Doğal lif takviyeli kompozitlerde lif
matris ara yüzey iyileştirme çalışmaları ve çevresel koşullara göre karakterizasyonu / Fiber / matrix interfacial improvement techniques and characterization due to environmental conditions for natural fiber reinforced composites
Yazar: MEHMET SAFA BODUR
Danışman: DOÇ. DR. MUSTAFA BAKKAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Makine Mühendisliği Bilim Dalı
Derece: Doktora, 2016
Konu: Makine Mühendisliği = Mechanical Engineering ; Mühendislik Bilimleri = Engineering Sciences ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology

Özet:
Doktora tezi kapsamında, öncelikle sentetik lif takviyeli kompozitlere alternatif olarak düşünülebilen "Doğal Lif Takviyeli Kompozit" granüllerin tasarlanması, üretilmesi, performans özelliklerinin incelenerek iyileştirme teknikleri irdelenmiştir. Ayrıca dış ortam ürünlerinde kullanımı göz önünde bulundurularak çevresel şartlar altında performansının irdelenmesi ve son aşamada mekanik performanslarının öngörülebilmesi amacıyla kullanılan analitik ve sonlu elemanlar yöntemlerinin uyarlanabilirliklerinin araştırılması gerçekleştirilmiştir. Kompozit malzemenin doğal lif takviyesi olarak kullanılan pamuk lifi, mevcut sistemde sınırlı bir toplama ve geri kazanımı bulunan ve sistemli olarak toplanmayan evsel ve endüstriyel tekstil atıklarından sağlanmıştır. Matris malzemesi olarak ekstrüzyon yöntemi ile şekillendirmeye uygun olabilecek Düşük Yoğunluklu Polietilen (DYPE) termoplastik malzemesi tercih edilmiştir. Bu tarz kompozitlerin özelliklerinin her yönüyle belirlenebilmesi ve karakterizasyonu amacıyla bir çok test ve analizden faydalanılmıştır. Mekanik testler olarak kompozitlere çekme, darbe, üç noktadan eğme ve Dinamik Mekanik Analiz (DMA) testi uygulanmıştır. Diğer taraftan Fourier Dönüşümlü Kızılötesi Spektrofotometresi (FTIR) ve Diferansiyel Taramalı Kalorimetri (DSC) analizleri kimyasal ve ısıl özelliklerin saptanması amacıyla tercih edilmiştir. Foto spektrometre cihazı, optik mikroskop ve Tarayıcı Elektron Mikroskobu (SEM) ise kompozitlerin fiziksel özelliklerini ve iç yapılarının irdelenmesi amacıyla kullanılmıştır.
Kompozit yapı içerisinde en uygun yapıyı oluşturmak macıyla lif matris ara yüzey iyileştirme çalışmalarına ağırlık verilmiştir. Bu amaçla çeşitli kimyasal iyileştirme işlemleri gerçekleştirilmiş ve çalışmalar neticesinde %70-80'lere kadar mekanik performansta artış elde edilmiştir. Mekanik performans bakımından en ideal kompozit yapının oluşturulmasıyla birlikte sıra kompozit granüllerin kullanım amacına yönelik olarak öngörülen ortam şartlarının, kompozit numunelerin mekanik, fiziksel, kimyasal ve ısıl özellikleri üzerinde etkileri uygulanan çeşitli yaşlandırma prosedürleriyle incelenmiştir. Çalışma kapsamında doğal lif takviyeli kompozitlerin kullanımı için en büyük engel olan nem yutum özelliğinin ve güneş ışınlarının (UV) kompozitlerin çekme ve darbe özellikleri yanında ısıl, kimyasal, fiziksel ve mikroyapısı üzerindeki etkileri incelenmiştir. 1000 saate kadar yapılan yaşlandırma işlemlerinin ideal özellikteki kompozitlerin üzerine etkileri irdelenmiş ve raporlanmıştır. Ayrıca çevresel şartlara karşı kompozitlerin özelliklerini koruyabilmesi amacıyla tercih edilebilecek kimyasal yöntemler çalışma kapsamında irdelenmiş ve olumlu sonuçlar elde edilmiştir. Buna göre lif ve matris ara yüzey durumunun iyileşmesiyle birlikte mekanik ve fiziksel özelliklerin korunabildiği ve nem yutum özelliklerinin de iyileştiği görülmüştür. Diğer yandan UV koruyucu katkıların türü ve oranlarına göre kompozitlerin mekanik veya fiziksel özelliklerinin korunmasında kayda değer bir şekilde etkili oldukları görülmüştür. Tez çalışmasının son aşamasında kompozitlerin çekme özellikleri göz önünde bulundurularak elastik bölgenin modellenmesiyle ilgili yürütülen analitik ve sonlu elemanlar yöntemleri öncü sonuçlar sunmuş ve ileride yürütülebilecek benzer çalışmalara yol göstermesi amaçlanmıştır.
Doktora tezinin sunmuş olduğu tüm sonuçlar doğrultusunda bu tür yeni ve özgün bir kompozit malzemenin alternatif olarak belli uygulamalarda tercih edilerek ekonomik anlamda, çevresel anlamda ve belli iş olanakları sağlayarak sosyal anlamda faydalar sağlayabileceği sonucuna varılmıştır.

Abstract:
Within the scope of The Doktoral Thesis, design, manufacturing and also techniques to improve the fiber/matrix interfacial adhesion were examined for "The Natural Fiber Reinforced Composite" granules as an alternative material to syntetic fiber reinforced composites. The performance under the environmental conditions were also studied taking into account the usage for outdoor applications. In the final stage, the adaptability of the analytical and finite element methods was investigated in order to predict the mechanical performance. The cotton fiber as natural fiber reinforcement in the composite material was collected from the domestic and industrial textile waste which results from the present system with a limited collectin and recycling and can not be collected systematically. As the matrix, low density polyethylene (LDPE) which may be suitable for forming by extrusion was preferred.  To determine every aspect of the characteristics of this type of composite, a lot of testing and analysis to characterization has been benefited. Tensile, impact, three-point bending and Dynamic Mechanical Analysis (DMA) testing were performed as mechanical tests while Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) analysis were preferred for chemical and thermal investigations respectively. Photo-spectrometer, optical microscope and Scanning Electron Microscopy (SEM) were also used to evaluate the physical and micro structures of this novel composites.
The improvement techniques for fiber matrix interfacial adhesion specifically was focused on in order to create the most appropriate composites structure. For this purpose, five different chemical methods namely, silane treatment, alkaline treatment, alkali–silane treatment, maleic anhydride, and alkali–maleic anhydride coupling agents were applied to evaluate the suitable process parameters (concentration, soaking time, ratio by weight) for the enhanced properties of T-FRP composites. Tensile, three-point flexural and impact tests were performed on both untreated and treated composites for comparison purpose. Treated and untreated T-FRP composites were characterized using scanning electron microscopy, differential scanning calorimetry, and Fourier-transform infrared spectroscopy to evaluate thermomechanical properties of composites. According to the results, approximately 70% and 45% rise for the tensile and flexural strength can be achieved by improving the poor interfacial adhesion between fiber and matrix for untreated composite. The best tensile and flexural strength values were obtained from the composites with maleic anhydride. Although, the mechanical properties were found to be increased about 30 - 40% by the alkaline and silane treatment alone, the combination of these treatments can be counted as more effective way to improve the interfacial adhesion. Surprisingly, it is interesting to note that the tensile and flexural properties were dramatically decreased approximately 40% by applying the alkaline treatment before addition of the maleic anhydride. In contrast to the tensile and flexural tests, impact test results showed that impact strength was somewhat negatively influenced with the chemical treatments compared to the untreated composite. Probably it is because of brittle fiber characteristics become more dominant on the properties of composites, as better adhesion and interfacial bonding between fiber and matrix occurred by chemical treatments. Thermal analysis showed that the melting behavior for all composite samples was observed to show slight changes compared to the pure LDPE matrix. However, addition of cotton fibers without any treatment (untreated composite) caused some increment in the crystallization temperature (Tc) and the degree of crystallinity (Xc) of LDPE matrix. Overall results shows that the T-FRP is a effective novel material for further property development via chemical treatments. 
