EP4214280A1

EP4214280A1 - A conductive alginate capsule encapsulating a healing agent

Info

Publication number: EP4214280A1
Application number: EP21785768.9A
Authority: EP
Inventors: Amir TABAKOVIC
Original assignee: Technological University Dublin
Current assignee: Technological University Dublin
Priority date: 2020-09-16
Filing date: 2021-09-14
Publication date: 2023-07-26
Also published as: GB202014576D0; WO2022058318A1

Abstract

A capsule (1) encapsulating a rejuvenator for viscoelastic materials such as bitumen/asphalt comprising rejuvenator containing compartments (21) surrounded by a matrix (2) in which the matrix (2) is formed from a biomaterial such as an alginate and comprises a conductive material (4) which can be inductively heated.

Description

Title

A Conductive Alginate Capsule Encapsulating a Healing Agent

Introduction

This invention relates to a capsule and more particularly to a capsule encapsulating conductive particles, such as iron powder, and a healing agent/rejuvenator for viscoelastic materials, such as asphalt bitumen. The invention also relates to a self- healing system for viscoelastic materials such as an asphalt self-healing system comprising the capsule and to a method of self-healing asphalt employing the system of the invention.

Background of the Invention

Viscoelastic or bituminous materials such as asphalt pavement (referred to as asphalt concrete in the US, Canada and Asia) are widely used across a variety of industries ranging from construction to transport to health. For example, asphalt pavement is widely used for the construction of roads, footpaths, car parks, runways and the like. However, due to traffic loading and environmental effects, asphalt deteriorates over time resulting in an increase in the stiffness and brittleness of the asphalt which causes the asphalt pavement to fail e.g. by fracturing. Moreover, studies have shown that cracking with subsequent potholing is one of the main causes of asphalt damage.

Accordingly, various solutions have been proposed to address asphalt failure and prolong the life of asphalt pavements. One method employs asphalt rejuvenators which reverse the deterioration process by restoring the lost properties of aged asphalt by replenishing the asphaltene and maltene content of the asphalt to restore its original properties and thus the self-healing capacity of the asphalt. However, rejuvenators applied to the surface of asphalt can reach no more than 2 cm into pavement structures so that asphalt adjacent microcracks deep within the pavement cannot be rejuvenated to aid self-healing.

An alternative known method of healing asphalt is induction heating, via an electromagnetic field generated by an induction coil which is passed over the surface of an asphalt pavement (e.g. a road) in which conductive particles have been incorporated during production of the asphalt. In this system, the heated asphalt causes the asphalt mastic to soften allowing bitumen to flow and heal cracks within the asphalt. However, it has been found that beyond five heating cycles the bitumen becomes brittle and the induction healing process loses its efficiency. Moreover, induction heating is less efficacious with aged asphalt as the mastic is stiffer and requires higher temperatures to flow.

Accordingly, other self-healing technologies have been proposed in which rejuvenator containing capsules are incorporated into the asphalt. The rejuvenator within the capsules is released into the cracks when the cracks within the pavement cause pressure to be exerted on the capsule which then breaks. Examples of materials that have been employed to encapsulate rejuvenators include epoxy materials, melamineformaldehyde materials and alginates.

In these systems, the released rejuvenator fills the crack and diffuses into the aged bitumen of the asphalt to soften the aged binder allowing it to flow thereby healing the crack.

However, the healing efficiency of the rejuvenator containing capsules has been found to be relatively poor. Similar deterioration problems are encountered with other viscoelastic or bituminous materials across the construction, transport and health industries. For example, airport runways typically include bituminous joints to connect the airport runway slabs and connect the runway to bridge decks and the like to adjacent roads etc. and which allow for thermal expansion/contraction movements. Similarly, traditional bridge expansion ‘nosing’ joints are constructed from steel with rubber elements or with a bituminous fill which also allow for thermal expansion/contraction movements. However, such joints are notoriously unreliable and often fail in their first year, requiring unscheduled maintenance and traffic congestion during the repair process.

Chinese Patent Specification No. 110655349 and European Patent Specification No. 3702411 both describe capsules for use in self-healing asphalt in which the capsules have an outer shell formed from a polymer such as an alginate and in which the matrix of the capsule contains asphalt or a rejuvenator. In these capsules, once the shell is cracked or broken, all of the contents of the capsule are released in a single dose in an uncontrolled manner so that the capsule can only effect an asphalt healing action once.

An object of the invention is to overcome at least some of the problems of the prior art.

Summary of the Invention

In its broadest sense, the invention relates to a capsule comprising: a matrix and at least one compartment in the matrix wherein the matrix comprises a conductive material and the compartment comprises a rejuvenator.

According to the invention there is provided a capsule comprising: a matrix formed from a biomaterial and at least one compartment in the matrix wherein the matrix comprises a conductive material and the compartment comprises a rejuvenator for viscoelastic materials.

Preferably, the rejuvenator comprises an asphalt bitumen rejuvenator. More preferably, the rejuvenator comprises an oil.

Preferably, the capsule comprises a plurality of compartments throughout the matrix.

More preferably, the matrix is porous.

In one embodiment, the shell comprises a biomaterial. Preferably, the biomaterial comprises an alginate.

Preferably, the conductive material is heated by induction energy. In a preferred embodiment, the conductive material comprises magnetically conductive particles. Suitably, the conductive particles comprise iron particles, hematite or magnetite. In another embodiment, the invention also extends to a viscoelastic material comprising a capsule as hereinbefore defined.

Preferably, the viscoelastic material is a bituminous viscoelastic material. More preferably, the viscoelastic material comprises asphalt pavement.

Advantageously, the capsule is distributed throughout the asphalt pavement.

