Indian Journal of Engineering & Materials Sciences Vo l. 10. October 2003 , pp. 4 19-423

Study of black and composite agglomerates of cork as thermal insulating material s

M A Oiaz-Oieza, A Oiaz-Parralejoa, A Macias-Garcia", P de la Rosa Blanco" & V G6mez-Serranob

"Departamento de Electron ica e Ingenierfa Elect romecanica, Escuela de Ingeni e rfas Industriales. Uni versidad de Extremadura. 0607 I Badajoz. Spain

"Departamento de Qufmi ca Inorgani ca. Facultad de C ienc ias , Universidad de Extremad ura. 0607 1 Badajllz. Spain

Received 16 October 2002; accepted 3 July 2003

The behaviour o f cork agg lo merates as thermal insul ating materials was studied . The products used were blac \.. agg lomerate (B A) and composite agglomerate (CA), which were prepared from bl ack regranul ate (BR ) of cork. In the preparation of the agg lo merates, the specimen thic kness for BA and the partic le s ize o f BR and the res in dosage fo r CA were varied. A procedure was proposed for measuring the thermal conducti vity of material s in a granul a r o r powder for ill . The thermal conducti vity o f BR. BA. and CA was determined using a Ketherm QTM-D3 fast mete r, provided wi th an adapte r QTM PA I. The behaviour as a therma l insulating material can be improved by decreasing the panicle size for I3R and by increasing the specimen thic kness fo r BA . In the case of CA, the best behavio ur is shown when the intermediate panic le ,ize o f BR and the highes t resin dosage a re used in its preparati on. Frequently. the in sulating be haviour is better for CA th an for BR, and also for BA than fo r CA .

III the housings, the building wall has two main functi ons: one is structural and the other is topo logical. The former has evolved in a parallel direction to the development of innovative building techniques and with the di scovery of new materials. The latter performs functi ons of defense against ex ternal agents such as rain, wind, temperature and nOises I. The facto rs that hinder a harmonious deve lopment of human life include the noise and the temperature changes, which cause the permanent alterati on of the physical ambient. To obtain a better quality of the thermal and acoustical ambient in public and pri vate premises, modern architecture has been induced to give priority to the insu lating field2-7 . Thi s fact , among others, has done that the study of the thermal insulating material s, whose main task-as is well-known is to avoid heat transmission between an enciosure and its environment, is a subject of growing interest 1-5 . Cork is a typi cal hot and cold insulating material , at present its main appli cation being in the

f· f fl" d 8~ manuacture 0 stoppers or t le wtne 111 ustry .. Therefore, the diverse use of cork as an insulator would surely result in an increase of its demand and abo in the price of the raw materi al.

The facto rs with influence on the thermal conducti vity of cork and cork-deri ved products incl ude the moisture content and the grain purity and

<' For correspondence (E-mai l: a m3cias@ materiaies. unex.es)

size. Moisture alters the true coefficien t of therlll ~t1

conductivity of these materi als, the effect on the insulating power being detrimental. Moreover. thc presence of extraneous matter in the cork') will al so affect in negati ve sense its behaviour as a therm al insulating materi al.

The particle size may act in opposite direc ti()n ~ (lJl the insulating power of cork . When it is sillali. the contact surface between particles is hi gher and th i" makes easy heat transmiss ion by conduction. However, heat transmission by convection. the so­called mi cro-convection, IS un-favoured in comparison to the particles wit h a large . ize. Thi ~ i~

so because in the fine parti cles their contact is Illorc perfect and this mitigates the formati on of air ho ll ow~.

which will have a lower facility of cycl ica l movemcnt in the interstiti al space in being between the parti c l c~

and hence of heat transmi ssion. On the other hand. the number of the typi cal lenticels (i.e., pores in being in the suberous bark ~\I1d that through .vh ich the cork trec takes up the oxygen needed to its breathing)lo pre<,cJlt in the cork parti cles increases wi th increasing parti clc size. Their wall s may covered with fore ign 1TI (lltcr (i. e., tiny particles of dust and tanning/' which al :,(l has a negative influcnce on lhe insulating ca paci ty of cork si nce it favours hem transmiss ion by conduction .

Of the above mentioned fac tors. whi ch influencc the insulating power of cork, the gra in purity i ... unavoidably of hard control. The other ones as a la 't

420 INDIAN J. ENG. MATER. SCI., OCTOBER 2003

resort depend on the particl e size. For example, the moisture content of cork will be higher for smaller sizes because of the large r ex ternal surface (i.e., this is so assuming a simil ar development of the porosi ty­internal surface- in the cork particles) that is available for the adsorpt ion of moisture.

