Viscoelastic behaviour of water-based pressure sensitive adhesives (PSAs)

Abstract

Water-based pressure sensitive adhesives (PSA) obtained by emulsion polymerization have peculiar characteristics compared with their solvent counterparts. In particular they contain significant amount of microgels, permeated by uncrosslinked molecules, and generate micronetworks after film formation. Viscoelastic characterisation of three PSAs, both in the linear limit and in uniaxial elongation is reported in this paper. Modelling of tensile stress-strain curves is proposed, based on a network theory accounting for the presence of entanglements (Ball et al. Polymer, 1981, 22, 1010–1018). A rate-dependence of some of the physical parameters of the model is introduced in order to describe experimental data.     

A new method to measure the high-shear bulk properties of pressure sensitive adhesives used in LCD devices

The wide application of pressure sensitive adhesives (PSAs) in thin film transistor liquid crystal display (TFT-LCD) devices is based on their advantages that PSAs provide sufficient bond strength and also are cleanly removable upon debonding. These two essential requirements are not obtainable without a proper understanding of the substrate as well as the rheological properties of the PSA. Unlike other substrates, the polarizer, the core unit of an LCD device, may exhibit a considerable dimension change with temperature and humidity. This imposes a large residual stress on the PSA layer, which often generates serious defects such as partial delamination. To overcome this problem, an accurate control of the rheological properties is required in the design of the PSA. In this study a new method is devised to measure the rheological behavior of thin film PSAs under high shear deformation. This method provided us with important information regarding the effects of molar mass and the level of crosslinking on the rheological behavior of the PSA. These findings along with the new method should be applicable not only to the LCD industry but also to other fields where an accurate control of PSA rheology is required.     

A new method based on application of cyclic strain to evaluate the durability of pressure sensitive adhesives

The development of PSAs suitable for specific applications requires an optimization of the two most essential properties: sufficient bond strength and clean removability. The latter is conveniently obtained by controlling the mode of debonding to an interfacial failure, yet the issue of bond strength is challenging since the optimum bond strength must be available in the same failure mode. More complication arises from the fact that the methods, such as peel and creep tests, used to evaluate the bond strength are often qualitative and the results obtained may not accurately reveal the effects of various PSA design parameters. In the present study, a new method, named cyclic strain application (CSA) test, is devised to evaluate the durability of PSAs. A key feature of this test is that by applying a controlled cyclic strain on a PSA/substrate interface we can quantitatively monitor the temporal stress variation as well as the time for interfacial failure (t _f). As a result, the effects of various PSA design parameters, such as molar mass, blending with low molar mass fractions, adhesion promoters, and the level of crosslinking on PSA durability can be clearly seen. Other factors being equal, higher molar mass samples showed greater t _f, while, blending with low molar mass fractions led to a reduced t _f. The use of a strong adhesion promoter, as anticipated, increased t _f. However, it should be noted that excessive bond strength may lead to a cohesive failure, which is detrimental in the design of good PSAs. From a comparison of t _f with peel strength, it was clearly shown that t _f, the key parameter obtained from the CSA test, had a strong correlation with peel strength. In addition, some other information, such as the detailed influences of the level of crosslinking and molar mass on PSA durability was also obtained from the CSA test. Lastly, some important PSA design concepts and other examples utilizing the CSA test are presented.     

On the work of adhesion and peel strength between pressure sensitive adhesives and the polymeric films used in LCD devices

Peel strength, a convenient measure of bond strength in adhesive/adherend systems, is known to be a function of various factors such as the thermodynamic work of adhesion, rate of measurement, thermal history, and temperature. Generally, it is believed that the work of adhesion is primarily involved in the first stage of adhesion through wetting phenomenon and beyond that its role diminishes in that the portion of thermodynamic contribution to actual bond strength is insignificant. In practice, however, we often observe that a suitable surface treatment increases the surface energy of the substrate, which further enhances the bond strength. One practical example is the surface treatment carried out in LCD industry to obtain sufficient bond strength between pressure sensitive adhesives and polymeric films. To further our understanding of the effect of surface treatment, we attempted to establish a possible correlation, if any, between the thermodynamic work of adhesion and peel strength. For this, we carefully measured the contact angles of water and diiodomethane against various polymeric films, and calculated the surface energy and the thermodynamic work of adhesion using the two widely used approaches: Young-Fowkes-Girifalco-Good, and Wu methods. Before establishing a correlation, some general aspects of the above two methods are discussed. The values of the work of adhesion obtained were compared with the measured peel strength. Indeed, we observed a clear correlation between the two quantities: the increase of the work of adhesion led to the increase of peel strength. As a reason for this correlation, we proposed that the increase of surface energy might be associated with the increase of various surface functional groups, which, in turn, contributed to the formation of chemical bonding with the PSA leading to the increase of peel strength.     

