The effect of post-mould curing on the mechanical properties of instant-setting poly(urethane-isocyanate) polymers

Materials Science and Engineering, 67 (1984) 31-37

31

The Effect of Post-mould Curing on the Mechanical Properties of Instant-setting Poly(urethane-isocyanate) Polymers M. N. BASSIM and E. SAHIN

Metallurgical Sciences Laboratory, Department of Mechanical Engineering, University of Manitoba, Winnipeg, Manitoba (Canada) (Received January 31, 1984)

SUMMARY

The effect of post-mould curing on selected properties of instant-setting poly(urethaneisocyanate) polymers was investigated. The changes in tensile, flexural and impact strength as a function of annealing, cooling rate and thickness were determined. Generally, the process of annealing was found to increase the tensile and structural strength by about 5%. The extent of this improvement depends on variables such as the thickness of the specimen, the heat treatment cycle and the composition of the polymer. It was found that annealing produces secondary crosslinking as a result of the reactions of isocyanate end groups with urethane groups and with themselves. It was also found that the relaxation of internal stresses as a result of annealing contributes to the measured mechanical properties.

by a decrease in toughness and ductility. In polystyrene and polycarbonate, an increase in tensile strength of up to 15% after annealing has been reported [3, 4]. It was suggested that heat treatment produces a more ordered structure within the amorphous regions of the polymer. In cross-linked or thermosetting polymers, partial curing may occur in the mould before the chemical reactions are nearly stopped by lowering the temperature. In such polymers the curing reaction rate will increase when the polymer undergoes a postmould heat treatment which results in a polymer with a higher degree of cross-linking [5, 61. In this study the effect of post-mould curing on instant-setting poly(urethaneisocyanate) polymers was investigated. These polymers are considered to be glassy and cross linked. The effect of heat treatment on these particular cross-linked polymers can be analysed under the following topics.

I . INTRODUCTION

Improving the mechanical properties of polymers by subjecting them to a heat treatment, or post-mould curing, procedure has been investigated. It was found that the mechanisms involved in these thermal processes depend on the type of polymer and are functions of structural factors within the polymer. Thus, when crystalline polymers such as polyethylene and polypropylene are slowly cooled from the melt, or annealed at temperatures close to the melting point, large crystallites as well as microcracks between these crystallites may form in the structure. This results in a decrease in tensile strength of up to 50% [1, 2]. In amorphous polymers, annealing and slow cooling result in an increase in strength accompanied 0025-5416/84/$3.00

1.1. Degree of cross-linking During polymerization, primary cross-linking reactions which are initiated by suitable catalysts and have high reaction constants take place, resulting in a cross-linked network of molecules. The theoretical mixing ratio and molecular position of active groups cannot be controlled throughout the polymer and there are always active ingredients in excess in the structure. When heat is subsequently applied, these groups react either with each other or with the reaction product if this is chemically allowable. This results in a change in the degree of cross-linking in the polymer [7]. 1.2. Density Heat treatment affects the branched segments of cross-linked polymers, resulting in © Elsevier Sequoia/Printed in The Netherlands

32 a more ordered structure in these segments similar to that in amorphous polymers. This reduces the free volume and increases the density. Also, secondary cross-linking reactions during heat treatments reduce the free volume in the structure and cause an increase in density. 1.3. Molecular orientation and relaxation of internal stresses Internal stresses are created during the polymerization process. Heat treatment may cause some groups in the polymer to break and re-form, resulting in stress relaxation controlled primarily by the degree of cross-linking, the heat treatment temperature and the bond energy of the relevant groups. 1.4. Degradation Polymer molecules are broken down by heat in processes such as scission of the main chain links or depolymerization. The extent of degradation due to heat treatment at temperatures close to the melting point affects the final mechanical properties of the polymer [8,9].

TABLE 1 T h e r m a l t r e a t m e n t s for ISP 1 0 0 a n d ISP 2 7 0

Process

ISP 1 O0 Air c o o l e d A n n e a l e d a n d air c o o l e d Die c o o l e d A n n e a l e d a n d die c o o l e d Insulation cooled A n n e a l e d a n d i n s u l a t i o n cooled

