PROCESS FOR THE ADHESION OF POLY-P-XYLYLENE TO SUBSTRATES AND MANUFACTURED ARTICLES

Abstract: A process for the adhesion of para-xylylene vapours to solid substrates characterized by the use of an unsaturated ester of phosphoric acid of formula:

Description: The invention relates to a process for the adhesion of poly-p-xylylene and derivatives thereof to solid substrates. More in particular, the invention relates to the use of adhesion promoters in order to cause poly-p-xylylene and derivatives thereof to adhere to various substrates. As is known, for the coating of solid substrates to be utilized in the electronic and electrotechnic fields, use is made of poly-p-xylylene or of its derivatives prepared by poly­ merization in situ of (2,2)-paracyclophane or derivatives there­ of of formula: wherein X may be a halogen, in particular chlorine, an alkyl, aralkyl or halogen-aralkyl radical, and m and n may be an integ­ er from 0 to 4, according to the well known vacuum vapour depos­ ition technique. An important problem encountered in the application of (2,2)-paracyclophane or derivatives thereof of formula (II) re­ sides in the difficult obtainment of a perfect and homogeneous adhesion of the corresponding polymer of p-xylylene to the subs­ trate to be coated. Actually, the adhesion of the p-xylylene polymers to very different surfaces to be coated, such as metal­ lic, plastic, ceramic surfaces and the like, is not quite satis­ factory. The problem of the adhesion is particularly important in the case of substrates to be utilized in electronics, where the adhesion of the p-xylylene polymers not only must be perfect, but also shall be attained without interfering with the (2,2)-­ paracyclophane deposition process, at the same time maintaining unaltered the electric and electronic properties of the subs­ trates. Thus, for example, the use of adhesives applied by spreading from a solvent do not offer a satisfactory solution of the problem because they adversely affect the vacuum vapor deposition processes. To solve this problem it was also suggested to use an adhesion promoter capable of acting as a bridge between the subs­ strate surface and the p-xylylene polymer. However, the known ad­ hesion promoters lead to satisfactory results only in a few types of substrates to be coated, wherefore the most suitable type of promoter is to be selected time by time. It has been now found by the Applicant that the p-xyl­ ylene polymers can be made to adhere to any type of substrate by using, as an adhesion promoter, an unsaturated ester of phosphor­ ic acid having formula : wherein at least one of radicals R₁, R₂ and R₃ is hydrogen and at least one of said radicals is : M - O - D - with M being methacrylate or acrylate and D a straight or ram­ ified alkylene radical, containing 2 to 6 carbon atoms, i.e. an ethylene,propylene,butylene,pentylene or hexylene radical,straight-­ chain or branched, optionally halogen-substituted. Said unsaturated esters of phosphoric acid having formula (I) not only perfectly adhere to any solid substrate, but also act as adhesion promoters for the p-xylylene vapors which, after deposition onto the substrate surface, form a poly­ mer. The present invention relates also to the substrate treated with a phosphoric acid unsaturated ester of formula (I) and coated with a p-xylylene polymer according to the vapor vacuum deposition technique. The divalent radicals of p xylylene or derivatives thereof can be prepared by pyrolysis, at a temperature higher than 400°C, of (2,2)-paracyclophane or of a derivative thereof of formula (II), so generating vapors of reactive radicals of formula: where X, m and n are the same as defined above. When m and n are equal and the substituents on each ring are equal, two moles of the same p-xylylene radical are formed, which, by condensation, give rise to a substituted or non-substituted p-xylylene homopolymer. Conversely, when m and n are different or when substituents X in each radical are dif­ ferent, their condensation leads to p-xylylene copolymers. The p-xylylene divalent radicals condense and polymer­ ize almost instantaneously on the substrate forming a compact and homogeneous coating film. Illustrative examples of phosphoric acid unsaturated esters of formula (I), which are useful as adhesion promoters in the present invention, are : methacrylethyl,phosphate, acryl­ ethyl phosphate, dimethacrylethyl phosphate, diacrylethyl phos­ phate, methacrylpropyl phosphate, acrylpropyl phosphate, dime­ thacrylpropyl