In one embodiment, the asphalt pavement comprises patch repair asphalt pavement.

In another embodiment, the invention also extends to a self-healing system for viscoelastic materials comprising a viscoelastic material as hereinbefore defined, and a heating source to heat the conductive material.

Preferably, the heating source comprises an external induction heating source. Suitably, the external induction heating source comprises a vehicle drawn induction heating source or induction coils embedded beneath asphalt pavement layer.

In another embodiment, the invention extends to a method of self-healing asphalt comprising employing the system of the invention as hereinbefore defined.

The invention also extends to method of making a capsule having a conductive biomaterial matrix and a rejuvenator for viscoelastic materials surrounded by the conductive biomaterial matrix comprising: mixing a biomaterial solution with conductive materials (CM) to form a biomaterial/CM mixture; combining the biomaterial/CM mixture with a rejuvenator to form a biomaterial/CM and rejuvenator mixture, and forming capsules from the biomaterial/CM mixture and rejuvenator mixture.

In one embodiment, the biomaterial comprises an alginate. Preferably, the alginate is a sodium alginate (SA) solution. In one embodiment, the conductive materials comprise conductive particles. Preferably, the conductive particles comprise conductive iron particles, magnetite or hematite.

In one embodiment, the proportion of conductive material (CM) to biomaterial employed to form the biomaterial/CM mixture is selected in accordance with the conductivity of the conductive material (CM).

Suitably, the sodium alginate (SA) is mixed with the conductive material (CM) in ratios of from about SA 80%:20% CM to about SA 20%:80% CM to form the SA/CM mixture. Preferably, the sodium alginate (SA) is mixed with the conductive material (CM) in a ratio of about SA 20%: 80% CM to form the SA/CM mixture.

In one embodiment, the SA/CM mixture and rejuvenator are combined in about 70/30 rejuvenator/alginate proportions.

Advantageously, the rejuvenator comprises a surfactant-rejuvenator solution in a proportion of about 1/1.5 surfactant/rejuvenator.

The Applicant has found that the capsules of the invention effectively deliver rejuvenator to viscoelastic/bituminous materials to replenish the asphaltene and maltene particles in the bituminous material. By incorporating or doping conductive materials such conductive iron particles into the inner matrix of the capsule, the capsules can be inductively heated so that the capsule is fractured to release the rejuvenator into cracks in asphalt pavement and the like. The rejuvenator therefore combines with the asphalt at the cracks. The heated conductive materials also serve to heat the asphalt so that the replenished bituminous material is sufficiently malleable to flow and fills the cracks in a self-healing process.

Due to the presence of the rejuvenator in the capsules of the asphalt pavement, the self-healing cycle can be repeated in excess of five times without deterioration of the asphalt from heating. In addition, where the capsules of the invention are distributed throughout the asphalt, the rejuvenator can be released from the capsules to rejuvenate the asphalt at fractures located deep within the asphalt pavement.

The incorporation of the conductive material in the capsule also obviates the need to separately include conductive materials such as steel fibres in the body of the asphalt during manufacture.

It should also be noted that the capsule of the invention can also be ruptured by propagating cracks to cause rejuvenator release and rejuvenation of aged asphalt binder I bitumen.

As the matrix of the capsule of the invention is formed from a biomaterial and the rejuvenator is contained within multiple compartments within the matrix, which allows multi stage healing, allowing multiple crack healing, rather than single stage healing, which is the case for the capsules with a single compartment. Release of the rejuvenator from the capsule takes place in a controlled manner as the capsule is ruptured e.g. as fractures propagate through the capsule. Accordingly, the release rate of rejuvenator can be in accordance with the level of deterioration of an asphalt pavement while not all the compartmentalised rejuvenator is necessarily released in a single dose so that the capsules of the invention can perform multiple asphalt healing actions over the lifetime of asphalt pavements. As the alginate employed to form the matrix of the capsules is a biomaterial, the capsules of the invention also enjoy several environmental and economic benefits, e.g. alginate is not harmful to the environment and the bio-matrix will deteriorate once exposed to oxygen which can be used as a secondary rejuvenator oil release mechanism. Biomaterials such as alginate are also relatively cheap and can be sourced locally around the world allowing the easy adoption of the technology for developing countries.

The capsules of the invention can be used for self-healing in any application where bituminous (viscoelastic material) is employed such as the construction, transport and health industries etc. However, a primary application of the conductive alginate capsule encapsulating rejuvenator is in asphalt self-healing systems for the damage repair and maintenance of asphalt pavement surfaces. In particular, the incorporation of the capsules and self-healing systems of the invention into road design can facilitate the development of the ‘Forever Open Road’ i.e. roads that do not need to be closed for road maintenance thereby avoiding costly traffic disruption and in turn improving the safety of road users and road repair crews.

The self-healing asphalt system of the invention can be incorporated into asphalt pavement during manufacture and before laying of roads and the like or, alternatively or in addition, can also be as applied as a self-healing asphalt pavement patch repair material to repair damaged roads and the like.

The self-healing system of the invention can also be employed in expansion joints such as bridge expansion joints and joints in airport runways and the like so that damaged joints can be repaired on demand within minutes without the need to close off the bridges or runways.

