Natural rubber is a highly versatile and valuable resource that is in high demand worldwide. This article provides a comprehensive guide to the harvesting and processing of natural rubber, as well as quality control and testing procedures. From an introduction to natural rubber to a breakdown of the various forms it can take, this article breaks down the key facts you need to know about this important material.
- Introduction: What is Natural Rubber?
- Harvesting and Processing of Natural Rubber
- Natural Rubber Processing
- Types of Natural Rubber
- Testing of Natural Rubber
- Compounding of Natural Rubber
- Applications of Natural Rubber
- Environmental Impact of Natural Rubber
- Conclusion: The Future of Natural Rubber
Introduction: What is Natural Rubber?
Natural rubber is one of the world's most versatile and valuable natural resources. Harvested from the rubber tree, Hevea brasiliensis, natural rubber latex is harvested and processed into natural rubber. Prized for its strength, durability, elasticity and natural waterproofing properties, most natural rubber is produced for tyres or other tyre-related products. Elsewhere, a significant quantity of natural rubber is used for countless consumer and industrial products, ranging from footwear to aerospace to pharmaceuticals.
Year on year, the global consumption of natural rubber is increasing, rising to 15.53 million tonnes in 2022. This is due to increasing consumer demand and a gradual shift from bias-ply tyres to radial tyres, which require more natural rubber to produce.
While there is a heavy focus on the recycling of end-of-life tyres and other rubber products, which cannot be recycled easily or indefinitely, there are also growing concerns about the environmental impact of natural rubber production. These concerns are centred around deforestation, the use of harmful chemicals and fair-trade practices.
As a primary component of tyres, natural rubber is superior to most synthetic rubber alternatives in several key areas and is often cheaper and more freely available in commodity markets. However, as a natural product, it is more vulnerable to external environmental events. This not only affects the price but also the material properties of the rubber. Alongside production processes, external factors are the other major contributor to inter and intra-batch differences. Therefore, rubber testing and quality control are key practices throughout the rubber industry, not only to maintain quality but to also ensure safety standards in the performance of the product.
Harvesting and Processing of Natural Rubber
The rubber tree, Hevea brasiliensis, is not the only plant capable of producing natural rubber latex, but it is the most prolific and productive. Native to the Amazon rainforest, rubber tree seeds were exported in the late 19th century to The Royal Botanical Gardens (Kew Gardens), England. This coincided with the beginnings of the automobile age, and the rapid need for rubber soon led to the creation of rubber plantations in Southeast Asia. With a similar subtropical climate to that of the Amazon rainforest, the plantations of Malaysia quickly overtook the wild cultivation of rubber as the leading producer of natural rubber.
Today, rubber tree plantations are found wherever favourable climate conditions permit. In general, Asia dominates production, with over 90% of natural rubber produced in Thailand, Indonesia, Malaysia and Vietnam combined. The rainforests of West Africa are also significant contributors to the global natural rubber supply.
To produce natural rubber, the trees must first be grown to maturity, which takes 5 – 7 years. Then, the rubber latex is extracted by ‘tapping’ the tree. Here, a knife is used to make a downward cut at about a 20°-30° angle to the horizontal plane at an approximate depth of 1mm. This action creates a hydrostatic pressure gradient that allows the latex to exude from the incision and flow into a collecting cup. This process is repeated daily, with new ‘taps’ created to encourage latex flow until the tree is exhausted. The cups are collected and the latex is combined to prepare for processing. Overall, the productive life of a rubber tree is around 25 years.
Natural Rubber Processing
Once collected, the latex is taken to a processing facility, often within a plantation or cooperative. The tapped latex consists of 30-35% rubber, 60% aqueous serum, and 5-10% other constituents such as fatty acids, amino acids & proteins, starches, sterols, esters, and salts.
