Microwave Assisted Preparation of Calcium Hydroxide

and Barium Hydroxide Nanoparticles and Their

Application for Conservation of Cultural Heritage

Khaled M. Saoud*, Imen Ibala, Dana El Ladki, Omar Ezzeldeen, Shaukat Saeed

Liberal Arts and Science Program, Virginia Commonwealth University in Qatar

Doha, Qatar

*Corresponding Author: Dr. Khaled M. Saoud; Email: s2kmsaou@vcu.edu , Ph. +974 6603 7810

Abstract. In this paper, we present the microwave-assisted synthesis and

characterization of alkaline nanomaterials for the preservation of cultural

heritage. Calcium and barium hydroxide nanomaterials suspended in aqueous as

well as alcoholic medium were applied on different samples to check their

effectiveness. The effect of treatment was measured using pH value and

alkaline reserve test. The alkaline reserve test results indicate improvement in

the alkaline buffer with significant reduction in acidity of the papers and

improvement in pH value of the surfaces. Ethanol was found as best solvent on

the basis of suspension of nanoparticles, pH value and ease of application

through spray gun. Accelerated ageing test revealed that the reinforcement

effect of these nanomaterials persisted throughout the ageing. All the results

converged in individuating these nanometric particles as an innovative,

completely compatible, and efficient material for the consolidation of old and

new paper surfaces.

Keywords: Cultural heritage, Nanomaterials, paper deacidification, alkaline

reserve, Calcium hydroxide, Barium hydroxide.

Introduction

The deterioration of cultural heritage objects is one of the most critical problems in

the humanities and social sciences today. Preserving and restoring the cultural

artifacts is crucial to maintain history and culture of nations and keeps it alive for

future generations.

Many of the hundred years old books are currently too fragile to handle due to the

presence of acidic content in the paper. In 1830, alum-rosin sizing was introduced to

the paper-making process to speed up production [1]. Unfortunately, sulfuric acid is

formed during this process which attacks the cellulose fibers in the paper [2]. There

are other factors such as oxidation, temperature and humidity, exposure to light, air

pollutants, and the amount of use also contribute to the destruction of the books [3].

Preservation involves a wide range of activities, including the general conservation

of important and rare books and documents, and handling procedures [4]. Many

techniques depend on manual operations are time-consuming and often require skilled

practitioners [5]. One of the strategies to deal with acid book preservation- problem is

the deacidification of the paper with suitable materials [6]. Sheets of paper are either

dipped in or sprayed with a solution containing one or more alkaline compounds [7].

Alkaline treatment usually magnesium or calcium carbonates and the solvents can be

aqueous or non-aqueous. Unfortunately, water dispersions of commercially available

Ca(OH)2 or Ba(OH)2 cannot be used since the particle sizes are too large and the

sedimentation rate is too fast, producing white glazing over painted surfaces. It has

been found that larger size of particles significantly reduce their effectiveness as

preservation or rehabilitation material. Some literature in this regard is reported in the

following paragraphs to highlight the role of size of particles especially the

nanoparticles.

Nanotechnology has gained recognition on the basis that particles of size below

100 nm impart new behavior and properties. In particular, by using these compatible

methods, it is possible to perform interventions without modification of the physico-

chemical and mechanical behavior of the materials, ensuring long-lasting effects. A

direct use of aqueous solutions of calcium or barium hydroxide is limited by the low

solubility of the salt (1.6 g/L) [8].

Dispersions of nanosized particles in non-aqueous solvents produce kinetically

stable systems and can also help to achieve a deeper penetration of the dispersion,

better stability and avoid the formation of white glazing on the treated surface [9-11].

