When buying PVC resin, the first number people ask about is the K-value. It is often treated as just a grade code, but the K-value largely decides how the resin runs on the machine and how the finished product performs.
What K-value is
K-value measures the solution viscosity of the resin, which reflects the average length of the PVC molecular chains. A high K-value means longer chains; a low K-value means shorter ones. Resin TDS sheets usually also list the average degree of polymerization, which describes the same thing on a different scale.
How it affects processing and the product
- Higher K-value: better tensile strength, elongation and heat aging, but melt viscosity rises, so processing needs slightly higher temperatures.
- Lower K-value: the material flows more easily and fuses faster at lower temperatures, which suits thin parts and clear products.
- For soft products, porosity matters just as much: the more porous the resin, the faster it absorbs plasticizer during dry blending.
Common grades by application
- K57: injection fittings, clear rigid and semi-rigid film, bottles (for example Vinora V-57).
- Rigid K65: pipe, conduit, profiles, ceiling panels, roofing and gutters, with high bulk density for extrusion output (for example Vinora V-65R).
- Soft K65: cable, hose, flexible film and footwear, with high porosity (for example Vinora V-65S).
- K70: high-performance cable, industrial hose, flooring and synthetic leather (for example Vinora V-70).
Beyond K-value, check these too
Bulk density affects extruder feeding and output. High volatile matter can cause voids and uneven surfaces. For film and hose, fish-eye level decides how smooth the surface is. All of these are listed in the TDS and in each batch's Certificate of Analysis.
If you change resin, do not swap it without a trial. A change in K-value or porosity usually needs matching adjustments to the stabilizer, lubricants and process temperatures. Our technical team can help plan the trial so the switch goes smoothly.