Adhesion property of novel polyimides containing fluorine and phosphine oxide moieties

Novel diamine monomers containing fluorine and phosphine oxide - bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine oxide (mDA6FPPO) and bis(3-aminophenyl)-4-(trifluoromethyl)phenyl phosphine oxide (mDAFPPO) - were utilized to prepare polyimides with dianhydrides such as 6FDA, BTDA or ODPA by the conventional two-step route, i.e. preparation of poly(amic acid) followed by solution imidization. The polyimides were characterized by FT-IR, NMR, DSC, and intrinsic viscosity measurements. The adhesion property of the polyimides was evaluated via a peel test with bare Cu foil, as well as silane/Cr-coated Cu foil, and failure surfaces were analyzed by SEM/EDX to elucidate the failure mechanism. The results were compared with those from the polyimides prepared from bis(3-aminophenyl)phenyl phosphine oxide (mDAPPO) containing only the phosphine oxide moiety, 1,1-bis(4-aminophenyl)-1-phenyl-2,2-trifluoroethane (3FDAm) containing only the fluorine moiety, and a commercial 3,3′-diaminodiphenylsulfone (mDDS). The polyimides with 3FDAm exhibited the highest T _g, followed by the mDAPPO-, mDA3FPPO-, and mDA6FPPO-based polyimides, but the mDAPPO-based polyimides exhibited the highest adhesion properties, followed by mDA3FPPO, mDA6FPPO, mDDS, and 3FDAm, which is attributed to the phosphine oxide and fluorine moieties.     

Adhesion between rubber compounds and ternary-alloy-coated steel cords,Part I: effect of cobalt plating amount in ternary-alloy-coated steel cords

The adhesion between a rubber compound and ternary-alloy-coated steel cords with different cobalt plating amounts (0, 2 and 4 wt%) was investigated to understand the role of cobalt in stabilizing adhesion to the rubber compound. The adhesion property of ternary alloy coated steel cords to the rubber compound did show significant enhancement after cure for the ternary alloy coated steel cord with 2 wt% cobalt plating. Further increase of cobalt plating in ternary-alloy-coated steel cord was responsible for the poor adhesion to the rubber compound. An improvement in adhesion durability after aging in various hostile environments was shown for the ternary-alloy-coated steel cord with 2 wt% cobalt plating. The interphase between the ternary-alloy-coated steel cords and the rubber compound studied using AES showed a stable adhesion interphase by optimum cobalt plating, resulting in enhancement of adhesion retention.     

Enhancing the Adhesion Retention by Controlling the Structure of the Adhesion Interphase Between Rubber Compound and Metal. Part I. Effect of Cobalt Salt

The adhesion between cobalt salt-containing rubber compounds and brass-plated steel cords was studied to understand the role of cobalt salt as an adhesion promoter. An improvement in adhesion was shown with the low loading of cobalt salt in the range 0.5–1 phr, while a significant decline in adhesion was observed with high loading at 2 phr and a long aging time of 15 days under humidity aging. The adhesion interphase between the rubber compound and the brass-plated steel cord studied using Auger electron spectroscopy (AES) showed stable depth profile by cobalt salt incorporation at low loading, resulting in the enhancement of the adhesion retention. For the high loading of cobalt salt, adhesion interphase grew excessively, especially ZnO layer formed under humidity aging, which resulted in reduced adhesion.     

Enhancing the Adhesion Retention by Controlling the Structure of the Adhesion Interphase between Rubber Compound and Metal. Part II. Effect of Zinc Borate

The adhesion between zinc borate-containing rubber compounds and brass-plated steel cords was studied to understand the role of zinc borate as an adhesion promoter. No improvement in adhesion after cure was shown with loading of zinc borate in the range 0.5–2 phr, while enhancement of adhesion retention after humidity aging was observed with loading of zinc borate in the range 0.5–2 phr. The adhesion interphase between the brass-plated steel cord and the rubber compound subjected to humidity aging treatment showed, using AES, a stabilized depth profile by a moderate loading of zinc borate, resulting in enhancement of adhesion retention. For the high loading of zinc borate, copper sulfide layer and zinc oxide layer grew excessively in the adhesion interphase under humidity aging.     

