Application of selected 2-methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics

Since their introduction half a century ago, acrylic pressure-sensitive adhesives have been successfully applied in many fields. They are used in self-adhesive tapes, label signs, marking films and protective films as well as in medical pharmaceutical applications for plaster, in dermal dosage systems and in a wide range of biomedical electrodes. In the last 15 years or so, the UV technology, especially UV-crosslinking, is well established in the market and allows the production of UV-crosslinkable pressure-sensitive adhesives (PSA) based on acrylics with interesting performance. So much so that the larger manufacturers of pressure-sensitive adhesive materials and their suppliers now use very expensive equipment to study pressure-sensitive adhesive behavior: tack, peel adhesion and shear strength. The balance between adhesive and cohesive strength after the crosslinking process is very important and critical for properties of acrylic PSA in form of self-adhesive films. In this work the cationic UV-crosslinking of acrylic PSA containing epoxy groups in their structure and additionally cationic photoinitiators based on 2-methylbenzothiazoles as photoreactive crosslinkers have been investigated using UV-lamp as ultraviolet sources. The investigated acrylic PSA were synthesized from 80 wt% of butyl acrylate, and 20 wt% of glycidyl methacrylate. The use of selected photoreactive crosslinkers: 1,5-bis[N,N׳-(2-methylbenzothiazolium)]pentane diiodide and 1,10-bis[N,N׳-(2-methylbenzothiazolium)]decane diiodide allows manufacturing of high quality PSA materials with interesting properties, such as high tack, high peel adhesion, and excellent shear strength.     

Mechanical behaviour of adhesively bonded polyethylene tapping tees

The mechanical properties of adhesively bonded MDPE joints were studied. The lap-shear joints were prepared using PE80 polyethylene gas pipe and four adhesive types; two acrylic and two epoxy resins. The key mechanical properties of lap shear strength and impact resistance were investigated as a function of adhesive type and surface preparation technique. Mechanical abrasion of the PE80 surface increased the strength of the bonds from 40 to 460% for the four adhesives, with the best performing acrylic adhesive having a lap-shear strength of 1.76 MPa and impact strength of 2.5 kJ/m². When used to bond PE80 tapping tees to PE80 gas pipe, the acrylic adhesive produced a gas tight seal at both the standard test pressure of 0.4 MPa and at an increased pressure of 0.8 MPa, and outperformed the PE80 tapping tee during shear testing and withstood a maximum of 10 cycles of 175 J during impact testing. These results highlight the potential of adhesive bonding as a method of joining PE80 tapping tees to PE80 gas pipe.     

Characterization of self-adhesive structural tapes modified with polyvinyl acetal resins

Thermally curable self-adhesive structural tapes (SATs) based on epoxy resin/acrylate copolymer composition and modified with three types of polyvinyl acetal resins (i.e. polyvinyl butyral-stat-acetal and polyvinyl butyrals) were prepared. The influence of polyvinyl resins addition on reactivity and self-adhesive features of SATs as well as on mechanical resistance of aluminum-SAT-aluminum joints was investigated. It was revealed that polyacetal resin addition (0.5 wt. part/100 wt. parts of adhesive composition) increases the adhesion of SATs to a steel substrate and, moreover, improves the overlap shear strength of thermally cured joints. The type and content of applied polyvinyl acetals influence the enthalpy of the photocrosslinking process, crosslinking degree as well as the epoxy groups conversion of SATs matrix.     

