NO178627B - Vinyl chloride (co) polymers suitable for use in paste PVC, as well as their preparation and use - Google Patents

Description

The application concerns vinyl chloride (co)polymers suitable for use in paste PVC, as well as the production and use of such.

Polyvinyl chloride (PVC) is the second most widely used plastic raw material in the world after polyethylene. The consumption of PVC in Western Europe is around 5 million tonnes. Major areas of application for PVC are plastic pipes, profiles, hoses, cable insulation, foils, bottles, etc. Paste PVC makes up about 10% of the total PVC production. Paste PVC is used in soft products in a mixture with plasticisers. Paste PVC is used, for example, in floor coverings, wallpaper, artificial leather and coated textiles such as tarpaulins and rain gear.

For the state of the art, reference is made to 'Encyclopedia of Polymer Science and Technology', 2nd edition, volume 17 pp. 329-376, 1982, where M.J. The bundle provides a detailed description of the production and use of paste PVC. Here, various production technologies, effects of various auxiliary substances in production are reviewed, as well as which principles apply to the various areas of application of this plastic raw material.

A disadvantage of the production of paste PVC is that all known production processes include the use of auxiliary substances such as ionic and non-ionic emulsifiers, buffer salts and catalysts, and some processes also use fatty alcohols. According to Bunten, these substances are known to give a number of undesirable properties to the product. The most important of these are increased water absorption in foils and coatings, reduced thermal stability, increased tendency for products to become discolored and 'fogging'

In recent technical literature, the use of supercritical extraction is mentioned more and more in connection with analytical and preparative chemistry, but also for use within more commercial industrial processes. Several processes are known for removing impurities, e.g. monomer, solvent, and oligomers from polymers using supercritical extraction.

Every pure substance has a so-called critical point. In a pressure-temperature phase diagram, there is a state of equilibrium between the substance's liquid and gas forms right up to the critical temperature. Above this temperature, the substance has only one phase: it is a supercritical fluid. Compared to ordinary solvents, supercritical fluids have much greater diffusivity, lower density and much lower viscosity. It has been shown that supercritical fluids have very interesting properties as extraction agents.

Carbon dioxide has particularly interesting critical parameters, with a critical temperature of 31 °C and a critical pressure of 7.38 MPa. This makes it possible to process thermally sensitive products under relatively mild conditions. Other extraction agents can also be used, such as nitrogen oxide or carbon dioxide in a mixture with lower alkanes or alcohols such as hexane, methanol, ethanol, isopropanol etc.

As the state of the art in this area, reference can be made to M.A. McHugh, V.J. Krukonis in Mark, Bikales, Overberger and Merges (eds): Encyclopedia of polymer science and engineering, Vol. 16, Wiley Interscience, New York 1989, pp. 368-399. The patent applications DE 39 21 630, DE 39 35 405 and EP 0 374 879 also describe known technology in this area. The use of supercritical extraction for the removal of solvents, monomers and oligomers from polymers is known according to these references. However, there is no description of the removal of excipients used in the production process itself, nor of quality improvement of the product.

The purpose of the invention is to achieve an improvement in the quality of paste PVC. This means a product which, among other things, has good thermal stability, reduced water absorption in plastic films and reduced emission of volatile substances. Another purpose is to develop a suitable method for removing auxiliary substances which are added during the production of paste PVC and which affect the aforementioned properties. It is an aim to be able to remove both polar and non-polar additives in an efficient way.

These and other purposes of the invention are achieved with the method described below and the invention is further defined and characterized by the accompanying patent claims.

The invention relates to a method for removing auxiliaries that are added during the production of PVC particles, especially for particles used for paste PVC. These include, among other things, ionic and non-ionic emulsifiers, buffer salts, catalysts and fatty alcohols. It was surprisingly found that it is possible to remove close to 100% of these by using an extraction agent in sub- and supercritical conditions. In order to be able to remove all these various excipients, it is essential to use an extraction agent that is made polar. This can be done by adding polar solvents such as e.g. water, methanol, ethanol or isopropanol. However, it is preferred to modify the polarity of the CG^-rich extraction medium in the final extraction step by adding PVC powder which has been suspended in a polar solvent part. The PVC powder can be added both in a dry and wet state, with a content of up to 50% water, preferably < 30%. The temperature of the solvent is kept in the range from 20 °C to the boiling point and the weight ratio between polar solvent/PVC is kept between 0.5:1 and 10:1, preferably 5:1. Suitable extractants are carbon dioxide, nitrogen oxide or mixtures of these with lower alkanes or alcohols. It is preferred to use carbon dioxide as extraction agent at 40-350 bar pressure, in a CCtyPVC ratio between 1:1 to 100:1, preferably 10:1 to 50:1 and temperature 15-60°C. PVC is produced by emulsion, microsuspension, miniemulsion or suspension polymerisation. The original content of excipients in the product is reduced by > 90%. The free-standing PVC is particularly suitable for use in products that must have a low water absorption, such as in cable insulation or roofing materials.

