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Radiation Effects & Defects in Solids

Vol. 161, No. 9, September 2006, 529535

Pre-irradiation-induced graft reaction of maleic anhydride

onto polypropylene

XIUMIN TAN*, YONGSHEN XU and CHONGLIN WANG

School of Chemical Engineering and Technology,Tianjin University, Tianjin 300072, China

Tianjin Institute of Technological Physics, Tianjin, China

(Received 19 May 2006; in final form 25 May 2006)

The radiation induced graft polymerization is a well-known method to obtain new materials. Untilrecently, only conventional radiation sources, such as Co-60 and electron beams, were used. Moreover,part of the damage induced in polymers by heavy ions can produce active sites (peroxides andhydroperoxides) that are useful to initiate grafting reactions.

Maleic anhydride (MAH) was grafted onto polypropylene (PP) wax with a number-averagemolecular weight (Mn) of 8000 by gamma pre-irradiation technique. Effects of total dose, monomerconcentration, reaction time, and temperature on percentage of grafting are studied in detail. It isshown that the optimum conditions for grafting are temperature of 70 C and total dose of 14.4 kGy.PP-g-MAH is characterized by infrared spectrum. Differential scanning calorimetry shows that the

compatibility of PP-g-MAH is better than that of PP.

Keywords: Graft copolymerization; Pre-irradiation; Polypropylene wax; Maleic anhydride

1. Introduction

The pre-irradiation grafting technique has been widely studied in order to modify the properties

of various materials. When organic polymers are subjected to ionizing radiation, the trapped

radicals or macromolecular peroxides and hydroperoxides, capable of initiating graft copoly-

merization reaction are formed [1, 2]. Unfortunately, this process is usually accompanied by

homopolymerization, an undesirable side reaction.

It has been reported that the grafting activity of preirradiatied polyethylene can be well kept

at lower temperature [2]. More recently, a result has been reported that the grafting activity of

radiation peroxidized polyethylene film can also be kept at room temperature for more than

three months [3].

Owing to the hydrocarbon nature of polypropylene (PP), and the absence of any reactive

sites, its use in certain applications, especially in fiber manufacture, faces some problems.

These include poor moisture absorption, poor uptake of dye, and poor photostability. One

*Corresponding author. Tel.: +86-22-27405629; Email: [email protected]

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530 X. Tanet al.

of the main routes to overcome these problems is through modification of PP by grafting it

with suitable monomers.Styrene, maleicanhydride (MAH),N-vinylpyrrolidone, acrylonitrile,

butadiene, ethylvinylether, methylmethacrylate, acrylamide, and methacrylic acid are typical

examples. Initiation of the graft copolymerization reaction is usually performed chemically,

photochemically, or by a radiation technique. The last was found to be the most promisingmethod, and the effect of solvent nature on the extent of grafting by-radiation was found to

be important.

By far, MAH modified PP [5] is the most important commercial functionalized polyolefin.

Because of the unique combination of the low cost of MAH reagent, high activity of succinic

anhydride moiety, and good processablity of MAH modified PP, it is the most popular choice of

material for improving the compatibility, adhesion, and paintability of PP. They can be found

in many important commercial products, such as glass fiber reinforced PP [6], anticorrosive

coating for metal pipes and containers, metalplastic laminates, multilayer sheets of article for

chemical and food packaging, and polymer blends such as PP/polyamide and PP/polyester,

as well as polymer/clay nanocomposites.The present study is an attempt to throw light on the modification of PP through its grafting

with MAH. The investigation studied the various parameters, which may affect the grafting

process and characterized the modified polymer.

2. Experimental

2.1 Materials

PP wax (wPP) with a number-average molecular weight (Mn) of 8000, a density of 0.89 g/cm3

was supplied by Qingdao Sinoplase I & Co., Ltd (P.R. China). MAH (Tianjin Chemical Co.,

Ltd., P.R. China) was used without any further treatment. Other chemicals were reagent grade.

2.2 Irradiation

Gamma irradiation of wPP by a 18.5 1015 Bq Co-60 irradiation unit (Tianjin Institute of

Technological Physics) was carried out at the dose rate of 0.3 kGy/h in the circumstance of air.

