highest level of treatment. The surface should never contact the reducing zone.
What does the flame colour tell you about the mixture?
A deep bluish violet, going almost transparent as gas is reduced, means the mixture is gas-lean -- that colour comes from excited CH radicals. Green means gas-rich, from excited C2 molecules. Increase the gas further and the radiation turns yellowish as carbon particles form. Ideal flame chemistry leaves an oxygen concentration of 0.1 to 0.5 percent in the plasma after the combustion reaction.
What are the process control variables?
Three: flame chemistry, the distance of the substrate from the flame, and the dwell time of treatment. The molar ratio of fuel to oxidiser is probably the single most important parameter. Stoichiometric combustion -- no excess oxygen or fuel -- is roughly 10:1 for natural gas and 24:1 for propane.
Ribbon burner or drilled port?
Ribbon burners are the later generation and the more widely adopted at industrial scale: crimped stainless steel ribbons set into a body, giving a large flame surface, good flame stabilisation, and a pattern customisable by slot width and ribbon configuration. They are the usual choice for treating polymer films. Drilled port burners are normally made in stainless steel and aluminium. Burner selection drives the uniformity and completeness of treatment, so part geometry and substrate both feed into it.
What are flashback and flame lift?
Two instabilities caused by an imbalance between the flow velocity of the fuel/oxidiser mixture and the burn velocity of that mixture. Flashback develops in a premixed burner when the burning velocity of the flame exceeds the stream velocity leaving the port, which lets the flame propagate back into the mixing chamber. A well-designed burner is stable enough that neither occurs.
