INDUSTRIAL APPLICATIONS OF ORGANOTIN COMPOUNDS

INDUSTRIAL APPLICATIONS OF ORGANOTIN COMPOUNDS

Alexander Ross Research Laboratories, M&T Cheniicals Inc., Rahway, N . J .

Although the first organotin compound was synthesized by Lowig’ in 1852, and Franklin2 instituted a rational study of the series of organotin compounds in 1853, the first industrial uses of organotins did not develop until 80 years later. In 1932, Standard Oil Development Co. was issued a patent3 for the use of tetra- alkyltin compounds as stabilizers of transformer oils. Yngve4 and his colleagues working in the Union Carbide Co. in the 1930’s are given the credit for the origi- nal development of organotin stabilizers for vinyl plastics. Since that time the commercial uses of organotins have developed rapidly until today it is a multi- million pound/ year business.

There are three major fields of utilization of organotins: ( 1 ) Heat stabilizers (primarily for polymers having halogen attached to car-

(2) Catalysts - here the variety is almost infinite as will be discussed later. (3) Biologically active agents for a wide variety of applications. As shown in TABLE 1, organotins are called upon to stabilize transformer oils,

polyvinyl and polyvinylidene chloride plastics, chlorinated rubber, chlorinated paraffins, and modified plastics. These are all materials which contain chlorine. In addition, other uses for organotins have been found in the stabilization of non- chlorinated products such as lubricating oil, hydrogen peroxide, polyolefins, and other plastics.

In the catalyst area (TABLE 2 ) organotins are finding use as esterification catalysts. This category includes transesterification and polyesterification. They are used as urethane catalysts, primarily for the production of “one-shot’’ urethane foams, but also for urethane elastomers. They are used to catalyze the room- temperature curing of silicones as well as for the curing of epoxy compounds. Many patents have been issued on the use of various organotins in the polymerization of olefins. In addition, there are many other catalytic uses under development.

One of the fastest growing fields of application for organotins is based upon their biological properties (TABLE 3) . They are extremely active as bactericides. This is of interest in hospitals for the control of Staphylococcus aureus, as well as to the textile industry. As fungicides the organotins have a very wide range of application. They are used to preserve paint, paper, textiles, wood, plastics, and anything else attacked by mildew or fungus. Industrial waters have been treated with organotins in paper mills, cooling towers, and secondary oil recovery. In agriculture, the use of organotins for the control of blight on sugar beets is quite prevalent in Europe today. Other organotins act as insecticides, herbicides, rodent repellants, and as anthelmintics in poultry, The range of organisms which organotins control extends far beyond these typical areas to marine organisms such as teredo and limnoria, two organisms responsible for the decay in wood in marine environments, as well as to barnacles and molluscs, which are very important in the fouling of ship bottoms.

Since the biocidal applications of organotins are to be discussed in another paper, this report will concentrate on the first two major categories: stabilizers and catalysts.

bon in the chain).

107

108 Annals New York Academy of Sciences TABLE 1

STABILIZER USES OF ORGANOTINS

Halogen containing products Nonhalogenated products

Transformer oils Lubricating oils Polyvinyl chloride plastics Hydrogen peroxide Chlorinated rubber Cellulose acetate Polyvinylidene halides Pol yamides Polymers modified with Polycarbonates

chlorinated materials Pol yolefins Chlorinated paraffins

TABLE 2 CATALYST USES OF ORCANOTINS

Esterification-transesterification polyesterification

Urethanes-foams elastomers

Silicone curing

Epoxy curing

Polymerization of olefins

Others

TABLE 3 BIOCIDAL USES OF ORGANOTINS

Bactericide Agriculture -

Hospitals Insecticide Textiles Fungicide

Herbicide Fungicide Rodent repellent

Paint Anthelminthic

Marine organisms Paper Textiles Wood Barnacles Plastics Molluscs

Teredo Limnoria Industrial water treatment

Paper mills Cooling water Secondary oil recovery

_ _ _ _ _ -_____ -

STABILIZER APPLICATIONS

Transfornirr Oils

One of the first stabilizer applications developed was for dielectric oils used in transformers. In the manufacture of transformers, traditional insulation materials were paper and mineral oil. The mineral oil gave many problems, one of which was oxidative decomposition to give a sludge. It was found very early that tetra- alkyls and tetraaryls of tin would prevent this sludge.:3

Ross: Organotin Compounds 109

The mineral type transformer oils had other drawbacks, however. They were limited to rather low temperatures to avoid decomposition; they were flammable; and the products of decomposition were potentially explosive. To overcome these difficulties, General Electric and others developed a series of chlorinated aro- matics as s~bs t i tu tes .~ Examples of the type of compound used are trichloro- benzene, pentachlorodiphenyl, and pentachlorodiphenyloxide. These materials also have difficulties. When arcing occurs in the transformer they decompose, evolving HCl. This highly corrosive acid dissolves in the oil and corrosion problems of the first magnitude can result. The paper insulation is attacked, and further decomposition of the oil is probably catalyzed by the HCI formed. The most efficient additives to overcome this effect are the organotins.6.' Generally, tetraaryl, tetraalkyl or the corresponding triaryl (alkyl) compounds are used. The compound of choice appears to be tetraphenyltin. It is proposed that these stabilizers act as efficient HCI scavengers, following the known chemistry of the aryltins (FIGURE 1 ) . The phenyl group is cleaved by the HCI to form benzene and triphenyltin chloride. Further HCI will react to split off another phenyl group, etc., until all the phenyl groups are cleaved. This means that one mole of tetraphenyltin could absorb as many as four moles of HCI, thus making it a very effective scavenger.

