Epoxies single-part

Adhesives. Two single part, rubber toughened hot cured structural epoxy adhesives were used throughout the current research. Both adhesives were provided by Dow Automotive,... 

Single part epoxy presents another option. Some materials, such as surface mount adhesives, have the curing catalyst locked in a phase change material that is released with heat. Cure temperatures range from 100 to 150°C. The material supplier controls the mixing process, and these materials have excellent shelf life. 

Some single part epoxies are premixed and frozen at temperatures ranging from -5 to -40°C. Freezing the material essentially stops the curing reaction. They are designed to have a working life of 4-24 h after thawing to ambient temperature. 

Epoxy-based systems are the most widely used structural adhesives. They are encountered everywhere in car manufacture, aircraft, building and construction industries, metal fabrication and the home. They are available as single-part and two-part. 

Single-part epoxy adhesives are available as liquid, paste or film and require heat to cure. The resin and hardener (or catalysed) are... 

Dicyandiamides. These are latent catalysts which are inactive at ambient temperatures but are released for reaction at about 90 °C at temperatures approaching 150 °C and above they react rapidly. They are primarily used in single part epoxy adhesives and require storage in cool conditions in order to remain stable for a reasonable shelf life. 

It will be noted that these HDT values lie in the range 34 to 48 C which for many civil engineering applications may not be much in excess of anticipated maximum service temperatures. For example, on the soffits of concrete bridges temperature extremes in the UK may lie between -20 °C and +38 °C (25). In steel bridges maximum temperature extremes of 60 to 65 "C may occur locally and this is one reason why single part hot cure epoxy products which have higher Tg values of the order of 100 °C and more are preferred in such situations. 

The above approach has not, to date, been extended to include steel, although considerable effort is expended in its pre-treatment prior to painting. The current practice is to weld bond oil steel using single-part epoxy adhesives curing during the first (electrocoat)... 

Figure 2.5 Stress/strain characteristics of two toughened adhesives. A is a toughened, single-part epoxy with a clearly delineated elastic limit (0). B is a highly compliant, toughened acrylic where the limit of the elastic zone is not clearly defined. Both adhesives fail at X. As the temperature rises, B flattens further and A takes on the B form. As the temperature falls, A becomes steeper and B takes on the A form. By choosing the correct formulation the modulus appropriate to the application may be obtained (see Figure 2.6).

Figure 2.9 In a (opposite) the proportionate relationship of load-bearing capacity with overlap length falls away (at about 10-15 mm in Figures 2.7 and 2.8). The actual performance of a rubber-toughened, single-part epoxy is given for a series of lap joints in b. The strength of the various joints, based on 16 gauge (approx 1 mm) steel is compared with the stress/strain performance of the steel itself.

With this restraint in mind, the most likely order of preference - taking general robustness into account - would be toughened adhesives of the following classes Single-part epoxies, Acrylics, Anaerobics, and Two-part epoxies... 

Toughened single-part heat-cured epoxies... 

An anaerobic adhesive is the best choice for these applications and only under extreme environmental conditions is it necessary to resort to the toughened variants or - in the final resort - the toughened, single-part epoxies (see p.33 Splines, keys and set screws and p.71 Co-axial - splined). 

Note 3 Normally adhesives (non-anaerobic) are not assessed on collar and pin joints -but for comparison with the mechanical and anaerobic techniques the performance of several typical adhesives has been assessed using this method. Acrylic This type of toughened adhesive does not show up well on joints of this size because it is too compliant to be effective on small joint areas. On a large lap joint, a 50 per cent increase could be anticipated. Two-part epoxy The many formulations available could fall anywhere in the band indicated according to their individual characteristics. Single-part epoxy These toughened adhesives offer the maximum perform ance possible and are usually easier to use than two-part types. 

Cam shafts may experience substantial forces, often in demanding environments, and only the toughened, single-part epoxies are generally capable of... 

When these are used, the simpler anaerobics may be adequate, so avoiding the curing cycle of the single-part epoxies. Indeed, it may even be possible to use medium-strength grades to facilitate maintenance. 

Care should be taken over any extreme environmental demands imposed by the fluids within the pipes. Contrary to popular belief, corrosive fluids excepted, water is much more damaging than many industrial chemicals the commonly met hydrocarbons cause little if any difficulty. Generally probably only the strongest and most durable of the anaerobics and single-part epoxies are worth consideration in these applications. Durability will be extended by incorporating the following features in the design ... 

Single-part epoxies will probably give the best performance on bonded copper tubes for use at elevated temperatures. 

This formidable list indicates quite clearly the care that needs to be taken in such a design. Nonetheless, the implementation of known successful designs has led to considerable savings and technical advantages. For example, unlike solder, the single-part epoxies and anaerobic adhesives do not creep at elevated temperatures and their performance can be better. 

Suitable plastics pulleys may be bonded satisfactorily with cyanoacrylate adhesives or, where large components are involved, with toughened acrylics -many of which are unsuitable for use on small parts. The stronger anaerobic adhesives and single-part epoxies should be used for all-metal assemblies. 

Finally, the temperature rise in a stalled motor must be considered. If temperatures are likely to exceed 120-150°C, then heat-resisting anaerobic grades or single-part epoxies should be used. 

As pointed out in Section 1.1, most anaerobic compositions are not true adhesives and, although very stable, they are not currently as robust or durable as the heat-cured, toughened, single-part epoxies. These epoxies are therefore used to replace solder on brass components when only a few threads are available for engagement. The softness of the alloy coupled with the low engagement area militate against anaerobics, especially when it is intended to use these at high temperatures. 

Stainless steel alloys have often proved difficult to bond, but the introduction of the toughened adhesives has alleviated the problem. Both the toughened acrylic and single-part epoxy types will bond these alloys well. Abrasion followed by a solvent wipe appears to be sufficient in most cases, though chemical treatments may be beneficial in more demanding environments. 

Single-part epoxy adhesives are available as liquid, paste or film and require heat to cure. The resin and hardener (or catalyst) are pre-mixed but curing does not occur because the hardener is inactive at room temperature. It only becomes reactive as the temperature is raised, usually above 100 C, forming a hard thermoset polymer which will not remelt on further heating. 

If the high modulus single part epoxy is selected for comparison, the end stresses increase as would be expected but the elastic limit is still too low to permit a completely elastic behaviour (Fig. 28). 

Figure 28 Single part epoxies generally display high elastic limits, but end stresses are...

We will consider a torsional load on a co-axial joint, with an overlap of 25 nun using a single part hot setting epoxy adhesive. The inner tube external diameter is 25 mm and both tubes are 1.5 mm thick steel. A torsional load of 100 Nm and a glue line of 0.05 mm gives a stress distribution as shown in Fig. 30. 

Figure 1.11 The steel bonnet of a tractor and its various stiffeners are bonded with a rubber-toughened, single-part, heat-cured, epoxy edhesive.

Figure 1.12 A modified single-part, heat-cured, epoxy adhesive is used in the productionline assembly of steel car bodies, and here the roof section is about to be bonded into place. 

Epoxy resins typically uses a two-part resin and hardener or single part cured by heat for large structural applications. 

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