A completely biodegradable and rain-dissolvable plastic film and its manufacturing method

Abstract

The present invention relates to a polyvinyl alcohol modified plastic film with a two-layer structure. The compositions of the upper and lower layers of the plastic film are both based on polyvinyl alcohol of grades 1788 and 1799, and sodium oleate, zinc stearate, glycerol, pentaerythritol, and oleic acid diethanolamide plasticizers are added. The co-extruded upper layer of the plastic film can adjust the dissolution time of the plastic film by rainwater (or irrigation water) by changing the microstructure (crystallinity and orientation). The weakly alkaline plasticizer contained in the co-extruded upper and lower layers of the plastic film can reduce the acidity of the soil and has the effect of improving soil acidification. During the natural degradation process of polyvinyl alcohol, it can wrap soil particles and lock the free water in the soil to avoid the damage caused by salt enrichment to the soil. This plastic film can achieve 100% biodegradation, and the degradation period is less than 1 year.

Description

Technical Field

[0001] The present invention relates to a plastic film based on polyvinyl alcohol and a manufacturing method thereof, belonging to agricultural plastic products. This plastic film can be 100% biodegradable and can be dissolved by rainwater or irrigation water (the dissolution time can be adjusted) and enter the soil. The intermediate generated during the degradation of the plastic film has the functions of retaining moisture in the soil, preventing the loss of calcium, magnesium, aluminum, manganese and other ions, reducing soil acidity, and improving soil moisture conditions. Background Art

[0002] Ordinary plastic films can effectively retain water, fertilizers and control weeds, and the usage amount is very large. For example, Xinjiang needs 255,000 tons of plastic films every year, and the whole country needs about 1.2 million tons of plastic films every year. The plastic film covered area exceeds 3.5 billion mu.

[0003] The residual film rate of ordinary non-degradable plastic films in farmland reaches about 40%. The degradation period of these non-degradable plastic films is generally 200 - 300 years. If plastic films are continuously used in farmland for ten years, then the residual rate of plastic films in the field reaches 262 kg / hectare, causing soil deterioration and a 10 - 23% reduction in crop yield.

[0004] With the understanding of the harmfulness of non-degradable plastic films by people, many biodegradable plastic films have emerged on the market currently, which has greatly promoted the development of modern agriculture. The plastic film manufactured by the present invention has the following characteristics: (1) The plastic film involved in the present invention has a double-layer structure; (2) The plastic film involved in the present invention can be 100% biodegradable; (3) The main polyvinyl alcohol substance in the plastic film involved in the present invention can be slowly dissolved by irrigation water or rainwater and enter the soil; (4) The time for the plastic film involved in the present invention to be dissolved by water can be controlled by the microstructure (crystallinity and orientation state) of the polyvinyl alcohol in the upper layer of the co-extruded plastic film; (5) The plastic film involved in the present invention and its degradation-generated intermediate can lock the free water in the soil, improve soil humidity, improve soil moisture conditions, and prevent salinization caused by soil water evaporation; (6) The main polyvinyl alcohol substance in the plastic film involved in the present invention can complex various metal ions such as calcium, magnesium, aluminum, manganese in the soil, prevent the loss of these metal ions, and prevent the occurrence of soil acidification; (7) The plasticizer sodium oleate in the plastic film involved in the present invention has weak alkalinity, can neutralize the acidity in the soil, and delay soil acidification; (8) The plasticizers in the plastic film involved in the present invention, sodium oleate, zinc stearate, glycerol, pentaerythritol, oleic acid diethanolamide, are environmentally friendly and non-toxic, and cause no harm to the soil and crops. Summary of the Invention

[0005] Polyvinyl alcohol with a grade of 1799 has a degree of alcoholysis of 99% and a degree of polymerization of around 1700. Each repeating unit contains a hydroxyl group. The hydroxyl group on one repeating unit can form hydrogen bonds with the hydroxyl group on the repeating unit of another molecular chain. For polyvinyl alcohol 1799, a large number of intermolecular hydrogen bonds will be formed in its aggregated state. The existence of these hydrogen bonds makes the crystallinity in its aggregated state high. Even for the molecular chains in the amorphous region, the chain spacing is very small, and the intermolecular force in the aggregated state is very large. Therefore, it is very difficult to process and form polyvinyl alcohol with a grade of 1799. Even for the 1788 resin with a degree of alcoholysis of 88%, a large number of intermolecular hydrogen bonds are formed, and it is also very difficult to carry out thermoplastic processing. Therefore, the prerequisite for the thermoplastic processing of polyvinyl alcohol is to break the intermolecular hydrogen bonds in its aggregated state (resin), and this is exactly the role of the plasticizer.

[0006] The plasticizer has a small molecular volume, and both glycerol and pentaerythritol contain more hydroxyl groups. They will slowly penetrate into the amorphous region of the polyvinyl alcohol resin, break the intermolecular hydrogen bonds between the polyvinyl alcohol (PVOH) molecules in the amorphous region. Moreover, these small plasticizer molecules with strong polarity will also slowly diffuse between the crystal planes of PVOH, break the intermolecular hydrogen bonds in the crystalline region, destroy the crystallization of PVOH, swell and dissolve the crystalline region. Glycerol or pentaerythritol forms new hydrogen bonds with the hydroxyl groups on the PVOH chain. In this way, the plasticizer with a small volume solvates the polyvinyl alcohol molecular chains, reduces the number of intermolecular hydrogen bonds between the polyvinyl alcohol molecular chains, and reduces the intermolecular force between the PVOH molecular chains, enabling the 1788 and 1799 resins to be successfully processed and formed at around 180 degrees, which is lower than the decomposition temperature of the hydroxyl group.

