Stabilization of poly(vinyl chloride) by β-dicarbonyl compounds

Eur. Polym. J. Vol. 25, No. 12, pp. 1245-1250, 1989

0014-3057/89 $3.00+0.00 Copyright ~ 1989 Pergamon Press plc

Printed in Great Britain. All rights reserved

STABILIZATION OF POLY(VINYL CHLORIDE) BY fl-DICARBONYL COMPOUNDS K. S. MINSRER and S. V. KOLESOV . Bashkir State University, Frunze St. 32, Ufa 450074, U.S.S.R. G. E. ZAIKOV Institute of Chemical Physics AS, Kosygina St. 4, Moscow 117334, U.S.S.R.

(Received 2 March 19893 Abstract--The possibilities of using fl-dicarbonyl compounds as thermal stabilizers of vinyl chloride polymers have been studied. The stabilizing properties are connected with the strongly marked CH-acidity shown by these compounds. Ideas on the mechanism of their interaction are presented.

elimination rate:

INTRODUCTION

It is usually thought that acids (carboxylic acids, alcohols, phenols, etc.) catalyse the decomposition of homo- and copolymers of vinyl chloride [1, 2]. It was discovered, however, that, for a wide range of compounds with proton-donating properties (carboxylic acids, OH-, CH-acids), inhibition of the thermal decompositions of poly(vinyl chloride) (PVC) and vinyl chloride (VC) copolymers is more common than catalysis of the decompositions [3-5]. Some examples are now given. CARBOXYLIC ACIDS

Even very strong acids (such as trifluoracetic, trichloracetic, formic) at concentrations of 10 -3 to 10-2mol/(base-mol) PVC do not accelerate but actually considerably inhibit the process of HC1 elimination as well as the discoloration during thermal decomposition, clearly shown for the case of formic acid (Fig. 1). The characteristic feature of PVC chemical stabilization by carboxylic acids is the increase of the stabilizing effect, expressed by the maximum value of the relative decrease of HC1

( V % - V . O I V % = V,c,I V % (Table 1) with the change o f p K acidity index (Fig. 2). In general, the dependence of Vnc~ on the initial content of carboxylic acid (Co), mixed with PVC, is marked. Strong carboxylic acids catalyse PVC thermal decomposition when in the macromolecules there are sequences of conjugated C-----C bonds. OH.ACIDS

Alcohols and phenols, possessing sufficient acidity, can considerably inhibit the thermal decomposition of PVC (Table 2). For example, the inhibitory effect of aliphatic alcohol on the decomposition changes in the following series: propyl <~ t-butyl < octyl < decyl alcohols. It directly correlates with the fact that, when there is no solvating effect of the solvent, the alcohol acidity increases with increase of polarization of alkyl groups, depending firstly on their volume and secondly on the branching of alkyl radicals at the -position. CH-ACIDS

3.0

Chemical compounds with strongly marked CHacidity (e.g. fluorene, benzonitrile, p-nitrotoluene, /3-carbonyl compounds, etc.) affect to some extent the decomposition of PVC (Tables 3 and 4). In this case as well as when carboxylic acids are used, there is direct dependence of the stabilizing effect on

] 100

EEl.

5o Y Table 1. The stabilizing effect of carboxylic acids on the thermal decomposition of PVC (10 Pa, 448 K, C o = 10.3 mol/mol PVC)

I"1

Compound

4O

8O Time (rain)

Fig. 1. HCI elimination and change of colour in the thermal decomposition of PVC (10 -2 Pa, 448 K) in the presence of formic acid C0. 102 mol/mol PVC: 1.9(1, 7); 5.0(2, 6); 10(3, 5): 0(4, 8).

