Polysulphides sealants

The need to maintain elasticity of rubber is of paramount importance under any serious and severe environmental conditions. The most stable rubbers in radiation environments are polyurethanes and phenyl silaxanes which are usable at well above 108 rads (106 Gy). Butyl rubber liquefies and neoprene evolves hydrochloric acid at similar dose levels. Most polyurethane rubber foams can be used at a dose level of 109 rads (107Gy) in vacuum at temperature levels of between -85°C to +250°C. Silicone and polysulphide sealants are probably less tolerant to ionizing radiation in a nuclear plant where chemical processes are being carried out. A schematic graphical representation of the tolerance of rubbers to ionizing radiation in nuclear plant is shown below in figure 7.4. 

Polyurethane sealants have taken a good share in today s sealant market. Polysulphide sealants are in greater use for water-retaining structures, and silicone sealants for structural glazing purposes but polyurethanes have a greater market share as construction sealants. These sealants were first introduced to the construction industry in the early 1970s as two-component materials. 

Details of various joints are given in the discussion on polysulphide sealants (section 7.3) and silicone sealants (section 6.3). The same criteria are applicable in the case of urethane sealants, with the exception of glazing joints. 

THERMOSETTING POLYMERS IN CONSTRUCTION APPLICATIONS 7.3 Polysulphide sealants... 

Polysulphide sealants were used in building structures for a long time before a standard specification was developed for such uses. ASA-A116.1 was the first specification for building sealants written by the Building Trade Specification Committee in 1956. Later, several specifications were designated by different societies for one-component and two-component systems. The specifications will be mentioned in the following. Apart from polysul-phides these specifications also refer to urethane and silicone sealants... 

High-performance characteristics are offered by polysulphide sealants with a maintenance-free service life in excess of 15 years. Good evidence exists to show that these sealants, when properly formulated and applied, have a real service life in excess of 40 years (Lowe, 1994a). 

Polysulphide sealants fulfil the requirements of expansion joints in concrete structures such as bridges, subways and high-rise buildings where a high level of joint movements occur. They are suitable for compression movement, extension movement and shear movement in butt joints as well as overlap joints. The only joint in which a sealant can accommodate more than +50% movement is in lateral shear in an overlap joint. In all other joint designs the sealant cannot take more than 25% move-... 

Figure 7.1 Joint movements associated with polysulphide sealants, by type of joint (a) butt joints showing (i) normal configuration, (ii) 25% compression, (iii) 25% extension, (iv) 25% vertical shear (b) overlap joints showing (i) normal configuration, (ii) 50% lateral shear compression, (iii) 50% lateral shear extension, (iv) 25% vertical shear...

Polysulphide sealants are of two types, namely two-component sealants and one-component sealants. These will each be discussed in turn in the following two sections. 

Polysulphide sealants used for horizontal joints are of a pourable consistency and are referred to as pouring-grade sealants . They have the capacity to flow sufficiently to give a reasonably smooth level surface when applied at ambient temperatures. In the case of vertical building joints, the sealants are made to have a non-sag consistency. These sealants are known as gun-grade sealants and permit application in a suitable joint of any aspect or inclination without appreciable slumping at ambient temperatures. 

Reference standards have been adopted for polysulphide sealants in countries such as Australia (Doc. 1572-1970), Canada (19 GP 13a-1970), Japan (JIS-5724-1969) and other countries. However, a good two-component sealant can meet British Standard BS 4254, US Federal Specification TT-S-00227 E and ASTM C-920. Fuel-resistant two-part polysulphide sealants comply with BS 5212-75 and US Federal Specification SS-S-200 D. Sealants used for water retaining and excluding meet the requirement of BS 6920. Polysulphide sealants used for fire protection comply with BS 4254-1983. Additionally these sealants are non-combustible to BS 476, part 4 1984. All these categories will be discussed under separate head-... 

Table 7.7 Reference standards for two-component polysulphide sealants...

Polysulphide sealants in general have good adhesion to steel, aluminium and glass, usually without a primer for porous substrates such as wood and masonry it is recommended to use a primer. 

Two-part polysulphide sealants in the cured state have elastoplastic and elastic characteristics. A movement accommodation factor of 25%-30% is possessed by low-modulus sealants and one of 10%-20% by high-modulus sealants. The life expectancy is 20-25 years. They are more suitable for sealing larger joints and also joints that are subject to a somewhat larger degree of movement than are one-component sealants. 

