The Role of Interfacial Adhesion in Polymer Composites Engineered from Lignocellulosic Agricultural Waste

This paper presents a comprehensive study about the application of a lignocellulosic agricultural waste, sunflower husk in different polymer composites. Two types of milled sunflower husk with different geometrical factors were incorporated into polypropylene, low-density and high-density polyethylene, polystyrene (PS), glycol-modified polyethylene terephthalate (PETG) and polylactic acid (PLA). The filler content of the composites varied between 0 and 60 vol%. The components were homogenized in an internal mixer and plates were compression molded for testing. The Lewis–Nielsen model was fitted to the moduli of each composite series, and it was found that the physical contact of the filler particles is a limiting factor of composite modulus. Interfacial interactions were estimated from two independent approaches. Firstly, the extent of reinforcement was determined from the composition dependence of tensile strength. Secondly, the reversible work of adhesion was calculated from the surface energies of the components. As only weak van der Waals interactions develop in the interphase of polyolefins and sunflower husk particles, adhesion is weak in their composites resulting in poor reinforcement. Interfacial adhesion enhanced by specific interactions in the interphase, such as π electron interactions for PS, hydrogen bonds for PLA, and both for PETG based composites.


Introduction
In recent years, sustainability has become a principle in many areas, including polymer science and engineering. As a result, more and more effort has been made to decrease the amount of fossil-based polymers and replace them with renewable, natural ones. However, these intentions are surrounded by a number of challenges since the processability and properties of natural polymers are inferior to those of petroleum-based plastics. Several approaches can be followed to eliminate these drawbacks, among which the preparation of polymer composites and blends is a relatively simple, efficient and economical option.
Sunflower husk is also a lignocellulosic waste material, and similarly to the above mentioned harvest wastes, it is available at low price and in large quantities. It consists of For the engineering of interfacial interactions in composites, another simple approach is actually the selection of a matrix polymer with adequate surface properties. Although there are numerous articles about lignocellulose reinforced composites, in most cases only one or two types of polymers are used as a matrix component. This means that quantitative analysis is limited and general conclusions about the role of interactions can hardly be drawn. Therefore, we selected several thermoplastic polymers having different moduli, different surface energies, and that are capable of forming different intermolecular interactions with the applied filler. We filled these polymers with milled sunflower husk in a wide composition range to study quantitatively the effect of interfacial adhesion on the mechanical properties of the composites. In the experiments, we used two types of sunflower husk filler with different size and aspect ratio to investigate the possible role of structure, as well.

Materials
Six commercially available thermoplastic polymer grades were used as matrix, namely polypropylene (PP), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), glycol-modified polyethylene terephthalate (PETG) and polylactic acid (PLA). All of these polymers were used in the form of pellets with 1-2 mm diameter. The type, source and the most important properties of the polymers used in this study are provided in Table 1. Two types of milled sunflower husk were applied as filler in the composites, and both of them were supplied by Bunge (Chesterfield, Missouri, USA). The SunPro Fiber (SPF) and SunPro 20 (SP20) grades are already milled and they contain slightly fibrous particles. The average particle length of SPF is 2600 µm, the aspect ratio is 3.3, and the density is 1.42 g/cm 3 , while these characteristics of SP20 are 1100 µm, 2.8, and 1.44 g/cm 3 .

Sample Preparation
Prior to sample preparation, sunflower husk fillers were dried in the air at 105 • C for 12 h in a ventilated oven, while PETG and PLA were kept at 200 mbar air pressure and 105 • C for 4 h in a vacuum oven to remove their humidity content. The components were homogenized in a Brabender W 50 EHT internal mixer at 42 cm 3 charge volume and 50 rpm. First, the polymer was melted within 1-2 min, then the filler was added and mixing was carried out for additional 10 min. Set temperature was 190 • C for the PS and PETG composites, 180 • C for the PP and PLA ones, as well as 160 • C for the LDPE and HDPE ones. The filler content of the composites increased from 0 to 30 vol% in 5 vol% steps, and from 30 to 60 vol% in 10 vol% steps.
Immediately after mixing, 1-mm-thick plates were compression molded from the still plastic materials using a Fontijne SRA 100 machine. The temperature of compression Polymers 2021, 13, 3099 4 of 16 molding was set at the same value as that of the internal mixer for each material. After the plates had been stored at room temperature and 50% relative humidity for one week, 5 tensile bars (type 1A ISO) were machined from each composite for further testing.

Characterization
The surface tension of unmilled sunflower husk and the polymers applied was determined by static contact angle measurements using the OWRK method [47][48][49][50]. Diiodomethane was used for the determination of the dispersion component of surface tension, while water, ethylene glycol and formamide were applied for the estimation of the polar component. The contact angle of 20 µL liquid droplets was measured at 23 • C and 50% relative humidity with a Ramé-Hart goniometer.
Mechanical properties (modulus, strength and elongation-at-break) were determined by tensile testing using an Instron 5566 universal testing machine. Gauge length was 115 mm and the cross-head speed was set at 5 mm/min. The structure of the composites was studied by digital optical microscopy (DOM) using a Keyence VHX 5000 apparatus. Micrographs were recorded on the compressed surface of the plates.

