Use of polyurea from urea for coating of urea granules
© Lu et al. 2016
Received: 21 February 2016
Accepted: 6 April 2016
Published: 14 April 2016
A new type of controlled release fertilizers coated with polyurea was prepared. The granulated urea was firstly changed into a liquid urea by heating as the coating liquid. By spraying uniformly the urea was coated with the polyurea synthesized by the reaction of isocyanates with a liquid urea. The effects of different modifiers on N release characteristics of polyurea-coated urea (PCU) were studied. The morphology and chemical structure of PCU coating materials was investigated by SEM and FTIR. We studied the nitrogen release characteristics of the PCU applied in both water and soil, and the biodegradability of PCU coating after buried in soil. The results showed that PCU reduced nitrogen release rate and exhibited excellent controlled release property. The PCU coating materials could biodegrade in soil. This indicated that the low cost PCU products from urea are expected to use in agricultural and horticultural applications.
Fertilizers are one of the most important products in agriculture. Crop yields and growth mainly depend on the use of fertilizers. But with the increase of the usage amount and time of fertilizers, environmental issue became more conspicuous. It was experimentally confirmed that the application of slow-release or controlled-release fertilizers (CRF), by coating urea granules with materials that exhibit excellent water resisting property, was an effective solution to reduce fertilizer losses and to minimize environmental pollution.
The slow-release or controlled-release fertilizers have been developed over the past decades, which focused on exploring polymer-coated fertilizers. For example, Zhang et al. reported that coating fertilizers nutrients with polyethylene or epoxy resin tended to reduce the nutrient release rate (Lu et al. 2013; Yang et al. 2012). Tomaszewska et al. found that polysulfone coated fertilizers had excellent controlled release property (Tomaszewska and Jarosiewicz 2006; Tomaszewska et al. 2002). However, polymer-coated fertilizers have a disadvantage that large amounts of non-degradable coating materials are left in soil when the nutrients are exhausted. So, degradable polymers coating materials, including grapheme oxide (Zhang et al. 2014), starch (Zhong et al. 2013), polyhydroxybutyrate (Costa et al. 2013), chitosan (Hussain et al. 2012) were widely studied. Although these degradable polymers coated fertilizers may reduce environmental pollution, these nutrient release longevity of natural polymer coating materials as CRFs is short, which will not meet nitrogen supply requirement for field crops. Moreover, these natural polymer coating materials were very expensive, which increased the costs of production. So, the development of low-cost, biodegradable polymer materials with excellent controlled release properties for CRF coating materials is the best solution.
Polyurea is a synthetic polymer from isocyanate and amine compound. It has high mechanical strength, excellent film-forming property, high electric energy density and water resistance property, which was utilized for various fields, including composited materials, aerogel, film, coating, microcapsules, dielectric materials, and steel plates (Holzworth et al. 2013; Hong and Park 1999; Komurlu and Kesimal 2014; Li et al. 2008; Nikishina et al. 1989; Wu et al. 2014). However, urea is rarely used as compound in the preparation of polyurea. Urea is a small molecular amine compound with two amino groups, and it also reacts with isocyanate to synthesize polyurea resin (Zhang and Lu 2014). However, urea was often utilized directly as fertilizer of plant, which has not been used for the preparation of polyurea according to the literature. Especially, little research has been done to explore urea-based polyurea coated fertilizers for agricultural applications.
Based on this background and our previous studies on polymers (Yang et al. 2013; Zhang et al. 2014), a series of novel polyurea-coated urea (PCU) fertilizers was prepared using polyurea synthesized by the reaction of isocyanates with a liquid urea as the main coating material. The morphology and chemical structure of PCU coating materials was investigated by SEM and FTIR. We studied the nitrogen release characteristics of the PCU applied in both water and soil, and the biodegradability of PCU coating after buried in soil. The polyurea derived from urea, which also provided N nutrients after the degradation of the coating materials in soil. We believe this new method will have great potential for developing CRF that provide plants with nutrients and ensure soil quality and crop productivity.
