Should you buy a cotton weighted blanket? In order to understand the items we choose to buy, it is important for us to consider the materials of which they are made. The sustainability of different materials is paramount when it comes to making eco-friendly choices about the things that we purchase and use around our homes. Materials science is often less than straightforward. But by analysing the life-cycle of different materials, we can begin to build up a picture that allows us to make the best possible choices. In this article, we will examine the example of cotton – a natural material – but not one which necessarily has the greenest of credentials.
Cotton Weighted Blanket Pros and Cons
Cotton is a natural material that has a range of properties that make it suitable for a weighted blanket. This is the most commonly used natural fibre fabric in the world. One of the reasons for the widespread use of the material in bedding is that it can be made into a fabric that is very soft and comfortable against the skin. Unlike synthetic fabrics like polyester and rayon, cotton is a breathable material, which will allow for good air flow. Those who have a sensitivity to texture often choose a fine cotton fabric for a weighted blanket, as the skin feel is comparable to that of high thread count sheets. Cotton is also a durable material that can cope with daily use and unlike synthetic materials, is biodegradable and so will not pose a waste problem at the end of its useful life.
As a natural fibre, cotton is a renewable resource. It conducts heat and dyes well, however, it can turn yellow and become weak when exposed to sunlight for prolonged periods, and has only a low level of flame retardance. In combination with other fibres, the shortcomings and natural weak points of cotton can be improved, which is why they are often used in conjunction with other natural fibres in weighted blankets, as well as clothing and fabrics for other applications.
The properties which make cotton a useful material for clothing, also makes it a useful material in weighted blankets. A number of cotton’s beneficial properties mean that it can enhance the health and well-being of users.
1. The Cotton Plant
Cotton is a soft, fluffy fibre, usually creamy white or yellowish in colour, that comes from plants that are natural to tropical and subtropical regions of the world.
Gossypium hirsutism (upland cotton) is the most common plant used for the world’s cotton production. It is native to Central America, Mexico, the Caribbean and the southern part of Florida, and accounts for around 80% of the world’s production. Three other related plants, Gossypium barbadense, Gossypium arborium and Gossypium herbaceum, are native to other parts of the world and account for the other 20% of world production (1).
The fibre of the cotton plant is the seed hair of the Gossypium plants. After the plants have flowered, they form an elongated pod or boll, within which the fibres begin to grow. As soon as the fibres have completed their growth cycle, the pods burst and the fibres emerge. Each cotton boll contains around 30 seeds, each one of which has between 2,000 and 7,000 fibres attached.
The Structure of Cotton Fibre
In order to understand the physical properties of cotton, and why it is used in weighted blankets and for a range of other applications in the textile industry, it is first necessary to develop at least a basic understanding of the structure of the cotton fibre. The structure of the raw fibre has a bearing on how it is processed and treated, as well as its physical properties as a finished fabric.
Cotton is interesting and has wide application in part due to the fact that it is the purest form of cellulose available in nature. Measured on a dry basis, cotton fibre is typically 94% cellulose, and generally within the range of 88-96%. Less than 10% of the fibre is made up of waxes, protein, pectate and minerals (2).
The fibre has a multi-layered structure that consists of a primary wall, a secondary wall, and a lumen. When viewed under a microscope, cotton fibre looks like a twisted ribbon or a twisted and collapsed tube. It is sometimes described as being kidney-shaped when viewed in cross-section.
The strength of cotton is largely down to its highly fibrillar and crystalline composition. One interesting feature of this structure is that its strength is increased by a quarter when it is wet. Another distinctive characteristic of cotton fibres is that they have a relatively high number of convolutions (twists). These twists give the fibre an uneven surface, which increases inter-fibre friction and makes it easier for fine, long cotton yarns to be spun.
The primary wall of cotton fibre is made up of cellulose and non-cellulosic materials. It has a non-structured orientation which means that the primary wall surface is unorganised and open, which gives the flexibility required during cell growth. The secondary wall is cellulose only, with a highly ordered and compact structure.
The highest quality cotton varieties are those that have the longest fibres, which are thin, with good resistance and elasticity – features that make it easy to spin and allow for the production of high-quality goods.
