S-LEC BH-8

S-LEC BH-8 is a poly(vinyl butyral) resin in the S-LEC B series manufactured by Sekisui Chemical. The material is supplied as a free-flowing white powder for use as a temporary organic binder in ceramic green tape, thick-film conductor paste, and glass-bonded electronic-paste systems. In the standard commercial form, the resin is characterized by a high degree of butyralization, a residual vinyl alcohol content that remains available for hydrogen bonding to oxide and glass surfaces, and a residual acetate content below 3 mol%. The glass transition temperature of the unplasticized resin is generally observed in the 65–75 °C range by differential scanning calorimetry according to ISO 11357-2:2020. The defining specification of S-LEC BH-8 relative to other S-LEC B grades is its higher solution viscosity at equal solids, which correlates with a higher weight-average molecular weight and with greater dried-film tensile strength at a given binder content.

Because the manufacturer assigns exact control limits by lot, acceptance testing normally includes the methods listed in Table 1. These methods are not product-specific reference values but are applied to poly(vinyl butyral) resins and should be requested with the certificate of analysis.

How does S-LEC BH-8 influence slurry rheology in nonaqueous tape-casting dispersions?

In 1–5 μm barium titanate dispersions formulated for multilayer ceramic capacitor tape, S-LEC BH-8 is typically dissolved in a mixed solvent system of methyl ethyl ketone, toluene, and ethanol before high-shear dispersion of ceramic powder. The resin content is commonly 6–12 parts per hundred ceramic weight, depending on the target green density and lamination performance. Slurries made with higher-viscosity PVB exhibit pseudoplastic flow, with a power-law index n generally between 0.7 and 0.8 over the shear-rate range 1–100 s⁻¹ when measured by rotational rheometry according to ISO 3219. The viscosity at 10 s⁻¹ can exceed 2000 mPa·s at 20 °C for a 72–78 mass% solids-loaded tape slurry. Because S-LEC BH-8 develops a continuous polymer network at lower addition levels, binder content can sometimes be reduced by 1–2 wt% relative to a medium-viscosity PVB grade without sacrificing green tensile strength. A reduction from 10 wt% to 8 wt% binder on a ceramic basis lowers the total organic loss measured by thermogravimetric burnout by approximately 15–20%, assuming constant residual solvent.

On a 300 mm tape-casting line with dual-zone air-float drying set at 55 °C and 75 °C, the substitution of a lower-viscosity grade with BH-8 typically requires a solvent dilution adjustment of 20–30% to maintain leveling under the doctor blade. If the solvent content is not increased, the higher native viscosity produces transverse thickness variation exceeding ±2 μm on a 25 μm wet tape and can reduce green density at the tape edges. Published data for this specific configuration is limited; the 20–30% adjustment should be confirmed by pilot-scale rheology and drying trials before production implementation.

Residual vinyl alcohol content in PVB is a critical batch-control factor. Variations of ±1 mol% in hydroxyl content can shift solution viscosity by more than 10% in ketone/toluene solvent systems because the hydroxyl groups form transient hydrogen bonds with the solvent and with ceramic surface silanols. The slip viscosity should therefore be rebalanced for each incoming lot when the lot certificate deviates from the qualification mean. Moisture above 1 wt% on the powder can produce similar shifts and should be controlled by pre-drying at 60 °C for at least 2 h at relative humidity above 60%.

Subsequent lamination of stacked green sheets is performed at 60–80 °C and 1–3 MPa for 30–120 s. The residual hydroxyl content of S-LEC BH-8 allows the binder to flow under pressure and form interlayer adhesion without plasticizer exudation. At hydroxyl levels below 18 mol%, lamination strength may fall below 0.15 N/mm peel force; at hydroxyl levels above 30 mol%, water uptake increases and the tape becomes moisture sensitive. Because BH-8 is positioned in the high-viscosity segment, the required lamination pressure may be 0.5–1.0 MPa higher than for lower-viscosity grades to achieve equivalent interlayer bond strength, especially when tape thickness exceeds 30 μm. Peel strength of laminated green tape is tested by ISO 527-3:2018 or equivalent internal methods.

Binder Burnout and Carbon Residue Control in Copper-Terminated MLCC Processing

Thermogravimetric analysis of poly(vinyl butyral) in air shows two main mass-loss stages: side-group elimination and partial oxidation beginning near 200–240 °C, followed by main-chain oxidation and char combustion above 350 °C. Under nitrogen, decomposition of S-LEC BH-8 is incomplete at 450 °C, and residual carbon can remain above 2 wt% unless a controlled oxygen-containing sweep is introduced during the debinding stage. This behavior is quantified by ISO 11358-1:2022 or ASTM E1131-08. For copper inner electrodes, binder removal must occur before the copper oxidation threshold is reached; therefore, a two-stage debinding profile is used: an initial hold at 250–280 °C in nitrogen removes the majority of low-molar-mass decomposition products, followed by controlled introduction of air or 0.1–1 vol% oxygen in nitrogen to oxidize residual carbon without oxidizing copper. The higher molecular weight of S-LEC BH-8 increases the temperature at which the dried tape loses mechanical integrity, which can be beneficial for shape retention but slows solvent and decomposition product diffusion. For tapes thicker than 50 μm, heating rates below 2 K/min and a hold at 150–180 °C for residual solvent removal are required to avoid blistering and interlayer delamination.

