Fundamentals of surface activation using plasma flame technique

Use of flame for increasing the surface tension of a substrate and making it suitable for adhesion, printing and painting is probably one of the oldest methods of the surface treatment used in polymer, automotive and aircraft industry. Although with the advancement in technology other methods of surface treatment like corona treatment have emerged, most manufacturers still prefer flame treatment over other techniques.

What is Flame Treatment?

Plasma flamePlasma flame is generated by simply combusting a fuel (composed of long hydrocarbon chains) under controlled temperature and pressure conditions. The flame is used to modify the surface tension of the substrate without changing its composition and properties. Plasma flame generation is an adiabatic process in which the temperature is set to 1816°C. Along with improving the surface tension of a material, flame treatment is also used for improving the quality and reliability of polyolefin. Plasma flame oxidizes the surface of the material, forming a 10nm thick oxygen film on the surface of the substrate, thus turning it into a hydrophilic material. This is the reason that plasma flame treated substances have better wettability than corona treated substrates. Another factor which makes plasma flame a more reliable option for surface treatment than corona is the composition of plasma flame.  Plasma flame is composed of a heterogeneous mixture of different functional groups which modify the chemical properties of the material being treated.

Chemical Reaction

For the production of plasma flame, a mixture of methane, molecular oxygen and gaseous nitrogen are combusted together in a chamber.
8N2 + Ch4 +2O2   →      Plasma flame + CO2 + 8N2 + 2H2O
The plasma flame consists of two cores, primary and secondary. The primary core is also known as reducing core, while the secondary/outer core is the oxidizing layer in which most of the usable plasma flame is concentrated. The outer layer of the plasma flame serves the purpose of flame treatment.

Factors Influencing Plasma Flame Treatment

Following are some of the factors which directly influence the effectiveness of flame treatment:

  • Presence of any organic residues on the surface of the material to be treated can change the composition of the material during the surface treatment.
  • The purity of the material is also an important factor in deciding the effectiveness of surface treatment, impurities present in the material form nonvolatile by-products which can permanently change the properties of the material being treated.
  • Exposure to high temperature, immediately after the treatment increases molecular chain mobility of the treated substance.

Considerations

Following considerations are critical to the success of flame treatment of metals and plastics:

  • Slightest variations in the pressure and temperature of the plasma generation chamber can alter the properties of the plasma flame.
  • Exposure time is an important factor for optimizing the effectiveness of surface treatment. Exposure time varies with the nature of the material to be treated.
  • The substrate should be placed at a distance of 12.7mm from the plasma flame.

Industries are relying on advanced plasma flame treatment systems which are configured to meet the international standards of surface treatment.