Cotton fibres range in length from 22-60mm and in diameter from 12-25μm. Higher quality fibres are referred to as long-staple fibres or extra-long staple. Long and extra long fibre varieties come from the cotton plant variety G. barbadense and account for around 10% of world cotton production. Commercially, cotton is usually divided into three different grades: top quality long and extra-long staple length varieties, the most common, medium length fibres, and low-cost, coarse, lower grade fibres. The highest quality fibres usually come from Egypt and the USA. Most of the world’s cotton production falls into the central, medium quality grade.
Fibre length is just one of the factors that determine the quality and price of cotton fibre. Fibre strength, fibre thickness, fineness of the fibre surface (micronaire value), leaf grade and colour grade are also all factors that determine these things, as well as the physical properties of a finished fabric.
2. The Physical Properties of Cotton:
We can gain a somewhat deeper understanding of cotton, its strengths, and weaknesses, by looking at the ways in which its basic structure affects its physical properties and the way in which it performs in different scenarios.
The Fibre Density of Cotton
Fibre density is one physical characteristic that plays a key role in determining the weight of fabrics. Density is the mass of unit volume and is usually expressed in grams per cubic centimeter (g/cm3). Internal void spaces in cotton can mean that it has a fibre density of around 1.35g/cm3, while lumen filled cotton generally has a fibre density of round 1.55g/cm3 when dry. Fibre density changes when a fibre absorbs water (3).
The fibre density of cotton, and its weight when compared to a number of other common textiles, means that it can be desirable for weighted blanket applications, where a heavier covering is required.
The Thermal Properties of Cotton
Other important physical properties for cotton and other fibres involve its thermal properties, including its specific heat. Cotton’s specific heat is 1.22-1.35 J/(g K). Another important factor is thermal conductivity, which determines how quickly heat will dissipate through the fabric. A pad of cotton fibre with a bulk density of 0.5g/cm3 has a thermal conductivity of 71 mW/ (m K). Another interesting area of thermal properties is thermal expansion and contraction. Cotton has been shown to have a coefficient of expansion per degree C of 4×10-4 (4). Comparing these figures with those of other fibres can help us understand how cotton will behave in certain scenarios, and whether it will lead to beneficial or negative results in an intended application.
While cotton is a desirable fabric for weighted blankets due to its soft and breathable qualities, cotton is also desired for its thermal properties. Cotton, due to its fibre structure, is a good conductor of heat, so can be a good choice where overheating could be a concern. However, cotton has a lower thermal conductivity than many other natural and synthetic fabrics, which makes it more difficult for body heat to escape. This can also make cotton a good choice where heat retention is a desirable property. The exact figures of thermal conductivity will depend on the properties of the exact fabric in question.
However, studies have been carried out on jersey materials containing cotton and a proportion of bamboo fibres. One study showed that the thermal conductivity of the fabrics generally decreased with an increase in the proportion of bamboo fibre (5). This could mean that weighted blankets containing bamboo fibres as well as, or instead of, cotton could be better for warmth retention. It is notable that the same study showed that fabrics containing bamboo fibres also performed better with regard to water vapour permeability and air permeability.
Water Absorption & Retention in Cotton
It has long been understood that fabrics take up water from the air. Interestingly, Leonardo Da Vinci has two drawings in his notebooks which indicate experiments which used a measurement of the weight of damp cotton ‘for knowing the quality and density of the air and when it will rain. Even earlier, Nicholas of Cusa had measured the increase in the weight of wool for the same purpose in the 15th Century. Detailed investigations into the subject of water absorption and retention in various fabrics were carried out from the late 19th Century and into the 20th.
The physical property of absorbing moisture is an important feature of materials. In clothing (and in blankets and other materials used close to the skin), a material’s ability to absorb moisture can help to keep the skin dry. Since the absorption of water also changes the thermal properties of a fabric, it can also allow it to act as a heat reservoir and keep the body insulated from sudden changes in external conditions. Another benefit of moisture absorbing fabrics is that, in damp conditions, static is far less likely to be a problem.
Cotton has good water absorption properties. When cellulose is first formed in the cotton plant, it is laid down in the presence of water. This favours the absorption of water, and the resulting structure has few crosslinks, giving rise to the high primary desorption curve. Cotton’s rates of absorption or regain relative to humidity are bettered only by wool, silk and certain types of viscose (such as bamboo fabrics) (6). It is important to note, however, that there can be a big difference in the absorption rates for different types of cotton fibre and cotton materials. The effects of processing can also have a marked effect, and temperature can also have a slight bearing on how much moisture a fabric will absorb.
