Sert Agrokimyasal Bariyer Şişelerde PVDC Değiştirilmesinden Sonra EVOH İşleme Sınırları

A six-layer extrusion blow moulding line producing rigid 1 L agricultural chemical containers typically replaces a chlorinated barrier core with ethylene-vinyl alcohol copolymer only after the entire melt-delivery system has been re-audited, because the thermal decomposition envelope, shear sensitivity, moisture tolerance, and interfacial rheology of EVOH are fundamentally different from those of PVDC. On an accumulator-head machine fitted with a 70 mm barrier-layer extruder and a spiral mandrel die, the melt inventory held in the adaptor, accumulator head, and screw channels can reach 4–6 kg, and the residence-time distribution is far broader than in continuous sheet or cast film processes. Where a PVDC barrier layer could be processed with barrel set points up to approximately 205 °C under disciplined shutdown protocols, EVOH begins to generate acetaldehyde, crotonaldehyde, and insoluble gel species when melt-film temperatures exceed 240 °C at residence times exceeding 15–20 min. The consequence is a characteristic failure mode in which brown or black specks of 0.2–0.7 mm appear first in the flash and pinch-off, then migrate into the sidewall, because stagnant material at the accumulator wall degrades before the main flow stream. These particles are not cosmetic; they act as discontinuities in the barrier layer and can initiate environmental stress cracking when the bottle is filled with xylene-based emulsifiable concentrates, then subjected to a UN packaging group II drop test from 1.2 m under the UN Model Regulations Chapter 6.1. Melt-flow comparisons between EVOH and PVDC are further complicated by the fact that commercial EVOH grades for rigid barrier packaging are generally characterised under ISO 1133-1:2022 at 210 °C with a 2.16 kg load, whereas PVDC was often not subjected to melt-flow testing at all because of its thermal instability. The removal of PVDC therefore shifts the burden of process control from chloride-corrosion and dioxin-related waste concerns to a set of narrow thermal, rheological, and moisture boundaries that must be enforced at every stage from pellet intake to regrind reincorporation.

From a process-control perspective, PVDC replacement introduces a moisture-management problem that the earlier chlorinated barrier layer did not impose at the same severity. PVDC melt layers and PVDC latex coatings are comparatively insensitive to residual moisture, but EVOH is hygroscopic and can pick up sufficient atmospheric water during storage and hopper residence to produce steam during melting. Resin suppliers typically require EVOH moisture content below 0.1 % by weight, and for high-gloss, void-free bottle sidewalls the practical upper limit is usually 0.05 %. Without a closed-loop desiccant dryer capable of maintaining a dew point of -40 °C or lower, EVOH pellets exposed to a plant environment at 25 °C and 60 % relative humidity for 2–4 h can sorb enough moisture to generate splay, microvoids, and irregular barrier-layer distribution. A hopper dryer alone, without a desiccant wheel, is not adequate because the equilibrium moisture content of EVOH under those ambient conditions can exceed 0.2–0.3 %. The practical result is moisture-induced viscosity reduction, in which the barrier-extruder melt pressure falls by 10–15 % and the layer thickness drifts below the lower control limit before the pressure transducer alarms. In a monolayer extrusion application, that deviation might be corrected by increasing screw speed, but in a six-layer coextrusion structure the EVOH layer is typically only 3–5 % of the total wall thickness, and the pressure signal cannot reliably distinguish between moisture-related thinning and simple throughput fluctuation. Processors who replace PVDC without installing dedicated EVOH drying capacity therefore observe variable oxygen transmission, delamination at the bottle shoulder, and intermittent gel formation that cannot be resolved by temperature adjustment alone.

What Melt Temperature and Residence-Time Boundaries Govern EVOH Barrier-Layer Integrity?

Barrier-layer integrity is governed by the interaction of melt temperature, residence time, oxygen availability, and moisture. EVOH decomposes by a free-radical degradation pathway accelerated by the presence of copper, iron, and other transition-metal residues; therefore, replacement of bronze bearings, copper bushings, and worn screw flights in the barrier extruder is a mandatory precondition after PVDC removal. The onset of measurable degradation in a 38 mol% ethylene EVOH grade is reported in resin supplier technical bulletins as an increase in yellowness index and methyl ethyl ketone-insoluble gel content after static heating at 230 °C for 20 min. At 250 °C, the same grade can generate visible gel species within 5–8 min. In an accumulator-head extrusion blow moulding operation, this imposes a practical maximum melt-temperature profile of 210–220 °C in the rear barrel zones, 215–230 °C in the metering zone, and 200–215 °C in the adaptor and head, depending on comonomer ratio and additive package. The indicated discharge thermocouple must not exceed 230 °C, but the actual melt film at the screw tip can be 5–15 °C higher because of solids-bed friction and viscous dissipation. Screws designed for PVDC, often with deeper channels and low compression ratios, are not suitable for EVOH because they create an excessively broad residence-time distribution. A barrier screw with a compression ratio of 2.8:1 to 3.5:1 and a length/diameter ratio of 24:1 to 30:1 is preferred, with water-cooled feed sections to prevent premature pellet compaction and bridging. The addition of a melt pump after the barrier extruder can stabilise layer thickness, but it adds a further residence-time volume that must be counted in the degradation calculation. Process records from conversion trials show that barrel set points above 240 °C, even for a brief start-up excursion, can generate enough decomposed material to contaminate the die and require purging with a high-viscosity polypropylene or rigid PVC purging compound for 30–60 min. Capillary rheometry performed according to ISO 11443:2014 provides the shear-viscosity data needed to set die temperatures, because the viscosity ratio between EVOH and the adjacent polyolefin layers determines whether the barrier layer remains centred or migrates.