At this stage, the liquid latex can be exported for use in latex-based products, such as gloves and condoms. For those suffering from a latex allergy, the protein, which is responsible for the reaction, can be removed via leaching or deproteinisation, to render the latex non-allergenic.
For all other pathways, the natural rubber latex is coagulated, either naturally or with acid, washed then dried in either the open air or a ‘smokehouse’. When dried with smoke and additives, the natural rubber is afforded some anti-oxidation protection.
Now in a solid form, the raw rubber can be prepared into six basic forms:
- Sheet rubber, technically specified
- Block rubber, technically specified
- Preserved latex concentrates
- Speciality rubber that has been mechanically or chemically modified.
This processed material typically contains 93% rubber hydrocarbon, 0.5% moisture, 3% acetone-extractable materials (sterols, esters and fatty acids), 3% proteins and 0.5% ash.
The flowchart below shows an overview of natural rubber processing and the various methods and pathways that lead to the six basic forms of natural rubber.
Types of Natural Rubber
Technically Specified Rubber (TSR)
Of these types of natural rubber, sheets, crepes, technically specified sheet rubber and technically specified block rubber are in a dry form and represent over 90% of the total natural rubber produced globally. Commercially, these dry forms are available in over forty different grades. Technically specified rubber (TSR) is the most widely produced form of natural rubber, where grading is based on the dirt content measured as a weight percentage.
The chart below shows an overview of the most popular types of technically specified rubber (TSR), including what they are produced from and the most common associated applications.
|TSR CV||Constant Viscosity||Stabilised field latex||Stabilised to a designated constant viscosity. Softer than other grades and used for high-quality products.|
|TSR L||Latex/Light||High-quality latex||A light-coloured rubber with high cleanliness, good heat-aging resistance, high tensile strength, modulus, and ultimate elongation and break.|
|TSR 5||0.05% Dirt||Fresh coagulum, ribbed smoked sheets or air-dried sheets||General-purpose friction and extruded products.|
|TSR 10||0.10% Dirt||Clean and fresh field coagulum or from unsmoked sheets||Generally good mixing characteristics. Typical applications incude tyres, raincoats and footwear.|
|TSR 20||0.20% Dirt||Feld coagulum, lower grades of ridged sheet rubber and unsmoked sheets||A large-volume grade with low viscosity and easy mixing characteristics. For use in tyres, conveyor belts, upholstery and other general products.|
|TSR 50||0.50% Dirt||Old, dry field coagulum or partly degraded rubber||Lowest grade available. Suitable for low-quality, high-volume applications.|
Chemically Modified Natural Rubber
In addition to TSR (technically specified rubber), natural rubber can also be bought and sold in a chemically modified form. This means that the rubber has undergone some form of chemical processing to modify either the properties or malleability of the rubber. Typically, these modifications have broad appeal for a variety of applications. By purchasing pre-modified natural rubbers, compounders can save valuable time and resources.
Examples of chemically modified natural rubber include:
|Liquid low molecular weight rubber||A combination of mechanical milling, heat and a chemical peptizer||Useful for flexible moulds||Liquid at room temperature|
|Methyl methacrylate grafting||Polymerisation of methyl methacrylate in the latex before coagulation||Adhesives||High hardness but can be blended with regular grades of natural rubber|
|Oil-extended natural rubber||Co-coagulation of oil and latex, Banbury mixing of oil and rubber or by the milling of oil-soaker rubber||Used in tyre treads||Improves ice grip and snow traction|
|Deproteinised natural rubber||Natural rubber latex is treated with an enzyme that breaks down the naturally occurring protein and other non-rubber material into water-soluble residues||Used in non-allergy medical gloves and condoms|
|Epoxidised natural rubber||Epoxy groups are randomly distributed along the polymer chain||Compatible with PVC||Improved oil resistance and low gas permeability|
|Thermoplastic natural rubber||Blended natural rubber and polypropylene|
|Superior processing rubber (SPR)||Mix of two natural rubbers, one with and one without cross-linking (vulcanised latex and diluted field latex)||Graded according to percent cross-linking, i.e. SP 20 is 20% crosslinked||A two-phase polymer system with high stiffness, good flow and good processability|
|Ebonite||Rubber vulcanised with very high levels of sulphur||Used in battery boxes, linings, piping valves, pumps and coverings for rollers, where chemical and corrosion resistance is required||Young’s modulus of 500MPa and Shore D hardness 75|
To enhance the processability of natural rubber, additional chemicals such as peptisers can also be incorporated. Peptisers can help to increase the productivity of mixers by lowering mixing temperatures, improving mixing uniformity, and reducing mixing energy. Furthermore, the choice of vulcanisation agents can significantly affect the curing properties of rubber. It is also possible to chemically alter the timings of the curing process with the use of accelerators.