2.3 Grafting procedure

The grafting experiment was performed in a glass ample having a cock. The xylene was added

first, followed by monomer. The irradiated wPP was immersed in the monomer solution,

purged by bubbling nitrogen. The grafting reaction was carried out by placing ampoules in

water bath, which was set at a relevant temperature.After grafting reaction, the grafted samples

were taken out from the monomer solution in glass ampoules and were washed with acetone

to remove the homopolymer and unreacted monomer.

2.4 Definition of grafting parameters

The grafted MAH onto PP, graft degree (G), was determined by chemical titration. PurifiedMAPP powder (0.1 g) was dissolved completely in 50 ml of xylene at 100 C, and the mixture

fl d f 1 h Addi d i 10 0 l f 0 1 M th l KOH l ti th i t

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Pre-irradiation-induced graft reaction 531

0.1 M ethanol acetic acid solution.A blank solution also was treated under the same conditions.

G(wt.%) was calculated by the following equation:

G =N(V

0 V )M

02 W 100

100%

whereNis the acid concentration of the ethanol acetic acid solution (mol/l),Wthe weight of

MAPP (g),V andV0the volumes (ml) of the ethanol acetic acid solution added to the MAPP

and blank solutions, respectively, and M0the molecular weight of MAH (98.06). The titration

was repeated for three times for each sample, and all samples show a very good reproducibility

inG.

2.5 Characterization of the grafted PP

2.5.1 Infrared spectroscopic analysis. Fourier-transform infrared spectroscopy (FTIR;

Nicolet, FTS3000) was used to obtain some qualitative information about the functional groups

and chemical characteristics of the organoclay. FTIR spectra were obtained from kBr pellets

at room temperature. FTIR spectra of PP and PP-g-MAH were also taken.

2.5.2 Differential scanning calorimeter. A PerkinElmer differential scanning calori-

meter DSC was used for the thermal analysis. The measurements were carried out at a heating

rate of 10 C/min under nitrogen gas. A 5 mg sample was placed in an aluminum pan before

being put in the sample cell. The cell was heated from the room temperature to 200

C toestimate crystallization temperature.

3. Results and discussion

3.1 Irradiation mechanism

When PP is irradiated in air, the free radicals and peroxides that are capable of initiating the

grafting reaction can be formed and kept long depending on the storage condition. However,

the yield of radicals and peroxides will be different. The schematic mechanism of reaction is

as follows [4]:

Irradiation and oxidation

R R. (1)

R. + O2 RO2. (2)

RO2. + RO2. ROOR + O2 (3)

RO2. + RH ROOH + R. (4)

RRO2 RCHO + RO. (5)

RROOH

RRO + OH R CHO + R + OH (6)

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532 X. Tanet al.

In these schemes, R represents PP, R represents the PP, and R. represents polymeric radicals,produced by irradiation. The dissociation of peroxides is possible as follows:

ROOR RO. + .OR (8)

ROOH RO. + .OH (9)

3.2 Investigations on grafting parameters

3.2.1 Effect of monomer concentration. The effect of MAH concentration on the per-

centage of grafting (radiation dose 7.2 kGy) is shown in figure 1. The results clearly show a

remarkable increase in the percentage of grafting with an increase in monomer concentration,

reaching a maximum value at 20%. The remarkable decrease in the percentage of grafting

at a high monomer concentration (20%) may be attributed to the increase in the probability

of homopolymer formation, which takes place at the expense of grafting of MAH onto wPP.

Moreover, the homopolymer formed may act as a barrier, which opposes the diffusion of themonomer towards the surface of the PP. This explanation seems to be reasonable, as graft-

ing was performed by-radiation on ampoules that were already charged and not through the

addition of monomer solutions to preirradiated PP. In the latter case, homopolymer production

should be minimized.

3.2.2 Effect of total dose. The extent of grafting as a function of dose is shown in figure 2.

The results reveal a continuous increase in the percentage of grafting with an increase of dose

up to 14.4 kGy. Greater increase in the dose rate is, however, accompanied by a slight decrease

in the percentage of grafting. This may be explained on the same basis as for the dependenceon monomer concentration, i.e., a great increase in the dose rate increases the probability of

homopolymer formation at the expense of grafting and at the same time, blocks the diffusion

of the monomer towards the surface of the films.

