EP0453917B1 - Coating consisting of two layers and method for making the same - Google Patents

Abstract

A method of coating to produce a two-layer, vibration-absorbing and anti-corrosive, abrasion-resistant coating for rigid substrates, in particular for sheet metals in the underbody region of motor vehicles, for reducing the noise caused by impacting particles, comprises an inner layer, facing the substrate, of a polymer A and a plasticizer P1, and a covering layer of a polymer B and a plasticizer P2, such that either the polymers A and B have a different chemical composition and the polymer A is essentially incompatible with the plasticizer P2 and the polymer B is essentially incompatible with the plasticizer P1, or both layers contain essentially the same concentration of one plasticizer (P1=P2), so that there is no risk of one layer being impaired by the plasticizer from the other layer.

Description

The invention relates to a two-layer, structure-borne sound absorbing and anti-corrosion, abrasion-resistant coating for rigid substrates, in particular for sheet metal in the underbody area of motor vehicles, which is also used to reduce noise caused by impacting particles.

Elastic structures, such as thin sheets of vehicle bodies or machine housings, emit high levels of airborne sound at different frequencies under airborne sound excitation or structure-borne noise due to their inadequate damping. It is known to apply viscoelastic damping foils or coatings to the sound-emitting sheets for damping.

Another cause of disturbing noises, particularly in motor vehicles, are impacting particles (stones and grit, sand, water) which are thrown by the wheels against the wheel arches and the vehicle floor. This noise is particularly annoying and uncomfortable because it contains a considerable amount of higher-frequency sound components. A known solution to this problem is to use a plastic front cover, for example made of polypropylene, in the wheel house. Such shells are attached floating in front of the wheel arch sheet at a distance of a few millimeters. While this measure is effective, it is also complicated and relatively expensive.

Another conceivable possibility would be to avoid the direct impact of the particles on the metal sheets in the underbody area of motor vehicles by applying suitable coverings to the sheet metal structure. Such a covering would have to be as soft and thick as possible in order to provide a "long braking distance" for the approaching particles in terms of space and thus also time. In this way it would be possible to achieve a considerable reduction in noise generation, in particular in the higher-frequency, particularly disruptive frequency range. A fundamental physical law, the Fourier transformation, states that the behavior of a system in the time domain is strictly linked to the associated behavior in the frequency domain: the faster a process runs in time, the more higher frequencies are required to describe this process in the frequency domain. Short abrupt processes involve more high frequencies than longer, less abrupt ones.

From DE-PS 28 52 828 it is known to produce a structure-borne sound-absorbing, at the same time protecting against corrosion, abrasion-resistant coating on a rigid substrate by successively applying two coating compositions which have a different modulus of elasticity after hardening. The inner layer facing the substrate is softer than the outer cover layer. With the help of these coatings, a significant improvement will be made structure-borne noise reduction. To produce the coating, two different plastisols are applied and gelled by heating and thereby hardened. The inner (softer) layer can also be foamed if necessary.

The problem of reducing noises caused by impacting particles is not addressed in DE-PS 28 52 828. In addition, it has been found that the coatings specified there are not stable over a long period of time, but that their mechanical and thus also their acoustic properties change over time. For use in motor vehicle construction, however, a coating must be designed in such a way that it meets the requirements not only at the beginning, but as far as possible over the entire life of the vehicle.

The invention is therefore based on the object of developing a coating for rigid substrates, in particular for sheet metal in the underbody area of motor vehicles, including the wheel arches, which has a structure-borne sound-damping effect and is corrosion-resistant and abrasion-resistant, and which at the same time also leads to a substantial reduction in those caused by impacting particles Leads to noises. It is also essential that the coating has the desired properties practically unchanged over long periods of time.

• a) die innere Schicht ein Polymer A und einen Weichmacher W₁ und
• b) die Deckschicht ein Polymer B und einen Weichmacher W₂

enthalten, wobei entweder die Polymeren A und B eine unterschiedliche chemische Zusammensetzung aufweisen und das Polymer A mit dem Weichmacher W₂ und das Polymer B mit dem Weichmacher W₁ im wesentlichen unverträglich sind, oder beide Schichten im wesentlichen dieselbe Konzentration an einem Weichmacher ( $\text{W₁ = W₂}$

) enthalten, so daß in keinem Fall eine Beeinträchtigung einer Schicht durch Weichmacher aus der anderen Schicht stattfindet.To achieve this object, a two-layer coating is proposed, which consists of an inner layer facing the substrate and a cover layer, wherein after gelling and / or curing the inner layer is softer compared to the cover layer and has a greater layer thickness; this coating is characterized in that

• a) the inner layer is a polymer A and a plasticizer W₁ and
• b) the top layer is a polymer B and a plasticizer W₂

contain, either the polymers A and B have a different chemical composition and the polymer A with the plasticizer W₂ and the polymer B with the plasticizer W₁ are essentially incompatible, or both layers essentially the same concentration of a plasticizer ( $\text{W₁ = W₂}$

) included, so that in no case is one layer affected by plasticizers from the other layer.

