The possibility of bonding brings many advantages for people. Joining two things together without screws, nails or rivets - this ability is what makes many products from cell phones to airplanes possible in the first place. Adhesives are also one of the most important components in the packaging industry.

After all, a sophisticated wrapper made of cardboard or plastic is of little use if it does not hold together at the crucial points. Just like the packaging materials themselves, adhesives today are also subject to the trend change towards more sustainability and the use of renewable raw materials.

We explain which types of adhesives are in focus for packaging and which sustainable alternatives are being developed or are already on the market.

Sustainable adhesives - an innovation with a long tradition

In the early history of adhesives, only sustainable formulations were used. In the Stone Age, our ancestors mainly used tree resin and wood tar for gluing. The glacier mummy Ötzi, for example, had an axe in his luggage, the blade of which was stabilized by leather straps and birch pitch in the yew wood handle.

Birch pitch is formed as a sticky residue when birch bark is burned and has been known as a primordial adhesive for at least 180,000 years. Much later, at the time of the Egyptians and Sumerians, people then began to cook glue from the bones and hides of farm animals.

Resins and glue made from bones, milk, flour and fish bladders remained the most important adhesives until the end of the 19th century. It was not until the invention of synthetic raw materials that a field of new adhesive types and uses opened up.

A century of synthetic adhesives: improved properties for more uses

In the 19th century, people increasingly sought to develop synthetic adhesives. Scientists wanted to chemically imitate substances with a natural adhesive power, for example tree resins or the molecular chains of starch or protein in glue (natural polymers).

Leo Hendrik Baekeland first achieved this with phenolic resin curing. Shortly afterwards, Fritz Klatte in Germany discovered the first artificial polymer: polyvinyl acetate. To this day, it is considered the most widely used plastic in adhesive production.

Synthetic resin and synthetic polymers opened up new processes and uses. Transparent adhesive tapes were invented, new curing processes using UV light and moisture were developed, and cross-material bonds were possible, for example between plastic and metal.

Meanwhile, the range of available adhesives spans from light tack PSA adhesives that give sticky notes their hold on book pages and objects to robust epoxy-based construction adhesives that bond aircraft parts together.

What requirements should adhesives meet today?

The latest developments are aimed at inventing adhesives that do not bond permanently but can be released again for the purpose of repairs and recycling. This mechanism can be triggered, for example, by the effect of temperature, electricity or a change in pH.

But apart from redissolubility, another property of an adhesive plays the more important role in recycling and environmental protection: the raw materials used and their carbon footprint. This is because both synthetic resins and synthetic polymers are (partly) produced from petroleum. Here, consumers and the adhesive-using industries are calling for environmentally compatible raw material sources to be tapped and alternative formulations to be developed.

Which adhesives play the most important role in packaging?

The following types of adhesives have the greatest importance in packaging:

1. Hotmelt adhesives

   These adhesives are also known as "hot melts" in technical jargon. They are characterized by their tackiness when heated, while they cure at room temperature. Hotmelt adhesives are used in the packaging industry for bonding cartons, bags and envelopes, but also for large-area bonding of packaging coatings. In detail, hotmelt adhesives today consist of

   • Polymers
   • Resin
   • Stabilizers and
   • Wax.
     

   The disadvantage: both polymers and resins in the adhesive are synthetically derived from petroleum products.

2. Pressure sensitive adhesives

   These adhesives remain more or less soft and tacky at room temperature. This makes it possible for Post-Its to be removed again or food bags to be resealed. Since pressure-sensitive adhesives develop their adhesive force through pressure, they are also referred to as PSAs (Pressure Sensitive Adhesives).

   Pressure-sensitive adhesives are frequently used in the packaging industry; for example, as stickers, labels, self-adhesive films and adhesive tapes.

   In earlier times, pressure-sensitive adhesives were obtained from natural rubber and resins. Nowadays, elastic plastics (elastomers) and synthetic resins are used instead, some of which are obtained from mineral oil.

3. Paste and glue

   Paste plays the main role in the production of corrugated board. In order to firmly bond the smooth and corrugated paper webs together, starch paste is used, which is usually obtained from corn. This mixture scores with adhesive strength and flexibility. The latter is important so that the layers of corrugated board do not break apart when rolled and folded during packaging production.

   Starch-based paste made from cereals is considered the most environmentally friendly and health-friendly type of adhesive. Glue for wood and labels, on the other hand, comes in different varieties. Some are based on synthetic ingredients, others use starch or milk components.
   
   

Adhesive type	Hot melt adhesive	Pressure sensitive adhesive	Glue and paste
Property	Liquefies on heating, solidifies on cooling.	Stays tacky, can be stuck on and peeled off multiple times.	Can be processed well due to its high viscosity.
Use	Bonding for cartons and bags	Stickers, adhesive tapes, self-adhesive bag closures.	Corrugated board production, labeling.
Possible disadvantage	Petroleum-based polymers and resins.	Petroleum-based elastomers and resins.	Dairy products, synthetic polymers.
Alternative solution	Resins and polymers made from renewable raw materials.	Resins and polymers made from renewable raw materials.	Bio-polymers from plants.

