S-LEC BM-2

S-LEC BM-2 is a polyvinyl butyral resin supplied as a free-flowing white powder and assigned to the medium solution-viscosity tier of the S-LEC B series. The resin is a terpolymer composed primarily of vinyl butyral, residual vinyl alcohol, and a low vinyl acetate fraction. This distribution of functional groups controls dissolution in ethanol/toluene and ethyl acetate/ethanol blends, governs hydrogen-bonding adhesion to glass, aluminium oxide, and corona-treated polymer films, and influences tensile elongation after solvent evaporation. In the S-LEC B range, BM-2 is positioned between low-viscosity BL grades and high-viscosity BH grades; the grade is selected where a formulation requires increased film cohesion relative to BL resins but still needs filtration and coating behaviour that a high-molecular-weight BH resin cannot provide. Primary end uses include ceramic green-sheet binders for multilayer ceramic capacitors, solventborne printing ink binders, and solventborne adhesion primers for metal oxides.

Composition and Standardised Property Ranges for S-LEC BM-2

The certificate of analysis for each lot of S-LEC BM-2 reports hydroxyl group content, acetate content, butyral content, volatile matter, ash residue, and solvent viscosity. These values are not fixed constants because industrial acetalization of polyvinyl alcohol with butyraldehyde is controlled by reaction time, catalyst removal, and washing; therefore, lot-to-lot variability exists in any PVB resin. Table 1 summarises the manufacturer-published typical range for BM-2. Incoming inspection on a production line usually repeats Brookfield solution viscosity and volatiles because solvent viscosity shifts coating thickness and drying load, whereas full compositional analysis is reserved for supplier qualification and root-cause investigations. The table values should be read as reference ranges; the lot-specific values on the current certificate of analysis take precedence for process adjustments.

The hydroxyl content is analytically expressed on a mass basis by the supplier; users should not convert directly to molar hydroxyl fraction without applying the molecular-weight basis of the vinyl alcohol repeat unit because the resin is a terpolymer. The residual acetate fraction is kept low to reduce plasticization and low-temperature water uptake, while the butyral fraction supplies the hydrophobic spacing between hydrogen-bonding sites that gives the resin its balance of ethanol solubility and film strength.

What Limits Binder Burnout in Multilayer Ceramic Capacitor Green Tape?

In MLCC green-tape processing, S-LEC BM-2 functions as the primary polymeric binder in a high-solids dispersion of barium titanate. A representative production slurry contains 65–75 mass% BaTiO₃, 25–35 mass% solvent, 0.5–1.5 mass% phosphate ester dispersant, 3.0–5.0 mass% S-LEC BM-2, and 0.5–1.5 mass% plasticizer such as dibutyl phthalate or benzyl butyl phthalate. The powder is pre-dried to remove surface moisture before addition because water competes with the dispersant and raises slurry yield stress. Dispersion is carried out in a high-shear dissolver or bead mill; the tip speed of the dissolver is held at 15–20 m/s for 30–60 min, and the slurry temperature is maintained below 35 °C to prevent solvent evaporation. After dispersion, the slurry is filtered through a 10 µm absolute-rated membrane to remove agglomerates and undispersed resin particles.

The filtered slurry is cast onto polyethylene terephthalate carrier film with a doctor blade gap of 100–250 µm. Drying in a three-zone oven at 70–100 °C reduces residual solvent to 0.2–1.5 mass%. The dried green tape is wound at line tension below 100 N/m to avoid blocking. Green tensile strength measured on 25 µm-thick tape by ASTM D882 is typically 1.5–3.0 MPa when binder loading is 3.0–5.0 mass%; below this range, punching and stacking operations generate edge cracking; above it, the binder leaves carbonaceous residue during co-firing. The burnout window is established by thermogravimetric analysis per ASTM E1131 in air at 10 °C/min; decomposition begins near 220 °C and multi-step oxidation is complete by 550 °C. The low ash value in Table 1 is a gate criterion because alkali residues in barium titanate layers shift capacitance-temperature behaviour and reduce insulation resistance.

On full-width tape-casting lines above 1.0 m, lot-to-lot Brookfield viscosity variation of ±1.0 mPa·s in the incoming BM-2 has been observed to move wet film thickness by 3–5% under constant doctor blade pressure. Inline solvent addition using a rotational viscometer operating at 20 Hz with feedback to the mix tank is used to hold coat weight within ±2%. When a slot-die coater is used instead of a doctor blade, the target apparent viscosity is typically 200–800 mPa·s at 10 s⁻¹; published data for S-LEC BM-2-specific slurry rheology at these shear rates is limited, so pilot-scale rheology scans are required for each new lot and powder supplier.

