K Minsker, G Zaikov, M Artsis - Achievements and research tasks for polyvinylchloride ageing and stabilization - страница 1

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CHEMISTRY & CHEMICAL TECHNOLOGY

Vol. 2, No. 3, 2008 Chemical Technology

Karl Minsker1, Gennady Zaikov2 and Marina Artsis2

ACHIEVEMENTS AND RESEARCH TASKS FOR POLYVINYLCHLORIDE AGEING AND STABILIZATION

1 Bashkirian State University, 32 Frunze str., 450074 Ufa, Bashkiriya 2 N. M. Emanuel Institute of Biochemical Physics, 4 Kosygin str., 119334 Moscow, Russia

Chembio@sky.chph.ras.ru

Received: November 29, 2007

Abstract. Perspectives of polyvinylchloride (PVC) production without labile groups in a backbone have been considered. It has been shown that such production provides drastic increase of an intrinsic stability of polymeric products, possibility of PVC processing with the minimum amounts or in total absence of stabilizers and other chemicals-additives and the opportunity of creation materials and products on a PVC basis with the essentially increased life time. Data allowing to create rigid, semi-rigid and flexible (plasticized) materials and products with the minimum amounts of chemicals-additives and prolonged life time of their service at exploitation under natural and special conditions are presented.

Key words: labile groups, chemical, structural-physical, solvatation, "echo" stabilization, non-toxic chemicals-additives, zeolites, modified clays.

1. Introduction

Polyvinylchloride (PVC) is one of the most known multi-tonnage and important polymeric products. Thousands of rigid, semi-flexible, and flexible (plasticized) materials and products based on PVC are widely used practically in all spheres of national economy and everyday life. First PVC was synthesized by E. Baumann in 1872, but its industrial production had begun much later - since 1935 in Germany according to literature data and in 1930 in the USA according to the DuPont company data.

The global PVC production is impressive: 220 thousand tons in 1950, about 1.5 million tons in 1960, more than 3 million tons in 1965, more than 5 million tons in 1970; now its production is estimated for more than 15 million tons.

The basic PVC problem is its low stability. Under the action of heat, UV-light, oxygen, radiations etc. it easily disintegrates according to the law of framing groups transformation with elimination of hydrogen chloride and formation of double carbon-carbon bonds in macromolecules with the appearance of undesirable coloration (from yellow up to black). Therefore, it is necessary to apply the set of methods increasing PVC sta­bility against action of various factors during its storage, pro­cessing and exploitation as well as during synthesis, storage and use of materials and products on its basis [1, 2].

It is logical to assume that among many aspects causing PVC low stability and rather short life time of materials and products on its basis, the knowledge of the reasons of abnormally high rates of its macromolecules disintegration compared with low-molecular weight models is of primary importance. This problem has appeared to be rather complex for understanding and, in essence, is still being discussed. To the present time the researchers of industrial centers of different countries can not find the mutual point of view concerning identification of a weak point in the structure of PVC macromolecule which determines its abnormally low stability. However, sometimes it is believed to be done purposely, though it is not so clear - what for?

2. Results and Discussion

2.1. What is responsible for PVC low stability?

The PVC low stability used to be connected with possible presence of labile groups in the macromolecules structure, which activate polymer disintegration. These labile groups are distinct from the sequences of regular vinyl chloride repeating units ~CH2-CHCl-CH2-CHCl-CH2-CHCl~. The overwhelming majority of researchers believe that such groups are: a) chlorine atoms bonded with tertiary atoms of the C-Cl (At) carbon; b) vicinal chlorine atoms in the macromolecule structure ~CH2-CHCl-CHCl-CH2~ (Av); c) unsaturated end-groups such as ~CH=CH2 and/or ~CCl=CH2; d) ^-chloroallyl groups ~CH2-CH=CH-CHCl~ (Ac); e) oxygen-containing hydroxy- and peroxy groups (A0) [1-8]. Meanwhile, even after brief consideration of PVC disintegration it becomes obvious that there is much less amount of labile groups (which can be considered as the reason of low PVC stability) in the macromolecules, because tertiary chlorine (At) and vicinal (Av) groups turn

into the //-chloroallyl ones and the hydroperoxide groups are transformed into carbonyl groups during PVC dehydrochlorin ation:

~H2C-

_~H2C-

I

C

I

CH3

-CH2~

- H2O

-CH2~

~H2C-

-CH2~

At

- ~H2C-

~H2C-

Moreover, the world practice of PVC investigations has shown that initial (freshly synthesized) PVC macromolecules do not contain di-(A2), tri-(A3) and/or polyene (А ) groups [2, 3, 9-14]. Internal peroxide groups ~CH2-CHCl-O-O-CH2-CHCl~ are not found as well. So if they are formed during PVC synthesis they would quickly collapse as a result of hydrolysis and/or homolytic break of О-О bonds. There are reliable experimental results, including those received during studying the thermal destruction of fractioned PVC, showing that although unsaturated end-groups are present in the structure of polymeric molecules, they do not affect the PVC disintegration rate [10, 13-15].

