CA1264214A

CA1264214A - Surface treatment of synthetic material for enhancement of bonding

Info

Publication number: CA1264214A
Application number: CA000505061A
Authority: CA
Inventors: Helmut Kaeufer; Guenter Schmack; Walter Brockmann
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-03-27
Filing date: 1986-03-25
Publication date: 1990-01-09
Also published as: DE3511126C2; EP0199126A3; EP0199126A2; JPS62121676A; DE3511126A1

Abstract

ABSTRACT OF THE DISCLOSURE

The surface or a portion thereof of an article made of synthetic plastic material is rendered viscoelastic and/or plastically deformable, by a die member having a large number (usually at least several thousand) of fibrous projections applied to the surface such that the fibrous projections become embedded in the synthetic material and the material adheres to the projections. The die member is applied for a short period during which the viscous material cools and increases its viscosity, whereupon the die is withdrawn from the surface. A number of fibres are drawn outwardly before the die member detaches itself from the synthetic material; thereafter the fibres are cooled to a solid nonviscoelastic and/or nonplastically deformable consistency. A
coating and/or adhesive material when applied will embed the fibres to thereby create a strong bond.

Description

~6~Z~I~

2 BACKGROUND OF THE INVENTION

4 The present invention relates to improving the surface of a synthetic material compound product or the like 6 so that it is readily amenable for bonding and for receiving 7 a coating, varnish, lacquer adhesi~e or the like that will 8 remain firmly adhering to that component. More particularly 9 the invention refers to increasing the ~urface roughness of such a synthetic ~omponent or part. Herein the invention 11 refers to both a method for obtaining a specifically 12 enhanced surface roughness as well a to methods for 13 immediate applicaiton and utilization of such a surface.

The present age of "plastics" uses large scale 16 mass production of parts, components etc. made of synthetic 17 material whereby injection molding, extrusion or calendaring 18 are the predominant methods of production. It is customary 19 here to add a certain lubricant and/or a separation material to the synthetic in order to improve their removabilîty from 21 the mold, die etc. Such inherent luhrication, howeve~, is 22 subsequently disadvantaged because it renders more difficult 23 a subsequent coating process, coating, lacquering, 24 varnishing or the like because such material will not or only inadequately adhere to such an inherently lubricated 26 surface. Within the large range of plastics there are 27 certain synthetic materials which on account of the~r 12~i42~4 2 chemical composition and nature as well as owing to their

3 solvability are not readily amenable for receiving a coating

4 in a firm adhering manner and al50 bonding of such a part to some other part i5 inherently difficult. This means that 6 durable bonding connection or the like cannot be expected.
7 An important example for this type of a synthetic is e.g.
8 polypropylene.

Therefore there exists a task to provide for some 11 method which treats in a general way such a synthetic so 12 that indeed a coating can be made to adhere to it. Well 13 known preparation methods in this regard are pickling 14 methods, plasma treatment and corona discharge treatment.
These methods are quite workable and improve the surface 16 roughness of such a component significantly but they have 17 the inherent drawback that there are process steps to be 18 applied subsequently, i.e; after the part has been made 19 otherwise and comp;eted; these preparation methods are not and in fact cannot be incorporated in the manufacture of the 21 part itself. Moreover these preparation methods are at least 22 in some instances prone to incurrence of large expenditures 23 and equipment which for reasons ~ust mentioned do not 24 constitute parts of the process equipment for making the parts themselves. Moreover, the coating and/or bonding of 26 such a synthetic part must usually follow ~ore or less 27 immediately after the preparation step because in some 2~

