• ISSN 1008-505X
  • CN 11-3996/S

添加纳米TiO2改善淀粉基聚酯聚氨酯复合材料的膜层质量和养分控释特性

董晶晶, 田虹雨, 石若琳, 杨云博, 武良, 房福力, 张民, 刘之广

董晶晶, 田虹雨, 石若琳, 杨云博, 武良, 房福力, 张民, 刘之广. 添加纳米TiO2改善淀粉基聚酯聚氨酯复合材料的膜层质量和养分控释特性[J]. 植物营养与肥料学报, 2023, 29(7): 1367-1378. DOI: 10.11674/zwyf.2022592
引用本文: 董晶晶, 田虹雨, 石若琳, 杨云博, 武良, 房福力, 张民, 刘之广. 添加纳米TiO2改善淀粉基聚酯聚氨酯复合材料的膜层质量和养分控释特性[J]. 植物营养与肥料学报, 2023, 29(7): 1367-1378. DOI: 10.11674/zwyf.2022592
DONG Jing-jing, TIAN Hong-yu, SHI Ruo-lin, YANG Yun-bo, WU Liang, FANG Fu-li, ZHANG Min, LIU Zhi-guang. Nano TiO2 addition improves the coating quality of starch based/polyester polyurethane and nutrient release characteristics of the coated urea[J]. Journal of Plant Nutrition and Fertilizers, 2023, 29(7): 1367-1378. DOI: 10.11674/zwyf.2022592
Citation: DONG Jing-jing, TIAN Hong-yu, SHI Ruo-lin, YANG Yun-bo, WU Liang, FANG Fu-li, ZHANG Min, LIU Zhi-guang. Nano TiO2 addition improves the coating quality of starch based/polyester polyurethane and nutrient release characteristics of the coated urea[J]. Journal of Plant Nutrition and Fertilizers, 2023, 29(7): 1367-1378. DOI: 10.11674/zwyf.2022592

添加纳米TiO2改善淀粉基聚酯聚氨酯复合材料的膜层质量和养分控释特性

基金项目: 国家重点研发计划项目(2022YFD1700603);山东省重点研发计划项目(2022SFGC0301);辽宁省2021年揭榜挂帅课题黑土地旱田地力提升关键技术示范与推广项目(2021JH1/10400039-2)。
详细信息
    作者简介:

    董晶晶 E-mail: dongjingjing1030@163.com

    通讯作者:

    田虹雨 E-mail: thy0634@qq.com

Nano TiO2 addition improves the coating quality of starch based/polyester polyurethane and nutrient release characteristics of the coated urea

  • 摘要:
    目的 

    淀粉基聚氨酯包膜缓控释肥施入土壤后,膜壳降解时间短,环境风险低,但因亲水性强、杂质多、致密性差等特点也降低了养分的控释性能,无法满足作物全生育期养分需求。我们优化了淀粉基聚氨酯膜材构成比例,并从膜材化学结构和膜壳物理性状角度,研究了该包膜工艺改善复合包膜尿素养分控释性能的机理。

    方法 

    本研究设计了以聚烯烃蜡为底涂层,淀粉基聚氨酯(BPU)为内涂层,纳米TiO2改性聚酯聚氨酯(PPU)为外涂层制备复合包膜尿素的工艺流程。采用响应曲面法设计了BPU、PPU、TiO2添加量三因素组合的20个处理,建立了养分初期释放率、释放期对BPU、PPU、TiO2添加量的响应模型;利用电镜扫描、原子力显微镜、傅里叶红外光谱、气相色谱/质谱,探究复合膜材形态与结构特征;通过热失重、Zeta电位、静水浸提方法,评价了膜材稳定性与养分释放动力学特征;利用土壤埋袋试验,研究了膜壳降解性能。

    结果 

    1)较BPU膜材,复合膜材(NBPU)在中性条件下Zeta电位绝对值提高34.8%,添加纳米TiO2后进一步提高了26.5%~64.6%,提升了其物理稳定性;膜壳在土壤中自然降解365天后,添加1.00%纳米TiO2的聚酯聚氨酯失重率为4.92%,与未添加TiO2的相比显著提高了3.3倍。2)复合膜材表面孔隙少、光滑平整,切面致密均匀;颗粒硬度为85.2 N,较BPU膜材提高了37.0%,膜表粗糙度降低了71.3%,肥料力学与耐磨性能均有所改善;BPU对前期养分释放具有显著调节作用,PPU添加量3%以内时添加量与养分释放期成正比,纳米TiO2则对3~28天的养分释放速度有显著影响。3)通过响应曲面法分别建立养分释放期、初期释放率与BPU、PPU、TiO2添加量的二次多项式数学模型(P<0.01),可用该模型对相应的指标进行分析和预测;明确设计变量对养分释放期的贡献度为PPU>BPU>TiO2,对初期释放率的贡献度为TiO2>BPU>PPU。

    结论 

    纳米TiO2可提高聚氨酯膜材中的微相均匀性,降低肥料养分初期释放率,且其光催化性可提高养分释放后残膜在土壤中的降解率;复合包衣可提升膜材结构稳定性,通过调控PPU、BPU和TiO2添加比例实现养分释放期调控;利用曲面中心设计法建立的肥料养分释放特征响应模型可为实际生产提供技术参考。

    Abstract:
    Objectives 

    Starch based polyurethane, a type of coating materials for controlled-release fertilizer production, is easy to degradation after entering soil, so results in low environmental risk. However, its strong hydrophilicity, high impurities and poor densification reduce the controlled release performance of nutrients. We optimized the composition ratio of starch based polyurethane with other additives from the chemical structure and physical properties of the coating materials.

