VeryGreen™ VG7233U Compostable Food Contact Approved Polylactic Acid is a hydrolytically sensitive aliphatic polyester; in monolayer trays and hinged clamshells produced from rigid sheet, the principal technical challenge is not base-resin compliance but the narrow window between sheet softening and incipient crystallisation. Sheet extrusion is normally run on a single-screw extruder with L/D 30:1 to 36:1, vacuum venting at -0.08 MPa to -0.09 MPa, and a gear pump to damp pressure variation. Drying before extrusion is compulsory: desiccant drying at 80°C for 4 h to 6 h with a dew point no higher than -40°C should reduce pellet moisture below 250 ppm. At ambient relative humidity above 60%, pellet open storage before hopper loading should be limited to 30 min; otherwise surface moisture uptake can lower molecular weight during extrusion and produce edge tear, die-lip build-up, and gauge variation. For amorphous sheet of 0.25 mm to 0.80 mm thickness, forming temperature should be controlled within 90°C to 110°C; a surface temperature spread greater than ±5°C across the sheet causes local draw differences, stress whitening, and non-uniform wall thickness. Below this range, edge tearing and web breaks occur at plug-assisted draw ratios above 1.5:1; above this range, sag exceeds 15 mm over a 300 mm span and spherulitic haze becomes visible. Regrind from skeleton trim can be used up to 20 wt% only if it is dried and blended in a closed-loop system; higher ratios lower sheet impact strength and broaden the forming window unpredictably. The finished part remains amorphous unless a separate annealing step is applied, so service temperature is limited and hot-fill performance must not be assumed from the food-contact approval alone. Alkaline residues from previous barrel purges, particularly amine-based purge compounds, should be avoided because alkaline hydrolysis accelerates chain scission even at melt temperatures below 220°C.

How Does Extrusion Coating onto Paperboard Alter Moisture and Adhesion Requirements?

When VG7233U is applied as a single-layer extrusion coating on cupstock, folding carton board, or moulded-fibre trays, moisture control becomes more critical than in sheet extrusion because residual water in the fibre-based substrate can flash into steam at the nip and create pinholes. Board moisture above 6 wt% is a known source of blistering, so preheating to lower surface moisture or using infrared drying immediately before the coating nip is common on production lines. Corona discharge or flame treatment is applied to bring the board surface energy to at least 38 mN/m to 44 mN/m, measured by ASTM D2578; untreated clay-coated board often gives inconsistent adhesion because the hot melt curtain is not in contact with the surface long enough for polar interactions to develop. The die gap and air gap are set differently from polyethylene coating: a shorter air gap of 150 mm to 250 mm reduces neck-in and melt curtain wobble, while melt temperature at the die is held between 200°C and 220°C. Because PLA melt strength is lower than low-density polyethylene, neck-in at the coating line is more severe; the die width is typically set 10% to 15% wider than the target coated width. Coating weight is measured by beta gauge or infrared gauge, with standard food-contact board structures in the range of 12 g/m² to 25 g/m². Adhesion to paperboard without primer is often below acceptable peel levels for cup forming and side-seaming; a starch-based or polyvinyl alcohol primer is therefore used where the final laminate must survive folding, scoring, and hot liquid contact. The food-contact approval of the resin does not eliminate the need for migration testing on the final laminate, because paperboards contain mineral fillers, coating binders, printing inks, and adhesives that can migrate through pinholes or exposed edge surfaces. Lot-to-lot variation in melt flow rate measured by ISO 1133-1:2022 should be tracked; a shift of more than 3 g/10 min from the supplier’s nominal value can change curtain stability and coating weight uniformity.

Cycle time in high-cavitation cold-runner moulds producing disposable cutlery and thin-wall food containers is governed by gate freeze and ejection stiffness rather than overall part cooling. Melt temperature at the nozzle is limited to 190°C to 210°C for thin-wall sections below 1.0 mm; higher settings are used only when filling long flow paths, but total residence time at melt temperatures above 220°C should not exceed 10 min, because lactide regeneration and hydrolytic chain scission accelerate rapidly. Mould temperature is held at 20°C to 35°C for amorphous parts, preserving clarity but leaving the part with low heat resistance; ejection can be difficult if the mould surface is too cold because PLA has higher surface tack than polypropylene. A screw cushion of 2.0 mm to 3.0 mm, back pressure no higher than 1.5 MPa, and screw decompression of 2 mm to 5 mm are used to prevent nozzle drool. Post-mould annealing at 90°C to 110°C for 15 min to 30 min raises heat deflection temperature but produces additional linear shrinkage in the range of 0.3% to 0.8%, measured by ISO 294-4, which must be compensated in tool dimensions. Drying discipline is equally important: pellets should be dried to below 250 ppm moisture and conveyed in sealed lines; otherwise the melt flow rate can drift upward by hydrolysis, causing short shots, flash, and contamination of the screw check ring. Hot-runner systems are possible, but the shut-off nozzle and manifold temperatures must not exceed 220°C for residence times beyond 5 min during cycle interruptions. In multi-cavity tools, fill imbalance exceeding 5% by part weight often appears as warpage difference between cavities; the cause is usually inconsistent gate land dimensions or thermal unevenness, not resin variability.

