Epoxy resin hybrids

Figure 35. Mechanical properties of carbon/carbon epoxy-resin hybrid composites, compared with the properties of the composite skeletons before resin impregnation (61,62). The composite skeletons were prepared from Sigrafil HM 3 PAN-based fiber, rigidized with a phenolic resin, and densified by four cycles with coal-tar pitch plus sulfur the carbonization temperature was 1000°C. (a) Young s modulus.

Figure 9.34 The composites frequency dependence of reflection coefficient at different filler s amount PANI-PTSA/epoxy resin composites (a), PANI-PTSA/FCjOyepoxy resin hybrid composites (b), PANI-PTSA (15%)/Fe30 (10%)/epoxy resin hybrid composites at different thickness (c), and the comparison of measured and calctdated reflection coefficient for PANI-PTSA (15%)/Fe30 (10%)/epoxy resin hybrid composite (d). Reprinted from Ref. [61] with permission from Elsevier.

Thermosetting-encapsulation compounds, based on epoxy resins (qv) or, in some niche appHcations, organosiHcon polymers, are widely used to encase electronic devices. Polyurethanes, polyimides, and polyesters are used to encase modules and hybrids intended for use under low temperature, low humidity conditions. Modified polyimides have the advantages of thermal and moisture stabiHty, low coefficients of thermal expansion, and high material purity. Thermoplastics are rarely used for PEMs, because they are low in purity, requHe unacceptably high temperature and pressure processing conditions. 

To improve the weatherabUity of epoxies, which normally chalk and yeUow, epoxy—polyester aUoys or hybrids are used. These powders with improved overbake resistance cure at temperatures as low as 130°C. They have film flexibiHty similar to epoxy resins, but their hardness is slightly decreased. Corrosion resistance is equivalent to epoxy powders in most cases, although solvent and alkaH resistance is inferior. 

L. Mascia and T. Tang, Ceramers based on crosslinked epoxy resins-silica hybrids low surface energy systems, J. Sol-Gel Sci. Technol., 1998, 13, 405. 

The early aerospace adhesives were primarily based on epoxy resin chemistry. However, unique applications requiring high temperatures and fatigue resistance have forced the development of epoxy-phenolic, epoxy-nitrile, epoxy-nylon, and epoxy-vinyl adhesives specifically for this industry. The aerospace industry has led in the development and utilization of these epoxy-hybrid adhesives. 

The epoxy resin is a primary component in any epoxy adhesive formulation, and it is often referred to as the base polymer. However, it is certainly not the only or even not always the predominant component in influencing desirable end properties. Epoxy resins by themselves are often too rigid to provide the required properties such as flexibility, peel and impact strength, and thermal cycling resistance. As a result, they are often modified with other components or hybridized with other types of polymeric resins to provide these functions. 

A variety of polymers, both thermosets as well as thermoplastics, can be blended and coreacted with epoxy resins to provide for a specific set of desired properties. The most common of these are nitrile, phenolic, nylon, poly sulfide, and polyurethane resins. At high levels of additions these additives result in hybrid or alloyed systems with epoxy resins rather than just modifiers. They differ from reactive diluents in that they are higher-molecular weight-materials, are used at higher concentrations, and generally have less deleterious effect on the cured properties of the epoxy resin. 

These blends can take a number of different forms. The added resin may be reacted with the epoxy resin, or it may be included as an unreacted modifier. The modifier may be blended into a continuous phase with the epoxy resin (epoxy alloys) or precipitated out as a discrete phase within the epoxy resin matrix (as is generally done in the case of toughening modifiers). Epoxy hybrid adhesives are often used as film (supported and unsupported) or tape because of the ease with which formulated systems can be dissolved into solvent and applied to a carrier or deposited as a freestanding film. Some systems, notably epoxyurethanes and epoxy-poly sulfides, can be employed as a liquid or paste formulation because of the low-viscosity characteristics of the components. 

These single-phase hybrids are very different from the two-phase toughened epoxynitrile adhesives that are discussed in Chap. 8. These two-phase adhesives have redefined structural adhesives to a great extent and have opened the door to many applications that were previously not possible because of the epoxy resin s inherent rigidity. The polymer mixtures that exist as separate phases provide significant increases in toughness but have only a small improvement in elongation at typical use levels. 

Blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines.18,19 These blocked prepolymers will initially react with the amines to form amine-terminated prepolymers that crosslink the epoxy resin. Urethane amines are also offered commercially for use with epoxy resins to develop hybrid adhesive systems.20... 

The number of possible hybrid systems that can be manufactured with epoxy resins is nearly infinite, and many adhesive formulations have been attempted in a quest to improve the main disadvantages of a cured epoxy brittleness and rigidity. 

Functionalized, liquid polybutadiene derivatives have also been developed as hybrid flexiblizers for epoxy resins. Carboxyl-terminated butadiene/acrylonitrile polymers, butadiene homopolymers, and maleic anhydride-amino acid grafted butadiene homopolymers have been used as flexibilizers to impart good low-temperature strength and water resistance to DGEBA-based epoxy adhesives. An epoxy system toughened by polybutadiene with maleic anhydride is claimed to provide a hydrophobic backbone, low viscosity, softness, and high tensile strength and adhesion (Table 7.10). 

Epoxy-silicone hybrid resins (Fig. 7.4) have generally been developed for use in the molding of microelectronic packages. These resins have moderate strength but exhibit a degree of elongation of about 60 percent at break. 

FIGURE 7.4 Chemical structure of epoxy-silicone hybrid resin.26... 

Another common method of flexibilizing epoxy adhesives is by blending the primary epoxy resin with other, more elastic polymers. Epoxy-nylon, epoxy-polysulfide, and to a certain extent epoxy-urethane hybrids use such a mechanism to provide flexibility. These flexibilizers are important additives for epoxy adhesives even though they may reduce certain... 

Typically tape or film epoxy adhesives are modified with synthetic thermoplastic polymers to improve flexibility in the uncured film and toughness in the cured adhesive. Epoxy resins can also be blended with phenolic resins for higher heat resistance. The most common hybrid systems include epoxy-phenolics, epoxy-nylon, epoxy-nitrile, and epoxy-vinyl hybrids. These hybrid film adhesives are summarized in Table 13.2, and structural properties are shown in Table 13.3. 

Figure 34. Comparison of the flexural strengths of unidirectional carbon/carbon composites (left-hand side) with those of hybrid composites in which the final impregnation is made with an epoxy resin (34) The composites were fabricated with high-modulus fibers rigidized with phenolic resin, and subjected to four densification cycles with coal-tar pitch plus sulfur.

Figure 36. Limiting oxygen index (LOI) of flammability resistance for carbon/carbon composites and related hybrid composites with epoxy resin impregnant, as functions of bulk density and resin content (64).

In a somewhat similar fashion, Ishii et alP- have demonstrated inkjet fabrication of polymeric microlenses for optical chip packaging. UV curable epoxy resin is deposited onto optical devices by inkjet printing. When the droplets hit the surface, they form into partial spheres due to their surface tension, and are UV-cured to form the microlens with diameters from 20 to 40 tm with /-numbers of 1.0 to 11.0. Their uniformity in a microlens array was measured to be within 1% in diameter and 3 tm in pitch (total count of 36 lenses). They have also demonstrated hybrid integration of inkjetted microlenses with a wire-bonded vertical-cavity-surface-emitting laser (VCSEL) with coupling efficiencies of 4 dB higher than without the microlens. 

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