linear thermal expansion of pet/pen fibers - Centrum Textil

World Textile Conference - 4 th AUTEX Conference Roubaix, June 22-24 2003 

LINEAR THERMAL EXPANSION OF PET/PEN FIBERS 

Jiří Militký 1 , OlgaPopelová 1 and Arun Pal Aneja 2 

1 Department of Textile Materials, Textile Faculty, TU Liberec Czech Republic, 

e mail: jiri.militky@vslib.cz 

2 E.I. DuPont, Polyester Unit, Kinson, North Carolina, USA 

ABSTRACT 

CODE O-FUN13 

PEN fibers are well known for their high performance properties, resistance and good tensile properties. Their potential 

markets are packaging, films and fibers. Blends of PET and PEN have been attracting increasing interest because they 

combine the superior properties of PEN wit the economy of PET. Mixture of these polyesters form due to transesterification 

during melts processing random copolymers. The glass transition temperature of PET/PEN increases linearly with volume 

fraction of PEN. It is therefore possible to control properties connected with Tg by including of PEN to PET. The main aim of 

this work is utilization of thermo mechanical analysis for investigation of PET/PEN fibers axial thermal expansion α as 

function of temperature. The temperature corresponding to the inflexion point T I and corresponding thermal expansion 

coefficient α are used for characterization of thermal expansion of various co polyesters. 

I 

. 

KEYWORDS 

Co polyesters, axial thermal expansion, thermo mechanical analysis, glass transition 

INTRODUCTION 

The polyethylene 2,6 naphtalate (PEN) is relatively new polyester having rigid naphthalene ring in its backbone on the market. 

This polyester exhibit higher glass transition Tg (about 123 0 C), higher crystallization temperature (194 0 C) and higher melting 

point (270 0 C) than PET. The elastic modulus of crystalline regions of PEN in direction parallel with chain axis is 145 GPa. 

This is about 40% higher than corresponding modulus for PET 108 GPa [6]. PEN fibers are well known for their high 

performance properties, resistance and good tensile properties. Their potential markets are packaging, films and fibers. 

Especially for tire yarns the response to cyclic dynamical deformation is interesting.[9]. 

Blends of PET and PEN have been attracting increasing interest because they combine the superior properties of PEN wit the 

economy of PET. Mixture of these polyesters form due to transesterification during melts processing random copolymers. The 

glass transition temperature of PET/PEN increases linearly with volume fraction of PEN [6]. It is therefore possible to control 

properties connected with Tg by including of PEN to PET. The addition of PEN improves gas barrier properties as well [3]). 

On the other hand due to higher stiffness of PEN [3], the low durability in repeated bending can be predicted. 

Blends of PET and PEN have been attracting increasing interest because they combine the superior properties of PEN wit the 

economy of PET. It is known, that introduction of even low level of 2,6,naphthalene units in place of terephtalate moieties acts 

to disrupt crystallinity, increase T g , improve static chain packing, and decrease local segmental mobility in the amorphous 

phase of copolymers [5-8]. The main aim of this work is utilization of thermo mechanical analysis for investigation of 

PET/PEN fibers axial thermal expansion α as function of temperature. The temperature corresponding to the inflexion point 

T I and corresponding thermal expansion coefficient α I are used for characterization of thermal expansion of various co 

polyesters. The glass transition temperature is estimated from DSC measurements and predicted by simple linear model. 

PROBLEMS OF POLYESTER FIBERS MODIFICATION 

Main activities in the development of synthetic fibers are in the area of the physical and chemical modifications. In this section 

the general aspects of fiber modifications focused to the polyester fibers are discussed. The term „modification“ is used to 

designate a deliberate change in composition or structure leading to an improvement in some fiber properties. The main aim of 

modification can be: 

•To obtain new properties, such as affinity for cationic dyes, 

•To enhance some positive properties, such as resistance to ultraviolet radiation, 

•To suppress some negative properties, such a reducing the pilling tendency. 

In spite of the great number of existing modification methods no consistent classification is available at yet. From the general 

viewpoint, however, it would appear advisable to classify the modification methods by the production steps at which they are 

applied. The following classification scheme results [2]: 

1. Modification in course of polymer preparation 

• Preparation of copolymers,


• Using additives, 

• Reducing the molecular mass. 

