CA2109706A1 - Surface treatment of polyoxymethylene moldings and moldings obtained thereby - Google Patents

Abstract

Described is a method of treating the surface of moulded polyoxymethylene articles by treating with aqueous acids, the method being characterized in that mixtures of aqueous hydrochloric acid (strength at least 20 % by wt.) with mono- or diethers of an aliphatic polyhydroxy compound are used, the components of the mixture being in the ratio 90:10 to 10:90 % by vol.

Description

~ 37 3S o.~. 0050/42489 Surface treatment of polyoxymethylene moldings and moldings obtained thereby The present invention relates to a process for the surface treatment of moldings based on polyoxymethylenes by treatment with aqueous acids.
The present invention further relates to moldings obtainable by this process and to the use thereof for metallization, ie. for coating with metals.
Polyoxymethylenes are engineering plastics and have found wide utility in many areas.
However, interesting applications in the field of coated, printed or metallized materials have hitherto been denied to polyoxymethylenes. The reason for this is said to be in particular the inadequate wettability and 1~ the poor adhesion of thin layers on polyoxymethylene.
~he subject is discussed in papers by U. Rossler, Plaste und Kautschuk 21 (1974), 132, and V. Zorll, Deutsche Farbenzeit~ng 32 (1978), 160.
Various proposals, in particular for the surface pretreatment, are known from W. Wuich Kunststoffberater 7/8 (1982), 24. Aqueous H3PO4 is recommended as a priming solution for polyoxymethylenes for the application of adheqives .
For the metallization of polyacetals it is known from Alex S. Forschirm ANTEC ~85, p. 1090, to use H~SO4/HCl mixtures in conjunction with palladium chloride and tin chloride.
Glazing processes for polyoxymethylenes are known from L. Dorn et al., Kunststoffberater 10 ~1982), 40. The ~0 priming bath here consists for example of perchloroethylene, para-toluenesulfonic acid, dioxane and diatomaceou~ earth.
Further pretreatments for coating polyoxymsthylene moldings are known from K. Heberlein, Metalloberflache 39 (1985~, 355, and H. Benninghoff, Metalloberflache 39 (198S), 10.
~:

7 D`'3 - 2 - O.Z. 0050/42489 The disadvantages of existing processes are in particular of an environmental and workplace nature (recycling, waste disposal, occupational hygiene and workplace safety).
In addition, the adhesion between the POM surface and the coating material is not satisfactory, since it is impossible to achieve uniform adhesion over the entire surface area.
Moreover, existing processes are greatly dependent on the nature of the polyoxymethylene (eg.
whether filler or glass fiber-reinforced or toughness-modified) and also on the nature of the coating material `~ (eg~ metal, lacquer or adhesive).
It is an object of the present invention to provide a proceæs for the surface treatment of moldings based on polyoxymethylene that is free of the above-described disadvantages and gives moldings which are subsequently readily coatable.
We have found that this object is achieved ~;~ 20 according to the invention by a process for surface ;~ treatment of moldings based on polyoxymethylenes by 5`.'~ treatment with aqueous acids, which comprises using a mixture of at least 20~ strength by weight aqueous hydrochloric acid with a mono- or diether of an aliphatic polyfunctional hydroxy compound in a mixing ratio of from 9Q:10 to 10:90 % by volume. The weight percentage is based on the HCl content at 20C, which should be at least 20% by weight, preferably at least 25% by weight and in particular at least 30% by weight.
The process of the invention is suitable for the surface treatment of moldings based on polyoxymethylenes, ie. consisting essentially of a polyoxymethylene homopolymer or copolymer. It may be modified with fillers, polymers which improve the impact toughness, ~other~polymers and customary additives.
Such polyoxymethylene homopolymers and copolymers arè known per se to the person skilled in the art and are ,/ .,,~, ~, `~ 7 ~ ,~

- 3 - o.Z. 0050/42489 described in the literature.
The homopolymers are generally prepared by polymerization of formaldehyde or trioxane, preferably in the presence of suitable catalysts.
For the purposes of the present invention preference is given to polyoxymethylene copolymers which as well as -CH2O- repeat units contain up to 50, preferably from 0.1 to 20, in particular from 0.3 to 10, mol% of repeat units I

