PVAc Paper Bag Adhesive

On multi-wall paper sack bottoming lines for cement, dry mortar and mineral fillers, the PVAc dispersion is not selected for ultimate shear strength alone. The emulsion is applied after a bottoming cylinder opens the stepped plies of three to six web layers, and the wet bond must tolerate transfer, stacking, filling, and drop loads of finished 25–50 kg sacks. Production bottomer units running at 200–450 bags/min apply 130–180 g/m² through engraved steel rollers. A typical PVAc homopolymer for this operation has solids 55–62 wt% and Brookfield viscosity 20,000–36,000 mPa·s at 20 °C per ASTM D1084, with pH held between 4.0–6.0. The adhesive must provide an open time above 8 s so that bottom ply alignment can be corrected, but it must not exceed 25 s because the sack enters stack compression almost immediately after bottoming. At relative humidity above 70 %, moisture uptake by the outer 90–110 g/m² extensible kraft retards film formation and can reduce the compression shear at 1 min by more than 30 % compared with the same adhesive at 50 % RH.

The dominant failure locus on these lines is internal ply delamination rather than adhesive-to-paper separation. When the bottom seam of a conditioned sack is dissected after 24 h at 23 °C and 50 % RH, acceptable results show more than 80 % fiber tear by hand peel. For instrumental comparison, a 25 mm wide strip cut from the bottom seam can be pulled at 300 mm/min according to ASTM D1876; converter specifications often set a minimum of 2.0–3.5 N/mm, but the numerical value is secondary to the failure locus. PVAc films remain moisture-sensitive after drying, and sacks stored under humid warehouse conditions above 60 % RH can lose drop-impact resistance because the film re-softens. To counter this limitation, a minor fraction of vinyl acetate-ethylene copolymer at 10–20 % of binder solids or a crosslinking monomer is blended into the dispersion. Drop testing for filled sacks is usually conducted according to ISO 2248 at ambient temperature, with the requirement that no bottom seam opening exceeds 5 mm after impact.

Why Do Side-Seam Nozzle Lines Demand Lower Viscosity Than Bottom-Paste Units?

Side-seam application on self-opening square-bottom grocery bags is a dynamically different operation from bottom pasting. A nozzle applies a continuous line of 2–4 mm width to the folded longitudinal seam while the web travels at 180–350 m/min, and the blank is subsequently folded, cut, and transferred to the bottom-forming station. The emulsion must show pronounced shear-thinning behaviour because the orifice diameter is only 0.4–0.8 mm; a bottom-paste product at 20,000 mPa·s would create excessive back-pressure and skip at the nozzle tip. Side-seam grades are therefore held to 2,500–6,500 mPa·s Brookfield viscosity at 20 °C and 45–52 wt% solids, with pH 4.0–6.0. The dispersing system is typically non-ionic or weakly anionic to avoid precipitation when process water hardness exceeds 300 ppm calcium carbonate equivalent.

The immediate bond requirement is wet tack rather than final strength. A spring-back defect occurs when the longitudinal seam opens more than 2 mm after the folder, and it is influenced by paper elastic recovery and water uptake into 70–90 g/m² machine-glazed kraft. In screening, a folded blank is dropped after 3 s and must remain closed under its own stack weight. If web moisture falls below 6 % measured by ISO 287-1, open time shortens to 3–5 s and nozzle skipping appears. Cohesive bond development is evaluated after 24 h by peeling a 25 mm strip at 300 mm/min per ASTM D1876; peak values of 1.5–2.5 N/mm with fiber tear are acceptable for a 10–15 kg grocery load.

After cord winding on premium retail shopping bags, the twisted paper handle cord is bonded with a filled PVAc formulation after the cord is formed from two or three spiralling plies of 30–50 g/m² unbleached kraft. The adhesive is applied to the cord flat before it is pressed into the bag side panel, usually by a doctor roller or a lift-and-transfer pad; the dry coat weight on the contact area is 20–40 g/m². The working dispersion is based on a PVAc homopolymer with solids 50–54 wt% and Brookfield viscosity 4,000–7,500 mPa·s at 20 °C. To prevent brittleness in cold retail conditions, 3–6 wt% of triethylene glycol dibenzoate or diethylene glycol dibenzoate on wet formulation mass is added; this shifts the PVAc film glass transition from approximately 28 °C to 5–12 °C. Plasticizer levels above 6 wt% produce blocking of folded bags in the stacker and visible strike-through on 110–130 g/m² kraft face sheets.

Bond performance is checked after 24 h conditioning at 23 °C and 50 % RH. A handle attached to a filled bag containing 5 kg is lifted cyclically; the required failure mode is cohesive tearing of the outer paper surface, not peel of the adhesive film. The same test at 5 °C detects plasticizer exudation, which appears as a waxy surface film and reduces friction. Because plasticized PVAc is not intrinsically biodegradable, bags destined for the European paper recycling stream are often limited to 2 wt% plasticizer if disintegration testing under EN 13432 is part of the converter's specification, although full compliance depends on the adhesive area relative to the fibrous furnish.

When a Valve Insert Requires Cold-Set Bonding Inside a Cement Sack

Valve sacks for cement and dry chemicals are filled through a pre-formed polyethylene valve insert that is fixed to the inner ply before bottoming. PVAc adhesive alone does not wet untreated low-density polyethylene; a corona or plasma surface treatment sufficient to raise wetting tension to at least 38 mN/m measured per ASTM D2578 is required. The adhesive is typically a 53–58 % solids emulsion with Brookfield viscosity 8,000–15,000 mPa·s, applied at 60–90 g/m² by transfer roller to the outside of the valve tube. The bond is then pressed for 2–5 s and allowed to set before the sack enters filling. Final peel strength is less critical than resistance to sliding during filling; high-shear viscosity at 1 s⁻¹ controls placement and squeeze-out.

