colouring agents. PVC is the only general purpose plastic that allows free, wide and seamless adjustment of the required physical properties of products such as flexibility, elasticity, and impact resistance, by adding plasticisers, additives and modifiers. Since the physical properties of end products are adjustable through compounding with additives, only a few types of resin are required to cover all applications (fibre, rigid and flexible plastic, rubber, paint, and adhesive). This controllability is also extremely beneficial for recycling.
4. PVC PROCESS
PVC manufacturing process
From a chemical viewpoint, PVC is a thermoplastic polymer based on chlorinated hydrocar-bons. This figure provides an overview of the production processes leading from raw materi-als to finished products.
PVC is produced from two primary raw materials, ethylene and chlorine. These react to form ethylene dichloride (EDC) which, upon cracking, yields vinyl chloride monomer (VCM). Free radical polymerisation is used to produce the PVC polymer itself.
The two most important polymerisation techniques are suspension polymerisation and emul-sion polymerisation, leading to the production of S-PVC and E-PVC, respectively. These two types of PVC polymer have different properties and are used for distinct applications. The S-PVC process yields granules of polymer of 100 to 200 microns in diameter, which are used in processes such as injection moulding, extrusion and PVC film manufacture. Here, S-PVC is analysed in more detail, as it is more commonly used. The differ-ence between S- and E-PVC is rather small, as the type of polymerisation has a low impact on the overall performance of the PVC over its life cycle.
Row Material
Row material for PVC industry are: Chlorine and Ethylene
Production of ethylene and chlorine
Definition: The raw materials for PVC are petroleum (which contributes to 43% of the polymer weight) and rock salt (57%). Petroleum is refined to naphtha, which is then cracked to form ethylene. Chlorine is obtained from rock salt through an electrolytic process.
Manufacture: Ethylene is produced, together with a variety of co-products, by the steam cracking of naphtha or natural gas liquids. The natural gas liquids contain ethane, butane and propane and when they are pyrolysed the products are ethylene and propylene. Ethylene production via steam cracking is a basic chemical process. The ethylene used as an interme-diate in PVC production is the minor portion of the chemicals produced during this process. Products from the steam cracker are used as intermediates for a variety of other products.
Thirty percent of the chlorine produced in Germany is used for Vinylchloride (VCM) and EDC (dichloroethane) production. Chlorine is produced by the electrolysis of a salt solution or brine. During electrolysis chlorine is produced at the anode while sodium or caustic soda and hydrogen are produced at the cathode. The products of the electrode processes must be kept separately; otherwise they will react to form unwanted by-products. Either chemical or physi-cal separation is employed. Today, there are three different commercial electrolysis proc-esses available: the amalgam process, the diaphragm process and the membrane process.
The amalgam process requires a higher consumption of electrical power than the diaphragm or membrane processes but produces a purer and more concentrated caustic soda solution. Substantial quantities of steam are required to concentrate the caustic soda solutions, pro-duced by the diaphragm and membrane cells, to the concentration levels that are produced by the amalgam process. The Best Available Technology (BAT) for chlorine manufacture is recognised to be the membrane process, which consumes less electricity compared to the amalgam and diaphragm processes.
Environmental impact: Emissions from ethylene production are released to air and water. They consist primarily of ethylene and propylene emissions to air, and methanol and pro-pane/butane emissions to water. Other emissions, such as Total Organic Compound (TOC), Sodium Hypochlorite (NaOCl), etc. are also present.
Environmental Impact of Ethylene Production
During chlorine production, emissions of chlorine and hydrogen are possible from all three processes (amalgam, diaphragm and membrane). Mercury can be emitted from the amalgam process only. Asbestos fibres within the diaphragm process are a potential workplace hazard but the existence of the fibres as such presents no environmental problem, since the fibres are chemically inert and no release during regular use appears. The diaphragm keeps the products of electrolysis separate while allowing the passage of electric current. A matter of concern is the handling and disposal of the diaphragm. Mercury disposal on-site can affect the water, air and soil in the plant neighbourhood, posing a hazard to plant operators and to the people and animal life living near the plant. Chlorine as a toxic gas has a potential environmental impact, if released to the environment.
Environmental Impact of Chlorine Production
These figures show the environmental profile of these materials in impact catego-ries. In the case of ethylene, naphtha occupies a major part in all impact categories, espe-cially Photochemical Ozone Creation Potential (POCP). This is mainly due to the processing of crude oil, which causes VOC emissions.
