Characterization method of real shear rheological properties of thin and soft adhesive layer material under constraint

Abstract

The application discloses a method for characterizing real shear rheological properties of a thin soft adhesive layer material under constraint conditions, and relates to the technical field of material mechanical property testing, and solves the technical problem that the rheological response of a micron-level thin soft adhesive layer deviates from the material body properties under real service conditions due to structural constraint conditions, and the rheological response is difficult to accurately measure; the method comprises the following steps: S1, preparing samples with different thicknesses of thin soft adhesive layers; S2, measuring rheological responses of each sample at different temperatures and frequencies as dynamic mechanical test data; S3, selecting a nonlinear function to establish a quantitative evolution relationship of the rheological response with the thickness of the adhesive layer; and S4, determining the influence range of the structural constraint on the shear rheological properties of the material by evaluating the characteristic thickness, and obtaining the real shear rheological properties of the soft adhesive layer material body; the method can reasonably consider the influence of the structural constraint effect on the response of the thin soft adhesive layer material and obtain the real shear rheological properties of the material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of material mechanical property testing, and in particular to a method for characterizing the true shear rheological properties of a thin soft adhesive layer material under constraint conditions. BACKGROUND

[0002] Soft adhesive materials, represented by pressure sensitive adhesive (PSA), optical clear adhesive (OCA), and hydrogel, are usually used as thin soft adhesive layers to connect high modulus adherends such as metals, glasses, or composite materials to form adhesive structures with significant constraint effects in practical service fields such as medical devices, flexible devices, and consumer electronics. The existing performance characterization methods do not fully consider the influence of structural constraint effects on the mechanical behavior of thin soft adhesive layers, resulting in significant differences between the obtained mechanical response characteristics and the test results in the free state. In the performance testing of micron-scale adhesive layers, structural constraint effects can cause two key problems: first, an excessively thin adhesive layer can cause systematic overestimation of test results, leading to overestimation of material performance, which may cause serious design errors in engineering applications; second, increasing the thickness excessively can reduce the influence of constraint effects, but it can also cause material waste, increased process difficulty, and other problems such as uneven coating, curing defects, and a significant increase in the risk of defects. SUMMARY

[0003] To solve the problems existing in the prior art, the present application provides a method for characterizing the true shear rheological properties of a thin soft adhesive layer material under constraint conditions, which solves the technical problem of accurately measuring the rheological response and material bulk properties of a micron-scale thin soft adhesive layer under real service conditions due to structural constraint conditions.

[0004] A method for characterizing the true shear rheological properties of a thin soft adhesive layer material under constraint conditions, comprising:

[0005] S1, preparing test samples with different thicknesses of thin soft adhesive layers;

[0006] S2, using a dynamic mechanical analyzer to apply dynamic loads with equal shear strain to the test samples, and measuring the rheological responses of each test sample at different temperatures and frequencies as dynamic mechanical test data, the rheological responses including storage modulus G’ , loss modulus G’’ , and loss tangent tan delta ;

[0007] S3, based on the dynamic mechanical test data, selecting a nonlinear function to establish a quantitative evolution relationship of the rheological response with the thickness of the adhesive layer;

[0008] S4, determine the influence range of structure constraint on material shear rheological property by evaluating the feature thickness, and obtain the real shear rheological property of soft adhesive layer material body.

[0009] Further, the test sample is a lap shear test sample.

[0010] Further, the thickness of the thin soft adhesive layer is in the range of 10-500 μm, and the modulus ratio of the base material of the test sample to the thin soft adhesive layer is greater than 100.

[0011] In the rheological test, after selecting the film stretching mode, the geometric size "width Width" and "thickness Thickness" should be set as the adhesive surface width and lap length of the lap shear test sample, and the "length Length" is the distance between the clamps at both ends of the test sample. At this time, the apparent modulus measured by the testing machine S and the shear modulus G have a conversion relationship , wherein h is the thickness of the adhesive layer.

[0012] Further, based on the nonlinear large deformation characteristics of the soft adhesive layer, the power-law stress singularity model of the traditional linear-elastic hard adhesive layer is no longer applicable. Therefore, the nonlinear function is suggested to use a weak singularity type function to describe the evolution law of the shear rheological response of the thin soft adhesive layer with thickness. This function form has the advantages of physical meaning and simple mathematical expression form. For higher precision fitting requirements, a type polynomial function with more fitting parameters can be used as an alternative solution.

