FURAN-CONTAINING MODIFIED ORGANOSILICON POLYMERS: PREPARATION, PROPERTIES AND SOME APPLICATIONS

Pergamon

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PII: S0014-3057(%)00138-3

Vol. 33, No. 7, 979-990, 1997 (C 1997ElsevierScienceLtd. All rightsreserved Printed

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REVIEW ARTICLE FURAN-CONTAINING MODIFIED ORGANOSILICON POLYMERS: PREPARATION, PROPERTIES AND SOME APPLICATIONS A. V. VORONKOV,’V. M. KOPYLOV2

IAN W. PARSONS3*

n=

=

n, a

a ~

(PS)have been the subjectof many studies [1–3].Interest in this class of materials derivesmainly from the fact that PS display a unique combination of properties, such as chemical and thermal stability, hydrophobicity and low glass transition temperature, together with very good UV resistance. Oligoethoxysiloxanes,(OES), represented by the followinggeneral formula

r

OEt

1

OEt

—SI-O-Si-OOEt

OEt

- .

are an accessiblevariety of PS. Theseoligomers,with different degrees of polycondensation,may easily be obtained by partial hydrolytic polycondensation of TES: TES + H,O + OES + EtOH *To

(1)

Currently, OES or their commercial analogues (ES-32, ES-40) are widely used, inter alia, in engineering parts, building construction and paper production [4-6]. One of the disadvantages of OES, on the other hand, is that they do not undergocatalytic or thermal polymerisation to form rigid and strong crosslinked polymers because they lack appropriate reactive functional groups. It is the purpose of this report to describe the preparation, properties and uses of OES materials bearing one such reactive grouping. As has long been known, one of the most reactive, albeit heat resistant, classes of organic compounds are furan compounds, i.e. those containing the furyl group:

0= o

(Fur)

A major advantage of using these compoundsis that they are derived from renewable sources, viz. the wastes from major world-scale agricultural crops

(maize,cotton, sugar-cane),so their production does not depend on world oil production or, at least in the first instance, on other fossildeposits. The properties of this radical mean that furan compounds can be easilypolymerized,usuallyyieldingrigid cross-linked polymers [7,8]. It is thus obvious that chemical modification of OES by incorporation of furyl units offers an opportunity for many improvements: ideally, the modified oligomers would combine valuable qualities of both silicones and furan compounds. The synthesis of tetrafurfuryloxysilane(TFS) has been reported earlier [9],using a route based on the transesterification of TES by furfuryl alcohol. The reaction was performed under vacuum (because of the high reactivity of TFS) at 5&100°C in the presenceof 0.3 wt% KOH as a catalyst, givinga yield of 85-89°/0.

However,this explanation cannot be the whole story; the by-product (distillate) consisted of only up to 5–8Y0 bisfurfuryI ether and 85–90V0of furfuryl alcohol. There was no explanation of these facts, apart from a vague suggestionof furfuryloxygroups reacting with the u-proton of the furan ring:

FurCH20’

Si(OEt), + 4FurCH,0H + (FurCH,O),Si + 4EtOH (2)

\

,Si–OH /

/

Partly substituted furfuryloxysilanes [(OEt)~ n(FurCHZO).Si]have also been obtained in the same way [l@-12]. The synthesis of alkyl/aryl/furfuryIoxysilaneshas also been reported, usingboth sodium methylate and furfurylate as catalysts. However, in these cases yields of the products were only about 50V0[13–15].It should be noted that there have been no earlier reports concerningtransesteriticationof the oligometric ethoxysiloxanes. R. Si(OEt),. n+ 4 – nFurCH,OH + RnSi(FurCHzO)~_.+ 4-nEtOH

+

(3)

The next step in the investigation of the furfurylsilanesinvolvedattempts to obtain oligomers based on these units. Most authors have concluded (in contrast with the ethoxysilanes) that it is impossibleto obtain liquid oligomersfrom furfurysilanes by methods involvinghydrolyticpolycondensation in a controlled manner [10, 16-18],and that the only way is thermal oligomerisation of furfurylsilanes. Thus, in an earlier study, the monomer was heated to temperatures of 24&300°Cfor 2–3hr in the presence of 0.1–1% ZnCl,, with evaporation of 12–25Y0of the low molecularweight by-products.In this way a black liquid material was obtained, which undergoes both thermal and catalytic curing. A mechanism was proposed for the oligomerisation [18,19];the main idea was that thermal polymerisation of TFS resulted in the generation of bisfurfuryl ether and polysiloxane:

