Stabilization of Polymers and Stabilizer Processes

1 Poly(vinyl chloride) Stabilization Mechanisms P E T E R P. K L E M C H U K

Downloaded by 79.172.193.32 on March 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch001

Geigy Research Laboratories, Ardsley, Ν. Y.

Investigation of the kinetics of the reaction of 4-chloro-2pentene, an allylic chloride model for the unstable moiety of poly(vinyl chloride), with several thermal stabilizers for the polymer has led to a better understanding of the stabili zation mechanism. One general feature of the mechanism is complexing of the labile chlorine atom by the metal atom of the stabilizer. A second general feature is substitution of the complexed chlorine atom by a ligand (either carboxylate or mercaptide) bound to the metal. Stabilization requires that the new allylic substituent (ester or sulfide) be more thermally stable than the allylic chlorine. The isolation of products from stabilizer-model compound reactions supports the substitution hypothesis of poly(vinyl chloride) stabiliza tion. *~r*he thermal degradation of poly (vinyl chloride) is now well understood to be caused by the ordered loss of hydrogen chloride, initiated at an unstable site, which results in the formation of long, colored polyene chains: — C H — C H — (CH —CH ) —CH —CH—X 2

2

n

2

CI |~ Cl CI — C H — C H — ( CH2=CH ) — C H = C H — X H C 1

9

"

(X = activating group)

n

k

The head-to-tail structure of poly (vinyl chloride) permits the continuous regeneration of an allylic chloride moiety as hydrogen chloride elimina tion proceeds along a chain. Thus, once initiated, loss of hydrogen chlo ride may proceed along a polymer chain without abatement. Allylic chloride (3, 6) and tertiary chloride (6) functionalities have been implicated as the unstable sites which initiate dehydrochlorination. Peroxide and hydroperoxide functionalities have also been implicated by 1

Platzer; Stabilization of Polymers and Stabilizer Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


2

STABILIZATION OF POLYMERS A N D STABILIZER PROCESSES

some investigators (11, 12, 13), and free radical degradation and stabili zation mechanisms have been postulated (JO, 13). The allylic chloride and groups result primarily from chain termination to monomer ( 4 ) . Chain termination by disproportionation may make a small contribution. About 60 terminal allylic chloride groups were estimated to be present among every 100 polymer molecules i n one poly (vinyl chloride) sample (3). Tertiary chloride groups, estimated i n one sample at 20 per polymer molecule, are postulated to arise from chain transfer via hydrogen ab straction between a polymer molecule and a growing radical chain ( 5 ) . A study of the thermal decomposition of model compounds led to the following order of increasing stability: tertiary chloride, internal allylic chloride, terminal allylic chloride, secondary chloride (1, 2). However, Baum and Wartman (3) conclude that hydrogen chloride loss is initiated primarily from terminal unsaturation and that initiation from tertiary chloride is much less important. Most thermal stabilizers for poly (vinyl chloride) are metal salts of carboxylic acids or mercaptans. The commonly used metals are cadmium, barium, zinc, lead, calcium, and dibutyltin. Originally it was assumed the metal salts act as scavengers for hydrogen chloride. However, Frye and Horst (7,8) found evidence for the introduction of ester groups i n the polymer from metal carboxylate stabilizers, which led them to postu late that thermal stabilizers function by substituting the unstable chlorine atoms with the ligands of the stabilizer to yield derivatives which are more thermally stable than the original chloride. This paper presents the results of a study of the reactions of several thermal stabilizers for poly (vinyl chloride) with an allylic chloride model and a tertiary chloride model. The findings of this study provide consid erable insight into the mechanism of stabilizer action. Experimental Materials. Chlorobenzene (Matheson) was washed with concen trated sulfuric acid, then with water, dried with anhydrous magnesium sulfate, and distilled. Dibutyltin dilaurate and dibutyltin maleate, commercial stabilizers Thermolite 12 and Thermolite 13 (Metal and Thermit), were used with out further purification. Dibutyltin β-mercaptopropionate (Advastab Τ 360, Advance Solvents ) was recrystallized twice from ethyl acetate. D i butyltin dichloride (Matheson) was recrystallized twice from n-heptane, m.p. 41.0°-43.0°C. The other dibutyltin compounds used were prepared by the reaction of dibutyltin oxide with the carboxylic acid or mercaptan in refluxing toluene with water being retained i n a Dean-Stark trap. Filtration of the resulting solution through filter aid, followed b y con centration under vacuum to constant weight, gave the desired compounds in virtually quantitative yield. Cadmium 2-ethylhexanoate was prepared in refluxing methanol from the reaction of cadmium oxide and the acid.


