Shorter test times for thermal- and radiation-induced ageing of polymer materials

Polymer Testing 20 (2001) 719–724 www.elsevier.com/locate/polytest

Shorter test times for thermal- and radiation-induced ageing of polymer materials 1: Acceleration by increased irradiance and temperature in artificial weathering tests Jo¨rg Boxhammer Atlas Material Testing Technology GmbH, Vogelsbergstrasse 22, D-63589 Linsengericht-Altenhasslau, Germany Received 27 December 2000; accepted 15 February 2001

Abstract In recent years, the lifetime of polymer materials and coatings has been extended continuously in many cases. In turn, this means that the times required for testing these materials have also increased. However, such extended testing times are very often unacceptable for economic reasons. Therefore it is an ongoing demand for material testing technology to shorten test times. One possible method of achieving this is to employ significantly increased levels of irradiance and temperature for testing under simulated conditions in weathering instruments. The applicability and limits of such measures are discussed on the basis of test results on a series of technical polymeric materials (mainly for automotive interior applications), using the change in appearance — measured as colour change — as the property of interest. The test results show that, for intensifying the irradiance by up to a factor of three, there is good proportionality between the change in material property and the amount of radiant energy. This confirms already available data for technical textile materials, implying that under specific conditions this option may be well viable to shorten test times while maintaining good correlation of the test results. From corresponding tests on irradiated samples in steps of increased temperature, it can be shown that the temperature dependency of radiation-induced material ageing is significantly dependent on the material/material formulation involved. Therefore, increasing the test temperature as a means of test acceleration must be considered very carefully on a case-by-case basis.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Acceleration; Artificial weathering; Correlation; Irradiance; Temperature; Automotive polymers

1. Introduction Polymer materials are often employed under ambient conditions where they are exposed to daylight or daylight behind window glass, as well as other weather factors such as heat and humidity/rain. Early recognition of the material ageing resulting from the effects of these environmental conditions represents a significant pre-

E-mail address: Boxhammer).

[email protected]

(J.

requisite for the optimal and economic utilization of these materials. Continuously improving material systems leads to increasingly extended test times, even for already employed, to some extent standardized, and often proven “short-term tests”. Therefore, for economic reasons, shortening test times represents an ongoing challenge to testing technology. To achieve this goal, various possible solutions are under discussion or are being systematically examined. Within the area of experimental testing technology, an additional reduction of test times on the basis of already applied and standardized procedures is being attempted

0142-9418/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 0 2 9 - 0

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by means of intensifying one or more weather factors acting on the surface of exposed materials. Current weathering instruments with filtered xenonarc radiation for carrying out tests in the laboratory have the technical prerequisites of varying the spectral irradiance (without changing the spectral energy distribution) and the material surface temperature over a wide range. Particularly within the area of testing techniques for the automotive industry, the application of increased irradiances in order to shorten test times has been under intense discussion for several years. The results of already existing studies are different. Based on weathering test methods with complex test cycles, the applicability of significantly increased irradiance levels seems to be critical [1]. On the other hand, promising results for irradiance increases up to three times the normal level are available for the lightfastness (colour change) of textile materials [2]. However, the complexity of technical polymer materials and material formulations as well as ageing mechanisms require further systematic material- and property-specific investigations of the applicability and limits of these types of measure. Therefore, this work discusses results from tests on a series of technical polymer materials essentially for automotive interior applications at various levels of irradiance and sample surface temperature, with the change in appearance — measured as colour difference — as the material property of interest.

called “black and white standard thermometers” (BST and WST) in weathering instruments [4]. Tests were conducted at high, mean and low levels of irradiance (low level equals the highest level of sun radiation at the earth’s surface behind window glass) — measured and controlled in the UV range of radiation (300–400 nm) — at unchanged material surface temperatures, based on operating conditions specified in the automotive standard ISO 105-B06 [5]. Tests in steps of increased temperature levels, characterized by the chamber temperature (controlled), WST and BST (adjusted), were conducted at an increased, controlled constant level of irradiance, partly using a filter system for simulating sunlight outdoors and partly based on the already cited automotive standard. 2.2. Test materials and material properties Various industrial polymer materials essentially for automotive interior applications [polypropylene (PP), poly(vinyl chloride) (PVC) and polyurethane (PU), partially in different formulations — Table 1] and some materials for general technical applications (partly used as standard reference materials in weathering tests as well) with significantly different ageing characteristics have been investigated at various levels of irradiance and sample surface temperature. The change in appearance — measured as colour difference (spectrocolorimeter, illuminant D65, 10° observer, in the CIELAB colour space) — was the parameter of interest.

