How are hemp plastics made?
Hemp plastics Innovations in hemp plastic technology are being released onto the market more frequently each year, and now even some of the largest companies (particularly car manufacturers) are incorporating hemp plastics into their products. There are various types of hemp plastic; here, we will briefly take you through how each is produced.
Cellulose is the most abundant organic polymer found on Earth, and is a fundamental part of the cell walls of plants and many species of algae. Although cellulose is primarily used to make paper, it is also used to make a range of different plastics including celluloid, cellophane and rayon. When plastics were first produced, they were all composed of organic, non-synthetic materials, and cellulose at that time was a fundamental part of the fledging plastics industry. Now, renewed interest is being paid to bioplastics for their various environmental advantages.
Cellulose is a long-chain polysaccharide with the chemical formula (C6H10O5)n that consists of hundreds or thousands of linked glucose units. It forms microfibrils (tiny, fibre-like strands) in the cell walls of plants, and also adopts several different crystalline forms, although its natural form is made up of crystalline sections along with some amorphous sections.
Cellophane, rayon & celluloid
Cellophane and rayon are both classified as regenerated cellulose fibres, as they are structurally identical to cellulose. They are produced similarly: cellulose is dissolved in alkali solution and extruded through a slit into a bath of sulphuric acid to make cellophane film, or through a spinneret to make rayon fibre. Celluloid is created by first producing nitrocellulose (cellulose nitrate) and then adding camphor, a widely used plasticiser, to produce a dense, solid thermoplastic that can be easily moulded when heated.
Hemp cellulose can be extracted and used to make cellophane, rayon, celluloid and a range of related plastics. Hemp is known to contain around 65-70% cellulose, and is considered a good source (wood contains around 40%, flax 65-75%, and cotton up to 90%) that has particular promise due to its relative sustainability and low environmental impact. Hemp is faster-growing than most tree species, and requires fewer pesticides than cotton or flax—although it does require significant fertiliser in some soils, and also has relatively high water requirements.
Other potential hemp cellulose products
Cellulose can be used to make a vast range of plastics and related substances. Much of the difference in physical properties is attributable to the length of the polymer chains and the extent of crystallisation. Cellulose is extracted from hemp and other fibre crops in various ways. The raw pulp can be hydrolysed (separated into its component parts through addition of water) at 50-90°C; it can also be soaked in a weak acid solution to separate the crystalline sections from the amorphous sections, to produce nanocrystalline cellulose.
It can be further subjected to heat and pressure to produce an intriguing form known as nanocellulose—a “pseudo-plastic” that appears gel-like and viscous in normal conditions, but becomes more liquid when shaken or subjected to stress. Nanocellulose has a range of potential applications, as a composite plastic reinforcing material, as a super-absorbent to clean up oil spills or make hygiene products, and even as a low-calorie stabiliser in food technology.
Zeoform is a hemp plastics producer offering a cellulose-based plastic, made using their patented process only from water and a range of natural cellulose fibres including hemp. Their website states that their technique “converts cellulose fibres into an industrial strength moulding material capable of being formed into an unlimited array of products” that range from “Styrofoam-light” to “ebony-dense”. Zeoform is advertised as being 100% non-toxic, biodegradable, and compostable, and a means of sequestering carbon in functional, attractive forms. It is not clear what their proprietary technique entails, but it apparently affords a fine degree of control over the extent of polymer length and enmeshment, allowing a diverse range of plastics to be produced.
Hemp composite plastics
Composite plastics are comprised of a polymer matrix, which could be cellulose-based or may be based on a range of other natural or synthetic polymers, and reinforcing fibres, which again may be natural (and primarily comprised of cellulose) or synthetic in origin. Natural polymers include tar, shellac, tortoiseshell, and many tree resins, while natural fibres include jute, sisal, cotton, and flax. Traditional inorganic fillers include talc, mica and glass fibre.
Biocomposites are generally defined as having at least one principal constituent that is organic in origin; although it is possible to produce plastics that are 100% organic, the majority are composed of some synthetic elements. Often, a natural fibre will be mixed with a synthetic polymer and labelled as a biocomposite. The various fibre and polymer combinations that can be used to make bioplastics vary greatly in density, tensile strength, rigidity, and a number of other factors that can be tweaked in the manufacturing process to create products suitable for a diverse range of applications—building and construction materials, furniture, musical instruments, boats, car panels, biodegradable shopping bags, and even in medicine, to make biocompatible ‘scaffolds’ in bone tissue reconstruction.
Hemp fibres are well known for their tensile strength, particularly the fibres from the female plant (male fibres are finer, softer, and often longer, but are also weaker). A 2003 study into polypropylene (PP) composites reinforced with natural fibre found that hemp, kenaf and sisal showed comparable tensile strength to traditional glass fibre composites, and that hemp outperformed its competitors in impact resistance. A 2006 study indicated that hemp fibre composites were slightly outperformed by glass fibresA 2007 study into PP composites reinforced with hemp fibres showed that using a form of PP known as maleated polypropylene (MAPP) increased the tensile strength and overall mechanical properties to within 80% of traditional glass fibre composites.
100% biocomposites made with hemp
Various biocomposites have been developed entirely from organic substances, and some of these contain hemp fibre as a filler. In 2003, a study into tensile strength of hemp fibre noted that if hemp fibre bundles were alkalised with dilute sodium hydroxide (NaOH) at concentrations of 4-6%, they exhibited significantly increased tensile strength and rigidity when combined with a cashew nut shell liquid polymer matrix to make biocomposite plastics.