According to a preferred embodiment, the inner layer is foamed, which both increases its softness and increases its layer thickness. In general, the inner layer has a layer thickness that is 2 to 20 times greater than that of the cover layer. The modulus of elasticity of the inner softer layer should generally be <10⁸ dynes / cm². The weight per unit area of the coating as a whole is preferably less than that of the substrate.

A coating that meets the requirements mentioned above must at the same time be very soft and as thick as possible so that it can effectively stop the impacting particles. Furthermore, it should show the best possible effectiveness as a structure-borne sound deadening material. For practical use, however, it is also essential that the coating offers good corrosion protection and, above all, shows high abrasion resistance. This is particularly true in the area of the wheel arches. High softness and good abrasion resistance are mutually contradictory requirements, which are however met by the two-layer coating according to the invention. The layer arranged on the sheet metal side is softer and relatively thick, so that it is the posed acoustic conditions met. Their thickness is generally in the range from 1 to 5 mm. The cover layer is thinner and relatively tough-elastic, so that it ensures abrasion resistance. A thickness in the range of 0.25 to 1 mm is preferred.

• Wenn die Polymeren in der inneren weichen Schicht und in der harten Deckschicht eine unterschiedliche chemische Zusammensetzung aufweisen, dann werden für die beiden Schichten Weichmacher ausgewählt, welche jeweils mit dem Polymeren der anderen Schicht nicht verträglich sind. Normalerweise ist ein höherer Weichmachergehalt für die innere Schicht erforderlich, um die gewünschte größere Weichheit zu erzielen, was aufgrund des Konzentrationsgefälles zu einer Wanderung des Weichmachers in die Deckschicht hinein führen kann. Wenn der Weichmacher der inneren Schicht mit dem Polymeren der Deckschicht unverträglich ist, wird eine Aufweichung der Deckschicht verhindert. Durch die Unverträglichkeit der Weichmacher mit dem Polymeren der jeweils anderen Schicht werden Beeinträchtigungen der mechanischen Eigenschaften der Schichten verhindert.
• Eine weitere Lösung des Problems besteht darin, daß man bei Verwendung desselben Weichmachers für beide Schichten im wesentlichen dieselbe Weichmacherkonzentration anwendet und auf diese Weise eine Wanderung des Weichmachers unterbindet. Die erforderliche höhere Weichheit der inneren Schicht wird in diesem Fall durch eine Aufschäumung erreicht.

In practice, it is crucial that the properties of the two layers are long-term stable, ie the top layer must be permanently elastic and abrasion-resistant, and the soft inner layer must remain permanently soft. This is ensured according to the invention in that the composition of the two layers is coordinated with one another in such a way that plasticizer does not migrate from one layer to the other or the migrating plasticizer does not impair the physical properties of the other polymer layer due to its incompatibility. This can be done in several ways:

• If the polymers in the inner soft layer and in the hard cover layer have a different chemical composition, plasticizers are selected for the two layers, which are each incompatible with the polymer of the other layer. A higher plasticizer content is normally required for the inner layer in order to achieve the desired greater softness, which can lead to a migration of the plasticizer into the cover layer due to the concentration gradient. If the plasticizer of the inner layer is incompatible with the polymer of the top layer, the top layer is prevented from softening. The incompatibility of the plasticizer with the polymer of the other layer prevents impairment of the mechanical properties of the layers.
• Another solution to the problem is that if the same plasticizer is used for both layers, essentially the same plasticizer concentration is used and in this way the migration of the plasticizer is prevented. In this case, the required higher softness of the inner layer is achieved by foaming.

These measures ensure the required long-term stability of the mechanical properties of the coating. In the above-mentioned DE-PS 28 52 828 there are no indications in this direction and the examples given there do not meet the conditions mentioned above: In Example 1, arylalkyl sulfonate is used as the plasticizer for the inner layer, which is combined with polyvinyl chloride, the polymer of the outer layer is compatible, so that a softening of the top layer occurs due to plasticizer migration. In example 2, the identical plasticizer is used in different concentrations for both layers, which also leads to migration of the plasticizer due to the concentration gradient.

