by Scott R. Sabreen, president, The Sabreen Group, Inc.
Flame plasma surface treatment is a highly effective process for improving adhesion of many hydrophobic polymers. Optimal treatment requires precise set-up of gas-burn combustion systems. Zero gas pressure regulators are used with premix burners systems, using venturi air/fuel mixers to maintain a constant air/fuel ratio regardless of the burner firing rate. Functionally, as combustion air flows through the orifice in the air/fuel mixer, it causes a pressure drop, which is sensed internally as a negative pressure through a downstream sensing tube in the zero gas pressure regulator. This exclusive Plastics Decorating article describes flame plasma pretreatment and proper specification and usage of zero gas pressure regulators.
Flame plasma surface treatment is a gas-phase pretreatment process in which the combustion of a hydrocarbon fuel, under controlled conditions, generates the flame plasma, which modifies the substrate surface without affecting the bulk properties of the polymer. The adiabatic flame temperature is approximately 3,300°F (1,816°C).
Flame treatment can offer unique process advantages in specific applications, particularly polymers, such as polyolefins. Since a greater extent of oxidation is concentrated near the outermost shallow surface region (5-10 nm), flame-treated surfaces often result in improved wettability and retains more stable aging (shelf-life) than corona-treated surfaces. Flamed polymers also may demonstrate improved wetting properties as a result of a different mix of chemical functional groups. Different oxidized functionalities will make varying contributions to the wettability of a surface-oxidized polymer.
Three key process variables are essential for optimized flame plasma treatment of 3D surfaces: 1) flame chemistry, 2) distance of the substrate from the flame and 3) dwell time of treatment. Optimal treatment occurs in the main reaction luminous zone as shown in the diagram.
The zero gas pressure regulator is designed to maintain zero pressure relative to atmosphere. When the negative pressure is sensed, the zero gas pressure regulator will open enough so that the positive pressure equals the negative suction pressure. As the combustion air flow increases or decreases, based on the energy requirements of the process, the suction will increase or decrease accordingly and the zero gas pressure regulator will open (with increasing combustion air flow) or close (with decreasing combustion air flow) to always maintain zero pressure. In this way, no matter what the combustion air flow is, the air/fuel ratio should stay constant.
Most zero gas pressure regulators are designed not to be adjusted, since the regulator spring is designed to precisely balance the weight of the valve internals. There are newer models available that allow for adjustment of the balancing spring. This provides added flexibility. When selecting a zero gas pressure regulator, consider turndown, maximum to minimum fuel flow. More is better. Also consider the size of the diaphragm, larger is better since it provides more precise control. In sizing zero gas pressure regulators, keep in mind that the pressure drop across the regulator is equal to the inlet pressure, since the outlet pressure is zero. Consult the manufacturers capacity charts for sizing. Flow capacity is a function of the pressure drop across the regulator.
Scott R. Sabreen is founder and president of The Sabreen Group, Inc., which is an engineering company specializing in secondary plastics manufacturing processes surface pretreatments, adhesion bonding, decorating and finishing, laser marking/laser welding and product security. He has been developing new technologies and solving manufacturing problems for more than 30 years. Sabreen can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.adhesionbonding.com.