U.S. patent application number 14/374000 was filed with the patent office on 2015-01-15 for method for producing matt copper deposits.
The applicant listed for this patent is Atotech Deutschland GmbH. Invention is credited to Philip Hartmann, Stefan Kretschmer, Bernd Roelfs.
Application Number | 20150014177 14/374000 |
Document ID | / |
Family ID | 47226182 |
Filed Date | 2015-01-15 |
United States Patent
Application |
20150014177 |
Kind Code |
A1 |
Kretschmer; Stefan ; et
al. |
January 15, 2015 |
METHOD FOR PRODUCING MATT COPPER DEPOSITS
Abstract
The present invention relates to a method for deposition of a
matte copper coating wherein a first copper layer is deposited from
an aqueous copper electrolyte which does not contain an organic
compound comprising divalent sulfur. A second copper layer is then
deposited onto the first copper layer from an aqueous copper
electrolyte comprising a first and a second water soluble
sulfur-containing additive wherein the first water soluble
sulfur-containing compound is an alkyl sulfonic acid derivative and
the second water soluble sulfur-containing additive is an aromatic
sulfonic acid derivative. The method provides copper layers with a
homogeneous and adjustable matte appearance for decorative
applications.
Inventors: |
Kretschmer; Stefan; (Berlin,
DE) ; Hartmann; Philip; (Berlin, DE) ; Roelfs;
Bernd; (Berlin, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Atotech Deutschland GmbH |
Berlin |
|
DE |
|
|
Family ID: |
47226182 |
Appl. No.: |
14/374000 |
Filed: |
November 27, 2012 |
PCT Filed: |
November 27, 2012 |
PCT NO: |
PCT/EP2012/073688 |
371 Date: |
July 23, 2014 |
Current U.S.
Class: |
205/182 |
Current CPC
Class: |
C25D 3/38 20130101; C25D
5/10 20130101 |
Class at
Publication: |
205/182 |
International
Class: |
C25D 3/38 20060101
C25D003/38; C25D 5/10 20060101 C25D005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2012 |
EP |
12152390.6 |
Claims
1. A method for deposition of a matte copper coating comprising, in
this order, the steps a. providing a substrate, b. depositing a
first copper layer onto the substrate from a first aqueous
electrolyte comprising a source of copper ions, at least one acid
and at least one polyether compound wherein said first electrolyte
does not contain an organic compound comprising divalent sulfur and
c. depositing a second copper layer onto the first copper layer
from a second aqueous electrolyte comprising a source of copper
ions, at least one acid, a first water soluble sulfur-containing
additive selected from the group consisting of alkyl sulfonic acid
derivatives and a second water soluble sulfur containing additive
selected from the group consisting of aromatic sulfonic acid
derivatives wherein a current density is applied to the substrate
during steps b and c.
2. The method for deposition of a matte copper coating according to
claim 1 wherein the at least one polyether compound in the first
electrolyte is selected from the group consisting of polyalkylene
glycols and polyglycerines.
3. The method for deposition of a matte copper coating according to
claim 1 wherein the at least one polyether compound in the first
electrolyte is selected from the group consisting of
poly(1,2,3-propantriol), poly(2,3-epoxy-1-propanol) and derivatives
thereof.
4. The method for deposition of a matte copper coating according to
claim 1 wherein the at least one polyether compound in the first
electrolyte is selected from the group consisting of compounds
according to formulae (1), (2) and (3): ##STR00009## wherein n is
an integer from 1 to 80; ##STR00010## wherein n is an integer
>1, m is an integer >1 with the proviso n+m is .ltoreq.30;
##STR00011## wherein n is an integer from 1 to 80; and wherein
R.sup.6, R.sup.7, R.sup.8 and R.sup.9 are identical or different
and are selected from the group comprising hydrogen, alkyl, acyl,
phenyl and benzyl.
5. The method for deposition of a matte copper coating according to
claim 4 wherein the molecular weight of the compounds according to
formulae (1), (2) and (3) ranges from 160 to 6000 g/mol.
6. The method for deposition of a matte copper coating according to
claim 1 wherein the concentration of the at least one polyether
compound in the first electrolyte ranges from 0.005 g/l to g/l.
