Hyun-Sung Do

The curing performance of UV-curable semi-interpenetrating polymer network structured acrylic pressure-sensitive adhesives

This article reports on the curing performance of UV-curable acrylic binders prepared with trifunctional monomers and a photoinitiator. The curing reaction was achieved by direct excitation of pressure-sensitive adhesives (PSAs) by irradiation with a 100-W high-pressure mercury lamp with different UV doses. The curing performance of PSAs was studied by photo-differential scanning calorimetry (photo-DSC), gel-fraction determination and Fourier transform infrared–attenuated total reflection (FTIR–ATR) spectroscopy. The reaction rate and extent of UV curing were found to be strongly dependent on the curing rates of the trifunctional monomers, trimethylolpropane triacrylate (TMPTA) and trimethylolpropane ethoxylated (6) triacrylate (TMPEOTA), which have different molecular weights. Exothermic areas increased with increasing acrylic acid concentration. Moreover, gel fractions sharply increased after UV irradiation and then remained constant with prolonged UV exposure. TMPTA blends had higher gel fractions than TMPEOTA blends because of TMPTAs fast curing rate. Also, the gel fractions of TMPTA blends showed no variation with acrylic acid concentration. However, the FTIR–ATR absorption peak areas representing the relative concentration of C=C bonds showed more conspicuous trends for the curing reaction. Although the gel fractions of TMPTA blends showed no differences, the relative concentrations of C=C bonds increased with increasing acrylic acid concentration. In addition, TMPTA blends showed higher relative concentrations of C=C bonds because of the faster curing rate of TMPTA.

Preparation and adhesion performance of UV-crosslinkable acrylic pressure sensitive adhesives

UV-crosslinkable acrylic pressure sensitive adhesives (PSAs) were synthesized by copolymerization of 2-ethylhexyl acrylate (2-EHA), vinyl acetate (VAc), acrylic acid (AA), 2-hydroxyethyl methacrylate (2-HEMA) and 4-acryloyloxydiethoxy-4′-chlorobenzophenone (unsaturated photoinitiator), with varying contents of 2-HEMA and photoinitiator, by solution polymerization. The UV-crosslinking behavior of the PSAs was studied by ATR–FT-IR spectroscopy, and PSA performance was characterized by probe tack, peel resistance and shear adhesion failure temperature (SAFT). As 2-HEMA acts as a good hydrogen donor to benzophenone, the efficiency of the photo-reaction was enhanced; thus with increasing contents of 2-HEMA and photoinitiator in the PSAs the incorporation of benzophenone groups in the PSAs, even at low UV doses, was quite fast. In addition, as the crosslinking reaction proceeds via photo-reaction mainly between 2-HEMA and photoinitiator, the probe tack and peel resistance of the PSAs having high concentrations of 2-HEMA and photoinitiator rapidly decreased in the early stage of UV irradiation due to increased crosslink density. These phenomena were also observed in the SAFT test, with the PSAs containing high levels of 2-HEMA and photoinitiator showing high SAFT values at low UV doses.

Preparation of SIS/SBS-based UV-cross-linkable pressure-sensitive adhesives using the thiol-ene reaction

Hot-melt pressure-sensitive adhesives (HMPSAs) usually contain styrene-isoprenestyrene (SIS) or styrene-butadiene-styrene (SBS) block copolymers, tackifier, plasticizer and other additives. However, these PSAs cannot be used in high-temperature applications or where solvent and chemical resistance properties are required. We, therefore, developed UV-cross-linkable HMPSAs by the thiol-ene reaction with the aim to increase adhesion properties at elevated temperature. For effective UV cross-linking, the selection of photoinitiator and photo-cross-linker is very important for thermal stability and fast curing. Adhesion properties, such as probe tack, peel strength and shear adhesion failure temperature (SAFT), were evaluated to measure PSA performance as a function of photoinitiator type.

Adhesion performance of UV-cured semi-IPN structure acrylic pressure sensitive adhesives