Characterization of particles packing in alumina green tape

Aqueous alumina slurry was prepared with a commercial powder of elongated particles, which has the aspect ratio ranging from 1 to 3.5 with the mean of 1.6, to examine the effect of forming conditions on the particle alignment in green tapes. The slurry appeared pseudoplastic with a yield stress, but showed no thixotropic behavior. Its flow curve fitted very well to the Herschel–Bulkley model approximation, which suggested shear-thinning constant of 0.54. Polarized microscopy with the liquid immersion technique was applied to examine the particle orientation through the direction along the tape thickness. In the absence of coquette flow, randomly oriented particles were noted in the tape. At the top surface, particles were aligned with their long-axes (a-axis) along the casting direction. The variation in the degree of orientation was 6.8 ± 1.2. In the area near the Mylar carrier, a-axis of particle made an angle to the carrier surface with the degree of orientation about 5.8 ± 1.0. As the combination of pressure flow and coquette flow, tape cast with casting velocity of 2.5 and 91.5 cm/min, which respectively resulted in shear rate of 1.38 and 50.8 s^?1, were observed. The orientation was significant near the top surface and was higher than that above the carrier surface. The a-axis of particles above the carrier surface was inclined to the surface at low shear rate (1.38 s^?1), but was nearly parallel at high shear rate (50.8 s^?1). Nevertheless, the orientation varies with the location in the tape prepared at the shear rate of 50.8 s^?1.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Enhanced transport critical current density of (Bi,Pb)-2223/Ag superconductor tapes added with nano-sized Bi2O3

Ag sheathed superconductor tapes with starting composition (Bi, Pb)-2223(Bi₂O₃)_0.01 were prepared. Bi₂O₃ with average size 150 nm was used in this work. The Bi₂O₃ amount was chosen based on our initial study on nano-sized Bi₂O₃ added pellets which showed an optimal superconducting property for 0.01 wt% addition. Non-added tapes were also prepared for comparison. The tapes were investigated by X-ray diffraction method, scanning electron microscopy and transport critical current density, J_c measurements (30 K to 77 K). The influence of different sintering times (50, 100, and 150 h) on J_c under applied magnetic field (0–0.75 T at 77 K) parallel and perpendicular to the surface of the tapes was also investigated. J_c of added tapes was found to increase significantly as compared with the non-added tapes. The Bi₂O₃ added tapes sintered for 150 h exhibited the highest J_c at 30 K of 57,900 A/cm² as compared with 19,400 A/cm² for the non-added tapes sintered for 100 h. The improvements in flux pinning and connectivity between grains due to nano Bi₂O₃ addition led to the enhancement of J_c.     

Contact time dependence of tack for crosslinked polyacrylic pressure-sensitive adhesives with two different molecular structures

The influence of molecular structure of pressure-sensitive adhesive on the wetting to adherend surface was investigated. For this purpose, crosslinked poly(n-butyl acrylate-acrylic acid) (A) and poly(2-ethylhexyl acrylate-acrylic acid) random copolymer (B) with an acrylic acid content of 5 wt% and various crosslinking degrees were used. Tack was measured by a probe tack test with a debonding rate of 10 mm/s and various contact times ranging from 3 to 30,000 s. The probe was made of stainless steel (SS). The tack increased with contact time and the degree of tack rising was B>A. The tack was A>B below the contact time of about 100 s, whereas it was B>A above 100 s. The order of molecular mobility was B>A from pulse nuclear magnetic resonance analysis, so the wettability to adherend surface became B>A. This is the reason why tack was B>A above 100 s. The interfacial tension at a water/toluene interface was decreased more effectively by A than B. This result indicates that the acrylic acid unit in A forms the interaction with the high energy surface such as SS in short contact time. The 2-ethylhexyl group is bulker than the n-butyl group. The bulky group promotes steric hindrance for the interaction of the acrylic acid unit. This seems to be the reason why tack was A>B below 100 s.     

A new carbon molecular sieve for propylene/propane separations

A new carbon molecular sieve (CMS) with a propylene/propane separation factor of approximately 27 was synthesized by a facile pyrolysis process from a gel-type strong acid cation exchange resin. The micropore shrinkage process during pyrolysis was investigated using a new high throughput adsorption technique with 48 parallel cells. This significantly reduced the characterization time. The ratio of propylene/propane adsorption rate in the CMS adsorbent changes from 1 to more than 150 when the final pyrolysis temperature changes from 550 to 1000 °C. The best performing CMS pyrolyzed at 850 °C was further characterized using a gravimetric adsorption method. The propylene and propane diffusivities are 1.0 × 10⁻⁹ and 1.1 × 10⁻¹¹ cm² s⁻¹ at 100 kPa and 90 °C. The high propylene/propane diffusivity ratio of 90 is similar to that in zeolite 4A, while the propylene diffusivity was more than 30 times higher than that in zeolite 4A. An effluent of 90 mol% propylene was obtained from a feed of 25 mol% propylene during adsorption/desorption tests using the CMS adsorbent pyrolyzed at 850 °C in a fixed-bed configuration. The new CMS adsorbent is a promising candidate for industrial scale propylene/propane separations.     