The more common production failure is cohesive separation of the corona-treated polyethylene skin from the underlying polymer, not debonding of the PVAc from the paper. Corona treatment introduces low-molecular-weight oxidized species that form a weak boundary layer if treatment is older than 24 h or if slip additives bloom to the surface. Converters therefore specify that treated film must be converted within 8–12 h of surface treatment and that the treated surface not contact idler rollers. The adhesive film remains thermoplastic; sacks filled with material hotter than 60 °C can show creep if the valve is under lateral load. In accelerated testing at 50 °C for 72 h, creep displacement of the valve beyond 3 mm is treated as a reject condition. No food-contact claim is made for this application; compliance is confined to REACH restrictions and occupational exposure limits for residual vinyl acetate monomer.

When bakery bags are destined for indirect food contact, the bottom fold and top tie-band are adhesively fixed with a PVAc dispersion that must satisfy FDA 21 CFR 175.105 for indirect food contact and, where the bag is used for moist bakery products, FDA 21 CFR 176.170 conditions of use. Residual vinyl acetate monomer in the dry adhesive film is normally limited to 0.1 % or less, and the formulation is free from dibutyl phthalate and other phthalate plasticizers. For bleached or glassine paper with grammage of 35–60 g/m², the dry adhesive coat weight is held to 2–4 g/m² to avoid oil or moisture strike-through. The emulsion itself has solids 46–50 wt%, viscosity 3,000–5,000 mPa·s, and pH 4.5–5.5. It is applied by nozzle or roller and pressed for less than 1 s.

Because PVAc films become stiff at low temperature, bags intended for frozen bakery transport require either a vinyl acetate-ethylene copolymer fraction or a plasticizer approved under the relevant food-contact framework. Without modification, a PVAc homopolymer film has a glass transition near 28 °C; at freezer temperatures as low as -20 °C the bonded fold can crack when opened. The most practical field check is a repeated fold test after 24 h at -18 °C, with no visible glue-line cracking. Organoleptic limits are set by the end user; if the adhesive contributes more than 0.5 mg/kg total volatile organic compounds to the package headspace in gas chromatographic screening, the grade is rejected even when the monomer and plasticizer levels are compliant.

Rheological Collapse in Recycled-Fiber Grocery Bags at Low Web Moisture

Grocery bags made from recycled liner or corrugated kraft impose a more difficult penetration control problem than virgin machine-glazed kraft. The recycled fiber fraction, often above 80 wt%, has higher surface roughness, higher ash content, and variable internal sizing; water from the adhesive is absorbed into the fiber lumens within 0.5–2 s. If the adhesive viscosity is not matched to this absorption rate, the side seam or bottom patch can undergo rheological collapse, in which the applied emulsion loses water so quickly that it does not level and leaves a discontinuous, stringy glue line. The defect appears as a skip pattern under 10× magnification and corresponds to T-peel values below 1 N/mm on 90 g/m² recycled kraft after 24 h.

Control on these lines is achieved by raising solids to 58–63 wt% while keeping Brookfield viscosity between 12,000 and 20,000 mPa·s. The higher solids reduces the amount of free water available to swell the sheet, and a small addition of 0.2–0.5 wt% carboxymethyl cellulose or a high-molecular-weight polyether thickener is used to keep the adhesive at the paper surface. Web moisture is measured offline according to ISO 287-1 and is normally held at 8–10 %. At web moisture below 6 %, open time falls to 2–4 s and stacker jams occur because the bottom fold opens before sealing. This is a shallow formulation zone, but production stops are disproportionately frequent when the incoming recycled furnish changes between suppliers.

Thermal Set Profiling Distinguishes Handle Attachment From Bottom Pasting

Although both bottom pasting and handle attachment use PVAc dispersions, their thermal set profiles are not interchangeable. Bottom pasting on multiwall sacks uses compression and residual paper moisture; the bottoming unit may run with heated compression belts at 70–95 °C, but the thick stepped plies act as an insulator, so the glue line does not reach the 100–120 °C set temperature demanded by a pure homopolymer. Handle attachment on retail bags instead presses the adhesive against a thin side panel or reinforcing patch with a heated jaw at 90–130 °C for 0.5–1.5 s, achieving more complete coalescence. This is why handle-attachment grades often use coalescing solvents such as butyl glycol acetate at 0.5–1.0 wt%, while bottom-paste grades rely on slower film formation, ambient drying, and compression dwell.

The practical consequence is that a bottom-paste formulation placed in a handle attachment line may produce glue-line starvation and stringing because its viscosity is too high, whereas a handle-attachment formulation transferred to bottom pasting may fail because its faster coalescence creates a surface skin before the plies are fully aligned. Table 2 summarises the process profiles observed on production-scale bag converting lines; the temperature values are measured at the press jaw or compression belt surface, not in the glue line itself.

For thin single-ply confectionery bags made from 20–30 g/m² bleached kraft, no independent reformulation is required; side-seam bonding uses the same 45–50 % solids, 3,000–5,000 mPa·s nozzle-grade PVAc with a 6 s open time at 100–150 bags/min.