Paint film formation process

1. Film Formation
Chapter 4
Marrion, A. Ed., The Chemistry and Physics of Coatings,
Second Edition, The Royal Society of Chemistry, 2004

2. Film Formation
Film Formation is the conversion of the
coating from a liquid into an integral
solid film after application so that the
coated article can start to be usefully
employed.

3. Stages of Film Formation
1. Touch Dry
2. Hard Dry or Block Resistant
Painting of a domestic window frame
• The first stage of film
formation is completed when
the window can be touched
without the paint transferring
to the decorator’s finger.
• The coating is said to be
‘touch dry’ in this condition.
• Should the window be
closed against its frame at
this stage then it is likely that
they would become stuck
together under the pressure
of closure.
• The process of film
formation must proceed
further to avoid this
problem and for the
coating to behave more
like a solid.
• When this condition is
achieved, the coating is
said to be ‘hard dry’ or
block resistant under
those particular conditions
of time, temperature and
pressure.

4. Film Formation Parameters
1. Rate of film formation
2. Extent of film formation
Film Formation process:
How quickly a coated article can be
1. Handled
2. Overcoated
3. Stored
Coating performance:
What properties have
developed by what time
after application.

5. Polymer glass transition temperature (Tg)
• The rationale for film formation behaviour in coatings is best
explained in terms of the polymer glass transition temperature
(Tg).
• The glass transition temperature marks a significant change in
the properties of a polymer, related to the mobility of the
segments from which all polymers are constituted.
• As the polymer is cooled to the T g and below, the co-operative
motions responsible for both translational and rotational motions
in the polymer backbone essentially become frozen, and the
macroscopic flow of the material (such as the transfer of paint to
the finger!) no longer occurs.
Polymer Tg (oC)
Polyethylene (LDPE) -125
Polypropylene (atactic) -20
Poly(vinyl acetate) (PVAc) 28
Poly(ethyleneterephthalate) (PET) 69
Poly(vinyl alcohol) (PVA) 85
Poly(vinyl chloride) (PVC) 81
Polypropylene (isotactic) 100
Polystyrene 100
Poly(methylmethacrylate) (atactic) 105

6. Glass Transition Temperature Tg
Glass Transition
•Property of the amorphous
region
•Below Tg: Disordered
amorphous solid with
immobile molecules
•Above Tg: Disordered
amorphous solid in which
portions of molecules can
wiggle around
•A second order transition
Melting
•Property of the
crystalline region
•Below Tm: Ordered
crystalline solid
•Above Tm: Disordered
melt
•A first-order
transition
•The glass transition is NOT the same as melting.
6

7. Glass Transition Temperature Tg
• Thermodynamic transitions are
classified as being first- or second-
order.
• In a first-order transition there is a
transfer of heat between system and
surroundings and the system
undergoes an abrupt volume change.
• In a second-order transition, there is no
transfer of heat, but the heat capacity
does change.
• The volume changes to accommodate
the increased motion of the wiggling
chains, but it does not change
discontinuously.
7

8. Glass Transition Temperature Tg
The motion that allows a polymer above its glass transition
temperature to be pliable is a long-range segmental motion.
Free volume is the space in a solid or liquid sample which is not
occupied by polymer molecules, i.e. the “empty-space” between
molecules.
Lowering of temperature will reduce the free volume. 8

9. Glass Transition Temperature Tg
• At temperatures well above Tg, 10 to 50 repeat units of
the polymer backbone are relatively free to move in
cooperative thermal motion to provide conformational
rearrangement of the backbone.
• Below Tg, the motion of these individual chains
segments becomes frozen with only small scale
molecular motion remaining, involving individual or
small groups of atoms.
Thus a rapid cooling rate or "quench"
takes rubbery material into glassy
behaviour at higher temperatures (higher
Tg).
9

10. Flow and Tg
• The ability of a material to flow depends on how close its
temperature is to the Tg.
• The larger the (T-Tg) – achieved by a low Tg, a high T or both–
then the greater the mobility or the lower the viscosity.
• If the temperature at which a material is stored is equal to its Tg,
so that (T-T g)=0,
• the mobility becomes very low indeed.
• To all intents and purposes the material is solid with a very high
viscosity (107–1014 Pa s).
• As a rule of thumb, a coating becomes touch dry, with no transfer
of paint to the finger, when Tg is within 20 C of the ambient
temperature.
• As the temperature increases above Tg, the restrictions on
polymer segment mobility become commensurately smaller as
the free volume increases.
• The material is able to undergo liquid flow.

