Flexural strength formulations

Composite formulations were prepared as follows The straw samples as received from INEEL were ground to 0.69 mm in a hammer mill and oven dried to 1.1% moisture. The dried straw samples were then blended with various amounts of high-density polyethylene (HDPE), lubricants, and maleated polyethylene blends (MAPE) (see Table 2). The mixed formulations were then extruded with a 35-mm Cincinnati Milacron Model CMT 35 counterrotating conical twin screw extruder (Cincinnati Milacron, Batavia, OH), which produced a 9.525 x 38.1 mm2 solid cross-section. Flexural strength, density, and water sorption were measured for the extruded samples according to ASTM Standard Methods (13,14). 

The strength properties more often specified for plastics materials are (1) tensile strength and elongation, (2) flexural strength, (3) Izod and Gardner impact, and (4) heat deflection temperature under load. Our purpose here is not to describe each test in detail but to point out some of the known effects that colorants and other formulation ingredients can have on these properties. Table 22.1 lists the ISO and ASTM test methods for most of the physical properties, and ref. 1 (pp. 7-112) describes each of the methods in detail. Table 22.2 lists typical values of the above cite four properties for selected thermoplastics. 

Sulfur foams with a wide spectrum of properties can be prepared by this process. Properties generally vary with density, which may range from 3 to 45 lb/ft3. However, while density is kept constant, properties such as compressive strength, flexural strength, and closed-cell content may be altered by formulation changes. Some of the more common properties are listed in Table II. 

Urethane and polyurea segments have been introduced into a polymer containing polyesters, polyethers, and casein. When blended into hair spray formulations containing upto 5 wt% solids, enhanced curl retention and flexural strength resulted. 

Reinforced fillers are added to improve the tensile and flexural strength of epoxies and fillers used with success are silica, asbestos, and alumina. In recent years, encapsulated fillers containing salts of Ni, Cu, Co and Fe have been found to give excellent reinforcing properties. Anti-corrosion fillers such as AI2O3 are also added to some epoxy formulations. 

However, adding of inorganic materials into a WPC formulation with a coupling agent can complicate an outcome. For example, addition of nanoclay to WPC (50% wood flour and 50% polypropylene) in the presence of Polybond 3200 has systematically decreased both tensile and flexural strength of the final material, for amounts of the nanoclay of 0, 2,4, and 10%. Effect of tensile and flexural modulus was more complex. The authors concluded that the nanoparticles disrupt adhesion between wood flour and the polymer matrix [24]. 

TABLE 5.16 Effect of metal-base (zinc stearate and SXT 2000) and nonmetallic lubricant (ST 3100) in formulated packages on flexural strength and flexural modulus of a WPC (61-63% wood flour, 34% HDPE, polybond 3029 as a coupling agent)... 

Generally, there is a certain correlation between density, on the one hand, and flexural strength and modulus, on the other, for many other materials, and that correlation is not related to porosity. For example, there is a strong correlation (R = 0.984) between density of all 38 polyethylene materials, listed in Table 7.49 of Chapter 7, including LDPE, LLDPE, HDPE, and their flexural modulus (Figure 6.1). Besides, mineral fillers in WPC materials increase density of the final product and also increase its flexural modulus. However, this chapter is mainly concerned about relationships between density and properties of WPC having the same formulation but produced at different regimes. 

When WPC materials are manufactured with the same formulation but at a different speed, using vented or nonvented extruders, different moisture content of cellulosic filler, and so on, the resulting profiles often have different density (specific gravity). As a rule, the higher the specific gravity, the higher the flexural strength (Table 7.23). 

Introducing zinc borate (in amounts up to 3% w/w) to a WPC formulation does not effect the flexural strength and slightly increases flexural modulus (stiffness) of the materials (Table 13.4), which is understandable, as ZnB is an inorganic material. Besides, zinc borate in this amount does not effect the water absorption, as well as the postmanufactured shrinkage and the coefficient of expansion-contraction (Table 13.5). 

Compression strengths of the solid formulation, determined by ASTM C39-49 test methods, were about 7000 psi. Flexure strengths of 1700 psi were determined by using ASTM C293-54T test procedures. 

VE polyurethane resins have mechanical properties similar or superior to those of conventional VE and epoxies. Characteristics include a heat distortion temperature of 120C (248F). Ultimate elongation of an unreinforced molding compound without fillers is 5.5% tensile strength is 80 MPa and flexural strength 150 MPa. The resins can be custom-formulated. Applications include customized automobile parts, recreational vehicles, outdoor equipment, tubs/showers and electrical parts. The resins are suitable for standard molding processes some were specifically developed for pultrusion, RIM, foam, adhesive, and polymer concrete applications. 

Re-formulated polyacid-modified composite resins have also been found to have properties that decline following water uptake and secondary acid-base reaction (Table 4.6). In one study that demonstrated this, Dyract AP was shown to have lower compressive and biaxial flexure strength values following soaking in water for 4 weeks than they did at 24 h [32] (Table 4.6). By contrast, when stored in dry conditions, strengths were not significantly different between 24h and 4 weeks. 

Fig. 2.11. Effect of formulation and cure temperature on flexural strength development of a two-part epoxy (Ref. 19). (a) Normal type, (b) Rapid type.

Strength than in increasing flexural modulus. For homopolymer, copolymer, and TPO composites, loadings of 30 wt% C-3000 mica increased flexural strength 14.5%, 16.6%, and 21.6%, respectively. The higher aspect ratio micas such as HiMod-270, L-140, and L-135 give less improvement than C-3000 except in the TPO formulation. In TPO, 30 wt% HiMod-270 improved flexural strength 30% vs. 21.6% for C-3000. 

Newer carbon nanocomposite PI formulations have even higher service temperatures up to 816 °C with high flexural strength. 

---