The Effect of Surface Treatment on the Strength and Adhesion Characteristics of Phoenix dactylifera-L(Date Palm) Fibers

ABSTRACT

There is a growing interest in the use of natural/bio-fibers as reinforcing components for thermoplastics and thermosets. However, they do suffer from a few limitations, such as lower compatibility with relatively hydrophobic polymer matrixes. Thus, improvement of the interface and interphase interactions in natural fiber–polyester composites is essential. In this research date palm (Phoenix dactylifera-L) fibers were modified by surface treatment using chemical method in order to improve their adhesion to polyester matrixes. Alkaline treatment, as an example of dissolution and treatment with silane coupling agents were performed. Furthermore, a combination treatment of alkaline and silane coupling agents was also carried out. Fiber modifications were monitored by Scanning Electron Microscopy (SEM). In addition to that, the improvement of adhesion and strength between date palm–modified fibers and polyester matrix was investigated by single filament pull-out test as well as tensile tests. It was found, from interfacial shear strength values, that substantial improvements in fiber-matrix compatibility have been achieved. According to single filament pull-out test results, interfacial shear strength increased for all treated fibers as compared to non-treated fibers. Particularly, combination of alkaline and silane coupling agents resulted in substantial adhesion improvement to the polyester matrix in comparison to the untreated fibers and fibers treated by alkaline and silane methods only.     

Effect of silane‐coupling agents on interfacial properties of CF/PI composites

In this article, to form a structure‐controlled interface, carbon fiber (CF) surfaces were first activated by plasma technique and then hydroxylated by LiAlH₄ treatment, and then were reacted with a suit of silane‐coupling agents terminated with desired functional groups to form thin films, which further reacted with polyimide (PI) resin to generate a strong adhesion interface. The morphology, structure, and composition of CF surfaces before and after treatment were investigated by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS), respectively. The results of FTIR and XPS analysis showed that silane‐coupling agents were successfully chemisorbed onto the CF surfaces by the hydrolysis and condensation reactions. The interfacial shear strength of the CF/PI microcomposites was evaluated by the microbond technique. The results showed that the types of the interfacial functional groups, especially the vinyl end groups in vinyltriethoxysilane (VS), which can react with PI resin, had very significant influence on the improvement of the interfacial adhesion properties of composites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

钛酸钾晶须及硫酸钙晶须改性环氧树脂

应用硅烷及钛酸酯等偶联剂对钛酸钾晶须及硫酸钙晶须进行表面处理，考察晶须对环氧树脂力学性能、工艺性等的影响。研究表明，钛酸钾晶须经硅烷偶联剂处理后，能很好地改善复合材料的性能，硅烷的表面处理效果较钛酸酯的好。钛酸钾晶须添加到环氧树脂中后，材料的弯曲强度随晶须含量增大逐渐增大，在晶须含量为8％时达到最大值，之后性能稍有下降；材料的弯曲模量随晶须添加量的增加逐渐增大，冲击强度稍有降低。体系中添加硫酸钙晶须后，材料的性能也得到一定程度的提高。硫酸钙晶须对环氧树脂工艺性的影响较钛酸钾晶须小。SEM表明，晶须经合适的偶联剂表面改性后，与树脂基体的界面粘接得到有效改善。     

Improving interfacial adhesion between thermoplastic polyurethane and copper foil using amino carboxylic acids

A chemical methodology to improve the adhesion between copper foil and a thermoplastic polyurethane (TPU) matrix is reported. The copper foil (0.127 mm thickness) was treated with aminocarboxylic acid‐based coupling agents such as 6‐aminohexanoic acid and 4‐aminobenzoic acid. 3‐Aminopropyl trimethoxysilane was also used as a conventional silane coupling agent for comparative studies. The interfacial adhesion between copper foil and laminated TPU was examined by means of peel adhesion test, scanning electron microscopy, and attenuated total reflection‐infrared spectroscopic methods. The treatment of copper foils with 6‐aminohexanoic acid resulted in improved adhesion, which was equal to that of the silane‐treated system. The mechanism of how the coupling agents strengthen the interfacial adhesion between TPU and copper foil is discussed. The solution concentrations of the coupling agents were optimized with respect to the peel adhesion of the interface. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of silane treatment on the Curaua fibre/polyester interface

