Micro polyester is a synthetic fibre that is commonly used in a range of household applications. It is one of a range of microfibre fabrics with fibre threads that have a diameter of fewer than 10 micrometres (finer than one denier). One denier, approximately the diameter of a strand of silk, is about one-fifth of the diameter of a human hair. Most microfibres are made from polyesters, polyamides or combinations of the two, sometimes also in conjunction with polypropylene. In this article, we will take a look at this synthetic fabric, which is often used in the formation of weighted blankets.
How and Why Micro Polyester is Used in Weighted Blankets
Synthetic fibres such as micro polyester are chosen for use in many weighted blankets due to the fact that combinations of synthetic fibres can be chosen to uniquely select for specific desirable characteristics. Characteristics of micro polyester that can make it useful in an application for specialist bedding include softness, toughness, absorption characteristics, water repulsion, and electrostatics characteristics. As a synthetic fibre, micro polyester can be used to create a wide range of finished fabrics, depending on the desirable characteristics that are required. It is often used to create plush or silky coverings for weighted blankets. Polyester fibre is sometimes also used as wadding within a weighted blanket for its warmth retaining characteristics.
As we will discover later in this article, the very characteristics that are desirable for certain applications can be detrimental in others. For example, while the lack of breathability of such fabrics can give excellent heat retention, it can also lead to overheating. What is more, there are a number of pressing environmental reasons why micro polyester may not be the best choice.
The History of Micro Polyester
Before we look more in depth at the pros and cons of this materials, let us take a brief look at the history of polyester and how microfibres of this sort came to be used in so many diverse applications.
For thousands of years, human beings relied on the inherent properties of fibres found in the natural world. We wore cotton, linen, silk or wool and that was pretty much that, right up until the 20th Century. In the 19th Century early attempts to create artificial threads for clothes were successful and the first commercial production of synthetic fabric (rayon) was achieved in France in 1891, but it was not until the 1900s that commercially viable ‘artificial silk’ companies were formed in the US and around the world.
Production of rayon grew over the first decades of the 20th Century to meet demand. By the mid-1920s rayon could be purchased by textile manufacturers for half the price of raw silk. This was one of the more successful of the early synthetic fabrics.
Research into large molecules and synthetic fibres began in the United States in 1926. Headed by W. H. Carothers, a group of scientists at E.I. Du Pont de Nemours and Co. developed the synthetic fibre nylon. Polyester was another of the polymers discovered by Carothers and co., but their research into this fibre was incomplete. The project looking to polyester was revived by British scientists Whinfield and Dickson, and they patented polyethylene terephthalate (PET) in 1941. PET forms the basis for polyester, as well as other synthetic fibres like Dacron and Terylene. Du Pont bought all the rights to this new fibre in 1946.
Polyester was first introduced to the general public in the United States in 1951. It was billed as a ‘miracle’ fibre that could be worn for 68 days straight without ironing and still look good. In 1958, another polyester fibre, Kodel, was developed by Eastman Chemical Products Inc. Inexpensive and durable, easy to wash and to care for, consumers caught on and the polyester market continued to expand throughout the 1950s and 1960s. Polyester replaced cotton as the most widely used and affordable fabric.
For clothing, polyester was in fashion until the notorious ‘double knit’ polyester clothing of the late 1960s, which led a negative public image of the fabric to arise in the 1970s. Polyester came throughout that decade to be regarded as a ‘cheap’ and uncomfortable fabric – but the rise of luxury polyester fabrics in recent decades is changing the reputation of the material.
The production of ultra-fine polyester fibres dates from the late 1950s. Melt blown spinning and flash spinning techniques were used to create fibres finer than 0.7 deniers. However, at this time, only fine staples of random length could be created and very few applications were discovered.
The most promising of subsequent studies into ultra-fine synthetic fibres were those undertaken in the 1960s by Dr. Miyoshi Okamoto and Dr. Toyohiko Hikota in Japan. They worked on producing ultra-fine fibres of a continuous filament type. Their discoveries led to many industrial applications for synthetic microfibres. One of these was Ultrasuede, one of the first synthetic micro-fibres, which came onto the market in the 1970s. Throughout the following decades, the varieties and uses of microfibres continued to increase, proliferating, especially throughout Europe, in the 1990s. Today, a range of different microfibres, including micro polyester, are used in clothing, bedding, and other textiles.
However, there has also been, in recent years, a growing awareness of the environmental problems and health concerns surrounding synthetic fabrics – issues that we shall cover in more depth below.
