Bionolle Starcla™ 25S is a commercial 50% bio-based polylactic acid/starch melt-compounded material. The 25S designation identifies a starch-modified polylactic acid grade in which gelatinized or plasticized starch domains are dispersed within a continuous PLA phase. The formulation is intended to raise renewable carbon content and reduce compound cost relative to unmodified PLA while retaining industrial compostability. Bio-based carbon is not inferred from mass balance alone; it is quantified by radiocarbon analysis under ASTM D6866 Method B, which reports the fraction of modern carbon in the total organic carbon. Lot-to-lot variation in melt flow rate, tensile modulus, tensile strength, notched Charpy impact, heat deflection temperature, and residual moisture should be confirmed against the supplier’s certificate of analysis, because starch source, plasticizer loading, and compounding history alter the final rheological and mechanical response.

Bionolle Starcla™ 25S is not a dry blend of PLA pellets and starch powder. A dry blend of those components tends to segregate during hopper conveying and produces variable starch content across the batch. Melt compounding disperses the starch phase and reduces, but does not eliminate, property drift. In comparison with unmodified PLA, the 25S grade generally exhibits lower tensile modulus, lower flexural strength, higher moisture uptake, and a narrower thermal processing window. In comparison with PBAT/starch blown-film compounds, the material is rigid and semi-brittle with much lower elongation; it is not a drop-in replacement for ductile film resins. Compared with mineral-filled PLA, the starch phase lowers density and raises bio-based carbon content but reduces modulus and the heat-deflection plateau. These distinctions confine the practical application range to rigid packaging, serviceware, trays, and other stiff compostable articles rather than flexible film or hot-fill containers.

What Limits the Drying and Melt-Temperature Window for the 25S Grade?

Residual moisture is the critical processing variable for this class of material. PLA undergoes hydrolytic chain scission at melt temperatures above 170 °C when water exceeds roughly 0.05 wt%, and the starch phase sorbs moisture faster than the surrounding PLA. Pellet drying is specified before extrusion, injection molding, or thermoforming. A desiccant dryer with a dew point at or below -40 °C, an air temperature of 60 °C to 80 °C, and a residence time of 4 h to 6 h is typical for PLA/starch compounds. The dried pellet moisture should be verified at or below 0.025 wt% by Karl Fischer titration using ISO 15512 or by a calibrated moisture analyzer. Published data specific to Bionolle Starcla™ 25S is limited; the stated limits are accepted practice for PLA/starch rigid compounds and should be confirmed against the supplier’s processing guide.

Melt temperature must be kept low to avoid starch darkening and PLA reversion to lactide. A melt range of 180 °C to 200 °C at the nozzle is commonly applied, with barrel profiles arranged to avoid local overshoot above 210 °C. Residence time above 210 °C should be minimized because the starch phase undergoes non-enzymatic browning and the PLA phase releases lactide and acetaldehyde. On production-scale injection molding machines, the failure signature is a progressive increase in melt volume-flow rate and the appearance of black specks in translucent parts; this indicates starch degradation rather than insufficient purging.

Converters running pre-compounded pellets are not required to perform reactive compounding, but they must protect the material from shear overheating. When regrind or post-industrial reclaim is added, a co-rotating twin-screw extruder with an L/D ratio between 36:1 and 44:1 and distributive mixing elements is preferred over severe kneading blocks. Kneading-block stacks can generate local viscous heating above 220 °C, causing starch degradation even when the barrel setpoint remains low. A flat or reverse temperature profile from feed to die is used, with the feed zone below the starch gelatinization onset and the die below 195 °C. Screw torque and specific mechanical energy are not fixed because they depend on screw design and throughput, but the process should be adjusted so that melt temperature measured at the die remains within the recommended envelope. Field production experience indicates that viscosity loss and black speck formation are the first observable indicators of excessive energy input.

Tensile, Impact, and Heat-Distortion Mapping for Grade Selection

Mechanical specification of Bionolle Starcla™ 25S must be performed on dry specimens because water plasticizes the starch phase and depresses the glass transition. Tensile modulus and tensile strength are determined according to ISO 527-2, and notched Charpy impact is measured under ISO 179-1. Unmodified PLA grades typically exhibit tensile modulus near 3.0 GPa to 3.5 GPa and elongation at break below 10%; starch modification tends to lower the modulus and reduce the elongation plateau, although the exact values depend on plasticizer and compatibilizer. The starch domains can reduce orientation-induced shrinkage anisotropy, which is beneficial in thin-wall molding. However, this occurs at the expense of lower heat-deflection performance. Heat deflection temperature is measured by ISO 75-2 Method A or B, and the result is sensitive to annealing. For PLA/starch compounds, unannealed heat-deflection values are generally below the PLA cold-crystallization exotherm; service temperatures should not exceed the measured value plus a safety margin.