CCI3COOH CF3COOH o-HOC6H4COOH HCOOH CH2CICOOH CH3COOH C6HsCOOH

1245

(DH-EA), (kcal/mo|) 10.5 13.8 18.5 19.0 19.0 22.0 23.1

o VHcl/Vnc I

0.69 0.63 0.44 0.56 0.47 0.37 0.38

1246

K.S. MINSKERet al. Table 3. The effect of CH-acids on the thermal decomposition of PVC (10Pa, 448 K, Co = 10 2 mol/mol PVC)

O.8

V.o/ V°cl

Compound 5

Fluorene Benzonitrile p-nitrotoluene Cyclopentadiene Phenylacetylene Triphenylmethane

5

4

6

1

l

l

l

-2

--1

0

1

PKroL

the acidity of the compound. For example, the inhibitory effect on PVC thermal decomposition [for Co= 10-2mol/(mol PVC)] increases in the series: diphenylmethane, trimethylmethane benzonitrile, p-nitrotoluene according to the increase of the strength of acidifying substituents [6]: Ph < CN < NOv Therefore, we can state that, during the thermal decomposition, the stabilizing effect of protondonating compounds (carboxylic and CH-acids) is the common feature of chemical additives of this class; the relative decrease of HCI elimination can be satisfactorily correlated with the "intrinsic" acidity of the compounds. It is evaluated by the difference between RH(DH) bond energy and the affinity of radical R for the electron (EA) in the gas phase independent of the origin of the acidity (the chemical origin of proton-donors) (Fig. 3). The (DH-EA) index reflects the "intrinsic" acidity of molecules in the gas phase. It has also been shown to characterize the investigated compounds for polymer degradation conditions without any solvent and in an inert atmosphere. It is rather important that, during the thermal decomposition of the predegraded PVC in which sequences of conjugated C------Cbonds have already formed, acids do not produce any stabilizing effect. Moreover, many compounds catalyse the dehydrochlorination of the predegraded polymer. Thus, depending on the conditions of the experiment and the chemical structure of PVC macromolecules, both stabilizing and accelerating effects of proton-donating compounds can be observed. The data show (Fig. 3) that the dependence is extrapolated to the value: VHcl/VOcl = 0.9. This means Table 2. The effect of OH-acids on the thermal decomposition of PVC (10Pa, 448 K, Co = 10 -2 mol/mol PVC)

Phenol Cresol lsobutyl alcohol Octyl alcohol Decyl alcohol

Table 4. The effect of #-dicarbonyl acids on the thermal decomposition of PVC (10 Pa, 448 K, Co = 10 2 mol/mol PVC) Compound

Fig. 2. The dependence of the stabilizing effect of carboxylic acids in the thermal decomposition of PVC (448 K, 10-2 Pa) on the strength of the acid (PKrej in CC14;standard benzoic acid): 2, CCI3COOH; 3, CH2CICOOH; 4, HCOOH; 5, CH3COOH; 6, CH3CH2COOH; 7, C 6 C s C O O H .

Compound

0.31 0.24 0.26 0. I 1 0.12 0.07

VHCI/VoHCl 0.10 0.19 0.07 0.15 0.19

(DH-EA), (kcal/mol)

VHcllV°cl

16.3

0.46

24.2 32.4 33.4

0.40 0.38 0.34

O~C~NH

/

I

CH 2

C~O

\

I

O----C~NH C6HsC(O)CH2C(O)CITH35 C2H~OC(O)CH2C(O)C:H5 CH3C(O)CHzC(O)OCH ~

of the compounds is increased with the accumulation of acidifying substituents in the molecule: --Ph, )C(O). In compounds of the C N - - C H R F - - C O - OR 2 and C H : - - C O - - C H C N - - C O - - C H C N - - C O - -

\

\

0.8

\

2 ~, O.e

j

42 o~.~_o 8

<~ 0.4

70~ON 12 16 "No 18 110 1 5 ~ ) ~ 19 20

0.2

140013 I I0

I 20

(DH-EA)

I

30 kcot/mot

I

40

"

21 0 ~0

Fig. 3. Relative change of PVC dehydrochlorination rate in the presence of proton-donors (448 K, 10-"Pa) depending on the acidity (DH-EA): 1, CCI3COOH; 2, CF3COOH; 3, see below; 4, o-HOC6H4COOH; 5, HCOOH; 6, CH2C1COOH; 7, CH3COOH; 8, C6HsCOOH; 9, C6HsC(O)CH2C(O)CI7H35;10, see below; 1I, CuH25OH; 12, C2HsOC(O)CH.,C(O)OC2Hs;13, CH3C(O)CH2COOCH3; 14, C6H5OH; 15, n-CH3C6HsOH; 16, see below; 17, C6H~CH2COOH; 18, n-CH3C~H5 NO,; 19, see below; 20, C6H~C~--~CH;21, C6HsCOCH3.

