Sulfur-Free Prevulcanization of Natural Rubber Latex by Ultraviolet

Article pubs.acs.org/IECR

Sulfur-Free Prevulcanization of Natural Rubber Latex by Ultraviolet Irradiation Nanthiya Hansupalak,†,‡ Sirinapa Srisuk,§ Panithi Wiroonpochit,*,∥ and Yusuf Chisti⊥ †

Department of Chemical Engineering, Faculty of Engineering, ‡Center of Advanced Studies in Industrial Technology, Faculty of Engineering, and §Interdisciplinary Graduate Program in Advanced and Sustainable Environmental Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand ∥ National Metal and Materials Technology Center (MTEC), 114 Thailand Science Park (TSP), Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand ⊥ School of Engineering, Massey University, Private Bag 11 222, Palmerston North 4474, New Zealand ABSTRACT: The possibility of prevulcanizing natural rubber latex via ultraviolet (UV) irradiation was investigated using 2-hydroxy-2-methyl-1phenylpropanone as the photoinitiator and 1,9-bis(acryloyloxy)nonane as the coagent. Effects of the following process variables were assessed on the tensile strength and the cross-link density of thin films prepared from the prevulcanized natural rubber (PVNR) latex: the duration of UV irradiation, the distance between the light source and the latex being irradiated, the mixing time of the latex with the coagent and the phtoinitiator, the depth of the latex pool, and the mass ratio of the photoinitiator to the coagent used. The PVNR films produced under optimal conditions had nearly 4-fold the tensile strength of the film produced without UV irradiation. Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance spectroscopy were used to assess the UV reactions, and a mechanism was proposed for UV-induced prevulcanization with the above identified photoinitiator and coagent. toxicity of the final product is lower than that of the conventionally made products. Therefore, UV-based prevulcanization is especially useful for making medical products as it reduces the chance of allergenic reactions occurring as a result of the use of the product. Allergenic reactions are associated with the sulfur- and nitrogen-containing chemicals used in the sulfur-based prevulcanization.6,7 The proteins present in the NR latex also contribute to the allergenicity of the NR products, but pretreatment processes have been developed to remove the allergenic proteins.8,9 Diverse radiation sources can be used to initiate prevulcanization. These include γ rays, UV light, and beams of electrons. UV light is nonionizing. UV irradiation has a lower operational cost and energy consumption compared to the other irradiation methods. Furthermore, the products made using UV prevulcanization show good skin compatibility and good physical and mechanical properties.7 UV irradiation does not generate any radiation residues.10 The UV light can vulcanize NR latex in an accelerator-free environment.7,11,12 UV irradiation is already well-established for curing acrylated oligomers such as polyesters, polyethers, polyurethanes, polysiloxanes, and polybutadiene.13 The photoinitiators used in UV-based curing include α-hydroxyphenyl ketone and 2,2dimethoxy-2-phenyl acetophenone.13

1. INTRODUCTION Although synthetic rubber is widely available, natural rubber (NR) obtained from the Hevea brasiliensis tree remains the material of choice in many applications. NR has a high molecular weight, and this results in a high strength and good elasticity of the objects made from it.1 NR latex is preferred especially for making dip-molded products such as medical examination gloves, condoms, dental dams, and teats of baby feeders.2 A partial vulcanization of NR latex prior to dipping the shaped molds into the latex is known as prevulcanization. Prevulcanization of the latex shortens the time of subsequent vulcanization of the shaped rubber product,3 and this cuts down on the total time of the production process and its energy requirements. The latex product of the prevulcanization process is known as prevulcanized natural rubber latex (PVNRL). PVNRL is homogeneous and has the fluidity of the latex. It is suitable for coating applications (e.g., shape formation by using dipping molds), and its use improves the uniformity of crosslinking in subsequent vulcanization. This directly contributes to the improved tensile strength of the final product.4 Compared to the conventional method of prevulcanization with sulfur, the alternative of using ultraviolet (UV) light to initiate prevulcanization has important advantages: lower doses of vulcanizing chemicals are needed and this reduces residual chemicals in the final product. In addition, the life cycle of the product does not involve emissions of sulfur dioxide, an environmental pollutant.5 Furthermore, the UV-based prevulcanization does not produce carcinogenic substances and the © XXXX American Chemical Society

