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"Self healing innovative elastomers for dynamic seals, damping and noise reduction"

Final Report Summary - SHINE (Self healing innovative elastomers for dynamic seals, damping and noise reduction)

Executive Summary:
During the SHINE project, Work Package 1 and 2 have been focusing on the development of new self-healing elastomers or further development of lab concepts of shelf-healing elastomers together with the development of self-healing elastomer (nano-)composite systems. This delivered 18 initial products. The development of new or updated analytical techniques and methods for the analysis of self-healing and fatigue functionalities, together with constitutive models, in WP3, delivered a good understanding of the potential applicability towards specific applications. This has resulted in the further up-scaling in WP 4 of 5 candidates for self-healing elastomers. In WP5 eventually 3 of the selected candidates have been successfully applied in 4 final applications:
1 a self-healing elastomer for asphalt application
2 a self-healing elastomer for elastomeric rail-mat systems
3 a self-healing elastomer for a rigid-support for rail systems
4 a self-healing elastomer for seals of bearing systems
The testing of the final applications resulted in promising results, but no self-healing elastomer which has the direct potential for final application. More work is needed to improve the functionality of the various elastomers in order to make final application possible.
During the project all deliverables have been met and reported.
In general the perception of the partners is positive in terms of the achievements, collaboration and progress made. All partners have gained new knowledge and insights into the complexity of self-healing principles of elastomers and the development of such elastomers into final applications. Relevant analytical methods have been developed which makes standardization of self-healing principles better possible and accessible in the future.

Project Context and Objectives:
The overall objective of the SHINE project has been to develop, scale up and validate a novel generation of recyclable elastomers of adequate mechanical properties that undergo spontaneous self-healing, leading to enhanced durability and reliability of the products made thereof such as dynamic seals, shock absorbers and anti-vibration systems used in machinery, vehicles, bridges, railroads and roads. The detailed objectives of each work package are listed below.

Work Package 1 (WP1) focused on development of a new generation of unreinforced self-healing elastomers with superior mechanical properties. Different starting materials, syntheses and self-healing strategies were selected and combined, in order to design elastomers with good mechanical properties and self-healing ability. Specific attention was paid to the synergism between the three chosen self-healing chemistries (hydrogen bonds, ionomeric interactions and reversible covalent bonds).
The hydrogen bond approach was based on the work of the team of Ludwik Leibler at ESCPI. Such works describe the ability to design elastomers undergoing self-healing at room temperature by forming both chains and crosslinks reversibly, via hydrogen bonds.
The ionomeric approach was based on the functionalization of polymeric materials with ionisable side-groups which could eventually exhibit the possibility of reversible crosslinking between polymer chains. These ionizable side-groups were co-polymerized during polymerization and/or attached by polymer analogous reaction at the polymeric backbone, existing side-groups or at the end of polymer chains. Different ionizable side-groups were tried.
The reversible covalent bond approach consisted on the incorporation of such dynamic covalent bonds into the polymer matrix, having the inherent ability to undergo dynamic formation and cleavage at room temperature, leading to a covalently crosslinked polymer network, where the crosslinks were in constant exchange.

The main objective of WP2 was the development and production of a new class of composite elastomers with self-healing properties, based on controlled interactions between the polymers and the surface of different nano-sized fillers and continuous and short-cut fibers. Usually elastomers need to be reinforced in order to have the required mechanical strength for industrial applications. Different starting materials, which were developed in WP1, syntheses and self-healing strategies were selected and combined, in order to create the desired composite elastomers with different mechanical properties and self-healing capabilities to be studied in WP3. Elastomers of different chemical nature were synthesized and partially filled with nano-sized fillers and short-cut fibers, respectively. The nano-fillers were either inorganic particles, surface modified fibers or microphase-separated polymers. The self-healing ability was implemented into the polymers by dynamic covalent bonds on the one hand and on the other hand by supramolecular stickers.

Work Package 3 (WP3) aimed at on the quantification and understanding of healing capabilities of neat and composite self-healing materials using different healing concepts and link the healing at molecular scale with healing at macromolecular level under dynamic and static conditions. For that purpose, characterization tools and methods to quantify healing kinetics and efficiency were developed.
Constitutive equations and simulations to predict material behavior were set up.
First generation protocols to quantify self-healing behavior and explore their standardization process were formulated.
The endeavor was to support the scientific work in work package one and two and to promote the consistent and comprehensible quantification of healing results within the scientific community as the characterization of self-healing materials, more specifically elastomers, is performed by the use of a portfolio of not uniform tests.

