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rubber composites: The influence of reactive diluents
Author’s Accepted Manuscript Dual-curable PVB based adhesive formulations for cord/rubber composites: The influence of reactive diluents Zehra Yildiz, Hacer Aysen Onen www.elsevier.com/locate/ijadhadh
PII: DOI: Reference:
S0143-7496(17)30106-9 http://dx.doi.org/10.1016/j.ijadhadh.2017.06.004 JAAD2017
To appear in: International Journal of Adhesion and Adhesives Received date: 16 June 2016 Accepted date: 14 April 2017 Cite this article as: Zehra Yildiz and Hacer Aysen Onen, Dual-curable PVB based adhesive formulations for cord/rubber composites: The influence of reactive diluents, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2017.06.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dual-curable PVB based adhesive formulations for cord/rubber composites: The influence of reactive diluents Zehra Yildiz1,2, Hacer Aysen Onen3 1
Istanbul Technical University, Institute of Sciences and Technology, Department of Polymer Science and Technology, TURKEY
2
Marmara University, Faculty of Technology, Department of Textile Engineering, TURKEY 3
Istanbul Technical University, Faculty of Arts and Sciences, Department of Chemistry, TURKEY
[email protected]
Abstract In this study, the effects of reactive diluent type on the adhesion strength of cord/rubber surfaces were investigated. For this purpose, a urethane acrylate oligomer was synthesized by the reaction of 2,4-toluene diisocyanate (TDI), 2-hydroxyethyl methacrylate (HEMA) and polyvinyl butyral (PVB) in the presence of di-n-butyltin dilaurate (T12) as catalyst. The structure of the oligomer was characterized by nuclear magnetic resonance (NMR) spectroscopy. Then the oligomer was included in adhesive formulations together with trimethylolpropane trimethacrylate (TMPTMA) and tricyclodecane dimethanol diacrylate (TCDDA)
as
reactive
diluents
and
thermal
and
photo
initiator
respectively.
Polyester/polyamide cord fabrics were dipped into the adhesive solution and cured by UVlight. Then coated fabrics were characterized by Fourier transform infrared (FTIR) 1
spectroscopy, differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Contact angle measurement was employed to investigate the wettability properties of the coated fabrics. Thermal curing between the coated fabric and rubber was performed under heat and pressure. The adhesion strength between the cord/rubber surfaces was determined by T-peel test. The highest adhesion strength of 100.4 N/cm with the lowest contact angle value of 70.2o were obtained in the sample containing TCDDA as reactive diluent, due to a higher functionality resulting in a greater crosslinking density. Key Words: UV-Curing; Adhesion; Polyvinyl butyral; Reactive Diluent; Cord Fabric 1. Introduction Cord fabric reinforced rubber composites have been widely used for manufacturing of conveyor belts, hoses, tires, membranes etc. Considering the industrial applications of cord/rubber composites, adhesion between the two components is the most important issue, determining the product life and quality of the material. Cord fabrics have been treated by some chemical formulations such as resorcinol-formaldehyde-latex [1, 2], cobalt boron acrylate [3], hydrated silica-resorcinol-hexamethoxymethylmelamine [4], prior to the adherence onto rubber surfaces. All these processes require the use of formaldehyde that can cause health and environmental problems because of its toxicity [5-7]. UV-curing technology is favored due to it being a solvent-free process with low energy consumption at low temperatures resulting in high productivity levels. It supplies to the surface a high scratch resistance property with high optical clarity [8]. Typically, UV-curable urethane acrylate oligomers can be synthesized by using an isocyanate such as TDI [9], hexamethylene diisocyanate (HDI) [10], isophorone diisocyanaate (IPDI) [11] and an acrylate functional monomer such as 2-hydroxy ethylacrylate (HEA) and HEMA. Acrylate 2
functionality in the structure, gives the ability to be cured by UV-light. During the reaction some of the isocyanate groups are intentionally left unreacted thus allowing reaction with a polyol to form a UV-curable urethane acrylate oligomer. Instead of a polyol, the isocyanate groups can be also reacted with PVB which gives a strong binding and adhesion property on many surfaces. In the literature, UV-curable urethane acrylate oligomers have been synthesized by using an isocyanate, acrylate functional monomer and a polyol but there has been no research considering the usage of PVB instead of a polyol as a co-reactant for isocyanate containing compounds. Reactive diluents are one of the most important components of a UV-curable formulation which are used to reduce viscosity, control crosslinking density, improve mechanical properties and chemical resistance etc. Acrylic based reactive diluents are commercially favored because of their high clarity and optical transparency with non-yellowing properties. In the literature a number of studies have been made considering the effects of the type and percentage of the reactive diluent in the formulations, on the properties of the end product. For instance, urethane acrylate oligomer has been synthesized and then mixed with 1,6hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA). The effects of reactive diluent functionality on coating properties has been considered. Results have shown that the higher crosslinking density of formulations with TMPTA, has increased the glass transition temperature and thermal stability of the coating material [12]. In further research, the effect of reactive diluent concentration on the thermal degradation properties of N-(4hydroxyl phenyl) maleimide derivatives has been studied. N-vinylpyrrolidone (NVP) has been chosen as a reactive diluent. The highest thermal stability with a 30 % char yield has been obtained in a 80 % maleimide derivative-20 % NVP composition [13]. The effect of a reactive diluent (dipropylene glycol diglycidyl ether) on the mechanical properties of MWCNT 3
