The Influence of Ultraviolet Radiation on the Surface Roughness of Prints Made on Papers with Natural and Bleached Hemp Fibers

Abstract

In the papermaking industry, cellulose fibers often undergo a bleaching process which affects the surface of the fibers, or their overall morphology. The surface of the produced paper, which is most often used as a printing substrate, depends on the production method, the arrangement of the cellulose fibers, and the quantity and fineness of the filler. The micro-irregularities caused by the uneven distribution of fibers and surface particles of the filler make the paper’s surface rough and affect the print quality and its stability when exposed to light from the moment of production to use. The unbleached cellulose fibers in the printing substrate contain natural pigments, lignin and hemicellulose that absorb UV radiation, as opposed to bleached fibers, which have higher whiteness and lightfastness. Therefore, the influence of UV radiation on the surface roughness of prints made on papers with natural unbleached and bleached hemp fibers was analyzed. This research confirmed that papers formed from unbleached fibers have rougher surface and that printed graphic products from bleached fibers have higher stability to UV radiation than those from natural, unbleached hemp fibers after 96 h of treatment in the Suntest chamber.

1. Introduction

Papermakers are constantly dealing with the growing demands on paper quality and the competitive global market [1], so evaluating paper’s properties in order to see whether they are or are not aligned with the intended end use and consumer needs is crucial. This resulted in numerous grades of paper whose composition changed and adapted over the years to convey information in written form. Even the popularity of electronic information transfer techniques did not stop the continuously growing production of paper and paperboard [2]. Namely, in comparison to the previous year, in 2022, there was an increase of over 4% in the global production of paper and paperboard, which reached approximately 414.09 million metric tons [3]. The continued high production of pulp and paper products has confronted manufacturers with the question of alternative raw materials, considering that wood, due to its strength and durability, is used as a raw material for other industries besides paper, ranging from construction and furniture production to renewable energy sources [4]. Besides wood, cellulose fibers for paper production can be obtained by chemical treatment of various types of annual plants such as grass, by-products of grain production and textile plants. The industrial hemp, as a non-woody plant, is an interesting source of fibers for papermaking because it is already grown for various purposes (food or textile industry) and can be used as a byproduct for the papermaking industry. Namely, the bleachability, the beatability and the tearing resistance of unbleached pulp are properties that make this plant a potential raw material for paper production [5].

Regardless of the type of cellulose fibers used in the production of paper of various categories, the surface and bulk properties must be refined and optimized to suit the required appearance, optical and printing properties, as the demands on paper quality are constantly growing [6]. A very important parameter for obtaining high-quality prints is paper surface roughness [7,8], and therefore this is one of the main problems in the papermaking industry [9] because it affects the major property of paper printing resolution and, consequently, the final quality of the printing papers. The surface roughness also has a significant effect on the optical properties of the paper, such as gloss, the amount of coating and ink absorption [10]. Smoothness and roughness are two complementary notations based on which the topography of the paper surface can be evaluated. Smoothness represents the degree to which the surface is free of inequalities and irregularities, while roughness indicates the degree of unevenness or irregularity over the surface. The micro-irregularities that make the paper surface rough are caused by the uneven distribution of the fibers and surface particles of the filler. Therefore, the papers have a heterogeneous composition and uneven smoothness on both sides of the sheet. This is especially emphasized for uncoated papers and laboratory-made papers. Profilometry provides raw data on structural surface parameters that are needed to characterize the paper’s surface [11].

