Synthesis of Citric-Acrylate Oligomer and its In-Situ Reaction With Chrome Tanned Collagen

20 Pages·2005·0.28 MB·English

8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
Synthesis of Citric-Acrylate Oligomer and its In-Situ Reaction
With Chrome Tanned Collagen (Hide Powder)

A. A. Haroun1; R. A. Masoud1; S. Bronco2 and F.Ciardelli2
1National Research Center/Dept. of Chemistry of Tanning Materials and
Leather Technology, Dokki, Cairo, Egypt
2University of Pisa, Dept. of Chemistry and Industrial Chemistry, Pisa, Italy

Summary
The purpose of this study is to formulate a new combined system of acrylic
and citric acids, which was prepared by free radical polymerization and
esterification reaction at the same time to form citric-acrylate CAC oligomer
through ester linkage and low molecular weight (M 2241) in comparison with
w
polyacrylic acid. The chemical structure and the reaction mechanism of this
oligomer were confirmed by different spectroscopic tools (1H,13C-NMR and
ATR-IR), gel permeation chromatography and thermogravimetric analysis
(TGA/DTA). The problem of the effect of masking agents in the chrome
tanning of the collagen and pickling of the hide was approached via the study
of the hydrothermal and mechanical properties, through the eco-friendly
oligomer (CAC), which was carried out in-situ treated/grafted chrome tanned
collagen (hide powder), and pickled hide. The microemulsion grafting
copolymerization of CAC using 2,2-azo-bis-isobutyronitrile (ABIN), via direct
coupling reaction, onto chrome tanned collagen showed that the free amino
groups of the collagen were considered to be potential sites for the in-situ
reaction with CAC oligomer. Also, using of citric-acrylate CAC oligomer,
during chrome tanning of leather, instead of the traditional strong acids
(sulfuric, hydrochloric and formic) resulted in a significant improvement in
chrome exhaustion and leather physical properties.

Key words: acrylic acid, citric acid, collagen, chrome tanning, grafting,
pickled hide
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT

1. Introduction
Leather making is constantly changing because it is expected to respond to the
demands of the market place, and the environmental legislations1. The temperature of
collagen denaturation (T ) that is close to 35°C reaches 66°C in the mature skin. Tanning,
d
by introducing cross-links, generally increases T , but its influence on the shrinkage
d
behaviour varies with its nature and the kind of the reactive involved groups2. The
hydrothermal stability of collagen fibers is particularly good measure of the strength and
quality of skin, leather and parchment materials and the degree of their deterioration. The
hydrothermal stability is characterized by the material shrinkage when heated in water at a
defined temperature3,4,5. To pickle the pelt (collagen) means its acidification in such away
to prevent its simultaneous swelling under the action of the acid. This is usually done by
salt addition. The art of pickling is to slow down the tanning action in the early stages, so
that the mineral tannins penetrate the cross-section of the hide and skin without difficulty.
This is achieved by modifying the reactive acidic (-COOH) and basic (-NH ) groups of the
2
collagen when the pelt is treated with a mixed solution of salts and acids6. Tanning is a
process of converting unstable raw hides into leather with adequate strength properties and
resistance to various biological and physical agents, this is accompanied by introduction of
additional cross links into collagen, which bind the active groups of the tanning agents to
functional groups of the protein7,8. The unique combination of properties that results from
successful tanning obviously requires much more than the simple introduction of cross
links, probably their number, size and location, as well as any change in the electrostatic
charge and hydrophilic/hydrophobic character of the collagen, which play a part of
determining the properties of the resultant leather9. Most leather production world wide is
still based on tanning of hides with chromium salts. There are, yet, no prospects of chrome
tanning being replaced generally by an alternative method. The conventional chrome
tanning viewed from the ecological aspects many problems such as: the enormous
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
quantities of chrome containing solid wastes (shavings and trimmings) occurring during
leather manufacture and the waste water10 . Synthetic tanning agents or syntans from a
generic group of tanning agents are characterised as synthetic polymers or resins with
chemically active groups capable of interacting with collagen typically via multiple
hydrogen bonding11. There are hundreds of multifunctional organic compounds capable of
introducing covalent cross links into collagen and, therefore, having the potential to serve
as tanning agents. Also tanning with organic substances is considered as an alternative to
chrome tanning12. The advantage of acrylic acid is being cheap and easy to polymerize
and react with different compounds to produce high molecular weight compounds13. The
acrylic polymers used for impregnation, base coat and top coat14 and the flexibility of
acrylic polymers in general is unmatched among polymeric materials, in addition to
flexibility, exceptional light fastness, absence of discoloration, pigment binding capacity,
moulding ability, water resistance and compatibility with other synthetic resins are some
of the striking operatives of acrylic emulsions which are highly desirable for use in leather
industry15. A new non formaldehyde durable press finishing system was developed based
on the combination of citric acid and a terpolymer of maleic acid, acrylic acid and vinyl
alcohol16, so this new system showed superior durable press performance, good laundering
durability and high fabric strength retention in the textile industry.

