Removing gaskets from the flange connection can be a tedious and labour intensive task which can result in flange surface or equipment damage. When using hammer and chisel to remove a gasket from the flange surface, one can damage the latter. The amount of time spent on cleaning flanges can be significant, and in many cases, the plant must remain off-line while the gaskets are being removed, creating a loss of valuable production time. It has an important impact on maintenance costs and plant efficiency. To avoid these problems using gasket materials with a proper anti-stick coating is recommended.

1. SPECIALLY
DESIGNED
ANTI-STICK
COATING
4AS

2. DONIT® | 4AS Specially designed anti-stick coating
The main goal of static gasket materials used in flange
assemblies is to assure the required level of tightness.
In order to fulfill this requirement especially at elevat-
ed temperatures the correct gasket material has to be
selected together with the best installation procedure in
order to obtain:
• sufficient compression on the gasket,
• sufficient deformation of the gasket in order to adapt
to the flange surfaces, compensating for any irregu-
larities.
In addition to these requirements technicians nowadays
demand that the gasket should be easily removed from
the flange surface during maintenance operations, with-
out destroying or even damaging the gasket and with no
remaining debris sticking to the flanges.
Figure 1 shows incorrectly selected gasket material
after removal from the flange.
Figure 1: Damaged gasket without anti-stick coating
WHY DO GASKET MATERIAL STICK TO THE
FLANGE SURFACES?
The rubber binders used in fibre gaskets are acting as
visco-elastic materials that tend to flow at elevated tem-
peratures and surface pressures. As the binders ‘wet
out’ and make direct contact with the metallic surface of
the flange, chemical adhesion, mechanical interlocking
and other modes of adhesion come into play.
WHY TO USE ANTI-STICK COATING?
Removing gaskets from the flange connection can be
a tedious and labour intensive task which can result
in flange surface or equipment damage. When using
hammer and chisel to remove a gasket from the flange
surface, one can damage the latter. The amount of time
spent on cleaning flanges can be significant, and in
many cases, the plant must remain off-line while the
gaskets are being removed, creating a loss of valuable
production time. It has an important impact on mainte-
nance costs and plant efficiency. To avoid these prob-
lems using gasket materials with a proper anti-stick
coating is recommended.
HOW TO SELECT THE RIGHT
ANTI-STICK COATING?
It is well known that PTFE, silicone and graphite are
excellent anti-stick coatings. However the problem with
these coatings is that they cause a reduction of the fric-
tion coefficient between the gasket and flange surface
in radial direction. This effect results in a decrease of
thermo-mechanical properties of the gasket material.
It can also lead to the reduction of gasket life time in
the flange assembly or to the destruction of the gasket
during assembly or in the first heating-up cycle. There-
fore it is recommended to use an anti-stick coating
which allows easy removal of the used gasket from the
flange surfaces and at the same time maintaining its
thermo-mechanical properties.
TESTING OF ANTI-STICK COATINGS
Friction test
Measuring normal and shear force, with the device
shown on Figure 2, we can obtain the value of friction
coefficient. With a lower value of friction coefficient, the
material is more prone to creep and extrusion even
during assembly at room temperature.
Figure 3 shows results of a friction coefficient test. Fric-
tion coefficients between gasket material with different
anti-stick coatings and flange surface are shown in
Figure 4.
Conclusion: 4AS anti-stick coating has negligible effect
on the friction coefficient.
Figure 2: Test rig for determining friction coefficient

3. Figure 3: Test of friction coefficient 
Figure 4: Friction coefficient for different coatings
Hot compression test
Resistance of gasket material treated with different an-
ti-stick coatings on temperature under constant surface
load is obtained with hot compression test. The test
rig is shown in Figure 5. From the graph (Figure 6) it is
evident that the gasket with 4AS anti-stick coating offers
the highest resistance under hot compression, while the
silicone coated gasket performs very poorly.
Figure 5: Test rig for hot compression test
Figure 6: Course of hot compression test
From the results of hot compression test (Figure 7) can
be seen that gasket with silicone coating will be crushed
at 50 MPa and at 80 °C, while gasket with 4AS at 50 MPa
remains still intact at 170 °C. The thermo-mechanical
properties are a little bit lower in comparison with the
material without anti-stick coating but otherwise still
sufficient.
Figure 7: Results of HCT tests
Figure 8 shows a visual comparison between gaskets
coated with silicone and 4AS. The gaskets were com-
pressed with surface pressure of 50 MPa and exposed to
temperature of 170 °C.
SILICONE 4AS
Figure 8: Gasket with silicone and 4AS coating
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8-45000
-40000
-35000
-30000
-25000
-20000
-15000
-10000
-5000
0
01 02 03 04 05 06 07 08 09 01 00 1101 20
Force[N]
Time [sec]
Normal Force [N]
Shear Force (r=125mm) [N]
0,0
0,5
1,0
1,5
2,0
2,5
silicone PTFE 2AS 4AS none
FC 0,13 0,75 2,00 2,28 2,35
Frictioncoefficient[/]
Type of AS coating
Friction coefficient for different AS coatings
0
200
400
600
800
1000
12000
10
20
30
40
50
60
30 60 90 120 150 180 210 240 270 300
Temperature [°C]
HCT test
Stress
none
2AS
4AS
PTFE
silicone
h [mm x 10-3]
0
60
120
180
240
300
3600
10
20
30
40
50
60
0 2 4 6

