Flexural Modulus and Strength

Modulus measurements carried out at higher span/depth ratios are less influenced by the penetration of the loading nose and supports into the specimen. A better technique is to measure the deflection at different span/depth ratios for the same load. 

Using the variable span method with 3-point loading  

C = softness constant due to machine softness, deformation of sample at load points 

Hence m can be obtained by the method of least squares and the value for the modulus can be determined. 

Note load roller Sf/h % span points may also be 5.3,10.7 and 13.3 mm span points may also be 8,16 and 20 mm  

Flexural modulus is a convenient measure of composite stiffness. Fillers can con-tribute significantly to a stifmess increase. 1 he 

Attempts to improve flexural strength by surface treatment of fillers have not, to date, been successful. A variety of silanes, titanates, and fatty acids and their derivatives have been used to coat magnesium hydroxide for use as a filler in polypropylene. Almost all composites had inferior flexural properties. In the few cases where some improvement was seen, it was 10% more then the unfilled material. 

Treatment of ultrafine talc with an acrylic modifier for use as a filler in rigid PVC always resulted in a gradual decrease of flexural modulus as the modifier concen- 

Better mixing methods and processing techniques which align fibers in the composite seem to be the most promising avenues to improve flexural modulus with filler additions. Mixing speed choice allows to increase flexural strength by 25%. 

Finding a way to balance often conflicting requirements is the most challenging aspect of product development work. If a material is formulated to be fire resistant, UV stable, or moisture resistant, it may have inadequate mechanical 

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Those stmctural variables most important to the tensile properties are polymer composition, density, and cell shape. Variation with use temperature has also been characterized (157). Flexural strength and modulus of rigid foams both increase with increasing density in the same manner as the compressive and tensile properties. More specific data on particular foams are available from manufacturers Hterature and in References 22,59,60,131 and 156. Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. The shear properties increase with increasing density and with decreasing temperature (157). 

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. 

Excellent mechanical properties with very high values for tensile strength, flexural strength and modulus in the flow direction. This applies to unfilled materials and may be further enhanced by incorporation of fibrous fillers. Quoted data for these properties are in the following ranges ... 

Curves showing change of tensile strength, flexural strength, and modulus with increasing temperatures or other environments. 

Amorphous polyarylates are light-amber transparent materials which exhibit mechanical properties comparable to that of unfilled PET in terms of tensile or flexural strength and modulus (Table 2.13) but are notably superior in terms of heat resistance (HDT = 174°C vs. 85°C for PET) and impact strength. 

Table 20.2 shows the effect on physical properties of a 2.5% letdown of a 70% Ti02 dispersion in liquid color. Tensile yield strength, Heat deflection temperature (HDT), and flexural strength and modulus are the only properties where there is some negative effect, essentially a plasticizing effect. Several properties in various resins are enhanced or unaffected. Consult this table before choosing a coloration system and decide what properties are important. 

Flexural strength and modulus of rigid foams increase with increasing density in the same manner as the compressive and tensile properties. 

In one further work on epoxy-POSS, comparative studies were conducted on epoxy/ladderlike polyphenylsilsesquioxane (PPSQ) blends and the associated nanocomposites [2]. The work revealed that, although a decrease in the flexural strength and modulus of epoxy/POSS nanocomposites in comparison to the neat epoxy resin was observed, only flexural strength deteriorated in the epoxy/PPSQ blends compared to the neat epoxy resin. Flexural modulus of epoxy/PPSQ blends was reported to be much higher than that of the epoxy resin and also increased with an increase in POSS content. It was... 

C-580 Flexural strength and modulus of elasticity on chemical-resistant mortars, grouts and monolithic surfacing. 

The outstanding characteristics of this material are its UL 94 V-0 and 5V flammability rating, heat-deflection temperature of 420 F (216 C) at 66 psi (0.45 MPa), high flexural strength and modulus, and excellent chemical and solvent resistance. PBT resins are especially suitable for applications requiring a combination of high heat endurance, sti ess, chemical resistance, and moderate creep (7) (9). 

FRP powder decreases flexural strength and modulus of elasticity but increases water absorption ratio in all cases. It may lead to durability problems. 

