Kinetics Study of Acid Hydrolysis of Waste Polyamide-6,6 using Sodium-1- Heptane Sulphonate Surfactant

Introduction

Today about 400 million tons of plastic products are being made. Due to its outstanding properties, plastics being used in huge quantity in our day-to-day life but only 20 to 30% of plastics is recycled and rest is waste. Polyamide waste are non-biodegradable material and they are very much used nowadays and hence after use gets collected in the environment in one or the other form and creates microplastic pollution, mega plastic pollution ¹. There are many things which are made up of polyamide such as rope, bottles, carpets, etc. ². Now a days many methods has been developed for the recycling of polyamide waste such as physical recycling, mechanical recycling where the polymer waste is washed, cut into small pieces, and then melted and another one is chemical recycling where many other sub processes are carried out to depolymerize polymers such as pyrolysis, solvolysis, partial oxidation, glycolysis, hydrolysis and many more^3-6. Recycled product obtained by mechanical method are of poor quality while that chemical recycled products are of good quality⁷. In recent past many efforts has been developed for monomerization of polymer waste by many methods ^8,9. 

Polyamides is a crystalline polymer which are obtained mainly as nylon6 and nylon-6,6¹⁰. Mostly polyamides are obtained in the form of thermoplastic polymer ¹¹. Based on the different experiments it is found that for depolymerization, temperature and time required are an important factor which led to the formation of monomer as a product. Nylon-6,6 when depolymerized gets converted to its monomers adipic acid and hexamethylene diamine. Many experiments have been performed by taking different solvents to convert nylon-6,6 into its monomer. At optimum condition the hydrolytic depolymerization of nylon waste in acidic medium gives an efficient result at 80⁰C ¹².   

Surfactants are organic compound which is made up of hydrocarbon chain attached to polar group. Surfactants are of many types depending upon its hydrophilic head as cationic, anionic, nonionic and amphoteric. There are many surfactants which plays an important role in pharmaceutical processes, metal-ligand formation, depolymerization of polymers etc. In one of the experiment cationic surfactants CTMB plays an important role for oxidation of L-glutamic acid by hexacyanoferrate (III) and due to its positive salt effect the rection rate first increases and then remains constant and shows first order kinetics using Ru (III), [Fe (CN)₆]^{3- 13}. In one experiment sodium lauryl sulphate surfactant is used for determining alpha lipoic acid in some pharmaceutical samples due to presence of 2 Sulphur alpha lipoic acid in SLS medium it forms chelate with Pd²⁺ hence SLS plays an important role to inhibit the process ¹⁴.

In this work, an attempt was made to depolymerize Nylon-6,6 waste to its derivative form in acidic medium with the help of catalyst which is sodium-1- heptane sulphonate, anionic surfactant in very small amount, plays an important role to give great amount of product dibenzoyl derivative of hexamethylene diamine (DBHMD).

Materials and Methodology

Chemicals and apparatus

The apparatus consists of 250 ml round bottom flask with water condenser and heating electric furnace. Nylon -6,6 waste was collected, washed, and cut into small pieces. Hydrochloric acid, benzoyl chloride, sodium chloride pallets were of analytical grades.

Viscosity method for nylon- 6,6 and CMC of anionic surfactant

Nylon waste molecular weight was determined by viscosity method using different concentration of nylon6,6 in m cresol which is used as solvent. For each concentration i.e from 0.0125% to 0.075% flow time was measured and graph was plotted between concentration on x axis and [𝜂𝑠𝑝]/𝐶 on Y axis. By using the formula [𝜂𝑠𝑝]/𝐶 = 𝐾𝑀^𝛼, where 𝐾 and 𝛼 value for the solvent is 2.41 × 10⁻³ and 0.6101 intercept and average molecular weight was found.

In this study sodium-1- heptane sulphonate (SHE) is an anionic surfactant which is used as a catalyst. By using drop count method critical miscelle concentration of the surfactant was obtained. 0.1% to 1 % concentration of surfactant was prepared by solubilizing surfactant in distilled water. Using stalagmometer number of drops of different concentration was counted and a graph was plotted between concentration (%) verses CMC and from graph CMC of sodium-1- heptane sulphonate was determined.