7. The method for deposition of a matte copper coating according to
claim 1 wherein the first water soluble sulfur-containing additive
in the second electrolyte is selected from the group consisting of
compounds according to formulae (4) and (5):
R.sup.1S--(CH.sub.2).sub.n--SO.sub.3R.sup.2 (4)
R.sup.3SO.sub.3--(CH.sub.2).sub.m--S--S--(CH.sub.2).sub.m--SO.sub.3R.sup.-
3 (5) wherein R.sup.1 is selected from the group consisting of
hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium
and ammonium, n ranges from 1 to 6, R.sup.2 is selected from the
group consisting of hydrogen, methyl, ethyl, propyl, butyl,
lithium, sodium, potassium and ammonium, R.sup.3 is selected from
the group consisting of hydrogen, methyl, ethyl, propyl, butyl,
lithium, sodium, potassium and ammonium and m ranges from 1 to
6.
8. The method for deposition of a matte copper coating according to
claim 1 wherein the concentration of the first water soluble
sulfur-containing additive in the second electrolyte ranges from
0.0001 to 0.05 g/l.
9. The method for deposition of a matte copper coating according to
claim 1 wherein the second water soluble sulfur-containing additive
in the second electrolyte is selected from the group consisting of
compounds according to formulae (6) and (7):
R.sup.4S.sub.y--X--SO.sub.3M (6) wherein R.sup.4 is selected from
the group consisting of ##STR00012## and hydrogen; X is selected
from the group consisting of ##STR00013## y is an integer from 1 to
4 and M is selected from the group consisting of hydrogen, sodium,
potassium and ammonium; and ##STR00014## wherein R.sup.5 is
selected from the group consisting of H, SH and SO.sub.3M and M is
selected from the group consisting of hydrogen, sodium, potassium
and ammonium.
10. The method for deposition of a matte copper coating according
to claim 1 wherein the concentration of the second water soluble
sulfur-containing additive in the second electrolyte ranges from
0.005 to 1 g/l.
11. The method for deposition of a matte copper coating according
to claim 1 wherein the second electrolyte further comprises at
least one carrier additive.
12. The method for deposition of a matte copper coating according
to claim 11 wherein the at least one carrier additive selected from
the group consisting of polyvinylalcohol, carboxymethylcellulose,
polyethylene glycol, polypropylene glycol, stearic acid
polyglycolester, alkoxylated naphtoles, oleic acid polyglycolester,
stearylalcoholpolyglycolether, nonylphenolpolyglycolether,
octanolpolyalkylenglycolether,
octanediol-bis(polyalkylenglycolether),
poly-(ethyleneglycol-ran-propylenglycol),
poly(ethylenglycol)-block-poly-(propyleneglycol)-block-poly(ethylenglycol-
) and
poly(propylenglycol)-block-poly(ethylenglycol)-block-poly(propylengl-
ycol).
13. The method for deposition of a matte copper coating according
to claim 10 wherein the concentration of the at least one carrier
additive in the second electrolyte ranges from 0.005 g/l to 5
g/l.
14. The method for deposition of a matte copper coating according
to claim 11 wherein the concentration of the at least one carrier
additive in the second electrolyte ranges from 0.005 g/l to 5 g/l.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for deposition of
matte copper deposits in the field of decorative coatings.
BACKGROUND OF THE INVENTION
[0002] Matte copper coatings in the field of decorative coatings
are required as a surface finish for e.g. sanitary equipment.
Another application of matte copper coatings is to replace matte
nickel layers ("satin nickel") as an intermediate layer in
decorative multilayer coating systems which becomes more demanding
due to the toxicity of nickel.
[0003] A homogeneous matte appearance is required for decorative
metal layers. The homogeneity of the matte appearance can easily be
achieved on substrates which have no complex shape because the
current density distribution during electroplating of matte copper
layers is within a narrow range. However, in cases where the
substrate to be coated has a complex shape, the current density
during electroplating is within a wide range. Typical substrates
having a complex shape which are to be coated with a matte copper
coating are for example shower heads and automotive interior
parts.
[0004] Another requirement for matte copper layers is that their
matte level should be adjustable in order to be able to manufacture
copper layers having different matte levels.
