Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F14/00—Homopolymers and copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen
  • C08F14/02—Monomers containing chlorine
  • C08F14/04—Monomers containing two carbon atoms
  • C08F14/06—Vinyl chloride

Definitions

• This invention relates to the suspension polymerization of vinyl chloride monomer (VCM), and more particularly to the use in such polymerization of an improved secondary protective colloid.
• VCM vinyl chloride monomer
• Polyvinyl chloride is one of the largest volume commodity thermoplastics manufactured industrially. More than 75 % of PVC is made using suspension polymerisation technology. The final physical form or morphology of the polymer is determined by a combination of factors in the polymerisation process. Two characteristics of the product which are particularly important are the particle size and the porosity of the particles.
• PVC particles with a mean grain size of the order of 100 to 200 ⁇ m, and a narrow particle size distribution such that no more than a small percentage of particles are smaller than 63 ⁇ m or larger than 250 ⁇ m.
• High porosity is important, because it facilitates the removal of unreacted VCM from the PVC particles.
• VCM is a known carcinogen, and the industry is therefore continually attempting to reduce the level of monomer to well below 1 ppm in the final resin. In the case of PVC-based foils for medical applications, residual monomer levels less than 50 ppb are required.
• High porosity means that unreacted VCM can be removed efficiently from the particles at lower temperatures, thus avoiding problems of thermal degradation of the PVC at higher temperatures. High porosity also facilitates the subsequent introduction of processing additives (such as plasticisers) into the particles.
• the final particle size and the amount of porosity in the polymer is determined by the polymerisation temperature, the prevailing agitation conditions in the polymerisation reactor, and by the use of certain additives which are variously referred to as “dispersants” or "protective colloids”.
• Protective colloids are categorised as either primary or secondary types. Primaries are largely responsible for the control of the size of the polymer particles and secondaries are largely responsible for controlling the porosity of the polymer particles.
• Primary protective colloids are typically water soluble polymers based on substituted cellulose ethers and / or high molecular weight partially hydrolysed polyvinyl acetates (PVA).
• Secondary protective colloids are predominantly based on lower molecular weight partially hydrolysed PVA, and to a lesser extent surfactant technology.
• the PVC industry is continually seeking to improve the suspension polymerisation process with the aim of improving polymer quality and improving the economics of production.
• the present invention overcomes some of the limitations of the existing commercially available secondary protective colloid technology.
• the partially hydrolysed PVAs which are used in the majority of industrial applications as secondary protective colloids are manufactured in a two stage process.
• vinyl acetate is polymerized to PVA.
• the PVA is partially hydrolysed to form a polyvinyl acetate - polyvinyl alcohol copolymer.
• the particular polymerization conditions used in the first stage have a profound effect on the properties of the final product, including higher order polymer characteristics such as the level of unsaturation, conjugated unsaturation, chain branching and the nature of end groups.
• the properties of the final product are also dependent on the hydrolysis method. For example, base hydrolysis leads to products having a "blocky" structure, i.e. polymers having sequences of hydroxyl groups interspersed among sequences of pendent acetate groups. Acid hydrolysis, on the other hand, leads to randomly hydrolysed polymers.
• the efficiency with which a secondary protective colloid introduces porosity into the PVC is significantly affected by the temperature at which the polymerisation is carried out.
• the porosity of the polymer is inversely proportional to the polymerisation temperature when all other polymerisation variables are kept constant.
• Low K value polymers prepared at high polymerisation temperatures generally have low porosity. It is therefore a still further aim of the present invention to provide secondary protective colloids which can produce high porosity at high polymerisation temperatures.
• a major concern for PVC manufacturers is the quality of the PVCs initial colour once it has been formulated into a melt state.
• a number of process variables are known to influence the initial colour, including the choice of additives used during the suspension polymerization process.
• Intense research efforts have been focused on understanding the factors which contribute to PVC thermal stability, with little regard to the inherent thermal stability of the protective colloid.
• great care must be taken to avoid burning the PVA particularly during the stripping stage to remove residual vinyl acetate monomer. Poor vinyl acetate stripping can result in dark or black contamination in the final protective colloid product.
• a further aim of this invention is therefore to provide secondary protective colloids with improved thermal stability compared to current technology.
• Acrylic based polymers have been developed which act as primary protective colloids in the suspension polymerisation of vinyl chloride.
