Preparation and Properties of Starch Modified Polyvinyl Formal Resin

Polyvinyl formal resin is widely used in construction, decoration, coatings, paper processing, and fiber processing due to its readily available raw materials, low price, and ease of use.1-3 However, it also suffers from issues such as high free formaldehyde content and poor water resistance.14-9 Therefore, its modification is essential. Furthermore, with the rising price of polyvinyl alcohol raw materials, reducing costs is also a top priority. This experiment proposes using inexpensive, renewable starch to partially replace polyvinyl alcohol in the reaction to reduce costs. Furthermore, the reaction of starch with formaldehyde in polyvinyl acetal resin reduces the amount of free formaldehyde in the system, minimizing environmental pollution. Furthermore, the reaction of starch with polyvinyl acetal resin forms a three-dimensional network structure, which improves the resin's water resistance. This study investigated the development of starch-modified polyvinyl formal resin using polyvinyl alcohol, starch, and formaldehyde as the main raw materials. This novel starch-modified polyvinyl formal resin offers advantages such as simple processing, short production cycle, low free formaldehyde content, excellent water resistance, and low cost.

1. Experiment
1.1 Materials and Instruments
Materials: Polyvinyl alcohol 1750 (PVA); formaldehyde; soluble starch; hydrochloric acid; sodium thiosulfate; sodium hydroxide.
Instrument: JJ-1 precision timed electric stirrer; HH series constant temperature water bath; infrared spectrometer.
12 Preparation of Starch-Modified PVA Formal Resin
Weigh a certain amount of starch into a 250 mL three-necked flask and heat to 60°C to completely dissolve the starch. Then, add 9.0 g of polyvinyl alcohol to the flask and raise the temperature to 90-95°C. Condensate and reflux until the polyvinyl alcohol is completely dissolved. After cooling the system to a certain temperature, adjust the pH with hydrochloric acid and aqueous ammonia. Then, add the measured amount of formaldehyde to the solution. Maintain the set temperature and allow the reaction to proceed for the specified time. Cool to approximately 50°C to adjust the pH to neutral. Discharge the solution to obtain a colorless, transparent, viscous liquid. 1.3 Determination of Viscosity
Measure the viscosity of the resin at 25°C using a 4-cup viscometer.
1.4 Determination of Water Resistance
Take a small amount of the finished product and apply it to a prepared paper sheet. Attach the paper sheet to a glass container. After it dries, place it in water. Record the time from the start of immersion in water to the time it falls off the glass container.
1.5 Determination of Formaldehyde Content
Determine the free formaldehyde in the resin according to the method in reference [6]. The free formaldehyde content was calculated according to the following formula:
r_(V₂-Y,) × 0.300N × 100%
Where F: free formaldehyde content, %; V₂: volume of 0.1 mol/L sodium hydroxide solution consumed by the blank, mL; V₁: volume of 0.1 mol/L sodium hydroxide solution consumed by the sample, mL; N: concentration of sodium hydroxide solution, 0.0978 mol/L; g: sample mass; 0.03004: mass of formaldehyde equivalent to 1 mL of 1 mol/L hydrochloric acid solution, g.
1.6 Infrared Spectroscopic Characterization
The infrared spectrum of the resin was measured using the KBr coating method.

2 Results and Discussion
2.1 Effect of Starch Dosage on Resin Properties
In this set of experiments, the pH value of the system was 2.0, the reaction temperature was 80°C, and the reaction time was 60 min. The starch dosage was varied, and the properties of the resulting resins are shown in Table 1.

Table 1 Effect of Starch Content on Resin Properties

With increasing starch content, the resin's viscosity and water resistance gradually increase. When the starch content is 4.0%, the product's viscosity reaches a maximum of 114.2s; at this point, the resin's water resistance reaches 257 min. This is because the active hydroxyl groups in starch can polymerize with the active groups (aldehyde, hydroxyl, and hydroxymethyl) in polyvinyl formal. As starch content increases, the reaction becomes more complete, forming a three-dimensional network structure. This increases the resin's viscosity and improves its water resistance. The free formaldehyde content in the resin decreased with increasing starch dosage. This is likely because, while starch is polymerizing with polyvinyl formal, it also chemically reacts with the free formaldehyde in the system, reducing the free formaldehyde content.
2.2 Effect of pH on Resin Properties
When preparing starch-modified polyvinyl formal resin, the pH of the system has a significant impact on the success of the experiment, as well as the resin structure and properties. When the pH is too low, the reaction is too rapid, resulting in gelation and failure. When the pH is too high, the reaction is too slow, causing some highly acetalized molecules to break down into less acetalized molecules, resulting in a low resin viscosity. In this experiment, a starch dosage of 3%, a reaction temperature of 80°C, and a reaction time of 60 minutes were used to vary the pH. The properties of the resulting resin are shown in Table 2.
The viscosity of the resin decreases with increasing pH. When the pH is too high, the viscosity decreases. When the pH is 1.5, the product has the highest viscosity. Because acid can catalyze the acetalization reaction, when the acidity in the system is low, the acetalization reaction is slow.
Table 2 Effect of pH on Resin Properties

The reaction is slow; however, if the acidity is too high, the reaction is too intense, which can easily lead to excessive acetalization in some areas and gel formation. Furthermore, as the pH value of the reaction system increases, the free formaldehyde content of the resin also increases. This is likely because as the pH value of the system increases, the polycondensation reaction between PVA and formaldehyde and the reaction between starch and formaldehyde slow down, resulting in an increase in the free formaldehyde content in the resin. In actual reactions, it is generally advisable to control the system pH between 1.5 and 2.0.
2.3 Effect of Reaction Time on Resin Properties
In this experiment, a starch dosage of 3%, a system pH of 2.0, and a reaction temperature of 80°C were selected. The reaction time was varied, and the properties of the resulting resins are shown in Table 3.
Table 3 Effect of Reaction Time on Resin Properties

With increasing reaction time, the viscosity of the resin gradually increases. This is because longer reaction times lead to more complete reactions, resulting in higher resin viscosity. Furthermore, the free formaldehyde content in the system gradually decreases with increasing reaction time. However, as the reaction time continues to increase, the free formaldehyde content increases. The content increases again. This is because the reaction between starch and formaldehyde is reversible. If the reaction time is too long, the reaction will proceed in the reverse direction, leading to an increase in the free formaldehyde content in the system.
2.4 Effect of Reaction Temperature on Resin Water Resistance
The starch dosage was 3%, the system pH was 2.0, and the reaction time was 60 minutes. The reaction temperature was varied. The experimental results are shown in Figure 1. With increasing reaction temperature, the resin's water resistance increases. The resin exhibits the best water resistance at 90°C. This is because as the reaction temperature rises, the reaction rate between the starch, PVA, and formaldehyde in the system accelerates, forming a network structure in the resin, thereby improving its water resistance.

2.5 Infrared Spectrum of Starch-Modified Polyvinyl Formal

A strong absorption peak at 3453.90 cm⁻ indicates the stretching vibration of the -OH group. This is due to the condensation reaction between the aldehyde group and the hydroxyl group of polyvinyl alcohol, which shifts the carbonyl peak in the polymer to 1633.42 cm⁻¹. The absorption peaks at 1384.40 cm⁻¹ and 1097.37 cm⁻¹ are characteristic of the reaction between starch and formaldehyde. Furthermore, the absorption peak at 617.70 cm⁻¹ is attributed to the methylene group.
3 Conclusion
Polyvinyl acetal resin can be modified with starch by changing the synthesis process conditions. This method has the advantages of simple process conditions and a short production cycle. In addition, the obtained resin has no irritating odor, is beneficial to environmental protection, and meets the current requirements for the development of synthetic resins towards environmental friendliness. The following conclusions were drawn through experimental research: (1) With the increase in starch dosage, the viscosity and water resistance of the resin were greatly improved. In addition, the free formaldehyde content in the resin was significantly reduced. (2) When starch was used to modify PVA formalin resin, the pH value of the reaction system had a great influence and should generally be controlled at around 1.5~2.0. (3) With the increase in reaction temperature, the water resistance of the resin also increased. The reaction temperature is preferably 90℃ and the reaction time is about 80min. References:
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