Influences of Laboratory and Natural Weathering on … · GLASS PERFORMANCE DAYS 2007 | Influences of Laboratory and Natural Weathering on the Durability of Laminated Safety Glass

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Influences of Laboratory and Natural Weathering on the Durability of Laminated Safety GlassFrank EnsslenSemcoglas Holding GmbH, Langebrügger Str. �0, D-26655 Westerstede, Germany

Keywords

�=Laminated safety glass (LSG) 2=PVB-interlayer 3=durability 4=weathering 5=shear modulus 6=adhesion characteristics 7=DIN EN ISO �2543-4

Summary

Beside known influences on the shear behavior of laminated safety glass (LSG) as interlayer temperature, loading rate and load duration, further effects result from climatic load (e.g. long-term exposure of ultraviolet radiation, moisture and air temperature), especially for outdoor applications. Experimental investigations show that moisture penetration of PVB-interlayer at the glass edge zones is a major influencing factor on the durability of LSG. Hence, shear behavior and adhesion characteristics change. Concerning large-scale architectural LSG panes carrying wind, snow and dead load no significant endangering of their structural safety occurs due to a local deterioration of the interlayer, only. In order to avoid visual damage of LSG in environments with access of high humidity (e.g. rain), it is recommended to protect edges thoroughly and effectively. Aging of the interlayer due to UV-radiation and air temperature is dependent on its intensity and duration, however, resulting in a stiffer material behavior, but not affecting the structural saftey.

1Introduction

Test methods for checking on the durability of LSG regarding UV-radiation, moisture and air temperature are generally dealt in the international standard DIN EN ISO �2543-4 [�]. That standard is partly used by PVB-interlayer manufacturers for their regular quality controls with respect to chemical, physical and optical characteristics of LSG, especially the PVB-interlayer. In addition, nowadays the standard gets increased attention focusing on test procedures required for the CE-marking of LSG in compliance with EN �4449 [2] obligatory to all members of the European Community beginning in March 2007.

Within the scope of manufactures’ quality control tests LSG samples after accelerated weathering in laboratory according to [�], or weathering under natural climatic conditions, are subjected to varies, partly non-standardized test methods (e.g. compression shear test,

Pummel test, bake and cook test). Basic test goals include assessing the adhesion character and possible delamination between interlayer and glass, and analyzing the shear strength. Of further interest are UV-transmission and moisture concentration of the PVB-interlayer. A visual inspection regarding color changes (e.g. yellowing) is also done.

Quantifying an important mechanical parameter, such as the shear stiffness, using the shear modulus (as a function of time), after long-term influence of enhanced climatic aging is missing within these quality control programs. Also, no statement on the load carrying behavior of weathered large-scale architectural LSG is made. Therefore, the main part of this publication is the investigation these open questions. Transferring the test results of weathering samples into practice is done by numerical parameter studies with the method of the finite elements (FEM).

2ExperimentalInvestigationsonSmall-ScaleLSGSamples

2.1 Samples for the Shear Tests

Due to economic advantages studying a large number of (climatic) parameters basic tests on small-scale samples were preferred.

Round LSG samples with a diameter of 3� mm were gained from laminates with a size of �00 x �00 mm by wet drilling using a position device, as can be seen in Figure � (left). These square

samples with a thickness of Float 4 mm/PVB 0,76 mm/Float 4 mm were the basic glass sheets for laboratory weathering. The basic glass sheets for natural weathering were 300 x 300 mm with a thickness of 2/0,76/2 mm. For preparation of the round samples, however, samples with �00 x �00 mm were cut from the latter basic sheets. The type of PVB-interlayer used was Trosifol MB.

As shown in Figure � (right), it was distinguished between round samples originally located at the edge and in the middle of the laboratory basic glass sheets, whereas the original location for the natural weathering basic sheets was classified by “edge and inside”. For the reference samples no difference between edge and middle (inside) was made.