3.2.3 Effect of reaction time. Figure 3 shows that the grafting yield is very small within

1 h, increases very quickly during 13 h, and then reaches a plateau. The grafting reaction gets

almost finished completely in 3 h, beyond which the improvement in G is very slight. The

very smallGwithin 1 h may be due to the severe phase separation between PP and MAH, and

the marked increasing and quickly reaching to the maximum ofG during 13 h means, the

grafting reaction occurs mainly at the first reaction stage.

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Figure 2. Effect of total dose on percentage of grafting. MAH: 20%,T = 70 C,t= 3 h.

Figure 3. Effect of reaction time on percentage of grafting. Total dose: 7.2 kGy, MAH: 20%,T = 70 C.

3.2.4 Effect of reaction temperature. Figure 4 shows that the grafting yield increases

rapidly with the increasing grafting temperature in the range of 3060 C. Beyond this range,

the curves appear to level out. It can be deduced that the grafting reaction occurs only at the

surface of the PP membrane under a low grafting temperature, whereas above 40 C, it tends

to graft the whole bulk.

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534 X. Tanet al.

Figure 5. FT-IR spectra of PP and PP-g-MAH.

3.3 Characterization of graft copolymerization

3.3.1 Fourier transform infrared spectra. When compared with the parents PP, the newabsorptions appeared at 1863 cm1 (asymmetric C=O stretching) and 1786 cm1 (symmetric

C=O stretching) in the spectra of the resulting sample, being the characteristics of MAH

grafted poly (propylene) figure 5.

3.3.2 Differential scanning calorimetry. It is reported that radiation resistance of PP

is remarkably affected by the morphology formed after molding. Figure 6 shows the melt

behavior of crystals with increasing temperature and crystallization behavior with decreasing

temperature after melting. Tm corresponds to the melt peak. The melt behavior is hardly

changed by the addition of PP-g-MAH. The PP-g-MAH has a lower Tmthan PP because of thelower crystallinity. In contrast, it can be seen that the crystallization behavior is affected by the

compatibilizer. The Tc of PP-g-MAH appears at a lower temperature when compared with

that of the pure PP, and the depression ofTcis larger in PP-g-MAH than in the PP, indicating

that the compatibility of the PP-g-MAH is higher than that for the PP. The depression of

crystallization temperature is becauase of the fact that the crystallization of the PP molecules

is retarded.

On the other hand, the copolymer (figure 6) shows a lower and broader melting

peak, indicating inhomogeneous polymer structures that are associated with polymer chain

degradation during the MAH modification.

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4. Conclusion

The grafting of PP to prepare its MAH-grafted copolymers has been carried out successfully

using the pre-irradiation technique. The grafting parameters were strongly dependent on vari-

ations in graft copolymerization conditions. FTIR spectra of the PP-g-MAH confirmed the

existence of a chemical link between the PP and the MAH. Characterization through DSC

revealed that the grafted and the ungrafted PP samples were clearly different. The glass tran-

sition temperature (Tg) peaks of PP and PP-g-MAH shift towards each other by increasing the

percentage of grafting.

References

[1] A. Chapiro,Radiation Chemistry of Polymeric System(Interscience, a division of John Wiley & Sons, New York,1962), p. 690.

[2] I. Ishigaki, T. Sugo, T. Okadaet al., J. Appl. Sci.27 1043 (1982).

[3] Y.C. Nho, J.H. Jin, The Fifth International Conference on Radiation Curing,1416389 (1995).[4] T.S. Dunn, B.J. Epperson, H.W. Sugget al., Radiat. Phys. Chem.14 625 (1997).[5] R.O. Mazzei, E. Smolko, A. Torreset al., Radiat. Phys. Chem.64 (2) 149 (2002).[6] R. Mazzei, D. Tadey, E. Smolkoet al., Nucl. Instrum. Methods Phys. Res. B 208 411415 (2003).

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