SiLicone, As an Important Building Material
Written on Tuesday, March 26, 2013 by Jose Sanchez Marquez
The following information was the result of a project conducted at Concordia University, Montreál, Canada called for the Master Course Modern Building Materials BLDG 6621 (Winter 2013).
SiLicone, As an Important Building Material. The main idea was present the types of test develop until now for the use of industrial silicone. Highlighting their common types of failures, properties and advantages of the structural sealant glazing as commercial product.
by Jose Sanchez Marquez,submitted .
(Note: the all project is presented here)
ABSTRACT
Sealant materials are very popular in our lives; they are used on a daily basis in our houses, in construction processes, in the industrial field, etc. The most known sealant is silicone and also therefore is the most used. The purpose of this project is to exhibit the silicone as an important construction material, analyzing its properties like, elastic recovery, moisture, curing, durability and behaviour to environmental factors (temperature, radiation) and mechanical loads but above all, to enlighten in what others circumstances the silicone can be used as a significant component of building facades.
INTRODUCTION
People generally use sealants, for example silicone, thinking of three main purposes, to separate the inside of buildings from the outside, to conserve energy by insulating walls and to stop air leaks [1]. Nevertheless, is imperative to recognize that, sealants are affected by temperature, especially in tropical countries, where the temperature is high almost all year. Furthermore, today there is not a reliable service-life prediction for sealant materials and - field studies indicate that 50 percent of building sealants fail within 10 years, and 95 percent fail within 20 years [1].
In order to avoid these failures, is necessary to start giving more significance to properties like, UV resistance, moisture and curing to finally being able to select the best sealant. Nowadays, the most commonly three generic sealants used on building facades are silicone, polyurethane and polysulphide. However, they can fail for the same reasons - user’s lack of knowledge of sealant properties; usage of unsuitable sealant material for a given substrate and conditions; improper workmanship; improper sealant dimension; quality control of sealant [2], - but in many cases, failures are believed to be associated with chemical or physical changes near the substrate-sealant interface, arising from the penetration of moisture into the joint [3].
Figure 1, Common types of sealant failures [2]
Finally, silicone is perhaps one of the most complete sealants materials able on the market. Thanks to previous research investigations, -silicone is the only material which do not absorb a significant amount of water over a long time period [3] and shows an excellent elastic recovery almost 100% values in one day under different conditions tested [4]. Also, - the stability of silicones is a result of the inherent resistance to cleavage of the SiC and SiO bonds, increasing flexibility over a wide range temperature range [5]. For all these reasons, silicone is a logical material to analyse and used on structural sealant glazing or curtain walls system able to develop a non-metallic material with the properties of glass [9].
SILICONE PROPERTIES
One of the most important roles that the sealants play as a building facade material is being the first line defense against air and water infiltration. - the resistance to weathering depends on the type of polymer, its composition and structure [2] reason why every building project need to analyzed the different weather conditions where the material is going to be placed and exposed.
1. Elastic Recovery Characteristics
Several efforts have been made in order to develop a test technique able to assess elastic recovery of sealants. Elastic recovery can be define as - the property of a sealant whereby the initial shape and dimensions of the material are wholly or partially restored on the removal of the forces causing deformation [2].
Chew et al [2] developed a portable, on-site testing technique in where they compare three of the most commonly used sealants in the market such as, silicone, polysulphide and polyurethane. Chew et al [2] analyzed one-part and two-part on polysulphide and polyurethane materials and one-part silicone.
Using samples of size 35x5x2 (mm) after 14 days of curing under standard conditions, they simulated weather conditions immersing the samples in water (25-30°C), hot water (70 °C), in the oven at 70 °C and QUV conditions- 4h UV (70°C); 4 h condensation (5O°C). It can be observed in Table2, the different results presented during the study.
Table 2, elastic recovery values of three generic sealants [2]
According to figure 2 and table 1, the silicone do not present significant changes compared polysulphide and polyurethane when it is exposed to these weather conditions. The most drastic change was presented when the material was immersed in hot water decreasing approx. 9.20% of the elastic recovery initial value [2].
Figure 2, Silicone elastic recovery [2]
At the end of the study it was observed that, in comparison with the other two sealants (polysulphide and polyurethane), the silicone is the one that presented the smallest changes on elastic recovery under different weather conditions and - is suitable for moving joints and harsh weather conditions and it is aesthetically acceptable [2].
