Binders shrinkage

Two types of carbon electrodes are in widespread use. Prebaked carbon electrodes (Fig. 5) are those made from a mixture of carbonaceous particles and a coal-tar pitch binder. The electrode is formed by extmsion or mol ding from a heated plasticlike mix and subsequently baked. Final bake temperature is sufficient to carbonize the binder, ie, about 850°C. At this temperature the binder is set, all volatiles have left, and a significant portion of the product shrinkage has occurred. 

Mortar mixed with plastic binder to fill the top cores (quality control and shrinkage can be problems). 

Internal destruction and volume shrinkage. Since destruction by the POC takes place at the interface between Ti02 and binder, therefore binder is not only decomposed at the surface of the paint film. There is also a certain amount of destruction inside and, with sufficient flexibility of the binder and sufficient adhesion between the pigment and the binder, this must lead to a shrinkage in volume. 

The linear shrinkage on drying is typically less than 2%, and on binder elimination and sintering less than 15%. 

In principle any diol/triol mixture which reacts with a diisocyanate will yield a network. However, to be useful as propellant binder, additional requirements must be met. The most important are low cure shrinkage, low reaction exotherm, rubbery characteristics down to arctic temperatures, good aging stability, and ease of handling during propellant manufacture. 

The major ingredients of composite propellants are ammonium perchlorate (AP-68%), metal powder (-16%) and polymeric binders like CTPB, HTPB and Thiokol. As total solids loading in composite propellants is -85-88%, polyesters which are commonly used for inhibition of DB propellants, have a relatively weak bond with composite propellants. This problem is further aggravated because of higher shrinkage in the case of polyesters. 

The physical panel size used resulted in more joints between panels than would have been the case in a Portland cement pour, owing to the surface finishing with a screed and the continuous obstruction of the existing support columns. Asphalt expansion strips were placed between panels and were provided with a removable top strip which was filled with a polyurethane compound. Shrinkage of pours in this work was negligible. Shrinkage in sulfur concrete was proportional to the amount of sulfur binder used. 

The uncured binder material should be a liquid which cures with minimum heat release and shrinkage and without evolution of gases. 

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