Chương 3.

Biến tính bề mặt

bằng lớp phủ (T5)
3.1. Lớp phủ hữu cơ
3.2. Lớp phủ vô cơ
4.3. Lớp phủ chức năng
 Coatings
Coatings are materials that are applied to a surface which form a
continuous film in order to beautify and/or protect the surface.

Sealants Paint Varnish

Enamel Shellacks Stains

 Basic Composition of Paint

Solvents (Liquids, 16%)

Pigments (45%)
Additives (4%)

Resins (Binder, 32%)

Water based paints may have considerably more liquid carrier

• Binder (nonvolatile)
• Principal ingredient
• Holds paint together and to the substrate
• Establishes chemical and physical properties
• Defines coatings generic type
• Solvent (volatile)
• It carries the pigment and binder (dissolve film former)
• Reduce viscosity for ease of application
• Control rate of cure
• Low molecular weight organic chemicals (aliphatic,
aromatic, and chlorinated hydrocarbons; alcohols;
ketones; esters; glycol ethers; and water)
Pigment (nonvolatile) Additives/ extenders
• Decorate, color, and hide • Anti-fungus
• Protect • Anti-skinning
✓Barrier • Plasticizers
✓Galvanic • Levelers
✓Inhibitive • Thixotropic agents
• Provide consistency • Accelerators
• Provide film thickness • Stabilizers
• Control vehicle penetration • …
into substrate
 Pigments are selected on the basis of
Particle size Particle shape Refractive Index
Tinting strength Lightfastness Hiding Power
Thermal Stability Chemical Reactivity Density (cost)

• Property Preference Reasons

•(1) Brilliance and Organic The most attractive, cleanest colours

• clarity of hue are obtained with organic pigments.

•(2) White and Inorganic The purest white pigment is TiO2

• black paints and the most jet black, carbon.

•(3) Non-bleeding Inorganic Inorganic compounds have

• negligible solubilities in
• organic solvents. Some organics
• are very insoluble.

•(4) Lightfastness Inorganic Inorganic compounds are generally

• more stable to UV than organics.

•(5) Heat stability Inorganic Very few organic compounds are

• stable above 300°C.
• Extenders provide no colour to a film, but their use is an
inexpensive method of improving adhesion, ease of sanding, film
strength and opacity.
• Calcium carbonate (whitewash)
• Aluminum silicate (clay)
• Magnesium silicate (talc)
• Barium sulphate (barytes)
• Silica
• Viscosity Modifiers
• silicates, clays, poorly soluble resins
• Dispersion Aids
• aid in pigment dispersion - chosen on a case-by-case basis
• Interfacial Tension Modifiers
• non-ionic surfactants, soaps
• Biocides
• insecticides, fungicides
 Solvent Selection Criteria

• Solvating Capacity:
Miscibility of polymer/solvent systems are dictated
by thermodynamics, as approximated by solubility
parameters and hydrogen bonding groupings.
• Viscosity:
Influenced by solvating capacity, but also a function
of the viscosity of pure solvent and additives.
• Volatility:
Rate of solvent evaporation influences drying time as
well as film aesthetic qualities. Decisions often based
upon boiling point/range.
• Toxicity and smell.
• Cost.
Type of Curing in Modern Coating Resins

• Solvent evaporation
• Oxidation
• Chemical reaction
• Catalyzed film former
• Thermoplastic
• Thermoset
• Emulsion - water based
 General Coating Formulations
Method Polymer Solids Polymer Dry rate Min. dry Handling/ Examples
M.W. structure (no heat) temp. Storage

Evaporation high (i) low, linear fast no practical good nitrocellulose

10-35% limit other lacquers;
solution (solutions) some emulsion
(ii) medium paints
40-70%
emulsion

Chemical low medium to crosslinked slow- very slow cans must decorative paints
reaction high, 35- moderate in cold be well some stoving
between paint 100% weather sealed enamels;
and air

Chemical low or medium to crosslinked fairly varies; two-pack or industrial stoving

reaction very high fast 10-15°C short shelf finishes; acid-
between low 30-100% life, unless catalysed poly-
paint stoving or urethane and
ingredients radiation polyester wood
curing type finishes
 Thermosetting Binders: Epoxy and Polyurethane Resins

• Epoxy resins are two-component paints formulated from epoxide

functionalized monomer and (usually) amine hardeners.
O O
H2N
CH2O OCH2 + NH2

O OH
CH2O OCH2 NH
NH2

• Reaction of diisocyanates with diols generates polyurethane

coatings whose structure/properties can be varied widely.

