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Üre-formaldehit reçinelerinin üretimi ve karakterizasyonu

Production and characterization of urea-formaldehyde resins

  1. Tez No: 39203
  2. Yazar: AZMİ ATEŞ
  3. Danışmanlar: PROF.DR. VAHDETTİN SEVİNÇ
  4. Tez Türü: Doktora
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1993
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 91

Özet

ÖZET Bu çalışmada, ağaç sanayiinde kullanılan üre-formaldehit reçinelerinin üreti mi ve mamul reçine özellikleri bütün boyutları ile incelenmiştir. Önce, hammadde olarak üre ve bir prekondanse aramamul olan formüreden, katı madde muhtevası % 55, % 65 ve % 66 olan muhtelif üre-formaldehit re çineleri hazırlandı. Bütün deneylerde genelde aynı reaksiyon şartları uygula narak, değişik formaldehit/üre mol oranlı reçineler üretildi ve mamul reçine özellikleri tespit edildi. Ayrıca imal edilen reçinelerin özelliklerindeki değişim ler, reçinenin şerbet formaldehit muhtevasına, depolama pH larına ve depo lama sıcaklıklarına bağlı olarak depolama süresince incelendi. Elde edilen sonuçlara göre, aynı şartlarda üretilen reçinelerde artan formal dehit/üre mol oranı, mamulün depolama ömrünü, serbest formaldehit muhte vasını, reakti vitesini ve viskozitesini artırdığı tespit edildi. Reçinelerde katı madde muhtevasının artışının ise depolama ömrünü azalttığı, viskoziteyi ar tırdığı, serbest formaldehit muhtevasını değiştirmediği ve reakti vitesinde faz la bir etkisi olmadığı gözlendi. Depolama için en uygun pH m pH=8.0-8.5 ol duğu tespit edildi. Ayrıca, üre-formaldehit reçinesine sonradan, mol oranını az ve çok değiştiren miktarlarda formaldehit çözeltisi ilavelerinin, reaktivite ve diğer reçine özelliklerine etkisi tespit edildi. vıı

Özet (Çeviri)

