Utjecaj vlakanaca konoplje u papiru na kemijsku stabilnost otisaka

Lahorka Malnar, Ivana Plazonić, Željka Barbarić-Mikočević

Sažetak

U posljednjih nekoliko godina zamjetan je porast upotrebe ne-drvnih vlakanca u industriji celuloze i papira. Bez obzira na podrijetlo vlakanaca, vrlo je važno da korištena vlakanca daju papir zadovoljavajuće kvalitete te otiske kao konačni grafički proizvod. Cilj ovog istraživanja je analiza nekih kemijskih, optičkih i površinskih karakteristika papira koja sadrže vlakanca konoplje, a na kojima su načinjeni otisci. Otisnuti papiri s vlakancima konoplje su analizirani na kemijsku stabilnost na vodu, alkohol i lužinu skladno standardu ISO 2836:2004. Procjena kemijske stabilnosti otisaka načinjena je mjerenjem Euklidove razlike u boji (Euklidova udaljenost od CIE L*a*b* koordinata, ΔE00 vrijednost) i hrapavosti površine otiska (Ra vrijednost).

Ključne riječi: vlakanca konoplje, sastav papira, otisak, kemijska stabilnost

The influence of hemp fibres in paper on chemical resistance of prints

Lahorka Malnar, Ivana Plazonić, Željka Barbarić-Mikočević

Abstract

In the last few years the consumption of non-wood fibres in pulp and paper industry has been showing the increasing trend. Regardless to the fibre origin, it is very important that used fibres provide good quality of the paper and their printability for the final graphic product. The aim of this research was the evaluation of some chemical, optical and surface properties of hemp fibre based papers which were used for making prints. Hemp fibre based papers were printed and the chemical resistance of prints has been analysed. In accordance with ISO 2836:2004 all prints were tested for resistance to water, alcohol and alkali. Evaluation of chemical degradation on prints was determined through the Euclid color difference (Euclid distance of CIE L*a*b* coordinates, ΔE00 value) and surface roughness (Ra value).

Keywords: hemp fibre, paper composition, print, chemical resistance

1. Introduction

1. Uvod

Nowadays, the paper industry is faced with a global deficiency of wood raw materials and alternative sources of virgin cellulose fibres are playing an important role in the paper production. Wood is still the most widely used raw material in the pulp and paper production in the world, but the consumption of non-wood fibres has been showing the increasing trend in the last few years. Many useful fibres could be obtained from various parts of plants including leaves, stems (bast fibres), fruits and seeds. Leading users of non-wood fibres in papermaking are Asia and the Pacific countries [1]. At the present time, the most commonly utilized non-wood fibre is straw, but other non-wood fibres such as cotton, hemp, sisal and kenaf are also becoming more important in the manufacture of pulp and paper. Regardless to the fibre origin, wood (hardwood and softwood) or non-wood (agricultural residues, industrial residues and naturally growing plants), it is very important that selected fibres provide good quality of the paper. Hemp (Cannabis sativa) as new lignocellulosic fibre resources for pulp and paper industries has a number of advantages (long and strong fibre, low lignin content). This is an annual plant reaching a height of 4-5 m and yielding 12-14 tons of dry matter/yr. ha. Only 80-150 days are needed for plants to be mature for fibre harvesting. Namely, 10-12 tons dry matter/yr ha can be harvested as fibre mass, 35% of which are long bast fibres and 65% are short core fibres. Hemp fiber for papermaking can have a fiber length from 15 to 55 mm. It is generally believed that the male plant is superior in respect of fibre compared to the female plant so male plants, therefore male hemp fibres are used in papermaking process [2, 3].
For achieving the high quality of the final graphic product, the printability of paper and the stability of those prints have an important role. Since image quality can be affected by paper characteristics in various ways, the research presented in this paper focuses on the evaluation of the paper with different mass ratio of hemp fibres and the chemical resistance of the prints made on those substrates. For that purpose hemp fibre based papers were printed and the chemical resistance of prints has been analysed. In accordance with ISO 2836:2004 all prints were tested for resistance to three different test liquid agents: water, alcohol and alkali [4]. Evaluation of chemical degradation on prints was determined through the Euclid color difference and surface roughness.

2. Experimental

2. Eksperimentalni dio

2.1. Materials
2.1. Materijali

Three types of commercially available hemp fibre based papers (grammage 90 g/m2) were used for making prints:
1. Handmade sustainable paper – paper made from 100% wildgrass hemp is unbleached, uncoated printer-compatible (Laser and Inkjet) paper in natural colour with natural deckled edge. Paper manufacturer is Distant Village, US.
2. Handmade sustainable paper – paper made from 100% hemp plant fibre is non-chlorine bleached uncoated printer-compatible (Laser and Inkjet) paper in cream colour with natural deckled edge. Paper manufacturer is Distant Village, US.
3. Hemp Heritage® – uncoated natural white office paper made from a mixture of 25% hemp and 75% post-consumer fibre. Paper manufacturer is Green Field Paper Company, US.
These papers were printed in full tone black colour by HP LaserJet 1320 printer and produced prints were samples for further analysis.

