PLASTICS JOINING 7.35 3. Plasma treatment (exposing the surface to ionized inert gas) 4. Metal-ion treatment (e.g., sodium naphthalene process for fluorocarbons) Surface preparation is most important for plastic parts that will be bonded with adhe- sives. Solvent and heat welding do not generally require chemical alteration of the surface; however, they do require cleaning. Welding procedures are discussed in another section of this chapter. As with metallic substrates, the effects of plastic surface treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some surface treat- ments, such as plasma, have a long effective shelf life (days to weeks) between treatment and bonding. However, some treating processes, such as electrical discharge and flame treating, will become less effective the longer the time between surface preparation and bonding. FIGURE 7.21 Types of angle joints and methods of reducing cleavage. 11 FIGURE 7.22 Reinforcement of bonded corner joints. FIGURE 7.23 Methods of joining flexible rub- ber or plastic. 11 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING 7.36 CHAPTER 7 Table 7.10 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics are to be joined with adhesives. Specific surface treatments for certain plastics and their effect on surface property characteristics are dis- cussed in Sec. 7.6. Details regarding the surface treatment process parameters may also be found in ASTM D-2093 and various texts on adhesive bonding of plastics. An excellent source of information regarding prebond surface treatments is the supplier of the plastic resin that is being joined. Chemical or physical surface treatments are especially required for structural bonding of low-surface-energy plastics. Low-surface-energy plastics include polyethylene, polypropylene, TPO, and fluorinated polymers. These surface treatments are designed to increase the critical surface tension and improve wetting and adhesion. In addition to in- creasing the critical surface tension, surface treatments are designed to remove contami- nants or “weak boundary layers,” such as a mold release. Abrasion and solvent cleaning are generally recommended as a surface treatment for high-surface-energy thermoplastics and for thermosetting plastics. Frequently, a mold-re- lease agent is present and must be removed before adhesive bonding. Mold-release agents are usually removed by a detergent wash, solvent wash, or solvent wipe. Common solvents used to clean plastic surfaces for adhesive bonding are acetone, tolu- ene, trichloroethylene, methyl ethyl ketone (MEK), low-boiling petroleum ether, and iso- propanol. A solvent should be selected that does not affect the plastic surface but is sufficiently strong to remove organic contamination. Safety and environmental factors must be considered when choosing a solvent. Solvent cleaning alone can be used for high- surface-energy plastics that do not require the maximum joint strength. The compatibility of cleaning solvents with plastic substrates is extremely important. Solvents can affect polymeric surfaces and provide unacceptable part appearance or even degradation of properties. Solvents that are recommended for cleaning plastics are shown in Table 7.11. Suppliers of mold release agents are the best source for information on sol- vents that will remove their materials. Abrasive treatments consist of scouring, machining, hand sanding, and dry and wet abrasive blasting. The abrasive medium can be fine sandpaper, carborundum or alumina abrasives, metal wools, or abrasive shot. Mechanical abrasion is usually preceded and fol- lowed by solvent cleaning. The choice is generally determined by available production fa- cilities and cost. Laminates can be prepared by either abrasion or the tear-ply technique. In the tear-ply design, the laminate is manufactured so that one ply of heavy fabric, such as Dacron, glass, or the equivalent, is attached at the bonding surface. Just prior to bonding, the tear- ply is stripped away, and a fresh, clean, bondable surface is exposed. Chemical surface treatments vary with the type of plastic being bonded. These pro- cesses can involve the use of corrosive and hazardous materials. The most common pro- cesses are sulfuric acid–sodium dichromate etch (polyolefins) and sodium-naphthalene etch (fluorocarbons). Both of these processes are described in ASTM D-2093. Flame, hot air, electrical discharge, and plasma treatments change the surface of the polymer both physically and chemically. The plasma treating process has been found to be very successful on most low-energy surface plastics. Table 7.12 shows that plasma treat- ment results in improved plastic joint strength with common epoxy adhesive. Plasma treat- ment requires vacuum and special batch processing equipment. Most optimized surface treatment processes require prolonged production time and provide safety and environmental concerns. One should be careful not to overspecify the surface treatment required. Only the minimal process necessary to accomplish the func- tional objectives of the application is required. Several new surface treatments and modifications of older, conventional surface treat- ments have been introduced over the last few years to provide alternatives to the common Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING PLASTICS JOINING 7.37 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING 7.42 CHAPTER 7 processes noted above. The driving factors for these developments have primarily been re- lated to environment and safety. Harsh chemicals and elevated-temperature processing as- sociated with conventional chemical and flame treatment methods have inhibited many from using such processes. TABLE 7.11 Common Degreasing Solvents for Polymeric Surfaces 13 Adherend Solvent Acetal (copolymer) Ketone Acetal (homopolymer) Ketone Acrylonitrile-butadiene-styrene Ketone Cellulose, cellulose acetate, cellulose acetate butyrate, cellulose nitrate Alcohol Fluorocarbons Chlorinated alcohol or ketone Polyamide (nylon) Ketone Polycarbonate Alcohol Polyolefins Ketone Polyethylene terephthalate, PET (Mylar) Ketone Polyimide Ketone Polymethylmethacrylate, methacrylate butadiene Ketone or alcohol Polyphenylene oxide Alcohol Polyphenylene sulfide Ketone, chlorinated solvents Polystyrene Alcohol Polyvinyl chloride, polyvinyl fluoride Ketone, chlorinated solvents Thermoplastic polyester Ketone Thermoset plastics Ketone TABLE 7.12 Typical Adhesive Strength Improvement with Plasma Treatment: Aluminum-to-Plastic Shear Specimen Bonded with a Conventional Epoxy Adhesive 15 Plastic Strength of bond, psi Control After plasma treatment Polyamide 846 >3956 Polyethylene 315 >3125 Polyethylene terephthalate 530 1660 Polypropylene 370 3080 Polystyrene 566 >4015 Polytetrafluoroethylene 75 750 Polyvinyl fluoride 278 >1280 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING PLASTICS JOINING 7.43 In addition to providing safer and environmentally friendly processes, these newer sur- face treatments have also been shown to provide for easier and faster processing. They promise a potentially tremendous positive impact on both manufacturing cost and perfor- mance properties. The reduced cost impact can be in the form of equipment costs, imple- mentation costs, operational costs, rework costs and storage/waste removal costs. 7.4.6 Adhesives Selection Factors most likely to influence adhesive selection are listed in Table 7.13. However, ther- mosetting adhesives such as epoxies, polyurethanes, or acrylics are commonly used for structural application. The adhesive formulations are generally tough, flexible compounds that can cure at room temperature. The reasons that these adhesives have gained most pop- ularity 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 temper- ature of the substrate relative to the adhesive. Special consideration is also required of polymeric surfaces that can change during normal aging or exposure to operating environ- ments. Significant differences in thermal expansion coefficient between substrates and the ad- hesive can cause serious stress at the plastic’s joint interface. These stresses are com- pounded by thermal cycling and low-temperature service requirements. Selection of a resilient adhesive or adjustments in the adhesive’s thermal expansion coefficient via filler or additives can reduce such stress. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING 7.44 CHAPTER 7 Structural adhesives must have a glass transition temperature higher than the operating temperature to avoid a cohesively weak bond and possible creep problems. Modern engi- neering plastics, such as polyimide or polyphenylene sulfides, have very high glass transi- tion 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 adhesive at very low temperatures could also manifest itself in poor impact strength. Generally, the best adhesive is one that will wet the substrate and, when cured, has a modulus and thermal expansion coefficient similar to the substrate or else has necessary toughness and elongation to accommodate stresses caused by thermal movements. Differ- ences in flexibility or thermal expansion between the adherends or between the adhesive and adherend can introduce internal stresses into the bond line. Such stresses can lead to premature failure of a bond. Thus, rigid, heavily filled adhesives are often chosen for bonding metals. Flexible adhesives are often chosen for bonding plastics and elastomers. Lower-modu- lus adhesives generally have the flexibility to bond well to plastic substrates. However, these are generally weaker in shear than more rigid adhesives. Fortunately, exceptionally high shear strength is often not required for an adhesive for plastic, since the plastic sub- strate itself is relatively weak. For many high-surface-energy thermosetting plastics, such as epoxies, polyesters, and phenolics, adhesive bonding is generally easy and can be accomplished with many of the same adhesives that are used on metal substrates. For thermoplastics, the surface energy is generally lower, the reactivity is greater, and the thermal expansion is higher than for ther- mosets. Therefore, when bonding thermoplastics, consideration must be given to the sur- face energy of the adhesive and the substrate, the compatibility of the adhesive with the substrate, and thermal expansion coefficients. There are numerous families of adhesives within the structural and nonstructural types. The most common chemical families of structural and nonstructural adhesive families for bonding plastics are identified in Table 7.14. Structural adhesives are those having bond shear strength on the order of 1000 psi or greater. This is often sufficient to cause failure of the plastic substrate when the bond is TABLE 7.14 Common Families of Structural and Non- structural Adhesives for Bonding Plastics Structural adhesives for bonding plastics • Cyanoacrylate • Epoxy • Polyurethane • Reactive acrylic • Light curing adhesive (acrylic and cyanoacrylate) Nonstructural adhesives for bonding plastics • Synthetic and natural elastomers • Thermoplastic hot melts • Resin latex adhesives • Silicone Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS JOINING [...]