Curing flexural strength

Heat resistance is an important characteristic of the bond. The strength of typical abrasive stmctures is tested at RT and at 300°C. Flexural strengths are between 24.1 and 34.4 MPa (3500—5000 psi). An unmodified phenoHc resin bond loses about one-third of its room temperature strength at 298°C. Novolak phenoHc resins are used almost exclusively because these offer heat resistance and because the moisture given off during the cure of resole resins results in undesirable porosity. Some novolaks modified with epoxy or poly(vinyl butyral) resin are used for softer grinding action. 

Waferboard, a more recent wood constmction product, competes more with plywood than particle board. Waferboard and strand board are bonded with soHd, rather than Hquid, phenoHc resins. Both pulverized and spray-dried, rapid-curing resins have been successfully appHed. Wafers are dried, dusted with powdered resin and wax, and formed on a caul plate. A top caul plate is added and the wafers are bonded in a press at ca 180°C for 5—10 min. Physical properties such as flexural strength, modulus, and internal bond are similar to those of a plywood of equivalent thickness. 

Carbon-Fiber Composites. Cured laminates of phenoHc resins and carbon-fiber reinforcement provide superior flammabiHty resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 15 shows the dependence of flexural strength and modulus on phenoHc—carbon-fiber composites at 30—40% phenoHc resin (91). These composites also exhibit long-term elevated temperature stabiHty up to 230°C. 

The properties of the laminate are dependent on the resin and type of glass cloth used, the method of arranging the plies, the resin content and the curing schedule. Figure 29.4 shows how the flexural strength may be affected by the nature of the resin and by the resin content. 

Figure 22 Influence of fiber content on flexural strength and fracture toughness of (O) softwood-cement composites and ( ) hardwood-cement composites (air-cured) [78].

The presence of the cement hydrate/polymer comatrix in LMM and LMC confers superior properties, such as high tensile and flexural strengths, excellent adhesion, high waterproofhess, high abrasion resistance and good chemical resistance, when compared to ordinary cement mortar and concrete. The degree of these improvements however depends on polymer type, polymer-cement ratio, water-cement ratio, air content and curing conditions. Some of the properties affected by these factors are discussed below [87, 88, 93-95]. 

Experiments on the effect of different curing conditions on the compressive and flexural strengths of plain and air-entrained concrete [35] showed that air-entrained concrete has less tendency to lose moisture under drying conditions, which means that if concrete curing conditions are not ideal, air-entrained concrete should develop strength more normally than plain concrete [43],... 

Table 3. Dynamic mechanical properties and flexural strength of diamine cured epoxy resins...

Fig. 18. Effect of the number of methylene groups (n) in the curing agent on the flexural strength.

With continuous ageing, degradation of the cross-links occurs resulting in loss of flexural strength in the cured polyester. Use of proper antioxidants improves the resistance to heat deterioration. 

Flexural strength. -10-pIy test panels 36 x 100 in. with no surface skins were cured at 300°F for ... 

In Fig. 2 and Fig. 3, flexural strength and flexural modulus of the cured resin depending on the PET content are shown. It is observed that flexural modulus of the cured resin made from glycolyses product using PG is higher than... 

In Fig. 4 and Fig. 5, flexural modulus and flexural strength of the cured resin depending on the adipic acid content are shown. It is found that flexural modulus of the cured resin decreased as the adipic acid content in the resin was increased. Introduction of adipic acid instead of maleic anhydride implies decrease of unsaturation of the resin. It seems that the decrease of flexural modulus resulted from decreased crosslink density due to lower unsaturation. It is of interest to note that flexural strength, on the contrary, tends to increase as the adipic acid content is increased. It is speculated that the cured resins of higher adipic acid content have more flexible chain and the flexural modulus are low compared with that of the cured resin of lower adipic acid content relatively. But, ultimate strength of the cured resin of higher adipic acid content is superior to the cured resin of lower adipic acid content due to possibly higher ultimate strain. 

Fig. 5. Flexural strength(FS) of the cured UPE resins depending on the adipic acid (AA) content in the resin.

Autoclave-cured specimens including FRP powder do not show superiority in flexural strength while water-cured specimens including 9.1% of FRP powder indicate excellent strength. 

Keywords asbestos-free mortar, autoclave curing, extrude, flexural strength, FRP powder, porosity, water curing... 

Some studies have been already conducted on low density mortar including FRP fine powder. These studies deal only with cast mortar of low flexural strength below 10 MPa cured in water or high temperature air [2] or humid air [3], No studies have been reported about extruded composites and properties of autoclave-cured specimens. 

Intrusion volume of all autoclave-cured composites was bigger than that of water-cured except PAO. It is noticeable that the pore diameters of autoclaved composites exceed in diameters of 1 micrometer and 0.01 micrometer while water-cured ones exceed in 0.1 micrometer. Among all cases, water-cured PA1 shows the minimum intrusion volume, which performed the maximum flexural strength. 

Composition Cure Bulk density (gr/cc) Flexural strength (MPa) Absorbed energy (J) Flexural modulus (GPa) Water absorption ratio (%)... 

Flexural strength of autoclave-cured specimens shows a drastic drop by including FRP powder (see Fig. 13). PA that contains only fine powder as pozzolanic materials shows the highest flexural strength. PAO that contains silica sand locates the next. 

On the other hand, the order of strength in the case of water-cured specimens differs from that of autoclave-cured specimens. PA1 shows highest flexural strength. The increase of FRP content means decrease of flexural strength in this case, too. However, PA and PAO that contain no FRP powder indicate lower strength. 

Flexural strength of all water-cured specimens showed lower strength than that of autoclave-cuned ones except PA1. This exception may be caused by the reinforcing effect of GF, only when the content of FRP powder is... 

Autoclave-cure did not contribute any significant improvements to the flexural strength because of degradation of GF and FRP. However, it may lead to least longterm additional degradation. 

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