Although black agglomerate maintains the own properti es of cork as insulating material, its efficacy as such depends on the purity of the raw material and on its parti cle size, extent of the cured process, dimensional uniformity of the product and densi ty . The norms in force establish fixed limits for these parameters in order to sa feguard the insulating power of thi s cork agglomerate. In additi on it is convenient, and so metimes necessary, to protect the insul ating power of the agglomerate against the ex ternal agents that are prone to cause its decrease. One such agent is moisture that , in spite of the low capacity of water adsorption of the material , has a marked importance in relation to the insulating behaviour of the agglomerate by virtue of the low specific weight of cork. For that pu rpose, a common practice is to use surface aspha lt coatings for the agglomerate pl ates (screens against steam), the aim of which is to prevent the product from its contact with moisture. In accordance with the large number of factors that may influence the thermal conductivi ty of the black agglomerate, the conditions used in its determination must be specified together with the reported value of conductivity.

From the above facts it becomes apparent that the method of preparation of cork agglomerates is an important factor in connection with the insulating propert ies of the resultant products. This study is aimed at investigating the behaviour as insulating materials of black and composite agglomerates of cork, a feasible use of which is the covering of ceilings and walls. With such an aim, the preparation of both kinds of agglomerates was undertaken first and thei r thermal conductivity was measured then . T he object of this work was further to compare the thermal insulating behaviour of both agglomeration products of cork.

Experimental Procedure Black agglomerate (BA) and composite

agglomerate (CA) were prepared from black regranulate (BR) of cork. This raw material is made lip of low-quality and residual product, which so far has no profitable application. In the preparation of the agglomerates, the as-received pieces of BR were first size- reduced and the resultant particles were then

agglomerated. The binding agent was suberin (i.e .. the own natural resin of cork) for BA and a polyurethanc resin (MC-7600- D, furni shed by Synhes ia ESpclllo la S.A.) fo r CA. This resi n was used ow ing to it s compatibility towards cork and to the presence in the binder of a small frac tion of free isocyanate group ... that favour the cured process of the agglomerate II. Details of the agg lomeration methods were g iven in a previous report l 2

. The specimen thickness was controlled for BA (Table I). In the case of CA. the particl e size and the resin dosage were varied (Table 2) . Three average particle sizes (mm) were used: 7.1, A; 4.2, B ; 1.4, C.

The thermal conductivity of I3R , BA, and CA was measured using a Ketherm QTM-D3 fas t meter. fitted a temperature probe QTM PA I. Thi method is based on the "princip le of the transitory hot wire". Its main advantages are that the restricti ons to the ex act specimen dimensi ons o nl y concern the minimum values and that it enables to ca lTY out conducti vity measurements for materi als in a granu late or powder form.

Measurements of the therma l conducti vity were effected after selecti ng the suitabl e elec trical heat in g. With such a purpose, once the proper working o f the eq uipment was veri fied in a prev ious operati on. the tested specimen was oven-dried until mass constancy was reached. After that, the temperature probe contacted with the specimen just in the measurement pl ace. The contact was mai ntained for 24 h to stabi li se the basis temperature of the probe and then the conductivity was measured. In view of the resu lts obtained, 0.5 A was chosen as the heater current. Subsequently, all thermal cond ucti vi ty measuremen ts were caITied out at room temperature

Table I - Dimensional characteri stics for the spec imens of black agg lomerate (BA) of corl. .

Dimensions. C ill Specimen

Length Wid th Thic kn~ss

BA-I 30.5 30.5 .:'i.0 BA-2 30.5 30.5 ) .0

BA-3 30.5 30.5 2.5 BA-4 30.5 30.5 2.5

Table 2 - Dosage and particle size for rhe spec i mens of composite agglomerate (CA)

Specimen Regranulate Resi n/cork dosage

CA-I A 80/ 100 CA-2 B 801100 CA-3 C 801100 CA-4 B 1201100 CA-5 B 150/100

DIAZ-DfEZ el al. : CORK AGGLOMERATES AS THERMAL INSULATING MATERIALS 42 1

Results and Discussion

Black regranulate of cork The values of the average thermal conductivity

(/\.01) for the three specimens of BR with a different particle size are given in Table 3. It is seen that Am decreases noticeably with decreasing particle size. These results are compatible with the fact that the presence of lenticels in the cork particles is more probable with increasing size. Thus, as stated above, on the walls of the lenticels is usually present foreign matter that for BR should be thermally conductor so that heat transmi ss ion is favoured. It is in line with the previously reported values of the bulk density for A and B (i.e., 64 and 63 kg m-3, respectively)ll . Thus, in spite of the significantly larger particle size of A, the density was a little higher for this regranulate specimen, which indicates that extraneous particles of high specific weight were found in the lenticels of BR. In view of these results it is likely that the lenticel effect on the uptake of foreign matter and on the associated increase in the thermal conductivity, may be because of a large number and size of the lenticels, is stronger than the surface effects both on the adsorption of moi sture and on the direct heat transmiss ion due to the contact between the BR particles. The latter effect may be balanced by the micro-convection. From the values of Am obtained for the BR specimens it is clear that for any possible application of BR as an insulating material its behaviour can be made better if one uses smaller particle sizes.