[0013] Further, the quantitative evolution relationship of the rheological response with the thickness of the adhesive layer is as follows:

[0014] (1)

[0015] wherein, and are defined as the real shear rheological properties of the soft adhesive layer material, which are related to the loading conditions such as frequency and temperature. The loading conditions include frequency and temperature , H’ and H’’ are the correction terms of the storage modulus and the loss modulus with respect to the thickness h , m 1 and m 2 are the power exponent parameter terms corresponding to the storage modulus and the loss modulus, n 1 and n 2 are the logarithmic correction terms corresponding to the storage modulus and the loss modulus.

[0016] Further, the influence law of thickness on shear rheological property suggests to describe by piecewise function: when , the relationship between shear rheological response and thickness is described by nonlinear function; when , the evolution curve of modulus with thickness tends to be horizontal, and the shear rheological response measured by the lap shear sample is close to the bulk property of the material.

[0017] When the thickness of the soft adhesive layer is greater than the characteristic thickness, the shear rheological response of the material tends to be stable, and the result of rheological response is close to the real property of the bulk material.

[0018] (2)

[0019] In the formula, is the thickness, is the characteristic thickness.

[0020] For the lap shear sample, based on the Saint-Venant principle, the influence range of the structural constraint effect is limited, so when the thickness of the soft adhesive layer is greater than a certain characteristic thickness, the shear rheological response of the material tends to be stable. In other words, when the actual thickness of the soft adhesive layer reaches the characteristic thickness h c , the influence of the structural constraint effect on the shear rheological property test result of the soft adhesive layer material can be ignored, and the measured rheological property (such as storage modulus G’ , loss modulus G’’ ) tends to the real value of the bulk material. The characteristic thickness can be considered to be related to the size of the dissipation zone of the adhesive layer along the thickness direction (i.e. theta is π / 2), which can be determined according to the rheological test of the lap shear sample of the soft adhesive layer with different thicknesses, and the critical thickness theoretical value can be approximately estimated based on the theoretical method under the plane strain condition to assist in verifying the rheological test result.

[0021] Further, the characteristic thickness can be calculated by the dimensionless parameter α (π / 2) 、 The shear modulus G , Poisson's ratio nu , intrinsic fracture energy gamma 0 and initial yield strength sigma y of the adhesive layer.

[0022] The beneficial effects of the present application include:

[0023] The present application proposes to use the rheological response test data of the lap shear sample containing thin soft adhesive layer with different thickness, based on the size-related structural constraint effect analysis method, to realize the quantitative characterization of the real shear rheological properties of thin soft adhesive layer material under the constraint condition. The method uses a dynamic mechanical analyzer system to determine the rheological parameters such as storage modulus and loss modulus of samples with different thickness, considers the size-related structural constraint physical meaning, establishes the evolution law of shear rheological response with the thickness of the adhesive layer, determines the optimal thickness that can characterize the real rheological properties, i.e. the so-called characteristic thickness, and thus obtains the real shear rheological properties of thin soft adhesive layer material under the constraint condition. Compared with the traditional test method of material rheological properties in free state, the present application is suitable for materials such as thin soft adhesive that is difficult or even impossible to prepare according to the size requirements of conventional methods, can reasonably consider the influence of structural constraint effect on the response of thin soft adhesive layer material and obtain the real shear rheological properties of the material, and helps to solve the key problem that the "laboratory test data" and "actual performance" are disconnected in engineering application, and provides an effective performance characterization means for the application design of soft adhesive materials in the field of flexible electronics. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The flow chart of the method for characterizing the real shear rheological properties of thin soft adhesive layer material under the constraint condition involved in the embodiments of the present application.

[0025] Figure 2 The schematic diagram of the lap shear sample involved in the embodiments of the present application.

[0026] Figure 3 The schematic diagram of the evolution law of shear rheological properties with the thickness of the adhesive layer, the nonlinear fitting function, the characteristic thickness and the bulk properties of the material involved in the embodiments of the present application.

[0027] Figure 4 The relationship between the storage modulus of the lap shear sample and the thickness of the adhesive layer under different frequencies involved in the embodiments of the present application.

[0028] Figure 5 The relationship between the loss modulus of the lap shear sample and the thickness of the adhesive layer under different frequencies involved in the embodiments of the present application.