1. Modification in course of fiber preparation 

• Drawing and setting conditions readjusting, 

• Speed of spinning changing, 

• Texturing, 

• Cross section geometry changing, 

• Fineness changing, 

• Bi-component and multi-component fibers production. 

1. Modification applied to commercial fibers 

• Grafting, 

• Plasma etching, 

• Controlled surface destruction. 

1. Combined modification 

(e.g. hollow microporous copolyester fibers containing additives) 

Details about these modifications are summarized in the book [2]. The description of modification effect on fiber properties is 

complicated by the fact that modification affects not only fiber structure and fiber properties but also conditions of fiber 

preparation. The main problems can be summarized to the following points: 

• It is difficult to measure structural parameters directly influencing given property (tie chain) 

• The properties are distinctly dependent on chemical composition of fibbers (chain flexibility) 

• Structural parameters are measured in static state whereas properties are usually determined in a dynamic state 

• Structure is changed during the measurement of some properties 

Influence of commoner on fiber properties can divided to the following categories 

A. Commoner has no effect 

B. Commoner has indirect effect 

C. Only the commoner amount matters (equilibrium melting point) 

D. It is the type of commoner that matters (Tg, dyeability) 

E. Commoner type and concentration have effect simultaneously 

Modification generally affect on the other technologically important characteristics as technology of fibre preparation, 

molecular mass of melt, degree of melt degradation and rate 

of crystallization. It is therefore difficult to separate effect of chemical modification from modification of technological 

parameters. Effect of combined physical and chemical modification on the basic structural characteristics and dyeability of 

modified fibers is described in the [1]). 

PET/PEN FIBERS 

It is known, that introduction of even low level of 2,6,naphthalene units in place of terephtalate moieties acts to disrupt 

crystallinity, increase Tg, improve static chain packing, and decrease local segmental mobility in the amorphous phase of 

copolymers. These results are supported by the geometrical structure of the two units (see fig.1) 

O 

C 

O 

C 

O 

C 

O 

C 

0.8 nm 

0.57 nm 

A 

Fig. 1 Dimensions of 2,6 naphthalene (A) and terephtalate (B) units 

B


The molar volume of PEN V PEN = 182.4 cm 3 /mol is higher than molar volume of PET V PET = 144 cm 3 /mol. The amorphous 

density of PEN ρ aPEN = 1327 kg/m 3 is lower than amorphous density of PET ρ aPET = 1333 kg/m 3 . The crystalline density of 

PEN ρ cPEN = 1407 kg/m 3 is lower than density of PET ρ cPET = 1440 kg/m 3 . 

The 2,6, naphthalene unit is substantially larger and does not fit into the unit cell of PET. The bulky kinked 2,6 naphtalene 

units in PEN are much less mobile than the terephtalate units. Amorphous phase free volume and local segmental mobility are 

reduced due to PEN presence. At the temperatures around 60 0 C or higher the motion of rigid naphthalene ring occur. One 

possible motion is hindered rotations of the naphthalene rings about the backbone. Another possible motion is interlayer 

slippage of the naphthalene rings. Both types of motion are schematically drawn on the fig. 2 [7]. 

During the deformation of PEN, the naphthalene rings are rapidly aligned parallel to the surface of the fibers and also occurs at 

highly localized regions. The subsequent slippage can leads to necking behavior during deformation. The naphthalene portions 

exhibit higher creep compliance. 

Fig. 2 Possible mechanisms of 2,6 naphthalene rings motion 

Co polyesters of PET/PEN have a lot of interesting properties influenced by the presence of naphthalene rings. For some 

applications is interesting to know the degree of endurance in bending. 