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64~06-206 instances the increase in adhesion produced by such a method is only a temporary one and may in fact decay rather rapidly.
Needless to say that the ~urface of such parts must not be soiled in any manner prior to applying to coating and that includes the requirement that these parts must not be touched by human hands since all these instances tend to reduce adhesibility of the part German printed patent 3,009,412 discloses the making and treatment of the surface of the synthetic material by rendering a relatively smooth surface of the part as it results from one of those prior manufacturing process, rough for example by means of sand blasting. Indeed such a roughened surface is be~ter suited for coating and bonding. However, this procedure of roughening the surface is hardly applicable to small surfaces and constructive bonding is still not possible unless there is a speclfic and chemically based adheslon that is really effective between the bonding agent and the æynthetic surface. Bonding of the roughen surface thereof is not possible if any speclfic adhesion is lost. US patent 3,557,407 discloses a drum with rigid pins extending radially therefrom which may pierce the softened surface of a plastic material and upon being withdrawn the pins pull individual strings of synthetic material from the surface.
One may obtain a very large clear pattern of fibres or filaments to extend from the surface of that material.
It is an object of the present invention to provide a new and improved method of preparing the surface of a synthetic part so as to be more readily amenable to coating, bonding, lacquering or the ~Zf~LZ~4 64s06-206 like whereby specifically such enhancement is adhesibility is to obtain even if there is no effective specific chemically based adhesion present between the bonding agent coating material or the like on one hand and the synthetic part on the other hand.
It is a related object to provide a new and improved method for coating in a general sense a synthetic part, component or the like.
In accordance with an aspect of the present invention, there isprovided an ar-ticle made of a synthetic plastic material having a generally smooth surface, at least a part of the said surface, having a fibrous surface texture comprising thereon outwardly-extending free~ending fibres at a density of the fibres of at least 1,000 per square millimeter, the said fibrous-textured surface being capable of being more readily bonded or coated than the said smooth surface. Preferably, the density of the fibres is at least five to six thousand fibres per square mm; more preferably at least 10,000 fibres per squares mm. The fibre length normally exceeds 30 micrometers and preferably at least 100 to 300 micro-meters. The fibre length is usually up to 3 mm but it was found that the range from 300 micrometers to 1 millimeter is preferred.
The effectiveness of the invention is believed to be attributable to a firm grip the coating, adhesive or the like will have as the fibres become embedded in the gel-dipping material, coating or the like.
It was found that, independently from the particular material involved in the synthetic part, a fibrous surface, i.e., a surface with a fibre density of several thousand fibres (at least 5,000 to 6,000) per square mm renders the surface readily %~

amenable towards adhesion to an adhes:ive coating, a lacquer or any other coatiny. Parts having been treated in this fashion readily adhere to each other. The constructive bonding obtained in this manner will be sufficiently strong even if there is no little or specific adhesion present such as a bonding agent. The effective-ness of the fibres as adhesion enhancement is not reduced through storage or even touching, and similarly the amenability to bonding or coating or the like is not affected by storage or contact.
In another aspect of the invention, there is provided a method of rendering at least a part of a surface of a synthetic plastics material amenable to bonding, coating or the like through the generation of a fibrous surface. It is assumed that the article (e.g., part or component) made of a synthetic plastics material has basically a smooth surface; at least there are no fibres. This article is now treated so that the surface is rendered viscoelastic or plastically deformable (unless such a state is already present in the first place). Next, this visco elastic or plastically deformable surface is put into contact with a die member having a number offlexiblefibrous projections of the stated fibre density, made of a material which will adhere to the viscoelastic or deformable surface of the article such that the fibrous projections of the die member will be embedded in plasticized material of that surface part. Contact of embedment is maintained for a short period such as a few seconds during which the embedding material cools and increases in viscosity towards solidification. Thereafter the die member is rapidly removed from the synthetic surface such that the material adhering ~ 64506-206 to the projec-tions of the die member will not immediately separate.
The material is carried along by the receding die fibres to thereby produce small fibres. Immedia-tely thereafter the visco-elastic and/or plastically deformable surface is treated such that it becomes solid i.e. is no longer viscoelastic and/or no longer plastically deformable.
The method is characterized by extreme simplicity in its application and it is particularly advantageous in that these fibres can be produced whenever it is practical during the manufacture and/or making of the synthetic part. In particular the method can be applied following immediately the removal from a mold, for example of an injec-tion molding machine, or upon emergence from an extrusion device or when removed from a calendar whereby particular use is made of the still existing residual viscoelastic and/or plastically deformable state. On the other hand the method can be applied at any later time for example immediately preceding bonding, lacquering, coating or the like.
Commensurate with the inventive concept t the die member should have at least 1,000, and preferably at least 5,000, more preferably at least 10,000 fibrous projections/square mm. The initially smooth nonfibrous surface of the part to be treated can be heated (unless still hot) to obtain viscoelastic and/or plastically deformable state in preparation of the die treatment and thereafter the part can be actively cooled or just permitted to cool. Heating can be achieved by, for example, infrared radiation.
This is particularly advantageous because only the surface needs to be rendered viscoelastic and/or plastically deformable can be heated.