Received: January 7, 2016 Revised: March 14, 2016 Accepted: March 21, 2016

A

DOI: 10.1021/acs.iecr.6b00076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research The prevulcanization of the NR latex using a photoinitiator in the presence of a polyfunctional thiol coagent has been reported.7,11,12 In these studies, the dry rubber content (DRC) of the NR latex was kept constant at 40% w/w and a thiolene addition reaction was involved in the cross-linking. As the thiol coagent contained sulfur, the products made using this type of prevulcanization have the potential to cause type IV allergic reactions on contact with human skin and other tissue. Furthermore, the use of 40% DRC in the NR latex restricts the applications of the products made. Although a 40% DRC of the NR latex is suitable for making dip molding products such as medical gloves and condoms, the manufacture of many other products (e.g., dental dams, tubes, baby feeder teats) requires the use of 60% DRC.2 The coagent used in vulcanization helps in improving the flexibility of the cross-links in the network of the cross-linked polymer chains.3 Acrylate coagents have been used in UVinduced cross-linking of many double-bond-containing polymers.13 Acrylate coagents do not contain sulfur. The use of acrylate coagents for cross-linking NR via UV irradiation has not been reported, but acrylate coagents have been used to cross-link NR in electron beam initiated processes.14−16 The present work attempts to use an acrylate coagent (1,9bis(acryloyloxy)nonane) in the UV-induced prevulcanization of high ammonia NR latex with a 60% DRC. Effects of the following factors on the tensile strength and the cross-link density of the PVNR films were assessed in order to determine optimal conditions for prevulcanization: the mass ratio of the photoinitiator to the coagent; the mixing time of the latex with the chemicals (i.e., the photoinitiator and the coagent); the duration of the UV irradiation; the distance between the light source and the surface of the latex pool being irradiated; the depth of the latex layer being irradiated. The PVNRL was characterized using attenuated total reflection Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance spectroscopy, for understanding the mechanisms involved in the cross-linking process.

Figure 1. Chemical structures of coagent 1,9-ND (a), photoinitiator Irgacure 1173 (b), and cis-1,4-polyisoprene, the repeating unit of natural rubber (NR) (c).

Each Petri dish was covered by a polyethylene cling film which absorbed less than 16% of the incident UV light in the wavelength range of 250−350 nm (UV spectrometer, Model U1900; Hitachi, Japan; www.hitachi-hta.com). While being stirred continuously (magnetic stirrer), the latex mixture in Petri dishes was irradiated from the top by a bank of six UV lamps (250−350 nm wavelength, 8 W) (Figure 2). The

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. High-ammonia NR latex (60% DRC) was purchased from Thai Eastern Rubber Co., Ltd., Rayong, Thailand. The photoinitiator, 2-hydroxy-2methyl-1-phenylpropanone (Irgacure 1173), was purchased from BASF (www.basf.com) and the coagent, 1,9-bis(acryloyloxy)nonane (1,9-ND), was procured from Tokyo Chemical Industry Co. Ltd. (www.tcichemicals.com), Japan. Potassium laurate solution, ammonium hydroxide solution (Sigma-Aldrich, Thailand), toluene (99% purity; Lab Scan Analytical Sciences, Ireland), and the other chemicals were used as received. The chemical structures of the coagent, the photoinitiator, and NR are shown in Figure 1. 2.2. Preparation of the Latex Mixture. A 50% emulsion of 1,9-ND was prepared by mixing 100 g of 1,9-ND with 99 g of deionized water and 1 g of potassium laurate for 30 min at room temperature in the dark. This emulsion, the photoinitiator Irgacure 1173, and the NR latex were mixed together for various lengths of time (0.5, 1.0, and 2.0 h) in the dark. For any specified mass ratio of 1,9-ND and the photoinitiator in the NR latex, the total quantity of the of the two reagents was always 4 g per 100 g of rubber in the latex, or 4 phr (parts per hundred rubber). 2.3. UV Irradiation. The prepared latex mixture was poured into Petri dishes to various depths (l) (=4, 6, 10, and 14 mm).

