Chemistry of Paint

Properties of a Coating

Desirable coating properties include:

 Chemical Resistance
 Water Resistance
 Ease of Application
 Adhesion to Substrate
 Cohesive Strength
 Flexibility and Elongation
 Impact Resistance
 Abrasion Resistance
 Temperature Resistance
 Dielectric Strength
 Classification of Coatings

Coatings are broadly classified as organic or inorganic.

 Organic coatings are those whose binders have been made from living or once-
living things. All organic coatings contain carbon bond.

 Inorganic coatings use inorganic binder materials, most commonly based on

either silicone or silicates.

The major difference between organic and inorganic coatings is heat resistance.
 Composition

Liquid-applied coating components are

characterized by the following terms: Resin

 Pigment
Vehicle

 Vehicle
o – Binder/Resin Solvent

o – Solvent
Pigment /
 Additives Additives

Coating Components
 Pigment

Pigments may be used to:

 Impart color

 Provide inhibitor protection

 Provide a form of cathodic protection

 Modify mechanical or electrical properties

 Additives

Additives are most commonly liquid components of a coating typically added in small
amounts to perform a specific function.

 Improve stability
 Minimize settling
 Reduce foaming
 Improving the flow out and wetting
 Increase pot life
 Increase UV resistance
 Increase or decrease gloss
 Solvents

 Solvents are added to liquefy the binder.

 Once the coating is applied and cured, solvents serve no purpose.

 Solvency Power: The ability to dissolve the resin.

 Volatility: Governs the evaporation rate (the speed at which the

solvent will leave the coating film during and after application).
 Binder

A coating typically gets its name from the binder used, such as: epoxy,
polyurethane, alkyd, acrylic, etc.

 Good wetting and adhesion

 Resist transmission of water, oxygen, and other chemical species
 Resist chemical and physical change in the service environment
 Dry within an acceptable period
 Form a stable film that maintains its characteristic properties
Alkyd Resin
(strength, hardness, flexibility)
 Modes of Protection

Corrosion control by coatings can occur by one of only three processes:

 Barrier coatings
 Inhibitive coatings
 Sacrificial (cathodic protection)
 Barrier Coatings

NaCl and
The barrier coating obstructs other ions

the ingress of oxygen, water

and soluble salts.
Oxygen

Water

Structure Surface (Steel)

Protective Coating

Structure
 Inhibitive Coatings

 Inhibitive coatings actively slow down the

reaction occurring at the anode, cathode, or
both:
Anodic and Cathodic
reaction Inhibited

 Must be in contact with the substrate. NaCl and

other ions

Oxygen

 Actually passivate the metal surface.

Water

Protective Coating

Inhibitive Primer
Structure Surface (Steel) Structure
 Sacrificial Coatings

Sacrificial coatings use a metal that is

anodic to steel that corrodes preferentially.

Holiday
Local Cathode
Sacrificial coatings: protection at holiday

 Usually contain zinc dust as the

predominant pigment. Structure Surface (Steel)

 Must have a minimum loading of zinc dust

to be effective.
 Adhesion

Strong adhesion is the key to coating performance and long life.

Adhesion can be:
 Chemical – Formed by a reaction between the coating and the substrate.

 Mechanical – Associated with surface roughness or anchor pattern.

 Polar – Most common for organic coatings. The resin acts as a weak magnet
on the substrate.

 Combination of all three

Coating Types and Curing
Mechanism
 Curing Mechanism

Curing is used to describe the way a coating transforms from a liquid to a solid state

There are two broad classifications for curing mechanisms:

o Nonconvertible – Cure by evaporation of the solvent with no chemical change

to the resin.

o Convertible – Undergo a chemical change during cure and cannot be returned

to their original state.
 Evaporation Cure

 Cure solely by solvent evaporation

Can be re-dissolved in the solvent

 Examples include vinyl and chlorinated rubber

 Coalescence

 Resin are dispersed in water.

 The water evaporates, the resin particles fuse (coalesce).

Typically known as latexes or acrylic latexes.

 Convertible Coatings

 Cure by one of several polymerization mechanisms

 Resins undergo a chemical change
 Not readily re-dissolved in the solvent
 Known as thermosetting materials
 Some examples of curing types are:
o Oxidation
o Co-reaction
o Hydrolysis
o Fusion
 Oxidation

Cure by reaction with atmospheric oxygen

 Unsuitable for immersion service

 Cannot withstand an alkaline environment, due to “saponification”

 Excessive film build may stop curing of the lower portion of the coating film

 Example: Alkyd
 Co-Reaction

Cure by polymerization reactions (cross-linking) between at least two chemical entities.