°7--NH)= 3.

I0.

16.

19.

Stabilization of PVC by fl-dicarbonyl compounds

O

II

CH3.

C.

CH

I

Ct

,

HCt /CH 3

1247

o

ct

I

I

c.3~ 1 / c % . 3 / c . ~ 5

CH

CH

I

Ct

- - CH

I

Ct

/cH3 CH

I

Ct

(A~ practically complete inhibition of the formation of polyenes on condition that Vna = Vs + Vp, where Vs and Vp are rates of the PVC dehydrochlorination along with the formation of isolated and conjugated C-------C bonds in macromolecules [7], and Vp/VHa ~ 0.9. The decrease of PVC dehydrochlorination rate in the thermal decomposition .carried out without catalytic agents is the result of chemical stabilization of the polymer, taking place due to the interaction with proton-donors for the destruction of active centres for polymer decomposition i,e. ~ - - ( C H ~ - - - - C H ) , - - C H C I - - ( n / > 1) Nvx

groups. This conclusion is indicated by the experimental data: for different samples of PVC, the values of VHc~, reached during PVC stabilization with protondonors, are proportional to the concentration of ~t,/Lenon/carbonylallyl/groups (CAG) g0 in the polymer macromolecules (Table 5). But it should be mentioned that the stabilizing effect of proton-donors cannot be referred to the well-known reactions of the chemical compound connected with the C------Cbond of ~t,fl-enon groups or to sequences of conjugated C-----Cbonds as in the case of mercaptan [8]. It cannot be referred to the substitution of mobile chlorine atoms at the allyl position by the C-----Cbonds, as in the case with phenols [9], imides [10], etc. Firstly, during interaction of PVC with protondonors (e.g. formic and acetic acids, malonic ester; Co = 10-2 mol/mol PVC, 353 K, 2-4 hr) the contents of the interchain groups with C------C bonds in the polymer do not change. Secondly, there is no increase of PVC stability/or the predegraded PVC, treated under the same conditions with proton-donors, such as t-butanol, malonic ester, benzonitrile; this is proved by the fact that after the complete removal of the residual stabilizer by ether extraction, the polymer is re-precipitated. The interaction of proton-donors with PVC seems likely to proceed by protonization of ~,fl-enon groups on carbonyl oxygen: O-~--~=C

H+ < =HO

I +=~--C=C

< - C - - - C ÷.

The can The this

stabilization of the strongly polar transition state produce the anionic proton-donating fragment. possibility of the stabilizing effect, represented in scheme, is shown by quantum chemical calcu-

lations, The calculations of (A) and (B) model structures for interaction with the simplest proton-donor viz. HCI shows that the (B) state is 746kJ/mol more advantageous energetically than the (A) state. The increase of the C5----C1 bond, which can serve as the reactivity index of (A) and (B) structures in the reaction of HCI elimination [11, 12] establishes the increase in stability of the (B) structure. In the weakly polar medium of solid polymer under the influence of ct,fl-enon group, strong polarization of proton-donor molecules occurs rather than the dissociation of compounds. Taking it into account, the index of the proton-donor "intrinsic" acidity should be seen not as the characteristic of the thermodynamic equilibrium of acid ionization (irrespective of the mechanism and rate of proton fission) but as an indication of the proton mobility in the protondonor molecule. It is important that this indication adequately shows the reactivity of compounds, serving as inhibitors of the polymer dehydrochlorination process; it can be the quantitative characteristic of the efficiency of compounds, which function as chemical stabilizers of PVC as related to their structure. From this point of view, fl-dicarbonyl compounds, containing the characteristic structural fragment - - - O - - ~ - - C H R--C-----O, where R is H or a hydrocarbon group, are of great interest as PVC stabilizers. In these compounds the acidity of the CH bond is rather strong and evidently causes the stabilizing effect in the thermal decomposition of VC polymers. This is illustrated by fl-dicarbonyl compounds stabilizing series, coinciding with the series of their "intrinsic" acidities (DH-EA). The stabilizing effect of fl-diketones on PVC results in the decrease of rate of polyene formation and polymer discolouration in thermal and thermal-oxidative decomposition (Fig. 4). It is clearly seen when using stearoyl benzoyl methane (Table 6). This substance also produces a stabilizing effect the decomposition of VC copolymers. In this case as well as when other chemical stabilizers are used, the stabilizing effect depends on the content of the second M2 monomer

