Synthesis and properties of new polymerisable antioxidants

PII: S0141-3910(98)00013-5

Polymer Degradation and Stability 62 (1998) 315±322 # 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/$Ðsee front matter

Synthesis and properties of new polymerisable antioxidants Jiang-Qing Pan, N. C. Liu & Wayne W. Y. Lau* Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent. Singapore 119260 (Received 17 November 1997; accepted 30 December 1997) Five new monomeric antioxidants containing hindered phenol have been synthesised by direct addition of 2,6-di-tert-butyl-4-hydroxy methyl phenol (DBHMP) to m-isopropenyl-,-dimethyl isocyanate (TMI) or allyl isocyanate (AI), and by controlled isocyanation in two steps, in which methacrylic acid (MAA) or acrylic acid (AA) reacts with toluene-2,4-diisocyanate (TDI) and hexamethylene diisocyanate (HMDI) in 1 to 1 mole ratio at 60 C, and this product as an intermediate was directly added to DBHMP in 1 to 1 mole ratio in presence of a catalyst dibutyltin dilaurate (DBTDL). These new monomeric antioxidants are 3,3di-tert-butyl-4-hydroxybenzyl, m-isopropenyl-,-dimethyl benzyl carbamate (A), 3,5-di-tert-4-hydroxy bezyl, allyl carbamate (B), 3,5-di-tert-butyl-4-hydroxy benzyl, (4-methacryloyl amino) tolyl carbamate-2 (C), 3,5-di-tert-butyl-4-hydroxy benzyl, (6-methacryloyl amino) hexyl carbamate-1 (D), and 3,5-di-tert-butyl4-hydroxy benzyl, (6-acryloyl amino) hexyl carbamate-1 (E). They were characterized by IR, NMR and elemental analyses. Their stabilizing action for protecting polypropylene against thermal oxidation and their polymerizability were examined. Results show that these new monomeric antioxidants possess stabilising e€ect against thermal oxidation and they are capable of copolymerizing with vinyl monomers. # 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

prepared by acrylation of aromatic amine with chlorides of , -unsaturated acids.2 In this paper, we report the synthesis and properties of ®ve new monomeric antioxidants prepared by direct addition reaction or controlled isocyanation of DBHMP.

Hindered phenol antioxidants are widely used e€ective primary antioxidants. They will displace aromatic amine antioxidants which are toxic and can cause discoloration.1±10 Antioxidants with low molecular weight (MW) are less e€ective owing to their poor thermal stability and low extraction resistance. One current trend for antioxidant development is to prepare antioxidants of higher MW for enhanced thermal stability and higher extraction resistance. Thus polymeric antioxidants have become attractive. Copolymerization, homopolymerization and grafting of functional monomers containing hindered phenol are the usual methods for preparing polymeric antioxidants. Preparation of monomers containing hindered phenol is a very important ®rst step for preparing polymeric antioxidants. Several monomeric antioxidants have been reported,2,7 many of them were

2 EXPERIMENTAL 2.1 Materials Toluene-2,4-disocyanate (TDI), hexamethylene diisocyanate (HMDI), methacrylic acid (MAA), and acylic acid (AA) were supplied by Aldrich Chemical Co. Inc. 2,6-di-tert-butyl-4-hydroxy methyl phenol (DBHMP) was supplied by Tokyo Chemical Industry Co. Japan; m-isopropenyl-, -dimethyl-benzyl-isocyanate (TMI) was supplied by American Cyanamid Co., and was distilled before use. Dibutyltin dilaurate (DBTDL), a catalyst, was supplied by Fluka Co., DBTDL was in form of a 80% solution in benzene.

*To whom correspondence should be addressed. Fax: 657791336; e-mail: [email protected] 315

