Poly(vinyl chloride) - ACS Publications - American Chemical Society

18

P o l y ( v i n y l chloride) 1

2

ROY T. GOTTESMAN and DONALD GOODMAN 1

The Vinyl Institute, A Division of the Society of the Plastics Industry, New York, NY 10017 Tenneco Polymers, Inc., Flemington, NJ 08822

Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch018

2

History of Production Manufacture of Vinyl Chloride Monomer Polymerization of Vinyl Chloride Copolymerization of Vinyl Chloride Characterization of Poly(vinyl chloride) Physical Properties Chemical Properties Classification of Poly(vinyl chloride) Resins Plasticizers Heat Stabilizers Processing Aids, Impact Modifiers, and Other Additives Applications for Poly(vinyl chloride) Vinyl Chloride Toxicity and Federal Regulations of the Poly(vinyl chloride) Industry

This chapter will cover the chemistry and technology of poly(vinyl chloride) (PVC) homopolymers having a repeating unit of -CH -CHCl-

as well as copolymers of vinyl chloride with smaller amounts of other unsaturated monomers such as vinyl acetate, ethylene, propylene, acrylates, and vinylidene chloride. 2

History of Production

Vinyl chloride was reported by Regnault in 1835 (1), and he reporte its first polymerization three years later (2). Actually, later work showed that the Regnault polymer was poly(vinylidene chloride). It was not until 1872 that PVC was prepared by Bauman, who carried out polymerizations of various vinyl compounds in sealed tubes (3). Industrial development of PVC resins began about 50 years ago. Full commercial scale production started in Germany in 1931 and in the United States in the late 1930s. U.S. production was sparked by the observation by Waldo L. Semon at B. F. Goodrich in 1933 that PVC, when heated in the presence of a high boiling liquid (a plasticizer), became a flexible material that resembled rubber or 0097^6156/85/0285-0383$14.70/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


384

APPLIED POLYMER SCIENCE

l e a t h e r . The annual p r o d u c t i o n of PVC exceeded 1 m i l l i o n l b i n 1935, and the p a t t e r n has been one of steady growth s i n c e then. Today v i n y l c h l o r i d e polymers rank with polyethylene and styrene resins among the most commercially s i g n i f i c a n t polymers. Uses of PVC are widespread—from automobile i n t e r i o r s , to the f l o o r s we walk on, the packaging of the food we eat, the f u r n i t u r e we s i t on, the v i n y l phonograph records we l i s t e n to, the garden hose we use, the shower curtains i n our bathroom, the d o l l s and toys our youngsters p l a y w i t h , and l a s t , but not l e a s t , the c r e d i t cards we use. In 1979, one hundred seven years a f t e r Bauman produced i t i n h i s l a b o r a t o r y , U.S. p r o d u c t i o n a l o n e was over 6 b i l l i o n l b . The s i g n i f i c a n t growth of t h i s polymer i s shown g r a p h i c a l l y i n Figure 1 (4). Figure 1 a l s o demonstrates the s e n s i t i v i t y of PVC production to economic c o n d i t i o n s . The s i g n i f i c a n t growth of PVC peaked i n 1979 and has s i n c e then shown s i g n s of weakness due to a depressed economy. The PVC producers i n the United States at the s t a r t of 1982 are l i s t e d i n T a b l e I . The name p l a t e c a p a c i t y of these p l a n t s i s i n the order of 8 b i l l i o n l b of PVC. The a c t u a l production rose from 5.5 b i l l i o n l b i n 1980 to approximately 6 b i l l i o n l b i n 1981. The overcapacity that was f e l t i n 1979 was accentuated during 1980 and 1981. P l a n t s were run at a p r o f i t a b l e 87% capacity i n 1979 but were l e s s p r o f i t a b l e i n 1982 when o p e r a t i n g at l e s s than 75% c a p a c i t y . C a p a c i t y i n c r e a s e s have peaked at a time of s l a c k demand. PVC consumption i s very much t i e d i n t o construction usages i n the United S t a t e s , perhaps more so than other t h e r m o p l a s t i c s . With i n t e r e s t r a t e s h i g h , c o n s t r u c t i o n a c t i v i t y i s depressed, and PVC use declines. Because of s i g n i f i c a n t o v e r c a p a c i t y and d e c l i n i n g p r o f i t margins, a p r e d i c t a b l e shakeout of PVC producers i s o c c u r r i n g . Considerable c o n s o l i d a t i o n of companies has taken place. Smaller companies with o l d plants are the most v u l n e r a b l e . Expansion plans have been postponed, and most construction a c t i v i t y i s l i m i t e d to simple replacement of outdated equipment. A l l of t h i s comes at a time when the v e r s a t i l i t y and energy s a v i n g s p o t e n t i a l of PVC i s most r e c o g n i z e d . A r e c e n t a r t i c l e demonstrated, f o r example, t h a t PVC i s s t i l l one of the l e a s t expensive p l a s t i c s , so that the cost per i n . of fabricated matter i s among the lowest (Table II) (5). The functional v e r s a t i l i t y of PVC i s reviewed i n Table I I I (6) i n which the numerous use areas for PVC are described. The energy required to produce PVC i s a l s o quite low (7). Much l e s s energy i s needed r e l a t i v e to metals production, p a r t i c u l a r l y aluminum.

Manufacture of V i n y l Chloride Monomer Two commercial feedstocks are used i n manufacture of v i n y l c h l o r i d e : ethylene or acetylene. Nearly 19% of the c h l o r i n e production i n the U n i t e d S t a t e s i s used i n v i n y l c h l o r i d e monomer (VCM) p r o d u c t i o n . Two other routes are not yet commercial.



18. GOTTESMAN AND GOODMAN

Poly (vinyl chloride)

V i n y l Chloride

i . i . i

Year Figure 1.

PVC and VCM Production:

1953 to 1981.


385


386

Table I .

PVC Producers and Capacities


In-place Nameplate Capacity as of 1/1/82 1000 Metric Tons

Supplier

Total

Suspension

B. F . Goodrich Tenneco Hooker Shintech Conoco Georgia-Pacific Borden Diamond Shamrock A i r Products Formosa Certain Teed Ethyl General T i r e & Rubber Pantasote Union Carbide Stauffer Goodyear Talleyrand Great American Keysor

780 425 365 330 325 320 228 194 165 107 100 81

700 405 295 330 325 320 205 160 165 80 100 65

70 65 65 65 45 45 30 30

70 65 65 65 45 30 30

Total

3835

3520

Dispersion

Additions, 1982 or Later

80 20 70

23 34

135

27

240

16 11 45

—

315

386


18.

Table I I .

387


GOTTESMAN AND GOODMAN

Cost Efficiency of Engineering Materials

Material

Dollars/lb

Cents/in.

3


Engineering Resins a

Acetal Nylon 6 / 6 Nylon 11 Polyarylate Poly(butylene terephthalate) Polycarbonate Poly(phenyl oxide) (modified) Polysulfone Reinforced PET, 30% Reinforced poly(phenylene s u l f i d e ) , 40% a

a

1.48 1.81 2.96 2.40 1.42-1.70 1.62 1.33-2.37 3.82 1.41 3.07-3.13

7.6 7.5 11.1 10.5 6.7-8.0 7.1 5.1-9.1 17.1 8.0 17.7-18.0

Other P l a s t i c Resins ABS, high impact Acrylic Polyethylene, high d e n s i t y Polypropylene Polystyrene, high impact P o l y v i n y l chloride) 3

3

0.97-1.14 0.87 0.55 0.44-0.50 0.50 0.31

3.7-4.3 3.7 1.9 1.4-1.6 1.9 1.3-1.5

Metals Aluminum SAE-309 (380) ingot Brass (#403) ingot Magnesium AZ-63A ingot S t e e l , CR carbon sheet ZincSAE-903 i n g o t a

Du Pont product b a s i s .

Source:

0.88-0.89 0.74 1.43 0.25 0.48-0.544 Du Pont (based on l i s t

8.4 22.7 9.3 7.0 11.6-12.8 prices).


388


Table I I I .

Major Markets for PVC (1981)


Market

Quantity

Calendering Building & construction Flooring Paneling Pool-pond l i n e r s Roof membranes Other b u i l d i n g Transportation Auto upholstery/trim Other upholstery/trim Auto tops Packaging: sheet E l e c t r i c a l : tapes Consumer & i n s t i t u t i o n a l Sporting recreation Toys Baby pants Footwear Handbags/cases Luggage Bookbinding Tablecloths, mats Hospital & health care Credit cards Decorative f i l m (adh.-back) Stationery, novelties Tapes, l a b e l s , e t c . Furniture/furnishings Upholstery Shower curtains Window shades/blinds/ awnings Waterbed sheet Wallcovering Other calendering ?otal calendering

68 11 16 9 2 32 8 5 36 4 10 15 2 14 11 9 2 17 7 8 5 2 6 34 5 5 4 14 8 369

3

Market

Extrusion Building & construction Pipe & conduit Pressure Water Gas Irrigation Other Drain/waste/vent Conduit Sewer/drain Other Siding & accessories Window p r o f i l e s A l l - v i n y l windows Composite windows Mobile home s k i r t Gutters/downspouts Foam moldings Weatherstripping Lighting Transportation Vehicle f l o o r mats Bumper s t r i p s Packaging Film Sheet Electrical: wire & cable Consumer & i n s t i t u t i o n a l Garden hose Medical tubing Blood/solution bags Stationery, novelties Appliances Total extrusion

Thousand metric tons.


Quantity

354 354 10 111 9 104 168 141 27 82 7 25 7 2 14 11 8 8 4 125 15 170 18 16 20 44 13 1473

18.