11. Increase in Tg
• It is implicit therefore, in order to convert a liquid
paint into a solid film at a given temperature, the Tg
of the coating must increase during the film formation
process to become equal to and usually exceed the
prevailing temperature.
• There are essentially two mechanisms by which the
increase in Tg can be obtained.
• These are solvent evaporation and chemical reaction.
• Note that some coatings (elastomeric systems) form
films by chemical reaction from components carefully
chosen to yield a product whose Tg is below the
ambient temperature.

12. THERMOPLASTIC COATINGS
• Reasons:
1. Many important polymer properties are achieved only when molecular
weights (MWs) become large enough for chain entanglement to occur.
The values of this entanglement MW, Me, vary significantly for different
polymers and ranges from a few thousand to in excess of 105 g/mol.
2. The viscosity of solutions of these polymers is relatively high and to
maintain application viscosity, the concentration of polymer has to be
low.
3. Many applications are necessary to build up the thick coatings necessary
for some end uses.
4. The environmental legislative pressure to reduce solvent emissions has
meant that such low solids coatings are no longer acceptable in many
industries.
Dissolve a polymer in a
volatile solvent, apply it to
the workpiece and allow the
solvent to evaporate
A traditional method of film formation used by
the coatings industries for
many years
Relatively few coatings technologies rely on simple solvent evaporation
nowadays.

13. Film formation from solutions of thermoplastics
• Occurs via the increase in Tg with loss of solvent.
• Initially, loss of solvent from these systems depends only on
1. The vapor pressure of the solvent
2. How quickly solvent vapor is removed from the immediate
environment and
3. The ratio of surface area to volume of the coating.
• If the application process involves the spraying of
atomised droplets of coating, the loss of solvent
during the transfer of the coating from the spray gun
to the workpiece can be very substantial.
• As the system Tg increases, the rate of evaporation
will become dependent on the rate at which the
solvent can diffuse through the solvent swollen
polymer.

14. • Solvent removal from the free surface sets up a solvent
concentration gradient across the coating to act as the driving
force for diffusion.
• The normal treatments for diffusion of small molecules through
polymers are made more complex for the coating systems where
the final Tg is above the film formation temperature.
• In these systems vitrification will occur at some stage when the
nominal Tg equals the drying temperature.
Rates of diffusion of solvents in the
glassy state are significantly lower
than in the liquid or rubbery phase
which means that glassy coatings
can retain significant amounts of
solvent for long periods – in some
cases for a number of years.
Problem: Coatings are in contact
with food or drink, such as in can
lacquers

15. Film Formation--Air dry (Lacquer)

17. Solvent Evaporation-Tensile Shrinkage Stresses
• Problem Statement: These internal stresses
cannot be easily relieved in thin films adhering
well to rigid substrates, and will weaken the
coating’s resistance to applied stresses or strains
as well as the strength of adhesion to its
substrate.
• Solution: To overcome the problems of solvent
retention described above, stoving at
temperatures considerably in excess of the coating
T g is carried out.
• Solution Limitations: This will not, however,
eliminate stresses from the system when coating
and substrate have substantially different
coefficients of thermal expansion.

18. Limitations of Thermoplastic Coatings
•Environmental pollution from
solvent evaporation
•Poor thermal or chemical
resistance
•Unstable solutions as paints for
semi-crystalline polymers

19. SOLVENTLESS CROSSLINKING SYSTEMS
• Film formation in electrostatic spray powder systems
1. Starts with a solid in particulate form on the
substrate.
2. On stoving, the individual particles soften and
flow so that coalescence takes place. During this
early stage in film formation, it is hoped that
complete wetting of the substrate occurs,
entrapped air is released, and an even film of now
liquid coating covers the surface. Special additives
are used to ensure this is the case.
3. Simultaneously, as the temperature rises in the
oven, the crosslinking reaction starts to occur
giving rise to MW and viscosity builds.
4. Near gelation, flow and levelling cease and the
surface appearance at that time is fixed in place.
Examples
Powder paints
1. Powder paints
2. Solvent-free liquid paints

20. SOLVENTLESS CROSSLINKING SYSTEMS
• There are two major classifications of powder coatings,
• Thermoplastic and Thermosetting.
Thermoplastic powders melt and flow with the application of heat.
• PVC,
• Nylon,
• Polypropylene,
• Vinyl, and
• Fluorinated resins
Thermosetting powders
• Epoxy,
• polyester,
• polyurethane, and
• acrylic and combinations thereof.