O C - C (Rs) -R R
where R1 to R4 are each independently of the others hydrogen, Cl-C4-alkyl or haloalkyl of from 1 to 4 carbon atoms, R5 is -CH2-, -CH2O- or C1-C4-alkyl- or Cl-C4-haloalkyl-substituted methylene or oxymethylene, and ~ is from 0 to 3. AdYantageously, these groups can be introduced into the copolymers by ring opening of cyclic ethers. Preferred cyclic ethers are those of the formula Rl--C O

R3--C (Rs ) n Rq where R1 to R5 and n are each as defined abo~e. Examples of cyclic ethers are ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepane, and examples of comonomers are linear oligo- or polyformals such as polydiox~lane and polydioxepane.
-~ It is also possible to use oxymethylene terpolymers prepared for example by reacting trioxane, one of the cyclic ethers described above and a third ; ~ ~

l U~ 7~ ~

- 4 - O.Z. 0050/42489 monomer~ preferably a bifunctional compound of the formula CH~ CH CH2 - Z - CH2 - CH - cH2 O o ~here Z is a chemical bond, -O-, -ORO- (R = Cl-C8-alkylene or C2-C~-cycloalkylene).
Preferred monomers of this kind are ethylens diglycide, diglycidyl ethers and diethers of glydicyls and formaldehyde, dioxane or trioxane in a molar ratio of 2:1 and also diethers of 2 mol of glycidyl compound and 1 mol of an aliphatic diol of from 2 to 8 carbon atoms, for example the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, cyclobutane-1,3-diol, - 1,2-propanediol and cyclohexane-1,4-diol, to name but a few.
Processes .for preparing the above-described hompolymers and copolymers are known to the person skilled in the art and described in the literature, so ~: that no details are required here.
~ The pr~ferred polyoxymethyl~ne copolymers have melting points o~ at least 150C and weight average molecular weight~ M~ within the range from 5000 to 150~000, p~eferably from 7000 to 100,000.
End group stabilized polyoxymethylene polymers which have C-C bonds at the chain ends are particularly preferred.
Additionally, the polyoxymethylenes may ccntain ~rom 0 to 50, preferably from 5 to 40, in particular from 10 to 30, ~ by weiqht of an impact modifying polymer.
Suitable for this purpose are in principle all kind~ of rubbers known per se, ~but graft rubbers having a butadiene-based grafting base and a graft shell of an aromatic vinyl monomer, (meth)acrylonitrile, ~:~ (meth)acrylic acid or (meth~acrylic esters have proved ~ L~ 7~

- 5 - o.Z. 0050/42489 particularly suitable in some cases. Appropriate products are described for example in DE 19 64 156, EP-A 156 285 and DE-A 34 41 547; suitable products are commercially available as Paraloid~ tRohm & Haas). For further details reference is made to the publications mentioned.
A particularly suitable graft rubber is a product having a grafting base of polybutadiene and small proportions (from 0.5 to 3~ by weight) of t-dodecylmercaptan, onto which a graft shell of a mixture of acrylic esters and optionally styrene and (meth)acrylonitrile have been grafted; in particular, the graft shell consists of a mixture of methyl methacrylate and n-butyl acrylate in a weight ratio of from 70:30 to 95:5, in particular from 85:15 to 95:5.
Polyurethanes for impact modification are described for example in EP-A 115 846, EP-A 115 847, EP-A 116 456, EP-A 117 664 and EP-A 327 384. Such products are commercially available for example- as Desmopan (Bayer AG) or Elastollan (Elastogran Polyurethane Elastomere GmbH).
Optionally, the polyoxymethylenes may contain from 0 to 40, preferably from 5 to 30, ~ by weight of fibrous or particulate fillers or mixtures thereof.
Examples of reinforcing fill~rs are potassium titanate whiskers, carbon fibers and preferably glass fibers, glass fibers being usable for example in the form of glass fabrics, mats, webs and/or rovings or chopped fiber made of low-alkali E-glass with a di~meter of from to 200 ~m, preferably from 8 to 50 ~m. Following incorporation, fiber fillers preferably have an average length of from 0.05 to 1 mm, in particular from 0.1 to 0.5 mm.
Other suitable fillers are for example woll-astonite, calcium carbonate, glass balls, quartz powder, silicon nitride, boron nitride and mixtures thereof.
Customary additives and processing aids for POM
are for example formaldehyde scavengers r plasticizers, ~,~