composites has also been studied [14]. The effect of reactive diluent type in adhesive formulations for cord/rubber composites has never been investigated before. Hence, this work aimed to study the effect of reactive diluent types on adhesion strength between cord/rubber surfaces. A urethane acrylate oligomer was synthesized based on the reaction of TDI-HEMAPVB. Then formaldehyde-free dual-curable adhesive formulations were prepared by using the oligomer, reactive diluent, thermal and photo-initiators. TMPTMA and TCDDA with different functionalities were used as reactive diluents. Adhesive formulations were applied on cord fabrics and then cured by UV-light. After thermal curing of the rubber, the adhesion strength between the cord/rubber surfaces was investigated. The influence of reactive diluent functionality on adhesion property of cord/rubber layers are discussed. 2. Experimental 2.1. Materials 2-hydroxyethyl methacrylate (HEMA), 2,4-toluene diisocyanate (TDI), trimethylolpropane trimethacrylate (TMPTMA), tetrahydrofuran (anhydrous), 1-hydroxycyclohexyl phenyl ketone (Irgacure 184), azobisisobutyronitrile (AIBN), di-n-butyltin dilaurate (T12) were all purchased from Sigma-Aldrich (Buchs, Switzerland). Polyvinyl butyral (PVB, Mowital B 60 H, hydroxyl content 18-21 %) and TCDDA (Genomer™ 1231) was supplied from Kuraray Europe GmbH (Frankfurt, Germany) and Rahn USA Corp. (Illinois, USA), respectively. Polyester/polyamide (PES/PA) (warp/weft) based cord fabrics (520 g/m2, 8 warp/cm, 4 weft/cm, 540 fibers/warp, 400 fibers/weft) and styrene butadiene rubber (SBR) sheets (60 shore A) were obtained from Izomas Co. (Izmir, Turkey) and Grainger Co. (Illinois, USA), respectively. 2.2. Oligomer synthesis 4
Preparation of the oligomer was carried out in a round-bottom flask equipped with a magnetic stirrer, a nitrogen gas inlet and a condenser. The TDI:HEMA molar ratio was set at 1:1. The reaction process can be seen in Figure 1. First, TDI was placed into the flask containing THF (60 wt.%), reactive diluent (TMPTMA or TCDDA) (50 wt.%) and T12 catalyst (0.03 wt.%). Then HEMA was added dropwise to the main flask over 0.5h in an ice bath and further reaction was carried out for 0.5h at 30 oC and another 1h at 70 oC. 1 wt.% PVB out of the total TDI mass was dissolved in THF in an ultrasonic bath and added dropwise to the main flask. After PVB addition, the reaction was proceeded for another 1h at 90 oC allowing the NCO groups in the 2 position to react with the hydroxyl groups of the PVB and to eliminate the HEMA residue. Finally, the PVB modified TDI-HEMA adduct was obtained. The sample codes with their compositions can be seen in Table 1. The sample designated S1 is untreated PES/PA cord fabric. Table 1. Sample codes of the coating formulations. Oligomer --
PVB Modified TDI-HEMA Adduct
Reactive Diluent Sample Codes --
S1
--
S2
TMPTMA, 50%
S3
TCDDA, 50%
S4
2.3. Preparation and application of coating formulations Coating formulations were prepared using a PVB modified TDI-HEMA adduct (30 wt.% out of the cord fabric mass), photoinitiator (3 wt.%), thermal initiator (3 wt.%) and THF. THF was used in order to obtain a better wettability of fabrics by lowering the viscosity of the 5
oligomer. For the dip-coating process, cord fabrics were cut into 2.5 cm x 7.5 cm dimensions that is required to make the T-peel test according to the appropriate ASTM standard [15]. Then, fabrics were dipped into the coating solution for 3 minutes followed by the removal of excess solution by a squeezing roller. THF was evaporated from the fabric in an oven at 70 oC for 10 min. Since oxygen of air acts as a radical scavenger during the UV-curing process, fabrics were placed into a zip-lock transparent polyethylene bag prior to the UV-curing process [16]. UV-light was applied on fabrics for 2 min from both sides of the fabric by using a OSRAM Ultra-Vitalux 300 W UV lamp. The thermal curing step was achieved in a heated press (Yasuda Seiki Seisakusho Ltd, Model 219) at 200 oC under 700 MPa pressure for 10 min, by placing one layer of dip-coated UV-cured cord fabric between two SBR layers. 2.4. Characterization The FTIR spectra was recorded in mid-infrared range (600-4000 cm-1), in both warp and weft yarn directions with a Perkin Elmer Spectrum-100 ATR-FTIR spectrophotometer, using a ZnSe ATR-crystal with a variable angle accessory. Omnic software was used to record the spectral data at a resolution of 8 cm-1 and 64 co-added scans. 1H NMR characterization was carried out on an Agilent VNMRS 500 MHz NMR instrument, with 3 s acquisition time, 20 ppm spectral width, 125 transients of 65k data points obtained over a 15 min data accumulation time, by using deuterated dimethylsulfoxide (DMSO-d6) as solvent and tetramethylsilane (TMS) as internal standard. Thermal properties were investigated by thermogravimetric analysis (TGA) (TA TGA Q50) under a 30 mL/min nitrogen flow rate with a heating rate of 10°C/min and differential scanning calorimetry (DSC) (TA DSC Q10) under 50 mL/min nitrogen flow rate with a heating rate of 10°C/min, over a 0-350°C temperature range. Contact angle measurements were conducted with a Gardco PGX+ goniometer on the
6
dip-coated and UV-cured fabric surfaces, by applying 3 µL water droplets. Each measurement was repeated three times from different fabric areas for reliability of the test. The adhesion strength between the cord fabrics and rubber surface was recorded on an Instron 4411 tensile testing instrument according to the T-peel test ASTM standard [15]. Optical microscopy images from the fabric surface were taken by an Olympus CH-2 microscope with Digital Microscopy software to see the rubber residue on fiber surface after peel testing. The optical microscopy images were processed in Matlab software in order to see the rubber residue clearly and quantitatively.