• US-A-4104457 describes acrylic acid copolymers to control particle size in bulk vinyl chloride polymerisations.
• US-A-4603151 and US-A-4684668 describe the use of cross-linked acrylic acid primary protective colloids which act as thickening agents.
• WO97/08212 describes the use of high molecular weight (> 100000) acrylic dispersants which have a cloud point above the polymerisation temperature. These polymers are stated to act as primary colloids when cross-linked, and as secondary protective colloids when not cross-linked.
• the use of high molecular weight acrylic acid copolymers generally requires the addition of a base to the suspension polymerisation process to neutralise the acrylic acid. It is therefore an aim of the present invention to provide protective colloids which do not require neutralisation in the suspension polymerisation process.
• EP-A-0483051, US-A-5155189 and US-A-5244995 describe the use of relatively low molecular weight polymers in suspension polymerization processes.
• the claimed additives are based on homo or copolymers containing more than 50% by weight of ⁇ , ⁇ - unsaturated ester(s) of acrylic acid and / or methacrylic acid having no ionic side groups. It is stated that terminal functional groups can be added to improve performance. These materials can be shown to act as secondary protective colloids increasing the porosity of the final PVC when compared to PVC produced without a secondary protective colloid.
• Suspension PVC production is a batch process and major developments in the last few years have focussed on improving the productivity of the polymerisation reactors.
• the process of making PVC can be split into the actual time polymerising VCM, referred to as the reaction time, and the time taken to charge and discharge the reactor, referred to as the non-reaction time.
• One way in which the non-reaction time can be reduced is to charge the reactor with hot water to reduce the non-reaction time. It is therefore an additional aim of the present invention to provide secondary protective colloids which can be used in hot water charge processes.
• a copolymer of (i) an alkyl acrylate or alkyl methacrylate and (ii) a hydroxyalkyl acrylate or hydroxyalkyl methacrylate is used as a secondary protective colloid in the suspension polymerization of vinyl chloride monomer.
• the present invention provides a polyvinyl chloride resin containing a copolymer of (i) an alkyl acrylate or alkyl methacrylate and (ii) a hydroxyalkyl acrylate or hydroxyalkyl methacrylate.
• the present invention provides a polyvinyl chloride resin having a K value of 60 or less, a cold plasticiser absorption value of 20% or greater (more preferably 25% or greater), and a ratio of median grain size (MGS): grain size spread (GSS) in the range 1.6 - 2.5.
• K value which is related to molecular mass, is determined by the method described in ISO 1628-2-1998 "Determination of the viscosity of polymer in dilute solution using capillary viscometers. Part 2 PVC resins". This method describes the values measured in cyclohexanone at a concentration of 5 g/L, and K value is calculated as follows:
• Cold plasticiser absorption value is determined in accordance with ISO4608 "Homopolymer and copolymer resins of vinyl chloride for general use. Determination of plasticiser absorption at room temperature”. The method determines the quantity of plasticiser absorbed by the resin at room temperature to give a dry mixture, and the results indicate the usefulness of a resin for the manufacture of dry blends. Detailed description of preferred embodiments
• the secondary protective colloids used in the present invention are copolymers of (i) an alkyl acrylate or alkyl methacrylate and (ii) a hydroxyalkyl acrylate or hydroxyalkyl methacrylate, which may be prepared by standard methods in the art, such as those described in "The Encyclopaedia of Chemical Technology" Volume 1, 4th Edition (Kirk
• copolymers are prepared in solution to aid in the control of the molecular weight.
• a chain transfer reagent can be used to further control the molecular weight of the resulting copolymer.
• Preferred copolymers have a peak average molecular weight (Mp) of from 5000 to 50000, preferably from 6000 to 20000, and most preferably from 8000 to 14000. Peak average molecular weight is determined by reversed phase liquid chromatography. Briefly, the method involves gel permeation chromatography at 35°C using THF as solvent, and polystyrene standards as calibrants.
• Mp peak average molecular weight
• the alkyl moiety in the alkyl acrylate or methacrylate is preferably a Ci to C 20 alkyl group, which may be a straight chain or branched. More preferably, the alkyl group is a C 2 to C 20 up. Particularly preferred are C 3 to C 16 alkyl groups, such as n-butyl, i-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl groups. Most preferred are 2-ethylhexyl acrylate and 2-ethylhexyl methacrylate.