Work Package 4 (WP4) was dedicated to the scale up of a selection of self-healing elastomers (SHE) developed during work package 1 (WP1) at a kg-scale. These SHE were either epoxy-amid (Arkema) or polyurethane based (Cidetec) or the result of an ionic modification of NBR / SBS resins (Fraunhofer Umsicht). As second target, these SHEs have to be formulated with fillers and/or fibres to prepare compounds with improved mechanical properties. In particular fibres could have been especially tuned in WP2 to improve their compatibility with the organic matrices and improve the self-healing character of the compound.

The main objective of WP5 was to apply, validate and demonstrate the usefulness of the self-healing elastomers developed in former WP’s in proof-of-concepts and small scale prototypes.

First, the SH materials identified suitable for applications are subject to a Safety, Health and Environmental (SHE) assessment. The focus of this assessment is to clearly show that the self-healing technology is not involving any substances that are not allowed to be used in any part of the manufacturing processes, in harmony with the SHE procedures used internally by any of the industrial partners involved.

In parallel, simple elements (slabs, plaques, rings, etc.) are manufactured with a double purpose: to validate at a very small scale that different SH-materials and compounds could be used in controlled manufacturing process. Similarly, SH materials are used in assemblies into rigid supports because many elastomers are generally used as an interface component between rigid materials. The impact on the static and dynamic mechanical properties of fillers, pigments and/or composite reinforcements (carbon and glass fibers) are evaluated with those prototypes. Furthermore, their processability is also evaluated in order to assess their potential in view of a mass production. Eventually, combining the knowledge gained, prototypes of self-healing seals for vehicle and machinery are produced by SKF.

Second, other applications of self-healing elastomers and compounds are designed of noise and vibration abatement systems for railroads and asphalt mixtures for construction of roads. The mechanical and vibro-acoustic properties of both isolated block and embedded rail are assessed through specific tests by Acciona. Similarly, different mechanical properties at both rheologicas and mixture scale are assessed. All test are following European and/or national codes and regulations.

Third, some of the proof-of-concepts developed are simulated. The simulation of the healing process allows defining the severity and location of damage, establishing the healing protocol and assessment procedure, and estimating the expected performance of the healing solution. This work will continue in operating conditions. Further tests will be performed to measure the effectiveness of the self-healing properties in increasing the working life of the seals without compromising any fundamental function of the sealing mechanism.

Finally, using the previous developed solution for instrumentation of the case-studies a (quasi) real time assessment of the healing process of the developed proof-of-concepts is performed. At the end, a critical assessment of this methodology is done in order to verify its potential for industrial applications.
The prime objective of Work Package 6 (WP6) is dissemination of the project results to the public at large– via presenting the results from this project at scientific meetings and publishing in highly ranked scientific journals. Another prime objective is to explore the possibility of application of the various self-healing materials into applications of interest to the industrial partners by making various prototypes and to evaluate these prototypes with current benchmarks in order to assess potential commercial applications.

Work Package 7 deals with the management of the EU work according to the rules and regulations laid down in the Grant Agreement closed between the EC and the consortium and the coordination of work with EU-SHINE. The project management concerns financial, contract and intellectual property management of the project.

