Semi-rigid foams of calcium silicate (CaSiO 3 ) embedded in natural rubber latex

Abstract

Semi-rigid foams were prepared by NR latex with 0, 2, 5, 10, 20 and 30 phr CaSiO₃ added in a sulphur-curing system. The main composition of eggshell is made of 96.23%wt of CaCO₃ and 3.77%wt of other oxide compounds. The CaSiO₃ induced a spontaneous polarisation and reinforced the NR matrix due to ionic charge contents of Ca²⁺, Si⁴⁺ and O²⁻ in its structure. Furthermore, the CaSiO₃ was found to enhance the mechanical, thermal and physical properties of the composite foams. The suitable condition to prepare the semi-rigid composite foams was by adding 30 phr CaSiO₃ in the NR latex with the sulphur-curing system. The obtained bulk density, the relative composite foam density, the compressive strength, the contact angle with water, and acetone at the water to acetone ratio of 1:1 were found to be 1.2230 g cm⁻³, 0.3611, 588.10 kPa, 103.3°, 26.5° and 85.3°, respectively.

Keywords

Natural rubber latex Calcium silicate Adsorption Semi-rigid composite foams Cellular rubber Calcium carbonate Filler Mechanical properties

Introduction

Natural rubber (NR) latex is an important material which can be used to produce various products sponge products known as rubber foams for thermal insulation, structural uses and energy absorption; film products such as gloves and condoms; and specific foams used in medical and dental equipment.^{1, 2} Rubber foams can be prepared from NR latex or a dry solid rubber.³ The processing of rubber foams involves curing with vulcanising agents, foaming agents and other additives, namely filler, colour, oil and antioxidant. The specifications of sponge rubbers depend on the foam density, cell microstructure such as cell size, cell morphology, shapes of closed and open cell, mechanical and thermal properties. Rubber foam or cellular rubber can be produced to yield either open or closed cell structure.^1-3 There are many important ingredients which control the rubber foam properties: foaming agents such as potassium oleate; type and quantity of fillers such as calcium carbonate, silica, carbon black and other oxide compounds; and the vulcanisation process.⁴

Calcium silicate (CaSiO₃) is one kind of oxide compounds or inorganic materials which is considered to be a dielectric material having spontaneous polarisation and can be used as a good filler in the NR latex. CaSiO₃ can be made from eggshells.⁵ Eggs have been used primarily as foods, drugs, cosmetics and foodstuffs. However, eggshells are an egg product industry residue that contributes towards environmental pollution as it favours microbial actions disposed in landfills. The by-product eggshell represents approximately 11% of the total weight approximately 60 g of an egg.^{6, 7} Eggshell is one source of calcium carbonate (CaCO₃) known as calcite which is a stable polymorph at room temperature. An eggshell is also a rich source of mineral salts, mainly CaCO₃ (96.23%wt), calcium phosphate (1.19%wt), magnesium carbonate (1.12%wt) and organic substances (1.46%wt).^{6, 7} CaCO₃ can be used as a starting material to prepare dielectric materials (CaSiO₃, CaTiO₃, CaZrO₃, etc.), dental and bone implantation, catalysts, and also fillers used in many industries such as rubber, ink, foodstuff, cosmetic, pharmaceutical and fertilisation.^6-8 CaSiO₃ can be produced by various methods such as a conventional process, a sol–gel process, a chemical precipitation, a solid-state reaction and a microwave method.^{5, 8-11} For applications, CaSiO₃ can be used as a dielectric material, filler, catalyst and adsorbent when it is added into the NR latex to prepare the composite rubber foams.^{11, 12} The CaSiO₃ acts as a dispersed phase whereas the NR latex functions as a matrix phase. The advantages of adding CaSiO₃ into NR latex are to increase the physical, mechanical and thermal properties suitable for uses as adsorbent, catalyst and filters due to the presences of Ca²⁺, Si⁴⁺ and O²⁻ in its structure.