Representative processing windows and barrier properties for EVOH grades used in rigid coextrusion are compiled from publicly available resin supplier data and polymer science literature. The values vary with comonomer sequence distribution, degree of saponification, and additive package, but the comparative ranges are useful for setting initial process targets after PVDC removal.

The selection of the EVOH grade for agrochemical bottles involves a trade-off between oxygen barrier and processability. The 27 mol% ethylene grade offers the lowest oxygen transmission under dry conditions, but its melt-temperature window is the narrowest and it is most sensitive to moisture and thermal degradation. The 44 mol% ethylene grade is more forgiving in the accumulator head and has better flexural fatigue resistance in flat-sided bottles, but its oxygen transmission is approximately one order of magnitude higher. For most rigid agricultural chemical applications, the design begins with a 32 mol% or 38 mol% ethylene grade, with the final choice determined by sidewall oxygen transmission measured according to ASTM D3985-20, moisture ingress measured according to ASTM F1249-20, and the required drop-impact performance measured according to ASTM D2463-15 or the relevant UN drop test. The table values should not be treated as exact product specifications; published data for a given commercial grade may fall outside these ranges depending on saponification level and additive package. The critical processing boundary remains the melt temperature at the die entry, because the barrier layer experiences its highest thermal stress immediately before forming the parison, and any instability at that point is amplified in the bottle sidewall.

When PVDC Is Absent from the Die-Layer Sequence

When PVDC is removed from the die-layer sequence, the barrier-layer thickness, position, and interfacial shear environment change in ways that are not immediately apparent from the total wall-thickness profile. PVDC layers in rigid agrochemical bottles are typically buried as a core at 1.5–3.0 % of total wall thickness, whereas EVOH is often specified at 3.0–5.0 % to achieve comparable oxygen barrier at 0 % relative humidity. The absence of PVDC also removes a secondary moisture barrier, which means that the EVOH layer must be shielded by the outer HDPE skin, and the tie-resin layers must be continuous and defect-free. In a spiral mandrel die, the layer interfaces are formed under melt pressure, and the encapsulation forces are sensitive to the viscosity ratio between EVOH and the adjacent tie resin. When EVOH is processed at the same melt temperature as a maleic anhydride-grafted LLDPE tie resin, the tie resin initially has a lower viscosity, but its viscosity may rise if the tie resin is degraded by residual moisture or if it is a high-anhydride-content grade originally selected for PVDC adhesion. This can produce a viscosity inversion that drives the EVOH layer toward the outer wall of the parison, a phenomenon known as barrier-layer migration. On production lines, this is detected by taking a 5 mm cross-section of the flash line and measuring the EVOH layer position under a polarised light microscope; acceptable layer distribution for a 1 L bottle is usually within ±10 % of the target circumferential position. When migration exceeds that band, the sidewall oxygen transmission measured per ASTM D3985-20 can increase from 0.3 cm³·20 μm/(m²·day·atm) to more than 3.0 cm³·20 μm/(m²·day·atm) even though the total EVOH weight remains unchanged.

Before the EVOH conversion, the die must be inspected for PVDC-related corrosion and residue. PVDC melt processing generates trace hydrogen chloride, which corrodes unprotected tool steel and leaves chloride residues in the spiral mandrel channels. These residues can accelerate EVOH decomposition at melt temperatures that would otherwise be safe, and they can also reduce the effectiveness of the tie-resin adhesion system. The die should be disassembled, cleaned with a mild alkaline detergent, and passivated with a citric acid-based metal cleaner before the EVOH trial. The spiral mandrel clearances that worked for PVDC may require adjustment because EVOH has a higher viscosity at the same melt temperature and a sharper shear-thinning index. If the clearance is too small, excessive shear heating at the mandrel lands raises the melt-film temperature to 250 °C or higher even when the set-point thermocouple reads 215 °C. If the clearance is too large, the layer distribution becomes sensitive to parison programming and may thicken at the bottle neck and bottom where the parison is pinched. Those thickened regions act as reservoir zones for degraded material during the next shot and contribute to the intermittent appearance of gel particles in subsequent bottles. For flat-sided containers with strong wall-thickness variation, the radial wall-thickness programmer should be set to a barrier-layer-specific profile rather than the profile developed for PVDC, because the EVOH layer responds differently to parison draw and pinch-off compression.

