SMART GELS HYDROGELS and Hybrids
SONG PEI FEN 11019165 U30073 ADVANCED ARCHITECTURAL TECHNOLOGY NOVEMBER 2013
SMART GELS
2 | CONTENTS
CONTENTS INSIDE FEATURES 3 Introduction to Gels & the Origin of Hydrogels 6
BIOMIMETICS DRAWING INSPIRATION FROM NATURE
4
Current application of hydrogels and Properties of hydrogels
8 Exploration & Analysis of physical properties of Hydrogels & hydrogel composites 24 Exploration & Analysis of chemical properties of Hydrogels & hydrogel composites 33 The future of hydrogel as a building coating Inspiration from the spray concrete technology 36 References
The properties and application of chitosan, derived from chitin
GEL TO COAT BUILDINGS SPRAY GEL TECHNIQUE
32
The future of sprayed hydrogel technique to heal damages on structures and coat buildings
SMART GELS
3 | INTRODUCTION
INTRODUCTION WHAT IS A GEL A gel is defined as a colloid in which the solid particles are suspended in a liquid. By weight, gels are mostly liquid, yet they behave like solids due to a 3-dimensional cross-linked network within the liquid. In this gels are a dispersion of particles within a liquid in which the solid is the continuous phases and the liquid is the discontinuous phase.
WHAT IS A HYDROGEL Hydrogels are a class of smart gels that change shape and other properties in response to changes in its environment. Hydrogel describes three-dimensional network structures obtained from a class of synthetic and/or natural polymers which can absorb and retain significant amount of water (Rosiak & Yoshii, 1999). The hydrogel structure is created by the hydrophilic groups or domains present in a polymeric network upon the hydration in an aqueous environment. Depending on what a hydrogel interacts with, it will exhibit different behaviours and response to the external stimuli such as pH, temperature, salt concentration and mechanical forces. (MIT, 2000)
CLASSIFICATION OF HYDROGELS Hydrogels are broadly classified into two categories: Permanent / chemical gel: They are covalently cross-linked (replacing hydrogen bond by a stronger and stable covalent bonds) networks (Hennink & Nostrum, 2002). They attain an equilibrium swelling state depending on the polymer-water interaction parameter and the crosslink density (Rosiak & Yoshii, 1999). Reversible / physical gel: They are networks held together by molecular entanglements, and / or secondary forces including ionic, hydrogen bonding or hydrophobic interactions. In physically cross-linked gels, dissolution is prevented by physical interactions, which exist between different polymer chains (Hennink & Nostrum, 2002). All of these interactions are reversible, and can be disrupted by changes in physical conditions or application of stress (Rosiak & Yoshii, 1999). Hydrogels were first designed in the late 1950’s for use in soft contact lenses, an application for which they are still used today. In the 1970’s, Dr. Tanaka discovered that the physical properties of hydrogels are directly influenced by the changes in external conditions around a hydrogel such as water, temperature, & light. Today, hydrogels are designed out of a wide range of synthetic & biodegradable materials which make them more versatile for numerous applications.(Zabor, n.d.) Hydrogels are commonly found in soft contact lenses, nappies, wound dressings, hair gel, plant water storage crystals and drug delivery systems. Currently, research is being done to investigate the possible application of hydrogels in tissue engineering, growing human organs in hydrogels.
SMART GELS
4 | BIOMIMETICS
Chitosan is also used in many applications involving the controlled release of active agents, and also as constituent of functional coatings. Recent studies suggest the application of chitosan and its derivatives as environmentally friendly protective coatings, in metal implants aiming to control the rate of dissolution of active components and to increase the biocompatibility and corrosion resistance of metal.
SMART GELS
5 | BIOMIMETICS
BIOMIMETICS
C C
DRAWING INSPIRATION FROM CHITOSAN, A NATURAL SELF-HEALING MATERIAL
hitin is the second most abundant polysaccharide on Earth, after cellulose. It is an important structural element in crustaceans exoskeletons, squid stilettos, fungi, insects, arachnids, molluscs and some species of algae.
hitosan,the deacetylated unit of chitin has many interesting features, including healing wound properties. From a chemical point of view chitosan is a linear copolymer of β-(1-4)-2-amido-2-deoxy-D-glucan (glucosamine) and β-(1-4)-2-acetamido-deoxy-D-gluc an (N-acetylglucosamine) that can be produced from chitin by means of a partial alkaline deacetylation. (Carneiro, 2012) Chitosan has unique physicochemical properties namely biocompatibility, antimicrobial activity, biodegradability and excellent film-forming ability. Due to its high molecular weight of 50–2000 kDa, chitosan exhibits a positive charge and film-forming and gelation characteristics (Alemdarog, 2006) Chitosan has been applied successfully in several fields such as biotechnology, pharmaceutics, biomedicine, packaging, wastewater treatment, cosmetics and food science.
