Microvoids glasses

In investigations of the failure of fiber compositions (PETP — short glass fibers) [251] it was found that the main process responsible for composite failure under load is the rupture at the matrix-fiber interface. The author of [251] observed formation of microvoids in loaded samples, both at the interphases and in the bulk. The microvoids, or cavities) grow in size and become interconnected by microcracks, and this results in fiber separation from the binder. However, when the matrix-fiber bond is strong enough, the cavities appear mostly in the bulk of matrix, the failure of the specimen does not over-power cohesion and traces of polymer remain on the fibers. 

Ramesh, N. S., Rasmussen, D. H., and Campbell, G. A., The Heterogeneous Nucleation of Microcellular Foams Assisted by the Survival of Microvoids in Polymers Containing Low Glass Transition Particles. Part 1 Mathematical Modeling and Numerical Simulation, Polym. Eng. ScL, 34, 1685 (1994)... 

The flexibility and extensibility of a crosslinked epoxy network are determined by the available glassy-state free volume. If the free volume is insufficient to allow network segmental extensibility via rotational isomeric changes then the brittle mechanical response of the epoxy glass is not controlled by the network structure but rather by macroscopic defects such as microvoids. For epoxies with sufficient free volume that allows plastic network deformation the mechanical response is controlled by the network structure. 

Ramesh NS, Rasmussen DH, Campbell GA (1994) The heterogeneous nucleation of microcellular foams assisted by the survival of microvoids in polymers containing low glass-transition particles. 1. Mathematical-modeling and numerical-simulation. Polym Eng Sci 34 1685-1697... 

The damage process is accompanied by numerous microvoids, the development of which has been utilized for waterproof and/or breathable films. Other interesting segments are fibers, glass-fiber reinforced PP, thermo-formable grades and piping systems. 

In this paper we discuss (1) small main-chain motions and their effect on the flow processes, (2) the embrittlement of polycarbonate, (3) the formation of microvoids from sample preparation and their effect on the brittleness of polymer glasses, and (4) the modification of the degree of brittleness of polymer glasses at the filler interface in polymer composites. 

Microvoid Formation during Sample Preparation of Polymer Glasses. 

This is a reasonable conclusion for highly oriented crystalline polymers where the amorphous regions are probably composed largely of tie molecules. It is unlikely to be true of microvoids and craze cavities formed in amorphous glasses or poorly oriented crystalline polymers. 

Typical fillers calcium carbonate, barium sulfate, talc, kaohn, mica, quartz, sand, glass spheres, silica, titanium dioxide, aluminum hydroxide, carbon fiber, glass fiber, aramid fiber, aluminum, copper, silver, iron, graphite, molybdenum disulfide, zirconium silicate, hthium aluminum silicate, vermiculite, slate powder, titanium boride, ground rubber, iron oxide, microvoids... 

Pace and Datyner ( 5, 5) have also proposed a model for the absorption (solution) of small molecules in polymers applicable at temperatures above and below Tg, which incorporates the dual-mode sorption model for the glassy region. The presence of microvoids is assumed for rubbery polymers as well as for polymer glasses. "Hole filling" is suggested as an important sorption mode above as well as below Tg, with one crucial difference between the sorption mechanism in the rubbery and glassy regions hole saturation does not occur in the rubbery state because new microvoids are formed to replace those filled with penetrant molecules. 

Above the glass transition temperature, i.e. in the rubbery state, the mobility of the chain segments is increased and frozen microvoids no longer exist A number of physical parameters change at the glass transition temperanire and one of these is the density or specific volume. This is shown in figure V - 26 where the specific volume of an amorphous polymer has been plotted as a function of the temperature. 

Typically, a fiber bundle has hundreds to thousands of fibers in an elliptical cross-section with a width of a few nuflimeters. Considering that a glass or carbon fiber has a diameter of 10 microns approximately, and if all the fibers are densely packed in a bundle, the gap between the fibers is only in the order of microns (i.e., 10 m). This is much smaller empty space than the empty space between the fiber bundles which is typically of the order of millimeters (i.e., 10 m). These two types of empty spaces give rise to two scales of permeabilities encountered by the resin flow, and they may result in microvoid entrapment inside the fiber bundles. 

The cross-section microscope picture shows a microvoid in a random E-glass fiber/unsaturated polyester FRP sample manufactured with VARTM. Also note that there are two different scales of flow pathways for the resin infusion (i.e., the large flow channel between fiber tows and the small flow channel between individual fibers in a fiber tow). 

Lead-free assembly can stress the resin-to-glass bond, and even cause some level of resin decomposition, which can create microvoids within a PCB. These phenomena can make it easier for pathways for CAF formation to be created. The use of more thermally stable resin systems may be required when CAF resistance is needed in products that will experience lead-free assembly. 

Mizoguchi et al. [48] have studied the crystallization of amorphous polyethyleneterephthalate induced by sorbed CO2 using WAXD, IR, and density measurement at temperatures between 35°C and 85°C and at pressures up to 50 atm. The sorption of CO2 by PET reduces the glass-transition temperature of PET, as evidenced by the reduced values of crystallization half-times. However, the amorphous density of PET crystallized in the presence of CO2 was less than that for thermally crystallized PET. This observation was attributed to the presence of microvoids. 

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