materials Article Tribology of Polymer Blends PBT + PTFE Constantin Georgescu 1,* , Lorena Deleanu 1,*, Larisa Chiper Titire 1 and Alina Cantaragiu Ceoromila 2 1 Department of Mechanical Engineering, Faculty of Engineering, “Dunarea de Jos” University of Galati, 800008 Galati, Romania; [email protected] 2 Department of Applied Sciences, Cross-Border Faculty, “Dunarea de Jos” University of Galati, 800008 Galati, Romania; [email protected] * Correspondence: [email protected] (C.G.); [email protected] (L.D.); Tel.: +40-743-105-835 (L.D.) Abstract: This paper presents results on tribological characteristics for polymer blends made of polybutylene terephthalate (PBT) and polytetrafluoroethylene (PTFE). This blend is relatively new in research as PBT has restricted processability because of its processing temperature near the degradation one. Tests were done block-on-ring tribotester, in dry regime, the variables being the PTFE concentration (0%, 5%, 10% and 15% wt) and the sliding regime parameters (load: 1, 2.5 and 5 N, the sliding speed: 0.25, 0.5 and 0.75 m/s, and the sliding distance: 2500, 5000 and 7500 m). Results are encouraging as PBT as neat polymer has very good tribological characteristics in terms of friction coefficient and wear rate. SEM investigation reveals a quite uniform dispersion of PTFE drops in the PBT matrix. Either considered a composite or a blend, the mixture PBT + 15% PTFE exhibits a very good tribological behavior, the resulting material gathering both stable and low friction coefficient and a linear wear rate lower than each component when tested under the same conditions. Keywords: polybutylene terephthalate (PBT); polytetrafluoroethylene (PTFE); blend PBT + PTFE; block-on-ring test; linear wear rate; friction coefficient Citation: Georgescu, C.; Deleanu, L.; Chiper Titire, L.; Ceoromila, A.C. Tribology of Polymer Blends 1. Introduction PBT + PTFE. Materials 2021, 14, 997. Due to a longer sequence of methyl groups in the monomer, the molecular chains of https://doi.org/10.3390/ma14040997 polybutylene terephthalate (PBT) are more flexible and less polar than those of polyethy1ene terephthalate (PET), inducing a lower melting temperature T (224–230 ◦C) and the glass Academic Editor: Sergey Panin transition temperature Tg (22–43 ◦C). This lower Tg allows for a fast crystallization when molding and shorter molding cycles with faster molding speed [1–4]. PBT, a semicrystalline Received: 10 January 2021 engineering polymer, is included in the polyester class of resins and has a set of properties Accepted: 16 February 2021 Published: 20 February 2021 that recommends it in many special applications: rigidity and strength, combined with very good heat aging resistance. PBT-based materials (composites and blends) have better Publisher’s Note: MDPI stays neutral dimensional stability with uniform shrinkage behavior, stiffness, and heat resistance, low with regard to jurisdictional claims in water absorption and high chemical resistance by incorporating fillers, reinforcing materi- published maps and institutional affil- als, and additives; material properties are tailored for user’s interest. They are processed iations. mainly by injection molding [5]. PBT has restricted processability because of its processing temperature near the degradation one [6]. Lin and Schlarb [7] tested a hybrid material, short carbon fiber-filled PBT, with and without graphite as a solid lubricant, using a pin-on-disc tribotester, in dry regime. PBT with nanoparticles without graphite exhibits very good tribological performance, under Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. moderate and severe load, with better mechanical characteristics as compared to the same This article is an open access article material filled with graphite. Under 3 MPa and 2 m/s, the friction coefficient is 0.18 and −6 3 distributed under the terms and wear rate is 0.8 × 10 mm /Nm. conditions of the Creative Commons Dechet et al. [8] presented a laboratory technology for producing spherical polymer Attribution (CC BY) license (https:// blend particles made of PBT and polycarbonate (PC) for selective laser sintering (SLS), creativecommons.org/licenses/by/ including cogrinding and thermal rounding. The size distribution, shape and morphology 4.0/). of polymer constituents in this PBT + PC blend were analyzed. Materials 2021, 14, 997. https://doi.org/10.3390/ma14040997 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 19 including cogrinding and thermal rounding. The size distribution, shape and morphol- Materials 2021, 14, 997 ogy of polymer constituents in this PBT + PC blend were analyzed. 