Processing for PVC

The next step would be to build a large scale plant of 50 ktonnes per year. However, such an investment needs financing and a commitment of the European PVC industry to this initiative (i.e., a choice for this technology as the feedstock recycling process for PVC waste). Building the plant would take about 5 years. At this stage, it seems that within the PVC industry there is more support for the Linde and Vinyloop processes. 

CLOSED-LOOP RECYCLING PROCESS FOR PVC IS BASED ON SOLVENTS... 

In the results the emissions of mercury appear to have a very substantial contribution for the human toxicity impact score. These emissions are caused by the coproduction of chlorine and sodium hydroxide by electrolysis using a mercury cell. However, this technique is phased out. Therefore, the process descriptions in the Ecoinvent database do not represent up to date technology. In the Ecoinvent database the process for PVC production, in which chlorine is used as one of the compounds, is an aggregated processes based on, seemingly outdated, data from PlasticsEurope. These outdated data also influence the impacts related to waste treatment by incineration because sodium hydroxide is necessary for the waste incineration process. 

Chisso VCM Removal PVC slurry with unreacted VCM Stripping process for PVC plants to recover VCM from PVC slurry VCM can be reused without deteriorating polymer quality 36 2000... 

PVC, over the years, has been subjected to scrutiny on health and environmental grounds. One issue is the level of residual vinyl chloride monomer in the material that may migrate into food. Vinyl chloride monomer has been determined to be a carcinogen, at least under some conditions. In the polymerization process for PVC, less than 100% of the vinyl chloride monomer (VCM) is converted to polymer. This means that relatively high values of VCM may remain unreacted and trapped in the resin. To remove this residual monomer, the resin is subjected to repeated applications of vacuum. In this manner, VCM concentrations in the resin are reduced substantially. PVC packaging resins currently produced have much lower levels of residual vinyl chloride (under 10 ppb) than those used in containers in the mid-1970s when this concern first surfaced. 

Polymers that are insoluble in their monomers will precipitate during polymerization. The resulting fouling problems that occur in bulk polymerization are greatly reduced by using suspension polymerization. This is one of the reasons for choosing a suspension process for PVC manufacture, as discussed previously (see Section 5.1.5). Manipulation of polymer precipitation, inside the drops, during... 

GroBe-Aschhoff, M. Optimizing the Compounding Process for PVC Window Profile Formulations, Kunststofi e piast europe, 89 (1999) 2, p. 20-21... 

Early demand for chlorine centered on textile bleaching, and chlorine generated through the electrolytic decomposition of salt (NaCl) sufficed. Sodium hydroxide was produced by the lime—soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for chlorine for PVC manufacture led to the production of chlorine and sodium hydroxide as coproducts. Solution mining of salt and the avadabiHty of asbestos resulted in the dominance of the diaphragm process in North America, whereas soHd salt and mercury avadabiHty led to the dominance of the mercury process in Europe. Japan imported its salt in soHd form and, until the development of the membrane process, also favored the mercury ceU for production. 

Raw Materials. PVC is inherently a hard and brittle material and very sensitive to heat it thus must be modified with a variety of plasticizers, stabilizers, and other processing aids to form heat-stable flexible or semiflexible products or with lesser amounts of these processing aids for the manufacture of rigid products (see Vinyl polymers, vinyl chloride polymers). Plasticizer levels used to produce the desired softness and flexibihty in a finished product vary between 25 parts per hundred (pph) parts of PVC for flooring products to about 80—100 pph for apparel products (245). Numerous plasticizers (qv) are commercially available for PVC, although dioctyl phthalate (DOP) is by far the most widely used in industrial appHcations due to its excellent properties and low cost. For example, phosphates provide improved flame resistance, adipate esters enhance low temperature flexibihty, polymeric plasticizers such as glycol adipates and azelates improve the migration resistance, and phthalate esters provide compatibiUty and flexibihty (245). 

RigidPoly(vinylchloride) Foam. The techniques that have been used to produce rigid vinyl foams are similar to those for the manufacture of flexible PVC foams. The two processes that have reached commercial importance for the manufacture of rigid vinyl foams (246) are the Dynamit-Nobel extmsion process and the Kleber-Colombes Polyplastique process for producing cross-linked grafted PVC foams from isocyanate-modified PVC in a two-stage mol ding process. 

Cost bilizers. In most cases the alkyl tin stabilizets ate particularly efficient heat stabilizers for PVC without the addition of costabilizers. Many of the traditional coadditives, such as antioxidants, epoxy compounds, and phosphites, used with the mixed metal stabilizer systems, afford only minimal benefits when used with the alkyl tin mercaptides. Mercaptans are quite effective costabilizets for some of the alkyl tin mercaptides, particularly those based on mercaptoethyl ester technology (23). Combinations of mercaptan and alkyl tin mercaptide ate currendy the most efficient stabilizers for PVC extmsion processes. The level of tin metal in the stabilizer composition can be reduced by up to 50% while maintaining equivalent performance. Figure 2 shows the two-roU mill performance of some methyl tin stabilizers in a PVC pipe formulation as a function of the tin content and the mercaptide groups at 200°C. 

By far the most common lead salt used for PVC stabilization is tribasic lead sulfate. It can be found either alone or combined with another lead salt in almost every lead-stabilized PVC formulation. Many of the combinations are actually coprecipitated hybrid products, ie, basic lead sulfophthalates. Dibasic lead stearate and lead stearate are generally used as costabilizers combined with other primary lead salts, particularly in rigid PVC formulations where they contribute lubrication properties dibasic lead stearate provides internal lubrication and lead stearate is a good external lubricant. Basic lead carbonate is slowly being replaced by tribasic lead sulfate in most appHcations due the relatively low heat stabiHty of the carbonate salt which releases CO2 at about 180°C during PVC processing. 

Fig. 7. Model for PVC fusion, accounting for molecular weight effects and processing temperature effects (a) unfused PVC primary particles (b) partially melted PVC primary particles (c) partially melted then recrysta11i2ed high molecular weight PVC, showing strong three-dimensional stmcture and (d) partially melted then recrysta11i2ed low molecular weight PVC, showing weak three-dimensional stmcture.

At processing temperatures, most polymers emit fumes and vapors that may be irritating to the respiratory tract. This is also tme for PVC and its additives. Such irritation may extend to the skin and eyes of sensitive people. Processing emissions exposure can also be greatly reduced or eliminated by the use of properly designed and maintained exhaust ventilation. 

The decomposition temperature should be suitable for the polymer. For example, a decomposition temperature for a blowing agent system for PVC should not be above the maximum possible processing temperature that can be used if significant degradation is not to occur. 

In the 1960s materials became available which are said to have been obtained by chlorination at lower temperatures. In one process the reaction is carried out photochemically in aqueous dispersion in the presence of a swelling agent such as chloroform. At low temperatures and in the presence of excess chlorine the halogen adds to the carbon atom that does not already have an attached chlorine. The product is therefore effectively identical with a hypothetical copolymer of vinyl chloride and symmetrical dichloroethylene. An increase in the amount of post-chlorination increases the melt viscosity and the transition temperature. Typical commercial materials have a chlorine content of about 66-67% (c.f. 56.8% for PVC) with a Tg of about 110% (c.f. approx. 80°C for PVC). 

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