PHOTOPOLYMERIZATION. 2.1 Description of photopolymerization; applications 1-5

Transcription

1 HAP. 2 PHTPLYMEIZATIN 2.1 Description of photopolymerization; applications 1-5 The transformation of a reactive liquid into a solid, by UV-radiation, leading to polymerization and cross-linking is termed photopolymerization or UV-curing. UV-curing is defined as: FAST TANSFMATIN F 100% EATIVE, SPEIALLY FMULATED, LIQUIDS INT SLIDS BY UV PHTNS. Photons generated by UV-light are absorbed by the chromophoric site of a molecule in a single event; this molecule generates radicals or protons, the initiating species that promote the fast transformation (time range s) from the liquid into the solid. As a result of the curing process, a solid polymer network, totally insoluble in the organic solvents and very resistant to heat and mechanical treatments, is formed from a 100% reactive liquid. The entire process is schematized in Fig. 2.1.

2 HAP. 2 PHTPLYMEIZATIN Photoinitiator UV radiation eactive species (radicals or ions) rosslinked polymer Multifunctional monomer Fig. 2.1: Schematic representation of photocuring process. As shown in Fig. 2.1, a UV-curable formulation is made of three basic components: 1. photoinitiator, which absorbs the incident light and readily generates reactive radicals or ions; 2. functionalized oligomer, which, by polymerizing, will constitute the back-bone of the three-dimensional polymer network formed; 3. a mono- or multifunctional monomer, which acts as a reactive diluent and will be incorporated into the network. The photoinitiator is the key of all process, because it determines both the rate of initiation and the penetration of the incident light into the sample, governing in this case also the depth of cure. Depending on the photoinitiator used, the reactive species generated can be radicals or ions, so the process can be named radical or cationic photopolymerization. As described in the following paragraphs, radical and cationic photopolymerizations are very different not only for the active species that start the reaction, but also for the types of monomers used and for the cure mechanism and experimental conditions in which the process is performed. In Fig. 2.2, the differences in the reactive species generated and the initiation step for the two processes are schematically illustrated. During the initial part of the reaction, polymerization rate depends on the reactivity and concentration of the functional group as well as on the viscosity of the matrix medium. ther important parameters are chemical micro-structure and functionality of monomers and/or oligomers: they will determine the final degree of polymerization, physical, and chemical characteristics of the final polymer. 17

3 HAP. 2 PHTPLYMEIZATIN H 2 H' H 2 H' (I) H + H 2 H H 3 H + (II) Fig. 2.2: Initiation step for radical (I) and cationic (II) photopolymerization. APPLIATINS Nowadays, UV-curing technology is well established in many industrial fields and in particular applications, it offers new possibilities of development. The principal industrial use of UV-curing technology is in the coating industry for the surface protection of all kind of materials, due to high speed process and good energy yield. A typical industrial line UV processor is made of two parts: the coating machine, where the UV-curable resin is applied on the substrate, and the UV oven, where the liquid resin is dried within a fraction of a second by passing under a powerful lamp. In Fig. 2.3 an industrial processor for the UV-curing of organic coatings is schematically presented. Fig. 2.3: UV-curing industrial processor for coatings. 18

4 HAP. 2 PHTPLYMEIZATIN Acrylate resins, cured with radical photopolymerization, are the most widely used UV-systems, with a total annual production of approximately tons, while cationic-type resins, cured with cationic photopolymerization, represent a minor part, i.e. about 2000 tons, but in continuous growth (10-12% per year). Here are reported the main industrial fields in which UV-curing technology is employed 1,2. Graphic arts/oatings Adhesives Electronics Stereolithography Dental composite materials Graphic Arts UV-curing is used both in the pre-press part to produce the printing plate as well as in the printing process itself, thanks to the development of fast-drying UV-curable inks. The printing process consists of the rapid transfer through an ink of a given image from a printing plate to the substrate (usually paper), thus allowing a fast production of prints. The main printing processes in which UV-curing is involved are: letterpress, gravure, flexography, screen printing and lithography. n the other side new UV inks have been developed. They present a number of advantages over conventional solvent-based inks: the higher viscosity allows several colours to be applied successively; their solvent-free formulations lead to a better print definition and high gloss images; the UV process is more economic because it requires less energy and achieves a higher productivity; the entire process is performed at ambient temperature, without any solvent emission, which makes it a environmental friendly procedure. Finally, for some specific applications, it is necessary to further improve surface properties of printed material (ex. gloss, smoothness, and abrasion and scratch resistances, weathering resistance). This can be achieved by applying a thin layer of a UV-curable varnish, which is known to give high gloss and smooth surface. 19