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2.2 Synthesis 2.2.1 Direct isocyanation of DBHMP with isocyanates 2.2.1.1 3,5-di-tert-butyl-4-hydroxy benzyl, m-isopropenyl-a,a-dimethyl bezyl carbamate (A). To a three-neck ¯ask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer, was added 4.720 g (0.020 mol) DBHMP and 15 ml benzene at room temperature (22 C), followed by addition of 4.020 g. (0.020 mol) TMI under stirring. The liquid system became cloudy. The exothermic reaction started when 0.01 ml catalyst DBTDL was added. The reaction liquid turned to a bright yellow solution. The reaction process was monitored by IR to determine the consumption of isocyanate at 2255 cmÿ1. The reaction took 6 h to complete. Benzene was removed under vacuum. After precipitating and washing with pentane (350 ml), ®ltering and drying, a white powder product (A) was obtained with 97.30% yield. 2.2.1.2 3.5-di-tert-buty hydroxy benzyl ally carbamate (B). Into a three-neck ¯ask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer was placed 4.720 g (0.020 mol) DBHMP and 10 ml dry benzene with stirring under nitrogen at room temperature (22 C), followed by addition of 1.660 g (0.020 mol) AI. The liquid system turned cloudy. The reaction started when 0.01 ml catalyst DBTDL was added. Reaction was strongly exothermic and the reaction liquid turned bright yellow. The reaction was monitored using IR to determine the consumption of isocyanate at 2255 cmÿ1. The reaction took 2 h to complete. After removal of benzene, precipitating and washing with pentane (350 ml), ®ltering and drying, a white powder product (B) was obtained with 94.80% yield. 2.2.2 Controlled isocyanation It is well known that isocyanates can easily react with compounds containing active protons

including alcohol, amine, acid and water.11±14 When , -unsaturated acids such as MAA or AA reacts with diisocyanates (TDI, HMDI) in 1 to 1 mole ratio, the isocyanate group at the para-position is converted to carry the C=C bond and so an intermediate is formed which carries an isocyanate group and C=C bond. New monomeric antioxidants can then be obtained through further isocyanation of this intermediate with DBHMP in 1 to 1 mole ratio. We have applied this idea to prepare three new monomeric antioxidants. Typical preparation of new monomeric antioxidant (C) using this scheme is described below. 2.2.2.1 3,5-di-tert-butyl hydroxy benzyl (4-methacryloyl amino) tolyl carbamate-2 (C). Step 1. To a three-neck ¯ask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer, was added 3.480 g (0.020 mol) TDI and 5 ml benzene at 60 C under stirring; then 1.720 g (0.020 mol) MAA was added dropwise. The reaction was monitored using IR to determine the consumption of carboxyl group in MAA at 1700 and 2650 cmÿ1. After 3 h the carboxyl group (1700, 2650 cmÿ1) in MAA disappeared. A white precipitate as an intermediate was formed, which was ®ltered and washed with pentane (350 ml). Step 2. This intermediate was added to 4.720 g of DBHMP (0.020 mol) and 8 ml benzene in another similar reactor at 60 C. The contents were well mixed under strong stirring. The controlled isocyanation of DBHMP with the intermediate began when 0.01 ml catalyst of DBTDL was added. Reaction was monitored using IR to determine the consumption of isocyanate in the intermediate at 2255 cmÿ1. After 48 h, a white precipitate was formed, which was ®ltered, washed with pentane (350 ml) and dried to give a white powder product (C), with 90.45% yield. Preparations of the other new monomeric antioxidants (D, E) followed a similar procedure described above, and are summarised in Table 1.

Table 1. Preparation of new monomeric antioxidants by controlled isocyanation Step 1a

C D E

Acid (g)

Isocyanate (g)

MAA 1.720 MAA 1.720 AA 1.447

TDI 3.480 HMDI 3.360 HMDI 3.366

At 60 C, benzene 5 ml. Catalyst used, 0.01 ml DLTDL.

a b

Step 2b

Yield (%)

Time (h)

DBHMP (g)

Benzene

Time (h)

3.0

4.720

8 ml

60 C/48 h

90.45

2.5

4.720

6 ml

60 C/18 h

90.40

2.0

4.720

6 ml

80 C/7 h

88.00

New polymerisable antioxidants

2.3 Analysis IR spectra were recorded using a Shimadzu FT-IR 8001 Fourier Transform Infra-Red Spectrophotometer. C. H. N. analytical values were obtained using a Perkin±Elmer 2400 C. H. N. Elemental Analyser. Proton-NMR spectra were recorded using a FX-90 FT-NMR, DCCl3 as solvent and TMS as an internal standard. TGA were performed using a Du-Punt 9900 Thermal Analyser, at 20 C/min and 75 ml/min nitrogen ¯ow.