The objective of this work was to fabricate composite rubber foams from NR latex used as a matrix phase. The CaSiO₃ was produced from hen eggshells composing of CaCO₃ and SiO₂ at the ratio 1:1 via the sol–gel process to function as a dispersed phase. Potassium oleate was used as the foaming agent in the sulphur-curing process to prepare the composite foams suitable to be used as an adsorbent, filter or catalyst products. The obtained composite foams possessed good mechanical and thermal properties.

Experimental

Materials and methods

The NR latex, wingstay L (antioxidant), potassium oleate (surface-active agent and an emulsifier), sulphur, zinc oxide (ZnO), zinc diethyl dithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT), polyethylene glycol (PEG) and sodium silicofluoride (SSF) (vulcanisation agents) was supplied by the Rubber Research Institute of Thailand, the Ministry of Agriculture, Bangkok, Thailand.

CaSiO₃ preparation⁵ employed chicken eggshells as a starting material. Raw hen eggshells were washed with tap water until the egg white was completely removed and dried at room temperature. The raw and clean hen eggshells were broken into small pieces, crushed by a porcelain mortar and pestle, ground by a high speed mill with a porcelain ball mill for 10 min into a fine powder and then calcined at 900°C for 2 h to obtain CaO. The obtained CaO was mixed with SiO₂ at the CaO:SiO₂ ratio of 1:1 by the sol–gel process at room temperature.⁵

The NR latex foam preparation

The weights of raw materials used for the composite foams preparation, namely the NR latex, ZnO, potassium oleate, CaSiO₃, sulphur, ZDEC, ZMBT, wingstay L, PEG and SSF, having the unit of parts per hundred rubber (phr) are as tabulated in Table 1. The sponge rubber samples were prepared in six formulae depending on the amount of CaSiO₃ powder added: 0, 2, 5, 10, 20 and 30 phr. The raw materials of composite foams were mixed together using a mechanical rapid stirrer. First, the NR latex was mixed with the potassium oleate in order to remove ammonia gas. Second, the CaSiO₃ was added at various contents: 0, 2, 5, 10, 20 and 30 phr as tabulated in Table 1. Third, sulphur, ZDEC, ZMBT and wingstay L were added into the compounds. Fourth, PEG, ZnO and SSF were added into the compounds which were casted into the rectangular shape plaster moulds and then heated at 100°C for 2 h. The composite foams were removed from the plaster moulds and rinsed with warm water at 70°C for 5 min. Then, the composite foam samples were dried in an oven at 70°C until the composite foams were completely dried. The composite foams were then vulcanised in a sulphur-curing system at 100°C for 2 h.

Table 1

Formulae of latex foams

Chemical composition	Formula 1 (phr)	Formula 2 (phr)	Formula 3 (phr)	Formula 4 (phr)	Formula 5 (phr)	Formula 6 (phr)
60% Concentrate latex (high ammonia, HA)	100	100	100	100	100	100
10% Potassium oleate	1.5	1.5	1.5	1.5	1.5	1.5
CaSiO₃	0	2	5	10	20	30
50% Sulphur	2.0	2.0	2.0	2.0	2.0	2.0
50% ZDEC	1.0	1.0	1.0	1.0	1.0	1.0
50% ZMBT	1.0	1.0	1.0	1.0	1.0	1.0
50% Wingstay L	1.0	1.0	1.0	1.0	1.0	1.0
50% ZnO	5.0	5.0	5.0	5.0	5.0	5.0
33% PEG	0.67	0.67	0.67	0.67	0.67	0.67
12.5% SSF	1.0	1.0	1.0	1.0	1.0	1.0

ZDEC is zinc diethyl dithiocarbamate (C₁₀H₂₀N₂S₄Zn) acting as a fast primary or secondary accelerator in natural rubber latex in a white powder form having density at 20°C of 1.48 g cm⁻³.

ZMBT is zinc 2-mercaptobenzothiazole (C₁₄H₈N₂S₄Zn) used in latex vulcanisation having a light yellow powder form, density at 20°C of 1.65–1.72 g cm⁻³, hardly soluble in acetone, benzene, ethanol and carbon tetrachloride, insoluble in water and gasoline and decomposed in strong acid and alkaline.

