What is PVC?

 

Polyvinyl chloride (PVC) is one of the most commonly used polymers and can be used in a number of applications, such as pipes, packaging and coated textiles. PVC is well known for its low cost and versatility. Pure PVC has a softening temperature of 80°C and a low thermal stability. By adding a plasticiser, the softening temperature can be lowered far below room temperature, giving the PVC a flexible behaviour, thus making it suitable for coated textiles .

The PVC polymer consists of repeated vinyl chloride monomers. The vinyl chloride monomer contains carbon chains with hydrogen and chloride. These units are linked together through polymerization. By adding heat and pressure, to the vinyl chloride monomers, they join together in the form of a very large molecule, a process called polymerization. The number of repeating units in the polymer chain may range from 800 to 1600. The configuration of the

chain could be either linear or three- dimensional networks, which also determines the plastic product properties.

There are two types of polymerization
reactions, condensation and addition. In condensation reactions two or more monomers react, with each other in a stepwise process. At each step of the reaction one small molecule is lost. Addition polymerization is a chain reaction where the monomer units are added to the chain one at a time.

Normally PVC is a rigid material because of the strong interactions between the polymer chains. These strong networks contribute to its characteristic properties, such as the high tensile strength. When PVC is heated these strong bonds are weakened and as a result the PVC gets more soft and workable. By adding plasticisers this property can be achieved at lower temperatures. When it is cooled these bonds are reformed and it returns to its originally stiffness. PVC most often appears in the form of a dispersed resin, which can easily be applied on the fabric trough coating. The dispersion resin consists of fine particles that have a size about one micron.

Polymerization of vinyl chloride is accomplished through free radical process, 93-98 % of PVC consists of amorphous areas and therefore belongs to amorphous polymers. The crystallites work as physical cross linkers, which prevents deformation of the material, during pressure, above Tg (glass transition temperature).This is a very valuable property and is why softened PVC can be used on textiles and the reasons to why it stays on the surface without penetrating the textile.

Tg is the temperature where an amorphous polymer transforms from being stiff to be more flexible and softer. For polymers that are partially crystalline it is only the amorphous areas that are affected by Tg, because it is only the amorphous areas that transforms. The polymer chains do not move, in relation to each other, while the temperature is below Tg. The warming and cooling rate of PVC are critical for Tg, a slow temperature change decreases Tg, this contributes to change in properties.For PVC without plasticisers Tg is 75-80°C. If plasticisers is added the Tg can be decreased below room temperature. One way to calculate the glass transition temperature of a polymer-plasticiser compound is through the following equation:

Tg= Tg2-kw1 t (Eq.1) Where:

Tg

Tg2 k

w1

Tg;glass transition temperature of polymer-Tg2:plasticiser(s) mixture glass transition K:temperature of unplasticised polymer plasticiser efficiency parameter

W1:weight fraction of plasticiser

The largest consumer of PVC is the building and construction industry, where half of the PVC within this area are used to make pipes and conduits. Applications, made with PVC, for the automobile area are instrument and door panels, upholstery, armrests, side body moldings and floor mats.

A plastic material can age according to either physical or chemical effects. The physical effect depends on a molecular structure change in the material. This can for example mean shrinking in volume and the material can get stiffer. Both physical and chemical ageing appear faster at high temperatures.