Market

Polyvinyl chloride)

Quantity

45 Blow molding: b o t t l e s Compression molding: sound records 449 Dispersion molding Transportation 16 Packaging: closures 14 Consumer & i n s t i t u t i o n a l 3 Toys Sporting/recreation 8 Footwear 6 Handles, grips 6 Appliances 6 I n d u s t r i a l : t r a f f i c cones 6 Adhesives, e t c . Adhesives 5 Sealants 4 Miscellaneous 5 Other dispersion 5 Total dispersion molding 84 Injection molding Building & construction Pipe f i t t i n g s 50 Other b u i l d i n g 3 Transportation: bumper parts 5 Electrical/electronics Plugs, connectors, e t c . 30 Appliances, bus. machines 17 Consumer & i n s t i t u t i o n a l Footwear 20 Hospital & health care 7 Other i n j e c t i o n 15 Total i n j e c t i o n molding 147

Market Dispersion coating Building: flooring Transportation Auto upholstery/trim Other upholstery/trim Anticorrosion coatings Consumer & i n s t i t u t i o n a l Apparel/outerwear Luggage Tableclothes,mats Hospital & health care Furniture/furnishings Upholstery Window shades/blinds/ awnings Wallcoverings Carpet backing Other Total dispersion coating

389

Quantity

61 12 2 5 6 4 4 3 8 6 6 7 15 139

Solution coating Packaging: cans Adhesives/coatings Total

4 21 25

V i n y l latexes; adhesives/sealants Export

25 195

Grand t o t a l


2551

390


From Acetylene. HgCl2 on charcoal HCsCH + HCl

> CH = CHC1 + Heat 2

90-140 °C


The gas phase r e a c t i o n of a c e t y l e n e w i t h hydrogen c h l o r i d e uses mercuric c h l o r i d e (8, 9) or other heavy metal h a l i d e s as c a t a l y s t . I t i s important t h a t the gas streams be dry and f r e e from a r s i n e , phosphine, or s u l f u r . Because e t h y l e n e i s p r i c e d s u b s t a n t i a l l y lower than acetylene, most recent processes substitute ethylene for a c e t y l e n e , and a c e t y l e n e - b a s e d v i n y l c h l o r i d e p l a n t s have been disappearing. From E t h y l e n e . T h i s process u t i l i z e s an o x y c h l o r i n a t i o n reaction w i t h e t h y l e n e and c h l o r i n e as f e e d s t o c k s . In the p r o c e s s , t h r e e d i s t i n c t reactions can be considered to be taking place: CH =CH + C l 9

9

(1)

> CH CHo-CHo -(jH

9

2

Cl Cl C] EDC (1,2-dichloroethane) (ethylene d i c h l o r i d e ) CH =CH + 2HC1 + 1/2 0 2

2

> jH^-jH^ + H 0

2

(2)

2

Cl C l The H C l consumed i n t h i s r e a c t i o n i s produced by decomposition of EDC: 480-510 °C CH -CH I I Cl C l 2

> CH =CH C l + HCl

2

2

pumice or hot tube

The o v e r a l l reaction can thus be seen to be 2CH =CH + C l + 2HC1 + l / 2 0 2

2

2

> 2CH -vj)H + H 0

2

2

Cl 2CH -CHo . 2 , 2 Cl Cl 9

2

2

Cl

> 2CH=CHC1 + 2HC1

2CH =CH + C l + l / 2 0 2

2

2

2

> 2CH =CHC1 + H 0 2

2

(3)

The f l o w sheet f o r a b a l a n c e d c h l o r i n a t i o n - o x y c h l o r i n a t i o n of ethylene to v i n y l c h l o r i d e monomer i s shown i n Figure 2. Currently t h i s process, with i t s v a r i a t i o n s i n v o l v i n g fixed and f l u i d beds and d i f f e r e n t methods of h e a t i n g and s e p a r a t i o n , d o m i n a t e s t h e commercial production of v i n y l c h l o r i d e with 93% of VCM being made by t h i s route.


18.



391


Balanced A c e t y l e n e - E t h y l e n e . An advantage of the acetylene-based process i s t h a t HCl i s not produced. Because H C l r e a c t s w i t h acetylene, a balanced acetylene-ethylene process has been described (10). This process represents a major step forward and reduces the amount of acetylene used with attendant raw material cost savings and use of by-product HCl. No new p l a n t s that use t h i s process are being b u i l t i n the United States. In 1970, Kureha Chemical Industry Company i n Japan brought a plant onstream i n which a self-developed process i s used to produce e t h y l e n e and a c e t y l e n e d i r e c t l y by cracking of crude o i l . These are used c a p t i v e l y i n VCM production. Thermal cracking of the intermediate EDC i s l i m i t e d to 50% to avoid excessive formation of high b o i l i n g products. Reactions i n v o l v e d are the f o l l o w i n g : CH =CH + C l 2

2

> CH =CH

2

2

Cl

2

Cl

f

480-510 C CH -CH | I Cl C l 2

> CH =CHC1 + HCl

2

2

pumice or hot tube

HC=CH + HCl

> CH =CHC1 2

CH =CH + HC=CH + C l 2

2

> 2 CH =CHC1

2

2

From Ethane. Ethane i s cheaper and more r e a d i l y a v a i l a b l e than e i t h e r e t h y l e n e or a c e t y l e n e . The "Transcat" process i n v o l v e s c r a c k i n g of a feedstock such as ethane to e t h y l e n e , which i s c h l o r i n a t e d , oxychlorinated, and dehydrochlorinated simultaneously. Copper o x y c h l o r i d e a c t s as an oxygen c a r r i e r i n t h i s process and a l s o functions i n the recovery of H C l : C u C l + hP 2

2

Cu Cl + C l 2

2

^

z

2

CuO*CuCl

•> 2CuCl

C u C l + 2HC1 + *s0 2

2

2

2

2

> 2CuCl + H 0 2

2

High purity v i n y l c h l o r i d e i s produced i n an o v e r a l l y i e l d of 80 mol% based on ethane. The feed can contain ethane, ethylene, mixed e t h y l e n e - c h l o r i n a t i o n products, and HCl i n various mixtures, and can thereby a l l o w r e c o v e r y of v a l u e s from such m a t e r i a l s . The f l o w s h e e t f o r t h e s i m u l t a n e o u s c h l o r i n a t i o n , o x i d a t i o n , and d e h y d r o c h l o r i n a t i o n f o r producing v i n y l c h l o r i d e by the Transcat process i s shown i n Figure 3. V i n y l c h l o r i d e i s a c o l o r l e s s , pleasant-sweet s m e l l i n g gas at normal temperatures. I t i s s o l u b l e i n a l i p h a t i c and a r o m a t i c hydrocarbons, e s t e r s , ketones, e t h e r s , a l c o h o l s , and c h l o r i n a t e d s o l v e n t s , but i s e s s e n t i a l l y i n s o l u b l e i n water. V i n y l c h l o r i d e may be stored i n ordinary s t e e l c y l i n d e r s , tank cars, and storage tanks. The monomer must be stored under pressure to maintain a l i q u i d state. Vessels are loaded or unloaded by use of i n e r t gas pressure or most commonly by using pumps. In e a r l i e r p r o d u c t i o n y e a r s , p o l y m e r i z a t i o n problems were encountered because of the presence of impurities such as acetylene, In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


392

Cl,

r

CH, = CHDCE Cho l rn i ator Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch018

2

OaO il.H 1• N

JT

ird l h iVo in iye

1

Fra tu ilm nann tig FraCcoo tiulm nann tig I l_ Strp Ccoo i p e r C rarcnkn igcse-— Scrubber Dryer Fu a

CautaC C yls,It

U1

Separao tr

CH, = CO H,xyCh o lrn iator

Figure 2.

Y

Crar.kinn}— 1 » I

oylpcohuo lnrd os cP om

r*-

Recycel DCE

Flowsheet: balanced c h l o r i n a t i o n - o x y c h l o r i n a t i o n of ethylene to v i n y l c h l o r i d e .

. Vn i yl Cho lrd ie H,0, CO, N,

H,0, co, Fra tu ilm nann tig Ccoo lhrn io iao n , OC xyhco la rn ito tin & DehydR roehaacolo tgrenaoitn

leesn PE rocfu snitg

O iaaco tio Rxed tn r

Cu O. CuC,I Figure 3.

Flowsheet: simultaneous c h l o r i n a t i o n , o x i d a t i o n , and dehydrochlorination for v i n y l c h l o r i d e production (Transcat p r o c e s s ) .


18.



393

i r o n , H C l , oxygen, water, and mercaptans, but these impurities have l a r g e l y been e l i m i n a t e d by improved manufacturing p r a c t i c e s . Properties of v i n y l c h l o r i d e monomer are given i n Table I V . Polymerization of V i n y l Chloride


V i n y l c h l o r i d e can be polymerized by suspension, emulsion, bulk, or s o l u t i o n techniques. The f i r s t two methods are the most important i n the U n i t e d S t a t e s . About 78% of the PVC produced by the U n i t e d States i s made by the suspension process, and nearly a l l of the rest i s produced by the e m u l s i o n process. In Europe, suspension and emulsion processes are used with approximately equal frequency. Suspension Polymerization. Suspension polymerization, a l s o c a l l e d g r a n u l a r or p e a r l p o l y m e r i z a t i o n , i n v o l v e s the f r e e - r a d i c a l c a t a l y z e d p o l y m e r i z a t i o n of d i s c r e t e d r o p l e t s of v i n y l c h l o r i d e monomer suspended i n water by a g i t a t i o n and a p r o t e c t i v e c o l l o i d such as g e l a t i n , m e t h y l c e l l u l o s e , and p o l y ( v i n y l a l c o h o l ) . Other suspending agents such as maleic anhydride-vinyl acetate copolymers, e t h y l c e l l u l o s e , c a r b o x y m e t h y l c e l l u l o s e , a l g i n a t e s , or other synthetic gums are a l s o used. The quantity and type of suspension system i s very important i n d e t e r m i n i n g p a r t i c l e s i z e , p a r t i c l e shape, porosity, and s i z e d i s t r i b u t i o n . L e v e l of suspending agent i s i n the range of 0.02 to 0.5%, but d e t a i l s c o n c e r n i n g these systems are c l o s e l y held commercial secrets. Modifying agents such as buffers and pH adjusters must be c a r e f u l l y selected. In a t y p i c a l suspension p r o c e s s , the suspending agent i s d i s s o l v e d i n d e i o n i z e d and deaerated water and added to e i t h e r a g l a s s - l i n e d or s t a i n l e s s s t e e l reaction v e s s e l . The remainder of the water, the c a t a l y s t or i n i t i a t o r , which i s g e n e r a l l y a monomers o l u b l e organic peroxide, and other a d d i t i v e s such as chain transfer agent (used to c o n t r o l chain length or molecular weight), which are g e n e r a l l y chlorinated s o l v e n t s such as t r i c h l o r o e t h y l e n e , are then added. The r e a c t o r , which i s j a c k e t e d and a g i t a t e d , i s s e a l e d and evacuated to remove oxygen, and l i q u e f i e d monomer i s pumped i n . The c l o s e d system i s heated to c a . 120-130 °F (50 °C) w i t h concomitant i n c r e a s e i n pressure to c a . 125 l b / i n . . Temperature c o n t r o l to ±1 °F i s necessary i n order to r e g u l a t e the m o l e c u l a r weight. P o l y m e r i z a t i o n i s continued at c o n s t a n t temperature u n t i l the pressure drops about 10-30 l b / i n . because of depletion of monomer at which p o i n t the excess monomer i s r e c o v e r e d , and the polymer s l u r r y i s s t r i p p e d to low r e s i d u a l VCM c o n t e n t , then c o o l e d , c e n t r i f u g e d , and d r i e d . T h i s p r e s s u r e drop corresponds to a c o n v e r s i o n i n the 80-92% range. Polymerization i s generally continued beyond the p o i n t at which 75% of the monomer has been c o n v e r t e d to polymer as i n d i c a t e d by p r e s s u r e drop, and d i f f e r e n t grades of r e s i n are .pa produced by s t r i p p i n g off excess monomer at different conversion l e v e l s . A s i m p l i f i e d f l o w sheet f o r a suspension polymerization process i s shown i n Figure 4. A l t h o u g h the batch r e a c t o r s o r i g i n a l l y used i n suspension polymerization were i n the 1000- to 2200-gal capacity range, plants were then b u i l t with l a r g e r sized reactors having 3750-gal and 5000g a l capacity. Recently, S h i t - E t s u i n Japan pioneered e x c e e d i n g l y l a r g e r e a c t o r s (36,000 g a l ) of unique design c a p a b l e of safe operation, and t h i s technology i s being used i n the United States by



