Osmotic cracking

In polyesters, the existence of an osmotic cracking process (see below), gives importance to other structural factors, especially the concentration of dangling chain ends. 

Aging does not modify the value of E significantly (except in the close vicinity of Tg), so that aging effects on fracture properties are relatively low in the brittle regime, except if defects are created. This is the case in polyesters (osmotic cracks), where durability is controlled by this process rather than by chain scission or any other structural change at the molecular or macromolecular scale. 

The problem of osmotic cracking is well known for polyester boat and swimming pool users. After a more or less long time of exposure in water, blisters and cracks appear at the surface. Their formation can be detected sometimes gravimetrically (Fig. 14.10). It can be shown that the induction time of osmotic cracking, tb, is (Gautier et al., 1999) ... 

Figure 14.10 Shape of a mass variation-time curve in the case of osmotic cracking (a) end of the diffusion transient, (b) onset of cracking, and (c) percolation threshold of the crack network. 

Osmotic cracking is influenced by the same factors as weight loss. The following mechanistic scheme can be proposed ... 

Any water-soluble additive is liable to be dissolved and leached out from a laminate over a period of time when exposed to wet environments, and if the additive is a particulate soUd, dispersed in the resin, the result can be widespread porosity. Before the additive is washed out altogether, the trapped pockets of additive solution constitute osmotic pressure sites, which induce microcracks in the matrix. Figure 2.8 shows a cross-section through a unidirectional glass epoxy laminate, in which the dissolving of residual unused dicyandiamide hardener by a hot, wet atmosphere caused porosity and osmotic cracks. Prolonged exposure at 100°C caused fragmentation of the laminate [15]. 

In the case of glass fibre/unsaturated polyester matrix composites, the kinetic curve of weight changes can present a peculiar shape revealing the presence of osmotic cracking (see Fig. 12.23). Such behaviour can be explained as follows ... 

Note that the phenomena of sorption and cracking are distinguishable if the sample thickness L is low enough to have < Iq, where = L D is the characteristic time of diffusion, D is the diffusion coefficient of water in the material, and Iq is the characteristic time of osmotic cracking, mainly linked to the hydrolysis rate and independent of thickness (Gautier et al., 1999). 

Osmotic cracking in a polyester for boat hulls, as revealed by the kinetic curve of weight changes after immersion in boiling water (according to Mortaigne et al., 1992). 

Tangelder GJM, Dubbleman M, Ringens PJ. Sudden reversible osmotic lens damage ( sugar cracks ) after initiation of metformin. N Engl J Med 2005 353 2621-2. 

Concentration-independent diffusion only occurs at low temperatures and/or low penetrant "activities". At high penetrant activities over a range of temperatures well below Tg the transport of penetrant into the polymer is accompanied by solvent crazing or cracking the osmotic stresses produced by the penetrant are sufficiently large to cause local fracture of the material. 

Osmotic transport through pores, cracks, or drilled holes... 

Gel-type ion exchangers are much less resistant to osmotic shock than macroporous ones. For example, after the application of 70 cycles of osmotic shock (by treating sulfonic acid resins alternatively with 76% HNOg and wastewater from an ammonium nitrate processing plant), the number of cracked beads in the case of a gel-type resin (Wofatit KPS) was 78.3%, but for macroporous resins the number varied from only 1.2% (Amberlite 200) to 7.6% (Duolite C26) [24]. Similar results have been obtained for anion exchangers. Thus, the number of the... 

These forces arising in the pores and cracks result, in particular, from repulsion of double electric layers of their walls, as well as from disruption of the water structure. Besides, osmotic pressure may also contribute because the ion concentration inside the pores increases due to adsorption relative to that in the outside solution [3]. 

At 25 "C, 1.0 mol/L of solution has an osmotic pressure of 84.8 cm/mol/deg jr 1.0 L/mole x 298 K = 2.5 x 10 cm of water. What this means is that a 1.0 M solution exerts about 25 atmospheres of pressure Fortunately, few systems have a 1 M solution on one side of a membrane and 0.0 M on the other. However, you know from experience that osmotic pressure can be quite large. How many times have you seen a concrete sidewalk or driveway raised up and cracked by tree roots In the days of wooden ships if cargoes of dried beans got wet, they were thrown overboard at once. The wet beans would swell so much and the force would be so great that it could push apart the hull of the ship. 

Loss of resins. Ultimately, the exchange capacity of the resin will diminish to the point where it is no longer effective. In a system where resin is recirculated, loss by attrition is superimposed on the other losses, which also include cracking of the resin by osmotic pressure. 

Other possible interfacial degradative mechanisms include the build up of osmotic pressure at the oxide/adhesive interface(6) (akin to the phenomenon of paint blistering by osmotic gradients), disbonding by alkali produced by the cathodic reaction in metallic corrosion(107), and the imposition of stress leading to bond stress-corrosion cracking( 108-110). 

Water penetration of coatings is usually destructive. Diffusion of water through the binder or penetration through submicroscopic cracks in the film allows water to accumulate at the coating-substrate interface. If transport of the water is driven osmot-icaUy, the resulting build up of pressure will disrupt the coating and lead to failure (see Durability fundamentals). 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

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