PIGMENTS CONCENTRATE PASTES

lovinggudgeonΜηχανική

22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

80 εμφανίσεις

PIGMENTS CONCENTRATE
PASTES




Marco Zuccadelli, Elisa Tarasco, Marco Canini


AB Chimica S.r.l., Italy



Introduction



WHAT ARE THEY
?


Preparations containing

PIGMENT
S, BINDER,
SOLVEN
T
and ADDITIVES.

Pigments are supplied
by the industries

in the form of dry powders. These
can be named
primary
particles
,

which are sized to give the best results in paint films. For exa
mple, white pigments
(Titanium D
ioxide


TiO2
) scatters light most e
fficiently (whiteness and opacity) when the particle
size is 0.22 microns. Some pigments are much smaller, some larger.

The small primary particles of pigment clump together whe
n the powder is dried and form
re
latively large clumps (a
clump

of Titanium Di
o
xide may contain 5,000,000 primary particles)
.


These clumps a
re known as aggregates and in this aggregated form
the pigments are supplied. To
produce good p
igments dispersions
, it is necessary that the pigment should be at its primary particle
size when
mixed with the vehicle. Otherwise most of the expensive coloured pigment
s

could not be
used efficiently, and the dry film will not be smooth, glossy, opaque or have the correct colour.

It is therefore necessary to break down the aggregates of pigment part
icles in the presence of the
vehicle and to prev
ent them from re
-
aggregations
. For
this reason, it is not
possible simply to st
ir
the pigment into the vehicle. R
ather, work has to be done on the mixture in order to break down the
aggregates and ensure that

the resin coats the small particle surfaces. This process i
s known as
pigment dispersion

and the following processes must occur:


-

Wetting

of the
pigment surface by the vehicle

-

De
-
aggregation of the pigment aggregates to small primary particles

-

Stab
ilisation

of these small particles by the r
esin to prevent re
-
aggregation


PIGMENTS


They are substances that impart coloration to the support in which they

are incorporated or to which
have overlapped. They distinguishes from

the
colorants

which act on th
e sub
-
layer being in it
dissolved.

They are classified through the
Colour

Index number (international

method of classification).

The Pigments can have both

natural origin and synthesis and both

organic and inorganic nature.


Some

pigments

nature
:


Colo
ur



Inorganic Pigments


Organic Pigments


Black



Iron Oxide



Carbon Black

Yellow



Iron Oxide




Azoic Pigments





Lead Chromates


Isoindolinone

Blue



Ultramarine Blue


Phthalocy
anine

Violet



Cobalt Blue



Quinacridone

Green



Chrome Oxide


Phthaloc
y
anine

Red



Iron Oxide



Manganese Toner




Molibdate Red


Quinacridone

White



Titanium Dioxide






BINDER



The b
inder can be a resin or a mixture of resins. For chemical reaction or for simple evaporation of
the volatile substances a resinous mixture
is formed, polymeric, resistant, endowed with filming
properties, or rather able to give or
igin to a continuous film. The b
inder is
loosened

or dispersed in
volatile substances and this solution o
r dispersion is called
Vehicle
. It is defined water or solve
nt
vehicle depending if the volatile substance is made of water or organic volatile substances.

The type
of resin used can be varied to enable the paint to have whatever final properties are required.


Commonly used resins
:


Solvent
-
base




Water
-
base


Lo
ng
-
oil Alkyd resin



Water
-
soluble Acrylic resin

Medium
-
oil Alkyd resin


Aqueous mix of Polymeric Surfactants

Short
-
oil Alkyd resin

Ketonic

resin

Epoxy resin


SOLVENT


It is a substance that in the liquid state it is able to solve other

substances. D
issolv
e or suspend the
binder and give the paint the correct consistency (viscosity) for easy application.
It

must also
evaporate after application to allow the paint to dry
.