For solventborne flexographic and gravure inks, S-LEC BM-2 is dissolved at 10–20 mass% in ethanol/ethyl acetate or ethanol/n-propyl acetate using a high-torque dissolver. Dissolution temperature is held at 40–50 °C because higher temperatures accelerate solvent loss without eliminating microgels. At a printing viscosity of 20–25 s measured by ISO 2431 with a 4 mm cup at 25 °C, a BM-2-containing gravure ink transfers with acceptable cell emptying from 60–80 lines/cm engravings, but the higher solution viscosity relative to low-viscosity PVB can require 0.2–0.5 mass% of a silicone-free flow agent to maintain leveling on corona-treated polyester. Adhesion results are influenced by the hydroxyl fraction; the same hydrogen-bonding hydroxyl groups that bond to glass and aluminium oxide also interact with oxidised polymer surfaces.

Hydroxyl-to-Butyral Ratio as a Solubility and Adhesion Boundary

The residual hydroxyl group content of S-LEC BM-2 is the main compositional variable controlling room-temperature solubility in anhydrous ethanol and adhesion to silanol-bearing surfaces. When the hydroxyl content rises above approximately 25 mass%, interchain hydrogen bonding reduces cold solvent solubility and can produce a yield stress at high solids. When the hydroxyl content falls below approximately 17 mass%, ethanol dissolution improves but adhesion to glass and aluminium oxide declines because fewer proton-donating hydroxyl groups are available for hydrogen bonding. The 20.0–23.0 mass% range in Table 1 places S-LEC BM-2 inside the adhesion/solubility window required for ceramic binders and polar film primers. On aminosilane-primed soda-lime glass, a cast film of BM-2 from a 10 mass% solution dried at 60 °C can exceed 2.0 N/mm T-peel adhesion when measured by ISO 8510-2; this value is substrate-limited and should be re-verified after any change in glass washing, silane concentration, or cure temperature. The butyral groups provide hydrophobic spacing that limits water sensitivity; the acetate groups are kept low to avoid plasticization and moisture gain at elevated relative humidity.

When S-LEC BM-2 Replaces S-LEC BL-1 in Solventborne Ink Systems

Substitution of S-LEC BM-2 for a low-viscosity PVB such as S-LEC BL-1 at equal resin solids raises ink viscosity and dry film tensile strength but lowers re-solubility and increases minimum drying-web temperature. In a representative flexographic ink pigmented with carbon black at 15 mass% on 12 µm corona-treated polyester, print trials showed tape adhesion per ASTM D3359 improve from 4B to 5B when BM-2 replaced BL-1, while static coefficient of friction increased from 0.25 to 0.32. The higher glass transition temperature of BM-2 requires the hot-air web temperature to be raised by 5–8 °C to achieve residual solvent below 5 mg/m²; otherwise blocking occurs at the rewind. Comparative typical values are listed in Table 2.

The viscosity increase when moving from BL-1 to BM-2 is the primary formulation difference. Ink makers compensate by reducing total resin solids by 1–3 mass% and increasing slow solvent by 5–10% of the solvent mass to maintain open time. If the formulation is not adjusted, gravure cells fail to empty at press speeds above 200 m/min, and foaming becomes more persistent in the ink pan. The higher molecular weight of BM-2 improves pigment binding and rub resistance but also makes dried ink films less readily re-dissolvable on press parts; cleaning cycles with a solvent blend containing 20–30% methoxypropyl acetate may be required.

Under Regulation (EC) No 1272/2008, S-LEC BM-2 is not classified as a hazardous substance in the supplied solid form. Dust from bag handling is controlled as organic nuisance dust; an occupational exposure limit of 3 mg/m³ total particulate is typically applied according to the ACGIH PNOS fraction, but local regulation must be checked. Registration under REACH Regulation (EC) No 1907/2006 is maintained by the manufacturer for industrial use. Food-contact status is not automatic; users must verify compliance with 21 CFR 175.105 or 21 CFR 177.1670 for the intended article because migration and extractive limits depend on the final packaging structure and coating thickness. S-LEC BM-2 should be stored in sealed polyethylene-lined bags at 10–30 °C and relative humidity below 60%; at relative humidity above 60%, pre-drying at 50–60 °C for 2–4 h is required before dissolution. High-acid-number additives with acid values above 5 mg KOH/g should be avoided because acid-catalyzed hydrolysis of the acetal groups degrades molecular weight and causes solution viscosity to drift downward over storage.