Figure 1. Synthesis of urethane acrylate oligomer.
3. Results and discussion 3.1. FTIR analysis Figures 2 and 3 show the FTIR spectra of untreated and adhesive formulation treated UVcured polyamide (weft) and polyester (warp) fibers. Accordingly, in Figure 2, characteristic 7
polyamide peaks of N-H bending at 686 and 1560 cm-1, C=O vibration at 1666 cm-1, -CHstretching vibration at 2850 cm-1, -CH2- stretching vibration at 2913 cm-1, N-H stretching vibration at 3090 and 3298 cm-1 can be seen in all samples. The carbonyl group peak of the carbamate unit in the coating formulation is overlapping with the PA characteristic peaks of N-H and C=O within the 1560-1666 cm-1 region, therefore there is no shift or displacement in primary amide peaks, proving that the interaction between the fiber and coating layer didn’t occur via primary amide functional groups [17, 18]. After the coating process, the existence of the urethane acrylate coating layer on the fiber surface can be observed from several peaks at 812 cm-1 (C=CH2) belonging to the methacrylate group of HEMA, at 1132 cm-1 (C-O-C) referring to butyral ring of PVB and at 1732 cm-1 (C=O) showing the ester peak, respectively [19-22].
Figure 2. FTIR spectra of untreated and adhesive formulation coated UV-cured PA fibers.
8
Figure 3 shows the FTIR spectra of dip-coated UV-cured PES fibers. Characteristic PES peaks of C=O stretching at 1732 cm-1, C-C-O asymmetric stretching at 1267 cm-1 involving the carbon in aromatic ring, O-C-C asymmetric stretching at 1099 cm-1 due to the ethylene glycol unit and C-H bending at 722 cm-1 can be observed in all samples [23-26]. New peaks emerged after the application of coating formulations onto the fiber surface such as the peak of C=CH2 at 812 cm-1 which belongs to the methacrylate group of HEMA. N-H bending and stretching peaks at 1560 and 3298 cm-1 demonstrates the existence of the carbamate ester in the oligomer structure. The other newly formed peaks are the stretching vibration peaks of CH2- and -CH- at 2913 and 2850 cm-1 respectively, proving the PVB structure, and C=O at 1666 cm-1 supporting the carbamate group in the oligomer, respectively [19-22].
Figure 3. FTIR spectra of untreated and adhesive formulation coated UV-cured PES fibers.
3.2. 1H NMR spectroscopy 9
Figure 4 shows the 1H NMR spectra of the urethane acrylate oligomer before and after modification with PVB. The characteristic peaks due to the methacrylate group of HEMA can be observed at 1.79-1.83 ppm (-O-CH2-CH2-O), 5.61-5.72 ppm (C(CH3)=CH2 trans) and 6.10-6.18 ppm (C(CH3)=CH2 cis). The methyl and phenyl ring protons of TDI can be seen at 2.14-2.17 ppm and 7.10-7.92 ppm, respectively. The carbamate group (-NH-COO) at 4.83 ppm supports the reaction between the isocyanate and hydroxyl groups. The intensity of the carbamate group increased after modification with PVB. Furthermore, the existence of PVB can be proven by the appearance of the peaks at 0.87 ppm (methyl group), 1.25-1.75 and 2.21 ppm (methylene group), 3.32-3.71 ppm (-O-CH-CH2), and 4.4 ppm (O-CH-O, dioxymethine proton) [12, 27-29]. The peaks at 6.51 and 6.78 ppm are due to possible side reactions of the urethane linkage with secondary amine functional groups, resulting in the formation of arylalkylalkyl urea [30].
10
Figure 4. 1H NMR spectra of urethane acrylate oligomer with/without PVB addition.
3.3. Thermal properties The TGA thermograms and the summary of TGA data of UV-cured free films are given in Figure 5 and Table 2. There was no significant decomposition till 200 oC for all samples. The urethane group starts degrading into an isocyanate group and an alcohol around 200 oC [31]. After that, in the 350-450 oC region, the aromatic group of the TDI-HEMA adduct starts to
11
decompose. Thermal degradation is completed at around 500 oC for all samples. In the case of the S3 and S4 samples, it is obvious that the inclusion of reactive diluent to the structure increases the thermal stability due to the increase in crosslinking density. After the addition of reactive diluents into the formulation, the reactive diluents act as crosslinking agents and more than half of the material remains stable up to 400 oC. Thermal stability increment is much higher in the S4 sample due to the bulky and cyclic structure of TCDDA that inhibits the movement of polymer chains [32]. According to Table 2, a char yield is formed with pyrolysis as a result of carbonization under the nitrogen atmosphere.
Figure 5. TGA curves of free films of coating formulations. Table 2. Thermogravimetric data of free films of coating formulations. Sample Codes
Temperature at 5% Weight Loss
Temperature at 50% Weight Loss
Residue at 400 oC (%)
Char Yield (%)
S2
157
339
40
9.7
S3
182
423
59
10.4
S4
191
445
64
13.5
12
Figure 6 shows the DSC thermograms of the UV-cured free films. The urethane acrylate oligomer shows a glass transition temperature (Tg) at 65 oC. After inclusion of reactive diluents to the formulation, crosslinking density increased so glass transition temperatures increased to 82 oC for S3 and 175 oC for S4, respectively. The glass transition temperature of TCDDA (around 186 oC) is much higher than TMPTMA (around 80 oC) as the bulky and tricyclic nature causes a higher crosslinking density followed by a higher Tg value. This structure, which is associated with a small free volume in the polymer matrix, slows down oxygen diffusion thus increasing the thermal stability and glass transition temperature in the S4 sample [33].