With the purpose of obtaining the best performances from this kind of product, some

synthesis methods have been recently developed, which allow further reduction of the

average size of hexagonal, calcium hydroxide particles to the nanometric scale [12-

14]. One of such procedure is based on the hydrolysis of calcium chloride solutions in

1,2-ethanediol by addition of aqueous sodium hydroxide at elevated temperature (150

°C), and subsequent peptizing of the particles’ agglomerates in propan-2-ol, by means

of an ultrasonic bath [15-17]. Alcohols are environmentally friendly, volatile, and,

compared to other solvents, have a low toxicity. Surface tension is small enough to

ensure optimal wetting that is responsible for high penetration of the dispersions

within the porous structure of papers and wall paintings [10, 11]. We in this study

found ethanol as best solvent compared to water and propanol, and opted to use

ethanol as solvent for dispersing calcium and barium hydroxide nanoparticles on the

paper samples using a spray gun. The reason being poor solubility of these salts in

water, and flexibility of ethanol for dispersion of these salts through brushing or

spraying. Spray produces uniform thin layer on the surface that ensures equal

treatment all over surface.

The major importance of this research was to develop materials that can be used to

support cultural institutions in Qatar while also supporting the development of

innovative technologies.

Materials and Methods:

Materials: All chemicals, like calcium Nitrate (Ca(NO3)2; 98%, Labchem), Barium chloride

(BaCl3; 99%, Labchem), Sodium Hydroxide (NaOH, Sigma Aldrich), were used as

received without further purification. Distilled water and pure ethanol were used in all

the preparation. Whatman No.1 filter paper manufactured from 100% cellulose and

containing no additives was used in the study as reference. Old paper from book

published ~ 100 years ago, new paper manufactured during 2-5 years and aged

(environmentally or hydrothermally) newspapers was also used.

Methodology

The work presented here can be broken down into following three steps: a)

Microwave assisted preparation of nanoparticles; b) Application of Nanoparticles to

Filter Paper; c) Application of Nanoparticles to Old and Aged Paper.

Synthesis Procedure:

Synthesis of Hydroxide Nanoparticles

In a typical synthesis process of hydroxide nanoparticles, a precursor solution was

prepared by dissolving 5g of barium chloride or calcium chloride in 100 ml of

distilled water. In a 250 ml reaction flask, precursor solution along and 1% solution of

CTAB (a surfactant) were mixed with addition of alcohol at elevated temperature.

NaOH solution (1M) was drop-wise added to the vigorously stirring solution until

white translucent precipitates were formed. The solution was subsequently placed in a

microwave chemical reactor (MCR-3) operated at a power of 266W for 10-15

minutes. Finally, the precipitated particles were filtered and collected after washing

thoroughly several times with ethanol and distilled water alternatively. The flow sheet

diagram of the process is shown in Figure 1. The dried powder samples were heated

in oven at 300 oC to remove the CTAB. Consequently, ethanol suspension of these

particles way sprayed on the test samples.

Figure 1: Flow sheet diagram showing synthesis of Barium and Calcium hydroxide

nanoparticles

Methodologies:

Aging Process:

Following steps were involved in aging the filter paper: a) The filter paper was

soaked in H2SO4 acid having pH~3, b) The sample was placed in an oven set to 90°C,

c) A piece of the filter paper was tore to perform pH test every hour.

Accelerated Aging Tests

Aged filter paper, b) Applied material on filter paper, c) Dried filter paper,

d) Placed a small piece of that filter paper in a really small amount of distilled water,

Added ethanol to allow the pH electrode to immerse, Tested the pH in hot and cold

extracts. Alkalinity and Alkaline Reserve test was performed based on T 553 om-10

Characterizations

X-ray diffraction (XRD) of the Nanoparticles samples used in this study

were recorded by using a Rigaku MiniFlex600 (CuKα radiation, wavelength =1.54 Å)

operated at 40 kV, 15 mA. High Resolution Transmission electron microscopy

(HRTEM; model: JEOL JEM-2100F) was used for the morphological and

crystallographic characterization of the nanoparticles. Surface area of the

nanoparticles was measured using BET Surface Area Analyzer nitrogen

adsorption/desorption measurements are carried out at 77 K (Quanta chrome

Instruments, Nova 2200e). Scanning electron microscopic images are obtained with a

HITACHI SU-70 SEM with an accelerating voltage of 5 keV.