For agrochemical formulations based on xylene, cyclohexanone, N-methyl-2-pyrrolidone, and chlorinated phosphate esters, the barrier performance of EVOH is controlled by the plasticising effect of the solvent on the polyolefin skins as well as the partial solvation of the EVOH layer. EVOH is resistant to many nonpolar solvents, but it is swollen by polar solvents and water; the oxygen transmission of a 32 mol% ethylene EVOH layer at 20 °C rises from 0.2–0.5 cm³·20 μm/(m²·day·atm) at 0 % relative humidity to 12–20 cm³·20 μm/(m²·day·atm) at 85 % relative humidity. Published data for this specific configuration is limited because agrochemical formulations vary widely in solvent polarity, surfactant loading, and co-solvent content. The accepted industrial approach is to rely on the outer HDPE layer for moisture barrier and to position the EVOH layer as close to the outside surface as possible without exceeding the tie-resin adhesion limit. When PVDC was present, that layer provided an additional moisture barrier, so the EVOH layer position was less critical. After PVDC replacement, the layer order must be validated by measuring the moisture content of the EVOH layer after filling. A non-destructive near-infrared spectroscopic method calibrated against Karl Fischer titration can estimate the EVOH moisture content through the bottle wall at intervals of 7, 28, and 84 days to establish the moisture-ingress rate. In a humid coastal warehouse, empty bottles may gain enough moisture in the EVOH layer within 3–6 months to reduce the effective oxygen barrier during subsequent use, even though the bottle appears unchanged. The design must therefore include a maximum empty-bottle storage time and a controlled storage atmosphere where the EVOH layer is not exposed to prolonged high humidity. The drop-impact and environmental stress crack resistance of the filled bottle are assessed after chemical contact using ASTM D543-21 and ASTM D1693-21, because the solvents that swell the polyolefin skins can reduce the stress-cracking threshold of the HDPE and expose the EVOH layer to localised strain.

Process Control Limits and Statistical Monitoring of EVOH Layer Thickness

Statistical process control for EVOH-containing barrier bottles requires direct measurement of the barrier-layer thickness rather than inference from total bottle weight, because the EVOH layer is a small fraction of the total wall and its thickness can vary independently. A capacitance-based thickness gauge or a confocal optical gauge installed after the bottle trim station can measure total wall thickness at a resolution of 0.1 mm, but the EVOH layer position and thickness require destructive cross-section analysis. A production-scale sampling plan therefore takes one bottle every 60 min, sections the sidewall at 45°, 135°, 225°, and 315° around the circumference, and measures the EVOH layer thickness under a microscope at 50× magnification. For a 1 L bottle with a target EVOH layer thickness of 0.10 mm, the control limits are typically set at 0.07 mm and 0.13 mm. Below the lower limit, the oxygen barrier falls below design specification, and above the upper limit, the excess EVOH can create interfacial stresses that reduce drop-impact resistance. The measured values must be correlated with the barrier-extruder screw speed and the die-gap setting, because these are the primary manipulated variables for layer thickness. When a thin-layer deviation occurs, the corrective action for EVOH is not the same as for PVDC. Increasing the barrier-extruder speed to correct a thin layer can raise the melt temperature by shear heating and initiate decomposition, so the preferred response is to increase the barrier-extruder temperature slightly, adjust the parison programmer to reduce total wall thickness in non-critical areas, or elevate the adaptor temperature within the safe window. Direct thickness correlation with oxygen transmission measured per ASTM D3985-20 on sidewall samples cut from production bottles provides the validation that the control chart is tracking a true functional property rather than an indirect process variable.

The limits on EVOH after PVDC replacement also include a reduced ability to use post-consumer regrind in the middle layer. Regrind from EVOH-containing bottles can be reprocessed at 15–30 % by weight in the regrind layer, but the regrind stream contains degraded EVOH from trim flash and rejected bottles. Repeated heat histories raise the gel content and reduce the impact strength of the finished bottle. A processing audit should measure the melt flow rate of the regrind layer material per ISO 1133-1:2022 at 230 °C with a 5.00 kg load to detect crosslinking or chain extension. If the melt flow rate falls by more than 20 % relative to the virgin polyolefin, the regrind content must be reduced or the regrind diverted to a non-barrier application. The use of a separate vented regrind extruder is recommended to remove moisture and residual volatile compounds from the recycled EVOH; PVDC regrind was not always vented because PVDC degrades more readily and was often excluded from regrind loops entirely. With EVOH, the regrind loop can be maintained if the trim is kept clean, dried to below 0.05 % moisture, and protected from recontamination before reintroduction into the middle layer.