Curing limitation

Fly ash increases the density, decreases the permeability, and increases the leaching resistance of Ordinary Portland Cement (OPC). It is a truism that The leach resistance of solidified cement-waste systems can be improved by any process which accelerates curing, limits porosity, or chemically bonds fission product or actinide elements. (Jantzen et al., 1984). Supercritical C02 treatment of a modified Portland cement is expected to further increase the density over the untreated material, so that a reduced porosity and improved leachability should result. In addition, the high silica content of fly ash, with its well-known sorbent properties toward actinides and certain other radionuclides, enhances the immobilization characteristics. 

Brittle when cured limited thmnal and hydrolytic stability (-50°C to +80X)... 

Though these materials were designed to combine properties of glass-ionomers with those of composites [39], their handling is entirely that of a typical composite. Through cure limitations, coupled with the need for photo-initiation, means that inCTonental... 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

CeUulose phosphate esters are also produced by treatment with sodium hexametaphosphate [14550-21-1] by the pad-dry-cure technique. These treated fabrics have high retention of breakiag and tearing strength (61). The reaction products contain more than 1.6% phosphoms and are iasoluble ia cupriethylenediamine [15243-01 -3] iadicating that some ceUulose cross-linking occurs. However, siace durable-press (DP) levels and wrinkle recovery values are low, it seems reasonable that only limited cross-linking takes place. 

In this case, the components are mixed, the pH adjusted to about 6.0 with sodium hydroxide, and the solution appHed to the textile via a pad-dry-cure treatment. The combination of urea and formaldehyde given off from the THPC further strengthens the polymer and causes a limited amount of cross-linking to the fabric. The Na2HP04 not only acts as a catalyst, but also as an additional buffer for the system. Other weak bases also have been found to be effective. The presence of urea in any flame-retardant finish tends to reduce the amount of formaldehyde released during finishing. 

Sodium Nitrate and Sodium Nitrite. Nitrates and nitrites ate used in meat-curing processes to prevent the growth of bacteria that cause botulism. Nitrates have been shown to form low, but possibly toxic, levels of nitrosamines in certain cured meats. For this reason, the safety of these products has been questioned, and use is limited (80). 

Several methods have received considerable research attention as alternatives to salt curing. These include use of sodium bisulfite as a disinfectant to allow preservation with or without decreased salt in a brine cure use of disinfectants such as quatenary amines for temporary preservation in direct shipping to the taimery from the packing plant (see Disinfectants and antiseptics) preservation of hides by radiation sterilization (see Sterilization techniques) and substitution of materials such as potassium chloride for sodium chloride. These methods have found only limited commercial success. 

Alkaline Catalysts, Resoles. Resole-type phenoHc resins are produced with a molar ratio of formaldehyde to phenol of 1.2 1 to 3.0 1. For substituted phenols, the ratio is usually 1.2 1 to 1.8 1. Common alkaline catalysts are NaOH, Ca(OH)2, and Ba(OH)2. Whereas novolak resins and strong acid catalysis result in a limited number of stmctures and properties, resoles cover a much wider spectmm. Resoles may be soHds or Hquids, water-soluble or -insoluble, alkaline or neutral, slowly curing or highly reactive. In the first step, the phenolate anion is formed by delocali2ation of the negative charge to the ortho and para positions. 

Cure kinetics of thermosets are usually deterrnined by dsc (63,64). However, for phenohc resins, the information is limited to the early stages of the cure because of the volatiles associated with the process. For pressurized dsc ceUs, the upper limit on temperature is ca 170°C. Differential scanning calorimetry is also used to measure the kinetics and reaction enthalpies of hquid resins in coatings, adhesives, laminations, and foam. Software packages that interpret dsc scans in terms of the cure kinetics are supphed by instmment manufacturers. 

Dynamic mechanical analysis provides a useful technique to study the cure kinetics and high temperature mechanical properties of phenoHc resins. The volatile components of the resin do not affect the scan or limit the temperature range of the experiment. However, uncured samples must be... 

Different phenoHc resins are used for different types of wood for example, plywood adhesives contain alkaline-catalyzed Hquid resole resins. Extension with a filler reduces cost, minimizes absorption, and increases bond strength. These resins have an alkaline content of 5—7% and are low in free phenol and formaldehyde. Because many resins have a high water content and limited storage stabiHty, they are frequently made at or near the mill producing the plywood product. The plywood veneers are dried, coated with resin, stacked for pressing, and cured at 140—150°C. 

An important aspect of this is the splitting of the polymer chain with thiol (eq. 5) or mercaptide ion (thiol + base catalyst). In fact, sodium sulfide or organic monothiols, eg, mercaptoethanol or decyhnercaptan, are utilized to lower the molecular weight of polysulftdes or to limit the extent of curing reactions. 

The production of alkylphenols exceeds 450,000 t/yr on a worldwide basis. Alkylphenols of greatest commercial importance have alkyl groups ranging in size from one to twelve carbons. The direct use of alkylphenols is limited to a few minor appUcations such as epoxy-curing catalysts and biocides. The vast majority of alkylphenols are used to synthesize derivatives which have appUcations ranging from surfactants to pharmaceuticals. The four principal markets are nonionic surfactants, phenoUc resins, polymer additives, and agrochemicals. 

However, conventional systems ia natural mbber do provide better flex life than EV cures, and this is one of the limitations of EV curiag. The short monosulftde bonds are less able to rearrange to reheve localized stresses which build duriag flexing, whereas the longer S bonds can. This abiUty for stress rehef is thought to be the mechanism for the superior flex life of conventional cures. 

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