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Tribological Properties of Blends ofMelamine-Formaldehyde Resin WithLow Density PolyethyleneWitold Brostow, Tea Datashvili, Bernard HuangLaboratory of Advanced Polymers and Optimized Mater ials (LAPOM), Department of Materials Scienceand Engineering and Department of Physics, University of North Texas, Denton, Texas 76203-5310Tribological properties of blends of melamine-formalde-hyde resin (MFR) with low density polyethylene (LDPE)containing 1, 5, 10, 20, 25 wt% MFR were investigated.We have determined sliding wear by multiple scratchingalong the same groove using a micro scratch tester. In-stantaneous penetration depth is lowered by the MFRaddition to LDPE. However, there is less viscoelastic re-covery and the residual (healing) depths increase withincreasing MFR concentration. Microindentation hard-ness increases along with the MFR content. Since MFRis only partially miscible with LDPE, MFR-rich islands inthe PE matrix offer more interfaces and so increasehardness. Friction determined with a pin-on-disk tribom-eter using silicon nitride balls as a function of MFR con-centration shows a minimum. The result is explained interms of surface morphology seen in scanning electronmicroscopy. At the same time, all blend friction valuesare lower than for neat LDPE. Wear determined in thepin-on-disk tribometer decreases along with the MFRconcentration increase. Thus, pin-on-disk wear and fric-tion show different faces of blends tribology. Blendingcan be used to improve tribological properties ofLDPE.POLYM. ENG. SCI., 48:292–296, 2008.ª2007 Society ofPlastics EngineersINTRODUCTIONPolymers have widespread applications in industry, andcontinue to gain increasing importance as technologyadvances because of their unique characteristics includinglow density and advantageous electrical and mechanicalproperties. However, some adverse effects limit theirpractical applications. In particular, low scratch and wearresistance and also environmental degradation have hin-dered many important applications. Thus, there is a needfor improved understanding of polymer tribology [1–12].On the other hand, polymer blends—subsequentlycrosslinked or not—have attracted significant scientificand technical interest as they can provide properties unat-tainable in pure components [13–29].Since low density polyethylene (LDPE) belongs to themost widely used polymers, we have decided to useblending to improve its properties. To this end we havereported the miscibility behavior and therm al propertiesof LDPE þ melamine-formaldehyde resin (MFR) blendscontaining 1, 5, 10, 20, 25 wt% MFR [26]. MFRs havebeen synthesized and blended with a LDPE. Thermalproperties have been analyzed by differential scanningcalorimetry and thermogravimetric analysis measure-ments. A detailed study on the miscibility behavior ofLDPE þ MFR blends has been made by using Fourier-transform infrared spectroscopy, environmental scanningelectron microscopy (ESEM), and atomic force micro-scopy (AFM). The observations were correlated withthe properties of the composites. Thermal analysis, AFM,and ESEM support the occurrence of a partial compatibi-lization.In this work we have focused on the tribological prop-erties of LDPE þ MFR blends. A variety of techniqueshave been used including a Micro Scratch tester (MST), amicrohardness, and a Nanovea pin-on-disk tribometer todetermine the tribology of the blends as potential designmaterials for the plastics industry. Effects of compositionvariation have been evaluated and the results connected tothe morphology of the blends.EXPERIMENTAL PARTMaterialsLDPE was supplied by Aldrich Chemicals. Melamine,C3H6N6(2,4,6-triamino-1,3,5-triazine); formaldehyde CH2O;and sodium hydroxide, NaOH used were supplied by Flukaand Sigma Chemicals, respectively.Synthesize of Melamine-Formaldehyde ResinMFR was synthesized from melamine by polycondensa-tion reaction with formaldehyde in a basic medium. TheCorrespondence to: Witold Brostow; e-mail: [email protected] grant sponsor: Robert A. Welch Foundation, Houston; contractgrant number: B-1203; contract grant sponsor: Georgian Research Devel-opment Foundation (GRDF), Tbilisi.DOI 10.1002/pen.20898Published online in Wiley InterScience (www.interscience.wiley.com).VVC2007 Society of Plastics EngineersPOLYMER ENGINEERING AND SCIENCE—-2008molar ratio melamine/formaldehyde was between 1 and 3.pH of formaldehyde (37.0% by weight in water) wasadjusted to 7.5–8.0 by adding 10 wt% NaOH aqueous solu-tion. The resulting solution was placed in a beaker and thor-oughly mixed with 20 g melamine; the components werethen stirred for 40 min at 1208C. The resulting structure is(1)The final product was subjected to evaporation. To achievecomplete water removal, the evaporation was carried out at708C and the residual pressure 13–16 kPa, followed by dry-ing in an oven at 808C for 24 hr.Blending and Sample PreparationBlends of dried PE and MFR were prepared by meltmixing in a C.W. Brabender D - 52 Preparation Station atthe rotation speed of 80 rpm and at 1608C. The resultingblends were pelletized and dried. The blends contained inturn 1, 5, 10, 20, and 25 wt% MFR.Subsequently, the blends were dried for 8 hr at 1008Cbefore compressing them in a Carver compression mold-ing machine at 1608C at the compression pressure 20.7 103kPa.Sliding Wear DeterminationThe first tribological test performed for each blend con-sisted using a Micro-Scratch Tester (MST) from CSEM,Neufchatel, Switzerland, utilizing the CSEM Scratch Soft-ware Version 2.3, which appl ies a constantly increasingforce from 0 to 25.0 N to the samples, or else a constantforce. Multiple scratching following the same groove pro-vides us with sliding wear determination (SWD) results.For each sample 15 scratches were performed. The parame-ters used in the tests were the following: load 10.0 N,scratch length 5.0 mm, scratch velocity 5.0 mm/min atroom temperature. A conical diamond intender with 200mm of diameter and the cone angle of 1208 was used. Theresults consist of the penetration (instantaneous) depth Rpand the healing (recovery) depth Rhdetermined 5 min later.Microhardness MeasurementsThe Vickers microhardness hVickersof each blend wasdetermined with a dynamic microhardness measurementdevice, HMV-M Shimadzu Micro Hardness Tester; modelM3, from Shimadzu, Kyoto, Japan.Loads of 100, 200, and 300 g were used to makemicroindentations. The holding time after completion ofthe indentation was 5 s. Five indentations were


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