NO301282B1 - Polymers and resins based on vinyl chloride and with smooth surface and improved internal homogeneous structure - Google Patents

Abstract

Polymers and resins based on vinyl chloride whose homogeneity of the internal structure is improved. They exhibit an excellent pourability and processability as well as very good degassing aptitude. They are prepared by suspended emulsion polymerisation.

Description

The present invention relates to vinyl polymers (copolymers of vinyl chloride) whose grains exhibit a smooth covering as well as a good packing homogeneity, a regularity in the distribution of the cavities and of the elementary particles in the interior of the grains.

The vinyl resins according to the invention are copolymers based on vinyl chloride which are insoluble in the roonomer mixture from which they originate.

According to this, the present invention relates to vinyl polymers and resins obtained from a "mixture of monomers consisting of at least 50% by weight of vinyl chloride, up to 5% by weight of acrylamide and optionally at least one additional monomer selected from vinylidene chloride, tetrafluoroethylene, chlorotrifluoroethylene, alkyl acrylates such as methyl acrylate , alkyl methacrylates such as methyl methacrylate, and olefins such as ethylene or propylene, provided that the copolymer obtained is substantially insoluble in the unreacted monomer mixture, in the form of a powder consisting of individual grains, and these polymers and vinyl resins are characterized by the linear roughness coefficient for the grains is equal to or less than 1.2 for a mean filter diameter equal to 7 and that the packing heterogeneity coefficient for the grains is equal to or less than 1.2 for a mean filter diameter equal to 15.

According to the invention, one can more particularly use polymers originating from monomer mixtures containing up to 5% by weight of acrylamide and selected from vinyl chloride alone; mixtures based on vinyl chloride and at least one olefin such as ethylene, propylene, butene-1, butene-2, isobutene, methyl-4-pentene-1; mixtures based on vinyl chloride and vinyl acetate; mixtures based on vinyl chloride and vinylidene chloride, provided that the copolymer formed is essentially insoluble in the monomer mixtures. As examples of copolymers according to the invention, mention should be made of those originating from: comonomer mixtures based on vinyl chloride containing up to 5 wt-# of acrylamide and at least one olefin comprising 0.1 to 30 and preferably 0.1 to 10 wt-# of the olefin, comonomer mixtures of based on vinyl chloride containing up to 5 wt-# of acrylamide and comprising 0.1 to 30 and preferably 0.1 to 15 wt-# of vinyl acetate, comonomer mixtures based on vinyl chloride containing up to 5 wt-# of acrylamide and comprising 0.1 to 30 and preferably 0.1 to 20 wt # of vinylidene chloride.

The polymers or vinyl resins are available in the form of a powder consisting of individual or agglomerated grains whose surface and shape vary in particular as a function of the polymerization method used (mass, suspension, supported emulsion).

The flowability of the resins is strongly related to the external morphology of the grains.

The flowability of the resins is assessed by, for example, measuring the time it takes for a pre-determined amount of powder to flow from a container of known dimensions.

For powders with the same average diameter, it is easy to understand that those that have a spherical shape and a smooth surface (with a minimum of irregularities and reliefs) drip better.

The flowability of such resins allows a non-negligible time gain during the transformation, for example at the level of the resin feed funnels, but also when it comes to the forming apparatus itself.

US-PS 4,435,524, 4,603,151, 4,607,058 and 4,684,668 describe a method for producing PVC resins of spherical type by suspension polymerization in the presence of special colloids. However, these resins, although spherical, exhibit a somewhat fluted surface reminiscent of a golf ball.

The individual or agglomerated grains that make up the vinyl resin itself consist of elementary particles that are more or less interconnected in the form of packets and voids that correspond to zones that are not occupied by the material.

The ability to transform for the vinyl resins depends on the morphology of the grains.

Thus, a resin whose grains consist of elementary particles and voids distributed homogeneously from the periphery to the core of the grains will exhibit a better behavior in transformation than a resin whose elementary particles are grouped in packages of a larger or smaller type and cohesively.