In the following, the 7 test series and their weathering conditions are explained:

Series �: Reference samples for series 2 through 7. Until testing (½ year later) samples wrapped in plastic foil were stored in a dark room.Series 2 to 4: Solarium weathering of basic glass sheets with a duration of 4, 8 and 20 weeks with �6 UV-bulbs (Osram UltravitalUx®, 300 W) following [59][�] at trOsifOl facilities. The distance between bulbs and the basic material was around � m, see Figure 2 (right). Surrounding temperature was approx. 70 to 80 °C (in [�[59]]: 45 °C) with a relative humidity of approx. �0 % and below.Series 5: Cycled weathering of basic glasses in a climatic chamber (Weiss

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Figure 1

Drilling of round glass samples with a positioning device (left); Depiction of samples originally located at the edge, middle and inside of basic glass sheets (right)

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WK � �80) for �2 weeks at Trosifol facilities. In total �4 cycles with �6 hours at 80 °C (95 % rel. humidity), and 8 hours at -30 °C (50 - 60 % rel. hum.) were performed. After that, samples rested for �4 days in a dark room. Afterwards, above mentioned procedure was repeated twice, which is a threefold weathering dosage compared to the regular quality control program.Series 6: The 2 years natural weathering of the basic glasses in Phoenix, Arizona (USA), featured solar exposure with a high average UV-radiation (≈ �07 kWh/m²a) and at the same time during the day high and dry environmental conditions (av: ≈ 28 °C, ≈ 43 % rel. hum.). At night, temperature could drop by the hour until the frost line, whereas up to 46 °C were possible, by day.Series 7: The 2 years natural weathering of the basic glasses in Miami, Florida (USA), featured solar exposure with a high average UV-radiation (≈ ��9 kWh/m²a) in combination with high daily temperatures (av: ≈ 27 °C) and a permanent high relative humidity (av: ≈ 72 %), which means extreme

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climatic environmental conditions for LSG. As opposed to Arizona, day and night temperatures are more balanced (av: ≈ 24 °C).

Figure 2 (left) presents the correlation between the intensity and duration of radiation for the UV-A-spectrum (3�5 through 380 nm) in the solarium - for the exposure on a horizontal surface - in comparison with Arizona/Florida, and Freiburg (Germany), respectively, determined from over �0 years-old averaged climatic data in [3]. Sample arrangement inside the solarium is shown in Figure 2 (right).

Figure 2 (left) illustrates that the amount of UV-A-radiation naturally taken into the interlayer over a period of 2 years exceeds the amount of that for 20 weeks in solarium. However, the solarium radiation was nonstop without any interruptions throughout the night with hot and very dry environmental conditions.

In general, the amount of the terrestrial solar radiation (approx. 6 to 7 % of the total solar energy) that hits a glass pane lessens, on the one hand, through reflexion and refraction. Important for the angle of refraction are the position of the sun and atmospheric

factors, like e.g. dust, haze. On the other hand, ordinary sheet glass (except quartz glass) completely absorbs the UV-B-radiation (280 to 3�5 nm) but lets through the UV-A-radiation up to a sheet thickness of 5 mm non-restrictive [5]. At larger glass thicknesses this radiation is (almost) fully absorbed, too.

The PVB-interlayer is able to (almost) completely absorb the UV-radiation with increasing thickness. According to the PVB-interlayer manufacturer the solarium samples with 4/0,76/4 mm have a UV-Transmission ≤ 0,5 %, only.

2.2 Test and Measuring Setup for the Shear Tests

Figure 3 presents the test setup. The testing device contains two similarly built steel pieces, in which the round shear LSG sample was clamped on both sides between adjustable aluminum pieces. This kind of fastening shorts moments generated due to the excentric load introduction inside the glass sample. Consequently, a pure shear loading of the interlayer was guaranteed.

