Pyrolysis of polypropylene in the presence of halogenated paraffin and bismuth salt: Some aspects of the degradation mechanism

Polymer Degradation and Stability | 1 (1985) 243 249

Pyrolysis of Polypropylene in the Presence of Halogenated Paraffin and Bismuth Salt: Some Aspects of the Degradation Mechanism Guido Audisio & Alberto Silvani Istituto di Chimica delle Macromolecole del C.N.R., Via E. Bassini 15/A, 20133 Milano, Italy

(Received: 26 June, 1984)

ABSTRACT A study is reported oJ the condensed-phase reactions which occur during the thermal degradation oJ mixtures oJ polypropylene and ,[tame retardant additives. Using the pyrolysis-gas chromatographic-mass spectrometric technique, various mixtures of polypropylene, chloroparaJfin and bismuth salt were studied. By comparing the pyrograms with that oJ pure polypropylene, two eJJects were noted. Firstly there was a decrease in the amount oJ low boiling degradation products, which suggests that the unzipping reaction becomes less javoured. Secondly, there was a change in the ratio between the diastereoisomers of tetramer and pentamer, i.e. a reduction oJ the isomerization caused by hydrogen transjer along the polymer chain.

INTRODUCTION The number of applications of polymeric materials is continuously increasing and their organic structure makes them very degradable and inflammable. More resistant materials are required and for this purpose, in the polyolefin field, the most common additives are various chlorocompounds in the presence of small amounts of antimony and bismuth salts. 243

Polymer Degradation and Stability 0141-3910/85/$03.30 ,~ Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

Guido Audisio, Alberto Silvani

244

While the action of hydrochloric acid in flame poisoning 1 is well known, the effect of these additives on the condensed phase reactions, in other words how they can interact to change the primary degradation mechanism, has been little studied. 2 Recent work in this field has given some evidence of these interactions. 3'4 We now report a study using the flash pyrolysis-gas chromatographic technique on these condensed phase reactions. This tool is one of the most useful, because it allows the control of experimental conditions (temperature, atmosphere and time of degradation) and the analysis of volatilized degradation products. EXPERI M E N T A L Materials

Commercial polypropylene (PP), containing 300 ppm of BHT antioxidant and supplied by Montepolimeri S.p.A., was sieved at 140 mesh. The chloroparaffin (CP) Cereclor 70 (ICI PLC, Great Britain) was dissolved in chloroform and the solution was slowly evaporated under vacuum in the presence of the polymer. The basic bismuth carbonate (BiO)2CO 3 (A.C.F. Chemiefarma NV, The Netherlands) was mixed with PP and CP in a ball mill. The compositions of the mixtures used are given in Table 1. Method

A CD 190 Pyroprobe with a coil probe was used. The mixture (1-2 mg) was placed in a small quartz tube in the coil. The pyrolysis temperature TABLE 1 C o m p o s i t i o n s of Mixtures

Mixtures

PP (wt ~0)

CP (wt %)

(BiO)2CO 3 (wt ~)

A B C D E

95 70 75-7 87"6 92-6

5 30 4 4.6 4.9

-20-3 7.8 2-5

Cl/Bi (molar ratio)

1 3 10

Pyrolysis oJpolypropylene: some aspects oJ the degradation mechanism

245

was 900 °C for 20 s. The rising temperature rate was about 60 °C per ms. The pyrolysis products were eluted on a 50m SE 54 fused silica capillary column. The oven temperature program started at 40°C for 5min, thereafter rising at 3 °C per min to 270°C. A 3700 Varian gas chromatograph with Vista 401 integrator was used.

RESULTS AND DISCUSSION

High and low boiling products Sugimura e t a l . 5 showed that the products of pyrolysis of PP at 650°C consist of a large range of compounds up to C28. The distribution of the pyrolysis products at 900°C is very similar to that at 650°C. Figure l shows the areas of the pyrogram peaks against their retention time (RT) for PP alone and for PP mixed with CP and bismuth salt. There is an increase in the peak area to an RT which corresponds to that ofn-pentane. This was found by injecting a standard solution of n-alkanes under the same gas chromatographic conditions. Figure 1 also shows how the presence of the additives decreases the amount of low boiling pyrolysis products and increases the amount of

25 20

15 10 5

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5

Fig. 1.

i

10

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15

20

25

30

w

35

!

40

45

so

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,

55 so

v

RT

Peak areas (".},) a g a i n s t their r e t e n t i o n time. - , Pure p o l y p r o p y l e n e ; p o l y p r o p y l e n e w i t h additive.

246

Guido Audisio, Alberto Silvani

high boiling products. This fact shows that the additive influences the PP degradation mechanism. In order to study this interaction we have defined as low boiling products all those with an RT lower or equal to that of n-pentane and as high boiling products all those with a higher RT. The results in Table 2 were obtained from the analysis of the mixtures listed in Table 1. These are percentages of the total volatilized products and the reported values are the average of several analyses. According to Table 2, an increase in the amount of CP from 5 ~o to 30 ~o results in less low boiling products being formed. The presence of the bismuth salt shows a synergistic effect in decreasing the low boiling pyro~sis products; but this effect occurs only when the molar ratio between chlorine and bismuth is 3 or higher. TABLE 2

Percentages of Low and High Boiling Products

Low boiling High boiling

PP

A

B

C

D

E

55 45

40 60

30 70

45 55

36 64

29 71

Isomerization The mechanism of thermal degradation of PP was clarified by Tsuchiya and Sumi. 6 It consists of two steps after the first homolytic cleavage of the carbon-carbon bond. (a) Unzipping reaction to form propylene. Polymer