On the other hand, the distribution state of the material in the grains can influence the degassing ability of the resins. During degassing of the monomers, it has been established that it is all the more difficult to degas vinyl resins the more cohesive element particle packs the grains contain.

The process for degassing vinyl resins generally consists of applying a negative pressure and raising the temperature of the reaction medium. Considering the sensitivity of the vinyl resins to temperature, it is preferred to limit the maximum thermal treatment temperatures to which the grains are subjected, that is to say in practice to limit the duration of the thermal treatment and to work at a temperature that is as low as possible.

A resin containing few or no packets can thus easily be degassed at a lower temperature or during a shorter period of time than a resin containing a larger number of packets.

To recognize the surface aspect of the grains, one can calculate their linear roughness coefficient by analyzing images obtained by examining powder grains by scanning electron microscopy, MEB.

The observed MEB image is obtained by capturing secondary electrons emitted by material, after excitation of the atoms by the incident beam. The secondary electrons have a very low energy (equal to approx. 50 electron volts) which are frequently captured by the material if they exit from a cavity on the surface, the probability of capture is greater the deeper the cavity is.

All the electrons that escape can be captured by the detector. In this way, a scanning electron microscopy image is obtained from the detection of secondary electrons which gives an image of the topography of the observed object, the voids appearing as darker and darker gray as their depth increases and the peaks in gray and clearer and clearer as the height increases .

Connecting the electron microscope to an image analysis allows analyzing on a screen a digitized image obtained by integrating the video signal on a number of images fixed by the operator. This digitized image can be decomposed into pixels (or image points) for which the greyness level is determined by the analyzer, with values between 0 (equal to black) and 255 (equal to white). For the sake of comparison, it should be mentioned that the human eye is not sensitive to more than a fortieth of the greyness level.

It is thus possible to process this image to extract the essential structures that can characterize the object.

The linear roughness coefficient is defined based on the knowledge of the surface contour line, which describes each line connecting two aligned points on the surface. The length of the contour line is determined in pixels based on the gray level histogram that is characteristic of the relief from which it originates. The obtained value is then divided by the length in pixels of the projection in the screen plane of the contour line, i.e. the straight segment connecting the two points. The relationship obtained is the linear roughness coefficient. It is greater than or equal to one. If there is a completely smooth surface, it is equal to one and the value increases with the roughness of the surface. This measure is carried out on several contour lines (one vertical, one horizontal and two slanted). However, only the value of this coefficient is not sufficient to characterize the roughness, thus the sensitivity of the analyzer is such that the gray level variations corresponding to a very small height variation are detected so that even a micro-roughness of an order of magnitude much smaller than the particle diameter can be detected (due to for example the particle itself or due to background noise). If this microporosity is the measure of the roughness of the relief, the effect of an image filtering by means of an intermediate filter is necessary.

The application of such a filter consists in shifting a square matrix of variable size on the screen, pixel by pixel, which is defined in advance by the operator. The calculator ranks in ascending order the gray level values contained in the matrix and replaces the central pixel with one of the values contained in the matrix and determined by the operator. In general, a value in the middle is chosen.

For a square matrix of size three, nine gray levels are ranked in ascending order and the fifth value becomes the central new pixel value.

The more the filter size increases, the coarser the objects that disappear. The variations in non-dotted reliefs are thus observable while the filter size remains small.

In order to recognize the internal structural homogeneity of the resins, the filling ability of the grains is measured by also using scanning electron microscopy and image analysis.

To determine the packing homogeneity of a grain filled with elementary particles, one uses the gray level value on some straight line on the surface. This surface is achieved by cold planing using a glass knife made of PVC powder embedded in a resin, which allows working on grain cross-sections. If the grain is very packed and homogeneous, there will be a predominantly gray level (clear) corresponding to the recovery of secondary electrons emitted by the primary sectioned particles, i.e. emitted in the section plane itself. The intraparticle holes are represented by shallower gray levels, but still relatively clear because when grains are homogeneously packed, the element particle in the surface layer is at a shallow depth.

