Cure shrinkage

Laminating resins have been offered by Akzo (Diacryl 101), Dow (Derakane Vinyl Esters) and Showa (Spilac). Typical of these is Diacryl 101, which is manufactured by esterification of the addition product of ethylene oxide to bis-phenol A with methacrylic acid. They exhibit lower curing shrinkage than the polyester laminating resins during cure. The structure of Diacryl 101 is... 

Such reactions allow chain extension and/or cross-linking to occur without the elimination of small molecules such as water, i.e. they react by a rearrangement polymerisation type of reaction. In consequence these materials exhibit a lower curing shrinkage than many other types of thermosetting plastics. 

Fillers are used in tooling and casting application. Not only do they reduce cost but in diluting the resin content they also reduce curing shrinkage, lower the coefficient of expansion, reduce exotherms and may increase thermal conductivity. Sand is frequently used in inner cores whereas metal powders and metal oxide fillers are used in surface layers. Wire wool and asbestos are sometimes used to improve impact strength. 

The polysulphides are frequently used in casting mixes and to a less extent in coating, laminating and adhesive applications. Their value in casting and encapsulation lies mainly with their low curing shrinkage and flexibility in the cured state. Their tendency to corrode copper and the somewhat inferior electric insulation properties of the blends does lead to certain limitations. 

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. 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

Nearly all polymeric materials (including adhesives and sealants) shrink during solidification. Sometimes they shrink because of escaping solvent, leaving less mass in the bond line. Even 100 percent reactive adhesives, such as epoxies and urethanes, experience some shrinkage because their solid polymerized mass occupies less volume than the liquid reactants. Table 3.6 shows typical percentage cure shrinkage for various reactive adhesive systems. 

Major limitations Cure shrinkage Dermatitis, difficult to mold release Colon limited... 

Thermomechanical analysis (TMA). In this technique, information on changes in the size of a sample is obtained, e.g. thermal expansion and coefficient of thermal expansion, cure shrinkage, glass transition, thermal relaxations, any phase transformation involving volume change in the material. We describe the measurement of the coefficient of thermal expansion in detail later in this section. 

Figure 2.5 Planarity of SOG film after cure. Shrinkage in the film due to curing results in a loss of planarity.

Shrinkage is the reduction in volume during cure. Epoxies have a low curing shrinkage. Shrinkage may be reduced through the use of fillers. 

This cure shrinkage stress build-up is another point favoring the application of this type of membrane over a hot asphalt membrane on concrete. The asphalt has sufficient cold flow to relieve much of the stresses in the resin squares during the 7-day cures. 

When a monolithic surfacing is applied to a concrete substrate, in most cases much of the cure shrinkage of the concrete has already taken place. However, the concrete contractor will have, perhaps, installed expansion joints in the slab, and should have cut in or formed control joints as well, so that any future shrinkage will occur at these intentionally weakened parts of the slab rather than in random locations. In addition, there will be "cold seams"—locations where a concrete pour was completed one afternoon and another pour made a day or so later, or fresh concrete was poured against dry, partly cured concrete. This joint is also weak, and with shrinkage of concrete over the years will probably also crack. 

Where cure shrinkage is expected to be greater than 0.05 to 0.1%, more frequent and larger expansion joints should be planned, with distances between joints reduced proportionately. 

This also means that stress relief joints will be required on this type of floor to break the lines of stress from the slow growth of the brick or tile, and so prevent disbonding, followed by heaving, just as the stress relief joints in a monolithic floor provide for the cure shrinkage of the monolithic. It should also be remembered that, unlike brick on a membrane, if brick (or tile) are bonded to the substrate, and if the substrate cracks, so will the tile or brick structure. 

Figure 45-4 Examples of floors laid with high bond, high strength furan mortar in which cure shrinkage of the mortar has caused the brick to break. The cracks so created have been filled with expansion joint sealant. See Chapter 25.

Cure shrinkage of mortar Strongly bonding mortars, as they shrink on curing, can pull brickworic apart. 

The distance between expansion joints or stress-relief joints was too great. (Cure shrinkage results in accumulation of stresses in the topping. When these exceed the bond strength, the topping disbonds and cracks.)... 

Note that if disbonding occurs, cracking due to stresses from cure shrinkage will almost certainly follow. 

Figure 52-16 Use of too tightly woven, hard, reinforcing glass cloth, possibly with incorrect surface treatment. The resin in the surfacing has failed to wet it, causing delamination, then cracking due to cure shrinkage.

Specific volume Cure shrinkage measurement (Dilatometry, etc.) A—>C 35,36... 

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