Statistical Analysis
The modulus of the sunflower husk particles was estimated by applying the model of Lewis and Nielsen [51]. The model was fitted to the moduli of the composites by nonlinear regression using the Generalized Reduced Gradient Nonlinear algorithm. The iteration steps were done by the Solver add-in of Microsoft Excel (Version 2016, Microsoft, Redmond, WA, USA).
Analysis of covariance (ANCOVA) was performed to determine the statistical significance of structure and interfacial adhesion in the reinforcing effect of sunflower husk. The level of significance was set at 0.05, thus a factor was considered to be significant in case its p-value was smaller than 0.05. Calculations were carried out by means of Statistica software (Version 13.3, TIBCO Software, Palo Alto, CA, USA).

Results and Discussion
The results are presented and discussed in several sections. Firstly, the factors limiting the modulus of the composites are studied. Secondly, the composition dependence of strength and elongation-at-break is presented, which expresses the performance of the composites at failure. Thirdly, the reinforcing effect of sunflower husk is analyzed quantitatively, and eventually, it is related to interfacial adhesion and structure.

The Limiting Factors of Modulus
A number of applications are subjected to static loading; thus, they must be engineered with adequate stiffness to maintain their dimensions. In many cases, the modulus of neat polymers is too low, thus their particulate filled composites are used instead. Many papers have shown that the incorporation of lignocellulosic fillers can enhance the modulus of polymers [4,9,[20][21][22][23][24]; however, the limiting factors are rarely discussed. In Figure 1, the modulus of the composites is plotted as a function of sunflower husk content. Young's modulus increases with increasing filler content for all the composite series studied since the lignocellulosic particles of sunflower husk have a higher modulus than the polymer matrices. For a better understanding of the tendencies, the semiempirical model of Lewis and Nielsen [38] was fitted to the measured moduli by nonlinear regression. This model can be expressed by the following equations.
where E, E m and E f are the Young's moduli {GPa} of the composite, the matrix and the filler, respectively, ν m is the Poisson's ratio {mm/mm} of the matrix, ϕ is the filler content {cm 3 /cm 3 }, and ϕ max is the maximum packing fraction {cm 3 /cm 3 } of the filler. The two parameters, A and Ψ, are related to the structure of the composite; however, they are not very well defined [52]. Parameter A can be related to filler anisotropy, through the relation where k E is the Einstein's coefficient, but the relation has not been thoroughly investigated and verified yet. Parameter Ψ is the function of the maximum packing fraction, thus it is related to anisotropy, but it is affected also by the formation of an interphase. Despite these uncertainties, the Lewis-Nielsen model is quite frequently used to predict the modulus of particulate filled composites [52]. In order to estimate ϕ max , the sunflower husk particles were assumed to be ellipsoids having aspect ratios between 2.8 and 3.3, and being in maximally random jammed state. Based on the simulations of Donev et al. [53], the maximum packing density is approximately 0.67 for such particles, therefore we used this value as ϕ max .          The brittleness of the composites studied is well demonstrated by their elongationat-break values which are presented as a function of sunflower husk content in Figure 5. Deformability is decreased significantly by the incorporation of the rigid filler particles results. The highest elongation-at-break values were measured for the polyolefin based composites, while the lowest values were determined for the PS based systems. This observation implies that the tendencies are mostly determined by the deformability of the matrix polymer, which is proved by Figure 6 showing the correlation between the elongation-at-break of the matrix polymer and that of the composites at 10 vol% and 40 vol% filler loading, respectively. The data are plotted on a logarithmic scale since they cover a range of 3-4 orders of magnitude.  interphase. If hydrogen bonds can develop between the components, the extent of reinforcement increases further. As a result, the reinforcing effect of the lignocellulosic fillers can be utilized mostly in PETG and PLA. This statement contradicts somewhat the considerable decreasing tendencies of strength for these two polymers ( Figure 4). A possible explanation is that we can expect low inherent strength for the sunflower husk particles applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of the reversible work of adhesion. In this work, the reversible work of adhesion was calculated from the surface tension of the components. According to the theoretical assumptions of Fowkes [72], the surface free energy is the sum of contributions from the different intermolecular forces at the surface, thus the surface tension can be divided into a dispersion and a polar component. This latter includes also specific forces, such as π electron interactions and hydrogen bonds. As a result, the reversible work of adhesion can be estimated also by the following formula [ these results as from the BlnσTm values shown in Table 3, and even the order of the data is the same. This observation is visualized by the linear correlation between them in Figure  9.  