Flexible Adhesives Research Articles

Detection Experiments of High-Barrier and High-Temperature Sterilization and Easy Tear Flexible Plastic Packaging Materials Applied in Food Packaging

In order to study the characteristics of a new kind of high-barrier and high-temperature sterilization and easy tear flexible plastic packaging materials applied in food packaging, the PET/AL/CPP was chosen as flexible plastic packaging material structure, different PET, CPP, alcohol inks, adhesives and other raw materials were selected for making a series of processing technology experiment and detection analysis. Then comparing the data obtained with the requirements, it was concluded that new flexible plastic packaging materials were extremely high resistance oxygen and resistance wet, high-temperature sterilization and good one-way easy tear, and etc.

Role of surface chemistry and topology of chemoselectively tailored embossed films on shear adhesion

Surface topology has been shown to play a crucial role in the adhesive ability of gecko-inspired, embossed films. Herein, we report the use of Pattern Replication In Non-wetting Templates (PRINT) to design micro- and nano-embossed films in order to explore the relationship between surface chemistry, feature size, feature geometry, and shear adhesion. Ketone-containing elastomers were synthesized from various amounts of poly(ethylene glycol) diacrylate, 2-hydroxyethyl methacrylate, and 2-(methacryloyloxy)ethyl acetoacetate. Due to the presence of the ketone, elastomer functionalization was demonstrated with various oxyamine-, hydrazine-, and hydrazide-terminated ligands. Films were also shown to be non-cytotoxic for future applications in biomedically related fields. This system may allow for the design of flexible adhesives that can be selectively tailored for a range of applications.

THERMAL MANAGEMENT SOLUTIONS FOR THE LED MARKET

High-brightness light emitting diodes (LEDs) are challenged with thermal management issues due to increased power and reduced surface area. This has led to the need for new materials with higher thermal conductivity that can quickly remove the heat from the active layer. LORD Corporation has developed two new thermal management materials, a “no pump-out” thermal grease and a low modulus die attach adhesive, as solutions to the heat dissipation problems facing LED manufacturers. These innovative technologies will help engineers solve complex fundamental thermal management problems. A new 4 W/mK silicone thermal grease has been developed with significant resistance to in-package bleed-out or pump-out eliminating the reliability problem most commonly encountered with traditional thermal greases. A new 10–20 W/mK thermal epoxy adhesive has also been developed creating a new class of flexible adhesives with high adhesion. This combination allows the new LORD die attach adhesive to not only effectively transfer heat out of the package, but also to dissipate the stress caused by thermal expansion and contraction during thermal cycling thereby affording improved package reliability.

Simulation of single-lap bonded and hybrid (bolted/bonded) joints with flexible adhesive

Balanced single-lap bonded and hybrid (bolted/bonded) joints with flexible adhesives have been studied using finite element analysis. The two-dimensional plane strain and three-dimensional analyses have been carried out. Geometrical and material nonlinearities were taken into account. Flexible adhesives were modelled using hyperelastic Mooney–Rivlin potentials. Joint stiffness, as well as adhesive stress distribution, in the overlap has been investigated. The sensitivity of mechanical response to the compressibility of the adhesive material has been demonstrated. Numerical analyses of hybrid (bolted/bonded) joints showed their fatigue life is longer than corresponding bolted joints.

Mechanical Characterization of Flexible Adhesives

In this paper, the performances of two different adhesive types—a polyurethane and a high temperature thixotropic adhesive sealant, room temperature vulcanizing (RTV) silicone rubber—were studied through adhesive joint tests. The standard Thick Adherend Shear Test (TAST) was performed in order to measure the shear properties of the adhesives. Single lap joints (SLJs) were fabricated and tested to assess the adhesive performance in a joint. The influence of temperature on the lap shear strength of the adhesives was investigated. It is shown that the lap shear strength of both adhesives is affected by variation of temperature. The effect of bondline thickness and overlap length on the lap shear strength of the adhesives was studied. The reduction of failure load with increasing the bondline thickness is a very common situation when dealing with structural adhesives. For the low strength flexible adhesive Sikaflex® 552 the failure load as well as the overall stiffness of the SLJs decreases as the bondline gets thicker, whereas for AS1805 RTV adhesive the failure loads increase as the bondline gets thicker. Also, in contrast to joints with brittle adhesives, the failure loads of joints with flexible adhesives increase almost proportionally with increasing overlap length. Fatigue tests were also performed and show a low variability in the results.

