|Ivana Plazonic||Faculty of Graphic Arts, University of Zagreb, Getaldi?eva 2, Zagreb, 10 000, Croatia|
|Vesna Džimbeg-Malcic||Faculty of Graphic Arts, University of Zagreb, Getaldi?eva 2, Zagreb, 10 000, Croatia|
|Irena Bates||Faculty of Graphic Arts, University of Zagreb, Getaldi?eva 2, Zagreb, 10 000, Croatia|
|Željka Barbaric-Mikocevic||Faculty of Graphic Arts, University of Zagreb, Getaldi?eva 2, Zagreb, 10 000, Croatia|
This research observes changes in the properties of commercially available hemp office papers exposed to photo-oxidation for artificial aging. In order to accelerate the changes that naturally occur in paper, photo-oxidation was performed using a Xenon light source. The electromagnetic radiation was administered in doses of 550?W/m2 over 24 hours with a temperature of 60°C. Three types of hemp office papers that differ in chemical composition and manufacturing process were used as samples. The strength properties (tensile index, elongation at break, tear index), surface properties (Bendtsen roughness, Bekk smoothness), pH of paper extracts, and optical properties were observed. The listed properties of artificially aged samples were compared to those of unaged commercial paper samples. The results show that, after only 24 hours, accelerated aging with a Xenon arc lamp affects all observed paper properties. For all analyzed office papers, the values of the tensile index, Bekk smoothness, and pH of paper extracts decreased, while the values of elongation break, tear index, and Bendtsen roughness increased due accelerated aging. Regarding the CIE L*a*b* color space value, lightness (L*) generally decreased after the aging treatment, and all paper samples became less reddish (a* decreased) and more yellow (b* increased). The obtained results of this research reveal that pulp with hemp fibers can produce higher-quality office paper that is more stable against light and temperature influences than those made with post-consumer fibers. However, to increase the durability of manufactured paper, hemp fibers must be bleached.
Accelerated aging; Hemp; Office paper; Xenon light source
About 92% of global paper production processes depend upon wood as a traditional raw material (Fahmy et al., 2017). However, wood fiber sources are insufficient, and additional fiber is needed in the paper industry (Miao et al., 2014). Many non-wood plants are used as alternative sources only on an experimental basis, although some are used in commercial paper mills. Generally, straw is the largest source of non-wood ?ber in the world, but cotton, hemp, sisal, and kenaf are becoming increasingly important sources as well. Despite the wide variety of cellulose fibers from different origins, it is very important that the fiber selected for production provides paper of the required quality. Therefore, many researchers have blended fiber pulp to achieve the desired properties of paper (Tripathi et al., 2013; Nassar et al., 2015).
Due to its low tetrahydrocannabinol content (? 0.2%), Cannabis sativa has potential for hemp cellulose pulp and paper production (Danielewicz and Surma-?lusarska, 2010; Miao et al., 2014). The hemp used for the pulp and paper industries features strong and long ?bers with low lignin content. In total, 35% of fibers are classified as long bast ?bers, and 65% are short core ?bers. Hemp fiber for paper production must have a fiber length of 15–55 mm (Azeez, 2018). The hemp plant has numerous advantages over wood as a raw material. Compared to one hectare of forest, one hectare of hemp can produce four times more paper. Hemp can be reused just after four months of cropping, while trees require 20–80 years (Ma?achowska et al., 2015). This plant can reach a height of 4–5 meters in only 80–150 days when it is classified as mature for ?ber harvesting. Industrial hemp, as an annual plant, achieves a yield of 12–14 tons of dry matter per hectare, from which 10–12 tons can be harvested as ?ber mass. In whole hemp stems, the content of cellulose is approximately 47% and lignin is 18% (Danielewicz and Surma-?lusarska, 2017). Industrial hemp provides a high pulping yield, which is of great importance for pulp and paper mills (Danielewicz and Surma-?lusarska, 2010). The chemicals involved in making hemp paper are much less poisonous than the chemicals used in making wood pulp paper; hemp paper can be whitened with hydrogen peroxide, while wood paper requires toxic bleaching substances such as chlorine or dioxins (Danielewicz and Surma-?lusarska, 2010). However, this raw material also has some disadvantages. Due to the macroscopic structure, there are great differences along the plant from top to bottom in terms of morphological structure and chemical composition. Therefore, hemp cannot provide fibers, pulp, and, consequently, paper of a consistent quality.
Regardless of the origin of the cellulose fibers used in the paper industry, from the moment of its production, paper is subject to a natural aging process. The aging process can be defined as a sum of all the irreversible chemical and physical processes that occur in organic materials slowly over time. The stability of paper during the aging process is largely determined by its composition and manufacturing process. Paper components can be classified by their origin, chemical structure, and function as fibers (composed mainly of cellulose but also of lignin, hemicelluloses, and other minor constitutes), mineral particles (calcium carbonate, kaolin, talc, etc.), natural sizing agents (e.g., starch or rosin) or synthetic ones (e.g., alkyl ketene dimer and alkenyl succinic anhydride), colorants, and other substances (Area and Cheradame, 2011). Only light that is absorbed by a molecule is effective for producing chemical changes. During the natural process of paper aging, pure cellulose does not absorb visible light (over 400 nm), but strongly absorbs ultraviolet (UV) light (under 200 nm). Residues of hemicellulose and other impurities are always present in the cellulose fibers used for commercial-grade papers. Hemicellulose exhibits similar behavior to cellulose, and lignin strongly absorbs in the UV and visible regions (Zervos, 2010). After light absorption, paper, as a printing substrate, may generate chromospheres (e.g., formaldehyde, furan, methoxy quinone, and stilbene) in acid and undergo alkaline hydrolysis and photo-oxidation degradation, which result in paper discoloration (Cui and Chen, 2011). To evaluate the permanence and durability of paper based on its chemical composition, different methods of accelerated aging have been used by researchers. Most of these methods include elevated temperature and constant humidity (moist or dry, depending on the method). Temperatures are between 60°C and 105°C; higher temperatures cannot be applied for testing since the higher the temperature, the more uncertain becomes the extrapolation to ambient conditions (Eckhart, 2014). However, accelerated aging of paper can just partially reproduce the natural aging mechanisms such as hydrolysis, oxidation, and photodeterioration (Zervos, 2010).
The main conclusions are drawn from the experimental results obtained from exposing hemp office papers to artificial aging by photo-oxidation. The analysis results indicate that hemp fibers can produce high-quality office papers that are more stable against light and temperature influences than those made with post-consumer fibers. However, to increase the durability of manufactured papers, hemp fibers must be bleached. A main purpose of bleaching the pulp is to remove the residual lignin and chromophores inside the pulp, which efficiently absorb UV radiation and cause paper degradation.
work was supported by the University of Zagreb.
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