2. Curing Characteristics
Field studies indicate that 50 percent of building sealants fail within 10 years, and 95 percent fail within 20 years [1], many of these failures are commonly associated to - user’s lack of knowledge; improper workmanship; improper sealant dimension [2], or in many cases, failures are believed to be associated with - chemical or physical changes near the substrate-sealant interface, arising from the penetration of moisture into the joint [3]. This phenomenon is probably more common than people think and is very possible that start due to, a non-properly curing process.
The curing process is up to the sealant material, it could take a week or several months and there are different methods with variable speeds, like Acetoxy, Oxime, Alkoxy, Benzamide, Amine and Aminoxy method. According to international standards, all sealants must be cure “statically” for a period of 28 days, before be subjected to any kind of strain, stress or weather conditions. In practice, sealants are installed and then subjected to the conditions mentioned above, in this case the curing is called “dynamic curing” and premature failures can take place, due to the polymer chains do not have enough time to crosslink, the material is going to reduce its elasticity, strength and elastic recovery properties [3].
Chew et al [3] analysed one-part and two-part on polysulphide and polyurethane materials and one-part silicone, table 3 shows the generic types of sealants used in the study.
Table 3, Generic materials used in the study [3]
One-part component simply means open the tube cartridge or pail and the material will cure. Normally, the one-part component reacts with the moisture in the air to become rubber - they are based on polymers where the main chain consists of alternating silicone and oxygen atoms (siloxane) in repetition [3].
In the study, the samples were cast in moulds of 300x100x15 mm and cured statically under standards conditions at 26+/-2°C and relative humidity of 65+/-5% for a period of five months, during this time elastic recovery was tested on each sample.
At the end it was observed that for one-part polysulphide materials the average fully cures were reached after 85 days and for the polyurethane materials fully cure were reached after 7days. Instead, the silicone after just one day (100%) of elastic recovery was reached indicating high resistance against stress during curing [3] – also develop a skin within 5 to 10 minutes , becoming tack free within one hour and curing to an elastic rubber about 1/10 inch deep in less than one day [6]. It was also observed that for two-part systems (polysulphide and polyurethane) after one day of curing values around (98 - 100%) of elastic recovery was reached [3].
In practice the one-part silicone and the two-part polysulphide and polyurethane are ideal for rapid installations. Figure 3 shows the curing behaviour of the one-part silicone and the two-part polysulphide and polyurethane.
Figure 3, Curing behaviour of sealant materials [3]
3. Moisture Influence
High humidity and excessive water are two of the most important conditions that affect sealant durability in building joints, all because - chemical and physical changes near the substrate-sealant interface, arising from the penetration of moisture into the joint, affect the adherence between the sealant and the cement mortar (CM) or glass-reinforced concrete (GRC) [4].
Aubrey et al [4] developed a testing method using concrete blocks of 50x50x5 mm prepared with Portland cement and cured for 28 days. Figure 4 shows the joint models of the cement mortar (CM) and glass-reinforced concrete (GRC).
Figure 4, Joint Models construction [3]
Five of the most commonly used sealants after full statically cure, using one-part (silicone, polysulphide and polyurethane) and two-part systems (polysulphide and polyurethane) sealants to conform the building joints samples [4]. Table 4 shows the generic sealant materials used in the study.
Table 4, Generic materials used in the study [4]
The joints were immersed in water at (20 - 25°C) for several days (14, 24 and 46 days), and dried to the ambient for (6-16 days) at (20 - 35°C) [4]. Table 5, shows the approximate percent weight gain due to water.
Table 5, Weight gain due to water after immersion [4]
At the end of the study it was observed that, the one-part silicone was the only sealant that for short and long time did not absorb a significant amount of water and the modulus did not change. For the rest of the sealants a reduction on their modulus was detected suggesting that - water weakened the joint by reducing the tear strength. This phenomenon was more common on joints made up with glass-reinforced concrete (GRC) due to a higher porosity, higher permeability [4]. Figure 5 shows the stress relaxation on wet and dry conditions.