Structural,electrical and optical properties of molybdenum-doped TiO2 thin films

Molybdenum doped TiO₂ (MTO) thin films were prepared by radio frequency (RF) magnetron sputtering at room temperature and followed by a heat treatment in a reductive atmosphere containing 90% N₂ and 10% H₂. XRD and FESEM were employed to evaluate the microstructure of the MTO films, revealing that the addition of molybdenum enhances the crystallization and increases the grain size of TiO₂ films. The optimal electrical properties of the MTO films were obtained with 3 wt% Mo doping, producing a resistivity of 1.1×10^?3 Ω cm, a carrier density of 9.7×10²⁰ cm^?3 and a mobility of 5.9 cm²/Vs. The refractive index and extinction coefficient of MTO films were also measured as a function of film porosity. The optical band gap of the MTO films ranged from 3.28 to 3.36 eV, which is greater than that of the un-doped TiO₂ film. This blue shift of approximately 0.14 eV was attributed to the Burstein–Moss effect.     

Improving the electrical properties of niobium-doped titania sputtering targets by sintering in oxygen-deficient atmospheres

Niobium-doped titania (TNO) film can be used as a transparent conductive oxide (TCO) film due to its excellent conductivity and visible transparency. The performances of TNO sputtering targets are thus critical issues in optimizing sputtered films. This study clarifies the influences of inert and reducing atmospheres on the microstructure, densification, crystal structure, and electrical properties of TNO sputtering targets. The results indicate that a sintering atmosphere of 90% Ar–10% H₂ can result in a lower sintered density, larger grain size, and lower resistivity than can an atmosphere of Ar, followed by one of air. Sintering in 90% Ar–10% H₂ or Ar obviously decreases the resistivity of TiO₂, from >10⁸ Ω cm to <10⁻¹ Ω cm, and the TNO target, from >10¹ Ω cm to <10⁻¹ Ω cm. The resistivity of TNO target sintered at 1200 °C in 90% Ar–10% H₂ is as low as 1.8×10⁻² Ω cm.     

Synthesis of high-quality AZO polycrystalline films via target bias radio frequency magnetron sputtering

The deposition rate, transmittance and resistivity of aluminium-doped zinc oxide (AZO) films deposited via radio frequency (r.f.) sputtering change with target thickness. An effective method to control and maintain AZO film properties was developed. The strategy only involved the regulation of target bias voltage of r.f. magnetron sputtering system. The target bias voltage considerably influenced AZO film resistivity. The resistivity of the as-deposited AZO film was 9.82×10⁻⁴ Ω cm with power density of 2.19 W/cm² at target self-bias of −72 V. However, it decreased to 5.98×10⁻⁴ Ω cm when the target bias voltage was increased to −112 V by applying d.c. voltage. Both growth rate and optical band gap of AZO film increased with the absolute value of target bias voltage – growth rate increased from 10.54 nm/min to 25.14 nm/min, and band gap increased from 3.57eV to 3.71 eV when target bias voltage increased from −72 V to −112 V at r.f. power density of 2.19 W/cm². The morphology of AZO films was slightly affected by the target bias voltage. Regulating target bias voltage is an effective method to obtain high-quality AZO thin films deposited via r.f. magnetron sputtering. It is also a good choice to maintain the quality of AZO film in uptime manufacturing deposition.     