The tensile load was applied by a screw-driven testing machine (Schenck-

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16 x 300 W Ultravitalux Bulbs Solarium at Trosifol

Figure 2

Intensity of UV-radiation as a function of the du-ration [127][3], [4][133] (left); Weathering of ba-sic glass sheets inside so-larium at Trosifol (right)

Figure 3

Small-scale testing device and round glass sample (right and middle); Mounting of LVDT at outer faces of shear sample (left)

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Trebel) with a maximum capacity of 250 kN. Measuring the mutual displacement of the LSG panes was done by two LVDTs with a maximum measuring length of ±2 mm. The measuring devices were fixed to the outer glass faces in order to measure displacement at the shear sample directly (cf. Figure 3).

2.3 Shear Test Performance

One of the LVDTs was used to regulate a constant shear velocity of v� = 0,� mm/min und v2 = �,0 mm/min over the complete testing time (stroke-control mode). The shear velocities selected delivered suitable test durations and the most reliable test results. Also, a wide range of the time scale (� s - 70 s) was preferably covered (cf. Section 2.6.3), which is normally important for characterizing the mechanical behavior of a polymeric material. When a shear angle of γ = �,0 rad was reached or sample damage occurred, the tests were stopped (not for the hystereses). During testing the temperature of the samples was 26 °C.

2.4 Determination of Moisture Content of PVB-Interlayer

Measuring the moisture content of the PVB-interlayer (exactness 0,02 %) was done with an Infrared-Spectrophotometer (Pier-Elektronik GmbH). The water concentration of the interlayer was determined (through the top glass sheet) at the basic units (�00 x �00 mm and 300 x 300 mm) - beginning from the glass edges up to the middle - by �0 mm steps.

2.5 Results of Moisture Measurements

Pre-tests in [6] showed that the generally surrounding temperature significantly influences the moisture desorption/absorption processes of the hygroscopic PVB-interlayer. At a temperature of +80 °C moisture processes are accelerated, dependent on the environmental relative humidity, especially at the edge zones of the samples. At room temperature (≈ 2� °C), however, mechanisms are slowed down. In this stage only moderate changes of the moisture content occur compared to the reference.

Artificial weathering using the climatic chamber (series 5) meant a particular kind of weathering procedure for LSG. Figure 4 demonstrates a large increase of the moisture concentration for the samples located at the edge and in the middle after weathering of 2 or 3 cycles in the climatic chamber. The moisture content of the middle samples increased after �2 weeks up to �,28 % (at edges up to �,66 %) compared to the initial values of 0,4 - 0,5 %. Glass corrosion in form of haze was detected on the outer faces of the samples.

The interlayer of the small-scale samples completely dried out during 20 weeks of solarium weathering (series 4).

In the results for natural weathering presented in Figure 4 no significant change of the moisture concentration of the PVB-interlayer of Arizona samples after 3 years can be observed. Values for both - edge and inside samples - stay around the initial moisture values acc. to manufacturer’s specification. In Figure 4 Florida samples show a substantial moisturing of the edge zones accelerated through high average air temperature.

For a better understanding, Figure 5 presents the results of the PVB-interlayer moisture distribution for the naturally weathered basic glass sheets. Additional lines in Figure 5 sketch the location of the round samples in basic glass sheets.

For penetrating depths of 90 to �00 mm after 3 years of extremely muggy climatic conditions no significant changes of the moisture concentration are recognizable. Dry conditions in Arizona only lead to marginal moisture changes at the ultimate glass edges.

At the ultimate edge of the glass sheets (< �0 mm) permanent moisturing and drying processes inside the PVB-

interlayer happen depending on relative humidity, temperature, and duration. Thus, damaging of the glass-interlayer adhesion in these zones can easily occur. Low temperatures can lead to freezing of water components resulting in molecular alteration.

2.6 Results of Shear Tests

2.6.1 Shear Behavior after Accelerated Laboratory WeatheringIn general, solarium samples (series 2 to 4) tested with a shear velocity of v� = 0,� mm/min (Figure 6 - left) and v2 = �,0 mm/min (Figure 6 - right) have a steeply increasing curve progression compared to the reference samples. Consequently, a stiffer, or more brittle, respectively, material behavior is observed.

More significant tendencies are observed for a longer duration of radiation. Altogether, the lower shear speed generates lower test curves. In addition, for the beginning of the working lines of the 20-weeks-solarium samples a more stiffening influence can be detected (circled in Figure 6 - right).

