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Powder Technology, 13, Jan-Feb. 1, pp. 49-56, 1976-01-01

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Use of compacts to study the mechanical properties of sulfur

Beaudoin, J. J.; Sereda, P. J.

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Ser

TH1

N21d

no.

666

National Research

Conseil national

Council Canada

de recherches Canada

BLDG

/

USE OF COMPACTS TO STUDY THE MECHANICAL

PROPERTIES OF SULFUR

I

by

J.J.

p u d o i n and

P.J.

Sereda

Reprinted from

,

Powder Technology,

Vol.

13,

No.

1, 1976 p. 49.56

DBR Paper

No. 666

Division of Building Research

Price

25

cents

1

BUILDI*JC

RCCFARCH

1

-

LIBRARY

-

I I

APR

14

1976

1,

I

1

NATIONAL RESEARCH COUNCIL

1

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SO MMAIRE

On effectue d e s m e s u r e s d e module dt61asticit6 e t d e m i c r o d u r e t 6 s u r d e s c o m p r i m 6 s de q u a t r e Cchantillons d e s o u f r e d e diffCrente c r i s t a l l i n i t 6 e t morphologie, a v e c un l a r g e & a r t de porosit6. L e s comprimCs f a i t s de C r y s t e x ( s o u f r e haut polym'ere) ont d e s propriCt6s mkcaniques i n f 6 r i e u r e s 'a c e l l e s d t a u t r e s Bchantillons examinCs. L e f a i t de r e c u i r e l e s c o m p r i m 6 s aug- m e n t e de beaucoup l e u r m i c r o d u r e t 6 . On effectue d e s m e s u r e s de module dt61asticit6 e t d e m i c r o d u r - e t 6 s u r d e s c o m p r i m 6 s d e s o u f r e r e n f o r c 6 d e f i b r e s d e poly6thyl'ene t6rCphtalate pour un l a r g e 6 c a r t de porosit6.

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Powder Technology, 1 3 ( 1 9 7 6 ) 49 - 56

@ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

Use of Compacts t o Study the Mechanical Properties of Sulfur

J. J. BEAUDOIN and P. J. SEREDA

Building Materials Section, National Research Council, Division o f Building Research, O t t a w a K I A O R 6 (Canada) 1

(Received January 20, 1 9 7 5 )

SUMMARY

Modulus of elasticity and microhardness measurements were made on compacts of four sulfur preparations having different crystallinities, morphologies and a wide range of porosities. Compacts made from Crystex -

a high polymeric sulfur - had mechanical properties inferior to those of other prepara- tions studied. Annealing compacts resulted in a large increase in microhardness. Measure- ments of modulus of elasticity and microhard- ness of sulfur compacts reinforced with poly(ethy1ene terephthalate) fibers were made for a wide range of porosities.

INTRODUCTION

There is, a t present, considerable interest in the utilization of sulfur as a structural material [ l

-

31. Studies [4] have been initiated t o assess the feasibility of substituting sulfur for portland cement as the binder in conventional concrete; concrete impregnated with sulfur has shown potential as a structural material. As the sulfur binder or impregnant forms a porous network which is a continuum of uniform porosity, it is important t o deter- mine the mechanical properties of sulfur as a

I

function of porosity. Sulfur samples of controlled uniform porosity can be prepared

by compacting sulfur powder. Liquid sulfur

t which is cast into a mould and cooled t o room

temperature forms a non-porous solid only if cast in small increments; if the increments are too large, the porosity is irregular due t o entrapped air.

Experimental data for engineering design (e.g. modulus of elasticity) are scarce and the data available are often conflicting. Previous work [ 5 , 6 ] involving measurement of mech- anical properties has been subject t o difficulties

in specimen fabrication. Specimens cast into horizontal moulds shrink and must be topped up; the interface formed is a potential weak plane. Specimens cast into vertical moulds provide a means of overcoming this problem; sawing of the sample end may introduce microcracks into the sample. These results and difficulties, however, apply t o the non- porous systems.