1.

KLEMCHUK

3

Poly(vinyl chloride) Mechanisms

Zinc 2-ethylhexanoate was prepared i n refluxing toluene from the reac tion of 2-ethylhexanoic acid with zinc oxide. The lead salt was prepared in methanol by the reaction of lead acetate with 2-ethylhexanoic acid. The reaction of barium oxide with 2-ethylhexanoic acid i n toluene pro vided the barium salt. 4 - C H L O R O - 2 - P E N T E N E . 1,3-Pentadiene ("Baker"; 100 grams, 1.47 moles) reacted with 60 grams (1.65 moles) of anhydrous hydrogen chloride i n 100 ml. of anhydrous' ether for three days at ambient tempera ture. The ethereal solution was washed quickly with water and aqueous sodium bicarbonate and dried quickly with magnesium sulfate. The ether was removed at atmospheric pressure. The colorless concentrate was distilled at 150 mm. i n a Nester and Faust spinning band column. 4-Chloro-2-pentene was collected at 55°C. (150 m m . ) ; n 1.4307 [litera ture value, b.p., 58°C. at 155 mm.; n 1.4328 ( 9 ) ] . D


D

DODECYL 4-PENT-2-ENYL

2 5

2 0

SULFIDE.

D i b u t y l t i n oxide

(21.37 grams,

0.08589 mole) reacted with 34.7 grams (0.1715 mole) of dodecyl mercaptan i n 150 ml. of toluene at reflux for 39 minutes when the evolution of water had ceased. The reaction product was charcoaled, filtered, and concentrated to constant weight at 55 °C. and < 1 m m . The yield of colorless dibutyltin bis(dodecylmercaptide) was 54.7 grams (theory 54.53 grams). This mercaptide, 1.5023 grams (0.00494 mole) of d i butyltin dichloride, and 17.96 grams (0.1718 mole) of 4-chloro-2-pentene reacted i n 250 m l . chlorobenzene solution for 7.2 hours when analysis of a sample of the reaction product for chloride showed the reaction to be virtually completed. Distillation i n the spinning band column gave chlorobenzene col lected at 57°C. (58 m m . ) . The sulfide was purified by elution from silica gel with n-hexane ( over-all yield 6 2 % of theory); n 1.4680. The structure of the sulfide was confirmed by N M R and infrared spectroscopy. Analysis: calculated for C i H S : C , 75.48; H , 12.67; S , 11.85; found: C , 75.20; H , 12.45; S , 11.56. D

7

4-PENT-2-ENYL LAURATE.

2 5

3 4

T W O chlorobenzene solutions, one 65 m l . ,

the other 70 ml., from virtually completed reactions of 0.086M dibutyltin dilaurate with 0.1717M 4-chloro-2-pentene were combined and concen trated under vacuum. The resulting liquid concentrate was chromatographed on 240 grams Woelm neutral alumina ( 3 % water added). Elution with n-hexane gave 1.99 grams (32% ) of 4-pent-2-enyl laurate; n 1.4437. The structure was confirmed by N M R and infrared spec troscopy. Analysis: calculated for C i 7 H 0 : C , 76.06; H , 12.02; found: C, 76.36; H , 11.84. Kinetic Measurements. A l l experiments were conducted i n a con stant temperature o i l bath controlled within ± 0 . 1 ° C . The stabilizer was dissolved i n chlorobenzene i n a volumetric flask of suitable volume, usually 100 ml., and placed i n the bath. After about 7 minutes the volume was adjusted to about 95 ml., and 4-chloro-2-pentene was added from a pipet with swirling; for 100 m l . of solution generally 2.00 m l . (average wt. 1.7958 grams) of chloride was added to give a 0.172N solution i n chloride. The volume was adjusted quickly to nearly 100 m l . with chloro benzene, and the contents were mixed thoroughly. Final adjustment to 100 m l . was made when thermal equilibrium was reached. D

