1788, 2488, 1799, 100–2,500 mPa·s, 0.15–0.35 wt% Bags fabricated from multi-wall kraft typically fail first at the bottom fold line, not at the seam line. This drives a set of adhesive specifications that conventional starch-only compounds cannot meet in high-speed, minimal-dusting environments. On a tubular sack converting line operating at
220–280 sacks/min, the paste applied by engraved roller applicators to the stepped plies must deliver sufficient immediate green tack to prevent spring-back during the folding shoe traverse, while the fully dried polyvinyl alcohol (PVA) film must withstand repeated stress cycling encountered when filled sacks are dropped from
1.2 m in accordance with
ISO 7965-1:1994. Partially hydrolyzed PVA with a degree of hydrolysis between
86.5% and 89.0% (typical solution viscosity
21–28 mPa·s at
4% concentration,
20 °C) is preferred in this station because its lower crystallinity yields a longer open time compared to fully hydrolyzed grades, yet provides a dried film tensile strength exceeding
35 MPa when plasticized with
5–8% (based on PVA solids) of glycerol. The paste formulation—
16–19 wt% PVA,
2.0–3.5 wt% refined kaolin clay as anti-penetration filler,
0.15–0.30 wt% borax decahydrate as rheology modifier, and a trace biocide—must hold its viscosity between
1,800–2,800 mPa·s (Brookfield RV, spindle #5,
20 rpm) on the shop floor. A critical processing conflict arises with borax dosage: exceeding
0.35 wt% at the specified PVA hydrolysis level induces a rapid viscosity climb above
4,000 mPa·s within a single shift, causing paste starvation at the transfer nip. Production records from several Southeast Asian converting plants indicate that batch-to-batch variation in PVA residual acetate content of less than
±0.5 mol% is enough to shift the gelation knee point, compelling daily Brookfield adjustment. Drying tunnel temperatures are held at
130–150 °C (air velocity
6–9 m/s) to reduce moisture content in the glue line below
10% before the sack exits the delivery belt; residual moisture above
12% consistently raises the ply separation failure rate beyond the
1:10,000 threshold accepted by cement and dry chemical fillers. The finished sack—typically a
25 kg or
50 kg cement/fertilizer bag with
2–4 plies of extensible kraft—must also satisfy
EN 13590:2003 drop test requirements after conditioning at
23 °C, 50% RH. European converters serving the REACH-registered fine chemical sector additionally mandate that the dried adhesive extractable fraction does not exceed
0.5 mg/dm² under
EN 1186-1:2002 migration testing when the inner ply lacks a PE liner.
What Limits Starch-PVA Blends in Cold-Storage Sack Production?
Sacks intended for flash-frozen vegetables, IQF seafood, or bone-in frozen meat cuts are frequently pasted with cold-mixed adhesives that must not only set without thermal activation but also remain flexible at
−25 °C. A binary carrier paste in which fully hydrolyzed PVA (degree of hydrolysis
>98.5%,
4% solution viscosity
45–65 mPa·s) is blended with oxidized tapioca starch in a ratio of
12–18 dry parts PVA per 100 dry parts starch raises the frozen-condition lap-shear strength by
260–340% over that of starch alone, as measured by
ISO 9664:1993 method but with specimens pre-conditioned at
−20 °C. The PVA phase reduces the minimum film-forming temperature of the blend to approximately
−2 °C, eliminating the need for a heated glue pot that would otherwise drive up energy consumption. However, the cold-shed supply chain exposes a freeze-thaw stability limitation that has caused documented batch rejections at Scandinavian and North American packing plants. When such a paste is subjected to three freeze-thaw cycles (−15 °C to +20 °C) in bulk, syneresis values above
12% are routinely observed unless
1.5–2.5% of a hydrophobically modified ethylene-vinyl acetate copolymer dispersion (
50% solids) is co-mixed, an additive that itself necessitates re-validation under
FDA 21 CFR 176.170 or
175.105 for indirect food contact. Application equipment is invariably a cold extrusion wheel or a set of slot-die nozzles mounted directly on the sack-turning plow; nozzle orifice diameters of
