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I read that sodium sulphate crystals are used to make sponges. The crystals are mixed in with some other stuff, then put into a mold. The mold is heated, causing the sodium sulphate crystals to melt and leak out through holes in the bottom, leaving the pores.

Is there a reason sodium sulphate specifically is used and not some other substance? Thanks

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  • $\begingroup$ Source where you read this? What kind of sponges? There's more than one kind of sponge. I assume you mean cellulose? If so, sodium sulphite is already an ingredient used in wood and paper making. $\endgroup$
    – DKNguyen
    Nov 15, 2022 at 17:02
  • $\begingroup$ @DKNguyen, okay, I understand now. Yes, it is cellolouse, I should have made that clear $\endgroup$ Nov 15, 2022 at 17:10
  • $\begingroup$ Conve3nient temperature range? Check data sheets to find out what T sodium sulphate melts at... $\endgroup$
    – Solar Mike
    Nov 15, 2022 at 17:16
  • $\begingroup$ @SolarMike Presumably the decahydrate (32C), not the anhydrous (884C) because the cellulose would be toast (300C). $\endgroup$
    – DKNguyen
    Nov 15, 2022 at 23:39
  • $\begingroup$ @DKNguyen then hopefully the OP knows the difference… $\endgroup$
    – Solar Mike
    Nov 16, 2022 at 5:04

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Other chemicals may be used but sodium hydroxide is preferred since it is an effective swelling agent.

Source: http://www.madehow.com/Volume-5/Sponge.html

  • "As it cooks, the sodium sulphate crystals melt, and drain away through openings in the bottom of the mold. It is their melting that leaves the characteristic pores in the finished sponge. The size of the pores is determined by the size of the sodium sulphate crystals. A rough sponge used for washing a car, for instance, is made with coarse crystals, while a fine sponge of the type used for applying makeup is made with very fine crystals. As the celluolose mix cooks, then cools, it becomes a hard, porous block.".

Source: https://patents.google.com/patent/US3476844A/en

"... sodium sulphate decahydrate, which is very soluble in water and easily fusible

"The porosity of the sponges obtained depends on the size of the hydrated sodium sulphate crystals used as pore producing material. Large crystals are obtained by slow crystallization of solutions of sodium sulphate and are very often composed of an agglomeration of relatively small crystals, thus producing cavities and irregularities which make the resultant mass rather fragile.".

"... Other dimensions and shapes could be used for the agglomerates, for example cubic crystals could be produced, although the tetrahedron gives better orientation effects, In place of hydrated sodium sulphate, any other fusible or easily soluble product can be used which is capable of producing under pressure, masses which are sufficiently solid to survive without breaking the various processes in the preparation of the paste used for the manufacture of regenerated cellulose sponges.".

Source: https://patents.google.com/patent/US3131076A/en

"The pore forming agent is conventionally sodium sulfate. Glaubers salt (Na2SO4.10H2O) is normally used, though the anhydrous salt may also be used if desired. There is, however, no advantage in using the anhydrous material. In place of sodium sulfate, other fusible, water soluble salts that do not readily regenerate viscose may be used, as may gas forming substances such as aluminum powder.
The cellulose swelling agent may be any of a large variety or materials including the alkali metal hydroxides, the alkali metal salts of weak acids, e.g. sodium zincate, salts of weak bases and strong acids such as zinc chloride and zinc nitrate and the like. Sodium hydroxide is, however, much preferred since it is a very effective swelling agent and introduces no new elements into the sponge making mixture.".

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