Doğal elyaf takviyeli tabakalı karma malzemelerin otomotiv uygulamalarında kullanılması
Utilization of natural fiber reinforced laminated hybrid composites in automotive applications
Yazar: MEHMET SAVAŞ
Danışman: DOÇ. DR. MUSTAFA BAKKAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Cam elyaf kompozitler = Glass fiber composites ; Elyaf = Fibre ; Kompozit malzemeler = Composite materials ; Plastik kompozitler = Plastic composites ; Plastik malzemeler = Plastic materials ; Termoplastik malzemeler = Thermoplastic materials

Özet:
Azalan doğal kaynaklarla birlikte bu kaynakların verimli kullanımının öneminin artışı ve yükselen çevresel bilinç, sentetik malzemelerin yerlerini çevresel etkileri daha az olan doğal, biyolojik malzemelerin almasını sağlamıştır. Kyoto protokolü başta olmak üzere emisyon oranlarının düşürülmesine yönelik yaptırımların neticesinde doğal malzemelere yönelik ilgi arttırmıştır.Karma yapılarda kullanılan elyafların, doğal malzemeler kaynaklı kullanılması yönünde bir eğilimi mevcuttur. Daha çok otomotiv sanayinde emisyon oranlarının düşürülmesine yönelik olarak araçların hafifletilmesi ve bunun için de dayanım/ağırlık oranları yüksek olan doğal elyaflar kullanımı gelişmektedir. Henüz başlangıç aşamasında olan bu tip malzemeler üzerine araştırmaların genişletilerek yaygınlaşması gerekmektedir.Çalışmada, otomobillerin tampon parçalarının mekanik özelliklerinin geliştirilmesi bakımından, laboratuar ortamında doğal elyaf ve sentetik elyafların bir arada kullanıldığı karma malzemeler incelenmiştir. Doğal elyaf olarak atık pamuklu kumaştan elde edilen pamuk ipliçikleri kullanılmıştır. Bu sayede hem atık kumaş, kullanılarak ekonomiye tekrardan kazandırılması sağlanmış hem de sentetik elyaf ve polimer kullanımı en aza indirilerek doğada çözünemeyen madde miktarı düşürülmüş ve gereken toplam maliyet değeri azaltılmıştır.Yapılan çalışmada, önceden belirlenen pamuklu kumaş ağırlık oranına sahip polimer matrisli karma malzemenin üretimi ekstrüder vasıtasıyla gerçekleştirilmiştir. Homojen bir yapı elde edebilmek amacıyla öncelikle kumaş parçacıkları bir ön işlemden geçirilerek pamuk iplikçikleri halini almış, sonrasında yine homojen özellikler elde edebilmek için üretilen malzemelere geri dönüşüm işlemi uygulanmıştır. Bu geri dönüşüm işleminde üretilen malzemeler küçük parçalara ayrılmış, bu parçalar bir kırıcı vasıtasıyla granül haline getirilerek tekrar ekstrüdere beslenmiştir. Bu işlemler 3 defa tekrar edilmiştir. Sonrasında ekstrüderden çıkan malzemeler pres altına alınıp soğuma sırasında malzemede olabilecek şişmeler engellenmiş, sabit bir malzeme kalınlığı elde edilmiştir. Oluşturulan malzemelerden numuneler alınıp mekanik özellikleri incelenmiştir.Ekstrüderde oluşturulan karma malzemeler, daha sonra sentetik bir elyaf olan cam elyafın farklı formlarında ve farklı düzenlerde yerleştirilmeleriyle oluşturulan tabakalı yapılar elde edilmiştir. Tabakalı yapılar, iki doğal elyaflı karma malzeme tabakası arasında cam elyaf olacak şekilde üç katlı olarak oluşturulmuştur. Tabakalı yapılar; ekstrüzyon işleminden sonra cam elyafın iki karma malzeme tabakası arasına serilmesi, oluşturulan yapının etüvde bir süre yüksek sıcaklıkta bekletilmesi ve sonrasında soğumasının pres altında bekletilerek yapılması şeklinde üretilmiştir. Bu malzemelerden de ilgili testler için numuneler kesilip mekanik özellikleri incelenmiştir.Elde edilen her iki grup malzeme türünün mekanik özellikleri, mevcut kullanılan Ford Transit Connect marka otomobil tampon malzemesinin yine laboratuar çalışmaları sonucu elde ettiğimiz özellikleriyle karşılaştırılmıştır.Çalışmanın ikinci aşamasında, ilk aşamada en iyi özellikleri veren malzemenin özelliklerinin geliştirilmesi amaçlanmıştır. Bu amaçla örme cam elyaflı tabakalı karma malzeme, tabakalarının ara yüzeylerindeki tutunma miktarını arttırmak amacıyla dört farklı yüzey işlemi malzeme üzerinde uygulanmıştır.Farklı yöntemler uygulanan malzemeler ayrılma testine tabi tutularak tabakalı malzemelerin ara yüzey tutunmalarını en çok arttıran yöntem olarak pamuk takviyeli karma malzemelerin tabakalı yapıyı oluşturmadan önce etüvde bir süre bekletilmesi ve sonrasında polyestre reçine uygulanması tespit edilmiştir. Bu yöntem uygulanarak elde edilen plakalardan ilgili deney numuneleri kesilmiştir. Geliştirilen bu malzemenin çekme ve darbe deneyleri yapılarak elde edilen sonuçlarla malzemenin ilk durumundaki değerleri karşılaştırılmıştır. İlave olarak geliştirilen malzemeye üç noktadan eğme testi de uygulanarak eğilme özellikleri de tespit edilmiştir.

Abstract:
