Using of dielectric spectroscopy method for diagnostics of cross-linking reaction course.
Biopolymers prove as very perspective and sought-after materials, which are due to their positive properties practically ideal for environment. Recently, the applications of natural materials markedly enlarge. Collagen hydrolyzate is natural polymer which is secondary product of tanning waste processing (Dalev et al., 2000). This biological material is prepared from insoluble collagen and subjected to controlled enzymatic hydrolyse. During the hydrolyse polymers and monomers are ruptured into polypeptides thereby the collagen solubility is increased. Collagen hydrolyzate has a positive property with respect to environment: it is easily biodegradable (Taylor et al., 1998). That is the property which is very demanded at the present conditions. Knowledge of reaction kinetics and chemical processes is necessary for automatic control of continuous production of various polymer or composite materials. There is another interesting link between biodegradation of cross-linked collagen under anaerobic conditions and the cross-linking degree (Kupec et al., 2003). The first application of dielectric spectroscopy was publish by Kiele and Race in 1934. They described usage of this method for study polyesterification reactions. Many research papers has been written since 1958, most of them dealt with epoxy resins, less about polyesters, polyamides and other resins. Progress in this field was mainly caused by industry needs especially aeronautics. Next but not less important fact was improvement in measurement devices. A method based on dielectric spectroscopy has been used for monitoring the reaction, but there have been problems due to the high conductivity of the collagen hydrolyzate solutions (Hedvig, 1977). Moreover, many cross-linking type reactions that occur are accompanied by a change in colour so it is convenient to monitor the reaction of glutaraldehyde by an optical method.
Dielectric spectroscopy method has been evaluated for its potential at monitoring the degree of cross-linking that can occur. Dissipation factor have been chosen because it is neither dependent on distance between electrodes nor its surface. It depends on angular frequency which remains constant, on permittivity and conductivity of material. Change of permittivity is caused by falling quantity of dipoles. Cross-linking reaction also has an impact on conductivity. Steel electrodes have been used for practical realization. To prevent their pollution by tested sample have been covered by aluminum foil on the side of contact with a sample. This solution also helps with weighing of sample because sample placed on the foil could have been weighing only with it not with much more heavier steel electrode which was overweight for our balance. A piece of plastics with "U" shape have been put between electrodes like a spacer. Its purpose was to avoid short-circuit.
2.1 Chemicals and Materials
Epoxy resins--5 min binary epoxy cement, produced by Lachema a.s., Czech Republic
Collagen hydrolyzate, 55 % aqueous solution, produced by Tanex, Jaromer, Czech Republic
Glutaraldehyde, 25 % aqueous solution, produced by Merck, Schuchardt, Germany
2.2 Used instruments
LCR Meter 4284A (20 Hz to 1 MHz)--Hewlett Packard Universal oven UNP 200--Memmert
2.3 Used software
Agilent VEEpro 8.0
User program was created because writing of data from LRC meter manually is impossible if the data has to be written for example every second. Automated acquisition system is needed. VEE Pro 8.0 was chosen for this task. The graphically oriented language is very efficient for these types of jobs. Created application was designed for measuring of dissipation factor at three frequencies. These frequencies can be changed by user in specific text fields. Used LCR meter can be set for frequency range from 20 Hz to 1 MHz.
2.4 Scheme of measurement
The main used measuring device is LCR meter HP4284A which is connected with personal computer via GPIB interface. The process of measuring is controlled by program written in Agilent VEE Pro 8.0.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
2.5 Dissipation factor measurement
Dissipation factor have been chosen because is not dependent on distance between elec-trodes and its surface. It depends on angular frequency which remains constant, on permit-tivity and conductivity of material. Change of permittivity is caused by falling quantity of dipoles. Cross-linking reaction also has an impact on conductivity. Steel electrodes shown in Fig. 1 have been used for practical realization. This solution also helps with weighing of sample because sample placed on the foil could have been weighing only with it not with much more heavier steel electrode which was overweight for our balance. A piece of plastics with "U" shape have been put between electrodes like a spacer. Its purpose was to avoid short-circuit.
2.6 Cross-linking reaction
Cross-linking reactions of epoxy resins are quite well described so this is the reason why they have been chosen to verify used method. Measuring was done at three frequencies 250Hz, 500Hz and 1000Hz which have been changing in the loop during the measuring. It was repeated several times. This measurement was repeated at different temperatures (at 22[degrees]C, 30[degrees]C, 40[degrees]C and 50[degrees]C) to probe influence of temperature. Obtained data has been processed in Matlab. Because we were not interested in absolute value of dissipation factor, only in its maximum value in time, simple modification has been done. All measured data has been transformed into 0 to 1 range. So the dependence of dissipation factor on time could be easily compared. This modification is done via this equation:
[D.sub.o] - min([D.sub.o])/max ([D.sub.o] - min([D.sub.o])) (1)
where, [D.sub.o] is set of raw data. The main scope of this research was collgen hydrolyzate. Solution from powdered hykol and distilled water was prepared (spare 56% aqueous solution of collagen). 0.5g of the sample was placed on aluminum foil, put on electrode and a certain amount of the cross-linking agent (25% aqueous solution of glutaraldehyde) added from micropipette. Then the sample was mixed by plastic stick and second electrode was put on.
[FIGURE 3 OMITTED]
The program for real-time measuring of dissipation factor has been created in Agilent VEE Pro. This program allows measuring dissipation factor at three frequencies which are changing in the loop during the measurement. From obtained results is obvious, that the gel was created gradually during the crosslinking process. Velocity and quallity of the crosslinking reaction depended on amount of agent and it was inversely proportional. A phase transition exists between 4% and 5% of added amount of glutaraldehyde. Behaviour of system was very stochastic in this interval. An influence of the water content will be contents our next examination. From automatic control point of view, it is very useful to explore and determine the relationship between amount of cross-linker and cross-linking degree. Knowledge of reaction kinetics and chemical processes may allow for designing an automatic control method. Suggestions for further research in this field can be applying mentioned method during treatment of recyclable composites and bio-composites, trying other cross-linking systems, for example, glyoxal, acetaldehyde or enzymatic cross-linking such as trans-glutaminase.
This work was accomplished with financial support by Research Projects of the Ministry of Youth, Education and Sports of the Czech Republic: MSM 7088352102.
Dalev P. G., Patil R. D. et al. (2000). Biodegradation of Chemically Modified Gelatin Films in Soil. JAPS, Vol. 78, pp 1341-1347
Hedvig P. (1977). Dielectric spectroscopy of Polymers. Akademiai Kiado, Budapest
Kupec J., Charvatova K., Navratil M., Kresalek V., Kresalkova M. (2003). Effect of Cross-Linking Waste Protein with Dialdehydes on Its Biodegradation under Anaerobic Conditions. Journal of Polymers and the Environment, 11, 3, pp 93-100, ISSN 1566-2543
Taylor M. M., Cabeza L. F., Kolomaznik K. et al. (1998). Functional Properties of Hydrolysis Products from Collagen. Alca, Vol. 93, pp 40-50
Dalev P. G., Patil R. D., Mark J. E., Vassileva E. and Fakirov S. (2000). Biodegradation of chemically modified gelatin films in solid. J. Appl. Polym. Sci. 78, pp 1341-1347
Dusek K. (Ed.) (1987). Epoxy Resins and Composites IV., Proceedings Polymer science, Akademie Verlag, Berlin
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|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2009|
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