Modern Plastics Handbook 2011 Part 12 pps

9.50 Chapter Nine P=Paar F=Fair G=Gaad Figure 9.25 Types of angle joints and methods of reducing cleavage.13 Stress direction - Corner joints for relatively flexible adherends, such as sheet metal, should be designed with reinforcements for support. Various corner- joint designs are shown in Fig. 9.26. With very thin adherends, angle joints offer low strengths because of high peel concentrations. A design consisting of right-angle corner plates or slip joints offers the most sat- isfactory performance. Thick, rigid members, such as rectangular bars and wood, may be bonded with an end lap joint, but greater strengths can be obtained with mortise and tenon. Hollow members, such as extrusions, fasten together best with mitered joints and inner splines. Flexible plastics and elastomers. Thin or flexible polymeric substrates may be joined using a simple or modified lap joint. The double strap joint is best, but also the most time-consuming to fabricate. The strap mater- ial should be made out of the same material as the parts to be joined, or at least have approximately equivalent strength, flexibility, and thickness. The adhesive should have the same degree of flexibility as the adherends. If the sections to be bonded are relatively thick, a scarf joint is acceptable. The length of the scarf should be at least 4 times the thick- ness; sometimes larger scarves may be needed. When bonding elastic material, forces on the elastomer during cure of the adhesive should be carefully controlled, since excess pressure Finishing, Assembly, and Decorating 9.51 will cause residual stresses at the bond interface. Stress concentra- tions may also be minimized in rubber-to-metal joints by elimination of sharp comers and using metal thick enough to prevent peel stress- es that may arise with thinner-gauge metals. As with all joint designs, polymeric joints should avoid peel stress. Figure 9.27 illustrates methods of bonding flexible substrates so that the adhesive will be stressed in its strongest direction. Rigid plastic composites. Reinforced plastics are often anisotropic materi- als. This means their strength properties are directional. Joints made from anisotropic substrates should be designed to stress both the adhe- sive and adherend in the direction of greatest strength. Laminates, for example, should be stressed parallel to the laminations. Stresses nor- mal to the laminate may cause the substrate to delaminate. Single and joggle lap joints are more likely to cause delamination than scarf or beveled lap joints. The strap-joint variations are useful when bending loads may be imposed on the joint. 9.5.5 Surface preparation Many plastics and plastic composites can be treated prior to bond- ing by simple mechanical abrasion or alkaline cleaning to remove surface contaminants. In some cases it is necessary that the poly- meric surface be physically or chemically modified to achieve acceptable bonding. This applies particularly to crystalline thermo- plastics such as the polyolefins, linear polyesters, and fluorocar- bons. Methods used to improve the bonding characteristics of these surfaces include 1. Oxidation via chemical treatment or flame treatment 2. Electrical discharge to leave a more reactive surface 3. Plasma treatment (exposing the surface to ionized inert gas) 4. Metal-ion treatment (for example, sodium naphthalene process for fluorocarbons) Stress direction ~ Poor -Poor lGOOd I-Good Figure 9.27 Methods of joining flexible rubber or plastic.13 ~l.1:J~ Poor Poor Good 9.52 Chapter Nine Table 9.11 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics are to be joined with adhesives. Solvent and heat welding are other methods of fastening plastics that do not require chemical alteration of the surface. Welding procedures will be discussed in another section of this chapter. The effects of plastic surface treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some common plastic materi- als that require special physical or chemical treatments to achieve ade- quate surfaces for adhesive bonding are listed in the following sections. Fluorocarbons. Fluorocarbons, such as polytetrafluoroethylene (TFE), polyfluororethylene propylene (FEP), polychlorotrifluoroethylene (CFE), and polymonochlorotrifluoroethylene (Kel-F), are notoriously difficult to bond because of their low surface tension. However, epoxy and polyurethane adhesives offer moderate strength if the fluorocar- bon is treated prior to bonding. The fluorocarbon surface may be made more "wettable" by exposing it for a brief moment to a hot flame to oxidize the surface. The most satisfactory surface treatment is achieved by immersing the plastic in a sodium-naphthalene dispersion in tetrahydrofuran. This process is believed to remove fluorine atoms, leaving a carbonized surface