3.3 Effect of temperature on crystal structure and crystallinity of different waxy materials
Figure 3 shows the crystal morphology of paraffin, beeswax, FHSO, HCO, EGMD and Estercoat at 0, 25 and 50 °C. In agreement with what was previously reported by Fei et al. [8] and Hwang et al. [20], paraffin, beeswax, EGMD and Estercoat all showed needle-like and fibrous crystal, however, their sizes and networks were different. It seemed that paraffin, EGMD and Estercoat all had dendritic crystals which are highly interconnected and formed junction points, while beeswax has much finer crystals. FHSO crystallized into more ordered and larger crystals, while HCO seems to have mixed crystal morphology (rosette, fibrous, and irregular) at 25 °C which agree with what was reported by Yang et al. [21]. As shown in Figure 3, when temperature was increased to 50 °C, the crystal type of paraffin, beeswax, EGMD and Estercoat was not significant altered, however, the crystal number and density seemed to be decreased. When the temperature was lowered from 25 to 0 °C, crystal morphology of paraffin was not significant changed, while beeswax had denser crystals, and EGMD’s and Estercoat’s crystal density was reduced. For FHSO, temperature did not have significant impact on their crystal structures. For HCO, observation of the crystals became difficult when temperature was lowered to 0 or increased to 50 °C. The changes in crystal structure may correlate to the changes in coefficient of friction of these materials as they significantly influence the physical properties such as hardness of waxes. It was observed that the lower crystal density would lead to higher surface coefficient of friction as paraffin, beeswax, EGMD and Estercoat all have increased friction coefficient when temperature was increased from 25 to 50 °C. The increased temperature resulted in lower crystal density and led to a softer material, which subsequently negative affected transfer film forming and increased the real contact area at the interface, thus increase the friction coefficient. For FHSO, no significant changes in crystal structure was observed with temperature changes, thus, the coefficient of friction of FHSO was quite consistent at low and high temperatures. Although the crystal structure of HCO at 0 and 50 °C was not clear for unknown reasons, it is reasonable to speculate that it was not significantly changed as its friction coefficient was not significantly affected.
Table 3 shows the enthalpy and relative crystallinity (RC) of the tested waxes. In general, the RC of the materials decreased as the temperature increased. However, RC of several materials including FHSO and HCO were not sensitive to the variation of the equilibration temperature. This is probably another reason for why their coefficient of friction was not affect by the environmental temperature (Table 1c). Paraffin, beeswax, EGMD and Estercoat all had the highest RC at 0 °C, and their RC significantly decreased when the temperature was increased from 25 to 50 °C. Correspondingly, their coefficient of friction and wear loss all significantly increased (Table 2c). Beeswax had significantly higher RC at 0 than 25 °C, and accordingly, lower coefficient of friction and wear loss was observed at 0 °C. However, EGMD and Estercoat had slightly higher RC at 0 °C compared to 25 °C, but their coefficient of friction and wear loss were actually higher at 0 °C. Factors other than the crystallinity such as surface topography at different temperature may also have played a role on the surface properties of the EGMD and Estercoat. There is very limited study on the possible physical property changes of waxes when they are conditioned under various temperature after solidification. Studies on the surface characteristics under various temperatures using laser scanning microscopy could be helpful for a better understanding of surface property variations with temperature.
Conclusions
Surface friction coefficient of waxes is shown to be significantly influenced by normal load, sliding velocity and environmental temperature. The wear loss of these waxes also varies significantly with different sliding conditions. The transfer film forming ability and hardness of these materials significantly affect their friction and wear behaviors. Paraffin wax was susceptible to both normal load and sliding velocity. Larger normal load resulted in lower friction coefficient while higher sliding velocity led to higher friction coefficient of paraffin. FHSO, HCO, beeswax, EGMD and Estercoat were less sensitive to normal load and sliding velocity, and their friction coefficient were almost consistent. Higher environmental temperature decreased crystal density and crystallinity of paraffin, beeswax, EGMD and Estercoat, and subsequently increased their friction coefficient. Increased normal load, sliding velocity and environmental temperature, in general, resulted more severe wear of all the tested materials. Overall, the soybean oil-based Estercoat is confirmed to have friction and wear properties comparable or better than paraffin under tested conditions and can be a good ecofriendly substitute of paraffin. The information provided in this study can also be used as a reference for correctly designing systems for coating, conveying, packaging operations, transporting and storing of papers and paperboards coated with these materials.