In the present authors’ opinion, the processeswhich could proceed during polymerisation of TFS are complicatedenough to preclude any preciseestimate of the exact structure of the final oligomer, albeit it is not just a mechanical blend of furan and silicon oligomers. It is obvious that using monomeric silicon compounds for the modification results in the formation of polymers with an almost uncontrolled structure. Therefore, the reproduceability of the properties of such polymersis too poor to encourage either their study or their use. On the other hand, the applicationsof oligomericsiliconcompounds,suchas OES, as oligomericmatricesof definitestructures and compositions offers a great opportunity to modify and improve the final materials’ properties by incorporation of furan compounds in controlled ways. Despite this, no systematic study has been reported concerning the dependence of the compositions of OES on the conditions employedin partial hydrolytic polycondensation of TES. Thus, in the present reviewwe report the results of a number of studies carried out along the followinglines: ●

a

\

Fur %

/ O

‘o

o t’k

/ \

~

●

the

OES by furan compounds, and studies of the properties of the modified oligomers and polymers thus obtained; ● investigations of opportunities to apply the modifiedoligomersas a binder for compositesand to study their properties.

TES

and condenser bearing a graduated collector. The flaskchargedwith the reaction mixturewas immersed in an oil thermostat, and heated up to the reaction temperature. The appearance of the first drop of ethanol was taken as indicating the start of the reaction. After cooling, the reaction mixture was filtered(for the evaluationof heterogeneouscatalysts) and residual volatiles were removed under vacuum (50°C/0.27kPa). The reaction was followed via the amount of ethanol collected, and the degree of substitution of ethoxy groups by furfuryloxygroups confirmed by ‘H-NMR spectroscopy[24].

AND DISCUSS1ON

of It was found that the course of the reaction and n composition of the OES mainly depends on n. Up to n = 0.4, the reaction mixture contains only L2 and L3 in detectable amounts. L4 appears at n = 0.4, C4 at n = 0.5, C5 at n = 0.6, L5 at n = 0.8, and C3 at n = 1.0 (see Fig. 1, for n = 1.0, where all the monitored ethoxysiloxanes are present). Both the time of the initial detection (and thus presumably of the formation) of ethoxysiloxanes, and of their concentration achieving a stationary state increases with their molecular weight. The rate of n= the consumption of TES and the ultimate degree of conversionincrease with increasingn. At n >0.8, m= TES is consumed completely. Dependent on n, in p= the course of the reaction the concentration of the ethoxysiloxaneseither increase and stabilize, or a exhibit an initial growth to a maximum, with a subsequent decrease to the stable concentration. Thus, the concentration of L2 passes through a maximum of n = 0.6, that of L3 at n = 0.7, of L4 at a n = 0.8 and L5 at n = 1.4 (see Fig. 1). 5 5N The concentrations of the cyclosiloxanes,as with the linear compounds, also exhibit two types of time dependency. The appearance of a maximum in the time dependence of the concentration of cyclosiloxa anes indicates that these products, formed in the course of the reaction, also participate in polycondensation, and that the polymerchain contains not only 2 linear segments, as stated in a number of papers [5,25],but somecyclosiloxanefragmentsas well.The consumption of the cyclic products is not due to polymerisation, as the cyclosiloxanes in ethanol solution of 0.05N HC1 do not undergo polymerisation over long time periods. The finalcompositionof the products of hydrolytic polycondensations shows that for each individual siloxane there exists an optimum n at which it is (EtO),.,.SiO~+(4– 2rz)FurCH,0H formed with the maximum yield (Fig. 2). The linear and cyclic ethoxysiloxanescontaining from 2 to 5 + (FurCHzO)d-z.SiOn+ (4 – 2n)EtOH (5) silicon atoms, the yields of which can be determined by GLC, appear as the main components of the reaction mixtureup ton = 0.6, wherenearly 100°/0of Initial studies concerned the effects of the tempera- the TES is consumed in their formation (Fig. 3). On ture, and of catalyst type and concentration on the further increase of n, the concentrations of these decreasein parallel with the formation of compounds rate and completenessof the reaction. The required initial quantities of OES, furfuryl of higher molecular weight, and at n = 1.5 their As at the initial stages of alcohol and catalyst were introduced into a amounts decreaseto 1OO/O. three-necked flask fitted with stirrer, thermometer hydrolysis the conversion of TES considerably