Functions of the s
olvent in a coating:


-

It solves the b
inder resulting in
the vehi
cle in which the p
igment is

dispersed

-

It wets the support on which the coating product will be applied (i.e. in the case of the wood it also



has to penetrate)

-

It influences the formation of the film that
,

according to the demands
,

has to be r
esistant, compact,


homogeneous, continuous and smooth

-

It evaporates completely in the time and under the desired conditions


Commonly used solvents:


White spirit

Xylol

Water

PMA

Butyl acetate


ADDITIVES


Commonly used additives:


Defoaming agents
, used in water
-
based paints for the foam formation prevention;

Dispersing and
wetting agents
,
active in the pigment particles demolition, helping the bonding with
the resin;

Coalescent
,
help the formation of an homogeneous film during the application;

An
tiskin
,
avoid the formation of superficial skin in solvent
-
based systems;

Anti
-
sedimentation
, avoid the formation of sedimentation;

Hardeners
,
added to the finished paint, help the exsiccation;

Preservatives
,
used

into the water
-
based paints, avoid the pro
liferation of bacteria and mildews.



Meaning



WHAT ARE THEY FOR
?


Semi
-
processed preparations for the production of varnishes, industrial, construction and
do
-
by
-
yourself enamels, wood stains and printing inks.

They simplify and facilitate

the productive trial, guaranteeing to the user constant and continuous
respect of the specifics and high quality level
.



PRODUCTIVE TRIALS:
THE PIGMENTS DISPERSION

PHASE


Whoever produces varnishes, for whatever type and use, is today facing a

big dilemm
a:


P
roduci
ng from the powdered pigments
or from the concentrate pastes?


The dispersion phase implicates the use of m
achineries, called mills, with employment of
manpo
wer both for the production and maintenance and the stock

of numerous raw materials, wi
th
the relative

problems of logistic, control and management.


THE CONCENTRATE PASTES ADVANTAGES




R
eduction of the production time, avoiding the grinding phase, including the relative costs,
difficult to calculate, i.e. the expenses of cleaning the machine
ries and general manpower



R
eduction of money investments dedicated to the machines used for the dispersion



R
eduction of the pollution inside and outside the business premises



I
ncreasing of the productive rhythm without relative costs



R
eduction of the wareh
ouse costs, avoiding the production of

an entire batch
, purchasing only
the necessary quantity for the immediate use




R
eduction of the raw materials quality control costs


QUALITY, CONTROL AND
RELIABILITY


Th
e numerous and accurate tests
do
ne

on all the en
tering raw materials, on

the preparations in phase
of production

and on th
e final product, guarantee the
constant acquiring of the followings

principal characteristics:




Maximum grinding fineness



High covering



Tone purity by the high saturation value



Lo
w flocculation tendency



Respect of the quality standards required



Assured conformity to the specifications



Problems of the pigments preparations



Being

high pigments concentrate preparations, the pastes have few problems related mainly with the
way the
users handle them. For instance, the
shock
of the pigments once suddenly
diluted into a low
concentrated

solution is a common factor influencing the reaction between the paste and the
converter. A pre
-
dilution of the paste with a 1:1 ratio is suggested, al
l the time.

The characteristics of the different pigments have to be considered using the preparations, any
known problem will affect the rheology and the stability
of the paste, as well as the finished paint.

For example, the Phthalocyanine Blue and the C
arbon Black pigments have the tendency to
flocculate, so a 1:1 pre
-
dilution with
the
converter is warmly suggested; the heavy inorganic
pigments (i.e. Iron Oxides) could form a pigment/solvent stratification and so a constant agitation of
the preparation i
s suggested.

In few word
s,

we can summarize that any pigment problem is a paste problem, but being the latter
an high

pigment
s

concentration preparation, that problem become critical.