phosphate, methacrylethyl-methacrylpropyl phos­ phate, etc. Among these, methacrylethyl phosphate is preferred. The phosphoric acid unsaturated esters of formula (I) are applied to the substrate as a solution. The solvent may vary as a function of the utilized ester. Suitable solvents may be : water; an alcohol having a low number of carbon atoms, such as methyl alcohol, ethyl alcohol, isopropyl alcohol, etc.; a halogenated hydrocarbon such as trichloroethylene; water-alcohol, mixtures, such as water-ethyl alcohol, or water-methyl alcohol, water-isopropyl alcohol, etc., in any volume ratio. In the solution, the amount of phosphoric acid un­ saturated ester of formula (I) may range from 0.05 to 50% by weight, depending on the utilized solvent; the preferred concen­ trations are those between 0.1 and 5% by weight. The substrate can be treated with such solutions ac­ cording to conventional techniques such as direct immersion of the substrate into the solution, or by means of spraying, etc. The solvent is then removed from the substrate by evaporation at room temperature or at higher temperatures, de­ pending on the characteristics of the employed solvent. The subs­ trate to be coated according to the process of the invention may be any solid of organic or inorganic nature, which can assume any geometrical shape, such as foils, fibres, particles and the like. The substrate is subjected to washing and to degreasing before undergoing the process of the present invention. Illustrative examples of organic or inorganic subs­ trates are : the metal substrates such as those of aluminium, iron, copper, steel, molybdenum, etc.; the substrates of metal­ lic and non-metallic oxides such as those of aluminium, titani­ um, lead, copper, beryllium, manganese, tungsten, vanadium, etc. and silicon oxides; the organic solid substrates such as the epoxy compounds and the thermoplastic and thermosetting com­ pounds of different composition. For the generation and the deposition of the vapors of reactive divalent radicals of p-xylylene onto the treated sur­ face of a substrate it is possible to use any known apparatus such as the one described for example in Kirk-Othmer - Encyclopaedia of Chemical Technology - Third Edition - Vol. 24, pages 746-747. Generally, said apparatus comprises a chamber suited to contain the substrate to be coated, which is maintained at a temperature below 200°C or in general below the condensation temperature of the particular type of p-xylylene utilized. Said chamber is in communication with a vaporization-pyrolysis furn­ ace through a labyrinth passage. The furnace is maintained at a temperature which is high enough to cause the evaporation and pyrolysis of (2,2)-paracyclophane or derivatives thereof. Said (2,2)-paracyclophane or derivative thereof is left in the furnace over a period of time as is sufficient for the evapor­ ation and the pyrolysis in order to form the corresponding di­ valent radicals of p-xylylene. These radicals pass to the de­ position chamber and polymerize when coming into contact with the surface of a substrate prepared according to the process of the present in­ vention,thus forming a coating film consisting of the p-xylylene polymer or of a derivative thereof. It has been found that when it is operated according to the process of the invention, the p-xylylene polymers exhibit and improved adhesion to the substrate at room temperature and, after treatment in water, both at room temperature and at a higher temperature, with respect to the adhesion attained with an untreated substrate. The invention is further elucidated by the following examples, which are to be construed as merely illustrative and not limitative of the scope of the invention. Example 1 A substrate consisting of an epoxy resin foil having a copper printed circuit was degreased by immersion in iso­ propyl alcohol for about 10 minutes. The substrate was then dried in air to remove the solvent. A solution was prepared by dissolving 5 g of methacryloethyl phosphate (a product placed on the market by AKZO under the trademark NOURYCRYL MP 12) in 500 cc of isopropyl alcohol and 500 cc of distilled water. The solution was allowed to stand overnight. The substrate was im­ mersed into the solution and left there during 30 minutes, then it was dried in air for 30 minutes and in hot air at 70°C during further 30 minutes. The substrate was put into the vapor deposition chamber of an apparatus for the polymerization of dichloro-(2,2)-paracyclo­ phane. 