Materials and Design 49 (2013) 842–849

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Silkworm cocoon as natural material and structure for thermal insulation

J. Zhang a, R. Rajkhowa a, J.L. Li a, X.Y. Liu b, X.G. Wang a,c,⇑a Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, VIC 3217, Australia b Biophysics and Micro/Nanostructures Lab, Department of Physics, Faculty of Science, National University of Singapore 117542, Singapore c School of Textile Science and Engineering, Wuhan Textile University, Wuhan 430073, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 December 2012 Accepted 3 February 2013 Available online 20 February 2013

Keywords:Biological composite Silk cocoon Thermal insulation Protection

0261-3069/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2013.02.006

⇑ Corresponding author at: Australian Future FibrCentre, Institute for Frontier Materials, Deakin Univer+61 3 52272894; fax. +61 3 52272539.

E-mail address: xwang@deakin.edu.au (X.G. Wang

Silkworm cocoons are important biological materials that protect silkworms from environmental threat and predator attacks. Silkw orm cocoons are able to provide significant buffer against temperature changes outside of the cocoon structure. We present our investigation of the thermal insulation proper- ties of both domestic and wild silkworm cocoons under warm conditions. Wild cocoons show stronger thermal buffer function over the domestic cocoon types. Both the cocoon walls and the volume of inner cocoon space contribute to the thermal damping behaviour of cocoons. Wild silkworm cocoons also have lower thermal diffusivity than domestic ones. Calcium oxalate crystals affects the thermal behaviour of wild silkworm cocoons, by trapping still air inside the cocoon structure and enhancing the thermal sta- bility of the cocoon assembly. The research findings are of relevance to the bio-inspired design of new thermo-reg ulating materials and structures.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Silkworm fibres are outstanding natural materials that have long been used for textiles. Silk-based materials have recently found applicati ons in a growing number of areas including biode- gradable medical scaffolds, implantable functional devices and tis- sue products. While the properties of silk fibres and silk proteins have been extensively studied [1–3], there are substanti al knowl- edge gaps in understanding how a wild silkworm cocoon provides the essential survival utility for the pupa residing inside. In the wild environment, a thin and light-weight cocoon can protect the silkworm from predator attacks and extreme weather conditions,while supporting its metaboli c activity. Understanding the rela- tions between structure, property and function of this important biological material will provide a conceptu al platform to design and develop novel bio-inspired textiles for thermal regulation applications . Thermal properties of fabrics and clothing are increasingly studied aiming to provide better comfort and safety [4,5]; this work will be of particular importance to future design of thermal textiles.

The primary function of the clothing assembly is to maintain hu- man body in an acceptable physiological state with respect to ther- mal balance, core and skin temperat ure, and perspiration under all types of environm ental conditions and for all degrees of body activ-

ll rights reserved.

es Research and Innovation sity, VIC 3217, Australia. Tel.:

).

ities [6]. Thermal comfort is the most essential factor in maintain ing the health and satisfaction of the wearer [7]. Human body itself reg- ulates temperature by internal metabolism, blood flow, physical activities or involuntary muscle contraction in shivering and cloth- ing fabrics act as barriers or buffers for the free heat exchange be- tween the wearer and environment [8,9]. Similarly, silkworm cocoons would provide thermal regulatio n function to extend the surviving chance of silkworms . The cocoon is a multilayer composite material formed by continuous twin silk filaments bonded by silk gum. The process of silk spinning and cocoon building has evolved over thousands of years through natural selection [10,11]. As a fineproduct of these fully developed processes, silkworm cocoon pro- vides strong protection against both natural predators, environm en- tal and physical adversaries, including extreme environmental temperat ure, during the immobile phase of life cycle when the silk- worm enters diapause and metamor phosis [12,13]. Compared with domestic species, wild cocoons reared in an open environment re- quire much higher level of protectio n.

Limited research has recently been conducte d to study the silk- worm cocoon-related propertie s and functions. For example, Zhao et al. investigated the mechanical propertie s of Bombyx mori co-coons and found that the elastic modulus, strength and thermo- mechanical parameters of the cocoon vary along its thickness direction in a manner to yield enhanced ability to resist possible physical attacks from outside [14,15]; Chen et al. recently studied a range of silkworm cocoons to correlate mechanical propertie ssuch as tensile and compress ive performance to their structure and morphologies. This work showed that the architectural arrangem ent of the cocoon is far more important than the material

(a) (b) (c) (d)

Fig. 1. Morphologies of silkworm cocoons: (a) B. mori , (b) S. cynthia , (c) A. pernyi , and (d) A. mylitta cocoons. The outer and inner surface morphologies of each cocoon type are shown in the second row and the third row, respectively.

