Solvent influence on the radical grafting of maleic anhydride on low density polyethylene

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Pergamon

0014-3057(94)E0045-6

Eur. Polym. J. Vol. 30, No. 9, pp. 1047-1050, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057/94 $7.00 + 0.00

SOLVENT I N F L U E N C E ON THE RADICAL G R A F T I N G OF MALEIC A N H Y D R I D E ON LOW DENSITY POLYETHYLENE A. PRIOLA, R, BONGIOVANNI a n d G. GOZZELINO Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, 10129 Torino, Italy

(Received 7June 1993; accepted in final form 12 October 1993) Abstract--The maleinisation of LDPE in various aromatic solvents in the presence of radical initiators was studied. A strong influence of the solvent on the reaction was observed and attributed mainly to the presence of benzylic hydrogens in the solvent molecule. It was confirmed that, during the polymer maleinisation, the benzylic hydrogens take part in the reaction with MA. The pure xylene isomers were investigated as solvents and the MA content in the polymer was found to depend on the kind of isomer employed as a solvent in the order: o-xylene < m-xylene < p-xylene. Under these conditions, most of the reacted MA is linked with the solvent molecules. Using solvents free of benzylic hydrogens (t-butylbenzene or o-dichlorobenzene) all the reacted MA is linked with the polymer. Working in the same solvents but under more severe conditions, intermolecular coupling reactions occur, giving rise to gelled polymers. GPC analysis of LDPE maleinised in xYlene solution shows a MWD similar to that of the initial polymer.

INTRODUCTION It is well k n o w n t h a t polyethylene, as a consequence o f its apolar character, exhibits very low compatibility a n d a d h e s i o n towards several substrates such as metals a n d polar polymers. T h e i n t r o d u c t i o n of polar groups into the polymer chain increases the compatibility a n d the possibility of interaction giving rise to the f o r m a t i o n o f a d h e s i o n b o n d s with the m e n t i o n e d substrates, thus allowing interesting applications. F o r this purpose, radical grafting of maleic a n h y d r i d e ( M A ) o n the polymers seems a very interesting reaction [1]. Papers t h a t describe grafting o n polyethylene [2, 3], E P M copolymers a n d polypropylene [4, 5], b o t h in solution a n d in the melt, have been published. It was also shown, by working o n model c o m p o u n d s , t h a t M A enters the polymer chains as a single succinic a n h y d r i d e ring [6, 7]. P E maleinisation has been performed in xylene (isomers mixture) solution. This solvent can either react with M A in the presence of peroxides [8, 9] or f o r m c h a r g e - t r a n s f e r complexes with it [1]. Therefore it seems interesting to investigate the influence of this a n d o t h e r solvents o n the maleinisation.

EXPERIMENTAL Materials As starting material, low density polyethylene (LDPE) was used: the sample (d = 0.924 g/ml, )~tv = 33,000, [rt] = 0.95 dl/g in xylene at 75 °) was kindly supplied by Enichem Anic (Milan). Maleic anhydride (MA) (C.Erba > 9 9 % ) was used as received; xylene (isomers mixture: o-xylene m-xylene 30%, p-xylene 14%, ethylbenzene 8%) was a C.Erba product. Other solvents (pure xylene isomers,

48%,

t-butylbenzene, o-dichlorobenzene) were Fluka products ( > 99%). All the solvents were distilled under nitrogen before use. Radical initiators viz. benzoyl peroxide (BPO, Fluka), t-butylperbenzoate (TBP, Fluka) and di-cumylperoxide (DCP, Akzo Chemie), were used as received.

Procedure Maleinisation was performed under N 2 in a three-necked flask, thermostatted at 135°C equipped with a magnetic stirrer. At the end of the reaction, the polymer was recovered by coagulating the solution with a 5-fold excess of acelone; unreacted MA was removed from the polymer by extraction with acetone for 18 hr in a Soxlet apparatus. Analyses The total MA conversion during maleinisation was determined by GLC using a C.Erba instrument equipped with a Carbowax 20, 5% column, 2 m long, at 160°C in the presence of t-butylbenzene or o-dichlorobenzene as internal standard. The content of grafted MA was determined by titrating the polymer solution in xylene with an alcoholic solution of KOH, using thymol blue as indicator, slightly modifying the published method [10]. Intrinsic viscosity measurements were performed in xylene at 75°C with a Cannon-Fenske viscometer [2]. GPC analyses were performed at 135°C using 1,2,3-trichlorobenzene as solvent. The instrument, a Waters HPLC, was equipped with a refractive index detector.

