International Journal of Adhesion & Adhesives

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1 International Journal of Adhesion & Adhesives 34 (2012) Contents lists available at SciVerse ScienceDirect International Journal of Adhesion & Adhesives journal homepage: An attempt to quantitatively predict the interfacial adhesion of differently surface treated nanosilicas in a polyurethane coating matrix using tensile strength and DMTA analysis M. Rostami a,b, M. Mohseni a,n, Z. Ranjbar b a Amirkabir University of Technology, Department of Polymer Engineering and Color Technology, 424 Hafez Avenue, P.O. Box , Tehran, Iran b Department of Surface Coatings and Corrosion, Institute for Colorants, Paint and Coatings (ICPC), 5 Lavizan, Hosainabad Sq., P.O. Box , Tehran, Iran article info Article history: Accepted 13 December 2011 Available online 23 December 2011 Keywords: Polyurethane Interfaces, surface treatment Adhesion by chemical bonding abstract Surface treatment of nanosilicas with silane coupling agents is a common method by which the interfacial interaction of these particles can be enhanced. This is because of interactions taking place between the silane and silica, as well as the interactions between the organic part of the silane with the polymeric matrix. Therefore, interfacial interaction of silane grafted silica plays a key role to ensure a better reinforcing effect. The present work is an attempt to quantitatively predict the interfacial bonding strength between differently amino silane treated nanosilicas and a polyurethane coating matrix. This was based on the data deduced from tensile strength and dynamic mechanical thermal (DMTA) experiments of differently loaded untreated and treated nanosilicas loaded films. Using a predefined linear model taking into account the yield stresses of the particle loaded polyurethane and that of the matrix itself, an interaction bonding strength parameter was obtained. It was shown that this parameter was directly proportional to the amino silane content on nanosilica. However, for higher loadings of silicas the model best fit the data deviated from linearity and obeyed a second order equation, in which the second power term attributing the extent of interfacial strength was systematically increased. These results were in good agreement with the storage modulus and glass transition temperature values revealed by DMTA analysis. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Fumed silicas, due to their very fine particle size and high specific surface area, are one of the most important reinforcing fillers for polymers and coatings [1]. Apart from geometry, surface chemistry of silica greatly affects the degree of reinforcement. As silica contains a large number of silanol groups on its surface in the form of vicinal, geminal and isolated, it can be considered as highly polar filler [2] and is less compatible in the non polar media [3]. Moreover, the surface silanol groups have a great affinity to form hydrogen bonding with each other, resulting in a strong filler filler interaction [4]. This can be prevented by the aid of coupling agents, which modify the hydrophilic nature of silica surface. Such a surface modification not only improves the wettability of silica in organic media, but also functionalizes the particle to interact chemically with the coating media. The formation of chemical bonds between the inorganic and organic phases is expected to guarantee a durable interaction between n Corresponding author. Tel.: þ ; fax: þ address: mmohseni@aut.ac.ir (M. Mohseni). the two incompatible phases. For the modification of silica surfaces, organosilanes are most commonly used [5]. Recently, much attention has been given to use organosilanes to couple fillers to an organic matrix [6]. Silane coupling agents are generally selected as the modification agents for nanosilica particles since the hydroxyl groups at the surface of colloidal silica particles can react with siloxane groups of the silane molecule. Generally, silane coupling agents may possess two functional active groups, i.e., an alkoxy group capable of reacting with surface silanol, and an organic functional group, which generally contains amino, epoxy and acrylic functionality in its structure. These may participate in the curing process leading to a possible strong linkage between silica and the polymer [7,8]. Consequently, a silane coupling agent can function as a bridge between silica and polymer, thereby enhancing the interaction between these two. Many types of silane coupling agents, varying in chemistry and reactivity, are available, of which amino silanes are among those usually used for silica modification [7]. The attachment of silane molecules to colloidal silica particles can prevent the aggregation of the particles due to the steric repulsion of grafted organic groups. Moreover, some new functional groups such as alkyl, amino, epoxy and vinyl can be also introduced [9 11] /$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi: /j.ijadhadh

2 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) As nanosilica has been known for many years to mainly impart mechanical integrity to polymeric matrices, there are numerous studies in which the behavior of different types of this filler in the resulting polymeric nanocomposite have been discussed. Most of these studies, as far as coating materials are concerned, have been focused on polyurethane systems. The reason for this is that polyurethane coatings have good weathering stability together with appropriate mechanical properties. As this polymer is flexible in nature, further increasing its mechanical integrity using hard particles like nanosilica with its low refractive index is reasonable. Zhou et al. [12] studied the effect of colloidal silica prepared by an in situ synthesis of different silane precursor on various properties of a polyurethane matrix. In another work Chen et al. [13] investigated the mechanical and optical properties of a polyurethane coating matrix using tensile experiment, dynamic mechanical thermal analysis as well as transmission electron microscopy. Jalili and Moradian [14] separately incorporated a hydrophilic and a hydrophobic silica in a polyurethane coatings and found that the optical clarity