Table 1Geometrical param eters of silkworm cocoons.

Cocoon type Longer diameter of cocoon (mm)

Longer diameter of compact cocoon shell (mm)

Shorter diameter of cocoon

Shorter diameter of compact cocoon shell

B. mori 31.57 ± 0.19 – 19.01 ± 0.17 – – –S. cynthia 49.15 ± 3.31 31.79 ± 1.44 14.07 ± 1.24 (24.01 ± 0.49)a 12.43 ± 0.40 (17.63 ± 0.37)a

A. pernyi 47.21 ± 0.27 – 24.55 ± 0.28 – –A. mylitta 46.55 ± 0.29 – 30.78 ± 0.57 – – –

a The cross section of the S. cynthia cocoon was irregular, so both the longest diameter and the shortest diameter were measured.

J. Zhang et al. / Materials and Design 49 (2013) 842–849 843

properties of silk fibres themselves to cope with different local environment conditions [16,17]; Roy et al. observed preferential gating of carbon dioxide from the cocoon inside to outside to im- part a conducive environm ent for the survival of the pupa [13].Aiming at learning from nature to design new bio-inspir ed ther- mo-regulating materials, this work is focused on the thermal insu- lation properties of silkworm cocoons. We have studied four silkworm cocoon types: the mulberry (from domestic B. mori ), eri (from domestic Samia cynthia ) and tasar (from wild Antheraea per- nyi and Antheraea mylitta ) silkworm cocoons. Abbrevia ted names of B. mori , S. cynthia , A. pernyi and A. mylitta , respectively , are used in this work, respectively.

2. Experimental details

2.1. Materials

B. mori and S. cynthia cocoons were purchased from silk rearing houses in Northeast India; A. pernyi cocoons were collected from Northern China and A. mylitta cocoons were collected from Central India. They were received as stifled cocoons, commonly used prior to reeling silk filament for textile applicati ons. Calcium oxalate crystal powder was purchase d from Sigma Aldrich and used as received.

2.2. Methods

2.2.1. Scanning electron microscopy (SEM)The cocoon walls were cut into square specimens with the

dimension of 5 mm � 5 mm, which were then attached to

conductive carbon tape on aluminium stubs. The specimens were observed by a Supra 55 VP scanning electron microscop e after sputter coated with gold. Both the cocoon inner and outer surfaces were investigated. The single silk fibre diameter was calculated from 50 measureme nts on SEM micrographs using image analysis software (Image J). The average single fibre diameter was deter- mined from the histogram graphs of size distribut ion.

2.2.2. Temperature monitoring Tempera ture was measured both inside and outside of the co-

coons (3–5 mm close to the outer surface) using two needle-type temperat ure probes (ICT SFM). Each temperature probe is 1.3 mm in diameter and 35 mm in length, with two sensors located 15 mm apart (the first sensor is 7 mm from the needle tip). For each measureme nt, one temperature probe was placed into the co- coon through the proximal end from which the moth usually es- capes and the other probe was attached onto the outer surface of the cocoon by a small piece of tape, with sensors deliberately uncovere d. Four types of thermal conditions (I–IV) were tested.For condition I, a gradual rise of temperature was introduced by placing the cocoons into an oven (Binder) with isothermal temper- ature setting (at 37 �C, 45 �C and 50 �C, respectivel y); for condition II, a sudden drop of temperature was introduce d by transferring the warmed cocoons that have nearly reached equilibriu m from the oven to air. The other two thermal conditions were applied by heating a tube furnace (Lindberg/BlueM) with two different ramp rates (2 �C/min for thermal condition III and 4 �C/min for thermal condition IV) from ambient temperature to 50 �C, respec- tively, while the cocoons were located inside. The temperat ure profiles were established based on the average data from

Table 2Geometrical parameter s and nomina l density of silkworm cocoon walls.

Cocoon type

Thickness of cocoon wall (lm)

Nominal density of cocoon wall (kg/m3)

Single fibre width on outer surface (lm)

Single fibre width on inner surface (lm)

B. mori 393 ± 21 377 ± 15 12.64 ± 1.61 8.98 ± 0.72 S. cynthia 243 ± 31 449 ± 56 16.29 ± 1.63 14.93 ± 3.60 A. pernyi 387 ± 31 711 ± 44 35.68 ± 3.36 29.07 ± 4.19 A. mylitta 260 ± 26 693 ± 54 34.63 ± 5.03 31.97 ± 4.90

(a) (b) (c)

Fig. 2. Calcium oxalate crystals on the outer surface of cocoons: (a) S. cynthia , (b) A. pernyi , and (c) A. mylitta .

