Study on light fastness of waterborne Ink-jet Printing ink on Spreader paper and non-Spreader paper

Inkjet printing technology has a commercial history of more than 10 years. With the advent of desktop printing, various coatings and additives have been applied to paper to improve the lightfastness of colors in office documents. Several studies have been done showing that additives and pigmented inks have a good protective effect on image quality under different exposure conditions. The ANSI IT9.3 Color Image Stability Subcommittee is developing testing standards for indoor light stability and outdoor durability. Additionally, the subcommittee is developing standards for moisture fastness, ozone fading, and thermal degradation/dark stability.

Today, general business reports, bills and data centers have begun to use full-color inkjet printing technology. These documents require low-cost paper to reduce running costs. In addition, due to the high speed of high-speed digital printing equipment, it is necessary to use azo-type dyes to meet its requirements. In general, transactional documents and bills require less long-term lightfastness than full-color photo-quality images produced by desktop inkjet devices. However, certain types of documents require long-term archivability.

For this new technology, inkjet printing, some basic research is needed to determine the relative lightfastness of uncoated or uncoated papers versus low-cost coated papers. In this study, we report the lightfastness of uncoated and coated papers printed with Scitex Digital Printing's VersaMarkTM full-color digital printing system. Gradual fade rates were compared using a xenon light source and compared to Florida sun exposure tests and indoor sunlight through window glass tests. These results were also compared to those obtained with a cool white light source and a purely fluorescent office light environment.

fluorescent light

Historically, photostability testing using high output cool white fluorescent lamps has been used for color photographic (ANSI IT9.9) testing. For example, standard test conditions of one day under 450 lux/12 hours of low wattage cool white fluorescent light, 60% relative humidity, and 70°F room temperature do not accurately simulate printing of various computer-generated images with inkjet inks. The final environment to use for the image. While the output of cool white fluorescent lamps may reproduce low light or museum environments to some extent, the light spectrum of these tubes is limited. By their very nature, the output of these lamps does not exactly reproduce the spectral power distribution of sunlight passing through window glass. For products whose main application environment is a lighted showcase or purely indoor fluorescent lighting environment, cool white fluorescent lamps can be used for testing. However, this test cannot accurately predict the lifespan of an image in a typical indoor environment (ie, in a home or office). Images placed near windows, sliding glass doors, skylights, etc. receive up to 50,000 lux of full spectrum sunlight (ultraviolet, visible and infrared) on a clear morning. Figure 1 shows the spectral output of a cool white fluorescent lamp compared to sunlight through a window glass.

Figure 1 - Comparison Between Cool White Fluorescent Lighting and Sunlight Through Window Glass

xenon arc lamp

In 1954, Germany began to use xenon arc lamps for accelerated aging tests. Xenon arc Test Chambers, such as the Q-SUN xenon arc Test Chambers, are best suited for testing the lightfastness of materials because they simulate well the full spectrum of sunlight available: ultraviolet, visible, and infrared all included. Xenon lamp aging Test Chambers use filters to simulate the corresponding spectrum (such as outdoor sunlight or sunlight through window glass).

Xenon lamps must be equipped with filters to reduce unwanted radiation. A filter of the type Windowpane simulates sunlight passing through a windowpane. It is usually used to test the main service life of the product indoors. Figure 2 shows the spectral power distribution of a xenon lamp equipped with a window glass filter compared to midday summer sunlight passing through the glass.

Figure 2 - Xenon lamp of the Q-SUN Test Chamber and sunlight through the window glass

test

Material

Materials used in this study were provided by various paper companies in the United States, Europe, and Japan. These paper products are divided into coated paper (CP) and uncoated paper (UC). The basis weight (paper product weight or BW) of CP1 type and CP2 type coated paper is 54 pounds per ream (80 g/m ² ) and 72 pounds per ream (107 g/m ² ), respectively. CP1 papers have a lighter coat weight than CP2 papers and the type of coating used is different. CP3 type paper is a single side coated (C1S) satin paper with a basis weight of 50 BW (74 g/m ² ). UC1 and UC2 are uncoated papers with basis weights of 50 BW (74 g/m ² ) and 60 BW (89 g/m ² ), respectively. UC1 is a basic uncoated paper, while UC2 is specially formulated to enhance the durability of inkjet inks when exposed to water. UC3 is a machine-made paper with a basis weight of 60 BW (90 g/m ² ), better for inkjet graphics