of the films containing hydrophilic silica was lower. They attributed this behavior to the interfacial interaction of nanosilica with the matrix. The interaction of different surface treated nanosilicas with a polyurethane coating was also investigated by Ranjbar and Rastegar [15]. They utilized optical studies and conventional mechanical tests to differentiate the behavior of various silicas. Barna et al. [16] used more novel techniques including micro- and nano-scratch experiments to discuss the variation in surface chemistry of amino silane treated nanosilica, nanoalumina and nanotitania. In a recent work reported by Dastmalchian et al. [17], other characteristic performances of hydrophilic and hydrophobic nanosilica were investigated. They reported that not only the rheological, morphological and surface energy of the silica containing films were affected, but the electrochemical behavior of the films were also changed. Turunc et al. [18] studied various amounts of carbonated silica particles directly added into carbonated soybean oil and carbonated polypropylene glycol resin mixture to prepare polyurethane silica nanocomposite coating compositions by nonisocyanate route. Cupping, gloss, impact and taber abrasion tests were performed on aluminum panels coated with those nanocomposite formulations and tensile tests, thermogravimetric were conducted on free films. In all these studies it has been claimed that the interfacial interaction between silica nanoparticle and the polymeric matrix in which it has been dispersed plays a crucial role in the mechanical properties. Hence, an effective way to enhance the mechanical integrity of composites is to improve the interfacial interaction between the matrix and the reinforcing nanoparticle. Quantification of such an interfacial interaction has been a challenge in recent years. Interfacial interactions significantly influence the properties of heterogeneous polymer systems including blends, filled polymers and advanced composites. Their importance is clearly shown by the widespread research activities in this field. A general conclusion based on microparticulate-filled polymers is that the strength of composite materials can be maximized when the interfacial adhesion between the filler surface and the polymer matrix is optimized [10]. However, very few studies have been performed to study the relationship between the mechanical properties of nanocomposites and their interfacial strengths. Unlike fiber-reinforced composites, the interfacial interactions in particulate filled composites are hard to measure directly, for example, by the fiber pull-out approach. However, there are large number of publications, including review papers, on the use of mechanical attributes of particulate polymeric coatings to interpret the interfacial bonding of the filler and the matrix. There is a detailed list of these attempts as reported by Perera [19] in which the relationship between the mechanical attributes deduced from tensile experiments and the extent of interfacial strength has been elegantly discussed. However, these almost deal with the interfacial bonding in a more qualitative manner. Pukanszky [20] proposed that the dimension of interface and strength of the interaction significantly influence the ultimate tensile properties of the composites. Pukanszky and co-workers [21] also developed a model to predict the variation in the composite s yield stress with the particulate filler content. In an attempt to obtain a more rigorous approach to describe quantitatively the compositional dependence of tensile yield stress in filled polymers, they proposed a semi-empirical relationship that successfully fits the experimental data of many different composites. Their approach is particularly interesting due to a dimensionless parameter, B, which is directly related to filler matrix adhesion and can be used to characterize the interfacial adhesion in a more quantitative way. We have recently conducted different surface treatments on nanosilica [22,23] using different types of coupling agents. Results revealed that not only the physical and chemical properties of treated particles varied with surface treatment procedures, their interactions with the polymeric media in which they are introduced also changed noticeably. These included changes in morphological properties and dispersibility of treated nanosilica and their nanocomposite films from which they are prepared. One important question left is that how these interfacial interactions can be quantified. Therefore, the aim of this paper is to make use of tensile experiments and DMTA analysis to give information on the interfacial strength. To this end, different loadings of amino silane treated nanosilicas into a polyurethane clear coat were prepared. Deduced data obtained from tensile strength experiments were used to calculate the interfacial bonding parameter. 2. Experimental 2.1. Materials Functionalized nanosilicas were previously produced by surface treating a hydrophilic silica having a specific surface area of 200 m 2 /g (Aerosil 200 from Degussa Co). The pristine silica was 12 nm in diameter. Using amino propyl triethoxy silane coupling agent surface grafting of nanosilica was performed at different conditions, the procedure of which have been previously reported in Refs. [22,23]. In brief, the coupling agent was hydrolyzed at different ph ranging from very acidic to alkaline. Surface grafted particles with various contents of amino functionality were then obtained. Table 1 lists the characteristics of three grafted particles together with that of the pristine nanosilica (UnT). Using C, N and H analysis of treated particles, the percentage of grafting has been found, the procedure of which has been explained in Ref. [23]. Each mole of silane used for surface treatment contains one mole nitrogen. Therefore, by dividing the N contents obtained in elemental analysis to the atomic number of nitrogen (14) one Table 1 Characteristics of different nanosilica. Sample UnT n 29 nn -Si 45-Si 58-Si ph value Density (g/cm 3 ) C-content N-content Grafting (%) E The explanation for elemental analysis of particles has been reported in Ref. [16]. n UnT¼untreated silica. nn Numbers denote the percentage of silane grafting.

