Preparation of low ice adhesion coatings: Effect of chain length of organic modifier on epoxy resin properties during hydrophobization

For more than a century, resins have been widely used in a variety of industrial sectors such as coatings, adhesives, automobiles, and more
.
Due to their chemical structure, their low cost and maneuverability are very useful
in manufacturing increasingly advanced materials.

A common problem today is the tendency of various materials to wetting, which may eventually lead to further damage to the structure due to atmospheric conditions coatingol.
com
coatings.

Author | G.
Morgiante, M.
Piłkowski, J.
Marczak

Today, scientists are trying to find better ways to improve the properties of coatings or composites, such as hydrophobicity or icing resistance
.
One of them is a fluorinated organic compound, which has good binding properties
to the substrate.

Therefore, in order to improve the properties of the basic epoxy resin, its chains were modified with hydrophobic compounds of different chain
lengths.
The effect
of molecular size of the modifier on the hydrophobicity and ice adhesion properties of the prepared epoxy resin were tested.

The chemical composition
of the prepared paint was analyzed by infrared spectroscopy (FTIR).
Its thermal stability was studied using TG and DSC
.
In addition, the wettability
of the sample was analyzed with a goniometer.
In addition, their adhesion test with ice is carried out
on a specialized device.

The devastating effects of icing occur in many industrial sectors, such as aviation, wind turbines or electric traction
.
Icing can reduce the efficiency of equipment, causing equipment damage and possibly even
crashing aircraft.

Because ice accumulation affects the ability of solar equipment or wind turbines to generate electricity, many of these devices require the use of de-icing systems
.
De-icing can be divided into two categories
: active and passive.

The active solution is to remove ice after ice has been deposited (de-icing system): these technologies include mechanical scraping, heat treatment and the use of de-icing fluids
.
Unfortunately, they all need electricity to work
.

In addition, they are characterized by low efficiency and emit large amounts of carbon dioxide or toxic substances
into the environment.
3Therefore, seek new energy-saving solutions
.
Passive solutions include the possibility of treating surfaces before use to prevent the adhesion of ice (anti-icing systems).

Active methods are currently widely used, but passive methods are cheaper and more environmentally friendly
.
3-5 There is currently no substance that can completely prevent ice or snow from accumulating
on its surface.
Future solutions should have better mechanical properties, zero power consumption and efficiency in all weather conditions, and should come
in the form of durable coatings.

One possible option is to use hydrophobic coatings
.
Epoxy resins have become the most popular coating materials
in engineering fields due to their excellent bonding strength, mechanical properties and water and oil resistance.
6Modified epoxy anti-icing coatings based on low surface free energy have also attracted the attention
of researchers.
7-11

◼Daniel et al.
prepared bio-based epoxy resin
by reacting completely bio-based cashew phenol-modified epoxy resin with furfurylamine.
By adding amino silicone oil as a low surface energy modifier, epoxy anti-icing coatings that are easy to de-icing are prepared, and the strength of the ice coating can be as low as about 40 kPa
.
12

The use of epoxy resin with high bond strength to introduce hydrophobic nanoparticles (such as PTFE and SiO2 fluorinated) into the coating system, and the preparation of superhydrophobic coatings can also achieve anti-icing and de-icing effects
.
9,13

In addition, Rolére et al.
introduced the study
of perfluorinated carboxylic acid as an epoxy resin modifier.
14 They also modified different resins with perfluorinated carboxylic
acids.

In their research, they focused on exploring tetrafunctional epoxy resins and their surface properties
after different curing processes.
The results show that the curing process is the main factor affecting the performance of
the composite.
14,15

In the literature, we can find articles that describe only the wettability of fluoric acid-modified epoxy resins, without ice adhesion analysis
of the prepared material.