Results of mechanical properties for four sulfur preparations are presented in this paper. The specimens were prepared by a compaction technique that has been used previously for preparing specimens of inorganic building materials other than sulfur t o measure mech- anical properties [ 7 ] . It is important t o recognize that cold pressing sulfur powder a t room temperature is similar t o hot pressing for many other systems owing t o the proximity of the pressing temperature and the melting point.

EXPERIMENTAL Materials

The following materials were used: (i) Preparation 1

Sulfur flowers (orthorhombic form) having 99.8% purity and average particle size of 5 pm, obtained from Stauffer Chemical Co.,

Westport, Connecticut. (ii) Preparation 2

Crystex, a microfine metastable polymeric sulfur 99.5% pure with an average particle size of 3 pm.

(iii) Preparation 3

Sulfur flowers heated t o 130 "C and air quenched at 25 "C t o provide samples contain- ing large amounts of monoclinic phase. The

(5)

Fig. 1. M o d u l u s o f e l a s t i c i t y us, p o r o s i t y f o r c o m p a c t s o f v a r i o u s s u l f u r preparations.

quenched sulfur was ground t o an average particle size of 5 pm and stored a t -20 "C before compacting.

(iu) Preparation 4

Sulfur flowers heated t o 200 "C and air quenched at 25 "C to provide samples contain- ing large amounts of amorphous sulfur. The quenched sulfur was ground t o an average particle size of 5 pm and stored at -20 "C before compacting.

( u ) Preparation 5

Poly(ethy1ene terephthalate) fibers (typical diameter 0.022 mm) were prepared with an aspect ratio of approximately 120.

The hardness and modulus of elasticity of sulfur compacts, fabricated from the four different preparations and with a wide range of porosities, were measured. These compacts included a series containing 1.45, 2.90 and 7.25% by volume of poly(ethy1ene

terephthalate) fibers. All specimens made were circular discs 1.25 in. (32 mm) in diameter and 0.050 in. (1.25 mm) thick. The compacts containing fibers were made by intimately mixing measured quantities of fiber and sulfur.

Porosity

The solid volume of the samples was measur- ed by helium comparison pycnometry. The porosity was determined by utilizing the solid volume and the apparent volume calculated from sample geometry. Porosity calculations for the fiber-reinforced compacts were made

1000 I I - - 8 0 0 - - - - P R E P A R A T I O N - 0 1 - . 2 0 3 1 4 -. - - Fig. 2. M i c r o h a r d n e s s us. p o r o s i t y f o r c o m p a c t s o f v a r i o u s s u l f u r preparations.

using specific gravity values for sulfur flowers and poly(ethy1ene terephthalate).

Young's modulus and microhardness

Young's modulus of the circular disc samples was determined by measuring the deflection of a specimen loaded a t the center and supported a t three points located at the circumference of a circle 25 mm in diameter [ 8 ] . A Leitz microhardness testing machine with a Vickers indenter was used for the micro- hardness measurements [ 9 ] . Ten hardness measurements were made on each disc and three discs were tested for each porosity condition.

Annealing

A series of compacts prepared from sulfur flowers were subjected t o the following heat

treatments: 3 hours at 5 0 "C; 4 hours a t 7 5 "C;

1

4 hours a t 95 "C; and 4 hours a t 1 0 0 "C. This

was done t o ascertain the effect annealing q

would have on the mechanical properties of the compacts as well as t o accelerate the effect of aging.

Pore size distribution

A mercury porosimeter having a maximum

pressure of 1 0 0 MPa was used t o determine pore size distribution in samples selected from the four sulfur preparations.