~ 14 64506-206 As tests have shown, a variety of materials can be used as dies. For example the die member may he a fibrous mesh made of natural or synthetic fibres, metal or mineral fibres, leather or fel-t. Also can be used sinter material made of materials, metals or organic materials. Depending upon the type of material, the adhesion can be obtained as described. Minerals, metals or organic materials having a natural surface roughness or a porous surface may also be used.
As the fibres are produced, they are in effect tension stretched as the die and its fibres are removed. The fibres as they are being drawn inherently have oriented molecules which is an additional advantage; their strength is in fact greater than the strength of the material at large. If the synthetic material to be surface treated in the stated manner has a partially crystalline surface, then the degree of nucleation of the synthetic, particularly in the surface but mostly in the fibres is being changed thereby improving mechanical parameters and strength values. The method according to the invention can be carried out such that, upon making fibres on the surface of polyolefines, functional groups can form in the surface.
According to the invention, a coating can be applied following the formation of the fibres, without a specific adhesive between the bonding or coating material and the plastics material article.
DESCRIPTION OF THE DRAWINGS:
While the specification concludes with claims particularly pointing out and distinctly claiming the sub~ect matter which is regarded as the invention~ it is believed that the invention, the ~2~214 objects and features of the invention and further objects, features and advantages thereof will be better understood in connection with the accompanying drawings in which:
Figure 1 illustrates somewhat schematically a flow chart for carrying out a preferred embodiment of the method according to the present invention;
Figure 2 is a raster electron microscope photograph of a surface of a polypropylene surface part showing fringes; and Figure 3 is also a raster electron microscope photograph showing a die-treated surface and an untreated surface.
Figure 1 illustrates steps for applying a preferred embodiment of the method of the present invention to the surface of a component (e.g. a flat part 3) which has been made some time ago. However, the method may equally be applied to a part that has just been made or transported or otherwise delivered. This will occur in those instances in which the synthetic part to be made is to be subject for bonding, lacquering or other coating.
If such coating or the like is to be carried out immediately upon making the part then the device and equipment used for practising the method should be part of the assembly line or process equipment or be positioned direct~y downstream from it so that the still elevated temperature of that part can be utilized directly in practising the process; the surface may still be viscoelastic or plastically deformable.
Alternatively the part is passed first through an infrared radiation heating device whereby the surface of the part is heated to a temperature such that the surface strata assume a viscoelastic or plastically deformable consistency.

_ 9 _ ~264Z~4 64506-206 Next a die having a plurality of microscopic flexible, fibrous projections which by way of example are shown in a much enlarged structure of Figure 3. The die is either made of a material or has a surface stratum of a material such that the surface material of the part to be treated will embed the fibrous projections of the die whenever the material engages and flows around them. The die may be pressed into the surface by means of plunger action or the like. The die is left in place for a certain period of time such as 5 to 10 seconds during which the softened material of the part to be treated embedded with the fibrous projections slightly solidifies or becomes a little less viscous.
Thereafter the die is pulled off. Hence, these fibrous projections of the die tend to pull material which adheres to the fibrous projections outwardly from the surface. The speed of pulling may be selected to determine the rate of disengagement of the die from the part to be treated; a very rapid receding of projections occurs from the original surface if the relative speed of separation as defined is high. In this way the adhering material is not capable of detaching itself right away from the projections but will in fact be carried along by the receding projections and thereby fibres are drawn outwardly until the ends "snap". Thus the surface 6 obtains a fibrous consistency.
Subsequently the part to be treated may be passed through a cooling duct channel operating for example with cold air or regular air, and the surfaces and the newly formed small fibres projecting from the surface will cool so that their state and consistency changes to a non-viscoelastic and/or non-plastically 1~64Z~

deformable consi,stency.
In principal the die does not have to be of a particular configuration. Examples of die configurations are plate shaped or ribbon shaped dies.
The degree and extent of fibres may be controlled so that the adhering die may meet different consistencies as far as the viscosity of the surface of the part is concerned. The effective viscosity and plasticity in this regard must be expected to be different for different synthetic materials because the interaction as far as adhesion is concerned will differ. The length of the fibres drawn and pulled up from the surface of the part and there-fore the depth extension of fibre generation may be adjusted in that the thic~ness and/or viscosity of that part of the substrate which participates in the fibre generation on account of being rendered viscoelastic and/or plastically deformable, may vary.
Through this variation, in fact the length and extent of the fibres can be adjusted. Another variable and operating parameter is the residence time of the die fibres in the embedding surface of the synthetic. The number of fibres to be drawn as the die fibres recede is directly dependent on the number of die fibre projections i.e. of points engagement between die and part.
Figures 2 and 3 illustrate fibres as well as fibre ' making, on the surface of a polypropylene part as stated above.
Raster electron microscope photographs are illustrated here. At the lower right hand corner, the scale is indicated to be app]ied directly to the illustration. It can be readily seen that on account of the very dense distribution of fibres anything that is lZ~4;Z~4 made ~o adhere, be it a coati.ny or a bonding layer, to such a fibrous sur:Eace is no longer directly in contact with what is conventionally termed the surface but only or by far predominantly with the fibres themselves. The fibres become intimately embedded in any bonding agent, lacquer or othe:r coating material. Owing to the intimate interdigitation and embedment of the fibres within the solidifyiny bonding lacquer or coating material, on solidifi-cation -thereof -this rnaterial is in fact shrunk upon the fibres and holds or grips them very firmly in an embedding relationship.

1 ~ 41Z~ ~ 6~506-206 Fiyure 3 illustrates in partlcular detail how the surface of the part is being treated. The figure shows particularly in the lower lef-t hand corner an untrea~ed surface, and beyond the untreated surface one can see a dense package of more or less upwardly extending fibres. Beyond these fibres one can see a die and partial removal of the die from intimate contact.
The invention will be explained further in relation to comparative examples in that the bonding of syntheti.c in accordance with the state of the art is compared with bonding as it obtains after such a part has been treated in accordance with the invention and that its critical bonding surface is rendered fibrous.
Generally speaking polypropylene is a kind of material which by its nature is difficult to bond or even not at all bondable. One could pretreat the polypropyl~ne part in chromium sulphuric acid which results in improved adhesibility of polypropylene. In this regard the shear tension strength was measured with regard to single overlapping metal-to-synthetic bonding under utilization of an epoxy bonding agent for about 8 N/square mm as far as its polypropylene surface is concerned. The treatment involves particularly dipping the synthetic part for about 3 minutes, partially or comple~ely, into a chromium sulphuric acid bath having a temperatuxe of about 90 degrees C.
Thereafter the part has to be rinsed with water and drying has to follow. The thus treated surface must not be touched and soiling ~Z~42~4 64506-2~6 must be prevented up un~il bonding occurs. Therefore the bonding must follow as soon as possible after this pretreatment. The treatment was shown to exhibit very little and actually hardly any visible changes in surface texture and structure of that part.
Therefore one relies here completely upon the specific adhesion between the bonding agent and the polypropylene part. If reliance proves unjustified then the bonding will fail.
The described example should be compared with bonding of a part whose surface to the subject to bonding has been rendered fibrous as per the inventive method. If such a fibrous surface i5 obtained a shear ~ension, strength of likewise 8 N/square mm will be obtained. The use life is considerably improved as compared with ~he chemically pretreated synthetic part. If the inventive method is used to obtain a fibrous surface of a polypropylene part then a tension shear strength of 8 N~square mm is also obtainable but with a considerably better use life.
The fibrous surface structure and texture obtained is actually qualitatively new and different from conventionally roughened surfaces owing to the very large number of fibres. This renders the surface radially amenable to bonding to other parts including parts having been treated in like manner. The interesting aspect here is that any bonding agent has really only secondary function as the fibres of the several parts may intermesh. The parts surface treated as per the invention are quite insensitive against touching and soiling; they can be stored and used without subsequent ox supplemental treatment for purposes 12~Z14 6~506-206 of bonding or for purposes of receiving a coating at a much later time. Even if there is no ~pecific adhesion bet~een bonding agent and the polypropylene part, the strength of the bond obtains through ~he embedment of the fibre~ in the adhesive and the strong grip the adhesive has on the fibres on account of shrlnkage. Thls ensures the strength of the adhesion. The polypropylene part may be provided with a fibrous surface on a continuous basis whereby the fibres will be produced in each surface increment within approximately ~0 seconds. On the other hand chemical treatment requires about 5 minutes. Upon the invention undesired by-products on account of any pickling or the like are not produced.
Finally it should be mention that the term "die member" does not have to be made in its entirety of a material that may or may not adhere to the synthetic. Rather, the die needs to have only a coating or outer stratum of such a material.
The invention i5 not limited to the embodiments described above but all changes and modifications thereof, not constituting departures from the spirit and scope of the invention, are intended to be included.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An article made of a synthetic plastic material having a generally smooth surface, at least a part of the said surface having a fibrous surface texture comprising thereon outwardly-extending free-ending fibres at a density of the fibres of at least 1,000 per square millimeter, the said fibrous textured surface being capable of being more readily bonded or coated than the said smooth surface.

