Evaluation of Conditioning Results: Visual and instrumental measurements

Proper quantification of the results of any exposure testing program is important. Often, customers are concerned with what property changes and/or how much their materials change during exposure. Changes in certain properties of materials, such as color or gloss, can be measured with instruments. Changes in other properties such as cracking, peeling, chalking, blistering, and rusting may be assessed visually or by ASTM, or other technical organization standard measurement scale.

frequency of evaluation

The performance of concern is usually measured or rated prior to exposure. Afterwards, the tests are regularly evaluated and measured. For outdoor exposure testing, assessments are typically performed on a monthly or quarterly basis. For accelerated lab tests, assessments are made every 100 hours or 250 hours. Evaluation is used to understand the aging process of materials over time. Sample evaluation is an important part of a comprehensive testing program.

visual inspection

Although visual inspection is somewhat subjective, it is important because it is also used by users to judge whether the appearance of the material is satisfactory. Visual inspections need to be repeatable and consistent with other personnel's visual inspections. Consistency is very important in visual inspection, which needs to be carried out by trained and experienced appraisers.

When the customer requests a visual inspection of the test sample, the appraiser checks for all observed defects listed in Table 1. The appraiser is to rate the variation of the sample according to the appropriate scale. Visual reports of aging effects are usually graded according to the ASTM standard grade system from grade 10 to grade 0, see Table 2 (shown on the next page).

The Federation of Coatings Technology (FSCT) book Standard for Graphical Defects in Coatings contains descriptions1 of visual graphics that can be used to directly compare test samples to derive a defect rating. This facilitates repeat evaluations by appraisers and helps clients understand sample defects in reports.

Visual ratings are obtained by visual inspection under standard lighting conditions. Microscopic evaluation (10X magnification) is used for confirmation or other necessary cases, such as microscopic observation.

Instrumental Measurement of Appearance Properties

For many products and materials, cosmetic properties are very important. Customers' evaluation of quality is often based on gloss, color, brightness, smoothness, or some other appearance characteristic. The useful life of a product often depends on these characteristics. Some products have other good performance, but they are often scrapped because the appearance does not meet the customer's aesthetic requirements.

Since appearance is, by definition, a visual or aesthetic property, it may seem paradoxical to measure it instrumentally. However, appearance is usually some surface property (eg, smoothness, reflectivity, color, etc.). After years of exploration, instrumental measurements have been very close to people's visual results.

Instrumental measurements of appearance are objective and have several advantages over subjective visual inspections. Instrumental measurements provide quantitative test results of performance. It eliminates the possibility of human bias, because if all conditions are the same, the instrument's test data can be repeated every time. In addition, the value measured by the instrument is continuous and can be used for statistical analysis. In the field of weathering, gloss, distinctness of image (DOI) and color are measured using optoelectronic instruments. These instruments are used to test the surface appearance properties of exposed and unexposed parts of the material to determine the service life of the product.

Sample cleaning

For outdoor weathering tests, it is strongly recommended that the instrumented portion of the exposed sample be tested on the cleaned surface. All of these measurements are very sensitive to surface buildup such as dirt and mold. These particles form an insulating layer on the surface of the test sample, preventing the instrument from measuring the actual sample surface. If dirt is present on the surface, the instrument measures the dirt and not the actual sample surface.

Gloss measurement

Gloss measurement is an important part of measurement evaluation, especially for coatings and plastics. Gloss is one of the first properties to be affected in weathering exposure.

It is also one of the first appearance properties that observers notice. Measuring gloss with instruments is a common way of optoelectronic measurement of surface appearance properties. Gloss is measured instrumentally to measure the gloss of a surface. Gloss measurement shines a beam of light on a surface at a fixed angle and then measures the percentage of incident light reflected at the angle of reflection (excluding scattered light). A very smooth surface would have a gloss value of 100 if it reflects all light rays at the angle of incidence.

Any surface roughness or other imperfections will cause the light beam to scatter, which will reduce the reflected light and therefore the gloss value. The gloss angle refers to the angle (relative to the normal) at which the beam of light hits the surface, and common gloss angles are marked in the chart below.

Refer to ASTM D523 or ISO 2813 for details on gloss measurement.

Distinctiveness of Image ( DOI )

When we look at the coating of a car, we see reflected images. The sharper the image, we consider the better the sharpness of the topcoat. Visibility of image is easily affected by surface degradation such as fine lines, orange peel, fine scratches, fogging, etc.

