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Antibacterial Powder Coatings

R. Siva
Marpol Pvt Ltd
Margao, Goa
siva@marpolind.com
 

Abstract

This article provides basic information about antibacterial powder coatings and its importance. Potential applications for antibacterial powder and selection criteria for suitable antibacterial agent are described. Mechanism of silver technology and the advantageous of silver additive are briefly described. Guidelines for designing antibacterial powder and factors which need to be considered while formulating and manufacturing antibacterial powder coatings are discussed.

Introduction

Powder coatings has gained widespread acceptance in the coatings industry for its performance characteristics which is equal to liquid coatings and the environmental benefits (free of VOCs) of powder make it far superior and user friendly. However there are increasing demands for new product developments and the major driving force for new product developments come from the market requirements combined with environmental compliance. End users are more concern to know about the technological improvements and about the value added products. Marpol is continuously striving to offer value added products to the customers. One of such requirement is Antibacterial powder coatings. End users are more concern about the coatings surface which inhibits the bacterial growth so coatings that repel microorganism are always in demand. Antibacterial powder coatings fulfill these needs.

Why Antibacterial powder coatings?

Antibacterial and anti mould properties are required when powder coatings are used to such applications that coated objects are contacted with number of people and in the fields of foods, medical care and sanitation etc. Antibacterial powder helps to fight against disease and infection; the system also stops the growth of bacteria and fungi. The advantages of this technology are

a. It doesn’t require any additional capital investment and the conventional extrusion and dry blending operation can be followed.

b. The cured film exhibits enhanced resistance to bacteria and fungi growth.

c. The cured film provides excellent mechanical properties, corrosion resistance with good flow and leveling properties.

There are bright scopes for antibacterial powder coatings in the following segments

1. Domestic appliances: Interior of refrigerators and air blown humidifiers can be powder coated with AB Powder coatings where humidity welcomes the growth of bacteria and fungi.
Refrigerators, Washing machines and Air conditioners etc. can be powder coated successfully.

2. Medical care: Hospitals and clinics are looking into using antibacterial powder coated objects (e.g.) Instrument trays, sterilization equipments, lighting fixture, cabinets and cots etc.

3. Sanitation: Kitchenware, bathroom wears, dish washers and shopping carts etc.

4. Food equipments: Mixing bowls, serving trays, sinks, cooler, display cases, food processing equipments, and storage bins etc.

5. Steel furniture industry: Steel shelving, storage cabinets and trays (used in shopping malls) etc.

6. General industry: Escalators and elevators etc

This article describes about the manufacturing of antibacterial powder coatings using silver additives and important factors which need to be considered while designing the formulation, manufacturing and during the application stage.

Selection of Antibacterial agent

Number of antibacterial agents and fungicides are available for antibacterial coatings. However it can be broadly classified into three types, each type of additive has its own advantages and some disadvantages.

Natural antibacterial agent: They are difficult to handle as they posses
relatively low boiling point.

Organic antibacterial agent: Though these additives are compatible with the resins they have a problem of low stability so that their efficiency is lost during the melting stage.

Inorganic antibacterial agent: It has high heat stability but shows poor solubility in the binder and some discoloration into water. It depends on the type of additive.

So while selecting the appropriate antibacterial agent for antibacterial powder the following points can be considered;

1. Efficiency at low dosage level
2. Solubility/compatibility with the binder
3. Antimicrobial effect against wide range of microbes such as bacteria, mold & fungi etc.
4. Rate of discoloration
5. Particle size of antibacterial agent (it plays an important role in migration of additive to the top surface of the coatings also it plays an important role in gloss reduction of the coatings)
6. Stability of antibacterial agent during processing & sustainability
7. AB agent should not produce visible bubbles in the coatings (indication of volatilizing of the active element)
8. Price/performance ratio
9. Environmental compliance & Regulatory guidance

Mechanism of silver technology

For the present study inorganic antibacterial agent silver type is used. Five different types of silver additives are tried. The silver additives used for the present study is a composite of active silver ions, sodium ions and other ions
(e.g. Zn). In some additives silver is embedded into a base material such as zeolite & glass matrix etc.
Silver ion is a natural inorganic antibacterial agent which is effective in fighting against bacteria, fungi& moulds. Silver ion is generally preferred for its

a. Bacterial resistance
b. Environmental compatibility/ compliance
c. Thermal stability
d. Effective life expectancy
e. It can be used for both indoor and outdoor applications.


Silver ion inhibits the process of cell division by which the bacteria replicates. The action of silver additive is based on the slow and continuous leaching of super fine silver ions that interact with the metabolism of the microorganisms in various ways. Moisture in the air accelerates the property of ion exchange process by which silver is released at a continuous rate and maintaining an antibacterial surface. In presence of moisture the zeolite pumps out silver ions in exchange for sodium ions from the environment. As the environment becomes more humid bacterial growth would be more, the zeolite discharges more silver ions thereby long term protection is ensured. Antimicrobial action of silver comes from the chemical properties of its ionized form Ag+. When microbes come in contact with antimicrobial treated coatings surface, Ag+ is released accordingly to the surface of the treated product to inhibit the bacterial growth.