(6)

TABLE 1

Regression analysis o f modulus of elasticity, and microhardness versus porosity data

- -- E = E o exp (-bEP) Preparation E o b~ r* (MPa x lop3) 1 13.90 0.0400 0.96 2 4 . 5 3 0.0377 0.97 3 1 0 . 8 2 0.0368 0.98 4 12.20 0.0541 0.99 1, 3 and 4 11.97 0.0424 0.83 H = Ho exp (-bHP) Preparation Ho b~ r (MPa x 1 0 - l ) 1 71.62 0.0636 0.99 2 20.47 0.0539 0.98 3 37.50 0.0636 0.94 4 38.55 0.0580 0.94 1 , 3 and 4 45.39 0.0552 0.84 *Correlation coefficient. X-ray diffraction

The d spacings for sulfur flowers (orthorhombic sulfur) were in accord with crystallographic data, as expected. Results also indicated satisfactory stability for Crystex (high polymeric sulfur). Samples prepared by heating elemental sulfur t o 130 "C and air quenching at 25 "C yielded X-ray spectra indicating the presence of both monoclinic and orthorhombic forms. Samples prepared by heating elemental sulfur t o 200 "C and quenching at 25 "C gave X-ray spectra indicating some conversion t o orthorhombic phase. These samples are probably mixtures of orthorhombic and amorphous sulfur. No monoclinic sulfur was detected.

Inherent in X-ray diffraction examination of sulfur are difficulties with conversion of monoclinic t o orthorhombic phase owing t o handling or exposure t o sunlight [6]. To aid in overcoming these difficulties, finely divided high purity starting materials were used. Grinding was used only for the two sample preparations that were quenched from 200" and 130 "C respectively.

TABLE 2

Mean slope ratio, bE/bH

Preparation 1 2 3 4 Combined

RESULTS

Porosity

The logarithm of the moduli of elasticity and microhardness when plotted against porosity indicated a linear relationship for all preparations tested (Figs. 1 , 2). The results of a linear regression analysis are recorded in Table 1. Preparations 1 , 3 and 4 had similar moduli of elasticity. Table 1 also presents the results obtained when these data are grouped together.

Values of moduli of elasticity and micro- hardness at zero porosity decrease in magni- tude according to the following preparation sequence: 1, 4, 3 and 2. Previous work [ l o ] on the mechanical properties of gypsum - all crystalline modifications - showed that the log mechanical property-porosity curves tended t o converge at zero porosity. As the sulfur systems studied did not show this behaviour, it is possible that the differences are due t o basic differences between different crystalline phases or crystalline and amorphous phases present.

Microhardness data for Preparation 1 lie significantly above data for Preparations 3 and

4. Data for Preparation 2 lie significantly below data for the other preparations. Only Preparation 1 was used after a long period of aging. Because compressive strength increases with time [ I ] , it is possible that aging may account for improved mechanical properties at all porosities.

The modulus of elasticity and microhard- ness for all sulfur preparations obeyed the general relationship

where E and H refer t o modulus of elasticity and microhardness respectively and P is the porosity.

Table 2 lists values of mean slope ratio, bE/bH, for each of the four sulfur preparations. This ratio is different for each preparation.

(7)

M I C R O H A R D N E S S . M P a

Fig. 3. Modulus of elasticity us. microhardness for

compacts of various sulfur preparations.

Fig. 4. Compaction pressure us. porosity for compacts

o f various sulfur preparations.

1 0

Equation (1) implies that porosity influences

the properties E and H in a similar way.

Figure 3 is a plot of log modulus of elasticity

versus log microh&dness. The curve is a

straight line, which is in agreement with eqn.

(1).

In Fig. 4 the logarithm of compacting pres-

sure versus porosity for four sulfur preparations is plotted. The compaction pressure for

Preparations 1 and 2 a t equal porosity is

approximately equal, a s is that for Preparations

3 and 4; the compaction pressure for Prepara-

tions 1 and 2 is greater than that for Prepara-

tions 3 and 4 a t all porosities. The curves diverge from linearity a t low porosity levels. The divergence may be due t o an increased

I I I I I

I

C O r v l P A C T O N P R E S S U R E . M P a

0 10 2 0 3 0 4 0 5 0 6 0 7 0 P O R O S I T Y . "b

Fig. 5. Microhardness us. compaction pressure for

compacts o f various sulfur preparations.

