This article presents the development of a method to prepare nanocrystalline cellulose (NCCs) from laser-printed waste paper using factorial design 23 experiments by controlled acid hydrolysis of the waste paper. The method applies high gain ultrasound and subsequent flotation, washing, and bleaching stages. Characterization of the raw material, prepared pulp, test sheets, and NCCs is presented. Optimum conditions to obtain high quality NCCs were found to be 65% acid, for 40 min time of treatment, and residual load in the range of 130 to 570 mmol/kg of NCCs. The obtained NCCs were 80 to 700 nm long depending on the acid hydrolysis conditions. They exhibited high values of whiteness (90.3% Elrepho), a-cellulose contents (95%), degree of polymerization (731), viscosity (9.59 cP), and chemical compositions similar to that of Whatman paper.
Isolation and Characterization of Cellulose Nanocrystals Created from Recycled Laser Printed Paper
Rogelio Ramírez-Casillas,a Karen Fátima Báez-Rodríguez,a Ricardo Herbe Cruz-Estrada,b Florentina Dávalos-Olivares,a Fernando Navarro-Arzate,a and Kestur Gundappa Satyanarayanac,*
This article presents the development of a method to prepare nanocrystalline cellulose (NCCs) from laser-printed waste paper using factorial design 23 experiments by controlled acid hydrolysis of the waste paper. The method applies high gain ultrasound and subsequent flotation, washing, and bleaching stages. Characterization of the raw material, prepared pulp, test sheets, and NCCs is presented. Optimum conditions to obtain high quality NCCs were found to be 65% acid, for 40 min time of treatment, and residual load in the range of 130 to 570 mmol/kg of NCCs. The obtained NCCs were 80 to 700 nm long depending on the acid hydrolysis conditions. They exhibited high values of whiteness (90.3% Elrepho), -cellulose contents (95%), degree of polymerization (731), viscosity (9.59 cP), and chemical compositions similar to that of Whatman paper.
Keywords: Laser printing; Waste paper; Environmental safety; Acid hydrolysis; Cellulose nanocrystals
Contact information: a: Department of Wood, Cellulose and Paper, “Engineer Karl Augustin Grellmann” University Center of Exact Science and Engineering, University of Guadalajara, Zapopan, Jalisco, 45000, Mexico; b: Center for Scientific Research of Yucatan (CICY), Yucatán, 97205 México; c: Poornaprajna Institute of Scientific Research, Poornaprajnapura, Bidalur Post, Devanahalli, Bengaluru 562110 (Karnataka- India); *Corresponding author: firstname.lastname@example.org
Environmental impacts have reached a critical stage, causing serious consequences to the natural environment all over the world. This is due to the industrial development of great world powers, developing countries, and even third world countries (Alliot et al. 2004). With the new age of computer technology and the growth in popularity of personal computers, there has been an increase in the consumption of paper due to all types of printers, including laser printers (Lee et al. 2013). It has been reported by the U.S. Environmental Protection Agency that approximately 300 million tons of paper annually are produced all over the world.
The worldwide consumption of paper during the last four decades has increased by 400%, with the USA being the largest consumer of paper (annual consumption of more than 227 kg) (http://www.thepaperlessproject.com/facts-about-paper-the-impact-of-consumption/). The same source reveals that during the last two decades, the country’s consumption has grown approximately 126% from 92 million tons, with only 5% of world’s population represented. According to official figures (Desdelared 2012; Semarnat 2018), Mexico generates an estimated waste of approximately 111,000 tons per day, of which 14% is waste paper. This type of waste paper can certainly impact the environment all over the world and, therefore, there is need for its proper disposal, i.e., recycling and reuse in some way. Such efforts for the disposal of this waste material in any form (whether as landfill or as a raw material) would have many environmental, economic, and social advantages not only in the USA and Mexico, but also in other countries.