Depolymerization of nylon-6,6 at various time interval using sodium -1- heptane sulphonate

Hydrolysis of 3g of Nylon 6,6 was done using 50 ml of 5N HCl and sodium-1- heptane sulphonate (SHE) of various amount (0.01g to 0.06 g) in 250 ml round bottom flask and refluxed using water condenser for 2 hr at 80^oC.The reaction process was carried out at different temperature but it was found that 80^oC was an optimum temperature where the rate of reaction is fast, hence after 2 hrs the reaction mixture was cooled and filtered and neutralized using required volume of 5N sodium hydroxide solution till red litmus turns to blue colour. This neutralised solution is hexamethylene diamine was then treated with benzoyl chloride till the odour of acid chloride gets disappeared. Hexamethylene now gets converted to the product dibenzoyl derivative of hexamethylene diamine (DBHMD) which is recrystallised with ethanol, dried, and weighed. It is found by calculation that by taking 0.03g of surfactant the highest amount of product is obtained hence further process was carried out by using same procedure by taking 0.03g of surfactant for depolymerization of 3g of nylon-6,6 at 80 ^OC but at various time interval (30 min to 180 min) and found that highest yield of the product DBHMD was obtained by taking 0.03g surfactant at 80⁰C by heating the reaction mixture for 120 min. Melting point of DBHMD was determined and found to be 154^OC and through FTIR analysis DBHMD was characterised. Using different parameters kinetic study is obtained.

Result and Discussion

Determination weight average molecular mass and CMC by surface tension method

Weight average molecular mass of Nylon-6,6 by using Ostwald’s viscometer was determined by plotting a graph between [𝜂𝑠𝑝]/C (x axis) and concentration (%) (y axis) is 27500, by using formula [𝜂𝑠𝑝]/𝐶 = 𝐾𝑀^𝛼 where 𝐾 and 𝛼 value of the solvent are 2.41 × 10⁻³ and 0.6101, respectively and intercept obtained by plotting intrinsic viscosity vs concentration is 1.2230.

0.1% to 1% of concentration of sodium-1- heptane sulphonate was prepared in distilled water. Number of drops were recorded using stalagmometer and surface tension of various concentrations was calculated by the formula,

γ sol = ρ sol × η H2O × η sol × γ H20

Where, γ_sol Surface tension, ρ_sol and ρ_H20 are density of solution and density of water, η_H2O and η_sol are no. of drops of water and solution respectively. CMC obtained by plotting a graph between concentration (x axis) and surface tension (y axis) was 0.302 mol/dm³. It was found that there is no effect of CMC on reaction process.

Escalation parameter for depolymerization of nylon-6,6 at different time interval using different amount of sodium-1- heptane sulphonate

Reaction mixture containing 3g nylon -6,6, 50 ml 5N HCl and sodium -1- heptane sulphonate (0.01g to 0.1g) in 250 ml round bottom flask attached with thermometer and stirrer was depolymerised at 80^OC using water condenser for 2 hrs and result obtained shows that highest yield obtained by taking 0.03g of surfactant shown in figure no. 1.

Figure 1: Percentage yield of various amount of sodium-1- heptane sulphonate Click here to View Figure

From figure no. 1 it is seen that by taking 0.03g of sodium-1- heptane sulphonate was taken as catalyst and the same amount of reaction mixture was refluxed at 80⁰c for different time interval (30 min to 210 min) and hence the result obtained showing highest % yield of product in 120 min shown in figure no. 2. Due to very less concentration of surfactant, pH does not change during whole reaction process. In many experiments we have seen that in absence of surfactant ¹² the rate of reaction is very slow and got less amount of product but due to addition of even small quantity of surfactant the rate of reaction is fast and product obtained is high.

