張 坤， 徐 靜*， 張 民

(1山東農(nóng)業(yè)大學(xué)化學(xué)與材料科學(xué)學(xué)院，山東泰安 271018； 2 山東農(nóng)業(yè)大學(xué)資源與環(huán)境科學(xué)學(xué)院，山東泰安 271018)

PPC/PBS包膜尿素膜材料降解特征

張 坤1， 徐 靜1*， 張 民2*

(1山東農(nóng)業(yè)大學(xué)化學(xué)與材料科學(xué)學(xué)院，山東泰安 271018； 2 山東農(nóng)業(yè)大學(xué)資源與環(huán)境科學(xué)學(xué)院，山東泰安 271018)

包膜材料； 聚碳酸亞丙酯; 聚丁二酸丁二醇酯; 降解

利用不同包膜材料成膜孔徑的大小、化學(xué)或生物分解特點來控制尿素溶解與養(yǎng)分釋放速度，是延長肥效期，提高氮素利用率的有效措施之一。為了減少因普通樹脂包膜所帶來的潛在白色污染，研制生物可降解樹脂包膜控釋肥，已經(jīng)成為世界上控釋肥領(lǐng)域研究的熱點[1-5]。在土壤中，包膜材料的降解速度與特征直接關(guān)系到包膜肥料的施用性能、作用效果和土壤生態(tài)環(huán)境效應(yīng)，不同膜材、在不同類型土壤中的降解速度不同，對包膜肥料的應(yīng)用性能影響較大[6-8]。

PPC與PBS樹脂均屬于可降解材料，在堆肥情況下可自行降解，最終完全降解為二氧化碳和水[9-10]。但是包膜肥料是在種植土壤中施用，其殘膜若在土壤中降解太慢，長期施用也會造成土壤污染或影響土壤團(tuán)粒結(jié)構(gòu)[6]。本課題組前期曾經(jīng)研究了兩種生物可降解包膜材料的適宜比例[11], 本研究以PPC/PBS包膜控釋肥料為應(yīng)用模型，采用失重率法、紅外光譜法、熱失重法及顯微觀察法研究了PPC/PBS復(fù)配膜材料在土壤中的降解規(guī)律和特征，為其作為肥料控釋膜材料提供參考數(shù)據(jù)。

1 材料與方法

1.1 材料和設(shè)備參數(shù)

掃描電子顯微鏡(日本電子，JSM6380LV)，包膜材料斷面和表面分別噴金處理，加速電壓40 kV, 放大2000 倍。

1.2 試驗地點和方法

稱取16 g包膜的尿素肥料，每個樣品2 g左右，均用0.15 mm孔徑紗網(wǎng)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(未種植作物)，每月1日取出一個樣品，計算肥料養(yǎng)分釋放量。將肥料搗碎，用蒸餾水洗去膜內(nèi)尿素，將膜放在濾紙上陰干待測。采用FTIR、TG、SEM等分析方法檢測包膜肥料的膜材料降解特征。

設(shè)12個月為降解的最長周期，以膜材料在普通種植土壤中的質(zhì)量損失率為評價其在土壤中降解性的重要指標(biāo)之一。將PPC/PBS樹脂(質(zhì)量比7 ∶5)復(fù)配溶液鋪膜，厚度約70 μm，剪成大小為6 cm×6 cm的12個正方形薄片，精確稱量每一樣品材料的重量、厚度，用0.15 mm孔徑紗網(wǎng)包裹每一片材料，埋入6 cm深度的土壤中，每月取一個樣品，蒸餾水洗凈，烘干至恒重，稱量，計算膜材料重量減少量。

膜化合物分子結(jié)構(gòu)和化學(xué)組成采用紅外光譜(FTIR)分析方法[6]；將純PPC和PBS樹脂以及所得包膜肥料的膜層洗凈烘干后，利用熱重分析法研究膜材料土埋前后的熱穩(wěn)定性變化[12]；采用掃描電鏡(SEM)對降解前后膜表面和包膜肥料壁材的斷面進(jìn)行形貌分析[13]。

2 結(jié)果與分析

2.1 包膜肥料在土壤中的降解

圖1 PPC與PBS樹脂紅外譜圖Fig.1 FTIR spectra of PPC and PBS resins

圖2 不同土埋時間下PPC/PBS包膜尿素膜FTIR譜圖Fig.2 FTIR spectra of degraded coatings after buried for different months[注(Note)： A—降解1、2、3個月后 After one, two, and three months；B—降解3個月后放大圖 Enlarged drawing after three months.]

