When Wood Flour Introduces Water and Hydroxyl Groups into a PLA Melt

In rigid biocomposite injection moulding, wood flour or hardwood fibre at 20-40 wt% loading reduces the melt-phase cohesion of PLA because the lignocellulosic surface exposes a high density of hydroxyl groups that do not wet the aliphatic polyester backbone. The addition of Compoline G-PLA 01, a maleic anhydride grafted polylactic acid compatibilizer, at 2.0-5.0 wt% of total compound introduces cyclic anhydride functionality to the PLA phase. During compounding, the anhydride ring opens and reacts with surface hydroxyl groups to form ester linkages across the fibre-matrix boundary. This reaction competes directly with hydrolysis: wood flour must be pre-dried in a desiccant dryer at 90°C for 4 h to below 0.5 wt% moisture, and PLA must be dried at 80°C for 4 h to below 250 ppm water, because residual water hydrolyzes the anhydride to a dicarboxylic acid before fibre surface reaction can proceed. Amine-based lubricants or primary amines should be excluded from the formulation, as they consume anhydride groups prematurely and reduce the available graft function for interfacial coupling. On production lines, batch-to-batch variation in wood flour moisture above 0.7 wt% is observed as screw torque spikes and melt pressure variability at constant screw speed.

Compounding is performed on a co-rotating twin-screw extruder with L/D 40:1 and screw diameter of at least 25 mm, using a side feeder at barrel 7 and a vacuum vent at barrel 9 operating at -0.08 MPa. The barrel profile is set from 165°C at the feed throat to 185°C at the die, with screw speed held between 250 rpm and 350 rpm. The compatibilizer is pre-blended with dried PLA pellets in a low-shear tumble mixer before gravimetric feeding, and wood flour is side-fed after the polymer melt seal to limit shear heating and fibre attrition. Barrel temperatures above 200°C are avoided because PLA depolymerization accelerates, releasing lactide and causing viscosity shifts that destabilize the die pressure profile. Injection moulding is conducted with melt temperature 180-195°C, mould temperature 25-40°C, injection velocity 50-80 mm/s, hold pressure 60-80 MPa, and back pressure 0.5-1.0 MPa. The low mould temperature range is selected to shorten cycle time while reducing PLA secondary crystallization after demoulding; the compatibilized compound is conditioned at 23°C and 50% RH according to ISO 291 before mechanical evaluation.

Mechanical evaluation is anchored to ISO 527-2 for tensile properties, ISO 178 for flexural properties, ISO 179-1/1eU for unnotched Charpy impact, and ISO 75-2 method B for heat deflection temperature at 0.45 MPa. Comparative control samples are moulded at identical fibre loading without the anhydride compatibilizer. Interfacial adhesion is assessed by scanning electron microscopy of cryofractured surfaces, where fibre pull-out length serves as the qualitative indicator; coupled formulations show shorter pull-out lengths and matrix-covered fibre surfaces. Food contact suitability is not automatically granted by the presence of the compatibilizer; the final formulation must be tested under Commission Regulation (EU) No 10/2011 for overall migration and specific migration of any residual maleic anhydride or its reaction products. Industrial compostability certification for rigid biocomposite packaging requires compliance with EN 13432:2000 or ASTM D6400-23, with disintegration, biodegradation, and ecotoxicity evaluated on the finished article rather than on the raw compatibilizer.

Blown film lines running PLA/starch compounds encounter phase separation and bubble instability when starch domains coalesce at the die lip. Starch loadings between 20 wt% and 35 wt% are common in compostable bag formulations, but unmodified starch has significantly higher hydrophilic character and lower melt strength than the PLA continuous phase. The use of Compoline G-PLA 01 at 3.0-7.0 wt% of total compound provides reactive anhydride groups that form ester linkages with starch surface hydroxyls. The addition is preferably made as a separately compounded masterbatch rather than as direct powder addition to the starch stream, because direct addition in the presence of starch-bound water can trigger premature anhydride hydrolysis. Starch must be pre-dried to below 0.5 wt% moisture, and plasticizers such as glycerol or sorbitol are introduced separately to avoid creating a low-viscosity starch phase that migrates to the film surface.

