Wet white tannage
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The properties of the leather are adjusted mainly by
the amount and type of syntan used
and by the process parameters. A wide variety of
different types of leather are thus available,
e. g.:
Automotive leathers Upholstery leather Garment leather Shoe upper leather
Shavings
and cuttings of wet white leather are free of
chrome.
chrome-free leather
has many demonstrable advantages:
* no chromium in
the effluent
* fully recyclable
shavings and end-products (shavings as nitrogenous fertiliser or components
for
leather fibres, FOC
leather combined with natural latex as sole material for fully recyclable
footwear)
* no risk of Cr(VI)
formation
* excellent
shrinking behaviour
* brilliant
dyeings, especially for fashion items
* leather free of
heavy metals for allergic persons
* improved scope
for sorting at the pretanned stage
The key factor in
the manufacture of chrome-free leather is pretanning the skins into wet-white.
This mode of pretanning has now been largely accepted in the industry. After
pickling, 2.5-3% glutardialdehyde1 (25% modified) is added
gradually at pH2.8-3.1. Key factors in ensuring 100%
through tanning,
with unsplit raw material in particular, is complete deliming and an adequate
pickle.
.
Decisive advantages
can be gained by pretanning in combination with selected syntans and
polymeric tanning agents. Combination pretanned wet-whites:
* are lighter in
colour and outstandingly suited for conversion into pastel shade leathers
* achieve shrinking
temperatures of at least 70°C
* allow faster
tanning and retanning with reduced amounts of chemicals
* make possible
more uniform final thickness and greater shaving accuracy through improved
hydroextraction in samming and reduced plumping of the leathers in final
tanning because the fibres
are more firmly
fixed
* ensure greater
reliability in item-specific sorting
* enhance fullness
and grain pattern2
For conventionally
and combination pretanned wet-white, the basic differences as against
wet-blue
or chrome crust are
retained:
* the shrinking
temperature of the pretanned wet-white is between 68 and 78°C, and therefore
below that of
wet-blue
* improved
shrinking behaviour and dimensional stability versus chrome tanned leather
* dried wet-white
is more difficult to wet back than wet-blue
* wet-white
requires suitable preservation
There are also a
number of important differences between wet-white and wet-blue in terms of
dyeing properties.
The reflectance
curve for undyed wet-white is more linear than that for wet-blue (the whiter
the material, the greater its reflectance). Wet-white tanned and retanned
with synthetic products
therefore yields
clearer, whiter leathers for dyeing; this results in more brilliant shades.
Particular care
must be taken to select chrome-free dyes and fixatives with a low
formaldehyde
content. FOC
leathers have a higher anionic charge, hence the fastness properties of the
dyes should
be accurately
tested.
Wet-white can be
tanned and retanned by various methods.
Retanning with
chrome or chrome syntans gives so-called semi-chrome leathers. The advantages
are chrome-free shavings and reduced consumption of tanning and retanning
chemicals.
Retanning can also
be performed with vegetable extracts (mimosa, chestnut, quebracho etc). When
heavily topped with vegetable tanning agents, semi-vegetable tanned wet-white
acquires properties closely similar to those of conventional full vegetable
tanned split hide.
Conventional soft
upper, saddle and fancy leathers can be produced efficiently in this way.
Pretanning with glutardialdehyde normally results in softer types of leather.
Greater firmness can be obtained by adding selected polyphosphates3 and
polymeric tanning agents4.
It is very
important here to choose suitable fatliquors. In upper leather production
from wet-white, fatliquors based on animal fats such as lanolin and lard oil
in combination with selected synthetic fatliquors have been found to give the
best results.
Because final
tanning with vegetable and synthetic tanning agents gives a fairly dry fibre,
great care should be taken to conserve the tensile strength. The remedy is to
add products containing
phosphoric acid
esters. Combination fatting with highly concentrated fatliquors containing
lecithin
leaves nubuck or
suede with a silky feel.
Retanning with just
syntans converts wet-white into very pale coloured crust leather which can be
dyed to brilliant
shades.
Wet-white is an
all-round product which can be turned into a wide variety of finished
leathers and leathergoods. Given the increased demand for chrome-free leather
on the part of both consumers
and processors,
leather from wet-white could also have a crucial role to play in footwear
manufacture.
Upper leather of
this type can be:
* upper leather for
leisure footwear, full pigmented/embossed
* floaters/drawn
grain leather for leisure footwear
* tight grain,
natural colour semi-aniline leather for shoes and handbags
* upper leather in
brilliant shades for children's shoes
* fashionable upper
and fancy leathers with various effects and embossing
* but also
conventional leather for fancy goods
Currently the
amount of shoe upper leather produced from wet-white is fairly small. Fifteen
years ago however, the automobile industry among others could not have
imagined the present importance of
such leathers.
Degradable materials obviously have an enormous advantage for industry and
the consumer. An ecological marketing concept and the advantageous properties
unquestionably characteristic of leather from wet-white have given a
tremendous boost to demand.
Wet-white yields a
leather which differs in character from that of wet-blue origin and comes
with its
own specification
and fastness properties. In marketing, these differences should be
emphasised, not concealed. Appropriate appreciation by the consumer can only
be gained through optimum
positioning of
these different leathers.
