Recycled paper for the paper industry is of great importance as a basic raw material. Offset printing is a widely used printing technique, and nearly all recycled paper is printed by the means of offset lithography (Barbaric et al. 2013). Lithographic inks, usually called offset oil‒based inks, are based on water-resistant vehicles (a carrier medium) and pigments that do not dissolve in water or alcohol. The inks are used to print newspapers, glossy magazines, books, and photographic reproductions, and they are an important portion of waste paper. In lithography, the print process can be divided into two main categories based on the ink‒setting mechanism: (a) inks are simply absorbed by the pores of paper at room temperature, leaving the pigment behind on paper surface without drying, which happens in the case of offset‒cold oil‒based inks (60% vegetable oil) and (b) inks are physically dried using mineral oils evaporated into the atmosphere at a temperature below 60°C, which happens in the case of offset‒hot oil‒based inks (35–45% mineral oil). However, petroleum-based oils are known to release volatile compounds harmful to both the environment and humans, which poses a challenge in recycling print substrates (Nie et al. 1998; Aydemir et al. 2018).
Increasing environmental and quality demands have resulted in the development of printing inks based on vegetable oils, such as soy beans, rapeseed, castor, and linseed. Vegetable oil–based inks have started receiving widespread attention due to their low volatile organic compound (VOC) content and ecofriendly, sustainable, and renewable nature (Kandile et al. 2017). The use of vegetable oils in non-food applications has gained considerable interest due to their renewable character, biodegradablity, and aptitude to facile chemical modification. From the traditional mineral oil‒based inks to the soybean oil‒based inks, and then to the 100% pure vegetable oil‒based (soybean oil, flaxseed oil, and tung oil) inks, offset printing inks have developed in the direction of a high technical and high vegetable oil content (Chen et al. 2021). However, vegetable oils have shortcomings with respect to dryness and printability behaviors. Currently, with the rapid growth of various of new printing techniques, the ultraviolet (UV)‒curing technique, due to its advanced efficiency, energy saving aspect, cost-effectiveness, and being environmentally friendly, has been widely used in industrial applications such as inks (Mendoza et al. 2015; Roberta 2019), coatings (Liang et al. 2021; Paraskar et al. 2020), and adhesives (Bednarczyk et al. 2020; Kim et al. 2021). In the recent past, light emitting diode ultraviolet UV (LED‒UV) cured printing inks and varnishes have drawn a great deal of interest because of their outstanding properties such as environmental friendliness, high print speed, quick drying, low VOC emission, and ability to provide an excellent quality in the printed layers, such as high gloss. Different from ordinary offset printing inks, LED–UV cured printing inks are based on formulations containing acrylates, photoinitiator systems, pigments, and other additives (chelating agents, anti‒oxidants, surfactants, and biocides) (Marina et al. 2018) that help improve processing and achieve certain properties of the printed layers. Package printing is one of the main application areas of LED‒UV cured printing inks that are used especially in high‒quality consumer goods or products with expensive designs (Mirschel et al. 2014). However, LED–UV cured inks also have drawbacks such as residual monomers, pungent odor, and allergic potential, especially the monomers and prepolymers derived from acrylic acid. Therefore, combining the advantages of vegetable oil‒based inks and LED–UV technology, a hybrid LED‒UV product named Montage α was developed. Montage α is a new series of offset inks, which is not just a simple mixture of vegetable oil‒based offset and LED‒UV inks but a new design based on a resin system with dual curing capability. Vegetable oil‒based varnishes served as the biological raw material, and LED‒UV was introduced into the system to bring in rapid curing of the hybrid LED‒UV ink. The fabrication of hybrid LED‒UV presented in this paper not only solves the problems associated with the traditional vegetable oil‒based inks but also improves their properties and curing speed. Currently, the main development trend of printing materials is to improve their environmental performance by choosing energy-saving, non-toxic, low consumption, no pollution, and the easy degradation of pollution-free packaging material. Therefore, the volume of these environmentally friendly inks being printed will be increasing, and LED‒UV inks have been very popular for their high quality magazine covers. They even have seen considerable growth in use for food packaging applications. Hence, the recycled pulp potentially is of high quality due to the high quality of substrates used with these inks.
