Requirements on secondary insulating glass sealants

Requirements on secondary insulating glass sealants Dr. Dieter Lange, AkzoNobel, Functional Chemicals, Greiz/Germany Abstract Nowadays there are various kinds of insulating glass available on the global market. The most popular type is the double sealed insulating glass unit (IGU). Polysulfide, polyurethane and silicone based sealants are the premium elastic sealants used as secondary seal. Provided they are formulated to the state-of-art these sealants meet all requirements on manufacture and service properties of well performing insulating glass units. In this paper processing and sealant properties of the leading secondary sealants are discussed by using test results of secondary insulating glass sealants commercially available on the European market. It could be shown that chemically based differences of the processing and service properties between the leading secondary sealants are the decisive factors for the selection of the secondary sealant type for a particular application of the insulating glass unit. Individual internal manufacturing standards established by the IGU producers which consider the specific properties of the different secondary sealants - mainly curing behavior, moisture and gas permeation - guarantee the manufacture of high quality insulating glass. 1. Introduction According to current standards elastically sealed insulating glass units are double sealed: the inner or primary sealant is mainly thermoplastic polyisobutylene (PIB) or butyl rubber which is applied to reduce the water vapor and gas permeability of the edge seal but also acts as processing aid to hold the spacer bar in place during manufacture. In some units a double-sided adhesive tape is used as processing aid, but this tape is not a vapor control check such insulating glass units are not dual-sealed. The outer or secondary sealant functions as an adhesive which holds the unit together and restricts moisture transmission into the unit and gas permeation out of the unit. Secondary sealants are based on different materials the most important are polysulfide (PS), polyurethane (PUR) and silicone polymers (SI). In 2007 more than 450 mln m² insulating glass was manufactured globally by using app. 65 mln liters of polysulfide, 30 mln liters of PUR and 15 mln liters of silicone based insulating glass sealants. Based on this data it is obvious that polysulfide based secondary insulating glass sealants are still the market leader among the elastic IG sealants. This outstanding position is not only a consequence of the unique combination of processing and product properties of polysulfide IG sealants it is also related to the long-term experience of nearly 50 years and to the current types of glazing/construction. In general, the ultimate choice of secondary sealant material should take into account the performance characteristics required of the edge seal the protection afforded by the glazing system the environment to which the secondary sealant will be exposed to during service In daily practice the above criteria led to more or less specialized application areas for the different secondary sealants: the preferred type of sealants used for commercial glazing applications (glass façades, structural glazing) is silicone based, polyurethane sealants are mainly used on automatic production lines where units of nearly the same size and shape designed for ordinary applications are manufactured in large numbers. Polysulfide sealants can be used for all applications (with the restriction that in glass façades the edge of the IGU has to be protected). This specialization is the result of the development process of IGU and sealant design during the last decades and is related to the properties of the cured sealant (e.g. special advantages) as well as to the processing properties of the sealant. Page 1

In practice IGU manufacturers and IGU users have a different perspective referring to the sealant properties: apart from the costs both have in common the goal to produce high quality IGUs. Additionally the IGU manufacturer is also interested in easy handling, easy processing and zero failure quota, i.e. the processing properties are very important. Both groups of properties will be discussed in this paper. Data of AkzoNobel s testing program in which commercially available sealants of the leading European manufacturers are included as well as data from the literature. Processing properties are only briefly described. They are discussed in various articles and presentations (e.g. [1], [2]). The main emphasis is directed at the properties of the cured sealants. 2. Processing properties The use of automatic production lines has led to additional requirements for IG sealants: high output, low wear and tear, easy handling, reduction of waste, recycling etc. Consequently, the base polymer and sealant manufacturers have nowadays to place more emphasis upon the processing properties of their products. Modern IG-sealant compounds must exhibit [2]: special rheology Good wetting of the substrate (glass and spacer) by the sealant is a precondition for good adhesion. The sealant should be continuously applied around the full perimeter of the unit including its corners. Therefore sealant material with low viscosity but non-sag flow behavior is required. The flow behavior of all tested secondary IG sealants can be characterized as pseudo-plastic; all types of secondary IG sealants met that requirement. high reactivity Sufficient pot life and fast curing are desired by the IGU manufacturer. In the following table the curing behavior of different commercially available secondary IG sealants is summarized: Table 1 Curing of secondary IG sealants Sealant PS 14 PS 23 PS 30 SI 25 SI 72 SI 73 PUR 28 PUR 32 Pot life min 30 29 20 40 33 20 35 20 tack free time min 50 45 35 240 150 50 90 50 Shore A after 1 h 13 20.5 16 7 3.5 4 6 11 curing time of 1.5 h 28 31.5 28 14 6.5 5 14.5 15.5 2 h 34 42 33.5 16 11.5 7.5 17 20.5 3 h 45 45 36 27 18 11 25 26 4 h 45 48.5 39 31.5 23 20 31.5 30 24 h 47 50 45 42.5 36 37 43 46 168 h 48 50 45 48 45 45 46 51 Well formulated secondary polysulfide IG sealants can achieve 80 % of their final hardness after 4 hours curing at room temperature. This unique curing behavior of polysulfide secondary IG sealants is the result of the oxidative curing with activated manganese dioxide. Products based on other polymers cure more slowly which already was demonstrated in [2]. The final hardness of app. 50 Shore A is achieved after 24 hours by most of the IG sealants. quick evolution of adhesion Quick evolution of adhesion is a major concern. The faster adhesion is achieved the fewer problems appear during early transportation of the IG units. Well formulated polysulfide IG sealants achieve full adhesion within 4 hours of curing at room temperature. The results of the adhesion tests described in EN 1279-6 (adhesion is Page 2

