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Behavior of polysulphide rubber (thiokol A) in organic acid solutions
Materials and Design 21 Ž2000. 211᎐215
Behavior of polysulphide rubber ž thiokol A/ in organic acid solutions C.A. BaahU , J.I. Baah1, P. Gianadda, C. Fisher School of Chemical Engineering, Uni¨ ersity of Natal, Durban 4014, South Africa Received 6 June 1999; accepted 19 July 1999
Abstract This paper involves the behavior of a polysulphide rubber Žthiokol A. in different organic acid solutions. The rubber samples Žin acid solutions. were left in a water bath for a period of 1 month, after which time they were taken out, dried and weighed to determine the percentage weight change of the samples over the period. The original acid solutions and the solutions in which the rubber pieces were suspended were then titrated with a standard sodium hydroxide solution and the change in acid strength determined for each. The mass of the thiokol A specimens tested was found to decrease in all cases Žmostly greater than a 3% decrease.. In terms of the solutions in which the testing took place the pH was found to increase while the molarity showed a general decrease. 䊚 2000 Elsevier Science Ltd. All rights reserved. Keywords: Thiokol A; Chemical resistance
1. Introduction Many corrosion problems arise from metal᎐acid contact and the use of a chemically resistant polymeric material as a lining is sometimes a solution to the problem. The acids selected were acetic, citric and propionic acids mainly because of the lack of detailed information on the behavior of the mentioned organic acids on thiokol A in the literature w1᎐5x. A material’s solvent resistance potential lies in its ability to withstand conditions imposed upon it without having a substantial change in its properties over the time of exposure. In addition, a material with good solvent resistance should also not alter the characteristics of the environment to which it is exposed. Hence, where solutions of acids are involved, pH and molarity
U
Corresponding author. Department of Chemical Engineering, Technikon Mangosuthu, P.O. Box 12363, Jacobs 4026, South Africa. Tel.: q27-31-260-3119; fax: q27-31-260-118. 1
also become important in determining the solvent resistance potential of the material.
2. Experimental procedure For the purposes of determining the effect of acid concentration and temperature on the solvent resistance properties of the specimens, each acid was tested at three different concentrations and each concentration at three different temperatures Žsee Table 1.. Starting with the 30% Žby mass. solution of each acid, three bottles Žper acid solution. were filled. The volume of each bottle is 50 ml. The original acid solution was then diluted to 20% and another three bottles for each acid filled with this diluted solution. Finally the original solution was diluted to 10% and the procedure repeated as before. Water was also used and three bottles, one for each temperature, were filled. It should be noted that water can be thought of as representing 0% acid.
0261-3069r00r$ - see front matter 䊚 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 9 9 . 0 0 0 5 3 - 9
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
212
Table 1 Test conditions: solvent concentrations and temperatures Solvent
25⬚C
35⬚C
45⬚C
Citric acid Ž%. Acetic acid Ž%. Propionic acid Ž%. Water
10, 20, 30 10, 20, 30 10, 20, 30 1 sample
10, 20, 30 10, 20, 30 10, 20, 30 1 sample
10, 20, 30 10, 20, 30 10, 20, 30 1 sample
Thirty rubber specimens, each approximately 4 cm = 1 cm = 0.5 cm, with a hole punched through each were used. A piece of fishing gut was tied to each piece, and the rubber Žwith gut. weighed. Each specimen was then suspended in a separate, sealed 50-ml bottle containing one of the acid solutions. A set of 10 bottles Žone containing water, and the other nine containing the three different acid solutions at the three different acid concentrations . were placed in a water bath at approximately 25⬚C, another set of 10 in a bath at 35⬚C and the remaining set at 45⬚C. The rubber samples were left in the water baths for a period of 1 month, after which time they were taken out, dried and weighed to determine the percentage weight change of the rubber over the 1-month period. The original acid solutions and the solutions in which the rubber pieces were suspended were then titrated with a standard sodium hydroxide solution and the change in acid strength determined for each. The change in pH of the solvents over the 4 weeks
was also determined by measuring the pH of each sample solution and that of the original acid solutions.
