Polysulphide adhesives

Adhesives. Epoxy-polysulphide adhesives are elastomeric in nature. Therefore, they can withstand movement and vibration. They reduce stress between dissimilar bonded materials. They are used as... 

Two-part polysulphide adhesives have been used where large areas are to be bonded together and a semi-flexible bond is required, and where it is not practical to use an elevated temperature cure due to the risk of distortion and where a very high facility cost would be involved for a relatively low production rate assembly. 

The polysulphides are frequently used in casting mixes and to a less extent in coating, laminating and adhesive applications. Their value in casting and encapsulation lies mainly with their low curing shrinkage and flexibility in the cured state. Their tendency to corrode copper and the somewhat inferior electric insulation properties of the blends does lead to certain limitations. 

In this book I have confined discussion to those polymeric materials which are cured by chemical reaction and which have found widespread application in the construction industry. As such, the book covers materials based on epoxies, polyurethanes, silicones, polysulphides, alkyds and polyesters. In addition, there is a chapter on hybrid polymer systems and one on acrylics. It is true that acrylic emulsions are not strictly thermosetting polymer systems, but their widespread use and importance made their exclusion difficult. These materials find use as coatings, sealants, adhesives, grouts, flooring compounds, repair compounds and waterproofing agents. 

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. 

The practice of modifying epoxy resins with liquid polysulphides may not be new but the method of doing so has advanced significantly in recent years to the extent that these systems are being given serious consideration in the design of specialist coatings, primers, adhesives and encap-sulants (Morton International, undated a Rees and Wilford, 1994 Rees et al., 1994). 

Adhesion. Owing to improved wetting properties, excellent adhesive characteristics are exhibited for coatings and adhesives with relatively small additions of liquid. Polysulphides are required to produce significant improvements in the lap shear strength of the system (Rees et al., 1994). 

Chemical resistance. Unmodified epoxy resins have excellent resistance to dilute acids, alkalis and certain solvents. The incorporation of an LP component into the epoxy resin extends this resistance to a wide variety of oils, aromatic and aliphatic hydrocarbons, esters and ketones. Coatings and adhesives based on polysulphide-epoxy resins exhibit reduced swell and maintenance of flexibility and adhesion when immersed in these fluids. In view of this, LP-modified epoxies are used on oilrigs, fuel-storage installations and chemical plants (Figure 10.4). 

Epoxy-polysulphide coatings exhibit excellent adhesion to steel, aluminium and concrete without the use of primers (Morton International,... 

Secondary and tertiary amines. Tertiary amines can be used as sole curing agents but in adhesive formulations they are more normally found in blends, for example as accelerators with polyamide systems(4) and as an essential part of polysulphide systems. The cured resins tend to be less temperature and chemical resistant than the amines described above. Secondary amines react first like primary then as tertiary amines. 

Table 2.5 summarises typical values of these bulk mechanical properties at around 20 °C for a range of epoxy adhesives. Of particular note is the low strength and stiffness of the epoxy polyamides and polysulphides as compared to those with aliphatic polyamine hardeners. For the design of bonded assemblies, joint tests are often used to determine the relevant mechanical properties. It must be remembered, however, that the results will be highly dependent on the specimen geometry and testing conditions and... 

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. 

Polysulphides Moisture insensitive quick set time flexible Odour poor performance at elevated temperature Adhesives, sealants... 

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. 

Reactive adhesives for elastic bonding are available as one-part and two-part systems, based on polyurethane (PUR), polysulphide, silicone or silane-terminated polymers, such as PUR-Hybrids and Modified Silicones (MS). These adhesive systems can he applied cold or warm or mixed with a water-based Booster component. 

Elastic sealants based on synthetic elastomers such as polysulphides, silicones or polyurethanes allow the periodically alternating movements of building structures caused by temperature fluctuations to be followed in a consistent manner. It has gone unnoticed by some that owing to the development of modem elastomaic sealants, a new field of application has emerged - the load-bearing adhesive joint... 

EHT88 Modified polysulphide/cyclo-aliphatic amine 1.07 Thixotropic 1 0.75 30 mins Gelcoats adhesives... 

The choice of vulcanisation system for the rubber can have a dramatic effect on adhesion. Typically sulphur cured rubbers are easier to bond to than sulphur-free or peroxide cured rubbers. This is believed to be due to the interaction of sulphur with key curative materials in the adhesive. The more sulphur that is present, the more interactions that are available, and hence the better the chance of getting good adhesion. SEV (semiefficient vulcanisation) and EV (efficient vulcanisation) cure packages are typically more difficult to bond because of their lower free sulphur contents. EV refers to cure systems which give predominantly monosulphidic or disulphidic crosslinks whereas conventional sulphur cure systems produce mostly polysulphidic crosslinks. SEV systems fall somewhere between EV and conventional systems in the type of crosslinks produced. Vulcanisation proceeds at different rates and with different efficiencies in different types of polymers, so the amount of sulphur needed to produce an EV cure system will also vary. For example, in NR, an EV system will generally contain between 0.4 and 0.8 phr of sulphur, while in NBR the sulphur level will generally be less than 0.3 phr of free elemental sulphur. 

It has been demonstrated previously that such additions can increase the strength of rubber-brass adhesion considerably. In this research it was established that polysulphides are only weak crosslinking agents for unsaturated rubber by themselves. In the presence of sulphenamide accelerators, such as OBTS, polysulphides, in amounts of 0.5 - 1 phr, activate the sulphur vulcanisation. However, the reversion process (crosslink breakdown) is not accelerated. A favourable effect on the physicomechanical properties of the vulcanisate was also reported. 

The generation of strong adhesive forces can be considered to result from chemical interaction between the metal and NBR. This generally results from the reaction of the metal with sulphur compounds and carboxyl compounds to give their metal salts, which have a high bond energy. Polysulphide reactivity is high for copper and brass plates. 

Epoxy formulations with amine hardeners, cured at room or elevated temperature, have a tendency to become hard and brittle. The addition of polysulphide modifiers increases elasticity, although at the expense of generally lower bond strength. These adhesives permit a fairly thick cement line . 

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... 

Tertiary amines also catalyse interaction between the hydroxyl groups formed and remaining epoxy groups and a cross-linked structure is built up. Epoxy-polysulphide compositions have good impact strength and find use in adhesives and castings. 

Epoxy-polysulphide Polyvinyl acetate > Wood adhesives — 0 101 0 35 Krueger (1962)... 

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