Frontal processes

Front polymerization processes are of interest primarily for formation of massive articles from materials which undergo severe shrinkage during reactive processing. This method holds the promise of reducing the level of residual stresses and to form massive monolithic items. There are several versions of frontal processes used in engineering practice at present, such as zone polymerization and polymerization with continuous build-up of polymeric layers. 

A typical example of frontal polymerization is the polymerization of methyl methacrylate (or an oligomer), placed inside a long aluminum tube 249 these tubes continuously dip into a bath with a liquid heated up to temperature of 70 - 80°C. The part at the tubes above the bath are cooled so that the reactive material does not polymerize. Polymerization shrinkage is compensated by continuous injection of a monomer or oligomer into the reaction zone. The appropriate combination of injection rate, velocity of tube movement through the reaction zone, and tube diameter are chosen according to experimental studies of the process. 

The possibility of feeding additional reactive material into the reaction zone is a determining factor in processing defect free articles. This is true not only for reactive processing but also for 

1 - insulation cover 2 - plane heater 3 - bottom 4 - safety-valve. 

The most favorable conditions for reactive processing of monolithic articles are created when the frontal reaction occurs at a plane thermal front. For example, a frontal process can be used for methyl methacrylate polymerization at high pressure (up to 500 MPa) in the presence of free-radical initiators. The reaction is initiated by an initial or continuous local increase in temperature of the reactive mass in a stationary mold, or in a reactor if the monomer is moving through a reactor. The main method of controlling the reaction rate and maintaining stability is by varying the temperature of the reactive mass.252 

---

Fig. 2 Precipitation triggered by orographic, convective, and frontal processes [6]...

At higher concentrations of fibres or at intermediate concentrations when a few fibres around the crack tip are orientated perpendicular to the notch plane, the loading curve increases linearly up to a maximum load Pi as the load is transferred onto the fibres at the crack front and a process zone develops. Fracture of the fibres lying normal to the notch plane results in unstable crack propagation until it is arrested by a packet of fibres favourably orientated then the applied load must be increased to create a new frontal process zone. Tlierefore the successive unstable crack extensions result in a saw-tooth like loading curve behaviour (types 3 and 3 loading curves in Table II, associated with Figures I OB and lOE, I OF respectively). 

Fig. 18 Evolution of the frontal process zone size as a function of the fibres concentration for the 8 wt% rubber-toughened composites.

A finite nonnegligible fracture process zone in front of the crack tip gives rise a deviation from the size-effect law prescribed in Eqs. (3a) and (3b). When the characteristic dimension of the frontal process zone Cf becomes significant compared to the characteristic dimensions of W and ao. Eqs. (3a) and (3b) must include the nonlinear contributions of the microscopic deformation and fracture in the process zone, and be modified through the replacement of the crack length ao with its effective value aeff( = oo + c/). This modification in Eqs. (3a) and (3b) results in the following formulae of failure strength in their intrinsic expressions ... 

A fracture mechanics estimate of the characteristic size of frontal process zone, Cf, can be made by recasting Eqs. (4a) and (4b) into the following equations [9 12] ... 

Ceramics have low fracture toughness because dislocations are difficult to move in ceramics especially at room temperature. It is, therefore, hypothesized that a frontal process zone (FPZ) ahead of a crack tip is composed of many nano-cracks rather than dislocations as in metals. To overcome the inherent brittleness of ceramics, a new microstructural design concept must be developed. The design concept of nanocomposites is a new, and significantly improved strengths are achieved with moderate enhancement in fracture toughness. The typical microstructure of nanocomposites consists of second-phase nano-size particles dispersed within the matrix grains. Thermal expansion mismatch between the matrix and second-phase particles improves several mechanical properties of nanocomposites. 

Figure 7. Schematic drawing of a frontal process zone and bridging in a wake for polycrystalline ceramics with R curve behavior.

H. Awaji, S M. Choi, and D. D. Jayaseelan, Indirect Estimation of Critical Frontal Process Zone Size Using a Single Edge V Notched Beam Technique, J. Ceram. Soc. Jpn., 109, 591 595 (2001). 

---