Figure 6. DSC curves of free films of coating formulations.
3.4. Wettability of samples
13
The images of water droplets on dip-coated UV-cured fabric surfaces with contact angle values can be seen in Figure 7. Functional groups of an oligomer cause a higher crosslinking density so the surface energy increases. In other words, when the functionality of the adhesive formulation increases, contact angle value of the surface decreases [34]. There was no contact angle value recorded in the S1 sample because the water droplet was immediately absorbed by the untreated fabric surface. The lowest contact angle value of 70.2o was obtained in the sample of S4. This result can be explained with the higher crosslinking density of TCDDA, which gives a more hydrophilic character to the surface due to the tricyclodecane units in the structure.
Figure 7. Images of water droplets on dip-coated UV-cured fabric surfaces with contact angle values. 3.5. Peel testing Adhesion strength values between the cord/rubber surfaces can be varied in the range of 3258 N in the literature depending on the testing procedure (T-peel test, H-pull test etc.), rubber type (SBR, ethylene methyl acrylic rubber, nitrile rubber, ethylene-propylene-diene rubber etc.), raw material of the fabric (aramid, polyester, polyamide etc.), cord fabric properties (thickness, weight, warp/weft densities), additives in the adhesive formulations and 14
so on. All these variations should be considered for an exact comparison [35-37]. The T-peel test results of cord/rubber composites can be seen in Table 3. When the untreated cord fabric was adhered onto the rubber surface by means of heat and pressure the peel strength value was recorded as 24.1 N/cm. The peel strength value for S2, which was treated with just the urethane acrylate oligomer, was approximately twice that of the untreated system (47.3 N/cm). The acrylate functional groups of the oligomer can be reacted with the double bonds of SBR during the thermal curing process [38]. The NH groups of the carbamate ester units in the oligomer structure make very strong hydrogen bonding with carbonyl groups of the PES cord fabric. Additionally, carbonyl groups of the oligomer may also interact with NH groups of the PA cord fabric via hydrogen bonding. After the addition of reactive diluents to the coating formulation, peel strength values increased. Thus, reactive diluent improves the binding properties of the surfaces and acts as an adhesion promoter. The highest peel strength value of 100.4 N/cm was found in the case of the S4 sample with the addition of TCDDA as reactive diluent. This result stems from the cycloaliphatic, fused-ring nature of TCDDA, which promotes high binding characteristics to the surface [39]. Table 3. Peel strength values between the rubber and PES/PA cord fabrics.
Sample Codes
S1
S2
S3
S4
Peel Strength (N/cm) 24.1 47.3 58.9 100.4
The cord fabric surfaces were observed by an optical microscopy after T-peel testing. The obtained images were converted into binary grayscale images by using an image processing technique in order to see the rubber traces on the fiber surface clearly and quantitatively (Figure 8). The black regions are indicative of the rubber material that remained adhered to 15
the fiber surface after the peel test. It can be clearly observed that the black regions are much greater in the S4 sample compared to the other formulations. In other words, the adhesion of rubber to the cord surface was sufficiently high so as to restrict complete peeling of rubber from the cord surface.
Figure 8. Images of cord fabrics after peel testing.
4. Conclusions The aim of this study was to prepare formaldehyde-free adhesive formulations for cord fabric and rubber composites. For this purpose, thermal and UV-curable urethane acrylate oligomers were synthesized by using PVB and included in adhesive formulations together with acrylate based reactive diluents. The effects of reactive diluent type on adhesion strength between cord 16
fabric and rubber surfaces were investigated. 1H NMR spectroscopy was used to characterize the oligomer structure. The prepared adhesive formulations were applied on cord fabrics by dip coating and then cured by UV-light. The existence of the coating layer on the cord fabric surfaces was proven by FTIR spectroscopy. Thermal analysis was performed on UV-cured free films for each formulation. The highest thermal stability with the highest glass transition temperature was obtained in the S4 sample containing TCDDA as reactive diluent. The bulky and cyclic structure of the tricyclodecane unit in TCDDA gives to the coating formulation high thermal stability and high rigidity. Contact angle measurement showed that the lowest contact angle value of 70.2o was obtained in the sample of S4 due to it having a high functionality and hence a higher crosslinking density. Dip-coated UV-cured cord fabrics were adhered onto rubber surfaces by means of heat and pressure. According to T-peel test results the highest peel strength value of 100.4 N/cm was recorded for the sample S4, when TCDDA was used as a reactive diluent. The adhesion mechanism between the cord/rubber surfaces is due to strong hydrogen bonding, observed between the NH groups of the carbamate ester units in the oligomer with carbonyl groups of the PES cord fabric and carbonyl groups of the oligomer with NH groups of the PA cord fabric. During the thermal curing process, the acrylate functional groups of the oligomer can be reacted with the double bonds of SBR. The hydrophilic behavior of the surface is an important factor affecting the adhesion properties of the surfaces. Considering all results, a consistent behavior was observed between the contact angle and peel test values. As mentioned before, the bulky and cyclic nature of TCDDA, limits the movement of polymer chains, thus promoting a higher crosslinking density resulting in strong binding between the cord/rubber surfaces. Acknowledgement
17
This study has been financially supported by Istanbul Technical University Research Fund. Many thanks to Ms. Zuhal Nart from Izomas Co. for her assistance in cord fabric supplement.