Characterization of the additives

Table 1
 

 
 


Antibacterial powder can be prepared by conventional manufacturing method.
In method 1¸ the antibacterial agent can be homogeneously mixed with powder composition during the premixing stage and this homogeneous premix is extruded and pulverized into a fine particles.
Method 2 is, base powder is prepared separately and the antibacterial agent is dry blended with the base powder.
Method 3, Antibacterial agent can be bonded with pre-formed powder particles. Formulations 1 to 3 were cured at 180°C/15min and all the formulations show good mechanical and corrosion properties.
For antimicrobial activity test, samples were prepared with different dosage of silver additive along with blank samples (containing no silver additive).The influence of light on effectiveness of silver additive can be tested by weatherometer. As continuous washing can reduce the performance of the coated film leaching test can be performed to understand the effectiveness.
Formulation 1: Polyester Clear Glossy

Table 2
 

Addition of Antibacterial Agent in formulation 1 by dry blending method
 

Formulation 2: Hybrid Silver Metallic

Table 3

 

Addition of Antibacterial Agent along with metallic pigments in formulation 2 by dry blending method

 

Formulation 3: Hybrid White Glossy

Table 4
 

 
Note: “In formulation 3B and 3C silver additive is incorporated during the premixing stage. The dosage of silver additive incorporated in formulation 2&3 is high and optimum dosage level can be decided by way of conducting experiments with different dosage level.”

Addition of Antibacterial Agent in formulation 3A by dry blending method

 

Test for antimicrobial activity

Antimicrobial activity of the powder coatings is tested as per
JIS Z2801:2000 -Test for antimicrobial activity& efficacy.
Japanese industrial standard JIS Z 2801 was designed to quantitatively measure the ability of an antimicrobial surface to kill or inhibits the growth of microorganism. This method tests for both bacteriostatic (growth-inhibiting) and bactericidal (bacteria-killing) properties. Microbial concentrations are standardized and bacteria are provided with the nutrients during the incubation period, which provides them with ample opportunity to grow if surfaces aren’t sufficiently antimicrobial. The test microorganism is prepared usually by growth in a liquid culture medium. The suspension of test micro organism is standardized by dilution in a nutritive broth. Control test surface are inoculated with microorganisms and the microbial inoculum is covered with thin sterile film. Inoculated, covered control and antimicrobial test surfaces are allowed to incubate undisturbed in a humid environment for 24 hours. After incubation, microbial concentrations on are determined.
Reduction of microorganisms relative to initial concentrations and the control surface is calculated. Each test sample is inoculated with a suspension of the test organism i.e. Methicillin Resistant Staphylococcus aureus (NBRC 12732) &
Escherichia coli-E coli (NBRC 3972)

Test procedure
Standard: JIS Z2801
Inoculating solution: Bacterial suspension initial count of 105 CFU/ml with a
1/500 Nutrient broth
Inoculating volume: 0.4 ml
Storing temperature: 35 degree C
Storing humidity : > 90%
Storing time : 24 hrs

Antimicrobial activity criteria

Difference in the number of survived bacterium is expressed by a logarithm of an inverse number of a ratio of a number of bacterium in a film containing antibacterial agent and to a number of bacterium in a film containing no antibacterial agent. Therefore higher value shows higher efficiency.

A: Bacterial viable cells immediately after inoculation of the control
B: Bacterial viable cells of the control after 24 hrs
C: Bacterial viable cells of the untreated test piece (Blank)
D: Bacterial viable cells of the antimicrobial test piece


Log C/D >= 2.0


An antimicrobial effect is accepted when the above conditions are passed.




Performance of samples against Staphylococcus aureus & E-Coli

Table 5 (Ref: Antibacterial test result of Formulation 1)

Note: CFU-Colony forming unit is a measure of viable bacterial numbers. The results are given as CFU/ml colony forming units per milliliter

Table 6 (Ref: Antibacterial test result of Formulation 2)
 

 

Table 7 (Ref: Antibacterial test result of Formulation 3)

Key factors

Though silver additives exhibit excellent antimicrobial activity, the performance of the final product depends on many parameters and some of them are discussed here. Combination of all will give the best result.

Dry film thickness of coatings

Silver additive is necessary to expose appropriately on the top surface of the coated components for better antibacterial performance. Migration of antibacterial agent to the surface of the coatings is most important to achieve the best antibacterial protection. However this migration depends on number of factors such as thickness of the coatings, binder content, recipe of formulation, dosage level of antibacterial agent, degree of cross linking, cross reaction of antibacterial agent with other ingredients(i.e. disturbance caused by other ingredients present in the formulation) etc.
In general lower dry film thickness is preferred to achieve good antibacterial performance. Migration of silver additive is easier at low thickness and as per our experiments we suggest 40-55 micron thickness for better antibacterial performance. However better migration of antibacterial agent can also be achieved by selecting appropriate particle size of silver additive, recipe of formulation etc. Extrudable grade version of antibacterial powder shows good performance even at higher thickness.

