Blocked diisocyanate-modified epoxy resin: Mechanism and mechanical properties

The modification of bisphenol-A-based epoxy resin with isocyanates was investigated. To increase the crosslinking density by a urethane reaction and to conduct the normal cross-linking reaction between the epoxide and the amine during the cure cycle, blocking the isocyanates with active hydrogens is necessary, which may be deblocked and reacted at elevated temperature. The modified reactions involving diisocyanates and hydroxyl groups generated from epoxy cured with an amine system were studied. This mechanism can be identified by the variations of infrared spectra in the carbonyl group stretching region. Furthermore, the effect of the blocked isocyanate incorporation with amine on the reactivity of epoxy was also studied. The thermal and the mechanical properties were characterized. It was found that the glass transition temperature increased with an aromatic blocked-diisocyanate content. The mechanical properties were improved by blocked diisocyanate. The morphology was also investigated, which showed that a homogeneous structure existed in the modified system. © 1996 John Wiley & Sons, Inc.     

13C NMR investigation of the reaction in water of UF resins with blocked emulsifiable isocyanates

A solid state ¹³C NMR study of hardened networks obtained by the reaction of blocked and nonblocked isocyanates (pMDI) with urea‐formaldehyde (UF) resins in water showed different results according to the temperature of the reaction. At high temperature, in water, both a nonblocked or an emulsifiable, blocked isocyanate, appear to crosslink with UF resins through the formation both of traditional methylene bridges connecting urea to urea and of urethane bridges. The latter have been confirmed by ¹³C NMR to form in water by reaction of the isocyanate ? N?C?O group with the hydroxymethyl groups of the UF resin. At ambient temperature, UF/pMDI resins where the pMDI is a emulsifiable blocked isocyanate, do not appear to form urethanes to any great extent but rather to crosslink through the usual UF resin urea to urea methylene bridges. Even in this case, when urethane bridges appear to be absent, evidence of crosslinking in water through reaction of the isocyanate with the ? NH₂ and ? NH? amide of the UF resin has not been observed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 589–596, 2006     

特种聚氨酯改性环氧树脂的研究

研究了在环氧树脂体系中,引入特种聚氨酯预聚体作为改性剂制备聚氨酯改性环氧树脂。通过红外图谱来表征改性的环氧树脂表明,改性是由环氧树脂上的羟基和聚氨酯预聚体上的异氰酸根两者交联来实现的,聚氨酯改性的环氧树脂其主体是环氧树脂,改性后环氧基团未发生变化。由性能测试可知,与改性前环氧树脂相比,经改性后环氧树脂基本性能未发生变化,但具有良好疏水性能。     

Synthesis and properties of urethane elastomer-modified epoxy resin having hydroxymethyl group

Synthesis and properties of urethane elastomer-modified epoxy resins were studied. The urethane elastomer-modified epoxy resins were synthesized by the reaction of a 4-cresol type epoxy compound having hydroxymethyl groups (EPCDA) with isocyanate prepolymer. The structure was identified by IR, ¹H NMR and GPC. These epoxy resins (EPCDATDI) were mixed with a commercial epoxy resin (DGEBA) in various ratios. The mixed epoxy resins were cured with a mixture of 4,4′-diaminodiphenylmethane and 3-phenylenediamine (molar ratio 6:4) as a hardener. The curing behaviour of these epoxy resins was studied by DSC. The higher the concentration of EPCDATDI, the higher the onset temperature and the smaller the rate constant (k) of the exothermic cure reaction were. It was considered that the ratio of hydroxymethyl group to epoxide group was very small and the molecular weight of EPCDATDI was large. Therefore, the accelerating effect of the hydroxymethyl group on the epoxide–amine reaction was cancelled by the retardant effect of increased molecular weight and viscosity, and decreased molecular motion. Toughness was estimated by Izod impact strength and fracture toughness (K_1C). On addition of 10 wt% EPCDATDI with low molecular weight (M?_n 6710, estimated by GPC using polystyrene standard samples), Izod impact strength and K_1C increased by 70% and 60%, respectively, compared with unmodified epoxy resin. Glass transition temperatures (T_g) for the cured epoxy resins mixed with EPCDATDI measured by dynamic mechanical spectrometry were the same as those of unmodified epoxy resin. The storage modulus (E′) at room temperature decreased with increasing concentration of EPCDATDI. Toughness and dynamic mechnical behaviour of cured epoxy resin systems were studied based on the morphology.     