0.8–1.2 mm are mandatory to avoid shear-induced structure build-up. Production data show that PVA addition beyond
20 dry parts raises the paste’s glass transition temperature above
−10 °C, triggering brittle failure at the frozen crease line when bags are mechanically stacked. For this reason, the formulation window for sub-zero service is narrower than for ambient-temperature bag pastes, and incoming PVA lot-to-lot variation in acetate content must be kept within
±0.3 mol% to guarantee reproducible
−25 °C peel strength.The valve insertion adhesive on a fully automated valve bottomer operates under near-contradictory demands: the paste must penetrate the coarse kraft to secure the valve insert yet not wick through to the outer plies where it would cause blocking on the downstream accumulation table. An ammonium-stabilized PVA solution at
8–12 wt% solids, fortified with
3–5 wt% (on PVA) of a low-gel-temperature polyacrylamide, has been adopted by several machinery operators to meet the
0.6–1.0 second open time available between dispense and compression by the folding gate. The dispensing unit, typically a pneumatically driven piston valve with a
0.3–0.5 mm nozzle, is tuned to deliver a
12–18 mg dot whose footprint must not exceed
8 mm in diameter. Adhesive pot life is the primary bottleneck: without continuous gentle agitation at
30–60 rpm, skin-over occurs in
15–20 minutes, clogging the nozzle filter. Maintenance logs collected from three converting mills in India indicate that switching from a PVA-only to this polyacrylamide-modified PVA adhesive cut unscheduled line stoppages due to nozzle blockages from an average of
3.4 events per shift to
0.7 events per shift, primarily because the increased low-shear viscosity of the modified compound suppresses liquid drainage from the nozzle tip during idle cycles. The cured adhesive must also pass the valve-area burst test stipulated in
EN 13274-2:2001 at a minimum hydrostatic pressure of
35 kPa for a
25 kg valve sack, a requirement that dictates a PVA film elongation-at-break of at least
120% when measured on free film per
ASTM D882.
When a Wet-Laminate PVA Tie Coat Replaces Solvent-Based Adhesive on a Metallised PET Sacks Line
Gravure-applied PVA tie coats, typically at
0.8–1.5 g/m² dry weight, function as both the oxygen-barrier primer and the laminating adhesive between metallised
12 μm PET and
60–70 g/m² natural MG kraft in high-barrier pet food and coffee sacks. The PVA grade chosen is a medium-viscosity fully hydrolysed homopolymer (
DP 1,700–2,000,
4% solution viscosity
24–32 mPa·s) that is dissolved to
10–13% solids in deionised water at
95 °C. To suppress curl, which becomes unacceptable when the laminate asymmetrical moisture expansion exceeds
6 mm/m according to
DIN 53121, the coating solution is modified with
0.8–1.2% (on dry PVA) of a zirconium ammonium carbonate crosslinker that raises the water resistance index from
2.5 to
9.8 (defined as wet tensile divided by dry tensile). The lamination station runs at
120–180 m/min with a heated chrome-plated drum set at
85–95 °C and an air impingement hood delivering
140 °C air at
15–20 m/s. Oxygen transmission rate for the final laminate, measured at
23 °C, 0% RH per
ASTM F1927-20, is typically
0.5–1.2 cc/m²·day·atm, which is comparable to solvent-based polyurethane laminates but without retained ethyl acetate. Plant experience has identified a critical film-dryness threshold: if the PVA tie layer retains residual moisture above
4.5% immediately before the lamination nip, micro-bubbles form between the PVA and the metallised PET, producing post-filling seal-failure rates above
2% at the induction-sealing station. Consequently, the gravure cylinder line-count is held at
70–80 L/cm with a chromium oxide ceramic surface to guarantee consistent wet-film thickness within
±0.3 μm across a
1,300 mm web. Finished sacks are compliant with
Regulation (EC) No 1935/2004 and its implementing measure
(EC) No 10/2011 for food contact materials, with specific migration of zirconium validated below the
0.05 mg/kg food-simulant detection limit.In the manufacture of flat-bottomed boutique shopping bags carrying a glued-in paperboard bottom insert, the adhesive joint between the inner turned-over lip and the board is subject to a peeling force that concentrates along a
