ijct published 2015

Indian Journal of Chemical Technology Vol. 22, Jan-Mar 2015, pp. 48-55

Synthesis and application of formaldehyde free melamine glutaraldehyde

amino resin as an effective retanning agent

Rashid Saleem1, Ahmad Adnan1 & Fahim Ashraf Qureshi2*

1Department of Chemistry, GC University, Katchery Road, Lahore 54000, Pakistan 2Office of Research, Innovation and Commercialization, Comsats Institute of Information Technology,

ChakShahzad Campus, Park Road, Islamabad 45600, Pakistan.

E-mail: [email protected]

Received 12 June 2013; accepted 19 June 2014

Novel melamine based free formaldehyde resin using glutaraldehyde as a condensing agent rather than formaldehyde has been synthesized under optimum conditions for use as a retanning agent. Characteristics and effects of the polymer as a retanning agent have been investigated against conventional melamine formaldehyde resin. Tear strength, tensile strength, elongation at break, and scanning electron microscopy (SEM) of experimental retanned leather have been studied in comparison with commercial melamine formaldehyde retanned leather and are found to be in better performance. Effluent emission of both retanning baths have been evaluated and found to contain less effluent load in experimental bath, thus less impact on the environment. Glutaraldehyde alone affects dying process and produces problem in leveling of shade. In this study, dispersing and leveled dying property of glutaraldehyde have also been improved after condensing with melamine. Both experimental and conventional melamine resins have shown good dispersing and leveling property in dying process of retanned leather. Structural elucidation of the experimental resin has been carried out by FTIR technique.

Keywords: Glutaraldehyde, Melamine, Retanning, Scanning Electron Microscopy

Recently, leather production with ECO friendly labels

has gained importance due to growing demands

from customers. Among harmful substances, free formaldehyde content is the leading one.

Formaldehyde has been enlisted in carcinogens

category 3 by European Union1,2.

Formaldehyde is used in the production of syntans which causes

the finished leather to contain free formaldehyde3.

This has major constraints on such products for use among global consumers

4,5. Syntans are

manufactured primarily by condensing formalin with

naphthalene, phenol, dicyandiamide and melamine to form polymerized condensed products having

retanning properties for different types of leather6,7

.

At present, tanners have a technical challenge to produce leather of high quality, meeting ECO

standards from skins of low quality and low grade8.

Thus, retanning, dyeing and fatliquoring require selective chemicals with specific pH. However,

the choice of improper chemical combination

with respect to syntans produces a differential pH across the skin making improper filling of the

collagen fibers9. Protein hydrolysates and different

combinations of tanning agents have been worked out in retanning as fillers to open new perspectives

10,11.

Ecolabelling concepts have created awareness to

produce formaldehyde free leathers. For this purpose

several investigations have been carried out to produce leather with desired properties using syntans

based on protein hydrolysates and vegetable tannins12

.

In the present work, formaldehyde was replaced with glutaraldehyde, an industrially available

aldehyde used as protein crosslinking agent and

disinfecting agent13

, so that melamine-glutaraldehyde resin was obtained. Glutaraldehyde is relatively less

harmful than formaldehyde. LD50 value (oral, rat) for

glutaraldehyde is 1470 mg/kg14

, while for formaldehyde is 100 mg/kg

15.

Glutaraldehyde has unique properties that make

it most effective protein crosslinking agent16

. Glutaraldehyde has been proposed as environmental

friendly replacement of chrome tanning to minimize

the environmental effects17

of chrome tanning. Leather produced by oxazolidine has shrinkage

temperature similar to that of glutaraldehyde but

is less hydrophilic and less full, because of low molecular weight of oxazolidine than glutaraldehyde

in polymerized form18

. Its usage is growing due to

decline in the formaldehyde use. Glutaraldehyde tanned leather is hydrophilic and “plumpy” as tanned

SALEEM et al.: SYNTHESIS OF FORMALDEHYDE FREE MELAMINE GLUTARALDEHYDE RESIN

49

with formaldehyde. However, the leather color is

yellow cast, which turns into orange. The orange

color causes problem in obtaining desired shade of leathers

19. There are various available aldehydes

with mono and multifunctionalities, which may be

utilized for tanning, however, only glutaraldehyde and its various derivatives have commercial acceptance

20.