Sustainability: Why are alternative adhesives now in demand for packaging?

The quest for environmentally compatible adhesives is consistently continuing the developments in sustainable packaging materials. With regard to climate protection and the avoidance of plastic waste, the focus here has for years been on cardboard, bioplastics, and materials made from rapidly renewable raw materials such as hemp paper and grass paper.

The next logical step should be to ask: What adhesives are used to assemble our environmentally friendly packaging and what sustainable alternative products are available for petroleum-based adhesives?

State of the art: Research is in full swing in the development of alternative adhesives

Essentially, two synthetic adhesive components have to be replaced or reconstituted from environmentally friendly raw materials: Synthetic resins and synthetic polymers. Scientists currently see the following alternative solutions:

Synthetic resin from vegetable oil

Petroleum-based epoxy resins are now an integral part of heavy-duty adhesives. The mixture of monomers and a chemical hardener can no longer be deformed once they have been heated and cooled. Researchers at the Fraunhofer Institute for Microstructure of Materials and Systems IMWS have succeeded in developing synthetic resin adhesives based on vegetable oil.

In this process, the fatty acids are epoxidized by the addition of carbon and oxygen so that, in combination with hardeners, they form a solid plastic. At the same time, useful properties of the biosynthetic resin adhesive can also be adjusted, such as electrical conductivity or the addition of antibacterial substances.

The resulting adhesive is solvent-free and based on 86 percent organic raw materials. This ratio is high, considering that adhesives with 35 percent organic content are already considered sustainable.

Polymers from vegetable raw materials

Polymers are long molecular chains that combine to form solid materials. In adhesives, they are first converted into liquid form by solvents or heating before they develop their adhesive strength during drying or cooling. In order to replace synthetic polymers, adhesives researchers are experimenting with different plant-based materials.

Starch from potatoes and cereals, for example, is particularly suitable as a polymer for paper adhesives. Lactic acid, which is not obtained from milk but from plants, serves as a raw material for adhesive resin and potting compounds.

The fiber lignin, which along with cellulose is responsible for the structure of wood and plant material, can act as a primer in adhesives after certain treatment. Even the properties of a pressure-sensitive adhesive that can be stuck on and removed again can currently be replicated with plant-based lactic acid polymers.

What alternative solutions are currently already on the market?

Environmentally compatible adhesives are not just dreams of the future. For example, you can already find hotmelt adhesives in stores that contain up to 90 percent bio-based ingredients, and paste that is 100 percent biodegradable and environmentally compatible.

Vegan adhesives - where do we need to consistently replace animal products?

During the past decades, animal products such as bone meal and fish bladders have been completely replaced by synthetic resins and polymers as adhesive components. However, one type of glue still exists today that is based on milk proteins: casein glue. This adhesive is popular as a temperature-resistant and waterproof wood glue.

It also plays an important industrial role as a label adhesive. The adhesive meets the high demands of the industry: it is easy to handle in automated processes, gives the labels a firm hold even on difficult surfaces, and can be removed without leaving any residue, for example on deposit bottles.

The problem: Aspects of animal welfare and CO2 reduction demand that we reduce the scale of industrial dairy farming and replace dairy products with plant-based products in many fields. Consumers are also sensitized: Those who eat a vegan diet want to rule out the possibility that the labels on vegan spreads and drinks are attached with casein glue.

How does a label glue without dairy products work?

Vegan labels currently pose a challenge to the adhesives industry. Some suppliers are developing alternatives here that contain fully synthetic polymers, which means nothing other than petroleum-based plastics.

However, fossil raw materials do not offer a better solution than milk proteins in environmental terms. Especially in view of the fact that the carriers of the labels, returnable glass bottles and screw jars, are considered to be a particularly environmentally friendly and sustainable packaging solution.

A more suitable adhesive alternative could be vegan starch-based glue. Unlike the fully synthetic glue formulation, no fossil raw materials are used here. From the point of view of "environmental protection and sustainability," this is just as important as the increased avoidance of animal products. The starch required can also be obtained in a resource-conserving way from waste products of the food industry, for example potato starch from wastewater from French fry production.

Take a closer look at sustainable adhesives: Currently, adhesives with 30 to 90 percent bio-based ingredients are on the market, all of which carry the "sustainable" label.

Conclusion: Alternative adhesives - first solutions are usable and much is in motion!

Adhesive strength, flexibility and good processability - the industrial demands placed on adhesives today are high. At present, bio-adhesives are not 100 percent convincing in some fields of application. However, researchers and specialist institutes are working hard to fill this gap in order to be able to replace petroleum-based components 100 percent. There are good prospects for our "bonding" technology to be labeled "environmentally compatible" in the future.

Take a look at our website and be inspired by our diverse offers and information with a view to environmental compatibility and sustainability. If you have any questions or suggestions, we look forward to hearing from you at any time!