tested after 24 h curing at room temperature; the sample has to resist against a certain load for at least 10 minutes) demonstrates clearly that most of the secondary IG sealants perform well. Table 2 Test results of initial adhesion according to EN 1279-6 Sealant PS 14 PS 23 PS 30 SI 25 SI 72 SI 73 PUR 28 PUR 32 Glass passed passed passed passed passed passed passed passed Aluminium spacer AF: adhesion failure passed passed passed 70 % AF after 3.4 min passed passed 80 % AF after 1.25 min 60 % AF after 1.5 min With respect to process properties secondary polysulfide IG sealants have some advantages in comparison to sealants based on other polymers but most of the investigated European products perform well and meet the requirements. 3. Function and properties of the IGU edge seal 3.1 Theoretical aspects The IG unit is exposed to various loads caused by handling (opening and closing), by wind and by changes in temperature and barometric pressure. The loads lead to deformations of the unit (Fig. 1); the sealant is stretched, compressed and sheared. Fig. 1 Loads and resulting deformations of an IG unit The ability of the sealants to accommodate those deformations under the additional exposition to humidity, UV radiation and heat determines the service life of the IG unit. An IGU has reached the end of its service-life when moisture condensation (in case of gas filled units also the loss of gas) occurs within the inter-pane space. There are some important general aspects of the permeation of moisture and gases which has also to be considered in case of IGU: The mass transport through polymeric materials, in contrast to mass transport through porous materials like filter paper, occurs by activated diffusion [3]. In principle, there are two possible ways for diffusion: through the secondary and the primary sealant or along the interface between glass and sealants. Diffusion along the interface can be much higher than through the sealant [4]. In case of double-sealed units the diffusion resistance is the sum of the individual resistances. The rate of permeation through the sealant is always proportional to its area and usually inversely proportional to its thickness if equilibrium conditions are established. Page 3

If equilibrium has not been reached, then the time needed to reach equilibrium is roughly proportional to the square of the thickness (Fick s and Henry s law). Thus the thickness of the sealant improves its barrier properties much more for the preequilibrium period than it does after reaching equilibrium. Loose networks e.g. plasticized or swollen structures - increase permeability. 3.2 Permeation of moisture (moisture vapor transmission rate (MVT)) In case of perfect adhesion between glass and sealants moisture can only enter the interpane space through the sealants. In case the adhesion of the primary sealant to glass fails the secondary sealant remains as the only barrier against the penetration of moisture. If the adhesion of the secondary sealant also fails then the unit cannot be used any longer and has to be replaced. Early failure of IGUs is mainly caused by manufacturing mistakes or bad sealant quality or both. Table 3 summarizes the moisture permeation through different sealants and through double sealed IGU (PIB + one of the secondary IG sealants in table 3). Table 3 Moisture vapor transmission MVT rate [g/m²d] MVT [%] DIN 53 122-3 mm sealant sheet EN 1279-4 double sealed IGU 20 C 60 C 23 C 23 C chapter 5.1 DIN 52 344 Source [5] [5] [6] [6] [7] [6] Sealant based on Polysulfide 4-5 20-30 3-6 5 5.8-7.0 < 1.2 Polyurethane 3-6 20-30 2-4 4 2.6-3.5 < 1.2 Silicone (two part, neutral) 7-16 40-70 15-20 15 9.2 < 1.2 Polyisobutylene 0.1-0.2 The data show clearly that the MVT rate depends on the polymer type and increases proportional to the temperature. One of the poorest barrier materials to gases and water is silicone rubber. This is interesting since silicone rubber swells only slightly in water. The consequence, material intended for water barriers cannot be chosen on the sole basis of swelling in water, as is often done [3]. PIB is a strong barrier for moisture vapor and determines the diffusion resistance of the IGU with the consequence that the MVT rates of all double sealed IGUs are similar. 3.2 Permeation of noble gas IGUs are filled with gases e.g. noble gases like Argon or Krypton - in order to improve their heat and/or sound insulation. Their diffusion depends on the temperature and the partial pressure difference within the IGU and the surrounding air. The molecule diameter of Argon is in the range of the rugosity (unevenness) of the glass surface i.e. diffusion is possible along the interface between glass and the primary PIB sealant - its adhesion to glass is physical [5]. As a consequence, the secondary sealants have an important barrier function against the diffusion of noble gases. In table 4 the results of the gas permeability tests according to EN 1279 are shown [7]. Page 4

