Surface Chemical Crosslinking Modification Of Thermoplastic Starch/poly(Vinyl Alcohol) Blend Film

As a kind of natural polymer, starch is renewable, inexpensive, biodegradable and extensively existing in plants, which makes it become the preferred raw material of biodegradable plastics. Due to its biodegradability, good film-forming capability and water solubility, poly(vinyl alcohol) (PVA) is well suited for making blends with natural polymer starch. Compared with pure starch films, TPS/PVA blend film presented a higher strain value. However, the hydroxyl groups on starch and PVA molecules lead to a high hydrophilicity of TPS/PVA blend film, which hinders its further application. The hydrophobic characteristic of the blend film still needs to be improved.In this paper, glutaraldehyde, sodium hexametaphosphate and borax were selected to crosslink the surface of TPS/PVA blend film to improve the water-resistance of TPS/PVA blend film, as well as reduce the sensitivity of its mechanical property to the humidity of environment. The surface crosslinking modification of TPS/PVA blend films was carried out through heating. The hydroxyl groups on starch and PVA molecules are responsible for the hydrophilicity of TPS and PVA. Crosslinking agents can react with the hydroxyl groups in starch or PVA for modifications, so crosslinking is an efficient way to increase water resistance of starch and PVA. The effects of concentration of crosslinking agents aqueous solution, soaking time, reaction temperature, reaction time and other factors on the surface crosslinking reaction were investigated. Moisture absorption and mechanical properties of control film and the surface crosslinked ones were measured to characterize the influence of the surface crosslinking modification.The specimens, modified by glutaraldehyde, sodium hexametaphosphate and borax respectively, were immersed in DMSO in which the TPS/PVA blend film is originally completed soluble. After 48 hours, the insoluble part (swollen film) was filtered out, which suggested that the specimens were crosslinked by glutaraldehyde, sodium hexametaphosphate and borax. When filtering out the insoluble part of the specimens soaked in DMSO, the swollen films divided into two pieces held together by their edges. This indicated that the specimens were surface crosslinked, and the inner part of specimens was dissolved.The surface of TPS/PVA blend films can be modified by crosslinking through soaking the films in glutaraldehyde aqueous solution and then heating in an oven. The results of crosslinking kinetics showed that, the crosslinking degree of the specimens increased with increasing reaction time and the increase rate was initially fast and then slowed down. For the films soaked in 1.0% glutaraldehyde aqueous solution for 30 s and heating at 20℃, 3 hours was the optimal choice, and for the films heating at 35℃and 50℃, 2 hours was the optimal reaction time. A higher temperature gave rise to a faster reaction rate, but the influence of reaction temperature on final crosslinking degree was not so remarkable. The crosslinking density of the surface layer increased with prolongation of soaking time in a given glutaraldehyde aqueous solution. However, the effect of concentration of glutaraldehyde aqueous solution on the crosslinking density of the surface layer depends on soaking time. When soaking time was shorter than 60s, higher concentration of glutaraldehyde aqueous solution gave rise to a higher crosslinking density in the surface layer. The results of moisture absorption measurements indicated that the surface crosslinking modification significantly reduced the hydrophilic characteristic of the TPS/PVA blend films, and the hydrophily reduced with increasing of crosslinking degree. The results of mechanical properties measurements showed that the surface crosslinking modification increased tensile strength and Young's modulus but decreased elongation at break of the TPS/PVA blend films. The influence of soaking time and reaction temperature was significant. Longer soaking time and higher reaction temperature gave rise to higher values of tensile strength and Young's modulus, and lower value of elongation at break. Besides, surface crosslinking modification by glutaraldehyde reduced the sensitivity of the blend film's mechanical property to the humidity of environment. When the humidity of environment increased, tensile strength and Young's modulus of control film and surface crosslinked ones decreased, elongation at break of crosslinked films increased, while that of the control one reached the highest value at 75% relative humidity.Sodium hexametaphosphate was used to crosslink the surface of TPS/PVA blend films. The results of crosslinking kinetics showed that, the crosslinking degree of the specimens increased with increasing reaction time and temperature, but the influence was no longer obvious after reaction time reached 3 hours. The crosslinking degree of TPS/PVA blend films presented an increasing trend with increasing concentration of sodium carbonate aqueous solution, and declined slightly when the concentration of sodium carbonate aqueous solution was too high. Longer soaking time in sodium carbonate aqueous solution gave rise to a higher crosslinking degree of the film. The crosslinking degree of TPS/PVA blend films initially increased and then reached stable values with increasing concentration of sodium hexametaphosphate aqueous solution. However, prolongation of soaking time in sodium hexametaphosphate aqueous solution reduced the crosslinking degree of TPS/PVA blend films, which was inconsistent with the expected results. For sake of care, experiments were performed in triplicate, and the results still presented the same changing trend. Further research is needed to figure out the reason. The best treatment conditions in the surface crosslinking modification process with sodium hexametaphosphate were as follows: the specimens were soaked in 4.0% sodium carbonate aqueous solution for 100 seconds and soaked in 5.0% sodium hexametaphosphate aqueous solution for 10 seconds. Soaking time in sodium carbonate aqueous solution was found to be the primary parameter to effect the crosslinking degree of the TPS/PVA blend film, soaking time in sodium hexametaphosphate aqueous solution was the secondary one and followed by concentration of sodium hexametaphosphate aqueous solution and concentration of sodium carbonate aqueous solution. The surface crosslinking modification by sodium hexametaphosphate reduced the hydrophilic characteristic of the TPS/PVA blend films remarkably, and the hydrophily reduced with increasing of crosslinking degree. The results of mechanical properties measurements suggested that the surface crosslinking modification increased tensile strength and Young's modulus, and decreased elongation at break of the TPS/PVA blend films.The crosslinking degree of TPS/PVA blend film modified by borax increased with increasing concentration of borax aqueous solution and soaking time in borax aqueous solution. The results of crosslinking kinetics showed that, for the films heating at 50℃for 2 hours, soaking in 10.0% borax aqueous solution for 60 s gave rise to a most efficient surface crosslinking in the investigated ranges of concentration of borax aqueous solution and soaking time. Prolongation of reaction time increased the crosslinking degree of TPS/PVA blend films, but the influence was no longer obvious after 3 hours. The surface crosslinking modification by borax increased the hydrophobicity of the TPS/PVA blend films, and the hydrophobicity increased slightly with increasing concentration of borax aqueous solution and soaking time. After the surface crosslinking modification by borax, tensile strength and Young's modulus of the TPS/PVA blend films increased while elongation at break decreased.