COATED PARTICULATE FILLERS

Abstract: Filler particles, e.g. calcium carbonate particles, are coated by the application of a C6-C10 aliphatic carboxylic acid, such as n-octanoic acid or n-decanoic acid, in the presence of water and an alcohol. The coated particles may be incorporated into polymers, such as unsaturated polyester resins or polyvinyl chloride.

Description: u ; COATED PARTICULATE FILLERS Field of the Invention The present invention relates to coated particulate fillers, to a process for their production and to com¬ positions comprising an organic polymer and such a filler. Background to the Invention Inert particulate fillers are frequently mixed with polymers, for example unsaturated polyester resins, in order to reduce the cost of the product and to reduce shrinkage where the polymer is for use in a moulding composition. It has been found to be desirable to put a surface coating onto such fillers to reduce the increase in viscosity that generally occurs when fillers are mixed with synthetic resins. Such a surface coating may also act to increase the stability of the filler-in-resin dispersion and to prevent separation and settling of the filler. A conventional treatment involves coating the mineral filler particles with εtearic acid (usually in an industrial grade containing only about 50% by weight of the C.g acid) at a rate of from 1 to 2% by weight on the filler. There is still a need, however, for a further reduction in the viscosity of iller/polymer compositions. For example, in the "spray-up" process for the manufacture of glass-fibre reinforced polyester compositions the polyester/filler mixture must pass through a spray gun and it is generally true that the lower the viscosity of the mixture the better will be the spray and roll-out characteristics thereof. The spray-up method is used, for example, in the production of bath-tubs by spraying a polyester/glass fibre composition onto a thermoformed shell of acrylic resin, any glass fibres that project from the sprayed-on coating then being forced back into the polyester mass by a "roll-out" operation. There is also a need for fillers that can be readily blended with such polymers as polyvinyl chloride to give fusion times that are shorter than those obtained with conventional fillers. Summary of the Invention The present invention now provides a particulate filler that comprises filler particles to the surface of which there has been applied an aliphatic carboxylic acid having from 6 to 10 carbon atoms in its molecule. (The particulate fillers of this invention are also referred to herein as "coated" fillers. However, this is not meant to imply that the particles are necessarily entirely covered with a layer of the aliphatic carboxylic acid, nor that the layer is necessarily of uniform thickness. ) The present invention also provides a process for the production of a particulate filler which comprises applying to the surface of filler particles an aliphatic carboxylic acid having from 6 to 10 carbon atoms in its molecule, said acid being applied in the presence of water and an alcohol. The present invention further provides a polymer composition comprising a matrix of an organic polymer having distributed therein a particulate filler according to this invention. Description of Preferred Embodiments Any filler may be used in the present invention, provided that it has a surface to which the aliphatic carboxylic acid may be bound: fillers having an acid- reactive surface are especially preferred. The fillers are, in general, mineral fillers, which may be synthetic¬ ally prepared or of natural origin. Suitable fillers may be selected from the oxides, hydroxides, carbonates and silicates of alkaline earth metals, titanium, silicon, aluminium and zinc. A particularly preferred filler is calcium carbonate in any of its various forms, in particular chalk whiting, ground limestone or ground calcite. Precipitated calcium carbonate may also be used. Other useful fillers include aluminium hydroxide (A1(0H) 3 ), which is commonly referred to as "aluminium trihydrate" or "ATH", wollastonite (a form of calcium 'silicate) , magnesium carbonate and barium carbonate. Although the particles may have any suitable size and morphology, it is preferred for many applications that they should be irregularly shaped and have a median particle size of from 2 to 20 μ . It is, of course, possible to employ a mixture of minerals as the filler component. The aliphatic carboxylic acid applied to the filler surfaces has from 6 to 10 carbon atoms, a C g acid being preferable to a C.