High High Moderate Moderate Low Low

I S P 2 7 0 with a mixing ratio o f 1.6 Die cooled Air cooled

I~ow High

ISP with a mixing ratio o f 1.1 Die c o o l e d Air cooled

Low High

140 hJ

120 I00

W

o_ 60 ~

40

0

0

2.1. Materials Two different cross-linked instant-setting poly(urethane-isocyanate) polymers designated ISP 100 and ISP 270 were used. (1) ISPIO0. This polymer has t w o main ingredients, namely p o l y o x y p r o p y l e n e triol and toluene diisocyanate. The mixing ratio of isocyanate to triol is 2.65. (2) ISP270. The ingredients in this polymer are amine polyol and polymethylene polyphenyl isocyanate. T w o mixing ratios of isocyanate to polyol were investigated, namely 1.6 and 1.1. The thicknesses used in this study were 6.4, 12.7, 25.4 and 50.8 mm for ISP 100 and 12.7 mm for ISP 270. Bars having these thicknesses were moulded from the t w o materials and cut into standard testing specimens depending on the t y p e of test used. 2.2. Heat treatment Six different heat treatment sequences were studied for ISP 100, while two cooling rates were investigated for ISP 270. These

oled

~ 80 W

2. E X P E R I M E N T A L P R O C E D U R E S

Relative cooling rate

I

I

I

I

I

I

I

I

I

I

2

5

4

5

6

7

8

9

TIME (h)

Fig. 1. T y p i c a l cooling processes a n d r a t e s for a s p e c i m e n t h i c k n e s s o f 12.7 ram.

heat treatments are shown in Table 1. All heat treatments were performed immediately after moulding. The annealing process was carried o u t at 95 °C for 2 h in an electric oven. The cooling process was performed by letting the specimen cool either in air (air cooled) or in the mould (die cooled) or by covering the specimen while cooling with a fibreglass blanket 50 mm thick (insulation cooled). Approximate cooling rates are shown in Fig. 1. 2.3. Test methods Tensile, flexural and impact tests were performed on the polymers. These tests were carried out according to ASTM specifications D 638-60 [10], D 790M-82 [11] and D 256-81 [12] for the tensile, flexural and impact properties respectively.

33 Density measurements, following ASTM specification D 792066 [13], were also performed. In addition to the above-mentioned tests, IR spectroscopy, differential thermal analysis and determination of internal stresses were performed. A Perkin-Elmer IR spectrometer was used for the IR spectroscopy. A differential calorimeter was employed for the differential thermal analysis. Powdered samples of mass approximately 20 mg were used and the temperature varied from 30 to 150 °C. The scanning rate was 2.5 °C min -1. The determination of residual stresses was carried out following ASTM specification D 1939-72 [14]. The test was modified for ISP 100 and ISP 270 using immersion in methanol. The number and severity of cracks, after a certain time interval, depending on thickness were recorded.

3. EXPERIMENTAL RESULTS

3.1. ISP 100 3.1.1. Tensile properties The tensile test results obtained for ISP 100 as a function of thickness and heat treatment are given in Fig. 2. In all cases except die cooling, annealing improved the tensile strength by 1.5%-13.5%. For the unannealed

specimens the die-cooled specimens seem to have a higher tensile strength than those which were air cooled or insulation cooled. Also, variations in the results appear to exist as the thickness increases.

3.1.2. Flexural properties As shown in Fig. 3, annealing improves the flexural yield strength for all cooling processes. The increase is between 0.6% and 10%. For the unannealed specimens, die cooling results in a relatively better yield strength than do the other cooling processes.

3.1.3. Impact properties The impact test results are shown in Fig. 4. Generally, the impact fracture energy decreases as the test temperature is increased. At low temperatures, annealing decreases the impact fracture energy by about 5%. This trend was not observed at higher temperatures. For the unannealed specimens, insulation-cooled specimens show higher impact fracture energies for all temperature and thickness ranges.

3.1.4. Density Annealing was found to cause a slight increase in density for all thicknesses and cooling rates tested. This increase amounts to only 0.05%-0.26%.

3.1.5. IR spectroscopy IR spectra for annealed and unannealed ISP 100 specimens are shown in Fig. 5. It is evident that there is a decrease in isocyanate groups (--NCO) as a result of annealing. This

675 650 625 600

875 !

b-

575 850

550 525

/ 800

b~

500

~ /

475

o125 0'.5

,*

2'

zso

{/~ ,4

7"25

O 25

THICKNESS (in)

Fig. 2. Tensile strength Otensil e a s a function of thickness and heat treatment for ISP 100:/% air cooled; A, annealed and air cooled; ~, insulation cooled; m, annealed and insulation cooled; ®, die cooled; $, annealed and die cooled.

/

I/} 3~

I

0.5

1

I

I

THICKNESS

2

{in)

Fig. 3. As for Fig. 2 but the flexural strength ofy for ISP 100.