2 5

a 2

2



4


A t suitable intervals 5.00-ml. samples of the reaction product were removed using a pipet which had been preheated to the bath tempera ture. The reaction was quenched by cooling the samples i n an ice bath. The sample was concentrated under vacuum, finally at 50 °C. at aspirator vacuum (12 mm.), to remove unreacted 4-chloro-2-pentene which reacts modeartely readily with silver nitrate. The residue from carboxylate stabilizers was dissolved with acetone and titrated potentiometrically with 0.1N silver nitrate in the presence of 100 ml. of water and 5 ml. of concentrated nitric acid. The residue from mercaptide stabilizers was dissolved with chloro benzene, and it was allowed to react with 0.4 ml. of 40% peracetic acid for 10-15 minutes. The reaction product was added with acetone rinsing to a solution of 0.50 gram of sodium sulfite in 100 ml. of water. After adding 5 ml. of concentrated nitric acid the chloride ion was titrated potentiometrically with 0.1N silver nitrate. Suitable control experiments had been carried out to establish that dibutyltin dichloride is not lost from the residue at 50 °C. and 12 mm. and to establish that the procedure for mercaptide removal (peracetic acid oxidation) d i d not interfere with the accuracy of the chloride analysis. Determining Substitution-Elimination Ratios. A 5.00-ml. aliquot of the reaction solution containing dibutyltin stabilizers was concentrated as usual, and the concentrate was titrated in methanol with 0.1N potas sium hydroxide in methanol. This titration gave total acidity: dibutyltin dichloride, dibutyltin dicarboxylate plus organic acid from the reaction of hydrogen chloride with dibutyltin dicarboxylate. Excess acidity (free organic acid) was calculated by substracting from the meq. of alkali consumed the quantity arising from dibutyltin dichloride and dibutyltin dicarboxylate. The chloride content of the same aliquot used for total acidity was determined by potentiometric titration with 0.1N silver nitrate solution. ^ . . ,„ meq. excess acidity % Elimination (E) = — x

% Substitution (S) = 100 - Ε X 100 Substitution elimination ratios for the other metal stabilizers were determined on reaction mixtures which had gone to completion. Aliquots were freed of metal chloride before titration with 0.1N sodium hydroxide by filtration (Pb) or extraction with water (Zn, C d ) . The titration gave the meq. of excess acidity directly. Results Kinetic investigation revealed that thermal stabilizers replace the allylic chlorine at rates which are much faster than the unimolecular decomposition rate of the allylic chloride. This clearly establishes that the stabilizers do not act only as acid acceptors but that they are capable


1.

KLEMCHUK

5

Polyvinyl chloride) Mechanisms

of rapidly replacing labile chlorine atoms with a more stable group, thereby "healing" the polymer and interrupting the zipperlike elimination of hydrogen chloride. It was especially gratifying to find a qualitative agreement between the effectiveness of dibutyltin stabilizers and their rates of reaction with the allylic chloride model. The rate expression for the reaction of the allylic chloride with dibutyltin stabilizers is considered to have four terms: -dRCl dt

fci[RCl]

+ fe [RCl] [Bu SnX ] + fc [RCl] [Bu^SnXCl] 2

2

3

+ fc [RCl] [Bu SnCl ] 4


2

2

2

(1)

The first term represents the unimolecular decomposition of allylic chlo ride. The first and fourth terms are responsible for hydrogen chloride ehmination and do not result i n stabilization. The second and third terms are responsible for stabilization. This work has provided an esti mate of the rate constants for dibutyltin dilaurate and dibutyltin bis(monobutyl maleate). The reaction half-time for 4-chloro-2-pentene, the allylic chloride model, with dibutyltin 0-mercaptopropionate is about 1/20 that for 2-chloro-2-methylbutane, a teri-chloride model, with the same stabilizer. This result supports the choice of an allylic chloride as the most impor tant unstable functionality of poly (vinyl chloride). Barium-cadmium synergism is postulated to be caused by a rapid exchange of chlorine from cadmium to barium accompanied by the trans fer of stabilizer ligand from barium to cadmium. The importance of this exchange lies in postponing as long as possible the formation of cadmium chloride which, as a strong Lewis acid, is capable of initiating polymer degradation. THscussion Allylic Chloride vs. ferf-Chloride Reactivity. There is some question in the literature as to whether the allylic chloride moiety or ferf-chloride group is more responsible for the thermal instability of poly (vinyl chloride) (J, 2). To shed some light on this problem we compared the relative reactivities at 100 °C. in chlorobenzene of 4-chloro-2-pentene and 2-chloro-2-methylbutane with dibutyltin β-mercaptopropionate. Data are summarized i n Table I. The half-time for the reaction of the allylic chloride with the stabilizer mercaptide group was less than 15 minutes, whereas the half-time for the fert-chloride was nearly 20 times longer. The greater reactivity of the allyl chloride suggests that it is the more important functionality i n polymer degradation. However, these results on rates of chlorine substitution are not necessarily an exact measure of thermal instability.


6


Table I.

Allylic Chloride vs. tert-Chloridc

Reaction with B u S n ( 0 C C H C H S ) Stabilizer 2

2

2

2

Chloride

Cone, M

Stabilizer Cone, M

Half-time, hours

CH CHCH=CHCH

0.172

0.0858

Stabilization of Polymers and Stabilizer Processes

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