Important parameters in wet white tanning:
–
Thorough deliming and bating of the pelts
–
Intensive washing
–
Pickling through cross section
–
Type and concentration of aldehyde
–
pH
–
Running time
–
Basificaion
–
Use of syntans
Flow chart of
wet white tanning
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ZIRCONIUM
TANNAGE
Basic Zirconium
salts have definite tanning action with a shrinkage temperature of
90-95 0C. Such leather is of firm, full substance and has
excellent white apparence. As with chromium, the sulphate, rather than the
chloride, is the prefered starting material for making the tanning
salt. It would find wide application provided high cost of tannin salts
could be diminished. Large amounts of zirconium salts are needed (more than
double the amount required for chromium tannage) to produce satisfactory
leather, this is particularly with solutions of 33 % basicity and may
be due to large particle size involved. Zirconium tannage most probably
is a salt formation involving anionic zirconium complexes and basic groups in
collagen.
Zirconium is Zr+4 and has coordination number 8. Neither the zirconyl group, Zr=O, nor the Zr-Zr group is found in solution.
Single OH s may
be relaced by acid residues or carbonate residues. Formation of insoluble
zirconium compounds starts when NaOH is added already at pH=1.5.
Masking: monocarboxylic acids have no effect, hydroxy acids show masking effect. Mechanism is thought to be a ‘multipoint attachment of zirconium to collagen. 1)Binding of anionic sites of zirconium complexes to amino groups 2)Polar binding of cationic sites of complexes to carboxyl groups 3)Covalent bonding of neutral sites and oxgen atoms of nonpolar carboxyl groups of collagen.
ALUMINIUM
Aluminium gives
white colour to the leather so that we
call the leather wet white.Reactions of aluminium with carboxyl groups
isless strong than chromium reactions.So that aluminium does not have strong
bonds with collagen than it can easily remove
with washing so their washing fastness is not good.Tanned leathers
with aluminium are more hard,firm and not washing prof.Shrinkage temperature
is about 70 C.When comparing with chromium its cheaper and more ecofriendly
and gives white leathers it has advantages in dyeing process.Aluminium uses
in tanning as Kal(SO4)2.12H2O or as Al(SO4) or AlCl3 form.Because of some
dissadvantages of tanning leathers only with aluminium its more useful to
make combinations with other tanning materials like
chromium,vegetable,aldahydes or zirconium.We can prepeare
basic aluminium sulphate (%33
basicty) like as chromium sulphate
.For example 10 kg Al2(SO4)3 and 10 kg water mix . One night waiting
gives better results. The next day 700 gr sodium cytrate adding and mixing. In addition another solution
prepared (20 kg water and 1.750 gr
sodiumcharbonate) and adding into
the first solution. SO we have Al2O3.
Another Aluminium salt is AlCl3 .AlCl3 has less tanning power than Al2(SO4)3.
Aluminium can’t occur stabile
acid complex with carboxyl groups of collagen so that washing and shrinkage
temperature less than wet white tanned leathers.
ALDEHYDE TANNAGE
Formaldehyde (a
pungent smelling gas) is water soluble and its solution is known as
formalin(toxic and may develop acidity). Presumably tanning was observed for
the first time when meat together with the skin were
smoked. Formaldehyde is probably the only tanning gas. Stabilized formalin(containing
8-10 % MeOH) contains 40 % formaldehyde and is used for tanning white,
washable leathers with the grain split or shaved off.
The skins are prepared to a pH 4 or 5 and drummed in 3% formalin with the least possible amount of water. A temperature of 30 C is beneficial. After runtime of 4-5 hours, they are left in the closed drum overnight and then 'ashed up' (1-1.5 % soda ash, 50-100% water) until pH is not less than 8. At this pH formaldehyde rapidly fixes to the skins. At higher pHs over tannage of grain side (with no penetration) occurs. If production of grain leather is intended various(modified) alkali systems are recomended (use of Mg salts) to avoid this danger. The amount of aldehyde being attached to the hide is small, from 0.2 to 2 %. Part of this may remain unbound. Aldehydes combine with the basic amino group of skin protein. In alkali some condensation(aldol) to larger molecules give fullness to the leather. Aldehyde tanned leathers have reduced ability to fix acid (basic groups have reacted).Similarly they can reduce fixation of some vegetable tans and dyes. Aldehyde tannage reduces isoelectric point of hides, so that at any pH it has a lower cationic charge than raw skin, and mineral tanned leather. This can reduce fixation of anionic sulphated oils so that such fatliquors penetrate better, but may washout more easily. Ts is raised only to 70 C. Leather becomes whiter as exposed to light and readily absorbs water. RNH3+ + CH2O --> R-NH-CH2OH + H+ RNH-CH2OH + NH2CO-R --> RNH-CH2 - NH-CO-R + H2O
Gluteraldehyde: (OCH-CH2CH2-CHO)
Under equivalent conditions it can give higher degree of tannage and increase of Ts than formaldehyde at lower pHs. Attention has been given to the phenomena that some degree of gluteraldehyde particularly on mineral tannages improves leathers resistance to perspiration. Gluteraldehyde forms semiacetal bonds with hydroxyls of hydroxyproline, hydroxylysine and serine. With phenols it yields insoluble compounds, so can not be used with vegetable tannins.
With amino groups
it my react in 3 ways:
Gluteraldehyde in
25-50% aqueous solution is found to oligomerize (3-5 molecules).This may be
prevented by addition of alcohol(at low temperature).
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WET-WHİTE TANNİNG
BY;EGEMEN
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