Thermal postcure

Since phenolic resins for wood bonding harden only thermally, postcuring during hot stacking is very important. In contrast to UF-bonded boards, PF-bonded boards should be stacked as hot as possible to guarantee a maximum postcuring effect. The... 

Other workers have reported on the use of urethane (meth)acrylates for the recording of holograms. In some cases, thermal postcuring of an epoxide was used for fixation. This work has been reviewed in Ref. 

Knowledge of structure and reactivity of trapped radicals is especially important for the design of thermal postcuring schedules as used with the manufacturing of lenses. This knowledge may also provide a starting point for the study of long term behavior. 

It has been established by a variety of techniques that aromatic cyanate esters cyclotrimerize to form cross-linked cyanurate networks.1 Analogously, the fluoromethylene cyanate monomers cure to cyanurate networks. In addition to the 19F-NMR spectra shown in Figure 2.3, evidence includes an up-field shift of the methylene triplet (1H-NMR, 0.21 ppm 13C-NMR, 9.4 ppm), the disappearance of the cyanate functional group (IR, 2165 cm4 13C-NMR, 111.9ppm) and the appearance of the cyanurate functional group (IR, 1580 and 1370 cm4 13C-NMR, 173.6 ppm).9 Typically, monomers are advanced to prepolymers by thermal treatment at 120°C or just above the melting point. The prepolymers are then cured at 175°C and are postcured at 225°C. 

Since the polymerization of TEGDA can easily be studied with DSC as well as with dynamic mechanical thermal analysis (DMTA) we have repeated our study with this monomer in order to see whether or not mechanical properties depend on the intensity rather than on the dose of UV irradiation. DMTA also reveals whether or not postcuring occurs during thermal after-treatment, similar to what has been observed with other thermosetting materials (8). 

Figure 9.13 shows the evolution of conversion and temperature profiles in the part. An almost uniform cure takes place up to the time at which Tw reaches the plateau at 177°C. At 132 min, both conversion and temperature attain the maximum values at the adiabatic boundary (z =L). At this time, Tmax exceeds Tw by about 90°C, which may produce an incipient thermal degradation. At 168 min, the final conversion profile is attained (T is almost uniform in the part and equal to Tw). The maximum conversion of the material located close to the metallic plaque is xm = 0.872 due to the effect of vitrification (and the assumption of Rc = 0 when T < Tg). Therefore, vitrification produces a conversion profile in the cured part. A postcure step would be necessary to completely cure the composite material. 

As discussed in Chapter 10, network polymers - as linear polymers - obey the time-temperature equivalence principle in the domain where they are stable, both chemically (no postcure, no thermal degradation), and physically (no orientation relaxation, water desorption, physical aging, etc.). 

The most general and specific aspect of thermal aging of polymer networks is the existence of a postcuring effect, often predominant in the early time of exposure (Fig. 14.19). The curves exhibit a maximum (for the ultimate stress of unsaturated polyesters, P can increase by more... 

Optimum cures with AEP catalyzed DGEB A are obtained using 20 to 22 pph. However, the crosslink density is not great, and the cured product does not attain a high degree of thermal or chemical resistance. The pot life and exotherm are similar to those of DETA and TETA, but a postcure (2 h or more at 100°C) is required to develop properties fully. 

Compared with other catalysts that homopolymerize epoxies, the imidazole offers improved thermal properties and retention of mechanical properties at more elevated temperatures. The cured resin has a heat distortion temperature between 85 and 130°C, which can be further increased by a postcure to about 160°C. 

Suitable curatives for the polysulfide-epoxy reaction include liquid aliphatic amines, liquid aliphatic amine adducts, solid amine adducts, liquid cycloaliphatic amines, liquid amide-amines, liquid aromatic amines, polyamides, and tertiary amines. Primary and secondary amines are preferred for thermal stability and low-temperature performance. Not all amines are completely compatible with polysulfide resins. The incompatible amines may require a three-part adhesive system. The liquid polysulfides are generally added to the liquid epoxy resin component because of possible compatibility problems. Optimum elevated-temperature performance is obtained with either an elevated-temperature cure or a postcure. 

After the failure criterion has been defined, the various processes that could cause this failure must be analyzed. For example, an increase in modulus could occur by thermal oxidation, increased postcure crosslinking, or the loss of plasticizer. Whatever the mechanism, each possible process needs to be identified and its rate characterized separately. Only then can interactions between different mechanisms be considered for life prediction. 

Quenched specimens given a brief thermal annealing at 140 °C for 10 minutes were found to exhibit toughness similar to that observed for as-postcured specimens (Table 1). Even though the strength for 10 minutes annealed specimens was not... 

Thermal analysis by differential scanning calorimetry on as-received materials indicated that the matrix was not fully cured an exotherm was detected the first scan from room temperature to 300 °C. A 250 °C/16 hours was therefore necessary for the specimens in order to complete all the crosslinking reactions. After the postcuring, differential scanning calorimetry confirmed a fully cured system which exhibit a regular step-function increase in heat capacity at Tg. The calorimetry results are shown in Fig. 8. 

In order to study the effect of physical aging on the carbon-fiber reinforced epoxy, the freshly quenched materials were then sub-Tg annealed at 140 °C. After annealing for only 10 minutes at that temperature, the toughness of the composite was restored to a level comparable to that of the postcured material (see Fig. 7). It is likely that residual thermal stresses resulted from the quenching were annealed away during this 10 minutes thermal aging at 140 °C. 

Figure 18 shows the effects of thermal history on the mechanical dispersion peaks in Fiberite 934 epoxy composites. The as-fabricated materials show by far the largest damping which spans from —100 °C to 20 °C. Postcuring completes the crosslinking and results in significantly lower-damping material. 

Figure 28 shows the thermal expansivity of epoxy above its Tg as a function of thermal history. Rubbery-state expansivity is generally an order of magnitude larger compared to the glassy-state expansivity (see Table 2). As-cast epoxy has an expansivity above Tg of 3.22 x 10-4 K-1. With postcuring and quenching, this parameter tends... 

To improve properties such as mechanical, thermal, and dimensions of certain molded TSs, also certain TPs, they are exposed to a postcure. The part is literally baked in an oven. Experience or a material supplier s recommended times and temperature profiles required to enhance properties are used. Baking also improves creep resistance and reduction in stresses. This postcuring is also used with certain TPs after IM or extrusion to improve their performance. 

After isothermal cure, temperature scans are conducted in order to measure the Tg after cure and Tg . However, due to thermal degradation, postcures can lead to lower glass transition temperatures than those obtained after cure. Thus, the determination of Tg , for high T, systems is a difficult problem. One approach is to establish a relationship between Tg and theoretical crosslink density for systems of lower Tgoc and similar chemical structure, and extrapolate to the system with higher crosslink density, thereby obtaining an estimate of Tg, ... 

The mixture of liquid resin and liquid amine was stirred and degassed in a 3-necked flask, just as for the thermal castings, but an ice bath was used to control any exotherm in the flask, so that rapid viscosity increase due to reaction would not occur to interfere with degassing. After pouring into the mold, the casting was kept at room temperature for at least 14 days before demolding. When a mild postcure was desired, at least 7 days/room temperature curing preceded it the postcure was 6 hr./100 C. 

Tensile tests were performed on neat epoxy resins In the following conditions as-cast, as-postcured, as-quenched, and aged at decade Increments from 10 to 10 minutes at 140°C In nitrogen while stored In darkness. A summary of the observed resin stress-strain behavior Is shown In Figure 2. As can be seen, the epoxy polymer was found to be extremely sensitive to thermal history. 

---