Yeni Yönetmeliklere Göre Gıdalarla Temasa Uygun Emaye Yüzeylerin Geliştirilmesi
Yeni Yönetmeliklere Göre Gıdalarla Temasa Uygun Emaye Yüzeylerin Geliştirilmesi
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Tarih
2016-06-29
Yazarlar
Kadakal, Gizem
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Institute of Science and Technology
Özet
Emaye, içerdiği inorganik oksit bileşenleri ile ve çeşitli yöntemlerle metalik taban malzeme üzerine, bir veya birkaç kat olarak uygulanan ve 500-870°C sıcaklıklarında pişirilerek taban malzeme üzerinde yapışması sağlanan camsı görünüme sahip bir kaplama malzemesidir. Kaplama kalitesini etkileyen en önemli faktör kaplamanın taban malzemeye yapışma performasıdır. Emaye kaplamalar kaplandığı metale estetik bir görünüm sağlamaları; sert, pürüzsüz bir yüzeye sahip olmaları, çizilme ve aşınmaya karşı dayanıklı olmaları; kaplandığı metali korozyondan koruyarak uzun süreli kullanımı sağlamaları; ısıl dirençlerinin yüksek ve termal şoklara dayanıklı olmaları; toksik olmamaları gibi özellikleri sebebiyle birçok endüstriyel alanda kullanılmaktadır. Beyaz eşya sektöründe pişirici cihazlarda, özellikle de fırın şasi ve tepsilerinde kendisine yaygın olarak kullanım alanı bulmuştur. Emayenin pişmesi sırasında gerçekleşen kimyasal reaksiyonlar sayesinde emaye ile taban malzeme arasında yapışma sağlanır. Yapışma olayı, birbiriyle bağıntılı olarak etkileşim noktalarında oluşan elektrostatik çekim kuvvetinden kaynaklanan mekanik bağlanma ve camsı tabakanın tutunmayı sağlayan metal oksitler tarafından doyurularak oluşan kimyasal bağlanma ile gerçekleşmektedir. Temelde yapışma mekanizması, emayedeki camsı yapı içerisindeki küçük bir bölgenin demir oksit (FeO) tarafından doyurulması ile sağlanmaktadır. Emayenin pişme sıcaklığında demirin yükseltgenmesi sonucunda oluşan FeO’in camsı faz içerisine difüzyonu gerçekleştiğinde emaye-taban malzeme ara yüzünde bir oksit ara katman oluşur. Arayüzde bulunan bu metal oksitler çelik yüzeyinde oluşturdukları yoğun pürüzlülük ile emayenin taban malzemeye yapışmasını sağlamaktadır. FeO oluşumunun yanı sıra emaye içinde bulunan kobalt oksit ve nikel oksit (CoO ve NiO) gibi tutunmayı sağlayan metal oksitlerin demir ile gerçekleştirdikleri elektrokimyasal reaksiyonlar sonucunda oluşan FeO da emaye içerisine dendritik yapılar oluşturarak mekanik tutunma sağlamaktadır. Tezin literatür kısmında, yaygın bir kullanım alanına sahip emaye kaplamaların özellikleri, yapısı, emayeleme çeşitleri ve emaye kaplama yöntemlerinden bahsedilmiş olup tez kapsamında incelenen emaye kaplamaların taban malzeme üzerine tutunma mekanizmaları hakkında genel bilgiler detaylı olarak verilmiştir. Deneysel çalışmalarda, Arçelik A.Ş. Pişirici Cihazlar İşletmesi’nden temin edilen DC04EK emaye kalite saclar ve Gizem Frit firmasından tedarik edilen çelik kalite toz emaye fritleri kullanılmıştır. Deneysel çalışmalara, taban malzeme olarak kullanılan levhaların yüzey temizliğinin yapılmasının ardından, emaye kaplamaya uygunluğunun belirlenmesi amacıyla kimyasal bileşimi, yüzey pürüzlülüğü ve ıslatma açısı değerlerinin belirlenmesi ile başlanmıştır. Taban malzeme karakterizasyonu tamamlandıktan sonra emaye kaplama işlemine geçilmiştir. Çift kat tek pişirim olarak yaş sprey yöntemi ile kaplanan numunelerde astar kat kalınlığı sabit tutulmuş; üst kat için MiniTab programında gerçekleştirilen deney tasarımına göre 2, 5 ve 8 kat emaye kaplama uygulaması gerçekleştirilmiştir. Kaplama işleminden sonra numuneler 810, 820 ve 830°C sıcaklıkta ve 2, 3,5 ve 5 dakika sürelerde pişirilmiştir. Tez kapsamında, emaye kaplamaların yapışma mukavemeti üst kat kaplama kalınlıklarına bağlı olarak incelenmiştir. Emaye-taban malzeme arayüzünde gerçekleşen kütle transferi, üst kat emaye içerisine katılan geçiş (trace) elementinin emayenin pişmesi sırasındaki difüzyonu ile ortaya konulmuştur. Bunun yanı sıra emaye kaplamaların pişirme sıcaklığı ve süresinin optimizasyonu gerçekleştirilerek arayüzdeki kimyasal