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additional toxicological information
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Acceptable well documented publication which meets basic scientific principles Read-across is justified on the following basis: The family of zinc borates that include Zinc Borate 500, Zinc Borate 2335 and Zinc Borate 415 (also known as Zinc Borate 411). Zinc borate 500 is anhydrous Zinc Borate 2335 and Zinc Borate 415 has different zinc to boron ratio. Zinc borate 2335 (in common with other zinc borates such as Zinc borate 415 and 500) breaks down to Zinc Hydroxide (via Zinc oxide) and Boric Acid, therefore the family of zinc borates shares the same toxicological properties. Zinc borates are sparingly soluble salts. Hydrolysis under high dilution conditions leads to zinc hydroxide via zinc oxide and boric acid formation. Zinc hydroxide and zinc oxide solubility is low under neutral and basic conditions. This leads to a situation where zinc borate hydrolyses to zinc hydroxide, zinc oxide and boric acid at neutral pH quicker than it solubilises. Therefore, it can be assumed that at physiological conditions and neutral and lower pH zinc borate will be hydrolysed to boric acid, zinc oxide and zinc hydroxide. Hydrolysis and the rate of hydrolysis depend on the initial loading and time. At a loading of 5% (5g/100ml) zinc borate hydrolysis equilibrium may take 1-2 months, while at 1 g/l hydrolysis is complete after 5 days. At 50 mg/l hydrolysis and solubility is complete (Schubert et al., 2003). At pH 4 hydrolysis is complete. Zinc Borate 2335 breaks down as follows: 2ZnO • 3B2O3 •3.5H2O + 3.5H2O + 4H+ ↔ 6H3BO3 + 2Zn2+ 2Zn2+ + 4OH- ↔ 2Zn(OH)2 ____________________________________________________________ Overall equation 2ZnO • 3B2O3 •3.5H2O + 7.5H2O ↔ 2Zn(OH)2 + 6H3BO3 The relative zinc oxide and boric oxide % are as follows: Zinc borate 2335:zinc oxide = 37.45% (30.09% Zn) B2O3 = 48.05% (14.94% B) Water 14.5% Zinc borate 415: zinc oxide = 78.79%; (63.31% Zn) B2O3 = 16.85% (5.23% B) Water 4.36% Zinc borate, anhydrous: Zinc oxide = 45 % B2O3= 55% (17.1 % B)

Data source

Reference
Reference Type:
publication
Title:
Structural Characterization and Chemistry of the Industrially Important Zinc Borate, Zn[Bs04(OH)3]
Author:
Schubert, D.M., Alam, F., Visi, M.Z. and Knobler, C.B.
Year:
2003
Bibliographic source:
Chem. Mater. 2003, 15, 866-871

Materials and methods

Type of study / information:
The structure of Zn[B3CXi(OH)3] was determined for the first time by single-crystal X-ray diffraction
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
The structure of Zn[B3CXi(OH)3] was determined for the first time by single-crystal X-ray diffraction
GLP compliance:
not specified

Test material

Constituent 1
Reference substance name:
Zinc borate
IUPAC Name:
Zinc borate
Test material form:
solid: crystalline
Details on test material:
- Name of test material (as cited in study report): Zn[B3CXI(OH)3]

Results and discussion

Any other information on results incl. tables

Description of Structure.

Zn[B3CXI(OH)3] is a complex inoborate (containing a borate structural unit that is an infinite chain) composed of linked triborate moieties, interconnected by coordination with pseudo-tetrahedral zinc centers and a network of H-bonds. Interatomic distances and angles for Zn[B3CXI(OH)3] are given in Table 2. The asymmetric unit in Zn[B3CXI(OH)3] includes one zinc atom and one triborate moiety. There are three unique boron sites, present as one groups share oxygen vertexes to form a B3O3 boroxyl ring. One BO4 group has one attached OH group and the other has two, producing the B3O4(OH)3 subunit. These triborate moieties link into infinite chains parallel to [101] by sharing an exocyclic oxygen (O4/O4A) between BO3 and BO4 polyhedra. The BO2-(OH)2 group is not involved in chain extension. Each zinc atom is pseudotetrahedrally coordinated by oxygens of three different polyborate chains. Two zinc coordination positions are satisfied by one endocyclic boroxyl oxygen (O1) and one hydroxyl oxygen (O7) that is attached to the adjacent triborate moiety in the same chain, producing the six-membered ZnB2O3 rings.

