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Temperature (°C)

6th International Conference on Structural Adhesive Bonding, AB2021 Porto, Portugal 8 – 9 July 2021

• Three endothermic peaks occur when the paste samples are heated. The first and the second peaks are due to the decomplexation reaction of the solvent (N-methyl-2-pyrrolidone, NMP). The third peak is relative to the imidization reaction. The thermal profile of the cured paste sample (red curve) no longer shows any peak, indicating the curetakes place completely during the first heating run.

• TGA/FT-IR analysis of uncured sample BR® 680-3 detected the presence of solvent molecules (ethanol, EtOH and NMP) and H2O from condensation reactions (imidization),whereas they are absent in the cured polyimide. After 48 min of analysis in both samples it is observable the absorbance of CO2 molecules due to decomposition reaction of thepolymer chain.

• At the end of the degradation process the mass of BR® 680-3 sample residue is equal to 39 wt.% highlighting that the polyimide-based adhesive has good thermal stability.

• Ea obtained for BR® 680-3 is equal to 34 kJ/mol, a lower energy value in comparison to obtained for the BR® 680-3 at higher conversion values (124 kJ/mol).

Object: study of the thermal stability of high-temperature-resistant polymeric adhesives for the innovative hybrid friction stir welding (FSW)-bonding technique. For the FSW-bonding technique it is necessary to use polymeric adhesives with a high thermal resistance. The best ones to this purpose are (mono- or bi-component) epoxy-based andpolyimide-based systems.

In particular, polyimides (PI) are characterized by high thermal, thermo-oxidative and chemical stability as well as good mechanical properties. Various polyimides are used asstructural adhesives for high and low temperature applications (automotive, aerospace, shipbuilding, etc) for bonding metals such as stainless steel, titanium, aluminum. Theyshow excellent properties also as coating and thin film.

The high-temperature-resistant polymeric adhesives investigated were characterized by thermal analytical techniques. PI-based adhesives during the polymerization processrelease volatile substances which were analyzed through thermal gravimetric analysis, TGA, coupled with FT-IR spectroscopy. Indeed, TGA measures the change in weight of asample as a function of temperature or time. The accurate measurement of the loss of weight provides one piece of information, but it does not identify what is changing. In thehyphenated technique FT-IR spectroscopy is used to identify the evolved gases and help in determining sample characteristics.

KIN

ETIC

MO

DELS

FIN

AL R

EM

ARKS

-5,0

-4,0

-3,0

-2,0

-1,0

0,0

50 150 250 350 450 550 650 750

DTG

(%

/°C

)

Temperature (°C)

20°C/min15 °C/min10 °C/min5 °C/min1 °C/min

Heating rate

(°C/min)

T5% (°C) Tonset (°C) Tp (°C) m (%) W (%)

20 121 540 586 61 39

15 120 539 576 60 40

10 117 534 569 60 40

5 98 524 550 62 39

1 85 503 522 63 37

0,0

0,4

0,8

1,2

1,0 1,3 1,5 1,8 2,0 2,3 2,5

Log β

1000/T (K-1)

-3,0

-2,5

-2,0

-1,5

-1,0

1,0 1,3 1,5 1,8 2,0 2,3 2,5

Log

dα/d

t

1000/T (K-1)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0

40

80

120

160

0,0 0,2 0,4 0,6 0,8 1,0

Ea (

kJ/m

ol)

α

OFW Friedman

• uncured sample (black curve)• cured paste (red curve)

Sample Tpeak

(°C)

ΔH

(J/g)

BR® 680-3

I peak104

II peak165

III peak~210°C

80

67

The sample cured at 200 °C shows a low intensity peak (dotted line) due to the imidization reaction in the range of temperature between 200°C and 250 °C

Sample Tg

(°C)

BR® 680-3 cured 280°C 350

BR® 680-3 cured 250°C338

0,0

0,1

0,2

0,3

0,4

25,0 125,0 225,0 325,0 425,0

He

at F

low

(W

/g)

Temperature (°C)

cured 280°C cured 250°C

cured 210°C cured 200°C

Tg

Tg

-4

-3

-2

-1

20

40

60

80

100

0 10 20 30 40 50 60 70

De

riva

tive

we

igh

t (%

/m)

We

igh

t Lo

ss %

Time (min) RMS Intensity Profile (PI_10°Cmin_2rep.spp)

Name Description

20 40 60

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

0,045

0,050

0,055

Mins

Abs

PI uncured

4000 3500 3000 2500 2000 1500 10000

10

20

30

40

50

60

70

Wavenumber

Min

s

-0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

Abs

CO2

H2OC-H

-C=O

-C-O

Sample TD onset

(°C)

Tv max

(°C)

Residual

(p%)

BR® 680-3

I step96

III step550

I peak 114

II peak 184

III peak 575

37

-5

-4

-3

-2

-1

0

20

40

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0 10 20 30 40 50 60 70

De

riva

tive

we

igh

t (%

/m)

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igh

t Lo

ss %

Time (min)

RMS Intensity Profile (PI cured_10.spp)

Name Description

20 40 60-0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

Mins

Abs

PI cured

Sample TD onset

(°C)

Tv max

(°C)

Residual

(p%)

BR® 680-3I step

563I peak 593 57

4000 3500 3000 2500 2000 1500 10000

10

20

30

40

50

60

70

WavenumberM

ins

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0,18

0,20

0,22

0,24

Abs

CO2

-C-O

condensation reaction: solvent(NMP, EtOH) and/or volatile products

polyimide decomposition

Determination of the energy of activation Ea forthe degradation of BR® 680-3 sample according tothe OFW and Friedman equations.

Ea ~34 kJ/mol

Ea ~124 kJ/mol

This work was realized thanks to the PhD scholarship granted by the Regione Liguria under the Operational Program LiguriaRegion - European Social Fund POR-FSE 2014-2020.

Acknowledgement:

DSC traces after curing at differenttemperature

BR® 680-3 uncured BR® 680-3 cured

30

50

70

90

We

igh

t Lo

ss (

%)

20°C/min15 °C/min10 °C/min5 °C/min

The sample was heated from 50°C to 340°C at aheating rate of 10 °C/min

The sample was heated at a rate of10 °C/min from 40 to 700 °C under aN2 flow and from 700 to 850°Cunder a O2 flow.

BR® 680-3 sample was heated at five differentheating rates: 1, 5, 10, 15 and 20 °C/min.