Deproteinized natural rubber film reinforced with silane-modified silica nanoparticles

Abstract

Mechanical properties and morphology of the deproteinized natural rubber (NR) films reinforced with modified silica (SiO₂) nanoparticles were studied in this research. In the step of deproteinization, a chemical method followed by centrifugation was used to reduce the amount of protein from 0.42 wt% to 0.11 wt%. Structure-modified nanosilica with Silane A174 was used as an additive to reinforce the deproteinized films. The addition of 1 phr of the modified nanosilica exhibited better tensile strength, tensile force at break, modulus at 500% and elongation at break which were improved significantly compared to those of bare films. The modified nanosilica dispersed uniformly in the rubber matrix with the nanoparticle size less than 100 nm observed from the scanning electron microscope. The prepared rubber films containing the modified SiO₂ nanoparticles have great potential to produce the NR gloves having very tiny amounts of protein for medical use which still provide the good mechanical strength compared to the standard ones.

Keywords

Deproteinized natural rubber natural rubber latex film nanosilica modified silica nanoparticle medical glove

Introduction

Natural rubber (NR) field latex, obtained from Hevea brasiliensis, is a white milky liquid that contains about 30–40% rubber fraction and 6% non-rubber components such as proteins, carbohydrates, lipids and ash. All compositions are dispersed in water as an emulsion.^1,2 Structural studies of NR by nuclear magnetic resonance spectroscopy showed that the rubber molecule consists of two trans-1,4 isoprene units, approximately 4000–5000 cis-isoprene units and two ending units such as ω-terminal and α-terminal.^3,4 At the terminal ends, some of the proteins are attached and present as a charged layer covering the rubber latex particle to stabilize the latex particles against coagulation.^5,6 Moreover, proteins are essential components that affect to cure characteristics of the rubber and play an important role in physical properties.⁷ These proteins, however, are allergenic.^8

–12 Hev b1 and Hev b3 are the two major allergenic proteins from 14 NR latex allergenic proteins (Hev b1–Hev b14). The soluble proteins are usually removed during the process of creaming or centrifugation to produce a concentrate from fresh latex. However, the remaining allergenic proteins can cause allergenic contact dermatitis. They could be further reduced by deproteinization of NR. Therefore, the protein-free NR is widely known as deproteinized natural rubber (DPNR).

Several rubber deproteinization methods are available to reduce the protein content of the NR latex, such as the use of enzyme, urea, surfactant and alkali. Proteolytic enzymes get rid of proteins by cutting them off into small peptides. Unfortunately, a long incubation time and strict temperature control is necessary for enzymatic deproteinization.^13

–19 Urea is used as a protein denaturant in the presence of a surfactant.^20

–23 Washing NR latex with non-ionic surfactants such as sodium dodecyl sulphate (SDS)²⁴ and polyethylene glycol²⁵ efficiently reduce proteins from the rubber particles by transferring them to the serum phase. Alkali such as sodium hydroxide (NaOH) also acts as a denaturant to hydrolyze proteins.²⁶

It is important to investigate the properties of vulcanized latex films in order to apply DPNR latex as rubber products such as medical-use gloves because NR exhibits excellent elasticity, flexibility, good formability and biodegradability.^27
–29 However, DPNR latex products, compared with those from NR latex, have major drawbacks on low tensile strength and poor tear resistance, especially for medical gloves and condoms. Many attempts used traditional reinforcement materials such as starch,³⁰ carbon black,³¹ ultrafine calcium carbonate³² and modified montmorillonite³³ to reinforce dry rubber or latex. However, these materials are not so effective for the latex.

Polymer-inorganic nanocomposites exhibit a combination of benefits of polymer (such as flexibility, toughness and an ease of processing) and those of inorganic material (such as durability, hardness and thermal stability).³⁴ Peng et al.³⁵ prepared an NR/silica (SiO₂) nanocomposite by employing poly(diallylmethylammonium chloride) as an inter-medium. The tensile strength, tensile modulus and tear strength of the nanocomposite are significantly improved at SiO₂ loadings of 2.5–4 wt%. Moreover, the thermal and thermo-oxidative ageing resistances of the prepared nanocomposite increased 10–25°C above those of the pure NR.

This present work aims to prepare DPNR using three different methods, that is, surfactant washing, urea treatment and saponification in the presence of urea. A series of experiments were conducted to explain the influence of deproteinization treatment on the protein content of NR latex. Moreover, the physical properties of DPNR latex and mechanical properties of DPNR latex reinforced with inorganic reinforcing material, that is, silane-modified nanosilica, were investigated. Furthermore, the morphology of the nanocomposite films was also reported.

