Sticking Troubles of O-Rings andCountermeasures
1. Introduction
The rubber O-ring is a seal used for both static seal
products （gaskets） and dynamic seal products２-1）Sticking of O-rings（packings）. It is made with soft elastomer, and so fits well

When a rubber O-ring contacts with its opposing face,to opposing faces and allows sealing with little tightening

such as a metal surface, under pressure, it sometimes

force. The O-ring also has the following other benefits:

sticks to the face. When the sticking strength of this• It can be used in various conditions, from highphenomenon is small, the phenomenon is sometimes

pressure to low pressure.

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2. Sticking Phenomenon andits Causes

called adhesion. Depending on the usage condition, it• It requires little space for mounting.

may stick sufficiently strongly to cause material• The structure of the mounting section is simple,

destruction of the rubber upon removal. In the case of

making mounting easy.

static seal products, the sealing properties themselves• It is standardized, readily available, and low cost.

rarely cause problems even if sticking occurs.

Thanks to these benefits, the rubber O-ring is the mostHowever, sticking may result in a）a very large force

widely used general seal product in various equipment

required to open flanges at changing O-rings, and b）

and devices in a wide range of industries including thein the case of a large-bore O-ring, the flanges cannot

automobile, hydro-pneumatic, machine-tool, food,be opened due to the large contact area with thesemiconductor, and aerospace industries.

opposing face, which creates substantial sticking force.

Rubber is an inherently elastic material and so recovers

Also, maintenance problems may occur, such as

to its original shape. In addition to elasticity, rubber has

cleaning difficulties if the sticking rubber ruptures:

non-slip, sticky properties. With these useful

the ruptured rubber sticks to the grooves andcharacteristics, rubber is used to produce tires, hoses,

opposing faces when flanges are opened.

belts, and so on. On the other hand, these characteristics

On the other hand, in the case of dynamic seal

could cause malfunction cases when handling products,

products, sticking may cause O-rings to detach from asuch as difficulties in mounting and dismounting. In thegroove and directly affects their sealing properties. In

case of the rubber O-ring, these problems cause

addition, sticking may cause equipment toproducts to stick to each other and to stick to theirmalfunction, including operational delays, abnormalopposing faces.

noise, and abnormal oscillation. Thus, sticking may

This report describes the causes of O-ring sticking anddirectly result in equipment malfunctions.

countermeasures, and introduces the products thatValqua has developed to reduce sticking troubles.

２-2）Mechanism of Sticking
Sticking is considered to occur by the following
process: when rubber contacts its opposing face,molecules of a rubber segment spread and stick to the43Technical Papers

opposing face, forming an interface through secondary

are categorized in Figure 1.

bonds（including hydrogen bonds and Van der Waals
Anti-sticking
methodsforce）. Thereafter, the initial sticking （physicalsticking）occurs. There are no clear categories ofsticking, and there are various theories about sticking.
However, it is sometimes called adhesion when thedegree of sticking is small. Also, the initial sticking
may be affected by the anchor effect, vacuum effect,and so forth. It is considered that primary bonds

Compounds

Surface
improvement- Oil bleeding- Additives- Coating- Surface finish

Figure1

Polymer
design- Molecular weight andcomposition- Polymer alloy

Categorization of sticking-prevention methods（including chemical bonds）subsequently start to form

３-1）S ticking-prevention Using Material Combination

at the interface and the sticking strength graduallyThis method reduces sticking by mixing aincreases, leading to strong sticking （chemicalcombination of the following selected internal1,2）

sticking + physical sticking）.

additives to rubber: solid lubricant including PTFE,
lubricating components, releasing components, or２-3）Factors Affecting Sticking

other chemicals. Generally, these formulated chemicals

The sticking strength varies depending on the type ofaffect the physical properties of rubber, compression

rubber and the material of the opposing face.

set, and others, so the characteristics of each additive

However, other major factors affecting sticking

should be balanced.

and humidity conditions, and the surface roughness of３-2）Sticking-prevention Using Surface Modification

the rubber and its opposing face. Other factors

This method reduces sticking by making the surface３）of the rubber resistant to sticking. There are two

Based on these reports, major factors are categorized

approaches for doing this: coating to form a sticking-affecting sticking strength have been reported.
as follows:４）• Sticking strength decreases with increase ofhardness of rubber.
• Sticking tends to occur with increasing temperature,
although this is not always true.

