INTRODUCTION:

Polymers prepared from renewable inexpensive natural resources such as carbohydrates, starch, proteins and even fats and oil have become increasingly interesting because of their low cost, ready availability and possible biodegadrability^1,2. Groundnut oil is a biodegradable triglyceride with unsaturation. Its polyunsaturation is not only beneficial for health, but it also makes it to polymerise the oil into useful solid materials³. In recent years, biodegradable polymers have received considerable attention because of large number of mass production application in packaging, paper coating, fibers, films and other disposable articles, as well as in biomedical application such as resorbable surgical sutures, implants and controlled drug delivery systems^4-8.

Pressure sensitive adhesives are visco-elastic materials with flow properties playing a key role in the wetting and debonding process⁹. The balance of these properties governs their time temperature dependent responses and adhesion strength¹⁰. Adhesive performance of pressure sensitive adhesives may be described in terms of both bonding and debonding.

Test methods have been developed to characterize the bonding properties such as tack tests and debonding properties such as peel and shear tests¹¹. Pressure sensitive adhesives present an interesting class of products that offers a potential to produce a novel generation of self adhesives with a great number of excellent properties. The main advantage of pressure sensitive adhesives compared with other types of adhesive are those of convenience of use. There is no storage problem; often the bond is readily reversible. They adhere by simply applying pressure to a huge variety of materials. These can be used in the production of single and double sided self adhesive tapes, foil labels, carrier free tapes, self adhesive bioelectrodes, and decorative PVS signs and making film. The latest application of water soluble acrylic pressure sensitive adhesives is in the field of medical products, where neutral electrodes and adhesive tape for securing operating theatre sheets are of special important¹².

A pressure sensitive adhesive requires a balance of cohesive strength and visco elastic properties. These characteristics are based on three parameters tack, adhesion and cohesion¹³. Test methods have been developed to characterize the debonding properties such as peel and shear tests¹¹. Tack is the dominant property of PSAS and is defined by ASTM as the ability of an adhesive to form a bond of measurable strength to another surface under conditions of low contact pressure and short contact time. This paper deals with the studies of the preparation and development studies of pressure sensitive adhesive from acrylated epoxy resin using the comonomer butylmethacrylate. The different materials were characterized by physical parameters, spectral methods, adhesive properties, bacterial adhesion studies and antimicrobial activities.

Fig.1: Synthesis of acrylated epoxidised Groundnut Oil

Fig.2. UV Spectra of (a) Groundnut oil (b) Epoxy resin (c) Acrylated epoxy resin

Fig.3. IR Spectra of a) Groundnut oil b) Epoxy resin c) Acrylated epoxy resin

Fig.4. NMR Spectra of a) Groundnut oil b) Epoxy resin

c) Acrylated epoxy resin

Fig.5(a) Shearing time against the weight of the adhesive resin at different curing time

EXPERIMENTAL:

Materials:

The oil used in this study was food grade (Gold winner) refined groundnut oil commercially available in supermarkets. Epoxidised groundnut oil was prepared by epoxidation using 30% hydrogen peroxide (AR Rankhem). Acrylic acid (E.Merck India) was used for acrylating the epoxy resin. The catalyst used was triethylamine (E.Merck India). Butylmethacrylate purchased from Sigma Aldrich was used as the comonomer.

Analysis of groundnut oil:

Groundnut oil was subjected to extensive analysis such as determination of saponification value, iodine value and specific gravity. The epoxy content of the epoxidised groundnut oil was determined.

