Characterization and Application of Naturally Occurring Mineral Based Pigment in Surface Coating

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 1101-1107

Published Online November 2012 (http://www.SciRP.org/journal/jmmce)

Characterization and Application of Naturally Occurring

Mineral Based Pigment in Surface Coating

Shweta Umale, Prakash Mahanwar

Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India

Email: [email protected]

Received July 8, 2012; revised August 13, 2012; accepted August 27, 2012

ABSTRACT

New naturally occurring mineral based pigment of general formula Hg2S have been processed and characterized for its

application in surface coating. Various analytical protocol like XRD, FT-IR, SEM and CIE 1976 colour coordinate sys-

tem have been performed for complete analysis of pigment. Characterizations using XRD, and CIE 1976 colour coor-

dinate assessment reveal the formation of pigments displaying colours ranging from brick-red to dark-brown. The typi-

cal designed pigment samples have been evaluated for their mass tone/hiding power, tinting strength and weather resis-

tance by coating on an MS steel panel. Optical, Chemical, Mechanical and performance properties of coating have been

evaluated for its application. The results demonstrated that the dark-brown pigment obtained in the present study was

found to be an interesting alternative to the existing classical toxic inorganic red pigments for surface coating applica-

tions.

Keywords: Light Fastness; Colour Value; Surface Coating; Dispersability; Refractive Index

1. Introduction

Natural inorganic pigments have been known since pre-

historic times, for example the drawings in the Pech-

Merle caves in the south of France, Northern Spain, and

Northern Africa were made with charcoal, ochre, man-

ganese brown and clays [1,2]. About 2000 BC, natural

ochre was burnt, sometimes in mixtures with manganese

ores, to produce red, violet, and black pigments for pot-

tery [3]. Arsenic sulphide and Naples yellow (a lead an-

timonate) were the first clear yellow pigments. Ultrama-

rine and artificial lapis lazuli (Egyptian blue and cobalt

aluminium spinel) were the first blue pigments [4].

Cinnabar is red color pigment which can be largely

used in surface coating application but there is large con-

troversy on utilization of mercury containing pigment. But

Mercury in traditional Chinese medicines mainly comes

from cinnabar, deliberately included for therapeutic pur-

poses based on Pharmacopeia of China [5]. Chinese Mi-

nistry of Health has paid close attention to the mercury

contents in traditional Chinese remedies. The allowable

amounts of cinnabar in these preparations have been de-

creased by as much as 65%, from a daily allowable dose

of 0.3 - 1.5 g in the 1977 Pharmacopeia to 0.1 - 0.5 g in

the 2005 Pharmacopeia of China [6], but the mercury in

these traditional medicines can still be thousands-folds

higher than what is considered safe in Western countries

including the USA The analysis showed that cinnabar is

insoluble and poorly absorbed from the gastrointestinal

tract [7-10]. Absorbed mercury from cinnabar is mainly

accumulated in the kidneys, resembling the disposition

pattern of inorganic mercury [11-13].

Use of inorganic pigments are paints, plastics, printing

inks for paper and textiles, leather decoration, building

materials, floor coverings, rubber, cosmetics, ceramic

glazes and enamels. When choosing a pigment for a par-

ticular application, several points normally have to be

considered [14-16]. The coloring properties are important

in determining the application efficiency and hence eco-

nomics. The following properties are also important:

1) General chemical and physical properties: Chemi-

cal composition, moisture and salt content, content of wa-

ter soluble and acid soluble matter, particle size, density

and hardness.

2) Stability properties: Resistance towards light, wea-

ther, heat and chemicals, anticorrosive properties and re-

tention of gloss.

3) Behaviour in binders: Interaction with the binder

properties, Dispersability, compatibility and solidifying

effect.

So the main objectives of this study was

1) To process and characterize mineral based pigment

collected from geographical place.

2) To study physico-chemical properties of the pig-

ment.

3) To formulate paint by using same pigment and to

study performance properties of the pigment.

