Subcritical failure of soft acrylic adhesives under tensile stress

The mechanisms of failure over time of a series of model acrylic pressure-sensitive adhesives under a moderate level of stress has been investigated with a probe method. Two competing mechanisms of failure have been observed: a progressive nucleation of cavities under stress and the propagation of existing cavities at the interface between the probe and the adhesive layer. Homogeneous creep of the adhesive was never observed as the only failure mechanism. In situations where the resistance to crack propagation was good relative to the resistance to cavitation, extensive nucleation of cavities was observed until a material-dependent and stable value of stress was achieved. On the other hand in situations were the resistance to crack propagation was weak, propagation led invariably to a complete failure of the adhesive bond. In addition to the stress relaxation, the energy dissipation was studied allowing to distinguish the different adhesives even further. This allowed determination of the optimal amount of a comonomer (acrylic acid) that had to be added to improve the long-term resistance of the adhesives under study. Further more we investigate the compliance of the confined adhesive layers and compare the obtained results to predictions from theoretical models.     

Molecular and nanoscale factors governing pressure‐sensitive adhesion strength of viscoelastic polymers

Pressure‐sensitive adhesives (PSAs) are finding increasing applications in various areas of industry and medicine. PSAs are a special class of viscoelastic polymers that form strong adhesive joints with substrates of varying chemical nature under application of light external bonding pressures (1–10 Pa) over short periods of time (1–5 s). To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, and high cohesive strength and elasticity, which are necessary for resistance to debonding stresses and for dissipation of mechanical energy at the stage of adhesive bond failure under detaching force. For rational design of novel PSAs, molecular insight into mechanisms of their adhesive behavior is necessary. As shown in this review, strength of PSA adhesive joints is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms. At the molecular level, strong adhesion is the result of a narrow balance between two generally conflicting properties: high cohesive strength and large free volume. These conflicting properties are difficult to combine in a single polymer material. Individually, high cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. Evident correlations are observed between the adhesive bond strengths of different PSAs, and their relaxation behaviors are described by longer relaxation times. Innovative PSAs with tailored properties can be produced by physical mixing of nonadhesive long‐ and short‐chain linear parent polymers, with groups at the two ends of the short chains complementary to the functional groups in the recurring units of the long chains. Although chemical composition and molecular structure of such innovative adhesives are unrelated to those of conventional PSAs, their mechanical properties and adhesive behaviors obey the same general laws, such as the Dahlquist's criterion of tack. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

The effect of heat-moisture aging on elastic-deformation properties and the type of failure of adhesive joints in testing for cracking resistance

Results of testing of two groups of film adhesives that differ in their chemical natures, service temperatures, and elastic-deformation characteristics, such as VK-3, VK-50, and VK-26M phenolic rubber adhesives and high-strength VK-51A epoxy adhesive, are reported. The locus of failure of adhesive joints after wedge tests in the initial state and after heat-moisture aging is studied using a scanning electron microscope. It is revealed that, as result of heat-moisture aging, the locus of failure of adhesive joints changes for VK-51A (with and without EP-0234 primer) and VK-3 adhesives, whereas it remains unchanged for VK-50 (with EP-0234 primer) and VK-26M adhesives.     

高效液相色谱-质谱法测定压敏胶及其制品中9种邻苯二甲酸酯

采用高效液相色谱-质谱( HPLC - MS)法,对压敏胶及其制品中的9种邻苯二甲酸酯进行同时测定.以乙酸乙酯为溶剂溶解压敏胶,甲醇沉淀高聚物,以3 000 r/min离心5min,清液定容后,用HPLC -MS分析检测.采用Xterra-Phenyl色谱柱,流动相为乙腈-水,流速为1 mL/min,选择离子监测.在优...     

