DESEALING COMPOSITIONS

Abstract: A composition for use in removing polysulfide sealants or coatings. The composition comprises a thiolate such as an alkyl or phenyl thiolate with an alkali metal or Quaternary ammonium ion in a solution of dimethyl formamide or dimethyl acetamide either alone or in admixture with an aromatic solvent such as toluene or xylene. In some circumstances a disulfide compound such as diphenyl disulfide or tert butyl disulfide in the composition provides a synergistic improvement in the desealing rate.

Description: DESEALING COMPOSITIONS This invention relates to a composition for removing sealant compositions particularly polysulfide sealants. Polysulfide sealants such as those formulated from Thiokol Corporation prepoly ers have found extensive industrial applications. Approximately 35 tonnes of prepoly er are imported annually into Australia. These sealants are used in the building industry where durability is important. They are also suited to specialized applications as in the sealing of integral aircraft fuel tanks where there is a requirement for both durability under variable temperature conditions and resistance to degradation by fuels. There are occasions when such sealants need to be removed and often it is difficult to remove them by mechanical means particularly when the sealant is located in a relatively inaccessible location. Also the highly valued solvent resistance of the polysulfide sealants creates difficulties in using chemical means. Two proposed methods of desealing polysulfides are disclosed in US Patents 4431457 and 4362570. The first (Reed et al) relates to the use of a polyamine containing compound to remove poly (arylene sulfide) polyamides from metallic surfaces. The second (Elwell) provides a mixture of dichloromethane and a minor amount of chlorotri- methylsilane as a solvent for removing polysulfide coatings. Commercial desealants for polysulfides are available. These contain a swelling agent in addition to the reactive constituent thiophenol which is the key component in the desealant. The function of this thiol is to act as a depolymerizing agent for the polysulfide polymer. The process involved is known as a thiol-disulfide interchange interchange reaction (eq. 1). R'SH + RSSR ^=≥ RSSR' + RSH (1) The polysulfide polymers are essentially disulfides and are prepared from dithiol ether monomer units. [SCH 2 CH 2 OCH 2 OCH 2 CH 2 S-SCH 2 CH 2 OCH 2 OCH 2 CH 2 ] n The reaction with a monofunctional thiol can lead to the fragmentation of the polymer chain into low molecular weight units terminated by the monfunctional thiols and oligomers of the original polymer (eq 2). R'SH + [SCH 2 CH 2 OCH 2 OCH 2 CH 2 S_ n R'S[SCHJCHJOCHJOCHJCHJS] SR' HS[SCH 2 CH 2 OCH 2 OCH 2 CH 2 S] SH (2) However thiols and thiophenol in particular have an objectionable odour. Other desealants which also act to leave the disulfide bond are tertiary phospines chloramine T, the hydroxyl ion, metal ions such as copper II and cobalt II and tetrathiomolybdate ion. However, all of the above reagents when studied in homogenous phases have slow reaction rates and elevated temperatures are normally required to adhere acceptable reaction times. Depolymerization reactions are required to take place in a heterogeneous phase with the polymer being insoluble in the reagent solution. In addition to the requirements of a rapid reaction, both the reagent(s) and the reaction products should be odourless and soluble in a commercially available solvent if such a formulation is to be of practical use. In the case of copper (II), the resultant bridged thiolate complexes generally have low solubility in most common solvents. The thiol-disulfide interchange reactions, however, appear to meet the necessary criteria. It is therefore an object of this invention to identify a re ' agent having a similar effectivness to thiols in depolymerization of polysulfides which do not have the thiol odour problem. To this end the present invention provides a composition for use in removing polysulfides comprising a solvent and 0.0025M to 0.04M of one or more thiolates alone or in conjunction with a disulfide, said solvent being able to dissolve the thiolate, swell the polysulfide and dissolve the depolymerization products of the polysulfide. Preferably the thiolate is selected from potassium thiophenolate, tetramethyl ammonium tert-butyl thiolate or tetraethyl, tetramethyl or tetrabutyl ammonium thiophenolate. The thiolates may be alkyl or aryl thiolates. The presence of a disulfide preferably diphenyl disulfide is optional as it has the effect of increasing the rate of depolymerization. This is particularly useful when the thiolate is less active as in the case of tetramethyl ammonium thiophenolate which is to be used in conjunction with disulfide. As well as the polysulfide polymer most sealants also contain a filler typically calcium carbonate. Care must be taken to ensure that any reaction of the filler with the depolymerizing agent does not reduce its effectiveness. Another factor to be taken into consideration is the selection of the solvent for the thiolate reagent. A solvent which swells the polysulfide polymer and is a solvent for the depolymerized product is required. The performance of the solvent, in effectively suspending the filler, is also an important consideration in ensuring that all of the sealant is removed. Preferred solvents are N,N dimethyl formamide (DMF), Dimethyl acetamide (DMA) or mixtures of either of these solvents with aromatic hydro¬ carbons such as toluene or xylene. Both DMA and DMF are easily absorbed by the skin (TLV, 10 ppm) hence an alternative solvent, which is less toxic, would be desirable. From a toxicity point of view, water would be an ideal solvent. Unfortunately its inability to penetrate the disulfide polymer together with the possibility of promoting corrosion and its incompatibility with aircraft fuel, makes this solvent unacceptable. Chlorinated hydrocarbons are generally good solvents for polymers but they can give rise to hydrogen embrittlement when in contact with metals. This arises from the liberation of small quantities of hydrogen chloride and its subsequent reaction with metals. Thus the aromatic