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Figure 4

Moisture content of PVB-interlayer of series 1 and series 4 to 7

Figure 5

Moisture distribution in PVB-interlayer for naturally weathered LSG (to show tendencies of the mois-ture distribution of series 6 and 7)

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As a result of the intense weathering conditions in the solarium (~ 70 °C, ~ �0 % rel. hum.), three possible influencing factors on the shear behavior were assessed:

Disappearing of softener components, which volatilize over free glass edges.Drying out of PVB-interlayer.As a third influencing effect the UV-radiation energy plays an important role. During the production of the interlayer, UV-absorber with a high dosage is added to the polymer matrix. As is generally known, this additive inhibits intermolecular degrading for a certain period of time. It could not clearly be determined how much UV-absorber was spent due to the nonstop and intense UV-exposure in the solarium, and thus, molecular degradation in the interlayer occurred. Also, because of the high temperature level during solarium weathering UV-degradation could not be determined, separately.

Principally, all three factors caused the same effect on the shear modulus. The elusion of softener, reduced moisture content and the UV-radiation itself lead to a restricted molecular mobility, and, as a result of that, affected an increased resistibility of the polymer chains. This, however, results in a stiffer shear behavior. Which one of the three factors acting in combination dominates is not known and requires further investigations.

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The τ-γ curve progressions of the samples weathered in the climatic chamber (series 5) are flat, partly bent downwards, for both shear velocities. Basically, the original location in the basic glass sheet (cf. Figure � - right) plays a significant role. The graphs of the edge samples are far lower than for the middle samples. Partially, loss of adhesion was reduced and delamination could be observed during testing, as shown in Figure 7 (right). These phenomena can be explained through two mechanisms:

Moisture enters into laminate over free glass edges. As a result, the polymeric interlayer gets softer (decreasing of shear modulus), because water generally acts like a softener.An increasing moisture concentration on the interlayer leads to a reduction of the adhesion between glass surface and interlayer. Delamination of edge samples predominately occurred with the faster testing speed (v2 = �,0 mm/min).

High temperatures inside the solarium (≈ 70 °C) caused a yellowing of the interlayer at sample’s edge zones, as can be seen in Figure 7 (left, middle). Enhanced yellowing could be observed with longer weathering duration, but seem to have only secondary influence on general shear behavior. Other visual deterioration could not be detected. Figure 7 (right) shows the delamination of the interlayer.

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2.6.2 Shear Behavior after Two Years of Natural Weathering in Arizona and FloridaThe existing climatic conditions in Florida (series 7) are one of the most intense weathering forms of LSG and can be compared with a moderate test using the climatic chamber. Also, the samples’ original location (cf. Figure � - right) in the basic glass sheet is important for the test results obtained. The curves from the edge samples are far lower than from the inside. As shown in Figure 8 (left, right), all curves including reference’s correspond very well with each other for both shear velocities. Partially, delamination occurred at Florida edge samples with the lower testing speed.

When inspecting Arizona curves (series 6) in Figure 8 (left, right) no major deviations between edge and inside samples can be noticed for both shear rates. For a speed of v� = 0,� mm/min all curves show hystereses. Generally, lower curves were obtained with the lower shear rate.

2.6.3 Secant Shear Moduli as a Function of Time for Weathered LSGFigure 9 (left) displays shear test results evaluated as time-dependent secant shear moduli after laboratory weathering (Series 2 to 5). Stiffening tendencies can be observed for the solarium samples, while shear stiffness for the climatic chamber samples is reduced. For the latter, a larger difference between weathered sample and reference can be found for a

Figure 6

Exemplary τ-γ chart for different artificially weathered LSG with v1 = 0,1 mm/min (left) and v2 = 1,0 mm/min (right)

Figure 6 left Figure 6 + 8.xls 8.5.2007 10:40

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Figure 7

Basic glass sheets after 20 weeks of weathering in solarium (left and middle); Delamination of edge sample after weathering in climatic chamber (right)

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shorter loading time (�� s - 77 s). More details about the evaluation using a time-dependent secant shear modulus can be found in [6].