394


Table IV.

Properties of V i n y l Chloride Monomer

Property

Value

B o i l i n g point Freezing point Density at -20 °C at 20 °C V i s c o s i t y at -10 °C at -20 °C

-13.8 °C -153.8 °C 0.983 g/mL (8.21 l b / g a l ) 0.910 g/mL (7.60 l b / g a l ) 0.25 cps 0.274 cps

Index of r e f r a c t i o n , n™*15 Surface tension, -20 °C Molecular weight Heat of polymerization Volume decrease on polymerization Flash point Vapor pressure at 0 °C at 20 °C at 40 °C Explosive l i m i t s i n a i r

1,398 22.27 dyn/cm 62.50 23 kcal/mol 35% -78 °C 25.1 l b / i n . 49.2 l b / i n . * 87.6 l b / i n . 4-22% by volume

VCeM W iht Vn iM yolno C he o lrrd ie . j •g ^ m i So trage Tank an

C aa targ yle st C Bh om b

1

2

CruM de So traVgC e Tank T odRR eceocvyece ryl an

rgke ae sodler S Tu an SW epa rtaro trGa H /

Compressor Condensor Dusltctor Ceypco ln S a raeo tr Coe osirit "MA 4

D ana de D ee aoien razie teddW tr

Dryer R o trF yalsA ri 2-S trgC ea oa om bnh iaoitnDryer

To PVC Bagger „ „ or Bulk storage Figure 4.

Flowsheet: chloride).

suspension p o l y m e r i z a t i o n of p o l y ( v i n y l



18.


Polyvinyl chloride)

395

Tenneco at Pasadena, Texas; S h i n t e c h at F r e e p o r t , Texas; and F i r e s t o n e (now Hooker Chemical Corp., a s u b s i d i a r y of O c c i d e n t a l Petroleum) at Baton Rouge, L o u i s i a n a . Cheraische Werke H u l s i n Germany i s r e p o r t e d to be o p e r a t i n g r e a c t o r s h a v i n g more than a 50,000-gal capacity. The heat of polymerization of v i n y l c h l o r i d e to polymer i s 22.9 k c a l / m o l c o r r e s p o n d i n g to the e v o l u t i o n of 659.5 B t u / l b . Thus, a major factor i n the successful operation of PVC manufacturing plants i s the m a x i m i z a t i o n of removal of exothermic heat b a l a n c e d by k i n e t i c c o n t r o l . T h i s f a c t o r l a r g e l y depends on the c h o i c e of c a t a l y s t systems to a s s i s t i n obtaining uniform conversion rates. A t y p i c a l c o n v e r s i o n c u r v e f o r suspension p o l y m e r i z a t i o n i s shown i n Figure 5 as given by Barr (11). The conversion rate peaks a f t e r 12.25 h at a p p r o x i m a t e l y 20% c o n v e r s i o n / h . The r e s u l t i s a peak heat l o a d , a s o - c a l l e d "heat k i c k " t h a t i s d i f f i c u l t to dissipate through the w a l l s of large reactors when c o o l i n g water i s used. This heat load can be removed by use of refrigerated water or v i a a r e f l u x condenser. Large reactor technology has advanced such t h a t one h a l f t o one t h i r d o f t h i s t i m e i s r e q u i r e d f o r polymerization. As i n d i c a t e d e a r l i e r , monomer s o l u b l e i n i t i a t o r s are used to c a t a l y z e t h i s f r e e - r a d i c a l polymerization. Among these are l a u r o y l peroxide, di(2-ethy1hexy1)peroxy dicarbonate, and 2 , 2 ' azobisisobutyronitrile. By properly s e l e c t i n g the i n i t i a t o r i t i s p o s s i b l e to reduce or e l i m i n a t e the i n d u c t i o n p e r i o d f o r the polymerization and obtain a uniform reaction rate. In addition to a quick s t a r t , the s e l e c t i o n of i n i t i a t o r s i s governed by the reaction temperature. U s u a l l y the 10 h h a l f - l i f e temperature i s a good i n d i c a t o r of t h i s s e l e c t i o n . A t a b u l a t i o n of c u r r e n t i n i t i a t o r s used i n PVC p r o d u c t i o n and t h e i r h a l f - l i f e d e c o m p o s i t i o n temperatures i s g i v e n i n T a b l e V (12). Optimum r e a c t o r o p e r a t i o n i n v o l v e s s t a r t i n g the r e a c t i o n f a s t , m a i n t a i n i n g an a c t i v e c o n t r o l l a b l e i s o t h e r m a l p o l y m e r i z a t i o n , and f i n i s h i n g without the c h a r a c t e r i s t i c exotherm ("heat k i c k " ) . To do t h i s , i t i s u s u a l l y necessary to use two or more i n i t i a t o r s . Once a b a l a n c e of i n i t i a t o r s i s found for a s p e c i f i c reaction temperature, the c o o l i n g water demand i s predictable and c o n t r o l l a b l e . The molecular weight of the polymer formed i s a function of the r e a c t i o n temperature used d u r i n g the p o l y m e r i z a t i o n . Lower temperatures favor increased m o l e c u l a r w e i g h t s . The r e l a t i o n s h i p between polymerization temperature and molecular weight of r e s i n (as expressed by r e l a t i v e v i s c o s i t y ) i s shown i n F i g u r e 6. In commercial practice, the molecular weight i s c o n t r o l l e d by reaction temperature, and the reaction rate i s c o n t r o l l e d by the s e l e c t i o n of the i n i t i a t o r and i t s concentration. Low molecular weight PVC homopolymer i s made at higher reaction temperature. Generally l e s s i n i t i a t o r , u s u a l l y l a u r o y l peroxide or a z o b i s i s o b u t y r o n i t r i l e (AIBN) i s needed, and there i s improved heat removal c a p a c i t y because of the i n c r e a s e d d i f f e r e n t i a l w i t h the c o o l i n g water. Low molecular weight polymers are more d i f f i c u l t to s t r i p of r e s i d u a l VCM and to dry down to low v o l a t i l e l e v e l s . A l s o , as p o l y m e r i z a t i o n temperature increases, the q u a l i t y of the polymer begins to suffer, and 160-170 °F i s g e n e r a l l y considered the maximum safe operating temperature. I f a s t i l l lower molecular


396



110

Time (hours) Figure 5.

V a r i a t i o n of c o n v e r s i o n and p r e s s u r e w i t h time i n a t y p i c a l suspension polymerization.

Table V. Decomposition Rates for PVC I n i t i a t o r s

3

Product

Chemical Name

Lupersol 188M75 Lupersol 228Z Lupersol TA-46 Lupersol 47M75 Lupersol 10M75 Lupersol 223 Luperox IPP Lupersol 221 Lupersol 225 Lupersol TA-54 Lupersol 11 Lupersol 219M75 Decanox F Alperox F

a-Cumyl peroxyneodecanoate Acetyl cyclohexylsulfonyl peroxide t-Amyl peroxyneodecanoate a-Cumyl peroxypivalate t-Butyl peroxyneodecanoate Di-2-ethylhexyl peroxydicarbonate Diisopropyl peroxydicarbonate Di-n-propyl peroxydicarbonate Di-sec-butyl peroxydicarbonate t-Amyl peroxypivalate t-Butyl peroxypivalate Diisononanoyl peroxide Didecanoyl peroxide Dilauroyl peroxide

Half-life. °C 10 h 1h 38 40 46 47 49 49 50 50 50 54 57 61 63 64

56 56 64 65 66 66 67 66 67 74 75 78 80 81

'Measured as 0.2 molar solutions in trichloroethylene.


Activation Energy, kcal/mol 26.6 29.9 28.7 28.0 28.5 30.5 29.6 30.7 27.6 25.7 29.7 31.1 31.6 31.2

18.