• Polyurethanes afford superiour abrasion and chemical resistance,

as well as a fast, low-temperature cure.
 Polyurethanes
• One part polyurethane
• Moisture cured polyurethane
• Acrylic polyols-aliphatic linear isocyanate two part
polyurethanes
• Polyester polyols-aliphatic isocyanate two part
polyurethane

component 1 component 2 n-m

m
O O
OH NH
n OCN R NCO R
OH NH
O O O
OH

m n-m
 Why do we need paints?

Paints are surface coatings generally suitable for site use,

marketed in liquid form.
They may be used for one or more of the following purposes:
• To protect the underlying surface by exclusion of the
atmosphere, moisture and insects.
• To provide a decorative easily maintained surface.
• To provide light- and heat-reflecting properties.
• To give special effects (for example, inhibitive paints for
protection of metals; electrically conductive paints as a
source of heat; condensation- resisting paints)
 Painting specific materials
Ferrous metals
• Steel forms the largest bulk of metals used in building and is one
of the most difficult to maintain.
• The best time to paint steel is immediately after production,
though mill scale (iron oxide film produced during hot rolling)
should be removed because:
• It behaves cathodically to the bare metal and may lead to
local corrosion.
• It may eventually flake off due to differential movement.

BTVN#2
Hot Roll Steel
Cold Roll Steel
 Non-ferrous metals

• Zinc and aluminium are the non-ferrous metals most likely to

require surface coatings and each provides a poor key for
paint, unless surface treatment is first carried out.
• Zinc, in particular, reacts with most oil-based paints, forming
soluble salts which reduce adhesion.
• Zinc should be degreased with white spirit, followed by
roughening of un-weathered surfaces with emery paper.
• Primers containing phosphoric acid are available for this; they
often also contain an inhibitor, such as zinc chromate.
• Other suitable primers contain calcium plumbate, zinc dust or
zinc oxide.
 Wood
• Protect as soon as possible after the manufacturing process
is completed since the surface is quite rapidly affected by
weathering/ultraviolet light, as a result of which the paint
adhesion properties significantly deteriorate.

• A primer is essential to penetrate and yet block the pore

structure.

• Undercoats are unsatisfactory here, since they often do not

penetrate the wood and may flake off later.
 Plastics

• Most plastics do not require painting, and paint coats, once

applied, cannot be removed by normal techniques.

• Paints, on the other hand, will reduce the rate of degradation of

plastics such as polyethylene.

• Adhesion is poor unless the surface is first roughened to give a

mechanical key.

• The impact strength of some plastics, such as PVC, may be

adversely affected, if painted, by migration of solvent into the
paint.
Painting systems
Provides aesthetic and 1st
defence protective properties
from the environment

Builds film thickness to

provide extra protection

Inhibits corrosion and creates

good adhension propeties

Appropriate surface
preparation should always
be carried out to ensure the
success of the project
 Aesthetic Properties of Dried Film Coatings

• Opacity
✓Extent of substrate coverage, as determined by pigments,
extenders and other occlusions in the film.
✓Dependent on refractive index of fillers relative to the
polymeric binder.
• Surface Finish: Gloss is a function of surface irregularity, as
determined by the film formation process and dispersion of
pigments/fillers.
• Color: Inorganic and organic colourants that are soluble or
dispersed in the film (may or may not provide opacity).
 Engineering Properties of Dried Film Coatings

• Properties: • Tests:
• Hardness • Indentation, Scratch (Pencil)
• Impact (& Chip) Resistance • Drop tests, Gravelometer
• Flexibility • Elongation, Bend
• Abrasion Resistance • Falling sand test
• Solvent Resistance • MEK (methylethylketone) double rub
• Adhesion • Scraping, Crosscut Adhesion
Issues:
• Properties are a complex function of many factors: Tg, MW, crosslink density,
pigmentation, static stress-strain behaviour, transient (creep) behavior
• What do you test? Free films? Coating with substrate?
• Weatherability and UV resistance – How does it hold up over time?
 Solvent Emissions From the Organic Coating Process ?

