Chemate Group / PVC Resin Technical Guide
What Is K Value of PVC Resin
When buying PVC resin, manufacturers often specify a K value before discussing price, packaging or delivery. Requests such as “PVC K67 for pipes” or “K70 resin for flexible compounds” are common because this parameter helps narrow the selection of suitable materials.
However, K value does not describe every aspect of PVC resin. Two grades with the same nominal K value can behave differently during mixing, fusion and production.
What Is K Value of PVC Resin
The K value of PVC resin is a dimensionless parameter calculated from dilute-solution viscosity measurements. It indicates the resin’s relative molecular weight and helps manufacturers assess its likely processing behavior.
For comparable PVC resins tested under the same conditions, a higher K value generally indicates a higher molecular weight. A lower K value generally indicates a lower molecular weight.
The relationship is useful for comparing materials, but K value is not a direct molecular-weight measurement. ISO 1628-2 describes the determination of PVC resin K value and explains that comparisons depend on consistent test conditions and polymer composition.
In practical purchasing terms, K value helps answer an initial question: Does this resin belong to the molecular-weight range normally considered for our product and processing method?
It does not answer whether the resin will perform successfully in a specific formulation without further evaluation.
Why Does K Value Matter in PVC Processing
PVC manufacturers must balance material flow, fusion behavior and finished-product performance. Molecular weight influences that balance.
Under otherwise comparable conditions, a lower-K resin generally flows more easily during processing. A higher-K resin generally has greater melt viscosity and can offer advantages where molecular-chain entanglement and melt strength matter.
These differences affect how a compound behaves inside an extruder, injection molding machine or calendering line.
However, production behavior also depends on:
- Resin particle structure and porosity.
- Plasticizer type and concentration.
- Stabilizers, lubricants and processing aids.
- Filler loading.
- Equipment design and operating conditions.
Common PVC Resin K Values and Their Applications
| Nominal K value | General selection direction | Applications commonly considered |
|---|---|---|
| K57–K60 | Lower molecular weight and easier flow | Injection-molded fittings and selected rigid products |
| K65 | Intermediate balance of flow and performance | Selected pipes, profiles and rigid sheets |
| K67 | Widely used range for rigid extrusion | Pipes, profiles and sheets |
| K68 | Close to K67, with somewhat higher molecular weight under comparable testing | Selected rigid extrusion grades |
| K70–K71 | Higher molecular weight | Flexible profiles, tubing, cable compounds and films |
| K75 and above | Higher molecular-weight specialty grades | Selected demanding flexible applications |
Note: These are application directions, not universal specifications.
PVC K65 vs K67 vs K68 vs K70
PVC Resin K65
K65 is often considered when manufacturers need a balance between manageable processing viscosity and suitable finished-product performance.
Compared with a higher-K resin in an otherwise comparable formulation, K65 may offer easier flow. Whether that improves production depends on the equipment, formulation and product requirements.
PVC Resin K67
K67 is a common starting point for rigid PVC extrusion, including many pipe, profile and sheet applications.
Its popularity makes it a useful reference grade when discussing PVC supply. Nevertheless, “K67” is an incomplete purchase specification. Buyers should also identify the producer, exact grade, acceptance range and other resin characteristics.
PVC Resin K68
K68 is close to K67 in nominal K value, but the two labels do not establish automatic interchangeability.
Before switching between them, compare the specification ranges. A grade sold as K67 and another sold as K68 may have overlapping acceptance limits, or they may represent meaningfully different products.
Particle morphology and additive interactions can also create processing differences that are not apparent from K value alone.
PVC Resin K70
K70 is commonly considered for plasticized PVC applications such as flexible tubing, cable compounds and films.
Its higher molecular weight can be useful where the formulation requires stronger molecular-chain entanglement. However, K70 resin is not inherently “soft PVC.” Flexibility depends substantially on plasticizer content and the complete compound formulation.
Is a Higher PVC Resin K Value Always Better
No. The best K value is the one that fits the product, formulation and processing equipment.
A higher-K resin may provide useful mechanical or melt-strength characteristics, but it can also increase resistance to flow. This may require adjustments to formulation or processing conditions.
A lower-K resin may improve filling or flow in a suitable application, but it may not provide the performance required by a product originally designed around a higher-molecular-weight resin.
How Is PVC Resin K Value Measured
PVC K value is determined through laboratory testing of a dilute polymer solution.
In general, the procedure involves preparing a resin solution under specified conditions, measuring its viscosity using a capillary viscometer and calculating K value from the measured solution behavior.
This is a solution-viscosity test, not a direct measurement of molten PVC flow inside an extruder.
ISO 1628-2:2020 specifies conditions for determining PVC resin viscosity number and K value. Its published scope also states that solution concentration affects comparability. Buyers should therefore compare results obtained using consistent methods and conditions.
When reviewing a technical data sheet or certificate of analysis, check:
| Item | What to confirm |
|---|---|
| Test method | The standard and method used |
| Reported value | Whether it is nominal, typical or an acceptance limit |
| Specification range | The permitted variation for the grade |
| Batch result | Whether a measured result is available for the supplied batch |
| Material identity | The producer and exact resin grade |
A typical value describes a product. An acceptance range defines what the supplier agrees to deliver.
Is K Value the Same as Degree of Polymerization
K value and degree of polymerization are related, but they are different parameters.
Degree of polymerization describes average polymer-chain length in terms of repeating units. K value is derived from solution-viscosity behavior and serves as an indirect indicator of molecular weight.
