Friction behaviour of flocked seals

· Abstract

The flocking of sealings to minimize the frictional forces in the contact is a common method in sealing technology. Flocked seals are commonly used in automotive, for doors, glove boxes, hat racks, and storage compartments. They have a great influence on the perception of the quality of the vehicles by the vehicle passengers. Today‘s customers demand that, for example, moving door windows slide almost frictionless and noiseless. Despite the optimizations made in this area, under certain mechanical and climatic conditions, unwanted friction-induced noise phenomena repeatedly occur in the contact of the seals. The lecture should address this scientifically and economically significant problem and provide results through systematic, friction-based analyses, which characterize the flocked seal as a source of noise. So far, it is unknown what effects the characteristic of the flock and the properties of the friction partner have on the friction behaviour.

In recent years, noise in vehicle interiors has become a considerable economic problem. Since unpleasant noises such as squeaking or creaking have a major impact on the perception of vehicle quality and comfort, as well as representing a safety risk, automotive manufacturers attach great importance to noise prevention. Noise interference occurs when contacting materials with unfavourable friction behaviour are set in relative motion, e.g. by the jerking motion when driving or a functional motion such as when opening a window. In the case of seals, friction behaviour can be improved for noise prevention, for example, with the aid of flock, which has the advantage over a bonded coating of higher impact and noise absorption. Flocked seals are frequently used in vehicles, for example in the door and window area, for glove boxes, parcel shelves and storage compartments.

Figure 1: examples of application

Despite the optimizations made in this area, unintentional friction-induced noise phenomena repeatedly occur with flocked seals, depending on the installation situation as well as the mechanical loads and climatic conditions. To solve these problems in the short term, a bonded coating, in special an anti friction coating, is currently applied to the flocked seal. Although this can reduce the disturbing noise, other positive properties of the fibrous structure are masked and manufacturing costs are increased.

It would make more economic and ecological sense if the seal could be designed from the outset in such a way that it does not generate interference noise under any of the loads and conditions typical in a vehicle. At present, however, there are no well-founded studies that could be used for targeted material optimization. For example, the interrelationships between the adjustable parameters of flocking (e.g. with regard to base materials, manufacturing conditions and possible post-treatments) and the friction behaviour of the seal achieved with them are as yet unknown. The influencing factors are very diverse and, moreover, not independent of each other. The aim of the research is to address this scientifically and economically important problem and to describe and evaluate the factors influencing noise generation during the manufacture and application of flocked seals. The aim of the project is to analyse and describe the frictional behaviour of flocked automotive seals and to develop ways of influencing it in a targeted manner.

· Economic significance

The use of flock fibres, especially in the automotive industry, is steadily increasing. A distinction is made here between flocking for optical enhancement (design) and technical flocking, which is mainly used for noise insulation (noise). Further economically relevant areas of application are sealing, tolerance compensation, avoidance of vibration noise, easy sliding, avoidance of condensation, protection against contact at high tem- peratures, high absorbency and slip resistance. Warranty costs due to noise are estimated to be 10% of total warranty costs (worldwide > 10 billion € per year). Therefore, the optimization of materials with respect to perceptible noise during relative motion is considered an important component of successful vehicle develop- ment. Currently, there are no generally applicable standards that cover the friction behaviour of flocked seals. The project results provide the basis for this and thus enable material optimization based on an objective test procedure. Knowledge of the fundamental relationships between flock characteristics and the resulting surface properties enable a targeted and thus effective approach to product development. The investigations and developments in the field of flock post-treatment open up additional possibilities and areas of application on the technical side.

Assuming the tribological system as it is mentioned in the following figure the samples we have to consider the sample structure, which consists of substrate, bonding agent and the flock fibres as well as the mating material and certain particles. In addition to the properties of the flock fibres themselves, the structure and characteristics of the flock (e.g. fibre density, fibre orientation, regularity, etc.) can also have a major effect on the frictional behaviour.