[0007] The reasonable selection of the plasticizer is the key to the processing and forming of polyvinyl alcohol resin. First, a plasticizer with high polarity should be selected because PVOH has a very high polarity; PVOH contains hydroxyl groups, so we choose glycerol or pentaerythritol containing hydroxyl groups. In addition, compared with pentaerythritol, glycerol has the smallest molecular volume. Therefore, the smaller-volume glycerol will diffuse fastest in the PVOH resin prior to other plasticizers, with the greatest depth of diffusion and the greatest degree of damage to the crystalline region of PVOH. Pentaerythritol also contains more hydroxyl groups on its molecule. It migrates into the PVOH resin along with glycerol, and pentaerythritol can replace glycerol and form new hydrogen bonds with the hydroxyl groups on the PVOH molecular chain. In this way, pentaerythritol replaces glycerol to form hydrogen bonds with the hydroxyl groups on PVOH. The larger-volume pentaerythritol increases the chain spacing of the PVOH molecular chains, further weakening the intermolecular force between the PVOH molecular chains and reducing the melting temperature of PVOH more.

[0008] Oleic acid diethanolamide contains -N-H groups, which can also break the intermolecular hydrogen bonds of PVOH. The hydroxyl group -OH in PVOH forms new hydrogen bonds with -N-H in oleic acid diethanolamide. Oleic acid diethanolamide contains a weakly polar fatty chain, which is located between the PVOH molecular chains, greatly weakening the intermolecular force of PVOH and being very beneficial to reducing the melting temperature of PVOH. The weakly polar oleic acid diethanolamide needs to be added after glycerol. Only after the molecular chain spacing of PVOH that has been swollen (expanded) by glycerol and pentaerythritol can it diffuse smoothly into the PVOH resin and play a plasticizing role.

[0009] The melting temperature of sodium oleate is about 232 °C. When compounded with other plasticizers, it can play an auxiliary plasticizing role. In addition, when PVOH melts and cools to form a mold, it can also play a role in heterogeneous nucleation, which is beneficial to the production of fine PVOH crystals. After sodium oleate is inserted between the PVOH molecular chains, the low-polarity fatty chain is also conducive to reducing the intermolecular force of PVOH. The plasticizing effect of sodium oleate is demonstrated during the melt processing.

[0010] The melting point of zinc stearate is about 120 °C. It melts at a lower temperature, forms a liquid film between PVOH particles, reduces the extrusion resistance, is beneficial to reducing the melt pressure during PVOH processing, and after the PVOH resin melts, the zinc ions in the zinc stearate molecule can form a permanent complex with the hydroxyl groups on the PVOH molecular chain, reducing the crystallization ability of PVOH. The role of zinc stearate is demonstrated in the molten state.

[0011] To facilitate the diffusion of the plasticizer in the PVOH resin particles, the PVOH of grades 1788 and 1799 is first in powder form. First, about 2 / 3 of the total amount of glycerol is mixed with the PVOH resin powder and swollen at room temperature for 24 hours. Let the glycerol with a smaller molecular volume diffuse into the amorphous region of PVOH, and slowly break the intermolecular hydrogen bonds in the crystalline region, destroying the crystalline region and increasing the intermolecular gap of PVOH molecules to facilitate the entry of other large-molecule plasticizers later.

[0012] After the PVOH powder is swollen by glycerol for 24 hours, the remaining glycerol and the mixture of other plasticizers come into contact with PVOH. Plasticizers with larger molecular volumes such as pentaerythritol and oleic acid diethanolamide replace glycerol, form new hydrogen bonds with PVOH, further increase the intermolecular distance of PVOH, weaken the intermolecular force of PVOH, reduce the melting temperature of PVOH, and enable PVOH to melt at about 180 °C, which is lower than the thermal cracking temperature of the hydroxyl group.

[0013] During the melt blending process of PVOH and the above-mentioned composite plasticizer, the auxiliary plasticizing effects of diethanol oleamide, zinc stearate, and sodium oleate are fully demonstrated. They reduce the number of hydrogen bonds between PVOH molecules, increase the distance between PVOH molecular chains, reduce the intermolecular force, lower the crystallization ability of PVOH, and decrease the PVOH crystal grain size. In this way, the plasticized and modified PVOH resin is obtained.

[0014] Using this plasticized and modified resin to blow films, a mulch film product is obtained. The PVOH with the grade 1799 has a large crystallization ability, and the obtained mulch film has poor cold water solubility, which results in a reduced degradation rate of the mulch film in the soil and affects the cultivation of the next season. The mulch film prepared from PVOH with the grade 1788 is quickly soluble in cold water and is not suitable as a mulch film. Therefore, the mulch film involved in this patent has a two-layer structure, where the co-extruded upper layer can contact rainwater but will not be immediately dissolved, and the lower layer contacts the ground.

[0015] In the plasticized resin used for the co-extruded upper layer of the mulch film, 1799 accounts for a relatively large proportion. The ratio of the two grades of polyvinyl alcohol, 1799 and 1788, is 90:10 to 70:30, and the dosage of the composite plasticizer is also relatively small, which are respectively: 10 - 20 parts of glycerol, 5 - 10 parts of pentaerythritol, 5 - 10 parts of sodium oleate, 2 - 5 parts of zinc stearate, and 2 - 5 parts of diethanol oleamide. The crystallinity of the co-extruded upper layer of the prepared composite film is 20 - 30%. The higher the crystallinity, the longer the time required for water molecules in the air, rainwater, or irrigation water to swell and dissolve the film when the mulch film is in use, the longer the film covers the soil, and the later the film enters the soil. Under natural conditions, the time for the film to disappear is proportional to the time for the film to dissolve in 50°C hot water. Generally, the time for the film to dissolve in 50°C hot water is 1 - 10 minutes, corresponding to the dissolution time in cold water (rainwater) of 10 - 200 days (when the co-extruded upper layer is 20 microns thick). The co-extruded upper layer of the mulch film is responsible for regulating the dissolution (invisible to the naked eye) time of the mulch film in the natural state, which is achieved by changing the microscopic structure of the co-extruded upper layer's aggregate state. The influencing factors on the water-soluble temperature in the microscopic structure of the co-extruded upper layer's aggregate state include crystallinity, crystal grain size, amorphous region ratio, amorphous region chain spacing, and the degree of solvation of the amorphous region by the plasticizer (the number of molecules of the plasticizer that wrap the hydroxyl groups on the PVOH molecular chain). The smaller the crystallinity, the smaller the crystal grain size, the larger the amorphous region ratio, the larger the PVOH molecular spacing, and the higher the degree of solvation, the shorter the time required for the co-extruded upper layer to be dissolved by cold water.

