HPMO MYLAR

MYLAR ® HPMO

A NEW HIGH PEPFORMANCE FILM FOR THE ELECTRIC MOTOR INDUSTRY

by C.C. WALKER of DuPont

SUMMARY

Mylar ® HPMO is DuPont's designation for its high performance-low oligomer polyethylene terephthalate film for electric motor insulation. The following describes an accelerated hydrolytic stability test whcih: 1) accurately differentiates films with improved hydrolytic and thermal performance and 2) clearly demonstrates the superiority of Mylar® HPMO over all current offerings of competitive high performance - low oligomer films whcih have UL RTIs as high as 140 ºC (electrical) and 130 º C (mechanical)

I. OLIGOMER CONTENT (EXTRACTABLES)

The oligomer content of polyethylene terephthalate(PET) film is measured by determining the weight loss of film after extraction for 24 hours in boiling xylene. HPMO film contains < = 0.5% extractables. Standard film typically contains 1.0 -1.3% extractables. Approximately 90% of the extracted material is cyclic trimer. a specific PET oligomer common in all PET films.

II. HYDROLYTIC STABILITY TEST

In 1959, McMahon, birdsall, Johnson, and Camilli published a very detailed study devoted to the hydrolytic stability of PET film and how hydrolysis affects the integrity of PET film as an electrical insulator (1). This work showed that PET film is no longer a reliable electrical insulator when the intrinsic viscosity, (n), as measured in TCE/phenol, 0.60/1.00 wt/wt, reaches approximately 0.30. quoting from the Bell study "...the critical (n) of film (is) namely 0.3042 (and) samples degraded to this level... are all brittle and essentially without tensile strength (p.74); the data indicate that hydrolyzed film does not lose its dielectric strength until it is brittle and physically weak" (p.72). At this low (n) the film has become so brittle it cannot be folded upon itself without fracture. These observations regarding brittleness and (n) have been consistently reconfirmed over the intervening years by continuing studies within DuPont.

Although one may vary temperature and water concentration to evaluate the extent to which one film may be superior to another with regard to hydrolytic stability, a favored procedure in this laboratory is to simply immerse the film under study in boiling distilled water and periodically withdraw samples for appropriate analysis. Such an environment provides a very constant temperature and water concentration. It has been our experience, that although possible, water measurement and mnaangement are difficult to control in sealed tube tests. Further, we find that such sealed tube tests are no more valuable in predicting relative improvements, even when well run, than the simple boiling distilled water test.

1) W. McMachon, et.al., J. Chem, and Engg. Data, Vol.4, p56-79, 1959

Assuming that boiling distilled water is employed as the test environment for hydrolysis, the next criteria to establish is what constitutes the failure point?The electric motor industry and PET film manufacturers have used a varity of failure criteria, these are: 1) the point at whcih 50% of the original elongation remains, 2) the point at which 50% of the original strength remains, 3)when elongation reaches an absolute value of 10% or less; other criteria are: 4) when the intrinsic viscosity reaches a critical value (about 0.3 from the Bell work), 5) when film cannot be folded upon itself without fracture (the flex test) and 6) when a critical density is reached.

Actually criteria 1 above fails to accurately represent a failure point since many films still exhibit complete electrical integrity at the point that 50% of their original elongation remains. Michael Minnick of G.E. empolys the attainment of an elongation of 10% or less as a failure point in sealed tube tests (2). This is more realistic as we will see. Mylar® HPMO was examined versus the standard motor films as well as an "improved performance" competitive low oligomer product. This latter film has been given a RTI of 140 º C (electrical) and 130 ºC (mechanical) by Underwriters Laboratory (UL). We examined the relative performance of these films in boiling distilled water employing all of the noted criteria. This work shows that all of these criteria are useful for asscessing relative performance levels in insulation in motor applications and that the boiling water enviornment that promotes hydrolytic degradation provides an accurate ranking based on UL RTI values.