S. UMALE, P. A. MAHANWAR

1102

2. Material and Methods

The material was obtained from various Geographical

places; it is of red colour type (Figure 1). For conveni-

ence red colored sample is referred as (RS). The sample

was crushed by using mortar and pestle and then ground

so as to obtain fine powder. The powder material was

then ball milled to convert into fine powder. Particle size

was determined by using particle size analyzer. Various

analytical protocols have been used for material identifi-

cation such as X-ray diffraction, FT-IR, Scanning elec-

tron microscope (SEM-EDX) and Differential Scanning

colorimetry (DSC). XRD analysis of samples shows the

presence of certain amount of mineral phases. An ele-

mental analysis can be performed by using energy dis-

persive X-ray fluorescence. The physico-chemical pro-

perties were studied.

2.1. Characterization and Testing

2.1.1. Oil Absorption Value (ASTM D 281-31)

Oil absorption value as per ASTM D281-31 was evalu-

ated using alkali linseed oil. Oil absorption value is ex-

pressed as mg of oil/100 gm of pigment. Pigment type,

pigment size, and particle size distribution these are the

factors which affect oil absorption value.

2.1.2. Moisture content of pigment (ASTM D 280-7 5 )

Moisture content refers to the water left in the pigment

after it has dried. Generally, the moisture present account

for a very small fractions and is mainly due to the water

entrapped within different particles of pigment. Here, a

known weight of a pigment is dried in an oven at 105˚C

for 2 hrs. From the loss in weight, the moisture content is

determined.

2.1.3. Bleedi ng of Pigme n ts (ASTM D 279-02)

Bleeding is the term used to describe the discoloration or

staining which sometime occurs when a white or light

coloured paint is applied over a deeper colour. Bleeding

property of pigment is evaluated as per ASTM D 279-02.

Pigment is tested for toluene, Xylene, Butanol and MEK.

2.1.4. Hiding Power of Paint (AST M D 344-97)

Hiding power of pigment was evaluated according to

Figure 1. Photographs of red pigment collected from geo-

graphical place.

ASTM D 344-97. Chequers board with alternative black

and white blocks was used for testing. Pigment was dis-

persed in medium and then applied on board. Amount

required to hide the measured space is calculated.

2.1.5. Resistance to Heat (IS: 3493)

Resistance to heat of pigment was evaluated according to

IS 3493. Heat resistance was tested up to 180˚C.

2.1.6. Density (AST M D 1 3)

Specific gravity of pigment was determined according to

ASTM D 13.

2.1.7. Formulation of Paint by Usi ng RS

Paint was formulated by using pigment RS. Various bat-

ches of paint were formulated by varying pigment vo-

lume concentration. Each batch was characterized for its

solid content, drying time, gloss and skinning properties.

The detailed material data sheet is mentioned in Table 1 .

The components of formulation are mentioned in Table 2.

The characterization of each batch is given in Table 3.

2.2. Performance Properties of Coating

Four coating formulation panels (F1, F2 and F3) were

tested for different mechanical and chemical resistance

properties as per the following standard test methods.

The coated panel with formulation is shown in Figure 2.

Table 1. Material data.

Product name Raplakyd-515-60)

% Non- volatile content at 120˚C60 ± 2

Solvent Mineral Turpentine

General ingredient Linseed oil penta

Oil length (%) 64

Acid value (max) (mgs. of

KOH/gm) 10

Dilution solvent % for testing

color & viscosity M.T/50

Color (Gardner) 7

Table 2. Formulation 1.

Raw material Part by wt wt%

Binder (raplakyd-515-60 )12 52

Pigment (RSO) 5 22

Solvent (MTO) 5 22

W & D agent 0.2 8.8

Antisettling agent 0.1 4.4

Drier 0.1 4.4

S. UMALE, P. A. MAHANWAR 1103

Table. 3 Characterization of batch.

PVC- 15.25

Vehicle non-volatile- 60%

Brushing - Excellent

Levelling- Very good

Settling- Hard

Skinning- High

Gloss- High

Drying time Touch dry—4 - 5 hrs

Hard dry—9 - 10 hrs

Figure 2. Photograph of painted MS panels with pigment.

2.2.1. Sc r atch Hardness

Scratch hardness was carried out as per ASTM D-2197.

The scratch resistance of the samples was determined

using an automatic scratch hardness tester having a hard

end steel hemispherical point of 1 mm diameter as a

scratching needle. The test was moved at a fixed speed of

3 cm/sec beneath the point of scratching needle. The

scratch resistance is reported as the weight on the stylus

to tear off the coating. This test simulates scratching by

sharp objects.

