International Journal of Adhesion & Adhesives 31 (2011) 735–742

Contents lists available at ScienceDirect

International Journal of Adhesion & Adhesives

0143-74

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ijadhadh

Synthesis and properties of polyamide derived from piperazine and lowerpurity dimer acid as hot melt adhesive

Pravin G. Kadam, S.T. Mhaske n

Department of Polymer Engineering, Institute of Chemical Technology, Matunga, Mumbai 400019, Maharashtra, India

a r t i c l e i n f o

Article history:

Accepted 27 June 2011Polyamide hot melt adhesive was synthesized from lower purity dimer acid (composition: �23% trimer

acid, �75% dimer acid and �3% monomer acid), sebacic acid, ethylenediamine and piperazine. The

Available online 27 July 2011

Keywords:

Hot melt

Lap-shear

Peel

Rheology

96/$ - see front matter & 2011 Published by

016/j.ijadhadh.2011.06.019

esponding author. Tel.: þ91 022 3361 2412;

ail address: st.mhaske@ictmumbai.edu.in (S.T

a b s t r a c t

effect of piperazine and dimer acid concentration on properties of polyamides such as thermal

properties: fusion temperature (Tf), heat of fusion (Hf), crystallization temperature (Tc), heat of

crystallization (Hc), softening point (Ts) and glass transition temperature (Tg), mechanical properties:

tensile strength and hardness, adhesion properties like lap shear strength (LSS) and T-peel strength

(TPS), and rheological properties were investigated. Concentration of piperazine was varied from

12.5 to 37.5 mol% while that of dimer from 37.5 to 42 mol%. Piperazine has one hydrogen atom on each

of its two nitrogen atoms in the ring structure. When it undergoes reaction with acids to form

polyamide, these hydrogen atoms get consumed, making the amide linkage unable to form hydrogen

bonding with the neighboring polyamide polymer chains. This leads to decrease in crystallinity of the

polyamide. Thus, as the mole percentage of piperazine in the polyamide increases, it becomes more

amorphous, decreasing Tf, Hf, Tc, Hc, Ts, Tg, tensile strength, hardness, LSS, TPS and viscosity. Dimer acid

and trimer acid are bulky compounds. As their percentages in the polyamide increase, it becomes

difficult for the neighboring polyamide chains to come closer. Thus, inter-molecular hydrogen bonding

decreases. This leads to decrease in crystallinity of the polyamide, lowering Tf, Hf, Tc, Hc, Ts, Tg, tensile

strength, hardness, LSS, TPS and viscosity.

& 2011 Published by Elsevier Ltd.

1. Introduction

Adhesives that harden by cooling from a melt condition arecalled as hot melt adhesives. These are thermoplastic adhesivesthat soften and melt when heated, and harden on subsequentcooling. The hot melt system must have a relatively low viscositywhen in the molten state to achieve wetting; and it must not cooltoo rapidly or it will not have time to completely wet the surfaceof the substrate. Once the hot melt adhesive is applied in themolten condition, the substrates must be joined immediately.When hardened, the hot melt adhesive can have various degreesof tackiness depending on the formulation.

Materials that are primarily used as hot melt adhesive, includeethylene and vinyl acetate copolymers, polyvinyl acetates, poly-ethylene, amorphous polypropylene, block copolymers(styrene butadiene rubber), polyamides, and polyesters. In gen-eral, hot melt adhesive formulations are solid at temperaturesbelow 80 1C. Typical application temperatures are 150–200 1C.Hot melt adhesives are non-structural adhesives. They are used

Elsevier Ltd.

fax: þ91 022 3361 1020.

. Mhaske).

for the applications requiring lower strength or temporary fasten-ing. [1]

The use of polyamides as hot-melt adhesive was found as earlyas 1959 [2]. Polyamide hot melt adhesives have lower meltingpoints than the polyamides used as engineering plastics (nylons’).However, they have greater temperature resistance than ethylenevinyl acetate adhesives and require fewer additives. But, they aregenerally more expensive [1]. Polyamides synthesized from dimeracids and diamines have low crystallinity and wide range ofmelting temperatures, which make them particularly suitable tobe used as hot melt adhesive [3].

