Synthesis and properties of new UV-absorbers with higher MW

Polymer Degradation and Stability 53 (19%) 153-159 @ 1996 Elsevier Science Limited ELSEVIER

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Synthesis and properties of new UV-absorbers with higher MW Jiang-Qing

Pan,“? Wayne W. Y. Lau,b* Z. F. Zhang” & X. Z. Hu”

“Institute of Chemistry, Academia Sinica, Beijing 100080, China bDepartrnent of Chemical Engineering, National University of Singapore, 10 Kent Ridge Cresent, Singapore 0511, Singapore

(Received

11 October 1995; accepted 3 January 1996)

Four new UV-absorbers with higher MW, 2-hydroxy-4-benzophenonyl noctadecyl carbamate (A, MW = 509), 2-hydroxy-4-benzophenonyl hexamethylene dicarbamate (B, MW = 596), 2-hydoxy-4-benzophenonyl toluene 2,4 dicarbamate (C, MW = 602), and 2-hydroxy-4-benzophenonyl methylene diphenylene dicarbamate (D, MW = 678). have been synthesized by direct addition reaction of 2,4-dihydroxy benzophenone (DHBP) and isocyanates. The structures of these UV-absorbers have been characterized by IR, NMR, UV, MS and elemental analysis. The photostabilizing effectiveness of these new UV-absorbers for polypropylene (PP) has been determined. Results show that increasing the MW of the absorbers can decrease the loss of absorber from PP film during heat treatment but not in prolonged UV-irradiation. These four UV-absorbers are less efficient owing to their poor compatibility with PP. 0 1996 Elsevier Science Limited

octadecyl isocyanate (01), hexamethylene diisocyanate (HMDI), toluene 2,4-diisocyanate (TDI) and methylene diphenylene diisocyanate (MDI) were supplied by TCI (Tokyo Chemical Industry Company, Japan), dibutyltin dilaurate (DBTDL), a catalyst, was from Fluka AG, prepared as an 80% solution of DBTDL in benzene.

1 INTRODUCTION It is well known that UV-absorbers based on orrho-hydroxy benzophenone are an old, but still widely used, cheap, effective light stabilizer.‘” They protect polymers against photooxidation by a mechanism of screening by rapid tautomerism.‘-’ The protecting effectiveness of a UV-absorber would be increased by maintaining a high enough concentration in a polymer substrate.4 Loss of a UV-absorber from a polymer substrate through migration and evaporation can be reduced for UV-absorbers of higher MW.4 Synthesis of UV-absorbers of higher MW is, therefore, an effective way to increase their effectiveness. In this paper, the synthesis and properties of four new UV-absorbers with higher MW are reported.

2.2 Synthesis 2.2.1 2-Hydroxy-4-benzophenonyl, carbamate

In a three-neck flask equipped with a condenser, a nitrogen inlet, an addition funnel and a magnetic stirrer was placed 5.0 g (0.02 mol) 2,4-dihydroxy benzophenone and 15 ml dry benzene with stirring under nitrogen. The flask was heated with an oil bath to 70°C and then 6.9g (0.02mol) 01 in 5 ml benzene was added dropwise; then 0.1 ml 80% solution of DBTDL in benzene was added under stirring. After about 20min, the oil bath temperature was raised to and maintained at 80°C for 6 h, an orange solution was obtained. Upon removal of the solvent (benzene), a fine white powder was obtained. The crude product was purified by

2 EXPERIMENTAL 2.1 Materials 2,4-Dihydroxy benzophenone plied by Sigma Chemical

(DHBP) Company

n-octadecyl

was supInc., n-

* Author to whom correspondence should be addressed. t Research Scientist at the National University of Singapore, Singapore. 153

154

J.-Q. Pan,

extraction using pentane (3 X 50 ml) and methanol (3 X 50 ml). After drying, a fine white powder product (A) was obtained with 86.84% yield. 2.2.2 2-Hydroxy-4-benzophenonyl, hexamethylene dicarbamate 7.65 g (0.036 mol) DHBP and 40 ml dry benzene

were added in a reaction flask. With stirring under nitrogen, the flask was heated with an oil bath to 7O”C, 3.Og (0.018mol) HMDI was then added dropwise. After about 20min, 0.1 ml DBTDL in benzene was added with stirring under nitrogen. The oil bath temperature was raised to and maintained at 80°C for 6 h. A pasty substance was obtained. 50ml benzene was added to the paste. Upon cooling precipitation occurred. Upon reprecipitation, a fine white powder product (B) was obtained with 96% yield. 2.2.3 2-Hydroxy-4-benzophenonyl