[0005] Plating bath compositions comprising at least one
polyglycerine compound for producing matte copper layers during
manufacture of printed circuit boards are disclosed in US
2004/0020783 A1. It is neither possible to obtain a homogeneously
matte copper deposit on a substrate having a complex shape nor to
adjust the matte level of such a copper deposit when using the
electrolyte disclosed therein.
OBJECTIVE OF THE INVENTION
[0006] It is the objective of the present invention to provide a
method for depositing copper layers which have a homogeneous and
adjustable matte appearance, especially on substrates having a
complex shape.
SUMMARY OF THE INVENTION
[0007] This objective is solved by a method for deposition of a
matte copper coating, comprising, in this order, the steps [0008]
a. Providing a substrate, [0009] b. Depositing a first copper layer
onto the substrate from a first aqueous electrolyte comprising a
source of copper ions, at least one acid and at least one polyether
compound wherein said first electrolyte does not contain an organic
compound comprising divalent sulfur [0010] and [0011] c. Depositing
a second copper layer onto the first copper layer from a second
aqueous electrolyte comprising a source of copper ions, at least
one acid, a first water soluble sulfur-containing additive selected
from the group consisting of alkyl sulfonic acid derivatives and a
second water soluble sulfur-containing additive selected from the
group consisting of aromatic sulfonic acid derivatives.
[0012] The copper coatings obtained by the method according to the
present invention have a homogeneous matte appearance on substrates
having a complex shape. Furthermore, the matte appearance of the
copper coating can be adjusted during deposition of the individual
copper layers.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The method for deposition of a matte copper coating
comprises deposition of two individual copper layers onto a
substrate from two individual copper electrolytes which are herein
denoted first electrolyte from which the first copper layer is
deposited and second electrolyte from which the second copper layer
is deposited onto the first copper layer.
[0014] The first electrolyte comprises a source of copper ions, at
least one acid and at least one polyether compound. The first
electrolyte does not contain an organic compound comprising
divalent sulfur, e.g., sulfides, disulfides, thiols, and
derivatives thereof.
[0015] Copper ions are added to the first electrolyte in the form
of a water-soluble copper salt or an aqueous solution thereof.
Preferably, the source of copper ions is selected from copper
sulfate and copper methane sulfonate. The concentration of copper
ions in the first electrolyte preferably ranges from 15 to 75 g/l,
more preferably from 40 to 60 g/l.
[0016] The at least one acid in the first electrolyte is selected
from the group comprising sulfuric acid, fluoroboric acid and
methane sulfonic acid. The concentration of the at least one acid
in the first electrolyte preferably ranges from 20 to 400 g/l and
more preferably from 40 to 300 g/l.
[0017] In case sulfuric acid is used as the acid, it is preferably
added in form of a 50 to 96 wt.-% solution. More preferably,
sulphuric acid is added to the first electrolyte as a 50 wt.-%
aqueous solution of sulfuric acid.
[0018] The at least one polyether compound in the first electrolyte
is selected from the group consisting of polyalkylene ethers and
polyglycerine compounds.
[0019] Suitable polyalkylene ethers are selected from the group
consisting of polyethylene glycol, polypropylene glycol,
stearylalcoholpolyglycolether, nonylphenolpolyglycolether,
octanolpolyalkylenglcolether,
octanediol-bis(polyalkylenglycolether),
poly(ethylenglycol-ran-propylenglycol),
poly(ethylenglycol)-block-poly(propylenglycol)-block-poly(ethylenglycol)
and
poly-(propylenglycol)-block-poly(ethylenglycol)-block-poly(propylengl-
ycol).