Black agglomerate of cork

Table 4 lists the values of Am for the tested specimens of BA . From these Am data it follows that the specimen thickness appreciably influences the thermal conductivity of BA . Thus, Am is lower for the specimens with a larger thickness. This behaviour was

Table 3 - Val ues of Am for the BR specimens

Reg ranulate Partic le size, mm

A 7.1 B 4.2 C 1.4

0.0447 0.0428 0.0371

Table 4 - Va lues of Am fo r the BA spec imens

Specimen

BA- I BA-2 BA-3 BA-4

0.0355 0.0358 0.0374 0.0391

expected since the thermal conductivity and the thickness are two inversely proportional quantiti es. In fact, a more marked gradient of heat flow shou ld be established inside the specimens with increasing specimen thickness. The mean values of Am are 0.03565 kcal mol hoi °e' for BA-l and BA-2 , 0 .03825 kcal mol hoi °e' for BA-3 and BAA, and 0.03695 kcal mol hoi °e' for the whole series of BA specimens.

The testing method used in the present study only enabled us to obtain the Am value at the mean temperature of the measurement probe (Tm). In o rder to report the A values at three temperatures. as recommended by a standard norm (UNE 56-037-74). an approximative method was used . It is based on the application of the following expression , which was given by Andrades '3:

A = 0.00015T m+ 0.03133 ... ( I )

According to thi s expression, the A increase corresponding to the rise in Tm of 1 °C is 0.00015 kcal mol hoi Del. The resultant A values for the four BA

specimens at the three T m values (Table 5) show that A increases with increasing temperature. The mean values of A for the tested specimens at a given Till value are 0.0332 kcal mol hoi °e' at 0 DC, 0.0362 kcal mol hoi °e' at 20°C, and 0.0392 kcal mol hoi °e' at 40 dc.

In the aforementioned reference norm is also established that A must be below 0.034 kcal m-I h-I °e l

at 0 DC, 0.036 kcal mol hoi °e' at 20°C, and 0.038 kcal mol h-I °e' at 40°C. Therefore, only for 0 °C the

calculated mean value of A falls under the demanded Ie limit. For 20 and 40°C, however, the A values are higher than those specified by the norm. The dev iation produced in the A values is attributable to the large number of factors (i.e., density, moi sture and quality) that may influence the thermal conductivity for BA. Moreover, the method of Andrades was tested under ideal conditions of density and moisture, which arc different from those under which the A values reported here were obtained. On the other hand, the presence of carbonised particles and of other impurities in BA mu st

Table 5- Vari a'ion o f?-. with Trn for the BA s pec illlc n ~

Specimen Am. kca lm-I h-I °C l

O°C 20°C 4()OC

BA-I 0.03 18 0.0348 0.0378 BA-2 0.0322 0.0352 0.0382 BA-3 0.0337 0.0367 0.0397 BA-4 0.0352 0.0382 0_04 12

422 INDIAN J. ENG. MATER. SCI., OCTOBER 2003

lower its quality and hence have a detrimental effect on the insulating power of the materi al. It should also be noted that for the three 7~1l values A, as All I is lower for BA-I and BA-2 than for BA-3 and BA-4 Cfable 5). Therefore, A decreases with increasing specimen th ickness. Although the variations of A are practically or the same magnitude as those that occur for the specimens with the same thickness. This shows clearly that \lot on ly the specimen thickness but also other factors inf"luenced the thermal conductivity of BA.

Composite agglomerate of cork

The va lues of Am for the CA specimens are li sted in Table 6. Across the series CA-I, CA-2, and CA-3, Am first decreases and then increases. Accordingly, it see ms that for these CA specimens, which were prepared by varying the particle size, two factors at least that acted in opposi te direction, influenced Am. One of them is likely the presence of lenticels, which should be greater for CA-l as the particle size of BR used in its preparation was the largest one. Moreover, the relative increase produced in Am for CA-3 is attributable the particle size. Probably, owing to a closer contact between the BR particles, heat transmission by conduction was favoured for such a CA spec imen, as stated above. The fact that this behaviour is shown by CA unlike by BR can be accounted for in terms of a differen t separation di stance between the BR particles in both material s. For CA it is poss ible that by binding effect the particle~ of BR can come closer together (i.e., it would be dealt with a more or less forced approximation of the particles by the binder action) than when they are free from binder in BR. As a result, the contact between the particles would be better for CA than for BR.

