Note: Descriptions are shown in the official language in which they were submitted.
~ 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.
~:
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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
,/ .,,~, ~,
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- 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
R2
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
R2
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
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- 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,
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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
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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\
R1 R2 R3 R4
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,
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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
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- 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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~ 10 ~ O.Z. 0050/4248
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~1~37~
O. Z . 0050/42489
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7 ~ ,!
- 12 - O. Z . 0050/42489
~ , .
7 ~ ~
- 13 .- O.Z. 0050/42489
EXAMPLES 7 TO 13
Etching medium, pret~eatment temperature and time and
coating method corresponded to Example 2 in accordance
with the invention. Different polyoxymethylene
compositions were used.
TABLE 2
~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;~
~ __ . __ .
~ hormopla~tie polyurothan- (Dc~mop~n from B~y-r AG, L-vorku--n~
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