Polyurethanes photocurable

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

As discussed in Chapter 7, radical polymerization can induce marked inhomogeneities, easily observable by DMTA. For example, in photocured polyurethane-acrylate networks, two a peaks corresponding to distinct phases, separated by about 40-100 K, depending on composition and cure conditions, can be observed in the DMTA dissipation spectrum at 10 Hz (Barbeau et al., 1999). 

Polymerization of epoxy monomers, requiring probably the simultaneous participation of the phenolic hydroxy group, and the production of polyurethanes are the main applications of the abovementioned catalysts. In particular, their role in the trimerization of the i.scx yanate group " appears quite relevant and several investigations of the synthesis of epoxy resins-- " - and polyurethanes- arc reported. A different type of catalyst is constituted by acetophenone-derived p-aminoketones employed with photocurable resins. -- ... 

For photocured polymer materials, the Persoz hardness was shown to depend mainly on the chemical structure of the prepolymer chain, on the functionality of the reactive diluent, on the photoinitiator system and on the duration of the UV exposure. The hardness of tack-free coatings thus continues to increase with further irradiation, as shown by Figure 6 for polyurethane-acrylates. This slow hardening process is likely to result from some further polymerization of the unreacted acrylic functions since their concentration was found to decrease concomittantly. 

Vinyl ethers (VE) are amongst the most reactive monomers that polymerise cationically, and they are increasingly used in photocurable resins, either neat [63] or in association with epoxides [64,65], acrylates [66], maleimides [67] or unsaturated polyesters [68]. Vinyl ether end-capped polyurethane, polyether, polysiloxane and polyesters were shown to undergo fast and extensive polymerisation when exposed to UV-radiation in the presence of an aryliodonium salt, with the formation of tridimensional polymer networks. The photoinitiated catioiuc polymerisation of a divinyl ether can be formally written as follows ... 

Bonnet has described photocurable, colored urethane - acrylate resins from sequential copolymerization of (XIl) with diisocyanates and multifunctional acrylates.Polyurethanes (XIIl) and polyureas (XIV) beaxing triphenylmethane and anthreiquinone dyes may be used for dyeing of natiaral and synthetic fibers or as electrophotographic toner materials. 3... 

Polyurethane acrylate resins cured by two different methods, thermally and photochemically, are investigated by means of differential scanning calorimetry and thermally stimulated depolarisation currents. Even if both curing methods lead to the same material from a chemical point of view, important differences exist between the thermocured resin and the photocured resin in terms of molecular relaxation behaviour. 8 refs. 

A typical formulation of a photocurable composite resin contains four basic components a radical-type photo initiator, an acrylate functionalized oligomer, a reactive diluent and the clay mineral filler. The photoinitiator is usually an aromatic ketone which cleaves into two radical fragments upon UV exposure. The telechelic oligomer consists of a short polymer chain (polyurethane, polyether, polyester) end-capped by the very reactive acrylate double bond. An acrylate monomer is generally used as reactive diluent to reduce the resin viscosity. Figure 7.2 shows some typical compounds used in UV-curable acrylic resins. Different types of phyllosilicates were selected as mineral filler an organophilic clay (Nanomer I-30E from Nanocor), native hydrophilic clays (montmorillonite KIO and bentonite) and a synthetic clay (beidellite). 

---