Today, secondary fibers represent an important source of lignocellulosic material for the paper and paperboard industries in developed countries. However, the recycling and reuse of paper presents important challenges for its use due to the aging of fibers, the presence of contaminants (additives used in the manufacturing of the original product, like fillers, adhesives, etc.), type of uses such as toner type, injection type, and others, as well as the conditions of use and their final disposal (Monte et al. 2012). The removal of non-lignocellulosic materials that adhere to the waste paper and were generated from printing, by cleaning the fibers is done through a process known as deinking. In general, deinking involves several stages: (i) pulping and disintegration, (ii) cleaning and screening, (iii) flotation, (iv) washing, and (v) bleaching (Beneventi 2000).
Of the various stages of the traditional deinking process, the flotation and washing stages have the greatest impact on the elimination of ink from the fibers (Álvarez and Abril 2006). The flotation step is a selective removal of ink particles achieved by the addition of surfactants. The addition of these compounds enables the ink particles to be dispersed, avoiding their deposition on the fiber and favoring their removal from the fiber slurry. It is reported that the speed and efficiency of the flotation process determine the particle size. Therefore, particle sizes outside of the 25 μm to 100 μm range would lead to a decrease in the efficiency of the flotation system (Ramirez et al. 2004).
In contrast, during the washing process, a cell is used for the physical removal of the contaminants. In this process, ink particles are dispersed as finely as possible so that they can be removed together with the water and other components through the washing medium. This washing process uses filters with a mesh of approximately 200 threads per inch. It has been reported that washing is effective for particle sizes of 1 μm to 10 μm. For larger particles the washing efficiency decreases considerably, requiring a further series of additional processes such as flotation, purification, or classification (Ramírez 2004; Ramírez et al. 2004). It has also been reported that toner inks are difficult to remove by conventional deinking processes as they contain thermoplastic binders that undergo polymerization and fuse onto the paper during the printing process at high temperatures (Ramírez et al. 2004). Ink particles involved in printing (toner, ultraviolet (UV), etc.) exhibit strong intermolecular cohesion interactions as well as strong bonds with the paper surface, specifically in the fiber. Therefore, they are difficult to separate and, even if it is possible to separate them from paper, they form large platelets, making them extremely difficult to remove with a traditional deinking process (Grossmann et al. 2010).
Deinking of laser printing paper has been studied by various researchers using different techniques (Norman et al. 1994; Sell and Norman 1995; Tatsumi et al. 2000; Gaquere-Parker et al.2009; Pauck et al. 2012). Based on these results, it can be said that: (i) the ink particles obtained had a range of sizes depending on the frequency of the generator used with piezoelectric transducers and (ii) ultra sound technique particularly with more stages showed greater efficiency particularly with low consistencies, good recovery of the retention value of water (WRV) of the secondary fiber and near environ temperatures seems to be better method for deinking process which gave better brightness; (iii) low-power, low-gain ultrasound equipment showing limitations of applying to pulps treated in batch processes.
Realizing the fact that large amounts of waste paper are generated globally especially with the use of computers and printing as mentioned above affecting the environment, one of the methods of disposal can be finding new uses for their reuse. However, one possible method is to extract one of the constituents (such as cellulose) from this paper, which is generally produced from fibers with low degree of purity. It should be noted that cellulose is insoluble in most solvents, including strong alkalis, making it difficult to isolate from lignocellulosic materials, as it is intimately bound with lignin and hemicelluloses (Alén 2000). However, in the case of soluble grade cellulose, the fiber is required to have high cellulose content (greater than 90%) and must have very low hemicelluloses, residual lignin, and extractable contents.
The cellulose thus obtained is reported to have very special properties, such as a crystalline arrangement and high levels of whiteness and viscosity (MacDonald 2011). Cellulose, due to its structure, is a compound that has a high potential for use as a nanomaterial, considering its abundance in nature and its nano fibrillar structure. This polysaccharide has very special characteristics that make it an exceptional material, such as its excellent mechanical properties, high rigidity, low cost, and biodegradability (Beck-Candanedo et al. 2005). In fact, a number of studies have reported on the production of nanocellulose in the form of crystals using various natural fibers; most such studies used acid hydrolysis as the first digestion method of choice for obtaining the pulp, with organic acids, sulfuric, phosphoric, and hydrochloric acids (Dong et al. 1998; Camarero Espinosa et al. 2013; Yu et al. 2013; Guo et al. 2016) in optimum conditions of their concentrations, temperatures, and time. Even oxalic acid has been used at various concentrations (Chen et al. 2016) as well as microwave assisted conditions (de Melo et al. 2017; Matharu et al. 2018). Recently, a review has also been published covering the use of environmentally friendly bio-renewable materials to produce nanocelluloses using different methods and their applications particularly in the form of crystals (Trache et al. 2017).