... Terms of Use as given at the website Source: Handbook of Plastics Technologies CHAPTER 8 PLASTICS RECYCLING AND BIODEGRADABLE PLASTICS Susan E Selke Michigan State University East Lansing, Michigan 8.1 INTRODUCTION Plastics recycling continues to grow around the world, although it has declined to some extent in the United States in the last several years Interest in and availability of biodegradable plastics. .. sustainability of products and processes is becoming a concern, fueling increased interest in biobased products of various kinds, including plastics These biobased plastics may or may not be biodegradable, just as biodegradable plastics may or may not be biobased This chapter begins with an overview of municipal solid waste and the contribution that plastics of various types make to it Recycling of plastics. .. percent of the parent plastic can be obtained Solvent cements also offers aesthetically pleasing bonds with reasonable assembly time and cost The major disadvantage of solvent cementing is the possibility of stress cracking or crazing of the part and the possible hazards of using low-vapor-point solvents When two dissimilar plastics are to be joined, adhesive bonding is generally desirable because of solvent... Terms of Use as given at the website PLASTICS JOINING PLASTICS JOINING 7.61 Nylon parts can be mechanically fastened by most of the methods described in this chapter Mechanical fastening is usually the preferred method of assembly, because adhesives bonding and welding often show variable results, mainly due to the high internal moisture levels in nylon Nylon parts can contain a high percentage of absorbed... resin surfaces to their sealing temperature of 223°C Contact times as short as 0.5 s are sufficient for small parts Pressures of 300 to 400 psi are generally adequate Parts my be heat treated for stress relief at 120 °C for several hours after welding However, this stress relief step is often unnecessary and may lead to degraded impact properties of the parent plastics Methylene chloride is a very fast... energy director, or small triangular tip in one of the parts, is necessary All of the ultrasonic energy is concentrated on the tip of the energy director, and this is the area of the joint that then heats, melts, and provides the material for the bond Ultrasonic welding is considered a faster means of bonding than direct heat welding Ultrasonic welding of parts fabricated from ABS, acetals, nylon, PPO,... of the bond Pressure during cooling should not be greater than 100 psi Polycarbonate parts having thickness of at least 40 mils can be successfully hot gas welded Bond strengths in excess of 70 percent of the parent resin have been achieved Equipment should be used capable of providing gas temperature of 315 to 648°C As prescribed for the heated tool process, it is important to adequately predry (120 °C)... methods of heating because of the joint design or nature of the final product Infrared radiation (IR) is generally supplied by high-intensity quartz heat lamps IR can penetrate into a polymer without contact with the surface of the part The depth of penetration depends on many factors, and it varies strongly with polymer formulation Laser welding is also a noncontact process for welding thermoplastics... to form the bond The bond strength is determined by diffusion of polymer from one surface into another instead of by the wetting and adsorption of an adhesive layer It is possible to weld plastics of different types However, for both thermal and solvent welding, the success of the process will be heavily determined by the compatibility of the polymers being joined With thermal or solvent welding, surface... psi A mixture of 95 percent chloroform and 5 percent carbon tetrachloride is the best solvent system for general-purpose bonding, but very good ventilation is necessary Ethylene dichloride offers a slower rate of evaporation for large structures or hot climates 7.6.13 Polyphenylene Sulfide (PPS) Being a semicrystalline thermoplastic, PPS is not ideally suited to ultrasonic welding Because of its excellent . with one of the plastic parts, and the other part is fixed firmly. The horn and the part to which it is in contact vibrate suf- ficiently fast to cause great heat at the interface of the parts being. is determined by diffusion of polymer from one sur- face into another instead of by the wetting and adsorption of an adhesive layer. It is possi- ble to weld plastics of different types. However,. of 1000 psi or greater. This is often sufficient to cause failure of the plastic substrate when the bond is TABLE 7.14 Common Families of Structural and Non- structural Adhesives for Bonding Plastics Structural