Thermal shock resistance epoxies

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

Borosilicate glass is a range of glasses based on boric oxide, silica, and a metal oxide. It has excellent thermal shock resistance and chemical resistance. A recent patent claimed the use of borosilicate glass powder (50-100 phr) in conjunction with expandable graphite (100 phr) or vermiculite in polyolefin, epoxy, or elastomers to achieve good fire retardancy (as evidenced by the Cone Calorimeter test at 35 kW/m2).99... 

The improved flexibility results in improved adhesion and thermal shock resistance, but at the sacrifice of elevated-temperature performance. A TETA cured FIGURE 6.4 Chemical structure of dibutyl epoxy (EEW = 190) plasticized with 17 pph phthalate. 0f chbutyl phthalate exhibited a heat distor-... 

The monofunctional epoxy diluents are essentially chain stoppers since they inhibit crosslinks from forming. The extent to which the cured properties are affected is directly dependent on the concentration of the diluent added to the epoxy resin. The general effect is to reduce viscosity and improve the impact and thermal shock resistance while slightly reducing the thermal resistance. The thermal expansion of the cured resin is increased, as it is also with nonreactive diluents. This can lead to internal stress on the bond line depending on the thermal expansion of the substrate material. 

Hybrid resins have been used to improve the flexibility, thermal shock resistance, elongation, heat distortion temperature, and impact strength of unmodified epoxy adhesives. However, there can also be some sacrifice in certain physical properties due to the characteristics of the additive. These alloys result in a balance of properties, and they almost never result in the combination of only the beneficial properties from each component without carrying along some of their downside. 

Although most epoxy adhesives have good weather resistance, optimum properties are generally achieved when the adhesive has a combination of good water resistance and thermal shock resistance. Figure 11.6 illustrates the retention of tensile shear strength of copper and aluminum strips bonded with an amidoamine cured epoxy after 2 years of weathering in a temperate climate. 

Other anhydrides such as dodecyl succinic anhydride (DDS A) or adducts of DDS A with polyglycols, can also be used for formulating heat cured epoxy adhesives. These have excellent electrical properties and good thermal shock resistance. Anhydride cured epoxies are also useful for bonding plastics, notably polyester such as Mylar.8... 

Chapters 8 and 9 consider the mechanical properties of rubber- and ceramic-particle toughened-epoxy materials. The importance of rubber cavitation is highlighted in Chapter 8. It is well known that this mechanism can relieve the high degree of triaxiality at a crack tip in the material and enable subsequent plastic hole growth of the epoxy resin, which is a major toughening mechanism. We return to rigid particles in Chapter 9, which examines their use to increase the thermal shock resistance of epoxy resins. 

We proposed a new test method to evaluate the thermal shock resistance of epoxy resin (7). This test method uses a notched-disk specimen, and the thermal shock resistance can be evaluated analytically on the basis of linear fracture mechanics (8). In our previous studies, we reported on the use of our proposed thermal shock test and evaluation methods (8, 11) to determine the thermal shock resistance of toughened epoxy with a soft second phase (9, 10), and also with hard particulates (11). 

This chapter discusses the behavior, under thermal shock conditions, of epoxy resins toughened with ceramic particulates. Alumina Al203 and silica Si02, which are usually used as filler for insulation materials, and the new ceramic materials silicon carbide SiC and silicon nitride Si3N4 are employed. For these toughened epoxy resins, the thermal shock resistance is evaluated by using fracture mechanics. The difference between experimental and calculated values of the thermal shock resistance is discussed from a fractographic point of view. 

A comparison of critical temperature differences of resins filled with several ceramic particulates is shown in Figure 4. The volume fraction of all these composites is 34.2%. The critical temperature difference of epoxy filled with hard particulates was classified into three groups on the basis of thermal shock resistance. Composites filled with a strong particulate, such as silicon nitride or silicon carbide, showed high thermal shock resistance. Some improvement in thermal shock resistance was recognized for silica-filled composites. Composites filled with alumina or aluminum nitride showed almost comparable or lower resistance compared with the neat resin. 

The effects of ceramic particles and filler content on the thermal shock behavior of toughened epoxy resins have been studied. Resins filled with stiff and strong particles, such as silicon nitride and silicon carbide, show high thermal shock resistance, and the effect of filler content is remarkable. At higher volume fractions (Vf > 40%), the thermal shock resistance of these composites reaches 140 K, whereas that of neat resin is about 90 K. The highest thermal shock resistance is obtained with silicon nitride. The thermal shock resistance of silica-filled composites also increases with increasing filler content, but above 30% of volume fraction it comes close to a certain value. On the contrary, in alumina-filled resin, the thermal shock resistance shows a decrease with increasing filler content. 

A series of hydro- and bicyclic derivatives of phthalic anhydride have been employed as hardeners in epoxy resins used for optical recording disks <88JAP(K)638370I >, and in a urethane composition to prepare a wire coating with good insulating, thermal shock resistance and adhesion <86JAP(K)61200164>. 

The thermal shock resistance in the above work was improved by increasing the volume fraction of a hard filler such as silicon carbide or nitride, but silica and glass beads had only a moderate beneficial effect and alumina had the opposite effect, whether the particles were spherical or angular. Fractography showed that the interface in alumina-filled epoxies was particularly weak. 

Uses Epoxy curing agent also in polyamide, polyurea, modified urethane resins, in adhesives, elastomers, foam formulas intermediate for textile and paper treatment chemicals promotes adhesion and thermal shock resist. 

Microcrystalline quartz is obtained by pulverizing quartz sands and is a hard solid (7 Mohs). It increases the thermal shock resistance in brittle resins - some filled thermosetting resins are cracked by relatively few thermal cycles between, say, ambient temperature and 100°C - when added at high concentrations (typically 100-200 parts per hundred by weight). It can be surface treated with an aminosilane to enhance adhesion, when used in epoxy compositions to improve flexural modulus, electrical insulation or thermal properties, and in the case of unsaturated polyesters, it can be treated with a methacrylic silane. 

Evans et al. [43] carried out 4 MeV electron irradiations of 14 different epoxy resins at 77 K which were selected from a large number of resin systems after screening tests on thermal shock at cryogenic temperatures [44]. The results of flexural tests show that most of these irradiated resins possess only moderate resistance to radiation. Takamura and Kato [45] tried to irradiate the bisphenol-A type epoxy resins with various hardeners at 5 K in a fission reactor and reported that the compressive strength of these epoxy resins decreased sharply after a combined neutron and y-ray irradiation equivalent to a dose of about 107 Gy. 

The room temperature cured epoxy adhesives discussed thus far exhibit a general lack of flexibility, especially when considered next to elastomeric sealants. Flexibility is generally desired when the performance requirements include high peel strength, impact strength, and resistance to thermal shock or thermal cycling. 

Non-reactive diluents. These diluents are low-viscosity materials which do not have any reactive sites and thus do not react with the epoxy systems. These diluents generally impart flexibility and improve the impact resistance, giving better thermal mechanical shock resistance. However, there is a sacrifice in physical strength, chemical resistance and high-temperature performance. Addition of 30 parts non-reactive diluent to 100 parts epoxy resin usually does not affect the physical properties of the system. Commonly used non-reactive diluents are nonyl phenol, furfuryl alcohol and dibutyl phthalate. 

Epoxy systems with flexibihsers and properly-selected fillers exhibit high resistance to rapid changes in temperature and do not show signs of cracking or shattering. Rigid epoxy systems can cause severe problems. Better formulated systems can withstand repeated thermal shock cycles from 180°C to -75°C without failure. 

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