FLOCCULATION


Flocculation or reagglomeration refers to the congregati
on of already dispersed pigment particles. It
is a consequent of the high energy state of the pigment particles with their large surface areas,
combined with inadequate stabilization. Destabilizing forces of attraction, depending on the
chemical character
of the pigment surface, include:




Van der Waals forces (dispersion forces, London forces, dipolar interactions
, induced
dipolar interactions)



Hydrogen bridge bonds



Ioni
c interactions (Coulomb forces)


Flocculation is usually an undesirable state since the
colour of flocculated pigments is less intense
than that of their finely divided (flocculated Red Iron Oxide has a bluer tint than its finely divided
equivalent), or cause flooding. There are some perspective, however, from which a certain, or
selective, f
locculation is actually desirable, to prevent hard sediment, for example.


SEDIMENTATION (Settling
-
out)


The sedimentation problem is due to the following main characteristics:




Pigment density: the higher it is, the more
quickly the pigment settles
-
out



S
ystem viscosity: the greater it is, the more s
lowly a sedimentation is formed



Particle radius: large, un
-
grinded or flocculated particles sink much more quickly

than small
ones


Agglomerates or flocculates are very large
particles and hence they settle
-
out

more quickly but form
only a soft sediment. Fine particles sink more sl
owly to the bottom but form a t
ightly compacted,
solid sediment that in extreme cases can be virtually free from binder.


P
H VALUE

The pH value is closely linked to electrical conducti
vity but is not the only
factor

that influences it.
As a measure of the concentration of H
+

ions, i.e. for the acid/base behaviour of aqueous solutions
in the ranges:




pH < 7: acid behaviour



pH = 7: neutral



pH > 7: alkaline, basic behaviour


the pH value i
s particularly important for the stability of the dispersion or emulsion binders
, but also
for the dissolved film formers, for which the following principles must be taken into consideration:




If binder or coatings/paints are acid
-
neutralized, i.e. acid st
abilized, their resistance is
usually increased (to a certain limit) due to an additional acid excess, i.e. a lower pH value; a
rise in the pH value due to the addition of alkali, on the other h
and, leads to rapid
coagulation



Conversely, alkali
-
neutralized

binders and coatings (neutralized with amines or alkali
solutions) can readily tolerate a somewhat higher pH value but definitely not an addition of
acid


The colour intensity and stability of pigments as well as the subsequent weather
-
proofing and
corros
ion protection action on the coating system also depend on the correct pH value.


Measurement errors in pH determination are unfortunately a common occurrence and one of the
most frequent sources of error in the development, production and application of w
ater
-
borne
coatings.




Materials and Methods



The following are the values
and tests
every manufacturer of pigments pastes should verify batch
after batch, in order to guarantee the constant compliance to the given specifications:




Reduction with white
for the tinting streng
th (spectrophotometer)




Full shade for the brightness and covering characteristics (spectrophotometer)



Viscosity with
a (
Brookfield
)

viscosimeter



Specific weight by pycnometer



pH value, fo
r water
-
based products, by a pH
-
meter



Grinding

fineness with a grind
-
meter


When the grinding fineness value is the lower obtainable, depending on the pigment type, further
tests are done to verify the colour strength, shade and brightness. If they are under the
specifications, it is necessary to veri
fy the viscosity, the specific weight and, in case of water
-
based
products, the pH value.


COLOUR STRENGTH


Colour intensity is not only important for supply agreements, it is also a good means to determine
a
n optimum dispersion time

and other dispersion
parameters, since it increases sharply during the
dispersion process as a result of the growing number of particles, until it reaches a limiting value
which thereafter remains unchanged.


As a consequence of over
-
dispersion and possible subsequent floccul
ation
, the colour intensity may
even decrease again over a continued dispersion period.



Dispersion Curve
0
10
20
30
40
50
60
70
80
90
100
1
2
3
4
5
6
7
8
9
10
Dispersion Time (h)
Colour Strength (%)


The slope and course of the dispersion are very much dependant on the coherence and wetting
ability of the pigments particle size and shape.

Finely divided, organ
ic pigments are usually significantly more difficult to disperse than inorganic
pigments containing coarser particles. Accordingly, micronised, Transparent Iron Oxides pigments
are likewise more difficult to disperse than conventional, opaque Oxide pigment
s
.


