Resin hydrostatic pressure

Let us first consider the synergistic elfect that water has on void stabilization. It is likely that a distribution of air voids occurs at ply interfaces because of pockets, wrinkles, ply ends, and particulate bridging. The pressure inside these voids is not sufficient to prevent their collapse upon subsequent pressurization and compaction. As water vapor diffuses into the voids or when water vapor voids are nucleated, however, there will be an equilibrium water vapor pressure (and therefore partial pressure in the air-water void) at any one temperature that, under constant total volume conditions, will cause the total pressure in the void to rise above that of a pure air void. When the void pressure equals or exceeds the surrounding resin hydrostatic pressure plus the surface tension forces, the void becomes stable and can even grow. Equation 6.5 expresses this relationship... 

Resin flow models are capable of determining the flow of resin through a porous medium (prepreg and bleeder), accounting for both vertical and horizontal flow. Flow models treat a number of variables, including fiber compaction, resin viscosity, resin pressure, number and orientation of plies, ply drop-off effects, and part size and shape. An important flow model output is the resin hydrostatic pressure, which is critical for determining void formation and growth. 

The second potential problem is illustrated in Figure 10.15. With only vacuum pressure applied during the initial part of the cure cycle, the hydrostatic pressure on the resin can be extremely low, even negative. This is an ideal condition for void formation and growth if allowed to persist to high enough temperatures. 

Decreasing the water absorption mechanically , i.e. by coating the external surface of an article, produces good results. It was shown that coating of epoxy syntactic foams with a thin layer of epoxy resin reduces the water absorption more than tenfold, even under high hydrostatic pressure 140). 

The third and perhaps most important class of water repellents consists of materials appHed to the surface of concrete for above-grade stmctures or others where water pressure on the concrete is small. This includes damp-proofing in which treatments cannot be subjected to continuous or even intermittent hydrostatic pressure (83). Repellents that may be used are oils, waxes, soaps, resins, and sihcon-based systems (84). 

Revision A of this specification was published 10 March 1957. There are two types. Type I for 4,5000 psig hydrostatic pressure and Type II for 10,000 psig. The QPL was cancelled on 31 March 1986. The syntactic buoyancy material consists of a low-density filler such as hollow-glass microspheres in a resin matrix such as epoxy resin. 

ACC-4 still has about 3-4% voids and fissures, which are detrimental to the performance of the material. In its use as a missile nose cone or rocket nozzle, the extremely hot gas environment can result in rapid degradation of the part because the chemically reactive gases can rapidly permeate the structure via the interconnecting fissures and voids and attack the carbon fibers. Furthermore, in the bow shock wave of a missile nose cone that reenters the ionosphere, the atomic oxygen that is formed can similarly permeate and attack the fibers. Further densification of ACC-4 by another PIC cycle (to reduce the voids to below the 3% level) is virtually impossible because the resin or pitch cannot be forced into the microcracks and pores of the composite even under extremely high hydrostatic pressure of 700-1,500 bar, because of viscosity and surface tension considerations. 

Cartridge type filters are operated at relatively low hydrostatic pressures. Their useful life is limited due to plugging of the membrane pores by retained solutes. The actual cartridge is made by pleating a membrane sheet and potting the ends by an appropriate resin or hot-melt-glue as indicated in Figure 1.33 (a). 

Fig. 34.15 shows the thermal expansion of the RT-cured adhesives under several pressures measured using the bellows vessel. The adhesives expanded greatly, even though they had been cured. The maximum volume expansion under atmospheric pressure was about four-fold at 90 °C. Above 100 °C, it seems that volume increased again. However, the volume expansion was caused by leaked gas it was confirmed that the gas leaked from the specimen by thermodilatometry with the glass syringe (Fig. 34.4). Since the microcapsules in the adhesive were expanded about eight-fold, only 1-2 MPa of hydrostatic pressure was apphed from the microcapsules to the matrix resin, as shown by the intersection in Fig. 34.14.

The vinyl ester resins are the most corrosion resistant of any of the monolithic surfacing systems, and they are also the most expensive and difficult to install. They are used when extremely corrosive conditions are present. The finished flooring is vulnerable to hydrostatic pressure and vapor moisture transmission. Refer to Table 18.5 for their resistance to atmospheric corrosion and Table 18.10 for their resistance to selected corrodents. 

In Ref [62], the studied object was an epoxy polymer on the basis of resin UP5-181, cured by iso-methyltetrahydrophthalic anhydride in the ratio by mass 1 0.56. Testing specimens were obtained by the hydrostatic extrusion method. The indicated method choice is due to the fact, that high hydrostatic pressure imposition in deformation process prevents the defects formation and growth, resulting to the material failure [64]. The extrusion strain was calculated according to the Eqs. (14.10) and (4.39) and makes up 0.14, 0.25, 0.36, 0.43 and 0.52. The obtained by hydrostatic extrusion specimens were annealed at maximum temperature 353 K during 15 min. 

Some high performance laminates consisting of carbon fiber webs and epoxy resins are cured in autoclaves. An autoclave is a pressure chamber in which the pressure is appHed hydrostatically. 

Fig. 3. Liquid water uptakes for a TGDDM-DDS neat resin at increasing applied hydrostatic relative pressures. (22)...

The transfer of autoclave pressure to the resin in the laminate does not occur hydrostatically because the resin is not enclosed in a constant-volume system. Flow can occur initially both vertically (thickness direction) and horizontally. Furthermore, the network of fibers can also eventually act as a network of springs to which the vacuum bag and bleeder assembly transfer the stress from the autoclave pressure. This stress can then be transferred... 

The Dave model considers a force balance on a porous medium (the fiber bed). The total force from the autoclave pressure acting on the medium is countered by both the force due to the springlike behavior of the fiber network and the hydrostatic force due to the liquid resin pressure within the porous fiber bed. Borrowing from consolidation theories developed for the compaction of soils [23,24], the Dave model describes one-dimensional consolidation... 

An appreciation of the importance of hydrostatic resin pressure must be developed to understand void growth fully. Because of the load-carrying capability of the fiber bed in a composite layup, the hydrostatic resin pressure needed to suppress void formation and growth is typically only a fraction of the applied autoclave pressure. The hydrostatic resin pressure is critical because it is the pressure that helps to keep volatiles dissolved in solution. If the resin pressure drops below the volatile vapor pressure, then the volatiles will come out of solution and form voids. 

In the early stages of the cure cycle, the hydrostatic resin pressure should be equal to the applied autoclave pressure. As resin flow occurs, the resin pressure drops. If a laminate is severely overbled, then the resin pressure could drop low enough to allow void formation. Thus, the hydrostatic resin pressure is directly dependent on the amount of resin bleeding that occurs. As the amount of bleeding increases, the fiber volume increases, resulting in an increase in the load carrying capability of the fiber bed. 

To investigate the potential pressure gradients that exist within a laminate during autoclave processing, miniature pressure transducers (Fig. 10.5), which are capable of measuring the hydrostatic resin pressure, were embedded at multiple locations within several laminates to study the effects of vertical and horizontal pressure gradients [9]. 

Void formation and growth in addition curing composite laminates is primarily due to entrapped volatiles. Higher temperatures result in higher volatile pressures. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the resin (i.e., the hydrostatic resin pressure) while the resin is a liquid (Fig. 10.9). The prevailing relationship, therefore, is ... 

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