In short, the capsules and associated self-healing systems of the invention ensure that damaged asphalt and the like can be repaired rapidly on demand with no disruption to traffic flows on roads, runways and bridges etc.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings and following non-limiting Examples in which:

Figure 1 is a schematic representation of a conductive capsule of the invention produced in accordance with Example 1 having a porous or sponge-like matrix formed from an alginate network defining voids which act as rejuvenator compartments and having a magnetically conductive material distributed throughout the matrix;

Figures 2(a) to 2(e) are a series of ESEM (environmental scanning electron microscope) images of a conductive capsule of the invention formed from sodium alginate (SA) 50% and conductive material/particles (CM) 50% in the form of iron powder/particles showing the inner morphology of the capsule and demonstrating that the body of the capsule is much more dense than the alginate only capsule - iron particles are well distributed throughout the body of the capsule with rejuvenating oil pockets, within the capsule structure;

Figure 3 is a schematic illustration of the asphalt pavement self-healing system of the invention incorporating the capsule of Figure 1 during the induction heating and rejuvenation steps when self-healing asphalt pavement of the invention with the asphalt pavement shown in the macro scale as damaged aged asphalt pavement before heating via induction energy, in the macro and micro scales during rejuvenation and in the macro scale as healed asphalt pavement after heating;

Figure 4 is a graph of the viscosities of alginate solutions containing varying Iron (Fe) powder content;

Figure 5 is a graph of capsule size distribution;

Figure 6 shows SEM capsule images: a) Alg 70: Fe30; b) Alg 50: Fe 50; c) Alg 30: Fe 70; d) Alg 20: Fe 80;

Figure 7 shows SEM images of rejuvenating oil pockets within varying capsule mixtures: a) Alg 70:Fe30; b) Alg 50:Fe50; c) Alg 30: Fe 70 and d) Alg 20: Fe 80;

Figure 8 shows SEM images of Iron Powder (bright areas) distribution throughout the capsule: a) Alg 70:Fe30; b) Alg 50:Fe50; c) Alg 20: Fe 80;

Figure 9 is an image showing the capsule magnetism;

Figure 10 is a graph of the resistance of the capsules;

Figure 11 is a graph of the thermogravimetric analysis (TGA) test results, (a) ramp temperature load, (b) ramp temperature load, up to 160°C and held for an hour;

Figure 12 is a graph of the thermal effect on the capsule Contact Pressure (P_c); Figure 13 is a graph of the thermal effect on the capsule Normal Stress at middle of the capsule (on);

Figure 14 is a graph of the thermal effect on the capsule Radial Stress at middle of the capsule (o_r);

Figure 15 is a graph of the effect of water, salt and humidity on capsule compressive strength;

Figure 16 is a graph of the inductive heating (temperature rise vs time) of Alg 20: Fe 80 capsules;

Figure 17 shows images of the capsule (mortar mix test specimen containing 10% capsule by weight) conductivity testing; (a) bitumen mix indirect tensile loading, (b) disintegrated test specimen, (c) induction heating/healing (5 min), (d) restored test specimen;

Figure 18 shows the bitumen ITS and Induction healing test results;

Figure 19 is a graph of the mortar mix strength for ITS test specimens containing 5% capsules recovery after 5 minutes of induction healing, and

Figure 20 is a graph of the mortar mix strength for ITS test specimens containing 10% capsules recovery after 5m ins of induction healing.

Detailed Description of the Invention

In its broadest sense, the present invention relates to a conductive capsule. Typically the capsule can have a size ranging from about 1 mm to about 3mm, and preferably from about 1 mm to about 2mm, and encapsulates a rejuvenator/healing agent for viscoelastic materials, such as: bitumen/asphalt, which is contained in compartments defined within a porous matrix. The matrix is further provided with a conductive material distributed throughout the matrix which can be heated by an external source e.g. via induction.

Where the capsule of the invention is adapted for use in self-healing asphalt pavement and the like, the conductive capsule can be made up of a matrix formed from a biomaterial containing the conductive magnetic material. The biomaterial can be a material such as alginate which can also include additives such as starch which has been found to increase capsule density and reduce moisture content. In this embodiment, the amount of alginate and water in the capsule mix would be reduced and in turn will reduce heating time and energy use to dry out/ dehydrate the capsules. The conductive magnetic materials can be any ferrous material including recycled ferrous materials, hematite and magnetite. However, conductive iron particles are preferred. The proportion of conductive magnetic material employed in the capsules of the invention is selected at least in part in accordance with the conductivity of the conductive material e.g. the use of a more efficient material such as magnetite in place of a less efficient material such as iron may result in reduced levels of conductive material being required. A suitable size range for the conductive particles is up to about 50 microns with a range of from about 1 micron to about 6 microns being typical. In one embodiment, the matrix incorporates the conductive material or particles, e.g. conductive iron particles.

As discussed further below, the capsule of the invention allows for self-healing of asphalt pavements by releasing the rejuvenator into the asphalt pavement upon heating to replenish the asphaltene and maltene content of the asphalt so that the heated and softened bitumen is better able to fill cracks within the asphalt pavement thereby self-healing the asphalt. Typical known rejuvenators are petroleum based products which vary according to producer. Alternatively, bio-rejuvenators such as vegetable oils (sunflower oil, rapeseed oil, etc), waste cooking oils, biomass based oils (such as: lignin, algae oils, etc), and even low viscosity bitumen (100 - 200pen bitumen) can be used.

The conductive capsules of the invention once embedded within asphalt pavements enable the pavement to ‘self-heal’ by heating the pavement (between 80°C and 90°C) e.g. by passing a heating source, such as an external induction heating source, over the pavement. Alternatively, the heating source can be induction coils embedded beneath asphalt pavement layer. Suitable induction heating sources can be magnetic field developing coils which can be portable one person operating apparatus or attached to vehicles. It is estimated that healing can be achieved within three minutes of an induction heating source passing over the asphalt pavement. The conductive particles incorporated into the capsule react with the electromagnetic current produced by the induction machine and heat up so that the capsule fractures to release the rejuvenator from the core of the capsule to replenish the bitumen and improve the malleability of the bitumen - i.e. the rejuvenator once released diffuses into the aged binder, replaces the lost maltenes and saturants and rejuvenates the binder thus facilitating an optimal self-healing process in which heating of the conductive material of the capsule also causes the pavement to heat up so that once a conductive material temperature of 80°C is reached bitumen starts to soften and flow closing the cracks to repair (i.e. self-heal) the pavement.