Project Results:
The main goal of Work Package 1 (WP1) has been to develop a new generation of self healing elastomers with superior mechanical properties. The reversible connections required in the self-healing process have been achieved either via non-covalent bonds (so-called supramolecular chemistry) with associations caused by molecular interactions, or by covalent bonds that have the ability to be formed and broken reversibly under equilibrium control (dynamic covalent bonds). The self-healing properties of the elastomers were introduced via three different chemical approaches and a combination thereof:
i) multiple hydrogen bonds,
ii) ionic interactions,
iii) reversible covalent crosslinking, and
iv) combination of the above mechanisms.
During the evolution of the project different materials based on the above mentioned approaches raised as potential candidates to be pursued towards applications. However, only a few of them were selected attending to material availability in kg scale, material production costs and adequate mechanical performance.
Selected materials are listed below:
Epoxy based rubbers HM and HR.- These self-healing materials based on multiple hydrogen bonds were synthesized using epoxy monomers (DEGBA for HR and epoxydized sojabean oil for HM) crosslinked with a bio-based mixture of multifunctional acids called SC6 along with 50% of UDETA (mono-functional amine containing a H-bonding group).
Self-healing Poly(urea)urethane (thermoset and thermoplastic versions).- These self-healing materials based on the reversible covalent crosslinking approach were synthesized using polypropylene glycol polyurethane prepolymers with different functionalities and molecular weights combined with an aromatic diamine (4,4’-aminophenyl disulfide) containing the dynamic covalent bond (aromatic disulfide bond).
Self-healing Silly Putty modified SBS.- This self-healing material is based on a blend of a well known silicone-based self-healing polymer (called Silly Putty) developed by Dow Corning during World War II and a thermoplastic SBS commonly used as bitumen additive in asphalts. The self-healing mechanism of this material consists in the transesterification of borate esters, thus making use of the dynamic covalent bonds approach.
Ionic NBR.- This self-healing rubber was synthesized by covalently attaching carboxylate side groups into the NBR backbone by co-polymerization or polymer analogous reaction. Subsequent neutralization step with the proper metallic salt rendered a material with good mechanical behavior and self-healing properties based on the ionomeric interaction approach.
These materials were extensively characterized in WP3 and pursued towards different applications within WP4 and 5.

Within Work Package 2 (WP2) different concepts were examined to increase the mechanical properties of self-healing polymers either by incorporation of nano-sized fillers or fibers, which could act as crosslinks or by the use of two different kinds of crosslinks of varying strength within one polymer network (dual network concept). Several model systems were developed based on self-healing polymers combined with crosslinking methods like transient and covalent crosslinking or the use of inorganic nanoparticles, micro-phase separated polymers or polymer fibers.

Model systems based on polyisoprene, polybutadiene and NBR used hydrogen bonding urazole groups as supramolecular stickers. Additional covalent crosslinking resulted in dual networks which show increased mechanical properties and an increased breaking strain compared to simple covalent networks. Structural analysis by IR spectroscopy and small angle neutron scattering (SANS) as well as small angle x-ray scattering (SAXS) gave further inside in these systems. While the dual network concept could be used to increase the mechanical properties of conventional elastomers the nature of the covalent crosslinks prevents any self-healing of the polymers.

To combine the mechanical advantages of fillers with the self-healing abilities of the polymers investigated in WP 1, several fillers were synthesized/prepared like:

PI-PS-PI triblock copolymers
iron oxide nanoparticles
silica nanoparticles
silver nanoparticles
aramid fibers

Based on the interest of one industrial partner (Teijin Aramid) aramid fibres in different shapes and forms were coated with conventional or custom developed coatings (e.g. urazole, RFL, epoxy, polyallylamine, hexanediamine, thiolate, CVD, Reverlink, NCO, NH2, OH, SF, AA2, WF, AF2...) as well as the various nanoparticles were coating with various surface functionalization coatings to increase the interaction with the polymer matrix either by supramolecular attractions or dynamic covalent bonds. Mixtures of the fibers or nanoparticles suspended in a self-healing polymer matrix of e.g. poly(urea)urethane, ionic NBR, Reverlink HR, Reverlink NR and polyisoprene were investigated with the adhesion test protocol developed in WP3-T3.2.1 for mechanical properties and self-healing. Most combinations showed a significant increase in mechanical properties of up to 200%, typically accompanied by a decrease in self-healing ability to 30-70%.

Work Package 3 was to understand and describe healing capabilities of the materials developed within the project (WP1 and 2). The involved partners therefore applied standard test, modified existing testing routines or developed new test that could be suitable for the reproducible and reasonable testing of healing characteristics in soft materials.
It was first agreed among the partners to not concentrate on specific applications but to emphasize the healing performance of elastomeric materials that could be quantified by easy to perform lab-tests. Materials were exchanged among the partners to compare results and describe the practicability of the methods applying various materials. The mechanical measurements were supported by simulation tools and the creation of constitutive equations describing the healing process and the kinetics linked to healing processes (D 3.4 and D 3.5)..
Besides the support and the close linkage to work packages 1 and 2, the goal was to create protocols and in a further step, the submission of documents for a pre-standardization process.

