Article pubs.acs.org/IECR
Sulfur-Free Prevulcanization of Natural Rubber Latex by Ultraviolet Irradiation in the Presence of Diacrylates Panithi Wiroonpochit,† Kittaporn Uttra,‡ Kanokwan Jantawatchai,∥ Nanthiya Hansupalak,*,∥,§ and Yusuf Chisti⊥ †
National Metal and Materials Technology Center, 114 Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand ‡ Interdisciplinary Graduate Program in Advanced and Sustainable Environmental Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand ∥ Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand § Center for Advanced Studies in Industrial Technology, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand ⊥ School of Engineering, Massey University, Private Bag 11 222, Palmerston North, New Zealand ABSTRACT: Natural rubber (NR) latex was prevulcanized by ultraviolet (UV) irradiation in the presence of different diacrylate coagents. Prevulcanization was affected by the aliphatic hydrocarbon chain length of the coagents. The following diacrylate coagents with increasing aliphatic hydrophobic chain lengths were used: 1,4-butanediol diacrylate (BTDA), 1,6-hexanediol diacrylate (HDDA), and 1,9-nonanediol diacrylate (NDDA). Oxygen inhibition of the freeradical reactions of prevulcanization and the effects of the inert CaCO3 filler (concentration range = 0−40 parts per hundred rubber) on properties of the prevulcanized NR films were investigated. The hydrocarbon chain length of the coagent influenced its hydrophobicity and therefore its affinity toward NR, and this affected the prevulcanization outcomes. HDDA showed the greatest affinity for NR and thus was the most suitable coagent for prevulcanization of NR. Prevulcanization via UV irradiation was insensitive to oxygen and the filler tested. The CaCO3 filler proved to be satisfactory and could be added either before or after UV irradiation.
1. INTRODUCTION Natural rubber (NR) latex produced by the rubber tree (Hevea brasiliensis) is widely used to make dip molded products such as medical examination gloves. Products made from the pristine NR latex typically have a low mechanical strength and poor wear resistance.1 To extend usability, the pristine NR latex is commonly subjected to a prevulcanization treatment. Prevulcanization enhances the mechanical properties of the products made from the treated latex.2 Prevulcanization transforms the highly thermoplastic raw rubber into a more elastic product through cross-linking of rubber molecules.3 In general, cross-linking of NR chains occurs via mechanisms based on free radicals. A cross-linking agent (or coagent) first generates free radicals that attack the double bonds in NR molecules to form cross-links.4,5 In conventional prevulcanization, the cross-linking agent is sulfur that can form monosulfide, disulfide, and/or polysulfide cross-links. The length of sulfur cross-links affects the tensile and tear strength, fatigue properties, thermal stability, and compression set of the final product.6 In radiation prevulcanization, NR chains can be crosslinked without using a coagent, but the rubber produced this way is quite brittle. A coagent with sufficiently long molecules results in the cross-links being flexible, and this reduces © 2017 American Chemical Society
brittleness of the cross-linked product. Therefore, the selection of a suitable coagent is essential,7 and this requires studies of the influence of the type of coagent on the mechanical properties of the cross-linked product. A suitable coagent for radiation prevulcanization must have appropriate functional groups that generate free radicals on irradiation. Coagents with mono-, di-, or poly-functional reactive groups with either abstractable hydrogen or double bonds can be used to aid the free radical production.8 The coagent radicals react with the carbon−carbon double bonds in NR molecules to form covalent cross-links.9,10 Although sufficiently energetic radiation (e.g., electron beams and γ rays) can generate free radicals directly from a suitable coagent,11−13 the use of such radiation is unattractive because of safety considerations and the expensive equipment needed. An alternative is to use weaker radiation such as ultraviolet (UV) light to generate radicals.14 This requires the presence of a suitable photoinitiator in addition to the coagent. A Received: Revised: Accepted: Published: 7217
