Short communicationAlkoxysilane functionalized polycaprolactone/polysiloxane modified epoxy resin through sol–gel process
Introduction
Epoxy resins were known for their high performance, especially with respect to thermal and dimensional stabilities combined with high stiffness and strength at low creep. As a consequence of their highly cross-linked structure, these materials tended to suffer from brittle behavior, poor crack resistance, and low fracture toughness. A well-known procedure to toughen such brittle polymers was to incorporate discrete modifiers into the rigid matrix. Among the modifiers, elastomers, including carboxy-, amine-, or hydroxyl-terminated acrylonitrile butadiene rubbers, functionally-terminated acrylates, polyurethane [1], [2], [3], [4], [5], [6], had been applied with great success to enhance the toughness of epoxy resins or related matrix resins without sacrificing other useful properties such as dimensional stability, stiffness, and strength.
Silicone rubbers, especially poly(dimethylsiloxane) (PDMS), exhibited a number of attractive properties including high chain flexibility, high thermal and oxidative stability, low temperature flexibility and good hydrophobic behavior [7], [8]. This combination of excellent features provided the necessary conditions for the application of PDMS as an elastomeric modifier to make the modified epoxy resin with improved properties [1], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, pure PDMS had very little use as a toughening agent because of the poor compatibility between soft segments of PDMS and polar hard segments in epoxy which largely resulted from the lack of hydrogen bonding. These materials, which usually exhibited separate Tg values due to thermodynamic incompatibility, were either macroscopically immiscible or exuded from the cross-linked matrix during curing procedure in the conventional introduction of siloxane into polymers through blending methods [14], [15], [16], [17], resulting in poor thermo-mechanical properties and compositional heterogeneity resulting from poor segmental compatibility, this limited the use of larger silicone concentrations [14], [17], [18], [19], and hampered their use in surface coatings [16]. To overcome this limitation and improve the interaction between PDMS and epoxy matrices with improved thermo-mechanical properties and toughness, several techniques were reported in the literature, including using silane coupling agents [20], [21] and chemically incorporating PDMS into the main chain of epoxy to form the interpenetrating network [15], [22], but there still existed some drawback in these modified systems, such as the thermo-mechanical properties’ declining swiftly with an increase of the PDMS soft segment.
Introduction of polar organic segment such as polycaprolactone (PCL) to PDMS via chemical blocking [19] to compatibilize epoxy-immiscible PDMS with epoxy-miscible organic segments had been also proved to be an interesting attempt, where polycaprolactone-block-poly(dimethylsiloxane)-block-polycaprolactone triblock copolymer-modified epoxy resin had shown a significantly improved toughness without adversely affecting the thermal and mechanical properties at silicone liquid rubber contents <10 wt%.
Compared with chemical modification procedure as above, sol–gel was a method for preparing organic/inorganic hybrids at a low temperature, which offered several advantages, the micro- and macrostructure of a hybrid composite could be controlled by optimizing the synthetic parameters, such as pH, concentration, water/alkoxide ratio, temperature, pressure, type of catalyst, and solvent. And incorporating an inorganic silica network into epoxy matrix to produce an inorganic/organic hybrid material had been extensively studied because it exhibited a wide range of improved multifunctional properties, including toughness, impact strength, tensile strength and thermal degradation [23], [24], [25]. These hybrid materials could be easily prepared by using the sol–gel process. In the sol–gel process, a silicone alkoxide such as tetraethoxysilane (TEOS), organofunctional silane [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] and alkoxysilane-functionalized (trialkoxy silyl-terminated) elastomer [33], [34], [35], [36] were commonly used as a precursor of silica to modify epoxy. Especially, the silyl-terminated elastomer-modified epoxy resins had shown a significant increase of mechanical properties such as elongation and peel strength, fracture energy [33], [34], [35], and the tuning of the mechanical properties (i.e., strength from the epoxy and high impact resistance from the elastomer) could be also easily achieved by adjusting the ratio and selection of the silylated elastomer and the epoxy resin. Similarly, in our studies, alkoxysilane-functionalized polycaprolactone (PCL–Si)-modified epoxy resin (DGEBA) had also shown excellent adhesion strength and fracture energy, when used as an adhesive for coatings, especially, the modified systems had a significant improvement in acid-, and basic-resistance, and thermal degradation when a suitable content of PCL–Si was used [37], [38]. So this modified epoxy appeared promising as new high performance and highly functional materials.
