Halojen içermeyen, alev geciktirici kauçuk karışımlarının geliştirilmesi ve karakterizasyonları
Development and characterization of non-halogenated flame retardant rubber mixtures
Yazar: EDA KARAMAN
Danışman: PROF. DR. ESMA SEZER
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Kimya = Chemistry

Özet:
Elastomerler, camsı geçiş sıcaklığı ve sıvılaşma sıcaklığı aralığında esnek özelliğe sahip polimerlerdir. Pratikte, sıvılaşma sıcaklığı çapraz bağlanma ile yükseltilebilir ve polimer zincirleri seyrek çapraz bağlandığında elastomerik özellik daha belirgin hâle gelmektedir. Kauçuk hammaddesinin vulkanizasyonu ile elde edilen, doğal veya sentetik kaynaklı olabilen, elastik yapıdaki polimerik malzemeler, en eski ve yaygın elastomerlerdir. Günümüzde farklı kullanım amaçlarına hizmet eden çeşitli elastomer türleri vardır. Bunlardan en yaygın kullanım alanına sahip olanlardan biri de etilen propilen dien monomer (EPDM) kauçuk bazlı elastomerlerdir. EPDM kauçuklar, genel olarak yüksek oranda dolgu ve yağ katılabilme özelliği sayesinde maliyetleri kolaylıkla düşürülebilen, ozon, UV, oksijen, radyasyon ve ışıkla yaşlanmaya, yorulmalara karşı çok dayanıklı olan dolayısıyla endüstride yaygın olarak kullanılan malzemelerdir. Ancak pek çok uygulamada kullanılan EPDM kauçukların yanmaya karşı dayanımları yetersizdir. Bu sebeple özellikle yüksek yanma dayanımının gerekli olduğu elektrik, elektronik, yapı, inşaat, tekstil, ulaşım vb. endüstrilerde amaca uygun yeni kauçuk reçetelerinin geliştirilmesi konusunda çalışmalar yapılmaktadır. Malzemeye yanma dayanımı sağlamak amacıyla kauçuk karışımlarına çeşitli katkılar ilave edilmektedir. Alev geciktiriciler olarak adlandırılan bu katkılar halojen, fosfor veya metal hidrat gibi çeşitli yapıları içerebilmektedir. Halojen içeren katkılar sıklıkla kullanılmasına ve iyi alev geciktirici özelliğe sahip olmasına rağmen aleve ya da ısıya maruz kaldığında serbest halojen radikallerinin salınımına yol açmaktadır. Bu radikaller, gelişim bozuklukları, tiroid ve kanser gibi ciddi sağlık sorunları ve hastalıklara, hatta ölümlere sebebiyet verebilmektedir. Bu sebeple Amerika ve Avrupa'da halojen esaslı malzemelerin kullanımlarına çeşitli kısıtlamalar getirilmiştir. Bu kısıtlamaların önümüzdeki yıllarda artması öngörülmektedir. Bu durum, halojensiz alev geciktirici malzeme geliştirilmesi konusunda yapılan çalışmaları hızlandırmıştır.
Bu çalışmada da kauçuk endüstrisinde yaygın olarak kullanılan EPDM kauçuğuna halojen içermeyen katkılar ile alev geciktirici özellik kazandırılması amaçlanmıştır. Bu amaçla yedi farklı kauçuk reçetesi hazırlanmış ve karşılaştırmalı olarak incelenmiştir. 1 numaralı reçete alev geciktirici dolgu içermemektedir ve referans karışım olarak kullanılmıştır. 2, 3, 4 ve 5 numaralı reçeteler aynı oranlarda sırasıyla aluminyum trihidroksit (ATH), çinko borat 4015, çinko borat 4013 ve huntit içermektedir. Daha sonra 5 numaralı reçetenin alev geciktirici özelliğini iyileştirmek amacıyla daha yüksek oranlarda huntit minerali içeren 6 ve 7 numaralı karışımlar geliştirilmiştir.
Kauçuk karışımları aynı şartlarda hazırlanmış ve karakterizasyonları reolojik, fizikomekanik ve yanma testleri ile gerçekleştirilmiştir. UL94 yatay yanma testine göre, 2 ve 5 numaralı karışımlardan elde edilen numunelerin yanmaya dayanıklı olduğu ve HB sınıflandırmasına dâhil edilebileceği belirlenmiştir. Ancak her iki karışım da UL94 dikey yanma testinin şartlarını sağlayamamış ve V0, V1, V2 sınıflarından hiçbirine dâhil edilememiştir. Bu karışımların LOI değerleri de yanmazlık sınıfı için yeterli görülmemiştir. 
