Bond line temperature

Bond line temperatures are best measured by thermocouples placed very close to the adhesive. In some cases it may be desirable to place the thermocouple directly in the adhesive for the first few assemblies being cured. 

If a cure of 60 min at 150°C is recommended, this does not mean that the assembly should be simply placed in a 150°C over for 60 min. The temperature is to be measured at the adhesive bond line. A large part will act as a heat sink and may require substantial time for the adhesive in the bond line to reach the necessary temperature. In this example, total oven time would be 60 min in addition to whatever time is required to bring the adhesive up to 150°C. Bond line temperatures are best measured by thermocouples placed very close to the adhesive. In some cases, it may be desirable to place the thermocouple in the adhesive joint for the first few assemblies being cured. 

There is more than one correct cure cycle for EVA. Like most polymers with peroxide promoters, it is good practice to raise the bond-line temperature rapidly to avoid peroxide decomposition before an adequate cure has been obtained. One cycle that has been proven uses two steps, one at 100 C for evacuation and adhesion, the other at 150°C for long-term oven cure. This cycle provides a high throughput with only one laminator. 

Overlap shear (OLS) made by bonding 20 mil etched aluminum to 63 mil etched aluminum using 160°C bond-line temperature, 2 secs dwell, 14 lbs gauge pressure. (2) Peel bonds made by bonding 4.5 mil aluminum foil to test substrates using 320°F (160°C) bondline temperature, 2 secs dwell, 14 lbs gauge pressure. (3) Adhesion tests done at 2 in./min for peel, 2 in./min for OLS. 

The load-bearing capacity of an adhesive reduces rapidly as its Tg (glass transion temperature) is exceeded, so only a few individual formulations - even the toughened ones - can perform well when bond line temperature exceeds 120°C. 

Cured rubbers may be bonded to metals or to cured or uncured dissimilar rubbers with the aid of Chemosil bonding agents. Bonding takes place using heat and pressure, and bond line temperatures in excess of 100 °C are usually required. 

The component assembly should be heated to give a bond-line temperature of between 100 °C and 160 °C for 15 to 60 minutes. Individual cure cycles are determined by the component size and the method of heating. Experimentation is often required to identify the most effective bonding conditions. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

If blocking agents come off too quickly, foaming in the bond line can result, especially if the substrates are non-porous. One exception to this rule is E-caprolactam. It remains in the adhesive and can act as a plasticizer, which can aid adhesion at elevated temperatures. 

Two-component systems consist of (1) polyol or polyamine, and (2) isocyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components, they can be used without addition of solvents. The mass ratio between the two components determines the properties of the bond line. Linear polyols and a lower surplus of isocyanates give flexible bond lines, whereas branched polyols and higher amounts of isocyanates lead to hard and brittle bond lines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature and the addition of catalysts. The pot life can vary between 0.5 and 24 h. The cure at room temperature is completed within 3 to 20 h. 

Plasticizers soften the film and increase the adhesion and the setting speed. The most common are phthalates, adipates and benzoates. The amount added can be in a broad range of 10-50%. They affect the swelling and softening of the PVAc emulsion particles, ensure film formation at room temperature, and the tack of the still wet adhesive. They also provide improved moisture resistance of the bond. Disadvantages are the lower resistance of the bond line against heat, possible migration of the plasticizers and enhanced cold flow. 

Bonded silver linings are fabricated for mild steel or copper vessels. They are soldered in situ to the walls of the vessel by means of a special tin-silver solder. The melting point of this solder is approximately 280°C, and 200°C is recommended as the maximum continuous operating temperature for linings bonded with it. Since the whole of the silver is firmly adherent to the vessel, bonded linings are suitable for operation under vacuum conditions, and provide excellent heat-transfer characteristics. 

