ZEOLITE-FILLED NATURAL RUBBER LATEX FOR GAMMA RAYS RADIATION SHIELDING APPLICATION

  

ZEOLITE-FILLED NATURAL RUBBER LATEX FOR GAMMA RAYS RADIATION

SHIELDING APPLICATION

1) 1) 2) 2)

  Ihda Novia Indrajati , Indiah Ratna Dewi , Dolly Gusrizal , Ahmad Roisus Syifa’

  1)

  Center for Leather, Rubber and Plastics, Jl. Sokonandi No. 9 Yogyakarta 55166, Indonesia

  2)

  Sekolah Tinggi Teknologi Nuklir, Jl. Babarsari, Sleman, DIY 55281, Indonesia email:

  ABSTRACT The objective of this research was evaluating the application of zeolite-filled

natural rubber latex as a gamma ray shielding material. Parameters studied were the

_{L ), effectiveness of shielding by calculating half value} linear attenuation coefficient (μ

layer (HVL) and the effect of irradiation on rubber crosslink density performed by

swelling experiments. The samples were prepared either by dipping or by casting

₁₃₇

process with various zeolite concentrations. Gamma ray radiation used Cs as the

source with the energy of 780 keV. The result findings showed that the linear

attenuation coefficient was dependent on zeolite concentration for the dipped

samples, while for casted samples were independent. Irradiation greatly affected the

crosslink density of casted samples, while dipped samples showed little changes or

remains unchanged.

  Keywords: zeolite, latex, gamma ray, shielding, attenuation

  INTRODUCTION

  High energy radiation (e.g. X-ray and gamma ray electromagnetic radiation) are often employed in a wide range of fields, e.g. laboratories, hospital, healthcare industries, power plant and aerospace (Onjun et al. 2014; Mheemeed et al. 2012; Nambiar and Yeow 2012). These radiation will interact with media and causes ionization (Abdel-Aziz et al. 1995). Long-term exposure of these kinds of radiation may cause potential health hazard or even a death. Thus, a radiation protection is crucially needed in order to protect the people and its surrounding. Radiation protection can be achieved in many ways; radiation shield is one of them. Principally, a radiation shield is a kind that creates a barrier between a person and the radiation source, such that the radiation can be reduced to a lower level (Mheemeed et al. 2012; Abdel-Aziz et al. 1995). The shielding effectiveness of a given material largely depends on the type of radiation and the range of energy associated with the radiation (Nambiar & Yeow, 2012). Radiation shielding materials and its component should possess good mechanical properties, long-term reliability, good fabrication and joining properties, low density, high radiation tolerance and high energy absorption capabilities (Chen et al. 2014).

  High atomic number (Z) materials often use to attenuate high-energy radiation, with the lead (Pb, Z=82) being the most popular. Unfortunately, the use of lead remains many problems because of high cost, high density and toxic in nature (Mann et al. 2015). Exposure to lead or its salt (e.g. lead oxide, lead acetate, etc.) may result in accumulation of the heavy metal within the body, which in turn may lead a serious or fatal health problem, such as neuronal disorders, kidney failure, reduced hemoglobin level, and red blood cells, etc. (Nambiar et al. 2013). Alternatively, investigations have been made to produce a lightweight lead-equivalent composite by employing high-Z filler incorporated in a comfortable polymer matrix. In the composite, high-Z filler play an important role in attenuating the radiation, while the polymer matrix reduces the overall weight of the composite compared to conventional lead shielding materials. The attenuation is obtained by combination of the filler and structural material (Nambiar & Yeow, 2012). Zeolite (Clinoptilolite) is a natural volcanic porous composed by hydrated alumina silicates (Akkurt et al. 2010). Zeolite is known as molecular sieve since its structure allows separation chemical substances passed by. Zeolite containing rocks are used in wastewater purification to remove anthropogenic radionuclides and also heavy metals (G alamboš et al. 2012). These abilities make zeolite potentially to apply as high-Z filler of polymeric composite for gamma rays shielding. Several studies have been conducted utilizing zeolite in different media for this purpose (Akkurt et al. 2012; Kurudirek et al. 2010; Puiso et al., 2015).

