Polyethylene microhardness

F. J. Balta -Calleja, C. Santa Cruz, R. K. Bayer, and H. G. Kilian, Microhardness and Surface Free Energy in Linear Polyethylene The Role of Entanglements, Colloid Polymer Sci., 268,440 (1990). 

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... 

Figure 1.2. Typical microhardness values of polymers compared with data for metals. LDPE, low-density polyethylene HDPE, high-density polyethylene PA, polyamides POM, polyoxymethylene CEPE, chain-extended polyethylene CF-composite, carbon-fibre composite PS, polystyrene PEN, poly(ethylene naphthalene-2,6-dicarboxylate. (From Balta Calleja Fakirov, 1997.)...

The microhardness of LDPE fractions and of high-density polyethylene (HOPE) samples has been studied as a function of molecular weight in the range of about 2 X 10 up to 4 X 10 (Baltd Calleja et al, 1997). Details of the lamellar structure were determined by TEM. The observed decrease of microhardness with increasing molecular weight is mainly due to the increase in thickness of the interlamellar layers (i.e. a decrease of crystallinity). After chemical treatment with chlorosul-phonic acid and with OSO4, samples show a drastic increase in microhardness. 

Equation (2) suggests that the volume fraction of crystalline material controls the microhardness value of a pol5mier. However, it was soon recognized the large influence of the crystalline lamellar thickness U upon microhardness in case of chain-folded and chain-extended polyethylene (PE) (4). Based on a thermodynamic approach, the dependence of hardness on the average crystal thickness was derived (15) ... 

Hardness measurements such as Rockwell or Vicker s indentation properties are time-dependent as a result of the viscoelastic flow and relaxation processes (236) (see Hardness). Microhardness measurements have been used to correlate with other properties such as Yoimg s modulus and compressive yield stress in polyethylenes (237) and glass-transition temperature of amorphous plastics (238). Scratch resistance in polyproplyene studies was found to have shear yielding as the main cause of plastic flow scratch pattern with tensile tear effects on the surface and shear-induced fracture on the subsurface (239). 

Hence, in the present chapter the Hill and Marsch equations fractal analogs are obtained, which has shown, that cross-linked epoxy polymers microhardness is defined by their structure only, characterized by its fractal dimension. Tabor s criterion is only fulfilled for Euclidean (or close to them) solids. The cross-linking degree enhancement results to loosely packed matrix loosening and corresponding reduction of cross-linked epoxy polymers microhardness. The similar results obtained for linear polyethylene and nanocomposites on its basis, filled with organoclay [16]. 

Kozlov, G. V, Beloshenko, V. A., Aloev, V. Z., Varyukhin, V. N. (2000). Microhardness of Ultra-High-Molecular Polyethylene and Componor on its Basis Produces by Solid-Phase Extrusion Method. Fiziko-Khimicheskaya Mekhanika Materialov, 36(3), 98-101. 

Figure 5.17 Surfaces cracks in polyethylene (arrows indicate indentation of microhardness measurement) [651]...

Peneva Y, Tashev E and Minkova L (2006) Flaimnability, microhardness and transparency of nanocoinposites based on functionalized polyethylenes, Europ Polym J 42 2228-2235. 

Perena J M, Martin B and Pastor M (1989) Microhardness and dynamic mechanical measurements in polyethylene near the betarrelaxation, J Mater Sci Lett S S49-S51. 

Fakirov S and Krasteva B (2000) On the glass transition temperature of polyethylene as revealed by microhardness measurements, J Macromol Sci Phys B39 297-301. 

Fakirov S, Krumova M and Rueda D R (2000) Microhardness model studies on branched polyethylene, Polymer 42 3047-3056. 

Rueda D R, Balta Calleja F J and Bayer R K (1981) Lifluence of processing conditions on the structure and surface microhardness of injection-molded polyethylene, J Mater Sd 16 3371-3380. 

Rueda D R, Bayer R K and Balta Calleja F J (1989) Microhardness and mechanical anisotropy of elongatioual flow-injection molded polyethylene, J Macromol Sci Phys 828 267-284. 

Figure 6.33 The dependences of the microhardness on the extra energy localisation regions dimension D for EP-1 (1), EP-3 (2), polyarylate (3), ultra-high molecular polyethylene (UHMPE) (4) and Componor UHMPE-bauxite (5) [87]...

The introduction of bauxite as a filler in ultra-high molecular polyethylene (UHMPE) results in two-fold growth of the extra energy localisation regions dimension from D 4 (UHMPE) to Df 8 (UHMPE-bauxite). In addition the microhardness increases approximately twice (Figure 6.33). 

Balth Calleja F J, Santa Cruz C, Bayer R K and Kilian H G (1990) Microhardness and surface free energy in linear polyethylene the role of entanglements, Colloid Polym Sci 268 440-446. 

The loeal eompressive strength of polyethylene is of more interest commercially than its bulk compressive modulus. Local compressive strength is normally referred to as hardness, microhardness or microindentation hardness (MH). Microhardness is important in terms of the retention of a good surface finish on molded articles. 

Figure 19 Plot of microhardness as a function of degree of crystallinity (from density) for high density polyethylene, low density polyethylene, and linear low density polyethylene. (From Ref 5.)...

Table 3 Microhardness of Various Types of Polyethylene and Selected Polymers...

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