2.3.2. Hydroxyl value
The hydroxyl content of synthesized bio-polyol was measured according to ASTM D1957-86. Prepared bio-polyol first was added to 5 mL pyridine/acetic anhydride mixture. The mixture was refluxed for an hour, then ten mL water was added to system. After reaction completed mixture was cooled to room temperature and 1 mL of 1% phenolphthalein was added. Residual acetic anhydride was titrated with 0.5 N potassium hydroxide to a faint end point. Obtained hydroxyl value of obtained bio-polyol was 17.365 mg KOH/g sample.
2.3.3. Modification of synthesized bio-polyol molecular structure by silane
The reaction was carried out in the 500 ml three-neck flask. The flask was fitted with a stirrer and equipped with a condenser and thermometer. 100 g of synthesized bio-polyol and 1 g tetraethyl orthosilicate were added into the flask and after charging the reaction was mixed continuously at 80 °C for 10 h (see scheme 2).
2.3.4. Synthesis of ZnO NPs
ZnO NPs were provided by a precipitation method (Seyed Dorraji et al., 2009).
3. Results and discussion
3.1. Characterization of ZnO NPs
The crystalline structure of synthesized ZnO NPs were specified using X-ray diffraction. In
Fig. 1a, the diffraction peaks at 2? of 31.8o, 34.6o, 47.7o, 56.7o, 63.4o, 66.2o, 68o and 68.9o are attributed to hexagonal crystal system of ZnO NPs (JCPDF 36-1451) 31.
The cellular morphology of prepared ZnO NPs was studied by SEM (See Fig. 1b). Fig. 1b shows ZnO NPs have spherical morphology with an average size of 50 nm.
3.2. Characteristics of prepared bio-polyol
The achievement of ring opening reaction associated with the formation of hydroxyl group on the synthesized bio-polyol. FTIR spectra of prepared bio-polyol and ESBO are compared with one another in Fig. 2 to confirm the presence of hydroxyl group on obtained bio-polyol.
As Fig. 2 shows at vicinity of 821 cm-1 polyol curve is wavy but shows lower absorbance than ESBO curve. The decrement of the band at 821 cm-1 in bio-polyol spectrum indicates that epoxy group have been consumed and the appearance of broad peak at 3469 cm-1 which isn’t seen in the spectrum of ESBO is assigned to –OH group absorption.
Strong absorption at 1742 cm-1 region corresponds to carbonyl group of bio-polyol. The peak at 1375 is related to C-H bending of the end methyl group and aliphatic C-H gives an absorption at 2800-3000 cm-1.
Fig. 3 indicates 1H NMR spectra of ESBO and synthesized bio-polyol respectively.
The resonances associated with CH-O of the ester groups and the protons of epoxy groups were appeared at 5.27–5.45 ppm and within 2.86–3.13 ppm respectively in 1H-NMR spectrum of ESBO.
In 1H-NMR spectrum of bio-polyol the peaks of CH-O related to ester groups still appear but the peaks within 2.86–3.13 ppm which are assigned to epoxy group protons don’t exist. Also, the peaks in the range of 3.4-4.0 ppm are the characteristic of methine -CH- and hydroxyl groups (HC-OH) that are absent in the 1H-NMR spectrum of the ESBO, this proves that the epoxy groups on carbon atom in the ESBO were changed to hydroxyl groups.
The bands at 4.1–4.34 ppm correspond to the protons of CH2O of ester groups appeared as same as those of ESBO.
3.3. Characteristics of modified bio-polyol
Fig. 4 illustrates 1H NMR spectra of tetra ethyl orto silicate and modified bio-polyol respectively. Due to the reaction of synthesized bio-polyol and tetra ethyl orto silicate (see scheme 2) the ethoxy group of silane removes from the reaction environment as alcohol. As shown in the Fig. 4 the peak of -CH2 within 3.5-4 ppm decreased but the band of -CH3 overlapped with another peaks. Therefore, results show that modification of bio-polyol was performed by tetra ethyl orto silicate successfully.
3.4. Lap shear strength
In this study assessment of influential factors were considered for factors of ZnO NPs, TEG and DBTDL catalyst.
Lap shear strength was determined and experiments were considered in Table 2 testing were conducted using L9 array and after 27 sample preparing with three times repetition samples.
3.4.1. Effect of operational factors on the lap shear strength
In order to specify the effects of each factor on lap shear strength in Taguchi method, ANOVA was used.
Table 3 indicate ANOVA, it shows the effects of factors on the response and so the last column helps to know the most effective factors among others.
ZnO NPs had the most influence assigned 62.83% of the total contribution of all the factors. Furthermore, TEG and DBTDL concentration were placed in next places and the contribution of them were 27.203 and 2.088 respectively. Also, the effect of error was 7.879.
To preparation the bio-adhesive, the amount of TEG and ZnO NPs between factors, have a major role to control the response, which it was obviously observed in the ANOVA table.
3.4.2 Effect of factors levels on lap shear strength
Taguchi method selects the optimum conditions in such a way that the uncontrollable factors (noise) will have minimum effect on response. In order to find if factors have impact on variation, the signal to noise ratios (S/N) were applied at Taguchi method.
The effect of each level of ZnO NPs was illustrated in Fig. 5 (a).
As diagram shows, level 2 has the most amount of S/N ratio. Hence it has the biggest effect on lap shear strength than other levels.
It is possible that when ZnO NPs are loaded between PU chains gap, an interactive force may be created against PU chains. So, when an external force is imposed on the adhesive it goes to rupture easily through the strong interaction force generated between PU chains and ZnO NPs. Furthermore, the aggregation of ZnO NPs can be happened in the dry film and leading to the decline of mechanical property.