2.2. Methods
2.2. Metode

2.2.1. Analysis of hemp fibre based papers used for making prints
2.2.1. Analiza papira s vlakancima konoplje korištenih za izradu otisaka

The chemical composition was determined in accordance with the standard T412 om-94 (moisture content) and T413 om-11 (ash content).
The optical properties were evaluated based on ISO Brightness and CIE L*a*b* values. The measurements were performed in accordance with the standard ISO 2470 (ISO Brightness, R457) with a Color Touch 2 spectrophotometer at D65/10º and CIE L*a*b* values with a spectrophotometer X-rite DTP 20 at D65/10º.
The average surface roughness (Ra) was measured with a Surface Roughness Tester TR200.

2.2.2. Analysis of prints
2.2.2. Analiza otisaka

The optical properties and average surface roughness (Ra) of the prints were performed with the same equipment as on the hemp fibre based papers.
The research objective was to evaluate the effect of hemp fibres in paper on chemical resistance of prints. The prints were tested for resistance to the following agents in accordance with International Standard ISO 2836:2004 [4]. In the field of printing industry this standard defines methods of assessing the resistance of prints to liquid and solid agents, solvents, varnishes and acids. Used test liquid agent and contact time of print with defined test agent is presented in table 1.

Table 1. Used test liquid agent and print contact time
Tablica 1. Korištene tekućine i vrijeme tretmana otiska

According to this standard method of assessing the resistance of prints is not equal for all used test liquid agents. The print resistance to water and alkali were analysed by the same procedure, which is different from the procedure defined for print resistance to alcohol.
The method of assessing the resistance of prints to water and alkali
Print sample was placed on a glass plate and sandwiched between four strips (two each side) of filter paper soaked in the test liquid agent. Another glass plate was placed on top and the sample was put under pressure by applying a 1 kilo mass on top of the plate and left for 24 hours.
The method of assessing the resistance of prints to alcohol
Print sample was place in a glass tube containing ethanol and immersed for five minutes.
When the contact time expired, treated print samples were dried in an oven at 40ºC for 60 minutes prior to assessment.

Dried treated print samples were compared to untreated print samples and changes in optical and surface properties caused by treatment were noted and discussed. Chemical resistance of prints was monitored through changes in the optical properties (Euclidean difference, ΔE00) and the surface roughness (ΔRa) of chemically treated samples.
The difference between print colour before and after chemical treatment is calculated according to equation 1 [5]:

                          (1)

Where:

The difference in the print surface roughness before and after chemical treatment was calculated according to equation 2:
                                                   (2)
Where:

	–	average surface roughness of untreated print
	–	average surface roughness of chemically treated print

3. Results and discussion

3. Rezultati i diskusija

Properties of hemp fibre based papers used for making prints are substantially influenced by the characteristics of fibres, fillers and additives used during papermaking process.
The papers were imaged using an Olympus BX51 System Microscope at 100x magnification. Microscopic images and measured chemical, optical and surface properties of those papers are presented in Table 2.

Table 2. The properties of hemp fibre based papers
Tablica 2. Karakteristike papira s vlakancima konoplje

In the microscopic images the individual hemp fibres and their colour are clearly visible (natural colour of hemp fibres in unbleached Paper 1and bleached hemp fibres in Paper 2). The results of chemical properties have shown how Paper 1 have the lowest content of ash (0.735%) and the highest content of moisture (4.980%). Paper 1 is handmade paper consisted only from natural unbleached hemp fibres with very small share of inorganic components. Therefore it can be concluded that chemical treatment of fibres and the addition of chemicals during papermaking process, affecting the absorption of moisture. Namely, Paper 1 absorb the most moisture from the air comparing to the other papers what is the consequence of the smallest share of ash (indicates the smallest addition of chemicals). Unlike him, the Paper 3 (industrial paper made of hemp fibres with a share of 25%) has the largest proportion of ash (4.078%) and the lowest moisture content (3.092%). The chemical composition of the paper and the papermaking method (handmade/industrial) significantly affect the moisture content and ash content in the paper.
Measured brightness value is the lowest for Paper 1 (37.698) and the highest for Paper 3 (81.586). In the CIE L*a*b* colour space the value L* represents the lightness of the colour, while the value +a* represents redness or the value –a* represents greenness and the +b* value represents yellowness or the value –b* represents blueness. For all analysed papers the colour values (a* and b*) are positive, which gives printing substrates the appearance of red-yellow colour. The highest values of +a* and +b* are measured on Paper 1 (unbleached hemp fibres), significantly lower on Paper 2 (bleached hemp fibres) and the lowest on Paper 3 (mixture of hemp and post-consumer fibres). These results confirm that optical properties of paper are in direct dependence on papermaking processes i.e. bleaching.
The roughness of paper surface is connected with paper composition and papermaking process. The highest average surface roughness is measured on Paper 1 (5.695 µm), lower on Paper 2 (4.770 µm) and lowest Ra value is measured on Paper 3 (4.395 µm).