FIGURE 1 . Tetraphenyltin as an HCl scavenger.

PVC Stabilizers

Without doubt, the use of organotins as stabilizers of vinyl chloride resins is the largest application for this class of materials. Vinyl chloride is polymerized by a free radical reaction, generally in suspension or in an emulsion. Polyvinyl chloride is generated as a nice white powder. In order for it to be converted into useful products, this powdery resin must be heated up to its fusion temperature. For an unplasticized resin, this temperature is in the neighborhood of 175-2OO0C. Unfortunately, polyvinyl chloride tends to decompose well below a useable fusion temperature. It has been reported to start losing HCI at 100OC. In order to be fabricated into products, PVC must be stabilized against thermal degradation.

There are many chemicals that have been used as stabilizers for PVC (TABLE 4). Organic materials have been tried and one time diphenylthiourea had some popularity, but it is quite a poor stabilizer. Lead compounds are used quite broadly for certain applications, primarily in the electrical insulation field. Barium and cadmium soaps find their greatest use in plasticized PVC products. Calcium soaps are poor stabilizers that are currently being used in some applications where toxicity is important and FDA approved nontoxic materials must be used. Organotins are the best stabilizers known and are used where the highest processing temperatures are required. This is generally found in the area of rigid,

110 Annals New York Academy of Sciences TABLE 4

STABILIZERS FOR PVC

Class Example

Organic materials Lead compounds Barium compounds Cadmium compounds Calcium compounds

Diphenylthiourea Dibasic lead phthalate Barium stearate Cadmium 2-ethylhexoate Calcium stearate

Organotin compounds Dibutyltin maleate

nonplasticized vinyls. PVC is not a polymer that becomes very fluid once it reaches the fusion point. Instead, it gradually loses viscosity as the temperature is raised. Thus, for faster processing, one must go to higher temperatures and this is the tendency of the industry today. The organotins also have the property of being compatible with the resins, thus giving clear products, whereas the solubility of the other materials in the resin is somewhat less and opaque products are generally obtained. A complete listing of various stabilizers available on the market is to be found in a pair of papers by Perry and Bruins.8 More recently, the various stabilizers used in vinyl compounds was discussed by R. Darby of Monsanto Chemical Co. at the 20th Annual Technical Conference of the Society of Plastics Engineers in Atlantic City in January, 1964. These papers discuss the various reasons for using the different types of stabilizers. This author will merely point out the reasons for using organotins (TABLE 5 ) .

Organotins are often the materials of choice because they stabilize for the longest period of time at the highest processing temperatures. This is especially important when much scrap is generated that must be reprocessed. In general, the organotins are compatible with the resin giving clear, rigid products.

Some of the other stabilizers tend to carbonize and blacken as 'soon as their stabilization power is gone. With the organotins, however, there is only a gradual coloring and decomposition even after the initial stabilization period.

Sulfur is very often present in and around PVC products, e.g., in the sulfite paper of the packaging materials, in rubber vulcanized with sulfur, etc. Other stabilizers, lead, for example, form colored products in the presence of sulfur. The organotins do not form colored sulfide derivatives.

Cost is the one big drawback of the organotins. They are generally priced in the neighborhood of $2.50-3.OO/lb. which is somewhat higher than the cost of the cheaper stabilizers. They are therefore used only where their distinctive properties are required.

TABLE 5 FACTORS IN THE USE OF ORGANOTIN STABILIZER

1. Stabilization at highest processing temperatures. 2. Compatibility with resin, giving clear products. 3. Gradual coloration after stabilization period. 4. Nonstaining in the presence of sulfur. 5. cost.


Although organotin compounds of almost any structure have been claimed as PVC stabilizers in various patents, the general run of stabilizers available on the market today is a dialkyltin compound, usually a dibutyltin derivative. Within this class, however, there is a large selection of compounds with varying types of stabilizer activity (FIGURE 2) .