bileşim ve mikroyapı incelenmiş; bunların tutunma mekanizması üzerindeki etkileri incelenmiştir. Bu kapsamda emayenin taban malzemeye tutunması darbe testi ile; arayüzde gerçekleşen kimyasal reaksiyonlar, elementel difüzyon ve oluşan dendritik yapı ise SEM-EDS analizleri sonucunda belirlenmiştir. Sonuç olarak, emaye-taban malzeme tutunma mekaniğini etkileyen üst kat kaplama kalınlığı, pişirme süresi ve sıcaklığı parametreleri üzerine çalışılmış ve en optimum parametrelerin 2 üst kat kaplama ve 810 ve 830°C’de 5 dakika pişirme koşulları olduğuna karar verilmiştir. Çalışmada, emayenin pişirilmeden de optik profilometre cihazı kullanılarak kaplama kalınlığının ölçülebileceği ortaya konulmuş ve kaplama kalınlığı ölçüm cihazı olan elkometre ile ölçüm sonuçları arasındaki ilişki gösterilmiştir. Üst kat kaplama kalınlığı, SEM analizinde Zn difüzyonu incelenerek belirlenmiştir. Emayenin pişme sıcaklığında demir elementinin oksitlenerek FeO, emaye içerisindeki kobalt oksit ve nikel oksitin indirgenerek metalik kobalt ve nikel oluşturması ile oluşan Fe-Co-Ni metalik yapıları, emaye ile taban malzeme arasında tutunma sağlanmaktadır. Arayüz bölgesinde gerçekleşen elementel difüzyon ve kimyasal reaksiyonlar sonucu bu yapılardan oluşan dendrit bölgesi SEM-EDS analizinde incelenmiştir. Emaye kaplamaların arayüzlerinde gerçekleştirilen çizgisel elementel analizler ile taban malzemeden emaye yüzeyine sonrasında da emaye bölgesine doğru gidildikçe Fe konsantrasyonun düştüğü; Si konsantrasyonunun arttığı gözlenmiştir. Bu olay kimyasal teoride belirtilen reaksiyonlar sonucunda, tabandan gelen demir elementinin emayenin pişme sıcaklığında oksitlenerek Fe+2 katyonunu oluşruması ve bu katyonun emaye içerisinde ilerlemesi ile açıklanmaktadır. Bu durum literatürde difüzyon teorisi olarak yer almaktadır. Çalışmada incelenen üst kat kaplama kalınlığı, pişirme süresi ve sıcaklığı parametrelerinin tutunma üzerindeki etkisi darbe testi ile belirlenmiş ve tutunmaları kıyaslanmıştır. Test sonucunda, en iyi tutunma performansını 2 üst kat kaplanmış 810 ve 830°C’de 5 dakika ile 5 üst kat kaplanmış 820°C’de 3,5 dakika pişirilen numuneler göstermiştir. Pişirme kapları, fırın tepsileri, sofra eşyaları gibi emaye kaplı yüzeyler, kullanım alanları ve şartları sebebiyle farklı gıda maddeleri ile sürekli etkileşim halinde bulunmaktadır. Gıda maddesinin özelliğine göre emaye yüzeyinden bir takım çözülmeler gerçekleşmektedir. Bu çözülmeler sonucunda etkileşimde bulunulan maddeye metal migrasyonu gerçekleşmektedir. Emayeden metal migrasyon miktarı ve kullanıma uygunluğu gıdalarla temas eden malzemeler için uluslararası regülasyonlar, “1935/2004 EC & EDQM Metals and Alloys Guidelines & 84/500 EC”’da yer alan testler ile belirlenmektedir. 2013 yılına kadar emayelerden metal migrasyonu için sadece kurşun ve kadmiyum geçişlerine bakılırken, 2013 yılından itibaren “iz (trace) element” olarak adlandırılan gümüş (Ag), alüminyum (Al), kobalt (Co), krom (Cr), bakır (Cu), demir (Fe), magnezyum (Mg), mangan (Mn), molibden (Mo), nikel (Ni), kalay (Sn), titanyum (Ti), vanadyum (V), çinko (Zn), arsenik (As), berilyum (Be), kadmiyum (Cd), lityum (Li), kurşun (Pb), civa (Hg), antimon (Sb), talyum (Tl) gibi 21 elementin migrayonu takip edilmektedir. Gıdalarla temas halinde bulunan emaye kaplı malzemelerden gıdalara geçen metal miktarı yemek ortamı simule edilerek belirlenmiştir. 