The local clays used were sourced from two different locations, Obowo and Ihitte-Uboma Local Government Areas within the South-Eastern region of Nigeria. Analytical grade of titanium tetrachloride (TiCl₄), hydrochloric acid and ammonia solution from BDH Laboratory were used. The core-shell extender pigments were prepared using local clays of 0.150 mm particle size which was found to be the major size present in both clays.

Titanium tetrachloride (TiCl₄) was added in three different concentrations of 2, 4, and 6 cm 3 to 100 cm³ hydrochloric (HCl) acid contained in three different 500 cm³ beakers. The Obowo or Ihitte-Uboma clay was then immersed in the three different solutions respectively, was gradually stirred and allowed for some time for the clay to settle, while the HCl formed an envelope on top of the settled clay. Ammonia solution was added to the impregnated clays in drops to adjust their pH till the extender pigments are completely precipitated. The paste formed was filtered through a Buchner funnel and washed with distilled water, dried, and calcined at 650˚C. Three different concentrations of titanium dioxide, and consequently, of titanium, was expected to be deposited on the clay surface.

Alkyd paint samples were prepared using the prepared core-shell extender pigments and TiO₂. Paints prepared using TiO₂ were for reference purpose. Xylene was used as the solvent while a medium oil alkyd resin based on soya bean oil purchased from TonyKelv Nigeria Ltd., Onitsha, Nigeria was used in this study as the binder. The different alkyd paint formulations studied are shown in Table 1. Lead and Cobalt naphthanate with metal contents of 36% Pb and 12% Co were used as driers. Cobalt naphthanate was used for surface dry, while lead naphthanate drier was used for through dry.

A given quantity of the prepared paint sample was accurately weighed, and calculated amounts of lead napthanate and cobalt naphthanate driers were added. The paint-drier mixture was mixed thoroughly using a glass rod. The amount of driers added represents 0.50 wt% lead (Pb), and 0.05 wt% cobalt (Co) which are the amount of these metals normally used in the surface coatings industry [23] . The amount of driers giving the above percentage of metals in the paint-drier mixtures was calculated as follows:

The American standard testing methods (ASTM) was used to evaluate the prepared alkyd paint samples for viscosity (ASTM D 1200-10), specific gravity (ASTM D 1475-13), adherence to surface (ASTM D 6677-07, 2012), settling and skinning tendencies (ASTM D 869-85). The settling tendencies of the samples were rated in accordance to the degree of settling on a scale that range from 10 to 0 (Table 2). Intermediate conditions were given appropriate odd numbers. The Nigerian Industrial Standard (NIS) procedure was used to study the mildew formation resistance (NIS 278:1990), media resistance and chalking tendency (NIS 268:1989) of the prepared alkyd paint samples. Mild steel panels were used as the coating surfaces for the casting of paint-drier mixtures. The panels were prepared according to ASTM D 609-00 procedure, and were taped to 0.40 mm thickness for the casting of the paint-drier mixtures. The paint-drier wet film thickness was assumed the same as the thickness of the tape layer used. The paint-drier mixtures were cast immediately after their preparations on the taped-off portions of the mild steel panels; the wet paint-drier films were leveled off using a short glass rod. The coated plates were placed on a level surface and were allowed to dry at room temperature. Subsequently, the drying properties (surface (dust-free) dry, tack free dry and through dry) were studied.

Note: TiO₂/LCC (2), TiO₂/LCC (4), and TiO₂/LCC (6) are the three different core-shell extender pigments prepared for each local clay that contain 2, 4, and 6 cm³ of titanium tetrachloride.

The X-ray fluorescence (XRF) determinations on the core-shell extender pigments presented in Table 3 shows that two major constituents of the prepared extender pigments in order of abundance are silica, (SiO₂) and aluminum oxide (Al₂O₃). The titanium dioxide (TiO₂) content of the core-shell extender pigments are between 7.0 and 13.0 wt% while Weaver [24] reported that TiO₂ values in kaolin commonly range from 0.20% to 2.50%. The high TiO₂ content in the prepared core-shell extender pigments is an indication of the presence of titanium dioxide and consequently titanium deposited on the clays.

The particle morphology of the core-shell pigments obtained from scanning electron microscope (SEM) shown in Figure 1 and Figure 2 indicates that the Obowo and Ihitte-Uboma core-shell extender pigments are nodular (a rounded irregular shape) shaped structures of different sizes. It can be inferred that as titanium is deposited on the clay surfaces, new structures are formed which have the clays appearing as clouds in the background of the micrographs and this may be attributed to the purity and fine particle nature of the clays. Therefore, the combination of the clay and titanium dioxide structure possibly lead to the formation of core-shell structures of different plate sizes without any change in the plate shapes.