ABSTRACT

Good adhesion at fibre/matrix interface of lignocellulosic fibres is crucial when substituting synthetic fibres in polymer composites. The great variability presented by those fibres requires diverse characterisation studies for better insights on fibre surface treatments and resin systems interactions. In this work, Curaua fibres were treated using silane coupling agents to improve their interfacial properties with polyester. The fibres were pre-treated using 4?wt% solution of NaOH and then treated with 5?wt% solution of (3-aminopropyl) trimethoxysilane (AMPTS) or triethoxymethylsilane (TEMS). Characterisation of the treated fibres was carried out using infrared spectroscopy, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Fibre wettability and adhesion towards polyester was investigated using contact angle measurements and pull-out tests, respectively. The AMPTS treatment yielded a significant result of 20.2?MPa in interfacial shear strength (≈2.5 times that of the untreated fibre), attributed to the increase in availability of binding sites with polyester.     

Using a localized fluorescent dye to probe the glass/resin interphase

A novel technique has been developed to study the buried polymer/substrate interfacial regions by localizing a fluorescent probe on the substrate surface. Epoxy functional multi‐layers of silane coupling agents were deposited on glass and doped with small amounts of a fluorescently labeled silane‐coupling agent (FLSCA). When the dye‐doped silane layers were immersed in an epoxy/amine cured resin, a blue shift in the emission maximum was measured after resin cure. Silane layers of varying thickness were tested. Thicker layers showed smaller fluorescence shifts during cure, suggesting incomplete resin penetration into these layers. The fluorescence sensitivity to the interfacial reaction was verified with external reflection Fourier Transform Infrared Spectroscopy (FTIR) of the silane layers immersed only in the amine hardener.     

Secondary neutral mass spectrometry studies of germanium-silane coupling agent-polymer interphases

This communication reports the results of an investigation of the germanium-silane-polymer interphase using secondary neutral mass spectrometry (SNMS). The goal of these studies was to gain an understanding of the factors which control the adhesion of polymers to metals or semi-metals. Interdiffusion of the silane coupling agent and the polymer is believed to play an important role in the promotion of adhesion of polymers using silane coupling agents. In this study we have obtained evidence of the interdiffusion of a silane coupling agent, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and a polymer, polystyrene, by probing the germanium-silane-polymer interfacial region with this emerging spectroscopic technique.     

Wood‐thermoplastic composites from wood flour and high‐density polyethylene

High‐density polyethylene/wood flour (HDPE/WF) composites were prepared by a twin‐screw extruder. The effects of WF, silane coupling agents, polymer compatibilizers, and their content on the comprehensive properties of the WF/HDPE composites have been studied in detail, including the mechanical, thermal, and rheological properties and microstructure. The results showed that both silane coupling agents and polymer compatibilizers could improve the interfacial adhesion between WF and HDPE, and further improve the properties of WF/HDPE composites, especially with AX8900 as a compatibilizer giving higher impact strength, and with HDPE‐g‐MAH as a compatibilizer giving the best tensile and flexural properties. The resultant composite has higher strength (tensile strength = 51.03 MPa) and better heat deflection temperature (63.1°C). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Role of silanes in adhesion. Part II. Dynamic mechanical properties of silane-treated glass fiber/polyester composites

