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Tablet hardness testing
Tablet hardness testing
Tablet hardness testing
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Tablet hardness testing is a laboratory technique used by the pharmaceutical industry to determine the breaking point and structural integrity of a tablet and find out how it changes "under conditions of storage, transportation, packaging and handling before usage"[1] The breaking point of a tablet is based on its shape.[2] It is similar to friability testing,[1] but they are not the same thing.

Tablet hardness testers first appeared in the 1930s.[3] In the 1950s, the Strong-Cobb tester was introduced. It was patented by Robert Albrecht on July 21, 1953.[4] and used an air pump. The tablet breaking force was based on arbitrary units referred to as Strong-Cobbs.[3] The new one gave readings that were inconsistent to those given by the older testers.[3] Later, electro-mechanical testing machines were introduced. They often include mechanisms like motor drives, and the ability to send measurements to a computer or printer.[3]

There are 2 main processes to test tablet hardness: compression testing and 3 point bend testing. For compression testing, the analyst generally aligns the tablet in a repeatable way,[2] and the tablet is squeezed between a fixed and a moving jaw. The first machines continually applied force with a spring and screw thread until the tablet started to break.[3] When the tablet fractured, the hardness was read with a sliding scale.[3]

List of common hardness testers

[edit]

There are several devices used to perform this task:

  • The Monsanto tester was developed 50 years ago. The design consists of "a barrel containing a compressible spring held between 2 plungers". The tablet is placed on the lower plunger, and the upper plunger is lowered onto it.[1][5]
  • The Strong-Cobb tester forces an anvil against a stationary platform. Results are viewed from a hydraulic gauge.[5] The results are very similar to that of the Monsanto tester.[6]
  • The Pfizer tester compresses tablet between a holding anvil and a piston connected to a force-reading gauge when its plier-like handles are gripped.[5]
  • The Erweka tester tests a tablet placed on the lower anvil and a weight moving along a rail transmits pressure slowly to the tablet.[5]
  • The Dr.Schleuniger Pharmatron tester operates in a horizontal position. An electric motor drives an anvil to compress a tablet at a constant rate. The tablet is pushed against a stationary anvil until it fractures. A reading is taken from a scale indicator.[5]
  • Kraemer Elektronik's tablet testing system was the first automatic tablet hardness testing system for auto-regulation at tablet presses, invented by German mechanical engineer Mr. Norbert Kraemer in Darmstadt, Germany. The tablets are separated by a patented feeder chute and moved on a horizontal starwheel through different testing stations. The Kraemer Elektronik automatic tablet testing system measures weight, thickness, diameter/length, width and hardness of tablets and capsules.[7]

Units of measurement

[edit]

According to the International System of Units, the units of measurement of tablet hardness mostly follow standards used in materials testing.

  • Kilogram (kg) – The kilogram is recognized by the SI system as the primary unit of mass.
  • Newton (N) – The Newton is the SI unit of force; the standard for tablet hardness testing. 9.807 Newtons = 1 kilogram (at one G, earth surface gravity).
  • Pound (lb) – Technically a unit of force but can also be used for mass under earth gravity. Sometimes used for tablet strength testing in North America, but it is not an SI unit. 1 kilogram = 2.204 pounds.
  • Kilopond (kp) – Not to be confused with a pound. A unit of force also called a kilogram of force. Still used today in some applications, but not recognized by the SI system. 1 kilopond = 1 kgf.
  • Strong-Cobb (SC) – An ad hoc unit of force which is a legacy of one of the first tablet hardness testing machines.[4] Although the SC is arbitrary, it was recognized as the international standard from the 1950s to the 1980s. 1 Strong-Cobb represented roughly 0.7 kilogram of force or about 7 newtons.[8] Although the Strong-Cobb unit is arbitrarily based on the dial reading of the Strong Cobb hardness tester, it became an international standard for tablet hardness in the 1950s until it was superseded by testers using SI units in the 1980s.[6] The Strong-Cobb is a unit with a very unusual name for a unit of measurement since it is named after the company, Strong-Cobb Inc. The inventor of the hardness tester was Robert Albrecht,[4] the plant engineer for the Strong-Cobb Company. He sold the patent to the company for $1.00.