P R E P A R A T I O N - 0 N O A N N E A I I N G A N N E A L E D 3 H R S O A N N L A L t D I A N N E A L E D n A N N E A L E D 4 H R S U H R S 4 H R S I I I I I I

1

o 1 0 2 0 30 4 0 i n 60 7 0 P O R O S I T Y %

Fig. 6. Effect o f annealing o n modulus o f elasticity o f sulfur compacts.

amount of sublimation and recrystallization at particle boundaries. It may also be an

indication that particle deformation is requir-

ed t o attain low values of porosity - this may

or may not result in strain energy stored in the particle. Density measurements of Preparation 4 compacts a t zero porosity

yielded density values up t o 2.128 X

lo3

kg/m3. Difference between this density value

and the published value of 2.07 X

lo3

kg/m3

may be due t o the inaccessibility of pore space t o helium, i.e. discrete pores trapped within solid sulfur particles. Further work is required t o establish the significance of this apparent increase in solid density due t o compaction pressure of 700 MPa.

(8)

1 0 1 I I I I I 1 I I 0 5 LO 1 5 2 0 25 30 3 5 4 0 4 5 5 0

P O R O S I T Y . %

Fig. 7 . Effect of annealing on microhardness of sulfur compacts. 1 0 . 0 0 0 . C O M P A C T I O N P R E P A R A T I O N P R E S S U R E . h l P a x . o o o - - 6 . 0 0 0 4 . 0 0 0

I

Ill I 0 0 I 0 01 P O R E U 1 h : i f l l S . ,m 1 i I I I I I - P R E P A R A T I O N 1 - - - O A N N E A L E O A T I O O ' C 1 6 H R S -

-

- . R O O M T E M P E R A T U R E . N O A N N E A L I N G

-

Fig. 8. Pore size distribution of selected sulfur compacts.

Compaction pressure

In Fig. 5 log microhardness is plotted versus

log compaction pressure. A relation between microhardness and compaction pressure is expected, as both processes are essentially the same. Any deviation from equality probably results from friction between the piston and cylinder walls of the mould in the case of compaction. The magnitude of the microhard- ness property increases monotonically with compaction pressure. Preparations 1, 3 and 4 appear t o require similar compaction pressure t o achieve a given value of microhardness. Preparation 2 required a greater compaction pressure t o achieve the same value. This can be due t o a dependence of friction effects and particle deformation on morphology.

P R E P A R A T I O N 1 F I B E R V O L U M E , '3 M * - - - a - 4 1 5 C---. 3 0 b,- a 7 5 I I I I I I 0 5 1 0 1 5 2 0 25 30 3 5 P O R O S I T Y '2

Fig. 9(a). Modulus of elasticity us. porosity for

sulfur compacts containing poly(ethy1ene terephthalate) fibers. P R E P I 2 4 0 0 0 1 0 0 " - RF 4 6 -0 1 0 2 0 3 0 4 0 5 0 P O R O S I T Y , 5

Fig. 9(b). Compaction pressure us. porosity for sulfur

compacts containing poly(ethy1ene terephthalate) fibers.

Effect of annealing

In Fig. 6 log modulus of elasticity is plotted

versus porosity for four different heat treat- ments of Preparation 1 followed by quenching in air at 25 "C. As the annealing temperature increases from 50" t o 100 "C, the modulus of elasticity increases for a given porosity. In Fig. 7 log microhardness is plotted versus

porosity. For the porosity range tested, there is more than a 100% increase in microhardness due t o annealing. Porosity determinations were made before and after annealing. There was a small decrease in porosity subsequent t o anneal- ing (less than 2%).

(9)

Fig. 10. Scanning electron micrographs of fractured sulfur compacts: ( a ) Preparation 1, ( b ) Preparation 2, ( c ) Preparation 3, ( d ) Preparation 4.

that experienced with halite, indicating that strain hardening plays a secondary role to re- crystallization by sublimation at particle boundaries. This effect has been observed with gypsum compacts when exposed t o high relative humidity [ l l ]

.

