Curcumin as a natural compound in the synthesis of rigid polyurethane foams with enhanced mechanical, antibacterial and anti-ageing properties

Journal Pre-proof Curcumin as a natural compound in the synthesis of rigid polyurethane foams with enhanced mechanical, antibacterial and anti-ageing properties Natalia Sienkiewicz, Sylwia Członka PII:

S0142-9418(19)30493-3

DOI:

https://doi.org/10.1016/j.polymertesting.2019.106046

Reference:

POTE 106046

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Polymer Testing

Received Date: 19 March 2019 Revised Date:

18 July 2019

Accepted Date: 16 August 2019

Please cite this article as: N. Sienkiewicz, S. Członka, Curcumin as a natural compound in the synthesis of rigid polyurethane foams with enhanced mechanical, antibacterial and anti-ageing properties, Polymer Testing (2019), doi: https://doi.org/10.1016/j.polymertesting.2019.106046. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

1

Curcumin as a natural compound in the synthesis of rigid polyurethane foams with enhanced mechanical,

2

antibacterial and anti-ageing properties

3 Natalia Sienkiewicz1*, Sylwia Członka1

4 5 6 7

1

8

12/16, 90-924 Lodz, Poland

Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego

9 10 11

* Corresponding author.

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E-mail address: [email protected]

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Abstract:

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This paper presents the effect of natural compound (E,E)-1,7-bis(4-Hydroxy-3-methoxyphenyl)-1,6-heptadiene-

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3,5-dione (curcumin) on the antibacterial, anti-aging, mechanical and morphological properties of polyurethane

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(PU) rigid foams. Concentrations of 1 wt.%, 2 wt.% and 5 wt.% of antibacterial compound was used, and the

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infection reduction capacity was evaluated for different types of bacteria (Escherichia coli (G-) and

20

Staphylococcus aureus (G+). In order to determine all assumed properties the synthesized PU foams were

21

characterized by analytical (Fourier Transform Infrared Spectroscopy, FTR), morphological (Scanning Electron

22

Microscopy, SEM), bactericidal effects (Disc Diffusion Method), thermal characteristic (Thermogravimetric

23

Analysis, TGA), mechanical techniques (three-point bending test and compressive test) and physical properties

24

(apparent density, dimensional stability, contact angle, water uptake, color characteristics). Compared to the

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reference foam, compositions modified with 1 and 2 wt.% of the curcumin showed greater compressive strength

26

(increase of 10 and 7%, respectively), higher flexural strength (increase of 17 and 7%, respectively), less water

27

uptake (decrease by 12 and 10%, respectively), as well as, better thermal properties. On the other hand, it has

28

been shown, that in all cases, curcumin can be used as a natural anti-ageing additive for polymers. The addition

29

of curcumin in the amount of 1, 2 and 5 wt.% considerably improved the stabilization of the polymer. Moreover,

30

based on the microbiological results, it has been shown that the addition of 5 wt.% of the extract is suitable for

31

the manufacturing of PU foams with enhanced antibacterial properties.

32 33 34 35 36 37 1

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1.

INTRODUCTION

39 40

Nowadays, the polymer foams material has been used in a wide variety of application such as packaging of food,

41

cushioning of furniture and also in wide isolation application. Polyurethane (PU) foam is one of the most

42

universal materials and is widely used in industry and everyday life applications quality and excellent efficiency

43

[1]. PU rigid foams are the most important class of PU and highly energy-efficient materials of PU product and

44

are good at its physical strength, mechanical and chemical properties both at high and low temperatures [2–5].

45

PU foams have low thermal conductivity and really advantageous mechanical and unique useful properties as

46

compared to other polymer foams that have been used in the manufacturing world.

47 48

Recently, there is a growing interest in using polymers to obtain materials that have antibacterial and antifungal

49

properties. These materials can be used as antibacterial touch surfaces, in places where hygiene and sterile

50

conditions are particularly required (e.g. in healthcare, cosmetology pharmacology or food industries) and thus

51

can become an alternative to commonly used disinfectants, which mostly show high toxicity to the environment

52

and the human health. This is particularly important because, as research shows, about 80% of infectious

53

diseases are transmitted through touch. The largest clusters of colon Escherichia coli, the stick of pneumonia

54

Klebsiella pneumoniae and Staphylococcus aureus which cause the most common infections in hospitals and

55

other medical places occur on objects located directly next to patients, such as bed rails, chairs, and cabinets so

56

there are constitute tactile surfaces, from which bacteria can easily transfer to patients, employees or other

57

people in hospitals. Therefore, the type of used materials has great importance in the preparation of final

58

products that should primarily inhibit the growth of bacteria, viruses, and fungi.

59 60

The main advantage of polymer with antibacterial properties should be to inhibit the growth of bacteria and

61

prevent their accumulation. They also should show bacteriostatic activity, that is, preventing the development of

62

bacterial colonies in their nearness or work bactericidal and actively destroying nearby bacteria, by affecting the

63

course of their cell processes. The next parameter is the efficiency and time of surface use. Antibacterial surfaces

64

should be made of materials which properties withstand a long period of use with simultaneous high efficiency

65

of action. In addition, they should be resistant to external factors so that the process of leaching the germicide

66

does not occur as a result of chemical or mechanical action and it should also show the possibility of permanent

67

bonding to different surfaces.

68 69

Polyurethanes are the only class of polymers widely used in the production of devices that directly come in

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contact with a human in medical application. The ability of micro-organism growing on the polyurethanes can

71

cause a human health problem during the usage and storage of polyurethanes. In recent years, polyurethanes

72

have been widely used as biomaterials for biomedical applications due to their mechanical properties and

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beneficial hemocompatibility. There is not much information in the literature about the method of obtaining

74

polyurethane foams with antibacterial and antifungal properties for special applications. The antibacterial and

75

anti-adhesive properties of polyurethanes were added by immobilizing chitosan and heparin on the samples of

76

polyurethanes via a stepwise process [6]. Soybean-oil-based cationic polyurethane coatings with antibacterial

77

properties have been prepared with a range of different molar ratios of soy polyols and hydroxyl groups from an

2

78

amine diol. All of the cationic polyurethane dispersions and films exhibit inhibitory activity against three

79

foodborne pathogens: Salmonella enterica ssp. enterica ser. Typhimurium, Listeria monocytogenes, and

80

Staphylococcus aureus. It is generally observed that increases in the ratio of ammonium cations improve

81

antibacterial performance [7]. Zwitterionic polyurethanes show great potential applications in the biomedical

82

field and there are humidity-responsive self-healing systems for renewable shape memory applications. Results

83

show that sulfobetaine groups are successfully grafted onto the based polyurethanes what improve cell

84

biocompatibility of polyurethanes [8]. Degradable polyurethanes have been synthesized with fluoroquinolone

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antibiotics and have shown an ability to kill bacteria when released following degradation of the polymer chains

86

by the macrophage-derived cholesterol enzyme. Since cholesterol enzyme has specificity for hydrophobic

87

moieties, it is desirable to alter the formulation of the polyurethanes to incorporate long hydrophobic monomers

88

immediately adjacent to the ciprofloxacin molecule [9]. Polyurethane foams were prepared by using functional

89

quaternary ammonium monomers to endow the porous materials with antimicrobial properties. For this purpose,