[0016] A relatively large proportion of 1788 is contained in the plasticized resin used in the lower layer of the plastic film. The ratio of two grades of polyvinyl alcohol, 1799 and 1788, is from 5:95 to 30:70. The proportion of the composite plasticizer is higher than that of the co-extruded upper layer. The proportions of the plasticizers are as follows: 10 - 20 parts of glycerol, 5 - 10 parts of pentaerythritol, 15 - 25 parts of sodium oleate, 2 - 5 parts of zinc stearate, and 5 - 15 parts of diethanolamide oleate. The crystallinity of the lower layer PVOH is 7 - 15%, and the dissolution time in cold water at 50°C is 0.2 - 1 minute, corresponding to a dissolution time in cold water (rainwater) of 1 - 5 days (when the co-extruded lower layer is 20 microns thick). About 18% of vinyl acetate monomer units are contained in the molecular chain of the PVOH of 1788. Its existence reduces the regularity of the molecular chain and the number of hydrogen bonds between PVOH molecules. Coupled with the action of the composite plasticizer, the crystallinity of the lower layer of the plastic film is relatively low. When in use, the lower layer of the plastic film is easily dissolved by cold water (water vapor in the air or in the soil) and enters the soil.

[0017] Before preparing the film, the co-extruded upper and lower layer resins need to be melt-blended using a twin-screw extruder. After the two PVOH powders of 1788 and 1799 are mixed evenly and swollen by the composite plasticizer in sequence, the upper and lower layer resins are respectively placed in the twin-screw extruder for melt-blending to obtain the plasticized resin.

[0018] After obtaining the plasticized resin, it is immediately formed on a multi-layer co-extrusion blown film machine. When blowing the film, the outer layer of the plastic film is set as the inner layer of the film bubble. The advantage of this setting is that the inner layer of the film bubble does not directly contact the air ring (cold air) of the blown film machine. After coming out of the die of the blown film machine, the temperature of the inner layer of the film bubble drops more slowly than that of the outer layer. Therefore, the inner layer PVOH (1799 resin) has more time for recrystallization to obtain an outer layer that is as water-resistant as possible (with a long water-soluble time).

[0019] During the use of the plastic film, under natural environments such as continuous rainfall, irrigation, high-humidity air, dew, and condensation of water volatilized from the soil, the inner layer will first dissolve and enter the soil. The strength of the outer layer of the plastic film will gradually decrease, the stiffness will gradually decline, the thickness will gradually become thinner, and it will gradually break and finally dissolve in the soil.

[0020] The plastic film will be 100% biodegradable in the soil within 1 year. The oligomer intermediates formed during the degradation of the PVOH main chain have the function of retaining water and moisture. The molecular chains will be adsorbed on the surface of soil particles, and the hydroxyl groups on the molecular chains will also complex metal ions such as calcium, magnesium, aluminum, and manganese in the soil to prevent the loss of these ions due to the decrease in groundwater level; the plasticizers glycerol and pentaerythritol in the plastic film will be quickly biodegradable into fertilizers; sodium oleate is slightly alkaline, and it will neutralize with H+ generated due to excessive application of chemical fertilizers in the soil to form water and neutral organic acids, reducing soil acidity and preventing soil acidification. The plasticizers diethanolamide oleate and zinc stearate will also be biodegradable into fertilizers. The matrix material PVOH in the plastic film, its main chain will be biodegraded into intermediates containing aldehyde groups and carboxyl groups, and the main chain will gradually be broken to generate oligomer fertilizers with smaller and smaller molecular weights. Eventually, all components in the plastic film will be decomposed into carbon dioxide and water, and the nitrogen element in diethanolamide oleate will be converted into a small amount of ammonium salts (fertilizers).

[0021] Technical characteristics

[0022] In the upper layer of the co-extruded plastic film, the ratio of the two grades of polyvinyl alcohol, 1799 and 1788, is 90:10 to 70:30. Preferably, the ratio of the two is selected to be 85:15 to 75:25, and more preferably, the ratio of the two is selected to be 80:20; the dosage of the composite plasticizer (percentage of the weight of the co-extruded upper layer resin) is 10 - 20 parts of glycerol, 5 - 10 parts of pentaerythritol, 5 - 10 parts of sodium oleate, 2 - 5 parts of zinc stearate, and 2 - 5 parts of diethanolamide oleate. Preferably, 13 - 18 parts of glycerol, 6 - 9 parts of pentaerythritol, 6 - 9 parts of sodium oleate, 2.5 - 4 parts of zinc stearate, and 2.5 - 4 parts of diethanolamide oleate are selected. More preferably, 15 parts of glycerol, 8 parts of pentaerythritol, 8 parts of sodium oleate, 3 parts of zinc stearate, and 3 parts of diethanolamide oleate are selected.

[0023] In the lower layer of the plastic film, the ratio of the two grades of polyvinyl alcohol, 1799 and 1788, is 5:95 to 30:70. Preferably, the ratio of the two is selected to be 10:90 to 20:80, and more preferably, the ratio of the two is selected to be 15:85; the ratios of the plasticizers in the lower layer are respectively: 10 - 20 parts of glycerol, 5 - 10 parts of pentaerythritol, 15 - 25 parts of sodium oleate, 2 - 5 parts of zinc stearate, and 5 - 15 parts of diethanolamide oleate. Preferably, their respective ratios are 12 - 18 parts of glycerol, 6 - 9 parts of pentaerythritol, 17 - 22 parts of sodium oleate, 2.5 - 4 parts of zinc stearate, and 7 - 13 parts of diethanolamide oleate. More preferably, their ratios are respectively 15 parts of glycerol, 8 parts of pentaerythritol, 20 parts of sodium oleate, 3 parts of zinc stearate, and 10 parts of diethanolamide oleate.