[0020] Suitable polyglycerine compounds are selected from the group
consisting of poly(1,2,3-propantriol), poly(2,3-epoxy-1-propanol)
and derivatives thereof which are represented by formulae (1), (2)
and (3):
##STR00001##
wherein n is an integer from 1 to 80, preferably from 2 to 30;
R.sup.6, R.sup.7 and R.sup.8 are identical or different and are
selected from the group consisting of hydrogen, alkyl, acyl, phenyl
and benzyl, wherein alkyl preferably is linear or branched C.sub.1
to C.sub.18 alkyl and acyl preferably is R.sup.10--CO, wherein
R.sup.10 is linear or branched C.sub.1 to C.sub.18 alkyl, phenyl or
benzyl; alkyl phenyl and benzyl in formula (1) may be
substituted;
##STR00002##
wherein n is an integer >1, m is an integer >1 with the
proviso n+m is .ltoreq.30; R.sup.6, R.sup.7, R.sup.8 and R.sup.9
are identical or different and are selected from the group
consisting of hydrogen, alkyl, acyl, phenyl and benzyl, wherein
alkyl preferably is linear or branched C.sub.1 to C.sub.18 alkyl
and acyl preferably is R.sup.10--CO, wherein R.sup.10 is linear or
branched C.sub.1 to C.sub.18 alkyl, phenyl or benzyl; alkyl phenyl
and benzyl in formula (2) may be substituted;
##STR00003##
wherein n is an integer from 1 to 80; preferably from 2 to 20; and
wherein R.sup.6, R.sup.7 are selected from the group consisting of
hydrogen, alkyl, acyl, phenyl and benzyl, wherein alkyl preferably
is linear or branched C.sub.1 to C.sub.18 alkyl and acyl preferably
is R.sup.10--CO, wherein R.sup.10 is linear or branched C.sub.1 to
C.sub.18 alkyl, phenyl or benzyl; alkyl phenyl and benzyl in
formula (3) may be substituted.
[0021] Polyglycerine compounds are produced according to known
methods. Indications on the conditions of production are disclosed
in the following publications for example: Cosmet. Sci. Technol.
Ser., glycerines, page 106 and U.S. Pat. No. 3,945,894. Further
details on the syntheses of polyglycerine compounds according to
formulae (1), (2) and (3) are disclosed in US 2004/0020783 A1.
[0022] Most preferably, the at least one polyether compound in the
first electrolyte is selected from compounds according to formulae
(1), (2) and (3).
[0023] The concentration of the at least one polyether compound or
all polyether compounds together in case more than one polyether
compound is added preferably ranges from 0.005 g/l to 20 g/l, more
preferably from 0.01 g/l to 5 g/l.
[0024] During operation, the temperature of the first electrolyte
is preferably adjusted to a value in the range of from 30 to
60.degree. C., more preferably from 40 to 50.degree. C.
[0025] The current density applied to the substrate during copper
deposition from the first aqueous electrolyte preferably ranges
from 0.5 to 5 A/dm.sup.2, more preferably from 1 to 3
A/dm.sup.2.
[0026] Optionally, the substrate is rinsed with water before
depositing the second copper layer from the second electrolyte.
[0027] Copper ions are added to the second electrolyte as a
water-soluble copper salt or an aqueous solution thereof.
Preferably, the source of copper ions is selected from copper
sulfate and copper methane sulfonate. The concentration of copper
ions in the second electrolyte preferably ranges from 15 to 75 g/l,
more preferably from 40 to 60 g/l.
[0028] The at least one acid in the second electrolyte is selected
from the group comprising sulfuric acid, fluoroboric acid and
methane sulfonic acid. The concentration of the at least one acid
in the second electrolyte preferably ranges from 20 to 400 g/l and
more preferably from 40 to 300 g/l.
[0029] In case sulfuric acid is used as the acid, it is added in
form of a 50 to 96 wt.-% solution. Preferably, sulfuric acid is
added as a 50 wt.-% aqueous solution of sulfuric to the second
electrolyte.
[0030] The second electrolyte further comprises a first
water-soluble sulfur-containing additive and a second water-soluble
sulfur-containing additive.
[0031] The first water-soluble sulfur-containing compound is an
alkyl sulfonic acid derivative. Preferably, the alkyl sulfonic acid
derivative comprises divalent sulfur.
[0032] The second water-soluble sulfur-containing compound is an
aromatic sulfonic acid derivative. Preferably, the aromatic
sulfonic acid derivative comprises divalent sulfur.