Application of Vibrating Beam Method to Determine Dynamic Properties of Flexible Adhesives

Nowadays, ductile adhesives are entering the arena of structural applications due to their high strains to failure that is able to tolerate possible stress—strain concentrations, and due to their high damping performance, which is useful for energy dissipation. In this article, dynamic properties of such an adhesive are investigated using a vibrating beam technique with free—free end conditions. For the measurements of flexural modulus and damping values of the adhesive, the free layer beam configuration with different layer thicknesses was used. It is shown that the technique is able to give the consistent results and the data can be used for design purposes. The results suggest that the dynamic properties of the adhesive are frequency dependent at room temperature, which means the adhesive has a viscoelastic behavior.

Urinary Bromide and Breathing Zone Concentrations of 1-Bromopropane from Workers Exposed to Flexible Foam Spray Adhesives

1-Bromopropane (1-BP) has been marketed as an alternative for ozone depleting solvents and suspect carcinogens and is in aerosol products, adhesives and solvents used for metal, precision and electronics cleaning. Toxicity of 1-BP is poorly understood, but it may be a neurologic, reproductive and hematologic toxin. Sparse exposure information prompted this exposure assessment study using air sampling, and measurement of urinary metabolites. Mercapturic acid conjugates are excreted in urine from 1-BP metabolism involving removal of bromide (Br) from the propyl group. One research objective was to evaluate the utility of urinary Br analysis for assessing 1-BP exposure using a relatively inexpensive, commercially available method. Complete 48 h urine specimens were obtained from 30 workers on two consecutive days at two facilities using 1-BP adhesives to construct polyurethane foam seat cushions and from seven unexposed control subjects. All of the workers' urine was collected into composite samples representing three daily time intervals (at work; after work but before bedtime; and upon wake-up) and analyzed for Br ion by inductively coupled plasma-mass spectrometry. Full-shift breathing zone samples were collected for 1-BP on Anasorb carbon molecular sieve sorbent tubes and analyzed by gas chromatography-flame ionization detection via NIOSH method 1025. Geometric mean (GM) breathing zone concentrations of 1-BP were 92 parts per million (p.p.m.) for adhesive sprayers and 11 p.p.m. for other jobs. For sprayers, urinary Br concentrations ranged from 77 to 542 milligrams per gram of creatinine [mg (g-cr)(-1)] at work; from 58 to 308 mg (g-cr)(-1) after work; and from 46 to 672 mg (g-cr)(-1) in wake-up samples. Pre-week urinary Br concentrations for sprayers were substantially higher than for the non-sprayers and controls, with GMs of 102, 31 and 3.8 mg (g-cr)(-1), respectively. An association of 48 h urinary Br concentration with 1-BP exposure was statistically significant (r(2) = 0.89) for all jobs combined. This study demonstrates that urinary elimination is an important excretion pathway for 1-BP metabolism, and Br may be a useful biomarker of exposure.

Measurements and models for design with modern adhesives

In many applications, adhesively bonded joints are increasingly required to sustain large deformations before failing and modern adhesives are formulated to provide this performance. There is a growing trend for the use of predictive design calculations, such as finite element modelling, to replace prototyping in the product development cycle. However, for modern non-linear adhesives, the modelling and data requirements are not straightforward. Research under the DTI Performance of Adhesive Joints programme has sought to improve understanding of these requirements for rubber-toughened structural adhesives and flexible adhesives. Toughened adhesives are elastic–plastic materials whose behaviour can be characterised at large strains by plasticity theory. The yield stress for rubber-toughened polymers is influenced by cavitation of the toughening phase in the glassy matrix. Functions are proposed to predict the contribution from this yield mechanism, and experimental methods to obtain the required parameters are outlined. Comparisons with test data show that the cavitation model accurately predicts the behaviour of these complex materials. Large strain elastic properties are required to characterise the behaviour of rubber-like flexible adhesives. Such behaviour can be modelled using hyperelastic material models developed for natural and synthetic rubbers. Available evidence suggests that first-order hyperelastic models show most promise. The results shown in this work suggest that the material property data required to model flexible adhesive behaviour can be obtained from uniaxial tension tests without recourse to the full set of tests specified for rubbers.