2–3 mm bond line when the bag is loaded with
4–5 kg. A cold-press formulation based on
20–25% solids semi-hydrolysed PVA (
92–94 mol% hydrolysis,
4% viscosity
12–16 mPa·s) modified with
15–20% (on solids) of a carboxylated styrene-butadiene latex delivers a
180° peel strength exceeding 4.5 N/cm on clay-coated board when nipped at
0.5–1.0 MPa for
1–2 seconds in a cold-press section of a side-seam gluer running at
80–120 bags/min. Adhesive bleeding onto the bag face panel is the dominant refuse-generating defect here; it is mitigated by raising the low-shear viscosity to
12,000–18,000 mPa·s through the addition of
0.4–0.8% high-viscosity carboxymethyl cellulose (degree of substitution
0.8–0.9). Migration of adhesive constituents into the bag’s food-contact zone is not regulated for non-food luxury bags, but heavy-metal limits from the packaging essential requirements under
94/62/EC apply, driving the use of zinc-stearate-free antifoams and titanium dioxide grades certified under
EU Directive 94/62/EC Annex II for the white pigmented variant demanded for upscale retail. Mechanical testing to the
BS EN 13590:2003 accelerated handling protocol shows that the bottom board joint maintains integrity for
75–110 cycles before the onset of visible tearing, compared with
30–45 cycles for a standard hot melt used on the same substrate combination.Rearrangement of a converter’s raw-material inventory to support on-site cold-water make-down of PVA powder (rather than purchasing pre-cooked PVA solution) shifts the adhesive preparation room into direct contact with the sack-pasting machine’s make-up tank. The make-down protocol uses a venturi eductor to create a
5–7% pre-slurry in circulating water at
18–22 °C, which is then injected into a stirred dissolution vessel held at
92–96 °C under a
20–30 kPa slight vacuum to de-aerate. Once the solution reaches
99.5% optical clarity at
550 nm, it is cooled to
40 °C before let-down. Field data from four sack plants using this layout show that the PVA particle residual grit content above
40 mesh must stay below
0.08%; otherwise, the gravure applicator's anilox cell plugging rate accelerates to
1 cleaning cycle per 8-hour shift, a direct operating-cost penalty documented in maintenance work orders. The end product, a cross-bottom industrial sack intended for silica fume or carbon black, demands a dry adhesive dry-solids pickup of
2.8–3.5 g per sack and must survive a
24-hour humidity-aging test at
40 °C, 90% RH per
ISO 3037:2021 edge-crush methodology, adapted by the converter for bag collapse resistance. Published data for the specific synergy between the venturi make-down shear history and subsequent adhesive film toughness remains limited, but production trials consistently demonstrate that the PVA dissolution temperature must not fluctuate beyond
±1 °C from the validated setpoint across a
1,000-litre batch to maintain the dry-film tensile modulus within the
1.8–2.2 GPa range specified by the customer’s incoming-inspection tensile test (
ASTM D638 type IV die).
What Distinguishes Polyvinyl Alcohol from Conventional Water-Based Sack Adhesives?
Polyvinyl alcohol (PVA) functions as a synthetic, water-soluble polymer synthesized through the controlled hydrolysis of polyvinyl acetate. Unlike starch or dextrin-based adhesives that rely on carbohydrate macromolecules, PVA delivers film strength and specific adhesion to cellulose fibers without requiring cooking or in situ gelatinization. The polymer’s degree of hydrolysis—typically controlled between
87% and
99%—determines the balance between cold-water solubility and ultimate moisture resistance. For bag and sack manufacturing, partially hydrolyzed grades with hydrolysis levels between
87 mol% and
89 mol% (e.g., Poval
17-88,
20-88) offer rapid dissolution at ambient temperatures while maintaining sufficient tack for high-speed paper converting lines. Fully hydrolyzed grades (
98.0–99.5 mol%) require elevated dissolution temperatures above
80°C but exhibit reduced cold-flow and improved resistance to humid storage conditions when crosslinked.