In our study, glutaraldehyde has been condensed

with melamine and sulfonated with sodium sulfamate to produce a stabilized water soluble resin that

imparts leather with very little color and has

no disturbance in dying, and also assist in leveling of dying as well.

Experimental Section

Chemicals and apparatus

Melamine (99.8% purity, powder) was taken

from Royal DSM and was processed as received.

Technical grade glutaraldehyde (50% w/w) was used without purification. Commercial sulfonated

melamine formaldehyde resin and sulfamic acid of

technical grade (powder, 99.8%) was also processed

as received. Commercial Pakistani wet blue of raw buffalo

hides were used for this study and were received

from Siddique Leather Works. Commercial grade chemicals were used for leather processing and

analytical grade chemicals were used for spent

liquors. Viscosity was determined by Brookfield

viscometer LV DVE 230 at 25°C. FT-IR spectrum of the resins was recorded by Bruker IFS 48.

Preparation of sulfonated melamine glutaraldehyde based resin

Melamine reacts with glutaraldehyde very

rapidly and forms a crosslinked polymer that has nearly zero water solubility. The polymer was

synthesized by condensation of amino group of

melamine with aldehyde group of glutaraldehyde in a basic medium. Various reaction parameters were

investigated for optimization of the required polymer

reaction. Primary product of the reaction is

methylolated melamine that is converted to polymer by further condensation. By further condensation,

the resin converts into a crosslinked insoluble resin.

The polymer was modified chemically by reaction with sodium sulfamate which acts as a sulfonating

agent to form a soluble product, like sulfonated

melamine formaldehyde condensate21

. Optimum

conditions were worked out to synthesize the stable retanning resin.

In a three necked flask fitted with condenser, stirrer

and thermometer, 105 g water (5.83 moles), 118.92 g

sulfamic acid (1.22 moles), 97.60 g 50% strength sodium hydroxide (1.22 moles) were mixed to form

sodium sulfamate. An amount of 51.44g melamine

(0.40 mole) was added and heated to 45°C. Afterward, 326.63 g 50% strength of glutaraldehyde (1.63 moles)

was added and temperature was raised to 85±2°C to obtain a clear resin solution. The reaction temperature

was maintained for ten minutes and then cooled to

60°C for further condensation of the resin at 60°C for 30 min. The reaction mixture was allowed to cool to

room temperature after 30 min. The solid content of

the resin was about 45±1%. The resin, free from

formaldehyde, was spray dried to obtain a powder form that was used in all leather retanning experiment.

Schematic route for the synthesis of sulfonated

melamine glutaraldehyde resin is given in Fig. 1.

Characterization of resin

Estimation of solid content

Solid content of liquid resin was determined by

weighing known quantity of the resin in an empty petri

dish and drying at 103-105°C for one hour as per standard procedure

22. Solid contents of the product were

calculated on dried weight basis and was found 45±1%.

Fig. 1—Schematic route for the synthesis of sulfonated melamine glutaraldehyde resin

INDIAN J. CHEM. TECHNOL., JAN-MAR 2015

50

Viscosity determination

Viscosity of liquid resin was determined by

Brookfield viscometer LV DVE 230 at 25°C and was found 58 cp.

Evaluation of the product as retanning agent

Retanning properties of sulfonated melamine

glutaraldehyde condensate were assessed by comparison against leather developed by commercial

melamine formaldehyde based retanning agent.

For comparative post tanning application of control and experimental resins, two similar buffalo wet blue

swatches were processed separately in comparison

rotating drums as below.