    Methods 

    The composite coating materials (NBPU) were composed of starch-based polyurethane (BPU) and nano-TiO2-modified polyester polyurethane (PPU). The coating procession included three steps, i.e. polyolefin wax as the base coating, BPU as the internal control layer, and PPU as the external control layer. Total 20 combination schemes of BPU, PPU and TiO2 were designed using the response surface method, and the response models of initial nutrient release rate and release period to the BPU, PPU and TiO2 ratio were established. Electron microscope scanning, atomic force microscopy, Fourier infrared spectroscopy, and GCMS were used to investigate the morphology and chemical structures of the composite coating materials. Thermogravimetric, Zeta potential and hydrostatic extraction method were used to evaluate the stability and nutrient release kinetics. Soil embedding bag test was used to evaluate the degradation performance of coating materials.

    Results 

    1) Compared with BPU, the absolute Zeta potential of designed NBPU increased by 34.8% under neutral condition, and that was further increased by 26.5%−64.6% through addition of nano-TiO2, indicating the improvement in physical stability of the NBPU After 365 days of natural degradation in soil, the weight loss rate of polyester polyurethane with 1.00% TiO2 increased to 4.92%, which was 3.3 times higher than that without 1.00% TiO2. 2) The surface of the composite coating layer was smooth and flat, with few pores, and the section surface was compact and uniform. The particle hardness of PPU was 85.2 N, which was 37.0% higher than that of BPU, the surface roughness was reduced by 71.3%, and the fertilizer mechanics and wear resistance were improved. BPU played a significant role in regulating the early stage of nutrient release. When the added amount of PPU was less than 3%, the nutrient release period was gradually extended with the increase of the added amount. Nano TiO2 had a significant effect on the nutrient release rate of 3−28 days. 3) Response surface method was used to establish quadratic polynomial mathematical models of nutrient release period, initial release rate and addition amount of BPU, PPU and TiO2 (P<0.01), the model could be used to analyze and predict the corresponding indicators. The contribution of design variables to nutrient release period was determined as PPU>BPU>TiO2, the contribution to the initial release rate was TiO2>BPU>PPU.

    Conclusion 

    Nano TiO2 can improve the microphase uniformity of polyurethane coating materials and reduce the initial release rate of fertilizer nutrients, and its photocatalysis can improve the degradation rate of residual coating materials in soil after nutrient release. The composite coating can improve the structural stability of the coating materials, and the nutrient release period can be controlled by adjusting the addition ratio of PPU, BPU and TiO2. The characteristic response model of fertilizer nutrient release based on curved surface center design can provide technical reference for practical production.

  • 化肥在确保粮食安全中具有不可忽视的作用,但不合理施肥致使我国粮食作物化肥利用率仅为40.2%。过量的氮肥通过径流和淋洗作用进入河流、湖泊导致水体富营养化[12]。NH4+与土壤中的盐基离子发生置换和淋洗导致土壤酸化[3],降低土壤功能微生物群落多样性,扰乱土壤微生物种群功能和结构特性、降低共发生网络稳定性[4],导致氨基酸、糖类在作物体内过量存积,使作物的抗病虫害能力自然减弱,易遭病虫侵染,最终导致作物营养吸收失衡,品质、产量降低,造成食品安全问题[5]

    包膜控释肥能降低养分释放速度,提高养分利用率、改善土壤环境。其作用机理为外界水分子渗透到膜内,养分溶解后渗透压增大,在压力作用下养分渗透到外部供作物吸收[6]。膜材是限制养分释放的重要因素[7],前人多关注性质稳定的聚乙烯、环氧树脂、醇酸树脂等。然而聚乙烯膜材制备过程中需添加反应溶剂且采用的立式流化床包膜工艺能耗较高,成膜后仍存在少量孔隙[8]。环氧树脂的环氧基可与多种含有活泼氢的化合物交联固化形成三维网状结构,这种结构使材料质脆易裂,抗冲击损伤性差,限制了环氧树脂的应用和发展[9]。醇酸树脂膜材虽综合性能好,但其生产原料、助剂的成本较高,且反应在溶剂中进行,在行业中使用较少[10]

    聚氨酯膜材由多元醇(软段)和多异氰酸酯(硬段)在颗粒表面原位反应形成[11]。由于制备简单、易于成型、养分释放稳定,是目前的主流膜材[12]。但分子链长、结构复杂,在自然界中降解极慢,存在一定的土壤污染风险[13]。使用易降解的生物质材料制备绿色膜材是当前的发展趋势,这类膜材主要由木质素、纤维素、淀粉等农业废弃物为原料制备生物基多元醇[14],再与多异氰酸酯聚合而成。其中,淀粉因来源广泛、成本低,是主流液化原料,但淀粉基聚氨酯的水性基团较多[15],最大养分控释天数仅14天[16],达不到缓释肥料国家标准(GB/T 23348—2009)要求。

    研究膜材改性技术,优化包衣模式,开发绿色环保、成本低廉、控释性能优异的膜材成为研究热点。Tian等[17]利用聚烯烃蜡、生物基聚氨酯、环氧树脂复合包衣实现了养分精准控制,但外控层环氧树脂刚性强、降解性能差、粘度大不利于产业化应用[18]。Khalina等[19]将交联淀粉作为内涂层,丙烯酸和丙烯酰胺作为外涂层制备包膜肥料,可以显著延缓水分进入肥料,但是丙烯酸的降解性差且丙烯酰胺存在生物毒性,具有一定的安全风险。

    采用天然高分子材料制备生物基聚氨酯,结合纳米粒子改性技术制备复合包衣,是解决现存问题的基本思路。本研究将淀粉液化制备生物基聚氨酯作为内控层,利用化学性质稳定、具有光催化性能的纳米TiO2[20]对聚酯聚氨酯进行改性作为外控层,制备出纳米TiO2/淀粉基/聚酯聚氨酯复合包膜尿素并探究其膜层及养分释放特性,利用响应曲面中心设计法探究复合包膜工艺中不同膜材用量对养分初期释放率和控释期的贡献度,为生产提供技术参考。

