Striving to meet customer satisfaction: manufacturers are constantly under pressure to meet all...

ABSTRACT

Manufacturers of coatings for durable products like automobiles for example, are constantly challenged to satisfy appearance demands, both for 'showroom appeal', as well as for the in-service maintenance of original appearance; a challenge that is compounded by the ever-increasing

array of exterior coatings materials and colours. Weathering tests, with an increasing reliance on accelerated laboratory tests, are an important part of ensuring customer's long-term satisfaction. Consequently, ongoing improvements in laboratory weathering instruments are mandatory to keep abreast of demands. This paper reports on an important innovation that records the temperatures of specimens under test, in real time. A capability that allows laboratory weathering tests to be conducted in unprecedented ways to provide more reliable accurate and reliable test results.

INTRODUCTION

Weathering is the phenomenon caused by environmental stresses that in combination cause chemical and physical changes to exposed organic material which are generally unfavourable. The main weathering stresses are light, and moisture in any of its forms. Heat is also extremely important since any reaction initiated by light is driven kinetically and therefore tends to significantly increase with temperature.[1]

Importance of specimen temperature is suggested by the 'rule of thumb' which states that reaction rates double for every 10 degrees JbC increase. In fact, reaction rate increases are material dependent. Fischer et al, found factors ranging from 1.2 to 1.8, per 10[degrees]C increase. [1] Temperature of materials under the influence of light and moisture will cause change at a rate [K.sub.d], determined by the Arrehenius equation:</p> <pre> Kd = Ae--r/R T Where: E = Activation Energy A = Pre -exponential constant

relating to test conditions and material characteristics R = Ideal gas or Boltzman's

constant, depending on equation

units T = Temperature in degrees Kelvin </pre> <p>LIMITATIONS OF CURRENT LABORATORY WEATHERING TEST

For temperature characterisation, laboratory weathering devices are equipped with a 'black panel' which could be of any number of designs in current use, though most comply to either of the designs generically described by ASTM G 151 [2]

During the course of a weathering test, the black panel is intended to provide an estimate of the worst-case sample surface temperature. This assumes that the panel, primarily because it is coated with an efficient, black solar absorber, will record a higher surface temperature than most, if not all, other 'real' specimens.

Researchers have used the instrument-provided black panel temperature to estimate temperatures of the specimens of real interest. [1] However, attempts to estimate specific surface temperatures of real specimens based on black panel temperatures are error prone for three main reasons. 1. Operational variability amongst different designs of black panel is well documented. [3] 2. Furthermore, the properties of black panel coatings change during the course of their service life, effectively creating a moving baseline. 3) Also, changes in specimens under testing cannot be expected to parallel changes occurring in black panels.

The need for knowledge of critical temperatures of specific materials under test is repeatedly expressed throughout the literature, [4] but until now, presumably because of the inherent challenges, laboratory instrumentation has been incapable to provide this information.

SPECIFIC SAMPLE SURFACE TEMPERATURE (S3T OR S3T) MEASUREMENTS

Atlas Material Testing Technology LLC, has recently developed proprietary technology to measure and provide the specific sample surface temperature of materials under test. The system has been thoroughly evaluated for its ability to provide accurate surface temperature measurements for a variety of materials under a wide range of weathering test conditions.

GENERIC SYSTEM DESCRIPTION

A schematic and photograph of the system are shown in Figure 1.

[FIGURE 1 OMITTED]

Following is a brief, generic description of the proprietary S3T's features:

* Non-condensing sensor head--the system has a facility to keep the non-contact IR sensor-head free of any condensation for proper performance;

* Light insensitive--The IR sensor was chosen to respond in a spectral range exclusive of the xenon output wavelength range;

* Operating air temperature range: 0-85 [degrees]C--This encompasses all known weathering test methods;

* Operating relative humidity range 0-100%;

* Small volume sensor--is necessary to be minimally obtrusive to test specimens;

* Spot size and systems response time appropriate

PROOF OF CONCEPT EVALUATION

The S3T was successfully tested in an Atlas MTT Ci4000 WeatherOmeter for the following performance criteria:

* Sensor sensitivity and sample tracking capability;

* Performance under various test conditions;

* Performance on various types of materials and different surface finishes;

* Performance using different sensor configurations--sensitivity to variations in physical set-up.

EFFECTS OF PRESET EMISSIVITY

A body's emissivity depends on its temperature, its specific material property and wavelengths of its emmitance. Thus, a single, constant emissivity setting for IR temperature measurements of all materials is, in theory, not ideal and a potential source of error. Fortunately, for most frequently tested materials, the emissivity falls in the fairly narrow range of 0.85 ~ 0.96 for temperatures 0 to 100[degrees]C.

