Polyvinyl Alcohol (PVA) for Biodegradable Agricultural Mulch

A twin-screw extruder with a 40:1 L/D ratio and counter-rotating intermeshing screws is typically configured for the compounding of partially hydrolysed PVA (87–89 mol% hydrolysis, DP 1700–2000) with glycerol and water as temporary co-plasticisers. The base formula consists of 70 wt% PVA, 20 wt% glycerol, and 10 wt% deionised water, pre-mixed in a high-speed mixer at 40 °C for 20 min to achieve homogeneous absorption. The extruder barrel temperatures are set from 140 °C in the feed zone to 175 °C at the die, with a melt temperature maintained below 190 °C to prevent thermal decomposition. A vacuum vent in the devolatilisation zone removes excess moisture, targeting a final water content of 2–4 wt% in the pelletised resin. The compounded pellets are fed to a single-screw blown film line equipped with a grooved feed section and a spiral mandrel die of diameter 150 mm with a die gap of 0.8–1.2 mm. Die temperatures are held at 175–180 °C, and a blow-up ratio of 2.5–3.0 is applied. Nitrogen purging of the die lip zone is required to inhibit oxidative degradation. The resulting film, with a thickness of 10–15 µm, exhibits a tensile strength of 30–38 MPa (ASTM D882) and an elongation at break exceeding 250%. As a finished mulch, this transparent film raises soil temperature by 3–6 °C versus bare ground and is used for early-season vegetable production such as lettuce and cucumber. Complete biodegradation in soil must comply with EN 17033:2018, specifically a ≥90% absolute biodegradation within 24 months per ISO 17556, and the ecotoxicity tests per OECD 208 must show plant emergence and biomass not less than 90% of the control soil. Operational boundaries: pre-drying of PVA granules to <0.5% moisture before compounding is mandatory at ambient relative humidity above 60%, and residence time in the extruder should not exceed 120 seconds to avoid molecular weight reduction that compromises film strength.

How Does Starch Addition Modify the Biodegradation Rate and Mechanical Compliance of PVA Mulch Films?

Incorporating thermoplastic starch (TPS) derived from maize or potato into PVA matrices reduces raw material cost and accelerates soil biodegradation, but introduces rheological and mechanical trade-offs. TPS is prepared by mixing native starch (30 wt% of the final blend), glycerol (12 wt% on starch), and water in a co-rotating twin-screw extruder at 70–80 °C until a gelatinised melt is obtained. This TPS is then blended with partially hydrolysed PVA (60 wt% total) and a reactive compatibiliser—citric acid at 0.5 wt%—to suppress phase separation. The final compound is pelletised and subsequently processed on a conventional blown film line, but with a reduced blow-up ratio of 1.8–2.2 and a lower die temperature of 165–175 °C to cope with the reduced melt strength. The resulting film thickness is typically 12–20 µm. For use as a short-term mulch in maize or potato cultivation, the film retains sufficient mechanical integrity for the first 30–40 days, after which biodegradation accelerates. The table below presents property evolution with increasing starch content in PVA-based films tested according to ASTM D882 and ISO 17556 (soil burial at 25 °C).

Compliance with EN 17033 demands that any blend still achieves the 90% biodegradation threshold within the 24-month window; TPS accelerates the process, but if starch exceeds 35 wt%, the film loses the minimum 15 MPa tensile strength required for mechanical laying on many standard mulch layers. Additionally, starch-rich films are prone to microbial attack during storage at relative humidity above 70%, necessitating sealed packaging. Ecotoxicity assessment per OECD 207 (earthworm acute toxicity) must also account for residual lactic acid or other metabolites from starch fermentation in soil, though published data for this specific configuration is limited. The finished mulch is typically deployed for organic vegetable production where regulatory acceptance for soil-biodegradable inputs under EU Regulation (EC) No 2019/1009 (Fertilising Products Regulation) is being mainstreamed.

Pigmented Photodegradable PVA Films for Weed Suppression and Controlled Decay

Carbon black-loaded PVA films combine the physical weed-blocking function of opaque mulch with a predetermined photodegradation rate triggered by ultraviolet radiation after a service period. The masterbatch is prepared by dispersing furnace-grade carbon black (primary particle size <50 nm) at a loading of 3–5 wt% in PVA via a twin-screw compounding step, using a side feeder to avoid agglomerates that can puncture the film bubble during blown film extrusion. An iron carboxylate-based photodegradation catalyst (0.2–0.4 wt%) is added to initiate Norrish-type chain scission upon UV exposure. The final film, with a gauge of 15–20 µm, is blown at a die temperature of 170–180 °C and requires a 200-mesh screen pack to filter carbon agglomerates; die head pressure typically reaches 18–25 MPa. As a finished agricultural mulch, this black film achieves a photosynthetically active radiation (PAR) transmittance below 5%, effectively suppressing Amaranthus and Echinochloa weeds in strawberry and tobacco fields. Mechanical properties are comparable to unpigmented PVA film, with MD tensile strength at 32–36 MPa and elongation >200% (ASTM D882). The photodegradation pathway must be validated under EN 17033 to ensure that UV-induced fragmentation does not leave recalcitrant microplastic residues. Instead, the combined action of sunlight and soil microbiota must convert the fragments to CO₂ and water within 24 months. Eco-toxicity screening per OECD 208 with powdered film residues at 1% w/w soil concentration has been a mandatory part of certification dossiers. Processors note that melt viscosity stability during film blowing is sensitive to the carbon black’s oil absorption number (typically 60–100 mL/100g); a high-structure black reduces extrudate swell, leading to unstable bubble formation. Furthermore, the photodegradation catalyst must not contain cobalt carboxylates exceeding 100 ppm in the final article if marketed in the EU under REACH Annex XVII restrictions on CMR substances.

