Free-Radical Bulk-Photopolymerization Process as a Method of Obtaining Thermally Curable Structural Self-Adhesive Tapes and Effect of Used Type I Photoinitiators

A new fabrication method for thin (120 µm) thermally curable structural self-adhesive tapes (SATs) was demonstrated by utilizing a series of acrylic syrups (ASs) modified using Bisphenol A-based liquid epoxy resin. The acrylic syrups containing poly(butyl acrylate-co-butyl methacrylate-co-glycidyl methacrylate-co-2-hydroxyetyl acrylate-co-4-acryloyloxy benzophenone) were synthesized via free-radical bulk-photopolymerization (FRBP) process. Influence of different type I radical photoinitiators (PIs), i.e., α-hydroxyalkylphenones (HPs), acylphosphine oxides (APOs) and its mixtures (HPs/APOs and APO/APO) on selected physico-chemical features of obtained ASs was studied. It turned out that APO-type PIs are more effective in the FRBP process (NMR studies). Self-adhesive tests of SATs revealed that the monomers’ conversion in ASs have a significant influence on adhesion and tack. Moreover, the polymer structures formed at the UV cross-linking stage of SATs significantly affect the cross-linking degree of SATs during thermal curing (differential scanning calorimetry method). The highest values of overlap shear strength were achieved by SATs based on ASs with monomers’ conversion on the level 50–60%

Bulk photopolymerization of (met)acrylates

Meth(acrylate) polymers are prepared by polymerization according to the radical mechanism. This is possible due to the occurrence of unsaturated double bonds (vinyl groups) in monomers. Meth(acrylate) polymerization reaction is usually carried out in organic solvents using thermal initiators which decompose at elevated temperature to give radicals. An appropriately selected initiator or electromagnetic radiation (e.g. UV light) can be used to initiate the polymerization process. In this case the process is called photopolymerization. Due to the fact that the radical photopolymerization mechanism of met(acrylates) has already been well understood, and the advantages of lightinduced polymerization offer enormous possibilities, this technology still attracts the attention of researchers and industrialists, which results in its continuous development. Meth(acrylates), which are very reactive, are suitable compounds for this technology. Light induced process of transformation these molecules into polymer can be carried out in a very short time. In addition, a wide range of monomers allows to obtain products with various properties. Typically, photopolymerization is associated with the cross-linking of polymers that form part of photocurable compositions, as a result of which, for example, varnish coatings, dental fillings or self-adhesive materials are produced. In this article, contemporary literature on the photopolymerization of (meth)acrylates with reference to the photopolymerization in bulk method is also pointed out. This technique allows to obtain polymer syrups - a polymer solution in unreacted monomers. These viscous liquids are very interesting semi-finished products (free of solvent and almost ready for application) for the preparation of various polymer materials, especially coatings and adhesives

Wpływ fotoinicjatorów na właściwości poliakrylanowych syropów i klejów samoprzylepnych

Pressure-sensitive adhesives (PSAs) were prepared via UV-irradiaton of an acrylic syrups with reactive diluent (functional diacrylate) and photoinitiator. The acrylic syrups were received via free radical bulk polymerization process (FRBP) and influence of radical photoinitiator (α-hydroxyketones, acylphosphine oxides or mixtures thereof) was investigated. It was revealed that the mixture of bisacylphosphine oxide (Omnirad 819) and α-hydroxyketone (Omnirad 184) was the most effective in FRBP (the highest monomer conversion), but preferable features of acrylic syrups (viscosity and molecular weight of copolymers) and self-adhesive properties of PSAs (especially cohesion) was achieved due to α-hydroxyketones using.Z syropów akrylanowych oraz wielofunkcyjnego monomeru i fotoinicjatora rodnikowego otrzymano poliakrylanowe kleje samoprzylepne. Syropy akrylanowe uzyskano metodą fotopolimeryzacji w masie monomerów z zastosowaniem różnych fotoinicjatorów rodnikowych (z grupy α-hydroksyketonów, tlenków acylofosfiny i ich mieszanin). Stwierdzono, że najbardziej efektywna w procesie fotopolimeryzacji jest mieszanina fotoinicjatorów: tlenku bisacylofosfiny (Omnirad 819) i α-hydroksyketonu (Omnirad 184) (największa wartość konwersji monomerów), jednak większą lepkością oraz większymi ciężarami cząsteczkowymi cechowały się syropy akrylanowe z udziałem α-hydroksyketonów (Omnirad 127 lub Omnirad 184), a uzyskane z nich kleje wykazywały lepsze cechy samoprzylepne, zwłaszcza kohezję