Figure 5, Stress relaxation on wet and dry conditions [3]
On dry conditions the joints made up with cement mortar (CM) or glass-reinforced concrete (GRC) and different sealants showed a similar behaviour [4].
According to the study results is possible to conclude that, silicone will fit better on wet conditions and tropical countries due to, water absorption is less than the one presented on the others sealants and the combination with cement mortar (CM) can make better building joints.
4. Fatigue Resistance
It is well know that, every sealant has a different response when - is under a number of cycles load until the adhesive integrity of the bond is changed on the surface or in the bulk of it. Block, S.R et al [5] developed a study to understand the fatigue behaviour in glazing sealants, using low modulus silicone, high modulus silicone, acrylic latex and polyurethane materials on commercial windows made up with aluminium, polyvinyl chloride (PVC) and wood (pine). Table 6, shows the modulus (Mpa) of the generic sealants used in the study.
Table 6, Generic materials used in the study [5]
The samples dimensions were 12x12x5 mm, every specimen was cured under standard conditions for a range of hours or days (1/2, 2, 4, 8, 24, 7 and 30 days). A cyclic movement was induced in extension only at movement amplitudes of 10%, 25% and 50 % of the joint width, and then a cyclic of loading was also induced at a frequency of 10 cycles per minute. A maximum of 100.000 cycles (10 days) was allowed for each specimen on every frame material (aluminum, PVC and wood) [5]. Figure 6 shows the sealants behaviour when joint percentage movement is increase on different frames.
Figure 6, Sealants in different frames material and joint movements [5].
According to figure 6, the low modulus silicone showed the best response to high movements with a short cure period (less than 1) and in just 7 hours was capable to reach a maximum of 100.000 cycles with the aluminum frame, indicating a strong failure resistance compare with the high, latex and polyurethane materials or what it means, a more durable window seal [5]. Table 7 shows the different response obtained changing the joint movement percentage and the frame material.
Table 7, Sealant material under different conditions [5]
According to table 7, the following material with strong failure resistance was the high modulus silicone, then the acrylic latex and the one with the lowest failure resistance was the polyurethane. Finally, aluminum showed to be the best component for frames, following by the PVC and then pine wood.
5. Durability
Sealant’s durability depends on the polymer properties, cure chemistry (cross linker and catalyst), fillers and additives. Silicone exhibit one of the best behaviour at different temperatures and weather conditions, making it the ideal sealant in tropical countries, where hot temperatures and raining season are common. The reason of its outstanding performance is – a direct result of the unusual organic/inorganic hybrid nature of their hydrocarbon pendant groups and siloxane polymer backbone, this combination helps to form excellent adhesion, water repellence, flexibility, ultraviolet resistance, etc [6].
5.1High Temperatures Although silicones presents a high thermal resistance, long time heat exposure develop ageing process that can affect service properties like,- siloxane bond rearrangement, oxidation of hydrocarbon groups, etc. causing degradation on the performance. The only property that it is not affected is the adhesion (according to formulation) at temperatures of (100-120°C) is improving [6].
5.2 Permeability Generally silicones are also well known for their slow solubility (absorb small quantities of water) and water repellence, - generally around of 0.2 -8 % absorption after 90 days of water immersion. However, long exposure periods in water can induce chemical reactions, especially when the silicone is exposed to strong acids and bases affecting the sealant adhesion due to, the dipolar nature of the siloxane bonds may induce hydrolysis. In order to avoid this chemical reaction is possible to pre-treat the silicone with a primer to minimise the impact [6].
5.3 Solar Radiation Silicones offer a very high oxidation resistance due to, the amount of ultraviolet radiation (UV) absorbed over time is very small. Actually it has being shown that in countries with sunny climates, silicones do not present important physical changes. On the other hand, infrared energy (IR) radiation is not enough to produce any chemical changes on silicones [6].
5.4 Adhesion and bond Durability A good sealant is the one who develop a high degree of adhesion to the substrate surfaces or primer layer to accomplish it, it’s necessary to have two situations, good molecular attraction between the sealant and the substrate surfaces and absence of contamination (dust) at the interfaces.
