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Gluing of Engineering Plastics Gluing Guide 2 Gluing Guide 2 Gluing Guide Cover

Gluing of Engineering Plastics Gluing Guide 2 Gluing Guide 2 Gluing Guide Cover photo courtesy Henkel 3 Table of contents 1. Introduction 2. Solvent bonding 2.1 Principle 2.2 Solvents 2.3 Procedure 2.4 Design for solvent bonding 3. Adhesive bonding 3.1 Principle 3.2 Adhesive types 3.3 Compatible adhesives and substrates 3.4 Adhesive properties 3.5 Pre-treatments 3.6 Recommendations for DSM products 3.7 Design for adhesive bonding 3.8 Quick adhesive test 3.9 How to choose the best adhesive? 4. Double-sided tape 5. Surface wetting 6. Cleaning 4 Gluing Guide 1. Introduction Gluing for assembly of plastic parts is an effective method of making permanent connections. This method produces esthetic clean looking joints with low weight and sufﬁ ciently strong connections. This is a very effective joining method for heat sensitive plastics that would normally deform if welded. In order to achieve a strong bond, it is important that the selection of the glue, the application technique and the shape of the connection are all integral parts of the design process. There are three different gluing processes: solvent bonding, adhesion bonding and bonding with double sided tape. These will be discussed in more detail in the chapters 2, 3 and 4. Surface wetting and cleaning, which are important for all gluing processes, are discussed in the chapters 5 and 6. 2. Solvent Bonding 2.1. Principle Solvent bonding or solvent welding is a process in which the surfaces of the parts to be joined are treated with a solvent. This swells and softens the surface and by applying pressure to the joint and with the evaporation of the solvent, the two surfaces bond. Adhesives are not used. The process is commonly used with amorphous thermoplastics such as Xantar. Speciﬁ c advantages of solvent bonding are: - homogeneous distribution of mechanical loads - good esthetics / no special requirements to hide the bond - economic assembly - low weight, no heavy screws, bolts and nuts - able to join heat sensitive constructions or materials which welding would distort or destroy - good sealing and insulating properties. Potential limitations are: - entrapment of solvent in the joint - stress cracking or crazing - dissimilar materials can only be joined if both are soluble in a common solvent or in a mixture of solvents - differences in thermal expansion of components are not compensated in a thick adhesive layer if dissimilar materials are bonded - reproducibility / process control - curing time - no disassembly possible - assembly hazards such as ﬁ re or toxicity. 5 2.2. Solvents Suitable solvents for bonding selected DSM products are given in the table 1. Arnite and Arnitel are generally bonded by other techniques such as adhesive bonding. Different solvents can be mixed to produce a mixture with optimal properties. For instance, if two dissimilar materials are to be joined, a mixture of two miscible solvents speciﬁ c to the different polymers can be used. A mixture of methylene chloride and ethylene dichloride is sometimes used for Xantar polycarbonate and polycarbonate blends. Methylene chloride evaporates faster than ethylene dichloride. A longer assembly time is therefore required if ethylene dichloride has been added. Table 1. Suitable solvents for some DSM products. A slurry made of solvent and up to 25% of the base resin can be used to produce a smooth ﬁ lled joint when the mating parts do not ﬁ t perfectly. Adding base resin makes the solvent easier to use. It is important to consult the Material Safety Data Sheet of the solvent used, for health and safety information and for proper handling and protection equipment. 6 Gluing Guide 2.3 Procedure Good wetting of the surface with the solvent is a requisite to achieve good solvent bonding. Chapter 5 describes the key parameters that are involved in surface wetting. The mating surfaces must be clean and free of grease before bonding. Cleaning with a suitable solvent may be necessary, see par. 6. Parts that have a single joining surface are simply pressed against a sponge or felt pad that has been impregnated with solvent. The quantity of solvent used should be kept to a minimum to avoid drips and crazing. More complex multiplane joining surfaces require contoured solvent applicators made from wood or a metal. It may be necessary to allow a few seconds to ensure sufﬁ cient swelling. The parts are then clamped together with a moderate pressure. The parts are removed from the clamping equipment and must not be used for a period of 24 to 48 