Unfortunately, while this does have advantages, as described above, it can also mean that cotton has the disadvantage that it can be prone to problems with mildew, and is slow to dry. Moisture absorption and then slow drying can also cause problems with permanent sets and creasing.
One interesting thing to note is that the moisture absorption of cotton is not only technologically important but also commercially relevant. In 100kg of raw cotton, there may be up to 12kg of water. It is expensive to pay for this at the price of raw cotton, and so it must be allowed for in calculating the weight for which a purchaser will be charged. For commercial transactions, a set of values of the recommended allowance have been agreed. The recommended allowance for cotton is generally 8.5%.
Another important factor in the absorption rates of fabrics is how much heat this process generates. The greater the amount of moisture the fabric absorbs, the greater the amount of heat that will be generated. This can bring a physiological advantage that can help a cotton weighted blanket, or an item of cotton clothing, to provide heat in response to changes in temperature and keep users at a more consistent temperature.
In hygroscopic materials, like cotton, a considerable part of any external temperature change is delayed in its passage through the material, and so the human body is given time to adjust. This benefit is not found in non-hygroscopic fibres, with which any delay to the transmission or temperature change will only be due to the fabric’s insulative properties.
Mechanical Properties of Cotton
The mechanical properties of a material are amongst the most important physical properties since they contribute not only to the behaviour of fibres during processing but also to the performance of finished fabrics. The mechanical properties of fibres cover a large number of effects, all of which combine to determine the overall character of that fibre. The most important of the mechanical properties of fibres are their tensile properties – in other words, how they behave under forces and deformations applied along the fibre axis. By measuring how far a fibre can be stretched before it will break, it is possible to determine its tensile strength.
The tensile strength of cotton has been shown in experimentation to vary considerably between different cotton types and varieties (7). As mentioned above, fibre strength is one of the characteristics used to determine the quality (and price) of various kinds of cotton. When considering strength, both the ability to withstand a force and the elasticity displayed before breaking point, are important. It is also worth noting that temperature and humidity also play roles in determining tensile strength and elasticity of cotton and other materials.
Another important element in mechanical properties is elastic recovery. This refers to the extent to which a fibre becomes permanently deformed when it is stretched. In cotton, elastic recovery is almost independent of the variety. However, coarse cotton shows less recovery from given stress than higher quality varieties. Compared to that of other fibres, the recovery of cotton is only moderate. Unfortunately, even small strains can leave an appreciable proportion of permanent deformation (8).
One final, important thing to consider when it comes to the physical characteristics of a given material is what will happen to that material at the end of its useful life. Cotton is a natural material and will biodegrade in a natural environment. Like other natural fibres, cotton is broken down through biotic processes. Microorganisms have evolved certain enzymes which attack bonds within the fibre’s natural polymers. Through this process, they release monomers that can be used as carbon and energy sources to further microbial growth. The microorganisms have not evolved enzymes to break down synthetic materials, which is why these fabrics endure and accumulate in the environment.
Cotton, however, as mentioned above, is highly crystalline in structure. (Aourn 70% crystalline and 30% amorphous). Amorphous sections are more susceptible to biodegradation than crystalline regions, and so cotton will take longer to break down than fibres with a higher amorphous content.
Cellulose is, as described above, the major component of cotton. Cellulose is broken down by microorganisms as follows:
Cellulose ⇒ oligomers ⇒ cellobiose ⇒ glucose (9).
Cotton can be broken down through aerobic or anaerobic decomposition. The breakdown of cotton has been studied in soil, and in vitro conditions. Studies have also been done to study the effect of Mercerization (the treatment of cotton fibres to improve their strength, uniformity, luster, and affinity for dyes) on decomposition. In general, untreated cotton was less susceptible to enzymatic attack than cotton that was treated with ammonia and/or sodium hydroxide. This was due to the fact that Mercerization treatment increased the proportion of amorphous regions in the fibres.
