Rub resistance of ink jet prints on laboratory substrates with wheat pulp

Mehanička otpornost ink jet otisaka na laboratorijskim podlogama s pšeničnom pulpom

Maja Rudolf, Ivana Plazonić, Katja Petric Maretić, Irena Bates, Valentina Radić Seleš
University of Zagreb, Faculty of Graphic Arts

Abstract

Each printing ink is formulated according to a specific printing technique in order to give the print a high degree of rub resistance to maintain equal quality during transport, storage and end use. Rub resistance is a very important parameter, which depends on the printing ink, printing substrate and printing conditions. In this paper, the rub resistance of laboratory substrates with variable content of wheat pulp printed with ink jet printing technique was analyzed. The ink jet printing technique is increasingly used for small print runs due to its variability and easy print preparation. Resistance to rubbing was evaluated according to BS 3110 standard on environmentally friendly laboratory substrates printed with magenta and cyan ink. The print stability was observed based on the color change of the prints shown through the Euclidean color difference. The analysis results showed greater rub resistance of prints on laboratory substrates with a lower share of wheat pulp with respect to all the analyzed samples. All samples printed with magenta ink show a higher degree of rub resistance than the samples printed with cyan ink.

Keywords: Euclidean color difference, wheat pulp, ink jet print, rub resistance

Sažetak

Svaka tiskarska boja se formulira prema specifičnoj tehnici tiska kako bi otisak imao visok stupanj otpornosti prema otiranju radi održavanja jednake kvalitete pri transportu, skladištenju i krajnjem korištenju. Otpornost prema otiranju je vrlo važan parametar koji ovisi o tiskarskoj boji, tiskarskoj podlozi i uvjetima tijekom otiskivanja. U ovom radu, analizira se otpornost prema otiranju laboratorijskih podloga s varijabilnim sadržajem pšenične pulpe otisnutih s ink jet tehnikom tiska. Ink jet tehnika tiska se sve više primjenjuje za male naklade radi moguće varijabilnosti i jednostavne pripreme. Otpornost prema otiranju ispitivana je prema BS 3110 standardu na ekološki prihvatljivijim laboratorijskim podlogama otisnutim s cijan i magenta bojom. Stabilnost otisaka promatrana je temeljem promjene boje otiska prikazane kroz Euklidovu razliku boja. Rezultati analize pokazali su bolju otpornost prema otiranju kod otisaka na laboratorijskim podlogama s manjim udjelom pšenične pulpe u odnosu na sve analizirane uzorke. Svi uzorci otisnuti s magenta bojom pokazuju veći stupanj otpornosti na otiranje od uzoraka otisnutih cijan bojom.

Ključne riječi: Euklidova razlika boja, pšenična pulpa, ink jet tisak, otpornost na otiranje

INTRODUCTION
Uvod

This research is based on usability of papers produced with addition of non-wood fibres obtained from agricultural residues, straw of the most common crop species in Croatia. According to Croatian Bureau of Statistics (Table 1.), the most common straw producing crop is wheat [1]. Only maize is a crop produced in larger quantities than wheat, but maize does not produce straw.

Table 1. Representation of crop species in Croatia with production in tone per year since 2011 (Source: Croatian Bureau of Statistics)
Tablica1. Prikaz vrsta žitarica u Hrvatskoj prema godišnjoj proizvodnji u tonama od 2011. godine (Izvor: Hrvatski zavod za statistiku)

Crop species	Production per year (t)
	2011.	2012.	2013.	2014.	2015.	2016.	2017.	2018.	2019.
Wheat	782.499	999.681	998.940	648.917	758.638	960.081	682.322	738.363	789.950
Rye	2.949	2.426	2.955	2.800	3.356	4.646	2.566	4.100	6.912
Barley	193.961	235.778	201.339	175.592	193.451	263.165	260.426	227.520	275.397
Oats	77.223	94.542	60.178	56.555	71.743	80.414	68.333	44.827	57.585
Maize	1.733.664	1.297.590	1.874.372	2.046.966	1.709.152	2.154.470	1.559.638	2.147.275	2.298.316
Triticale	35.149	54.356	47.855	61.316	54.595	81.393	68.648	62.010	66.125
Other cereals	2.052	2.172	2.238	2.656	5.860	10.264	6.068	6.830	5.108