Spring 2017

strength include the hardness of rubber, temperature

resistant thin film on the surface of rubber, andsurface treatment using chemical immersion orplasma/radio irradiation. The types of coating include
the following: a coating that physically adheres to thesurface and a coating that forms a thin film through• The effects of humidity vary depending on thereaction with the surface of the rubber to enhancecombination of rubber and its counterpart material.

adhesion to the base rubber. The thin film should• Sticking strength decreases with increasingexhibit chemical affinity with the base rubber. A

roughness of the surface of both rubber and metal.

typical surface treatment is surface hardening: rubber

Contact area affects this phenomenon.

is immersed in chemical solution containing avulcanizing agent and then heat is applied for

3. Sticking-prevention Methods

hardening. However, the area modified by thisSimple methods of preventing sticking include

surface, and frictional wear of the modified layer

greasing and lubrication. However, these processes

results in the loss of this preventive effect. Thiscan contaminate products and mounting sections. To

approach should be used with care when applied toshorten the operation time as well to as prevent

dynamic seal products.

approach is limited to the area close to the top

contamination, O-rings which inherently prevent
sticking are required.

３-3）Sticking-prevention Using Polymer Design

Major methods of preventing the sticking of O-rings

This method reduces sticking by improving the44No.32

Sticking Troubles of O-Rings and Countermeasuresmaterial polymer of the O-ring itself. Generally,• Prevention of twisting and damage in O-rings

polymers with high molecular weight are considered
chains and end groups（including -COOH, -OH, and>CO）.
Also, this method can be performed by giving apolymer a structure which inhibits molecular-chain- Untreated- New Rubberflon

Bonding strength (gf)to stick less, as well as polymers with fewer sidemotion（e.g. a structure with a high glass transition
which has a structure to reduce sticking while
retaining the elasticity of rubber. Further moleculardesign investigations are expected by new polymer
design and polymer alloys with new structures.
Valqua has developed various methods to prevent andreduce sticking in its products. Some of our key
products are outlined below.

4. New Rubberflon
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New Rubberflon （Rubberflon is our registeredtrademark）is an example of our products that aredesigned to prevent sticking.
New Rubberflon is produced using the surfacemodification method, not just a coating. Because ofNo.32

reactive treatment, the treated surface layer adhere
with matrix rubber, thus not easy to remove.
Moreover, the method does not affect the physicalproperties of the base rubber, so most rubber

Test method: tack testing device
The strength required to detach the probe from the test pieces, after
being in contact under certain conditions, is measured.
Test piece: acrylic rubber sheet of 2 mm thickness; weight: 100 gf;
probe: stainless probe, Φ 5 mm; velocity of probe removal: 600
mm/minFigure2 Sticking strength of a rubber treated with New Rubberflon

Coefficient of kinetic frictionpoint）. However, it is difficult to develop a polymer- Untreated-New
Rubberflon

Test method: ball indenter test, testing velocity: 60 mm/min
Test piece: rubber sheet of 2 mm thickness, weight: 200 gf, ballindenter: SUS Φ 6SBR: styrene-butadiene rubber; NBR: nitrile rubber;
FKM: fluorocarbon rubber; IIR: isobutylene-isoprene rubber;
EPDM: ethylene propylene rubber; CR: polychloroprene rubber;
ACM: acrylic rubber; U: polyurethane rubber

Figure3

Coefficient of dynamic friction of a rubber treatedwith New Rubberflon

materials such as nitrile rubber and fluororubber
except for silicone rubber can be processed with thismethod. New Rubberflon offers both low sticking andfriction. Thus, the product not only prevents sticking