Each stabilizer listed is representative of a group of stabilizers currently being marketed. The first and oldest one known, dibutyltin dilaurate, represents the class of fatty acid derivatives of dibutyltin oxide. This is a stabilizer with excellent lubricating properties. It should be pointed out here that the rheological properties of the resin are extremely important in its processing. The rigid PVC products are generally processed either by extrusion or by calendering. In either case they are submitted to a high degree of shear. Frictional heat is generated which often causes local hot spots and unexpected degradation. Furthermore, the resin might stick to the rolls of the calender or mill. Lubricatibn of the resin is therefore a problem. The dilaurate is one way to solve this problem. Unfortunately, it is not as good a stabilizer as some of the other organotins.

Dibutyltin maleate is shown as a cyclic structure (FIGURE 2) but is most likely a polymer. It is an extremely good stabilizer. It has poor lubrication, unfortunately. One of the approaches taken in the problem of lubrication of an organotin maleate stabilizer involves the half alkylation of maleic anhydride. The resulting half-acid esters can then be reacted with dibutyltin oxide to give ester maleates

DIBUTYLTIN DILAURATF YNGVE V. U.S. PAtENT 2.307,092(1943)

DlBUTYLTl M AT

QUATTLEBAUM 4 NOFFSINGER U.S. PATENT 2,307,157(1943)

T,

QUATTLEBAUM 0 NOFFSINGER IBlD;

DIBUTYLTIN BIS- BEST, C. E. U.S. PATENT 2,731,484

CAURYL MtRC APTIDF

DIBUTYLTIN BIS- ISOOCTYLTHIOGI YCOl ATF

WEINBERG JOHNSON U.S. PATENTE2,648,650(1953)

f;' P CHg, ,O-C-CH=CH-C-OR

Q Hg' \O-t-CH=CH-$-OR Sn

0 0

FIGURE 2. Major dibutyltin stabilizers.

112 Annals New York Academy of Sciences

Bu2Sn(OR)2

Bu3SnOR

DlALKOXlDE

TRIALKYLTIN ALKOXIDE

/ococII H13 Bu2 Sn,

sue sn(

o c o y OCOCH

ococlI H13 MIXED CARBOXYLATE-MALEATE

-5n-0-y-0 Cf: z I. POLYMERIC ACETAL

Bu2Sn - [CH(COOC2H512]2

MALONATE (ACTIVE HYDROGEN)

Bu2Sn S or ( B u ~ S ~ ) ~ S SULFIDES

Bu2Sn-(SCH 2-1 CNHC8Hl+2 v

MERCAPTOACETAM IDE

0 CYCLIC MERCAPOPROPIONATE

CLEVERDON STAUDINGER U.S. PATENT 2,307,157(1953)

U.S. PATENT 2,745.820(1956) MACK, G.

GLOSKEY, C. R. U. S. PATENT 2,826,957(1953)

CHURCH, JOHNSON, EI RAMSDEN

U.S. PATENT 2,591,675 (1952)

MACK EI PARKER

U.S. PATENT 2,604,483 (1952)

WEINBERG 8 RAMSDEN U.S. PATENT 2,746,946 (1956)

LEISTNER EI HECKER US. PATENT 2,704,756 (1955)

GREGORY, W. A. U.S. PATENT 2,636,891 (1953)

FIGURE 3. Proposed organotin stabilizers.

as exemplified by dibutyltin bis-alkylmaleate, where R can vary from methyl to lauryl.

The compounds that have by far the best heat stability are the organotins which contain sulfur. An example of the mercaptide group is dibutyltin bis- laurylmercaptide. An even better stabilizer is the dibutyltin bis-isooctylthio- glycolate in which the sulfur is attached to a carbon alpha to a carboxylate ester.

There has been a great deal of research work in attempts to find improved organotin stabilizers some of which have achieved a modicum of success in the field (FIGURE 3 ) . Generally speaking, a dibutyl or tributyl grouping is maintained o n the tin and the negative groups are altered broadly. Some of the structures that have received attention are the alkoxides. The mixed carboxylate-maleate type is another approach to lubricating maleates. Acetals have been prepared by the condensation of dialkyltin oxides with aldehydes. Condensation products of dibutyltin oxide with active a-hydrogen compounds have been proposed. Among the


sulfur compounds, the simple sulfides have been patented as well as the dibutyltin bis-mercaptoacetamide compounds. The cyclic mercaptopropionate has been an item of recent interest.

There are literally hundreds of patents in this area with too many structures to enumerate. A recent paper by RiethmayeP discusses most of the organotin compounds proposed as PVC stabilizers.

Historically, PVC has not been used as a packaging material. In recent years, however, there has been an increasing interest in the use of vinyl for packaging, especially for foods. This application introduces problems of toxicity. In the past, when one wanted to make a vinyl which would be acceptable for food use, stabilizers based upon stearates of magnesium, calcium, zinc, etc., were used. These soaps have F D A approval but they are very poor stabilizers, thus severely limiting processing conditions. Today, in order to keep up with high speed packaging operations, there is a growing tendency to use organotins in food grade PVC. Actually, organotins have been used in a related area for some years now. The National Sanitary Foundation, which has a responsibility for monitoring water pipe, has approved the use of a number of PVC compounds containing organotin stabilizers. This clearance was given on the basis of extraction studies which show that the dibutyltins are practically nonextractable from rigid vinyl pipe. Actually, the extractability figures are so low that there is reason to believe that the dibutyltins could, if the matter were pursued, be accepted by the F D A for use in food packaging materials. However, with the established levels of oral toxicity of the dibutyltins coupled with the extreme wariness of the F D A , and the limits of accuracy of extraction data, there is sufficient reason to hesitate to institute expensive, long-term feeding and toxicology studies. Instead, the dioctyltin stabilizers are currently being promoted for food grade PVC.