Figure 2. Experimental setup. The prepared latex contained in a covered Petri dish was irradiated using an overhead bank of UV lamps.

intensity of the incident radiation at the surface of the Petri dishes was 0.7−1.8 mW/cm2. The irradiation period was 0.5, 1, and 2 h in different experiments. The distance between the lamps and the surface of the latex, or the irradiation distance (x, Figure 2), was 30, 45, and 60 mm in different experiments. All experiments were conducted in a dark room at 28 °C. After the UV irradiation, PVNRL was diluted to 45% (w/w) dry rubber content with an aqueous solution of ammonia (0.6% w/w ammonia). A 1 mm thick film of PVNR was produced by pouring the PVNRL suspension on a glass plate and drying to a constant weight in air at room temperature. Around 36 g of PVNRL suspension was required on a glass plate of dimensions 13 cm × 13 cm to form the final 1 mm thick film. The dry PVNR film could be peeled from the glass surface. 2.4. Characterization. 2.4.1. Equilibrium Swelling Test. This test was carried out at room temperature in the dark. A sample of the PVNR film (2 cm × 2 cm) was immersed in toluene until the weight of the swollen film no longer increased, indicating attainment of equilibrium. The film was taken out of the solvent, dabbed with a filter paper to remove the adhering solvent from the surface, and weighed. The film was then dried to constant weight in an oven at 60 °C, cooled to room B


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Industrial & Engineering Chemistry Research

Figure 3. Tensile strength and cross-link density of the irradiated PVNR films produced under various conditions: (a) effect of the mass ratio of the photoinitiator to the coagent (R); (b) effect of the mixing time; (c) effect of the irradiation distance (x, Figure 2); (d) effect of the depth of the latex pool (l, Figure 2); (e) effect of the irradiation time. For each study, only the factor of interest was varied while the other factors were fixed at the following base values: the photoinitiator to coagent mass ratio of 1; a mixing time of 1 h; an irradiation distance of 45 mm; a latex pool depth of 10 mm; an irradiation time of 1 h.

reflection Fourier transform infrared spectroscopy (ATR-FTIR Spectrum Spotlight 300; PerkinElmer, Waltham, MA, USA). Since the UV-irradiated PVNRL and the NR latex (both as liquids) were in liquid form, containing a large amount of water and ammonia molecules, the excessive 1H signals from water and ammonia dominated the 1H signals of the latexes, coagent, and phtoinitiator that needed to be investigated. Therefore, the UV-irradiated PVNRL and the NR latex were only characterized using 13C nuclear magnetic resonance spectroscopy (Bruker Biospin 500 13C NMR operating at 500 MHz). The samples for 13C NMR were prepared and measured at 25 °C.17

temperature in a desiccator, and weighed. The volume fraction ϕp of the polymer in the swollen sample was calculated as follows: ϕp =

1 1+

ρp ρs

(

WSW WAD

)

−1

(1)

where WSW is the weight of the solvent-swollen film, WAD is the weight of the oven-dried film, ρp (=0.93 g/cm3) is the density of the polymer, and ρs (=0.87 g/cm3) is the density of the solvent. The cross-link density (ρc, mmol/cm3) was then calculated by using the Flory−Rehner equation:7 ρc =

3. RESULTS AND DISCUSSION 3.1. Effect of UV Prevulcanization Factors on Film Strength and Cross-Link Density. The mass ratio R of the photoinitiator to the coagent affected the film properties as shown in Figure 3a. An increase in the ratio R from 0 to 1 caused an increase in both the tensile strength and the crosslink density of the films. Once the R-value exceeded 1, both the tensile strength and the cross-link density dropped substantially. This phenomenon coincided with the appearance of an oil layer on the surface of the films just after they were formed and being air-dried. This change of the film surface, namely blooming,18 was caused by the migration of the excess photoinitiator in the rubber. The low values of the tensile strength and the cross-link density seen at R > 1 (Figure 3a) are not explained by blooming alone. The following reactions occurring in the rubber during UV irradiation were also involved: (1) the recombination of the free photoinitiator radicals to regenerate the photoinitiator under conditions of a high concentration of the photoinitiator and, therefore, its radicals, in the rubber; (2) the early termination of the propagation step owing to a massive content of monoradicals

−(ln(1−ϕp) + ϕp + χϕp2)

(

Vs ϕp1/3 −

ϕp 2

)

(2)