1

4.00

,-1.50

Log( tinm/h)

Log( tinlch)

n=

exceeds the

With increasingn, irrespectiveof the increaseof the rate of TES consumptionand degreeof its hydrolysis, TES and a delay in the arrival of the reaction system at the ethoxysiloxanes.The experimental data lead to the stationary state is observed.Under our conditions,on conclusion that at n <0.5, (EtO)sSiOH is formed increasingn from 0.2 to 1.5,the time of attaining the predominantly, leading on to the production of stationary concentrations of OES is increased by a linear siloxanes. At n >0.5, considerable amounts factor of 28 (from 96 to 2700hr). of (EtO)JSi(OH) are formed; the latter reacts with The effect of the concentration of the catalyst on TES or siloxanes, forming ethoxysiloxanols at the the course of the reaction was studied for n = 0.5 in beginning and later ethoxycyclosiloxanesby inter- the presenceof various concentrations of HC1(from molecular condensation, or products of higher 2.5 x 10-3to 5 x 10-2mol/L). It was found that the molecular weight by co-condensation with silox- final composition of the reaction mixture is anes and TES. The decrease of the yield of practicallyindependentof the content of the catalyst, ethoxycyclosiloxanes at n > 1.2 is caused by an but with increasingconcentration of the catalyst the increase of the yield of (EtO)Si(OH)j, which is time of the transition of the reaction system to the expected to lead to the formation of branched and stationary state is sharply decreased(from 1300hr at 2.5 x 10-3mol/L to 96 hr at 5 x 10-2mol/L). polcyclic products. m

A

1

2

0.50

Loo n (Seetext)

n n.

1.50

100.0

monitoring the concentration of TES in products with n >0.8, the appearance of L2, L4 and L5 in all products and finally by redistribution of the concentrations of all linear ethoxysiloxanes.(In the course of the disproportionations, the cyclosiloxanes were consumed completely.) In all cases the final compositions found by experiment agree with the compositions calculated by eqn (4). It means that OES, independently of the n value, undergo structural rearrangement in the presence of nucleophilic reagents and that their final compositionsare statistical. Finally OES were characterized by density, refractivity,specificviscosity,elemental composition and molecular weight (Table 1). Some information was also extracted from IR spectra of the products (Fig. 4). Based on these experimental data, we have calculated the overall composition of OES as a function of the n value:

~ 1.0

1.5

n (Seetext) Fig. 3. Consumption of TES to give ethoxysiloxanes with 2–5 silicon atoms as a function of n.

It should be noted that dependence on n of the content of the linear ethoxysiloxanes in the final products, (where x is the number of siliconatoms in the linear siloxanemolecule,~= 4, p = and the initial concentration of TES), deviates considerably from the statistical value, calculated by means of the Flory equation [26](Fig. 2):

[(EtO),-,. SiO.]~ where n = 0.2–1.5 and m = 2–75. Thus, the composition and structure of the products of hydrolyticpolycondensationof TES have been established as a function of the degree of conversion of the functional ethoxy groups, and the requirements for obtaining, in a controlled manner, OES with different degrees of condensation as starting materials for modification have been laid down.

(f x —X)!f ‘“y= (x –

+ -2.r+2).qno

(4)

The observed deviations can be explained on the bases of the formation of ethoxycyclosiloxanesand differences in the reactivity of TES and of the siloxanes formed during the reaction. On the other hand, it has been established earlier that individual ethoxysiloxanescan undergo disproportionation in the presenceof nucleophiliccatalysts (KF, Me,NOH, Me,SiONa), even at room temperature [21].The general form of the disproportionation of ethoxysiloxanescan be represented as:

There were no reports concerningthe use of alkali metal fluorides as transesterification catalysts. It was therefore of interest to study the activity of these compounds in this regard, for comparison with KOH which is widely employed for this purpose. In addition to fluoridesof alkali metals, two very strongly nucleophilicreagents—tetramethylammonium fluoride and hydroxide (Me,NF, Me,NOH)-and sodium trimethyl silanolate (Me,SiONa) were used in amounts between0.15 and 2.5 wt% [24,28].