[0029] REFERENCE NUMERALS

[0030] 1-high modulus substrate, 11-clamping area, 12-lap area, 2-thin soft adhesive layer. DETAILED DESCRIPTION

[0031] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0032] Embodiment 1

[0033] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. Figure 1 The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0034] A method for characterizing the true shear rheological properties of a thin soft adhesive layer material under constraints, comprising:

[0035] S1, preparing a sample with a thin soft adhesive layer of different thicknesses;

[0036] S2, using a dynamic mechanical analyzer to apply a dynamic load of equal shear strain to the sample, and measuring the rheological response of each sample at different temperatures and frequencies as dynamic mechanical test data, wherein the rheological response includes storage modulus G’ , loss modulus G’’ and loss tangent tan delta ;

[0037] S3, based on the dynamic mechanical test data, selecting a nonlinear function to establish a quantitative evolution relationship of the rheological response with the thickness of the adhesive layer;

[0038] S4, determining the influence range of the structural constraints on the shear rheological properties of the material by evaluating the characteristic thickness, and obtaining the true shear rheological properties of the soft adhesive layer material itself.

[0039] The sample is a lap shear sample. The thickness of the thin soft adhesive layer ranges from 10 to 500 μm, and the modulus ratio of the base material of the sample to the thin soft adhesive layer is greater than 100.

[0040] Specifically, the thin soft adhesive layer material in the embodiment is an optically transparent adhesive with viscoelastic properties, which exhibits liquid flowable properties before curing; the base material is high modulus inorganic glass (the thickness of the single piece of glass at the lap joint area is 0.95 mm). The size of the adhesive surface of the lap shear sample is 10 mm*10 mm, and multiple samples of different thicknesses (ranging from 30 to 300 μm) are prepared to characterize the influence of the structural constraint effect.

[0041] Liquid optically clear adhesive was applied to the surface of the glass substrates, roll coating was suggested to achieve uniformity, after bonding the two glass substrates, a UV curing lamp (power 24 W) was used to irradiate for at least 120 s to ensure the curing of the bonding layer.

[0042] After the completion of the specific curing, a single lap shear sample was obtained, as shown in Figure 2 , including two high modulus substrates 1, the high modulus substrate 1 includes a clamping area 11 and a lap area 12, the clamping area 11 is used for mechanical analyzer clamping, and the lap area 12 is the overlapping area of the two high modulus substrates 1, corresponding to the thin soft adhesive layer 2, which is also the area of shear stress concentration. In the figure L 0 The distance between the clamps at both ends of the sample, L 1 The lap length, L 2 The clamping length, b The substrate width, H The total thickness, F is the dynamic load applied by the subsequent mechanical analyzer.

[0043] After the completion of the curing, the total thickness between the upper and lower substrates of the bonding layer was measured using a digital micrometer after standing for at least 1 hour (as shown in Figure 2 The total thickness minus the thickness of the upper and lower substrates can obtain the thickness of the bonding layer, the thickness of the bonding layer of each sample was measured at least three times and the average value was taken.

[0044] Rheological test test process:

[0045] Rheological test was carried out by dynamic mechanical analyzer, thin film tensile mode was selected to apply dynamic load. In the software parameter setting, the "width Width" and "thickness Thickness" of the geometric size should be set as the bonding surface width and lap length of the lap shear sample (as shown in Figure 2 ), and the "length Length" is the distance between the clamps at both ends of the sample L 0 It can be directly determined by the "Measure" function of the instrument.

[0046] Rheological tests were performed by using a temperature step-multiple frequency sweep mode (Temp Step / Freq Sweep) through the DMA Multi-Frequency-Strain module. The frequency sweep range was from 0.02 Hz to 200 Hz (4 test frequency points were set for each decade), the initial temperature was set at 30 ℃, and the temperature was increased by 10 ℃, and the isothermal equilibrium time at each temperature point was 2 min. The dynamic load amplitude was adaptively adjusted according to the thickness h of the adhesive layer of the lap shear sample to ensure that different samples had approximately the same shear strain, and the shear strain in this embodiment was set to about 0.15.