toluene 2,4-

dicarbamate 7.4g DHBP

(0.034mol) and 40 ml dry benzene were placed in a reaction flask, then 3.Og (0.017 mol) TDI was added with stirring under nitrogen, followed by addition of 0.1 ml DBTDL. After about 20min, the oil bath temperature was raised to and maintained at 80°C for 6 h, a yellow paste substance was obtained. This crude product was purified by extraction using acetone and precipitation. After filtering and drying, a yellowish powder product (C) was obtained with 63% yield. 2.2.4 2-Hydroxy-4-benzophenonyl

methylene diphenylene dicarbamate 5.14 g DHBP (0.02 mol) and 25 ml dry benzene

were placed in a reaction flask, then 3.Og (0.012mol) MD1 in 5 ml benzene was added. With stirring under nitrogen 0.1 ml DBTDL was added. The oil bath temperature was raised and maintained at 80°C for 6 h. A yellowish precipitate was observed. It was washed with benzene, and this crude product was purified by acetone extraction (3 X 50 ml). After drying, a yellowish powder, product (D), was obtained with 80% yield. 2.3 Characterization of products IR spectra were recorded by a Shimadzu FT-IR 8101 Fourier Transform Infrared Spectrophotometer. ‘H-NMR spectra were obtained using a

et al. Jeol FX-900 FTNMR, DCCl, as solvent, TMS as an internal standard. MS spectra were obtained using a MICROMASS 7035E, UV-spectra of products were obtained using a Unicam-1750 UV-spectrophotometer, ethanol as solvent. UVspectra of PP film were obtained using air as a reference. 2.4 Photo-oxidation 110 p thick test PP samples containing UVabsorbers were placed in a weather-o-meter, in which two 1000 W high pressure mercury lamps were used as UV-light source.’ A Pyrex glass filter was used to cut off light of wavelength shorter than 2900 A. Test temperature was set at 45”C, photooxidation of samples was monitored by using a Perkin-Elmer 782 IR Spectrophotometer at wavenumber of 1710cm-‘. Induction period was determined from a plot of carbonyl absorption vs irradiation time.’

3 RESULTS AND DISCUSSION 3.1 Synthesis It has been reported’@‘3 that isocyanates can readily react with simple primary alcohol in stoichiometric quantities. Our results show that the reaction of isocyanate with 2,4-dihydroxy benzophenone was also stoichiometric. It was a slow reaction without catalyst at 80°C and a mild reaction in the presence of DBTDL which gave a higher yield. This is associated with the hydroxy in DHBP connected to the benzene ring. It is known that the reactivity of aromatic hydroxy to isocyanate is much less than that of aliphatic hydroxy.‘” Results of IR, NMR and elemental analysis show that pure products can be obtained using these experimental conditions. The catalyst DBTBL is a wax-like substance at it is therefore most conroom temperature; venient to add DBTDL as a 80% solution of DBTDL in benzene in this synthesis. 3.2 Product structure characterization IR spectra of the product A are shown in Fig. 1. It can be seen that after reacting at 80°C for 6 h

New W-absorbers

with higher MW

155

1640 cm-‘, -OH, 34OOcm-l, benzene ring, 3040 cm--1).14~15Based on IR results and consideration that reactivity of 4-hydroxy in DHBD with isocyanate is much higher than that of 2-hydroxy due to association with hydroxy-bond and steric effect,l’ one can conclude that the following reaction had taken place under these experimental conditions: Cl$+Ctl,+CH,-N=C=0

+

NV HO&*

101) cy+cH,~cy-~-r;-o IJ:- CiBP Iproduct-A)

. .

HO-

/

I

2000

I

I I

I

I

I

1000

1500

I

The ‘H-NMR spectra of product A are shown in Fig. 2. It can be seen that product A contained a new carbamate group (3.38 N-CH2-, 5.02 NH), benzene ring from benzophenone (7.5-7.66, 6.7-0.88), methyl (0.937) and methylene (1.25) both from OI.15,16Thus these results from NMR further support this conclusion. Results of IR and NMR of products B, C, and D are shown in Table 1. After reaction of the isocyanates (HMDI, TDI, MDI) with DHBP, the isocyanate absorption (2260 cm-‘) disappeared, new carbamate had formed. Therefore, similar to

I

100

Wave number,cm-’ Fig. 1. IR spectrum of product A.

the isocyanate in 01 (2260cm-‘) had disappeared. A new carbamate group (1740cm-l, 33OOcm-‘) had formed and the product contained 0-hydroxy benzophenone (carbonyl,

If)

PPM

8

fel

7

Id1

6

5

lb) la1

Id

I

3

2

Fig. 2. ‘H-NMR spectrum of product A.

