Interesting fact: On average, adhesives make up about 2-3% of a disposable diaper by weight. Of course, that amount varies depending on product design, which determines:
- The number of places where adhesives are used
- The adhesive and substrates used
- The adhesive performance requirements
Product design is often based on consumer needs and how to fulfil them. For example:
An incontinent adult may choose to wear a thicker diaper that has higher levels of absorbency at night, when product changes are less frequent. During the day, they may prefer a thinner pant diaper that provides greater discretion.
Each design requires different amounts of adhesive add-on so the overall product can perform as intended. This makes decisions regarding which adhesives are best suited to use in each diaper—and the appropriate add-on levels—extremely important. They really should be made early in the design development process with adhesive experts like those at Bostik.
Why testing add-on is important
Too much—or too little—adhesive is not good. Remaining on-target for add-on levels helps support positive results in many ways. Reasons for testing include:
- Confirming adhesive add-on levels are correctly calibrated
- Supporting consistency of product quality and performance
- Reducing waste and the risk of manufacturing defective or rejected products due to applying too much or too little adhesive
- Controlling costs to help avoid using more adhesive than is necessary to achieve the desired level of performance
Other reasons for regularly testing add-on include line optimisation assessments and product performance audits. In a recent North American market study focusing on disposable diapers for adult incontinence, we measured the use of elastic adhesive in a sampling of products from four different manufactures. This audit helped us compare the consistency of add-on levels and elastic creep performance, as well as to draw some interesting conclusions.
When it comes to test methods for measuring add-on in disposable hygiene products, three very different methods are commonly used by the industry. The first two occur on the production line. The third, the solvent test, is performed offline.
Measuring adhesive add-on with the cup test
Diaper producers do not typically perform add-on analysis using the cup test themselves. Instead, they prefer to employ the tools and technology of trained adhesive testing experts, like those at Bostik, to verify that nozzles are applying the intended add-on in various applications during production.
A carrier is used to collect adhesive from the nozzles for designated amounts of time, and those amounts are then individually measured. This provides a flow in grams per minute. To calculate adhesive consumption in grams per diaper, the flow rate is divided by the number of diapers that would be produced during that time period, and the intended application area per diaper.
Ideally, the test would be completed regularly for all application points of the diaper, but it can be a safety risk to collect samples on hard-to-reach nozzles. In addition, cup tests create interruptions in operations. Also note that the above calculations assume continuous flow applications. For intermittent adhesive flow situations calculations are more complex, and the process is exposed to additional sources of error.
Automated add-on sensors on the line
Some diaper producers utilise intelligent adhesive application systems which can measure the adhesive flow rate. When combined with a camera, it is possible to check that desired adhesive flow is spread on the expected surface. However, if no camera is used, it is not possible to state that the measured flow equates to well-placed adhesive.
The solvent test for adhesive add-on measurement
In order to test amounts of add-on present in finished products, a solvent test is conducted in a controlled laboratory such as those at Bostik. The idea is straightforward:
- Cut and weigh several samples from the finished product
- Remove the adhesive using a solvent
- Allow to dry and weigh again
- Compare the before and after values
In reality, however, many factors can impact the accuracy of solvent-based testing. Due to the small amount of adhesive used, weights are measured to the nearest 0.0001g. At this level of precision, even the smallest irregularities can skew results. Because of this, testing multiple samples is required.
With the solvent test, there are also a few areas of risk that may impact results, but they can be avoided when performed by a highly trained team:
- Incomplete removal of adhesive (This risk can be reduced by selecting the appropriate solvent and protocols)
- The presence of another adhesive in addition to the one being measured
- Fluff fibres being washed away or otherwise affecting testing for core adhesives
- Interaction of substrates with the solvent (Testing the substrates alone can help identify this issue)
Moisture being absorbed from the air into the sample while the solvent is evaporating (A controlled testing environment can help minimise this risk)
Inconsistent procedures when different companies are hired for testing
Don’t settle for issues such as inconsistent product performance, reduced adhesion, or unnecessary expense. You need accurate, consistently dependable results to keep you on track. Rely on well-trained teams using agreed-upon best practices, such as those at Bostik’s various labs worldwide.
We offer a wide range of testing and product audit services, as well as training in market trends, testing, and more. Schedule a meeting with Bostik to discuss product audits and other unique services.