Álcool polivinil (PVA) para ligantes e revestimentos para comprimidos

Partially hydrolysed polyvinyl alcohol (PVA) conforming to Ph. Eur. 3.1.7 /USP-NF Polyvinyl Alcohol, with a degree of hydrolysis in the range 87.0–89.0 mol% and a 4% aqueous solution viscosity of 4.0–6.0 mPa·s at 20 °C (determined per ISO 2555:2018, Brookfield LVDV, spindle 2, 60 rpm), serves as a high-efficiency wet-granulation binder for tablet manufacture. Granulating fluids are prepared by dispersing the polymer in cold water, heating to 80–90 °C under agitation until fully dissolved, and cooling to room temperature; solutions remain processable for up to 48 hours if preserved, as unpreserved systems risk microbial proliferation. Binder addition levels of 2–5% w/w dry basis on total tablet weight are typical, corresponding to a 5–15% w/w PVA solution. The binder mechanism relies on the formation of crystalline hydrogen-bonded junctions upon drying, which confer interparticulate bridges with a film tensile strength of 40–60 MPa (ASTM D882), significantly surpassing the cohesive strength generated by povidone or pregelatinised starch at equivalent use levels. The FDA Inactive Ingredient Database lists a maximum potency of 60 mg per oral tablet for this grade. In direct compression, a 2–8% w/w dry binder loading further exploits PVA’s compressibility, with accepted commercial grades commonly designated by a two-number code—for instance, “4-88”—where the first digit approximates the 4% viscosity in mPa·s and the second indicates the mol% hydrolysis.

In high-shear wet granulation equipment (Diosna P1-6, impeller speed 200–400 rpm, chopper 1500–3000 rpm), a 10% w/w PVA binder solution is added via a pressure pot over 2–4 min, followed by wet massing for an additional 1–2 min. Granulation end-point is reached when the power consumption plateau indicates a granule size d50 of 150–300 µm; prolonged massing yields overgranulated, friable granules that generate tablets with capping tendency. PVA powder is hygroscopic; for accurate dispensing when ambient relative humidity exceeds 60%, the material must be pre-dried at 60 °C for 2 hours to reduce moisture content below 1.0% w/w. This pre-treatment avoids batch-to-batch viscosity drift in the binder solution, a factor that directly influences granule density and final tablet hardness.

What Differentiates PVA from Cellulosic Binders in Immediate-Release Tablet Formulation?

The binding performance advantage of PVA over hypromellose (HPMC E5) and povidone (K30) becomes measurable in placebo dicalcium phosphate dihydrate (Emcompress®) tablets prepared under matched granulation conditions—identical granulating fluid apparent viscosity of 150 mPa·s at 20 °C, 500 mg target tablet weight, and compression force of 10 kN on a Korsch XL 100 rotary press. Table 1 illustrates crushing strength data (mean ± SD, n = 10) obtained on a Schleuniger 6D hardness tester following Ph. Eur. 2.9.8. The data confirm that PVA delivers consistently higher compact hardness at each binder level, reducing the amount of binder needed to achieve a target breaking force of, for example, 80 N. At 5% w/w binder, PVA exceeds HPMC by approximately 33 N and povidone by 47 N. Despite the higher crushing strength, disintegration times remain comparable: tablets with 5% w/w PVA disintegrate in 8–12 min in water (Ph. Eur. 2.9.1), similar to HPMC-based formulations. The underlying cause is not solely film strength; PVA solutions also display Newtonian flow behaviour up to 80 °C without thermal gelation, unlike HPMC E5, which forms a three-dimensional gel above 30 °C. Consequently, a 20% w/w PVA 5-88 solution exhibits a viscosity of 180–220 mPa·s (ISO 2555:2018) and remains sprayable, whereas a 20% w/w HPMC E5 solution exceeds 1500 mPa·s and undergoes plug-flow behaviour, limiting its applicability in high-solids coating or binder spraying. This rheological distinction, combined with the absence of a gel-sol transition, permits the use of PVA as a binder in both low-shear and high-shear granulation without the risk of premature gelation at localized hot spots in the granulator.

Tablet weight 500 mg, compression force 10 kN, n=10; hardness measured with Schleuniger 6D.