Experimental

Materials

High ammonia natural rubber (HANR) latex with 60% dry rubber content (DRC), potassium hydroxide, sulphur (S), zinc diethyldithiocarbamate, zinc 2-mercaptobenzothiazole, Wingstay-L, zinc oxide, calcium nitrate (Ca(NO₃)₂·4H₂O), deoxychloric acid, trichloroacetic acid, phosphotungstic acid, Vultamol and Bentonite clay were received by the Rubber Authority of Thailand. SDS, NaOH, calcium chloride, acetic acid (CH₃COOH) and urea were supplied from Mehta Group Trading Ltd (Thailand). Selenium powder, zinc metal, anhydrous potassium sulphate (K₂SO₄), cupric sulphate (CuSO₄·5H₂O), boric acid, sodium carbonate, bromocresol green and methyl red indicators were purchased from TTK Science Co., Ltd (Thailand). SiO₂ (99.8% purity) nanoparticles with a diameter of 12 nm and surface area of 175–225 m² g⁻¹ and 3-(trimethoxysilyl)propyl methacrylate (Silane A174: 98% purity) were purchased from Sigma-Aldrich Co., Ltd (Thailand). All materials were used as received without further purification.

Preparation of silane-modified nanosilica

The SiO₂ nanoparticles were surface-modified by Silane A174 in accordance with the Dow Corning Method. Silane A174 was applied to SiO₂ surfaces as a dilute aqueous solution. CH₃COOH was used to adjust the pH of water (800 mL) to be 3.5–4.5. Then, Silane A174 (4 g) was added and stirred for a minimum of 30 min before it hydrolyzed and formed a clear homogeneous solution. Dry SiO₂ nanopowder (13.33 g) was then added to the solution while stirring for a further 30 min. In the last step, the mixture was vacuum dried at 120°C for 12 h.

FTIR spectral analysis of silane-modified nanosilica

Functional group analysis of bare nanosilica and silane-treated nanosilica was carried out using Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer: Spectrum 100, Waltham, USA). All samples were dried thoroughly and kept in a desiccator before analysis. The specimens were taken over a frequency range of 500–4000 cm⁻¹.

Preparation of DPNR latex

Three types of DPNR latex were made from HANR latex using SDS, urea and NaOH, respectively, through a batch process. In the researchers’ previous work,³⁶ the effects of centrifugation, reagent concentrations, incubation temperature and time on the protein content were evaluated. The DPNR latex prepared with SDS was derived from 100 parts per hundred of rubber (phr) of HANR (60% DRC) latex and 100 phr distilled water in the presence of 1.0 phr SDS under the shaking incubator at 150 r min⁻¹ under 30°C for 60 min. The centrifugation was controlled at 10,000 r min⁻¹ under 25°C for 30 min and used to separate the latex into a cream fraction, which was re-dispersed in a 1.0 wt% SDS solution to make 30% DRC latex. Then, the latex was second centrifuged and re-dispersed with the SDS solution to make 60% DRC. The obtained latex was called S-DPNR. The DPNR latex prepared using urea was designated as U-DPNR latex. The incubation of HANR latex was performed with 0.1 phr urea in the presence of 1.0 phr SDS at 50°C for 120 min. The double centrifugation was applied at 10,000 r min⁻¹ under 25°C for 30 min. The last type of DPNR latex was prepared using a strong base NaOH. This called B-DPNR latex was performed using exactly the same method as that from U-DPNR latex except the use of 2.0 wt% NaOH solution instead of distilled water.

Measurement on physical properties of DPNR latex

The total solid content (TSC) and DRC of the latex were measured as described in ISO 124:2014 and ISO126:2005, respectively. For the TSC, 2.0 g of latex was weighed exactly (m ₀) into a Petri dish and dried at 70°C in a hot air oven for 16 h. The dry latex was cooled in a desiccator to ambient temperature and weighed. The sample was returned to the oven for 30 min at 70°C, allowed it to cool as before and reweighed (m _TSC). The drying procedure was repeated for a period of 30 min until the loss in mass between two successive weightings was less than 0.5 mg. The percentage of TSC was calculated as equation (1). Ten replications were measured.

TSC = \frac{m_{TSC}}{m_{0}} \times 100

For the DRC, 10 g of latex was weighed exactly (m ₀) into a Petri dish and diluted with 20 mL of distilled water. The dish was carefully rotated to dilute the latex and ensure homogeneity. While the 2% v/v CH₃COOH was being added, the dish was slowly rotated to allow the rubber to coagulate thoroughly. The dish was covered with a watch glass and heated on a steam bath for 30 min. The coagulum was washed with distilled water, made into a 0.2-mm thin sheet by roller, and dried at 70°C in a hot air oven for 16 h. The dry latex sheet was then cooled in a desiccator to room temperature and weighed. The process of drying was repeated and reweighed (m _DRC). The percentage of DRC was calculated as equation (2). Ten replications were measured.