844 J. Zhang et al. / Materials and Design 49 (2013) 842–849

temperature readings (every second) recorded from two sensors (of each thermal probe) located inside and outside of cocoons,respectively .

2.2.3. Modulated temperature differential scanning calorimet ry (DSC)The thermal conductivity of four types of cocoon walls was ob-

tained using TA Q200 DSC through the modulated DSC method in accordance with the procedure stated in ASTM: E 1952-11. A de- tailed illustration of the principle can be found in ref [18]. Temper- ature was modulated with an amplitud e of ±1 �C and a period of 60 s. Nitrogen gas flowed at a rate of 50 mL/min while the average test temperat ure and heat capacity were recorded. Polystyrene discs were used for calibration [19]. Firstly, the observed thermal conductivity (k0) was determined by:

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k0 ¼ ð8LC2Þ=ðCpmd2PÞ ð1Þ

where L is the specimen thickne ss, C is the apparen t heat capacity,Cp is the specific heat capacity, m is the mass and d is the diamete rof the tested thick specimen. Heat capacity is the amount of heat re- quired to change the temperat ure of a substanc e by a given amount.The specific heat capacity, Cp, is the heat capacity per unit mass of amater ial and the apparent heat capacity, C, is the measured speci- men heat capacity, which is dependent on both the sample mass and the modulatio n freque ncy of modu lated DSC. The therma l con- ductivit y calibration constant (D) was then calculate d using follow- ing equation:

D ¼ ðk0krÞ1=2 � kr ð2Þ

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J. Zhang et al. / Materials and Design 49 (2013) 842–849 845

where kr is the reference therma l conduct ivity. Finally, the therma lconducti vity of the test specimen was determine d using following equation:

k ¼ k0 � 2Dþ ðk20 � 4Dk0Þ

12

h i.2 ð3Þ

The observed thermal conductivity, k0, is the measured thermal conductivity of the specimen without calibration; the specimen thermal conductivi ty, k, is the calibrate d thermal conductivity.The calibration constant, D, is a factor which is applied to correct the effect of the heat loss through the sides of the tested specimen when the modulated DSC method is used. Based on the measured thermal conductivity values for cocoon walls, three other thermal property parameters were determined for comparison between different cocoon types. The thermal diffusivity (a) of the cocoon walls was defined by the relationship:

a ¼ k=ðqCpÞ ð4Þ

where q is the density of cocoon wall. Thermal resistance (R) was then determine d by:

R ¼ L=k ð5Þ

The thermal absorptivi ty (b) was expresse d as:

b ¼ffiffiffiffiffiffiffiffiffiffiffikqCp

qð6Þ

3. Results and discussion

3.1. Morpholo gies of silkworm cocoons

The morphologies of four types of silkworm cocoons are shown in Fig. 1. The silkworm cocoon wall is a multi-layer composite structure that can be considered as a porous matrix of sericin rein- forced with randomly oriented continuous fibroin fibres [15]. In comparis on with the other three cocoon types, S. cynthia has a dis- tinctive cocoon structure where a compact inner cocoon wall is surrounded by loosely formed outer fibrous structure. The cocoon geometri cal parameters are presented in Table 1. The domestic co- coons B. mori and S. cynthia (Fig. 1a and b) are smaller than the wild cocoons A. pernyi and A. mylitta (Fig. 1c and d). Based on these mea- suremen ts, the volume of the inner space surrounded by different cocoons was calculated assuming the short diameter of the cocoon equals to the diameter of the half spheres at both ends. The volume of inner space is 7 cm 3, 3 cm 3, 18 cm 3 and 27 cm 3 for B. mori , S. cyn- thia, A. pernyi , and A. mylitta cocoons, respectively . The inner space

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846 J. Zhang et al. / Materials and Design 49 (2013) 842–849

volume has been shown to play an important role in affecting co- coon’s thermal function, ascribed from the air space and the mass of silkworm inside, in addition to the cocoon wall’s contribution [20].