It had been reported by Luijten and van der Kerklo that increasing the size of the alkyl groups on tin decreases the fungicidal activity. Mammalian toxicity of the organotins roughly follows the same pattern. It was further demonstrated that the dioctyltin stabilizers are virtually equivalent to their dibutyltin homologues in stabilizing activity.'I Subsequent toxicity studies have shown the octyltins to be a full order of magnitude safer (in terms of an acute oral LD5") than the dibutyltins.12J3 This has led to the increasing interest in the dioctyltins as stabilizers.

There are three countries in Europe which have already cleared some of the octyltin compounds for use in food (FIGURE 4). i n Germany, dioctyltin maleate and dioctyltin mercapto esters, e.g., dioctyltin isothioglycolate have been ~ 1 e a r e d . l ~ In Italy, they have cleared the dioctyltin 2-ethylhexylthioglycolate which is very similar to the German compound, except that the alkyl radical on the ester grouping has been more clearly specified, as well as dioctyltin bis-2-ethylhexylmaleate. Austria appears to follow Germany and has approved the same stabilizers.

These dioctyltins are extracted from PVC in amounts about equal to those of the butyltins. However, with their lower toxicity levels, they have a higher thera- peutic index. This makes tbem much more acceptable to governmental agencies charged with the responsibility of safeguarding foodstuffs. Rapid clearance is possible in Europe. Because of the long-term feeding and toxicology tests required in the U.S., however, it will be a while before we will see any large scale food packaging markets developed for these products in this country.

In the meantime, one other organotin compound has been cleared for food use in Germany. This is a butylthiostannoic anhydride,15 formed by the reaction of butyltin trichloride with sodium sulfide. The result is a cross-linked polymeric material with alternate tin and sulfur bonds. The material in itself is not a very good stabilizer; however, it is apparently completely nontoxic in acute oral tests.


DIOCTYLTIN MALEATE+

0 C H 0-8,

'8 "I7 '-$ 0

"\s,/ CH / \ A H

? 0 C e H q ,O-d-CH=CH-8-0CeH17

C&7' 'O-$-CH-CH-$-OC8H17 ** sn DlOCTY LTI N

BIS- 2- ETHYLHEXYL-MALEATE

0 0

BUTY LTH IOSTANNOIC ANHYDRIDE*

n

FIGURE 4. Dioctyltin compounds for food grade PVC. *Cleared for food packaging use in Germany. **Cleared for food packaging use in Italy.

Up to 20 g./kilo has been fed to rats without mortality.16 It has utility only in a special grade resin which must be calendered on a special, very expensive calender which operates at relatively low temperatures. Sheets of PVC made in this way have been introduced in the U.S. and may take up the need for stabilized food grade PVC until the octyltins come into their own.

The mechanism of the decomposition of PVC and the mechanism of its stabilization have been studied almost from the time that the polymer has been discovered. As a result there is a mass of conflicting and confusing literature on the subject. To this day the subiect is not completely understood and much work is still going on. To study the mechanism one must first consider the structure of PVC.

Normally, one considers polyvinyl chloride as a linear polymer with chlorines on every other carbon (I) . It has long been

IjI CI H CI H CI I I l l

-cH-cH-AH-cH-cH-cH- I

established, however, that PVC is not truly linear. It has a branched structure. Studies by Cotman17 indicate that there are approximately 20 branches per PVC

Ross : Organotin Compounds 115

molecule in a typical resin. This branching creates a tertiary chlorine on the main chain (11).

y c1 €/ Ct* p 71 I

-CH-CH-CH-C-CH-CH- I

H-C-H HA-CI

I1

This, then, is considered a weak point for degradative attack. Likewise, in chain terminations, generally by disproportionation, an allylic halide is created in the chain (111). This has also been

If CI H yl* 7 71 I

-CH-CH-&H-CH-C=CH I11

considered to be a weak point and subject to attack. It has been generally accepted that where there is a tertiary or allylic chlorine

which can be extracted, a hydrogen atom on the adjacent carbon is lost with it to form an unsaturation (IV).

- C H d H - C H = CH-CH = CH- 4- HCl IV

This makes the next chlorine an allylic one, thus facilitating the loss of another HCI and starting a chain reaction for successive loss of HCl molecules. As a result, the decomposition proceeds apace with the loss of HCI and the formation of a polyene chain. This conjugated chain is considered to be responsible for the early color formation in decomposing PVC resins.

