Thermal and Mechanical Analysis of Nylon 6 and Nylon 66 Blends

7/23/2019 Thermal and Mechanical Analysis of Nylon 6 and Nylon 66 Blends

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San Jose State University

SJSU ScholarWorks

Masters Theses Masters Theses and Graduate Research

1997

Thermal and mechanical analysis of nylon 6 and

nylon 6,6 blends

Daniel Vroom

San Iose State University

Follow this and additional works at: http: / /scholarworks.sjsu.edu/etd_theses

Recommended Citation

Vroom, Daniel, "Thermal and mechanical analysis of nylon 6 and nylon 6,6

blends" (1997). Masters Theses. Paper 1479.

This Thesis is brought to you for free and open access by the Masters Theses and

Graduate Research at SJSU ScholarWorks. It has been accepted for


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inclusion in Master's Theses by an authorized administrator of S_]SU

Scholarworks. For more information, please contact Library-sCholarWorllOo,000 g/mol) (3).


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Nylons form products that have high tensile and compressive strengths, however

their

impact strengths may be less than what is needed for certain applications. They are

alsonotch sensitive, which can lead to failure at the site of nicks, scratches, or chemical

degradation (4). These properties are due to the high degrees of crystallinity. For

this

reason, nylons often have to be modified in some manner, such as reinforced,

plasticized,

or blended, in order to produce parts of suitable impact strength (4).

1.3 IMPORTANCE OF POLYMER BLENDS

The blending of polymers has steadily increased in scope and importance in the

last

several decades (3). The mixing of two or more polymers. which have different

properties, to give a product with some desirable intermediate property, is not only

a

technically sound practice but usually less costly and time consuming than

synthesizing

a new polymer (3).

Some of the benefits of poly-blending include enhanced processability, increased

modulus for elastomers, decreased modulus for thermoplastics, improved thermal


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properties, and improvement in impact resistance (3). By blending polymers in

certain compositions, one can tailor the end product to meet the end needs, in terms

ofmechanical and thermal properties.

Polymer blends also can have advantages over other types of polymer modifiers,

such

as small molecule plasticizers. Many brittle plastics, such as polyvinyl chloride

(PVC),

often have to be plasticized to prevent cracking and enhance processability.

Addition of

small molecule modifiers (plasticization) can decrease the modulus enough to

prevent

brittleness. However, the plasticizer may be leached or vaporized out of the

polymer due

to contact with certain solvents or excessive heat. This can again lead to brittleness

and

cracking. If a brittle polymer can be plasticized with another compatible polymer,

however, this problem can be avoided.

1.4 SELECTION OF BLEND COMPONENTS

For a blend of two polymers to be miscible, the excess Gibbs energy must be


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negative. The expression for the free energy for a mixture (where the subscript m

denotes

mixture, is given by equation [3].

AG, =AHm - TAS,,, [3].

For a two polymer system, the change in entropy AS, while positive, is usually

very small

due to the fact that the system contains relatively few large molecules, resulting in

relatively few possible configurations. Thus, for most polymer blend systems, the

free

energy is basically a function of the heat of mixing, which must be near zero or

negative

for spontaneous mixing but is usually positive for polymer/polymer systems.

Equation [4] by Hildebrand and Scott (3) is a widely used equation for calculating

AH.

AH... = V...[(& -52):] vi 1': [4]

where V, is total volume of the mixture, 5 is the solubility parameter and is the

molar

volume of the components. Inspection of equation [4] reveals that it will only

calculate

positive values for the heat of mixing. Thus, according to [4], all polymer blends

will


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exhibit phase separation. However, miscible systems have been identified. The

discrepancy arises from the fact that equation [4] does not take into account

possible

strong interactions between molecules such as dipole/dipole interactions orhydrogen

bonding. Such strong interactions between molecules can lead to negative values

for the

heat of mixing and thus miscibility. This suggests that selecting a polymer system

in

which strong interactions between the polymer chains can occur can possibly lead

to

miscibility.

