Polyurethane simultaneous IPNs

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

A more selective approach consists in trying to influence the kinetics of formation of at least one network in this case, the two networks are formed more or less simultaneously, and the resulting morphology and properties can be expected to vary to some extent without changing the overall composition. The same system as previously studied, PUR/PAc, has been utilized in order to prepare a series of in situ simultaneous IPNs (SIM IPNs), by acting essentially on two synthesis parameters the temperature of the reaction medium and the amount of the polyurethane catalyst. Note that the term simultaneous refers to the onset of the reactions and not necessarily to the process. The kinetics of the two reactions are followed by Fourier transform infra-red (FTIR) spectroscopy as described earlier (7,8). In this contribution, the dynamic mechanical properties, especially the loss tangent behavior, have been examined with the aim to correlate the preceding synthesis parameters to the shape and temperature of the transitions of the IPNs. 

By setting the temperature of the reaction medium at 60 C from the beginning of the IPN formation, the PUR synthesis is accelerated, and that of the methacrylic system begins after the usual inhibition period. The competition between the two processes can still favour the complete formation of PUR before appreciable radical copolymerisation may have taken place, though the kinetic curves may change or even cross. For this reason, a second factor, the content of PUR catalyst, is varied too with less stannous octoate, the formation of the first network is more or less delayed, even at 60 C, and counterbalances to some extent the effect of temperature. In such a case, the conversion of the methacrylic phase may proceed further before higher or even post-gel conversions are reached for polyurethane. Thus, IPNs in which both networks have been formed more or less simultaneously, are obtained by this... 

Xie et al. (92, 93) synthesized simultaneous IPN from castor oil polyurethane and copolymers of vinyl monomers, including styrene, methyl methacrylate, and acrylonitrile, without cross-linker using a redox initiator at room temperature and both the formation kinetics of cross-linking and grafting on phase separation were examined. It was demonstrated that the resulting materials were mainly grafted IPN... 

In addition, PET can be used to form semi-IPNs with other naturally functionalized triglyceride oils, such as vemonia oU (31). The procedures for PET/vemonia semi-IPNs are essentially the same as those of PET/castor ones, but with important differences. For PET/castor mixtures, the diisocyanate cross-hnker was added at 240°C and the mixture was poured into the molds rapidly before the castor gel point had been reached. In this case, PET/castor polyurethane semi-IPNs were formed, in which crystallization and gelation occurred simultaneously resulting in a single, broad glass transition temperature. For PET/vernonia, the sebacic acid was added at 280°C, which reduced the temperature to about 250°C, where the mixture was held for another 5 min, then poured into a preheated mold and allowed to cool, during which time the PET crystalhzes. In this case, PET/vernonia polyester network was formed, and the PET crystallized prior to network formation because the latter... 

Simultaneous IPNs are formed by homogeneously mixing together monomers, prepolymers, linear polymers, initiators, and crosslinkers, The monomers and prepolymers are simultaneously polymerized by independent reactions that differ enough to avoid interfering with each other. For example, a polyure-thane/polymethacrylate and a polyurethane/polystyrene were made in a process in which both monomers were prepolymerized, dissolved together, and reacted to form an IPN. Another urethane system was made from castor oil reacted with toluene diisocyanate and sebacic acid polyesters. The resultant urethane prepolymer was then mixed with polystyrene to form an IPN. 

A series of castor oil polyurethane/poly(methyl methacrylate) interpenetrating polymer networks (IPNs) and gradient IPNs, cured at room temperature, were prepared by a simultaneous IPN method, and nanocomposites with BaTiOs superfine fibre were reported for the systems. A dose-dependent improvement in thermoelectric and mechanical properties was observed in the nanocomposites compared to the pristine systems. 

Simultaneous IPNs involve monomers or reactive oligomers and crosslinkers of two or more reactive systems. These systems are generally chosen such that the reaction of one component does not interfere with or is involved with the reactions of the second component. Otherwise, grafting reaction would compete with interlocking ring formation as the method of compatibilization. An example of a simultaneous IPN is the reaction of free radical polymers (such as polyacrylates) in the presence of condensation polymers such as polyurethanes, as has been the subject of many investigations [171-174]. A PU/PMMA simultaneous IPN exhibited transparency and showed only limited phase separation below 30% PMMA [171]. This IPN... 

