Polysiloxanes

34 Polysiloxanes J. J. LEBRUN and H. PORTE Rhone Poulenc, Saint Fons, France 34.1 INTRODUCTION

593

34.2 ·POLYCONDENSATION OF SILANOLS AND SILOXANOLS 34.2.1 Reactivity of Silanols 34.2.2 Condensation Reactions of Silanols 34.2.2.1 Thermal condensation 34.2.2.2 Homogeneous catalysis 34.2.2.3 Other polycondensation catalysts

594 594 594 594 594 598

34.3 CONDENSATION OF SILANOLS WITH ORGANOSILANES 34.3.1 Introduction to the Preparation of Elastomers 34.3.2 Chemistry of Crosslinking 34.3.2.1 Crosslinking via acyloxy groups 34.3.2.2 Crosslinking via alkoxy groups 34.3.2.3 Crosslinking via alkenyloxy groups 34.3.2.4 Crosslinking via amino groups 34.3.2.5 Crosslinking via ketiminoxy groups 34.3.2.6 Crosslinking via amido groups 34.3.2.7 Crosslinking via aminoxy groups 34.3.3 Conclusion

598 598

605 606

34.4 REFERENCES

606

600 600 602 603 603 604 604

34.1 INTRODUCTION The synthesis of poly(dimethylsiloxane) macromolecules having =SiOH groups at the end of the chain may be carried out by the following methods. (i) Either by ring-opening polymerization (equation 1),for example in a basic medium using water as a transfer agent during polymerization or as a deactivating agent at the end.

'AI

)S[O~_

~

•

Me Me \/

Me Me \/

HOSiV'V'VVV"SiO- Mt " . _

x.

x= 3-6

--

\/

-~

Propagation

.

H

/\

Me Me

\/

1\

H

(1)

Me Me

(ii) Or bypolycondensation catalyzed by acids or bases (equation 2), starting with et,w-siloxanediololigomers resulting from the hydrolysis of Me 2SiCI 2 •

H+

or

OH-·

Y \ 7;

HO ( SiO\ H

+

(2)

Me Me

By either of these processes, average degrees of polymerization of a few thousands may be obtained. 593

Synthesis by Step Polymerization

594

Although there have been many studies relating to the anionic or cationic polymerization of cyclosiloxanes, the polycondensation of linear cx,w-siloxanediol derivatives has formed the subject of only a few published fundamental investigations. Nonetheless, this reaction is widely utilized in silicone chemistry on an industrial scale for the production of macromolecules which form part of elastomeric formulations. In the latter case, the reactions mainly employed involve the use of silanol or siloxanol groups with polyfunctional organosilicon derivatives R nSiX4- n, where n=O, 1 or 2, the group X being chosen so as to have a =:Si-X bond which is easily hydrolyzable: i.e. acyloxy, alkoxy, amino, amido and the like.

34.2 POLYCONDENSATION OF SILANOLS AND SILOXANOLS 34.2.1 Reactivity of Silanols Condensation reactions involving silanols are governed by the polarity of the Si-O and O-H bonds. This depends on the electronic nature of the substituents of silicon. Silanols are stronger acids than their hydrocarbon homologues; this is demonstrated by the formation and the characterization of complexes of silanol and electron-donating compounds: ethers,' - 8 amines, 8,9 ketones.' and the like. The bond between the silicon and the oxygen is (J and pn-dt: in nature, which increases the polarity of the O-H bond. 1,4,5,9-11 This property of the silanol bond depends on the nature of the silicon substituents. For example, the acidity of silanol decreases when the inductive effect + I of the silicon substituents increases. In contrast to the chemistry of hydrocarbon compounds, the basicity of silanols is not inversely proportional to their acidity. 7,12

34.2.2 Condensation Reactions of Silanols Like all reactions of this type, the condensation of silanols is an equilibrium reaction (3). (3)

It will therefore be possible to promote the formation of a siloxane bond by decreasing the partial pressure of water in the medium.

34.2.2.1

Thermal condensation

Because of their acid nature, silanols may be made to undergo homocondensation by heat treatment. This is so, for example, in the case oftrimethylsilanol which undergoes condensation from room temperature upwards.!" The heat stability of tris(organo)silanols increases with the steric hindrance of the silicon substituents. Thus, tris(isoamyl)silanol undergoes condensation from 270°C upwards.I" whereas triphenylsilanol is stable up to 300°C. 16 The reactivity of silanols increases with the number of hydroxyl groups on the silicon.13 Diorganosilanediols are much more heat unstable than triorganosilanols: dimethylsilanediol must be stored at low temperature 17-18 and the impurities present in the glass catalyze its condensation. 19 As in the case of the triorgano derivatives, the stability of diorganosilanediol increases with the size of the substituents: diethylsilanediol is stable at room temperature.i" diisopropylsilanediol undergoes condensation from 130°C upwards;" and dicyclohexylsilanediol at 200°C. 21 Linear oligomers of formulae HO[Me2SiO]nH (with n=3, 4 or 5) are much more stable and do not react even at 200°C;22 therefore the condensation reaction must be catalyzed.