The invention will be further described in the examples with reference to the drawings figures 1-3, where

Figure 1 shows a flow chart for the removal of excipients from PVC.

Figure 2 shows the extraction yield as a function of the ratio between the amount of solvent and the amount of PVC added. Figure 3 shows a flow chart for the removal of auxiliaries from PVC where methanol washing is

including.

Production of paste PVC starts with the production of a PVC latex. This consists of small particles of typically 1 fim that are dispersed in water. The latex has a solids content between 45 - 55%. The method used in our examples to produce PVC latex is a micro-suspension polymerization according to the so-called mini-emulsion technique. According to Bunten, this is a widespread method for producing paste PVC. One of the first patents to describe this technique is Norwegian patent no. 147 756.

A mixed emulsifier system consisting of anionic emulsifier and long-chain alkanol in a ratio of 1:1 to 1:3 is heated to above the melting point of the alkanol. This predispersion is added to the reactor together with the required amount of water and other polymerization aids. After exclusion of oxygen, vinyl chloride monomer is added, which spontaneously emulsifies into a stable monomer emulsion with finely divided, small droplets. The polymerization takes place in these droplets where monomer is converted to polymer. The polymerization is terminated by blowing off unreacted monomer when the conversion has reached 85-95%. A PVC latex has then been obtained which has a primary particle distribution within the size range 0.2-2 jttm.

Common emulsifiers such as alkali or ammonium salts of alkyl sulphates, alkyl sulphonates, alkyl ether sulphates, alkyl aryl sulphonates or fatty acids are often used together with a post-stabiliser which is often a non-ionic emulsifier of the type alkylphenol ethoxylate or fatty alcohol ethoxylate. This gives the latex an increased mechanical stability which means that it can withstand stresses during pipe transport, pumping and filtering or screening. Examples of suitable alcohols are myristyl alcohol, cetyl alcohol, stearyl alcohol and dodecanol. Alkanols are used in amounts from 0.2 to 2% based on the monomer.

The most common method of drying PVC latex is by spraying. The dried powder consists of particles that are both agglomerates and free primary particles. In our examples, disc atomizers are used to dry the latex in a conventional way. far less widespread are techniques that use coagulation of the latex by means of acidification or flocculating agents. Coagulated latex is centrifuged before drying in a hot air oven.

As already described, the known technique for the production of paste PVC gives the product weaknesses because the auxiliaries which are necessary during the manufacturing process become part of the final product. It is known that the properties of paste PVC depend on the content of anionic emulsifiers and also hydrophobic additives. Increased water absorption in plastic films and reduced thermal stability are caused by the presence of emulsifier. Fatty alcohols are relatively volatile and cause increased fogging.

Manufacture of latex

PVC latexes can be produced by a number of known methods. The latex that was used in the following examples was produced according to the method described in Norwegian patent no. 147 756. The additives here consist of a mixture of fatty alcohol and anionic emulsifier. In addition, a non-ionic emulsifier was added to the finished polymerized latex. The same latex was used for all experiments.

Production of PVC powder

PVC latexes are conventionally dried using atomization dryers (spray drying) where the water is evaporated in an atomizer tower. With this technique, all added auxiliary substances remain in the finished powder. Alternatively, the PVC latex can be filtered, and the filter cake is dried in a flash dryer or other suitable hot air oven Both of these methods are used as references to the new method for removing excipients.

Analysis methods

The extract

The extracted amount is weighed and the percentage of extractable material is calculated from the total amount of PVC weighed into the reactor in relation to the weight of the extract. The effects of amount and type of solvent as well as pressure and temperature on the extraction yield can be seen in the examples. The proportion of PVC in the extract has been found by measuring the chlorine content with X-ray diffraction (XRF). All anionic emulsifiers used contained a known amount of sulphur. Proportion of fatty alcohol was found using GPC analyses.

Powder analyses

Content of anionic emulsifier in extracted powder was found by two-phase titration with Hyamine 1622 (Kebo.Lab, CAS 121-54-0) and acidified indicator of dimidium sulphide disulfine blue in weak sulfuric acid. Amount of fogging from powder was measured using standard method DIN 75 201.