• the copolymer may comprise more than one, e.g. two or three, different alkyl acrylates or alkyl methacrylates having different alkyl groups.
• the hydroxyalkyl moiety in the hydroxyalkyl acrylate or methacrylate is preferably a C 1 to C 6 hydroxyalkyl group, and more preferably a C 1 to C 4 hydroxyalkyl group.
• 2- Hydroxyethylacrylate and 2-hydroxymethacrylate are particularly preferred.
• the copolymer may comprise more than one, e.g. two or three, different hydroxyalkyl acrylates or hydroxyalkyl methacrylates having different hydroxyalkyl groups.
• the copolymers used in the present invention will generally comprise from 20% to 80% by weight of alkyl acrylate or methacrylate moieties, with the balance being hydroxyalkyl acrylate or methacrylate moieties.
• minor amounts generally less than 20% by weight, preferably less than 10% by weight, and most preferably less than 5% by weight
• Such other monomers include styrene, vinyl chloride monomer, vinylidene chloride, acrylonitrile, butadiene and isoprene.
• the copolymers of the invention comprise from 20% to 80% by weight of alkyl acrylate or methacrylate moieties and from 80% to 20% by weight of hydroxyalkyl acrylate or methacrylate moieties, more preferably from 20% to 50% by weight of alkyl acrylate or methacrylate moieties and from 80% to 50% by weight of hydroxyalkyl acrylate or methacrylate moieties, e.g. from 25% to 50% by weight of alkyl acrylate or methacrylate moieties and from 75% to 50% by weight of hydroxyalkyl acrylate or methacrylate moieties.
• the above-described secondary protective colloids are used in the suspension polymerization of vinyl chloride monomer to form particulate polyvinyl chloride homopolymers or copolymers.
• Such polyvinyl chloride copolymers generally comprise less than 50% by weight, and preferably less than 20% by weight of comonomers other than VCM.
• Comonomers which may be used include acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, acrylamides, vinyl esters such as vinyl acetate, vinyl ethers, and the like.
• the secondary protective colloids are generally used in an amount of from 50 to 2000 ppm by weight of monomer, preferably from 100 to 1500 ppm, and most preferably from 200 to 1000 ppm. A mixture of two or more secondary protective colloids may be used if desired.
• At least one primary protective colloid will generally also be used.
• Suitable primary protective colloids include partially hydrolysed PVA, cellulose derivatives such as methyl cellulose and hydroxypropyl methyl cellulose, calcium phosphate, gelatin, starch, poly vinyl pyrrolidone and vinyl ethers. Partially hydrolysed PVA and cellulose ethers are particularly preferred. "Partially hydrolysed PVA primary protective colloids typically have a molecular weight of from 50000 to 250000, more usually from 75000 to 200000, and a degree of hydrolysis of from 70% to 90%, e.g. from 75% to 90%.
• the suspension polymerization is carried out under conditions to give high productivity, which would cause an unacceptable deterioration in the morphology of the resulting PVC if conventional secondary protective colloids were used.
• use of the secondary protective colloids of the invention permits the greater use of a reflux condenser (or equivalent means) to remove heat from the reaction. This allows higher rates of reaction for a given reaction temperature, or lower reaction temperatures for a given reaction time.
• heat removal may begin at a very early stage in the reaction. For example, heat removal may begin at less than 15% monomer conversion, and preferably at less than 10% (e.g. 5%) monomer conversion. It is especially preferred to begin partial heat removal via a condenser from the start of the reaction.
• the secondary protective colloids of the invention are also suitable for use in hot water charge processes.
• the temperature of the water charge in such processes lies between 60° and 120°C, and more preferably between 70° and 90°C.
• the volume of solvent used was approximately double the total monomer volume.
• Initiators bis (4-tert-butylcyclohexyl) peroxydicarbonate and dilauroyl peroxide (0.45 weight percent on total monomer weight) and mercaptoethanol (1.6 weight percent on total monomer weight) was added as chain transfer reagent.
• the polymerisation was started by heating to 75° C (and controlled ⁇ 5° C) and'continued for a minimum of 3 hours. After maintaining the reaction at temperature the resulting copolymer product which was in the form of a mobile liquid was cooled to room temperature. No further purification was necessary before use in suspension polymerisation experiments.
• a suitable chain transfer agent was used to control the molecular weight.