Table 1: Analytical data

Properties	Groundnut Oil	Epoxy resin	Acrylated epoxyresin
Specific gravity (gm/c.c) at 30^oC	0.99	1.21	0.64
Saponification value	187.4	142.38	121.17
Iodine value	102.3	4.86	36.79
Epoxy Content	--	0.282	0.0243

Synthesis of acrylated epoxidised groundnut oil:

Epoxidised groundnut oil was prepared using hydrogen peroxide, glacial acetic acid and 49% sulphuric acid¹⁴. Groundnut oil was weighed into a three necked flask fitted with a Leibig’s condenser and ground joint thermometer. Groundnut oil was mixed with glacial acetic acid and 49% sulphuric acid. Hydrogen peroxide was added drop by drop through a dropping funnel for about 2 h. The reaction mixture was refluxed with constant stirring for about 10-12 h at 70-80^oC. The epoxised resin was separated from the unreacted glacial acetic acid by washing with warm water. There are considerable side reactions (via) oxirane ring opening, leading to diols, hydroxy esters and other dimers which are believed to be catalyzed by the presence of a strong mineral acid¹⁵.

The epoxidised groundnut oil was then acrylated using acrylic acid. Triethylamine was used as catalyst and benzene as the solvent. The reaction mixture was again refluxed for about 3-5 h at 80º-100^oC. The iodine value, saponification value, specific gravity and viscosity of groundnut oil, epoxy resin, acrylated epoxy resin were determined according to the ISI standard 840-1964. The epoxy content of epoxy resin was determined as per the ASTM standard D1652-04. It was also analysed by ultraviolet spectral analysis (0.05% hexane solution). IR spectra (KBr pellet method) were taken in a Perkin Elmer FT-IR spectrometer. ¹H NMR spectra were recorded in CDCl₃ using FT-NMR spectrometer (brukker) at 300 M Hz.

Synthesis of PSA by adding butylmethacrylate:

The pressure sensitive adhesive was prepared by reacting acrylated epoxy resin with the comonomer butylmethacrylate (BMA). Different samples were prepared based on the different concentration of butyl methacrylate (ie) 100%, 80%, 60% and 40% BMA and labelled as PSA-5, PSA-6, PSA-7 and PSA-8 respectively. The adhesion properties are assessed by shear strength and peel strength. The shear strength was determined as per ASTM standard D1002 and peel strength as per ASTM standard D903-78.

Table 2: Peel Strength of pressure sensitive adhesive tapes coated on polyethylene film substrate glass.

Sample Code	Speed (inch/min)	Peel Strength (gm)
PSA-5	50	480.6
	100	572.5
	150	592.5
	200	410.8
	250	308.4
PSA-6	50	442.8
	100	505.4
	150	485.7
	200	420.5
	250	250.7
PSA-7	50	375.5
	100	427.2
	150	442.3
	200	366.4
	250	200.8
PSA-8	50	342.7
	100	415.3
	150	383.6
	200	204.4
	250	155.6

Analysis:

Shear strength:

Tests were done with aluminium foils and sheets as a substrate material¹⁶. The shear strength was measured by coating 0.5, 0.75 and 1.00 g of the adhesive resin on a steel tape of known area (3 inch) and cured in the oven for about 1hour at 100 ºC. After curing a weight (0.05 kg) is hung on a tape bonded over a known area to a steel test plate (4”×1”). The time taken for the tape to move a known distance was taken as the indication of shear strength. The test was carried out at room temperature for all samples¹⁷.

Peel strength:

The peel strength was measured by coating the adhesive on a thin polyethylene film having thickness of 5”×1”. The adhesive film was cured in the oven for about 1 h at 100ºC. The film was then pasted on the glass plate (6”×2”). Common peel tests include the T-peel test, the 180^opeel test and 90^o peel test^18,
19. The peel strength of the pressure sensitive adhesive was determined at a separation angle of 180º using a bioadhesive tester AR-1000.

Biodegradability:

Biodegradability was investigated by aging in saline solution. 1 g of the sample was kept in 50 ml normal saline solution. The degree of absorption of saline solution was noted for a month with the duration of 5 days. The degree of weight loss was noted for about two months with the duration of 10 days.

The degree of biodegradation was estimated from the weight loss analysis of samples as

Weight loss (%) = {(W₀-W₁)/W₀} x 100

Where W₀is the weight of the adhesive taken and W₁is the weight of the residual adhesive after degradation at different intervals of time²⁰.

Table 3: Viable count/sample for PSA.

