The Effect of Acid Hydrolyzed Sago Starch on Mechanical Properties of Natural Rubber and Carboxylated Nitrile Butadiene Rubber Latex

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ScienceDirect Materials Today: Proceedings 17 (2019) 1047–1055

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RAMM 2018

The Effect of Acid Hydrolyzed Sago Starch on Mechanical Properties of Natural Rubber and Carboxylated Nitrile Butadiene Rubber Latex S. Dauda , Y. S. Youa, and A. R. Azuraa* a

School of Materials &Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia

Abstract Sago starch is an organic filler that provide biodegradable properties to natural and synthetic rubber latex thin film products upon disposal. There is antagonism effect between biodegradability and mechanical properties of sago filled XNBR latex films where it prone to agglomerate and incompatible with latex system which, eventually reduced the mechanical properties of the latex films. The objective of this research is to develop a small particle size sago starch to improve the mechanical properties and biodegradability of latex films upon disposal. The native sago starch (NSS) was hydrolyzed with different hydrolysis condition at different temperature, acidity and duration. The results showed that the optimum synthesis parameters of preparation acid hydrolysis sago starch was at 2.18 M sulphuric acid under non heated condition (27o C) for 7 days. The particle size of native sago starch is 1.233 µm and it reduced to 0.313 µm under same condition. It was found that the AHSS has improved mechanical properties and swelling resistance of latex films as compared to NSS filled NR latex films and NSS filled XNBR latex films. The biodegradation assessment by mass loss analysis indicates AHSS filled NR latex and XNBR filled latex films has higher degree of biodegradation rate. Overall addition of AHSS has increased the mechanical and biodegradable properties of both type of latex films (NR and XNBR) compared to NSS. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: carboxylated nitrile butadiene rubber, natural rubber, mechanical properties, starch

* Corresponding author. Tel.: +604-599611; fax.; +604-5996907 E-mail address: [email protected]

2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.

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1. Introduction Gloves are the world most produced latex-based products which are made of either from NR latex or synthetic latex. Each year, the global demand for latex gloves increase despite many environmental issues regarding the latex products and rubber product generally. The single application of medical and general purposes gloves has caused major disposal problems. The most effective approach to this problem is by incorporating degradable material into the latex system to speed up the degradation process. Starch is one of commonly used polysaccharide for biodegradable latex film. Starch is composed of 20-30% of amylose content and 70-80% of amylopectin content [1]. Amylose is a linear polysaccharide with α- 1-4-linked D-glucose. Amylopectin is a high molecular weight polymer having a similar structure as amylose joined by α-1-6- glucan branches [2]. Sago starch (Metroxylon sagu) was chosen as filler because it is can be easily found locally and inexpensive. Sago starch has the highest amylose content (27%) compared to other types of starches [3]. Amylose content in starches is important criteria for biodegradable material as it can become sources of nutrient for microorganisms to initiate biodegradation process. However, starch without some modification is rather useless. Physical or chemical modifications are compulsory to meet specific properties of latex films. Chemical modification of native sago starch can be achieved via a few methods; for instance, modification by chemical treatment [4]. Among other modification approaches, acid hydrolysis becomes one of the most effective method to reduce particles size of the starch. Acid hydrolysis induces the formation of sulphate ester group on the surface of the starch that can increase the interaction between rubber and starch [5]. Strong acids such as hydrochloric acid (HCl) and sulphuric acid (H2 SO4) are commonly used for starch hydrolysis [6]. Rohana et al [4] have reported on the positive effect of AHSS on mechanical properties of NR latex films, on the contrary, impart poor biodegradability properties. This research is conducted to study the potential of AHSS on mechanical and biodegradability properties of XNBR latex films in comparison to NR latex films. 2. Methodology 2.1. Materials The materials used in this experiment such as high ammonia natural rubber latex (HA-NR latex) was purchased from Zarm Scientific and Supplies (Malaysia) Sdn Bhd. The XNBR latex was supplied by Synthomer Sdn. Bhd. The native sago starch used was supplied by Sago Link Sdn Bhd. Sulphuric acid (H2 SO4) used as hydrolysis agent was purchased from Merck (USA) Ltd with molar mass 98.08 g/mol and concentration of 18 M. Suphur, zinc oxide, antioxidant, potassium hydroxide and diethyldithiocarbamate were all purchased from Farben Technique (M) Sdn. Bhd. 2.2. Preparation of acid hydrolysed sago starch (AHSS) The acid hydrolysis procedure was adapted from Angelier et al [7] with some modification. Native sago starch was mixed with aqueous H2SO4 solution. Then, after hydrolysis, the suspension was filtered using filter paper and washed several times with distilled water until pH 7. The suspension was centrifuged at 3,000 rpm for 15 minutes until tested neutral. AHSS then dried in an oven at 50o C for 30 minutes. The native sago starch (NSS) and acid hydrolysis (AHSS) were prepared by mixing the mixture for 30 minutes under 300 rpm to ensure complete homogenization dispersion of the filler. The NSS and AHSS dispersions were treated by ball mill for 24 hours under speed of 200 rpm. 2.2.1. Preparation of latex films The NR latex was sieved before addition of compounding ingredients to remove flocs. The latex compound was stirred using mechanical stirrer under 200 rpm for 1 hour. The mixed latex mixture was subjected to 80oC water bath and constantly stirred under 200 rpm for prevulcanization process. The NR latex compound was matured for 24 hours at room temperature before dipping process. The compounding of XNBR latex was carried out similar to NR latex except that the maturation period was 48 hours at room temperature before dipping process. Prevulcanized latex compounds were stirred for 15 minutes at 200 rpm prior to dipping process. Clean aluminium plates dipped