1

0

156

J.-Q. Pun, et al. Table 1. Characterization IR

Product Carbamate

-_-

of products

A

‘H-NMR

Benzophenone

Carbamate

Benzophenone

3.38 5.02 CH,-O.937

7.5-7.66 6.7-6.86

A

1740 2925

1600 1640 3300 3040

B

1730

1600 1640 3300 3040

1.48 3.27 5.1

6.7-6.9 7.5-1.66

C

1730 1760

1600 1640 3300 3040

2.31 1.25

6.1-6.9 7.5-7.66

D

1740

1600 1640 3300 3040

3.95 7.26

6.7-6.9 1.5-1.67

product A, the following reactions occurred:

should have

350

ton

150

500

(nm) Fig. 3. UV-spectra

of reaction products others (in ethanol).

A, B, C, D and

A HMO1 -0HBP (product-81

N=C=O (OHBPI

ml

(iOl-DHtlPj (poduct-c1 MO

O=C=N-@CY++N=C=O

IMOII

+ 2 Ho-@-a*

I

UIHEPJ

UV-spectra of the products are shown in Figs 3 and 4 together with spectra of DHBP and UV-531, a commercial UV-absorber of the following structure: I

250

I

I

300

350

1

100

1

150

I 500

(t-d Fig.

4. UV-spectra

of reaction products others (in PP film).

A, B, C, D and

157

New W-absorbers with higher MW Table 2. CHN content of products Product

Calc. (%)

Found (%) C

H

N

C

H

N

A

75.97

9.57

2.39

B C D

68.15 69.18 69.78

5.81 3.92 4.40

5.10 5.11 4.24

75.44 68.46 69.77 72.57

9.23 5.37 4.32 4.42

2.75 4.70 4.65 4.13

The results of MS show that the molecular weights of products are 509 (product A), 596 (product B), 602 (product C) and 678 (product D).

From Figs 3 and 4, it can be seen that products

A, B, C, D and their precursor DHBP, as well as

Am5

UV-531, possess similar UV-spectra to orthohydroxy benzophenone compounds.1,2,7*sJ7J8 Products A, B, C and D had inherited the structural from unit of ortho-hydroxy benzophenone DHBP through these synthesis reactions. These UV-spectra are consistent with the conclusions drawn based on IR and NMR analyses. Results of elemental analysis are shown in Table 2, which shows close agreement between measured and calculated values. 3.3 Product properties Products A and B are in the form of a fine white powder, while products C and D are yellowish powders. Their mp’s are: 79-80 (product A), 163-164 (product B), 171-174 (product C) and 295°C (product D). They are difficult to dissolve in water, alcohol and common organic solvents. This may be associated with the higher polarity of these new products owing to the presence of carbamate groups. UV-spectra show that these new products strongly absorb UV-light up to 390nm. Obviously, the UV-absorptions at 280-290 and 325-330nm are associated with the structural group ortho-hydroxybenzonphenone.1*2,7~sJ7 This structure can effectively protect polymers against photooxidation by a mechanism of screening action and rapid tautomerism.‘” 3.4 Photostabiliig

effectiveness

Photostabilizing effectiveness of these four new products for polypropylene is shown in Fig. 5 and Table 3, it can be seen that their photostabilizing

Irradiation Fig. 5. PP

time

Photostabilizing effectiveness of UV-absorbers film (C = 0.3%): a-product D, b-product c-product A, d-product C, e-DHBP.

for B,

effectiveness at 0.3% concentration for PP film was less compared to UV-531, indicating that the amount of these UV-absorbers at this concentration (0.3%) is not enough to screen off the UV-light for this thin film (110 CL).It is necessary to increase the concentration of these UVabsorbers in this thin film for better photostabilizing effectiveness. Figure 6 shows the photoprotecting effectiveness of these products for PP film at 1% concentration. It can be seen that at higher concentration their photoprotecting effectiveness was increased. It is interesting to note that the photoprotecting effectiveness of UV-531 increased significantly when its concentration was increased from 0.3 to 1%. This was associated

158

J.-Q. Pun, et al.

Table 3. Photooxidation induction period (b) of PP film containing UV-absorbers at different concentrations

0.3% 1%

Blank

A

B

C

D

UV-531

19.5 19.5

30 61

21 40

37 50

24 32

44 520

DHBP

with the good compatibility of UV-531 with PP. A solubility study showed that products A, B, C, D did not dissolve easily in common organic solvents and they are not compatible with PP, especially at 1% concentration of the product. They did not distribute evenly in PP, and they exuded to the substrate surface. In contrast, good compatibility of UV-531 with PP was observed even at concentration higher than 1%. This is associated with the presence of long alkyl groups in UV-531. Compatibility of a stabilizer with its polymer substrate is indeed a very important factor to its photostabilizing effectiveness. Figure 7 shows the loss of the UV-absorbers in PP film during UV-irradiation at 45°C. It can be seen that all of them were depleted quickly. It has been reported*%‘* that UV-absorbers based on ortho-hydroxybenzophenone can photolyze

I

.

