Intelligent adhesives boost filter resistance

5 min read

When comparing a purely thermoplastic to a reactive adhesive, the reactive system is exhibiting a much better performance, with an outstanding flexibility, a very long open time and high heat resistance. All of these are properties which the reactive adhesive achieves due to the chemical crosslinking reaction that takes place after application..

The product range for air filters is vast and covers all kinds of special variations. Even in terms of the relatively limited spectrum of air filters for vehicles, this is still a product range where structural forms, sizes and material choices are almost unlimited.

Adhesives are responsible for the optimum connection of the most diverse structural components and material combinations and are used for flat surface bonds of the filter medium, for the strength of small surfaces in pleating, and for flexibility of the frame assembly. Specially formulated adhesives facilitate high-speed manufacturing cycles, problem-free engineering and application technology, and accurate production of the highest quality for each intermediate product that undergoes further processing, up until to the final step.

There are a number of adhesive systems which increase the reliability and resistance of filter systems, such as the thermoplastic Jowat-Toptherm and the reactive PUR adhesive system Jowatherm-Reaktant.

These products are made for a flat surface compound of activated charcoal filter media and used during the laminating process and for the end product. 

The adhesives enhance the qualities of the filter, because the performance of the activated charcoal filter is substantially determined by the adsorption capacity of its filter medium. The new Jowat laminating adhesives reach good strength levels of the activated charcoal medium in the laminating process, even when the adhesive grammages are low. This ensures that the open surface of the activated charcoal remains as large as possible with only minimal reduction of adsorption and air permeability – even when the filter media consist of several layers. When bonding the activated charcoal onto the carrier material, a low adhesive grammage is crucial for maintaining the actual function of a filter – the filtration and constant supply of clean air – to a high level.

New adhesives also increase efficiency because they are based on polyurethane or polyolefin, have good spraying properties and the high green strength allows fast downline steps, thereby supporting the manufacturing processes. The open time of the adhesives (see Table 1) is adapted to the individual manufacturing methods when the filter fleece materials are coated with activated charcoal. Even when the adhesive bond is subject to downline stress resulting from operations like reeling and unreeling, cutting and pleating of the coated fleeces, the flexibility of the Jowat adhesives will also withstand these mechanical stress factors.

The selection of the appropriate adhesive system must be made taking into account the manufacturing process and the characteristics required from the end product when in actual use. The table below shows a comparison of the relevant process parameters of both adhesive systems.

Table 1: Relevant process parameters and characteristics by comparison (source: Jowat)

Thermoplastic adhesive

Reactive system

Sprayability

 

 √

Flexibility

 √

 √

High green strength

  limited

Long open time >40 s

Short open time <40 s

Heat resistance

 limited

Low fogging

Processing temperature

High processing temperature

Low processing temperature

Following the basic choice of either thermoplastic or reactive system, the specific adhesive is selected according to the requirements of the manufacturing processes and the performance criteria of the filter media which have to be met.

Table 2: Processing parameters of selective Jowat adhesives for the lamination of activated charcoal filter media (source: Jowat)

Jowat-Toptherm® 238.75

Jowatherm-Reaktant® 614.18

Polymer basis

Polyolefin

Polyurethane, reactive

Viscosity at 120 °C

approx. 5,500 mPas

Viscosity at 190 °C

approx. 12.000 mPas

Processing temperature

160-180 °C

110-130 °C

Open time

at processing temperature

approx. 35 s

approx. 40 s

When comparing a purely thermoplastic to a reactive adhesive, the reactive system is exhibiting a much better performance, with an outstanding flexibility, a very long open time and high heat resistance. All of these are properties which the reactive adhesive achieves due to the chemical crosslinking reaction that takes place after application.