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tri- and tetra-hydroxyl functional quaternary ammonium compounds were synthesized and used as a triol and

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tetraol monomer in a polyurethane foam formulation. By this method, the cationic ammonium group is

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chemically incorporated into the polyurethane backbone. The antimicrobial properties of the cationic

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polyurethane foams were evaluated toward Escherichia coli and Staphylococcus aureus Gram-negative and

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Gram-positive bacteria, respectively. Polyurethane foams containing more than 20% of quaternary ammonium

95

component display high antimicrobial activity and can be used for several applications requiring antimicrobial

96

properties [10]. Polyurethane foams modified with colloidal copper (CuNP) were electrochemically synthesized

97

by means of the so-called sacrificial anode electrolysis (SAE) technique. Functionalization of industrial

98

polyurethane foams was carried out by their impregnation in diluted CuNP colloids. Samples were

99

morphologically and spectroscopically investigated and characterized. Their antimicrobial activity was tested

100

towards three model microorganisms (Staphylococcus aureus, Escherichia coli and Kluyveromyces marxianus),

101

demonstrating CuNPs capability of strongly inhibiting bacterial growth and spread of bacteria [11].

102 103

The present work focuses on the use of curcumin as a natural compound to produce composites with improved

104

antibacterial, anti-aging, mechanical and physical properties. PU foams modified with natural extract were

105

characterized by means of the microbiological test (Disc Diffusion Method), color characteristic (CM-3600d

106

Spectrophotometer), mechanical and thermal methods. The influence of the different amount of curcumin on

107

chemical structure (Fourier Transform Infrared Spectroscopy, FTIR), thermal properties (Thermogravimetric

108

Analysis, TGA), physico-mechanical properties (compression strength, three-point bending test, apparent

109

density, water absorption) and morphology of obtained PU composites was examined. The results obtained in the

110

present paper, indicate that the addition of curcumin in the range of 1-5 wt.% influences the morphology of

111

analyzed foams and consequently their further mechanical and thermal properties. Depending on the amount of

112

natural compound in foam mixture, obtained composites exhibit improvement or deterioration of

113

abovementioned properties.

114 115 116 117 3

118 119

2.

EXPERIMENTAL

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2.1 Materials

121

The base PU foam samples were produced with two-component (Izopianol A 40/30/C and Purocyn B) supplied

122

by Purinova Sp. z o.o. Applied Izopianol 40/30/C is a fully formulated mixture which contains polyester polyol,

123

catalyst (Diethanoloamine), flame retardant (Tris(2-chloro-1-methylethyl)phosphate) and chain extender (1,2-

124

propanediol) (Producer information). A used isocyanate which is a polymeric diphenylmethane 4, 4′ diisocyanate

125

(Purocyn B, Purinova Sp. z o.o.) containing 31 wt.% of free isocyanate groups. Both commercial components

126

were combined in a ratio of 100:160 (ratio of OH:NCO groups, in pursuance of the information provided by the

127

producer of the component). The blowing agent in the foam there was a carbon dioxide generated in the reaction

128

of water and isocyanate groups. Curcumin - a natural phenolic compound (Sigma-Aldrich) was used as an

129

antibacterial and antifungal additive with antiaging properties to polyurethane foams. The chemical structures of

130

curcumin is presented in Fig. 1. Since curcumin molecules have polar groups and hydrophilic properties, strong

131

interfacial interaction, such as hydrogen bonding, can be formed between the curcumin molecules and isocyanate

132

leading to the formation of a cross-linked structure. Hydroxyl groups present in curcumin molecules can react

133

with isocyanates even in the absence of catalyst [12–15]. A urethane bond is formed with this reaction. The

134

generalized reaction scheme of isocyanate and curcumin reaction is shown in equation (1).

135

136 137 138

Fig. 1. Chemical structure of curcumin.

139

140 141 142

2.2 Sample preparation

143

PU foams for special application were produced by a single step method from a two commercial component

144

(Izopianol 40/30/C and Purocyn B) which was modified with an addition of natural phenolic compound in the

145

amount of 1, 2 and 5 wt.% in relation to the total polyol mass. Firstly, Izopianol 40/30/C (containing polyester

146

polyol, catalyst - Diethanoloamine, flame retardant - Tris(2-chloro-1-methylethyl)phosphate, chain extender -

147

1,2- propanediol, surfactant - polyether polydimethylsiloxane copolymer) and curcumin as antibacterial additive

148

were weighed out in the required proportions and placed in a 1000 ml polypropylene forms. Then, the mixture

149

(component A with curcumin) was homogenized with an overhead stirrer at 500 rpm under ambient conditions

150

(temperature: 21°C) for precisely 30 s. In the next step, the Purocyn B (polymeric diphenylmethane 4, 4′ -

151

diisocyanate) was added to the previously prepared mixture and all components together were stirred for 15 s

4

152

with an overhead stirrer at 2000 rpm. After that, the prepared reaction mixture poured into an open mould which

153

allowed to expand foam freely in the vertical direction. The prepared samples were conditioned within 24 hours

154

in the ambient temperature. The formulations of the PU foams are shown in Table 1. A schematic figure of the

155

synthesis of PU foams is presented in Fig. 2.

156 157

Table 1. Foam formulations. Mass content [by weight] Sample code

Comments

PU-0 PU-1 PU-2 PU-5

Izopianol 40/30/C

Purocyn B

Curcumin

Reference foam (unfilled) Foam reinforced with 1 wt% of curcumin

100 100

160 160

0 1

Foam reinforced with 2 wt% of curcumin Foam reinforced with 5 wt% of curcumin

100 100

160 160

2 5

158 159 Component A: Izopianol 40/30/C + curcumin (1; 2; 5 wt.%) Component B: Purocyn B

160 161 162 163

Fig. 2. Schematic procedure of synthesis of PU foams.

164

2.3 Methods

165

2.3.1 Antibacterial test

166

Disc diffusion method for antimicrobial susceptibility testing was carried out according to the National

167

Committee for Clinical Laboratory Standards [16] to assess the presence of antibacterial properties of the PU

168

foams with plant extracts. Selected bacteria were used to the disc diffusion tests: Escherichia coli (G-) and

169

Staphylococcus aureus (G+). About 3 mL of diluted bacterial suspension was incubated with 0.05 g of powdered

170

PU foam in the plates and incubated at 37 °C for 4 h. A 3 mL bacterial suspension without any PU foam was

171

used as control. Then 10 µL of the bacterial suspension was dropped onto the culture plate containing agar

172

medium. The Petri dishes were incubated in a thermostat at 37°C for 24 hrs. After this time, the images of

173

samples were made by an optical microscope and the results were measured – the bacterial growth inhibition

174

zone around the tested samples. The test was repeated three times to ensure reliability.

175 176

2.3.2 Characterization techniques

177

The average size of curcumin was measured using a Zetasizer NanoS90 instrument (Malvern Instruments Ltd,

178

UK). The size of particles in polyol dispersion (0.04 g l-1) was determined with the dynamic light scattering DLS

179

method.

5

180

The absolute viscosities of used polyol and polyol premixes were determined corresponding to ASTM D2930

181

(equivalent to ISO 2555) using a rotary Viscometer DVII+ (Brookfield, Germany). The torque of samples was

182

measured as a range of shear rate from 0.5 to 100 s-1 in ambient temperature.