.

.

1

.

*

50

25

Irradiation time (h) Fig.

7. Retention of UV-absorbers in PP film during UV-irradiation in air at 45°C: 9-product B, lO-product C, ll-product A, 12-product D, 13-DHBP, 28-531.

through the following reaction: POOH

2 Q-t

-

OH0 &-OR

hv

PO’+‘!lH

+

PO‘ _ .OH

PO 2 @C&JR

NV-absorber)

Photosensitizer can be formed by abstraction of a proton from the o&o-hydroxy hydrogen bond of

Irradiation Fig. 6. Photostabilizing

.

0

time (hl

effectiveness of UV-absorbers for PP film (C = 1%): l-product B, 2-product C, 3-product A, 5-531, 6-DHBP at l%, 7-DHBP at 0.4%, &PDS at 0.3%, 0-PP blank.

4-product

D,

New W-absorbers

505 0

of UV-absorbers

1.59

200

100

Heating Fig. 8. Retention

with higher MW

time (h)

in PP film during heating in air at 45°C: 9, 10, 11, 12 for products B, C, A, D respectively.

the UV-absorber in the above reaction. Photolysis of these UV-absorbers could have occurred which explains their rapid depletion. of these UVFigure 8 shows depletion absorbers in PP film during heating at 45°C. It can be seen that the depletion by heating at 45°C was much less than by UV-irradiation. This result further supports the conclusion that depletion of these UV-absorbers in UV-irradiation was associated with a photolysis reaction. It is also noted that while practically no loss of these new products was observed during heating at 45”C, UV-531 did show a small loss. Loss of DHBP was the highest. These observations can be explained based on the MW of absorbers (MW: 590 (A), 596 (B), 602 (C), 678 (D), 326 (UV-531), 214 (DHBP)). Higher MW of a UV-absorber makes it more heat resistant.

Academy of Sciences and the National University of Singapore for their support of this study.

REFERENCES 1. Rabek, J. F., Photostabilization of Polymers. Elsevier Applied Science Publishers, NY, 1990. 2. Ranby, B. & Rabek, J. F., Photodegradation, Photooxidation and Photostabilization of Polymers. John Wiley and Son Ltd., London, 1975. Hawkins, W. L., Polymer Degradation and Stabilization. Verlay, Berlin, 1984. Bailey, D. & Vogl, O., Macromol. Sci. Rev., C14-2 (1976) 267.

Motonobu

Minagawa,

Polym.

Degrad.

Stab.,

25(2)

(1989) 121.

Lau, W. W. Y. & Pan, J. Q., Polymeric light stabilizers, In Functional Polymers for Emerging Technologies, ed. Reza Arshady. Submitted to ACS Books, 1995. 7. Pan, J. Q. & Cao, C., Polym. Degrad. Stab., 37(3) (1992) 195.

4 CONCLUSION Four new UV-absorbers of higher MW based on o&o-hydroxybenzophenone were synthesized. These UV-absorbers are less effective for protecting PP against photooxidation owing to their poor compatibility with the polymer. They also deplete quite rapidly under UV-irradiation through photolysis. However their higher MW renders them more lasting in heat treatment processes.

ACKNOWLEDGEMENT The authors

would like to thank

the Chinese

8. Pan, J. Q., Zhang, W. & Cui, S., Photographic Sci. Photochem., l(l990) 14. 9. Pan, J. Q. & Lau, W. W. Y., Polym. Degrad. Stab., 44(l) (1994) 85. 10. Institute of Chemical Industry, Polyurethane Elastomer, 3rd edn. Chem. Ind. Publ., Beijing, 1991. 11. Dexter, R. W., Saxan, R. & Fiori, D. E., J. Coat. Technol., 58(l) (1986) 731.

12. Zhou, G. & Smid, J., J. Polym. Sci. Chem., 29 (1991) 1097.

13. Oerter, G., Polyurethane Handbook. Hanser Publishers, NY, 1985. 14. Bellamy, L. J., The Infrared Spectra of Complex Molecules. Chapman and Hall, NY, 1980. 15. Clerc, J. T., Pretsch, E. & Seibl, J., Structural Analysis of Organic Compounds. Elsevier Science Publishing Company, NY, 1981. 16. Abraham, J. & Loftus, P., Proton and Carbon-13 NMR Spectroscopy. John Wiley and Sons, NY, 1985. 17. Lin, S. A., Additives for Rubber and Plastics. Chem. Ind. Publ., Beijing, 1982. 18. Allen, N. S., Polym. Degrad. Stab., 7 (1984) 86.