Akzo Nobel Functional Chemicals GmbH & Co. KG Liebigstraße 7 D-07973 Greiz DL-148 Insulating Glass Energy saving without noble gas filling? The combination of Low-E coatings and noble gas filling in today s Insulating Glass Units (IGU) give the best overall properties in a window. These windows provide excellent protection against the environment, and allow day light and ventilation, but it is the additional energy savings and noise protection benefits that truly set them apart. It is the energy savings in particular that is of increasing importance; nearly one third of all energy consumption is associated with buildings [1]. Consequently, energy saving has to now be counted among the primary requirements of a window. The comparison of today s commercially available glazing types in table 1 [2] demonstrates the considerable advantages of Low-E, noble gas-filled IGUs. In Europe where heating is the main issue these advantages can be expressed in terms of oil: low-e, argon-filled IGUs need only around 50 % oil of air-filled IGUs, and triple pane IGUs just less than 25%. According to various studies (e.g. [3]), decreasing of the Ug-value by 0,1 W/(m² K) leads to oil saving of about 1,2 liters per m² per year. This ratio is also valid in the US [4] savings of more than 0.12 USD/ft²/y are possible. Consumers are becoming more aware of this, and R-5 windows are enjoying growing acceptance. It is easy to predict that consumer demand will go beyond R-5 in the not distant future. Tab. 1 Single-Pane Double-Pane air-filled Double-Pane Low-E, argon-filled Triple-Pane Low-E, argon-filled U g -value (SI) W/(m² K) 5,60 2,80 1,30 1 0,65 U-value (US) BTU/(h ft² F) 0.99 0.49 0.23 0.11 R-value (h ft² F)/BTU 1 2 4.3 9.1 Oil consumption l/(m² y) 60 30 15 7 U g -value, U-value R-value U g =thermal transmittance value of the glass unit; in general U describes how well a building element conducts heat; U = 1/R Measure of thermal resistance; R is the inverse of U, measured as [W/(m² K)] or [(hr ft² F)/BTU] in the US, i.e. US R-values are equivalent to approximately six times SI R-values: 1 (K m²)/w = 5.678263 (h ft² F)/BTU So what part does the noble gas filling play in this exceptional performance and what is the consequence of gas loss in a sealed IGU? 1 for comparison: Low-E, air filled IGU: U g = 1,70 W/(m² K)=0.3 BTU/(h ft² F) C:\Documents and Settings\Renzd\Local Settings\Temporary Internet Files\Content.Outlook\2B3KRKJ3\Energy saving_usversion_finaldl.docx
The improvement of the U-value by argon-filling can be achieved by less than 1 % of the raw material cost of a window according to [4]. An estimation of the cost of gas filling is made in [5]. The effect of noble gas filling on the U g -value is shown in figure 1 (data based on [6]) the greater the level of gas, the greater the benefit. The converse is also true the more gas that is lost from a unit, the worse the performance. Should an IGU lose its noble gas entirely the resulting increase in U g -value would be around 0.07 BTU/(h ft² F), and oil consumption goes up by 0.1 gallon/ft²/y. Fig. 1 U-value[BTU/h ft² F] 0,35 0,30 0,25 0,20 0,15 0,10 Impact of Argon Gas Filling on U-value (4/12/4 Low-E) U = 0.07 0,05 0,00 >90 80 60 40 20 0 Argon Filling in % Therefore it is important to minimize gas loss over time. This can be achieved through the diligent choice of sealing material and careful manufacturing practices. Almost all of the 7 billion ft²igus manufactured world-wide last year are dual sealed: the inner sealant is based on polyisobutylene (PIB), and the outer or secondary seal has to be an elastic, rubber-like material which functions as an adhesive, holding the glass unit together and keeping it tight during the service life. It is clear that the secondary sealant fundamentally determines the quality and durability of the insulating glass unit. It is often promoted in advertisement that the UV resistance, weatherability, temperature resistance, adhesion, mechanical properties and even shrinkage are the most important features of secondary IG sealants, whereas resistance against permeation of moisture vapor and noble gas are regarded as insignificant or ignored altogether. This argument is generally based on the belief that the excellent barrier properties of the primary PIB sealant are sufficient to protect the IGU cavity against moisture and gas permeation. If this assessment was true, then all IGUs equipped with PIB primary sealants should show similar test results in gas and moisture penetration, regardless of which secondary sealant type was applied. Standard tests at various testing institutes performed with IGU sealed with PIB and the premium secondary sealants - polysulfide, polyurethane, and silicone - show the opposite: 2