The boroxyl oxygen involved in zinc coordination lies between the BO3 and BO4 polyhedra involved in chain extension. The remaining two zinc coordination sites are occupied by hydroxyl oxygens of BO2(OH)2 groups in two separate polyborate chains. In this way, all hydroxyl groups are involved in zinc coordination.

Zn[B3CXI(OH)3] exhibits a complex higher order structure. Not only does each zinc center link three separate polyborate chains, but also these chains are interconnected by H-bonds. All three OH hydrogens are involved in H-bonding with oxygen atoms of adjacent chains. Chains are connected via O5-H5—O3, O6-H6---O4, and O7-H7--O2 hydrogen bonds. Notably, all three H-bonds involve B—O—B acceptor oxygen atoms and not B—O—H oxygens, which may explain the relatively high dehydration onset temperature of Zn[B3CXI(OH)3]. Details of H-bonds are given in Table 3. The three hydrogens atoms in Zn[B3CXI(OH)3] are observed in the 'H MAS NMR spectrum. Owing to their distinct differences in H-bonding strengths, the three proton sites yield clearly resolved resonances at 4.6, 7.0, and 8.6 ppm. There are regular 4-fold centrosymmetric connections between alternating complementary pairs of triborate moieties in neighboring chains. These interactions involve two zinc centers one in each chain, that link adjacent, triborate rings via 2-fold O6- Zn-O1 coordination (coroner sharing of zinc tetrahedu and borate polyhedra). The same triborate moieties are also linked by pairs of O7-H7"-O2 H-bonds. Zinc atoms lie very roughly in layers positioned between repeating pairs of polyborate chains.

Borate structures can be described in terms of compact, insular groups, referred to as fundamental building blocks (FBBs), forming the basis of classification schemes for crystalline borate compounds. These schemes define FBBs according to the number of boron atoms, the number of trigonal BO3 and tetrahedral BO4 groups, and the mode of polymerization between the FBBs, to give isolated, modified isolated, chain, modified chain, sheet, modified sheet, and three-dimensional network structures. Using the classification scheme devised by Christ and Clark, the borate structural unit of Zn[B3CXI(OH)3] is described as ∞:3( A + 2T), indicating a three-boron FBB containing one trigonal and two tetrahedral boron centers.Using the more recent classification scheme proposed by Burns and Hawthorne, the structural unit in Zn[B3CXi(OH)3] is described as 1∆2□:< ∆2□>, where and refer to BO3 and BO4 polyhedra, respectively.

Table 2. Bond Lengths (Â) and Bond Angles (deg) for Zn[B3CXI(OH)3]
Zn(l)-0(5) 1.919(2) 0(5)-Zn(l)-0(6) 105.91(9)
Zn(l)-0(6) 1.9473(19) 0(5)-Zn(l)-0(7) 110.00(9)
Zn(l)-0(7) 1.965(2) 0(6)-Zn(l)-0(7) 117.66(8)
Znd)-O(l) 1.9663(17) 0(5)-Zn(l)-0(l) 119.48(8)
0(1)-B(1) 1.386(4) 0(6)-Zn(l)-0(l) 108.04(8)
0(1)-B(3) 1.526(3) 0(7)-Zn(l)-0(l) 96.24(8)
0(2)-B(l) 1.361(3) B(l)-0(1)-B(3) 120.6(2)
0(2)-B(2) 1.474(3) B(l)-0(1)-Zn(l) 119.39(16)
0(3)-B(3) 1.430(3) B(3)-0(1)-Zn(l) 118.42(15)
0(3)-B(2) 1.440(3) B(l)-0(2)-B(2) 123.8(2)
0(4)-B(l)l#l] 1.363(3) B(3)-0(3)-B(2) 120.3(2)
0(4)-B(3) 1.465(3) B(l)[#l]-0(4)-B(3) 134.8(2)
0(5)-B(2)[#2] 1.484(3) B(2)t#2]-0(5)-Zn(l) 116.02(16)
0(6)-B(2)[#3| 1.482(3) B(2)(#3)-0(6)-Zn(l) 130.81(16)
0(7)-B(3)[#4] 1.477(3) B(3)[#4]-0(7)-Zn(l) 126.62(16)
B(3)-0(7)f#l] 1.477(3) 0(3)-B(3)-0(4) 114.3(2)
B(l)-0(4)[#4] 1.363(3) 0(3)-B(3)-0(7)[#l] 112.4(2)
B(2)-0(6)[#3] 1.482(3) 0(4)-B(3)-0(7)[#l] 108.2(2)
B(2)-0(5)[#5] 1.484(3) 0(3)-B(3)-0(l) 110.6(2)
0(4)-B(3)-0(l) 104.5(2)
0(7)[#1]-B(3)-0(1) 106.2(2)
0(2)-B(l)-0(4)[#4] 117.9(2)
0(2)-B(l)-0(l) 119.5(2)
0(4)[#4]-B(l)-0(l) 122.6(2)
0(3)-B(2)-0(2) 112.9(2)
0(3)-B(2)-0(6)[#31 108.6(2)
0(2)-B(2)-0(6)[#3| 109.3(2)
0(3)-B(2)-0(5)[#5| 114.2(2)
0(2)-B(2)-0(5)[#5| 103.7(2)
0(6)[#3]-B(2)-0(5)[#5] 107.9(2)
" Symmetry transformations used to generate equivalent atoms:      x+ V2,-y +3/2, 2+ V2; [#2]x+ V2,-y +3/2, z- V2; [#3]-x, -y +2,-z +2; [#4Jx -V2, -y +3/2,z -V2; [#5]-x -V2, -y +3/2,2+ V2.

