Preparation And Performance Analysis Of PVFM Negative Pressure Seepage Material

Negative pressure irrigation is a water-saving irrigation mode for timely irrigation, according to the water needs of the crop. Negative pressure irrigation system usually consists of water storage, water pressure control valve, negative pressure connection pipe, and emitter. Emitter is at the water-soil interface, which directly contacts with the soil and crop root. It plays a vital role in efficient operation of the entire device. The former researches on the negative pressure irrigation are less, the exploration for the emitter is slow. Their researches are mainly focused on inorganic clay or ceramic materials, which are fragile, poor toughness, higher prices, not suitable for large area promotion.Through cross boards researching, there is a kind of microcellular foam materials of polymer materials. It can be divided into the open cell foam and closed cell foam according to the connectivity of the pores. However, the micron level and open pores are what we need. If the pore size is too small, seepage rate would be slow. If the pore size is too large, the capillary attaction would be so small that could not reach a higher bubble point. A kind of hydrophilicity microcellular foam plastics-- polyvinyl formal foam(PVFM) was found after screening. It was made by acetalation of polyvinyl alcohol(PVA) and formaldehyde, had soft, high toughness, adjustable pore structure, and good water permeability performance. In this research, PVFM was improved on this basis. Pore size distribution, bubble point, tensile strength, hardness, accumulate in the soil infiltration capacity, infiltrationrate, wetting front transportation were tested, so that the effects of reaction conditions, additives, specifications for PVFM could be studied, then expounded the feasibility of making PVFM into negative permeability materials. The main conclusions of the paper are following:(1)The performance of PVFM could be affected by the ratio of raw materials. Apparent density reached a minimum and water absorbency reached a maximum when H2 O ratio was 8.75. Bubble point was largely different between, maximum of 73.3 kPa when the ratio was 1:7.50:0.50 and minimum of 26.7 kPa when the ratio was 1:8.75:0.50. Generally, the bubble point could reach a high level at H2 O ratio of 7.50 or formaldehyde ratio of 0.75. Under-5kPa and-10 kPa, water exudation rate was higher when H2 O ratio was larger than 7.50. In theory, the PVFM with PVA: H2O: formaldehyde ratio of 1:7.50:0.75, whose mean pore size was small, bubble point was 65.3kPa, saturated hydraulic conductivity was 7.45×10-4cm?s-1, could be used as a novel negative pressure seepage material in negative pressure irrigation.(2)PVFM performance could be affected by the reaction conditions. Apparent density, water absorbency, porosity had a higher performance, When the reaction temperature was 55℃, reaction time was no less than 10 h, and mixer speed was at 2000r?min-1,1500r?min-1, 1000r?min-1 respectively. Bubble point and the rate of soil water reached a high level when the reaction temperature was 55℃, or reaction time was no less than 10 h, or mixer speed was no more than 1500 r.min-1.In summary, bubble point and cumulative infiltration would reach a high level when the reaction temperature was 55℃, reaction time was 10 h, mixer speed was1500 r?min-1,while physical performance was also excellent under these condition. So this condition could be the best reaction condition for preparing PVFM.(3)Different additives had different effects on the performance of PVFM. On the one hand, the absorption rate showed a decreasing trend and the bubble point, apparent density and the porosity showed an increasing trend. On the other hand, mechanical properties and cumulative infiltration of PVFM showed a decreasing or increasing trend in different extent. Taken together, silicone oil and kaolin together could enhance obviously the apparent density and porosity. At the same time, they could also improve the hardness, tensile strength and break elongation, especially bubble point. However, it was not obvious that silicone oil and kaolin improved infiltration. In general, performance of silicone oil and kaolin was better than that of other additives.(4)The performance of PVFM cups could be affected by physically specifications. Negative pressure seepage capacity of hollow PVFM was better than the solid. Increasing the length of solid PVFM has a negative influence on the cumulative infiltration. Negative pressure seepage capacity of 15×3cm hollow PVFM were more excellence than Ceramic cup. Under various negative pressures, the cumulative infiltration per unit area of hollow PVFM cup were higher, and soil water transported much faster in the same time. A broader soil moisture space can be controlled by hollow PVFM cup than by Ceramic cup. The moisture characteristic curve determined with PVFM cup was similar to Ceramic head. So, hollow PVFM could be used to determine soil water characteristic curve. 15×3cm hollow PVFM could more easily satisfy the crop demand for water under-5~-10 kPa which was suitable for crop growth.