Wpływ UV na właściwości samoprzylepne silikonowych klejów samoprzylepnych

The paper presents the results of research on the UV radiation influence on the self-adhesive properties of silicone adhesives. The adhesives were obtained by cross-linking commercial resins (PSA 590, Q2-7566) at the temperature of 110°C, using different amounts (0–3 wt%) of bis(2,4-dichlorobenzoyl) peroxide. Self-adhesive properties of adhesives such as adhesion, tack, and durability before and after aging were determined. The adhesives showed stable self-adhesive properties, however, the PSA 590 adhesive had a higher maximum operating temperature than the Q2-7566 adhesive.W pracy przedstawiono wyniki badań dotyczące wpływu promieniowania UV na właściwości samoprzylepne klejów silikonowych. Kleje otrzymano poprzez sieciowanie w temperaturze 110°C żywic komercyjnych (PSA 590, Q2-7566), stosując różne ilości (0-3% mas.) nadtlenku bis(2,4-dichlorobenzoilu). Oznaczono właściwości samoprzylepne klejów takie, jak adhezja, przyczepność oraz trwałość przed i po starzeniu. Kleje wykazywały stabilne właściwości samoprzylepne, przy czym klej otrzymany na bazie żywicy PSA 590 miał wyższą maksymalną temperaturę użytkowania niż klej na bazie żywicy Q2-7566

Adhesive Films Based on Benzoxazine Resins and the Photoreactive Epoxyacrylate Copolymer

UV-cross-linkable and thermally curable self-adhesive structural tapes (SATs) were compounded using solid commercial benzoxazine resins (Araldite MT 35700 and Araldite MT 35910) and a photoreactive epoxyacrylate copolymer (EAC). As initiators of benzoxazine resin polymerization and epoxy component cationic polymerization, two kinds of latent curing agents (LCAs) were tested, i.e., amine type and ionic liquid type. The influence of the benzoxazine resin and the LCA type on the UV-cross-linking process, the self-adhesive features and thermal curing behavior of UV-cross-linked tapes, as well as the shear strength of cured aluminum/SAT/aluminum joints and thermal stability of adhesives were investigated. It was found that the amine additive and the benzoxazine resin take part in the UV-cross-linking process of the EAC as hydrogen donors, which is confirmed by an increase in cohesion (+86%) and a decrease in adhesion (−25%) of SATs. The highest results of adhesion to steel (47 N/25 mm) and overlap shear strength (11.1 MPa) values were registered for SATs based on Araldite MT 35910 and contained 7.5 wt. parts of the amine-type hardener. The formation of a polyacrylate-benzoxazine network has a significant impact on the course of the thermal curing process and the thermomechanical properties of adhesive joints, which was also confirmed by the Cure Index calculation

Influence of Acrylic Acid on Kinetics of UV-Induced Cotelomerization Process and Properties of Obtained Pressure-Sensitive Adhesives

A new environmentally friendly method of photoreactive pressure-sensitive adhesives (PSAs) preparation was demonstrated. PSAs based on n-butyl acrylate (BA), acrylic acid (AA) and 4-acryloyloxy benxophenone (ABP) were prepared via the UV-induced cotelomerization process in the presence of a radical photoinitiator (acylphosphine oxide) and telogen (tetrabromomethane). Hydroxyterminated polybutadiene was used as a crosslinking agent. Influence of AA concentration (0–10 wt %) on kinetics of the cotelomerization process was investigated using a photodifferential scanning calorimetry method, selected physicochemical features of obtained photoreactive BA/AA/ABP cotelomers (molecular masses, polydispersity, monomers conversion and dynamic viscosity) and self-adhesive properties of obtained PSAs (adhesion, tack and cohesion) were studied, as well. It turned out that AA content is the important factor that influences monomers conversion (thereby the volatile parts content in prepolymer) and PSAs’ properties. As the acrylic acid content increases, the reaction rate increases, but the total monomers conversion and the solid content of the prepolymer decreases. Additionally, the adhesion and cohesion of PSAs were grown up, and their tackiness decreased. However, the AA content has no effect on molecular weights (Mw and Mn) and polydispersity (c.a. 1.5) of photoreactive cotelomers. The optimal AA content necessary to obtain a prepolymer with low volatile parts content and good PSA properties was determined

Synthesis of Epoxy Methacrylate Resin and Coatings Preparation by Cationic and Radical Photocrosslinking

This work involves the synthesis of hybrid oligomers based on the epoxy methacrylate resin. The EA resin was obtained by the modification of industrial-grade bisphenol A-based epoxy resin and methacrylic acid has been synthesized in order to develop multifunctional resins comprising both epoxide group and reactive, terminal unsaturation. Owing to the presence of both epoxy and double carbon–carbon pendant groups, the reaction product exhibits photocrosslinking via two distinct mechanisms: (i) cationic ring-opening polymerization and (ii) free radical polymerization. Monitoring of EA synthesis reactions over time using PAVs, MAAC and NV parameters, and the FT-IR method reveals that esterification reactions proceed faster at the start, exhibiting over 40% of conversion within the initial 60 min, which can be associated with a relatively high concentration of reactive sites and low viscosity of the reaction mixture at the initial reaction stage. With the further increase in the reaction time, the reaction rate tends to decrease. The control of the EA synthesis process can guide how to adjust reactions to obtain EAs with desired characteristics. Based on obtained values, one can state that the optimum synthesis time of about 4–5 h should be adopted to prepare EAs having both epoxy groups and unsaturated double bonds. The structure of the obtained EA was confirmed by FT-IR and NMR methods, as well as the determination of partial acid value and epoxy equivalent. Samples at various stages of synthesis were cured with UV radiation in order to study the kinetics of the process according to cationic and radical polymerization determined via photo-differential scanning calorimetry (photo-DSC) and real-time infrared spectroscopy (RT-IR) and then the properties of the cured coatings were tested. It turned out that the cationic polymerization was slower with a lower conversion of the photoreactive groups, as compared to the radical polymerization. All the obtained EA coatings were characterized by good properties of cured coatings and can be successfully used in the coating-forming sector