22. SOLVENTLESS CROSSLINKING SYSTEMS
• The viscosity and
temperature profile
during stoving might
typically be as shown in
Figure 4.8.
• The so-called ‘viscosity
well’ describes how fluid
the system becomes and
for how long. It is
determined by a number
of factors including
1. Rate of temperature rise,
2. Crosslinking reaction kinetics,
3. Molecular Weight
4. MW build characteristics prior to gelation (which depends on the
functionality of the system) and extent of reaction at gelation.
5. Viscosity
6. Pigment volume concentration of the starting materials.

23. SOLVENTLESS CROSSLINKING SYSTEMS
• Powder coatings are single pack systems where it is assumed each
particle contains all formulation ingredients in the right proportion.
• Storage prior to use places requirements both on physical and
chemical stability.
• To avoid physical sintering, the Tg of the powder needs to be above
about 50 C and depends on storage temperature and hydrostatic
pressure.
Traditional methods of manufacturing powder coatings
Require the use of extruders to mix and disperse the ingredients into
the polymer matrix.
Reaction will inevitably occur in the extruder operating at typically
100–120 C or even higher.
• Degradation of the flow and levelling and hence appearance of the
final coating.
• The problem is exacerbated by the demand for powder coatings
which cure at even lower temperatures for use on temperature-
sensitive substrates.
• Currently, the lower limit on stoving temperatures is about 120–140
C.

24. DISPERSE PHASE POLYMER SYSTEMS
• The problems of using solutions of high MW thermoplastics in
coatings have been successfully overcome by using disperse
phase polymer technology.
Advantages of Stabilised Dispersions of Polymer Particles in an Aqueous
Continuous Phase
1. Low viscosity
2. Low VOC
3. The possibility of using very high MW polymers
The viscosity of a dispersed phase in a
continuous liquid is more or less independent
of the MW of the disperse phase polymer but
depends on the volume fraction of the
disperse phase.

25. Film formation from a latex
• Film formation from a latex is
commonly divided into three steps.
• STEP 1: Firstly, after application of
these coatings to a non-porous
substrate, water and organic co-
solvents begin to be lost by
evaporation, the solids content rises
and particles approach each other
more closely.
• STEP 2: Secondly, for successful film
formation, the particles must
coalesce and this involves
overcoming the hitherto effective
repulsive interparticle stabilisation
forces.
• STEP 3: Finally, film formation
requires diffusion of polymer chains
across the particle interface which,
for ambient systems, means that the
Tg of the polymer in the interfacial
regions must be below ambient.

26. Polymer-in-water dispersion
Close-packing of particles
Water loss
Dodecahedral structure
(honey-comb)
Deformation
of particles
Idealised View of Latex Film Formation
Interdiffusion
and coalescence
Homogenous Film
T > MFFT
T > Tg
Optical Clarity

27. Stages of Latex Film Formation
Dark field optical
microscopy
Atomic force microscopy
TEM

28. Hot
Cold
Clear
Cloudy
Minimum Film
Formation Temperature
(MFFT)
+10°C
-10°C

29. Measuring MFFT

30. Factors affecting Minimum Film
Formation Temperature MFFT
• Lateral and vertical drying
• Particle packing
• Fundamental driving forces for particle
deformation
• Diffusion and particle coalescence
• Factors influencing surfactant distribution

31. Typical Morphologies
AFM Images
5 mm x 5 mm
Randomly-packed array of deformable particles in dry film
Source: A. Tzitzinou et al., Macromolecules, 33 (2000) 2695.

32. Particles are flattened at their boundaries in dry film
Typical Morphologies
AFM Images
Source: A. Tzitzinou et al., Macromolecules, 33 (2000) 2695.