  Rubber composites have been used as shielding material by employing either lead or nonlead filler (Onjun et al., 2014; Mheemeed et al., 2012; Gwaily, 2002; Abdel-Aziz & Gwaily, 1997). Radiation proceeds to chemical changes of the rubber molecule chains, e.g. crosslinking (intermolecular bonds) or degradation (chain scission of bonds in the main polymer chain and inside chains). Crosslinks produced by this technique similar to those obtained by sulfur vulcanization. However, the net effects, while similar, are not identical (Samar

  žija-Jovanović et al. 2014; Khalid et al., 2010). Once exposed to a given dose of radiation, the C=C bonds in thermoset resin chain will be partially cross-linked to form a 3D stable gel. As a result, the mixture is no longer viscous liquid, but becoming viscoelastic gel (Martínez-Barrera et al. 2013). Radiation crosslinking produced carbon-carbon crosslinks that improve the mechanical properties especially in high temperature (Samaržija-Jovanović et al., 2014). Several studies had been successfully employed radiation to cure the rubber. Naseri & Jalali-Arani (2015) compared the effect of gamma irradiation and sulfur cure system on SBR/EPDM blends in the presence of nanoclay. (Mohamed et al. 2012) conducted a research on radiation vulcanization of NR/SBR blends containing silica, carbon black and titanium dioxide. Senna et al. (2012) and Almaslow et al. (2013) studied electron beam irradiated on natural rubber. Abou Zeid et al. (2008) investigated gamma irradiation on EPDM composites. The effect of radiation onto butyl rubber had been investigated by Scagliusi et al. (2012), and Khalid et al. (2010) evaluated the effect of radiation dose on the properties of natural rubber filled by CNT.

  The present investigation is concern with the study of natural rubber latex that was prepared either by dipping or casting process, utilizing natural zeolite as filler in various concentrations. The prepared samples were subjected to gamma irradiation

  137

  using Cs as the source. The linear attenuation coefficients were calculated and effectiveness of the shielding material was evaluated from its half value layer (HVL). The effect of radiation on swelling ratio and crosslink density were also studied.

  MATERIALS AND METHOD Materials

  Concentrated natural rubber latex (NRL) with dry rubber content 60% and the additives including natural zeolite (purchased from local market), Ionol, zincdiethyldithiocarbamate (ZDEC), sulfur, ZnO, Darvan, Ammonium Chloride (NH

  Additives Dispersion Preparation

  Compounding is a mixing process between NRL and its additives. In order to get a homogeneous compound, both of them should be in the same phase. NRL is an emulsion, while the additives are powder. Thus, the additives should be emulsified before added into NRL. The emulsions or dispersions were prepared by using ball mill in the concentration of 50% w/w. The procedure to get 100 g, 50% w/w dispersion as follows: 50 g of each additive were mixed with 4 g Darvan (dispersing agent) and 46 g aquadest in a sealed-polyethylene bottle, which were filled by ceramic balls. Ceramic balls play a role as a grinding media. It is recommended to use three different diameters ceramic ball (e.g. 1, 2 and 3 cm) with the ratio of 1:2:1. The mixes in the sealed PE bottle were ball-milled for 24 h. The surface speed of ball mill was maintained on 60 rpm.

  Compounding of NRL

  The compounds for casting and dipping process were similar. The differences only laid on the zeolite concentration. Additives dispersion was incorporated into NRL based on the composition as seen on The mixes gently stirred manually for 15 minutes in order to produce homogenous compound. However, avoid making froth during stirring. Then, the compound stored at a closed chamber and matured for 72 h. Basic compound (0 phr zeolite) was used for both dipping and casting process.