March 18, 2017 June 3, 2017 June 8, 2017 June 16, 2017 DOI: 10.1021/acs.iecr.7b01133 Ind. Eng. Chem. Res. 2017, 56, 7217−7223
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Industrial & Engineering Chemistry Research
Thailand. Irgacure 1173 was used as the photoinitiator. The following coagents (Figure 1) were used in different experi-
commonly used photoinitiator that responds to UV light is Irgacure 1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone).4,15 The coagents commonly used in UV-initiated cross-linking are thiols, amines, and acrylates.16,17 Thiols are effective coagents, but they contain sulfur, give off an unpleasant odor, and have poor storage stability.18 Similarly, amines may produce bad odors, are moisture sensitive,16 and can stain rubber products with their yellow color. In contrast, acrylates are colorless, less odorous, and free of sulfur. Therefore, acrylates are potentially useful coagents for making hypoallergenic products. Acrylates used as coagents in UV-initiated cross-linking include n-butyl acrylate (nBA), 1,4-butanediol diacrylate (BTDA), 1,6-hexanediol diacrylate (HDDA), 1,9nonanediol diacrylate (NDDA), trimethylolpropane trimethacrylate (TMPT), and trimethylolpropane triacrylate (TMPTA). Among acrylate coagents, polyacrylates form chemical crosslinks between rubber chains. In contrast, a monoacrylate simply attaches to a rubber chain at a single site and therefore does not produce cross-links. Instead, sufficiently long monoacrylate molecules attached to a polymer chain facilitate its entanglement with other nearby chains to produce a physical “crosslinking” effect. A high reactivity has been reported for diacrylate coagents in photopolymerization,19 but not all diacrylates are suitable for use with NR because they are poorly soluble in it. In practice, achieving a balance between reactivity and solubility of a coagent in the NR matrix is essential for effective UV-initiated prevulcanization. Because natural rubber chains are hydrophobic, a hydrophobic coagent is required for compatibility. Hydrophobicity of a coagent may be increased, for example, by having a long straight-chain alkyl group between the two reactive ends of the coagent.20 Because UV-initiated polymerization has a low energy requirement, it is potentially affordable for industrial use. Our previous work has shown promising results with UV-based prevulcanization of NR latex.4 Here we report on the effects of the aliphatic chain length of diacrylate coagents on the UVbased prevulcanization of NR. Mechanical properties of the product made using different coagents are discussed. The hydrophobicity-based interactions between a coagent and NR are probed by contact angle measurements, a quantification of the coagent uptake by NR, and an assessment of the coagent conversion during prevulcanization. In addition, the effects of calcium carbonate (CaCO3) filler on the mechanical properties of the films made from NR prevulcanized with a suitable coagent are discussed. Fillers are used to increase the volume and mass of rubber products and improve their processability.21 The commonly used fillers in the rubber industry are carbon black, silica, and CaCO3.22 In work with prevulcanized NR latex (PVNRL), CaCO3 is mostly preferred. This is because CaCO3 is easily mixed with the latex and also because a natural light color is desired in many products (e.g., medical examination gloves, rubber thread) made with PVNRL. An inert filler such as CaCO3 is cheap, and its use reduces the cost of production. An opaque filler such as CaCO3 reduces UV-light penetration in the latex during prevulcanization; therefore, an assessment of its effect is important.
Figure 1. Chemical structures of diacrylate coagents: (a) 1,4butanediol diacrylate (BTDA), (b) 1,6-hexanediol diacrylate (HDDA), (c) 1,9-nonanediol diacrylate (NDDA).