In eq 2, χ is the Flory−Huggins parameter for polymer−solvent interaction, and Vs is the molar volume of the solvent. 2.4.2. Viscosity. The viscosity of the latex and the mixture was measured by using a Brookfield viscometer (Model LVDVI Prime, spindle S21, 60 rpm; www.brookfieldengineering. com). 2.4.3. Tensile Strength Test. The tensile strength of the PVNR films was measured in accordance with the ASTM Standard D412-98a. The specimens were stamped from 1 mm thick PVNR films that had been kept in vacuo at room temperature. The tensile properties were measured at room temperature using an Instron tensile tester (Instron 5566, Canton, MA, USA) at a crosshead speed of 50 mm/min and a load of 1 kN. 2.4.4. Chemical Characterization. The PVNR film (1 mm thick) samples were characterized by using attenuated total C


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and this in turn lowered the extent of vulcanization and affected the mechanical properties as mentioned earlier. In Figure 3c, an irradiation distance of 45 mm proved to be optimal as it reduced the water evaporation, but it provided a sufficient radiation intensity to form films with the highest level of cross-link density and tensile strength. The water evaporation at the gas−liquid interface was strongest at the shortest irradiation distance of 30 mm, yielding coagula suspended in the medium. In contrast, at the highest irradiation distance (60 mm), the water evaporation was low, but so was the intensity of the UV light. This led to PVNR films with a low cross-link density and tensile strength. In Figure 3d, the cross-link density and therefore the tensile strength of the film increased as the depth of the latex mixture increased from 4 to 10 mm. A further increase in the depth of the latex pool caused a dramatic reduction in the cross-link density and the tensile strength. The decline in these film properties over the 10−14 mm depth range is a reflection of the limited ability of the UV light to penetrate a deep pool of suspended opaque particles.21 In view of Figure 3d, a 10 mm depth of the latex mixture was optimal for attaining the maximum strength of the PVNR films. The effects of the irradiation time on the properties of the films are shown in Figure 3e. An irradiation time of up to 0.5 h was clearly insufficient to achieve any cross-linking. An irradiation time of 1 h maximized the cross-link density and the tensile strength of the films. A further increase in the irradiation time to 2 h actually reduced the cross-link density and the film strength. In these experiments (Figure 3e), the highest amount of coagulum formation was observed at an irradiation time of 2 h. As explained earlier, the high amount of coagulum in the latex layer limited the penetration of UV light, leading to less vulcanization of the rubber particles and poorer properties for the films made from them. There was also the possibility that a 2 h exposure to the UV light degraded the rubber particles22 to adversely affect the cross-link density and the tensile strength. The data in Figure 3 suggest that the cross-link density and the tensile strength are strongly correlated. This agrees well with the literature on other polymers.4 The maximum values of the cross-link density and the tensile strength were 0.17 mmol/ cm3 and 11 MPa, respectively. Based on the data (Figure 3), the optimal conditions for maximizing the cross-link density and the tensile strength were the following: a photoinitiator to the coagent mass ratio of 1; a mixing time of 1 h; an irradiation distance of 45 mm; a latex pool depth of 10 mm; an irradiation time of 1 h. 3.2. FTIR and NMR Analyses. The FTIR spectra were recorded for each sample before and after irradiation. Thus, the spectra were recorded for the following: films of the pure NR latex (without any reagents); films of the pure NR latex after irradiation for 2 h; films of the mixed latex prior to irradiation; films of the mixed latex after irradiation of separate samples for 1 and 2 h. Different film treatment methods were explored for the FTIR measurements. These were (1) the use of air-dried film; (2) the use of films that had been washed for 30 min in a continuously stirred bath of an aqueous solution of a surfactant (2% w/w sodium dodecyl sulfate), rinsed in deionized water, and dried overnight at 60 °C; and (3) the use of films that had been swollen in toluene at room temperature for 3 days and dried (60 °C, overnight). The objective of treatment 3 was to leach any small molecules and residual reagents through the swollen