Modification of epoxy resins with polysiloxane TPU for electronic encapsulation. II

Polyol or polysiloxane thermoplastic polyurethanes (TPU) were used to reduce micro-cracking in cresol–formaldehyde novolac epoxy resin cured with phenolic Novolac resin for electronic encapsulation application. A stable dispersion of TPU particles in an epoxy resin matrix was achieved via the epoxy ring opening with isocyanate groups of urethane prepolymer to form an oxazolidone. The effects of structure and molecular weight of TPU in reducing the stress of electronic encapsultant were investigated. The mechanical and dynamic viscoelastic properties and morphologies of TPU modified epoxy networks were also studied. A “sea-island” structure was observed via SEM. The dispersed polysiloxane TPU rubbers not only effectively reduce the stress of cured epoxy resins, by reducing flexural modulus and the coefficient of thermal expansion, but also increase the glass transition temperature because of the rigid oxazolidone structure formation. Electronic devices encapsulated with the polysiloxane TPU modified epoxy molding compounds exhibited excellent resistance to the thermal shock cycling test and resulted in extended device life. © 1996 John Wiley & Sons, Inc.     

Kinetic study of the urethane and urea reactions of isophorone diisocyanate

Kinetic studies of the catalyzed urethane reactions between isophorone diisocyanate (IPDI) and alcohols and of the urea reactions between an isocyanate‐terminated prepolymer [IPDI–PPG2000–IPDI, where PPG2000 is poly(propylene glycol) with a number‐average molecular weight of 2000 g/mol] and water in the bulk state were performed with Fourier transform infrared (FTIR) spectroscopy. Dibutyltin dilaurate was used as the catalyst for the urethane reaction, and various tertiary amines were used as catalysts for the urea reactions. The reactions were followed through the monitoring of the change in the intensity of the absorbance band for NCO stretching at 2270 cm^?1 in the FTIR spectra; the activation parameters were determined through the evaluation of the kinetic data obtained at various temperatures (within the range of 30–60°C). The kinetic data indicated that the catalyzed isocyanate/alcohol and isocyanate/water reactions both followed second‐order kinetics during their initial stages but later followed third‐order kinetics resulting from the autocatalytic effects of hydrogen bonding between the hydroxyl groups and the newly formed urethane and urea groups. Furthermore, activation energies of 64.88 and about 80 kJ/mol for the isocyanate/alcohol and isocyanate/water reactions, respectively, indicated that the urea‐forming reactions were more sensitive to the reaction temperature than the urethane‐forming reactions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Hybridization of aqueous PU/epoxy resin via a dual self‐curing process

Amino‐terminated and carboxyl‐containing polyurethane (PU) is prepared by an isocyanate‐terminated PU prepolymer process. Carboxyl‐containing epoxy resin is obtained from a half‐esterification of epoxy resin with maleic anhydride. These two aqueous resins are obtained after neutralization with triethylamine and dispersion into water phase, respectively. A latent curing agent (TMPTA‐AZ) is prepared by a Michael addition of aziridine with trimethylolpropane triacrylate (TMPTA). A self‐curing system of PU/epoxy hybrid is obtained from a blending of these two aqueous resins with latent curing agent. PU/epoxy hybrid is derived from two self‐curing reactions on drying. The first curing for hybridization between PU amino groups with oxirane groups of epoxy resin is via a ring‐opening reaction and the secondary curing takes place on carboxyl groups of PU/epoxy hybrid with aziridine of TMPTA‐AZ. The final properties of these dual self‐cured PU/epoxy hybrids are reported. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Hydrogen bonding effects on aspartate ester reactions