5 numaralı karışımın yanmazlık özelliğini geliştirebilmek amacıyla huntit miktarı artırılarak 6 ve 7 numaralı karışımlar hazırlanmıştır.  Bu karışımlardan elde edilen standart numuneler UL94 testine tabiî tutulmuş ve V0 koşullarını sağladığı görülmüştür. Huntit miktarının artırılmasıyla LOI değeri, 22,1'den 24,7 değerine artırılabilmiştir. 6 ve 7 numaralı karışımlardaki dolgu miktarı yüksek olduğundan mekanik özelliklerinde düşme gözlenmesine rağmen, daha az dolgu içeren 2, 3 ve 4 numaralı karışımlara oldukça yakın bir davranış sergilemektedir. Sonuç olarak bu çalışmada halojen içermeyen huntit minerali kullanılarak EPDM'e alev geciktirici özellik kazandırılabileceği belirlenmiştir.

Abstract:
Elastomers are polymeric materials obtained by vulcanization of rubber. They can be of natural or synthetic origin. Natural rubbers are obtained from a tropical tree called 'Hevea Brasiliensis'. Synthetic rubbers, which are widely used as natural rubbers, are petrolium-based materials.Today, there are various types of elastomers that serve different purposes. Polybutadiene rubber, styrene-butadiene rubber, acrylonitrile butadiene rubber, chloroprene rubber are some of these elastomers. 
In elastomer mixtures, there are general compounding ingredients which are rubbers, fillers, oils, vulcanization system, age resistors and other ingredients such as colorants. Rubber can be selected as natural or synthetic rubbers. Vulcanization system is a general name for sulphur, peroxide, resin. Age resistors are antioxidants and antiozonants. The selection and optimization of these ingredients has very imporant role on the performance of the elastomer product. Fillers are generally used as reinforcing and cheapening materials. Also, they can used as flame retardants. 
Ethylene propylene diene monomer (EPDM) is one of the most useful synthetic elastomers. EPDM rubbers are widely used in industry because they can be easily cheapened due to their high level of filler and oil additive properties, and are very resistant to aging and fatigue by ozone, UV, oxygen, radiation and light. However, EPDM rubbers, which are used in many applications and fields such as construction, electrical insulation, automotive etc., have insufficient fire resistance. It is a flammable material with a very low limit oxygen index (LOI) value.  Therefore, the flame retardant performance of EPDM materials should be enhanced. For this reason, especially in electrical, electronic, building, construction, textile and transportation industries, various types of non-flammable elastomer mixtures are being developed. 
In order to provide fire resistance to the material, alternative additives are added to rubber mixtures. These additives can be named as flame retardants and they can be halogen-based, phosphorus-based and metal hydrated flame retardants. Halogenated flame retardants such as decabromodiphenyl oxide improve flame retardancy of EPDM. Although halogen-containing materials are frequently used and have good flame retardancy, these types of materials cause toxic gas release when exposed to flame or heat. Inhaling these gases causes serious health problems and even deaths. For this reason, several restrictions have been imposed on halogen-based materials in America and some European countries. These restrictions has accelerated the development of halogen-free flame retardant materials.
For elastomers, there are various types of characterization methods. Determination of rheological properties, hardness values, mechanical and flame retardant behaviours of rubber mixtures is very important. 
To indicate flame retardancy of polymeric materials, there are different methods such as UL94 horizontal burning test, UL94 vertical burning test and LOI measurement. UL94 horizontal burning test is applied for the calculation of burning rate of the polymer. If burning criterias are suitable, material is classified as UL94 HB. Another test method for observing burning behaviour is UL94 vertical burning test. For vertical testing, there are three classifications which are V-0, V-1 and V-2. V-0 rating is the best result that a polymer can receive. 
In addition to UL94 horizontal and vertical burning tests, LOI test is another widely used test method. LOI test is applied for determining the minimum oxygen concentration required to support the combustion of materials. For LOI test, oxygen/nitrogen mixture is used. Since the oxygen concentration of the air is 21%, materials which have LOI values less than 21% are classified as highly flammable. A self-extinguishing material should have at least 21% LOI value.
In this study, non halogenated rubber compounds are developed and their physico-mechanical, rheological and flammability tests are applied and compared. For this purpose, seven different rubber recipes were prepared in the same conditions and the mixtures are numbered from 1 to 7. The first rubber mixture does not contain flame retardant filler. It contains 63 phr of carbon black and it is used as a reference mixture. The second mixture contains 75 phr of ATH, the third mixture contain 75 phr of zinc borate 4015 and the fourth mixture contains zinc borate 4013, which has bigger particule size than 4015. The fifth mixture contains 75 phr of huntite. Sixth and seventh mixtures are prepared in order to improve non-fammability properties of fifth mixture and they contain 125 phr and 150 phr of huntite, respectively. 