RESULTS AND DISCUSSION

Peroxide effect First, a c o m p a r i s o n of the activities of some radical initiators was made, performing M A grafting on L D P E in xylene (mixture of isomers) solution. As in

1047

1048

A. PRIOLA

Table 1. Radical grafting of MA on LDPE with various initiators. Experimental conditions: 50 ml xylene mixture; LDPE 5.0 g; MA 25mmol T = 135°C, except for BPO ( T = 100°C); reaction time 60 min Peroxide Peroxide concentration t t / 2 (M x 10 -3) (hr) BPO TBP DCP

0.8 1.0 1.4

MA content (%w/w)

[q] (dl/g)

My

2.1 4.3 3.7

0.71 0.49 0.55

20,700 11,600 14,100

0.4 0.55 1.8

Solvent influence As already mentioned, several studies on maleinisation have been performed using a mixture of xylene isomers as solvent. We monitored the reaction path in the various pure xylene isomers and the M A content vs time is plotted in Fig. 1. It is evident that the solvent has a strong influence on the M A content. Its value increases in the order:

o-xylene < m-xylene < p-xylene. These results can be explained considering previous results of Scheckter on the peroxide catalysed addition of M A to aromatic compounds containing benzylic hydrogens [8]. The reaction gives rise to the formation of addition products having a succinic anhydride structure (A) according to the following chain mechanism: (1)

R" + Ar-CH 3- * Ar-CH~ + RH

(2)

Ar-CH~ + MA --0 Ar-CH2-MA'

(3)

Ar-CH2-MA" + Ar-CH 3 Ar-CH2-MA-H + Ar-CH~. (A)

(4)

4 m

f g <

1

10

20

30

40

50

100

~"

80 60

other studies [1-5], a mixture of xylene isomers was used (its composition is given in the experimental section), due to the excellent solubility of the polymer in it and its availability. The results are shown in Table 1 together with the values of ll/2 for the initiators. DPC was chosen as a typical initiator of the reaction, considering its good catalytic activity and its long tl/2. The same initiator was employed in a previous work on polyethylene maleinisation [3].

I - - , 2R"

et al.

60

Time (rain) Fig. 1. MA content of LDPE (weight %) vs reaction time in various xylene isomers: o-xylene (l-I); m-xylene (O); p-xylene (O). Experimental conditions: solvent volume 25 ml, LDPE 2 g, MA 6 mmol, DCP 0.15 mmol, T = 135°C.

g

40

~

20

If 0

I

I

I

I

I

I

10

20

30

40

50

60

Time (min) Fig. 2. MA conversion vs reaction time in the absence of the polymer in xylene isomers (same symbols and conditions as for Fig. 1). Scheckter [8] calculated the kinetic chain length of the reaction employing di-t-butylperoxide as radical initiator and found that it varied for the different xylene isomers in the order: o-xylene < m-xylene < p-xylene. This order was attributed by the same author to the steric hindrance of the methyl group towards the hydrogen transfer reaction (step 4) or the addition of the xylyl radical to MA (step 3). With the aim of confirming Scheckter's results, we performed the MA-solvent reaction under the same experimental conditions as used in the LDPE maleinisation experiments. In Fig. 2 the plots of the M A conversion vs time in the xylene isomers are shown. The kinetic curves agree with Scheckter's data and show that MA conversion is nearly complete in 1 hr in the presence of para- and meta-isomers, but is rather low in the ortho-isomer. Considering the results of Fig. 1, we notice that the more reactive the solvent the higher the M A content in the polymer. This is an important feature of the reaction; it suggests that the polymer through its mobile hydrogens takes part in the chain reactions (steps 3 and 4) involving the benzylic hydrogens of the solvents. The higher the chain length in the solvent, the higher the MA content in the polymer. We can note that, working in meta- or para-xylene, nearly all the MA reacts. In the case of p-xylene, about 10% links with the polymer and the remainder, with the solvent molecules. The chain reaction mechanism involving LDPE can be represented as Scheme 1 [4]. It was therefore very interesting to investigate the use of solvents having no benzylic hydrogen. We chose t-butylbenzene and o-dichlorobenzene for this purpose. At first we checked that, in these solvents, in the absence of the polymer and under the experimental conditions of Fig. 2, no reaction occurred involving MA and its concentration remained constant according to GLC analyses. Then we performed LDPE maleinisation under the same conditions in these solvents. The results of reactions performed in t-butylbenzene or o-dichlorobenzene solutions are reported in Fig. 3 and compared to those obtained using p-xylene.

Solvent influence on the radical grafting of maleic anhydride ~

+

s

'

-SH

-

1049 +MA

. . . .