-. acid which, in turn, is generally preferable to a C g acid. It is preferred that the acid be straight-chained and that it be saturated. A mixture of suitable aliphatic carboxylic acids can, of course, be employed in the practice of this invention. Particularly preferred acids are n-octanoic acid and n-decanoic acid, as well as mixtures thereof. These acids can be obtained in quite good purity (a purity of 95% is adequate), for example from the firπi of Akzo Chemie. These coating materials are generally employed in the form of the free acid, rather than in the form of their salts. i is important in resin-based systems to avoid the migration of free acid from the filler surface, since this may interfere with the surface properties of the moulded article. It is therefore necessary to - A - employ a coating technique in which all, or virtually all, of the acid is chemically bonded to the surface of the filler. Thus, for example, in the case of a chalk whiting, it is necessary to achieve complete or nearly complete conversion of the acid in the coating into the calcium salt. In order to obtain a satisfactory coated filler using a C 8 -C 10 aliphatic carboxylic acid, in accordance with the present invention, it appears to be essential to apply the acid in the presence of both water and an alcohol. As discussed in greater detail below, the applicant's research has indicated that satisfactory results are not obtained if the acid is applied either alone or in the presence of only an alcohol. Furthermore, attempts to apply the fatty acids in a manner analogous to the methods used for applying coatings of stearic acid did not give good results; this is surprising, for stearic acid is solid whereas the fatty acids preferred herein are liquid and might therefore be expected to be amenable to convent- ional coating techniques. The use of low-boiling alcohols is preferred; excess alcohol has to be removed from the coated particles and a high-boiling alcohol would require a correspondingly high temperature for its removal by evaporation, with the consequent risk of yellowing or charring of the organic coating. The preferred alcohols include tert.- butyl alcohol, propan-1-ol, propan-2-ol, eτhanol and methanol, the last-mentioned being especially preferred. In addition to having low boiling points, these preferred alcohols are all readily miεcible with water. A mixture of alcohols can, of course, be use d . The quantity of alcohol used should be sufficient tc dissolve the acid completely to give a clear mixture free of any precipitate. However, it is clearly desirable to use, as far as possible, the minimum serviceable quantity of alcohol, since excess alcohol has to be removed after the coating operation. Preferably, from 0.5 to 2 parts by weight of water are employed per part by weight of alcohol. It is also preferred to employ from 0.5 to 2 parts by weight of the alcohol/water mixture per part by weight of aliphatic carboxylic acid. It must be mentioned that within these preferred ratios, certain combinations may be encountered which are impractical due to insolubility problems. However, such impractical combinations would come to light in preliminary trials and an appropriate adjust¬ ment may then readily be effected. A typical preferred weight ratio of carboxylic acid: alcohol:water is 1:0.26:0.33. Using a ratio of this order and a preferred coating technique (such as that described below in Example 1), then the uncombined acid in the coating on the filler may be as low as 0.7 to 2% of the acid used. In contrast, experiments have shown that if the acid is added either in the absence of water and alcohol or in the presence of methanol alone, the uncombined acid in the coating may be as much as 15 to 30% of the acid used. Although it is possible to add the acid in admixture with both the alcohol and the water, the preferred procedure comprises adding the water first to the filler, mixing these components and raising the temperature to, say, 50°C, adding the acid and the alcohol (preferably as a pre-formed mixture) to the mixture of filler and water, and then mixing thoroughly to ensure a good dispersion of the acid over the filler-particle surfaces. The excess water and alcohol are then removed, con¬ veniently by evaporation, and the residual coated filler is cooled, whereupon it is suitable for collection, bagging and transporting. Preferably, the amount of carboxylic acid in the coating will be from 0.25 to 3.0% by weight of the filler; in many applications, an amount of carboxylic acid of up to 2% by weight of the filler will be satisfactory. The coated fillers of the present invention can be incorporated into a wide variety of organic polymers in order to form composites. When the matrix of the composite is selected from unsaturated polyester resins, for example such resins that are liquid at room temperature (15-20°C) up to the polymerisation temperature (typically 70°C), it has been found that the coated fillers do not cause a large increase in viscosity and the