Alternatives to Natural Rubber
As the common rubber tree, Hevea Brasiliensis is just one of many plants that produce rubber latex, other plants can be sources of natural rubber. One of the most promising sources is Guayule, a shrub native to the southern region of the US and Mexico. With a dry weight of approximately 20% resinous rubber, Guayule is commonly touted as a replacement for natural rubber.
However, the primary alternative to natural rubber is synthetic rubber, which is formulated from petrochemicals. Derived from crude oil, synthetic rubber is chemically synthesised isoprene. As an artificial polymer, polyisoprene is more uniform than natural rubber and therefore has higher mixing efficiency.
The most popular synthetic rubber is styrene-butadiene rubber (SBR). Common uses for SBRs include applications that require high tensile strength, resilient tear strength and abrasion resistance.
However, synthetic rubber cannot replace natural rubber in all applications. With a lower cost, ample supply and specific desirable material properties, natural rubber remains a top choice for many industrial applications, particularly in the automotive sector.
Testing of Natural Rubber
As shown, the various processing methods and amount of dirt present can result in big differences in the end natural rubber product. Therefore, natural rubber testing is an essential accompaniment to processing. Rubber testing can take place either at the production facility or before transportation and shipping. As the price of natural rubber is dependent on the quality of the raw rubber, many of the tests performed are standardised to allow buyers to make a fair comparative assessment.
The composition testing of natural rubber involves using rudimentary laboratory equipment to determine the various building blocks and contaminants that make up any typical sample of natural rubber. These simple tests are used to measure the quality of both the rubber and the initial processing procedures that take place soon after harvesting.
The percentage dirt of natural rubber is measured by dissolving a small sample of rubber in a rubber solvent, usually alongside a small amount of peptiser, at 125°C until the rubber has dissolved. This process takes about 3 hours. Then, the rubber solution is filtered through a 325-mesh screen. The remains are dried at 100°C and weighed. The percentage dirt is equal to the ratio of these remains to the original sample weight.
Name: Percentage Dirt
Instrument/Equipment: Rubber Solvent, Sieve, Drying Oven, Weighing Scales
Test Type: Natural Rubber
Material: Natural Rubber
Standards: ISO 249, ASTM D1278
The percentage ash of natural rubber is measured by heating a sample of natural rubber inside a crucible within a furnace at 550°C until the contents turn to ash. When ashing is complete, the crucible is cooled and the contents are weighed. The percentage of ash is equal to the ratio of the ash remains to the original sample weight.
Name: Percentage Ash
Instrument/Equipment: Crucible, Furnace, Weighing Scales
Test Type: Natural Rubber
Material: Natural Rubber
Standards: ASTM D1278
Volatile Matter Content
To measure the volatile matter content of natural rubber, the rubber is first prepared on a laboratory mill to achieve sample strips of a maximum width of 2.5mm and maximum thickness of 1.25mm. The natural rubber strips are dried in either a circulating air oven set to 100°C or a desiccator. After warming, the dried samples are weighed and compared to the original sample weight.
