This invention relates to a process for making liquid polysulfides, the liquid polysulfides made thereby, and sealants made therefrom.
Liquid polysulfides (LP's) have been available commercially for over thirty years. They are known to be polymers whose repeat units each contain an organic group and two adjacent sulfur atoms, represented by the chemical structure —(—S—S—R—)— where R is an organic group. The pair of adjacent sulfur atoms in this structure is called a “disulfide link.” Details of suitable organic groups are described below. LP's include the usual variety of copolymers, branched structures, and end groups found in polymers of all types. Because they are liquids, they can be conveniently mixed and compounded with other materials, such as for example curing agents, cure accelerators or retarders, fillers, plasticizers, thixotropes, and adhesion promoters as appropriate for the application contemplated by the practitioner. LP's are used in a wide variety of applications, including for example in the manufacture of sealants for aircraft, insulating glass, and other items. The structure, the current methods of making LP's, the usual applications of LP's, and the corresponding formulations have all been described in “Polymers Containing Sulfur (Polysulfides)” by D. Vietti and M. Scherrer, in volume 19 of the Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, Wiley (1996).
LP's of the present invention are different from polymers known as “poly (aliphatic sulfide)” polymers or “aliphatic polysulfide” polymers or “poly (alkylene sulfide)” polymers or similar names, whose repeat units contain organic groups and sulfur atoms that are connected only to carbon atoms. That is, poly(aliphatic sulfide)s have repeat units such as for example —(—R—S—)— or —(—R—S—R′—S—)— where R and R′ are organic groups. These polymers are described, for example, in Chapter 3 of Polymer Synthesis Volume III by S. R. Sandler and W. Karo (Academic Press, 1980). Such poly (aliphatic sulfide)s have been made in the past by reacting metal sulfides with dihalo organic compounds in the presence of a phase transfer catalyst, as reported for example in Japanese Patents JP04046931, to T. Tozawa et. al., and JP56090835, to Y. Kazuya; in Y. Imai et. al., journal of Polymer Science, volume 17, pages 579-583, 1979; and in M. Ueda et. at., Macromolecules, volume 15, pages 248-251, 1982. Both Tozawa and Ueda report that the presence of the phase transfer catalyst leads to an increase in the molecular weight of the polymers they produce.
The monomer units of the liquid polysulfides of the present invention are known to predominantly contain disulfide links. A liquid polysulfide polymer molecule may contain a small number of the aliphatic sulfide type monomer units. Generally, liquid polysulfide polymers are believed to have 80% or more of their total weight made of monomer units with disulfide links. Most samples of liquid polysulfide are believed to have 95% or more of their total weight made of monomer units with disulfide links.
In the past, liquid polysulfides have been produced, as described in U.S. Pat. 5,430,192, by first making a solid polysulfide polymer and then, in an extra step, converting the solid polymer to a liquid. During the making of the solid polysulfide polymer, an inorganic salt such as magnesium chloride is used. It is believed that the inorganic salt reacts with the sodium polysulfide to form colloidally suspended particles, on which the solid particles of organic polysulfide polymer grow. The resulting solid polysulfide polymer is thought to have relatively high molecular weight. The dispersion of solid polymer must be washed with water to remove impurities, which produces significant quantities of waste water. Next, the extra step converts this solid polymer to a liquid, by reacting the polymer with sodium dithionite and caustic or, more commonly, with sodium hydrosulfide (NaSH) and sodium sulfite (Na2SO3). This reaction is thought to reduce the molecular weight of the polymer, though it is also thought to be difficult to carefully control the precise value of the reduced molecular weight. After the molecular weight reduction, the extra-step process also requires a so-called “strip” step, in which the liquid polysulfide is reacted with more sodium sulfite, in order to remove labile sulfur from the polymer. Labile sulfur is sulfur that can be removed from the polymer by a relatively mild chemical reaction, such as for example the reaction with sodium sulfite. Then, to purify the product, the magnesium must be converted to a soluble salt by acidifying the reaction mixture, commonly with acetic acid or sodium bisulfite. Then the mixture must be washed with water to remove the soluble salts, producing further significant quantities of waste water. This extra-step process has the disadvantages of requiring extra time, effort, and materials, and of producing large amounts of waste water.
The problem addressed by the present invention is the provision of a simplified polymerization process for making liquid polysulfides directly, so that the extra step of converting a solid polysulfide to a liquid is no longer necessary. One further advantage of the present invention is that the elimination of the extra step also eliminates a significant amount of waste water from the LP manufacturing process. A second further advantage of the present invention is that elimination of the extra step also eliminates the need for the “strip” operation to remove labile sulfur, thus simplifying the manufacturing process and reducing the amount of sodium sulfite that must be removed from the liquid polysulfide. A third further advantage is that the present invention allows the practitioner to control the molecular weight of the LP without using the historical extra-step process.