For ultra-thin biodegradable films serving as temporary moisture barriers in drip irrigation agriculture within arid regions, PVA grades with a higher degree of polymerisation (DP 2400–2600) and 98–99 mol% hydrolysis are dissolved in demineralised water to a concentration of 12–15 wt% solids. The solution is heated to 90 °C and agitated until homogeneous, then a non-ionic surfactant (0.1 wt% of solution) and a nano-clay dispersion (montmorillonite, 1.0–1.5 wt% on PVA solids) are blended in using a high-shear rotor-stator mixer. The casting solution is de-aerated under vacuum and applied onto a chrome-plated chill roll via a slot die, with a wet film thickness calibrated to yield a dry film of 6–8 µm. The temperature of the chill roll is maintained at 15–20 °C, followed by multi-zone drying at air temperatures of 60, 90, and 110 °C to prevent skin formation. The cast film is then heat-treated at 120 °C for 3 min to induce partial crystallisation, raising water dissolution resistance without compromising ultimate biodegradability. The end product, supplied in rolls, is applied as a mulch for melon and cucumber crops under drip lines; the thin film maintains soil moisture for the first 30–45 days and then gradually dissolves and biodegrades through combined hydrolytic and microbial attack. Water-soluble fractions must comply with OECD 301B ready biodegradability criteria (>60% within 28 days) to avert groundwater contamination concerns. Bursting strength is lower than blown film equivalents—approximately 18–22 MPa tensile—but the elongation at break surpasses 300%, allowing the film to be laid mechanically without tearing. Restrictions: the storage of the film requires sealed packaging with a silica gel desiccant to maintain moisture content below 8%, otherwise pre-hydration can lead to premature tackiness and blocking on the roll.

When Mechanical Strength Retention Must Exceed 45 Days in High-UV Environments, Coextruded PVA/PBAT Multilayers Are Formulated

A three-layer coextrusion structure—PBAT skin layers and a PVA core layer—addresses the inherent limitation of PVA films losing tensile strength rapidly in regions with intense solar radiation and extended cropping cycles, such as cotton cultivation in Xinjiang or the Texas High Plains. The outer PBAT layers (poly(butylene adipate-co-terephthalate)) with a melt flow index of 3–5 g/10min (190 °C/2.16 kg, ISO 1133-1) incorporate a hindered amine light stabiliser (HALS) at 0.3 wt% and a UV absorber at 0.2 wt%. The core layer comprises PVA (87–89 mol% hydrolysis, DP 1700) plasticised with a blend of glycerol and sorbitol (25 wt% total on PVA) to lower the melt processing temperature to 175–185 °C. Each layer is fed by a separate single-screw extruder to a spiral mandrel die with three concentric channels. Die temperature is set to 180 °C for the core and 155 °C for the skins. The total film thickness is 20–25 µm, with a layer thickness ratio of 20/60/20 (skin/core/skin). In field trials, these trilayer films retained 70% of their initial machine-direction elongation after 60 days of exposure, versus 15% retention for monolayer PVA controls. The table below compares key properties.

Full compliance with EN 17033 is achievable if the entire film structure meets the 90% biodegradation threshold in soil within 24 months. However, the PBAT skins degrade more slowly than the PVA core, which often hydrolytically disintegrates first, leaving PBAT fragments that may require longer biodegradation times in temperate soils. To ensure no persistent particulate remains, a reactive compatibiliser such as PVA grafted with maleic anhydride (0.5 wt%) is added during core pelletisation to strengthen interlayer adhesion, preventing premature exfoliation of the skin layers. Ecotoxicity testing per OECD 208 with milled film residues must pass both the 90% emergence rate and 90% biomass endpoint. This coextruded mulch is targeted for longer-season crops: cotton, processing tomatoes, and organic pumpkin production, where the mulch must remain intact for 12–16 weeks before soil incorporation. Storage requirements: the film must be kept in dry, UV-shielded packaging because both PVA and PBAT are susceptible to pre-deployment hydrolysis under high-humidity warehouse conditions.