The two-part water-based polyurethane process is used in automotive clear varnishes

Polyurethane products are used extensively in high-performance coatings. Years of success have come from outstanding properties of polyurethane chains, such as good solvent resistance and mechanical properties (hardness
/
flexibility balance), very good adhesion on different substrates, rapid film-forming and drying at room temperature. If they do not contain aromatic structures in polymer compositions, they also have very good weather resistance. These properties make polyurethane-based coatings ideal candidates for applications that require high-quality coating appearance and resistance

such as automotive patch paints and
OEM
coatings
)
.
solvent-based coatings have dominated the market over the past few decades. However, with tougher regulations on
VOC
emissions in most countries, paint manufacturers and raw material suppliers are being forced to develop new technologies to replace solvent-based polyurethane products that are greener but offer the same level of performance. As a result, water polyurethane coatings
(single- and two-part systems) that emerged in the late80s of the

80s are now used in many ways.
two-part water-based polyurethane:
technology challenge
two-group water-based polyurethane system was a real technical challenge 30 years ago
30
and seems unconquerable. For the two-part water-based polyurethane system
, the
unencumbated polycyanate curing agent is dispersed in a polymer network structure that contains polyhydroxyquine (polyols (similar to solvent systems) and is removed from water and co-solvents (Figure
1
). The challenge is to avoid important side reactions between polycyanate phases and water during a "reasonable" time period, which means that there is sufficient time for construction personnel to paint and construct. Fortunately, this problem is avoided because the
NCO
group reacts faster than the hydroxyl hydroxyl group polyols with water, as shown in Table
1
.
the emulsification of the chemical curing agent can be obtained through a high-speed shear mixture of conventional hydrophobic polycyanate. However, this method requires special equipment, and the preferred method is to use an appropriate surfactant system to modifie polycyanate, which can achieve the effect of "spontaneous emulsification" when it is added to a water-based medium. Spontaneous emulsification occurs when two insoluble liquids are put together and no additional energy (stirring and temperature) is required to obtain the emulsion, as shown in figure
2
. The knowledge about the spontaneous emulsification of "oil" phase into water is incomplete, and thorough research is still needed in order to better understand the process and related parameters. Spontaneous emulsification is generally believed to be the formation of a double molecular layer structure (layer phase or bubble) by an organic solution through a surfactant, which, when shaken, leads to the formation of the emulsion
2
.
Although water-based coating technology represents the latest technological level of primers and substrates in the automotive industry, there are limitations to being used as transparent varnishes, which are currently available only in a very small commercially available system. This is due to the limitations of the system originally developed: slower film-forming speed due to the volatilization of water, sensitivity of curing agents to water, surface defects (pinholes, microblisters), etc.
3
. However, a new generation of raw materials and special hydrophosphonates can help us overcome most of our shortcomings so that we can design a high-performance system that truly competes with conventional solvent-based systems in terms of final performance.
the purpose of this article is to improve the performance of different generations of solvent-based transparent varnishes used in the automotive
OEMs
the performance of acrylic melamine coatings and two-part solvent-based polyurethane coatings in automotive patch paints with those obtained using two-part water-based systems.
test
and coating preparation
table
2
describes the different types of transparent varnishes studied and their curing conditions. Varnish
A
is a commercially available acid catalytic acrylic
/
melamine system. Transparent varnishes
B (
high temperature baking system
) and

C (
low temperature baking system
)
for pre-dual-part solvent-based polyurethane systems are designed for
OEM
paint and automotive finishes, respectively. In this study, transparent varnish
B
was considered as a reference for wear resistance: in fact, previous work in our laboratory showed that the use of transparent varnish
B
was subjected to
1,500
hours of accelerated climate conditions
(
Aging Tester
)
test, followed by a car cleaning test with a loss of light rate of less than
20%
. Transparent Varnish
D (
Cryogenic Baking System
)
is a coating based on a two-part water-based formulation designed specifically to meet automotive repair requirements
(
viscosity, drying dynamics and final product performance
)