- 6 - O.Z. 0050/42489 lubricants, antioxidants, adhesion promoters, fillers, light stabilizers and pigments. The proportion of such fillers is in general in the range from 0.001 to 5% by weight (stabilizers, pigments and lubricants) or from 5 5to 40% by weight (fillers).
Appropriate compounds are known to the person skilled in the art and described for example in EP-A 327 384.
The moldings can be produced ~rom the above-10described polyoxymethylene molding compositions in a conventional manner, for example by extrusion, injection or blow molding. The processing conditions will of course depend on the composition of the polyoxymethylene material and are known to the person skilled in the art.
15The surface treatment of moldings based on polyoxymethylenes is effected for example with the aid of a mixture of at least 20% strength by weight hydrochloric acid and a mono- or diether of an alipha~tic polyfunctional hydroxy compound in a mixing ratio of from 2090:10 to 10:90, preferably from 30:70 to 70:30, in particular 50:50, % by volume. The duration and temperature of the treatment are related; the higher the temperature, the shorter the treatment time. An increase in the concentration of the HCl content likewise results 25in a shorter treatment time.
Preference is given to a temperature of from 10 to 80C, in particular from 15 to 45C, and very particularly frQm 18 to 35C.
The treatment time is preferably within the range 30from 0.5 to 25 minutes, in particular from 1 to 15 minutes, especially from 5 to 10 minutes.
It has been found to be advantageous if, given a hydrochloric acid concentration of from 25 to 37% by weight, the product of treatment temperature in degrees 35Celsius and the treatment time in seconds is within the range from 1000 to 21,000, since the coating of so pretreated moldings gives particularly good crosshatch 7 D ~
7 - o.Z. 0050/42489 results.
The pretreatment mixture contains as a further ingredient a mono- or diether of an aliphatic polyfunctional hydroxy compound of the general formula I

H H
O (C ~ (I)y O\

where Rl is hydrogen or C1- to C10-alkyl, R2 is hydrogen, Cl- to Cl0-alkyl or hydroxyl, R3 i~ hydrogen or C1- to C10-alkyl, R4 is hydrogen, Cl- to C1O-alkyl or -(CH2~n-oR5, R5 is hydrogen or C1- to C10-alkyl, x is an integer from 1 to 10, y is 0 or 1, and n is an integer from 2 to 4.
Preferred ~adicals Rl are methyl, ethyl, propyl and n-butyl, and R2 i5 preferably hydrogen, methyl or hydroxyl - and if it is hydroxyl x should be at least 2.
R3 is preferably methyl or hydrogen, R4 is hydrogen or a (CH2)noR5 group, where n is preferably 2 or 3, and Rs is preferably methyl.
Preferred compounds of the general formula are l-methoxy-2-hydroxypropane (propylene glycol l-methyl ether), dipropylene glycol monomethyl ether, ethylene glycol monomethyl ether and diethylene glycol monomethyl ether and also di- or monomethyl ethers of glycerol.
Such additives are ~enerally known and commercially available (eg. Solvenon~ or methylglycol).
After the pretreatment the moldings are advantageously freed of adhering acid residues by rinsing in water.
Thereafter the moldings can be metallized in a conventional manner.
In general the sequence is activation, reduction, 7 ~ ~