—Glass fiber/unsaturated polyester composites, prepared by impregnating glass braid with varying thickness coatings (from 200 Å up to 1600 Å thick) of polyester resin, were tested with a DuPont Dynamic Mechanical Analyzer. The effects of the polyester resin thickness and silane treatments on the dynamic mechanical properties of the composites were evaluated. The results are supported by Fourier transform infrared spectroscopy of the composite materials. It is shown that both the concentration and the organo-functional group of the silane coupling agent influence the damping, storage, and loss moduli as well as the glass transition temperature (T_g) of the matrix resin in the closest vicinity to the glass/resin bondline. In the absence of a silane inner layer, a low T_g, 'soft' boundary layer exists due to inhibition of the polyester resin cure by the glass surface. It is noted that a reactive silane, such as γ-methacryloxypropyltrimethoxysilane, promotes the formation of a 'soft' or 'rigid' (high T_g) boundary layer, depending on the concentration of the silane in the treating solution. On the other hand, a non-reactive silane, such as methyltrimethoxysilane, produces a 'rigid' interphase in the entire range of concentrations of the silane solution. An attempt was made to correlate the dynamic mechanical properties of the boundary layer with the fiber/polymer interfacial shear strength. Upon pretreatment of glass fibers with silane coupling agents, the relative magnitude of the loss modulus, E", and the nature of the boundary layer (T_g) seem to be better indicators of efficient stress transfer from the polymer to the glass fiber in the composite system than tan δ. Efficient stress transfer is characterized by a low value of E" and 'soft' boundary layers. The results suggest that the mere presence of glass/polyester chemical bonding is insufficient to ensure effective stress transfer. A strong bond results from the synergistic effect of glass/silane/polymer chemical bonding and a 'soft' boundary layer.     

Effects of the Treatment of Al(OH)3 by a Silane Coupling Agent on the Tensile Properties of PVC/Al(OH)3 Composite

The present work deals with the study of a composite based on poly(vinyl chloride) (PVC) and aluminum hydroxide (Al(OH)₃) which is treated with different concentrations of the silane coupling agent N-(2-aminoethyl)-3-aminopropyltrimethoxysilane.

The composites containing untreated Al(OH)₃ and those treated with the coupling agent were prepared by melt mixing using a two-roll mill.

Analysis of the treated filler by means of Fourier Transform Infrared spectroscopy (FTIR) showed the formation of oligoaminosilanes resulting from the condensation of the silane coupling agent.

The tensile properties of the PVC/Al(OH)₃ composite reflected the effect of the addition of the mineral filler and also the influence of the chemical treatment on the interfacial adhesion. The incorporation of aluminum hydroxide into PVC resulted in an increase of Young's modulus and the yield stress.

From the calculation of a parameter B which was used to quantify the state of adhesion between the polymeric matrix and the filler, it was concluded that the surface chemical treatment of the filler with the silane coupling agent leads to higher reinforcement as a result of the interfacial interactions developed between PVC and Al(OH)_3.     

The prediction of adhesion between polymer matrices and silane-treated glass surfaces in filled composites

Extending the database and the analysis of work reported earlier, the practical adhesion between a glass filler, modified by various silane coupling agents, and different polymeric matrices is measured and compared with predictions based on a generalized thermodynamic criterion. The criterion used is the magnitude of the (negative) molar Gibbs free energy of mixing, (-ΔGmix)_0.5, for a pseudo-solution consisting of equal molar amounts of the repeat units of the polymer matrix and the organo-functional group of the silane coupling agent. It is computed using the group contribution method of UNIFAC, in which molar volumes, molar areas, and molar interaction energies are constructed from contributions of the functional groups which make up the molecules of the solution. Measurements leading to the values of the adhesion strength are carried out using the singleparticle composite method, in which a rectangular polymer specimen containing a single silane-treated glass bead is subjected to increasing uni-axial tensile stress until interfacial failure, as observed using a microscope, occurs at one of the poles of the sphere. Earlier work reported a good correlation between the local stress at the pole computed from such measurements and the value of (-ΔG _mix)_0.5 computed using UNIFAC for ten different organo-silane-modified spheres imbedded in a poly(vinyl butyral) matrix. The present work extends the database to two additional matrices, viz. poly(methyl methacrylate) and poly(ethyl methacrylate). In addition, elastic fracture-mechanics theory is used to deduce specific adhesion energies in all cases. The relative values of the latter are all found to be in good correlation with the predictive thermodynamic criterion.     