33. Voids in randomly-packed array of particles:
Yet film is optically transparent
Typical Morphologies

34. Property Thermoplastic Thermoset
MW prior to application High Low
MW as dry film High Very high
Crosslink density Very low Moderate-v.high
Hardness Poor-good Moderate-Excellent
Solvent resistance Poor-good Excellent
Chemical resistance Poor-Excellent Moderate-Excellent
Permeability, H2O Very low-high Very low
Gloss Low-high High-very high
Recoatability Excellent Poor-good
Low VOC use Poor Excellent
Film Formation-Comparison

35. SOLUTIONS OF CROSSLINKING POLYMERS
The Crosslinking Process:
• Film formation from solutions of crosslinking polymer systems
combines two processes.
1. Solvent evaporation takes place as described for
solutions of thermoplastic polymers.
2. At the same time chemical reactions take place between
functional groups on polymeric and/or monomeric species
which result in the build-up of MW of the system.
• There are two main classes of crosslinking processes.
1. step-growth reactions
2. chain-growth reactions.
We will consider mainly the
step-growth reactions
involving two different types
of functional groups which
are mutually reactive, in
an alternating
copolymerisation.
At least one of the starting components must have
more than two functional groups per molecule.

36. Importance of Functionality
• The functionality is very important as it determines a limit to the
chemical conversion up to which the polymer system is still
processable (i.e. flows).
• In a typical system, consisting of an A-functional polymer or
oligomer with a B-functional crosslinker, random reactions
between A and B groups will lead initially to chain extension and
branching.
Figure: Formation of a network of
difunctional A species and
trifunctional B species:
(a) before reaction;
(b) forming branched chains;
(c) beyond the gel point;
(d) when reaction is completed

37. Chemically reactive systems
Main Resin Cross-linker
Polyols Amino Resins
Polyols (Poly)isocyanates
Polycarboxylic acids Epoxies
Polycarboxylic acids Carbodiimides
Polycarboxylic acids Aziridines
(Poly)amines, amides Epoxies
Acetoacetate (active methylene) Amino resins
Acetoacetate (active methylene) (Poly)isocyanate
Polyols and polycarboxylic acids Siloxanes
Miscellaneous Combinations

39. Formation of a waterborne 2K PU coating

40. Two of the most important transitions are gelation and
vitrification;
-The resin transforms from a liquid to a rubbery state.
-There is a drastic increase in viscosity.
Gelation: -Point at which covalent bonds begin to connect between
linear chains, forming regions of large networks.
-The reaction continues at a significant rate.

41. Crosslinking Systems
• The mechanical properties of the film depend strongly upon the
1. Tg of the crosslinked polymer
2. Degree of crosslinking
Physical properties such as
•Water and oxygen permeability
•Solvent and chemical resistance
are affected by the degree of crosslink density.

42. Gel Point
The largest molecule in the system progressively
becomes larger and larger until at a certain critical
stage of the reaction its dimensions span the whole
of the reaction vessel. The achievement of this state
is known as gelation.
• Achievement of the gel state leads to the loss of solubility: the
system will swell in a thermodynamically good solvent, but the
crosslinks prevent the chains moving apart and dissolving.
• Beyond the gel point, further reaction leads to the bonding of the
remaining finite molecules to the gel, increasing the crosslink
density.
• Unreacted functional groups on the gel can also react with other
unreacted groups on the gel.
• Eventually, all the precursor molecules may become part of the
network, if the system has been formulated correctly.

44. Crosslink Terminology
• A number of extreme changes accompany crosslinking
• Soluble Insoluble
• Flow Severely reduced flow
• Glass Transition Temperature Increase in Tg

45. Gel Point
• The gel point occurs at a particular extent of reaction dependent
on the ratio of the numbers of functional groups (stoichiometry)
and the effective functionality of the components.
• According to the theory of Flory and Stockmayer, given some
assumptions, the gel point in an A + B copolymerisation is given
by Equation (4.1):

46. Gel: A two-phase structure.
at Gelation:
Liquid
Crosslinked
networks
Catalysed
solution
Vitrification: -Occurs when the glass transition temperature
of the curing resin increases to the current resin
temperature.
-The rate of the cure reaction is significantly reduced,
as further crosslinking requires diffusion of molecules
through the network.
-The final physical phase depends on the temperature
the process has been held at.