3 26 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) obtains the moles of nitrogen on the treated silica surface, which is in turn equal to the number of moles of silane molecules grafted. The initial mole of silane at the starting bath was for 100 g silica [23]. This is assumed to be the stoichiometric amount (One mole silane to each mole hydroxyl group). By dividing moles of nitrogen to 0.458, the percentage of grafting is obtained. The results revealed that the grafting percent was varied between 29 and 58, meaning that at different conditions used not more than 58% of grafting can be reached. Particle 29-Si in Table 1 has the lowest grafted amino silane at its surface, while particle 58-Si has the greatest. The presence of amino functionality at the surface of these particles results in interaction of isocyanate hardener with the particle and formation of urea linkage [24], the extent of which is proportional to the grafting percent. The purpose of this paper is then to quantify the interfacial interactions of these grafted particles using tensile strength and dynamic mechanical thermal analysis. Accordingly, different silica loaded PU films were prepared. Tacryl 1-392H, an acrylic polyol supplied by Tak Resin Co. (Iran) and Desmodur N75 BA/X, an aliphatic polyisocyanate cross-linker, provided by Bayer Co.(Germany) were used to prepare PU loaded nanosilica films. Specifications of polyol and hardner are given in Table 2. The ratio of polyol to hardener was 1:1 of molar (6.69:1 wt/wt). Treated and un-treated nanosilica particles were sonicated in toluene:mek (50:50) for 10 min and then added to the acrylic polyol. The contents of silica in the final cured film were set at 2, 4 and 6 wt%. Grinding of particles was accomplished by pearl milling using 0.5 mm zirconia beads. The ratio of beads to polyol was 1:1 wt/wt. The duration of milling and the shear rate were 2 h and 1200 rpm, respectively. This milling time was found to be adequate for obtaining a proper dispersion, the quality of which was evaluated by clarity of the resultant films. Sample 58-Si in Table 1 showed to be well dispersed at the first hour of milling, judging from the higher clarity of the films prepared. The greater clarity may indicate higher dispersibility, which coincides with greater silane content and lower hydrophilicity. Sample UnT was the most difficult one to disperse, as expected. The resin containing nanosilica particle was mixed with the isocyanate curing agent and then applied on pre-cleaned glass substrates using an applicator to obtain nanocomposite films. Films were left one week to cure at ambient temperature. The dried film thickness was 3373 mm. Samples were named according to the type of silica incorporated. For treated silica containing films, X-PU was used, where X denotes 29-Si, 45-Si and 58-Si particles according to Table 1. Pristine silica loaded PU (neat silica) is named as UnT-PU. Unloaded PU with no silica (blank) is shown as PU Films properties To compare the optical and mechanical behavior of different grafted particles in polyurethane matrix, different testing Table 2 Specification and typical properties of acrylic polyol and aliphatic polyisocyanate. Property Tacryl H N75 BA/X NCO content Not applicable % C, mpa s Solids (wt%) Flash point (1C) E25 33 Monomeric HDI maximum % Not applicable 0.5 Equivalent wt. (as supplied) Not applicable avg. 255 Specific 25 1C, g/cm Acid value (mg KOH/g) 5 10 Not applicable Hydroxyl content (%) 1.8 Not applicable methods were utilized. Optical quality of films was evaluated by a Gretag Macbeth spectrophotometer Color-Eye 7000A according to ASTM D1003. The haze values measured reflect the optical transparencies of films. The lower the hazes value the greater are the films transparency. To investigate the interfacial behavior of particles in the matrix, tensile strength experiments were carried out on free standing films using a Zwick tensile machine (Germany). The specimens for tensile were cut 10 mm 120 mm from films and clamped to the cross head of instrument. The pulling rate was 5 mm/min. A 20 mm bench mark and the original cross-section area were utilized to calculate their tensile properties. The tensile experiments were conducted at room temperature. The average of at least five measurements was then reported. To quantify the interfacial adhesion of particle with the polyurethane matrix, parameter B known as interfacial interaction parameter was calculated using an empirical equation developed by Pukanszky and his co-workers [20,21] according toeq.