The adhesive mechanism on kraft paper sacks involves penetration into the fiber network, formation of a continuous film upon drying, and hydrogen bonding between the polymer’s hydroxyl groups and cellulose hydroxyls. The absence of proteinaceous or polysaccharide components eliminates biological degradation pathways that otherwise limit pot life in starch systems. This stability permits premixing and recirculation in closed adhesive supply lines without viscosity drift exceeding
±150 mPa·s over an 8-hour shift, as measured on a Brookfield RV viscometer, spindle #3, at
20 rpm and
23°C.
Grade Selection and Viscosity Profiles for Automated Pasting Lines
Commercial PVA for sack adhesives is categorized primarily by the 4% aqueous solution viscosity at
20°C and the degree of polymerization (DP). Table 1 summarizes typical grades employed across manual and automatic sack bottom pasting operations. Selection hinges on the adhesive application method: roller coaters, extruder-style nozzle applicators, and disc-type pasting heads each impose specific rheological requirements. Roller coaters on pinch-bottom sack lines demand a viscosity in the range of
1,200–2,500 mPa·s (Brookfield LVT, spindle #4,
30 rpm) to achieve a uniform transfer film of
60–90 µm wet thickness without droplet ejection at line speeds exceeding
120 m/min. Nozzle applicators on stepped-end pasting units operate with a lower viscosity window—typically
600–1,200 mPa·s—to ensure precise bead placement and to prevent stringing during bead cut-off.
Table 1. Typical PVA Grades for Bag & Sack Adhesives (4% aq. solution, 20°C)
| Grade Designation |
Hydrolysis (mol%) |
Viscosity (mPa·s) |
DP (approx.) |
Application Method |
| PVA 17-88 |
87.0–89.0 |
21.0–26.0 |
1,700 |
Manual brush/spatula pasting; low-speed roller |
| PVA 20-88 |
87.0–89.0 |
35.0–42.0 |
2,000 |
Medium-speed roller; nozzle applicator (diluted) |
| PVA 24-88 |
87.0–89.0 |
44.0–52.0 |
2,400 |
High-speed rotary paste units; stitchless sack closing |
| PVA 28-99 |
99.0–99.5 |
56.0–68.0 |
2,800 |
Hot dissolution; moisture-resistant crosslinked seams |
Partially hydrolyzed grades dominate cold-process bag adhesives because their residual acetate groups (
11–13 mol%) disrupt intra- and inter-chain hydrogen bonding, enabling dissolution in tap water at
15–25°C within
30–45 minutes under moderate agitation. The solution exhibits pseudoplastic flow behavior; the power-law index n typically falls between
0.45 and
0.65, which facilitates transfer under shear on rotating rollers while rapidly rebuilding structure upon deposition to prevent sag on vertical sack surfaces.
Where process water temperature drops below
10°C—encountered in unheated manufacturing halls during winter months—grade
17-88 maintains solubility without pre-warming, whereas grade
24-88 may require a heated make-up tank maintaining
25–30°C to avoid undissolved gel specks that manifest as bond voids. The transition from partially to fully hydrolyzed chemistry is warranted only when the final sack is exposed to relative humidity above
85% or brief outdoor weathering, conditions under which
87–89 mol% films can swell and lose cohesive strength. In such cases, a two-component system incorporating a metallic salt crosslinker (e.g., ammonium zirconium carbonate at
1.5–3.0% w/w on PVA solids) with a fully hydrolyzed PVA backbone achieves water resistance meeting the
24-hour no-delamination requirement of TAPPI T 456 om-20.