Consequently, a K value should not be treated as a universally exact conversion into degree of polymerization.
If two suppliers report different parameters, ask for the relevant test methods and supporting technical documentation before concluding that the materials are equivalent.
PVC Resin K Value and SG Grade Comparison
PVC resin is often identified by SG grades in Chinese specifications and by K values in international trade. The table below provides a commonly used reference for comparing these designations.
SG grades correspond to approximate K-value ranges rather than single fixed values. The actual specification should always be confirmed against the producer’s technical data sheet.
| PVC resin grade | Common reference K-value range |
|---|---|
| SG1 | 75–77 |
| SG2 | 73–74 |
| SG3 | 71–72 |
| SG4 | 69–70 |
| SG5 | 66–68 |
| SG6 | 63–65 |
| SG7 | 60–62 |
| SG8 | 55–59 |
These ranges are commonly published in commercial SG-series specifications. However, manufacturer-specific classifications can differ.
Is PVC SG5 Equivalent to K67
SG5 is commonly associated with the K66–68 range, making K67 a useful nominal reference. However, the two descriptions are not complete technical equivalents. An SG5 specification for Zhongtai PVC, for example, lists a K-value range of 66–68.
When evaluating an SG5 resin against a K67 requirement, buyers should also compare particle-size distribution, bulk density, plasticizer absorption, volatile matter and other application-specific requirements. Matching the K-value range is an initial screening step. Final approval should depend on the complete specification and production performance.
Why Can Two PVC Resins with the Same K Value Process Differently
K value describes one important characteristic, but it does not capture the complete resin structure or quality profile.
Particle Size and Distribution
Differences in particle-size distribution can affect powder handling, mixing and compound consistency.
Porosity and Plasticizer Absorption
For flexible PVC, resin porosity and plasticizer absorption characteristics influence dry blending and the preparation of a consistent compound.
Bulk Density
Bulk density affects how much resin occupies a given volume. Differences can influence storage, conveying and volumetric feeding.
Fusion Behavior
Resins with the same nominal K value may respond differently to a particular lubricant, stabilizer and processing-aid package.
Impurities and Appearance
Visible contaminants, gels or other defects may affect finished-product appearance even when K value meets specification.
For buyers, the practical implication is clear: matching K value is an initial screening step, followed by a full specification comparison and production trial.
How to Choose the Right K Value for Your Application
Define the Product Requirements
Identify whether the product is rigid or flexible and specify the properties that matter. These may include impact resistance, tensile performance, appearance, dimensional stability or compliance with a product standard.
Identify the Processing Method
Injection molding, extrusion and calendering place different demands on material flow and processing behavior.
The resin must suit the production process as well as the finished product.
Use the Existing Approved Grade as a Benchmark
If your production line already runs successfully, record the producer, grade, K-value range and other relevant resin specifications.
An approved material provides a more useful benchmark than a generic application recommendation.
Compare the Full Formulation
A formulation containing substantial plasticizer or filler may respond differently to a resin change than an unfilled rigid compound.
Review stabilizers, lubricants and processing aids before expecting a substitute grade to run under identical conditions.
Conduct a Controlled Trial
Keep the trial conditions documented and compare measurable results.
Useful observations include:
- Mixing and feeding consistency.
- Extruder torque or motor load.
- Melt pressure and output.
- Surface appearance.
- Dimensions and mechanical test results.
- Scrap rate and production stability.
The final approval should depend on repeatable production performance and finished-product compliance.
FAQs About PVC Resin K Value
What does K67 mean in PVC resin?
K67 identifies a PVC resin with a nominal K value around 67. It indicates a viscosity-related molecular-weight range rather than a complete product specification. Buyers should confirm the producer’s acceptance range, test method and application guidance before approving a particular grade.
Which K value is suitable for PVC pipes?
K67 is a common starting point for many rigid PVC pipe applications, but suitability depends on the exact grade, formulation, equipment and pipe specification. Pressure pipes require particular attention to applicable performance requirements. A nominal K value alone does not establish compliance.
Can K65 replace K67 PVC resin?
It may be possible in a particular application, but it should not be assumed. The change can affect flow, processing behavior and product properties. Compare full specifications and conduct a controlled production trial before making a routine substitution.
What is the difference between K67 and K68?
They are close nominal values, with K68 generally indicating slightly higher molecular weight under comparable testing. However, the commercial specification ranges may overlap. Buyers should compare actual grade specifications and batch data before deciding whether the difference matters for production.
Is K70 PVC always flexible?
No. K70 describes a resin characteristic, while flexibility depends heavily on the compound formulation, particularly plasticizer type and loading. K70 resins are often considered for flexible applications, but K value itself does not define finished-product hardness.
Does a higher K value mean better PVC quality?
No. A higher K value indicates a different molecular-weight range, not a higher quality ranking. Quality means consistently meeting the agreed specification and producing the required finished product. A lower-K resin may be the correct choice for a process that needs easier flow.
Can K value predict PVC thermal stability?
K value alone cannot establish thermal stability. Resin characteristics, stabilizers, other additives, processing temperature and residence time all influence degradation behavior. Thermal stability should be evaluated separately using relevant resin or compound tests.
Discussing PVC Resin Requirements with Chemate Group
For manufacturers sourcing PVC resin from China, an effective inquiry combines the K-value requirement with application and production details.
When contacting Chemate Group, share your current resin grade, intended application, required quantity and destination port. If you are considering an alternative material, include the existing technical data sheet and any production issues you want to address.