Sources

[edit]
  1. ^ a b c Joseph Price Remington (2006). Remington: The Science And Practice Of Pharmacy. Lippincott Williams & Wilkins. ISBN 0781746736.
  2. ^ a b "Tablet hardness testing". Sotax. Archived from the original on 5 January 2013. Retrieved 16 February 2013.
  3. ^ a b c d e f "Some Information on Tablet Hardness Testing". Engineering Systems. Retrieved 16 February 2013.
  4. ^ a b c US 2645936, Robert, Albrecht, "Tablet hardness testing machine", published 1953-07-21, assigned to Strong Cobb & Company Inc. 
  5. ^ a b c d e "Quality control of solid dosage form". Scribd. Retrieved 16 February 2013.
  6. ^ a b McCallum, A.; Buchter, J.; Albrecht, R. (1955). "Comparison and correlation of the Strong Cobb and the Monsanto tablet hardness testers". Journal of the American Pharmaceutical Association. 44 (2): 83–85. doi:10.1002/jps.3030440208. PMID 14353719.
  7. ^ Charles, Ischi AG (2017). "Automated tablet & capsule testing technology". Scribd.
  8. ^ Russ Rowlett (September 1, 2004). "How Many? A Dictionary of Units of Measurement". University of North Carolina. Retrieved 16 February 2013.

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Tablet hardness testing

View on Grokipedia
from Grokipedia
Tablet hardness testing, also referred to as tablet breaking force testing, is a standardized procedure in that measures the compressive force required to fracture a tablet along its , thereby assessing its mechanical integrity and internal bonding strength. This test evaluates the tablet's ability to resist breakage during production, packaging, shipping, and consumer handling, ensuring consistent dosage delivery and product stability. The importance of tablet hardness testing lies in its role as a key indicator of quality and efficiency, where insufficient can lead to and tablet crumbling, potentially causing content loss or uneven drug release, while excessive may hinder disintegration and . Regulated primarily by the general chapter, the test complements related assessments like (USP <1216>), which measures weight loss from abrasion, to provide a comprehensive evaluation of tablet robustness. In product development, data informs compression force optimization during and tableting, influencing factors such as selection, moisture content, and to achieve balanced mechanical properties. Modern hardness testing employs automated instruments that apply a constant loading rate of no more than 20 N per second or a constant platen movement of no more than 3.5 mm per second, using parallel platens to diametrically compress at least six tablets per batch, with results expressed in newtons (N) and an average value reported. For cylindrical tablets, tensile strength—a more precise metric of internal strength—can be derived from breaking force (F), diameter (D), and thickness (H) using the equation σ = 2F / (π D H). These methods ensure compliance with international pharmacopeial standards, supporting real-time release testing and process analytical technology (PAT) for efficient quality assurance.

Overview

Definition

Tablet hardness testing is a critical quality control measure in that evaluates the mechanical strength of compressed tablets by determining the force required to cause diametric under compression. This process applies a gradually increasing load to the tablet's flat faces until breakage occurs, quantifying the tablet's resistance to crushing and thereby assessing its structural integrity. The concept of tablet hardness testing originated in the 1930s, with the first dedicated apparatus patented by Smith and Grosch in 1934 to address the need for standardized mechanical strength evaluation in early pharmaceutical production. By the mid-1930s, mechanical testers were introduced specifically for this purpose, enabling manufacturers to verify that tablets could endure the rigors of packaging, shipping, and handling without fracturing prematurely. While tablet hardness focuses on compressive fracture resistance, it is distinct from friability testing, which measures a tablet's susceptibility to abrasion and edge chipping during mechanical agitation, and from disintegration testing, which assesses the time required for a tablet to break apart into particles upon exposure to aqueous media. These parameters collectively inform tablet formulation to balance durability with , ensuring effective .