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into coagulant tanks for 10 seconds and then dried in an oven at from 100 oC for 5 minutes and allowed to cool at room temperature for 5 minutes before dipped into latex dipping tank for 10 seconds and cured in an oven at 100 oC. The NR latex was cured for 10 minutes while XNBR latex was cured for 90 minutes. 2.3. Particle size and zeta potential test The NSS and AHSS was diluted to 10% by weight in distilled water. Both specimens were used as a reference for the NSS/NR, AHSS/NR, NSS/XNBR and AHSS/XNBR latex dispersions. The latex dispersions were diluted with distilled water to 0.1 % by weight and all the specimens were subjected to sonication to achieve a homogenous dispersion. The zeta potential for all specimens was analysed using Malvern ™Zetasizer 2.4. Swelling Test The swelling test was carried out according to ASTM D471 where a test piece weighing about 0.2 g was cut from NR and XNBR latex films. The films were immersed in pure toluene in universal bottle before transferred into a water bath and heated for 48 hours at 40°C. The test piece was taken out, the loose liquid rapidly removed by blotting with filter paper and the swollen weight was immediately measured by using equation 1. Swelling index = [(Average swollen diameter – unswollen diameter) / Unswollen diameter] x 100

(1)

2.5. Testing 2.5.1. Tensile Test The tensile properties of both NR and XNBR latex films determined by using Instron Universal Tensile machine according to ASTM D412. The latex films were cut into dumbbell shape by using Wallace die cutter. The crosshead speed was set to 500 mm/min. The results of tensile strength, modulus (at 100 and 300 % elongation) elongation at break of the latex films were obtained. The results were reported from the average of 5 samples. 2.5.2. Tear Test Tear test pieces were prepared using crescent cutter for both type NR and XNBR latex films. The test was carried out by pulling the crescent test piece using Instron Universal tensile testing machine according to ASTM D624. The tear test was carried out with the crosshead speed of 50 mm/min. the results were reported from the average of 5 samples. 2.5.3. Mass Loss The mass loss test was carried out for 3 weeks. The films were buried in soil. The withdrawn films were washed off and dried overnight. The initial mass of both latex films before (Mi) and after soil burial, (Mf) were weighed and recorded. The mass of both latex films, before after soil burial is measured and recorded. Mass loss was calculated according to equation 2. [Mi – Mf) / Mi] x 100%

(2)

2.6 Scanning Electron Microscope (SEM) The morphology of NSS and AHSS were examined by Scanning Electron Microscopy (SEM) Zeiss Supra 35 VP. The filler powders and latex films were sprinkled, attached on the carbon tapes, mounted on an aluminium stubs and coated with gold using Emitech K575x sputter coating for 30 seconds before subjected to SEM analysis.

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3. Results and Discussion 3.1. Particle Size Fig. 1 shows the average particle size of NSS and AHSS. The particle size of NSS measured by zeta nanosizer is 1233 nm. After acid hydrolysis, the particle size decreased significantly. The reaction time used from 3, 5, 7 and 9 days at room temperature for 2.18 M H2SO4. The particle size of sago starch reduced from 444 nm (3 days), 346.50 nm (5 days) to 313.10 nm (7 days) and then increased to 447.7 nm (9 days). The amorphous region of the starch granules is susceptible to strong acid such as H2SO4 which caused scission to its glaucosidic linkages. Partially removal of the amorphous region of starch granules reduces the particle size of the AHSS [8]. The particle size of AHSS had increased due to swelling of the opened and disordered structure of sago starch granules after 9 days of prolonged treatment as a result of increased water absorption [9].

