Journal of Sol-Gel Science and Technology 26, 1239–1242, 2003c© 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.

Sol-Gel Derived Binder for Inorganic Composites

ELIN NILSENDepartment of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway

JANNE PUPUTTI AND MIKA LINDENDepartment of Physical Chemistry, Abo Akademi, 20500 Abo, Finland

JEAN LE BELLDepartment of Physical Chemistry, Abo Akademi, 20500 Abo, Finland; Paroc OY AB, 21600 Pargas, Finland

MIKAEL PERANDERParoc OY AB, 21600 Pargas, Finland

MARI-ANN EINARSRUD∗

Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, NorwayMari-Ann.Einarsrud@chembio.ntnu.no

Abstract. A new low cost inorganic binder system for large volume products like fiber insulation, buildingmaterials, etc. has been developed based on sol-gel technology. The precursor for the binder system is an amorphousmineral raw material containing silica as the major component. The sol was prepared by dissolving the amorphousmineral material in formic acid and the mineral was dissolved in a few hours dependent on the molarity of the formicacid. The sol stability was dependent on the solids content and the pH. The gel formation was studied using lightscattering and NMR. The results show a growing particle size of particles mainly consisting of silica while the othercations were dissolved in the pore liquid. During the drying of the wet gels, salts of these cations were crystallizedin the pores and further decomposed during heating. The derived binder shows good wetting properties to mineralfiber surfaces and a good strength of paper-binder composites. The new binder system applicable to approximately800◦C has a great potential as a substitute for some traditional organic systems.

Keywords: precursor chemistry, gel structure, inorganic binder, wetting properties, mechanical strength

1. Introduction

There exists a need for adhesives and binders to re-place presently used organic systems, especially in ap-plications where high temperature resistance is cru-cial. Organic systems usually have upper applicationtemperatures up to 250◦C. In addition, health and en-vironmental requirements demand the search for new

∗To whom all correspondence should be addressed.

inorganic water based systems. Important requirementsfor such a binder system are to provide a properbinding to inorganic surfaces, give sufficient mechan-ical strength and have operating temperatures up to8–900◦C, as well as, being resistant against humidityand water.

The sol-gel technology is beneficial for develop-ment of inorganic binder systems [1]. There is, how-ever, one serious drawback. The precursors tradi-tionally used in sol-gel systems are fairly expensive,

1240 Nilsen et al.

thus preventing their use in many bulk applications.Expensive precursors have however been used to de-velop inorganic binder systems [2–4]. The motivationof the present work was to apply sol-gel technologyto develop an inorganic binder system based on a lowcost amorphous mineral raw material containing silicaas the major component. The dissolution of the min-eral, stabilization of the sol and the gelation mechanismwere studied. Further the drying of the wet gels and theproperties of the derived sol as binder was evaluatedby studying the wetting properties on mineral fiber aswell as the strength of the paper-binder composites.

2. Experimental

An amorphous mineral raw material with the molar ra-tio between the major components Si:Al:Ca:Mg equalto 2.27:1.11:1:1.10 was used. The raw material alsocontained minor amounts of transition metals. Crushedmineral (normally 4.6 wt%) was dissolved in formicacid (MERCK AG, p.a.) at room temperature undercontinuous stirring. The dissolution time and pH of thesolution were measured as a function of the molarity ofthe formic acid (0.5–10 M). The composition of the so-lution during dissolution was measured by DCP-AES(Direct Current Plasma-Atomic Emission Spectropho-tometer, SpectraSpan 7 DCP, Applied Research Lab-oratories). In order to determine particle growth rateand sizes, dynamic light scattering measurements wereperformed at 25◦ with an argon laser (λ = 488 nm) anda Brookhaven BI 9000 correlator. Mathematical treat-ment of the measured intensity correlation functionswere done by using inverse Laplace transformationalgorithm, based on a constrained regularization pro-gram REPES [5]. Measurements were accomplishedin two different measuring angles, 60◦ and 120◦. 29Siand 27Al MAS NMR spectra were recorded on aVarian INOVA-300 spectrometer using a home-builtCP/MAS NMR probe and spinning speeds of 2.0 kHz.For heat treated gels spinning speeds of 7.0 kHz wasused.

The drying of the wet gels was studied by thermo-gravimetric analysis (TG, Netzsch STA 449 C) with aheating rate of 10 K/min and the crystallization of thegels during heat treatment was studied by x-ray diffrac-tion (XRD, Philips PW 1025/50). The wetting proper-ties to mineral fiber were studied by scanning electronmicroscopy (SEM, Zeiss DSM 940). The strength of thebinder was evaluated by making paper-binder compos-ites. Sheets of paper (20 × 120 mm2) were impregnated

with sol prior to drying at different temperatures(RT-150◦C) for 18 hours. The strength is presented asthe load in tension at which the paper-binder compos-ite was teared off. A load frame, LLOYD LR5K, withtension speed of 10 mm/min was used.

3. Results and Discussion

The dissolution time versus molarity of the formic acidused of 4.6 wt% solutions of the mineral is given inFig. 1. The figure shows that the dissolution time iswithin a few hours which is acceptable for the devel-opment of a sol-gel process. The pH value increasesduring the dissolution of the mineral and the pH valueafter the dissolution is completed is also included inFig. 1. The highest sol stability is obtained using 2.5 Mformic acid and this molarity is used in the subsequentstudies. The kinetics of the dissolution is presented inFig. 2 where the concentration of the major cations areplotted as a function of time for 4.6 wt% mineral mate-rial dissolved in 2.5 M formic acid. In agreement withFig. 1 the mineral dissolves fast in formic acid. The stepin the concentration of Si after about 5 hours is dueto a bimodal particle size distribution of the crushedmineral.