Increasing energy demands and environmental pollution have become the most serious problems of the 21st century. The environmental impact of paper and its production on the environment is significant. In 2020, the production of paper and paperboard is estimated to reach approximately 500 million tons (Rita et al. 2019). The paper and pulp industry accounts for 34% of municipal waste and is considered as the third largest polluter of air, water, and land (Rourke 2019). Hence, the recycling of waste paper has attracted extensive attention as it is a means of reducing global environmental problems such as carbon dioxide emissions, deforestation, and destruction of natural resources (Jiang et al. 2020; Adediran et al. 2021; Tao et al. 2021; Stevulova et al. 2021). Recycling one ton of waste paper can produce approximately 0.8 tons of recycled pulp, can replace approimately 30 eucalyputs trees, and save 7000 gallons of water, 400 kWh of electricity, 380 gallons of oil, and reduce toxic waste emissions (Kumar et al. 2021). Paper is a felted sheet of cellulose fibers formed during the papermaking process. Inks of various compositions are absorbed by or fused with the cellulose fibers to form prints and images (Nie et al. 2021). Therefore, the conversion of this relatively abundant and inexpensive raw material into high-quality products requires the development of effective methods to remove contaminants. Deinking is necessary step in recycling some kinds of waste paper. It can be achieved by various means, which include chemical (Tsatsis et al. 2019; Allix et al. 2011), enzymatic (Sango et al. 2021; Nathan et al. 2020), and physical methods (Tatsumi et al. 2000; Fricker et al. 2006). These methods can be used to deink different types of inks. Among these methods, flotation deinking is the most commonly adopted by the paper industry due to its simple, easy operation, short preparation period, and relatively high yield of fibers. It also is a critical step in the process of waste paper recycling and a widely adopted standard practice for ink removal in Europe, North America, as well as many other countries (Vashisth et al. 2011). This method is the oldest technology for deinking wastepaper and has been used commercially for several decades. In the last few years, many studies regarding waste paper deinking placed much more importance on the enzymatic technology (Chee et al. 2013; Feng et al. 2018; Mustafa et al. 2020). Although there have been considerable advances in the application of biotechnology to paper recycling, enzymatic deinking processes still face problems that have limited their commercialization.
In the deinking flotation process, the bubble size, pH, temperature, printing process used, ink thickness, size of ink particles, and the age of printed products were the main areas of concern. Heindel (1999) described that flotation mainly removes the haydrophobic contaminants and ink particles in the size range 20‒300 µm. However, the optimum sizes for flotation deinking can be dependent upon the type of ink and added chemicals. Dorris and Page (1997) found that the optimal size for a high flotation efficiency for photocopier and aser print ink was between 60 and 100 µm. Additionly, it has also been reported that the thermal ageing of ink occurs during the summer season, and this affect ink detachment and fragmentation (Castro et al. 2002). Hence, the age of printed products has been well known to cause tremendous affects that offset ink deinkability (Marina et al. 2016). The magnitude of this problem depends on the type of ink used in the printing process. Nevertheless, effective deinking of printed paper based on some new offset inks, e.g., vegetable oil‒based inks and LED‒UV inks, especially hybrid LED‒UV inks, could not be achieved, even in recent years.
The overall objective of this study is to investigate the deinkability of hybrid LED‒UV ink, LED‒UV inks, and vegetable oil‒based inks using the flotation deinking method. The particle size distribution (PSD) was used to analyze the size of the ink particles. Fourier-transform infrared spectra (FT‒IR) was employed to realize the structure of vehicles. Furthermore, the deinking efficiencies of hybrid LED‒UV, LED‒UV, and vegetable oil‒based inks on the same paper using the flotation method are proven, and the changes in the physical properties of the paper before and after deinking are studied.