(1). d c ¼ d 0 ½ð1 j f Þ=ð1þ2:5j f ÞŠexpðBj f Þ where d c and d 0 aretheyieldstressofthecompositeandtheblank polyurethane matrix, respectively. j f is the volume fraction of fillers as obtained by density measurements. By taking [d c /d o [(1þ2.5j f )/(1 j f )] as Q and plotting Ln Q vs. j f, straight lines are obtained for each set of particle loaded PU films. The slopes give rise to B values. To study the mechanical properties of silica loaded films a Tritec2000 DMTA instrument was used. T g and storage modulus of coatings were measured at frequency of 1 Hz and heating rate of 5 1C/min according to ASTME1640. The analysis was conducted in tensile mode. The dimension of films were 2 cm 5 cm. 3. Results and discussion Silanization of particles leads to less hydrophilic functionalized nanosilicas composed of various amounts of amino functional groups at the surface. From the carbon and nitrogen contents given in Table 1, the grafting percentage of silane at the surface were then calculated [23]. It is expected that the dispersibility of these particles depends on the grafting content. The greater the grafting content, the less hydrophilic particles become and a greater interaction may be achieved between particles and the PU matrix. Hence, it is expected to observe clearer films from particles carrying higher siloxane structure at the surface, which coincides with the presence of higher amino functional groups. In fact, according to Table 1 the order of hydrophobicity of particles is 58-Si445-Si429-Si4UnT. This means that particle 58-Si should be dispersed better and UnT should be dispersed less. Fig. 1 illustrates the haze values measured from films containing different loadings of silica for each set of particles. As can be seen, UnT-PU has the highest haze value among other silica loaded films. There is a systematic decrease in haze value as the grafting content increases. 58-Si- PU has the lowest haze value at different concentrations of silica. All films including the untreated and treated silica as well as PU itself were highly transparent meaning that there was reasonably a good dispersion achieved. As the percent transmission was above 90, haze transmission instead of percent transmission was used in order to be able to distinguish their differences. The optical properties of polyurethane/silica composite films are largely related to the dispersion of particles in resin. This is also affected by the interfacial interaction of particle and the media in which it is dispersed. It could be seen that among silica loaded films sample 58-Si has the lowest haze number (0.62). The lowest haze value of all samples corresponds to PU, as expected. So, it can be observed that haze value of sample sample 58-Si is close to ð1þ

4 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) Fig. 1. Haze transmission of polyurethane containing treated and untreated silica. that of the blank PU. It means that this sample with its higher silane grafted, has better interaction with resin. Moreover, as the acrylic polyol is acidic in nature, it may be discussed that these hydroxyl groups give a stronger interaction with the basic amine functional groups on treated particles. Therefore, the results of haze transmission values can be simply utilized to differentiate the effect of interfacial interaction of particles with the media. This suggests that there is a higher interfacial interaction between this particle and PU matrix. Sample 45-Si-PU, which has lower grafting content stays in the second order with respect to the haze value. Other samples stand in further ranking depending on the content of silane grafted. Hence interaction of particles is expected to be 58-Si445-Si429-Si4UnT. Moreover, transmission electron microscopy (TEM) studies of all particles showed that [16] the particle sizes lied between 12 nm and 16 nm. While the average particle size of the pristine nanosilica is roughly 12 nm, it was observed that the increase in particle size as shown by the TEM technique was in the range of 2 4 nm. However, small angle x-ray scattering technique was performed to measure the particle sizes more precisely [22]. It was found that the increase in particle size was between 0.36 nm and 1.7 nm. Therefore, it can be assumed that the difference between the particles sizes of untreated and treated nanosilicas is not significant and that the variations in mechanical and optical properties of different nanosilicas arise from their difference in surface chemistry rather than morphology. The greater interfacial interaction also results in improved mechanical properties as measured by DMTA analysis [24]. The higher storage modulus of 58-Si loaded film compared with other samples indicates a better interfacial interaction. The behavior of these particles in PU films are in good agreement with the results of dispersibility of particles in liquid state polyol [23]. Samples containing higher grafting percent showed superior interaction in polyol as revealed by Nephelometric Trbidity Unit (NTU) values as previously reported in Ref. [23]. In brief, it was found that the surface treated samples could interact better in solvents with high g H (hydrogen bonding component of Hansen solubility parameter) and g P (polar component of Hansen solubility parameter). It can be predicted that the amino groups on the treated surface can better interact with the media having high g H compared with silanol groups. The reason for such an observation can be explained by the higher nitrogen contents of 58-Si sample, in which a higher grafting has occurred. In addition the lowest grafting of sample 29-Si was further confirmed by its inferior interaction with 2-propanol, which is closer to the behavior of untreated silica. The increased glass transition temperature and enhanced cross-linking density as obtained from DMTA results also explains the greater interfacial interaction of particles with matrix being in order of increasing the grafting content [24] (Fig. 2). To quantify this interaction a semi-empirical model was used from data deduced from tensile experiments as shown below Quantification of interfacial interaction There are various methods and mathematical models by which the interfacial interaction of filler and polymeric matrices can be explained [25,26]. Although interfacial adhesion between the matrix and filler is important for load transfer between these phases, this interfacial adhesion is difficult to measure directly by these mathematical models. In that way, filler matrix adhesion is often inferred only qualitatively by examining the interfacial aspects at fracture surfaces or simply by measuring whether or not the mechanical properties of the composite are greater than those of the matrix. In an attempt to obtain a more rigorous approach to describe this property quantitatively and to elaborate the compositional dependence of tensile yield stress in filled polymers, a semiempirical relationship that successfully fits the experimental data of many different composites [20,21]. This equation is particularly interesting due to a dimensionless parameter B, which is directly related to filler matrix adhesion and can be used to characterize the interfacial adhesion in a more quantitative manner. This approach can be also extended to describe the ultimate tensile strength of a series of particulate composites. Values of the B parameter were obtained for the composites tested and used to judge how surface treatment affected filler matrix adhesion. According to the shear Lag theory [21], when a stress is loaded to composites, the fracture causes two mechanisms: (i) the fracture happens at the interface of particles and polymer and, (ii) the fracture appears in the form of shear yield [27]. In the latter case, the cohesion of composites is lower than that of the adhesion [28]. In other words, if the interface could transfer the stress from composite to particles, the interface is good and could increase the yield stress. Table 3 reports typical mechanical attributes of nanocomposite films deduced from tensile experiments for the samples containing 4 wt% particle loadings. These include Maximum force, force at Yield, % elongation at Yield, Young s modulus and work of break. Figs. 3 and 4 represent the tensile at yield and elongation at yield of different samples at various silica contents. According to

5 28 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) Fig. 2. Tan d as a function of temperature for different silica loaded nanocomposite films. Table 3 Tensile strength, elongation at break and Young s modulus of polyurethane with 4 wt%. silica. Sample Maximum force std¼0.05 Force at yield std¼0.05 % elongation at yield std¼0.05 Young s modulus std¼0.1 Work of break std¼120 PU UnT-PU Si-PU Si-PU Si-PU std¼standard deviations. Fig. 3. Tensile at yield of different silica loaded nanocomposite films. Fig. 4. Elongation strength of different silica loaded nanocomposite films. these figures, in UnT-PU, because of high surface energy of UnT silica and its affinity to form aggregate, the dispersion of particles is low. In the other words, the compatibility of UnT with polyurethane seems lower than that of the treated particles. Thus, this sample has low tensile strength, low Young s modulus and higher elongation than treated ones. Also, the polyurethanes