[MPa]
Time [min]
h [mm x 10-3]
0
50
100
150
200
silicone 2AS PTFE 4AS none
Tmax [°C] 80 120 140 170 200
Temperature[C]
Type of AS coating
Tmax at 50 MPa

4. 4AS Specially designed anti-stick coating | DONIT®
Stress-strain test
Stress-strain test shows resistance of gasket materi-
al against surface load under constant temperature.
Stress-strain tests were done for gaskets treated with
different anti-stick coatings at room temperature,
100 °C, and 200 °C (Figures 9, 10, 11).
Figure 9: Stress-strain test at room temperature
Figure 10: Stress-strain test at 100 °C
Figure 11: Stress-strain test at 200 °C
Figure 12 shows the load capacity of gasket material
with differently treated surfaces at room temperature,
100 °C, and 200 °C. 4AS offers the best mechanical
resistance trough all temperature ranges compared with
2AS, PTFE and silicone coatings.
Figure 12: Overview of stress-strain tests
Anti-stick test
With anti-stick test we determine the force required for
flange separation from the gasket. In Figure 13 we can
see anti-stick tests results for gaskets with different
anti-stick coatings.
Figure 13: Force required to separate flanges 
Figure 14: Flange separation
4AS anti-stick coating offers very low separation force
while maintaining its thermo-mechanical properties
compared to silicone or PTFE coating.
0
100
200
300
0
100
200
300
0 0,5 1 1,5 2 2,5 3 3,5 4
Temp. [°C][MPa]
Time [min]
Stress‐strain test at RT
Surface stress [MPa]
Temperature [°C]
0
20
40
60
80
100
0 20 40 60 80 100 120 140 160 180 200 220 240
h [%]
 [MPa]
PTFE silicone none 2AS 4AS
[MPa]
[MPa]
[MPa]
0
50
100
150
200
250
silicone PTFE
2AS
4AS
none
Max surface stress [MPa]
silicone PTFE 2AS 4AS none
[MPa] 30 38 44 50 58
[MPa] 51 65 76 81 118
[MPa] 108 172 238 240 240
Load capacity of gasket material
with differently treated surfaces
VO RT
BO 200°C
BO 100°C
BO 200°C
BO 100°C
VO RT
0
50
100
150
200
250
silicone PTFE 4AS 2AS none
F [kN] 10 16 22 112 220
Force [kN]
Type of AS coating
Force needed to separate the flange surfaces
0
100
200
300
0
100
200
300
0 0,5 1 1,5 2 2,5 3 3,5 4
Temp. [°C] [MPa]
Time [min]
Stress‐strain test at 100°C
Surface stress [MPa] Temperature [°C]
0
20
40
60
80
100
0 20 40 60 80 100 120 140 160 180 200 220 240
h [%]
 [MPa]
2AS PTFE 4AS silicone none
0
20
40
60
80
100
0 20 40 60 80 100 120 140 160 180 200 220 240
h [%]
 [MPa]
4AS silicone PTFE 2AS none
0
100
200
300
0
100
200
300
0 0,5 1 1,5 2 2,5 3 3,5 4
Temp. [°C] [MPa]
Time [min]
Stress‐strain test at 200°C
Surface stress [MPa] Temperature [°C]
FF

5. DONIT® | 4AS Specially designed anti-stick coating
Furthermore, with this test we can try to remove the
gasket from the bottom flange. The state of the gas-
ket after removal depends on how strongly the gasket
sticks to the flange surface. Figure 15 shows the state
of the flange surfaces and the gasket after removal.
The gasket was without anti-stick coating. Because the
gasket stuck to the flange surfaces the chisel was used
to remove it. Gasket was damaged during removal and
some remains were still on the flange surfaces.
Figure 15: Gasket without anti-stick coating
Figure 16 shows the flange surfaces and gasket after
removal of gasket with 2AS anti-stick coating. Gasket
could be removed in one piece with minor damage on
the outer edge. There was only slight residue on the
flange surfaces.
Figure 16: Gasket with 2AS anti-stick coating
When removing gasket with 4AS anti-stick coating
(Figure 17) there is no trace or remaining debris on the
flange surfaces. Gasket could be easily removed in one
piece without any damage to the gasket.
Figure 17: Gasket with 4AS coating
WHY 4AS?
Because 4AS is a specially designed anti-stick coating which meets all requirements of good sealing performance
and maintenance practice. It does not affect thermo-mechanical and sealing properties of the gasket material and it
enables easy removal from flange surfaces during maintenance operation.
The quality of 4AS is achieved by a special selection and combination of ingredients in combination with a special
process for applying this coating to the sheet material surface. This requires the knowledge of the interrelations
between the surface properties and functional characteristics of the gasket material. Intensive research has been
performed to determine the influence of the composition of anti-stick coatings with regard to the adhesion on metal
surfaces and in relation to functional characteristics of sealing materials.

6. Copyright © DONIT TESNIT, d.o.o.
All rights reserved
Date of issue: 07.2017 / 4AS-05-2015
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