Abstract

Statistical tests were used to check the normality of the mechanical properties, and the homogeneity of the variances for the populations of the groups of the properties, of three plastic films before performing the one-way ANOVA test. The tensile strength, the tearing resistance, both in the MD and the CD direction and the thicknesses were the properties of three samples tested. Plastic film samples 2 and 3 MD tensile strength data were found normal using the Shapiro-Wilk test of normality. For plastic film 1 we proceeded with the Spearman’s non-parametric linear correlation test and it was found that the MD tensile strength of film 1 had a weak negative linear correlation with the MD tensile strength of sample 3. Plastic film samples 1, 2 and 3, MD tearing resistance data were found normal using the Shapiro-Wilk test of normality. Plastic film sample’s 1 thickness data was found normal using the Shapiro-Wilk test. For plastic film samples 2 and 3, the Spearman’s non-parametric linear correlation test was applied and the thickness data of sample 2 was correlated weakly and negatively linearly with the thickness of sample 3. Levene’s statistic test of homogeneity of variances for the nine categories of MD mechanical properties and thickness of the films was found statistically significant thus there was no equality between the population variations. The one-way ANOVA test was found statistically significant (F (8,114) =905.319>1.83) (p<0.001) and thus at least two of the nine mean values of the MD mechanical properties, and thickness’s of the three films differed. A post hoc test was then performed; the multiple comparisons Bonferroni test and we detected the values of the mechanical properties between which differences occurred. The same procedure was followed for the CD tensile strength and the CD tearing resistance data, and the thickness’s of the three plastic films. The CD tensile strength data of all three plastic films was found normal. Also, the CD tearing resistance data for all three film samples was found normal. The Levene’s statistic=22.562, p=0.000 test was found statistically significant and no equality of variance was observed. The one-way ANOVA test was also statistically significant (F(8,122)=269.487>1.83) (p<0.001) and at least two, of the mean CD values and the thickness values, differed. The Bonferroni test showed that the CD tensile strength differed significantly, in terms of the mean value, with the CD tearing resistance of all three film samples. Anisotropy was observed on the tensile strength and the tearing resistance of the plastic films 2 and 3, in contrast with film 1 for which isotropy was detected.

Keywords:Thin plastic films; Tensile strength; Thickness; Tearing resistance; Shapiro-Wilk test of normality; Spearman’s nonparametric correlation test; One-Way ANOVA test; Multiple comparisons Bonferroni test