How do you make hydroxypropyl starch?

Hydroxypropyl starch (HPS) is synthesized through the reaction of starch with propylene oxide and serves as a critical set-up agent in the construction sector alongside cellulose ethers. The integration of this agent significantly impacts the rheological properties of plaster, enhancing its workability. Specifically, the incorporation of the set-up agent boosts the stability of spray-applied plaster and aids in the efficient formation of slurries, which involves the seamless distribution of tiny particles throughout the plaster.

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The effectiveness of plaster adhesion is closely linked to how quickly the HPS dissolves, with optimal agents achieving significant thickening within roughly 10 seconds. Additionally, maintaining this thickening stability is essential, as the initial viscosity established needs to be preserved for approximately 15 minutes.

There are established methods for producing HPS, primarily utilizing a dry process where starch is reacted directly with catalytic amounts of a base and propylene oxide (refer to U.S. Patent Nos. 2,516,632, 2,516,633, 2,516,634, and 2,733,238). The achieved molar substitution (MS) degrees range from 0.05 to 1.5. A more refined method involves etherification in the presence of an inert solvent (U.S. Patent No. 2,845,417), such as various alcohols. This method typically yields MS values between 0.4 and 0.9 (U.S. Patent No. 3,652,539). However, one notable drawback is that separating the inert reaction medium post-etherification can be challenging, leading to subpar performance of the resulting HPS.

The innovative process described here introduces a technique to produce hydroxypropyl starch with maximum MS values of 1, where the starch undergoes etherification with minor water presence and significant amounts of propylene oxide.

The HPS produced through this approach is particularly well-suited for applications as a set-up agent for both gypsum and cement plaster formulations.

Surprisingly, this process is capable of yielding hydroxypropyl starch exhibiting lower MS values, ideally between 0.4 and 0.8. This contrasts with the known methods for creating hydroxypropyl cellulose, which tend to produce higher MS values of 3 to 5 (U.S. Patent No. 3,278,520). Hence, it was logical to expect similar elevated MS values when fabricating HPS, but this innovative process defies those anticipations.

Clarification of the term "MS-value":

In the molecular structure of cellulose or starch, each anhydroglucose unit is comprised of three OH groups. While the degree of substitution (DS) quantifies the average number of substituted OH groups per anhydroglucose unit, the MS indicates the average number of moles of reactants bonded to each unit. For hydroxyl derivatives, the MS is generally higher than the DS, as introducing a hydroxyalkyl group also generates an additional OH group, allowing for further hydroxyalkylation. This interaction can create lengthy side chains. The relationship of MS to DS reflects the average length of these chains, with an upper limit of 3 for DS but potentially much higher values for MS.

Typically, the etherification process is executed within a temperature range of 50°C to 75°C, preferably between 60°C and 70°C, while using 0.01 to 0.1 mole of sodium hydroxide per mole of starch, with water content maintained at 10% to 20% by weight (optimally at about 15% to 16%), alongside propylene oxide utilized at a 3 to 10 mole ratio (ideally around 4 to 5 moles) per starch mole. Upon combining these materials, one can either immediately reach the etherification temperature and maintain it for approximately four hours or initially alkalize for an hour at room temperature (25°C), then heat and etherify at the specified temperature for three hours.

Under the conditions described in this innovative etherification approach, only 10% to 15% of the excess propylene oxide is transformed into byproducts (mostly propylene glycols), which allows for the recovery of about 75% of the excess propylene oxide for reuse in subsequent batches.

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Neutralization of the sodium hydroxide can be accomplished using either a mineral or organic acid. The minimal alkali content results in negligible salt formation, and often the end product is suitable for use without any extensive purification. For plaster applications, particularly gypsum-based mixtures, the optimal addition of this hydroxypropyl starch ranges from 0.005% to 0.08%, with preferred concentrations between 0.02% and 0.05% in relation to the overall plaster composition.

Examples of Hydroxypropyl Starch Production

In Example 1, a total of 6,466 g of corn starch (with a moisture content of 9.8%) is placed in a 40-liter reactor, followed by adding 8,352 g of propylene oxide and 226 g of water. A uniform solution of 745 g of 9.66% sodium hydroxide is added while stirring. After one hour of alkalization at room temperature, the mixture is heated to 70°C for three hours under pressure of 2 to 2.5 bars. Following etherification, 97.4 g of 85% formic acid is introduced under pressure for alkali neutralization. The excess propylene oxide is removed, recovering 6,500 g, which will be reused with 1,852 g of fresh propylene oxide for future batches. The resultant product exhibits an MS of 0.76.

In Example 2, using the same amount of corn starch and propylene oxide as Example 1, after alkalization, the temperature is raised and maintained at 70°C for three hours. The neutralization and recovery stages mirror those in Example 1. The final MS recorded is 0.57.

For Comparison Example 3, 3,232.8 g of corn starch, 2,320 g of isopropanol, and 2,240 g of water are processed with 288 g of 50% sodium hydroxide after an hour of alkalization. Post alkalization, isopropanol and propylene oxide are introduced, followed by heating at 63°C for three hours. The neutralization process aligns with previous examples. The drying yields an MS of 0.43. In a practical test as a set-up agent, a mixture of 70 g of plaster of Paris, 26 g of lime sand, 3.7 g of calcium hydroxide, and 0.25 g of methyl cellulose is combined with 0.05 g of hydroxypropyl starch and 39 g of water. Initial observations determined that gypsum plaster mixes with HPS from Examples 1 and 2 thickened swiftly (within about 10 seconds) and maintained a high stability over fifteen minutes of stirring. In contrast, the mixture containing HPS from Comparison Example 3 was inadequate for use as a set-up agent due to insufficient thickening effect.