183

The FTIR absorption spectrums were recorded using BIO-RAD 175C (USA) spectrophotometer in an air

184

atmosphere. FTIR spectra were collected for the wavelength range of 3500 – 400 cm-1 at a resolution of 4 cm-1.

185

FTIR was performed with a DGTS/KBr detector. Data processing was performed using OMNIC 3.2 software

186

developed by Thermo Scientific Products (Thermo Fisher Scientific, Germany). The average of 64 individual

187

scans was obtained and the average spectrum was presented.

188

The morphology and cell size distribution of foams were examined from the cellular structure images of foam

189

which were taken using JEOL JSM-5500 LV scanning electron microscopy (JEOL Ltd., USA). All microscopic

190

observations were made in the high-vacuum mode and at the accelerating voltage of 10 kV. The samples were

191

scanned in the free-rising direction. The morphology and cell size distribution of foams were examined from the

192

cellular structure images of foam which were taken using a Leica MZ6 optical microscope. The average pore

193

diameters, walls thickness and pore size distribution of samples were calculated using ImageJ software (Media

194

Cybernetics Inc.).

195

The apparent density of selected foams was measured accordingly to ASTM D1622 (equivalent to ISO 845). The

196

densities of prepared two samples were measured and averaged.

197

To determine the compressive strength (σ10%) of foams agreeably to the ASTM D1621 (equivalent to ISO 844)

198

applied Zwick Z100 Testing Machine (Zwick/Roell Group, Germany) with a load cell of 2 kN and the speed of 2

199

mm min-1. For the purpose of the test, the foams had to be cut into specimens of a certain size with a band saw

200

in a direction perpendicular to the foam growth direction. In the next step, the prepared sample of foam was

201

placed between two plates and the compression strength was measured as a ratio of the load causing 10%

202

deformation of sample cross-section in the parallel and perpendicular direction to the square surface. The result

203

of the test was averaged of 5 measurements per each sample.

204

Three-point bending test was carried out using Zwick Z100 Testing Machine (Zwick/Roell Group, Germany) at

205

room temperature, according to ASTM D7264 (equivalent to ISO 178). The tested samples were bent with

206

testing speed 2 mm/min. Received flexural stress at break (εf) effects for each sample of foam were expressed as

207

a mean value. The result of the test was averaged of 5 measurements per each sample.

208

The thermal characteristics of the prepared foams were evaluated by TGA measurements performed using the

209

STA 449 F1 Jupiter Analyzer (Netzsch Group, Germany). A sample of 10 mg was placed in the TG pan and

210

heated in an argon atmosphere at a rate of 10 K min−1 up to 600°C. The initial decomposition temperatures, T10%,

211

T50% and T80% of mass loss of sample were determined.

212

Surface hydrophobicity was analyzed by contact angle measurements using the sessile-drop method with a

213

manual contact angle goniometer with an optical system OS-45D (Oscar, Taiwan) to capture the profile of a pure

214

liquid on a solid substrate. A water drop of 1 µL was deposited onto the surface using a micrometer syringe fitted

215

with a stainless steel needle. The contact angles reported are the average of at least ten tests on the same sample.

6

216

Water absorption of the RPUFs was measured according to ASTM D2842 (equivalent to ISO 2896). Samples

217

were dried for 1 h at 80°C and then weighed. The samples were immersed in distilled water to a depth of 1 cm

218

for 24 h. Afterward, the samples were removed from the water, held vertically for 10 s, the pendant drop was

219

removed and then blotted between dry filter paper (Fisher Scientific, USA) at 10 s and weighed again. The

220

average of 5 specimens was used.

221

Changes in the linear dimensions were looked into with accordance to the ASTM D2126 (equivalent to ISO

222

2796). The samples were conditioned at a temperature of 70°C and -20°C for 14 days. Change in linear

223

dimensions was calculated in % from equation (2).

224 225

(2) ∆l=((l-lo)/lo)·100

226 227

where lo is the length of the sample before thermostating and l is the length of the sample after thermostating.

228

The average of 5 measurements per each type of composition was reported.

229

The color of the obtained materials was measured using a CM-3600d spectrophotometer (Konica Minolta

230

Sensing, Japan). The wavelength range was 360–740 nm. Color characteristics of the PU foams were defined

231

by the colorimetric coordinates: brightness (L*), red-green component (a*), blue-yellow component (b*),

232

and total change of color (∆E*). ∆E* was calculated according to equation (3).

233 234

(3)

=

( ∗) + ( ∗) + ( ∗)

235 236

where L* is 100, which represents an ideally reflecting diffuser. The minimum value for L* is zero, which

237

corresponds to the color black. A positive value for a* is red; a negative a* is green; a positive b* is yellow;

238

and a negative b* is blue.

239 240

3.

RESULTS AND DISCUSSION

241 242

3.1 Characteristic of the curcumin

243

An important value that characterizes fillers is the size of their particles. The size of curcumin particles was

244

measured in a polyol dispersion (0.04 g l-1). The particle size distribution of the sample measured at the

245

beginning (5 minutes after the ultrasonic mixing) and at the end (25 minutes after ultrasonic mixing) of the

246

measurement are presented in Fig. 3a,b. Presented results clearly indicate that the size of the particles of the filler

247

increases with time, indicating a tendency of the filler to agglomeration. It can be seen that in the second case,

248

the particle size distribution shows two bands. The first one covers diameters of primary aggregates, while the

249

second band evidences the presence of agglomerates.

250

7

251 252 253

Fig. 3. Distribution of particles size of the curcumin measured a) 5 minutes and b) 25 minutes after ultrasonic mixing.

254

3.2 Impact of curcumin on PU mixture viscosity

255

Fig. 4a shows the measured viscosity of the mixed polyol without and with curcumin. The rheological properties

256

of polyol premixes are presented as the viscosity versus shear rate. Compared to the control polyol system, the

257

dynamic viscosity of the mixture slightly increases with 1 and 2 wt.% of curcumin and dramatically increases

258

with 5 wt.% of curcumin, as a result of the presence of filler particles interacting with the polyether polyol

259

through hydrogen bonding and van der Wall’s interaction [17]. 5000

4.5 PU-0 PU-1 PU-2 PU-5

4000

b) 4 log(viscosity η) [mPa∙s]

Viscosity η [mPa∙s]

a)

3000 2000 1000

3.5 3 2.5 2

0 0

260

PU-0 PU-1 PU-2 PU-5

10

20 30 Shear rate γ [1/s]

40

0

50

0.3 0.6 0.9 log (shear rate γ) [s-1]

1.2

261 262 263

Fig. 4. a) Viscosity as a function of shear rate and b) log-log plot of the viscosity vs. the shear rate for the polyol premixes

264

The rheological properties of the polyol premixes with curcumin are shown as the viscosity versus shear rate in

265

Fig. 4a. In all systems, the viscosity is generally reduced at increased shear rates. The viscosity of the samples

266

initially decreases sharply and then significantly slower to reach a relatively stable value, due to the fact that

267

particles of liquids reach the best possible arrangement. Such a phenomenon is typical for non-Newtonian fluids

268

with a pseudoplastic nature and is quite often found in many previous works [18,19]. To further analyze the data,

269

the graph of viscosity versus shear rate is converted to log viscosity versus log shear rate form as shown in Fig.