For the production of the two-layer coatings according to the invention, plastisols are used, such as have been used in automotive construction for some time as underbody protection, weld seam sealing, adhesives and the like, and as is already known in principle from DE-PS 28 52 828. The coating can be applied particularly advantageously in such a way that the two layers are formed in successive spraying processes (wet-on-wet application), with the gelation of the layers taking place simultaneously and together by a subsequent heat treatment. However, it is also possible to pre-gel the inner layer after application by heating and, if appropriate foam up and only then apply the top layer. To gel the plastisols, the mixture is heated to about 100 to 180 ° C. for 10 to 60 minutes.

As already stated, the required different softness of the layers is achieved either by a higher plasticizer content in the inner layer and / or by foaming the inner layer.

Particularly suitable polymers for the plastisols are powders of polyvinyl chloride homo- and / or copolymers, e.g. with vinyl acetate. Also suitable are powders of (meth) acrylate homo- and / or copolymers, as described in DE-PS 24 54 235 and 25 29 732. So-called core / shell acrylic polymers according to DE-AS 27 22 752 can also be used with advantage.

Numerous suitable plasticizers such as phthalates, phosphates, adipates and citric acid esters are known for polyvinyl chloride homopolymers and copolymers. Dialkyl phthalates such as e.g. Diocytl phthalate and dinonyl phthalate because they are incompatible with (meth) acrylate homo- and copolymers. For the latter, dibenzyl ether, dibenzyl toluene, diphenyl methane and diphenyl ether are particularly suitable as plasticizers because, in turn, they are not compatible with vinyl chloride homo- and copolymers. In contrast, e.g. Arylalkyl sulfonates are compatible with both groups of polymers, so that they can only be used if the plasticizer concentration in both layers is essentially identical.

Suitable blowing agents which are activated when the plastisols are gelled are known to the person skilled in the art. Examples include azodicarbonamide, azoisobutyronitrile, dinitrosodimethyl terephthalamide and optionally also water.

The mechanical properties of the layers, in particular the hardness and also the abrasion resistance, can be influenced in a manner known per se by adding inorganic fillers. Suitable fillers are, for example, calcium carbonate and oxide, barium sulfate, carbon black, graphite, titanium dioxide, talc and organic or inorganic hollow spheres.

The following examples are intended to explain the invention in more detail.

example 1

The plastiol for the production of the inner softer layer had the following composition: Polymethyl-n-butyl methacrylate (with 2% vinylimidazole) 20% Dibenzyltoluene 50% Calcium carbonate 28% Calcium oxide 2%

1% azodicarbonamide was optionally added to produce foamed layers.

A plastisol of the following composition was used for the top layer: Polyvinyl chloride (K value 70, paste type) 30% Dinonyl phthalate 30% Calcium carbonate 38% Calcium oxide 2%

The two plastisols were sprayed wet on wet onto a metal sheet and baked by heating at 160 ° C. for 30 minutes. Two-layer coatings were obtained which not only had excellent abrasion resistance, but also showed very good damping behavior, and which are able to largely absorb the energy from impacting particles and to bring about a very considerable reduction in noise, see that in Example 3 results given for experiments C, D, E and F.

Example 2

A foamable plastisol for the production of the inner softer layer had the following composition: Polyvinyl chloride 30% Dinonyl phthalate 30% Calcium carbonate 36% Calcium oxide 2% Polyaminoamide 1 % Azodicarbonamide 1 %

The same plastisol as in Example 1 was used for the top layer.

Example 3

The coatings specified below in the APAMAT® were examined to determine how effectively sound generation can be reduced by stone chipping, splashing water, etc. In principle, however, only more or less flat samples can be examined in the APAMAT®. However, the wheel house sheets are by no means flat, so that their rigidity is considerably increased compared to a flat sheet. In order to approximate this increase in stiffness, flat 1 mm thick steel plates (84 x 84 cm) were braced, whereby 3 rivets were used to fix the struts. The actual fixation was carried out with TEROKAL® 4520-34 (Teroson GmbH), an extremely adhesive and strongly curing one-component adhesive.