Michael addition reactions were utilized to form AB₂ type monomers and esterified to form polyaspartate esters (PASPE). FTIR and NMR spectroscopic methods indicate that a relatively linear architecture was produced that contrasts with an expected hyperbranched (HB) structure. Linear chains formed due to a reactivity difference between the hydrogen bonded (H-bonded) and non-H-bonded esters. The esterification reaction was directed toward the H-bonded ester as shown by 2D NMR correlation spectroscopy. A model is proposed to explain the observed increase in reactivity of the H-bonded ester. Hydrogen bonding (H-bonding) in both the monomeric and polymeric aspartate (ASP) esters was analyzed by 2D NMR spectroscopy. The difference in chemical shifts of the methylene protons before and after reaction was attributed to the geometrical effects of the five-membered ring formed from H-bonding. Monomeric aspartate esters (ASPE) were found to have the greatest difference in chemical shift, while the in-chain H-bonded protons of the PASPE were observed to have the least difference in chemical environments. Internal H-bonding in the ASPE affected its reaction with an aliphatic isocyanate (NCO) due to varying reactivities of the primary hydroxyl (OH) compared to the secondary amine (NH). Internal hydrogen bonding of the OH groups with the amine may also explain unexpected relative reactivities with isocyanates. Both NMR and FTIR spectroscopy indicated that the OH group was exclusively consumed with no NH reaction product detected. A lack of hydantoin ring formation upon further heating the NCO/ASPE reaction product proved that the OH group was more reactive. In an equimolar reaction of ASPE with cyclohexyl isocyanate, NMR and FTIR measurements showed quantitative formation of the urethane adduct instead of the expected urea.     

Coatings of PDMS-modified epoxy via urethane linkage: Segmental correlation length,phase morphology,thermomechanical and surface behavior

Modification of an aliphatic epoxy resin with silicone was carried out through urethane route. Hydroxy-terminated polydimethylsiloxane (HTPDMS) was reacted with toluene diisocyanate to form an isocyanate-capped prepolymer. The –NCO group of the prepolymer was further reacted with –OH group of an aliphatic epoxy adipate resin, obtained by reaction of adipic acid with an epoxy resin, to obtain silicone-modified epoxy resin. Silicone-modified epoxy resins containing 15 and 30 wt% of silicone prepolymer have been synthesized by this procedure. TGA analysis of the silicone-modified epoxy resin showed considerable improvement in thermal stability over the unmodified epoxy resin. Modulated differential scanning calorimetry (MDSC) was used to find the segmental correlation length of the silicone-modified polymers. The cured films of silicone-modified resins showed biphasic morphology as evidenced from scanning electron microscopy. Further, the thermomechanical property of the samples was investigated by dynamic mechanical analysis. The surface properties of the silicone-modified matrices were studied by atomic force microscopy and contact angle goniometry. The silicone-modified samples showed considerably lower surface energy, surface roughness and contact angle hysteresis compared to the unmodified resins.     

Synthesis of aralkyl novolac epoxy resins and their modification with polysiloxane thermoplastic polyurethane for semiconductor encapsulation

A series of phenol‐based and naphthol‐based aralkyl epoxy resins were synthesized by the condensation of p‐xylylene glycol with phenol, o‐cresol, p‐cresol, or 2‐naphthol, respectively, followed by the epoxidation of the resulting aralkyl novolacs with epichlorohydrin. The incorporation of stable dispersed polysiloxane thermoplastic polyurethane particles in the synthesized epoxy resin's matrix was achieved via epoxy ring‐opening with the isocyanate groups of urethane prepolymer to form an oxazolidone. The mechanical and dynamic viscoelastic properties of cured aralkyl novolac epoxy resins were investigated. A sea‐island structure was observed in all cured rubber‐modified epoxy networks via SEM. The results indicate that a naphthalene containing aralkyl epoxy resin has a low coefficient of thermal expansion, heat resistance, and low moisture absorption, whereas phenol aralkyl type epoxy resins are capable of imparting low elastic modulus result in a low stress matrix for encapsulation applications. Modification of the synthesized aralkyl epoxy resins with polysiloxane thermoplastic polyurethane have effectively reduced the stress of cured epoxy resins, whereas the glass transition temperature was increased because of the formation of the rigid oxazolidone structure. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1905–1916, 1999     