84/500/EEC düzenlemesine göre gıda maddelerini simüle edebilecek en yakın kimyasallar hacimce % 4’lük asetik asit ve % 0,5’lik sitrik asit olarak belirlenmiştir. Emaye gibi camsı yapılarda migrasyon, gıda-emaye arayüzünde gerçekleşmektedir. Emaye kaplı malzemelerin gıda maddeleri ile temas ettikleri yüzey alanı, gıda maddelerinin özellikleri (asitli, yağlı vb.), temas süresi ve ortam sıcaklığı metal migrasyon miktarını etkilemektedir. Buna göre regülasyonlara uygun olarak migrasyon testleri hem asidik hem alkali ortamı simule etmesi açısından %0,5’lik sitrik asit çözeltileri ile yapılmıştır. Testler, 100°C sıcaklıkta gerçekleştirilmiştir. Deney süresi 2 saat olarak belirlenmiştir. Test için emaye-taban malzeme tutunma mukavemetleri en yüksek olan 2 üst kat kaplanmış, 810 ve 830°C’de 5 dakika pişirilen numuneler kullanılmıştır. Etüvden çıkarılan numunelerin muamele edilerek üretilen çözeltiler ICP-MS cihazında analiz edilerek emayeden asit çözeltisine migrasyona uğrayan metal miktarları ölçülmüştür. Yapılan migrasyon testlerinde gıdalarla temas edern malzemeler regülasyonlarında yer alan Li, Co, Mn ve Ni elementlerinin analizleri gerçekleştirilmiştir. Analiz sonuçları, berlirli migrasyon limitleri (SRL) ile karşılaştırılarak numunelerin gıdalarla temasa uygunluğu belirlenmiştir. Sonuç olarak her iki numune için de sadece Mn elementinin migrasyon limit değerinin altında kaldığı; Li, Co ve Ni elementlerinin ise bu limitlerin üzerinde kaldığı görülmüştür
Enamel is defined as a glassy inorganic coating bonded to a metal substrate for one or more coat by thermal fusion at 500-870°C. At these heating tempreratures, adhesion is provided between metallic substrate and enamel. The most important factor affecting the quality of the coating performance is adhesion mechanics. Enamel represents smooth durable coatings with many excellent properties including abrasion, wear and chemical resistance, high hardness and brilliant colours. The properties such as high thermal resistance and thermal shock endurance provide the usage in many industrial area. Enamel coated materials are widely used in cooking appliances sector especially in furnace chasis and baking tray. During the enamelling process, the most important factor affecting the functionality is the achievement of a good adhesion between enamel and metal substrate. Chemical composition of enamel, the type of steel, the roughness of the metal surface, the heating process and the temperature of glazing, the atmosphere of the oven are the factors affecting the interface reactions and so adhesion mechanism between glassy coating and metal substrate. Many of the researches have been investigated the enamelling process on a steel surface from various points of view. Good adherence of enamel to the metallic substrate is ensured with different pre-treatment methods such as pickling using dilute sulfiric acid, high power diode laser surface treatment and electrochemical processing. With the aim of modifying structure and composition of the steel-enamel interface, doping enamels with selected transition metal elements or pre-treatment of the steel surfaces. Most commonly used elements include Ni, Co and Ti. Doping of enamel with Co or pre-treatment of the steel surface with Co was observed to be more effective than procedures with Ni, Ti or taht without any transition elements. Transition metal pre-treatment leads more complex oxidation of the steel surface. There are three basic adhesion mechanisms of the enamel coating which are chemical, mechanical and diffusion theory. It is indicated in the chemical theory that a continuous transition of the type of bond must be achived in the region of the phase boundary from the metallic bond of the base metal via an oxide adherence layer to the ionic bond of the enamel layer. The precondition of good adherence is roughening of the interface surface leading to a tight mechanical clinging of the enamel to the steel surface which is based on the mechanical theory. When the attack on the iron in the microscopic ranges is not uniform, concentrated localized attack occurs and at the end such roughening surface obtains. In addition, there are other theories, which are used to analyze the adherence of the enamel and metallic matrix, such as electrolytic and dentritic theories. Nevertheless, there are not any theory that can adequately describe the adhesion mechanism for all system. The adhesive metal oxides called cobalt and nickel oxides (CoO and NiO) play important role which is complicated and diffucult to explain