Chemical degradation of polyurethanes2. Degradation of flexible polyether foam by dimethyl phosphonate

Polymer Degradation and Stability 67 (2000) 397±405

Chemical degradation of polyurethanes 2. Degradation of ¯exible polyether foam by dimethyl phosphonate K. Troev*, A. Tsekova, R. Tsevi Institute of Polymers, Bulgarian Academy of Sciences, 1113 So®a, Bulgaria Received 21 December 1998; accepted 18 February 1999

Abstract Flexible polyether polyurethane foam based on toluene diisocyanate and polyether polyol has been lique®ed by dimethyl phosphonate (CH3O)2P(O)H at 160 C. The degraded products have been studied by 1H, 13C and 31P NMR spectroscopy and shown to be phosphorus-containing oligomers. # 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction This paper presents our further studies on the new method for chemical degradation of polyurethanes by treating them with esters of phosphonic and phosphoric acids [1]. Here we report on the chemical degradation of ¯exible polyether polyurethane foam based on toluene diisocyanate (Desmodur T-80) and polyether polyol with dimethyl phosphonate. The primary aim of these studies is to develop a new method for converting ¯exible polyether polyurethane foam waste and used ¯exible polyether polyurethane foams into reusable products which can be used in the preparation of polymers including polyurethanes with reduced ¯ammability. 2. Experimental part 2.1. Materials and methods Flexible polyether polyurethane foam based on: polyether polyol TDI-T-80 H 2O Sn-octanoate B-Y (amino catalyst) Sil. stab. BF-2370 The ¯exible polyether polyurethane foam was cut into small pieces and placed into the glass apparatus. Dimethyl * Corresponding author. Tel.: +359-2-979-2203; fax.:+359-2-707523. E-mail address: [email protected] (K. Troev).

phosphonate available.

(CH3O)2P(O)H,

Fluka,

commercially

2.2. Instruments 1

H, 13C and 31P NMR spectra were recorded on a Bruker apparatus, 500 MHz, in CDCl3 solvent. The viscosity was measured on a Brook®eld LV viscometer. Phosphorus content was determined on a Specol spectrophotometer, 420 nm wave length. 2.3. Method for chemical degradation of ¯exible polyether polyurethane foam by dimethyl phosphonate Into a three- necked ¯ask equipped with a stirrer, thermometer and re¯ux condenser were put 362 g of dimethyl phosphonate and 120 g of ¯exible polyether polyurethane foam cut into small pieces (3±5 mm in size). The degradation proceeded at 160 C. Then the temperature was lowered to 60 C, the unreacted dimethyl phosphonate was removed under vacuum (0.1 mm Hg) and reused for degradation. Experiments on the chemical degradation of ¯exible polyurethane foam were performed with heating for 1, 2 and 3 h. The results from the degradation are presented in Table 1. 3. Results and discussion When heating a mixture of ¯exible polyurethane based on toluene diisocyanate and polyether polyol and dimethyl phosphonate (a 1:3 ratio) at 160 C the polyurethane

0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00106-8

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K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

Fig. 1. 31P{H} NMR spectrum of ¯exible polyether polyurethane foam degraded by dimethyl phosphonate Ð upper layers obtained after degradation: aÐ1 h; bÐ3 h.

K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

is lique®ed. Two liquid layers are formed after removing the unreacted dimethyl phosphonate. We have studied the structure of the products in both layers obtained after 3 and 1 h of degradation by means 31P, 1H and 13C NMR spectroscopy. 3.1. Study on the upper layer The 31P{H} NMR spectra of the upper layer products obtained after degradation for 1 (Fig. 1a) and for 3 h (Fig. 1b) have signals at =1.68; 2.34; 3.60; 5.08; 6.19± 6.38; 7.83; 9.67; 9.80; 10.52; 11.01; 11.72; 32.30; 34.45 ppm. The signals at 7.83; 9.81; 10.52, 11.01 and 11.71 are of the highest intensity. As seen from 31P NMR spectrum of the upper layer obtained after degradation

399

for 3 h (Fig. 2) the signal at =11.71 ppm is a doublet of septets with 3J(P,H)=11.78 Hz and 1J(P,H)=698.5 Hz which is typical for the phosphorus atom in the molecule of dimethyl phosphonate [2]. The content of dimethyl phosphonate in the upper layer is 4.3%. The signal at 7.83 ppm is a doublet of quartets with 3 J(P,H)=12.0 Hz and 1J(P,H)=657.4 Hz. The signal could be assigned to the phosphorus atom in product 1 (Table 2). The signal at 5.09 ppm is a doublet with 1 J(PH)=665.2 Hz which could be assigned to the phosphorus atom in product 2 (Table 2). The signal at 9.81 ppm is a doublet of quintets with 3J(P,H)=11.9, 9.2 Hz and 1J(P,H)=703.4 Hz and could be assigned to the phosphorus atom in product 3 (Table 2). The signal at 9.66 ppm is a doublet of sextets with 3J(P,H)=11.4 Hz

Table 1 Conditions for ¯exible polyether polyurethane foam (PU) chemical degradation by dimethyl phosphonate (DMP) DMP, g

PU, g

360 362 360

Fig. 2.