This theory is not universally held, however. Winkler18 puts forth a strong case against blaming the allylic chlorines for the instability of the PVC. He maintains that the mechanism is properly a free radical one, involving abstraction of a hydrogen from a methylene group (V). The molecule then stabilizes itself by the

y . 71 y 71 -CH-CH-CH - CH-CH-CH-

y 5." \1 y y1 -CH-CH-CH = CH-CH-CH- 4- C1 V

loss of a chlorine atom to form the unsaturation. The free chlorine atom can then abstract another hydrogen from a methylene group to propagate the chain mechanism.

A great deal of work has been done on the evolution of HCI. It has been studied in various systems under nitrogen and under oxygen, with and without stabilizers; yet its role in the decomposition of PVC is still not understood. For a long time it was considered that the loss of HCI was autocatalytic. However, more recent work by Arlmanlg has indicated that such autocatalysis does not occur. There are further complications in the picture of PVC degradation. It has been found that


cross-linking occurs as the decomposition progresses. Furthermore, in the presence of oxygen, carbonyl compounds are produced. With such a complicated picture of the polymer itself and of its mechanism of degradation, it is little wonder that there are many different theories for the action of stabilizers (FIGURE 5).

SCAVENGING OF HCI

RADICAL INHIBITION - KENYON, A. S.

R * + Bu2 Sn(OAcI2 -R-Bu + Bu Sn (OAcIZ - y!

DIELS-ALDER ADDITION - FOX, HENORICKS AND RATTI

CARBALKOXYLATION - FRY€ AND HORST

H F' Bu2Sn (OAc& + 2 (-CH-CH -C= CH-)

OAc H I

Bu2SnCI2 +2( -CH-CH-C =CH-)

ANTIOXIDANT ACTIVITY - MACK, G.

FIGURE 5 . Theories of mechanism of PVC stabilization.

The scavenging of HCI, the traditional concept, is now considered only a small part of the story. A. S . KenyonZ0 has suggested that the alkyl groups directly bonded to the tin are used to alkylate the polymer radicals, thus stopping the chain decomposition (VI). Fox, Hendricks, and R a t t P have suggested that an organotin maleate adds to the double bond system of the polyenes in a Diels- Alder type of reaction (VII). More recently, Frye and HorsP have suggested that organotins operate by replacing labile chlorines with acid moieties, thus reducing the tendency to go over to a polyene system (VIII). Mackz3 has suggested that a good organotin stabilizer must also be an antioxidant and prevent the polyenes from being oxidized to carbonyl compounds. Of course, in a field as complicated as this, there are arguments for and against each theory. A critical evaluation will not be attempted here.

Stabilization of Other Chlorinated Products The technology developed for the stabilization of polyvinyl chloride resins

applies to other products to which PVC is added (TABLE 6 ) . Thus, nitrile rubber


modified with P V C has been stabilized with organotin compounds. The same thing is true of acrylics modified with halogenated materials.

There are also patents describing the stabilization of polyvinylidene chloride with organotin compounds (TABLE 6) . Chlorinated rubbers which are similar in structure to PVC have been stabilized with organotins. Latex paints with organotin stabilizers are sold in Great Britain.

Chlorinated paraffin, and to a lesser extent, chlorinated polyethylene and chlorinated rubber are often used as flame retardants in plastics as well as in other substrates. If these products are subject to any appreciable heat history, they must also be stabilized, and often organotins are used for this purpose (Church & Johnson, TABLE 6) .

TABLE 6 OTHER STABILIZATION USES OF ORGANOTINS (CHLORINATED PRODUCTS)

Modified nitrile rubbers (with PVC). ABRAMS, W. J. 1962. Rubber Age 91: 255.

Modified acrylics (with PVC, Saran, etc.) WALTER, A. T. 1959. U S . Patent 2,868,756.

Polyvinylidene chloride DAVIE & PATTERSON. 1959. French Patent 1,184,249.

Chlorinated rubber-(used in latex paints) GAY, P. S. 1953. Tin and Its Uses 29: 6. GAY & HEDGES. 1954. British Patent 714,100.

Chlorinated paraffins CHURCH &JOHNSON. 1952. U. S. Patent 2,580,730.

TABLE 7 OTHER STABILIZATION USES OF ORGANOTINS ( NONHALOGENATED PRODUCTS)

Lubricating oils-( antioxidants and sludge inhibitors)

__._________ __.

STANDARD OIL Co. 1947. British Patents 591,717 & 9. MCDERMOTT, J. P. 1957. U.S. Patents 2,876,812-3-4.

Hydrogen peroxide WOOD & HILL. 1958. British Patent 788,951.

Kubber HART, E. J . 1949. U. S. Patent 2,476,661. M & T CHEMICALS. 1957. A series of 10 patents.

Cellulose acetate FROMMFELDT, H. 1960. German Patent 1,103,014. SESOKO, M. 1960. Japanese Patent 9430/60.