Figure 2: Percentage yield of DBHMD at various time interval. Click here to View Figure

Kinetics of nylon-6,6 waste depolymerisation

Based on benzoyl derivative of hexamethylene diamine the kinetic study was carried out at escalate parameter. The amount of reacted and unreacted nylon-6,6 was obtained by the amount of dibenzoyl derivative of hexamethylene diamine. From the kinetic study we get to know that it shows pseudo first order reaction, expressed by the equation given below ¹⁵;

Due to excess amount of HCl and H₂O it gets negligible and very small amount of SHE is used as catalyst and hence concentration of SHE also gets negligible hence the above reaction can now be expressed as;

If we take the initial amount and some amount after some time interval as [Nylon-6,6] _a and [Nylon-6,6]_b then the integral of equation 2 can be written as

The above equation can be expressed as [a/(a-x)] where a is initial weight and (a-x) is weight after some time and finally above equation can be written as;

The optimum conditions set earlier and the order of reaction was determined by doing depolymerization of nylon-6,6 using catalyst. From table no.1 the amount of product was obtained at different time intervals and data tabulated in table no.1 shows that it is pseudo first order kinetics because hydrochloric acid is taken in excess amount and catalyst used is negligible, since the rate equation for first order reaction ^15-16 is given as K= (2.303/t) log(a/(a-x), if this equation is rearranged then we get, log(a/(a-x)) = Kt / 2.303.  By plotting graph between time (min) on x axis and log(a/(a-x)) on y axis we get a straight line passes almost through origin and slope gives rate constant value 6.68 × 10^-3 min^-1 shown in figure no. 3.

Table 1: Depolymerization of nylon-6,6 waste using SHE as catalyst

Time (min)	Amount of nylon-6,6 waste (gram) [a]	amount of SHE (gram)	Amount of DBHMD (gram)	Amount of nylon-6,6 reacted (gram) [x]	Amount of nylon-6,6 unreacted(gram) [a-x]	log (a-x)	a / (a-x)	log(a/a-x)
0	3.00	0.03	0.00	0.00	0.00	0.47	1.00	0.00
30	3.00	0.03	0.51	0.36	2.64	0.42	0.13	0.54
60	3.00	0.03	1.40	0.97	2.03	0.30	1.48	0.17
90	3.00	0.03	2.50	1.77	1.23	0.08	2.45	0.38
120	3.00	0.03	3.40	2.35	0.65	-0.18	4.50	0.66
150	3.00	0.03	3.06	2.14	0.86	-0.06	3.50	0.54
180	3.00	0.03	2.95	2.07	2.95	-0.03	3.22	0.50

 

Figure 3: Graphical presentation. Click here to View Figure

FTIR spectra analysis

The FT-IR spectral data ¹⁷ of the product obtained DBHMD and the controlled DBHMD was recorded and found that various peaks obtained which was found that most of the peaks of product DBHMD were same as that found in controlled DBHMD shown in figure 4 and 5 and table no 2. From figure 5 we see that there is presence of hydrogen bond shown by sharp bands. Bands obtained at 3017 Cm^-1, 1423 Cm^-1, 1629 Cm^-1, 3309 Cm^-1 indicate the presence of C-H, C-C, C=O, N-H. There are many bands between 1600 to 400 Cm-1 which indicate presence of stretching bands.

Figure 4: FTIR Spectra of controlled DBHMD 12. Click here to View Figure

Figure 5: FTIR spectra of product DBHMD. Click here to View Figure

Table 2: FTIR spectra values of DBHMD.

Compound	Wavelength (Cm -1)
C-H	3071
C-C	1423
C=O	1629
N-H	3309

 

Conclusion

Result reveals that anionic surfactant sodium-1- heptane sulphonate (0.03g) which is used as catalyst helps to degrade nylon-6,6 waste at 80⁰C of 120 min heating and gives better result. it was found that there is no relation between CMC and the amount of sodium-1- heptane used in the reaction. Melting point and FTIR spectra reveals that product obtained is dibenzoyl derivative of hexamethylene diamine. It shows pseudo first order kinetics and rate of reaction is 6.68×10^-3 min^-1. In future, other then anionic surfactant other type of surfactant can be used for depolymerization and get different results.

Acknowledgement

We express our heartfelt thanks to SAIF, FT-IR spectrum, Cochin, Kerala for their support in conducting the characterization of our work.

Conflict of Interest

The authors declare no conflict of interest.

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