Test and Evaluation of Light Resistance Aging of Printing Products and Printing inks

【Abstract】In this paper, the lightfastness of printed matter and printing ink is studied by the xenon lamp accelerated aging test method, and the interpretation of GB/T 22771-2008 is carried out. Firstly, the principle of light aging test of printed matter and printing ink is introduced, and then 13 kinds of offset printing inks are exposed to light by using Q-Sun B02 xenon lamp Test Chamber . Finally , the test results are analyzed on the basis of interpreting GB/T 22771-2008 standard, and It is pointed out that the GB/T22771-2008 standard may have inaccurate evaluation problems and solutions when exposing a series of samples.

【Key words】   printed matter; printing ink; light aging resistance; xenon lamp test; GB/T 22771-2008

1 GB/T 22771-2008 Standard Background

    More and more inks require light fastness testing, for example: jet printing inks and inks for packaging materials. The GB/T 22771-2008 standard "Assessment of Lightfastness of Printed Matters and Printing Inks by Xenon Arc Lamps in Printing Technology" was modified to adopt the international standard ISO 12040:1997[1]. Light aging test method, this standard is also the first national standard for printing ink light aging test. Applying this standard can evaluate the light fastness level of different printing and ink samples.

    The following part of this article will demonstrate how to implement the standard through specific light aging tests to evaluate the light fastness level of printed matter and printing inks. At the same time, it points out the deficiencies of the standard and the places that need to be improved.

2 The principle of light aging test of printed matter and printing ink

2.1 Outdoor aging factors

Aging damage is mainly caused by three factors: light, temperature and humidity. Any one of these three factors will cause material aging, and their combined effect is greater than the harm caused by any one of them.

2.1.1 Lighting

The chemical bonds of polymer materials have different sensitivities to different wavelengths of sunlight in sunlight, generally corresponding to a threshold value. The short-wavelength ultraviolet rays of sunlight are the main reason for the aging of most polymer physical properties. For example, the action threshold of CN (carbon-nitrogen) bond is 393nm. However, some prints and ink systems can be damaged by long-wave UV or even visible light, causing discoloration or fading.

2.1.2 Temperature

The higher the temperature, the faster the chemical reaction. The aging reaction is a photochemical reaction, and the temperature does not affect the photoinduced reaction speed in the photoinduced chemical reaction, but affects the subsequent chemical reaction speed. Moreover, the effect of temperature on material aging is often nonlinear.

2.1.3 Humidity

Water will directly participate in the material aging reaction. Relative humidity, dew and rain are the main manifestations of water in nature. Studies have shown that outdoor materials will be in a humid state for a long time every day (up to 8-12 hours per day on average [2]. Dew is the main cause of outdoor humidity. Dew is more harmful than rain because it adheres to the material The longer the time on the surface, the more serious moisture erosion will be formed. For indoor materials, generally only the influence of relative humidity is considered.

2.2 Xenon lamp accelerated light fastness test

2.2.1 Sunlight simulation

As the light source of the xenon lamp Test Chamber [3], the xenon lamp can produce ultraviolet, visible light and infrared rays, which can well simulate full-spectrum sunlight. The spectrum generated by the xenon lamp needs to be filtered before being used for testing to reduce the unwanted part of the ultraviolet spectrum. Different spectra can be obtained by using different types of glass filters. The use of filters depends on the material being tested and the end-use conditions of the product. Different filters have different cut-off points for the short-wave UV rays, which will largely affect the speed and type of aging. There are three types of filters that are commonly used: daylight filters, window glass filters, and extended UV filters. Window glass filters are generally selected in printing and printing ink tests. As shown in Figure (1), it is the difference between the spectrum of Q-Sun xenon lamp Test Chamber equipped with filters and the spectrum of sunlight passing through glass by Q-Lab Company. Compare.

2.2.2 Irradiance Control

    The new xenon lamp Test Chamber needs to be equipped with an irradiance control system. For example, the Q-Sun xenon lamp Test Chamber of Q-Lab Company uses a solar eye closed-loop control system to provide stable light intensity.