Pore size distribution

Figure 8 shows pore size distributions for selected sulfur compacts. Preparations 2, 3 and

4

at approximately equal total porosity have dissimilar pore size distributions. Preparation 2

contains pores well distributed between 0.7 and 0.014 pm; in contrast Preparations 3 and 4 contain the majority of pores between 3 and 0.7 pm. For Preparation 2 the nature of the pore size distribution changes as the compac- tion pressure increases from 70 t o 280 MPa. At 70 MPa the pores are essentially all 0.7 pm while at 280 MPa the pores are uniformly distributed.

Fiber-reinforced sulfur

(10)

plotted versus porosity for a series of Prepara-

tion 1 compacts containing 1.5, 3.0 and 7.5%

by volume poly(ethy1ene terephthalate) fibers with an aspect ratio of 120. As the fiber content increases, the elastic modulus for the compact decreases at a given porosity, reflect- ing the contribution of the lower modulus fiber. In Fig. 9(b) compacting pressure is plotted versus porosity. The presence of fibers does not make a significant difference t o the compaction pressure required for a given porosity for porosities greater than 10%.

Particle shape

Scanning electron micrographs (Figs. 10(a)

-

(d)) show relative differences in particle shape and obvious differences in morphology for the four sulfur preparations examined. Figure

10(a) shows that a compact of Preparation 1

(12% porosity) has rounded particles with rough surface texture. It appears that fracture occurred through particles. Figure 10(b) shows that a compact of Preparation 2 (12% porosity) is composed essentially of rounded particles with smooth surfaces. Fracture shows very few points of solid contact, indicating a potentially weak material. Figure 10(c) shows that a compact of Preparation 3 (12% porosity) is composed of irregularly shaped particles. Figure 10(d) shows that for a compact of Preparation 4 (approx. 12% porosity) the particles form an amorphous mass with few distinct particle boundaries and glass-like fracture characteristics.

DISCUSSION

Values of modulus of elasticity of plastic- elastic sulfur threads have been reported by

Dale [6] as varying from 4.05 X

lo3

MPa t o

1220 X

lo3

MPa. Values of modulus of

elasticity at zero porosity for Preparations 1 , 2, 3 and 4 as given in Table 1 fall within this range, but as data are scarce and the range of published data wide, comparison of the latter with present data is difficult. Extrapolation of the log mechanical property-porosity curve t o zero porosity was necessary to obtain modulus of elasticity and microhardness values for

Preparations 1 and 2; the accuracy of these

values, therefore, is subject to the accuracy of the extrapolation. Preparation 4 undergoes an increase in modulus of elasticity from

10.7 X

lo3

MPa to 13.8 X

lo3

MPa at zero

porosity, as compaction pressure increases from 420 t o 700 MPa. This anomalous behaviour may be due t o sublimation and re- crystallization at particle boundaries.

Published data [6] for hardness are values

expressed as Shore B-2 hardness numbers for various allotropes at two melt temperatures,

130" and 187 "C. This makes comparison difficult with the present data, as no precise relationship exists between the Vickers and Shore B-2 hardness numbers. Approximate relationships are subject to variations in materials and mechanical or heat treatment.

The mechanical properties of compacts made from Preparation 2 (Crystex, a long chained, high polymeric, amorphous sulfur

with molecular weight 100,000

-

300,000) are

inferior to those of compacts made from Preparations 1 , 3 and 4 at every porosity. This is probably due t o the relative ease with which the long chains undergo alignment, deforma- tion and translation with respect t o one another.

The mechanical properties of compacts fabricated from Preparation 2 may be poorer than those of compacts from other prepara- tions as the number of interparticle contacts per unit area appears t o be less for Preparation 2 compacts. This might possibly result from the spherical shape and smooth surface tex- ture of the particles. It is likely that crystalline phases deform more readily under pressure t o produce stronger particle t o particle bonds.

CONCLUSION

(1) Compacts of powdered sulfur of differ- ent crystallinity and morphology provide a convenient means for measurement of modulus of elasticity and microhardness of porous sulfur bodies and, by extrapolation, even that of non-porous sulfur bodies.