Table 4 Gas permeation Gas permeability in g/m²*h (EN 1279) Sealant based on Test temp. Argon Krypton Nitrogen Polysulfide 23 C 4.3-6.8 8.0-24 1.5-1.7 30 C 5.5-8.5 13-26 1.8-2.3 Polyurethane 23 C 40-75 135-210 10-12 30 C 63 170-205 13-15 Silicone 23 C 700-800 n. d. 340 30 C n. d. n. d. 360 3.3 IGU design sealant dimensions Although silicone based secondary IG sealants show the lowest resistance against the diffusion of gases and moisture they can be used to manufacture gas-filled insulating glass but that fact has to be considered in the design of the IGU. Therefore many of the European IGU manufacturers established internal standards to meet the requirements of EN 1279 with gas-filled IGUs: the thickness of the sealant and the consumption of PIB are strictly fixed in dependence of the type of secondary sealant. In fig. 3 and table 5 the corresponding data are summarized. Fig. 3 Sealant dimensions Table 5 Sealant consumption (internal standards of European IGU manufacturers) secondary IG sealant based on consumption of PIB [C in fig. 3] thickness of secondary sealant [B in fig. 3] g/m spacer (single side) PS or PUR Silicone 2.5 3.5 mm 2.5-3 4-5 As discussed in the previous chapters lower resistance against diffusion of gases and moisture of silicone based sealants can be compensated by the application of more material and careful sealing of the ends of the spacers (with PIB) to manufacture high quality gas-filled IGUs. Noble gas diffusion can be used to estimate the service life of an IGU: The Argon loss rate of polysulfide/polyisobutylene sealed IG units was determined by HOLLER [6] to be in the range of 1 8*10-3 /y whereas for the system polyurethane/polyisobutylene values between 6-25*10-3 /y were found. FELDMEIER and SCHMID [8] anticipate 20 years of service life of an IGU if the Argon loss rate is around 1 % per year. According to these Page 5

calculations polysulfide/polyisobutylene sealed units with loss rates of regularly less than 1 %/y can be expected to offer service lives of 30 to 40 years. 3.4 Strength, relaxation, adhesion It is often discussed that secondary sealants have only the function of an elastic adhesive - the barrier function is provided by the primary sealant. In the previous chapters it could be shown that also the permeability of moisture and gas through the secondary sealant is of importance for the performance of an IGU. Nevertheless strength and adhesion to glass and spacer relates to the ability of the sealant to hold the glass panes together and to prevent moisture vapor (and inert gas) to pass the interface between glass and sealants. Both strength (and relaxation) and adhesion are dependent on the polymer type and under the aspect of cost reduction even more important to mention on the sealant formulation. The polymer/plasticizer/filler ratio determines the properties not only in case of polysulfide sealants. As mentioned permeability of gases and moisture decreases with higher polymer content and stress recovery increases proportional to the polymer content. At the same time the plastic deformation component of the secondary sealant decreases. The influence of the sealant composition on the overall sealant performance has been described in the literature and many presentations (e.g. [5], [9], [10]). 4. Conclusions Good processing properties and the ability to handle various loads when exposed to environmental influences are the most important criteria for secondary IG sealants. Because of individual advantages a specialized application of different seals was developed. Long-term practical experience has been used to improve not only the quality of the sealants but also the quality of all components of the insulating glass unit and its design has also been optimized. In case of secondary IG sealants it is approved by field data that all types of sealants meet the requirements on high performing IGUs provided that they are formulated according to the best state-of-art. In this context it is important to mention the result of an investigation by MOGNATO et al. [11]. They concluded from their investigation of IGUs that only efficient checking during manufacture and the strict adherence to regulations lead to low failure quota. In order to achieve this high quality level inspection by third parties appears indispensable to ensure good product quality. 5. Literature [1] Lange, D., GPD 99 Conference Proceedings, p. 102-106 [2] Lange, D., GPD 03 Conference Proceedings, p. 598-601 [3] Lebovits, A., Modern Plastics, (1969)1, p.139-213 [4] Zisman, W. A., Ind. Eng. Chem. (1963) 57, p. 28 [5] Garvin, S. L. et al., Building Research Establishment Report 1995 (BRE publication) [6] Holler, G., in Mehrscheibenisolierglas, Expert Verlag 1995, p. 68-99 [7] Wittwer, W., Kömmerling, unpublished report [8] Feldmeier, F., Schmidt, J., Bauphysik 14 (1992), p. 12-17 [9] Lange, D., Intelligent Glass and Architecture, 2 (2007) 08, p. 76-82 [10] Lange, D., Fenestration Days, Moscow 2005, presentation [11] E. Mognato et al., Proceedings of Glass Performance Days 2007, p. 606-609 Page 6