The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p = 0.8912). Although the particle size was quite different in the two types of milled sunflower husk used in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our previous results about lignocellulosic composites showed that the aspect ratio of a filler particle is a more relevant factor than particle size [62]. Therefore, the negligible effect of filler structure may be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for SP20, and 3.3 for SPF). The average modulus of the filler particles was an output of the nonlinear regression, thus its value determined the fitted curves. When all moduli were involved in the nonlinear regression, the Lewis-Nielsen model did not fit adequately to the data. The deviation between the observed and fitted data was the highest at large filler contents, which indicates the appearance of a factor being neglected by the model. The validity range was determined by removing the observed moduli one by one from 60 vol% to lower filler contents, and then by re-fitting the model to the remained data. For both fillers, the results of the best fits are summarized in Table 2, while the goodness-of-fit is demonstrated by Figure 2 showing the estimated moduli plotted against the observed moduli from the validity interval. The modulus of the two filler types does not differ significantly from each other, therefore the Lewis-Nielsen model was re-fitted to the data of all the composite series studied, and the fitted curves were also placed in Figure 1. The common modulus of sunflower husk particles was found to be 7.50 ± 0.71 GPa, which is close to the modulus of lignocellulosic filler materials with similar size, anisotropy and composition [54][55][56], but it is inferior compared to those with a more fibrous structure and higher cellulose content [56].        The brittleness of the composites studied is well demonstrated by their elongationt-break values which are presented as a function of sunflower husk content in Figure 5. eformability is decreased significantly by the incorporation of the rigid filler particles sults. The highest elongation-at-break values were measured for the polyolefin based omposites, while the lowest values were determined for the PS based systems. This obrvation implies that the tendencies are mostly determined by the deformability of the atrix polymer, which is proved by Figure 6 showing the correlation between the elonation-at-break of the matrix polymer and that of the composites at 10 vol% and 40 vol% ller loading, respectively. The data are plotted on a logarithmic scale since they cover a nge of 3-4 orders of magnitude. interphase. If hydrogen bonds can develop between the components, th forcement increases further. As a result, the reinforcing effect of the lign can be utilized mostly in PETG and PLA. This statement contradicts so siderable decreasing tendencies of strength for these two polymers (Figu explanation is that we can expect low inherent strength for the sunflow applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the the reversible work of adhesion. In this work, the reversible work of adh lated from the surface tension of the components. According to the the tions of Fowkes [72], the surface free energy is the sum of contributions f intermolecular forces at the surface, thus the surface tension can be divid sion and a polar component. This latter includes also specific forces, su interactions and hydrogen bonds. As a result, the reversible work of adh mated also by the following formula [73] where Wmf is the reversible work of adhesion between the matrix and t surface tension, the subscripts m and f represent the matrix and the fil while the subscripts d and p denote the dispersion and polar component o respectively. The dispersion and polar surface tension of sunflower husk 2 2 ) PETG; ( Polymers 2021, 13, x FOR PEER REVIEW 1 these results as from the BlnσTm values shown in Table 3, and even the order of the d the same. This observation is visualized by the linear correlation between them in F 9.  The results of statistical analysis, namely ANCOVA, also corroborated the dom role of adhesion in the determination of reinforcement. The effect of the reversible of adhesion was statistically significant (p = 0.0028), while that of the filler type was n = 0.8912). Although the particle size was quite different in the two types of milled flower husk used in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our p ous results about lignocellulosic composites showed that the aspect ratio of a filler pa is a more relevant factor than particle size [62]. Therefore, the negligible effect of structure may be related to the similar aspect ratio of the lignocellulosic fillers (2 SP20, and 3.3 for SPF).
The limited validity of the Lewis-Nielsen model can be attributed to the surface properties of the components. Firstly, we can assume poor interfacial adhesion between the milled sunflower husk and the polymers possessing low surface energy, such as poliolefins and PS, which could result in the debonding of matrix/filler interface around larger particles even at very small deformations where the modulus of the composites was determined [57,58]. Secondly, we can also assume that the wettability of the sunflower husk is poor, which results in the physical contact of their particles at higher filler contents as shown by the DOM images of Figure 3. Since these associations are held together only by weak interactions, they can be easily disrupted [57,59]. This effect can induce the formation of voids around the filler particles resulting in lower modulus than expected.

