PPC/PBS包膜尿素膜材料降解特征

张 坤， 徐 静*， 张 民

(1山东农业大学化学与材料科学学院，山东泰安 271018； 2 山东农业大学资源与环境科学学院，山东泰安 271018)

PPC/PBS包膜尿素膜材料降解特征

张 坤1， 徐 静1*， 张 民2*

(1山东农业大学化学与材料科学学院，山东泰安 271018； 2 山东农业大学资源与环境科学学院，山东泰安 271018)

包膜材料； 聚碳酸亚丙酯; 聚丁二酸丁二醇酯; 降解

利用不同包膜材料成膜孔径的大小、化学或生物分解特点来控制尿素溶解与养分释放速度，是延长肥效期，提高氮素利用率的有效措施之一。为了减少因普通树脂包膜所带来的潜在白色污染，研制生物可降解树脂包膜控释肥，已经成为世界上控释肥领域研究的热点[1-5]。在土壤中，包膜材料的降解速度与特征直接关系到包膜肥料的施用性能、作用效果和土壤生态环境效应，不同膜材、在不同类型土壤中的降解速度不同，对包膜肥料的应用性能影响较大[6-8]。

PPC与PBS树脂均属于可降解材料，在堆肥情况下可自行降解，最终完全降解为二氧化碳和水[9-10]。但是包膜肥料是在种植土壤中施用，其残膜若在土壤中降解太慢，长期施用也会造成土壤污染或影响土壤团粒结构[6]。本课题组前期曾经研究了两种生物可降解包膜材料的适宜比例[11], 本研究以PPC/PBS包膜控释肥料为应用模型，采用失重率法、红外光谱法、热失重法及显微观察法研究了PPC/PBS复配膜材料在土壤中的降解规律和特征，为其作为肥料控释膜材料提供参考数据。

1 材料与方法

1.1 材料和设备参数

扫描电子显微镜(日本电子，JSM6380LV)，包膜材料断面和表面分别喷金处理，加速电压40 kV, 放大2000 倍。

1.2 试验地点和方法

称取16 g包膜的尿素肥料，每个样品2 g左右，均用0.15 mm孔径纱网包裹防止漏出。于2013年1月1日一次性埋入6 cm深度的土壤中(未种植作物)，每月1日取出一个样品，计算肥料养分释放量。将肥料捣碎，用蒸馏水洗去膜内尿素，将膜放在滤纸上阴干待测。采用FTIR、TG、SEM等分析方法检测包膜肥料的膜材料降解特征。

设12个月为降解的最长周期，以膜材料在普通种植土壤中的质量损失率为评价其在土壤中降解性的重要指标之一。将PPC/PBS树脂(质量比7 ∶5)复配溶液铺膜，厚度约70 μm，剪成大小为6 cm×6 cm的12个正方形薄片，精确称量每一样品材料的重量、厚度，用0.15 mm孔径纱网包裹每一片材料，埋入6 cm深度的土壤中，每月取一个样品，蒸馏水洗净，烘干至恒重，称量，计算膜材料重量减少量。

膜化合物分子结构和化学组成采用红外光谱(FTIR)分析方法[6]；将纯PPC和PBS树脂以及所得包膜肥料的膜层洗净烘干后，利用热重分析法研究膜材料土埋前后的热稳定性变化[12]；采用扫描电镜(SEM)对降解前后膜表面和包膜肥料壁材的断面进行形貌分析[13]。

2 结果与分析

2.1 包膜肥料在土壤中的降解

图1 PPC与PBS树脂红外谱图Fig.1 FTIR spectra of PPC and PBS resins

图2 不同土埋时间下PPC/PBS包膜尿素膜FTIR谱图Fig.2 FTIR spectra of degraded coatings after buried for different months[注(Note)： A—降解1、2、3个月后 After one, two, and three months；B—降解3个月后放大图 Enlarged drawing after three months.]

图3 PPC与PBS树脂的热失重图Fig.3 Thermal gravimetric curves for PPC and PBS resins

图4 PPC/PBS包膜尿素膜热失重图Fig.4 Thermal gravimetric curves of PPC/PBS coatings burried in soil[注(Note)： A—降解1、 2、 3个月后 After one, two, and three months；B—局部放大图 Partial enlarged detail.]

图5 埋入土壤之初和1、2、3个月后包膜表面SEM电镜图Fig.5 SEM images of coating surface at initial, one, two and three months later after buried

2.2 树脂材料的降解

由图7可以看出，树脂膜材料在土壤微生物作用下降解，初期降解速度较慢，随着时间的延长, 试样生物降解度逐渐增加，在第5个月达到5%失重，之后失重速率明显增快，显示出良好的降解性，说明内部结构已经遭到破坏，开始大幅度降解，随后降解速度持续上升， 到10月份已达到78%的失重率。

图6 包膜肥料膜降解前后截面SEM电镜图Fig.6 SEM images(cross section) of coating membrane before and after buried for different months

图7 PPC/PBS在土壤中随时间的质量累积损失率Fig.7 Cumulative weight loss rate of PPC/PBS membrane vs. burring time in soil

10月份后降解开始减缓，据推测估计是因为进入十月份天气开始干燥，且气温降低，影响微生物对材料进一步降解。虽然降解速度减慢，但是到12月份的时候，由于膜材料破碎程度比较严重，无法将其与土壤分离取样，所以可将其视为降解完全。因此本研究选择PPC与PBS作为尿素包膜壁材，一年内在土壤中能够完全降解，不会对土壤造成污染。