Chemicals were taken on the basis of shaved weight of hide in post tanning application. Two hides,

each of 125 g were washed with cold water for

fifteen minutes in comparison rotating drums, followed by addition of 187.5 g of water, 1.875 g of

sodium formate and 1.25 g of sodium bicarbonate to

neutralize the hides upto pH 5-5.2. After ninety minutes mixing, water was drained out and additional

water (250 g) was added in the drum for washing

and drained after fifteen minutes. Water (125 g) was

added for retanning, dying and fatliquoring process. Melamine glutaraldehyde based amino resin (12.5 g)

and commercial melamine formaldehyde resin

(12.5 g) were added in retanning comparative drums and mixed for forty five minutes. Synthetic fatliquor

(5 g) was added in retanning drums and mixed

for sixty minutes. Acid dye (4 g) was added and

mixed for thirty minutes. To adjust pH upto 3.8, formic acid (1.875) was added slowly in

one hour. Water was drained off after complete

exhaustion of bath. The leather swatches were washed with water and hooked to dry. They were conditioned

and staked.

Physical testing and hand evaluation of leathers

Samples for physical testing were obtained from control and experimental leather as per standard

IUP method23

. Samples were conditioned 80±4F

temperature and 65±2% of relative humidity for 48 hours period. Tensile strength and percentage

elongation at break of retanned leathers were

performed by Tensile testing machine (STM 566F)

by standard procedure24

. Tear strength was performed by tear testing

machine (STM 566ST) by standard procedure25

and grain strength was evaluated by lastometer as per standard procedure

26. Assessment for softness,

fullness, roundness, grain tightness, and dye leveling

properties of control and experimental leathers

were made by hand and visual examinations.

Rating of leathers for each functional property was experienced by three persons on a scale of 0-5 points,

where higher point indicates better property.

Analysis of spent liquor

Spent liquors of post tanning from experimental

and control trials were analyzed for Total solids

(drying at 103-105°C for 1.5 h) and Chemical oxygen demand (COD) as per standard procedure

27. Emission

loads per metric ton of processed wet blue of buffalo

hides were estimated by multiplying the concentration

(mg/L) with total volume of effluent (L). Free Formaldehyde analysis in leather

Free formaldehyde content was determined from the leather swatches by standard procedure

28.

The standard procedure is specific for the

determination of released and free formaldehyde in leathers. The method is primarily based on

colorimetric analysis. Reflectance measurements

The basic principle is measuring the amount of reflected light from opaque specimen surface

at wavelengths of visible spectrum as a fraction

of reflected light by white standard illuminated

identically. This is called reflectance factor. White standard is perfect reflecting diffuser that

shows 100% reflectance at every wavelength.

Reflectance measurement of Specimens of control and experimental leathers were determined by

Spectraflash SF 550 (Data color). Colour measurements

Parameters for colour measurement such as L, a, b

for control and experimental dyed crust leathers were measured using Spectraflash SF 550 (Data color). ∆L,

is lightness difference; ∆a and ∆b, shows difference in

a and b values, whereas a represents red and green axis, and b is representing yellow and blue axis.

∆L, ∆a, ∆b and ∆C are calculated by subtracting

corresponding values of experimental leather from the control leather. Scanning electron microscopic analysis

Samples from control and experimental dye

crust leathers were taken from the standard position

of sampling23

. Specimens of leather were cut with uniform thickness and washed with acetone.

They were coated with 300oA thickness of gold


51

using Ion sputtering device, Model JFC 1500, Jeol

Japan. A Jeol JSM 6490 analytical scanning electron

Microscope embedded with Energy dispersive X-ray analyzer was used for analysis. Micrographs

of grain and cross section of fibers were obtained

by operating SEM at high vacuum and voltage of

15 KV with higher magnification levels. Light Fastness

Resistance of color of experimental and control

dyes crust leathers to an artificial light, Xenon

arc lamp, was determined by using standard test procedure

29. Specimens of dyed crust leathers

of experimental and control leathers were exposed

to light under xenon arc lamp along with blue

wool cloths as a standard. Assessment of fastness was carried out by comparing fading of dyed

crust leather with that of standard and rating of 1-4 is

given, where 1 represents very low light fastness and 4 represents very high light fastness.

Results and Discussion Melamine based amino resin was synthesized

using glutaraldehyde as a condensing agent in

replacement of formaldehyde. The required solubility

was achieved through sulfonation by sodium sulfamate. The synthesized resin was water miscible

like commercial melamine formaldehyde resin.