Interfacial Adhesion of Polyamide 66 Fibres to an Aqueous Polyurethane–Acrylic Hybrid Polymer Adhesive

In this work, the interfacial adhesion between a polyamide 66 fibre, and an aqueous polyurethane–acrylic hybrid polymer adhesive was investigated. Silane and air plasma treatments were introduced to modify the surface of the polyamide 66 fibre. The surface chemistry was characterised using X-ray photoelectron spectroscopy (XPS). There were more oxygen-containing functional groups, –OH or –COOH, introduced by air plasma and silane treatments on the surface of polyamide fibre to increase its chemical activity. The microbond test was used to measure the interfacial shear strength (IFSS) between the waterborne polyurethane–acrylic hybrid polymer adhesive and a polyamide fibre. It has been found that air plasma and silane surface treatments can be used to improve interfacial adhesion. IFSS at 8.7 and 5.9 MPa, respectively, were higher than that of the control, 5.0 MPa. After water immersion at 50°C for 48 h, IFSS dropped to 7.0 MPa for air plasma-treated specimen and to 4.4 and 4.1 MPa for silanised and control specimens, respectively. Air plasma surface treatment is more effective than silane treatment to improve the interface adhesion in the polymer fibre–polymer composite.     

The role of additional silane coupling agent treatment in oxygen plasma-treated UHMPE fiber/vinylester composites

Ultra-high modulus polyethylene (UHMPE) fiber was treated with oxygen plasma and a silane coupling agent in order to improve the interfacial adhesion between the UHMPE fiber and vinylester resin. The oxygen plasma and γ-methylmethacryloxypropyltrimethoxysilane (γ-MPS)-treated UHMPE fiber/vinylester composites showed a slightly higher interlaminar shear strength than the oxygen plasma-treated UHMPE fiber/vinylester composites. The interfacial adhesion of the oxygen plasma-treated UHMPE fiber/vinylester composites in this study is mainly due to mechanical interlocking between the micropits formed by the oxygen plasma treatment and the vinylester resin. The γ-MPS molecules adsorbed onto the UHMPE fiber surface neither affected the morphology of the UHMPE fiber surface, nor reduced the extent of mechanical interlocking. The improved interfacial adhesion by the γ-MPS treatment is due to enhanced wettability and chemical interaction through the chemically adsorbed γ-MPS molecules, as detected by Fourier-transform infrared (FT-IR) spectroscopy. The γ-MPS molecules adsorbed onto the ultra-high molecular weight polyethylene (UHMWPE) plate surface also reduced the aging effect of the oxygen plasma-treated UHMWPE surface.     

The structure of γ‐glycidoxypropyltrimethoxysilane on glass fiber surfaces: Characterization by FTIR,SEM, and contact angle measurements

The purpose of this article is to determine the structure of γ‐glycidoxypropyltrimethoxysilane (γ‐GPS) on glass fiber surfaces. The interfacial adhesion of glass fiber–polymer can be improved by the silane treatment of the glass fiber. To change the composition of the glass and regenerate to the hydroxyl groups, activation pretreatment of heat cleaned woven glass fabric was performed using a 10% (v/v) hydrochloric acid aqueous solution for different durations before silane treatment. The treatment of silanization of heat cleaned and acid activated glass fibers with (γ‐GPS) were conducted at various time intervals. These fibers would be used to quantify the relationship between contact angle of glass fiber surface and the interfacial shear strength of the fiber–polymer interface. The effect of acid activation on glass surface and the interaction between glass fibers and silane coupling agent were examined using Fourier transform infrared spectroscopy. The experiments, in conjunction with electron photomicrographs of glass surfaces treated with coupling agent, are interpreted in an attempt to explain the stability of coupling agent‐glass interfaces. From SEM analysis, it was clearly observed that agglomerations of silane agent in the cavities among the heat cleaned fibers are available. However, this case was not observed for the silanization of acid activated glass fibers. In addition, contact angle measurements on glass fibers were performed to evaluate surface structure. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Organofunctional silanes as adhesion promoters: direct characterization of the polymer/silane interphase

Organofunetional silanes can be used as coupling agents to promote the adhesion of organic matrices to inorganic substrates. Silane coupling agents are of the general structure YSi(OR)₃, where Y is an organofunctional group selected for bonding to organic polymers while (OR) is a hydrolyzable group on silicon which can react with surface hydroxyl groups on the substrate. One mechanism of adhesion promotion by silanes has been postulated to be the interdiffusion of the coupling agent and the polymer to form an interpenetrating network. This paper reviews some earlier results using X-ray photoelectron spectroscopy to probe the polymer/silane interphase region and gives new results obtained using the technique of sputtered neutral mass spectrometry (SNMS). It was demonstrated that SNMS had the sensitivity necessary to detect the polymer/coupling agent interphase. It was found that interdiffusion of the coupling agent with the polymer was maximized when the solubility parameters of the polymer and coupling agent were matched.     