Table 5. The dependenceof the efficiencyof the PVC stabilization by stearoylbenzoylmethaneon CAG concentrationin PVC "7o"10 4 V~-~t~b.106 (tool/toolPVC)

[mol HCl/(mol V C sec)]

VHcl"~b106

0.8 1.0 1.2 1.8 2.2

0.68 0.75 0.98 1.43 1.64

0.40 0.49 0.60 0.81 1.05

1248

K.S. M]NSKEget 0

/

0

"NH--4

0 ::::::::~ CH2 > \ NH-----~

0

0

c.,-4

> p h - - ~ H2

O (DH-EA)

al.

> C2H50~

O

2

> CH30~

0

16.3

2

ph~

H2

O

32.4

O

33.4

45.6

kcal/mol

[13]. The inhibition of the dehydrochlorination (VHCI) of VC1 copolymers with low content of comonomeric units (2-5mo1%), containing stearoyl benzoyl methane decreases as the number of M2 units increases and practically disappears in copolymers containing M s/> 10 mol% (Fig. 5). Stabilizers of the dicarbonyl compounds class do not accept HCI evolved in the polymer decomposition but, in combination with HCI acceptors, considerably increase the thermostability time of polymer compositions (Table 6) due to the inhibitory effect during the polymer thermal decomposition. In experiments using stearoylbenzoylmethane for PVC samples with different levels of stability, the stabilizing effect depends directly on the content of ~,fl-enon groups showing that their interaction with the proton-donor is the main cause of inhibition of VC polymers decomposition (Table 5). The interaction of fl-diketones with ct,fl-enon groups is likely to proceed according to the reaction of the type: 1,4-addition of donor, containing reactive methylene group, to the double bond of ct,fl-unsaturated carbonyl compound (the Michael reaction).

_L/L-o --

C o mmot/PVC

8 I

mot

16 I

5

24 I

2.0

0.8 (L ~

>~'~

3

1.0

o.6g

67 0.4 E

E

a

9 0.5

-10,2 ~)"

5 I 20

J 40

Time

i 60

>=

(rain)

Fig. 4. Effect of some fl-dicarbonyl compounds on the elimination of HCI and PVC dehydrochlorination rate 00 -2 Pa, 448 K): l,~-acetoacetate; 2,7--ester of malonic acid; 3,8--stearoylbenzoylmethane; 4,9--malonylurea; 5--without additives.

+ [RICOCH2COR2T-~ Rj COCHCOR2 ] Z---* R1COCH(COR2)---

I

I

I

Another reaction path is possible.

+ ,COCH CO

E

7 - - - R j C O C H ( C O R 2 ) - - - ~ - - - - ~ - - - - ~ - - O H : - - - R, COCH(COR2 )---~--CH--C------O

I fl-Diketones with active methylene groups seem suitable donors in the Michael resection which is favoured by the large volume of substituents at the a-carbon atom and the high stability of the anion of the enol produced from it (which is the result of charge delocalization). VC polymer stabilizers may be fl-dicarbonyl compounds of various structures (Table 7). The efficiency OR2 types [14], the ---C~=N group serves as the acidifying substituent with the electronegative beteroatom, fl-Diketone stabilizers for VC polymers have been produced by Rh6ne-Poulenk. Rhodiastab-

I

I

f

Table 6. Thermal stability of PVC compositions stabilized by stearoylbenzoylmethane (448 K, air, Co = 10 mol/mol PVC, HCI-acceptor concentration--5 mol/mol PVC) Composition Thermal stability (min) PVC 1 PVC + CaSt: 50 PVC + BaSt2 160 PVC + CdSt: 94 PVC + Rhodiastab-50 12 PVC + CaSt2+ Rhodiastab-50 61 PVC + BaSt: + Rhodiastab-50 290 PVC + CdSt: + Rhodiastab-50 185