ments: 1,4-butanediol diacrylate (BTDA, C10H14O4, molar mass = 198 kg kmol−1; BASF, Ludwigshafen, Germany), 1,6hexanediol diacrylate (HDDA, C12H18O4, molar mass = 226 kg kmol−1; BASF, Ludwigshafen, Germany), and 1,9-nonanediol diacrylate (NDDA, C15H24O4; molar mass = 268 kg kmol−1; Tokyo Chemical Industry, Co., Ltd., Tokyo, Japan). Ground calcium carbonate (Filmlink 100; size range 0.07−3 μm) for use as a filler was purchased from IMERYS Pigments Pte Ltd., Singapore. All chemicals were used as received. 2.2. UV Prevulcanization of NR Latex. A mixture of NR latex, the coagent, and the photoinitiator (the latex mixture) was prepared as previously described.4 The mixture was poured into Petri dishes (diameter = 95 mm; total depth = 15 mm) and prevulcanized by UV irradiation while being continuously mixed under a normal air atmosphere unless specified otherwise. The prevulcanization was conducted in a dark room at 28 °C, under the previously established optimal conditions: a premixing time of 1 h; an irradiation time of 1 h; an irradiation distance (i.e., the distance between the UV lamps and the surface of the latex pool) of 45 mm; and a latex pool depth of 10 mm.4 The latex mixture was irradiated from the top by a bank of six UV lamps (250−350 nm wavelength; 8 W).4 Petri dishes were always tightly covered with a polyethylene cling film to minimize evaporation. The cling film absorbed less than 6% of the incident UV light. The intensity of the incident radiation at the surface of the Petri dishes was 1.2 mW cm−2. For prevulcanization in an oxygen-free environment, the amount of coagent plus photoinitiator was fixed at 4 phr (parts per hundred rubber). The Petri dishes were prepared as noted above. Before UV irradiation was initiated, argon was bubbled through the latex mixture in the Petri dishes for 30 min. The Petri dishes were then covered with a polyethylene cling film. The prevulcanized NR (PVNR) films (1 mm thick) were made by pouring 36 g of the prevulcanized NR latex (PVNRL) suspension on a glass plate (13 × 13 cm) and drying to a constant weight in air at room temperature. 2.3. Mechanical Properties and Cross-Link Density of PVNR Films. The tensile strength of the cross-linked PVNR films (1 mm thick PVNR films) was measured using an Instron tensile tester (Instron 5566, Canton, MA) at a crosshead speed
2. EXPERIMENTAL SECTION 2.1. Materials. High ammonia NR latex (60 wt % dry rubber content; NR particle size range of 100 nm to 5 μm) was obtained from Thai Eastern Rubber Co., Ltd., Rayong, 7218
DOI: 10.1021/acs.iecr.7b01133 Ind. Eng. Chem. Res. 2017, 56, 7217−7223
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Industrial & Engineering Chemistry Research of 500 mm min−1 and a load of 1 kN. The measurements were in accordance with ASTM Standard D412-98a. The cross-link density (ρc, mmol cm−3) was determined using the equilibrium swelling measurements and the Flory− Rehner equation. A PVNR film (2 × 2 cm) was immersed in toluene until equilibrium was attained. The weight of the toluene-equilibrated film was measured after wiping the excess solvent with a lint-free absorbent tissue. The film was then dried to a constant weight in an oven at 60 °C, cooled to room temperature in a desiccator, and weighed. The experiment was repeated three times. The volume fraction ϕp of the solventswollen polymer was calculated using the following equation: ϕp =
h in the dark. The film was then removed, the excess coagent was blotted with a lint-free paper, and the weight (wa) was measured. The coagent uptake (%) was calculated using the following equation: w − wb Percent uptake = a × 100 wb (4) The experiment was repeated at least three times with each coagent. 2.4.4. Effect of Calcium Carbonate (CaCO3). The PVNR films with various contents of CaCO3 were characterized in terms of tensile properties and morphology. The tensile properties were measured as explained above. The morphology was examined using a Hitachi S-3400N scanning electron microscope (Hitachi, Japan).
1 1+
ρp ρs
(
WSW WAD
)
−1
(1)
where WSW is the weight of the solvent-swollen film, WAD the weight of the oven-dried film, ρp (= 0.93 g cm−3) the density of the polymer, and ρs (= 0.87 g cm−3) the density of the solvent. The cross-link density (ρc, mmol cm−3) was calculated using the following equation: ρc =
