What is difference between PVC and PVDC

The most basic material for the forming web is polyvinyl chloride (PVC). The principal advantages of PVC are the low cost and the ease of thermoforming. The main disadvantages are the poor barrier against moisture ingress and oxygen ingress. In the case of blister packaging the PVC sheet does not contain any plasticizer and is sometimes referred to as Rigid PVC or RPVC. In the absence of plasticizers, PVC blisters offer structural rigidity and physical protection for the pharmaceutical dosage form. On the other hand, the blister cavity must remain accessible by the push-through effect and the formed web may not be too hard to collapse when pressed upon; for this reason the PVC sheet thickness is typically chosen between 200μ to 300μ depending on the cavity size and shape. Most PVC sheets for pharmaceutical blisters are 250μ or 0.250 mm in thickness. Typical values for the Water Vapor Transmission Rate (WVTR or MVTR) of a 250μ PVC film are around 3.0 g/m2/day measured at 38 °C/90% RH and the Oxygen Transmission Rate (OTR) is around 20 mL/m2/day. In order to overcome the lack of barrier properties of PVC film, it can be coated with PVDC or laminated to PCTFE or COC to increase the protective properties. Multi-layer blister films based on PVC are often used for pharmaceutical blister packaging, whereby the PVC serves as the thermoformable backbone of the structure. Also, the PVC layer can be colored with pigments and/or UV filters. The European Pharmacopoeia (Ph Eur) references the requirements for PVC blister packs for pharmaceutical primary packaging in the monograph EP 3.1.11 “MATERIALS BASED ON NON-PLASTICISED POLY(VINYL CHLORIDE) FOR CONTAINERS FOR DRY DOSAGE FORMS FOR ORAL ADMINISTRATION”. In order to be suitable for pharmaceutical blister packs, the PVC formulation also needs to comply with the US Pharmacopoeia <661>; EU food legislation; US 21.CFR and Japanese food contact requirements.
Polyvinylidene chloride (PVDC) can be coated onto a PVC film to obtain very high moisture and oxygen barrier properties depending on the coating weight. PVDC coated blister films are the most common and prevailing barrier films used for pharmaceutical blister packs. PVDC coatings are also the most economical method for adding water barrier and oxygen barrier properties to a PVC film. PVDC blister films are available in 2 or 3 layer specifications referred to as duplex or triplex. Since the PVDC is applied by a coating process, the coating weight is expressed in grams per square meter (gsm). Duplex structures are typically PVC/PVDC films, ranging from 250μPVC/40gsmPVDC to 250μPVC/120gsmPVDC with WVTR from 0,65 to 0,25 g/m2/d and OTR from 1 to 0,1 cc/m2/d. For very deep draw thermoformed cavities, the triplex specifications are used : PVC/PE/PVDC, where the PE layer assists when forming deeper cavities. The PE (polyethylene) forms a soft intermediate layer between the rigid PVC and PVDC layers. Triplex specifications exists in similar coating weights as duplex specifications: 250μPVC/25μPE/40gsmPVDC up to 250μPVC/25μPE/120gsmPVDC. In order to obtain high barrier properties, PVDC is always applied using an emulsion coating process using a PVDC resin dispersed in water. The film producer applies the coating in several steps, drying-off the water between each coating station.
PVDC grades are available in 2 types of polymer: (I) the historic grades offering medium to high barrier properties and (II) a super barrier coating grade offering the highest barrier. The super barrier coating grade has over two times the barrier to moisture and oxygen per gram coating weight compared to the historic grades. The most common structures using the super barrier PVDC are triplex configurations 250μ PVC/25μ PE/120gsm PVDC up to 250μ PVC/25μ PE/180gsm PVDC, with WVTR of 0,11 down to 0,06 g/m2/day and available from various suppliers.
*The above information is provided by Wikipedia