Nylon solubility parameter

For example, nylon 66 will dissolve in formic acid, glacial acetic acid, phenol and cresol, four solvents which not only have similar solubility parameters but also are capable of acting as proton donors whilst the carbonyl groups on the nylon act as proton acceptors (Figure 5.6). 

Polyformaldehydes (polyoxymethylenes, polyacetals) These are physically similar to general purpose nylons but with greater stiffness and lower water absorption. There are no solvents, but swelling occurs in liquids of similar solubility parameter. Poor resistance to u.v. light and limited thermal stability are two disadvantages of these materials. 

Considering Table 1.16, only the first polymer, polyethylene, has non-polar contributions alone the next three have also polar components and the last, nylon-6,6, has contributions from all three forces. The largest solubility parameter for this polymer also corresponds to the highest melting point and stiffness, reflecting the importance of cohesive energy density as a measure of intermolecular forces. 

In terms of polymer matrices for composite materials, there will be a compromise between solvent and water resistance. Thus non-polar resins are likely to be less resistant to hydrocarbon solvents, which have low polarity, but more resistant to moisture absorption. Polar resins behave in the opposite way. Strongly polar solvents, such as dimethyl sulphoxide or similar, can interact with polar structures in the resin and are difficult to resist. Crystalline thermoplastic polymers are often better for such applications. For example, polyethene will only dissolve in hydrocarbon solvents (of similar solubility parameter) at temperatures above the crystalline melting point. Polar semi-crystalline polymers such as the polyamides or nylons can be dissolved in highly polar solvents, such as cresol, because of a stronger interaction than that between molecules within the crystallites. High performance thermoplastic polymers such as polyether ether ketone (PEEK) have been promoted for their resistance to organic solvents (see Table 3.5) [12], The chemical resistance of unsaturated polyester and vinyl ester and urethane resins is indicated in Table 3.6 [15]. 

They are only soluble in a few solvents (formic acid, glacial acetic acid, phenols and cresols), of similar high solubility parameter. Nylons are of exceptionally good resistance to hydrocarbons. Esters, alkyl halides, and glycols have little effect on them. Alcohols can swell the polymers and sometime dissolve some copolymers. Mineral acids attack the nylons but the rate of attack depends on the type of nylon and the nature and concentration of the acid. Nitric acid is generally active at all concentrations. The nylons have very good resistance to alkalis at room temperature. Resistance to all chemicals is more limited at elevated temperature (Brydson, 1989). 

Because of their crystallinity, the nylons are soluble at room temperature only in liquids capable of interaction with the polymer. Thus nylons dissolve in strong proton donors such as acetic acid, formic acid and phenols. Liquids of similar high solubility parameter, e.g., alcohols, have some swelling action and even readily dissolve some copolymers. Nitrobenzene, benzyl alcohol and glycols are effective solvents at elevated temperatures towards most other organic solvents, fuels and oils nylons show outstanding resistance. 

According to the criteria for steric stabilization, the dispersant tail must be soluble in, or at least compatible with, the polymer. Solubility is governed by the principle that like dissolves like. In more scientific terms, it means that the dispersant tail and the polymer must be of similar polarity, or more precisely of similar solubility parameter. Therefore a perfluoroalkyl tail would be well suited to PTFE, an alkyl dispersant tail would be ideal for PE and polypropylene, and a high polarity PE oxide type tail would work in polymers such as PVC, PET, and nylons. 

The highest losses in mechanical properties of a polymer take place when the solubility parameters of a polymer and solvent match. The same statement can be made when the polarities of a solvent and a primary polymer bond match. A close match of the solubility parameters or polarities results in the incompatibility of the polymer and the solvent. To be clear, incompatible here means that the solvent attacks the polymer. Selection of a polymer for a given chemical environment must be made by evaluating the materials that have the largest solubility differences or polarity differences with the chemical environment. For example nylon 6/6 resists cleaning solvents such as carbon tetrachloride, and polystyrene and ethylene glycol are incompatible. Nylon 6/6 has polar amide bonds, while carbon tetrachloride is a non-polar solvent. In comparison, water is a polar liquid and is absorbed by nylon 6/6. Similarly polystyrene and ethylene glycol are both polar, and thus interact. 

---