.

Table 3. Details of Hydrogen Bonds for ZN[B2CXI(OH)3]
D-H-A d(D-H) (À) d(H-A) (Â) d(D-A) (Ä) Z(DHA) (deg)
0(5)-H(5)-0(3)[#6] 0.61(3) 2.09(3) 2.705(3) 175(4)
0(6)-H(6)-0(4)[#21 0.74(3) 2.03(3) 2.766(3) 174(3)
0(7)-H(7)-0(2)|.#7] 0.69(3) 2.39(3) 3.073(3) 170(3)
" D and A signify donor and acceptor oxygens, respectively. Symmetry transformations used to generate equivalent atoms: [#2]x +V2,-y +    z -V2; [#6]-x+ V2,y -V2,-z+rV2; [#7]-x- V2,y- V2,-z + %.
Comparison with Mineral Structures.

The structure of Zn[B3CXI(OH)3] bears similarities to some borate minerals, notably the industrially important colemanite, Ca[B3O4-(OH)3]-H2O, and the lesser known studenitsite, Ca[B304-(OH)3]. The industrial mineral hydroboracite, CaMg-[B3O4(OH)3]2-3H2O, is also related. These minerals also contain infinite polytriborate chains of the ( ∆2□) type that are cross-linked by coordination with metal cations. However, significant differences exist between the structures of these minerals and Zn[B3CXI(OH)3] resulting from the different coordination demands of their metal cations.

In colemanite, calcium atoms link together by sharing oxygen to form chains running parallel to the polytriborate chains (parallel to [100]). These calcium-containing chains share oxygen of the polytriborate chains and thereby interconnect them into infinite sheets. These sheets are connected by H-bonding and a relatively small number of Ca—O bonds, resulting in the perfect [101] cleavage characteristic of colemanite. In contrast, the tetrahedral zinc centers in Zn[B3CXI(OH)3] do not form chains by sharing oxygen between zinc and instead interconnect the polyborate chains into a three-dimensional network rather than sheets. Consequently, Zn[BeCXI(OH)3] is a much less friable material than colemanite.

The closest mineral analogue to Zn[B3CXI(OH)3] in composition is studenitsite, Ca[B3O4(OH)3]. In contrast, however, this mineral contains corrugated calcium oxide sheets lying between and interconnecting parallel polytriborate chains. Some notable differences between studenitsite and colemanite are the absence of interstitial water and a helical rather than translational extension of borate chains in the former, with B3O4(OH)3 groups within a chain related by a 2-fold screw axis parallel to the fo-axis of the crystal. Although colemanite contains one water in the coordination environment of calcium, this water uses one hydrogen to H-bond to adjacent water with the result that the total number of bonds from the interstitial complex, [Ca(H2O)]2+, to the borate structural unit is the same as that for Ca2+ in studenitsite if each metal center is assigned the same coordination number.