By use of more than one dicarboxylic acid and diamine,crystallinity, thermal properties, mechanical properties and adhe-sion properties can be adjusted. Diamine piperazine is a specialkind of diamine monomer, decreasing crystallinity, while lowmolecular weight dicarboxylic acids such as sebacic acid or adipicacid mixed with dimer acids, can improve the tensile strength ofthe polyamide [4].

Leoni et al. synthesized polyamide hot-melt adhesive fromhigher purity polymeric fatty acid (composition: 2% monomeracid, 97% dimer acid and 1% trimer acid), ethylenediamine andpropylenediamine. Polymeric fatty acid was reacted with 50 mol%ethylenediamine and 50 mol% propylenediamine. The prepared

Fig. 1. Molecular structures of (a) linoleic acid, (b) dilinoleic or dimer acid and

(c) trinoleic or trimer acid.

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742736

polyamide hot-melt adhesive had a softening point 100 1C, tensilestrength 8.5 MPa and T-peel strength 1.2 N/mm. Rossini et al. [5]studied synthesis of polyamide hot-melt adhesives from poly-meric fatty acid containing 15% trimer acid, 82% dimer acid and3% monomer acid. Formulation contained 44 mol% polymericfatty acid, 6 mol% adipic acid, 30.75 mol% ethylenediamine and19.25 mol% piperazine. Acids and amines were reacted in 1:1molar ratio. The formed polyamide hot-melt adhesive had asoftening point 101 1C, T-peel strength 0.72 N/mm and tensilestrength 3.7 MPa [6]. Wroczynski et al. prepared polyamidehot melt adhesive using polymeric fatty acid (25 mol%; composi-tion: 0.4% monomer acid, 95.6% dimer acid and 4% trimeracid), 1,18-octadecane dicarboxylic acid (25 mol%), piperazine(32.5 mol%) and ethylenediamine (17.5 mol%). The synthesizedpolyamide had the tensile strength of 1859 psi and steel lap shearstrength of 1568 psi [7]. Chen et al. studied the synthesis ofpolyamide hot melt adhesive derived from higher purity dimeracid (composition: 2% monomer acid, 97% dimer acid and 1%trimer acid), reporting mechanical, adhesion and thermal proper-ties [5]. Hot melt adhesives are applied through glue gun, when inmolten state. Thus, viscosity of the adhesive has an important roleto play, affecting throughput and thus the application and settingrate of the adhesive [8]. Current study deals with polyamidessynthesized from lower purity of dimer acid (composition: �23%trimer acid, �75% dimer acid and �3% monomer acid). The effectof the content of piperazine and dimer acid on the properties suchas softening point, tensile strength, elongation at break, hardness,glass transition temperature, lap shear strength, T-peel strengthand viscosity were studied.

Fig. 2. Molecular structures of (a) sebacic acid, (b) ethylenediamine and (c) piperazine.

Table 1Molecular weight of the compounds used.

Sr. No. Compound Molecularweight (g/mol)

1 Trimer acid 840

2 Dimer acid 560

3 Linoleic acid 280

4 Sebacic acid 202

5 Ethylenediamine 60

6 Piperazine 86

2. Experimental

2.1. Materials

Dimer acid (Pripol 1017), with an acid value of 197 mg potas-sium hydroxide (KOH)/g, was supplied by Croda Chemicals, Mum-bai, India. The contents were as follows: monomer acid �3%, dimeracid �75%, trimer acid �22%. Ethylenediamine was obtained fromAnkita Chemicals, Mumbai, India, sebacic acid was procured fromAmrut Industrial Products, Mumbai, India; and piperazine fromArihant Chemical Corporation, Mumbai, India. All chemical com-pounds were used as obtained without purification.

2.2. Preparation of polyamides

Figs. 1(a–c) and 2(a–c) show the molecular structures of thereactants used in the study; while their molecular weights aregiven in Table 1.

In order to obtain high molecular weight and molecularuniformity, the ratio of total acid equivalent present in Pripol1017 (consisting dimer acid and trimer acid) and sebacic acid tototal amine equivalents present in ethylene diamine and piper-azine was kept at, approximately, 1.