270

4b. From this graph, it can be seen that the curvatures of viscosity versus shear rate can be made close to linear

271

using this log-log format with regression of 0.968-0.988. Thep ower-law index (n) was calculated from the

272

slopes. All results are presented in Table 2. For the system PU-5, the power-law index is lower than that of their

273

PU-1 and PU-2 counterparts, leading to highly non-Newtonian behavior.

with curcumin.

274 275 8

276

Table 2. Dynamic viscosity and logarithmic plot of the fitting equations for polyol premixes. Dynamic viscosity η [mPa·s] Fitting equation

Power law index (n)

R2

380

y = -0.060 + 0.305

0.345

0.968

885

772

y = -0.061 + 0.308

0.321

0.988

1934

1207

1053

y = -0.059 + 0.310

0.318

0.979

3342

2086

1819

y = -0.058 + 0.308

0.306

0.968

Sample code 0.5 RPM

5 RPM

10 RPM

PU-0

628

424

PU-1

1418

PU-2 PU-5

277 278

3.3 The influence of the curcumin on the maximum temperature (Tmax) of the reaction mixture during the

279

foaming process

280

The reaction of the synthesis of PU is highly exothermic [20,21]. The rate of increase in temperature determines

281

the activity of reaction mixture, what is associated with the reactivity of the components of the mixture. As

282

shown in Table 3, the introduction curcumin into the PU system decreases the activity of reaction mixture which

283

is confirmed by a decrease in the Tmax during the foaming process in each case. The Tmax decreases from 122 to

284

112°C with the addition of 1-5 wt.% of curcumin. Kairyte et al. [22] have shown that the addition of waste ash

285

decreased the maximum temperature, which can be attributed to the assumption that the fillers absorb part of the

286

heat generated during the foaming effect. Basically, the analog tendency has been also observed by other authors

287

in previous works [23–25].

288 289

Table 3. Selected properties of PU foams. Sample code

Temperature [oC]

Cream time [s]

Extension time [s]

Tack-free time [s]

Cell size [µm]

Wall thickness [µm]

Apparent density [kg m-3]

PU-0 PU-1

125 122

43 ± 4 46 ± 2

277 ± 10 312 ± 11

341 ± 14 330 ± 12

472 ± 10 456 ± 8

62 ± 4 63 ± 2

38 40

PU-2 PU-5

115 112

47 ± 2 49 ± 2

408 ± 8 424 ± 9

321 ± 12 312 ± 10

442 ± 6 412 ± 8

66 ± 3 68 ± 2

41 43

290 291

3.4 Foaming kinetic of PU foams

292

The foaming process was determined by measuring the characteristic processing times like cream, extension and

293

gelation time. The cream time was measured from the start of mixing of components to a visible start of foam

294

growth, extension time elapsing until reaching the highest volume of the foam and gelation time was determined

295

as the time when the foam solidifies completely and the surface is no longer tacky [26].

296 297

The results presented in Table 3 indicate a slight increase in cream and extension time for the PU foams

298

containing curcumin in each amount. This dependence is mostly related to the fact that well-dispersed filler in

299

the reaction mixture acts as a nucleating agent in the nucleation process, leading to a greater bubble formation

300

and prolonged cream time [27]. Moreover, further growth of the resulting cells seems to be hindered by the

301

increase in viscosity of modified systems (see Table 2) leading to prolonged cream and extension times, as was

302

also noted by other researchers [28]. Compositions modified with the addition of curcumin are also characterized

303

by a shorter tack-free time, indicating that curcumin particles act as a curing accelerator. The total characteristic

304

times measured for the compositions with curcumin are higher than those measured for the PU-0, but still in the

9

305

range of operating conditions for preparing PU foams [29,30]. Contrary results have been obtained by Liu et al.

306

[19] who determined that compositions modified with waste ash were characterized by a longer tack-free time as

307

well, indicating that the waste ash did not act as a curing accelerator. The authors have stated that this might be

308

related to the fact that not all fillers determine the same reaction manner that is related to the chemical

309

composition and particle size distribution leading to different foaming kinetics.

310 311

3.5 FTIR Analysis

312

The chemical structure of used curcumin was specified by FTIR analysis. The FTIR spectrum of the curcumin is

313

shown in Fig. 5. The characteristic peaks are observed at 3448 cm-1 (-OH stretching vibration), 2920

314

cm-1 (-CH2 stretching vibration). The strong peak at 1630 cm-1 is due to the mixed vibration of (C=O) and (C=C)

315

[31]. The shoulder peak at 1602 cm-1 is assigned to the symmetric aromatic ring stretching vibration (C=C) [31].

316

The wide absorbance area from 1600 to 1400 cm-1 represents the (C-H) stretching vibration of the aromatic

317

region [32,33]. The other peaks at 1281 cm−1, 1033 cm-1 and 959 cm-1 are assigned to enol (C-O), (C-O-C) and

318

benzoate trans (-CH) vibration, respectively [34]. Curcumin

O-H C=O C=C C-H

C-O-C C-O C-H

-1

Transmitance [cm ]

PU-0 N-H

C-H C=O

PU-1 C-O

PU-2

PU-5

4000

3500

3000

2500

2000

1500

1000

500

319 320

Fig. 5. FTIR spectra of curcumin and PU foams.

321

Fig. 5 presents the spectrum ready foam with the addition of the antibacterial additive. The FTIR analysis

322

verified the presence of functional groups characteristic for the urethane moieties (as characteristic absorption

323

bands). The intense bands in the range of 1700–1770 cm-1 represented the characteristic (C=O) stretching

324

vibration [35,36]. The band in the range 3200-3600 cm-1 indicate the (N-H) stretching vibration and (N-H)

325

bending vibration of the (N-H) of the urethane moieties are present, what suggests that the urethane segments

326

were formed as a result of the reaction of the polyol with the isocyanate despite the presence of curcumin

327

[37,38]. The other characteristic bands are presented in Table 4. It should be noted that the addition of the extract

328

only slightly affected the chemical structure of the foam. Small changes are observed only in the intensity and

10

329

the values of wavenumbers of the characteristic bonds, which were appointed from FTIR spectra of all tested

330

samples. The signal at 1712 cm-1 corresponding to (C=O) stretching vibration from free urethane indicates some

331

otherness in the spectra of analyzed foams what can be caused with increasing content of curcumin in the

332

reaction mixture the carbonyl peak slightly decreases to value 1705 cm-1. This can be explained by the fact that

333

OH groups of curcumin chemically reacted with the part of the isocyanate, leaving fewer isocyanate groups

334

accessible for the reaction with the polyol component.

335 336

Table 4. Analysis of signal displacements in FTIR spectroscopy of characterized samples. Wavenumber [cm-1] Bond PU-0

PU-1

PU-2

PU-5

3346

3345

3342

3333

N- H stretching

2275

2275

2275

2275

N=C=O stretching

1712

1710

1708

1705

C=O stretching

1593

1593

1592

1595

Ar-H deformation

1506

1507

1507

1507

N-H bending

1411

1412

1411

1411

PIR deformation

1308

1306

1306

1306

C-N stretching

1226

1225

1225

1226

C-N stretching

1076

1073

1073

1073

C-O stertching

337 338

It is also worth noting that with the increase of the curcumin amount the absorbance peak 2275 cm-1 (stretching

339

vibration of residual isocyanate groups (-N=C=O), becomes more pronounced. This can be caused by the

340

hydroxyl groups of curcumin which can modify the isocyanate index (RNCO/OH) defined as the number of moles

341

of NCO groups of the isocyanate per OH mole of the polyol in this way they affect on the consumption of NCO

342

groups.