hydrocarbons remain the most acceptable alternatives. Toluene and xylene have TLVs of 120 ppm. However, only tetrabutyl ammonium thiophenolate is sufficiently soluble in these solvents. The concentration range of the thiolate varies with - reagent and solvent. For tetraethyl ammonium phenyl thiolate a most preferred range is 0.01 to 0.024 M in N,N dimethyl formamide while for tetrabutyl ammonium phenyl thiolate the most preferred concentration range is 0.06 to 0.01 M in N,N dimethyl formamide. When a disulfide is added the concentration range is generally within the range of 0.01 M to 0.4 M. Any suitable disulfide may.be used. Preferred disulfides are diphenyl disulfide or tert-butyl disulfide. In order to reduce the toxicity of the solvent used it is preferred to use mixtures of aromatic hydrocarbons and dimethyl formamide. Preferred solvent mixtures are toluene and at least 10% by weight of dimethyl formamide or xylene with at least 15% by weight of dimethyl formamide. The invention will be further explained in relation to the following examples. The desealants of this invention were evaluated in the apparatus as illustrated in figure 1. The test specimen 1 of polysulfide polymer is suspended in the desealing solution 5 by a thin wire 2 from a glass rod 3 resting on a top loading balance 4. The specimens were totally immersed in a 35cm3 desealing solution. The change in weight of the immersed specimens with time was recorded. The results are expressed as a percentage loss of weight with time and plotted. The sealant specimens were formulated and prepared for casting according to the manufacturer's specifications. They were cast on a Teflon (Reg T.M.) sheet to a thickness of 3-4 mm. The sealants were allowed to cure at 24°C for 48 hours and then maintained at 40°C for 7 days. Test specimens were cut from the sealant sheets using a 20.6 mm diameter die. The sealant commonly used is a commercial sealant PR-1750-B1/2 from the Thiokol Corporation which contains approximately 30% calcium carbonate and the balance poly sulfide polymer. Example 1 - Prior Art The variation in the time taken for 75% weight loss by the sealant PR-1750-B specimens when subjected to thiophenol (PhSH) in N,N-dimethyl formamide (DMF) was measured. Figure 2 shows the variation as a function of thiophenol concentration. This shows that there is an optimum concentration range. Example 2 The same tests as in example 1 were carried out using Potassium thiophenolate (PhSK) and figure 3 shows the variation in achieving 75% weight loss with changing concentration. The solvent is DMF. An optimum concentration range for depolymerizing this particular sealant is observed. Example 3 Example 2 was repeated with the addition of 0.131 M diphenyl disulfide solution to the Potassium thiophenolate. Again the time to achieve 75% weight loss was plotted against Potassium thiophenolate concentration as shown in figure 4. The rate of depolymerization is increased by the presence of the diphenyl disulfide. Example 4 The procedure of the previous examples was repeated using tetramethyl ammonium thiophenolate with 0.127 M diphenyl disulfide solution. Figure 5 plots the time to achieve 75% weight loss against the concentration of tetramethyl ammonium thiophenolate. Example 5 In this example a 0.008M solution of tetramethyl ammonium thiophenolate solution is used with varying concentrations of diphenyl disulfide (PhSSPh) and this is plotted against the time to achieve 75% weight loss in the specimen as shown in figure 6. This demonstrates that the effect of the diphenyl 10 disulfide reaches a maximum at a concentration of about 0.2M. Example 6 The same procedure as in example 5 was repeated using tetrabutyl ammonium thiophenolate (PhSNBu.). The plot - j _5 of time to achieve 75% weight reduction plotted against PhSNBu. concentration is shown in figure 7 . Example 7 The procedure of example 6 was repeated using tetraethyl ammonium thiophenolate. The results are shown in 20 figure 8. Example 8 Using the same technique as in the earlier examples and 0.01M of tetrabutyl ammonium thiophenolate the effect of varying DMF (N,N dimethyl formamide) concentration with 5 toluene is shown in figure 9. A concentration minimum for DMF is shown. Example 9 The procedure of example 7 was repeated except that xylene was substituted for toluene. As in example 7, a DMF 0 minimum concentration is exhibited in figure 10. Example 10 The procedure of example 8 was repeated using dimethyl acetamide DMA instead of DMF. The results are shown in figure 11 which 5 illustrates that DMA is similar to DMF. From the above description and examples it can be seen that the present invention provides an effective and safe method of removing sealants.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS; 1. A composition for use in removing polysulfides comprising a solvent 0.0025M to 0.04M of one or more thiolates alone or in conjunction with a disulfide, said solvent being able to dissolve the thiolate, swell the polysulfide and dissolve the depolymerization products of the polysulfide. 2. A composition as claimed in claim 1 wherein the thiolate is selected from alkali metal or ammonium alkyl or arythiolate. 3. A composition as claimed in claim 2 wherein the thiolate is selected from Potassium thiophenolate, tetramethyl ammonium tert-butyl thiolate or tetraethyl, tetramethyl or tetrabutyl ammonium thiophenolate. 4. A composition as claimed in claim 1 wherein the solvent comprised N,N Dimethyl formamide. Dimethyl acetamide or mixtures of either of these solvents with an aromatic solvent. 5. A composition as claimed in claim 4 wherein the aromatic solvent is selected from toluene or xylene. 6. A composition as claimed in claim 1 wherein a disulfide selected from diphenyl disulfide or tert-butyl disulfide is present in a concentration of 0.01 to 0.4 Molar. 7. A method of removing polysulfide coatings or sealants wherein a composition as defined in claim 1 is applied to the coating for a time sufficient to effect dissolution and break up of the polysulfide. mag/1:11

Inventors: MAZUREK WALDEMAR, MAZUREK, WALDEMAR
Applicants: COMMW OF AUSTRALIA, THE COMMONWEALTH OF AUSTRALIA
Application year: 1986
Main classification: C11D7
IPC codes: C11D7