hours to ensure that full strength has been attained. Heat can be used to accelerate the overall rate of evaporation and reduce the cycle time. 2.4 Design for solvent bonding The load on the assembly can be applied in several ways as indicated in ﬁ gure 1. Figure 1. The load can be applied in several ways. General design guidelines are: - design for lap-shear loads - maximize the bonding surface; for instance, use a scarfed or a dovetail joint - avoid stress concentrations at thick-thin sections - ensure that there is sufﬁ cient venting. Scarf or dovetail joints should be relatively shallow, so that solvent entrapment is avoided. Entrapped solvent can cause crazing over time and lead to part failure. The parts should therefore be molded with a minimum of internal stress. Gates should be located away from the areas to be bonded. Caution should also be exercized when working with “closed” parts, to avoid getting solvent trapped inside the part. The designer should take account of the fact that the material strength in the bond between two parts made of glass ﬁ bre reinforced materials will never exceed the material strength of the matrix material, because the glass ﬁ bres do not bridge the gap between the two parts. 7 3. Adhesive Bonding 3.1. Principle The main criteria for achieving good adhesive bonding are surface wetting and curing of the adhesive. Important variables for the application of an adhesive and distribution on a substrate are surface wetting, adhesive viscosity and chemical resistance of the substrate to the adhesive. The principles of surface wetting are described in more detail in paragraph 5. In general, adhesion is based on various mechanisms as shown (see ﬁ gure 2). - Mechanical interlocking can contribute to the strength of the bond if the substrate surface is rough, thus enabling the glue to ﬂ ow into the micro holes. - The same physical forces of the wetting process also play an important role for the adhesion of the cured glue to the substrate. A good adhesion between the molecules of the substrate and the glue contributes to the bonding strength. When polair groups (e.g. –COOH, -C=O, -NH, -NH2) are present at the surface of a substrate a strong interaction is possible between the glue and the substrate. - In some cases there is a chemical reaction between the reactive groups of the glue and the substrate. Very high bonding strength can be achieved with a relatively small amount of these chemical bonds. - Molecular interdiffusion can also occur on a molecular level. An “interpenetrating network”. This could happen when the surface of a plastic dissolves in the glue. A kind of third phase is formed between the substrate and the glue where the polymer chains are mixed on a molecular level (interdiffusion). Figure 2 Different adhesion mechanisms. 8 Gluing Guide Molecular interdiffusion is limited by crystallites, therefore it is more difﬁ cult to achieve good adhesion on semi-crystalline thermoplastics compared to amorphous ones. Adhesion on non-polar thermoplastics, e.g. polyoleﬁ ns, will improve considerably when the surface is pretreated using corona, UV, plasma or ﬂ ame treatments. Poor bonding occurs when the adhesive layer does not stick properly to the substrate. Pretreatment may be helpful, e.g. cleaning, degreasing and sanding, see paragraphs 3.4 and 5. Speciﬁ c advantages of adhesive bonding are: - application on various substrates like thermoplastics, thermosets, elastomers and metals - homogeneous distribution of mechanical loads - differences in thermal expansion of components can be compensated by using a thick adhesive layer - good esthetics / no special requirements to hide the bond - economic assembly - low weight, no heavy screws, bolts and nuts - able to join heat sensitive constructions or materials, which welding would distort or destroy - no thermal stresses introduced - good sealing and insulating properties - in many cases cheaper (no high investment costs, no additional costs of metal parts). Potential limitations are: - long term behavior may not be very good - stress cracking or crazing of the plastic may occur - dissimilar materials can only be joined if both are compatible with the adhesive - reproducibility / process control - curing time can be long, depending on the adhesive - no disassembly possible - assembly hazards such as ﬁ re or toxicity - sometimes requires a complex process (pretreatment, special equipment, curing) 9 3.2 Adhesive types A wide variety of adhesives are commercially available. Epoxy Various epoxy adhesives are available, with different characteristics and properties. The uploads/Geographie/ gluing-guide 2 .pdf