Name: Volatile Matter Content
Instrument/Equipment: Drying Oven, Weighing Scales
Test Type: Natural Rubber
Material: Natural Rubber
Standards: ISO 248, ASTM D1278
Dry Rubber Content
To calculate the dry rubber content, samples of natural rubber are cut and dried for a pre-determined length of time. Using a moisture content balance, the dry rubber content is calculated as the difference in weight before and after the drying process.
Name: Dry Rubber Content
Instrument/Equipment: Drying Oven, Moisture Content Balance
Test Type: Natural Rubber
Material: Natural Rubber
Standards: ISO 126
The nitrogen content of natural rubber is typically measured using the Micro-Dumas combustion method. In this method, the rubber sample is combusted at an elevated temperature in an oxygen environment. The nitrogen released during this process is separated and measured.
Name: Nitrogen Content
Instrument/Equipment: Micro-Dumas Combustion Method
Test Type: Natural Rubber
Material: Natural Rubber
Standards: ISO 19051, ASTM D1278
The Mooney Viscosity of rubber is a specific measurement of the viscosity of unvulcanised rubber performed on a Mooney Viscometer. The measurement uses a unique empirical scale, Mooney Units, which is difficult to map to alternative measurements of viscosity that use standard units. In the Mooney Viscometer, an unvulcanised rubber sample is placed above and below a rotor. As the rotor rotates, the resultant resistance of the rubber to the shearing force generates a torque signal that is measured and interpreted to calculate the Mooney Viscosity. The test is performed at an elevated temperature that is less than the ideal curing temperature of the material.
The main information obtained from a Mooney viscometer includes:
Mooney peak: the initial peak viscosity, which is a function of green strength and a measure of compounded factory shelf life
Mooney viscosity: typically measured at 100°C using a ML 1 + 4 test (i.e., Mooney large rotor, with 1-minute conditioning and 4-minute test duration); it provides a measure of ease of processing and depends on molecular weight and molecular weight distribution and molecular structure; generally ranges from 45 to over 100
Delta Mooney: measured at 100°C after 15 minutes it indicates the ease of processing compounds that are milled before being extruded calendered (e.g., hot feed extrusion systems)
Two additional tests can also be performed on a Mooney Viscometer. Stress Relaxation measures the decay in torque after the rotor is stopped abruptly midway through a Mooney Viscosity test. As the sample relaxes, the subsequent decrease in resistant torque is measured as a function of time. Secondly, Scorch Time is used to determine the onset of cure at a given temperature and is interpreted as the time in which a rubber compound can be processed before curing.
Name: Mooney Viscosity, Stress Relaxation, Scorch Time
Instrument/Equipment: Mooney Viscometer
Test Type: Cure/Rheological Testing
Material: Uncured Rubber
Standards: ISO 289, ASTM D1646
The plasticity of natural rubber is measured using a Plastimeter. The plasticity is measured by compressing a small sample under a known load for a set amount of time.
An important measurement is the Plasticity Retention Index (PRI), which is used to indicate the oxidation resistance of raw natural rubber at a specified temperature. This is because natural rubber is susceptible to oxidation, which can both negatively affect the processing of the compound and the mechanical properties of the final product. Natural antioxidants are available to offer some protection against this degradation. The extent of the oxidation can be quantified by the change in the plasticity of the rubber over time.
In this method, rubber samples are divided into two batches, one of which is tested immediately, and the other is aged in an oven, typically at 140°C for 30 minutes. The Plasticity Retention Index (PRI) is a ratio of the aged plasticity to its original value, expressed as a percentage.
Name: Plasticity Retention Index (PRI)
Test Type: Natural Rubber
Material: Natural Rubber
Standards: ISO 2930:2017
For more information on standard test methods for rubber, view our recent article on rubber testing methods: Understanding Rubber Testing Methods: A Comprehensive Guide.