Chitosan enhances the functions of inflammatory cells such as polymorphonuclear leukocytes, macrophages and fibroblasts; thus, it promotes granulation and organization. Therefore, chitosan can be used for large open wounds. It also accelerates wound healing and permits regeneration of tissue elements skin wounds. Chitosan gel has excellent tissue-adhesive quality. A study showed that chitosan-treated wounds were epithelized when compared with wounds of the control group after the treatment. (Alemdarog, 2006) Chemical structure of A) chitin and B) chitosan
SMART GELS
6 | APPLICATION OF HYDROGELS
CURRENT APPLICATION OF HYDROGELS Soft Contact Lens Hydrogel is flexible and can adapt to the global ocular curvature of the eye for optimal comfort Hydrogel has good permeability of oxygen, which is dissolved in the water and diffused to the cornea Hydrogel has high retention of water (70%-80%) to maintain moisture Hydrogel is composed of biostable/biodegradable materials
Drug delivery systems Hydrogel can release an antibiotic at a controlled rate to the body tissue beneath. The hydrogel is called a carrier when it is loaded with a drug. As the swelling of the hydrogel increases, the chains of the cross linked network move further apart and the drug can diffuse quickly through the hydrogel to the skin.
Wound healing Hydrogel has the ability to conform to the wound bed and keep the wound moist. It stops the wound drying out and protects it from infection. The hydrogel can control bleeding and does not stick to the surface so it can be removed easily without damaging the skin. Hydrogel can be formed in situ and therefore has the potential to be used for creating patient injury-specific wound dressings.
Tissue engineering Hydrogel is being tested to develop a scaffold that mimics the matrix of the tissue toeb repaired. Cartilage consists of 3 basic components : water, a matrix, and cells. This natural material possesses remarkable mechanical and biochemical properties, which together make it very difficult to mimic. Though hydrogels could mimic biological behaviour of cartilage, it is poor in mechanical strength, and therefore scientists have added reinforcing materials to hydrogels to enhance its structural performance.
SMART GELS
7| PROPERTIES OF HYDROGELS
PROPERTIES OF HYDROGELS
Smart hydrogels are environment stimuli-responsive. They demonstrate volume transformation in response to changes in external environment conditions, such as solution pH, electric field, temperature, solvent composition, glucose/carbohydrates, salt concentration or ionic strength, light/photon, pressure, coupled magnetic and electric fields. Properties of hydrogels are determined by the monomer composition, cross-linking density, and polymerization conditions. Physical Properties of smart hydrogels Weak and brittle Poor mechanical strength Poor toughness Limited extensibility a nd recoverability Chemical Properties of smart hydrogels Reversible swelling/deswelling behaviour High Absorption capacity High water retention High ionic conductivity High environmental sensitivity Permeability Surface property Biological Properties of smart hydrogels Biocompatible - do not stimulate an immune reaction Ability to support cell growth Wound healing properties Cooling properties
Physical appearance and qualities of hydrogel : Squishy, synthetic, flexible, mostly water, almost as tough as rubber Hydrogels change shape (or other properties) in response to changes in its environment. The cross-linking density between polymer chains strongly affects the physical properties of the gel.
Classes of hydrogels : Natural hydrogels come from proteins or polysaccharides Artificial hydrogels are produced by materials such as poly(lactic) and poly(glycolic) acid, poly(ethylene) oxide, poly(vinyl) alcohols, polydiol citrates, and polyhydroxyethyl methacrylate. Common hydrogels: Hydrogels composed of alginate – obtained from marine algae and various bacteria composed of 2 acidic monomers Agarose – neutral polysaccharide extracted from marine red algae, a physical gel because it is governed by hydrogen bonds
To improve hydrogel properties for load-bearing biomedical applications have included the introduction of composite materials such rubber or glass, the use of cross-linking agents such as glutaraldehyde, and the use of freeze-thawing procedures to induce partial crystallinity.