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PII: DOI: Reference:
S0143-7496(17)30106-9 http://dx.doi.org/10.1016/j.ijadhadh.2017.06.004 JAAD2017
To appear in: International Journal of Adhesion and Adhesives Received date: 16 June 2016 Accepted date: 14 April 2017 Cite this article as: Zehra Yildiz and Hacer Aysen Onen, Dual-curable PVB based adhesive formulations for cord/rubber composites: The influence of reactive diluents, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2017.06.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dual-curable PVB based adhesive formulations for cord/rubber composites: The influence of reactive diluents Zehra Yildiz1,2, Hacer Aysen Onen3 1
Istanbul Technical University, Institute of Sciences and Technology, Department of Polymer Science and Technology, TURKEY
2
Marmara University, Faculty of Technology, Department of Textile Engineering, TURKEY 3
Istanbul Technical University, Faculty of Arts and Sciences, Department of Chemistry, TURKEY
[email protected]
Abstract In this study, the effects of reactive diluent type on the adhesion strength of cord/rubber surfaces were investigated. For this purpose, a urethane acrylate oligomer was synthesized by the reaction of 2,4-toluene diisocyanate (TDI), 2-hydroxyethyl methacrylate (HEMA) and polyvinyl butyral (PVB) in the presence of di-n-butyltin dilaurate (T12) as catalyst. The structure of the oligomer was characterized by nuclear magnetic resonance (NMR) spectroscopy. Then the oligomer was included in adhesive formulations together with trimethylolpropane trimethacrylate (TMPTMA) and tricyclodecane dimethanol diacrylate (TCDDA)
as
reactive
diluents
and
thermal
and
photo
initiator
respectively.
Polyester/polyamide cord fabrics were dipped into the adhesive solution and cured by UVlight. Then coated fabrics were characterized by Fourier transform infrared (FTIR) 1
spectroscopy, differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Contact angle measurement was employed to investigate the wettability properties of the coated fabrics. Thermal curing between the coated fabric and rubber was performed under heat and pressure. The adhesion strength between the cord/rubber surfaces was determined by T-peel test. The highest adhesion strength of 100.4 N/cm with the lowest contact angle value of 70.2o were obtained in the sample containing TCDDA as reactive diluent, due to a higher functionality resulting in a greater crosslinking density. Key Words: UV-Curing; Adhesion; Polyvinyl butyral; Reactive Diluent; Cord Fabric 1. Introduction Cord fabric reinforced rubber composites have been widely used for manufacturing of conveyor belts, hoses, tires, membranes etc. Considering the industrial applications of cord/rubber composites, adhesion between the two components is the most important issue, determining the product life and quality of the material. Cord fabrics have been treated by some chemical formulations such as resorcinol-formaldehyde-latex [1, 2], cobalt boron acrylate [3], hydrated silica-resorcinol-hexamethoxymethylmelamine [4], prior to the adherence onto rubber surfaces. All these processes require the use of formaldehyde that can cause health and environmental problems because of its toxicity [5-7]. UV-curing technology is favored due to it being a solvent-free process with low energy consumption at low temperatures resulting in high productivity levels. It supplies to the surface a high scratch resistance property with high optical clarity [8]. Typically, UV-curable urethane acrylate oligomers can be synthesized by using an isocyanate such as TDI [9], hexamethylene diisocyanate (HDI) [10], isophorone diisocyanaate (IPDI) [11] and an acrylate functional monomer such as 2-hydroxy ethylacrylate (HEA) and HEMA. Acrylate 2
functionality in the structure, gives the ability to be cured by UV-light. During the reaction some of the isocyanate groups are intentionally left unreacted thus allowing reaction with a polyol to form a UV-curable urethane acrylate oligomer. Instead of a polyol, the isocyanate groups can be also reacted with PVB which gives a strong binding and adhesion property on many surfaces. In the literature, UV-curable urethane acrylate oligomers have been synthesized by using an isocyanate, acrylate functional monomer and a polyol but there has been no research considering the usage of PVB instead of a polyol as a co-reactant for isocyanate containing compounds. Reactive diluents are one of the most important components of a UV-curable formulation which are used to reduce viscosity, control crosslinking density, improve mechanical properties and chemical resistance etc. Acrylic based reactive diluents are commercially favored because of their high clarity and optical transparency with non-yellowing properties. In the literature a number of studies have been made considering the effects of the type and percentage of the reactive diluent in the formulations, on the properties of the end product. For instance, urethane acrylate oligomer has been synthesized and then mixed with 1,6hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA). The effects of reactive diluent functionality on coating properties has been considered. Results have shown that the higher crosslinking density of formulations with TMPTA, has increased the glass transition temperature and thermal stability of the coating material [12]. In further research, the effect of reactive diluent concentration on the thermal degradation properties of N-(4hydroxyl phenyl) maleimide derivatives has been studied. N-vinylpyrrolidone (NVP) has been chosen as a reactive diluent. The highest thermal stability with a 30 % char yield has been obtained in a 80 % maleimide derivative-20 % NVP composition [13]. The effect of a reactive diluent (dipropylene glycol diglycidyl ether) on the mechanical properties of MWCNT 3