Paper, depending on its intended application, is created by an interaction of individual cellulose fibers obtained by different treatments, forming a network with a complex hierarchy and porosity [12]. During the pulping process, the lignin is removed from the middle lamella of lignocellulosic biomass, while during the bleaching process, its removal from cell walls is carried out in multiple stages. The aim of the bleaching process is to increase the brightness of the pulp by removing or modifying the colored components, i.e., the residual lignin. The fiber pulp must then be washed several times to remove the dissolved lignin and all the chemicals it contains. Due to global trends and pressure to reduce the production of chlorinated organic compounds in pulp production, ECF (elemental chlorine-free) and TCF (total chlorine-free) are seeing increasing use as bleaching methods that are more ecologically friendly [13]. During this chemical process, pulp becomes whiter to improve its printing properties and its ability to absorb liquids [14,15]. The pulp of bleached fibers is used for products in which high purity is required and yellowing is not desired—mainly high-quality writing and printing papers [16]. If cellulose fibers do not undergo a bleaching process and are used in their natural form in paper production, pigments will stay in paper structure, as well as lignin and hemicellulose, which absorb UV radiation. However, it is important to emphasize that lignin, which is usually present at 15–25% in lignocellulosic material, absorbs the UV and visible light more intensely than cellulose and hemicellulose [17]. This logically leads to the conclusion that the stability of the paper during the aging process, which starts from the moment of its production, is largely determined by the composition of the paper and the manufacturing process [18]. The process of material aging can be described as the sum of all the irreversible chemical and physical processes that occur in organic materials slowly over time. Artificial light sources are often used to imitate the contents of natural solar radiation, or some part of the spectrum emitted by the sun. Although it makes up only 4.6% of the solar spectrum (ranges between 280 nm and 400 nm), UV radiation causes the most significant damage to polymetric materials. The most aggressive part is the UV B range between 280 nm and 315 nm. Devices with artificial light sources are generally used to accelerate degradation in order to evaluate the resistance of materials to outdoor weathering. Filtered long-arc xenon, fluorescent, metal halide lamps and carbon arc lamps are examples of frequently used sources, while mercury vapor and tungsten lamps are used less frequently. However, it has been proven that among all artificial UV sources, the best simulation of natural light is gained by xenon lights [19].

In addition to the paper products, the prints obtained by printing the paper are also exposed to numerous factors that impair the surface area and quality of the print. Since light causes gradual degradation of paper and prints [20], in this research, the influence of UV radiation on the roughness and stability of flexographic prints on a printing substrate made of bleached hemp fibers will be examined and compared with prints made on a printing substrate made of natural, unbleached hemp fibers.

2. Materials and Methods

2.1. Printing Substrate

Two types of office papers with a standard grammage of 80 g/m² were used. Their characteristics, which were determined in our previous research [18,21], are listed in

Table 1. Characteristics of papers used as printing substrate with natural, unbleached hemp fibers (HN) and a printing substrate with bleached hemp fibers (HB).

Characteristics	Printing Substrates
	HN	HB
Composition	100% hemp plant fiber
Production	handmade
Coating	uncoated
Bleaching	natural, unbleached	non-chlorine bleached
ISO Brightness	37.70 ± 0.21	70.79 ± 0.77
Thickness, mm	0.18 ± 0.01	0.20 ± 0.01
Ash900 [%]	1.34	1.72
Moisture [%]	4.95 ± 0.39	5.17 ± 0.37
Tensile index [Nmg−1]	55.52 ± 5.43	58.64 ± 3.94
Elongation at break [%]	2.04	2.48
Tear index [mNm2g−1]	24.96 ± 1.57	15.15 ± 1.25
Roughness (Bendtsen method) [mL min−1]	920.50 ± 204.28	646.50 ± 139.58
Smoothness (Bekk method) [s 10 mL−1]	1.98 ± 0.13	3.24 ± 0.21

.

Optical microscopy of both hemp papers used as printing substrate was performed on a Zeiss, Axio Zoom V16 microscope (Göttingen, Germany). Images were taken using a Zeiss Axiocam 208 color digital camera and processed using ZENcore 3.3 software.

As the hemp papers used in this research are made from different types of hemp fibers (unbleached and bleached) the behavior of water absorption was analyzed by water absorption tests in accordance with the international standard ISO 5637 [22]. Briefly, the samples were dried in an oven at 105 ± 2 °C during 24 h. All paper samples were weighed (mass m₁) prior being immersed in a water bath for 5 min at a temperature of 23 °C. Then, samples were drained by gravity for 2 min and weighed again (mass m₂). Relative water absorption was calculated using Equation (1).

$$Relativewaterabsorption(\%)={\displaystyle \frac{m_2-m_1}{m_1}}\times 100$$

(1)

2.2. Printing of Analyzed Substrates

The RK PrintCoat Instruments Ltd. Esiproof print tester (Litlington, UK), which simulates flexographic printing, was used for the manual printing of pre-cut hemp papers Printing was performed in full tone with the inks cyan (C), magenta (M), yellow (Y) and black (K) with the trade name Iroflex 917 from the manufactured Sun Chemical (Wien, Austria) (

Table 2. Flexographic prints on a printing substrate with natural, unbleached hemp fibers (HN) and a printing substrate with bleached hemp fibers (HB).

