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), Microcrystalline cellulose (MCC, pH 6.4, Bulk density (untapped 0.3 g/cm³, CAS No. 9004-34-6) were acquired from Sigma Aldrich.

Polymer emulsions were synthesized in a flat-bottom glass reactor equipped with 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.

PVA and MCC 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 used as buffer. The 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 the temperature was maintained at 90˚C for 1 hr. Then, the emulsion was cooled down to 25˚C and added Isothiazolinones preservative.

PH PVA + MCC solution was used as a protective colloid for stabilization of the emulsion. Sodium lauryl sulphate was used as an anionic surfactant. Sodium bicarbonate 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.

The 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,

Solid   content ( % ) = ( 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. The 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.

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 cast 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.

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

The viscosity and pH of the prepared samples are shown in Table 2. The presence of MCC in PVAc Homo (MCC-01 and MCC-02) sample enhances the pH and makes the system less acidic in nature as compared to PVAc Homo. Compared to the PVAc Homo, increase in viscosity was observed in with increase in the concentration of MCC in PVAc Homo. This is due to the grafting of VAc monomer on hydroxyl group of cellulose and resulted in an increased in viscosity as compared to PVAc Homo.

The pencil hardness of emulsion polymer film evaluates the mechanical strength property. With the addition of MCC in PVAc Homo (MCC-01 and MCC-02), hardness of the film was increased as compared to PVAc Homo as shown in Table 2. Due to grafting of VAc on MCC resulted in a dense structure. Slip from sharp pencil due to needle like structure increased, hence the MCC-01 and MCC-02 samples showed higher values of pencil hardness.

In general water contact angle correlates with the water resistance properties of the samples. Moreover, to correlate with the water resistance properties of MCC-01

and MCC-02 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 1 and the values are labelled in Table 2. It is observed that the contact angle value is higher for MCC-01 and MCC-02 as compared to PVAc Homo. Sight reduction in strength of bonded wood specimen in wet condition in PVAc Homoprovides evidence for the same observation (Reason for water resistance).

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. From Table 2, it was observed that by the addition of MCC in PVAc Homo CC, water resistance was increased. After 24 hr immersion in water, PVAc Homohas lower strength as compared to MCC-01 and MCC-02 as shown in Figure 2. This is due MCC fibers which act as a barrier for water or moisture through adhesive which was validated in contact angle testing. In addition to MCC, dry strength increased in MCC-01 and MCC-02 as compared to PVAc Homo this was due to cellulosic hydroxyl having a high affinity towards wood cellulose due to secondary force which resulted in a strong bond on wood substrate. Dry strength was shown in Figure 3.