34.2.2.2 Homogeneous catalysis ( i) Catalysis by acids

Many cationic catalysts bring about the condensation of silanols, among which are those mentioned in Table 1.

Polysiloxanes

595

Table 1 Acids

Refs.

H 2S04 HCI

23-32 26, 33--42 43 23,32,44 45--47 33

HBr H 3P0 4 H 3B04

HN0 3 CF 3C02H Aryl S03H MeS0 3 H CF 3S0 3H

48

49-51 52 52-54

Studies on the kinetics of cationic polycondensation of silanediols were carried out in the 1960s.55 The results obtained depend on: the nature of the silanediol employed; the type of catalyst chosen; and the possible presence of additives. Taking the condensation rate of dimethylsilanediol as reference, the reactivity of different monomers in the decreasing order is as follows: Me 2Si(OH)2 > Me(CH=CH 2)Si(OH)2 > (CICH2)MeSi(OH)2 > (CI2CH) MeSi(OH)2; and in the aromatic series: Me 2Si(OH)2 > PhMeSi(OH)2 > Ph 2Si(OH)2· Increasing the electronegative groups on the silicon causes a decrease in the electron density on the oxygen of the silanol bond and renders attack by a proton more difficult.P An increase in the size of the substituent causes a decrease in condensation rate by a steric effect.58 Many catalysts have been studied: HCI, HBr, H 2S04 and HCI0 4.56,67 It is possible to demonstrate the following common trends. (i) Second order kinetics with respect to silanediol and first order kinetics with respect to the catalyst. 56-67 (iijExistence of an induction period. 60-62,64 (iii) Significant modification of the progress of reaction by the introduction of water: elimination of the induction period and acceleration of the rate of condensation catalyzed by HCI and H 2S0 4;60,61,63 decrease in the rate of condensation catalyzed by HBr and HCI0 4;63 and increase in activation energy in some cases (Table 2). (iv) Similarity in behaviour of condensations in the presence of water and of alcohol.P" Table 2

H 2S04

Ea(kcal mol- 1 ) HBr

HCI0 4

6.1

14.1

4.5

0.15 0.35 0.50

11.1

7.8

14.2

14.0

Monomer=Et 2Si(OHh and solvent = dioxane (1cal = 4.19J).

The mechanism proposed 59 is the sum total of two elementary processes: (a) production of silanol to give a primary oxonium ion; and (b) reaction of a silanol with the oxonium ion formed (Scheme 1). ==SiOH ==SiOH

+

+

(a)

HX ~ ==SiOH;X-

==SiOHi"X-«

~ ==SiOSi==

+

HX

+

H 20

(b)

Scheme 1

However, it should be noted that there is no experimental evidence for the existence of this oxonium ion. It is only very recently that cationic polycondensation of siloxanes has been studied by using models for the formation of reaction products. 52 The monomer employed is decamethylpentasiloxane-J,9-diol and the catalysts are methanesulfonic and trifluoromethanesulfonic acids. The condensation leads to the formation of two categories of products (Scheme 2).

596

Synthesis by Step Polymerization Me

---.

~Ii I s

Cyclic

Me

~

Me

HO---.L~inLH ~I~ Me

Linear

Scheme 2

The nature of solvent employed has a significant effect on the nature of products formed: in dioxane, cyclic oligomers are predominantly formed and a difference in order with respect to the monomer is observed (first order for the formation of cyclic molecules and second order for the formation of linear oligomers); whilst in dichloromethane, the main compounds formed are linear. The author proposes a reaction mechanism which involves a nucleophilic assistance of dioxane towards silanols, a role which cannot be fulfilled by dichloromethane. (ii) Catalysis by bases

The polycondensation reaction of silanediols and of c<,w-siloxanediols may also be catalyzed by adding strong bases: KOH, NaOH or LiOH, at concentrations of the order of 10- 3 moll-1.68-7o A base-catalyzed condensation passes through a preliminary stage of silanolate formation (equation 4). (4)

Considering the respective concentrations of silanol and the base, it can be assumed that the silanolate concentration in the medium is equal to the initial concentration of the catalyst, when Mt + = Li +, Na + or K + in a polar solvent. The propagation stage may be approximated to a nucleophilic attack by the silanolate on a silanol with hydroxyl regeneration, as in equation (5), which determines the kinetics of the polycondensation. (5)