The Pros and Cons of Micro Polyester
One of the main benefits of micro polyester fibre is that it can be used to create a number of different fabrics with an incredibly soft skin feel, silky and soft, and with a pleasant texture. Micro polyester is also considered to be a useful fabric where extra warmth is required, though its warmth retention characteristics (more information on which can be found below) does mean that it can easily cause those who run hot at night to overheat, so this is worth bearing in mind when deciding whether or not a micro polyester blanket is a right choice.
Studies have suggested that the bed climate temperature (temperature below the blanket) and relative humidity should be maintained at around 32°C to 34°C, 40% to 60% relative humidity (1). Synthetic blanket materials are often unable to provide this optimal temperature and humidity range and, due to the poor breathability – overheating is a common problem. Low breathability also means that toxin release from such synthetic fabrics is poor, which can have an effect on health.
What is more, some smooth and silky synthetic fabrics made from micro polyester fibres can be slippery, which can cause them to slide off during the night. This is another factor which can determine whether they are a good choice for the weighted blanket application.
Unfortunately, there is another major negative to using micro polyester fabrics: micro polyester, as a synthetic fabric, is not an environmentally sustainable choice.
In order to understand why micro polyester (and other synthetic fabrics) have a negative impact, it is first essential to look at the different stages of the life cycle of the fibre and fabric:
- Micro polyester production
- Processing and treatments
- In-use environmental impact
- Waste disposal problems.
Unfortunately, as you will discover below, there are environmental concerns associated with micro polyester (and other synthetic fabrics) at each of the above life-cycle stages.
Micro Polyester Production:
Polyester makes up about 18% of world polymer production and is the fourth-most-produced polymer after polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC). More than 60% of global PET production is for synthetic fibres (2).
How Micro Polyester is Made
PET (polyester) is derived from a reaction between ethylene glycol (derived from crude oil) and terephthalic acid (derived from ethanol). The reactions involved in the process of creating this synthetic material are idealized as follows:
The Dimethyl Terephthalate Process (DMT):
C6H4(CO2CH3)2 + 2 HOCH2CH2OH → C6H4(CO2CH2CH2OH)2 + 2 CH3OH
n C6H4(CO2CH2CH2OH)2 → [(CO)C6H4(CO2CH2CH2O)]n + n HOCH2CH2OH
The Terephthalic Acid Process
n C6H4(CO2H)2 + n HOCH2CH2OH → [(CO)C6H4(CO2CH2CH2O)]n + 2n H2O
Of course, there is an environmental concern inherent in the use of finite and polluting fossil fuels, from which the core ‘ingredients’ for polyester are derived. For this reason, fibres derived from natural polymer types (cellulose and proteins) are preferred by environmentalists over those derived from the four synthetic polymer types that comprise most of the rest of the textiles market: polyester, polyolefin, polyamide, and vinyl (including acrylic).
Petroleum is not only used in the feedstock (as raw material) to make polyester fibre, but also to generate the energy required to make it. More than 70 million barrels of oil are used to make polyester each year (3). Though less intensive to produce than nylon, polyester still requires more than twice the energy of conventional cotton to produce.
Most polyester is produced in countries like China, Indonesia, and Bangladesh, where environmental regulations can be lax and pollution is rife. Unfortunately, the harmful chemicals produced in the manufacture of this fibre are often emitted untreated to air and water and cause significant environmental damage.
The water used in production is lower than that used for natural fibres. However, run-off can often be mismanaged, is often polluted, and further increases environmental harm in the industry.
The polymers themselves are theoretically unreactive and so unharmful – but the same is not true of the monomers from which they are constructed. These are often carcinogens and poisons which can have severe effects on human health. Unfortunately, within any such material, a small proportion of the monomers will not be converted into the polymer but will be trapped within the polymer chain. These can later be off-gassed into the air or can, as water-borne monomers, find their way into your body. The worrying thing is that these monomers are so toxic that it takes only a tiny amount of these substances to be harmful to human (and animal) health.
Scientific studies suggest that unnatural fabrics derived from polymers can disrupt the endocrine system, the immune system and affect reproductive health, as well as potentially being instrumental in a range of cancers and other illnesses (4). Polyester bedding has also been shown to cause respiratory problems, and potentially cause or worsen skin complains like eczema and dermatitis. (These may be due to the environmental conditions created by the fabrics physical properties and by chemical treatments and dyes on the fabrics involved, as well as the composition of the fabric and monomers trapped therein.)