Performance at Failure
The practical relevance of a composite material is demonstrated not only by its modulus but also by the mechanical properties measured at failure, such as strength and deformation-at-break. In Figure 4, tensile strength is plotted against sunflower husk content. In all cases, strength decreases with increasing filler loading, but the gradient of the tendencies is quite different. On an absolute scale, the strength of PLA based composites decreases drastically, while that of LDPE and PS based ones changes slightly. The decrease of strength is often considered to be a consequence of weak interfacial adhesion between the matrix polymer and the filler. However, strength is affected not only by interfacial interactions but also by matrix [60] and filler properties [61], as well as by structure [62],

Performance at Failure
The practical relevance of a composite material is demonstrated not only by its modulus but also by the mechanical properties measured at failure, such as strength and deformation-at-break. In Figure 4, tensile strength is plotted against sunflower husk content. In all cases, strength decreases with increasing filler loading, but the gradient of the tendencies is quite different. On an absolute scale, the strength of PLA based composites decreases drastically, while that of LDPE and PS based ones changes slightly. The decrease of strength is often considered to be a consequence of weak interfacial adhesion between the matrix polymer and the filler. However, strength is affected not only by interfacial interactions but also by matrix [60] and filler properties [61], as well as by structure [62], thus a proper analysis of the tendencies requires the application of adequate models (see next section).  The brittleness of the composites studied is well demonstrated by their elongationat-break values which are presented as a function of sunflower husk content in Figure 5. Deformability is decreased significantly by the incorporation of the rigid filler particles interphase. If hydrogen bonds can develop between the components, the extent of reinforcement increases further. As a result, the reinforcing effect of the lignocellulosic fillers can be utilized mostly in PETG and PLA. This statement contradicts somewhat the considerable decreasing tendencies of strength for these two polymers ( Figure 4). A possible explanation is that we can expect low inherent strength for the sunflower husk particles applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of these results as from the BlnσTm values shown in Table 3, and even the order of the data is the same. This observation is visualized by the linear correlation between them in Figure  9.  The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p ) PLA; empty symbols: SP20; full symbols: SPF. The brittleness of the composites studied is well demonstrated by their elongationat-break values which are presented as a function of sunflower husk content in Figure 5. Deformability is decreased significantly by the incorporation of the rigid filler particles results. The highest elongation-at-break values were measured for the polyolefin based composites, while the lowest values were determined for the PS based systems. This observation implies that the tendencies are mostly determined by the deformability of the matrix polymer, which is proved by Figure 6 showing the correlation between the elongation-at-break of the matrix polymer and that of the composites at 10 vol% and 40 vol% filler loading, respectively. The data are plotted on a logarithmic scale since they cover a range of 3-4 orders of magnitude.

Reinforcing Effect of Filler
As was mentioned earlier, the strength of a composite is affected by many factors including interfacial adhesion, matrix and filler properties, as well as structure [60][61][62]. The application of the model proposed by Pukánszky et al. [63] offers the possibility to study these factors quantitatively. This semi-empirical model is based on the Nicolais-Narkis model [64], but it uses an effective load-bearing cross-section of the matrix, as well as it takes into account the influence of interfacial interactions. The composition dependence of tensile strength is expressed by the following formula [63].
where σ Tred is the reduced tensile strength {MPa}, σ T and σ Tm are the true tensile strength {MPa} of the composite and the matrix, respectively (σ T = σλ and λ = L/L 0 , where L is the ultimate and L 0 the initial gauge length {mm} of the specimen), n is a parameter taking into account strain hardening {dimensionless}, ϕ is the volume fraction {cm 3 /cm 3 } of the filler, and B is related to its relative load-bearing capacity {dimensionless}, i.e., to the extent of reinforcement. Parameter B is determined by the size of the interface between the matrix and the filler and by the properties of the interphase [65] where A d and ρ d are the specific surface area and density of the filler, while and σ i are the thickness of the interphase and its strength, respectively. Since the thickness of the interphase ( ) depends on the strength of interactions, parameter B can provide information also about interfacial adhesion. If we take the logarithm of the two sides of Equation (5), we receive the following linear form where the dependent variable is the natural logarithm of reduced tensile strength, the independent variable is the volume fraction of the filler, the intercept is the natural logarithm of the matrix strength, and the slope is equal to parameter B. The reduced tensile strength of PP/SP20 and PETG/SP20 composite series is plotted this way with the fitted linear curves in Figure 7, while the fitting results are summarized in Table 3 for all the series studied. Based on the fitted B values, two major groups can be distinguished. For the poliolefin based composites, parameter B is quite small implying the presence of only weak van der Waals forces in the interphase. The other composites have higher B values, which can originate from the formation of specific interactions in the interphase, such as π electron interactions for PS [66][67][68][69], hydrogen bonds for PLA [70], and both for PETG based systems [66][67][68][69][70].

Reinforcing Effect of Filler
As was mentioned earlier, the strength of a composite is affected by many factors including interfacial adhesion, matrix and filler properties, as well as structure [60][61][62].
where E, Em and Ef are the Young's moduli {GPa} of the composite, the matrix and the filler, respectively, νm is the Poisson's ratio {mm/mm} of the matrix, φ is the filler content {cm 3 /cm 3 }, and φmax is the maximum packing fraction {cm 3 /cm 3 } of the filler. The two parameters, A and Ψ, are related to the structure of the composite; however, they are not very well defined [52]. Parameter A can be related to filler anisotropy, through the relation A = kE − 1, where kE is the Einstein's coefficient, but the relation has not been thoroughly investigated and verified yet. Parameter Ψ is the function of the maximum packing fraction, thus it is related to anisotropy, but it is affected also by the formation of an interphase. Despite these uncertainties, the Lewis-Nielsen model is quite frequently used to predict the modulus of particulate filled composites [52]. In order to estimate φmax, the sunflower husk particles were assumed to be ellipsoids having aspect ratios between 2.