317

From Fig. 1, it can be seen that the intermediateI (Fig. 1(c)) containing an anhydride (1780 cmÿ1) and a carbamate group ( 1725, 3300 cmÿ1) was formed with consumption of the carboxyl group (1700, 2650 cmÿ1) in MAA. This intermediate-I is unstable, as shown in Fig. 1(c) and (d), and it converts to methacrylic amide containing isocyanate:

2.4 Thermal oxidation Preparation of polypropylene (PP) samples containing these new monomeric antioxidants followed a standard procedure reported elsewhere.15±17 Thermal oxidation of PP samples was carried out in an oven at 140 C in air and was monitored using a Shimadzu FT-IR 8001 Fourier Transform Infra Red Spectrophotometer at 1710 cmÿ1. Induction period was determined from plot of carbonyl absorption versus ageing time.15±17 3 RESULTS AND DISCUSSION

Experimental results show that the reactions between isocyanates and MAA or AA are stoichiometric, in agreement with literature.11±14 No unreacted MAA or isocyanate was found in the ®ltrate, and the intermediate, as a ®lter cake (over 90% yield), contains methacrylic amide (1650 cmÿ1) and isocyanate group (2255 cmÿ1), indicating that only one of the two isocyanate groups in the TDI or HMDI molecule took part in the addition reaction with MAA or AA at a 1 to 1 mole ratio. Thus, the unreacted isocyanate group in the intermediate can be used for further isocyanation with DBHMP. Structural analyses

3.1 Synthesis Isocyanates can readily react with simple primary alcohols in stoichiometric quantities.11±14 Our experimental results show that reactions of isocyanates with DBHMP are stoichiometric. Without catalyst the reaction was slow at room temperature, but faster and exothermic in presence of DBTDL at room temperature. The reactions between isocyanates and DBHMP are faster than that of isocyanates with 2,2,6,6-tetramethyl-4piperidinol,11±14,18±20 The latter reaction in fact is very slow at room temperature even in the presence of catalyst DBTDL.18±20 Results of structural analysis show that isocyanation of DBHMP always occurs ®rst at the 4-hydroxy methyl group, indicating that the reactivity of alcohol (4-hydroxy methyl in DBHMP) is higher than that of the phenol group in DBHMP.11±14 The use of a re¯ux condenser was necessary to avoid solvent loss through evaporation during the exothermic reaction. DBTDL is a wax-like substance at room temperature; it is therefore more convenient to add DBTDL in form of a solution in benzene. Experimental results also show that the reactions between isocyanates and MAA or AA are direct addition reactions, shown in Fig. 1.

Fig. 1. IR spectra of reaction products of MAA with TDI.

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con®rmed that the other new monomeric antioxidants, D and E, were also successfully prepared using this two-step isocyanation. 3.2 Structural analysis of new monomeric antioxidants Typical results of IR spectra of the antioxidants are shown in Figs 2 and 3 from which it can be seen that the isocyanate absorption in TMI or AI at 2255 cmÿ1 had disappeared after reaction at room temperature for 6 h, and a new carbamate group (absorption at 1720±1695, 3300 cmÿ1) was formed. This indicates that the following reactions have occurred:

at 2.14 ppm). The Ar±CH2±OH in DBHMP (CH2 4.54, 4.60 ppm, two peaks, ±OH 1.73 ppm, three peaks) had been converted to carbamate Ar±CH2± O±C(=O)±NH± (±CH2± 4.93 ppm. single peak, ±NH±, 5.06 ppm). These NMR results further support the conclusions drawn from IR results. From corresponding NMR spectra of product B, it was observed that B contains the new carbamate group (NH 4.74 ppm, CH2±Ar 5.02 ppm), hindered phenol (Ar-OH 5.26 ppm, two butyl 1.44 ppm, benzene ring 7.18 ppm), and a double bond (from AI, 5.06, 5.15 and 5.71 ppm). These NMR results also support the conclusions drawn from IR results. Typical IR spectra of product C prepared by this two-step controlled isocyanation are shown in Fig. 4. From Fig. 4, it can be seen that product C contains phenol (3650 cmÿ1) from DBHMP and a new carbamate group (1710, 3300 cmÿ1). Based on information from Figs 1 and 4, one can conclude that the following reactions have occurred:

From corresponding NMR spectra of product A, it was observed that A contains the new carbamate group (NH 5.06, ±CH2±Ar 4.93 ppm), hindered phenol (from DBHMP Ar±OH 5.21 ppm, two tert-butyl 1.44 ppm, benzene ring 7.14 ppm), a double bond (from TMI 5.31, 5.18 ppm and methyl

Fig. 2. IR spectrum of product A produced from TMI and DBHMP.