(i.e., the photoinitiator radicals) having only one active site per molecule. In view of the results in Figure 3a, an R-value of 1 was chosen as an optimal value and used in the following experiments. Figure 3b shows the effects of mixing time on tensile strength and cross-link density values of the PVNR films. Both the tensile strength and cross-link density increased with increasing mixing time up to a mixing time of 1 h. A mixing time of 1 h provided films with the highest tensile strength and the crosslink density. A further increase in mixing time adversely affected both the tensile strength and cross-link density (Figure 3b). An increase in mixing time to 1 h likely enhanced the uniformity of distribution of the photoinitiator and the coagent in the latex, as previously reported.14 The reduction in cross-linking density and tensile strength at a mixing time of >1 h (Figure 3b) can be explained as follows. In the absence of UV irradiation, a mixing time of 2 h did not significantly change the viscosity of the pure NR latex (no photoinitiator and coagent present). The viscosity of the latex before and after being continuously mixed for 2 h was about 60.0 mPa·s. In the presence of the coagent and the photoinitiator, the viscosity of the nonirradiated latex mixture notably increased from an initial value of 60.0 mPa·s to a value of 77.5 mPa·s at 2 h. Thus, this viscosity increase is strongly suggestive of chain entanglement induced by the presence of the coagent and the photoinitiator.19 Both these compounds are known to be hydrophobic liquids. In the NR latex, they diffuse into the rubber particles to swell them. The resulting increased volume of the particles raises the frequency of particle−particle interaction during agitation and hydrophobic rubber particles coming in contact adhere to form relatively large coagulated particles. The particles are larger if the mixing time is longer. As UV light is unable to penetrate any opaque object, irradiation of the larger coagulated particles formed after mixing times of >1 h (Figure 3b) affected only the particle surface, and thus reduced vulcanization. This explains the lower cross-linking density and, therefore, a lower tensile strength at a mixing time of 2 h compared to the data at a mixing time of 1 h. The effect of the distance between the UV radiation source and the surface of the latex pool, or irradiation distance, on the tensile strength and the cross-link density of the PVNR films is shown in Figure 3c. Increasing the irradiation distance from 30 to 45 mm caused a certain increase in the tensile strength and the cross-link density. Further increase in the irradiation distance to 60 mm dramatically reduced both the tensile strength and the cross-link density of the film. The results can be explained by the strong effect of the irradiation distance on the intensity of the irradiation20 of the latex pool and therefore on the coagulum formation during irradiation. As the irradiation distance was decreased, the intensity of radiation intensified and this enhanced the efficacy of UV radiation in dissociating the photoinitiator into free radicals that initiate the polymer cross-linking process. This could potentially lead to a higher cross-linking density and better tensile strength. Furthermore, evaporation of the water from the latex layer was certainly increased by reducing the proximity of the UV lamps to the surface of the latex. Evaporation of water from the surface of the latex tended to dry out the surface to leave behind a thin surface film. This film was fragmented by the shear forces associated with the mixing of the Petri dishes via the magnetic bars, resulting in the formation of small fragmented pieces of the film in the medium. These suspended opaque fragments reduced the penetration of light in the latex, D


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The absorption peak at 1730 cm−1 occurred only in the spectra of the films made from the latex mixture (spectra c−e, Figure 4). This band is presumably associated with the CO functionality and possibly with the ester (R−COO−R′) functional groups in the coagent and the ketone (R−CO− R′) groups in the photoinitiator.7 Thus, this peak was not seen in the spectra of the films made of pure NR latex and the 2 h irradiated pure NR latex (spectra a and b, Figure 4), as they did not contain any photoinitiator and coagent molecules. As the surfactant solution wash and swelling with toluene had no effect on the FTIR spectra of the films, it was concluded that both the coagent and the photoinitiator molecules in these films were strongly attached to the NR in the films. The 13C NMR spectra are shown in Figure 5. The presence of the coagent and the photoinitiator in the liquid latex mixture prior to irradiation (spectrum c, Figure 5) was evidenced by the following: (1) appearance of the signals characteristic of C1, C2, C3, and C4 of the coagent at 131.5, 132.7, 168.0, and 67.1 ppm, respectively; and (2) the appearance signals characteristic of the carbon atoms in the benzene ring of the photoinitiator at 130.7 and 132.3 ppm. However, no signal of C14 (the carbonyl group of the photoinitiator) appeared at around 196 ppm (spectrum c, Figure 5). In addition, the signals corresponding to the double bonds (C19 and C21) and the methyl carbon (C20) in isoprene units were seen at 127.7 and 137.2 ppm, respectively.23 After irradiation, the signals corresponding to the photoinitiator and the isoprene in the latex mixture were detected, while the signal from the coagent disappeared (spectrum d, Figure 5). This suggests that the coagent molecules were completely consumed. A comparison of the 13C NMR spectrum of the latex mixture after irradiation for 1 h (spectrum d, Figure 5) with the spectra of the nonirradiated latex mixture (spectrum c, Figure 5) and the pure NR latex (spectrum b, Figure 5) showed an absence of the previously mentioned signals of C1 and C2 of the coagent. Cross-linking did not produce new signals, but this occurs commonly in cross-linking of NR. In addition, the UV irradiation for 1 h did not produce any new NMR signals, but it did decrease slightly the 13C NMR intensity ratio of the C19 signal to the C20 signal (Figure 5). Furthermore, the