The pH of solution at 10% concentration was 7.85. As there were no such functionalities in the

synthesized resin that could be oxidized under

light so colour of dyed leather did not changed due

to good light fastness. The particular advantage of glutaraldehyde modified resin was the absence

of formaldehyde which is considered health hazard

and carcinogen. Organoleptic properties

Organoleptic properties such as fullness and

softness of leather fibers, roundness and tightness

of leather grain, and colour uniformity after dying for control and experimental crust leathers were

comparatively visually evaluated. An average rating

to each functional property of the experiment was given in Fig. 2. Better property was expressed

by higher number. Fullness, grain tightness and

softness of experimental retanned leather was higher than control melamine formaldehyde retanned

leather where as roundness and color uniformity

of retanned leathers after dying were comparable in

control and experimental leathers.

Physical characteristics of leathers

Tear and tensile strength of dyed crust leathers

were performed both along and perpendicular to backbone line. Resulting values for each side corresponding to along and perpendicular to backbone and given in Table 1. Grain crack strengths for all dyed crust leathers were carried out. Mean values corresponding to every experiment was averaged and results are given in Table 1. The results are showing that all experimental results in leather have comparable tensile strengths, % elongation at break, tear strength and grain cracking with that of control leathers. Increase in tensile strength and tear strength of experimental resin is due to strong compositing

effect of non-formaldehyde melamine resin with collagen fibers of the leather. A higher value of % elongation of non-formaldehyde retanned leather is due to more flexibility character of melamine glutaraldehyde condensate as compared to melamine formaldehyde resin.

Free formaldehyde analysis in leather

Experimental and control retanned leathers have

been evaluated for free formaldehyde by using standard procedure and results have been given in

Table 1. There was no detectable free formaldehyde

in experimental retanned leather; while control retanned leather contained free formaldehyde at the

rate of about 145 mg/kg. Experimental retanned

leather showed no detectable free formaldehyde

because it was synthesized without formaldehyde.

Fig.2—Organoleptic properties of leathers retanned with melamine glutaraldehyde resin and commercial melamine formaldehyde resin


52

Table 1—Physico chemical characteristics for leathers retanned with non formaldehyde and commercial

melamine formaldehyde based retanning agents

Leather made by using the products Physicochemical properties

Non formaldehyde melamine resin

Commercial melamine formaldehyde resin

Tear strength (N/cm) Parallel to backbone 500 496

Tear Strength (N/cm) Perpendicular to backbone 675 630

Distension at grain cracking (mm) 7.75 7.35

Distension at Burst (mm) 11.25 10.75

Tensile strength (N/cm2) parallel to backbone 1920 1420

% Elongation Parallel to backbone 50 50

Tensile strength (N/cm2) perpendicular to backbone 1720 1514

% Elongation perpendicular to backbone 45 41

Free formaldehyde content N.D 145

Light fastness 2.5 2.5

Table 2—Characteristics of waste water for commercial melamine and nonformaldehyde melamine retanned leather

Parameters Non formaldehyde melamine retanning

Commercial melamine formaldehyde retanning

Chemical Oxygen Demand (ppm) 13610 15320

Total solids (ppm) 18555 20678

Volume of effluent (L/ton of shaved hide) 1385 1385

COD based emission load (kg/ton of shaved hide) 18.84 21.21

Total solids based emission load (kg/ton of shaved hide) 25.69 28.63

Spent liquor analysis

Liquid effluent generation has been one of the major problems of the leather tanning industry.

These effluents contain large amounts of organic

matter, chlorides and sulfates. The resulting waste water of tannery has high salinity which cannot be

easily corrected. With evolving of industry in last

few decades, there has also been a growing awareness

of need to keep environment safe. This has been promoted by enforcement of legislations, which have

been progressively restrictive to control the wastes

and their disposal30

.

The spent liquors from experimental and control processes were collected. Total solids (TS) and

chemical oxygen demand (COD) are two parameters,

which were chosen to analyze the environmental

impact. Observed value of total solids and chemical oxygen demand may not give direct correlation with

environmental consequences. So their values have

been converted into emission loads. Values for total solids and chemical oxygen demand, and calculated

emission loads are given in Table 2. It has been observed that reduction in TS and COD load has been

obtained in formaldehyde free melamine based

retanned leather.

