Dual curing

Polyols used for this purpose include trimethylol propane and propylene glycol. Systems containing urethane-acrylate oligomers bearing doubly-functionalized isocyanate groups are commercially available [24]. The chemical structure 

Chart 11.4 Chemical structures of oligomers bearing acrylate and isocyanate groups isocyanato-allophane acrylate (left) and isocyanato-urethane acrylate (right) [24]. 

Chart 11.5 Part of a three-dimensional network formed by UV irradiation and thermal treatment of a dual-cure acrylic urethane system [27]. 

Another mode of dual curing involves the simultaneous occurrence of free radical and cationic radiation-induced cross-linking polymerization of formulations containing appropriate initiators [20, 23, 28]. This method, which is called hybrid curing, leads to coatings with imique properties. A typical hybrid-cure system contains a diacrylate and a diepoxide, the former polymerizing by a free radical and the latter by a cationic mechanism. Exposure of the system to in- 

Chart 11.6 Segments of network structures formed by the radical polymerization of a diacrylate (top) and the cationic polymerization of a biscycloaliphatic diepoxide (bottom) [27]. 

---

Dual cure sites Ductile iron Ductile particles Duct silencers Dugro [7681-76-7]... 

In order to enhance the reactivity of the chlorine atom, a second reactive monomer can be adopted giving dual cure sites. According to the Hterature, the second monomer can contain carboxyl (22—24), cyanoalkyl (25), hydroxypropyl (26), or epoxy groups (27,28). 

New efficient vulcanization systems have been introduced in the market based on quaternary ammonium salts initially developed in Italy (29—33) and later adopted in Japan (34) to vulcanize epoxy/carboxyl cure sites. They have been found effective in chlorine containing ACM dual cure site with carboxyl monomer (43). This accelerator system together with a retarder (or scorch inhibitor) based on stearic acid (43) and/or guanidine (29—33) can eliminate post-curing. More recently (47,48), in the United States a proprietary vulcanization package based on zinc diethyldithiocarbamate [14324-55-1]... 

Blocked isocyanates are particularly helpful in dual cure mechanisms. In one instance, UV light first polymerizes an acrylate polymer containing hydroxyl groups. The system also contains a malonate ester-blocked isocyanate. The one-component system is heated, which starts the polymerization of the acrylate. Higher temperatures unblock the isocyanate, permitting the cure of the urethane to proceed [15]. 

Goodman, D.L. and Byrne, C.A., Phase-separated dual-cure elastomeric adhesive formulations and methods of using the same. US Patent 5,997,682. 

Another development has been the advent of the dual-cure resin cements. These are hybrids of glass polyalkenoate cements and methacrylates that set both by an add-base cementation reaction and by vinyl polymerization (which may be initiated by light-curing). In these materials, the solvent is not water but a mixture of water and hydroxyethylmethacrylate which is capable of taking dimethacrylates and poly(acrylic add)-containing vinyl groups into solution. In the absence of light these materials set slowly and... 

One of the most interesting recent developments has been the advent of the resin glass polyalkenoate cements (Antonucci, McKinney Stansbury, 1988 Mitra, 1989 Wilson, 1989, 1990 Mathis Ferracane, 1989 Minnesota Mining Manufacturing Company, 1989 Albers, 1990). They are dual-cure hybrids that set by a combination of acid-base and polymerization reactions, and there are several types. Polymerization is effected by either chemical or light initiation. 

Dual Photo/Thermal Initiation Studies. A series of studies were performed using reactive formulations containing both a photoinitiator and a thermal initiator dissolved in the Derakane resin. The objective of these studies was to investigate a dual cure strategy in which the heat liberated by the photo-induced polymerization leads to the production of additional active centers by the dissociation of a thermal initiator. In this way, the dual cure strategy could offer both the temporal control of the start of the reaction afforded by the photopolymerization, as well as enhanced reaction rate and completeness of cure provided by the thermal initiation. 

Sheet molding compound (SMC) is a composite material that has been used as an alternative to steel for more than 30 years. SMC consists of a glass-fiber-filled unsaturated polyester and vinyl ester resin that has been compacted into a sheet. These sheets are then placed into compression molds and formed into body panels, and these panels are then coated mainly by dual-cure coatings. ... 

Recently a solvent-free dual-cure laminating adhesive based on the combination of polyurethane chemistry and EB cure was developed that exhibits improved properties compared to laminates prepared by either method alone. ... 

Dual-cure resins for wood composites. These resins (heat and EB irradiation systems) are based on urethane-acrylate thermoplastic adhesive demonstrate efficacy to bond wood. Eow amounfs of EB radiation were needed to produce a wood bond that survived shipping and that improved the composite modulus. The system demonstrated promise and a new direction for wood adhesives. 

