Hisun PLA by Zhejiang Hisun Biomaterials: Biobased PLA with BPI & EN13432 Certifications
Zhejiang Hisun Biomaterials Co., Ltd. produces polylactide resins from starch-, sugar-, or dextrose-derived lactic acid through fermentation, lactide formation, and ring-opening polymerisation. The resulting polylactide is a linear aliphatic polyester whose repeating unit is lactic acid. The material is supplied in pellet form and, for grades certified under the Biodegradable Products Institute (BPI) certification and EN 13432:2000, the polymer is formulated so that aerobic industrial composting achieves mineralisation, disintegration, and ecotoxicity endpoints. The certification status is grade-specific, not universal across all Hisun PLA grades. Typical chain-structure control parameters for crystalline packaging grades include a D-lactide fraction below 2 mol% and residual lactide below 0.3 wt%. The renewable carbon fraction of polylactide is generally above 95% when measured by ASTM D6866 Method B because the organic carbon source is microbial sugar fermentation. Melt flow indices reported under ISO 1133-1:2022 at 190°C and 2.16 kg depend on molecular weight and additive package; typical moulding grades fall within 6–30 g/10 min.
Compostability Certification Matrix and Test Method Alignment
European and North American compostability certification schemes are separate but share numerical endpoints. EN 13432:2000 is the European standard for packaging recoverable through composting and biodegradation, and is used to demonstrate compliance with relevant EU packaging provisions. BPI certification under ASTM D6400 provides evidence for label claims in North America. Both schemes require aerobic biodegradation measured as carbon conversion to CO₂ under controlled composting conditions, disintegration after a defined time in thermophilic compost, and absence of toxic effects on plant growth. The standards are not interchangeable for labelling; a BPI certificate does not automatically constitute European conformity and vice versa. Technical data must be reviewed against the specific certificate number and the current listing for the Hisun grade in question. The table below summarises the main test architecture.
| Requirement | EN 13432:2000 test method | ASTM D6400/BPI test method | Endpoint |
|---|---|---|---|
| Chemical characterisation | EN 13432:2000 annex provisions for heavy metals and volatile solids | ASTM D6400 referenced heavy-metal limits | Metals below regulatory thresholds; no prohibited substances |
| Aerobic biodegradation | ISO 14855-1:2012 or ISO 14855-2:2018 | ASTM D5338-15 | ≥90% organic carbon to CO₂ within 180 days |
| Disintegration | ISO 16929:2021 or ISO 20200:2015 | ISO 16929:2021 or ISO 20200:2015 as referenced by BPI | ≤10% dry mass remaining on 2 mm sieve after 12 weeks |
| Ecotoxicity | EN 13432:2000 Annex E; OECD 208 | OECD 208 as referenced by ASTM D6400 | No significant germination or biomass inhibition |
| Compost quality | EN 13432:2000 Annex E chemical parameters | No separate requirement beyond disintegration and ecotoxicity | Compost supports plant growth |
Published data for the exact Hisun grade submitted in a given certificate is limited to the certificate holder and not reproduced generically. Users should request the current BPI certificate circular and EU declaration of conformity from the resin supplier. Additives, masterbatches, printing inks, and adhesives added downstream may change compostability test outcomes; a final article can be certified only after testing the full formulation. In practice, converters must also control regrind content and processing history because the biodegradation test specimen is melt-processed, and inappropriate drying can alter the carboxylic acid end-group profile before the sample reaches the compost reactor.
Thermal Degradation Pathways During Residence-Time Extension in PLA Melt Processing
Molten polylactide degrades through hydrolytic chain scission, intramolecular transesterification, oxidative degradation, and thermal depolymerisation to lactide. The hydrolytic pathway is the most sensitive to residual moisture and autocatalytic because hydrolysis produces new terminal carboxyl groups that accelerate further ester cleavage. At pellet moisture above 250 ppm, injection moulding at 205°C commonly produces silver streaks, lowered melt viscosity, and reduced notched impact strength. Pre-drying is therefore mandatory. A desiccant wheel dryer with a dew point of -40°C or below, air flow near 0.5 m³/h per kg/h, and a bed temperature of 80°C for 4 h reduces moisture below 250 ppm. For warehouse relative humidity above 60%, exposed pellets can rehydrate within 10–15 min; dried resin should be conveyed under dry air to the feed throat. Vacuum drying at 80°C for 4 h is used for small hoppers and filament extrusion. Moisture analysis with a direct-loss instrument or coulometric Karl Fischer titration at 160°C can determine residual moisture; the latter is preferred for low-level quantification. The melt residence time is limited because thermal degradation occurs even in the absence of moisture. At melt temperatures 185–210°C, total residence time from first melting to die exit should remain below 8–12 min. In a twin-screw extruder with L/D 40:1 and modular screw elements, hold-up in kneading blocks and reverse elements can extend local residence times; screw design should avoid stagnant zones near the vent. Shutdowns above 230°C cause rapid visbreaking, yellowing, and accumulation of lactide oligomers on die lips. For production-scale compounding, a side feeder is used for temperature-sensitive or shear-sensitive additives.
