PVC Information

PVC Information

agricultural plastic film

The polymerization of VCM is started by compounds called initiators that are mixed into the droplets. These compounds break down to start the radical chain reaction. Typical initiators include dioctanoyl peroxide and dicetyl peroxydicarbonate, both of which have fragile O-O bonds. Some initiators start the reaction rapidly but decay quickly and other initiators have the opposite effect. A combination of two different initiators is often used to give a uniform rate of polymerization. After the polymer has grown by about 10x, the short polymer precipitates inside the droplet of VCM, and polymerization continues with the precipitated, solvent-swollen particles. The weight average molecular weights of commercial polymers range from 100,000 to 200,000 and the number average molecular weights range from 45,000 to 64,000.

Once the reaction has run its course, the resulting PVC slurry is degassed and stripped to remove excess VCM, which is recycled. The polymer is then passed through a centrifuge to remove water. The slurry is further dried in a hot air bed, and the resulting powder sieved before storage or pelletization. Normally, the resulting PVC has a VCM content of less than 1 part per million. Other production processes, such as micro-suspension polymerization and emulsion polymerization, produce PVC with smaller particle sizes (10 μm vs. 120-- 150 μm for suspension PVC) with slightly different properties and with somewhat different sets of applications.

Microstructure

The polymers are linear and are strong. The monomers are mainly arranged head-to-tail, meaning that there are chlorides on alternating carbon centres. PVC has mainly an atactic stereochemistry, which means that the relative stereochemistry of the chloride centres are random. Some degree of syndiotacticity of the chain gives a few percent crystallinity that is influential on the properties of the material. About 57% of the mass of PVC is chlorine. The presence of chloride groups gives the polymer very different properties from the structurally related material polyethylene.

Additives to finished polymer

The product of the polymerization process is unmodified PVC. Before PVC can be made into finished products, it always requires conversion into a compound by the incorporation of additives (but not necessarily all of the following) such as heat stabilizers, UV stabilizers, plasticizers, processing aids, impact modifiers, thermal modifiers, fillers, flame retardants, biocides, blowing agents and smoke suppressors, and, optionally, pigments. The choice of additives used for the PVC finished product is controlled by the cost performance requirements of the end use specification e.g. underground pipe, window frames, intravenous tubing and flooring all have very different ingredients to suit their performance requirements. Previously, polychlorinated biphenyls (PCBs) were added to certain PVC products as flame retardants and stabilizers.

Phthalate plasticizers

Most vinyl products contain plasticizers which dramatically improve their performance characteristic. The most common plasticizers are derivatives of phthalic acid. The materials are selected on their compatibility with the polymer, low volatility levels, and cost. These materials are usually oily colourless substances that mix well with the PVC particles. About 90% of the plasticizer market, estimated to be millions of tons per year worldwide, is dedicated to PVC.

Bis( 2-ethylhexyl) phthalate was a common plasticizer for PVC but is being replaced by higher molecular weight phthalates
Low and high molecular weight phthalates.
Phthalates can be divided into three groups based on their molecular weight.

Low molecular weight phthalates have 6 or 7 carbon atoms in their alcohol chain. Medium molecular weight phthalates have 8 or 9 carbons in their alcohol chain. High molecular weight phthalates have from 10 to 13 carbons in their alcohol chain. Because of high volatility at pvc processing temperatures, low molecular weight phthalates are no longer used in the US. The most common medium molecular weight phthalates are DOP (dioctyl phthalate, also known as DEHP, di-2-ethylhexyl phthalate) and DINP (diisononyl phthalate). High molecular weight phthalates have a limit of 12 carbons (if linear) or 13 carbons (if branched) in the alcohol chain because phthalates with longer alcohol chains are incompatible with pvc resin. The more common high molecular weight phthalates are DIDP, DPHP and DTDP, all from branched alcohols. There are phthalates from linear alcohols from 8 to 12 carbons which are used when the flexible pvc compound is to have superior low temperature flexibility and improved weathering. Because of possible health effects, there are movements to replace some phthalates such as DOP/DEHP with safer alternatives in Canada, the European Union, and the United States. In Europe DEHP and DBP are undergoing the REACH Authorisation process Registration, Evaluation, Authorisation and Restriction of Chemicals with results expected in 2014. Since no applications for BBP and DIBP were received by the European Chemicals Agency (ECHA), the use of these substances will be phased out in the EU by 21 February 2015.

Mid-high molecular weight phthalates today represent over 85% of all the phthalates currently being produced in Europe (2011). On 31 January 2014 the European Commission published its conclusions regarding the re-evaluation of the restrictions on DINP and DIDP in toys and childcare articles which can be placed in the mouth.The Commission confirmed the main conclusions presented in August last year by the ECHA, which was asked in September 2009 to review any newly available scientific information relative to these two high phthalates.

The European Commission concluded that "no unacceptable risk has been characterised for the uses of DINP and DIDP in articles other than toys and childcare articles which can be placed in the mouth". With regard to the latter, the existent restrictions are nevertheless maintained, based on the precautionary principle. DINP and DIDP are therefore safe for use in all current consumer applications. The European Chemicals Agency also concluded that no further risk management measures are needed to reduce the exposure of children and adults to DINP and DIDP.

Heat stabilizers

One of the most crucial additives are heat stabilizers. These agents minimize loss of HCl, a degradation process that starts above 70 ° C. It is autocatalytic once dehydrochlorination starts. Many diverse agents have been used including, traditionally, derivatives of heavy metals (lead, cadmium). Increasingly, metallic soaps (metal "salts" of fatty acids) are favored, species such as calcium stearate. Addition levels vary typically from 2% to 4%. The choice of the best heat stabilizer depends on its cost effectiveness in the end use application, performance specification requirements, processing technology and regulatory approvals.

Rigid PVC applications

Regular PVC (polyvinyl chloride) is a common, strong but lightweight plastic used in construction. It is made softer and more flexible by the addition of plasticizers. If no plasticizers are added, it is known as uPVC (unplasticized polyvinyl chloride), rigid PVC, or vinyl siding in the U.S. In Europe, particularly Belgium, there has been a commitment to eliminate the use of cadmium (previously used as a part component of heat stabilizers in window profiles) and phase out lead based heat stabilizers (as used in pipe and profile areas) such as liquid autodiachromate and calcium polyhydrocummate by 2015. According to the final report of Vinyl 2010 cadmium was eliminated across Europe by 2007. The progressive substitution of lead-based stabilizers is also confirmed in the same document showing a reduction of 75% since 2000 and ongoing. This is confirmed by the corresponding growth in calcium-based stabilizers, used as an alternative to lead-based stabilizers, more and more, also outside Europe.

Tin based stabilizers are mainly used in Europe for rigid, transparent applications due to the high temperature processing conditions used. The situation in North America is different where tin systems are used for almost all rigid PVC applications Tin stabilizers can be divided into two main groups, the first group containing those with tin-oxygen bonds and the second group with tin-sulphur bonds. According to the European Stabiliser producers most organotin stabilisers have already been successfully REACH registered. More chemical and use information is also available on this site.

Flexible PVC applications

Flexible PVC coated wire and cable for electrical use has traditionally been stabilised with lead but these are being replaced, as in the rigid area, with calcium based systems.

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