• the reaction was allowed to proceed to high conversion.
• the subsequent copolymer solution was used directly in the suspension polymerisation of PVC without further purification.
• a suitable chain transfer agent was used to control the molecular weight.
• the reaction was allowed to proceed to high conversion.
• the subsequent copolymer solution was used directly in the suspension polymerisation of PVC without further purification.
• a suitable chain transfer agent was used to control the molecular weight.
• the reaction was allowed to proceed to high conversion.
• the subsequent copolymer solution was used directly in the suspension polymerisation of PVC without further purification.
• a suitable chain transfer agent was used to control the molecular weight. The reaction was allowed to proceed to high conversion. The subsequent copolymer solution was used directly in the suspension polymerisation of PVC without further purification.
• Suspension polymerisations were carried out in pilot plant scale reactors of 160 litre and 250 litre capacity fitted with turbine type or Pfaudler type agitators.
• Vinyl chloride monomer was production grade monomer supplied by EVC (UK).
• Demineralised water was used in all polymerisations. Demineralised water quality limits were set between pH 5 to pH 9 with a conductivity below 10 uS. Reactions were initiated with organic peroxide free radical initiators chosen from;
• Primary protective colloids 1, 2, 3 and 4 were partially hydro lysed PVAs, having the following properties:
• Primary protective colloid 5 was a hydroxypropyl methyl cellulose containing 28.5% MeO, 6 % HPO and having a viscosity of 40 - 60 mPa/s.
• Primary protective colloid 6 was a methyl cellulose containing 30% MeO and having a viscosity 12 - 18 mPa/s.
• Reactors were first prepared by the application of Evicas 90 (Trade Mark) antifouling agent. Agitation was set at a lower speed than the reaction speed while adding demineralised water and initiator. The reactor was sealed and purged with Nitrogen before applying a vacuum. Protective colloids were added to the reactor followed by VCM. Stirrer speed was increased to reaction speed and the reactor heated via a jacket. Heat of polymerisation was removed by the jacket or via a reflux condenser where stated. Additional demineralised water was injected depending on the recipe to ensure good heat transfer. At a specified conversion indicated by a drop in polymerisation pressure the remaining VCM was vented from the reactor before stripping the residual monomer under vacuum and high temperature.
• Evicas 90 Trade Mark
• K70 PVCs was prepared using Primary Protective Colloid 1 (500 ppm) and various secondary protective colloids (900 ppm). The results were as follows:
• MGS Median grain size
• b Grain size spread is defined as the difference between the particle size at 85% of the distribution and the size at 15 % of the distribution.
• Particle size distribution measured by laser diffraction analysis on a Beckman Coulter LS230 instrument.
• d Cold plasticiser absorption (CPA) test conforms to ISO 4608, and represents the porosity 5 in the S-PVC.
• e Apparent density (AD) test method conforms to ISO - 60 1977.
• the prefix 414 in the Run number refers to a polymerisation conducted in the 250 litre reactor, whereas 421 refers to a polymerisation conducted in the 160 litre reactor.
• the secondary protective colloid according to the present invention gives a much higher porosity.
• monomers used to make the improved secondary protective colloid are added directly to the polymerisation (Run 421,593) no improvement in particle size or porosity is observed.
• 2-ethylhexyl acrylate homopolymer (Run 421,570) and 2-hydroxyethyl acrylate homopolymer (Run 421,567) are used, no advantage is shown. 0
• Example 1 was repeated using secondary protective colloids prepared from different acrylates, with the following results:
• the preferred copolymer is formed between 2-ethylhexyl acrylate and 2-hydroxyethyl acrylate.
• Example 1 was repeated using secondary protective colloids formed from different comonomers. The following results were obtained:
• Example 1 was repeated using secondary protective colloids having different hydroxyalkyl acrylate content. The results were as follows:
• Example 5 The purpose of this Example was to test an additive of the kind described in EP-A- 0483051.
• K70 PVCs were prepared using a combination of Primary Protective Colloid 2 (160 ppm), Primary Protective Colloid 3 (500 ppm), Primary Protective Colloid 5 (70 ppm) and various secondary protective colloids (400 ppm), including SK Oligomer UMB 2005B (Trade Mark, a low molecular weight poly butyl acrylate based polymer functionalized at the polymer chain end with hydroxyl groups).