that can be wet easily. Fluorocarbon films treated for adhesive bonding are available from many suppliers. A formulation and description of the sodium-naphthalene process may be found in Table 9.11. Commercial chemical products for etching fluorocarbons are also listed. Polyethylene terephthalate (Mylar). A medium-strength bond can be obtained with polyethylene terephthalate plastics and films by abra- sion and solvent cleaning. However, a stronger bond can be achieved by immersing the surface in a warm solution of sodium hydroxide or in an alkaline cleaning solution for 2 to 10 min. Polyolefins. These materials can be effectively bonded only if the sur- face is first oxidized. Polyethylene and polypropylene can be prepared for bonding by holding the flame of an oxyacetylene torch over the plastic until it becomes glossy or else by heating the surface momen- tarily with a blast of hot air. It is important not to overheat the plas- tic because it causes deformation. The treated plastic must be bonded as quickly as possible after surface preparation. Polyolefins, such as polyethylene, polypropylene and polymethyl pentene, as well as polyformaldehyde and polyether, may be more effectively treated with a sodium dichromate-sulfuric acid solution. This treatment oxidizes the surface, allowing better wetting by the adhesive. Another process, plasma treatment, has been developed for treating hard-to-bond plastics such as polyolefins. This process works in various Finishing, Assembly, and Decorating 9.53 TABLE 9.11 Surface Preparation Methods for Plastics 1. Abrasion. Grit or vapor blast, or For general- medium-grit emery cloth purpose followed by solvent degreasing bonding 2. Etch in the following acid For maximum solution: bond strength. Parts by wt. ASTM D 2093 Potassium dichromate 75 Distilled water 120 Concentrated sulfuric acid (96%, sp. gr. 1.84) 1500 for 10 s at 25°C. Rinse in distilled water, and dry in air at RT Acetone 1. Abrasion. Sand with 280A-grit For general- emery cloth followed by solvent purpose bonding degreasing 2. "Satinizing" technique. Immerse For maximum the part in bond strength. Parts by wt. Recommended Perchloroethylene 96.85 by DuPont 1,4-Dioxane 3.00 p-Toluenesulfonic acid 0.05 Cab-o-Sil (Cabot Corp.) 0.10 for 5-30 s at 80-120°C. Transfer the part immediately to an oven at 120°C for 1 min. Wash in hot water. Dry in air at 120°C Acetal (homopolymer) Acrylonitrile butadiene styrene Acetone 1. Abrasion. Grit or vapor blast, or 220-grit emery cloth, followed by solvent degreasing 2. Etch in chromic acid solution for Recipe 2 for 20 min at 60° methyl pentane Cellulosics: Cellulose, cellulose acetate, cellulose acetate butyrate, cellulose nitrate, cellulose propionate, ethyl cellulose Methanol, isopropanol 1. Abrasion. Grit or vapor blast, or For general 220-grit emery cloth, followed by bonding solvent degreasing purposes 2. After procedure 1, dry the plastic at 100°C for 1 h, and apply adhesive before the plastic cools to room temperature Diallyl phthalate, diallyl isophthalate Acetone, Abrasion. Grit or vapor blast, or methyl ethyl lOO-grit emery cloth, followed by ketone solvent degreasing Steel wool may be used for abrasion 9.53 TABLE 9.11 Surface Preparation Methods for Plastics ( Continued) Degreasing solventAdherend Remarks Method of treatment Epoxy resins Sand or steel shot are suitable abrasives Abrasion. Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing Prime with epoxy adhesive and fuse into the surface by heatin:g for 30 min at loo°C Acetone, methyl ethyl ketone Abrasion. Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing Ethylene vinyl acetate Furane Acetone, methyl ethyl ketone Alumina (180- grit) is a suitable abrasive Ionomer Acetone, methyl ethyl ketone Abrasion. Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing Melamine resins Acetone, methyl ethyl ketone For general. purpose bonding Abrasion. Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing Methyl pentene Acetone 1. Abrasion. Grit or vapor blast, or 100-grit emery cloth, followed by solvent degreasing 2. Immerse for 1 h at 60°C in Parts by wt. Sulfuric acid (96% sp. gr. 1.84) 26 Potassium chromate 3 Water 11 Rinse in water and distilled water. Dry in warm air 3. Immerse for 5-10 min at 90°C in potassium permanganate (saturated solution), acidified with sulfuric acid (96%, sp. gr. 1.84). Rinse in water and distilled water. Dry in warm air Coatings (dried) 4. Prime surface with lacquer based offer excellent on urea-formaldehyde resin bonding surfaces diluted with carbon tetrachloride without further pretreatment Phenolic resins phenolic melamine resins Acetone, methyl ethyl ketone detergent Steel wool may 1. Abrasion. Grit or vapor blast, or be used for abrade with 100-grit emery abrasion. Sand cloth, followed by solvent or steel shot are degreasing suitable 