A first overview on methods, including 19 different tests for mechanical testing and ageing experiments was set up. A respective deliverable (D 3.1) presenting the testing methods and the gathered results by the partners was formulated and submitted. Three of the methods investigated thereby were exclusively for the interface characterization of fiber reinforced elastomers.
Accordingly, scientific work, using these testing methods was undertaken by the partners. By the exchange of materials it became clear what materials were suitable for further investigation by not only revealing positive healing characteristics by applying only one specific testing method. The according overview on materials is given within the description of WP 1 and 2.
Model systems were successfully characterized by microscopic interactions showing the dynamic behavior of the polymeric systems leading to restoration of the elastomeric structure (D 3.2).
For a subsequent standardization process (D 3.3) three characterization methods were chosen:

Tensile properties recovery (TU Delft) and in comparison with
Fracture properties recovery (TU Delft)
Tensile testing using cylindrical test specimen

Methods 1 and 2 were performed and compared using varying healing parameters (time, temperature) by the use of a soft elastomer. A TTS-mastercurve for healing was created.
Method 3 was presented by the use of different types of NBR-based elastomers. Healing was compared between the use of cylindrical testing specimen and elastomeric tensile strips.
A positive feedback from the Dutch Standardization Institute was given to the submitted documents.
The results of work package 3 were presented in different scientific articles and scientific conferences and symposia.
Concerning the scale-up of resins explored into the WP1, at least 7 materials have been produced at a kg-scale or even more. Among them:
- 45 Kg of “rigid” epoxy-amid based SHE (Reverlink HR). Grade with high glass transition (Tg ~25°C)
- 60 Kg of “malleable” epoxy-amid based SHE (Reverlink HM). Grade with lower Tg (~ 10°C). Both of them prepared in two steps, first one is production of an intermediate called SC6 (amid part) containing hydrogen bounds donors/acceptors (bringing the self-healing properties) that is then blended with suitable epoxides (bis phenol A diglycidyl ether (BADGE) for the 1st grade, epoxydised soyabean oil for the 2nd one) to give the two grades of epoxy-amid based SHEs. Furthermore as curing of HM is quite long, a screening of catalyst has been set up and a catalysed composition has been disclosed, reducing the 48h curing time of WP1 for the second grade to only 6h. This proved the scalability of this process.
- 10 kg of a 2K PU (prepolymer and crosslinker) containing dynamic covalent SHE roduced from commercially available poly(propylene glycol) (PPG - average molecular weight of 6 and 2 kDa) treated with isophorone diisocyanate (IPDI) in the presence of dibutyltin dilaurate (DBTDL) as a catalyst, to obtain tris- and bis-isocyanate-terminated urethanes respectively. As 2K PU, this part A has been furnished to the WP5 partners together with the aminated and disulfure bound (dynamic covalent bonds, source of the self-healing properties) containing crosslinker (part B). These volumes, together with the knowledge that the process used is very similar at larger scale, gave proof of the full scale scalability of this product.
- Additionally CIDETEC has also performed the scaling up of 1kg of Silly-Putty modified SBS, containing both dynamic borate ester bonds. Also there, based on the fact that the large scale process in very similar to the applied process, this gave proof of the full scale scalability of this product.
- 6,3 kg of self-healing zinc ionic NBR version have been produced according what has been technically reported in WP1 (co-polymerization of carboxylate side groups into the NBR backbone and subsequent neutralization step with the zinc metallic salt). The mixer process is very well scalable to large scale, indicating that full scale production is feasible.

