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.
