Thermal dehydrochlorination and stabilization of poly(vinylchloride) in solution: Part IV—Synergistic effects of β-diketone compounds and metal soap stabilizers

Polymer Degradation and Stability 24 (1989) 89-111

Thermal Dehydrochlorination and Stabilization of Poly(vinyichloride) in Solution: Part IV--Synergistic Effects of ~-Diketone Compounds and Metal Soap Stabilizers N a i m a Bensemra, Tran Van Hoang, Alain Guyot CNRS, Laboratoire des Materiaux Organiques, BP 24-69390, Vernaison, France

Michel Gay & Louis Carette Rhone-Poulenc Recherches Specialty Chemicals, Centre des recherches de Saint-Fons, BP 62-69192 Saint-Fons cedex, France (Received 20 June 1988; accepted 7 July 1988)

ABSTRACT Thermal degradation of poly(vinylchloride ) ( P VC ) has been carried out in trichlorobenzene ( T C B ) solution at 187°C in the presence ~f zinc and calcium stearates and fl-diketone compounds ( BDCs). BDCs are able to react alone with P VC through C alkylation, but the reaction is strongly enhanced by the metal soap system. In the initial stages of the reaction, both BDC residues and stearate groups are grafted onto the P V C backbone and some crosslinking may occur due to the double reaction of BDC compounds. The initial dehydrochlorination of P VC, as well as the development oJpoO'ene sequences are strongly inhibited in the presence of the total system, but later, the degradation ~4f the polymer is accelerated. The rates of all the processes are strongly dependent on the concentration of structural defects initially present in the poO'mer. Coordination complexes between BDC and zinc compounds and re-equilibrium keto-enolJorms of the BDC are observed in the presence ~/" Ca compounds.

INTRODUCTION /%Diketone (BDC) compounds were introduced some years ago as secondary stabilizers for initial coloration improvement in the metal soaps 89

Polymer Degradation and Stabilio' 0141-3910/89/$03-50 ~~ 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

90

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

stabilizer system for poly(vinylchloride) (PVC). 1 The mechanism of their action has been studied previously, 2 using benzoyl acetone as their model compound and chlorohexene as a model compound for allylic chlorine atoms, which are believed to represent the weak structures in the initial polymer or to be at the end of a growing polyene sequence during the dehydrochlorination (DHC) pro6ess. It has been observed that BDCs are able to react with these allylic chlorine atoms through C alkylation in the presence of ZnC12; this reaction is competitive with D H C which remains the main reaction except at the beginning of the proces s and in non-polar media. Benzoyl acetone was shown to be grafted to the polymer in a test carried out at 180°C in a rolling mill and in the presence of mixtures of calcium and zinc stearates; in addition, benzoyl acetone was shown to cause acceleration of the D H C process. More recently, using stearoyl benzoyl methane (Rhodiastab 50 ®)in combination with Ba stearate, Minsker et aL 3 concluded that the observed stabilizing function of this system does not arise from the substitution of allylic chlorine atoms. The purpose of this paper is to present a more complete study of the reactions of various BDCs on PVC in trichlorobenzene solutions and their synergistic action with the calcium zinc stearate stabilizer systems.

EXPERIMENTAL Most of the experimental methods (DHC, coulometric titration, infrared and UV spectroscopy) have been described in previous papers. 4'5 The PVC samples are those designated XII and XIII in the IUPAC working party. 6 A careful analysis of their structural defects has been carried out in our laboratory 7 and the results are recalled in Table 1. Five different ~-diketones have been used, namely:

Stearoyl benzoyl methane (BDC 1) white powder (Rhodiastab 50")

C 17H35 - - C - - C H 2 - - C - - ~ x ~ TI O

I) O

x_5__/

Phenyl-l-methyl-5-hexane 1,3-dione (BDC 2) CH3--CH--CH2--C--CH2--C~r~ I II

CH 3

O

O

yellowish liquid (Rhodiastab 82")

91

Dehydrochlorination and stabilization of P VC

Dibenzoyl methane ( BDC 3) yellowish powder (Rhodiastab 83 :~) 0

0

Allyl-3-benzoyl acetone (BDC4) 0 ~~--(~n--

O

If[- - c n 3

CH2--CHzCH2

bis-fl-Diketone (BDC 5)

