This research has been partially supported by the Spanish Ministry of Science and Innovation [Grant MTM2014-52876-R], [MTM2017-82724-R] and by Xunta de Galicia (Unidad Mixta de Investigaci&#243;n UDC-Navantia [IN853B-2018/02]). We would like to thank TA Instruments for the image showing the scheme of the rheometer used. This image is included in the Table of Materials of the article.&#160;We also would like to thank Journal of Thermal Analysis and Calorimetry for its permission for using some data from reference [16],&#160;and the Centro de Investigaciones Cient&#237;ficas Avanzadas (CICA) for using its facilities.

1. Checking the manufacturer curing conditions
<ol>
	<li>Cure the adhesive sample following the manufacturer recommendations, and then evaluate it by a TGA and a DSC test. Record the specific curing conditions.</li>
	<li>TGA test of cured sample
	<ol>
		<li>Perform thermogravimetric tests in a TGA or in a simultaneous DSC+TGA equipment (SDT).</li>
		<li>Carry out a thermogravimetric test of the cured sample following the next steps&#160;to determine the inorganic filler content and the temperature at which the material starts to degrade. Do not exceed that temperature in further tests.</li>
		<li>Open the air stopcock. Switch on the SDT (or TGA) apparatus. Open the SDT control software.</li>
		<li>Open the furnace of the SDT and place two empty capsules: one will be the reference capsule and the other will contain the sample.</li>
		<li>Close the furnace and press the bottom Tare.</li>
		<li>Open the furnace and place a sample size of 10-20 mg in the sample capsule.</li>
		<li>Fill the information about the sample in the tab Summary.</li>
		<li>Open the tab Procedure and click Editor. Drag the segment type Ramp to the Editor screen. Establish the ramp as 10 or 20 &#176;C/min to 900 &#176;C. Click OK.</li>
		<li>Open the tab Notes. Choose Air as the purge gas and establish a flow rate of 100 mL/min. Click Apply. 
		NOTE: TGA test has two objectives: 1) to determine the inorganic filler content and 2) to determine the temperature at which the material starts to degrade. For the first objective the test has to be performed in an air atmosphere. For the second one, an air atmosphere represents the most common situation in normal use.</li>
		<li>Close the furnace.</li>
		<li>Start the experiment.</li>
	</ol>
	</li>
	<li>DSC test of cured sample
	<ol>
		<li>Carry out the DSC tests on a standard DSC or on a modulated temperature DSC (MTDSC) instrument working in standard mode, use aluminum crucibles. Carry out a DSC test of the cured sample following the next steps to study the following parameters: the Tg of the material, a possible residual curing and the Tg&#8734; of the sample.</li>
		<li>Open the nitrogen stopcock. Switch on the DSC apparatus. Open the control software of the DSC instrument.</li>
		<li>Click Control | Event | On. Then click the tab Tool | Instrument Preferences, choose DSC and establish a standby temperature of 30 &#176;C.</li>
		<li>Click Apply. Click the tab Control | Go to Standby Temperature and wait for at least 45 min before starting any experiment.</li>
		<li>Open the tab Summary. Click Mode and choose Standard.</li>
		<li>Open the tab Procedure, click Test and choose Custom. Click Editor.</li>
		<li>Drag an Equilibrate segment indicating the temperature at which to start the experiment (that temperature should be relatively low, for example -80 or -60 &#176;C).</li>
		<li>Drag the segment type Ramp to the Editor screen. Introduce a heating rate of 10 or 20 &#176;C/min and the final temperature into the command editor window. The final temperature is tentatively chosen to allow for a complete cure and must be lower than the degradation temperature obtained from the previous TGA test. 
		NOTE: These recommended heating rates are proposed as a starting point that will probably work fine in most cases. However, these heating rates can be modified to improve sensitivity or resolution.</li>
		<li>Drag the segment type Ramp to the Editor screen. Similarly, to the previous step, introduce a 10 or 20 &#176;C/min cooling rate to a temperature tentatively below the glass transition.</li>
		<li>Drag the segment type Ramp to the Editor screen. Introduce a 10 or 20 &#176;C/min heating rate to a temperature slightly below the degradation temperature.</li>
		<li>Open the tab Notes. Choose Nitrogen as the flow gas and establish a flow rate of 50 mL/min. Click Apply.</li>