RESULTS OF THE HYDROLYTIC STABILITY STUDY

The films employed and their identifying numbers are: 250 micron (10 mil) Mylar® HPMO (#1), 250 micron (10 mil) Mylar® MO (#2), 250 micron (10 mil) "standard" competitive (#3) 250 micron (10 mil) "improved performance" competitive (#4). Films 2 and 3 are "standard" motor films and film 4 has made claims for superior hydrolytic and thermal stability which were confirmed by the UL RTI ratings. Film 1, as noted earlier, is DuPont's entry into the high performance filed. Precut tensile specimens of these films were placed in boiling distilled water and samples periodically removed. Tensile properties, density and intrinsic viscosity (n) were determined on the samples. Let us now see how these properties are affected in this test and how the test reflects UL ratings and the relative performance of these films.

ERCENTAGE OF STRENGTH REMAINING (Table1)

Using the 50% criteria we assign failure to "standard" competitive and MO sometime close to, but after 216 hours and well before 360 hours. At 216 hours MO and "standard" competitive have 57% and 60% respectively, of their original strength remaining and 0% at 360 hours. HPMO reaches this failure point close to, but sometime after 504 hours where it has 56% of its remaining tensile strength, and before 640 hours where its strength has been reduced to 25% of its original value. "Improved performance" competitive fails sometime after 360 hours (57%) but well before 504 hours (0%). It is possible to rank these films from the longest to the shortest lived with the approximate failure time in parenthesis:

HPMO (~504 hrs) > #4 (~360 hrs) > #3 (~216 hrs)

Table 1 Percentage of Original Tensile Strength & Percentage of Original Elongation Remaining, TS/E
	0 HR	72 HR	144 HR	216 HR	360 HR	504 HR	640 HR
1) HPMO	100/100	91/108	91/98	88/110	75/94	56/55	25/3
2) MO	100/100	86/102	75/70	57/11	0/0	0/0	0/0
3) "Standard Competitive"	100/100	84/89	72/64	60/4	0/0	0/0	0/0
4) "Improved Performance" Competitive	100/100	89/129	89/98	82/91	57/12	0/0	0/0

PERCENTAGE OF PERCENT ELONGATION REMAINING (Table1)

Using 50% as the failure point and applying the same line of reasoning as described above we have the following rankings:

HPMO (~504 hrs) > #4 (> 216 hrs < 360 hrs) > MO ~ #3 (> 144 hrs < 216 hrs)

INTRINSIC VISCOSITY (n) AND THE FLEX TEST (Table 2)

These tests results are given in Table 2 and are grouped together to show the remarkable agreement of flexibility and (n). While considering this data, the reader is reminded to recall that the Bell study infers that the end of the useful life of biaxially oriented PET as an electrical insulator is approached at (n) = 0.30. Let us now examine the results of the viscosity/flexibility tests:

The first film to fail the flex test is "standard" competitive at 216 hours. It is significant that at this time the intrinsic viscosity of this film was 0.33. This value is very near to the failure level of 0.30 noted by the Bell Laboratory work. Note also, that at this same time (216 hours), that DuPont's MO film was still flexible with (n) = 0.36. at 360 hours MO is brittle with (n) = 0.36 while "improved performance" competitive film is still flexible with (n) = 0.38. At 504 hours the "improved performance" competitive film is brittle with an (n) = 0.38 while HPMO is still flexible with an (n) = 0.39. If we rank these films from the longest to the shortest life based on flexibility we have :

HPMO (> 504 hrs < 640 hrs) > #4 (> 360 hrs < 504 hrs)

> MO (> 216 hrs < 360 hrs) >#3 (>144 hrs < 216 hrs)

