The properties of a paint film are closely related to how the binder dries or cures. The four most common drying / curing mechanisms are:

• Physical Drying (solvent evaporation)
• Oxidative Curing (reaction with oxygen in the air)
• Chemical Curing (chemical reaction between base and hardener)
• Coalescence (evaporation of water followed by merging of the paint droplets)

 

Physical drying

1. The solvents evaporate
2. Molecules pack and melt together to form a paint film
3. No chemical reactions

 

Oxidative curing (drying)

1. The solvents evaporate
2. Oxygen enter the paint and the reaction starts
3. The binder molecules link together through a chemical reaction with oxygen

 

Chemical curing

1. The solvents, if present, evaporate
2. After mixing the separate components together the individual reactive ingredients react together
3. A three dimensional network is formed which gives the solid paint film

 

Proper mixing is a pre-requisite to obtain proper curing. Insufficient mixing makes it impossible for all the reactive ingredients to come in contact in the right proportions with each other and a three dimensional network will not be formed.

 

Coalescence  (Physical drying waterborne paints)

1. Water evaporate
2. Dispersed droplets melt together
3. Co-solvents evaporate to form a paint film
4. No chemical reactions

 

Factors influencing the drying process

• Ventilation
• Temperature
• Film thickness
• Number of coats
• Evaporation rate of the solvents, if present

 

For water borne paints high relative humidity slows down the drying process

 

Curing mechanism of various binders

Physically drying:

• Vinyl
• Acrylic

 

Oxidatively curing:

• Alkyd
• Drying oils
• Epoxy ester

 

Chemically curing:

• Epoxy
• Polyurethane
• Polyester
• Vinyl ester
• Silicate
• Polysiloxane

The generic type refers to the ingredient responsible for the basic properties of the paint. In most cases the binder dictates the properties, and defines the generic type.

 

Common generic types of paint

• Alkyd
• Vinyl
• Acrylic
• Epoxy
• Zinc rich epoxy
• Polyurethane
• Polysiloxane
• Inorganic zinc silicate
• Polyester
• Vinyl ester
• Silicone

 

Sometimes paint may be referred to as “modified”, e.g. modified epoxy paint. This may involve either a chemical modification of the binder or blending smaller amounts of an additional binder into the paint.

A. Alkyd paint

Advantages

• Good application properties
• component
• Good weather ability
• Good wetting properties
• Good recoatability
• Good leveling properties
• Good gloss retention
• Heat resistant up to 120 ˚C

 

Limitations

• Poor chemical resistance (especially against alkalis)
• Never to be used on Zinc
• Limited water resistance (submerged)
• Limited solvent resistance
• Limited film thickness per coat

 

B. Acrylic paint-Solvent borne

Advantages

• Physically drying
• Easy to recoat
• One component
• Good water resistance
• Relatively good chemical resistance
• Good weather resistance

 

Limitations

• High VOC
• Poor solvent resistance
• Low solid content
• Relatively poor wetting properties
• Thermoplastic

 

C. Unmodified epoxy paint

Advantages

• Very good chemical resistance
• High alkali resistance
• Moderate resistance to acids
• Good adhesion
• Very low permeability
• High mechanical strength
• Dry heat resistant up to 120˚C

 

Limitations

• Chalking
• Temperature affects cure time
• Over coating time affected by temperature. Some have short maximum overcoat times

 

D. Epoxy mastic paint (modified epoxy)

Advantages

• Surface tolerant
• High solids
• High build

 

Limitations

• Not UV - resistant
• Temperature dependent for curing
• Sweating-non reacted cure components

 

E. Zinc rich epoxy paint

Advantages

• Chemically curing
• Good corrosion protection
• Good adhesion
• Requires min. Sa 2½
• Good mechanical strength
• May be recoated with all types of paint, except alkyd based or very high build coatings >500µm
• Dry heat resistant up to 120˚C

 