[0033] The first sulfur-containing additive is more preferably
selected from the group consisting of compounds according to
formulae (4) and (5):
R.sup.1S--(CH.sub.2).sub.n--SO.sub.3R.sup.2 (4)
R.sup.3SO.sub.3--(CH.sub.2).sub.m--S--S--(CH.sub.2).sub.m--SO.sub.3R.sup-
.3 (5)
wherein R.sup.1 is selected from the group consisting of hydrogen,
methyl, ethyl, propyl, butyl, lithium, sodium, potassium and
ammonium, more preferably R.sup.1 is selected from the group
consisting of hydrogen, methyl, ethyl, propyl, sodium and
potassium; n is an integer from 1 to 6, more preferably n is an
integer from 2 to 4; R.sup.2 is selected from the group consisting
of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium,
potassium and ammonium, more preferably, R.sup.2 is selected from
the group consisting of hydrogen, sodium and potassium; R.sup.3 is
selected from the group consisting of hydrogen, methyl, ethyl,
propyl, butyl, lithium, sodium, potassium and ammonium, more
preferably R.sup.3 is selected from the group consisting of
hydrogen, sodium, potassium and m is an integer from 1 to 6, more
preferably m is an integer from 2 to 4.
[0034] The concentration of the first sulfur-containing additive in
the second electrolyte preferably ranges from 0.0001 to 0.05 g/l,
more preferably from 0.0002 to 0.025 g/l.
[0035] The second sulfur-containing additive in the second
electrolyte is more preferably selected from the group consisting
of compounds according to formulae (6) and (7):
R.sup.4S.sub.y--X--SO.sub.3M (6)
wherein R.sup.4 is selected from the group consisting of
##STR00004##
and hydrogen; X is selected from the group consisting of
##STR00005##
y is an integer from 1 to 4 and M is selected from the group
consisting of hydrogen, sodium, potassium and ammonium; and
##STR00006##
wherein R.sup.5 is selected from the group consisting of hydrogen,
SH and SO.sub.3M and M is selected from the group consisting of
hydrogen, sodium, potassium and ammonium.
[0036] Most preferably, the second sulfur-containing additive is
selected from compounds according to formula (6).
[0037] The concentration of the second sulfur-containing additive
in the second electrolyte preferably ranges from 0.005 to 1 g/l,
more preferably from 0.01 to 0.25 g/l.
[0038] Optionally, the second electrolyte further comprises one or
more carrier additive selected from the group consisting of
polyvinylalcohol, carboxymethylcellulose, polyethylene glycol,
polypropylene glycol, stearic acid polyglycolester, alkoxylated
naphtoles, oleic acid polyglycolester,
stearylalcoholpolyglycolether, nonylphenolpolyglycolether,
octanolpolyalkylenglycolether,
octanediol-bis(polyalkylenglycolether),
poly(ethylenglycol-ran-propylenglycol),
poly(ethylenglycol)-block-poly(propylenglycol)-block-poly(ethylenglycol)
and
poly(propylenglycol)-block-poly(ethylenglycol)-block-poly(propylengly-
col).
[0039] The concentration of the optional carrier additive in the
second electrolyte preferably ranges from 0.005 g/l to 5 g/l, more
preferably from 0.01 g/l to 3 g/l.
[0040] During operation, the temperature of the second electrolyte
is preferably adjusted to a value in the range of from 20 to
50.degree. C., most preferably of from 25 to 30.degree. C.
[0041] The current density applied to the substrate during copper
deposition from the second aqueous electrolytes preferably ranges
from 0.5 to 5 A/dm.sup.2, more preferably from 1 to 3
A/dm.sup.2.
[0042] The matte level of the copper surface may be tailored by
adjusting the thicknesses of the first and second copper layer by
simple experimentation. A more matte appearance may be achieved
with a thinner second copper layer, whereas a less matte appearance
may be achieved with a thicker second copper layer.
[0043] The following examples further illustrate the present
invention.
EXAMPLES
Substrates
[0044] Both ABS (acrylonitrile-butadiene-styrol-copolymer) and
brass substrates having a complex shape were used throughout all
examples.
[0045] The ABS substrates were etched in chromic acid, activated
with a palladium containing colloid and metallised by electroless
plating of nickel from an acidic hypophosphite-based electrolyte
prior to copper deposition.
[0046] The brass substrates were degreased prior to deposition of
copper.
Test Methods:
[0047] The matte appearance of copper coatings was tested by visual
inspection of the copper plated substrates throughout all
examples.
Example 1
Comparative
[0048] Copper was deposited on ABS and brass substrates having a
complex shape from an aqueous acidic electrolyte comprising 80 g/l
CuSO.sub.4.5H.sub.2O, 240 g/l sulfuric acid, and 1 g/l of a mixture
of polyglycerin compounds according to formula (1) with n=2 to
7.