Based on the findings in Table 2, it appears that film which has an (n) >0.35 remains flexible, but below this value embrittlement can occur. Note for example that MO was flexible at 216 hours with an (n) = 0.36 and #3 is brittle at 216 hours with an (n) = 0.33. Similar obervations were recorded by C. Heffelfinger of this laboratory during his study of the hydrolysis of motor film in 1967. Quoting from his report "All biaxially, oriented heat set films (PET) fail when viscosity has decreased to about 0.34." Heffelfinger's failure point was when the film was brittle (failed the flex test). If we rank these films according to the estimated time to reach (n) = 0.35, we have from the longest to the shortest times:

HPMO (> 504 hrs < 640 hrs) > #4 (> 360 hrs < 504 hrs)

> MO (> 216 hrs < 360 hrs) >#3 (>144 hrs < 216 hrs)

Table 2 FLEX TEST ( PASS - FAIL ) & (n) AT AND JUST PRIOR TO FAILURE
	0 HR	72 HR	144 HR	216 HR	360 HR	504 HR	640 HR
1) HPMO	P	P	P	P	P	P(0.39)	F(0.29)
2) MO	P	P	P	P(0.36)	F(0.26)	F	F
3) "Standard Competitive"	P	P	P	F(0.33)	F(0.24)	F	F
4) "Improved Performance" Competitive	P	P	P	P	P(0.38)	F(0.28)	F

DENSITY (Table3))

The use of density as a criteria for failure has not been empolyed very often, howeverm a study of the increase in density as hydrolysis proceeds has provided some interesting informaiton. Unlike viscosity, there is no general agreement as to what density value must be achieved to render the film no longer functional. An examination of earlier work leaves little doubt that film with a density of 1.4100 g/cc is quite brittle and no longer functional. From Table 3 we see that film with a density of 1.400 g/cc is functional. Note that #3 has a starting density of 1.4008 very close to 1.400. For purposes of discussion let us assume the critical value for density is between 1.4100 (brittle) and 1.4000 (flexible). A value half way between these values would be a logical choice, so let us use 1.4050 as the critical density value. Looking at Table 3 it is clear that #3 is the first film to reach a density of 1.4050 as the critical density value. Looking at Table 3 it is clear that #3 is te first film to reach a density of 1.4050 g/cc and does so sometime beyond 144 hours but before 216 hours. Mylar® MO follows, but does so very close to 360 hours, well beyond #3. Continuing the analyses in this fashion we rank the films based on the time required to reach the 1.4050 g/cc density value. Ranking from longest to shortest times are:

HPMO (~ 640 hrs) > #4 (>360 hrs < 504 hrs)

> MO (~ 360 hrs) > #3 (> 144 hrs < 216 hrs)

The changes in density for MO, #3 and #4 as these samples hydrolyze, are fairly well behaved showing regular increases. The HPMO samples do not all show such regular increases in density, although they do show a slow upward trend. In spite of the fact that the HPMO density data is not well behaved, it is clear that the rate of increase in density is much slower for HPMO than #4, #3 and MO. These results suggest that HPMO crystallizes much more slowly than the other films studied. Films exhibiting slow rates of crystallization are expected to remain flexible longer than those with nore rapid rates.

Table 3 DENSITY g/cc
	0 HR	72 HR	144 HR	216 HR	360 HR	504 HR	640 HR
1) HPMO	1.3938	1.3941	1.3923	1.3929	1.3976	1.4005	1.4050 *
2) MO	1.3977	1.3961	1.3982	1.4003	1.40055 *	1.4186 *	ND *
3) "Standard Competitive"	1.4008	1.4023	1.4032	1.4077 *	1.4099 *	1.4182 *	ND *
4) "Improved Performance" Competitive	1.3970	1.3996	1.3986	1.3997	1.4027	1.4070 *	1.4115 *

* Fails Flex Test
ND - Not Determined

% ELONGATION (Table 4))