Pripol 1017 consists of 75% dimer acid, 22% trimer acid and 3%monomer acid (linoleic acid). Basis for the reaction was 200 g ofPripol, which according to its composition had 150 g of dimeracid, 44 g of trimer acid and 6 g of monomer acid. This means ithad 0.27 mol of dimer acid, 0.05 mol of trimer acid and 0.02 molof monomer acid. The functionality of trimer acid is 3 while thatof all amines used is 2, so the number of moles of amine requiredas equivalent to trimer is higher, and it would be required in theratio 1:1.5 (trimer acid : amine), so as to equalize the function-ality of the two compounds (trimer acid and amines), making allthe functional groups undergo reaction. Number of moles ofamines, thus required, will be accordingly distributed between

ethylenediamine and piperazine. Indirectly, number of moles oftrimer acid can be considered 1.5 times its original quantitypresent, so that calculation and distribution among amines(ethylenediamine and piperazine) becomes easy. Thus instead of0.05 mol of trimer acid in the above formulation, it will beconsidered 0.08 mol of trimer acid. So the composition of Pripol1017 becomes 0.08 mol trimer acid, 0.27 mol dimer acid and0.02 mol monomer acid. Total moles of acids present in Pripol1017 are 0.37. Sebacic acid content is kept constant at 14.14 gi.e. 0.07 mol. Thus, total mole of acids present in the reactionsystem is 0.44. Number of moles of ethylenediamine and piper-azine required will vary according to formulation, taken on thebasis of Pripol 1017.

The four necked 500 ml flask was equipped with stirringsystem (moon shaped teflon blade stirrer attached to the stirringmotor), condenser (Double walled with water as coolant. Waterflow rate was maintained at 1 l/min), nitrogen gas inlet (nitrogen

Table 2Monomer composition of SET 1 polyamides.

Sr. No. Monomers g

(mole fraction)

S11 S12 S13 S14 S15 S16

1 Dimer acid 200.0 (37.5%) 200.0 (37.5%) 200.0 (37.5%) 200.0 (37.5%) 200.0 (37.5%) 200.0 (37.5%)

2 Sebacic acid 24.85 (12.5%) 24.85 (12.5%) 24.85 (12.5%) 24.85 (12.5%) 24.85 (12.5%) 24.85 (12.5%)

3 Ethylenediamine 7.36 (12.5%) 10.32 (17.5%) 13.32 (22.5%) 16.26 (27.5%) 19.26 (32.5%) 22.20 (37.5%)

4 Piperazine 31.82 (37.5%) 27.61 (32.5%) 23.31 (27.5%) 19.09 (22.5%) 14.79 (17.5%) 10.58 (12.5%)

Table 3Monomer composition of SET 2 polyamides.

Sr. No. Monomers g

(mole fraction)

S21 S22 S23 S24 S25 S26

1 Dimer Acid 200.00 (42%) 200.00 (42%) 200.00 (42%) 200.00 (42%) 200.00 (42%) 200.00 (42%)

2 Sebacic Acid 14.14 (8%) 14.14 (8%) 14.14 (8%) 14.14 (8%) 14.14 (8%) 14.14 (8%)

3 Ethylenediamine 6.60 (12.5%) 9.24 (17.5%) 111.88 (22.5%) 14.52 (27.5%) 17.16 (32.5%) 19.80 (37.5%)

4 Piperazine 28.38 (37.5%) 24.60 (32.5%) 20.81 (27.5%) 17.03 (22.5%) 13.24 (17.5%) 9.46 (12.5%)

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742 737

gas flow rate was maintained at 500 ml/min) and temperaturecontroller. To the flask was added 200 g (0.27 mol dimer acid,0.08 mol trimer acid and 0.02 mol linoleic acid) of Pripol 1017,14.14 g (0.07 mol) of sebacic acid, 20.81 g (0.24 mol) of piperazineand 11.88 g (0.20 mol) of ethylenediamine. Order of addition ofreactants is: pripol 1017, sebacic acid, piperazine and ethylenedia-mine. The reactants were heated gradually from 220 to 240 1C whilestirring speed was maintained constant at 1200 rpm. Acid value wascontinuously noted at an interval of half hour to know the progress ofreaction. Product was removed from the reactor when the acid valueof the polyamide reached approximately 10 or below. Time requiredto reach the desired acid value is about 4–5 h. Synthesized polyamide,in the molten condition, was poured in the teflon mold (havingdimension: 15�15�3 cm3) and allowed to cool at roomtemperature.