397


weight i s desired, use of chain transfer agents (regulators) at higher temperatures i s required.

the

E m u l s i o n P o l y m e r i z a t i o n . As noted with suspension polymerization, emulsion polymerization a l s o i n v o l v e s the dispersion of VCM i n an aqueous medium. As d i s t i n g u i s h e d from suspension polymerization, however, the emulsion process i n v o l v e s the use of a surface a c t i v e agent or soap as the e m u l s i f i e r and a w a t e r - s o l u b l e c a t a l y s t or i n i t i a t o r instead of the monomer-soluble c a t a l y s t used i n suspension processes. Although a g i t a t i o n i s necessary, i t i s not as important as i n the suspension processes because the emulsion i s maintained by use of the soap and p r o t e c t i v e c o l l o i d s to insure l a t e x s t a b i l i t y . The e m u l s i f i e r plays a s i g n i f i c a n t r o l e i n emulsion polymerizations. At the concentrations used, most of the e m u l s i f i e r e x i s t s i n the form of m i c e l l e s t h a t c o n s i s t of 50 to 100 m o l e c u l e s of emulsified soap. These m i c e l l e s s o l u b i l i z e a portion of the v i n y l c h l o r i d e t h a t i s n o r m a l l y o n l y s l i g h t l y water s o l u b l e (0.09%) at 20 °C. The remainder of the monomer e x i s t s o u t s i d e the m i c e l l e s . The reaction s t a r t s when a free r a d i c a l generated from the i n i t i a t o r e n t e r s t h e m i c e l l e , meets t h e monomer t h e r e i n , and r a p i d p o l y m e r i z a t i o n o c c u r s . As the r e a c t i o n c o n t i n u e s , the polymer p a r t i c l e s grow l a r g e r than the raicellar core and become engulfed by monomer molecules d i f f u s i n g out from the monomer droplets. Harkins (13) has t h e o r i z e d t h a t the monomer d r o p l e t s are not the l o c i of p o l y m e r i z a t i o n . Rather, they s u p p l y monomer to the growing c h a i n r a d i c a l i n i t i a t e d i n the soap m i c e l l e . Smith and Ewart (13a, 13b) quantified the Harkins theory by the equation R = k^M N/2 where R i s the rate of propagation, k i s the r a t e constant f o r p r o p a g a t i o n , M i s the monomer c o n c e n t r a t i o n i n growing c h a i n p a r t i c l e s , and N i s the number of polymer p a r t i c l e s per unit volume. I f M i s the constant, t h i s equation i s reduced to Rp = k N. Thus, the rate of emulsion polymerization should s o l e l y be a function of the number of polymer p a r t i c l e s . In a c t u a l i t y , the r e a c t i o n r a t e i n c r e a s e s up to 20-25% c o n v e r s i o n because of the i n c r e a s e i n the number of growing r a d i c a l c h a i n s ; then the r a t e s t e a d i e s as does the number of polymer p a r t i c l e s up to 70-80% c o n v e r s i o n . Beyond t h i s p o i n t , the r a t e drops o f f because of low monomer c o n c e n t r a t i o n . Thus, as T a l a m i n i (13c, 13d) has noted, a v a i l a b l e evidence indicates that emulsion polymerization of v i n y l c h l o r i d e does not resemble true emulsion polymerization as described by Smith and Ewart, but shows the general behavior of heterogeneous polymerization. In the U n i t e d S t a t e s e m u l s i o n p o l y m e r i z a t i o n s are g e n e r a l l y c a r r i e d out i n equipment q u i t e s i m i l a r to t h a t used f o r batch suspension p o l y m e r i z a t i o n . A f t e r monomer r e c o v e r y , the r e s i n i s g e n e r a l l y i s o l a t e d by spray d r y i n g . T h i s process amounts to approximately 13% of the PVC produced i n the United States. Outside of the United States, and p a r t i c u l a r l y i n Germany, most emulsion PVC i s produced i n continuous p o l y m e r i z e r s by use of t a l l r e a c t i o n towers. In a t y p i c a l tower, which i s the s u b j e c t of a patent to BASF, the tower has a diameter of 5 f t 3 i n . and i s some 23 f t t a l l , and the r e a c t o r i s a g i t a t e d o n l y at the top by a 4 - f t by 1 8 - i n . paddle agitator at 50 r/min to avoid l a t e x coagulation. In a t y p i c a l emulsion polymerization, e m u l s i f i e r , i n i t i a t o r , and buffer are d i s s o l v e d i n deionized water and are fed continuously to 1

p



398


the top of the reactor while monomer enters above the l i q u i d l e v e l . These reactors are operated i n p a r a l l e l with 18-24 h residence time i n each stage. The reaction temperature i s maintained between 100 and 125 °F by use of brine c i r c u l a t i o n i n the tower jacket. One of the reputed advantages of the tower reactor i s the large L/D r a t i o i n which monomer d r o p l e t s c a r r i e d down i n t o more or l e s s stagnant section of the tower w i l l separate and coalesce r i s i n g to the top of the tower where t h e r e i s a g i t a t i o n and the p o s s i b i l i t y f o r e m u l s i f i c a t i o n . T h i s advantage i s g i v e n as a major p l u s f o r the p a r a l l e l tower type of o p e r a t i o n as c o n t r a s t e d w i t h a c o n t i n u o u s s e r i e s of cascade reactors. At the c o n c l u s i o n of p o l y m e r i z a t i o n , unreacted monomer i s r e c o v e r e d by vacuum s t r i p p i n g , then i s compressed, condensed, and p u r i f i e d for r e c y c l e i n the process. A s t a b i l i z e r , u s u a l l y sodium carbonate, i s then added to the l a t e x at a l e v e l of about 0.4%, and the s t a b i l i z e d l a t e x i s spray dried. A l t e r n a t i v e l y some processes i n v o l v e drum drying f o l l o w i n g by grinding. In these procedures that i n v o l v e t o t a l d r y i n g o f t h e l a t e x , any c a t a l y z e r e s i d u e s , e m u l s i f i e r , b u f f e r , or other a d d i t i v e s d u r i n g the process end up with the product. P a r t i c l e s from emulsion processes are about 1 ym i n diameter, about 1/100 of those encountered i n suspension polymerization. A l t e r n a t e processes t h a t f i n d l i t t l e use i n the U n i t e d S t a t e s i n c l u d e c o a g u l a t i o n of the l a t e x by use of added e l e c t r o l y t e s , a l c o h o l s , ketones, heat, or shear. The coagulum i s dewatered by any convenient f i l t r a t i o n apparatus (thereby to remove the bulk of the e m u l s i f i e r ) and dried. Emulsion p o l y m e r i z a t i o n l e a d s to a v e r y narrow p a r t i c l e s i z e d i s t r i b u t i o n , which i s s u i t a b l e for use i n p l a s t i s o l a p p l i c a t i o n s . R e s i n s h a v i n g both a monodisperse and bimodal p a r t i c l e s i z e d i s t r i b u t i o n are produced. B l e n d i n g of l a t e x e s i s sometimes p r a c t i c e d w i t h the e x t e n s i v e use of s e e d - l a t e x t e c h n i q u e s . A s i m p l i f i e d schematic f l o w sheet f o r a c o n t i n u o u s e m u l s i o n polymerization process i s given i n Figure 7. Bulk Polymerization. Bulk p o l y m e r i z a t i o n processes, unlike suspension and e m u l s i o n processes are conducted i n the absence of d i l u e n t s . Removal of heat was a d e f i n i t e problem u n t i l work by P r o d u i t s Chimiques P e c h i n e y - S t . Gobain s o l v e d t h i s problem by breaking up l a r g e r polymer blocks with heavy b a l l s i n a h o r i z o n t a l , r o t a t i n g autoclave (14). The o r i g i n a l Pechiney-St. Gobain process has s i n c e been improved and l e s s than 10% of the t o t a l U.S. PVC c a p a c i t y i s produced by the l i c e n s e e s of the p r o c e s s : B. F. Goodrich Co. at P e d r i c k t o w n , New J e r s e y and a t P l a q u e m i n e , L o u i s i a n a ; Hooker Chemical Corp., a s u b s i d i a r y of O c c i d e n t a l Petroleum Corp. at Burlington, New Jersey; and Certain-Teed at Lake Charles, Louisiana. The polymerization i s a two-step batch operation i n i t i a t e d with 2 , 2 - a z o b i s i s o b u t y r o n i t r i l e (AIBN) or l a u r o y l p e r o x i d e or w i t h a mixture of AIBN and a c e t y l c y c l o h e x y l s u l f o n y l peroxide (ACSP). The process has the o b v i o u s advantage of e l i m i n a t i n g the need f o r p r o t e c t i v e c o l l o i d s , e m u l s i f i e r , buffers, and other a d d i t i v e s . The reaction i s started i n a v e r t i c a l reactor equipped with a high-speed a g i t a t o r o p e r a t i n g at 130 r / m i n at 140 °F and r e q u i r e s 3 h. The reaction i s continued to 10% conversion, when the batch i s dropped ,




Figure 6.

399


Polymerization temperature-relative r e l a t i o n s h i p on PVC homopolymers.

viscosity

V i yR lC iryeaM m Ton ech oo vlerd ndonoR eecrycel n

Spray Dre ir T

Toyse Pro.dIu ctl dR covery S iegS C a l tsm sfe i rsncau ndn o trage Figure 7.