+SH +S C H A C O

l

"CH--

CH--CO

x o

l

CO/

CH2--

\ o CO/

SH = solvent

Scheme 1 4 ~

.-.----o -

3 ~

2 o <

I

I

I

I

I

I

I

10

20

30

40

50

60

Time (rain) Fig. 3. MA content of LDPE (%w/w) vs reaction time in p-xylene (@); t-butylbenzene (O); o-dichlorobenzene (~).

It is evident that the MA contents of the polymers maleinised in t-butylbenzene or o-dichlorobenzene are nearly the same. MA conversion is very low but it is completely linked to the polymer; when working in p-xylene, MA, as already stated, reacts mainly with the solvent. The kinetic curves in t-butylbenzene or o-dichlorobenzene (Fig. 3) stop at 30 min; if the reaction time is longer or the conditions are even more severe (higher concentrations of MA and of the initiator) crosslinking becomes important and gives 100

-

(A)

~*~

6

5

-

(B)

~

. -

.....

4

3

80 60 40 20

(%)

0 100 80 60 40 20

/

,

2

~-----

_1~-(

h,.,,, ,

h,.,,, ,

6

5

4

I."n"~ 3

l,mtL,

2

log MW

Fig. 4. GPC chromatograms of LDPE before (A) and after maleinisation (B). MA content = 3.1%.

rise to gelled polymer. This fact is to be attributed to polymeric radical coupling reactions as discussed in other papers [3, 10]. In Fig. 4 the GPC spectrum of the LDPE polymer maleinised in xylene solution is reported and compared with that of the initial polymer. The chromatograms are very similar, showing that under these conditions the polymer MWD does not change during the maleinisation. Thus the GPC data show that important coupling reactions do not occur when the maleinisation is performed in xylene solution. Moreover one can observe that the curves have their maxima at slightly different MWs: the one referring to the grafted polymer is found at lower MW in agreement with the viscometric data (Table 1) which indicate that the higher the MA content the lower the intrinsic viscosity. These results may be due either to a true MW reduction as a consequence of chain scissions as found for ethylene-propylene copolymers [4] or to the hydrodynamic polymer-solvent interactions, that can be different when MA is introduced into the polymer chain.

CONCLUSIONS

The radical grafting of MA on LDPE in aromatic solvents is strongly dependent on the kind of solvent. If benzylic hydrogens are present in the solvent molecule, they take part in the reaction with MA. Using the various xylene isomers, the MA content in the polymer increases according to the order: o-xylene < m-xylene < p-xylene. The same order was found in the reactivity of MA towards the solvent molecules. This fact indicates that the polymer, through its mobile hydrogens, takes part to the chain reaction involving the benzylic hydrogens of the solvents. Using solvents free of benzylic hydrogens, such as t-butylbenzene and o-dichlorobenzene, MA conversion is low but nearly all is bound to the polymer. LDPE samples having low MA contents are obtained. Working under more severe conditions, intermolecular coupling occurs and a gelled polymer is formed. In solvents having benzylic hydrogens, these intermolecular coupling reactions are excluded. In fact, GPC data show that the maleinised polymers obtained in these solvents have MWD similar to that of the initial polymer.

A. PRIOLAet al.

1050

Acknowledgements--We thank Enichem Anic S.p.A. for kindly supplying LDPE samples and performing GPC analyses and acknowledge the Italian CNR (contract No. 91.03150.03) and MURST for financial support.

REFERENCES 1. B. C. Trivedi and B. M. Culbertson. Maleic Anhydride, p. 459. Plenum Press, New York (1982). 2. D. Braun and V. Eisenlohr. Angew. Makromol. Chem. 55, 43 (1976). 3. N. G. Gaylord and M. Mehta. J. Polym. Sci. Part ,4 26, 1189 (1988).

4. G. De Vito, N. Lanzetta et al. J. Polym. Sci. Polym. Chem. Ed. 22, 1335 (1984). 5. F. Ide and A. Hasegawa. J. appl. Polym. Sci. 18, 963 (1974). 6. K. E. Russell and E. C. Kellusky. J. Polym. Sci. Part A 26, 2273 (1988). 7. A. Sipos, J. McCharty and K. E. Russell. J. Polym. Sci. Polym. Chem. 27, 3353 (1989). 8. H. Scheckter and H. C. Barker. J. org. Chem. 21, 1473 (1956). 9. W. K. Bikford and H. Fisher et al. J. Am. Oil. Chem. Soc. 25, 251 (1948). 10. N. G. Gaylord and M. Mehta. J. appL Polym, Sci. 33, 2549 (1987).