PARTICLE SIZES


The modern methods of determining particle size distributions are now indispensable in the
development, production and quality control of pigments, binder dispersions and even
f
or coating
materials. The most important methods are:




Co
ulter Counter



Photon Correlation Spectroscopy



Laser Light Diffraction


These three methods can only be used with extremely diluted dispersions, in which the particle sizes
can quite easily deviate somewhat from their original state.


RUB
-
OUT TEST


To revea
l flocculation or flooding, a RUB
-
OU
T test is performed by rubbing
the film with a finger
while it is still damp but already starting to dry. In doing so the flocculated or flooded, i.e.
separated, pigments are rubbed together and the colour of the homogen
eous ble
n
d
is
reinstated by
means of a kind of
re
-
dispersion
. The difference in colour intensity from the unrubbered film
indicates the extent of the problem.

If a RUB
-
OUT test on white/coloured pigmented syst
ems shows the colour intensity
of the rubbed
co
ated surface to be greater than that of the rest of the surface, we refer to a
positive

RUB
-
OUT
effect, and in the opposite case to a
negative
effect.



Tinting Systems



The demand of quality products is increasing year after year and not only for industr
ial coatings. At
today even the final user for do
-
by
-
yourself applications is demanding more and more quality, in
terms of constancy of the colorimetric characteristics and environmental safety matters.

The market demand of water
-
based varnishes for wood,
industrial coatings and road signs is
sensibly increasing, the new European regulations for the VOC’s emissions and the high costs for
the research and development departments are forcing more and more paint producers to take the
deci
sion of quitting the d
ispersion

phase into the productive trial, quitting the costs of the lab,
avoiding to do colorimetric compliance tests as a routine job and quitting to stock numerous raw
materials like the pigments, reducing the warehouse
stock
cost and manpower.

Also, th
e difficulties to find well prepared young technical persons and the reluctancy that most of
the senior technical managers demonstrate

in training youngsters, make the

demand of complete
tinting system
s

enormously increasing in the last few years.

At today

it is possible to find in the market supplier
s

of tinting systems for all the
different
coating
applications
.

It is necessary to highlight the differences between the two main tinting systems way to operate:
volumetric

and
gravimetric
. Years ago

the total
ity of the tinting machines dedicated to the small
shops were calibrated to work exclusively with the volumetric system, being easier for the
workers
dose and sell the finished products without making extra
activit
ies like involving balances,
taking
into c
onsideration the specific weight data

and making calculations
. The numerous problems
created by the sudden viscosity data variations (i.e. in
-
stock temperature and humidity data
variations)

affect the colorimetric characteristics

of the pigments concentrat
e pastes

and the
formulations have to
be settled time by time.

On the opposite way, the gravimetric controlled pigments dispersions guarantee a reliable constancy
in maintaining their colorimetric characteristics unchanged in the time. The modern way to pr
oduce
tinting machine has included the possibility to switch the system from volumetric to gravimetric
even in
the smaller machines, just adding

a good balance and asking to the supplier a dedicated

gravimetric

operating software.



Conclusions



The trad
itional production methods are more and more expensive and

therefo
re anti
-
economic and
inadequate. I
t is necessary to specify that
the pigments pastes

are able
, at today,

to
satisfy any
particular demand;
these preparations are formulated to be compatible
with the most used resins
having the purpose to produc
e industrial and resale paint
s
,

not altering

but increasing the quality
specific
s required by the final users.
Their complete adaptability to the modern industrial tinting
systems fully

satisfies
all
th
e production

requisites. The introduction of these

preparations is an
occasion
to revolutionize the traditional productive method, furnishing the means with

which the
paint

manufacturers can economize the procedures satisfying the always greater deman
d of
ready
products, either in
different packaging and in different colo
u
rs. Besides, the use of
the pastes

allows
to give a va
lid technological answer to the
legislation in subject of environmental and ecological
respect.



Bibliography


[1] T. Brock, M. Grote
klaes, P. Mischke: European Coatings Handbook (2000)