Climate Change
Carbon Footprint

✓ Paints based on organic solvents

✓ Carbon derived from fossil sources
✓ Used as a carrier in paints, evaporates and adds to
the environmental carbon compound concentration
▪ Powder coating is a dry finishing process, using fine particles
of paint, which are electrostatically charged and sprayed
onto a workpiece.
▪ Once the powder is applied, the part is cured, causing the
powder to adhere to the surface.
 Painting Process
▪ Fine particles are fluidized in a feed hopper

▪ Powder is vacuumed into the spray gun

▪ A second burst of air increases the particle velocity

▪ Particles are then charged by high velocity friction

▪ Spray gun applies powder to the grounded workpiece

▪ Coated part is then oven cured

• Oven temperature: 300 °F

• Time for complete cure: 20 minutes
• Curing causes a chemical reaction that bonds the powder
coating to the workpiece.
 cold substrate cross-
substrate heated linked

electrical polyester melts solid

adhesion TGIC diffuses film
Tribostatic Gun Layout
Higher Quality, Extremely Durable, Environmentally
Friendly, Cost Efficient
▪ No solvents are used
Advantages ▪ Over-spray (up to 98%) can be reused
▪ More resistant to chipping, cracking, and fading
▪ Corrosion and chemical resistant
▪ Coating does not run, drip, or sag
▪ Thick coatings are easily done
▪ Simple clean-up and maintenance
Disadvantages
❑ Thin coatings are difficult to produce
❑ Storage and handling of the powder requires special climate controls
❑ Color matching is somewhat more difficult
❑ Cure temperatures may be too high
❑ Difficult to coat sharp corners
Methods for applying organic Coatings

• Powder Spray coatings

• Electrocoating
• Fluidized Bed
• Dip coating
• Spray coating
• Spin coating
Powder Spray coatings
The basic principle of electro coat is that materials with opposite electrical charges
are attracted to each other. In an electro coat bath a specific charge is applied to
the part that is immersed in paint particles that have an opposite charge.

The paint particles are attracted to the part and are deposited on the part to form
an even, continuous film over every surface until the coating reaches the desired
thickness. Electro coating is complete when attraction stops.

Electro coating is classified as either anodic or cathodic depending on the polarity

of the charge.
Anodic Cathodic
In anodic electro coating, the part to be In cathodic electro coating, the
coated is made the anode (a positive workpiece is made the cathode (a
electrical charge) and attracts the negative charge) and attracts the
negatively charged paint particles in the positively charged paint particlesType
paint bath. your paragraph here.

Major advantage of electro coat over most other systems is its covering ability.
Thickness is easily controlled using simple voltage adjustment. Electro coating lines can
be highly automated, closed-loop systems with excellent productivity and low operating
costs so no direct labour is required.
Fluidized Bed Coating
• Polymer powder in air generated fluidized bed
• Normal or electrostatic FB coater
• PVC, polyethylene copolymers
• Protective, decorative coatings
• Safety glass bottles

Heat part, then dip into FB coater

100 OC for PE copolymer
 Dip Coating

PVC
Spin Coat
Process Summary:
• Wafer is held onto vacuum chuck Photoresist
dispenser
• Dispense ~5ml of photoresist
• Slow spin ~ 500 rpm
• Ramp up to ~ 3000 to 5000 rpm
• Quality measures:
– time
– speed
– thickness
– uniformity
– particles and defects

Vacuum chuck

To vacuum pump
Spindle connected
to spin motor
Spray coating
Quality Control of Industrial Painting Operations

• Industry standards for coating application QC

• Developing a quality control plan for painting
• Navigating a Technical (Product) Data Sheet
• Measuring ambient conditions and surface temperature
• Witnessing mixing, thinning and application procedures
• Calculation and measurement of wet film thickness
• Dry film thickness measurement
• Post-application testing
➢Cure/hardness
➢Holiday/pinhole detection
➢Adhesion
➢Identifying application defects
Industry Standards for Coating Application
• SSPC-PA 1 Shop, Field and Maintenance Painting of Steel
• SSPC-PA 2 (frequency and tolerance of coating thickness
measurements on steel)
• SSPC-PA 9 (frequency and tolerance of coating thickness
measurements on concrete)
• ASTM E337 (use of whirling/aspirating psychrometers)
• ASTM D4414 (wet film thickness measurement)
• ASTM D7091/D6132 (dry film thickness measurement)
• ASTM D5402/D4752/D3363/D1640 (drying, curing, hardness)
• ASTM D5162/D4787 (holiday/pinhole detection)
• ASTM D3359/D6677/D4541/D7234 (adhesion)