Polysulphone prepared from 2,2-bis(4-hydroxyphenyl)propane and 4,4-dichlorodiphenylsulphone is particularly stable to electron irradiation [381]. Retention of flexural strength and modulus are retained... 

It is often observed that the higher the fiber content, the higher the flexural strength and modulus of the WPC. For example, increase of wood fiber content from 20 (w/ w) to 40, 50, and 60% (w/w) in polypropylene led to a systematic increase in flex strength and modulus, resulting in their overall increase by more than 200% [139],... 

TABLE 3.13 Flexural strength and modulus of composite 2X6 deck boards, filled with rice hulls (57% by weight) and those filled with rice hulls (32% w/w) and Biodac (29% w/w)... 

In reality an amount of VOC produced due to decomposition of rice hulls (or wood flour for that matter) in the extruder hardly exceeds 0.25-0.50%. However, even in this case porosity of the composite material, and decrease of its density (and, hence, flexural strength and modulus) can be significant. In a modified above example, 200 kg of the composite material, containing 50% (w/w) rice hulls and having a total volume of 175 L, would accumulate 500-1000 g of VOC, that is,... 

TABLE 4.5 Effect of talc on flexural strength and modulus of wood-flour-polypropylene composite material in the presence of different amounts of a lubricant (data were provided hy Luzenac America)... 

Table 4.5 shows that although flexural strength and modulus increase with the increase of the amount of talc with respect to that of wood flour, the lubricant deaeases the effect. 

Effect of talc on flexural strength and modulus of plastics is often higher compared with that of calcium carbonate. For example, polypropylene filled with 58% CaC03 had flex strength and modulus of 4430 270 and 365,000 44,000 psi, respectively, but the same polypropylene filled with 20% CaCOs and 30% talc had flex strength and modulus of 6125 + 100 and 486000 18000 psi, respectively. 

Tables 4.28 and 4.29 show effect of a montmorillonite nanoclay employed as a masterbatch (PolyOne Nanoblend 1001) in polypropylene on polypropylene-based composite containing 50% (w/w) maple wood flour and on flexural strength and modulus of the WPC.

Unlike WPC, neat polypropylene filled with the same nanoclay significantly increases its flexural strength and modulus (Table 4.30). 

One can see that nanoclay consistently decreases tensile and flexural strength and modulus of the polypropylene-based composite material, and only slightly (less than 10%) increases tensile and flexural modulus of the composite. 

Metablen A3000 is made in a small granular form (50 mesh size), and at 2-5% w/w, it reportedly improves dispersibility and flowability of hot melts and improves the impact resistance of WPC profiles (ratio of wood powder to polypropylene 80 20 data of Mitsubishi Rayon America). Examples of effect of Metablen A3000 on flexural strength and modulus of a WPC are shown in Tables 5.7 and 5.9. 

Dover Chemical Corporation produces resinous chlorinated paraffins under a name Chlorez . All Chlorez grades have a physical form of white powder (particles smaller than 50 mesh) with chlorine content around 70%. The manufacturer recommends Chlorez as flame retardants (in a combination with antimony trioxide) and lately as a nonreactive coupling agent under a brand name Doverbond (such as Doverbond DB 4300 or Doverbond 3000). The manufacturer claims that Chlorez in the amount of 10% along with 3% of a lubricant in the WPC shows an effect of a coupling agent and increases the flexural strength and modulus of the product, as well as the UV and moisture resistance (private communication, Dover Chemical Corp.). DB 4300 lists at 1.50/lb [12,13]. 

Polypropylene-based maleated coupling agents can be used in HDPE-based WPCs, particularly if a small amount of polypropylene is added to the system. Table 5.11 compares two HDPE-based composites, containing rice hulls and Bio-dac as cellulosic/mineral fillers, with Polybond 3009 (HDPE-based) and Polybond 3200 (polypropylene-based). As one can see, in all the cases both flexural strength and modulus are significantly increased. 

Water absorption in the presence of coupling agents are shown in the presence of different lubricants. The lowest water absorption was with the same lubricants that provided the best increase in flexural strength and modulus, though the latter effects were only 19 and 23%, respectively (data by Ferro Corporation). 

TABLE 6.1 The effect of density (specific gravity) of GeoDeck composite pickets of the railing system on their flexural strength and modulus (stiffness)... 

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