DRC = \frac{m_{DRC}}{m_{0}} \times 100

The determination of volatile fatty acid (VFA) number was done by ISO 506:1992. Fifty grams (m ₀) of latex and 50 mL of ammonium sulphate were accurately added and mixed into a beaker. Then, the beaker was placed on the steam bath, maintained at 70°C, and continued stirring the latex until it coagulated. The beaker was left on the steam bath for 15 min and decanted through a dry filter paper for serum. The 25 mL of serum was acidified by 5 mL of 50% wt/wt sulphuric acid and mixed well. The 10 mL of acidified serum was conducted with the distillation process until 100 mL of distillate had been collected. Any dissolved CO₂ from the distillate was eliminated by passing through a stream of air free from CO₂. Finally, it was titrated with barium hydroxide solution with known molarity (M) and volume in mL (V). The VFA number was calculated with the following equation, where ρ is the density of the serum which is equal to 1.02 mg m⁻³.

VFA number = (\frac{134 .64 \times M \times V}{m_{0} \times TSC}) \times (50 + \frac{m_{0} (100 - DRC)}{100 \times ρ})

The determination of sludge content was carried out by ISO 2005:2014. Then, 40–45 g (m ₀) of latex was added and weighed into two centrifuge tubes. The centrifugation was operated under 2300 r min⁻¹ for 20 min. The cream layer was scooped off followed by drawing off the supernatant liquid to approximately 10 mm above the top of the sludge. The ammonia–alcohol solution was added and the process of centrifugation was re-operated for 25 min. The procedure was repeated until the supernatant liquid was clear after centrifuging. The sludge was dried at 70°C in a hot air oven for approximately 1 h and weighed (m _sludge). The percentage of sludge content was calculated as follows:

Sludge content = \frac{m_{sludge}}{m_{0}} \times 100

The protein content was determined by the Kjeldahl method as described in ASTM D3533. Briefly, strong sulphuric acid solution and the catalyst mixture of K₂SO₄, copper (II) sulphate and selenium powder were used to digest the dry rubber sample and converted into ammonium ion. The distillation process was used to convert ammonium ion into ammonia gas. The nitrogen content (% nitrogen) could be found via titration with sulphuric acid and calculation using equation (5), where V ₁ and V ₂ are volumes in cubic centimetre of sulphuric acid for titrations of the sample and blank, respectively. M is the concentration in molarity of sulphuric acid. W is the weight of a sample used. The protein content (% protein) was calculated from the nitrogen content using equation (6):

% Nitrogen = \frac{(V_{1} - V_{2}) M \times 0 .0140 \times 100}{W}

% Protein = 6.25 (% nitrogen)

Particle size distribution and zeta potential analysis of latex

The determination of particle size distribution and zeta potential values were carried out using a NANO Plus 3 particle analyzer (GAT Scientific, Malaysia). The rubber latex was dispersed in de-ionized water to make about 10–15% v/v of rubber latex before the analysis. The latex was maintained colloidal stability via the surface charges having either lower of −30 mV or higher of +30 mV.³⁷

Preparation of compounded latex films

The compounded DPNR latex was prepared by compounding of the NR latex (HANR or DPNR latex) with all components according to the medical glove formulation shown in Table 1. Each ingredient was added one by one, followed by stirring for 3 min in each addition to obtain the homogenization. After all components were added, the mixture was stirred for further 30 min and incubated at room temperature for 72 h.

Table 1.

Formulation of compounded latex.

Ingredients	Weight
Ingredients	phr	g
60% HANR or DPNR latex	100	167
10% KOH	0.4	4
50% Sulphur	0.5	1
50% ZDEC	0.75	1.5
50% ZMBT	0.25	0.5
50% Wingstay-L	1	2
50% ZnO	0.5	1
Distilled water	30	30
10% modified nanosilica	0, 0.25, 0.5, 1, 1.5	0, 2.5, 5.0, 10, 15

HANR: high ammonia natural rubber; DPNR: deproteinized natural rubber; KOH: potassium hydroxide; ZDEC: zinc diethyldithiocarbamate; ZMBT: zinc 2-mercaptobenzothiazole; ZnO: zinc oxide; phr: parts per hundred of rubber; g: gram.