Tests with various materials were conducted with pre-selected emissivity settings of 0.85, 0.90 and 0.95 while all other test conditions were kept constant (BPT - [degrees]60, CT - 38[degrees], RH - 50%). Minimal temperature differences were observed between the various emissivity settings. These results indicate that a single, nominal emissivity setting will provide acceptable accuracy on a wide variety of materials.

MEASUREMENT OF VARIOUS MATERIALS AND SURFACE FINISHES

A cross-section of typical weathering samples, including painted surfaces (coatings), textiles and polymers, was used to test the S3T's sensitivity to different materials. Here, only the time profiles of the coated specimens are shown in Figure 2. However, means and standard deviations are summarised in table 1, where the type of material is indicated by the letter suffix following the colour. For example, T for textile, C for coatings and P for plastic.

[FIGURE 2 OMITTED]

VALIDATION

A replicate set of the coloured PVC coatings on aluminum substrates panels, with embedded thermocouples, described by Fischer, et al, in 'Surface Temperatures of Materials in Exterior Exposures and Artificial Accelerated Tests' [1] was provided courtesy of the 3M authors. The surface temperatures of these panels were measured by S3T and simultaneously by the method using embedded thermocouples described in the paper. The comparisons were conducted under the widely disparate conditions of the following test methods; SAEJ1885 [5], SAEJ1960 [6], AATCC169-1[7], GM 3414TM, [8] as well as the following International Standards Organisation Weathering Standards ISO195-B02, ISO105-BO6 and ISO11341-1.

The averages and standard deviations were plotted for the two measurement methods. The data for the SAE J1885 test method shown in Figure 3 was representative of all the test methods. Typically, there was good agreement between the average temperatures as measured by both techniques, while the S3T measurements were more repeatable, indicated by lower standard deviations.

[FIGURE 3 OMITTED]

The average temperature readings for all test methods are complied and compared in Figure 4.

[FIGURE 4 OMITTED]

CONCLUSION

By providing the capability to record in situ specimen surface temperatures in real time, the S3T system can redefine how weathering tests are conducted. The system makes it possible for researcher to conduct investigations in materials durability testing that was either very difficult or impossible before.

References

[1.] Fischer, R M, and Ketola, W. D, 'Surface Temperatures of Materials in Exterior Exposures and Artificial Accelerated Tests.' Accelerated and Outdoor t Durability Testing of Organic Materials, ASTM STP 1202, Warren D. Ketola and Douglas Grossman, Eds., American Society for Testing and Materials, Philadelphia, 1994.

[2.] Annual book of ASTM International Standards, ASTM 100 Barr Harbor drive, W. Conshohocken, PA 19428, US.

[3.] W.D. Kelola, RM. Fischer, K.P. Scott, R. Quinn, 'Surface Temperatures of Materials exposed in a Xenon-arc device, controlled with Black Panels of different designs' Prepared for ISO/TC61/WG2- September, 2002.

[4.] D. Bauer, 'Wish List For Automotive Paints Testing' Journal of Coatings Technology, Vol. 74, No. 924, January 2002.

[5.] Society of Automotive Engineers, Method J1960, Accelerated Exposure of Automotive Interior Trim components using a Controlled Irradiance Water-Cooled Xenon-Arc Apparatus, SAE, Warrendale, PA, US.

[6.] Society of Automotive Engineers, Method J1960, Accelerated Exposure of Automotive Exterior Materials using a Controlled Irradiance Water-Cooled Xenon-Arc Apparatus, SAE, Warrendale, PA, US.

[7.] American Association of Textile Chemist and Colorists (AATCC), Method 16 E, Colorfastness to Light, Technical Manual, Research Triangle Park, NC 27709, US.

[8.] General motors worldwide Engineering Standards--Test Method Materials.

Kurt P. Scott, Director, Research 8 Development, Atlas Material Testing Technology LLC, 4114 N. Ravenswood Avenue, Chicago, IL Ph: (773) 289-5770. e-mail--kscott@atlasmts.com

Zhijun Zhang, PhD, Illinois Institute of Technology 3300 S. Federal St. Chicago IL, 60616

William Buttner, PhD, Illinois Institute of Technology 3300 S. Federal St. Chicago IL, 60616

Table 1. 53T temperatures during stable stage for various
materials ([degrees]C)

Sample           BPT      IR-BPT  Black  Green  Red C  Blue C  Yellow
             (reference)            C      C                     C

Mean            90.38     92.59   88.50  86.33  83.71  81.69   78.88

STDEV           0.13      0.40    0.22   0.18   0.20   0.20    0.20

Relative
uncertainty     0.15%     0.43%   0.25%  0.21%  0.24%  0.25%   0.25%

Sample         Mixed-    Gray T   Purple  Blue T   PS P   PC P
              colour-T

Mean            87.92     85.58    73.68   78.98  65.28  65.84

STDEV           0.97      2.21     0.33     0.42  0.18   0.25

Relative
uncertainty     1.10%     2.59%    0.45%   0.54%   0.28%  0.39%