    试验材料:玉米淀粉,纳米TiO2 (50 nm,上海阿拉丁),聚烯烃蜡(天津凯通),硫酸(97%,分析纯,天津凯通),二甘醇(分析纯,天津凯通),多亚甲基多苯基多异氰酸酯(PAPI,质量分数31.1% –NCO,烟台万华),二氯甲烷(色谱纯,上海阿拉丁),聚酯多元醇(上海玉高),尿素(直径3~5 mm,N 46.4%,山东华鲁恒升)。

    供试设备:包膜机(yb-600,鸿泰机械),天平(YP30001B,力辰科技),水浴锅(HH-2,力辰科技),双层玻璃反应釜(GR-30,上海科圣),油浴锅(HH-4,力辰科技)。

    向三颈烧瓶中添加113.60 mL二甘醇、4.07 mL硫酸,与冷凝器、温度计、搅拌器相连。冷凝回流,加热至120℃后加入15 g淀粉,在800 r/min的转速下回流90 min获得淀粉基多元醇。将淀粉基多元醇与PAPI按35%∶65% (w/w)混合,反应生成淀粉基聚氨酯(BPU)。

    聚酯多元醇与PAPI按1:1 (w/w)混合反应制得聚酯聚氨酯 (PPU)。

    通过 Design Expert V8 软件,将BPU用量范围设置为尿素质量的1.66%~3.00%,TiO2添加量为外控层PPU质量的0.16%~1.84%,PPU用量为尿素质量的1.66%~3.34%,采用Central Composite Design中心复合设计法,基于二次多项式模型自动生成20组处理方案(表1)。

    表  1  基于响应曲面法的复合包膜材料用量设计(%)
    Table  1.  Coating materials design based on Response Surface Methodology
    序号
    Code
    淀粉基聚氨酯 (BPU)
    Starch-based polyurethane
    TiO2聚酯聚氨酯(PPU)
    Polyester polyurethane
    12.000.502.00
    23.000.502.00
    32.000.503.00
    43.000.503.00
    52.001.502.00
    63.001.502.00
    72.001.503.00
    83.001.503.00
    91.661.002.50
    103.341.002.50
    112.501.001.66
    122.501.003.34
    132.500.162.50
    142.501.842.50
    152.501.002.50
    162.501.002.50
    172.501.002.50
    182.501.002.50
    192.501.002.50
    202.501.002.50
    下载: 导出CSV 
    | 显示表格

    制备包括底涂层、内控层和外控层3个过程:

    1)底涂层:向包衣转鼓中加入1.0 kg尿素,转鼓转速设置为45 r/min,在(80±2)℃下加热10 min,加入5.0 g聚烯烃蜡,反应5~10 min形成尿素底涂层,用于后续不同内控层和外控层包膜尿素的制备。

    2)内控层:按照表1中BPU 的添加量,分两次喷涂至尿素表面

    3)外控层:将喷涂了内控层的尿素,按照表1中的用量将纳米TiO2和PPU混合,分两次喷涂制成肥料外控层。

    以上方法共制备了20组复合包膜尿素样品。

    对照样品制备包膜量为5%的无TiO2包膜尿素 (0% TiO2-NBPU)、BPU包膜尿素 和PPU包膜尿素。制备方法为:在底涂层尿素上,按照2)、3)条件和步骤,制备内涂层量2.5% BPU和外涂层量2.5%的无TiO2的PPU对照样品;制备包膜量5% 的BPU包膜尿素和PPU包膜尿素按照2)的条件和方法,在涂底层尿素外分别喷涂BPU和PPU。图1为制备的3个对照样品和3个用于后续研究的处理样品。

    图  1  不同包膜方案制备的包膜尿素
    注: PPU—聚酯聚氨酯单一包膜尿素,BPU—淀粉基聚氨酯单一包膜尿素,0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指添加了TiO2 0%、0.16%、1.00% 和1.84% 的复合膜材NBPU,其中,BPU用量均为2.5%。
    Figure  1.  Coating-urea prepared with different coating designs
    Note: BPU and PPU are starch based polyurethane coating materials and polyester polyurethane coating materials. 0%TiO2-NBPU, 0.16% TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating materials with 0%, 0.16%, 1.00% and 1.84% TiO2 added to polyester polyurethane, among them, the amount of BPU is 2.5%.

    将BPU、PPU和NBPU包膜的尿素分别称取100 g装入0.074 mm尼龙网袋,每个样品重复3次,于2019年10月埋入山东省泰安市山东农业大学南校区试验基地(36°9′40″N、117°9′48″E)的耕层土壤,土壤类型为棕壤,在“中国土壤系统分类”中为普通简育湿润淋溶土。供试地点属暖温带大陆性季风型气候,年均气温13℃,年均降雨量13.2 mm。网袋间距为10 cm。埋入365天后(2020年10月)回收样品,用去离子水冲洗干净后,在60℃烘箱中烘干至恒重,测定膜壳失重率。

    使用红外光谱仪(Nicolet-380)分析淀粉、膜材的傅里叶红外光谱。使用扫描电镜(JSM-5800,SEM)观察膜表及断面的微观形貌。利用原子力显微镜(Bruker AXS)测定膜表的起伏状况。将包膜尿素浸泡在去离子水中,待养分释放完全后,洗除肥料膜壳表面残存的养分,烘干磨碎后取5.0 mg在5.0 mL二氯甲烷溶液中浸泡24 h,采用气相色谱/质谱法检测膜中组分。称取5 mg膜壳浸泡于5 mmol/L氯化钠溶液中静置12 h,过0.22 μm滤膜后,用Zeta电位分析仪(Zetasizer Nano ZS)测量其Zeta电位。

    使用热重分析仪(DTG-60A,日本岛津)评估包膜尿素膜壳的热稳定性,取5~10 mg膜壳,放入内径为6 mm的氧化铝坩埚中。在50 mL/min氮气氛围保护下,以10℃/min的升温速率从25℃升高至800℃。