Although, in most of the cases the silicone displays and exhibits excellent bonds to most conventional substrates including glass, glass coatings, ceramic frits and powder coated paints, conversion-coated and anodized aluminum. However, some finishes may require a primer; their use aids to increase adhesion and help to insulate the sealant from the alkalinity of the concrete [6]. To use it properly, surfaces must be clean also and the application time must be short otherwise the material becomes inert. Figure 7 shows a representation of the adhesion between sealant and substrate with and without primer.
Figure 7, Adhesion between sealant and substrate with and without primer [6]
STRUCTURAL SEALANT GLAZING (SSG)
Most of the time silicone is used thinking on three main uses, - separate the inside of buildings from the outside and conserve energy by insulating walls and stopping air leaks [1]. Silicone is one of the most complete materials able on the market and for the reasons mentioned previously; reason why, is a logical material to be used as a structural sealant glazing in curtain wall system [9].
The structural sealant glazing (SSG) systems are basically the adherence between the surface glass or other materials, to a framing system such as anodized aluminum, painted metal or certain stones using a specifically formulated structural silicone sealant as both an adhesive and a sealant [9]. It can be observed in figure 8 a cladding system SSG detail.
Figure 8, Cladding system SSG detail [9]
1. Sustainability
Several are the advantages of the SSG system; the aesthetic appeal could be one the most highlight, the ability to connect the inside with the outside environment. Also due to, there is no need to use mechanical fasteners such as screws, bolts or clamps the SSG can provide a full glass facade without protuberances picking up less dirt and can use better the daylight, reducing energy consumption inside buildings especially on corridors, hallways, atriums, etc. leading to energy-efficient buildings.
Additionally, full glass facade elements aids to decrease the Heat island effect in urban areas due to the sunlight reflection of the glass, helping to reduce the city temperature, also the - nitrous oxides in the air can trigger dangerous ozone buildup and smog as well as require even more energy consumption to cool buildings [10]. It can be observed in figure 9 an example of the influence of the city density on the late afternoon temperatures.
Figure 9, Heat island effect [11]
2. SSG highlights and commercial Products
It is necessary highlight the fact that, structural silicone glazing and frames must work together as a one element and the curtain wall design must take into account the weatherproofing sealant capability, besides the data sheets given for the manufactory company.
In the market, several are the available products to achieve a curtain wall system. Nevertheless, is important know how to choose the proper material and the types of product that market is offering nowadays. The selection can be done in a better way, if is dividing in two stages, paying attention to the most relevant product characteristics.
2.1. Design stage: according to the joint design, wind and death load is important to focus on the next factors and the commercial range of values on the datasheets given by the manufactory companies,
a. Hardness – Durometer = (24 - 42)
b. Ultimate tensile strength (Mpa) = ( 1.3 – 2.35)
c. Ultimate elongation (%) = (116 – 715)
d. Heat resistance (some cases 149°C)
e. Joint movement capability = (+/- 12.5 to +/- 50%)
f. Service temperature range after cure = (-48°C to 121°C)
2.2 Installation stage: Installation process has the same importance as the correct sealant selection, according to weather condition, time curing or joint movement capability. The stage depends entirely on the building location, weather conditions, building settlement, the installer’s experience and the commercial range of values on the datasheets given by the manufactory companies,
g. Consistency - paste
h. Partial Cure time = (1-3 days)
i. Full cure time = (7 – 14 days)
j. Installation temperature range = (4°C to 50°C)
k. Work life – tooling time = (10 – 45 min)
l. Weathering and UV resistance – excellent
m. Minimum sealant thickness = (6 – 7 mm )
In the market is possible to find structural sealant glazing as one or two component products. The principal differences between them are showed on table 8.
Table 8, Principal differences [13]
A rapid installation especially on one-part silicone due to, the limited application life is very important. For the case of the two-part sealants the application life is subjected to the A/B ratio where, A is the base component and B is the catalyst component, each component should be weighed to the desired A/B ratio and mixed. At the end, the final result must be the same whether if used one or two component products; reach a good adhesion to guarantee long- term bond durability in the joint.
Though, in most of the cases the silicone displays and exhibits excellent bonds to most conventional substrates including glass, glass coatings, ceramic frits and powder coated paints, conversion-coated and anodized aluminum; corrosion on the surfaces is not allowed and is important clean up all dust with a clean brush, vacuum or by compressed air. Also, on porous surfaces besides the dust sometimes is necessary make a deep cleaning whit wire brushing, grinding, blasting or solvents to guarantee a mechanical interlocking between the substrate and the sealant [12]. Finally, the direct application of sealant must be done on dry surfaces with a temperature range of (4 - 50°C).