		<li>Fill the information about the sample in the tab Summary.</li>
		<li>Click Control | Lid | Open. Place a reference pan and a pan with a sample of 10-20 mg weight inside the DSC cell.</li>
		<li>Launch the experiment by clicking Start.</li>
	</ol>
	</li>
</ol>
2. DSC analysis of a fresh sample
<ol>
	<li>Prepare a fresh sample of the adhesive using the ratios and procedures recommended by manufacturer and immediately subject it to the following tests.</li>
	<li>Ramp curing test
	<ol>
		<li>Perform a heating-cooling-heating test as indicated below to obtain the curing enthalpy of the adhesive, the final glass transition on heating and to establish the range of temperatures where the curing process starts.</li>
		<li>Open the tab Summary. Click Mode and choose Standard.</li>
		<li>Click the tab Tool | Instrument Preferences, choose DSC and establish a standby temperature of 10 &#176;C. Click Apply. Click the tab Control | Go to Standby Temperature,</li>
		<li>Open the tab Procedure, click Test and choose Custom. Click Editor. Drag the segment type Equilibrate at -80 &#176;C to the Editor screen. Drag the segment Ramp and establish 10 or 20 &#176;C/min to (a temperature slightly below the degradation temperature obtained from the TGA test).</li>
		<li>Insert the segment Equilibrate at -80 &#176;C. Then drag the segment Ramp, establish 10 or 20 &#176;C/min to (the same temperature as before). Click Ok.</li>
		<li>Fill the information about the sample in the tab Summary.</li>
		<li>Click Control | Lid | Open. Place a reference pan and a pan with the freshly prepared sample of 10-20 mg weight inside the furnace.</li>
		<li>Start the experiment.</li>
	</ol>
	</li>
	<li>Isothermal curing test
	<ol>
		<li>Taking into account the DSC plot of the curing in ramp, choose several temperatures at the beginning of the exotherm to execute the isothermal experiments. 
		NOTE: The isothermal experiments will allow to evaluate the maximum degree of curing that can be obtained at each temperature.</li>
		<li>Open the Summary tab. Click Mode and choose Standard.</li>
		<li>Open the Procedure tab, click Test and choose Custom. Click Editor. Drag the segment type Ramp to the Editor screen. Introduce a 20 &#176;C/min to the chosen isothermal temperature.</li>
		<li>Introduce an Isothermal segment for time enough to complete the cure at this temperature. It is possible, for example, to establish 300 min, but the test can be stopped when the heat flow curve is flat.</li>
		<li>Introduce a command segment Equilibrate at 0 &#176;C. Add a Ramp segment, establish a heating rate between 2 and 20 &#176;C/min (in the example 2.5 &#176;C/min was chosen) to the maximum temperature, which was chosen from the TGA test in order not to compromise the thermal stability of the adhesive.</li>
		<li>Drag the Mark end of cycle segment to the editor window. Insert another Equilibrate segment, this time with a temperature of -80 &#176;C. Add another Ramp segment with a heating rate between 2 and 20 &#176;C/min (in the example 2.5 &#176;C/min was chosen) to the same temperature indicated before. Click Ok. 
		NOTE: A set of heating rates are suggested. Probably, most of them work correctly and depending of the nature of the curing process, mainly its kinetics, and the sensitivity and resolution required, some of these heating rates could be better. If the evaluation is done with comparative purposes the same conditions should be used for each studied adhesive system. In order to minimize the time elapsed from mixing the components to the beginning of the isothermal experiments, the temperature of the DSC cell should be adjusted to a temperature lower than the isothermal temperature before mixing both components.</li>
		<li>Click the tab Tool | Instrument Preferences, choose DSC and establish a temperature lower than the isotherm temperature of the experiment. Click Apply. Click the tab Control | Go to Standby Temperature.</li>
		<li>Fill the information about the sample in the tab Summary.</li>
		<li>Click Control | Lid | Open. Place a reference pan and a pan with the sample of 10-20 mg weight inside the furnace.</li>
		<li>Start the experiment.</li>
	</ol>
	</li>
</ol>
3. Rheological analysis
<ol>
	<li>Perform the rheological tests on a rheometer, using a 25 mm parallel plate geometry.</li>
	<li>Logarithmic strain sweep test
	<ol>
