by Scott R. Sabreen, president,
The Sabreen Group, Inc.
High-performance polymers, including polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyetherimid (PEI), polyimide (PI) and polytetrafluoroethylene (PTFE), are among the most difficult plastics to bond. These plastics are used widely for applications needing high-temperature stability, broad chemical resistance, stiffness, strength and creep resistance at elevated temperature. Designers select such materials for their light weight, replacing metals, thermosets and ceramics. The success of an adhesive bonding operation is reflected in the strength of the joint generated and in its ability to retain useful joint strength for long periods in the operational environment.
High-temperature materials are divided into two main categories, semicrystalline and amorphous, based on their difference in molecular structure. Semicrystalline materials have a highly ordered molecular structure with sharp melt points. They do not gradually soften with a temperature increase, instead, semicrystalline materials remain solid until a given quantity of heat is absorbed and then rapidly change into a low viscosity liquid. Amorphous high-temperature resins have a randomly ordered molecular structure that does not have a sharp melt point; instead, amorphous materials soften gradually as the temperature rises. These materials change viscosity when heated but seldom are as easy flowing as semicrystalline materials. Amorphous resins lose their strength quickly above their glass transition temperature (Tg). See table.
Polyphenylene sulfide (PPS), arguably, is one of the most challenging polymers to bond to itself or dissimilar materials. Within industry, PPS is known as the plastic that performs like metal. To successfully bond 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% and 40%) 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 unfilled, 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; therefore, they 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°F to 300°F are required. When PPS is molded below 275°F, the moldings are amorphous, or semicrystalline, 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 as 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 unremoved 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. Some electrical pretreatments do not always remove/clean all poly-aromatic hydrocarbons; thus, it may be necessary to continue solvent cleaning. Electrical atmospheric plasma can remove/clean dirt/debris and most hydrocarbons. 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°F. Therefore, the oven cure temperature can range safely between 300°F and 350°F. It’s 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, ensuring full cross-linking 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% 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°F to 300°F;
- 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°F to 350°F.
Additional benefits are gained if product surfaces are textured.
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.