5 HAP. 2 PHTPLYMEIZATIN oatings oatings are applied to a surface. They can be divided into: Functional coatings, improving the surface by: protecting it from abrasion, scratch, mar, chemicals; providing different properties such as release, slip, adhesion, electrical conductivity or insulation, antifogging, flame retardance; acting as a barrier to various liquids or gasses. Decorative coatings are applied to: change appearance ( colour, gloss or mat finish, texture); hide surface (imperfections, electrical circuitry, etc.). Usually coatings are classified according to the substrate they are applied to: paper and paperboard wood plastics metal glass and ceramic miscellaneous. This type of employ of UV-curable varnishes is increasingly used to obtain highly resistant coatings to protect any substrate: wood, plastic, metal, glass, optical fibres, paper, leather, fabrics, etc.; the film thickness is of the order of µm to assure a long-lasting protection. Adhesives adiation curing has two main areas of application in the field of adhesion: 1. to bond together two parts of a laminate, acting as a quick-setting glue. In this case the use is limited by the UV-transparency of one of the two parts of the laminate. The whole process is divided in three steps: applying of the adhesive in the liquid state; assemblage of the two parts; exposure of the assembly to UV-light. UV-cured laminates show a great potential because they are produced by a process that is faster, cheaper and easier to work out than the usual thermal cure carried out for hours under high pressure. 20

6 HAP. 2 PHTPLYMEIZATIN 2. To produce pressure-sensitive adhesives and release coatings. It consists in a rapid photoinitiated crosslinking producing a viscoelastic system with predetermined properties. Electronics Here UV-curable systems have found applications as photoresists in the imaging step, fast drying adhesive and conformal coatings. Stereolithography This new technology is based mainly on the capability of UV-curable systems to give three-dimensional solid objects, by scanning the surface of a resin with a laser to form a thin solid pattern, and building up the model step-by-step by adding one layer on top of another. omplex parts can be obtained faster, with great precision, and more flexible processing than with conventional modelling techniques. Besides it allows the direct use of digital design information to guide the formation of a model that closely represents the original design. Dental omposite Materials Adding mineral fillers such as glass or silica particles to UV formulations is possible to obtain extremely hard and abrasion-resistant composite materials. These types of resins present a number of advantages over conventional systems: immediate readiness for use, extended working time, higher polymerization rate, and short setting time, better adhesion of the filler particles to the matrix. The curing of these systems has to be performed at visible light and it is necessary to take into account that inert filler can be up to 60% in volume, so the penetration of light in these composite resins is limited therefore it has to be carried a multiple step process. PINIPAL ADVANTAGES/DISADVANTAGES 1,2 The main advantages of UV-curing technique are better understood if compared with the traditional thermal-curing polymerization (Tab. 2.1). 21

7 HAP. 2 PHTPLYMEIZATIN Tab. 2.1: omparison of UV and thermal curing 1. Parameter UV Thermal ommercial apital cost + - perational cost + - Formulation cost - + Floor space + - ure speed + - Skill level required 0 + Environmental No solvent release + - Energy consumption + - Technical hemical resistance + - Formulation variety 0 + uring of pigmented films - + No substrate damage + 0 Low cure temperature 0 - Sensitivity to oxygen + + Health & safety Fire hazard + - adiation hazard 0 + Irritant raw materials = advantage - = disadvantage 0 = intermediate 22

