Thermosetting adhesives requirements

These are materials that cannot be heated and melted after the initial cure. Curing takes place by chemical reactions at room temperature or at an elevated temperature, depending on the type of adhesive. Some thermosetting adhesives require considerable pressure, while others require only contact pressure. Solvents are sometimes added to facilitate application. These adhesives are usually available as solvent-free liquids, pastes, and solids. 

Shelf-Life - the period for which an adhesive may be stored before use. Manufactures provide guidance on the appropriate storage conditions. Some adhesives (notably one-part thermosetting adhesives) require refrigerated storage to prevent them from degrading. 

Wood Composites—these are resin-bonded composite boards where the particles are wood shavings, flakes, chips, or fibers bonded with thermosetting adhesives that can be urea formaldehyde, melamine formaldehyde, phenol formaldehyde, or diisocyanate. In recent years, the markets for OSB and MDF board have been rapidly increasing. Most particle board production uses urea-formaldehyde as a binder that is acid setting. Hence, sodium borates (alkaline) can interfere with the setting. As a result, boric acid has been the major boron compound used as the flame retardant in particle board.28 29 Typically, a loading of 12%-15% of boric acid in MDF is required to meet the ASTM E-84 Class A rating. If sodium borate is used as a flame retardant, phenol-formaldehyde binder, that is compatible with alkaline chemicals, is commonly used. 

The use of formaldehyde-free adhesive resins from whey and whey byproducts for manufacturing construction-quality boards could resolve these problems simultaneously. The demand for formaldehyde-based thermosetting adhesive resins in the United States was estimated to be 1.9 billion pounds in 1983 (5). The anticipated requirement for resins and the potential availability of raw materials from whey are a fortuitous combination. 

Quite frequently, copolymerization is used to optimize the properties of polyacrylates. For example, copolymers of ethyl acrylate with methyl acrylate provide the required hardness and strength, while small amounts of comonomers with hydroxyl, carboxyl, amine, and amide functionalities are used to produce high-quality latex paints for wood, wallboard, and masonry in homes. These functionalities provide the adhesion and thermosetting capabilities required in these applications. Monomers with the desired functional groups most often used in copolymerization with acrylates are shown in Table 15.8. 

Thermosetting matrices, such as epoxies and thermosetting hot-melt adhesives, are used where increased reliability is required. Repair of anisotropically conductive interconnections assembled with thermoset adhesives is problematic, however, as the adhesive matrix must be removed completely from the substrate and device prior to reassembly. 

Most of the glued components produced in Europe for non-structural purposes such as joinery use urea-formaldehyde (UF), poly (vinyl acetate) (PVAc) or polyurethane (PUR) adhesives. These adhesives must meet the performance requirements specified in EN 204 (2002) [15] for thermoplastic adhesives or EN 12765 (2002) [16] for thermosetting adhesives when tested in accordance with the following test methods ... 

Thermosetting adhesives are provided as one- and two-part systems. The one-part systems usually require elevated temperature cure and have a limited shelf life. The two-part systems have longer shelf lives and can usually be cured slowly at room temperature, or somewhat faster at moderately higher temperatures. A disadvantage is their need for careful metering and mixing to make sure that the prescribed proportions are blended and that the resultant mixture is homogeneous. Once the adhesive is mixed, the useful life is limited. ... 

Adhesive, one-component—An adhesive material incorporating a latent hardener or catalyst activated by heat. Usually refers to thermosetting materials, but also describes anaerobic, hot-melt adhesive, or those dependent on solvent loss for adherence. Thermosetting one-component adhesives require heat to cure. 

Thermosetting adhesives are materials that cannot be heated and softened repeatedly after the initial cure. Adhesives of this sort cure by chemical reaction at room or elevated temperatures, depending on the type of adhesive. Substantial pressure may also be required with some thermosetting adhesives, and others are capable of providing strong bonds with only contact pressure. Thermosetting adhesives are sometimes provided in a solvent medium to fadhtate appUcation. However, they are also commonly available as solventless liquids, pastes, and Saftds. 

Silver-coated solid and hollow spheres find wide acceptance in applications in which EMI/RFI and ESD control is required. Silver-coated spheres can provide high compressive strength and excellent processing durability for applications requiring conductive thermoplastics. Other uses include conductive thermosets, adhesives, coatings, caulks, and composites. 

Some adhesives solidify simply by the evaporation of a carrier liquid (water or solvent). Others harden as a result of going from a molten liquid to a cooled solid. Still others solidify by means of a chemical reaction. Some adhesive systems may require several mechanisms to harden. For example, thermosetting waterborne adhesives require the water carrier to first evaporate and then chemical cross-linking occurs before a final bond is achieved. 

Table 61.20 compares the materials. Pressure-sensitive adhesives (PSAs) provide a low-cost solution due to their simple application process. A problem of PSA is creep characteristics under mechanical stress, as PSA cannot maintain the exact positions of the stiffener boards under the stress. Thermosetting adhesives provide a reliable bonding between the layers, but they require a long processing time in a heat press with suitable press pad materials and impose a higher processing cost. 

The requirements that perhaps most clearly define the characteristics needed in a truly structural adhesive are those specifying maximum limits on creep under sustained load. MMM-A-132, for example, specifies that lap shear creep at 75°F can not exceed 0.015 inch in 192 hours under a 1600 psi load. Limits set on creep at higher temperatures help define the service temperature limitations of the specific adhesive. Similarly, Mil-A-25463 sets stringent limits on creep deflection under load on bonded sandwich structure. For all practical purposes the need for creep resistance makes it mandatory that thermosetting adhesive be utilized in aircraft structure. Thermoplastics having the required resistance to creep even at 180°F have melting points that are so high that they would be very difficult to process. 

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