2 of 19 Materials based on blends of PBT + PET with flame retardant agents (a new formu- lated agent, expandable graphite (EG), added separately or in a mixture of both) were testedMaterials for determining based on the blends influence of PBT of + PETflame with retardant flame retardant [9]. Results agents demonstrate (a new formulated that in- agent,corporation expandable of the graphitemixture (EG),in PBT added + PET separately blends could or in be a mixturerecommended of both) as were potential tested ap- for determiningplications for the electronic influence household of flame retardantdevices, products [9]. Results and demonstrate automotive that components, incorporation but ofresearch the mixture should in be PBT continued + PET blends for their could mechanical be recommended and tribological as potential behavior. applications for electronicIn machine household design, devices, components products made and exclus automotiveively of components, polytetrafluoroethylene but research should(PTFE) beare continued rare, even for if theirthey mechanicalmaintain their and properti tribologicales at behavior.the initial values, independent of the manufacturingIn machine method design, components[1], including made chemical exclusively properties of polytetrafluoroethylene that remain unchanged (PTFE) for a arelong rare, time even (stability if they in maintainchemically their aggressive properties environments, at the initial insolubility, values, independent weather stability of the manufacturingand anti-adhesion). method The [properties1], including practically chemical unaffected properties are that flexibility remain unchangedat negative fortem- a longperatures, time (stabilitythermal stability, in chemically a low dielectric aggressive constant environments, and high insolubility, resistance at weather electrical stabil- arch ity[10]. and All anti-adhesion). experimental works The propertiesunderlined practically a very low unaffected friction coefficient are flexibility [11,12] at and negative high temperatures,wear rate, especially thermal in stability, dry regime a low [13–15], dielectric but constantadding reinforcements and high resistance as short at electrical fibers or archpowders [10]. reduces All experimental the wear by works a factor underlined of 100 or a veryeven lowmore friction [13], especially coefficient in [ 11lubricated,12] and highcontacts wear [16,17]. rate, especially Producers in and dry users regime prefer [13– 15to ],add but PTFE adding in reinforcementsother polymers asbecause short fibers of its orpoor powders wear resistance, reduces the being wear more by a factor efficient of 100 as orsolid even lubricant more [13 [18]], especially and rarely in lubricatedused as a contactsmatrix [19].[16 ,17]. Producers and users prefer to add PTFE in other polymers because of its poorPolymer wear resistance,blends have being been more developed efficient for as their solid sets lubricant of characteristics, [18] and rarely the used mixture as a matrixresulting [19 in]. several new and improved properties or different from those characterizing each Polymercomponent blends alone have [20]. beenThe notion developed of a poly formeric their setsblend of refers characteristics, to materials the artificially mixture resultingcreated, rationally in several combining new and improved different comp propertiesonents or to different improve from one thoseor more characterizing characteris- eachtics and component to diminish alone those [20]. that The are notion not, ofbase a polymericd on theoretical blend models, refers to laboratory materials artificiallytests and, created,finally, prototype rationally combiningresults. At differentpresent, po componentslymer alloys, to improve polymer one blends or more and characteristics their compo- andsites torepresent diminish over those 80% that (by aremass) not, of basedthe total on polymer-based theoretical models, materials laboratory [21]. tests and, finally,In prototypetribology,results. there are At present,several polymer alloys,blends polymerused for blends their andgood their characteristics, composites representespecially overusing 80% PTFE (by as mass) a solid of thelubricant. total polymer-based Polymer blends materials are attracting [21]. the attention of specialistsIn tribology, through there a areset several of particular polymer blendsproperties, used forsuch their as goodlow characteristics,specific mass espe-and ciallystrength-to-mass using PTFE asand a solid stiffness-to-mass lubricant. Polymer ratios blends that areare attractingsuperior theto attentiontraditional of specialistsmaterials, throughtribological a set properties of particular [22], properties, resistance suchto aggressive as low specific environments, mass and electrical strength-to-mass and thermal and stiffness-to-massproperties, which ratios led thatto the are