~

C - - C H 2--C--{CH 2)7--C--C H 2 C ~ k / Ji N [I 11 k~_/ O

O

O

O

From their N M R spectra (Bruker 80MHz) in a 1/1 mixture of dichloroethane and dimethylsulfoxide as solvent the percentages of enol structures are, respectively, 78-5 (BDC 1), 90.1 (BDC 2), 94-0 (BDC 3), 10-0 (BDC 4) and 88"9 (BDC 5). The grafting of these BDCs on PVC was studied using size exclusion chromatography (SEC) in tetrahydrofuran (THF) as liquid carrier, with sets of three microstyragel columns (Waters) either for high molecular weight compounds (106, 104, 500 A) or for low molecular weight compounds (104, 500, 100A) and with two successive detectors (UV and refractometer). Capillary viscometric experiments were carried out in THF solutions at 25°C and the data treated according to the Mark-Houwink law, [q] = 1"63 x 10 -4 M 0"766 dliters/g, previously given by Bohdanecky et al. 8 Raman spectra were obtained using a DILOR midrodi128 apparatus with TABLE 1

Characterization of PVC Samples Samples

XII Xlll

Mn

37 500 43 300

Mw/Mn

2.4 2.2

De['ects [br 1000 monomer units Internal unsaturation (ozonolysis)

Chain end unsaturation (1H- NMR)

Labile CI (phenolvsis)

0-52 0-24

0-8 0.75

1.4 1.0

92

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

an Argon laser tuned at 514.5nm (green). Analysis were carried out by comparing the band at 1500-1510 c m - 1 (7(C~C),;I n = 12-13) with that at 1425 cm-~ (6 CH2); a 32-h heating treatment at 65°C was carried out in order to reduce the fluorescence, but this treatment does not eliminate it and no quantitative data can be obtained. Complexes between BDC and zinc or calcium stearate have been prepared by heating an equimolar mixture at 187°C for 30 min. Calcium chloride was heated under vacuum at 120°C for 3 h before use and zinc chloride was purified by sublimation. Their complexes with BDCs were prepared by cogrinding the solids. Infrared spectra were obtained using KBr pellets. Studies of PVC blends (in the solid state) were carried out using a plastograph (Haake Rhecord, Heraeus-France). The temperature in the reactor was about 174°C and the rotor rate was 50rpm.

R E S U L T S A N D DISCUSSION In the absence of metal soaps, BDC shows a stabilizing effect by decreasing the rate of D H C (Fig. 1) and leading to a short induction period before the appearance of yellow coloration. SEC experiments with UV detection in addition to the usual refractometer, demonstrate that the dibenzoylmethane (BDC 3) used in these experiments becomes gradually grafted on to the PVC (Fig. 2); however, this grafting process, which probably occurs through C alkylation, is not well enough developed to be easily detected by infrared spectroscopy; the main band which is observed is that at 1620 c m - 1 assigned to the stretching vibration of the double bonds resulting from the DHC. The ketone bands at 1680 or 1720 are not observed. The distribution of polyene 20 "C15

"b U5 T

0

0

25

50

75

100 125 Time (rain.)

I

I

I

150

175

200

Fig. 1. Evolution of HCI from 0.16mol/liter of PVC XII heated at 187°C in trichlorobenzene solution alone (+), in the presence of 4 weight % of a mixture of Zn and Ca stearates (molar ratio Zn/Ca = 0"32) and BDC 3 (0.9 mmol/liter) (©) and in the presence of BDC 3 (0.9 mmol/liter) only (11).

Dehydrochlorination and stabilization of P VC

93

p~

f

\

\ l\ \

J

lOmin

30min

50rain

75rain

Fig. 2. SEC chromatogram with UV ( - - - ) and refractive index ( ) detection of PVC XII solution in TCB (0.16mol/liter) in the presence of BDC 3 (0.9mmol/liter) after various heating times at 187°C. 15

0

n:6

/

10

n~7

;4 Z

n:8 n:9 n:lO n:11

(a)

20

40

60

Time(min)

•

/

n6

/ / .'