q.Et[OSi(OEt)&OEt + +“

“‘

+

I+ o

+qt.+ ,)Et[OSi(OEt)z]n+ I + c.0 + qtEt[OSi(OEt)J (5) are the molar concentrations of the siloxanes. The composition of the products of disproportionation agree, within the limits of experimental error, with statistical expectation, calculated using eqn (4). Thus, it is obviouslypredictable that the products of hydrolytic polycondensation of TES will also undergo disproportionation, and this has been experimentallyverified.This study was very important from the point of view of following the transesteritications of OES by furan compounds, which proceed in the presence of nucleophilic catalysts. Indeed, the disproportionation of OES has been observed [27]. It has been followed by Here ql . . .

Vx10.2,cm-l

TES

n=

a

He e

-

H3C

\ ~-CH2 tib

C

I/

Ht)

‘\\ Hd

c—o

a

H2

o

a

2 ,—,—

t

~1

7

8

5

6

is noteworthy that BaFz is practically not a catalyst for this reaction and MeqNOH decomposes upon heating. In the presence of KOH at any of the temperatures investigated, there was incomplete substitution of ethoxy groups. Up to 150”Cin the presence of fluorides in any concentration studied there was also incomplete substitution. However, when the reaction was performed at 160°C,complete substitution was observedin the presence >0.5 wtO/O of fluoride catalysts (Fig. 5). Me,SiONa showed some differences in activity: even at a low concentration (0.05wtO/O)85.2°/0 substitution was observed, and 0.15wtO/O,gave complete substitution (but over long reaction times). At a

4

— I

I

1

3

2

1

0

and the optimal concentration is 1.5wt% (Fig. 6). The rate of the transesterificationreaction depends strongly on the degree of conversion of the initial OES. It increases for the materials in which conversion increases from P = O [((EtOd)Si] to P = 50% [(EtO)2SiO),]and then decreases (Fig. 7). This phenomenon can be explained by the more complicatedstructure of the initial OES oligomersat P > 50Y0and the consequent steric difficulties. TFS and OFS materials with overall formulae corresponding to an average composition [(FurCH,0)4- ,. SiO.]., where n = 0.5-1.5 and 2-75, have been obtained with a yield of 94-97% [29]. These materials were viscous transparent liquids, dark brown in colour, and were soluble in acetone, toluene and tetrachloromethane. The materials were characterizedby density, specificviscosity,elemental composition and molecular weight (Table 2). IH-NMR spectraof OFS wereanalogousto Fig. 5(2). IR-spectroscopyalso confirmedthe proposedcomposition of the OFS (Fig. 8).

Thermal polymerisation of OFS at temperatures >200”C has been reported to

< KF < CSF < Me,NF

Table 1. Some characteristicsof OES materials Elementalanalysis(%)

n 0.2 0.5 1.0 1.5

&“

n;”

Specific viscosity”

0.96 1.04 1.18 1.25

1.38 1.39 1.40 1.42

0.0062 0.0123 0.0198 0.0367

c Found 44.72 42.1I 34.90 25.23

Calc.’

H Found

45.92 42.27 34.21 24.75

9.32 8.77 7.14 5,32

.l O/O solution in toluene. bCalculatedfor OES with overall formula [(EtO)+*,,)Si O,,].,.

Calc.

Si Found

Calc.

M. Found

Calc.

9.12 8.52 6.95 5.15

14.49 16.37 20.78 26.61

14.35 16,25 20.84 28.62

260 340 675 7300

193 170 134 97

Polycondensation degree m 1.3 2.0 5.0 75

2.50

1.67

A 0.87

—

0.02 25.0

50.0

75.0 %

Fig. 6. Illustrating the different

1

2

3

4

5

yield black cross-linked polymers [30];in this study the reaction was monitored by measurements of the gel content. The time to complete cure (hardening) decreaseswith increasingdegreeof polycondensation in the initial oligomers.At 200”Cthis time was 45 hr for TFS and 15hr for OFS with n = 1.5. At 260°C, the corresponding times were 4 hr and 0.3 hr, respectively. The formation of volatiles during the polymerisationof OFS has been observed;analysisof these volatiles has shown that they mainly consist of furfuryl alcohol, which corresponds to earlier published data for polymerisation of TFS [19].We also observedthe formation of water in the course of the polymerisation.However,it should be noted that formation of the volatiles was observed only for oligomers with n <0.8. For n > 1.0 there was no \