Fig. 5 (b) illustrates the effect of the second factor levels on the lap shear strength. The level 2 of the second factor has the greatest effect on the lap shear strength. It is observed that the amount of %1 for this factor is the best amount.
It can be said that when TEG as chain extender is added to polyol because of crosslinking in adhesive matrix the lap shear strength is improved but when its amount increases because of increasing in crosslinking, plasticizing effect can happen so lap shear strength can decrease.
The effect of third factor levels is illustrated in Fig. 5 (c). As it shows all levels of the third factor have constant effect on lap shear strength.
The reason can be that, due to DBTDL can cause crosslinking via OH group and improve lap shear strength but because the amount of NCO group was more than OH group so it couldn’t made crosslinked structure in adhesive matrix and it just can decrease the curing time.
3.4.3. Confirmation of experiment design
For the any experiment design it would be necessary to confirm it, which is obtained by comparing the experimental and estimated results.
A comparison of the estimated results with the experimental values is illustrated in Fig. 6.
in Fig. 6. It can be observed that the experiment design was able to predict the lap shear strength of adhesive to a reasonable degree of accuracy.
3.5. Optimization of formulation
The main aim of this study was the determination of optimized bio-adhesive to achieve maximum lap shear strength.
After studying the different levels of each factor by the Taguchi method (Table 1 and ANOVA table), below levels were chosen as optimum level of each one:
? The level 2 of the ZnO NPs (wt=0.1%)
? The level 2 of the TEG (wt=1%)
? The level 1 of the DBTDL (wt=0%)
3.5.1 FTIR spectra of optimum adhesive
Fig.7 shows FTIR spectra of optimum adhesive. The peak at 1740 cm-1 corresponds to carbonyl stretching that confirms the presence of a urethane linkage. The characteristic of –NH stretching and bending vibrations gives two absorptions at 3362 cm-1 and at the vicinity of 1530 cm-1, respectively. The peak observed at 2275 cm-1 indicates the presence of unreacted NCO due to the higher NCO/OH ratio.
The C-H stretching of methylene and methyl groups in the soft segment give absorption peak at 2924 and 2854 cm-1 respectively. On the other hand, the bands between 1000 cm-1 and 1200 cm-1 in the finger print region were related to the single bond CO-O-C stretching vibration. The peaks centered at around 1537 cm-1 and 1597 cm-1 are due to conjugated double bonds in the aromatic ring of the hard segment which Indicates the presence of hard segment and reaction with polyol. The formation of PU
adhesive is clearly confirmed by disappearing the –OH absorption band and the presence of a –NH band.
3.5.2 SEM images of optimum adhesive
The surface morphology of optimized bio-adhesive and distribution of ZnO NPs in adhesive matrix was investigated by (SEM Fig. 8). The SEM image shows a good dispersion of NPs.
3.5.3. modulus and Lap shear strength of optimum adhesive
The lap shear strength of optimum adhesive was measured. The amount of lap shear strength was almost the same amount as the result that Taguchi method had predicted for optimum condition.
The Taguchi predicted amount was 7.173 MPa and the obtained lap shear amount for prepared adhesive in optimum condition was 6.478 MPa.
Obtained amount is approximately in accordance with predicted amount and indicates the experimental design in this work was correct and applicable.
Modulus of optimal adhesive and blank adhesive were evaluated according to the stress–strain curves (Fig. 9) as Fig. 9 shows the modulus amount of optimum adhesive and blank adhesive are 0.368 GPa and 0.266 GPa respectively. So, this result can be obtained that incorporation of additive improved the lap shear strength.
3.5.4. Thermal properties (TGA/DTG) analysis of optimum adhesive
Thermal behavior and stability of the optimum adhesive was characterized by TGA/DTG analysis.
As shown in Fig. 10 there are three main decomposition stages for optimum bio-adhesive. The initial 20% (by weight) of PU adhesives’ decomposition starts at approximately 220 ?C that can be due to the breaking of urethane bonding. The second stage of adhesives decomposition which occurred from 340 to 430 °C was due to the polyol backbone and the thermally stable isocyanurates’ hard segments decomposition. Finally, the last stage was recorded from 440 to 550 °C and was due to the decomposition of the char that was formed in the previous decomposition steps. This analysis showed that the prepared optimum bio-adhesives was thermally stable up to about 220°C.
3.5.5. contact angle
In order to investigate the effect of additive on hydrophobicity of optimum and blank adhesive, contact angle analysis was used. According to the results shown in Table 4, the contact angle values of water on the surface of the optimum adhesive including additive is lower than the blank sample (Fig. 11).
These results show that additives could enhance the wettability of the adhesive due to their higher polarity compared with blank adhesive without any additives.
To preparation of the PU bio-adhesive with high lap shear strength, ESBO was hydroxylated and modified using citric acid and tetra ethyl ortho silicate, respectively. Hydroxylation and modification of ESBO were confirmed by FTIR and H-NMR spectrum. Also, to get better results, effect of ZnO NPs, TEG and DBTDL as three possible influential factors on lap shear strength were investigated. For this purpose, the Taguchi method was applied for optimization of effectual factors. Under optimized conditions (weight percentage of ZnO NPS = 0.1, TEG=1 and DBTDL=0), the lap shear strength of optimal sample was equal to 6.478 MPa. This strength value was in good agreement with the predicted value (7.173 MPa) by Taguchi method. Among all factors ZnO NPS was determined to be the most effective factor by 62.83% contribution.
It also showed good thermal resistance for synthesized bio-adhesive at optimum conditions.
This work was supported by University of Zanjan. The authors are grateful to University of Zanjan for financial and other supports.