3 讨论与结论

全生物降解材料由于能够在自然条件作用下(生物酶、水、温、光等)自行降解，并最终转化为二氧化碳和水，环保无污染，逐渐成为了包膜控释肥包膜材料发展和选择的趋势[5, 14-15]。而降解材料在种植土壤中表现出的降解特性直接决定了该类材料作为包膜层对肥料养分的控释效果[6-8, 16]。

热重分析技术自60年代起在高聚物性质的研究中开始广泛应用，既可以测定聚合物的热稳定性，同时也可以分析聚合物共混物的组分和含量。聚合物中的支链、分子量较小的有机小分子，在等速升温的过程中会先行分解，而对于结构相对稳定的聚合物主链则需要在较高温度下才会发生分解。通常采用比较初始分解温度高低的方法对材料热稳定性进行评价。本实验中，对降解前后的膜材料进行了氮气氛围下的热重分析，根据图谱可以发现，土埋一个月后膜材料的初始分解温度略有增加，降解三个月后，膜材料的热分解温度提高了9℃。多项研究表明，多组分复配的降解材料在降解前后，其初始分解温度出现略微增高或不变的现象。Weng等研究发现[12]，生物降解树脂PBAT/PLA不同配比复合膜材料在土壤中降解后，部分材料热分解温度略有增加。周庆海等[9]通过研究生物降解树脂PPC/PBS共混物的热稳定性，发现PBS的引入提高了共混物的热分解温度，其原因是组分中热稳定相对较高的组分含量增加，能够抑制热稳定性差的材料的热分解。本试验结果证明了这一点。因而我们推断在种植土壤环境下膜材料中热稳定性稍差的PPC树脂先行降解，且随着降解程度的加深，PBS树脂在膜材料中所占的比重越来越多，同时也发挥了其抑制前者热降解速率的作用。

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Degradation behavior of PPC/PBS as urea coating in soil

ZHANG Kun1, XU Jing1*, ZHANG Min2*

(1CollegeofChemistryandMaterialScience,ShandongAgriculturalUniversity,Tai’an,Shandong271018，China;2CollegeofRecoursesandEnvironment,ShandongAgriculturalUniversity,Tai’an,Shandong271018,China)

【Objectives】 Biodegradable resin has been used as coating materials in manufacture of controlled/slow-released fertilizers in order to prevent the persistent polyolefin resin from potential damage to soil. Different coating materials have different degradation behavior in soils, which influence the final application effects of coated fertilizers. Therefore, the study of the degradation characteristics of the coating materials will provide directly support for the correct application of coated fertilizers and the safety of soil ecological environment. 【Methods】 Two kinds of biodegradable polymers, poly(propylene carbonate)(PPC) and poly(butylenes succinate)(PBS)(mass ratio, PPC ∶PBS=7 ∶5) were used as coating materials for urea in a burial soil experiment. Infrared spectroscopy(FTIR), scanning electron microscopy(SEM), and thermo-gravimetric analyses(TGA) were used to investigate the degradation characteristics of the PPC/PBS film. The degradation behavior of the coating material in soil including the weight loss, structure change, thermal properties and the surface morphology of polymer films was simulated. 【Results】 1) The FTIR analysis shows that the absorption strength of -OH around 3480 cm-1increases firstly and then decreases, and the carbonyl absorption peaks are strengthen at 1849-1543 cm-1significantly. The asymmetry stretching vibration absorption peaks of the cumulative double bonds, -C=C=C and -C=C=O, appear at 2363 and 2331 cm-1, respectively, indicating that the molecular chain of the coating materials is broken up firstly and then oxidized, forming new compounds. The random cleavage of the ester bonds are initiated in the biodegradable resin structure. 2) The thermo-gravimetric analysis shows that the thermal decomposition temperature of the degraded membrane moves to higher temperature compared to the initial, which demonstrates that the PPC resin in the membrane is degraded firstly in the soil and then the PBS section with larger proportion plays more inhibitory effect to reduce the further decomposition of PPC. 3) The SEM analysis shows that the PPC/PBS film is found suitable to the rugged surface of urea and combined tightly on the surface of urea. The initial surface is smooth with uniform density and no holes on it. As the buried time elongated, the membrane surface becomes rough and a few of micropores of 3-5 μm appears. With the degree of degradation increasing, the membrane structure becomes loose, and the micropores gradually grow up in the internal surface and then extend into the surrounding structure. The pore size increases to about 20 μm and the thickness of the film is thinned to 40-50 μm. However, the PPC/PBS coating still keeps the original frame, which shows that the degradation process of the membrane material needs a certain time. 4) The tested degradable films have good biological degradabilities with a low initial degradation rate. As the buried time extension, the sample biodegradation degrees increase gradually, about 5% of weight loss is found within five months, after that, the weight loss rate is significantly accelerated and reaches to 78% in October, decomposed completely within 12 months. 【Conclusions】 The degradation behavior of PPC/PBS as urea coating material in soil is initiated on the coating surface caused by the cleavage of chemical structure of film, then followed by biodegradation of the ester bonds at random. The degradation rates of the coating materials are slow within the first five months, then increased dramatically within the following five months, and biodegraded completely in 12 months.

coating material; PPC; PBS; degradation

2014-02-24 接受日期： 2014-10-24 网络出版日期: 2015-02-13

国家自然科学基金青年科学基金项目(31000938)； 山东省教育厅项目(2013GZX20109)资助。

张坤(1978—)，男，山东肥城人，博士研究生，讲师, 主要从事可生物降解材料在农业领域的应用。E-mail: zhangk@sdau.edu.cn * 通信作者 E-mail: jiaxu@sdau.edu.cn; minzhang-2002@163.com

S145.9

A

1008-505X(2015)03-0624-08