34 12

,'7

E

\

\

-)

\ \ \

c~ w

z

(.9 n~ W Z W

k

IO

w

,

h

.\ \ \

b3

ILl

b.-

S~ t-C.)

9

L6

b_

8

t.) n I

7

I

I

-20--10

(a)

0

I0

I

I

I

I

20

30

40

50

5

I

(b)

(°C)

TEMPERATURE

I

-3o-2o-~o

I

I

I

I

1

1

o

to 20 30 40 50

TEMPERATURE (°C)

A

,~ E

IO

v >(..9 n,I..iJ

9

N, E t~

z

ILl

IO )C9 c~ UJ

8

W

W ,j

W r~

7

2

8

6

<~ n~ h ~_

7

n,' t.l. (..)

O.

5

}

I

-30-zo-~o

(c)

I

l

I

I

I

I

0 ~0 20 30 40 50 TEMPERATURE (°C)

--

6

i

I

-30-20-~0

(d)

I

I

I

I

I

I

0 to 20 30 40 50 TEMPERATURE (°C)

Fig. 4. A s for Fig. 2 but the impact fracture energy as a f u n c t i o n o f t e m p e r a t u r e for ISP 1 0 0 : (a) thickness, 6 . 3 5 ram; (b) thickness, 12.7 m m ; (c) thickness, 2 5 . 4 m m ; (d) thickness, 5 0 . 8 ram.

66.0

--

57,8 W

(..3 Z < I-l-

q9.6

--

41.4

cO Z < el" t--FZ W rr" w n

i

J"

--

i 33.2

--

25.0

--

16,8

--

8.S--

l 4910

4580

I 4250

I

I 3920

5590

3260

2930

2600

2270

1940

1610

[280

950

620

i 290

WAVENUMBER

Fig. 5. IR s p e c t r o s c o p y o f ISP 1 0 0 :

, annealed s p e c i m e n ; - - - ,

unannealed s p e c i m e n .

35 is shown at the characteristic absorption frequency of isocyanate groups at a wavelength of 2270 A. 3.1.6. Differential thermal analysis The differential thermal analysis of ISP 100 is shown in Fig. 6(a). This figure shows that there are two processes which determine the thermal response of this polymer. An exothermic reaction starts at 85 °C and has a maximum at 120 °C. Another endothermic reaction occurs over the whole temperature range and is dominant for temperatures higher than 120 °C. This is evident in Fig. 6(b), which shows an isothermal analysis at temperatures of 100, 120 and 140 °C.

creased the tensile strength and flexural strength by 20% and 13% respectively for a mixing ratio of 1.6. The opposite effect is observed for a mixing ratio of 1.1. In this case, die cooling reduces the tensile strength and flexural strength by 29% and 19% respectively. The impact tests on ISP 270 with a mixing ratio of 1.6 reveal similar tendencies as in the tests on ISP 100, namely high impact energies at low temperatures and low impact energies at high temperatures. Also, slow cooling rates result in higher impact energies for all the temperatures tested.

A

3.1.7. Internal stresses A comparison between the internal stresses for specimens which had been annealed as opposed to those which had not been annealed shows that the magnitude and intensity of internal stresses are greatly reduced by the annealing process. This is evident in Fig. 7.

g E E

I

E

w

3.2. ISP 270 Table 2 shows the tensile strength and flexural strength of ISP 270 as a function of cooling rate and mixing ratio. Die cooling in-

I 50

~

I 80

(a)

i

I ItO

I

I 140

I

1 170

i I I l I10 140 Temperolure (°C)

I 170

Temperature(°C)

A

,9 E

0

E ..p

o w

I,u I

E

(b)

.c

I 50

I

(a)

I 80

I

I I10

i

I 140

i

I

I 80

I

Fig. 7. Differential thermal analysis of ISP 270: (a) mixing ratio, 1.6, (b) mixing ratio, 1.1.

o

w

I 50

I 170

Temperature (°C)

TABLE 2

~

o.i T=I40oC

J

T= 120°C --T=

~°°lr

(b)

-0

i

I 20

L

I 4O

Tensile strength and flexural strength of ISP 270 (thickness, 12.7 ram) Mixing ratio

Process

Tensile strength (MPa)

Flexural strength (MPa)

1.6 1.6 1.1 1.1

Air cooled Die cooled Air cooled Die cooled

43.6

73.5

52.4 55.0 42.0

83.0

IO0°C I

Time(rain)

Fig. 6. Thermal analysis of ISP 100: (a) differential thermal analysis; (b) isothermal analysis.