343 344

The FTIR spectra confirmed the successful incorporation of curcumin in the synthesized samples. A weak peak

345

is observed in the same region at 3770 cm-1 in the spectra of all the samples containing curcumin are attributed to

346

the mixed stretching vibrations of (C=C), (C=O) and phenyl rings of curcumin. By increasing the curcumin

347

contents from 1 to 5 wt.% in PU foams, the absorption peak intensities at 3770 cm-1 are increased which

348

confirmed the incorporation of curcumin in the PU structure.

349 350

The hydrogen bonding index (R) and the content of rigid PU segments may be used to calculate phase separation

351

degree (DSP) from the FTIR results [39]. The indicator (R) is analyzed based on the intensity of the

352

characteristic bands derived from the carbonyl groups present in the urethane and urea formations. For this

353

purpose, a baseline for carbonyl vibration (1770–1630 cm-1) was determined. The hydrogen bond ratio was

354

calculated using equation (4).

355 356

(4) R = (A1+A2)/(A3+A4)

11

357

where A1 and A2 are a range of bands corresponding to hydrogen bonded carbonyl bonding in urea groups (1640-

358

1686 cm-1) and urethane groups (1705-1724 cm-1), while A3 and A4 are a range of bands corresponding to

359

carbonyl not bound by hydrogen in urea groups (1690-1702 cm-1) and urethane groups (1732-1760 cm-1). Based

360

on the hydrogen bonding index (R) value the phase separation degree (DSP) parameter has been calculated from

361

equation (5).

362

(5) DSP = R/(R+1)

363

The data presented in Table 5 indicate that the introduction of curcumin resulted in the formation of more

364

hydrogen bonds compared to the PU-0, while the phase separation degree remains almost unchanged.

365 366

Table 5. Hydrogen bonding index (R) and phase separation degree (DPS) of PU foams. Sample code

R DSP [%]

PU-0

PU-1

PU-2

PU-5

1.57 61

1.60 62

1.59 61

1.56 61

367 368 369

3.6 Cellular structure of PU foams

370

The SEM images show the surface area of samples cut out perpendicular to the foaming direction. The

371

morphologies of the PU foams are shown in Fig. 6. As observed from the micrograph of the reference foam (Fig.

372

6a), the cell size and cell distribution are nearly uniform and the foam consists of closed cells with a negligible

373

amount of cells with broken walls.

374 375

As expected, the incorporation of solid filler particles into the polymer matrix resulted in increasing porosity of

376

the resultant products. With the addition of curcumin, the overall cell structure becomes less uniform and the

377

number of broken cells is increased. In the case of PU-1, the closed-cell structure is well preserved, and the cell

378

size is uniform (Fig. 6b) which indicates that application of 1 wt.% of curcumin enhances the formulation of

379

smaller and more regular cells. When the contents of curcumin exceed 2 wt.%, damaged cells become visible

380

(Fig. 6c). The highest number of damaged cells is shown in Fig. 6d, which corresponds to the PU-5. Higher

381

content of open cells in the case of PU foams modified with the highest amount of curcumin can be connected

382

with poor interfacial adhesion between the filler surface and the polymer matrix, which promotes earlier cell

383

collapsing phenomena and increases a high possibility of generating open pores [40]. Moreover, the possible

384

interphase interactions between curcumin and PU in cell struts disturbed formulation of stable foam structure

385

[41] which results in the coalescence of crowded cells. The dispersions of the particles of used fillers in PU

386

foams are presented in Fig. 7a-c. It is clearly visible that for both series of modified foams, filler particles are

387

attached to the cell wall. Some dots and projections also become detectable in the cell void and a coarse surface

388

can be seen in the cell struts.

389

12

a) PU-0

390 391

c) PU-2

b) PU-1

d) PU-5

Fig.6. Morphology of a) PU-0, b) PU-1, c) PU-2 and d) PU-5 observed at the same magnification. a) PU-1

a) PU-2

c) PU-5

392 393

Fig. 7. Morphology of a) PU-1, b) PU-2 and c) PU-5 observed at the same magnification.

394 395

The values of the cell size of the foams were statistically analyzed by means of ImageJ software from SEM

396

images. In comparison to the PU-0, the modified PU foams are characterized by a wider cell size range and

397

higher cell distribution frequency (Fig. 8). It is also observed that the pore size decreases with increasing filler

398

content (Table 3), so it can be concluded that curcumin has an effect on reducing the cell size. This may be due

399

to the increased viscosity of the system after the addition of the filler which restrains the expansion of the cells.

400

Moreover, the addition of the fillers can change the nucleation mode from homogenous to heterogeneous and

401

reduce the nucleation energy, which in turn promotes the formation of large numbers of small cells [40]. Similar

402

observations were described by other authors [42–44].

403 404 405

Fig. 8. Cell size distributions of a) PU-0, b) PU-1, c) PU-2 and d) PU-5.

13

406

3.7 Apparent density of PU foams

407

Apparent density is an important parameter that influences the properties and performances of PU foams. The

408

values of density of prepared foams are presented in Table 3. In general term, the apparent density tends to

409

increase when the curcumin is added. The reference foam is characterized by an apparent density of 38 kg m-3.

410

The apparent density for PU composites increases from 40 to 43 kg m-3 with an increase of curcumin. This effect

411

can be explained by an analysis of the role of filler particles on nucleation and cell growth. The curcumin

412

particles act as nucleation sites promoting the formation of bubbles, and this is an increasing trend with particles

413

content, but, at the same time, the growth process of the resulting cells is hindered by the increase of the gelling

414

reaction speed, revealing in bigger viscosity. This results in bubble collapse and higher density foams. Moreover,

415

it should be pointed out that another factor affecting the density of PU foams is a higher density of curcumin (ca.

416

1.6 g cm-3) comparing to the PU foam matrix. This resulted in an increase in the apparent density of studied

417

composites, which is also in agreement with the results reported in the literature [45,46].

418 419

3.8 Compressive test

420

Another important parameter that impacts performance characteristics is the compressive strength, and the

421

change in its value is presented in Table 6.

422

Table 6. Mechanical properties of PU foams. Sample code

Specific compressive strength (parallel) [kPa kg-1 m-3]

Specific compressive strength (perpendicular) [kPa kg-1 m-3]

Flexural strength εf [MPa]

Elongation [%]

PU-0

6.6

3.8

0.402

11.2

PU-1

7.3

4.4

0.469

10.2

PU-2

7.0

3.9

0.432

10.8

PU-5

5.0

3.1

0.407

11.5

423 424

Fig. 9 shows the compression stress-strain curves for selected specimens. By comparing the obtained results in

425

both directions, it is observed that the foams offer better mechanical properties in a free-rising direction relative

426

to the transverse direction. This may be due to the suitable orientation of polymer in free-rising direction and

427

deforming mechanisms in different directions. Like all cellular materials, PU composites exhibit three stages of

428

deformation in compression (the linear elastic region on the stress-strain curve, the plateau region, and the

429

densification region). The increase in brittleness caused by the reinforcements determines a more abrupt

430

transition from the elastic region to the plateau, in contrast to the smooth transition observed in the case of the

431

PU-0. The elongation at break of the PU composites increases with increasing filler content, implying that

432

curcumin addition makes the PU matrix more flexible. This is a common result in PU composites reinforced by a

433

conventional, biodegradable filler [23,47,48].