[0047] As shown in Figure 3 , the shear rheological response of the soft adhesive layer, such as the storage modulus and the loss modulus, presents a three-stage evolution characteristic with the change of the thickness: in the constraint-dominated stage (h h < h c ), the modulus rapidly rises with the decrease of the thickness; when approaching the characteristic thickness, it enters the transition stage (h h ≈ h c ), the modulus changes tend to be gentle; and finally, in the bulk property-dominated stage (h h > h c ), the modulus tends to a stable value, reflecting the true properties of the soft adhesive layer material.

[0048] The experimental results of the shear rheological response data of the storage modulus and the loss modulus at different frequency conditions with the evolution of the thickness are shown in Figures 4-5 . Due to the stress concentration induced by the constraint effect near the interface forming a singular field, the adhesive layers of different thicknesses present significantly different rheological responses. In view of the nonlinear large deformation characteristics of soft materials, the stress distribution of the thin soft adhesive layer along the thickness direction deviates from the power-law singularity prediction of the traditional linear elastic theory. Therefore, it is suggested to use a nonlinear function containing a logarithmic term to fit the relationship between the adhesive layer thickness and the structural rheological response, and the function has the following form:

[0049] The nonlinear function uses a type function containing a logarithmic term, and the quantitative evolution relationship of the rheological response with the adhesive layer thickness is as follows:

[0050] (1)

[0051] wherein, and are defined as the true shear rheological properties of the soft adhesive layer material, and are related to the loading conditions, including the frequency and the temperature , H’ and H’’G' and G" are the storage modulus and loss modulus, respectively h G' and G" are the storage modulus and loss modulus, respectively m G' and G" are the storage modulus and loss modulus, respectively m G' and G" are the storage modulus and loss modulus, respectively n G' and G" are the storage modulus and loss modulus, respectively n G' and G" are the storage modulus and loss modulus, respectively G' and G" are the storage modulus and loss modulus, respectively

[0052] It is particularly pointed out that: based on the Saint-Venant principle, the influence range of structural constraint effect is limited, therefore, when the thickness of soft adhesive layer is greater than the characteristic thickness, the shear rheological response of the material will tend to be stable. In other words, when h is greater than or equal to the characteristic thickness h c , the rheological response result is close to the real performance of the material body, at this time, and both satisfy the value of 1. That is:

[0053] (2)

[0054] In the formula, is the thickness, is the characteristic thickness.

[0055] Therefore, the real shear rheological performance of the soft adhesive layer can be measured by the rheological test using the lap shear sample with a thickness greater than the characteristic thickness.

[0056] The characteristic thickness of the soft adhesive layer material is closely related to the influence range of the structural constraint effect, in addition to being determined by the above experimental method, for the lap shear configuration and other cases conforming to the plane strain assumption, the characteristic thickness can be estimated based on the following theoretical formula:

[0057] (3)

[0058] wherein, the shear modulus G , Poisson's ratio nu , intrinsic fracture energy gamma 0 and initial yield strength sigma Y are material parameters of the soft adhesive layer, the dimensionless parameter α ( theta ) is related to the stress triaxiality of the soft adhesive layer under the structural constraint condition, when theta is π / 2, the value range is 1.25 to 5. In order to conservatively estimate the characteristic thickness to measure the material body performance, the parameter is recommended to be 2~5. Based on the above method, the constraint effect and the soft adhesive layer material body performance can be separated by the thickness evolution law, which effectively avoids the risk of overestimating the performance of the thin layer material by the traditional method.