rubber network. The different treatment methods were found to not affect the FTIR spectra (data not shown). Subsequently, the spectra were recorded only on thin films that had been washed with the surfactant (treatment method 2, above). These FTIR spectra are shown in Figure 4.

Figure 4. FTIR spectra of nonirradiated pure NR latex (a), 2 h irradiated pure NR latex (b), NR latex mixed with the photoinitiator and the coagent (no irradiation) (c), 1 h irradiated latex mixture (d), and 2 h irradiated latex mixture (e).

All thin films prepared from the latex mixture exhibited FTIR spectra similar to the spectrum of the film made from the pure nonirradiated NR latex (Figure 4). The spectra had the characteristic peaks of NR: absorption bands at around 3033 cm−1 assigned to C−H stretching in CCH; the 2853−2962 cm−1 bands attributed to C−H stretching in −CH3 and −CH2−; the 1664 cm−1 bands due to CC stretching; the 1450 cm−1 bands associated with the bending of C−H in −CH2−; the 1375 cm−1 absorption bands due to C−H bending in −CH3.7

Figure 5. Latex-state 13C NMR spectra of the coagent (a), pure NR latex (b), nonirradiated latex mixture (c), and 1 h irradiated latex mixture (d). E


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Figure 6. Proposed mechanism of UV prevulcanization.

without the photoinitiator and the coagent had tensile strengths comparable to the thin films prepared without irradiation from the latex mixture containing the photoinitiator and the coagent. The NMR data and the above comparative assessment of the films formed under different conditions implied that the UV irradiation, the photoinitiator, and the coagent used in this study worked well together. A lack of either UV irradiation or the coagent/photoinitiator, or both, prevented the formation of the chemical cross-links. UV irradiation creates electrondeficient free radicals25 which chemically react with molecules possessing double bonds (i.e., polyisoprene chains, 1,9-ND, Irgacure 1173) and with molecules that have extra lone pairs of electrons (as in 1,9-ND and Irgacure 1173). Based on the results presented, the key observations are the following: Both the tensile strength and the cross-link density of the cross-linked rubber films are higher than those of the untreated rubber films (Figure 3e). The signals for the carbonyl (CO) and vinyl (CC) groups were seen only in the spectra of the films of NR mixtures and not in the spectra of pure NR films (Figure 4). As both the coagent and the photoinitiator have the same carbonyl group, the FTIR spectra suggest the presence of these compounds in both the UV-irradiated and nonirradiated films made from the NR mixtures.

intensity ratio was unchanged for the nonirradiated latex mixture (spectrum c, Figure 5) compared to the signal for the pure NR latex (spectrum b, Figure 5). This ratio decreased from 0.68 for the nonirradiated latex mixture (spectrum c, Figure 5) to 0.65 (spectrum d, Figure 5) for the 1 h irradiated latex mixture. In view of this 4% reduction in the intensity ratio (spectra c and d, Figure 5) as a result of irradiation, the amount of CC in the films made from nonirradiated NR latex was unaffected by the presence of the coagent and the photoinitiator. In other words, cross-linking of the rubber chains cannot occur in the presence of the photoinitiator and the coagent, unless UV irradiation is used. In nonirradiated films, the coagent and the photoinitiator in the different samples did not generate chemical cross-linking. Thus, within a few hours of the swelling test, the films became gels and could not retain their shape. Furthermore, the tensile strength of the nonirradiated films containing the coagent and the photoinitiator was comparable to the green strength of the NR film (tensile strength = 1.57 MPa),24 as shown in Figure 3e. In view of these results, the reduction in the intensity ratio of the CC signal (Figure 5) upon irradiation is indicative of a loss of some CC in isoprene units through cross-linking. As previously noted, the cross-linking reactions consumed all of the coagent molecules upon UV irradiation and UV irradiation proved to be essential for prevulcanization. The thin films prepared from the pure NR latex irradiated for 2 h F