Compounding of Natural Rubber
As the natural rubber supply chain is global, natural rubber can be sourced from anywhere in the world. On the open market, natural rubber is sold in sheets or bales that are graded by dirt content, Mooney viscosity and plasticity. These three parameters are enough for rubber compounders to purchase the best raw rubber for their application. However, some compounders may undertake further natural rubber testing before mixing to ensure the quality of the raw rubber.
Several production techniques can affect the characteristics of natural rubber, especially the viscosity. These contributory factors include:
|Name||Effect on Mooney Viscosity|
|Latex dilation||Small effect|
|Ammonia preservative||Proportionate; up to 10 units difference with increase from 0.01% to 0.50% ammonia preservative|
|Type of coagulation method (natural, bacterial, formic acid, heating, pH)||Ranges from 65 to 85 Mooney units; highest values for natural coagulation|
|Maturation||Oxidation during handling and storage causes an increase in viscosity (hardening)|
|Drying temperature||Mooney viscosity increases above 60°C|
|Baling temperature||Baling while still hot leads to an increase in cross-linking that raises the gel content|
|Plantation||Differences in tree age, climate and yield stimulants can vary the viscosity|
|Storage temperature||Rubber will reversibly crystallise under strain when stored at low temperatures (from -20°C to -30°C)|
Consistency and Uniformity
For any rubber product, the consistency of the final compound is determined by the uniformity of the base ingredients. For natural rubber, the lack of uniformity can cause variation in mixing specifications, extrusion profiles, tack and the end rubber properties.
The two most important parameters for assessing the consistency of natural rubber are the Plasticity Retention Index (PRI) and Mooney viscosity. The PRI is used to assess the processing properties of natural rubber, particularly in relation to hardening during storage, which can occur inconsistently. The most used way to assess consistency using Mooney viscosity is to compare the mean, minimum and maximum values across a range of samples.
Contamination and Dirt
Significant efforts have been made to reduce the level of dirt in natural rubber at the plantation level, leading to improved contamination rates across all grades. Typically, contaminants include foreign materials like bark, wood, twigs, leaves, and leaf stems, originating from the field. The most significant issue caused by these contaminants is final product failure, as large foreign particles do not disperse during compounding and can create sites for crack formation.
While the level of dirt can be measured experimentally by its residue, this method does not account for light-matter contamination, such as wood chips or plastic. At the processing factory, the washing and cleaning of natural rubber involves a sedimentation process that separates heavy materials from the floating light rubber crumb. This does help to reduce dirt content but it does not satisfactorily separate the light, floating contaminants. As such, contamination by foreign matter is caused by matter that floats, and therefore, it cannot be controlled by washing or measured by its dirt residue.
When fatty acids exist in rubber compounds at excessive levels, they can rise to the surface, causing component separations and affecting the vulcanisation kinetics. The bloom of these fatty acids, such as palmitic, oleic and stearic acids, can be controlled using synthetic polyisoprene, additional polymers or tack-inducing resins.
The amount of fatty acid in a rubber is determined by the amount of washing the raw rubber undergoes before baling and shipping. The natural rubbers that require the least washing tend to contain higher levels of fatty acids.
Storage and Handling
To prevent moisture penetration and mould growth, bales of natural rubber must be wrapped properly. This maintains the quality level of the rubber at the point of purchase and prevents any further contamination during transit or storage. It is recommended to transport rubber bales in metal, rather than wood, containers to prevent wood contamination. Rubber must also be allowed to cool completely before baling, as an increase in temperature can lead to an increase in cross-linking.
Natural rubber also tends to harden during processing and storage, both at the plantation processing factory and during shipping. This storage hardening is caused by the oxidation of the polymer chain. This mechanism causes the partial hydrolysis of the protein and amino acids, subsequently increasing cross-linking to form a gel. As the gel content increases, this can also lead to an increase in bacterial action and pH level. To suppress this, additional chemicals can be added to the natural rubber. This method is also used as the basis for the development of constant viscosity (CV) natural rubber.