‘/

~

/

\

(6)

\

—

—Si-O~Si—

{}/ /

+

/

FurCH2-O-Si _

I

+

\

/

\

furan compounds. The main contributing cause is that the final polymers are intractable, being non-melting and insoluble products, and this has limited the opportunities for structural studies. Characterisation of the final structure of the polymersobtained was not the main aim of our work, but experimentalresults and literature data allow us to make some assumptions concerning possible mechanisms of thermal polymerisation. Polymerisation mainly proceeds using the double bonds of the furan rings [31], but the formation of volatiles indicates active polycondensation processes, which presumably proceed side by side with addition polymerisation. Thus, the formation of water and furfuryl alcohol can be explained by the following reactions:

+-FurCH,OH

~

\

detectable formation of volatiles at any temperature (Fig. 9). In point of fact, there is no general unanimity concerning the mechanism of polymerisation of

/

\

of The thermal polymerisationof OFS has some disadvantages.One of them is the difficultyof controlling the reaction, and another significant difficulty lies in the very

Table 2. Some characteristicsof TFS and some OFS materials

&

[(EtO)4-!,,siO,l,

1.226 1.282 1.303 1.425

n= m = O n = 0.5, m = 2 n= I m= 5 n = 1.5,m = 75 “1O/.solution in

Specific viscosity Found 0.022 0.033 0.045 0.065

c

56.84 54.78 51,68 41.62

Elementalanalysis(%) H Calc. Found Calc.

Found

Calc.

57.68 55.06 50.41 40.27

400 680 1200 11300

416 654 1190 11175

4,84 4,59 4.20 3.36

toluene,

stressed structure of PFS formed in this way. The latter leads to low crackingresistanceand thus to low strength. An investigation of the literature survey indicated another possibility, using electrophilic catalysis using either Bransted or Lewis acids [10]. Pilot experimentsshowedthat in the presenceof these catalysts, solidcross-linkedpolymerscan be obtained from OFS even at room temperature. Phenyl sulphonic acid (BSA), p-toluene sulphonic acid (P-TSA) and tin(IV) chloride were chosen as initial catalysts in the polymerisation of OFS [30].It was found that the amount of catalyst required for the complete hardening of the polymers increased with increasing degree of condensation in the initial oligomers. Thus, at 20”C, a polymer based on TFS needs 0.5 wtO/O,sulphonic acid, oligomers with n = 0.5 need 1wtO/O and oligomerswith n >0.8 need

.

4.80 4,26 4.21 3.28

M.

2 wt%. At 50°C, TFS and oligomers with n <0.8 need O. S”/o, and those with n >0.8 need 10/..In the presence of SnCl, at all temperatures studied these values are: 10/0for TFS and 2.5°/0for all oligomers. The effectivenessof the catalysts is as follows: p-TSA=BSA > SnCl, The time of complete setting of polymersincreases with increasingpolycondensationdegreeof the initial oligomers. This latter phenomenon is explained by the decreasing content of furfurloxy groups in the oligomers and extension of the silicone chain with increasingn (it was confirmedthat the initial siloxane chains do not undergo any changes in the range of temperatures investigated). The kinetics of the catalytic polymerisations of TES and OFS have been studied by DTA, using

‘@ 1

‘Y

2

,

4 ‘C=C-H

-

‘\ (““2v‘=C-v

Ring

v

‘*/

o

-.,

vxlf,cm-’ Fig.

n=

n=

1

0

I

0

I

0.5 n

1.0

1.5

I

I

0.00 o

I

300 C

n n=

enthalpies of the reaction as the yardstick [32].This method allows monitoring of the reaction progress under isothermal conditions and can locate even minor thermal effectswith a precision of up to 0.05 degrees. Reactions were performed at 50”C in the presence of 1, 2 and 3 wt% BSA. It was found the polymerisation reaction was first order in catalyst, and in concentration of furfuryloxy groups. Also, the rate and thermal effect of the reaction decrease with increasing n in the initial oligomer (Fig. 10). This phenomenon can be explainedstraightforwardlyon the basisof increasing complication in the structure of the initial oligomers and the consequent steric inhibition. It is interesting that the activities of the various oligomers vary in opposite ways in the courses of thermal and catalytic polymerisations. This can be