2. Experimental
2.1. Materials
i) Hide powder was provided by E. Merck with specific characteristics
(pH 5.2-5.6, Cr O : below 0.5%, ignition residue: below 0.9%).
2 3
ii) Pickled hide was supplied by a local tannery at Cairo.
iii) Citric acid (99.5%) was provided by Biosar, s.d. Fine Chem. LTd.
iv) Acrylic acid (99%) was provided by E. Merck.
v) All other chemicals were obtained from local commercial sources.
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
2.2. Methods
2.2.1) Formulation of the CAC oligomer:
7.2 G of acrylic acid (0.1mol) was mixed, drop wise, with 19.2g citric acid (0.1mol) at
65°C, for 3 hours in liquor 200 ml distilled water, in presence of sodium
bisulfite/potasium persulfate (1:2) as an initiator.
2.2.2) Structure elucidation of the synthesized CAC oligomer:
I) Spectroscopic techniques:
I.1) 1H, 13C-NMR: Using Varian Unity 300 MHz.
I.2) ATR-IR: using Perkin Elmer- Attenuated reflected infrared spectrometer.
II) Gel permeation chromatography (GPC) :
Using Techno Service, Jasco GP, with the following conditions: ultrahydrogel linear
2×30cm Column; eluent: 0.8ml/min LiNO 0.1M; refractive index detector; pullulan as
3
standard, temperature 35°C.
III) Thermogravimetric analysis: (TGA/DTA)
Using Mettler Toledo Stare system TGA/DTA 85le module, scanning from 30°C to
600°C in N atmosphere of 60ml/min and heating rate of 10.0°C/min.
2
2.2.3) Application of the synthesized CAC oligomer:
I) Treatment of hide powder:
2 Ml of sodium hydrogen phosphate solution was added to 5g of hide powder for
10min, followed by 5g of CAC for 30 min, at 40°C, dried at 80°C for 15 min followed by
washing with dist. water for 5 min and filteration.
II) Treatment of pickled hide:
5% of CAC and 1% of sodium hypophosphite solution and 1.5% sodium chloride
solution were added to x g of pickled hide at 35°C for 30 min (liquor ratio 1:10).
III) Microemulsion grafting copolymerization using ABIN17,18:
The microemulsion solution of a mixture of 5g of hide powder and 2.5g of sodium
dodecyl sulphate (SDS) in 100 ml distilled water and the required amount of 2,2-azo-bis-
isobutyronitrile (0.096g) in 50 ml 2-butanone was added in a flask equipped with stirrer,
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
thermometer, condenser and nitrogen inlet at 70°C for a period 1 hour, so as to enable the
complete dispersion of the initiator as micro globules in aqueous medium stabilized by the
surfactant (SDS)-cosurfactant (2-butanone) system. 5.28g (0.1mol) of the synthesized
CAC oligomer was dissolved in 50 ml mixed solvent of distilled water and methanol and
added drop wise to the microemulsion solution. The copolymerization reaction was
completed in 4 hours, after which, the contents were filtered, washed with distilled water,
then dried in vacuum at 40°C. The homopolymer was removed from the grafted hide
powder by exhaustive Soxhlet extraction with absolute methanol for 18 hours.

IV) Using of CAC as pretanning agent:
x G of pickled hide samples were impregnated in float 100% water containing
35% of sodium chloride, 1% of sodium lauryl sulfate and different concentrations of
CAC (1%, 2%, 3%, 4% and 5%) at 35°C for 40 min, the treated samples were
washed by CO –free distilled water very well followed by addition 6% of basic
2
chromium sulfate (33%basic) to each treated pickled hide samples at 35°C over
night then add 1% of formic acid as masking agent for 30 min.
V) Using of CAC as masking agent:
To x g of pickled hide samples in float 100% add 6% of basic chromium sulfate
(33% basic) and 35% of sodium chloride at 35°C overnight, then add to each sample
different concentrations of AC (1%, 3%, 5%, and 8%) for 40 min.
VI) Using of CAC as retanning agent:
x G of pickled hide samples were tanned with 6% of basic chromium sulfate
(33%basic) in presence 35% of sodium chloride at 35°C overnight, then add to