As second achievement of the WP4, compounds of SHE previously prepared have been formulated. Pigments (red, orange and yellow - powder of vanadium bismuth and different organics molecules from Bruchsaler Farben), fillers (Carbon black N550, silica Ultrasil VN2, microcrystalline cellulose BE 600-10 TG) as well as aramid fibres (Twaron 1097 and 1099) have been introduced. Reinforcement leads to a decrease in self-healing percentage however this tendency is conjugated with an increase of the strength at break. One of the best results is obtained with 10 phr of silica and HR SHE. A stress at break > 5 MPa and elongation of 300% is obtained with preserved self-healing properties (>50%). In last attempts, use of silanized silica appears to improve again the mechanical properties.
Introduction of 0.5 phr of aramid 1097 Twaron pulp fibres with a specific surface treatment developed during the WP2 (fibre/matrix coupling agent with hydrogen bound bearing moiety) was also an achievement of WP4. This introduction allow an increase of 16% of self-healing strength and 18% of strain healing with a Reverlink HM matrix compared to aramid fibre with classical denacol treatment.
All in all this work gave good indications and directions of compounding, filler and fibre functionalisation and composite functionality in relation to self-healing performance of elastomers.

In Work Package 5 the following products were assessed on their Health, Safety and Environmental characteristics by people from HSE departments of the involved end-users based on the Safety Data Sheet documents that have been provided by the producers of the materials:

All the assessments indicated that a moderate risk level is identified for all products and it has been verified that none of the products contain any substance forbidden in the various steps of the representing manufacturing processes.

In relation to the development of simple parts in the form of plaques, rings or slabs it turned out that application of the Reverlink materials into conventional production molds, due to the long curing process of the Reverlink materials, will not be possible. Also both Reverlink materials have a very low shear strength compared to natural rubbers, although when combined with other additives (Reverlink HM-M10), it may stick better on metals (especially aluminum) and thermoplastics enhancing the shear bond with those materials. A mechanical test assessment performed on HR and HM matrix indicated that HR presents better mechanical and healing properties than HM-M10. The major drawback of HR is that it presents a very large compression set at room temperature, which make it unusable in sealing applications. At high temperatures, the compression set decreases largely to an admissible value. In any case, both types of Reverlink materials show very low mechanical properties making them too weak for sealing applications. Nevertheless, its noise attenuation and dampening properties properties shows exceptional performance. Their high healing performances make them nonetheless a promising solution in other areas such as damping applications.
IHNBR was assessed being a potential SH-elastomer for the sealing applications of SKF. The work in WP5 made clear that this product can be molded using conventional production molds. The difficulty arising with iHNBR is that it adheres very well to the metals, making the demolding impossible or greatly affecting the aspect of the plates by making them irregular. The adhesion to a rigid support of iHNBR was studied as well, but it is actually so high that the glue failed instead of the material.
After various assessments and tests done with the different elements produces, only the Fraunhofer/UMSICHT iHNBR material, modified with Zn2+ as the ion and containing 40 phr silica, was successful in providing the pre-requisite physical and molding properties to produce 2 prototype seal designs: a simpler seal ring design and a more complex Hub Bearing Unit (HBU) seal design that had a more detailed seal lip. The adhesion to the metal body was excellent and was probably achieved in part by the addition of ionic groups on the HNBR chain. This can be seen as an added benefit to using an ionically modified elastomer.
The sensitivity of this molded elastomer to applied force at elevated temperatures creates a problem that a prolonged duration is required before the mold cools enough to allow removal of the part. This also suggests that creep occurs at higher temperatures, which would need to be eliminated as a materials property. During the length of the project is was not possible to show if all the required properties of this material provide acceptable sealing performance and self-healing ability. Potential further testing of specific seal designs by SKF, in combination with Umsicht may attain this.
More detailed physical testing was performed for Fraunhofer/UMSICHT iHNBR* that was more focused on the suitability for dynamic sealing application. After the assessment was completed, the main conclusion is that iHNBR showed physical properties that were acceptable such as Tg and hardness but the sensitivity of this molded elastomer to applied force at elevated temperatures creates a problem that a prolonged duration is required before the mold cools enough to allow removal of the part. This also suggests, along with RPA data, that creep occurs at higher temperatures, which would need to be eliminated as a materials property. This means it would not yet be suitable for a dynamic sealing application.
In relation to applications of self-healing elastomers and compounds for the design of noise and vibration abatement systems for railroads and asphalt mixtures for construction of roads results obtained showed very interesting patterns that make application possible. From the assessment of the self-healing potential by looking into the resting behaviour it can be perfectly noticed that the resting periods enhance the fatigue performance of the mixes. PMB samples had a better performance than SHINE ones. Nevertheless, healing performance is evident because resting periods “activates” the healing capacity of the mixes.
Reverlink HM and a SH-PU from CIDETEC were used in prototypes for railway applications. Proper dosages ranges where previously identified in the framework of WP4. The trends observed from the outcomes of the tests were not as good as expected due to the evolution observed of both dynamic and static stiffness values in time. This is critical, because even if the stiffness values are out of range, adequate values might be achieved as a continuously increase of the stiffness may lead to a malfunction of the track systems in the long term.
Nonetheless, a potential opportunity for application is worth considering where the phenomena observed can provide valuable benefits from the perspective of a mechanical performance. The embedded track system has two fields for improvements were a SH-elastomer may play a key role: the first is a design requirement for lateral deformations and the second is the potential failures that appear inside the system. Considering the performance observed on both the SH-PU where practically the damaged samples recover its integrity, but also the increase in stiffness is a constant, it seems very reasonable to explore this application.
CMT proved that proper FEM simulations of proof-of-concepts can be helpful in assessing a material that showshealing versus a conventional material that does not heal. A material testing apparatus that is capable of measuring the dynamic response of elastomeric materials in the relevant frequency domain have been developed. This new apparatus will allow a more controlled (and automated) experimental setup as it will be possible to measure the actuator force magnitude and prevent double strike phenomena during transient excitation. Also the measurement of dynamic response can be done with both accelerometer (compression setup) and laser vibrometer (Oberst bar), increasing the accuracy of dynamic measurements.