Fig. 1. Average particle size of AHSS with treatment duration

Table 1 shows the zeta potential values of unfilled, NSS-filled and AHSS-filled latex compounds of both NR and XNBR latex. This test was conducted to investigate the stability of the colloidal dispersion for NSS and AHSS particle dispersions in NR and XNBR latex. In comparing NR latex compound, AHSS filled NR latex compound is least stable. This could be due to acid residual in the AHSS that could destabilize the latex compound. The presence of a trace of acid also reduced particles polarity in latex medium which makes it prone to agglomeration [10]. However, the XNBR latex does not influence by AHSS significantly due to lack of protein and some constituents commonly present in NR. Table 1. The effect of treatment condition on AHSS zeta potential Latex Compound Unfilled NR

Zeta Potential (mV) -65.5

NSS filled NR

-63.7

AHSS filled NR

-49.5

Unfilled XNBR

-48.2

NSS filled XNBR

-46.2

AHSS filled XNBR

-39.2

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3.2. Swelling Properties Swelling values can provide a piece of information on the capacity of the crosslinked polymer to absorb different liquids [11]. Fig 2 shows the swelling percentage of unfilled (control), NSS and AHSS filled NR and XNBR latex films. The swelling percentage of unfilled (control) NR latex films is the highest. The excessive toluene uptake during swelling for unfilled NR latex films is easily diffusion of solvents through its segmental mobility and a large amount of free volume between polymer chains [12]. The entanglement between NR and glycosidic links (starch) prevent the penetration of solvent to NR latex film. Moreover, contribute to the reduction in penetration of solvents into the latex films and restrict the chain mobility, thus, reduces the swelling percentage of starch filled-NR latex film [13]. The strong filler-filler interaction due to the hydrophilic nature of starch caused agglomeration and also prevent the penetration of solvent into NR latex films. The addition of AHSS has reduced the swelling percentage of NR latex films compared to NSS and control NR latex films. The improvement was contributed by the increase surface activity due to small particles size of AHSS and presence of sulphate ester which enables the formation of C-O-C bonds between AHSS and NR thus resulted in improve rubber-filler interaction of the latex films after vulcanization [14]. Whilst, the swelling percentage of unfilled (control) XNBR latex film is the lowest followed by AHSS and NSS-filled XNBR latex films. The swelling percentage of XNBR latex films for control, NSS and AHSS filled XNBR films were lower compared to NR latex films due to inherent properties of XNBR with bulky groups.

Fig. 2. Swelling percentage (%) of control, NSS and AHSS–filled NR or XNBR latex films

3.3. Mechanical properties latex films The mechanical properties of unfilled (control) NR latex, filled (NSS and AHSS) NR latex, unfilled (control) XNBR and filled (NSS and AHSS) XNBR latex are presented in Table 2. The unfilled NR latex showed higher tensile strength than filled NR latex because of its ability to strain-induced crystallization upon stretching [15]. The decrement in tensile strength of NR latex films after addition of NSS due to weak interfacial adhesion between hydrophilic nature of sago starch and hydrophobic nature of NR latex. The non-reinforcing nature of starch also contribute to a decrease in tensile properties of NR latex films. As for AHSS filled NR latex films, the AHSS/NR latex films showed slightly higher tensile strength due to the smaller particle size of AHSS. The smaller particle size promotes better interaction with the latex matrix [16]. The elongation at break (EB) of unfilled NR latex films is the highest followed by AHSS and NSS filled NR latex films. The flexibility of the chain molecules of NR decreased with the addition of sago starch which makes the films become stiffer and enabled the film to break at shorter elongation or lower force. However, the AHSS filled NR latex films has higher EB than NSS filled NR latex films because AHSS has lower crystallinity which induces the formation of stress-induced crystallization region of polymeric chain to reduce the crack propagation [17]. The incorporation of sago starch in NR latex increased the moduli (M100 and M300) in general. However, the moduli for NSS/NR latex films are lower than AHSS/NR latex films.

