ESTIMATION OF UHMWPE CRYSTALLINITY DEGREE VARIATION. 1. Differential Scanning Calorimetry (DSC) in tests on polymers

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1 Journal of Applied Matheatics and Coputational Mechanics 2014, 13(4), ETIMATION OF UHMWPE CYTALLINITY DEGEE VAIATION Michał obociński Institute of Mechanical Technology, Czestochowa University of Technology Częstochowa, Poland Abstract. The paper presents theral analysis as far as tests on polyers are concerned. The obtained results, after being atheatically elaborated, differentiate theral processes and define noral heat. It also consists of atheatical elaboration of the tests as well as an explanation of the graphical results. The paper also presents the epirical results of the tests conducted on polyers crystallinity variation subitted to one of the factors influencing the change of the aterial paraeters. Keywords: theral analysis, degree of crystallinity 1. Differential canning Calorietry (DC) in tests on polyers Differential canning Calorietry is one of the basic ethods of theral analysis widely used in tests on polyers. The DC ethod is used in tests on: Polyers phase processes Physical processes: glass transition T g, softening point T Mixtures ingredients and copolyers Theral transition T d (teperature of destruction) of polyers, oxidation and cobustion Heat: cristalisation, polierisation, dissolution, absorption and desorption Melting enthalpy and crystallinity degree The purpose of Differential canning Calorietry (DC) is to define the change of heat streas inducted into the saple and to the reference saple occurring under the influence of the given teperature to the saples, and an attept to estiate the iportance of the teperature [1, 2]: where: - saple heat strea, - reference saple heat strea. = (1)

2 118 M. obociński The results of DC easureents is the curve line DC - the dependence of the easured heat streas on the tie/teperature. The ost coon teperature regie is the change of it with a constant speed T β = const (2) t foreseen in advance, independent on the saple paraeters change, coonly called an average speer of heating. An exaple of such dependence is shown in Figure 1. Fig. 1. The curve line DC and exaple of physical processes occurring while heating the saple [2] The crystallinity phase in polyer is defined as crystallinity degree. Crystal polyers have particular teperature of decoposition T t. Heating the polyer causes the decrease of crystallinity degree, as the crystal phase changes into the aorphic one. The elted polyer has only the aorphic phase. Polyers crystality degree can be defined as fractional consistence of crystality phase in the polyer saple. The above definition is based on a so-called double phase odel. In fact, it applies ostly to hoopolyers. But it is also possible to use it for hoopolyer ixtures and copolyers. Cheical reactions and processes where the heat is used or eitted are visible on the curie line DC as a reflex up or down fro the zero line. Those going down stand for egzotheral processes (i.e. cristalisation), and those going up stand for endotheral ones (i.e. elting). Defining crystallinity degree by DC ethod is one of the ost coonly used epirical ethods and is arked as ω c,h. The curve line obtained in the DC test is presented as dependence:

3 Estiation of UHMWPE crystallinity degree variation 119 dq dt = cp (3) where: - ass of the saple, c p - specific heat with constant pressure (theral capacity for ass unit), dq - heat provided or eitted fro the syste in tie unit, dt - heating or cooling speed. pecific heat c p is one of the basic theral paraeters of each aterial. Precisely, c p can be arked as differential function: c 1 dq dt ( T) = (4) While easuring, when the heating speed is constant (β = const), it is assued that the heat streas to the saple and fro the reference saple are settled and of the values: = C = C dt C dt C where: C, C - theral capacity of the saple/reference saple. In order to properly conduct the analysis it is iportant to obtain the so-called correction signal of geoetrical asyetry of calorieter by easuring the blanks. (5) N, = = 0 N, = const 0 ( T) = ( T) ( T) = c ( T)β (6) Finally, the specific heat is defined as [3]: c ( T) = ( T) ± c ( T) N, N (7)

4 120 M. obociński When using the DC ethod it is advised to apply the following cristality degree definition: h where: h t - elting teperature of the saple, h t,c - elting teperature of the saple totally cristal. t ω c, h = (8) ht, c To define crystallinity degree the Differential canning Calorietry Method DC was used. The calorieters work basing on easures of the heat, used to aintain the tested saple and the cheically neutral pattern of the sae teperature. Usually the heating or cooling is applied with constant speed and range. The area between a curve line DC and the zero line is proportional to the change of enthalpy of the tested saple. In the ideal exaple, the zero line should be parallel to the tie axis (Q = 0). Practically, it is usually turned up or down due to the specific heat changes (or ass) of the saple. 2. Testing of the polyethylene structure with the Differential canning Calorietry The UHMWPE polyethylene used for edical purposes has a coplex structure and consists of aorphic and crystal phase. These two phases define echanical features of polyethylene, and any changes ay prove any destructive processes. In endoprostheses cups the change of cristality degree of polyethylene can be caused by: Friction and long tie stresses tg radiation Any changes caused by long-ter usage processes [3]. Due to the above reasons the following work presents the analysis of the change of crystallinity degree of the saples under the influence of one of the factors [4]. Polyethylene saples in the shapes of cuboid 10 x 10 x 20, were subitted to tg radiation. The saples of tg radiated UHMWPE were tested on the change of crystallinity degree. To do that there was used Differential canning Calorietry Method DC [6]. Figure 2 shows the easureents results for UHMWPE saple radiated with 90 cgy, while Figure 3 for the saple radiated with 150 cgy. Figure 4 additionally shows the results of DC tests for the saple of Chirulen 1020 not subitted to ionic radiation. Crystallinity degree for particular saples was: aple treated with 90 cgy %, aple treated with 150 cgy %, aple not treated at all %.

5 Estiation of UHMWPE crystallinity degree variation 121 Fig. 2. DC results for the saple treated with 90 cgy Fig. 3. DC results for the saple treated with 150 cgy Fig. 4. DC results for the saple treated with no radiation

6 122 M. obociński The difference between the results of crystallinity degree is presented in Figure 5. Basing on the above results it can be stated that the difference between crystallinity degree of the saple treated with 90 cgy (the lowest dosage) and the saple treated with 150 cgy (the highest dosage) is ca. 5%. Very siilar difference of crystallinity degree (5.4%) was noticed between not treated saple and the saple treated with 90 cgy. Fig. 5. Crystallinity degree of tested saples eferences [1] Pielichowski K., Flejtuch K., Zastosowanie odulowanej różnicowej kaloryetrii skaningowej (MDC) w badaniach właściwości polierów, Poliery 2002, 47, [2] Balcerowiak W., óżnicowa kaloryetria skaningowa i terograwietria - aspekty teoretyczne i praktyczne, V zkoła analizy tericznej, Zakopane [3] Gierzyńska-Dolna M., Biotribologia, Wydawnictwo Politechniki Częstochowskiej, Częstochowa [4] Projekt badawczy nr 4TO8A04422, Teoretyczno-doświadczalna analiza obciążeń układu ruchowego endoprotez stawu biodrowego ze szczególny uwzględnienie badań własności tribologicznych ateriałów, [5] zuera M., Charakterystyka wybranych etod tericznych, LAB 2013, 1, 18. [6]