each sample different concentrations of CAC (2%, 4%, 6% and 8%).
2.2.4) Measurements:
I) Determination of the carboxyl content as follows:
i) 0.5 G of citric acid was dissolved in 50 ml CO –free distilled water then titrated
2
against 0.01N standard solution of sodium hydroxide using acid-base titration19 .
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
The carboxyl content was calculated by the following equation:
0.01× ml NaOH × 0.21
Carboxyl content ═ mmole/gm
0.5 × n
Where n is the number of the carboxyl groups in citric acid.
ii) After treatment of hide powder with CAC, it was filtrated and titrated against 0.1N
standard sodium hydroxide solution.
iii) The washing of the treated hide powder cake by CO –free cold distilled water was
2
titrated, to calculate the unreacted carboxylate.
iv) The washing of the treated hide powder cake by CO –free hot distilled water
2
after drying was titrated to calculate the unbound carboxylate.
II) Shrinkage temperature20,21:
All samples under investigation were measured according to the official methods of
analysis IUP16 (International Union Commission Testing for Physical Properties of
leather).
III) Mechanical Properties (tensile strength and elongation)22:
All samples under investigation were measured according to DIN (Deutsches Institute
Fuer Normung e.v.)

3. Results and Discussion
The final chemical structure of acrylic/citric mixture was formulated according to scheme
1, which is confirmed by different spectroscopic tools as follows:
Fig.1-ashows 1H-NMR spectra of CAC oligomer. The data show that the following peaks

at (d, 2.72), (d, 2.9), (t, 2.48), (t, 3.7), and (m, 4.3-4.4) ppm corresponding to –CH (citric
2
acid), –CH (citric acid), -CH , –CH -C=O and (CH=C) groups respectively.
2 2 2 71
Fig.1-b shows 13C-NMR spectra of CAC oligomer. The data show that the following
signals at 35, 38, 42, 45, 59, 62, 73, 173, 178 and 179 ppm corresponding to the carbon
atoms numbers 3, 7, 6, (8 and 9), 5, 10, 4, 1 and 2 respectively.
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
Fig.2 shows ATR-IR spectra of CAC oligomer. We can observe that the characteristic
peaks at 3433, 2967, 1720, 1648, 1638 and 1223 cm-1 corresponding to -OH (stretching), -
CH aliphatic, -C=O ester, -C=O carboxylic acid, -C=C- and C-O (stretching) groups
2
respectively.
Fig 3 shows ATR-IR spectra of the untreated hide powder (blank) and the grafted one. It
was observed that, the primary bands of collagen arise from the peptide bond vibrations23-
24 amide I, II and III centred at 1654, 1515 and 1238 cm-1 respectively. The grafted hide
powder spectra showed both the characteristic bands at 1654 and 1238 cm-1 disappeared
and new other bands at 1233 and 1203 cm-1 were formed. This may be due to in-situ
grafting reaction which takes place more preferably at amide I and III than that at amide II
through the polypeptide chains of the collagen molecule.
Also, the peak at 3280 cm-1 corresponding to –NH group stretching in the free form
2
disappeared, after grafting process, due to in-situ reaction with the carbonyl group of the
acrylic acid to form amide cross linking. The –NH stretching absorption peak takes place
at the region (3500-3000 cm-1 ), however, when –NH group participates in hydrogen
bonding, the frequency of its vibration decreases and its intensity increases. It can be
concluded that, the free amino groups of the collagen were considered to be a potential site
for the in-situ reaction with CAC.
It is noticed that the spectral feature of highly cross-linked collagen (chrome tanned
collagen) sample was modified.
We can suppose that the spectral features are affected by both of the cross linking and
other factors. One of this, is the characteristic microstructure development of the CAC-
collagen. Some big differences between a natural drying sample and freeze drying one,
from the degree of the fibrils arrangement of the CAC-collagen composite. So, we think
that the IR spectra of the cross linked CAC-collagen samples include the effect of this
characteristic microstructure development.
From the GPC analysis of CAC oligomer, the synthesized compound has polydespersity
about 1.04 and average molecular weight about 2241. It can be concluded that, the only
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
possible route to formulate CAC, which agree with the above characterizations, was
proceeded firstly by esterification of the acrylic acid via ester linkage, followed by free
radical polymerization up to low molecular weight, this may be due to the presence of
citric acid in the reaction medium which causes rapid termination of the polymerization
reaction by migration of the vinyl protons and vinyl chain transfer mechanisms25, as
shown in scheme 1. In other words, citric acid acts as inhibitor during the free radical
polymerization of acrylic acid