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The addition of sago starch reduced the tensile strength of XNBR latex films. The formation of starch gelatinized region prevents coalescence of XNBR latex and colloidal particles during the formation of films [18]. As a result, fewer ionic crosslinks and monosulphic linkages formed in XNBR latex films [17]. The elongation at break of XNBR latex films decreased with the incorporation of NSS and AHSS. The flexibility of the chain molecules in NBR latex films can lead to a decrease in an inability to elongate further before break [19]. The stiffening effect and improved rubber-filler interaction of XNBR films contributed by the presence sulphate ester on AHSS particles. The influence of AHSS in XNBR latex films are more significant in elongation at break compare to tensile strength. The effect of AHSS on modulus is relatively significant in XNBR films compared to NR latex films due to the smaller particle size of AHSS and presence of sulphate ester group. The tear strength of XNBR and NR latex films showed a similar trend. The good rubber-filler interaction reduced the crack propagation of the film. Table 2. The tensile and tear properties of control, NSS and AHSS-filled NR or XNBR latex films Properties

Control

NSS

AHSS

NR latex films Tensile strength (MPa)

22.47

15.12

18.67

Elongation at break (%)

1192.33

832.77

878.33

0.71

1.46

1.52

M100 (MPa) M300 (MPa)

1.43

2.75

2.52

Tear strength (N/mm)

72.83

51.87

65.11 11.46

XNBR latex films Tensile strength (MPa)

15.91

12.04

Elongation at break (%)

577.23

516.67

467

M100 (MPa)

2.15

2.59

3.24

M300 (MPa)

5.4

5.45

6.57

36.76

29.31

30.98

Tear strength (N/mm)

3.4. Mass loss Fig. 3 shows the mass loss of unfilled, NSS and AHSS –filled NR and XNBR latex films after soil burial for 3 weeks period. AHSS filled NR latex films showed the highest percentage of mass loss followed by NSS and unfilled NR latex films. Mass loss trend for NR latex film is similar to XNBR latex films. Both unfilled NR and XNBR latex films have a lower mass loss. The presence of compounding additives namely accelerator, activator, antioxidant and sulfur inhibit the biodegradation rate of latex films [20]. However, incorporation of sago starch attracts soil microorganism to consume the biodegradable starch and secrete an enzyme that can break down rubber molecular chains [21]. The consumption of starch by microorganisms formed porosity in latex films and caused mass loss to latex films [22]. The AHSS filled NR latex films and AHSS filled XNBR latex films showed a high mass loss. This could be attributed to the reduction of amorphous region after acid hydrolysis of sago starch which leads to ease of microorganisms attack on rubber and glycosidic chains [23].

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Fig. 3. The mass loss (%) of control, NSS and AHSS-filled NR or XNBR latex films after 3 weeks

3.4. Morphological Properties Fig. 4 shows the images of NSS and AHSS. The SEM micrograph displays the shape and size of sago starch particles before and after acid hydrolysis. The average granule size of NSS is 1.233 µm, after acid hydrolysis reduced to 0.313 µm. The surface of the AHSS granules become more porous, rougher and mostly eroded. The morphology of biodegradation of NR and XNBR latex films with incorporation of NSS and AHSS are shown in Fig. 5. The dark spots are observed more severely on the surface of the NSS and AHSS filled NR and XNBR latex films samples compared to control samples indicate the presence of bacterial growth. The presence of micro voids also signifies the consumption of starch by the soil microorganisms.

(a)

(b)

Fig. 4. SEM images of (a) NSS and (b) AHSS at 500x of magnification

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Fig. 5. SEM images of (a) control XNBR film (b) NSS filled XNBR film (c) AHSS filled XNBR film and (d) control NR film (e) NSS filled NR film (f) AHSS filled NR film after 3 weeks soil burial

4. Conclusion The tensile properties of NR latex films decreased with addition of NSS due to weak interfacial adhesion between hydrophilic nature of sago starch and hydrophobic nature of NR latex. However, the incorporation of AHSS in latex films shows an improvement in mechanical properties and swelling resistance of NR and XNBR latex films compared to NSS. This could be due to better compatibility of AHSS in NR and XNBR latex films compared to NSS in NR and XNBR latex films. The low amorphous, smaller particles size, and presence of sulphate ester contribute to increased rubber-filler interaction between AHSS and rubber matrix. The AHSS filled NR latex films showed the highest percentage of mass loss followed by NSS and unfilled NR latex films. Mass loss trend for NR latex film is similar to XNBR latex films. Both unfilled NR and XNBR latex films have a lower mass loss. Acknowledgement The authors would like to acknowledge the School of Materials and Mineral Resources Engineering of the Universiti Sains Malaysia for support and research facilities. This work is supported by the Fundamental Research Grant (Grant No. 203 / PBAHAN / 6071353). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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