		<li>Do an exploratory logarithmic strain sweep test&#160;following the steps below to set up the strain amplitude to be used in the curing study of the adhesive in the rheometer. Perform the test with a fresh sample (before curing).</li>
		<li>Open the air stopcock. Switch on the rheometer apparatus. Open the rheometer control software.</li>
		<li>Place the specific geometry on the rheometer.</li>
		<li>Click Zero Gap.</li>
		<li>Click the tab Geometry. Choose the specific geometry.</li>
		<li>Open the tab Experiment.</li>
		<li>Fill the information about the sample in the tab Sample.</li>
		<li>Click the tab Procedure. Choose Oscillation Amplitude. This experiment can be performed at room temperature (the actual temperature is annotated), and with a frequency of 1 Hz and a logarithmic sweep from 10-3 to 100% of strain. 
		NOTE: To prepare a sample of the two-component system, weigh components at room temperature, about 20 &#176;C to the exact proportions recommended by the manufacturer. Then mix both components.</li>
		<li>Place the sample on the bottom plate with the upper plate separated about 40 mm from the lower plate. Lower the upper plate until a gap of about 2 mm is observed between both plates. Trim off the excess adhesive.</li>
		<li>Start the experiment.</li>
	</ol>
	</li>
	<li>Isothermal multifrequency curing test 
	NOTE: This test shows if there is or not gelation and, in case of gelation, it provides the gelation time. In addition, the contraction and the evolution of G&#8217; and G&#8217;&#8217; can be observed along the curing process.
	<ol>
		<li>Follow the subsequent procedure to monitor the curing of the adhesive.</li>
		<li>Click the tab Procedure. Choose Conditioning Options. Establish the Mode Compression, Axial Force 0 N and Sensitivity of 0.1 N. Click Advance and establish a Gap change limit of 2000 &#181;m in the up and down directions.</li>
		<li>Insert a new step of an oscillatory time sweep. This experiment can be performed at room temperature (the actual temperature is annotated), the duration of the test as a function of the estimated curing time based on the Data Sheet of the adhesive, and the percentage of Strain which is chosen from the result of the previous logarithmic strain sweep test. Choose Discrete and then set the frequencies 1, 3 and 10 Hz for all samples.</li>
		<li>Remove the previous sample, do the Zero Gap and place a new sample. Then proceed as in step 3.2.9.</li>
		<li>Start the experiment. 
		NOTE: Do not remove the sample at the end of experiment. It will be used in the next experiment.</li>
	</ol>
	</li>
	<li>Torque sweep test
	<ol>
		<li>Once the curing test ends, proceed to the torque sweep test&#160;following the steps below to find out the linear viscoelastic range for the previously cured material. 
		NOTE: The extension of LVR can be determined either by applying strain sweep test, mostly in controlled-strain rheometers, or torque or stress sweep test, mostly in controlled-stress rheometer. However, in some rheometers both methods can be used.</li>
		<li>Click the tab Procedure. Choose the Oscillation Amplitude. This experiment can be performed at room temperature (the actual temperature is annotated), with a frequency of 1 Hz and a logarithmic sweep from 10 to 10000 &#181;Nm of torque. 
		NOTE: Use the same sample that was left in the instrument from the previous experiment.</li>
		<li>Start the experiment. 
		NOTE: Do not remove the sample at the end of experiment. It will be used in the next experiment.</li>
	</ol>
	</li>
	<li>Temperature scan test
	<ol>
		<li>Perform a temperature scan test following the steps below to verify the cure is complete.</li>
		<li>Click the tab Procedure. Choose Temperature Ramp. Initiate the experiment from room temperature, establish a ramp rate of 1 &#176;C/min, which ensure a uniform distribution of temperature into the sample without consuming too excessive time, a frequency of 1 Hz and a given Torque amplitude, which is chosen from the previous Torque sweep test. 
		NOTE: Use the same sample that was left in the instrument from the previous experiment.</li>
		<li>Close the furnace of the rheometer. Open the air stopcock of the furnace.</li>
		<li>Start the experiment. 
		NOTE: If the next experiment is needed, do not remove the sample at the end of experiment. In that case it would be used for the next experiment.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