Sample		Number of bacteria adhered per sample
Sample		*E.coli* ATCC 25922	*S.aureus* ATCC 25923
PSA-5	Unit 1	2.31 x 10⁶ cfu	1.26 x 10⁶ cfu
	Unit 2	2.72 x 10⁶ cfu	1.92 x 10⁶ cfu
	Unit 3	2.24 x 10⁶ cfu	2.1 x 10⁶ cfu

Fig.5(b) Shearing time against curing time

Fig.6 Peel Strenth of pressure sensitive adhesive tapes

Bacterial studies:

Bacterial adhesion study and antimicrobial activity test were done. 0.1% gentamycin was added to the adhesive during curing process. For bacterial adhesion study samples of 1 cm square pieces were used and for antimicrobial activity spherical disc of 10 mm diameter were used. Both test samples were sterilized by autoclaving before performing the test and finished by agar diffusion method. The tests were done in triplicates. Gentamycin (10µg/disc) of positive control was used for antimicrobial activity testing.

RESULTS AND DISCUSSION:

The synthesis of acrylated epoxidised groundnut oil was given in Fig.1. The structure of groundnut oil from the triglyceride molecule, the major component of natural oils²¹.

Table 4: Zone of inhibition

Zone of inhibition
Sample code	Units	*E.coli* ATCC 25922	*S.aureus* ATCC 25923
PSA-8	Unit 1	Nil	5 mm
	Unit 2	Nil	6 mm
	Unit 3	Nil	6 mm
Gentamicin – 10 mcg		19mm	25mm

Fig.7(a) Effect of saline solution on the percentage of absorption

Fig.7(b) Effect of saline solution on the percentage of weight loss

Analysis of groundnut oil, epoxy resin and acrylated epoxy resin was carried out and data are presented in table 1.

The iodine value represents the degree of unsaturation. The high iodine value of groundnut oil indicates the high degree of unsaturated component in the parent oil. In the epoxy resin the iodine value is decreased showing that the double bond is replaced by the epoxy group. The epoxy content was found to be 0.282.

The iodine value is increased significally in the acrylated epoxy resin due to the unsaturated acrylated side chain. Accordingly epoxy content is reduced 0.0243 in the acrylated epoxy resin.

Saponification number of fat or oil is used for the determination of the size, average molecular weight of the fatty acids and to estimate the non-fatty impurities if present. The saponification value reveals that the molecular weight of the epoxy resin and acrylated resin has increased compared to the oil.

Spectroscopic characterization:

Ultraviolet Spectroscopy:

The UV spectrum was shown in Fig.2. Groundnut oil sample shows an electronic absorption band at 224 nm²². Epoxy resin exhibited peak at 220 nm. In acrylated epoxy resin the peak again shifts to 230 nm. The substantial red shift in electronic absorption exihibited in acrylated samples would indicate the presence of a conjugated double bond band (K-band) in the fatty acid molecule²³.

Infrared Spectroscopy:

IR Spectral analysis of groundnut oil, epoxy resin and acrylated epoxy resin was given in fig.3. A change in quality and the extent of deterioration of the oil samples were observed spectroscopically. The room temperature infrared data was determined and reported previously by Obaleye and Orjiekwe^24,25. The groundnut oil sample shows a very strong and sharp band at 1740 cm^-¹ due to the ester carbonyl group. The epoxy group shows three characteristic absorption bands. The epoxy group gave the bands at 1250 cm^-1 and 950 cm^-1 because of the symmetric and asymmetric ring stretching²⁶. The region of the first band is small and it lies almost at about 1240-1250 cm^-¹.The peak at 1240 cm^-¹ correspond to the epoxy group (fig 3 a) .The particular peak is disappeared in the acrylated epoxy resin . The region of other two bands are broader, the portion of the maximum depending on the structure of the epoxide .These absorption bands appear between 950 and 860 cm^-¹ and between 865 and 785 cm^-¹. These particular bands obtained at 908 cm^-¹ and 722 cm^-¹ in the epoxy resin correspond due to the stretching vibration of -CH- and -CH2- groups. Bands in the far IR region are reported by Hummel²⁷. This particular band in the far IR region is observed at 587 cm^-¹ in the epoxy resin but absent in the oil as well as in the acrylated epoxy resin. The epoxy resin shows the carbonyl band of triglyceride ester group at 1740 cm^-¹ and in the acrylated epoxy resin at 1725 cm^-¹. The weak band near 1370 cm^-¹ in groundnut oil and epoxy resin belong to the vibration of the -OH of the carboxylic group.