圖3 PPC與PBS樹脂的熱失重圖Fig.3 Thermal gravimetric curves for PPC and PBS resins

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

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

2.2 樹脂材料的降解

由圖7可以看出，樹脂膜材料在土壤微生物作用下降解，初期降解速度較慢，隨著時間的延長, 試樣生物降解度逐漸增加，在第5個月達(dá)到5%失重，之后失重速率明顯增快，顯示出良好的降解性，說明內(nèi)部結(jié)構(gòu)已經(jīng)遭到破壞，開始大幅度降解，隨后降解速度持續(xù)上升， 到10月份已達(dá)到78%的失重率。

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

圖7 PPC/PBS在土壤中隨時間的質(zhì)量累積損失率Fig.7 Cumulative weight loss rate of PPC/PBS membrane vs. burring time in soil

10月份后降解開始減緩，據(jù)推測估計是因為進(jìn)入十月份天氣開始干燥，且氣溫降低，影響微生物對材料進(jìn)一步降解。雖然降解速度減慢，但是到12月份的時候，由于膜材料破碎程度比較嚴(yán)重，無法將其與土壤分離取樣，所以可將其視為降解完全。因此本研究選擇PPC與PBS作為尿素包膜壁材，一年內(nèi)在土壤中能夠完全降解，不會對土壤造成污染。

3 討論與結(jié)論

全生物降解材料由于能夠在自然條件作用下(生物酶、水、溫、光等)自行降解，并最終轉(zhuǎn)化為二氧化碳和水，環(huán)保無污染，逐漸成為了包膜控釋肥包膜材料發(fā)展和選擇的趨勢[5, 14-15]。而降解材料在種植土壤中表現(xiàn)出的降解特性直接決定了該類材料作為包膜層對肥料養(yǎng)分的控釋效果[6-8, 16]。

熱重分析技術(shù)自60年代起在高聚物性質(zhì)的研究中開始廣泛應(yīng)用，既可以測定聚合物的熱穩(wěn)定性，同時也可以分析聚合物共混物的組分和含量。聚合物中的支鏈、分子量較小的有機小分子，在等速升溫的過程中會先行分解，而對于結(jié)構(gòu)相對穩(wěn)定的聚合物主鏈則需要在較高溫度下才會發(fā)生分解。通常采用比較初始分解溫度高低的方法對材料熱穩(wěn)定性進(jìn)行評價。本實驗中，對降解前后的膜材料進(jìn)行了氮氣氛圍下的熱重分析，根據(jù)圖譜可以發(fā)現(xiàn)，土埋一個月后膜材料的初始分解溫度略有增加，降解三個月后，膜材料的熱分解溫度提高了9℃。多項研究表明，多組分復(fù)配的降解材料在降解前后，其初始分解溫度出現(xiàn)略微增高或不變的現(xiàn)象。Weng等研究發(fā)現(xiàn)[12]，生物降解樹脂PBAT/PLA不同配比復(fù)合膜材料在土壤中降解后，部分材料熱分解溫度略有增加。周慶海等[9]通過研究生物降解樹脂PPC/PBS共混物的熱穩(wěn)定性，發(fā)現(xiàn)PBS的引入提高了共混物的熱分解溫度，其原因是組分中熱穩(wěn)定相對較高的組分含量增加，能夠抑制熱穩(wěn)定性差的材料的熱分解。本試驗結(jié)果證明了這一點。因而我們推斷在種植土壤環(huán)境下膜材料中熱穩(wěn)定性稍差的PPC樹脂先行降解，且隨著降解程度的加深，PBS樹脂在膜材料中所占的比重越來越多，同時也發(fā)揮了其抑制前者熱降解速率的作用。

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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 網(wǎng)絡(luò)出版日期: 2015-02-13

國家自然科學(xué)基金青年科學(xué)基金項目(31000938)； 山東省教育廳項目(2013GZX20109)資助。

張坤(1978—)，男，山東肥城人，博士研究生，講師, 主要從事可生物降解材料在農(nóng)業(yè)領(lǐng)域的應(yīng)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