resultant filler/resin composites are particularly advantageous in manufacturing processes which require the composite to be sprayed. The coated fillers of this invention are also useful in polyester dough-moulding compounds, which are used in injection or compression dough-moulding machines. The filler imparts dimensional stability to the moulded composition and helps to prevent undue shrinkage and warping as well as moderating the exotherm. Such com¬ posites, owing to their good heat-resistance properties, may be used, for example, in the covers of sandwichtoasters. The present coated fillers also find application in other resins, for example polyvinyl chloride (PVC) resins. The coated fillers of the present invention can be distributed in the polymer matrix by methods customary in the art. In general, from 5 to 150 parts by weight of filler will be employed per 100 parts by weight of the matrix polymer. Typically, the amount of filler will be 5-20 pphr (parts per hundred parts of resin, by weight) in rigid PVC, 50-100 pphr in flexible PVC and 30-150 pphr in unsaturated polyester resins. In a number of applications (especially in dough-moulding applications, which do not require the use of a spray gun or nozzle) the amount of filler may be even higher, for example up to 300 parts per 100 parts of polymer. The present invention is illustrated in and by the following specific Examples. Example 1 One litre of water was added to 200 kg of chalk whiting (2.7 jjm median diameter) contained in a 630 litre high-speed mixer operated at slow speed, during which addition the temperature rose to 50°C, and then 3 kg of n-octanoic acid mixed with 780 ml of methanol were run into the mixer. The temperature of the mixture was allowed to rise to 110°C (this rise in temperature being due solely to the energy of the mixing process) and was maintained there for a brief period in order to permit the water and the methanol to evaporate off. • The coated whiting filler remaining in the mixture was then discharged into a cooler. The level of free acid in the coating was deter¬ mined to be 0.2% by weight of the filler, whereas the level of combined acid (i.e. the acid that was chemically bonded to the filler surface) amounted to 1.45% by weight of the filler. The viscosity of a 1:1 mixture (by weight) of the coated filler and the polyester "Crystic 196" (from Messrs. Scott Bader) was 1842 cP. Example 2 Using a procedure analogous to that described in Example 1, a chalk whiting filler (Snowcal 7M produced by Blue Circle Industries PLC), having a weight median particle diameter of 2.7 jam, was coated with various quantities of n-octanoic, n-decanoic and, for the purposes of comparison, stearic acid, using coating levels of from 0.25 to 3% by weight of the filler. Examination of this wide range of coating levels was necessary, for the optimum coating level had to be determined for each particular resin/filler combination. The resin used in these experiments was the unsaturated polyester resin Crystic 196, this being a general purpose resin suitable for use in hand lay-up or machine moulding applications. The viscosities of the coated fillers mixed with the Crystic 196 resin at a ratio of 1:1 by weight were measured using a rotating cylinder- viscometer with a shear rate of 0 to 112 sec " . The minimum viscosity was found to occur at a coating level of 1.5% by weight for the n-octanoic and n-decanoic acid treatment and at 1.0% for the stearic acid treat¬ ment. Both the n-octanoic acid and n-decanoic acid treatments gave viscosities of the filler/resin mixture which was 25% below that of the mixture containing the filler treated with stearic acid and 48% below that of the uncoated filler, as shown in the following table: Table 1 Uncoated Snowcal 7ML/polyester mix 3,150 cps n-decanoic acid coated Snowcal 7ML/ polyester 1,500 cps n-octanoic acid coated Snowcal 7ML/ polyester 1,500 cps Stearic acid coated Snowcal 7ML/ polyester 2,000 cps Example 3 Experiments similar to those described in Example 2 were carried out using a calcium carbonate filler having a weight median particle diameter of 16 um (Calcite T30 from Blue Circle Industries PLC) . The resin used in these experiments was the unsaturated polyester resin "Stypol" (from Messrs. Freeman), this being a typical resin for use in dough moulding compounds, Using 150 parts by weight of filler per 100 parts by weight of the polyester resin, the minimum viscosity was found to occur at a coating level of 0.75% by weight of the filler using n-octanoic acid, 0.50% using n-decanoic acid and 0.5% for the stearic acid tested for comparison purposes. In these experiments the reduction in viscosity achieved with the use of the n-octanoic acid coated filler was 50% greater than that achieved using stearic acid coated filler