Lastly, the storage temperature of natural rubber must also be taken into consideration. At extremely low temperatures, between -20°C and -30°C, rubber will crystallise when under strain. Unlike storage hardening, this process is reversible. However, the crystallisation of any non-rubber components can affect material properties such as fatigue resistance, green strength, tensile strength and abrasion resistance.
Standard Test Methods
There is a large variety of standardised test methods utilised throughout the rubber industry, each focussing on one aspect of rubber characterisation. These areas include cure/vulcanisation properties, chemical analysis, physical properties and environmental testing. Rubber technologists will use any number of tests to pass quality control checks and to ascertain future processing and performance properties.
With this information, changes to compound formulation or mixing can be implemented. As there can be large variation between not only batches but also individual natural rubber bales, it is necessary to continuously test rubber compounds as they are produced, to detect any inter and intra-batch differences.
For a more extensive look into the standard test methods for rubber, view our recent article on rubber testing methods: Understanding Rubber Testing Methods: A Comprehensive Guide.
Applications of Natural Rubber
Before the mass production of synthetic rubber, natural rubber was used for all rubber-based products. Well-known examples of these early consumer products include waterproof Macintosh ‘Mac’ coats and crepe-soled Desert boots. As the production of synthetic rubber began to supersede that of natural rubber, many products shifted away from the use of natural rubber.
However, natural rubber is still used extensively today, particularly in the tyre industry where a typical passenger tyre contains over 40% natural rubber. As a versatile material, natural rubber can be used in injection moulding, calendering and extrusion applications to fabricate any number of semi-finished and finished rubber products. While natural rubber is light beige, many may not be aware of its presence in an abundance of everyday items due to the addition of carbon black.
Environmental Impact of Natural Rubber
One of the ongoing concerns around the plantation method of cultivation is the clearing of forests, often in areas of key biodiversity. With the vast majority of rubber grown by smallholders, there is limited traceability in the rubber supply chain, as many areas of cultivation have not been mapped until very recently. With the extent of the problem not yet fully understood, and a market driven by consumption, there are presently no regulations about traceability standards.
However, despite these concerns, natural rubber plantations are showcased as examples of sustainable production and are seen as a key part of a circular rubber economy. This is because rubber trees are water-efficient, require low levels of fertilisation and rarely require tilling. In addition, rubber plantations can support other plants and crops, such as fruit trees. Together, this reduces the carbon footprint and rebuilds some lost biodiversity. What is more, after latex production capacity is spent, the rubber tree wood can be repurposed for fuel, building materials or furniture.
In addition, the use of chemical coagulants in the processing of natural rubber is harmful to human health. While there are generally many regulations around their use, the concerns remain that at small plantations and cooperatives, this safety advice may not be followed. This is coupled with the physical toil of prolonged agricultural work, which by nature is associated with hazards and physical injuries.
Lastly, the rubber trees that make up much of today's global supply can be traced back to the original stock of seeds that were taken from Brazil in the late 19th century. The lack of genetic diversity has contributed to reliable and productive trees but also leaves open a vulnerability to external pathogens, moulds and blights that could devastate natural rubber production.
Conclusion: The Future of Natural Rubber
Modern scientific studies have successfully recorded the genome of the rubber tree, both for posterity and as a basis for further genetic research into making the plant more resilient to external events. Listed as a critical raw material, the future of rubber is dependent on the successful cultivation of the natural rubber tree, Hevea brasiliensis, for many years to come.
This also coincides with a global drive for more sustainable and environmentally friendly agricultural practices. By framing natural rubber as a sustainable, circular product, the entire rubber supply chain can benefit from these improved practices, increasing the green credentials of many rubber products, including tyres.
While there remains work to be done, it is clear that natural rubber is an essential raw polymer that cannot, if ever, be entirely supplanted by a synthetic alternative.