Composites of alkaline poly(6-hexanelactam) with solid lubricants: one-step synthesis, structure, and mechanical properties

Abstract

One‐step preparation of composites of alkaline poly(6‐hexanelactam) (PCL) with solid lubricants (SL), such as graphite and molybdenum disulfide, is an energy‐saving process as additional polymer fusion and melt mixing are not necessary. The initiator/activator system used were: (i) sodium dihydrido‐bis(2‐methoxyethoxo) aluminate (SYN)/cyclic trimer of phenyl isocyanate (PIC); (ii) sodium/hexamethylene‐1,6‐diisocyanate (HMDIC); (iii) SYN/HMDIC. These systems have been found suitable for the composite synthesis since they are relatively insensitive to traces of humidity and other low‐molar‐mass compounds which may be adsorbed on lubricant surfaces. Incorporated graphite (up to 20 wt.‐%) and MoS2 (up to 40 wt.‐%) do not perceptibly affect glass transition temperature, melting temperature and crystallization temperature while polymer yield, polymerization degree and rate decrease. The crystallinity assessed by DSC passes through a maximum at 20% of MoS2 in contrast to that detected by X‐ray diffraction (XR) measurements. As expected, the mean spherulite diameter drops profoundly. Both SL affect mechanical properties of produced composites: (i) increase the modulus; (ii) decrease the compliance, the time dependence of which remains close to that observed for the matrix; (iii) reduce the yield strength; (iv) slightly lower the impact strength of composites. A tentative hybrid composite containing 15 wt.‐% of graphite, 5 wt.‐% of MoS2, and 5 wt.‐% of mineral oil (as a liquid lubricant) shows a modulus high enough and its propensity to creep is not enhanced. The observed changes in mechanical properties of PCL caused by the incorporation of SL do not preclude applications analogous to those of unfilled PCL. Friction properties of the composites were beyond the scope of this work.

Gradient Composites of Alkaline Poly(6-hexanelactam) with Graphite: One-Step Synthesis, Structure, and Mechanical Properties

“…Differences in the crystallinity of various samples are not significant, which holds for both graphite-free (49-56%) and graphite-rich (60-64%) layers. As far as the ascertained crystallinity is concerned, one should take into consideration the fact [6,12] that DSC indicates higher values of crystallinity than XR because the heat of fusion encompasses also contributions from less ordered regions which are not detected in the XR method. [12] The melting temperature of graphite-free and graphite-rich layers is practically identical in all samples.…”

“…ref. [12] ). Differences in the crystallinity of various samples are not significant, which holds for both graphite-free (49-56%) and graphite-rich (60-64%) layers.…”

“…As far as the ascertained crystallinity is concerned, one should take into consideration the fact [6,12] that DSC indicates higher values of crystallinity than XR because the heat of fusion encompasses also contributions from less ordered regions which are not detected in the XR method. [12] The melting temperature of graphite-free and graphite-rich layers is practically identical in all samples. The 2 nd scan indicates ( Table 2) that the conditions for crystallization during fast cooling are much less favorable than those in the course of the isothermal polymerization; thus, the crystalline fraction achieves only 33-35%, but is always somewhat higher in the graphite-rich layers.…”

“…[6,12] These composites are suitable for preparation of bulky castings such as cylindrical, rectangular, sheet and tube-shaped products. However, the sheetlike products with a uniform distribution of a filler or lubricant may not be suitable enough for applications where only one side of the sheet is exposed to wear conditions, while the opposite side is expected to have properties ensuring sufficient interfacial adhesion in fixing the sliding part to a machine.…”

“…The incorporated filler underwent sedimentation due to the gravity during initial stages of the polymerization process. Initiators and activators of anionic polymerization were used which had been proved [5,6,10,12] to be rather insensitive to the presence of water and impurities adsorbed on fillers and solid lubricants.…”