(2) Modulus of elasticity and microhard- ness of sulfur compacts can be expressed as simple exponential functions of porosity.

(3) Modulus of elasticity, E, can be express-

ed as an exponential function of microhard- ness, i.e.

(11)

and Ho are modulus of elasticity and micro- hardness a t zero porosity respectively; bE/bH is the ratio of the slopes of the log mechanical property versus porosity function.

(4) Porosity affects modulus of elasticity and microhardness measurements in a similar way; the relation between the two properties is independent of porosity.

(5) Heating sulfur compacts t o 100 "C and subsequent quenching at 25 "C increases bond area and thus increases the modulus of

elasticity.

(6) Compacts made from Crystex - a high polymeric sulfur - exhibit lower values for modulus of elasticity and microhardness for the porosity range studied. Difference in mechanical behaviour may be attributed t o the presence of long-chain molecules and seemingly poor interparticle bond.

(7) Compacts of sulfur appear t o have potential as structural models for studying the effects of fiber reinforcement on the mechanic- al properties of sulfur composites.

ACKNOWLEDGEMENTS

The authors wish t o acknowledge the valu- able assistance of J. Wood, who performed most of the experiments. Thanks are also due t o E. Quinn and G. Aarts for their assistance. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.

REFERENCES

1 B. R. Gamble e t al., Civil engineering applications of sulfur-based materials, Civil Eng. Res. Rep. No. CE74-2, Univ. of Calgary, February 1974. 2 R. E. Loov, A. H. Vroom and M. A. Ward, Sulfur

concrete - a new construction material, J. I Prestressed Concr. Inst., 1 9 ( 1 ) (1974) 86

-

95.

3 A. C. Ludwig, Sulfur-reinforced systems for structural applications, Proc. Int. Conf. o n

Materials Technology, May 1968, pp. 367 - 370. i

4 J. J. Beaudoin and P. J. Sereda, Freeze thaw

durability of sulfur concrete, Building Res. Note No. 92, Division of Building Research, National Research Council of Canada, June 1974.

5 J. M. Dale, Determination of the mechanical properties of elemental sulfur, Mater. Res. Stand., (Jan.) (1961) 23

-

25.

6 J. M. Dale and A. C. Ludwig, Mechanical properties of sulfur, in Elemental Sulfur (Ed. Beat Meyer), Interscience, New York, 1965, pp. 1 6 1

-

178. 7 I. Soroka and P. J. Sereda, The structure of

cement stone and the use of compacts as structur- al models, Proc. 5th Int. Symp. on the Chemistry of Cement, Tokyo, 1968, Part 111, Vol. 111, pp. 67

-

73.

8 P. J. Sereda, R. F. Feldman and E. G. Swenson,

Effect of sorbed water on some mechanical I

properties of hydrated portland cement pastes and

compacts, Highway Res. Board Special Report 90, I

pp. 5 8 - 73.

1

9 P. J. Sereda, Significance of microhardness o f porous inorganic materials, Cem. Concr. Res., 2

(1972) 717.

~

1 0 I. Soroka and P. J. Sereda, Interrelation of hard-

~

I ness, modulus of elasticity, and porosity in various

gypsum systems, J. Am. Ceram. Soc., 5 1 (6) (1968) 337 - 340.

Figure

Fig.  1.  M o d u l u s  o f  e l a s t i c i t y   us,  p o r o s i t y   f o r  c o m p a c t s   o f  v a r i o u s  s u l f u r  preparations
Fig. 5. Microhardness  us.  compaction pressure  for  compacts o f  various sulfur preparations
Fig. 7 .  Effect  of annealing on microhardness of  sulfur compacts. 1 0 . 0 0 0  .  C O M P A C T I O N   P R E P A R A T I O N   P R E S S U R E
Fig. 10. Scanning electron micrographs of fractured  sulfur compacts: ( a )  Preparation  1,  ( b )   Preparation  2,  ( c )   Preparation 3, ( d )  Preparation 4

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