“Many packaging mills that produce recycled containerboard are facing challenges in achieving the desired strength targets for their products,” recognizes Vladimir Grigoriev, Sr. Manager, Application Excellence, Packaging & Hygiene Solutions Americas at Kemira.

Dry strength of board is an important functional requirement and there is an increasing demand for high performance grades in the packaging value chain.

“The issue is that maintaining and improving strength properties in recycled packaging grades cost-effectively with the available means is a challenging task.”

Here are 3 key reasons why dry strength is a challenge in recycled board production.

1. Deteriorating recycled fiber quality

The first challenge for strength is the availability of high-quality recycled fiber raw materials. The quality of the furnish has a direct impact on the board producers’ ability to build mechanical strength in the recycled board.

“This has been a major challenge in recycled board production in Europe for some time, and the downward trend of recycled fiber quality is visible in other regions as well. There are less virgin fiber materials coming into the recycling process and together with the increasing number of cycles in the fiber recycling, the raw material quality deteriorates,” says Mikko Virtanen, Sr. Manager, Market Development, Packaging & Hygiene Solutions EMEA at Kemira.

Additionally, particularly in North America, board producers are increasingly incorporating mixed waste paper sources into their furnish. These mixed sources have varying quality levels and contaminants, which create additional challenges for maintaining consistent board strength properties.

To compensate for the weaker fibers, the recycled board producers are forced to add higher quality fibers to their furnish, maintain extra sheet grammage or use more strength aids such as starch. This can quickly add to the production cost, hurting profitability and competitiveness in the market.

In many cases, the use of starch as a dry strength additive is already at a limit and increasing the dosage is neither effective nor economical.

“In many cases, the use of starch as a dry strength additive is already at a limit and increasing the dosage is neither effective nor economical. Starch slows down sheet dewatering and drying, which lead to a decrease in machine productivity or an increase in energy consumption.”

2. Difficult and demanding process conditions

The second major challenge in the recycled board production are the demanding wet end conditions at a board machine. The mills often operate with largely closed water loops, recirculating and reusing the process water. As a result, the colloidal and mineral contaminants that come from recycled fibers and mixed waste paper accumulate in the water loop, negatively impacting the process conditions.

“When recycled fibers are used in paper production, it leads to higher conductivity and increased ash levels in the process water. These conditions damage the effectiveness of traditional dry strength chemicals.”

“For example in Europe, the recycled fibers often contain 15–20% of mineral filler and the fresh water consumption can easily be as low as three cubic meters per ton of paper, which can lead to conductivity of up to 7mS/cm. No wonder the traditional dry strength additives lose their efficiency,” explains Mikko Virtanen.

3. Need to reduce the packaging weight

The demand for sustainability in the packaging value chain puts focus on lighter weight packaging materials. The requirement for reducing the unit weight while maintaining high protection functionality is another challenge for dry strength in the recycled packaging grades.

“In recycled containerboard production, lightweighting is currently not as high on the agenda as it is for some of the other grades, e.g. cartonboard. But in the long run, the overall trend will increasingly impact also the recycled packaging materials. These grades are typically the ones that are used in secondary packaging when shipping goods around the world, so reduced grammage and lower weight also support the overall reduction of fuel consumption and CO2 emissions,” points out Grigoriev.

Currently, it is not uncommon that recycled board producers resort to overweighing in order to achieve the needed dry strength. The thicker the board, the better the strength properties.

“Maintaining the strength properties with reduced basis weight enables both economical and ecological benefits. The overweighing is not the most cost-effective way to solve the problem,” states Virtanen.