Like Miccio et al.
, Glaris et al.
also described a modification method to improve the hydrophobicity of epoxy resin with fluorinated acid16, Miccio et al.
correlated
the fluorine content in epoxy resin with the water contact angle obtained by different combinations of epoxy resin/curing agent and fluorinated acid.
17

As we all know, fluorinated polymers have the characteristics of
low surface free energy and large water contact angle.
18-21In addition, an epoxy resin modification method
using fluorine compounds is introduced.
22,23

Few articles have proposed a correlation between the chemical structure of fluorinated epoxy resins and ice adhesion properties, which is important
for the preparation of low-ice adhesive materials in anti-icing applications.

The most important scientific problem that needs to be solved is reducing the adhesion of ice
.
Therefore, the preparation and characterization
of low ice adhesion coatings are mainly studied.
The coating is based on the chemical modification of commercial epoxy resins to improve their hydrophobicity and ice adhesion properties
.

The effects of fluorination modifiers and their molecular chain length on the wettability and ice adhesion of modified epoxy resins were studied, and the influence of
surface chemical composition on surface free energy was also confirmed.

In addition, FTIR, DSC and TG measurements were performed to evaluate the chemical composition and thermal parameters
of the obtained materials.

Experimental methods and materials

Part.
1

material

Epoxy resin was prepared using commercially available Epidian 5 resin
and IDA curing agent supplied by CIECH Sarzyna S.
A.

The modifiers used in this study were perfluoroheptanoic acid "C7" (98%, ABCR), heptadecyconic acid "C9" (>95%, TCI Chemicals) and perfluorodecanoic acid "C12" (96%, ABCR).

These samples are named after the abbreviations of the modifiers used (Figure 1).

Part.
2

Epoxy coating preparation

Epoxy coatings
are prepared by mixing epoxy resin (Epidian 5) with each modifier at a specific mass ratio (4% by weight of modifier).
Modification is carried out in a water bath at 80 °C for 2 h
.
The prepared modified epoxy resin is cooled
at room temperature.

Then the curing agent is added to the modified resin according to the mass ratio of 0.
5:1 and mixed
.
The prepared mixture is then deposited on microscope coverslips and cured at room temperature for 48 h
.

The performance
of the resulting paint sample is then checked.
For the reference sample, an unmodified epoxy resin was used for further investigation, which was prepared by mixing Epidian 5 resin with a hardener IDA at a 0.
5:1 mass ratio, named "Reference Material"
in this paper.

Figure 1 Structure of the fluoroalkyl modifier used: (a) perfluoroheptanoic acid - "C7", (b) heptadecycinolnonanoic acid - "C9", (c) perfluorodecanoic acid - "C12"

Part.
3

Characterization

The wettability of the sample was determined on a goniometer and the anti-icing
of the sample was measured on a special ice adhesion device.
The correctness of the modification was investigated by infrared spectroscopy, and the thermal performance of the obtained sample was characterized
by DSC and TG tests.

Part.
4

Water Contact Angle (WCA) measurement

and Surface Free Energy (SFE)

At room temperature and humidity of about 50%, the wettability
of the paint was determined by a goniometer (Data Physics OCA 35).

The contact angle (CA) is measured using the sessile drop method, in which 5L of distilled water droplets are placed at 5 different points of the sample to ensure surface uniformity and maintain statistically correct results
.

In addition, the surface free energy (SFE) is calculated
by the van OSS–Good method.
Calculations were determined by standard liquid settings: distilled water, diiodomethane (99%, Sigma-Aldrich CAS: 75-11-6), and glycerol (99.
5%, Sigma-Aldrich CA: 56-81-5).

Part.
5

Ice adhesion determination

The measurement of ice adhesion is based on a simple tensile test method
.
To keep the room temperature stable (19°C, 39.
7% humidity), each test was performed under closed conditions (Figure 2).

Figure 2 Ice adhesion measuring device and its components: (1) tensile meter; (2) Ice cubes (2×2 cm2); (3) Samples; (4) Peltier module; (5) Cooling system
.

