PU foam waste obtained from Hamilton India Pvt Ltd. was grinded and converted into small pieces. Diethanol amine and sodium hydroxide were of analytical reagent grade procured from SD Fine Chemicals Ltd., Mumbai. Isocyanates cross linkers were obtained from Bayer Material Science Ltd.

Calculated quantities of PU foam, DEA and NaOH were charged in a four neck glass reactor equipped with stirrer, reflux condenser, temperature indicator and nitrogen inlet (see Table 1). PU foam and DEA were reacted in 1:1 (PUDE C1) and 1:2 (PUDE C2) (w/w) ratios along with 0.2% NaOH catalyst on weight basis of PU foam. The aminolysis reaction was carried out at 180˚C under reflux and nitrogen atmosphere for 10 hours. The depolymerised product was characterized by hydroxyl value, amine value and ATR analysis.

In case of DEA, the steric hindrance at the hydrogen bond may contribute to weakening the association between the amino group of DEA and the carbonyl group of urethane. The weakness of the association between DEA and carbonyl may partially cause the hydrogen bond between the amino group of DEA and the ether of the urethane group, which will produce the Isocyanate/DEA urea adduct in the polyurethane foam decomposition process [6] . Due to longer reaction times taken by DEA, microwave radiations as a source of energy was chosen. The reaction time was reduced from 10 hours to 30 min. In order to study the effect of various parameters in microwave, variation in reaction time and microwave power were carried out and the depolymerised product was characterized for hydroxyl and amine value to study the reaction mechanism. The details of the reactions carried out are tabulated (see Table 2 and Table 3)

Depolymerised product with hydroxyl value 315 mgKOH/g and amine value 22 mgKOH/g prepared in the above steps was cured with Hexamethylene diisocyanate [HDI biuret], Isophorone diisocyanate [IPDI] curing agent. Xylene: MEK (70:30 v/v) mixture was used as solvent. The films were applied on mild steel panels (150 mm ´

PU-Polyurethane Foam/DE-Diethanol Amine/C?Conventional.

PU-Polyurethane Foam/DE-Diethanol Amine/T-Reaction with different times

PU-Polyurethane Foam/DE-Diethanol Amine/M-Reaction with different microwave power.

100 mm ´ 0.5 mm) and cured at 120˚C for 30 minutes. The coatings were further evaluated for mechanical and chemical properties.

From the tabulated results (see Table 4), we observe that hydroxyl value increased as the amount of DEA increased. This is due to the fact that DEA solvolyses the urethane bond and disintegrates the polymeric network into mixture of smaller molecular weight oligomers and monomers. Amine value also showed an increase with the increase in the quantity of DEA in the depolymerisation reaction. This must be due to the generation of amine simultaneously along with the formation of hydroxyl moiety, as side reactions in the depolymerisation reactions. Further the effect of time and microwave power on the depolymerization reaction of PU foam was also studied (see Table 5). The conventional method takes long hours, i.e. 10hours for depolymerization reaction and also side reactions take place which leads amine formations and further requires purification steps to remove excess amines which involves energy and cost.

Thus conventional method is not a sustainable method to be used.

Depolymerisation reaction when carried out conventionally with DEA takes long hours to depolymerise polyurethane foam, due to the fact that the stearic hindrance at the hydrogen bond may contribute to weakening the association between amino group of DEA and carbonyl group of urethane. Hence the depolymerization was also studied using the assistance of microwave with varying time and varying microwave power. Microwave heating is based on the ability of a particular substance such as a solvent or substrate to absorb microwave energy and effectively convert the electromagnetic energy to heat energy. Molecules with dipole moment attempt to align themselves with the oscillating electric field of the microwave irradiation leading to rotation and hence microwave reduces the stearic hindrance and hence the reaction time reduces.

From the above mentioned results in Table 5, we can observe that there is an increase in the hydroxyl value which is due to the disintegration of the PU foam into low molecular weight fractions as well as monomers, which contributes to the increase in hydroxyl value as time increases from 10 min to 30 min. Amine value also increased, but it may be observed that there was an decrease in the amine value when compared with the conventional method. This proved that there were less side reactions taking place in microwave reactor which can be beneficial to use depolymerised product without further purification.

When microwave power was increased from 400 kW to 600 kW to 800 kW, we observed that there was an increase in hydroxyl as well as amine value, due to formation of low molecular weight hydroxyl species along with amine moieties due to increase in internal energy of the reaction mixture by absorption of the microwave energy at different microwave power (see Table 6).

ATR spectra in Figure 1, for depolymerised product using DEA, shows a transmission band at 3400 cm⁻¹,

PU-Polyurethane Foam/DE-Diethanol Amine/C-Conventional.