[0049] A homogenous, strongly matte copper surface was obtained
which is too matte for decorative applications.
Example 2
Comparative
[0050] Copper was deposited on ABS and brass substrates having a
complex shape from an aqueous acidic electrolyte comprising 80 g/l
CuSO.sub.4.5H.sub.2O, 240 g/l sulfuric acid, and 0.5 mg/l of a
first sulfur-containing additive according to formula (5) with m=3
and R.sup.3=sodium, 80 mg/l of a second sulfur-containing additive
according to formula (6) with
##STR00007##
and M=sodium and 200 mg/l polyethylene glycol.
[0051] The copper surface obtained has a homogenous technical gloss
which is not desired for decorative applications.
Example 3
Comparative
[0052] A first layer of copper was deposited onto ABS and brass
substrates having a complex shape from the electrolyte used in
example 2. Thereon, a second copper layer was deposited from the
electrolyte used in example 1.
[0053] A homogenous, strongly matte copper surface was obtained
which is too matte for decorative applications.
Example 4
Comparative
[0054] A first copper layer was deposited onto ABS and brass
substrates having a complex shape from the electrolyte used in
example 1. Next, a second copper layer was deposited thereon from a
second electrolyte comprising 80 g/l CuSO.sub.4.5H.sub.2O, 240 g/l
sulfuric acid, and 0.5 mg/l of a sulfur-containing additive
according to formula (5) with m=3 and R.sup.3=sodium. The second
electrolyte did not contain a second sulfur-containing additive
selected from compounds according to formulae (6) and (7).
[0055] The resulting copper surface has a non-homogeneous matte
appearance which is not acceptable for decorative applications.
Example 5
Comparative
[0056] A first copper layer was deposited onto ABS and brass
substrates having a complex shape from the electrolyte used in
example 1. Next, a second copper layer was deposited thereon from a
second electrolyte comprising 80 g/l CuSO.sub.4.5H.sub.2O, 240 g/l
sulfuric acid, and 80 mg/l of a sulfur-containing additive
according to formula (6) with
##STR00008##
and M=sodium. The second electrolyte did not contain a first
sulfur-containing additive selected from compounds according to
formulae (4) and (5).
[0057] The copper surface obtained has a matte appearance with
burnt areas (shady black appearance) which is not acceptable for
decorative applications.
Example 6
[0058] The first copper layer was deposited onto the ABS and brass
substrates from the electrolyte used in Example 1. The second
copper layer was deposited thereon from the electrolyte used in
Example 2.
[0059] The copper surface obtained has a homogeneous matte
appearance which is desired for decorative applications.
Example 7
[0060] The first copper layer was deposited from a first
electrolyte comprising 80 g/l CuSO.sub.4.5H.sub.2O, 240 g/l
sulfuric acid, and 1 g/l polyethylene glycol. The second copper
layer was deposited thereon from the electrolyte used in Example
2.
[0061] The copper surface obtained has a homogeneous matte
appearance which is desired for decorative applications.
TABLE-US-00001 TABLE 1 First and second electrolytes used in
Examples 1 to 6. Example 1* Example 2* Example 3* Example 4*
Example 5* Example 6 Example 7 First Containing none Containing
Containing Containing Containing Containing electrolyte mix of
additives with mix of mix of mix of polyethyleneglycol;
polyglycines divalent sulfur polyglycines polyglycines polyglycines
No additive according to according to according to according to
according to with divalent formula (1); formula (5) formula (1);
formula (1); formula (1); sulfur No additive and (6) No additive No
additive No additive with divalent with divalent with divalent with
divalent sulfur sulfur sulfur sulfur Second none Containing
Containing Containing Containing Containing Containing electrolyte
additives with mix of additive with additive with additives with
additives with divalent sulfur polyglycines divalent sulfur
divalent sulfur divalent sulfur divalent sulfur according to
according to according to according to according to according to
formula (5) formula (1); formula (5); formula (6); formula (5)
formula (5) and (6) No additive No additive No additive and (6) and
(6) with divalent acc. to acc. to sulfur formula (6) formula (5)
Optical - - - - - + + appearance** *comparative examples; **+ =
good; -: not sufficient
* * * * *