2. Experimental design

2.3. Evaluation increments

2.1. Weathering instruments — varied irradiance and temperature

All tests were conducted as a function of radiant exposure.

All tests were carried out in weathering instruments with xenon-arc lamps as laboratory light sources. In devices with xenon-arc radiation, the spectral distribution of sunlight (reference spectrum: CIE No. 85, Table 4, [3]) is simulated well in the ultraviolet (UV) and visible wavelength range by the use of appropriate filters (for daylight and daylight behind window glass). Since it is primarily the UV range of the radiation that is responsible for photochemical processes, the UV irradiance (at sample level) is adjusted and controlled to be constant in these instruments. In materials exposed to radiation, the ageing processes primarily take place on or near the surface area of the material. While measurement of the surface temperatures of individual samples being irradiated is technically possible, it is complicated and, in weathering practice, particularly in devices, is not practical. Thus, in order to characterize the temperature level of exposed samples, reference temperatures are measured by using the so-

3. Results and discussion 3.1. Test acceleration through increased irradiance levels The effect of increasing the irradiance level is to deposit the same amount of radiant energy on the specimen surface in a shorter length of time, thus decreasing the length of the test. Starting from a base value for accelerated tests [approximately 50 W/m2 (300–400 nm)], the UV irradiance (radiation behind window glass) was increased by a factor of 2 and 3 in two separate steps. In doing this, the setpoint temperatures — 65°C chamber temperature (CT) and 100°C BST — (as well as a relative humidity of 20%) were kept constant. The actual surface temperatures of the different exposed specimens during these tests are supposed to remain approximately con-

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Table 1 Specimen identification Sample

Material

Colour

Remark

PU1/1 PU1/2 PU2 PU3 PVC PP1 PP2 PP3

Polyurethane Polyurethane Polyurethane Polyurethane Poly(vinyl chloride) Polypropylene (stabilized) Polypropylene (not stabilized) Polypropylene (stabilized)

Grey Grey Grey Black Light grey Tan Black Light grey

Foam behind (4 mm) Fabric behind Woven fabric behind

stant as well, lying between the cited BST of 100°C and the WST of 82°C measured under the described conditions. The test results confirm the well-known [6], quite different ageing behaviour (colour difference) of the materials investigated. The reaction rates increase significantly with increasing irradiance but without adverse effects concerning the ageing characteristics (Fig. 1 shows the total colour difference as a function of exposure time for two polyurethane materials as an example). The proportionality between the change of total colour difference and radiant exposure H is quite satisfactory (Fig. 2). The correlations between the test results for all materials examined at the lowest and highest irradiance level for two test increments are illustrated in Fig. 3. In this example, the empirical correlation coefficients are close to r=1. The empirical correlation coefficients for the results of corresponding assessments from all tests — devices: Xenotest Alpha (48, 96 and 144 W/m2) and Xenotest Beta (48 and 96 W/m2) — are summarized in Table 2 (with the data from Fig. 3 shown in bold). Overall, the empirical correlation coefficients lie between approximately 0.8 and 0.99.

Woven fabric behind Foam behind (2 mm) Foam behind (2 mm)

Fig. 2. Colour difference versus radiant exposure H as a function of irradiance E (300–400 nm) for two example polyurethane material systems; temperature 65/82/100 (CT/WST/BST).

Fig. 3. Comparison of test results (all materials); Pearson’s correlation coefficient for ⌬E∗ and two test increments (example).