The properties of the core-shell extender pigments presented in Table 4 shows that the prepared extender pigments are slightly alkaline having values ranging from 7.45 - 7.56. It is important to note that the pH of the prepared core-shell extender pigments were not much affected by progressive increase in titanium dioxide deposited on the pigments. The specific gravity of the core-shell extender pigments were found to increase with increase in titanium dioxide present in the pigments. From the present study, the core-shell extender pigments have lower specific gravity than titanium dioxide. Interestingly, lower specific gravity of pigments leads to a considerable cost saving in coatings [25] .

The refractive indices of the prepared core-shell pigments determined using an Abbe’s refractometer also presented in Table 4 illustrates that the pigments have refractive index lower than that of TiO₂ pigment but higher than other extenders. It is important to note that all extenders have refractive index that fall in the range between, 1.45 and 1.65 [26] . Therefore, the core-shell extender pigments have shown a remarkable improvement in the refractive index of extenders and can be considered to impart opacity to polymer coatings. The refractive indices of the prepared core-shell extender pigments were found to increase with increase in titanium dioxide deposited on the extender pigments.

The core-shell extender pigments’ oil absorption properties were determined using the spatula rub-out technique and results obtained are shown in Table 4. The various prepared core-shell extender pigments exhibited roughly the same oil absorption values, and which is higher than the oil absorption TiO₂ pigment. Less oil absorption indicates less resin demand without compromising other coating properties [27] . The oil absorption depends mainly on the physical structure (i.e. size and shape of the extender particles), which also affects several other coating properties including flow characteristics, setting tendency, film durability [28] .

The specific gravity values of core-shell extender pigments and titanium dioxide (TiO₂) formulated alkyd paints are shown in Table 5 and illustrated graphically in Figure 3. The table shows that the TiO₂ formulated alkyd paint has the highest specific gravity when compared to those formulated using the core-shell pigments. This is

Abbreviations: OBC = Obowo clay core-shell pigment; TC = Titanium tetrachloride; IHC = Ihitte-uboma core-shell pigment; 2, 4, 6 = Quantity (cm³) of TC in OBC or IHC.

to be expected since TiO₂ pigment has the highest specific gravity value compared to the core-shell pigments used in this study [22] . The alkyd paints formulated using the core-shell pigments exhibited low values of specific gravity. Obviously, more of the core-shell pigments can be incorporated into the alkyd paints with considerable cost saving when compared to the incorporation of TiO₂ in alkyd paint. Specific gravity of paints is used to determine the coverage of paint products on a substrate, that is, the volume occupied by a known weight of paint product [29] .

The study shows that alkyd paints formulated using Ihitte-Uboma core-shell extender pigments prepared using 2 cm³, and 4 cm³ of TiCl₄, as well as Obowo core-shell extender pigments prepared using 4 cm³ and 6 cm³ TiCl₄ exhibited the same specific gravity (1.35) in paints. The alkyd paint formulated using Obowo core-shell pigments that was prepared using 2 cm³ TiCl₄ exhibited the least specific gravity in the prepared paints.

The viscosity values of the formulated alkyd paint samples are presented in Table 5 and illustrated graphically in Figure 4. Titanium dioxide (TiO₂) formulated alkyd paint exhibited the least viscosity value among the alkyd paints studied. The present study has shown that the viscosity of alkyd paints prepared using the core-shell extender pigments was observed to decrease as the quantity of TiO₂ they contain increased, and vice-versa. The range of viscosities observed for the paint samples makes them suitable for application by spray or brush. Tiwari and Saxena [26] who studied fly ash extended coatings obtained viscosities that ranged between 2.0 and 2.58 Pa.s, and noted that the formations were suitable for application by brush or spray. The interaction of various constituents in any coating system determines its viscosity [30] . Viscosity affects the application and flow properties of a coating and is generally adjusted to the intended application [31] [32] . According to the Nigerian Industrial Standard NIS 268:1989, the minimum viscosity for gloss paint shall be 22 centipoise.

The thickness of a coating is of importance since it profoundly affects the other properties of coatings such as the extent of protection of a substrate by the applied paint. The data obtained on the thickness of the dried paint films in this study are presented in Table 5 and illustrated graphically in Figure 5. From the figure, it is evident that TiO₂ formulated paint exhibited the least paint dry film thickness (0.24 mm). Though the thickness of the

alkyd paint films formulated using the core-shell extender pigments were generally observed to decrease with increase in TiO₂ content, the Ihitte-Uboma core-shell extender pigments formulated paint samples exhibited the highest film thickness.

According Tan Tian Aik, [33] , weight loss due to weathering is proportional to film thickness up to 20 μm, above which the rate of weight loss becomes indifferent of the thickness. Thus, a film thickness of more than 20 μm performs well as a barrier resistant to weathering. The results obtained in this study on the film thickness of the samples studied are in the desired range to be considered for corrosion protection.

The drying properties of the prepared paint samples given in Table 5 are illustrated graphically in Figure 6. The drying properties investigated are dust-free dry, tack-free dry, and through-dry.

The chalking tests carried out on the formulated paint samples indicated that none of the paint samples showed any sign of chalking. A good paint should not chalk. The results of the above test showed that the core-shell extender pigments performed equally well in coatings just like the paint prepared using the conventional TiO₂ pigment.

The formulated paint samples exhibited no sign of skin formation after eight months of storage. Various degrees of settling were observed in the samples after eight months of storage as shown in Table 5 and illustrated graphically in Figure 7. The settling tests shows that paints formulated using the core-shell extender pigments from both local clays showed moderate settling tendencies when compared to the formulation containing TiO₂ pigment which almost showed no settling tendency. However, the settling tendencies of the core-shell extender pigment formulated alkyd paints were observed to be independent on TiO₂ content though at the highest TiO₂ content of the core-shell pigments, they showed an improved settling tendency.

Generally, some properties of pigments affect their settling tendencies. These include particle size, particle shape, specific gravity, and the activity which includes such properties as basicity, wettability, flocculating, dispersing and gel forming tendencies. The properties of the binder also influence the settling tendencies. It is probable to reason that the difference in settling tendency observed for the core-shell extender pigments and TiO₂ formulated paint samples may be due to the difference in their particle structure.

Furthermore, specific gravity is known to be related to the sedimentation and granulation of paints and affects the rate at which solid particles settle in liquids. The core-shell pigments were reported [22] to have specific gravity that was lower than that of TiO₂. The proportional difference between the specific gravities of these extender pigments and the binder may have led to the slight settling noticed in the core-shell extender pigments based paint formulations. Oil absorption of pigments also relates to its settling tendencies. Pigments with high oil absorption will tend to absorb the paint vehicle and form coat after an extended period of storage. TiO₂ pigments were reported [22] to have oil absorption value that was lower than those of the core-shell pigments. This could have also contributed to the various levels of settling observed in the paint samples.

Data on the adhesion properties of the prepared paint samples are shown in Table 5, and illustrated graphically in Figure 8. The figure shows that all the alkyd paint samples exhibited good adhesion properties. According to Nigerian Industrial Standard, NIS 268:1989, gloss paint should not exhibit more than 50% removal of the dried paint film. The paint samples prepared using the core-shell extender pigments of Ihitte-Uboma clay, core-shell extender pigments of Obowo clay (with the exception of the pigment containing 6 cm³ TiCl₄), exhibited particularly good adhesion properties. For the paint samples formulated using the Obowo core-shell extender pigments, the % adhesion loss of paint dry film was observed to increase with increase in TiO₂ content. Alkyd paints formulated with Obowo clay core-shell extender pigment prepared using 6 cm³ TiCl₄ exhibited the highest % adhesion loss of dry paint film (8.5%) followed by the TiO₂ pigment formulated paint sample. All the prepared paint samples exhibited % adhesion loss in the range, 1 - 10. Adhesion according to ASTM D 907 (2012) is defined as the state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking action or both. For the most part, organic coatings are removed in service by abrasion, chipping, coins, and other instruments, picking away at exposed edges, corrosion of the substrate, impact or impingement by stones, etc.

All the formulated alkyd paint samples did not show any sign of mildew formation after seven months of exposure to rain, and sunlight. This is an indication that the core-shell extender pigments performed equally well in coatings just like the TiO₂ formulated paint samples. On the other hand, the alkyd paint formulated samples shaded their color after seven months of exposure to rain and sunlight. The paint formulations containing TiO₂ exhibited slight rust while all the core-shell extender pigment based paint formulations resisted rust as shown in Figure 9. This is an indication that the core-shell extender pigment formulations can withstand environmental challenges, and therefore can be used in corrosive environment.

The visual changes that occurred on the formulated paint films after immersion in 2% NaCl, 2% H₂SO₄, and 2% Na₂CO₃ and distilled water for 24 hours are presented in Table 6. The performance of the paint samples shown in Figure 10 were rated according to the method of Muralidharan [34] as follows: 0―No change; 1―Very slight effect; 2―Effect; 3―Light effect; 4―Definite effect; 5―Bad effect.

From Table 6, it is evident that all the formulated paint samples performed satisfactorily in 2% Na₂CO₃ and distilled water with the exception of TiO₂ formulated paint sample. The paint samples also performed well in 2% sulphuric acid. The performance in 2% NaCl was good; except for slight salt effect on paint samples formulated using TiO₂. The good performance of the core-shell pigments based paints in acidic as well as 2% Na₂CO₃ could be attributed to the presence of inert oxides they contain [22] [26] .

It is important to note that water in one way or the other is the common enemy to most materials of construction. The absorption of water by paint films can lead to loss of components from the film, and subsequent deterioration

0―No change; 1―Very slight effect; 2―Effect; 3―Light effect; 4―Definite effect; 5―Bad effect.

of paint films. The stability of paint samples formulated using the core-shell pigments in NaCl solution means that the paints can be applied where high durability against corrosive environment is required.