Processing and characterization of hybrid composites based on high molecular weight polyethylene

Predmet naučnog istraživanja je unapređenje svojstava hibridnih kompozitnih materijala korišćenjem polietilena visoke molarne mase (eng. Ultra High Molecular Weight Polyethylene – UHMWPE) u obliku vlakana, koji imaju široku primenu u vojnoj, građevinskoj i pomorskoj industriji. Izrada ovakve vrste kompozita predstavlja izazov kada je potrebno istovremeno uspostaviti kompatibilnost između nekoliko vrsta materijala različitih karakteristika i kao krajnji rezultat dobiti materijal očekivanih svojstava. Međutim, uprkos dugom nizu godina ispitivanja i poboljšavanja svojstava, još uvek postoje nove mogućnosti njegovog unapređenja. Primarni cilj doktorske disertacije je sinteza hibridnog kompozita unapređenih mehaničkih karakteristika i njegova primena u izradi užadi. Na osnovu procene uticaja komercijalnih neorganskih punioca i sintetisanih čestica aluminijum oksida dopiranih gvožđe oksidom na mehanička svojstva hibridnog kompozita sa polimernom matricom na bazi kopolimera etilena i vinil-acetata ojačanog UHMWPE vlaknima, definisano je koji kompozit ima najbolje karakteristike koje zadovoljavaju postavljene zahteve. U ovom istraživanju je korišćen kopolimer etilena i vinil-acetata kao matrice, koja je kompatibilna sa UHMWPE vlaknima i kasnije dodavanim puniocima. Kao ojačanja su korišćena UHMWPE vlakna, komercijalne nano čestice i viskeri aluminijum oksida, kao i čestice aluminijum oksida dopirane gvožđe oksidom. UHMWPE vlakna su impregnirana kopolimerom etilena i vinil-acetata (eng. Poly(ethylene-co-vinyl acetate – EVA)) sa različitim vrstama i masenim udelima punioca. Radi poboljšanja adhezivnih svojstava, matrica je delimično hidrolizovana u cilju stvaranja većeg broja reaktivnih OH grupa koje su imale funkciju obezbeđenja bolje interakcije sa puniocima i ojačanjem. U izradi i ispitivanju hibridnih kompozita korišćena su dva tipa UHMWPE vlakana – netretirana i površinski modifikovana vlakna hromnom kiselinom. Površina čestica Al2O3 je modifikovana nehidrolizovanim i hidrolizovanim GLYMO silanom (3-glicidoksipropiltrimetoksilan). Prezentovana istraživanja daju novi pristup u ispitivanju međupovršinskih efekata, interakcija i stepena ojačanja između UHMWPE vlakana i matrice, uz dodavanje različitih vrsta čestica aluminijum oksida kako bi se ispitao njihov uticaj na povećanje termičke stabilnosti i zatezne čvrstoće dobijenih hibridnih kompozitnih materijala. U cilju krarakterizacije hemijskog sastava materijala, u radu su korišćene infracrvena spektroskopija sa Furijeovom transformacijom (eng. Fourier-transform infrared spectroscopy FT-IR) i Raman spektroskopija. Karakteristike matrice na atomskom nivou, odnosno stepen hidrolize EVA kopolimera definisane su primenom nuklearne magnetne rezonance (NMR spektroskopija). S obzirom na inertnost UHMWPE vlakana i slabu polarnost, ispitivanje kompatibilnosti i međupovršinskih interakcija između UHMWPE vlakana i EVA matrice vršeno je određivanjem ugla kvašenja. Merenja su sprovedena na uzorcima EVA/UHMWPE kompozita sa netretiranim i oksidovanim UHMWPE vlaknima u kombinaciji sa netretiranom i delimično hidrolizovanom EVA matricom. Na osnovu dobijenih rezultata utvrđeno je da su oksidovana UHMWPE vlakna, usled povećanja površinske slobodne energije, ostvarila najmanji kontaktni ugao sa delimično hidrolizovanom EVA matricom što je ukazalo na bolju međupovršisku adheziju odnosno kompatibilnost. Termogravimetrijska analiza dala je uvid u efekat uticaja dodatih čestica na termičku stabilnost hibridnih kompozita kod kojih su tokom procesa zagrevanja, do temperature od 600 °C, praćene fizičke i hemijske promene, odnosno proces degradacije EVA matrice i UHMWPE vlakana. Takođe, kao jedan od bitnih faktora u ispitivanju stabilnosti kompozitnih materijala je i određivanje temperature topljenja primenom DSC analize i merenje entalpije endotermskog procesa EVA matrice i UHMWPE vlakana u kompozitnom materijalu...The subject of scientific research is to improve