× z

/ n7

,,

2

// /

(b)

50 Time(rain)

100

Fig. 3. Amount of polyene sequences of various length (between 4 and ll) formed on thermal degradation of purified PVC XII (a) alone and (b) in the presence of BDC 3.

94

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

sequences is not very much changed in the presence of BDC 3, as compared with PVC degraded in the absence of any stabilizer (Fig. 3). It seems, however, that long sequences (longer than 10) are formed less easily. All these data show that temperature is an important parameter in these reactions, because it has been observed previously 2 that no reaction takes place with the model c o m p o u n d s at 60°C either in dichloroethane or in THF; here, at 187°C in TCB, a limited reaction takes place which is efficient enough to delay the development of the coloration due to the presence of long polyene sequences. As shown in Fig. 1, the presence of a mixture of metal soaps drastically changes the D H C process. After an induction period, when the evolution of HC1 may be limited due to reaction with the metal soaps, a very strong acceleration of D H C takes place, as already reported in the case of the model c o m p o u n d benzoyl acetone. 2 So the ternary systems PVC, BDC, metal soaps has been studied more extensively. The first point to be studied was the effect of the relative amounts of zinc and calcium, for which BDC 1 was chosen. As shown in Fig. 4, for the same total and relative a m o u n t of metal carboxylates, the addition of BDC 1 causes an acceleration of the DHC. The induction period before HC1 has reacted fully with the metal stearates is a little shorter and the slope of the D H C curve is also slightly steeper for the lower Zn/Ca ratio. However, the main effect of the Zn/Ca ratio is not changed, the reaction being faster for the higher ratios. In the following experiments the molar ratio of Zn to Ca stearates was fixed at its lowest value; namely 0"32. Another point which is not changed by the presence of BDC, is the effect of the structural defects in the polymer. As shown in Fig. 5, sample XIII is much more stable than sample XII. Actually, with PVC XIII, the solution remains colorless for more than 2 h at 187°C. In addition, after the induction 40

// /,,,' / /I I,,' ,+// ~

,-

/

/ ~

z /

',.) 10

I / I.I

~

/f o 0

,

/

/ /

/

/ I

/

I

4:

/

/

~

/

//

,./

/

+,

___,-_,-_-_iJL_ ~ _ _ # _ _ ~ i L . - - J + ~ - " 25

:50

75

100 Time(rain)

150

150

175

Fig. 4. Evolution of HCI from PVC XII (0.16 mol/liter) in the presence of mixtures of zinc and calcium stearate of different compositions. Molar ratio Zn/Ca: 0.32 (+); 0'96 (11); 3' 17 (A): without ( - - - ) or with 0.5 mmol/liter of BDC 1 ( ).

Dehydrochlorination and stabilization of" P VC

95

!

20~

,;

J

,/

~ 15

/'

"6 E 10 v

u

5

I

0

..... L-------+=J-,---,--,--,/, 0

50

100

150 Time (min)

,

200

250

300

Fig. 5. Evolution of HC1 from 0.16 mol/liter of PVC XII ( + ) and XIII ( i ) in the absence ( - - - ) and in the presence of 1 mmol/liter of B D C 3, molar ratio Zn/Ca = 0.32.

period in the evolution of HC1 (220 min for PVC XIII and 70 min for PVC XII), the color rapidly becomes black for PVC XII, but is only green for PVC XIII. The synergistic effect of BDC 3 for the initial coloration has been clearly demonstrated. In the following experiments, only PVC XII was used. The structure of the BDC is an important parameter in the process. Figures 6 and 7 illustrate the following effects. Ternary systems containing BDC are more efficient in inhibiting completely the evolution of free HC1 during the induction period. The duration of the induction period can vary by a factor of 2 on changing from BDC 3 to BDC 4. On the other hand, the shorter the induction period, the faster is the DHC rate after the complete consumption of the stearates. A notable exception is substituted BDC 4. Large differences are observed also during the induction period. Thus, with BDC 2 and 3, no C1 ion can be titrated during the first 30 min, while, using BDC 1, the formation of C1 ions seems to start immediately and continuously increases. On the other hand, an S-shaped curve is observed using BDC 2. Increasing the amount of BDC causes a shortening of the 20

/,//

I

/

4-

~",...1 5 ~o v

D 5 t 0

0

215

i

50

Jl"

i

75

i

m

106" Time (min)

I

125

150

175

Fig. 6. Evolution of HC1 from PVC XII in the absence of BDC (A) and in the presence of 1 mmol/liter of BDC 1 (O), B D C 2 ( , ) , B D C 3 ( + ) and BDC 4 ( 5 ) ; molar ratio Zn/Ca = 0.32.