PEG is polyethylene glycol (H–(O–CH₂–CH₂) _n –OH) which is soluble in water, methanol, ethanol, acetonitrile, benzene and dichloromethane, and insoluble in diethyl ether and hexane.

SSF is sodium silicofluoride (Na₂SiF₆) acting as an accelerator.

Wingstay L is an antioxidant for latex compounding.

Sulphur was used as a vulcanising agent dosage at 0.3–2.5 phr on a dry basis in the form of 50% dispersion in water.

ZnO was used to vulcanise natural latex products with higher translucency.

Potassium oleate functioned as a surface-active agent, emulsifier and foam promotor due to high ability to dissolve in strong alkali.

CaSiO₃ was used as an inorganic filler to reduce cost, to stiffen the products and to improve mechanical, thermal, physical, electrical properties.

Characterisation

X-ray diffraction (XRD) was taken and analysed using a Bruker AXS analyser (D8 Discover) with VANTEC-1 Detector. Samples were analysed using a double-crystal wide-angle goniometry. Scans were measured from 5° to 80° 2θ at a scan speed of 5° 2θ/min at 0.05° or 0.03° 2θ increment using the CuK_α radiation (λ = 0.15406 nm). Peak positions were verified with those of the International Center for Diffraction Data Standard (JCPDS) patterns to identify crystalline phases.

Cumulative and fractional distribution was measured by using a particle size analyser (Malvern, Mastersizer S Long Bed, Polydisperse 2.19). The samples were dispersed in a water medium and vibrated in an ultrasonic bath for 20 min.

True density of samples was measured by a gas pycnometer (Quantachrome, Ultra pycnometer 1000) and calculated using equation (1) (1) where ρ is the true density (g cm⁻³); D is the sample dry weight (g); and the true volume is the volume of the solid component only (cm³). It was determined by crushing the sample into powder form so that all pores were destroyed.

Bulk density of samples was measured and calculated using equation (2) (2) where ρ is the bulk density (g cm⁻³); D is the sample dry weight (g); and the apparent volume is the volume of sample including open and closed pores (cm³).

Relative foam density of samples was measured and calculated according to ASTM D 3575 and ASTM D 1056 using equation (3) (3)

SEM micrographs were obtained from a scanning electron microscope (SEM, JEOL-5200). The samples of CaSiO₃ powder and NR latex foams with/without adding CaSiO₃ powder were mounted onto stubs using a carbon paste and were sputter-coated to ∼0.1 µm with gold to improve conductivity. An acceleration voltage of 13 kV with magnifications of 200, 500 and 2000 times were used.

Contact angle was measured by using a contact angle metre (Kyowa, model DM-CE 1).

Surface tension of liquid medium for contact angle measurement was measured by an automatic surface tensiometer (Kyowa, model DY 300) in terms of DuNuoy Ring (Platinum ring) or Wilhelmy plate (Platinum plate). The measurement was taken three times at room temperature (25°C).

The compressive strength was measured with a universal testing machine (UTM Hounsfield, model H 10 KM). The samples were prepared in the cubic square shape with the dimensions of 2.0 cm in width, 2.0 cm in length and 1.0 cm in thickness.

Results and discussion

Characteristics and physical properties of raw materials and composite foams

The NR latex was measured for its total solid content, dry rubber content, non-rubber content, ammonia content on total weight and on water phase, volatile fatty acid number, mechanical stability time, magnesium content on solid, KOH number, pH, specific gravity, coagulum content, viscosity and colour according to the ASTM and ISO methods; they are equal to 60.69%wt, 60.10%wt, 0.59%wt, 0.67%wt, 1.70%wt, 0.0173, 750 s, 19.90 ppm, 0.20, 10.29, 0.94, 0.005%wt, 91.0 cps and white colour, respectively, where data are as tabulated in Table 2.

