Barium sulfate improves stiffness and acoustic property while having relatively little effect on surface finish.

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From: Polypropylene, 1998

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Y. Nonomura, in Cosmetic Science and Technology, 2017

14.2.1.1.5 Barium Sulfate

Barium sulfate, BaSO4, is made by reacting barium hydroxide and other barium sources with sulfuric acid and has a long history as a translucent white pigment. Barium sulfate can be formed in various shapes such as planar, starred, or spherical structures depending on the formulation condition, especially the supersaturation of its barium source, and the internal pore size also changes.18 Planar barium sulfate shows especially high lubricity, and when applied to the skin it not only has a smooth feeling but also has a high light-scattering property, showing a soft-focus effect that makes small wrinkles and pores less visible.19


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Group 16 (O, S, Se, Te) Alkaline Earth Compounds


R.C. Ropp, in Encyclopedia of the Alkaline Earth Compounds, 2013

Barium Sulfate


Barium sulfate has the molecular formula of BaSO4 and the molecular weight of 233.3896 g/mol. It can be prepared by the reaction of barium carbonate and sulfuric acid:


BaCO3 + H2SO4 ⇒ BaSO4 + CO2 + H2O

Barium sulfate is a soft crystalline solid. It is a rhombic crystal. The pure salt is white but the color of the mineral “barite” can vary between red, yellow, gray or green, depending on impurities. Its density is 4.50 g/cm3 and its refractive index is 1.64. It melts around 1580 °C but decomposes above 1600 °C. Its hardness is 4.3 to 4.6 Mohs. It is virtually insoluble in water (285 mg/l at 30 °C) and insoluble in alcohol. Its Ksp is 1.1 × 10–10. It is soluble in concentrated sulfuric acid. The crystal structure of BaSO4 is known to be rhombic, with a space group pnma. The lattice parameters are: a = 8.896 Å, b = 5.462°, c = 7.171 Å, V = 348.4 Å3. Its three-dimensional structure is shown in Fig. 3.33.



FIGURE 3.33.


Strontium sulfate has an identical structure. Thernodynamic constants are given by the following table. These values are for 293 K (Table 3.14).


TABLE 3.14.


Thermochemical properties
Δ = –352.3 kcal/mol
ΔGf = –325.7 kcal/mol
S0 = 31.6 cal/degree mol
Cp = 24.3 cal/degree mol
ΔHfusion = 9.71 kcal/mol

Natural barium sulfate is widely distributed in nature and occurs as the mineral “barite” (also known as barytes or heavy spar). It often associated with other metallic ores, such as fluorspar. Barites containing over 94% BaSO4 can be processed economically. It also contains silica, ferric oxide and fluoride impurities. Silica is the prime impurity that can be removed as sodium fluorosilicate by treatment with hydrofluoric acid followed by caustic soda. Very pure barium sulfate may be obtained by treating an aqueous solution of a soluble barium salt with sodium sulfate:


BaCl2 + Na2SO4 ⇒ BaSO4 + 2NaCl

Barium sulfate is one of the most insoluble salts of the alkaline earths. It does not undergo double decomposition reactions in aqueous phase like its Mg homologue. It dissolves in concentrated H2SO4 to form an acid sulfate that breaks down to BaSO4 upon dilution. Reduction with coke under heating produces barium sulfide:


BaSO4 + 3C ⇒ BaS + 2CO + CO2

The accidental discovery of this conversion many centuries ago led to the discovery of the first luminescent material in 1803. The sulfide, unlike the sulfate, is water soluble. Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in a series of compounds like barium sulfate, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for the existence of “atoms”.

BaSO4 reacts violently when heated with aluminum or explosively when mixed with potassium. Sulfamic acid, HSO3NH2 is a moderately strong acid. Water solutions are unstable and slowly hydrolyze to NH4HSO4. It has been used to produce nanosized barium sulfate particles.

Barium sulfate has many commercial applications. It is used either as natural barite, or precipitated BaSO4. The precipitated salt in combination with equimolar amount of co-precipitated zinc sulfide formerly was used as a white protective coating pigment, known as “lithopone”. Similarly, in combination with sodium sulfide, it is used to produce fine pigment particles of uniform size, known as “blanc fixe”. Natural barite, however, has greater commercial application than the precipitated salt. It is used as an additive in drilling mud in crude oil, well drilled to lubricate and cool the drilling bit, and to plaster the walls of the drill hole to prevent caving. It is used as a filler in automotive paints, plastics and rubber products. It also is used as a filler in polyurethane foam floor mats, white sidewall rubber tires and as a flux and additive to glass to increase the refractive index.

