Flame Plasma Pretreatment Improves Adhesion of Polymers
By Joseph DiGiacomo and Scott R. Sabreen
Adhesion problems are widespread throughout the plastics industry and are increasing as metal components are being replaced by advanced engineering polymers. Many plastics have chemically inert and nonporous surfaces with low surface tensions that make bonding difficult. Most plastics are hydrophobic and are not naturally wettable. Surface pretreatments solve many adhesion problems while increasing bond strength performance. Direct flame treatment can offer many cost and process benefits.
Basics of Surface Wetting
As a general rule, acceptable bonding adhesion is achieved when the surface energy of a 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 “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 onto a shape having the least surface area. Therefore, the higher the surface energy of the solid substrate relative to the surface tension of a liquid, the better its “wettability” will be, and the smaller the contact angle (Figure 1). Since fluids are dynamic forces, not static, it is important to consider both the advancing and receding contact angle measurements on surface modified materials. The advancing contact angle is most sensitive to the low-energy (unmodified) components of the surface, while the receding contact angle is more sensitive to the high-energy, oxidized groups introduced by the surface treatments.
Gas-phase surface oxidation pretreatment processes are used to increase surface energy and improve the wetting and adhesive properties of polymeric materials including polyolefins, Teflon, silicones, vinyls, TPO’s, TPU’s, and elastomers. Common pretreatment processes used are flame, electrical (corona) discharge, low pressure cold gas plasma (Microwave/RF), and UV radiation/ozone. More recently developed for treating thin-film applications is chemical plasma (corona discharge of inert gases such as helium). All of these processes are characterized by their ability to generate “gas plasma” – an extremely reactive gas consisting of free electrons, positive ions, and other chemical species. In the sciences of physics and chemistry, the mechanisms in which these plasmas are generated are different but their effects on surface wettability are similar. Plasmas can be conceptualized as a fourth state of matter. If sufficient energy is supplied, solids melt into liquids, liquids vaporize into gases, and gases ionize into plasmas.
Free electrons, ions, metastables, radicals, and UV radiation generated in plasma regions can impacta surface with energies sufficient to break the molecular bonds on the surface of most substrates. This creates very reactive free radicals on the polymer surface which, in turn, can form, cross-link, or in the presence of oxygen, react rapidly to form various chemical functional groups on the substrate surface. Polar functional groups that can form and enhance bondability include carbonyl (C=O), carboxyl (HOOC), hydroperoxide (HOO-), and hydroxyl (HO-) groups. Even small amounts of reactive functional groups incorporated into polymers can be highly beneficial to improving surface characteristics and wettability.
Flame Plasma Surface Treatment
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 3300°F (1816°C). Flame treatment can offer unique process advantages in specific applications, particularly polyolefins. Since a greater extent of oxidation is concentrated near the outermost shallow surface region (5-10nm), flame treated surfaces often result in improved wettability and retain more stable aging (shelf-life) than corona treated surfaces. Flamed polymers also may demonstrate improved wetting properties. Different oxidized functionalities will make varying contributions to the wettability of a surface-oxidized polymer 1. Three key process variables are essential for optimized flame plasma treatment of 3-Dimensional surfaces:
1. Flame Chemistry
Premixed flames consist of an inner or primary core (reducing zone) and a secondary or outer core (oxidizing zone). The optimum flame plasma resides in the secondary core. For natural gas, the following describes the combustion reaction:
CH4 + 2O2 + 8N2 → CO2 + 2H2O + 8N2 + flame plasma
This reaction is exothermic, i.e., produces heat. The ideal air/fuel ratio is the exact amount of oxygen present to burn the fuel completely. There is no excess oxygen or fuel. This is called the stoichiometric air/fuel ratio. A lower air/gas ratio is called sub-stoichiometric, or “rich”, containing more fuel than there is oxygen. A higher ratio is “lean”, containing excess air. Flame chemistry is determined by the air/fuel ratio. In general, the stoichiometric ratio is approximately 10:1 for natural gas and 24:1 for LPG (Propane). The optimum flame chemistry is that which provides for an O2 concentration in the flame plasma that is, after the combustion reaction, of 0.1% – 0.5%. The use of a Plasma Analyzer can help ensure that the preset optimum air/fuel ratio is constantly maintained regardless of changes in temperature, humidity, or gas composition.