Importance

Tablet hardness testing plays a crucial role in by ensuring the mechanical integrity of tablets, thereby preventing breakage during shipping, storage, and dispensing. Insufficient hardness can lead to tablet , resulting in fragmentation or powdering that compromises dosing accuracy and . For instance, broken tablets may deliver inconsistent active amounts, potentially causing under- or overdosing with serious therapeutic consequences. Optimal tablet , measured as the diametrical force required to break the tablet, is essential for maintaining . Excessively hard tablets may resist disintegration in the , delaying drug release and reducing absorption efficiency, while overly soft tablets disintegrate too rapidly, potentially leading to suboptimal therapeutic effects. This balance ensures timely dissolution and enhances overall drug efficacy, as supported by studies showing that higher hardness correlates with slower drug release profiles. Economically, rigorous hardness testing minimizes waste from defective batches by enabling early detection of formulation issues, thus optimizing production yields and reducing material losses. Regulatory compliance, as outlined in FDA guidelines and USP <1217> standards, mandates hardness specifications for batch release, helping manufacturers avoid costly recalls and enforcement actions—such as warning letters issued for hardness non-conformance—that can exceed millions in remediation expenses. By tying hardness to quality assurance, this testing supports sustainable manufacturing practices and safeguards public health.

Principles

Physical Basis

Tablet hardness testing is fundamentally based on the principles of diametrical compression, a method that applies a controlled compressive force to a tablet to induce tensile failure. The tablet is positioned between two parallel, rigid platens, with the force directed perpendicular to the tablet's flat faces along its . As the load increases, the resulting stress distribution within the tablet generates a maximum tensile stress at the center of the loaded , promoting crack initiation and transverse across the midplane. This failure mode allows the breaking load—the force at —to serve as a proxy for the tablet's mechanical integrity, as higher loads indicate greater resistance to deformation and breakage. The underlying material science of tablet hardness centers on the tensile strength, which quantifies the interparticle bonding established during compaction. These bonds form through intimate particle contacts created by deformation under , where the bonding depends on the balance between bonding area (the total interfacial contact) and bonding strength (the intermolecular forces at those interfaces). Particle behavior during compression varies: deformation enables permanent reshaping and enlargement of contact areas, fostering robust van der Waals, , or electrostatic bonds; in contrast, brittle shatters particles into fragments, potentially increasing the number of contact points but often yielding weaker tablets if subsequent flow is limited, as fragmentation alone does not guarantee sufficient cohesion. To relate the observed breaking load to intrinsic material properties, tensile strength σ\sigma is calculated via the approximation σ=2PπDt,\sigma = \frac{2P}{\pi D t}, where PP is the diametral breaking load (in newtons), DD is the tablet (in meters), and tt is the thickness (in meters). This equation derives from Hertz's 19th-century theory of elasticity for a thin cylindrical disk under uniform diametral compression, assuming linear elastic, isotropic behavior up to failure. The derivation involves solving the biharmonic stress function for conditions, yielding a tensile stress σxx=2PπDt\sigma_{xx} = \frac{2P}{\pi D t} at the disk center (along the loading axis), which is three times the magnitude of the adjacent compressive stress but opposite in sign; failure is presumed to occur when this central tensile stress exceeds the material's tensile limit, with the factor of 2 arising from the nonuniform relative to the average load over the cross-section. While this model idealizes tablets as brittle solids, real compacts exhibit viscoelastic effects that may introduce shear contributions to fracture.