From the scanning electron microscopy (SEM) images shown in Fig. 1, it can be observed that silkworm cocoon has a non-woven structure with twin silk fibroin fibres coated with sericin. The co- coon outer surface (the outmost layer) and inner surface (the inner most layer) are remarkably different in terms of the silk fibre mor- phology, fibre width and the porous structure created by silk fibres.The fibre width is generally smaller for the cocoon inner surfaces than the outer surfaces. The geometrical parameters and density of cocoon walls were obtained based on three measure ments and are summarised in Table 2. The silk fibres are significantly (100–300%) larger in size and the cocoon structure is more compact for wild cocoons (A. pernyi and A. mylitta ) than the domestic ones (B. mori and S. cynthia ). The nominal density of the cocoon walls was obtained by dividing the mass of a 10 mm � 10 mm piece by the volume (where the thickness was measure d with a vernier cal- iper). The nominal density is 711 kg/m 3 for the A. pernyi cocoonwalls and 693 kg/m 3 for the A. mylitta cocoon walls, which are also much higher than the 377 kg/m 3 for the B. mori cocoon walls and 450 kg/m 3 for the S. cynthia cocoon walls. On the outer surfaces of S. cynthia , A. pernyi and A. mylitta cocoons, cubic crystals can be clearly visualised; they deposit on the surface of silk fibresand stack in the pores (Fig. 2). These crystals are calcium oxalates [13,17] and have unique function for preferential gating of CO 2from cocoon inside to outside [13]. There are no crystals on the outer surface of the B. mori cocoon.

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Fig. 7. The maximal difference in heating rate for sensor locations inside and outside of cocoons, under thermal conditions III and IV. Four types of cocoons are:(1) B. mori , (2) S. cynthia , (3) A. pernyi , and (4) A. mylitta .

3.2. Thermal insulation of cocoons under steady state conditions

Figs. 3 and 4 show the temperat ure profiles of locations inside and outside of cocoons (3–5 mm distance from cocoon outer sur- face) when cocoons were moved into an isothermal oven and then

moved out to air (under thermal condition I, II). The temperat ure change inside of the cocoon was slower than the outside when asudden temperat ure change occurred in the environment, which indicates a certain degree of temperature buffer from these co- coons. The maximal temperature lag caused by cocoon insulation was calculated and shown in Fig. 5. The wild cocoons (A. pernyi and A. mylitta ) exhibited stronger thermal damping than the domestic ones (B. mori and S. cynthia ). For example, the tempera- ture lag of the A. mylitta cocoon was 400% higher than the B. mori cocoon (Fig. 4 and 5), for the case when cocoons were heated in the oven with an isothermal setting of 45 �C. It also took a considerabl ylonger time for the wild cocoons to reach an equilibrium tempera- ture than the domestic cocoons. For example, after 23 min, the temperat ure inside of the A. pernyi cocoon remained 2 �C lower than the outside in a 45 �C oven (Fig. 4c), in contrast to the equal

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J. Zhang et al. / Materials and Design 49 (2013) 842–849 847

temperature between inside and outside of the B. mori cocoon after 14 mins’ heating (Fig. 4a).

The wild cocoons exhibited better thermal damping than domesticate d ones at higher oven temperature s (45 �C, 50 �C).With the use of 45 �C oven, the A. pernyi and A. mylitta cocoonsdemonstrat ed the best thermal buffer capability, which might be related to the natural habitat conditions under which the silk- worms are required to defence against. Significantly, when the 45 �C oven was used, the inside temperat ure of the A. pernyi andA. mylitta cocoons reached 10 �C and 12 �C lower than the outside during heating, and 15 �C and 17 �C higher than the outside during cooling, respectively .

3.3. Thermal insulation of cocoons under non-steady state conditions

Fig. 6 shows the temperature profiles of cocoons when the host tube furnace was heated at two ramp rates: 2 �C/min and 4 �C/min(thermal condition III, IV). The temperature of the inner space of cocoons increased at a slower rate than the outside environment.The higher the furnace heating rate, the larger the difference in the temperature ramp rates of the inner space and outside environ- ment, except for the S. cynthia cocoon. The S. cynthia cocoon has the smallest volume of inner space as noted in Section 2.1 (only 3 cm 3),therefore the small area of the cocoon walls and the limited vol- ume of air entrapped inside might have limited thermal buffer capability under the extreme temperature changing condition (ata heating rate of 4 �C/min). Fig. 7 presents the maximal heating rate differenc e for different types of cocoons. At the furnace heat- ing rate of 2 �C/min, the ramp rate difference was 0.22 �C/min,0.22 �C/min, 0.78 �C/min and 0.99 �C/min for the B. mori , S. cynthia ,

