PVC Resin HS-700(SG-8)

PVC Resin HS-700 (SG-8) is a suspension-polymerised polyvinyl chloride homopolymer supplied as a white, free-flowing powder. The SG-8 designation places the resin at the low-molecular-weight end of the classification system in GB/T 5761-2006. Lot certificates generally report a Fikentscher K value in the 55–59 band, a viscosity number from 73 cm³/g to 86 cm³/g when tested according to ISO 1628-2, and an average degree of polymerisation of 650–750. The direct processing consequence is a lower melt viscosity at a given shear rate and a lower gelation temperature than SG-7 and SG-5 resins. This positions HS-700 for thin-wall injection moulding, rigid foam extrusion, and calendered sheet where melt pressure, cycle time, or die temperature is the controlling process constraint.

Raw resin properties are assessed under the suspension PVC test framework. Apparent density is determined by ISO 60, volatile matter by ISO 1269, residual vinyl chloride monomer by ISO 6401, and particle size distribution by ISO 1624. The table below shows representative SG-8 ranges used for incoming inspection; these are not lot-specific certified limits and procurement should require the actual certificate of analysis for each batch.

HS-700 is a suspension homopolymer and is not interchangeable with emulsion or microsuspension paste PVC resins. Suspension PVC median particle size is typically in the 100–150 μm range, whereas paste PVC is commonly 1–40 μm; the particle size and particle morphology make HS-700 unsuitable for plastisol formation without further grinding. It is also not a vinyl chloride–vinyl acetate copolymer; the grade does not provide the additional melt-flow reduction and lower heat distortion temperature associated with vinyl acetate comonomer.

Moisture uptake becomes a processing defect source when storage relative humidity exceeds 60%. Pre-drying in a desiccant hopper at 80 °C for 1–2 h is a standard line-side corrective action before dry blending. The resin is incompatible with strong amines and alkali metal hydroxides; these substances accelerate dehydrochlorination and can cause early yellowing in hot runner channels and screw dead spots.

What Distinguishes SG-8 from SG-7 and SG-5 When the Dry Blend Reaches the Melt Phase?

The classification bands in GB/T 5761-2006 separate SG-8 from higher-viscosity grades. SG-8 has a lower average degree of polymerisation than SG-7 and substantially lower than SG-5; SG-5 is the common pipe extrusion grade with higher chain entanglement and higher melt strength. The following comparison is used for raw-material selection, but it does not directly predict compound mechanical properties.

At the same torque rheometer bowl temperature of 180 °C and rotor speed of 60 min⁻¹, an SG-8 dry blend reaches fusion torque earlier and at a lower maximum torque than an SG-5 control. The trend is observable in a Brabender Plasti-Corder or Haake PolyLab, although direct HS-700-specific fusion curves are limited. The underlying cause is the reduced chain entanglement density in the lower-molecular-weight homopolymer; this lowers the energy required to convert the dry blend into a fused, homogeneous melt. The same reduction in melt viscosity produces a longer spiral-flow length in thin-wall injection moulding and lower die pressure in extrusion.

PVC melt rheology data should be generated on a stable formulation using a capillary rheometer with a 10:1 die length-to-diameter ratio and melt temperature of 190 °C. Because PVC is thermally sensitive, apparent viscosity at shear rates of 100–1,000 s⁻¹ is influenced by both wall slip and degradation; capillary data for SG-8 are therefore best used as a relative ranking against SG-5 rather than as absolute values. SG-8 compounds generally exhibit lower pressure drop across the die at the same screw speed, but direct HS-700 data are limited.

In rigid PVC injection moulding of thin-wall fittings with wall sections below 3.2 mm, production machines with clamp forces of 1,200–2,000 kN and barrel zones set between 165 °C and 190 °C are typical. An SG-8-based compound can fill multi-cavity tooling at lower injection pressure than an SG-5 control. Screw recovery time and cooling time may be reduced because the material fuses more quickly and can be processed at lower melt temperature. Actual cycle-time improvements depend on gate geometry, hot-runner pressure drop, and mould cooling efficiency; published HS-700-specific production data are limited. Production-scale failure modes observed in SG-8 injection moulding include gate blush, jetting in thicker sections, and post-demoulding warpage when orientation relaxes unevenly. Balanced runner systems, vented moulds, and the absence of sharp corner transitions reduce these defects.

