Fibrous reinforcement of glass-ionomer cements

Clinical Materials 7 (1991) 313-323

Fibrous Reinforcement C.W. B. Oldfield School

of Materials,

of Glass-Ionomer

Cements

& Bryan Ellis

University

of Sheffield,

Abstract:

Glass-ionomer

Northumberland

Road,

Sheffield SlO 2TZ, UK

cements (GICs) are used very successfully as dental

restorative materials. However, their mechanical properties can be improved by the incorporation of fibrous reinforcement, allowing other, more stringent applications of these materials to be possible. Such a potential use for materials with higher elastic moduli and strength is as a bone cement for hip prothesis. In this paper methods of incorporating carbon and Safil fibres into GICs are discussed. The optimum compositions for these reinforced cements has been determined. Data is presented which shows that it is at least possible to double the Young’s modulus and strength of GICs by the incorporation of a modest volume fraction of reinforcing fibres. Extensive fibre pull-out occurs with carbon fibres, with the fibres bridging the crack. However, with Safil fibres brittle fracture occurs and there is little l&l-out.

1

INTRODUCTION

aration of carbon fibre reinforced GICs. The incorporation of substantial volume fractions of fibrous reinforcement required a concm-rent reduction in the volume fraction of glass. It was possible to incorporate approximately a volume fraction of Qf = 0.2, that is 20 volume per cent of an HMS (high modulus with surface .treatment) carbon fibre. Details were given of the dependence of Young’s modulus, maximum fracture stress and fracture toughness, Krc, on the volume fraction of carbon fibre in GIC prepared from MP4-glass. Essentially, these properties h(ad maxima in the region of 10-15 volume per cent of carbon fibre, with an increase on the elastic modulus of 400% and strength of 1000% relative to the non-fibrous reinforced GICs. From fractographic analysis it was established that fibre pull-out occurred with fibres bridging the crack. The purpose of the present paper is to extend the authors’ previous report to include data on the reinforcement of GICs based on G338 glass (see later), and a comparison with the GICs-MP4 glass. Data on reinforcement of GICs with Safil fibres is also included.

of glass-ionomer cements (GICs) The properties have been. studied by several research groups and their development has been discussed in detail by Alan Wils#on who invented these unique materials, and John McLean who implemented their applications in clinical dentistry.’ Even though they have been investigated intensively, their detailed structure is still the subject of current research.2 The GICs have mechanical properties which are more than satisfactory for applications such as dental restorations with specific roles. However, they have relatively low Young’s moduli and fracture strength. Their low moduli and strengths restrict th.e use of GICs, and an improvement in their properties would extend their possible application to, for example, the cementation of hip prothesis or other joint replacement.334 The mechanical properties can be increased by the incorporation of fibrous reinforcement. There is an extensive literature5,$ on the properties of fibrous composites, and the effects of various fibrous reinforcements, on the flexural strength of GICs have been reported briefly with some details of the increased strengths that can be attained.’ Little detail was given about the methods of fabrication of GICs witlh fibrous reinforcement. Recently the authors have reported’ on the prep-

2 EXPERIMENTAL Ground glass, carbon, Safil and alumina (ICI Ltd, Cheshire, UK) and poly(acrylic 313

Clinical Mmferials 0267-6605/91/$03.50

0 Elsevier

Science Publishers

Ltd, England

fibres acid)

Table 1. Composition

SiO, A&O, CaO CaF, AlF, AIPO, Na,O Na,AIF,

of the ion-leachable

glasses

Iwp4

6338

(w/o)

(w/o)

