Epoxy adhesives levels

Table 12.8 shows the effect of three commercially available substituted ureas on shelf life, cure rate, exotherm, and glass transition temperature of a dicyandiamide cured epoxy adhesive. The accelerators are compared at use levels of 1, 3, and 5 pph in a one-component adhesive consisting of 100 pph of DGEBA epoxy, 8 pph of dicyandiamide, and 3 pph of... 

Room temperature curing, two-part epoxy adhesives are usually considered to have a lower level of outdoor performance than the heat cured adhesives. Their performance, however, can be improved by heat treatment following the room temperature cure. 

Adhesive compounding also occurs at the end-user level. This is generally done when the end user cannot find a product to meet his or her individual needs, to reduce cost associated with a middleman, or to make use of other materials (e.g., scrap resin, fillers, etc.) that are generated during the course of business. Epoxy adhesives are more conducive to the end user or in-house processor than other polymeric adhesives because of the ease with which the liquid resins can be mixed with other ingredients. Many of these operators buy raw materials from various suppliers and do their own blending and formulating. 

The following section looks at the processes and equipment commonly used in the compounding of epoxy adhesive systems. Since they can be utilized by either the formulator or the end user, there will be no strict division of the discussion by user. Hazards and safety issues related to these materials and processes are discussed somewhat in this chapter, but a more thorough discussion is found in Chap. 18. Test methods, quality control processes, and standards that are commonly used at the formulation level are addressed in Chaps. 19 and 20. 

There are a number of processes that can be used to limit the exposure to chemicals that are commonly used in formulating epoxy adhesives. In general, the employer is required to protect his or her workers from being exposed to any hazardous chemical over the permissible exposure level. 

The next generation of adhesives (1970s) saw further improvements in purification of epoxy adhesives in which the chloride, potassium, sodium, and other ions were reduced to levels below 20 ppm. Outgassing was also reduced by changes in the hardeners used to cure the epoxies and by optimizing the cure schedules. The first specification for the qualification of adhesives was issued during this period by NASA s Marshall Space Flight Center in 1978. 

High levels of chloride ions, above 600 ppm, and sodium ion levels above 100 ppm were reported for the early first generation epoxy adhesives. The epoxy resin itself is a source of both chloride and sodium ions if not properly purified. Most epoxy resins are synthesized from epichlorohydrin and bisphenol-A in the presence of sodium hydroxide and produce sodium chloride as a byproduct (see Chapter 3). 

Fig. 3. Toughening of Epoxy Adhesive with CTBN at Different Level of Cure Agent (cold rolled steel, cured at 120 C for 3 hrs.)...

Figures 10.3 and 10.4 show the degradation of the epoxy adhesive bond due to the effect of temperature on the anodic aluminum joint at 95% RH. The plots show that those joints exposed at 23 C and 95% RH degraded rapidly when the stress level exceeded a force of 3914 N, which would be...

Fig. 1. XPS survey scan of the surface of an epoxy adhesive cured in contact with PTFE. The PTFE film became detached in a peel test at very low load, but some transfer to the epoxide has taken place. Characteristic core level peaks (e.g. F Is), Auger peaks (F KLL) and valence band peaks (VB) are all present in this spectrum...

Corrosion resistance can be improved in epoxy adhesives by the addition of suitable fillers. DGEBA-resin-based adhesives show improved resistance to anodic debonding when used to bond steel/polyolefin laminates by the incorporation of aluminum oxide.Levels of aluminum oxide can be varied from 3-40 weight % of the total adhesive composition. 

One drawback with addition-type polymers is that the cured adhesive tends to be brittle. Attempts to toughen addition polyimides have met with some success, though improvements in this area are needed. The development of systems exhibiting the same level of toughness as today s state-of-the-art epoxy adhesives is a goal for the future. 

A configuration frequently used in cyclic stress testing is the reduced area adhesive bond (RAAB) specimen. This is similar to the blister detection specimen, but is 1/2 inch wide and a 1/4-inch hole is drilled through the center of the overlap region this provides more environmental exposure of the bondline. Potter has reported using RAAB specimens to compare the durabilities of several modified epoxy adhesives.Specimens subjected to cyclic stressing failed in fewer hours than specimens held at a constant stress, even when cycled at a lower stress level. He also found that a slow cyclic rate was more damaging than a fast cyclic rate. 

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