Flowsheet: continuous e m u l s i o n poly(vinyl chloride).

p o l y m e r i z a t i o n of



400


to a h o r i z o n t a l a u t o c l a v e a g i t a t e d w i t h s l o w l y r o t a t i n g r i b b o n agitators as shown i n Figure 8. The agitator system comprises two r i b b o n s wound i n s p i r a l s of d i f f e r e n t diameters and of o p p o s i t e hands. The l a r g e r diameter s p i r a l serves as a conveyor screw at the end of the r e a c t i o n c y c l e and conveys the polymer to the product discharge port. Autoclaves ranging from 4200 to 21,000 g a l are used i n commercial o p e r a t i o n s , and i t i s understood t h a t s t i l l l a r g e r autoclaves are being designed. PVC i s i n s o l u b l e i n VCM. The p r e c i p i t a t e d polymer tends to c o a g u l a t e i n the c o n v e r s i o n range of 1-8%. As the c o n v e r s i o n continues, the precipitated polymer absorbs more and more monomer, and at a 15-20% c o n v e r s i o n , the r e a c t o r c o n t a i n s s o l i d polymer s w o l l e n w i t h monomer i n a monomer atmosphere. The h o r i z o n t a l autoclave prevents the formation of large polymer blocks by breaking them up. Advantages c l a i m e d f o r t h i s process are the h i g h e r b u l k d e n s i t y , improved p a r t i c l e s i z e d i s t r i b u t i o n , and more r a p i d p l a s t i c i z e r absorption. T a l a m i n i and c o w o r k e r s (15) h a v e demonstrated that the bulk p o l y m e r i z a t i o n process i s k i n e t i c a l l y equivalent to suspension polymerization. The autoclave i s operated at an a g i t a t i o n speed of 30 r/min and i n i t i a l l y i s c o o l e d t o m a i n t a i n the 140 °F p o l y m e r i z a t i o n temperature and a 130 l b / i n . v i n y l c h l o r i d e pressure. As the r e a c t i o n proceeds, hot water or c o l d water i s c i r c u l a t e d i n the j a c k e t of the a u t o c l a v e t o keep the a u t o c l a v e p r e s s u r e a t 130 l b / i n . . A p p r o x i m a t e l y 9.5 to 10 h are r e q u i r e d f o r the second stage of the p o l y m e r i z a t i o n f o r a t o t a l r e a c t i o n time of 12.5 to 13 h per batch. At t h i s point, the autoclave i s vented through a cyclone-bag f i l t e r combination (to remove entrained fine polymer) to the monomer r e c o v e r y system. The r e s i d u a l monomer adsorbed i n the polymer i s removed by evacuating the autoclave twice and breaking the vacuum each time with nitrogen. The monomer stripped product i s discharged through a port and transferred to product f i n i s h i n g where oversize material i s reduced by grinding and m i l l i n g to desired p a r t i c l e s i z e specifications. 2

S o l u t i o n Polymerization. S o l u t i o n polymerization i s over 45 years o l d , but o n l y about 3% of the PVC produced i n the U n i t e d S t a t e s i s made t h i s way. The s o l u t i o n process d i f f e r s from the other processes a l r e a d y d i s c u s s e d i n t h a t a s o l v e n t i s added to the p o l y m e r i z a t i o n system. The system may be heterogeneous, i n which case the monomer i s s o l u b l e but the polymer i s i n s o l u b l e . Examples are the use of hexane, butane, e t h y l a c e t a t e , or c y c l o h e x a n e as s o l v e n t s . A f t e r a d d i t i o n of a peroxide i n i t i a t o r and h e a t i n g to 40 °C, the p o l y m e r i z a t i o n s t a r t s and polymer p r e c i p i t a t e s out of s o l u t i o n as formed. In homogeneous r e a c t i o n s , both monomer and polymer are s o l u b l e t h e r e i n . Examples are the use of d i b u t y l phthalate and tetrahydrofuran as s o l v e n t s . S o l u t i o n p o l y m e r i z a t i o n i s used almost e x c l u s i v e l y f o r p r o d u c t i o n of copolymers of v i n y l c h l o r i d e and v i n y l a c e t a t e and t e r p o l y m e r s c o n t a i n i n g maleates. The v i n y l a c e t a t e copolymers contain 10-25% acetate, are h i g h l y uniform with a narrow molecular weight range, and are v a l u a b l e m a i n l y because of t h e i r unique s o l u b i l i t y and f i l m - f o r m i n g c h a r a c t e r i s t i c s . They f i n d use as


18.



401

s o l u t i o n c o a t i n g r e s i n s where t h e i r high q u a l i t y and u n i f o r m i t y command a premium price, Copolymerization of V i n y l Chloride In the c o p o l y m e r i z a t i o n of two monomers, propagation steps possible are k

M *

+ M

Mj*

+ M

x


12 > M *

2

2

k

+ M

x

M *

+ M

2

21 > M * x

k 2

four

> Mj* k

2

2

l l

x

M *

M-^ and M , the

2 2

> M * 2

where M ^ * and M * are growing polymer c h a i n s w i t h M^ and M , r e s p e c t i v e l y , at the a c t i v e growing ends. The polymer composition i s given by the r e a c t i v i t y r a t i o s : 2

2

r

r

k

l l

k

12

k

22 _

-

i

2

—

= k

12 I f r | and r are l e s s than 1, the polymers tend to a l t e r n a t e . I f r^ i s g r e a t e r than 1 and r i s l e s s than 1, Mj predominates i n the polymer. To prevent forming a polymer with a wide d i s t r i b u t i o n of composition, i t i s necessary that the more r e a c t i v e monomer be added during the course of the polymerization. The r e a c t i v i t y r a t i o s for v i n y l c h l o r i d e (Mj) and other monomers (M ) are shown i n Table V I . Copolymers w i t h a l l of the comonomers i n T a b l e VI have commercial value, but the copolymer containing v i n y l acetate i s the most important and i s p l a s t i c i z e d i n t e r n a l l y . Major uses include coatings, f l o o r coverings, and phonograph records for which low melt v i s c o s i t y , lower p r o c e s s i n g temperatures, freedom from e x t e r n a l p l a s t i c i z e r s , and s o l u b i l i t y are required. Copolymerization with v i n y l acetate reduces t e n s i l e strength, heat d i s t o r t i o n temperature, a b r a s i o n r e s i s t a n c e , c h e m i c a l r e s i s t a n c e , and heat s t a b i l i t y . However, l e s s heat s t a b i l i t y i s required because these copolymers can be processed at lower temperatures. The composition of a v i n y l c h l o r i d e - v i n y l a c e t a t e copolymer produced from a m i x t u r e of monomers i s shown i n F i g u r e 9. Because r | , the monomer r e a c t i v i t y r a t i o , i s g r e a t e r than one, and r is l e s s than one, the copolymer i s r i c h e r i n Mj ( v i n y l c h l o r i d e ) . Thus, i f one were to copolymerize a mixture of monomers comprising 60% v i n y l c h l o r i d e , the r e s u l t a n t copolymer would c o n t a i n approximately 75% v i n y l c h l o r i d e . 2

2

2

2


402



Coolant

Product discharge

Figure 8.

Horizontal autoclave: polymerization process.

Table V I . M

Pechiney

S t . Gobain b u l k

Monomer Reactivity Ratios, M^ = V i n y l Chloride

2

Acrylonitrile Butadiene Dibutyl maleate Diethyl fumarate Diethyl maleate Isobutylene Maleic anhydride Styrene V i n y l acetate Vinylidene chloride JV-Vinyl pyrrolidone

r

l

0.02 0.035 1.4 0.12 0.8 2.05 0.296 0.067 1.68 0.2 0.53

r

2

3.28 8.8 0.0 0.47 0.0 0.08 0.008 35.0 0.23 1.8 0.38



18.


100

tO


60

40

20

403

0

Wt. % V i n y l Chloride Monomer Figure 9.

Composition of copolymer produced from a v i n y l c h l o r i d e v i n y l acetate monomer mixture.



404


In attempts to reduce p r o c e s s i n g temperatures and i n c r e a s e s o l u b i l i t y , copolymers of v i n y l c h l o r i d e and v i n y l i d e n e c h l o r i d e have been developed. These are s o l u b l e i n ketone s o l v e n t s , can be d i l u t e d w i t h aromatic hydrocarbons, and are thus used i n c o a t i n g s a p p l i c a t i o n s . Such copolymers are u s e f u l as extender r e s i n s i n p l a s t i s o l a p p l i c a t i o n s for which rapid fusion at high temperatures i s required. Copolymers with 15% of maleate and fumarate esters afford good p r o c e s s i n g p r o p e r t i e s w i t h o n l y a m i l d impairment of p h y s i c a l properties. Heat d i s t o r t i o n temperature i s r e l a t i v e l y high. Copolymers w i t h i s o b u t y l v i n y l ether were d e v e l o p e d by BASF. These copolymers are s o l u b l e i n toluene and xylene, compatible with n i t r o c e l l u l o s e and o t h e r r e s i n s , and f i n d use i n c o a t i n g s a p p l i c a t i o n s . They provide strong f i l m s with good adhesion to metal and possess e x c e l l e n t l i g h t s t a b i l i t y and weathering properties. Other copolymers that have improved processing c h a r a c t e r i s t i c s at lower processing temperatures include a series of v i n y l c h l o r i d e p r o p y l e n e copolymers t h a t were i n t r o d u c e d by A i r Products and Chemicals and c o n t a i n 3 to 10% p r o p y l e n e . The lower p r o c e s s i n g temperatures permit the use of lower quantities of s t a b i l i z e r s , the use of less-expensive nontoxic s t a b i l i z e r s , and h i g h e r p r o c e s s i n g rates. Such copolymers have Food and Drug Administration approval for food a p p l i c a t i o n s . S i m i l a r copolymers having l i k e advantages and based on v i n y l c h l o r i d e - e t h y l e n e have been commercialized by Union Carbide. The p o s s i b i l i t i e s f o r copolymers with s p e c i a l i z e d performance c h a r a c t e r i s t i c s depend on the a b i l i t y to e f f e c t c o p o l y m e r i z a t i o n with the other monomer. To date efforts to effect copolymerization with such monomers as styrene and butadiene have been unsuccessful (r =35 v s . r ^ O . 0 6 7 f o r s t y r e n e , and r =8.8 v s . r ^ O . 0 3 5 f o r butadiene). The p o s s i b i l i t i e s nevertheless e x i s t for modifying the basic polymer to effect such d e s i r a b l e properties as c r o s s - l i n k i n g , improvement i n impact resistance, c o m p a t i b i l i t y with processing a i d modifiers, and UV s t a b i l i t y . 2

2

Characterization of P o l y ( v i n y l chloride) The p o l y m e r i z a t i o n process r e s u l t s i n polymer m o l e c u l e s w i t h a v a r i e t y o f c h a i n l e n g t h s or m o l e c u l a r w e i g h t s . T h u s , any measurement by e x p e r i m e n t a l means l e a d s to an average v a l u e . We should d i s t i n g u i s h between two types of molecular weight averages: 1. 2.

Mn = number average m o l e c u l a r weight. T h i s q u a n t i t y i s a measure of the number of m o l e c u l e s i n a known mass and can be a r r i v e d at by osmometry and g e l permeation chromatography. Mw = weight average molecular weight. This quantity i s measure of the e f f e c t t h a t the mass of the m o l e c u l e has on the average and i s obtained by u l t r a c e n t r i f u g a t i o n or g e l permeation chromatography. Most PVC r e s i n s i n commercial use have Mn ranging from 50,000 to somewhat above 100,000.

