GB2054541A - Coated limestone filler for resin systems - Google Patents

Abstract

A limestone filler comprises particles of ground limestone having an average diameter of from 0.25 mu m to 44 mu m coated with benzoic acid in an amount of from 0.05 to 0.50 weight %. The coated limestone may be used as a filler for various systems, and reduces binder demand and increases the pot life of the resin-filler mixture.

Description

SPECIFICATION Coated limestone filler for resin systems This invention relates to inorganic fillers of the type commonly used in various resin systems. More particularly, it relates to ground limestone fillers suitable for use in polymerizable resin formulations including such materials as styrenated polyester resin, epoxy resin and linseed oil. Moreover, the present invention relate to a method for increasing pot life and reducing binder demand for a ground limestone filler.

Binder demand is generally considered to be the amount of binder or resin required to completely wet out the filler used to produce a resin-filler mixture of a given viscosity. Pot life, on the other hand, is the time period between mixing the binder or resin and filler and the time when that mixture has become too viscous to be further worked.

Products based on thermosetting and thermoplastic resin formulations normally contain a considerabie proportion of filler. The commonly used fillers are much less expensive than the associated resin systems.

Consequently, there is a strong incentive to increase filler proportion to a maximum which is commensurate with the required strength of the cured product. In many systems, the optimum proportion of filler cannot be achieved because the resin-filler mixture becomes too viscous to be worked before that optimum can be attained. Therefore, methods to reduce binder demand are constantly being sought.

With highly filled resins, pot life is often reduced to the point that insufficient time is afforded for necessary processing such as wetting of reinforcing fiberglass or release of entrapped air.

Prior attempts to solve the foregoing problems having included such approaches as alteration of the particle size distribution of the filler, surface attrition, compaction, and surface coating of the filler. These attempts have met with varying degrees of success. An attempt to get better dispersion of filler is disclosed in Japanese patent publication 50/40399. In that publication there is described the coating of particles of, interalia, calcium carbonate with various organic acids including, for example, chlorobenzoic acid. However, even further increased pot life and reduced binder demand is desirable.

The present invention provides an economical method of achieving lower binder demand and increased pot life for the commonly used ground limestone fillers, e.g. calcitic limestone and dolmitic limestone.

The present invention is based on the discovery that a filler comprising ground limestone coated with 0.05 to 0.50% by weight of benzoic acid, the ground limestone having an average diameter of from 0.25 sum to 44 lim, achieves increased pot life and reduced binder demand in an economical fashion. Preferred forms of ground limestone include calcitic and dolomitic limestones. Preferred binders include styrenated polyester resins, epoxy resins and linseed oil.

The heart of the present invention is the discovery that benzoic acid coated particles of ground limestone lower initial viscosity of resin-filler mixtures and delay development of such high viscosity that the mixture can no longer be worked. Lower initial viscosity at equal proportion of filler represents lower binder demand.

Delay in developing too high viscosity is called increased pot life. The two effects are related in that a lower initial viscosity normally results in a long pot life because of the longer time needed for increased resin polymerization to produce the greater viscosity change from a lower starting point to the same unworkable end point viscosity. In the present invention, the initial rate of viscosity development is also reduced.

Therefore, pot life is extended even beyond that expected from lowered initial viscosity.

Coating of the selected particle size of ground limestone can be effected in any of a variety of known ways.

However, the preferred technique entails dry blending benzoic acid with the ground limestone by intense rotary impact agitation involving, for example, agitator peripheral speeds of from 20 to 60 mis. During this type of intense blending friction raises the batch temperature. Generally, blending in this fashion for 10 to 20 minutes is sufficient and results in a filler temperature of at least 1 20"C unless the blender is specifically cooled.