The inkjet inks used in this study were Scitex Digital Printing's Cyan #6092001, Magenta #6092002, Black #6092003 and Yellow #6092004. These inks are all water-based inks, except that the cyan dyes are prepared from phthalocyanine-based dyes, and the others are prepared from azo-type dyes. These dyes are known to have relatively good lightfastness when used in inkjet images advertised by manufacturers. These inks are formulated with other ingredients for use in Scitex continuous inkjet printers, and the inks have a viscosity of 1.1 centipoise.

print

All paper and inks are used on Scitex Digital Printing's VersaMarkTM commercial color press, which runs at 500 feet per minute. The continuous inkjet printhead uses a 9-inch wide 9500 series printhead, PS-90 print station and fluid system. All images are printed single-sided, with sufficient ink saturation for "better" image quality, but without causing the ink to fully bleed through the paper and appear on the unprinted side. The printed images in this research project are all 0.5 cm2.

Natural exposure and accelerated aging test

Both natural exposure and accelerated aging tests were performed at the Q-Lab Weathering Research Service Center. The readings of the instrument Colorimeter and densitometer were read in 4 areas of the sample every 10 h. For 6 samples of printed coated paper and non-coated paper, 3 replicates of each sample (18 samples in total), the following tests were carried out:

I. Outdoor exposure under a glass frame, in a well-ventilated exposure box facing south at 45° in Florida for 72 h (3 days).

II. In the QUV UV aging Test Chamber, the cool white fluorescent lamp exposure test was carried out according to ASTM G154 standard, the irradiance at 420 nm was 0.60 W/m ² , the chamber temperature was 31-35°C, and the test time was 40 h.

III. Xenon arc lamp exposure test was carried out in Q-SUN xenon lamp aging Test Chamber equipped with window glass filter according to ASTM G155 standard, the irradiance at 340 nm was 0.35 W/m 2 , and the black panel temperature was according to ASTM ^D3424 standard Method 3 is 63°C.

IV. The commercial document exposure test of Scitex Digital Printing Company was carried out in the inner corridor of the building (indoor pavilion), using fluorescent lamps and sunlight for 30 weeks or 5000 h. V. Scitex Digital Printing Co., Indoor (indoor fluorescent light) exposure test, using 2000 h of fluorescent light.

Measurement of LAB and ΔE value

All L*A*B* values ​​are measured using a Gretag SPM 50 Spectrophotometer according to ASTM D2244. The measurement conditions are D65 light source, 10°observation angle, including specular reflection. Three replicate samples of each paper type were prepared, all with color images printed on them, and measurements were taken on these samples at specified time intervals. ΔE values ​​are calculated from LAB measurements on printed images kept as dark as possible.

Densitometry

Density was measured using a Macbeth TR927 densitometer. R, G, B and Othro filters are used to print cyan, magenta, yellow and black images respectively

Test Results and Analysis

Exposure Test Results

Q-SUN 氙灯老化试验箱氙弧灯暴露测试初步显示涂布纸(CP)出现了相当大的ΔE值。众所周知，在染料及喷墨行业，品红色和黄色油墨经氙弧灯照射40 h后，会出现相当大的褪色，如图 3 所示。对于CP1和CP3来说，在氙弧光照射下，青色油墨褪色的 ΔE值小于品红色和黄色油墨，而黑色油墨的ΔE值是最小的。CP2的值在CP1和CP3之间。ΔE值大于10表明油墨或染料褪色程度相当高。

图3 – 紫外线光源下涂布纸的褪色情况

这些 ΔE值也与72 h佛罗里达玻璃框下太阳光曝晒的ΔE值进行了比较。从图 3 可以看出，这些值与40 h氙弧灯暴露测试得出的ΔE值相差无几。为了得到进一步的比较结果，图3中的这些数值再与2000 h到5000 h在室内大厅的自然室内光和荧光测试所得的数值进行了比较。同样，室内大厅暴露测试的 ΔE值与40 h的氙弧灯暴露测试和72 h佛罗里达太阳光曝晒测试的结果相当。