Mw i s always g r e a t e r than Mn except i n the case of monodisperse polymer for which Mw = Mn. The r a t i o of Mw/Mn i s taken as a measure of the p o l y d i s p e r s i t y or the breadth of molecular weight d i s t r i b u t i o n of the polymer. The



18.



405

molecular weight d i s t r i b u t i o n for PVC produced commercially tends to be narrower than for many other polymers produced v i a f r e e - r a d i c a l p o l y m e r i z a t i o n because the broadening i n d i s t r i b u t i o n caused by chain transfer to p r e v i o u s l y formed polymer m o l e c u l e s i s l a r g e l y minimized by the p r e c i p i t a t i o n of polymer from s o l u t i o n . Mw/Mn u s u a l l y l i e s between 2.0 and 2.3 and s e v e r a l i n v e s t i g a t o r s have shown r a t i o s i n t h i s range. Pezzin and coworkers (16) examined the m o l e c u l a r weight d i s t r i b u t i o n on s i x samples taken at degrees of conversion ranging between 4 and 94% i n a suspension polymerization and concluded t h a t the d i s t r i b u t i o n does not change d u r i n g the course of the polymerization. The molecular weight i s g e n e r a l l y determined by measurement of v i s c o s i t y of d i l u t e s o l u t i o n s . The most widely used or preferred s o l v e n t s f o r PVC are t e t r a h y d r o f u r a n (THF), cyclohexanone, and methyl e t h y l ketone (MEK). PVC s w e l l s i n aromatic hydrocarbons, but i s unaffected by a l c o h o l s or a l i p h a t i c hydrocarbons. Benzyl a l c o h o l at 155.4 °C and THF/water at 30 °C are the r e p o r t e d (17) t h e t a s o l v e n t s f o r PVC, t h a t i s , one i n which the dimensions of the molecular c o i l are unperturbed by s o l v e n t effects. For such d i l u t e s o l u t i o n s , the v i s c o s i t y and m o l e c u l a r weight are r e l a t e d by the Mark-Houwink-Sakurada equation [ri] = K M , where [n,] i s the i n t r i n s i c v i s c o s i t y and M i s the molecular weight. K and a are constants that are determined by measurement of i n t r i n s i c v i s c o s i t y and molecular weight (obtained by an absolute technique such as l i g h t s c a t t e r i n g , u l t r a c e n t r i f u g a t i o n , or osmometry). The i n t r i n s i c v i s c o s i t y [r|] i s obtained by p l o t t i n g the reduced v i s c o s i t y v e r s u s c o n c e n t r a t i o n and e x t r a p o l a t i n g to i n f i n i t e d i l u t i o n . Reduced v i s c o s i t y i s g i v e n by the e x p r e s s i o n r|-r|o/rioC where n, i s the v i s c o s i t y of s o l u t i o n , r\o i s the v i s c o s i t y of s o l v e n t , and C i s the concentration i n g/dL. The f o l l o w i n g r e l a t i o n s h i p s have been determined by Bohdanecky and coworkers (18): a

[n] = 1.38 x l O ^ i y ) ^ (Cyclohexanone, 25 °C) 4

[ri] = 1.63 x 10" M 0.77 (THF, 25 °C) W

and these p r o v i d e r e l i a b l e c o r r e l a t i o n s f o r narrow f r a c t i o n s of l i n e a r PVC where m o l e c u l a r weights are l e s s than 10 . Above 10 , discrepancies are noted, and these are attributed to aggregation. McKinney (19) has developed the f o l l o w i n g expressions r e l a t i n g number average and weight average molecular weights with i n t r i n s i c viscosity: M = 0.5 [n] x 10

5

n

\

= 0.9 [n] x 10

5

I f the molecular weight d i s t r i b u t i o n d i f f e r s from s l i g h t l y greater than 2 , these equations lead to erroneous r e s u l t s . Commercially, s e v e r a l types of v i s c o s i t y measurements are used to determine the m o l e c u l a r weight of PVC. These i n c l u d e , i n a d d i t i o n to i n t r i n s i c v i s c o s i t y as p r e v i o u s l y d e f i n e d , the following:


406 1. 2. 3.


4.


I n h e r e n t v i s c o s i t y (ASTM D l 2 4 3 - M e t h o d A ) — v i s c o s i t y i s determined at 15 °C on a 0.2% s o l u t i o n of cyclohexanone. S p e c i f i c v i s c o s i t y (ASTM D 1243-Method B ) — v i s c o s i t y i s determined i n nitrobenzene at 0.4% at 25 °C (used w i d e l y i n Japan). R e l a t i v e viscosity—determined on a 1% s o l u t i o n i n cyclohexanone at 25 °C. Fikentscher K value—0.5% s o l u t i o n i n cyclohexanone or ethylene d i c h l o r i d e (used widely i n Europe).

The i n t e r r e l a t i o n s h i p s between these v i s c o s i t y methods are shown i n F i g u r e s 10, 11, 12, and 13. In a d d i t i o n to m o l e c u l a r weight and m o l e c u l a r weight d i s t r i bution, branching and c r y s t a l l i n i t y of PVC merit consideration. Branching i n PVC, which occurs by a chain transfer mechanism, i s dependent on polymerization c o n d i t i o n s . Thus, i n a heterogeneous system having high l o c a l i z e d concentrations of polymer, branching might be expected to be r e l a t i v e l y high. At lower polymerization temperatures, the degree of branching i s low. The degree of branching i s e s t a b l i s h e d by IR a n a l y s i s on the p o l y e t h y l e n e produced by hydrogenation of PVC. The IR absorption bands for a methyl group (which would occur at a branch p o i n t ) at 1378 cm"" and a methylene group at 1370 cm" are used f o r t h i s determination. In general, branching i s not very prevalent i n PVC polymerized below room temperature (19a). The degree of branching i n commercial polymers has been found to range between 0.4 and 1.1 per 100 carbon atoms. Changes i n p h y s i c a l p r o p e r t i e s such as d e n s i t y , m e l t i n g point, and glass t r a n s i t i o n temperature have often been a t t r i b u t e d to branching, but i n r e a l i t y they are due to the s y n d i o t a c t i c i t y or c r y s t a l l i n i t y of the polymer. I t was Staudinger (19b) who f i r s t r e c o g n i z e d the concept of s t e r e o r e g u l a r i t y of v i n y l polymers. PVC may be r e p r e s e n t e d as a c h a i n i n which the carbon atoms l i e i n the p l a n e of the s l i d e , and atoms shown with a hatched l i n e l i e below the plane and those with the b o l d l i n e l i e above the p l a n e . Three c o n f i g u r a t i o n s can thus result: 1

1

1.

Atactic—No order to the configuration.

2.

I s o t a c t i c — A l l chlorine atoms are i n the same r e l a t i v e p o s i t i o n .

3.

Syndiotactic—Chlorine atoms are i n an a l t e r n a t i n g sequence.

Although the i s o t a c t i c and syndiotactic configurations are regular and a l l o w c r y s t a l l i z a t i o n of the polymer chain to occur, most PVC i s a t a c t i c , and the r e g u l a r i t y necessary f o r c h a i n s to pack and therefore c r y s t a l l i z e does not e x i s t . Thus, PVC i s not g e n e r a l l y c l a s s i f i e d as a c r y s t a l l i n e polymer. As formed i n conventional commercial processes, PVC i s h i g h l y amorphous with a very low degree of c r y s t a l l i n i t y associated with i t s s y n d i o t a c t i c sequences. Thus, PVC produced i n the normal fashion contains about a 10% degree of c r y s t a l l i n i t y . However, i f the polymerization i s carried out at very low temperatures, there i s a marked increase i n c r y s t a l l i n i t y . Thus, polymer prepared at





18.




408

J 1 40

1 50

l

I 1 60

1

I

1 70

1 80

1

I

I

1

I

I

I

1 90

2 00

2 10

2,20

2.30

2 40

2.50

1

L

2 60

2 70

2.80

Relative Viscosity 25 °C, 1% w/w in Cyclohexanone Figure 12.

R e l a t i v e v i s c o s i t y v s . K-value for PVC r e s i n s .


18.

Poh (vinyl chloride)


2

409

c

5| CM O

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Figure 13. D i l u t e s o l u t i o n v i s c o s i t y c o r r e l a t i o n c h a r t f o r PVC resins.