Owing to their long curing period, one-component polysulphide sealants are not generally used in joints with dynamic movements. When a joint opens and closes under stress and under variations in temperature, a slow-curing sealant may experience cohesive failure as well as deformation. 

Completely cured one-component polysulphide sealants are elasto-plastic and elastic in character. They have a movement accommodation factor of 20%-25%. These types of sealants are particularly suited for the perimeter sealing of window joints [Figure 7.2(a)], joints between precast concrete panels [Figure 7.2(b)], movement joints in brickwork where the movement is likely to be relatively slow, stone claddings, etc. The life expectancy for one-component sealants is, like that of two-component sealants, 20-25 years. 

One-component polysulphide sealants comply to British Standard BS 5215, US Federal Specification TT-S-00230 C and Canadian Research Council Standard 19 GP 13a. The merits of one-component polysulphide sealants are as follows ... 

Flgnre 7.2 Use of polysulphide sealant for (a) perimeter seating of window joints (b) in joints between precast concrete slabs. Cross-hatched area = one-part polysulphide sealant ... 

Polysulphide sealants represent a good technology for environmental protection. Such sealants comply with the requirements of BS 5212 and US Federal Specification SS-S-200 D. They have a high polymer content and low plasticiser content. They possess excellent flexibility and movement tolerance and are resistant to penetration of stones and hard debris. They have prolonged hfe because of their high age-hardening resistance. 

Figure 7.3 Joints in concrete slabs applied with fuel-resistant polysulphide sealants to prevent contamination of groundwater...

This is an important and growing application for chemical-resistant and oil-resistant polysulphide sealants. As a result of a survey commissioned in Germany by the Association for Mineral Oil, Natural Gas and Coal (Deutsche Wissenschaftliche Gesellschaft fiir Erdbl, Erdgas und Kohle, 1989, 1991) it was found that less volatile diesel fuels permeate into the ground and contaminate the water-table. It was roughly estimated that one litre of diesel can spoil a million litres of water. The problem is more serious in areas where the water-table is near to the surface. 

Table 7.9 Chemical-resistant polysulphide-sealant (specimen according to DIN 52 455, part 1, with acid-resistant clinker) (a) thixotropic (b) self-levelling. Source Morton International Ltd, Coventry, UK...

Bacterial attack and fungal growth are the two biochemical effects which influence the durability of a sealant. Bacterial attack results in the chemical breakdown of the sealant. Fungi attach themselves to the surface of the sealant and live as parasites, feeding on the organic matter present. The use of polysulphide sealants in the water industry has proved to be a versatile method for overcoming the above-mentioned problems. 

The long-term testing of specialised polysulphide sealants in the sewage environment has led to formulations not only for that environment but for universal applications in water, whether contaminated, clean or for potable use. 

Thermal degradation studies of polysulphide sealants have shown the evolution of gases such as hydrogen sulphide, ethylene, sulphur dioxide and carbon dioxide, but the rate of evolution of these compounds was found to be below the level considered hazardous (Berenbaum, 1969). 

Morton International (1992) Evaluation of LP Liquid Polysulphide Sealants in Finham Sewage Treatment, year 8 status, Morton International, Coventry. 

Morton Thiokol Corp. (1987) Bioresistance of LP based Polysulphide Sealants, Morton Thiokol Corporation, Coventry. 

For more information see articles on Silicones properties. Silicone adhesion and specialized reviews on silicone and polysulphide sealants Sealants in double glazing are the subject of a separate article. 

As the base volume cylinder and hardener cylinder reciprocate, they displace the two material components in the required ratio to the outlet ports. This proportioner may be used to meter and mix silicones, polyurethanes, epoxies, adhesives or polysulphide sealants. 

ASE 85.Adhesives, Sealants Encapsulants Conference Day 3 Environmental Effects, Applications Off-Shore Applications Assembly. Kensington, London, 5-7 Nov. 1985, p.52-64. 6A INTERPRETATION OF THE EFFECTS OF DIFFERENT CURING SYSTEMS ON THE WATER RESISTANCE OF POLYSULPHIDE SEALANTS... 

The results are reported of a study of the disparate swell behaviour in hot water of polysulphide sealants produced with different curing agents (manganese dioxide and ammonium dichromate). Sealants tested were commercial aircraft sealants PR-1750, Pro-Seal 899 and PR-1422 and laboratory formulations prepared using Thiokol liquid... 

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