Spectroscopic study of the molecular structure of a lignin-polymer system

Polymer Degradation and Stability 37 (1992) 125-129

Spectroscopic study of the molecular structure of a lignin-polymer system A . M . A . N a d a , ~ M . Fadly b & M . D a w y ~ a Cellulose & Paper Department, National Research Centre, h Physics Department, Faculty of Science, Helwan University, c Physical Chemistry Department, National Research Centre, Cairo, Egypt (Received 15 May 1991; accepted 30 May 1991)

The molecular structure of phenol-lignin formaldehyde resin was studied by means of infrared and electronic spectra. The effect of the phenol : lignin ratio, temperature and the adduct (phenol-lignin):formaldehyde (PL:F) ratio on the absorption spectra of the polymer produced were investigated. The new band which appeared at 2850 cm-~ in the infrared spectra of P L - F resins was assigned to the methoxyl group. More information about the molecular structure was obtained from a qualitative study of the frequency and intensity changes of the absorption bands.

INTRODUCTION

resins produced are lignin concentration, temperature and the phenol-lignin:formaldehyde ratio. Differential scanning calorimetry has shown great promise in the characterization of thermoset resin cure. 8 Thus, the evaluation of the structure-property relationships in lignin based polymeric materials becomes a technical possibility. 9 In this present work, infrared (IR) and ultraviolet (UV) spectroscopic studies are used to obtain information about the molecular structure and reactions of the phenol-lignin formaldehyde resin.

Phenol-formaldehyde resins are widely applied as insulating materials, foundry resins and wood adhesives. Acid (novalac) or base (resol) catalysed phenolic resins are the preferred adhesives in plywood production. The opportunity to incorporate lignin, an abundant polyphenolic pulping residue, which is very similar in structure to a phenolic resin, into a phenol-formaldehyde adhesive has been of interest. Most early attempts to incorporate lignin into thermosetting phenolics ~ were limited to small proportions. There has been great interest in enhancing the chemical reactivity and the physical properties of lignin fractions to be used as prepolymers in phenolic resins. 2,3 In the pulping of rice straw, which is used in many pulp mills at Rakta in Egypt, a large amount of black liquor from soda pulping is wasted. Most previous work in this area which claims that phenol-lignin can be used as a raw material for phenol-formaldehyde resins does not mention reaction conditions. In our previous studies 6'7 it was shown that the most important factors affecting the yield and properties of the

EXPERIMENTAL

Lignin was precipitated from Rakta black liquor by acidification with sulphuric acid. Phenollignin formaldehyde resin was prepared in two stages. In the adduct stage, the lignin reacted with phenol in the presence of oxalic acid as catalyst, according to the condition used by Vorher.10 The reaction was carried out in a small autoclave. In the polymerization stage the phenol-lignin (PL) or phenol (P) and formaldehyde (F) in the required ratio, with oxalic acid as catalyst were heated under reflux in a

Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 125

126

A. M. A. Nada, M. Fadly, M. Dawy

three-neck flask. The resin was then separated and purified as stated elsewhere, n The resulting novalac resin was dissolved in alcohol and purified by reprecipitation. IR spectra were obtained using a Beckman 4250 spectrophotometer and the KBr disc technique. The frequencies of the bands are reproducible to within + 1 cm-1. Electronic spectra were obtained in methanol solution using a Beckman 5260 spectrophotometer.

60 50 t~

o

4o

.o .<

3O

2O 0

I

I

I

I

1

2

3

&

Time

(hr)

Fig. 2. Effect of heating time on the absorbance of (O) OH phenolic and (O) OCH3 groups in the adduct (-) and polymerization ( - - - ) stages of the formation of phenol lignin-formaldehyde resins.