From a comparison of the condensation by acid catalysis with that by basic catalysis, it is observed that the reactivity ·of the different silanediols is reversed: Me 2Si(OH)2 < (CH 2=CH)MeSi(OH)2 < (ClCH 2)MeSi(OH)2 < (CI2CH)MeSi(OH)2 and (Me)2Si(OH)2 < MePhSi(OH)2 < Ph 2Si(OH)2· The nucleophilic attack by the base on the silicon atom is facilitated by the electronegativity of the substituents. It is also interesting to note that in the case of basic catalysis, the electronegativity prevails over the steric effect of the substituent.72 The condensation rate of trimethylsilanol in a methanolic medium is 500 times faster with hydrochloric acid than with potassium hydroxide. 70 Kinetic studies on polycondensation in methanol have been carried out with dimethylsilanediol or with methylphenylsilanediol, restricting the conversion level to 100/0. 71 Beyond this value, the formation of =SiOMe groups interferes with the determinations. The condensation rate ofsilanols is proportional to the catalyst concentration, but does not depend on the nature of the cation. The limiting stage of the reaction would be the action of the anion on the silanol"?: 73 and its kinetics are first order with respect to the silanol."! When the silanol is condensed in acetone using di(n-hexyl)ammonium 2-ethylhexanoate, the reaction is first order with respect to silanol, but it is of second order with acetonitrile as solvent. 74 A study of the kinetics of condensation of 1,4-bis(dimethylhydroxysilyl) benzene and 1,1,3,3-tetramethyl-l,3-dihydroxydisiloxane in toluene under conditions of azeotropic distillation of water, in the presence of potassium hydroxide; indicates a second order with respect to the monomer and an order of 0.5 with respect to the catalyst. 75 In this case, it is possible that the silanolate in a solvent of low polarity might be in the form of an associated ion pair (1), and that propagation occurs predominantly on non-associated silanolate. ==Si-=-O-K + I I :+ I

K~O-Si==

(1)

Polysiloxanes

597

More recently, Chojnowski?" has proposed a polycondensation mechanism involving an elimination reaction with the intermediate formation of a silicon-containing derivative carrying a double bond (Scheme 3, where R = organic residue on polymer chain). Me

Me I

ID-~iid­

R-Si-OH

I

R-

--+

I Me

Me

+

Me I

Si=O

I Me

Scheme 3

The formation of such a silanone had been proposed previously."? This intermediate is unstable and undergoes rapid change, as in equation (6).

". M(

Me

Me

SI==O

+ ROH

I

RO-Si-OH

---.

(6)

I

Me

Thus, the anionic polymerization of cyclosiloxanes and the polycondensation catalyzed by bases could involve this reaction intermediate, the formation of which would explain the results obtained for the model polycondensation of decamethylpentasiloxane-l,9-diol. This study was carried out in 1,4-dioxane in the presence of 20/0 of water and of NaOH. The reactions expected areas in Scheme (4). Me Me

XHof;S~otH Me Me.

~~

+

~ H" ( SiO"\ H

y\

'7;

H2 0

Cyclization

+

Chain extension

H2 0

Me Me

Scheme 4

Logically, at the beginning of the reaction, the production of compounds in which n = 5 andye 10 may be expected. However, at low conversion levels (100/0), (2) and (3) are essentially formed, but no cyclic derivative D5 such as' (4) nor (5).

Hoi;Si~H Me \ie

HOfSiO~H /\ );

HO-fSiO'\ H 1\ ~ Me Me

Me Me (3)

(2)

(5)

(4)

This is confirmed even for a high conversion level (~o%). However, from the kinetic determinations, it is not possible to decide between the conventionally proposed mechanisms (Scheme 5) and the mechanism put forward by Chojnowski (Scheme 6). Me'~1

I »: -SiOMt +

I Me

"s·1-0f'! si.I + HOI

1

SchemeS

\/

\ /

Ho-SiO-Si;

Scheme 6 PS5.-T*

1

I

I

I

HO-Sio-Sio-Ji-

I

598

Synthesis by Step Polymerization

Nevertheless, assuming the passage through a silanone, it would be possible to explain some secondary reactions encountered in anionic polymerization and written previously (Scheme 7; R=organic residue or polymer chain; Mt=Li, Na, K and the like). This mechanism can hardly be considered because of the proximity of the negative charges. Me

Me

-~i-O-Mt+ + R-~i-O-Mt+ I

I

Me

Me

Scheme 7

On the other hand, the mechanism in Scheme 8 appears to be more realistic. Me I

-Si-O-

I

Me

Me I + Si=O ~ I Me

Me

I I

R-Si-O-Mt+

Me

Scheme 8

In fact, irrespective of whether it is by the formation of a silanone or by the intramolecular or intermolecular .reaction of the siloxanolate with the polysiloxane chain, it is difficult to restrict the redistribution reactions taking place during polycondensation performed to high conversion levels. These secondary reactions result in an increase of the fraction of volatile compounds with low molecular weights. Additionally, the activation of the polycondensation reaction using suitable catalysts generally results in an acceleration of the secondary reactions as well. However, the use of agents. which sequester the cations and free the anion enables this mechanism to be avoided from a kinetic point of VIew. in the presence of cryptands.?" polyheteromacropolycyclic compounds such as (6) or of the sequestering agent 79 tris(oxaalkylamine) (7), it is observed that the rate of polycondensation catalyzed by strong bases is greatly accelerated while the formation of volatile compounds of low masses is restricted at the same time.