Potential Impact:
The final result from WP1, a joined effort of CIDETEC, ESPCI, ARKEMA and Fraunhofer ÜMSICHT, will be a set of fully characterized self-healing elastomers implementing different self-healing technologies. This set of materials have proven to overcome most of the specifications in different applications such as dynamic seals, dampening systems like elastomeric rail mats or embedded tracks, asphalts, etc. The development of these materials has been and will be of great value in future material designs. The newly developed materials will also be used in other research projects.

From WP 2 it was shown, that without the addition of fillers some of the self-healing polymers developed did not match the criteria for industrial application. Composite elastomers are widely used in industrial applications like e.[*]g[/*]. in the production of seals and dampeners.
Despite their importance, little is known about the interactions of fillers with the transient or dynamic bonds in self-healing elastomeric polymers. It is therefore of paramount importance to investigate the supramolecular interactions within self-healing composite materials and the effect of different filler material, geometry, structure and coating on the macroscopic properties of the polymer composites. It could be shown that most composite materials show an increase in mechanical properties accompanied by a decrease of self-healing abilities. However, both of these effects are highly dependent on the nature of the filler, the coating and the polymer used. An increased microscopic analysis coating structures and the nature of the interaction with the polymer matrix could be helpful for the future development of self-healing composite materials.

In WP3 the endeavor of the SHINE consortium was to compare different approaches of qualification and quantification methods and present those results as suggestion and advices leading to more comparable results in the future when healing efficiencies are displayed to close this gap. In the submitted protocols, a first generation portfolio of suitable testing methods and ways to qualify elastomers has been created that could be a benefit for the further scientific as well as industrial quantification of healing capabilities. Moreover, strong research on fiber adhesion and interface healing was done in collaboration between scientific and industrial partners offering innovative approaches to characterize fiber reinforced elastomers.

WP4 was an opportunity to demonstrate in particular for Arkema, Cidetec and Fraunhofer what an industrialisation of new technology involves in term of equipment, input from end users, time-lines and financial requirement. Furthermore results obtained in formulations have found direct application into WP5 and proved the self-healing concept for reinforced materials.

The purpose of WP5 was to look after industrial applications of SH materials developed in the framework of SHINE project. As many of the partners involved in the project come from industry it was easy to define the specifications of the applications and set up protocols for prototype development and perform medium scale tests that allow understanding the healing performance in close to real conditions. All these have provided the partners of the consortium with new insights in how to collaborate in an applied research setting, with a strong focus on industrial applications. Although the final results of the project will not directly be applicable into final products the experience gained in this research has allowed partners to better assess the limits and possibilities of SH elastomeric materials. This is an important foundation which, together with the analytical and simulation methods developed, have a high potential fields for further improvement of the performance and may also lead to new market opportunities not previously foreseen at the beginning of the project.

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