Table 2

Characteristics of natural rubber latex

Characteristics	Test methods	Test results
Total solid content (%wt)	ISO 124	60.69
Dry rubber content (%wt)	ISO 126	60.10
Non-rubber content (%wt)	…	0.59
Ammonia content (on total weight, %wt)	ISO 125	0.67
Ammonia content (on water phase, %wt)	…	1.70
Volatile fatty acid number (VFA number)	ISO 506	0.0173
Mechanical stability time 55% TS (s)	ISO 35	750
Magnesium content on solid (ppm)	…	19.90
KOH number	ISO 127	0.20
pH of latex	ISO 976	10.29
Specific gravity (25°C)	BS 6057	0.94
Coagulum content (%wt)	ISO 706	0.005
Viscosity (Spindle 1, no.60 rev min⁻¹, 60%TS, cps)	ISO 1652	91.0
Colour of latex	…	White

The particle size distribution or cumulative mass percent finer (CMPF) of CaSiO₃ at 10, 50 and 90% CMPF was equal to 10.53, 31.92 and 73.34 µm, respectively. CaSiO₃ had a surface area of 2.04 m² g⁻¹, a total pore volume of 0.0034 cm³ g⁻¹ and an average pore diameter of 6.65 nm.¹³

Physical properties (true density, bulk density and relative foam density) are measured and calculated according to equations (1), (2) and (3), as tabulated in Table 3. The true density values of CaSiO₃ powder and NR latex are 2.440 and 0.947 g cm⁻³, respectively. The bulk density values of the composite foams with 0, 2, 5, 10, 20 and 30 phr CaSiO₃ powder added are equal to 0.1684, 0.6146, 0.6397, 0.6876, 1.0751 and 1.2230 g cm⁻³, respectively. The bulk density value of the composite foams increases with increasing CaSiO₃ content. The relative foam density values of the composite foams with 0, 2, 5, 10, 20 and 30 phr CaSiO₃ added are 0.0497, 0.1815, 0.1889, 0.2030, 0.3174 and 0.3611, respectively. The composite foam relative density value also increases with increasing CaSiO₃ content.

Table 3

Physical properties of composite foams

Samples	True density (g cm⁻³)	Bulk density (g cm⁻³)	Relative composite foam density
CaSiO₃ powder	2.440	…	…
NR latex	0.947	…	…
Composite foams (no added CaSiO₃)	…	0.1684	0.0497
Composite foams (added 2 phr CaSiO₃)	…	0.6146	0.1815
Composite foams (added 5 phr CaSiO₃)	…	0.6397	0.1889
Composite foams (added 10 phr CaSiO₃)	…	0.6876	0.2030
Composite foams (added 20 phr CaSiO₃)	…	1.0751	0.3174
Composite foams (added 30 phr CaSiO₃)	…	1.2230	0.3611

Remark: … means not measured.

FTIR spectra of CaSiO₃ powder, NR latex and composite foams with 30 phr CaSiO₃ added were measured in the range of wave number 400–4000 cm⁻¹ as shown in Fig. 1 and the peaks are as tabulated in Table 4. The FTIR spectrum of CaSiO₃ powder shows peaks at 682 and 714 cm⁻¹ belonging to the strong ʋ(Ca–O) and ʋ(Ca–O–Si); the peak at 1646 cm⁻¹ corresponds to the strong ʋ(C=C) and a broad peak at 3400 cm⁻¹ can be referred to the ʋ(O–H) consistent with previous work.^{5, 14} The FTIR spectrum of the NR latex shows peaks at 1376 cm⁻¹ due to the strong ʋ(S=O), ʋ(C–O) and ʋ(C=S); at 1390–1800 cm⁻¹ due to the ʋ(C=O), CH₃ asymmetric deformation and ʋ(C=C); at 2855, 2917 and 2959 cm⁻¹ belonging to the δ(C–H), ʋ(=C–H) and ʋ(CH₂); at 3015 cm⁻¹ due to the ʋ(=C–H); and a broad peak at 3400 cm⁻¹ due to the ʋ(O–H). The FTIR spectrum of composite foams with 30 phr CaSiO₃ added shows peaks consistent with the peaks of the NR latex and the CaSiO₃ powder: at 682 and 714 cm⁻¹ due to the strong ʋ(Ca–O) and ʋ(Ca–O–Si); at 1015 and 1061 cm⁻¹ due to the ʋ(O–Si–O); at 1376 cm⁻¹ ʋ(S=O), ʋ(C–O) and ʋ(C=S); at 1390–1800 cm⁻¹ due to the ʋ(C=O) and CH₃ asymmetric deformation; at 1646, 1654 and 1662 cm⁻¹ due to the strong ʋ(C=C); at 2855, 2917 and 2959 cm⁻¹ due to the δ(C–H), ʋ(=C–H) and ʋ(CH₂); at 3015 cm⁻¹ due to the ʋ(=C–H); and at 3200–3600 cm⁻¹ due to the strong ʋ(O–H).