Organophilized Montmorillonites as Fillers for Silicone Pressure-Sensitive Adhesives

In the presented work, organophilized montmorillonites (OMMT) with selected quaternary ammonium compounds (QAC) with different chemical structure ((trioctylmethylammonium chloride—A336, dimethyloctadecyl[3-trimethoxysilyl)propyl]ammonium chloride—D, cetyltrimethylammonium bromide—CTAB, 2-methacryloxyethyltrimethylammonium chloride—MOA) were obtained and used as fillers for physically modified silicone pressure-sensitive adhesives (Si-PSA). Before OMMT addition into Si-PSA matrix, they were analyzed via TGA and XRD techniques. Type of chemical structure of QAC affected d-spacing of OMMT. New self-adhesive materials were obtained based on prepared Si-PSA compositions by adding the obtained fillers to the polymer matrix. New tapes exhibit a good level of useful properties as adhesion, cohesion, and tack—the values did not change or slightly decreased; in addition, the tapes with addition of OMMT showed high thermal resistance reaching the measuring limit of the test equipment—to 225 °C

Synthesis of Hybrid Epoxy Methacrylate Resin Based on Diglycidyl Ethers and Coatings Preparation via Cationic and Free-Radical Photopolymerization

A series of difunctional epoxy methacrylate resins (EAs) containing at least one epoxy and at least one methacrylate group were synthesized by means of an addition reaction between epoxy-terminated diglycidyl ethers and methacrylic acid. In order to investigate the impact of polymer architecture on the course of addition reactions and further coating properties, several different types of diglycidyl ethers, i.e., linear, containing aliphatic or aromatic rings, with a short or polymeric backbone, were employed in the synthesis. The carboxyl–epoxide addition esterification reactions have been found to, in a relatively straightforward manner, control the extent of acrylation depending on the substrate feed ratio and reaction time. The structure of obtained pre-polymers was evaluated by FT-IR and NMR methods. At the same time, the extent of addition reactions was validated via quantitative analysis, including non-volatile matter content (NV), acid value (PAVs), and epoxy equivalent value (EE) analysis. The modification was carried out in a manner likely to create a compound with one epoxy and one carbon–carbon pendant group. Hence, due to the presence of both functionalities, it is possible to crosslink compositions based on synthesized EAs via two distinct mechanisms: (i) cationic polymerization or (ii) free-radical polymerization. Synthesized epoxy methacrylate pre-polymers were further employed for use in formulate photocurable coating compositions by the cationic or radical process. Furthermore, the photopolymerization behavior and properties of cured coatings were explored regarding some structural factors and parameters. The investigated polymeric materials cure in a short time to obtain coatings with good properties, which is why they can be successfully used to produce protective and decorative coatings for many industries

Influence of Methacrylate and Vinyl Monomers on Radical Bulk Photopolymerization Process and Properties of Epoxy-Acrylate Structural Adhesives

In this paper, epoxy-acrylate structural adhesives tapes (SATs) were obtained from Bisphenol A-based liquid epoxy resin and epoxy acrylic resins (EARs). A new method of EARs preparation, i.e., the free radical bulk photopolymerization process (FRBP), was studied in detail. The influence of methacrylic monomers (methyl methacrylate, ethyl methacrylate, butyl methacrylate, lauryl methacrylate, (2-acetoacetoxy)ethyl methacrylate) and vinyl monomers (N-vinylpyrrolidone and styrene) on the FRBP process of base monomers (i.e., butyl acrylate, glycidyl methacrylate and 2-hydroxyethyl acrylate) was investigated. The kinetics of photopolymerization process was monitored by photo-differential scanning calorimetry method. The properties of the obtained EARs (viscosity and average molecular weights), as well as monomers conversion using 1H NMR, were determined. It was revealed that styrene significantly decreases the photopolymerization rate and increases the final monomers conversion (+27%). However, the resulting tetrapolymers BA-co-GMA-co-HEA-co-STY have low molecular weights and low polydispersity (2.2). Methacrylate monomers with shorter aliphatic chains (4) also decrease the rate of photopolymerization due to the length of the aliphatic chain increasing. Surprisingly, the best results of adhesion to steel and shear strength were obtained for SAT based on epoxy acrylate resin with styrene (11 N/25 mm and 20.8 MPa, respectively). However, the thermomechanical properties of SAT with styrene were weaker than those with methacrylates