Thus, PVC gross-dehydrochlorination of VHCl with sufficient proximity can be described by Scheme 1, where ocg- the content of regular vinyl chloride ~CH2-CHCl~ groups; KCI, K,, Kv, Kc, Kp - rate constants of the corresponding PVC dehydrochlorination reactions; Krr -rate constant of polyene chains destruction.

Following the scheme: VHCl - Knlan + К А + K A with real values of Ka = 10-8 -10-7 s-1 and ag = 1 mol/mol PVC; Kt = 10-4 s-1 and [At] = 103 mol/mol PVC; К = 10-4 -105 s-1 and [Ac] = 10-4 mol/mol PVC; Кv= 103­-10-4 s-1 and [A ] = 10-5 mc)l/mol PVC; K = 102 s1 (448 K).

a0"

Ac

A2:

At

-It

k A*

Scheme 1

One can see that Scheme 1 assumes the concept of ^-chloroallyl activated disintegration of PVC accepted by the majority of researchers, but without convincing proofs [1-5]. However, this postulate is in the contradiction with many experimental facts [16, 17]. In particular:

1. Calculated values of VHCl drastically differ from the experimental ones.

2. The-chloroallyl activation of PVC disintegration assumes an autoacceleration of PVC gross-dehydrochlorination process in time [16-18]. The linear dependence is observed experimentally (Fig. 1). A gross-rate constant of PVC disintegration, according to experimental data and Fig. 1 at Kc = 10-4-10-5 s-1 should contain the expression with Kp @ 10~2 s-1 (at 448 K) from the very beginning of PVC thermal destruction. However, according to the data of thermodisintegration of low molecular weight model compounds [19-21] this is observed only at destruction of the model compounds containing a chlorine atom in a / -position to conjugated (C=C)n bonds (at n > 2), i.e. at occurrence of the effect of the adjacent group of the long-range order (Table 1).

IHC1]*10J, mol/mol PVC 100

100 240

Time, min

Fig. 1. Kinetic curves of PVC dehydrochlorination.

ft-chloroallyl activation: calculated data (1), experimental data (2), (448 K, 10-2 Pa)

Thus, even the primary analysis of experimental results concerning the concept of //-chloroallyl activation of the PVC dehydrochlorination, does not sustain criticism and has no right for existence. It is the erroneous point of view.

On the basis of theoretical consideration of PVC thermal disintegration and taking into account all available experimental data it should be noted that if internal /-chloroallyl groups (as well as tertiary chlorides and vicinal ones) are present in the macromolecules structure, they do not contribute much to the process of PVC gross-dehydrochlorination due to their sufficient relative stability. It was assumed and proved that an oxovinylen (carbonylallyl) conjugated dienophile group is such a group, in which the unsaturated bond is activated by the adjacent

electrophilic group C=O (-C(O)-CH=CHCl-CH2-)

apparently present in PVC macromolecules in rather small amounts g @ 10~4 mol/mol PVC, but disintegrates at the rather high rate (K @10 2 s-1) with HCl elimination [14,

17, 22-24].

It is extremely important to emphasize that the concept of oxovinylene activation of PVC disintegration does not contradict any known experimental facts. Meantime, new (including the original ones) proofs of existence of the basic groups in the structure of PVC

Cl Cl

~HC-CH~

OOH Cl

C

CHCl~

CHCl~

HCl

Table 1

Dehydrochlorination rate constants at thermal destruction of low molecular weight model compounds

No.

Compound

Temperature area where the compounds start to degrade with a noticeable rate, K

Groups index

Decomposition rate constant,

K, s-1

1

2,4-dichloro-pentane

563-593

ao

2.6-10-9

2

raeso-2,4-di-chloropentane

563-593

ao

1.9-10-9

3

3-ethyl-3-chloropentane

488-553

At

7.9-10-6

4

4-chlorohexene-2

433-463

Ac

5.1-10-4

5

4-chlorodecene-2

438-468

Ac

5.0-10-5

6

7-chloronona-diene-3,5

343-369

ap

3.4-10-2

7

6-chloroocta-diene-2,4

360-386

ap

2.6-10-2

macromolecules have been recently received. In particular, oxovinylene groups present in PVC macromolecules are easily disintegrated during alkaline hydrolysis (5 % aqueous solution КОН, 5 % solution of PVC in cyclohexanone) under mild conditions [13, 14]. It is a characteristic reaction for a, ^-unsaturated ketones [25]:

о

II

H2O          // II ~CH=CH-C(O)~ ~c'       +   H3C-C~ (1)

KOH \

Using this reaction it is easy to estimate the contents of labile oxovinylene groups in the macromolecules structure (g0) by a decrease of PVC average viscous molecular weight [13-17].