28.0 350 260

25.2 14.2 12.8 4.5 24 B

11.0 19.2

(PAA), were provided by the Laboratory of the Government Chemist (Middlesex, UK). (C )-Tartaric acid was purchased from BDH Chemicals Ltd, Poole, UK; it had a minimum assay of 99.5 %. The ground glass had been sieved and had a nominal particle diameter of about 30 ,um. It was stored in airtight glass jars in a dry cool condition. The compositions of the glasses are given in Table 1. The MP4 glass was manufactured by Pilkington Bros (Lancashire UK) and G338 was prepared by LGC. There was phase separation in G338 glass with at least two phases present, A is rich in silicon and B is much richer in calcium. The chopped HMS carbon fibre was manufactured by Courtaulds Grafil (Coventry, UK) and had an epoxy compatible surface treatment with 1 weight per cent size. The fibre diameter was c. 7 pm uncoated and 8 ,um coated, with chopped lengths of 0.25 mm and 1-Omm. The fibre has a Young’s modulus of 380-400 GPa and a failure strain of 0.8 %. The Safil had a diameter of 3 pm and was O-5mm long, with a Young’s modulus of 3000 GPa and a strength of 2000 MPa. The GICs were prepared by dry blending the glass powder, chopped fibre and freeze dried PAA in a mechanical mixer until an even distribution was achieved. Liquid was added to this mixture in a cooled petri dish and stirred with a cooled metal spatula until ‘thickening’ was noted. Initial setting was then imminent. Cylindrical test pieces were fabricated with a nominal diameter of 4.5 mm by loading the cement into modified I cm3 syringes. The cement was stored in the syringe prior to testing to prevent damage. To avoid damage to the test pieces they were not removed from the syringe, which was placed in containers at 100 % RH until required for the measurement of stress-strain and fracture properties. The mechanical properties were measured in a four point bending jig fitted to a Mayes tensile testing machine.

For ~~act~gra~bic study the fracture surface was

scope.

Compositio

ation of g~ass-io~~~e~ cements requires at least three able glass, a water-sol side groups sue water, it is necessary t rapidly because the cements will start to ‘set’ as soon as the ions are leache glass The nature of the setti occurs due to side groups of the ~~l~~~r chain. ate of setting, it is necessary to incl

100%

Fig. area

Triangular initial ~~rnposit~~~ agrare!. ~~~~t~~~~~ the cement forming regiorn with P4 glass; m-->limits of cement forming region with G338 glass.

Class deficient

region:

Liquid deficient region:

Polymer

deficient

region;

(/,/!,d(,, ,,m: ‘:I’ il:;lill;il~l,l,~1,~;i~: uIILIILILIIIIIIIyyI. ‘.. -: ‘ \ 1. .: -Y-:.1,

__ _~ I= _zzYG>

Fibrous reinforcement of glass-ionomer cements diagram and has been reported previously for cements formed from MP4 glass.‘,’ In Fig. 1 the region of cement formation is shown for cements formed from G338 glass together with that for MP4 glass. It is important to appreciate that the boundaries in Fig. 1 will depend on the quantities of cement being mixed. The area of cement formation will be larger when small amounts are mixed, such as for a de:ntal restoration. The mixing of the larger quantities required for the fabrication of test specimens can present considerable problems due to the rapidity of setting of some cements. It is obvious that a deficiency of any one of the main components will either prevent a cement being formed or yield a cement which has low strength and elastic modulus. These factors have been discussed in previous reports,‘,’ but it is appropriate to give slome further details and consider the differences between cements prepared from MP4 and G338 glasses. GIC-MP4 glass sets more slowly than GICG338 glass which is ‘stiffer’ in the fluid stage. In the region aldjacent to the glass polymer (GP) boundary, Fig. 1, cement formation cannot occur because of a deficiency of water, which is required to dissolve the PAA and provide a medium for acid leaching and/or ion exchange of the glass. The role of water is discussed by Wilson and McLeanlO who point out that it is important for the setting reactions and final structure of a GIC. Also, it was found that. with insufficient water it was not possible to obtain a uniform mix, that is, small regions were completely starved of water, and hence were incapable of bearing a significant tensile stress. Eventually, a critical limit is reached when the ’starved ’ regions form an interconnected structure which provides a facile fracture path. These ‘cements’ ar’e useless. Finally, with even less water present only a ‘crumbly ’ mix can be obtained and then it is not possible to mould the material. From Fig. 1 it can be seen that more water is required to form homogeneous cements from G338 glass than for MP4 glass. This is due to either the higher rate of ion exchange with G338 than MP4, and/or the higher rate of glelation of the PAA with the ions leached from G338. It is surprising that a glass powder with a composition such as MP4 will form adhesive attachments between the particle when water is added. This effect is even more pronounced when tartaric acid is present as well. Thus, along the GL boundary in Fig. 1 powder agglomerates are formed. They fail :in tension due to a deficiency of bonding