Figure 2: tribological system (left) and different flock properties (right)

However, the properties of the flocked surface influence not only friction, but also other functions of the material.For example, a seal should primarily serve to separate two spaces and prevent foreign substances such as dust, water, air or similar from being exchanged between the spaces. The sealing effect as well as the friction behaviour, is influenced by the characteristics of the flocked surface (in particular fibre type, density, length and orientation), so that, depending on the characteristics, foreign substances can enter the friction gap in the worst case (figure 2, left). This must be prevented and therefore always taken into account when optimizing the seal in terms of friction behaviour.

We want to address the following questions:

1. What is the relationship between the material properties of flocked seals and the friction properties?

2.How can the friction and noise behaviour of flocked materials be specifically influenced?

3.Can processes for the subsequent modification of flock be profitably used to improve the frictional properties of flocked seals?

So first we have to find out how we can characterize the flocked seals. Of course the main parameters are the length, the diameter (dtex) and the material type of the fibres. A special parameter regarding the process is the density of the fibres. Therefore the amount of fibres in an area of a square millimetre is calculated or counted. During the project we have analysed the surfaces and have counted the amount by using computer aided picture analytics.

Figure 3: representation of different fibre density and diameter (left) and counting fibres

by picture analytics (right)

Another interesting fact which is related to frictional behaviour is the effect of adhesion. It is important how the fibres interact with the mating material. The interaction is often discussed with the surface energy and its polar and dispersive parts. Surface energies can be calculated using a drop shape analysis with different liquids. Comparing the ratio between the dispersive and the polar part of the surface energy/tension for two phases allows for a prediction of the adhesion between these two phases. The closer the ratios match the more interactions are possible between the phases and the higher the adhesion which is to be expected.

Figure 4: Drop shape analysis of coated or uncoated flock surfaces

In case of uncoated surfaces the drop, in this case a water droplet, moves directly into the surface. Coated surfaces are mostly hydrophobic and the droplet remains on the surface. Now it depends on the properties of the mating surface whether the adhesion is high which will result in higher friction or vice versa.

Additionally the deformation behaviour or hardness of the samples as well as the properties of the bonding agent and substrate were measured and taken into account.

To find a first answer for the question related the correlation between friction and fibre properties we plot the calculated fibre density vs the coefficient of friction (COF).

· Correlation of fibre ends to friction

There is a strong correlation of the coefficient of friction to the amount of fibres/mm² . This is related to the resulting contact area. A high amount of fibres is related to a small gap between them. Therefore, these fibres interfere with each other when bending and tend to stay upright. Therefore, the fibre ends are more likely to come into contact with the mating surface. This leads to a smaller contact surface and thus to less friction.

Figure 6: fibres and its contact surface

This leads to some short conclusions.

A smaller diameter (dtex) should lead to more flattening. This flattening is related to a bigger contact area and results in higher friction. The same should be present if we increase the normal force or the pressure. Higher pressure is related to a higher flattening. Also if we extend the fibre length it is easier to bend them. This will lead to more deformation and so to higher friction.

Coating of fibres enhances the hydrophobicity. This is good to reduce friction, but always dependent on the mating surfaces (delta surface energy).

But what about alternatives to coating?

In the research project three different approaches were tested (see figure 7).

Using the atmospheric pressure plasma process, very thin polymer layers should be deposited on the flocked surfaces. This is already used in different applications to reduce friction.

A process that is still little used is the subsequent structuring of the flocked surface by means of laser techno- logy. In this way, defined structures can be introduced into the flocked surface. Since the surface structure generally has a demonstrable influence on the friction behaviour, this variant is tested for its effectiveness/ applicability with flocked seals. In terms of optimisation with regard to friction and stick-slip, the focus will be on reducing the contact area.

And third, the application of textile finishing agents was tested to find an alternative to coat the surfaces.

Figure 7: approach for alternatives to improve friction behaviour.

· Post-treatment of the materials via plasma polymer coating

In order to save and optimise the current coating process, alternatives are to be examined within the project in order to save money and  influence the friction properties. The first is coating by means of a plasma polymerisation process. For this purpose, a SiOx layer was deposited on the fibres via a plasma nozzle (Figure 8).

Figure 8: Plasma nozzle process schematic.