Blown Film Stalk Geometry, Frost Line Position and Tear Anisotropy

Neat PLA has lower melt strength than polyethylene, so bubble stability is the limiting factor in blown film, not throughput. Published data for neat PLA in conventional air-cooled blown film lines is limited; most commercial compostable films use cast film, double-bubble orientation, or polymer blends where food-contact approval permits the added polymers. For trials on VG7233U, a die gap of 0.8 mm to 1.2 mm, blow-up ratio of 1.5:1 to 2.2:1, and frost line height of 3 to 6 die diameters are reasonable starting points, but tear anisotropy is pronounced. Machine-direction tear strength can be 2 to 3 times lower than transverse-direction tear strength when measured by ASTM D1922, unless orientation is balanced by adjusting stalk height; a tall frost line often worsens this imbalance by increasing machine-direction orientation. Bubble cooling should use a dual-lip air ring rather than a single-lip ring, because the low melt strength of PLA can permit diameter fluctuations when air is not uniformly distributed. Gauge variation caused by bubble instability is commonly above ±10% on narrow air-cooled lines, which makes such film unsuitable for high-speed horizontal form-fill-seal packaging without online thickness gauging. Heat-seal testing per ASTM F2029 is required because the seal initiation temperature of PLA is higher than polyethylene; seal strength is also sensitive to dwell time and jaw temperature. Without adequate quench and controlled crystallisation, film blocking can occur on rewind at roll pressures above 0.5 MPa. For the food-contact status of the final film, the complete formulation, including slip and antiblock concentrates, must fall within the supplier’s approved additive envelope; conventional amide slip agents can introduce migration issues and are not presumed acceptable for this grade.

Because compostability certification under EN 13432:2000/Amd 1:2005 or ASTM D6400-23 does not transfer automatically to finished articles unless the construction remains within the approved formulation limits, the entire article must be treated as a system. A compliance matrix should record the resin grade, colourant masterbatch, nucleating or clarifying additives, coatings, adhesives, printing inks, and any processing aids, because each component can alter the ultimate biodegradation, disintegration, or ecotoxicity result. For VG7233U, the food-contact approval and compostability certification are separate technical claims; a moulded cup may pass industrial compostability but still exceed overall migration limits if the wrong masterbatch or adhesive is used. Under EU 10/2011, overall migration into assigned food simulants must not exceed 10 mg/dm², and the finished article must be tested under the time-temperature conditions assigned to the intended food contact. For US applications, the grade-specific FDA food-contact notification or 21 CFR listing should be referenced in the certificate of compliance for the exact resin lot. The table below is a working compliance matrix for finished articles.

Disintegration testing under ISO 16929:2021 is not a simple weight-loss check; compost fragments are sieved through a 2 mm sieve after 12 weeks, and the threshold of 90% is based on dry matter relative to initial dry matter. The result is sensitive to sample thickness; articles above 2 mm may need longer residence time in industrial composting or may not meet the standard’s timeframe, even if the base resin is certified. This is a critical operational boundary for thick-walled cutlery or dense injection moulded items. Plant toxicity testing per OECD 208 uses the compost generated after disintegration; a failed ecotoxicity result cannot be corrected by further processing. Therefore, no claim of industrial compostability should be placed on a finished article unless the complete formulation has been tested in the final geometry, because migration kinetics in the polymer matrix and additive partitioning into food simulants are affected by thermal history, part thickness, and surface area-to-volume ratio.

When Annealing Converts Amorphous Sheet into Crystallised Food Service Articles

A two-step production line can use the same thermoformed sheet to produce transparent amorphous lids or, with a post-forming annealing tunnel, opaque crystallised trays for hot-fill and short reheat exposure. Annealing at 90°C to 110°C for 15 min to 30 min raises the crystalline fraction from below 5% to above 30%, measured by differential scanning calorimetry per ISO 11357-3. This shifts heat deflection temperature into the 85°C to 110°C range for PLA homopolymer grades, but notched Izod impact strength measured by ISO 180 can fall by 20% to 40% compared with the amorphous part. Dimensional change is not uniform: post-annealing shrinkage of 0.5% to 1.5% in length and width must be anticipated, and the part must be supported during annealing to prevent warpage. Infrared tunnel ovens with air temperature uniformity within ±3°C are preferred over convection ovens because local overheating can distort rim areas. The annealed tray can tolerate 80°C continuous contact and short excursions to 100°C where the part is not under mechanical load; it is not suitable for boiling water immersion, oven baking, or steam sterilisation. If the tray is used with a lidding film, the heat-seal temperature must exceed 120°C to allow seal initiation against the crystallised substrate, but the tray cavity must not reach its deflection temperature during sealing. This limit creates a practical processing conflict on high-speed tray-sealing lines: the seal dwell time must be long enough to soften the lidding film but short enough to avoid rim deformation. Published data for this specific grade in annealed tray configurations is limited; production trials should start with small temperature increments and measure rim deformation after sealing, because crystallinity level and heat-seal performance are not independent variables.