Mechanical and thermal studies of intercross-linked networks based on siliconized polyurethane-epoxy/unsaturated polyester coatings

https://doi.org/10.1016/j.porgcoat.2003.09.018Get rights and content

Abstract

Two different intercross-linked networks (ICN) of siliconized polyurethane-epoxy/unsaturated polyester (UP) coatings were developed. Epoxy and UP resins were modified with polyurethane (PU) prepolymer and hydroxyl terminated polydimethylsiloxane (HTPDMS) using γ-aminopropyl triethoxysilane (γ-APS) and vinyltriethoxysilane (VTES) as silane cross-linkers and dibutyltindilaurate (DBTL) as catalyst. Aromatic polyamine adduct (A) and aliphatic amine (B) were used as curatives for epoxy and methyl isobutyl ketone peroxide (C) as curative for UP resin. The coating materials were obtained in the form of tough films and characterized for their mechanical properties such as tensile strength and impact test as per ASTM methods. The thermal stability of the ICN coatings was studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) and compared with unmodified epoxy/UP systems. Mechanical properties were enhanced with incorporation of PU (10 wt.%) and silicone (10 wt.%) due to the toughening of brittle epoxy and UP systems. The introduction of PU into unmodified epoxy/UP coating systems reduces the thermal stability due to the presence of thermally weak urethane linkages whereas, the incorporation of 10 wt.% silicone into PU modified epoxy/UP systems improves their thermal stability due to the partial ionic nature, high bond energy and thermal stability of –Si–O–Si– linkage.

Introduction

Numerous coating systems based on epoxy and unsaturated polyester (UP) resins are available for industrial and submerged structures, but they are not completely satisfactory from the viewpoint of prevention of corrosion and fouling. Epoxy and UP resins possess good adhesion and chemical resistance but their brittle behavior with low elongation restricts their utility for high performance applications [1], [2], [3], [4]. A systematic and detailed study is required to propose suitable chemical modifiers for epoxy and UP resins to obtain coating materials suitable for corrosion resistant applications under marine and industrial environments. Hybrid coating materials resulting from intercross-linked network (ICN) mechanism enhance the thermal stability, impact behavior and hydrophobicity characteristics compared to those obtained from blending technique [5], [6], [7]. The technology of intercross-linked polymer network as coating material in which combinations of two or more polymers are chemically cross-linked with those of the other is presently in a state of emergence.

The effect of blending polyamide, polyacrylates, chlorinated and nitrile rubber with thermoset epoxy/UP resins improves compatibilization, corrosion resistance and mechanical properties with only modest reductions in other important properties compared to unblended resins [8], [9], [10], [11], [12], [13]. Among the elastomeric modifiers, polydimethylsiloxane, commercially known as silicones is regarded as one of the suitable materials to modify epoxy/UP resins, owing to its versatile behavior like resistance to weathering and aging, good wetting and film forming ability, low temperature flexibility resulting from –Si–O–Si– linkage and good hydrophobic behavior [14], [15], [16]. Polyurethane (PU) is another class of favored polymeric modifier for coating applications owing to its superior impact strength, resistance to UV light, abrasion resistant and anti-microbial characteristics [17], [18], [19], [20].

The hybrid ICN of siliconized PU-epoxy and siliconized PU-UP are expected to possess improved toughness, better thermo-mechanical properties, better corrosion and microbial resistance than conventional coating materials. Hence in the present work, an attempt is made to improve the thermo-mechanical characteristics of epoxy and UP coatings by forming ICNs with PU prepolymer and silicone.

Section snippets

Materials and sample preparation

The commercially available epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) GY 250 (epoxy equivalent∼180–190; Vantico, India) and UP resin (isophthalic acid based, with 30% styrene, viscosity at 25°C=1400 cP) was used as base material. Hydroxyl terminated polydimethylsiloxane (HTPDMS, Mw=18,000) as modifier, γ-aminopropyl triethoxysilane (γ-APS) and vinyltriethoxysilane (VTES, Aldrich, USA) as silane cross-linking agents and dibutyltindilaurate (DBTL, Merck, Germany) as catalyst were used as

Test methods

Tensile properties are studied using dumbbell-shaped specimens as per ASTM D 3039 using Instron testing machine (model 6025 UK), at a cross-head speed of 2 mm/min. Impact resistance of the coating systems are measured by falling weight impact tester according to ASTM specification G1477. Thermogravimetric technique is the most important method to assess the thermal stability of polymeric materials. The thermal stability of the cured coating systems was assessed by thermogravimetric analysis

Formation of silicone modified PU-epoxy ICNs

The polymer network formation of silicone modified PU-epoxy system proceeds in three steps and is ascertained from FT-IR spectra. In the first step, the isocyanate group of polyurethane prepolymer (PU) reacts with secondary hydroxyl group of the epoxy resin. The disappearance of isocyanate peak at 2270 cm−1 and formation of CO and N–H peaks of urethane group at 1680–1630 and 1581–1518 cm−1, respectively, are used to ascertain the completion of the reaction (Fig. 1a). The second step involves

Conclusions

Siliconized polyurethane-epoxy and siliconized polyurethane-UP ICN coatings were developed and their thermo-mechanical characteristics studied. The introduction of PU (2.5–20%) and silicone (10%) into the epoxy and UP increased their viscosity and decreased the cure schedule due to the formation of ICN. The mechanical properties were enhanced with incorporation of PU (2.5–10%) and silicone (10%). This may be due to long flexible and resilient nature of PU chain and the stress relieving nature

Acknowledgements

The authors thank Prof. V. Mohan, Dean of Technology, Anna University, Chennai, for his help. One of the authors, A. Anand Prabu thank the Council of Scientific and Industrial Research, Government of India, New Delhi, for awarding Senior Research Fellowship.

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      The curing reaction between the epoxy function and amine group of the curing agent is further supported by the disappearance of the peak at 914 cm−1, which is characteristic of the epoxide ring in the uncured resin. The carbonyl group of the polyester and the epoxide ring of epoxy resin having a strong intermolecular hydrogen bonding and the carbonyl absorption frequency is shifted from 1728 to 1738 cm−1 (Fig. 2) [28]. The absence of epoxy signal at 914 cm−1 in the cured sample of 5 wt% UP toughened epoxy matrix is confirmed.

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      Hence, modification is needed to eliminate such deficiencies and improve the properties. Various approaches have been taken into account like blending either physically or through chemical reactions with other materials, e.g. polyester [25–28], polyetherimide [29–33], polyurethane [34–40], liquid elastomers [41–45], siloxane based polymers [46–51], inorganic fillers and nano particles [52–60], to enhance barrier properties. Among these classes of materials, nano particles have attracted much more attention due to their effectiveness and appropriateness [15].

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