For hardness, density, mechanical and flame retardancy tests, rubber mixtures are vulcanized at 150°C for 20 minutes. After vulcanization process, test samples were prepared. Hardness values of mixtures from 1 to 7 are 64 ShoreA (ShA), 57 ShA, 58 ShA, 58 ShA, 60 ShA, 71 ShA and 77 ShA, respectively. From this result, it is observed that black fillers (carbon black) increases hardness values of mixture more than white fillers (flame retardants). Also, it is understood that the amount of huntite mineral required to enhance the hardness value of the mixture by 1 ShA is approximately 4.5 phr.
Density values of the mixtures are determined as 1.068 g/cm3, 1.19 g/cm3, 1.216 g/cm3, 1.214 g/cm3, 1.196 g/cm3, 1.306 g/cm3 and 1.402 g/cm3, respectively. Since the filler amount of sixth and seventh mixtures are higher than other mixtures, their densities are higher as expected.
According to the rheological results of mixtures which have similar hardness values, huntite mineral extends the time of vulcanization. However, t90 value, the optimum time period required for vulcanization, decreases as the amount of huntite filler enhanced.  Also, mixtures 6 and 7, prepared by increasing the filling ratio and having the highest hardness value, gave the highest mooney values as expected.
While the effect of white fillers on burning is generally positive, their effect on the mechanical properties of the material tends to be negative. In order to examine the effects of fillers on the mechanical properties of the mixtures, their mechanical values were measured. From mixture number 1 to 7, the tensile test results are 16.4, 12.3, 13, 12.2, 16, 12.3 and 12, respectively. The results showed that reference mixture, number 1, had the highest tensile strength value. Among the mixtures number 2, 3, 4 and 5, the mixture with the closest tensile strength value to the reference mixture was mixture containing huntite. Also, mixtures number 6 and 7 have tensile strength close to the mixtures number 2, 3 and 4 despite their high filler ratio.
Elongation test results are 385%, 422%, 467%, 464%, 435%, 535% and 513% respectively. These results show that white fillers have a positive effect on elongation of EPDM. In addition to these tests, tearing strength values are measured as 5.5, 3.4, 4, 4.1, 6.9, 11.1 and 11.5 respectively. It is concluded from these results that huntite mineral has a positive effect on the tear strength of EPDM.
In order to measure the burning rates and flammibility behaviours of mixtures, UL94 horizantal and vertical burning tests were performed of mixtures. 
Horizantal burning tests are applied to mixture number 2 and 5. For both mixtures, the flame is extinguished spontaneously before reaching the 25 mm line and the burning rates of the samples did not exceed the 40 mm/ min limit. Thus, according to UL94 standard, both mixtures are included in the HB classification. Since these two mixtures succeed in the test, the horizontal burning test of the 6 and 7 mixtures was not performed.
In addition to horizantal burning test, vertical test is applied to the mixture 2 and 5. The results showed that neither of these mixtures could be included in the UL94 vertical classification because they could not fulfill the requirements specified in the standard. After these results, mixtures 6 and 7 are developed and tested. As a result, mixtures number 6 and 7 both fulfill the V-0 classification criteria, which is the best non-flammability result of UL-94 vertical test. As expected, the increase in the amount of huntite in the mixtures affected the combustion properties positively.
A material's LOI value below 21% means that the material is highly prone to burning. In order to determine the combustibility of mixture number 2, 5, 6 and 7, their LOI values are measured. The results are 22.1, 22.1, 22.8 and 24.7 respectively. LOI values of the mixture number 2 containing 75 phr ATH and the mixture number 5 containing huntite in equal proportions were measured the same. Accordingly, the oxygen ratios that these two mixtures need to keep burning are equal. As it is understood from the results, the increase in the amount of huntite mineral has enhanced the LOI value and positively affected the flame retardant properties of the mixtures.
As a result of this study, it was observed that huntite mineral is a flame retardant compatible with EPDM. Huntite has been a promising filler in the development of a flame retardant mixture, as it gives EPDM rubber a flame retardant property, even if its high rate of addition does not significantly reduce its mechanical properties. The study will be continued by adding different fillings that can create a synergistic effect with the huntite mineral and determinating the mechanical and combustibility properties of different mixtures.