They used water ice cubes (2×2 cm2, frozen at −19 °C with 5 ml of deionized water, using a silicone mold), hung on a stretch drive, and then frozen on the cooled sample surface (2 mm thickness) and held for 10 min
.

The sample is first placed in a cooling system
maintained at about −10°C throughout the process.
Press the ice cubes (density 830 kg/m3) against the surface for 30 sec to eliminate possible void distances
between the force detector and the sample.
Then, the separation force separating the cube after a set time is calculated as the adhesion force by a simple formula:

where Fs is the separation force [N] required to separate the ice from the surface, and A is the interface area [mm2]
of the ice on the specimen.

Each sample is measured at least three times to obtain statistically correct results
.
The tensile meter (S2M/10N) is used in the equipment and the accuracy level is 0.
02
.
The loading rate is 0.
5cm/s
.
The cohesive strength of the ice cubes is always higher than the bonding strength
of the interface.
The cube has no cracks and shows no signs of
being crushed.

Part.
6

Fourier transform infrared (FTIR) spectrometry

The structural changes of the modified epoxy resin chain were characterized
by Bruker FTIR Tensor 27.
An infrared spectrum
of the absorption emission of each modifier was obtained using an infrared spectrometer equipped with the MIRacle TM ATR accessory.
At room temperature, each coating is scanned
in the range of 450-4000 cm−1.
The results obtained are used to determine whether the modification occurred
correctly.

Part.
7

Thermogravimetric determination (TG)

Thermogravimetric analysis of the cured resin was performed with Mettler ToledoTGA2 to determine the thermal stability
of the sample.
Weigh 5 mg sample and place in
a corundum crucible.
The measurement is carried out under nitrogen protection and the temperature range is 25 °C ~ 460 °C
.
The heating rate is set to 10 K/min
.

Part.
8

Differential Scanning Calorimetry (DSC)

DSC analytical measurements
were performed on the curing reaction using the differential scanning calorimeter (Mettler Toledo DSC1).
Approximately 3.
5 mg of sample is placed in a crucible and heated
.
Under the protection of nitrogen, the curve
was recorded at a heating rate of 10k/min in the range of 0~100 °C.

Results and discussion

Part.
1

Water Contact Angle (WCA) measurement

and Surface Free Energy (SFE)

In this paper, epoxy resin (Epidian 5) was modified by fluoric acid with different molecular chain
lengths.
The effects of
material modification on their final hydrophobicity and ice adhesion were studied.
All prepared paint samples were analyzed and chemical modification was analyzed
for their hydrophobicity.
An unmodified epoxy resin was used as a reference sample
.

First, measure the water contact angle
.
The results obtained are shown
in Figures 3 and 4.
The modified sample with "C12" has the highest contact angle of water (106±4°) and the lowest surface free energy (17.
9±5.
2mJ/m2).

It was also observed that the modification process ended successfully, with significantly higher results for WCA and lower results for SFE in all modified samples compared to reference coatings (unmodified epoxy), although only "C12" showed hydrophobic properties (WCA greater than 90°).

It can be seen (Figures 3 and 4) that the molecular hydrophobic properties of the modified sample increase
as the length of the modifier chain increases.
17 It is worth emphasizing that the "AB" component in the surface free energy of the modified sample decreases with the increase of the molecular chain length of the modifier
.

In addition, the SFE component "AB" of the modified sample was lower than the "LW" (Lifshitz–van der Waals interactions) component, indicating a greater
proportion of long-term interaction between the coating and the liquid.
This means that hydrogen bonds are less likely to form
.

The weaker interaction between the paint and the liquid also affects the cleaning performance of the resulting coating, which is improved after modification while the "LW" component remains at a similar level
as all modified coatings.

The "AB" change may be due to the addition of hydrophobic fluoridation chains to the resin backbone, resulting in a decrease
in polar groups in the resin.
WCA and SFE vary almost linearly, indicating that longer fluorinated modifier molecular chains enhance the hydrophobicity
of modified samples better than shorter fluorinated modifier molecular chains.