As a general rule of thumb, cotton is compostable in a home setting and will break down within a number of months. A number of different factors, however, will determine how long it takes for cotton to break down, including not only what treatments have been used on the fabric but also its exact composition and level of degradation. Of course, where and how cotton biodegrades – ie in the soil, with or without oxygen etc. – will also have a bearing on how long it will take. If sent to landfill, cotton can often end up decomposing anaerobically, and in the process will release methane – one of the most potent greenhouse gases, and contributing to global warming.
3. The History of Cotton
Cotton was domesticated in the Old and New Worlds in antiquity and has been used to make fabric since prehistoric times. Fragments of cotton fabric have been found in the Indus Valley which date from the 5th Millennium BCE. The oldest examples of cotton textiles were all found in areas with dry climates since in these regions, conditions allowed for the fragments to be preserved. Some of the oldest cotton bolls in the world were discovered in a cave in Mexico. These have been dated to around 5500 BCE, though some doubt has been cast on these estimates. It is certain, however, that cotton was being grown and processed in this region by 3000 BCE.
The ancient Greek historian, Herodotus, mentions Indian cotton in the 5th Century BCE, and when Alexander the Great invaded India, his troops switched from their woolen clothes to more comfortable cotton ones. There were frequent mentions of the quality and vibrancy of Indian fabrics throughout the ancient period.
The spinning wheel was invented in India sometime between 500 and 1000 CE and was used in the Islamic world by at least the 11th Century. It was introduced to Europe in around 1350. Handheld roller cotton gins were used in India from the 6th Century CE and were introduced to other countries from there. Between the 12th and 14th Centuries CE, further advancements occurred, and dual-roller gins were used in both India and China. This innovation had spread throughout the Mediterranean region by the 16th Century CE, by which time, the cotton industry was widespread and trade in cotton had become extremely lucrative in a number of different societies. Christopher Columbus, when he explored the Bahamas and Cuba, found local natives wearing costly and handsome cotton. This may have contributed to his incorrect belief that he had landed on the coast of India (10).
During the 16th, 17th, and 18th Centuries CE, Indian cotton production increased, both in terms of raw cotton and cotton textiles. Agrarian reforms introduced in the Mughal Empire included a new revenue system, which provided state incentives to grow lucrative cash crops like cotton. Cotton textile manufacturing was the largest manufacturing industry in the Mughal empire, and was responsible for a large part of the empire’s international trade, especially as demand for cotton cloth grew in European urban markets during the Renaissance and the Enlightenment. A lopsided trade arrangement developed, and Europeans, with little to offer Mughal India for their cotton, exported large quantities of gold and silver in order to pay for the imports they wanted (11).
New innovations such as the spinning jenny, water frame, and the spinning mule, revolutionized cotton textile manufacture in Britain in the late 18th Century. In the early 19th Century, Egypt had the fifth most lucrative cotton industry in the world in terms of the number of spindles per capita. Steam engines were introduced to the industry here during this time period. In the United States, the invention of the modern cotton gin in 1793 by Eli Whitney greatly expanded the US cotton industry and led cotton to overtake tobacco as the main cash crop in the south. By the 1830s, the US produced the majority of the world’s cotton. Cotton cultivation led greatly to the expansion of slavery in the United States.
It was the British Empire that drove cotton’s rise to global importance. The East India Company imported vast quantities of cotton textiles into Britain, and Indian textiles dominated the market until the 19th Century processes of industrialisation provided labour savings for the British cotton textiles industry, and protectionist tariffs and bans halted Indian dominance. A situation developed in which Britain developed a monopoly over India’s large cotton resources, and surpassed India as the world’s leading cotton textile manufacturer in the 19th Century.
Under the British commercial empire, the global cotton industry continued to grow. Cotton cultivation was expanded to West India and successful trade established with British colonies in the Americas and elsewhere. However, attempts to grow cotton in the Caribbean and West Africa failed due to bad weather and poor soil.
The current global cotton industry was strongly shaped by the influence of British empiricism, which established a strong market for the materials cotton could create, and by the industrial revolution, which shaped the mass industry (often on the backs of slaves, child labour and other forms of exploitation.). During the industrial revolution, various technological innovations and inventions turned cotton into the common, ubiquitous material that it still is today.