When PET films are subjected to hydrolysis in an environment saturated with water the % elongation usually increases in the early stages due to the plasticization f the filmby water. This increase is then followed by a steady decline. It has been our experience that PET film whose elongation has fallen below 10% has become embrittled and cannot usually be folded upon itself without breakage. It is this failure criteria which Minnick employed in his study (2) and was also demonstrated in the earlier Bell study, page 76, (1). since the Bell study reported that, "hydrolyzed film did not lose its dielectric strength until it degraded to the point where it is brittle and physically weak" we should expect a loss in dielectric integrity at or below 1-% elongation. Let us now examine our test results using 10% elongation, or less, as a failure criteria. These results are in Table 4. Also in this table we note whether or not the film failed the "flex" test, that is, whether or not it was brittle. Using the 10% elongation criteria let us now examine the results in Table 4. The first film to fall below the 10% level is #3. At 216 hours it has 7% elongation and also failed the "flex" test. At 360 hours Mylar® MO had 0% elongation and was brittle, but the prior sample at 216 hours had 18% elongation and was flexible. It would appear that the <10% elongation criteria is a reasonable predictor of flexibility and functionality. It is possible to rank these films in the order of those taking the longest time to reach <10% elongation to those taking the shortest time. The ranking is:

HPMO (> 504 hrs < 640 hrs) > #4 (> 360 hrs < 504 hrs)

> MO (> 216 hrs < 360 hrs) > #3 (> 144 hrs < 216 hrs)

Table 4 HPMO VS OTHER FILMS " AVERAGE " ELONGATION
	0 HR	72 HR	144 HR	216 HR	360 HR	504 HR	640 HR
1) HPMO	157	170	154	172	147	87	4 **
2) MO	159	162	112	18	0 **	0 **	0 **
3) "Standard Competitive"	P	P	P	F(0.33)	F(0.24)	F	F
4) "Improved Performance" Competitive	146	188	143	133	17	0 **	0 **

** Fails Flex Test

A SUMMARY OF RESULTS AND CONCLUSION (Table 2)

Asummary of the film rankings is provided it Table 5. Regardless of the technique employed one may draw the following conclusions:

Table 5 SUMMARY OF RANKINGS (HRS) BY EACH CRITERIA
% STRENGTH REMAINING	HPMO (~504) > #4 (~360) mo ~ #3 (~216)
% ELONGATION REMAINING	HPMO (~504) > #4 (>216 <360) >MO ~#3 (>144 <216)
FLEX TEST	HPMO (>504 <640) >#4 (>360 <504) >MO (>216 <360) >#3 (>144 <216)
( n )	HPMO (>504 <640) >#4 (>360 <504) >MO (>216 <360) >#3 (>144 <216)
DENSITY	HPMO (~640) > #4 (>360 <504) >MO (~360) > #3 (>144 <216)
% ELONGATION	HPMO (>504 <640) >#4 (>360 <504) >MO (>216 <360) >#3 (>144 <216)

1) DuPont HPMO film consistently shows longer hydrolytic life than the "improved performance" competitive and all other films tested.

2) The "improved performance" competitive product exhibits improved hydrolytic stability vs Mylar® MO and a "standard" competitive product.

3) Mylar® MO exhibits improved hydrolytic stability vs. the "standard" competitive product in four of the six tests and equivalent performance the other two.

As was noted earlier the "improved performance" competitive product has been granted a RTI of 140 oC (electrical) and 130 oC (mechanical) by UL; the extended lie of this film in the hydrolytic stability test, using any criteria for failure, vs. that of Mylar® MO and the "standard" competitive product which have 105 oC ratings, clearly demonstrates the validity of the hydrolytic stability test to assess relative hydrolytic/thermal endurance. The preceding ntoed validation strengthens the contention that Mylar® HPMO provides superior performance to the "improved performance" competitive product and all other polyester films carrying similar elevated (140 oC electrical, 130 o mechanical) UL RTI ratings. All of these results are in agreement with a number of other tests carried out by DuPont as well as certain US and Asia-Pacific electric motor manufacturers.


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