434

14

400 a)

Compresive strength [kPa]

Compressive strength [kPa]

400

300

200 PU-0 PU-1 PU-2 PU-5

100

0

PU-0 PU-1 PU-2 PU-5

b) 300

200

100

0 0

4

8

12

0

4

Strain [%]

8

12

Strain [%]

435 436 437 438

The general trend for compressive strength is to decreases with filler concentration (Fig. 10). A significant

439

improvement in compressive strength is observed with the addition of 1 wt.% of curcumin. The value of

440

compressive strength measured parallel to the foam rise direction increases to 291 for sample PU-1. No further

441

improvement is observed with increasing filler content. The value of compressive strength decreases to 285 and

442

217 kPa for sample PU-2 and PU-5, respectively. As it is shown in Table 6, the decreased tendency of specific

443

compressive strength is a similarity with compressive strength, which indicated that besides density there is

444

another factory influenced the compressive strength. Such changes in the mechanical properties of composite

445

samples can be explained in terms of characteristic features of their structure. As presented in Fig. 6, reference

446

foam has mostly spherical and equally distributed cell structure. With increased filler content it could be

447

observed that foam cell structure becomes more distorted and less uniform distribution. At this time if there is an

448

application of loading, bending and shrinkage of cell walls occur and results in the development of microcracks

449

[27]. The reduction of the content of closed cells with increasing addition of curcumin also contributes to these

450

results due to the contribution to the modulus coming from the stretching of the closed-cell walls, which is

451 452

decreased with curcumin addition.

Fig. 9. Compression behaviors of PU foams measured a) parallel and b) perpendicular to the foam rise direction.

50

350 40 250 35

Compressive strength [kPa]

45

30 PU-0

PU-1

PU-2

50 b)

Apparent density

150

453

300

Compressive strength (parallel) Apparent density [kg m-3]

Compressive strength [kPa]

a)

Compressive strength (perpendicular) Apparent density

250

45

200

40

150

35

100

PU-5

Apparent density [kg m-3]

450

30 PU-0

PU-1

PU-2

PU-5

454 455 456

Fig. 10. Effect of apparent density on compressive strength of PU foams measured a) parallel and b) perpendicular to the foam rise direction.

457

Moreover, the reason for the decreasing mechanical strength at a high filling rate might also be related to non-

458

uniform dispersion of particles in polyol mixture. High tendency to aggregate filler particles, noticeable in the

15

459

structure, leads to a weakened interfacial adhesion between the filler and effective active surface. In

460

consequence, PU foams are characterized by microphase separation of the structure, which leads to the failure of

461

samples in an unexpected manner at random locations in the samples. A non-uniform concentration of the filler

462

in some regions contributed to the embrittlement effect of polymer structures, inhibiting the enhancement of

463

mechanical properties of PU foams. By increasing the content of the curcumin up to 5 wt.% the negative effects

464

of the filler such as disruption of the formation of hydrogen bonds, disruption of the reaction stoichiometry, the

465

probability of the agglomeration of nanoparticles due to the increase of viscosity and inappropriate distribution

466

of particles is increased. Therefore, the interaction of the particles with PU macromolecules is decreased and the

467

mechanical properties are weakened. The poor interfacial adhesion between some particles, especially the loose

468

ones as discussed above, the polymer matrix and the uneven dispersion of the filler may lead to the above results,

469

as proven by other authors [49–52].

470 471

3.9 Three-point bending test

472

As in the case of compression results presented in Fig. 9, no correlation between flexural strength (σf ) and

473

apparent density is observed as well (Fig. 11). It can be also seen that incorporation of curcumin filler affects the

474

σf of PU composites. 0.8

50

0.7

Apparent density 45

0.6 40 0.5 35

0.4

0.3

475

Apparent density [kg m-3]

Flexural strength [MPa]

Flexural strength

30 PU-0

PU-1

PU-2

PU-5

476 477

Fig. 11. Effect of apparent density on flexural strength of PU foams.

478

Compared to the PU-0, σf is improved by the addition of curcumin in the amount of 1 and 2 wt.%. The value of

479

tensile strength of PU-1 and PU-2 increases from 0.402 to 0.469 and 0.432 MPa, respectively, as compared to

480

the PU-0. The incorporation of curcumin in the amount of 5 wt.% leads to a deterioration of σf. The value of σf

481

decreases to 0.407 MPa, as a result of greater elasticity, connected with the cellular morphology of PU foams

482

(see Fig. 6). Due to an uneven distribution of the filler in the PU matrix and many clusters present in the

483

structure of PU composites, the mechanical properties of the resulting materials are reduced. The lack of

484

reinforcing effect with the incorporation of the filler was also observed in previous studies [53,54].

485 486

Fig. 12 shows the stress-strain curves for the PU foams. All samples exhibit a linear elastic behavior in the low-

487

stress region and plastic deformation in the high-stress region, pointing at a comparable mechanical performance

488

of reinforced foams. The incorporation of curcumin in the amount of 1 and 2 wt.% reduces the elongation at

16

489

break (εf) of PU foams. With increasing content of curcumin, the foams exhibit an extended range of εf as a

490

result of the higher content of open cells and greater flexibility of the PU matrix [53,54].

491

Flexural strength [MPa]

0.6 PU-0 PU-1 PU-2 PU-5

0.4

0.2

0 0

492 493

5 Elongation [%]

10

15

Fig.12. Flexural stress-elongation curves of PU foams.

494 495

3.10 Dynamic-Mechanical Analysis of PU foams

496

The dynamic mechanical behavior of PU foams as a function of the temperature is shown in Fig. 13. The results

497

presented in Fig. 13a and Table 7, indicate that the incorporation of the curcumin to the PU matrix affects the

498

value of Tg, which corresponds to the maximum value of the curve loss tangent (tanδ) versus temperature.

499

Compared to the PU-0, PU-1 and PU-2 are characterized by higher Tg, however, with increasing content of the

500

filler, Tg value of modified PU foams decreases to 112°C. Wu et al. [55] have shown that the Tg of PU foams

501

reflects the rigidity of the polymer matrix which is a function of the isocyanate index, cross-link density and

502

aromaticity level of the foams. Given that the isocyanate index has been held constant in this study, the decrease

503

in the Tg must be a reflection of the decreased aromaticity and cross-link density due to the presence of the

504

curcumin [56]. Silva et al. [57] stated that the larger the concentration of the filler in PU system, the lower is the

505

Tg, indicating that the presence of rice ask hush increases the chain mobility of the high cross-linked zones of the

506

polymeric matrix, which is associated with a larger fraction of free volume in the network due to lower

507

interaction between the polymer chains. A similar effect was noticed by Silva et al. [27] in the case of foams

508

modified with cellulose fibers. They stated that the cellulose fibers could induce a decrease in the reactivity of

509

the components of their rigid polyurethane foams since the OH groups of cellulose chemically reacted with part

510

of the isocyanate, leaving fewer isocyanate groups available for the reaction with the polyol component.