PVC paste is produced by mixing plasticizer (DOP), possibly dispersing agent (nonylphenol ethoxylate or isopropyl palmitate) in a Hobart mixer according to standard method ISO 4612. The viscosity of the PVC paste is measured with a Brookfield RVT viscometer at spindle speed 2.5, 10 and 50 rpm. Thermostability is measured by spreading a film of 0.6 mm on a release paper. The film is plasticized in a Verner Mathis oven at 160°C for 2 minutes. The film is then split into standardized strips which are further heat treated at 190°C in 3 minute intervals. The yellowing index on the film is measured with a colorimeter from Dr. Lange. The water absorption in films with a thickness of 1 mm and a diameter of 50 mm is measured by weight gain after the films have been immersed in water at 50°C for 2 days.

To illustrate the method and the properties achieved, several experiments were carried out which will be described below. In the examples, pure PVC was used, but the method is also suitable for PVC copolymers, such as copolymers of PVC and vinyl acetate, ethylene methyl methacrylate etc.

Example 1

Extraction of spray-dried powder was carried out with equipment as shown in figure 1. An autoclave 1 is filled with a bed of PVC particles through which the extraction liquid is pumped 2 from a reservoir 3. The extract is easily removed in a series of separators 4, 5, 6 where the pressure is relaxed, which causes the extract to precipitate out of the solvent. The solvent is recycled in the process. The equipment also includes a heating bath 7, a cooling bath 8, condenser 9 and heating device 10.

The process conditions can be varied over a very large area in this apparatus. Both normal and supercritical conditions can only be used by controlling the temperature and pressure conditions. At 20°C and 200 bar, the conditions are below the critical limit, while at 40-55°C and 300 bar, work is done at super-critical conditions. The effectiveness of the solvent can be easily varied in this process, either by changing only the temperature, or by mixing different liquids. In such a way, polar as well as non-polar auxiliaries can be extracted from paste PVC using one process step.

Several experiments were carried out under different experimental conditions and with different extraction agents. Tables 1-3 show test conditions and results for these. It is the extract that is analyzed to obtain information about which substances are removed. The extract's content of PVC is due to the solvent having carried some fine-grained PVC through the mechanical seals. This is a problem that can easily be solved during the construction of the equipment.

This example shows that different extractants can be used. In order to remove as much excipients as possible, it is preferred to use a polar solvent such as, for example, by adding methanol to CO2.

In Figure 2, the extraction yield is shown as a function of the ratio between the amount of extractant (solvent) and the amount of added PVC. The temperature is varied between 20 and 55°C and the pressure between 200-300 bar. Carbon dioxide is used as a solvent. For ratios above 100, the amount extracted is approximately constant.

Example 2

The efficiency of the process and thus also the economy can be improved by modifying the procedure as described in example 1. As a polar medium is necessary for the removal of anionic emulsifier, experiments were carried out where PVC latex was filtered and where a filter cake with a certain water content was obtained. This filter cake was treated with hot methanol in a stirred tank at low pressure. Methanol was then decanted or filtered from the PVC/methanol suspension. Solid matter (moistened with methanol) was transferred to a high-pressure reactor and was further extracted with sub- or supercritical carbon dioxide, see Figure 3. In this method, CO2 is approximately saturated with methanol. The dissolution properties of CO2 are thereby changed. In this way, the polarity of the solvent can be controlled and controlled. At the same time, methanol is stripped from the PVC particles. With this method, a separate step for adding methanol is avoided since methanol is already present in the powder.

Carbon dioxide leaving autoclave 1 expands in one or more stages (see Figure 1) to recover additives and methanol. Carbon dioxide from the separators is compressed and recycled to the autoclave. If very clean product is to be produced, CO2 can be passed through an adsorbent where methanol is removed from the liquid at the end of the purification step.

Also other solvents such as e.g. water, methanol, ethanol or isopropanol can be used to modify the polarity of the CO2-rich supercritical medium in the final extraction step. Significantly milder conditions and a smaller amount of extraction agent can then be used.

The effect of treatment was monitored by measuring the residual amount of emulsifier (anionic) and the amount of fogging from the powder after treatment. Conditions and results are shown in tables 4-6.

In table 4, the effect of methanol washing is shown in supercritical extraction of moist PVC. As can be seen from the table, temperature and methanol/PVC ratio are the most important parameters for achieving high extraction yields.

Sample no. 8 was processed further with C02 extraction and the results are shown in table 5.