The high consumption of electricity is the main characteristic of the electrolysis process for the production of chlorine (see Figure 4-4), causing CO2 and SO2 emissions, which result in a high Global Warming Potential (GWP) and Acidification Potential (AP). In comparison to the other process steps, the electricity production of the electrolysis process also results in a high Human Toxicity Potential (HTP). The German Environmental Protection Agency considers the mercury emission from the production of chlorine with the amalgam process and the elec-tricity consumption (its production and supply for the electrolysis) as the main environmental factor of concern for the chlorine production process. New installations of electrolysis processes have be operated with the membrane process.
A step-by-step shift to the membrane process for chlorine electrolysis is in progress. This is seen as a major step to further reduce environmental impacts of chlorine production.
Risk characterisation: In the case of PVC, the production of chlorine plays a very important role and it is a continuing topic of discussion in the assessment of risk. Information regarding exposure: limits and areas of exposure, risks to the environment and health should be men-tioned. Moreover, the production of the petrochemical part of PVC, as for all plastics, includes various risks: sea pollution from drilling to shipping, and risks associated with explosive gases such as ethylene.Risk characterisation is not a primary task within LCA, as LCA deals with potential impacts of regular operation. Today, initial approaches are in development that address the integration of risk aspects into LCA. Nevertheless, existing methods, such as Risk Assessment, are more suitable to answer specific questions related to the risks of chlorine.
Chemical Process
The chemical process for making PVC involves taking the simplest unit, called the monomer, and linking these monomer molecules together in the polymerization process. Long molecular chains are formed called polymers (which are also called macromolecules).
This is the case for PVC, which is made from vinyl chloride monomer known usually by its initials VCM through polymerization. Some monomers exist in the form of reactive gaseous chemical substances, and some of these may cause health hazards when in direct contact with humans. In these cases they are manufactured and processed under strict control for health, safety and environmental protection. On the other hand, polymers such as PVC, which are manufactured from monomers through polymerization, are solid and chemically stable substances, therefore do not affect human health. VCM, which is the raw material for PVC, is a gas at ambient temperature but is usually stored in liquid form under pressure. Ethylene and chlorine are raw materials for PVC. Upstream industries are those that provide these materials and include producers of basic petrochemicals (sometimes known as feedstock’s’), which supply ethylene, and the chlor-alkali (caustic soda) industry, which supplies chlorine.
At a first stage in the PVC production process ethylene and chlorine are combined to produce an intermediate product called 1.ethylene dichloride; this is then transformed into vinyl chloride, the basic building block of polyvinyl chloride or PVC. The process of `polymerization' links together the vinyl chloride molecules to form chains of PVC. The PVC produced in this way is in the form of a white powder. This is not used alone, but blended with other ingredients to give formulations for a wide range of products.
PVC is a thermoplastic made of 57% chlorine (derived from industrial grade salt) and 43% carbon (derived predominantly from oil / gas via ethylene). It is less dependent than other polymers on crude oil or natural gas, which are nonrenewable, and hence can be regarded as a natural resource saving plastic, in contrast to plastics such as PE, PP, PET and PS, which are totally dependent on oil or gas.
PVC production
The process that take place are
First, the raw material VCM is pressurized and liquefied, and then fed into the polymerization reactor, which contains water and suspending agents in advance. Through high-
speed agitation within the reactor, small droplets of VCM are obtained. Next, the initiator for polymerizations is fed into the reactor, and PVC is produced by reaction under a few bar at 40 - 60°C.
PVC obtained through suspension polymerization is suspended in water as particles of
50~200 μm diameter (in slurry form). Thereafter the slurry discharged from the polymerization reactor is stripped of residual monomer, dehydrated, dried and the particle size controlled by screening to yield PVC in the form of a white powder. The un-reacted VCM is entirely recovered through the stripping process, and after purification, recycled as raw material for reuse in this process. PVC resin produced via this ‘suspension’ process is referred to within the industry using the abbreviation S-PVC.
Emulsion polymerization and bulk polymerization are alternative, much less extensively employed, technologies to manufacture PVC. Emulsion polymerization produces finer resin grades having much smaller particles, which are required by certain applications. This type of resin is sometimes called ‘paste’ PVC and referred to within the industry using the abbreviation
P-PVC to distinguish it from S-PVC.
5. APPLICATION
PVC is one of the most used plastic materials in the world. At global level, demand for PVC exceeds 35 million tonnes per annum and it is in constant growth (+5% on global average), with higher growth rates in the developing countries.
In Europe (EU-32), the production of PVC products – including exports – totals about 8 million tonnes per year. European PVC resin consumption totals some 6.5 million tonnes per year, or 15% of all plastics use in Europe, with an average growth of 2-3% per year.
6. GREEN PROCESS
The concept of sustainability is often stated in a number of ways but four core principles appear throughout: Protect the environment, promote human health, conserve resources, and assure social and economic well-being to the global population. PVC resin has intrinsic properties that allow finished products to meet all four of these objectives. PVC resin and products compare favorably to other materials in life cycle assessments when reviewing key impacts of resource and energy conservation, and greenhouse gas emissions.
