Short communication
Alkoxysilane functionalized polycaprolactone/polysiloxane modified epoxy resin through sol–gel process

https://doi.org/10.1016/j.eurpolymj.2007.12.014Get rights and content

Abstract

Incorporating elastic polysiloxane and/or an inorganic silica network in epoxy resin could result in the enhancement of physico-chemical properties due to the existence of Si–O bonds. To improve the compatibility between polysiloxane and epoxy matrices and intensively strengthen the properties of the modified system, here polysiloxane was introduced into epoxy resin through compatibilizing epoxy-immiscible polysiloxane with epoxy-miscible polycaprolactone segments via a sol–gel process. To fulfill the process, a blend containing alkoxysilane-functionalized polycaprolactone/polydimethylsiloxane (PCS–2Si) was firstly synthesized using direct nucleophilic addition between –OH groups of polydiol and –NCO of a silane. And then a series of modified epoxy resins were prepared in different epoxy/PCS–2Si weight ratios. All the modified composites were characterized by conventional methods, and their morphological, thermal degradation and surface properties were studied. The results showed that increasing the PCS–2Si content caused the changes of miscibility between epoxy and polysiloxane. Also, the thermal stability of the modified composites was greatly improved. As for the temperature value at 5% weight loss, it reached to 308.5 °C for the composite containing 50–60% (wt%) PCS–2Si, over 150 °C higher than that for neat amine-cured epoxy resin. Similarly, the modified composites showed good hydrophobicity. The improvement of these properties came from the improved interaction between PCS–2Si and epoxy, the forming of Si–O–Si network and the enrichment of siloxane chains on the surface of films. Therefore, it is believed that this modified epoxy appears promising as new high performance and highly functional materials.

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)

  • T. Okamatsu et al.

    Effect on the toughness and adhesion properties of epoxy resin modified with silyl-crosslinked urethane microsphere

    Polymer

    (2002)
  • J.L. Chen et al.

    Temperature-dependent phase behavior in poly(ε-caprolactone)-epoxy blends

    Polymer

    (2001)
  • P.M. Remiro et al.

    The effect of crosslinking and miscibility on the thermal degradation of an uncured and amine-cured epoxy resin blended with poly(ε-caprolactone)

    Polym Degrad Stab

    (2002)
  • Y. Ni et al.

    Influence of intramolecular specific interactions on phase behavior of epoxy resin and poly(ε-caprolactone) blends cured with aromatic amines

    Polymer

    (2005)
  • M. Copuroglu et al.

    Effect of preparation conditions on the thermal stability of an epoxy-functional inorganic/organic hybrid material system with phenyl side group

    Polym Degrad Stab

    (2006)
  • L.B. Longuet et al.

    Epoxy networks toughened by core-shell particles: influence of the particle structure and size on the rheological and mechanical properties

    J Appl Polym Sci

    (1999)
  • S. Sankaran et al.

    Chemical toughening of epoxies-I. Structural modification of epoxy resins by reaction with hydroxyl-terminated poly(butadiene-co-acrylonitrile)

    J Appl Polym Sci

    (1990)
  • K.P.O. Mahesh et al.

    Preparation and characterization of chain extended bismaleimide modified polyurethane-epoxy matrices

    J Appl Polym Sci

    (2003)
  • K.P.O. Mahesh et al.

    Mechanical, thermal, and morphological behavior of bismaleimide modified polyurethane-epoxy IPN matrices

    Polym Adv Technol

    (2003)
  • L. Rey

    Enhancement of crack propagation resistance in epoxy resins by introducing poly(dimethylsiloxane) particles

    J Mater Sci

    (1999)
  • M. Alagar et al.

    Mechanical properties of e-glass fiber reinforced siliconized epoxy composites

    J Polym Comp

    (2000)
  • R.P. Kambour

    Flammability resistance synergism in BPA polycarbonate-silicone blocks polymers

    J Appl Polym Sci

    (1981)
  • S.T. Lin et al.

    Preparation and structural determination of siloxane-modified sulfone-containing epoxy resin

    J Polym Sci Part A: Polym Chem

    (1996)
  • G.H. Hsiue et al.

    Synthesis, characterization, thermal and flame-retardant properties of silicone-based epoxy resin

    J Appl Polym Sci

    (1999)
  • W.C. Shih et al.

    Tetrafunctional aliphatic epoxy 1. Synthesis and characterization

    J Appl Polym Sci

    (1998)
  • Cited by (0)

    View full text