in the electrolytic theory. According to this theory, during firing process the base metal reduces the metal oxides in the enamel to the metallic state. These adherence promoting metal oxides are formed galvanic cells that strongly corrode the base metal. Other theory promoting the adherence is the dendritic theory which have the dentrites with tooth like apperance and play an essential role in the adherence at the interface. The dentrites are formed in which metal normally crystalizes from the melt, aqueous solutions or any nonmetalic liquid. The compond of the metal to the enamel tends to attach the enamel mechanically to the base metal in the dendritic region after cooling. Shortly, the adhesion between enamel and metal substrate is provided by means of chemical reactions during firing of the enamel. Mechanical interlocking of the steel with enamel was suggested to occur via “anchor points” at the steel–enamel interface and originating from the alloying of diffused iron with Ni, whereas chemical bonding is usually limited by the adherence of metal oxides. At the firing temperature of the enamel, iron is oxidised to FeO and diffused to the glassy phase and formed an oxide layer at the steel-enamel interface. In the interface, due to the fact that these metal oxides are roughened the steel surface the adhesion between steel and enamel is provided. As well as the FeO formation, the adhesive metal oxides such as CoO and NiO achieve an electrochemical reactions with iron and form Fe-Co-Ni metallic structure. These structures called dendrites provide mechanical adhesion. In the literature section of the thesis, the properties of enamel coatings, types of enamelling and enamel coating methods are mentioned. In the scope of the thesis, adhesion mechanism types of enamel coatings on metal substrastes are given in detail. In the experimental studies, 100×100×8 mm sized DC04EK enamelled quality steel plates which obtained from Arçelik Cooking Appliances Factory and enamel powders which supplied from Gizem Frit Company are used in enamelling process. At the beginning of the experiment, after chemical treatment, the compliance of steel plates such as chemical composition, surface roughness and contact angle measure is tested with optical emission spectrometer, optical profilometer and contact angle measurement device respectively. At the end of the characterization of base materials, enamel coating process is proceeded with wet spray coating method. In the studies, the samples are coated as two coat one fire (2C1F) method. The coating thickness of the base coat keeps constant while top coat is covered with 2, 5 and 8 coats according to the experimental design. After coating process, the samples are fired at temperatures 810, 820 and 830°C and times 2, 3,5 ve 5 minutes. Adhesion strength of enamel coating according to the top coat thickness is investigated within the scope of the thesis. For this purpose, top coat enamel suspension including ZnO as a trace element is applied and masss transfer mechanism at the steel-enamel interface is presented with the diffusion of Zn while firing of the enamel coating. In addition, optimization of firing temperature and time of the enamel coating is achieved with observing the chemical composition and microstructure at the interface. In this context, the adhesion degree of the enamel to the steel is identified with impact test while chemical reactions, elemental diffusions and formation of dendritic structure at the enamel steel interface are clarified with SEM-EDS analysis. As a result of the experimental studies, optimum top coat coating thickness, firing temperature and firing time affecting enamel-steel adhesion strength is decided as 2 top coat, 810°C and 5 minutes respectively. With this study it is verified that enamel coating thickness is measured by using optical profilometer without firing. Elcometer is the device measuring the coating thickness with leaving a mark