SMART GELS
8 | PHYSICAL EXPLORATION
PHYSICAL EXPLORATION OF HYDROGELS Mechanical Capacity of Hydrogel Studies have been performed to investigate the implementation of hydrogels in tissue engineering to generate cartilage. Though hydrogels are highly bio-compatible and could support cell growth, their capability is limited structurally. Limitation of pure hydrogels Insufficient mechanical strength compared to articular cartilage, for advancement in tissue engineering. This severely limits the use of hydrogels as substitutes for natural tissues or for load-bearing applications such as articular cartilage. Attempts to enhance mechanical property of hydrogels The design of hydrogels with a good mechanical performance is of crucial importance in many existing and potential application areas of soft materials. To improve hydrogel properties for load-bearing biomedical application composite materials such as rubber or glass, the use of cross-linking agents such as glutaraldehyde, and the use of freezethawing procedures to induce partial crystallinity are introduced. The nanoscale dispersion of layered silicate or clays in polymer networks is one of the techniques offering significant enhancements in the materials properties of hydrogels. The physical tests recorded in this report investigate the elastic behaviours and compressive ability of pure hydrogels and hydrogel composites, to observe the standard of each gel composite.
SMART GELS
9 | PHYSICAL EXPLORATION
PHYSICAL TESTING OF HYDROGELS Elasticity Behaviour of Pure Hydrogel
Weight of load : 6.83g
Weight of load : 66.84g
SMART GELS
10 |PHYSICAL EXPLORATION
Elasticity Behaviour of PVA Hydrogel
Weight of load : 6.83g
Weight of load : 66.84g
SMART GELS
11 | PHYSICAL EXPLORATION
Elasticity Behaviour of Alginate Hydrogel
Weight of load : 6.83g
Weight of load : 66.84g
SMART GELS
12 | PHYSICAL EXPLORATION
Elasticity Behaviour of Collagen Hydrogel
Weight of load : 6.83g
Elasticity Behaviour of Pure Collagen
Weight of load : 6.83g
SMART GELS
`13| PHYSICAL EXPLORATION
Elasticity Behaviour of Cement Hydrogel
Weight of load : 6.83g
Elasticity of Hydrogels The elastic modulus of hydrogels depend on the cross-link and charge densities of the polymer network as well as on the cross-linked polymer concentration after the gel preparation. The higher the initial monomer concentration, the larger the effective cross-link density of the hydrogels and the smaller their swelling capacity. Increasing number of ionic groups in hydrogels is known to increase their swelling capacities. This is mainly due to the simultaneous increase of the number of counter-ions inside the gel, which produces an additional osmotic pressure that swells the gel (Flory 1953).
Deduction from the tests For the force of 668.4N, all the tests reached their breaking point and gave way to the weight, so there was no clear observation of deformation and elasticity. For the force of 683N, the cement hydrogel showed the least elasticity because of the dominant brittle and inelastic property of cement. On the other hand, the alginate hydrogel showed the most elasticity because of the dominant elastic property of alginate. In these experiments, we could observe that the performance of hydrogel composite is influenced by its the behaviour of its constituents.
SMART GELS
14 | PHYSICAL EXPLORATION
PHYSICAL TESTING OF HYDROGELS Compressive Properties of Pure Hydrogel
2 1.8 Thickness (cm)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1200
1400
SMART GELS
15 | PHYSICAL EXPLORATION
Compressive Properties of Plaster Hydrogel
1.4
Thickness (cm)
1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
1200
1400
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
Deduction Latex hydrogel shows the least deformation when load is applied to it. Thus, it has the highest compressive strength and highest resistance to vertical force. This is due to the high elastic property of latex and its high resistance to permanent deformation.