Example 1 - Capsule Production

A 6 wt.% solution of sodium alginate in de-ionized water was prepared (6g of alginate in 94g/ml of de-ionized water). At the same time, a 2.5 wt.% poly(ethylene-alt-maleic- anhydride) (PEMA) polymeric surfactant solution was prepared by dissolving the copolymer in water (2.5g of PEMA in 97.5g/ml of de-ionized water) at 70°C and mixing it for 60min. After the PEMA had been dissolved in the water it was allowed to cool to room temperature (20±2°C) and was combined with rejuvenator, forming a healing agent solution, in PEMA/rejuvenator 1/1.5 proportion.

The following Tables illustrate various suitable ingredient proportions in the capsules of the invention:

Table 1

An example of the Alginate Solution Weight proportion, target wight 100q. Table 2

An example of the PEMA Solution Weight proportion, target wight 50q.

Table 3 An example of the Capsule Solution Weight proportion, starting with Alginate- Fe powder solution target wight 50g. a) Capsule SA: Fe ratio = 80:20 b) Capsule SA:Fe ratio = 70:30 c) Capsule SA: Fe ratio = 50:50 d) Capsule SA:Fe ratio = 30:70 e) Capsule SA: Fe ratio = 20:80

In the present Example, the rejuvenator employed was vegetable oil and more specifically sunflower oil.

The sodium alginate (SA) was then mixed with conductive material (CM) (iron particles in the present example in ratios of from about SA 80%:20% CM to about SA 20%:80% CM to form SA/CM mixtures. The SA and rejuvenator solutions were then combined in 70:30 rejuvenator/alginate proportions, which has been found to be an optimum ratio and facilitates the optimal/maximal amount of rejuvenator for encapsulation in the alginate capsules. The above Tables show capsule constituent proportions for varying SA: CM ratios.

The solution was then poured into a pressurised vessel (glass funnel, syringe, or similar), and allowed to drop into a Calcium Chloride (CaCI₂) bath to form a porous matrix of the capsules. The forming capsules were left in the CaCI₂ bath for 1 -2h and stirred at a very low stirring rate, allowing them to coagulate in a process in which positively charged Ca ions attached themselves to long alginate molecules forming the matrix of the capsules whilst trapping the iron particles in the alginate matrix and encapsulating the rejuvenator droplets in compartments defined in the porous matrix. The size of the compartments in the porous matrix can be controlled by increasing the amount of rejuvenator oil in the mix and/or by using a slower mixing speed when preparing the capsule solution. The capsules were then sieved out of the bath and left to dry for at least 12 hours at 40°C in order to achieve a stable mass.

Figure 1 shows schematic diagram of the capsule produced in accordance with Example 1 while Figures 2(a) to 2(e) show a series of ESEM (Environmental Scanning Electron Microscope) images of a conductive capsule of the invention. As shown in the drawings, the capsule is generally indicated by the reference numeral 1. The capsule 1 is substantially spherical in shape and is made up of a porous body or inner matrix 2 formed from an alginate network 3. The porous matrix 2 therefore resembles a honeycomb or sponge structure defining voids in the form of compartments or cells 21 formed/distributed throughout the matrix 2. The matrix 2 of the capsule 1 is rendered conductive by a magnetically conductive material 4 held in the matrix 2. The conductive material 4 is distributed throughout the alginate network 3 of the matrix 2 and in the present embodiment is in the form of conductive iron particles 4 supported in the matrix 2 with the capsule having an SA:Fe ratio = 80:20. The alginate network 3 of the matrix 2 further encapsulates a rejuvenator/healing agent 22 also distributed throughout the matrix 2 the form of droplets 23 contained within the compartments 21 .

In use, the capsules 1 of the invention can be incorporated into a bitumen self-healing system such as an asphalt pavement self-healing system made up of the conductive capsules 1 of the invention, an asphalt material incorporating the conductive capsules and, optionally, a heating source such as an induction heating source for effecting heating of the conductive capsules 1. In such systems, the capsules 1 can be incorporated into asphalt pavement to be used in repairing a road or, as shown in Figure 3, the capsules 1 can be incorporated into the asphalt pavement of the road during manufacture. Table 4 below describes various detailed asphalt mixes containing varying amounts of bitumen and capsules.

Table 4

Asphalt Mixes As shown in Figure 3, capsules 1 of the invention formed in accordance with Example 1 are incorporated into asphalt pavement 5 during manufacture which is subsequently laid as a pavement 6 which as shown in the drawings is in the form of a road 7 which is subject to wear and tear caused by environmental factors 8 such as sun, heat, precipitation and traffic using the road 7. The asphalt pavement 5, excluding the capsules 1 of the invention, can be conventional in composition e.g. can be an asphalt mortar formed from bitumen 9, suitable fillers 10 and aggregates/stone 11 with the bitumen 9 being in the form of a mastic 12 at the micro scale.