120 120 120

31

Duration of degrdation, h

1 2 3

Reacted DMP, g

109 114 122

Unreacted DMP, g

251 246 238

Degree of degradation, %

100 100 100

Yield g

223 225 234

Upper layer, g

62 63 78

Lower layer, g

153 149 134

Solid product, g

8 13 22

P, % Upper layer

Lower layer

3.69 3.65 4.08

4.29 19.40 16.20

P NMR Spectrum of ¯exible polyether polyurethane foam degraded by dimethyl phosphonateÐupper layerÐduration of degradation 3 h.

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K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

Table 2 Phosphorus containing products from the chemical degradation of ¯exible polyether polyurethane foam by dimethyl phosphonate N Structure

31

p NMR , ppm; J, Hz

Content, % Upper layer

1

7.83, dq 3J(P,H)=12.01J)P,H)=657.4

22

Lower layer 66.4

2 5.09, d 1J(P,H)=665.2

3.3

10.3

3 9.81 dquintets 3 J(P,H)=11.9, 9.2 1 J(P,H)=703.4

2;15

<1

9.66, dsex 3 J(P,H)=11.4 1J(P,H)=693.5

11

<1

11.01, dq 3 J(P.H)=11.11 1J(P,H)=715.0

15

<1

10.50, dquintets 3J(P,H)=11.86 1 J(P,H)=708.5

16

<1

4

5

6

7 3.59, septet 3 J(P,H)=11.10

0.8

4.1

2.34, quintet 3J(P,H)=11.0

1.4

5.0

m, 32.03-32.46

9.6

10.3

8

9

and 1J(P,H)=693.5 Hz which could be assigned to the phosphorus atom in product 4 (Table 2). The signal at 11.01 ppm is a doublet of quartets with 3J(P,H)=11.11 Hz and 1J(PH)=715.0 Hz which could be assigned to the phosphorus atom in product 5 (Table 2). The signal at 10.50 ppm is doublet of quintets with 3J(P,H)=11.86 Hz and 1J(PH)=708.5 Hz and could be assigned [2] to the phosphorus atoms in product 6 (Table 2). The signal at 3.59 ppm is a septet with 3J(P,H)=11.10 Hz and could be assigned [3] to the phosphorus atoms in product 7 (Table 2). The signal at 2.34 ppm is a quintet with 3J(P,H)=11.0 Hz which could be assigned [2] to the phosphorus atom in product 8 (Table 2). The signal at 32.03±32±46 ppm is a multiplet which is due to the phosphorus atom in dimethylmethyl phosphonate±product 9 (Table 2).

The 1H NMR spectrum (Fig. 3) shows signals which can be attributed to the protons of products above. There are seven types of P-H protons at: 6.67 ppm with 1 J(P,H)=658.8 Hz Ð product 1; 6.70 ppm with 1J(P,H)= 664.7 Hz Ð product 2; 6.80 ppm with 1J(P,H)=703.6 Ð product 3; 6.81 ppm with 1J(P,H)=700.2 Hz Ð product 4; 6.91 ppm with 1J(P,H)=715.6 Hz Ð product 5; 6.85 ppm with 1J(P,H)=709.3 Hz Ð product 6; 6.68 ppm with 1 J(P,H)=698.6 Hz which can be assigned to P-H protons of dimethyl phosphonate. The signals in 13C NMR spectrum (Fig. 4a) could be assigned to: 17.72 ppm to the CH3CH carbon atoms; 51.62 ppm to carbon atoms in CH3O-C(O) group; 63.97 to carbon atoms in the P-O-CH2 group; and 63.97 ppm to carbon atoms in the P-OCH(CH3) group. In the 13C

K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

401

Fig. 3. 1H NMR spectrum of ¯exible polyether polyurethane foam degraded by dimethyl phosphonate-upper layer-duration of degradation 3 h.