A. pernyi and A. mylitta cocoons, respectivel y. A remarkable case was that the A. mylitta cocoon demonstrated the highest thermal damping. While the outside temperature of cocoon increased at aramp rate of 6.9 �C/min, the temperature of its inner space merely increased at a ramp rate of 4.7 �C/min, which was 31% slower than the temperature change of the outside environment. At the slower furnace heating rate (2 �C/min), the A. pernyi and A. mylitta cocoonsexhibited relatively greater thermal buffer capacity over the domestic cocoons. However, at the faster furnace heating rate (4 �C/min), the advantage was only maintain ed by the A. mylitta co-coon, which has the largest volume of inner space (27 cm 3).

In nature, the main function of insect cocoons is to provide pro- tection for the pupa occupant to survive against environm ental damage or enemy attacks. The above experimental results showed that while there was a tendency for the inside temperat ure of co- coons to approach the temperature of the surroundings , the inside temperat ure did not change instantaneously when cocoons were exposed to a sudden temperature change. Fluctuations in cocoon temperat ure has critical effect on the developmen t and survival of insect pupae (e.g. spiderlin gs [21]), and it is exceedingly neces- sary for them to have cocoons to regulate temperature . Further- more, insects were observed to place their cocoons in modifiedthermal environment [22] (e.g. in protected microclimate) to les- sen the temperature influence, in addition to the cocoon’s intrinsic insulation function. The cocoon, the air space it created, together with the pupa mass have demonst rated significant thermal damp- ing against short-term radiant loads in the field experiments [20].Cocoons with smaller pupa masses would have limited ability to manage extreme temperatures in the wild. Although the experi- mental condition s used in this work are not what the pupa would experience in real life, the results indicate significantly reduced change rate of temperature before reaching equilibriu m, which may inspire improved thermal regulation function in designing new protectiv e materials and structures. Previous studies have also revealed thermal-rel ated functions from other insect cocoons by absorbin g solar heat through dark pigments [12,23] or regulating temperat ure through thermophotov oltaic reactions in silk fibres[24,25].

3.4. Thermal insulation of deminera lised cocoon

To further study the influence of calcium oxalate crystals on the thermal insulation properties, the A. pernyi cocoon was deminera- lised by ultrasonicat ion method. The temperature profiles for loca- tions inside and outside of the demineral ised cocoon under both steady and non-steady state thermal condition s are shown in Fig. 8. When the oven temperature was set as 45 �C (Fig. 8a),although the cocoon maintained thermal insulation, the maximum temperat ure differenc e was 5 �C during heating and 8 �C during cooling. The thermal buffer provided by the demineralised cocoon reduced from the original A. pernyi cocoon, which had a maximum temperat ure lag of 10 �C during heating and 15 �C during cooling.The ramping rate difference between the inside and outside of the demineral ised cocoons also became smaller, when a ramping rate of 2 �C/min of the tube furnace was applied (Fig. 8b). This fur- ther indicates the effect of crystals on the thermal insulation prop- erty of cocoons.

3.5. Thermal properties of silkworm cocoon walls

Silkworm cocoon walls contribute to the thermal insulation of cocoons. To further understand the thermal properties of cocoon walls, temperature modulated different ial scanning calorimetry (DSC) was used to measure the thermal conductivity, thermal resistance, thermal diffusivity and thermal absorptivi ty at the tem- perature of 7 �C, 20 �C, 27 �C, 47 �C, 67 �C and 87 �C. The obtained

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fusi

vity

(m

2 s-1)

temperature (ºC) temperature (ºC)

temperature (ºC)temperature (ºC)

(a)

(b)

(d)(c)

Fig. 9. Thermal properties of silkworm cocoon walls: (a) thermal conductivity, (b) thermal resistance, (c) thermal diffusivity, and (d) thermal absorptivity.

848 J. Zhang et al. / Materials and Design 49 (2013) 842–849

thermal property data are shown in Fig. 8. Thermal conductivity is the materials’ ability to conduct heat. Dry heat transfer through insulation fabrics may involve the conduction in the solid phase constituting the insulation, the radiation in the material and the heat transfer in the air confined in the insulation [26]. Because the complexity of the fibre orientation, fibre construction, porosity and bulk density, the thermal conductivity of textiles can hardly be predicted and only be measure d directly. As shown in Fig. 9a, the thermal conductivity values of cocoon walls are within the range from 0.0106 Wm �1 K�1 to 0.0653 Wm �1 K�1; the calcium oxalate crystal pellet was also measured and its thermal conductivity is higher than 0.2 Wm �1 K�1, which can be as high as 27 times of that of the cocoon walls.

