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Apr 29, 2010

Revising Chemistry

POLYVINYL ALCOHOL (PVOH, PVA, or PVAL) is a water-soluble synthetic polymer. Polyvinyl alcohol has excellent film forming, emulsifying, and adhesive properties. It is also resistant to oil, grease andsolvent. It is odorless and nontoxic. It has high tensile strength and flexibility, as well as high oxygen and aroma barrier properties. However these properties are dependent on humidity, in other words, with higher humidity more water is absorbed. The water, which acts as a plasticiser, will then reduce its tensile strength, but increase its elongation and tear strength. PVA is fully degradable and is a quick dissolver. PVA has a melting point of 230°C and 180–190°C for the fully hydrolysed and partially hydrolysed grades, respectively. It decomposes rapidly above 200°C as it can undergo pyrolysisat high temperatures. PVA is an atactic material but exhibits crystallinity as the hydroxyl groups are small enough to fit into the lattice without disrupting it. ACETATE An acetate (IUPAC name: ethanoate) is either a salt or an ester of acetic acid. Its formula is written both as CH3CO2− and C2H3O2−. Chemists abbreviate acetate as OAc− and AcO−. Thus HOAc is the abbreviation for acetic acid, NaOAc for sodium acetate, and EtOAc for ethyl acetate.[1] Acetate is a common anion in biology. HYDROLYSIS Hydrolysis is a chemical reaction during which molecules of water (H2O) are split into hydrogen cations (H+) (conventionally referred to as protons) and hydroxideanions (OH−) in the process of a chemical mechanism.[1][2] It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization. Such polymer degradation is usually catalysed by either acid, e.g., concentrated sulfuric acid (H2SO4), or alkali, e.g., sodium hydroxide(NaOH) attack, often increasing with their strength or pH. Hydrolysis is a chemical process in which a certain molecule is split into two parts by the addition of a molecule of water. One fragment of the parent molecule gains a hydrogen ion (H+) from the additional water molecule. The other group collects the remaining hydroxyl group (OH−). The most common hydrolysis occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water autoionizes into negative hydroxyl ions and positive hydrogen ions. The salt breaks down into positive and negative ions. For example, sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with hydroxyl ions whereas acetate ions combine with hydrogen ions to produce neutral acetic acid, and the net result is a relative excess of hydroxyl ions, causing a basic solution. However, under normal conditions, only a few reactions between water and organic compounds occur. In general, strong acids or bases must be added in order to achieve hydrolysis where water has no effect. The acid or base is considered a catalyst. They are meant to speed up the reaction, but are recovered at the end of it.

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