In 2007, a biocomposite produced from organically-derived polylactic acid (PLA; an important biodegradable thermoplastic polyester) reinforced with hemp fibres was announced by Korean researchers. The researchers also found that treatment of the hemp fibres with dilute alkali increased their tensile strength; the biocomposites themselves exhibited greater strength and rigidity compared to PLA-only plastics.
In 2009, researchers at Stamford University announced development of a hemp fibre-reinforced composite made with bio-polyhydroxybutyrate (BHP), a polymer that can be derived from bacteria species (including Bacillus)that are subjected to physical stress such as nutrient deprivation. These hemp-BHP composites are strong, smooth and attractive, and durable enough to be used in construction, furniture, and flooring.
100% bio-polyethylene terephthalate (PET)
Polymer resin-based plastics may be comprised of a single molecule, whereas composite plastics require the addition of a resin or resins to cause the fibres to adhere together and take on their final plastic form. While most bioplastics comprise a number of different materials, it is possible to produce a variety of plastics, even composites, which are entirely organic.
In 2011, PepsiCo announced that they had produced a 100% plant-based plastic water bottle made from polyethylene terephthalate (PET), a thermoplastic polymer resin with the chemical formula (C10H8O4)n that is usually produced from petrochemicals, but in this case was produced from a range of plant sources including switchgrass, pine bark and corn husks.
However, it does not appear that the 100% bioplastic bottle is currently available, although PepsiCo and various other soft drink manufacturers have begun to incorporate bioplastics into their mass-produced bottles—such as Coca-Cola, who distributed 2.5 million of its 30% plant-based PlantBottle in the first two years of production, accounting for 68 million kilograms of bio-PET. The companies involved in the initiative have stated that their goal is to ensure that 100% bio-PET bottles are in commercial use by 2018.
Bio-MEG and bio-PTA can be made from bio-ethanol
PET is produced by combining 32.2% monoethylene glycol (MEG) with 67.8% purified terephthalic acid (PTA) in an esterification reactor and converting the result into a polycondensation reactor. It is possible to produce MEG from bio-ethanol: the ethanol is catalytically dehydrated to form ethylene, which is then oxidised; ethylene oxide then reacts with water to form MEG. MEG can also be produced from vegetable oils, although this is not currently conducted on a large scale. Bio-ethanol and bio-oil can be produced from hemp and other crops, and PET comprised of around 30% plant-based material can be produced from it; but producing PTA from natural sources seems to be somewhat trickier.
Bio-PTA is not yet commercially available, although it has been created in laboratory conditions—in 2011, Winsconsin bioproducts firm Virent announced that they had successfully created it from plant sugars from sugar cane, corn and woody biomass. Several companies are attempting to realise mass-production techniques; with a potential high-value contract from the soft drinks manufacturers, there is a great deal of incentive to get it done.
In order to make bio-PTA, it is first necessary to make bio-paraxylene (an aromatic carbon typically made from petroleum); like bio- can be produced from ethylene. The ethylene can be produced directly from plant glucose (or from bio-ethanol, which is the end product of the fermentation of plant glucose), and is then catalytically dehydrated to produce paraxylene.
Hemp as a source of bio-ethanol
Hemp is often overlooked as a source of ethanol, but various studies have assessed its potential and found favourable results. While sucrose-containing crops like sugar cane and starchy crops like corn are higher overall producers of ethanol, they are more energy-intensive, more environmentally damaging, and in higher demand for other purposes such as to provide food.
It seems that one of the biggest arguments against use of hemp to produce bioethanol is cost-effectiveness, but these issues are far more deeply linked with economies of scale, as the hemp industry is tiny compared to that of the cotton, corn or sugarcane industry. As hemp cultivation continues to increase, these obstacles should diminish accordingly—and hemp does appear to have important advantages over other crops, particularly in terms of its low environmental impact.
Hemp has been demonstrated on various occasions to be a biomass source with a relatively good yield of ethanol. A 2009 study found that hemp could produce 141g of ethanol per kg¹ of dry hemp hurds, while 2010 study found that hemp was capable of yielding up to 171g/kg?¹ of ethanol from dry hemp matter.
Getting ethanol from hemp cellulose
It is thought that the most important properties of herbaceous crops such as hemp for bioethanol production are high availability of biomass and high glucose yields. In photosynthetic plants the usual mechanism is to produce glucose from starch (another polymer made up of linked glucose units, but with bonds that are more easily broken down); it is glucose from starch that provides the basis for corn ethanol, but glucose can also be produced from cellulose on an industrial scale.
It is widely held that cellulose biomass presents a better solution for long-term sustainability, as cellulose products are often waste materials, where starchy products like corn are wastefully being diverted to make ethanol instead of foodstuffs. However, producing glucose from cellulose—especially from high-lignin plants such as hemp—is difficult and costly, and the hunt is on for new, improved techniques that maximise efficiency and yield. Pretreating the dry matter with steam for a short while prior to subjecting the material to enzymatic hydrolysis for example appears to optimise the glucose yield from hemp and other high-lignin plants.
An Estonian study into biomass crops found that the hemp specimens they tested performed best in terms of cellulose content (at 53.86% compared to the lowest, Jerusalem artichokes, at 21-26%) and glucose yield (312.7g kg?¹ against the lowest, 122.7g kg?¹ from sunflower). They concluded that of the seven plants tested, hemp presented the best candidate.
It is clear that research still remains to be done to achieve the best sustainable alternatives to petroleum-based plastics. However, the pace of new research is picking up as governments and nations throughout the world gain better understanding of the need to drastically reduce petrochemical usage, and hemp is increasingly being recognised as having great potential in our natural ‘toolbox’ of promising bioplastics crops.