8 HAP. 2 PHTPLYMEIZATIN Arguments in favour of the replacement of thermal curing by UV-curing are mainly lower capital and running costs, lower floor space requirements, higher running speeds, less substrate heating, the high quality of the cured coating or ink, no solvent release during curing and the development of new curable formulations having less or no skin irritant raw materials. There are also economic and ecological factors that encourage the continuous growth of radiation curing technology such as: aw materials containing a low amount of volatiles, less or no skin irritant, have been developed and increase the range of formulation variety. Low-viscosity monomer-free oligomers and water reducible oligomers can be used in spray coating applications. New applications in metal and glass coatings are possible thanks to oligomers that adhere well to critical substrates. Weather resistant products are available for outdoor applications. More reactive photoinitiators allow lower concentrations in formulations or less powerful UV sources to be used. Photoinitiator-free UV-curable systems appear on the market. New monochromatic UV-sources were introduced. n the other side thermal curing still holds a strong position due to the advantage in formulation costs and variety, the avoidance of radiation and the lower skill level required. Moreover the thickness of the sample that can be photocured is normally very thin if compared to a thermal cured one. Mainly for this reason UV-cure technology is still not widespread in the composites industry. 2.2 adical photopolymerization 1,2,4 The radical polymerization mechanism can be schematically represented in Fig. 2.4: H 2 H' H 2 H' Fig. 2.4: adical polymerization mechanism. 23

9 HAP. 2 PHTPLYMEIZATIN is the active specie generated by photodecomposition of the initiator. It should be pointed out that it is only the initiation step, radical formation from the photoinitiator, which is different from thermal polymerization. adical photoinitiators can be divided into two groups according to the way the active species are generated 2 : 1. by photocleavage, if radicals are generated by a intramolecular scission ; 2. by hydrogen abstraction, if radicals are generated by the abstraction of an atom of hydrogen from a donor molecule. In Fig. 2.5 are illustrated the two ways of radicals generation. A-B* A + B homolitic cleavage A* + H AH + hydrogen abstraction Fig. 2.5: Mechanism of radicals generation in radical photopolymerization. 1. Photocleavage: in this class we found aromatic carbonyl compounds that undergo to homolytic - bond scission upon UV exposure, with the formation of two radical fragments; the benzoyl radical was shown to be the major initiating species. The process is schematized in Fig. 2.6; examples of photoinitiators belong to this class are: benzoin ethers derivatives, benzilketals, hydroxyalkylphenones, α-amino ketones, and acylphosphine oxides. X hv X Fig. 2.6: adical formation reaction for aromatic carbonyl compounds. 2. Hydrogen abstraction: this is a typical reaction of some aromatic ketones, like benzophenone, thioxanthone, or camphorquinone. Under UV irradiation, they do not undergo fragmentation, but abstract a hydrogen atom from an H-donor molecule to generate a ketyl radical and the donor radical. 24

10 HAP. 2 PHTPLYMEIZATIN The process is schematized in Fig. 2.7: * hv H H Fig. 2.7: adical formation reaction for aromatic ketones. In this case initiation of polymerization occurs through the H-donor radical. The most frequently used H-donor molecules are tertiary amines, because of the high reactivity of the α-amino alkyl radical towards the double bond, as shown in Fig. 2.8: Ar 2 N H 2 hv Ar 2 H N H H 2 H N H H 2 Fig. 2.8: adical formation reaction in case of tertiary amine used as co-initiator. This latter class of photoinitiators have also the advantage of reducing the inhibition effect of oxygen because they promote a peroxidation mechanism that consumes the oxygen present in the monomer. In Fig. 2.9 are listed the principal classes of radical photoinitiators. benzoin derivatives benzil ketals ' ' benzophenone derivatives thioxanthone derivatives S hydroxyalkylphenone ' acylphosphine oxides P H Fig. 2.9: adical photoinitiators commonly used. 25

11 HAP. 2 PHTPLYMEIZATIN The main classes of resins that can be cured with radical system are: acrylate and methacrylate monomers, thiol-ene systems, and unsatured polyester resins. Acrylate and methacrylate monomers are by far the most used in industry because they are very reactive and can be used to create a large variety of crosslinked polymers with tailor-made properties. Their polymerization is very fast at the beginning, but progressively slows down when gelification and vitrification occur; for this reason there are always some residual unreacted insaturations trapped in the polymer network. They can be divided into: functionalized oligomers mono- or poly-functional monomers The most important types of functionalized oligomers are: epoxy acrylic resins urethane acrylic resins polyalkylene glycol diacrylates polyester diacrylates The most important monomers are: diethylene glycol diacrylate hexanediol diacrylate trimethylolpropane triacrylate. In Fig is presented the typical reaction scheme for this class of monomers. Epoxy acrylates are highly reactive and produce hard and chemically resistant coatings, so they are used in wood finishing applications, varnishes for paper, and cardboard as well as for hard coatings 2,4. Polyesters acrylates are often applied in wood coatings, varnishes, lithographic and screen inks. Methacrylates monomers have similar reactivity to acrylates monomers, but with a lower propagation rate 2,4. 26

