Bonding, adhesive heating equipment

Heating equipment. Many structural adhesives require heat as well as pressure. Most often the strongest bonds are achieved by an elevated temperature cure. With many adhesives, trade-offs between cure times and temperature are permissible. But generally, the manufacturer will recommend a certain curing schedule for optimiun properties. 

The types of equipment available to assist in the bonding operation, ie jigs, fixtures, adhesive rollers, ovens, other types of heating equipment, presses, etc. 

Such adhesives are required to remain stable at elevated temperatures over periods of several hours (during normal daily operation of production equipment) and formulations that decompose under such conditions or in which pronounced changes of viscosity occur are not suitable. In order to achieve the bond strength required the adhesive must wet properly the substrates as soon as it is applied—so the temperature of the substrates can be important (if too cold they may absorb heat, cause cooling of the adhesive prematurely—before the surfaces are wetted—and impede bonding). 

Epoxies are especially reliable when used with epoxy-based composites because they have similar chemical characteristics and physical properties. Room temperature curing adhesives are often used to bond large composite structures to eliminate expensive fixtur-ing tools and curing equipment required of higher-temperature cure adhesives. However, room temperature epoxies require long cure times, so they are not suitable for large, highspeed production runs. Some of the lower-temperature composite materials are sensitive to the heat required to cure many epoxies. Epoxies are too stiff and brittle to use with flexible composites. 

Films. Both structural and nonstructural adhesives are commonly available in film form. Adhesives applied in the form of dry films offer a clean, hazard-free operation with minimum waste and excellent control of film thickness. However, the method is generally limited to parts with flat surfaces or simple curves. Optimum bond strength requires curing under heat and pressure, which may involve considerable equipment and floor space, particularly for large parts. Film material cost is high in comparison to liquids, but waste or material loss is the lowest of any application method. 

After the adhesive is applied, the assembly must be mated as quickly as possible to prevent contamination of the adhesive surface. The substrates are held together under pressure and heated if necessary until cure is achieved. The equipment required to perform these functions must provide adequate heat and pressure, maintain constant pressure during the entire cure cycle, and distribute pressure uniformly over the bond area. Of course, many adhesives cure with simple contact pressure at room temperature, and extensive bonding equipment is not necessary. 

Both induction and dielectric heating involve relatively expensive capital equipment outlays, and the bond area is limited. Their most important advantages are assembly speed and the fact that an entire assembly does not have to be heated to cure only a few grams of adhesive. 

Plasticized sulfur is a hot-melt adhesive and fulfills more of these requirements than any presently available, commercial glue. However, modified sulfur cannot be applied with present production equipment, and sulfur lacks many important, frequently required qualifications, among them fire resistance and resistance to heat. Thus, substantially more work would be necessary to make elemental sulfur a viable commercial bulk glue. However, we found that elemental sulfur can be polymerized in situ with formaldehyde resins (5,33), and this yields bonds which have good mechanical properties, moisture resistance, and promising high-temperature behavior. Such glues can be handled with presently available equipment and presently common process conditions (35,36). These materials are described separately (33). 

Ease of fabrication is one of the many advantages of polyethylene foam. It can be skived to precise thickness, cut and shaped to form custom parts, and joined to itself or to other materials without major investment in complex equipment. It can also be vacuum formed. Expanded polyethylene will adhere to itself by the use of heat alone. Hot air, or a plate heated to approximately 350°F (177°C) can be used to simultaneously heat the surfaces of two sections of foam to be joined. Upon softening, the two pieces are quickly joined together under moderate pressure, and an excellent bond formed, with only a short cooling period required. Release of the melted foam is aided by a coating of fluorocarbon resin or silicone dispersion on the heating surface. The foam may also be bonded to itself and to other materials by the use of solvents or commercially available adhesives (6). 

In the heat of solubility method for adhesion determination, the film material is chemically dissolved from the substrate, e.g. by an exothermic reaction. The total energy thus converted consists of the energy released by the dissolving of the pure film material less the energy required to break the bonds between film and substrate. To enhance the contribution from the adhesion the ratio of the number of molecules on the film/substrate interface should be as high as possible in relation to the total number of molecules in the film. The thermal yield of the reaction is determined with a microcalorimeter, although tests with coated metal films on NaCl substrates have shown the sensitivity of such equipment to be, as yet, insufficient for exact measurements [74,75]. 

Assembly of silicon chips onto substrates with anisotropically conductive adhesives uses specialized equipment, initially developed for ffip-chip solder and TAB inner lead bonding. Heat and pressure are transmitted to the adhesive through a thermode attached to a robotic arm or a high-precision linear translator. Equipment requirements are more demanding than for solder assembly, as no self-alignment can occur. A minimum placement accuracy of 0.0005 in. is required. Coplanarity between the substrate and die is critical one study reports maintaining coplanarity to within 0.00004 in. [19]. The pressure required to achieve interconnection depends on the size of the die, the type of conductive particle used, and the viscosity of the adhesive at the bonding temperature. 

Laminates are composites made by combining two or more natural or artificial materials to maximise the useful properties of the components and minimise the weaknesses of individual components. A laminate consists of one or more sheets of fibres of one or more materials permanently bonded together by heat, pressure, welding, or adhesives. Different laminate stractmes that are used in cut/stab resistant personal protective equipment (PPE) are discussed below. 

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 ... 

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