    将10 g包衣肥料放入装有200 mL去离子水的塑料瓶中,放置在(25±0.5)℃的恒温培养箱中,利用折光率仪(RX-5000α,ATAGO)定期测定溶液的折光率,并根据折光率−氮素曲线,折算释放量,直至氮累计释放量超过80%[2122]

    图2显示了膜材原材料淀粉(Starch)、淀粉基多元醇(LS)和异氰酸酯(PAPI)及6组膜材的傅里叶红外光谱。从单个原料(图2 左)看,淀粉主要是由葡萄糖分子聚合而成,在2920 cm−1出现的中等强度的峰与−CH2的反对称伸缩振动有关,1157 cm−1附近出现的吸收峰归属为C−O及C−C键的伸缩振动[23]。在3315 cm−1出现较弱的−OH的伸缩振动峰。淀粉液化制备为多元醇后(图2-LS),在1218和 1123 cm−1处分别出现了C−O−C和C−O键伸缩振动的吸收峰,在2870 cm−1处出现甲基的伸缩振动峰[24],3345 cm−1处峰值增大且波形变宽的吸收峰与O−H伸缩振动带有关,表明淀粉中的醚键在硫酸作用下发生了水解,产生大量的羟基,有利于聚氨酯的合成[16]。在异氰酸酯(图2 左 PAPI)中,1107 cm−1 的吸收峰与C−O 键的伸缩有关,2240 cm−1附近的强吸收峰与−N=C=O 键有关[17],这是 PAPI参与成膜的关键基团。而成膜后(图2右),PAPI的−N=C=O 键消失,因为PAPI中的−N=C=O键已与多元醇中的−OH反应固化为氨基甲酸酯键。此外,PAPI中的1408,1507和1595 cm−1峰与芳香族化合物C=C的伸缩振动有关[25],而反应完成后膜材中也出现了相同的峰,说明PAPI中的苯环成功引入到BPU中,膜材的反应完成。成膜后(图2 右),含有液化淀粉的膜材(BPU和NBPU)均在2932~2800 cm−1范围出现了强烈的甲基和亚甲基中C−H伸缩振动,证实液化淀粉中的C−H键已接入膜材中。对于TiO2改性膜材,由于TiO2的吸收谱带通常在中红外的低频区和远红外区,吸收峰位置(729 cm−1)与−CH2的面内摇摆距离较近,因此在FTIR谱图中难以分辨。

    图  2  包膜原料(左)和不同工艺包膜材料(右)的傅里叶红外光谱图
    注:Starch—玉米淀粉;LS—淀粉基多元醇,PAPI—多亚甲基多苯基多异氰酸酯。BPU指尿素内涂层材料淀粉基聚氨酯,PPU为尿素外涂层材料聚酯聚氨酯。NBPU指内涂层为BPU、外涂层为PPU的复合包膜尿素,0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指外涂层膜材PPU中添加了0%、0.16%、1.00% 和1.84% TiO2的复合包膜尿素。
    Figure  2.  Fourier infrared spectra of coating raw materials (left) and prepared composite coating materials (right)
    Note: Starch—Corn starch; LS—Starch-based polyols. BPU is starch based polyurethane used for the inner coating of urea, PPU is polyester polyurethane used for the outer coating of urea, NBPU is the composite coating urea with BPU and PPU for inner and outer coating materials, 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers.

    以二氯甲烷浸提TiO2添加量为0.16%、1.00%、1.84%的3个复合膜材,浸提液中测定出了异氰酸酯单体(24.5 min,图3),这可能是−NCO过量导致的。聚氨酯是一种由硬段和软段交替组成的聚合物,多元醇属于软段,有助于提高聚氨酯的柔性,异氰酸酯属于硬段,决定了聚氨酯的硬度[26]。液化产物含有多元醇、微量水等消耗异氰酸酯的氢化物,当异氰酸根过剩时,形成的膜材表面光滑成膜效果更好[16]

    图  3  添加不同TiO2比例复合膜材的GC–MS谱图
    Figure  3.  GC-MS spectra of composite coating materials containing different TiO2 ratios

    图3还可看出,TiO2添加量为0.16%、1.00%、1.84%的3个复合膜材浸提液的主峰保留时间基本相同,表明TiO2添加量对复合膜材的浸出物组成没有显著影响。浸出物中主要为长链烷烃,如对环芳烷(30 min)、二十八烷(23.60 min、26.79 min)和三十烷(34 min),还有2,2'-氧基双乙醇(6.8 min)。随着纳米TiO2的增加,在29.2 min、30.7 min、35.3 min处产生新峰(长链烷烃)且30~35 min范围内的峰值升高,表明纳米TiO2受到激发后从价带跃迁到导带中,形成了具有强还原能力的光生电子和强氧化能力的光生空穴,聚氨酯碎片在这种强氧化还原作用下分解形成大量长链烷烃[27],加快了膜材的降解。因此,TiO2的添加主要作用是促进了复合膜材的降解。

    图4为不同膜材制备的包膜尿素表面和横截面电镜扫描图。放大50倍时,BPU包膜尿素表面粗糙不平(a1),放大至100倍时,可观察到BPU膜材表面存在大量孔隙(a2),100倍截面存在许多重叠和裂纹(a4)。可能是因为液化水解过程会产生少量的水,与异氰酸酯结合分解产生二氧化碳气体,致使膜表凹凸不平,膜壳与肥料之间的边界不明显,这些存在的重叠和裂纹增加了水分子与膜壳的接触,因此养分释放速度加快[28]

    图  4  不同材料包膜尿素表面与截面SEM图
    注:a,BPU单一包膜尿素;b,PPU单一包膜尿素;c,BPU和PPU复合包膜尿素;d,BPU和1.00%TiO2-PPU复合包膜尿素。 1、2图分别为放大50倍、100倍包膜尿素表面图,3、4分别为放大50倍、100倍包膜尿素截面图。
    Figure  4.  Electron scanning morphologies of surface and cross section of urea with different coating materials
    Note: a, BPU single coating urea; b, PPU single coating urea; c, BPU and PPU composite coating urea; d, BPU and 1.00%TiO2-PPU composite coating urea. The 1 and 2 were the surface images under 50 and 100 fold magnification, and 3 and 4 were the cross section images under 50 and 100 fold magnification, respectively.