RESULTS A N D DISCUSSION

The IR absorption spectra of lignin (L), phenol-formaldehyde (P-F) and phenol-lignin formaldehyde (PL-F) in the region 2004000 cm -~ are shown in Fig. 1. Close comparison of the spectra of lignin with those of PL-F and P-F shows a new strong band at 755 cm -1 in the IR spectra of both P-F and PL-F. Some bands show detectable intensity changes such as methoxyl, hydroxyl and the C------Cstretching vibration. The integrated intensities of these bands for the three samples are shown in Table 1. From these data it is clear that the intensity of the

characteristic absorption bands of the methoxyl of lignin at 2850 cm -1 is lower in PL-F resin than in lignin itself. However, P-F does not absorb in this region as shown by Fig. 1. The decrease in the intensity of the methoxyl stretching vibration of PL-F is attributed to the hydrolysis of this group during the adduct and polymerization stages. These results can be confirmed by considering the effect of time and temperature on the absorbance of the methoxyl and phenolic hydroxyl groups, as shown in Figs 2 and 3. These figures show that the intensity at 2850 cm-' of the methoxyl group of the PL-F resin decreases with increasing time and temperature of the adduct and polymerization stages. On the other hand, 60 v

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30

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200

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500

1000

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1500

i

i

2000 Wave

2020

3000

number

I

60

/*000

I

I

J

100

I

1~,0

Teml~eralure (*C)

(cm -1)

Fig. 1. The IR absorption spectra of (1) lignin, (2) phenol-formaldehyde, and (3) phenol lignin-formaldehyde resins.

Fig. 3. Effect of temperature on the absorbance of (O) OH phenolic and ( 0 ) OCH 3 groups in the adduct (-) and polymerization ( - - - ) stages of the formation of phenol lignin-formaldehyde resins.

Table 1. Integrated band intensity of various groups in lignin, phenolformaldehyde and phenol lignin-formaldehyde Substance L P-F PL-F

(25% L)

I

180

3300 cm-1

2850 cm 1

1600 cm-1

1200 cm-1

50 67 46

65 -39

43 39 38

34 50 37

755 cm -16 11.5

Molecular structure of a lignin-polymer system

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40

50

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Concent rat ion ('/o)

Fig. 4. Effect of lignin concentration on the absorbance of (O) OH and (O) OCH3 groups in phenol ligninformaldehyde resins. the intensity of the phenolic hydroxyl groups (at 1200cm -1) of the resin is increased. This confirms the hydrolysis of the methoxyl group of the lignin during the polymerization stage. However, by increasing the lignin content of the PL-F resin, the intensity of OCH3 group absorption increases (Fig. 4), and that of OH phenolic groups decreases. The intensity of the phenolic OH band (at 1200cm -1) of lignin is lower than that of phenol. This is attributed to

the lower molecular weight of phenol (94), which has one phenolic hydroxyl group, than that of lignin (324) with one phenolic hydroxyl group. For this reason the band intensity of the phenolic hydroxyl groups of the resin decreases with increasing lignin concentration. The new strong band, which appears at 755 cm ' in the IR spectra of PL-F and P-F, can be assigned to a methylene group. The appearance of this band is due to the reaction of phenol in the ortho or para position (or its phenolic hydroxyl group) with the ot-hydroxyl group of the propane side chain on the aromatic ring of the lignin molecule. The intensity of this band increases with increasing time of the adduct stage in which phenol is reacting with ligin. The ~ C stretching vibration of lignin at 1450cm -l shows a detectable increase in intensity and broadening in the spectra of P-F and PL-F resins. The broadening and intensification of this band may be due to the coupling of the vibrational band of the CH2 group, produced from the reaction between phenol or phenol-lignin with formaldehyde, with the C--C stretching vibration, which can be represented as follows:

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OH

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Phenol lignin-formaldehyderesin OH

OH + CH20

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~ Phenol-formaldehyderesin

CH2,~

128

A. M. A. Nada, M. Fadly, M. Dawy

_.~0.3 _o u

50.2

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I 40

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Lignin conc.(*l,)

Fig. 6. Effect of lignin concentration on the molecular

absorption coefficient of phenol lignin-formaldehyde resin. \\ • \ '~

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x~.