非涂布纸(UC)的暴露测试如图 4 所示。从图中可以看出，UC1和UC2的ΔE值和相对褪色程度要低得多。在大多数情况下，青色、品红色和黄色油墨的ΔE值不会超过图3中所示涂布纸的50%。尤其是与佛罗里达玻璃框下太阳光曝晒和室内大厅暴露测试存在较大差异。对于室内大厅暴露测试的非涂布纸来说，经5000 h后，黄色油墨以外的所有油墨的ΔE值均都在10 以下。

图4 – 紫外线光源下非涂布纸的褪色情况

本项目的主要研究重点在于说明，对于不同光源在相同的暴露时间下，涂布纸相比非涂布纸的褪色更厉害。需要考虑的主要因素是涂布纸的初始密度比非涂布纸高，如表1所示。从表中可以看出，对于大多数颜色来说，涂布纸(CP)的初始密度比非涂布纸(UC)的高。这种密度差异可以归因于一个事实，即油墨主要是由涂布纸的表层所吸收。

表1 – 涂布纸和非涂布纸的初始密度值

涂布纸上一般都涂有涂料，含有二氧化钛、碳酸钙和其它物质，能迅速吸收油墨的组分，从而使得染料保留在纸张的表面上，增加了油墨和染料颜色的光密度。CP2 的涂层最厚，所以除了品红色外，这种涂布纸的光密度值都是最高的。相比较而言，在非涂布纸或胶版纸上印刷低粘度的水性喷墨油墨时，墨水和染料很快会被纸张的内部结构吸收，使纸张表面的光密度变得较低。在UC1这种基本胶版纸上就可以看到这种情况。UC2和UC3经过了机械加工，除黑色外，光密度都比UC1型纸高。

氙弧灯试验的结果表明，各种涂布纸的褪色程度显著高于非涂布纸，从中可得出结论，初始光密度是一个影响因素。这个因素可以归因于一个事实，即涂布纸的表面上有更多的染料分子。所以，这些染料分子能更直接地接触到光源。而对于非涂布纸来说，整张纸都被较高浓度的染料分子渗透了。因此，纸纤维和配方组分可保护若干部位上的染料免受光源的直接照射。

在QUV试验箱中进行冷白荧光灯暴露测试，如图5和图6所示。从图5可以看出，在QUV荧光灯下暴露40 h后，涂布纸上黄色油墨的褪色程度ΔE值大于10。这些值与通常在办公环境中使用的室内荧光灯照明的值进行比较。CP3型纸上品红色油墨在该类光照下褪色程度很高。与图3的氙弧灯褪色相比，荧光光源下的褪色程度很轻。但与室内大厅暴露(5000 h)相比，其中也包括荧光光源，CP1黄色、CP3黄色和CP3品红色经高达2000 h的室内荧光灯照射后结果相当。对于黑色油墨，CP2的值在CP1和CP3之间。

图5 - 荧光光源下涂布纸的褪色情况

非涂布纸的情况如图6所示，在荧光光源照射下褪色程度非常低，而UC3黄色再次是一个例外。荧光照射2000 h后的ΔE约为30，与UC3在室内大厅(未示出)暴露2000 h后的ΔE为38.63 这一结果相差无几。黑色染料在荧光灯下的褪色程度非常低。对于黑色油墨，UC2的值在UC1和UC3之间。

图6 - 荧光光源下非涂布纸的褪色情况

排序相关性

对于大多数材料来说，将实时(自然)曝晒与实验室结果联系起来是非常困难的一件事(X小时自然曝晒= Y小时实验室加速暴露)。一些有用的方法之一是对相对排序进行比较。斯皮尔曼等级顺序是一种统计方法，可提供一整套性能排序的数值。例如，如果比较两组数据，排序表示两组数据之间匹配的紧密程度。数值为1. 0表示完全相关。随机的相关性表示值为0。负相关性表示值为-1.0。斯皮尔曼等级相关系数(rs)常用在有关的老化测试上。参见下面的表2。