3. Results and discussion Considering the change in mass of the thiokol A specimens the first thing that can be said is that the mass decreased in all cases, regardless of the solvent, temperature or concentration. Very generally, it seems that the mass of the polysulphide specimens decreases more with increasing acid concentration, and that the mass decrease is more pronounced at higher temperatures, regardless of the type of acid. This is reflected in Figs. 1᎐3. In analyzing the decrease in mass, two possibilities arise. Firstly there is the possibility that the decrease in mass was due to the dissolution of the rubber by the solution, or secondly, that the mass decrease was due to small ‘loose’ particles being detached from the parent specimen due to the solution, this not being a result of the actual dissolution of the rubber. The ‘loosening’ of these particles may have arisen due to the method by which the specimens were cut Ži.e. industrial clippers.. Some of the specimens were slightly porous, and that this porosity was not uniform amongst all specimens. These pores within the material contribute to the total surface area exposed to the fluid and hence the surface
Fig. 1. Plot of percentage mass decrease vs. acid concentration for different temperatures for acetic acid.
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
213
Fig. 2. Plot of percentage mass decrease vs. acid concentration for different temperatures for citric acid.
area also depends on the proportion of these pores in a particular specimen. Ideally, measurements of surface area exposed to the fluid would be more useful to determine the dissolution trends involving temperature and acid concentration, however, since there was no accurate means of determining this surface area Žbecause of the porosity.
the easiest and most accurate measurement that could be made was that of mass. And while there is a direct correlation between mass and volume, no such correlation exists for the surface area. Hence, if surface area is the governing quantity that determines the degree of dissolution, the results for the different specimens cannot really be comparable on a mass basis, i.e. the
Fig. 3. Plot of percentage mass decrease vs. acid concentration for different temperatures for propionic acid.
214
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
dissolution of a particular specimen is only applicable to another specimen with the same surface area. In terms of the pH change between the start and end of the trial, it is observed that the pH increased in all cases. The interesting thing is that the pH change for the water was much greater Žalmost by a factor of 10., than that of the acid solutions, as can be seen in Fig. 4. In compiling the relevant figures, it was felt that data for water should also appear on the same figure as data for the acid solutions, since water can be thought of representing a 0% acid concentration. In effect this means that on such a figure the variations of pH change with respect to acid concentration Žfor acid concentrations greater than 0%. are greatly ‘attenuated’ by the scaling of the figure. Thus, in fitting a trend line to this figure, all four points are incorporated and since the pH change for the water is so much larger than the others Ži.e. nonzero concentrations of all acids. a similar trend line is produced for all acids. Since the pH change for water is of the same order of magnitude for all three temperatures, the graphs generated for the different temperatures reflect the same characteristic shape Žsee Fig. 4.. The fact that there was an increase in pH in all cases seems to indicate Hq ions are involved in the dissolution process, but that the pH change for water should be greater than that of the acid solutions is unexpected. However, when one considers that in water, for every one Hq ion that becomes involved in the dissolution process, one OHy ion is produced and remains in
solution ᎏ as a result the pH increases. In the case of the acid, there is an excess of Hq ions ᎏ these become involved in the dissolution process without significant formation of OHy ions; hence the change in pH of the acid solutions should be less, as was the case. The molarity of the acid solutions showed a slight decrease over the course of the trial in most cases. This too can be attributed to the involvement of the Hq ions in the dissolution process. The method of determining the molarity of the solution was via titrimetry, using sodium hydroxide base. The question that arises though is that it is not known what type of complex is formed between the Hq ions and the dissolved rubber particles, nor is there any indication of the strength of this complex. It is thus a possibility that the sodium hydroxide Žas a strong base. could displace the Hq ions from the complex, thereby yielding a non-representative result. Having considered all the various aspects of the results all that remains to be discussed is the applicability of the results to situations beyond the bounds of the 50-ml bottles within which the tests were performed. What can be said is that the results are applicable to thiokol Type A in situations involving stagnant solutions at the temperatures of exposure and concentrations of the acid solutions used during the testing. If classification of the thiokol Type A in terms of its solvent resistance to the solutions tested had to be based on this report, the overall finding would be that all of the acids tested fell into the Non-Resistant category w6x Žthe category for mass loss greater than 3%..