Synthesis and photosensitive properties of water soluble and photocrosslinkable polymers from epoxy phenolic resin

Water soluble, photocrosslinkable prepolymers were synthesized from epoxy phenolic resin (epoxy novolak resin) via ring‐opening reaction with acrylic acid, followed by ring‐opening reaction with aqueous solution of triethylamine hydrochloride. No phase transfer catalyst was needed to accelerate the second reaction, since the product formed can act as a phase transfer catalyst. The prepolymer obtained contains both photocrosslinkable acrylate groups and hydrophilic quaternary ammonium salt groups. Optimum conditions for these reactions were studied. The photosensitive properties of the prepolymer were also investigated. The effect of different photoinitiators, different crosslinkable diluent monomers and amine accelerator on the photosensitive properties of the prepolymer were compared. The photoinitiator of hydrogen abstraction type is still effective without using amine or alcohol as accelerator, because the prepolymer contains αH beside the OH groups formed in the ring‐opening reactions.     

An Infrared Spectroscopic Study of Chemically Modified Chemithermomechanical Pulp

FT-IR spectroscopy was used to characterize chemically modified wood pulp and to judge the efficiency of different pulp modification processes. Treatment of wood pulp with aliphatic anhydrides was shown to be more effective in a polar solvent such as DMF compared to a non-polar one typified by xylene. The esterification of hydroxyl group associated with the cellulosic material was indicated by the characteristic absorption bands of the resulting cellulosic esters; the degree of hydroxyl conversion was determined by the ratio of peak intensity of hydroxyl and carbonyl stretching vibrations. The so-called “Infrared Acetyl Index” was shown to give a linear correlation with the gravimetrically determined anhydride uptake. Isocyanate modification of wood pulp yielded more complex infrared spectra, because of various secondary reactions of the reactive isocyanate component. Various aromatic isocyanates, used for chemical treatment of the wood pulp, led mainly to urethane formation, as evidenced by the spectra of the modified pulp. The presence of polyurea or urea as the main reaction product of aromatic isocyanates, postulated by a number of authors, could not be confirmed. Nevertheless, phenylisocyanate treatment, at high concentration of isocyanate and without purification of the modified pulp, led to the appearance of at least one more reaction product in the pulp spectra, which is presumed to be either triphenylisocyanurate, the trimerization product of phenylisocyanate, or carbanilide.     

Novel water-dispersible glycidyl carbamate (GC) resins and waterborne amine-cured coatings

Water-dispersible glycidyl carbamate (GC) functional resins were synthesized and crosslinked using a water-dispersible amine to form coatings. GC functional resins are synthesized by the reaction of an isocyanate functional compound with glycidol to yield a carbamate (urethane) linkage (–NHCO–) and reactive epoxy group. The combination of both functionalities in a single resin structure imparts excellent mechanical and chemical properties to the coatings. Previous studies on the development of GC coatings have focused on solvent-borne coating systems. In this study, GC resins were modified by incorporating nonionic hydrophilic groups to produce water-dispersible resins. To determine the influence of the content of hydrophilic groups on dispersion stability, aqueous dispersions were made from a series of hydrophilically modified GC resins and characterized for particle size and dispersion stability. The composition of a typical, dispersed GC resin particle was predicted using Monte Carlo simulations. Stable GC dispersions were used to prepare amine-cured coatings. The coatings were characterized for solvent resistance, water resistance, hardness, flexibility, adhesion, and surface morphology. It was observed that GC resins were able to be dispersed in water without using any surfactant and by minimal mixing force (hand mixing) and produced coating films with good properties when crosslinked with a compatible waterborne amine crosslinker.     