2.2. How can we identify carbonylallyl groups?

It is important to specify that both ^-chloroallyl and polyene groups are inert to an alkaline hydrolysis but they easily decomposed during oxidative ozonolysis in the presence of hydrogen peroxide [13]. The ozonolysis method allows to estimate a complete amount of internal unsaturated (^-chloroallyl, chloropolyenyl, and oxovinylene) groups in the PVC macromolecules structure by a decrease of PVC molecular weight. Thus, it is experimentally shown that practically all internal unsaturated groups present in PVC initial macromolecules are oxovinylene ones and PVC dehydrochlorination rate is linearly connected with the content of the internal labile oxovinylene groups in polymeric molecules [14, 26], determined by the alkaline hydrolysis (Fig. 2). It is significant that the polymeric products synthesized in the absence of oxygen were always noticeably more stable than PVC produced in industry in the first case due to the presence of rather stable internal ^-chloroallyl (not oxovinylene) groups (oxidative ozonolysis) in PVC macromolecules structure. Generally, the real process of HCl elimination during PVC disintegration via

transformation of framing groups is complex, since all abnormal groups contained in the macromolecules structure contribute to the process. However, the contribution of different reactions varies and in some cases may be neglected.

V*10',HCI/(molPVC*s)

2.5 -

2.0 -

0.2 0.6 1.0 i,4

Г«*104, mol/mol PVC

Fig. 2. Dependence of PVC dehydrochlorination rate on the content of carbonylallyl groups in polymer molecules (448 K, 10-2 Pa)

The kinetic analysis, when taking into account the real content of characteristics, including abnormal groups in PVC and rate constants of their disintegration (Table 2), has shown [14, 17, 24, 27] the ratio of rate constants КГ,,:К:К:К is 1:100:100:100000. For this reason PVC own

Cl     c     t p

thermal stability is determined by an effect of the adjacent group of the long-range order (conjugation effect) and the total elimination rate of HCl from PVC is described by the simple equation with sufficient accuracy:

vhci = = Kcia0 + Kpg0 = vci + vP (2)

Taking into account PVC disintegration with participation of tertiary chloride (At) and j8-chloroallyl (Ac) groups the contribution of the expression Vp = KPgo is about 90 % and more of total gross-rate of PVC dehydrochlorination, that precisely points to oxovinylene (not ^-chloroallyl) activation of the gross-process of PVC thermal disintegration.

Table 2

Rate constants of dehydrochlorination of haracteristic groups and their content in

the initial PVC structure

Group

Content in PVC

Disintegration rate constants at 448 K

 

Index

Amount, mol/mol PVC

Authors

Index

Value, s 1

Authors

~CO-CH=CH-CHCl~

g0

~10-4

K.Minsker, 1978 E.Sorvik, 1984 G.Zimmerman, 1984

Kp

10-1-10-2

K.Minsker, 1977 W.Starnes, 1985

(~CH2)CCl-CH2-CH2Cl~

ACl0

0

K.Minsker, 1978 G.Zimmerman, 1984

KCl

10-5-10-4

Z.Meyer, 1971 B.Troitsky, 1973 W.Starnes, 1983

-CCl-CH2CH2Cl

At0

~10-3

E.Sorvik, 1984 A.Caraculaku, 1981 V.Zegelman, 1985

Kt

10-4

W.Starnes, 1983 Z.Meyer, 1971

~CH2-(CH=CH)n>1-CHCl~

ap

0

K.Minsker, 1976

Kp

~10-2

Z.Meyer, 1971 K.Minsker, 1984

~CH2-CHCl-CH2-CHCl~

«0

1

-

KCl

10-7-10-8

Z.Meyer, 1971 K.Minsker, 1972

The development of the concept of oxovinylene activation of PVC thermodestruction appeared to be an important mark in the theory and practice of PVC chemistry and objectively defines the necessity of the new specific approach to the investigation of various aspects of PVC destruction and stabilization.

In particular, new characteristic reaction for unsaturated ketones, confirming the presence of oxyvinylene groups in PVC structure, is the interaction of ~C(O)-CH=CH-CHCl~ groups with organic phosphites P(OR)3 [29-33] and dienes [34, 35].

2.3. Principal ways for PVC stabilization

Organic phosphites under mild conditions (290­330 K) easily react with oxovinylene groups in the presence of proton donors with formation of stable ketophosphonates:

H H

~C-C^C-CH~    + P(OR)3

(3)

=P(OR)3

Reaction kinetics of organic phosphites interaction with oxovinylene groups are shown in Fig. 3. The formation of ketophosphonate structures according to the reaction (3) results in disappearance of internal unsaturated C=C bonds in a PVC structure. As a result, neither oxidizing ozonolysis of a polymeric product nor alkaline hydrolysis in particular results in disintegration of macromolecules and decrease of PVC molecular weight.