Most of the structural silicone manufactory companies emphasize that, the best way to reach full cure is under standards conditions, (relative humidity of 50 ±5% at an air temperature of 23 of ±1°C), the average products can attain - a cured thickness of 3-4 mm per 24 hours if the material have access to atmospheric moisture (open spaces), otherwise in confined spaces with limited access to atmospheric moisture will cure only from that surface which has access to the atmosphere [13]. Finally, is important to remind that, as temperature decreases, the cure rate slows down (and vice versa) [13].
Figure 10 shows a real example of SSG system, is possible to see the combination between the aluminum frame – glass and the glass light reflection.
3. The past, present and the Future of (SSG)
In the past, thank to innovation practices made in the 1960s silicone sealants were used as a part of structural sealant glazing systems for exterior enclosure of buildings. The SSG system started in 1970 as two sides adhered to a metal framing, and then in 1971 four sides applications, where silicone sealants were only utilized to attach glass to metal [14]. Then the development of a structural joint started to improve the system viability in 1980s.
The basic joint design is based on a trapezoidal loading developed by ASTM (American Society of Testing Materials); the design depends on the wind load; sealant strength and short span length of rectangular glazing unit.
Joint dimension (bite): [14]
However, since that time few changes have been made on this loading concept, the opposite of what happened with the silicone sealant technology; new improvements on structural silicone glazing are conducted on two main aspects; decrease the bulk dimension of the joint without affect pressure resistance in hurricane zones where wind pressure are very high or is increasing; and develop a slender curtain wall framing reducing the amount of aluminum; other benefits can be:
× Visual area increase,
× Cost reduction,
× Lighter units, easy installation,
× Thermal performance
Figure 11 shows the typical SSG configuration in where is possible to see the structural silicone and the others components of the system such as, glass, weatherseal component, the spacer between the glass and the structure.
Figure 11, Typical SSG configuration [13]
Clift et al [13] using a software program called ANSYS; conducted a finite element modeling of a structural silicone, detecting that in - a typical curtain-wall assembly, where the silicone is adhered in a square cavity (rectangular shape), the finite rotation of the glass at the perimeter seal under negative load will induce the greatest movement at the edge of the silicone joint. Reason why, the proposed design allows that the silicone at the perimeter joint have additional movement. Figure 12 shows the conventional design and Clift et al [14] design in where glass can rotate more freely, the principal difference between both is their shape.
Figure 12, Glass rotation for conventional and trapezoidal joint [14]
Two computer models were generated by ANSYS software; for the proposed and conventional the specimen sizes were 23.81 x 6.35 x 12.7 mm and 50.8 x 12.7 x 12.7 mm respectively. The specimens were tested on a 1905 x 1524 mm glass model and loaded to 9.6 kPa.
The test results showed that a stress reduction can be achieved by allowing the silicone to rotate with the glass under large wind loads [14]. Table 9, shows the maximum values obtained in the gross area of the two joint specimens under a 9.6 load.
Table 9, Stresses values [14]
Figure 13, shows the stress behaviour at Mid-span, red color indicates the maximum value of each specimen at the edge corner (proposed: 0.64 – conventional: 0.91), blue colors indicates lowest values. It can be inferred also that the proposed design have a uniform stress behaviour compare with the conventional joint.
Figure 13, Stress behaviour at Mid-span of the proposed and conventional design [14]
CONCLUSIONS
Nowadays, the silicone is one of the most complete sealant materials available in the market making of, the most suitable construction material.
Compared with others sealant materials, the silicone proved to have higher properties becoming in the most used material for the construction industry and for building rehabilitation. Thanks to previous research studies it was possible recognized the outstanding properties of the silicone starting from elastic recovery to curing characteristics, moisture influence and durability properties. For the case of elastic recovery, silicone showed an exceptional behaviour, after 35 days under simulated extreme conditions such as, immersed the silicone in hot water or within an oven at 70°C their losses were only 9.20% and 7% respectively, of the elastic recovery. Meanwhile, for the others sealants (polyurethane and polysulphide) the values obtained were almost 20% and 17% respectively making of the silicone, the most suitable material to conform a moving joint and with best aesthetic appearance.