Straw is not considered to be waste but valuable raw material in many industries and fields of human interests such as: biofuel, construction industry, agriculture and livestock, applied arts etc. The significant raw materials for pulp and paper industry are fibres. To produce pulp fibres from cellulosic raw material (wood or non-wood) the most used pulping process is soda process where fibres are separated from plant tissues.  However, with contrast to wood fibres which are still the main source of raw materials for papermaking process, wheat straw fibres are characterized as shorter in average fibre length and narrow in width [2]. Therefore, straw pulp is often blended with pulp from other species if there is need for papers of high mechanical strength. In this research straw pulp was added in different ratios to pulp of recycled wood fibres to enrich shortened fibres by recycling in disintegration stage of paper production [3]. Based on the analysis of the results which will be determined by the characteristics of paper with wheat pulp, the recommendations of optimal amounts that can be used without losing paper quality could be given.

As paper is the most commonly used substrate in most sectors of the printing industry, it is of high importance to choose adequate printing substrate which will provide desired quality of the print by used printing technique. In our previous researches we observed that printed paper substrates with addition of wheat pulp up to 30% provide required characteristics for quality printing [4, 5].

Rub resistance is a property of printed substrate that depends on ink absorbance, paper surface and printing conditions. Stability of printed ink could be defined as resistance to fading during handling and touching the surface of paper during shipment, storage or handling by end users [5]. One of the ways to determine rub stability of prints is rub resistance test, which is important during evaluation of suitability of certain print technique on certain printing substrate [6]. The aim of this work was to evaluate the rub resistance of laboratory substrate with variable amounts of wheat pulp printed with two process UV curable inks using the digital ink jet printing technique.

Phases of the experiment were as follows:

1. Forming laboratory sheets of paper with variable content of wheat pulp
2. Printing with ink jet UV curable process inks
3. Testing for rub resistance and spectrophotometric measurements
4. Analysis and interpretation of testing results

EXPERIMENTAL PART
Eksperimentalni dio

1. Forming laboratory sheets of paper
1. Formiranje laboratorijskih listova papira

Laboratory sheets of paper were made on device Rapid Köthen (FRANK PTI) sheet former with grammage of 42.5 g/m². Raw materials that were used for production of paper were recycled paper pulp and wheat straw pulp. In total, four different types of laboratory paper samples were formed for research purposes: reference sample with 100% of recycled paper pulp and samples of this pulp blended with 10%, 20% or 30% of wheat pulp [7]. Table 1. shows sample marks that were used for each type of sample paper and ratio between pulps in the substrate.

Table 2. Laboratory paper substrates composition and their marks
Tablica 2. Sastav laboratorijskih papirnatih podloga te njihove oznake

Sample mark	Wheat pulp %	Recycled paper pulp %
N	0	100
1NP	10	90
2NP	20	80
3NP	30	70

2. Ink jet printing
2. Otiskivanje ink jet tehnikom tiska

All samples of laboratory paper were printed with digital ink jet technique on EFI Rastek H652 printer in full tone with magenta and cyan UV curable printing inks. Printing was done over the whole sample in "High quality" mode, 8 passes, with 600 dpi resolution and printing speed of 12.1 m²/h.

3. Rub resistance testing
3. Test na otiranje

Mechanical resistance test (ie. rub resistance test) was done on Hanatek T4 Rub and Abrasion Tester according to BS 3110 standard [8]. Original printed samples were cut to smaller round pieces with diameter of 5 cm. Printed samples were put together with unprinted samples on device discs and rub resistance test was performed with the pressure of 0.23 kg (0.5 lb) with circular motions of 20, 40 and 60 rotations at the constant speed of 1 rotation per second.

4. Spectrophotometric measuring
4. Spektrofotometrijska mjerenja

Before and after rub resistance testing spectrophotometric measurements were taken from all printed samples to determine the degradation of color presented by Euclidian difference. Spectrophotometer SpectroEye was used with illuminate D50 and 2° observer.  Euclidian difference (ΔE₀₀) was calculated based on colorimetric CIE L*a*b* values of 30 measurements made along each printed sample and rub resistance test conditions according to CIEDE2000 formula (1).

