Introduction to nonwovens
At NIRI we are sometimes asked, “What is a nonwoven material?”.
And as with most things relating to advanced fibrous materials, the answer isn’t always a simple one.
In this article we’ll try to highlight the key features of a nonwoven material, the basic types that exist, how they differ from traditional fabrics, how they are produced and what their applications are.
We will also introduce some of the environmental considerations relating to nonwovens and what the future may hold for the nonwoven sector.
The true definition of a nonwoven
A nonwoven is a fabric made from fibres or filaments that is bonded together by mechanical (frictional), chemical (adhesive) or thermal (fusion) means, sometimes in combination. Unlike conventional textile fabrics, produced by weaving or knitting yarns together, nonwovens have more complex geometric structures, and the majority don’t contain yarns. This means the physical properties of nonwovens are quite different to woven or knitted fabrics, and they are suited to a different range of applications, which include single use, e.g., hygiene products, and long-lasting, durable products, e.g., automotive headliners.
ISO standard 9092 and CEN EN 29092 provide formal definitions of nonwovens, and are widely accepted by the industry. The trade associations representing the nonwovens industry, such as EDANA, play a key role in evolving the standards, and the following helpfully defines a nonwoven in a single sentence:
“A nonwoven is an engineered fibrous assembly, primarily planar, which has been given a designed level of structural integrity by physical and/or chemical means, excluding weaving, knitting or paper making.”
How do nonwoven materials differ from traditional fabrics?
Nonwovens are not made from yarns that are woven or knitted together like traditional fabrics. Instead, they are made by forming a web of fibres or filaments (or both), followed by bonding to give the structure strength using mechanical, chemical, or thermal processes. A variety of different web forming and bonding processes exist, each of suited to different raw materials, and by combining the processes in different ways, it is possible to manufacture fabrics with a range of useful properties, depending on the intended application. Nonwovens also look and feel different to conventional textile fabrics largely because they have different structures.
“Nonwoven materials are made by forming a web of fibres or filaments (or both), followed by bonding to give the structure strength using mechanical, chemical, or thermal processes.”
The nonwoven trinity
Nonwovens can be broadly divided into three categories: drylaid, wetlaid and spunlaid, which relate to the nature of the processes used to produce the web.
Drylaid Nonwovens
Drylaid processes include carding, carding and crosslapping, carding and vertical lapping, and airlaying. These processes are designed to work with staple fibres, rather than filaments, and together represent one of the largest categories of web forming technologies.
Drylaid processes are compatible with both natural and man-made fibres of different fibre lengths and diameters, as well as blends of different fibres and recycled fibres, although there are technical limits.
In nonwovens, worker-stripper carding machines are employed to produce thin webs that are collected on a conveyor belt prior to optional lapping processes (cross-lapping or vertical lapping), which enable production of a thicker structure, before bonding.
Airlaying refers to a family of process technologies some of which are compatible with short cut (<15 mm) fibre, wood (fluff) pulp and even particles, e.g., superabsorbent polymers. Other airlaying systems are designed for longer fibres, and to convert mechanically recycled fibre waste into webs. Airlaid processes involve dispersing and transporting fibres in air, followed by fibre deposition on a conveyor to form a web. Compared to carded webs, airlaid webs tend to have more isotropic properties due to differences in fibre orientation.
Wetlaid nonwovens
Wetlaid processes follow similar principles to papermaking, but the technologies are distinctively different in their design and operation. They are designed to work with staple fibres and not filaments. Wetlaid nonwovens are different to paper because at least 50% by mass of the fibres have a length to diameter ratio >300, or >30% of the content consists of fibre in fabrics with a density <0.40g/cm3.
Wetlaid nonwovens are particularly well suited to short fibres that are difficult to process using drylaid methods. They can be made from wood pulp and natural fibres, a variety of man-made fibres, including high modulus and inorganic fibres, as well as mechanically recycled waste fibre. Wetlaid web formation involves suspending and transporting short fibres in water to maintain their separation, followed by fibre deposition on a conveyor to form a web.
Wetlaid processes also allow very light-weight webs to be produced, with excellent web weight uniformity. Wetlaid webs commonly have isotropic properties, but as in most nonwoven web processes, fibre orientation can be manipulated to some extent, depending on requirements.
Spunlaid Nonwovens
Spunlaid processes include spunbond, meltblowing and combinations of the two, collectively described as spunmelt nonwovens. Unlike drylaid and wetlaid materials, spunlaid webs primarily consist of filaments, and the processes involve extrusion of liquid polymers (normally polymer melts), drawing, solidification and finally filament deposition on a conveyor to form a web, as part of one integrated process.
Most spunlaid systems use melt extrusion to convert polymer chip into filament, which requires thermoplastic polymer feedstocks, e.g., PP, PE, PET, PA, PLA, although the range of compatible materials is gradually being expanded.
Spunbond and meltblowing systems employ different polymer grades, and mechanisms for drawing the extruded liquid polymer, which affect the diameter, morphology and mechanical properties of the final filaments, and the two processes are commonly used in sequence to make composite nonwoven webs, e.g., SMS (spunbond-meltblown-spunbond). Masterbatch additives combined with the extruded polymer allow a variety of physical properties to be modified, depending on final product performance requirements.
A variety of other process technologies e.g., flash-spinning, electrospinning, solution blowing and centrifugal spinning, involving liquid polymer extrusion (solutions and melts) are in operation for specific applications, including for micro- and nanofibre nonwoven fabric production.