The Structure of Micro Polyester
With regard to the structure of polyester, it is interesting to note that unlike natural fibres, which have a well-defined structure that is not easily changed, melt-spun synthetic fibres such as polyester have a much more flexible structural mobility. The solid and liquid states of such fibres are only weakly separated by comparatively small temperature differences. Some structural mobility occurs well below the melting point. The nature of the cooling process and subsequent heat treatments means that there can be differences in the structure of finished fibres and fabrics.
While the structure of natural materials like cotton can be described, only broad indications can be given of the structures that are found in melt-spun fibres. Evidence of structure in analytical studies is strictly only applicable to the particular sample being studied and often the provenance is not well specified (5).
Various different bonds determine the structure of synthetic and natural fibres including hydrogen bonding. In polyester fibres and others that are based on aromatic polymers, there is an interaction between benzene rings that also affects the properties of the material.
Polyester, like other synthetic fibres used in the textiles industry, is a linear polymer. With regard to the chemical composition of such fibres, it is important to note that in addition to the basic chemical repeat unit, there may also be small proportions of other groups inserted within chains, at chain ends or within chain branches which are inserted to modify fibre properties such as dyeability.
In order to make synthetic polymers suitable for fibres, they must be partly crystalline. Chains must be regular so that a favourable packing at one point is repeated all along the chains. Chains must also have a shape that allows them to pack together closely and for attractive forces between them to be effective. These attractive forces must also be inherently strong enough. Finally, there must also be a suitable degree of flexibility. All of these requirements are satisfied in the best fibre forming polymers.
PET does not crystallise as readily as nylon. After slow-speed spinning, the PET is amorphous. However, as the chains are pulled into alignment during the drawing of the fibre, they lock into a crystalline register. The crystallinity of the drawn fibre is about 50%. The crystallites are more elongated than in nylon, usually with a length/width ratio of two or more. The structure of polyester fibres is generally micellar with chain folding or more uniform order/disorder, similar to that found in polyamide fibres (6).
The development of high-speed spinning has increased the efficacy and potential of polyester fibre production. As wind up speeds increases, crystallinity increases. Above about 5000mm/min it is possible to produce polyester yarns that can be used in textiles without further processing. Information on the effect of spinning speed on polyester fibre formation, structure and properties can be found in a number of research studies (7).
The mode of formation and history of a synthetic fibre play an important role in determining the fibre’s structure and in turn, in developing its physical properties and their practical utility.
The Physical Properties of Micro Polyester
One of the most important physical properties of micro polyester is, of course, the fineness of the fibre that can now be produced. The development of direct spinning methods have taken polyester filaments to 0.1 dtex (8). Using conjugate spinning techniques and ‘island-in-a-sea’ fibres gives even finer fabrics (9).
The formation of ultra-fine fibre has resulted in the formulation of fabrics that are incredibly soft to the human touch, which feel extremely luxurious, and which are desirable from a human comfort point of view and for skin-feel.
Thermal Properties of Polyester
Polyester (PET) has a density of 1.39 g/cm3, a specific heat of 1.03 J (g K) and thermal conductivity of 140 mW/(m K). These figures can be used to compare this material both with natural fibres and with other synthetic polymers. These figures determine how the fabric behaves at certain temperatures, as well as how heat transfers through the fabric. Broadly speaking, polyester is relatively good at preventing heat from escaping, but as mentioned above, this can have both positive and negative effects. Another interesting feature of polyester fabric is that it contracts rather than expands when heated, with a negative coefficient of expansion of -10 x 10-4 above 80 degrees Celsius.
Moisture Properties of Polyester
Polyester fabrics, like most synthetic fabrics, regain much less moisture relative to humidity than natural fabrics such as cotton. While the recommended allowance or commercial regain figure for cotton is 8.5%, polyester has a commercial regain figure of just 1.5 or 3%. However, polyester beats other polymer fibres in this regard, as many have zero moisture absorption. None of the groups in the composition of polyester attract water strongly, which accounts for the low water absorption.
The ability of cotton and other natural materials to absorb moisture means that they can help keep the skin dry. Since polyester cannot absorb as much water, it can keep water (and sweat) trapped close to the skin, which means that is can be less healthy than more absorbent and breathable materials.
Interestingly, the heat of wetting for polyester fibres is proportional to the surface area (per unit mass). This suggests that the moisture taken up by such fibres is present on the surface of the fibre (10).
Again, that the moisture is held on the surface of the material means that it may remain in contact with the skin rather than being helpfully wicked away.
Low heat gain from wetting also means that polyester is less able to temper the effect of external temperature changes on the human body which it protects. A natural material such as wool, or cotton, for example, will transmit temperature change far more slowly than polyester.