Results and Discussion:

XRD Analysis

Figure 1 shows X-Ray Diffraction pattern (XRD) of Ba(OH)2 and Ca(OH)2

nanoparticles used in this study. The X-ray diffraction Pattern are consistent with the

X-ray diffraction pattern of crystalline Barium Carbonate (BaCO3) and Ca(OH)2

Nanoparticles respectively. The XRD pattern of Ba(OH)2 was verified using JCPDS

(Joint Committee on Powder Diffraction Standards) card no. 00-045-1471 which

shows that the as prepared initially thought to be Ba(OH)2 is in fact react with CO2 in

air to form BaCO3 which is a well-known reaction reported by many researchers

exhibiting maximum diffraction from (111) crystal plane at 2 Theta of 23.891

degrees. Also, the XRD pattern of Ca(OH)2 nanoparticles, which was verified using

JCPDS (Joint Committee on Powder Diffraction Standards) card no. 1000045 and

2100992, shows that the as prepared Ca(OH)2 Nanoparticles exhibiting maximum

diffraction from (101) crystal plane at 2 Theta of 33.94 degrees. However, Calcium

hydroxide XRD contains small impurities of Calcium carbonate which result from the

reaction with CO2 in air. Thus the data reported in Figure 1 and discussed above,

suggests successful preparation of the desired materials to be used for preservation of

paper in this study.

Figure 2: XRD patterns of Ba(OH)2 and Ca(OH)2 Nanoparticles.

SEM Analysis

Figure 2 shows micrograph of Scanning Electron Microscope (SEM) and Energy

Dispersed X-Ray (SEM-EDX) spectra obtained of Ba(OH)2 and Ca(OH)2

Nanoparticles. It is evident from the micrograph that there is a significant difference

in shape and morphology of the two samples. The micrograph also indicate

agglomeration of the particles in the case of Ba(OH)2. The particles are in the form of

ribbons having a width greater than 500 nm and length apparently around 10 µm. In

the case of Ca(OH)2, the particle seems to be consist of small flaks which may be

outcome of an interconnected network of small particles (˂ 100 nm).

The element distribution maps revealed by SEM-EDX analysis shows that the

elements are mostly Barium (Ba) and calcium (Ca) along with oxygen in case of

Ba(OH)2 and Ca(OH)2 nanoparticles respectively, which is consistent with element

present in these materials.

Figure 3: Scanning electron micrograph (SEM) and SEM-EDX spectra with the

distribution of elements obtained of Ca(OH)2 and Ba(OH)2 Nanoparticles.

Effect of Solvents:

The nanomaterials prepared using microwave assisted synthesis method for

deacidification treatment showed that these hydroxides are suitable for the paper

deacidification. However, published literature suggests that many of the group II

materials have a high pH values and could potentially damage the paper. Also, some

of these materials have been reported to be not stable. Table No. 1 summarize data of

pH values of the materials synthesized in this research work and applied on filter

paper as suspension in different solvents.

Table No. 1: pH value of hydroxides applied on Whitman Filter paper dispersed in

different solvent

Material ↓ / Solution→

pH (in specified solvent)

Ethanol Propanol Distilled

Water

Ba(OH)2 Powder 6.02 4.50 7.24

Ca(OH)2 Powder 7.44 6.87 10.16

Physical mixture of

Ca(OH)2 and Ba(OH)2 7.70 8.65 7.10

Ca(OH)2+ Ba(OH)2 8.60 6.21 11.75

CaO 11.11 8.42 12.3

Ca(OH)2 (Heating Ca(OH)2) 7.92 7.42 11.71

One pot synthesis of Ca(OH)2 9.04 6.49 9.85

We have also performed experiments in this project to explore the effect of the

various solvents normally used in the preservation treatment process. Our results

indicate that Ethanol is the most suitable solvent compared to distilled water and

propanol as shown in Table No. 1. Water is known to decrease the strength of the

paper compared with alcohol due to its lower molecular weight. Also, lower boiling

points of alcohol can ensure faster and lower temperature drying of the paper.