2. Removal of surface layer of one abrasives. ply of fabric previously placed Glass-fabric on surface before curing. Expose decorative fresh bonding surface by laminates may tearing off the ply prior to be degreased bonding with detergent solution 9.54 TABLE 9.11 Surface Preparation Methods for Plastics ( Continued) Degreasing solvent Adherend Remarks Method of treatment Polyamide (nylon) Acetone, methyl ethyl ketone, detergent 1. Abrasion. Grit or vapor blast, or Sand or steel abrade with 100-grit emery cloth, shot are suitable followed by solvent degreasing abrasives 2. Prime with a spreading dough Suitable for based on the type of rubber to be bonding bonded in admixture with polyamide isocyanate textiles to natural and synthetic rubbers 3. Prime with resorcinol- Good adhesion formaldehyde adhesive to primer coat with epoxy adhesives in meta1-plastic joints Polycarbonate, allyl diglycol carbonate Methanol, Abrasion. Grit or vapor blast, or isopropanol, lOO-grit emery cloth, followed by detergent solvent degreasing Sand or steel shot are suitable abrasives Fluorocarbons: Polychloro- trifluoroe- thylene, polytetra- fluoro- ethylene, polyvinyl fluoride, polymono- chlorotri- fluoro- ethylene Trichloro- ethylene 1. Wipe with solvent and treat with Sodium-treated the following for 15 min at RT: surfaces must Naphthalene (128 g) dissolved in not be abraded tetrahydrofuran (11) to which is before use. added sodium (23 g) during a Hazardous stirring period of 2 h. etching Rinse in deionized water, and solutions dry in water air requiring skillful handling. Proprietary etching solutions are commercially available (see 2). PTFE available in etched tape. 2. Wipe with solvent and treat as ASTM D 2093 recommended in one of the following commercial etchants: Bond aid (W.S. Sharnban and Co.) Fluorobond (Joelin Mfg. Co.) Fluoroetch (Action Associates) Tetraetch (W. L. Gore Associates) 3. Prime with epoxy adhesive, and fuse into the surface by heating for 10 min at 370°C followed by 5 min at 400°C 4. Expose to one of the following Bond within 15 gases activated by corona min of discharge: pretreatment Air (dry) for 5 min Air (wet) for 5 min Nitrous oxide for 10 min Nitrogen for 5 min 5. Expose to electric discharge from Bond within 15 a tesla coil (50,000 V ac) for min of 4 min pretreatment 9.55 TABLE 9.11 Surface Preparation Methods for Plastics (Continued) Degreasing solvent Remarks Adherend Method of treatment Polyesters, polyethyl- ene tere- phthalate (Mylar) Detergent, acetone, methyl ethyl ketone 1. Abrasion. Grit or vapor blast, or For general- loo-grit emery cloth, followed by purpose bonding solvent degreasing 2. Immerse for 10 min at 71) 95°C For maximum in bond strength. Parts by wt. Suitable for Sodium hydroxide 2 linear polyester Water 8 films (Mylar) Rinse in hot water and dry in hot air Chlorinated polyether Acetone, methyl ethyl ketone Etch for 5-10 min at 66-71°C in Suitable for film Parts by wt. materials such Sodium dichromate 5 as Penton. Water 8 ASTM D 2093 Sulfuric acid (96%, sp. gr. 1.84) 100 Rinse in water and distilled water Dry in air Low-bond- strength applications Acetone, methyl ethyl ketone 1. Solvent degreasing 2. Expose surface to gas-burner flame (or oxyacetylene oxidizing flame) until the substrate is glossy 3. Etch in the following: For maximum Parts by wt. bond strength. Polyethylene, polyethylene (chlorinated), polyethyl- ene ter- ephthalate (see polyesters), polypropylene, polyformalde- hyde Sodium dichromate 5 ASTM D 2093 Water 8 Sulfuric acid (96% sp. gr. 1.84) 100 Polyethylene and polypropylene 60 min at 25°C or 1 min at 71°C Polyformaldehyde 10 s at 25°C 4. Expose to following gases Bond within 15 activated by corona discharge: min of Air (dry) for 15 min pretreatment. Air (wet) for 5 min Suitable for Nitrous oxide for 10 min polyolefins. Nitrogen for 15 min 5. Expose to electric discharge from Bond within 15 a tesla coil (50,000 V ac) for 1 min of min pretreatment. Suitable for polrolefins. Abrasion. Grit or vapor blast, or 100- For maximum grit emery cloth, followed by solvent strength relieve degreasing stresses by heating plastic for 5 h at loo°C Polymethyl methacrylate, methacrylate butadiene styrene Acetone, methyl ethyl ketone, detergent, methanol, trichloro- ethylene, isopropano 9.56 Finishing, Assembly, and Decorating 9.57 TABLE 9.11 Surface Preparation Methods for Plastics ( Continued) PoIy- phenylene oxide Methanol Solvent degrease Plastic is soluble in xylene and may be primed with adhesive in xylene solvent Polystyrene Methanol, Abrasion. Grit or vapor blast, or isopropanol, lOO-grit emery cloth, followed by detergent solvent degreasing Suitable for rigid plastic Polysulfone - Methanol Vapor degrease 1. Abrasion. Grit or vapor blast, or Suitable for 100-grit emery cloth followed by rigid plastic. solvent degreasing For maximum