The vulcanization level of the compounded latex is necessary to be checked before forming the latex film. Chloroform was mixed with the latex sample (1:1) to check latex coagulation, which is depending on the incubation time. Three days of incubation usually obtains a good result in chloroform test. The preparation of latex film was performed by a dipping method. The dipping was achieved by using a glass tube as a mould. The mould was dipped into the 10% w/v Ca(NO₃)₂·4H₂O as the coagulant for 5 s followed by drying at room temperature for 3 min. The dried mould was dipped into the compounded latex and soaked for 15 s. The soaked latex was allowed to coagulate at room temperature for 30 min before the vulcanization in an oven at 100°C for 20 min. In the case of washed latex gloves, the mould was soaked into hot water at 70°C for 1 min before the vulcanization process.

Measurement on extractable protein content in compounded latex films

The water-extractable protein content was followed by ISO 12243:2012 using the modified Lowry method. The extraction was carried out in triplicate using three gloves from a given lot. The purification and concentration of each extract and the subsequent determination were run singly. A solution of ovalbumin protein was used as a standard solution at different concentrations. The UV spectrophotometer (Shimadzu: UV-1280, USA) was used to determine the absorbance at 280 nm. The calculation was given by equation (7), where E is the extractable protein content (µg g⁻¹ of glove), V is the volume of extractant used (cm³), c is the protein concentration in the redissolved-protein solution (µg cm⁻³), F is the concentration factor and m is the sample mass (g).

E = \frac{V × c ×5}{F × m}

The water-extractable protein content in the compounded latex films was determined in both unwashed and washed films. The washed process was performed by dipping the films into 70°C water for 1 min before cutting them into pieces of rubber film sample.

Mechanical properties of films

Mechanical properties such as tensile strength, modulus and elongation at break were determined using a universal testing machine (Instron, USA). The determination of tensile properties was followed by the ASTM D412 standard. Ten pieces of dumb-bell shaped specimens were cut from each type of compounded film. The thermal ageing properties of the films were studied by placing the specimens in an ageing oven at 70°C for 168 h prior to the determination of tensile properties.

Morphology

The film specimens were fractured after immersion into liquid nitrogen and the fracture surfaces were sputter-coated with conductive gold before observation. Scanning electron micrographs (SEM) were taken with a high-vacuum microscope (ThermoFisher Scientific: Model Quanta 450, FEI, USA) at an accelerated voltage of 10 kV with a magnification up to 20,000.

Results and discussion

FTIR spectral analysis of silane-modified SiO₂ nanoparticles

In the FTIR spectrum of bare nanosilica (Figure 1(a)), the most intensive absorption band at 1100 cm⁻¹ together with the less intensive absorption bands at 810 cm⁻¹ are attributed to the vibrational absorption of Si–O–Si groups. Moreover, the absorptions at 1630 and 3435 cm⁻¹ are indicated that the SiO₂ contains hydroxyl groups (O–H) on the surface. After the introduction of silane (Figure 1(b)) to nanosilica (Figure 1(a)), FTIR of the silane-treated nanosilica (Figure 1(c)) exhibited the additional absorption bands at 1731 and 2923 cm⁻¹ ascribed to C=O and C–H from silane, respectively. This indicates that silane reacts with the silanol groups of SiO₂,^38,39 as shown in Figure 2, to introduce a non-polar end group onto the surface of SiO₂, which can easier reinforce with the non-polar NR latex.

Figure 1.

FTIR spectra of (a) fumed nanosilica, (b) Silane A174 and (c) silane-treated nanosilica. FTIR: Fourier transform infrared; Silane A174: 3-(trimethoxysilyl)propyl methacrylate.

Figure 2.

Functionalization of SiO₂ by Silane A174.

Physical properties of latex

The physical properties, that is, TSC, DRC, sludge content and VFA number of HANR and DPNR latex were tabulated in Table 2. After deproteinization with various chemicals and centrifugation for several times, %TSC and %DRC of DPNR latex were lower than those of HANR latex. The sludge contents of all types of DPNR latex were significantly reduced because of the centrifugation process. This means that the non-polymeric substances were substantially removed by this method of rubber deproteinization. VFA number in rubber is referred to the degradation of proteins by bacteria present in the latex. VFA numbers of DPNR were twice lower than that of HANR. This may be due to the corresponding decrease of protein contents in DPNR latex. The protein content, determined by the Kjeldahl method, decreased from 0.42 wt% to 0.11 wt% after incubation with urea for 2 h at 50°C in the presence of NaOH and SDS solutions. It demonstrated that the proteins attached to the rubber with weak attractive forces could be detached with urea and NaOH.

Table 2.