Decreasing resource quantities causes increasing environmental conscious and increasing importance of using natural resources efficiently. With their minimum environmental harmful effects, natural materials take the place of synthetic materials gradually. Law sanctions like Kyoto protocol are also increased the interest of natural materials as well as obligatory of decreasing carbon emissions.In ancient times, first composite samples have shown on bricks that are made of clay and straw. Clay and straw are not functional materials individually, but their composite material form, brick is a functional structure in building industry. Clay keeps together the straws and straws can stop the cracks on the clay, so their conjuction has good mechanical properties.The natural fiber studies are in a preliminary period in world, so all mechanical, physical and thermal properties of fibers have not known enough yet. A great deal of scientific researches have been studied about natural fibers. Common trend of these researches are that related fibers have been generally growed in domestic areas where the researches are studied. Composite materials have been investigated in their mechanical properties with tensile testing, impact testing, bending testing, melt flow index analyses, water absorbtion testing, etc. In studies, there have not been only natural fiber composite materials, but also hybrid composite materials with synthetic fibers such as glass fibers, carbon fibers, and aramid fibers.In most articles, to increase the mechanical properties, different chemical treatments have been applied fibers such as sodium hydroxide, acetilation, permanganate, peroxide, maleic anhydride, etc. These treatments were applied to fibers in conposite materials for increasing the bonding forces and also applied to fiber laminates or composite laminates to increase interlaminar bonding strengths.Composite materials have a huge range area of usage because of their various advantages such as high tensile and bending strength, good molding performance, easy forming properties, good conductive and good insulation materials individually, high resistance to corrosion and chemical attacks, permanent coloring features, high vibration damping rates, etc.Beside lots of advantages of composite materials, they are not perfect materials, so they have some disadvantages. Composite materials have air voids, so their fatigue strengths are not enough according to metals. They have different mechanical and physical properties in same material, and variable parameters for different directions. They are not manufactured in accurate dimensions, and operations such as hole punching, cutting. They are so brittle according to forging metals. They will have additional damage, if they are disassembling. They have high moisture absorbtion rates, so they need special drying conditions before any operation.Composite materials are classified according to structure elements and using areas. The matrix material classes are polymer matrix, ceramics matrix, and metal matrix. Fiber reinforced composites, particle reinforced composites, and layer reinforced composites are the reinforcement classes of these materials. Additional classifications are according to structures (metal organic, metal inorganic, etc.), component phases (matrix system composites, laminar composites), component distributions (permanent composites, transient composites), and functions (electrical composites, structural composites).Various types of production methods are used for composite materials. Production parameters such as molding material geometrical form, size, resin type and properties, reinforcement type, expected material features are the main items for selecting production method. The types of production methods are pultrusion, resin transfer molding (vacuum assisted or not), hand lay up, surface preparation and bonding, laminate construction, compression molding, and filament winding.Strength and stiffness are the main properties of natural fibers in composite materials. Cellulose is a natural polymer raw material that gives high strength and stiffness characteristics to composites in lower mass loads. They are used for long natural fibers. These structural cells are existed in plant stems, leaves, and seeds. Generally, fibers are comprised from wood cores surrounded around the stems. Fiber bunchs consisted different fibers and fiber filaments are existed in stems.Fibers used in composite materials have been an inclination to made of natural sources. Especially in automotive industry, utilization of natural fibers which have good strength/mass ratios, are improved to reduce carbon emission quantities.Researches about natural fibers are increased because of their competitive properties due to synthetic fibers like glass fiber. Automotive is an important sector that composite materials made of these type fibers are used. Using rates