Thermal resistance is directly related to thermal insulation. In Fig. 9b, it can be seen that the B. mori cocoon wall had the highest thermal resistance thus the best thermal insulation property. Ther- mal resistance is directly proportional to the sample thickness and inversely proportional to thermal conductivi ty. Since the thermal conductivity of cocoon walls is not significantly different from each other, the thickness becomes the dominant factor governing ther- mal insulation. Because natural fibres usually have only around one tenth of the thermal resistance of still air, thermal insulation of textiles is caused primarily by the still air captured within the fibrous structure. Furthermore, when two textiles are of equal thickness, the one with lower density has the better insulation.Similarly here, the thickest B. mori cocoon, which also has the low- est density (Table 2), showed the highest thermal resistance under the experimental conditions.

It is worthwhile pointing out that wind is a critical factor that can alter thermal insulation, in particular, in natural environment where wild silkworms survive. The cocoons that have excellent insulation properties relying on still air trapped in the cocoon walls could lose the function easily under windy conditions. Wind can remove the relatively motionless insulating air on the outer surface and in the interior of the cocoon wall therefore impair its intrinsic insulation function. However, the cocoons with crystals on the out- er surfaces could lessen wind penetration and therefore effectively maintain thermal insulation under windy conditions. As men-

tioned previousl y, the S. cynthia , A. pernyi and A. mylitta cocoonshave cubic calcium oxalate crystals on the outer surfaces; these crystals may provide a certain degree of wind-proof to help main- tain the thermal insulation function. In fact, the crystals have shown a lower diffusion rate in comparison with the cocoons with- out crystals in a gas diffusion test [16]. The more compact fibrearchitectur e for the A. pernyi and A. mylitta cocoons will also help to prevent wind penetration and to maintain good thermal insula- tion in harsh environment.

Thermal diffusivity is the measure of thermal inertia. When asubstance has high thermal diffusivity, heat moves through the substance rapidly because it conducts heat quickly relative to its volumetr ic heat capacity and does not require much energy trans- fer to or from its surroundi ngs to reach thermal equilibrium [27].Fig. 9c shows the thermal diffusivity of cocoon walls. It is clear that the A. pernyi and A. mylitta cocoon walls had much lower thermal diffusivit y than the B. mori and S. cynthia cocoon walls before the temperat ure increased to 47 �C, mainly due to the higher density of their cocoon walls. This partially contributes to the lower tem- perature changing rate and the longer equilibriu m time experi- enced by the wild cocoons.

Thermal absorptivity is a surface structure-related property and provides an objective measure ment of the warm-cool feeling of textiles. If the thermal absorptivity is high, the textile fabric gives a cooler feeling at first contact with the skin [28]. Although this is not directly related to cocoon biological functions, it is interest- ing to observe that the cocoons walls of A. pernyi , A. mylitta and S.cynthia types exhibited higher thermal absorptivity values (asshown in Fig. 9d), which may correlate to the comparatively ‘‘super’’- thermal conductive crystals that deposited on the outer surfaces of these cocoons.

4. Conclusion s

The thermal insulation properties of both domestic (B. mori andS. cynthia ) and wild silkworm cocoons (A. pernyi and A. mylitta )were investigated. Under both steady and non-stead y state ther-

J. Zhang et al. / Materials and Design 49 (2013) 842–849 849

mal condition s, the inner temperature of silkworm cocoons showed significant thermal damping characterist ics against sud- den changes in outside temperature. However, the wild cocoons exhibited a higher level of thermal buffer than the domestic ones.Thermal property measureme nts were conducted on cocoon walls by a temperature modulated DSC method. The lower thermal diffu- sivity of wild cocoons partially contributes to the lower tempera- ture changing rate and longer equilibriu m time experienced by A.pernyi and A. mylitta cocoons. Calcium oxalate crystals affect the thermal behaviour of silkworm cocoons, by keeping the still air trapped inside the cocoon structure and enhancing the thermal stability of the cocoon assembly .

Acknowled gements

The authors would like to acknowled ge the funding support from the Australian Research Council (ARC) through a Project DP 120100139. The assistance from Ms. Jasjeet Kaur with the demin- eralisation of silk cocoon is appreciated.

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