A comparison of internal self-repair with resin injection in repair of concrete

A comparison is made between the two modes of repair of internal cracks in concrete: resin injection, which is done manually after cracking, and self-repair in which the adhesive for repair is already present in the matrix at the time the cracking occurs. The purpose of the project was to study the effect of various adhesives used for self-repair on the repair of concrete considering factors such as stress transfer capability, varying crack width, and damping. Different release modes and a similar range of adhesives with varying modulus of elasticity were used in the experiments. The purpose for varying the modulus of elasticity was to investigate the penetration ability of the adhesives. It is known that an adhesive's relative stiffness is directly related to its modulus of elasticity, and consequently, its ability to repair cracks is influenced by this factor. Finally, a comparison was made with studies undertaken by others on resin injection repair regarding stress, strain, and modulus of elasticity. Self-repair adhesives with higher modulus of elasticity (stiffer adhesives) transferred stresses well across the crack width allowing the crack to sustain as much, if not more, as the original loading as measured by specimen strength. The adhesives with lower modulus of elasticity (more flexible adhesives) also transferred the stresses.

Respiratory Effects of Toluene Diisocyanate in the Workplace: A Discussion of Exposure-Response Relationships

Toluene diisocyanate (TDI) is an important industrial intermediate used in manufacturing flexible polyurethane (PUR) foams, surface coatings, cast elastomers, sealants, and adhesives. In this review long-term trends in workplace exposures to TDI are assessed in both the producing and using industries, and respiratory health effects of TDI are evaluated in relation to workplace TDI concentrations. The key respiratory health effects associated with repeated or long-term TDI exposure are bronchial asthma and an accelerated rate of decline in lung function. In the early years of the industry, annual incidence rates of occupational asthma (OA) due to TDI ranged from 1% to as high as 5 to 6%, depending on the extent of engineering and work practice controls in the various workplaces. Since the mid-1970s, annual OA incidence rates have been <1%, where 8 h TDI concentrations have been maintained below 5 ppb as determined by personal monitoring, even where short-term TDI concentrations above 20 ppb and less frequently above 40 ppb were routinely detected. In these latter settings, there is evidence that the majority of OA cases may be attributable to TDI concentrations well above 20 ppb associated with overexposure incidents. Further study is needed regarding the role of such incidents in inducing respiratory sensitization. Cross-sectional and longitudinal studies of lung function have indicated that continued exposure after development of work-related respiratory symptoms can lead to transient or accelerated fixed declines in forced expiratory volume in 1 sec (FEV1). These findings are congruent with the FEV1 declines demonstrated in general population studies of persons with persistent bronchial hyperresponsiveness or nonoccupational asthma. More recent longitudinal studies in settings with ongoing medical surveillance have provided no consistent evidence of accelerated FEV1 loss among employees exposed up to 5 ppb TDI on an 8 h time-weighted average basis.

Use of the thick adherend shear test for shear stress-strain measurements of stiff and flexible adhesives

Five commercial structural adhesives were tested using the thick adherend shear test (TAST). These adhesives have mechanical properties ranging from those of high-strength, heat-cured epoxies to ductile, acrylic-based materials. Consideration was given to the adherend selection and dimensions to approach a uniform shear stress-strain in the bonded area, so that the test could be used with both stiff and flexible adhesives. Comparison of the TAST results was also made with those obtained using the butt torsion test. The TAST extensometry has been shown to be suitable for measuring the shear strain properties of the adhesives tested without modification. From the shear behavior of the five adhesives measured using the TAST method, and from the results presented in this paper, it can be seen that the TAST method is repeatable and reproducible for a wide range of adhesive types and adhesive properties. From these results, it is possible to generate comprehensive adhesive shear data. Also, the curves from the butt torsion test and the TAST were found to be consistent and give the same behavior of the adhesives tested.

USE OF THE THICK ADHEREND SHEAR TEST FOR SHEAR STRESS-STRAIN MEASUREMENTS OF STIFF AND FLEXIBLE ADHESIVES

Five commercial structural adhesives were tested using the thick adherend shear test (TAST). These adhesives have mechanical properties ranging from those of high-strength, heat-cured epoxies to ductile, acrylic-based materials. Consideration was given to the adherend selection and dimensions to approach a uniform shear stress-strain in the bonded area, so that the test could be used with both stiff and flexible adhesives. Comparison of the TAST results was also made with those obtained using the butt torsion test. The TAST extensometry has been shown to be suitable for measuring the shear strain properties of the adhesives tested without modification. From the shear behavior of the five adhesives measured using the TAST method, and from the results presented in this paper, it can be seen that the TAST method is repeatable and reproducible for a wide range of adhesive types and adhesive properties. From these results, it is possible to generate comprehensive adhesive shear data. Also, the curves from the butt torsion test and the TAST were found to be consistent and give the same behavior of the adhesives tested.