65.2

55.0

36

For a mixing ratio of 1.1, the impact fracture energy increases as the temperature increases. Also, a high cooling rate results in higher impact energies. There were no significant changes in density or IR spectroscopy for ISP 270 as a

result of variation in the mixing ratio and thermal treatment. The differential thermal analysis revealed that an exothermic reaction dominates over the whole temperature range and becomes more significant at higher temperature, as shown in Fig. 8. No improve-

(a)

(e)

~

Fig. 8. Results of test on residual stresses for ISP 100, in w h i c h the specimens were immersed in methanol for 5 days (thickness, 6.35 ram): (a) air cooled; (b) annealed and air cooled; (e) die cooled; (d) annealed and die cooled; (e) insulation cooled; (f) annealed and insulation cooled.

37

ment in the elimination of residual stresses resulted from the thermal treatment of ISP 270.

4. DISCUSSION

The above-mentioned results provide a rather complete characterization of the effect of post-mould curing on the mechanical and structural properties of instant-setting polymers. Overall, post-mould curing appears to be beneficial to this class of polymers since it provides an increase in strength. Because such polymers are being used as replacements for conventional metal parts, an increase in strength is most desirable. From a structural viewpoint, the IR spectroscopy analysis has shown that the amount of isocyanate groups was reduced as a result of annealing the ISP 100. These groups may be involved in three basic reactions: (i) the reaction of isocyanate with water to produce --NH2 end groups and urea links; (ii) the reaction of isocyanate with urethane groups; (iii) the reaction of isocyanate with itself to produce an isocyanate dimer. Annealing during the post-mould curing promotes the disappearance of isocyanate groups by increasing the rate constants for these reactions and maintains some degree of mobility of chain ends, which gives a better o p p o r t u n i t y for these reactions to take place. All reactions involving free isocyanates are exothermic. The exotherm starts at 85 °C and has a maximum at 120 °C. Density measurements showed that annealing increases density. This is due to the reduction in free volume as a result of cross-linking reactions as well as ordering in the branched segments. The increase in cross-linking also causes an increase in tensile strength and flexural strength. For the impact fracture energy, increased cross-linking results in a decrease in impact strength, especially at low temperatures. Another beneficial aspect of annealing is the reduction in residual stresses. This was demonstrated in this study. In fact, the structure of instant-setting polymers contains some groups (allophenates and urethanes) which could break and re-form as a result of temperature, causing a relaxation of the internal stresses.

The effect of thickness on the mechanical properties can be discussed in terms of a change in bubble concentration during moulding as well as a reduction in cooling rate as the thickness increases. For ISP 270, no significant change in chemical structure was observed during cooling. The main contribution is thus believed to be due to the ordering of branched segments of the polymer as well as to probable degradation. The effectiveness of these factors is related to the degree of cross-linking. For ISP 270 with a mixing ratio of 1.1, there is a relatively small extent of cross-linking and any damage to these cross-links will greatly affect the mechanical properties. In contrast, for ISP 270 with a mixing ratio of 1.6, the abundance of cross-linking is not significantly affected by the cooling procedure. Thus, consistently, the slow cooling rates improved the tensile strength and flexural strength of ISP 270 with a mixing ratio of 1.6 while it resulted in a decrease in mechanical properties for ISP 270 with a mixing ratio of 1.1.

ACKNOWLEDGMENT

Financial support by the Natural Sciences and Engineering Research Council of Canada is acknowledged.

REFERENCES 1 Y. F. Fu and R. Ullman, J. Polym. Sci., 60 (1962) 50. 2 F. J. Padden and H. D. Keith, J. Appl. Phys., 30 (1959) 1479. 3 S. Raha and P. B. Bowden, Philos. Mag., 22 (1970) 463. 4 J. H. Golden, B. L. Hammant and E. A. Hazel, J. Appl. Polym. Sci., 11 (1967) 1571. 5 W. Wrasidlo, J. Polym. Sci. A, 2 (9) (1971) 1603. 6 A. S. Kenyon and L. E. Nieben, J. Macromol. Sci. A, 3 (1969) 275. 7 J. H. Saunders and K. C. Frisch, Polyurethane, Part I, Chemistry, Wiley-Interscience, New York, 1962, pp. 63-118. 8 G.J. Mol, Thermochim. Acta, 10 (1974) 259. 9 E. Dyer and G. C. Wright, J. Am. Chem. Soc., 81 (1959) 2138. 10 A S T M Stand. D 638-60, 1980. 11 A S T M Stand. D 790M-82, 1982. 12 A S T M Stand. D256-81, 1981. 13 A S T M Stand. D 792066, 1979. 14 A S T M Stand. D 1939-72, 1978.