TSC, DRC, sludge content, VFA number, nitrogen content and protein content of HANR and DPNR latex.

Sample	TSC (%)	DRC (%)	Sludge content (wt%)	VFA number	Nitrogen content (wt%)	Protein content (wt%)
HANR	60.92	60.15	0.0062	0.0312	0.0672	0.42
S-DPNR	60.63	59.83	0.0020	0.0204	0.0448	0.28
U-DPNR	60.41	59.74	0.0018	0.0183	0.0224	0.14
B-DPNR	60.16	58.91	0.0015	0.0145	0.0176	0.11

TSC: total solid content; DRC: dry rubber content; VFA: volatile fatty acid; HANR: high ammonia natural rubber; S-DPNR: sodium dodecyl sulphate-treated deproteinized natural rubber; U-DPNR: urea-treated deproteinized natural rubber; B-DPNR: base sodium hydroxide-treated deproteinized natural rubber.

Particle size distribution and zeta potential analysis of latex

The particle size distribution of B-DPNR latex is shown in Figure 3. The zeta potential values of HANR and DPNR latex are tabulated in Table 3. The charges of all rubber samples were negative at pH 8.3–9.8. The zeta potential values of S-DPNR, U-DPNR and B-DPNR latex were lower than that of HANR. This is because the protein molecules covered on HANR particles contain both carboxyl and amino groups. After the protein removal, the negative charge of rubber (DPNR) particles was reduced. The mean diameter of particle size of all samples is also given in Table 3. It was found that the particle diameter of all DPNR latex was larger than that of HANR latex. This may be due to the reduction of zeta potential values of DPNR latex. Nevertheless, the DPNR latex still maintains stability without self-coagulation after the removal of a negative charge (proteins). This is confirmed that SDS can be used to stabilize the latex particles against coagulation.

Figure 3.

Particle size distribution of B-DPNR latex.

Table 3.

Average diameter of particle and zeta potential values of NR latex.

Sample	Average diameter of particle (nm)	Zeta potential (mV)	pH
HANR	447.0	−58.78	9.8
S-DPNR	620.8	−45.95	8.3
U-DPNR	634.1	−44.67	8.4
B-DPNR	618.8	−46.70	8.4

NR: natural rubber; HANR: high ammonia natural rubber; S-DPNR: sodium dodecyl sulfate-treated deproteinized natural rubber; U-DPNR: urea-treated deproteinized natural rubber; B-DPNR: base sodium hydroxide-treated deproteinized natural rubber.

Extractable protein content of films

The amount of water-extractable protein contents of rubber compounded films are given in Table 4. The unwashed HANR film exhibited the protein content of 858.80 μg g⁻¹ of rubber, which was very much higher than that of the washed HANR film (260.95 μg g⁻¹). This confirmed that the water-soluble protein on the surface of rubber film could be reduced by immersing it into hot water. Nevertheless, all methods of deproteinization using chemicals could further remove proteins in the rubber latex. The results confirmed that the urea treatment in the presence of NaOH solution (B-DPNR) was quite effective to prepare a low protein rubber glove (16.33 μg g⁻¹). Moreover, the washing process with hot water could get rid of more proteins on the surface of the glove (7.62 μg g⁻¹).

Table 4.

Extractable protein contents in unwashed and washed compounded latex films.

Compounded films	Water-extractable protein content (μg g⁻¹)
Compounded films	Unwashed films	Washed films
HANR	858.80	260.95
S-DPNR	82.66	17.18
U-DPNR	20.18	16.92
B-DPNR	16.33	7.62

HANR: high ammonia natural rubber; S-DPNR: sodium dodecyl sulfate-treated deproteinized natural rubber; U-DPNR: urea-treated deproteinized natural rubber; B-DPNR: base sodium hydroxide-treated deproteinized natural rubber.

Mechanical properties of films

Table 5 lists the mechanical properties of the compounded films before and after heat ageing. Before the thermal ageing, most of the mechanical properties except elongation at break of HANR film were higher than those of DPNR films. This may be due to the fact that the DPNR latex could lose its mechanical properties by multi-centrifugation, so most tensile properties of the compounded DPNR films decreased as expected. Among the three types of DPNR films, their tensile properties were close to each other. In addition, it should be noted that the deproteinization process was found to increase the elongation at break of HANR film. Plus, heat ageing could enhance the stiffness and strength of both HANR and DPNR films. This is might be because of the increasing cross-linking density of latex films after obtaining additional curing.

Table 5.

Mechanical properties of compounded films before and after thermal ageing.