composites has also been studied [14]. The effect of reactive diluent type in adhesive formulations for cord/rubber composites has never been investigated before. Hence, this work aimed to study the effect of reactive diluent types on adhesion strength between cord/rubber surfaces. A urethane acrylate oligomer was synthesized based on the reaction of TDI-HEMAPVB. Then formaldehyde-free dual-curable adhesive formulations were prepared by using the oligomer, reactive diluent, thermal and photo-initiators. TMPTMA and TCDDA with different functionalities were used as reactive diluents. Adhesive formulations were applied on cord fabrics and then cured by UV-light. After thermal curing of the rubber, the adhesion strength between the cord/rubber surfaces was investigated. The influence of reactive diluent functionality on adhesion property of cord/rubber layers are discussed. 2. Experimental 2.1. Materials 2-hydroxyethyl methacrylate (HEMA), 2,4-toluene diisocyanate (TDI), trimethylolpropane trimethacrylate (TMPTMA), tetrahydrofuran (anhydrous), 1-hydroxycyclohexyl phenyl ketone (Irgacure 184), azobisisobutyronitrile (AIBN), di-n-butyltin dilaurate (T12) were all purchased from Sigma-Aldrich (Buchs, Switzerland). Polyvinyl butyral (PVB, Mowital B 60 H, hydroxyl content 18-21 %) and TCDDA (Genomer™ 1231) was supplied from Kuraray Europe GmbH (Frankfurt, Germany) and Rahn USA Corp. (Illinois, USA), respectively. Polyester/polyamide (PES/PA) (warp/weft) based cord fabrics (520 g/m2, 8 warp/cm, 4 weft/cm, 540 fibers/warp, 400 fibers/weft) and styrene butadiene rubber (SBR) sheets (60 shore A) were obtained from Izomas Co. (Izmir, Turkey) and Grainger Co. (Illinois, USA), respectively. 2.2. Oligomer synthesis 4
Preparation of the oligomer was carried out in a round-bottom flask equipped with a magnetic stirrer, a nitrogen gas inlet and a condenser. The TDI:HEMA molar ratio was set at 1:1. The reaction process can be seen in Figure 1. First, TDI was placed into the flask containing THF (60 wt.%), reactive diluent (TMPTMA or TCDDA) (50 wt.%) and T12 catalyst (0.03 wt.%). Then HEMA was added dropwise to the main flask over 0.5h in an ice bath and further reaction was carried out for 0.5h at 30 oC and another 1h at 70 oC. 1 wt.% PVB out of the total TDI mass was dissolved in THF in an ultrasonic bath and added dropwise to the main flask. After PVB addition, the reaction was proceeded for another 1h at 90 oC allowing the NCO groups in the 2 position to react with the hydroxyl groups of the PVB and to eliminate the HEMA residue. Finally, the PVB modified TDI-HEMA adduct was obtained. The sample codes with their compositions can be seen in Table 1. The sample designated S1 is untreated PES/PA cord fabric. Table 1. Sample codes of the coating formulations. Oligomer --
PVB Modified TDI-HEMA Adduct
Reactive Diluent Sample Codes --
S1
--
S2
TMPTMA, 50%
S3
TCDDA, 50%
S4
2.3. Preparation and application of coating formulations Coating formulations were prepared using a PVB modified TDI-HEMA adduct (30 wt.% out of the cord fabric mass), photoinitiator (3 wt.%), thermal initiator (3 wt.%) and THF. THF was used in order to obtain a better wettability of fabrics by lowering the viscosity of the 5
oligomer. For the dip-coating process, cord fabrics were cut into 2.5 cm x 7.5 cm dimensions that is required to make the T-peel test according to the appropriate ASTM standard [15]. Then, fabrics were dipped into the coating solution for 3 minutes followed by the removal of excess solution by a squeezing roller. THF was evaporated from the fabric in an oven at 70 oC for 10 min. Since oxygen of air acts as a radical scavenger during the UV-curing process, fabrics were placed into a zip-lock transparent polyethylene bag prior to the UV-curing process [16]. UV-light was applied on fabrics for 2 min from both sides of the fabric by using a OSRAM Ultra-Vitalux 300 W UV lamp. The thermal curing step was achieved in a heated press (Yasuda Seiki Seisakusho Ltd, Model 219) at 200 oC under 700 MPa pressure for 10 min, by placing one layer of dip-coated UV-cured cord fabric between two SBR layers. 2.4. Characterization The FTIR spectra was recorded in mid-infrared range (600-4000 cm-1), in both warp and weft yarn directions with a Perkin Elmer Spectrum-100 ATR-FTIR spectrophotometer, using a ZnSe ATR-crystal with a variable angle accessory. Omnic software was used to record the spectral data at a resolution of 8 cm-1 and 64 co-added scans. 1H NMR characterization was carried out on an Agilent VNMRS 500 MHz NMR instrument, with 3 s acquisition time, 20 ppm spectral width, 125 transients of 65k data points obtained over a 15 min data accumulation time, by using deuterated dimethylsulfoxide (DMSO-d6) as solvent and tetramethylsilane (TMS) as internal standard. Thermal properties were investigated by thermogravimetric analysis (TGA) (TA TGA Q50) under a 30 mL/min nitrogen flow rate with a heating rate of 10°C/min and differential scanning calorimetry (DSC) (TA DSC Q10) under 50 mL/min nitrogen flow rate with a heating rate of 10°C/min, over a 0-350°C temperature range. Contact angle measurements were conducted with a Gardco PGX+ goniometer on the
6
dip-coated and UV-cured fabric surfaces, by applying 3 µL water droplets. Each measurement was repeated three times from different fabric areas for reliability of the test. The adhesion strength between the cord fabrics and rubber surface was recorded on an Instron 4411 tensile testing instrument according to the T-peel test ASTM standard [15]. Optical microscopy images from the fabric surface were taken by an Olympus CH-2 microscope with Digital Microscopy software to see the rubber residue on fiber surface after peel testing. The optical microscopy images were processed in Matlab software in order to see the rubber residue clearly and quantitatively.