5. D2370, a Non-adhesive  Fluororubberbetween products or to the opposing face, but also

Fluororubber has excellent heat and chemical resistance,

reduces insertion friction when mounting O-ringsand so is used in a broad range of industries. Generally,

Figure2 showed

fluorine is considered to be an adhesion-resistant,values obtained by experiment to measure New

slippery material. However, contrary to expectations,

Rubberflon’s sticking strength. Figure3 showed values

fluororubber often sticks to the opposing face, and so isobtained by experiment to measure friction coefficient.

not an adhesion-resistant material.

The treatment of O-rings with New Rubberflon isD2370 was developed using our unique compounding

expected to provide the following benefits:

technique. The product reduces sticking while• Prevention of sticking between O-rings

maintaining the characteristics of conventional• Prevention of sticking to the opposing face

fluororubbers including heat resistance and physical• Improvement in fit of O-rings（greaseless）

properties.

including cylindrical-surface seals.

45５,６）

Technical Papers

Table1 showed the physical properties of D2370, andimportant factor which affects maintenance seriously.

Figure4 showed its sticking strength measured byHowever, there also are cases where sealing

experiment.

properties are sometimes retained and extended

Table1 L ist of physical properties of D2370（a non-adhesive fluororubber）
D2370
Non-adhesive
fluorocarbon rubber

D0270
Standard Valqua
fluorocarbon rubber

Hardness (Shore A)

70

71

Tensile strength (MPa)

14.0

13.9

Stretch (%)

190

230

Compressed
permanent deformation＊）rate（％）

1416

200°C/70 hours, 25% compression, Φ 29 mm disc
The values in the table are observed values, not standard values.

＊）

because of the sticking characteristics of O-rings insite of beyond limitation for sealing by permanent
deformation of O-ring. In such cases, prevention
sticking might not an only way to solve problems but
also prevention methods while considering various
operating conditions were required.
Also, various factors including the usage environmentand conditions affect sticking, so it is difficult to solve
sticking problems only with seal materials. Therefore,
it is important to precisely identify the conditions in

Bonding strength (N)which O-rings are used.
We welcome feedback and honest opinions from users,
to help us make full use of the characteristics ofrubber materials, develop high-value-added products,
and introduce new products.

Test method: The test piece was placed in contact with thecounterpart under certain conditions and cooled in an ambientenvironment for 4 hours, then the strength required to detach thecounterpart from the test pieces is measured.
Test piece: rubber sheet of 2 mm thickness; contact pressure: 5.88
MPa; heating conditions: at 120℃ for 20 hours
Counterpart: ring-shaped SUS 304 of Φ 25 x Φ 19; detachingvelocity: 50 mm/min

Figure4 Sticking strength of D2370 measured by experiment

D2370 can be used as an alternative to common
fluororubber in various valves which use a common
fluororubber O-ring including for door seals and gate

１）Mori, K. et al., Journal of the Society of Rubber Scienceand Technology, Vol. 60, 7th issue, 1987
２）Handbook for the Rubber Industry, 4th ed., Societyof Rubber Science and Technology, Japan, 1994
３）Mori, K. et al., Journal of the Society of Rubber
Science and Technology, Vol. 60, 4th issue, 1987
4）Sagan I., Valqua Review, Vol. 38, 8th issue, 1994
5）Sagan I., Okazaki, M., Valqua Review, Vol. 41, 1stissue, 1997
6）Sagan I., Tribology, March 1998

valve seals. We consider that D2370 can reduce
operational malfunctions at opening and closing points
by decreasing sticking strength of seal materials. Inaddition, the product is expected to prevent sticking
to the opposing face including flanges.

6. Conclusion
This report explained sticking factors and methods forpreventing the sticking of O-rings, and introduced oursticking-prevention products.
In the case of static seal products, sticking is one ofSpring 2017

7. ReferencesMasanori Okazaki
Corporate Research andDevelopment Group
Development Division
46

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