| Platform | Melt temperature | Drying condition | Tooling or downstream condition | Critical control point |
|---|---|---|---|---|
| Injection moulding | 185–205°C | 80°C for 4 h, residual moisture ≤250 ppm | Mould 25–110°C; injection pressure 70–120 MPa | Residence time 12 min |
| Sheet extrusion | 195–210°C | 80°C for 4–6 h | Chill roll 15–40°C; die gap 0.5–1.0 mm | Surface temperature for forming |
| Blown film | 175–195°C | 80°C for 4 h | Die gap 0.6–1.2 mm; BUR 2:1–4:1 | Bubble stability |
| Cast film | 190–215°C | 80°C for 4 h | Quench roll 15–30°C; die gap 0.3–0.8 mm | Web tension and quench |
| 3D printing filament | 180–200°C | 60–80°C for 4 h | Water quench 45–60°C; diameter 1.75 ± 0.05 mm or 2.85 ± 0.10 mm | Diameter variation and moisture |
For injection moulding of Hisun PLA in BPI/EN13432-certified packaging, mould temperature is a dominant variable for crystallinity and post-mould stability. A cold mould at 25–40°C quenches the part to an amorphous state with high transparency but low heat resistance; a hot mould at 100–110°C with a hold time of 30–90 s increases crystallinity and produces heat-deflection temperatures above 90°C under 0.45 MPa. The change is not free: mould shrinkage increases, and the part can become more brittle. Mould temperature control requires water or oil thermolators with uniformity of ±1°C across the cavity surface. The screw should provide low compression and a check ring; nozzle shut-off valves prevent drool because PLA has low melt viscosity at high shear. Hot runner systems need open-flow channels without dead spots; stagnation creates acetaldehyde and lactic acid, which appear as splay and odour. Typical barrel temperature profile is 185–205°C, injection pressure 70–120 MPa, holding pressure 40–70 MPa, back pressure 1–3 MPa, and screw speed 50–150 min⁻¹. If regrind is used, batch-to-batch variation in molecular weight shifts melt flow index and should be controlled by sieve analysis and mix ratio. The use of external mould release agents should be avoided in certified articles because migrating aliphatic hydrocarbons can interfere with compost ecotoxicity testing.
What Constrains Continuous Service Temperature in Annealed Compostable Rigid Packaging?
Continuous service temperature in annealed compostable rigid packaging is not determined by the glass transition alone. The amorphous PLA matrix exhibits a glass transition at 55–60°C as measured by differential scanning calorimetry at 10 K/min. An unannealed injection-moulded article under load may distort above 50°C. Annealing or mould-temperature crystallisation creates α-form crystallites whose melting endotherm occurs from 150°C to 175°C. At crystallinities of 25–35%, the heat-deflection temperature under 0.45 MPa by ISO 75-2/B or ASTM D648-18 can exceed 100°C. The cold-crystallisation exotherm of PLA typically peaks around 105–125°C; this means thermoforming and annealing operations operate within a narrow surface-temperature window. If the sheet surface reaches 130°C before forming is complete, premature crystallisation causes haze, loss of drawability, and incomplete cavity fill. If the mould remains below 90°C, the part stays largely amorphous and heat resistance remains low. Nucleating agents such as talc or PDLA-based stereocomplex agents at 1–2 wt% reduce spherulite size and increase crystallisation rate, but at loadings above 5 wt%, the melt viscosity and die pressure increase and film or sheet optical clarity degrades. The operational boundary is therefore a processing window of approximately ±5°C in surface temperature for crystalline thermoforming. Long-term service above 60°C in wet conditions remains limited by hydrolytic attack; PLA can absorb up to 0.5 wt% water at 50% RH, and at elevated temperature this moisture hydrolyses the ester backbone and reduces molecular weight over months. Published data for the exact service lifetime of a specific Hisun PLA grade in a particular hot-fill application is limited; prolonged exposure trials are required.