After the ideal composite structures were created in terms of mechanical performance, effects of anticipated environmental conditions for the purpose of usage on the chemical, physical, mechanical and thermal properties of composite granules were investigated with the implementation of various aging procedure. For this purpose, the effect of water sorption which is the biggest obstacle for the use of natural fiber reinforced composites and UV radiation on the thermal, chemical, physical and microstructure properties addition to tensile and impact properties of composite samples. This study elaborates and present the detailed information about the effects of aging process up to 1000 hours on the composite samples with the ideal characteristics. Degradation of physical, mechanical and chemical properties of T-FRP composites was evaluated through common chemical treatments such as maleated coupling, alkaline treatment, silane treatment and alkali-silane treatment. Untreated and chemically treated T-FRP composites were subjected to water uptake and UV exposure up to 1000 hours. Tensile and impact properties were mechanically examined, and the changes on the physical properties due to water uptake, swelling and color change were investigated. In addition, Fourier Transform Infrared Spectrum (FTIR) analysis was performed in order to evaluate the chemical changes after exposure. Results show that reduction about 10-15% in TS values for all composite samples were found (less than reduction in TS of pure LDPE) after 1000h UV exposure time. Because, composite samples have a lower amount of polymer subjected to the photo oxidative degradation. The highest overall drop in TS values was observed for composites with chemical treatments. This result is likely due to degradation of fiber/matrix interfacial adhesion which is more predominant on the reduction in TS compared to control sample.  For impact strength, it is interesting to note that chemically treatment lead the composites to be less affected samples after UV exposure.  FTIR results showed that the chemically treated composites have lower stability than the untreated T-FRP composites with longer UV exposure.  All composite samples have shown the surface degradation after 500h of UV exposure and then the cracks seemed to be increased with longer UV exposure. Samples with chemical treatments showed a lower rate of water absorption around 0.2~0.6% after 1000 h of water immersion. The extent of drops varies and depends on the chemical treatment.  Composites with maleic anhydride exhibit the best tensile properties after 1000h of soaking time establishing a strong interface between cotton fiber and polymer matrix. The change in IS with water immersion was found to be more complex and also likely to be more dependent on the treatment type compared with the results for TS with water absorption. Chemical treatment with MA or Si is very sufficient to provide water uptake resistance. On the other hand, under UV exposure higher drops might be occurred in mechanical properties for the chemically treated composites which should be taken into account specifically for the outdoor applications exposed to sun light too much.
In addition, chemical methods which may be preferred in order to protect the properties of composites against environmental conditions have been analyzed in the scope of this work and positive results have been obtained. According to test results, the mechanical and physical properties can be protected as well as water sorption behavior recovered as the improvement of fiber and matrix interface was observed.On the other hand, the UV protective additives showed a significant manner to be effective on the retention of mechanical or physical properties of the composites according to the types and proportions. The effect of photostabilizers on the mechanical, thermal and physical properties of textile fiber reinforced polymer (T-FRP) composites was investigated for unweathered and accelerated ultra violet (UV) weathering conditions. Three different type of photostabilizers were chosen at varying concentrations in order to obtain the best formulations. The first phase of this study examined the effect of incorporating different concentrations of ultra violet absorber (UVA), hindered amine light stabilizers (HALS) and antioxidants (AOs) into T-FRP composites for unweathering condition. Tensile, flexural and impact tests were performed for the mechanical properties as well as differential scanning calorimetry (DSC) analysis for thermal properties. According to test results, photostabilizers do not influence mechanical and thermal properties significantly. In the second phase of the study, the influence of the photostabilizers on the durability performance of T-FRP composites under accelerated UV weathering was determined by tensile test, thermal analysis and color measurements. The results showed that the efficiency of the photostabilizers is highly dependent on the type, concentration and also weathering time. According to test results, tensile properties of T-FRP with photostabilizers were influenced badly in higher photostabilizer concentrations for all chemically stabilized composites as well as flexural properties were seemed to be changed in the same manner with tensile testing results while impact strength of T-FRP composite were not affected by either photostabilizer types or concentrations. DSC results show that almost no change has been observed in the melting temperature for the composites independent to photostabilizer type or concentrations. On the other hand, stabilized T-FRP composites were found to have generally lower crystallinity values compared to T-FRP composite for the unweathering condition.
Composites with the addition of the Chimasorb and Tinuvin at the concentrations of 0.5wt% and 0.2 wt% respectively exhibited lower mechanical properties drop and showed higher durability during 240 h of UV weathering. However, it is worth noting that photostabilizers at the concentrations of  0.4 wt% and 1wt% for Tinuvin and Irganox should be taken into account as promising photostabilizers and concentrations for the long term UV weathering. The crystallinity value for unstabilized T-FRP composite (control) was found to be increased for the first stage of accelerated UV exposure until a drop was reached with 240 h of UV exposures. For stabilized composites, gradual increase in crystallinity % was observed during 120 h of UV weathering, followed by almost same crystallinity % values up to 240 h. These results from DSC supports the ability and efficiency of the stabilizers. Color measurements overally show that, the effectiveness is highly dependent to the type and concentration of the stabilizers. Although AOs are not as effective as HALS on the retention of mechanical properties during weathering, they may be considered to be an alternative photostabilizer for the physical retention at the adequate concentrations.
In the final stages of thesis, the finite element methods and analytical works are carried out regarding the modeling of elastic behavior as the tensile properties of composite and presented pioneering results and it is intended to show the way for similar studies which may be conducted in the future.
In accordance with all the results that have been presented in this Doctoral Thesis, it is concluded that the new and unique composite material presented as an alternative material can provide economic, environmental and social benefits by providing certain job opportunities with the preference for in some applications.