The 12 Principles of Green Chemistry
Those principles designated with an ‘*’ illustrate at least seven areas with which PVC resin production conforms.
Principle No.2, “Atom Economy”, refers to the conversion of raw materials into finished products. Overall conversion efficiency for ethylene dichloride to vinyl chloride to PVC is estimated to be 94% , which puts it among the highest for polymer resins. Key to achieving this is steam stripping of PVC resin that removes unreacted monomer and recycles it back to the front of the polymerization process which drives the polymerization (step 5) to nearly complete utilization of monomer feedstock.
As shown in Figure 1, PVC has one of the lowest embodied energy profiles for a polymer because of its composition with chlorine, use of catalysts in feedstock preparation, and exothermic monomer polymerization reaction, which all combine to adhere to Principle No.6 “Design for Energy Efficiency.” A further look at embodied energy is warranted since it should be basic to any material sustainability evaluation due to the amount of GHG emitted to produce the material. In the case of PVC, replacement of ethylene with chlorine in the polymer matrix creates an interesting tradeoff between the electrical energy intensive derivative chlorine (total energy for process and transportation = 17.67 MJ/Kg including salt mining) versus the fuel and electrical energy intensive derivative ethylene (total energy for process and transportation = 17.73 MJ/Kg), with the net result of PVC having a low total embodied energy. It should be pointed out that comparisons of polymers or other materials on a unit basis would only be an indication of certain intrinsic attributes, and may not be representative of actual impacts of any material when a complete cradle to grave life cycle assessment (LCA) is performed.
Figure 1: Resin GHG Emissions vs. Embodied Energy, Data Reference: ACC Cradle to Gate LCI Report
Principles No. 3 and 12, “Less Hazardous Chemical Syntheses” and “Inherently Safer Chemistry for Accident Prevention” intend to reduce environmental and workplace hazards through chemical selection. But all organic raw materials, whether in a solid or liquid form, can present combustible dust or flammable liquid hazards which require responsible care during handling and processing, and whether derived from biomaterials or fossil fuels, cause issues when released into the environment. Alternatively, process safety practices and workplace monitoring are ways to accomplish the same end result. The PVC industry has a history of implementing safeguards and a proven record of achieving both of these aforementioned green chemistry principles through sophisticated process monitoring and controls. Year over year reductions in criteria emissions for vinyl chloride and dioxin are shown in Figures 2 and 3 respectively.
PVC plants are equipped with an elaborate array of continuous air samplers that alert employees quickly in the event of any monomer leak. These monitors and the continual emphasis by the industry for safety has resulted in an illness and injury rate that is one-third that of the overall chemical industry, and one-sixth that of overall manufacturing, as recorded in Figure 4.
Figure 2: PVC Industry Cradle to Resin Vinyl Chloride Emissio Figure 3: PVC Industry Cradle to Resin Dioxin Emission
Principle No. 8 “Reduce Derivatives” can be shown to be met by the examining the block flow diagram in Figure 5 which depicts the relatively simple route to manufacture PVC resin in just 5 steps.
Several steps incorporate catalysts which drive reactions more effectively as required by Principle No. 9 “Catalysis.”
Each step is computer controlled and continuous process parameter monitors assure optimal operations, which meets the requirement of Principle No.11 “Real-time Analysis of Pollution Prevention.”
Figure 4: Illness and Injury Rate, PVC Resin
Figure 5: Cradle-to-Resin Flow Diagram of PVC Production
7. ECO-PROFILES AND LCA
Life Cycle Assessment (LCA): Method to evaluate the magnitude of resources consumption and impacts on the environment for a material or product throughout its whole life cycle (production, use, waste disposal and recycling) .Rather than focusing on the environmental load alone, it evaluates from a comprehensive viewpoint. It has been highlighted as one of the guidelines for material comparison and green procurement.
By using LCAs correctly it can be ensured that:
- Product systems are not erroneously favored or condemned on the basis of individual results.
- Developments do not inadvertently shift or even increase environmental impact but in fact reduce it.
- The resources available are directed in such as way that they achieve the greatest environment benefit for the entire system.
PVC product systems have been investigated using LCA methodology in almost all significant application areas in terms of production quantity.
Eco-profiles
Eco-profiles form part of Life-Cycle Assessment (LCA). Whereas LCAs are cradle-to-grave analyses of the environmental impact of a product, eco-profiles stop at the factory gate ("Cradle-to-gate"). The Eco-profiles of PVC were fully updated in 2006. They can be downloaded .
The key impacts calculated in these eco-profiles are aggregated according to a standard methodology and published in Environmental Declarations.
- BENEFITS OF USEING PVC
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