Flexographic Prints	Printing Substrates
	HN	HB
Cyan
Magenta
Yellow
Black

). Many inks are created by combining different pigments to achieve the desired shade, and the resistance of the resulting mixture of pigments tends to be determined by the least resistant pigment. The pigments most commonly used in flexographic printing inks have a color index: PY 3, 5, 98 (monoazo yellow), PY 12, 13, 14 (diaryl yellow), PO5 (permanent red 2G), PR 53 (lake red C), PR 49 (lithol red), PR 5 (permanent red R), PB 15 (phthalocyanine blue), PB 62 (ultramarine), PR 101 (iron oxide) and PB 7 (carbon black) [23]. The following conditions were used during the printing process: a ceramic anilox roller of 40 lin/cm (spread angle 60°) and a total cell volume of 39.10 cm³/m², as well as a temperature of 23 °C and a relative humidity of 50%.

2.3. UV Radiation Treatment

CMYK prints on both types of printing substrate (HN and HB), as well as unprinted papers, were placed in the Suntest XLS+ test chamber, supplied with a daylight filter, which generates visible and near-ultraviolet electromagnetic radiation in the wavelength range from 290 nm to 800 nm. The artificial aging was carried out according to ASTM D 6789-02 [24]. The level of light intensity was (765 ± 50) W/m², the temperature was kept at 22.6 °C and the relative humidity was 50%. The duration of UV radiation was set at 96 h, which corresponds to 89 days of natural aging (~3 months) [25].

2.4. Roughness Analysis

Roughness profiles of the samples’ surfaces were measured with a tactile Mahr MarSurf PS 10 profilometer in order to study the correlation between the surface characteristics of prints on papers with natural and bleached hemp fibers and UV electromagnetic radiation. This profilometer has a mechanical stylus that “senses” the surface of the sample by moving 4.0 mm (evaluation length) across the surface with a measurement force of 7.5 × 10⁻⁴ N. The roughness parameters determined for each sample, paper and prints, were as follows: R_a—the arithmetic average of the profile’s deviation; R_z —the height of the profile’s irregularities at 10 points; and R_max—the greatest height of the profile [26].

The results are presented in tables as the average value of the measurements at ten different positions of each sample, and XLMiner Analysis ToolPak in Excel 365 Microsoft Office version 2410 was used for statistical analysis using analysis of variance (ANOVA). For the graphical presentation of the results, the difference in the most used roughness parameter (ΔR_a, ΔR_z, ΔR_max) of all samples before and after exposure to UV radiation was calculated according to Equations (2)–(4).

$${\mathsf{\Delta }R}_a=R_{aAUV}-R_{aBUV}$$

(2)

$${\mathsf{\Delta }R}_z=R_{zAUV}-R_{zBUV}$$

(3)

$${\mathsf{\Delta }R}_{max}=R_{maxAUV}-R_{maxBUV}$$

(4)

where AUV—after UV treatment and BUV—before UV treatment

2.5. Graininess Analysis

As the paper surface roughness has a major influence on ink pigment distribution during the printing process, the analysis of graininess on each flexographic print was performed with a PIAS-II digital microscope (Billerica, MA, USA) using software that complies with the standard ISO 13660 [27]. The graininess of the printed surface was observed by the irregular variation in the optical density with a spatial frequency smaller than a tile size. It was calculated according to the ISO-13660 standard, on the basis of which the observed photographed surface was divided into 100 equal tiles (1.27 mm × 1.27 mm), on which 900 reflectance measurements (%) were made [28]. The graininess was calculated according to Equation (5).

$$Graininess=\sqrt{{\displaystyle \frac{\sum _{i=1}^n{\mathsf{\sigma }}_i^2}{n}}}$$

(5)

where σ_i—the standard deviation within a tile; i—the tile number; and n—the total number of tiles.

3. Results

The microscopic images of hemp paper used as printing substrates depicted below (Figure 1) were created using an optical microscope Zeiss, Axio Zoom V16 (Göttingen, Germany).