NMR Spectra:

Although less specific parameters such as iodine value, which reflects the total amount of unsaturated fatty acids in an oil or fat can also be determined by¹H NMR²⁸. The ¹H NMR spectum was shown in Fig.4. In the ¹H NMR spectra of groundnut oil the peak at 5.3 ppm corresponds the unsaturated groups (i.e) vinylic hydrogen. This particular peak is disappeared in the epoxy product but shifted in the acrylated epoxy and have moved down field 5.7-6.3 ppm upon conjugation. In groundnut oil peak at 4.1-4.3 ppm originate from the proton in the methylene group of the triglycerides²⁹. The peak at 2.75 ppm which corresponds to the proton in the CH₂groups between two carbon-carbon double bonds completely disappears in the epoxy product. The strong peak at 1.3 ppm is attributed to the long chain (more than five methylene groups) of the side chain³⁰. The peak at 0.86 to 2.3 ppm is due to the long aliphatic side chain.

Shear test:

Shear test is one of the test to characterize the adhesion properties of tapes for the assessment of shear strength. Shear test gives the ability of a tape joint to resist a load applied in the shear mode. The shear strength was determined for the acrylated epoxy resin crosslinked with different percentages of butyl methacrylate. The relation between shearing time against the weight of the adhesive was given in Fig. 5a. The relation between shearing time and curing time was given in fig. 5b.

PSA-5 has higher shear strength compared to the other composition. There is only a slight increase in shearing time when the weight of the adhesive resin is increased. It is also noted that as the curing time increases, shearing time also increases to a large extent.

Peel test:

Peel adhesion is one of the characteristics of pressure sensitive adhesives. This is evaluated by measuring the tensional force required to remove the adhesive tape. The peel strength depends on various factors such as visco elastic properties of the adhesive, stiffness of the adherend rate of separation, temperature etc. The response of the pressure sensitive adhesive to stress is of a visco elastic nature. At cohesive failure, the failure is due to predominant liquid like viscous response. At the adhesive failure the response is predominantly rubber like elastic. The characteristics of good PSA for biomedical application are

(i) Options of skin irritating ingredients.

(ii) Good water resistance so that the bond is maintained when skin perspires.

(iii) High cohesive strength to allow clean removal of the tape from the skin.

(iv) High water vapor transmission rate to decrease the occlusitivity of the tape.

(v) Rheological properties to accommodate the skin movement without losing the bond and without excessive skin irritation mechanically.

The high peel strength in the cohesive failure region is important in the performance of PSA tape. Different speeds were chosen because the difference in peel force in the fast and slow peel test has shown to depend on the cross linking density and can help to characterize the adhesive¹⁶. The peel strength determined for the present acrylated epoxy resin crossed linked with different percentages of butyl methacrylate was given in table 2 and the relation between peel strength (gm/cm) on the peel rate (inch/min) was given in Fig.6.

Peel forces are often measured at one standard peel rate and the adhesive performance is inferred from these results³¹. PSA-5 gives higher peel strength (480.6 gm/cm) at 50 peel rate in comparison to all other composition. At lower peel rate both samples PSA-5 and PSA-8 exhibit cohesive failure at the peel of 150 inch/min on the other hand. Both PSA-6 and PSA-8 exhibit cohesive failure at the peel rate of 100 inch/min. Both PSA-5 and PSA-7 exhibit the characters of a soft adhesive which would split and removes and leave a residue. On the other hand, PSA-6, the adhesive shows a cohesive failure at low peel rate which ensure that the adhesive will fail only when its maximum elongation is reached.