Units of Measurement

Tablet hardness is commonly measured using the kilopond (kp), newton (N), or Strong-Cobb unit (SCU). The newton serves as the SI unit of force and is the preferred standard in modern pharmacopeial testing. Historically, the kilopond and Strong-Cobb unit predominated before the , with the Strong-Cobb unit emerging as a global standard in the 1950s for hydraulic testers. A transition to the newton occurred in the and beyond, driven by international standardization in pharmacopeias like the (USP) and (EP) to align with SI units. Conversions between units are essential for interpreting historical data or equipment outputs, with 1 kp approximately equal to 9.81 N and 1 SCU approximately equal to 7.00 N. For example, a tablet with a hardness of 5 kp equates to roughly 49 N. The following table provides key equivalents for reference:
From UnitTo Newtons (N)To Kiloponds (kp)To Strong-Cobb Units (SCU)
1 kp9.8111.40
1 SCU7.000.711
1 N10.100.14
In calculations such as tensile strength, the breaking force is expressed in newtons to ensure consistency with SI principles.

Equipment

Types of Testers

Tablet hardness testers are broadly categorized into manual and automated types, each designed to apply diametral compression force to measure the breaking point of tablets. Manual testers, such as the and Strong-Cobb models, represent early designs that rely on mechanical mechanisms for force application. The tester, a lever-based instrument invented over 50 years ago, consists of a barrel with a compressible spring positioned between two ; is placed on the lower plunger, and the upper plunger is released to apply until breakage, with hardness read from a scale typically ranging from 0 to 20 kg. These testers offer advantages like low cost, portability, and simplicity for small-scale operations, but they suffer from drawbacks including operator variability in force application, inconsistent results due to subjective handling, and mechanical wear over time that reduces precision. Similarly, the Strong-Cobb tester employs an anvil mechanism powered by a manual hydraulic or air pump that pushes against a stationary platform, with force measured via a hydraulic meter and results expressed in arbitrary Strong-Cobb units (approximately 1 SC = 7 N). Known for its rugged construction and user-friendly interface, it was introduced in the 1950s and remains suitable for basic quality control, though it shares manual testers' limitations of labor-intensive operation and potential for human error in placement and reading. Automated testers, in contrast, incorporate electronic controls and sensors for higher precision and throughput, often featuring digital displays for and capabilities to measure multiple parameters like alongside . Examples include the VK 200, a microprocessor-controlled model that processes up to 250 tablets per run with constant-speed compression and automatic recording of minimum, maximum, and average values, supporting load ranges from 4 to 350 N. These systems provide benefits such as reduced operator variability, efficient data analysis, and compliance with standards like USP <1217>, though they require more initial investment and maintenance. The Erweka TBH 28 exemplifies semi-automated designs, capable of testing tablets from 3 to 30 mm in diameter with a force range up to 500 N, including features like LCD displays and automatic zeroing for enhanced reproducibility in pharmaceutical settings. Specialized automated testers further differentiate by compression mechanism: linear systems apply force along a straight path for standard diametral testing, while rotary models use rotating feeders for high-throughput environments, handling hundreds of samples sequentially with load cell ranges typically between 50 and 500 N to accommodate diverse tablet formulations.

Operation and Calibration

Tablet hardness testers operate by compressing a tablet between two opposing platens or jaws until it fractures, measuring the peak force required to achieve breakage. The tablet must be aligned consistently, typically with its perpendicular to the platens for round tablets, ensuring the force is applied uniformly across the compression axis to induce tensile . Platens are precision-ground, smooth, and parallel, with faces larger than the tablet's contact area to prevent . In manual testers, the operator manually adjusts the knob or lever to apply force, while automated models use a motor-driven system for controlled platen movement. The is ramped up at a constant rate of no more than 20 N/s, or equivalently, platen movement not exceeding 3.5 mm/s, to maintain controlled loading and minimize variability in results. Upon detecting the —via a sudden drop in or displacement in automated systems—the tester automatically records the maximum breaking and halts the compression. This shutdown criterion ensures the measurement captures only the true fracture point without excessive deformation. Operators should verify tablet positioning before each test to avoid inconsistencies. Calibration of tablet hardness testers involves verifying the accuracy of the force measurement system using certified reference standards. Standard procedures include zero-load verification to ensure no baseline offset, followed by applying known forces with calibrated weights (e.g., 5 kg, 10 kg, 20 kg) traceable to national institutes like NIST, checking against nominal values with tolerances of ±2%. Alternatively, certified reference tablets with known breaking forces can be used for dynamic calibration, simulating actual testing conditions. The or sensor must achieve precision to within 1 N, with full-range verification up to 800 N for comprehensive coverage. Calibration should be performed periodically, typically every 3 to 6 months or after repairs, and documented to maintain through an unbroken chain to SI units. Routine ensures reliable performance and prevents errors. Platens and jaws require regular with a dry lint-free cloth or soft to remove tablet fragments, residues, or that could alter contact surfaces; a damp cloth may be used sparingly for stubborn buildup, followed by drying. integrity is verified during , but daily visual inspections for damage or wear are recommended. Common issues, such as misalignment of platens leading to uneven force distribution and false low readings, can be troubleshot by realigning components per manufacturer specifications and re-verifying with standards. Batteries in integrated thickness or modules should be checked and replaced as needed to avoid indirect impacts on overall testing accuracy.