Particle growth in the sol is displayed in Fig. 3. Themaximum in particle size distributions is normalizedto one. The particle size shows a steady increase fromapproximately 25 nm 7 hours after the completion ofthe dissolution to more than 1000 nm after 73 hours.Results obtained by 29Si MAS NMR are presented inTable 1. The number of Qn-species with the higher

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e of fiber (h)

Molarity (M)

Figure 1. Dissolution time and pH after the dissolution is completeversus molarity of formic acid solution for the dissolution of 4.6 wt%mineral material.

Sol-Gel Derived Binder for Inorganic Composites 1241

0 5 10 15 20 25

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10000 Si Ca Al Mg

ppm

Time (h)

Figure 2. Development of concentration (in ppm) of cations duringdissolution of 4.6 wt% mineral material in 2.5 M formic acid. Themineral material used here had a coarser grain size compared to thematerial used in Fig. 1.

10 100 10000.0

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Figure 3. Particle growth versus time after dissolution of 4.6 wt%mineral material in a 2.5 M formic acid.

n-number increases from the sol to the gel and to thedried gel. This is in accordance with enhanced con-densation as the temperature increases. The Qn-speciesdistribution is independent of the drying temperature

Table 1. Q-species distribution in percentage for the sols beforegelation, the wet gels and gels dried at 70 and 200◦C. The sol wasmade by dissolving 4.6 wt% fiber in 2.5 M formic acid.

Species Sol Gel Dried gel (70◦C) Dried gel (200◦C)

Si Q2 12 7 4 5

Si Q3 48 40 36 36

Si Q4 40 53 60 59

in the range studied. In both the sol and the wet gelthe coordination number for Al was six, determinedby 27Al MAS NMR, showing that Al is not incorpo-rated into the silica network. EDS analysis of washed(4 times in water:HCOOH mixture at pH 2) and driedgels shows that all the cations except Si are removedfrom the wet gel. These observations show that the gelnetwork mainly consists of silica with the other cationsdissolved in the pore liquid.

XRD of dried gels shows the formation of crystallinephases. At 70◦C, the diffraction pattern is complex anda complete interpretation is difficult, however it is clearthat Ca-format and probably Al-format are present. Thecrystallization is occurring because the concentrationof the different cations dissolved in the pore liquid willincrease as the drying takes place and finally the sol-ubility limit will be reached. By heating the sampleabove 200◦C the Ca-format is decomposing formingCaCO3.

TG measurements of a semi-wet (dried at RT) gelis given in Fig. 4. Significant weight losses occur inthe temperature regions around 150, 200 and 300◦C.The weight loss around 150◦C is believed to be due toevaporation of water and formic acid while the decom-position of formats, carbonates and hydroxides are tak-ing place at higher temperatures. No significant weightloss is occurring above approximately 400◦C.

The precipitation of Ca-format in the pores is disad-vantageous due to the fact that the crystals can act ascenters for crack formation [4]. The crystal growth canbe suppressed by using a high drying rate or by dryingvery small gel films or droplets (which is the case when

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s (%

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Figure 4. TG data (10 K/min) obtained from a gel prepared bydissolving 4.6 wt% mineral material in 2.5 M formic acid.

1242 Nilsen et al.

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paper-binder composite

paper

Loa

d (N

)

Drying temperature (oC)

Figure 5. Load of failure of paper-binder composites preparedusing different drying temperatures.

Figure 6. SEM micrograph illustrating the wetting and bindingproperties of the derived binder on mineral fibers.

the sol is used as binder for fiber insulation). However,the most advantageous for the application of the presentsol, is to remove Ca from the sol by precipitation of anon-soluble Ca-salt followed by filtration.

The mechanical strength of the binder tested aspaper-binder composites is presented in Fig. 5. Priorto this test Ca was removed from the sol by precipita-tion of Ca-phosphates with phosphoric acid followedby filtration. As can be seen the load at which frac-ture occurs shows a maximum at drying temperaturesof approximately 120◦C. Included in the figure is alsothe results from testing paper without any binder. Thepaper shows a constant strength throughout the tem-perature interval of interest. The maximum in strengthis believed to be due to increased strength due to in-creased condensation followed by a decrease causedby decomposition of precipitated salts.

The wetting properties of the binder tested by spray-ing the sol (no Ca removed) onto mineral fibers areillustrated in Fig. 6. Excellent wetting properties tothe mineral fibers are obtained and no cracking or frac-ture of the binder was observed. This result shows thatthe present precursor has a great potential as a low-costbinder for large volume products.

4. Conclusions

An amorphous mineral raw material has shown promis-ing properties as low cost precursor for binders forlarge volume products like fiber insulation and build-ing materials. The derived new binder system showsgood wetting properties to mineral fiber surfaces and agood binding and has a great potential as a substitutefor some traditional organic systems.

References

1. C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics andChemistry of Sol-Gel Processing (Academic Press, New York,1990).

2. H. Schmidt, G. Jonschker, S. Goedicke, and M. Mening, J. Sol-Gel Sci. Techn. 19, 39 (2000).

3. D.E. Valette and D. Yarwood, US Patent 5223030 930629, June29, 1993.

4. M. Mening, G. Jonschker, and H. Schmidt, European Patent No.EP 0 642 475 B1, 06-19-96.

5. P. Stepanek, Dynamic Light Scattering: The Method and Some Ap-plications, edited by W. Brown (Clarendon Press, Oxford, 1993).