As Figure 9 (right) illustrates, Arizona samples (series 6) show only slight deviations in total shear behavior between edge and inside sample under consideration of the time-dependent secant shear modulus.

As opposed to that, after two years of Florida weathering (series 7) two circumstances are presented: While under relevant testing parameters no major influences on the shear modulus of the inside samples are observed, the shear modulus of the edge samples was considerably lower (25 - 35 %) due to the moist state of the PVB-interlayer compared to the reference.

3NumericalParameterStudiesonLarge-ScaleLSGPanes

3.1 Influence of High Edge Moisture Content on Load Bearing Behavior

Based on cognitions won, high moisture concentration of PVB-interlayer can be a major safety and design factor for the impairment of the load bearing performance of architectural LSG. Because of that reason, high moisture concentrations are specified here as a significant aging attribute. First, the relevant bond area carrying the load is reduced. Second, a deteriorated adhesion of glass-interlayer shows a bad

Figure 8

Exemplary τ-γ chart of naturally weathered LSG with v1 = 0,1 mm/min (left) and v2 = 1,0 mm/min (right)Figure 8 left Figure 6 + 8.xls 8.5.2007 10:40

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post breakage behavior and reduced binding of glass splinters.

In order to clarify, whether moisture that enters into the PVB-interlayer over the edges has an influence on the bending behavior of uniaxially and biaxially carrying large-sized LSG panes, numerical parameter studies were carried out using the general FEM program ansys 7.0 [7]. Independent from an interlayer product type, moisture penetration at the edges was simulated and the total bearing behavior due to a short, spontaneous loading was analyzed. Modifications of the glass characteristics over time were neglected.

3.1.1UniaxiallySupportedPlateSimulationofastrongmoisturepenetration of the PVB-interlayer at the edges of a uniaxially supported plate (��00 x 360 mm) was done, as a worst case scenario, by extremely reducing the shear modulus. Hence, in main bearing direction the edge zones from the edge up to �00 mm inside of the plate were stepwise allocated with a modified,

very low shear modulus (0,05 N/mm²), as shown in Figure �0. For the non-aged PVB-interlayer an exemplary shear modulus of �,0 N/mm² was selected.

For calculation a linear elastic material behavior of the interlayer and an unaffected interface between glass and interlayer were assumed. Modeling of the plate was done with real LSG dimensions measured with 3,86/0,70/3,86 mm. The modulus of elasticity of the glass was 70000 N/mm², and Poisson’s ratio ν = 0,23. A non-linear calculation with volume element sOlid 45 [7] three-layered was performed. Calibration of the reference FE model (non-aged interlayer) was done by re-calculating test results of four-point-bending tests on LSG plates with the same dimensions. Additionally, reliability of results was verified by analytical approaches in the Literature.

Figure �� presents a non-linear regression curve (polynom of 2. order) for the increasing of the mid-span deflection as a function of the ratio of deteriorated edge zone and relevant

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Secant shear modulus as a function of time - series 2 to 5 (left), series 6, 7 (right)

Figure 10

Element grid of quarter plate and demonstration of different shear moduli of PVB-interlayer at the edge zone (here: 100 mm deteriorated zone) and inside

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total bond area (�000 x 360 mm). The trend line is generally independent from the load step. It can be seen that for a reduced bond area of �5 % the mid-span deflection increases about 5 %. Hereafter, the deflection increases with diminished bond area, disproportionately.

3.1.2BiaxiallySupportedPlateIn the following, the influence of strong moisture penetration of the PVB-interlayer at the edges of a biaxially supported plate (2000 x �000 mm) is examined. Analogous to the previous section, the plate’s edge zone was stepwise allocated from the edge up to �00 mm inside of the plate with a modified, very low shear modulus (0,05 N/mm²), as shown in Figure �2. For the non-aged PVB-interlayer the shear modulus was �,�� and 3,0 N/mm². The latter shear modulus represents a monolithic material behavior. Model calibration was done by re-calculating test results of area loaded LSG plates. Results’ reliability was verified by analytical approaches in the Literature.