For rigid PVC foam extrusion, HS-700 is selected primarily because lower gelation temperature helps the chemical blowing agent, commonly azodicarbonamide, decompose before the melt front leaves the die. Counter-rotating twin-screw extruders with L/D of 24:1–30:1 and screw diameter 65–90 mm can process SG-8-based foam formulations at die temperatures 10–15 °C lower than equivalent SG-5 formulations. Barrel settings are commonly 155–175 °C in the feed section and 170–185 °C at the die, depending on whether the line uses chemical blowing agent or gas injection. Lower die temperature reduces thermal stabiliser demand, but the lower melt strength of SG-8 can lead to cell coalescence if the pressure drop at the die lip is too rapid. An acrylic processing aid at 2–6 phr is often added to increase melt strength and prevent cell wall rupture. Foam density measured by ASTM D792-20 is commonly in the 0.45–0.65 g/cm³ range for free-foam PVC board, but the exact value depends on blowing agent loading and die gap.

Calendered rigid PVC film and sheet in thickness 0.2–1.0 mm may incorporate HS-700 at 20–40 wt% of the total resin to reduce calender roll torque and improve gauge control. Four-roll calenders with roll surface temperatures of 170–190 °C are typical. The viscosity reduction shortens residence time in the calender gap, but line speed must be limited to avoid draw resonance because the lower molecular weight reduces melt strength. The same flow benefit is used in non-pressure thin-wall electrical conduit, where SG-8 can be blended with SG-5 to improve output without increasing melt temperature. Foam-core pipe is another co-extrusion application in which SG-8 is used in the core layer and SG-5 skins provide surface impact resistance; the low melt viscosity of HS-700 permits the core layer to fill the annular space at lower die pressure, but skin-core melt strength differences must be balanced by die design.

Balance of Flow Enhancement and Loss of Long-Term Mechanical Integrity

HS-700 is not a direct substitute for SG-5 in pressure pipe, pressure fittings, or other articles where hydrostatic design basis and long-term creep rupture strength govern the specification. The lower chain length of SG-8 reduces tensile yield strength and notched impact resistance relative to SG-5 when tested under ASTM D638-14 and ISO 179-1:2023. Pipe-grade substitution should not be attempted without conformance testing to ISO 1452 or ASTM D1785. Impact modifiers such as methacrylate-butadiene-styrene or chlorinated polyethylene can recover some notched impact strength, but the long-term strength boundary remains. Published hydrostatic test data for HS-700 are limited.

The processing window for SG-8-based compounds is narrower than for SG-5 at high melt temperatures. Above 200 °C, dehydrochlorination accelerates sharply; in injection moulding the stable operating window around a set point of 185 °C is commonly no more than ±5 °C for tin-stabilised rigid compounds. This demands closed-loop barrel temperature control, a screw compression ratio of 2.0:1–2.5:1, and minimisation of dead spots in check rings and hot-runner manifolds. General-purpose SG-5 screws can generate excessive shear heating and push the melt temperature beyond the degradation threshold.

Thermal stabiliser selection must be matched to the lower heat stability of SG-8. Tin mercaptide stabilisers are typically used at 1.0–1.5 phr to maintain early colour hold; calcium-zinc systems require lower melt temperatures and acid scavengers to avoid hot-runner deposit formation. Dynamic thermal stability tests at 190 °C show a shortened induction time before dehydrochlorination relative to SG-5. Published activation energies for dehydrochlorination of suspension PVC are commonly in the 100–150 kJ/mol range, but the practical ranking of stabiliser packages should be confirmed by torque rheometry. With lower melt viscosity, the external lubricant concentration for SG-8 should be reduced relative to an SG-5 control under the same equipment; otherwise plate-out on calender rolls and screw surfaces may occur.

Batch-to-batch variation should be monitored through incoming K value and apparent density testing. Experience on production-scale twin-screw extrusion lines shows that SG-8 lots near the upper K-value band produce a smaller melt pressure reduction than lots near the lower band. Adjusting screw speed alone cannot fully compensate for K-value drift; feed throat temperature and die temperature must also be adjusted. When storage conditions are outside the recommended range, conditioning is required because free moisture influences apparent density and can alter dry blend lubrication. Bulk density below 0.50 g/cm³ may require a crammer feeder in single-screw extrusion; counter-rotating twin-screw machines are less sensitive to feed density but should still be monitored for throughput variation.

Blending SG-8 with SG-5 is a common industrial technique to manage melt viscosity while retaining enough mechanical strength for non-pressure articles. Blends of 10–30% SG-8 with SG-5 are used in electrical conduit and non-structural profile extrusion; the effect on tensile strength and impact resistance should be measured by ASTM D638-14 and ISO 179-1:2023 on the target compound. Direct HS-700 blend data are limited and should be generated on the production line. Food contact status and regional compliance of the finished article must be established by migration testing of the final compound; bulk resin conformity to the above test methods does not confer food contact approval under EU REACH or other regional regulations.