Borate and polyborate structural units have associated basicities that are approximately proportional to the percentage of tetrahedral boron in their FBBs. This is also a function of the solution pH prevailing during borate crystallization since specific borate anions are stable only within a given pH range. Empirical methods based on the analysis of borate mineral structures were developed recently to estimate Lewis basicities of borate structural units. According to the valence matching principle, to have a stable structure, the Lewis basicity of the (anionic) borate structural unit must match closely the Lewis acidity of the (cationic) interstitial complex. Correlations of structural features found in minerals suggest that borate structural units adjust to varying acid—base conditions, within stability ranges, by changing the average coordination number of oxygen atoms (O—CN) in the structural unit, counting hydrogen bonds and bonds to cations. Higher average O—CN values are associated with higher borate basicities. These correlations depend heavily on the assignment of coordination number to cations since this largely defines average O—CN.

Colemanite, studenitsite, and Zn[B3CXI(OH)3] have chemically equivalent borate structural units and thus have the same Lewis basicities. However, Zn2+ has substantially higher Lewis acidity than Ca2+. The structure of colemanite was analyzed by others with the conclusion that it has an average O—CN of 3.6, considering calcium to be 8-coordinated, and examination of the studenitsite structure indicates that it also has an average O—CN of 3.6. The O-CN value of these minerals correlates well with the overall pattern for borate mineral structures. Zinc favors four-coordination and they consider the zinc atom in Zn[B3CXI(OH)3] to be four-coordinate. Each structural unit oxygen in Zn[B3CXI(OH)3] is coordinated once by either an H-bond or zinc, resulting in an O—CN of 3.0. This O-CN value is significantly below the range reported for borate minerals. Only by considering the coordination number of zinc to be higher than 4 can the O—CN be higher.

For metal coordination, some authors have included all oxygens having a metal—oxygen bond valence greater than 0.05 vu with metal—oxygen distances less than the metal—metal distance, with consideration of distance gaps. Application of the distance rules alone can overcount the number of oxygens actually involved in bonding interactions with the metal. In Zn[B3CXI(OH)3], there are 24 oxygen atoms having Zn—O distances shorter than the Zn—Zn distance (4.391 A). The immediate coordination environment around zinc contains four oxygen atoms, at distances from 1.919 to 1.966 A, with geometry close to an idealized tetrahedron. Valence analysis gives a sum of bond valences for zinc of 3.8 vu, counting only these four Zn-O bonds. There is a large gap between the closest four oxygens and the next two, which have Zn-O distances of 2.707 À (O2) and 2.918 A (O7). Beyond these, the next closest oxygen lies at a distance of 3.179 A (O4).

For the borate structural unit in colemanite, studenitsite, and Zn[B3CXI(OH)3], it has been argued that the boroxyl oxygen linking the two tetrahedral boron atoms should receive two additional bonds, either from the metal or H-bonds, to satisfy its valence requirements. In colemanite this oxygen receives two H-bonds and in studenitsite it is bonded to two calcium atoms. However, the corresponding oxygen in Zn[B3CXI(OH)3], O(3), receives only one H-bond and is not within the tetrahedral coordination environment of zinc. The shortest three O(3)—Zn distances are 3.282, 3.771, and 3.885 A. Valence analysis, counting one H-bond and two [4,B—O bonds, gives a sum of valences for O(3) of 1.81 vu, somewhat less than the 1.90 vu calculated for colemanite. The two '4|B—O bonds to these oxygen atoms are both slightly shorter by 0.021-0.022 A in Zn[B3CXI(OH)3] than in colemanite.

The related synthetic monomeric triborate 6, Zn-(H2O)[B3O3(OH)5], is an analogue of the mineral mey-erhofferite, Ca(H2O)[B3O3(OH)5]. Here, the tetra-coordination of zinc also results in a significantly lower average O—CN in the synthetic material compared to that in the mineral. Explanations offered for why synthetic borates may not match the patterns observed for borate minerals refer to the greater range of options available in nature compared with more restricted synthetic systems. Hydrolytic stability under geologic conditions may also be a factor. Clearly, geological conditions, as well as those used to crystallize most anhydrous borates, favor more thermodynamically stable products. Nevertheless, improved understanding of structure—stability relationships is important to the development of synthetic methodologies for borates having useful properties.

Solubility and Hydrolysis.