• Table 1. Composition of NRL compounds (in phr )

  Casting Dipping Ingredients B

  C1 C2 C3 C4 C5 D1 D2 D3 D4 D5 NRL DRC 60% 100 100 100 100 100 100 100 100 100 100 100 50% disp. ZnO 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

  5.0

  5.0 50% disp. Ionol 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

  2.0

  2.0 50% disp. ZDEC 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

  1.2

  1.2 50% disp.sulfur 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

  1.0

  1.0 50% disp. zeolite 0.0 5.0

  10

  15 20 25 2.5 5.0 7.5 10.0 12.5

• phr: part hundred rubber

  Casting Procedures

  Samples prepared by casting process were made into two different thicknesses, i.e. 0.5 and 1.0 cm. The procedures of casting process as follow: 20% v/v ammonium chloride aqueous 20% (w/v) was added into NRL compound (prepared in the previous step) and gently stirred. Immediately those mix casted into the square-mold such that the desired thickness achieved, and then let them hardened. Casted samples were vulcanized by steam 100ºC for 60 minutes. The samples thickness was measured using dial gauge three different area of sample.

  Dipping Procedures

  Coagulant dipping was employed to prepare the film sample using 30% ethanolic-calcium nitrate (Dipcal®) as coagulant and 30% calcium carbonate as parting agent. Cleaned and dried glass former were immersed in the coagulant solution for 10 s, followed by air-drying. The dried coated former were then dipped into the NRL compound with 10 s dwell time. After that, the former was withdrawn and the deposits on the surface were allowed to dry. Repeat the same procedure for double dipping. Vulcanization was conducted by steam at 100ºC for 30 minutes. The cured NRL film then stripped out from the former and stored in a sealed plastic. The samples thickness was measured using dial gauge at three different area of sample.

  Attenuation Performance Measurement

  The rubber specimens were exposed to gamma ray radiation for 30 s at room

  137

  temperature by Cs source with activity of 1 μCi and energy of radiation 780 keV.

  L ) was calculated using Equation 1,

  The linear attenuation coefficient (μ

  ……………………………………. (1) , I ,

  

I t μ and x are the initial intensity, intensity after traversing material, linear

  attenuation coefficient and thickness of the material, respectively. The can be μ L obtained from the slope of plot ln (I /I ) versus x. Single way ANOVA was carried out _t to evaluate the effect of zeolite concentration onto L . The confidence limit level is

  μ taken to be 95% (α=0.05).

  The effectiveness of shielding material can be evaluated by its Half Value Layer (HVL) (Mheemeed et al. 2012). HVL describe the thickness of the material at which the intensity of the radiation after interacting with the absorber material is reduced to half of its energy before interaction (Eren Belgin et al., 2015). HVL is calculated after obtaining the L using Equation 2.

  μ

  …………………………………… (2)

  Swelling Test and Crosslink Density Measurement

  Swelling experiment was performed on 2x2 cm cut sample prepared by either

  3

  the casting or dipping. The samples were immers ) at ed in toluene (ρ=0.867 g/cm 25°C for 24 h. after the period was completed; the samples were taken out and blotted with filter paper to remove liquid on the surface. Swelling ratio is defined on Equation 3,

  ………………………………….. (3) where w and w are the mass of the test pi _{t ece before swelling and after time “t”} immersion, respectively. The mass of the sample weighed using electronic balance having precision 0.0001 g.

   Crosslink density of rubber vulcanizates were determined from the equilibrium

  swelling measurements, and calculated using the Flory-Rehner (Equation 4) as follows: ……………………… (4)

  Where M c = the number average molecular weight of the rubber chains between crosslink; p = density of the polymer; V s = molar volume of the solvent (for toluene

  ρ

• 1

  interacting with a material per unit path length. This parameter is the most important quantity characterizing the penetration and diffusion gamma radiation in medium (Medhat, 2009). The magnitudes of measured

  CASTING 0.0 5.058 0.1370 5.0 5.200 0.1333

  10.0 1.478 4.6887 12.5 1.385 5.0036

  0.0 1.133 6.1165 2.5 1.660 4.1747 5.0 3.040 2.2796 7.5 2.038 3.4003

  (cm) DIPPING

  ) HVL

  (cm

  μ L

  Zeolite loading (phr)

  μ ^{L and HVL of prepared samples}

  Table 2.