The effect of varying the pressure is shown in this table (Table 5) and also in Table 1. An increase in the pressure from 100 to 300 bar was expected to give better extraction yields as CO2 normally has better properties as a solvent at higher pressure. When surprisingly equally good results were obtained at 100 bar, this may be caused by the less good dissolution properties at lower pressure being compensated for by higher linear velocity and lower viscosity of CO2. This means that at lower pressures it is easier to penetrate the microporous structure of the particles.

Sample no. 11 was processed further with C02 extraction and the results are shown in table 6.

These experiments clearly show that the described extraction technique removes both anionic emulsifier and fatty alcohol very effectively. Experiments 8-11 show that an increased amount of methanol in relation to PVC increases the washing efficiency. Experiments 12-16 show that when the medium is polar, the amount of anionic emulsifier is also reduced during the supercritical extraction with C02. Likewise, fogging is significantly reduced in this operation, proving that hydrophobic, volatile additives are effectively removed.

Fogging can be reduced from 16.30 mg down to 1.03 mg (- 93.7%) and the amount of anionic emulsifier can be reduced from 16.0 mmol/kg down to 0.4 mmol/kg (- 97.5%). This shows that the described process is very efficient and leads to an improvement in the quality of the product.

Example 3

The polar modification of C02 can also be done with solvents other than methanol. Experiments were carried out in the same way as described in example 2, but where methanol has been replaced with hot water. The effect of water washing is shown in Table 7.

Samples 17 and 18 were treated further with C02 and the results are shown in Table 8.

Example 4

An additional process step in the production of paste PVC is required according to the described method. In order to minimize the financial consequences of this, experiments were also carried out with very mild conditions during the extraction. Experimental conditions and results are shown below. Subcritical conditions were used for carbon dioxide.

The results show that even with mild conditions, the remaining amount of emulsifier in the powder has been reduced from 16 mmol/kg down to less than 0.93 mmol/kg (-94.2%).

Example 5

Three pasta mixtures were made, using respectively extracted PVC, untreated PVC and spray-dried PVC.

Recipe for the pasta mixture was:

IPP = isopropyl palmitate

LZ616 = barium zinc thermostabilizer from Lankro

phr = parts per hundred resin

Foils were rolled out and the properties measured. The results are shown in table 10.

The results show that the water absorption in foil is greatly reduced as a result of the extraction, and likewise the development of yellow color in the foil as a function of the time it is exposed to heat is greatly reduced. The paste viscosity is also lower than the standard samples. All this shows that the described extraction method leads to quality improvement.

With this method, up to 100% of excipients are removed. The result depends on the polarity of the solvents. When non-polar solvents are used, mainly hydrophobic excipients are removed. The best yield with respect to anionic emulsifier was obtained in those experiments where methanol was mixed in to increase polarity.

Claims (10)

1. Polyvinyl chloride or polyvinyl chloride copolymers suitable for use in paste PVC, characterized in that the polymer particles have been treated in a polar solvent in a first step with subsequent removal of the solvent, after which the particles in a second step have been treated with an extraction agent in sub -, or supercritical condition, whereby the original content of polymerization aids has been reduced by > 90%. 2. Procedure for removing excipients from polyvinyl chloride (PVC) or PVC copolymer, characterized in that the PVC particles are treated with a polar solvent in a first step with subsequent removal of the solvent, after which the particles are treated in a second step with an extraction agent in a sub- or supercritical state. 3. Method according to claim 2, characterized in that the solvent is removed by decantation or filtration. 4. Method according to claim 2, characterized by the use of dry PVC or PVC with a water content of up to 50%, preferably < 30% dry powder. 5. Method according to claim 2, characterized in that the polar solvent is selected from methanol, ethanol, isopropanol, water or mixtures thereof. 6. Method according to claim 2, characterized in that the temperature in the polar solvent is kept between 20 °C and the boiling point. 7. Method according to claim 2, characterized in that the polar solvent/PVC weight ratio is kept between 0.5:1 to 10:1, preferably 5:1. 8. Method according to claim 2, characterized in that carbon dioxide, nitrogen oxide or mixtures with lower alkanes are used as extraction agent. 9. Method according to claim 2, characterized in that carbon dioxide is used as extraction agent at 40-350 bar pressure, temperature 15-60°C and a ratio between C02/PVC of between 1:1 and 100:1, preferably 10:1 to 50:1. 10. Use of PVC or PVC copolymers produced according to claims 2-9, for products where low water absorption is required, such as cable insulation and roof covering.