Similarly, it was possible to identify that, under standard conditions weather (relative humidity of 50 ±5% at an air temperature of 23 of ±1°C) the average structural silicone products are going to have a cure rate of 3-4 mm per 24 hours. Additionally, silicone exhibited a constant and small water absorption quality, between 24 to 46 days of water immersion the material only gained 2.2% of the total weight water and their modulus did not change in comparison with the other sealants, a reduction on their modulus was detected by Aubrey et al [4], suggesting that water weakened the joint by reducing the tear strength. The small moisture influence on silicones, make them ideal to work with in tropical countries where raining seasons are very common.
In addition, it was possible to recognize that, with a low modulus silicone when is combined with aluminum frame is able to develop the strongest failure resistance with a short cure time (7 hours) meaning that, in practice at the time to choose the best sealant for installation process, the silicone is going to be the first alternative compare with acrylic latex or polyurethane materials.
Furthermore, silicones have shown that it can be use it as an important component on curtain walls system, helping to develop energy-efficient buildings and improving the cities sustainability. Besides, their use is helping people to change the main concept about what a building is, before people commonly perceived buildings as, big elements isolated from the outside world, now with glass facades implementation, people are realizing that is possible to connect the inside with the outside environment with an aesthetic appeal.
Finally, many are the improvements that can be continue developing on structural silicone for SSG systems such as, allowing the silicone to rotate with the glass under wind loads can generate a reduction and then uniform stress behaviour compare with the conventional joint, generating at the end, a percentage reduction in the amount of aluminum of the entire facade.
REFERENCES
1. Christopher C. White, “How Long Will It Last? Predicting the Durability of Building Construction Sealants”, pp. 38 - 41. ASTM Standardization News- (January /February) 2012. Available Online at www.astm.org.
2. Michael Y. L. Chew and Lee Der Yi, “Elastic Recovery of Sealants”, Building and Environment, Vol. 32, No. 3, Pp. 187-193, 1997.
3. M.Y.L. Chew, “Curing Characteristics and Elastic Recovery of Sealants”, School Of Building and Real Estate, Faculty of Architecture & Building, National University of Singapore.
4. D. W. Aubrey and J. C. Beech, “The Influence of Moisture on Building Joint Sealants”, Building and Environment, Vol. 24, No. 2, Pp. 179-190, 1989.
5. A.T Wolf, “Evaluation Of The Fatigue Resistance Of Glazing Sealants During Cure – Window Glazing Sealant Selection”, Dow Corning S.A, Midland, USA. Rilem Report 21 - Durability Of Building Sealants (1999). Google Book.
6. A.T Wolf, “Durability Of Silicone Sealants”, Dow Corning S.A, Seneffe, Belgium. Rilem Report 21 - Durability Of Building Sealants. Google Book.
7. D.F.Bergstrom, R.D. Dashner and R.H. Krahnke, “Effects of Environmental Exposure Conditions and Sealant Composition on Silicone Sealant Properties – Effects on Mechanical Properties and Apparent Corss-Link Density”, Dow Corning Corporation, Midland, USA.
8. Flackett, Dale, “One Part Silicone Sealants”, Gelest Inc, USA. Google Book.
9. Thomas F. O’Connor, “Holding It Together, Structural Sealant Glazing Proves It’s worth”, pp. 34 - 37. ASTM Standardization News- (January /February) 2012. Available Online At www.astm.org.
10. A.R. Hutchinson and T.G.B Jones, “Global Considerations and Performance Requirements of Sealants”, Joining Technology Research Centre, Oxford Brookes University, United Kingdom M.A. Lacasse. Rilem Report 21 - Durability Of Building Sealants (1999).
11. Sherry A. Boyd, “Getting Into Green” Concrete Decor, the Journal of Decorative Concrete, Volume 12 (May/June) 2012. Available online at http://ww.concretedecor.net/decorativeconcretearticles/vol-12-no-4-mayjune-2012/getting-into-green/.
12. European High Quality Low Energy Building (EULEB). “Microclimate – Heat Island Effect”. Available Online at http://www.acca.it/euleb/en/glossary/index.html.
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