Barium sulfate is frequently used clinically as a contrast agent for X-ray imaging and other diagnostic procedures. It is most often used in imaging of the gastrointestinal tract. It is administered, orally or by enema, as a suspension of fine particles in an aqueous solution. Although barium, and its water-soluble compounds are often highly toxic, the extremely low solubility of barium sulfate protects the patient from absorbing harmful amounts of the metal. Barium sulfate is also readily removed from the body, unlike prior compounds, which it replaced. Its absorbance of X-rays is also higher.

Barium sulfate mixtures are used as white pigment for paints. In oil paint, barium sulfate is almost transparent, and is used as a filler or to modify consistency. One major manufacturer of artists’ oil paint sells “permanent white” that contains a mixture of titanium white pigment and barium sulfate. Barium sulfate itself is called blanc fixe (French for “permanent white”).

A scientific-grade, barium sulfate-based paint is offered for sale that exhibits near-perfect diffuse reflectance at levels up to 98% in the UV-VIS-NIR wavelength range. It is applied by spray painting to almost any substrate (metals, plastics, glass) for use in integrating spheres, laser cavities, lamp reflectors and display backlights. It is characterized by a near-perfect Lambertian (i.e. diffuse) reflectance of up to 98% in the spectral range from 250 to 2500 nm.

Other chemical applications of barium sulfate are used as a pigment for photographic paper. It is also used to prepare many other barium salts. It is available in many forms commercially.



The effect on the level of X-ray absorption (as evidenced by increasing amounts of observable image contrast) of adding greater amounts of barium sulfate to PEEK can be seen in a standard configuration shown in Fig. 3.9. Here the PEEK compounds containing 4%, 6%, and 20% by weight of barium sulfate powder have been molded into rectangular pieces at different thicknesses ranging from 2 to 10 mm in 2 mm increments and assembled in the order shown. Next to these is a machined aluminum “stepped wedge” of the same dimensions by way of comparison. It can be seen that the compound containing 20% barium sulfate matches very closely the radiopacity of the metal at like-for-like thicknesses. By selecting compounds containing 4%, 6%, or 20% barium sulfate, the implant developer can, therefore, tailor the amount of radiopacity of the device to achieve an optimized level of contrast. The radiographs shown in Fig. 3.10 illustrate this for PEEK-OPTIMA polymer spinal fusion cages that have been made with no additive and 6% and 20% image contrast additive, respectively, compared with a metallic cage.




Substantially, as a consequence of their shape, powders do not enhance the tensile strength of polymer materials compared with fiber reinforcements, although there are physical changes to the base polymer that occur as a result of their addition. Table 3.2 compares the mechanical properties of barium sulfate-filled PEEK with unfilled PEEK. It can be seen that increasing the amount of filler actually reduces the tensile strength of the material from 100 through 95 MPa, to 90 MPa with the highest filler loading. This strength reduction is offset by the beneficial gain in X-ray contrast, which, as has been illustrated, increases with increasing amounts of additive.


PropertyTest MethodUnitsPEEK-OPTIMA UnfilledPEEK-OPTIMA Image Contrast Grade (Low Radiopacity)PEEK-OPTIMA Image Contrast Grade (High Radiopacity)
Tensile strengthISO 527MPa1009590
Tensile elongationISO 527%202015
Flexural modulusISO 178GPa43.84.5
Flexural strengthISO 178MPa170150150
Notched Izod impactISO 180kJ/m27.678
Specific gravityISO 1183g/cc1.31.361.49

As an alternative method to adding radiopaque powder, metallic wires (markers) may also be added to PEEK (particularly to PEEK composites) to make them visible radiographically. This will be described in more detail in Section 3.5.2.2, although it is, perhaps, appropriate to show a radiograph in this section of this form of PEEK composite material. Figure 3.11 compares the image contrast achieved with CFR PEEK (containing wire markers) with that for titanium and CFR PEEK (with no markers) under X-ray inspection.



A.G. Abel, in Paint and Surface Coatings (Second Edition), 1999

3.12.11.4 Barium sulphate (natural — barytes; synthetic — blanc fixe)

Colour Index — Barytes, CI Pigment White 22; blanc fixe, CI Pigment White 21.

Formula — BaSO4.

Barium sulphate is very inert, insoluble and stable to light and heat. The natural form is obtained as the mineral ‘heavy spar’. After being crushed, washed and dried it is usually micronized, reducing its particle size from 25 µm to 2–10 µm, thus aiding its dispersibility. The synthetic version is made by reacting available barium compounds with sulphuric acid or soluble sulphate salts, and has a finer texture than the natural grades, giving it a higher oil absorption.