Pot lives of DETA and TETA adhesives are on the order of 20 to 30 min at room temperature. When mixed with DGEBA epoxy resins in large batches, the exotherm can be significant due to the reactivity. This generally limits the amount of mixed adhesive that can be prepared at one time, and it also limits the amount (mass) of adhesive that can be applied to a joint [although often thin bond line and the thermal conductivity of the substrate (e.g., metals) will diminish exotherm effects]. 

The type and amount of fillers are chosen so that a practical bond line thickness will result after application of the necessary pressure (usually only contact pressure, approximately 5 psi) during cure. Ordinarily, the objective is a bond line thickness of 2 to 10 mils. Consideration, of course, must be given to the curing temperature. Viscosity of the formulation could drastically be reduced at elevated temperatures, and unless there is a furrow designed into the joint to contain the adhesive, much of the adhesive could flow out of the joint area before the adhesive is completely cured. 

Benzoquinone tetracarboxylic acid dianhydride (BTDA) has been found to provide epoxy adhesives with excellent high-temperature properties, in both the short and long terms. The formulation described in Table 12.12 provides good resistance to 260°C. This two-part adhesive can be cured 2 h at 200°C. The disadvantage of BTDA is that relatively high cure temperatures are required that result in a high degree of internal stress within the bond line. 

The high-temperature resins described above provide the main elements in the adhesive formulator s recipe. However, there are also additives, fillers, etc., that can further enhance the thermal properties of more conventional epoxy adhesives. These additional components improve thermal resistance by providing oxidation resistance, toughening, and control of bond line stress. 

The amount of the applied adhesive and the final bond line thickness must be monitored because they can have a significant effect on joint strength. Curing conditions should be monitored for pressure, heat-up rate, maximum and minimum temperatures during cure, time at the required temperature, and cool-down rate. The primary concerns are to ensure the following ... 

Increase pressure. Check that clamps are seated properly Modify clamps or check for freedom of moving parts Use higher curing temperature. Check that temperature at the bond line is above die minimum specified throughout the curing cycle... 

The primary amine then can react with another isocyanate to produce a urea. Thus one mole of water consumes at least two moles of isocyanate, builds molecular weight, and liberates carbon dioxide in the process. If this happens in a closed container such as a drum of adhesive, the result can be explosive, particularly as the reaction mass rises in temperature because of the exothermic reactions. On the other hand, these reactions can be useful in a bond line because under the proper conditions desirable urea linkages can be introduced into the curing adhesive through the reactions of small amounts of water normally present on the adherend surfaces. 

According to this model, the temperature dependence of molecular motions for adsorbed and non-adsorbed chain units in filled PDMS containing hydrophilic Aerosil is shown in Fig. 9 [9]. The lowest temperature motion is a C3 rotation of the CH3 groups around the Si-C bond (line 1 in Fig. 9). The rate of the a-relaxation (points 2 in Fig. 9) in filled PDMS is close to that for unfilled sample (line 2 in Fig. 9). It has been proposed that independence of the mean average frequency of a-relaxation process on the filler content in filled PDMS is due to defects in the chain packing in the proximity of primarily filler particles [7]. Furthermore, the chain adsorption does not restrict significantly the local chain motion, which is due to high flexibility of the siloxane main chain as well as due to fast adsorption-desorption processes at temperatures well above Tg. 

The objectives of this test pattern is to analytically resolve these problems into three manageable segments. The first task will be to define the viscoelastic kinetic properties of a material as a function of various reaction temperatures. These properties (viscosity, viscous modulus, elastic modulus, tan delta) define the rate of change in the polymers overall reaction "character" as it will relate to article flow consolidation, phase separation particle distribution, bond line thickness and gas-liquid transport mechanics. These are the properties primarily responsible for consistent production behavior and structural properties. This test is also utilized as a quality assurance technique for incoming materials. The reaction rates are an excellent screening criteria to ensure the polymer system is "behaviorally" identical to its predecessor. The second objective is to allow modeling for effects of process variables. This will allow the material to undergo environmental... 

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