In a xenon arc test system, irradiance control is very important. The spectral range of xenon lamp extends from 295nm to 3000nm. The new standard requires that the control of light intensity is based on point control (such as ISO11341-2004[4], ASTM G 155-05a[5], ISO4892-2:2003[6], etc. ), the selection of the control point is based on the difference between the simulated environment and the performance of the detection material. For the outdoor environment, the physical properties of the detection material are generally used. The 340nm control point is generally used; for the indoor environment, the discoloration and fading of the detection material is generally used. The 420nm control point . In some standards, 300~400nm TUV control is also used.

2.2.3 Temperature control

In xenon arc test equipment, temperature control is also important because temperature affects the rate at which materials age. The xenon lamp Test Chamber generally uses a black panel thermometer or a black standard thermometer to accurately control the surface temperature of the sample. Generally, most of the ISO standards in Europe use black standard thermometers, while some standards in the United States use black panel thermometers. Some types of xenon lamp Test Chambers can also control the air temperature of the chamber at the same time, so as to meet the requirements of comprehensive control of the exposure environment temperature.

2.2.4 Humidity simulation

    The xenon arc Test Chamber can simulate the effect of humidity on light fastness through water spray or humidity control system. Water spray can simulate the thermal shock and stress corrosion of rainwater on outdoor products. Humidity will affect the type and speed of aging of certain items (such as certain textiles or inks, etc.), and it is recommended to control relative humidity in the test standards for such materials.

Figure (1) Comparison between Q-Sun spectrum equipped with window glass filter and solar spectrum through glass

3 Printed matter and printing ink xenon lamp aging test and evaluation

3.1 Test conditions and process

    In our opinion, there are two exposure methods included in the section "4.4.1 Exposure" of GB/T 22771-2008.

    One is the exposure test for only one sample. This test is relatively simple, as long as the test sample is exposed to light according to the prescribed method (that is, the test conditions required in the standard) until obvious changes occur. The standard also points out that "the so-called obvious change is equal to or less than grade 3 of the gray scale card for assessing discoloration in accordance with GB/T 250. The observer should ensure that no visual fatigue occurs during the test, and the visual observation should have standard Observation conditions for prints".

    The other is to conduct simultaneous and systematic experiments on the same series of prints. This test uses a continuous covered exposure method to test the lightfastness of prints with different degrees of lightfastness in one test. Specific steps are as follows:

    Step 1: Expose the sample until the grade 3 blue wool standard shows obvious changes corresponding to grade 3 of the gray sample card for assessing discoloration;

    Step 2: cover a quarter of the exposed part of the printed matter and the blue wool standard, and continue to expose until the blue wool standard of grade 5 shows obvious changes corresponding to grade 3 of the gray scale for assessing discoloration;

    Step 3: Cover a quarter of the part that is continuously exposed to the sun, and continue to expose until the blue wool standard of grade 6 shows an obvious change corresponding to grade 3 of the gray scale for assessing discoloration;

    Step 4: Cover more and continue to expose to the sun until the changes that can just be observed visually occur in the blue wool standard of grade 7 (grade 4 of the gray scale for evaluating discoloration).

Because under normal circumstances, there are many test samples in the ink factory or printing factory, if the first exposure method is used, it is not realistic to test only one sample at a time. Therefore, we chose the second exposure method for this test, and carried out xenon lamp accelerated lightfastness aging test on 13 different offset printing ink samples at the same time. At the same time, we also carried out the exposure and the blue wool standard of SDC1 to 7. The specific test conditions are shown in Table (1) .

Table (1) Test conditions for xenon lamp accelerated light aging test

3.2 Test results and analysis

3.2.1 Test results

The "4.4.2 Evaluation" section of GB/T 22771-2008 also gives the method for evaluating the lightfastness level of the sample, specifically: determine the blue wool standard sample that has the same obvious change as the test sample within the same time, Use the grade number of the blue wool standard sample to indicate the lightfastness grade of the test sample.

In order to verify whether the visual inspection results are correct, we also obtained the color change △E* of the exposed samples and the blue wool standard by instrumental measurement. We use CIE L*a*b* color units, D65 light source and 10°observation angle.