Some of the raw materials from which CNCs can be obtained include a marine animal-tunicate pulp (Samir et al. 2004; Samira et al. 2008), eucalyptus kraft pulp (BEP) (Beck-Candanedoet al. 2005; Wang et al. 2014; Chen et al. 2016), sisal fibers (Moran et al. 2008), and cellulose whiskers from waste from banana rachis (Musa cavendish) (Bolio-Lopez et al. 2011), sugarcane bagasse (Behin et al. 2008; Natthapon et al. 2018), softwood kraft pulp (Hamad and Hu 2010), rice straw (Lu et al. 2012), recycled pulp (Filson et al. 2009; Guo et al. 2016), and cotton linter fibres (filter paper) (Lorenz et al. 2017).
Regarding waste paper, particularly the laser printed paper, some authors have found that the laser-printed ink particles present on paper are flat. Furthermore, the chemical composition of these particles are functional groups of polymers (polyethylene and polypropylene), which are not very susceptible to chemical treatments. This is because the size of these particles is not reducible; also their adhesion to the paper cannot be easily minimized (Norman et al. 1994; Sell and Norman 1995).
Considering the above mentioned facts in respect to finding a new method of disposing waste paper, including that generated with laser printing and the possibility to produce nanomaterials from this waste paper, the main objectives of this study are: (i) to develop a new method to reuse/recycle the waste laser-printed paper and (ii) to characterize the raw material, pulp, and the nanomaterials produced by the waste paper.
In order to meet the other objective to produce nanocellulose crystals (NCCs), a raw material (paper used in laser printing) having high cellulose content (>90%) and very low content of low removable hemicelluloses was used in this study. Optimum conditions have been arrived to obtain high quality NCCs with chemical composition very similar to standard cellulose. It is hoped that this study will be a forerunner for the utilization of other lignocellulosic wastes generated all over the world.
Printed sheets of Maxbrite bond paper (waste laser printed paper) with a letter size of 75 g/m² obtained from Office Max, Mexico City, Mexico were used in this study. The characteristics of the original paper used without impression are: α-cellulose of 84.2%, β-cellulose of 12.6%, -cellulose of 3.2%, whiteness of 87.7%, and values of L*, a*, and b* being 95.0, 0.1 and 0.1, respectively. It may be noted that L* represents the lightness of different colors, starting from from zero for black to 100 for perfect white; a* represents a red color when the number is postive, green when it is negative, and zero when it is grey. Finally, b* represents a yellow color when positive, blue color when it is negative, and is zero for grey as per TAPPI T 527 om-94 (1994).
A number of steps were involved in obtaining nanocrystals of cellulose using the printed paper (waste paper). These are shown schematically in Fig. 1.
To homogenize the amount of toner deposited on each sheet, sheets of bond paper were printed on one side with the following legend: “Deinking of printing paper Laser, applying sequences with ultrasound of intensive action, oriented towards the obtaining of cellulose of high purity”, using a laser printer (Make: Hewlett Packard; Model: HP Laser Jet 1022, México City, México). In fact, it is proper to use HP Laser Jet 1022 toner cartridge with the selected paper. The characteristics of the toner used are: styrene acrylate copolymer 55 to 65%, iron oxide 30 to 40%, and salicylic acid chromium chelate 1 to 3%. The specified test was repeated several times, until a total of 45 rows were filled. The text was printed in the Times New Roman font (size 12), bolded, and double spaced. This allowed for the deposit of an approximate amount of 0.1872 g of toner/sheet.