The effect of different ammonium structure of quaternary ammonium resin on the electrochemical properties

The quaternary ammonium resin is synthesized by the ring-opening reaction of an epoxy resin with a tertiary amine in the presence of a proton donor in the solution. This kind of resin can be dispersed in the water phase to form a conductible milky dispersion. In this study, different kinds of tertiary amines including the full-alkyl group and the ester group-containing or urethane group containing tertiary amine are reacted with DGEBA-type epoxy resin to synthesize the quaternary ammonium resins. The resin characteristics and the electrochemical properties of its emulsion are investigated. In addition, properties of the emulsion prepared from tertiary amine salt resin are also measured for comparison. It is found that if the substituted groups of the ammonium structure of the quaternary ammonium resin are all alkyl groups, rupture voltage of the emulsion is very low. But when one of the substituted group contains an ester group or an urethane group, rupture voltage increases remarkably and the resin can be used for electrodeposition. Meanwhile, the electrodeposition theory of quaternary ammonium resin and its thermal cross-linking reaction and the electrochemical properties of coemulsion prepared by mixing the quaternary ammonium resin and the tertiary amine salt resin are also discussed. © 1993 John Wiley & Sons, Inc.     

Study of film properties of some urethane oils

In this study, sunflower oil-based urethane oils were obtained from three kinds of isocyanate components: toluene diisocyanate (TDI), hexamethylene diisocyanate (HMDI) and poly(1,4-butandiol) toluene 2,4-diisocyanate (PBTDI) terminated prepolymer. The polymers were prepared at four different ratios of isocyanate component/oil. Sunflower and linseed oil alkyd resin samples were also prepared as the comparative samples. The results suggest that the viscosity and the film properties of urethane oils depend on the amount and type of isocyanate component. The increase in isocyanate content of the urethane oils caused high viscosity. In comparison with the samples having the same oil content, PBTDI-based samples showed the highest viscosity. Viscosity of the polymers can dramatically affect some film properties. For example, high polymer viscosity caused short drying time. In comparison of alkali and water resistances of urethane oils with those of alkyd resins, better results were obtained depending on the structure of the urethane oils. On the other hand, alkyd resins and TDI-based polymers exhibited the best hardness properties. Chemical Engineering Dept, Maslak 80626Istanbul, Turkey; email: guners@itu.edu.tr.     

Formation of cross-links during prepolymerisation reaction of methylene-bis (4-phenylisocyanate) with poly (tetramethyleneoxide) in N,N-Dimethylacetamide

An attempt was made to investigate the cross-linking reaction which occurs as a side reaction when an excess of methylene-bis(4-phenylisocyanate) (MDI) is reacted with polytetramethyleneoxide (PTMO) (number-average molecular weight 2010) in N,N-dimethylacetamide (DMAc) prepolymerisation reaction) to give an isocyanate-terminated prepolymer having urethane groups. Cross-links existing in the prepolymer were shown to be of the biuret type by high performance liquid chromatography (HPLC) analysis on a product prepared by the raction of excess phenylisocyanate with n-butanol in DMAc. The biuret-forming reaction, as well as the urethane reaction (i.e. the main reaction), is accelerated remarkably by an N,N-dimethyl-N'-phenyl(phenylcarbamoyl)acetamidine derivative formed from a side reaction of MDI with DMAc, and the former reaction is suppressed dratically by neutralising the corresponding amidine with, for ecample, H₂SO₄. A novel kinetic model of the prepolymerisation reaction is proposed and reaction orders and rate constants are derived. Three heterocompetitive reactions are involved: (1) urethane groups formed from MDI with PRMOin the prepolymer, and subsequent formation of (2) biuret type cross-links between isocyanates and urea groups formed from MDI with DMAc as a side reaction, and (3) the corresponding amidine derivative.     