From Figure 1b, it is obvious that hemp paper formed using a bleaching process has single fibers in its structure, while in Figure 1a, some hemp fibers form bundles. The structure of hemp papers influences the water absorption. The relative water absorption values of both hemp papers used as printing substrates are presented in

Table 3. Relative water absorption of hemp papers (HN and HB).

Paper	Relative Water Absorption (%)
HN	340.72 ± 5.10
HB	280.77 ± 10.33

.

Since the roughness of the paper surface of both papers (HN and HB) influences the print quality and its stability during a long period of time, the surface roughness was measured. The profilometers measure the real topography of a paper surface and profilograms that visualize the results of measurement were captured during stylus line travel on the paper. The data obtained on both hemp papers before UV radiation treatment are presented in Figure 2. The total measured length of one measurement is 4.0 mm divided into five 0.8 mm sections. Based on the ten obtained measurements, the average results of the roughness parameters (R_a, R_z, R_max) of the paper surface and their standard deviations are summarized in

Table 4. Parameters of surface roughness of hemp papers (HN and HB).

	Parameters of Surface Roughness (μm)
	Ra	Rz	Rmax
HN	5.575 ± 0.402 a	31.081 ± 1.966 a	39.461 ± 4.645 a
HB	4.302 ± 0.233 b	24.529 ± 1.328 b	29.878 ± 2.427 b

^a,b Different superscript letters within the column indicate significant differences between samples at p < 0.05.

.

The greater surface irregularities are visible in the surface roughness profilogram measured along the length of 4.0 mm on paper with unbleached hemp fibers (HN) compared to the profilogram of paper with bleached hemp fibers (HB) (Figure 2). The results of all roughness parameters clearly show that paper with unbleached hemp fibers has a significantly rougher surface than paper with bleached hemp fibers (Table 4).

The change in surface roughness of the unprinted paper with unbleached hemp fibers (HN) and unprinted paper with bleached hemp fibers (HB) after UV radiation treatment for 96 h is presented through differences in ΔR_a, ΔR_z and ΔR_max values in Figure 3. Based on the R_a roughness parameter value, it was observed that the surface roughness of the HN printing substrate increased by 17% after UV radiation treatment, while the effect of UV radiation on the roughness of the printing substrate with bleached hemp fibers (HB) was significantly lower: only 4%.

Table 5. Parameters of surface roughness of flexographic prints measured before UV radiation treatment.

Flexographic Prints		Ra (μm)	Rz (μm)	Rmax (μm)
Cyan	HN	5.782 ± 0.385 a	30.876 ± 2.130 b	38.010 ± 3.388 b
	HB	4.110 ± 0.299 c	24.653 ± 1.739 e	30.585 ± 2.582 d
Magenta	HN	5.519 ± 0.299 a	30.260 ± 2.113 c	36.100 ± 2.675 b
	HB	4.208 ± 0.298 c	23.219 ± 2.241 e	29.054 ± 5.415 e
Yellow	HN	5.506 ± 0.342 a	29.405 ± 3.272 c	36.932 ± 5.467 b
	HB	4.483 ± 0.205 b	26.623 ± 2.115 d	32.464 ± 3.878 c
Black	HN	5.808 ± 0.532 a	33.816 ± 3.860 a	43.350 ± 7.051 a
	HB	4.101 ± 0.213 c	23.686 ± 1.632 e	30.397 ± 6.703 e

^a–e Different superscript letters within the column indicate significant differences between samples at p < 0.05.

summarizes the findings on the determined roughness parameters of flexographic prints with cyan, magenta, yellow or black ink before UV radiation treatment.

Prints with cyan, magenta, yellow and black ink on papers with natural, unbleached hemp fibers (HN) do not have significant differences in surface roughness parameter R_a but differ from the R_a values of prints on papers with bleached hemp fibers (HB). Other surface roughness parameters R_z and R_max of flexographic prints show significant difference in values regarding type of ink and printing substrate. For a better insight into the changes caused by UV radiation in surface roughness of flexographic prints on papers with natural, unbleached and bleached hemp fibers, the changes in all roughness parameters are shown graphically in Figure 4, Figure 5 and Figure 6.