For the series CA-2, CA-4, and CA-5, Am first increase and then decreases (i .e., just the opposite variation to that noted for CA-I, CA-2, and CA-3). Therefore, the resin dosage has also a marked influence on Am. Tn the case of CA-2, CA-4" and CA­S, the particle size used in their preparation was the intermediate one (i.e., 4.2 mm, B), and hence the

Table 6-Values of A.m for the CA specimens.

Specimen

CA-I CA-2 CA-3 CA-4 CA-5

0.0427 0.0383 0.0413 0.0451 0.0405

effect of the presence of lenticels in the BR panlck, will be less significant than for the largest pani ck size. Then , the relati ve decrease produced i \l I'm for CA-2 as compared to CA-4 may be merely associ;llCci with the hi gher resin dosage used in th preparation pi thi s specimen. As a result, the binding between the BR particles would be more effecti ve and thi s would propitiated a closer contact between Ih e particle" and heat transmiss ion for CA-4. The behav iour ex hibil l'c1 by CA-4 and CA-5 suggests however that. \",hen thl' resin dosage was too hi gh in the agg lomeration medium, the prese nce of a larger am()unt of hinci er between the BR particles in the res ultant procill ct~

somehow to a certain extent caused their sep~rati()n. then being unfavourabl e the effect 0 11 their COllt;lct and on heat transmission .

A Am bar-graph for the specimcns of S R ~ncl CA i~ shown by Fig . I. It follows that for a largcr number () f CA specimens, Am is noticeably lower th an 1'01' tI H)~l'

of SR. The opposite applies only to CA-3 and CA--L

0.06,.--.-~---""'-~--.-------r-~--.------'

0.05

0.04

J-' :.c

0.03 'E ro u 6 0.02 ""E

0.01

AfCA-1

00-428 c---

0383

'R« / ~~~

00..1l ~

o~ !~~~_

8/CA-2 C/CA-3

I: I B~ c:=JCA

8/CA-5

Fig. I-Compari son o f the thermal insul at ing behaviour (li' HR and CA

0.0451

BA-l CA-2 BA-4 CA-4

Fig. 2-Comparison of the thermal insul ating behaviour 01' IlA and CA

DIAZ-DlEZ el al.: CORK AGGLOMERATES AS THERMAL INSULATING MATERIALS 423

Accordingly , the agglomeration of BR had a beneficial effect on the insulating behaviour of the material, except for when using the smallest particle size of BR and the intermediate resin dosage in the preparation of CA. Nevertheless, the average values of Am are 0.04153 kcal m-I h-Io C l for BR and 0.04158 kcal m-lh-loC I for CA . Therefore, regardless of the particle size for BR and of the method used in the preparation of CA, both cork materials exhibit a very similar insulating behaviour. This is quite an int erest ing finding as it means that BR can be reclaimed and used again as an agglomerate product of cork without signi ficant detrimental changes in the insulating properties of the material occur.

Comparison of both agglomeration products of cork

Fig. 2 compares the insulating behaviour of SA and CA. The selected specimens have been the two couples

for which Am presents the lowest and highest values (i.e., SA-l and CA-2, on the one hand, and BAA and CA-4, on the other hand)_ It is infen'ed that, clearly, BA behaves as a better insulating material than CA. The mean values of Am are 0.03695 kcalm-I h-I Del for BA

and 0.04158 kca l n,-I h-I Del for CA. Therefore, Am is significantly lower for SA than for CA.

Conclusions From the above results, the following conclusions

may be drawn: The thermal conducti vi ty depends on the particle

size of SR, the speci men thickness of SA, and the partic le size of S R and the resin dosage for CA.

The thermal conducti vity of BR decreases noticeably with decreasing particle size, which ha been connected with the less probable presence of lenticels contai ning conductor matter for a smaller particle size.

As far as SA is concemed, the conductivity is lower for the larger thickness specimens. Furthermore, it decreases with decreasing temperature.

The thermal conductivity of CA fi rst decreases with decreasing particle size of SR and then increases. The oppos ite variation of the conducti vi ty is noted with increasing resin dosage in the preparation of CA. Accordingly , the agglomeration of BR to yield CA, enables one to improve the insu lati ng behaviour of the mate ri al provided th at both the particle size and the resin dosage are con trolled . SA behaves as a better insulating mate ri al than CA.

Acknowledgements The authors wish to express their thanks to the

lnstituto del Corcho, la Madera y e l Carbon ( ICMC ) and to Corcho Merida for the availabi lity of their laboratories and material means when carry ing OLit the experimental work of this study.

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