Fig. 4. Plot of acid pH change vs. acid concentration for different acids at a constant temperature of 25⬚C.
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
4. Conclusion The rubber was classified as being Non-Resistant w6x to the three organic acids tested, at all concentrations and temperatures. The solvent resistance of the thiokol A was measured in terms of its change in mass over the period of exposure and also in terms of the effect it had on the environment in which it was tested. It was found to be non resistant in both cases, that is, there was an appreciable change in mass as well as a change in the testing environment Žsolvent properties pH and molarity.. While the results are valid on a qualitative basis, none of the numerical data presented here within can be quoted with any degree of certainty since many unquantified variables are present having the effect of undermining the accuracy of these figures.
5. Recommendations Testing at intermediate temperatures and concentra-
215
tions might be of interest ᎏ this will result in more data points being generated for the various graphs allowing trend lines to be more representative of the true picture. An investigation at intermediate temperatures and concentrations is underway.
References
w1x Morton M. Introduction to rubber technology. New York: Van Nostrand Reinhold, 1962:363᎐382. w2x Roff WJ, Scott JR. Fibres, films, plastics and rubbers ᎏ handbook of common polymers. England: Butterworth, 1971:436᎐445. w3x Morton M. Rubber technology. 2nd ed New York: Van Nostrand Reinhold, 1973:349᎐367. w4x Schwartz SS, Goodman SH. Plastics materials and processes. New York: Van Nostrand Reinhold, 1982:458᎐459. w5x Mukhlyolnoz IT. A practical course in chemical technology. Moscow: MIR, 1982:58᎐63. w6x Waterman NA, Ashby MF. Elsevier materials selector, vol 3. London: Elsevier, 1991:1545᎐1577.
Behavior of polysulphide rubber ž thiokol A/ in organic acid solutions C.A. BaahU , J.I. Baah1, P. Gianadda, C. Fisher School of Chemical Engineering, Uni¨ ersity of Natal, Durban 4014, South Africa Received 6 June 1999; accepted 19 July 1999
Abstract This paper involves the behavior of a polysulphide rubber Žthiokol A. in different organic acid solutions. The rubber samples Žin acid solutions. were left in a water bath for a period of 1 month, after which time they were taken out, dried and weighed to determine the percentage weight change of the samples over the period. The original acid solutions and the solutions in which the rubber pieces were suspended were then titrated with a standard sodium hydroxide solution and the change in acid strength determined for each. The mass of the thiokol A specimens tested was found to decrease in all cases Žmostly greater than a 3% decrease.. In terms of the solutions in which the testing took place the pH was found to increase while the molarity showed a general decrease. 䊚 2000 Elsevier Science Ltd. All rights reserved. Keywords: Thiokol A; Chemical resistance
1. Introduction Many corrosion problems arise from metal᎐acid contact and the use of a chemically resistant polymeric material as a lining is sometimes a solution to the problem. The acids selected were acetic, citric and propionic acids mainly because of the lack of detailed information on the behavior of the mentioned organic acids on thiokol A in the literature w1᎐5x. A material’s solvent resistance potential lies in its ability to withstand conditions imposed upon it without having a substantial change in its properties over the time of exposure. In addition, a material with good solvent resistance should also not alter the characteristics of the environment to which it is exposed. Hence, where solutions of acids are involved, pH and molarity
U
Corresponding author. Department of Chemical Engineering, Technikon Mangosuthu, P.O. Box 12363, Jacobs 4026, South Africa. Tel.: q27-31-260-3119; fax: q27-31-260-118. 1
also become important in determining the solvent resistance potential of the material.