In SET 1, molar ratio of dimer acid and sebacic acid was heldconstant; dimer acid 37.5%, sebacic acid 12.5%; while mole ratioof piperazine to ethylenediamine was varied from, piperazine37.5% and ethylenediamine 12.5%, to piperazine 12.5% andethylenediamine 37.5%.

In SET 2, molar ratio of dimer acid and sebacic acid was heldconstant; dimer acid 42%, sebacic acid 8%; while mole ratio ofpiperazine to ethylenediamine was varied from, piperazine 37.5%and ethylenediamine 12.5%, to piperazine 12.5% and ethylenedia-mine 37.5%. Compositions of SET 1 and SET 2 are shown inTables 2 and 3 respectively.

2.3. End group analysis

2.3.1. Acid value determination

50 ml neutralized benzene–alcohol (1:1 ratio on volume basis)mixture was added to dissolve 2 g sample in 125 ml erlenmeyerflask, warming in water bath if necessary. Solution was cooled (ifnecessary) and a few drops of phenolphthalein indicator wereadded. Solution was titrated against 0.5 N alcoholic potassiumhydroxide solution (alc. KOH) to a faint pink color which remainsfor 30 s or more. The acid value was calculated as:

Acid value¼ ð56:1� volume of alcoholic KOH� normality of alcoholic KOHÞ=ðweight of sampleÞ

2.3.2. Amine value determination

50 ml neutralized iso-propyl alcohol was added to dissolve 2 gsample in 125 ml erlenmeyer flask, warming on a heat source ifnecessary. Solution was cooled (if necessary) and a few drops ofbromocresol green indicator were added. Solution was titrated

against, 0.5 N standardized hydrochloric acid (HCl), to yellowishgreen color which remains for 30 s or more. The amine value wascalculated as:

Amine value¼ ð36:6� volume of HCl

�Normality of HClÞ=ðweight of sampleÞ

2.4. Experimental techniques

Softening point was determined in accordance to ASTM E-28 ata heat rate of 5 1C/min. Differential scanning calorimetry (Q100DSC, TA Instruments) characterization was done to investigate thecrystallization and melting behavior of the prepared polyamides.Two consecutive scans were obtained to minimize the influenceof possible residual stresses in the material due to any specificthermal history. A scanning rate of 10 1C/min was used for theboth exo- and endo-thermal cycle with the nitrogen purge at50 ml/min. The heat of fusion (Hf) and fusion temperature (Tf)were determined from the second heating scan, whereas the heatof crystallization (Hc) and crystallization temperature (Tc) weredetermined from first cooling scan.

The FTIR spectra were recorded with PerkinElmer, SpectrumGX equipment. 1–2 wt% solution of polyamide was dissolved inchloroform for the characterization and was scanned with aresolution of 2 cm�1 and the scan range of 450–4000 cm�1.Samples were recorded with 4 scans.

Tensile strength of compression molded sheet was measuredin accordance with ASTM D638 with the crosshead speed of50 mm/min. 1Shore D hardness was determined in accordancewith ASTM D2240 (test sample dimension: 20�20�6 mm3).The adhesive joint was obtained by pressing a polyamide film(obtained by compression molding) between two pieces ofsurface treated aluminum adherent at a temperature of 30 1Chigher than softening point of the polyamide and at a pressure of30 kPa for 20 min, followed by cooling at room temperature for24 h. Lap shear strength was determined according to ASTMD1002-01 with a crosshead speed of 1.3 mm/min (see Fig. 3 fortest specimen dimension and area of overlap). T-peel strengthwas measured in accordance with ASTM D1876-01 with a cross-head speed of 254 mm/min (see Fig. 4 for test specimen dimen-sion). Average thickness of adhesive layer for lap and T-peel jointswere kept within 0.035 mm (0.001 in.).

