Kumaş Takviyeli Polietilen Malzemenin Şekil Hafızası Özelliklerinin İncelenmesi

Abstract

Şekil hafızalı malzemeler geçtiğimiz yüzyılın ikinci yarısından itibaren çeşitli bilim dallarının dikkatini çekmiş, özellikle de malzeme biliminde kendine çok önemli bir yer edinmiştir. Diğer akıllı malzemelerle karşılaştırıldıklarında, bu malzemelerin sınaî kullanım alanlarının genişliği, araştırmacıların üzerinde yoğunlaşmalarına neden olmuştur. İlk kez 1950’li yıllarda, bazı alaşımların östenitik-martenzitik faz dönüşümlerinde rastlanılan bu özellikle ilgili yapılan çalışmalar, yapılan araştırmalar sonucunda sınaî ve tıbbî uygulamalara öncüllerinden çok daha uygun olan Nitinol (Ni-Ti)’ün keşfiyle ivme kazanmıştır. Şekil hafızalı alaşımların açtığı yeni ufuklar bilim insanlarına, bu tür malzemelerin daha geniş alanlarda kullanılabilmeleri için itki sağlamış, mevcut şekil hafızalı alaşımlardan daha ucuz ve daha kolay bir şekilde imal edilebilecek yeni malzemelerin arayışı içine sokmuştur. 1980’li yıllarda polimerler üzerine yapılan çalışmalarda, bu malzemelerin ve kompozitlerinin de benzer özelliklere sahip oldukları ortaya konmuştur. Şekil hafızalı alaşımlarınkine göre çok farklı bir mekanikle, çapraz bağlı polimerlerin hemen hemen tümünde “şekil hafızası” gözlemlenmiştir. Polimer ailesinin çeşitliliği, alaşımlara göre hammadde ve imalat maliyetlerinin düşüklüğü ve bazı mekanik ve kimyasal özellikler açısından üstünlükleri, bu malzeme üzerine ilgiyi arttırmış, şekil hafızalı polimerlerin uygulama alanlarının arttırılabilmesi için bu malzemelerin, içerikleri ve üretim yöntemleriyle oynanarak, özelliklerinin yeniden biçimlendirilmesi için yapılan çalışmaların hızlanmasına neden olmuştur. Özellikle tıbbî uygulamalarda, alaşımlara görece yüksek kimyasal kararlılıkları nedeniyle polimerler, gün geçtikçe daha çok tercih edilir olmuştur. Dikişlerden, protezlere, kalp stentlerinden kulak kanalı kalıplarına pek çok cerrahî uygulamada artık şekil hafızalı polimerler kullanılmaya başlanmıştır. Bu tez çalışması İstanbul Teknik Üniversitesi bünyesinde geliştirilen bir malzemenin, içerik olarak kumaş takviyeli polietilen kompozitin şekil hafızası özelliklerini belirlemek için yapılmıştır. Söz konusu malzemeden alınan, aynı standartta, fakat hazırlanma biçilerinde farklılık gösteren 3 ayrı numune üzerinde yapılan deneylerde, bu malzemenin hafıza geçiş sıcaklığı ve hafıza mekanizmasının özellikleri ortaya çıkartılmıştır. Deneyler, bir ısıl oda içerisinde numuneler standart döngülere sokularak yapılmış ve ısıl çiftler yardımıyla sıcaklık değerleri ölçülmüştür. Bu deneylere göre kullanılan kumaş takviyeli polietilenin hafıza geçiş sıcaklığı 42 oC, sabitlik oranın % 60-70 arası ve boyutsal geri dönme kararlılığına da yaklaşık % 95 olarak elde edilmiştir. Ayrıca deneylerden çıkartılan bir başka sonuç da, malzemenin ekstrüzyon yönünün veya yüzey işleminin bu özelliklerde önemli bir değişikliğe yol açmıyor olmasıdır. Bu çalışmada elde edilen verilerin bu tip malzemelerin hafıza davranışlarının modellenmesinde ve bu malzemenin türevlerinin hafıza özelliklerinin ortaya çıkartılmasında faydalı olması umulmaktadır.