CH COOH
2
CH2 CH COOH HO C COOH
CH COOH
Acrylic acid 2
Citric acid
Free radical Esterification
Polymerization Reaction
3 1
O
CH COOH
2
7 6 4 2
CH2 CH2 C O C COOH
5 3 1
CH COOH
2
O
CH COOH
2
8 9 10
(C CH ) C O C COOH
71
CH2 COOH
Scheme 1. Formulation of CAC oligomer
As shown in Table 1, the remaining carboxyl content after cold washing of the treated hide
powder is 0.00007 mmol/gm, while in case of hot washing, it is 0.0001 mmol/gm.
Consequently the percentage of the reacted carboxyl anions of CAC with hide powder is
about 97%.
Table 2 shows the effect of CAC oligomer treatment on the thermal stability of the treated
pickled hide. It was observed that, the shrinkage temperature of the treated pickled hide
was slightly altered relative to the untreated one. On the other side, the thermal stability of
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
the treated pickled hide with polyacrylic acid was higher than that of the untreated and
CAC treated pickled hide. This may be due to the changing in the collagen structure
dimensional due to introducing high/low molecular weight compounds across the
polypeptide chains of collagen. This means that CAC oligomer has low molecular weight
in comparison with polyacrylic acid (PAA). Consequently, CAC oligomer can be used in
the pretanning step without any harmful effect on the fiber of the pickled hide.
The integral of the peaks in the TGA/DTA curves as shown in Figs 4-a, 4-b and 4-c were
summarized in Table 3 . The results show that, the first step occurs at 90°C and is
attributed to the loss of adsorbed water. The dehydration of the treated hide powder takes
place up to 267°C, both of hygroscopes and water were highly lost in comparison with the
untreated hide powder and all of the dehydration/dehydroxalation steps were endothermic.
Dehydration of the bound water in two steps was attributed to the differences in the
bonding position of water in the structure of hide powder. Between 267°C and 335°C
bound water (co-ordination water) was lost. Stabilization of the collagen takes place
through electrostatic and hydrophobic interactions, so hydrogen bonded water plays a big
part in the stabilization of the molecule. Consequently all these non-covalent bonding
breakdown on heating and this breakdown starts at the weakest points of the helix between
the stabilizing clusters. It can be concluded that, at low moisture content, one single
endothermic peak corresponding to the denaturation process of leather and the evaporation
of water was detected. On the other side, at higher moisture, two endothermic peaks were
detected, the first one corresponds to denaturation and the second to evaporation of water.
Tables 4 and 5 show the effect of CAC oligomer as pretanning and masking agents on the
mechanical properties of the pickled hide. The data show that, both tensile strength and
elongation at break were slightly altered, when the concentration of CAC oligomer was
increased.
Table 6 shows the effect of CAC oligomer as retanning agent on the mechanical properties
of the chrome tanned leather relative to basic aluminum sulfate. The data show that, 5%
CAC gives the same effect in compared with 8% basic aluminum sulfate. This indicates
8th ARAB INTERNATIONAL CONFERENCE ON
POLYMER SCIENCE & TECHNOLOGY
27 – 30 November 2005, Cairo-Sharm El-Shiekh, EGYPT
that, CAC oligomer could be used as retanning agent up to 5% concentration to exhibit the
same effect of the traditional mineral retanning agents such as basic aluminum sulfate.
Fig 5 shows the effect of CAC oligomer as masking agent on both thermal stability and
chrome exhaustion percentage during chrome tanning of pickled hide. The curves
illustrate that, the chrome exhaustion was increased with increasing CAC concentration,
while the thermal stability of the chrome tanned collagen was levelled off with increasing
CAC concentration. This may be due to the effect of low pH, at high CAC concentration,
on the rate of the chromium complex formation between polypeptide chains of the
collagen in the pickled hide.
Fig 6 shows the effect of CAC oligomer as pretanning agent on both thermal stability and
chrome exhaustion of the pickled hide. The curves illustrate that, at 4% CAC, the
optimum chrome exhaustion can be achieved while the thermal stability was slightly
altered with increasing CAC concentration. This means that the optimum pH of the
pickled hide which gives the optimum condition for tanning process is below 4-5.
Fig 7 shows the effect of CAC oligomer as retanning agent on thermal stability of chrome
tanned leather relative to the traditional retanning agents. The data show that, the thermal
stability of CAC retanned leather was increased with increasing CAC concentration and
the optimum shrinkage temperature was done at 4-5% CAC, while in case of aluminum
retanned leather, the optimum shrinkage temperature was obtained at 6-8% basic
aluminum sulfate.
Fig 8 shows the effect of CAC oligomer during chrome tanning of the pickled hide on
both thermal stability and chrome exhaustion percentage. The curves illustrate that, both
the shrinkage temperature and chrome exhaustion were gradually decreased with
increasing CAC concentration and the optimum CAC concentration is 1%. This may be
due to the highly acidic medium of the chrome tanning bath.