The present invention relates to dyeing of a wood material in the
form of sheets of wood intended for the production of veneering, wood panels, a
multilaminar wood product and others. Dyeing refers in particular here to the obtaining
of even colouring throughout the thickness of the wood material.
The dyeing method according to the present invention can be applied
to sheets of wood of various wood types, both coniferous and broad-leaved, such
as for example poplar, beech, oak, pine, fir, ash, birch, cherry, Nigerian Whitewood,
"koto", "ilomba" (Pycanthus kombo) etc., which can be used for veneering or for
preparing multilaminar wood or forforming wood-based panels.
For dyeing wood use has hitherto been made of dyes belonging to classes
defined as acid dyes, basic dyes and direct dyes where the chromophorous molecules
penetrate into the fibres of the wood, providing it with the appropriate colour
effect, but without binding to the fibres themselves except via "weak" bonds (e.g.
Van der Waals bonds).
The traditional dyeing system referred above causes low light fastness
to be obtained due to phenomena of photodegradation under exposure to light, phenomena
which vary according to the tone, dye and substrate.
The colour change of dyed woods exposed to the light is due mainly
to the formation of free radicals generated by the photolysis of some chemical
bonds both in the molecules of the wood (lignin, cellulose, hernicellulose etc.)
and in those of the dyes, particularly if small in size, as are those mentioned
above normally. The action of light oxidation caused by the radicals is also accelerated
by various factors such as atmospheric oxygen and humidity.
In JP-A-56 056 810 a process for dyeing wood using vat dye solutions
under pressure is described.
The general object of the present invention is that of obtaining a
dyed wood with high fastness to light.
This object is achieved thanks to dyeing of sheets of wood by a method
comprising the phases hereinbelow indicated in the independent claim 1.
Present invention concern also a dyed sheet of wood obtained with
the above cited method of dyeing wood by using a dye chosen in the "vat dye" class.
By dyes belonging to the class of vat dyes we refer to those indicated
under the item "Vat Dyes" of the Color Index published by the American Society
of Dyers and Colourists, such as for example Indanthren from BASF, Cibanone from
Ciba Geigy, Sandotrene from Sandoz and others.
The wood sheets dyed with vat dyes and the materials which can be
obtained therefrom have brilliant colours which are also very fast to light.
All vat dyes are normally insoluble in water, but can be dissolved
in a form defined as "leuco" by reduction of some functional groups such as for
example the ketone groups.
To perform dyeing the fibre is immersed in the solution containing
the leuco form of the dye, stabilised if necessary, and, when it has diffused,
the dye is restored by oxidation of the leuco form absorbed by the fibre.
The dyes belonging to the vat class normally consist of large molecules
with intrinsic features of resistance to attacks by radicals.
The present invention will in any case be made clearer on reading
the following with reference to the accompanying drawings, in which:
- Figure 1 represents a block diagram schematising particular embodiments of
the method for obtaining dyed sheets of the present invention;
- Figure 2 is a diagram illustrating an example of a method for manufacturing
a wood product such as multilaminar wood comprising, as intermediate processing
phase, the method of dyeing sheets of wood which forms the main object of the present
- Figure 3 is a graphic representation of the effect obtained on the surface
of a sample of material coloured with a vat dye according to the present invention,
after which it was subjected to a controlled test of exposure to light (Xenotest);
- Figure 4 is a graphic representation of the effect obtained on the surface
of a comparison sample coloured with a traditional acid dye after the latter has
been subjected to a test similar to the one performed for the sample of Figure
According to a first preferred embodiment of the dyeing method of
the invention, the sheets, if necessary purged of chromophorous impurities and
otherwise, are placed in a dyebath containing at least one vat dye previously reduced
into a soluble form (leuco) (see preparation 1 of Fig. 1), together with a certain
amount of agents for reducing and correcting the pH level, suitable for preventing
oxidation of the dyes themselves during the process of impregnation of the wood.
The quantity of reducing agents is to be determined on each occasion
in such a way that said quantity is sufficient for neutralising the oxidating
effect of the oxygen present both in the air above the dyebath and in the solvent,
and such as to neutralise the oxidating effect of the wood being dyed.
The constant control on the state of reduction and of the pH of the
dyebath, with possible restoration of the appropriate conditions, allows the solubilised
dye molecules to penetrate as far as the deepest layers of the sheet.
According to the working temperature, it is at times advantageous
to use substances to stabilise the reducing environment, already known forthe
textile process, such as for example glucose and sodium nitrite.
Penetration of the vat dye into the wood fibre is facilitated by adding
These substances that facilitate or improve the penetration of vat
dye are surface-active agents, such as a wetting surfactant agent. These substances
facilitating penetration of the vat dye can also be electrolytes (NaCl, Na2SO4
Moreover into the dye bat are also added impregnating agents.
The time and temperature required for dyeing with the dye in reduced
form will depend on the type and thickness of the wood used; purely by way of
an example, purged broad-leaved sheets with a thickness smaller than 1 mm require
times varying from 30 minutes to 12 hours, with temperatures of between 60° and
According to the invention hydrosulphite is normally chosen as a
reducing substance, even if other substances can be used such as for example bisulphite
with sodium borohydride.
After the phase of absorption of the dye in leuco form, the phase
of fixing of the same by oxidation is performed.
Having discharged the dyebath, said oxidation process is performed
by the formation of a bath containing substances such as for example hydrogen
peroxide, hypochlorite or gaseous oxygen at a temperature of between 40° and 105°
for a time sufficient for restoring the insoluble form of the dye (30 minutes -
According to a further embodiment, which however has a considerably
longer duration, said oxidation process can also be carried out by exposure to
There then follows a phase of neutralisation of the dyebath to restore
the pH to values compatible with the chemical nature of the wood, using for example
acetic acid, propionic acid and others, if necessary with impregnating substances
added, such as for example ethylene glycol, glycerol or others.
