Uncured resin, temperature

Uncured BCB/BMI Blend Resin Properties. The uncured resin is a bright yellow powder at room temperature. LC of each of the two components indicated that the BMI was greater than 98X monomer and the BCB was greater than 90% monomer prior to blending. To make the resin blend, a 1 1 molar mixture of BCB and BMI was completely codissolved in methylene chloride, and the solvent was then stripped... 

In general, the elastomer must be prereacted (adducted) with the epoxy for the toughening effect to take place. Adducts reduce the likelihood of early phase separation and maintain the solubility of the elastomer in the uncured resin system. For CTBN the reaction is carried out at high temperatures (150 to 160°C) and usually in the presence of a catalyst, such as tris-dimethylamino phenol or piperidine. The resulting epoxy-CTBN adducts are available from several suppliers, and they can be easily formulated into epoxy adhesives. 

Carboxy-terminated curative, such as CTBN, provides excellent toughening in part due to its miscibility in many epoxy resins. Phase separation during cure is required to obtain toughening, and generally the phase separation requires an elevated-temperature cure. However, by prereacting the CTBN with a portion of the epoxy to obtain an adduct, a room temperature curing toughened epoxy is possible. Adduction reduces the likelihood of early phase separation and maintains the solubility of the elastomer in the uncured resin system. 

Fig. 3 The CTP diagram, a modified version of the TgTP proposed by Wang and GillhamJ C = Tg — Tgo)/ (Tgoo — Tgo). where Tgo is the initial glass transition temperature of the uncured resin Tgoo, is the glass transition temperature of the fully cured resin Tg is the glass transition temperature of the partially cured resin. (From Ref...

Because conventional UV resins cure only when exposed to UV light, resin underneath components will not cure and is unacceptable for automotive use. The aerobic UV coatings have a unique secondary heat curing ability (as low as 85°C), which makes them attractive. Simply raising the temperature of the boards for 5 min allows the uncured resin to shadow cure. ... 

The rate of cure is temperature dependent and many formulations stop curing altogether below a temperature of about 5 °C. If carefully formulated the change in volume between the uncured resin-hardener system and the fully cured polymer can be very low. This property, together with their relatively high strength and claimed resistance to moisture and chemical attack, forms the basis of the use of epoxy resins as structural adhesives. 

There are far fewer options for preparing the second-phase particle distribution by precipitation within the uncured resin matrix. This approach, again, is not ideal for solvent-based adhesives. Any such toughener must be soluble in the base resin system at elevated temperature and then give an even precipitation, without agglomeration, as the matrix cools back to ambient temperature. 

Unfortunately, DPC instruments do not have sufficient response time to follow the very fast rate of free radical photochemical reactions encountered in rapid processing environments. As shown in Fig. 2.80 the time to 90% cure of a UV curable resin at a UV intensity of 400mW/cm is about one second at room temperature. This fast kinetic measurement was carried out by a technique known as real time infrared spectroscopy (RT-IR). By this method one follows quantitatively the development of cure by tracking the disappearance of unsaturation as a decrease in the area of an absorbance band that was initially associated with the uncured resin (Decker and Moussa, 1988). The data in Fig. 2.80 indicates that a power compensation DSC begins to give accurate conversion data when UV intensities are of the order of lOmW/cm or less for this resin. Importantly, the data also show that the free radical polymerization of this UV curable resin can be fitted to a linear log-log plot of time versus intensity over a range of intensities of nearly five orders of magnitude. It is... 

Tf TgQ is a dimensionless temperature with being the glass transition temperature of the freshly mixed uncured resin, = E /RT, and g(7 g ) is the degree of conversion at the dimensionless glass transition temperature, T = T g/T go- Note that at T < TgQ, no reaction occurs because the reactive species are immobilized in the glassy state. In order to estimate the value of t - from Eq. (14.4), one must have information on a, the degree of conversion at T. ... 

Figure 14.2 Schematic showing the degree of cure of an epoxy at glass transition g as afunction of dimensionless temperature T /T, where denotes the glass transition temperature, gO denotes the glass transition temperature of uncured resin, denotes the temperature at which gelation and vitrification cross each other, and the solid line is the best fit to Eiq. (14.6). (Reprinted from Enns and Gillham, Journal of Applied Polymer Science 28 2567. Copyright 1983, with permission from John WUey Sons.)...

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Dynamic mechanical analysis provides a useful technique to study the cure kinetics and high temperature mechanical properties of phenoHc resins. The volatile components of the resin do not affect the scan or limit the temperature range of the experiment. However, uncured samples must be... 

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