- 8 - O.Z. 0050~42489 chemical metallization and, finally, electro-metallization.
The products obtained have a particularly high adhesion and resistance in the event of temperature changes and also an altogether smooth, hardly roughened surface. Not only high but also uniform adhesion is achieved over the entire surface area of the molding, substantially regardless of the nature of the polyoxymethylene used.
The effect of the treatment with HCl is to modify the ether groups of the polyoxymethylene present in the surface layer, particularly good results being obtained if at least 5 mol% of the ether groups originally present are modified in a surface layer of not more than 100 ~m thickness.
It is clear from the above that the moldings of the invention are suitable in particular for subsequent metallization. Moldings treated by the process of ~the invention are also readily lacquerable and printable. The metallized moldings thus obtained can be used in many areas, for example in the automotive sector or in the field of sanitary technology and consumer products (eg.
cosmetics containers).
EX~MPLES
A 2 r~m thick sheet of a polyoxymethylene copolymer from trioxane and from 2.5 to 3~ by weight of butanediol formal, based on the total monomer content (Ultraform N 2320 from Ultraform Gm~H with a melt index of 7.4 g/10 min (190C/2.16 kg1) was precleaned with isopropanol and then treated with an etching medium.
(Composition and also mixing ratios, temperature and treatment duration see Table 1.) Thereafter the acid residues were removed by an ultrasou~d treatment tabout 10 min) in saturated sodium-carbonate solution, followed by rinsing with distilled water and drying.
Coating followed in a conventional manner with an h 1 U ~ 7 ~ ~
- g - o.z. 0050/42489 epoxy-based solution of adhesive-forming components.
The adhesion was assessed after 144 hours by the crosshatch test of German Standard Specification DIN 53 151. This standard lays down the following classification of crosshatch ratings:
_ _ _ Crosshatch Description rating _ Gt ~ The edges of the cuts are completely smooth; no spalling.
_ .
Gt 1 Small splinters have spalled away at the intersections of the cross-cut lines;
spalled area about 5% of the squares.
_ ._ _ _ _ Gt 2 The coating has spalled away along the edges of the cuts and/or the intersections of the cross-cut lines; spalled area about 15% of the squares.
_ _ Gt 3 The coating ha~ partially or completely spalled away in broad strips along the edges of the cuts and/or the coating has partially or completely spalled away from individual squares; spalled area about 35%
of the squares.
: _ _ .
Gt 4 The coating has completely or partially spalled away in broad strip~ along the edgss of the cuts and/or from individual squares; spalled axea about 65% of the . squares .
Gt 5 Spalle~ area more than 65% of the squares.
_ _ .

The test of the temperature change resistance was carried out in accordance with German Standard Specification DIN 53 496.
For comparison (C), identical polyoxymethylene sheets were identically pretreated with other concentration ratios of the acid or other aqueous acids, and coated.

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- 13 .- O.Z. 0050/42489 Etching medium, pret~eatment temperature and time and coating method corresponded to Example 2 in accordance with the invention. Different polyoxymethylene compositions were used.

~x mpl- Polyoxym-thyl-n- compo-ition Cro-~batch Tomp-raturo cb~ngo r~ting to-t . _ _ 7 90~ by weight of POH copolym-r of ~xampl- 2 0 pao-10~ by woight of gl~-~ fibor 8 90~ by waight of PO~ copolym-r 0 pa~-25~ by w-ight of gla-- fibor 9 80~ by w ight of PO~ copolymor 0 pa~-20~ by w~lght of wolla-tonlt-~- 20 lO 96 5~ by wolght of PO~ copolyolor pab-_ 3.5~ by w lght of ebnlk ~- ~ 11 80~ by w-~ght of PO~ copolym r 0 pa--20~ by w ight of earbon fib-r ~ ~ 25 .
12 9~ by w-lgbt of P0~ eopolym-r 0 pa--; _ 10~ br w ight of TPU~
13 70~ by w-ight Qf PO~ eopolym-r 0 pao-~0 30~ by w--igbt of TPU;~
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~ hormopla~tie polyurothan- (Dc~mop~n from B~y-r AG, L-vorku--n~

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Claims (7)

We claim:

1. A process for surface treatment of moldings based on polyoxymethylenes by treatment with aqueous acids, which comprises using a mixture of at least 20% strength by weight aqueous hydrochloric acid with a mono- or diether of an aliphatic polyfunctional hydroxy compound in a mixing ratio of from 90:10 to 10:90 % by volume.

2. A process as claimed in claim 1, wherein the treatment is carried out at from 10 to 80°C.

3. A process as claimed in claim 1 or 2, wherein the treatment is carried out for 0.5 to 25 minutes.

4. A process as claimed in claim 1 or 2 or 3, wherein the mono- or diether of an aliphatic poly-functional hydroxy compound has the general formula I:

I .
where R1 is hydrogen or C1- to C10-alkyl, R2 is hydrogen, C1- to C10-alkyl or hydroxyl, R3 is hydrogen or C1- to C10-alkyl, R4 is hydrogen, C1- to C10-alkyl or -(CH2)n-OR5, R5 is hydrogen or C1- to C10-alkyl, x is an integer from 1 to 10, y is 0 or 1, and n is an integer from 2 to 4.

5. A molding obtainable as claimed in claim 1 or 2 or 3 or 4.

6. A polyoxymethylene molding in which in a surface layer of not more than 100 µm thickness at least 5 mol %
of the ether groups originally present have been modified by reaction with HCl.

7. A method of using a molding as claimed in claim 5 or 6 as a coating, printing, metallization, cementing or flocking substrate.