Afield in which this invention is especially applicable involves formulating fiberglass reinforced sheet molding compounds (SMC) and bulk molding compounds (BMC). Atypical SMCformuiation would involve: Ingredients Parts low shrink polyester resin 100 t-butyl perbenzoate (catalyst 1 magnesium hydroxide (maturation control additive) 4 zinc stearate (mold release agent) 5 benzoic acid coated ground limestone 150 (filler) The foregoing admixture of ingredients (72% by weight) is sheeted with chopped fiberglass (28% by weight).

The formed sheet is compacted between polyethylene sheets by rollers collected and matured at about 30"C until the viscosity increases to above about 20 x 106 cp. This normally takes from one to two days. The matured sheet can then be compression molded at about 1500C into its final form.

The present invention is illustrated by the following examples: Example I Six samples of ground limestone were coated with different proportions of benzoic acid by the preferred method of intense dry blending. A seventh sample was treated identically, but without benzoic acid (to be a control). Each sample was made as follows, only the quantity of benzoic acid having been varied for each different sample.

A 1,000 g quantity of ground limestone with average spherical diameter of 5.5 Fm (Pfizer Inc.'s Vicron 25-11) was charged to a model 2 JSS Prodex-Henschel high intensity mixer. The desired proportion of benzoic acid was added, e.g. two grams, to make a 0.2 % benzoic acid coated product. Then the charge was agitated for 10 minutes at 3800 rpm (blade tip speed 42 m/s).

Portions of each sample were used to make a representative SMC resin-filler mixtures. A total of 240 g of Rohm & Haas Paraplex P-19D resin was weighed into a 600 ml beaker. This resin is a mixture of isophthalic acid based polyester and a thermoplastic polymer (as disclosed in U.S. Patent 3,701,748) in styrene solution.

Then 360 g of a coated limestone filler was added. This proportion of resin plus filler is designated 60% filler loading or 150 phr. This mixture was immediately agitated until completely dispersed. Agitation caused the temperature to increase to 85 F., at which temperature the viscosities were measured.

A brookfield model HBT viscometer fitted with a number 6 spindle was used to measure the viscosity at 5 rpm. In taking a measurement, the spindle was first immersed in the sample up to the spindle notch and then attached to the viscometer. The viscometer was turned on at 5 rpm and a reading taken on the second revolution. The viscosity in centipoise equals the reading multiplied by 16,000. The results are tabulated below.

Weight % benzoic acid on Initial viscosity limestone: dry mixed 10 min. (cps) 0 28,800 0.10 20,800 0.20 16,000 0.25 19,200 0.30 20,800 0.35 24,000 0.50 25,600 These data show that a significant viscosity lowering can be achieved with benzoic acid addition as low as 0.10%. Economic considerations indicate little incentive to add more than 0.5% benzoic acid.

Example Il Example I shows a maximum effect of coating at about 0.2% benzoic acid. This level of coating was repeated, but using a solution coating method instead of the dry method of Example I. A charge of 1500 g of ground limestone (Vicron 25-11) was placed into a Patterson-kelley V-cone blender Model LLB-7754 with intensifier bar rotating at 1,725 rpm. A solution of 3.0 g benzoic acid in 8.0 ml ethanol was added. This mixture was blended for 6 hours to thoroughly distribute the acid and evaporate the ethanol. The result was a filler of limestone particles coated with 0.20% of benzoic acid. A sample of the coated filler and the uncoated limestone were added to resin as in Example I and initial viscosity determined.The results were: Wt. % benzoic acid Initial viscosity by solution coating (cps) 0 50,000 0.20 38,200 Example III The presence of benzoic acid results in extended pot life of resin-filler mixtures as is shown in this example. Representative SMC resin-filler mixtures were made up as follows. A charge of 240 g of the Paraplex P-19D resin of Example I was placed in a 600 ml beaker. While stirring, 360 g of limestone filler, 12 g zinc stearate, and 2. 4 g t-butyl perbenzoate were added. The temperature of the mixture was brought up to about 30"C by the effect of mixing and 9.6 g of magnesium hydroxide, maturation thickening agent, was added. Stirring was continued and the temperature increased to about 32"C. The mixture was poured into a pint can, and the viscosity measured after different time intervals by the procedure of Example I.

