Bionolle Starcla™ 40S is a 50% bio-based polylactic acid/starch compostable compound supplied as opaque pellets for injection molding and sheet extrusion. The bio-based carbon content is determined by radiocarbon analysis under ASTM D6866-21 or ISO 16620-2:2019. The 40S grade designation identifies a specific rheology and starch/PLA ratio within the Starcla series; the numeral is not a direct starch mass fraction. Application areas include rigid, short-service-life articles such as plant pots, disposable cutlery, trays, thin-wall packaging inserts, and cosmetic packaging. The material is intended for industrial aerobic composting environments. It is not automatically suitable for long-term aqueous contact, microwave reheating, dishwasher exposure, or steam sterilization unless the finished article has been validated under those specific conditions.

Formulationally, the starch phase reduces dependence on petrochemical carbon and accelerates composting attack relative to neat PLA. It also increases equilibrium moisture uptake, lowers melt strength, and narrows the thermal processing window. The compound is hygroscopic rather than merely moisture-sensitive. At 50% relative humidity, pellet surface moisture increases within minutes to hours. Desiccant drying is therefore mandatory before melt processing. Hot-air box dryers without desiccation do not create the required humidity differential in humid production halls and are generally inadequate for lot-to-lot reproducibility.

What Are the Critical Drying Limits Before Twin-Screw Compounding and Injection Molding?

Residual moisture content is the primary control variable for this compound. For injection molding, residual moisture should be reduced below 0.025% (250 ppm). Above 0.1%, hydrolysis of the PLA matrix becomes measurable as molecular-weight loss and reduced impact toughness. Above 0.3%, feed-throat bridging, screw slippage, splay, and steam-induced surface pitting are commonly observed on production-scale equipment. Drying at 70 °C to 80 °C for 4 h to 6 h is normally effective when the dryer is correctly sized and the air supply dew point is below -40 °C. Dried pellets should be conveyed in dry-air systems. At 50% relative humidity, ambient exposure of dried pellets can re-establish surface moisture within 30 min to 60 min.

For twin-screw compounding of regrind or rework, the moisture limit is equally strict because water reacts with ester linkages at melt temperatures above 170 °C. Vented barrels are recommended. If the extruder is unvented, moisture cannot be removed effectively at standard screw speeds, and hydrolytic degradation shifts melt flow rate upward while reducing melt strength. A single-screw extruder with L/D 24:1 to 30:1 and a two-stage vented screw is the minimum configuration for sheet extrusion. A corotating twin-screw extruder with L/D 40:1 is preferred for compounding high-rework fractions above 20%.

Barrel temperature setpoints are constrained by a narrow window. Injection molding profiles of rear 140–155 °C, center 150–165 °C, front 160–175 °C, nozzle 165–180 °C, and mold temperature 20–40 °C are typical for this compound class. Back pressure below 1.0 MPa and moderate screw rotation are used to limit shear heating. For sheet extrusion, melt temperature should not exceed 190 °C because the starch phase darkens and generates acidic volatiles that accumulate at die lips. Below 150 °C, melt homogenization is poor, back pressure rises, and surface flow lines appear. The practical processing window on unfavorable machine configurations is therefore narrow, sometimes within ±5 °C to ±10 °C around the mid-range. Residence time at melt temperature should remain below 10 min, and shot size should fall between 30% and 70% of barrel capacity to limit thermal aging.

Injection molding screws with compression ratios of 2.0:1 to 2.5:1 and constant-taper designs reduce local temperature peaks. Hot-runner systems should maintain internal temperatures below 190 °C. Valve-gated hot runners can accumulate degraded starch deposits if the gate is not purged during production pauses. Sheet extrusion is performed on conventional three-roll calendering stacks. Sheet should be cooled below 50 °C before wind-up because residual heat promotes blocking and static charge. Thermoforming surface temperatures of 80–110 °C are common for PLA/starch sheet; above 110 °C, localized thinning and starch-phase degradation risk increase.