PRODUCTION AND CHARACTERIZATION OF UREA-FORMALDEHYDE RESINS SUMMARY In the modern age of industry, urea-formaldehyde (UF) resins have an important share in the field of thermo-setting amino resin applications. First commercial production of urea-formaldehyde resins had implemented by Pollack in Austria, by Goldschimdt in Germany and by Ellis in the United States, in the years 1920s. In Turkey, the production of urea-formaldehyde resins had started in late 1960s and today the total production capacity reached about 290.000 mt/y. Being cheaper, urea-formaldehyde resin is mainly used in the production of particle board, fiberboard and plywood also in veneering. Other fields of application are paper lamination, textile, leather tanning and etc. In this work, preparation and identification of urea-formaldehyde resin, used in wood industry was examined in detail. Basically UF resin is produced by reacting urea and formaldehyde up to a certain degree of resinification at two stages of the reaction, methylolation and condensation. The methylolation step is in general performed under neutral or mildly alkaline conditions and at temperatures between 25 and 80°C and if the amount of formaldehyde is enough, theoretically tetramethylolurea can be attained. In the second stage of the reaction, in acidic medium at 80-1 00°C methylolurea compounds are condensed by liberating either water or formaldehyde, forming methylene and ether bonds. At the end-use, the reaction is completed as polycondensation and polymerization by means of a catalyst in which ending with a three dimensional solid network, a noncombustible and insoluble final structure is produced. At the first stage of the reaction, there is a nucleophilic attack on to urea mo lecule by formaldehyde: O© O U0 =C© + H2N - C ^ N ? H2N - C - N - CH2OH \h ^H I H monomethylolurea VIII -By the same reason, monomethylolurea (MMU) molecules can react with other formaldehyde molecules and produce dimethylolurea (DMU): O H O H2N- C- N - CH2OH + HCO -? HOCH2- NH- C- N- CH2OH (DMU) In the presence of excess formaldehyde methylolation can proceed further and trimethylolurea (TMU) with a trace amount of tetramethylolurea may be formed. The rates of methylolation reactions decrease as going from monomethylolation to tetramethylolation. In other words, as formaldehyde/urea mole ratio increase the yield of MMU decreases as the yields of DMU and TMU become larger. In the production, higher formaldehyde/urea mole ratio at neutral or mildly alkali conditions is preferable, since DMU formation is lessened where as condensation rate is increased. In advanced methylolation steps, pH becomes more important and additional methylol groups to be introduced, can be placed into the structure merely at high pHs. There is no significant effect of temperature on methylolation reactions. In the stage of condensation, changing the phase to acid, the true polycondensation is made by elimination of water or formaldehyde and the molecular weight increases by forming dimer, polymer or a more complex network. Starting with monomethylolurea the structure attained is as follows: H-N C=0 NH CH2OH CH2- N C=0 NH, CH2OH With the presence of dimethylolurea, the condensation product is formed as; H-N C=0 NH CH2OH N- CH, C=0 NH !_CH2OH N- CH2OH C=0 NH I CH2OH - IXn, shows the polymerization degree, that is suitably adjusted, at the same level of pH and temperature, by the length of the polycondensation stage. It is proposed that advanced heavy molecules are also produced by -N=CH2 groups. In addition to these two main reactions, methylolation and condensation, a number of other reactions are also important in the manufacture and application of amino resins. For example, two methylol groups may combine to produce a dimethylene ether linkage and liberate a molecule of water. The methylene ether so formed is less stable than the diamino-methylene bridge and may rearrange to form a methylene link and liberate a molecule of formaldehyde. pH variations during the reaction is attributed to the oxidation of formaldehyde partly by air and much more by Cannizaro reaction. In the reaction of urea with formaldehyde the important variables are formaldehyde/urea mole ratio, pH, temperature, reaction time and the concentration of the reactants and the stages of the reaction are controlled by the first four of them. In general, in the preparation of urea-formaldehyde resins, conditions for the first part of the reaction are selected to favor the formation of methylol compounds. In the ensuing stage of the reaction, these methylol derivatives are condensed at acidic pH with evolution of water or formaldehyde and the reaction is stopped to give a stable syrup. At the end-use of this syrup, further condensation, by similar reactions leads to the final stage, polymerization, where a three dimensional network is formed by high molecular condensation products. In this work, starting with urea and formurea (a precondensate mixture of urea and formaldehyde at a mole ratio of F/U = 5.0/1.0) various urea- formaldehyde resins were prepared at solid contents selected as 55%, 65% and 66% with formaldehyde/urea mole ratios differing between 1.10/1.0 and 1.75/1.0, using the same reaction conditions and procedure. A special UF resin with a low F/U mole ratio was also prepared by gradual addition of formaldehyde solution and urea, to the reaction mixture. Each sample obtained, was stored and its resin properties were observed and recorded during its storage life. In the second part of the experiments some properties of the resins which prepared beforehand, have been changed externally and the effects of those alterations were observed. First, enough amount of resin was produced in apilot plant, from which samples having storage pH s between 1 and 14 were prepared. In the end of this experiment, it was found that samples having pH 1-4 and 12-14 were out of use immediately. Each other sample, having a storage life less or more, showed a pH change where acidic ones increased up to about pH=7,5 where as alkali ones decreased down to about pH=8.0, in the course of their storage life. In a similar way, resin samples having different free formaldehyde contents were prepared by external addition of formaldehyde solution in specified quantities and the changes in resin properties of the samples were examined. In the context of the results obtained, increasing formaldehyde/urea mole ratio of a urea-formaldehyde resin, which means more methylol groups in the resin, resulted in viscosity increase owing to extended intramolecular actions. On the other hand, increasing formaldehyde/urea mole ratio, caused to increase free formaldehyde content of the resin as expected. It was found that, because of active methylol groups, the reactivity and the storage life of the resin increased with higher formaldehyde/urea mole ratio. Increasing the solid content of a UF resin, caused the viscosity to increase and the storage life to decrease, due to increase of the concentration of reactants, without a substantial change in free formaldehyde content and the reactivity of the resin. Besides, the viscosity increased because of increasing degree of condensation either by the proceeding reaction in the coarse of storage life or by prolonged reaction time. It was determined that, during the storage life, the jell-time of the resin became longer with respect to the initial value in the period of time. This is more obvious at low formaldehyde/urea mole ratios. The reason of this phenomenon is increasing polymerization degree and diminishing methylol groups during the storage. It was observed that, higher storage temperature resulted in lower storage- life, owing to increasing reaction rates. As a result of our experiments, it was determined that ideal storage pH for urea-formaldehyde resins is in the range of 8.0-8.5. The descending pH trend of resins having alkali storage pH, can be explained by Cannizaro reaction and the ascending pH trend of resins of acidic storage pH, can be ascribed to the hydrolysis of urea in the resin. XIAddition of formaldehyde solution to urea-formaldehyd resins caused to increase the reactivity owing to increased free formaldehyde content. In the course of time, the partial increase in solid content and density of the resin showed that additional formaldehyde partially joined up with the polymeric structure. On the other hand, although methylol groups increased, the decrease in free formaldehyde content which was seen by the analysis, explains the decrease of the reactivity. This shows that free formaldehyde content of a resin is more effective on jell-time. XII -

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