Another advantage of using ethanol is the fast evaporation after the treatment which is

beneficial in terms of shortening the time needed for preservation treatment.

Effect of solvent on pH value of Whitman Filter paper coated with the primary

materials synthesized in this study was also explored. The values of pH on both

extreme are not desirable in the deacidification treatment as it may initiate

degradation process. It has been found that paper with neutral or mild alkaline pH

deteriorates more slowly than paper with lower pH value.

Another negative effect of using water as a solvent is its effect on the strength of

the paper. It has been reported that most papers retain about 10 per cent of their dry

strength when wet by wateri. This reduction in strength seems to be too much to

handle the paper during its treatment. Thus, such solvent is desired which do not

cause significant reduction in strength of paper. In this study we have found that

Ethanol is the most suitable solvent for the treatment process. Our results confirming

all these positive effects in mind, we chose ethanol as solvent for the subsequent

experiments.

We used accelerating aging process to simulate the deterioration process in the

paper to generate data that may be used to predict the life time of the paper. In this

way we want to test the effectiveness of our treatment process. For this purpose we

used an aggressive aging method to shorten the time normally required to study the

aging process. Thus the sample paper to be aged was initially soaked in Sulfuric acid

solution, dried in oven with hot air flow, and then subjected to aging process which

involved keeping the paper samples at 90°C under ambient conditions for 1–6 hours.

In our opinion, this ageing process is sufficiently rigorous in comparison with the

process where the paper sample is normally exposed to heating only for longer

durations without its pretreatment with any acid or base. Each paper sample was taken

out after its exposure to high temperature for desired time to get specified aging

treatment i.e., taken out after one hour and treated with the nanoparticle solution then

after two hours and so on. The pH measurements were performed on the hot and cold

water extracts [19]. The procedure for these tests is given by the Technical

Association of the Pulp and Paper Industry (T43S-S2; 1952) [20]. The data has been

depicted in Figures 3.

However, the pH determinations of aqueous extract were carried out in accordance

with the method described in TAPPI T 435 [21-22].

Figure 4: Accelerating Aging Test of 50-50 Physical mixtures of Ca(OH)2 and

Ba(OH)2 dispersed in ethanol and applied to Whatman Filter paper compared to

Ca(OH)2 and Ba(OH)2 nanoparticles Using a) Cold extract. b) Hot extract

We also studied the effect of solvent on the pH value of Whatman Filter paper

coated with Ba(OH)2 nanoparticles combined with the accelerating aging. We found

that the pH values of hot and cold extracts using ethanol as solvent follow the same

trend after three hours of aging while in the case where propanol was used as solvent;

no trend could be extracted as shown in Figure 4.

Figure 5: Effect of solvent on the accelerating aging of Whatman Filter paper coated

with Ba(OH)2 nanoparticles: a) with Propanol, b) with Ethanol.

Ca(OH)2 nanoparticles has been explored by many researchers and have been

regarded as the best solution for paper deacidification in the cultural heritage

preservation in the today’s market. We, in this study, adopted a new synthesis strategy

to prepare Ca(OH)2 nanoparticles as a solution for paper preservation problem. We

prepared Ca(OH)2 nanoparticles using microwave synthesis and we applied this

material to 100 years old paper and pure cellulose filter. Our results indicate that

Ca(OH)2 nanoparticles not only naturalize the acidic content in the paper but also

increase the alkaline reserve of the paper. However, in spite of these positive effects,

Ca(OH)2 nanoparticles caused damage to the old paper as well as the filter paper by

interacting with underlying substrate. The micrograph shown in Figure 5 reflects

these effects.