要把加速老化测试和实时曝晒联系起来，需要将暴露于这两种环境中的材料的性能等级作比较，从而建立起测试之间的联系强度。

具体测试等级排序的讨论结论

暴露测试前后对所有测试样品进行仪器测色。以ΔE值记录每个试样的颜色变化。排序的相关性是按如下方法测定的：通过一个暴露测试得出C, M, Y 和 K(黑色)各类型样品在单个衬底上的ΔE值，同时得出另一个暴露测试在相同衬底上的ΔE值，然后将两组ΔE值作比较。例如，样品CP1在美国佛罗里达玻璃框下曝晒后的ΔE值(Y值)与CP1在Q-SUN氙灯老化试验箱中测试40 h后的ΔE值(Y值)相比较。

6种纸型衬底和油墨颜色中每一种的ΔE值都与另一种暴露测试中的相应值(例如，室内大厅暴露与QUV冷白荧光灯暴露)进行比较。然后According to有效数据确定斯皮尔曼相关系数。这些相关性的例子如图7 – 10所示。

图7 – 黄色油墨印刷品在QUV与室内荧光灯暴露下的排序

图8 – 品红色油墨印刷品在室内荧光灯与室内大厅暴露下的排序

图9 – 品红色油墨印刷品在Q-SUN与室内大厅暴露的排序

图10 – 青色油墨印刷品在Q-SUN与佛罗里达玻璃框下暴露的排序

图7显示了QUV老化试验箱暴露测试的结果与室内荧光灯褪色暴露测试结果之间的良好相关性，表明QUV老化试验箱能很好地确定室内办公环境中印刷品褪色的耐久性。图8表明品红色染料的强褪色表现与光源无关。图9中的差异可能是 Q–SUN老化试验箱和典型的室内办公环境之间的温度差异造成的。然而，在图10中较高的排序相关性表明氙弧灯暴露测试与佛罗里达玻璃框下曝晒可很好地应用于喷墨成像材料。

测试中的问题

进行耐光性测试时有多个需要考虑的问题，包括：光照、温度、材料的温度敏感性、湿度、黑暗稳定性、褪色的线性度、协同失效、气体(臭氧)褪色，纸张发黄及lux与W/m²之间的关系。由于有这么多参数，喷墨印刷品的耐光性测试结果可能会有差异。因此，只有对这些参数进行充分的调查研究，才能对使用寿命作出准确的预测。

结论

从本项研究可以得知，印刷在涂层衬底上的喷墨油墨比印刷在胶版衬底或非涂层衬底上的更容易受到紫外线照射的影响而退化。

带窗玻璃滤光器的氙灯老化试验箱可以预测带窗玻璃的室内环境中的油墨/衬底的耐光性。另外，配备冷白荧光灯管的QUV 紫外老化试验箱可以模拟室内环境的加速效果，适用于室内办公或零售环境中打印商业文件的喷墨油墨和衬底。佛罗里达玻璃框下自然曝晒可以作为加速试验，以模拟带窗玻璃的室内环境。本项研究中的差异性，可能与温度或湿度的差异有关。

室内大厅暴露，室内荧光灯暴露，实验室加速暴露(Q-SUN氙灯试验箱 和 QUV紫外老化试验箱)及佛罗里达玻璃框下太阳光暴露测试之间的排序相关性极好。自然和加速测试，都能区分产品耐光性的好坏。

本项研究表明，黑色喷墨油墨受自然和加速光稳定性测试的影响最小。黑色油墨使非涂布纸和涂布纸都有优良的可归档性。

Further testing of inkjet inks, coated and uncoated substrates should include simulation of ambient humidity, temperature and ozone. Currently, the repeatability and reproducibility of different exposure methods is unclear. Variations due to sample preparation and color measurement techniques are also unknown. It would be useful to establish an appropriate exposure test baseline for future studies based on actual real-world use. A consideration in research is to select a reference material of known lightfastness to be subjected to the exposure test along with the specimen. The data from the test samples can then be normalized to the properties of the reference material.