Viscosity vs shear rate was determined at temperature of200 1C up to maximum shear rate of 100 1/s. Viscosity vs timewas determined at temperature of 200 1C at constant shear rate of

Fig. 3. Dimension of test specimen for Lap Shear Strength (according to ASTM D1002-01).

Fig. 4. Dimension of test specimen for T-peel Strength (according to ASTM D1876-01).

Scheme 1. Structure of polyamide.

Table 4Characterization of SET 1 polyamides.

Sample S11 S12 S13 S14 S15 S16

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742738

1 1/s for 250 s. Viscosity was determined using rheometerMCR101, Anton Paar, Germany.

Acid number (mg KOH/g) 10.3 11.2 9.6 9.9 10.0 8.2

Amine number (mg KOH/g) 7.7 6.1 7.1 7.3 5.1 6.1

Table 5Characterization of SET 2 polyamides.

Sample S21 S22 S23 S24 S25 S26

Acid number (mg KOH/g) 10.5 11.6 11.7 10.6 9.7 10.0

Amine number (mg KOH/g) 7.3 6.6 7.1 8.5 8.2 8.7

3. Results and discussion

3.1. Characterization of the polymerization

Wheeler and Milun [9] expressed the structure of polyamidessynthesized from dimer acid, sebacic acid, ethylene diamine andpiperazine, as shown in Scheme 1.

Acid number and amine number of SET 1 and SET 2 polyamidesare listed in Tables 4 and 5 respectively. Since acid number andamine number of the prepared polyamides (SET 1 and SET 2) donot vary significantly, it is supposed that the number of acid andamine functional groups on all polyamides is approximatelysame. Thus, it is the molecular structure and thus the monomercomposition of the polyamide affecting the properties and not thefunctional groups.

FTIR spectrum of polyamide S14 and S24 shows following:

S14: FTIR spectrum of the sample is shown in Fig. 5. Smallpeak at 3441.9 cm�1 appeared due to OH stretch of water.Small band for the amide I (–NH2 in primary amides)groups appeared at 3320.8 cm�1. Bands at 2924.2 cm�1 and2853.5 cm�1 are due to asymmetric and symmetric stretchingof CH2, respectively. Small peak at 1739.4 cm�1 is caused dueto CQO stretching. The carbonyl peak (–CONH2) of polyamideis at around 1651.4 cm�1 (steep peak). Amide II band/CH2

asymmetric deformation is indicated by the peak at wave-number 1542.2 cm�1. N–H deformation/CH2 scissoring peak isindicated at wave number 1456.0 cm�1. Peak at 1369.0 cm�1

is due to amide III band/CH2 wagging. The peak at1243.9 cm�1 and 1215.0 cm�1 corresponds to C–N stretchingvibration bond. C–C stretching is depicted by small peak at1019.6 cm�1. N–H wagging/CH2 rocking is depicted by small

peak at 773.6 cm�1. FTIR analysis thus proves that compoundformed is an amide. Similar method and raw materials wereused for preparing other polyamides (S11 to S16), thus, theyare also amides.S24: FTIR spectrum of the sample is shown in Fig. 5. Small bandat 3432.3 cm�1 appeared due to OH stretch of water. Small bandfor the amide I (–NH2 in primary amides) groups appeared at3316.8 cm�1. Peaks at 2924.0 cm�1 and 2853.3 cm�1 are due toasymmetric and symmetric stretching of CH2, respectively.Small peak at 1739.4 cm�1 is caused due to CQO stretching.The carbonyl peak (–CONH2) of polyamide is at around1651.6 cm�1 (steep peak). Amide II band/CH2 asymmetric defor-mation is indicated by the peak at wave-number 1551.9 cm�1.N–H deformation/CH2 scissoring peak is indicated at wavenumber 1455.0 cm�1. Peak at 1369.2 cm�1 is due to amide IIIband/CH2 wagging. The peak at 1244.0 cm�1 and 1215.0 cm�1

corresponds to C–N stretching vibration bond. C–C stretching isdepicted by small peak at 1018.6 cm�1. N–H wagging/CH2

rocking is depicted by small peak at 773.8 cm�1. FTIR analysisthus proves that compound formed is amide. Similar methodand raw materials were used for preparing other polyamides(S21 to S26), thus, they are also amides.