96

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette 100

75

o o

25_

5O c 0 U

0

I

0

25

50

I

75 100 Time (min)

I

I

I

125

15Q

175

Fig. 7. Consumption of metal soap stabilizers (molar ratio Zn/Ca = 0"32) in the absence (A) and in the presence of 1 mmol/liter of BDC 1 (O). BDC 2 ( , ) and BDC 3 (+).

induction period and an acceleration of the consumption of the stearates (Figs 8 and 9 for BDC 3). However, a surprising result is observed with BDC 1; for the lower amount of BDC, the curve for stearate consumption is S shaped as in the case of BDC 2 (Fig. 10). The accumulation of metal chlorides during the induction period proves that BDC 1 is more effective in substituting the allylic chlorines than either BDC 2 or BDC 3, which give complexes with HC1. In fact, this grafting reaction of C-alkylation, demonstrated for model compounds, 2 gives rise to HC1 which consumes metal soaps to give metal chlorides. The grafting reaction is expected to lead to carbonyl absorption in the infrared spectra. However, in a conventional IR spectroscopy experiment nothing is observed except a transient band at 1540-1580 which was assigned 5 to residues of metal soap stearate complexes not separated from the polymer on precipitation with methanol. Using Fourier Transform IR the substitution becomes clear, as shown in Fig. 11 where the characteristic

/ o/

40

"C 3 0

~ 2c "-" IC (D I 0

Fig. 8.

0

I

25

I

J

50 Time ( m i n )

I

I

75

1O0

Evolution of HCI from PVC XII (0" 16 mol/liter) in the presence of different amounts of BDC 3; 0.5 (O) and I mmol/liter (+).

Dehydroehlorination and stabilization o/ PVC 100

97

+

Ao 75 o

g g 25

0

25

0

50

75

100

Time(min)

Fig. 9.

Consumption of metal soap stabilizers for different amounts of BDC 3 (same notation as in Fig. 8).

ketone bands at 1 7 2 3 - 1 7 2 7 c m -~ (aliphatic ketone) and at 1675cm -~ (aromatic ketone) grow with increasing heating time. These spectra are rather similar to that of B D C 4, which shows two bands at 1719 and 1675cm -1 and in which the enol form is a minor part (10%) of the compound. They are different from the spectra of the initial B D C 1 which shows essentially a broad band at 1625 and a smaller one at 1575cm 1, or B D C 3 where the main bands are located at 1597 and 1542 c m - 1 and are to be assigned to the enol structure. In the case of BDC 3, two bands are observed at 1697 and 1 6 6 5 c m - ~ in the grafted polymer. Because the stabilizer system includes metal stearates, the esterification reaction is competing with the C alkylation of the labile chlorine atoms. In the three spectra the ester band at 1 7 3 4 c m - ~ 5 can be observed, at least as a shoulder, but it seems very difficult to obtain a quantitative estimate of the competition. 10C

75 m L

~ 5c

g U

25 /

/

0 0

Fig. 10.

25

50

75 1O0 Time (rain)

150

I 175

Consumption of metal soap stabilizers for different amounts of BDC 1; 0.5 (©) and 1 mmol/liter ( + ).

o,,,, ...

.~ ½ i ,r-%.

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~',.

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\

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(b)

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,,

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10~]0 16~50 1770 1740 17i0 10"80 1050 1725 10¢J~ 10(55 1755 1725 1(5~ 10~ Wavenumber Wavenumber

{

(c)

i

\A

17'70

1755

17~10

° 1710 1080 1050 1725 ltS~g~ 10~ Wave numl~er

Fig. 11. Fourier transform infrared spectra of the carbonyl bands of purified PVC XII after various times of heat treatment in TCB solution at 187°C in the presence of a mixture of Zn and Ca stearates (molar ratio Zn/Ca = 0-32) and 1 mmol/liter of,BI;)C 1 (a), BDC 2 (b), or

BDC 3 (c).