[0059] In this embodiment, rheological tests were carried out on multiple lap shear specimens with a thickness of the soft adhesive layer ranging from 30 μm to 230 μm, and the storage modulus and loss modulus results of the soft adhesive layer under different frequency conditions were obtained (e.g. Figures 4-5 As shown). The test results show that: when the thickness of the adhesive layer is thin ( h <50 μm), due to the strong constraint of the substrate on the deformation of the material, the storage modulus and loss modulus show a rapid growth trend with decreasing thickness, among which the modulus value measured at a thickness of 30 μm is more than 80% higher than that of the thicker layer area; when the thickness of the adhesive layer is thicker ( h >50 μm), the modulus change tends to be stable, and the shear rheological response fluctuation range is small after exceeding the characteristic thickness. This platform value can be considered to be close to the true shear rheological properties of the material. The experimental results of the characteristic thickness are estimated to be ~50 μm. It is worth noting that this staged transition process from the dominance of the constraint effect to the dominance of the material's bulk properties occurs over a wide frequency range covering four orders of magnitude ( Figures 4-5 The results show that the 7 frequency points (0.02 Hz, 0.2 Hz, 2 Hz, 6.3 Hz, 20 Hz, 63 Hz and 200 Hz) are consistent, which fully verifies the reliability of the characteristic thickness criterion. From the perspective of theoretical model analysis, based on the material parameters of the soft adhesive material (shear modulus is 27 kPa, Poisson's ratio is 0.48, intrinsic fracture energy is about 200 J / m 2 , the initial yield strength can be estimated to be 280 kPa), calculated by the theoretical model under the plane strain assumption, when the dimensionless parameter α ( theta ) is 2, the theoretically predicted characteristic thickness is ~56.3 μm, which is close to the experimentally estimated characteristic thickness. In practical engineering applications, it is recommended to use lap shear specimens slightly larger than the characteristic thickness (e.g., 1.5 times the characteristic thickness) for testing. This approach minimizes the risk of uneven coating and curing defects while ensuring that the effects of structural constraints are minimized, while balancing material economy and process feasibility. This combined experimental testing and theoretical prediction method provides a reliable technical solution for accurately characterizing the true rheological properties of micron-thin soft adhesive layer materials under constrained conditions.

[0060] The above-described embodiments merely represent specific implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of protection of the present application. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the technical concept of the present application, and all such variations and improvements fall within the scope of protection of the present application.

Claims

1. A method for characterizing the true shear rheological behavior of thin, soft adhesive layer materials under constraints, characterized in that, The method comprises the following steps: S1, preparing a sample with a thin soft adhesive layer of different thickness; S2, using a dynamic mechanical analyzer to apply dynamic load of equal shear strain to the sample, measuring rheological response of each sample at different temperatures and frequencies as dynamic mechanical test data, the rheological response including storage modulus G’ , loss modulus G’’ , and loss tangent tan δ ; S3, based on the dynamic mechanical test data, selecting a nonlinear function to establish a quantitative evolution relationship of rheological response with the thickness of the adhesive layer; S4, through the evaluation of the characteristic thickness, determining the influence range of the structural constraint on the shear rheological properties of the material, and obtaining the true shear rheological properties of the soft adhesive layer material body; When the thickness of the soft adhesive layer is less than the characteristic thickness, the quantitative evolution relationship of the rheological response with the thickness of the adhesive layer is as follows: ； wherein, and the real shear rheological properties of the soft adhesive layer material are defined as a function of the loading conditions, including the frequency and the temperature , H’ and H’’ are the correction terms of the storage modulus and the loss modulus, respectively, as a function of the thickness h , m 1 and m 2 are the power exponent parameter terms of the storage modulus and the loss modulus, respectively, n 1 and n 2 are the logarithmic correction terms of the storage modulus and the loss modulus, respectively. When the thickness of the soft adhesive layer is greater than the characteristic thickness, the shear rheological response of the material tends to be stable, and at this time, the result of the rheological response is the true performance of the material body, and the expression is as follows: ； In the formula, is the thickness, is the characteristic thickness; The characteristic thickness is given by the dimensionless parameter α (π / 2) 、 Shear modulus of the adhesive layer G Poisson's ratio ν Intrinsic fracture energy Γ 0 and initial yield strength σ y Calculated: ； wherein the shear modulus G , the Poisson's ratio ν , the intrinsic fracture energy Γ 0 and the initial yield strength σ Y is a material parameter of the soft adhesive layer, a dimensionless parameter α θ is related to the stress triaxiality of the soft adhesive layer under the structural constraint conditions, and when θ its value ranges from 1.25 to 5.​ 2. The method of claim 1, wherein the thin, soft adhesive layer material is characterized under a constraint condition. The sample is a lap shear sample.

3. The method for characterizing the true shear rheological properties of a thin soft adhesive layer material under constraint conditions according to claim 1, characterized in that: The thickness of the thin soft adhesive layer ranges from 10 to 500 μm, and the modulus ratio of the base material of the sample to the thin soft adhesive layer is greater than 100.

4. The method of claim 1, wherein the thin, soft adhesive layer material is characterized under a constraint condition. The relationship between the thickness of the adhesive layer and the structural rheological response was fitted using a nonlinear function of the form containing a logarithmic term. The relationship between the thickness of the adhesive layer and the structural rheological response was fitted using a nonlinear function of the form containing a logarithmic term.

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