FTIR spectra of composite foams

Table 4

FTIR spectra of composite foam

Wave number (cm)⁻¹	Functional groups
682 and 714	ʋ(Ca–O), ʋ(Ca–O–Si)
1015 and 1061	ʋ(O–Si–O)
1376	ʋ(S=O), ʋ(C–O) and ʋ(C=S)
1390–1800	ʋ(C=O) and CH₃ asymmetric deformation
1646, 1654 and 1662	Strong ʋ(C=C)
2855, 2917 and 2959	δ(C–H), ʋ(=C–H) and ʋ(CH₂)
3015	ʋ(=C–H)
3200–3600	ʋ(O–H)

The XRD peak patterns comparison of the CaSiO₃, calcined eggshell or CaO, and CaCO₃ powder are as shown in Fig. 2. The XRD peak pattern of the eggshell is consistent with the JCPDS file 00-005-0586 of calcite or CaCO₃ in the rhombohedral phase formation at 2θ: 29.41°, 39.40° and 43.15°. The XRD peak pattern of CaO obtained from the eggshell calcination at 900°C for 2 h shows the cubic phase formation consistent with the JCPDS file No. 00-048-1467, and the peaks at 2θ: 37.36°, 32.2° and 53.86° are consistent with a previous work.⁵ The XRD peak pattern of the CaSiO₃ shows the monoclinic phase formation consistent with the JCPDS file nos. 01-089-6463 and 00-043-1460 with the peaks at 2θ: 27.52°, 26.04° and 25.95°. Furthermore, the comparison of the XRD peak patterns between the CaSiO₃ and the composite foams with 0, 5 and 30 phr CaSiO₃ added is as shown in Fig. 3. The XRD peak pattern of CaSiO₃ is of a crystalline phase, whereas the XRD peak pattern of the composite foam without CaSiO₃ added shows an amorphous phase formation. The XRD patterns of the composite foams with 5 and 30 phr CaSiO₃ added show a semi-crystalline phase.

XRD peak patterns of eggshell, CaO₃ and CaSiO₃ powders

XRD peak patterns of CaSiO₃ powder and composite foams with/without adding CaSiO₃ powder

The SEM micrographs of CaSiO₃ powder, pure NR latex and composite foams without CaSiO₃ powder with magnifications of 100, 500 and 2000× are as shown in Fig. 4. The SEM micrographs of CaSiO₃ show a needle or platelet shape, whereas the SEM micrographs of NR latex show smooth and non-porous surfaces. The SEM micrographs of the composite foam without CaSiO₃ added show a uniform porosity distribution, consistent with the SEM micrograph results of catalyst foams.¹⁵ and the SEM micrograph results of NR foams.⁴ The composite foam contains both open and closed cells. Therefore, the cell morphology characteristics of the obtained composite foam are potentially suitable as an adsorbent to separate polar and non-polar matters. Furthermore, the SEM micrographs of the composite foams with 0, 2, 5, 10, 20 and 30 phr CaSiO₃ added with the magnification of 200× are as shown in Fig. 5. It can be seen that the shape of open and closed cell morphology of the composite foams with 0, 2, 5, 10, 20 and 30 phr CaSiO₃ added varies from a nearly spherical shape to irregular shape with increasing amount of CaSiO₃ added.