The deposited layers are still too thick at this stage (figure 9). A rougher structure can also be seen on the coated fibre surface. This could have a positive effect on the friction properties. Further experiments are being carried out here on varying the process parameters (e.g. lower precursor flow, faster treatment speed). The thickness results in a very low deformation behaviour which has a negative effect on friction up to now.

Figure 9: comparison of the uncoated and coated flock fibres.

Second the laser treatment was taken into account. Defined structures were developed and applied to the samples using a laser system. Depending on the laser intensity, the fibres can be ablated completely or only partially. The fibres are melted by the heat input and thus can exhibit different properties.

Figure 10: Laser treatment and modified samples

The samples were then subjected to a friction analysis. Different forces and speeds were taken into account. As an example, the coefficient of friction for a selection of fibre surfaces is shown in the following diagram.

Figure 11: Coefficient of friction in dependence of laser treatment and fibre length

The structures shown here have a significantly lower coefficient of friction than the original shown in red. This proves that laser structuring can lead to an improvement in the friction properties. It can also be seen that longer fibres (0.8 mm) cause somewhat higher friction values. This can be explained by the above-mentioned approach of bending longer fibres and the associated higher contact area.

Thirdly, textile finishing processes were tested. Various finishing materials were distributed on the flock surfaces. The substances can be divided into four groups: oil and water repellent finish, fluorocarbon-free hydrophobing agent, softeners and silicone-free softeners.

Figure 12: Coefficient of friction of different flock surfaces with textile finishing

The individual equipped materials were also subjected to the friction test. The diagram (figure 11) shows the determined friction coefficients compared to the untreated sample (red). It can be shown that textile finishes can be successfully used to optimise the friction properties.

To investigate the behaviour of flocked seals in practical application, a test rig is to be set up that reflects the sealing, sliding and cleaning properties. Up to now, the practice of car manufacturers is only a static leak test of the windows under simulated rain conditions at the end of production. So far, no cycles with opening and closing of the side windows have been carried out. This can now be done with this new test rig.

Figure 13: new test rig for wear and leakage und der rain conditions

· Summary

1.Influence of the substrate material: This influence essentially relates to the formation of the deformation properties of the flocked complete geometry. Depending on the viscoelasticity, a contact area of  varying size is formed. The higher the contact area, the higher the friction.

2.Influence of the adhesive: The choice of adhesive influences the strength of the bond. The adhesive must both adhere to the substrate and absorb the flock fibre. Depending on the viscosity of the adhesive, the penetration of the fibre is thus influenced and indirectly, via the layer thickness and elasticity/ stiffness, the sliding strength of the flock fibres. This strength influences the sliding of the fibres against elevations of the counter material and is thus connected with the frictional force.

3.Influence of the fibre characteristic: The fibre characteristic is described by the parameters fibre stiffness, fibre length, fibre diameter and fibre density. Depending on the variation, this influences the deformation properties and the resulting contact area. The friction force is essentially determined by  the fibre density.

4.Influence of post-treatment: There are two successful possibilities for post-treatment. The contact surface to the mating material can be influenced by subsequent structuring. By coating or functional finishing of the fibres, the adhesion tendency of the fibres to the counter  material can also  be controlled. The reduction of the adhesion leads to a reduction of the susceptibility to stick-slip.

5.Influence of the counter material: The counter material or the friction partner has a high influence on the friction behaviour. A flocking should therefore work for several different friction partners. Therefore, the compatibility consideration  regarding the chemical component, which is related to the adhesion  properties,  and  the  surface  structure  component, which describes the contact area,  is essential for an optimisation of the materials.

6.Influence of loads:  The different types of  loads  (mechanical and thermal)  influence the  material properties both on the surface (adhesion) and in the material itself (stiffness/damping/deformation). This thus has an impact on the cleaning (tightness) as well as the friction behaviour. Here, both contact surfaces and adhesion properties change.

The aforementioned effects on the contact surface, stiffness, damping, adhesion properties and deformation not only influence the strength of the friction, but also have an influence on the stick-slip behaviour.

This article is reprinted with permission from the European Flocking Association. Unauthorized secondary distribution is prohibited