Cooling adhesive bonding

The second component of the internal stresses is thermal stress caused by differences of the coefficients of linear thermal expansion of the adhesive and the substrate. They appear in the coiu se of heating or cooling of the adhesive-bonded joint. The mechanism of the internal stresses occurring in adhesive-bonded joints does not generally differ from that in coatings, but because there are two sohd surfaces the magnitude of the stresses in the first case appears to be substantially greater. 

Spruce (Picea abies Karst) lamellas were used to investigate the influence of the heat treatment of wood on the shear strength of the phenol-formaldehyde adhesive bond. Lamellas were heat treated at two temperatures 180°C and 220°C. The process of heat treatment in a vacuum, developed by Rep and co-workers [36], was used. Prior to heat treatment, all lamellas were planed to dimensions of 350 mm x 100 mm X 18 mm, and oven dried at 103°C. The treatment was performed in a vacuum chamber (Kambic, Laboratory Equipment d.o.o., Semic, Slovenia), where an absolute pressure of 5 kPa was achieved. The lamellas were heated to the desired temperatures, which took about 1 h, and then treated for 3 h at a constant temperature. The lamellas were then left to slowly cool down to room temperature. The lamellas were oven dried before and after heat treatment to determine mass loss. Mass loss (ML) after heat treatment was estimated according to the formula ... 

Thermoplastic fusible adhesives (see Hot melt adhesives) may be used in the production of adhesive-bonded non-wovens, as powders, fibres or sheet films. The formation of strong bonds between the fibres occurs as the softened thermoplastic adhesive material resolidifies on cooling. The amount of adhesive incorporated in the web governs the strength of the fabric. 

Two-shot injection molding. Two-shot or overmolding refers to a process wherein either different colors or different materials are molded into one part. In this process, the first material or color is injected, then the mold is rotated, and the second shot is injected as depicted in Fig. 1.67. An alternative method is to use a retractable core." In this case, the first material is injected and cooled to solidify, and then the core is retracted to allow injection of the second material as shown in Fig. 1.68. Bonding is accomplished through either strictly mechanical means or by adhesion between the two components through diffusion of the chains. This can result in parts with two combined materials without the need for an additional adhesive bonding step." In the case in which direct adhesion of the materials is desired, proper selection of compatible materials is required. Table... 

Thermoset plastics are cross-linked by chemical reaction so that their molecules cannot slip by one anotber. They are rigid when cool and cannot be softened by the action of heat. If excessive heat is applied, thermoset plastics will degrade. Consequently, they are not weldable. Because of their chemical resistance, they cannot be solvent cemented. Thermoset plastics are usually joined by either adhesive bonding or mechanical fastening. Typical thermosetting plastics are epoxy, urethane, phenolic, and melamine formaldehyde. 

The process of adhesive bonding can be divided into four steps surface preparation, placing the adhesive, application of heat and/or pressure, and curing (or cooling for thermoplastic adhesives) of adhesive. 

Tables II and III summarize the total adhesive testing done during the overall program. Since at CEL-NBS both gradual- and rapid-cooling tests were to be conducted at -323 F, the comparative effects of different types of thermal stress could be investigated. For a discussion of thermal stresses in adhesive bonds at low temperatures refer to McClintock and Hiza [2].

The total stress on an adhesive bond while cooling to the test temperature is assumed to be the sum of transient stresses due to a thermal gradient across the adhesive layer and interface, plus residual stresses caused by a difference in thermal expansions of adhesive and adherend. The transient stress will vanish after the lower temperature has been attained. This leaves only residual stresses which are present at any temperature except that at which the bond is cured. If this assumption is correct, then the cooling time is not a significant measure of the transient thermal stress. The maximum cooling rate, however, is extremely... 

Free mono- and multilayer films may be adhesive- or extmsion-bonded in the laminating process. The bonding adhesive may be water- or solvent-based. Alternatively, a temperature-dependent polymer-based adhesive without solvent may be heated and set by cooling. In extmsion lamination, a film of a thermoplastic such as polyethylene is extmded as a bond between the two flat materials, which are brought together between a chilled and backup roU. 

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