47. m
(Degree of cure)
0 1
Gel
Point
For some manufacturing processes, it is important to consider
viscosity of the resin as the cure reaction progresses.
m
Time
Initially the increase will be relatively slow, significantly
quickening as the “gel point” is reached.
During the early stages of cure, the pre-polymeric chains are
combining, and the average molecular weight of the mixture
increasing. The viscosity therefore increases.

49. Thermoset resins may assume various physical forms, or phases
during the cure reaction, depending on the temperature history.
TTT diagram Depicts:
• Important temperatures
• Transitions between states
Kind of like a phase diagram
for metals… remember..
.
Phase Transformations – (Gel and Vitrification)
The influence of temperature and time is best appreciated through
consultation of the Time-Temperature-Transformation (TTT) diagram.

50. Tg is the glass transition
temperature of the fully
crosslinked polymer.

TgGEL is the temperature at
above which gelation occurs
before vitrification.
Tg0 is the glass transition
temperature of the unreacted
components.
Below Tg0 the catalysed solution will be a glassy
solid. The crosslinking reaction can only occur
very slowly, by diffusion (months or years).
Thermosets used in prepregs are stored below Tg0 .

51. Between Tg0 and TgGEL, the catalysed solution is
in a liquid state. Crosslinking occurs until
vitrification, where a transition to a glassy solid is
made. The reaction is very slow thereafter.
Between TgGEL and Tg , the liquid catalysed
solution gels first, ending the possibility for flow.
Crosslinking continues at a good rate until
vitrification.

Above Tg , the liquid catalysed solution gels, but
will never vitrify. The polymer remains in a
rubbery state throughout the curing process. Once
temperature is brought below Tg , vitrification
occurs.



52. At vitrification the resin will not necessarily be 100% cured. Some
amount of the unreacted components remain, and the reaction will
continue very slowly from this point.
The full cure line on the TTT
diagram denotes when the
cross-linking operation is
completed.
Some parts are post-cured
at a higher temperature, for
a significant time, to ensure
full cure, and the best
properties (long times though).
Full
Cure

53. T(z)Q Q Q
Tair Tair
Tair Tair
This heat must be
removed from the part.
The Thermoset Cure Reaction
Not only are these reactions exothermic, but their reaction rates
are also affected strongly by the local temperature of the resin.
Consider the “slab” of thermoset resin curing below:
Thermoset cure reactions are highly exothermic, generating
significant heat during the crosslinking process.

54. Semi-crystalline
thermoplastic
Thermoset
Temperature
HDT Tg
Tm Td
HDT Tg
Td
(Tm)
Hard, stiff Leathery Liquid
Degraded, Char
Degraded
Hard, stiff Semi-rigid

55. Gel Point
• There are a number of methods for identifying the gel point.
• One commonly used (although not universal) rheological
definition of gelation is the point at which the elastic and viscous
components of shear modulus are equal.
In more practical terms gelation is marked by
1. The rapid increase in viscosity and
2. The appearance of a pronounced elastic solid
behavior from a previously viscous liquid.

56. Gel Point
• Film formation in crosslinking systems might be thought to be
complete at gelation since the viscosity has diverged to infinity at
this point.
• This is certainly not the case.
• At gelation there is a large proportion of low MW sol material
and a residual solvent content which depends on the kinetics of
crosslinking and speed of evaporation of solvent. Typically the
system
• Tg will be below the test temperature and the material will
behave as a weak, soft or swollen rubber and may well be so
fragile as to fail to resist the indentation and blocking tests
described earlier.
• Further reaction and loss of solvent will then be necessary before
defined criteria of film formation are met.
• Knowledge of the various rate processes is therefore required to
deepen our understanding of film formation.

57. Kinetic Aspects of the Crosslinking Process
• Many coatings performance parameters depend critically on the
extent of cure which is achieved in the various timescales
available
• In some industries the conditions of time and temperature for
curing of crosslinking coatings are tightly specified.
The reasons for this may be economic
1. Production line speed,
2. Size and rating of ovens,
3. The sensitivity of the substrate to elevated temperatures
4. The traditional practice of an established process.
• Thus, it is more usual for the coatings formulator to tailor his
product to an existing cure schedule.
Any paint manufacturer hoping to introduce a new product in these circumstances
would have to offer significant improvements in cost or performance or both to
induce the customer to make major changes in his operation.