Due to wear and tear, the road 7 becomes damaged as indicated by the reference numeral 13 so that cracks such as microcracks 14 form in the bitumen 9 of the road 7. In order to cause release of the rejuvenator 22 from the core 3 to replenish the aged bitumen 9 and effect self-healing of the asphalt pavement 5, the conductive particles 4 in the matrix 2 of the capsules 1 are inductively heated by drawing a coil 15 over the surface of the road 7. The coil 15 can be drawn by a maintenance vehicle 16. Accordingly, as shown in the induction healing step in the drawing, the asphalt pavement 5 is in turn heated by the conductive particles 4. As indicated above, the heating step causes the capsule 1 to rupture 18 to release rejuvenator 22 into the microcracks 14 from the compartments 21 as indicated by the reference numeral 17. It should be noted that the capsule of the invention can also be ruptured by propagating cracks to cause rejuvenator release and rejuvenation of aged asphalt minder. The rejuvenator assists in replenishing the bitumen 9 to enhance the malleability of the bitumen at the microcracks 14 so that, when the bitumen 9 reaches a temperature of about 80°C from the heat transferred from the inductively heated conductive particles 4, the microcracks 14 are filled by the flowing of bitumen 9 into the microcracks 14. Accordingly, following cooling of the asphalt pavement 5, the microcracks 14 are closed by the rejuvenated bitumen indicated by the reference numeral 19 so that the road 7 is healed 20.

Example 2 - Capsule Production

All of the chemicals used in the conductive alginate capsule encapsulating rejuvenator production process of this Example were purchased from Merck Group, Sigma Aldrich, Ireland, except for the rejuvenator (vegetable oil), which was purchased from the local supermarket. The conductive alginate capsules encapsulating a bitumen rejuvenator were prepared using a drop process from an emulsion of rejuvenator suspended in a water solution of sodium alginate. For this, a 6 wt.% solution of sodium alginate in deionized water was prepared. At the same time, a 2.5 wt.% poly(ethylene-alt-maleic- anhydride) (PEMA) polymeric surfactant solution was prepared by dissolving the copolymer in water at 70 °C and mixing it for 60 min. After the PEMA was dissolved in the water, it was allowed to cool to room temperature (20 ± 2 °C), and was combined with the rejuvenator. For this study, a vegetable oil of 0.9 g/cm3 density at a room temperature (20 ± 3 °C) was used, forming a bitumen healing agent solution, in a PEMA/rejuvenator at a ratio of 1/1.5 proportion by weight. The sodium alginate solution was mixed with iron powder (40 pm particle size), at 700 rpm for 1 h to allow for uniform iron particle dispersal within the alginate mix. As before, five different alginate (Alg)Ziron powder (Fe) mix ratios were prepared in proportions of the weight of dry constituents ratios of 100:0, 70:30, 50:50, 30:70, and 20:80. After the sodium alginate and iron solution was fully mixed, the PEMA and rejuvenator (oil) solution was added to the alginate-iron powder solution mix with a ratio of 70% rejuvenator to 30% sodium alginate. The full capsule solution was mixed at 700 rpm for 20 min. For small capsule volume solution mixtures, it was sufficient to mix the constituents for 2 min at 200 rpm. As the capsule production was scaled up from 20 mL to 20 L, it was necessary to increase the mixing time and rate in order to allow for suitable oil and iron particle distribution throughout the capsule solution. The capsules were produced using a drip production process using a 20 L capacity pressurised system. The pressurised cylinder was equipped with a pneumatic stirrer, which was used to agitate the solution within the pressurised cylinder during the capsule production process in order to prevent iron segregation at the bottom of the cylinder. The stirring rate was controlled during the process via an air pressure valve. The stirring rate during the production process was kept at 200 rpm. A shower head with 61 capillary openings of 1 mm diameter was used as the drip system. The production rate was 0.222 L/min. A 101 bath with an inserted sieve containing a 0.45 M solution of CaCI₂'6H2O was used for the capsule coagulation. After production, the capsules were dried in the oven for 12 h at 40 °C. Bitumen and Mortar ITS

The bitumen used in the preparation of the indirect tensile strength (ITS) test specimens was 70/100 Pen bitumen supplied by Lagan Materials Ltd. The bitumen and mortar ITS test specimens were prepared using a Struers FixiForm non-stick mould with dimensions of a depth max (h) of 24 mm and diameter (d) of 30 mm. For this study, the diameter specimen height ratio was 2:1 (29 mm(d):14.5 mm(h)). The bitumen and capsules were preheated to 160 °C and then mixed together, and finally, the mixture was poured into the mould and left to cool down to room temperature (20 ± 3 °C) for 24 h. Table 5 summarises the test specimen constituent weight for both the bitumen and mortar test specimens. For the induction healing process, a cylindrical (ring) coil was used with a 100 mm diameter and 50 mm height. The mould with the test specimen was placed in the centre of the ring, allowing for even induction heating throughout the test specimen. A Zwick Roell ZwickiLine Z5.0 TN for a Flexible Low- Force Testing machine was used to carry out the ITS test. The tests were carried out at a loading rate of 0.1 mm/s and test temperature of 20 °C. The bitumen test specimens were very difficult to test at room temperature. For this reason, the test specimens were submerged in liquid nitrogen for 10 s before testing. The bitumen test specimens containing 5% and 7% capsules completely deformed when extracted from the mould. As a result, only test specimens containing 10% and 20% capsules were tested.

The ITS test was conducted by applying a vertical compressive strip load to a cylindrical specimen. The load was distributed over the thickness of the specimen through two loading strips at the top and bottom. The combination of specimen geometry and boundary conditions induced tensile and compressive stress along both the vertical and horizontal diameters. The tensile stresses, which developed perpendicular to the direction of the load, were of a relatively constant value over a large portion of the vertical diameter. This would be expected to cause failure of the specimen by splitting along the vertical diameter. The critical stresses and strains within the indirect tensile specimen were computed using an analytical formulation based on linear elastic theory. This theory assumes that the material is homogenous and isotropic, that it only experiences plane stress conditions, and that the loading strips are simplified to line loads.

