Temperature and Humidity Effect on Creep Behavior of Polybutylene Succinate

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1 Temperature and Humidity Effect on Creep Behavior of Polybutylene Succinate Satoshi SOMIYA and Tatunori SAKAI Department of Mechanical Engineering of Keio University, Hiyoshi Kohhoku-ku Yokohama, Japan Abstract Polybutylene Succinate (PBS) is a typical biodegradable polymer and is hoped to use as an engineering-polymer material to protect the environment on earth in stead of synthetic high polymer. As an engineering material, this polymer has wanted to have high durability in sever air and water condition. The purpose of this research is to make clear the mechanical properties and the visco-elastic behavior in high temperature condition and several humidity conditions. The bending and tensile properties of PBS at some temperature and humidity conditions were researched. Especially, the applicability on Time-temperature superposition principle and Humidity-creep time superposition principle into creep deformation were discussed to estimate the long term durability. The test temperature conditions were selected from 0 C to 70 C and test humidity conditions were selected from 0 to 90% on the relative humidity. To compare the effect of absolute immersed water, the specimen which was completely immersed in pure water was prepared. Finally, for this material, it was found that the ability of applications of these two principles was recognized and the relationship between humidity and temperature was recognized. 1. Introduction To prevent the progress the failure of the weather and the circumstance on earth, the development of biodegradable materials has important role for it objection. There are two kinds of biodegradable polymers. One is the natural base products and the other is synthetic materials made from oil products. Usually, the former is low strength material and the latter is high strength material. Some biodegradable polymer products were developed and introduced into the engineering field, but this polymer is using in the field such as agriculture and household goods because the mechanical properties of this material were lower than them of some engineering plastics used in parts of machine. It is well known that they usually have lower mechanical and chemical properties than usual engineering polymers. Polybutylene Succinate (PBS) is a synthetic biodegradable material. But as a mechanical engineering material, there were very few report, especially there were few data for very hard circumstance such as high temperature and very high humidity especially visco-elastic behavior. In this study, the tensile creep phenomenon of PBS polymer was researched. Especially the effect of temperature and humidity on creep behavior was focused to make clear the long term usefulness of PBS as a material for machines. 2. Material and Test instrument Used Polybutylene Succinate was a typical biodegradable material and the commercial name was BIONOLLE #1020, which was made by SHOWA HIGHPOLYMER Co. Ltd. In Japan, this is a typical synthetic biodegradable resin and is a practical engineering material. The material was made by injection molding method and the shape of specimen for tensile test and creep test was strip types, which were 10mm in width, 150mm in length and 10mm in thickness. They were kept in standard temperature and moisture condition such as RH 50% and 20 C after molding. Before creep test, the specimen was kept in a box holding the absolute dried air for days to avoid the effect of water immersion. After that, the specimens were prepared by water absorption treatment in some relative humidity condition at 0%, 60%, 75% and 90% and also immersed in pure water for 500hours. The static mechanical properties were measured by Instron-type tensile test machine (Max loading: 500N). The tensile creep test has been done by the creep test machine in hot-humidity oven, which can load until 50N (Max loading) and can expose them in the temperature from 0 C to 80 C and humidity from 0% to 90%. To accelerate