Emulsion of polyurethane having thermosetting properties. I. Preparation

A method of preparation of an urethane emulsion which provides thermosetting film is described here. By reacting a urethane prepolymer containing terminal isocyanate groups with diethylenetriamine in the presence of ketones, poly(urethane-urea-amine) was prepared. In other organic solvents, however, gelation occurred immediately and preparation of polyurethane-urea-amine was not successful. The function of ketone was ascribed to the tentative formation of a Schiff base between the primary amino group of diethylenetriamine and ketone in situ, and the reaction of primary amine with isocyanate was partly masked to prevent gelation. Then the free amino group is generated when the polymer is treated with epichlorohydrin and the reaction occurs between them. After the solution was neutralized with an aqueous acid, the solvent was removed in vacuo to give a stable self-emulsifiable thermosetting urethane emulsion. Typical mechanical properties of film from this urethane emulsion are also given here.     

Novel moisture-cured hyperbranched urethane alkyd resin for coating application

A hyperbranched polyol (HBP) was synthesized using dipentaerythritol as a core material and 2,2-bis(methylol) propionic acid as a chain extender. This was reacted with varying concentrations of soya fatty acid to make hyperbranched alkyd (HBA) resins. The HBA resins containing unreacted hydroxyl groups were reacted with isophorone diisocyanate at NCO/OH ratio of 1.6:1 to make high solid hyperbranched urethane alkyd (HBUA) resins. The excess NCO content in the HBUA resins was used to cure with atmospheric moisture, and thus moisture-cured HBUA coatings were formed. The resins were characterized by FTIR, and ¹³C NMR spectroscopic analysis. A series of such resins were made using different fatty acid/isocyanate ratios with respect to the hydroxyl groups present in the HBP. The effect of compositions on the mechanical and weathering properties of the cured resins was investigated. It was observed that there was an optimum fatty acid–isocyanate ratio in terms of the requirements of solvent, mechanical and weathering properties of the resin. The requirement of solvents for formulating HBUA coatings is much lower compared to linear alkyd-based coatings. The present study reveals that the moisture-cured HBUA resins can be used as a binder material in the field of low-pollution weather-resistance coatings.     

15N CP/MAS NMR Study of the Isocyanate/Wood Adhesive Bondline. Effects of Structural Isomerism

Structurally isomeric ¹⁵N-labeled polymeric methylenebis(phenylisocyanate), pMDI, adhesives were synthesized. One resin had a high content of 4,4'-MDI, and another was prepared with a high content of 2,4'-MDI. Both resins were cured with wood (Liriodendron tulipifera) for various times and temperatures and then analyzed using ¹⁵N CP/MAS NMR. It was found that resin polymerization occurs via the reaction of isocyanate with wood moisture to form polyurea. Biuret formation and isocyanate dimerization were detected. Urethane formation probably also occurs; however, signal overlap of urea and urethane signals prevents a definitive conclusion. These findings are similar to previous ones; however, subtle differences are noted. The structurally isomeric resins displayed similar chemistries. Of the two resins, the resin prepared with a high content of 2,4'-MDI cured more slowly, and resulted in a network that was more mobile in the midkilohertz frequency range. This leads to the prediction that resins high in 2,4'-MDI may have a superior performance in impact loading.     

Microwave assisted synthesis and determination of chain branching in solid epoxy resins using H NMR spectrometry

The degree of branching in solid epoxy resins (epoxy value EV=0.11 mol/100 g) prepared under microwave and conventional conditions has been investigated by ¹H NMR spectrometry. The applied method involves rapid reaction between trichloroacetyl isocyanate and hydroxyl groups incorporated in the epoxy resin structure. Solid epoxy resins were prepared by polyaddition of Bisphenol A (BPA) to a lower-molecular-weight (L-M) epoxy resin (Rütapox 0162, EV=0.57) in the presence of 2-methylimidazole (2-MI) as a catalyst. All the microwave reactions were performed in the multimode microwave reactor Plazmatronika-Poland with microwave frequency of 2.45 GHz and maximum of microwave power of 600 W. The results show that the degree of branching in resins synthesized under microwave irradiation is comparable with those obtained under conventional heating (i.e. 6–12%) and is not influenced by the reduction of reaction time.