Capsule composite characterisation

- Viscosity

The A&D Vibroviscometer SV-10 was used to measure the viscosity of the capsule alginate solutions. The viscosity values for solutions containing varying ratios of Alginate : Iron powder (100:0; 70:30; 50:50; 30:70 and 20:80) were measured. The solution containing alginate and rejuvenating oil only, without iron, (Alg. 100 : Fe 0) was used as the control solution.

- Relative Density

Following the procedure described in EN 162:2013, a beaker of 50ml volume filled the capsules to capacity. The mass of the capsules in the 50ml beaker was then measured. One representative sample for each capsule mix was used to determine the apparent relative density of each capsule mix. The relative density was determined as follows: where: po = relative density (g/cm³) m = total mass of the capsules (g)

Vo = volume (cm³) - Scanning Electron Microscope (ESEM)

Scanning Electron Microscopy (SEM) was used to evaluate the morphology of the rejuvenator compartments within the sodium alginate fibres. For this purpose, a Hitachi Sll-70 FESEM system was employed. Low accelerating voltage of 2kV and a beam current of less than 1 nA were used to limit the electron beam damage on the heat sensitive polymeric capsules.

- Thermogravimetric Analysis (TGA)

The thermal stability characterization of the conductive alginate capsule encapsulating a bitumen rejuvenator (oil) was performed using a Shimadzu DTG-60 Simultaneous DTA-TG system, at a scanning rate of 6.5 °C/min, under a nitrogen gas (N2) at a flow of 50 mL/min.

- Electrical Resistivity

The capsule electrical resistivity was measured using an ITT Matrix MX 545 digital multimeter. The capsule electrical resistivity was measured by placing the conductivity pins on the opposite sides of the capsule.

- Uniaxial (Compression) Strength Test

A Zwick Roell ZwickiLine Z5.0 TN for Flexible Low-Force Testing machine was used to carry out the ITS test, Figure 3d shows the ITS system set up. Tests were carried out at a loading rate of 0.1 mm/s and test temperature of 20 °C. The effect of (a) temperature (b) humidity, (c) moisture, and (d) moisture and salt (brine) on the capsule strength was measured. The influence of temperature on the capsule strength was evaluated by placing capsules in the desired temperature for 3 h, and then left to condition for 1 h at room temperature (20 ± 2 °C) prior to testing. The conditioning temperatures were -19 °C, 6 °C, 20 °C, 40 °C, 80 °C, 120 °C, 160 °C, and 200 °C. The effect of moisture on the capsule mechanical properties was carried out by placing a capsule in a dish with a salt water solution (100 g of water and 36 g of sodium chloride). The capsules with a water/salt solution were sealed and left for 14 days to condition. The water/salt solution created a humid environment of 75% humidity. Finally, the effect of salt on the mechanical strength of the capsules was tested by submerging the capsules in brine (salty water) for 72 h. In order to calculate the capsule compression strength, the Hertz Theory of elastic contact between a steel plate and elastic sphere was adopted.

- Induction

The induction heating was carried out using an Abrell EKO 10/100C, PWR, CNTRL EKOHEAT® 10/100C, ES, solid state induction power supply CE rated with an input of WYE configured, 360-520 VAC, 50/60 Hz, three phase, and an output of 10 kW terminal, 50-150 kHz. A solenoid coil was used to apply the induction heating to the test specimen.

Results

Alginate solution viscosity

Error! Reference source not found, shows the variation in viscosity of the alginate solution with increasing Iron powder content. The results show that iron content affects the solution viscosity. Increasing the iron content from 0% to 80% by weight, increases the viscosity 3.4 times. It is also expected that capsules containing higher amounts of Iron will be much denser than alginate capsules encapsulating oil only.

Capsule Size Distribution

The capsule size distribution was recorded by taking a random sample of 120 capsules (30 capsules from four Alg:Fe ratios, ranging from Alg 70: Fe 30 to Alg 20: Fe 80) and measuring the diameter manually using the digital Vernier callipers. Figure 5 shows the dry capsule size distribution, with the results showing capsule dimeter ranges between 1.5 mm to 2.4 mm, with over 50% of capsules of 2.1 mm in diameter, 10% of 2 mm, and 29% of 2.2 mm. The results show that capsules were of a very regular and consistent size.

Capsule Relative Density

Table 6 summarises the relative density values for each capsule mix. The results show that the density of the capsules increases with the increase of iron powder in the capsule mix.

Capsule SEM

Figure 6 shows the cross-sectional image of the capsules with varying alginate (Alg)/iron powder (Fe) ratios. The images show that capsules are much denser than some known capsules.

Figure 7 illustrates the bitumen rejuvenating pockets within the capsule structure. It is clear from the images that the alginate/iron power ratio has no significant influence on the size of the oil pockets within the capsule - capsule size is dependent on the mixing rate and less on the oil content. The reduction in the oil content in the mix is due to the reduction of alginate in the mix. With increasing the iron powder content, the alginate content is reduced and, as a result, the oil content is also reduced. Figure 8 shows the iron powder patterns within the capsule alginate structure. The images show that the patterns change between the lower iron content capsules (Alg 70: Fe 30 and Alg 50: Fe 50) and higher iron content capsules (Alg 20: Fe 80).

Capsule Resistivity

From the start, the capsules showed the magnetic properties. All of the capsule mix types were attracted to the magnetic field, see Figure 9. However, additional tests were necessary to determine the efficiency in conducting the induction energy, given that alginate could act as an insulator, thus preventing the induction energy flow. Figure 10 summarises the results from the capsule resistance test. The tests show that the mixtures had resistance to the current flow, indicating their ability to conduct inductive energy. As expected, the capsules containing the highest concentration of iron powder (Alg 20: Fe 80) showed the highest resistance. The resistance gradually increased with the increase of the iron powder content in the mix, except for the mix containing equal amounts of alginate and iron powder (Alg 50: Fe 50). It is not precisely clear why this occurred, however, two reasons are proposed. Firstly, the capsules might not have been fully dehydrated and thus had a higher moisture content. However, the TGA test data, see Figure 11 , did not confirm this hypothesis. The second reason might be a structure imbalance at this mix ratio. Figure 12 and Figure 15 show similar behaviour, where the capsule strength is lower than that of the capsules containing a higher mount of alginate.