Each sheet metal braced in this way was first measured as such and then with the coating, the following arrangement being used for the measurement:
APAMAT® reception cabin with microphone stiffened steel sheet investigated coating APAMAT® ball slingshot
The third-octave sound pressure spectrum of the untreated sheet, determined in the receiving cabin of the APAMAT®, was saved and used as a reference. The reference spectra of all braced empty sheets were practically identical. The third-octave spectra with the respective coating were also determined and then subtracted from the reference spectrum of the carrier plate. The difference spectra thus formed are a measure of the effectiveness of the respective coating. They are shown in Figures 1 and 2.

• A TEROTEX® 3105-147 (Teroson GmbH) in einer Schichtdicke von ca. 1,2 mm (Beschichtungsgewicht ca. 1 kg). Es handelt sich um ein handelsübliches PVC-Plastisol für den Unterbodenschutz von Kraftfahrzeugen.
• B vorgelagerte Platte aus Polypropylen/EPDM mit einer Dicke von 2 bis 3 mm (3,4 kg). Diese Variante entspricht einer Radhausauskleidung gemäß Stand der Technik.
• C Es wurde eine zweischichtige Beschichtung gemäß Beispiel 1 aufgebracht. Die innere Schicht war geschäumt und wies eine Dicke von ca. 5,5 mm auf, die Dicke der äußeren Schicht lag bei ca. 1 mm. Das Gesamtbeschichtungsgewicht betrug 3 kg.
• D Es wurde eine zweischichtige Beschichtung gemäß Beispiel 1 aufgebracht. Die innere Schicht war geschäumt und wies eine Schichtdicke von ca. 4 mm auf, die Schichtdicke der Deckschicht betrug ca. 0,5 mm. Das Gesamtbeschichtungsgewicht lag bei 2,4 kg.
• E Es wurde eine zweischichtige Beschichtung gemäß Beispiel 1 aufgebracht, wobei die innere Schicht geschäumt war und eine Schichtdicke von ca. 7 mm aufwies. Die Dicke der Deckschicht betrug ca. 1 mm, das Gesamtbeschichtungsgewicht lag bei 3,3 kg.
• F Es wurde eine Beschichtung gemäß Beispiel 1 aufgebracht, wobei die innere Schicht nicht aufgeschäumt war und eine Schichtdicke von ca. 3 mm aufwies. Die Schichtdicke der Deckschicht lag bei etwa 1 mm. Das Gesamtbeschichtungsgewicht betrug 3,7 kg.

The braced steel sheets as described above (each 84 x 84 cm; 6.7 kg weight) were provided with the following coatings:

• A TEROTEX® 3105-147 (Teroson GmbH) in a layer thickness of approx. 1.2 mm (coating weight approx. 1 kg). It is a commercially available PVC plastisol for the underbody protection of motor vehicles.
• B upstream plate made of polypropylene / EPDM with a thickness of 2 to 3 mm (3.4 kg). This variant corresponds to a wheel house lining according to the state of the art.
• C A two-layer coating according to Example 1 was applied. The inner layer was foamed and had a thickness of approx. 5.5 mm, the thickness of the outer layer was approx. 1 mm. The total coating weight was 3 kg.
• D A two-layer coating according to Example 1 was applied. The inner layer was foamed and had a layer thickness of approx. 4 mm, the layer thickness of the cover layer was approx. 0.5 mm. The total coating weight was 2.4 kg.
• E A two-layer coating according to Example 1 was applied, the inner layer being foamed and having a layer thickness of approximately 7 mm. The thickness of the top layer was approx. 1 mm, the total coating weight was 3.3 kg.
• F A coating according to Example 1 was applied, the inner layer not being foamed and having a layer thickness of approximately 3 mm. The layer thickness of the top layer was about 1 mm. The total coating weight was 3.7 kg.

The results of the APAMAT® measurements with ball excitation are shown in diagrams 1 and 2. The acoustic effectiveness of the coverings depends on their softness and thickness. Soft and thick coverings drastically reduce the structure-borne noise excitation above a certain cut-off frequency. This cutoff frequency shifts to lower frequencies as the base becomes softer or thicker. The hard cover layers, which are necessary to ensure sufficient abrasion resistance, somewhat reduce the effectiveness of the underlying soft covering. The cover layers should therefore not be thicker than absolutely necessary.

• Die einschichtige Beschichtung aus abriebfestem PVC-Plastisol ist allen anderen Varianten deutlich unterlegen (A).
• Die Polypropylenplatte (B) ist vor allem bei höheren Frequenzen den erfindungsgemäßen Beschichtungen deutlich unterlegen.
• Die erfindungsgemäßen Beschichtungen C und D zeigen insbesondere bei etwas höheren Frequenzen ein ausgezeichnetes Verhalten.
• Der Vergleich der Beschichtungen E und F zeigt die gute Wirksamkeit einer Beschichtung, bei der die innere Schicht nicht aufgeschäumt ist; durch Aufschäumung wird jedoch die Wirksamkeit noch weiter verbessert.