2 creep behavior, changed test temperatures were 0 C, 40 C, 50 C, 60 C and 70 C. In creep test, the short term test method has been used to prevent the effect of physical aging. Applied load was 10 percents of tensile strength. Static mechanical properties were measured by the specimen, which contained the saturated water amount on each humidity condition.. MECHANICAL ENJINERRING PROPERTIES IN HUMID CONDITION.1 Preparation method of material On some engineering plastics and green composites, the effect of absorbed water in materials on the static mechanical properties and the long term deformation have been researched and reported. On the other hand, for biodegradable polymers, there was not so much report. For the case of used PBS, the saturated water absorption was 1.25% when material immersed in water. To discuss the effect of water absorption on mechanical properties, the specimens was immersed or exposed in pure water and in several relative humid air until the amount of water reached to saturation. The relative humidity of treated conditions was 0, 60, 75 and 90%. Fig. 1 shows the process on water absorption in humid and water at 50 C. The absorption speed increased by the relative humidity. When it was over about 15 hours, all curves reached to the constant values. The saturated values were raised by increasing humidity. And also, the water absorption curve in pure water was drawn in the same graph. This curve shows high speed absorption than in humid conditions. The maximum saturated water gain of this material was about 1.25%, which was higher than that of other engineering plastics Water 0%RH Immersion time (hour 1/2 ) Fig. 1 Weight gain behavior by relative humid and water condition at 50ºC of PBS resin.2 Static mechanical properties in moisture condition The effect of humidity on mechanical properties of biodegradable plastic and its composites have been researched and reported [1-].The influence of water storage in biodegradable polymer on mechanical properties has been researched. They were affected by water absorption. Fig. 2 shows the bending strength dependency of amount of water gain. From these results, bending strength of water absorbed materials was decreased on a straight line according to the increase of the weight gain. Fig. shows the bending modulus of immersed in humid condition. And the value of modulus was decreased by weight gain. The amount of decrease of modulus reached to about 20 % between absolute dry and absolute water-gain. Furthermore, the dependency of tensile strength and tensile modulus of PBS on water gain were researched too. Fig. 4 shows the tensile strength and modulus decreased by the water absorption increasing as same as bending results. From these results, Mechanical properties of PBS were strongly depended on water absorption. Creep behavior is one important parameter for durability on an engineering material for high temperature and very long term condition. In this research, the purposes of this study are to understand the creep behavior, and to make the effect of temperature clear on creep behavior of Polybutylene Succinate (PBS). The application of Time-Temperature super position principle for creep deformation was discussed. Because the glassy temperature of this resin was -0 C, mechanical behavior at the standard room temperature such as 20 C might be rubbery. But it was found that this material shows clearly visco-elastic behavior at test temperature range from 0 C to 70 C.

3 Bending strength (MPa) Dry 0%RH Bending modulus (MPa) Dry 0%RH Weight gain (%) Fig. 2 Wate absorption dependency of bending strength in humidity condition at 50 ºC Tensile strength (MPa) Weight gain (%) Fig. Relationship between weight gain and bending modulus in humidity condition at 50 ºC Tensile strength Tensile modulus Tensile modulus (MPa) Relative humidity (%RH) Fig. 4 Tensile strength and modulus in different relative humidity at 50ºC 4. Temperature dependency of Creep behavior on PBS The creep phenomena on biodegradable polymers have researched and reported [4-5]. But there were few reports for the discussion on the time- temperature super position principle. To prevent the effect of the water gain in specimen it was kept in absolute dry. Creep deformation was accreted. The shape of creep deformation curves (t) usually was divided into three step such as Step one, Step two and Step three. According to rise the temperature, the creep strain grows sharply and quickly. To evaluate creep deformation on Step one and Step two, creep compliance value Dc (t, T) was calculated with following equation. ( t, T ) Dc( t, T ) (1) ( t) where t and T are real Time and Temperature relatively. (t) and (t) means creep strain and applied stress at real time t. In this creep test, (t) was constant value 0, that was 10% of static strength. The change of

4 creep compliance was drawn in Fig.5. In this figure, the horizontal axis shows the creep compliance value and the vertical axis shows the real creep time. In Fig. 5, the creep compliance curves obtained at each temperature condition were plotted. The values Dc(t,T) were increased by progressing creep time and rising temperature. These shapes of curves do not the same pattern but all curves were moved to not only horizontal direction but also the vertical direction, it was found that these curve made an almost one curve. This curve was sometime was called as master curve of creep compliance. To discuss the relationship between time and temperature on creep deformation, not only the drawing one master curve of creep compliance was tried by moving these creep compliance to the referent curve but also the relationship between the shift-factor and temperature was discussed. The reference temperature was 0 C to decide the position of moving curves. Fig. 6 shows the master curve for creep compliance of used PBS material from 0 C to 70 C. On this figure, the vertical axis shows the compliance value and the horizontal axis shows physical time on logarithmic scale. The curve shows very smooth line and the creep compliance value was increased by the progress of physical time ºC 60ºC 50ºC 40ºC 0ºC Real time log t (min) Fig. 5 Creep Compliance curves of PBS resin in different temperature condition Physical time Log t' (min) Fig. 6 Master curve of creep compliance on PBS resin by short term test Log a To /T (10 - /K) 1.5 Fig. 7 Time-Temperature shift factor for PBS resin for short term test 1