2. Distance of the Substrate from the Flame
The optimum distance (position) of the substrate from the flame (just above the inner cones, i.e., reducing zone) is typically between ⅜” – ½” (9.5mm – 12.7mm). Since it may not always be possible to achieve this exact distance due to variations in the substrate geometry, approximately ⅜” (9.5mm) should be considered the minimum distance between the flame and the innermost surface to be treated. The actual treating portion of the flame extends approximately 1½” (38.1mm) beyond the flame tip, with about ½” (12.7mm) producing the highest level of treatment. The surface to be treated should never come in contact with, or below, the inner flame cone because this is the reducing zone (sub-stoichiometric) of the flame. (Figure 2)
3. Dwell Time
Dwell time is an important factor in optimizing the surface treatment. The part must be in contact with the flame plasma for sufficient time for the reaction kinetics to be maximized. Contact time is controlled by how fast the part is passed through the flame and is dependent on burner width, plasma output, and type of resin. For example, polypropylenes may require different dwell time than polyethylenes. Over-treatment should be avoided.
Surface Pretreatment Factors
As with all gas phase surface pretreatments, the degree or quality of treatment is affected by the cleanliness of the plastic surface. The surface must be clean to achieve optimal pretreatment and subsequent adhesion. Surface contamination such as silicone mold release, dirt, dust, grease, oils, and fingerprints inhibit treatment. Material purity also is an important factor. The shelf life of treated plastics depends on the type of resin, formulation, and the ambient environment of the storage area. Shelf life of treated products is limited by the presence of low molecular weight oxidized materials (LMWOM) such as antioxidants, plasticizers, slip and antistatic agents, colorants and pigments, and stabilizers, etc. Exposure of treated surfaces to elevated temperatures increases molecular chain mobility – the higher the chain mobility, the faster the aging of the treatment. Polymer chain mobility in treated materials causes the bonding sites created by the treatment to move away from the surface. These components may eventually migrate to the polymer surface. It is highly advantageous to bond, coat, paint, or decorate the product as soon as possible after pretreatment.
Modern industrial flame treating combustion systems are safe and designed to meet national standards such as NFPA 89 in the US, EN 746-2 in the European Community, and CAN/CSAB149- 3. Flame treating, unlike corona discharge, does not produce ozone and thereby does not require ozone removal systems. Dyne solution testing and Goniometers (contact angle meters) are the most common techniques for measuring polymer surface energies. The use of ESCA (electron pectroscopy by chemical analysis), XPS (X-ray photoelectron spectroscopy), SSIMS (secondary ion mass spectroscopy), and AFM (atomic force microscopy) are advanced techniques for analyzing the degree of surface oxidation and topography. (Figure 3 – Atomic Force Microscopy)
The global need to achieve robust adhesive bonding on plastics demands robust surface pretreatment systems such as flame treatment. Each gas-phase surface oxidation pretreatment method is application-specific and may possess unique advantages and/or limitations. Careful evaluation of all process factors as well as a thorough understanding of these alternative technologies is essential.
References:
1. Strobel, M., Walzak, M.J., Hill, J.M., Lin, A., Karbshenski, E., Lyons, C. S., “A Comparison of Gas Phase Methods of Modifying Polymer Surfaces,” J. Adhesion Sci. Tech. 9(3):365 (1994)
2. Sabreen, Scott, “Surface Treatments for Electronic Components – Solutions for Adhesive Bonding Problems”, NEPCON West (1993)