Properties of adhesives and adhesive materials used in aviation industry

Methods for developing adhesives and adhesive material used in the structures of almost all units of aviation machinery are described step by step. The properties of high-strength film-forming adhesives; adhesive prepregs; sandwich-type aluminum-polymer materials (SIALs); cold- and hot-setting adhesives intended for gluing metals, different nonmetallic materials, and combined joints; and self-adhesive film materials of ZPPK and FAS grades are given. It is also shown where and for which purposes these adhesives are applied.     

The applicability of adhesives in Arctic conditions

Analysis of properties of cold-setting epoxy adhesives and various types of film adhesives is carried out with reference to their applicability in Arctic conditions. Data on waterproofness, water absorption, and hygroscopic properties of the set adhesive films are determined. The application of adhesive joints at–60°C is considered.     

A reversible water-based electrostatic adhesive

Commercial adhesives typically fall into two categories: structural or pressure sensitive. Structural glues rely on covalent bonds formed during curing and provide high tensile strength whilst pressure-sensitive adhesives use physical bonding to provide weaker adhesion, but with considerable convenience for the user. Here, a new class of adhesive is presented that is also reversible, with a bond strength intermediate between those of pressure-sensitive and structural adhesives. Complementary water-based formulations incorporating oppositely charged polyelectrolytes form electrostatic bonds that may be reversed through immersion in a low or high pH aqueous environment. This electrostatic adhesive has the advantageous property that it exhibits good adhesion to low-energy surfaces such as polypropylene. Furthermore, it is produced by the emulsion copolymerization of commodity materials, styrene and butyl acrylate, which makes it inexpensive and opens the possibility of industrial production. Bio-based materials have been also integrated into the formulations to further increase sustainability. Moreover, unlike other water-based glues, adhesion does not significantly degrade in humid environments. Because such electrostatic adhesives do not require mechanical detachment, they are appropriate for the large-scale recycling of, e.g., bottle labels or food packaging. The adhesive is also suitable for dismantling components in areas as varied as automotive parts and electronics.     

Surface properties and abhesion of undecyl oxazoline block and homopolymers

The surface properties of three undecyl oxazoline homopolymers and two phenyl/undecyl oxazoline block copolymers (as comparison) were studied. After coating on glass slides and annealing, all films had a low critical surface energy of 21 dynes/cm. Water contact angles were higher than 107° for the most hydrophobic films. The deduction that the polymer surfaces contained close-packed methyl groups was further confirmed by electron spectroscopy chemical analysis (ESCA) angle profiling on an annealed undecyl oxazoline homopolymer film. A model was developed for the variation of elemental ratios as a function of photoelectron take-off angle. This verified that the polymer films had the polymer backbones parallel to the surface with the undecyl tails oriented toward the surface. When these block and homopolymers were coated on copy paper and glass slides, the peel strengths of pressure-sensitive adhesives with these surfaces were very low for short dwell times at room temperature. At long dwell times or at elevated temperatures, the peel strengths remained low for the homopolymers but increased greatly for the block copolymers to values higher than those in the tape on glass. After 24 h at 70°C, ESCA analysis showed that the adhesive diffused into the phenyl block domains of the diblock copolymer, generating high peel strength and cohesive failure. However, under the same annealing conditions, the triblock copolymer showed adhesive failure while peel strength increased. ESCA analysis showed very litle diffusion of the adhesive into the triblock copolymer. The homopolymers were stable toward vinyl acetate type adhesives even at elevated temperature; they were abhesive up to 100°C with no interdiffusion.     

Scaling properties of contact area of pressure-sensitive adhesives

Two new devices were developed to provide accurate measurements of both the contact area and the tack strength of pressure-sensitive adhesives (PSAs). The first one is the "mechano-optical tack tester" (MOTT), which was designed to apply controlled contact pressure by means of a quartz prism probe, for determined contact times, onto the surfaces of PSA samples. The contact area is measured by the reflection of light at the quartz probe surface, which is in contact with the adhesive. The second device is an "acoustic contact tester" (ACT) that measures the tack strength and the contact area between a silicate glass and an adhesive by the reflection of an acoustic wave. Two ultrasonic sensors of different acoustic wavelengths have been built in order to study the scaling effects of the contact kinetics between an adhesive and the probe. MOTT and ACT experiments on a commercial PSA show that the contact area is the main parameter that governs the tack properties of the PSA. The experiments and the modeling point out that the contact area depends on the compression stress, the roughness, the thickness, and the Young's modulus of the PSA. However, comparison of contact kinetics from MOTT and ACT experiments show that the contact area is a subjective parameter that depends on the wavelength of the reflected beam used for its measurement.     