315

matrix between the particles. It has been reported that tartaric acid, the optically active isomer, is a cement former ‘in its own right ‘, but the mechanism of its action has not yet been completely elucidated.ll With an excess of a 10 weight per cent of tartaric acid (see the bottom left-hand corner of Fig. 1) not all of the liquid is relained by the glass particles. However on drying out, a glassy film is formed which surrounds the weakly bonded particles. Within this glassy film are a few crystalline inclusions which from their morphology have been tentatively identified as calcium tartrate. Further work with these compositions would be warranted, although the materials formed at the left-hand bottom corner of Fig. 1 do not have a practical application at present. This is because there may be a competition for the formation of complexes between aluminium and calcium ioniic species with the tartaric acid, and the preferential formation of calcium tartrate contrasts with the observation of Prosser et aZ.l”of the formation of strong aluminium tartaric acid complexes. From Fig. 1 it can be seen that the ‘left-hand’ boundaries for the formation of a GIC from either MP4 or G338 glasses are approximately identical, except that it is not possible to mix cements with large volume fractions of G338 glass’ (i.e. > 0*64), whereas with MP4 it is possible to mix compositions with up to Q, = 0.75. This is due to the increased reactivity of the G338 glass decreasing the setting time, thereby depriving the operative of the time necessary to obtain a uniform mix. This means ‘dry inclusions’ are more likely to exist, with cement failure the result Films of partially neutralized PA.A have been studied as water based coatings and the influence of metal ions investigated.13s14 Also, the effect of silica, aluminium oxide and a calcium silicate as fillers on the glass transition behaviour of a PA.A matrix have been measured.15 It was reported that neither silica nor alumina affect the r, of PAA bult with calcium silicate there was a significant increase in T, with increase in volume fraction of filler, up to Qf = 0.2. This was attributed to reaction of calcium ions from the calcium silicate with the highly polar carboxylic groups of the PAA. The absence of a T, increase with either silica or alumina is claimed to be expected on the basis of a weak interaction between PAA and these relatively acidic hers. The relative extents of interaction between calcium and aluminium ions require fuller study t0 fully confirm such an explanation. However, this study illustrates the complexities of the interactions between cationic

316

0.05 HMS 100%

G

carbon

fibre B mm long. /---~--I the fibre pull-out.

= 23 pm. Note

100% P+L

Fig. 2. Triangular initial composition diagram for the formation of carbon fibre GIC with MP4 glass. Unhatched area is the cement forming region. Glass deficient

Liquid

deficient

region :

region :

&;z=

species and the carboxylic acid side groups of PAA. For the preparation of GICs it is readily appreciated that the region adjacent to the EP line is deficient in cations and hence the cements when formed are weak; the boundaries for MP4 and G338 glasses are very similar. In summary it can be seen from Fig. 1 that there is a central compositional region which allows formation of GIC and that this region is more restricted for G338 than for MP4 glasses. The initial composition of GICs containing carbon fibres can also be displayed in the form of a triangular diagram, Fig. 2. It should be noted that the variables are now the volume fractions of glass powder, Q,, fibre, Q, and “matrix former’, that is the polymer, tartaric acid and water, Qp + I. Similar considerations apply regarding the compositional range that allows formation of cements, to those discussed previously for the unreinforced cements with reference to Fig. 1. However, it has been found that to incorporate larger volume fractions of carbon fibre, i.e. Q, > 0.05, it is necessary to reduce the volume fraction of glass powder. Fractography Figure 3 is a SEM photomicrograph of the fracture surface of a GIC-G338 glass reinforced with a

of fracture surface of GIC-MP4 Fig. 4. SE i = 0.15 HMS carbon fibre. /p,

volume pull-out

fraction is clearly

glass with Q, := 12.5

pm.