UV-curable solvent-free pressure sensitive adhesives (PSAs) are gaining importance in the area of adhesives because of increasing environmental concerns and the goal to reduce volatile organic compounds (VOCs) in work areas and consumption places. These PSAs have advantages such as low emission of VOCs, a solvent-free process, a fast producton rate at ambient temperature and only a modest requirement for operating space. In this study, UV-curable PSAs were investigated by measuring their adhesion performance in terms of probe tack, peel strength, shear adhesion failure temperature (SAFT) and holding power. PSAs were synthesized from 2-ethylhexyl acrylate (2-EHA), acrylic acid (AA) and vinyl acetate (VAc), using variations in AA concentration to control the glass transition temperature (T g) of the prepared PSAs. In addition, two types of trifunctional monomers, trimethylolpropane triacrylate (TMPTA) and trimethylolpropane ethoxylated (6) triacrylate (TMPEOTA), which have different chain lengths, were used to form semi-interpenetrating polymer network (semi-IPN) structures after UV exposure. With increasing AA concentration in the PSAs, both the T g and viscosity increased. Also, probe tack and SAFT increased, but peel strength decreased. After UV irradiation, probe tack decreased, and SAFT and peel strength increased as AA concentration increased in the PSAs. In most cases, cohesive failure changed to interfacial failure after UV exposure. Also, TMPTA increased the cohesion of PSAs; however, TMPEOTA affected the mobility of PSAs due to the different chain lengths of the two types of trifunctional monomer in a different way. The increase of TMPEOTA content diminished the cohesion of PSAs. Consequently, the adhesion performance of the PSAs was closely related to the T g of the PSAs, and the two types of trifunctional monomer showed different adhesion performances.

Effect of grafting of acrylic acid onto PET film surfaces by UV irradiation on the adhesion of PSAs

To improve the peel strength between a pressure-sensitive adhesive (PSA) and its substrate, grafting of acrylic acid (AA) onto the surface of poly(ethylene terephthalate) (PET) film was carried out. After AA was coated onto the surface of PET films using a spin coater, the coated PET films were irradiated by UV. To investigate the surface chemistry and topography of the PET-g-AA films, the grafted surface of the PET films was characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning probe microscopy (SPM). From these investigations, the effects of grafting of AA at the surface of PET by UV irradiation were discussed. In addition, to determine the effect of grafting on the adhesion between PSA polymer and PET-g-AA films, peel strength was measured after the PSA/PET-g-AA system was cured at various temperatures. As the esterification between PSA polymer and PET-g-AA films occurred in the interfacial region, the peel strength of the PSA/PET-g-AA system generally increased with increasing curing temperature.

Curing Behavior and Adhesion Performance of UV-Curable Styrene–Isoprene–Styrene-Based Pressure-Sensitive Adhesives

This article reports on the curing behavior and adhesion performance of ultraviolet (UV)-curable, polystyrene–polyisoprene–polystyrene (SIS)-based pressure-sensitive adhesives (PSAs) blended with di- or tri-functional monomers and a photoinitiator. The curing reaction was achieved by direct excitation of PSAs by irradiation with a 100 W high-pressure mercury lamp at different UV doses. The curing behavior of the PSAs was studied by gel fraction determination, rigid-body pendulum type physical properties test (RPT) and Fourier transform-infrared–attenuated total reflection (FT-IR–ATR) spectroscopy. The adhesion performance was determined by probe tack, peel strength and shear adhesion failure temperature (SAFT) measurements. The reaction rate and extent of UV curing reaction were found to be strongly dependent on the curing rate for the following five multifunctional monomers studied: three di-functional monomers, ethylene/glycol dimethacrylate (EGDMA), triethylene glycol dimethacrylate (TEGDMA) and poly(ethylene/glycol)(400) dimethacrylate (PEG(400)DMA), and two tri-functional monomers, trimethylolpropane triacrylate (TMPTA) and trimethylolpropane ethoxylated (6) triacrylate (TMPEOTA). In addition, the adhesion performance was affected by the semi-interpenetrating polymer network (IPN) structure formed depending on the curing rate and degree of cross-linking.

Viscoelastic and adhesion properties of EVA/tackifier/wax ternary blend systems as hot-melt adhesives

Two types of wax were added to a ethylene vinyl acetate (EVA) copolymer/aromatic hydrocarbon resin (tackifier) blend in the molten state and the miscibility, viscoelastic and adhesion properties of ternary blends as hot-melt adhesives (HMAs) were investigated. Miscibility and viscoelastic properties were studied using differential scanning calorimetry (DSC), Brookfield viscometry and dynamic mechanical thermal analysis (DMTA), and their adhesion strength was determined in terms of single lap shear strength. DSC thermograms of both types of waxes showed their melting peaks in a similar region to that of EVA/tackfier blend. It was difficult to evaluate the miscibility of ternary blends using DSC because the melting peaks of the waxes overlapped with those of the EVA/tackifier blend, although the glass transition temperature (T g) of the ternary blend systems slightly increased with increasing wax concentration. However, their storage modulus (E′) increased slightly and loss tangent (tan δ) showed different peaks when two types of wax were added to the EVA/tackifier blend. Therefore, the miscibility of EVA/tackifier blend altered with addition of waxes. In addition, their melt viscosity decreased with increasing wax concentration. Furthermore, the adhesion strength of the ternary blends decreased with increasing wax concentration, despite the increment of storage modulus. These results suggested that the ternary blends of EVA/tackifier/wax were heterogeneous.