    放大50倍的PPU包膜尿素表面光滑平整(b1),放大100倍后也未见孔隙(b2),100倍截面(b4)致密紧实没有缝隙。这有利于阻止水分子的渗透,使养分从膜内缓慢释放[12]

    相较于以上两种膜材,复合膜材NBPU和1.00% TiO2-NBPU放大50倍后的表面光滑、质地均匀(c1、d1),放大100倍的截面存在明显的分层结构(c4、d4)。这是因为喷涂的聚烯烃蜡填充了核芯颗粒表面的孔隙,促进了外部聚氨酯与核芯颗粒的粘合。本研究中最外层的PPU能够很好的填补内层BPU成膜时产生的孔隙,形成光滑平整的表面,减缓水分子的渗透。

    聚氨酯的稳定性受形貌和两相间分离程度的影响,粗糙度值与微相分离程度呈正比[29]。BPU和PPU的粗糙度分别为34.8、6.75 nm,因而PPU膜材表面较为光滑,而复合包膜材料的粗糙度为10.0 nm,较BPU显著降低了71.3%,加TiO2后膜表面粗糙度(11.3 nm)变化不大(图5)。与前人研究[30]不同的是,在复合膜材的表面未观察到纳米凸起,这是因为所添加的纳米TiO2分散在包膜液中,在包膜过程中,随着颗粒间或颗粒与转鼓内壁挤压,纳米粒子嵌入膜内或形成的纳米凸起被磨平,从而使改性膜材表面依然平整。

    图  5  原子力显微镜下的膜壳表面形貌图 (A) 和相位图 (B)
    注: BPU—淀粉基聚氨酯膜壳,PPU—聚酯聚氨酯膜壳,0%TiO2-NBPU、1.00%TiO2-NBPU分别指外膜层材料中添加了0%、1.00% TiO2的复合膜壳。
    Figure  5.  Morphology (A) and micro-phase diagram (B) of coating crusts under atomic force microscope
    Note: BPU and PPU are the coating materials of starch based polyurethane and polyester polyurethane. 0%TiO2-NBPU, 1.00%TiO2-NBPU refer to the coating crusts of polyester polyurethane containing 0% and 1.00% TiO2.

    相位图显示了膜材的微相分离程度(图5B)。聚酯多元醇由于粘度高、分子量大、官能度低导致硬段与硬段间通过氢键作用形成硬段微相区[31],复合包膜改善了PPU的相分离程度,而TiO2也使微相分离更加均匀。均匀的微相能够抵御水分子进入聚氨酯膜层,避免无定形硬段相微区的瓦解[32],能够控制养分缓慢释放。

    对膜材热稳定性的测试结果表明,随温度的升高,BPU膜壳的热分解分3个阶段(图6),在190℃~200℃主要是内部残留水减重,250℃~400℃出现因BPU膜材中的聚合物分解产生的失重主峰[17],此时失重率为45%,400℃~580℃的失重主要是灰分形成及残余共价键分解,此时失重率约为5%。

    图  6  不同膜材的热失重曲线
    注: a,膜材耐受温度(25℃~100℃)热失重图;b,25℃~800℃热失重图;c,25℃~800℃热失重速率图。0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指复合包膜尿素外涂层材料中添加了0%、0.16%、1.00% 和1.84% 的TiO2。BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素。
    Figure  6.  Thermal weight loss curves of coating materials
    Note: a is the thermogravimetric diagram of the production temperature tolerance (25℃−100℃), b is the thermogravimetric diagram of 25℃−800℃, and C is the diagram of the thermogravimetric loss rate of 25℃−800℃. 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers. BPU and PPU refer to the urea only coated with starch based polyurethane and polyester polyurethane, respectively.

    25℃~100℃是膜材形成过程的耐受温度,在此范围内BPU热稳定性最差,100℃内分解率1.07%,而PPU热稳定性较好,100℃分解率为0.72%,复合包膜较BPU提高了膜材的热稳定性。而添加TiO2后进一步提高了膜材的热稳定性。添加0.16%、1.00%和1.84% TiO2复合膜材失重5%的温度(Td5%)分别为218℃、230℃和243℃,说明纳米TiO2粒子含量的增加能够改善膜材生产环境(<100℃)下的热稳定性。可能是因为添加的纳米材料在膜材表面均匀分布,减缓聚氨酯软段与硬段的运动与断裂[33],降低聚氨酯的热分解速率。

    当官能团在表面上解离或离子从溶液吸附到表面上时产生表面电荷形成Zeta电位,Zeta电位绝对值越大,粒子表面电荷密度越大,粒子间静电作用越强,颗粒越难聚集,混合体系越稳定[34]。结果表明,膜材的Zeta电位在pH 3~5范围内变化较小(图7)。随pH增大,膜材的Zeta电位绝对值升高。这可能是由于膜壳内存在未反应完全的异氰酸根基团和羧基脱质子化产生负电荷,在酸性条件下这些负电荷与H+中和使膜材的电位性较低。

    图  7  不同pH环境下包膜尿素的Zeta电位
    注: 0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指复合包膜尿素外涂层材料中添加了0%、0.16%、1.00% 和1.84% 的TiO2。BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素。
    Figure  7.  Zeta potential of coated urea under different environmental pH
    Note: 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers. BPU and PPU refer to the urea only coated with starch based polyurethane and polyester polyurethane, respectively.

