Polyethylene yield stress

The plastic deformation of such polymers is a major research area and has a triennial series of conferences entirely devoted to it. The process seems to be drastically different from that familiar from metals. A review some years ago (Young 1988) surveyed the available information about polyethylene the yield stress is linearly related to the fraction of crystallinity, and it increases sharply as the thickness... 

Figure 3 Chemometric calibration of the elastic modulus (left) and the yield stress (right) of a series of polyethylenes. Reproduced from Gabriel et al. [23], Copyright 2003, with permission from Wiley-VCH Verlag GmbH.

Harden s (27) market survey of the growth of polyolefin foams production and sales shows that 114 x 10 kg of PE was used to make PE foam in 2001. The growth rate for the next 6 years was predicted as 5-6% per year, due to recovery in the US economy and to penetration of the automotive sector. In North America, 50% of the demand was for uncrosslinked foam, 24% for crosslinked PE foams, 15% for EPP, 6% for PP foams, 3% for EVA foams and 2% for polyethylene bead (EPE) foam. As protective packaging is the largest PE foam use sector, PE foam competes with a number of other packaging materials. Substitution of bead foam products (EPP, EPE, ARCEL copolymer) by extruded non-crosslinked PE foams, produced by the metallocene process was expected on the grounds of reduced costs. Compared with EPS foams the polyolefin foams have a lower yield stress for a given density. Compared with PU foams, the upper use temperature of polyolefin foams tends to be lower. Eor both these reasons, these foams are likely to coexist. 

In a way similar to that described for polyethylene fere-phthalate (Sect. 4.2), some antiplasticiser small molecules with a specific chemical structure are able to affect the ft transition and the yield stress of epoxy resins, but they do not have any effect on the y transition. In the case of HMDA networks, an efficient antiplasticiser, EPPHAA, whose chemical structure is shown in Table 8, has been reported [69]. The investigation of such antiplasticised epoxy networks by dynamic mechanical analysis as well as solid-state NMR experiments [70] can lead to a deeper understanding of the molecular processes involved in the ft transition and of their cooperativity. 

Another motivation for measurement of the microhardness of materials is the correlation of microhardness with other mechanical properties. For example, the microhardness value for a pyramid indenter producing plastic flow is approximately three times the yield stress, i.e. // 3T (Tabor, 1951). This is the basic relation between indentation microhardness and bulk properties. It is, however, only applicable to an ideally plastic solid showing no elastic strains. The correlation between H and Y is given in Fig. 1.1 for linear polyethylene (PE) and poly(ethylene terephthalate) (PET) samples with different morphologies. The lower hardness values of 30-45 MPa obtained for melt-crystallized PE materials fall below the /// T cu 3 value, which may be related to a lower stiff-compliant ratio for these lamellar structures (BaM Calleja, 1985b). PE annealed at ca 130 °C... 

Oxidation raised the yield stress, increased elongation and reduced the stress at break. Figure 5 shows that the linear polyethylene was brittle at room t perature after an... 

Figure 5. Stress-strain behavior of the linear polyethylene in Figure 3. The yield stress increases with oxidation.

Figure 8.4 A tensile neck in a polyethylene Tensar soil stabilising grid. Curvature of the principal stress directions increases the average yield stress on the section AA.

Tensile Properties Similar to polyethylene, the stress-strain curve of JSR RB has a yield point. Above the yield point, the stress-strain curve continues to increase with elongation, then breaks. This kind of stress-strain curve is similar to EVA and indicates a characteristic property lying somewhere between amorphous and crystalline polymers. The dynamic properties of JSR RB can be improved by stretch-... 

When an initially isotropic polymer is drawn or extruded to a high deformation ratio under suitable conditions, it develops appreciable anisotropy which is apparent in mechanical tests at all stresses up to and beyond the yield stress. A typical example of the anisotropy of yield observed is shown in Fig. 2 in which the tensile yield stress of oriented polyethylene terephthalate (PET) sheets is shown as a fiinction of the an e 6 between the tensile axis (TA) and the initial draw direction (IDD). This large anisotropy is somewhat similar to that observed in cold-rolled metal sheets, for which theories of anisotropic plasticity were suggestedby Hill, Yoshimura and others. A modification of the theory... 

Fig. 2. The variation of tensile yield stress with 0, the angle between the initial draw direction and the tensile axis. The material is oriented polyethylene terephthalate sheet, draw ratio 5 1. Equation (8) was used to construct the full-line and is a good fit to the available data (after Brown et...

Fig, 5. The tensile and compressive yield stresses of (a) oriented polyethylene terephthalate, (b) oriented polypropylene plotted as a function of extension ratio. The stress axis is parallel to the orientation direction. It is clear that for both materials, only the tensile yield stress is appreciably dependent on degree of orientation. 

To probe the models for nucleation-controlled plastic flow we compare the predicted temperature dependence of the tensile plastic resistance with the tensile-yield-stress experimental results of Brooks and Mukhtar (2000). For comparison the polyethylene PE3 of average molecular weight = 131000 with a crystallinity of only 0.673 and lamella thickness of 34.3 nm is chosen. For the predictions of the temperature dependence, eqs. (9.26)-(9.28) of Section 9.4.3 are used, where we take in the denominator of eq. (9.25) the factor (1 + K), since the experiments were performed in tension. Noting that the lamella thickness of this polymer type is 1 = 34.3 nm, which is thicker than that for mode A of monolithic-screw-dislocation nucleation, we consider only modes B and C involving nucleation of screw-dislocation half loops and edge-dislocation half loops, and, together with the results of Fig. 9.21, we state the expected tensile yield stress Oy to be... 

Brooks, N. W. J. and Mukhtar, M. (2000) Temperature and stem length dependence of the yield stress of polyethylene, Polymer, 41, 1475-1480. 

Since polyethylene mainly crosslinks, the effect of irradiation is to generally enhance the physical properties. For example, for HOPE an increase in both the yield stress and secant modulus at 0.5% strain is observed [52], The effect on the properties above the melting point have been extensively studied [82] and there is a direct relationship of the elastic modulus measured at 160 °C to the dose. For irradi-... 

As an example, the tensile deformation of polycarbonate/polyethylene blends is similar for a range of concentrations except for the magnitude of the yield stresses (Yee 1977). In this blend polycarbonate matrix undergoes strong yield shearing, and the decisive factor is the shear resistance of polycarbonate. 

Calcitrm carbonate in polyethylene has two mutually opposite effects the reinforcement and the nucleating effect. The reinforcement effect increases the btrlk crystalhnity and modttlus. The nucleating effect decreases the spherulite size and has a negative influence on yield stress, neutralizing the positive irtfluence of increase in percentage crystallinity. ... 

Maleated or acrylic acid grafted polypropylene can also be used as a coupling agent for these types of compounds. The use of maleated polyethylene and polypropylenes in this application is widely documented. Jancar (36) conducted a study of maleated polypropylene in Mg(OH)2 filled polypropylene. Their findings indicated a critical concentration of carboxylic acid groups. At low levels of addition, the composite yield stress increased sharply. However, at a certain concentration based on filler content, very little increase in yield stress was observed with increased addition of PPgMAH. The possible explanations give for this were... 

---