Performance at Failure
The practical relevance of a composite material is dem ulus but also by the mechanical properties measured at fai formation-at-break. In Figure 4, tensile strength is plotted ag In all cases, strength decreases with increasing filler loa tendencies is quite different. On an absolute scale, the stren decreases drastically, while that of LDPE and PS based ones of strength is often considered to be a consequence of weak the matrix polymer and the filler. However, strength is a interactions but also by matrix [60] and filler properties [61

Reinforcing Effect of Filler
As was mentioned earlier, the strength of a composite is affected by many factors including interfacial adhesion, matrix and filler properties, as well as structure [60][61][62].
) PS; ( siderable decreasing tendencies of strength for these two polymers (Figure 4). A possible explanation is that we can expect low inherent strength for the sunflower husk particles applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of the reversible work of adhesion. In this work, the reversible work of adhesion was calculated from the surface tension of the components. According to the theoretical assumptions of Fowkes [72], the surface free energy is the sum of contributions from the different intermolecular forces at the surface, thus the surface tension can be divided into a dispersion and a polar component. This latter includes also specific forces, such as π electron interactions and hydrogen bonds. As a result, the reversible work of adhesion can be estimated also by the following formula [73] = 2√ • + 2√ • where Wmf is the reversible work of adhesion between the matrix and the filler, γ is the surface tension, the subscripts m and f represent the matrix and the filler, respectively, while the subscripts d and p denote the dispersion and polar component of surface tension, respectively. The dispersion and polar surface tension of sunflower husk was found to be 41.3 mJ/m 2 and 1.8 mJ/m 2 , respectively, while the calculated Wmf values with the measured γ values of the polymers are collected in Table 4. We can draw the same conclusions from  The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p = 0.8912). Although the particle size was quite different in the two types of milled sunflower husk used in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our previous results about lignocellulosic composites showed that the aspect ratio of a filler particle is a more relevant factor than particle size [62]. Therefore, the negligible effect of filler structure may be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for SP20, and 3.3 for SPF).
) PLA; empty symbols: SP20; full symbols: SPF. One trend line represents the LDPE composites having the highest deformability, and the other one represents the much more rigid PLA composites.

