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1、1 Epoxy Adhesives That Cure Below Freezing Temperatures SpecialChem | Edward M Petrie - Apr 18, 2011 One of the distinct advantages of epoxy adhesives is that they can be cured at room temperature or even at lower temperatures. Epoxy adhesives are often divided into room temperature curing types and
2、 elevated temperature curing types. Once mixed, the two-part room temperature curing epoxy adhesives are designed to react at ambient conditions and temperatures near room temperature (20 - 25 C) or lower. However, in many cases the cure time may be accelerated with heat, but this is not a requireme
3、nt. Once mixed, the crosslinking reaction begins immediately, and the resulting working life is limited. The cure rate and working life will be dependent on the specific formulation as well as the ambient temperature. Although the adhesive will harden to a handling strength in usually a short period
4、 of time, it will continue to develop strength for a much longer period. Property General Purpose Fast Setting High Performance Viscosity, Pa-sec Resin Hardener Mixed 50 35 45 260 160 250 91 103 54 Mix Ratio, weight 100 resin / 80 hardener 100 resin / 100 hardener 100 resin / 71 hardener Specific gr
5、avity Resin Hardener 1.17 0.92 1.48 1.44 1.36 0.97 Gel time 2 hrs 4 min 1 hr Cure schedule 24 hrs at 25 C or 30 min at 100 C 4 hrs at 25 C 5 days at 25 C or 2 hrs at 88 C Elongation at break, % - - 11 Aluminum tensile shear strength, psi, at -60 C 25 C 82 C 121 C 2900 2600 300 - 1500 2900 1200 - 420
6、0 4500 2600 1000 Tensile shear after: 30 days at 60 C 30 days after humidity aging 2200 1700 (40 C, 92% RH) 3000 1000 (54 C, 95% RH) 3800 2900 (54 C, 95% RH) T-peel strength, lb/in - - 11 Table 1: Properties of Two-Component Epoxy Adhesives.1 2 Depending on the type of curing agent chosen, epoxy adh
7、esive systems can reach full strength in minutes to several days. The most common types of epoxy adhesives will reach handling strength in several hours and full cure in five to seven days. The faster curing agents can reach handling strength in as little as five minutes at room temperature. These f
8、ast cure formulations also provide practical cure times at low temperatures and they are well suited for outdoor application in winter. Table 1 shows typical physical property and performance data for several two-component epoxy adhesives. As can be seen from the property data, the adhesives can be
9、formulated to meet a broad range of application and performance property requirements. Characteristics of Fast Curing Epoxy Adhesives Highly reactive two component epoxy adhesives have several advantages. These adhesives can come to a practical handling strength in minutes at room temperatures and c
10、an develop useful bond strengths at ambient temperatures as low as -20 C. The various uses for these adhesives include: 1. Bonding difficult to clamp fixtures such as surface mounting electrical receptacles, brackets, and racks to masonry walls; 2. Rapid production line assembling, such as bonding t
11、ogether sections of reinforced plastic car bodies; 3. Bonding traffic buttons to highways when minimum traffic delay is important; 4. Emergency repair of leaks; and 5. Cold weather construction uses such as pressure grouting, pipe joint sealing, bridge support structure repair, etc. These fast curin
12、g systems also have difficulties, however, that are directly related to their reactivity. Generally, the disadvantages must be balanced against the relative merits of using these adhesives. The major disadvantages of using fast curing epoxy adhesives include the following. 1. Shrinkage on polymeriza
13、tion is greater than with slower curing adhesives, and the increased shrinkage can result in increased stresses within the joint 2. Highly reactive epoxy adhesives are generally more brittle than slower reacting systems, and this results in lower peel and impact strengths 3. Highly reactive adhesive
14、s are more difficult to mix and apply because of their short working life resulting in inaccuracies and material waste; automated metering, mixing, and dispensing equipment may be required 4. There may be insufficient time to properly align the substrates, and misaligned parts can result in waste 5.
15、 Many of the more reactive curing agents and accelerators cause skin irritation and toxicological problems. Thus, fast cure is often looked upon as a liability rather than as a benefit. Because the fast acting catalysts react so quickly with epoxy resins and because they are used in very small propo
16、rtions, homogeneous mixing is difficult, and variation in crosslinking density can occur throughout the adhesive. Another consequence of high reactivity is a high exotherm. A high exothermic temperature can be degrading to the final epoxy adhesive properties and even dangerous if large masses of mixed adhesives overheat. Curing Agents