Table 3. The properties of prints
Tablica 3. Karakteristike otisaka

By printing the hemp fibre based paper in full-tone of black colour, L* value is significantly reduced for all samples (Table 3). The values of a* and b* measured after printing are still positive (the prints are still in the red and yellow part of the spectrum), but their values in relation to the unprinted papers are significantly smaller. In fact, their values are still at highest on prints made on Paper 1. Generally, the surface roughness of the prints is increased in relation with unprinted paper sample. The highest measured Ra value is on Print 1.

The influence of hemp fibres on chemical resistance of prints was evaluated through Roughness difference (Figure 1) and Euclidean color difference (Figure 2).

Figure 1. The influence of the test liquid agents on prints Euclidean colour difference
Slika 1. Utjecaj kemijskih sredstava na Euklidovu razliku boja otisaka

By comparing spectrophotometric values (L*a*b*) of prints before and after the treatment with test liquid agents, the difference between print colour i.e. ΔE00 is calculated according to equation 1. All used test liquid agents influenced on the CIE L*a*b* values of prints. For all prints treated with all test liquid agents ΔE00 is in range from 0.35 to 2.25 which are not visible to an average observer (Figure 1). If we compare the influence of all test liquid agents on prints Euclidean colour difference, the water had the minor influence on ΔE00 values. The Print 1 and Print 2 have shown the same trend in ΔE00 changings after treatment with test liquid agents as follows: water ˂ alcohol ≈ alkali. For Print 3 ΔE00 values are similar for water and alcohol, and slightly lower for alkali. Print made on Paper 3 shows the greatest stability in all tested liquid agents. From these results it could be assumed how chemical composition of paper has important influence on chemical resistant of prints. Paper 1 and Paper 2 are consisted only from hemp fibres, which are bleached in Paper 2 i.e. unbleached in Paper 1. Paper 3 is a mixture of hemp (25%) and post-consumer fibre. Beside fibres, the chemicals additives in paper have an important influence on chemical resistant of prints. Namely, by analysis of ash content (which is equivalent to inorganic content), it is observed how ash amount in Paper 1 is negligible, slightly higher in Paper 2 and the highest in Paper 3 (Table 2). These results are in accordance with published results Bates et all, which have proven that coated papers and boards show greater chemical stability on alcohol, alkali and water from recycled paper [6].
In Print 1 and Print 2 the ink is in direct contact with hemp virgin fibres, unlike in Print 3 where the ink is in contact mainly with chemicals additives. Therefore the highest chemical resistance shows industrially made Paper 3. On the other hand, bleached hemp fibres in handmade Paper 2 give prints with the lowest chemical resistant regardless to test liquid agent used in this research.

Figure 2. The influence of the test liquid agents on prints surface roughness difference
Slika 2. Utjecaj kemijskih sredstava na hrapavost površine otisaka

It is noticed that the average surface roughness of all prints increased after treatment in water and alkali, while treatment in alcohol has positive influence on surface roughness of all prints (Figure 2). This result can be explained by contact time of used test liquid agent and print, where contact with alcohol was only 5 minutes i.e. 24 hours with water and alkali. Print 2 and Print 3 treated with alcohol have negligible changes in surface roughness, while Print 1 showed significantly changes in decreasing surface roughness (positive ΔRa value). Regardless to the same duration of contact time, ΔRa value is higher after treatment with alkali then treatment with water, especially for Print 1 and Print 3.

4. Conclusion

4. Zaključak

The aim of the research was to point out the influence of hemp fibres in substrate on chemical resistance of prints. Taking into account all obtained results, the following could be concluded:

• Prints made on the handmade hemp fibre based papers show lower chemical resistance than prints made on industrial made paper.
• Unbleached virgin hemp fibres provide better chemical resistance of prints than bleached virgin hemp fibres.
• Water as a test liquid agent has the smaller influence on print degradation, then alcohol and alkali.

5. Literature

5. Literatura

[1] Pande, H; Non-wood fibre and global fibre supply. Unasylva - No. 193, An international journal of the forestry and food industries - Vol. 49 - 1998/2; University of Toronto, Canada, p. 44-50
http://www.fao.org/docrep/w7990e/w7990e08.htm#non%20wood%20fibre%20and%20global%20fibre%20supply
[2] Chandra, Mudit; Use of nonwood plant fibers for pulp and paper industry in Asia:
potential in China. p.91. Blacksburg, Virginia
[3] Bowyer, Jim L.; Hemp (Cannabis sativia L.) as a papermaking raw material in Minnesota : Technical, Economic and Environmental consideration, 2001
[4] International Standard ISO 2836:2004(E), (2004), Graphic Technology- Prints and printing inks- Assessments of resistance to various agens, Geneva, Switzerland
[5] Yu, Lufei; Tolerance Equivalency between ΔE*ab and ΔE00 Metrics, 2015. Thesis. Rochester Institute of Technology. Accessed from
[6] Bates, Irena; Mihić, Josip; Vuksanović, Antonia; Radić Seleš, Valentina; Analiza kemijske stabilnosti otisaka na prehrambenoj ambalaži; In: Proceedings of 18th International conference of Printing, Design and Graphic Communication Blaž Baromić 2014; ed. Mikota, Miroslav. Zagreb: Hrvatsko društvo grafičara, 2014. 13-24