on the surface is used only fired enamel samples. Thus, coating thickness measurement with different methods are indicated in the scope of the study. Moreover, top coat coating thickness is verified with surveying Zn diffusion in SEM analysis. At the enamel firing temperature, iron element is oxidized to FeO while CoO and NiO in the enamel is reduced to metallic Co and Ni. With this electrochemical reaction, Fe-Co-Ni metallic structures are formed at the enamel-steel interface, which provides adhesion of enamel to the steel. Elementel diffusion and dendritic structure is observed with SEM-EDS analysis. Interface area of enamel coating is tested with line scan elemental analysis. Iron concentration from steel to the enamel surface is decreasing while Si concentration increasing in the other direction at the interface. Iron concentration close to the dendritic reigon is higher than the other parts of the enamel. As mentioned in the chemical theory, iron in the base metal is oxidized to Fe+2 cations and diffused to the enamel at the firing temperature of enamel. This phenomena is illustrated diffusion theory in the literature. In this experimental study, parameters influcing adhesion are top coat coating thickness, firing temperature and time are examined with impact test and the adhesion mechanics are compared. According to the impact test result, the best performances of adhesion are the samples having 2 top coat coated and fired at 810 and 830°C firing temperature and 5 minutes firing time. In addition, 5 top coat coated and fired at 820°C and 3,5 minutes samples also shows excellent adhesion performance. Enamel coated surfaces such as cooking utensils, baking trays and tablewares are interacting with different kind of foodstuffs because of the usage area and conditions. According to the speciality of foodstuffs, some dissociations occur from enamel surfaces. In consequence of these dissociations, metal migration is recognised from enamel surface to the solution interacting with enamel. The amount of metal migration and the convenience of utilization is identified with the tests in the international Food Contact Materials Regulations “1935/2004 EC & EDQM Metals and Alloys Guidelines & 84/500 EC”. The quantity of metal migration is identified with simulating the real food enviroment. With regard to the 84/500/EEC regulation, the closest chemicals simulating the real foodstuffs are adjusted as 4% acetic acid and 0,5% citric acid. Since 2013, only lead and cadmium transitions are controlled for enamel coating. However, with new regulations in 2013 21 element migration is inspected, which are silver (Ag), aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), magnesium (Mg),manganese (Mn), molibdenium (Mo), nickel (Ni), tin (Sn), titanium (Ti), vanadium (V), zinc (Zn), arsenic (As), beryllium (Be), cadmium (Cd), lithium (Li), lead (Pb), mercury (Hg), antimony (Sb), thallium (Ta). Migration is taken place at the food-enamel interface for glassy materials such as enamel coatings. The surface area of foodstuffs contacting with the enamel, the speciality of foodstuff (acidic, fatty etc.), contact time and ambient temperature affect the quantity of metal migration. According to the regulations, the tests are performed with 4% v/v acetic acid and 0,5% v/v citric acid at 100°C. The test period is presented as 1 hour for acetic acid solution and 2 hours for citric acid solution. Two samples having the best adherence performance, which is coated two top coat and fired at 810-830°C for 5 minutes, are used for migration experiments.The solutions treating with enamel samples are analysed with ICP-MS device. Lithium, cobalt, manganese and nickel migrations are analysed according to the food contact material regulations. The migration results are compared with the specific release limits and specified the food contact suitability. In conclusion, only the amonut of manganese is under migration limits for both samples.