Fig. 3. IR spectrum of product B produced from AI and DBHMP.

New polymerisable antioxidants

From corresponding NMR spectra of product C, it can be seen that C contains a double bond (CH3 1.60±1.80 ppm. two proton 6.45, 6.67 ppm), hindered phenol (benzene ring, 7.21; Ar±OH, 5.27; two butyl, 1.45 ppm; Ar±CH2±O 5.09 ppm) and a tolyl group (CH3 2.18 ppm, benzene ring 7.26 ppm). These NMR results further support the conclusions drawn from IR results. The main features in the IR, NMR spectra of these new monomeric antioxidants C±E) and elemental analyses of A±E are summarized in Tables 2 and 3. Based on information from Tables 2 and 3, the following reactions can be written:

Fig. 4. IR spectrum of product C produced by controlled isocyanation of TDI with MAA and DBHMP.

319

3.3 Properties The properties of these new monomeric antioxidants are shown in Tables 4 and 5. It is interesting to observe that monomeric antioxidant B produced from AI and DBHMP is unstable even at room temperature. We observed that this monomeric antioxidant was initially a needle-like crystal, but after one week it turned to a yellowish powder, and then turned to a brown powder in 26 days. Carbon dioxide was released during this conversion process. When product-B was stored in nitrogen or hexane, its decomposition was retarded. The changes in IR spectra of monomeric antioxidant B with storage time at room temperature are shown in Fig. 5, from which it can be seen that the carbamate in product B (1695, 3300 cmÿ1) had disappeared after 1 week. From corresponding NMR spectra of product B, it was observed that the decompositon product of B (after 26 days) contained double bond (5.17, 5.30 and 5.80 ppm, 3H), phenol (benzene 7.16, Ar±OH 5.05, and butyl 1.43 ppm), and new peaks at 3.49 ppm (N±CH2± Ar, single peak, 2H) and at 3.01, 3.08 ppm (double peaks, =CH±CH=N±, 1H). Therefore, the following decomposing reaction had occurred:

The other monomeric antioxidants (A, C±E) prepared in this work and the monomeric light stabilizers prepared from AI and hindered piperidinol do not undergo this change, some of them having been stored at room temperature for 5 years.18±20 The decomposition mechanism of

320

J.-Q. Pan et al. Table 2. Characterization of new monomeric antioxidants (C±E) IR change (cmÿ1)

Reaction

C D E

NMR (ppm)

New peaks in product

Acid and isocyanate in reactants

Phenol

Carbamate

C=C

1710 3300 3650 1710 3300 3650 1710 3300 3650

1700, 2650 and 2255 (disappeared)

7.21 5.27 1.45 7.18 5.90 1.42 7.14 5.04 1.42

4.104.60

1.68 6.45 6.67 1.80 5.70 5.83 6.22 5.60 5.65

MAA+ TDI+ DBHMP MAA+ HMDI+ DBHMP AA+ HMDI+ DBHMP

1700, 2650 and 2255 (disappeared) 1700, 2650 and 2255 (disappeared)

product B needs further study, we think a combined e€ect of degradation chain transfer of the allyl group and oxidation of the hindered phenol in product B plays a very important role in the decomposition.21,22 3.4 Stabilizing action of new monomeric antioxidants for protecting PP against thermal oxidation

Table 3. Elemental analysis

A B C D E

3.403.70

2.18 3.20-3.40 3.20±3.40

result of their low thermal stability related to their low MW and the low content of hydroxyl group in phenol. Therefore, these monomeric antioxidants are not suitable for application as antioxidant but they can be used for preparing polymeric antioxidants and polymer bound antioxidants.2,23 3.5 Polymerizability

The changes in carbonyl absorption at 1710 cmÿ1 versus heating time and thermal oxidation induction (IP) of PP-®lms containing 0.10% of the new monomeric antioxidants are shown in Fig. 6 and Table 6, which show that these new anti-oxidants performed better than a common commercial antioxidant BHT (MW=220) and a precursor hindered phenol DBHMP (MW=236). Their lower e€ectiveness is associated with their lower thermal stability (BHT, 117 C and DBHMP, 140 C) and their low MW. The e€ectiveness of these monomeric antioxidants (A-E) is still low, which is a

