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Characteristics of polyvinyl alcohol
Polyvinyl alcohol (PVA) is a synthetic polymer that can only be processed within a certain temperature range due to its thermoplastic properties. As well as being highly flexible, the material is characterized by excellent mechanical properties, optimal film-forming capability and a high resistance to solvents.
Whether it is used as a coating, binder, surface finishing or material in paper or packaging production – polyvinyl alcohol has become an integral part of the industrial sector.
PVA is a thermoplastic that starts off as a white to yellowy powder. This powder is odorless and non-toxic but may undergo thermal pyrolysis at excessively high temperatures.
Polyvinyl alcohols are soluble polymers synthesized through alcoholysis of polyvinyl acetate. The process can result in different kinds of PVA that differ mainly in terms of their molecular weight and amount of residual acetyl groups.
Besides high adhesive strength and bonding capacity, PVA also has excellent film-forming, emulsifying and adhesive properties as well as resistance to other substances, such as grease, oils and solvents. All these characteristics make PVA a versatile polymer with many applications.
Polyvinyl alcohols (PVA or PVOH) are used in a wide range of applications, e.g. as binders in paints and coatings systems, in protective and strippable paints , for heat-sealable paper coatings, wood primers, and the modification of dispersion adhesives.
PVA is used to make aqueous feed solutions for adhesives. Compared with natural products, such as casein and its by-products (dextrin), polyvinyl alcohol has high adhesive strength even at small quantities thanks to its uniform chemical structure.
Besides being a raw material for making adhesives, PVA is also used in the paper industry as a rewettable water-activated adhesive on envelopes and labels. This type of adhesive is made using PVA solutions (with a concentration of around 30 percent) that contain added preservatives and defoamers.
Alcohol or acrylic dispersions are added to accelerate the drying process. However, drying temperatures must be kept under 130°C (266°F) to ensure optimal results. PVA is also used in many wet bonding applications in the packaging industry.
Aqueous PVA solutions can be added to polymer dispersions to modify the properties of dispersion adhesives. The addition of PVA increases an adhesive’s open time and setting time. This is very useful particularly when the adhesive is applied by machine.
The type of polyvinyl alcohol used is determined by the required viscosity of the adhesive. Furthermore, the addition of PVA prevents skin formation in dispersion adhesives, which can negatively affect the operation of applicator wheels or rollers.
PVA still has many other applications. For example, it is used as a food additive, a carbon dioxide barrier in PET bottles, or a coating agent for pharmaceutical tablets.
One special property of PVA: The polymer’s water solubility can be modified during the conversion of its precursor polyvinyl acetate (PVAc), with solubility decreasing as the degree of hydrolysis increases. Depending on the type of alkaline hydrolysis, the compounds can dissolve in warm or cold water and subsequently be broken down into water and carbon dioxide by microorganisms.
The industrial sector makes use of this reaction to manufacture water-soluble packaging, such as soluble films used to wrap dishwasher tablets, chlorine tablets for water purification, or laundry tablets for industrial washing machines
PVA solutions are produced by stirring the polymer in cold water and then heating the mixture up to 90-95°C (194-203°F). Heating can also be done using hot steam. The solution is then continuously stirred and left to cool in a corrosion-resistant vessel to prevent skin formation.
Partially hydrolyzed PVA dissolves more quickly than its fully hydrolyzed counterpart. Defoamers are added to prevent foam formation during stirring or transportation. Heating and stirring also help the solution maintain its viscosity. The viscosity may rise during prolonged periods of storage, thus causing the dissolution rate to fall.