Figure 3 WCA values
of the resulting reference sample of the coating and unmodified epoxy resin Epidian 5.
The epoxy resin Epidian 5 was modified with perfluoroheptanoic acid "C7", heptafluorononanoic acid "C9" and perfluorodecanoic acid "C12"
.

Figure 4 The surface free energy value
was calculated using the components of the SFE calculation, "AB" (acid-base component) and "LW" (Lifshitz–van der Waals interaction).
The values of the resulting SFE and its components were edited
with the unmodified, cured epoxy Epidian 5 control sample.
The epoxy resin Epidian 5 was chemically
modified by perfluoroheptanoic acid "C7", heptadecyfluorononanoic acid "C9" and perfluorodecanoic acid "C12".

Part.
2

Ice adhesion

The hydrophobic properties of the modified epoxy resin coating were improved, so ice adhesion tests were performed
on samples with ic-thinning potential.
The ice adhesion measurement results are shown in
Figure 5.

Figure 5: Results of ice adhesion test results for the resulting coating

Hejazi et al.
point out that to determine whether the material is ic-resistant, its adhesion to the surface ice layer must be less than 100 kPa
.
25

As can be seen from the values in the figure, although the efficiency and performance of the reference sample still needs to be improved, it is considered to meet the requirements
of slight icing.
It can be further found that with the increase of the molecular chain length of the fluorine compound modified by epoxy resin, the ice adhesion value of the obtained sample decreases
almost linearly.
This correlates with WCA and SFE measurements of the supplied sample (Figure 3, Figure 4).

The ice adhesion value of "C12" modified paint differs from that of unmodified epoxy resin paint by more than 15 kPa
.
The modification process improved the hydrophobicity and icing resistance of the resulting sample, and it can be seen from the values in Figures 3 and 5 that the longer the modifier molecule, the higher the
hydrophobicity and icing resistance.

Mirshahidi et al.
point out that untreated steel and aluminum substrates are characterized by high ice adhesion values (about 1300 kPa and 1400 kPa, respectively).

The measurement method is similar to the one discussed in this article, but the size and shape of the test material and ice cubes are completely different
.
The results obtained cannot be compared equivalently, but differences between
unmodified and modified samples can be seen.
The ice adhesion value of the unmodified surface was significantly higher than that of the hydrophobic modified sample
.
26

Part.
3

Fourier transform infrared (FTIR) spectroscopy

The correctness of the chemical modification process was verified by FT-IR spectroscopy
.
Figure 6 shows FTIR spectra
of unmodified and modified epoxy resin coatings.
Colors have been calibrated on each sample, from the reference coating (black) to "C7" (red), "C9" (green), and "C12" (blue).

Figure 6 FTIR spectra of the obtained coatings: 1) control sample-cured unmodified epoxy resin Epidian 5; epoxy resin Epidian 5 modified by 2) perfluoroheptanoic acid "C7", 3) heptadecycnonanoic acid "C9", 4) perfluorodecanoic acid "C12" modified

The peaks observed around 3000-2850 cm−1 correspond to C-H in the -CH, -CH2, and -CH3 groups, which are expected to be strongly present
in the spectrum given the structure of the epoxy resin used in the study sample.
27,28 about 1735~1750 cm−1 region (A peak) and 1300-1400 cm−1 region (C peak) can correspond to C=O
extending from the ester group.

The peaks listed as punctate (B peaks) in the range of 1550 to 1600 cm−1 are characteristic of the C=C extension of aromatic ring absorption, and also the characteristics of N-H bending (1570 cm−1), as a marker of the amine group in the curing agent,29 indicating that crosslinking has been completed
.

Samples 2, 3, and 4 peaked between the 1130 and 1160 cm−1 ranges (labeled D) and have been calibrated as C-F
from -CF2 and -CF3.
This means that the modification has successfully occurred because the reference sample has no peak at
that wavelength.
The spectrum of the modified resin shown (Figure 6) shows that the correct modification process has occurred, which is later checked
with DSC analysis.