So this adhesive will not easily debond when in use. At higher peel rate, this adhesive shows an adhesive failure and therefore will come off clearly when removed. The failure of all these samples suggest the transition from cohesion to adhesive failure suggesting that the acrylated epoxy resin has copolymerised with butylmethacrylate to a high molecular weight.

Degradation studies:

Invitro degradation of the polymers was investigated by monitoring the change in the weight loss during degradation³². Inorder to study the biodegradation of the adhesive hydrolytic degradation in saline water was carried out. The effect of saline water on the copolymerised acrylated epoxy resin was evaluated. The percentage of absorbtion and weight loss increases with time was given in Fig 7. The degradation of the adhesive was a slow process and negligible weight loss was observed for short time interval.

Adhesion studies:

In bacterial adhesion study the gram negative bacteria E.coli ATCC 25922 adhered to the test material at the rate of 2.42 x 10⁶ cfu/sample in PSA-5 and 2.26 x 10⁶ cfu/sample in PSA-8. The viable bacteria showed no change in colony morphology. The gram positive S.aureus ATCC 25923 adhered on the test material at the rate of 1.79 x 10⁶ cfu/sample in PSA-5 and 1.46x10⁶ cfu/sample in PSA-8. The viable bacteria showed a reduction in colony size indicating some inhibition to normal growth. The bacterial adhesion in both species was maximum in PSA-5 and minimum in PSA-8. The bacterial adhesion study shows that the pressure sensitive adhesive adhere bacteria to a large extent and so can serve as a bacteriostat to stop bacteria from reproducing, while not necessarily harming them³³. A detail of viable count /sample in triplicates was given in table 3.

Antimicrobial studies:

In antimicrobial activity testing, the test material does not contain any antimicrobial activity against gram positive S.aureus ATCC 25923 and gram negative E.coli ATCC 25955 under the test conditions for PSA-5, PSA-6 and PSA-7. But in PSA-8, the test material exhibited a little antimicrobial activity against gram positive S.aureus ATCC 25923 and the zone of inhibition varied from 5mm -6mm in size inclusive of the material. But the test material does not exhibit any antimicrobial effect against gram negative . E Coli ATCC 25922 and under test conditions.As the concentration of the comonomer BMA was decreased the test material exhibits antimicrobial activities. Zone of inhibition in antimicrobial activity testing was given in table 4. Photographs of culture plates was given in Fig.8.

CONCLUSION:

The adhesive properties of the pressure sensitive adhesive were carried out. Degradation studies shows that the pressure sensitive adhesive was biodegradable. Pressure sensitive adhesive in PSA8 formulation having a very low concentration of butylmethacrylate exhibit small antimicrobial activity against gram positive S.aureus ATCC 25923 and gram negative E.coli ATCC 25922 was negative. Comparing the peel strength of present formulation, PSA-6 may be suitable for non-occulusive PSA tapes. Non-occulusive PSA tapes are much useful to decrease the level of skin irritation considerably and also minimize the incidence of bacterial infection. Non-occulusive PSA tapes allows the migration and transmission of moisture. Bacterial adhesion studies show that the pressure sensitive adhesive adhere bacteria to a large extent and hence can serve as a bacteriostatic. Bacteriostats are often used in plastics to prevent growth of bacteria on the plastic surface. This is in contrast to bactericidies which kill bacteria.

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Received on 03.08.2009 Modified on 06.10.2009

Asian J. Research Chem. 3(1): Jan.-Mar. 2010; Page 40-46

Development Studies of Biodegradable Pressure Sensitive Adhesives from Groundnut Oil and Butylmethacrylate

Fig.1: Synthesis of acrylated epoxidised Groundnut Oil

Properties

Sample Code

PSA-5

Sample

Number of bacteria adhered per sample

PSA-5

Fig.6 Peel Strenth of pressure sensitive adhesive tapes

Sample code

PSA-8

Gentamicin – 10 mcg

NMR Spectra:

REFERENCES: