PVC Resin SG-8 (K 55-59): Specification and Applications

SG-8 is the lowest molecular weight grade we supply, and the one that flows furthest. Where a thin wall, a long flow path or a surface that has to come out clean is the limiting factor, this is the grade. Transparent sheet, foam board, pipe fittings, shrink film, thin-wall injection.

We ship SG-8 as a first-tier distributor out of Shandong, China. We do not polymerise it. The batch certificate is issued by the producing plant, and we obtain it for you.

Quick answer: SG-8 is defined by GB/T 5761-2018 as a viscosity number band of 73–86 mL/g, which is the same thing as a K value of 55–59 and an average degree of polymerisation of 650–740. It has the highest flow and the highest apparent density requirement of the four grades we supply, and the lowest plasticiser absorption requirement. It is bought for transparent sheet, foam board, pipe fittings, shrink film and thin-wall injection.

1. What SG-8 actually is

SG-8 is the bottom of the range we hold. The band in three expressions:

• Viscosity number: the standard defines SG-8 as 73–86 mL/g.
• K value: 55–59. Buyers outside China write K57, K58 or K60 loosely; the band is what matters.
• Average degree of polymerisation: 650–740.

A word about "high strength". SG-8 is sometimes sold as a grade for "high-strength injection mouldings". That phrasing is misleading, and it is worth understanding why before you buy on it. Mechanical strength in a finished PVC part comes from molecular weight, and SG-8 has the least of it of any grade in this range. What SG-8 gives you is flow: it fills a long, thin, intricate cavity that a higher-K grade would short-shot on, and it does so at lower melt temperature, which reduces degradation and keeps the part's colour clean. If a part is described as high-strength, ask what the strength is actually coming from. Usually it is the formulation, the wall design, or a filler and reinforcement package, not the resin grade. Buy SG-8 for flow and surface, and specify the strength you need on the drawing.

Why transparency lines use it. Low molecular weight means fewer and smaller gel particles and better melt homogeneity, so light passes through more cleanly. Combined with the highest apparent density requirement in the range (at least 0.50 g/mL) SG-8 also feeds evenly, which is what keeps a transparent sheet free of streaks and flow marks.

And the trade-off. Lower molecular weight means lower impact strength, lower creep resistance and a lower heat-distortion temperature. Sheet and fittings made on SG-8 need an impact modifier if they are going to take a knock, and they are unsuitable for pressure applications that a SG-5 part would handle. The plasticiser absorption requirement, at least 12 g per 100 g, confirms this is a rigid-product grade.

2. Specification

Values below are the requirement figures for the SG-8 band under GB/T 5761-2018.

Three notes on reading this table.

This is the widest K value band of the four. K 55–59 spans five units, against two for SG-5 and SG-3. A resin at 56 and a resin at 59 differ noticeably in melt viscosity, and on a thin-wall injection job that difference is the difference between a full part and a short shot. If the job is flow-limited, quote the viscosity number you need and hold the supplier to it. "SG-8" leaves too much room.

These are the SG-8 band requirements. The lot's own measured values are on its certificate. Your incoming inspection should compare against your own acceptance range.

Batch COA comes from the producing plant. Tell us at enquiry stage and we will obtain the certificate for the lot we are offering and send it before you commit. We do not issue certificates ourselves.

Standards note: GB/T 5761-2018 remains in force. The official record shows it was reviewed on 1 July 2025 with a conclusion of continuing validity. GB/T 3402.1-2026 and GB/T 3402.2-2026 take effect on 1 February 2027 in place of the 2005 and 2010 editions, changing the test methods written on a certificate. What a buyer should now check is written up here.

3. Where SG-8 goes, and how it is processed

One practical warning that applies across all of these: because SG-8 is the most heat-sensitive grade in the range on account of its low molecular weight, the temperature profile and residence time on the line matter more than they do on SG-5. A barrel that a SG-5 job tolerates will degrade an SG-8 job. If you are moving a line down to SG-8, review the temperature profile and the shut-down procedure before the first run, not after.

4. Naming it correctly on a purchase order

5. Packaging, marking and documents

How to read the markings on a PVC resin bag covers the batch number, grade marking and the fields your incoming sheet should record.

6. What to send us for a firm offer

1. Grade and position in the band: SG-8, plus the viscosity number or K value you need. On a flow-limited job this is the most important line on the enquiry.
2. End product and process: transparent sheet, foam board, fitting or thin-wall part; extrusion, foam, injection or calendering.
3. Wall thickness and flow length: the geometry is what decides whether SG-8 is enough or too much.
4. Clarity or cell structure requirement: this sets the fish eye and impurity particle limits that go on the order line.
5. Quantity: first order and expected monthly volume. FCL and LCL are both workable.
6. Packaging: 25 kg bags, jumbo bags, palletised, or private label.
7. Discharge port: name it and we will quote FOB, CFR or CIF against it.
8. Documents you need: Certificate of Origin, Form E, third-party inspection at loading.
9. Payment terms: so we can show you the total cost of the order.

7. Frequently asked

Is SG-8 a "high-strength" grade? No, and it is worth being careful about that description. SG-8 has the lowest molecular weight of the four grades we supply, so it has the lowest impact strength and creep resistance of the four. What it gives you is flow and surface quality. If a part needs strength, the strength comes from the formulation and the wall design. Buy SG-8 for the flow and specify the strength separately.

SG-7 or SG-8, which one for transparent sheet? SG-8, in most cases. Its lower molecular weight means fewer gel particles and better melt homogeneity, which is what clarity depends on. SG-7 is the better choice where the section is thicker and you need a little more toughness. Tell us the thickness and we will tell you which we would ship.

Can I get the COA before I place the order? Yes. Tell us the quantity you are considering and we will obtain the certificate for the lot we are offering from the producing plant and send it for review, including the viscosity number measured on that lot, which is the figure that decides whether your thin-wall job runs.

Do you supply in jumbo bags, or with our own bag design? Both are possible. 800 kg jumbo bags and palletised loading on request; private-label bag printing for volume orders.

What does the price depend on? The feedstock route the lot came from, volume and packaging, delivery basis, payment terms, and timing. PVC quotes hold for a shorter window than most plastics.