PVC Resin Grades Explained: SG-5 vs SG-3 vs SG-7 vs SG-8
What the K-value and degree of polymerization actually tell you, with the GB/T 5761-2018 grade chart and the end product each grade is bought for.
Most PVC enquiries that go wrong go wrong at the grade, not at the price. SG-5 and SG-3 are both described as "suspension PVC resin", the bags look identical, and the certificate numbers are in the same format. Then the compound behaves differently on the line, and three weeks later somebody is arguing about whose fault the reject rate is.
This is the chart and the reasoning behind it.
Quick answer: the K-value is the single number that decides how the resin behaves. SG-5 (K 66–68) is the general-purpose workhorse for pipe, fittings, profiles and rigid sheet. SG-3 (K 71–72) is for flexible products — films, leather, cable, footwear. SG-7 (K 60–62) and SG-8 (K 55–59) are the lower-viscosity end, bought for rigid sheet, transparent sheet and injection-moulded parts.
One number decides almost everything
The K-value is the industry's shorthand for molecular weight. It is measured in cyclohexanone at 25 °C, and it moves inversely to temperature in the reactor: run the reactor hotter and you get a shorter chain, a lower K-value, and a resin that flows more easily.
Once you know that, the rest follows mechanically:
- Higher K-value → longer chains → higher melt viscosity, higher fusion torque, slower fusion, and higher finished strength, stiffness and heat-distortion resistance.
- Lower K-value → shorter chains → better flow, easier processing, lower energy per tonne, but lower mechanical performance.
Neither direction is "better". A resin that is too high in K-value for your line will not fuse properly and you will see it as poor weld strength or surface defects. A resin that is too low will run beautifully and fail a pressure test.
The full GB/T 5761-2018 grade chart
Chinese suspension PVC is graded SG-1 to SG-8. Each grade is a bucket defined by viscosity number, with the K-value and average degree of polymerization as equivalent expressions of the same thing.
| Grade | Viscosity number (mL/g) | K-value | Average degree of polymerization |
|---|---|---|---|
| SG-1 | 156–144 | 77–75 | 1785–1536 |
| SG-2 | 143–136 | 74–73 | 1535–1371 |
| SG-3 | 135–127 | 72–71 | 1350–1250 |
| SG-4 | 126–119 | 70–69 | 1250–1150 |
| SG-5 | 118–107 | 68–66 | 1135–981 |
| SG-6 | 106–96 | 65–63 | 980–846 |
| SG-7 | 95–87 | 62–60 | 845–741 |
| SG-8 | 86–73 | 59–55 | 740–650 |
Two things about this table are worth internalising.
First, the ranges are wide. SG-5 spans a viscosity number of 107 to 118. A resin at the bottom of that band and a resin at the top are not the same material in your extruder. This is why "we buy SG-5" is not a complete specification — say where in the band you want to sit, and hold the supplier to it.
Second, the grade number is not a quality ranking. SG-8 is not "worse" than SG-3. It is a different molecular weight aimed at a different product.
Grade by grade: what each one is bought for
SG-5 — K 66–68, DP roughly 980–1135
The volume grade, and by a wide margin the one most buyers mean when they say "PVC resin". The molecular weight sits at a comfortable balance: enough strength for a pressure pipe, enough flow for high-speed extrusion.
Bought for: water supply and drainage pipe, electrical conduit, irrigation pipe, uPVC window and door profiles, pipe fittings, rigid sheet and board.
If you are adding impact modifier or processing aid and running a standard pipe or profile line, this is almost certainly your grade. It is also the most liquid grade in the market, which matters more than people admit — availability and consistent repeat supply are worth more than a few dollars a tonne.
SG-3 — K 71–72, DP roughly 1250–1350
The highest molecular weight of the four grades most commonly traded. Long chains mean high plasticizer absorption, which is exactly what flexible formulations need.
Bought for: flexible and semi-rigid products, artificial leather, films, cable insulation and sheathing, footwear, hose, and foamed material.
If your formulation is built around a plasticizer, SG-3 gives the resin more capacity to take it without sweating out. That is the whole point of buying it.