Adhesion between rubber compounds and ternary-alloy-coated steel cords. Part II: Effects of sulfur and cobalt salt in rubber compounds

The adhesion properties between rubber compounds with different amounts of cobalt salt and sulfur and ternary-alloy-coated steel cord with 2 wt% cobalt plating amount were investigated to understand the effects of cobalt salt and sulfur in rubber compounds on their adhesion characteristics to the ternary-alloy-coated steel cord. The adhesion properties of the rubber compounds to ternary-alloy-coated steel cord were largely dependent on the amounts of both cobalt salt and sulfur in the rubber compounds. The pull-out force of adhesion sample increased significantly with increasing concentration of cobalt salt in the rubber compound with constant sulfur loading, while it decreased slightly with increasing sulfur into the rubber compound with constant cobalt salt loading. Adhesion retention after various hostile aging treatments improved in the rubber compounds incorporating both cobalt salt and high loading of sulfur. The interphases between the rubber compounds and the ternary-alloy-coated steel cord studied using AES showed stable adhesion patterns by incorporating cobalt salt, resulting in enhancement of the adhesion retention.     

The interrelationships between electronically conductive adhesive formulations,substrate and filler surface properties,and joint performance. Part I: the effects of adhesive thickness

Electronically conductive adhesives (ECAs) have received a great deal of attention for interconnection applications in recent years. Even though ECAs have excellent potential for being efficient and less costly alternative to solder joining in electronic components, they still possess a number of problems with respect to durability and design to meet specific needs. One of the issues that requires understanding is regarding the optimum adhesive thickness (AT) to be used. This study addresses this issue in relation to the formulations of the conductive adhesives and their interactions with adherend surfaces. For this purpose, two different adherends varying in surface characteristics were utilized along with three different conductive adhesive formulations with varying particle loadings, and shapes and sizes of conductive nickel fillers. Joints were also prepared with two different AT values, to gain insight into the influence of AT on the joint strength, deformation and joint conductivity. As the AT was increased, only a small reduction in failure load and ultimate displacement values were observed with unetched adherends. With etched adherends, however, a small increase in joint stretchability was evident with higher adhesive thickness tested at a lower crosshead speed. When the AT was increased, we also noted a corresponding increase in the initial joint resistance.     

Adhesion Performance and Microscope Morphology of UV-Curable Semi-interpenetrated Dicing Acrylic PSAs in Si-Wafer Manufacture Process for MCP

UV-curable acrylic pressure-sensitive adhesives (acrylic PSAs) have many applications in industry. As the Si-wafers become thinner, the acrylic PSAs for MCP need to show proper adhesion and leave little residue on the Si-wafer after UV irradiation when released from the dicing tapes. Strong adhesion is required in the dicing process to hold the Si-wafer before UV irradiation. On the other hand, weak adhesion strength is required after UV irradiation to prevent damage to the Si-wafers during the pick-up process. This study employed semi-interpenetrating polymer network-structured dicing of acrylic PSAs in the Si-wafer manufacture process. The binder PSAs contained 2-ethylhexyl acrylate (2-EHA) and acrylic acid (AA). The adhesion performance of the peel strength on a Si-wafer was examined as a function of the UV dose. The results showed that the abovementioned two requirements were achieved using semi-IPN dicing acrylic PSAs using a hexafunctional acrylate monomer and a UV-curing system. FE-SEM and XPS revealed little residue on the wafer after removing the tape. This paper suggests the optimal conditions for the curing agent, the additional hexafunctional monomer, photoinitiator and the coating thickness.     

Adhesion Between Metals and Polymers as a Three-Dimensional System

Adhesion science in a technical sense is the study of reactions in boundary layers. From a macroscopic point of view the result is the adhesive joint strength dependent on the magnitude of the adhesion forces without hints on the nature of these forces. So the question of the nature of adhesion has at least to be answered for technical applications by using other measurement techniques. From the microscopic point of view adhesion is of interdisciplinary nature, where molecules or atoms act with each other across the interface. Mainly adhesive bonds are based on these interactions of different bodies like metals and polymers or other material discontinuities. So far we can speak about a “chemical adhesion”. But in practice there we realize a “technical adhesion” with more or less sharp discontinuities.     

Adhesion Between Metals and Polymers as a Three-Dimensional System

Adhesion science in a technical sense is the study of reactions in boundary layers. From a macroscopic point of view the result is the adhesive joint strength dependent on the magnitude of the adhesion forces without hints on the nature of these forces. So the question of the nature of adhesion has at least to be answered for technical applications by using other measurement techniques. From the microscopic point of view adhesion is of interdisciplinary nature, where molecules or atoms act with each other across the interface. Mainly adhesive bonds are based on these interactions of different bodies like metals and polymers or other material discontinuities. So far we can speak about a “chemical adhesion”. But in practice there we realize a “technical adhesion” with more or less sharp discontinuities.     