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PVNRL. Second, the current prevulcanization treatment was carried out in vessels open to the atmosphere. As oxygen is known to rapidly react with free radicals to generate inactive products,13 the efficacy of the process may have been lower than if it were carried out in a closed reactor and without any dissolved oxygen in the rubber latex. Potentially, all dissolved oxygen in the latex can be removed by purging it with an inert gas such as nitrogen. 3.3. Potential for Process Scale-Up. The optimal processing conditions identified here can be potentially implemented in a scaled-up commercial process. Other than the specified ratios of the different chemicals, the relevant conditions are as follows: a temperature of 28 °C; a mixing time of 1 h; an irradiation time of 1 h; an irradiation distance of 45 mm; a latex pool depth of 10 mm. An air-conditioned humidity controlled production hall can easily provide the necessary temperature. A reactor of a rectangular channel configuration is envisaged for this process. An open-top rectangular channel can be easily constructed such that the latex flows in at one end of the channel and flows out at the other end. The depth of the latex in the channel can be readily controlled at the required 10 mm by having an overflow baffle (height of about 10 mm) installed at the exit end. The residence time of the latex in the irradiated channel needs to be 1 h. For a channel width of 0.6 m and a relatively slow latex flow velocity of 0.1 m/s in the channel, a mean residence time of 1 h is easily attained by having a channel length of 360 m. A meandering configuration of this channel can be accommodated in a production hall of 860 m2 floor area. Either the UV lamps can be mounted 45 mm above the surface of the latex in the channel, or the channel could be made with a clear quartz base so that the lamps can be mounted 45 mm below the base of the channel. Multiple low-speed magnetically driven mixing impellers can be installed at the bottom along the length of the channel to continuously mix the latex as it flows from the entrance of the channel to its exit. All this is technically feasible. Such a system with a specified latex flow rate of 0.1 m/s and a channel width of 0.6 m will generate a volumetric flow rate of about 2.2 m3/h of the prevulcanized latex. In a 24 h operation, nearly 50 m3 of prevulcanized latex could be produced using this process. The irradiated channel must be preceded by a stirred mixing tank to mix the latex and the reagents. For the required mixing time of 1 h and a latex flow rate of 2.2 m3/h, a tank of about 2.8 m3 will suffice.

The disappearance of only the bands attributed to the coagent in the NMR spectra of the PVNR film (Figure 5) suggests that the chemical structure of the coagent was altered during the cross-linking process. Spectral evidence points to a complete consumption of the coagent. The above together with the observed increase in tensile strength and cross-linking density on irradiation suggests that the rubber chain cross-linking reaction dominated and the coagent took part in the formation of the cross-links. On absorbing the UV light, the photoinitiator 2-hydroxy-2methyl-1-phenylpropanone is known to produce benzoyl and isopropanol radicals by homolytic cleavage of the C−C bonds.13 Consequently, the UV prevulcanization initiated by the photoinitiator used can only occur through a process involving free radicals. The proposed mechanism for the prevulcanization is shown in Figure 6. According to the mechanism in Figure 6, absorption of UV light results in the formation of benzoyl and isopropanol radicals via homolytic cleavage of 2-hydroxy-2-methyl-1phenylpropanone, the photoinitiator. These radicals (P• in Figure 6) initiate prevulcanization by attacking the CC bonds of the nearest molecules of polyisoprene (rubber chains) and the coagent. The reaction between the radicals of the photoinitiator and the polymer chains produces polymer radicals, whereas the reaction between the radicals of the photoinitiator and the coagent (1,9-bis(acryloyloxy)nonane, Figure 6) generates products with one or two radical centers. These are the monoradical and the biradical, respectively (Figure 6). These coagent radicals can also react with polyisoprene to form cross-links between the polyisoprene chains (Figure 6) and produce the polymer radicals. The latter may further react with other molecules of the coagent and other segments of the polyisoprene chains. Such reactions happen repeatedly and randomly to form the cross-linking network between the rubber chains with the coagent molecules acting as cross-linkers.13 In the present study, the cross-linking process carried out under the above identified optimal conditions led to an impressive increase in the tensile strength of the NR vulcanizates. The tensile strength of the cross-linked films was 10−12 MPa (Figure 3e), or 4-fold the tensile strength of the green films (i.e., the films of pure NR). Although the crosslink density of 0.17 mmol/cm3 achieved under the optimal treatment conditions was comparable to the literature values,11 the tensile strength was 2−3 times lower than the literature values. Regardless of the low tensile strength values, the PVNR product would be suitable for making objects such as the dental rubber dams used in orthodontics.26 The products made with the prevulcanizate prepared by the method used here would not contain sulfur and the accelerators and, therefore, are expected to be safer than the products made using the conventional sulfur vulcanization process. Applications that require such safe products include the rubber dams used in dental treatment, the examination gloves used by surgeons, and the teats used on baby feeders. The prevulcanization used here posed two problems. First, a rubber coagulum was produced within 2 h of mixing prior to UV irradiation, but a much shorter mixing time may not be sufficient to homogeneously distribute the photoinitiator and the coagent in the rubber latex and within the rubber particles. This problem may be circumvented by diluting the latex prior to mixing, but this may be unacceptable for making products that require a high content of the dry rubber solids in the