12 HAP. 2 PHTPLYMEIZATIN Initiation H 2 H H 2 H Propagation H 2 H monomer H 2 H H 2 H H 2 H H H 2 H H 2 H H 2 H H 2 Termination P n P m P n P m P n vitrification Fig. 2.10: Polymerization reaction of acrylates. The most important advantages of acrylate formulations are high reactivity and adjustable viscosity. apid cure speed and low viscosity combined with brittleness and poor adhesion are obtained when acrylate monomers are used; acrylate oligomers have higher viscosity and lower reactivity than monomers, but they guarantee a broad range of coating property requirements. Therefore radiation curable formulations usually consist of monomers as reactive thinners and oligomers as binders. Thiol-ene systems are used in many applications such as coatings, adhesives, sealants, etc. Their polymerization reaction can be represented as follows (Fig. 2.11): Ar 2 SH hv Ar 2 H S S H 2 H ' S H 2 H ' S H 2 H ' SH S H 2 H 2 ' S Fig. 2.11: Polymerization reaction of thiol-ene systems. Using multifunctional monomers it is possible to obtain a three-dimensional network in which connecting chains are made of alternating copolymer. It should be noticed that 27

13 HAP. 2 PHTPLYMEIZATIN thiol-ene systems are less sensitive to air inhibition that other radical systems because peroxy radicals are also capable to extract H from the thiol (Fig. 2.12) forming the thiil radicals which continue the polymerisation process. S H 2 H SH P P 2 P 2 H S Fig. 2.12: Hydrogen abstraction from thiol molecule. Unsatured polyester resins are mainly employed in the wood finishing industry; the radical-initiated crosslinking occurs by direct addition copolymerization of the vinyl monomer with the unsaturations at the polyester backbone, as shown in Fig. 2.13: H H H H 2 crosslinked polymer Fig. 2.13: Polymerization of unsatured polyesters. In Fig the principal classes of radical monomers are listed. polyester/styrene H H H H 2 thiol/ene ( SH) 4 H 2 H ' H H 2 acrylates (H 2 H H 2 H H 2 ) 3 H 2 H 2 H 3 H H 2 = polyester, polyether, polyurethane, polysiloxane Fig. 2.14: adical monomers commonly used. 28

14 HAP. 2 PHTPLYMEIZATIN 2.3 ationic photopolymerization 6-9 The cationic polymerization mechanism is schematically represented in Fig. 2.15: H + H 2 H H 3 H + Fig. 2.15: ationic polymerization mechanism. H + is the active specie generated by photodecomposition of the initiator. Photoinitiators for cationic photopolymerization can be divided into three groups: Aryl diazonium salts Ferrocenium salts Diaryliodonium/triarylsulfonium salts. The latter are named onium salts and are nowadays the photoinitiator class most used in cationic polymerization. They are stable crystalline compounds, readily soluble in a wide variety of common polar solvents and cationically polymerizable monomers and absorb strongly in the UV region. In Fig. 2.16, their structure is represented. I S MtX n (I) MtX n (II) Fig. 2.16: General structure of onium salts : diaryliodonium (I) and tryarylsulfonium (II) salt. Under UV light, they are subjected to photolysis through a quite complex mechanism. In the case of diaryliodonium salts, one can have photoexcitation of the salt and after the decay of the resulting excited singlet with heterolytic and homolytic 29