N-f-CHRl-CHR2-0+CHR3-CHR4-0-hRs] 3 (7)

(6)

34.2.2.3 Other Polycondensation catalysts Many acid, basic and metal catalysts have been described and claimed for the polycondensation of silanols. Table 3 gives a non-exhaustive list thereof.

34.3 CONDENSATION OF SILANOLS WITH ORGANOSILANES 34.3.1

Introduction to the Preparation of Elastomers

Reactions involving the use of a silanol group with another silicon derivative containing the labile group =SiX have especially been studied in the formulation of silicone elastomers. Silicone macromolecules have very specific properties, among which a weakness of their cohesive forces and a high freedom of rotation of the SiOSi bonds are noteworthy; these result in a relatively low value for their glass transition temperature ( < -100 DC). Therefore, the preparation of elastomeric materials having superior mechanical properties requires the crosslinking of linear polysiloxanes into which reinforcing fillers have been incorporated.

599

Polysiloxanes Table 3 Catalyst

Ref

80-82 82-84 85 82,86

Amines Amine salts + carboxylic acid Amine salts + sulfonic acid Amine salts + phosphoric acid BF 3' MeC0 2H + pyridine Alkali metal alkylarylsulfonate Carboxylate of Na, K, Li, Pb, Hg and the like Iron or tin octoate (ROhVO Alcoholate of AI, Mg, Na and the like Phenolate Metal halides Ion exchange resins Zeolites and the like

87

88 89,90 91,92 93 94,95 96,97 98 99 100

Elastomers are generally classified according to the method of crosslinking employed: hot vulcanizable elastomers (crosslinking by radical reactions); single component cold vulcanizable elastomers (crosslinking by condensation reactions); and two-component cold vulcanizable elastomers (crosslinking by hydrosilylation or by condensation). Most of the elastomeric compositions which can be vulcanized at room temperature contain the following components: an cx,w-hydroxylated polysiloxane (8); a crosslinking agent of the RSiX 3 or SiX4 type, where X is a group which can be hydrolyzed (acyloxy, amino, ketiminoxy, enoxy and the like); a filler (silica); a catalyst e.g. a metal salt having the property of a Lewis acid or a base; various additives e.g. colouring agents, fungicides and the like. HO ( SiO' H

y\7;

Me Me (8)

The structure of the chemical network of the elastomer depends on a number ofstages which may be sequential or simultaneous: (i) Functionalization 'of the linear chains (equation 7).

I

I

HOSi-SiOH

+

I I

2RSiX3

~

I I

X2RSiOSi-SiOSiX 2R+

I I

2HX

(7)

(ii) Hydrolysis (equation 8).

I I

X2RSiOSi-SiOSiX 2R

I I

+

H 20

~

I I

HOXRSiOSi-SiOSiX 2R

I I

+

HX

(8)

(iii) Crosslinking by condensation (equation 9). I

vvSiX

I

+

I

HOSivv

I

I I I I

- - - . vvSiOSivv +

HX

(9)

Among the most commonly employed organosilicon groups =SiX, those in Table 4 may be mentioned. Cold vulcanizable elastomers produced by single component or two-component formulations find their uses as flowing materials for applications in coating, adhesive bonding, moulding and electrical insulation or as non-flowing thixotropic materials in the fields of adhesive bonding and sealing (building and public works). The economic investment of this class of material is increasingly important. For thirty years, there has been a succession of innovations which are described exclusively in the form of patents.

600

Synthesis by Step Polymerization Table 4 Organosilicon Groups

Formula

==SiOCOR ==SiOR

Acyloxy groups Alkoxy groups

/R

1

==SiN

Amino groups

\R z

==SiN-C-R 2

Amido groups

I

I

R1 0

==Si-O-C= CHR 2

Alkenyloxy (or enoxy) groups

I

R1 R

==Si-O-N

Aminoxy groups

/1 \

RjRt

==Si-O-N=C

Ketiminoxy groups

-.

The first single component formulations provided have employed an acyloxysilane-based crosslinking system. The elimination of the undesirable acid release has led to the production of a second generation of elastomers. The use of silane carrying hydrolyzable groups, such as alkoxy, amino and the like, has enabled this problem to be overcome. The current trend consists of improving the environment for the crosslinking system, for instance by adding: 'scavengers' which increase the stability during storage; and catalysts and crosslinking agent (but the problem of their. toxicity becomes very important). The analysis which follows is not exhaustive, but tends to illustrate each category of elastomer. The classification is carried out according to the type of crosslinking employed.

