Polybutylene terephthalate (PBT; CAS 24968-12-5) is a semicrystalline thermoplastic polyester obtained by polycondensation of 1,4-butanediol with terephthalic acid or dimethyl terephthalate. The procurement specification for Polybutylene Terephthalate Packaging and MOQ: Bags, Octabins and Bulk is applied to unmodified injection moulding grades, glass-fibre-reinforced compounds, and flame-retardant formulations because the selected package directly controls moisture ingress, lot identity, and contamination risk before melt conversion. Unfilled PBT has a density of 1.30–1.32 g/cm³ under ISO 1183-1:2019, a crystallite melting peak of 223–225 °C by ISO 11357-3:2018, and a melt volume-flow rate of 10–25 cm³/10 min at 250 °C and 2.16 kg load under ISO 1133-1:2022. Intrinsic viscosity is typically 0.70–1.20 dl/g when measured according to ISO 1628-5:2020. The equilibrium moisture content at 23 °C and 50 % relative humidity is below 0.10 wt%, while saturated water absorption under ISO 62:2008 is approximately 0.2 wt%. Published data for specific lot-to-lot variability can be limited because grades differ in molecular weight, carboxyl end-group content, and additive package.

The material is specified for electrical connectors, power-distribution parts, automotive underhood components, and industrial housings because it crystallises rapidly and withstands fuels, weak acids, and aliphatic hydrocarbons at service temperatures up to 120–140 °C for short peaks. Unreinforced PBT exhibits a flexural modulus of 2,300–2,800 MPa per ISO 178:2019; a 30 wt% short-glass compound raises flexural modulus to 8,000–10,000 MPa and the heat deflection temperature at 1.8 MPa to 200–220 °C per ISO 75-2:2013. Electrical grades typically exceed a comparative tracking index of 400 V under IEC 60112:2003. Flame-retardant PBT is classified UL 94 V-0 at 0.8 mm or 1.6 mm depending on formulation. Notched Izod impact strength for unreinforced PBT at 23 °C is commonly 3–6 kJ/m² per ISO 180:2019. These properties are recorded on dry granulate; moisture-induced hydrolytic chain scission changes the polymer backbone rather than acting as a simple surface contaminant, so packaging barrier performance is a technical processing variable.

Moisture control is the primary technical driver for bag, octabin, and bulk selection. PBT pellets are dried before processing to a residual moisture level below 0.02 wt% measured by Karl Fischer titration under ISO 15512:2019. At the melt-processing range of 250–270 °C, water reacts with ester linkages through hydrolytic chain scission; each hydrolytic event increases chain ends and reduces weight-average molecular weight and intrinsic viscosity. A residual moisture level above 0.02 wt% can cause measurable intrinsic-viscosity loss in a single compounding pass, reduced melt strength, and surface splay on moulded parts. Bags with a continuous internal PE liner and heat-sealed closure suppress moisture diffusion when stored at 10–30 °C and relative humidity below 60 %; octabins provide the same barrier at higher unit mass with a single extraction point; bulk silos require desiccant breathers and closed-loop transfer because condensation on silo walls is a known field failure mode in humid production environments.

What Is the Maximum Safe Residual Moisture Before Melt Processing?

The safe threshold is not uniform across processing methods. Injection moulding and direct extrusion of unmodified PBT normally require pellet moisture below 0.02 wt%; for thin-wall electrical parts moulded on clamp forces above 800 kN, the stricter target of 0.010 wt% is often enforced because shear heating raises the melt temperature and accelerates hydrolysis. Desiccant drying with a dew point of −35 °C or lower and air temperature held at 120 °C for 4–6 h is the standard procedure for 25 kg bags and 1,000 kg octabins unsealed within 2 h of processing. Drying above 140 °C initiates thermo-oxidative yellowing and aldehyde byproduct formation; drying below 110 °C leaves diffusive water in semicrystalline domains that is not removed within a practical cycle. The usable air-temperature window is therefore 110–140 °C, but the more critical parameters are residence time and air dew point. Air flow should be maintained at 1.0–1.5 m³/h per kg/h of resin throughput to avoid bed channelling in large drying hoppers. On production lines with L/D 40:1 twin-screw extruders and vacuum venting at −0.08 MPa, residual moisture above 0.05 wt% is associated with outgassing at the die, strand breaks, and a shift in melt volume-flow rate greater than 15 % relative to dried granulate. Published data for specific screw geometries is limited.

PBT pellets are produced on strand or underwater pelletizers. Strand pelletizers yield cylindrical granules of 2.5–3.0 mm diameter and 3.0–4.0 mm length; underwater die-face pelletizers yield lenticular granules with similar bulk density. Pellet geometry influences the minimum drying air velocity and hopper outlet: for 3–4 mm pellets, a desiccant dryer with bed depth of 30–60 cm and air velocity of 0.5–1.0 m/s provides uniform drying. In octabin and bulk systems, pellet-surface moisture rather than internal diffusion controls initial drying rate; glass-fibre-reinforced compounds may require an additional 30–60 minutes because the fibre interface can retain moisture after pellet surface reaches target dew point. These values are approximate and depend on grade, additive package, and pellet porosity.