511

17

1

400000 PU-0 PU-1 PU-2 PU-5

0.8

tan(δ)

0.6 0.4 0.2

300000

200000

100000

0

0 50

512 513

PU-0 PU-1 PU-2 PU-5

b) Storage modulus [MPa]

a)

100

150

Temperature

50

200

[oC]

100

150

200

Temperature [oC]

Fig. 13. a) Tanδ and b) storage modulus as a function of temperature plotted for PU foams modified with curcumin.

514 515

In Fig. 13b, it is also notable that PU foams modified with 1 and 2 wt.% of curcumin are characterized by higher

516

storage modulus (E’) as compared to PU-0. It can be concluded that the addition of curcumin has significantly

517

increased the E’ of PU and consequently the stiffness of studied composites is also enhanced. This is due to the

518

presence of filler in the PU matrix as well as higher viscosity of the modified systems, which imposes serious

519

limits on the mobility of polymer chains, affecting their higher stiffness. With increasing content of the filler up

520

to 5 wt.% the value of E’ slightly decreases. This decrement in E’ is attributed to the beginning of a thermal

521

transition, which is associated with hard segments phase. The changes observed around 100°C are attributable to

522

the presence of a high concentration of hydrogen-bonded aromatic urethane groups in the poly(ether-urethane)

523

phase and hard-segment domains which act as macroscopic cross-links. Deterioration of dynamic-mechanical

524

properties as a result of the incorporation of the filler was also observed in previous works. For example, Silva et

525

al. [58] reported that the addition of rice husk ash leads to samples with reduced storage modulus respect to the

526

unfilled foam, mainly due to the relaxation of zones of higher cross-link density, but also to the decreasing level

527

of physical filler-matrix interactions developed in the system, as the result of the increased viscosity of the

528

reactive liquid mixture due to increasing additions of ash.

529 530

Table 7. Thermal properties of PU foams. Sample code

Tg [°C]

T10 [°C]

T50 [°C]

T80 [°C]

Char residue [%]

PU-0

127

265

454

591

28

PU-1

137

234

439

577

22

PU-2

138

224

400

570

20

PU-5

112

239

444

576

22

531 532 533

3.11 Thermogravimetric Analysis of PU foams

534

The thermogravimetric analysis (TGA) was used to characterize the thermal stability of the prepared samples of

535

PU foams. The results for the tested foams presented in Fig.14 and Table 7 show really similar degradation

536

pattern which consists of three stages. The first step is connected with dissociation of urethane bond at a

537

temperature between 150 and 340°C (corresponding to the temperature at 10% of total weight losses) [59,60].

538

The second step of the decomposition of samples is noticeable in the temperature between 350 and 400°C and is

18

539

imputed to the distribution of soft polyol segments (corresponding to the temperature at 50% of total weight

540

losses) [61]. The last step of degradation, assign to the degradation of the fragments generated during the second

541

step, which means to loss of weight by about 80%, appear at the temperature of 510°C [62]. 0

120

b)

a) Deriv weight [%/oC]

Weight loss [%]

90

60 Turmeric extract PU-0 PU-1 PU-2 PU-5

30

200

-0.004 Turmeric extract PU-0 PU-1 PU-2 PU-5

-0.006

0 0

-0.002

400

-0.008

600

0

Temperature [oC]

542

200

400

600

Temperature [oC]

543 544

Fig. 14. a) TGA and b) DTG curves for PU foams.

545

The addition of the curcumin which is the natural sources of antioxidant has an impact on the thermal stability of

546

the prepared foams. The samples with curcumin in various amounts are relatively more thermally stable, start to

547

degrade with the first degradation peak at 224-239°C, second degradation peak at 439-444°C and the third

548

degradation peak at 570-576°C compared with the standard sample without modification. The results also

549

confirm that the more extract present in the foams the degradation rate of the samples decreases.

550 551

3.12 Microbiological results

552

Table 8 and Table 9 presents the microbiological results obtained for the PU foam with 1, 2 and 5 wt.% of

553

curcumin. The bactericidal compounds of natural extract presented significant results after 6 h of exposition

554

compared to the negative control. It has been shown that in all cases the addition of the curcumin to the PU

555

foams affected E. coli and S. aureus. The highest antimicrobial behavior against S. aureus and E. coli was

556

obtained for samples with 5 wt.% of curcumin. The significant elimination of microorganisms for PU-5 was

557

observed after 12 h of exposure. Microbiological test results clearly indicate that the growth of the colonies of E.

558

coli and S. aureus was acceptably inhibited in the presence of curcumin after 24 h. In fact, curcumin particles

559

begin to release the reactive species into the bacteria that prevent the growth of the cell which leads to the

560

distortion and permeation of the cell and finally leads to the bacteria cell death.

561 562

Table 8. Antibacterial activity of PU foams against E.coli. E.coli (CFU/ml) Time (h) Bacterial suspension

0 6 12 24

74 x 106

PU-0

PU-1

PU-2

PU-5

74 x 106

74 x106

74 x106

74 x106

74 x 106

58 x105

12 x105

82 x105

6

4

4

15 x103

74 x 10

34 x10

<74 x 106

16 x 104

563 19

76 x10

31 x 104

1.4 x 103

564

Table 9. Antibacterial activity of PU foams against S. aureus. S. aureus (CFU/ml) Time (h) Bacterial suspension

PU-0

PU-1

PU-2

PU-5

0

74 x 106

74 x 106

74 x 106

74 x 106

6

6

5

5

87 x104

74 x 106

12 24

74 x 10

18 x10

18 x10

74 x 106

42 x104

63 x104

19 x103

74 x 106

23 x 104

12 x 104

10.8 x 103

565 566 567

3.13 Color Characteristics of PU foams

568

The composite samples were examined optically to observe the change in color values. The total color difference

569

(∆E*) was measured using a Konica Minolta color spectrophotometer and calculated using the CIE Lab system,

570

where L* indicates the brightness of a color, a* describes the red-green content and b* the yellow-blue content.

571

The mean values and the standard deviations of the total color differences (∆E*) taken from each of the tested

572

composite samples are recorded in Table 10.

573 574 575

Table 10. Color coordinates of the PU foams before thermo-oxidative ageing (L* is the degree of lightness difference, a* is the red/green parameter, b* is the yellow/blue parameter and ∆E* is the total color change). Sample code

Colorimetric Parameters L*

a*

b*

∆E*

PU-0 PU-1

11.75 24.34

22.45 73.54

-5.12 -3.85

5.04 17.05

PU-2 PU-5

47.58 59.57

74.64 78.39

-3.56 0.06

26.69 28.41

576 577

Curcumin is a diarylheptanoid, belonging to the group of curcuminoids, which are natural phenols responsible

578

for curcumin's yellow color [31,32,63]. Curcumin undergoes characteristic changes under the action of various

579

environmental factors [64,65]. Therefore, we tested the influence of curcumin on the color of PU foams and

580

examined the color change of these materials as a function of ageing time (Table 11). A significant difference in

581

color can be seen between the PU-0 and curcumin-modified PU samples. With increasing concentration of

582

curcumin, PU composites show a decrease in the L* value. The results of L* clearly indicate that samples with a

583

higher content of curcumin (PU-5) possess more intense color. Based on the results of a* and b* value, all

584

samples are characterized by more yellow and red shades, as compared to the PU-0.