• Primary Protective Colloid 2 160 ppm
• Primary Protective Colloid 3 500 ppm
• Primary Protective Colloid 5 70 ppm
• various secondary protective colloids 400 ppm
• K70 PVCs were prepared using Primary Protective Colloid 1 (500 ppm) and various secondary protective colloids (900 ppm). The results were as follows:
• WO97/08212 teaches that a low molecular weight primary protective colloid having a low degree of hydrolysis (such as Primary Protective Colloid 3) should be used with low molecular weight acrylate secondary protective colloids.
• Primary Protective Colloid 3 When the primary colloid is replaced with one which is less effective at increasing porosity (Primary Protective Colloid 1), UMB 2005B is shown to be less effective compared to the improved secondary protective colloid of the invention.
• K70 PVCs was prepared using a combination of Primary Protective Colloid 2 (200 ppm), Primary Protective Colloid 3 (500 ppm) and Secondary Protective Colloid B or 6 (400 ppm)
• the improved secondary protective colloid of the invention is shown to give higher porosity compared to standard PVA based secondary protective s colloid technology.
• polymer made using the improved secondary protective colloid of the invention lose VCM at a much faster rate than the polymer formed using standard secondary protective colloid technology.
• the combination of high porosity and excellent internal structural homogeneity can not be achieved using standard PVA based secondary protective colloid technology.
• Example 8 Low molecular weight rigid PVC resins are used in a number of critical film and blow molding applications where it is essential to achieve low levels of residual monomer in the final product. Higher temperature monomer stripping can be used to achieve very low levels of VCM, but thermal degradation of the PVC may result.
• K70 PVCs were prepared to show the effect of using the improved secondary protective colloid of the invention in combination with a standard PVA based secondary colloid.
• Primary Protective Colloid 1 was used in an amount of 500 ppm, and the total amount of secondary protective colloid was 900 ppm.
• Example 10 The initial colour of PVC is influenced by a number of process variables. It is known that the thermal stability of standard secondary protective colloids must be controlled during production to avoid variation in colour resulting from thermal degradation of the colloid. A comparison of the inherent thermal stability of the improved secondary colloid and standard technology is shown below. A thin film of secondary colloid was placed in an aluminium dish and dried at 180 0 C for 1 hour.
• Formulations were first milled on a two roll mill then compression molded. The Yellow index of the final plaque was assessed.
• Example 13 A similar comparison to example 12 was carried out using K57 PVC samples. The colour performance was assessed in an unplasticised Tin based stabilised bottle formulation.
• the amount of water absorbed by the polymer is very important. Where PVC is used in contact with water, it is desirable that water is not absorbed, turning the PVC translucent or hazy and therefore losing the good optical properties of PVC.
• the amount of water absorbed by suspension polymerized PVC is influenced by a number of process variables.
• Samples were prepared on a 2 roll mill (160 °C / 150 0 C), and then immersed in demineralised water at 50 0 C. After certain periods of time, samples were removed and the amount of light transmitted through the sample was measured on a UWVisible spectrometer at 600 nm. Results are expressed as a % light transmitted relative to the amount of light transmitted before immersion in water.
• US-A-5977273 and EP-A-705847 describe the use of ionically modified protective colloids, having a high cloud point temperature, which can be used in high productivity hot water charge processes.
• the following example shows that the improved secondary protective colloid of the present invention, which has no discernable cloud point in the temperature range used in the commercial production of PVC by suspension polymerisation, gives excellent performance in hot water charge processes.
• the example shows data from a K70 polymerisation. The reactor was charged with a small quantity of the charge water, protective colloids and free radical initiator and VCM. The remaining water was charged to the reactor as hot water at 85 0 C in a 2 minute time period.
• a necessary requirement of any improved secondary protective colloid for the suspension polymerisation of vinyl chloride is the ability to perform under high productivity conditions.
• Suspension polymerisation is generally carried out as a batch process, so the reaction time of each batch must be reduced to a minimum to improve the plant's productivity.
• Using a single initiator at a fixed temperature the evolution of heat tends to peak towards the later stages of polymerisation, when all free liquid VCM is consumed.
• a more even rate of heat evolution can be achieved by the use of a combination of initiators with different half life temperatures. It is common practice to remove the heat of polymerisation by the combination of a cooling jacket or coil on the reaction vessel and an external reflux condenser.

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