Partially hydrolyzed polyvinyl alcohol (PH PVA) (CAS No. 9002-89-5, % degree of hydrolysis 87% - 89%, Molecular weight 99,000 - 105,000 g/mol) was obtained from Kuraray Cooperative Limited, India. Vinyl acetate (VAc, CAS No. 108-05-4), sodium lauryl sulfate (SLS, CAS No. 68585-34-2), sodium bicarbonate (SBC, CAS No. 144-55-8), potassium persulfate (PPS, CAS No. 7727-21-1), 2-ethyl hexanol (2 EH, CAS No. 104-76-7) and Isothiazolinones preservative (CAS No. 1003-07-2) were acquired from Sigma Aldrich. Calcium carbonate (CaCO₃, CAS No. 471-34-1, % Moisture content of 1% and Oil absorption value of 19.5 - 20.5 gm/100gm) clay (CAS NO. 1332-58-7, % Moisture content of 1% and Oil absorption value of 37 - 45 gm/100gm) was purchased from 20 Microns Limited. Deionized water (DIW) was used which was prepared and tested in the lab.

Polymer emulsions were synthesized in a flat-bottom glass reactor equipped with a condenser, stirrer, temperature indicator, metric pump and glass funnel for the addition of additives. Semi-continuous emulsion polymerization process was adopted with continuous supplying thermal energy using a water heating bath. Table 1 shows the polymer emulsion recipes.

PVA was first put into a glass reactor. The water was added to them, and the mixture was kept for continuous stirring at 175 rpm. The temperature was slowly raised up to 94˚C - 96˚C and the final temperature was kept at 94˚C - 96˚C. The cooking was carried out for 2.5 hr. Then, the solution was cooled down to 25˚C and added isothiazolinones preservative. It is used in PVAc Homo.

Fillers like Calcium carbonate in PVAc Homo CC and clay in PVAc Homo CL and PVA were first put into a glass reactor. The water was added to them, and the mixture was kept for continuous stirring at 175 rpm. The temperature was slowly raised up to 94˚C - 96˚C and the final temperature was kept at 94˚C - 96˚C. The cooking was carried out for 2.5 hr. Then, the solution was cooled down to 25˚C and added isothiazolinones preservative.

PH PVA solution was used as a protective colloid for the stabilization of the emulsion. Sodium lauryl sulphate was used as an anionic surfactant. Sodium bicarbonate is used as buffer. Initiator solution was prepared by dissolving potassium persulfate in deionised water. 2 ethyl hexanol was used to control foam generation in batch. Polymerization was carried out by controlled addition of vinyl acetate (VAM) monomer. After the complete addition of monomer, potassium persulfate solution in water was added and temperature was maintained at 90˚C for 1 hr. Then, the emulsion was cooled down to 25˚C and added Isothiazolinones preservative.

PH PVA + filler solution was used as a protective colloid for stabilization of emulsion. Sodium lauryl sulphate was used as an anionic surfactant. Sodium bicarbonate is used as buffer. Initiator solution was prepared by dissolving potassium persulfate in Deionised water. 2 ethyl hexanol defoamer was used to control foam generation in batch. Polymerization was carried out by controlled addition of monomer VAM. After the complete addition of monomer, potassium persulfate solution in water was added and temperature was maintained at 90˚C for 1 hr. Then, the emulsion was cooled down to 25˚C and added isothiazolinones preservative.

Solids content of emulsion was checked by heating sample (around 1 g) at 120˚C for 120 minutes in a hot air oven. Measurements were done in triplicate to minimize error. To measure the solid content the following equation was used,

Solidcontent ( % ) = ( W 3 − W 2 ) / ( W 2 − W 1 ) × 100 %

W₁ = Weight of empty weighing pan.

W₂ = Weight of Pan with sample before heating.

W₃ = Weight of pan with residue after drying till constant weight.

A Brookfield Viscometer RVT was used for calculating the viscosities of the samples. The values were taken at 20 rpm and at a temperature of 30˚C. pH of the samples was analysed using a digital pH meter, CL 54 + Toshcon Industries, India. To reduce the error, viscosity and pH measurements were taken 5 times.

A Perkin Elmer instrument Q 100 DSC has been used for estimating the glass transition temperature (T_g) of the samples. During the measurement, an oxygen-free nitrogen stream of 40 mL/min was maintained through the cell. Sample film casted on Teflon was dried at 105˚C for 90 min. 5 mg film was analysed directly on DSC by cyclic heating-cool-heat method. The DSC analyses were done within a range of −10˚C to 200˚C at a heating rate of 10˚C∙min⁻¹.

Using an applicator, 200-micron films were casted over a glass plate and the time for drying was measured using a stopwatch. Time taken for full drying of films is noted at 30˚C.

The pencil hardness test was done using a QHQ-A portable pencil hardness tester, having lead pencils of varying hardness numbers. Using an applicator, 200-micron films were casted over a glass plate. After the films are dried at 28˚C for 1 day, the test was conducted following ASTM D 3363 standard.

The water contact angle was measured using a Rame-Hart Goniometer, Germany at 28˚C and 60% relative humidity. To measure the contact angle, the samples are cast over glass substrates with an applicator of 100 micrometers. The test was conducted 5 times to reduce the error. Contact angle measurement gives an idea of the behaviour of prepared samples with respect to water. Moreover, variations in the hydrophobicity in formulations can be analysed using contact angle measurements.

Shear strength was evaluated at different intervals in joints prepared by gluing beech specimens in a normal climate. Bonded assemblies were tested after keeping them at 23˚C and 60% relative humidity after 4 hr, 6 hr, 24 hr and 7 days. In this case, strength was calculated in MPa. Wet strength tests were performed according to condition D3-3 of EN 205 after 7 days in a normal climate, specimens were soaked in water for an additional 24 hr and then tested water resistance.

Solids content of emulsion was checked by heating the sample (around 1 g) at 120˚C for 120 min in oven. It was calculated that the solid for both the samples are approximately the same (Table 2).

Viscosity and pH of the prepared samples are shown in Table 2. Presence of calcium carbonate in PVAc Homo CC sample enhances the pH and makes the system less acidic in nature as compared to PVAc Homo CL. Compared to the PVAc Homo CC, increase in viscosity was observed in PVAc Homo CL containing clay-PVA as a protective colloid. High oil absorption value and smaller particle size of clay resulted in increased in viscosity as compare to PVAc Homo CC.

DSC thermograms quantify the thermal stability of samples. The overlaid DSC thermograms for PVAc homo and PVAc Homo CC, PVAc Homo and PVAc Homo CL are shown in Figure 1. Both the samples were disintegrated at about their T_g, mainly attributed to the decompositions of polymer and homopolymer chains. Due to secondary forces in PVAc Homo CL led to the increment of T_g as compared to PVAc homo CC, due to the complex network of hydrogen bonding formation. The same is observed from the viscosity analysis.

Drying time test was conducted on glass plate at controlled conditions (50% relative humidity, temperature of 29˚C - 31˚C). From the result shown in Table 2, the drying time for PVAc Homo CC was more than that of PVAc Homo and PVAc Homo CL. The acidity of the filler influenced the drying time on glass plate as shown in Table 2.

Pencil hardness of polymeric film evaluates the mechanical property. As the addition of clay in PVAc Homo CL, hardness of film was increased as compared to

PVAc Homo and PVAc Homo CC as shown in Table 2. Clay has high acidic nature hence adhesive formulated from them have quick film formation and faster drying when compared with calcium carbonate. Due to faster drying, slip from sharp pencil due to platelet like structure increased, hence the PVAc Homo CL sample showed a higher value of pencil hardness.

In general water contact angle correlates with the hydrophobic or hydrophilic nature of the samples. Moreover, to correlate with the water resistance properties of PVAc Homo CC and PVAc Homo CL with the PVAc Homo, water contact angle measurement was studied. Distilled water is made in contact with the emulsion samples casted over glass slides and contact angle values were noted. The contact angle of the sample with water is shown in Figure 2 and the values are labelled in Table 2. It is observed that the contact angle value is higher for PVAc Homo CL as compared to PVAc Homo and PVAc Homo CC. Clay has high acidic nature hence emulsion synthesis from them has quick film formation and faster drying when compared with calcium carbonate. Sight reduction in strength of bonded wood specimen in wet condition in PVAc Homo and PVAc Homo CC provides evidence for the same observation.

The bonding strength of adhesives was tested on the substrates in dry condition and another set of samples in wet conditions as per EN 204 for water resistance, represented in Figure 3. From Table 2, it was observed that by the addition of calcium carbonate in PVAc Homo CC, water resistance was slightly decreased as compared to PVAc Homo CL. After 24 hr immersion in water, PVAc Homo CC has lower strength as compared to PVAc Homo CL as shown in Figure 4. This is due to platelet like structure of clay particle which act as a barrier for water or moisture through adhesive which was validated in contact angle testing. With the addition of clay and calcium carbonate, dry strength increased in PVAc Homo CC and PVAc Homo CL as compared to PVAc Homo this was due to hardness introduced by calcium carbonate and clay which resulted in faster setting of bond on wood substrate. Handling strength was more due to the high hardness of PVAc Homo CL as shown in Figure 5.