58. • In contrast, there are examples where curing conditions are not
subject to the same control.
Coating products applied to large outdoor industrial structures, such
as
• ships,
• bridges or
• chemical plants,
must achieve acceptable levels of film formation and performance
whether in
• winter or
• summer and
• high or
• low humidity
within production time scales which probably do not change
significantly with the season.
• This can be very challenging.
Kinetic Aspects of the Crosslinking Process

59. Third Aspect: Storage stability of the product before application.
• The customer or user of paint would like indefinite stability in the can,
irrespective of storage conditions, as well as fast complete cure after
application.
For ambient temperature crosslinking systems involving the simple reaction
of functional groups on polymer and curing agent
• The constraints of storage and reactivity are virtually impossible to
overcome –
• In some circumstances it is not unknown for the reaction temperature to
even be lower than the storage temperature.
• Successful strategies have been to separate the reactive components into two
or even three packs which are immediately mixed prior to use, thus negating
the need for long storage times.
• Alternatively, a crosslinking chemistry may be employed for single pack
systems which become operative only by the action of an atmospheric
component such as
1. oxygen,
2. water,
3. or more recently a vaporised catalyst.
Kinetic Aspects of the Crosslinking Process

60. CONSEQUENCES OF VITRIFICATION
• The transformation of the liquid coating to the solid
film has many consequences for the performance
properties of films
1.Internal Stress
2.Solvent Retention
3.Physical Ageing

61. Internal Stress
epoxy–amine coating
• Solvent evaporation
• Chemical reaction.
coatings will tend to shrink during film
formation
volume reduction due to chemical
reaction is 5%.
• While the Tg of the coating is still below the film
formation temperature, there is still plenty of mobility in
the system for the polymer segments and molecules to
accommodate the shrinkage.
• Once the coating has vitrified this is no longer the
case.
• Stresses start to develop in the coating arising from the
adhesion of the film to the rigid substrate.

62. Internal Stress
Environmental factors can also lead to stresses:
• A temperature decrease will lead to shrinkage (tensile) stresses
• A temperature increase will lead to expansive (compressive) stresses.
• Swelling in water or other liquids can lead to stresses as long as the Tg
remains higher than the prevailing temperature.
• As solvent evaporates or reaction occurs, the lateral
movement of the segments and molecules to
accommodate the requirement for a smaller volume
cannot occur as the polymer system adheres to the
substrate.
• The film then behaves as if it is in tension.
• The greater this tensile stress, the less external
perturbation is required to promote unwanted effects
such as delamination or cracking in the coating.

63. Solvent Retention
• The solvent evaporation and crosslinking processes are highly
interdependent.
• In the early stages of film formation when solvent evaporation is at
its most rapid, loss of solvent leads to higher functional group
concentrations and consequently faster chemical reaction.
• Loss of solvent, however, also leads to increased Tg, and lower
species mobility.
• Once the system approaches vitrification, both the chemical reaction
and the evaporation of solvent are slowed considerably.
The result can be
Significant solvent retention – particularly for fast reacting
systems
which can impact on the performance of the coating in a variety of
ways including,
1. Reducing hardness,
2. Increasing water sensitivity (leading potentially to blistering and
corrosion),
3. An inhomogeneous distribution of residual solvent (with many
possible consequences).

64. Physical Ageing
• This is a phenomenon common to all vitrified materials and
arises from the very restricted ability of the polymer segments to
undergo any structural reorganisation in the glassy state.

66. Physical Ageing
• As a material passes into the glassy state, e.g. by cooling, the rates
of structural reorganization (relaxation) are much lower than the
rate of the temperature reduction.
• The system cannot adjust its structure in response to the lowered
temperature.
• It is therefore in a non-equilibrium state and possesses excess
volume.
• During storage in the glassy state, there will be a very slow
reduction in volume depending on how close the storage temperature
is to the Tg.
• This results in an increase in Tg, and thus the process is self-
retarding.