表(2)给出了几个具有代表性的样品的试验结果，包括它们的△E*数值。一般来说，曝晒样品的△E*数值与SDC蓝色羊毛标准的△E*。数值进行比较可以客观给出样品的耐光等级。

表(2)代表性样品的试验结果

3.2.2 试验结果分析

在进行加速耐光试验之前，我们已经大概知道，这13个样品中有的耐光性很好，有的很差，有的一般，各样品之间的差异性很大。试验结果显示，确实如预测的一样，这就说明，使用Q-SunB02氙灯试验箱可以区分样品的不同耐光性。如表2中的样品l就是耐光性很好的样品，而样品2和样品7是耐光性很差的样品，样品10是耐光性一般的样品。

对于表(2)中的评级，其实GB/T 22771-2008标准中没有具体的说明。我们是按照自己对标准的理解进行评级。先是在不同曝晒阶段分别对样品进行评级，最后再给出一个综合耐光等级。这个所谓的“综合耐光等级”是在样品出现明显变化的那个阶段评定的耐光等级。如样品2，它在第一阶段(曝晒2l小时)和第二阶段(曝晒68小时)未出现明显变化，而在第三阶段(曝晒156小时)出现了明显变化，所以我们把在第三阶段评定的耐光等级作为综合耐光等级。

3.2.3 GB/T 22771-2008评定方法存在的问题

从表(2)的试验数据，我们可以看出，应用GB/T 22771-2008中的第二种曝晒方法对样品进行耐光等级评定时，有时会不准确。例如：表(2)中的样品2和样品7，在每个曝晒阶段进行评级时，样品2在前三阶段好于样品7，在最后阶段与样品7相当，但是最终的综合评级却是样品2为1级，样品7为2级。为了形象地解释这种奇怪的情况，我们作出了两个样品的△E*与曝晒时间之间的关系图，如图(2)所示。

从图(2)中我们发现，样品的△E*随时间的变化并不是线性的。有的样品，如样品2，一开始颜色变化比较缓慢，而曝晒到了一定时间，颜色则开始发生快速变化。而有的样品，如样品7，则在曝晒开始阶段就显示快速的颜色变化，相反，等曝晒了一段时间之后，颜色变化趋于平缓。样品2的耐光等级比样品7差的原因在于：样品2在前两个曝晒阶段(分别曝晒21小时和68小时)未出现明显变化，所以只能将发生明显变化的第三个曝晒阶段的耐光等级作为最终的综合耐光等级；而对于样品7，它在第二个曝晒阶段就出现了明显变化，所以在这个时候就可以评定样品的综合耐光等级。

图(2)样品2(方形)和样品7(圆点)的△E*与曝晒时间之间的关系

所以确定合适的曝晒终点尤为重要。GB/T2277l-2008标准中确定的这四个曝晒阶段能大体评定样品的耐光等级，但是有时也是存在上述问题。

4结论及建议

我们通过具体试验分析了13种不同胶印油墨样品的耐光老化性能，同时也分析解读了GB/T2277l-2008的部分内容，尤其是曝晒、耐光等级评定等章节。试验结果显示，按照GB/T2277l-2008标准，可以区分曝晒样品耐光性的好、中、差。

Although the GB/T 2277l-2008 standard is a commonly used standard for evaluating the light aging resistance of printed materials and printing inks, the overall evaluation results are true and objective, but sometimes controversial test results are also given. When the test results are in dispute, we recommend applying the first exposure method in the standard, that is, to expose each sample and a set of blue wool standards separately, and when the sample changes significantly, compare it with the blue wool standard, and use The grade number of the blue wool with the same obvious change as the sample indicates the lightfastness grade of the test sample.

【references】

【1】ISO 12040:1997 "Evaluation of Lightfastness of Printed Matters and Printing Inks by Filtered Xenon Arc Lamps in Printing Technology".

【2】Douglas M．Grossman, The right choice-UV fluorescent testing or xenon arc testing. PCI, March 10, 2006.

【3】Zhang Heng. Comparison of two effective methods for testing weather resistance and light stability, QUV and Q-Sun. Automobile Technology and Materials, 2006, 8.

【4】ISO 11341:2004 "Paints and varnishes - Artificial aging and exposure - filtered xenon arc radiation".

【5】ASTM G 155-05a "Xenon Arc Test Apparatus for Exposure of Non-metallic Materials".

【6】ISO 4892-2:2003 "Plastics - Exposure methods to laboratory light sources - Part 2: Xenon arc light sources".

 (Chen Aijun/Shanghai Peony Ink Co., Ltd.)

 (Zhang Heng and Sun Xinglei／US Q-Lab Company CHINA Representative Office)

 (Fan Hanqing/Director of National Ink Industry Information Center)