  μ ^{L and HVL of the dipped and casted} samples are listed at

  μ ^{L ) defines as the probability of a radiation}

  Linear attenuation coefficient (

  RESULT AND DISCUSSION Linear Attenuation Coefficient

  solvent. Swelling test and crosslink density measurement were performed before and after being radiated.

  w ^{1 ,} ρ ^{1 are the weight and density of rubber, while w 2 ,} ρ ^{2 are the weight and density of}

  ………… .........……………. (5)

  parameter (for NR-toluene=0.42). The term V _r was calculated by using the relation as expressed on Equation 5.

  Χ=polymer-solvent interaction

  ), V _r is molar volume of rubber,

  mol

  3

  =106.2 cm

  V _s

• 1

  10.0 5.394 0.1285 15.0 5.442 0.1273 20.0 5.539 0.1251 25.0 5.731 0.1209 It is clearly seen that incorporation of zeolite into NRL matrix significantly affects the attenuation coefficient of dipping prepared samples. ANOVA result confirmed this finding since F>F crit as shown atThe P-value lowers than

  α (0.05) means that there is a strong correlation between zeolite loading and . The is increased

  μ L μ L

  nearly three times with the incorporation of 5 phr zeolite, and then starts to decrease with further addition. In contrast, of casted samples were independent to zeolite

  μ L loading and it has confirmed by ANOVA (Error! Reference source not found.).

  Table 3. Single-way ANOVA Source of

  SS df MS F P-value F crit Variation

  DIPPING Between

  110.5748 1 110.5748 10.10754 0.009832 4.964603 Groups Within Groups 109.3983 10 10.93983 Total 219.9731

  11 CASTING Between Groups 151.4928 1 151.4928 3.460421 0.092481 4.964603 Within Groups 437.7872 10 43.77872 Total 589.28

  11 Gamma rays strike the material and their penetration are affected by the photon energies, thickness, density (Martínez-Barrera et al. 2013) and chemical composition (Erdem et al. 2010). The denser material will attenuate or absorb much radiation energy. Thus, homogeneous dispersion of the filler particle with less coagulation in the composites becomes the most important requirement. Coagulation or agglomeration of filler particle will generate a free volume in the NRL matrix. illustrates this phenomenon, where I o and I t are the input and output intensity of the shielding material, respectively. Free volume reduced the composite density. In turn, it will decrease the composite ability to attenuate the radiation energy. Figure 1. Free volume creation as a result from agglomeration In this study, lower zeolite concentration provides better filler particles dispersion. For dipped samples, critical zeolite loading was 5 phr, which provides better filler dispersion with less coagulation. Above this point, probability of coagulation is higher exhibited by decreasing . Effective shielding material is provided by 5 phr zeolite

  μ L since it exhibit lowest HVL, while in casted samples HVL is found to be comparable.

  Zeolite dispersion was added into NRL in the concentration of 50% during compounding. This dispersion might be highly concentrated so that homogeneous compound was difficult to reach especially for higher loading. High-speed stirring can help homogenizing the NRL compound, but it is avoided because of frothing tendency. This explains the independency of zeolite loading on L of casted samples.

  μ

  Thus, it is recommended to reduce the zeolite dispersion concentration into lower level.

  Swelling Ratio and Crosslink Density

  The swelling phenomenon occurred in rubber can be describe as follows; the molecular chains in vulcanized rubber are usually loosely gathered that create a gaps in the molecular network (Lv et al. 2015). Thus, swelling test is often applied to predict the crosslink density of vulcanized rubber. depict swelling ratio (Q t ) of dipped and casted samples respectively.

  Figure 2. Swelling ratio of dipped samples The Q of the un- and irradiated of dipped samples show similar trend and the values _t remain unchanged. It means that the gamma ray irradiation did not change the crosslink density of the vulcanizates, and confirmed by its changes as seen on

  At the same zeolite concentration, single dipped samples provide denser crosslink than double dipped. The single dipped samples had lower thickness so that within the same vulcanization period will build denser crosslink in the rubber matrix, while the double dipped was insufficient.