Bonds with either Jowat-Toptherm or Jowatherm-Reaktant allow good functionality of the filter medium due to the formulation ingredients which are low in VOCs and neither of the adhesives has any recognizable odour in the finished product. These are characteristics which are expected especially when they are used for the manufacture of passenger compartment filters. Beyond this, low adhesive consumption also results in lower energy requirements, and in consequence reduced manufacturing costs.

Pleating and filter frame assembly

The thermoplastic Jowat filter adhesives for pleating and frame assembly have been developed for sealing and forming properties and to promote a high level of process safety. Apart from good adhesion to paper and fleece materials, structural properties are also required. This means the shape of the filters must remain unchanged over their entire life, even when the material compound is exposed to mechanical forces or high temperatures. The special adhesives for pleating and frame assembly ensure that the filter elements have a lasting product life and a high quality.

Pleating

Diverse filter materials and manufacturing methods require adhesives for the pleating step with special processing characteristics: Jowat adhesives support fast manufacturing and are adapted for this, while the high green strength data are maintained, which ensures fast downline production cycles of the pleated filter media. The adhesives also have a wide spectrum of adhesion for safe bonding of substrates made of paper, fleece and other materials like nano-fibres. When papers impregnated with phenolic resins are used a high temperature resistance is required since curing takes place after pleating, and usually at temperatures above 150°C.

Filter frame assembly

The engine compartments of cars are becoming increasingly more compact and a consequence of that is that the air filter placement often permits only a filter exchange under heavy deformation.

Adhesives for pleating and frame assembly, major quality factors for the filter performance.

(©Jowat SE)

This in turn demands frame bonds with high levels of strength and excellent flexibility. The adhesive, or the bondline, cannot fail and the filter pockets must not develop leaks, which means that the performance of the filter element must not be impaired. Apart from this, the assembly inside the engine compartment also requires superior levels of temperature resistance - depending on the placement of the filter, the filter bond may be exposed to temperatures from 80 to 130°C. The processing characteristics and their impact on the process times can be tailored to the requirements of the manufacturing process (see Table 3). 

Table 3: Examples for formulations with different viscosities, open times and processing temperatures (source: Jowat)

Jowatherm® 214.86

Jowatherm® 262.20

Jowat-Hightherm® 262.90

Jowat-Hightherm® 263.15

Polymer basis

Copolyester

EVA

Polyolefine

Polyolefine

Viscosity at 200 oC

approx. 25,000 mPas

Viscosity at 190 oC

approx. 28,000 mPas

Viscosity at 170 oC

approx. 4,200 mPas

Viscosity at 150 oC

approx. 2,000 mPas

Processing temperature

190-210 oC

160-180 oC

140-160 oC

180-200 oC

Open time

at processing temperature

approx. 15 s

approx. 5 s

approx. 30 s

approx. 15 s

In order to provide adequate stability to the filter media after pleating, the adhesive is applied in bead form for the lateral bonding of the filter pockets, and to hold the folds in place. When assembled into the frame, the adhesive can also be applied with a flat slot nozzle. The necessary strength, flexibility and tightness on the frame are given by the embedding of the ends of the pleats in the adhesive layer. The products listed in Table 3 have proven to be successful for pleating as well as for assembly into the frame. 

Conclusion

The filters used in vehicles play a significant role in terms of its ecological footprint. All air filters have a series of properties in common: the optimum air filter is tailored to the surrounding component architecture, it is tightly sealed and creates minimal differential pressure in spite of its superior capacities of particle removal and absorption.

The filter should constantly maintain the highest possible performance up to the time of exchange. When the adhesive is perfectly matched to the substrates used and their surface properties, to the production processes and the future wear in actual operation of the filters, major savings in processing costs are possible without loss of quality of the end product. Jowat filter adhesives meet the stringent heat resistance demands by car manufacturers, and allow the manufacture of vehicle compartment cell filters and engine intake air filters in OEM quality. Using modern adhesives with special formulations which have no detrimental impact on the performance of the filter is highly desirable both from an ecological and an economic perspective.