Stabilization of PVC by 3-dicarbonyl compounds

(a)

/

z

1249

Table 8. Thermal stability and transparency of rigid PVC film, stabilized by Rhodiastab-50 Components, indicators

~

PVC C--5868PG Dioctylphthalate Ca stearate Epoxom Polygard BTA--3N Modifier Glycerin Zn stearate Zn chloride Rhodiastab-50

8 i

I

I

20

40 Time

1

2

3

100 3 1 2 I 12 0.3 0.5 -

100 3 I 2 1 12 0.3 -0.01 1.0

100 3 1 2 1 12 0.3 091 1.0

Static thermostability, (rain. 175 C)

44

68

70

Transparency

30.5

29.5

44.2

50 mixtures of/J-diketones, containing as their main component stearoylbenzoylmethane and also Rhodiastab-82-phenyl-l-methyl-5-hexanediene and Rhodiastab-83-dibenzoylmethane are available for industrial use. They are non-toxic or slightly toxic substances which in synergistic combination with organic stabilizers, containing metal, ensure the production of thermostable, transparent PVC materials of stable colour. The preferred ingredients are Ca and

60

{mini

Fig. 5. The effect of stearoylbenzoylmethane--l.5 10 -2 mol/mol PVC (6--10) on the change of (a) HC1 elimination rate and (b) colour stability (Y) in the thermal decomposition (10 -2 Pa, 448 K) of VC polymers: 1,6--PVC: 2 , 7 - - V C M A , 2.2; 3 , 8 - - V C M A , 5.8; 4,9--VCP, 3; 5 - 1 0 ~ B C A , 11.8.

Table 7. Structure of ~'-dicarbonyl compounds--stabilizers of VC polymers General formula

R1

RI--c--CHR2--C--R

II

3

R2

CH 3

H

H

CI--C17

©

II

0

0

~

C(O)

CH3

CH3~

R3 C6~CI3

O

O

! ] ~ C H R 1- -

!~OR

2

CH3C(O}

II

1

C~CH2~C--OR

CH3C(O)

CH3C{O)

C12--Cl8

H

Ctt2C(CH2OHt 3

~

O

{;sHlt

H

CH3CtO)

O

C2H5OC(O)

C2~C18 C(O)

C2H5

C8~Ct8 CH2C(CH2OH} 3

O RI O ~

II

C~

0 R10 ~

II

C~

O

II

0 CH2~

t

CHR 2 - C ~ O R

II

C~

0 CH 2 -

II

(-~

OR I

C2~C 6

CH3C(O)

C2H5

CTHIsC[O)

C2H 5

~

C6H13

(CH3}3CC(O}

C1~C18

C(O)

1250

K . S . MINSKER et al. Table 9. The effect of combination of stearoylbenzoylmethane with polyols on PVC colour stability "Synmero" 10---score Etalon scale (Hungary) (448 K, air) Colour stability in test time (min) Polyol

0

15

30

45

60

--

1

1

1-2

2

4

1-2 1 1

2 1 1

2 I-2 I

2-4 I-2 1-2

4 2 1-2

Sorbitol Pentaerythritol Glycerin

75

90

10

--

4 6 2

~6 10 4

The composition: PVC, 100; Ca benzoate, 0.3; stearate, 0.4; antioxidant, 0.I; lubricant, 0.5; shockproof modifier, 3.0; polyol, 0.67; stearoylbenzoylmethane, 0.34.

