Thiol/polyene

It should finally be emphasized that the use of DPB is not restricted to acrylic compounds since it proved to be also a very efficient photoinitiator for the polymerization of vinyl monomers, like N-vinylpyrrolidone (NVP). In addition, DPB appeared to be particularly well-suited to photo-cure systems that need hydrogen abstraction type photoinitiators, like the thiol-polyenes resins (2), since it is then to be compared to the poor-performing benzophenone. 

Keywords Acrylates Crosslinking Dual cure Epoxides Glass laminates Interpenetrating polymer networks Isocyanates Photo initiated cationic polymerisation Photoinitiators Photopolymerisation Pressure sensitive adhesives Release coatings Structural adhesives Thiol/polyene UV radiation curing Vinyl ethers. 

The curative mechanism of heat stabilizers accounts for a reduction in the rate of formation of polyene sequences -[CH=CH]n- and deactivation of hydroperoxides in PVC (Andreas, 1990). Organotin maleate 44 or thiol 97 released from 45 were reported to stop the growth of polyenes by adding to the C=C bond in PVC. 

Cases in which allyl radicals display sufficient reactivity to participate successfully in radical chain reactions include the addition of bromotrichloromethane to butadiene the reaction of cyclopentadiene with tosyl cyanide, the addition of thiols , stannanes " and hydrogen halides . All these reactions follow the simple two-step radical chain mechanism depicted in Scheme 1, and the low reactivity of the intermediate allyl radicals can be compensated by using the trapping agent in excess or even as the solvent. In chain reactions with three or more chain-carrying radicals, this compensation is not possible anymore, because the concentration of the reaction partners has to be chosen such that the selectivity requirements for all intermediate radicals are satisfied. Complex radical chain reactions with polyenes as one of the reactants are therefore not known. 

The Ink formulations usually contain an unsaturated monomer or prepolymer, a radical precursor and/ or an actinic sensitizer, a polymer vehicle base, and pigmentation. Recent formulations Include polyunsaturated polyenes, and polyacrylates with thiol or benzoin radical sources.. The acid or hydroxyl content of the resin base is usually varied to adjust the water or oleophillic wetting action of the composition. 

The thiol group has not seen regular use as a terminating group in polyene cyclization, but now Saito et al. have shown that it behaves quite satisfactorily giving tetrahydrothlophene annulated... 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

Hydrogen chloride is bound with metallic (Zn, Ca, Ba) organic acid salts with formation of the metal chloride and the corresponding free fatty acids. The formation of polyene sequences can be prevented by combination reactions with thiol or mal-eate derivatives. Although efficient in blocking the degradation, most of the inor-... 

Recently, Starnes and Zaikov have reported that thiol esters such as 2-ethylhexyl 3-mercaptobenzoate function as efhcient stabilizers both by displacement of labile chloride and addition to polyene sequences. In this case, good performance was noted with high-shear dynamic stability testing, using 3 phr of thiol plasticizer/stabilizer. Although it is difficult to visualize such a system being competitive with low-cost tin stabilizers used at levels as low as 0.3 phr, it will, nevertheless, be of interest to follow commercialization of this approach. 

All of the ester thiols retard the development of color upon heating. Their ability to do so also is consistent with the prevention of polyene prodnction via reaction 1, where R CI is either a growing polyenyl chloride [i. e., -(CH=CH) CHC1-] or an unstable stmctural defect in the imdegraded polymer. 

Thiols, in general, are well-known to imdergo additions to alkene double bonds by a free-radical mechanism or in an ionic process that is promoted by strong acids such as HCI. During PVC thermolysis, such additions of ester thiols would tend to inhibit coloration by shortening polyene lengths. They also would tend to inhibit polyene growth if they occurred, for example, as in equation 2,... 

In a related kinetics experiment, PVC was degraded at 180°C for 2.3 h and then treated with an excess of 2. This addition immediately caused a great increase in the rate of evolution of HCI. Yet, after another hour of heating, the rate decreased to about half of its value prior to thiol inclusion. The initial rate increase produced by 2 can be attributed to the formation of HCI in reaction 1, while the subsequent rate deceleration is consistent with the ability of that reaction to deactivate labile structures, such as -(CH=CH) CHC1- that would lead to continued polyene growth. (Their concinrent deactivation via reaction 2 may have oconred as well.)... 

They destroy thermally labile structural defects. These defects are tertiary ehloride, internal allylie chloride, and polyenyl chloride groups. Their destruction involves the nucleophilic displacement of chloride and possibly the addition of the thiols to polyene double bonds (as in reaction 2, for example). 

They inhibit coloration by shortening polyene sequences. Even if the addition of thiols to polyenes does not stop their growth, it will tend to make them less colored by decreasing their lengths. Long, intensely colored polyenes may actually be converted into colorless structures by multiple thiol additions. 

---