Scanning electron microscopic analysis

Fullness of retanned leathers can be evaluated

by viewing the grain surface and cross section of retanned leather fibers using scanning electron

microscopy. Micrographs of retanned leathers

showing grain and cross section are given in

Fig. 3. The experimental and controlled retanned leathers show comparable compactness in fiber

structure throughout cross section representing

uniform filling of both retanning agents. Formaldehyde free melamine based retanned leather

is showing more compactness.

Colour difference studies

Colour measurement values for experimental and

control retanned leathers are given in Table 3.

Experimental leathers show negative value of ∆L


53

Fig. 3—Scanning electron micrographs of grain (X50) and cross section of fiber structure (X500). 3(a) Grain surface of experimental leather; 3(b) Grain surface of control leather; 3(c) Fiber cross section of experimental leather; 3(d) Fiber cross section of control leather

Table 3—Colour difference measurements of leathers

Commercial melamine formaldehyde based retanned leather

Illuminant L a b

D65 73.59 -0.2 29.93

Non formaldehyde melamine based retanned leather

Illuminant L a b ∆L ∆a ∆b

D65 70.26 1.37 36.08 -3.33 1.57 6.15

Distinction of experimental leather Darker Red Yellow

which is showing for darker in shade. Experimental retanned leather has over all colour difference value

of 3.33 in comparison to control leather expressing

increase of shade strength between experiment and control leather. Both retanned leathers have uniform

shade of dye which clearly shows equal dispersing

and leveling property of resins.

Structural elucidation

Structure of powder resin was characterized

by Infra red spectrum as given in Fig. 4 using

DRS accessories 8000 by diluting with KBr in range of 4000-500 cm

-1. A broad band at 3359.84 cm

-1

is attributed to NH and OH bonds of amine

and alcohols. Two signal at 2942.67 cm-1

and 2868.22 cm

-1 show antisymmetric and symmetric

vibrations of methylene group. Peak at 1566.07 cm-1

indicate carbonyl functionality of the resin.

Peak at 1409.87 cm-1

present the scissoring vibrations of the methylene group. Absorption at

1194.41cm-1

indicates stretching vibration of the

C-S and S=O functionalities of R-SO3- group in the resin. Sharp absorption at 814.16 cm

-1

shows deformative vibrations for 1, 3, 5 triazine

ring.


54

Fig. 4—FTIR spectrum of melamine glutaraldehyde condensate

Conclusion

Environmental regulations regarding formaldehyde

are not mostly met by formaldehyde based resins even when formaldehyde is used in minimum

concentration. Currently environmental legislations

require eliminating such products from leather

making process. In present work, it has been possible to completely replace formaldehyde in the

synthesis of melamine resin as a retanning agent.

The condensation of melamine is made with glutaraldehyde which is stabilized by sulfonation

through sodium sulfamate under optimum conditions.

There is no detectable free formaldehyde in experimental retanned leather in contrast to control

leather. Melamine formaldehyde type retanning

agents can be completely replaced by this product as

observed from physicochemical properties of retanned leathers. Tensile and tear strengths of experimental

retanned leather are better than control. Experimental

retanned leather is darker in colour in comparison to control leather as shown by colour difference

measurement, which is also in agreement, assessed by

visually. Glutaraldehyde alone affects dying of leather and produce uneven shade on the leather, but after

condensing with melamine, dispersing and leveling

property of glutaraldehyde based melamine resin

has been improved just like conventional melamine formaldehyde resin. In specific, experimental

retanned leather processed with non formaldehyde

melamine based retanning agent possesses better

performance in properties than control retanned

leather.

Acknowledgements

The research was supported by Pakistan Higher Education Commission, Government of Pakistan and

the support is gratefully acknowledged. The authors

also acknowledge Shafi Reso Chemicals, Private Limited for providing laboratory facilities to study

application performance of formaldehyde free

melamine resin.

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