Figure 6: Scanning electron micrograph (SEM) of old paper and Whatman filter

paper with and without Ca (OH)2 nanoparticles coatings used in this study

The BET Surface Area and nitrogen adsorption/desorption measurements of

Ba(OH)2, nanoparticles measured using Quanta chrome Instruments, Nova 2200e

shows that the surface area of Ba(OH)2 is 4.558 m2/g. HR-TEM of Ba(OH)2 nano rods

synthesized using microwave in this work have been shown in Figure 6. The rods

have a thickness of 20-30 nm and have length of few hundred nanometers.

Figure 7: TEM of Ba(OH)2 nano rods synthesized using microwave method.

The nanomaterials that we applied on the paper samples in this study was expected

to supply the paper with an alkaline reserve to combat the acidic content present in the

samples and to act as reserve to provide protection from further attack by acids during

its future life. As we know, for a paper to have a life expectancy of at least 100 years

it must have an alkaline reserve of 2% or more (or it should be >400 mmol/kg for a

life expectancy of 500 - 1,000 years depending on the nature of paper).

We adopted the ISO standard test (ISO 10716: 1994) to determine the amount of

alkali reserve present in our samples. The results depicted in Figure 7. It is evident

from the data that by applying only 0.1 g of our nanomaterials, we obtained

approximately 6, 11 and 15 times higher values (in comparison with 400 mmol/kg)

for the filter paper treated with Ba(OH)2 and Ca(OH)2 suspensions respectively.

However, for old paper sample, only Ca(OH)2 could give positive values for a similar

treatment. It is important to note here that if we take -242 mmol/kg as zero, then 70

mmol/kg are equivalent to 312 mmol/kg, which is close to the value required for a life

expectancy of at least 500 years. Thus it may be concluded that Ca(OH)2

nanoparticles stands out to be most promising of the two.

Figure 8: Comparison between the Alkaline Reserve tests performed on a) Whitman

Filter Paper b) 100 Years Old paper

The effect of application of nanomaterials on the old paper sample was studied by

measuring the surface pH value of the samples. The actual images of the test

performed on samples have been depicted in Figure No. 8. The pH value for old paper

sample, on the other hand, is 2.5 which rises to 8.5 in case of calcium hydroxide

sample. This data clearly shows the effectiveness of the nanoparticles in combating

the acidic content present in the old paper due to its aging process, whereas

stabilization in pH value around its maximum is indicative of its pure cellulosic

nature.

Figure 9: pH measurement of Reference old paper and old paper treated with

Ca(OH)2 nano-particles

Conclusions:

Calcium hydroxide and barium hydroxide nanoparticles were successfully

synthesized using novel microwave synthesis technique. The effectiveness of these

materials toward conservation of cultural heritage, specifically paper samples was

successfully studied. Characterization technique like XRD, SEM, HR-TEM were used

to elucidate the composition and structure of Ba(OH)2 and Ca(OH)2 nanomaterials.

BET Surface Area measurements of Ba(OH)2 nanoparticles showed that the surface

area of Ba(OH)2 is 4.558 m2/g. In order to find an appropriate solvent for dispersion

and application of these materials on actual samples, Ethanol was found to be the

most appropriate solvent compared to distilled water and propanol. Aggressive aging

procedure applied on the samples, to shorten the time spent on processing, was found

to be effective for evaluation of the materials synthesized in this project. Simple pH

measurements, using digital pH equipment and pH maker pen, and alkaline reserve

tests provided enough insight about the effectiveness of the prepared nano-materials.

Acknowledgement:

“This publication was made possible by a UREP award [14 - 139 - 2 -

047] from the Qatar National Research Fund (a member of The Qatar

Foundation). The statements made herein are solely the responsibility of the

author[s].”

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