34.3.2

Chemistry of Crosslinking

34.3.2.1

Crosslinking via acyloxy groups

The principle of this crosslinking is illustrated by equations (10) and (11). Hydrolysis Condensation

==SiOCOR ==SiOCOR

+ H 20 ( .

+ ==SiOH

~

~

==SiOH ==SiOH==

+ RC0 2H + RC0 2H

(10) (11)

The reaction between a polysiloxane carrying silanol ends and a methyltriacetoxysilane type crosslinking agent was described as early as in 1957 by Rhone Poulenc.""! The composition is stable during storage for a few months in a confined atmosphere and crosslinks by atmospheric moisture without catalyst in a few hours.l02-103 The condensation reaction may be catalyzed by metal salts and more particularly by tin salts, 104, 105 titanium salts"?" or even by a mixture of the twO.1 0 7 Catalysis by titanium requires much lower quantities than the quantities of tin employed. lOB The fillers employed are generally silicas'?" which have optionally undergone a treatment designed to make them hydrophobic. The production of functional macromolecules is carried out in situ or beforehand Section (34.2.2.) Different categories of materials have been employed (Table 5). It is worth noting that other types of functionalization may be employed, although they do not currently have significant industrial outlets (equation 12).106,110

601

Polysiloxanes Table 5

Category

Formula

Acyloxysilane

Acyloxy-/alkoxy-silane mixture Mixed acyloxyalkoxy compounds Others

Ref.

104 114

MeSi(OAch and EtSi(OAch MeSi(OAch + Me 2Si(OAch ViSi(OAch,a PhSi(OAch MeSi(OCOPhh MeSi(OAch + Si(OEt)4 MeSi(OACh + alkyl polysilicate (ButOhSi(OAch (EtO)xSi(OAc)4- x R4- a- bSi(OR)a(OCOR)b (AcOhSiOSi(OAch

I

140-142 109-112 111 115 116 113

I

Me Me Me

I

117

(AcOhSi(OSihOAc

I

Me

118,119

(ACO)3SiCH2CH2Si(OAc)2

I

Me

138

Silyl 2-ethylhexanoate a

Me

Vi = Vinyl group.

Me

Me

'-fc--siIt II

H-Si I Me

I" Me

O-Si-H Me

The crosslinking of the functionalized linear chains occurs through the hydrolysis of the acyloxysilanes under the influence of atmospheric moisture. The silanols formed condense with other acyloxysilanes. The crosslinking therefore takes place through the surface in contact with ambient air and spreads into the mass by diffusion of water vapour. This causes problems in the case of crosslinking in thick film. An improvement consisted of incorporating lime into the elastomer, which enables water to be released within the mass.120-124 This enables setting speeds to be accelerated very substantially (equations 13 and 14). ==SiOAc Ca(OHh

+ HyOfatm) +

2AcOH

==SiOH

+

AcOH

(13)

Ca(OAch

+

2H 20

(14)

---+ ---+

The intrinsic properties of silicone elastomers may be modified or improved by adding specific additives. One of the properties most commonly sought is the adhesiveness to supports of different kinds. Thus, a number of additives may be grouped together (Table 6). The use of silicone elastomers in a humid environment requires the addition of antifungal substances such as solvents.l " alkyl benzimidazolylcarbamate v'" or thiurams.P? Table 6

Additive Si(OR)x(OCOR')x Zirconium salt Epoxysilane Methyl ethyl silicate Silicone resin

Support Aluminum Aluminum, steel Glasses, metals Metals, PVC, concrete Metals

Ref.

115 133 125,131 134 129,132

602

34.3.2.2

Synthesis by Step Polymerization Cross/inking via a/koxy groups

The principle of this crosslinking is illustrated by equations (15) and (16). Hydrolysis Condensation

==SiOR ==SiOR

+

+

H 20 ~ ==SiOH

+

+

==SiOH ~ ==SiOSi==

(15)

ROH

(16)