In the case of a less well-packed grain, there are significant voids and poor distribution between the cut primary particles, placed in the cut plane. This is represented by highly contrasting gray levels.

The analyzer allows knowledge of the gray level histogram, measured along a line on the surface of the slice, the histogram length, represented by a broken line measured in pixels. By dividing this value by the length of the chosen straight line, the linear roughness coefficient value for the observed surface is obtained (here called packing heterogeneity coefficient because the observed surface is a grain section): the further the value lies from 1, the more heterogeneous the section.

The vinyl resins according to the invention can be produced by polymerization by suspended emulsion and preferably according to the method described in EP 0.286.476.

By polymerization in suspended emulsion is meant a polymerization in the presence of an initiator system of which at least one of the components is water-soluble, and of at least one monomer in an aqueous phase dispersed in finely divided form in the monomer.

The attached photographs show the various aspects of the invention and have been obtained using a scanning electron microscope.

Photograph No. 1 shows, at 400 times magnification, a grain of vinyl resin according to the invention;

photograph No. 2 shows at 400 times magnification a grain of vinyl resin produced by suspension;

photographs 3 and 4 show with 1100 times magnification the grain cross-section of the vinyl resin according to the invention; photograph No. 5 shows, at 1100 times magnification, the cross-section of a grain of vinyl resin produced by mass polymerization;

photograph no. 6 shows, at 1100 times magnification, the cross-section of a grain of vinyl resin produced by suspension polymerization.

The following examples shall illustrate the invention.

Examples 1 to 8 relate to vinyl resins polymerized in suspended emulsion according to EP0 0,286,474.

Example 1 (Comparison).

For a vertical reactor of 1 liter and made of stainless steel, equipped with a double jacket for circulation of heat exchange fluid, a turbine stirrer with 6 stirring plates working at 700 revolutions per minute. minute and a deflector, 550 g of vinyl chloride (VC) and an aqueous solution of 800 mg of potassium persulphate in 38 mg of water are added after applying negative pressure. The apparatus is flushed by degassing 50 g of VC.

The temperature in the reaction medium is brought to 56 °C within 45 minutes and then kept constant, this corresponds to a relative pressure of 8.2 bar.

After 2.5 hours of polymerization at 56°C, the resin is degassed and dried. 75 g of polyvinyl chloride is recovered,

PVC.

Example 2

The apparatus and the polymerization conditions are the same as in example 1 except that the potassium persulfate solution (800 mg) is prepared from 18 ml of water and that an aqueous solution of 1 g of acrylamide in 20 ml of water is introduced before continuing with the flushing of the apparatus by degassing 50 g VC.

After 1.5 hours of polymerization at 56°C, degassing and drying, 110 g of PVC is recovered.

Example 3 (Comparison).

One works under the same conditions as in example 2, but replaces acrylamide with methacrylamide.

After 2 hours of polymerization at 56°C, drying and degassing, 95 g of PVC is recovered.

Example 4 (Comparison).

The apparatus is the same as in example 1.

550 g of VC and an aqueous solution of 300 mg of potassium persulfate, 33 mg of iron (II) sulfate, 46.7 mg of hydrogen peroxide and 100 mg of ascorbic acid in 38 ml of water are added to the reactor, after negative pressure has been applied. The apparatus is then flushed by degassing 50 g of VC.

The temperature in the reaction medium is brought to 70°C within 45 minutes and then kept constant, this corresponds to a relative pressure of 11.5 bar.

After 15 minutes of polymerization at 70°C, degassing and drying, 95 g of PVC is recovered whose grain structure corresponds to that in photographs 1 and 3.

Example 5

Apparatus and experimental conditions are the same as in example 4, except that 1 g of acrylamide is added to the initiator system before introduction into the reactor.