Figure 1. Synthesis of urethane acrylate oligomer.
3. Results and discussion 3.1. FTIR analysis Figures 2 and 3 show the FTIR spectra of untreated and adhesive formulation treated UVcured polyamide (weft) and polyester (warp) fibers. Accordingly, in Figure 2, characteristic 7
polyamide peaks of N-H bending at 686 and 1560 cm-1, C=O vibration at 1666 cm-1, -CHstretching vibration at 2850 cm-1, -CH2- stretching vibration at 2913 cm-1, N-H stretching vibration at 3090 and 3298 cm-1 can be seen in all samples. The carbonyl group peak of the carbamate unit in the coating formulation is overlapping with the PA characteristic peaks of N-H and C=O within the 1560-1666 cm-1 region, therefore there is no shift or displacement in primary amide peaks, proving that the interaction between the fiber and coating layer didn’t occur via primary amide functional groups [17, 18]. After the coating process, the existence of the urethane acrylate coating layer on the fiber surface can be observed from several peaks at 812 cm-1 (C=CH2) belonging to the methacrylate group of HEMA, at 1132 cm-1 (C-O-C) referring to butyral ring of PVB and at 1732 cm-1 (C=O) showing the ester peak, respectively [19-22].
Figure 2. FTIR spectra of untreated and adhesive formulation coated UV-cured PA fibers.
8
Figure 3 shows the FTIR spectra of dip-coated UV-cured PES fibers. Characteristic PES peaks of C=O stretching at 1732 cm-1, C-C-O asymmetric stretching at 1267 cm-1 involving the carbon in aromatic ring, O-C-C asymmetric stretching at 1099 cm-1 due to the ethylene glycol unit and C-H bending at 722 cm-1 can be observed in all samples [23-26]. New peaks emerged after the application of coating formulations onto the fiber surface such as the peak of C=CH2 at 812 cm-1 which belongs to the methacrylate group of HEMA. N-H bending and stretching peaks at 1560 and 3298 cm-1 demonstrates the existence of the carbamate ester in the oligomer structure. The other newly formed peaks are the stretching vibration peaks of CH2- and -CH- at 2913 and 2850 cm-1 respectively, proving the PVB structure, and C=O at 1666 cm-1 supporting the carbamate group in the oligomer, respectively [19-22].
Figure 3. FTIR spectra of untreated and adhesive formulation coated UV-cured PES fibers.
3.2. 1H NMR spectroscopy 9
Figure 4 shows the 1H NMR spectra of the urethane acrylate oligomer before and after modification with PVB. The characteristic peaks due to the methacrylate group of HEMA can be observed at 1.79-1.83 ppm (-O-CH2-CH2-O), 5.61-5.72 ppm (C(CH3)=CH2 trans) and 6.10-6.18 ppm (C(CH3)=CH2 cis). The methyl and phenyl ring protons of TDI can be seen at 2.14-2.17 ppm and 7.10-7.92 ppm, respectively. The carbamate group (-NH-COO) at 4.83 ppm supports the reaction between the isocyanate and hydroxyl groups. The intensity of the carbamate group increased after modification with PVB. Furthermore, the existence of PVB can be proven by the appearance of the peaks at 0.87 ppm (methyl group), 1.25-1.75 and 2.21 ppm (methylene group), 3.32-3.71 ppm (-O-CH-CH2), and 4.4 ppm (O-CH-O, dioxymethine proton) [12, 27-29]. The peaks at 6.51 and 6.78 ppm are due to possible side reactions of the urethane linkage with secondary amine functional groups, resulting in the formation of arylalkylalkyl urea [30].
10
Figure 4. 1H NMR spectra of urethane acrylate oligomer with/without PVB addition.
3.3. Thermal properties The TGA thermograms and the summary of TGA data of UV-cured free films are given in Figure 5 and Table 2. There was no significant decomposition till 200 oC for all samples. The urethane group starts degrading into an isocyanate group and an alcohol around 200 oC [31]. After that, in the 350-450 oC region, the aromatic group of the TDI-HEMA adduct starts to
11
decompose. Thermal degradation is completed at around 500 oC for all samples. In the case of the S3 and S4 samples, it is obvious that the inclusion of reactive diluent to the structure increases the thermal stability due to the increase in crosslinking density. After the addition of reactive diluents into the formulation, the reactive diluents act as crosslinking agents and more than half of the material remains stable up to 400 oC. Thermal stability increment is much higher in the S4 sample due to the bulky and cyclic structure of TCDDA that inhibits the movement of polymer chains [32]. According to Table 2, a char yield is formed with pyrolysis as a result of carbonization under the nitrogen atmosphere.
Figure 5. TGA curves of free films of coating formulations. Table 2. Thermogravimetric data of free films of coating formulations. Sample Codes
Temperature at 5% Weight Loss
Temperature at 50% Weight Loss
Residue at 400 oC (%)
Char Yield (%)
S2
157
339
40
9.7
S3
182
423
59
10.4
S4
191
445
64
13.5
12
Figure 6 shows the DSC thermograms of the UV-cured free films. The urethane acrylate oligomer shows a glass transition temperature (Tg) at 65 oC. After inclusion of reactive diluents to the formulation, crosslinking density increased so glass transition temperatures increased to 82 oC for S3 and 175 oC for S4, respectively. The glass transition temperature of TCDDA (around 186 oC) is much higher than TMPTMA (around 80 oC) as the bulky and tricyclic nature causes a higher crosslinking density followed by a higher Tg value. This structure, which is associated with a small free volume in the polymer matrix, slows down oxygen diffusion thus increasing the thermal stability and glass transition temperature in the S4 sample [33].
