DUAL DENSITY NONWOVEN COMPOSITES FOR FILTRATION 

Conrad D’Elia
National Nonwovens
Easthampton, Massachusetts, U.S.A.

INTRODUCTION
The nonwoven industry continues to expand the limits of technologies to improve its ability to isolate fine particles with greater efficiency.  As this development of new products has accelerated, industry has maintained a parallel course to write as many patents as possible to protect its technology.

In focusing on these new technologies and patents, many customers have ignored established technologies and failed to consider their many positive attributes.  Our customers have now begun to assess the new products and realize that the new materials do not satisfy market demands.

This paper will explore the effect that needlepunch technology and products can have on dual density composite filters to enhance their moldability, pleatability, cutting, processability, durability, and cleanability.

CURRENT PRODUCT OFFERINGS
Nonwovens cover a broad range of manufacturing technologies that are established and maintain a core base of products for filtration.  Markets that continue to use these products are:  bag house filters, furnace filters, filters for the automotive industry, and liquid filtration media, to name a few.  Each product has been nurtured to ensure that they meet the changing demands of the market.

 With the introduction of several new melt blown technologies, the marketing efforts assured that the developing technology would provide a superior media:  a medium providing a highly efficient submicron particle filter.  This attribute, although paramount, was found to be insufficient to allow the use of this material as a homogeneous filter.  The lack of fiber entanglement and bond strength in a melt blown fabric results in: low tensile strength, poor abrasion resistance, and minimal durability. Combining a spun bonded nonwoven with a melt blown provided the support and protection to process the melt blown into a usable filter. This concept produced an efficient dual density filter for the following products:  face masks, clean room air filters, and HEPA filters.

What is currently being offered is an efficient melt blown nonwoven and a shell fabric.  This is today’s highly efficient dual density filter -- a dual density medium and a single component filter.

CURRENT NONWOVEN PROCESSING TECHNOLOGIES
The expansion of nonwovens into a diverse array of markets has promoted the development of many new processing technologies.  Our industry began with the development of wool felting and continues to expand.  The following methods are now used to produce nonwoven fabrics: 

            Wool Felting

            Chemically Bonded (wet laid)

            Needlepunch

            Chemically Bonded (dry laid)

            Spray Bonded (high loft)

            Spunbonded

            Thermal Bonded

            Hydroentanglement (spunlaced)

            Melt Blown.

Each method of manufacturing results in a defined set of attributes that overlaps with other processes but clearly characterizes each technology.   The design of a filter for a specific application will require a specific media. The chart, Nonwoven Filtration Processing Technologies and Attributes, outlines the qualities characterizing each type of nonwoven.

The values in the chart establish parameters for each processing technology.  The selection of the material for individual components of a dual density filter  results in meeting the customers’ needs. The chart, Market Requirements, outlines the current market requirements for dual density filters. 

Upon careful review, it is evident that needlepunch products are more diverse than other products.  They can provide a wide range in:  permeability, weight, density, width of production, and durability.  They have the ability to produce a wide range of depth loading capacity not only in volume but also in particle size.  The durability and amount of abrasion resistance that a product has will directly affect the degree of post-treatments and processing that can be applied to the fabrics.

POST TREATMENTS FOR FILTRATION FABRICS
A fiber matrix in any orientation may not be sufficient to provide all of the attributes for the required filter.  The addition of post-processing can enhance the capabilities of the filter to increase its:  efficiency, chemical resistance, durability, abrasion resistance, cleanability, and thermal stability.   The following post-treatments are essential and should be provided by the nonwoven manufacture for today’s filtration fabrics. 

            Chemical treatment -  PTFE (fluorocarbon)

                                               Silicone

                                               Hydrophilic & Hydrophobic finish    

                                               Latex impregnation

            Electrostatic charging

            Membrane, film addition

            Heat set

            Singe

            Glaze

            Press

Each of these treatments must be able to be applied to the fabric in various amounts.  The construction of each product must be mechanically stable to prevent any degradation of the formation, strength, and density while undergoing additional post-treatment and processing.   Clearly, needlepunch nonwovens provide the highest degree of product stability while offering a wide range of other attributes for filtration needs.

DUAL DENSITY COMPOSITES
The concept of a dual density filter has been gravely misused to describe a combination of a nonfiltering outer cover and a melt blown.  Today’s market requirements necessitate that dual density filters be engineered to be a dual density composite filter:  a filter where each component expands the filtering capabilities of each other component.   Needlepunch nonwovens and melt blowns can be incorporated into a composite filter through either needlepunching or laminating.  Both methods will enhance the attributes of the structure.  This is possible by the selection of a multifunctional needlepunch nonwoven. 