The selection of which polymers to blend is also a function of the properties that

are

needed for the end product. The most common blends involve those between

elastomers

and plastics, two amorphous plastics, crystalline and amorphous plastics, and less

commonly two crystalline plastics. In many instances, plastics can be too brittle for

many

applications. The addition of an elastomer can increase the impact strength to

acceptable

levels. Conversely, the addition of a rigid plastic to an elastomer can increase the

modulus of the elastomer.


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The change in properties of the major component polymer by the addition of the

minor component polymer is partly a function of the amount of minor component

added.

It is also a function of how well phases adhere to each other in immiscible systems.By

measuring the desired property as a function of blend composition, one can

determine the

optimum composition to achieve the desired results.

As previously discussed, the compatibility of the selected polymers is a major

concern. Most polymers tend to be thermodynamically incompatible, leading to

some

degree of phase separation. If the polymer pair is totally immiscible, mechanical

properties can suffer if there is poor adhesion at phase boundaries. For example,

fractures

can initiate at poorly joined phase boundaries. However, partially compatible, or

less

commonly, totally compatible pairs, can lead to significant improvements in

properties.

The better adhesion at phase boundaries, in partially miscible blends for example,

can

give rise to better mechanical performance.

1.5 ANALYSIS OF BLEND PERFORMANCE


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In analyzing the blends of two polymers, the level of compatibility of the blend and

the resulting effects on the physical properties are of interest. The major physical

parameter in determining miscibility is the glass transition temperature. A totally

immiscible system will exhibit two distinct glass transitions, while a miscible pair

will

exhibit a single glass transition that is usually the weight average of the pure

component

transitions (2).

The analysis of mechanical properties is also important in the analysis of blends. In

general, two component mixtures may be described by equation [5].

P = P,C, + PZCI + IP,P2 [5]

where P is a particular property of the blend, C is the concentration (subscripts

denote

component 1 and 2 respectively), and I is an interaction parameter which describes

the

thermodynamic compatibility of the two components (2). When I is greater than

zero,

the blend is termed synergistic and has property values better than those that would

be

calculated from the weighted average. Conversely, I values less than zero aretermed

nonsynergistic, and have property values less the weighted averages. When I

equals zero,


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property values reflect the weighted averages. This behavior is illustrated in Figure

1.

Miscible or partially miscible blends tend to display synergistic or non-synergistic

behavior over the entire composition range. Synergistic behavior may arise from

molecular interactions between chains, while non-synergistic behavior can be

caused when

a molecular interaction is disrupted due to the addition of a second polymer (2).

lmmiscible blends will usually exhibit synergistic behavior over part of the

composition

range, and non-synergistic behavior over the rest, depending on which component

is the

continuous phase (5). 0'

Hal! H v

Hanzyflerqlxtxc

(Ks)

Parr.-er Canczntruuan.

Figure 1: Property relationship versus blend concentration.(3)


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1.6 SCOPE OF STUDY

This study investigates the effects of blending two semi-crystalline polyamides,

nylon

6.6 as the major component (except for the 50% blend), and nylon 6 as the minor

component. and attempt to determine the compatibility of the blend and the

resulting

effects on the thermal and mechanical properties. This was done using thermal

analysis

(DSC) , dynamic mechanical analysis (DMA). and the measurement of Izod impact

strength of various blends.

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

While many investigations have been done in the field of polymer blends,

considerably less has been undertaken in the area of nylon/nylon blends. The trend

towards finding compatible polymer pairs has been of particular interest. The

ability to

predict miscibility in blends, either through the development of semi-empirical

relations,


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or the modification of expressions describing the thermodynamics of mixing for

polymer

systems has also been of interest.

2.2 ANALYSIS OF BLENDS

The analysis of blends is principally concerned with determining phase behavior of

the components. The presence of a single glass transition (Tg) is generally themain

determinant of miscibility. Different methods, some more sensitive than others, are

often

employed to determine Tgs of mixtures. In an early work on the miscibility

problem,

Sperling et al. (6) investigated the system polyethyl acrylate (PEA) and

polymethlymethacrylae (PMMA). Dilatometric measurements indicated the

presence of

a single Tg. The authors presented several possible interpretations for this result.