It was not until the 20th Century that the US south diversified from cotton and took up the cultivation of other crops. Interestingly, a cotton pest called the boll weevil which caused decades of harm to US cotton cultivation, is considered to have been almost as important as the Civil War as an agent for change in this region, forcing a range of economic and social changes. British cotton industry was at its peak in 1912 yet slumped, and during the interwar period, 345,000 workers left the industry and 800 British mills closed. By 1958, Britain had become a net importer of cotton cloth and by the 1980s, the textile industry in Britain had almost gone entirely.
Cotton textile mills have moved from Western Europe to lower wage areas in more recent years. Though synthetic fibres have increased in market share and taken over from cotton as the most used textiles, demand for cotton globally has still doubled since the 1980s (12).
4. How Cotton is Grown Today?
In order to grow cotton successfully, a long, frost-free period, plenty of sunshine and moderate rainfall are required. Recent advances have involved the use of genetically modified strains of cotton, which have greater resistance to pests and disease and can deliver greater yields, though which are sometimes considered to create problems as well as solving them. There has also been a growth in organically grown cotton in recent years. The market for organic cotton was estimated to be more than $7 Billion in 2010 and continues to grow. Turkey, India, Syria, Peru, the United States, Uganda, Tanzania, Israel, and Pakistan are the top producers of organic cotton today (13).
Unfortunately, the areas were cotton is generally cultivated today often do not get the levels of rainfall requisite for successful cultivation. This means that irrigation is often required, which can make cotton an extremely water-intensive crop. Severe water shortages in regions such as India are exacerbated by the cotton industry (14).
The global average water footprint for 1kg of cotton is 10,000 litres. Even when irrigation is used, US cotton uses just 8,000 litres per kg. The far higher water footprint for India’s cotton is due to inefficient water use and high rates of water pollution.
5. Why Choose Organic Cotton?
Unfortunately, much of the cotton produced around the world use high levels of pesticides and herbicides, which pollute water supplies and the wider environment. Aldicarb, parathion, and methamidophos are three insecticides which are deemed to be particularly hazardous to human health. Unfortunately, these are amongst the ten most commonly used insecticides in cotton production. Cotton accounts for 16% of global insecticide releases – more than any other single crop (15).
Choosing organically grown cotton is a far greener and more ethical choice. It is better for human health as well as for the planet. Organic production can also help to protect the soil, which has a knock on effect in protecting global food security.
6. Where Cotton Comes From
The largest global producers of cotton are China and India. The United States is the largest exporter of raw cotton (16). Where cotton is likely to come from depends on the characteristics of the cotton and its quality level. As mentioned above, cotton is usually grouped into different grades depending on its fibre length. Long-Staple cotton usually comes from Egypt or the United States. Medium staple cotton comes from Central Asia, West Africa, southern Europe, the Middle East, the US, Brazil, and Japan. Lower quality varieties come from Central Asia, the USA, and India.
Where the cotton used in weighted blankets comes from has a huge bearing on its environmental impact and sustainability credentials. While, as mentioned above, the cotton industry in India comes with a large environmental toll, the North American cotton industry is far less environmentally damaging.
Measures such as crop rotation (with wheat, corn, and soybeans), and conservation tillage (reduced tillage or no-tillage) help to protect the soil, and are, in part, responsible for the reduction in environmental impacts such as soil loss on US farms (17).
7. Cotton Harvest
A mono-crop cotton harvest takes energy in the form of polluting fossil fuels. Finite, polluting fuels are usually used in farm machinery. Most of the cotton produced in the United States, Europe, and Australia is harvested mechanically, with a cotton picker or cotton stripper. In developing countries, cotton often continues to be harvested by hand. This can often have a cost to human life and is another ethical concern regarding cotton production in these areas.
Unfortunately, in certain regions, Uzbekistan for example, the cotton industry is responsible for a range of human rights abuses (18).This is another reason to pay attention to where the cotton used to create weighted blankets comes from.
Fossil fuels are also used in lorries/ships used to distribute cotton all around the world for the next stage in the manufacture of the fabric. This is another reason why cotton, which may have to travel a long way to reach processing plants, and its final point of use, is not always an environmentally sustainable choice. Generally speaking, it is best to try to source cotton grown and manufactured as close as possible to where you live.