Examples include:
– Epoxies, Polyurethanes, Polyureas, Polyaspartics, Polysiloxanes and several others
 Co-Reaction

 For solvent bone coatings, first step is solvent evaporation and second step is
polymerization.

 Once mixed together, applicator has a short time to apply before it gels. The
time period when the mixed material is usable is called pot life.

 Some co-reaction cured coatings also require a period of time after mixing but
before application for the chemical reaction to start; this is referred to as the
induction time.
 Hydration

Coatings require some amount of water to complete the cure.

Examples include:

o Moisture cured polyurethane

o Solvent based inorganic zinc coating based on an ethyl silicate
 Fusion

 Forced heat curing

 May be single or two component materials

 Example is fusion-bonded epoxy (FBE)

Curing Mechanisms
 Selection of Coating
The selection of a coating is based on many factors that include:
 Service environment of the coating:
 Interior or exterior
 Immersion or atmospheric
 Chemical
 In-service temperature (plus typical range and upset conditions)

 Substrate being coated

 Size and configuration of item to be coated
 Surface preparation available and possible at job site
 Selection of Coating

 Application temperature and humidity

 Life expectancy of both item being coated, and coating
 Ability of applicators
 Availability of application equipment
 Critical safety requirements, e.g., a nuclear power plant, buried pipeline, or
commercial ship
 Budget
 Generic Coating Types

o Acrylic o Polysiloxane
o Alkyds o Polyurethane
o Chlorinated Rubber o Polyureas
o Epoxy o Silicones
o Latex (Emulsions) o Vinyl Esters
o Phenolic o Vinyl
o Polyaspartic o Zinc (Inorganic)
o Polyesters o Zinc (Organic)
 Acrylic

 Excellent UV and weathering resistance

 Can be applied as coalescence curing emulsions/ water dispersions

 Historically applied as decorative coatings rather than for corrosion resistance

 Alkyds

Oxidative curing, referred to as “oil based paints”

 Single package material and broad range of colors

 Can be very slow curing products, limited thickness per coat

Can be blended with other resins to improve properties e.g.

 With epoxies to make Epoxy Ester
 With silicones to make Silicon Alkyds
 Urethane alkyds by mixing with Isocyanates
 Chlorinated Rubber

 Evaporation curing

 Contains a large amount of VOC

 Eliminated in most parts of the world

 Excellent resistance to water, sunlight, and many chemicals

 Should not be over coated with two component coatings

 Epoxy

 Two components consist of an epoxy resin (base) and a curing agent (converter)

 Can be solvent-based, water-based, or solvent-free

 Excellent adhesion, chemical resistance, water resistance, and wet adhesion

 Amine cured epoxies are especially sensitive to amine blush

 Exhibit chalking with atmospheric (UV) exposure

 Latex (Emulsions)

 Resins normally thermoplastic resin types

 Coalescence curing
 Phenolic

Typically used where low pH environments and higher temperatures are factors

 Excellent resistance to acids

Usually use for internal tank lining of corrosive material storage tanks

Can be blended with epoxies to make Epoxy Phenolic

 Polyaspartic

 Use to achieve low- or near-zero-VOC systems

 Pot lives from five minutes to several hours

 Film builds up to 380 μm (15 mils) DFT in a single pass

Are aliphatic Polyureas

 Polyesters

 Have a short pot life

 Glass flake reinforced, high build coatings

 Excellent moisture resistance

 Exceptionally high abrasion resistance

 Polysiloxane

 Used in services with abrasion, chemicals, extreme UV, and high temperature

Three major categories :

o Inorganic Polysiloxane
o Epoxy-Polysiloxane Hybrids
o Acrylic-Polysiloxane Hybrids
 Polyurethane

Two major types

 Aliphatic
o More resistant to UV exposure, excellent gloss and color retention
 Aromatic
o Better chemical resistance in immersion but not UV resistant

 Main hazard is the isocyanate component

 Available with a variety of curing times

 Polyureas

 Very flexible materials

 Very short cure times

 Many require the use of an epoxy primer on steel

 Silicones

 Formed by chemical modification of quartz, sand, or silicon

 Excellent high temperature and UV resistance

 Most require heat to cure but some cure at ambient temperatures

 Also used as foul-release coatings in the marine industry

 Vinyl Esters

 Often referred to as linings

 Normally two-component coatings

Excellent resistance to most of the corrosive chemicals even at higher temperatures

 Have rather short pot life

 Excellent abrasion resistance with glassflake added

 Vinyl

One of the earliest industrial coatings

 Were used on highway bridges and extensively in the marine industry

 Banned from use in most countries due to high VOC

 Zinc (Inorganic)

 Widely used primer for steel structures

 Provides cathodic protection

 Very resistant to different chemicals and especially solvents

 Very high heat resistance with a max of 400°C (750°F)

 Disadvantages

o Can be difficult to apply

o Not for acidic environments
 Zinc (Organic)

 Very different from inorganic zinc

 Organic resin with zinc filler

 Examples epoxy, polyurethane, etc.