and 63% below the viscosity of a mixture of polyester resin with uncoated filler. The use of the decanoic acid decreased the viscosity of the mixture by 10% compared with stearic acid. The results are summarised in the following table: Table 2 Uncoated Calcite T30/polyester 14000 cps n-decanoic acid coated Calcite T30/polyester 9500 cps n-octanoic. acid coated Calcite T30/polyester 5180 cps Stearic acid coated Calcite T30/polyester 10360 cps Example 4 To evaluate further the performance of the coated fillers of the present invention, samples of coated fillers prepared as described above in Examples 2 and 3 were subjected to the following tests. The ability of the surface coatings to reduce the absorption of organic materials by the fillers was measured using a technique based on torque rheometry described in "The Journal of Paint Technology" Vol 46, No. 591, April 1974, pp 44-50. The liquid used for these determinations was di-octyl phthalate (DOP), more correctly termed bis-(2-ethylhexyl phthalate). The apparent density after compaction was measured according to British Standard BS 1460 : 1967 , and bulk density was measured according to BS 2782 : Part 6 :Method 621A. Both of these tests measure the degree of lubrication between particles . The results are summarised in the following table : Table 3 Absorption of Density Bulk POP (ml/lOOg) after Density Compaction (g/ml) Uncoated Snowcal 7ML 24.30 1.06 0.60 1.5% by weight n-octanoic acid coated Snowcal 7ML 11.45 1.19 0.85 1.5% by weight n-decanoic acid coated Snowcal 7ML 12.08 1.09 0.82 1.0% by weight stearic acid coated Snowcal 7ML 12.40 1.03 0.76 Uncoated Calcite T30 17.60 1.42 0.82 0.75% by weight n-octanoic acid coated Calcite T30 10.90 1.76 * 1.16 0.5% by weight n-decanoic acid coated Calcite T30 10.90 1.70 1.10 0.5% by weight stearic acid coated Calcite T30 11.30 1.67 1.03 The above results indicate that the C g acid performs slightly better than the C 0 acid, both of these, however, being clearly superior to the Ci.t 0 s acid (stearic acid) known from the prior art. In further experiments, a series of mixtures each comprising a coated filler and a polyester resin at a weight ratio of 1:1, using whiting fillers coated, respect¬ ively, with a C 4 , C 6 , C 8 , C 1Q , C 12 , C 14 , C 18 or C 2Q saturated, straight-chain fatty acid and using the resins Crystic 196 and "Uralam" (from Messrs. Uralite). The viscosity of each mixture was measured using a rotating viεcometer and it was found that the viscosity decreased with decreasing chain length of the fatty acid reaching a minimum value with the C g acid, after which the viscosity values increased with decreasing chain length. Fatty acids having a chain length greater than 10 carbon atoms appear to offer but little advantage over the stearic acid conventionally used, whereas the short-chain fatty acids give unsatisfactory coatings (possibly due to their very high reactivity) . Example 5 A number of resin/filler blends were prepared that had the following formulation: 100 parts by weight of unplasticized PVC (Breon S110 from B.P. Chemicals), 2.5 parts of a stabilizer/lubricant (Interstab L3651F from Akzo Chemie) and either 15 or 25 parts of filler. The filler was a chalk whiting (Snowcal 7ML) , uncoated or coated either with stearic acid or with n-octanoic acid in accordance with this invention. For each blending operation, the fusion time was ascertained and the torque response curves were obtained using a Haake Rheocord EU10 apparatus fitted with a Haake Rheomix 600 internal mixer. The uncoated Snowcal 7ML was difficult to disperse in the resin and was found to retard fusion. Coating the Snowcal 7ML with stearic acid was found to decrease the fusion time, giving a minimum value when 2% w/w of the coating agent was employed. Coating the Snowcal 7M1 with n-octanoic acid was found to give a greater reduction in fusion time at coating levels of from 0.5 to 4% w/w. The following table records resultε that were obtained in these tests. Table 4 The results indicate that by coating the filler in accordance with the present invention it is possible to obtain a substantial reduction in the fusion time of the resin blend without incurring an unacceptable rise in fusion torque. Moreover, * the present invention allowed the filler loading to be increased without an unacceptable increase in fusion time. It has also been found that when employing the present fillers with flexible PVC and with other resins (cf. Table 3 in Example 4), it is possible to obtain the desired characteristics, e.g. softness, using less DOP than would otherwise be required. With rigid PVC, the present fillers give better mixing and dispersion and can therefore improve the rate of fusion. It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope and spirit of the invention.