Dehydrochlorination and stabilization Of PVC

99

In Fig. 12 comparative data for PVC XII and PVC XIII demonstrate again that the amount of structural defects initially present in the PVC sample is one of the main parameters in the C alkylation reaction. The grafting of BDC moieties is more easily detected using the very sensitive UV detector after the SEC apparatus. The measurement is carried out at 320 nm, where the absorption of the solvent trichlorobenzene is very weak, while it is near its maximum for BDC (Fig. 13). Then, as shown in Fig. 14 for BDC 3, it is possible to follow both the grafting and the consumption of the BDC. Comparison with coulometric titration of C1 ions shows that the consumption of stearates and of BDC 3 are related (Fig. 15). The polymer peak in the SEC experiment becomes broader and broader so that the molecular weight distribution becomes broader; this point can easily be explained by the fact that the two protons of the central methylene group of

~l \ \

/

/

\

\

\

,.~ PVC"z'n"

/

\

\ \

't

p v C "xTn"

\,

//

L/ 1770

1742 1756

17 5 1728 1701 Wavenumber

16(50 1673

Fig. 12. Comparison between the FTIR carbonyl bands ofPVC XII and PVC XIII after the same heating time (60 min) in the presence of BDC 3 (1 mmol/liter) and metal soap stearate (Zn/Ca = 0"32).

100

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

?

I

I

l

'

I

500

320 380 26G wavelength ~nm)

Fig. 13.

U V spectrum of BDC 1 in tetrahydrofuran (c = 2 g/liter).

the BDC can be substituted by two successive C alkylations. Thus grafting of PVC on mono substituted BDC takes place, leading first to a grafted polymer and then to a crosslinked polymer. Microgels are actually formed, which are efficiently filtered by the filters and eventually by the column of the SEC apparatus so that a part only of the injected polymer is detected. A clear illustration is shown in Fig. 16. When the structure of the BDC is such that

II t t

/, r t

t~ I

I

I

[',,

I

t

iI

t /

\.

I

~

Xx

it

/

\ \

/

30min

50min

75min

Fig. 14. SEC chromatograms with U V ( - - - ) and refraction index ( ) detection of PVC XII (0.16mol/liter) in TCB heated for various times in the presence of metal soap stearate (Zn/Ca = 0.32) and BDC 3 (1 mmol/liter).

Dehydrochlorination and stabilization of PVC 100

°

75

L.

50

0 U

25

0

Fig. 15.

0

101

+

20

40 Time (rain)

60

80

Consumption ofBDC 3 (O) and metal soap stearates (+) upon heating PVC XII in TCB solution at 187°C.

only monoalkylation is possible (BDC 4), no retention of the polymer is observed (Fig. 17). In addition, there is no significant broadening of the polymer peak. However, the grafting efficiency is low in that case. The results of viscosimetric measurements of samples after different thermal treatments are reported in Table 2. For PVC, in agreement with published data, 9 the limiting viscosity number decreases first to a flat minimum and then increases slowly. In the presence of stearates (binary system) only a rather strong increase is observed, probably caused by the crosslinking effect of ZnClz.l o The minimum is again observed in the ternary system, at levels, however, which are strongly dependent on the nature of the BDC. It seems that the increase in the viscosity after the minimum is more rapid and this may be due to the crosslinking ability resulting from the double C alkylation. Actually the phenomenon of crosslinking after the minimum is smaller in the case of BDC 4. Rather surprisingly, high values of the viscosity are observed in the presence of BDC 1; possibly it might result from a micellization effect caused by the long alkyl chain of the molecule. In the other cases, the presence of BDC seems to moderate the crosslinking

/

/\ lOmin

Fig. 16.

/ /

\

/

40rain

SEC chromatograms of PVC X II (0'16 mol/liter) heated in the presence of stearates (Zn/Ca = 0.32) and BDC 2 (1 mmol/liter) for 10 and 40min respectively.

i02

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

f

/\

!