6 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) containing treated silicas have lower elongation and higher tensile and modulus. It can be seen that 58-Si-PU has the highest tensile strength and modulus and the lowest elongation. This arises from the highest silane content and better interaction with PU matrix, which restricts the mobility of the matrix when loaded in tensile experiment. In fact, this sample has the highest glass transition temperature [24]. The study reported by Chen and coworkers [8], showed that, the tensile strength of polyurethane containing silica treated with acrylic silanes was lower than that of the ones reported here. It may mean that silica treated with reactive functional groups, can interact more effectively with polyurethane thereby increasing the interfacial strength and cross-linking density of the matrix. The tensile yield stress of PU composites filled with the nanoparticles with different surface treatments at various weight ratios are given in Fig. 3. It is observed that the treated particles indeed could reinforce the PU matrix while the untreated silica adversely reduces the tensile strength of the composite. When the inorganic particles are introduced into the PU matrix, they would decrease effective cross-section area to bear the load, which would reduce the tensile yield stress of composites with increasing the content of nanosilica. On the other hand the interface could transfer stress from matrix to inorganic particles, which could improve the yield stress. The aggregation of inorganic particles in the matrix would reduce the contact area and create physical defect in composites, all of which will decrease the effective interfacial interaction, and the tensile yield stress of composites. Therefore, tensile yield stress of particulate filled polymers is mainly affected by the content and effective interfacial interaction. Table 4 Young s modulus, storage modulus (deduced from DMTA analysis ) and T g of nanocomposite films loaded with 4 wt% silicas. Sample Young s modulus std¼1.5 n Storage modulus (divided by 10 8 ) (Pa) std¼0.2 n PU UnT-PU Si-PU Si-PU Si-PU std¼standard deviations. n Values are obtained at room temperature. T g (1C) std¼0.5 Untreated silica sample aggregates severely in PU matrix due to its strong hydrophilic nature, which causes poor dispersion and low adhesion with PU matrix, as also revealed in Fig. 1. Consequently, the stress cannot be effectively transferred from the matrix to particles and the tensile yield stress of composites decreases with increasing particles content. Samples 58-Si and 45-Si have better dispersion in PU matrix compared to UnT due to their higher hydrophobic nature. The interface could therefore, transfer more stress from PU matrix to the inorganic particle. Furthermore, sample 58-Si has a higher strengthening effect in PU matrix compared with particle 45-Si. Table 4 also lists the Young s modulus and storage modulus (deduced from DMTA analysis ) together with the T g of nanocomposite films loaded with 4 wt% silicas. According to Table 4 it is evident that, the Young s modulus, storage modulus and T g of films containing treated silicas are higher than that of the PU itself and the one composed of untreated silica. The mechanism of this observation has been previously discussed in Ref. [24]. Jancar et al. [29] used parameter B to quantitatively characterize the interfacial adhesion of CaCO 3 /PVC composites. By introducing a relative tensile strength Q, which is defined as Q¼ d c /d 0 [(1þ2.5 j f )/(1 j f )] the effective interfacial interaction parameter can be obtained by plotting Ln Q versus j f. Fig. 5 illustrates the results of such plots. In this figure Ln Q has been plotted against volume fraction of particles in the films. The slopes of the lines give rise to B values. Table 5 reports the B values of samples at two cases. In the first, lines have been plotted for volume fractions up to 4 wt% from which B 0 4 values have been obtained. The second one reports the B values for volume fractions up to 6 wt% shown as B 0 6. At volume fractions up to Table 5 B values obtained from Fig. 5. Sample % graft Value of n B (0 4) Value of B (0 6) nn UnT-PU Si-PU Si-PU Si-PU n For weight percents up to 4%. nn For weight percents up to 6% y = x R 2 = y = x R 2 = Si-PU 58-Si-PU 45-Si-PU Ln Q y = x R 2 = UNT-PU Linear (29-Si-PU) Linear (58-Si-PU) y = x R 2 = Linear (45-Si-PU) Linear (UNT-PU) Volume Fraction Fig. 5. Ln Q vs. j f curves.