Figure 6. DSC curves of free films of coating formulations.
3.4. Wettability of samples
13
The images of water droplets on dip-coated UV-cured fabric surfaces with contact angle values can be seen in Figure 7. Functional groups of an oligomer cause a higher crosslinking density so the surface energy increases. In other words, when the functionality of the adhesive formulation increases, contact angle value of the surface decreases [34]. There was no contact angle value recorded in the S1 sample because the water droplet was immediately absorbed by the untreated fabric surface. The lowest contact angle value of 70.2o was obtained in the sample of S4. This result can be explained with the higher crosslinking density of TCDDA, which gives a more hydrophilic character to the surface due to the tricyclodecane units in the structure.
Figure 7. Images of water droplets on dip-coated UV-cured fabric surfaces with contact angle values. 3.5. Peel testing Adhesion strength values between the cord/rubber surfaces can be varied in the range of 3258 N in the literature depending on the testing procedure (T-peel test, H-pull test etc.), rubber type (SBR, ethylene methyl acrylic rubber, nitrile rubber, ethylene-propylene-diene rubber etc.), raw material of the fabric (aramid, polyester, polyamide etc.), cord fabric properties (thickness, weight, warp/weft densities), additives in the adhesive formulations and 14
so on. All these variations should be considered for an exact comparison [35-37]. The T-peel test results of cord/rubber composites can be seen in Table 3. When the untreated cord fabric was adhered onto the rubber surface by means of heat and pressure the peel strength value was recorded as 24.1 N/cm. The peel strength value for S2, which was treated with just the urethane acrylate oligomer, was approximately twice that of the untreated system (47.3 N/cm). The acrylate functional groups of the oligomer can be reacted with the double bonds of SBR during the thermal curing process [38]. The NH groups of the carbamate ester units in the oligomer structure make very strong hydrogen bonding with carbonyl groups of the PES cord fabric. Additionally, carbonyl groups of the oligomer may also interact with NH groups of the PA cord fabric via hydrogen bonding. After the addition of reactive diluents to the coating formulation, peel strength values increased. Thus, reactive diluent improves the binding properties of the surfaces and acts as an adhesion promoter. The highest peel strength value of 100.4 N/cm was found in the case of the S4 sample with the addition of TCDDA as reactive diluent. This result stems from the cycloaliphatic, fused-ring nature of TCDDA, which promotes high binding characteristics to the surface [39]. Table 3. Peel strength values between the rubber and PES/PA cord fabrics.
Sample Codes
S1
S2
S3
S4
Peel Strength (N/cm) 24.1 47.3 58.9 100.4
The cord fabric surfaces were observed by an optical microscopy after T-peel testing. The obtained images were converted into binary grayscale images by using an image processing technique in order to see the rubber traces on the fiber surface clearly and quantitatively (Figure 8). The black regions are indicative of the rubber material that remained adhered to 15
the fiber surface after the peel test. It can be clearly observed that the black regions are much greater in the S4 sample compared to the other formulations. In other words, the adhesion of rubber to the cord surface was sufficiently high so as to restrict complete peeling of rubber from the cord surface.
Figure 8. Images of cord fabrics after peel testing.
4. Conclusions The aim of this study was to prepare formaldehyde-free adhesive formulations for cord fabric and rubber composites. For this purpose, thermal and UV-curable urethane acrylate oligomers were synthesized by using PVB and included in adhesive formulations together with acrylate based reactive diluents. The effects of reactive diluent type on adhesion strength between cord 16
fabric and rubber surfaces were investigated. 1H NMR spectroscopy was used to characterize the oligomer structure. The prepared adhesive formulations were applied on cord fabrics by dip coating and then cured by UV-light. The existence of the coating layer on the cord fabric surfaces was proven by FTIR spectroscopy. Thermal analysis was performed on UV-cured free films for each formulation. The highest thermal stability with the highest glass transition temperature was obtained in the S4 sample containing TCDDA as reactive diluent. The bulky and cyclic structure of the tricyclodecane unit in TCDDA gives to the coating formulation high thermal stability and high rigidity. Contact angle measurement showed that the lowest contact angle value of 70.2o was obtained in the sample of S4 due to it having a high functionality and hence a higher crosslinking density. Dip-coated UV-cured cord fabrics were adhered onto rubber surfaces by means of heat and pressure. According to T-peel test results the highest peel strength value of 100.4 N/cm was recorded for the sample S4, when TCDDA was used as a reactive diluent. The adhesion mechanism between the cord/rubber surfaces is due to strong hydrogen bonding, observed between the NH groups of the carbamate ester units in the oligomer with carbonyl groups of the PES cord fabric and carbonyl groups of the oligomer with NH groups of the PA cord fabric. During the thermal curing process, the acrylate functional groups of the oligomer can be reacted with the double bonds of SBR. The hydrophilic behavior of the surface is an important factor affecting the adhesion properties of the surfaces. Considering all results, a consistent behavior was observed between the contact angle and peel test values. As mentioned before, the bulky and cyclic nature of TCDDA, limits the movement of polymer chains, thus promoting a higher crosslinking density resulting in strong binding between the cord/rubber surfaces. Acknowledgement
17
This study has been financially supported by Istanbul Technical University Research Fund. Many thanks to Ms. Zuhal Nart from Izomas Co. for her assistance in cord fabric supplement.
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