Four different limestone fillers were tested, a ground limestone of average spherical diameter 3.5 Fm (H. T.

Campbell & Sons Co's Camel-Wite), plus blends of Camel-Wite with a 0.50% benzoic acid coated ground limestone (Vicron 25-11) made by the process described in Example I. the results were: Total Wt. % benzoic Viscosity (cps) After time of Filler acid 0 min 5 min 30 min 60 min 100% Camel-Wite, 0% coated vicron (control test) 0 30,400 30,400 52,800 73,600 75% Camle-Wite, 25% coated vicron 0.125 27,200 17.200 33.600 51,200 50% Camel-Wite, 50% coated vicron 0.250 28,800 27,200 28,800 40,000 25% Camel-Wite, 75% coated vicron 0.375 25,600 27,200 27,200 40,000 Example IV Benzoic acid was found to lower initial viscosity in SMCformulation when applied with ground calcitic limestone and dolomitic limestone. Samples of these fillers were dry coated by the method of Example I.

They were then formulated by the method of Example I and initial viscosity measured. The control samples with no coating were agitated in the Henschel mixer for the same time (10 minutes) as were the coated samples. The results were: Filler Wt. % benzoic Initial Av. particle Surface acid as coating viscosity Composition size (tom) area (m2/g) on filler (cps) calcitic limestone 5.5 2.8 0 28,800 (vicron 25-11) 0.20 16,000 calcitic limestone 8.0 1.6 0 25,600 (vicron 31-6) 0.11 16,000 calcitic limestone 7.0 2.2 0 19,200 (vicron 41-8) 0.16 11,200 dolomitic limestone 10.00 1.2 0 28,800 (dolocron 32-15) 0.085 20,800 Note: Vicron and Dolocron are trademarks used by Pfizer Inc. for special grades of ground calcitic limestone and ground dolomitic limestone respectively.

Example V A sample of ground limestone filler (Vicron 25-11) was coated with 0.2% by weight of benzoic acid by the method of Example I, except the intense agitation was continued for 15 min. and the batch temperature rise was not controlled. Final coated product temperature was 145 C. The uncoated control sample rose to 1900C during the same 15 min. agitation. Portions of these two fillers were used to make resin-filler mixtures with two different resins, by the procedure of Example I (which used a styrenated polyester resin). Inititial viscosities were measured with the following results.

Filler Loading Initial viscosity** Resin Filler % phr (cps) Linseed oil uncoated 67 200 7360 0.2% benzoic 67 200 3250 Epoxy* uncoated 60 150 8320 0.2% benzoic 60 150 6080 * General Mills' Genepoxy Ml80, a modified liquid epoxy resin of epoxide equivalent 178-186, and containing 12% reactive diluent.

** Viscosity measured with a number 4 spindle.

Claims (10)

1. A filler comprising ground limestone coated with 0.05 to 0.50% by weight of benzoic acid, the ground limestone having an average diameter of from 0.25 m to 44 m.

2. A filler as claimed in claim 1 wherein the ground limestone is calcitic limestone.

3. Afiller as claimed in claim 1 wherein the ground limestone is dolomitic limestone.

4. A composition comprising a coated limestone filler as claimed in any one of claims 1 to 3 in admixture with a polymerizable resin.

5. A composition as claimed in claim 4 wherein the resin is a styrenated polyester resin.

6. Acomposition as claimed in claim 4 wherein the resin is an epoxy resin.

7. A composition as claimed in claim 4 wherein the resin is linseed oil.

8. A method of producing a coated ground limestone filler which comprises coating particles of ground limestone having an average diameter of from 0.25 cm to 44 Fm with benzoic acid in an amount of from 0.05 to 0.50% weight %.

9. A method as claimed in claim 8 wherein said particles are dry blend coated by intense rotary impact agitation with the benzoic acid.

10. A method as claimed in claim 8 substantially as hereinbefore described in any one of Examples I to V.