Procedures

Testing Methods

Tablet hardness testing begins with careful to ensure representative and accurate results. A typical batch sample consists of 10 to 20 tablets selected randomly from the production lot to account for variability, with a minimum of 6 recommended for statistical reliability. Tablets are first dedusted to remove any loose particles that could affect measurements, and their physical dimensions—such as for round tablets and thickness—are measured using a caliper or integrated tester for normalization purposes, often to calculate tensile strength from the breaking force. The execution of the test follows a standardized to maintain consistency. Tablets are placed between the platens of the hardness tester in a consistent orientation, typically diametral compression for round tablets (perpendicular to the flat face) or along the longest axis for oblong shapes, to simulate real-world mechanical stress without bias from selection order. Force is applied at a constant rate, such as a loading speed not exceeding 20 N/s or a platen movement of 3.5 mm/s, until the tablet fractures; the peak force at the point of breakage is recorded in newtons () with a precision of at least 1 . This process is repeated for each tablet in the sample. Post-testing calculations involve computing the of the breaking forces across the sample, along with the standard deviation to assess variability and ensure batch uniformity. For enhanced analysis, the breaking force may be normalized using measured dimensions to derive tensile strength via established formulas, such as σ=2PπDT\sigma = \frac{2P}{\pi D T} for cylindrical tablets, where PP is the breaking force, DD is the , and TT is the thickness. Variations in testing methods accommodate different laboratory setups and throughput needs. Single-station testers are used for manual or low-volume testing, where each tablet is loaded and tested individually, while multi-station automated systems allow simultaneous testing of multiple tablets for efficiency in high-production environments. These automated setups often integrate hardness testing with complementary assessments, such as weight variation and dimensional measurements, in a single to streamline .

Standards and Guidelines

The (USP) General Chapter <1217> provides standardized guidelines for tablet breaking force testing, emphasizing the measurement of mechanical strength to ensure product during development and manufacturing. This chapter specifies that breaking force should be measured using calibrated instruments reporting in kiloponds (kp) or newtons (), with 1 kp equivalent to 9.8 , and requires a minimum of six individual tablets to be tested for statistical reliability. While no universal acceptance range is mandated, typical breaking force values for most conventional tablets fall between 4 and 8 kp to balance durability against friability and disintegration concerns. The (Ph. Eur.) section 2.9.8 on resistance to crushing of tablets aligns closely with USP <1217> in but requires testing of 10 tablets (denoted as N units) to determine the mean, minimum, and maximum forces needed to crush them, expressed in newtons with a precision of 1 N. The procedure mandates consistent tablet orientation—such as diametrically for round tablets or along the longest axis for oblong shapes—and the use of jaws with flat, perpendicular surfaces larger than the contact area to ensure uniform application of force. The (WHO) promotes global harmonization of pharmaceutical specifications through its guidelines on of medicines, recommending hardness testing as part of routine evaluations for solid dosage forms like chewable and dispersible tablets to verify physical integrity alongside , disintegration, and thickness. These guidelines reference international pharmacopeial standards (such as USP and Ph. Eur.) for methodological details, ensuring consistency in testing across regions without prescribing unique parameters for breaking force. Revisions to USP <1217> in the 2020s have incorporated enhanced apparatus specifications, shifted terminology from "" to "tablet breaking force" for clarity, and extended applicability to chewable and lozenge formats while integrating considerations for in routine testing. These updates emphasize through compliance with FDA regulations like 21 CFR Part 11, which governs electronic records and signatures in computerized systems used for testing, requiring secure audit trails, validation, and controls to prevent unauthorized alterations. Similar harmonization efforts in Ph. Eur. and WHO guidelines support automated systems to maintain and reliability in global manufacturing.