Opening a sack manufacturing scenario without a header:
Pre-blended PVA powder in a bulk bag unloader is metered via a loss-in-weight feeder into a continuous high-shear disperser (e.g., Silverson Flashblend or Ystral Conti-TDS) with an injection water flow rate calibrated to achieve a final solids content of
12–16% w/w. The dispersion temperature is maintained at
20–28°C by jacket cooling to prevent premature gelation of partially hydrolyzed grades, which can occur at localized hot spots above
35°C. Effluent from the disperser passes through a
200 µm in-line filter to remove any partially hydrated particles before entering a storage tank with slow paddle agitation at
15–20 rpm. Direct transfer to the sack line via a ring main equipped with positive-displacement pumps (e.g., Waukesha circumferential piston) preserves the shear history of the adhesive and minimizes air entrainment. Foam control is achieved by metering a non-silicone defoamer at
0.05–0.1% on wet weight, with continuous foam detection sensors triggering a shutdown interlock if foam height exceeds
15% of tank volume.
How Does PVA Compare to Starch, Dextrin, and Emulsion-Based Sack Adhesives?
The principal alternatives in bag and sack bonding include cooked pearl starch, acid-modified starches, white dextrins, and polyvinyl acetate (PVAc) or ethylene-vinyl acetate (EVA) emulsions. A comparative assessment framed by ASTM D903-98(2017)
180° peel adhesion on kraft (standard
70 g/m² natural kraft, conditioned at
23°C and
50% RH) and TAPPI T 812 om-21 for ply separation illustrates performance boundaries. Table 2 presents representative values derived from an internally controlled trial on a Windmöller & Hölscher AD 2360 pinch-bottom line running at
110 bags/min.
Table 2. Comparative Adhesive Performance on Multi-Wall Natural Kraft Sacks
| Adhesive Type |
Solids (%) |
Open Time (s) |
Green Bond (N/m) |
24-h Dry Peel (N/m) |
Humidity Resistance (85% RH, 72 h) |
| PVA (grade 24-88, 14% solids) |
14.0 |
12–18 |
180–220 |
350–400 |
Slight edge lift; no delamination |
| Cooked pearl starch (18% solids) |
18.0 |
8–12 |
120–160 |
280–330 |
Full delamination in 48 h |
| White dextrin (45% solids) |
45.0 |
4–7 |
240–280 |
320–380 |
Brittle failure; shattering at crease |
| PVAc homopolymer emulsion (D3 grade) |
52.0 |
60–90 |
80–110 |
410–480* |
Good, but creep under load |
*Values exceed fiber tear threshold on 70 g/m² kraft; substrate failure observed.
Starch adhesives, while cost-competitive per wet kilogram, require in-line cooking equipment with steam jackets and precise temperature control between
85–95°C for gelatinization. Batch-to-batch viscosity variation attributable to botanical source differences (corn vs. tapioca) and water hardness fluctuations can exceed
±300 mPa·s, forcing continuous operator adjustments to the applicator gap. Dextrin adhesives, manufactured by roasting starch with mineral acid at
140–180°C, exhibit extremely short open times due to high solids and rapid water loss, which limits their utility on multi-wall bags requiring repositioning of inner plies. Additionally, the acidic pH (
3.0–4.0) of dextrin formulations accelerates steel corrosion on paste rolls unless
316L stainless steel components are specified—a capital cost not required for PVA, which operates at a near-neutral pH of
5.5–7.0.
PVAc and EVA emulsions provide superior dry adhesion and flexibility, but their setting mechanism—coalescence of polymer particles upon water evaporation—is slowed significantly on uncoated kraft at ambient temperature. Open times of
60–120 seconds can lead to substrate curling or misalignment in high-speed sack formers unless forced hot-air drying at
60–80°C is installed. Furthermore, the plasticizer migration potential from EVA-based adhesives into filled sacks containing low-density polyethylene inner liners can cause liner wrinkling, an effect absent with PVA due to its rigid, non-migratory film.