Fig. 5. FTIR spectra of SET 1 (sample S14) and SET 2 (sample 24) polyamides.

Fig. 6. Glass transition temperature of SET 1 polyamides.Fig. 7. Heat of fusion of SET 1 polyamides.

Fig. 8. Heat of crystallization of SET 1 polyamides.

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742 739

3.2. Thermal properties

Graphs of glass transition temperature (Tg), heat of fusion (Hf)and fusion temperature (Tf), heat of crystallization (Hc) andcrystallization temperature (Tc), of polyamides of SET 1 are shownin Figs. 6–8, respectively, while that of SET 2 are shown inFigs. 9–11, respectively. Values of Tg, Tf, Hf, Tc, Hc and softeningtemperature (Ts), of polyamides of SET 1 and SET 2 are shown inTables 6 and 7, respectively.

Intermolecular forces of attraction play an important role onthermal properties. In the formulation four types of reactantswere used. Adjusting the concentration of the reactants, Tg, Tf, Hf,Tc, Hc and Ts can be varied. Tg, Tf, Hf, Tc, Hc and Ts decreased as themolar percent of piperazine in the particular SET increased. Thisindicates that there is a decrease in crystallinity of the material aspiperazine content increases. Piperazine has one hydrogen atomeach on two of the nitrogen atoms present in the ring structure.When it undergoes reaction with acids to form the amide linkage,these hydrogen atoms get consumed. This makes the amidelinkage unable to form hydrogen bonding with the neighboring

Fig. 9. Glass transition temperature of SET 2 polyamides.

Fig. 10. Heat of fusion of SET 2 polyamides.

Fig. 11. Heat of crystallization of SET 2 polyamides.

Table 6Thermal properties of SET 1 polyamides.

Sample S11 S12 S13 S14 S15 S16

Tg, 1C �12 �6 �5 �4 �3 1

Tf, 1C 97.8 99.7 125.6 138.7 140.3 165.8

Hf, J/g 1.3 4.2 5.5 6.8 7.3 8.8

Tc, 1C 48.7 55.3 89.3 101.7 105.6 112.0

Hc, J/g 0.9 2.2 4.9 5.8 5.8 7.2

Ts, 1C 103.0 123.0 135.0 149.0 163.0 181.0

Table 7Thermal properties of SET 2 polyamides.

Sample S21 S22 S23 S24 S25 S26

Tg, 1C �8 �8 �7 �6 �6 �5

Tf, 1C 88.1 90.3 91.4 99.8 109.4 120.2

Hf, J/g 0.8 1.3 2.0 2.9 4.4 5.0

Tc, 1C 28.8 36.4 44.2 51.0 59.5 68.9

Hc, J/g 2.1 6.0 7.4 8.2 11.1 12.6

Ts, 1C 94.0 104.0 105.0 122.0 135.0 141.0

Table 8Mechanical properties of SET 1 polyamides.

Sample S11 S12 S13 S14 S15 S16

Tensile strength (MPa) 1.2 3.5 6.7 11.3 14.1 16.5

Elongation at break (%) 79 65 62 41 23 15

1Shore D hardness 24 31 37 50 52 62

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742740

polyamide molecules. Thus, as the concentration of piperazine inthe polyamide is increased the inability of the amide linkage toform hydrogen bond increases, decreasing crystallinity. As theconcentration of Pripol 1017 in the polyamide increases fromSET 1 to SET 2 Tg, Tf, Hf, Tc, Hc and Ts decreases. This is again due to

decrease in crystallinity of the material. Dimer acid and trimeracid are bulky molecules compared to sebacic acid. The bulkystructure resists the neighboring polyamide molecules to comecloser through hydrogen bonding, decreasing the intermolecularforces of attraction. Thus, dimer acid and trimer acid are notcapable of forming intense and strong hydrogen bonds as that arepossible with sebacic acid, decreasing crystallinity.

