Durable and light weight polymer composites for extreme wear conditions - Abrasive wear and scratch resistance of polymer composites

The thermoplastic polymer compounds and polymer composites for enhanced abrasive wear resistance were developed based on commercially available polymers, compounded with micron-sized and nanosized fillers. PA1010, HDPE, UHMWPE were used as base polymers in the DURPOL compounds. Thermoplastic polyurethane (TPU) compounds with enhanced tear and scratch resistance, and glass fibre reinforced PA12 were also developed for enhanced scratch resistance.
The abrasive wear of polymers was determined as mass loss during the sand abrasion tests. The tests followed the standard ASTM 65-04 concerning the overall test setup, two loading levels was used to study the wear performance of polymers in different loading conditions. For the scratch resistance evaluation, the reciprocating scratch testing was performed on VTT REC tester. The influence of the scratches on the visual appearance of the polymer surface was measured with a gloss meter and 2D profilometry. For evaluation of tear resistance of TPU materials, the specific test samples were processed by injection moulding and by stamping the sample to specific form. The samples were tested according the ISO 34-1 standard (Method B). The polymers were characterised by optical microscopy and SEM after the wear tests. The indentation measurements were used to determine the plane strain modulus, indentation modulus and indentation hardness of materials. Atomic force microscopy, DSC and DMA analysis, tensile tests and Charpy tests were also carried out for selected materials.
In the cases where the polymer has no special requirements for strength or stiffness, the lowest wear can be gained with the soft elastomers. However, it was found out that when the TPUs were modified e.g. by preparing TPU-PA blends to increase the modulus, the abrasive wear of the developed material increased.
Concerning the neat polymers, PA1010 exhibits lower wear compared to PEEK, and high molecular weight tube extrusion grade HDPE polymer has similar abrasive wear compared to PEEK, but the modulus of these polymers is clearly lower compared to PEEK, around 2 GPa and 1.2 GPa, respectively. The PA compounds were developed in this project by using different fillers to increase the modulus of PA1010. The addition of 15 wt% silanized glass flakes, or 15 wt% halloysite nanotubes increased the indentation modulus from the 1.9 GPa range to 2.3 GPa level, while retaining the abrasive wear resistance of the PA1010 polymer. Indentation modulus and tensile modulus of PA1010 was increased also by electron beam crosslinking.
The semi-aromatic polyamide has high modulus combined with reasonable wear resistance, but the polymer is brittle. The polymer was toughened by blending it with UHMWPE and PA, which did not have a major influence in the wear performance. The lowest abrasive wear was achieved by PA soluble plasticizer additions to the semi-aromatic polyamide.
For the PE polymer the HDPE/UHMWPE blends were developed. UHMWPE polymers typically have high wear resistance, but they cannot be processed by injection moulding. In DURPOL project, we succeeded to develop injection mouldable UHMWPE and HDPE blends. In addition, different fillers were used in the blend to improve the performance. The UHMWPE/HDPE with fillers, such as γ-Al2O3, graphene oxide and fumed silica provided improved abrasive wear resistance with slightly reduced modulus. Addition of glass flakes increased the modulus of the blend while wear performance remained on the same level as for HDPE. Increase of modulus was also achieved with ultrafine precipitated calcium carbonate (PCC), interestingly reducing also the abrasive wear. In our preliminary studies the electron beam cross-linking raised the room temperature indentation modulus of UHMWPE/HDPE blend containing nanofillers, by roughly 15%. The observed abrasive wear resistance measured at ambient room temperature was similar, and typically slightly worse compared to the non-crosslinked reference blend.
For the development of soft polymers, in order to improve tear resistance and scratch resistance, the TPU composites with cellulose microfibre and hemp fibre reinforcements combined with the “soft touch” were developed. These new materials were processed without plasticizers using commercial mineral filler additives.