Claims: CLAIMS 1. A particulate filler that comprises filler particles to the surface of which there has been applied an aliphatic carboxylic acid having from 6 to 10 carbon atoms in its molecule. 2. A filler according to claim 1, characterised in that the filler particles are of a mineral selected from the oxides, hydroxides, carbonates and silicates of the alkaline earth metals, titanium, silicon, aluminium and zinc. 3. A filler according to claim 2, characterised in that the filler particles are of calcium carbonate. 4. A filler according to claim 1, 2 or 3, characterised in that the aliphatic carboxylic acid is a straight-chain, saturated acid. 5. A filler according to claim 4, in which the acid is selected from n-octanoic acid, n-decanoic and mixtures thereof. 6. A process' for the production of a particulate filler which comprises applying to the surface of filler particles an aliphatic carboxylic acid having from 6 to 10 carbon atoms in its molecule, said acid being applied in the presence of water and an alcohol. 7. A process according to claim 6, characterised in that the alcohol is selected from methanol, ethanol, propan-1-ol, propan-2-ol and tert-butyl alcohol. 8. A process according to claim 6 or 7, character¬ ised in that 0.5 to 2 parts by weight of water are employed per part by weight of alcohol. 9. A process according to claim 6, 7 or 8, characterised in that 0.5 to 2 parts by total weight of alcohol and water are employed per part by weight of aliphatic carboxylic acid. 10. A process according to any one of claims 6 to 9, characterised in that the aliphatic carboxylic acid is applied to the filler particles in an amount of from 0.25 to 3.0% by weight of the filler. 11. A process according to any one of claims 6 to 10, characterised in that water is added to the filler particles and the aliphatic carboxylic acid is subsequently added in admixture with the alcohol. 12. A process according to any one of claims 6 to 11, characterised in that after the aliphatic carboxylic acid has been applied to the filler particles, the excess water and alcohol is removed by evaporation. 13. A process according to any one of claims 6 to 12, characterised in that it is applied to the production of a filler according to any one of claims 2 to 5. 14. A particulate filler whenever produced by a process according to any one of claims 6 to 13. 15. A polymer composition comprising a matrix* of an organic polymer having distributed therein a part¬ iculate filler according to any one of claims 1 to 5 and 14. 16. A polymer composition according to claim 15, characterised in that the matrix comprises an unsaturated polyester resin or polyvinyl chloride. 17. A polymer composition according to claim 15 or 16, characterised in that from 5 to 300 parts by weight of filler are present per 100 parts by weight of the matrix polymer.

Inventors: WRIGHT PETER JOHN, WRIGHT, PETER, JOHN
Applicants: BLUE CIRCLE IND PLC, BLUE CIRCLE INDUSTRIES PLC
Application year: 1985
Main classification: C08K9
IPC codes: C08K9, C09C3