5 The range of experimental creep time was almost figures on logarithmic scale on real time, but the width of the master curve on time scale expanded into 11 figures on logarithmic scale for the case on physical time. From these results, it was assumed that the long term deformation was able to estimate from the data obtained by the short span test. This shows the creep behavior on each temperature was a part of continuous deformation process. If there was the linearity between time and temperature, creep deformation on several test temperature was able to estimate. Fig. 7 shows the relationship between shift factor and temperature. The value of shifting on time axis was plotted on an Arrhenius type figure. It means the horizontal axis shows the vertical value of test temperature. Because all plotted data made a straight line on it, the applicability of Time-Temperature superposition principle was confirmed. It means using this mater curve of creep compliance and the shift factor line on Arrhenius type figure, the process of creep deformation at arbitral time and temperature is able to calculate. 5. Effect of Humidity on Creep behavior on PBS It was well known that humidity affects the visco-elastic deformation for plastics [5-6]. For discussion of very long term creep behavior, it is very useful method that test temperature changed widely as shown in previous section, but we could not change the temperature condition for creep test keeping the same amount of water in material. To keep the amount of water in polymer during creep test, test temperature could not change because not only relative humidity but also the absorbed water in resin might change by the temperature condition. The calculated creep compliance values were plotted to real time. Fig. 8 shows the creep compliance curve D c (t) to creep time. The phenomenon for creep progress was looks like the behavior caused by the change of temperature. Rising humidity, creep compliance values were increased as the same as the change of temperature. Humidity action to visco-elastic behavior look like change of temperature. Next, the effect of humidity on creep behavior have been discussed as same as the temperature. A master curve of creep compliance for humidity was discussed and the relationship between shift factor and humidity was researched. The creep curve for the dried specimen at RH 0% was selected as a reference curve for shifting and the other curves were moved to horizontal direction until over-lapping on each other as same as the creep behavior at high temperature. Finally, it was obtained the master curve of creep compliance as shown in Fig. 9. This figure shows the curve was very smooth as same as the master curve for the case of changing temperature. Furthermore, the amount of shift factor for humidity was plotted to humidity as shown in Fig. 10. In this figure, the vertical axis shows the shift factor and the horizontal axis shows the humidity. All shift factors made a smooth curve and the value decrease by the humidity. Compare the amount of shift factor, effect of humidity was smaller than the temperature. When temperature changed from 0 C to 70 C, the change of shift factor was 8 figures on time axis but when humidity changed from absolute dry to perfect water immersed condition, the amount of shift factor on creep deformation was 1.8 figures on logarithmic scale Water 0%RH Dry Real time log t (min) Fig. 8 Creep Compliance curves of PBS resin at 50ºC by each condition (C=57%)

6 Log a MA Log t' (min) Fig. 9 Master curve of PBS resin at 50ºC by different humidity (C=57%) Weight gain (%) Fig. 10 Time-Moisture Absorption shift factor for PBS resin (C=57%) 6. Conclusions The water absorption phenomenon on PBS was researched and the dependency of mechanical properties on the amount of water has been researched. The tensile creep behavior of Polybutylene Succinate (PBS) has been researched at high temperature condition and wide humid conditions. It was found that the creep behavior of PBS shows an Arrhenius type visco-elastic behavior. And also the applicability of Time-Temperature superposition principle and the applicability of Humidity-Time superposition principle were confirmed. The equality effect of temperature and humidity on creep phenomena was recognized. Acknowledgment We would like to thanks for Grant-in-Aid for Scientific Research ( C ) in Japan. The number of this research is Reference [1] HO K-L G., Pometto A.L., and Hinz P. N., Effects of temperature and relative Humidity on Polylactic Acid Plastics degradation Journal Environ. Polym. Degrade. Vol. 7 No.2 p (1999) [2] E. A. R. Duek, C.A.C. zavaglia, and W. D. Belangero, In vitro study of Poly(lactic) pin degradation Polymer. Vol. 40. No. 2 p (1999) [] Riggs P.D., Braden M., Tilbrok D.A., Swai H., Clarke R.L., Pater M.P., The water uptake of poly(tetrahydrofurfuryl methacrylate)., Biomaterials Vol. 20 No.5 p , 1999 [4] Biswas K.K. Somiya., Creep Behavior of Metal Fiber-PPE Composite and Effect of Test Surrounding Mechanics of Time-Dependent Materials,. 1 P [5] Martucci J.F., Influence of the fiber content and the processing conditions on the flexural creep behavior of Sisal-PCL-Starch Composite J. Thermoplastics Composite Materials Vol. 5 No. P25-265,2002 [6]Zhou S-M, Tashiro k., Confirmation of time-humidity Superposition Prince pale for Various Water-Absorbable Polymers through Dynamic Visco-elastic measurement under Controlled Conditions of relative Humidity and Temperature. J. polymer Science Part B Vol9 No [7] Sen A., Bhacharya M. Stelson K. A. and Voller V.R., Creep in Injection molded Starch/Synthetic Polymer Blends. Material Science Engineering A. P