The problems associated with the multifunctional curing agents for CTPB and the resultant aging behavior of the cured polymers have led to a practical solution for curing binders and propellants—i.e., using mixed aziridines or a mixture of an aziridine and an epoxide. Such mixtures, when appropriately balanced, usually provide satisfactory mechanical behavior and high temperature stability. In dual curing systems such as MAPO and BITA or MAPO and a suitable multifunctional epoxide,... 

Madhusoodhanan S, Nagvekar D. (2008) Dual cure digital inks for industrial printing. RadTech UV/EB Technology Expo and Conference 2008, Chicago. 

The polyols can have a functionality of two or greater. Higher functionality results in increased viscosities (or reduced molecular weights) and increased cross-link density. Likewise, the isocyanate can have a functionality of two or greater. Epoxy systems have been incorporated into the polyol component to offer dual curing mechanisms and increased product-resistance properties. 

Klosterman et al. (2003) examined the electron-beam curing of a dual-curing (the monomer contains both acrylate and acetylene reactive groups) liquid-crystal (LC) resin. It... 

Timpe classified the onium salts as bivalent initiators , that is, capable of initiating both radical and cationic cures simultaneously, hence capable of so-called dual cures of mixtures of radical and cationically polymerizable monomers [18]. This theme has been developed in the more recent review by Rabek [19]. More recently, use of photogenerated Bronsted or Lewis acids to effect decomposition of acid-labile polymers has found utility in photoimaging and been reviewed [20],... 

Dual Cure Photopolymerization The dual cure technique is a two-step process involving two different types of chemistry [ 1 i]. It usually consists in combining a UV irradiation and a thermal drying process and usually appears as a two-pack material, typically based on a polyisocyanate and an acrylate (10.87). The acrylate is... 

One of the more interesting possibilities of developing truly unique photocurable films lies in the ability of certain onium salts to generate both a reactive radical species and a strong acid upon photolysis. Indeed, several papers describe dual radical/cationic photocurable systems (16-18). The possibilities afforded by dual cure systems provide a unique mechanism for generating cured films with expanded properties for a number of applications. 

The experimental work presented in this paper is intended to show the utility of AM oligomers in UV cure formulations and the effect combined acrylamide content has on the UV cure properties of acrylated melamines. This work will consider three cure processes UV, thermal, and dual cure. In this case dual cure is UV followed by a thermal bump. 

Dual Cure. Films were prepared for Dynamic Mechanical Analysis (DMA). All films were cast on release paper with a 4.5 mil draw down bar, and partially cured with two 200 watt/inch lamps at half power and a belt speed of 200 ft/min. The films were intentionally under cured to facilitate cutting with minimal flaws. After the films were cut into ii-inch test pieces, they were cured with two 200 watt/inch lamps at 100 ft/min, equal to 260 millijoules/cm dose. The instrument used for the DMA work was a Rheometrics RSA II Solids Analyzer. All tests were made at a frequency of 11 hz with a nominal strain of 0.05%, under nitrogen. Both temperature scans, at 2°C/minute, and isothermal runs were made. 

Table 7 shows a comparison of other properties between the 40% AM and 25% melamine containing films. With the exception of condensing humidity, the AH film has comparable properties. The lower humidity resistance of the AH film might be due to residual unsaturation. It is likely that under thermal cure conditions and with a lack of free radicals there was little reaction of the double bonds. The purpose of this experiment was to show that condensation cure was possible with AM. Under dual cure conditions an effort would be made to cure both functionalities of the AM. 

Demonstration of Dual Cure. To demonstrate thermal cure after UV cure three sets of films were prepared for Dynamic Mechanical Analysis (DMA). In the first set, four formulations were based on AM 1, AH 2, the epoxy acrylate, amd the urethane acrylate. Formulations used the test resin at 65% in TPGDA, and included 3% Darocur 1173 as a source of free radicals. DMA plots for the epoxy acrylate and urethane acrylate are shown in Figure 9. The elastic or storage modulus (E ) in the rubbery region is often used as a measure of crosslink density. The maximum of the tan delta plot, not shown, is... 

Properties of Dual Cured Films. The properties of dual cured films containing the oleamide modified AH and epoxy acrylate were compared. The formulations used oligomers at 65% in TPGOA and included 3% Darocur 1173 and 3% PTSA. They were applied to clay coated board over black lithographic ink with a 3 rod and cured by a UV exposure of 250 mj/cm followed by a thermal cure of 5 minutes at 150 C. The... 

---