4. CONCLUSIONS NR latex can be effectively prevulcanized using the UV irradiation method demonstrated here. Using 2-hydroxy-2methyl-1-phenylpropanone as the photoinitiator and 1,9bis(acryloyloxy)nonane as the coagent, the optimal conditions for prevulcanization at room temperature (28 °C) are as follows: a photoinitiator to the coagent mass ratio of 1; the total amount of the coagent and the photoinitiator in the latex, of 4 phr; a mixing time of 1 h; an irradiation time of 1 h; an irradiation distance of 45 mm; a latex pool depth of 10 mm. The PVNR films produced under these conditions had a tensile strength of 10−12 MPa and a cross-link density of 0.17 mmol/ cm3. The coagent molecules were shown to be involved in the cross-linking process. A mechanism for the cross-linking of the NR chains in a UV initiated prevulcanization was proposed. G


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(16) Makuuchi, K.; Yoshii, F.; Gunewardena, J. A. G. S. G. Radiation vulcanization of NR latex with low energy electron beams. Radiat. Phys. Chem. 1995, 46, 979. (17) Ukawa, J.; Kawahara, S.; Sakai, J. Structural characterization of vulcanized natural rubber by latex state 13C NMR spectroscopy. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 1003. (18) Ramcharan Company. Blooming in rubber: An overview with analytical path. Ramcharan Prod. Dev. Inf. 2013, 3, 1. (19) Hasan, M. R.; Molla, M. A. I.; Sarker, M.; Masum, S. M.; Rana, A. A.; Sultana, S.; Haque, M. E.; Karim, M. M. Determination of protein content in gamma ray irradiated and non-irradiated natural rubber latex film. Int. J. Basic Appl. Sci. 2011, 11, 34. (20) Maag, K.; Lenhard, W.; Löffles, H. New UV curing systems for automotive applications. Prog. Org. Coat. 2000, 40, 93. (21) Decker, C. The use of UV irradiation in polymerization. Polym. Int. 1998, 45, 133. (22) Yahya, Y. S. R.; Azura, A. R.; Ahmad, Z. Effect of curing systems on thermal degradation behaviour of natural rubber (SMR CV 60). J. Phys. Sci. 2011, 22, 1. (23) Silverstein, R. M.; Webster, F. X.; Kiemle, D. Spectrometric Identification of Organic Compounds; Wiley: New York, 2005. (24) Amnuaypornsri, S.; Sakdapipanich, J.; Tanaka, Y. Green strength of natural rubber: The origin of the stress−strain behavior of natural rubber. J. Appl. Polym. Sci. 2009, 111, 2127. (25) Tamboli, S. M.; Mhaske, S. T.; Kale, D. D. Crosslinked polyethylene. Indian J. Chem. Technol. 2004, 11, 853. (26) Four D Rubber Co., Ltd. Technical Data Sheet: Latex-free dental dam; 2014. www.fourdrubber.com.

AUTHOR INFORMATION

Corresponding Author

*Tel.: (66) 2564-6500. Fax: (66) 2564-6446. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the Natural Rubber Focus Unit (NRFU), National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), 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 (TAIST-Tokyo Tech), Japan.

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REFERENCES

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Sulfur-Free Prevulcanization of Natural Rubber Latex by Ultraviolet

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