8. Cotton Fabric Manufacturer
Cotton fibres reach a processing plant where they are carded and woven on a machine (often guzzling tons more non-renewable energy) into a rough, greyish fabric. But this is not the end of the journey and the cotton fabric must undergo further ‘wet’ processing. It is often treated with heat and polluting chemicals, perhaps being bleached, dyed or printed before it reaches the desired softness, finish, and colouring. The ways in which cotton is turned into a finished fabric not only have a bearing on the environmental credentials of the product it is used for, but also has an impact on the physical properties of the cotton, as described above, and its suitability for its intended application.
Dyes & Treatments for Cotton
The toxicity of synthetic dyes often used on cotton are another source of environmental concern in the cotton industry (19). Many textiles are manufactured with harmful chemicals like banned Azo colourants, formaldehyde, pentachlorophenol, cadium, nickle, and lead.
Consumers looking to make an environmentally conscious choice when looking at weighted blankets should also consider the dyes and other treatments added to cotton during the final stages of fabric production. Natural dyes are often used as an alternative to synthetic ones by organic cotton producers. One of the benefits of using a natural fabric rather than a synthetic one is that they will take a natural dye more effectively.
However, it is important to look not only at the types and toxicities of the dyes and treatments used but also at how responsibly companies are ridding themselves of waste water and avoiding pollution of the surrounding environment.
9. Standards & Certifications in the Cotton Industry
We have discussed above the reasons to opt for ‘organic cotton’. But ‘organic cotton’ is not always what it says that it is. It is important to choose certified organic cotton as if it is not certified, the claim can be meaningless. It is very important to look for the certification and to determine by whom it was certified. Certification bodies will depend on your location. For example, in the United States, one common certification body is the United States Department of Agriculture’s National Organic Program. OneCert also provides organic certification worldwide.
In addition to considering certification for organic cotton production, it is also important to understand and seek out other certifications that cover various elements of the entire supply chain and manufacture. Green certifications include Oeko-Tex, Greenguard and SMART. Consumers could also consider looking for a Fair Trade mark, to ensure that producers receive a fair payment and air treatment as they undertake their work.
Cotton can be a desirable material in weighted blankets but can come at a cost. Choosing cotton sourced from sustainable producers and manufacturers can help to make sure that you are making an environmentally conscious and ethical choice.
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(2) ‘Handbook of Natural Textiles Volume 1: Types Properties and factors affecting breeding and cultivation’, The Textile Institute, Woodhead Publishing
(3)‘Physical Properties of Textile Fibres’ The Textile Institute, Woodhead Publishing
(4)‘Physical Properties of Textile Fibres’ The Textile Institute, Woodhead Publishing
(5)Prakash Chidambaram, Ramakrishnan Govindan and Koushik Chandramouli Venkatraman, ‘Study of Thermal Comfort Properties of Cotton/ Regenerated Bamboo Knitted Fabrics’, African Journal of Applied Sciences, 2012
(6) ‘Physical Properties of Textile Fibres’ The Textile Institute, Woodhead Publishing
(7) S. Kawabata. Proc. 4th Japan–USA Conf. Composite Materials, 1988, p. 253.
(8) R. Meredith. J. Text. Inst., 1945, 36, T147.
(9) ‘Biodegradable & Sustainable Fibres’, The Textiles Institute, Woodhead Publishing
(10) Yafa, Stephen (2005). ‘Cotton: The Biography of a Revolutionary Fiber’. Penguin Group.
(11) Karl J. Schmidt (2015), ‘An Atlas and Survey of South Asian History’, page 100, Routledge
(12) “The Cotton Chain – The Facts”.New Internationalist. 399. 1 April 2007.
(13) Vipan Bhat, Rupesh Choudari, ‘Organic cotton vs BT cotton, Colourage, January 2012, pp. 46-48.
(14) ‘The Cost of Cotton in Water Challenged India’, The Guardian, March 2015
(15) EJF, ‘The Deadly Chemicals in Cotton’, Environmental Justice Foundation, London, UK
(16) ‘Cotton-production-worldwide-by-top-countries’, www.statista.com
(17) ‘Perspectives on Cotton: Land Use and Cotton Production’, Cotton Incorporated
(18) EJF. ‘White Gold: the true cost of cotton’. Environmental Justice Foundation, London, UK
(19) ‘The Impact of Dyes’, https://www.trustedclothes.com/blog/2016/06/23/impact-of-dyes/