As far back as 1991 it was shown [7] that gas leakage rates 2 L i <0.5%/y of argon-filled dual pane insulated glass units could only be achieved by using polysulfide secondary sealants. Silicone sealed units show a wide variation from L i <1%/y to L i >10%/yr. Slight deviations in the IGU manufacturing process could not be compensated by the silicone sealants because of their high gas permeability. Polysulfide sealants are more tolerant against such deviations. This is demonstrated by a smaller variation, between <0.5%/y and <2%/y. The majority of tested polysulfide sealed IGUs had L i <1%/y. Similar tests of argon permeation of double sealed IGUs (submitted by various European IGU manufacturers) performed between 2008 and 2010 at ift Rosenheim [8] provide analogous results (Tab. 2): Tab. 2 Secondary sealant type Number of tests % of failed tests criterion L i 1,0 %/y (EN 1279-3) Polysulfide 121 22 Polyurethane 93 27 Silicone 111 34 Statements about IGUs applied in low-energy-houses confirm that gas leakage rates between 0.2 and 0.6 %/y could be easily achieved with PIB/polysulfide sealed IGUs whereas it was difficult to meet L i 1.0 %/y with PIB/silicones [9]. Tests according to EN 1279-2 and 3 on IGUs sealed with PIB and silicone or polysulfide secondary sealants, respectively, were performed by AkzoNobel in cooperation with IG sealant and IGU manufacturers in 2010 in order to investigate the influence of sealant thickness and PIB consumption (among other parameters) on both noble gas (gas leakage rate L i ) and moisture permeation (average moisture penetration index 3 I av ). The results published in [10] confirm the well known truth: the thicker the sealant layer and the more PIB that is applied, the lower the moisture penetration and argon permeation; but there are differences between the silicone and polysulfide sealed IGUs: o moisture penetration of silicone sealed IGUs varies significantly (2%<I av <14%) dependent on the IGU features whereas I av of polysulfide sealed IGUs was only between 4.5 and 7.5 %. All units passed the test, but results show that IGUs with silicone sealants are more sensitive to sealant thickness and PIB application than those sealed with polysulfides. o the gas leakage rate L i before and after the EN 1279-3 test of the polysulfide sealed IGUs was nearly the same and independent of the sealant thickness and applied PIB whereas the silicone sealed units vary considerably (Tab. 3): 2 L i is the gas leakage rate defined by EN 1279-3 as the proportion expressed as a percentage by volume of gas i leaking from a gas filled unit per year 3 I av is the moisture penetration index defined by EN 1279-2 as the amount of drying capacity consumed after standardized ageing conditions 3
Tab. 3 PIB/Polysulfide secondary sealant PIB/Silicone secondary sealant L i (initial) L i (EN 1279-3) L i (initial) L i (EN 1279-3) [%/y] [%/y] [%/y] [%/y] average 0.55 0.67 0.57 1.22 standard deviation ± 0.07 0.02 0.47 0.90 Obviously, there is a considerable impact of the secondary insulating glass sealant type on gas and also on humidity tightness. Permeation properties are material properties and are related to the polymer type and the sealant formulation. Consequently, it is not arbitrary which secondary sealant type is chosen. Data received of membrane testing according to EN 1279-4 performed by independent institutes confirm the different moisture and gas permeability of the premium secondary IG sealants (Tab. 4). Tab. 4 Moisture Vapor Transmission Rate [g/(m² day)] Permeation of Argon [10-3 g/(m² h)] Source [6] [11] [6] [8] [12] Polysulfide 7-9 3 6 5 8 4.5 4.3 6.8 Polyurethane 1 4 2 4 30 50 36 40 75 Silicone 15 20 15 20 500 1000 888 700 800 In case the primary seal fails, the secondary sealant is left as the only barrier and therefore it should possess the best permeation properties. In regards to gas tightness the right choice of secondary sealant is essential because the molecule diameter of argon is in the range of rugosity (unevenness) of the glass surface 4. Since the PIB adhesion to glass is physical, not chemical, diffusion is possible along the interface between the glass and PIB. Polysulfide sealant s excellent barrier properties against gas loss and humidity penetration should make it the first choice for noble gas filled IGUs in order to provide maximum tightness. Maximum tightness also translates to maximum safety because in contrast to moisture penetration into an IGU cavity which is visible, loss of gas from the cavity is invisible and can only be non-destructively measured with expensive specialized instruments by experts. The window owner s sole way to recognize gas loss on his own is through increasing energy bills. Unfortunately it can be difficult for the owner to reliably attribute increasing energy bills to IGU gas loss as there can be many other causes of increasing bills, such as weather and energy cost variability. In conclusion, the question posed by this article s headline can be answered with No : energy saving without noble gas filling is ineffective and does not correspond to the state-of-the-art of insulating glass. Polysulfide secondary sealants ensure that noble gas remains in the cavity and therefore they contribute essentially to the longevity of insulating glass units. 4 the tin-side of the float glass surface has an unevenness of 1.2 to2 nano meters; the Argon diameter is 0.34 nano meters 4
Literature [1] EU Commission 2006; Glaswelt, 07(2011), p. 13 [2] W. Feist; Passivhaustagung, Oct. 31, 2006 [3] Announcement of BF-Bundesverband Flachglas; July 22, 2009 [4] Jackson, R.; Window & Door, Oct./Nov. 2010, p. 65 [5] Almasy, J.; USGlass, vol. 38, May 2003, p.32 [6] E. Mognato; GPD Proceedings, 2005, p. 235 [7] Feldmeier, F., Schmid, J.; IfT-Sript 01/91, p. 16 [8] Lieb, K; ift Rosenheim, unpublished data 2002-2010 [9] energiesparhaus.at; powered by tripple.net [10] Lange, D; 09/2012, Chin. Glass Ass. Proceedings (Guangzhou) [11] Holler, G.; Mehrscheibenisolierglas, Expert Verlag 1995, p. 68-99 [12] Wittwer, W., Lange, D.; unpublished report, 2009 D. Lange August 20, 2012 5