Shape memory materials have made an attractive domain for scientists from miscellanious branches, starting by the second half of the twentieth century. Especially on materials science, they have had a great place for their unique characteristics. Compared to the other smart materials, these have a broad fields of use in many industries, and because of that reason scientists and engineers from different fields have been interested in this new phenomenon. First time, in 1950s, those materials are introduced as Au-Cd alloys, thanks to their austenitic-martensitic phase transition which permits them to “remember” their fore shape by the change of crystalline structure. But since recovery strains of those materials were not significant, they could not prove a significant importance at first step. And then, by the discovery of Nitinol (nickel – titanium alloy), shape memory alloys could reach an important point among the smart materials. Nitinol’s formidable mechanical characteristics for shape memory alloy uses in practical life paved the way of shape memory materials to a further horizon. Starting with the defence industry, Nitinol got used in various industries and became an important material in medical uses due to its rare properties. Thanks to Nitinol’s use in different technologies, scientists looked forward to find new materials with better characteristics, lower costs of production and cheaper raw materials. Since shape memory alloys were neither cheap, nor easy to manufacture, people turned their eyes to other materials with similar properties. During the 1980s, in the studies performed on polymers and their composites, scientists discovered that these materials have a similar characteristics as well. Of course different by the mechanics of those alloys, almost all of the cross linked polymers, copolymers and composites could prove a “shape memory”. As mentioned above, metal alloys had this characteristics, since they could change their crystalline structure during their phase transitions, as martensitic-austenitic transition. But in the polymers’ case, they “remember” their former shape thanks to their latent energy locked in the cross links when they are cooled. Basically, when polymers are cooled below their melting or glass-transition point, they show different characteristics and their viscoelastic behavior changes or gets halted. So, a cross linked polymer material deformed in a heated medium, cooled to a temperature below one of these transitions gets fixed and can not recover its original shape until it is reheated above this transition temperature. Since polymers form a vast material family, and it is possible to observe this shape memory characteristics, merely in all of them, they offer us a vast diversity of options in use. In addtition, their chain structure property enhance tayloring of their properties, such as adding new polymers with different transition points, may result with a new copolymer with multishape memory i.e. by producing a copolymer with different polymer components, it is possible to produce a shape memory material that can recover more than one shape (up to four as far as known). Hence it gives us flexibility of different characteristics, such as fixicity rate, recovery rate and recovery strain. Compared to the shape memory alloys, shape memory polymers have many advantages. As they are polymers, first of all, they are low-weighted, thus this makes them more applicable in many ways. They are usually more elastic than many other materials and that makes their recovery strains greater, although not proportionally. But above all they are much cheaper and easier to produce. To produce a Nitinol or any other industrially used shape memory alloy part, costs mostly circa $100, while most of the polymers cost less than $10 per pound, and to produce polymers do not require a high level metallurgy as shape memory alloys do. Another advantage of the shape memory polymers was their superior biocompatibility when compared with shape memory alloys. Since most of the metal alloys are active materials for in vivo applications, shape memory alloys, as well, are not very compatible for every medical operation. On the other hand polymers are chemically much more stable and that makes them more biocompatible and some of them are biodegradable as well. Thus they are more useful for medical purposes where shape memory effect is desired. Because of all these advantages that we listed above, in many areas shape memory polymers are getting preferred as this technology improves. Especially in surgical operations they started to replace shape memory alloys rapidly. From sutures to enclose wounds quickly to blood clot removal operators, they proved their succes in medical applications. In this field, it is another advantage of polymers that they are chemically more stable than alloys, so their in vivo applications are less dangerous for human health in long term. Since some of the shape memory polymers are biocompatible and biodegradable at the same time, unlike the shape memory alloys, biomedical industry inclined on them now. In this master’s thesis study, we have observed shape memory characteristics of fabric(made of cotton) reinforceed polyethylene. This is a composite material extruded in 180 oC, then rolled, and finally hot pressed in order to cross-link the polymer chains. Cotton whiskers in the material are (assumed that) randomly distributed. The material itself, is first introduced in a graduate study in Istanbul Technical University in order to introduce an alternative material for various products. In our experiments we prepared 3 different samples from the same material plate with different surface processes and extrusion orientations according to the ASTM D638 standard. Then we exerced cycles of loading and unloading under a thermally controlled medium. For these work, we have used a tensile testing machine and a thermal chamber. Temperature is observed via thermocouples and elongation values are recorded with a computer program compatible to our testing device. In our test results, we observed that our material has a fixicity rate between 60-70% and a recovery rate of around 95%. Also we recorded that transition temperature fort his material is 42 oC. These results yielded that neither extrusion orientation, nor surface processes do not have any significant effect on material’s shape memory characteristics(so nor on their mechanical properties). We hope that our study will be helpful for those who will research similar materials’ shape memory characteristics and will be modelling type of materials shape memory behavior.