Bag, Octabin, and Bulk Packaging Configurations

Table 1 lists the nominal pack mass, dimensions, barrier configuration, minimum order quantity, and transfer equipment for the three packaging formats used for PBT. The values reflect industrial packaging norms and should be confirmed against supplier-specific commercial terms.

For bag packaging, a standard pallet comprises 40 bags of 25 kg, giving a net mass of 1,000 kg. Each bag is printed with the product name, grade, lot number, production date, and net weight. A heat-sealed inner polyethylene liner of minimum thickness 0.08 mm is used for injection-moulding grades; flame-retardant and glass-filled grades may be packed in aluminium-foil laminates when high-humidity transit exceeds 4 weeks. Pallets are stretch-wrapped and corner-protected. ISO 9001:2015 lot traceability requires each bag to carry a unique batch number that links to the production log, the masterbatch addition record, and the retained sample. The practical minimum order for bag-packed PBT is one homogeneous lot of 1,000 kg. Smaller laboratory or trial quantities are repacked only under controlled conditions because opening a sealed bag breaks the moisture barrier and may compromise the supplier’s certificate of analysis.

Octabins in PBT supply chains are usually 1,000 kg net mass with a 1,200 × 1,000 mm base footprint and sidewalls of 7-ply corrugated board. The format is selected when a moulder consumes more than 1,000 kg per week of a single grade but does not operate a bulk silo. A single octabin replaces 40 small bags, reducing liner waste and manual cuts at the machine throat. Discharge is made through a side spout or by lifting the octabin onto a tilting station; the PE liner must be secured to the discharge cone to prevent liner draw-in. The minimum order quantity for octabin packaging is generally 5,000 kg for a standard unfilled PBT grade and 10,000 kg for a colour-matched or flame-retardant compound, reflecting cleaning and setup time on the compounding and packaging line. Lot-to-lot moisture variance in octabin-packed material is usually lower than in bags because one large liner limits repeated atmospheric contact; however, if the liner is punctured during forklift handling, the entire octabin must be re-dried before melt processing.

In automotive underhood parts such as sensor housings and connectors, injection moulders often specify octabin packaging because the production lot is larger and the material is consumed rapidly enough to avoid repeated opening of small bags. For prototyping or small-batch electrical parts, bag packaging with the 1,000 kg minimum order is the standard entry point. Bulk packaging is reserved for dedicated production cells with silo-mounted dryers and automated conveying; in that configuration the packaging minimum order quantity of 18,000 kg is balanced against reduced labour and packaging waste. These packaging choices do not change the polymer’s upper service temperature or flammability classification; they change only the moisture history and contamination probability at the moulding machine.

When Bulk Silos Replace Octabins at Injection Moulding Plants

Bulk packaging of PBT is limited to converters consuming at least 18,000 kg per month of one grade. This threshold aligns with carryover risk between campaigns in a shared silo and the moisture-control burden of large-scale handling. Bulk delivery uses pressure-differential tankers or walking-floor trailers discharging into bolted or welded steel silos with internal surfaces prepared to a roughness of Ra ≤ 0.8 µm to prevent pellet hang-up. Silos must be vented through desiccant drums sized for the daily air exchange caused by temperature cycling, and the fill pipe should include a cyclone or screen to remove pellet fines generated during dense-phase conveying at 2–3 bar. For glass-fibre-reinforced PBT, bends in pneumatic lines should be limited to a radius of no less than 10 pipe diameters and conveying velocity kept below 30 m/s to minimise fibre attrition and dusting. The bulk minimum order quantity is typically 18,000 kg, equivalent to a full tanker; partial bulk loads are not recommended because vessel headspace permits condensation and can cause localized moisture gain. Published data for specific moisture gain rates in partially filled bulk trailers is limited. The material is sampled at delivery with an automatic sampler at the fill line or from the silo after discharge of the first 50 kg; the retained sample is tested for melt volume-flow rate, moisture, and, for flame-retardant grades, UL 94 vertical burn classification at 0.8 mm thickness.

Regulatory controls affect packaging and minimum order decisions for PBT. RoHS 2011/65/EU restricts cadmium, lead, mercury, hexavalent chromium, PBB and PBDE, and the supplier must provide a conformity declaration for each lot shipped. REACH Regulation (EC) No 1907/2006 requires substance registration and communication of SVHC content when applicable. For indirect food contact, migration limits are assessed under (EU) No 10/2011 or applicable national positive lists; the user is responsible for verifying that the specific PBT grade and packaging liner do not transfer substances above the prescribed overall migration limit of 10 mg/dm². Because the packaging liner itself is a potential contaminant, bag and octabin liners must be made from food-grade polyethylene and should not be reused across different resin families.

PBT should not be dried or processed in equipment that previously handled polyamide at high residence time, because amine end groups can accelerate ester interchange and mimic hydrolytic viscosity loss. Storage near open steam vents, hot water lines, or outdoor exposure is incompatible with the moisture limits described for bag, octabin, and bulk packaging. Do not combine PBT regrind with wet virgin pellets without uniform blending, because local moisture pockets at the feed throat can cause screw slip and burn marks even when the average lot moisture is below 0.02 wt%. The same prohibition applies to reintroducing dust from bag collection systems into an injection moulding hopper without prior drying, because fines have a larger surface area and adsorb moisture more rapidly than intact pellets.