SEM micrographs of CaSiO₃ powder, pure NR latex and composite foams without CaSiO₃ powder with magnifications of 100, 500 and 2000×: a and a-1 CaSiO₃ powder; b and b-1 pure NR latex; c and c-1 composite foams without adding CaSiO₃

SEM micrographs of composite foams with/without adding CaSiO₃ powder with magnification of 200×: a 0 phr; b 2 phr; c 5 phr; d 10 phr; e 20 phr; and f 30 phr added into composite foams

Mechanical properties of composite foams

The mechanical properties (compressive strength and compressive force) of the composite foams with 0, 2, 10 and 30 phr CaSiO₃ added are as tabulated in Table 5. The compressive strength values of the semi-rigid composite foams with 0, 2, 10 and 30 phr CaSiO₃ added are 10.00 ± 0.01, 383.90 ± 0.10, 493.60 ± 0.07 and 588.10 ± 0.04 kPa, respectively. The improvement in the compressive strength is by a factor of 50. Therefore, the CaSiO₃ can act as a good filler to increase the mechanical properties of the semi-rigid foams.

Table 5

Mechanical properties of composite foams

Samples	Compressive strength (kPa)	Compressive force (N)	Thickness (mm)	Width (mm)
Composite foams (no added CaSiO₃)	10.00 ± 0.01	2.67 ± 1.40	9.91 ± 0.80	26.50 ± 1.00
Composite foams (added 2 phr CaSiO₃)	383.90 ± 0.10	61.33 ± 5.85	6.14 ± 0.85	26.67 ± 0.35
Composite foams (added 10 phr CaSiO₃)	493.60 ± 0.07	78.20 ± 0.90	5.47 ± 0.23	29.25 ± 2.41
Composite foams (added 30 phr CaSiO₃)	588.10 ± 0.04	59.87 ± 2.42	4.08 ± 0.16	24.97 ± 0.32

Contact angles and adsorption of composite foams

The percentages of porosity and contact angles of water and acetone on composite foam surfaces are as tabulated in Table 6. The composite foams with 0, 2, 5, 10, 20 and 30 phr CaSiO₃ added possess the porosity percentages equal to 21.29, 17.75, 17.75, 17.15, 14.20 and 13.61%/cm⁻², respectively. With increasing amount of CaSiO₃ added, the porosity percentage decreases by nearly a factor of two. The contact angle values were measured by using water, acetone and water–acetone solution at a weight ratio of 1: 1. Water is a liquid with low wetting and a high contact angle due to its high polarity index of 10.2 and surface tension of 59.66 ± 1.11 mN m⁻¹. However, the composite foams with 10, 20 and 30 phr CaSiO₃ added are more suitable for non-polar solvent adsorption than polar solvent adsorption due to the obtained contact angles of 97.8 ± 2.3°, 101.8 ± 1.0° and 103.3 ± 0.5°, respectively, consistent with the results reported by Maestro et al. The contact angle of liquid plays a role analogous to the hydrophilic and lipophilic balance correlated with ϴ. The ϴ < 90° means a hydrophilic behaviour while ϴ > 90° is a hydrophobic behaviour.^16-18 On the other hand, acetone is a liquid with high wetting and a small contact angle due to its low polarity index of 5.1 and a low surface tension of 16.87 ± 0.21 mN m⁻¹. Therefore, the contact angles of acetone on all composite foams are lower than 90° indicating a hydrophilic behaviour. The water and acetone mixture solution at a ratio 1: 1 possesses a medium wetting with a moderate contact angle due to it polarity index in the range of 5.1–10.2 and a surface tension of 25.30 ± 0.49 mN m⁻¹. The contact angles of the mixture solution on all samples are slightly higher values (ϴ < 90°) than acetone adsorption. Therefore, the data suggest that the composite foams can be potentially used as an adsorbent to separate solvents or substances due to different polarities. The other factor for the composite foams to act as an efficient adsorbent is the CaSiO₃; it is a dielectric material containing Ca²⁺, Si⁴⁺ and O²⁻ charges with spontaneous ionic, electronic and interfacial polarisations.

