New criteria for evaluating the scratch-resistance properties of polypropylene

28 July 2017
Han Jiang, Chengkai Jiang, Guozheng Kang, Qianhua Kan, Jianwei Zhang, and Yonghua Li
Several suitable measures of scratch resistance are assessed, and polypropylene with large yield strength and small elastic modulus is found to exhibit the best quality.

Polypropylene (PP) is widely used in the automotive industry for applications that require pleasing aesthetics (i.e., a smooth surface) and good structural integrity. The formation of scratches on PP, however, lessens the aesthetic nature of its surfaces. It is therefore critically important to find a suitable criterion for assessing the scratch-resistance behavior of polymers (i.e., in an effort to design PP with good scratch-resistance properties).1–3

In the past, researchers have used a variety of evaluation approaches to investigate how the mechanical properties of PP (such as elastic modulus and yield strength) affect its scratch characteristics.4, 5 Techniques that have been proposed and implemented include the optical method,1 the critical normal force,6, 7 the tangential force,5 as well as geometric deformation parameters.5, 8–11 However, few studies have been focused on verifying the suitability of these different measures—i.e., through an integrated approach of a scratch test and finite-element (FE) analysis—even though the effect of a single mechanical parameter on scratch resistance has been investigated using the FE method. For instance, the effects of elastic modulus, yield strength, Poisson's ratio, and the coefficient of friction on PP scratch behavior have previously been studied with the use of a simplified model,5 but the coupling effect of material parameters on scratch behavior has received little attention thus far.

In our work, we therefore adopted an integrated approach in which we combined a scratch test with a FE simulation (using an elastic–perfectly plastic model) to assess the suitability of different criteria for evaluating the scratch resistance of PP. We then used our identified criteria to experimentally and numerically evaluate the coupling effects of elastic modulus (E) and yield strength (σy) on the scratch performance of PP.12 For our tests, we obtained nine synthesized PP systems from Kingfa Science and Technology Co., Ltd (China). In addition, we modeled the geometry of a scratch groove according to the parameters illustrated in Figure 1.


Schematic cross section of a scratch-induced groove. D: Residual scratch depth. H: Groove shoulder height. W: Groove shoulder width. A: Groove shoulder area.

The E and σy values that we obtained from the tensile tests on the nine PP systems are given in Table 1. We also present values for the critical normal load (Fc)—which gives rise to ‘fish-scale’ damage—that we measured during the scratch tests for each PP sample. We find that a reduction of E does not necessarily cause a decrease in Fc. Unfortunately, it is impossible to test how σy affects Fc because these two parameters change simultaneously.

Elastic modulus (E), yield strength (σy), and critical normal load (Fc) of the nine PP samples. Data provided by Kingfa Science and Technology Co., Ltd (China).

Material No.E (MPa)σy (MPa)Fc
113503313
212503313
31200309
411003110
510003213
69002613
78002110
86502010
9550238

We also calculated the tangential force (Ft) in the FE analysis for each of our PP specimens. These results are shown in Figure 2 compared with our Fc results, and we see that PP samples with higher Fc exhibit smaller Ft. Indeed, we calculated a strong negative correlation between these two parameters (with a Spearman correlation coefficient of −0.91). Our results, therefore, indicate that Ft could provide a suitable index for evaluating the scratch-resistance performance of PP. To that end, the correlations between Fc and values for various geometrical deformation parameters (as shown in Figure 1, and calculated in the E simulations) are illustrated in Figure 3. We have thus also confirmed that the residual scratch depth (D) and groove shoulder area (A) measures are potentially suitable for assessing the scratch resistance of PP.


Comparison of the critical normal load (Fc) and tangential force (Ft) of the nine polypropylene (PP) samples (as listed in Table 1). ρ: Spearman correlation coefficient.


Comparisons of Fc and geometrical deformation parameters (as shown in Figure 1) for the nine PP samples. Comparisons with Fc are shown for (a) the residual scratch depth, (b) groove shoulder height, (c) groove shoulder width, (d) groove shoulder area.

The results of our numerical study into the coupling effects of E and σy on the scratch behavior of PP are shown in Figure 4. We find—Figure 4(a)—that there is no obvious change in D with increased E, whereas σy has a significant effect on D. Furthermore, we observe that a larger σy value causes smaller D and gives rise to a better scratch resistance. For a small σy value (i.e., 20MPa)—see Figure 4(b)—we find that A increases rapidly with increasing E. This trend, however, is less significant for a larger σy (33MPa). Nonetheless, the effect of σy on A is consistent for all values of E. We calculate that the PP sample with the largest σy (33MPa) and the smallest E (550MPa) values has the smallest A. From the results in Figure 4(c), we also observe a slight reduction in Ft with increased σy and a slight Ft increase with greater E. Although the coupling of E and σy does affect Ft, this influence is not as strong as on D and A. It is clear that E and σy have coupling effects on the scratch behavior of PP no matter which criterion is used, and materials with larger σy and smaller E exhibit remarkably good scratch resistance.


Coupling effect of E and σyon the scratch behavior of PP. Results are shown for (a) D, (b) A, and (c) Ft. All data are non-dimensionalized by their corresponding maximum values. For example, D/Dσ20E550means that D is normalized to the case where σyis 20MPa and E is 550MPa.

In summary, we have identified new criteria—the tangential force, residual scratch depth, and groove shoulder area—for evaluating the scratch resistance of PP. Within the elastic modulus and yield strength ranges that we have investigated, we find that PP with a large yield strength and a small elastic modulus exhibits the most preferable scratch-resistance performance. Our findings are therefore helpful for guiding the design of PP with improved scratch resistance. In our future work we plan to adopt a suitable constitutive model for our FE simulations that will be able to deal with the complexity of polymers. With this model we will be able to consider a number of factors, including the rate of deformation and its temperature-dependent behavior. We believe that this approach will enable us to more fully understand the damage mechanism behind scratches in PP.


Authors

Han Jiang
School of Mechanics and Engineering, Southwest Jiaotong University

Han Jiang received his PhD from Texas A&M University and he is now a faculty member of Southwest Jiaotong University. His research interests are focused on measures of polymer mechanical behavior (e.g., experimental investigations, constitutive modeling, surface scratch deformation, and cyclic loading behavior).

Chengkai Jiang
School of Mechanics and Engineering, Southwest Jiaotong University

Guozheng Kang
School of Mechanics and Engineering, Southwest Jiaotong University

Qianhua Kan
School of Mechanics and Engineering, Southwest Jiaotong University

Jianwei Zhang
School of Mechanics and Engineering Science, Zhengzhou University

Yonghua Li
Kingfa Science & Technology Co., Ltd.


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DOI:  10.2417/spepro.006927