Reinforcing Effect of Filler
As was mentioned earlier, the strength of a composite is affected by many factors including interfacial adhesion, matrix and filler properties, as well as structure [60][61][62].     The brittleness of the composites studied is well demonstrated by their elongationat-break values which are presented as a function of sunflower husk content in Figure 5. Deformability is decreased significantly by the incorporation of the rigid filler particles results. The highest elongation-at-break values were measured for the polyolefin based composites, while the lowest values were determined for the PS based systems. This observation implies that the tendencies are mostly determined by the deformability of the matrix polymer, which is proved by Figure 6 showing the correlation between the elongation-at-break of the matrix polymer and that of the composites at 10 vol% and 40 vol% filler loading, respectively. The data are plotted on a logarithmic scale since they cover a terphase. If hydrogen bonds can develop between the components, the extent of reinrcement increases further. As a result, the reinforcing effect of the lignocellulosic fillers n be utilized mostly in PETG and PLA. This statement contradicts somewhat the conderable decreasing tendencies of strength for these two polymers (Figure 4). A possible planation is that we can expect low inherent strength for the sunflower husk particles plied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of e reversible work of adhesion. In this work, the reversible work of adhesion was calcuted from the surface tension of the components. According to the theoretical assumpns of Fowkes [72], the surface free energy is the sum of contributions from the different termolecular forces at the surface, thus the surface tension can be divided into a disperon and a polar component. This latter includes also specific forces, such as π electron teractions and hydrogen bonds. As a result, the reversible work of adhesion can be esti-  Table 3, and even the order of the data is s observation is visualized by the linear correlation between them in Figure   ispersion and polar surface tension components of the polymers used in the study, ble work of adhesion in their composites.  lts of statistical analysis, namely ANCOVA, also corroborated the dominant on in the determination of reinforcement. The effect of the reversible work as statistically significant (p = 0.0028), while that of the filler type was not (p ough the particle size was quite different in the two types of milled sunsed in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our previout lignocellulosic composites showed that the aspect ratio of a filler particle vant factor than particle size [62]. Therefore, the negligible effect of filler be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for  According to Equation (6), there is a linear correlation between parameter B and the logarithm of matrix strength, which is corroborated by Figure 8. This implies that B values contain not only the effect of interfacial adhesion but also that of matrix properties, and more reliable conclusions could be drawn from the B values if they were made independent of the matrix properties. For this purpose, we used a correction proposed in our previous paper [71] to give a more accurate estimation of reinforcement, thus parameter B was multiplied by the natural logarithm of the true tensile strength of the matrix polymer (BlnσTm). These values are also listed in Table 3, and, indeed, they show a slightly different picture of reinforcement. The lowest BlnσTm values, i.e., the smallest extent of reinforcement, still belong to the polyolefin based composites. The reinforcing effect of filler particles is increased somewhat in the PS matrix as π electron interactions may develop in the  According to Equation (6), there is a linear correlation between parameter B and the logarithm of matrix strength, which is corroborated by Figure 8. This implies that B values contain not only the effect of interfacial adhesion but also that of matrix properties, and more reliable conclusions could be drawn from the B values if they were made independent of the matrix properties. For this purpose, we used a correction proposed in our previous paper [71] to give a more accurate estimation of reinforcement, thus parameter B was multiplied by the natural logarithm of the true tensile strength of the matrix polymer (BlnσTm). These values are also listed in Table 3, and, indeed, they show a slightly different picture of reinforcement. The lowest BlnσTm values, i.e., the smallest extent of reinforcement, still belong to the polyolefin based composites. The reinforcing effect of filler particles is increased somewhat in the PS matrix as π electron interactions may develop in the  According to Equation (6), there is a linear correlation between parameter B and the logarithm of matrix strength, which is corroborated by Figure 8. This implies that B values contain not only the effect of interfacial adhesion but also that of matrix properties, and more reliable conclusions could be drawn from the B values if they were made independent of the matrix properties. For this purpose, we used a correction proposed in our previous paper [71] to give a more accurate estimation of reinforcement, thus parameter B was multiplied by the natural logarithm of the true tensile strength of the matrix polymer (BlnσTm). These values are also listed in Table 3, and, indeed, they show a slightly different picture of reinforcement. The lowest BlnσTm values, i.e., the smallest extent of reinforcement, still belong to the polyolefin based composites. The reinforcing effect of filler particles is increased somewhat in the PS matrix as π electron interactions may develop in the )) PETG/SP20. Empty symbols represent data omitted from fitting. According to Equation (6), there is a linear correlation between parameter B and the logarithm of matrix strength, which is corroborated by Figure 8. This implies that B values contain not only the effect of interfacial adhesion but also that of matrix properties, and more reliable conclusions could be drawn from the B values if they were made independent of the matrix properties. For this purpose, we used a correction proposed in our previous paper [71] to give a more accurate estimation of reinforcement, thus parameter B was multiplied by the natural logarithm of the true tensile strength of the matrix polymer (Blnσ Tm ). These values are also listed in Table 3, and, indeed, they show a slightly different picture of reinforcement. The lowest Blnσ Tm values, i.e., the smallest extent of reinforcement, still belong to the polyolefin based composites. The reinforcing effect of filler particles is increased somewhat in the PS matrix as π electron interactions may develop in the interphase. If hydrogen bonds can develop between the components, the extent of reinforcement increases further. As a result, the reinforcing effect of the lignocellulosic fillers can be utilized mostly in PETG and PLA. This statement contradicts somewhat the considerable decreasing tendencies of strength for these two polymers (Figure 4). A possible explanation is that we can expect low inherent strength for the sunflower husk particles applied due to their disadvantageous geometrical factors [61]. interphase. If hydrogen bonds can develop between the components, the extent of reinforcement increases further. As a result, the reinforcing effect of the lignocellulosic fillers can be utilized mostly in PETG and PLA. This statement contradicts somewhat the considerable decreasing tendencies of strength for these two polymers (Figure 4). A possible explanation is that we can expect low inherent strength for the sunflower husk particles applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of the reversible work of adhesion. In this work, the reversible work of adhesion was calculated from the surface tension of the components. According to the theoretical assumptions of Fowkes [72], the surface free energy is the sum of contributions from the different intermolecular forces at the surface, thus the surface tension can be divided into a dispersion and a polar component. This latter includes also specific forces, such as π electron interactions and hydrogen bonds. As a result, the reversible work of adhesion can be estimated also by the following formula [73] = 2√ • + 2√ • where Wmf is the reversible work of adhesion between the matrix and the filler, γ is the surface tension, the subscripts m and f represent the matrix and the filler, respectively, while the subscripts d and p denote the dispersion and polar component of surface tension, respectively. The dispersion and polar surface tension of sunflower husk was found to be 41.3 mJ/m 2 and 1.8 mJ/m 2 , respectively, while the calculated Wmf values with the measured γ values of the polymers are collected in Table 4. We can draw the same conclusions from ung's moduli {GPa} of the composite, the matrix and the sson's ratio {mm/mm} of the matrix, φ is the filler content mum packing fraction {cm 3 /cm 3 } of the filler. The two pato the structure of the composite; however, they are not er A can be related to filler anisotropy, through the relation ein's coefficient, but the relation has not been thoroughly arameter Ψ is the function of the maximum packing fracpy, but it is affected also by the formation of an interphase. Lewis-Nielsen model is quite frequently used to predict d composites [52]. In order to estimate φmax, the sunflower