Distinction of image is a specialized, more accurate form of gloss measurement that uses a 30-degree reflection angle. Distinctness of image measurements use a narrow, highly focused beam of light. Typical gloss measurements use a wide beam. The sensor in the distinctness of image measurement measures the sharpness of the edge of the light beam on a scale of 100, with a score of 100 being very good. For details on the measurement of distinctness of image, refer to ASTM E430.

color measurement

Measuring color instrumentally is the most complex of all optoelectronic measurement methods. Color measurement is to shine a beam of light on the surface of the sample, and then collect the reflected light of the sample. The reflected light spectrum is used to quickly record the color of the measured object.

Gloss and distinctness of image are the properties most susceptible to aging effects.

L* a* b* color space

Most color instrumentation methods use the L* a* b* color space to define color. The L* a* b* method uses three numbers to represent the color. The value of each of the three numbers represents a corresponding coordinate.

Lightness factor (L*).

The lightness or darkness of the measured object is expressed by the numerical value of L*, from 0 to 100. That is: 0=black, 50=gray, 100=white.

Red/Green Factor (a*)

Objects appear red or green, indicated by the value of a*. When a* is a positive number, the color of the measured object is red. The larger the value, the redder the object's color. When a* is negative, the color of the measured object is green. The larger the value of this negative number, the greener the color of the object. When a* is 0, the color of the object is between red and green.

Blue/yellow factor (b*)

物体呈现黄色或蓝色，用b* 的数值来表示。当 b* 是正数，被测物体的颜色是黄色。数值越大，物体的颜色越黄。当 b* 是负数，被测物体的颜色是蓝色。这个负数的数值越大，物体的颜色越蓝。当b* 是0 时，物体的颜色介于黄色和蓝色之间。

任何颜色都可由 L* a* b* 的坐标数值来表示。例如，紫色的 L* 值可能大于 50，a* 是正数(红色)，b* 值是负数(蓝色)。

颜色测量问题

不像光泽和鲜映性测量，进行颜色测量时，所用的测试条件存在很大差异。另外，有几种常用的不同类型的颜色测量仪器，它们的测量结果之间会存在细小的差异。

进行对比测量时，以下的测试参数需要保持一致，应该在老化测试开始时对其加以规定。

测量结构(反射式的还是积分球式的)

颜色测量是把一束光照射到样品上，分析反射光的光谱，确定 L* a* b* 的数值。光传感器视仪器类型不同而不同，仪器是反射式元件还是积分球式元件。反射元件通常由测量点的入射/反射角来表示，例如，45/0 或 0/45

观察角

这是新、老技术之间一个重要的差异。老的颜色测量方法中只使用 2° 观察者。新的仪器使用 10° 观察者。许多新的仪器可供选择，以与老的仪器进行比较。

光源

在测量中可选择几种不同光源中的一种对样品进行照射。不同的光源可能给出不同的结果。D65 光源代表中午日光，是常用的一种光源。

光谱成分

光谱成分指漫反射光的光谱成分。在积分球光度计中，光谱成分既可能包含反射光也可能不包含反射光。当使用反射光度计时(例如，0/45 形状的)，光谱成分一般不包含反射光。

色差计算

通过比较暴露前后 L* a* b* 数值的不同来计算颜色的变化。用△来表示数值之间的不同，也就是用现在的测量值减去初始值。△的数值越大，颜色变化越大。△L 代表光亮度的变化，△a 和 △b 也是以同样的方式来表示颜色变化：

+∆L 表示颜色变得更亮

-∆L 表示颜色变得更暗

+∆a 表示红色加深(绿色变浅)

-∆a 表示绿色加深(红色变浅)

+∆b 表示黄色加深(蓝色变浅)

-∆b 表示蓝色加深(黄色变浅)

All three of these factors are combined to calculate the total color change, ΔE, and is widely used to represent the standard of discoloration of exposed materials.

color coordinates.

The color of the measured object can usually be given by one of several recognized color scales CIE L* a* b*, Hunter L* a* b* or CIE XYZ. The CIE and Hunter Lab scales are widely used and calculated similarly. The XYZ scale is not used much these days. Specially specified color indices are used to determine specific changes. The yellowness index (YI) and whiteness index (WI) are used to measure the color of opaque materials that are close to white, such as TiO2 pigments. These scales were originally used for visual measurements, but have now been converted to instrumental measurements and used for comparison with older visual data.

Refer to ASTM D2244 for details on instrumental measurements of color. Good publications on the theory and application of color measurement are available from all major color Measurement Instrument manufacturers.

in conclusion

Because the useful life of a material is often influenced by aesthetic requirements or other visual factors, it is important to measure any appearance characteristic of the exposed test specimen. In this regard, there are many techniques that can be employed. For visual inspection, specialists can give good qualitative and quantitative grade results. Instrumental measurements are also particularly useful because of their objectivity, if possible. Whether it is visual inspection or instrument measurement, it is a very important part of a comprehensive test.