Thermal toughening

Increase of strength is possible by chemical or thermal toughening, but it is a rather expensive step and in contradiction to the pharmaceutical requirement of delivering stress-free glass [6], Nevertheless in special applications it might be useful. 

Low-expansion glass ceramics, however, show an excellent thermal shock resistance and a high thermal load capacity. For that reason they are used as table and oven ware, oven plates, or fire resistant glasses. With all these applications, low-expansion glass ceramics are exposed to relatively high temperatures. In most of these cases, compressive stresses due to thermal toughening would be reduced. 

The low compressive stresses obtainable by thermal toughening and the reduction of prestressing in daily use are the reasons why low-expansion glass ceramics are usually not prestressed by thermal toughening. 

The addition of Vamac B-124 to ethyl cyanoacrylate has a more pronounced effect on peel strength, both at ambient temperature and after thermal exposure. After 24 h at ambient temperature, the peel strength of the rubber-toughened formulation is almost 40% greater than the control formulation A without rubber. After heating the test specimens for 2 h at 121°C, the peel strength of formulation A, is almost non-existent, while that of C has increased significantly, as seen in Fig. 7. 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

We most often encounter polystyrene in one of three forms, each of which displays characteristic properties. In its pure solid state, polystyrene is a hard, brittle material. When toughened with rubber particles, it can absorb significant mechanical energy prior to failure. Lastly, in its foamed state, it is versatile, light weight thermal insulator. 

Thomas, R. N., Univ. Safety Assoc. Safety News, 1981, 15, 16-17 Failure of the product (0.5 g) to crystallise out from the aqueous DMF reaction liquor led to vacuum evaporation of the solution at 60-70°C. Dining evaporation the mixture exploded violently, shattering the fume cupboard sash of toughened glass. The product may well be thermally unstable, but reaction of DMF with excess warm perchloric acid, possibly in near-absence of water, may also have been involved. 

The effect of radiation on the thermal expansion of this toughened composite (T300/CE 339) is shown (191 in Figure 24. The thermal strains measured during the cool-down portion of the first thermal cycle (cooling from RT to -150°C) are shown for the baseline composite (no radiation exposure) and for samples exposed to total doses as high as 10 0 rads. Radiation levels, as low as 10 rads... 

Figure 24. Effect of radiation fluence on thermal expansion of elastomer-toughened specimens during cool down from room temperature. (Reproduced from reference 19.)...

To assess the effect of elastomer degradation on composite performance, additional composites were fabricated with the same 121°C cure epoxy without any addition of the elastomer (211. The expansion behavior of the modified epoxy composite was similar to the toughened material. For electron doses less than 10 rads the CTE of the toughened and untoughened composites were essentially the same which suggests that the epoxy matrix and not the elastomeric component controls the thermal expansion behavior. 

---