Compared with ethylcellulose, S-LEC BH-8 leaves a lower ash residue under identical complete-combustion conditions but requires a more aggressive oxygen exposure to reach complete burnout. Acrylic binders, in contrast, often begin decomposition at lower temperatures and can produce sharper burnout exotherms. In multilayer stacks, a broad decomposition interval is generally preferred because it avoids delamination caused by rapid gas evolution. S-LEC BH-8 provides a broader debinding interval than many low-molecular-weight acrylics and poly(vinyl alcohol) binders in the 200–400 °C range. This is one of the primary reasons it is selected for co-fired barium titanate bodies with nickel or copper electrodes.

Adhesion of PVB to glass and indium tin oxide is dominated by acid-base interactions between residual hydroxyl groups and surface silanol or metal hydroxyl sites. S-LEC BH-8 exhibits higher adhesion at higher humidity than non-hydroxyl acrylic binders, but the same hydroxyl content can cause water uptake. The equilibrium water absorption of PVB at 50% RH is commonly 1.5–4 wt%, depending on grade; higher hydroxyl content increases water sensitivity and can reduce dielectric performance in uncured thick films. Pre-drying is therefore recommended before high-frequency electrical testing.

When S-LEC BH-8 replaces ethylcellulose in thick-film silver conductor paste

In screen-printable silver conductor pastes, the binder must provide pseudoplastic flow, clean screen release, and sufficient green adhesion to alumina, glass ceramic, or polymer dielectric surfaces. Ethylcellulose is widely used in this role because it dissolves in terpineol-based solvents and leaves minimal residue after firing. S-LEC BH-8 can replace ethylcellulose when the paste is processed through fine-mesh stainless-steel screens of 200–325 mesh and when printed line width below 100 μm must retain edge definition before drying. The hydroxyl groups of PVB interact with the substrate, increasing green-state wire-bond pull strength in dried prints, but the higher solution viscosity of BH-8 raises the paste viscosity at 10 rpm on a Brookfield viscometer. If the paste formulation exceeds 30 000 mPa·s at 25 °C, high-speed automatic printing may exhibit skipping or dog-ear defects. Solvent or plasticizer adjustment is therefore required when migrating from ethylcellulose to BH-8. In laser ablation tests on dried paste deposits, PVB-bonded films show less edge chipping than ethylcellulose-bonded films, but published data for this specific configuration is limited.

In direct comparison within the S-LEC B series, lower-viscosity grades such as BM-2 and BM-5 are preferred for quick-setting solvent-release coatings and low-solids ink vehicles. BH-3 occupies a middle position; it provides moderate green strength and lower solution viscosity than BH-8. BH-8 is selected when the binder must survive aggressive debinding or compression without tearing; however, its use reduces maximum ceramic solids loading in the slurry. The trade-off is not linear, because above a critical molecular weight the solution viscosity rises steeply with only marginal gains in green tensile strength. This threshold corresponds approximately to the transition between BH-3 and BH-8 in solvent systems with low polarity. Product-specific molecular weight data should be taken from the certificate of analysis.

The high native viscosity of S-LEC BH-8 narrows the usable solids window in gravure and flexographic inks

Gravure and flexographic ink vehicles formulated with poly(vinyl butyral) binders typically require low viscosity at the application shear rate and fast solvent release after transfer. S-LEC BH-8 is not the first-choice grade for low-viscosity ink systems because its molecular weight places it above S-LEC BL-1 and S-LEC BM-2. In laminating inks for flexible packaging, BH-8 is used only when the print must withstand aggressive downstream lamination or hot-fill conditions. At equal solids, BH-8 increases the apparent viscosity at 1000 s⁻¹ by roughly 2–4 times relative to a low-viscosity PVB grade. To maintain viscosity below 100 mPa·s at 25 °C, the resin concentration may need to be reduced to 5–8 mass% of the vehicle, which in turn lowers the adhesion to polar substrate surfaces. In contrast, S-LEC BL-1 can be formulated at 10–15 mass% while maintaining equivalent transfer viscosity. The selection of BH-8 in ink systems is therefore limited to applications where its higher film toughness outweighs the loss of solid loading. The exact viscosity ratio is system dependent and should be measured with ISO 3219 using the specific solvent blend.

The solubility window of S-LEC BH-8 is narrower than that of BM-2 due to higher molecular weight. It dissolves readily in methyl ethyl ketone, cyclohexanone, ethanol/toluene mixtures, and selected glycol ethers; it is insoluble in water and aliphatic hydrocarbons. Ketone-only solvents produce lower viscosity than aromatic/alcohol blends, but drying rates are slower. In ceramic slurries, mixtures of methyl ethyl ketone and ethanol with 20–40 vol% aromatic hydrocarbon are used to match evaporation profiles. A change from 10 mass% ethanol to 20 mass% ethanol in a ternary solvent may lower viscosity by 15–25% while increasing moisture uptake. These solvent effects are measured with Brookfield and cone-plate rheometers according to ISO 3219.

Operational boundaries for the resin include storage below 30 °C in low-humidity packaging. The material should be pre-dried at 60 °C for at least 2 h when moisture absorption exceeds 1 wt%. S-LEC BH-8 is not recommended for aqueous alkaline formulations above pH 8 because the ester and acetal groups undergo hydrolysis; strongly acidic phenolic resin blends can similarly reduce molecular weight during storage. The powder is combustible as a finely divided organic solid and should be handled with dust-control equipment and grounding to dissipate static charge. No direct food-contact use is claimed unless the specific grade is covered by applicable food-contact regulations for the target application.