strength, prime witlf nitrile- phenolic adhesive 2. Solvent wipe with ketone Suitable for plasticized material Polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride Styrene acrylonitrile Trichloro- ethylene Solvent degrease Urea for- maldehyde Acetone, Abrasion. Grit or vapor blast, or methyl ethyl lOO-grit emery cloth, followed by ketone solvent degreasing SOURCE: Based on the following: N. J. DeLolis, Adhesives for Metals Theory and Technology, Industrial Press, New York, 1970; C. V. Cagle, Adhesive Bonding Techniques and Applications, McGraw-Hill, New York, 1968; W. H. Guttmann, Concise Guide to Structural Adhesives, Reinhold, New York, 1961; "Preparing the Surface for Adhesive Bonding," Bulletin Gl-600, Hysol Division, Dexter Corporation; A. H. Landrock, Adhesive Technology Handbook, Noyes Publications, Park Ridge, N. J., 1985; and J. Schields, Adhesive Handbook, CRC Press, Boca Raton, Fla., 1970. Polyurethane Acetone Abrade with 100-grit emery cloth methyl and solvent degreaser ethyl ketone 9.58 Chapter Nine ways depending on the type of plastic being treated. For most poly- olefins, plasma treatment cross-links the polymeric surface byexposing it to an electrically activated inert gas such as neon or helium. This forms a tough, cross-linked surface that wets easily and is adequate for printing and painting as well as bonding. Table 9.12 shows the tensile-shear strength of bonded polyethylene pretreated by these various methods. Elastomeric adherends. Vulcanized-rubber parts are often contaminated with mold release and plasticizers or extenders that can migrate to the surface. Solvent washing and abrading are common treatments for most elastomers, but chemical treatment is required for maximum properties. Many synthetic and natural rubbers require "cyclizing" with concentrated sulfuric acid until hairline fractures are evident on the surface. Fluorosilicone and silicone rubbers must be primed before bonding. The primer acts as an intermediate interface, providing good adhesion to the rubber and a more wettable surface for the adhesive. 9.5.6 Adhesives selection Factors most likely to influence adhesive selection are listed in Table 9.13. However, thermosetting adhesives, such as epoxies, polyurethanes, or acrylics, are commonly used for structural applica- tion. The adhesive formulations are generally tough, flexible com- pounds that can cure at room temperature. The reasons that these adhesives have gained the most popularity in bonding of plastics are summarized in this section. The physical and chemical properties of both the solidified adhesive and the plastic substrate affect the quality of the bonded joint. Major elements of concern in selecting an adhesive for plastic parts are the thermal expansion coefficient and glass transition temperature of the TABLE 9.12 Effect of Surface Treatments on Polyethylene 15 Relative bond strength* 18.9 >20 2.9 4.7 1.0 1.0 1.0 High-density polyethylene Polypropylene Valox 310 polyester (General Electric Company) Silicone rubber *Results normalized to the control for each material. SOURCE: Branson International Plasma Corporation. Finishing, Assembly, and Decorating 9.59 TABLE 9.13 Factors Influencing Adhesive Selection Stress Chemical factors Exposure Temperature Biological factors Working properties Tension Shear Impact Peel Cleavage Fatigue External (service-related) Internal (effect of adherend on adhesives) Weathering Light Oxidation Moisture Salt spray High Low Cycling Bacteria or mold Rodents or vermin Application Bonding time and temperature range Tackiness Curing rate Storage stability Coverage substrate relative to the adhesive. Special consideration is also required of the polymeric surface which can change during normal aging or exposure to operating environments. Significant differences in the thermal expansion coefficient between substrates and the adhesive can cause serious stress at the plastic's joint interface. These stresses are compounded by thermal cycling and low-temperature service requirements. Selection of a resilient adhe- sive or adjustments in the adhesive's thermal expansion coefficient via filler or additives can reduce such stress. Structural adhesives must have a glass transition temperature higher than the operating temperature to avoid a cohesively weak bond and possible creep problems. Modern engineering plastics, such as polyimide or polyphenylene sulfides, have very high glass transition temperatures. Most common adhesives have a relatively low glass transition temperature so that the weakest thermal link in the joint may often be the adhesive. Use of an adhesive too far below its glass transition temperature could result in low peel or cleavage strength. Brittleness of the adhe- sive at very low temperatures could also manifest itself in poor impact strength. Plastic substrates could be chemically active, even when isolated from the operating environment. Many polymeric surfaces slowly [...]