Found (%)

3.503.90

Other

Calc. (%)

C

H

N

C

H

N

76.44 71.43 70.19 69.92 69.40

9.71 10.39 8.24 9.84 9.32

4.08 4.70 5.26 6.72 6.48

76.89 71.44 71.65 69.51 68.17

8.92 9.09 8.02 10.48 11.00

3.20 4.39 6.19 6.90 6.91

Vinyl monomers of -methyl styrene moiety and those having an allylic structure are hard to homopolymerize by free radical initiation due to steric hindrance by the methyl group24±26 and degradation chain transfer (autoinhibition) of Table 5. Solubilitiesa of new monomeric antioxidants Solvent C5H12 C6H14 Cyclo-hexane Toluene Ether acetate THF Benzene HCCl3 Acetone Propanol Ethanol Methanol Water

b

A

B

C

D

E

7.0 7.4 8.2 8.9 9.1 9.1 9.2 9.3 9.9 11.9 12.7 14.5 23.4

x x hs s s s es es es s s s x

x x hs es es es es es es hs hs hs x

x x hs es es es es es es es s s x

x x x es es es es es es es s s x

x x x es es es es es es es s s x

a s, soluble; x, does not dissolve; hs, hard to dissolve; es, dissolve easily. b , solubility parameter of organic solvent, (cal/ml3)1/2.

Table 4. Properties of new monomeric antioxidants Monomer MP ( C) Thermal stability ( C) Appearance

A

B

82 180 180 Paste Crystal

C

D

E

BHT

DBHMP

128±129 180 White powder

95±97 200 Brown powder

67±69 193 White powder

69-70 117 Crystal

140 140 Yellow powder

New polymerisable antioxidants

Fig. 5. Change in IR spectrum showing change in product B with storage time at room temperature.

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allylic monomer free radicals.21,22 Our results show that monomeric A and B do not homopolymerize, but can be copolymerized with vinyl monomers such as styrene and MMA, which is consistent with ®ndings reported in the literatures.5,6,27,28 On the other hand, the monomers with methacrylate and acrylate moieties are easily homopolymerized and copolymerized by free radical initiation. Our results show that the new monomeric antioxidants (C±E) which contained , -unsaturated amide and hindered phenol moieties can be copolymerized with vinyl monomers such as styrene, but the rate of copolymerization was slower than that with MAA and acrylic acid. It was observed that monomeric antioxidants (C±E) can also be homopolymerized by free radical initiation. However the rate of homopolymerization was much slower than that the rate of copolymerization with styrene. Obviously, the presence of hindered phenol, which is as an e€ective inhibitor in these monomers, can be expected to a€ect this homopolymerization.29

Fig. 6. Stabilizing e€ectiveness of new monomeric antioxidants for protecting PP-®lm against thermal oxidation at 140 C in air. O± blank (PP-®lm) and A±E are PP-®lm samples each containing 0.1% of one of the new monomeric antioxidants A, B, C, D and E, respectively; sample BHT is a PP-®lm containing 0.1% of antioxidant BHT and sample DBHMP is a PP-®lm containing 0.1% of DBHMP.

Table 6. Thermal oxidation induction periods (IP) of PP-®lm containing 0.1% of the new antioxidants by weight Sample

Blank

A

B

C

D

E

BHT

DBHMP

IP(h) MW OH%a

1.0

18.15 437 3.89

23.2 319 5.33

4.75 452 3.76

14.1 446 3.82

6.85 437 3.94

1.55 220 7.73

1.2 236 7.20

a

OH% is the hydroxyl content in the phenol group of antioxidant.

322

J.-Q. Pan et al.

4 CONCLUSIONS 1. Five new monomeric antioxidants containing hindered phenol were synthesised by direct addition of DBHMP to TMI or AI and by controlled isocyanation of diisocyanate with MAA or AA and DBHMP. 2. These monomeric antioxidants possess some stabilizing e€ect in protecting polypropylene against thermal oxidation. 3. These monomers can be copolymerized with vinyl monomer by free radical initiation for the preparation of polymeric antioxidants. 4. Monomeric B produced from AI and DBHMP is unstable at room temperature.

ACKNOWLEDGEMENT This work was supported by a National University of Singapore grant, RP 960694.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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