Capsule TGA

The biggest concern for the capsules is whether they can withstand the asphalt mixing temperatures, during asphalt manufacturing process, which are between 160°C and 180°. For this reason, TGA tests were used to replicate these conditions. Two tests were designed; one that simulates the asphalt mixing temperatures and a second that simulates the asphalt transport temperature (heat and hold for 1 h). Error! Reference source not found, a) shows the TGA test results of capsules containing varying Alginate : Iron Powder ratios. The capsules were subjectedto ramp heating from room temperature (20 ± 3 °C) to 200 °C at rate of 10 °C/min. The results show that the maximum weight loss is 12% for the capsules of Alg:Fe with a ratio of 70:30, and the minimum is 5% for the 20:80 Alg:Fe capsule ratio. This is because the capsules hold the moisture in the alginate rather than the iron. As result, after the drying process, it is expected that the capsules with the higher alginate content will experience a greater weight loss. Nevertheless, capsules containing a high amount of iron powder (Alg 20: Fe 80) showed a weight loss of only 5% at 200 °C. The heat and hold test in Figure 11 b showed similar results, with the highest weight loss for mixtures with higher alginate and lower iron powder content. The weight loss continued while the temperature was held at max 160 °C, losing an additional 3-4% of the weight. This is expected, as capsules will keep losing weight while moisture evaporates from the capsules. These results indicate that the conductive alginate capsules encapsulating a bitumen rejuvenator of the invention have a suitable thermal resistance to withstand the standard asphalt production process. Capsule Compressive Strength

Another very important question in the capsule design process is whether they can withstand harsh (thermal and loading) conditions during the asphalt (bituminous) mix production. For this reason, the effect of the temperature, moisture, humidity, and salt on the compressive failure strength of conductive alginate capsules encapsulating a bitumen rejuvenator were investigated. Figure 12 illustrates the compression test results from the thermal effect test. The results show that temperature does indeed have a strong effect on the capsule strength, with Alg 20: Fe 80 capsules showing the highest variation across the temperature range (-19 °C-200 °C). However, the pressure stabilised after 80 °C. It is believed that moisture residue within the capsule affects the capsule strength at lower temperatures (80 °C. Figures 13 and 14 show the normal and radial stress within the capsule at the maximum pressure contact, at varying temperatures. Similarly as for contact pressure, capsules experience high stress at lower temperature and as temperature increases, up to 80 °C, stresses decrease, which is expected because residual moisture within the capsules soften the alginate. As the temperature passes 80 °C, the stresses increase again up until 160 °C. This is because excess moisture in the capsule evaporates and capsules become more brittle. However, at 200 °C, the capsule stress starts to fail. At this point, the alginate within the capsule starts to disintegrate and the capsule loses its structural integrity.

Figure 15 shows the effect of water, brine, and humidity on the capsules’ compressive strength. A dry capsule was used as the control sample. The results show that all three methods of pre-conditioning have a significant effect on all capsule mixtures, except the Alg 50:Fe 50 capsule mix. The results show that the capsule compressive strength improves with exposure to the salt water (brine). However, the strength of the capsules decreases by more than 50% for mixtures Alg 70: Fe 30; Alg 30: Fe 70, and Alg 20: Fe 80 when subjected to water, brine and humidity. This could possibly be because the capsules absorb the moisture when exposed to a moist environment, becoming softer and thus experiencing a reduction in the strength. Table 7 shows the capsule test samples weights before and after conditioning. The results confirm that there was an increase in the weight gain of the capsules, indicating that the test samples did absorb moisture during the conditioning phase, thus resulting in softer capsules with reduced strength. However, this will not have great effect on the capsule healing performance, as the heat generated by the induction radiation will increase the capsule temperature and dry it, allowing it to recover its initial (dry) strength.

Induction

Capsule induction was carried out in order to determine whether capsules can optimally conduct induction energy and what the maximum temperature is each capsule mix can reach. The test was performed at 5.6 kW and a 109 Hz induction machine energy output. A solenoid coil was used for energy induction and the capsules were placed in a glass beaker with a 9 mL volume. The maximum time allowed for the test was 300 s (5 min). Table 8 summarises the test results for all four capsule types. From the data, the Alg 20: Fe 80 ratio capsule reached an optimal temperature (>80 °C). Figure 16 shows the capsule Alg 20: Fe 80 temperature rise. The results show that temperature reached a maximum temperature at 97 °C. Accordingly, the Alg 20:Fe 80 capsule design was adopted for inclusion in the bitumen and mortar mixes.

Indirect tensile strength of Mortar and Bitumen test specimens

The bitumen and bitumen mortar mix ITS - Induction (healing) tests demonstrated the potential of the capsules to repair crack damage in a bitumen and bitumen mortar mix within 5 minutes of being subjected to induction heating, at an energy output of 759V and a frequency of 109kHz. Error! Reference source not found, shows healing of a completely disintegrated bitumen test sample containing 10% capsules, within 5 minutes of healing time.

