Material Testing Weathering Tests for Polymers – Why Black Panel Thermometers (BPTs) are Redundant and Misleading

How do polymers behave with temperature changes that occur in their life cycle? This is a decisive factor for product development. For this reason, there are several standards for weathering tests. This article provides an overview of why some of these tests are unsuitable.

Polymer degradation by solar radiation usually depends on specimen surface temperature. Therefore, this temperature needs to be measured in irradiation and weathering tests

Up to the 1980s in international and national standard weathering test procedures a variety of differently designed temperature sensors, so-called black panel thermometers BPT, were allowed to control the thermal conditions in the test devices. Fact was, that the surface temperature of a test specimen could vary by up to 20 K at the same set temperature in different weathering devices.

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To solve this problem a precisely defined „black standard thermometer BST” was proposed in 1993 and afterwards recommended in many ISO weathering standards. At present, simple BPTs for temperature control appear again more frequently in the standards, which means that the actual surface temperature of the irradiated polymeric materials is significantly higher compared to the temperature indicated by BPT and e.g. the rate of photooxidation may increase by up to 100%.

The experimental results which led to the introduction of BST in 1993 have been confirmed by many new studies.

Introduction

It is needless to say that the specimen temperature is a very important parameter concerning the influence on physical properties of polymers including its photo degradation behaviour. In photo degradation processes the surface temperature of the polymer or coating specimen is the most influencing factor.

Precise measurement of the surface temperature of irradiated polymers is an unsolved problem.

In the following we refer to the important series of standard weathering tests ISO 4892 Plastics — Methods of exposure to laboratory light sources, part 1 to 3, editions from 1980 till 2016 (last edition), which are the basis for numerous other international and national weathering standards for plastics, coatings and other specific polymeric materials.

50 year ago different black panel thermometers (BPTs) were state of the art to determine – as indicated in the standards – the maximum surface temperature of irradiated specimen surfaces in artificial weathering devices with xenon or carbon arc lamps. But there was a variety of technical design which fell far short to fulfil this task. At the same black panel temperature of 45 °C in different xenon arc weathering devices the actual maximum surface temperature was significantly higher. The temperature difference varied between +7 K and +25 K and caused large differences in ageing results after a same level of radiant exposure. About 40 years ago first tests started with a new design of a black panel thermometer, called „black standard thermometer (BST)“, which, as required by the international weathering standards, should show an actual maximum surface temperature and which allows repeatable and uniform thermal conditions in different weathering devices. The basic principles are described in section "Normung” of [1].

In 1993 the authors [2], representing the manufacturers of Xenotest and Weatherometer weathering devices and the German Federal Institute for Materials Research and Testing, published a summary on this item. It explained in detail the theoretical and experimental basis for measuring the temperature of irradiated polymer surfaces and the limits of the experimental determination. The authors presented a BST with the aim to determine the maximum surface temperature of a black polymer of 5 mm thickness, see figure 1. The advantage of this BST was obvious. Consequently, since 2001, the editions of ISO 4892-2, have included several exposure cycles with a temperature control using the black standard thermometer BST. A warning was noted in Clause 6.2.1 Black-standard / black-panel temperature:

„For reference purposes, Table 3 specifies black standard temperatures. Black panel thermometers may be used in place of black standard thermometers. However, different set-point temperatures must be used to compensate for the differences in thermal conductivity between the two thermometer types (see ISO 4892-1, subclause 5.2.3). If a black panel thermometer is used, then the type of thermometer, the way in which it is mounted on the specimen holder and the selected temperature of operation shall be stated in the test report“.

Requirements on BST / BPT in International Standards

A fundamental requirement according to ISO 4892-1: 2016, clause 5.2.1 is „the black panel shall be mounted within the specimen exposure area so that it is in the same plane and orientation and receives the same radiation and experiences the same cooling conditions as a flat test panel surface“.

This important requirement is met only by a thermometer with a flat panel which is free from superstructures like temperature sensors: the black standard thermometer BST.

Current developments

What's nearly unbelievable: New designs of BPTs were published during the last decades and there were even patent applications. The most popular argument for BPTs is that they more accurately reflect the maximum surface temperature of black coated steel panels (who exposes black coated steel panels according to an ISO 4892-2-cycle for plastics?). This may be true in some cases, but the important task of a black coated thermometer is to control uniform thermal conditions in different weathering equipment. The black panel surface of the new BPTs is partly covered by a sensor housing and with small bars which act like cooling fins, see the variety of designs in figure 3 [3]. Heating by radiation as well as cooling by both moving air and the emission of thermal radiation are hindered. In some cases, there is also poor thermal contact between the temperature sensor incorporated in a small black tube and the black panel surface. Not to mention the backside of the panels, which can be metallic or coated leading to differences in radiation cooling [4].