3. RESULTS AND DISCUSSION As shown in an earlier publication,4 the UV-mediated crosslinking of NR chains in the presence of a diacrylate coagent occurs via mechanisms based on free radicals. First, the absorption of UV light by the photoinitiator results in its homolytic cleavage to produce two radicals. These radicals initiate prevulcanization by attacking the CC bonds of polyisoprene chains of NR and the coagent. This generates polyisoprene radicals and the coagent radicals. These radicals randomly attack other NR chains to produce a network of NR chains cross-linked by the coagent. The present work focused on selection of a suitable diacrylate coagent for UV-mediated prevulcanization of NR latex so as to maximize the tensile strength and the cross-link density of the product. 3.1. Effect of Alkyl Chain Length. The tensile strength and cross-link density data for PVNR films made using the different diacrylate coagents are shown in Figure 2. The highest
−[ln(1 − ϕp) + ϕp + χ ·ϕp2]
(
Vs ϕp1/3 −
ϕp 2
)
(2)
where χ is the Flory−Huggins parameter for polymer−solvent interaction and Vs is the molar volume of the solvent. 2.4. Characterization of Coagent. 2.4.1. Conversion of the Coagent. A 3 × 3 cm piece of the PVNR film was cut into smaller pieces (about 1 × 1 mm). The unreacted coagent in the sample was extracted with acetone in a Soxhlet apparatus for 24 h at 60 °C.23 After extraction, all acetone was evaporated at room temperature to leave behind a residue of the unreacted coagent. This residue was dissolved in d-chloroform for analysis by 1H NMR (Bruker Biospin 500 1H NMR operated at 500 MHz; www.bruker.com). Dichloroethane was used as an internal standard. The percent conversion of the coagent during UV prevulcanization was calculated as follows: Percent conversion =
N1 − N2 × 100 N1
(3)
where N1 is the amount of coagent in the film prior to extraction and N2 is the amount of the coagent extracted after the prevulcanization treatment. N1 was calculated using the amount of the coagent initially added and the mass of the film used in extraction. N2 was quantified using the 1H NMR spectra as explained above. 2.4.2. Wettability of Pristine NR Films with the Coagent. Wettability of a polymer film with a liquid is a measure of the mutual compatibility of the polymer material and the liquid. The wettability of the pristine NR film (i.e., films made from NR latex in the absence of coagents, photoinitiators, and UV irradiation) with the coagent was evaluated using the contact angle method at ambient open-air conditions. Contact angles were measured using a goniometer (Ramé-Hart 100; RaméHart Instrument Co., United States). At least 20 separate measurements were made, each time by placing a drop of a coagent on the surface of the film.24 The data were averaged. The measurements were reproducible to ±2°. 2.4.3. Coagent Uptake by Pristine NR Films. The uptake of a coagent by pristine NR films was measured by immersing a piece of the film with a known weight (wb) in the coagent for 1
Figure 2. Tensile strength and cross-link density of the PVNR films made with various coagents. The coagent-to-photoinitiator ratio was kept constant at 2 phr:2 phr.
tensile strength and cross-link density were found in films made with HDDA, the coagent with an intermediate hydrophobicity. Therefore, HDDA was the most effective coagent. This was unexpected. The expected order of tensile strength and crosslink density was NDDA > HDDA > BTDA. Both reactive functional groups of all three coagents were identical (Figure 1), and the only difference among them was the length of the alkyl chain linking the two reactive functional groups. Based on the lengths of the hydrophobic alkyl chains, the coagent 7219
DOI: 10.1021/acs.iecr.7b01133 Ind. Eng. Chem. Res. 2017, 56, 7217−7223
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The high affinity of HDDA for NR was further confirmed by calculation of the solubility parameter (δ) of the coagents via an evaluation of their cohesive energy density. The values of enthalpy of vaporization of BTDA, HDDA, and NDDA, predicted by ACD/Laboratories Percepta Platform−PhysChem Module, were 50.6, 54.2, and 58.5 kJ mol−1, respectively. Thus, the δ-values at 25 °C were 15.88, 15.20, and 14.32 (MPa)1/2 for BTDA, HDDA, and NDDA, respectively. NR is polyisoprene. The δ-value of isoprene at 25 °C was 15.18 (MPa)1/2.26 Therefore, the δ-value of HDDA was the closest to the δ-value of NR, making the two most compatible. Figure 3 shows the percent uptake of the three coagents by 1 g of dry rubber. Using this data, the masses of the different coagents taken up by 1 g of rubber were as follows: 0.51 g of BTDA, 1.71 g of HDDA, and 0.68 g of NDDA. These masses could be converted to moles of these compounds through division by the relevant molar mass. Thus, the amounts of the coagents taken up were as follows (mmol/g rubber): 2.575 for BTDA, 7.566 for HDDA, and 2.537 for NDDA. On mole basis, the uptake of the coagents relative to HDDA was, therefore, as follows: 0.340 for BTDA, 1 for HDDA, and 0.335 for NDDA. Each mole of a coagent taken up provided two moles of the reactive functional groups (i.e., the