The results of the measurements show the following:

• The one-layer coating made of abrasion-resistant PVC plastisol is clearly inferior to all other variants (A).
• The polypropylene plate (B) is clearly inferior to the coatings according to the invention, especially at higher frequencies.
• The coatings C and D according to the invention show excellent behavior, in particular at somewhat higher frequencies.
• The comparison of coatings E and F shows the good effectiveness of a coating in which the inner layer is not foamed; however, the effectiveness is further improved by foaming.

Example 4

The table below illustrates the long-term behavior of a coating according to the invention compared to a coating similar to Example 1 of DE-PS 28 52 828. In contrast to Example 1 according to the invention, the plastisol for the inner layer contained a mixture of arylalkyl sulfonate and dibenzyl toluene instead of dibenzyl toluene in the comparative experiment in a 1: 1 ratio as a plasticizer. The arylalkyl sulfonate is compatible with both the methacrylate polymer and PVC.

The table below shows the values for the tensile strength of the two coatings after a longer storage period of up to 8 weeks. With the coating according to the invention according to Example 1 it can be seen that the tensile strength slowly increases in the course of storage; this is the typical behavior of normal PVC plastisols during aging. In contrast, the comparison test shows a decrease in the tensile strength values due to migration of the alkylarylsulfonate from the methacrylate layer into the PVC layer, as a result of which the latter becomes softer. The tensile strength of the 2-layer systems is essentially determined by the tough elastic PVC plastisol layer, so that their change for the deterioration observed is responsible.

Claims (12)

1. Two-layer, sound-absorbing, abrasion-resistant and also anticorrosive coating for rigid substrates, in particular for sheet metals in the underbody region of motor vehicles, for reducing the noise caused by impacting particles, comprising an inner layer facing the substrate and a covering layer, wherein after gelling and/or curing the inner layer is softer than the covering layer and has a greater layer thickness, characterised in that

   a) the inner layer contains a polymer A and a plasticiser P₁ and

   b) the covering layer contains a polymer B and a plasticiser P₂,

   wherein either the polymers A and B have a different chemical composition and the polymer A is essentially incompatible with the plasticiser P₂ and the polymer B is essentially incompatible with the plasticiser P₁, or the two layers contain essentially the same concentration of a plasticiser (P₁ = P₂), so that in no case does there occur impairment of one layer by the plasticiser from the other layer.
2. Coating according to claim 1, characterised in that the inner layer is foamed.
3. Coating according to claim 1 or 2, characterised in that the inner layer has a layer thickness two to twenty times greater than that of the covering layer.
4. Coating according to one of claims 1 to 3, characterised in that the inner layer has a modulus of elasticity < 10⁸ dyn/cm².
5. Coating according to one of claims 1 to 4, characterised in that the weight per unit area of the coating as a whole is less than that of the substrate.
6. Coating according to one of claims 1 to 5, characterised in that the polymer A is a (meth)acrylate homopolymer or copolymer.
7. Coating according to one of claims 1 to 6, characterised in that the polymer B is a vinyl chloride homopolymer or copolymer.
8. Coating according to one of claims 1 to 7, characterised in that the plasticiser P₁ is dibenzyltoluene or a diphenyl ether.
9. Coating according to one of claims 1 to 8, characterised in that the plasticiser P₂ is a dialkyl phthalate.
10. Coating according to one of claims 1 to 9, characterised in that one or both layers also contain fillers.
11. Method for the application of a coating according to claims 1 to 10, whereby two plastisols of different composition are applied in succession to the substrate and the layers are either simultaneously or in succession gelled by heating to an elevated temperature and optionally foamed, characterised in that two plastisols are used which contain

    a) for the inner layer a polymer A and a plasticiser P₁ and

    b) for the covering layer a polymer B and a plasticiser P₂,

    wherein either the polymers A and B have a different chemical composition and the polymer A is essentially incompatible with the plasticiser P₂ and the polymer B is essentially incompatible with the plasticiser P₁, or the two layers contain essentially the same concentration of a plasticiser ( $\text{P₁ = P₂}$

    

    ), so that in no case does there occur impairment of one layer by the plasticiser from the other layer.
12. Method according to claim 11, characterised in that a plastisol containing a blowing agent suitable for foaming is used for the inner layer.

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