The bituminous tests containing 10% and 20% of capsules were successfully extracted from the moulds and tested. However, as the test specimens were too soft to be tested at room temperature of 20 ± 3 °C, they had to be submerged in liquid nitrogen for 10 s. As a result, all of the test specimens disintegrated at the maximum load. Figure 17 shows the testing and healing process. Figure 18 shows the summary ITS and induction healing test results, and the results show a very high specimen strength recovery. Test specimens containing 10% capsules recovered 94% of their initial strength, whereas specimens containing 20% capsules experienced 118% strength recovery. These results indicate that conductive alginate capsule encapsulating a bitumen rejuvenating oil is suitable the healing system of the bituminous mixtures.

The next step was to perform the tests on the ITS mortar mix containing Conductive Alginate Capsule Encapsulating Bitumen Rejuvenating Oil. Figure 19 and Figure 20 show the mortar mix, containing 5% and 10% capsules in the mix, strength recovery 66% and 67% respectively, after full breakage. These results indicate that conductive alginate capsules encapsulating rejuvenator can successfully repair cracks in a bituminous mix. In summary, it has been demonstrated that the capsule of the invention is highly efficacious as a bitumen mix extrinsic self-healer with the capsule having an Alg 20: Fe 80 ratio giving optimal results. Accordingly, capsule having a SA:CM ratio of at least 20:80 provide optimal induction heating. However, where conductive materials having greater conductivity than Fe are employed in the capsules of the invention, e.g. magnetite, lower amounts of conductive material can still provide optimal results. As shown in Figures 18 to 20, the capsules of the invention reinforced the bitumen and bitumen mortar mixtures, i.e. , increasing the amount of capsules in the bitumen and bitumen mortar results with a higher initial strength. The results also show that conductive alginate capsules encapsulating a bitumen rejuvenator can successfully repair the damage (close cracks) in a bitumen and bitumen mortar mix with an efficiency of up to 118% for the bitumen specimens and 67% for the mortar specimens. The results also show that the capsules of the invention have sufficient physical, mechanical, and thermal strength to be included in an asphalt mix design as an extrinsic self-healing system.

Claims

1. A capsule comprising: a matrix formed from a biomaterial and at least one compartment in the matrix wherein the matrix comprises a conductive material and the compartment comprise a rejuvenator for viscoelastic materials.

2. A capsule as claimed in Claim 1 wherein the rejuvenator comprises an asphalt bitumen rejuvenator.

3. A capsule as claimed in Claim 2 wherein the asphalt bitumen rejuvenator comprises an oil.

4. A capsule as claimed in any of Claims 1 to 3 wherein the capsule comprises a plurality of compartments throughout the matrix.

5. A capsule as claimed in any of Claims 1 to 4 wherein the matrix is porous.

6. A capsule as claimed in any of Claims 1 to 5 wherein the biomaterial comprises an alginate.

7. A capsule as claimed in any of Claims 1 to 6 wherein the conductive material is heatable by induction energy.

8. A capsule as claimed in any of Claims 1 to 7 wherein the conductive material comprises magnetically conductive particles.

9. A capsule as claimed in Claim 8 wherein the conductive particles comprise iron particles, hematite or magnetite.

10. A viscoelastic material comprising a capsule as claimed in any of Claims 1 to 9.

11. A viscoelastic material as claimed in Claim 10 wherein the viscoelastic material is a bituminous viscoelastic material.

12. A viscoelastic material as claimed in Claim 11 wherein the viscoelastic material comprises asphalt concrete.

13. A viscoelastic material as claimed in Claim 12 wherein the capsule is distributed throughout the asphalt concrete.

14. A viscoelastic material as claimed in Claim 12 or Claim 13 wherein the asphalt concrete comprises patch repair asphalt concrete.

15. A self-healing system for viscoelastic materials comprising a viscoelastic material as claimed in any of Claims 10 to 14, and a heating source to heat the conductive material.

16. A self-healing system as claimed in Claim 15 wherein the heating source comprises an external induction heating source.

17. A self-healing system as claimed in Claim 16 wherein the external induction heating source comprises a vehicle drawn induction heating source or induction coils embedded beneath asphalt pavement layer.

18. A method of making a capsule having a conductive biomaterial matrix and a rejuvenator for viscoelastic materials surrounded by the conductive biomaterial matrix comprising: mixing a biomaterial solution with conductive materials (CM) to form a biomaterial/CM mixture; combining the biomaterial/CM mixture with a rejuvenator to form a biomaterial/CM and rejuvenator mixture, and forming capsules from the biomaterial/CM mixture and rejuvenator mixture.

19. A method as claimed in Claim 18 wherein the biomaterial comprises an alginate.

20. A method as claimed in Claim 19 wherein the alginate is a sodium alginate (SA) solution.

21 . A method as claimed in any of Claims 18 to 20 wherein the conductive materials comprise conductive particles.

22. A method as claimed in Claim 21 wherein the conductive particles comprise conductive iron particles, magnetite or hematite.

23. A method as claimed in any of Claims 18 to 22 wherein the proportion of conductive material (CM) to biomaterial employed to form the biomaterial/CM mixture is selected in accordance with the conductivity of the conductive material (CM).

24. A method as claimed in any of Claims 19 to 23 wherein the sodium alginate (SA) is mixed with the conductive material (CM) in ratios of from about SA 80%:20% CM to about SA 20%:80% CM to form the SA/CM mixture.

25. A method as claimed in Claim 24 wherein the sodium alginate (SA) is mixed with the conductive material (CM) in a ratio of about SA 20%:80% CM to form the SA/CM mixture.

26. A method as claimed in any of Claims 19 to 25 wherein the SA/CM mixture and rejuvenator are combined in about 70/30 rejuvenator/alginate proportions.

27. A method as claimed in any of Claims 19 to 24 wherein the rejuvenator comprises a surfactant-rejuvenator solution in a proportion of about 1/1.5 surfactant/rejuvenator.