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Sodium TreatmentSodium treatment is carried out by immersing the substrate in anaggressive etching solution containing either a sodium-naphthalenecomplex dissolved in tetrahydrofuran or a sodium-ammonia complexdissolved in ammonia. The etching process results in the dissolutionof the amorphous regions of the substrate’s surface, consequentlyincreasing the substrate’s surface roughness and potential formechanical interlocking. Moreover, sodium treatment introducesunsaturated bonds, carbonyl groups, and carboxyl groups to thesubstrate’s surface, which increases the substrate’s reactivity andwettability. Due to carbonaceous residue which results from thedefluorination of the surface, sodium treatment darkens the surfaceto an approximate depth of 1 micrometer. The on-part life of thetreatment is very long (years), however, heating and UV exposurerapidly degrade the treated surface. Major disadvantages of usingsodium treatments are extended exposure to the solution will resultin a substantial degradation of the substrate’s surface, the etchantsare highly hazardous, and the solution degrades very rapidly in thepresence of oxygen.Common uses: FluorocarbonsSurface GraftingSurface grafting is accomplished by grafting a chemical species to thesubstrate’s surface which increases the substrate’s bondability. Forexample, polyethylene can be exposed to gamma radiation in thepresence of vinyl acetate monomer, which then becomes chemicallygrafted to the polyethylene surface.Common uses: Vinylic Compounds on PolyolefinsSurface RougheningSurface roughening is a simple, low cost method of increasing thebondability of many plastics. Surface roughening increases thebondability by dramatically increasing the number of mechanicalinterlocking sites.Common uses: Effective for many plasticsThermal TreatmentThermal treatment increases the bondability of plastics by exposingthe plastic to a blast of hot air (approximately 500°C), which oxidizesthe surface. This mainly introduces carbonyl, carboxyl, and amidegroups to the surface, but some hydroperoxide groups are alsoformed. Very similar to flame treatments, this process also utilizes afree radical mechanism accompanied by chain scission and somecrosslinking. The improved bondability results from increasedwettability, due to the introduction of polar groups, and interfacialdiffusivity, caused by chain scissions.Common uses: PolyolefinsTranscrystalline GrowthTranscrystalline growth improves bondability of a plastic by moldingadherends against a high energy metallic substrate that inducestrancrystalline growth in the plastic’s surface regions. The metallicsubstrate induces the formation of crystallites at the plastic’s surfaceand results in rod-like or columnar spherulites that form inward fromthe interface. This is thought to strengthen the surface by driving lowmolecular weight material into the interior. In addition, some metallicsubstrates may oxidize the plastic’s surface, resulting in a substantialincrease in the reactivity and wettability of the plastic’s surface. Theeffectiveness of this treatment is dependent on such moldingconditions as the cooling rate and mold surface.Common uses: Polyolefins, Polyamides, PolyurethanesUV ExposureUV exposure increases the bondability of plastics by irradiating themwith high intensity UV light. However, the effectiveness of UV exposureis very dependent on the wavelength of light being used. For example,light with a wavelength of 184 nm will crosslink the surface ofpolyethylene, while light at 253.7 nm will not. UV irradiation causeschain scissions, crosslinking and oxidation of the polymer’s surface,even in inert gases. Many different mechanisms describing why UVexposure increases the bondability of plastics have been proposed,including: increasing the wettability; strengthening the plastic’sboundary layer through crosslinking; and inducing hydrogen bonding.The predominant view is that the bondability is improved by theformation of polymeric scission products, which promote interfacialflow, interdiffusion, and polar interactions.Common uses: Polyolefins76The Loctite ® Design Guide for Bonding Plastics, Volume 4
Adhesive Joint DesignIntroductionIn this section, the terms and concepts related to joint design aredivided into three categories which include:• Types of Joints• Joint Stress Distribution• Design GuidelinesSCARF JOINT: A scarf joint is an angular butt joint. Cuttingthe joint at an angle increases the surface area.Before looking at different types of joints, a few termsneed to be explained:Joint: A joint is the location where an adhesive joins two substrates.Joint Geometry: Joint geometry refers to the general shape of anadhesive bond. Is the shape of the bond long and narrow, short andwide, thick or thin?STRAP JOINT (SINGLE OR DOUBLE): A strap joint is a combinationoverlap joint with a butt joint.Types of JointsThe specific types of joints which will be examined in thissection include:• Lap/Overlap• Joggle Lap• Butt Joint• Scarf Joint• Strap/Double Strap• CylindricalLAP/OVERLAP JOINT: A lap joint, also called an overlap joint, isformed by placing one substrate partially over another substrate.CYLINDRICAL JOINT: A cylindrical joint uses a butt joint to join twocylindrical objects.JOGGLE LAP JOINT: The joggle lap joint is an offset joint andis very similar to the lap joint.BUTT JOINT: A butt joint is formed by bonding two objects end to end.The Loctite ® Design Guide for Bonding Plastics, Volume 4 77
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Cyanoacrylate AdhesivesGeneral Desc
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Silicone AdhesivesGeneral Descripti
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Page 77: Surface TreatmentsAdhesive Abrading
Page 81 and 82: TitleDesign GuidelinesEngineers mus
Page 83 and 84: Shortening bond overlaps reduces bo
Page 85 and 86: Softening and weakening of the surf
Page 87 and 88: Determining The Test MethodThe lap
Page 89 and 90: Block Shear Test Method1. Assemblie
Page 91 and 92: ViscosityViscosity is a product pro
Page 93 and 94: Index of Trade NamesTrade Name Plas
Page 95 and 96: Trade Name Plastic Type Manufacture
Page 97 and 98: AcknowledgmentsThis manual would no
Page 99: For more information, please call 1
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