585 586 587

Table 11. Color coordinates of the PU foams after thermo-oxidative ageing (L* is the degree of lightness difference, a* is the red/green parameter, b* is the yellow/blue parameter and ∆E* is the total color change). Sample code

Colorimetric Parameters L*

a*

b*

∆E*

PU-0 PU-1

10.75 23.34

24.29 75.47

-4.21 -3.14

12.51 24.05

PU-2 PU-5

45.58 57.57

75.18 79.21

-2.54 0.08

27.69 29.41

20

588 589

The results revealed that curcumin could be used as a natural color indicator of polymer ageing time. The color

590

changes are discussed primarily in terms of the color coordinates measured in the CIE-Lab space. Fig. 15 shows

591

the changes of the ∆E* parameter after the ageing process.

592 593

It appears that the tendency to change color (parameters a* and b*) is similar for all samples. In contrast, a more

594

pronounced difference is observed for total color change. As shown in Fig. 15, the greatest ∆E* is observed for

595

sample PU-0, without curcumin addition. With increasing concentration of curcumin the difference in ∆E* is

596

lower, as compared to the sample before the aging. This result, clearly indicates that the use of this curcumin, as

597

a natural antioxidant, can protect PU foams from the negative effects of high temperatures. Based on the results,

598

we conclude that curcumin can be used as a natural anti-ageing additive for polymers. The addition of curcumin

599

considerably improves the stabilization of the polymer.

600 40 Before aging

ΔE* [-]

30

After aging

20

10

0 PU-0

601 602

PU-1

PU-2

PU-5

Fig. 15. Total color differences ΔE* of PU foams before and after 10 days of thermo-oxidative ageing.

603 604

3.14 Contact angle, water absorption, the dimensional stability of PU foams

605

Hydrophobicity is very important at a wide variety of applications which is gained from the value of contact

606

angle of water with the surface. The contact angle of a water droplet for hydrophilic and hydrophobic samples is

607

<90° and >90°, respectively. The terms of hydrophilic and hydrophobic describe the apparent attraction and

608

repulsion between water and surfaces. The water-contact angles (θ) of the PU-0 and PU foams with various

609

weight percentages of curcumin are shown in Fig. 16. It can be observed from the obtained results that, all PU

610

foams are hydrophobic. Hence, it is expected that by increasing the curcumin content, due to its hydrophobic

611 612

nature [63], the water contact angle of PU foams is increased.

a) PU-0

613 614

b) PU-1

c) PU-2

d) PU-5

Fig. 16. Contact angle on the surface of the a) PU-0, b) PU-1, c) PU-2, d) PU-5.

21

615

To widen the characterization of application properties, the water absorption of PU foams was measured. Water

616

uptake depends mainly on the cellular structure of foams as well as the hydrophobic nature of the used fillers

617

[66–68]. As presented in Fig. 17, with increasing content of curcumin, PU foams absorbed more water, as a

618

result of a more open structure. In this case, the cells are broken and are interconnected to accommodate more

619

amount of water. Thus, it can be concluded that in this case, the cellular morphology of PU foams is the

620

dominating factor affecting water sorption of analyzed materials. Beside this dependence, it should be pointed

621

out that water absorption for all modified materials is decreased in comparison with the reference foam what can

622

be attributed to the hydrophobic character of curcumin [69] as well as to the fact that the filler particles can act as

623

a barrier, preventing the penetration by water and leading to a limited water uptake by modified foams. A similar

624

trend of fillers, preventing penetration by water can be also found in previous works [23,25]. In this case, water

625

absorption is slightly higher, however, this result is still considered satisfactory for the use of PU foams as

626

insulating materials and construction components [70]. 150

16 Water absorption

140

Contact angle [o]

Water absorption [%]

Contact angle 14

130 12

120

110

10 PU-0

PU-1

PU-2

PU-5

627 628 629

Fig. 17. Effect of contact angle on water absorption of PU foams.

630

The % linear changes in length, width, and thickness after exposure at 70 and -20°C for up to 14 days for PU

631

foams are presented in Fig. 18. The dimensional stability of PU foams indicates that the addition of curcumin

632

resulted in negligible changes of dimensional stability of the modified foams in relation to the PU-0. In all cases

633

the variations in the sample’s dimensions after the special treatment are random and thus they can be attributed

634

mostly to experimental errors while measuring. According to the industrial standard, PU panels tested at 70oC

635

should have less than 3% of linear change [71]. In each case, the dimensional stability of PU foams is thus still

636

considered to be mild and within commercially acceptable limits [71]. 4

4 Width

Length

Thickness

b) Dimensional stability [%]

Dimensional stability [%]

a) 3

2

1

0

637 638

Width

Length

Thickness

3

2

1

0 PU-0

PU-1

PU-2

PU-5

PU-0

PU-1

PU-2

Fig. 18. Dimensional stability of PU foams after exposure at a) 70°C and b) -20°C.

22

PU-5

639

4.

CONCLUSION

640

PU foams were successfully modified using curcumin as a natural compound. The impact of curcumin on

641

antibacterial properties, thermal properties, dynamic-mechanical properties, physicomechanical properties

642

(compressive strength, three-point bending test, apparent density, dimensional stability), foaming parameters and

643

morphology of PU foams was examined. The presented results indicate that the addition of curcumin in the

644

range of 1–5 wt.% influences the morphology of analyzed foams and consequently their further mechanical and

645

thermal properties. It was noticed that PU foams modified with curcumin are characterized by smaller and less

646

regular cells. Compared to the reference foam, compositions modified with 1 and 2 wt.% of the curcumin

647

showed greater compressive strength (293 and 285 kPa, respectively), higher flexural strength (0.469 and 0.432

648

MPa, respectively), less water uptake (10 and 11%, respectively), as well as, better thermal properties. The

649

results obtained in this study confirm that the addition of curcumin over a certain optimal level has a negative

650

effect on cell morphology. The addition of curcumin in the amount of 5 wt.% led to samples with reduced

651

compressive strength, thermal transitions and storage modulus respect to the PU foams containing 1 and 2 wt.%

652

of the filler, mainly due to detrimental changes induced by the filler. On the other hand, it has been shown, that

653

in all cases, curcumin can be used as a natural anti-ageing additive for polymers. The addition of curcumin in the

654

amount of 1, 2 and 5 wt.% considerably improves the stabilization of the polymer. Moreover, based on the

655

microbiological results, it has been shown that the addition of 5 wt.% of the extract is suitable for the

656

manufacturing of antimicrobial PU foams.

657 658

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859

29

1. Turmeric extract has an influence on foaming process of polyurethane system 2. Morphology of modified foams is significantly affected by the turmeric extract content 3. Foams containing turmeric extract in amount of 1 and 2 wt.% are characterized by better mechanical and thermal properties properties 4. Introduction of turmeric extract over a certain optimal level (5 wt.%) leads to deterioration of physico-mechanical properties of the modified foams 5. Foams containing turmeric extract in each amount are characterized by better antiageing and antibacterial properties