6’

I

I

o n value n

OFS

I

n = 1.0

n=

a

The thermal behaviourof PFS was studied using therrnogravimetric analysis (TGA). The TGA runs were carried out in air, heating samples from 20 to 1000”Cat 50/rein [24]. The main conclusion is that PFS shows high thermal resistance and further, that degradation of PFS noticably reduces with increasing n. Thus, the temperature of 10’-%. weightlossincreasesfrom 390°C (n= 0.5) to 500”C(n = 1.5)(Fig. 11);the amount of residue increases (28 and 60°/0,respectively). It is interesting to note that the thermal stability of PFS is significantlyhigher than that of the polymer based on monomeric TFS (Fig. 11). The most reasonable explanation of this phenomenon is the presence in PFS of a three-dimensionalnetwork, including two kinds of polymer chains generated during polymerisation: hydrocarbonchains, formedby polymerisation of furfuryIoxygroups;and siloxanechains, whichare present in the initial oligomers.The siloxane chains, on this thesis, act to reduce degradation. However,it is not possible to say that the observed increase of residue results entirely from an increasingportion of silicone fragments in the polymer. According to the experimental data, increasing n in the initial oligomers’formation affects the amount of residue after thermal processing more than is explained by the content of siloxane sections (Fig. 12).Therefore, the observedeffectis due to stabilization of the furan polymer by silicone fragments in the three-dimensional net. Furan and furansilicone compounds have found use in the manufacture of glues, varnishes, coating compositions and glass-reinforced plastics [33]. Nevertheless,the verystressedstructure formedin the course of the polymerisation of furan compounds demands high filling to generate the best properties

40.0

r .

20.0

0.0

1

I

1.0

0.5

I

I.5

n value

n W 4

from such polymers. Polymerconcretes are one potentially important variety of such highly-filled 100VO composites, and are now well developed[34].At the moment, one of the most widely used binders for polymerconcretes is the so-called furfuryl alcoholacetone monomer (FAM) [35]. FAM–polymerconcretes are used for the manufacture of items intended for use in aggressive environments (anticorrosive pavements, acid-proof lining, pickling baths). At the same time, the FAM composites suffer from the major disadvantages of insufficient strength and thermal resistance. The structure and properties of the PFS allow the prediction that OFS as binders should significantlyimprovethe strength and thermal stability of polymerconcretes. For the determination of the correlation between material properties and length and structure of siloxane fragments, TFS (l), and the following oligomers—[(FurCHzO)3 SiOo.5]2(2), [(FurChzO)z SiO]~(3) and [(FurChzO)SiO,.,],5(4>were chosen as binders [36]. The traditional composition for the FAM–polymerconcretes was used [35]. This comprises combined fillers, 90?’. (road-meta1–55.8 V0, quartz sand–20.5Y0,andesite powder–13.7V0)and binder, 10%. BSA was used as catalyst for cold hardening the binder, at levelsof 2, 4, 5 and 6% (of 3 and 4, weight of binder) for binders 1, respectively.(These levels were selected as yielding

a

6 5

3

Table 3. Some propertiesof the polymerconcretesreportedin this work

Binder I 2 3 4 FAM

Strength(MPa) Tensile Compressive 69.8 76.9 89.6 121,6 71,4

5.7 6.5 8.3 12.1 6.1

“After exposureto a 10wtO/osulphuricacid solution, ~angent of dielectricloss angle.