of shown. Also,

erefore,

fraction of reinforci When ~~alysi~g S

the present results

Fibrous reinforcement of glass-ionomer cements possible to estimate the volume fractions of voids present. Such voids are always present in GICs and are mainly due to air between the glass matrices and fibres not being displaced by liquid during the mixing process. The lower the volume fraction of the liquid phase, the higher the volume fraction of voids. Work is in progress (Ellis, B. & Oldfield, C. W. B., .unpublished) on methods of estimating the volume fraction of voids using a mercury porisimeter and a mercury balance. From inspection of Figs 4 and 5 it can be seen that the fibres are not aligned, and previously fibre orientation has been quantitatively estimated and shown to be essentially random.’ Since the fibres are not aligned it is clear that they must bridge the crack at oblique angles. This has different effects on carbon and Safil fibres as will be discussed subsequently since it has a major effect on the load deformation behaviour of fibrous reinforced GICs. Further evidence of the topography of the fracture surface is clearly shown in Fig. 6 in which the ‘crossing’ of fibres can be seen, which has important consequences for the behaviour of the composite, as will be clear when the behaviour of “oblique’ fibres, that is fibres that are not perpendicular to the fracture surface, are considered (see later). From Fig. 6 it may be concluded that there is little, if any, pull-out of Safil fibres. The broken end can be seen in the ‘plane’ of the fracture surface. This is due to the brittleness of the Safil fibres compared with the carbon fibres. Failure of this type could be d.ue to either of two factors or a combination of both For fibres approximately perpendicular to the crack plane, high adhesion between the matrix and the fibre surface will limit

Fig. 5. SEM of fracture surface of GIC-MP4 glass with Q, = I = 11 pm. Another 0.1 HMS carbon fibre 1 mm long. Iexample of fibre pull-out.

Fig. 6. SEM of fracture surface of GIC-G338 0.05 of Safil fibres 0.5 mm long. I--

glass with Q, =

j = 1.5 pm.

fibre pull-out and brittle fracture can occur, essentially at the fracture surface. However, for oblique fibres either the matrix must yield and/or the fibres ‘bend’ through a sharp radius. These effects are illustrated in Fig. 7. In Fig. 6 the Safil fibre end is clearly seen in the distorted, elliptically shaped hole. This indicates that some matrix deformation has occurred before fracture of an oblique fibre. Dehydration shr:inkage would produce an approximately cylindrical hole. The bending of an oblique fibre will be limited by its ‘flexibility’ which is defined by lH~11~~ as the minimum radius of curvature, pmin, which the fibre can sustain before fracture occurs, which can be calculated from : Ed Pmin

=

2a,

where E is Young’s modulus, d is the diameter and ~~ is the fracture strength of the fibre. Insertion of the appropriate values into this equation shows that the minimum radius of curvature of the carbon fibre is about 1/5th of that of Safil. That is, the carbon fibre is 5 times more flexible, wlhich partially accounts for the fibre pull-out of carbon fibres, which contrasts with the brittle fracture of the Safil fibres. In Fig. 8 it can be seen that the crack path is tortuous, and it was observed for this sample that the load had not decreased to zero although the crack had penetrated about 95 % of the thickness of the test piece. This is due to fihres bridging across the crack which is illustrated in Fig. 9 for GIC-MP4 glass.

318

C. W. B. OEdfieid,Bryan Ellis

(a)

SEM of crack in GIG-MP4 carbon

fibre

glass with Q, = 0.15 HMS

1 mm long. Note the fibres bridging acrm~ the crack. /PI

= 70 ,Um

(b) !i?iI

-i

Fig. 7. (a) Pull-out

of oblique fibres; fibres.

(b) fracture

of brittle

Load-deformation curves for cylindricai Fig. -G338 glass diameter 4.5 mm, 4 point of (a) Q, = 0; (b) Q, = @OS HMS carbon fibres 1 mm (c) Q, = 045 Safil fibres 0.5 mm long.

long;

Mechanical properties to fibrous

reinforcement on the of e is a large incr stress for GICs reinforced with carbon fibre 1 mm long, and a dra the area enclosed by the load-deformation curve, i.e. the fracture energy increases. There is also an increase in Young’s modulus as would be expected with fibrous reinforcement. The large increases in both fracture load and energy is due to the fi reinforcement, with the latter attributable large extent of fibre pull-out discussed previously. Reinforcement with the com~a~t~ve~y 6brittle ’ ulus an Safil fibres increases both the Young’s . n behaviour

Fig. 8. SEM of the tortuous crack path in GIC-G338 with Q, = 0.05 HMS carbon fibre 1 mm long. )-I= 1 mm.

glass

319

Fibrous reinforcement of glass-ionomer cements 50%

F 50% F

(a)

100% P+L

50% G

100% P+l_

Fig. 11. Young’s modulus contour plots on triangular initial composition diagrams. Iso-modulus lines, units GPa. (a) GIC-MP4 glass HMS carbon fibre 1 mm long; (b) GICG338 glass HMS carbon fibre 1 mm long. Samples stored at 100% RH for 24 h.