A preliminary TGA test of each adhesive is always a fundamental step as it gives information about the temperature range at which the material is stable. That information is crucial to correctly setting up further experiments. In addition, TGA may also inform about the filler content, which can be very insightful to understand that storage and loss modulus may not to cross along the cure.
On the other hand, DSC allows to study the cure of most thermosetting systems but not of those whose cure ...

Nowadays there is an increasing demand of adhesives. Today&#39;s industry demands that adhesives have increasingly varied properties, adapted to the growing diversity of possible new applications. It makes the selection of the most suitable option for each specific case a difficult task. Therefore, creating a standard methodology to characterize the adhesives according to their properties would facilitate the selection process. The analysis of the adhesive during the curing process and the final properties of the cured system are crucial to decide whether an adhesive is valid or not for a certain application.
Two of the most commonly used experimental techniques to study the behavior of adhesives are Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). Rheological measurements and thermogravimetric tests are also widely used. Through them, the glass transition temperature (Tg) and the residual heat of curing, which are related to the degree of cure1,2, can be determined.
TGA provides information about the thermal stability of the adhesives3,4, which is very useful to establish further process conditions, on the other hand rheological measurements allows the determination of the gel time of the adhesive, analysis of the curing shrinkage, and the definition of the viscoelastic properties of a cured sample5,6,7, while the DSC technique allows measurement of the residual heat of curing, and discernment between one or more thermal processes that can take place simultaneously during the curing8,9. Therefore, the combination of DSC, TGA and rheological methodologies provide detailed and reliable information to develop a complete characterization of adhesives.
There is a number of studies of adhesives where DSC and TGA are applied together10,11,12. There are also some studies that complement the DSC with rheological measurements13,14,15. However, there is not a standardized protocol to address the comparison of adhesives in a systematic way. That comparison would all to better choose the right adhesives in different contexts. In this work, an experimental methodology is proposed for doing a characterization of the curing process through the combined use of the thermal analysis and rheology. Applying these techniques as an ensemble allows to gather information about the adhesive behavior during and after the curing process, also the thermal stability and the Tg of the material16.
The proposed methodology involving the three techniques, DSC, TGA and rheology is described in this work using three commercial adhesives as an example. One of the adhesives, hereinafter referred to as S2c, is a two-component adhesive: component A contains tetrahydrofurfuryl methacrylate and component B contains benzoyl peroxide. The component B acts as an initiator of the curing reaction by causing the tetrahydrofurfuryl methacrylate rings to open. Through a free radical polymerization mechanism, the C=C bond of the monomer reacts with the growing radical to form a chain with tetrahydrofurfuryl side groups17. The other adhesives, T1c and T2c, are the one- and two-component versions from the same commercial house of a modified silane polymer adhesive. The curing process begins by the hydrolysis of the silane group18, which can be initiated by ambient humidity (as in the case of T1c) or by the addition of a second component (as in the case of T2c).
Concerning the application areas of these three different systems: the adhesive S2c was designed to substitute, in some cases, welding, riveting, clinching and other mechanical fastening techniques and it is suitable for high strength fastening of concealed joints on different types of substrates including top coats, plastics, glass, etc. The T1c and T2c adhesives are used for elastic bonding of metals and plastics: in caravan manufacturing, in the railroad vehicle industry or in shipbuilding.