SMART GELS
16 | PHYSICAL EXPLORATION
Compressive Properties of PVA Hydrogel
2 1.8 Thickness (cm)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
1200
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
Deduction PVA hydrogel shows the most deformation when load is applied to it. Thus, it has the lowest compressive strength and lowest resistance to vertical force. This is due to the dominant weak compressive strength in PVA
1400
SMART GELS
17 | PHYSICAL EXPLORATION
Compressive Properties of Alginate Hydrogel
2 1.8 Thickness (cm)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1200
1400
SMART GELS
18 | PHYSICAL EXPLORATION
Compressive Properties of Latex Hydrogel
1.4
Thickness (cm)
1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
1200
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1400
SMART GELS
19| PHYSICAL EXPLORATION
Compressive Properties of Clay Hydrogel
1.4
Thickness (cm)
1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1200
1400
SMART GELS
20 | PHYSICAL EXPLORATION
Compressive Properties of Agarose Hydrogel
1.4
Thickness (cm)
1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1200
1400
SMART GELS
21 | PHYSICAL EXPLORATION
Compressive Properties of Silicone Hydrogel
2 1.8 Thickness (cm)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1200
1400
SMART GELS
22 | PHYSICAL EXPLORATION
Compressive Properties of Collagen Hydrogel
1.4
Thickness (cm)
1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
1200
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
After loading
1321.93g
1400
SMART GELS
23 | PHYSICAL EXPLORATION
Compressive Properties of Cement Hydrogel
1.4
Thickness (cm)
1.2 1 0.8 0.6 0.4 0.2 0
0
200
400
600
800
1000
Weight of load (g)
205.75g
Before loading
413.25g
687.55g
1321.93g
After loading
1200
1400
SMART GELS
24 | CHEMICAL EXPLORATION
CHEMICAL EXPLORATION OF HYDROGELS Absorption and Desorption Capacities of Hydrogel Hydrogel Composition The volume transition behaviour of the smart hydrogels is influenced by various parameters such as ionic additives and temperature. The ability of hydrogel to absorb water arises from the hydrophilic functional groups attached to the polymer backbone while their resistance to dissolution arises from cross-links between network chains. Water inside the hydrogel allows free diffusion of some solute molecules, while the polymer is a matrix that holds the water together. Incorporating more hydrophilic or hydrophobic monomers in hydrogel composition is a useful approach to regulate the volume transition behaviour of the hydrogel. The increase of charged monomer contents of the hydrogels increases the degree of volume transition. This is because of the simultaneous increase of counterions inside the hydrogels which generates an additional osmotic pressure that expands the hydrogels. Hydrophilic gels called hydrogels are crosslinked materials absorbing large quantities of water without dissolving. Softness, smartness, and the capacity to store water make hydrogels unique materials. (Tanaka 1981; Shibayama and Tanaka1993)
SMART GELS
Testing of parameters that may affect the rate of swelling of hydrogels The following experiments investigate the effects of temperature of solvent, pH of solvent, salt & sugar concentration of solvent and density of solvent on the rate of swelling of hydrogels. The effect of pH of subtrates, salt and sugar subtrates on the rate of deswelling of hydrogels are also tested Through these tests, analysis could be formulated regarding the response of hydrogels to different environmental stimuli and the reasons underlying the behaviours.
25 | CHEMICAL EXPLORATION
SMART GELS
26 | CHEMICAL EXPLORATION
CHEMICAL TESTING OF HYDROGELS
Rate of expansion of hydrogels (g/min)
Effect of Temperature of Solvent (water) on the Rate of Expansion of Hydrogel
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
10
20
30
40
50
60
Temperature of solvent (°C) SET A
Sets/Variables Original weight of hydrogels (g) Weight of water (g) Time taken for absorption (min)
SET B
SET C
A
B
C
2.28
2.28
2.28
39.684
39.684
39.684
15
15
15
Temperature of solvent (°C)
19°C
27°C
49.3°C
Final weight of hydrogels (g)
9.402
11.933
12.504
Rate of expansion (g/min)
0.475
0.644
0.682
SMART GELS
27 | CHEMICAL EXPLORATION
Rate of expansion of hydrogels (g/min)
Effect of Salt and Sugar Concentration of Solvent on the Rate of Expansion of Hydrogel
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 6.4
6.6
6.8
7
7.2
7.4
pH of solvent corresponding to concentration of salt and sugar in solvent SET A
Sets/Variables
SET B
SET C
SET D
A
B
C
D
Original weight of hydrogels (g)
2.275
2.275
2.275
2.275
Temperature of solvent (°C)
20°C
20°C
20°C
20°C
49.736
54.331 50.126
54.201
7.32
7.16
Weight of salt water (g) Weight of sugar water (g) pH of salt water
6.70
6.453
pH of sugar water Time taken for absorption (min)
15
15
15
15
Final weight of hydrogels (g)
5.037
4.980
9.830
10.527
Rate of expansion (g/min)
0.184
0.180
0.504
0.550
SMART GELS
28 | CHEMICAL EXPLORATION
Rate of expansion of hydrogels (g/min)
Effect of Acidity and Alkalinity of Solvent on the Rate of Expansion of Hydrogel
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
pH of solvent SET A
Sets/Variables
SET B
SET C
SET D
A
B
C
D
Original weight of hydrogels (g)
2.665
2.665
2.245
2.245
pH of solvent
5.70
3.05
8.75
9.56
15
15
15
15
Final weight of hydrogels (g)
12.665
11.110
9.025
8.855
Rate of expansion (g/min)
0.667
0.563
0.452
0.441
Time taken for absorption (min)
SMART GELS
29 | CHEMICAL EXPLORATION
Rate of expansion of hydrogels (g/min)
Effect of Density of Solvent on the Rate of Expansion of Hydrogel
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Density of solvent (g/ml) SET A
Sets/Variables
SET B
A
SET C
B
C
Original weight of hydrogels (g)
2.255
2.280
2.255
Density of solvent (g/ml)
0.789
1.000
1.260
Time taken for absorption (min)
15
15
15
Final weight of hydrogels (g)
6.160
11.933
5.862
Rate of expansion (g/min)
0.260
0.644
0.204
SMART GELS
30 | CHEMICAL EXPLORATION
Effect of Acidity and Alkalinity of Additives on the Degradation of Hydrogel BEFORE
AFTER
OBSERVATION & ANALYSIS Additive to hydrogel : Acid pH 2 Observation No apparent changes to the hydrogel after 15 minutes
SET A - pH of additive : 2 (extremely strong acid)
Additive to hydrogel : Alkali pH 11 Observation No apparent changes to the hydrogel after 15 minutes
SET B - pH of additive : 11 (extremely strong alkali)
Effect of Salt and Sugar Additives on the Rate of Degradation of Hydrogel Additive to hydrogel : 1 teaspoon of table salt Observation : Hydrogel melts instantaneously Hydrogel dissolves completely after 5 minutes SET C - 1 teaspoon of table salt
Additive to hydrogel : 1 teaspoon of brown sugar Observation : Sugar melts into the hydrogel Sugar completely dissolves into the hydrogel after 40 mins SET D - 1 teaspoon of brown sugar
SMART GELS
31 | CHEMICAL EXPLORATION
Why do hydrogel dissolve when salt is added to it When salt is added to hydrogel, the chains start to change their shape and water is lost from the gel. The cross-links in the hydrogels uncoil and the protein chains attract water molecules by hydrogen bonding. As more salt (sodium chloride) is added to the hydrogel, the positive sodium ions are attracted to the negative oxide ions, making less space for the water molecules. This makes the hydrogel lose some of its water content. The negative charges along the chain repel each other less in the presence of the sodium ions and so the chains become more coiled up. This also forces out water from the hydrogel. The result is that a small change in salt concentration can have a significant effect on the amount of water leaving the hydrogel.
SMART GELS
32 | FUTURE OF HYDROGELS IN BUILT ENVIRONMENT
SPRAYED CONCRETE TECHNOLOGY History of spray concrete In 1895, the curator of Field Museum of Natural Science in Chicago USA, Dr Carlton Akeley developed a single chamber pressure vessel which sprayed mortar mixture onto skeletal frames to create the body of prehistoric animals. This could not be achieved by conventional trowelled mortar. Thus, he designed a ‘Cement Gun’ which used compressed air to force the mortar mixture along a delivery hose. While in the hose, the mixture was hydrated by a water spray fitted at the nozzle of the hose. The sprayed material was known as “Gunite”. This method was patented in 1911 and acquired by the Cement Gun Company, a British owned company. Properties & Advantages of Sprayed Concrete Low Water/Cement Ratio compared to poured concrete gives the sprayed concrete excellent selfsupporting quality and compressive strengths
The Selfridges Building in Birmingham & the Darwin Cocoon at the Natural History Museum are structural buildings using reinforced C40 spray concrete.
Additives to enhance wet and dry process shotcrete : Silica Fume Air-Entraining Admixtures Fibers Accelerators Methods of sprayed concrete application : Wet Mix - All ingredients, thoroughly mixed with water, are pumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material on the form. Dry Mix - Pre-blended dry or damp materials are channeled through a hose by compressed air at high velocity to the nozzle, where water is added. The high-impact velocity consolidates the material on the form.
High velocity of application results in dense compaction and high output. Compressive strengths are 30% higher than conventionally placed concrete. High Density, Low Permeability & Low water absorption results in durable homogenous material with excellent freeze/thaw resistance and abrasion resistance and low surface cracking. A good surface preparation of sprayed concrete results in excellent bond strength, eliminating the need for bonding agents and coatings. In comparison with conventionally poured concrete, sprayed concrete requires far less formwork. Application Structural repair and strengthening for concrete damaged by corrosion, fire damaged structures, bridges and towers. Protective coatings to steel framed structures, tunnel and refractory linings and for other structures such as swimming pools, river walls, domes and shell structures. Free-formed structures such as zoological structures, swimming pools, sculptures and landscaping. Surface finishes to a structure to eliminate finishing costs, such as white sprayed concrete.
SMART GELS
33 | FUTURE OF HYDROGELS IN BUILT ENVIRONMENT
SPRAYED HYDROGEL TECHNOLOGY An overview of sprayed hydrogel technology Like the sprayed concrete technology, hydrogel has a great potential in application in the built environment, as a building coating which is responsive to the environment as well as self-healing. Hydrogel coating is a composite smart polymer gel formulated with composites for enhanced properties and performance. It incorporates several additives such as cross-linking polymers, pigments, thixotropic agents, promoters and inhibiters. Components of hydrogel coat system Because pure hydrogels have been tested and proved to have structural limitations. Thus, the properties will be enhanced with the addition of the following compounds : Polymer A saturated acid is important in the making of hydrogel spray - isophthalic. Isophthalic provides greater levels of water (acid) and chemical resistance, weather and corrosion resistance with a higher degree of flexibility Monomer Monomers such as styrene and methacrylate react and cross-link with sites in the polymer to create a cross-linked thermoset product. They also lower the hydrogel’s viscosity to help with the application. Most gel coats are at a lower viscosity to allow for better and smaller particles being sprayed, and ultimately, a better end result. Fillers Fillers are needed to alter physical properties of a hydrogel coat to increase its resilience to water and other environmental conditions. Thixotropic Agents Thixotropic agents such as fumed silica and organoclays have a viscosity that is dependent on shear rate, crucial in hydrogel coating. The viscosity of thixotropic agents decreases at higher shearing rates. The purpose is to hold the hydrogel coat on a vertical surface, yet allow easy breakup for good spraying properties.
Pure hydrogels do not have the optimum qualities to be a building coating and has very little mechanical strength on its own. Therefore, additives are compounded in hydrogels to reinforce the hydrogel for structural integrity and better control of reactions. Hydrogel coat is best applied by an airless spray application with traditional methods using a pressure pot or catalyst injection system.
During spraying, a high shear operation, the thixotropic agents lowers the viscosity of gel coats for easy application onto a surface. To decrease a risk of sag under low shear, the viscosity of hydrogel coat is increased once deposited onto the mold. This allows the coating to flow and level out before thickening. Promoters and Inhibitors Promoters and inhibitors influence the cure behavior, including solidification time and lay-up time, of hydrogel coats. Solidification time must be sufficient for spraying, air release and leveling. Lay-up time must be short enough to ensure production rates Peroxide initiators/catalysts initiate most gel coats curing, and may be applied to the hydrogel coating. Promoters such as cobalt are needed to split the peroxide catalyst into free radicals, and accelerating the cure rate. Inhibitors are used to provide self life stability and help control gel time during storage of hydrogel coats.
SMART GELS
34 | FUTURE OF HYDROGELS IN BUILT ENVIRONMENT
Factors affecting quality of hydrogel coating
Addition of catalyst to the hydrogel mixture
• Gel time • Viscosity • Weathering properties • Reactivity with external environmental stimuli and physical context • Evaporation rate • Flash point • Thixotropic Index (the dependence of the gel coat’s viscosity on the shear rate) • Density of the gel coat/Weight per gallon • Color, Sag, Porosity, Pigment Separation. • Additives for hardening/curing • External environmental conditions – light, temperature, humidity, salt content in moisture • Catalyst delivery rate and ratio
“Hot Pot” System A measured amount of catalyst is stirred by hand directly into a pressure pot which is then sprayed from a pressure feed tank within the permitted gel time period.
