by Scott Sabreen, The Sabreen Group, Inc.
Question: We’re having adhesion failures when trying to bond a molded Nylon-66 automotive part using epoxy adhesives. The part has been wiped with alcohol and we’ve increased the epoxy thickness, hoping this will help. Can you suggest how to solve the problem?
Answer: The good news is that your adhesion bonding problem is definitely solvable. Based upon the information you provided, there appears to be multiple process issues contributing to adhesion failure, assuming a proper joint design. By definition “adhesion failure” occurs at the interface between the adhesive and the adherend (substrate). Visually, there is residual adhesive remaining at any location on only one surface and not the second substrate adherend.
Nylon-66 (Polyamide) is a semi-crystalline polymer. Nylons are inherently difficult to bond because they are hydrophobic, chemically inert and possess poor surface wettability (i.e. low surface energy). Further, nylons are hygroscopic and will absorb moisture in excess of three percent of its mass of water from the atmosphere. Moisture, in and of itself, creates adhesion problems. The rate of moisture absorption is dependent upon time, relative humidity and temperature. Therefore, it is important to perform bonding processes as soon as possible following molding operations or package the parts tightly in non-poly bags with a desiccant. Using proper drying procedures for nylon resins is critical to processing and part performance.
Surface Cleaning and Pretreatment Processes
Surface preparation is critical to achieve high bond strength in all bonding applications. Surfaces must be clean and free of contamination from dirt, grease and oils. Low molecular weight materials (LMWM) such as silicones, mold release and anti-slip agents inhibit bonding. Alcohol (your current cleaning solution) only removes superficial dirt but not hydrocarbon contaminates. To clean Nylon-66 surfaces and remove LMWM materials, I suggest cleaning with Toluene, Xylene, Acetone or MEK (in accordance with your company’s solvent policy and state law). Proper technique must be used at all times, including lint-free cloths and wearing powder-free hand protection. Excess solvent can create weak boundary layers of unremoved chemicals leaving a haze build-up which will further inhibit bonding.
Due to their hydrophobic nature, nylon bonding applications frequently require plasma surface pretreatment to increase the surface energy and provide chemical functionality. Plasma pretreatments for three-dimensional parts include electrical corona discharge, atmospheric blown ion, electrical air plasma (spot treatment) and RF Cold Gas (low pressure). As a general rule, acceptable bonding adhesion is achieved when the surface energy of the substrate is approximately 10 dynes/cm greater than the surface tension of the liquid or adhesive. The surface energy of untreated nylon is approximately 40 dynes/cm. Therefore, the desired post-treatment surface energy needs to be in the range of 50-54 dynes. In this situation, the liquid is said to “wet out” or adhere to the surface. Electrical pretreatments do not remove/clean all poly-aromatic hydrocarbons ,so it may be necessary to continue solvent cleaning prior to plasma pretreatment. RF Cold Gas pretreatment will remove hydrocarbons, thus additional cleaning is not necessary. A common method for measuring surface energy “wetting” is the use of calibrated dyne solutions in accordance with test method ASTM D2578.
Using Epoxy Adhesives
Your decision to use structural epoxy adhesives is an excellent selection. Your question does not state whether you chose a one-component or two-component epoxy. I suggest using a two-component epoxy which consists of mixing an epoxy resin plus a hardener or procuring “duo-pak” applicators. For epoxy resin adhesives, the process of polymerization is termed “curing” in which an exothermic temperature (heat) increase reaction occurs. Typically, full cure time is in the range of seven days at 75 degrees Fahrenheit but is variable between applications and environmental conditions. The speed of cure can be accelerated by adding heat (120-210 degrees Fahrenheit) which results in additional polymerization and can give the epoxy better properties. Bake time and temperature is application specific. One-component epoxies eliminate the need for mixing, but cure only with the addition of heat (normally 250-300 degrees Fahrenheit). By comparison, the working life of two-component epoxies is limited.
Your question states that the thickness of the applied epoxy bond line has been increased. This may likely be contributing to adhesion failures, rather than helping to prevent them. Epoxy adhesives have relatively high coefficients of thermal expansion. Bond lines need to be kept thin and uniform in thickness (typically 3-5 mil for maximum shear strength). The adhesive supplier can provide guidance for optimal bond line thickness for your application. Pre-measured “duo-pak” applicators will eliminate mixing problems and provide a more exact mix ratio. They also provide better stoichiometric chemical reactions — resulting in better strength and outgassing properties.
Relative to the surface texture of the part to be bonded, as molded, a textured surface will increase mechanical interlocking adhesion in addition to solvent cleaning and plasma pretreatment. Texture can be accomplished within the mold tool or manually using a Scotch-Brite pad. For example, NTMA mold cavity finish “40-Diamond buffed 1200 grit” will likely improve bond strength versus finish “10-Fine Diamond 8000 grit” (0-3 micron range). Even slightly textured surfaces will be beneficial.
In summary, to achieve high-strength adhesion bonding of Nylon-66 and epoxy adhesives, I recommend the following:
- Ensure the nylon-66 resin is properly dried before molding.
- Conduct the bonding process as soon as possible following molding.
- Clean the part surface with an alternative solvent which will remove surface debris and hydrocarbon contaminates.
- Consider implementing a plasma surface pretreatment to increase surface wetting/chemical functionalization.
- Apply a thin, uniform adhesive bond line (optional heat cure for two-component epoxy). Additional benefits can be gained if the product has a textured surface.
Scott Sabreen is the founder and president of The Sabreen Group, Inc., a plastics engineering consulting firm. He is a board member for the Society of Plastics Engineers Decorating/Assembly Division, technical editor for Plastics Decorating and expert engineer for Omnexus/SpecialChem, Intota-Guideline and Nerac. Sabreen may be reached via email at [email protected].