Limitations

• Temperature dependent curing
• Film thickness: 25 - 90 um
• Not acid- and alkali resistant
  • (Resistant between pH 5-9)

 

F. Polyurethane paint

Advantages

• Very good weather resistance
• Excellent gloss retention
• Very good chemical resistance
• Very good solvent resistance
• Cures down to 0 ˚C

 

Limitations

• May cause skin irritation
• May create toxic fumes in a fire

 

G. Zinc ethyl-silicate paint

Advantages

• Very good solvent resistance
• Very high heat resistance, up to 400˚ C (max 540 ˚C for short periods)
• Very high mechanical strength
• Very good adhesion to blast cleaned steel
• Can be overcoated with a wide range of coatings (not alkyd)

 

Limitations:

• Requires humidity in excess of 50% RH for curing
• DFT: 125 µm
• Mud-cracking or checking at higher DFT

 

H. Glass-flake reinforced polyester

Advantages

• Quick curing
• Variable curing time
• Application with airless spray
• Excellent mechanical strength
• Glass-flakes reduce shrinkage, increases mechanical strength and water resistance
• Very good chemical resistance

 

Limitations

• Short pot life
• Limited shelf life
• Recoating interval: 2 to 12 hours
• Should not be applied over zinc primers and galvanized surfaces

 

I. Acrylic waterborne paints

Advantages

• Very low VOC
• Water is relatively cheap and readily available
• Quick surface drying
• Simple cleaning of equipment
• Low risk of fire

 

Limitations

• Can not be used below 0˚C
• Dries slowly at high relative humidity
• Needs long time to reach full hardness
• Thermoplastic
• Poor solvent resistance

 

 

Common curing agents for Epoxy Paints

Polyamide

• Unmodified & modified epoxy paints

 

Amine and amine adduct

• Improved chemical resistance
• E.g. Tank coatings

 

Surface tolerance

• Surface tolerant paints adhere well and offer long term protective properties on substrates where optimum surface preparation is not possible or desired
• The surface tolerance depends largely on the wetting (penetrating) properties of the binder
• Most paints can be applied to steel blast cleaned to Sa 2½
• Not all paints possess the required wetting property for successful application on uneven substrates
• Surface tolerant paints give a longer life time on such substrates

Fouling is the settlement and growth of marine plants and animals on surfaces in the sea. There is an estimated 4-5.000 fouling species and these can be classified into:

 

1. Micro-fouling

• Generally referred to as slime, a complex viscous mixture of bacteria and microscopic organisms

 

2. Macro-fouling

• Which includes animals and plants

 

Examples of fouling in seawater

Micro-fouling

• Biofilm or slime
• Bacteria
• Blue green algae
• Diatoms
• Protozoa
• Other micro-organisms
• Up to 1 mm thick

 

Macro fouling - algae

• Grass
• Green algae
• Brown algae
• Red algae
• Grow only in presence of light

 

Macro fouling - animals

• Barnacles
• Mussels
• Tubeworms
• Hydroids
• Bryozoa
• Tunicates

 

How does fouling happen?

• Free floating or free swimming spores and larvae follow the currents, tides and waves
• They look for the opportunity to contact suitable surfaces
• Fouling happens when these larvae or spores settle and develop into an adult, small at first, but steadily growing
• Any man-made structure in the sea will be subjected to fouling (marine growth)
• The challenge is to create a surface which is not attractive or conducive for such growth

 

Fouling on ships’ hulls  leads to:

• Increased frictional drag
• Increased fuel consumption
• Increased emissions to air
• Possible transmigration of non-indigenous species
• High cost of removal

 

The purpose of antifouling paints

• To prevent or reduce growth
• To provide better fuel economy over the sailing period
• To avoid growth penetrating the coating and thereby damage the corrosion protection

 

The composition of antifoulings

Antifouling paints contain the same main groups of raw materials as other paints plus additional biocides to prevent or reduce the settlement of marine growth.