... resonates a metal horn The horn is in contact with one of the plastic parts and the other part is fixed firmly The horn and the part to which it is in contact vibrates sufficiently fast to cause great heat at the interface of the parts being bonded With pressure and subsequent cooling, a strong bond can be obtained with many thermoplastics Rigid plastics with a high modulus of elasticity are best Excellent... be necessary Certainly, the parts should be clean, and all mold release and contaminants must be removed by standard cleaning procedures It may also be necessary to dry certain polymeric parts, such as nylon and polycarbonate, before welding so that the inherent moisture in the part will not affect the overall quality of the bond It may also be necessary to thermally anneal parts, such as acrylic, before... There are two types of vibration welding: linear, in which friction is generatedby a linear motion of the parts, and orbital, in which one part is vibrated using circular motion in all directions Vibration welding has been used on large thermoplastic parts such as canisters, pipe sections, and other parts that are too large to be excited with an ultrasonic generator.An advantageof vibration welding over... bondedareas, such as on aircraft parts, are usually cured in an autoclave The parts are mated first and covered with a rubber blanket to provide uniform pressuredistribution The assemblyis then placed in an autoclave, which can be pressurized and heated This method requires heavy capital equipment investment Vacuum-bagging techniquescan be an inexpensivemethod of applying pressureto large parts A film or plastic... for mechanical fastening operations.Ultrasonic energy can be used to apply threaded inserts to molded plastic parts and to heat stake plastic studs Vibration welding is similar to ultrasonic welding, except that it uses a lower frequency (120 to 240 Hz) of vibration In this way, very large parts can be bonded The process parameters affecting the strength of the resulting weld are the amplitude and... ferromagnetic particle-filled thermoplastics These adhesivescan be 9.76 Chapter Nine shapedinto gasketsor film that can easily be applied and will melt in an induction field The advantage of this method is that stresses causedby large metal inserts are avoided Induction welding is less dependentthan other welding methodson the properties of the materials being welded It can be used on nearly all thermoplastics... assembly guide to the various methods of assemblies that have been found appropriate for specific plastics 9.7.1 Acetal homopolymer and acetal copolymer Parts made of acetal homopolymer and copolymer are generally strong and tough, with a surface finish that is the mirror image of the mold surface Acetal parts are generally ready for end use or further assembling with little or no postmold finishing... quick drying to prevent moisture absorption, yet slow enoughto allow assemblyof the parts The recommended cure time is 12 to 24 h at room temperature The time can be reducedby curing at 130 to 150°F.A cement can be made by dissolving ABS resin in a solvent of up to 25% solids This type of cement is very effective in joining parts that have irregular surfaces or areas that are not readily accessible Becauseof... generally good chemical resistance, parts made from polyamide or resin (or nylon) are generally more difficult to finish and assemble than other plastic parts However, nylons are used in virtually every industry and market The number of chemical types and formulations of nylon available also provide difficulty in selecting fabrication and finishing processes Nylon parts can be mechanically fastened... apparatus touch with the other stationary substrate so that the surfaces melt without damaging the part Sufficient pressure is applied during the processto force out excessair bubbles The rotation is then stopped, and pressure is maintained until the weld sets Rotation speed and pressure are dependenton the thermoplastics being joined The main processparameters are the spin of rotation, weld or axial pressure, . maximum the part in bond strength. Parts by wt. Recommended Perchloroethylene 96.85 by DuPont 1,4-Dioxane 3.00 p-Toluenesulfonic acid 0.05 Cab-o-Sil (Cabot Corp.) 0.10 for 5-30 s at 80 -120 °C. Transfer the. (Cabot Corp.) 0.10 for 5-30 s at 80 -120 °C. Transfer the part immediately to an oven at 120 °C for 1 min. Wash in hot water. Dry in air at 120 °C Acetal (homopolymer) Acrylonitrile butadiene styrene Acetone 1 for treating hard-to-bond plastics such as polyolefins. This process works in various Finishing, Assembly, and Decorating 9.53 TABLE 9.11 Surface Preparation Methods for Plastics 1. Abrasion. Grit