NMR spectrum there are no signals for the carbon atoms of the urethane group at 152±155 ppm as well as signals for aromatic carbon atoms at 119±135 ppm. These data give us grounds to assume that the upper layer consists mainly of phosphorus-containing polyether alcohols.

upper layer. This data allows for the assumption that the lower layer is comprised mainly of products 1, 2, 7 and 8. The polymer chain of the ¯exible polyether polyurethane foam is built by the following repeating units:

3.2. Study on the lower layer The 31P{H} NMR spectra of the lower layer products obtained after degradation for 1 (Fig 5a) and 3 h (Fig. 5b) have the same signals as the upper layer di€ering only in their intensities. The measured integral intensities show that the highest content in the product with a phosphorus atoms have a chemical shift at 7.83, 4.67 and 32.18 ppm, while the other signals have lower intensities. The 1H NMR spectrum of the lower layer shows signals for three types of P-H protons: at 6.67 ppm with 1J(P,H)=657.6 Hz for P-H protons of product 1; 6.68 ppm with 1 J(P,H)=698.1 Hz for P-H protons of dimethyl phosphonate and at 6.70 ppm with 1J(P,H)=668.3 Hz for P-H protons of product 2. It should be noted that in the 13C NMR spectrum (Fig. 4b) there are signals at 153.2 ppm for NHC(O) carbon atoms and at 110±146 ppm for aromatic carbon atoms with considerably higher intensity when compared to the signals for the same carbon atoms in the

The data from the NMR studies performed on the products from the chemical degradation of ¯exible polyether polyurethane foam with dimethyl phosphonate gives us grounds to suggest that product 1 was formed as a result of alkylation of urethane or urea groups (water is used as blowing agent) by dimethyl phosphonate [1]. When the carbon atom of the methoxy group of product 1 participates in the same reaction with urea or urethane groups, the resulting product is 2. The formation of the product 2 was proven by the change in the intensities of the signals for the phosphorus atoms at 7.83 and 5.09 ppm depending on the duration of the degradation. The intensity of the signal at 7.83 ppm (product 1) for 1 h of degradation lowers from 49.8 to 42.3% when the degradation lasts 3 h. Meanwhile the intensity of the signal at 5.09 ppm increases from 4.8 to 9.8%. It is known [3±5] that the

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K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

Fig. 4. 13C NMR spectrum of ¯exible polyether polyurethane foam degraded by dimethyl phosphonateÐ(a)Ðupper layer; (b) lower layer; (duration of degradation 3 h).

K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

403

Fig. 5. 31P{H} NMR spectrum of ¯exible polyether polyurethane foam degraded by dimethyl phosphonateÐlower layers obtained after degradation: a 1 h; b 3 h.

404

K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

nitrogen atom being a nucleophilic center prefers to attack the weaker electrophilic center in the molecule of dialkyl esters of phosphonic acid, namely the a-carbon atom of alkoxy group. Products 3±6 are formed as a result from the exchange reactions with the participation of dimethyl phosphonate ester groups and the urethane groups [1].

Products 7 and 8 probably result from the radical processes in which the phosphonate radicals take part (Scheme 1). Dimethylmethyl phosphonate, product 9, forms as a result of thermal degradation of dimethyl phosphonate [6]. Running the above processes while heating the ¯exible polyether polyurethane foam with dimethyl phosphonate

Scheme 1.

Fig. 6. Viscosity vs. degradation time for ¯exible polyether polyurethane foam degradation by dimethyl phosphonate Ð upper layer.

Fig. 7. Viscosity v. degradation time for ¯exible polyether polyurethane foam degradation by dimethyl phosphonate Ð lower layer.

K. Troev et al. / Polymer Degradation and Stability 67 (2000) 397±405

leads to a decrease in molecular weight of polyurethane. The determined viscosities of the upper (Fig. 6) and lower layers (Fig. 7) con®rmed the decrease in the molecular weight of polyurethane when treated with dimethyl phosphonate. Phosphorus content (Table 1) in the lower layer is considerably higher if compared to that in the upper layer. The di€erence might be explained by the structure of the products formed during the degradation of the phosphorus-containing products. The lower layer is built up chie¯y of phosphorus-containing aromatic compounds Ð products 1, 2 (Table 2) which form via alkylation reaction. It was established that during the degradation of ¯exible polyether polyurethane foam with dimethyl phosphonate an increase in the amount of solid product formed can be correlated to the duration of the degradation (Table 1). 31P NMR studies of the solid product showed that it did not contain phosphorus. The results obtained show that the degradation of ¯exible polyether polyurethane foam with dimethyl

405

phosphonate results in phosphorus-containing oligomers. 4. Conclusions It has been shown that dimethyl phosphonate can be used as a degradation agent for ¯exible polyether polyurethane foam. The degradation proceeds at 160 C. The products obtained are phosphorus-containing oligomers. References [1] Troev K, Atanassov Vl, Tsevi R, Grancharov G, Tsekova A. Polym Degrad Stab, 2000;67:159. [2] Troev K. Heteroatom Chem 1993;8:165. [3] Thoung N. Bull Chem Soc France 1983;3:928. [4] Troev K, Borisov G. Phosphorus and Sulfur 1987;29:129. [5] Troev K, Tashev E, Borisov G. Phosphorus and Sulfur 1982;13:359. [6] Beach L, Drogin R, Schewmaker J. Prod Research and Dev 1963;2:145.