Although each of all PDMS and silica from sol–gel gave modified epoxy resin-outstanding performances as above, it was noteworthy that only few published papers dealt with incorporating PDMS into epoxy resin through a sol–gel process. Considering the provided characteristics of PDMS, it should be much interesting to co-modify epoxy resin with PDMS and silica from sol–gel to obtain an optimal performance. In this work, alkoxysilane-functionalized polydimethylsiloxane, i.e., silyl-terminated PDMS (PDMS–Si), was tentatively introduced into epoxy resin through compatibilizing epoxy-immiscible PDMS with epoxy-miscible polycaprolactone (PCL) segments, which were attached to each other through a sol–gel process between PDMS–Si and PCL–Si. Here, the PCL–Si and PDMS–Si blend were prepared using direct nucleophilic addition, and a series of modified epoxy resin were then prepared at different PDMS–Si/PCL–Si (PCS–2Si) blend contents. All the modified epoxy resin composites were characterized by conventional methods, including scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimetric analysis (TG), contact angle and X-ray photoelectron spectroscopy (XPS). Their morphological, thermal degradation and surface properties were studied, and the effect of modifier content on the properties of the modified system was analyzed.
Section snippets
Materials
Commercially available polycaprolactone diol (PCL 210, Mn = 1000, OH value = 114.1 mgKOH/g, Daicel), α,ω-bis(3-hydroxypropyl) polydimethylsiloxane (HPDMS, Tego® HSi 2311, OH value = 44.9 mgKOH/g, Degussa), diglycidyl ether of biphenol A type epoxy resin (DGEBA, epoxy equivalent ∼ 500 g/Eq, Wuxi resin plant, China), Aradur® 250 (polyamidoamine type curing agent, amine value = 425–455 mgKOH/g, Huntsman Co.), and isocyanatopropyltriethoxysilane (ICPTES, Shinetsu), dibutyltin dilaurate (DBTDL, 95%, Aldrich)
FT-IR spectra analysis
As previously reported [36], synthesis of silyl-terminated elastomers was directly carried out via nucleophilic addition between –OH groups of polydiol (polycaprolactone diol) and α,ω-bis(3-hydroxypropyl)polydimethylsiloxane and –NCO groups of ICPTES (shown as Scheme 1). And DBTDL was chosen as a catalyst to ensure the complete reaction between the –OH and –NCO. Fig. 1 shows the IR spectra of the ICPTES, PCL-210/HPDMS mixture and the resultant of PCL-210/PDMS and ICPTES after the reactive
Conclusions
Using direct nucleophilic addition between –OH groups of polydiol and –NCO of a silane, silyl-terminated polycaprolactone/silyl-terminated polydimethylsiloxane was successfully synthesized. Combined with epoxy resin at different compositions, a series of PCS–2Si modified epoxy resin composites were prepared and then cured by amino-glycidyl reaction and the sol–gel process. The morphology and thermal stability of the composite system was characterized as a function of PCS–2Si content. It was
References (51)
- et al.
Function and performance of silicone copolymer. Part IV. Curing behavior and characterization of epoxy-siloxane copolymers blended with diglycidyl ether of bisphenol-A
Polymer
(2000) - et al.
Epoxy urethane acrylate
Eur Polym J
(2000) - et al.
Controlled synthesis of vinylmethylsiloxane–dimethylsiloxane gradient, block and alternate copolymers by anionic ROP of cyclotrisiloxanes
Polymer
(2002) - et al.
Epoxy resin possessing flame retardants elements from silicon incorporated epoxy compounds cured with phosphorus or nitrogen containing curing agents
Polymer
(2002) - et al.
Synthesis, characterization and development of high performance siloxane-modified epoxy paints
Prog Org Coat
(2005) - et al.
Thermal degradation of epoxy-silica organic–inorganic hybrid materials
Polym Degrad Stab
(2006) - et al.
Nanostructured sol–gel derived conversion coatings based epoxy- and amino-silane
Prog Org Coat
(2003) - et al.
Thermal stability and degradation kinetics of novel organic/inorganic epoxy hybrid containing nitrogen/silicon/phosphorus by sol–gel method
Thermochim Acta
(2007) - et al.
Triggered release of molecular additives from epoxy-amine sol–gel coatings
Prog Org Coat
(2005) - et al.
Functionalized inorganic/organic nanocomposites as new basic raw materials for adhesives and sealants
Int J Adhesion Adhesive
(2004)