Its refractive index (1.64) is higher than other extenders, which gives it some pigmentary properties. Its high density is also useful for paints sold by weight. It is used in primers, undercoats, and industrial finishes, where it hardens the film. Its high density leads to it having a tendency to settle. In spite of it being a barium salt, its insolubility ensures it is non-toxic.


Clive Maier, Teresa Calafut, in Polypropylene, 1998

4.3 Barite

Barium sulfate (Figure 4.3), commonly called barite, is found in the hydrothermal veins of cavity fissures in limestones, sandstone, shales, or clays or as surface deposits resulting from limestone weathering. Barite brightness depends on origin; brown buff barite (brightness 80–85) is found in Nevada, Missouri, Georgia, Illinois, and Mexico, while pigment grade white barite (brightness 92–94) is obtained almost exclusively from China. <942>

Barites are the most chemically resistant of the commonly used minerals in polypropylene (calcium carbonate, talc, mica, and barite; Table 4.1), with excellent resistance to acids, alkali, and all known organic solvents. Due to a 60% higher specific gravity, loadings are low compared to other mineral fillers, and the effect on physical properties of the filled resin is not as pronounced. Barite has a high refractive index than other minerals. It is used in heavier parts or for applications that require sound deadening or corrosion resistance. <942>


Andreas Höpe, in Experimental Methods in the Physical Sciences, 2014

6.6.2.2 BaSO4-Based Standards


John Murphy, in Additives for Plastics Handbook (Second Edition), 2001

7.1.3.2 Barium sulphate (‘blanc fixe’)

Precipitated barium sulphate (‘blanc fixe’) is an inert white filler, resistant to acid and alkalis, and has very good weathering resistance. It does not absorb light from the ultraviolet to the infra-red range and so does not impair the brilliance of colour pigments. Particle sizes range from 0.7 to 3.0 μm. Dispersability and lack of grit are high: hardness and stiffness of plastics are improved without effect on surface quality (especially gloss and colour brilliance). It is also used to increase density and X-ray opacity, especially for toys and medical articles, and improves sound insulation values. Special grades increase light scattering without absorption in semi-opaque compounds such as lampshades, PC and PMMA sheets, and PVC film. Ultrafine particle grades (less than 0.2 μm) have been developed as nucleating agents for partially crystallized thermoplastics.

Natural barium sulphates (barytes) are inert and allow very high loadings: fine-particle grades are preferred to increase the density of a plastics compound, while coarse particles are better for acoustic applications, especially in automobiles.

Blanc fixe micro is a white inorganic powder for plastics and coatings, comprising barium and sulphate. It is practically insoluble in water, organic solvents, and acids/alkalis. It is produced from barytes, with removal of impurities, achieving a narrowly defined particle size distribution. Titanium dioxide production technology is used for finishing. Its particles are almost as fine as those of titanium dioxide pigments (barytes, 4 μm; synthetic barium sulphate, 3 μm; blanc fixe micro, 0.7 μm; titanium dioxide, 0.3 μm).

In use it is notable for low binder replacement, ready dispersability, extreme fineness, low agglomerate content, and (in coatings) high gloss. It can also act as a ‘spacer’ between white or coloured pigments, potentially reducing titanium dioxide by 5–15%, or reducing pigment costs, or raising solids content. Cost can be reduced by about 5% without detriment to properties.


W. Engewald, J. Pörschmann, in Journal of Chromatography Library, 1991

5.7.2 Modification With Non-porous Ionic Adsorbents

Disregarding barium sulphate, boron nitride and molybdeneum disulphide, non-porous inorganic adsorbents are rarely used in the pure form as column packings. Their specific surface area is low, and so is their capacity. In order to increase the surface area, they are coated on silica gel, aluminium oxide or inert solid supports. Regarding the chromatographic superiorities of (pure!) silica as type II adsorbents, we shall deal only with the modification of this material. Particularly alkali and alkaline-earth metal halides and salts of the transition metals have been applied as modifiers. They are coated on the adsorbent as follows. An aqueous solution of the salt is mixed with the silica gel whilst heating until the water has evaporated. In order to obtain a homogeneous coverage, the mixture is heated to the melting point of the salt. It can be assumed that a chemical reaction proceeds between the surface silanol groups and the metal chlorides, forming a qualitatively new surface <561>, which is more homogeneous than the surface of the salts themselves.