Flexural

Abrasion resistance (glcm’)

Acid resistance index, K.,,

Flammability index, K~

tan&

9.7 14.6 16.1 20.2 10.4

0.18 0.09 0.05 0.04 0.26

0.97 0.98 1.0 1.02 0.73

0.20 0.14 0.12 0,10 0.28

0.034 0.021 0.028 0.015 0.06

Modified organosilicon polymers

989

using 3–5 times less of the hardening catalyst. Large amounts of BSA in FAM-based materials result in by their sensitiveness to moisture, because BSA is washed out by the water. In addition, use of OFS in comparison with FAM makes the process of manufacturing the materials more convenient.Thus, the hardening catalyst is introduced by absorption from acetone solution on to the sand surface. The moulds expand more easily because of the materials’ lack of adhesion to metals and glass (siloxaneunits). REFERENCES Further, the environmental conditions of manufacturing improve because OFS’ oligomers do not volatilise under the initial conditions or during the The Chemistry and Technology of Silicones. hardening process. Concerning the dependenceof the materials’ final Siloksanovaya properties on the structure of the initial OFS, it was suyaz (Siloxane Bond). Nauka, Novosibirsk, 1976. found that almost all properties studied improved 4. Takatany, T., J. Chem. Soc. Japan, 1953, 74, 947–950. with increasing degree of polycondensation in the and Kuznetzov, A. I., J. prikl chimii, 5. initial oligomer,whereasproperties of the TFS-based 1983, 56(l), 233-235. material were substantially worse (Table 3, Fig. 13). 6. Krannish, R., Silikattechnik, 1961, 2, 78–80. 7. The Fwans. The improvement in the stress–strain properties of the PFS materials with increasing polycondensation Furanovie smoli (Furan Resins). can be explained by the increasing flexibilityof the polymer chains, arising from the reduction in the quantity of cross-linksand knots in the three-dimensional polymer network. Subsequently, improvements have been made in J. Am. the compositionsof the PFS-basedpolymerconcretes. Thus, by introducing special fillers (silicon carbide, Furansilani (Furansilanes). ceramic, ashes, diabase powder) and by using FeCl~ and SnClz as hardening catalysts, mechanical Izu. A.N. Latu. SSR, properties of the polymerconcretes have been 1961, 59. improved significantly (strength:compression–upto Docl. A.N. SSSR, 150MPa, tensile28 MPa; abrasion-O.01g/cm2)[38]. 15. 1962, 145, 806. OFS have also been used as binders and additives 16. for pastes, protective and decorative coatings and artificial marbles. The introduction of OFS into the formulations of these result in improvementsin their strength, thermal stability and corrosion resistance Polymer, 1973, 14, 420. [39]. Sci. It should be noted, finally,that two picklingbaths USSR, from PFS–polymerconcrete (2 x 1.5 x 1.5m) have Docl. Konfer, been manufactured for industrial use; they have been Poiym. Sci. in service since 1988without any problems. USSR, 1985, 27, 395. 24. CONCLUSIONS Chemical modification of organosilicon polymers by furan compounds yields a novel class of materials, which can be readily polymerised in the presence of electrophilic catalysts to form cross-linked polymers with a three-dimensional structure. These polyfurfuryloxysiloxanes exhibit a combination of the more valuable properties of both the initial classes of compounds. Highly-filled composites prepared from these materials by cast cold moulding display high thermal stability, together with excellent mechanical properties and unique corrosion resistance, especially in acid media at high temperatures. Therefore, wide application of these materials in corrosive environments will certainly bring benefits. Application of the transesterification reaction to oligomeric ethoxysiloxanes with regular composition and structure as a method of direct chemical modification opens practically unlimited possibilities

Izv. A.N. SSR, Ser. Neorg. Mater., 1978, 14, 935. 26. Physical Chemistry of High Polymeric Substances. Visokhomol Soed, 1984,

789.

28. procesi ipolimeri. Nalchik, 1985; A. IY Vses Konf,

Polkondensacionnie Docl

N Noviypolimer (Novel Polymer). Ruk Depon v NIITEChIM, 1985, N 898. 31. Kamenskii, L, PhD thesis, Moscow Mendeleev Institute, 1978; H. Peres, PhD thesis, Moscow Mendeleev Institute, 1978. 32. Sazonov, V. and Kandirin, L., Chimiya i Tehnologiya

Organicheskich Proiz, M, 1977, 7. 33.

et al.

990 34. 35.

Stroiizdat, 1977.

Stroitelnie I Specmateriali Na Osnove Organominerabrich Kompozicii—Novocherkassk, 1988.

Stroiizdat, 1981. 36.

Modljicirovanniy polinrerbe~on(Madijied Polymerconcrete). Docl. konferencii,

I

Inform listok Minsk, “Polymia,” 1988;