15 HAP. 2 PHTPLYMEIZATIN cleavages of carbon-iodine bond. Free-radicals, cationic and cation-radical fragments are produced according to the scheme reported in Fig Ar 2 I MtX n hv Ar 2 I MtX n * ArI MtX n Ar ZH ArI Z HMtX n Fig. 2.17: Photolysis of diaryliodonium salt under UV light. Protonic acids, denoted as HMtX n, derive from the reaction between the aryl cations and aryliodine cation radicals with solvents, monomers, or impurities. HMtX n is the real initiator of cationic polymerization, as shown in Fig HMtX n M H M + MtX n H M + MtX n nm H (M) n M + MtX n Fig. 2.18: Initiation mechanism for cationic polymerization. For triarylsulfonium salts the photolysis is similar, but the heterolytic cleavage is dominant on homolytic cleavage. The anion generally indicated as MtX - n must have non-nucleophilic characteristics because any cationic species generated during photolysis or by addition to a monomer would give combination with a nucleophilic anion and, as result, retardation or complete suppression of polymerization reaction. According to their nonnucleophilicity, the most useful anions are: PF - 6, AsF - 6, and SbF - 6. The type of anion determines also the strength of the Brønsted acid generated via photolysis: bigger anions generate stronger acids, so the reactivity order is: SbF - 6 > AsF - 6 > PF - 6 > BF - 4. In Fig are shown the differences observed changing the anion on the kinetics of photopolymerization of cyclohexene oxide. 30

16 HAP. 2 PHTPLYMEIZATIN Fig. 2.19: Photopolymerization of cyclohexene oxide using 0.02% mol of ( 6 H 6 ) 3 S + X - salts 6. The onium salts show a very high degree of thermal stabilty due to their cation part which is stabilized by the resonance of benzenic rings and by the d-orbital of central atom. As a result of this stability, they undergo thermal decomposition at very high temperatures, as shown in Fig Fig. 2.20: TGA analysis of ( 6 H 6 ) 3 S + AsF - 6 in nitrogen and air during an heating ramp of a rate of 10 /min 6. In Fig are summarized the various critical functions that can be assigned to the cation and anion portion of an onium salt. 31

17 HAP. 2 PHTPLYMEIZATIN I MtX n ATIN DETEMINES PHTHEMISTY λ max molar absorption coefficient quantum yield photosensitization thermal stability ANIN DETEMINES PLYME HEMISTY acid strength nucleophilicity anion stability initiation efficiency propagation rate constants Fig. 2.21: Anatomy of an onium salt photoinitiator. Studies made on reactive systems using photo-calorimetric technique 6, i.e. photo- DS, have revealed that there are other parameters controlling the reaction: oncentration of photoinitiator, for each of them is possible to observe that there is a specific concentration for which is obtained an optimum cure rate. Further increase in photoinitiator level does not produce a corresponding increase in the cure rate, possibly due to the light screening effects by the triarylsulfonium salt itself or its photolysis products. UV-light intensity, because the system is limited by the absorption of the photoinitiator, so it is useless to have very high light intensities. At very low intensities there appears to be some type of inhibition effect. Temperature effect, it has been observed that in all cationic systems cure at the highest temperature the substrate give the highest cure rate, of course this is not always possible. 32

18 HAP. 2 PHTPLYMEIZATIN Water effect, because the presence of water (or other hydroxyl containing impurities) can change both the rate and the extent of polymerization of epoxy monomers. Two other classes of cationic photoinitiators have been mentioned above: Aryldiazonium salts Ferrocenium salts. Aryldiazonium salts were the first class of cationic photoinitiators developed in the 1970s. They can be used in the ring opening polymerization of epoxides through the reaction scheme represented in Fig Ar N 2 BF 4 hv Ar F BF 3 N 2 BF 3 H 2 n Fig. 2.22: Photolysis mechanism of diaryldiazonium salt and cationic polymerization of an epoxy monomer. This class of cationic photoinitiators had no success essentially for two reasons: 1. the thermal instability of aryldiazonium salt leads to poor latency so that the systems spontaneously gelled in few hours even in absence of light. 2. The generation of nitrogen gas as photolysis product leads to film defects. Ferrocenium salts are a very different class of cationic photoinitiators. They undergo photolysis to generate an iron-based Lewis acid with the loss of the arene ligand. This species coordinates to an epoxy monomer to give ring-opening polymerization as shown in Fig