Investigations by Scott <562> and Vidal-Madjar and Guiochon <563> in this field indicated that the modified adsorbents can interact specifically and charge-transfer interactions with π-electron systems take place. Varying the cations and anions allows considerable adjustment of the selectivity <564>. Adsorbents modified with BaCl2 (which has both Ba2+ and Cl− ions on all faces and on the surface) or with CoCl2 or NiCl2 (which exhibit a crystalline layer structure and faces and surfaces with mainly Cl− ions) actually show differences in the specificity of the intermolecular interaction, but by modification with these three salts a more homogeneous surface is created, consisting preferentially of Cl∼ ions, independent of the previous crystalline structure <561>. Owing to the greater specificity of BaCl2, the separation of butadiene from the isomers of butane and butene can be achieved on BaCl2-modified Silochrom at 50°C, whereas CoCl2-modified Silochroms require a column temperature below room temperature <565>.


SaltAdsorbentApplicationReferences
LiCl, NaClSilica gelUnsaturated hydrocarbons, Organohalides aromatics<566>
KCl, CsCl, Na2SO4, LaCl3Silica gel, graphitized carbon blackUnsubstituted and haloge-nated aromatics<567, 568, 569>
NaCl, LaCl3, Na2 MoO4Silica gel, aluminium oxideInert gases<570>
NiCl2,CoCl2, BaCl2Silica gelIsomers of different families<565>
MgCl2, CoCl2, ZnCl2Silica gelAromatics<571>
LiF, NaF, KF, CsFAluminium oxideUnsaturated/saturated compounds<572>
Na3PO4Silica gelC1–C5 hydrocarbons within 2 min<568>

Complex compounds have also been utilized for modifying the surface. For example, (Ag pyridine2)NO3 interacts specifically with olefins <573>, and silica gels modified with tetram-minecopper(II) sulphate, copper(II)bisethylenediamine sulphate or copper(II)bistriethanol-amine sulphate are suitable for the separation of low-molecular-weight aliphatics and aromatics <574>. Metal complexes of disubstituted organophosphorus acids have been used in the separation of compounds with π-electrons or heteroatoms <575>. Finally, it should be pointed that metal complexes, similarly to salts and other modifiers, coated on an adsorbent do not remove any heterogeneity completely but decrease the specific adsorption energy and increase the specificity even at small layer thicknesses.


G. Crapper, in Polymer Science: A Comprehensive Reference, 2012

10.29.4.3.5 Barium sulfate

Barium sulfate is extracted from mined barites, and undergoes grinding and classification to yield particles sizes suitable for use in coatings.

As mentioned previously, barium sulfate has a higher specific gravity (ρ = 4.5 g cm−3), and hence leads to higher specific gravity coatings, or lower coverage coatings (i.e., the coating covers a lower area for a given weight). The paint formulator should, therefore, only consider barium sulfate when other possible fillers are not suitable, for example, due to the poor acid resistance of calcium carbonate fillers.

Barium is a heavy metal and is toxic, and as such could be implicated in evaluations of powder coatings for suitability in end uses where it may come into contact with people and animals. However, the low solubility of barium sulfate means that the material is not biologically available. Indeed, barium sulfate is used in medicine for ingestion to increase contrast in X-ray applications. Such arguments lead the industry to consider barium sulfate as a nonhazardous filler.


1.

Formation of barium sulfate turbidity:i.Place a 100-mL sample (or a suitable portion diluted to 100 mL) into a 250-mL Erlenmeyer flask.

ii.

Add exactly 5.0 mL conditioning reagent (See section:5.7.5).

iii.

Mix in the stirring apparatus.

iv.

While the solution is being stirred, add a measured spoonful of BaCl2 crystals (See point 3 in section: 5.7.5) and begin timing immediately.

v.

Stir exactly 1.0 min at constant speed.

2.

Measurement of barium sulfate turbidity:2.1Immediately after the stirring period has ended, pour solution into absorbance cell.

2.2

Measure turbidity at 30-sec intervals for 4 min.

2.3

Record the maximum reading obtained in the 4-min period.

3.

Preparation of calibration curve:3.1Prepare calibration curve using standard sulfate solution (see point 6 of “Reagent” section).

3.2

Maintain increment of standards at 5-mg/L in the 0–40 mg/L sulfate range (Above 50 mg/L the accuracy decreases and the suspensions lose stability).

3.3

Check reliability of calibration curve by running a standard with every three or four samples.

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4.

Calculations: Estimate amount of SO42− from linear calibration curve, using the following relation: