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Adhesives Ingredients
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Adhesives Ingredients

Selecting Plasticizers for Adhesives and Sealants

Plasticizers are one of the commonly used additives in adhesives and sealants formulations. They improve the performance of adhesives by creating much-needed flexibility, optimizing viscosity, toughness, set time functionality, impact resistance, and even adhesion.

However, with increasing complexity of adhesives and sealant formulations every year, you need to select a particular plasticizer to change properties of your formulations in a certain direction to meet the requirements. Learn how to achieve desired flexibility by understanding the fundamentals of plasticizers used in adhesives and sealant products.

Overview

Need for Plasticizers

Need for Plasticizers

Plasticizers are used in all types of adhesive and sealant formulations. They are used primarily to the final bond and to provide improved low-temperature properties. However, plasticizers can also supply additional benefits that are important to both the formulator and end-user.

A large number of different resins and plasticizers are used with adhesives and sealants. There are over 38 general chemical families of plasticizers and multiple types within each family. This makes the process of selecting a plasticizer for a specific adhesive or sealant formulation a very difficult task.

Plasticizers must be considered with regard to:

  • The compatibility with the base polymer in the formulation
  • The end-properties that the formulator is trying to achieve, and
  • The type of processing that the adhesive will be subject to during compounding and application

Let’s begin by understanding how various properties of a formulation get affected by incorporating different plasticizers or its blends. Also, learn the basics of selecting the right plasticizer for your adhesives and sealant products.


Properties Affected by Plasticizers

Properties Affected by Plasticizers

By far, the largest application of plasticizers is as a flexibilizing additive in plastic products such as food packaging, wire and cable insulation and medical devices. In these markets plasticizers are mainly used with polyvinyl chloride (PVC), but many other rigid polymers are plasticized for practical use.

The table below identifies and appraises the various properties that can be affected by the addition of plasticizers. This gives the individual formulator, the ability to engineer properties for specific applications.

Property Effect
Processing
Plasticizers often increase melt flow during processing. They can also be used to improve the efficiency of compounding with other ingredients
Mechanical
Addition of plasticizer lowers hardness and strength and increases elongation and creep. It increases adhesion and coefficient of friction.
Damping
Plasticizers improve damping of vibrations and reduce the resulting noise.
Thermal
Addition of plasticizer slides the modulus vs. temperature curve toward lower temperatures. This lowers the stiffening temperature of flexible polymers. It also lowers the softening temperature of rigid polymers. It limits the maximum use temperature.
Flame resistance
The addition of plasticizers can reduce or increase the flame resistance of a polymer depending on the degree of flame resistance inherent in the plasticizers (e.g., halogen concentration).
Electrical
Plasticizer addition increases mobility of ionic impurities, which lowers electrical resistance. Plasticizer increases the mobility of polar groups in the polymer, which increases the dielectric constant and loss factor. The addition of plasticizer generally results in more efficient microwave heating.
Chemical
The addition of plasticizer increases the free volume and molecular mobility, which increase permeability of small molecules. Being low molecular weight species plasticizers have a tendency to migrate out of the polymer. This could reduce permanence (age resistance) and produce toxicity problems. Toxicity of some plasticizers has led to considerable environmental and political concern.

Use of Plasticizers in Adhesives & Sealants


Similar to diluents, plasticizers are non-volatile solvents for the base resin and by being incorporated into the formulation, they separate the polymer chains and enable their deformation to be more easily accomplished. In this way, plasticizers can improve the processing characteristics of the adhesive as well as the end-properties.

Plasticizers may also be one of the least expensive additives used in adhesives and sealants, and they can be used in relatively large concentrations depending on the application and formulation (up to 100 parts by weight per 100 parts of the base polymer). For these reasons, they are sometimes used as extenders for cost-reducing purposes.

Plasticizers generally affect the viscoelastic properties of the base resin; whereas, diluents would simply reduce the viscosity of the system. Whereas diluents ultimately result in brittle, hard adhesive systems, plasticizers result in increased flexibility and lower modulus. Plasticizers provide flexibility, but they also generally reduce the cohesive strength of the adhesive film. As a result, plasticizers are used to improve peel and impact properties (often at the detriment of a decrease in tensile strength).

Plasticizers have been used to give polymers a permanent adhesive character (pressure sensitivity), or to regulate their adhesion-cohesion balance to achieve removability. For example, plasticizers can be incorporated into peelable pressure-sensitive adhesive to soften the adhesive, and thus improve peelability and clean removal.

Plasticizers, arguably, are asked to provide more functions in adhesives and sealants than they do in plastic articles. In adhesives and sealants, there are also a large number of secondary functions for plasticizers that are driven by both:

  1. Expectations of the improvements of the original physical properties of the polymer
  2. Expectations for improvements in the processing properties of the formulation

For example, plasticizers can be used to provide softness and flexibility, but they are also used to improve melt processability, compounding efficiency, or to reduce the drying time of waterborne systems. In certain cases, they perform even additional functions such as flame retardance. Plasticizers are important to tailor the application and property requirements of these products.

Expectations from Plasticizers


Often the selection of a plasticizer is a compromise where some properties are improved at the sacrifice of other properties. This can be seen in the image below which describes the relative effects that plasticizers have on final physical properties.

Exceptions from Plasticizers
Expectations from Plasticizers



How do Plasticizers Work

How do Plasticizers Work

Plasticizers are organic liquids or solids that are incorporated by melt processing or diffusion into a compatible polymer to reduce interaction between molecules and improve molecular mobility.

  • There are several theories to explain plasticization; however, the main point in all of these is that plasticizers reduce polymer-to-polymer chain attractions and provide greater mobility to the polymeric chains.
  • The degree of plasticization is largely dependent on the plasticizer's chemical structure, including chemical composition, molecular weight and functional groups.

As a result, plasticizers lower the glass transition temperature of the base polymer.

Plasticizers that have low molecular weight and a small number of polar groups generally provide a higher degree of flexibility and plasticization. Unfortunately, low molecular weight also corresponds to a high degree of migration and volatilization problems that could be associated with health or safety risks.

Almost all types of polymers can be plasticized. However, the plasticizer must be compatible with the base polymer in a formulation to function efficiently.

Compatibility is the first and most important criterion for selecting a plasticizer.


Criteria to Select Plasticizers

Criteria to Select Plasticizers

Plasticizers that are used in adhesive and sealant formulations are generally chosen on the basis of the following criteria:

  • Compatibility with a given polymer or set of component ingredients
  • Compounding characteristics
  • Effect of plasticizer on the rheological properties of the polymer
  • Desired mechanical and thermal properties of the end formulation
  • Resistance to water, chemicals, UV, weathering, dirt, microorganisms & general aging
  • Toxicity
  • Cost analysis (volume required or plasticizing efficiency, price/pound, etc.)

Learn How to Select the Plasticizer According to the Following Parameters:


Base Polymer


Almost all types of polymers can be plasticized. However, the plasticizer must be compatible with the base polymer in a formulation to function efficiently. The plasticizer and polymer should have similar polarity to produce compatibility. Often the solubility parameter of the polymer and that of the plasticizer is matched to provide compatibility. In practice, however, the choice of plasticizer(s) is usually accomplished empirically – through trial and error.

Adipates Benzoates Citrates Phosphates Phthalates Trimellitates Others Use levels*
Acrylic 5-10
Aminoplasts & phenoplasts √ (b)
Chloroprene
Epoxy
Ethylene copolymers (emulsions) √ (a)
Ethylene copolymers (solids)
Fluoropolymers
Natural rubber
Natural polymers / cellulosics 10-40
Polyamide (b) <5
Polyvinyl chloride <100
Polyvinyl butyral (d)
Polyvinyl ether (c)
Polyester <3
Polyolefin <15
Polyurethane <20
Polyvinyl acetate and derivatives (emulsions)
Silicones (e)
Styrene copolymers
Synthetic elastomers
(a) Blend of esters
(b) Sulfonamides
(c) Rosin ester
(d) Glycerols
(e) Silicone oils
*Parts per hundred of the base polymer
Combinations of Base Polymer and Plasticizers Commonly Used in A&S Formulations


However, one must realize that there are many types of plasticizers within a given family and that will provide varying properties. The degree of plasticization and the concentration of plasticizer required (also shown in the table) are largely dependent on the plasticizer's chemical structure.

Plasticizer compatibility is the key to plasticizer performance in adhesives and sealants. In latex systems, compatibility can be assessed by observing the viscosity increase after the addition of the plasticizer and by observing the clarity of a cast film of the plasticized adhesives. A compatible plasticizer will give a greater viscosity response and will provide a clear dry film.

Viscosity Response


The key evaluation parameters in choosing a plasticizer are:

  • Viscosity and
  • Viscosity Response

Plasticizers will reduce the melt viscosity of hot-melt adhesives and 100% solids adhesives & sealants. However, the addition of certain plasticizers to a polymeric emulsion will usually result in increased viscosity. This effect can be used to formulate a waterborne adhesive having greater coating thickness build-up.

When higher viscosity is not required, the thickened emulsions are cut back to a specified constant viscosity, resulting in cost savings.

The degree of viscosity response can also be a predictor of compatibility (i.e., The greater the viscosity response, the greater the compatibility).

Rheology & Viscosity Made Easy


3. Glass Transition Temperature


Suppression of the glass transition temperature (Tg) is an important plasticizer function as the formulator can use this to control many of the final properties of the adhesive or sealant like:

  • Flexibility
  • Low-temperature resistance
  • Toughness
  • Adhesion and more

Plasticizers that have a greater effect on the glass transition temperature are considered to be more efficient and more compatible with the base polymer.

The glass transition temperature of the final adhesive can be estimated by using the Flory-Fox equation of simple mixtures.1

The glass transition temperature of plasticizers is sometimes provided in the suppliers’ literature, but the melting or freezing point can be a good approximation.

4. Adhesion


Peel strength is often used as a measure of adhesion to a substrate. Flexible adhesives generally have higher peel strength due to better distribution of stress and are preferred for flexible substrates such as packaging films & foils.

5. Set and Open Times


Plasticizers can significantly shorten the set time and extend the open time for waterborne or hot-melt adhesives. Both effects are looked upon as improvements by the formulator as they are valued in many end-use applications including packaging.

Set time is the time required for the adhesive to provide a measurable degree of bond strength after substrates are mated. Open time is the time that the adhesive can stand after being applied and before the substrates are mated. Longer open times may be required for substrate adjustment or production timing. Long open times are also indicative of a good wet tack.

6. Volatility and Migration


Another important criterion is the ability of the formulation to retain the plasticizer during the life cycle of the product. This is affected by some of the factors noted above (compatibility, aging, etc.) but can also be measured in terms of volatility and migration.

Plasticizer migration can lead to degradation of properties during the life of the adhesive or sealant.

  • It causes transfer of the plasticizer to unwanted surfaces (e.g., fogging of windshields in autos or contamination of electronic substrates) and
  • It can also adversely affect the environment (see Regulatory Compliance below).

ASTM and other professional organizations provide standards for measuring these criteria. Some migration of the plasticizing agent from or throughout the plasticized adhesive can be tolerated, such as minor separation due to composition equilibrium or temperature influence, but the plasticizing agent should not migrate to the extent of phase separation.

Volatility is typically a limiting factor for the use of many plasticizers. The fugitive nature of plasticizers is important because of:

  1. Volatile loss during processing and the resulting necessity to increase plasticizer concentration
  2. Degradation of properties with the time that the adhesive bond is in service, and
  3. Volatility is an indication of VOCs

Thermal gravimetric analysis (TGA) measures the volatiles lost in a plasticizer or adhesive formulation as a function of time at temperature. This is important in estimating migration potential as well as safety / toxicity issues. Many adhesives and coatings are processed at high temperatures (either during the compounding process or during application and cure).

A high degree of volatility would signal significant loss of plasticizer during these processes, resulting in the degraded final product. Volatility can be measured in a standard thermal oven by ASTM D-2369 and this has become part of EPA-24 in the U.S.


7. Regulatory Compliance


The European Chemicals Agency (ECHA) manages the pan-European chemical oversight program REACH. The following phthalate plasticizers have been cited as substances of very high concern (SVHC):


These chemistries have come under increased scrutiny under the REACH regulations because of their toxicity data in some animal species coupled with wide their use and high volumes. Of particular concern are BBP, DBP, and DEHP, which have been targeted by ECHA for the authorization process in 2013.

 »  Check Out the Current Regulations on Phthalates!

8. Chemical & Physical Properties


The main plasticizer families that are used in adhesive and sealant formulations are:


The average chemical and physical properties represented by plasticizers within these families are shown in the Table below.

Properties
Adipates
Benzoates
Citrates
Phosphates
Phthalates
Trimellitates
Average property values for all types within plasticizer family
Molecular weight,daltons
343
201
360
320
391
547
Specific gravity @ 25°C
0.927
1.1
1.045
1.16
0.977
0.984
Melting point, °C
-50
-25
-59
+8 to -90
-50
-38
Boiling point, °C
417
321
294
300-420
384
420
Solubility in water @ 25°C, wt%
<0.01
6.8x10-4
0.65
<0.001
<1x10-3
<0.001
Hildebrand solubility parameter, cal/cc0.5
7.61
10
9.02
9.3
16.83
9.00
Viscosity @ 20°C, mPas
13.7
80
40
15-100
78
194
Surface tension @ 20°C, mN/m
29
40.4
38
33
34
Plasticizer loss (24 hr @ 87°C), wt%
8.7
8.9
4.5
3.6
3.6
0.5
Outstanding property
1
2
3
4
5
6

The chemical and physical properties of common plasticizer families used in formulating adhesives and sealants are:

  1. Low-temperature resistance
  2. Stain resistance, low migration
  3. Considered safe in food and medical applications, some do not support fungal growth
  4. Flame retardant, low smoke
  5. Compatibility, cost effectiveness, good balance of properties
  6. Low volatility and migration, UV and heat resistance

Desired Performance


Another method of classifying plasticizers is based on their performance such as:

  • General-purpose plasticizers
  • High-performance plasticizers
  • Specialty plasticizers

Today, phthalates remain the most popular choice because they usually provide the best all-round performance and they are low in cost. However, phthalates are suspected carcinogens. As a result, replacements are generally being sought for phthalate plasticizers in most applications and not only in applications that have a high risk (i.e., children's toys, food packaging, etc.)

The table below shows the primary and secondary performance functions of the main plasticizers that are used in adhesive and sealant formulations.

  General purpose Performance plasticizers Specialty plasticizers
    Strong solvating properties Low temperature properties Low volatility Low diffusion Stability Flame resistance Safety (food contact approvals) 
Adipates β   α
        β
Benzoates β
β
β β
α
    β
Citrates     β
β       α
Phosphates   β β β
    α
 
Phthalates α
β β β
β
  β  
Trimellitates     β
β α
β    
α - Primary performance function
β - Secondary performance function


Types of Plasticizers

Types of Plasticizers

Plasticizers are commonly classified based on their chemical composition because it is easier to understand the influence of structural elements (e.g., different alcohols in a homologous series of phthalates, adipates, etc.) on the properties of plasticizers and their effect on base polymers.

One of the features that make the adhesives and sealants industry particularly complex is the large array of raw materials available for formulation. This complexity is increasing each year since companies are constantly introducing new materials.

Different plasticizers affect different physical and chemical properties of materials. Therefore, a formulator selects a particular plasticizer to change properties in a certain direction to meet requirements.

There are over 38 general chemical families of plasticizers that are used for polymer modification. The chemical classes listed below are those most commonly used in adhesive/sealant formulations. Within each chemical class there are many variations. For example, there are over 30 different types of phthalates on the market. Many products will also use blends of plasticizers to achieve the desired end properties.
Classes of Plasticizers for Adhesives and Sealants

Adipate Plasticizers


Adipate plasticizers present very good plasticizing efficiency. They provide superior extraction resistance to oil & fat, and also give good outdoor performance thanks to their UV resistance.

Di-2-ethylhexyl adipate (DOA) is less compatible than DOP and is considerably more volatile. DOA is approved by FDA for use in produce wrap and meat wrap films. The adipates do not lower the melting point of PVC as much as the corresponding phthalates do, but they flexibilize the amorphous regions of the PVC more efficiently, and they are lower in molecular weight and specific gravity. Hence, they impart higher flexibility weight for weight and better low-temperature properties.

Benzoate Plasticizers


Benzoate plasticizers are noted for fast fusion and stain resistance. In the adhesives and sealants industry these plasticizers are valued for:

  • High solvating power
  • Low moisture sensitivity
  • Excellent resistance to organic extraction and UV resistance

However, their high viscosity limits their application in some areas.

The primary applications of benzoates are phthalate replacements in adhesives and coatings, PVC plastics, and certain elastomers. In the adhesives and coatings sector, the primary usage is in emulsion adhesives, latex sealants, and hot melt adhesives. Most significant is that in the U.S. benzoates (principally dipropylene glycol dibenzoate) have replaced much of the C4 phthalate previously used in emulsion coatings and adhesives (e.g., PVAc and acrylic).

This has not occurred as quickly in Europe, primarily because benzoates have been relatively expensive and partly because of the availability of other "green" plasticizers.

Emerald's Plasticizers

Citrate Plasticizers


Citrate plasticizers are esters of citric acid. They have been used mainly as plasticizers with cellulosics, PVC, polyvinyl acetate, and other polymers used in medical plastics and food contact packaging. Some citrate esters find specialty use in blood bags and food wraps. Citrates are also used in toys produced by the plastisol process.

Citric acid based-plasticizers are one of the major contenders to replace phthalates (especially targeted to replace DOP). Some properties of materials plasticized with citrates can match those plasticized with DOP but the cost of their production is substantially higher.

Phosphate Plasticizers


Phosphate plasticizers are used in the adhesive and sealant industry when a secondary function of flame resistance is of value. Their price is prohibitive for many general-purpose applications.

They are primarily used as plasticizers for PVC plastisols, polyacrylates, cellulose derivatives, and synthetic elastomers. Phosphates have a long history of use as plasticizers. The first application was the substitution for camphor in nitrocellulose. They are generally considered to be equivalent in cost to general-purpose phthalates, and this has promoted them to their current status as a specialty plasticizer for flame resistance.

Phthalate Plasticizers


In general, the phthalic anhydride esters, or phthalates, have been used widely throughout the adhesive industry as general-purpose plasticizers. The significant quantities of phthalate plasticizers used are in large part, due to their cost-effectiveness. The most commonly known are the dibutyl phthalates and dioctyl phthalates.

The general-purpose phthalates are produced from phthalic anhydride and various alcohols. The length of the alcohol constituent ranges from normal butyl (C4) to undecryl (C11). Selecting a phthalate requires consideration of two variables in their side-chain structure: the length and the degree of branching. The effect of these variables on properties is described in the image below.

The phthalates used industrially range from C1 through C10 to C18. However, at least 90% of production and use of phthalates is in the C8 through C10 band, and most adhesives are used is in the C2 through C8. The main usage of these low carbon number plasticizers is C2 for cellulose acetate and C4 for polyvinyl acetate. The selection of the best phthalate plasticizer to use for a particular application is guided by economics, toxicological regulation (if required), ease of processing, and performance in end-use.

Eastmans Range of Phthalate Plasticizers

Trimellitate Plasticizers


Trimellitate ester plasticizers represent the state-of-the-art in low volatility monomeric plasticizers. As a result, they are primarily indicated for high-temperature applications and in automotive interiors where fogging due to outgassing can be a problem.

Their principal uses are at 90°C and 105°C rated electrical wire insulation & jackets, and other applications requiring plasticizers with volatility lower than is attainable with higher molecular weight phthalates.

Trimellitates present excellent water resistance and extreme low-temperature flexibility.


Other Commonly Used Plasticizers

Other Commonly Used Plasticizers

Sulfonamides


Sulfonamides include phenyl esters of sulfonated n-paraffins. Were it not for their higher prices, these materials could be used as alternatives for general-purpose phthalate plasticizers in a wide range of applications, including adhesives and sealants.

Sulfonamides have an advantage over phthalates for certain processes and aggressive service environments due to their chemical structure.

The plasticizer has low susceptibility to hydrolysis and this gives the product high resistance to degradation during exposure to weather, microorganism or alkaline media. Because of their good weather and chemical resistance properties, sulfonamide plasticizers are used predominantly in outdoor sealants.

One large application is in polyurethane direct glazing sealants for the automotive industry.

Oils


Oils (like paraffinic, naphthenic, and aromatic) are used mainly as plasticizers in non-polar adhesives. These are generally used in the hot-melt adhesives and sealants. Base polymers include the mid-blocks of styrene-butadiene copolymers and polyolefin-based adhesives.

Polymeric Plasticizers


Polymeric plasticizers are used frequently in adhesives and sealants. These are similar to the base polymers and do not have the volatility problems associated with traditional plasticizers. Polymeric plasticizers include:

  • Polybutylene
  • Butyl rubber
  • Polyisoprene and
  • Other synthetic elastomeric polymers

There are many more commercial and potential polymeric plasticizers available for use in adhesives and sealants.

The chief advantages of polymeric plasticizers over general purpose monomeric plasticizers are greater permanence and migration resistance. The chief disadvantages are higher cost, lower plasticizing efficiency, and very poor low-temperature properties. Practical plasticizers often contain mixtures of polymeric and monomeric plasticizers.


Challenges for Plasticizers

Challenges for Plasticizers

The table below summarizes some of the current technical challenges for the plasticizer industry. These challenges are fundamental to both plasticized plastic articles as well as adhesive and coating formulations.

Challenges
Description
Migration out of polymer
  • Can be due to leaching (removal via liquid extraction) or migration (any method of transfer to a gas, liquid, or solid) of plasticizer molecules.
  • Polymers fail to retain their flexibility and the plasticizers coming out of the polymers often pose health and environmental risks.
Retention of high temperature flexibility
  • Plasticizers generally have measurable vapor pressure at elevated temperatures. This encourages migration and results in discoloration, tackiness, and embrittlement.
  • Certain plasticizers will also degrade at elevated temperatures.
Retention of low temperature flexibility
  • The degree to which a plasticizer lowers the glass transition temperature, Tg, depends on the compatibility.
  • Many liquid plasticizers may solidify before the polymer passes through the Tg.
Health and environmental effects
  • Phthalate plasticizers have been a target of worldwide scrutiny especially in the case of medical devices, food packaging, household items, and toys.
Government regulation
  • Many government agencies are seeking to limit the amount of certain plasticizers that enter into the environment.
Flammability
  • Traditional plasticizers are hydrocarbons and make the products that they are incorporated into more susceptible to fire hazards.
  • Phthalates have also been noticed to increase smoke production, ease of ignition, and burning rate.
Compatibility with new polymers
  • Many new polymers are continually being developed. Plasticizers must evolve to be compatible with these new materials.
UV stability
  • Degradation by UV irradiation may also cause reduction of plasticizer efficiency.
  • It has been found that plasticized PVC degrades more rapidly than unplasticized PVC in the near-UV region of the spectrum.
Biodegradability
  • Plasticizers are used for biodegradable plastics such as polylactic acid, polycaprolactone, etc.
  • There may be more stringent regulation of these plasticizers since they will be released to the environment more easily.
  • Focus is on developing compatible plasticizers that are also biodegradable.
Improved permanence
  • By preventing plasticizer loss and degradation and by improving compatibility the effective lifetime of plasticizers in flexible products can be increased.

Although plasticizers have been used for many decades and continue to be a major part of the market, they also have some disadvantages. These generally arise due to toxicity and environmental regulation standards that apply to broader plastic parts. However, there are also many concerns, problems, and constraints on the use of plasticizers from a purely technical or performance standpoint.

Often these problems can be looked upon as opportunities for competitive products. Certain new products are likely to provide relief from these issues, especially those associated with environmental regulations. With research on developing products, we may see relief in other areas such as:

  • Migration reduction
  • Compatibility
  • Viscosity reduction

Several factors make plasticizers less than ideal as additives to a polymer system. Perhaps, the factor of most importance is the tendency to migrate out of the system.

Plasticizers migrate to the surface of plastics, and can then evaporate or leach into the surrounding environment.

This limits the usefulness of many plasticizers, as they eventually migrate out of the plastic entirely, and as a result, the plastic becomes brittle with time.

The release of plasticizer into the environment represents an environmental hazard as well. Because of the widespread use of phthalates, they have become one of the most abundant industrial pollutants in the environment. There is public concern about phthalates because of their widespread use and occurrence in the environment.

As the plasticizer industry has matured, a number of other technical challenges have been addressed to improve formulations, solve technical end-use problems, or meet new requirements.

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1 Comments on "Selecting Plasticizers for Adhesives and Sealants"
Firas K Jul 7, 2020
Thank you very much MR Edward for this valuable and nicely arranged information.

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