J

J

lOmin

40min

75min

Fig. 17. SEC chromatograms with UV ( - - - ) or RI ( ) detectors for PVC x i i solutions (0.16 tool/liter) containing stearates (Zn/Ca = 0-32) and BDC 4 (1 mmol/liter).

TABLE 2 Viscosity (THF, 25°C) of Polymer Samples from PVC XII

Stabilizer system

Time of heating at 187°C (min)

Limiting viscosity number (dl/g)

Mv

none

10 30 50 60

0'97 0'90 0'92 1"09

84 300 76 500 78 700 98 200

Zn/Ca = 0'32 no BDC

10 30 90

1.41 1"68 1"90

131 000 164000 193 000

Zn/Ca = 0-32 BDC 1

10 30 60

2'07 1"88 2"18

216 000 190000 242 000

Zn/Ca = 0"32 BDC 2

10 40 70

1"19 0.84 1.26

105 000 67 000 113 000

Zn/Ca = 0.32 BDC 3

10 30 50

1.04 0.92 1"25

88 000 75 000 112 000

Zn/Ca = 0.32 BDC 4

10 40 70

1"29 1"13 1.22

117 000 98 000 109 000

Dehydrochlorination and stabilization ~[ P VC

103

TABLE 3 Percentage of Gel in Two Blends (a) and (b) After Various Processing Times Blend b

Blend a Time (rain)

Percentage

Time (min)

Percentage 0 019 0'31

0"33

0

0"06

2"5

0'16

25

l0

0"23

12"8

8

78"00

9"8

96"60

efficiency of the Zn products probably due to their lower effectiveness in the grafting reaction. In order to verify the C alkylation of BDC compounds, we have compared the two blends (PVC XII, 50 g; DOP, 15 g; Zn stearate, 0'6 g; and Ca stearate, 1.2 g in the absence (blend a) and in the presence (blend b) of 1 g of the bis-/3-diketone (BDC 5)) treated at 174°C in the Haake Rheocord. The difference in the torque (Fig. 18), in the degree of solubility (Table 3), in the relative viscosity (Fig. 19) and in the formation of metal chlorides (Fig. 20) in these two blends can be attributed to the monoalkylation and eventually to the double alkylation. Furthermore, the FTIR analysis of these blends gives the same result as that obtained from studies in solution--the ester band (1734cm -~) and the ketone bands (1727-1676cm -~) are identified with low intensities. The effect of binary and ternary systems on the polyene sequence distribution has been studied by UV spectroscopy, using the quantitative methods described by Braun and Thalmaier ~~ and also Shindo and Hirai, 12 of THF solutions of purified polymers. The data are not reliable for very short sequences because of the absorption due to the grafted BDC. For example,

Z v

4

oI_ 0 I--

/ 1

2

o

lO

5 Time(min)

Fig. 18.

Plastograms of two blends containing (

) or in absence of (

t BDC 5.

104

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

2-0

16'

~~+\

+

1-2 (3,8 O'4

0 0

I

I

I

I

I

3

6

9

12

15

Time (min)

Fig. 19.

Relative viscosity of blend

a (+)

and blend

b (O) as a

function of treatment time.

using the more stable PVC XIII in the presence of both stearates and BDC 3, only one band at 340nm (apparently n = 5) is observed; it obviously corresponds to grafted BDC 3 because the UV spectrum of that compound shows one band centered at about 345 nm, slightly shifted from the spectrum of BDC 1 (Fig. 13). At variance with PVC alone and with the binary system PVC-BDC without stearates (Fig. 3), in the corresponding ternary system (Fig. 21) the formation of polyene sequences is delayed chiefly for sequences longer than 8; in some cases (Fig. 22) the number of short sequences goes through a maximum. A rough estimate of the total number of sequences produced shows that it may be assumed, following the original suggestion of Frye and Horst, 13 that the development of the sequences is stopped on substitution of the allylic chlorine atom at their end with either a carboxylate group from O alkylation by the stearate, or a BDC group from the C alkylation reaction. In the first case, the substituted group is not stable in the presence of HC1 and DHC may start again after consumption of most of the 80

+//

.60 ; 40

+

c 0

D 2C

0

~ 0

:k

i

5

Time (rain)

Fig. 20.