C==C + --~2---C

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C

+

.~c--c~2 I c

, C~----C+ ~ C ~ C

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C

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C

Pyrolysis of polypropylene: some aspects oJ the degradation mechanism

247

(b) Hydrogen transfer and/3-scission. C

C

C

C

C

I

I

i

I

I

(9)

(7)

(5)

(3)

(1)

C

C

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I

C

C

C

I

I

I

C

C

C

C

I

I

i

I

C

C

C

C

C

I

I

I

I

I

---~C~---C--C~---C~--C---C~

Htransfer

3-1

5-1

7-1

9-1

C==C~C~C

Oligomers

H transfer and//-~ission>

(dimer)

C==C~---C~

(trimer)

C~-------C--C~~C~C

(tetramer)

C~----------C~C~--C~--C~

(pentamer)

Many other oligomers are formed by this mechanism and can be analyzed on a capillary glc column up to the decamer. We shall consider the tetramer and the pentamer. These two compounds can exist in a number of stereoisomeric forms (as shown in Table 3), which can be resolved on a glc column. 7 TABLE 3 Stereoisomeric Forms of Tetramer and Pentamer

Tetramer Isotactic

Syndiotactic

C~-----~ C

C

C

C=C

C C

i

i

I

I

i

i

i

I

1

C

C

C

C

C

C

C

C

C

C=C

C

C

Pentamer C

C

C

C

I

I

C ~C~C

C

C

C=C

C

C ~C

C

C

C

C

I

C -C -C

C-C

C C

C

i

I

I

I

i

I

C

C

C

C

C

C C

Heterotactic

i C=C

C

C

C

C

C

C

C

C

i

i

I

I

C

C

C

C

248

Guido Audisio, Alberto Silvani

The starting PP in these experiments was pure isotactic (98 ~) so that, if there were no isomerization during the thermal degradation, we should find only one diastereoisomer, namely that related to the isotactic structure of the polymer. The presence of the other isomers means that isomerization occurs at the tertiary carbon atoms of the polymer backbone. To a first approximation their relative amounts can be assumed to be a measure of the degree of isomerization,V i.e. the higher the isotactic isomer percentage the lower the extent of isomerization. The relative percentages of the tetranaer and pentamer isomers in the pyrolysis products are presented in Table 4. TABLE 4 Relative Percentages of Isomers in Pyrolysis Products PP

A

B

C

D

E

Tetramer Isotactic Syndiotactic

55 45

59 41

56 44

59 41

59 41

64 36

Pentamer Isotactic Heterotactic Syndiotactic

48 13 39

52 13 35

48 13 39

51 12 37

52 12 36

57 12 31

CONCLUSION During the degradation of PP in the presence of CP and Bi salt additives interaction of these additives occurs in the bulk of the polymer. This interaction reveals itself through two effects: the proportion of low boiling products of degradation decreases and isomerization occurs. One of the steps in the combustion of organic polymers is the thermal degradation of the polymer by the heat of the flame. The volatile products of this degradation act as a fuel for the combustion. Thus the decrease in the proportion of low boiling products helps the retardant flame function of the additives because it implies a decrease of the amount of fuel available to the fire. It is reasonable to believe that this effect is due to the presence of Ci radicals. If they react with an alkyl radical of the polymer chain they prevent hydrogen transfer and the subsequent 3-scission,'through which

Pyrolysis oJpolypropylene: some aspects oJ the degradation mechanism

249

the production of low boiling c o m p o u n d s occurs. The second effect, isomerization, is due to an intramolecular hydrogen transfer from the tertiary carbon atoms of the PP backbone to the radical site. It is known that a radical carbon atom has a planar configuration and when it is saturated it partially loses its former stereochemistry. Thus we can only say that a relationship exists between the decrease in the isomerization and the prevention of the intramolecular hydrogen transfer by C1 radicals, but at this time we do not know its mechanism. The C P additive acts during the combustion at two 'cooperative' levels: firstly the C1 radicals act as a flame-poison by inhibiting the chain propagation reaction, and secondly the C1 radicals, interacting with the polymer degradation in the condensed phase, restrict the production of volatiles and therefore the fuel for the autocombustion. From this point of view synergism by Bi salt can be seen as an increase in the efficiency of the C1 interactions both in the flame and in the bulk of the polymer.

ACKN OWLEDGEMENTS The authors are indebted to Mr A. Rossini for mass spectrometric measurements and to Mr L. Consonni for technical assistance. This work has been supported by the Consiglio Nazionale delle Ricerche through the Progetto Finalizzato Chimica Fine e Secondaria.

REFERENCES 1. R. J. Schwarz, in Flame retardancy o! polymeric materials, Vol. 2, W. C. Kuryla and A. J. Papa (Eds), New York, Dekker (1973). 2. R. R. Hindersinn and G. Witschard, in Flame retardancy oJ polymeric materials, Vol. 4, W. C. Kuryla and A. J. Papa (Eds), New York, Dekker (1978). 3. L. Costa, G. Camino and L. Trossarelli, Polym. Degrad. Stab., 5,355 (1983). 4. G. Audisio and A. Silvani, Atti del IV Convegno AIM, 246 (1981). 5. Y. Sugimura, T. Nagaya, S. Tsuge, T. Murata and T. Takeda, Macromolecules, 13, 928 (1980). 6. Y. Tsuchiya and K. Sumi, J. Polym. Sci. Part A-l, 7, 1599 (1969). 7. G. Audisio and G. Bajo, Makromol. Chem., 176, 991 (1975).