acrylate groups). Thus, the number of the reactive functional groups provided by HDDA was 3 times (1/0.340) the number provided by BTDA. Similarly, HDDA provided nearly 3 times (1/0.335) as many reactive groups as NDDA. The actual concentrations of the various coagents in the reaction mixtures were within ±15% of their average concentration (8.767 mM). Therefore, the large differences in the number of reactive functional groups provided by the different coagents can be explained only by the differences in uptake of the coagents by the rubber phase. The consumption of the different coagents in UV-mediated reactions with NR was examined by comparing the conversion of the coagents. For this, the 1H NMR spectrum of the residue recovered by extraction of the NR film that had been subjected to a prevulcanization treatment was measured. A representative spectrum is shown in Figure 4 for the residue extracted from PVNR film made with HDDA. The percent conversion was estimated using the ratio of the heights of the 1H NMR spectral
hydrophobicity increased in the following order: NDDA > HDDA > BTDA.20 A coagent with a greater hydrophobicity should accumulate more in the hydrophobic NR particles dispersed in the aqueous phase of the latex. This accumulation was expected to facilitate cross-linking of the rubber molecules during UV prevulcanization. The reactivity of the acrylate functional groups in the different coagents was likely to be comparable in view of their similar molecular structures (Figure 1). Also, the molar concentrations of the three coagents in different latex mixtures were similar: 7.45 mM for NDDA, 8.84 mM for HDDA, and 10.01 mM for BTDA. Given this, a plausible explanation for the results in Figure 2 is a higher affinity of HDDA for NR particles compared to affinity of the other coagents. NR latex is a multicomponent complex colloidal system consisting of water with dissolved components (e.g., salts, carbohydrates, proteins) and dispersed NR particles with adsorbed components (e.g., proteins, lipids) on their surface.25 These surface-adsorbed components actually make the NR particles less hydrophobic than they would be otherwise and help to stabilize the latex (i.e., prevent spontaneous coagulation of the dispersed phase). Therefore, HDDA, the coagent with an intermediate hydrophobicity, had the highest affinity for the dispersed NR particles. The affinity of the coagents for NR was further probed by (1) comparing their ability to wet or spread on pristine NR films; (2) measuring the uptake of the coagents in the pristine NR films; and (3) measuring the quantity of the unreacted coagent remaining in the UV prevulcanized product. As shown in Figure 3, of all the coagents, HDDA had the smallest contact angle on a rubber substrate, confirming its
Figure 3. Percent uptake of coagents by identical pristine NR films and the contact angles of the coagents with the films. The contact angle and spread of a drop of coagent on the NR film are shown in the photograph above the relevant bar.
higher affinity for NR relative to the other coagents. This suggested that a larger amount of HDDA penetrated the NR matrix compared to the other coagents. The measurement confirmed a high percent uptake of HDDA by the NR films (Figure 3), and this explained the high cross-link density of the PVNR films produced using this coagent (Figure 2). Furthermore, the above explanations are entirely consistent with the observed higher cross-link density of the PVNR films made using NDDA compared to films made using BTDA.
Figure 4. 1H NMR spectrum of the residue extracted from a PVNR film made with HDDA. Protons a−d are identified in the HDDA. Protons e of the internal standard (1,2- dichloromethane) are shown. 7220
DOI: 10.1021/acs.iecr.7b01133 Ind. Eng. Chem. Res. 2017, 56, 7217−7223
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can quench free radicals required in the prevulcanization process so that they are no longer reactive.16 In addition, oxygen can scavenge free radicals to produce new less reactive radicals.16 This reduces the number of active free radicals available for prevulcanization, and the efficiency of cross-linking deteriorates. As a consequence, the properties of the product are adversely affected.19,28 Therefore, testing for possible inhibition by oxygen was important for developing a suitable process for prevulcanization of NR latex. Prevulcanization using HDDA was carried out under two different atmospheres: the normal air atmosphere and an inert argon atmosphere. As shown in Figure 6, similar tensile
