# Cured Density and Volumetric Shrinkage in Adhesive Bonding: Key Considerations

Adhesive density must be considered in overall part design by product engineers. Cured density is also used to determine shrinkage calculations for reactive adhesives. It can be calculated by applying the ISO 1183-3 test standard.

## Testing and Simulation of Cured Density

Density refers to the mass of a material per unit volume. Cured density is determined in an adhesive after the curing (hardening) process.

Density measurements are useful for evaluating the uniformity of a material’s quality or to reveal any flaws such as the appearance of voids in bonded parts.

Measuring and determining adhesive density after the curing process provides valuable information that may be leveraged in later assessment protocols, such as volumetric shrinkage.

## Testing Procedure

Huntsman employs the ISO 1183-3 test standard when measuring the density of solid materials. This method utilizes a gas pycnometer instrument for measuring density by gauging any changes in gas pressure as displayed in the instrument upon introducing a specimen to a specific temperature range.

The specimen’s volume is accurately determined using the change in gas pressure displayed on the pycnometer. This, combined with the mass of the specimen, is what makes the calculation of the density possible. This volume measurement technique eliminates the negative impact of any large voids or irregularities in the specimen.

## Test Parameters

Test specimens correspond with ISO 1183-3 standard and may be comprised of powder, granules, pellets or flakes. Other materials can be shaped to any form that is compatible with the size of the pycnometer cell used. Huntsman testing frequently uses a 4 mm cubed specimen.

It is crucial that when cutting or preparing the specimen, any changes in density as a result of compressive stress are avoided. At least two specimens are tested as a minimum using this method, and temperature control remains consistent.

## Data Provided

Density (g/cm3) is determined by applying the following equation:

ρT = mapp / V

where ρT represents the mass per unit volume of a material at a specific temperature T, mapp stands for the mass of a body acquired by measuring its weight using a balance, and V depicts the volume of a body without pores in a three-dimensional space.

## Predict the Impact of Volume Contraction

Volumetric shrinkage testing is a key characteristic that allows engineers to understand and limit part deformation and internal stresses that can cause cracks and long-term failure of a bonded assembly. Huntsman’s testing method follows ISO 1183-3 and ISO 3521 standards.

### Testing and Simulation of Volumetric Shrinkage

Optimizing the curing process is crucial for determining the strength of high-quality adhesives. Reactive adhesives usually shrink (contract) over the course of the curing process as chemical bonds form.

The chemical composition of a reactive adhesive determines the extent to which it will shrink in relation to the cure temperature and speed.

Measurement of an adhesive's density prior to and after the curing process facilitates quantification of the volumetric shrinkage. This allows engineers to make any predictions regarding changes in bond line thickness for an adhesive with a specific set of cure conditions, providing valuable information that relates to internal stress and deformation (warping) in bonded parts.

### Testing Procedure

Huntsman's internal standard for measuring volumetric shrinkage is dependent on receiving accurate data related to an adhesive's density before and after the curing process.

• Step 1: Density analysis of the uncured system in liquid or paste form.
• Step 2: Determine the density of the cured system post-curing and conditioning.

Huntsman applies the ISO 1183-3 test standard for measuring the density of solids, as well as extending the same method to liquid and paste materials.

A gas pycnometer instrument is used to characterize density by measuring changes in the gas pressure when introducing a specimen.

The cure conditions of an adhesive will significantly impact shrinkage, so it is vital that measurements of the cured density are performed once the cure schedule has been applied for the intended application or in line with a customer’s specific requirements.

### Test Parameters

For the uncured state, conditioning of the liquid adhesive components is performed at the measurement temperature prior to mixing. This state is measured immediately to avoid progression of the curing reaction.

For the cured state, a mold is used for the preparation of a cured plate with 4 mm cubes cut for testing. At least two specimens are measured for each state.

### Data Provided

The overall volume shrinkage is determined by a percentage of the change in the density pre- and post-curing.

A positive value indicates volume shrinkage while a negative value signals volume expansion. To calculate volume shrinkage, the following relationship is used:

Shrinkage % = (D2 - D1 ) x 100 / D1

Where D1 represents density when uncured and D2 indicates density after curing.

### Tips for Modeling and Simulation

Volumetric shrinkage is a key data input when simulating process-induced stresses and deformation. Adhesive contraction-induced volumetric shrinkage during the curing process can bring about additional stresses and deformation in assembled parts.

Simulation of residual stresses requires concurrent cure kinetics, heat transfer, and mechanical analysis. The effectiveness of such a model is mostly dependent on the input data accuracy, which describes the volumetric contraction and progression of elastic modulus as a function of the degree of adhesive cure throughout the entire curing process.

This information has been sourced, reviewed and adapted from materials provided by Huntsman Advanced Materials.

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