Cross deformation and stress relaxation of biaxially oriented polystyrene films

When a biaxially oriented polystyrene film was stretched along one direction and subsequently stretched along the perpendicular direction, the film showed enhanced ductility with pronounced yield softening and extended strain hardening. In the forward deformation, at least two types of shear bands were observed. The bands at the early stages of yielding did not seem to contribute to the reduction of thickness. They were approximately 200 μm thick and had an intersection angle of approximately 120°. The bands developed in the later stages contributed to the thickness reduction. These bands were smaller and possessed an intersection of approximately 90°. In the cross deformation, new shear bands developed that were likely related to the reverse shearing of the existing bands. Stress relaxation showed a power‐law relationship between the stress rate and relaxation time. The internal stress of the cross deformation was significantly (ca. 3 times) lower than that of the forward deformation at the same strain. The enhancement in ductility may be attributed to the lowering of internal stress during the cross deformation. The internal stress increased with the applied stress and strain. Fracture occurred when the internal stress reached a certain level, about 57–68 MPa for deformation along both directions and approximately 44–47% of the final applied stress. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 687–700, 2003     

Transitions/relaxations in polyester adhesive/PET system

The correlations between the transitions and the dielectric relaxation processes of the oriented poly(ethylene terephthalate) (PET) pre-impregnated of the polyester thermoplastic adhesive have been investigated by differential scanning calorimetry (DSC) and dynamic dielectric spectroscopy (DDS). The thermoplastic polyester adhesive and the oriented PET films have been studied as reference samples. This study evidences that the adhesive chain segments is responsible for the physical structure evolution in the PET-oriented film. The transitions and dielectric relaxation modes’ evolutions in the glass transition region appear characteristic of the interphase between adhesive and PET film, which is discussed in terms of molecular mobility. The storage at room temperature of the adhesive tape involves the heterogeneity of the physical structure, characterized by glass transition dissociation. Thus, the correlation between the transitions and the dielectric relaxation processes evidences a segregation of the amorphous phases. Therefore, the physical structure and the properties of the material have been linked to the chemical characteristics.     

Main stages of gluing technology

The main stages of gluing technology—from surface preparation for gluing to control of the quality of adhesive joints—are considered. Methods of preparation for gluing of surfaces of polymer materials and metals are given. Recommendations for preparation and coating of adhesives are given. The main parameters of the adhesive curing and apparatus used are described.     

VISCOELASTIC BEHAVIOR OF THE NON-HOOKE DEFORMATION BEFORE YIELDING IN UNIAXIAL DRAWING OF AMORPHOUS POLY(ETHYLENE TEREPHTHALATE)*

Viscoelastic behavior of the non-Hooke deformation of amorphous PET film before yield was investigated in thetemperature region 74--80.5℃ around the glass transition temperature. The film specimen was drawn to yield point followedby unloading to zero stress, then the residual deformation was held constant, while the subsequent evolution of the stress wasrecorded. An induction period was found in the course of stress evolution fol1owed by a stress step-increase. The inductionperiod decreases with increasing drawing temperature with an activation energy of 1.10 MJ/mol·K, which is attributed tothe time needed for the relaxation of rubbery deformation through cooperative internal rotations. At temperatures lower than74℃, there is no stress increase or the induction period becomes too long to be observed. Thus the nature of anelasticity inthe non-Hooke region before yielding is attributed to stress induced rubbery deformation. The experimental results areinterpreted in terms of Perez' rheological model of a series connected Hooke spring and a Voigt element consisting of aparallel connected elastic spring and a dashpot.     