Commercial urea granules with a particle size in the range of 2–5 mm were used in the experiment, which were produced by Lanhua Coal Mining Group Co., Ltd. (Shanxi, China). The isocyanates (polymethylene polyphenyl isocyanate (PAPI)) with 30.03 wt% NCO group were obtained from Yantai Wanhua Co. Ltd (Shandong, China).All reagents used were CP or AR grade and easily obtained from commercial sources.
Preparation of PCUs
The granulated urea was firstly heated and changed into a liquid urea (LU). Then LU, PAPI and (or) modifier were mixed uniformly to obtain the coating liquid. Urea particles were heated to 60–90 °C in a rotary drum. Afterwards the above coating liquid was evenly sprayed on the surface of urea particles and cured in a few minutes to synthesize polyurea coating material. After spraying a measured amount of the coating liquid, the final polyurea-coated urea (PCU) products were obtained. The polyurea synthesized by LU and PAPI coated urea is denoted as PN. The polyurea that was synthesized by LU, PAPI and polyether amine coated urea is denoted as PNA. PNE (PCU including ethylene glycol), PNG (PCU including diethylene glycol) and PNX (PCU including octanol) were obtained in the same way.
N release behavior of PCUs
The N release behavior of PCUs in water at 25 °C was measured in accordance with the Chinese National Standard GB/T 23348-2009 (2009). For each sample, 10 g PCUs were sealed in a glass bottle containing 200 ml deionized water, and then incubated at 25 °C in incubator. After each incubated period (1, 3, 5, 7, 10, 14, 28, 42, 56, 84, 112 days), a certain amount of solution was taken out to measure the nitrogen content, then 200 ml deionized water was again added into the glass bottle and keep on incubating at 25 °C in incubator. The nitrogen concentration was measured using the Kjeldahl method (Bradstreet 1954). All samples were carried out in triplicate and the average value was taken as the nitrogen concentration of each sample. The N release longevity of PCUs is defined as the time when the cumulative N release reaches 80 % of the total N (Yang et al. 2013). The buried bag method was employed to determine the N release rate of PCU in soil (Hyatt et al. 2010). The buried experiments were accomplished according to the method of Yang et al. (2013).
Morphology of PNG
Analyses were performed with a JSM-5800 scanning electron microscope (SEM). They were split into two halves, and the cross-section or surface of PNG was adhered to sample holders with double-sided adhesive tape. Prior to observation, the samples were coated with gold in a sputtering device.
FTIR of coating materials
The PNG coating was analyzed using a Nicolet 380 FTIR spectrometer. The wavenumber range was from 500 to 4000 cm−1. Prior to analyses, Samples and KBr powder were ground and mixed uniformly for the preparation of tablets to obtain the FTIR spectra.
Results and discussion
Effect of different modifiers on N release characteristics
FTIR analysis of coating materials
Morphology of PNG
N release characteristics of PNG
Biodegradability of coating materials
A series of PCUs were prepared by coating urea with the polyurea synthesized by the reaction of PAPI with a liquid urea to reduce nitrogen release of coated fertilizers. The dense appearance and polyurea synthesis process for PCU coating were observed by SEM and FTIR. The PCU coating had excellent controlled-release ability, and the nitrogen release behavior was clearly improved with the addition of diethylene glycol. The PCU coating material had obvious trend of degradation in soil. These results show that the polyurea resin is an effective coating material of CRF.
PL and ZM designed the study; YZ, CJ, and YLcompleted the all experiments and provided the data analysis. PL wrote the manuscript, aided by ZM, while they contributed substantially to revisions. All authors read and approved the final manuscript.
This project is supported by National Natural Science Foundation of China (Grant No. 31301848) and, Science and Technology Development Project of Shandong Province (Grant No. 2014GNC112006), the Natural Science Foundation of Shandong Province (Grant No. ZR2013CQ013), and Science and Technology Development Project of Tai’an City (Grant No. 201440774).
The authors declare that they have no competing interests.
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