Part.
4

Thermogravimetric analysis (TG)

The data obtained by thermogravimetric analysis allows further DSC analysis and helps to characterize the thermal resistance
of the prepared coating.
Each resin sample is measured and then compared to each other to confirm the differences between the modified and unmodified cured resins and to check the threshold of
the DSC temperature range.
The curve is divided into two-step decomposition intervals to determine the local maximum decomposition temperature and the maximum decomposition temperature
after overall stabilization is reached.
The measurement results are shown
in Figure 7.

Figure 7 TGA and DTG comparison of coatings

Comparing the DTG results of the control sample and the modified resin, it can be seen that the increase of the chain length of the modifier may be a factor affecting the thermal resistance of the material, but the effect is not significant
.

This may be due to the increase in structural density, delaying the decomposition of "C7" (red), "C9" (green), and "C12" (blue), with the first ("C7") changing the most
.
It is clear that all samples exhibit similar thermal properties and two-step decomposition (Figure 7).

Even though the temperature of 13 - 15% mass has shifted by about 30 °C, the maximum decomposition (78 - 85% mass) temperature has been recorded at the same value as the other mixtures (369 °C
).

This means that the modifier used in the modification process can delay the onset of decomposition, but cannot be considered a complete decomposition inhibitor because all samples have been completely decomposed
at 460°C.

Part.
5

Differential Scanning Calorimetry (DSC)

DSC measurements are performed on unmodified and modified epoxy samples to check the curing process
.
The plot is divided into two parts to show the observed Tg change and compare the heating steps shown in Figures 8 and 9, as the latter is to check whether the previous heating and cooling are sufficient to determine the glass transition temperature Tg
.

Figure 8: DSC results of the first heating cycle of epoxy coatings

Figure 8 shows that the Tg value does not change significantly
.
The glass transition temperature of the control coating was 59.
63 °C, and the glass transition temperature of C7, C9 and C12 samples was 57.
60 °C and 57.
55 °C, respectively.
and 56.
82 °C
.

A slight decrease in Tg may be the effect of changes in crosslinking density, as described by Miccio et al.
, and an increase in the length of fluorinated alkyl chains grafted into the resin may be responsible
for this phenomenon.
17

After cooling, a reheating cycle is performed (Figure 9).

Since the test was performed for a multi-component system, the second heating showed the result of structural reorganization of the sample, which was achieved by breaking the Tg value in the first cycle (Figure 8).

There was a similar increase in the glass transition temperature (median ISO) for all coatings (e.
g.
, sample "C9" from 57.
55 to 61.
33°C), showing the structure of
an efficient crosslinked polymer.

Figure 9: DSC results of epoxy coatings after the second heating cycle

Conclusion

This article describes the effect of fluorinated acid on the modification of
epoxy resins.
In addition, the chain length of the modifier and its influence
on the hydrophobic process of coatings are studied.
The synthesis of matrix with fluorine-containing alkyl compounds was successfully performed and confirmed by Fourier infrared spectroscopy (FTIR) testing
.

• After chemical modification of epoxy resin (Epidian 5), its hydrophobicity, anti-icing and surface free energy were significantly improved
  .
  

• The higher the content of fluorine groups in the modifier molecule used in the modification process, the higher
  the hydrophobicity and icing resistance of the resulting sample.
  

• The prepared coating is characterized by a slight change in thermal stability due to an increase in the initial decomposition temperature
  , although its complete decomposition temperature remains constant.
  

• The disadvantage of adding fluorinated molecules to the resin backbone is that the crosslinking density and glass transition temperature decrease
  with the increase of the -CF group.
  Careful consideration of fluorine content should be given to avoid excessive glass transition temperature variations, especially when
  the material is operating at low temperatures.