Thermoplastics joint strengths

Thermoplastic adhesives can be apphed as a molten liquid to encapsulate the fabric and then harden by cooling. These heat-activated adhesives generate joint strength almost instantly after cooling. The most common sources of heating include hot air or heated... 

Hot melt adhesives These are linear thermoplastics which are applied to the substrate in the melt. Joint strength develops on cooling. They have low toxicity and are amenable to automation. Examples are copolymers of ethylene and vinyl acetate, some polyesters and some polyamides. 

Stainless steel heating elements can be used alone in welding thermoplastics. Stainless steel heating elements introduce foreign material into the joint but are used to minimize any potential for galvanic corrosion and to reduce fiber motion, which has a detrimental effect on joint strength. The heating element comprises the innermost portion of the weld stack (Fig. 14.36) used in... 

Butt Joint Strength of Thermoplastics from Induction Welding... 

FIGURE 27.8 T-peel strength values of dichloroisocyanurate (DCI)-treated styrene-butadiene rubber (SBR) (R2 rubber) and thermoplastic rubber (TR)/waterbome polyurethane adhesive/canvas joints. Influence of the dichloroisocyanurate (DCI) concentration. (From Cepeda-Jimenez, C.M., Pastor-Bias, M.M., Martfn-Martfnez, J.M., and GottschaUc, P., J. Adhes. Set Technol., 16, 257, 2002.)... 

An early study by Rudkin (1950) showed that substitution of OH groups with acetyl resulted in a significant decrease in bond strength between the wood and a UF resin in lap-joint tests. Vick and Rowell (1990) studied the adhesive bonding of acetylated yellow poplar, with 18 different thermoplastic and thermosetting adhesives. The effectiveness of the adhesives was examined by determination of bond shear strength (and wood failure) of 6 mm thick, bonded wood strips after conditioning at 27 °C and... 

Sulfur s thermoplastic property and its resistance to chemical attack led to its use as a decorative and functional coating (46,47). Unless additives are used to prevent reversion to a crystalline allotrope, the coatings develop pinpoint holes and lose strength when used as a jointing material. When pure sulfur was used to protect concrete for storing hot saline solutions, it failed in less than one year (48). 

Polymer pyrolysis to form advanced ceramics allows the production of highly covalent refractory components (fibers, films, membranes, foams, joints, monolithic bodies, ceramic matrix composites) that are difficult to fabricate via the traditional powder processing route [1-4]. Yajima was the first to demonstrate the feasibility of producing high-strength SiC-based fibers from pyrolysis of polycarbosilane [5]. In this process, a thermoplastic pre-ceramic polymer is first shaped into the desired form, cross-linked into a pre-ceramic network and finally converted into a ceramic material by a pyrolysis process in a controlled atmosphere (Fig. 1). A common feature of the polymer route is the formation of intermediates called amorphous covalent ceramics (ACC) [6]. These are formed after removal of the organic components and before crystallization that occurs at higher temperatures. 

They are used for fast assembly of structures designed to be only lightly loaded. Hot melts can be in the form of blocks, rods, granules, powder and film. The adhesive is heated and applied to one surface as a melt loining is then carried out immediately after application and the adhesive cools and develops strength by its consequent solidification. The quicker the surfaces are joined, the better for the bond. Because the bond strength is reached in seconds, the need for clamps of fixtures is eliminated. These adhesives can be used for joints which may subsequendy need to be detached and/or reattached due to their thermoplastic structure. In use however the bonded joint must not be heated up to its melting temperature. 

It is noted that the residual tensile strength depends primarily on the strength of the thermoplastic particles and the thickness of the thermoplastic layer within the narrowed crack. The reason is that the molten thermoplastic forms a thin film in the narrowed crack under the recovery pressure by the SMPFs, similar to the adhesive layer in an adhesively bonded joint (here the fractured two half beams serve as the adherends). It has been well demonstrated that the tensile and shear resistance of adhesively bonded joints highly depends on the adhesive thickness. A thinner adhesive layer usually leads to higher peel and shear resistance [28-32]. This is one reason why a 100% pre-strained SMPF leads to a higher healing efficiency because... 

Thermoplastic adhesives are not ordinarily recommended for use at above 66°C, although they can be used up to 90°C in some applications. These materials have poor creep resistance and fair peel strength. They are used mostly in stressed joints and designs with caps, overlaps, and stiffeners. The materials most commonly bonded are non-metallic material, especially wood, leather, plastics, and paper. With the exception of some hot-melt adhesives, thermoplastic adhesives are not generally used for structural applications. Examples of thermoplastic adhesives are shown in Table 4.2. ... 

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