Hardness of polymers

We test the hardness of polymers by applying an indenter to their surface with a known force and noting the depth to which the tip penetrates the sample. These tests typically fall into one of two categories. In the first, the depth of penetration is read directly from a dial on the instrument, calibrated in arbitrary hardness units. The farther the tip penetrates the sample, the lower is its hardness. The second type of test involves impressing a pyramidal indenter tip against the sample with a known force and measuring the depth to which it penetrates. In practice we measure the dimensions of the indentation and calculate the depth of penetration and compressive modulus based on the tip geometry. 

Given the great variety outlined above, only some general trends for the hardnesses of polymers can be discussed here. For more detail regarding thermoplastic polymers, the reader is referred to Balta, Calleja and Fakirov... 

Recent publications describe the effects of silica on the conductivity and mechanical properties of a polyethylene oxide/ammonium bifluoride complex containing propylene carbonate [36], as a foam stabilizer in polyester polyurethane foams, and on the properties of polylactic acid nanocomposites prepared by the sol-gel technique [37] (see also Chapter 24), on the mechanical properties and permeability of i-PP composites [38], on the surface hardness of polymers for biomedical devices [39], on enhanced properties of polymer interlayers that are used in multiple layer glazing panels [40]. 

The methods commonly used in determining the hardness of polymers are static indentation methods. Here, the indenter penetrates the test specimen at normal incidence under the application of a known force. Typical forces range from 10 to 1 kN, leading to indentation widths of a few millimeters. The hardness is determined from the optical imaging of the residual width of indentation. Pyramidal indenters are preferred because the contact pressure is independent of the load applied and the indentation is less affected by elastic release than other indenters (1). 

Most fillers are harder than polymers, so filler addition increases the hardness of polymers. The trends are similar to those for modulus and yield strength, namely that more anisotropic fillers are more effective at increasing hardness. As hardness is a surface measurement, similar to a room temperature Vicat test, the filler will only affect the hardness if it is present at or near the surface. If the filler is depleted at the surface, for example due to flow effects during injection molding, then it may have little or no effect on the measured hardness. 

In another study of PE and PP with wood fiber. Macro Valinte et al. [45] reported that the hardness of polymer matrix is foimd to increase with increasing wood fiber content for both LDPE and PP. In another study more filler content in low density polyethylene composites resulted in higher hardness. Hardness of PE composites also depends on the radiation treatment. This treatment produces a crosslinking network inside the polymer matrix resulting in an improvement of its hardness [46]. 

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