(1)

Where ∆L' represents difference in lightness between printed samples before and after rub resistance test, ΔC’ is the chroma difference between printed samples before and after rub resistance test, ΔH’ is hue difference between printed samples before and after rub resistance test. R_T is the rotation function, K_L, K_C, K_H are the parametric factors for variation in the experimental conditions and S_L, S_C, S_H are the weighting functions [9].

If the calculated Euclidean difference result is below 1, then the average eye of the observer cannot perceive the difference between two colors. Very small difference between colors can be perceived when the results are between 1 and 2, but it is tolerated, that is, the color difference up to 2 is acceptable [10].

RESULTS AND DISCUSSION
Rezultati i diskusija

Color difference between printed samples before and after rub test was measured and the results were divided into ranges of Euclidean color difference (ΔE₀₀) by 0.5 to observe the shift from negligible color differences to more significant. Table 3. shows the distribution of results of measuring color difference of four laboratory samples printed with magenta UV curable ink with respect to the number of rotations made with rub tester device (20, 40, 60). It is evident that for the majority of magenta printed samples calculated ΔE₀₀ was in the acceptable range from 0 –1.

Table 3. Distribution of ΔE₀₀ of printed samples with magenta ink after rub resistance test with 20, 40 and 60 rotations
Tablica 3. Distribucija ΔE₀₀ otisnutih uzoraka magenta bojom nakon testa otiranja s 20, 40 i 60 okretaja

magenta	N			1NP			2NP			3NP
ΔE00	20	40	60	20	40	60	20	40	60	20	40	60
0-0.5	57%	63%	57%	43%	43%	27%	37%	33%	23%	27%	43%	17%
0.5-1.0	40%	33%	33%	40%	47%	57%	50%	57%	60%	50%	30%	43%
1.0-1.5	3%	3%	7%	17%	10%	17%	10%	10%	17%	17%	10%	20%
1.5-2.0	0%	0%	3%	0%	0%	0%	3%	0%	0%	0%	13%	17%
2.0-2.5	0%	0%	0%	0%	0%	0%	0%	0%	0%	7%	3%	3%
2.5-5.0	0%	0%	0%	0%	0%	0%	0%	0%	0%	0%	0%	0%

For the referent laboratory sample N printed with magenta ink, 97% of measured differences were in the range from 0 to 1 for the rub resistance test performed with 20 rotations. With 40 rotations 96% of printed samples had color difference ΔE₀₀ from 0 to 1, while only 3% (one sample) was in the range from 1 to 1.5. With increasing abrasion on 60 rotations, 90% of magenta prints had color difference ΔE₀₀ between 0 and 1.   
Samples with added 10% and 20% of wheat pulp (1NP and 2NP) show similar results with slight decrease of printed samples with ΔE₀₀ in acceptable range from 0 to 1 regardless of number of rotations in rub testing – from 83% to 90%. Tests made on laboratory substrates with 30% added wheat pulp (3NP) show higher decrease of rub stability for magenta color. It was noticed that ΔE₀₀ in the range of 0 – 1 have 77% of samples during testing with 20 rotations, 73% with 40 rotations, and 60% with 60 rotations. On graphs in Fig. 1 the distribution of results is shown in respect with number of measurements on Y axis, and ranges of ΔE₀₀ by 0.5 on X axis. Vertical line between ranges 1.5 – 2 and 2.0 – 2.5 distinguishes the acceptable ΔE₀₀ ranges on the left, and noticeable color differences above ΔE₀₀ 2 on the right.

Fig 1. Distribution charts of ΔE₀₀ color difference of magenta prints on four types of paper before and after rub resistance testing with 20, 40 and 60 revolutions
Slika 1. Grafovi distribucije ΔE₀₀ razlike u boji magenta otisaka na četiri vrste uzoraka papira prije i poslije testa otiranja s 20, 40 i 60 okretaja

For the referent sample the largest number of ΔE₀₀ measurements made on magenta printed samples were in the range of 0 – 0.5. As the share of added wheat pulp rises in the paper substrate, a slight shift is observed toward the range of 0.5 – 1 and higher. On laboratory substrates with added 10% and 20% of wheat pulp majority of ΔE₀₀ results were in the range of 0.5 – 1, and results of ΔE₀₀ above 2 were not measured. For the magenta prints on paper substrates with added 30% of wheat pulp further degradation of rub stability was observed, while ~20% of ΔE₀₀ results were in the range above 1.5 and only 3% above 2.