Simultaneous IPNs Based on Polyurethane and Poly(urethane acrylate). . 77... 

Polyurethane-acrylic coatings with interpenetrating polymer networks (IPNs) were synthesized from a two-component polyurethane (PU) and an unsaturated urethane-modified acrylic copolymer. The two-component PU was prepared from hydroxyethylacrylate-butylmethacrylate copolymer with or without reacting with c-caprolactonc and cured with an aliphatic polyisocyanate. The unsaturated acrylic copolymer was made from the same hydroxy-functional acrylic copolymer modified with isocyanatoethyl methacrylate. IPNs were prepared simultaneously from the two-polymer systems at various ratios. The IPNs were characterized by their mechanical properties and glass transition temperatures. 

In this work, polyurethane (PU) and epoxy (EP) mixtures were selected for investigation because they are known to form partially miscible IPNs with broad glass transition temperatures. These were first prepared by Frisch et al(6) using a simultaneous polymerization technique in bulk. These materials showed the effects of cross-linking only one polymer component (pseudo-IPN) and intentional grafting between the component polymers. Klempner et al (2) also studied PU/EP IPNs for vibration attenuation. The polyurethanes in this work were chain extended and crosslinked with a 4 1 equivalent ratio of butanediol (BD) and trimethylol propane (TMP). 

Simultaneous PU/EP IPNs were prepared by a casting technique. The polyol was heated to 100-110 C at reduced pressure for one hour. It was cooled to 60 C and the isocyanate and epoxy resin were added. The preheated, premixed mixture of extender and crosslinking agents for polyurethane and epoxy were then added and mixed thoroughly. The mixture was poured into a preheated mold, allowed to gel, and cured for 16 hours at 110 C. Rods of approximately 3/16 diameter X 12 were also produced for dynamic mechanical testing. 

Other synthetic approaches to the kinetic problem have been taken. Variations in catalyst concentration for the formation of each component network from linear polyurethanes and acrylic copolymers have been used along with a rough measure of gelation time (5) to confirm the earlier (2-3.) results. Kim and coworkers have investigated IPNs formed from a polyurethane and poly(methyl methacrylate) (6) or polystyrene (7) by simultaneous thermal polymerization under varied pressure increasing pressure resulted in greater interpenetration and changes in phase continuity. In a polyurethane-polystyrene system in which the polyurethane was thermally polymerized followed by photopolymerization of the polystyrene at temperatures from 0 to 40 C, it was found (8.) that as the temperature decreased, the phase-... 

Two- and three-component interpenetrating polymer network (IPN) elastomers composed of polyurethanes (PU), epoxies (E), and unsaturated polyester (UPE) resins were prepared by the simultaneous technique. Fillers and plasticizers were... 

Although interpenetrating polymer networks (IPNs) are now beginning to be commercially exploited, little is known about many types of engineering behavior, such as fatigue. In this paper, energyabsorbing simultaneous interpenetrating networks (SINs) based on polyether-type polyurethanes (PU) and poly(methyl methacrylate)... 

The effect of fillers on the reaction of polymer formation was discussed in Chapter 4. It is evident that introducing a filler during IPN formation should also lead to its influence on the rates of the IPN formation. This influence should affect the possibility of microphase separation. This question was studied " for simultaneous semi-lPN based on a crosslinked polyurethane and linear PBMA. The ratio PU PBMA was 3 1, the ratio IPN flller was 60 40 and 80 20 by weight. It was established that the onset of auto-acceleration of the butyl methacrylate polymerization increases from 160 min without filler to 220 min in the presence of a filler (talc). After the onset of auto-acceleration, the reaction rate of butyl methacrylate pol5mierization decreases with the increase of amount of filler. The filler influence on the reaction kinetics was explained based on the so-called... 

Most recently, two other groups have undertaken the study of castor oil-based IPN s. Tan and Xie investigated the conditions for formation of castor oil polyurethanes with homopolymers or copolymers of styrene, methyl methacrylate and acrylonitrile. Simultaneous interpenetrating polymer networks were made which had high strength, good resilience and high resistance to abrasion and hydrolysis. 

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