Most

notably, that the single Tg was due to a miscible system. However the authors also

surmised that the transitions of each component may have been "run together" due

to pressure caused by unequal expansion coefficients. Their ultimate conclusion

was that

more sensitive measurements were needed to clarify the situation.


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Calorimetric and dynamic mechanical measurements, have proved to be more

sensitive in Tg detemiinations. Bernstein, et al. (7) studied the blends of

polyvinylidene

flouride (PVF) with polyvinyl acetate (PVAc). polyvinyl propionate (PVPr), and

polyvinyl

butyrate (PVBu). Samples of blends were subjected to differential thermal analysis

(DTA) and dynamic mechanical measurements (DMA). The systems PVF/PVPr

and

PVF/PVBu exhibited two alpha peaks on damping curves (loss modulus versus

temperature) indicating two glass transitions, therefore immiscibility. The system

PVF/PVAC, however, exhibited only a single alpha peak over the entire blend

composition

range. Furthermore, the crystallization of PVF is affected by the addition of PVAc.

DTA

results indicated that PVAc depressed the melting point of PVF, suggesting thatPVAc

is a miscible diluent for PVF.

DSC and DMA studies were used by Schurer (8) to investigate isotactic (i-) and

syndiotactic (s-) forms of PMMA blended with polyvinyl chloride (PVC). DSC

scans for

the i~PMMA/PVC system indicate two distinct endothermic shifts in the baseline,

indicating two phases. DMA damping curves confirm this result, showing two loss

maxima. The system s-PMMA/PVC shows only a single damping maxima for all


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compositions indicating miscibility.

MacKnight, et al. (9) studied the system poly(2,6-dimethylphenylene oxide)

(PPO)/polystyrene (PS). A single glass transition between the two single

component transitions indicated miscibility. However. DMA measurements,

showed a loss maxima

that was at least partially resolvable into two peaks. This would indicate that DMA

is

somewhat more sensitive than DSC for determining multiple glass transitions.Dielectric

relaxation measurements on this system revealed only a single, somewhat

broadened

maximum for the mixtures. The authors conclusions were that mixing did not

occur at

the segmental level, but mixing did occur to an extent between pure polymer rich

phases

such that distinct Tgs were not observed, although broadened ones were.

Since the glass transition is characterized by a sharp decrease in elastic modulus, it

seems reasonable to expect that mechanical measurements would be more sensitive

to this

mechanical event" than thermal measurements would. DSC may not be sensitive

enough

for determining transitions in blends in which the pure component Tg's are as close

as


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in the MacKnight study (9).

2.3 MISCIBILIT Y AND MECHANICAL PROPERTIES

The miscibility of a polymer system usually has a strong influence on the

mechanical

properties. Yee (10), studied blends of poly(2,6-dimethyl p-phenylene oxide)

(PMZPO)

and polystrene (PS). DSC and DMA measurements indicated a single Tg over the

entire

composition range, which lead to the conclusion of miscibility. Tensile stress

measurements of each blend showed positive blending effects, with a maxima at

the 10%

mixture. The author thus concluded that mixing had occurred on a segmental level.

Blending can have negative effects on mechanical properties as well as positive

effects. Mekhilef, et al. (1 1) investigated the behavior of polystyrene (PS) blended

with

styrene-isoprene-styrene (SIS) and ethylene-propylene rubber (EPR). The Izod

impact

strength of the PS/EPR system and the PS/SIS system both showed negativeblending

effects up to 20% modifier. SEM analysis showed voids and sharp boundaries

around


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the EPR rich phases (PS/EPR blend) resulting in a weak interphase and the

resulting poor

impact strength. The PS/SIS system showed a similar weak interphase.

Chin and Hwang (12) investigated the additions of polycarbonate (PC) and nylon 6

(PA 6) to ABS rubber. Blends of ABS 1000/PC and ABS3000/PC showed negative

blending effects for notched Izod impact tests over the entire composition range as

illustrated in Figure 2. However. tensile modulus measurements for the same

system

showed negative effects only below about 50%, with positive blending effects

above this

value. This behavior can be seen in Figure 3.

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Thermal and Mechanical Analysis of Nylon 6 and Nylon 66 Blends