After 5 minutes of polymerization at 70°C, degassing and drying, 140 g of PVC is recovered.

Example 6

To a 16 1 prepolymerizer and of stainless steel, equipped with a double jacket for circulation of heat exchange fluid, turbine agitator of the "Lightnin" type at 750 revolutions per minute and with 6 stirring plates, as well as a deflector, after applied negative pressure, 8.9 kg of VC and an aqueous solution containing 9.6 g of potassium persulfate, 1.06 g of iron(III) sulfate, 1.5 g of hydrogen peroxide, 3.2 g of ascorbic acid are placed and 12 g of acrylamide in 2.435 kg of water, after which the apparatus is flushed by degassing 0.9 kg of VC.

The temperature in the reaction medium is brought to 56 °C within 40 minutes and kept constant, this corresponds to a relative pressure of 8.2 bar in the prepolymer.

After 14 minutes of prepolymerization, the degree of conversion is close to 15$ and the reaction medium is transferred to a vertical polymerizer of 25 liters, also made of stainless steel, equipped with a double jacket for circulation of heat exchange fluid. The stirrer consists of a spirally wound screw-shaped band that runs close to the polymerizer walls and is attached by means of three carriers to a rotating arm that passes through the upper part of the polymerizer along its axis and at the lower end is fixed with an arm that is fixed at its lower end with an arm with a shape corresponding to the bent bottom of the polymerizer. The stirring speed is regulated to 100 revolutions per minute. The reaction temperature is brought to 56°C within 30 minutes and is kept constant, this corresponds to a relative pressure of 8.2 bar in the polymer istor.

After 106 minutes of polymerization at 56°C, degassing and drying, 6,320 kg of PVC is recovered whose grain structure corresponds to that of photographs 1 and 4.

Example 7

Apparatus and conditions for the polymerization are the same as in example 6 except that the speed of the turbine stirrer is 1000 revolutions per minute. minute.

After 100 minutes of polymerization at 56°C, degassing and drying, 5,840 kg of PVC is recovered.

Example 8

Apparatus and conditions for the polymerization are as in Example 6, except that the amount of VC added to the prepolymerizer is 7.9 kg, that the aqueous solution consists of 6.9 g of potassium persulfate, 2.12 g of potassium metabisulfite and 10.1 g of colloidal ethyl hydroxyethyl cellulose in 3.2 kg of water and that the temperature in the reaction medium in the polymeriser is brought to 450 C within 30 minutes and that, after transfer to the polymeriser, a further amount of VC equal to 4.9 kg is added.

After 4 hours of polymerization at 56°C, degassing and drying, 8.8 kg of PVC is recovered.

The resins in Examples 9 to 12 are commercial resins or samples whose characteristics are summarized in Table 1.

For example 9: the resin LACOVYL GB 1150, commercialized by the company ATOCHEM and produced by mass polymerization and whose structure corresponds to photo 5.

For example 10: the resin LACOVYL S 110, commercialized by the company ATOCHEM and produced by mass polymerization.

For example 11: the resin LACOVYL RB 8010, commercialized by the company ATOCHEM and produced by mass polymerization.

For example 12: resin sampled by the company Goodrich, prepared by suspension polymerization and in which the structure corresponds to photographs 2 and 6.

Assessment of the ability to transform.

The ability to reform the resin is assessed in 2 types of reforming apparatus:

1) a cylinder elter under the following conditions:

2) a brabender mixer under the following conditions:

tube 50 m^

The resin to be converted is mixed in advance with various components in the following amounts by weight:

In the case where the conversion takes place in a cylinder filter, the duration of conversion to foil is measured under the conditions described above.

When it comes to transformation in Brabender mixers, the mechanical moment of the powder is measured, which corresponds to the moment of gelation as well as the gelation time and the corresponding gelation temperature.

Table 2 summarizes the different conversion characteristics measured for the resins of Examples 5, 6, 8 and 9.