Details of the hydrolytic behavior of Zn[B3CXI(OH)3], which are important to industrial applications, have not been described previously in the literature. Zn[B3CXI(OH)3] is sparingly soluble in water and exhibits incongruent solubility. Less than 50 mg will dissolve in a liter of water at 20 °C (0.005 wt %). Concentrated aqueous slurries of solid Zn[B3CXI(OH)3] (e.g., 10 wt %) are stable indefinitely, the bulk of the solids persisting as crystalline Zn[B3CXI(OH)3] in contact with its solution. However, very dilute slurries of Zn[B3CXI(OH)3] hydrolyze at room temperature to boric acid and highly insoluble Zn(OH)2. Owing to the thermal instability of Zn(OH)2, hydrolysis at elevated temperatures (boiling water) gives boric acid and ZnO, the reverse of the reaction used for commercial production of Zn[B3CXI(OH)3]. Room-temperature hydrolysis is relatively slow. A stirred 10 wt % slurry of Zn[B3CXI(OH)3] required ~3 weeks to reach equilibrium, ultimately giving a supernatant solution that is saturated in boric acid (800— 850 ppm B) and has 10—15 ppm Zn. This solution has an approximately neutral pH, which is below the pH at which Zn(OH)2 exhibits minimum solubility. The stability of concentrated slurries of Zn[B3CXI(OH)3] is attributable to the high concentration of boric acid in solution resulting from partial hydrolysis. Incongruent solubilities can be expressed in various ways. If solubility is definded as the weight percent of the hypothetical anhydrous components, ZnO and B203, in solution at equilibrium, calculated from observed B and Zn concentrations, then the solubdity of Zn[B3CXI(OH)3] is »0.28 wt %. However, at very low slurry concentrations, the undissolved solids resulting from hydrolysis of Zn[B3CXI(OH)3] are primarily Zn(OH)2.

Under appropriate conditions, Zn[B3CXI(OH)3] can be converted to other crystalline hydrated zinc borates.

Thermochemistry.

The thermal behavior of Zn[B3CXI(OH)3] is important to industrial applications, particularly, uses in fire retardancy and ceramics manufacture. Dehydration of Zn[B3CXI(OH)3] commences upon heating above 290 °C, observed in TGA experiments as three overlapping endothermic events that are complete by «420 °C. Complete dehydration requires «445 J/g, as measured by DSC experiments. The dehydration sequence, involving loss of three molar equivalents of water through condensation of B—OH groups, yields a substantially amorphous material of composition 2ZnO-3B2O3. This material undergoes a sharp exothermic event («270 J/g) at «640 °C corresponding to crystallization of two anhydrous zinc borate phases, 3ZnO-B2O3 and 4ZnO-3B2O3, presumably in the presence of liquid B2O3, as determined by XED analysis of quenched samples.23'24 Upon further heating, 3ZnO-B2O3 decomposes at «870 °C to a liquid and crystalline 4ZnO-3B2O3, which subsequently melts at «960 °C.

Conclusions

Borate compounds exhibit considerable structural variability, having spatial arrangements directed by the demands of interstitial cations. Hydrogen bonding plays an additional important role in integrating and stabilizing structures. Borates differ from other oxide materials, such as aluminates and silicates, in which the main group element oxide components play a more dominant role in directing structure. Theories recently developed for borate minerals advance our understanding of borate materials, but do not consistently apply to synthetic borates. A current challenge is the extension and adaptation of these principles to synthetic systems. Finally, elucidation of the structure of 1 leads to a revision of the chemical formula from 2ZnO*3B2O3-3.5H2O, used to describe this material as an article of commerce for more than 30 years, to 2ZnO-3B2O3'3H2O.

Applicant's summary and conclusion

Executive summary:

Several unique crystalline zinc borates are known, a few of which find industrial use in significant tonnages. Although the most important of these has been a commercial product for more than 3 decades, it was never before structurally characterized. The structure of Zn[B3CXi(OH)3] was determined for the first time by single-crystal X-ray diffraction, revealing it to be a complex network consisting of infinite polytriborate chains cross-linked by coordination with zinc and further integrated by hydrogen bonding. The structure bears similarities to certain borate minerals, most notably, studenitsite (Ca[B304(OH)3]) and colemanite (Ca[B304(OH)3]-H20); however, significant differences are described. Hydrolytic and thermochemical properties of Zn[B3CXI(OH)3] are discussed. This compound illustrates the important role played by metal cations in directing the spatial arrangement of anionic polyborate structural units in metal borates. This new structural information leads to a revision in the chemical formula, 2ZnO-3B203-3.5H20, typically used to describe this material as an article of commerce, to 2ZnO*3B2O3'3H2O. Zn[B3CXI(OH)3] crystallizes in the monoclinic space group P2l/n with a = 6.845(2) A,b = 9.798(2) A, c = 7.697(2) A, beta = 106.966(4)°, V = 493.8 (2) A3, and Z = 4.

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