[0024] The temperature range for the melt blending and blown film extrusion of PVOH is relatively narrow. Generally, the maximum processing temperature is 180 °C. Therefore, when performing blown film extrusion or melt blending, the temperature of the screw heating section is set to 160 - 180 °C, the feeding temperature is set to 160 °C, and the temperature of the extrusion die head is set to 180 °C. The temperature of the film cooling air is 25 °C.

[0025] The molecular chains of polyvinyl alcohol are relatively rigid and prone to fracture and degradation at high shear rates. Therefore, the suitable screw rotation speed during melt blending and blown film extrusion is 3 - 20 rpm, preferably 5 - 15 rpm, and more preferably 10 rpm.

[0026] During blown film processing, the screw rotation speeds of the inner and outer layers respectively determine the thicknesses of the inner and outer layers and the total thickness; the total thickness of the film can be adjusted between 20 - 50 μm, where the thickness of the coextruded upper layer accounts for 10 - 50% of the total thickness, so as to achieve a dissolution time of the plastic film for farmland in cold water of 10 - 100 days.

[0027] The cold water mentioned here includes rainwater, irrigation water, dew, moisture in the air (relative humidity), and water droplets condensed after volatilizing to the lower layer of the film from the soil.

[0028] The performance test method of the plastic film for farmland of the present invention is as follows:

[0029] Water - soluble temperature resistance: Cut a piece of film with an outer dimension of 5×5 cm using scissors; place the film in hot water at 50 °C under magnetic stirring, start timing, and observe the time required when the volume of the film residue becomes less than 0.3 cm³.

[0030] Film crystallinity and crystal grain size: Cut the plastic film for farmland into 2 cm×2 cm squares, and use a Bruker (Beijing) Technology Co., Ltd. D8 X - ray diffractometer with a Cu target to scan in the range of 5 - 40° at a scanning speed of 6° / min. Obtain the X - ray diffraction curve of the film, and characterize the crystallinity of the PVOH film according to the ratio of the area of the diffraction peak of the (101) crystal plane around 2θ = 19.6° to the area of the amorphous region; in addition, calculate the diameter of the crystalline region (crystal grain size) according to the full width at half maximum of the diffraction peak of the 101 crystal plane.

[0031] Cut the dried plastic film for farmland sample into 2 cm×2 cm sizes, fix the film sample on the sample holder using a special fixture, and perform tests using a Nicolet 6700 FT - IR Fourier transform infrared spectrometer from Thermo Company of the United States. The test wavelength range is 4000 cm -1 ~400 cm -1 .

[0032] Mechanical property tests such as tensile strength, Young's modulus, and elongation at break were carried out using an Instron-5565A electronic tensile testing machine. According to "GB / T 1040-92 Test Method for Tensile Properties of Plastics" and "GB 13022-91 Test Method for Tensile Properties of Plastic Films", the tests were conducted at a temperature of (25 ± 2)°C. The dimensions of the biaxially oriented film specimen were 20 mm × 150 mm, the gauge length was 100 mm, and the test rate was 200 mm / min.

[0033] Analysis of the film cross-sectional structure: The film was immersed in liquid nitrogen for 5 minutes and then quickly broken. The cross-section was analyzed for its profile under an SS-550 (Shimadzu, Japan) scanning electron microscope.

[0034] The degradation performance of the mulch film was tested according to GB / T19277.1-2011 "Determination of the ultimate aerobic biodegradability of materials under controlled composting conditions - Method by measuring the evolved carbon dioxide - Part 1 General method" and GB / T20197-2006 "Definition of degradable plastics".

[0035] Effect of the dissolved matter of the mulch film on the soil: 1 g of the mulch film was added to 20 g of distilled water at a temperature of 50°C for standby. 200 g of flower soil was divided into two parts. 20 g of distilled water was added to one part of the flower soil, and the above-mentioned mulch film solution was added to the other part of the flower soil. The water (solution) and the flower soil were mixed evenly. The two parts of the flower soil were respectively placed in an open glass dish with a diameter of 10 cm and placed indoors at 25°C with a humidity of 50% for 10 days, and then the soil water content and pH value were measured. After the test, the soil was taken out and placed in a 500-ml beaker, 300 ml of distilled water was added, stirred for 15 minutes, filtered, and the calcium, magnesium, aluminum, and manganese ion concentrations in the filtrate were measured by chemical titration after concentrating the filtrate. Specific embodiments

[0036] The present invention will be further described below through the following specific examples and application examples. The following specific description is for the convenience of understanding the present invention and is not intended to limit the protection scope of the present invention.

[0037] Example 1

[0038] A mulch film made entirely of the upper-layer resin (only for performance testing)

[0039] (1) Swelling of the co-extruded upper-layer resin

[0040] 80 parts of PVA resin powder with the grade of 1799 and 20 parts of PVA resin powder with the grade of 1788 (parts by weight, the same below) were mixed in a 15103 - type high - speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes, and then 10 parts of glycerol were added under stirring conditions. After stirring for 10 minutes, the material was taken out and sealed in a polyethylene plastic bag with a thickness of 50 microns and left standing at room temperature of 25 degrees for 24 hours;

[0041] The above - mentioned material was taken out and put into the above - mentioned high - speed mixer. A mixture consisting of 5 parts of glycerol, 8 parts of pentaerythritol, 8 parts of sodium oleate, 3 parts of zinc stearate and 3 parts of diethanolamide oleate was added under stirring conditions. After mixing for 10 minutes, the material was taken out and sealed in a polyethylene plastic bag with a thickness of 50 microns and left standing in a drying oven at 40 degrees for 24 hours;

[0042] (2) Melt blending of the co - extruded upper - layer resin

[0043] The above - mentioned swollen material was taken out and added to the hopper of a twin - screw extruder. The four - section heating temperatures of the screw were set at 160 degrees, 170 degrees, 180 degrees and 180 degrees respectively, the extrusion die head temperature was 180 degrees, the screw speed was 10 rpm, air cooling was used, and pellets were obtained to get the co - extruded upper - layer plasticized and modified resin;

[0044] (3) Mulch film completely made of the upper - layer plasticized and modified resin

[0045] The above - mentioned plasticized and modified co - extruded upper - layer resin was taken out and added to the hoppers of the innermost and outermost extruders of the film bubble of a multi - layer co - extrusion blown film device. No material was added to the hoppers of other extruders (if the multi - layer co - extrusion is more than 2 layers). The four - section heating temperatures of the screw were set at 160 degrees, 170 degrees, 180 degrees and 180 degrees respectively, the extrusion die head temperature was 180 degrees, the screw speed was 15 rpm, air cooling was used, and the obtained longitudinal draw ratio was 4 and the transverse blow - up ratio was 2.5, and a 20 - micron - thick mulch film completely made of the upper - layer modified resin was obtained (only for testing performance).

[0046] Example 2

[0047] A mulch film completely made of the lower - layer resin (only for testing the performance of the lower - layer resin)

[0048] (1) Swelling of the co - extruded lower - layer resin

[0049] Mix 15 parts of PVA resin powder with the grade number 1799 and 85 parts of PVA resin powder with the grade number 1788 (parts by weight, the same below) in a 15103 type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes, then continue to add 10 parts of glycerol under stirring conditions, take out the material after stirring for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 microns and let it stand at room temperature of 25 degrees for 24 hours;

[0050] Take out the above material into the above high-speed mixer, add a mixture composed of 5 parts of glycerol, 8 parts of pentaerythritol, 20 parts of sodium oleate, 3 parts of zinc stearate and 10 parts of diethanolamide oleate under stirring conditions, take out the material after mixing for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 microns and let it stand in a drying oven at 40 degrees for 24 hours;

[0051] (2) Melt blending of the co-extruded lower layer resin

[0052] Take out the above swollen material and add it to the hopper of the twin-screw extruder. Set the heating temperatures of the four sections of the screw to 160 degrees, 170 degrees, 180 degrees and 180 degrees respectively, the extrusion die head temperature to 180 degrees, the screw speed to 10 rpm, air-cool and pelletize to obtain the co-extruded upper layer plasticized and modified resin;

[0053] (3) Mulch film completely made of the lower layer plasticized and modified resin

[0054] Take out the above plasticized and modified co-extruded lower layer resin and add it to the hoppers of the outermost and innermost extruders of the multi-layer co-extrusion blown film device. Do not add any material to the hoppers of other extruders (if the co-extrusion is more than 2 layers). Set the heating temperatures of the four sections of the screw to 160 degrees, 170 degrees, 180 degrees and 180 degrees respectively, the extrusion die head temperature to 180 degrees, the screw speed to 10 rpm, air-cool and draw to obtain a longitudinal draw ratio of 4 and a transverse blow-up ratio of 2.5, and obtain a mulch film with a thickness of 20 microns completely made of the upper layer modified resin (only for testing performance).

[0055] Example 3

[0056] Preparation of a mulch film with a co-extruded upper layer thickness of 10 microns and a co-extruded lower layer thickness of 10 microns

[0057] (1) Swelling and melt blending of the co-extruded upper layer resin

[0058] The formula and process are the same as those for the swelling and melt blending of the upper layer resin in Example 1;

[0059] (2) Swelling and melt blending of the co-extruded lower layer resin

[0060] The formula and process are the same as those for the swelling and melt blending of the upper layer resin in Example 2;

[0061] (3) Blown film forming of plastic film

[0062] Put the above co-extruded upper-layer resin after plasticizing modification into the hopper of the innermost extruder of the multi-layer co-extrusion blown film device; put the modified co-extruded lower-layer resin into the hopper of the outer extruder of the film bubble; do not add materials to the hoppers of other extruders (for co-extrusion devices with more than 2 layers); set the four-section heating temperatures of the two screws to 160 °C, 170 °C, 180 °C and 180 °C respectively, the extrusion die head temperature to 180 °C, set the rotational speeds of the two screws to 15 and 10 rpm respectively, air-cool, and the longitudinal draw ratio of the traction obtained is 4, and the transverse blow-up ratio is 2.5, to obtain a plastic film with an upper layer and a lower layer thickness of 10 μm and a total thickness of 20 μm.

[0063] Example 4

[0064] Preparation of plastic film with a co-extruded upper layer thickness of 10 μm and a co-extruded lower layer thickness of 10 μm

[0065] (1) Swelling and melt blending of co-extruded upper-layer resin

[0066] Mix 85 parts of PVA resin powder of grade 1799 and 15 parts of PVA resin powder of grade 1788 in a 15103 type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes, then continue to add 15 parts of glycerol under stirring conditions, take out the material after stirring for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 μm and let it stand at room temperature of 25 °C for 24 hours;

[0067] Take out the above material into the above high-speed mixer, add a mixture composed of 3 parts of glycerol, 9 parts of pentaerythritol, 9 parts of sodium oleate, 4 parts of zinc stearate and 4 parts of diethanolamide oleate under stirring conditions, take out the material after mixing for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 μm and let it stand in a drying oven at 40 °C for 24 hours;

[0068] Take out the above swollen material and add it to the hopper of the twin-screw extruder. Set the four-section heating temperatures of the screw to 160 °C, 170 °C, 180 °C and 180 °C respectively, the extrusion die head temperature to 180 °C, the screw rotational speed to 10 rpm, air-cool, and pelletize to obtain the co-extruded upper-layer plasticized modified resin;

[0069] (2) Swelling and melt blending of co-extruded lower-layer resin

[0070] Mix 10 parts of PVA resin powder with the grade number 1799 and 90 parts of PVA resin powder with the grade number 1788 in a 15103 type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes. Then, continue to add 10 parts of glycerol under stirring conditions and stir for 10 minutes. After that, take out the material, seal it in a polyethylene plastic bag with a thickness of 50 microns, and let it stand at room temperature of 25 degrees for 24 hours;

[0071] Take the above material into the above high-speed mixer, and add a mixture composed of 2 parts of glycerol, 6 parts of pentaerythritol, 17 parts of sodium oleate, 2.5 parts of zinc stearate, and 7 parts of diethanolamide oleate under stirring conditions. After mixing for 10 minutes, take out the material, seal it in a polyethylene plastic bag with a thickness of 50 microns, and let it stand in a drying oven at 40 degrees for 24 hours;

[0072] Take the above swollen material and add it to the hopper of a twin-screw extruder. Set the four-section heating temperatures of the screw to 160 degrees, 170 degrees, 180 degrees, and 180 degrees respectively, the extrusion die head temperature to 180 degrees, the screw speed to 20 rpm, and cool it with air to obtain co-extruded lower-layer plasticized modified resin by pelletizing;

[0073] (3) Blow molding of the plastic film for farmland

[0074] Take the above plasticized modified co-extruded upper-layer resin and add it to the hopper of the innermost extruder of the multi-layer co-extrusion blown film device; add the co-extruded lower-layer modified resin to the hopper of the outer extruder of the film bubble; do not add materials to the hoppers of other extruders (for co-extrusion devices with more than 2 layers); set the four-section heating temperatures of the two screws to 160 degrees, 170 degrees, 180 degrees, and 180 degrees respectively, the extrusion die head temperature to 180 degrees, set the rotational speeds of the two screws to 14 and 9 rpm respectively, cool it with air, and the longitudinal stretching ratio of the traction obtained is 4, and the transverse blowing ratio is 2.5 to obtain a plastic film for farmland with the upper and lower layers having thicknesses of 10 and 10 microns respectively and a total thickness of 20 microns.

[0075] Example 5

[0076] Preparation of a plastic film for farmland with a co-extruded upper layer thickness of 10 microns and a co-extruded lower layer thickness of 10 microns

[0077] (1) Swelling and melt blending of the co-extruded upper-layer resin

[0078] Mix 75 parts of PVA resin powder with the grade number 1799 and 25 parts of PVA resin powder with the grade number 1788 in a 15103 type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes. Then, continue to add 10 parts of glycerol under stirring conditions and stir for 10 minutes. After that, take out the material, seal it in a polyethylene plastic bag with a thickness of 50 microns, and let it stand at room temperature of 25 degrees for 24 hours;

[0079] Take out the above materials into the high-speed mixer above. Under stirring conditions, add a mixture composed of 3 parts of glycerol, 6 parts of pentaerythritol, 6 parts of sodium oleate, 2.5 parts of zinc stearate, and 2.5 parts of diethanolamide oleate. After mixing for 10 minutes, take out the materials and seal them in a polyethylene plastic bag with a thickness of 50 microns and let them stand in a drying oven at 40 degrees for 24 hours;

[0080] Take out the above swollen materials and add them to the hopper of the twin-screw extruder. Set the four-section heating temperatures of the screws to 160 degrees, 170 degrees, 180 degrees, and 180 degrees respectively, and the extrusion die head temperature to 180 degrees. Set the rotational speeds of the two screws to 18 and 12 rpm respectively for air cooling, and pelletize to obtain the co-extruded upper-layer plasticized modified resin;

[0081] (2) Swelling and melt blending of the co-extruded lower-layer resin

[0082] Mix 20 parts of PVA resin powder of grade 1799 and 80 parts of PVA resin powder of grade 1788 in a 15103-type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes. Then, continue to add 15 parts of glycerol under stirring conditions and stir for 10 minutes. After that, take out the materials and seal them in a polyethylene plastic bag with a thickness of 50 microns and let them stand at room temperature of 25 degrees for 24 hours;

[0083] Take out the above materials into the high-speed mixer above. Under stirring conditions, add a mixture composed of 3 parts of glycerol, 9 parts of pentaerythritol, 22 parts of sodium oleate, 4 parts of zinc stearate, and 13 parts of diethanolamide oleate. After mixing for 10 minutes, take out the materials and seal them in a polyethylene plastic bag with a thickness of 50 microns and let them stand in a drying oven at 40 degrees for 24 hours;

[0084] Take out the above swollen materials and add them to the hopper of the twin-screw extruder. Set the four-section heating temperatures of the screws to 160 degrees, 170 degrees, 180 degrees, and 180 degrees respectively, and the extrusion die head temperature to 180 degrees. Set the rotational speed of the screw to 20 rpm for air cooling, and pelletize to obtain the co-extruded lower-layer plasticized modified resin;

[0085] (3) Blow molding of the plastic film for farmland

[0086] Take the above plasticized modified co-extruded upper-layer resin and add it to the hopper of the innermost extruder of the multi-layer co-extrusion blown film device; add the co-extruded lower-layer modified resin to the hopper of the outer extruder of the film bubble; do not add materials to the hoppers of other extruders (for co-extrusion devices with more than 2 layers); set the four-section heating temperatures of the two screws to 160 degrees, 170 degrees, 180 degrees, and 180 degrees respectively, and the extrusion die head temperature to 180 degrees. Set the rotational speeds of the two screws to 12 and 7 rpm respectively for air cooling. The longitudinal stretching ratio of the traction obtained is 4, and the transverse blowing ratio is 2.5, to obtain a plastic film for farmland with upper and lower layer thicknesses of 10 and 10 microns respectively and a total thickness of 20 microns.

[0087] Example 6

[0088] Preparation of a co-extruded upper layer with a thickness of 10 μm and a co-extruded lower layer with a thickness of 10 μm plastic film

[0089] (1) Swelling and melt blending of the co-extruded upper layer resin

[0090] Mix 90 parts of PVA resin powder of grade 1799 and 10 parts of PVA resin powder of grade 1788 in a 15103 type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes, then continue to add 15 parts of glycerol under stirring conditions, take out the material after stirring for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 μm and let it stand at room temperature of 25 degrees for 24 hours;

[0091] Take out the above material into the above high-speed mixer, add a mixture consisting of 5 parts of glycerol, 10 parts of pentaerythritol, 10 parts of sodium oleate, 5 parts of zinc stearate and 5 parts of diethanolamide oleate under stirring conditions, take out the material after mixing for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 μm and let it stand in a drying oven at 40 degrees for 24 hours;

[0092] Take out the above swollen material and add it to the hopper of a twin-screw extruder. Set the heating temperatures of the four sections of the screw to 160 degrees, 170 degrees, 180 degrees and 180 degrees respectively, the extrusion die head temperature to 180 degrees, the screw speed to 3 rpm, air-cool and pelletize to obtain the co-extruded upper layer plasticized and modified resin;

[0093] (2) Swelling and melt blending of the co-extruded lower layer resin

[0094] Mix 30 parts of PVA resin powder of grade 1799 and 70 parts of PVA resin powder of grade 1788 in a 15103 type high-speed mixer produced by Beijing Huaxinke Plastic Machinery Co., Ltd. for 5 minutes, then continue to add 15 parts of glycerol under stirring conditions, take out the material after stirring for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 μm and let it stand at room temperature of 25 degrees for 24 hours;

[0095] Take out the above material into the above high-speed mixer, add a mixture consisting of 5 parts of glycerol, 10 parts of pentaerythritol, 25 parts of sodium oleate, 5 parts of zinc stearate and 15 parts of diethanolamide oleate under stirring conditions, take out the material after mixing for 10 minutes, seal it in a polyethylene plastic bag with a thickness of 50 μm and let it stand in a drying oven at 40 degrees for 24 hours;

[0096] Take the above-mentioned swollen material and add it to the hopper of a twin-screw extruder. Set the heating temperatures of the four sections of the screw to 160 °C, 170 °C, 180 °C, and 180 °C respectively, the extrusion die head temperature to 180 °C, the screw speed to 20 rpm, cool with air, and pelletize to obtain the co-extruded lower-layer plasticized modified resin;

[0097] (3) Blow molding of the plastic film

[0098] Take the above-mentioned co-extruded upper-layer resin after plasticizing modification and add it to the hopper of the innermost extruder of the multi-layer co-extrusion blown film device; add the co-extruded lower-layer modified resin to the hopper of the outer extruder of the film bubble; do not add materials to other extruder hoppers (for co-extrusion devices with more than 2 layers); set the heating temperatures of the four sections of the two screws to 160 °C, 170 °C, 180 °C, and 180 °C respectively, the extrusion die head temperature to 180 °C, set the speeds of the two screws to 20 and 15 rpm respectively, cool with air, and the longitudinal draw ratio of the traction is 4, and the transverse blow-up ratio is 2.5, to obtain a plastic film with upper and lower layer thicknesses of 10 and 10 microns respectively and a total thickness of 20 microns.

[0099] Example 7

[0100] Preparation of a plastic film with a co-extruded upper layer thickness of 5 microns and a co-extruded lower layer thickness of 5 microns

[0101] (1) Swelling and melt blending of the co-extruded upper-layer resin and the lower-layer resin, the same as in Example 3.

[0102] (2) Blow molding of the plastic film

[0103] Take the above-mentioned co-extruded upper-layer resin after plasticizing modification and add it to the hopper of the innermost extruder of the multi-layer co-extrusion blown film device; add the co-extruded lower-layer modified resin to the hopper of the outer extruder of the film bubble; do not add materials to other extruder hoppers (for co-extrusion devices with more than 2 layers); set the heating temperatures of the four sections of the two screws to 160 °C, 170 °C, 180 °C, and 180 °C respectively, the extrusion die head temperature to 180 °C, set the speeds of the two screws to 8 and 5 rpm respectively, cool with air, and the longitudinal draw ratio of the traction is 6, and the transverse blow-up ratio is 4, to obtain a plastic film with upper and lower layer thicknesses of 5 microns each and a total thickness of 10 microns.

[0104] Example 8

[0105] Preparation of a plastic film with a co-extruded upper layer thickness of 5 microns and a co-extruded lower layer thickness of 10 microns

[0106] (1) Swelling and melt blending of the co-extruded upper-layer resin and the lower-layer resin, the same as in Example 3.

[0107] (2) Blow molding of the plastic film

[0108] The above-mentioned plasticized and modified co-extruded upper-layer resin was added to the hopper of the innermost extruder of the multi-layer co-extrusion blown film device; the modified co-extruded lower-layer resin was added to the hopper of the outer extruder of the film bubble; no materials were added to the hoppers of other extruders (for co-extrusion devices with more than two layers); the four-stage heating temperatures of the two screws were set at 160 °C, 170 °C, 180 °C and 180 °C respectively, the extrusion die head temperature was 180 °C, the rotational speeds of the two screws were set at 9 and 12 rpm respectively, air cooling was used, the longitudinal draw ratio of the obtained film was 6.5, and the transverse blow-up ratio was 5, obtaining a plastic film with upper and lower layer thicknesses of 5 and 10 microns respectively and a total thickness of 15 microns.

[0109] Comparative Example 1

[0110] A polyethylene modified plastic film containing 20% starch purchased on the market, with a thickness of 15 microns

[0111] Comparative Example 2

[0112] A pure polyethylene plastic film purchased on the market, with a thickness of 10 microns

[0113] It was found from the infrared spectrum in Figure 1 that the position of the hydroxyl peak of the unmodified pure PVA film was at 3357 cm -1 , while the hydroxyl peaks of the plastic films of Example 3, Example 4 and Example 5 after plasticizing modification were located at 3348 cm -1 , 3343 cm -1 and 3334 cm -1 respectively. Compared with unmodified PVA, the position of the hydroxyl peak of the plastic film shifted towards lower wavenumbers, indicating that the hydroxyl groups on the composite plasticizer formed new hydrogen bonds with the hydroxyl groups on the molecular chain of polyvinyl alcohol, weakening the force between the oxygen atom and the hydrogen atom on the hydroxyl group of the polyvinyl alcohol molecular chain.

[0114] It was found from the scanning electron microscope photograph of the cross-section in Figure 2 that there were many dimples and shear bands on the cross-section of the plastic film prepared in Example 3, indicating that the plastic film had good toughness and non-brittle fracture. In addition, no solid particles of sodium oleate were found, nor were there any bubble traces, indicating that sodium oleate with a relatively high melting temperature had been well dispersed in the PVOH matrix with the help of plasticizers such as glycerol and pentaerythritol, and glycerol and pentaerythritol did not vaporize during the melt blending and blown film processing, and the prepared PVOH plasticized plastic film had a uniform texture and no defects in the aggregate structure.

[0115] From the X-ray diffraction analysis in Figure 3, the intensity of the crystallization peak of pure PVOH was large, the crystallinity was relatively high, and the grain size was also relatively large; while the diffraction intensity of the crystallization peak of the plastic film in Example 3 decreased, indicating that the composite plasticizer maximally destroyed the crystallization of PVOH, enabling melt blending and blown film to be achieved at 180 °C. The appropriate crystallinity enabled the plastic film to have a certain water resistance while being able to finally dissolve in cold water and enter the soil.

[0116] Table 1 lists the water-soluble time of the plastic film at 50 degrees and its mechanical properties; it can be seen from the test results that the plastic film involved in this patent not only has excellent mechanical properties but also has degradable properties compared with starch-modified polyethylene plastic film and pure polyethylene plastic film, and the degradation degree is greater than 76% within 180 days. After the plastic film dissolves and enters the soil, it can increase the pH value of the soil, neutralize the acidity in the soil, retain the moisture in the soil, reduce water evaporation, and has the effect of soil moisture conservation. In addition, the dissolved substances of the plastic film can also retain ions such as calcium, magnesium, aluminum, and manganese in the soil and reduce the loss of these ions with rainwater or irrigation water.

[0117] Those skilled in the art should understand that the above preferred embodiments are only specific descriptions of the present invention and do not constitute limitations on the present invention. Any changes and equivalent substitutions made to the technical solutions of the present invention without departing from the principles and scope of the technical solutions of the present invention should be covered within the scope of the claims of the present invention.

[0118] Performance Analysis of Modified PVOH Films in Table 1

[0119] Infrared Comparison between Pure PVOH and the Films of the Examples in Figure 1

[0120] Microstructure of the Cross-section of the Mulch Film in Example 3 in Figure 2

[0121] X-ray Diffraction Analysis of the Film in Example 3 in Figure 3

[0122] Properties of Plasticized Modified PVOH Plastic Films in Table 1

[0123]

Claims

1. A plastic film prepared from modified polyvinyl alcohol, characterized in that, It is a two-layer co-extruded film, and both the upper and lower layers are obtained by extruding a blend of polyvinyl alcohol of grades 1799 and 1788 and a composite plasticizer. In the upper layer component, the weight ratio of polyvinyl alcohol of grades 1799 and 1788 is 90:10 to 70:30, and the total weight of polyvinyl alcohol of grades 1799 and 1788 is 100 parts. The percentages of each component in the composite plasticizer accounting for the total weight of polyvinyl alcohol are: glycerol 10 - 20 parts, pentaerythritol 5 - 10 parts, sodium oleate 5 - 10 parts, zinc stearate 2 - 5 parts, and diethanolamide oleate 2 - 5 parts. In the lower layer component, the weight ratio of polyvinyl alcohol of grades 1799 and 1788 is 5:95 to 30:70, and the total weight of polyvinyl alcohol of grades 1799 and 1788 is 100 parts. The percentages of each component in the composite plasticizer accounting for the total weight of polyvinyl alcohol are: glycerol 10 - 20 parts, pentaerythritol 5 - 10 parts, sodium oleate 15 - 25 parts, zinc stearate 2 - 5 parts, and diethanolamide oleate 5 - 15 parts. The preparation method includes the following steps: After mechanically blending powdered 1799 and 1788, first add 2 / 3 of the total amount of glycerol plasticizer, then seal it airtight at room temperature for 24 hours, then add the mixture of the remaining 1 / 3 of the glycerol plasticizer and other plasticizers, fully mix, and then seal it airtight at 50°C for 24 hours; the co-extruded upper and lower layer resin mixtures are melt-blended with a twin-screw extruder, the temperature is set between 160 - 180°C, and the screw speed is 5 - 15 rpm.

2. A ground film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the upper layer, the weight ratio of polyvinyl alcohol of grades 1799 and 1788 is 85:15 to 75:

25.

3. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the upper layer, the weight ratio of polyvinyl alcohol of grades 1799 and 1788 is 80:

20.

4. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the upper layer component, the percentages of each component in the composite plasticizer accounting for the total weight of polyvinyl alcohol are: glycerol 13 - 18 parts, pentaerythritol 6 - 9 parts, sodium oleate 6 - 9 parts, zinc stearate 2.5 - 4 parts, and diethanolamide oleate 2.5 - 4 parts.

5. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the upper layer component, the percentages of each component in the composite plasticizer accounting for the total weight of polyvinyl alcohol are: glycerol 15 parts, pentaerythritol 8 parts, sodium oleate 8 parts, zinc stearate 3 parts, and diethanolamide oleate 3 parts.

6. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the lower layer, the weight ratio of polyvinyl alcohol of grades 1799 and 1788 is 10:90 to 20:

80.

7. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the lower layer, the weight ratio of polyvinyl alcohol of grades 1799 and 1788 is 15:

85.

8. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the lower layer component, the percentages of each component in the composite plasticizer accounting for the total weight of polyvinyl alcohol are: glycerol 12 - 18 parts, pentaerythritol 6 - 9 parts, sodium oleate 17 - 22 parts, zinc stearate 2.5 - 4 parts, and diethanolamide oleate 7 - 13 parts.

9. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, In the lower layer component, the percentages of each component in the composite plasticizer accounting for the total weight of polyvinyl alcohol are: glycerol 15 parts, pentaerythritol 8 parts, sodium oleate 20 parts, zinc stearate 3 parts, and diethanolamide oleate 10 parts.

10. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, The total thickness of the film is 20 - 50 microns, and the thickness of the co-extruded upper layer accounts for 10 - 50% of the total thickness.

11. A plastic film prepared from modified polyvinyl alcohol according to claim 1, characterized in that, It is prepared by using a multi-layer co-extrusion blown film equipment, wherein the innermost screw of the extruder extrudes the outer layer of the plastic film, so that the outer layer of the plastic film is located in the innermost layer of the blown film bubble; the co-extruded inner layer material is added to the screws of other layers of the extruder, and the inner layer of the extruded plastic film is located on the outer side of the bubble.

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