    pH=7的条件下BPU电负性最低(−13.44 mV),NBPU电负性(−18.10 mV)与PPU (−19.00 mV)类似,较BPU稳定性提高34.8%,而添加TiO2后膜材的悬浮稳定性较单一包膜和复合包膜进一步提高,说明TiO2提高了复合膜材的稳定性。TiO2添加量分别为0.16%、1.00%时,膜材电位分别为−22.9、−29.8 mV,较复合膜材NBPU分别提高26.5%和64.6%。

    在5%的包膜厚度下,BPU (图8)硬度最低(62.2 N),因为淀粉基多元醇内存在部分杂质,膜材的聚合程度较差,膜材内部存在大量的折叠和孔隙(图4a),导致颗粒耐磨性和抗压性较差,养分释放速率过快;PPU由于膜材致密紧实(图4 b),其包膜尿素硬度达103.3 N,硬度过高会影响养分溶解速度,使得养分释放缓慢影响作物施肥效果[17]。复合包膜尿素的硬度为85.2 N,较BPU高37.0%,较PPU低17.5%,包膜颗粒韧性和硬度适中,减少生产和运输过程中的磨损和挤出。

    图  8  不同TiO2添加量的复合包膜尿素颗粒压力强度
    注:0% TiO2-NBPU、0.16% TiO2-NBPU、1.00% TiO2-NBPU、1.84% TiO2-NBPU分别指外层膜材中添加了0%、0.16%、1.00% 和1.84% TiO2的复合包膜尿素。BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素。柱上不同小写字母表示不同处理间在0.05水平上差异显著。
    Figure  8.  Particle pressure intensity of composite coating urea containing different TiO2 ratios in the outer coating layers
    Note: BPU and PPU are urea only coated by starch based polyurethane and polyester polyurethane, respectively. 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers. Different letters above the bars indicate significant difference among treatments at the 0.05 level.

    TiO2在膜材中的添加比例也影响包膜尿素的硬度。0.16% TiO2-NBPU、1.00% TiO2-NBPU和1.84% TiO2-NBPU包膜尿素的颗粒硬度分别为79.5 N、80.2 N和92 N,颗粒强度随TiO2增加呈逐渐上升趋势(图8)。二氧化钛的添加提高了聚酯聚氨酯软段与硬段的微相均匀性(图5B),缓解了聚氨酯短链的移动。因此复合包膜工艺和TiO2改性均能提高肥料硬度,改善储运性能。

    在田间土壤内放置365天后,BPU、PPU膜壳的失重率分别为5.42%和1.15% (图9)。BPU中的淀粉基在土壤微生物作用下,发生了一定程度上的水解、氧化、破损,致使BPU软段中的化学键断裂而失重。PPU膜材致密,化学键不易断开,因此生物降解性能较差。当PPU膜材中添加1.00%的纳米TiO2后,失重率提高至4.92%,与BPU膜壳的失重率相当,较PPU显著提高3.3倍。可能是TiO2在光子能量作用下产生光电子和空穴,光电子被土壤水中的溶解氧捕获生成超氧自由基,空穴可氧化膜材表面基团,促进化学键断裂、矿化和降解[34]。因此通过TiO2改性PPU能够提高石化膜材的降解性。

    图  9  不同包膜尿素在土壤中放置365天后的失重率
    注:BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素,1.00%TiO2-NBPU指外层膜材中添加了1.00%TiO2的复合包膜尿素。柱上不同小写字母表示不同处理间在0.05水平上差异显著。
    Figure  9.  Weight loss ratio of coated urea products after buried 365 days in soil
    Note: BPU and PPU are urea only coated by starch based polyurethane and polyester polyurethane, respectively, and 1.00%TiO2-NBPU refers to the composite coating urea containing 1.00% TiO2 in the outer coating layers. Different letters above the bars indicate significant difference among treatments at the 0.05 level.

    当PPU用量为2.50%和TiO2添加量为1.00%时,BPU添加量分别为1.66%、2.50%和3.34%的样品 24 h初期释放率分别为0.10%、0.43%和0.17%;养分释放80%的天数分别为65、109及96天(图10a)。在BPU用量为2.50%和TiO2添加量为1.00%时,适当增加PPU用量可显著调控养分释放速度(图10b)。当PPU添加量分别为1.66%、2.50%和3.34%时,养分初期释放率分别为0.14%、0.43%和0.17%;释放天数分别为42、106和99天。在本研究条件下增加BPU或PPU用量均可延长养分释放期,因为提高材料用量后能形成更加致密均匀的膜层,填补孔隙与裂缝。但添加量从2.50%增大至3.34%,养分释放速度和天数相差不大,这是因为膜材软段中的极性基团如羟基与水分子发生作用导致膜材具有一定的吸水性,水分子渗入后发生溶胀现象,养分释放速度更易受膜内外的渗透压差影响,所以膜材厚度的影响降低[35]。且随着膜内与膜外的渗透压降低,后期养分释放速度减慢。

    图  10  不同BPU、PPU、TiO2添加量下复合包膜尿素的养分释放特征
    注: a:PPU用量为2.50%和TiO2添加量为1.00%,BPU添加量为1.66%、2.50%和3.34%的样品;b:BPU用量为2.50%和TiO2添加量为1.00%,PPU添加量为1.66%、2.50%和3.34%的样品;c:BPU、PPU用量为2.50%,TiO2添加量为0.16%、1.00%、1.84%的样品。
    Figure  10.  Nutrient release characteristics of composite coating urea under different BPU, PPU, TiO2 addition rates
    Note: a: Samples with 2.50% PPU and 1.00% TiO2, 1.66%, 2.50% and 3.34% BPU; b: Samples containing 2.50% BPU and 1.00% TiO2, 1.66%, 2.50% and 3.34% PPU; c: Samples with 2.50% BPU and PPU, 0.16%, 1.00% and 1.84% TiO2 addition.