New Developments in the Area of Solvent-Borne Acrylic Pressure-Sensitive Adhesives

Since their introduction half a century ago, acrylic pressure-sensitive adhesives have been successfully applied in many fields. In the last fifty years or so, acrylic pressure-sensitive adhesives (PSAs) have made tremendous strides from what was virtually a black art to what is now a sophisticated science. So much so that larger manufacturers of pressure-sensitive adhesives and even their polymer suppliers now use very expensive equipment to study pressure-sensitive adhesive behavior. The three properties which are useful in characterizing the nature of pressure-sensitive adhesives are tack, peel (adhesion) and shear (cohesion). The first measures the adhesive's ability to adhere quickly, the second its ability to resist removal by peeling, and the third its ability to hold in position when shear forces are exerted. The performances of pressure-sensitive adhesives, such as tack, peel and shear, based on polyacrylates synthesized through co-polymerization of acrylate monomers and formulated in organic solvents mixtures are, to a large degree, determined by the molecular weight of acrylic copolymer, polymerization method and especially by the type and quantity of the crosslinking agent added to the PSA. Newly developed solvent-borne PSAs are used in protective foils, removable and repositionable self-adhesive products, water-soluble PSAs and water-dispersible self-adhesive products, photoreactive UV-crosslinkable self-adhesive tapes, and dual-crosslinkable PSAs for self-adhesive tapes with post-crosslinking potential characterized by enhanced cohesion at higher temperatures. The mentioned water-soluble PSAs, water-dispersible self-adhesive products and photoreactive UV-crosslinkable self-adhesives are synthesized in organic solvents as solvent-borne acrylic PSAs.     

Growth of dense Ti3SiC2 MAX phase films elaborated at room temperature by aerosol deposition method

For the very first time, dense and thick films of Ti₃SiC₂, a popular MAX-phase material, were elaborated on glass substrates by the aerosol deposition method (ADM) at RT. The influence of some processing parameters on the deposition rate and morphology of the films was studied. The films revealed an adhesive interface with the substrate and a dense internal microstructure with nanocrystallites resulting from a high fragmentation of the initial powder at the impact. The film surface showed different types of structuration, from a flat to a rough one with the presence of craters, whose deepness and diameter were linked to the film thickness. The deposition rate and film morphology were both influenced by the distance of projection and the carrier gas flow. Films with thicknesses ranging from 0.1 to 16 μm were thus obtained with a high deposition rate reaching 4 μm min⁻¹, with a roughness, R_a, lower than 300 nm.     

Improved hydrophilicity of zinc oxide-incorporated layer-by-layer polyelectrolyte film fabricated by dip coating method

ZnO nanoparticles suspended in poly(acrylic acid) (PAA) were deposited onto layer-by-layer (LBL) polyelectrolyte (PET) films fabricated from poly(allylamine hydrochloride) (PAH) and PAA by dip coating method. Effect of etching time and concentration of ZnO suspension on hydrophilicity of the LBL-PET films before and after UV irradiation was examined using water contact angle measurement. 2.0 M PAH/PAA solutions with a dipping speed of 3.0 cm/min provided stable LBL-PET films with thickness sufficient for HCl etching. Glass substrates with the etched LBL-PET film dipped into 0.2 wt.% ZnO suspension exhibited the contact angle of 10° after irradiated by UV for 60 min.     

Adhesive potential of marama bean protein

Marama bean is an underutilised indigenous Southern African oilseed legume with protein content similar to soya bean. In this study, the adhesive potential of marama protein was explored. At 45% moisture content, marama protein was very sticky with a force of adhesion (6.5 N), which was about twice that of soya and 5 times that of gluten. Marama protein adhesive prepared using a standard procedure described for soya, had better adhesive properties when applied on a wooden substrate than did soya in terms of strength and resistance to delamination in water. The shear strength of marama protein (36–173 kg/cm²) was about 1.5 times higher than that of soya over the protein concentrations: 1.2–3.6 mg protein/cm². When 2.4 mg protein/cm² was applied on the wood, about 47% of glued wood pieces were delaminated for marama compared to 90% for soya after 2 cycles of 48 h soaking. Marama protein contained more β-sheet structure (54%) than soya (47% β-sheet), which was increased by approx. 12% in marama protein adhesive compared to 3% in soya protein adhesive. By AFM, marama protein adhesive presented a rough surface without holes compared to soya, which was rough but with many holes within its structure. Thus, the high β-sheet conformation and surface structure of marama protein are most likely responsible for its better adhesive behaviour compared to soya.     