On the other hand, since polyester has low water regain and water absorption, it is quick to dry, which can make it convenient for the householder.
Tensile Properties of Polyester
Most melt-spun synthetic fibres are moderately high in strength and have moderately high breaking tension. This means that they are tough and durable fibres. However, the tensile strength of a particular fabric will vary considerably relative to the amount of drawing and how exactly it was created and processed. Though breaking points are close together, polyester has a markedly higher initial modulus than nylon and polypropylene fibres, which has a practical effect on the handle of fabrics. Manufacturing processes have an important part to play in determining how strong and durable a micro polyester fabric will be, as well as on the molecular weight of the polymer. As the degree of orientation is increased by drawing, strength and stiffness increase and breaking extension decreases (11).
Since, generally speaking, high-quality polyester fabrics are strong, durable and relatively flexible, they tend to be suitable for a wide range of applications within the textile industry, and are often long-lasting. Unfortunately, many polyester textiles on the market today are cheap and of poor quality, and so do not last long.
Micro Polyester Processing & Treatments:
When considering the sustainability of a given material, it is important to consider not only the raw fibre itself but also the processes and treatments it has undergone before reaching the finished product. One of the down-sides of polyester, from an environmental standpoint, is that it generally cannot be dyed using low impact, natural dyes. Unfortunately, lower cost polyester textiles are also far more likely than natural materials to be contaminated with other concerning substances.
Azo-aniline dyes, which may be carcinogens, and which are toxic by means in inhalation of the vapour, ingestion or by absorption through the skin, have been banned in many countries. They are, however, still used on a range of textiles around the world. They can cause a range of skin reactions. Many other synthetic dyes are also considered to pose a threat to human health, as well as to the wider environment.
Heavy metals can also be used in the dyeing process, and these too can have a severe negative impact both on human health and the environment. Heavy metals such as lead, chromium, and mercury are all used in the textile industry and could serve as contaminants that are a danger to humans, and to the planet’s ecosystems.
Another thing consumers should look out for when buying synthetic blankets is that they could have been treated with formaldehyde. While washing blankets or other items thoroughly can remove much of the substance, the residue can still remain. The residue of substances used to clean fabrics, to make them resistant to staining, or anti-microbial, can also remain on finished products and may cause harm to human or environmental health.
While such substances can be found on many different textiles, synthetic fabrics, which are not compatible with more natural methods for dyeing and other treatments and processes, may be more likely to be contaminated with harmful substances or to have released such harmful substances into the environment during their manufacture.
In order to avoid purchasing products that include such harmful substances, it is a good idea to look for certifications, such as Oeko Tex standards, which ensure that harmful products have been avoided throughout a textile’s entire life cycle.
In-Use Environmental Impact: Microplastic Pollution:
Fibres from synthetic fabrics are, also, unfortunately, a major source of plastic pollution during the time that they are in use. While more and more people are becoming aware of the issue of plastic pollution in our oceans and waterways and throughout natural environments, most focus on the plastic packaging that daily enters and leaves our homes. Recycling such items, and reducing the amount of plastic waste that we generate, is, of course, important. But microplastics are a far more insidious and difficult problem.
Unfortunately, micro polyester fabrics are part of the problem, not part of the solution. When a synthetic fabric is washed in a washing machine, plastic fibres will be washed down the drain. Fabrics made from polyester, and other synthetic materials such as acrylic, polyamide, spandex, and nylon may shed up to 700,000 microfibres with each wash (12). They are one of the most insidious and problematic sources of plastic pollution. These micro-particles end up in the water cycle and are found in alarming concentrations throughout all the Earth’s ecosystems, doing huge harm to marine life and other wildlife.
Washing synthetic fabrics like micro polyester may also release other harmful substances, such as those used in dyeing, cleaning, and treatments as mentioned above, into the wider environment.
Micro Polyester’s Lack of Biodegradability:
Another downside to synthetic textiles such as micro polyester is that they contribute to our global waste problem at the end of their useful life. The fibres of synthetic clothing are not naturally biodegradable, meaning that they are a problem that is likely to stick around for generations to come. Natural fibres will biodegrade eventually, completing the cycle, and will not end up in a landfill. For this reason, many feel that they are preferable to successfully transition into a sustainable future for humanity on this planet.
Recycled Polyester Textiles
One of the positives of PET, when compared to other synthetic materials, is that it is fully recyclable. It is possible to reduce the environmental impact of choosing micro polyester textiles by sourcing products that are made from 100% recycled material.
