Tensile properties foams

Tensile strength and modulus of rigid foams have been shown to vary with density in much the same manner as the compressive strength and modulus. General reviews of the tensile properties of rigid foams are available (22,59,60,131,156). 

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). 

Medium to low tensile properties according to the tensile properties of the core. The tensile strength and modulus of a lOOkg/m foam are roughly as low as 3 MPa and 0.1 GPa, respectively. 

LDPE/EVA blend was irradiated using gamma-irradiation and then expanded by heat as a foamed material. The EVA content in the blend was optimised to form a gel. The effects of atmospheres and of irradiation dose rate were studied. The ETIR spectra of the foam revealed the oxidation level. The relations between gel fraction of LDPE/EVA blend, expansion ratio, apparent density, average cell diameter and tensile properties of the foam are discussed. 8 refs. 

While we are dealing mostly with open-cell foams, control of the degree of openness also has an effect on tensile properties. Saunders reported on the control of open-cell structures by the addition of a stannous catalyst. They showed maxima... 

FIGURE 3.11 Relationship of cross-link density to tensile properties of a foam. (Source From Saunders, J.H. and Frisch, K.C. ... 

In most cases, the factors that affect tensile properties also affect the compressive strength of a polyurethane. This is most easily depicted in a graph such as Figure 3.12 that shows compressive strengths of foams. 

Dow also developed polyurethane foams from polyols via hydroformylation of fatty acids. The foams have properties which are comparable to foams from petrochemicals in terms of density and flexibility. The advantages of using sustainable feedstocks in viscoelastic foams are increased load bearings and tensile and tear properties [39, 40]. The hydroformylation and consecutive hydrogenation of fatty acids derived from seed oil can also be used to form low viscosity polyester polyols. Therefore, fatty acid methyl esters are transesterified with diols, e.g., glycol (Scheme 12). The polymer contains chemically active hydroxy groups which can be used for polyurethanes in coating applications [41]. 

Since melamine is not soluble in the polyol or MDI, it should be very fine dispersed so that it does not interfere with the foaming process. The effect of melamine particle size on properties of flexible PU foams was studied by Kageoka et al.69 They reported that the foam with the finer particles showed higher hardness, better tensile properties, and less flammability than that with the larger ones. A flame-retarded foam with better physical properties can be manufactured by a polyol including melamine particles smaller than the strut thickness of the resultant foam. 

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

ASTM D 3574 - Test K covers dry-heat aging of flexible cellular materials. This test consists in exposing foam samples in an air-circulating oven at 140°C for 22 hours. The specimens are then removed and conditioned for not less than 12 hours at 23°C and 50 percent RH. As with the Steam Autoclave Tests (Jj and J2) physical tests are carried out before and after agiqg. According to Shah (1) tensile properties are usually studied after dry-heat aging. 

TABLE 17.11 Tensile Properties of Unfoamed and Batch MicroceUuIar Foamed PLA [2]... 

Figure 8. Tensile properties at maximum of DDC closed cell foams along the FD and of a commercial LDPE foam.

The results obtained in the field of thermoplastic starch in combination with polymers or copolymers of vinyl alcohol with aliphatic polyesters and copolyesters in terms of biodegradation kinetics, mechanical properties and reduced sensitivity to humidity make these materials ready for a real industrial development starting from film and foam applications. The present global market is around 12000 tons/year. Main producers are Novamont with Mater-Bi trade-mark, ENPAC and National Starch. The tensile properties of films made of two Novamont s Mater-Bi grades are reported in Table 3, in comparison to these of low density polyethylene (LDPE). Figs. 6-7 show applications of Mater-Bi starch-based materials now on the market. 

The main advantages of using foam concrete is that the density of the material can be varied between 300 and 1600 kg/m, and the foam provides good insulation properties and it is easy to handle by pumping. However, several properties still need to be modified in order to make foam concrete an even more competitive material. There is a need to decrease the shrinkage and the water uptake, and to increase the strength and the insulation ability. At present, the foam structures can be enhanced by the use of various additives, such as microsilica or superplasticizers which reduce the cement/water ratio and improve the stability. Polyester fibre additives enhance the hardening and also improve the tensile properties. 

Zeng C., Hossieny N., Zhang C., Wang B., and Walsh S., Morphology and tensile properties of PMMA carbon nanotube nanocomposite foams. Composites Science and Technology 2013, 82, 29-37. 

The dry heat test consists of exposing foam specimens in an air-circulating oven and observing the effect on physical properties of the foam. The tensile properties are determined using the same procedure described in the tension test for flexible foams. 

Mahfuz et al. [44] found that the addition of 3 wt% of TiO nanoparticles significantly increased (50-70%) the flexural strength and stiffness of PU nanocomposite foams. The tensile properties of extruded PS/CNF composites were investigated by Shen et al. [8]. They found that the tensile modulus of PS foams increased with increasing concentration of CNFs. When the fiber concentration reached 5% by weight, the tensile modulus of the CNF/PS nanocomposite foams with a density of 0.69 g/cm was 1.07 GPa, which is comparable to that of solid PS (1.26GPa). 

Throne has reported that the relationship between foam modulus and density can be generalised to other properties such as tensile strength, fatigue strength, creep properties as well as shear and compression modulus. Thus if X is the general material property then... 

We prepare force versus compression plots in a similar fashion to force versus elongation plots. We generally perform compressive testing over a much more limited range of strain than tensile testing. Samples typically take the form of thick pads, which do not break in the same manner as tensile specimens. The limit of compressive strain can approach 100% for low density foams, but is much less for other samples. The most common property that we obtain from this mode of testing is compressive modulus. 

Denning s three papers in the late 1960s (a.2-a.4) reviewed the development of closed-cell polyolefin foams, and their mechanical properties. Some of his predictions on materials development turned out to be true. In Part I he explains that non-crosslinked polyethylene (PE) foams have inferior creep properties to crosslinked foams this appeared to be the tensile creep of the melt, rather than compressive creep of the... 

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