Property	Before ageing				After ageing
Property	HANR	S-DPNR	U-DPNR	B-DPNR	HANR	S-DPNR	U-DPNR	B-DPNR
Tensile strength (MPa)	24.0	17.4	19.3	17.8	25.0	19.6	20.0	20.5
Tensile force at break (N)	18.8	9.4	9.9	10.6	21.1	10.2	10.3	11.5
Tensile modulus (MPa)
300% Elongation	1.4	1.0	1.1	1.0	1.6	1.1	1.4	1.2
500% Elongation	3.1	1.9	2.7	2.7	3.5	2.0	2.9	2.8
Elongation at break (%)	756	854	785	792	716	835	759	790

The effect of nanofiller loading

Compared with ASTM D3578, all DPNR glove films meet some specification requirements of the type-1 examination glove (elongation at break before ageing ≥ 650% and one after ageing ≥ 500%). However, some types of films were under tensile strength specifications (before ageing ≥ 18 MPa and after ageing ≥ 14 MPa), which is maybe due to the preparation step of deproteinization process. To improve the mechanical properties of DPNR film by reinforcing with nanofiller, the B-DPNR film having the lowest extractable protein content was chosen.

The incorporation of inorganic nanofillers into the rubber matrix leads to a significant improvement in the mechanical properties of the host rubber. Figure 4 shows the mechanical properties of the pure HANR, pure B-DPNR and B-DPNR/modified SiO₂ nanocomposites with different modified SiO₂ contents. HANR films show an excellent mechanical property compared to those of B-DPNR. The improvement in B-DPNR films, however, is found after the addition of nanofillers because of the effective reinforcement of modified SiO₂. The tensile strength, tensile force at break and modulus at 500% increased with the modified SiO₂ loading when less than 1 phr of modified SiO₂ is loaded. However, with further addition of modified SiO₂, the tensile properties gradually decrease. This might be because of the nanofiller aggregation.

Figure 4.

Mechanical properties of pure HANR, pure B-DPNR and B-DPNR/mod. SiO₂ nanocomposites: (a) tensile strength, (b) tensile force at break, (c) modulus at 500% and (d) elongation at break. HANR: high ammonia natural rubber; B-DPNR: base sodium hydroxide-treated deproteinized natural rubber; mod. SiO₂: modified silica.

The fillers usually reduce the flexibility of HANR due to the restriction in the molecular chain slipping along the filler surface. The influence of filler on elongation at break plays a critical role at high filler loading or more than 2.5 wt%.³⁵ In this current work, an incorporation of a small amount of modified SiO₂ gave a remarkable enhancement in elongation at break, which reached a peak at modified SiO₂ loading of 1 phr.

Morphology

SiO₂ has a number of hydroxyl groups on the surface as its nature. This leads to the strong interaction among SiO₂ particles, and therefore fumed SiO₂ nanoparticles have strong self-aggregation. As can be seen in Figure 5(a) and (b), the fumed SiO₂ could form spherical nano-clusters with the cluster-size up to around 50 μm. However, these aggregated particles cannot be broken down by the shear forces during the compounding process.⁴⁰ When the unmodified nanosilica was used as filler in the NR matrix, severe aggregation could be observed as shown in Figure 5(c). The dark phase represents the NR matrix and the bright phase corresponds to the SiO₂ nanoparticles. This might be because of the huge difference in polarity between non-polar rubber and polar SiO₂. Once the polar SiO₂ particles were surface modified with non-polar chain end of silane and used as nanofillers in the non-polar matrix, most of the spherical clusters of nanoparticles were individually distributed among the NR matrix as in Figure 5(d). This resulted in better mechanical properties as described in the previous section.

Figure 5.

SEM images of (a) nanosilica cluster (×250 magnitude), (b) nanosilica cluster (×3500 magnitude), (c) 1 phr unmodified nanosilica/B-DPNR film (×20,000 magnitude) and (d) 1 phr modified nanosilica/B-DPNR film (×20,000 magnitude).

Conclusions

Deproteinization of HANR was achieved in the latex stage with urea and NaOH in the presence of surfactant followed by centrifugation. This procedure was proved to be an effective method to prepare DPNR latex. Physical properties of the DPNR latex, that is, %TSC, %DRC, sludge content and VFA number were lower than that of HANR latex meaning that non-polymeric substances were removed by this method. The protein content was reduced from 0.42 wt% (HANR) to 0.11 wt% (B-DPNR). However, the average particle diameter in DPNR latex (618.8 nm) was found to be larger than that of HANR latex (447.0 nm) due to the reduction in the negative charge of DPNR particles. In addition, the amount of water-soluble protein in DPNR film was reduced to 7.62 μg g⁻¹, which was extremely reduced compared to that of HANR film (858.80 μg g⁻¹). Unfortunately, the removal of proteins in rubber resulted in reduction in mechanical properties. Some types of DPNR film were, therefore, under the tensile strength specification followed ASTM D3578.