2.2.2. Pe n cil Hardness

Pencil hardness of coating was determined as per ASTM

D 3363 using pencil hardness tester with calibrated set of

drawing leads ranging from B (the softest) to 6H (the

hardest). The process was started with softest pencil to

end with hardest pencil. A scratch hardness value of the

films is equal to the hardness value of the pencil that tear

the film.

2.2.3. Impact Resistance

Tubular impact resistance test was conducted as per

ASTM D2794 to predict the ability of the coating to re-

sist cracking caused by rapid deformation. It is reported

in terms of inch-pound (height-load). Tubular impact re-

sistance test was carried out using an indenter with

hemispherical head of diameter 0.625 inch and 2 lb load.

2.2.4. Flexibility

The Conical Mandrel Bend Test of the coatings was car-

ried out as per ASTM D522. The Mandrel Bend Test is a

measure of flexibility of coating. As applied coat was

dried enough the prepared panels were kept between the

mandrel and draw bar. The lever bar was drawn down at

uniform velocity to bend the specimen approximately of

135˚. The bent surface was observed for crack or other

surface defects.

2.2.5. Cross-Hatch Adhesi on

This test carried out as per ASTM D3359-83. Crosscut

adhesion tape test is used to assess the adhesion of coat-

ing films to metallic substrates. Cuts were made on the

coating in one steady motion with sufficient pressure on

the cutting tool having a cutting edge angle between 15˚

and 30˚. After making two such cuts at 90˚ the grid area

was brushed and a 2.5 cm wide semi-transparent pres-

sure-sensitive adhesive tape was placed over the grid.

After 30 seconds of application, the tape was removed

rapidly and the grid was inspected according to the stan-

dards. The amount of coated area retained under the tape

corresponds to the adhesion efficiency of the coating.

More the coated material removed by the tape, poorer the

adhesion of the coating to the substrate.

2.2.6. L, A, B Values

The L*, a*, b* values of the pigment system were evalu-

ated by using colour Eye 7000 spectrophotometer with

CIELab colour coordinate system.

2.2.7. Chemical Properties

Resistance to acid and alkali was determined by using

ASTM D-4274-88 standard while for detergent resis-

tance standard ASTM D-2248a was followed. For this

test, the coated panels were immersed in 5% solution of

HCL (acid), 5% solution of NaOH (alkali) and 5% solu-

tionof detergent. The immersed panels were maintained

at constant temperature. The panels were removed for

examination after 6, 12, 18 and 24 hours from the start of

the test and observed for loss of adhesion, blistering,

popping or any other deterioration of the film.

2.2.8. So lvent Res istance

The resistance of the coating towards the solvents like

Methyl Ethyl Ketone (MEK) and xylene was determined

as per the procedure given in ASTM D-5402-93. The

coated panels were rubbed with the cotton moist with

respective solvent and observed for any softness of the

film, peeling of the film and loss of gloss.

2.3. Optical Properties

2.3.1. Glos s

Gloss is a measure of ability of coated surface to reflect

S. UMALE, P. A. MAHANWAR

1104

light at a particular angle without scattering. Gloss was

determined according to ASTM D523-67. Gloss of the

cured sample was measured at angles 45˚ and 60˚ using a

digital mini gloss meter calibrated against internal stan-

dard i.e. refractive index (Komal Scientific Co. Mumbai,

India) and the results are reported in terms of gloss unit

(GU).

2.3.2. Colour Values

L*, a* and b* values were calculated as per the CIE 1976

method. The L*, a* and b* values of coating were deter-

mined using spectrophotometer. (Gretag Macbeth, Col-

ourEye-7000A, USA). L* determines the lightness or dark-

ness (higher the L* value, higher is the lightness and vice

versa), a* gives the redness or greenness (+ve a* = red-

ness and –ve a* = greenness) and b* indicates the yel-

lowness or blueness (+ve b* = yellowness and –ve b*=

blueness).

2.3.3. UV Resistan c e

QUV accelerated weathering tester of Q-panel lab prod-

ucts was used for weather ability testing. The panels

were kept in the weathering tester to study the weather-

ing resistance of the coatings. This test is conducted ac-

cording to ASTM G 154. The test was done in two alter-

nate cycles provided with 4 hours of UV radiation and 4

hours of condensation as per the standard. The samples

were tested for the optical properties to observe the

change in colour values. During the accelerated weather-

ing, the surface colour difference of the sample was

measured in order to record photochemical degradation.

The surface colour difference (ΔE*) was measured using

colour spectrophotometer and it was calculated using

CIE L*a*b* system.

ΔL* = Lf

*– Li

Δa* = af

*– ai

Δb* = bf

*– bi

L*, a*and b* represent the lightness, redness and ye-

llowness, respectively.

ΔE* = (ΔL*2 + Δa*2 + Δb*2)1/2

ΔL*, Δa* and Δb* represent the difference between the

initial (i) and present (f) values.

3. Result and Discussion

3.1. Charactrization

3.1.1. XRD Analysi s

XRD pattern of the material shows intense and sharp

peaks which reveal the crystalline nature of pigment Fi-

gure 3. The data obtained from diffract grams was then

analyzed for reflection angle to calculate the interatomic

spacing (D value in Angstrom units). The intensity is

measured to discriminate various D spacing and the re-

sult was compared with standard mineral data to identify

possible matches.

3.1.2. Elemental analysis

Elemental analysis of pigment was performed by induc-

tively coupled plasma—atomic emission spectroscopy

(ICP-AES) and Energy Dispersive X- ray fluorescence

(EDX). Result shows that pigment contain mercury as

major constituent in the form of mercury sulphide.

3.1.3. SEM Analysis of Sample

The morphology of the material was analyzed by using

SEM and were found to be more or less uniform Figure

4. The crystalline nature of pigments has also been no-

ticed from SEM micrograph of pigment. SEM micro-

graph of the sample shows that surface of pigment is

irregular and shows cristae.

3.1.4. FT-IR Spectrum of Pigment

FT-IR spectrum of pigment Figure 5 shows that no

characteristic peak was observed in the spectrum indi-

cating no organic functionality.

Figure 3. XRD pattern of pigment.

Figure 4. SEM micrograph of pigment.

Figure 5. FT-IR spectrum of sample.

S. UMALE, P. A. MAHANWAR 1105

3.1.5. Physicochemical Properties of Pigments

Physic-chemical properties of the pigment are mentioned

in the following Table 4. Which indicate that pigment

has excellent acid resistance but poor alkali resistance?

The pigments do not show bleeding in organic solvents.

They are neutral in nature. The oil absorption value of

this pigment is in the acceptable range considering the

inorganic nature of pigment.

3.2. Performance Properties of Paint by Red

Colour Pigment

3.2.1. Mechanic al Properties

The results of scratch resistance, pencil hardness, impact

resistance, flexibility, cross hatch adhesion for coating

formulations Figures 1 to 10 are shown in Ta ble 5. From

the table it is clear that impact resistance of paint is good.

The flexibility test was passed for all the pigment coating.

Cross hatch adhesion by tape was determined by apply-

ing cross cuts on the coated panel. All the coating for-

mulations passed across hatch adhesion test. Gloss of lac-

quer coating was compared with gloss of Figures 1 to 3

at 45 and 60. Gloss of formulation 1, 2 and 3 is higher

than lacquer. Result shows that in both the cases gloss

increases after dispersing pigment in lacquer. Improve-

ments in properties are attributed to surface characteris-

tics of pigment.

Table 4. Physico-chemical properties of the pigment.

Parameter Inference

Oil absorption value 29.7 mg/100 g pig.

Particle size 30 - 50 micron

Density 2.66 g/cm3

Chemical nature Neutral

Chemical resistance Acid—good

Alkali—good

Bleeding properties

Toluene—excellent

Xylene—excellent

Butanol—excellent

Methyl ethyl ketone—excellent

Table 5. Mechanical properties of pigment.

F1 F2 F3 F4 F5 F6 F7 F8 F9F10

Impact

resistance P P P P P P P P P P

Flexibility P P P P P P P P PP

Pencil

hardness 3B 2B 3B 3B 3B 2B 3B 3B 3B 3B

Scratch

hardness

(g)

200 300 300 300 300 300 300 300 300 300

Gloss at

60˚ 78 73 89 71.68 70 71 68 67 6870

Gloss at

45˚ 51 36 45 45 48 52 55 52 51 49

3.2.2. Chemical Resistance

The coating samples were tested for their chemical resis-

tance properties. The results of the tests are shown in

Table 6. Chemical inertness of pigment protects the sur-

face from various chemicals & reagents thereby resulting

in improvement in overall chemical & solvent resistant

properties. It was observed that acid has caused loss in

adhesion resulting in partial peeling off the coating also

alkali causes blistering of paint film. Film shows good

chemical resistance against organic solvents.