Dosage level of silver additive

Higher dosage level of silver additive improves the performance of coatings. However it has some adverse effect on the other properties of the coatings. So optimum dosage level to be decided based on the results of experiments and the optimum dosage level also depends on the recipe of formulation. Clear powder requires less dosage whereas highly pigmented versions may need higher dose of additives.
a. Higher dose of silver additive leads to gloss reduction. Clarity of the coatings (DOI) is less at higher concentration level of silver additive. Haze would be severe if the particle size of the additive is coarser and its proportion in the formulation is high.
b. Speckle type appearance can be noticed at higher dosage level. This effect would be severe if the particles size of silver additive is coarser and its proportion is very high in the formulation. Entrapment of moisture in the silver additive can also lead to speckle type appearance.

“But the interesting thing is this type of effect won’t occur if the silver additive is uniformly dispersed in the powder formulation during the premixing stage .The clarity of the coating is excellent when silver additive is mixed during the premixing stage and speckle type appearance not noticed even with coarser grade additives.”

c. Higher dose of silver additive can lead to discoloration.

Optimum dosage level of silver additive depends on various parameters such as type of silver additive, recipe of formulation etc. it is necessary to conduct thorough experiments to decide about optimum dosage level.

Presence of other ingredients

Presence of some ingredients in the formulation may influence the antibacterial performance of silver additives. Some antioxidants, imdidazole derivatives and fillers like calcium carbonate may influence the antibacterial performance of powder. One more suspect is barium sulphate which might spoil the activity of silver because sulphur has a feature which reacts with silver easily. As silver ions have a tendency to react with atmosphere sulphur and exchange the ions it may have some impact. However our present experiments show good performance with optimum proportion of good grade barium sulphate. It may also depend on the dosage and purity of the fillers. In some of the formulations the dosage of antibacterial agent was increased but it didn’t yield the desired result. Scanning electron microscope (SEM) observation had confirmed there were many impurities on the coated panels. So it is necessary to ensure that there is no cross-contamination or absence of contaminants otherwise which may lead to poor antibacterial performance. Some pigments can have detrimental effect on silver for e.g. pigment contains sulphur which will react with silver and spoil its activity.

Recipe of formulation

Recipe of formulation plays a predominant role to get the desired result. For an antibacterial powder to be effective requires that silver ions be able to diffuse through the coating to the surface. Matching between a silver additive and recipe of each powder is also important; coarser grade silver additives produce haze and gloss reduction can be noticed. In general it is better to maintain quite good amount of binder in the formulation. There is a correlation between biocide concentration and cross linking density of the powder. Higher cross linking may affect the migration of silver ions to the top surface of the coatings. If the cross linking rate is high then there could be a possibility for silver ions be covered or buried under the coating film it can lead to poor antibacterial performance.


Degree of dispersion & method of incorporation

Good dispersion of the additives to be achieved to get better antibacterial performance; it is necessary to uniformly disperse the silver additives in the powder formulation and remember good dispersion ensures good performance. Experiments show dry blending method is much better than pre-blending method (extrudable version); however our present extrudable version shows good antibacterial performance. A high contact area between silver additives and resin is necessary to ensure sufficient silver ions are liberated to diffuse to the surface.

Powder application
Good application practice helps to maximize the performance of the end product. Silver additives are hygroscopic and will absorb a significant amount of moisture if left exposed to the air and it makes difficult the spraying process. It leads to poor powder deposition efficiency and some time spurting type of phenomenon can be noticed during powder application. Antibacterial powder should be stored in a cool dry environment. In general end users apply powder at higher powder output rate in order to meet out their productivity. As particle size and specific gravity of silver additive and powder are different there could be a possibility for separation of silver additives from the base powder (if silver additive is dry blended) during application. It may lead to insufficient amount of silver ions on the end components and there could be a possibility for inadequate antibacterial performance. So it is necessary to ensure uniform fluidization, appropriate virgin/recycle ratio and spraying conditions etc.

Summary

The compositions reported in this article gives antibacterial film which exhibits good mechanical and corrosion properties. Silver additives based on silver ions exhibit an excellent antimicrobial effect in different powder chemistry such as pure polyester and epoxy-polyester etc. The optimum dosage level required to achieve good antibacterial performance vary from 0.5% to 1.0% however as this depends on recipe of formulation and type of additive thorough experiments need to be conducted to decide the optimum dosage level. The adverse effect of antibacterial agent on paint surface such as gloss reduction and haze can be reduced by way of selecting suitable particle size of silver additives and through process & product optimization etc.

References:

1. www.antimicrobialtestlaboratories.com

2. www.pfonline.com

3. The Technology, Formulation and Application of Powder Coatings
By David M Howell- “David Howell Consultancy”.