19 HAP. 2 PHTPLYMEIZATIN Fe X hv Fe X 1 Fe X ( ) 3 1 n 1 polymer Fig. 2.23: Photolysis mechanism of ferrocenium salt and cationic polymerization of an epoxy monomer. The use of this class of photoinitiator is limited to the monomers that can bond effectively with the photogenerated coordinatively unsatured ion center. ationic photopolymerization is used to cure monomers that are reactive towards cationic species. In Fig. 2.24, the most important monomers that can be UV-cured in the cationic way are scheduled. Among all the monomers presented, the most interesting classes for cationic photopolymerization are multifunctional vinyl ethers and epoxides because they are very reactive and commonly available. H 2 H 2 H 2 n H H 2 n S H 2 H 2 S n H H 2 n ationic Photoinitiators hv N H H 2 n (H 2 ) 5 N H 2 H 2 (H 2 ) 4 n n n Fig. 2.24: Polymerizable monomers with cationic photoinitiators. 34

20 HAP. 2 PHTPLYMEIZATIN The epoxy monomers can be UV-cured through the opening of the epoxy ring, catalyzed by the acid species generated by photolysis of the initiator. The reaction mechanism is presented in Fig X H H ' X H H ' oxonium ion monomer X ( H H) n ' H H ' Fig. 2.25: Polymerization scheme for an epoxy monomer. In presence of difunctional epoxides UV-curing leads to a crosslinked polymer. The reactivity of this class of monomers is quite broad, for example monomers containing the epoxycyclohexane group are much more reactive than glycidyl ethers or glycidyl esters, due to steric and electronic factors. Two examples of epoxy monomers commonly used are cyclohexane dimethanol diglycidylether, denoted DGE and 3,4-epoxycyclohexyl-3,4 - epoxycyclohexanecarboxilate, denoted E. Their structures are given in Fig H 2 H 2 H 2 H 2 DGE E Fig. 2.26: hemical structures of DGE and E. 35

21 HAP. 2 PHTPLYMEIZATIN Vinyl-ethers monomers are the most reactive towards cationic photopolymerization, giving a three-dimensional polymer network with a low number of residual insaturations. The high reactivity of these monomers is due to the presence of the double bond = that, with the oxygen atom, stabilizes the cation through the chain growth (Fig. 2.27). + H 2 H H 2 H H 2 H Fig. 2.27: Growth of the polymer chain and its stabilization by resonance. Even if vinyl ethers are ideally suited for cationic photopolymerization, their use in industry is limited by their high cost and the hazards of using acetylene under high pressure during their synthesis. PINIPAL ADVANTAGES/DISADVANTAGES Effect of oxygen ne of the main advantages of cationic-initiated polymerization, if compared to the radical induced process, is that the former is not sensitive to oxygen, thus allowing coatings to be cured rapidly even in the presence of air. Influence of film thickness In thin films the photopolymerization develops at the same rate, but as the film thickness is increased, the propagation rate value, p, drops, due to the UV filter effect of the top layer (Fig. 2.28). Film thickness has a pronounced effect also on the maximum conversion level. Moreover atmospheric oxygen will diffuse less rapidly in thick coatings. 36

22 HAP. 2 PHTPLYMEIZATIN Fig. 2.28: Influence of the film thickness on the photopolymerization of a cycloaliphatic diepoxy 7. Post-polymerization ne of the distinct features of cationic photopolymerization, compared with radicalinduced process, is the post-cure phenomena: it consists in a further and not negligible polymerization taking place once the light has been switched off. Such an important post polymerization is due to the fact that two cations cannot interact to undergo coupling or disproportionation, so that the living polymer chain continues to grow in the dark, until termination occurs by transfer reaction or bimolecular interaction with another species present in the polymerization mixture (as water, bases, or another portion of polymer chain). Fig shows some typical conversion vs. time curves recorded after exposure, compared to continuous irradiation: post-polymerization is relatively more important in the early stages of the reaction, but a significant increase of the degree of conversion could be noticed even after 20 minutes of storage in the dark. 37

23 HAP. 2 PHTPLYMEIZATIN Fig. 2.29: Polymerization profiles recorded after UV exposure of various durations for a cycloaliphatic diepoxy Why using cationic photopolymerization? The main differences between radical and cationic photopolymerization has been described and it becomes evident that the cationic UV-curing process offers many important advantages that are summarized here: the initiating species is a stable compound only consumed by anions or nucleophiles; after UV exposure, cationic polymerization continues for a long time; since no radicals are involved, cationic photopolymerization is not sensitive to oxygen; films made from cationic formulations show low shrinkage and good adhesion. 38