<table><tbody><tr><td>2960 SDT</td><td>TA Instruments</td><td></td><td>Simultaneous DSC/TGA device: Used to perform thermogravimetric tests.</td></tr><tr><td>Discovery HR-2</td><td>TA Instruments</td><td></td><td>Rheometer to perform rheological test.</td></tr><tr><td>MDSC Q2000</td><td>TA Instruments</td><td></td><td>Differential Scanning Calorimeter with optional temperature modulation. Used to peform DSC and MDSC tests.</td></tr><tr><td>Sikafast 5211NT</td><td>Sika</td><td></td><td>S2c: a two component system manufactured by Sika. It is based on tetrahydrofurfuryl methacrylate and contains an ethoxylated aromatic amine.&lt;br/&gt; The second component contains benzoyl peroxide as the initiator for the crosslinking reaction.</td></tr><tr><td>Teroson MS 939 FR</td><td>Henkel</td><td></td><td>T1c: manufactured by Henkel, which is a one component sylil-modified-polymer, whose cure reaction is triggered by moisture.</td></tr><tr><td>Teroson MS 9399</td><td>Henkel</td><td></td><td>T2c: a two component system manufactured by Henkel. It is a sylil-modified-polymer too but the second component is aimed to make the curing rate a little more independent from the moisture content of air.</td></tr><tr><td>TRIOS</td><td>TA Instruments</td><td></td><td>Control Software for the rheometer. Version 4.4.0.41651</td></tr></tbody></table>

evaluation of the curing of adhesive systems by rheological and thermal testing

The analysis of thermal processes associated to the curing of adhesives and the study of mechanical behavior once cured, provide key information to choose the best option for any specific application. The proposed methodology for the curing characterization, based on thermal analysis and rheology, is described through the comparison of three commercial adhesives. The experimental techniques used here are Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Rheology. TGA provides information about the thermal stability and filler content, DSC allows the evaluation of some thermal events associated to the cure reaction and to thermal changes of the cured material when subjected to temperature changes. Rheology complements the information of the thermal transformations from a mechanical point of view. Thus, the curing reaction can be tracked through the elastic modulus (mainly the storage modulus), the phase angle and the gap. In addition, it is also shown that although DSC is of no use to study the curing of moisture curable adhesives, it is a very convenient method to evaluate the low temperature glass transition of amorphous systems.

In order to show the application of the proposed method three adhesive systems are used (Table of Materials):
<ul>
	<li>S2c, a two-component system.</li>
	<li>T1c, a one-component silane-modified-polymer, whose cure reaction is triggered by moisture.</li>
	<li>T2c, a two-component system. It is a silane-modified-polymer too, but the second component is aimed to make the curing rate a little more independent from the moisture content of air.</li>
</ul>
The thermal stability and...

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Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing

An experimental methodology based on thermal and rheological measurements is proposed to characterize the curing process of adhesives with to obtain useful information for industrial adhesive selection.

The analysis of thermal processes associated to the curing of adhesives and the study of mechanical behavior once cured, provide key information to choose ...

evaluation-curing-adhesive-systems-rheological-thermal

Research

JoVE Journal

Engineering

7.0K Views.  Universidade da Coruña. Onze methodologie combineert thermische analyse en reologie om het uithardingsproces van een lijm te karakteriseren en nuttige informatie te verkrijgen voor industriële lijmselectie. Deze techniek maakt het mogelijk om een standaardgeleider te maken voor het uitharden van lijmsystemen, waardoor het gemakkelijker wordt om verschillende lijmen te vergelijken. Deze methode kan ook worden gebruikt als een aanvaardbaar criterium in de kwaliteitscontrole van lijmsystemen.Als u een thermogra...