Film blowing is typically run on a single-screw extruder with a barrier screw of L/D 30:1, die gap 0.8-1.2 mm, die temperature 165-180°C, blow-up ratio 2.5:1 to 3.5:1, and frost line height 2-4 die diameters. The melt temperature must not exceed 190°C because the starch phase browns and PLA degrades into lactide. In the absence of the compatibilizer, bubble instability appears as gauge bands that are measurable with ISO 4593 thickness scanning; the gauge tolerance for converter yield is generally maintained within ±10% of nominal thickness. The addition of Compoline G-PLA 01 modifies the elongational viscosity of the melt, and this effect is monitored indirectly through bubble diameter stability and frost line consistency. Film tensile properties are measured according to ISO 527-3:2018, and tear resistance is measured according to ISO 6383-2:1983. Final gauge range for compostable shopping bags is typically 20-50 µm, with thicker gauges required for organic waste sacks due to puncture demands.

Biodegradation and compostability claims for PLA/starch films are not inherent to the raw compatibilizer and must be proven on the final compound. The relevant framework is EN 13432:2000 for packaging recoverable through composting and biodegradation, or ASTM D6400-23 for the North American market. Ultimate aerobic biodegradation is measured under ISO 14855-1:2012 by carbon dioxide evolution, while disintegration is assessed under ISO 20200:2015 at laboratory scale. Because the compatibilizer is a PLA-based graft, it generally participates in the polyester hydrolysis and enzymatic attack that degrades PLA; however, ecotoxicity testing of the final compound is mandatory because anhydride reaction by-products may influence plant growth response. Production runs at ambient relative humidity above 60% require closed resin-hopper drying with a dew point of -40°C or lower to prevent moisture regain between the dryer and the extruder throat.

What Limits PBAT Domain Elongation in PLA Cast Film Without Reactive Coupling?

When PLA is blended with 20-40 wt% PBAT on a cast film line, the PBAT phase tends to form large spherical or ellipsoidal domains that reduce tear propagation resistance and increase haze. The low interfacial adhesion between PLA and PBAT becomes visible during film stretching as cavitation at the domain boundaries, followed by premature necking and web breaks. Compoline G-PLA 01 is introduced at 2.0-6.0 wt% of total compound to provide reactive anhydride functionality that can engage hydroxyl species generated by partial hydrolysis at PBAT chain ends and at the PLA/PBAT interface. This reduces interfacial tension and stabilizes PBAT domain elongation under the high extensional field of the cast film machine-direction orientation unit. The compatibilizer is pre-blended with PLA before PBAT addition because the anhydride function competes with moisture and must not be exposed to the higher moisture content of PBAT pellets. Both resins are dried to below 250 ppm moisture at 80°C for 4 h prior to extrusion.

Compounding is performed on a co-rotating twin-screw extruder with L/D 36:1, configured with distributive mixing elements rather than aggressive kneading blocks to limit shear heating of PBAT. The melt temperature is controlled between 160°C and 185°C, and the die temperature is set at 180°C. The compound is cast onto a chill roll maintained at 30-50°C to suppress PBAT re-crystallization and control film blocking. Cast film thickness for flexible packaging is typically 15-50 µm, depending on the required oxygen and water vapour transmission properties. Film tensile properties are measured according to ISO 527-3:2018, tear resistance according to ISO 6383-2:1983, oxygen transmission rate according to ASTM D3985-17, and water vapour transmission rate according to ISO 15106-2. Morphological verification is performed by scanning electron microscopy on cryofractured film cross sections; compatible formulations show PBAT domains elongated in the machine direction with lengths 2-4 times their transverse diameter, whereas uncoupled controls exhibit near-spherical domains with smooth boundary surfaces.