Figure �3 shows the increase of the mid-span deflection as a function of the ratio of deteriorated edge zone and relevant total bond area (�980 x 980 mm) for both shear moduli and the area load applied. The increase of mid-span deflection reduces with an increase of the deteriorated edge zone at a constant area load. This tendency is observed for both shear moduli and can be attributed to the development of membrane effects for a softer load bearing behavior.

When comparing the non-aged and aged interlayer in Figure �4, a change of the distribution of the maximum principal tensile stresses at the bottom side of the lower plate (quarter plate) can be registered for both shear moduli and, for instance, an area load of q = 2 kN/m². For the bottom side of the lower plate there is an increase of the stresses following the plate diagonal (increase of influencing membrane stresses), where as maximum stresses in plate middle stay almost constant. Hence, the level of the shear modulus has no significance on the level of the maximum principal tensile stresses in plate middle in comparison between a non-aged and aged PVB-interlayer. Further results of FE parameter studies are presented in [6].

4Conclusions–Summary

According to the author’s opinion, stiffening aging phenomena (e.g. embrittleness, drying out) due to UV-radiation and high air temperature and their influence on the general shear behavior can be neglected for a relevant shear angle in construction practice (γ ≤ 0,9 rad [8]).

The numerical parameter studies showed that moisture penetration of the PVB-interlayer at the edge zones of large-scale architectural LSG has to be regarded only as a local deterioration

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Figure 11

Increase of mid-span deflection of uniaxially supported plate vs. ratio of deteriorated edge zone and total bond area

Figure 12

Element grid of quarter plate and demonstration of different shear moduli of PVB-interlayer at the edge zone (here: 100 mm deteriorated) and inside

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Figure 13

Increase of mid-span de-flection of biaxially sup-ported plate vs. ratio of deteriorated edge zone and total bond area

Figure 14

Comparison of the distribution of the maximum principal tensile stresses (quarter plate) for non-aged and deteriorated edges (here: 100 mm) exemplary applying an area load of 2 kN/m²

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of the interlayer. A partial failure of the bond performance leads to marginal changes of bearing behavior for uniaxially carrying plates. Similarly, this can be considered for biaxially carrying plates under bending stress (wind, snow and dead load), while a softer bearing behavior is additionally compensated through membrane effects. Thus, a deterioration of the PVB-interlayer is restricted to optical changes and visual vitiations generating bubbling or delamination of the interlayer at the glass edges. As a further result, for both LSG structures, consequences concerning an endangering of the structural safety can be eliminated. As well, a domino effect for adjacent structural elements is unlikely. An immediate changing of deteriorated panes is not required.

With respect to structural elements under high compressive loading (e.g. columns) delamination can be significant due to possible instabilities (e.g. local buckling [9]). For glass constructions with point holders instabilities cannot be excluded in case of delamination,

but further investigations in that field are necessary. Considering LSG used for photovoltaic applications severe problems with functionality occur if moisture concentration exceeds specific values.

In order to avoid damages at the edges of architectural LSG especially in extreme climatic environmental conditions or, with access to high humidity (e.g. rain), a thorough and effective protection (e.g. sealing [�0]) of glass pane’s edges during building should be arranged.

The assumption of a general aging factor of LSG completely reducing adhesion of glass-interlayer can be abandoned according to the current state of knowledge.

References[�] DIN EN ISO �2543-4: „Verbundglas und

Verbund-Sicherheitsglas - Teil 4: Verfahren zur Prüfung der Beständigkeit“, Beuth Verlag GmbH, August �998

[2] EN �4449: „Glas im Bauwesen - Verbundglas und Verbund-Sicherheitsglas - Konformitätsbewertung/Produktnorm“, Beuth Verlag GmbH, Juli 2005

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[�0] Block, V., Davies, P.S.: „Enhanced edge stability with structural glass laminates”, Glas Processing Days 2005, Conference Proceedings Book, 2005

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