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/

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200

400 600 Wave number (nm)

B00

0

I

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I I 2 3 4 Polymerizotion time(hour)

Fig. 5. The electronic absorption spectra of (. . . . . ) lignin, ( . . . . ) phenol, (-) phenol-formaldehyde, and ( - - - ) phenol lignin-formaldehyde resins.

Fig. 7. Effect of polymerization time at 94°C on the oscillator strength (e) of phenol lignin-formaldehyde resin.

The electronic absorption spectra of lignin, phenol, P-F and PL-F in the spectral region 900-200 nm are shown in Fig. 5. It is clear that a strong absorption band appears at 280 + 5 nm. This band can be assigned to a locally excited n - n transition. The effect of substitution of phenol with lignin in the final resin product was studied. The effect of various parameters in the adduct and polymerization stages on the U V spectra will be discussed. The oscillator strengths per mole of the n-~r transition of the samples were calculated and are shown in Table 2. These data show that the

oscillator strength per mole of the n-~r transition decreases with increasing lignin concentration in the PL-F resin, as shown in Fig. 6. This indicates the increasing lignin substitution of low molar absorption coefficient (0.19) in place of phenol of high molar absorption coefficient (0.82). Figures 7-10 show that the oscillator strength (e) of the resin increases with increasing time and temperature as well as with the ratio of formaldehyde in the polymerization stages. This is attributed to the increase in cross-linking between phenol-lignin and formaldehyde, which is confirmed by the increase in the viscosity of the polymer on increasing the 0.3

Table 2. Oscillator strength (t) of the n - ~ transition (280_.+5 nm) of the samples investigated

Substance L P P-F PL-F (25% L) PL-F (40% L) PL-F (50% L)

Oscillatorstrength (e) 0-19 0-82 0.55 0.23 0-19 0-18

0.2

"6

=_ 0.1

g

i

i

i

i

1

2

3

4

Adduct

time ( h o u r )

Fig. ft. Effect of adduct time at 155°C on the oscillator strength (e) of phenol lignin-formaldehyde resin.

Molecular structure of a lignin-polymer system

These results show that both IR and UV spectroscopic studies give clear information about the reaction which occurs between phenol, lignin and formaldehyde.

0.4

tO

0.3 "6

Y

02

129

0

oi

i

6o

70

I

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80

90

100

Poly merizQtion ternp.(~)

Fig. 9. Effect of polymerization temperature for 3 h on the oscillator strength (e) of phenol lignin-formaldehyde resin.

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.--_ o2 0

o 05

i

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0.6

0.7

0.8

0.9

PL:F r a t i o

Fig. 10. Effect of phenol lignin:formaldehyde ratio on the oscillator strength (e) of the resin produced.

temperature stage .6

and time of the polymerization

REFERENCES 1. Nimz, H. H., Lignin-based wood adhesive. In Wood Adhesive, ed. A. Pizzi. Marcel Dekker, New York, 1983, pp. 247. 2. Clarke, M. R. & Dolenko, A., US Pat. 4113675 (1978). 3. Enkvist, T. V. E., US Pat. 3 864 891 (1975). 4. Rieche, A. & Redmger, L., Plaste Kautsch, 9 (1962) 131. 5. Shinjo, H., Japan Pat, 13 797 (62) (1959). 6. EI-Saied, H., Nada, A. M. A., Ibrahem, A. A. & Yousef, M. A., Angew. Makromol. Chem., 122 (1984) 169. 7. Nada, A. M. A., EI-Saied, H., Ibrahem, A. A. & Yousef, M. A., J. Appl. Polym. Sci., 33 (1987) 2915. 8. Cassel, B., Characterization of Thermosets. PerkinElmer Thermal Analysis Application Study, 1977 p. 47. 9. Glasser, W. G., Barnett, C. A. & Sano, Y., Appl. Polym. Symp., 37 (1983) 441. 10. Vorher, W., Schweers, W. H. M., Appl. Polym. Symp., 28 (1975) 277. 11. US Department of Commerce, Off. Tech. Serv. PB Rep., 25 (1945) 642.