Finite element and experimental studies on single-lap balanced joints in tension

Adhesively bonded balanced single-lap joints have been studied using finite element analysis and verified either experimentally or theoretically. In the finite element method, a geometrically nonlinear two-dimensional method was applied using ANSYS code Version 5.3. The effects of (i) plane strain and plane stress conditions; (ii) simply supported and fully fixed boundary conditions at the adherend ends; (iii) filleted and unfilleted overlap end geometries; and (iv) two different adhesive materials on the bending moment factor, k, and the adhesive stresses have been evaluated. The finite element simulations were compared with experimental results only for the filleted joint bonded with a relatively flexible adhesive. The results for unfilleted joints bonded with flexible and rigid adhesives were compared with the analytical solution proposed by Oplinger.

Structured latex particles with improved mechanical properties

Structured polymer latex particles are prepared by a swelling emulsion polymerization process, in which the initial particles are first swollen by ethylenically unsaturated monomers and the polymerization of the latter is then carried out. This special polymerization process leads to multi-phase particle morphology. Instead of a thermodynamically more favorable large-scale phase-separation, we obtain multiple, near-spherical domains dispersed within the particles. TEM analysis after selective staining reveals the size and distribution of the microdomains. Dynamic mechanical analysis of the polymer films confirms the absence of a distinct, large second phase and indicates that such microdomains interfere at the molecular level with the segmental mobility of the dispersing phase. We present examples of soft polymers as the continuous phase and hard polymers as the dispersed phase. The inclusion of hard microdomains into soft continuous phase increases dramatically both the films tensile strength and elongation, which means improved cohesive strength of the polymer material. The increase in tensile strength of the polymer film correlates directly with the hardness of the dispersed phase. Improvement in tensile strength and elongation is important in a number of industrial applications of polymers, such as flexible coatings, coalescent-free paints and pressure sensitive adhesives (PSAs). Examples are presented which demonstrate the potential applications of the swelling emulsion polymerization process.

Mechanical and Optical Properties of UV-Curable Modified Acrylic Adhesives

Abstract Ultraviolet (UV) curable modified acrylics constitute a unique class of polymers for optical bonding applications due to their transparency. To elucidate their adhesion characteristics two UV-curable modified acrylics, including rigid and flexible formulations, were evaluated. Two main aspects were investigated. First, durability in hot-humid environment, in combination with epoxy, amine and methacryl terminated silanes, was assessed. Secondly, the optical transparency of the modified acrylic adhesives in the visible and infra-red regions of the spectra was measured. Transmittance was evaluated for the neat adhesives and for the cases where silanes were used both as surface primers as well as additives incorporated into the adhesives. Experimental results show conclusively that enhanced mechanical properties are obtained when silanes are utilized as primers, especially for strength endurance in humid environments. Best performance for the rigid and flexible UV-curable adhesives was obtained when the methacrylic-terminated silane was used. Furthermore, when silanes were used as primers or directly incorporated into the adhesives, some improvement in transmittance was observed in all regions of the spectra. Generally, the UV-curable adhesives studied exhibited more than 90% transparency in the visible and near infra-red regions. Due to inherent absorbing bands, close to 3.5 and 5.5 μm and in the whole range of 8 to 12 μm, transmittance levels are low in these regions. These results indicate that acrylic adhesives in particular and polymer adhesives generally are inadequate optical adhesives in the latter region.

Polyurethanes for Flexible Surface Coatings and Adhesives

Polyurethanes can be tailor-made for a wide range of flexible surface coatings, e.g., for textile substrates and leather; and for a wide variety of adhesives. Polyurethanes give unexcelled performance in almost all of these applications. A prime target of coating textiles with polyurethanes is the achievement of the quality of natural leather. A highly water vapor permeable, but water repellent textile coating has recently become possible using a direct coating process. The application of a micro porous polyurethane film on split leather by a reaction process leads to prime quality leather for shoe uppers. For many years, elastomenc polyurethanes have been the top-ranking adhesives for highly plasticized PVC articles. More recently, polyure thanes have been introduced into the market as structural adhesives, especially for the bonding of automotive body parts. A new principle of heterogeneous cross-linking of polyurethanes which has potential for one-package structural adhesives is pre sented. Aqueous polyurethane dispersions can also be tailor-made for the whole range of two-dimensional applications. They complement the high solids and the powder polyurethane coatings, as well as the solvent-free polyurethane adhesives in the fulfillment of ecological and industrial hygiene requirements.