Enamel is defined as a glassy inorganic coating bonded to a metal substrate for one or more coat by thermal fusion at 500-870°C. At these heating tempreratures, adhesion is provided between metallic substrate and enamel. The most important factor affecting the quality of the coating performance is adhesion mechanics. Enamel represents smooth durable coatings with many excellent properties including abrasion, wear and chemical resistance, high hardness and brilliant colours. The properties such as high thermal resistance and thermal shock endurance provide the usage in many industrial area. Enamel coated materials are widely used in cooking appliances sector especially in furnace chasis and baking tray. During the enamelling process, the most important factor affecting the functionality is the achievement of a good adhesion between enamel and metal substrate. Chemical composition of enamel, the type of steel, the roughness of the metal surface, the heating process and the temperature of glazing, the atmosphere of the oven are the factors affecting the interface reactions and so adhesion mechanism between glassy coating and metal substrate. Many of the researches have been investigated the enamelling process on a steel surface from various points of view. Good adherence of enamel to the metallic substrate is ensured with different pre-treatment methods such as pickling using dilute sulfiric acid, high power diode laser surface treatment and electrochemical processing. With the aim of modifying structure and composition of the steel-enamel interface, doping enamels with selected transition metal elements or pre-treatment of the steel surfaces. Most commonly used elements include Ni, Co and Ti. Doping of enamel with Co or pre-treatment of the steel surface with Co was observed to be more effective than procedures with Ni, Ti or taht without any transition elements. Transition metal pre-treatment leads more complex oxidation of the steel surface. There are three basic adhesion mechanisms of the enamel coating which are chemical, mechanical and diffusion theory. It is indicated in the chemical theory that a continuous transition of the type of bond must be achived in the region of the phase boundary from the metallic bond of the base metal via an oxide adherence layer to the ionic bond of the enamel layer. The precondition of good adherence is roughening of the interface surface leading to a tight mechanical clinging of the enamel to the steel surface which is based on the mechanical theory. When the attack on the iron in the microscopic ranges is not uniform, concentrated localized attack occurs and at the end such roughening surface obtains. In addition, there are other theories, which are used to analyze the adherence of the enamel and metallic matrix, such as electrolytic and dentritic theories. Nevertheless, there are not any theory that can adequately describe the adhesion mechanism for all system. The adhesive metal oxides called cobalt and nickel oxides (CoO and NiO) play important role which is complicated and diffucult to explain in the electrolytic theory. According to this theory, during firing process the base metal reduces the metal oxides in the enamel to the metallic state. These adherence promoting metal oxides are formed galvanic cells that strongly corrode the base metal. Other theory promoting the adherence is the dendritic theory which have the dentrites with tooth like apperance and play an essential role in the adherence at the interface. The dentrites are formed in which metal normally crystalizes from the melt, aqueous solutions or any nonmetalic liquid. The compond of the metal to the enamel tends to attach the enamel mechanically to the base metal in the dendritic region after cooling. Shortly, the adhesion between enamel and metal substrate is provided by means of chemical reactions during firing of the enamel. Mechanical interlocking of the steel with enamel was suggested to occur via “anchor points” at the steel–enamel interface and originating from the alloying of diffused iron with Ni, whereas chemical bonding is usually limited by the adherence of metal oxides. At the firing temperature of the enamel, iron is oxidised to FeO and diffused to the glassy phase and formed an oxide layer at the steel-enamel interface. In the interface, due to the fact that these metal oxides are roughened the steel surface the adhesion between steel and enamel is provided. As well as the FeO formation, the adhesive metal oxides such as CoO and NiO achieve an electrochemical reactions with iron and form Fe-Co-Ni metallic structure. These structures called dendrites provide mechanical adhesion. In the literature section of the thesis, the properties of enamel coatings, types of enamelling and enamel coating methods are mentioned. In the scope of the thesis, adhesion mechanism types of enamel coatings on metal substrastes are given in detail. In the experimental studies, 100×100×8 mm sized DC04EK enamelled quality steel plates which obtained from Arçelik Cooking Appliances Factory and enamel powders which supplied from Gizem Frit Company are used in enamelling process. At the beginning of the experiment, after chemical treatment, the compliance of steel plates such as chemical composition, surface roughness and contact angle measure is tested with optical emission spectrometer, optical profilometer and contact angle measurement device respectively. At the end of the characterization of base materials, enamel coating process is proceeded with wet spray coating method. In the studies, the samples are coated as two coat one fire (2C1F) method. The coating thickness