The nanocomposites of B-DPNR/modified SiO₂ were successfully prepared by latex compounding. The incorporation of an appropriate amount of 1 phr modified SiO₂ apparently improved the tensile strength, tensile force at break, modulus at 500% as well as elongation at break of the B-DPNR films. This was supported by the morphology from SEM that most of the spherical clusters of modified SiO₂ nanoparticles were individually distributed among the B-DPNR matrix. The better the dispersion of modified SiO₂ in the rubber matrix, the greater the improvement in the mechanical properties.

Footnotes

Acknowledgements

The authors are grateful to the Research Institute of Rangsit University for financial support. We are indebted to the Rubber Authority of Thailand for chemicals and instrument supports throughout the work. We also wish to acknowledge the Department of Chemistry, Faculty of Science, Rangsit University for material support.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors are grateful to the Research Institute of Rangsit University for the financial support.

ORCID iD

Supat Moolsin

References

Wei

Zhang

, et al. A review on characterization of molecular structure of natural rubber. MOJ Poly Sci 2017; 1: 197–199.

Sing

Chung

, et al. The micromorphology and protein characterization of rubber particles in Ficus carica, Ficus benghalensis and Hevea brasiliensis . J Exp Bot 2003; 54: 985–992.

Tanaka

. Structural characterization of natural polyisoprenes: solve the mystery of natural rubber based on structural study. Rubber Chem Technol 2001; 74: 355–375.

Tanaka

. Structure and biosynthesis mechanism of natural polyisoprene. Prog Polym Sci 1989; 14: 339–371.

Mekkriengkrai

Sakdapipanich

Tanaka

. Characterization of terminal groups in natural rubber: origin of nitrogenous groups. Rubber Chem Technol 2006; 79: 366–379.

Tarachiwin

Sakdapipanich

Ute

, et al. Structural characterization of α-terminal group of natural rubber. 2. Decomposition of branch-points by phospholipase and chemical treatments. Biomacromolecules 2005; 6: 1858–1863.

Allen

Bristow

. The gel phase in natural rubber. Rubber Chem Technol 2001; 36: 1024–1034.

Yeang

Arif

SAM

Yusof

, et al. Allergenic proteins of natural rubber latex. Methods 2002; 27: 32–45.

Gaspari

Thatcher

Burns

, et al. Bacterial protease treatment of natural rubber latex alters its primary immunogenicity in a mouse model of sensitization. Clin Immunol 2002; 105: 9–16.

10.

Turjanmaa

. Allergy to natural rubber latex: a growing problem. Ann Med 2010; 26: 297–300.

11.

Hain

Longman

Field

, et al. Natural rubber latex allergy: implications for the orthodontist. J Orthod 2007; 34: 6–11.

12.

Levy

Charpin

Pecquet

, et al. Allergy to latex. Allergy 1992; 47: 579–587.

13.

Junoi

Chisti

Hansupalak

. Optimal conditions for deproteinizing natural rubber using immobilized alkaline protease. J Chem Technol Biotechnol 2015; 90: 185–193.

14.

Sakdapipanich

. Structural characterization of natural rubber based on recent evidence from selective enzymatic treatments. J Biosci Bioeng 2007; 103: 287–292.

15.

Perrella

Gaspari

. Natural rubber latex protein reduction with an emphasis on enzyme treatment. Methods 2002; 27: 77–86.

16.

Pichayakorn

Suksaeree

Boonme

, et al. Preparation of deproteinized natural rubber latex and properties of films formed by itself and several adhesive polymer blends. Ind Eng Chem Res 2012; 51: 13393–13404.

17.

Kawahara

Takano

Yunyongwattanakorn

, et al. Crystal nucleation and growth of natural rubber purified by deproteinization and trans-esterification. Polym J 2004; 36: 361–367.

18.

Guidon

Jr Perrin

Harrison

. Peptide bond cleavage site determination of novel proteolytic enzymes found in ROS 17/2.8 cell lysates. J Cell Physiol 1996; 166: 407–412.

19.

Prasertkittikul

Chisti

Hansupalak

. Deproteinization of natural rubber using protease immobilized on epichlorohydrin cross-linked chitosan beads. Ind Eng Chem Res 2013; 52: 11723–11731.

20.

Yamamoto

Nghia

Klinklai

, et al. Removal of proteins from natural rubber with urea and its application to continuous processes. J Appl Polym Sci 2008; 107: 2329–2332.