Assessment of rice yield

Using a chronometer, measure the time it takes for 383.1 cm<3> of resin to flow from a cone-shaped container of chromed stainless steel (largest cone diameter 92.8 mm, small diameter (container bottom): 9.5 mm; cone height: 114.3 mm).

The bottom of the container is closed with a plate and the container is filled with a resin by filling it along the wall in order to avoid accumulation of powder.

When the container is full, you level the powder on top of the container and then remove the plate that closes the bottom and start the stopwatch at the same time.

The powder is allowed to flow freely and the stopwatch is stopped when the container is empty, the time expressed in seconds corresponds to the resin's ability to drip.

Table 3 indicates the time elapsed for the resins of Examples 2, 6, 8, 9, 10, 11 and 12.

Assessment of appearance of the grain surfaces.

Grains from different vinyl resins are examined using a scanning electron microscope of the type SEM 505 from the company Philips. The images obtained are processed with an image analyzer IBAS 2000 K0NTR0N. The linear roughness coefficient is calculated as a function of the average filter diameter. The obtained curves are shown in Figure 1.

For the resins according to the invention (examples 2, 6 and 8) the linear roughness coefficient tends towards 1 for a filter diameter equal to 7, while for the resins according to examples 9 to 12 the same value is reached for a filter diameter equal to 15, which thus corresponds to a much less smooth look.

Assessment of the internal structural homogeneity of the grains.

The interior of the grains of different resins is examined using scanning electron microscopy (with a SEM 505 from the company Philips, connected to an image analyzer IBAS 2000 K0NTR0N

The powder is encased in wood glue. After curing, a planing is carried out with a glass knife at -60"C so that the elementary particles are not deformed by the heating due to the passage of the knife. Gold is metallized to limit the electron discharge phenomenon associated with the insulating nature of the vinyl resins, under observation by scanning electron microscopy .

The packing's heterogeneity coefficient is calculated as a function of the mean filter diameter. The obtained curves are summarized in Figure 2.

For the resins according to the invention (examples 2 and 6) it is determined that the packing heterogeneity coefficient is below 1.2 for a filter diameter equal to 15, while for the resin from examples 9 and 10 it is equal to 1.5. It is also determined that the reduction of the coefficient with the filter diameter is much more brutal for the resins according to examples 2 and 6.

Claims (5)

1. Vinyl polymers and resins obtained from a mixture of monomers consisting of at least 50 wt-$ vinyl chloride, up to 5 wt-sS acrylamide and optionally at least one additional monomer selected from vinylidene chloride, tetrafluoroethylene, chlorotrifluoroethylene, alkyl acrylates such as methyl acrylate, alkyl methacrylates such as methyl methacrylate, and olefins such as ethylene or propylene, provided that the copolymer obtained is substantially insoluble in the unreacted monomer mixture, in the form of a powder consisting of individual grains, characterized in that the linear roughness coefficient of the grains is equal to or less than 1.2 for a mean filter diameter equal to 7 and that the packing heterogeneity coefficient for the grains is equal to or less than 1.2 for a mean filter diameter equal to 15. 2. Vinyl polymers and resins according to claim 1, characterized in that they originate from a monomer mixture consisting of vinyl chloride and up to 5% by weight of acrylamide. 3. Vinyl polymers and resins according to claim 1, characterized in that they originate from a monomer mixture based on vinyl chloride containing up to 5% by weight of acrylamide and 0.1 to 30 and preferably 0.1 to 10% by weight of at least one olefin. 4. Vinyl polymers and resins according to claim 1, characterized in that they originate from a monomer mixture based on vinyl chloride containing up to 5 wt-lb of acrylamide and 0.1 to 30 and preferably 0.1 to 15 wt-lb of vinyl acetate. 5. Vinyl polymers and resins according to claim 1, characterized in that they originate from a monomer mixture based on vinyl chloride containing up to 5% by weight of acrylamide and 0.1 to 30 and preferably 0.1 to 20% by weight of vinylidene chloride.