, i5

10

Consumption of metal soap stabilizers in the two blends

a

( + ) a n d b (0).

Dehydrochlorination and stabilization of PVC

tO

o ~2

•

/

Z

105

n:6

I /

/ /

/

/

n:7

/ " •

/~e/o

n:8

n:9 i

I

1OO

5O Time(rain)

Fig. 21. Evolution of the polyene sequences in purified PVC XII (c = 2.5 g/liter tetrahydrofuran) after various heating times (187°C in TCB solution) in the presence of Zn and calcium stearate ( z n / c a = 0.32) and BDC 3.

metal carboxylate. Then longer sequences are replacing the short ones. In the second case, a possible cause of the drop in the number of short sequences might be the elimination of grafted and crosslinked material as filtered microgels during the purification process. The molecules already grafted at the beginning of the process have more chance of being crosslinked by double C alkylation, and then to be physically eliminated. During the later stages, the C alkylation becomes less efficient and longer polyene sequences can be formed.

'F d

l

[ f

0

J

Z

i I

L

//

/

•

•

n:6

n:8

I 5O Time(rnin)

Fig. 22.

I

I00

Same conditions as in Fig. 21 except that BDC 2 is used.

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

106

f

I

i

I

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I

5 (CH 2 )

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~(C =C)12.13

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,

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.

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I

1.500

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1250

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1200

R a m a n spectrum of purified PVC XII after 30 min at 187°C in TCB solution in the presence of B D C 1 and Z n - C a stearates (Zn/Ca = 0"32).

Attempts have been made using R a m a n spectroscopy to try to detect the presence of long polyene sequences, following the work of Gerrard and Maddams 14 who showed that this technique is able to detect sequences with between 9 and 17 double bonds at D H C levels as low as 0.0001%. In the present case a small band at 1500cm-1 is observed (Fig. 23), showing the presence of sequences of 12 or 13 double bonds. Due to fluorescence problems, however, it is not possible to give precise quantitative data. It can be concluded nevertheless, that the ternary system including BDC strongly reduces the formation of long polyene sequences, at least in the initial stages of the degradation. All the above data demonstrate that the ternary system of metal carboxylates and BDC is a very efficient stabilizer system at the beginning of the thermal degradation process because it is able to inhibit both the D H C and the formation of long polyene sequences. However, in its presence, the rate of the D H C later increases very rapidly, reaching a value much higher than for PVC alone or for the binary system. The effect of BDC is not to be compared with that o f c i s - e n o n e structures, which have recently been shown by Lukas et al. 15"16 to be active as catalysts for DHC. Actually, in the absence of metal stearates, BDCs are able to retard the DHC, possibly because the C alkylation reaction takes place to some extent, interrupting with moderate efficiency the development ofpolyene sequences. Even in the presence of stearate, the long term stabilization of BDC can be observed if the metal chloride is eliminated. This is true when epoxidized soya bean oil is added to the reaction, the induction time being substantially increased as shown in Fig. 24.

Dehydrochlorination and stabilization of PVC + I I I

15

/ / /

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olc

E

107

/ /

/

0

/ /

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+ I I

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100 150 Time (rain)

200

25o

Fig. 24. Evolution of HC1 from PVC XII stabilized with a mixture of zinc and calcium stearate (molar ratio Zn/Ca = 0"32) and with BDC 2 (11) or BDC 3 ( + ) (1 mmol/liter each) in the absence () and the presence ( ) of epoxidized soya bean oil (5% in weight). ~

Zn (OCOR) 2

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LI

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1800

1675 1737

1550 1612

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1300 1362

Wavenumber

Fig. 25.

Infrared spectra of the complexes between BDC 1 and zinc compounds.