    BPU和PPU的添加量均为2.50%时,TiO2能调控3~28 天的养分释放速率(图10c)。TiO2的添加量分别为0.16%、1.00%和1.84%的复合包膜肥料的养分初期释放率分别为4.60%、8.30%、25.9%,养分释放80%时的天数分别为136、108和102 天。纳米TiO2对改善复合包膜尿素的初期养分释放率具有重要作用。这是因为添加纳米TiO2后增加了硬段和软段的结合,增加了局部的相容性,但是随时间的延长,纳米TiO2受到水或者养分渗出的影响而失去功能,对养分释放后期的调控效果减弱。

    结果表明,调控BPU、PPU和TiO2的添加量均能调节各阶段的养分释放速率,可利用复合包膜工艺,根据养分释放要求调整BPU、PPU和TiO2的添加量,生产更符合农业生产需求的肥料。与Tian等[17]制备的聚烯烃蜡/生物基聚氨酯/环氧树脂复合包膜尿素相比,采用聚烯烃蜡/淀粉基聚氨酯/TiO2改性聚酯聚氨酯复合包膜工艺,在相同包膜率(5%)条件下中心点释放期由56天提高至108天,提高了近1倍;在生产工艺方面用聚酯聚氨酯替代高粘度的环氧树脂,提高喷涂效率和均匀性;膜材降解性方面,相较于难降解的环氧树脂,用TiO2改性聚酯聚氨酯可提高降解率并与生物基膜材相当,具有环境安全性。

    根据响应值Y设计养分释放特征模型,估算BPU用量(A)、PPU用量(B)和TiO2加入量(C)对养分初期释放率(Y1)、养分释放天数(Y2)的影响(图11)。利用4次模式拟合响应模型:

    图  11  养分初期释放率及释放天数随BPU、PPU 和TiO2 添加量的变化
    Figure  11.  Changes of initial nutrient release rate and release days of composite coating urea with the addition of BPU, PPU and TiO2

    Y1=0.3901−0.0465A−0.0892B−0.1431C+0.066AB+0.1827AC+0.2556BC (1)

    Y2=106.87+16.11A+24.35B+7.07C−10.38AB+1.28AC−9.02BC−6.19A2+5.36B2+4.22C2 (2)

    R2表示模型与真实值的相关性,调整后的R2反映模型参数的变化[36]。对各模型进行方差分析,结果显示:Y1Y2P值均<0.01,R2分别为0.7673、0.8568,模型调整R2分别为0.6600、0.7278,表明模型调节因子R2可以很好地解释大多数反应值的变化,在研究的整个回归区域内拟合度较好。

    通过响应面法构建的拟合模型指出了3个试验变量对24 h养分释放率的影响顺序:TiO2添加量>BPU添加量>PPU用量,对养分释放天数的影响顺序为:PPU添加量>BPU添加量>TiO2添加量。通过对回归模型的分析可知,BPU、PPU和TiO2添加量对初期养分释放率及释放天数具有显著影响。

    纳米TiO2的添加对聚氨酯功能团的性质与数量无影响,但增加了聚氨酯中硬段和软段微相分离的均匀度,提高了膜材的热稳定性、物理稳定性,促进了复合膜材在土壤中的降解性。

    复合包膜工艺可提高肥料颗粒强度和膜材致密性;通过改变淀粉基聚氨酯、聚酯聚氨酯、纳米TiO2配伍比例,可制备出不同性能的复合膜材,实现对包膜尿素养分释放期的调控;聚酯聚氨酯外涂可提高复合膜材整体结构稳定性。

    综合控释肥行业标准要求、成本控制及回归模型分析得出,较为合理的BPU、PPU、TiO2添加量分别为2.5%、2.5%、1.00%。

  • 图  1   不同包膜方案制备的包膜尿素

    注: PPU—聚酯聚氨酯单一包膜尿素,BPU—淀粉基聚氨酯单一包膜尿素,0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指添加了TiO2 0%、0.16%、1.00% 和1.84% 的复合膜材NBPU,其中,BPU用量均为2.5%。

    Figure  1.   Coating-urea prepared with different coating designs

    Note: BPU and PPU are starch based polyurethane coating materials and polyester polyurethane coating materials. 0%TiO2-NBPU, 0.16% TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating materials with 0%, 0.16%, 1.00% and 1.84% TiO2 added to polyester polyurethane, among them, the amount of BPU is 2.5%.

    图  2   包膜原料(左)和不同工艺包膜材料(右)的傅里叶红外光谱图

    注:Starch—玉米淀粉;LS—淀粉基多元醇,PAPI—多亚甲基多苯基多异氰酸酯。BPU指尿素内涂层材料淀粉基聚氨酯,PPU为尿素外涂层材料聚酯聚氨酯。NBPU指内涂层为BPU、外涂层为PPU的复合包膜尿素,0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指外涂层膜材PPU中添加了0%、0.16%、1.00% 和1.84% TiO2的复合包膜尿素。

    Figure  2.   Fourier infrared spectra of coating raw materials (left) and prepared composite coating materials (right)

    Note: Starch—Corn starch; LS—Starch-based polyols. BPU is starch based polyurethane used for the inner coating of urea, PPU is polyester polyurethane used for the outer coating of urea, NBPU is the composite coating urea with BPU and PPU for inner and outer coating materials, 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers.

    图  3   添加不同TiO2比例复合膜材的GC–MS谱图

    Figure  3.   GC-MS spectra of composite coating materials containing different TiO2 ratios

    图  4   不同材料包膜尿素表面与截面SEM图

    注:a,BPU单一包膜尿素;b,PPU单一包膜尿素;c,BPU和PPU复合包膜尿素;d,BPU和1.00%TiO2-PPU复合包膜尿素。 1、2图分别为放大50倍、100倍包膜尿素表面图,3、4分别为放大50倍、100倍包膜尿素截面图。

    Figure  4.   Electron scanning morphologies of surface and cross section of urea with different coating materials

    Note: a, BPU single coating urea; b, PPU single coating urea; c, BPU and PPU composite coating urea; d, BPU and 1.00%TiO2-PPU composite coating urea. The 1 and 2 were the surface images under 50 and 100 fold magnification, and 3 and 4 were the cross section images under 50 and 100 fold magnification, respectively.