Fig. 3. The change of ~C(O)-CH=CH~ groups content in PVC during its interaction with tri-(2-ethylhexyl)phosphite (1-3). C0 = 10-2 mol/mol PVC. Temperatures: 289 K (1), 298 K (2), 448 K (3)

It is important to specify that organic phosphites do not react with-chloroallyl groups that is confirmed by the method of competing reactions of organic phosphites (trialkyl-, arylalkyl- and triarylphosphites) with a mixture (1:1, mol/mol) of methylvinylketone (model of an oxovinylene group) and 4-chloropentene-2 (model of a / -chloroallyl group) at 353 K. An organic phosphite practically quantitatively (regarding to proton donors) selectively reacts with methylvinylketone, while 4-chloropentene-2 is practically quantitatively segrerated after the reaction without changes, not counting some amount of its dehydrochlorination products (less than 7 wt %).

The CH3-C(O)-CH2-CH2-P(OR)2 is the main reaction

product (up to 75 wt %). In this reaction trialkyl- and alkylarylphosphites are more active than triarylphosphites.

Dienophylic oxovinylene groups react with conjugated dienes according to the Diels-Alder reaction:

CHCl~

о

C

CHCl~

H -C =

H

RHC

-CHCl- +

HC

CH

CHR-

O

HH

-C-C-CHCl~

/ \

RHC CHR-

\ /

(4)

This new, not known earlier reaction for PVC proceeds under mild conditions (353 K) with cyclopentadiene, piperylene, isoprene, 5-methylheptatriene-1,3,6 etc. and results (see reaction 4) in liquidation of internal unsaturated C=C groups in PVC chains similarly to organic phosphites.

PVC stabilization, i.e. a complex of methods used for the increase of polymer stability to the action of various factors (heat, light, oxygen etc.) during storage, processing and exploitation, is closely connected with a level of PVC development disintegration theory. Therefore, it is clear that significant change of theoretical developments about the reasons of PVC thermal instability (a presence of oxovinylene groups in the backbone), mechanism of the process (fundamental influence of the adjacent groups of the long-range order) and kinetics of their disintegration have shown the necessity and have given the possibility to create a new view while determing of new effective ways of PVC stabilization.

According to the reaction (3) it is impossible and not necessary to increase stability of PVC macromolecules due to the reduction of rate Vcl because this process is rather slow. According to the experimental data, the rate of PVC statistical dehydrochlorination Vcl (according to the random law) is constant and does not depend on the method of polymer synthesis and its molecular weight. Hence, it is the fundamental characteristic of PVC, showing that all parts in ~ВХВХВХ~ clusters participate in the process of HCl elimination according to the random law, whereas the rate of the conjugated systems formation VP may differ as a result of its linearly dependence on the content of oxovinylene groups in PVC initial macromolecules (go) (Fig. 2).

Thus, the basis of effective PVC stabilization which determines both operational properties and durability of rigid materials and products from PVC is the principle of self-stability increase of PVC macromolecules [17, 40­43]. This can be achieved first of all due to a chemical stabilization of PVC - destruction of labile oxovinylene groups present in initial PVC macromolecules via specific polymer-analogous reactions with even one reaction

center (1)-(3):

2

~C-CH=CH-CH~

HI 13

O Cl

The conjugation ~C(O)CH=CH~ has to be destroyed and/or labile chlorine atom has to be replaced

with a more stable framing group during interaction with the corresponding chemicals-additives (stabilizers). This principle is a basis of PVC stabilization in real composition during manufacturing rigid materials and products.

1. Polymer-analogous reactions with >C=O fragments of oxovinylenchloride groups:

R3SiH

 

 

 

R3GeH R3

о II

о

/\

'R-HCCH-R"

~C-CH=CH-CHCl~

 

 

 

 

OH OH

 

1 1

R'-CH-CH-R"

~CH-CH=CH-CHCl~

I

—Si

G.A.Razuvaev et al., 1969 [5]

~C-CH=CH-CHCl~

/\

0 O

1 I

R'-CH-CH-R"

K.S.Minsker, S.R.Ivanova, 1978 [36]

2. Polymer-analogous reactions with >C=C< fragments of oxovinylene groups:

P(OR)3

~C-CH2-CH-CHCl~

II I

O -,P(OR)3

~C=CH~ 04 ^,CH-CHCl~

K.S.Minsker, N.A.Mukmeneva et al., 1979 [31-33]

O

R'-CH=CH-CH=CH-R" ,

HC=CH

/ \

~c CHCl~

\ /

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