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Mechanical and Electrical Studies of Silicone Modified Polyurethane

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Mechanical and Electrical Studies of Silicone Modified Polyurethane Polymer Journal, Vol. 36, No. 10, pp. 848—855 (2004) Mechanical and Electrical Studies of Silicone Modified Polyurethane–Epoxy Intercrosslinked Networks y Arun ANAND PRABU and Muthukaruppan ALAGAR Department of Chemical Engineering, Anna University, Chennai-600 025, India (Received May 27, 2004; Accepted July 20, 2004; Published October 15, 2004) ABSTRACT: A series of intercrosslinked networks (ICNs) based on silicone modified polyurethane (PU)–epoxy resins were developed. In this study, epoxy resin (diglycidyl ether of bisphenol-A) was modified with PU prepolymer and hydroxyl-terminated polydimethylsiloxane (HTPDMS) using -aminopropyl triethoxysilane ( -APS) as silane cross linker and dibutyltindilaurate (DBTL) as catalyst to form ICNs. Aromatic polyamine adduct (A), diethylenetri- amine (B) and polyamidoamine (C) were used as epoxy curatives. The final products were obtained in the form of tough films. Changes in chemical structure during ICN formation, mechanical and electrical properties were investigated us- ing FT-IR spectra, tensile, impact and dielectric testing. The mechanical properties were enhanced with incorporation of PU (10 wt %) and silicone (10 wt %) due to the toughening of brittle epoxy matrices. Electrical properties showed a marginally decreasing trend with the incorporation of PU (0–20 wt %) influenced by the polar urethane linkages where- as silicone incorporation (10 wt %) showed an enhancement due to the presence of inorganic –Si–O–Si– linkage. Among the systems studied, the silicone (10 wt %) modified PU (10 wt %)–epoxy cured with ‘‘A’’ exhibited excellent mechanical and electrical characteristics and can be used as coatings and composites for industrial, electrical and ma- rine components. [DOI 10.1295/polymj.36.848] KEY WORDS Intercrosslinked Network / Epoxy / Polyurethane / Silicone / Coatings / Composites / Numerous polymeric resins based on epoxy, unsat- used in epoxy and polyurethane synthesis. These mate- urated polyester, phenolic and acrylics are available rials exhibit a high degree of phase separation resulting for composite and coating applications, but they are in poor mechanical properties and compositional het- not completely satisfactory from the viewpoint of me- erogeneity resulting from poor segmental compatibili- chanical and dielectric characteristics. Among them, ty.18 Several techniques were reported in literature to epoxy resin though possessing good adhesion and synthesize PDMS based polyurethane and epoxy res- chemical resistance, its brittle behavior with low im- ins. These techniques include the introduction of polar pact strength and dielectric properties restricts the util- functionality to PDMS,19,20 hard segment modifica- ity for high performance applications.1,2 Many at- tion21 and using silane-coupling agents.12,22 Introduc- tempts have been made in the past to toughen the tion of PDMS into the main chain of epoxy impedes epoxy resin.3–8 A systematic and detailed study is re- surface-enrichment and in order to meet the need for quired to propose suitable chemical modifiers for ep- surface modification, a lot of PDMS must be intro- oxy resin. Modification via blending or the addition duced. Concomitantly, the thermo-mechanical proper- of thermoplastic polymers to the thermoset epoxy is ties declined swiftly with increase of the PDMS soft a possibility. Among the elastomeric modifiers, poly- segment. To overcome this limitation, we previously dimethylsiloxane (PDMS) commercially known as sil- tried amino-terminated PDMS using amino silane cou- icone is regarded as one of the suitable modifiers ow- pling agents. Amino-terminated PDMS bonded to the ing to its inherent characteristics like good wetting and epoxy in the side chain to form a crosslinked struc- film forming ability, good dielectric strength and hy- ture.12 Consequently, the thermo-mechanical proper- drophobic behavior.9–12 Polyurethane (PU) is another ties showed an increasing trend with only a small class of favored polymeric modifier owing to its supe- amount (10 wt %) of silicone needed to meet the mod- rior impact strength, low curing temperature and abra- ification. Stanciu et al.23 reported that hybrid polymer- sion resistant characteristics. PU crosslinked with ep- ic materials resulting from intercrosslinked network oxy reduces phase separation and improves me- (ICN) mechanism in which two or more polymers chanical properties compared to unblended epoxy res- are chemically crosslinked in their main or side chains ins.8,13–17 Incorporation of a macrodiol such as PDMS to form a network structure enhance their mechanical into the epoxy matrix is generally difficult because of and hydrophobicity characteristics than those obtained its poor compatibility with conventional compounds by blending technique. The technology of ICNs as yTo whom correspondence should be addressed (Tel: +91-44-22203543, Fax: +91-44-22352870, E-mail: [email protected]). 