Film Coating Performance and Defect Mitigation

Aqueous PVA-based coating systems—available as fully formulated powders incorporating plasticizer, opacifier, and anti-tacking agent—are reconstituted to solids contents of up to 25% w/w. The low solution viscosity at moderate solids (15–20% w/w) supports higher spray rates and reduced processing time in perforated pan coaters. In a 24-inch Accela-Cota pan with a 5 kg tablet charge, the operational parameter set must remain within the ranges summarized in Table 2, which derive from commercial-scale coating campaigns and correlate with specific film defects. Failure to control these parameters leads to visual and functional defects such as orange peel, edge chipping, and twin formation. The glass transition temperature of the plasticized PVA film is typically 35–45 °C; maintaining the product bed temperature just above this range ensures film coalescence without blocking. Without adequate plasticizer—commonly PEG 400 at 10–20% of polymer weight—the elongation at break of the PVA film falls below 10% (ASTM D882), causing cracking along tablet edges. When correctly plasticized, the average surface roughness (Ra) remains under 1.5 µm (stylus profilometry), yielding glossy, aesthetically acceptable tablets. A processing limitation appears at high storage humidity: above 75% RH PVA-coated tablets develop surface tack due to water uptake exceeding 20% of the film mass, necessitating alu-alu blister packaging for moisture-sensitive products.

When applied as a subcoat at a weight gain of 2–4 mg/cm², a partially hydrolysed PVA layer functions as an efficient oxygen barrier. Cast films of 50 µm thickness exhibit oxygen transmission rates under 5 cm³/(m²·day·atm) at 23 °C and 50% RH (ASTM F1249), in stark contrast to HPMC-based films, which typically exceed 50 cm³/(m²·day·atm). This permeability profile eliminates the need for a separate oxygen-barrier coat when applying methacrylic acid copolymer enteric coatings over oxidation-prone actives; additionally, PVA does not require a post-coating thermal curing step, unlike aqueous ethylcellulose dispersions (Surelease) that demand approximately 1 hour at 60 °C for complete coalescence—a process that can degrade thermolabile drugs. Published moisture vapour transmission rates for plasticized PVA films range between 100–200 g/(m²·day) at 37 °C/90% RH, positioning the polymer as a functional moisture-barrier subcoat for hygroscopic APIs when combined with an outer enteric layer. Aqueous PVA solutions are incompatible with borates and strong chelators capable of crosslinking the polymer; any formulation containing sodium tetraborate as a buffer will induce immediate gelation and must be avoided.

When Rapid Disintegration Is Required in Orodispersible Tablets

Low-viscosity PVA (3.0–4.5 mPa·s at 4%) acts simultaneously as a dry binder and a disintegration enhancer in fast-melt tablets. Direct-compression mixtures incorporating 1–3% w/w PVA together with 5% w/w crospovidone yield tablets that disintegrate in under 30 seconds (Ph. Eur. 2.9.1) when compressed to a hardness of 20–30 N. The synthetic origin of PVA eliminates concerns regarding transmissible spongiform encephalopathy associated with animal-derived gelatin, and its performance is independent of the pH of saliva-like fluids. Dissolution testing performed in 900 mL of pH 6.8 phosphate buffer using USP Apparatus II at 50 rpm shows complete drug release within 5 minutes. Such formulations benefit from PVA’s low equilibrium moisture content, which retards hydrolytic degradation of moisture-sensitive active pharmaceutical ingredients during shelf storage at 25 °C/60% RH.

For continuous twin-screw wet granulation employing a co-rotating extruder (Thermo Scientific Pharma 16 HME, L/D 40:1, 16 mm barrel diameter), a 10% w/w aqueous PVA binder solution is metered into the powder feed at a liquid-to-solid ratio of 0.15–0.25. The screw configuration integrates conveying elements in the feed barrel followed by staggered 30°, 60°, 90° kneading blocks to densify granules without exceeding a melt temperature of 70 °C; local temperatures above this threshold initiate premature film formation that obstructs downstream milling screens. Granules discharged from the extruder are dried in a segmented fluid-bed dryer to a loss-on-drying specification of 1.5–2.5% w/w. The resulting granules display a Carr index of 12–18, indicating excellent flowability suitable for high-speed rotary tablet compression at outputs exceeding 100,000 tablets/hour. In this configuration, PVA provides markedly stronger granules than equivalent concentrations of hydroxypropyl cellulose (HPC-L), which frequently results in tablet hardness variability due to granule attrition during feed frame passage.