| The most of the plastic products that have been widely using are non-degradable or partially degradable. After use, these plastic items, particularly the single use plastic products, were disposed as waste and went into environment. Because of the difficulty in degradation, the waste plastics have give negative impact on natural ecological environment. Moreover, the raw materials of these traditional plastics mainly come from oil fuel which is facing depletion. Therefore, development of biodegradable plastics from renewable resources is undoubtedly important, and is one of the effective ways to solve the problem of "white pollution" and the "oil crisis".Natural polymer starch is one of the most abundant and inexpensive bio-materials having the characteristics of renewability and bio-degradability, which make it to be one of the most competitive raw materials for producing biodegradable materials. At present, thermoplastic starch (TPS) as one of the most potential and very promising biodegradable starch plastics has become a very active research field in the world. However, each of the glucose rings in starch molecules has three hydrophilic hydroxyl groups ( at the C2, C3 and C6 location) which are responsible for the hydrophilicity of starch materials, especially when the granule starch was converted into thermoplastic starch as the hydrogen bondings in the granule starch is destroyed and plasticizers exsit. So, the hydrophilicity of thermoplastic starch is substantially increased. It is the poor water resistance of thermoplastic starch, i.e., material swell and mechanical properties change with environmental humidity, seriously limits its widespread applications. In order to improve the water-resistant performance of thermoplastic starch and reduce the sensitivity of its mechanical properties to the environment humidity, it is necessary to modify these hydrophilic hydroxyl groups of starch molecules.In this work, the surface of the TPS was modified by surface photocrosslinking and a hydrophobic crosslinked layer on the surface was formed. The purpose of this work is to improve the water resistance of TPS without significantly changing its mechanical properties and performance of its products. Photosensitizer concentration, UV radiantion energy, plasticizers and other factors affecting crosslinking extent and water resistance of TPS materials were investigated. The crosslinking extent and effect of relative humidity on mechanical properties of TPS were analyzed. The surface morphologies of TPS sheets before and after surface modification were observed. It is expected that this work could provide a scientific basis for developing a technique method to modify TPS surface through photocrosslinking.The surface area normalized swelling degree and crosslinking mass were used to characterize the surface crosslinking degree respectively. In the studied two systems, starch/water and starch/glycerol, radiation energy and photosensitizer concentration solution have the same influence on the crosslinking degree basically. With UV radiation energy increasing, the surface area normalized swelling degree of the tested sample decreased sharply when the radiation energy less than 20J/cm2, while the surface area normalized crosslinking mass increased sharply. When UV radiation energy varied form 20J/cm2 to 40J/cm2, the changes of the surface area normalized swelling degree and crosslinking mass slowed down. No obvious changes in surface area normalized swelling degree and crosslinking mass were observed when further increaing radiation energy. For the starch/water system, the concentrations of photosensitizer solution varied form 0.1% to 2.0%, while for the starch/glycerol system, the concentrations of photosensitizer solution varied form 0.5% to 1.0%. It was found that the concentration of 0.75% of photosensitizer solution gave rise to the highest crosslink extent in the investigated range. There is no a simple proportional relationship between crosslinking extent and soaking time. Compared to the 10 sec and 90 sec soaking, 30 sec soaking gave rise to the hightest crosslinking extent. For the crosslinking reaction under lower UV radiation energy (radiation energy is less than 20J/cm2), water content in samples, varied form 2% to 20%, has no marked effect on the crosslinking extent. When the water content in samples is higher than 20%, the surface area normalized swelling degree of the test sample increased sharply and the surface area normalized crosslinking mass decreased rapidly with the increase of water content,. However, for the crosslinking reaction under higher UV radiation energy (radiation energy is more than 20J/cm2), the crosslinking extent was not affected by water content in samples. Under a same surface photocrosslinking modification conditions, the crosslinking extent in the starch/glycerol system is slightly better than that in the starch/water system. Compared to other factors, UV radiation energy is the primary important to the crosslinking extent, followed by the concentration of photosensitizer solution, soaking time and type of plasticizer.The relationship between the crosslinking extent and moisture (water) absorption of TPS, as well as the water contact angle and boil-resistance, were studied. For a given plasticizer, with the increase of the crosslinking extent, moisture (water) absorption and humidity sensitivity of TPS decreased gradually, and the time keeping its integrity in hot water (90°C) was prolonged. Compared to the starch/water system, the starch/glycerol system has higher moisture absorption and humidity sensitivity and shorter keeping its integrity in hot water (90°C) . After surface photocrosslinking modification, the surface water contact angles of both starch/water system and starch/glycerol system were obviously larger than those of the control ones. Thess results showed that after surface photocrosslinking modification the wetting of the TPS material surface was weakened and TPS became more hydrophobic. However, with increase of surface photocrosslinking extent, the contact angle showed a decreasing tendency. This tendency may be caused by surface roughness which increased in the process of modification.The effect of relative humidity and crosslinking extent on tensile properties of starch/glycerol system were investigated. As relative humidity increased, the tensile strength and elastic modulus decreased and the elongation at break increased gradually. The value of elongation at break reached its maximum, 28.961%, in the relative humidity of 75% , but fell to 14.456% when the relative humidity was 95% and moreover the tensile strength and elastic modulus sharply decreased. So the appropriate relative humidity to keep mechanical properties of TPS materials stable was 58% to 75%. For the photocrosslinking modified samples, when the UV radiation energy was less than 40J/cm~2, the tensile strength and elastic modulus of TPS increased gradually as the UV radiation energy increase, while the elongation at break was a declining tendency. When the UV radiation energy reached 40J/cm~2, the tensile strength and elastic modulus were 8.09MPa and 400.58MPa respectively and the elongation at break was 14.38%. However, further increased radiation energy, the tensile strength and elastic modulus slightly decreased while the elongation at break increased slightly. These changes may be due to the starch molecule chain session under the higher radiation energy of 80J/cm~2. |
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