7 30 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) wt% the linearity of the plots seems greater. At weight percents up to 6% deviation from linearity seems more obvious. In fact, as can be seen the regression values (r 2 ) are not high enough to consider the trend, as a perfect straight line. However, at both cases the corresponding B values deduced from the plots are in good agreement with previous results, i.e., there is a systematic increase in interfacial interaction as the grafting percentage increases. Sample 58-Si-PU shows the highest B value while UnT-PU gives the lowest. This is perhaps due to the more hydrophobic nature of sample 58-Si and its higher amine content enabling to react more with the PU matrix. For poor interfacial bonding, the particles do not carry any load, so B¼0. Hence, it can be expected that the presence of greater amounts of amino silane at the surface results in higher entanglement of amine functional groups with the polyurethane matrix. This leads to formation of urea groups as revealed by FT-IR spectroscopy [24]. Eq. (1) has also been applied to analyze the strength results of epoxy/glass bead composites in which glass beads were subject to different treatments [29]. It was found that B increased from 3.49 to 3.87 when the adhesion is improved by silane treatment of the glass beads compared to the untreated composites. As mentioned before, the interaction parameter reflects the strength of interface and directly depends on the interaction of particle with the matrix. Parameter B considers stress transfer between dispersed phase and matrix and indicates irregular structures or processes (aggregation, orientation of anisotropic filler particles), what can considerably influence the yield stress. Strong interfacial interactions lead to high values of B and consequently to high yield stress of corresponding system. Higher specific surface and anisotropy of filler is reflected in higher values of B, while agglomeration and surface treatment of filler have an opposite effect. Therefore, it can be discussed that the linearity of the equation proposed based on volume fraction of particles do not seem to work properly at large loadings perhaps due to the interaction of the strain fields around the particles. Eq. (1) may imply that the composite modulus is independent of particle size and predicts a linear relationship between yield stress and particle volume fraction. It is useful for low particle loading but not suitable for large loadings. The origin of this equation has come from Einstein equation [30], which is basically based on a linear correlation between the Young s modulus and volume fraction of particles in the composite. There are several modifications to Einstein s equation. One of these modifications is an old model suggested by Guth [31] who added a particle interaction term in the Einstein equation to derive equation (2). E c =E o ¼ aj 2 f þbj f þc where E c and E o are the Young s modulus of the composite and the matrix, respectively. The Einstein equation seems to be valid only at low concentrations of filler and assumes perfect adhesion between filler and matrix, and perfect dispersion of individual filler particles. This equation implies that the composite modulus is independent of particle size and predicts a linear relationship between E c and j f. However, as proposed by Guth [24], the relation between the mechanical attributes deduced from tensile experiment follows a second order equation if the interfacial ð2þ Fig. 6. E c /E 0 values vs. silica content B (0-6) B( 0-4) 12 B parameter Grafting Content Fig. 7. Interfacial interaction parameters at different grafting contents for B values reported in table.

8 M. Rostami et al. / International Journal of Adhesion & Adhesives 34 (2012) interaction also shows effective. He proposed that the linear term in Eq. (2), i.e., parameter b is the stiffening effect of individual particles and the second power term, i.e., parameter a is the contribution of particle interaction. Therefore, to see if this is applicable for the system used in this study, the relative Young s modulus in Eq. (2) (E c /E o ) was plotted versus volume fraction of nanosilicas used (j f ) and a second order equation was fit. Fig. 6 illustrates the results of such fittings. As can be seen, all curves represent a very good regression value (R 2 ¼1). The absolute values of the corresponding second order parameters (a) systematically increase by increasing the grafting content of silica. While UnT-PU has a very low a value (0.004), those of silane grafted silica containing PUs are 0.028, and for 29-Si-PU, 45-Si-PU and 58-Si-PU, respectively. This is in good agreement with the results found earlier using linear relation shown in Eq. (1). The increasing trend in second order parameters of Guth model, representing the strength of interface, is thus justified with respect to the silane content of silica. Fig. 7 is the plot of interfacial interaction parameters B reported in Table 5 versus grafting contents of silicas (29, 45 and 58). As can be seen there is more or less a linear correlation between the interfacial interaction parameters and grafting contents of particles. 