Fig. 1. Colour difference versus exposure time as a function of irradiance E (300–400 nm) for two example polyurethane material systems; temperature 65/82/100 (CT/WST/BST).

In confirmation and expansion of the experiences for certain textile materials mentioned previously, this indicates that, at least where the polymer material systems tested here are concerned, an increase in the irradiance by up to three times can be employed to shorten test

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Table 2 Influence of varied irradiance on correlation — Pearson’s correlation coefficients for ⌬E∗ and two test increments E (300–400 nm) (W/m2)

48

Radiant exposure, ⌬E∗=27.8 MJ/m2 48 1 96 0.927 144 0.901 48 0.886 96 0.87 Radiant exposure, ⌬E∗=88.2 MJ/m2 48 1 96 0.928 144 0.917 48 0.919 96 0.935

96

144

48

96

0.927 1 0.96 0.974 0.976

0.901 0.96 1 0.991 0.983

0.886 0.974 0.991 1 0.997

0.87 0.976 0.983 0.997 1

0.928 1 0.803 0.794 0.921

0.917 0.803 1 0.983 0.942

0.919 0.794 0.983 1 0.939

0.935 0.921 0.942 0.939 1

times while maintaining good correlation of the test results concerning the evaluation of colour changes. 3.2. Test acceleration through increased temperature Nearly all secondary ageing processes in polymer materials that follow the primary photochemical step induced by sun radiation are temperature-dependent. The reaction rates generally increase with increasing temperature [7]. The relationship between temperature and reaction rate may be described by the Arrhenius equation. In temperature ranges where the relationship is nearly linear, increased temperatures can, in principle, be employed to shorten test times. The influence of temperature on the radiation-induced material ageing was investigated for some polymer materials for general technical applications and for use as reference materials in artificial weathering tests. Again, the change in appearance measured as total colour difference was the parameter of interest and measurements of the irradiated specimens were carried out as a function of radiant exposure in steps of increased temperature. The tests were carried out in a device with xenonarc radiation and “sunlight outdoors” filter system at an already increased irradiance level [E(UV)=100 W/m2]. The surface temperature of the exposed specimens was increased step-by-step from the normal level used when performing weathering tests by increasing the ambient temperature (chamber temperature). By adjusting the air stream in the chamber, the difference between the CT and the WST and BST was kept constant at 9 K and 18 K, respectively. Thus, the temperature increase at the surface of the exposed specimens is assumed to correlate well with that of the chamber temperature. The test results show that the influence of temperature

on the ageing behaviour of irradiated samples depends significantly on the type of polymer material. Fig. 4 illustrates the colour change as a function of radiant exposure and temperature for an alkyd/melamine coating applied to aluminium (used, e.g., as reference material for weathering tests). Based on the quite good linear relationship between the reaction rate and the temperature according to the Arrhenius equation (inset in Fig. 4), the influence of temperature on the colour change of irradiated specimens can be calculated (Fig. 5) and this, in turn, can be employed as the basis for an assumption of the applicability of increased temperature in order to shorten test time. A different result is provided by a clear standard polystyrene that is also employed as a reference material — the criterion here being not the total colour difference, but the ⌬b∗ scale as a measure of yellowing — where a slope change in the Arrhenius diagram takes place

Fig. 4. Colour difference versus radiant exposure and temperature CT/WST/BST (curves calculated) for alkyd/melamine coating.

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Fig. 5. Influence of temperature on the radiation-induced material ageing (Arrhenius diagram of colour difference) (curves calculated) for the alkyd/melamine coating.

within the observed temperature range (Fig. 6), thus limiting the temperature range that can be employed to shorten the test time (indicating that care has to be taken with extrapolation of data evaluated in a limited temperature interval). Apparently, the influence of the glass transition temperature TG, above which the mobility of the chains and radicals increases while, simultaneously, the oxygen diffusion coefficient changes, already begins to take effect [7]. However, tests at still higher temperatures would be required in order to quantitatively describe this reaction range. A rather complex ageing characteristic was discovered during corresponding studies of commercial (general technical application) PVC (Fig. 7). The graphically determined temperature dependence of the colour change (Fig. 8) is already characterized at relatively low values by two nearly linear partial areas with significantly different ageing behaviour. This result confirms experiences gained in, for example, artificial weathering tests of PVC

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Fig. 7. Colour difference versus radiant exposure H and temperature (CT/WST/BST) for PVC material.