ROH

Alkoxysilane-based cold vulcanizable elastomers have undergone a great deal of development over the past 20 years and currently continue to form the subject of many investigations. The composition may be provided in a single component or a two-component form, with formulations which are very close. The first single component elastomers contained a functionalized oil, a crosslinking agent and a catalyst. The synthesis of functionalized oil, which has been known for a long time, 143,144 has been improved by the use of functionalization catalysts, such as organic derivatives of titanium.l"! alkoxyaluminum chelates 146 and N,N -disubstituted hydroxylamines. 147 The most efficient currently available catalysts are amine based.v'" The crosslinking and/or functionalization agents employed are alkoxysilanes such as: Si(OMe)4' MeSi(OMe)3' MeSi(OCH 2CH 20Me)3' ViSi(OMe)3' PhSi(OMe)3 and PhSi(OCH 2CH 20Me)3' In two-componentelastomeric compositions, it is possible to employ partially hydrolyzed substances called alkyl polysilicates.P" The crosslinking catalysts are chosen from amongst the following: (a) aliphatic or arylaliphatic amines; (b) organic compounds of titanium149-155 such as: titanium alkoxides, e.g. Ti(OEt)4' Ti(OPr n)4' Ti(OBu n)4' Ti(OCH 2CH 20Me)4' and Ti(OSiMe 3)4; and titanium chelates, e.g. (9), (10) and (11). (It is worth noting that other metals catalyze the crosslinking, e.g. Zr, 156 Al 157 and the like.) (c) Organic compounds of tin;158 among the many compounds that have been employed are tin carboxylates,159-160 tin chelates161-163 and tin oxides.P"

[( MehCHO]2 Ti

f° ! "

: : = ~\eCH~

o__

2

Me

(9)

(10)

(II)

The choice of the catalytic system will depend on the application aimed at (setting time). Thus, the catalysts will most frequently be employed in the form of mixtures, and tin is often included in the compositions in order to accelerate the crosslinking. This two-stage process (functionalization followed by crosslinking) has been simplified by the use of a direct mixture 165 of: a,w-dihydroxypolysiloxane oil; an excess of crosslinking agent; a functionalization catalyst (an amine); and a crosslinking catalyst. The main problem encountered in this type of composition relates to stability during storage. The principle of 'scavengers' consists in blocking the non-functionalized silanols, which are responsible for the instability of the mixture, by the crosslinking agent. A very large number of patents aimed at improving this stability by using 'scavengers' of silanol groups, 166-174 the main categories claimed being silazanes, 168,169,172,174 enoxysilanes,166,171 and isocyanates. The amines play a part important in elastomer composition, such as: catalysts for functionalization of oils; crosslinking catalysts; and adhesive agents. The amines employed for the functionalization of oils are generally aliphatic amines (n-butyl .. amine and amylamine), aliphatic polyamines or arylaliphatic amines (benzylamine and phenylethylamine).175-177 One of the improvements consisted of the use of organoaminosilanes.U'tP" which are capable of taking part in the crosslinking,182-185 e.g. NH2(CH2)3Si(OMe)3' NH2(CH2)3Si(OCH 2CH 20Me)3, NH2(CH2)2NH(CH2)3Si(OMe)3, NH2(CH2)30(Me)2CCH=CHSi(OMe)3 and the like. Another role of the amine consists in providing adhesiveness to the supports.186-189 Many other compounds may improve adhesive properties: isocyanate, hydroxyl, mercapto, 190,191 carbamatel"! and epoxy.i'"

603

Polysiloxanes

Among the different additives required to modify the intrinsic properties of elastomers, plasticizers (trimethylsilyl-blocked silicone oils 1 5 8 and organic polymers) 194, 195 and heat stabilizers (rare earth hydroxidesj' " may be mentioned.

34.3.2.3 Cross/inking via a/keny/oxy groups The principle of this crosslinking is illustrated by equations (17) and (18). R2 Hydrolysis

==SiO-C==C/

I

a, Condensation

+

-SiOH

,

+ H 20 ~ =SiOH

(17)

+

R3

/

== SiOy==C, R}

R2 -----. ==SiOSi

==

+

(18)

R3

The crosslinking of a,w-dihydroxysiloxane oils using tri- or tetra-functional alkenyloxysilane derivatives has been described by Shinetsu.l?" The reaction is carried out in the presence of a catalyst such as a metal salt of an acid, an Al alcoholate, a titanium ester and the like. The reagent systems consisting of alkenyloxysilane may comprise a curing accelerator containing the units (12).197

(12)

Instead of reacting alkenyloxysilane in contact with atmospheric moisture with a,w-dihydroxy oils at the time of using the composition, it is possible to functionalize the oils in a previous stage using a tin-based catalyst (equation 19).198 Me

I

Me

Me

I

Me

I I 2 Vi(SiOC=CH 2 h + HOSi4Q-Si)nOH ----. Vi-Si-O-Si--(O-Si)n-O-Si-Vi I ~e ~e ~e '0 M ~ 0/

dI

MeC

II

lee I CMe MeC

II

CH 2 CH 2

II

'0I

CMe

II

+ 2MeCOMe (19)

CH 2 CH 2

34.3.2.4 Cross/inking via amino groups The principle of this crosslinking is illustrated by equations (20) and (21).

(20)

Hydrolysis

Condensation

(21)