of natural fiber composites have exceeded half ratio of all types of composites. Last decade, european automotive manufacturers have used biological fibers in door panels, seat backs, inner floor layings, headliners, package trays, dashboards, trunk liners.Kyoto protocol is a commitment agreement which member countries have a responsibility about reducing the carbon emission rates. Number of this protocol members is ever increasing to protect world. Fuel consumption is the main factor of carbon emission, so with reducing fuel consumption, required emission rates can be caught. Automotive parts made of natural fibers which have good mechanical properties with minimum weights, will be assisted to decrease this comsumption.All researches about automotive performance are settled on to decrease material costs with supplying same quality level to be stay competitive in world automotive markets. In automotive industry, an important problem is what synthetic materials parts have been done after completion of their using life period. Recycling and biological degredation properties of natural sources and materials, become an advantage for this problem, so agriculture based natural fibers take petroleum based synthetic materials place in this industry. In terms of environmental conditions, the using natural materials cause less material wastes and less fuel consumption.In this study, natural fiber effects with using synthetic fibers are investigated together in composite materials. The resulted mechanical properties of composites have been compared with the front buffer material of Ford Transit Connect. Waste cotton fabrics are used as natural fibers in this study. This usage causes either same recycled materials have been used again in different area, or reduced utilization of synthetic fiber and polymer materials ratios in composite. Using less non-biodegradable material and reduce total costs are the other advantages of natural fibers.Firstly waisted mass ratios of cotton staples and polymer matrix have been produced in a single screw extruder. To supply homogenous properties in all material, cotton fabrics have been pretreatment process and they would be cotton staples in a textile machine. Then produced materials have been recycled operation. In recycling process, the product composite material have been cut into small pieces. The pieces of composites are put into granulator to make granuls for feeding extruder again. After that all composite material granuls have been fed into extruder, so the second production stage would be begun. This recycling process has been three times for all composites. After last stage, the composte materials have been taken to hydrolic press to prevent swelling and provide same thickness in all material plates. Samples which are cut in these plates, have been mechanical tested and acquired related property results.After extrusion process, laminar structures have been comprised and these structures have been kept waiting in incubator oven for a while, then immediately transferred to hydrolic press before becoming cold. Material in hydrolic press have been cooled down under hydrolic loads to prevent swelling or another inequalities. Samples have been cut in laminar materials and tested to determine the mechanical properties.The composite plates which were produced by extruder, have been also made laminar structures with different knitting types and different length dimensions of glass fibers. Laminar structure have been formed with three layers as one glass fiber layer between two composite material plates.Two groups of material?s mechanical properties have been compared to the leading buffer material of the Ford Transit Connect automobile mechanical properties, which have been tested in same laboratory conditions.On the second stage of this study, the aim is improving mechanical properties of material which have the best properties. Therefore four different surface treatments are used to increase the adhesion forces of laminates interfaces of the highest properties material, knitting glass fiber reinforced laminated hybrid composite.These four different kinds of materials are subjected to separation tests, to establish which have the highest adhesion force. The holding in incubator and applying polyester resin to laminates intefaces of knitting glass fiber reinforced laminated hybrid composite is the best method which have been the highest results. The tensile testing and impact testing are applied to this type of material specimens, and the results are compared with the prior condition that without surface treatment and polyester resin. Additionally three points bending test is applied to the material to establish the bending properties.