There are two means of recycling polyester. The first is mechanical. In this method, collected and crushed PET plastic bottles or polyester textiles can be used to make polyester fabrics. The plastic bottles are first crushed and purified and turned into polyester material (DMT). They are then turned into polyester chips through a process of re-polymerization and spun into polyester filaments. These are then cut into fibres and used to make the new textile material.
The second method is chemical recycling. In this process, waste plastic products are returned to their original monomers, which can then be re-used to create polymers as in the virgin process.
Choosing recycled polyester textiles can reduce the amount of plastic that ends up in a landfill. According to a study by the Swiss Federal Office for the Environment which was released in 2017, recycled polyester production also requires 59% less energy than the virgin product. WRAP estimates that recycled PET production reduces emissions compared to virgin polyester production by 32%. (Though still uses more energy than regular cotton, organic cotton, hemp or wool.)
It is important to note, however, that many recycled textiles are not themselves recyclable, and so these recycled products will often ultimately still end up contributing to the global waste problem. While some producers do produce textiles that can be infinitely recycled, these are not yet common or easy to access. Those textiles which use a blend of polyester and other materials cannot usually be recycled. Finishes that are applied to the fabric can also render them un-recyclable.
Mechanical recycling accounts for most polyester recycling currently undertaken in the industry. This process is cheaper than chemical recycling and involves the use of no chemicals other than detergents needed to clean input materials. Unfortunately, however, this process cannot usually be undertaken infinitely. The mechanical process degrades the fibre. It can lose its strength and often has to be mixed with virgin fibre. Heating degrades the material, so each subsequent iteration of the polymer is of a lower grade than that which came before.
Chemical recycling, in which polymers are broken back down into their original monomers, does allow for an infinite recycling stream. However, it is not yet widespread nor is it yet a truly closed loop system.
Even recycled polyester textiles cause environmental harm. Many of the harmful substances used to create virgin polyester are also required for recycled polyester and, of course, garments still shed microplastic particles when washed. Unfortunately, therefore, recycling is only ever going to be a part of the solution.
Still, if consumers do wish to take advantage of the beneficial heat retention and ease of washing and care that comes with a micro polyester fabric then opting for a recycled product is the most sustainable choice. If you wish to determine the proportion of recycled material within a product then standards and certifications can help. Look for the Global Recycle Standard’s 100% recycled label.
In conclusion, although polyester is in many ways a remarkable and useful material, its negative environmental impact generally outweighs any of the positive benefits. In order to transition to a more sustainable way of life, we must move away from such petroleum-based and polluting products and instead opt for more natural and environmentally friendly materials.
(1) Okamoto-Mizuno and K. Mizuno, Journal of Physiological Anthropology An official journal of the Japan Society of Physiological Anthropology (JSPA)2012 31:14
(2)Ji, Li Na (June 2013). “Study on Preparation Process and Properties of Polyethylene Terephthalate (PET)”. Applied Mechanics and Materials. 312: 406–410.
(3)James Conca, Making Climate Change Fashionable – The Garment Industry Takes On Global Warming, Forbes, Dec. 3rd 2015
(4)R.U. Halden, Plastics and Health Risks, Annual Review of Public Health, Vol.31:179-194, 21 April 2010
(5)E. Morton and J. W. S. Hearle, Physical Properties of Textile Fibres, Fourth Edition, Textiles Institute, Woodhead Publishing, 2008
(6)E. Morton and J. W. S. Hearle, Physical Properties of Textile Fibres, Fourth Edition, Textiles Institute, Woodhead Publishing, 2008
(7)Shimizu, N. Okui and T. Kikutani. In High-Speed Fibre Spinning, A. Ziabicki and H. Kawai (editors). Wiley, New York, 1985 429.
(8)Bianchi and R. Maglione. In Polyester: 50 Years of Achievement, David Brunnschweiler and John Hearle (Editors), The Textile Institute, Manchester, 1993, p. 196
(9)Okamoto. In Polyester: 50 Years of Achievement, David Brunnschweiler and John Hearle (Editors), The Textile Institute, Manchester, 1993, p. 108.
(10)F. H. Bright, T. Carson and G. M. Duff. J. Text. Inst., 1953, 44, T587.
(11)Marshall and J. R. Whinfield. In Fibres from Synthetic Polymers, R. Hill (Editor), Elsevier, Amsterdam, Netherlands, 1953, p. 437.
(12)Laura Paddison, Single clothes wash may release 700,000 microplastic fibres, study finds, Guardian, 27th Sept 2016