From Figure 4, it can be seen that differences in the R_a roughness parameter of flexographic prints are greater if the fibers in hemp paper are natural and unbleached. The highest value of ∆R_a is observed on hemp paper with unbleached fibers printed with yellow ink (∆R_a = 0.58 μm), while on the same paper printed with black ink, the value of ∆R_a is the lowest (∆R_a = 0.16 μm). For prints on hemp paper formed with bleached fibers, the values of ∆R_a are negligible, ranging from −0.03 μm to 0.13 μm. Comparing ∆R_a values of flexographic prints with unprinted hemp papers (Figure 3) ranging from 0.93 μm (∆R_a HN) to 0.16 μm (∆R_a HB), the values of ∆R_a decrease, which is to be expected because the paper’s surface is covered with the ink.

Unprinted paper made from natural, unbleached hemp fibers with a higher value of the height of irregularities of the profile at 10 points (R_z HN = 31.081 μm) in comparison with paper made from bleached hemp fibers (R_z HB = 24.529 μm) resulted in flexographic prints whose R_z value changes much more due to UV radiation. Again, the change in this roughness parameter under the influence of UV radiation for prints on HN paper was greatest on cyan (∆R_z HN = 4.02 μm) and yellow (∆R_z HN = 3.96 μm) flexographic prints, while it was significantly lower for black (∆R_z HN = 1.77 μm) prints. A negligible influence of UV radiation was observed on R_z values for flexographic prints on HB paper ranging from −0.84 μm to 0.21 μm. The differences in surface roughness values (∆R_z) of printed hemp papers are much lower than those of unprinted hemp papers (∆R_z HN = 7.16 μm; ∆R_z HB = 3.10 μm, as shown in Figure 3).

From Figure 6, it can be seen that UV radiation affects the R_max roughness parameter of flexographic prints on both analyzed hemp papers. For this parameter, a more significant effect of UV radiation was observed for unprinted paper with natural hemp fibers (∆R_max HN = 9.07 μm from Figure 3), where prints with all inks have decreased their value (cyan: ∆R_max HN = 6.69 μm; magenta: ∆R_max HN = 6.17 μm; yellow: ∆R_max HN = 4.72 μm; black: ∆R_max HN = 4.61 μm). A negligible effect of UV radiation, in the form of a decrease in the value of R_max, was observed on prints made on paper with bleached hemp fibers (cyan: ∆R_max HB = −1.57 μm; magenta: ∆R_max HB = −1.08 μm; yellow: ∆R_max HN = 0.11 μm; black: ∆R_max HN = −1.49 μm). The surface roughness of unprinted hemp paper itself showed a smaller change after UV radiation compared to paper made from unbleached hemp fibers (∆R_{max HB} = 6.92 μm from Figure 3). From the results presented in Figure 4, Figure 5 and Figure 6, it is evident that all flexographic prints made on hemp paper with natural, unbleached fibers (HN) become rougher after exposure to the UV radiation in Suntest XLS+ test chamber, which is demonstrated by the increased values of all three observed roughness parameters. The largest change in the surface roughness up to 11%, based on R_a roughness parameter value, was observed on the HN substrate printed with yellow ink. The next print with a R_a roughness parameter value that changed after UV radiation was a cyan flexographic print (~10%), followed by a magenta print (~8%), while the highest stability was observed for a black flexographic print, with a 3% change in this value. For printed HB substrate, the UV radiation influence was negligible. The measured values (∆R_a, ∆R_z, ∆R_max) which are near zero value can be considered as a device error.

The topographic information of unprinted papers and cyan and yellow flexographic prints before and after UV radiation treatment was taken by digital microscope PIAS-II, and 3D surface plots diagrams were created in ImageJ image analyzer (Figure 7, Figure 8 and Figure 9).

The graininess values for all prints before and after UV radiation treatment are summarized in

Table 6. Graininess values of flexographic prints before and after UV radiation treatment.

Flexographic Prints		Graininess (%)
		HN	HB
Cyan	BUV	2.3	1.8
	AUV	1.8	1.5
Magenta	BUV	1.6	0.6
	AUV	1.6	0.6
Yellow	BUV	2.0	1.3
	AUV	2.1	2.8
Black	BUV	2.0	0.7
	AUV	2.1	0.7

.