Contribution of Colloidal Forces in Non-contact Area to the Adhesion Between a Micro-probe and a Substrate Surface: Theoretical Analysis

Colloidal forces outside the microscopic probe (particle)–substrate adhesion contact area were analyzed theoretically. Equations describing the van der Waals, electrical double layer, and hydrophobic forces were derived for the non-contact area of a probe–substrate system assuming a simple sphere–flat geometry. Two cases were considered: particles freely resting on the substrate surface and particles pulling off the substrate. The results of modeling presented in this communication suggest that the adhesion of fine particles (microscopic and sub-microscopic particles) to flat surfaces can be affected by the forces acting outside the contact area. However, due to increased distance between the particle and substrate during separation, both the van der Waals and electrostatic forces acting outside the contact area are negligibly small compared to the short-range adhesion forces and they do not contribute to the measured pull-off force to any great extent for most systems. On the contrary, our calculations suggest that the long-range hydrophobic forces can contribute to the strength of adhesion between hydrophobic fine particles and hydrophobic substrates.     

Preparation and Thermal Degradation of White Fluorinated Polyimide/TiO2 Composite Films with Strong Shielding Performance

A series of white fluorinated polyimide/TiO₂ composite films were prepared by a solution mixing method. They showed high tensile strength beyond 95.1MPa and excellent insulting property with surface and volume resistances exceeding 1.9 × 10¹¹Ω and 2.3 × 10¹² Ω cm, respectively, coupled with water absorptions below 1.1%, water contact angles up to 95.9 and whiteness beyond 58.5. It was found that their optical transparency decreased dramatically with the increasing doping of TiO₂. The thermal degradation of pure fluorinated polyimide (FPI) and representative polyimide/TiO₂ composite film with 5 wt% of TiO₂ (PI/TiO₂-5 wt%) was also studied.     

硫醇甲基锡在PVC加工中的热稳定性及加工性能测试

通过PVC加工的动态试验,利用两辊塑炼机和转(扭)矩流变仪对硫醇甲基锡在PVC加工过程中的热稳定性及流变性能进行测试,试验结果表明,硫醇甲基锡在PVC加工过程中有良好的热稳定性及优越的流变加工性能。     

Design and Performance Evaluation of a Solar-Assisted Biomass Drying System with Thermal Storage

A solar biomass hybrid air heating system that does not require a conventional auxiliary heater but can still provide a daily load fraction exceeding 90% and supply hot air at a steady temperature and flow rate continuously for 24 h a day has been developed. The system, which combines an unglazed transpired solar collector, rock bed, and a biomass gasifier stove with heat exchanger, was evaluated by drying chilli using air at 60°C and 90 m³/h. The chilli was dried from 76.7% moisture (w.b.) to 8.4% over 32.5 h of continuous drying. The dryer reduced the drying time by 66% compared to open sun drying and provided 91.6% load fraction during the 24-h operation. The temperature of hot air supplied was stable at 60±3°C for about 21 h during the entire drying duration.     

Mechanical Characteristics and Thermal Decomposition Behaviors of Polytetrahydrofuran Binder with Glycerol Propoxylate (Mn=1500) as a Crosslinking Agent

Polytetrahydrofuran (PTHF) is an effective binder ingredient for improving propellant performance, although it is not an energetic material. PTHF becomes sufficiently rubbery for use as a binder when a triol is added as a crosslinking modifier. In this study, glycerol propoxylate (GPO), with a molecular weight of 1500, was used as a crosslinking modifier, and the curing behavior, tensile properties, and thermal decomposition behaviors of the PTHF binder with GPO were investigated. A PTHF (M _n=650)/GPO blend with a PTHF/GPO mole ratio (ξ ) less than or equal to 4 and a PTHF (M _n=1400)/GPO blend with ξ ≤1 were used as propellant binders. The curing behaviors and mechanical properties of the PTHF/GPO blends were influenced by the molecular weight of PTHF and ξ , while the thermal decomposition behaviors were not affected. It was found that the PTHF/GPO blends had higher initial viscosity, longer pot life, and unique mechanical properties compared to those of the PTHF blends supplemented with GPO (M _n=260).     

一种新颖的含黄酮结构可交联高分子染料的合成与染色性能研究(英文)

A yellow crosslinking polymeric dye was prepared by grafting the flavone moiety containing azo chromophore onto polyvinylamine backbone.The λ max of this polymeric dye in water is 382 nm.The polymeric dye is fixed to silk and cotton with a crosslinking agent,2-chloro-4,6-di(aminobenzene-4’-β-sulphatoethylsulphone)-1,3,5-s-triazine,which acts as a bridge between the fiber and dye molecules.The fixation of this polymeric dye reaches 99% and the dyed samples exhibit excellent rubbing and washing fastness.     

聚合物基蒙脱土纳米复合材料热稳定性能及阻燃性能

综述了近年来国内外聚合物基蒙脱土纳米复合材料热性能和阻燃性能方面的研究进展及机理,并对蒙脱土与其他绿色阻燃剂协同阻燃的前景进行了展望。