SG-7 — K 60–62, DP roughly 741–845
Lower molecular weight, better flow, higher stiffness in the finished part.
Bought for: rigid sheet and panel, transparent sheet, calendered products, and injection moulding where wall sections are thicker or flow paths are long.
Transparent sheet is the classic application — the shorter chain helps clarity and surface finish.
SG-8 — K 55–59, DP roughly 650–740
The lowest viscosity of the standard ladder. Made for machines that need flow more than they need strength.
Bought for: injection-moulded fittings and components, sheet and board, foamed board.
The rest of the ladder
SG-1, SG-2, SG-4 and SG-6 exist and are traded, but they sit between the main four. SG-4 and SG-6 in particular are worth knowing as substitution candidates: if SG-5 is short in the market, SG-4 (K 69–70) or SG-6 (K 63–65) may be a workable bridge for some formulations. Workable is not the same as equivalent — re-check your fusion and impact before you commit a full production run.
The other numbers on the COA worth checking
The K-value gets all the attention. These are the ones that quietly decide whether the batch runs like the last one.
| Item | SG-3 | SG-5 | SG-7 | SG-8 |
|---|---|---|---|---|
| Apparent density (g/mL, min) | 0.45 | 0.48 | 0.52 | 0.52 |
| Plasticizer absorption (g/100 g, min) | 26 | 19 | 12 | 12 |
| Volatile content (%, max) | 0.30 | 0.40 | 0.40 | 0.40 |
| Sieve residue, 0.25 mm (%, max) | 1.6 | 1.6 | 1.6 | 1.6 |
| Sieve pass, 0.063 mm (%, min) | 97 | 97 | 97 | 97 |
| Residual VCM (ppm, max) | 5 | 5 | 5 | 5 |
Apparent density drives how much material your feeder pushes per revolution. A batch that arrives at the low end means your output per hour quietly drops, even though the bag says the same grade.
Plasticizer absorption is the one to watch if you buy SG-5 for a flexible or heavily filled formulation. SG-5 at 19 g/100 g against SG-3 at 26 g/100 g is a real, measurable difference in how much plasticizer the resin will hold.
Volatile content matters for pre-mixing and for any formulation where you weigh accurately. Moisture also shows up as surface defects.
Sieve data and fish eyes are your early warning on the batch, not the grade.
Full grade data and packing options are on the specifications section. See also the product and grade overview.
Four mistakes that keep showing up
1. Buying SG-5 for everything. It is the cheapest thing to source and the easiest to quote, so it gets used for products that genuinely want SG-3 or SG-7. The saving on the resin shows up later as plasticizer migration, poor clarity or weld-line failures.
2. Ignoring plasticizer absorption when adding filler. Cost-down work pushes filler content up. Filler competes with plasticizer for the same space in the resin. If you are raising calcium carbonate and holding SG-5, you are pulling from a smaller margin than you think.
3. Chasing the bottom of the grade band. A supplier offering "SG-5" at the very bottom of the viscosity range is selling you a different processing window. Sometimes that is fine. Sometimes it is the reason your line runs slower.
4. Not putting the number on the purchase order. "SG-5" plus "K-value 66–68" plus the viscosity number, written into the PO and matched against the COA on arrival, is the difference between a specification and a hope.
How to specify it properly
On the PO, and in the enquiry before it, state:
- The grade (SG-5) and the K-value range you will accept
- Carbide-based or ethylene-based, if your formulation is sensitive to it
- The end product and processing method — extrusion, injection, calendering
- Packing format
- Destination and Incoterm
- Payment terms
If the route matters to you — and for transparent, cable and potable-water formulations it usually does — read Carbide-Based vs Ethylene-Based PVC Resin next. It explains why the same grade number can behave differently depending on how the VCM was made.
Still deciding? Send us the end product and the processing method and we will tell you which grade and which COA items to insist on. Get in touch — first reply usually within 12 hours, GMT+8.
Need SG-5 to land before your next production run?
Tell us the grade, the tonnage and the destination port. We come back with a price, a specification sheet and the full document list — usually within 12 hours, GMT+8.