Finally the dyed and stabilised sheets are dried in a wholly traditional
A second preferred embodiment of the method of the present invention
(cf. preparation 2 of Fig. 1) foresees preparation of the reduced form of the
dye, belonging to the vat class, directly in the dyebath, by adding a suitable
quantity of reducing agents at controlled pH (normally alkaline).
According to a third preferred embodiment, given as an example in
point 3 of Figure 1, the use of vat dyes known as "solubilised" is foreseen, in
other words those in which the soluble leuco form is prepared and stabilised in
a period of time prior to and independent of dyeing.
A fourth preferred embodiment finally foresees the reduction of the
dye to the soluble form electrochemically, and also possible maintaining of this
state in the dyebath itself, again via this method (cf. preparation 4 of Figure
Purely by way of a non-limiting example of the concept claimed, we
shall now give hereinunder some examples of accomplishment of the method for dyeing
sheets of wood of the present invention.
For the sheets produced in this way the result of a comparative test
with similar sheets dyed traditionally is also provided, both subjected to exposure
to the light rays from a xenon lamp by means of the Xenotest 150S (Heraeus) equipment.
Broad-leaved wood (Nigerian Whitewood) processed into sheets with
a thickness of 0.60 mm is purged at 100°C in a bath containing 66 cc/l of 35%
hydrogen peroxide, 4 g/l of sodium silicate with pH 10 through sodium hydroxide
until the undesirable substances are eliminated.
Separately, according to the preferred procedure no. 1, the leuco
form of the colour combination required is prepared by dissolving a sufficient
quantity of dye for obtaining a concentration in the dyebath of 1 g/l of Vat Yellow
2 (NGC Sandotrene Yellow from Sandoz), Vat Blue 66 (Clf Indanthren Blue from BASF)
and Vat Red 13 (6BMD Cibanone Red from Ciba Geigy) in a volume of water equal to
a sixth of the dyebath itself (preparation of the vat) with a mixture of sodium
bisulphite plus sodium borohydride with pH 13.5 through caustic soda until the
litmus paper for vat dyes (Vat Yellow 2) changes colour.
Dyeing is then performed by immersing the purged wood in a bath containing
the dye, previously prepared in the leuco form in the manner described above, with
pH 13 through NaOH with 8 g/l of wetting surfactant agent (Primasol NF from BASF)
added, constantly checking (every 30 minutes) the state of reduction of the bath
by appropriate litmus paper and if necessary restoring the suitable dyeing conditions
by sodium hydrosulphite and NaOH.
After 8 hours of dyeing at 80°C the bath is discharged and the dyes
are oxidated by adding 33 cc/l of 35% hydrogen peroxide for 1 hour at 95°C.
The wood is finally stabilised by neutralising the fibre via acid
treatment with acetic acid, with 2 g/l polyethylene glycol added, at 95°C for one
Broad-leaved wood (poplar) processed into sheets with a thickness
of 0.75 mm is dyed using 2 g/l of Vat Red 13 (6BMD Cibanone Red from Ciba Geigy)
reduced to the leuco form and maintained in this form by electrolytic reduction
in an ionic environment through sodium chloride at 99°C for 6 hours.
Having discharged the bath the dyes are oxidated by exposing the
individual sheets to the air for a time sufficiently long for regenerating the
In order to check on the increased fastness to light of the material
obtained in this way, other sheets of poplar and Nigerian Whitewood were dyed
by dyeing with acid pH through acetic acid with 1 g/l of Acid Yellow 17 (2G Lissamina
Yellow from ICI), 1 g/l Add Red 66 (Suprammina Red B from Bayer) and 1 g/l of Acid
Blue 40 (E-2GL Sandolan Blue from Sandoz), obtaining a colour similar to the one
The sheets dyed in the two ways described above are dried according
to known procedures, for example in a hot air tunnel.
Part of them are used for decorating a plywood panel while the remaining
ones are used for preparing a section of multilaminar wood (according to the illustrative
diagram of Fig. 2) obtaining in both cases a pleasing appearance.
Evaluation of the increase in fastness to light was performed on both
the natural material and on various simulations of "finished product" or covered
with polymeric films such as those in polyester, polyurethane, acrylic etc., according
to the method indicated in UNI 9427 for furniture surfaces, expressing the results
on the basis of the blue scale where 1 corresponds to very weak light fastness
and 8 very high light fastness.
The change in colour over time was evaluated by comparing the exposed
part and the unexposed part of the specimen, comparing it to the similar situation
on the blue scale.
Blue scale evaluation
Sample dyed with vat dyes
Comparison sample dyed with acid dyes
In order to represent better the advantageous effect of fastness to
light of the wood dyed using vat dyes according to the present invention compared
to woods dyed in a traditional way, examples are given in Figures 3 and 4 respectively
of the effect produced by performing a controlled test of exposure to light (Xenotest)
on a sample of material dyed with a vat dye according to the present invention
and on a comparison sample dyed with a traditional acid dye.
In particular it can be seen that said samples both have in the lower
zone a sector which has not been exposed to the light during the test of accelerated
ageing with the Xenotest equipment described previously. Above, bands can be identified
as corresponding to exposures equal to five, twenty and fifty hours respectively.
From the visual comparison of the two samples it is possible to perceive
immediately how the sample dyed in the traditional manner already shows after
five hours a marked variation up to complete degradation of the original colour
when exposure to the light is extended up to twenty hours or more.
On the contrary the sample of wood dyed according to the present
invention with a dye belonging to the vat class does not show any substantial loss
of colour even after exposure of over fifty hours.