848 Silicone Modified Polyurethane–Epoxy ICNs CH CH 3 a 3 CH3 NCO DBTL NHCOO CH2 O OCHN 3 + HO CH2 CH2 CH2 O H n n N2 700C NCO PPG Polyurethane prepolymer NCO NCO 2,4 - TDI b O O O OH O H2C CH HC CH2 H2C HC H2C H2C HC CH2 n n COO Epoxy resin HN R R NH O COO O H C CH HC CH2 c 2 n Polyurethane modified epoxy NH2 o 80 C (CH2)3 HO PDMS OH Si(OEt) 3 (HTPDMS) γ-APS O OH OH O H2C CH CH CH2 CH2 CH HC CH2 n N n PU (CH2)3 PU epoxy O Si O epoxy O PDMS Scheme 1. Formation of silicone modified PU–epoxy ICN. materials for engineering and electrical applications is (NCO/OH ratio 1.5) as epoxy modifier was prepared presently in a state of emergence. from toluene diisocyanate and polycaprolactone gly- 16 In the present study, an attempt has been made to col (Mw 1000) as reported earlier and shown in develop three different epoxy resins (capable of curing Scheme 1a. The resulting PU prepolymer was stored at <30 C): an unmodified epoxy; a PU–epoxy ICN in airtight containers for further usage. Hydroxyl ter- where NCO-terminated PU prepolymer was cross- minated polydimethylsiloxane (HTPDMS, Mw linked with pendant hydroxyl groups in epoxy; and 1;000) also as epoxy modifier (Elkay Silicones, India), a silicone modified PU–epoxy ICN where hydroxyl -aminopropyl triethoxysilane ( -APS, Aldrich, terminated polydimethylsiloxane (HTPDMS) was USA) as silane cross-linking agent and dibutyltindi- crosslinked with oxirane groups in PU–epoxy using laurate (DBTL, Merck, Germany) as catalyst were an silane coupling agent. Thermoplastic PU and sili- used as received. HTPDMS was dried at 90 C under cone are expected to behave as thermosets when inter- vacuum (0.01 Torr) for 4 h prior to usage to remove crosslinked with epoxy using silane-coupling agents volatile impurities. Aromatic polyamine adduct (A, to form ICNs with improved mechanical and electrical HY 2969), diethylenetriamine (B, HY 951) and poly- characteristics. amidoamine (C, HY 840), all obtained from VanticoÒ, India were used as curatives for the epoxy resin. The EXPERIMENTAL nomenclature and stoichiometry of curatives (in pa- renthesis), crosslinking agent and catalyst are present- Materials ed in Table I. The commercially available epoxy resin, ‘GY 250’ [DGEBA, Vantico (India); epoxy equivalent 180{ Development of Polyurethane Modified Epoxy Resin 190, viscosity 9000–12000 cP] was used as a base (X1) material and is referred as ‘X0’. PU prepolymer A typical formulation of epoxy/PU (100/10, AX1) Polym. J., Vol. 36, No. 10, 2004 849 A. ANAND PRABU and M. ALAGAR Table I. The stoichiometric equivalents of curatives, cross-linking agent and catalyst used in modified epoxy systems Epoxy/PU/silicone (w/w) Amine Viscosity Curatives used Code 100/0/0, X0 100/10/0, X1 100/10/10, X2 eq/kg at 30 C (cP) Aromatic polyamine AAXa (60)b AX (54) AX (48) 4.7–5.0 700–900 adduct, HY 2969 0 1 2 Aliphatic amine, BBX(10) BX (9) BX (8) 20.6 10–20 HY 951 0 1 2 Polyamidoamine, CCX(50) CX (45) CX (40) 6.6–7.5 10000–15000 HY 840 0 1 2 -APS — — — 1.0 — — DBTL — — 0.02 0.04 — — aNomenclature of modified epoxy systems. bStoichiometric equivalents of curatives added. was prepared by one-step polyaddition procedure as dumbbell-shaped specimens (150 Â 25 Â 3 mm) as follows: Calculated amount of PU prepolymer per ASTM D 3039 using an Instron testing machine (10 wt %) relative to the epoxy resin (100 w/w) were (Model 6025 UK), at a crosshead speed of 2 mm/ melt mixed for 10 min at 70 Æ 1 C in the presence of min. Izod impact strength of samples (63 Â 10 Â 3 0.02 wt % of DBTL catalyst, cooled to 30 C and mm) was determined as per ASTM D 256-88. Hard- cured using 54 g of HY 2969 (A). PU prepolymer con- ness of the material is measured using a Durometer- tent was varied as 2.5, 5, 10, 15 and 20 wt % in order Type D instrument as per ASTM D 2240. The abra- to optimize the PU content in epoxy resin. The inter- sion resistance was determined by Taber Abrader as cross linking reaction between PU and epoxy per ASTM D 4060 and expressed in terms of weight (Scheme 1b) has been already confirmed.17 Other loss per 1000 number of revolutions under a stated PU modified epoxy resins were prepared in similar load of 500 g. Dielectric strength of the specimen manner with varying the curatives ‘HY 951 and HY disks (100 mm in diameter and 4 mm in thickness) 840’ and referred as ‘BX1 and CX1’ respectively. was measured according to ASTM D 149-87 using Shearing Bridge, Vettiner, France (accuracy 0.08%) Development of Silicone Modified Polyurethane–Ep- at 250 V and 50 Hz. Arc resistance of the specimens oxy Resin (X2) (100 mm in diameter and 4 mm in thickness) was Optimization of HTPDMS incorporation into epoxy measured according to ASTM D 495 using Arc Re- has been reported earlier by the authors.12 A typical sistance Tester (Enamelled Wire Testing Equipments, formulation of epoxy/PU/silicone (100/10/10 by Pune, India) at 250 V and 50 Hz.
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