In sheet extrusion, a twin-screw or single-screw extruder with L/D 30:1 or greater supplies a coat-hanger die with polished lip lands. Melt temperature at the die is maintained at 195–210°C. The roll stack is set to 15–40°C for amorphous sheet; the lower range improves gloss and reduces blocking, while higher roll temperatures initiate crystallinity. Die gap is usually 0.5–1.0 mm for thermoforming sheet. Sheet thickness is controlled by roll speed and die bolt adjustment to ±0.05 mm. Thermoforming uses zoned infrared ovens with pyrometer feedback because the forming temperature window is narrow; the sheet surface is heated to 90–110°C. Plug-assist temperature and oven residence time must be balanced so that the sheet does not exceed the cold-crystallisation onset before forming. Mould temperature for in-mould crystallisation is held at 95–105°C. Wet sheet or high residual moisture produces blisters; sheet moisture should be kept below 250 ppm. Regrind levels up to 20–30 wt% are used on some lines, but every heat cycle reduces molecular weight and raises melt flow index; at 40 wt% regrind and above, oligomer deposition on die lips increases and sheet elongation at break may fall below 3%, limiting deep-draw ratios. Acetaldehyde and lactide volatiles are exhausted at the die and roll stack; local exhaust must maintain workplace exposure below applicable limits. A failure mode observed on production-scale thermoforming lines is non-uniform sheet temperature across the width caused by aging IR emitters; this produces differential crystallinity and warpage in trimmed parts.
When Blown Film Production Encounters Low Melt Strength at 190°C
At 190°C, unmodified PLA has low melt strength compared with LLDPE, and this constrains bubble stability in blown film. The practical process window is narrow: melt temperature 175–195°C, die gap 0.6–1.2 mm, blow-up ratio 2.0:1–4.0:1, and frostline height 250–500 mm. Below 175°C, cold crystallisation near the die lip can produce frost lines and die deposits; above 200°C, viscosity loss causes bubble sag and gauge variation. Extruders for PLA blown film typically use a single screw with L/D 24:1–30:1, barrier screw design, and grooved feed section. A gear pump between extruder and die is beneficial because PLA is shear-sensitive; pump suction pressure and die pressure are controlled to ±0.5 bar to avoid gauge bands. Melt-pumping sections without a gear pump exhibit larger pressure swings at high screw speeds. The addition of 5–15 wt% PBAT or PBS improves bubble stability and tear strength, but at 20 wt%, tensile modulus drops and the film loses stiffness. Cast film is an alternative for high throughput; cast lines use a flat die with 0.3–0.8 mm die gap and quench roll at 15–30°C. The film remains largely amorphous and is limited to service below 50°C. Water vapour transmission of a 25 µm PLA film at 38°C and 90% RH by ASTM F1249 is commonly above 300 g/m²/day, which limits high-moisture barrier applications. Barrier coating or lamination may be required. Blown film of certified PLA for compostable bags must be shelf-life tested because moisture uptake during storage can block the roll and degrade mechanical properties over time.
Impact modification of compostable PLA compounds is performed on twin-screw compounding lines with L/D 40:1 and side feeders for flexible polyesters. The dispersed-phase morphology depends on the shear rate and viscosity ratio. A dispersed modifier domain below 1 µm is generally associated with toughening; larger domains indicate phase coarsening and produce a plateau or decrease in impact strength. Notched impact strength measured by ISO 179-1/1eA can increase from 2.5 kJ/m² for unmodified PLA to 8 kJ/m² when a suitable biodegradable polyester modifier is present at 15 wt%; tensile modulus measured by ASTM D638-14 may fall by 25–35% over the same addition range. Above 20 wt%, phase inversion and coalescence become likely, and further addition does not guarantee improved impact behaviour. Plasticisers based on citrate esters at 3–10 wt% shift the glass transition temperature downward by 10–25°C as measured by dynamic mechanical analysis at 1 Hz; above 12 wt%, surface exudation and blocking during storage are known risks. Migration of low-molecular-weight additives in the polymer matrix follows Fickian diffusion; the diffusion coefficient increases with temperature and with decreasing plasticiser molecular weight. Additives with primary or secondary amines should be avoided in PLA because aminolysis of the ester backbone accelerates molecular weight reduction during melt processing and during service. Any additive package added to a certified Hisun PLA grade must be evaluated for its effect on compostability endpoints; powders that contain non-compostable inorganic residues can alter the heavy-metal profile of the final article and must be below the thresholds in EN 13432:2000 and ASTM D6400. The final compound, not the unfilled resin, must satisfy disintegration requirements because dispersed phases can reduce particle fragmentation and leave residues on the 2 mm sieve.