4. Conclusion Interfacial interactions of different amino silane treated silicas in a polyurethane coating matrix were studied using tensile strength experiment, dynamic mechanical thermal analysis and haze measurement. It was shown that predefined linear models from which interfacial strength of silica and the matrix can be obtained worked well at particle loadings less than 4 wt%. At higher loadings, however, a deviation from the linear model was found and the tensile behavior of the particle loaded films fit a second order equation. Increased Young s modulus and storage modulus together with enhanced glass transition temperatures were attributed to a better interaction between the amine containing silica and the matrix. This was also confirmed by higher transparency and lower haze values as the amino silane content of silicas increased. References [1] Zhou S, Wu L, Sun J, Shen W. The change of the properties of acrylic-based polyurethane via addition of nano-silica. Prog Org Coat 2002;45: [2] Fu S, Feng X, Lauke B, Mai Y. Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate polymer composites. Composites Part B 2008;39: [3] Xanthos M. Functional filler for plastics. Wiley-VCH; [4] Wypych G. Handbook of fillers. 2nd Edition. Toronto, NY, USA: Chem Tec Publishing; [5] Plueddemann EP. Silane coupling agents. NY, USA: Plenum Press; [6] Sun S, Li C, Zhang L, Du HL, Burnell-Gray JS. Effects of surface modification of fumed silica on interfacial structures and mechanical properties of poly(vinyl chloride) composites. Eur Polym J 2006;42: [7] Sepeur S, Goedicke S, Grob F. Nanotechnology technical basics and applications. Hannover, Germany: Vincentz Network; [8] Chen G, Zhou S, Gu G, Yang H, Wu L. Effects of surface properties of colloidal silica particles on redispersibility and properties of acrylic-based polyurethane/silica composites. J Colloid Interface Sci 2005;281(2): [9] Salon MB, Belgacem MN. Competition between hydrolysis and condensation reactions of trialkoxysilanes, as a function of the amount of water and the nature of the organic group. Colloids Surf., A 2010;366: [10] Solomon DH, Hawthorne DG. Chemistry of pigments and fillers. John Wiley and Sons; [11] Vogel BM, Delongchamp DM, Mahoney CM, Lucas LA, Fischer DA, Lin EK. Interfacial modification of silica surfaces through g-isocyanatopropyl triethoxy silane amine coupling reactions. Appl Surf Sci 2008;254(6): [12] Zhou R, Lu DH, Jiang YH, Li QN. Mechanical properties and erosion wear resistance of polyurethane matrix composites. Wear 2005;259: [13] Chen G, Zhou Sh, Gu G, Wu L. Modification of colloidal silica on the mechanical properties of acrylic based polyurethane/silica composites. Colloids Surf., A 2007;296: [14] Jalili MM, Moradian S. Deterministic performance parameters for an automotive polyurethane clearcoat loaded with hydrophilic or hydrophobic nano-silica. Prog Org Coat 2009;66: [15] Ranjbar Z, Rastegar S. The influence of surface chemistry of nano-silica on microstructure, optical and mechanical properties of the nano-silica containing clear-coats. Prog Org Coat 2009;65: [16] Barna E, Bommer B, Kursteiner J, Vital A. Innovative, scratch proof nanocomposites for clear coatings. Composites Part A 2005;36: [17] Dastmalchian H, Moradian S, Jalili MM, Mirabedini SM. Investigating changes in electrochemical properties when nano-silica is incorporated into an acrylic-based polyurethane clearcoat. J Coat Technol 2009, doi: / s [18] Turunc O, Kayaman-Apohan N, Vezir Kahraman M, Mencelog Y, Gungor A. Nonisocyanate based polyurethane/silica nanocomposites and their coating performance. J Sol Gel Sci Technol 2008;47: [19] Perera DY. Effect of pigmentation on organic coating characteristics. Prog Org Coat 2004;50(4): [20] Pukanszky B. Influence of interface interaction on the ultimate tensile properties of polymer composites. Composites 1990;21: [21] Pukanszky B, Turcsanyi B, Tudos F. Effect of interfacial interaction on the tensile yield stress of polymer composites. In: Ishida H, editor. Interfaces in Polymer, Ceramic and Metal Matrix Composites. NY, USA: Elsevier; [22] M Rostami, Surface Treatment of nanosilica using coupling agents, PhD thesis, Iran: Amirkabir University of Technology; [23] Rostami M, Mohseni M, Ranjbar Z. Investigating the effect of ph on the surface chemistry of an amino silane treated nanosilica. Pigm Resin Technol 2011;40(6): [24] Rostami M, Ranjbar Z, Mohseni M. Investigating the interfacial interaction of different aminosilane treated nanosilicas with a polyurethane coating. Appl Surf Sci 2010;257: [25] Rong MZ, Zhang MQ, Pan SL, Lehmann B, Friedrich K. Analysis of the interfacial interactions in polypropylene/silica nanocomposites. Polym Int 2004;53: [26] D Almeida JRM, Carvalho LHDE. An investigation on the tensile strength of particulate filled polymeric composites. J Mater Sci 1998;33: [27] Amdouni N, Sautereau H, Gerard JF. Epoxy composites based on glass beads. II. Mechanical properties. J Appl Polym Sci 1992;46: [28] Jiang ZX, Meng LH. Influence of coupling agent chain lengths on interfacial performances of polyarylacetylene resin and silica glass composites. Appl Surf Sci 2007;253: [29] Jancar J, Dianselmo A, Dibenedetto AT. The yield strength of particulate reinforced thermoplastic composites. Polym Eng Sci 1992;32(18): [30] Einstein A. Ann Phys 1906;19: [31] Guth E. J Appl Phys 1945;16:20 5.