410


-15 °C has about 57% c r y s t a l l i n i t y and that produced at -75 °C has 85% c r y s t a l l i n i t y . The c r y s t a l l i n i t y i s due to the presence of s y n d i o t a c t i c sequence. NMR has a l s o been used to study the degree of s y n d i o t a c t i c i t y i n PVC as, f o r example, t h a t prepared by the b u l k p o l y m e r i z a t i o n process at temperatures ranging from -78 to 120 °C. By the a n a l y t i c a l techniques used, an a t a c t i c polymer would have a f r a c t i o n of s y n d i o t a c t i c d i a d s , 6 e q u a l to a, the f r a c t i o n of i s o t a c t i c d i a d s , or e q u a l to 0.5. Polymer prepared at or near commercial temperatures had a s y n d i o t a c t i c d i a d f r a c t i o n $, e q u a l to 0.51 to 0.53. However, the s y n d i o t a c t i c i t y increased as the temperature was decreased as shown i n Figure 14. B r i e f mention should be made as to ordering i n the PVC polymer. I f we consider the methylene group i n v i n y l c h l o r i d e as the t a i l of the m o l e c u l e and the carbon atom b e a r i n g the c h l o r i n e atom as the head of the m o l e c u l e , three p o s s i b i l i t i e s e x i s t : head-to-tail, head-to-head, and t a i l - t o - t a i l . Addition polymerization leads preponderantly to the h e a d - t o - t a i l structure. Marvel (20) and coworkers c a r r i e d out d e c h l o r i n a t i o n of the polymer i n b o i l i n g dioxane with zinc dust. In t h i s procedure, c h l o r i n e i s removed to y i e l d cyclopropane rings and z i n c c h l o r i d e . The d e c h l o r i n a t e d m a t e r i a l c o n t a i n e d 13-16% r e t a i n e d c h l o r i n e . A l t h o u g h t h i s f a v o r s the h e a d - t o - t a i l arrangement, i t does not preclude a c e r t a i n amount of head-to-head or t a i l - t o - t a i l product. However, the r e s i d u a l c h l o r i n e content corresponds to that expected from the random attack of z i n c on a h e a d - t o - t a i l polymer structure. Physical Properties PVC i s a hard, b r i t t l e polymer at room temperature, and, as such, would have l i t t l e use were i t not f o r the f a c t t h a t i t r e a d i l y accepts a number of p l a s t i c i z e r s . The resultant f l e x i b l e p l a s t i c i z e d products possess a broad spectrum of d e s i r a b l e p r o p e r t i e s . T y p i c a l properties of unmodified PVC r e s i n are given i n Table V I I . Glass T r a n s i t i o n Temperature. The g l a s s t r a n s i t i o n temperature, T , i s that temperature at which an amorphous polymer changes from tne hard or g l a s s y s t a t e to a s o f t or rubbery m a t e r i a l . Near t h i s temperature c e r t a i n p r o p e r t i e s change, f o r example, the modulus abruptly changes. Mechanical l o s s ( r a t i o of energy d i s s i p a t e d to energy s t o r e d per c y c l e by a sample under o s c i l l a t o r y s t r a i n ) e x h i b i t s a maximum at T . A p l o t of mechanical l o s s and the e l a s t i c component of shear modulus (obtained w i t h a t o r s i o n pendulum at a frequency of c a . 1 c / s ) v e r s u s temperature as determined on a c o m m e r c i a l , s u s p e n s i o n - p o l y m e r i z e d product i s shown i n F i g u r e 15 (21). The i n f l e c t i o n point i n the modulus versus temperature curve can be used to measure T as with an Instron tester. From Figure 15 i t i s seen t h a t at room temperature and below, PVC i s hard and brittle. The effect on T of the polymerization temperature was studied by Reding, W a l t e r , and Welch (22). P l o t t i n g t h e i r t a b u l a r data r e s u l t s i n Figure 16, which shows that there i s e s s e n t i a l l y a l i n e a r dependence of T on polymerization temperature. The v a r i a t i o n of T with polymerization temperature i s affected more by s t e r e o r e g u l a r i t y than by changes i n head-to-head structure content.



18.

Table V I I .

411



Typical Properties of Unmodified PVC Resin

Property

Value

Tensile strength, l b / i n .

2

6500

Elongation at break, % Tensile modulus, l b / i n .

25 2

5

6 x 10 to 12 x 10

Shore hardness

D75 to 85

Heat capacity, 25 °C, cal/deg/g

0.226

Coefficient of thermal expansion, deg" (glass) (rubber)

5.2 x 10" 4 2.1 x 10"*

Glass t r a n s i t i o n temperature,

70 to 80

Refractive index at 20 °C

°C

1.54


5



412

Figure 15.

M e c h a n i c a l l o s s (0) and s h e a r m o d u l u s temperature for commercial suspension PVC.

(X)

vs.

110

Figure 16.

G l a s s t r a n s i t i o n temperature polymerization temperature.

as

a

function


of




413

P l a s t i c i z e r s have a marked e f f e c t on T . One would n o r m a l l y expect t h a t the p l a s t i c i z e d polymer would nave a lower s t r e n g t h , hardness, and modulus than the u n p l a s t i c i z e d m a t e r i a l . A c t u a l l y i n c o r p o r a t i o n of s m a l l amounts of a p l a s t i c i z e r such as d i ( 2 ethylhexyl) phthalate (DOP) causes an increase i n these values. At the 10-15% l e v e l of p l a s t i c i z e r , X-ray d i f f r a c t i o n studies and IR analyses show an increase i n the amount of c r y s t a l l i n i t y , probably due to an ordering of polymer chains. This r e s u l t i s the s o - c a l l e d " a n t i - p l a s t i c i z e r e f f e c t " and i s dependent on the t y p e o f plasticizer. As the amount of p l a s t i c i z e r i s i n c r e a s e d , however, these properties ( t e n s i l e strength and modulus) do decrease as shown i n F i g u r e 17. The e f f e c t on T as a r e s u l t of i n c r e a s i n g l e v e l s of DOP p l a s t i c i z e r i s shown i n Ffgure 18. An increase i n pressure r e s u l t s i n an increase i n T . When the polymer c h a i n s are subjected to p r e s s u r e , more therraaTL energy i s r e q u i r e d to a c t i v a t e the energy l o s s mechanisms. Heyderaann and G u i c k i n g (23) determined the s p e c i f i c volume of u n p l a s t i c i z e d and DOP p l a s t i c i z e d PVC by dilatometry over the temperature range of -80 to 150 °C at h y d r o s t a t i c p r e s s u r e s of 1-1000 atm. F i g u r e 19 shows t h e i r r e s u l t s . W i t h i n the l i m i t s of a c c u r a c y , T appears to be a l i n e a r function of applied pressure. M e l t i n g Point (Softening Point). As noted e a r l i e r , the t a c t i c i t y or c r y s t a l l i n i t y of PVC i s markedly a f f e c t e d by p o l y m e r i z a t i o n temperature, as i s the melting point. Because polymer decomposition i s too rapid at the elevated temperatures, the melting point of PVC cannot be determined by d i r e c t t e c h n i q u e s . Reding and coworkers (22) used an extrapolation technique i n which DOP was added to PVC at various concentrations, and s t i f f n e s s modulus was measured as a f u n c t i o n of temperature. The m e l t i n g t e m p e r a t u r e on each p l a s t i c i z e d sample was taken at the temperature at which the s t i f f n e s s modulus was 10 l b / i n . . By p l o t t i n g the m e l t i n g temperatures versus p l a s t i c i z e r content and e x t r a p o l a t i n g back to zero p l a s t i c i z e r , the m e l t i n g p o i n t f o r the polymer was o b t a i n e d . T h e i r r e s u l t s shown i n F i g u r e 20 are i n good agreement w i t h those obtained by Nakajima and coworkers (24) who extrapolated the melting p o i n t of p o l y m e r - d i l u e n t systems as a f u n c t i o n of thermodynamic i n t e r a c t i o n parameters. The melting point of an a l l - s y n d i o t a c t i c or c r y s t a l l i n e polymer would be very h i g h , but i s lowered by the amorphous c h a i n s of the polymer. When a p l a s t i c i z e r i s added, i t s o l v a t e s the amorphous chains to produce a sheet that i s f l e x i b l e at room temperature. The sheet i s s t r o n g and tough, however, because of the network of nonsolvated, high-melting c r y s t a l l i n e portions. D i f f e r e n t i a l Thermal A n a l y s i s . D i f f e r e n t i a l thermal a n a l y s i s (DTA) i s a technique i n which differences i n temperature between a sample under i n v e s t i g a t i o n and an i n e r t reference material are measured as both m a t e r i a l s are s i m u l t a n e o u s l y heated at a uniform r a t e . Endotherms and exotherms due to melting, phase changes, or chemical changes are thus seen. Changes i n heat c a p a c i t y at T are o f t e n noted. Matlack and Metzger (25) reported on a thermogram of PVC above room temperature, and t h e i r c u r v e i s shown i n F i g u r e 21. P o i n t A shows a change i n slope over the 65-80 °C range which i s associated




414

1.01

0

1

5

1

1

10

15

£

20

' 1.0

25

Percent DOP

Figure 17.

E f f e c t of p l a s t i c i z e r t e n s i l e strength.

on modulus of e l a s t i c i t y

80

-401 0

I

10

I

20

I

30

Percent DOP

Figure 18.

Glass t r a n s i t i o n temperature of p l a s t i c i z e d PVC.


and


18.



415

Atm x 10

Figure 19.

Effect of hydrostatic pressure on the g l a s s t r a n s i t i o n temperature of u n p l a s t i c i z e d and DOP-plasticized PVC.

350

-60

-20

20

60

100

Polymerization Temp., °C

Figure 20.

M e l t i n g p o i n t of PVC as a f u n c t i o n of p o l y m e r i z a t i o n temperature.


416


with T Point B, at which an endothermic response i s noted between 165 ana 210 °C, i s the m e l t i n g p o i n t . As noted e a r l i e r t h i s w i l l v a r y w i t h temperature of p o l y m e r i z a t i o n . P o i n t C, an exotherm at approximately 250 °C, i s the point at which o x i d a t i v e attack occurs and i s dependent on the s t a b i l i z e r system used. P o i n t D i s the temperature at which d e h y d r o c h l o r i n a t i o n occurs ( l o s s of H C l ) . Point E, an endotherm, i s postulated as being due to decomposition. In a d d i t i o n to DTA, other techniques used to d e t e c t thermal a c t i v i t y i n PVC include d i f f e r e n t i a l scanning calorimetry, s p e c i f i c heat, thermal c o n d u c t i v i t y , and thermal d i f f u s i t y .