Published data for this exact grade is limited. The following indicative property envelope is compiled from public literature on 50% bio-based PLA/starch compounds rather than reproduced from a manufacturer lot-release certificate. Manufacturers’ lot-release values may differ.

Incoming inspection should include melt flow rate testing under ISO 1133-1:2022 using dried pellets, moisture analysis by Karl Fischer titration, and bio-based carbon verification when customer claims depend on it. Batch-to-batch viscosity and color variation can occur if starch source, moisture content, or regrind fraction changes. Purging should be performed with a PLA-based or acrylic purging compound. Polyolefin residuals create visible contamination and interfacial weakness when equipment is not adequately purged.

Regrind addition up to 20% is common for noncritical parts. Higher regrind fractions increase melt-flow variability because each heat history partially degrades the PLA phase. Melt filtration with screens of 100 µm to 200 µm is used in sheet extrusion to remove starch aggregates and carbonized specks. Filtration upstream of the die reduces surface defects but the pressure differential must be monitored to avoid excessive shear heating.

Compostability Standards and Certification Matrix

Compostability is an article-level property. A pellet may be formulated to meet EN 13432:2000/AC:2005 or ASTM D6400-21, but certification applies to the finished part, sheet, or packaging item. For thin-wall injection molded or thermoformed articles below 1.0 mm thickness, industrial aerobic composting at 58 °C and 50–60% moisture content typically initiates starch solubilization and PLA hydrolysis. Thicker sections may require longer disintegration times. The following matrix identifies the primary standards used to demonstrate compostability.

The term “compostable” in this context means industrial aerobic composting under controlled time, temperature, moisture, and aeration conditions. It is not equivalent to soil, marine, or home-compost claims. Marine degradability is not claimed for this grade unless a separate marine biodegradation standard such as ASTM D6691-17 is applied and the finished article passes relevant test thresholds.

Bio-based carbon content is not the same as biodegradation percentage, compostability certification, or total renewable content. Processing aids, mineral fillers, colorants, or compatibilizers may contribute to the non-bio-based fraction. The 50% bio-based carbon value places Starcla 40S in the intermediate bio-based segment. It differs from 100% bio-based compounds that may avoid fossil-derived additives, but it may provide lower formulation cost while retaining industrial compostability.

If Heat Resistance Above 60 °C Is Required, the Starch Phase Becomes a Limiting Variable

Starcla 40S is not a high-heat grade. At 50% bio-based PLA/starch composition, heat deflection temperature under ISO 75-2:2013 method B generally remains below 65 °C in unstressed parts. The starch phase, especially when plasticized by residual moisture or monomer, further reduces dimensional stability under hot static loads. Applications that contact hot beverages, dishwasher cycles, microwave heating, steam sterilization, or automotive interior radiation above 60 °C should be rejected unless the final part is explicitly validated. This differentiates the grade from high-crystallinity PLA compounds with mineral fillers or high-heat PLA blends that can approach 80–100 °C HDT under comparable conditions.

Compared with unfilled PLA, the starch phase reduces formulation cost and improves industrial compostability but lowers tensile elongation and impact strength. Compared with PBAT-rich biodegradable compounds, Starcla 40S is stiffer and more dimensionally stable in ambient conditions but has lower elongation at break and is more sensitive to moisture. Compared with higher-bio-based PLA grades, the 50% bio-based carbon content may allow lower formulation cost while retaining processability on standard polyolefin injection molding and sheet extrusion lines. The main operational incompatibilities are prolonged high-temperature residence, hot-air drying, excessive shear, highly basic fillers that accelerate ester hydrolysis, and direct humid storage of opened bags.

Unopened bags should be stored indoors below 35 °C and below 60% relative humidity. Partially used bags must be resealed within 15 min to 30 min or transferred to moisture-barrier containers. Pallets should not be stacked in direct sunlight. After molding, parts may require conditioning at 40 °C and 50–60% relative humidity for 24 h before performance testing to stabilize dimensions and reduce variability in article-level measurements.