 | The solvents used for dissolving nitrocellulose are divided into 3 groups in accordance with their dissolving capabilities:
The dissolving capacity (solvency) decreases within a class of substances in proportion to the increase in relative molar mass. This results from the increasing residual hydrocarbon content, which is particularly evident with alcohols:
- Methanol is a true solvent, ethanol is a true solvent for the A grades and a latent solvent for the E grades; n-decyl alcohol is a non-solvent both for the E grades and the A grades. Similar behaviour can be observed with ketones and esters.
A mixture of a true solvent and a latent solvent can have the same or a partly better solvency than the true solvent. This is frequently observed with mixtures of some true solvents with for instance ethanol, isopropanol or butanol.
Sometimes a mixture of two latent solvents may have the dissolving capacity of a true solvent. An example of this is when ethanol and diethyl ether are mixed.
Non-solvents are generally added to the coating to reduce the cost of material. Another function of non-solvents is their effect on the coating’s evaporating behaviour.
The viscosity of a nitrocellulose solution depends not only on the molecular weight of the nitrocellulose grade, but also on the solvent or solvent mixture used. This behaviour is caused by the varying structure of the individual solvents, which can be described by, among other things, the solubility parameter, dipole moment and the capability of forming hydrogen bonds.
In general the following rules apply:
- Within a given class of solvent, the higher the relative molar mass of the solvent, the higher the viscosity of the nitrocellulose solution (the dissolving capability of the solvent decreases equally in relation to the increasing molar mass). Hence, the viscosity of a nitrocellulose solution increases when using acetates in the sequence ethyl acetate, butyl acetate, amyl acetate.
- Latent solvents can increase the solvency and cause a reduction in the viscosity of the nitrocellulose solution.
By adding ethanol as a latent solvent for E 510 to butyl acetate, the viscosity of the nitrocellulose solution will decrease. The viscosity reaches a minimum at around 30% ethanol and 60% butyl acetate and then increases again.
Viscosity of a 10% solution of E 510 (bone dry) in butyl acetate / ethanol mixtures
E 510 | Butyl acetate | Ethanol | Viscosity at 25°C (Höppler viscometer) |
10 % | 90 % | 0 % | 66.2 mPa s |
10 % | 80 % | 10 % | 48.8 mPa s |
10 % | 70 % | 20 % | 45.2 mPa s |
10 % | 60 % | 30 % | 44.8 mPa s |
10 % | 50 % | 40 % | 46.2 mPa s |
10 % | 40 % | 50 % | 47.0 mPa s |
10 % | 30 % | 60 % | 51.3 mPa s |
10 % | 20 % | 70 % | 54.9 mPa s |
10 % | 10 % | 80 % | 69.4 mPa s |
The use of non-solvents (diluents), which have no dissolving capability, causes the viscosity of the nitrocellulose solution to increase. Excessive addition of non-solvents may cause gelling and flocculation. This is shown in the following table.
Increasing the content of non-solvent (toluene) also increases the viscosity of the nitrocellulose solution, and at a content of 10 to 30 % by weight of toluene in the solution a kind of plateau is reached where the viscosity remains virtually constant. If the amount of toluene is further increased, the viscosity of the solution will rise sharply until flocculation finally occurs.
Viscosity of a 10% solution of E 510 (bone dry) in butyl acetate / toluene mixtures
E 510 | Butyl acetate | Toluol | Viscosity at 25° C (Höppler viscometer) |
10 % | 90 % | 0 % | 66.2 mPa s |
10 % | 80 % | 10 % | 72.3 mPa s |
10 % | 70 % | 20 % | 73.6 mPa s |
10 % | 60 % | 30 % | 75.0 mPa s |
10 % | 50 % | 40 % | 87.3 mPa s |
10 % | 40 % | 50 % | 110.8 mPa s |
10 % | 30 % | 60 % | Flocculation |
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