Zn carboxylats which, in combination with stabilizers of the Rhodiastab type, give synergistic increase of the thermostability time and stable colour of polymer compositions. High proportions of Ca/Zn salts (up to 2 : 1 and higher) providing thermostability of compositions and stable colour are permissible. It is recommended to use salts, such as Ca- and Zn-stearats at concentrations of 2-0.6 and 0.3-0.1% per PVC mass. It is shown experimentally that the concentration of Zn salt can be reduced to 0.01% and Zn stearate can even be replaced by Zn chloride (Table 8). The use of polyols, for example in rigid materials, is rather favourable as it considerably increases the colour stability of compositions (Table 9). However, the stabilizing effect depends to a great extent on both the chemical nature of the fl-dicarbonyl compound and on the chemical structure of the polyols. In all cases, a positive effect is obtained by introduction of epoxy compounds into the composition. There can be different compositions based on PVC containing fl-diketones. The compositions can be treated by various methods, such as casting, extrusion, calendering; they can be used in the production of rigid and plasticized films, sheets, wrapping material for food products and medicines, etc. 1. PVC/susp/ --100 Modifier shockproof --10 Epoxy soya bean oil --3 Ca stearate --I Stearate --0.7 Trinonylphenylphosphite -4).3 fl-diketone --0.25-0.64 Lubricant (Wax E) --1 2. PVC/susp., bulk/ --80 Copolymer BA-I 5 --20 Modifier shockproof --10 Epoxy soya bean oil --3 Ca stearate --0.5 Zn stearate ---0.25 /~-diketone ---0.244).43 Lubricant ---0.5 Ca stearate ---0.25-0.35 Zn stearate ---0.3--0.39 Rhodiastab-50 --0.25-0.35 3. PVC/susp., bulk/ --100 Modifier shockproof --8 Epoxy soya bean oil --2.5 Sorbitol --0.09-0. I Lubricant --0.45 Lubricant (Wax OA) ---4).6

4. PVC/susp., bulk/ Dioctylphthalate Epoxy soya bean oil Ba stearate fl-diketone Polyol--(D-mannitol D-xylitil) Cd stearate 5. PVC/susp., bulk/ Dioctylphtalat Ba stearate Cd stearate Trinonylphenylphosphite Rhodiastab---50

--100 --37 --8 --1 --0.05-5 --0.05-5 --1 --1 --100 --50 ~0.9 ---0.56 ~0.5 ---0.4

Thus, inhibition of the thermal dehydrochlorination of VC polymers is the common property of various compounds, exhibiting proton-donating properties. Stabilizers of the Rhodiastab-series belong to this group of very promising stabilizers.

REFERENCES

1. I. S. Troytskaya, V. N. Myakov, B. B. Troytsky and G. A. Razuvayev. V~sokomolek. Soedin, A 9, 2119 (1967). 2. E. H. Zilberman and V. B. Saltinova. Plastmassy 17 (1967). 3. K. S. Minsker, S. V. Kolesov, V. M. Yanborisov, M. E. Adler and G. E. Zaikov. Dokl. Akad. Nauk SSSR 268, 1415 (1983). 4. K. S. Minsker, S. V. Kolesov, V. M. Yanborisov and G. E. Zaikov. Polym. Deg. Stab. 15, (1986). p. 305. 5. S. V. Kolesov, A. M. Steklova, G. E. Zaikov, K. S. Minsker, VjJsokomolek. Soedin, A 28, 1885 (1986). 6. O. A. Reutov, I. P. Beletskaya and K. P. Butin. Nauka 247 (1980). 7. K. S. Minsker, AI. AI. Berlin, V. V. Lisitsky and S. V. Kolesov. V~sokomolek. Soedin, A 19, 32 (1977). p. 32. 8. R. C. Poller. J. Macromolec. Sci. A 12, 373 (1978). 9. T. Suzuki. Pure appl. Chem. 49, 539 (1977). 10. A. Michel. J. Maeromolec. Sei. A 12, 361 (1978). 1I. Yu. E. Eisner and B. L. Yerusalimsky. Nauka 54 (1976). 12. K. S. Minsker, S. V. Kolesov, V. M. Yanborisov, AI. AI. Berlin and G. E. Zaikov. Polym. Deg. Stab. 9, 103 (1984). 13. K. S. Minsker, S. V. Kolesov, R. B. Pancheshnikova and R. M. Ahmetkhanov. Dokl. Akad. Nauk SSSR 266, 370 (1982). 14. Avt. Svid. SSSR 719509; opubl, v Bull. Inform. 8, 217 (1980).