The first patents were filed by Wacker. 1 9 9 The aminosilanes claimed as crosslinking agents in single component formulae are of the type R nSi(NR'R")4-n or (13), where R' and R" = H, ethyl or butyl. (R"R'NhRSiLNHSilNHSiR(NR'R"h

~

/\h

R NR'R" (13)

Synthesis by Step Polymerization

604

By using tricyclohexylamine alkylsilane as the crosslinking agent, the adhesiveness of the material is improved and the amine released during the crosslinking has the advantage of being not very toxic. An identical result may be obtained by using dicyclohexylamine dimethylsilane/?'' as the chain extender. Bayer 2 0 1 proposes a mixed crosslinking system consisting of alkoxylated and aminated silanes, for example (14) or (15). (ROhSi-CH-NHR" I R'

(14)

(15)

The presence of aminosilane or of aminosilazane would enable the surface curing of conventional single component CVE composition (where CVE=Cold Vulcanized Elastomer), in the presence of carbon dioxide, to be accelerated.P?

34.3.2.5 Cross/inking via ketiminoxy groups The principle of this crosslinking is illustrated by equations (22) and (23).

(22)

Condensation

----+

==SiOSi==

+

(23)

As crosslinking agent for rt,w-dihydroxy oils, Wacker 2 0 3 uses a mixed system based on aminosilane and ketiminoxysilane (equation 24). (24)

In particular, the use of organotri(ketiminoxy)silane is proposed in the following: (a) a flame resistant CVE composition by Dow Corning.i?" (b) a cold vulcanizable formulation by Toray2 0 5 based on an rt,w-silaxanediol oil, a crosslinking agent which may be a ketiminoxysilane, a silicate or polysilicate, a zeolite and a metal salt; (c) a CVE formulation incorporating a fungicide, by Bayer; 206 (d) a CVE composition by Rhone Poulenc, which employs a mixed ketiminoxysilanejalkoxysilane crosslinking system; 207 (e) a tin-catalyzed elastomeric composition to form oil resistant low modulus seals; 218 and (f) single component systems catalyzed by organotitanium esters (Rhone Poulenc'P" and Toray '!").

34.3.2.6 Cross/inking via amido groups The principle of this reaction is illustrated by equations (25) and (26). Hydrolysis

I

II

+ H 20

~

==SiOH

+

==Si-N-C-R 2

~

==SiOSi==

==Si-N-C-R 2

+ R 1 NHCOR 2

(25)

RIO Condensation

==SiOH

I

I

+ R 1NHCOR2

(26)

RIO

The first patents date from 1964 and were filed by Bayer.i'" The compositions are single component formulations based on oils blocked by silanols or alkoxysilanes, filler and other additives. The amido derivatives may be a mixed alkoxyamidosilane of the type (16).

Polysiloxanes

605

(16)

In these compositions, the amidosilane derivative plays the role of crosslinking agent (x = 3) but more frequently the role of chain extender (x = 2). For example, in a low modulus composition, Dow Corning/?" employs a diorganodiacetamidosilane (17) in the presence of an oil having silanol end groups, and a polyfunctional aminoxysilane (18). CH 2=CH

"Sif N-C- Me' / I II )2 Me Me 0

y\

MeJSiO ( Si-O\ (Si-o \ SiMeJ

M/\

7;

Me Me Me ONEt 2

(17)

(18)

The acetamidosilane is obtained by reacting an alkali metal salt of an N -alkyl acetamide with an organohalosilane. Similar compositions were proposed by Toray210,211 and Toshibaf'? with bis(N-methylacetamido)methylvinylsilane as the chain extenderand a crosslinking agent based on aminoxysilane groups, also proposed by Shinetsu.216,21 7 The presence of acetamidosilane promotes the rapid formation of the surface crosslinking. Another method for the synthesis of amidosilane and amidosiloxanes consists of reacting an amide containing at least one active hydrogen with H nSiR4- n (n = 2, 3 or 4) or R mHpSiO(4-m- p)/2 where m=0-3 and p=0.005-2 in the presence of pt. 215

34.3.2.7

Cross/inking via aminoxy groups

The principle of this reaction is illustrated in equations (27) and (28).

Hydrolysis

(27)

Condensation

(28)

The early synthesis of aminoxysiloxanes and aminoxysilanes were described by Rhone Poulenc.P", StaufferP! and General Electric.V? The products wereobtained by the action of the desired hydroxylamine, most frequently Et 2NOH, on a halosilane, an alkoxysilane or a siliconcontaining derivative carrying the bond ==Si-H. 222 Aminoxysilanes are employed as chain extenders (bifunctional) or as crosslinking agents (functionality ~ 3) in elastomeric compositions vulcanizing by atmospheric moisture. Among the compounds most commonly encountered, cyclosiloxane derivatives may be mentioned, such as (19) and (20) (where R=-(CH 2)4 Me,223 -(CH2)7Me,225 -(CH2)3Me,226 _Me),227-230 and also organoaminoxysilane derivatives, such as R-Si(ONEt 2)3 or (21).230-232 Me S·/ 0....... 1'0 Me Et 2NO, / \ ~ SI SI Et2N~

