For polymerisation finishing plants, PP homopolymer and impact copolymer resins are dispatched as cylindrical or lenticular pellets with a nominal diameter of 3.0 mm to 4.5 mm and a length of 2.5 mm to 3.5 mm. The transfer format specified as PP Packaging: 25 kg Bags, FIBC & Bulk Handling determines not only logistics but also fines generation, moisture condensation risk, and electrostatic discharge behaviour at the converting line. A sealed 25 kg multi-wall sack provides different surge capacity and housekeeping demands than a 1,000 kg FIBC or a 24 t bulk road tanker discharge into a 40 m³ silo, yet all three formats must preserve the same downstream melt homogeneity.

What Limits Flow Reliability in 25 kg Bag Filling Lines?

Because pellet geometry controls sack fill volume, filling reliability in 25 kg sack lines is assessed first by apparent density rather than by rheology. Commercial PP pellet apparent density measured according to ISO 60 typically ranges from 0.52 g/cm³ to 0.56 g/cm³, giving a nominal fill volume of 45 L to 48 L per sack. A shift in pellet diameter of more than 0.3 mm changes bulk density by roughly 2 %, which can move gross bag weight outside a ±0.1 kg checkweigher tolerance on high-throughput impeller or belt-driven fillers. Multi-spout bagging machines with load-cell control commonly run at 8 to 15 sacks per minute per spout; published plant data correlating pellet shape to fill-rate variance in valve sacks is limited, and filler trials are normally required.

Pellet attrition during screw conveying into the bagger and drop impact from the weigh hopper creates fines below 500 µm. Residual fines content above 0.5 % by weight, screened through a 500 µm sieve in accordance with ISO 3310-1, increases dust liberation and can cause static cling to the sack inner wall. Because PP has very low equilibrium moisture uptake, moisture content measured by ISO 15512 method B typically remains below 0.05 % after sealed outdoor storage. Pre-drying is therefore not a normal requirement unless condensation occurs when cold pellets are moved into warm humid air at relative humidity above 60 %.

Static charge accumulation on freshly die-face-cut pellets remains a frequent cause of weight variability in sack filling. When pellet surface potential produces localised discharge, the fill stream can be disturbed and produce uneven settling. Bag fill points are fitted with grounded metal spouts or ionisation bars, and inner high-density polyethylene liners are specified with antistatic additive addition only when surface resistivity exceeds 1×10^11 Ω/sq; above this value, charge decay times measured by IEC 61340-2-3 can extend beyond 10 s and interfere with automatic palletising.

Valve sacks that are overfilled or misaligned can trap pellets between the valve and outer ply, releasing paper fibres into the hopper. On injection-moulding machines with clamp forces above 1500 kN, such fibre contamination is typically observed as nozzle filter pressure rise before visual defects appear. Crushed pellets from damaged 25 kg sacks increase fines that segregate in the feed hopper; on a corotating twin-screw extruder with L/D 40:1, this segregation can appear as motor-load fluctuation and feed-throat bridging.

When FIBC Liners Replace Rigid IBCs in Resin Transfer

Above 10 t/h, FIBC transfer of PP pellets introduces packaging risks that differ from rigid intermediate bulk containers because fabric walls are not airtight and the container geometry changes during lift and discharge. A four-panel FIBC with a safe working load of 1,000 kg may be tested under ISO 21898 for non-dangerous goods; the test sequence includes cyclic top-lift and topple testing, while UN-marked containers such as 13H2/Y are subject to additional stack and drop requirements when dangerous-goods classification applies. For PP resin, UN marking is not mandatory unless the grade is classified for transport, but the designation is often retained because it provides a defined siftproofness and liner compatibility record.

Electrostatic classification of FIBCs follows IEC 61340-4-4. Type B containers use low-breakdown-voltage fabrics to prevent propagating brush discharges but are not suitable where flammable gas or vapour atmospheres exist. Type C containers must be grounded to earth with a resistance below 1×10^8 Ω before filling and discharge; a single ungrounded Type C container in a filling bank defeats the protective circuit. Type D containers rely on corona-discharge fabric and do not require grounding, but they must not be used if conductive interior coatings block charge dissipation. Laminated foil or metallised polyester liners are specified when moisture barrier requirements exceed those of standard LDPE liners; during discharge, a folded liner across the spout can induce funnel-flow channeling in the receiving hopper, and the magnitude of the rate reduction is installation-dependent.

FIBC filling stations for PP often use a lance fill head suspended from a load-cell platform with ±0.5 kg accuracy. As the container fills, the lance raises to maintain a drop height below 500 mm; this reduces pellet bouncing and keeps the filling nozzle from burying in the product. Outdoor storage of FIBCs can reduce fabric strength through UV degradation; FIBC fabric condition is evaluated under ISO 4892-2 or by retained top-lift verification, not by the resin specification.

Bulk Density, Silo Shear, and Conveying Attrition

In silo discharge design, bulk density is insufficient to predict flow; wall friction and particle-size distribution govern hopper geometry. Pellet-wall friction angles under consolidation stresses corresponding to 10 m to 20 m of fill are measured in a Jenike shear cell according to ASTM D6128-22. For PP pellets on 304 stainless steel, the derived mass-flow hopper half-angle from vertical is typically in the range of 20° to 25°, with higher surface roughness increasing the allowable angle but also increasing particle surface scuffing. When a silo is converted from powder service, existing hopper liners may require replacement with 2B-finish stainless steel or UHMWPE to reduce wall friction and prevent stagnant zones.

Dilute-phase pneumatic conveying of PP at gas velocities of 18 m/s to 25 m/s creates fines through particle-wall impact, especially at elbows with centreline radius below 5× pipe diameter. Dense-phase plug conveying at 5 m/s to 10 m/s reduces attrition but increases pressure drop and requires a pressure-capable blow tank. Long-radius bends with 10× centreline radius and polished internal surfaces are used where pellet-size conservation is critical. The increase in fines depends on conveyor layout, bend count, and pellet hardness; published data for specific installations is limited because attrition is a system property rather than a resin property.

When PP is intended for food-contact articles, resin contact with packaging materials is not a regulatory afterthought. The base polymer is evaluated under FDA 21 CFR 177.1520, and the packaging liner must not introduce substances above the 10 mg/dm² overall migration limit of EU 10/2011. A change from one FIBC liner supplier to another is therefore subject to extractables screening, particularly for printing inks and slip agents that may migrate into pellets when stored above 40 °C. REACH compliance under Regulation (EC) No 1907/2006 requires a supply-chain declaration for phthalates and organotin stabilisers where applicable; PP homopolymer itself is generally not classified as a substance of very high concern.

Cross-contamination between PP grades must be controlled in bulk silo transfer lines. Equipment shared between homopolymer and random copolymer grades is purged with virgin product until the melt flow rate of the downstream sample, measured by ISO 1133-1:2022 at 230 °C/2.16 kg, is within 5 % of the target value. For high-clarity random copolymer lines, purge ratios of 3:1 to 5:1 by silo volume are validated by offline rheometry, and the required ratio is specific to the silo outlet geometry.