3.2.3. Optical Pro perties

The coated panels were tested for the accelerated weath-

ering testing up to 400 hours of exposure. During this

after every 100 hours gloss and colour values of the

tested panels were determined. From the L*, a* and b*

values the discoloration measurement is done by calcu-

lating ΔE* value. Table 7 shows the discoloration value

for the paint coating. We studied change in colour values

with respect to hours exposed for coatings. Result shows

that there is no drastic change in color values after expo-

sure to Quv weatherometer indicating good light fastness.

The color values of natural pigment were compared with

synthetic pigment (red iron oxide) shown in Table 8. A

lower value of DE* i.e., 15.12 indicate that natural pig-

ment has similar mass tone to that of synthetic pigment

and can replace synthetic one.

Table 6. Chemical resistance of coating.

Property F1 F2 F3

Acid Resistance 4 5 5

Alkali Resistance 1 3 3

Distilled water Resis-

tance 1 1 1

Detergent Resistance1 1 1

MEK 1 1 1

Solvent Re-

sistance xylene1 1 1

1Not affected, 2Loss in gloss, 3Blistering, 4Slight loss of adhesion, 5Film

partially removed, 6Film completely removed

Table 7. L*, a* and b* values for paint system during the

accelerated weathering.

S1 S2 S3

*a* b

* L

*a* b

* L

* a

* b

000 hrs40.84 36.06 22.1839.85 25.36 16.55 35.08 19.95 18.86

100 hrs40.76 36.05 22.1839.81 25.32 16.55 35.01 19.87 18.87

200 hrs40.73 36.04 21.1839.80 25.32 16.52 35.01 29.8718.8

300 hrs39.73 36.04 21.1840.20 25.32 16.52 35.01 29.8718.8

400 hrs38.76 36.05 22.1839.81 25.32 16.55 35.01 19.87 18.87

S. UMALE, P. A. MAHANWAR

1106

3.2.4. Eval ua ti on of Mass To ne/ Hi di ng Power of

Pigment

The hiding power and tinting strength was evaluated by

coating on an opacity chart at a thickness of 150 μm

(Figure 6). The CIELAB 1976 method of determination

of L*, a* and b* was employed to determine the hiding

power and tinting strength of the colorants by compari-

son of values when coated on a board with a checkered,

white and black background and the results are summa-

rized in Tables 9. The colour difference has been quanti-

fied on the CIELAB based colour difference (DEab),

which is calculated using the Equation (1) [CIE 1986;

CIE 2004; Ohno 2000]. For mass tone, the colour differ-

ence between the black and white region was found to be

DEab = 10.8. A lower value of DEab (3.3) for mass tone

on the black versus the white regions in the case of

Ce0.8Ti0.05Pr0.15O2 sample indicates the ability of the

dark-brown colorant to cover black and white regions

uniformly well.

Figure 6. Photograph of hiding power of natur al pigme nt.

Table 8. Comparison of c olour value of nat ural pigment with

synthetic pigment.

Synthetic pigment Natural pigment

L* 34.10 40.20

a* 23.96 33.47

b* 15.85 25.92

DL* 6.10

Da* 9.50

Db* 10.06

DE* 15.12

Table 9. Colour coordinate of the pigments after hiding

power analysis on white and black surface.

Surface L* a

* b

* DEab

Black 44.1 17.4 17.3 10.5

White 51.6 24.2 21.1 3.4

DEab = [(DL*)2 + (Da*)2 + (Db*)2]1/2

4. Conclusion

Studied natural pigment has good chemical resistance

and excellent mechanical properties. Naturally occurring

pigments shows good performance properties compared

with synthetic pigment, thus natural pigment can be good

alternative for synthetic pigment.

5. Acknowledgements

Author acknowledges to UGC-SAP for providing finan-

cial support and also to sophisticated analytical instru-

ment laboratory (SAIF, IIT. Bombay) for providing SEM

and ICP-AES facility for analysis.

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