PU-Polyurethane Foam/DE-Diethanol Amine/T-Reaction with different times.

PU-Polyurethane Foam/DE-Diethanol Amine/T-Reaction with different microwave power.

which indicates the OH stretch in the recycled polyol. The peaks at 2971 cm⁻¹ and 2872 cm⁻¹ represent the stretching vibrations of CH bonds in aliphatic carbons. Bending vibrations of methylene group appear in recycled polyol chain at 1376 cm⁻¹ and 1454 cm⁻¹. Bending vibrations of NH group in amines appear in 1602 cm⁻¹ and 1649 cm⁻¹. The absorption band at 1080 cm⁻¹ and 1277 cm⁻¹ are related to aliphatic ether group of recycled polyol. The peak at 1327 cm⁻¹ confirms the presence of aromatic amines in the depolymerised product. The peak at 1408 cm⁻¹ confirms the C?C in aromatic rings. Also the peaks at 616 cm⁻¹, 658 cm⁻¹ indicate the CH bend. C?H in aromatic rings is confirmed by the presence of peaks at 738 cm⁻¹, 772 cm⁻¹ and 843 cm⁻¹.

ATR spectra in the above Figure 2 confirmed the formation of polyurethane urea coating, the presence of peak at 3336 cm⁻¹ confirmed the NH stretch in urea. The peak at 1725 cm⁻¹ confirmed the presence of C = O in urethane. NH bends were confirmed by the presence of transmission bands at 1597 cm⁻¹ and 1528 cm⁻¹. The peaks at 1310 cm⁻¹, 1219 cm⁻¹, 1164 cm⁻¹ and 1066 cm⁻¹ confirmed the C?O stretch in ether. The NH wag was observed at 850 cm⁻¹. The peaks at 1463 cm⁻¹ and 1413 cm⁻¹ represent the C?C stretch in aromatic ring, while peak at 815 cm⁻¹ and 769 cm⁻¹ represent the CH in aromatic rings. The transmission band 685 cm⁻¹ and 585 cm⁻¹ confirmed the CH bend.

The coatings were applied using Xylene: MEK in ratio 70: 30 w/w and cured at 120˚C. A digital electromagnetic thickness gauge instrument was used to measure the dry film thickness (DFT) of each coated panel and all the films were of dry film thickness in the range of 60 - 80 microns. Each coating showed gloss which can be attributed to structure of raw materials used in preparation of coatings. Gloss of film is related to the polyether linkages of the polyol, urethane linkages due to which orderliness was incorporated across polymer backbone. Presence of oxygen in ether linkages could be the reason for inter-chain locking because of dipole interactions. This offers regularity in packing of chains and so does the crystallinity. Pencil Hardness of the film is equated with the pencil that just fails to damage the coating and Scratch hardness of the film is equated with the load at which needle penetrates the coating. Pencil Hardness and Scratch hardness of IPDI based coatings showed consent result. This is due to the fact that presence of polyurethane linkages in the film makes them tough and also difficult to notch off. Impact is the momentary shock which could be sustained if the energy of impact would be dissipated uniformly over the coating. For this free volume between the chains play a crucial role. Impact resistance was satisfactory. Each of the film could pass the impact test (both on surface of coating and reverse) when indenter was released from the height of 60 cm. This could be due to aliphatic nature of the film formed across the polymer backbone, with free volume available between adjacent chains. The adhesion of the coatings to the metal substrate was investigated by crosshatch method. It was observed that all the coatings showed excellent adhesion. This could be due to the polar nature of the polymer backbone containing urethane moieties in IPDI based coatings. The flexibility of the coated panels was evaluated using Conical Mandrel Bending test. It was observed that the coatings exhibited excellent flexibility. This could be due to the aliphatic nature of Polyether polyol and IPDI which increases the flexibility of final coating.

Acid and Alkali resistance were investigated according to ASTM D1308. Coatings showed excellent acid resistance, wherein no visible damage was done to any of the films kept in acid solution for 24 hours. In alkali solution slight loss of gloss was observed for IPDI coatings and minute cracks were observed on HDI-biuret based coatings. The excellent chemical resistance could be due to the presence of intermolecular hydrogen bonding in the cross-linked polymeric structure which helps in shielding the acid susceptible hydroxyl group resulting in very good acid resistance. The solvent resistance of coatings was evaluated by MEK and Xylene rub method to study the effect of polar and non-polar solvent rub on the coating properties. The coated films showed excellent solvent resistance even after 200 double rubs of Xylene. This could be due to highly cross-linked, dense and polar polymeric films which are highly adherent to the metal substrates. The polar nature of the polymeric film could be the reason for poor resistance to MEK, i.e. 110 - 120 MEK double rubs (see Table 7).