seen that the maximum modulus for GIC-G338 glass occurs with a higher concentration of glass than for MP4 glass. Similar contours are obtained for the maximum fibre stress and comparisons show that the maxima occur at somewhat different volume fractions of glass for both fracture stress and moduli. For more detailed discussion of the effects of composition on the mechanical properties of GICs

fracture stress, Fig. 10. However, the crack propagates w:ithout fibre pull-out and the load falls precipitately when it propagates across the test specimens. It is useful to present the effects of composition of GICs on the mechanical properties as contours on a triangular iagram. In Fig. 11 contours of Young’s modulus for carbon fibre reinforcement of GIC (MP4 and G338 glass) are plotted. It can be

50

40

30

20

10

Glass Volume fraction/%

Fig. 12. Young’s modulus

23

versus volume fraction of glass for 0, GIC-MP4 glass, and 0, GIC-G338 glass. The volume fraction WMS carbon fibre 1 mm long is constant Q, = pO.5, see line f to P in Fig. 13(b). ECM

of i

modl.deas

of

CQ

with volume fr models from which ielsen.“7 Ah of these mc increase of elastic ,' 3

i i

p' ,r,'

/

I i

/-+,

‘\\

1’ 1’

\

\ i

\

5

10 Fibm V&me

15

fraction/%

Fig. 13. Young’s modulus versus volume fraction of HMS carbon fibre 1 mm long. The volume fraction of glass is constant, Q, = 0.4, see line g to gl, Fig. II(a). ?? , GIC-MP4 glass; 0, GIC-G338 glass.

it is helpful to plot the property as a function of one of the compositional variables with the other held constant. This is equivalent to climbing over the hump by several different routes. Thus, in Fig. 12, the volume fraction of carbon fibres is constant at Q, = O-05 and th e volume fraction of glass varies along line f-f in Fig. 1l(b) from left to right with Q2, decreasing. Particulate fillers increase the elastic

n

k

F-

5

/’ /‘

r 3 P

4

Fig. 14. Maximum

!’ .’ ,’

-..‘\

on

particulate composites.

the

sdxx@h

of

fkn the strength decreases

-\ ‘\ ‘\ ‘\ ‘\ ‘\

Young’s modulus versus volume fraction of WMS carbon fibre I mm long. The volume fractions and water are variable. 0, GIC-MP4 glass; 0, GIC-6338 glass.

ofglass,polymer

322

C. W. B. Oldjeld, Bryan Ellis

both QgandQp+Lvary. Similar factors apply to the data in these graphs as for Fig. 13, but the actual values of the elastic modulus are somewhat higher. The maxima for cements prepared from either MP4 or G338 glass are at about a volume fraction of carbon fibre of Qr = O-05. This is considerably lower than the 12 volume per cent concentration of fibre for the maximum modulus of about 12.5 GPa for GIC-MP4 reported recently,’ Fig. 15. Consequently, the maximum modulus attained in the present series of experiments is lower than that attained previously. This is because in the previous work, test specimens were moulded with a pressure applied for 1 h during the setting of the cements. With a volume fraction of fibre of about Q, = 0.05 the optimum elastic modulus attained is about 3.5 GPa for the present samples compared with 7 GPa for the pressure moulded specimens. Thus, not only mixing variables but also moulding conditions affect the properties of these cements, which obviously has clinical implications However, it can be appreciated that even with only a low concentration of HMS carbon fibre, Q, = O-05,it is possible to at least double the elastic modulus. Also, GICs-G338 glass have higher moduli than those prepared from MP4, especially in the fibre concentration range which includes the maxima. CONCLUSIONS From the results presented in this paper it can be appreciated that it is possible to incorporate carbon fibres in glass-ionomer cements, and attain increases in both their elastic moduli and strength. Also the maxima for the two GICs are at different volume fractions of carbon fibre. With adjustment of the volume fractions of the other components, incorporation of higher volume fractions of carbon fibre has been achieved which, if these materials behaved similarly to other fibrous composites,21 would have even higher strengths. Since they do not, some mechanism such as an increase in the concentration of voids must override the effect of fibrous reinforcement. In conclusion :

(i> By adjusting the volume fraction of glass it is possible to incorporate both carbon and Safil fibres into GICs. (ii) Mixing is more facile for glasses based on MP4 glass than G338 glass because the rate of setting is higher with the latter. (iii) A major factor which increases the work of fracture of GICs reinforced with carbon fibres is due to fibre pull-out.