2. Experimental procedure For the purposes of determining the effect of acid concentration and temperature on the solvent resistance properties of the specimens, each acid was tested at three different concentrations and each concentration at three different temperatures Žsee Table 1.. Starting with the 30% Žby mass. solution of each acid, three bottles Žper acid solution. were filled. The volume of each bottle is 50 ml. The original acid solution was then diluted to 20% and another three bottles for each acid filled with this diluted solution. Finally the original solution was diluted to 10% and the procedure repeated as before. Water was also used and three bottles, one for each temperature, were filled. It should be noted that water can be thought of as representing 0% acid.
0261-3069r00r$ - see front matter 䊚 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 9 9 . 0 0 0 5 3 - 9
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
212
Table 1 Test conditions: solvent concentrations and temperatures Solvent
25⬚C
35⬚C
45⬚C
Citric acid Ž%. Acetic acid Ž%. Propionic acid Ž%. Water
10, 20, 30 10, 20, 30 10, 20, 30 1 sample
10, 20, 30 10, 20, 30 10, 20, 30 1 sample
10, 20, 30 10, 20, 30 10, 20, 30 1 sample
Thirty rubber specimens, each approximately 4 cm = 1 cm = 0.5 cm, with a hole punched through each were used. A piece of fishing gut was tied to each piece, and the rubber Žwith gut. weighed. Each specimen was then suspended in a separate, sealed 50-ml bottle containing one of the acid solutions. A set of 10 bottles Žone containing water, and the other nine containing the three different acid solutions at the three different acid concentrations . were placed in a water bath at approximately 25⬚C, another set of 10 in a bath at 35⬚C and the remaining set at 45⬚C. The rubber samples were left in the water baths for a period of 1 month, after which time they were taken out, dried and weighed to determine the percentage weight change of the rubber over the 1-month period. The original acid solutions and the solutions in which the rubber pieces were suspended were then titrated with a standard sodium hydroxide solution and the change in acid strength determined for each. The change in pH of the solvents over the 4 weeks
was also determined by measuring the pH of each sample solution and that of the original acid solutions.
3. Results and discussion Considering the change in mass of the thiokol A specimens the first thing that can be said is that the mass decreased in all cases, regardless of the solvent, temperature or concentration. Very generally, it seems that the mass of the polysulphide specimens decreases more with increasing acid concentration, and that the mass decrease is more pronounced at higher temperatures, regardless of the type of acid. This is reflected in Figs. 1᎐3. In analyzing the decrease in mass, two possibilities arise. Firstly there is the possibility that the decrease in mass was due to the dissolution of the rubber by the solution, or secondly, that the mass decrease was due to small ‘loose’ particles being detached from the parent specimen due to the solution, this not being a result of the actual dissolution of the rubber. The ‘loosening’ of these particles may have arisen due to the method by which the specimens were cut Ži.e. industrial clippers.. Some of the specimens were slightly porous, and that this porosity was not uniform amongst all specimens. These pores within the material contribute to the total surface area exposed to the fluid and hence the surface
Fig. 1. Plot of percentage mass decrease vs. acid concentration for different temperatures for acetic acid.
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
213
Fig. 2. Plot of percentage mass decrease vs. acid concentration for different temperatures for citric acid.
area also depends on the proportion of these pores in a particular specimen. Ideally, measurements of surface area exposed to the fluid would be more useful to determine the dissolution trends involving temperature and acid concentration, however, since there was no accurate means of determining this surface area Žbecause of the porosity.
the easiest and most accurate measurement that could be made was that of mass. And while there is a direct correlation between mass and volume, no such correlation exists for the surface area. Hence, if surface area is the governing quantity that determines the degree of dissolution, the results for the different specimens cannot really be comparable on a mass basis, i.e. the
Fig. 3. Plot of percentage mass decrease vs. acid concentration for different temperatures for propionic acid.