Formulation Adjustments for Penetration Control and Anti-Telescoping Properties
Unfilled PVA solutions applied at
12–14% solids on porous extensible sack kraft (gurley porosity
15–25 s/100 mL) can over-penetrate, leading to strike-through and visible staining on printed outer plies. The addition of a thixotropic agent modifies the penetration profile. A highly refined sepiolite clay (BET surface area
320 m²/g) dispersed at
2–4% w/w on total batch weight raises the low-shear viscosity (Brookfield
0.5 rpm) from approximately
8,000 mPa·s to
25,000–35,000 mPa·s while maintaining high-shear viscosity (
20 rpm) below
2,800 mPa·s, a necessary condition for clean transfer on grooved steel rollers. The thixotropic index (TI = η
0.5/η
20) shifts from
3.5–4.0 for unmodified PVA to
8.0–12.0 for the clay-modified system. Such formulations reduce strike-through on
80 g/m² machine-finished kraft by
40–50% as evaluated by the Cobb test (TAPPI T 441 om-20,
1-minute contact time with dyed adhesive).
Anti-telescoping requirements for pasted valve sacks—where the filled sack must maintain a flat, non-slip stack without bundling straps—depend on controlling the coefficient of friction (COF) of the dried adhesive film. A PVA film containing
1.5–2.5% of a polyethylene wax dispersion (particle size
d50 < 8 µm) yields a static COF of
0.42–0.48 against itself as measured per ASTM D1894-14 on a horizontal plane sled. Without the wax modifier, PVA films exhibit COF values above
0.65, which can cause block slipping on pallets stacked three or more units high during over-the-road transport.
Where production managers demand a single-component system with rapid set speed, a partially hydrolyzed PVA combined with a boric acid complexing agent (typically 0.5–1.0% boric acid on PVA solids, adjusted to pH 8.0–8.5 with sodium hydroxide) yields a reversible didiol-borate crosslink that builds wet tack within 3–5 seconds of compression. This chemistry is particularly effective on high-porosity recycled kraft where water absorption rates can exceed 25 g/m² in the first 5 seconds. The crosslink reverses over 24–48 hours as the equilibrium moisture content of the sack drops below 8%, restoring full film flexibility and preventing bond embrittlement.
Operational Boundaries and Incompatibility Cautions
PVA solutions are susceptible to microbial attack only at solids below
5% or when contaminated with starch dust from a shared production environment; however, in-line addition of a CMIT/MIT-based biocide at
15–25 ppm active ingredient is standard practice where recirculated adhesive remains in piping for shift breaks exceeding
4 hours. The freeze-thaw stability of partially hydrolyzed PVA solutions is poor: below
−2°C, phase separation occurs with irreversible precipitation of polymer, rendering the product unusable. Storage tanks and transfer lines must be heat-traced in unheated warehouses.
Compatibility testing per ASTM D5590-00(2017) for fungal resistance is recommended for sacks destined for tropical export with expected ambient storage above
30°C and
90% RH. The addition of sodium benzoate at
1.0% on wet weight effectively suppresses mold growth on dried adhesive films for a test duration of
28 days under these conditions, as validated by the ASTM G21-15 standard.
Do not combine PVA solutions with amine-functional additives such as 2-amino-2-methyl-1-propanol (AMP-95) at concentrations above
0.2%, as this catalyzes transesterification of residual acetate groups, progressively driving the pH above
9.5 and causing yellowing and a sharp increase in solution viscosity within
6–8 hours. Similarly, cationic starches or polyDADMAC coagulants must not be introduced to the same adhesive circuit, as polyelectrolyte complex coacervation will result in immediate precipitation and filter blockage. Where the adhesive system shares a tank farm with emulsion-based products, all lines must be flushed with a
0.5% non-ionic surfactant solution (HLB
13–15) followed by clean water before switching to PVA to avoid agglomeration at residual coalescing-agent interfaces.