Performance at Failure
The practical relevance of a composite material is demonstrated not only by its modulus but also by the mechanical properties measured at failure, such as strength and deformation-at-break. In Figure 4, tensile strength is plotted against sunflower husk content. In all cases, strength decreases with increasing filler loading, but the gradient of the tendencies is quite different. On an absolute scale, the strength of PLA based composites decreases drastically, while that of LDPE and PS based ones changes slightly. The decrease of strength is often considered to be a consequence of weak interfacial adhesion between osites studied is well demonstrated by their elongationnted as a function of sunflower husk content in Figure 5. ificantly by the incorporation of the rigid filler particles -at-break values were measured for the polyolefin based lues were determined for the PS based systems. This obncies are mostly determined by the deformability of the d by Figure 6 showing the correlation between the elonlymer and that of the composites at 10 vol% and 40 vol% data are plotted on a logarithmic scale since they cover a e. interphase. If hydrogen bonds can develop between the components, the extent of reinforcement increases further. As a result, the reinforcing effect of the lignocellulosic fillers can be utilized mostly in PETG and PLA. This statement contradicts somewhat the considerable decreasing tendencies of strength for these two polymers (Figure 4). A possible explanation is that we can expect low inherent strength for the sunflower husk particles applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of the reversible work of adhesion. In this work, the reversible work of adhesion was calculated from the surface tension of the components. According to the theoretical assumptions of Fowkes [72], the surface free energy is the sum of contributions from the different intermolecular forces at the surface, thus the surface tension can be divided into a dispersion and a polar component. This latter includes also specific forces, such as π electron interactions and hydrogen bonds. As a result, the reversible work of adhesion can be estimated also by the following formula [73] = 2√ • + 2√ • where Wmf is the reversible work of adhesion between the matrix and the filler, γ is the surface tension, the subscripts m and f represent the matrix and the filler, respectively, while the subscripts d and p denote the dispersion and polar component of surface tension, respectively. The dispersion and polar surface tension of sunflower husk was found to be 41.3 mJ/m 2 and 1.8 mJ/m 2 , respectively, while the calculated Wmf values with the measured γ values of the polymers are collected in Table 4. We can draw the same conclusions from ) PETG; ( the same. This observation is visualized by the linear correlation between them in Figure  9.  The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p = 0.8912). Although the particle size was quite different in the two types of milled sunflower husk used in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our previous results about lignocellulosic composites showed that the aspect ratio of a filler particle is a more relevant factor than particle size [62]. Therefore, the negligible effect of filler structure may be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for SP20, and 3.3 for SPF).

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination of the reversible work of adhesion. In this work, the reversible work of adhesion was calculated from the surface tension of the components. According to the theoretical assumptions of Fowkes [72], the surface free energy is the sum of contributions from the different intermolecular forces at the surface, thus the surface tension can be divided into a dispersion and a polar component. This latter includes also specific forces, such as π electron interactions and hydrogen bonds. As a result, the reversible work of adhesion can be estimated also by the following formula [73] where W mf is the reversible work of adhesion between the matrix and the filler, γ is the surface tension, the subscripts m and f represent the matrix and the filler, respectively, while the subscripts d and p denote the dispersion and polar component of surface tension, respectively. The dispersion and polar surface tension of sunflower husk was found to be 41.3 mJ/m 2 and 1.8 mJ/m 2 , respectively, while the calculated W mf values with the measured γ values of the polymers are collected in Table 4. We can draw the same conclusions from these results as from the Blnσ Tm values shown in Table 3, and even the order of the data is the same. This observation is visualized by the linear correlation between them in Figure 9. these results as from the BlnσTm values shown in Table 3, and even the order of the data is the same. This observation is visualized by the linear correlation between them in Figure  9.  The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p = 0.8912). Although the particle size was quite different in the two types of milled sunflower husk used in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our previous results about lignocellulosic composites showed that the aspect ratio of a filler particle is a more relevant factor than particle size [62]. Therefore, the negligible effect of filler structure may be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for SP20, and 3.3 for SPF).
nd Ef are the Young's moduli {GPa} of the composite, the matrix and the ely, νm is the Poisson's ratio {mm/mm} of the matrix, φ is the filler content φmax is the maximum packing fraction {cm 3 /cm 3 } of the filler. The two pad Ψ, are related to the structure of the composite; however, they are not ed [52]. Parameter A can be related to filler anisotropy, through the relation ere kE is the Einstein's coefficient, but the relation has not been thoroughly