“Using recycled or biodegradable materials and implementing sustainable production practices can help to minimise their environmental impact, and these are key areas that NIRI looks to innovate and drive change across the nonwoven sector.“
What bonding processes are used in the creation of nonwovens?
Web bonding follows web formation to produce a stable nonwoven fabric.
Mechanical bonding works by increasing the frictional resistance between fibres or filaments in the web, usually through entanglement. The most common mechanical bonding processes are needlepunching (fibres displaced and entangled by oscillating barbed needles), hydroentanglement (fibres displaced and entangled by high velocity water jets) and stitch bonding (fibres displaced and consolidated by stitching). Particularly for needlepunching and hydroentangling, satisfactory bonding depends to a large extent on the dimensions, modulus and other physical properties of the fibres or filaments in the web, and their ability to be rearranged and formed into entanglements.
Thermal bonding is a convenient method of bonding webs that contain a proportion of thermoplastic polymer, exploiting the fact that fibres or filaments made of these materials can melt, flow and then resolidify following cooling. Fused bond points can be formed either locally affecting groups of fibres, at fibre intersections, or across the full area of the web, depending on how the heat is introduced to the web. The most common thermal bonding methods are calender bonding and through-air bonding. Calender bonding uses at least two nipped and heated rollers, the upper one usually being engraved to enable a localised point-bonded structure to be produced, in which a proportion, rather than the entire area of the web is bonded. Consequently, the fabric remains air and liquid permeable, as well as flexible. Through-air bonding convects heat through the web while under minimal compression, by means of hot air in an oven system, rather than compressing the web between heated rollers. Consequently, fibres are bonded only where they intersect and are in direct contact, resulting in a weaker, but bulkier fabric structure compared to calender bonding. Various means of thermal bonding have been developed, some using means of heating, but calender bonding and through-air bonding are the most common.
Chemical bonding involves applying adhesive binders to webs to increase their strength. These are normally delivered as aqueous latex polymer formulations (rather than polymer solutions), produced by emulsion polymerisation followed by a drying and curing step. A variety of synthetic, bio-based and natural cross-linking binder polymers are selected depending on the cost, physical and chemical properties and biodegradation credentials of the polymers. Binder formulations also contain auxiliary chemistries such as surfactants, pH buffers and pigments to assist with either binder adhesion, or to influence bulk fabric appearance and properties. Fabric properties are also affected by the way in which the binder is applied to the web. Common methods include saturation (impregnation), spraying, foam application and printing, all of which affect the amount and distribution of binder throughout the thickness of the web prior to drying. Given another polymer in the form of a binder is added to the web during chemical bonding, its properties have a large influence on final fabric properties.
What are the environmental impact of nonwovens?
The environmental impact of nonwoven materials is a complex topic and cannot be oversimplified, as there are major challenges and opportunities.
As with other large manufacturing industries, there are significant environmental impacts associated, for example, with the extraction of non-renewable raw materials for nonwoven production, emissions during nonwoven processing, as well as impacts associated with the end-of-life disposal of used nonwoven items, particularly single-use hygiene products, of which billions are consumed each year – the majority of which is not reused or recycled. Currently, there is high dependency on petroleum-based plastics in the industry such as PP and PET, and an increasingly challenging regulatory landscape, together with consumer awareness, that is driving innovation. For example, significant technological developments are in progress relating to advances in melt-processable bio-plastics, fibre-to-fibre recycling and natural renewable feedstock materials that do not compete with food production. Significant innovations in raw materials, nonwoven processes and product design can be anticipated.
Efforts to improve the sustainability of nonwoven materials focus on reducing the environmental impact of nonwoven production and increasing the use of environmentally friendly materials and practices.
Nonwoven technologies now and in the future
At the same time, nonwoven technology is playing a crucial role in enabling sustainable development across many areas. To mention just a few examples:
- Food Security: Nonwovens are widely used to increase agricultural productivity by protecting crops from insect pests and weather-related damage, controlling plant irrigation and nutrient supply, managing plant pollination, as well as to support sustainable food production systems and enable long-term food storage and transport.
- Health and Well-Being: Nonwovens play an extensive role in infection control, packaging and logistics of drug delivery, and are used as important components in many medical devices. Low cost, absorbent hygiene products (AHPs), specifically for continence management and femcare are among the largest global applications for nonwovens.
- Clean water and Sanitation: Nonwovens are extensively used as filter media for the delivery of clean water, filtration of food quality liquids and drinks, as well as membrane supports in reverse osmosis to produce potable water. Various applications also exist in ultrafiltration, with filter media capable of removing sub-micron particles including microbial contamination.
- Sustainable Cities: Nonwovens are contributing to the construction of affordable, energy efficient housing, reducing carbon emissions by improving thermal and moisture management in walls and roofing systems within domestic and industrial buildings, as well as providing multiple new products used by the building and civil engineering industries.
Considering the environmental impacts of a nonwoven product throughout its entire life cycle, from raw material extraction and production through to disposal, recycling or reuse is now as important as understanding its primary function during use. Meeting the challenge of developing nonwoven products fit for a sustainable industry is a key objective of the NIRI team, as we look to improve upon and drive positive change across the nonwoven sector.
The future of nonwovens is incredibly bright, and we are keen to collaborate with partners and clients that have the same desire for change and innovation.