 The following functions characterize needlepunch nonwovens:

              High depth loading

            Durability

            High abrasion resistance

            Gradient density fiber orientation

            Fine denier blends ranging from .05 to 60 denier

            Engineered modulus in XY axis.

 The addition of the needlepunch nonwoven to the filter core ( a melt blown) is viewed as a prefilter.  This is the primary stage of filtration that must assume an expanded role in the composite for both current and future filter requirements.

PREFILTERS
An engineered prefilter must have the following attributes:            

            Isolate and collect large particles without affecting the efficiency of the melt

            blown

            Pleatable and moldable

            Ability to accept topical, mechanical, thermal, and chemical treatments

            Cost effective 

The use of needlepunching technology to combine prefilters with melt blowns can be accomplished with a wide range of needle penetration and fiber entanglement.  A successful product occurs with the proper understanding of needlepunching technology. The process of selectively interlocking specific amounts of fiber is possible when the proper needles and needling is applied.  The combination of multiple supported or unsupported prefilters with a melt blown produces a secondary composite substructure within the system.  The staging of several prefilters on either side of the melt blown produces a dual density nonwoven composite.

NEEDLEPUNCH DUAL DENSITY COMPOSITES
Needlepunching technology has evolved to a level where as many as 20 individual layers can be combined to form an engineered structure with a weight of 256 oz/yd2 at one inch thick.  This material is capable of being cut and machined into cylinders to be used for filtration and oil wicking.   The technology that makes this possible can be used to combine heterogeneous nonwovens into a multifunctional composite filter. 

Low density needlepunch composites can be interfaced with varying degrees of needling to create a complex prefilter that is laminated or needlepunched into a melt blown.  With the efficiency of needling, the use of staple fibers, three-dimensional orientation of fibers, and interlocking of layers, the needled composite can be stabilized to undergo further processing.   This allows the composite to be slit and/or dye cut into complex shapes without being deformed and or delaminated. 

Through needlepunching, the degree of interlaminate strength can be adjusted to allow isolated layers to move and elongate independently during molding.   Other attributes of needlepunching will enable the composite to maintain its dimensionality during use and cleaning, and provide increased depth loading capacity.  With the addition of new fiber technology, low melt, and splitable fibers to the fiber blend, it becomes clear that the technology of needlepunching will provide a bridging for today’s products to meet future needs. 

FUTURE NEEDS
The market will continue to look for the perfect filter.   This product can only be made with the development of more versatile and complex components than those currently available today.   It will be essential to increase the complexity and capability of the product to allow for its universal acceptance.  This will inevitably increase the cost of the material.  This cost increase must be offset by the product’s increased durability and ability to be cleaned, thereby increasing its lifespan.   

To provide a cleaner environment with toxin free air and water it will be necessary to improve the filtering efficiencies to filter nano particle sizes, gases, complex chemical compounds, and individual elements.  Expanding these limits will further enable the filtering of subatomic particles for research in space and here on earth. 

 Not all of the materials have been developed to accomplish these demanding tasks.  The technology needed to provide a foundation for future developments is needlepunch nonwoven dual and multiple density composites.  Currently, highly engineered needlepunch multiple density composites are being used for the filtering of hazardous gases, viruses, bacteria, and subatomic particles in accelerators.  The ability of needlepunch nonwovens to produce high performance and consistently uniform composites will demand the attention of engineers when designing solutions for our future needs.

SUMMARY
Dual density high efficient filters have been developed to allow the use of melt blown nonwovens in the filtration industry.  The market is now looking for products without the restrictions of a simple dual density filter with a one-filter component.  The need for multi-functional dual density composites will be the driving force for the development of high efficiency filters.  The multi-functional attributes of needlepunch nonwovens will provide the seeds for engineering products to meet the requirements of our current and future needs.

IN CONCLUSION
As composite designs continue to be developed for a wide range of materials, we will need to focus on fiber treatment.  Emerging technology is being developed to etch and/or graph chemicals onto the surface of synthetic and natural fibers.  Once stabilized, additional treatments can be applied to enhance the ability of the fiber to attract and chemically bond itself to the filtrate.   

Preliminary work has resulted in the establishment of reactive sites on fiber surfaces that enable them to crosslink to other polymers, add hydrophilic or hydrophobic properties, and establish a permanent charge.  This, in turn, will enable the filter to be more effective in absorbing gases and nano-sized particles, allowing the media to be cleaned and chemically recharged. 

            Market Trends and competition are creating a more demanding environment requiring more innovation and technology. The key to engineering new technology is with the commitment by material suppliers to generate new ideas that can be transformed into new products providing increased performance over traditional materials. The new emerging needlepunch dual density composite filters reflect our commitment to the filtration market. At National Nonwovens, we will continue to manage new technology to bridge today’s products and technology to meet the future market demands.