3.3. Mechanical properties

Tables 8 and 9 give the tensile strength, elongation at break and1Shore D hardness values of polyamides belonging to SET 1 andSET 2. It was found that, as the percentage of piperazine in theformulation increases, tensile strength and 1Shore D hardness ofpolyamide decreases, whereas elongation at break increases. Dueto the inability of piperazine to form hydrogen bonding withneighboring molecules after undergoing reaction with the acids,the intermolecular forces of attraction decreases. This leads todecrease in crystallinity of the material. More the concentration ofpiperazine in the polyamide, lesser is the crystallinity obtained ofthe material. Lesser the crystallinity, lesser is the packing of thepolyamide molecules. Thus lower are the values of tensile strengthand 1Shore D hardness. Elongation at break is inversely propor-tional to crystallinity. Accordingly elongation at break increasedwith increase in piperazine and Pripol 1017 concentration inpolyamide. This is in complete agreement with the thermalproperties obtained. Decrease in tensile strength and hardness,and increase in elongation at break are caused by decrease incrystallinity of the polyamide.

Similarly, when dimer acid content in the polyamide increases,crystallinity of the material decreases. This is so, as compared tobonding of sebacic acid to amine, bonding of dimer acid to aminecauses polar amide linkage to get separated by larger distance,decreasing intermolecular forces of attraction. Also the bulky

Table 9Mechanical properties of SET 2 polyamides.

Sample S21 S22 S23 S24 S25 S26

Tensile strength (MPa) 1.0 2.9 3.8 8.3 8.9 11.9

Elongation at break (%) 96 73 65 54 40 38

1Shore D hardness 11 23 28 37 46 52

Table 10Adhesion properties of SET 1 polyamides.

Sample S11 S12 S13 S14 S15 S16

T-peel strength (N/m) 0.86 1.01 1.14 1.22 1.38 1.45

Lap shear strength (MPa) 2.13 2.21 3.45 5.11 5.50 9.34

Table 11Adhesion properties of SET 2 polyamides.

Sample S21 S22 S23 S24 S25 S26

T-peel strength (N/m) 0.46 0.58 1.07 1.08 1.24 1.40

Lap shear strength (MPa) 1.52 1.82 1.87 2.74 4.44 5.27

Fig. 12. Graph of shear stress (Pa) vs shear rate (s�1) of SET 1 polyamides at

constant temperature of 200 1C.

Fig. 13. Graph of shear stress (Pa) vs shear rate (s�1) of SET 2 polyamides at

constant temperature of 200 1C.

Fig. 14. Graph of viscosity (Pa s) vs shear rate (s�1) of SET 1 polyamides at

constant temperature of 200 1C.

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742 741

structure of the dimer and trimer acid resists the polyamidemolecules to come closer, further decreasing the intermolecularforces of attraction. Decrease in intermolecular forces of attrac-tion, in turn decreases crystallinity of the material. Thus SET2 polyamides having more of dimer acid and trimer acid thansebacic acid have lower crystallinity compared to SET 1 polya-mides. Lower Tg, Tf, Hf, Tc, Hc and Ts values of SET 2 polyamidesthan SET 1 polyamides, proves this. Thus SET 2 polyamides havelower tensile strength and hardness but higher elongation thanSET 2 polyamides.

3.4. Adhesion properties

Tables 10 and 11 give the T-peel strength and lap shear strengthof joints made with SET 1 and SET 2 polyamides. As the molarfraction of piperazine or Pripol 1017 (dimer acid and trimer acid)increases, intermolecular forces of attraction in the polyamide hotmelt adhesive decreases; as proved through thermal and mechanicalproperties. Decrease in intermolecular forces of attraction allows thepolyamide molecules to flow past each other easily on application offorce in T-peel or lap shear joint, decreasing T-peel strength and lapshear strength of the polyamide hot melt adhesive. Thus T-peelstrength and lap shear strength decreases with increase in piper-azine concentration for any particular SET; and Pripol 1017 (dimeracid or trimer acid) concentration from SET 1 to SET 2.