Table 6

The percentage of porosity per surface area and contact angle of water and acetone of composite foams

Samples	The percentage of porosity on surface area^a (%)	Contact angle of water^b	Contact angle of acetone^c	Contact angle on composite foams of solution (water:acetone)^d 1:1
Composite foams (no added CaSiO₃)	21.29	79.8 ± 1.3	21.8 ± 0.8	69.9 ± 0.4
Composite foams (added 2 phr CaSiO₃)	17.75	81.2 ± 7.4	25.8 ± 0.6	84.9 ± 0.7
Composite foams (added 5 phr CaSiO₃)	17.75	90.5 ± 5.1	22.0 ± 0.4	84.1 ± 0.6
Composite foams (added 10 phr CaSiO₃)	17.15	97.8 ± 2.3	19.1 ± 0.6	66.2 ± 1.2
Composite foams (added 20 phr CaSiO₃)	14.20	101.8 ± 1.0	25.6 ± 0.7	76.6 ± 0.9
Composite foams (added 30 phr CaSiO₃)	13.61	103.3 ± 0.5	26.5 ± 0.4	85.3 ± 0.8

^aThe porosity percentage (%) on surface area of composite foams (cm²) was calculated in order to analyse quality and densification of composite foams.

^bPolarity index and surface tension of water were equal to 10.2 and 59.66 ± 1.11 mN m⁻¹, respectively.

^cPolarity index and surface tension of acetone were 5.1 and 16.87 ± 0.21 mN m⁻¹, respectively.

^dPolarity index and surface tension of mixture solution between water and acetone at a ratio 1:1 were in the range 5.1–10.2 and 25.30 ± 0.49 mN m⁻¹, respectively.

Conclusion

The semi-rigid composite foams with CaSiO₃ added were fabricated with a potential to be used as an adsorbent or filter to separate polar and non-polar substances. The CaSiO₃ is one dielectric material having spontaneous polarisations due to the presence of Ca²⁺, Si⁴⁺ and O²⁻ charges. The CaSiO₃ made from eggshell can act as a good bio-filler or a dispersed phase to reinforce and increase mechanical properties of the composite foams with the NR latex as the matrix phase. The semi-rigid composite foams with CaSiO₃ added adsorbed or separated the substances or solvents having different polarity index values. The large amount of filler particles added into composite foams is suitable for non-polar or hydrophobic solvent adsorption (ϴ > 90°), while the small amount of filler particles added is suitable for polar or hydrophilic solvent adsorption (ϴ < 90°). The bulk density, relative composite foam density, compressive strength and contact angle increased monotonically with increasing amount of CaSiO₃ added.

Footnotes

Acknowledgements

The authors would like to thank the following: The Petroleum and Petrochemical College, and the Scientific and Technological Research Equipment Centre, at Chulalongkorn University, Thailand; the Department of Materials Engineering at Kasetsart University for the use of their analytical equipment. AS also would like to acknowledge the financial supports from the Conductive and Electroactive Polymers Research Unit of Chulalongkorn University, the Thailand Research Fund (TRF) and the Royal Thai Government.

References

Najidha

, Saxena

N. S.

, Sreeja

, Unnithan

C. H.

and Predeep

: ‘Optical and electrical characterization of SbCl₅ doped cis-1,4-polyisoprene’, Mater. Lett. , 2005, 59, 3431–3436. doi: 10.1016/j.matlet.2005.06.021

Lowery

T. H.

and Richardson

K. S.

: ‘Solvent, mechanism and theory in organic chemistry’, 181; 1998, New York, Harper Collins. 1998, 181.