Video: Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing

Preparation and Testing of Plant Seed Meal-based Wood Adhesives

Recently, the interest in plant seed meal-based products as wood adhesives has steadily increased, as these plant raw materials are considered renewable and environment-friendly. These natural products may serve as alternatives to petroleum-based adhesives to ease environmental and sustainability concerns. This work demonstrates the preparation and testing of the plant seed-based wood adhesives using cottonseed and soy meal as raw materials. In addition to untreated meals, water washed meals and protein isolates are prepared and tested. Adhesive slurries are prepared by mixing a freeze-dried meal product with deionized water (3:25 w/w) for 2 hr. Each adhesive preparation is applied to one end of 2 wood veneer strips using a brush. The tacky adhesive coated areas of the wood veneer strips are lapped and glued by hot-pressing. Adhesive strength is reported as the shear strength of the bonded wood specimen at break. Water resistance of the adhesives is measured by the change in shear strength of the bonded wood specimens at break after water soaking. This protocol allows one to assess plant seed-based agricultural products as suitable candidates for substitution of synthetic-based wood adhesives. Adjustments to the adhesive formulation with or without additives and bonding conditions could optimize their adhesive properties for various practical applications.

To facilitate the effort in seeking more economic and environment-friendly formulations of natural product-based wood adhesives, this work demonstrates the preparation and testing of plant seed-based wood adhesives. This protocol allows one to assess plant seed-based agricultural products as suitable candidates for the substitution of synthetic-based wood adhesives.

Recently, the interest in plant seed meal-based products as wood adhesives has steadily increased, as these plant raw materials are considered renewable ...

Environment

A Testing Platform for Durability Studies of Polymers and Fiber-reinforced Polymer Composites under Concurrent Hygrothermo-mechanical Stimuli

The durability of polymers and fiber-reinforced polymer composites under service condition is a critical aspect to be addressed for their robust designs and condition-based maintenance. These materials are adopted in a wide range of engineering applications, from aircraft and ship structures, to bridges, wind turbine blades, biomaterials and biomedical implants. Polymers are viscoelastic materials, and their response may be highly nonlinear and thus make it challenging to predict and monitor their in-service performance. The laboratory-scale testing platform presented herein assists the investigation of the influence of concurrent mechanical loadings and environmental conditions on these materials. The platform was designed to be low-cost and user-friendly. Its chemically resistant materials make the platform adaptable to studies of chemical degradation due to in-service exposure to fluids. An example of experiment was conducted at RT on closed-cell polyurethane foam samples loaded with a weight corresponding to ~50% of their ultimate static and dry load. Results show that the testing apparatus is appropriate for these studies. Results also highlight the larger vulnerability of the polymer under concurrent loading, based on the higher mid-point displacements and lower residual failure loads. Recommendations are made for additional improvements to the testing apparatus.

The durability of polymers and fiber-reinforced polymer composites in service is a critical aspect for their designs and condition-based maintenance. We present a novel low-cost laboratory testing platform for the investigation of the influence of concurrent mechanical and environmental loadings, and may help design more efficient yet safer composite structures.

The durability of polymers and fiber-reinforced polymer composites under service condition is a critical aspect to be addressed for their robust designs ...

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline ...

Chemistry

Environmentally-controlled Microtensile Testing of Mechanically-adaptive Polymer Nanocomposites for ex vivo Characterization

Implantable microdevices are gaining significant attention for several biomedical applications1-4. Such devices have been made from a range of materials, each offering its own advantages and shortcomings5,6. Most prominently, due to the microscale device dimensions, a high modulus is required to facilitate implantation into living tissue. Conversely, the stiffness of the device should match the surrounding tissue to minimize induced local strain7-9. Therefore, we recently developed a new class of bio-inspired materials to meet these requirements by responding to environmental stimuli with a change in mechanical properties10-14. Specifically, our poly(vinyl acetate)-based nanocomposite (PVAc-NC) displays a reduction in stiffness when exposed to water and elevated temperatures (e.g. body temperature). Unfortunately, few methods exist to quantify the stiffness of materials in vivo15, and mechanical testing outside of the physiological environment often requires large samples inappropriate for implantation. Further, stimuli-responsive materials may quickly recover their initial stiffness after explantation. Therefore, we have developed a method by which the mechanical properties of implanted microsamples can be measured ex vivo, with simulated physiological conditions maintained using moisture and temperature control13,16,17. 
To this end, a custom microtensile tester was designed to accommodate microscale samples13,17 with widely-varying Young's moduli (range of 10 MPa to 5 GPa). As our interests are in the application of PVAc-NC as a biologically-adaptable neural probe substrate, a tool capable of mechanical characterization of samples at the microscale was necessary. This tool was adapted to provide humidity and temperature control, which minimized sample drying and cooling17. As a result, the mechanical characteristics of the explanted sample closely reflect those of the sample just prior to explantation.
The overall goal of this method is to quantitatively assess the in vivo mechanical properties, specifically the Young's modulus, of stimuli-responsive, mechanically-adaptive polymer-based materials. This is accomplished by first establishing the environmental conditions that will minimize a change in sample mechanical properties after explantation without contributing to a reduction in stiffness independent of that resulting from implantation. Samples are then prepared for implantation, handling, and testing (Figure 1A). Each sample is implanted into the cerebral cortex of rats, which is represented here as an explanted rat brain, for a specified duration (Figure 1B). At this point, the sample is explanted and immediately loaded into the microtensile tester, and then subjected to tensile testing (Figure 1C). Subsequent data analysis provides insight into the mechanical behavior of these innovative materials in the environment of the cerebral cortex.

Environmentally-controlled Microtensile Testing of Mechanically-adaptive Polymer Nanocomposites for ex vivo Characterization

A method is discussed by which the in vivo mechanical behavior of stimuli-responsive materials is monitored as a function of time. Samples are tested ex vivo using a microtensile tester with environmental controls to simulate the physiological environment. This work further promotes understanding the in vivo behavior of our material.

Implantable microdevices are gaining significant attention for several biomedical applications1-4. Such devices have been made from a range of materials, ...

Bioengineering

The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ethylene-propylene rubber, grafted with maleic anhydride, is thermo-reversibly cross-linked in two steps. The pending anhydride moieties are first modified with furfurylamine to graft furan groups to the rubber backbone. These pendant furan groups are then cross-linked with a bis-maleimide via a Diels-Alder coupling reaction. Both reactions can be performed under a broad range of experimental conditions and can easily be applied on a large scale. The material properties of the resulting Diels-Alder cross-linked rubbers are similar to a peroxide-cured ethylene/propylene/diene rubber (EPDM) reference. The cross-links break at elevated temperatures (&#62; 150 &#176;C) via the retro-Diels-Alder reaction and can be reformed by thermal annealing at lower temperatures (50-70 &#176;C). Reversibility of the system was proven with infrared spectroscopy, solubility tests and mechanical properties. Recyclability of the material was also shown in a practical way, i.e., by cutting a cross-linked sample into small parts and compression molding them into new samples displaying comparable mechanical properties, which is not possible for conventionally cross-linked rubbers.

A simple two-step approach involving rubber modification and cross-linking yields fully reworkable, elastic rubber products.

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

Additive Manufacturing of Functionally Graded Ceramic Materials by Stereolithography

An additive manufacturing technology is applied to obtain functionally graded ceramic parts. This technology, based on digital light processing/stereolithography, is developed within the scope of the CerAMfacturing European research project. A three-dimensional (3-D) hemi-maxillary bone-like structure is 3-D printed using custom aluminum oxide polymeric mixtures. The powders and mixtures are fully analyzed in terms of rheological behavior in order to ensure proper material handling during the printing process. The possibility to print functionally graded materials using the Admaflex technology is explained in this document. Field-emission scanning electron microscopy (FESEM) show that the sintered aluminum oxide ceramic part has a porosity lower than 1% and no remainder of the original layered structure is found after analysis.

This manuscript describes the processing of single multifunctional ceramic components (e.g., combinations of dense-porous structures) additively manufactured by stereolithography.

An additive manufacturing technology is applied to obtain functionally graded ceramic parts. This technology, based on digital light ...