21.

Klinklai

Saito

Kawahara

, et al. Hyperdeproteinized natural rubber prepared with urea. J Appl Polym Sci 2004; 93: 555–559.

22.

Kawahara

Klinklai

Kuroda

, et al. Removal of proteins from natural rubber with urea. Polym Adv Technol 2004; 15: 181–184.

23.

Bennion

Daggett

. The molecular basis for the chemical denaturation of proteins by urea. Proc Natl Acad Sci USA 2003; 100: 5142–5147.

24.

Nawamawat

Sakdapipanich

. Effect of deproteinized methods on the proteins and properties of natural rubber latex during storage. Macromol Symp 2010; 288: 95–103.

25.

Abhilash

Sabharwal

Dubey

, et al. Preparation of low-protein natural rubber latex: effect of polyethylene glycol. J Appl Polym Sci 2009; 114: 806–810.

26.

Amnuaypornsri

Sakdapipanich

Tanaka

. Highly purified natural rubber by saponification of latex: analysis of green and cured properties. J Appl Polym Sci 2010; 118: 3524–3531.

27.

Bode

Kerkhoff

Jendrossek

. Bacterial degradation of natural and synthetic rubber. Biomacromolecules 2001; 2: 295–303.

28.

Schwerin

Walsh

Richardson

, et al. Biaxial flex-fatigue and viral penetration of natural rubber latex gloves before and after artificial aging. J Biomed Mater Res 2002; 63: 739–745.

29.

Walsh

Schwerin

Kisielewski

, et al. Abrasion resistance of medical glove materials. J Biomed Mater Res B Appl Biomater 2004; 68: 81–87.

30.

Angellier

Molina-Boisseau

Dufresne

. Mechanical properties of waxy maize starch nanocrystal reinforced natural rubber. Macromolecules 2005; 38: 9161–9170.

31.

Busfield

JJC

Deeprasertkul

Thomas

. The effects of liquids on the dynamic properties of carbon black filled natural rubber as a function of pre-strain. Polymer 2000; 41: 9219–9225.

32.

Cai

Rian

, et al. Reinforcement of natural rubber latex film by ultrafine calcium carbonate. J Appl Polym Sci 2003; 87: 982–985.

33.

Arroyo

Lopez-Manchado

Herrero

. Organo-montmorillonite as substitute of carbon black in natural rubber compounds. Polymer 2003; 44: 2447–2453.

34.

Kong

Peng

, et al. Nanotechnology and its role in the management of periodontal diseases. Periodnotology 2006; 40: 184–196.

35.

Peng

Kong

, et al. Self-assembled natural rubber/silica nanocomposites: its preparation and characterization. Compos Sci Technol 2007; 67: 3130–3139.

36.

Moolsin

Dechruksa

. Deproteinized natural rubber prepared with urea and sodium hydroxide. In: PACCON conference 2016 (polymer chemistry), Bangkok, Thailand, 9–11 February 2016, pp. 1165–1170. Bangkok: PACCON.

37.

Promdsorn

Chaiyasat

. Heterocoaggulation of natural rubber latex and poly[styrene-co-2-(methacryloyloxy) ethyl trimethylammonium chloride] nanoparticles. Adv Mat Res 2012; 506: 299–302.

38.

Chuayjuljit

Boonmahitthisud

. Natural rubber nanocomposites using polystyrene-encapsulated nanosilica prepared by differential microemulsion polymerization. Appl Surf Sci 2010; 256: 7211–7216.

39.

Gao

, et al. Grafting of polystyrene onto a nanometer silica surface by microemulsion polymerization. Chin J Polym Sci 2001; 20:71–76.

40.

Kim

Ahu

Hirai

. Crystallization kinetics and nucleation activity of silica nanoparticle-filled poly(ethylene-2,6-naphthalate). Polymer 2003; 44: 5625–5634.

Deproteinized natural rubber film reinforced with silane-modified silica nanoparticles

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of silane-modified nanosilica

FTIR spectral analysis of silane-modified nanosilica

Preparation of DPNR latex

Measurement on physical properties of DPNR latex

Particle size distribution and zeta potential analysis of latex

Preparation of compounded latex films

Measurement on extractable protein content in compounded latex films

Mechanical properties of films

Morphology

Results and discussion

FTIR spectral analysis of silane-modified SiO2 nanoparticles

Physical properties of latex

Particle size distribution and zeta potential analysis of latex

Extractable protein content of films

Mechanical properties of films

The effect of nanofiller loading

Morphology

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References

FTIR spectral analysis of silane-modified SiO₂ nanoparticles