Further requirements and recommendations in ISO 4892-2

In the last edition of ISO 4892-2: 2013 two notes to clauses 4.4 and 6.2.1 describe the problem:

„The preferred maximum surface temperature device is the black standard thermometer“ and

„If a black panel thermometer is used, the temperature indicated will be 3 °C to 12 °C lower than that indicated by a black standard thermometer under typical exposure conditions“.

In clause 5.2.3 of ISO 4892-2: 2013, there is an additional recommendation:

„The actual difference between a black-panel temperature and a temperature measured with a black standard thermometer should, however, preferably be determined for each exposure condition“. Why this extra effort?

Results from practice

An example that proves the great importance of uniform thermal conditions in weathering tests is illustrated in figure 2 [5]. It shows the loss of gloss of an oil-modified polyurethane coating as a function of weathering duration t at three different specimen temperatures.

The following examples show that the requirements and recommendations in ISO standards are useful:

• Example 1. Eight black panel thermometers and one BST (bottom line, right) shown in figure 3 were exposed at a tilt angle of 30 ° South in Tsubuka, Japan [3]. The BST according to ISO 4892-1 showed the highest temperature of 67 °C. The temperatures indicated by all BPTs were in the broad range from 55 °C to 64°C, although the thermal load of the thermometers was the same.

• Example 2. The temperature differences indicated by BPT and BST were confirmed by an interesting comparison of two common weathering procedures by BASF Schweiz AG [6]. During the weathering cycle 1 of ASTM G 155, the temperature was controlled at 63 °C by a BPT and during cycle 1 of ISO 4892-2, the temperature was controlled at 65 °C by a BST. Although the irradiance level according to ISO 4892-2, cycle 1 was 50 % higher than in the ASTM G 155, cycle 1, the ASTM cycle was more effective in photo degradation of polymers.

It could be demonstrated that the surface temperature in ASTM G 155, cycle 1 was about 9 K higher than in ISO 4892-2, cycle 1. This higher temperature explains the higher effectivity of the lower irradiance level according to cycle 1 of ASTM G 155.

Result, discussion and conclusion

ISO Standards should represent the state of the art. Therefore, methods of temperature measurement, which were already recognized 40 years ago as incorrect, should not be used. Laboratories which are using BPTs (black panel thermometer) for controlling the thermal conditions in irradiation and weathering tests are strongly recommended to reflect very carefully on what was already published on the behaviour of BPTs and BSTs, see reference list.

Conclusions

Black panel thermometers BPTs are not suitable to control uniform thermal conditions in different weathering devices and therefore, really superfluous. In the last three decades several publications showed, that characterizing thermal conditions in a weathering device by controlling the temperature using the variety of differently designed BPTs makes no sense and means waste of time and money.

BPTs have no clear physical basis. Repeatable and reproducible uniform thermal conditions in weathering tests need a reliable basis: the black standard thermometer BST.

References

[1] Künstliche Bewitterung kompakt - ein Handbuch für Praktiker; ÖGUS Österreichische Gesellschaft für Umweltsimulation ; Facultas-Wien, 2020, ISBN: 978-3-7089-2030-6

[2] J. Boxhammer, D. Kockott, P. Trubiroha, Black Standard Thermometer – Temperature measurement of polymer surfaces during weathering tests, Materialprüf. 35 (1993), 5, 143 – 147

[3] T. Tomiita, N. Kashino, H. Hamasaki, Observations of various Black Panel Temperature Quantification of outdoor thermal degradation environment (Part 4); J. Struct. Constr. Eng., AIJ, No. 531 (2000), 21 – 26

[4] P. Trubiroha, V. Wachtendorf, A, Geburtig, Surface Temperatures of Irradiated Test Specimens - Decisive Physical Quantities and a Largely Unsolved Problem, 6th European Weathering Symposium “Natural and Artificial Ageing of Polymers”, ISBN 978-3-9813136-8-0, Ed. Th. Reichert, GUS eV, Pfinztal, Germany, 2013, 185 – 194

[5] W. Mielke, P. Trubiroha, Die künstliche Bewitterung von Polymerwerkstoffen bei ausgewählten Klimabedingungen, Materialprüf. 30 (1988), 10, 316 – 321

[6] M.C. Grob, ASTM G 155 CY 1 Versus ISO 4892-2 CY 1: so Similar and so Different, 6th European Weathering Symposium “Natural and Artificial Ageing of Polymers”, ISBN 978-3-9813136-8-0, Ed. Th. Reichert, GUS eV, Pfinztal, Germany, 2013, 247 – 261

* P. Trubiroha Scientific advisor, 14169 Berlin/Germany

* *D. Kockottt, UV Technik, 63456 Hanau/Germany

* **J. Boxhammer, Scientific advisor, 64807 Dieburg/Gerrmany

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