Structural and electrical properties of Sr2NaNb4O13 thin film grown by electrophoretic method using nanosheets synthesized from K(Sr2Na)Nb4O13 compound

Sr₂NaNb₄O₁₃⁻ (SNNO⁻) nanosheets were exfoliated from the K(Sr₂Na)Nb₄O₁₃ compound that was synthesized at 1200 °C. The SNNO⁻ nanosheets were deposited on a Pt/Ti/SiO₂/Si substrate at room temperature by the electrophoretic method. Annealing was conducted at various temperatures to remove organic defects in the SNNO film. A crystalline SNNO phase without organic defects was formed in the film annealed at 500 °C. However, a SrNb₂O₆ secondary phase was formed in the films annealed above 600 °C, probably due to the evaporation of Na₂O. The SNNO thin film annealed at 500 °C showed a dielectric constant of 74 at 1.0 MHz with a dielectric loss of 2.2%. This film also exhibited a low leakage current density of 9.0 × 10⁻⁸ A/cm² at 0.6 MV/cm with a high breakdown electric field of 0.72 MV/cm.     

Tribological study of amorphous BC4N coatings

Amorphous BC₄N thin films with a thickness of ∼ 2 μm have been deposited by Ion Beam Assisted Deposition (IBAD) on hard steels substrates, in order to study the wear behavior under high loads and the applicability as protective coatings. The bonding structure of the a-BC₄N film was assessed by X-ray Absorption Near Edge Spectroscopy (XANES) and Infrared Spectroscopy, indicating atomic mixing of B–C–N atoms, with a proportion of ∼ 70% sp² hybrids and ∼ 30% sp³ hybrids. Nanoindentation shows a hardness of ∼ 18 GPa and an elastic modulus of ∼ 170 GPa. A detailed tribological study is performed by pin-on-disk tests, combined with spectromicroscopy of the wear track at the coating and wear scar at pin. The tests were performed at ambient conditions, against WC/Co counterface balls under loads up to 30 N, with the sample rotating at 375 rpm. The coatings suffer a continuous wear, at a constant rate of 2 × 10^− 7 mm³/Nm, without catastrophic failure due to film spallation, and show a coefficient of friction of ∼ 0.2.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Effect of excess lead on the structural and electrical characteristics of sol-gel synthesized RuO2/Pb(ZrxTi1−x)O3/RuO2

In this study, we investigated the effect of excess lead on the structural and electrical characteristics of lead zirconate titanate [Pb(Zr_xTi_1−x)O₃, PZT] thin films using the sol-gel spin coating method. X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and field-emission transmission electron microscopy were used to study the structural, morphological, chemical, and microstructural features, respectively, of these films as functions of the growth conditions (excess lead concentrations of 10, 20, and 25 mol%). The PZT thin film prepared at the 20 mol% condition exhibited the best electrical characteristics including a lower leakage current of 6×10⁻⁷ A/cm² at an electric field of 50 kV/cm, a larger capacitance value of 1.92 μF/cm² at a frequency of 1 kHz, and a higher remanent polarization of 20.1 μC/cm² at a frequency of 5 kHz. We attribute this behavior to the optimal amount of excess lead in this PZT film forming a perovskite structure and suppressing the reaction of PZT film with RuO₂ electrode.     

Enhancement of electrical properties of (Gd,V) co-doped BiFeO3 thin films prepared by chemical solution deposition

Pure BiFeO₃ (BFO) and (Bi_0.9Gd_0.1)(Fe_0.975V_0.025)O_3+δ(BGFVO) thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. The improved electrical properties were observed in the BGFVO thin film. The leakage current density of the co-doped BGFVO thin film showed two orders lower than that of the pure BFO, 8.1×10^?5 A/cm² at 100 kV/cm. The remnant polarization (2P_r) and the coercive electric field (2E_c) of the BGFVO thin film were 54 μC/cm² and 1148 kV/cm with applied electric field of 1100 kV/cm at a frequency of 1 kHz, respectively. The 2P_r values of the BGFVO thin film show the dependence of measurement frequency, and it has been fairly saturated at about 30 kHz.