Polyamides UNION CARBIDE. 1960. French Patent 1,253,028.

Polycarbonates N. V. ONDERZOEKINGS INSTIUT (HOLLAND). 1960. French Patent

1,253,066. -__ ___. ___.____ - _____ -


Stabilization of Nonchlorinnted Products

It is interesting to note that the organotins have been used to stabilize materials where halogen is not involved (TABLE 7 ) . There are a number of patents describing the use of organotins as antioxidants and sludge inhibitors in lubricating oils. Decomposition of hydrogen peroxide has also been inhibited by the addition of organotins. Rubber can be protected from ‘ozone cracking by addition of organotins.

Many polymers have been reported to be more stable to heat and in some cases to light by the addition of organotins. Included in this list are cellulose acetates, pol yamides and pol ycarbonates. Pol yolefins, and more specifically, polypropylene, also have a thermal stability problem. Through 1961 at least 14 patents have been issued in the US. claiming organotins as efficient stabilizers for such polymers. Many of these are for the tin-mercapto compounds.

None of these latter uses have developed to the stage where their mode of action has been reported, or even to the point where one can relate activity to the structure of the compounds. As commercial interest in these materials increases, such information will, no doubt, be developed.

CATALYTIC APPLICATIONS Another major area of use of the organotins is as catalysts in various types of

organic reactions. Urethane Catalysts

Of these, the best known use is in the field of urethanes. Although organotins have been used for catalyzing the formation of urethane elastomers and urethane coatings, their greatest market penetration is in the field of urethane foams. Polyurethane foams were first considered commercially in the 1930’s. At that time, interest was centered primarily in the polyurethanes obtained by the reaction of diisocyanates with polyesters. These came into commercial production immediately after World War 11. They were rather high cost materials and had limited markets. It was known that polyols could also react with isocyanate to give urethane materials and since these polyols were available from low cost petrochemicals they were considered next. It was in 1957 that the first polyether urethanes became commercial.

The first catalysts to be used in this type of condensation reaction were amines. They were rather slow, however, and necessitated a two step process. The first step involved the preparation of a pre-polymer which contained terminal isocyanate groups. The pre-polymer was then reacted in the final stage with water and polyol to give the finished product. In 1958 intense research efforts were made to find systems which would allow the production of polyurethane foam directly from the basic ingredients. This was achieved in 1958 by the discovery of the activity of the organotin catalysts.

The basic reaction forming the polyurethane chain involves the addition of hydroxyls to isocyanates to give urethane functions along the growing chains (1x1 (FIGURE 6 ) . In the case of foams, water is generally added to give the gas reaction (X). Water reacts with an isocyanate to give the intermediate unstable carbamic acid. This decomposes to carbon dioxide which bubbles out as a gas producing the foam, leaving an amine group on the polymer. These terminal amine groups then react with other isocyanate functions to give a cross-linking reaction through a urea type of linkage (XI). The chemistry does not stop here. These urea compounds can react further with isocyanates to form biurets.

Ross: Organotin Compounds CHAIN EXTENSION

O=C=NdvvwN=CsO t H O J v v v w O H - - - - - - - - c

JV\hO-C-NmN-C-OJVVVVV\, t ; H H a

119

Is

GAS REACTION

m N + C = O + H20 +C% t x

t i ? ti CROSS LINKING O N c C= 0 + w N H 2 - - - - t W N - C - N W H FIGURE 6. Chemistry of urethane foams.

The problem in this series of reactions is to control the relative rates of reaction. Should the chain extension reaction occur too fast, a relatively unfoamed gel would result. Then, when the carbon dioxide is generated, excessive pressures result and the gel splits. On the other hand, if the gas reaction is too rapid, the gel is not strong enough to retain the gas as the bubbles form, and the foam collapses. The organotins are of great interest because they give a very rapid gel reaction in comparison to the amines as can be seen in TABLE 8.

In modern formulations, amines and other agents are added in addition to the organotins to assist in the gas reaction in order to keep the reaction rates properly balanced. The technology has developed to the point where the physical properties of the foams can be quite precisely controlled by the amount and type of catalyst added.

One of the early drawbacks of organotins lay in the observation that foams made with them degraded at elevated temperatures (ca. 285’F.). It was later found that this is an oxidative degradation which is controlled by the use of antioxidants. In the meantime, however, it was discovered that certain stannous compounds, such as the stannous octoate, also catalyzed the reaction quite well and did not suffer from this dry heat problem. Although the stannous compounds are not completely stable in air as are the organotins, and although they require

TABLE 8 RELATIVE ACTIVITY OF URETHANE FOAM CATALYSTSe4

Catalyst Relative activity

None N-methylmorpholine Triethylamine N,N,N’,N-tetramethyl-l,3-butanediamine Triethylaminediamine Stannous chloride Ferric acetylacetonate Tetraphenyltin Tri-N-butyltin acetonate Bis(2-ethylhexyl) tin oxide Di-N-butyltin diacetate Di-n-butyltin dilaurate Di-N-butyltin dichloride Di-N-butyltin dilaurylmercaptide Dimethyltin dichloride

1 4 8

27 120

2,200 3,100

9 31,000 35,000 56,000 56,000 57,000 7 1,000 78,000


longer times to cure, they have the additional advantage of being cheaper than the organotins. Consequently, they are widely used for flexible urethane foams. In many cases, combinations of organotins and the stannous compounds are used in a single catalytic system. Although many organotins are active, catalysts of choice appear to be the dibutyltin dioctoate and the dilaurate.

Silicone Catalysts

One of the growing applications of organotin compounds is in the field of silicone rubbers. In general, silicone rubbers are cured at elevated temperatures using peroxide catalysts. It has been found that silicones can be cured at room temperature through the use of organotin catalysts. Basically, a prepolymer with silanol end groups is caused to react with a dialkoxysilane joining two chains together (FIGURE 7) . To the extent that there are hydroxyl groups in the middle of the chain, cross-linking can occur.

Ma Ma Ma I I I

-S i -OH +RO-Si-OR'+ HO-Si-0 I I I Me Ma Me

Ma Me Ma I I I

I I I Me Me Me

-Si - -O -Si-O-Si-O-+22OR

FIGURE 7. Curing of silicone rubbers with organotins.25

The room temperature curing (RTV) silicones are finding many diverse applications, e.g., making dental impressions, encapsulating electronic parts, etc.

There are many patents in this field covering a variety of organotin compounds as silicone curing catalysts.26.27 The compounds of choice, however, appear to be the dibutyltin dioctoate and the dilaurate.

Esferificarion Catalysts

Another application of organotins which was developed during the 1950's is in the catalysis of esterification reactions. This work includes transesterification and polyesterification reactions as well. It has been found that organotin compounds, and more specifically dialkyltin derivatives, have several distinct advantages in this type of reaction.

( 1 ) They have a very high catalyst efficiency, at times exceeding that of sul- furic and the sulfonic acids that are generally used.

( 2 ) There is very little tendency to eliminate water from secondary alcohols to form olefins. This is always a problem in acid catalyzed esterifications.

Ross : Organotin Compounds 121

( 3 ) Acids tend to decompose the esters to some extent giving off-colors, whereas with organotin compounds colorless esters are produced.

(4) Because the organotins are neither strongly acid nor basic, they do not leave such residues in the polyesters.

( 5 ) Very often the presence of the organotin residue gives some degree of heat stabilization to the plastics.

(6) Physical and electrical properties of the product are often improved. Because of these properties, organotin esterification catalysts are used in some

cases, even though they are more expensive than other catalysts. Some of the fields for such uses are in the manufacture of plasticizers and polyester polymer~ .2~-~O

Lactones,3I pyrrolidone~,~2 and glycolic acid33 have also been polymerized using organotins as catalysts. The mechanism of operation is probably the same as for the polyesterification catalysis.

Epoxy Curing Catalysts

Another catalytic function reported for organotins is in the curing of epoxy resins.34 Here the problem has been that the best hardening agents react so fast with epoxy systems that the resin sets up into a very hard, cross-linked solid soon after mixing. In the vernacular, it has a short “pot life.” Organotins have been shown to give longer pot life and yet to very effectively and completely cure the epoxy resin when given sufficient time and/or temperature.

Catalytic Mechanism

It is interesting to speculate on the reasons why the organotins are such good catalysts for so many varied reactions. This author thinks common cause can be made of the urethane catalysts, the silicone curing catalysts, the esterification catalysts, the lactone and pyrrolidone polymerization catalysts, and the epoxy curing compounds. It should be remembered that tin, in the fifth row of Group IV of the periodic chart, has not only 5s and 5p orbitals, but also some low energy 5d orbitals that can be utilized in chemical bonding. The existence of hexacoordinate and pentacoordinate organotin compounds are good evidence to the participation of these orbitals. One can therefore postulate a coordination between the tin and the oxygen (or nitrogen) in the substrate, with the oxygen (or nitrogen) contributing its electrons to the coordinate bond (FIGURE 8) . This polarizes the carbonyl bond making the carbon more susceptible to attack by an electrophillic reagent, such as alcohol, as in the case of the transesterification shown in FIGURE 8. This approach could account for the catalytic action of organotins wherever oxygen or nitrogen compounds are involved.

I C”2 I

OAC F i c o R E 8. Mechanism of catalyst function of organotins.


Olefin Polymerization Catalysts

In the patent literature on the polymerization of olefins (and some other vinyl compounds) one finds many organotin catalysts described, especially in combina- tion with transition metal compounds, such as titanium or vanadium. The organotins probably operate by the alkyiation or reduction of the transition element and, hence, are replacements for the aluminum alkyls in the Ziegler type catalysts. Because the tins are much more stable and easily handled, they have some advantages over the aluminums. As the polymerization of olefins is the subject of another paper, the technology will not be discussed here. This area does have commercial potential for the organotins, however. There is at least one polyolefin plant operating with organotin catalysts.