Effects of interfacial adhesion on properties of polypropylene/Wollastonite composites

Wollastonite reinforced polypropylene (PP/CaSiO₃) composites were prepared by melt extrusion. A silane coupling agent and a maleic anhydride grafted PP (PP‐g‐MA) were used to increase the interfacial adhesion between the filler and the matrix. The increased adhesion observed by scanning electron microscopy (SEM) resulted in improved mechanical properties. A model was applied to describe the relationship between the interfacial adhesion and tensile properties of PP/CaSiO₃ composites. There is stronger interfacial adhesion between silane‐treated CaSiO₃ and polymer matrix containing PP‐g‐MA as a modifier. Results of dynamic mechanical thermal analysis (DMTA) showed that stronger interfacial adhesion led to higher storage modulus. The influence of CaSiO₃ particles on the crystallization of PP was studied by using differential scanning calorimetry (DSC). The introduction of CaSiO₃ particles does not affect the crystallization temperature and crystallinity of PP matrix significantly. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effective surface treatments for enhancing the thermal conductivity of BN‐filled epoxy composite

To improve the thermal conductivity of BN‐filled epoxy composite, admicellar polymerization was used to coat polystyrene and polymethyl methacrylate on the BN surface to improve the interfacial adhesion in the composite. The treated surface was characterized by FTIR and contact angle measurements. The results show that the admicellar treatment led to improved wettability of epoxy resin on the treated surface. Thermal conductivity of the composite increased from 1.5 W/mK for untreated BN to 2.69 W/mK when the admicellar‐treated BN was used, indicating improvement in the interfacial adhesion between BN and epoxy resin in the composite. The mechanical properties of the composite also improved significantly. The surfactant : monomer molar ratio of 1 : 10 was found to be the optimum condition for the admicellar polymerization process. The solubility parameter concept was used to explain the difference in the effectiveness of polystyrene and polymethyl methacrylate. When compared to the more conventional silane treatment, admicellar treatment was found to be more effective in improving the interfacial adhesion between the BN particles and epoxy resin. SEM micrographs of the fractured surface of the composite further confirm the improvement in the interfacial adhesion after the admicellar treatment. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Methods for improving the performance of silane coupling agents

The use of silane coupling agents in mineral- and glass-reinforced composites is well known. They impart improved initial mechanical properties, but, more importantly, they cause mechanical properties to be retained during the use of the composite. The main cause of loss of mechanical properties is attack of water at the interface. Recent research has focused on imparting more durable bonding of the silane coupling agent to both the polymer and the reinforcement. Improved silane coupling agent systems have been developed by utilizing several techniques: blends of hydrophobic silanes with hydrophilic silanes to give greater hydrophobic character; use of 1,2-bis-(trimethoxysilyl)ethane as an additive to give increased siloxane crosslinking; use of more thermally stable silanes such as phenyltrimethoxysilane and N-[2-(vinylbenzylamino)-ethyl]-3-aminopropyltrimethoxysilane to give increased thermal stability; and the use of a carboxy-functional silane with a carboxy-functional polymer and zinc salt to give ionomer bonds at the interface. The effectiveness of these new coupling agent systems was tested by measuring the flexural strength of composites and the adhesion strength of coatings on inorganic substrates. The results show that composites have increased flexural strength and better strength retention during thermal aging; coatings have greater adhesion strength; there is greater resistance of interfacial bonding to degradation by moisture; and thermoplastic composites have better properties after high shear processing.     

Silane coupling agents: the role of the organofunctional group

Some highlights of recent research on silane coupling agents are reviewed. Studies of silanes in solution, of chemical bonding in the substrate/matrix interphase, and of the interfacial bond in composites have provided new insights into the mechanism of effectiveness of silane adhesion promoters. Examples are presented of new chemical structures and of new concepts of bonding designed for optimal performance of silane coupling agents in advanced composites.