peaks at 5.8 and 3.7 ppm (Figure 4). This ratio was related to conversion via a calibration curve. This method has been welldocumented.23 The calibration curve was prepared by measuring the spectral peak heights at 5.8 ppm of different dilutions of a standard solution of a pure coagent. The peaks at 5.8 and 3.7 ppm corresponded to protons c in the unreacted acrylate group and protons e of the internal standard (Figure 4). A signal at 5.8 ppm in the extract residue implied the presence of the unreacted coagent. The percent conversion of the coagents (eq 3) was as follows: 90% for HDDA, 76% for NDDA, and 68% for BTDA. These data agreed well with the percent uptake data for the various coagents (Figure 3). Therefore, the coagent that was most taken up in the NR particles was also the most effective in the prevulcanization reaction. In view of the above data, only HDDA was used for all further work. Using HDDA as the coagent, the next objective was to determine the total amount of the coagent plus the photoinitiator for use with NR latex for obtaining a high mechanical strength. In all cases, the weight ratio of HDDA and photoinitiator remained fixed at 1:1. (In preliminary experiments, this ratio was varied from 1:0.4 to 1:10. A ratio of 1:1 consistently resulted in films with the highest tensile strength. Therefore, the ratio was fixed at 1:1 in all work shown here.) In different experiments, the total amount of HDDA plus photoinitiator was varied in the range of 2−6 phr. (Higher amounts could not be used because the latex mixture began to coagulate at a HDDA plus photoinitiator level of >6 phr.) The tensile strength and cross-link density of the PVNR films produced are shown in Figure 5. Based on the data, a HDDA
Figure 6. Tensile strength and cross-link density of PVNR films made with and without oxygen. The total amount of HDDA plus photoinitiator was 4 phr, and HDDA to photoinitiator weight ratio was 1:1.
strength and cross-link density were observed for the samples prevulcanized under different atmospheres. The evidence showed that the prevulcanization was insensitive to oxygen. This was in accordance with the literature.15 This is advantageous for practical use of the process because rigorous exclusion of oxygen is difficult to implement. 3.3. Effect of Filler. All PVNR films used in this study were made with a HDDA plus photoinitiator level of 4 phr and HDDA to photoinitiator weight ratio of 1:1. As seen in the SEM images of PVNR films made with various loadings of the CaCO3 filler (Figure 7), the size of the filler agglomerate was scarcely affected by filler loading, but the concentration of the agglomerates dispersed in the rubber matrix increased with increasing filler load. The effects of the various loadings of the filler on the tensile properties of the PVNR films are summarized in Table 1. Increasing the filler loading from 0 to 5 phr caused a notable increase in the tensile strength of the PVNR films (Table 1). With further increase of the loading, the tensile strength remained statistically unchanged. In the loading range of 0−40 phr, the elongation at break of these films barely changed, although an apparent drop of the elongation at break was noticed at a filler loading of 40 phr (Table 1). The relatively small effect of the filler on both elongation at break and tensile strength (Table 1) indicated that the filler was inert,29,30 i.e., it did not react with the rubber molecules. In view of the small influence of CaCO3 on tensile properties and its inert status, it was suitable for inclusion in PVNR films to reduce production cost without compromising mechanical properties. A low
Figure 5. Tensile strength and cross-link density of PVNR films made using various total amounts of HDDA plus photoinitiator (HDDA to photoinitiator weight ratio was 1:1).
plus photoinitiator amount of 4 phr was optimal. Weaker films were produced at a HDDA plus photoinitiator amount of 6 phr (Figure 5) as incipient formation of an opaque surface coagulum prevented access of the latex in deeper parts of the pool to the surface for exposure to the UV light. HDDA had a high affinity for NR, as previously noted. Adding a large amount of HDDA resulted in its absorption into rubber particles, and this rapidly increased their size and volume. During mixing, such swollen rubber particles frequently collided, and this triggered entanglement of the surface chains and eventually a coagulation of the particles. 3.2. Effect of Oxygen. Molecular oxygen is quite reactive and inhibits many reactions involving free radicals.16,27 Oxygen 7221
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Figure 7. Cross-sectional scanning electron micrographs of PVNR films containing various amounts of CaCO3: (a) 0 phr, (b) 5 phr, (c) 10 phr, (d) 20 phr, and (e) 40 phr. The horizontal scale bars represent a length of 100 μm. See Table 1 for tensile properties of these films.