108

Naima Bensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

On the other hand it should be noted that zinc stearate is not soluble in tetrahydrofuran and that the addition of an equimolar amount of any of the BDCs makes the mixture soluble. The same is true for zinc chloride and calcium chloride but not for calcium stearate. All the 1/1 complexes are soluble in tetrahydrofuran. An infrared analysis was carried out of the complexes obtained either by evaporating a methanol solution or by hot pressing powder mixtures between aluminium sheets at various temperatures. The corresponding spectra are shown in Figs 25-28. The main conclusion to be drawn is that the enol bands of the BDC are strongly shifted. In the case of zinc compounds the shift is observed to lower wave number (Figs 25-26); it may be argued that the complexation chiefly involves the ketone group and thus causes a decrease of the electron density at the erlol. Re-equilibration of the keto-enol system (Fig. 26 chiefly) follows. The

\

//:1

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i

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Infrared spectra of the complexes between BDC 3 and zinc compounds.

Dehydrochlorination and stabilization of PVC

109

r f'-~v~f'~ Ca(OCOR) 2 \

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t~1

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c~c,2

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1550 1425 1300 1612 1487 1362 Waveumber

Infrared spectra of the complexes between BDC 1 and calcium compounds.

calcium c o m p o u n d s are more ionic and the calcium atom is not able to form a coordination complex like zinc. The shift in this case mainly concerns the enol group and is towards higher wave numbers (BDC 3-CaC12 in Fig. 28) or its band is broader (Fig. 27). It is caused by the interaction of the H a t o m of the enol with the anion associated to the calcium. The withdrawing character of the second organic group (phenyl in BDC 3) of the BDC is also i m p o r t a n t in determining the magnitude of the shift:

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~-~Zn

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CI

110

NaimaBensemra, Tran Van Hoang, Alain Guyot, Michel Gay, Louis Carette

,4

\ \

\

\

/

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Wavenumber

Fig. 28. Infrared spectra of the complexes between BDC 3 and calcium compounds. The major cause of the enhanced ability of the BDC to be grafted through C alkylation is most probably the formation of its complex with Zn compounds. Possibly the strong complexation of ZnC12 might be responsible for the initial inhibition of its prodegradant ability. On the other hand, the exchange reaction between calcium stearate and zinc chloride, which is believed to explain the synergistic effect of the metal soap system, might also be made more difficult, owing to the complexation of both its constituents. However, our present study has been limited to the analysis of the complexes themselves, and not in the presence of PVC. A complementary study of the complexes and their changes in the conditions of the PVC experiments is required and we hope to be in a position to report on this later.

REFERENCES 1. Rhone-Poulenc Ind. French Pat. 2. 297227 Aug. 1976. 2. Michel, A., Tran Van Hoang, Perrin, B. & Llauro, M. F., Poly. Deg. and Stab., 3 (1980-81) 107. 3. Minsker, K. S., Kolesov, S. V., Yanborisov, V. M. & Zeikov, G. E., Poly. Deg. and Stab., 15 (1986) 305.

Dehydrochlorination and stabilization of P VC

111

4. Tran Van Hoang & Bert, M., Poly. Deg. and Stab., 16 (1986) 35. 5. Bensemra, N., Tran Van Hoang, Michel, A., Bartholin, M. & Guyot, A. (to be published). 6. Guyot, A., Pure and Applied Chem., 57 (1985) 833. 7. Llauro, M. F., Michel, A., Guyot, A., Waton, H., Petiaud, R. & Pham, Q. T., J. Macromol. Sci. Chem., A23 (1986) 221. 8. Bohdanecky, M., Sole, K., Kratochvil, P., Kolinsky, M., Ryska, M. & Lin, D., J. Polymer Sci. Part A2, 5 (1967) 343. 9. Kelen, T., J. Macromol. Sci. Chem., A13 (1978) 349-60. 10. Tran Van Hoang, Michel, A. & Guyot, A., Poly. Deg. andStab., 5 (1982) 365-77. 365 77. 11. Braun, D. & Thallmaier, M., Makromol. Chem., 99 (1966) 59. 12. Shindo, Y. & Hirai, T., Makromol. Chem., 155 (1972) 1. 13. Frye & Horst, J. Polym. Sci., 40 (1959) 419. 14. Gerrard, D. k. & Maddams, W. F., Macromolecules, 8 (1975) 54 and 10 (1977) 1221. 15. Svetly, J., Lukas, R., Michalcova, J. & Kolinsky, M., Makromol. Chem., 185 (1984) 2183. 16. Lukas, R. & Pradova, O., Makromol. Chem., 187 (1986) 2111.