3.5. Rheological properties

Figs. 12 and 13 show the variation of the shear rate as afunction of the shear stress at 200 1C for SET 1 and SET 2polyamides, respectively. As shear stress increases, the shearingof polyamide molecules increases, increasing shear rate of poly-amides of SET 1 or SET 2. Thus, Polyamide (SET 1 and SET 2)shows non-Newtonian behavior. It was found that shear stressrequired to shear polyamide molecules increased with decrease inpiperazine concentration for a particular SET and decrease inPripol 1017 (dimer acid and trimer acid) concentration from SET2 to SET 1. With decrease in piperazine concentration the numberof hydrogen bond forming amide linkages in the polyamideincreases, increasing intermolecular forces of attraction. This leds

to better packing of polyamide molecules, increasing crystallinity.Thus, less shearing of polyamide molecules take place at same shearstress. Yield point was also found to have increased appreciably withdecrease in piperazine concentration for a particular SET. Whereaswith increase in Pripol 1017 concentration, the intermolecular

Fig. 17. Graph of viscosity (Pa s) vs time (s) of SET 2 polyamides at constant

temperature of 200 1C.

P.G. Kadam, S.T. Mhaske / International Journal of Adhesion & Adhesives 31 (2011) 735–742742

distance between the polyamide molecules decreases due to sterichindrance exerted by the bulky dimer acid and trimer acid mole-cules. This leds to decrease in crystallinity of the polyamide andthus, increases the shearing of the material. Yield point was found tohave increased very slightly with decrease in Pripol 1017 concen-tration. Thus, polyamides undergoes shear thinning with increase inshear stress.

Figs. 14 and 15 show the variation of the viscosity as a functionof the shear rate at 200 1C for SET 1 and SET 2 polyamides,respectively. Increase in shearing with increase in shear stresscauses decrease in viscosity of the polyamide. But the shearingdecreased with decrease in piperazine concentration for particularSET and Pripol 1017 concentration from SET 2 to SET1. Thus,viscosity of polyamides increased with decrease in piperazineconcentration for particular SET and decrease in Pripol 1017 fromSET 2 to SET 1. All the polyamides showed slow decrease in viscosityup to the shear rate of 1 s�1. But, viscosity decrease was drastic afterthe shear rate of 1 s�1, showing higher shear thinning.

Figs. 16 and 17 show the variation of the viscosity as a functionof the time at 200 1C for SET 1 and SET 2 polyamides, respectively.A viscosity–time measurement during shearing was used to showtime-dependent effects. In general, it was found that viscositydecreased with increase in time at constant temperature. Thus theprepared polyamides showed thixotropic behavior. This can be dueto reversible changes involving the breaking and re-forming of thepolyamide molecular aggregates [10]. The breaking and re-formingbecame difficult with decrease in piperazine and Pripol 1017concentration in the polyamide. This can be due to increase in

Fig. 15. Graph of viscosity (Pa s) vs shear rate (s�1) of SET 2 polyamides at

constant temperature of 200 1C.

Fig. 16. Graph of viscosity (Pa s) vs time (s) of SET 1 polyamides at constant

temperature of 200 1C.

inter-molecular forces of attraction caused by increase in thenumber of amide linkages capable of forming hydrogen bondsand decrease in the steric hindrance caused by the bulky the bulkystructure of Pripol 1017 (dimer acid and trimer acid). Thusviscosity increased with decrease in molar percent of piperazineand Pripol 1017 in the polyamide for the same time.

4. Conclusion

Polyamide hot melt adhesives were prepared using Pripol1017 (dimer acid and trimer acid), sebacic acid,, piperazine andethyelendiamine. Molar fraction of piperazine was varied from12.5% to 37.5% and that of Pripol 1017 (dimer acid and trimeracid) from 37.5% to 42% and studied the effect of the same onthermal, mechanical, adhesion and rheological properties of thepolyamide hot melt adhesive. Crystallinity and thus the inter-molecular forces of attraction play an important role in affectingthe above said properties. Through thermal and mechanicalproperties it was seen that crystallinity of the polyamidesdecreased with increase in piperazine and Pripol 1017 concentra-tion. Lap shear strength and T-peel strength increased withdecrease in piperazine and Pripol 1017 concentration in thepolyamides. Shearing of polyamide molecules decreased withdecrease in piperazine and Pripol 1017 concentration leading todecrease in yield point and viscosity due to increase in inter-molecular forces of attraction. Time dependent study proves thatthe polyamide hot melt adhesive shows thixotropic behavior.Again the viscosity increased with decrease in piperazine andPripol 1017 content at the same time due to increase in inter-molecular forces of attraction.

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