4. Discussion

The difference in the surface roughness of hemp papers produced from natural unbleached hemp fibers (HN) and bleached fibers (HB) could be explained by the delignification and bleaching processes. Namely, fibers that have not undergone delignification and bleaching stay bundle-shaped, with a rugged surface (Figure 1a). After the bleaching process, the fiber bundle was released and formed hemp paper with a smoother surface. Arnata and his colleagues also reached these conclusions in their research [29]. Paper formed from bleached hemp fibers has a minor change in roughness after UV radiation treatment than paper formed from natural, unbleached hemp fibers, which can be clearly seen in Figure 3, as the standard deviation error bars of ΔR_a and ΔR_z do not overlap, while for ΔR_max, less overlap is observed. This result was to be expected, as natural, unbleached cellulose fibers contain natural pigments, lignin and hemicellulose which absorb UV radiation. This increase in surface roughness with UV radiation is consistent with the results of other researchers, who found that surface roughness increases after aging [30,31] and that, compared to coated papers, uncoated papers experience greater surface degradation in terms of roughness [30].

In general, flexographic prints on paper with bleached hemp fibers (HB), which are less rough than paper with natural unbleached fiber (Table 4), are more stable when exposed to UV radiation. Therefore, it could be concluded that bleached fibers provide paper with a less rough surface compared to natural unbleached fibers, and that papers with less surface roughness exhibit minor changes in roughness after being exposed to UV radiation. The greatest change in surface roughness of flexographic prints on paper with bleached fibers (HB) after UV radiation was observed for the yellow prints and was no greater than 3%, while the changes in other prints (cyan, magenta and black) were minimal and within the error of the measuring device. It is interesting that roughness parameters R_z and R_max on prints made on HB paper after UV radiation treatment were approximately the same or even slightly lower than the values measured before UV radiation treatment. Flexographic prints made on paper with bleached hemp fibers (HB) exhibited a minor change in roughness during exposure to UV radiation compared to flexographic prints made on paper with natural, unbleached hemp fibers (HN). It was also noticed, regardless of the paper’s composition as a printing substrate, that the greatest changes in ∆R_a value influenced by UV radiation occurred on the flexographic prints made with yellow ink, which have lightfastness of pigments with a value of five, while the flexographic prints with black ink, which have lightfastness of pigments with a value of seven (defined by the manufacturer Sun Chemical) are the most stable. Since the value of eight is the highest possible value describing strong lightfastness, it can be assumed that the UV radiation treatment has an influence on degradation of the pigments’ structure and, consequently, changes the surface roughness of the print.

The 3D surface plots diagrams present the topography of paper and flexographic prints (Figure 7, Figure 8 and Figure 9). It is obvious that papers with natural unbleached hemp fibers have a rougher surface than papers with bleached hemp fibers. From the 3D surface plots, no drastic surface changes were noticed after UV radiation treatment (Figure 7). Figure 8 and Figure 9 show that the topography of cyan and yellow flexographic prints does not undergo significant changes due to UV radiation treatment, and as such, it can be assumed that print quality will not be degraded. Namely, the graininess values of the prints made on papers with natural unbleached hemp fibers are higher than those produced on papers with bleached fibers, regardless of the ink used during the printing process (Table 6). The differences in roughness values (ΔR_a) measured on the paper surface with bleached fibers (HB), positively correlate (PCC = 0.9432, p = 0.056) with differences in graininess values calculated in the same way as the differences in roughness values (after UV treatment minus before UV treatment), while a negative correlation was found when comparing the same parameters with hemp paper made from unbleached fibers (PCC = −0.435, p = 0.565). UV radiation treatment did not appear to result in significant differences in the graininess values of any of the prints.

5. Conclusions

Based on the results of the surface roughness parameters of hemp papers and the surface roughness parameters of flexographic prints, it has been proven that bleached hemp fibers produce a paper and printed graphic product that exhibits minor changes in roughness values after exposure to UV radiation, while the same effect is not observed for natural, unbleached hemp fibers. It was noticed that natural unbleached hemp fibers form paper with a rougher surface than that of paper made with bleached hemp fibers. A positive correlation was found between the differences in roughness values (ΔR_a) and the differences in graininess values after UV radiation for prints made on hemp paper with bleached fibers, while UV radiation treatment did not appear to result in significant differences in the graininess value of any of the prints.