Because the starch phase raises hydrophilic character, storage and handling must prevent moisture regain. Pellets exposed to ambient air at relative humidity above 60% for extended periods should be re-dried before processing. The material is not recommended for hot-fill container applications unless the part design has been validated under the intended fill temperature and the heat-deflection behavior of the specific lot supports the load.

When 25S Replaces Neat PLA in Thin-Wall Injection Moulding

The compound follows standard PLA screw geometry but requires tighter melt-residence-time control. Thin-wall containers, cutlery, and single-serve items are molded with a general-purpose three-zone screw and a length-to-diameter ratio of 20:1 to 24:1. Hot-runner systems are often avoided because stagnant zones allow starch degradation and black speck formation. Valve-gated cold runners or heated sprue bushings with streamlined flow paths reduce dead spots. Mould temperatures from 20 °C to 40 °C are used to preserve cycle time; cooling time is governed by part thickness and the solidification of the PLA phase. Clamp force is calculated from projected part area and injection pressure, and no unusual clamp requirements are introduced relative to PLA. Back pressure should be set at the minimum level needed to homogenize the melt, typically below 10 bar; excessive back pressure increases shear heating and can push melt temperature above the degradation threshold.

The material is purged with a low-MFI PLA or a suitable acrylic purge before shutdown, and the barrel is heated for purge only when necessary. Production experience shows that abrupt temperature increases during start-up can generate high screw torque if the feed zone is hot enough to soften the starch before conveying; therefore, the machine is started with a cold hopper and a feed throat temperature below 50 °C to prevent pellet bridging. Dried pellets should be conveyed by closed dry-air systems, not open hoppers, if ambient relative humidity exceeds 60%.

Bio-Based Carbon Content Is Measured Against a Radiocarbon Reference

The 50% bio-based designation refers to the biogenic carbon fraction, not to the total renewable mass of the formulation. ASTM D6866 Method B measures the 14C signal of the sample relative to a modern oxalic acid reference, and the result is expressed as percent modern carbon. In a PLA/starch compound, both major polymer phases are derived from renewable feedstocks; a 50% bio-based carbon value therefore reflects the proportion of renewable to total carbon in the compound, including any fossil-carbon additives, plasticizers, or compatibilizers. The value does not quantify starch content directly, and it does not certify industrial compostability. Purchasers should request the supplier’s report that states the method, sample preparation, and reference standard used for the specific lot.

Because accelerator mass spectrometry distinguishes modern and fossil carbon, it is more robust than simple mass balance in detecting fossil-derived processing aids. However, mineral fillers that contain no carbon can dilute the organic carbon fraction and alter the calculated bio-based carbon in the formulation; this is a reporting and material-design issue, not a method failure.

Does the 25S Grade Satisfy Industrial Composting Scheme Requirements?

Compostability is a multi-tiered property requiring chemical characterization, aerobic biodegradation, disintegration, and ecotoxicity assessment. Under EN 13432:2000, the material must biodegrade by at least 90% relative to cellulose within 180 days under controlled aerobic composting as measured by ISO 14855-1, disintegrate by at least 90% through a 2 mm sieve after 12 weeks in a pilot-scale test, and show no adverse effects on compost quality. ASTM D6400 sets parallel criteria for North American industrial composting, using methods such as ASTM D5338 for biodegradation and ISO 16929 for disintegration. The starch phase in Bionolle Starcla™ 25S is readily accessible to microbial enzymes, so early-stage hydrolysis can be faster than unmodified PLA. Full mineralization is still controlled by the PLA phase, which requires hydrolysis and microbial attack under industrial composting conditions; home composting is not guaranteed.

Compliance with EN 13432:2000 or ASTM D6400 is a formulation-specific certification. The presence of a 50% bio-based PLA/starch designation does not by itself confer certification. The downstream converter must verify that the final article’s additives, printing inks, and conversion aids do not compromise the certified composition.

In sheet extrusion and thermoforming, the pellet is dried to the same residual moisture criterion and processed on a single-screw extruder with a barrier screw and gear pump. Die temperatures are held at the lower end of the PLA processing range to limit starch degradation, and polished rolls at 30 °C to 50 °C are used to control sheet gloss and thickness. Thermoformed trays are possible, but the sheet must be stored in moisture-barrier packaging because the starch phase increases equilibrium moisture uptake. Regrind levels above 30% are not recommended without verified retention of melt strength and impact properties, because repeated heat history degrades the starch interface. The compound’s operational boundary is industrial composting; it is not intended for marine or freshwater degradation, and it is not a soil biodegradable grade unless separately certified under ISO 17556.