  Q increased until reach a maximal value than start to decrease upon zeolite _t

  concentration. Q t maximum was achieved at 5 phr of zeolite for both single and double dipped samples. For single dipped samples, this finding is interesting since 5 phr of zeolite gave highest crosslink density. The denser crosslink on rubber matrices, the lower swelling because higher crosslink density will restrict swelling (Samaržija-Jovanović et al. 2014). Interaction between filler particles and matrix also contribute to the gaps creation that will affect the extent of swelling. Solvent molecules penetrate into these gaps and causes a change in volumes and weight (Lv et al. 2015). Swelling of the polymer matrix by solvent depends on diffusivity and solubility of penetrating solvent molecule; which is a rate-dependent process (Samaržija-Jovanović et al. 2014). Thus, at higher loading of zeolite Qt was going slower because at high filler loading the diffusion path of the penetrating solvent molecule will be longer and more tortuous.

  Figure 3. Swelling ratio of casted samples Table 4. Crosslink density of prepared samples

  Zeolite Concentration

  (phr) Crosslink Density

  (10

  3

  mol/cm

  3

  ) Change to initial value

  (%) Un- irradiated

  Irradiated

  DIPPING

  Single Dip 0.0 11.769 11.693 -0.643 2.5 11.846 11.857 0.092 5.0 15.130 17.557 16.037 7.5 13.674 13.576 -0.713

  10.0 11.886 12.966 9.088 Double Dip

  0.0 12.033 11.705 -2.731 2.5 11.913 12.308 3.319 5.0 10.102 12.832 27.025 7.5 11.213 11.805 5.277

  10.0 9.292 9.366 0.795

  CASTING

  0.5 cm 0.0 5.618 1.947 -65.344 5.0 8.724 3.178 -63.570

  10.0 25.516 5.855 -77.054 15.0 26.698 10.798 -59.554 20.0 29.773 11.048 -62.893 25.0 41.259 19.339 -53.129

  1.0 cm 0.0 5.582 5.128 -8.118 5.0 5.997 5.190 -13.457

  10.0 8.436 5.305 -37.118 15.0 7.495 5.220 -30.345 20.0 5.696 4.824 -15.300 25.0 5.060 4.756 -6.012

  Different thickness of casted samples gives different Q response upon zeolite _t increment as seen onQ t tends to increase upon zeolite concentration when the samples thickness is 0.5 cm, while 1.0 cm-samples firstly increases until obtaining an optimum value and starts to decrease with further addition of zeolite. Gamma rays radiation significantly reduces Qt and marked reduction in crosslink density are also found. In this case, radiation has led to network degradation through chain scission. Ionizing energy of gamma rays radiation absorbed by the polymer backbone initiates a free radical substance; subsequently polymer then undergoes a chain scission and crosslinking, both of which alter the polymer characteristic (Nambiar & Yeow, 2012). Typically chain scission and crosslinking are coexist, the prevalence of each depending on many factors. A key role parameter can be played by irradiation parameters, such as the total absorbed dose and irradiation dose rate, because they affect the concentration of the reactive species formed

  (Samaržija-Jovanović et al., 2014). Crosslink density reduction after irradiation probably resulted by chain scission which is predominant than other mechanism. In the other side, radiation provides better filler-polymer matrix interaction, thus in turn decreased the Q . _t

  CONCLUSION

  Zeolite-filled natural rubber latex film has been prepared by either dipping or casting process for gamma rays radiation shielding. The linear attenuation coefficient

  

L ) of the prepared dipped samples were dependent on the increment of zeolite

  (μ concentration, while of casted samples were independent. The most effective shielding material was given by 5-phr zeolite dipped sample. Agglomeration of filler particles in the polymer L . The concentration of zeolite dispersion matrix affected the μ used in this study was too high and it was recommended to reduce it in order to get homogeneous filler dispersion. Gamma rays radiation did not affect the swelling ratio (Q t ) and crosslink density of the dipped samples, and both parameters were affected by zeolite concentration. In contrast, the radiation greatly changed the Q t and crosslink density of the casted samples. Chain scission plays a dominant role than other mechanism and contributing to the crosslink density reduction.

  ACKNOWLEDGEMENT

  The author will express their thanks to the management of Sekolah Tinggi Teknologi Nuklir, Yogyakarta for facilitating the gamma ray radiation experiment.

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