Ariff

Z. M.

, Zakaria

, Tay

L. H.

and Lee

S. Y.

: ‘Effect of foaming temperature and rubber grades on properties of natural rubber foams’, J. Appl. Polym. Sci. , 2008, 107, 2531–2538. doi: 10.1002/app.27375

Najib

N. N.

, Ariff

Z. M.

, Baker

A. A.

and Sipaut

C. S.

: ‘Correlation between the acoustic and dynamic mechanical properties of natural rubber foam: effect of foaming temperature’, Mater. Des. , 2011, 32, 505–511. doi: 10.1016/j.matdes.2010.08.030

Tangboriboon

, Khongnakhon

, Kittikul

, Kunanuraksapong

and Sirivat

: ‘An innovative CaSiO₃ dielectric material from eggshells by sol-gel process’, J. Sol-Gel Sci. Technol. , 2011, 58, 33–41. doi: 10.1007/s10971-010-2351-1

Tsai

W. T.

, Yang

J. M.

, Lai

C. W.

, Cheng

Y. H.

, Lin

C. C.

and Yeh

C. W.

: ‘Characterization and adsorption properties of eggshells and eggshell membrane’, Bioresour. Technol. , 2006, 97, 488–493. doi: 10.1016/j.biortech.2005.02.050

Nys

, Gautron

, Garcia-Ruiz

J. M.

and Hincke

M. T.

: ‘Avian eggshell mineralization: biochemical and functional characterization of matrix proteins’, C. R. Palevol. , 2004, 3, 549–562. doi: 10.1016/j.crpv.2004.08.002

Deer

W. A.

, Howie

R. A.

and Zussman

: ‘Chain silicate’, 167; 1963, London, Longmans, Green and Co., Ltd.

Henmi

, Kawahara

and Henmi

: ‘The 37, 4T and 5Z polytypes of wollastonite from Kushiro, Hiroshima Prefecture’, Am. Miner. , 1983, 68, 156–163.

10.

Shieh

S. R.

, Duffy

T. S.

and Shen

: ‘Elasticity and strength of calcium silicate perovskite at lower mantle pressures’, Phys. Earth Planet In. , 2004, 143–144, 93–105. doi: 10.1016/j.pepi.2003.10.006

11.

Cho

Y. B.

and Seo

: ‘High activity of acid-treated quail eggshell catalysts in the transesterification of palm oil with methanol’, Bioresour. Technol. , 2010, 101, 8515–8519. doi: 10.1016/j.biortech.2010.06.082

12.

Toro

, Quijada

, Yaazdani-Pedram

and Arias

J. L.

: ‘Eggshell, a new bio-filler for polypropylene composites’, Mater. Lett. , 2007, 61, 4347–4350. doi: 10.1016/j.matlet.2007.01.102

13.

Tangboriboon

, Pakdeeniti

, Kunanuraksapong

and Sirivat

: ‘Calcium silicate (CaSiO₃) as alternative ionic coagulant and solid lubricant for ceramic molds in natural rubber latex film preparation’, Rubber Chem. Technol. , 2012, 85, (4), 645–660. doi: 10.5254/rct.12.88923

14.

Yoo

, Hsieh

J. S.

, Zou

and Kokoszka

: ‘Utilization of calcium carbonate particles from eggshell waste as coating pigments for ink-jet printing paper’, Bioresour. Technol. , 2009, 100, 6416–6421. doi: 10.1016/j.biortech.2009.06.112

15.

Twigg

M. V.

and Richardson

J. T.

: ‘Theory and Applications of Ceramic foam Catalysts’, Chem. Eng. Res. Des. , 2002, 80, 183–189. doi: 10.1205/026387602753501906

16.

Maestro

, Gusmán

, Ortega

and Rubio

R. G.

: ‘Contact angle of micro- and nanoparticles at fluid interfaces’, Curr. Opin. Colloid Interface Sci. , 2014, 19, 355–367. doi: 10.1016/j.cocis.2014.04.008

17.

Bink

B. P.

: ‘Particles as surfactants – similarities and differences’, Curr. Opin. Colloid Interface Sci. , 2002, 7, 21–41. doi: 10.1016/S1359-0294(02)00008-0

18.

Ravera

, Ferrari

, Liggieri

, Miller

and Passerone

: ‘Measurement of the partition coefficient of surfactants in water/oil systems’, Langmuir , 1997, 13, 4817–4820. doi: 10.1021/la962096+