(iv) Brittle fracture occurs with little, if any, fibre pull-out. (v) With only relatively low volume fractious of forcement it is po Young’s moduli

ACKNOWLEDGEMEN The authors are grateful to J. Ellis for many interestin Laboratory of the Govern nation of materials, and Engineering Research Council for ~n~~c~a~support (CASE award to CWBO).

EFERENGES 1.

. & McLean,

J. W., ~~a~s-~~~~~~~ Cement. Publishing Co Inc., Chicago, 1988. E. A. & Nicholson, J. W., A Study of the 2. Wasson, Relationship Between Setting Chemistry and Properties of Glass-Poly(alkenoate) Cements. &it. Polym. J., %3(1990) 197-83. C. J. & Strating, .1 The 3. Jonck, L, M., Grobbelar, Biocompatibility of Glass-Ionomer Cement in Joint placement Bulk Testing. Clin. 4. Jonck, L. M., Grobbelar, C. J. Evaluation of Glass-Ionomer Interface Material in Total Joint Replacement. A Screening ter., 4 (1989) 201-24. Bearing Fibre Composites. Pergamon CamHull, D., An ~ntroduetion to Composite Materids bridge University Press, Cambridge, 1981. Wilson, A. D. McLean, J. W., Composition. In GhssMonomer Cement. Quintessence Publishing Co Inc7 Cl% 21-42, see p. 28-9. 8. his, B., Howarth, L. G. , The Fracture of Glass-Ionomer Ce the Fractography of Glassesand Ceramics II, New York State College of Ceramics, Alfred University, New York, July 1990. To be published by The American Ceramic Society. & Bailey, J. W., The 9. IIowarth, L. G., Ellis, Seventh InterProperties of Glass Ionomer Cements. ~atio~~~ Conference Polymers, Churchill

on Defamation,

Yield and Fracture

oj

College, Cambridge, UK, April 1988. 10. Wilson, A. D. & McLean, J. W., The Setting Reaction and its Clinical Consequences. In ~l~s~-~o~~~e~ Ce’eaneplt. Quintessence Publishing Co Inc., Chicago, 1988, p. 43-56* see p. 51. 11. Wilson, A. D. & McLean, J. W., The Setting Reaction and its Clinical Consequences. In ~~~~s-~on~~e~ Cement, uintessence Publishing Co inc., Chicago, 1988, p. 4%56> see p. 53. Wilson A. D,, NM 12. Prosser, H. J.> Richards, L. P. ental Material II, The Acid in Glass-Ionomer Cements. J. Biomed iklater. Res., 13.

& Wilson,

A. D., Thermal

Fibrous reinforcement of glass-ionomer cements Films of Partially Neutralized Poly(acrylic acid), 1. Influence of Metal Ions. Bit. Polym. J., 19 (1987) 67-72. 14. Nicholson, J. W., Wasson, E. A. &Wilson, A. D., Thermal Behaviour of Films of Partially Neutralized Poly(acrylic acid), 3. Effect of Magnesium and Calcium Ions. &it. Polym. J., 20 (1988) 97-107. 15. Greenberg, A. R., Influence of Filler Chemistry on the Glass Transition Behaviour of a Polymer Matrix Composite Material. J. Mater. Sci. Lett., 6 (1987) 78-80. 16. Hull, D., Fibres and matrices. In An Introduction to Compos,ite Materials, Cambridge University Press, Cambridge, 1981, pp. 9-35. See p. 26.

323

Polymers. In Mechanical Properties of Polymers and Composites, Vol. 2. Marcel

17. Nielsen, L. E., Particulate-Filled

Dekker Inc., New York, 1974, pp. 379452, see pp. 3877394. 18. Nielsen, L. E., Particulate-Filled Polymers. In Mechanical Properties of Polymers and Composites, Vol. 2. Marcel Dekker Inc., New York, 1974, pp. 37945’2, see pp. 405515. 19. Nielsen, L. E., Fiber-Filled Composites and Other Composites. In Mechanical Properties of Polymers and Composites, Vol. 2. Marcel Dekker Inc., New York, 1974, pp. 453-510, see pp. 45465.