Table 4. Distribution of ΔE₀₀ of printed samples with cyan ink after rub resistance testing with 20, 40 and 60 rotations
Tablica 4. Distribucija ΔE₀₀ otisnutih uzoraka cijan bojom nakon testa otiranja s 20, 40 i 60 okretaja

cyan	N			1NP			2NP			3NP
ΔE00	20	40	60	20	40	60	20	40	60	20	40	60
0-0.5	57%	53%	23%	30%	30%	30%	17%	20%	23%	23%	20%	20%
0.5-1.0	30%	40%	40%	47%	30%	47%	43%	43%	47%	30%	50%	53%
1.0-1.5	13%	7%	27%	23%	40%	17%	33%	20%	17%	23%	17%	10%
1.5-2.0	0%	0%	10%	0%	0%	3%	3%	10%	10%	10%	10%	10%
2.0-2.5	0%	0%	0%	0%	0%	3%	3%	3%	3%	10%	0%	7%
2.5-5.0	0%	0%	0%	0%	0%	0%	0%	3%	0%	3%	3%	3%

Table 4. shows distribution of ranges of ΔE₀₀ values for samples printed with cyan ink on all laboratory formed paper substrates with different rub testing conditions (20, 40 and 60 rotations). Results confirm that the majority of samples had acceptable range of color difference (ΔE₀₀) between 0 and 1. For the referent substrate with no wheat pulp added, 87% of printed substrate had ΔE₀₀ in the range of 0 – 1 for 20 rotations, 93% for 40 rotations and 63% for 60 rotations. For the same ΔE₀₀ range printed substrate with 10% wheat pulp show lower percentage of measured ΔE₀₀ than referent samples: 77% for 20 rotations, 60% for 40 rotations and 77% for 60 rotations. Prints made on laboratory substrates with 20% and 30% of wheat pulp show further decrease of samples with lower ΔE₀₀ values, with similar percentages (from 53% to 77%) and show lower stability than printed magenta samples with the same percentage of added wheat pulp into paper substrate.

Fig 2. Distribution charts of ΔE₀₀ color difference of cyan prints on four samples of paper before and after rub resistance test with 20, 40 and 60 revolutions
Slika 2. Grafovi distribucije ΔE₀₀ razlike u boji cijan otisaka na četiri vrste uzoraka papira prije i poslije testa otiranja s 20, 40 i 60 okretaja

Graphs in Fig. 2 show the distribution of results in respect with number of performed measurements along the same sample on Y axis, and ranges of ΔE₀₀ by 0.5 on X axis for the cyan printed samples. Prints obtained on laboratory substrate with the mark N, used as reference sample in this research, gave mainly color difference (ΔE₀₀) results in the range of 0 – 0.5, which is defined as unnoticeable color difference for the average observer. Printed laboratory substrates with the addition of 10% wheat pulp have color difference slightly shifted to the range of 0.5 – 1 which is still acceptable and only two measurements of ΔE₀₀ above 1.5 were recorded. In prints on laboratory substrates with 20% and 30% of wheat pulp further shift can be observed, where some color differences are greater than 1.5 (less than 20% of measurements) but still most of measured ΔE₀₀ differences are in the range of visual acceptability. Cyan prints show greater degradation of the color and therefore less rub stability than magenta prints.

CONCLUSION
Zaključak

Based on the results of performed analysis in this research, it can be concluded that the rub resistance of all tested prints on laboratory substrates with wheat pulp was in the satisfactory range of Euclidian color difference (ΔE₀₀). Increased number of rotations in rub resistance test resulted in slightly increased color differences values for all observed prints. As well, better rub stability was noticed for prints on papers with lower share of straw pulp. However, our research has shown that printed papers with share of wheat pulp less than 30% provide good rub stability for prints and those laboratory substrates are recommended for use in graphic industry. In general, for prints obtained with magenta ink were noticed less color degradation due to different ink absorption into the substrate than on prints made with cyan ink.

LITERATURE
Literatura

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Korespodencija:

Maja Rudolf, Sveučilište u Zagrebu, Grafički fakultet
maja.rudolf@grf.unizg.hr

Acknowledgements:

This work has been supported in part by Croatian Science Foundation under the project “Printability, quality and utilization of substrates with non-wood fibers” (UIP-2017-05-2573)