The processing window for PLA/PBAT cast film is constrained by PBAT thermal degradation above 200°C and PLA melt strength loss caused by lactide formation. At PBAT contents above 40 wt%, phase inversion occurs and the compatibilizer dosage must be re-optimized because Compoline G-PLA 01 distributes predominantly in the PLA phase. Published data for this specific configuration is limited at production scale, and film converters should run statistically designed experiments with a minimum of three screw speed settings and three compatibilizer levels to map the tear-strength response surface. Food contact packaging based on PLA/PBAT cast film requires compliance with Commission Regulation (EU) No 10/2011 for overall migration from the final article; recyclability under mechanical recycling streams is evaluated separately because the presence of PBAT affects the sorting behaviour of PLA packaging.

Injection Moulding of Rice Hull/PLA Compounds at Low Melt Temperatures

Rice hull grades designated for polymer compounding contain approximately 15-20 wt% silica, which accelerates screw flight wear in addition to creating low-surface-energy filler interfaces that do not bond cleanly to PLA. The addition of Compoline G-PLA 01 at 2.0-5.0 wt% of total compound improves interfacial adhesion at the rice hull surface, but the silica fraction is primarily surface hydroxylated and less reactive than lignocellulosic fibre; therefore, the compatibilizer effect is concentrated on the organic portion of the filler. Rice hull must be pre-dried at 100°C for 3 h to below 0.5 wt% moisture, and PLA must be dried at 80°C for 4 h. The compound is run on a co-rotating twin-screw extruder with L/D 36:1, using bi-metallic screw and barrel construction with screw flight hardness above 60 HRC to resist abrasive wear from silica.

The barrel profile is held between 170°C and 185°C, with screw speed 200-300 rpm and a side feeder positioned after the PLA melt seal. A melt screen pack of 60/100/60 mesh is installed before the die to remove char particles and lignaceous agglomerates. Injection moulding is conducted with melt temperature 175-190°C, mould temperature 30-50°C, and clamp force selected to maintain a cavity pressure above 35 MPa. The mould surface temperature is kept higher than for unfilled PLA to reduce shrinkage at the filler-matrix interface and to limit sink marks around bosses and ribs. After moulding, parts are conditioned according to ISO 291 at 23°C and 50% RH for 48 h before mechanical testing. Tensile properties are measured under ISO 527-2, flexural properties under ISO 178, Charpy impact under ISO 179-1/1eA, and heat deflection temperature under ISO 75-2 method B at 0.45 MPa.

The operational boundary for rice hull compounds is the filler moisture sensitivity after drying: if the dried rice hull is exposed to ambient air above 55% RH for more than 30 min, moisture regain raises steam pressure at the die and creates surface voids in extrudate. The compound is therefore best processed with a hopper purge of dry air or nitrogen. Screw wear on production-scale machines is measurable after continuous campaigns; maintenance intervals are determined by monitoring melt temperature rise, motor load increase, and fill pressure drop at the side feeder. Because rice hull/PLA compounds are not inherently transparent and may contain trace organic volatiles, food contact use is subject to the same migration testing under Commission Regulation (EU) No 10/2011 that applies to other PLA-based reinforced articles.

In recovering post-industrial PLA trim that carries PBS or PBAT contamination, the recycled stream develops sharp interfacial boundaries that reduce weld-line strength and lower impact failure energy. The contaminants originate from multilayer film structures and are difficult to remove completely by standard dry sorting. Compoline G-PLA 01 is added at 1.5-4.0 wt% during re-extrusion to restore some interfacial compatibility between freshly dried PLA regrind and the residual polyester fraction. The recycled PLA regrind must be pre-dried at 80°C for 6-8 h to below 150 ppm water, because regrind has higher specific surface area and absorbs moisture more rapidly than virgin pellets. The target moisture level is lower than for virgin PLA due to the additional hydrolysis risk from repeated heat histories.

