Polyamide Resins
General
Polyamide resins come in two 'flavours', 'thermoplastic and 'thermoset' (reactive) types. Whereas the thermoplastic types are solids at room, and higher, temperatures, the thermoset varieties are either liquid or fairly soft at room temperatures. Similar to the alkyd resins, these polymers are formed by a condensation reaction, where an acid reacts with a base, resulting in the formation of a polymer, along with water as a by-product. The acids in this instance might be either natural in origin or synthetic, whilst the base is an amine.
Polyamide Types
The following table highlights the differences between these two polyamide types:
| Polyamide Type | Solvents Used | Application Method | Drying Mechanism | Drying Speed | Application Area | Uses | Durability |
|---|---|---|---|---|---|---|---|
| Thermoplastic | Variable | Glue Gun, Spray, Roller | None/Solvent evaporation | Variable | Hot Melt Adhesives, Inks | Sealants, Printing inks | Fair |
| Thermoset | None or strong | Brush/Roller/Spray | Solvent Evaporation/Reaction | Variable | Adhesives, Coatings | Metal primers, Topcoats, Grouts | Good |
- The solvents used in these polymers are dependent upon the end application. Solvents used to dissolve the resin for, say, printing inks, requires careful selection, particularly if the final product is to be applied to food packaging where only certain solvents are allowed depending on the requirements of the local health authorities.
- In other instances, where, for example the polyamide is going to be used in an adhesive or 'hot melt' application, the use of any solvent would most likely compromise the adhesive strength.
- Polyamides comprising part of a two component (i.e. catalysed) coating system have solvents chosen specifically for the intended application. Generally these are more polar molecules, such as the solvent esters, together with aromatic solvents and aliphatic or aromatic alcohols.
A few further details on these resins:
- The polyamide resins show very good to excellent adhesion on a variety of materials including wood, glass, metals, concrete and some plastics. They do have on Achilles heal, however, and that is their resistance to sunlight (particularly the Ultra Violet portion) is limited.
- The adhesive and elastic properties of Thermoplastic Polyamides are used to advantage in 'hot melt' applications. The resins are applied at temperatures greater than their softening point, using 'glue guns' or similar application methods, to bond dissimilar items together, for example rubber seals/gaskets onto metal canisters.
- Another use of Thermoplastic Polyamides is in printing inks, especially when it comes to difficult substrates to adhere to, for example polyethylene packaging that might be subject to refrigeration. In these applications, a variety of solvents might be used to reduce the viscosity to that required by the application process.
- The Thermoset Polyamides are combined with a second resin that 'crosslinks' with the polyamide at room or elevated temperatures. In the majority of cases this second resin is an epoxy which lends the common name to these products: "Two pack Epoxy Adhesive" or "Two pack Epoxy Paints". In these products, it is the polyamide itself that is termed the catalyst, even though, in certain systems, the polyamide component is the greater.
- The uses of the two pack Thermoset polyamide/Epoxy adhesives are legion. They are often supplied as 2 part, putties, or liquids, that need to be combined and used before the curing reaction starts. The bond strength that they exhibit in adhesives is quite exceptional. Cracks in dam walls (e.g. Kariba Dam in Zambia) have been repaired with 2 pack epoxy/polyamides and they also find use in bonding car panels together especially where the panels are themselves made from polymers (e.g. fibreglass) where it is not possible to weld or otherwise mechanically join the panels with confidence.
- Otherwise, Thermoset Polyamides together with epoxy resins, find application in solventless concrete floor coatings, interior pipe coatings and other metal primer systems where excellent corrosion resistance is required.
- The 'Touch dry time', of solvent based polyamide systems, is dependent on the specific solvent used and the temperature to which it is subjected during the drying process. Not all polyamide/epoxy systems are dried at ambient temperatures, in some cases 'forced drying' at elevated temperatures is used. The cure speed of the 2 component polyamide/epoxy systems is likewise dependent on the specific polyamide and epoxy used, and once again, the temperature to which the system is subjected during curing.
Chemistry
To be completed at a later date