Performance at Failure
The practical relevance of a composite material is demonstrated not only by its modulus but also by the mechanical properties measured at failure, such as strength and deformation-at-break. In Figure 4, tensile strength is plotted against sunflower husk content. ness of the composites studied is well demonstrated by their elongations which are presented as a function of sunflower husk content in Figure 5. is decreased significantly by the incorporation of the rigid filler particles ghest elongation-at-break values were measured for the polyolefin based hile the lowest values were determined for the PS based systems. This obies that the tendencies are mostly determined by the deformability of the r, which is proved by Figure 6 showing the correlation between the elonof the matrix polymer and that of the composites at 10 vol% and 40 vol% espectively. The data are plotted on a logarithmic scale since they cover a ders of magnitude. interphase. If hydrogen bonds can develop between the components, the extent of re forcement increases further. As a result, the reinforcing effect of the lignocellulosic fil can be utilized mostly in PETG and PLA. This statement contradicts somewhat the c siderable decreasing tendencies of strength for these two polymers (Figure 4). A poss explanation is that we can expect low inherent strength for the sunflower husk partic applied due to their disadvantageous geometrical factors [61].

Effect of Adhesion and Structure on Reinforcement
Another approach for the estimation of interfacial adhesion is the determination the reversible work of adhesion. In this work, the reversible work of adhesion was cal lated from the surface tension of the components. According to the theoretical assum tions of Fowkes [72], the surface free energy is the sum of contributions from the differ intermolecular forces at the surface, thus the surface tension can be divided into a disp sion and a polar component. This latter includes also specific forces, such as π elect interactions and hydrogen bonds. As a result, the reversible work of adhesion can be e mated also by the following formula [73] = 2√ • + 2√ • where Wmf is the reversible work of adhesion between the matrix and the filler, γ is surface tension, the subscripts m and f represent the matrix and the filler, respectiv while the subscripts d and p denote the dispersion and polar component of surface tensi respectively. The dispersion and polar surface tension of sunflower husk was found to 41.3 mJ/m 2 and 1.8 mJ/m 2 , respectively, while the calculated Wmf values with the measu γ values of the polymers are collected in Table 4. We can draw the same conclusions fr ) PETG; ( these results as from the BlnσTm values shown in Table 3, and even the order of the data is the same. This observation is visualized by the linear correlation between them in Figure  9.  The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p = 0.8912). Although the particle size was quite different in the two types of milled sunflower husk used in our experiments (1100 µ m for SP20, and 2600 µ m for SPF), our previous results about lignocellulosic composites showed that the aspect ratio of a filler particle is a more relevant factor than particle size [62]. Therefore, the negligible effect of filler structure may be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for SP20, and 3.3 for SPF).
The results of statistical analysis, namely ANCOVA, also corroborated the dominant role of adhesion in the determination of reinforcement. The effect of the reversible work of adhesion was statistically significant (p = 0.0028), while that of the filler type was not (p = 0.8912). Although the particle size was quite different in the two types of milled sunflower husk used in our experiments (1100 µm for SP20, and 2600 µm for SPF), our previous results about lignocellulosic composites showed that the aspect ratio of a filler particle is a more relevant factor than particle size [62]. Therefore, the negligible effect of filler structure may be related to the similar aspect ratio of the lignocellulosic fillers (2.8 for SP20, and 3.3 for SPF).

Conclusions
In this paper, a comprehensive study is presented about the role of interfacial adhesion and structure in polymer/lignocellulose composites. The lignocellulosic filler was sunflower husk which is an agricultural waste available at low price and in large quantities. The modulus of the composites is limited by several factors. In polymer matrices with low surface energy, the debonding of matrix/filler interface around larger particles may occur even at very small deformations. In addition, at larger filler contents, sunflower husk particles physically contact each other resulting in very weak associations. Interfacial adhesion was estimated quantitatively from the extent of reinforcement and the reversible work of adhesion. Both approaches provided concordant results about interfacial interactions. Only weak van der Waals forces act in the interphase of polyolefin based composites, which results in poor adhesion and reinforcement. For the PS based systems, the reinforcing effect of sunflower husk particles is increased, which can be related to by the formation of π electron interactions in the interphase. Among the polymers studied, interfacial adhesion is the strongest in the PLA and PETG based composites since hydrogen bonds can also develop in the interphase. Regarding filler structure, no difference was found between the reinforcing effects of the two types of milled sunflower husk used in our experiments, which can be explained by their similar aspect ratio. For all series, stiff and rigid composites are obtained at large sunflower husk loadings, which could be mitigated by the application of elastomers. The relatively low strength of the composites might be improved by increasing the inherent strength of sunflower husk particles. For this purpose, both the modification of filler geometry by further milling and the chemical treatment of the filler particles could be beneficial.