/

Me

\

/

Me "Si(ONEt 2 h / Vi

"-

0"' . . . . 0 R /S~ R Me (19)

(21) (20)

The condensation reactions between aminoxysilanes or aminoxysiloxanes and silanols may be catalyzed by: dibutyltin dilaurate; 224 butyl titanate; 228 or a boric acid ester. 229

606

Synthesis by Step Polymerization

34.3.3 Conclusion The choice of the elastomeric composition, related to the crosslinking agents, is made according to several important parameters for the desired applications. Among these parameters, one may list: the adhesion to different substrates; the kinetics of crosslinking; and the toxicity and the corrosion generated by the elimination of organic compounds. Table 7 summarizes the advantages and the drawbacks of the most common types of elastomeric compositions. Table 7 Crosslinking system of the composition

Alkoxy Acyloxy

Alkenyloxy Amino Ketiminoxy Amido Aminoxy

Advantages

Odourless Low toxicity Excellent adhesion properties No corrosion of substrates Good storage properties Good adhesion on glass Good thermal stability Short crosslinking time No corrosion Good properties for electronic applications Good adhesion properties Short crosslinking time Low toxicity Good adhesion

Drawbacks

Rather long crosslin king time Strong odour Corrosion of metallic substrates

Unpleasant odour Long crosslinking time Corrosion of metallic substrates (Cu) Long crosslinking time Rather low adhesion properties Rather long crosslinking time

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607

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608

Synthesis by Step Polymerization

102. L. F. Ceyzeriat and P. Dumont, Rhone Poulenc, Ger. Offen. 1121803 (1958): Fr. Pat. 1198749 (1955) (Chern. Abstr., 1961, 55, 7888f). 103. L. B. Bruner, Dow Corning, US Pat. 3035126 (1958) (Chern. Abstr., 1963, 59, 1545f). 104. L. B. Bruner, Dow Corning, US Pat. 3077465 (1959) (Chern. Abstr., 1963, 58, 14275d). 105. 1. R. Russel, Dow Corning, US Pat. 3061575 (1960) (Chern. Abstr., 1963,58, 11559d). 106. Dow Corning, US Pat. 3109013 (1960): Belg. Pat. 611888 (Chern. Abstr., 1963, 58, 9312c). 107. Rhone Poulenc, Fr. Pat. 1392648 (1964) (Chern. Abstr., 1965, 63, 1816d). 108. Rhone Poulenc, Eur. Pat. 102268 (1982) (Chern. Abstr., 1984,100, 211446w). 109. Dow Corning, US Pat. 3642692 (1970): Ger. Pat. 2117026 (Chern. Abstr., 1972,76, 73499y). 110. Bayer, US Pat. 3819674 (1971) (Chern. Abstr., 1975, 83, 30092n). 111. General Electric, US Pat. 3334067 (1966) (Chern. Abstr., 1967, 67, 74337v). 112. Dow Corning, US Pat. 3754967 (1966) (Chern. Abstr., 1974, 80, 4683z). 113. Toray, Fr. 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Abstr., 1986, 104, 6999v). 162. General Electric, US Pat. 4517337 (Chern. Abstr., 1985, 103, 38553r). 163. General Electric, US Pat. 4-554310 (Chern. Abstr., 1986,104, 110834y). 164. General Electric, Fr. Pat. 2240263; US Pat. 3839246 (Chern. Abstr., 1975,82, 32166t). 165. Rhone Poulenc, Eur. Pat. 21 859 (Chern. Abstr., 1981, 94, 158142y). 166. General Electric, US Pat. 4377706 (1981) (Chern. Abstr., 1983, 99, 5814x). 167. General Electric, US Pat. 4424157 (1982) (Chern. Abstr., 1984, 100, 210 124c). 168. General Electric, Eur. Pat. 110251 (1982) (Chem. Abstr., 1984,101, 92599g). 169. General Electric, US Pat. 4417042 (1982) (Chern. Abstr., 1984,100, 8241h). 170. General Electric, Fr. Pat. 2546396 (1983). 171. General Electric, Fr. Pat. 2546525 (1983) (Chern. Abstr., 1985, 102, 96790f). 172. General Electric, Eur. Pat. 139064 (Chern. Abstr., 1985, 102, 114957x). 173. General Electric, Fr. Pat. 2543562 (1983) (Chern. Abstr., 1985, 102, 47185d).

Polysiloxanes 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211. 212. 213. 214. 215. 216. 217. 218. 219. 220. 221. 222. 223. 224. 225. 226. 227. 228. 229. 230. 231. 232.

609

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