2. The article as claimed in claim 1, wherein the fibrous surface has at least 5,000 fibres per square millimeter.

3. The article as claimed in claim 1, wherein the fibrous surface has at least 10/000 fibres per square millimeter.

4. The article as claimed in claim l, 2 or 3, wherein the length of the fibres is at least 30 micrometers.

5. The article as claimed in claim l, 2 or 3, wherein the length of the fibres is at least 100 micrometers.

6. The articlesas claimed in claim ll 2 or 3, wherein the length of the fibres is from 30 micrometers to 3 millimeters.

7. The articlesas claimed in claim l, 2 or 3, wherein the length of the fibres is from 100 micrometers to l millimeter.

8. The article as claimed in claim l, wherein the surface has at least 10,000 fibres per square millimeter and the length of the fibres is within the range from 300 micrometers to 1 millimeter.

9. A method of making at least a portion of a generally smooth surface of an article made of a synthetic plastics material amenable to bonding or coating, which method comprises the steps of:
(A) applying a die member having a number of flexible fibrous projections to a viscoelastic or plastically deformable portion of the surface of the article such that the synthetic material will embed and adhere to the projections;
(B) withdrawing the die member from the portion such that a number of fibres are drawn outwardly from the portion while the fibres of the die are still embedded before the die member detaches itself from the article; and (C) causing the fibres to solidify thereby generating a dense layer of outwardly extending free ending fibres, wherein the density of the fibres is at least 1,000 per square millimeter.

10. The method as claimed in claim 9, wherein the die has at least 5,000 projections per square millimeter and as a result 9 the density of the fibres is at least 5,000 per square millimeter.

11. The method as claimed in claim 9, wherein the die has at least 10,000 projections per square millimeter and as a result, the density of the fibres is at least 10,000 per square millimeter.

12. The method as claimed in claim 9, 10 or 11, including the step of preheating the portion of the article so as to make the portion viscoelastic or plastically deformable before applying the die member.

13. The method as claimed in claim 9, 10 or 11, wherein the die member is made of at least one member selected from the group consisting of a natural fibre mesh, synthetic fibre mesh, a metal fibre mesh, a mineral fibre mesh, leather and felt.

14. The method as claimed in claim 9, 10 or 11, wherein the die member is made of a sintered mineralic or organic material.

15. The method as claimed in claim 9, 10 or 11, wherein the die member has a surface made of a porous mineral, metallic or organic material.

16. An article made of a polyolefin having a generally smooth surface, at least a part of the said surface having a fibrous surface texture comprising free ending thin fibres having a length of from 100 micrometers to 1 millimeter extending outwardly therefrom at a density of at least 5,000 per square millimeter, the said fibrous-textured portion being capable of being more readily coated than the said smooth surface.

17. The article as claimed in claim 16, which is made of polypropylene.

18. The method as claimed in claim 9, 10 or 11, wherein:
the synthetic plastic material is a polyolefin; and the viscoelastic or plastically deformable portion of the surface of the article is provided by processing the article while still hot immediately after it is removed from a mold or by heating the desired portion by infrared radiation.

19. A method as claimed in claim 9, 10 or 11, which further comprises applying subsequently an adhesive or coating to the portion such that the fibres become embedded in the coating or adhesive and, upon solidification of the adhesive or coating on account of shrinking, each of the fibres is firmly embedded in the coating or adhesive through shrinking.