Fig. 8. Colour difference versus temperature (CT) and radiant exposure H (marked section in Fig. 7) for PVC material; WST=(CT+9)°C, BST=(CT+18)°C.

Fig. 6. Influence of temperature on the radiation-induced material ageing (Arrhenius diagram of colour difference) for polystyrene material; partial curves (A and B) calculated.

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Fig. 9. Colour difference versus radiant exposure and temperature (tests in two xenon devices, A and B) for polymeric materials for automotive interior applications.

under standardized conditions, where it has been found that relatively small temperature differences can lead to distinctly different test results concerning the change in appearance. The examples for only a single material property described above already make clear the material-dependent nature of the extremely varying temperature dependence of the ageing behaviour under radiation. They show that, in order to employ increased temperature for the purpose of shortening test times, very careful investigations must be made on a case-by-case basis. Additional tests on automotive interior materials (already tested at varied irradiance) at 10°C above and below the standardized temperature level (CT=65°C; BST=100°C) confirm the conclusions cited above. These tests were performed in two xenon devices [A and B; E(UV)=100 W/m2]. The test results for three materials are shown in Fig. 9. The test results in the two instruments are rather close together. The effects of varied temperature (20 K difference) are nearly negligible for two materials (PVC and PP1) but significant for the polyurethane tested (PU3). The ageing curve calculated for the highest temperature — based on linear extrapolation of the ageing curves calculated for the lower temperatures and the Arrhenius equation — deviates distinctly from the measurement results, particularly at low test increments (Fig. 10). Aside from the “critical temperature limit” that can be recognized here, this result also indicates that even apparently minor differences in temperature requirements in standardized test methods [5] may lead to distinctly different test results. Overall, the quite different ageing behaviours of the materials/material formulations tested shows that a very careful decision must be made on a case-by-case basis

Fig. 10. Colour difference: measured data and calculated curves (see Fig. 9).

with regard to the applicability and limits of utilizing increased temperatures for the purpose of shortening test times in artificial weathering tests.

References [1] L. Crewdson, Correlation of outdoor and laboratory accelerated weathering tests at currently used and higher irradiance levels — Part III, in: 2nd International Symposium on Weatherability (2nd ISW), Tokyo, 27–29 September 1994, Material Life Society, Tokyo, Japan, 1994. [2] J.W. Stuck, Experiences with, and new options provided by, high speed exposure, in: Symposium on “Colourfastness Testing in Textile Industry”, DEK/Atlas-Xenotest, Gelnhausen, 22 October 1996, Atlas MTT GmbH, Gelnhausen, Germany, 1996. [3] Solar Spectral Irradiance, CIE Technical Report, Publ. No. CIE 85, CIE, Vienna, Austria, 1989. [4] J. Boxhammer, D. Kockott, P. Trubiroha, Black standard thermometer — temperature measurement of polymer surfaces during weathering tests, Materialpru¨ fung 35 (5) (1993) 143–147. [5] ISO 105-B06: Textiles – Tests for Colour fastness – Part B06: Colour fastness and ageing to artificial light at high temperatures: Xenon arc fading lamp test, ISO, Geneva, Switzerland, 1998. [6] J. Boxhammer, K. Scott, A comparison of new and established accelerated weathering devices in aging studies of polymeric materials at elevated irradiance and temperature, in: 3rd International Symposium on Weatherability (3rd ISW), Tokyo, 14–16 May 1997, Material Life Society, Tokyo, Japan, 1997. [7] U. Schulz, Der Einfluß von Temperatur und Feuchte auf die photochemische Alterung polymerer Werkstoffe, in: Seminar No. 510235004 — Natu¨ rliches und ku¨ nstliches Bewittern polymerer Werkstoffe, TA Wuppertal, 1994.