214
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
dissolution of a particular specimen is only applicable to another specimen with the same surface area. In terms of the pH change between the start and end of the trial, it is observed that the pH increased in all cases. The interesting thing is that the pH change for the water was much greater Žalmost by a factor of 10., than that of the acid solutions, as can be seen in Fig. 4. In compiling the relevant figures, it was felt that data for water should also appear on the same figure as data for the acid solutions, since water can be thought of representing a 0% acid concentration. In effect this means that on such a figure the variations of pH change with respect to acid concentration Žfor acid concentrations greater than 0%. are greatly ‘attenuated’ by the scaling of the figure. Thus, in fitting a trend line to this figure, all four points are incorporated and since the pH change for the water is so much larger than the others Ži.e. nonzero concentrations of all acids. a similar trend line is produced for all acids. Since the pH change for water is of the same order of magnitude for all three temperatures, the graphs generated for the different temperatures reflect the same characteristic shape Žsee Fig. 4.. The fact that there was an increase in pH in all cases seems to indicate Hq ions are involved in the dissolution process, but that the pH change for water should be greater than that of the acid solutions is unexpected. However, when one considers that in water, for every one Hq ion that becomes involved in the dissolution process, one OHy ion is produced and remains in
solution ᎏ as a result the pH increases. In the case of the acid, there is an excess of Hq ions ᎏ these become involved in the dissolution process without significant formation of OHy ions; hence the change in pH of the acid solutions should be less, as was the case. The molarity of the acid solutions showed a slight decrease over the course of the trial in most cases. This too can be attributed to the involvement of the Hq ions in the dissolution process. The method of determining the molarity of the solution was via titrimetry, using sodium hydroxide base. The question that arises though is that it is not known what type of complex is formed between the Hq ions and the dissolved rubber particles, nor is there any indication of the strength of this complex. It is thus a possibility that the sodium hydroxide Žas a strong base. could displace the Hq ions from the complex, thereby yielding a non-representative result. Having considered all the various aspects of the results all that remains to be discussed is the applicability of the results to situations beyond the bounds of the 50-ml bottles within which the tests were performed. What can be said is that the results are applicable to thiokol Type A in situations involving stagnant solutions at the temperatures of exposure and concentrations of the acid solutions used during the testing. If classification of the thiokol Type A in terms of its solvent resistance to the solutions tested had to be based on this report, the overall finding would be that all of the acids tested fell into the Non-Resistant category w6x Žthe category for mass loss greater than 3%..
Fig. 4. Plot of acid pH change vs. acid concentration for different acids at a constant temperature of 25⬚C.
C.A. Baah et al. r Materials and Design 21 (2000) 211᎐215
4. Conclusion The rubber was classified as being Non-Resistant w6x to the three organic acids tested, at all concentrations and temperatures. The solvent resistance of the thiokol A was measured in terms of its change in mass over the period of exposure and also in terms of the effect it had on the environment in which it was tested. It was found to be non resistant in both cases, that is, there was an appreciable change in mass as well as a change in the testing environment Žsolvent properties pH and molarity.. While the results are valid on a qualitative basis, none of the numerical data presented here within can be quoted with any degree of certainty since many unquantified variables are present having the effect of undermining the accuracy of these figures.
5. Recommendations Testing at intermediate temperatures and concentra-
215
tions might be of interest ᎏ this will result in more data points being generated for the various graphs allowing trend lines to be more representative of the true picture. An investigation at intermediate temperatures and concentrations is underway.
References
w1x Morton M. Introduction to rubber technology. New York: Van Nostrand Reinhold, 1962:363᎐382. w2x Roff WJ, Scott JR. Fibres, films, plastics and rubbers ᎏ handbook of common polymers. England: Butterworth, 1971:436᎐445. w3x Morton M. Rubber technology. 2nd ed New York: Van Nostrand Reinhold, 1973:349᎐367. w4x Schwartz SS, Goodman SH. Plastics materials and processes. New York: Van Nostrand Reinhold, 1982:458᎐459. w5x Mukhlyolnoz IT. A practical course in chemical technology. Moscow: MIR, 1982:58᎐63. w6x Waterman NA, Ashby MF. Elsevier materials selector, vol 3. London: Elsevier, 1991:1545᎐1577.