System of sprayed hydrogel application Selection of spray methods depends on : • • •
How the material is delivered to the gun How the catalyst is added & the solvent How the gel coat is atomized.
The most accurate system Uses a large quantity of clean-up solvent Catalyst Injection System Used especially for high production fabrication under uninterrupted spraying. The liquid catalyst is injected into the atomizing air supply by an adjustable venturi device with a flow meter. The ball settings of the catalyst flow meter are translatable into cubic centimeters or grams per minute so the catalyst feed can be matched to the measured weight output of Gel coat per minute and ratio established
SMART GELS
35 | FUTURE OF HYDROGELS IN BUILT ENVIRONMENT
SPRAYED HYDROGEL TECHNOLOGY
Hydrogels could be sprayed onto the damaged buildings due to environmental errosion in Bulgaria and earthquake in Nepal
Airless spray system for hydrogel coat application The airless spray system provides an excellent film leveling at high delivery rates and elimination of blow-back when spraying in closed areas. In a typical airless system, the gel coat does not come into direct contact with compressed air. The hydrogel coating is forced into a small orifice under high pressure by an air-activated high ratio pump. The high velocity fluid impacts with ambient air at the orifice and breaks up due to the sudden and extreme pressure differential. An air-activated pump feeds the nozzle which delivers the gel coat spray. A catalyst nozzle is located next to the fluid nozzle and delivers a catalyst spray which mixes by implosion into the gel coat fan. Airless systems are commonly used for coating large, typically open or flat parts.
Objectives & Contexts of sprayed hydrogel application in the built environment •
Repair existing damaged infrastructure with self healing properties
Applicable in Earthquake-affected countries such as Haiti and New Zealand Places with old historical buildings such as Greece and Italy Highly polluted industrial cities such as China Vandalised infrastructure such as scratched cars •
Aesthetic finishes and textures
Applicable in Existing old buildings as a new skin New buildings •
Free-form structures
Applicable in Large scale installations Temporary structures Make-shift shelters
SMART GELS
36 | REFERENCES
IMAGE REFERENCES
Cover page http://www.nisenet.org/image-collection/other-materials Contents page http://www.chemistryviews.org/details/news/3992181/A_DNA_Hydrogel_Capable_of_Metamorphosis.html http://www.pharmainfo.net/reviews/preparation-and-evaluation-chitosan-microspheres http://scotia-supplies.com/products-page/tarmac-building-products/ introduction http://cdn.arstechnica.net/wp-content/uploads/2012/06/hydrogels.jpg Biomimetics https://en.m.wikipedia.org/wiki/File:Prawn_(PSF).png http://www.fmap.ca/ramweb/media/rebuilding_fisheries/home.php Application of Hydrogels http://www.vwoptics.in/yahoo_site_admin/assets/images/589279_f520.233200154_std.jpg http://photos.tradeholding.com/attach/hash107/44395/opatrunek.jpg http://jlinlab.ecust.edu.cn/images/recent/drag%20del.jpg http://www.robaid.com/bionics/biocompatible-hydrogel-could-be-used-as-cartilage-replacement.htm Physical Exploration of Hydrogels http://www.gcsescience.com/hydrogel-hydrogen-bonding.gif dna gels http://www.nature.com/nmat/journal/v11/n8/images/nmat3392-f3.jpg Chemical Exploration of Hydrogels http://www.rpi.edu/dept/chem-eng/Biotech-Environ/PRECIP/precp.dena.gif Sprayed Concrete Technology http://upload.wikimedia.org/wikipedia/commons/2/2d/Selfridges_Exterior7.jpg http://www.shotcrete.co.uk/shotcrete_structual_buildings.htm http://www.sjrocksinc.com/services Sprayed gel technology http://www.exova.com/media/289803/spray-polyurethane-original.jpg http://www.graco.com/content/dam/graco/aftd/images/application/Application%20of%20Chop-117.Med.tif.imagep.59.0.1114.800.png http://2.bp.blogspot.com/-hXCAFf_Yzbg/Tkt982IokEI/AAAAAAAAAqQ/-pX76PgHkqw/s1600/concrete-crack.jpg http://blog.thecheaproute.com/everest-base-camp-trek-journal-days-2-3/crack-building-namche-bazaar-earthquake/ http://ursulagrobler.blogspot.co.uk/2012/08/cyrillic-and-cats-general-landscape.html