100 g of dichloro-(2,2)-paracyclophane were evaporated at 185°C and at 50 mm of Hg. The vapors were heated in a quartz tube at 665°C in order to pyrolize the dichloro-(2,2)-paracyclo­ phane and to form the corresponding chloro-p-xylylene divalent radicals. The radicals were introduced into the deposition chamber maintained at a temperature lower than 70°C, where they condensed on the substrate surface to form a poly-p-xylylene film of about 10 µm thickness. The adhesion of the film to the substrate, measured according to ASTM D 3359-78, was corresponding to classification 5. The adhesion remained unaltered both under normal atmospheric cond­ itions and after a 24-hour treatment in water at room temper­ ature. Example 2 Example 1 was repeated, but using an aluminium foil as a substrate. Also in this case, an excellent adhesion of the poly-chloro-p-xylylene film to the substrate was obtained, with classification 5 accord­ ing to standard ASTM D 3359-78. The adhesion remained unaltered under normal atmospheric conditions as well as after a 24-hour treatment in water at room temperature. Example 3 Example 1 was repeated but using a stainless steel foil as a substrate. A perfect adhesion of the film to the substrate was obtained (classification 5), and such adhesion remained un­ altered also after a 24-hour treatment in water at room temper­ ature. Example 4 Following the modalities of example 1, a glass sheet was coated. The adhesion of the poly-chloro-p-xylylene film was excellent and was corresponding to classification 5 according to ASTM D 3359-78. Example 5 Operating according to example 1, a glass sheet was first dipped into an aqueous solution containing 0.5% by weight of methacrylethyl phosphate and then coated with a poly-chloro-­ p-xylylene film. The adhesion of the poly-chloro-p-xylylene film was excellent and was corresponding to classification 5 of according to ASTM D 3359-78.

Claims: 1. A process for the adhesion of a p-xylylene polymer to or­ ganic or inorganic solid substrates, which comprises treat­ ting the substrate surface with an adhesion promoter and contacting said coated substrate with vapors of reactive divalent radicals of p-xylylene or derivatives thereof which polymerize after deposition onto the substrate surface, thus form­ ing a coating film, characterized in that said adhesion promoter is a phosphoric acid unsaturated ester having formula : wherein at least one of radicals R₁, R₂ and R₃ is hydrogen and at least one of said radicals is : M - O - D - M being methacrylate or acrylate and D being a straight or ramified alkylene radical containing 2 to 6 carbon atoms, optionally halogen-substituted. 2. The process according to claim 1, wherein the substrate sur­ face is treated with a solution of a phosphoric acid un­ saturated ester having formula (I). 3. The process according to claim 2, wherein the solvent of the phosphoric acid unsaturated ester having formula (I) is selected from water, an alcohol having a low number of car­ bon atoms, a halogen hydrocarbon and water-alcohol mixt­ ures. 4. The process according to claim 2 or 3, wherein the solution concentration ranges from 0.05 to 50% by weight, preferably from 0.1 to 5% by weight. 5. The process according to any of the preceding claims, where­ in the phosphoric acid unsaturated ester of formula (I) is methacrylethyl phosphate. 6. The process according to any of the preceding claims, where­ in the vapors of divalent radicals of p-xylylene or deriv­ atives thereof are obtained by pyrolysis, at a temperature higher than 400°C, of (2,2)-paracyclophane or derivatives thereof of formula : wherein X may be a halogen, in particular chlorine, an alkyl radical, an aralkyl radical or a halogen-aralkyl radical, and m and n may be an integer from 0 to 4. 7. An organic or inorganic substrate, particularly suitable for being utilized in the electronic and electrotechnic fields, treated with a phosphoric acid unsaturated ester having formula (I) and coated with a polymer of p-xylylene or a derivative thereof containing the repeating units : wherein X may be a halogen, in particular chlorine, an al­ kyl radical, an aralkyl radical or a halogen-aralkyl radic­ al, and m and n may be an integer from 0 to 4. 8. The substrate according to claim 7, wherein the unsaturated ester of phosphoric acid is methacrylethyl phosphate.

Inventors: CICUTA, ALDO, DR., FIORE, LEONARDO, DR.
Applicants: MONTEDISON S.P.A.
Application year: 1986
Main classification: B05D7
IPC codes: H05K3, C08G61, C08J7, H01B1, B05D7