Modulus: silicone sealants

In non-working joints the degree of movement is substantially less (5% or less). In these joints low-to-medium modulus silicone sealants are suitable. 

The service life of these sealants is 20 years or more. They have a range of movement from 25% (polysulfides) to +40/—25% (polyurethanes) to +100/-50% (low modulus silicone sealants) extension/compression. Their price can range as much as seven times that of some low movement caulks. 

Due to the low glass transition and melting temperatures of PDMS polymer, 100% silicone sealant do not substantially stiffen at lower service temperature. Typically, their Young s modulus is maintained within a 25% range over a temperature range of —40 to 80°C. 

The role played by the various ingredients in the composition of sealant, and in particular on the durability of adhesion has been discussed recently [77]. Inert plasticizers, such as trimethylsilyl-endblocked-PDMS, are typically added to silicone sealant compositions in order to adjust the rheology of the uncured sealant. They result in a reduction of the modulus and hardness of the cured sealant. Differences in the durability of silicone sealants are found to be due to differences in their cure chemistry, and more specifically to the nature and... 

Three different commercial formulations of silicone sealants from Dow Corning was used in the NSF sponsored studies. They were DC-790, DC-995, and DC-983, in the order of increasing modulus. Dumbbell test coupons (samples) were prepared as per the ASTM standards. Some test coupons were maintained at ambient conditions as control and the rest were subjected to simulated weathering. The weathered coupons were removed from the test layout at regular intervals of time and were tested for any changes in crosslink density due to exposure. 

Silicone sealants can be formulated to have a low modulus as well as a high modulus. Some manufacturers also supply an intermediate class described as mid-modulus. The choice of reactive silicone oils with different viscosities allows one to obtain sealants with variable elastic moduli. 

Low-modulus sealants are usually preferred when the joint elongation is high (over 25% of joint width) whereas high-modulus sealants are chosen for assembly where the elastic movement under strain is reduced to a minimum (Houde, 1993). Table 6.6 gives the properties and applications of silicone sealants depending upon their modulus. 

Table 6.6 Properties of silicone sealants, depending upon modulus...

Different types of joints are associated with silicone sealants. The curing system and modulus of a sealant are important factors for the joint application type. Figure 6.2 illustrates the type of joints and Table 6.7 lists suitable products for use with each type of joint. 

Low-modulus neutral cure silicone sealants are suited to all kinds of working joints, ensuring that minimum stress is placed upon the sealant at its point of adhesion to the substrates (Figure 6.6). 

Ca(0CCH3)2 may form (see Equation 1) and cause adhesion failure. Neutral and base-liberating sealants bond well to these cementatlous substrates with no loss of adhesion with time. Some of the more recent non-acid-curing silicone sealants bond effectively to most common substances. However, the 100% modulus of these sealants tends to be lower than typical acid-curing silicone sealants. 

Now that the ingredients are defined, the nature of vulcanization can be discussed. Vulcanization occurs through the cross-linking system. The system shown below is the system used by many major silicone sealant compounders around the world, to make high and medium modulus sealants. It consists of hydroxyl-ended poly-... 

The chemistry just outlined produces silicone sealants which cure to a fairly tough, resilient rubber. Generally, sealants made by the above route are the higher modulus, strong silicone sealants used in products like silicone glazing sealants, silicone adhesives and silicone bath tub caulk. 

For many applications these sealants are too high in modulus. For places sealed with silicone sealants that see tremendous movement (>25% expansion and contraction), like expansion joints in highways or buildings, the toughness of the above sealant results in undue stress on the bond line at high elongation. In these applications, either the sealant will fail adhesively or the sealant will pull the concrete surface (or similar surface) apart. 

Compared to a common thixotrope such as fumed silica, pulp provides equivalent viscosity at less than one tenth the weight in a typical epoxy resin. In addition, fluid viscosity is unaffected by further processing (agitation) or aging—in contrast to fumed silica modified resin where viscosity drops and is not fully recovered under similar conditions. Pulp can also provide reinforcement in an adhesive matrix as shown by the significant increase in tensile strength, modulus, and tear strength of both a PVC plastisol adhesive and a silicone sealant on the addition of pulp [137,140]. 

In an attempt to correlate aging-induced changes on the mechanical properties (modulus, tensile strength, and elongation at break) of silicone sealants, Lacasse and Parolil usedthe same series of sealants. The results from both the mechanical properties and the STA (TG/DTA) showed changes in the silicone sealants. However, the authors were not able to correlate the results obtained from the mechanical test with those from the STA. 

The study showed that silicone sealants exhibit very high dynamic moduli (10 dyn/cm ) compared to PU (lO dyn/cm ). Moreover, silicone is stiffer than PU sealants below -30°C (Fig. 5a-b). Malik reported that silicone and PU sealants behave similarly only above this temperature and that the modulus of the silicone sealant falls in the acceptable window. According to the author, the storage modulus for a good construction sealant should not exceed 10 dyn/cm at -40°C. Therefore, he indicated that silicone sealants may not be suitable for temperatures below -30°C because they are too rigid. 

Figure 5. a) Storage modulus vs angular frequency for silicone and polyurethane sealants at -40°C. (b) Storage modulus vs angular frequency for silicone and polyurethane sealants at -30 C. (c) Dynamic mechanical data for silicone sealant at 1 rad/sec. (Reproduced with permission.p Note 1 Dyn/cm = 0.1 Pascal (Pa). 

One Part Medium Modulus Oxime Cure Silicone Sealant... 

Most conventional low-modulus adhesives and sealants, such as polysulfides, flexible epoxies, silicones, polyurethanes, and toughened acrylics, are flexible enough for use at intermediate low temperatures such as -40°C. Low-temperature properties of common structural adhesives used for applications down to -129°C are illustrated in Fig. 15.9, and the characteristics of these adhesives are summarized in Table 15.12. 

Mid-to-high-modulus acetoxy-cured sealants are the most common. In certain applications, neutral cure silicones are also employed. Whatever the curing system, a sanitary sealant should be equipped with a fungicide. The sanitary field includes bathrooms (Figure 6.7), kitchens, refrigerators or cold storage rooms. 

---