of the base coat keeps constant while top coat is covered with 2, 5 and 8 coats according to the experimental design. After coating process, the samples are fired at temperatures 810, 820 and 830°C and times 2, 3,5 ve 5 minutes. Adhesion strength of enamel coating according to the top coat thickness is investigated within the scope of the thesis. For this purpose, top coat enamel suspension including ZnO as a trace element is applied and masss transfer mechanism at the steel-enamel interface is presented with the diffusion of Zn while firing of the enamel coating. In addition, optimization of firing temperature and time of the enamel coating is achieved with observing the chemical composition and microstructure at the interface. In this context, the adhesion degree of the enamel to the steel is identified with impact test while chemical reactions, elemental diffusions and formation of dendritic structure at the enamel steel interface are clarified with SEM-EDS analysis. As a result of the experimental studies, optimum top coat coating thickness, firing temperature and firing time affecting enamel-steel adhesion strength is decided as 2 top coat, 810°C and 5 minutes respectively. With this study it is verified that enamel coating thickness is measured by using optical profilometer without firing. Elcometer is the device measuring the coating thickness with leaving a mark on the surface is used only fired enamel samples. Thus, coating thickness measurement with different methods are indicated in the scope of the study. Moreover, top coat coating thickness is verified with surveying Zn diffusion in SEM analysis. At the enamel firing temperature, iron element is oxidized to FeO while CoO and NiO in the enamel is reduced to metallic Co and Ni. With this electrochemical reaction, Fe-Co-Ni metallic structures are formed at the enamel-steel interface, which provides adhesion of enamel to the steel. Elementel diffusion and dendritic structure is observed with SEM-EDS analysis. Interface area of enamel coating is tested with line scan elemental analysis. Iron concentration from steel to the enamel surface is decreasing while Si concentration increasing in the other direction at the interface. Iron concentration close to the dendritic reigon is higher than the other parts of the enamel. As mentioned in the chemical theory, iron in the base metal is oxidized to Fe+2 cations and diffused to the enamel at the firing temperature of enamel. This phenomena is illustrated diffusion theory in the literature. In this experimental study, parameters influcing adhesion are top coat coating thickness, firing temperature and time are examined with impact test and the adhesion mechanics are compared. According to the impact test result, the best performances of adhesion are the samples having 2 top coat coated and fired at 810 and 830°C firing temperature and 5 minutes firing time. In addition, 5 top coat coated and fired at 820°C and 3,5 minutes samples also shows excellent adhesion performance. Enamel coated surfaces such as cooking utensils, baking trays and tablewares are interacting with different kind of foodstuffs because of the usage area and conditions. According to the speciality of foodstuffs, some dissociations occur from enamel surfaces. In consequence of these dissociations, metal migration is recognised from enamel surface to the solution interacting with enamel. The amount of metal migration and the convenience of utilization is identified with the tests in the international Food Contact Materials Regulations “1935/2004 EC & EDQM Metals and Alloys Guidelines & 84/500 EC”. The quantity of metal migration is identified with simulating the real food enviroment. With regard to the 84/500/EEC regulation, the closest chemicals simulating the real foodstuffs are adjusted as 4% acetic acid and 0,5% citric acid. Since 2013, only lead and cadmium transitions are controlled for enamel coating. However, with new regulations in 2013 21 element migration is inspected, which are silver (Ag), aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), magnesium (Mg),manganese (Mn), molibdenium (Mo), nickel (Ni), tin (Sn), titanium (Ti), vanadium (V), zinc (Zn), arsenic (As), beryllium (Be), cadmium (Cd), lithium (Li), lead (Pb), mercury (Hg), antimony (Sb), thallium (Ta). Migration is taken place at the food-enamel interface for glassy materials such as enamel coatings. The surface area of foodstuffs contacting with the enamel, the speciality of foodstuff (acidic, fatty etc.), contact time and ambient temperature affect the quantity of metal migration. According to the regulations, the tests are performed with 4% v/v acetic acid and 0,5% v/v citric acid at 100°C. The test period is presented as 1 hour for acetic acid solution and 2 hours for citric acid solution. Two samples having the best adherence performance, which is coated two top coat and fired at 810-830°C for 5 minutes, are used for migration experiments.The solutions treating with enamel samples are analysed with ICP-MS device. Lithium, cobalt, manganese and nickel migrations are analysed according to the food contact material regulations. The migration results are compared with the specific release limits and specified the food contact suitability. In conclusion, only the amonut of manganese is under migration limits for both samples.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2016
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2016
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2016
Anahtar kelimeler
emaye,
kaplamalar,
gıdalarla temas eden malzemeler,
regülasyon,
enamel,
coating,
food contact materials,
regulation