 Limited cathodic protection factor

Environmental Testing
 Environmental Effects

Environmental, or ambient, conditions can greatly affect all phases of a coating

operation.
 Surface Temperatures

 Surface temperature is often different from air temperature.

Application at incorrect temperatures can cause defects.

 Minimum and maximum application temperatures should be recognized.

 Substrate should be at least 3°C (5°F) above the determined dew point.

Minimum Temperature Maximum Temperature

 Relative Humidity

 Measure of the amount of moisture in the air compared to saturation level.

 May affect the coating if too high or too low.

 Too high may cause solvent entrapment.

 Wind Speed

Wind speed can affect the coating job by:

Blowing abrasives

Causing excessive overspray

Accelerating solvent evaporation

Contributing to the formation of dry spray.

Dew Point
The temperature at which moisture will begin to form on a steel surface.
 Surface Temperature Instruments

Typical instruments you may encounter include:

o Digital Infrared Thermometers
o Mechanical Surface Contact Thermometer
o Electronic Surface Contact Thermometer
 Digital Infrared Thermometer

Positives
 Are very simple: point, shoot, and read
 Deliver quick reading

Negatives
 Steam, dust, smoke, and/or vapors can prevent accurate readings
 Cannot measure reflective surfaces accurately
 Mechanical Surface Contact Thermometer

Magnetic surface contact thermometers are one of the most common instruments

 Positives
o Simple and inexpensive

 Negatives
o Stabilization time can be minutes
o Easily lose their accuracy
Heated Surface
 Electronic Surface Contact Thermometer

Magnetic surface contact thermometers are one of the most common instruments

 Positives
o Quick and accurate

 Negatives
o More expensive than other surface
 Relative Humidity Instruments

 Electronic Hygrometers

 Sling Psychrometer

 Powered Air Flow Psychrometer

 Electronic Hygrometer

 Read and calculate the:

o Temperature
o Relative humidity
o Dew point
 Sling Psychrometer

 Sometimes called a whirling hygrometer

 Used to measure the ambient air temperature

 Dry-bulb temperature and wet-bulb temperature

used to calculate the dew point and relative humidity
 Powered Airflow Psychrometer

Operates in a similar way to the sling psychrometer, but air is moved using a
fan, rather than slinging the instrument.
 Chart Recording Ambient Conditions

Time
RH%
Dew Point
Ambient Temperature
Substrate Temperature
Ok to work Yes/No
 Wind Speed

 Wind speed can be a safety issue when work is being performed at heights.

 Monitoring wind direction can prevent damage to property from overspray.

Product Technical Data Sheet
And
Material Safety Data Sheet
 Product Technical Data Sheet

 Coatings Product Data Sheets (Paint Specs) provide users with valuable information
on the application aspect of a particular product.

 Communicate technical facts related to the specific material and its application
properties.

 Common to find a small section related to safety.

 Product Technical Data Sheet
 Information of critical importance to the project success:
o Surface preparation
o Storage
o Mixing and thinning
o Application procedure
o DFT requirements
 One should always consult Product Technical Data Sheets.
 It may come across discrepancies between the project specification and the product
data sheet. The holder of the specification (owner) must be contacted for clarification
 Product Technical Data Sheet

A thorough understanding of the

Product Technical Data Sheet cannot
be over emphasized!
 Material Safety Data Sheet
 Contains data regarding the properties of a particular substance.
 Provides workers and emergency personnel with critical information on composition,
handling, or working with that substance in a safe manner.
 Includes information such as melting point, boiling point, flash point, toxicity, health
effects, first aid, reactivity, storage, disposal, protective equipment, and spill handling
procedures.
 Provides information regarding the safety issues associated with any hazardous (or
potentially hazardous) material.
 Provides instructions for the correct action to take in the event of a spill, explosion,
fire, or hazardous exposure.
 Material Safety Data Sheet
 When coating materials are being transported, there is a constant risk of spillage or
exposure of others to potentially toxic chemicals.

 In some countries the law requires that MSDS be carried by transporters whenever
industrial quantities of coatings are moved by road, rail, or air.

 Inform emergency services of potential hazards in the event of a spill, fire, or other
hazard.

 Chemicals commonly used and judged to be hazardous are listed by various industry
or national authorities.