Properties of solvent based polyurethane adhesives containing fumed silicas

Five hydrophilic and two hydrophobic fumed silicas of different surface area and particle size were added to solvent based polyurethane adhesives. Silica addition produced a noticeable increase in the adhesive viscosity, imparted negative thixotropy, increased the storage modulus (G') and improved the green adhesion of chlorinated rubber/PU adhesive/chlorinated rubber joints. Those modifications were more pronounced in the adhesives which contain hydrophilic silicas.     

聚甲基乙烯基硅氧烷/炭黑复合体系的电阻弛豫行为

考察了聚甲基乙烯基硅氧烷/炭黑(CB)复合交联体系在恒定压应力作用下以及完全卸载后的导电行为,发现电阻在恒定压应力作用下及完全卸载后均随时间非线性下降,呈现典型的电阻弛豫行为.根据电阻弛豫时间以及最大电阻弛豫幅度,分析了导电网络结构变化与电阻弛豫之间的关联,讨论了炭黑含量对电阻弛豫的影响.结果表明,在恒定压应力2~4MPa下,电阻弛豫具有两个与CB含量无关的弛豫时间,对应不同尺寸导电网络的结构变化.完全卸载后,基体形变回复造成导电网络发生结构弛豫,弛豫时间随CB含量增大而延长.     

Bio-based and Degradable Block Polyester Pressure-Sensitive Adhesives

A new class of bio-based fully degradable block polyesters are pressure-sensitive adhesives. Bio-derived monomers are efficiently polymerized to make block polyesters with controlled compositions. They show moderate to high peel adhesions (4–13 N cm⁻¹) and controllable storage and loss moduli, and they are removed by adhesive failure. Their properties compare favorably with commercial adhesives or bio-based polyester formulations but without the need for tackifier or additives.     

Properties of adhesive materials used in repair-and-renewal operations

Advantages of repair technologies based on the use of adhesive materials are shown. Basic properties are given for adhesive materials used in repair operations, namely, epoxy-based metal-filled materials; film adhesives and adhesive prepregs; anaerobic repair compositions based on caoutchoucs and acrylates; and epoxy, acrylate, and urethane acrylates used for adhesion under conditions of increased humidity and under water. Some operation technologies for repair and renewal based on the use of adhesive materials are considered, namely, technologies that deal with breakdowns and preventive repairs on oil and gas pipelines, in the aviation industry (repair of honeycomb sandwiches included), in power engineering, and in the chemical and petrochemical industry, as well as those that solve relevant problems of car repair.     

Modification of adhesives based on poly(vinyl acetate) dispersion by the Sol-Gel method

It is shown that parameters of some properties of adhesives and adhesive joints can be increased by treating poly(vinyl acetate) dispersion with tetraethoxysilane or ethyl silicates. It is established that the mechanism of the effect of these ethers of orthosilicic acid are the sol-gel processes that lead to the formation of an organic-inorganic matrix.     

Condensation and relaxation/transformation of dense t-ZrO2 nanoparticles

Dense tetragonal (t)-ZrO(2) nanocondensates were synthesized under very rapid heating and cooling by pulsed Nd-YAG (YAG--yttrium aluminum garnet) laser ablation with oxygen background gas and characterized by electron diffraction. The t-ZrO(2) nanoparticles with a residual stress up to about 5 GPa tended to form deformation twins/faults upon local electron dosage. By contrast, the t-ZrO(2) nanoparticles formed at an-order-of-magnitude higher power have a residual stress above 6 GPa and tended to transform into a metastable cubic (c) phase vulnerable to amorphization. The relaxation/transformation of the self-constrained t-ZrO(2) nanoparticles can be rationalized by a lowering of internal energy under the influence of resolved shear stress and local electron heating.