Table 1. Effect of Filler on Tensile Properties of the PVNR Filmsa amount of CaCO3 [phr]
tensile strength [MPa]
CaCO3 Added after UV Irradiation 0 12.05 5 14.66 10 15.34 20 15.54 40 15.97 CaCO3 Added before UV Irradiation 20 15.33 a
± ± ± ± ±
1.42 0.61 1.01 0.28 0.38
± 0.37
elongation at break (%) 833 794 794 777 691
± ± ± ± ±
53 65 61 51 60
100% modulus [MPa] 0.45 0.47 0.52 0.52 0.57
854 ± 37
± ± ± ± ±
0.04 0.03 0.03 0.03 0.06
0.50 ± 0.04
300% modulus [MPa] 0.72 0.75 0.87 0.90 1.25
± ± ± ± ±
0.09 0.06 0.08 0.08 0.18
0.82 ± 0.05
500% modulus [MPa] 1.21 2.32 2.72 3.03 4.25
± ± ± ± ±
0.76 0.82 0.90 0.90 2.71
2.15 ± 0.40
All films were prepared with 2 phr HDDA and 2 phr photoinitiator.
interaction between CaCO3 and rubber led to poor filler dispersion, as seen in Figure 7b−e. In Table 1, the filler-filled PVNR films showed higher moduli (i.e., 300% and 500% elongation) than the unfilled films (Table 1). The moduli of the filler-filled PVNR films increased notably with increasing CaCO3 content (Table 1). Similar findings were reported elsewhere.30,31 The increase in moduli was due to the following phenomena: (1) the dilution effect caused by numerous clusters of a stiff filler in the rubber matrix and (2) the fact that the filler particles could serve as physical cross-link sites.32 The occurrence of many cross-link sites in the rubber matrix facilitates the crystallization of rubber chains at large elongations.31,32 Both filler particles and the strain-induced crystallites can hinder the mobility of rubber chains.32 This increases the resistance of the filled rubber samples to deformation. To determine if CaCO3 (an opaque filler) significantly reduced UV-light absorption in the latex during prevulcanization, a comparison of tensile properties was made between the PVNR films that contained the filler during UV irradiation and the films that were irradiated before the filler was added. The mechanical properties of PVNR films containing 20 phr CaCO3 were unaffected by the order in which the filler was added
(Table 1), whether before or after the UV irradiation. This implied that up to a loading of 20 phr the filler particles in the latex mixture did not interfere with absorption of UV light sufficiently to significantly hinder the cross-linking reaction.
4. CONCLUSIONS The UV-mediated prevulcanization of NR could be successfully accomplished by using diacrylate coagents with various lengths of aliphatic hydrophobic backbone chains. The results showed that HDDA was a good coagent as it provided the highest tensile strength and cross-link density of PVNR films. This was due to its suitable alkyl chain length that made HDDA highly compatible with NR. The optimum total amount of HDDA plus photoinitiator was 4 phr. This yielded a tensile strength of 12.05 MPa and a cross-link density of 0.052 mmol cm−3 in PVNR films. Furthermore, the prevulcanization process was not affected by oxygen at levels found in the air. The inert CaCO3 filler included at levels of up to 40 wt % barely affected the tensile properties of the PVNR films. Therefore, CaCO3 proved to be an effective filler for use in PVNR films. CaCO3 did not interfere with UV-mediated prevulcanization process at loadings of up to 20 wt %. All of these features suggest a 7222
DOI: 10.1021/acs.iecr.7b01133 Ind. Eng. Chem. Res. 2017, 56, 7217−7223
Article
Industrial & Engineering Chemistry Research
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strong commercial potential for UV-mediated prevulcanization of NR with suitable diacrylate coagents.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (66) 2579-2083. E-mail:
[email protected]. ORCID
Nanthiya Hansupalak: 0000-0002-9917-403X Yusuf Chisti: 0000-0002-0826-7012 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Natural Rubber Focus Unit (NRFU), National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), Thailand; Thailand Research Fund (Institutional Research Grant No. IRG5980004); the Graduate Scholarship Program of the Faculty of Engineering, Kasetsart University, Thailand; and Kasetsart University Research and Development Institute (KURDI), Thailand. In-kind support was provided by the Thailand Advanced Institute of Science and Technology and Tokyo Institute of Technology (TAISTTokyo Tech), Japan.
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REFERENCES
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DOI: 10.1021/acs.iecr.7b01133 Ind. Eng. Chem. Res. 2017, 56, 7217−7223