MISCELLANEOUS APPLICATIONS

A search of the literature, especially the patent literature, reveals a large number of other potential applications for organotins. For example, there is a Belgian patent that quotes the use of organotins in the preparation of luminescent ortho- phosphate powders;35 another that uses organotins for the activation of alumi-

A German report claims that organotins increase the yield of gamma isomer in crude c y ~ l o h e x a n e . ~ ~

Organotin halides have been suggested for the treatment of glass to form an electrically conductive film.3s

Antler of Ethyl Corp. has shown that organotin sulfides are excellent wear inhibitors in l~bricants.3~

SUMMARY

In summary, it can be concluded that the organotin compounds are, as a group, highly versatile materials. They probably owe this wide applicability to the unique position of tin in the periodic chart. Together with lead, silicon, and aluminum, the tin compounds constitute the largest tonnage organometallics being produced today. The many current and potential applications of organotins portent in- creased growth for this group of compounds in the future.

REFERENCES 1. LOWIC, C. 1852. Ann. 84: 308. 2. FRANKLIN, E. 1853. Ann. 85: 329; 1859. 11: 44. 3. MCCABE, W. L. & B. MEADE. 1932. U. S. Patent 1,857,761. 4. YNCVE, V. 1940. U. S. Patent 2,219,463. 5. CLARK, F. M. 1957. Chem. Eng. News 25: 2978. 6. CLARK, F. M. 1949. U. S. Patent 2,468,544. 7. ELLENBURG, A. M. 1951. U. S. Patent 2,573,894. 8. PERRY, M. & P. F. BRUINS. 1955. Chem. Tech. : 609,673. 9. RIETHMAYER, A. 1963. Kunstoff-Rundschau 10: 271; 345.

10. LUIJTEN, J. G. A. & G . J. M. VAN DER KERK. 1955. Investigations in the Field of Organo- tin Chemistry. : 59. Tin Research Institute. Middlesex, England.

11. LUIJTEN, J. G. A. & S. PEZARRO. 1957. Brit. Plastics 30: 183. 12. KLIMMER, 0. R. & 1. U. NEBEL. 1960. Arzneimittel-Forsch. 10: 44-8. 13. BARNES, J. M. & H. B. STONER. 1959. Pharmacol. Rev. 1.1(2): 211-31 (Part 1). 14. 1961. Bundesgesundheitsblatt 19: 310-312. 15. FARBWERKE HOECHST. 1962. British Patent 890,283. 16. KLIMMER, 0. R. 1963. Private communication. 17. COTMAN, J. D. 1953. Ann. N. Y. Acad. Sci. 57: 417. 18. WINKLER, B. E. 1959. J. Poly. Sci. 35: 3. 19. ARLMAN, E. J . 1954. J. Poly. Sci. 12: 543. 20. KENYON, A. S . 1953. Nat. Bur. Stand. Circular No. 52s: 81

Ross: Organotin Compounds 123 21. Fox, V. W., J . G. HENDRICKS & H. J. RATTI. 1949. Ind. Eng. Chem. 41: 1774. 22. FRYE, A. H. & R. W. HORST. 1959. J. Poly. Sci. 40: 419. 23. MACK, G. P. 1953. Mod. Plastics 31: 150. 24. MACK, G. P. 1960. Chem. Proc. : 30. 25. HUGHES, J. S. 1962. In Developments in Inorganic Polymer Chemistry. Lappert & Leigh,

Eds. : 149. Elsivier Publishing Co. New York, N. Y. 26. MITZSCHE, S. & M. WICK. 1957. Kunstoffe 47: 431. 27. MIDLAND SILICONES. 1960. British Patent 852,596. 28. B. F. GOODRICH. 1955. British Patent 810,381. 29. ALDERSON. T. 1953. U. S. Patent 2.658.055. 30. CALDWELL; J. R. 1955. U.S. Patent 2,720.507. 31. YOUNG, D. N., F. HOSTETILER & C. F. HORN. 1959. US. Patent 2,890,208. 32. GENERAL ANILINE & FILM. 1959. Belgian Patent 577,374. 33. HIGGINS, N. A. 1954. U.S. Patent 2,676,945. 34. CHEMISCHE WERKE ALBERT. 1957. British Patent 783,764. 35. 1960. Belgian Patent 589,051. 36. KOPPERS Co. 1959. British Patent 808,706. 37. LANGENBECK, W., G. LOSE & H. FURST. 1952. 38. LYTLE, W. 0. & A . E. JUNGE. 1952. US. Patent 2,566,346. 39. ANTLER, M. 1959. Ind. Eng. Chem. 51: 753.

Chem. Tech. (Berlin) 4: 247.

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INDUSTRIAL APPLICATIONS OF ORGANOTIN COMPOUNDS

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