Thermal Conductivities of Adhesives

Unfilled polymer adhesives are inherently thermal insulators. Their thermal conductivities range from 0.1 to 0.3 By formulating 

In addition to the type of filler, thermal conductivity depends on many other factors including the amount of filler used, whether it is by weight or by volume, its form and size, the surface treatment of the filler particles, the completeness of cure, and the test conditions used for measurement. Test methods are described in MIL-STD-883, Method 1012.1 and in ASTM C518.[ ][58] 

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Titanium or beryllium oxide also provides a degree of improvement in thermal conductivity to epoxy systems. Magnesium oxide and aluminum oxide have also been commonly used for this purpose, although the degree of improvement is not as great as with the fillers discussed above. The effect of various fillers on the thermal conductivity of cured adhesive is shown in Fig. 9.6. The incorporation of metal fibers with metal powders has been shown to provide synergistic improvement to the thermal conductivity of adhesive systems,... 

Table 2.7 Thermal conductivities of adhesives and plastics commonly used in electronic devices and assemblies ...

Overheating of devices resulting in electrical malfunctioning or reduction in the life expectancy of the device and circuit can be the direct result of poor conductivity and high thermal impedance of the adhesive. Thermal conductivity of adhesives becomes critical as devices are operated at higher speeds, consume more power, and dissipate more heat. Semiconductor junction temperatures must remain within normal limits. 

Table 2.7. Thermal Conductivities of Adhesives and Plastics Commonly Used in Electronic Devices and Assemblies... 

Alumina particles are one of the commonly used fillers for improving the thermal conductivity of adhesives in particular insulation adhesives. Aluminum and silver powders or flakes are used to improve the thermal and electrical conductivities for adhesives intended to be an electrical or thermal path. The filler volume content level is very important to get sufficient conductivity. However, excessive filler content might cause degradation in mechanical properties of the adhesives (Kahraman and Al-Harthi 2005 Kahraman et al. 2008). 

The heat isolation effect of the bonding layer is pronounced. The thermal conductivity of adhesive material can however be improved by mixing metallic particles in the adhesive. 

Denning (1969) Thermal Conductivity of Adhesives at Low Temperatures by H. Denning, Cryogenics 9, 282-283. 

The dimensional stability of low density, water blown rigid PU foams for pour-in-place thermal insulation applications was improved by the use of a phthalic anhydride based polyester polyol containing a dispersed cell opening agent. The foam systems obtained allowed some of the carbon dioxide to be released through the cell windows immediately after filling of the cavity, and to be rapidly replaced by air. Studies were made of the flowability, density, open cell content, dimensional stability, mechanical properties, thermal conductivity and adhesion (particularly to flame treated PE) of these foams. These properties were examined in comparison with those of HCFC-141b blown foams. 21 refs. 

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]. 

Thermal conductivity is also important in highly integrated electronic applications where the heat generated by components must be transferred to a heat pipe or by some other means outside the electronic package. Thermal conductivity within adhesive systems is also a means of reducing exotherm and stresses that could develop during the curing cycle or other excursions to elevated temperatures. 

TABLE 9.9 Thermal Conductivity of Metals, Oxides, and Conductive Adhesives at 25°C18... 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

These processes have an advantage in that the heat penetrates deeply into the joint and into the epoxy material itself. With conventional thermal energy processes, the heat must be conducted into the mass of the epoxy adhesive from outside the joint. This is hindered by the presence of the substrates, the substrate geometry, and the relatively low thermal conductivity of the epoxy itself. 

The thermal conductivity of the adhesive and the thickness of the bond line... 

In contact molding, the block, say 2" thick, can be attached to the inner mold, and joints between the blocks made with an elastomeric adhesive, and then the FRP laid up upon, and bonded to, the back of the block. Now when the molds are stripped, the liquid contact face will be the borosilicate block, which has a top surface operating temperature of 960°F. In these higher thermal ranges, the coefficient of thermal conductivity of the block ranges from 0.60 to 0.75. Thus, it is possible to operate a vessel so lined at, say, 600°F, while keeping the inner surface of the FRP unit at about 320°F. For greater thermal drop (cooler FRP), a thicker layer of block would be used. 

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

The thermal conductivity of adherends influences the temperature conditions in the glueline during adhesive curing. It plays a special role in the application of hot-melt adhesives on metals due to the quick solidification of the melt in the boundary layer zone and the possible impairment of the adhesion development. The thermal conductivity A is indicated in the dimension W/cm K (Watt per centimeter Kelvin). Values of certain materials ... 

Hot-melt adhesives, because of the high temperature of the melt and very low thermal conductivity of glass (risk of fracture). 

Figure 3.16 Effect of increasing amounts of silver filler and of temperature on the thermal conductivity of an epoxy adhesive. The concentrations are hy volume (23% hy volume corresponds to 80% hy weight). ...

Preventing degradation of the electrical or thermal conductivity of the filler or the filled cured adhesive. 

The main advantages of silver-glass adhesives are their very high thermal conductivities and thermal stabilities compared with filled polymer resin formulations. Thermal conductivities of 65 W/mK to over 80 W/m K are reported. ... 

Bjorneklett A, Halbo L, Kristiansen H. Thermal Conductivity of Epoxy Adhesives Filled with Silver Particles. Inti J Adhesion. Apr. 1992 12(2). 

In this approach, entire wafers or segments of wafers containing ICs are stacked. Interconnections are formed from vias in the silicon. In vertically stacking memory chips or other IC chips, the chips are first thinned to several mils (4 mils or less), then adhesive bonded, and electrically interconnected directly from chip-to-chip, either from the edges or through vias in the silicon. In these approaches, both electrically conductive and electrically insulative adhesives are used. Thermally conductive preform adhesives or thin thermoplastic films are used to bond and isolate the chips within the stack and to dissipate heat. Several processes are available to vertically stack and interconnect chips of the same size and function such as SRAM, flash, and DRAM memory chips. Other processes have been developed to vertically stack chips of different sizes and functions, or to horizontally interconnect different chips in one layer, then vertically stack the layers. ... 

Poor initial thermal conductivity of the adhesive chosen. 

Thermal-transfer adhesives that are electrically insulating also exhibit wide ranges of thermal conductivities, depending on the filler type and amount. The thermal conductivities of epoxies filled with boron nitride or diamond are approximately 4 W/m K and 12 W/m K, respectively, while those of the more common aluminum-oxide-filled adhesives range from 1 to 2 W/m K. 

Expanded polystyrene foam (EPS) has different applications, because of its physical form (beads) and properties (higher permeability to water and less effective adhesion to facing materials than polyurethane). The expansion gases, pentane and steam, escape fairly rapidly from the foam, so the thermal conductivity of the foam filled with air is about twice that of the best polyurethane foam—a 50 mm thick slab of foam has a U-value of 0.5-0.6 Wm K . EPS mouldings can be used as shutters (formwork) for pouring concrete in a composite wall. The two EPS layers are connected at intervals to fix the thickness of the concrete. Extruded polystyrene foam (XPS) is used in plank form for insulation imder the concrete floor of houses, and in roofing panels. 

The successful application of an adhesive at low temperatures is dependent upon the difference in the coefficient of thermal expansion between adhesive and polymer, the elastic modulus, and the thermal conductivity of the adhesive. Epoxy-polyamide adhesives can be made serviceable at very low temperatures by the addition of appropriate fillers to control thermal expansion. 

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