    图  5   原子力显微镜下的膜壳表面形貌图 (A) 和相位图 (B)

    注: BPU—淀粉基聚氨酯膜壳,PPU—聚酯聚氨酯膜壳,0%TiO2-NBPU、1.00%TiO2-NBPU分别指外膜层材料中添加了0%、1.00% TiO2的复合膜壳。

    Figure  5.   Morphology (A) and micro-phase diagram (B) of coating crusts under atomic force microscope

    Note: BPU and PPU are the coating materials of starch based polyurethane and polyester polyurethane. 0%TiO2-NBPU, 1.00%TiO2-NBPU refer to the coating crusts of polyester polyurethane containing 0% and 1.00% TiO2.

    图  6   不同膜材的热失重曲线

    注: a,膜材耐受温度(25℃~100℃)热失重图;b,25℃~800℃热失重图;c,25℃~800℃热失重速率图。0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指复合包膜尿素外涂层材料中添加了0%、0.16%、1.00% 和1.84% 的TiO2。BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素。

    Figure  6.   Thermal weight loss curves of coating materials

    Note: a is the thermogravimetric diagram of the production temperature tolerance (25℃−100℃), b is the thermogravimetric diagram of 25℃−800℃, and C is the diagram of the thermogravimetric loss rate of 25℃−800℃. 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers. BPU and PPU refer to the urea only coated with starch based polyurethane and polyester polyurethane, respectively.

    图  7   不同pH环境下包膜尿素的Zeta电位

    注: 0%TiO2-NBPU、0.16%TiO2-NBPU、1.00%TiO2-NBPU、1.84%TiO2-NBPU分别指复合包膜尿素外涂层材料中添加了0%、0.16%、1.00% 和1.84% 的TiO2。BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素。

    Figure  7.   Zeta potential of coated urea under different environmental pH

    Note: 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers. BPU and PPU refer to the urea only coated with starch based polyurethane and polyester polyurethane, respectively.

    图  8   不同TiO2添加量的复合包膜尿素颗粒压力强度

    注:0% TiO2-NBPU、0.16% TiO2-NBPU、1.00% TiO2-NBPU、1.84% TiO2-NBPU分别指外层膜材中添加了0%、0.16%、1.00% 和1.84% TiO2的复合包膜尿素。BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素。柱上不同小写字母表示不同处理间在0.05水平上差异显著。

    Figure  8.   Particle pressure intensity of composite coating urea containing different TiO2 ratios in the outer coating layers

    Note: BPU and PPU are urea only coated by starch based polyurethane and polyester polyurethane, respectively. 0%TiO2-NBPU, 0.16%TiO2-NBPU, 1.00%TiO2-NBPU, 1.84%TiO2-NBPU refer to the composite coating urea containing 0%, 0.16%, 1.00% and 1.84% TiO2 in the outer coating layers. Different letters above the bars indicate significant difference among treatments at the 0.05 level.

    图  9   不同包膜尿素在土壤中放置365天后的失重率

    注:BPU—淀粉基聚氨酯单一包膜尿素,PPU—聚酯聚氨酯单一包膜尿素,1.00%TiO2-NBPU指外层膜材中添加了1.00%TiO2的复合包膜尿素。柱上不同小写字母表示不同处理间在0.05水平上差异显著。

    Figure  9.   Weight loss ratio of coated urea products after buried 365 days in soil

    Note: BPU and PPU are urea only coated by starch based polyurethane and polyester polyurethane, respectively, and 1.00%TiO2-NBPU refers to the composite coating urea containing 1.00% TiO2 in the outer coating layers. Different letters above the bars indicate significant difference among treatments at the 0.05 level.

    图  10   不同BPU、PPU、TiO2添加量下复合包膜尿素的养分释放特征

    注: a:PPU用量为2.50%和TiO2添加量为1.00%,BPU添加量为1.66%、2.50%和3.34%的样品;b:BPU用量为2.50%和TiO2添加量为1.00%,PPU添加量为1.66%、2.50%和3.34%的样品;c:BPU、PPU用量为2.50%,TiO2添加量为0.16%、1.00%、1.84%的样品。

    Figure  10.   Nutrient release characteristics of composite coating urea under different BPU, PPU, TiO2 addition rates

    Note: a: Samples with 2.50% PPU and 1.00% TiO2, 1.66%, 2.50% and 3.34% BPU; b: Samples containing 2.50% BPU and 1.00% TiO2, 1.66%, 2.50% and 3.34% PPU; c: Samples with 2.50% BPU and PPU, 0.16%, 1.00% and 1.84% TiO2 addition.

    图  11   养分初期释放率及释放天数随BPU、PPU 和TiO2 添加量的变化

    Figure  11.   Changes of initial nutrient release rate and release days of composite coating urea with the addition of BPU, PPU and TiO2

    表  1   基于响应曲面法的复合包膜材料用量设计(%)

    Table  1   Coating materials design based on Response Surface Methodology

    序号
    Code
    淀粉基聚氨酯 (BPU)
    Starch-based polyurethane
    TiO2聚酯聚氨酯(PPU)
    Polyester polyurethane
    12.000.502.00
    23.000.502.00
    32.000.503.00
    43.000.503.00
    52.001.502.00
    63.001.502.00
    72.001.503.00
    83.001.503.00
    91.661.002.50
    103.341.002.50
    112.501.001.66
    122.501.003.34
    132.500.162.50
    142.501.842.50
    152.501.002.50
    162.501.002.50
    172.501.002.50
    182.501.002.50
    192.501.002.50
    202.501.002.50
    下载: 导出CSV
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  • 收稿日期:  2022-11-07
  • 录用日期:  2023-01-26
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