Re-extrusion is performed on a co-rotating twin-screw extruder with L/D 40:1 and vacuum venting at -0.08 MPa. The melt temperature is maintained at 175-190°C, and screw speed is set between 250 rpm and 350 rpm. The compatibilizer does not act as a chain extender; if the recycled PLA has undergone severe hydrolytic degradation, melt flow rate measured according to ISO 1133-1:2022 at 190°C/2.16 kg will remain high, and the addition of Compoline G-PLA 01 alone will not restore molecular weight. In such cases, the regrind fraction must be limited or blended with virgin PLA to keep the MFR within the processing range of the downstream moulding equipment. Mechanical properties are measured according to ISO 527-2 for tensile strength and ISO 179-1/1eU for Charpy impact. Recycled content claims are documented according to ISO 14021:2016 if the compound is marketed as containing post-industrial material.

The presence of printing inks, adhesives, or paper labels in the recovered trim introduces volatile degradation products and char. Melt filtration with a screen pack of 80/120/120 mesh is specified to protect the die and downstream tooling. The compatibilizer should be introduced only after the regrind has passed the vacuum venting zone, because early addition can cause anhydride hydrolysis at the wet regrind surface and reduce coupling efficiency. Operational limits must be set for contaminant concentration: above 5 wt% residual PBS or PBAT, the final compound may require higher compatibilizer dosage, but phase inversion and stiffness loss should be evaluated case by case using dynamic mechanical analysis.

Melt Elasticity Requirements in Thermoformed PLA/Jute Fibre Sheet

Thermoforming grade PLA/jute fibre sheet requires melt elasticity values that prevent sag and excessive thinning during the heating stage, yet the incorporation of bast fibres often depresses extensional viscosity and creates zones of premature local yielding. Compoline G-PLA 01 is incorporated at 2.0-5.0 wt% of total compound to couple the jute fibre surface to the PLA matrix and to reduce fibre-rich regions that initiate edge tear during plug-assisted forming. The jute fibre is pre-dried to below 0.5 wt% moisture at 95°C for 3 h, and PLA is dried at 80°C for 4 h. Sheet is extruded through a flat die onto a polished three-roll stack, with melt temperature 175-190°C and roll temperatures 40-60°C. Target sheet thickness is 0.8-2.0 mm, with thickness variation controlled within ±5% to maintain uniform heating and forming behaviour.

Thermoforming is conducted with sheet surface temperature 90-110°C, forming pressure 0.4-0.7 MPa, and plug assist speed selected to avoid tearing at the fibre-rich web. The forming window is narrow because PLA crystallizes slowly but may crystallize prematurely in heated zones if the jute fibre acts as a nucleating agent; this is monitored by measuring sheet surface temperature with an infrared pyrometer and by observing part whitening after forming. Mechanical properties of the formed article are measured according to ISO 527-2 for tensile strength and elongation, ISO 178 for flexural modulus, and ISO 6603-2 for puncture impact behaviour. Heat resistance under load is measured with ISO 75-2 method B at 0.45 MPa. The compatibilized sheet is conditioned at 23°C and 50% RH for 48 h before testing, because moisture content above 0.3 wt% in the sheet causes surface bubbles and delamination during heating.

Food contact trays thermoformed from PLA/jute fibre sheet must comply with Commission Regulation (EU) No 10/2011 for overall migration and with specific migration limits for any reaction by-products of maleic anhydride. Industrial compostability is verified under EN 13432:2000 or ASTM D6400-23 on the finished tray geometry, because wall thickness influences disintegration time. The jute fibre source should be selected for low pesticide residue and low ash content, as ash constituents may alter the hydrolysis rate of PLA during composting and may require additional ecotoxicity testing. Processing above 200°C must be avoided because jute fibre thermally degrades and PLA chain scission produces lactide that condenses on the roll stack, causing surface defects on the sheet.