• Binder
• Biocide
• Extenders
• Pigments
• Solvents
• Additives

 

The most common biocide used in antifouling paints is Cuprous oxide

 

Main types of antifouling paints:

Conventional antifoulings

• Soluble matrix paints
• Rosin as part of the binder
• Approximately 12 months protection
• Binders dissolves in water and biocide is released

 

NOTE: Danger of cracking and flaking if not quickly immersed in water

 

Long-life antifouling

• Insoluble matrix paints
• Only the biocides are released
• Effective protection is up to 24 months
• Leaves weak substrate for subsequent coat (leached layer)
• Sealer coat normally required
• Binder: Vinyl, acryl (possible to add small amount of rosin)

 

Self-polishing antifoulings

• Predictable performance
• Extended dry docking period
• Control of roughness and smoothing
• No “sandwich coatings” problems
• Lifetime directly proportional to dry film thickness

 

Antifouling for aluminium hulls

• Copper-containing antifouling must not be used directly on an aluminium hull
• Metallic copper in contact with aluminium will cause bi-metallic corrosion
• A full anti corrosive system must be applied on the aluminium hull before the antifouling

Shopprimer definition

A quick drying paint applied as a thin film to a metal surface after blast cleaning, to give corrosion protection during the period before and during fabrication

 

Requirement: Must not interfere with the speed or quality of cutting and welding of the steel during fabrication, so DFT must be low

 

Shopprimer history

• In the early days shopprimers were designed for only temporary protection and was removed by blast cleaning before the steel was painted
• Later on, improved quality was developed which required only re-blasting of welds and mechanical / thermal damages before the steel was painted
• Today’s technology using only UHPWJ treatment before painting
• Shopprimer may now be a part of the long term protective paint system

 

Shopprimers – types

While other paints generally are classified according to their type of binder, shopprimers are often classified according to their main pigment content:

1. Iron oxide shopprimers
2. Zinc rich shopprimers

 

Types / pigments	Binders	Typical DFT (µm)	Corrosion protection, typical (months)
Iron oxide	PVB(*)	15-20 µm	3
	Epoxy	15-20 µm	5
Zinc rich	Ethyl silicate	10-15 µm	10
	Epoxy	15-20 µm	8

(*) Poly Vinyl Butyral

 

Iron oxide shopprimers – characteristics

Types / pigments	Binders	Advantages	Limitation
Iron oxide	PVB(*)	– One component – Quick drying – Flexibility	– Low %-volume solids – Low heat resistance – Not compatible with cathodic  protection
	Epoxy	– Adhesion – Chemical resistance – Well suited with cathodic protection	– Short pot life – Low flash point

(*) Poly Vinyl Butyral

 

Zinc rich shopprimers – characteristics

Types / pigments	Binders	Advantages	Limitation
Zinc rich	Ethyl silicate	– Abrasion resistance – Corrosion protection – Allows high welding and burning speed – Few burning damages	– Needs high air humidity for curing – Forms zinc salts on the surface – Zinc fumes from welding and burning may cause zinc fever
	Epoxy	– Adhesion – Ease of over coating	– Forms zinc salts on the surface – Zinc fumes from welding and burning may cause zinc fever

 

Activities involved in shop priming

• Cleaning
• Blasting
• Application
• Handling

 

Controlling thickness and integrity of the film

• Use glass plates or smooth steel plates only
• Length of test panel must be sufficient to cover overlapping zones
• Glass plate: Hold against a light source to check film integrity (dry spray, pinholes, etc.)
• Steel panel: Thickness measurement (magnetic DFT gauge)
• Repair main steel plate if test panels were fixed to main plate

 

Coating over shop-primed steel

• Check for cleanliness / intact surface: OK
• If contaminated by zinc salts:
  • Remove by High Pressure Water
  • Cleaning or blast sweeping
• Rust, damage, welds should be treated according to specification
• Heat-affected areas (thermal damages) must receive treatment to specified standard