Applications and Considerations

Pharmaceutical Applications

In the formulation stage of tablet development, hardness testing plays a pivotal role in optimizing selection and ratios to achieve desired mechanical properties while ensuring . Binders such as (PVP) are commonly evaluated through iterative (R&D) trials, where hardness measurements guide adjustments to concentrations—typically 2-5% w/w—to enhance granule cohesion and produce tablets with target values around 5-7.5 kgf without compromising disintegration or drug release. For instance, in optimizing ferrous sulfate tablets, a D-optimal mixture design incorporated PVP at 3.13% alongside to attain a of 5.5 kgf, meeting (USP) dissolution criteria of over 80% release. This process ensures robustness by correlating data with performance metrics early in R&D. During scale-up from to production batches, testing monitors consistency in internal strength to anticipate and mitigate compression-related issues, such as capping or , across varying equipment scales. By quantifying variations to ensure low variability, manufacturers can adjust parameters like compression force or conditions to maintain uniformity between pilot and commercial runs. In continuous lines, real-time monitoring via has been integrated to track batch-to-batch reproducibility, preventing deviations that could arise from equipment differences. Recent advancements include near-infrared spatially resolved spectroscopy (NIR-SRS) for predicting tablet and other quality attributes in real time. In , hardness testing facilitates in-process checks during to verify mechanical integrity and support overall product specifications, with tablets ideally balanced to avoid excessive that risks poor handling or damage. For extended-release formulations, correlates inversely with dissolution profiles; higher values can prolong drug release by reducing and erosion rates, ensuring sustained therapeutic levels over 12-24 hours. This relationship is critical in matrix systems using polymers like hydroxypropyl methylcellulose, where optimization predicts release patterns aligning with in vivo performance.

Factors Influencing Results

Tablet variables such as , , and significantly influence measurements by altering force distribution and during testing. For instance, round tablets typically experience uniform force application in crush tests, whereas or caplet shapes may require tests to account for asymmetric stress, potentially leading to inconsistent breaking points if orientation is not standardized. Larger tablet s can amplify load-force values, making direct comparisons unreliable without normalization by or thickness. coatings further increase apparent by providing additional resistance to the applied force, which must be considered when evaluating core tablet integrity. Environmental factors, particularly humidity and temperature, affect tablet moisture content and thereby introduce variability in hardness results, often ranging from ±10-20%. Elevated humidity promotes moisture sorption, softening tablets and reducing hardness by up to 39% under combined high temperature (45°C) and relative humidity (75-100%) conditions, while low humidity can cause hardening through moisture loss and excipient recrystallization. Storage conditions exacerbate these effects; for example, permeable packaging like polycoated paper allows greater moisture ingress compared to protective barriers such as aluminum foil or PVC/PVDC, leading to accelerated changes in physical stability. Measurement errors stemming from operator technique and platen condition can further compromise result accuracy, with improper tablet alignment causing substantial deviations in readings. Worn or misaligned platens may introduce or torsion (exceeding 0.15 mm at 300 N), falsifying peak detection and overall hardness values. To mitigate these influences, ensures consistent orientation and application speed, while environmental controls stabilize and during testing, minimizing variability across batches.

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