by Scott Sabreen, The Sabreen Group, Inc.
Polyphenylene Sulfide (PPS) is a high-temperature, semi-crystalline engineering thermoplastic. Within the industry, PPS is known as THE plastic that performs like metal. Arguably, it’s one of the most challenging polymers to bond to itself or dissimilar materials. To be successful at bonding PPS requires an understanding of its chemical and physical properties, thus making resin grade moduli different and critical for each application.
Properties of PPS
The properties of PPS depend on its crystallization behavior. PPS is chemically inert with low surface energy and offers the broadest resistance to corrosives of any advanced engineering plastic. It is used in thousands of automotive, aerospace, medical and industrial applications where high temperature, solvent-proof, electrically-shielded parts are needed. While these characteristics are ideal for performance, poor surface wettability is the bonding challenge for manufacturers.
Two distinct forms of PPS exist, “Branched” molecular structure (Chevron Phillips Ryton®) and “Linear” (Ticona Fortron®). Glass-filled fibers (30 percent and 40 percent) and glass fiber/mineral mixtures to standard PPS allows for specialized and demanding applications. Designers carefully examine the selection of branched or linear, filled or un-filled, relative to field performance properties, joint-tool design and primary processing. Unfortunately, less emphasis is normally given to the impact of these selections upon secondary manufacturing operations, specifically adhesion bonding processes.
Proper processing of PPS is critical in order to achieve the stated properties of this material. PPS products are not hygroscopic, and therefore, do not experience dimensional expansion problems like nylon (polyamides). Yet it is important to use dry resin in molding parts. PPS should be dried in dehumidifying hopper dryers. To achieve a fully crystalline state, mold temperatures of at least 275 to 300 degrees Fahrenheit are required. When PPS is molded below 275 degrees Fahrenheit, the moldings are amorphous, or semi crystalline, and remain in this state until they are exposed to higher service temperatures (including heat curing of adhesives). Further, mold temperature has a dramatic effect on the surface appearance. Bonding processes should be performed as soon as possible following molding operations, or package parts tightly in non-poly bags.
Surface cleaning and pretreatment
For PPS products, surface cleanliness and plasma pretreatment are critical prerequisites to achieving high strength bonds. Surfaces must be contamination free from dirt, grease and oils. Low molecular weight materials (LMWM) such as silicones, mold release and anti-slip agents inhibit bonding. To solvent clean PPS surfaces and remove LMWM materials (in accordance with company policy and state law), acetone or methyl ethyl ketone (MEK) are suggested. Weaker solvents such Xylene, Toluene and Alcohol (IPA) can be used to remove superficial dirt but not hydrocarbon contamination. Avoid using excess solvent because it can create weak boundary layers of un-removed chemicals leaving a haze build-up inhibiting bonding. Use proper technique at all times including lint-free cloths and wearing powder-free protective hand gloves.
Due to its non-polar hydrophobic nature, PPS adhesion bonding applications normally require plasma surface pretreatment, immediately following solvent cleaning, to increase the surface energy and provide chemical functionality. Pretreatments for PPS include Electrical Corona Discharge, Atmospheric Blown Ion, Flame Plasma and RF Cold Gas (Low Pressure). Each method is application specific and possesses advantages and/or limitations. Electrical pretreatments do not remove/clean all poly-aromatic hydrocarbons so it may be necessary to continue solvent cleaning. RF Cold Gas pretreatment will remove hydrocarbons, thus pre-cleaning is not necessary.
As a general rule, acceptable adhesion is achieved when the surface energy of the substrate is approximately 8-10 dynes/cm greater than the surface tension of the liquid or adhesive. The surface energy of untreated PPS is approximately 38 dynes/cm. The surface tension of compatible epoxy resin adhesives is 45-50 dynes/cm. Therefore, the calculated post-treatment surface energy must be in the range of at least 48-54 dynes/cm. In this situation, the liquid is said to “wet out” or adhere to the surface. Practically, the most robust bonding of PPS is achieved when the surface energy is 60-70 dynes/cm. This higher plasma treatment level has an additional benefit of extending the pretreatment shelf-life. A common method for measuring surface energy “wetting” is the use of calibrated dyne solutions in accordance with test method ASTM D2578.
Adhesives and curing
Optimal joint design is critical in any adhesive bonding application. Bonded joints can be subject to tensile, compressive, shear, peel or cleavage forces, often in combination. For many PPS applications, two component, heat curable structural epoxy adhesives are ideal. Uniform, thin bond line thickness (0.002 to 0.007″) is preferred for optimal shear and tensile strength properties. For glass-filled PPS applications, the Heat Deflection Temperature in the crystalline state is >500 degrees Fahrenheit. Therefore, the oven cure temperature can safely range between 300 and 350 degrees Fahrenheit. Its important to note this is the temperature of the parts reached during curing, which may be different from the oven set point. Parts should remain at temperature until completely cured assuring full crosslinking of the adhesive. Insufficient cure (temperature/time) is one of the most common problems that results in adhesion failure.
In addition to solvent cleaning and plasma pretreatment, a textured surface, as molded, will increase mechanical interlocking adhesion. 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” likely will improve bond strength vs. Finish “10-Fine Diamond 8000 grit (0-3 micron range). Even slightly textured surfaces are beneficial. For connector products and other recessed-hole applications, etched core pins in the mold are highly effective.
In summary, to achieve high strength adhesion bonding of PPS (30-40 percent glass-filled fibers) and heat curable epoxy adhesives, I recommend the following:
- ensure the PPS resin is properly dried before molding and processed at 275 to 300 degrees Fahrenheit;
- conduct bonding processes as soon as possible following molding;
- solvent clean part surfaces;
- use plasma pretreatment to increase surface wetting and chemical functionalization;
- apply a uniform thin adhesive bond line; and
- oven cure at 300 to 350 degrees Fahrenheit.
Additional benefits are gained if product surfaces are textured.
Need Answers to Adhesion Bonding Questions? Call Scott at 972.820.6777.
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].