,
whose performance is described in the discussion below.
the properties of polycyanates used in polyurethane systems can be found in Table
3
,
EsaquaXD401
is a hydrophetamine-hydrophosphate (a composite structure based on
HDI/IPDI
derivatives) specifically designed to improve the drying performance of the two-component water system
4
. For the two-part polyurethane transparent varnish formulation, the ratio of
NCO/OH
is adjusted to
1.05 (
transparent varnish
B
and
C)
, transparent varnish
D
to
1.2 (
water-based system
)
.
all transparent varnishes are sprayed with the commonly used air gun
(DeVilbis SRI)
on a base coating called
black Onyx
" (dry film thickness
12-14um
). Apply the coating system to a metal substrate that has been de-oiled and coated with a commercially available two-part solvent primer two-way slurry (
Q-panel
aluminum plate)
(
dry film thickness
-35-40um)
.
experimental method
all cured systems were stored at
23
degrees C and with a relative humidity of
50%
before the test, the following tests were carried out:

Persuz hardness;

reverse impact;

gloss;

solvent resistance (butyl refracourse wipe);
.
micro-indentation and micro scratch properties;

weather resistance (artificial aging tester); and
-
acid corrosion resistance under
Jacksonville
conditions.
micro-indentation test
5
mechanical properties (elastic mods and hardness) of materials close to the surface. The indentation tester used in this study is a diamond
cone,
Berkovich
1150
. Micro-indentation and micro-scratch tests are conducted using
MTA
'
Nano-Indenter XP
XP.
micro scratch test is carried out according to the procedure of gradually increasing the load, using the standard load increase speed increased from
20
μ
N
to
160mN
, the scratch speed is
1
μ is
500
μ
m
, usually collecting the height profile before and after the scratch, minus the thickness of the undetected coating to obtain the depth of penetration and recording. The remaining scratches are then observed using an optical microscope.
this study, storage

1
months
)
and aging systems
(

with aging testers
1, under
23oC
and relative humidity
50%

) and
(
at23oC
Micro-indentation and micro-scratch test results and discussions on the 500
-hour artificial accelerated aging test
)General performance and final appearance
good mechanical balance (hardness
/
flexibility balance), controlled surface smoothness and brightness, good chemical resistance is a key requirement for automotive transparent varnish. Table
4
shows the mechanical and optical properties of different transparent varnish formulations, as well as the solvent resistance obtained from the test with butyl ketone refruitation wipes. The results show that the three polyurethane transparent varnish systems have very similar properties in terms of mechanical hardness, flexibility (impact resistance), optical appearance and solvent resistance. In particular, the difference between solvent-based and water-based transparent varnishes is not visible. However, transparent varnish
A (
acrylic melamine system
)
has a lower hardness
/
flexibility. This performance is a feature of different
in
example of transparent varnish and A-levels.
Inability
/
Scratchability
In the automotive industry, the damage associated with damage resistance
/
scratching is one of the most important ethical issues that consumers can feel, and it is related to coating durability. It is clear that
/
scratching performance can only be guaranteed by varnish layer.
the behavior of polymer materials during scratching is a complex phenomenon that requires special tools such as micro scratchers to analyze and understand overall performance. The mechanical properties of four different transparent varnishes before and after the artificial accelerated aging test were measured by micro scratch test, and the results were summarized in
5
5. In this table,
E's
is equivalent to the coating's equivalent elastic mould, obtained by the formula, where
E'
is the elastic mould of the material and υ is the Poisson coefficient.
H
corresponds to the hardness value measured with a
Berkovich
indentator at a load speed of
3 x 10-2s-1
.
values

H
are obtained at a depth of
1
μ
m
indentation.
values of
H/E are also reported to represent extreme stress, where plastic deformation is generated by indentation meters.
the value
H/E, the better the material is resistant to plastic deformation during mechanical indentation and scratching. Finally,
n
is the adhesive plasticity index in
Norton-Hoff
's law, which is related to the ratio of hardness and strain:
1/n s 1
: the material exhibits pure viscosity
1/n s0
: the material exhibits pure plasticity (the corresponding hardness ratio has no effect when hardness is measured).
's like
Bertrand-Lambotte
's proven
6
,
n
shows the material's toughness scratch healing ability: the lower the value of
n
, the faster the toughness scratches heal. Transparent varnishes resist plastic deformation and toughness scratch healing (
H/E
and
n
factors) results are reported in Figures
3
and
4
respectively.
results show that transparent varnish
The
system has a lower elastic mod and hardness than the A-level (acrylic melamine system) and lower
H/E
and
n
values. In fact, the transparent varnish
A
shows that the lower hardness and elastic mod are consistent with the previously measured Persuz hardness value. A low
H/E
indicates that transparent varnishes
A
are less resistant to plastic deformation when scratched. On the other hand,
n
value (adhesive plasticity index) implies higher toughness