the properties of hybrid composite materials based on ultra high molecular weight polyethylene (UHMWPE) fibers, which have wide application in military, construction and marine industries. The production of this type of composite is a challenge when it is necessary to establish compatibility between several types of materials of different characteristics and, at the same time, to obtain the material of the expected properties. However, despite many years of testing and improving properties, there are still new possibilities for its improvement. The primary goal of the doctoral dissertation is synthesis of the hybrid composite with advanced mechanical characteristics and its application in the production of ropes. Based on the assessment of the influence of commercial inorganic fillers and synthesized particles of aluminum oxide doped with iron oxide on the mechanical properties of the hybrid composite with a polymer matrix which is based on the copolymer of ethylene and vinyl acetate reinforced with UHMWPE fibers, it is defined which composite has the best characteristics that meet the set of requirements. In this study a copolymer of ethylene and vinyl acetate was used as a matrix, which is compatible with UHMWPE fibers and subsequently added fillers. As reinforcements, UHMWPE fibers, commercial nano particles and aluminum oxide whiskers were used, as well as aluminum oxide particles doped with iron oxide. UHMWPE fibers were impregnated with copolymers of ethylene and vinyl acetate (Poly (ethylene-co-vinyl acetate - EVA)) with different types and weight of fillers. In order to improve the adhesive properties, the matrix was partially hydrolyzed in order to create a large number of reactive OH groups that have the function of providing better interaction with fillers and reinforcement. In the preparation and testing of hybrid composites, two types of UHMWPE fibers were used - untreated and surface modified fibers with chromic acid. The surface area of Al2O3 was modified by non-hydrolyzed and hydrolyzed GLYMO silane (3-glycidoxypropyltrimethoxylane). The presented researches give a new approach in the examination of inter-surface effects, interactions and the degree of amplification between UHMWPE fibers and matrices, with the addition of different types of aluminum oxide particles in order to examine their impact on increasing the thermal stability and tensile strength of the obtained hybrid composite materials. In order to characterize the chemical composition of the material, an infrared spectroscopy with Fourier transform (FT-IR) and Raman spectroscopy were used. The characteristics of the matrix at the atomic level, like degree of hydrolysis of the EVA copolymer, are defined by the application of nuclear magnetic resonance (NMR spectroscopy). Considering the inertness of UHMWPE fibers and its poor polarity, compatibility testing and interfacial interactions between UHMWPE fibers and EVA matrix were performed by determining the angle of wetting. Measurements were performed on samples of EVA/UHMWPE composites with untreated and oxidized UHMWPE fibers in combination with untreated and partially hydrolyzed EVA matrix. On the basis of the obtained results it was established that the oxidized UHMWPE fibers, due to the increase in surface free energy, achieved the smallest contact angle with a partially hydrolyzed EVA matrix, which indicated better interconnecting adhesion and compatibility. Thermogravimetric analysis provided an insight into the effect of the influence of added particles on thermal stability of hybrid composites and physical and chemical changes ( process of degradation of EVA matrix and UHMWPE fibers). These changes were monitored during the heating process up to a temperature of 600 ° C. Also, as one of the important factors in testing the stability of composite materials is determination of the melting temperature using DSC analysis and measurement of the enthalpy of the endothermic process of the EVA matrix and UHMWPE fibers in the composite material..