Artificial Thermal Ageing of Polyester Reinforced and Polyvinyl Chloride Coated Technical Fabric

Architectural fabric AF9032 has been subjected to artificial thermal ageing to determine changes of the material parameters of the fabric. The proposed method is based on the accelerated ageing approach proposed by Arrhenius. 300 mm x 50 mm samples were cut in the warp and fill directions and placed in a thermal chamber at 80 &#176;C for up to 12 weeks or at 90 &#176;C for up to 6 weeks. Then after one week of conditioning at ambient temperature, the samples were uniaxially tensioned at a constant strain rate. Experimentally, the parameters were determined for the non-linear elastic (linear piecewise) and viscoplastic (Bodner&#8211;Partom) models. Changes in these parameters were studied with respect to the ageing temperature and ageing period. In both cases, the linear approximation function was successfully applied using the simplified methodology of Arrhenius. A correlation was obtained for the fill direction between experimental results and the results from the Arrhenius approach. For the warp direction, the extrapolation results exhibited some differences. Increasing and decreasing tendencies have been observed at both temperatures. The Arrhenius law was confirmed by the experimental results only for the fill direction. The proposed method makes it possible to predict real fabric behavior during long term exploitation, which is a critical issue in the design process.

Here, we simulate accelerated thermal ageing of technical fabric and see how this ageing process influences the mechanical properties of the fabric.

Architectural fabric AF9032 has been subjected to artificial thermal ageing to determine changes of the material parameters of the fabric. The proposed ...

Mechanoluminescent Visualization of Crack Propagation for Joint Evaluation

In this study, methods for the mechanoluminescent (ML) visualization of crack propagation and mechanical behavior to evaluate adhesive joints are demonstrated and explained. The first step involved sample preparation; an air spray was used to apply ML paint to the surface of the adhesive joint specimens. The performance of the ML sensor was described to examine the measurement conditions. The results of ML sensing during a double cantilever beam (DCB) test and a lap-shear (LS) test are demonstrated as these are the most frequently and widely used methods for evaluating adhesives. Originally, it was difficult to directly quantify the crack tip and strain/stress distribution and concentration because the crack tip was too small, and the effects of the strain could not be observed. The mechanoluminescence, crack propagation, and mechanical behavior during mechanical testing can be visualized via the ML pattern during the adhesive evaluation. This allows for the recognition of the precise position of the crack tips and other mechanical behaviors related to structural failure.

In this study, a protocol is presented that describes the use of mechanoluminescent (ML) visualization for monitoring crack propagation and mechanical behavior during adhesive joint evaluation testing.

In this study, methods for the mechanoluminescent (ML) visualization of crack propagation and mechanical behavior to evaluate adhesive joints are ...

Standard Test Method ASTM D 7998-19 for the Cohesive Strength Development of Wood Adhesives

The properties of cured wood adhesives are difficult to study because of the loss of water and other components to the wood, the influence of wood on the adhesive cure, and the effect of adhesive penetration on the wood interphase; thus, normal testing of a neat adhesive film is generally not useful. Most tests of wood adhesive bond strength are slow, laborious, can be strongly influenced by the wood and do not provide information on the kinetics of cure. Test method ASTM D 7998-19, however, can be used for fast evaluation of the strength of wood bonds. The use of a smooth, uniform, and strong wood surface, like maple face-veneer, and sufficient bonding pressure reduces the adhesion and wood strength effects on bond strength. This method has three main applications. The first is to provide consistent data on bond strength development. The second is to measure the dry and wet strengths of bonded lap shear samples. The third is to better understand the adhesive heat resistance by quickly evaluating thermal sensitivity and distinguishing between thermal softening and thermal degradation.

We present a procedure, ASTM D7998-19, for a rapid and more consistent evaluation of both dry and wet strength of adhesive bonds on wood. The method can also be used to provide information on strength development as a function of temperature and time or strength retention up to 250 &#176;C.

The properties of cured wood adhesives are difficult to study because of the loss of water and other components to the wood, the influence of wood on the ...

Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing

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