Chemical Properties PVC i s g e n e r a l l y i n s e n s i t i v e to c h e m i c a l a t t a c k i n comparison to other commercial polymers. Contact w i t h a c i d s , f o r example, produces no d e l e t e r i o u s e f f e c t s . However, r e a c t i o n w i t h aqueous a l k a l i at elevated temperatures or w i t h o r g a n i c amines a d v e r s e l y affects the s t a b i l i t y of PVC. Thermal D e h y d r o c h l o r i n a t i o n . PVC, per se, i s u n s t a b l e and, at moderate temperatures, undergoes decomposition w i t h l o s s of H C l . The successful commercial development of PVC i s l a r g e l y due to the f a i r l y complex technology i n v o l v i n g development of s t a b i l i z e r s that n e u t r a l i z e the e v o l v e d H C l and r e s u l t i n improved c o l o r of the compounded p o l y m e r so t h a t i t can be p r o c e s s e d l i k e o t h e r thermoplastics. The k i n e t i c s of the d e h y d r o c h l o r i n a t i o n have been s t u d i e d e x t e n s i v e l y , but there i s no agreement on the mechanism. There i s some support f o r a f r e e - r a d i c a l c h a i n mechanism, a l t h o u g h other i n v e s t i g a t o r s f a v o r a u n i r a o l e c u l a r decomposition mechanism. A r a d i c a l chain of any considerable k i n e t i c length has been r u l e d out. I t was proposed very e a r l y i n the h i s t o r y of PVC t h a t H C l was l o s t p r i m a r i l y from positions where the c h l o r i n e atom was e s p e c i a l l y l a b i l e because of i t s l o c a t i o n i n an a l l y l i e p o s i t i o n r e l a t i v e to the double bond. Such structures r e s u l t from the l o s s of HCl which creates a double bond and thereby makes the adjacent c h l o r i n e atom l a b i l e . Subsequent l o s s of H C l appears to be c a t a l y z e d by H C l i t s e l f and i s d e s c r i b e d as a u t o c a t a l y t i c and produces a l t e r n a t e double bonds. Thus, a " z i p p e r " r e a c t i o n occurs as shown i n F i g u r e 22. Bengough and Sharpe (26) measured the r a t e of d e h y d r o h a l o genation of laboratory-prepared samples i n which c a t a l y s t residues were removed by p u r i f i c a t i o n . N i t r o g e n gas was swept o v e r the degrading s o l u t i o n and thereby entrained the evolved H C l , which was measured a n a l y t i c a l l y . An increased rate of HCl l o s s was noted with lower molecular weight polymers and was proportional to the number of moles of polymer present. This r e s u l t suggests that the l o s s of HCl i s connected i n some way to the concentration of polymer chain ends. The " z i p p e r " mechanism i s confirmed by the work of Baura and Wartman (27) who found t h a t o z o n o l y s i s f o l l o w e d by h y d r o l y s i s of p a r t i a l l y degraded PVC d i d not a p p r e c i a b l y reduce i t s m o l e c u l a r weight. I f the HCl l o s s had occurred i n a random fashion within the polymer chain, a considerable drop i n molecular weight would have been expected. In addition, when the r e s i n was m i l d l y chlorinated


Polyvinyl chloride)



Figure 22.

Dehydrohalogenation of PVC ("zipper" r e a c t i o n ) .



418

to s a t u r a t e double bonds at the c h a i n ends but not to e f f e c t s u b s t i t u t i o n i n the c h a i n , the r e s u l t a n t polymer had a lower dehydrohalogenation rate. The presence of oxygen i n the environment of degrading PVC g e n e r a l l y accelerates the l o s s of H C l . This r e s u l t i s b e l i e v e d to be due to attack by oxygen at the conjugated polyene structures to b l e a c h them and s i m u l t a n e o u s l y form oxygenated s t r u c t u r e s t h a t further l a b i l i z e adjacent c h l o r i n e atoms. Chemical Dehydrochlorination. The treatment of PVC w i t h c e r t a i n c h e m i c a l r e a c t a n t s r e s u l t s i n complete l o s s of H C l and a product with the empirical formula (CH=CM) , a polyene. This r e s u l t occurs when PVC i s reacted with a l c o h o l i c or aqueous a l k a l i , with calcium hydroxide i n 2-ethoxyethanol or calcium methylate i n THF. Because l e s s than 0.5% of unreacted c h l o r i n e remains when a d i s p e r s i o n of sodium amide i n l i q u i d ammonia i s r e a c t e d w i t h a 3% s o l u t i o n of PVC i n THF, i t i s hypothesized t h a t the d e h y d r o h a l o genation does not occur i n a random f a s h i o n . On the b a s i s of mathematical p r o b a b i l i t y c o n s i d e r a t i o n s , c o n s i d e r a b l y h i g h e r r e s i d u a l c h l o r i n e would be expected i n a random a t t a c k . Thus, chemical dehydrochlorination appears to proceed i n a fashion s i m i l a r to thermal dehydrochlorination, that i s , v i a a "zipper" sequence.


n

R a d i a t i o n - I n i t i a t e d R e a c t i o n s . PVC i s a l s o degraded by a c t i n i c i r r a d i a t i o n r e s u l t i n g i n d i s c o l o r a t i o n , chain s p l i t t i n g , and crossl i n k i n g . HCl i s released during the process as measured d i r e c t l y or as e s t a b l i s h e d by the f a c t t h a t b a s i c s t a b i l i z e r s are g r a d u a l l y c o n v e r t e d to t h e i r c h l o r i d e form. I t has been found t h a t H C l i s only evolved when l i g h t of wavelengths shorter than 340 nm i s used, t h e amount o f H C l formed i n c r e a s i n g w i t h d e c r e a s i n g l i g h t wavelength. T h i s photodegradation p r o b a b l y f o l l o w s a r a d i c a l mechanism i n which t r a c e i m p u r i t i e s and c a t a l y s t r e s i d u e s are p r o b a b l y the source of i n i t i a l r a d i a t i o n a b s o r p t i o n and r a d i c a l formation. The unsaturated structures formed absorb more UV quanta and decompose, and thereby c o n t i n u e the decomposition. Oxygen affects t h i s reaction, and molecular weight of the polymer decreases p r o b a b l y because of c h a i n - s p l i t t i n g , w i t h the development of carbony1 bands i n the IR spectrum. High energy i r r a d i a t i o n may l e a d to H C l l o s s and formation of c o l o r . Other effects of i r r a d i a t i o n are c r o s s - l i n k i n g and/or chain s p l i t t i n g as w e l l as c a r b o n y l development i f oxygen i s p r e s e n t . C o n t r o l of the c r o s s - l i n k i n g must be maintained so t h a t c h a i n s c i s s i o n and c o l o r development are minimized. E l e c t r o n beam crossl i n k i n g and copolymerization of polymers i n c l u d i n g PVC have become i n c r e a s i n g l y important t o o l s i n the p l a s t i c s industry. Manufacturers of communication wire have found that thicker and more c o s t l y wrapped and lacquered wire can be s e l e c t i v e l y replaced with PVC c r o s s - l i n k e d i n s u l a t i o n . The process i n v o l v e s i r r a d i a t i n g m o d i f i e d PVC i n s u l a t e d w i r e i n a manner as d e p i c t e d i n F i g u r e 23 (28). Color Development. The absorption spectrum i n the v i s i b l e region of PVC subjected to heat d e g r a d a t i o n shows s e v e r a l maxima t h a t are r e l a t e d to the degree of c o n j u g a t i o n of the p o l y e n e s formed by d e h y d r o c h l o r i n a t i o n . Bengough and Varma (29) have s t u d i e d the


18.



419


v i s i b l e a b s o r p t i o n s p e c t r a of s o l u t i o n s of PVC subjected to degradation at 198 °C. Their findings are shown i n Figure 24. They concluded t h a t the a b s o r p t i o n maxima noted corresponded to the varying degrees of conjugated unsaturation. C r o s s - l i n k i n g . When i t i s heated i n s o l u t i o n , as described above, PVC not only loses HCl, but a l s o undergoes c r o s s - l i n k i n g and thereby r e s u l t s e v e n t u a l l y i n an i n s o l u b l e g e l . I n i t i a l stages of t h i s c r o s s - l i n k i n g reaction can be followed by measurement of i n t r i n s i c v i s c o s i t y . However, because of s i m u l t a n e o u s l y o c c u r r i n g c h a i n s c i s s i o n reactions and because the molecular weight of the branched m o l e c u l e s formed cannot be r e l i a b l y c a l c u l a t e d , i n t e r p r e t a t i o n of r e s u l t s i s very d i f f i c u l t . According to the Flory-Stockmayer (29a) theory of g e l a t i o n , the g e l point, corresponding to the formation of an i n f i n i t e network, i s reached when one c r o s s - l i n k per i n i t i a l polymer molecule present has formed. Hydrogenation. Approximately 97% of the c h l o r i n e atoms present i n PVC can be r e p l a c e d by hydrogenations i n the treatment of the polymer w i t h l i t h i u m aluminum h y d r i d e i n THF. By use of t h i s reaction, which leads to a polyethylene, the amount of branching i n the polymer can be determined by IR a n a l y s i s on the hydrogenated m a t e r i a l . The dehydrochlorinated r e s i n that r e s u l t s from treating PVC with ammonia under pressure can be hydrogenated. C h l o r i n a t i o n . C h l o r i n a t i o n of PVC r e s u l t s i n a r e s i n o u s product having about 73% c h l o r i n e , which corresponds to the introduction of one c h l o r i n e per carbon atom. Such postchlorinated polymers have a c h i e v e d some i n d u s t r i a l importance because they have lower s o f t e n i n g temperatures and i n c r e a s e d s o l u b i l i t y i n a v a r i e t y of s o l v e n t s . They have been used i n synthetic f i b e r s i n Germany and i n s o l u t i o n coatings. By IR and X - r a y a n a l y s i s , i t has been e s t a b l i s h e d t h a t most of the c h l o r i n e atoms substitute at methylene groups with only 25% of the i n i t i a l methylene groups remaining unmodified. Low-temperature l i g h t - a c t i v a t e d c h l o r i n a t i o n l e a d s to more s u b s t i t u t i o n of the methylene group. I f c h l o r i n a t i o n i s c a r r i e d out below 65 °C i n the presence of a d i l u e n t that s w e l l s but does not d i s s o l v e the polymer, the formation of products w i t h c h l o r i n e s on a l l adjacent carbons i s promoted. Dannis and Ramp (30) have shown t h a t products prepared by t h i s technique have h i g h e r T v a l u e s and g r e a t e r heat r e s i s t a n c e than products prepared at higner c h l o r i n a t i o n temperatures. The change i n T as a r e s u l t of i n c r e a s e d c h l o r i n e content i n such l o w temperature c h l o r i n a t i o n s i s shown i n Figure 25. The high T shown for p o l y ( v i n y l d i c h l o r i d e ) i s not that of p o l y ( v i n y l i d e n e chroride). C l a s s i f i c a t i o n of P o l y ( v i n y l chloride) Resins PVC r e s i n s are c l a s s i f i e d on the b a s i s of t h e i r p h y s i c a l form and the processing methods used. General Purpose Resins. Homopolymer and copolymer resins produced by suspension, emulsion, bulk, or s o l u t i o n techniques are included



420


Crosslinklng PVC Modified with Acrylics

§

Modified PVC

t

O

Crosslinked Insulated Wire

O

Electron Beam

Wire Insulation Extruder

Figure 23.

Schematic o u t l i n e of i n d u s t r i a l PVC i r r a d i a t i o n process.

i

3 SO

i 400

I 450

i 500

Poly(vinyl chloride) - ACS Publications - American Chemical Society

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