PET/PEN FIBERS PREPARATION 

The co polyester samples having various content of PEN were prepared under comparable conditions. Basic characteristics of 

these samples are given in the table 1.Melting behavior were investigated from DSC traces measured on the Perkin Elmer DSC 

6 apparatus. The melting peak has been characterized by melting temperature T m [ o C] (at peak maximum) and heat of fusion 

H f [J/g] (area under peak). Results are summarized in the table 1 

Table 1. Basic thermal properties of PET/PEN 

Sample PEN content [%] Tm [ o C] Tg [ o C] H f [J/g] α *10 

6 [deg 

−1 ] T I [ o C] 

A 0 254.77 77.62 51.35 0.44 57.88 

B 5 246.996 79.59 43.082 0.15 55.08 

C 10 236.235 82.52 49.16 0.63 71.74 

D 15 227.37 85.46 35.815 2.10 75.27 

E 20 214.016 87.4 36.479 5.44 77.48 

THERMAL ANALYSIS 

Thermal behavior were investigated from DSC traces measured on the Perkin Elmer DSC 6 apparatus. The melting peak has 

been characterized by melting temperature T m [ o C] (at peak maximum) and heat of fusion H f [J/g] (area under peak). Glass 

transition temperature Tg has been characterized by the point of inflection on the first endothermic bend on DSC trace. Results 

are summarized in the table 1. It is well known, that the Tg of random copolymers is lowered due to presence of modifying 

units. Because the PET/PEN forms random copolymer is glass transition temperature well described by the linear mixing rule 

(Gibbs-Di Marzio relation) [4] 

Tg( f ) = T * f + T * (1 − f ) (1) 

gPEN 

g PET 

I


where and Tg(f) is in units [ 0 C]. From published values T gPET =81 o C T gPEN =123 o C it can be predicted ideal Tg(f). In reality is 

Tg dependent on a lot of molecular (molar mass and distribution) and structural factors and therefore these parameters should 

be computed for real polymers. The experimental dependence of Tg on f is shown on the fig. 5. 

Fig. 3. Dependence of glass transition temperature Tg on PEN fraction f. 

The linearity of Tg on f dependence is clear. The least squares regression model has the form. 

Tg = 77.402 + 50.86*f (2) 

From these values the Tg of both polyesters has been computed Tg PET =77.43, Tg PEN -=128.29. 

THERMO MECHANICAL ANALYSIS 

Temperature dependences of co polyesters dimensional changes h(T) were measured on the TMA CX apparatus. Typical h(T) 

curve is shown on the fig. 4. The axial thermal expansion were computed from well known relation 

1 ∆h( 

T ) 

α = 

(3) 

l ∆T 

0 

where l 0 is initial sample thickness and ∆ h(T ) is first difference of dimensional changes. For computation of α (T ) and 

identification of the inflextion point the derivative analysis has beeen used. First of all the data were smoothed by the robust 

3R (three point repeating median) combined with Hamming windows [10]. Smoothed data were numerically derived by the 

three point central difference formula. Results for the sample No.E are shown on the fig. 5. The values T I and α 

I 

computed as 

coordinates of global maximum on the second derivative curve. The means from three repeating measurements are given in the 

last two columns of table 1.


Fig 4. Typical h(T) curve for sample No. E 

Fig 5 First and second derivatives of h(T) for sample No. E 

The dependence of 

α 

I 

on the volume fraction of PEN is given on the fig. 2.


Fig 2. Influence of PEN content on the axial thermal expansion of copolyesters. 

The marked no linearity of this dependence is probably valid in the restricted range of PEN concentration only. 

CONCLUSIONS 

A lot of PET/PEN fibers properties are connected with improved thermal and mechanical characteristics due to naphthalene 

rings presence. On the other hand the crystalline phase is lowered and mechanical relaxation caused by the motion of 

naphthalene rings appeared. As results of these changes the increasing of PEN content leads to increasing of thermal 

expansion coefficient α . 

I 

AKNOWLEDGEMENTS: 

This work was supported by the research project LN00B090 of Czech Ministry of Education 

REFERENCES 

[1] Militký, J.: Some theoretical problems of PET fibers modifications, Proc. Int. Conf. New Fiber, Ueda July 1999 

[2] Militký, J. et all. : Modified Polyester Fibers, Elsevier, Amsterdam 1991 

[3] McDowell C.C. et all.: Synthesis, physical characterization, and acetone sorption of random PET/PEN copolymers, J. 

Polym. Sci. B36, 2981 (1998) 

[4] Saleh Y.S., Jabarin A.: Glass transition and melting behavior of PET/PEN Blends, J. Appl. Polym. Sci. 81, 11 (2001) 

[5] Higashioji T., Bhusdan B.: Creep and shrinkage behavior of improved ultra thin polymeric film, J. Appl. Polym. Sci. 84, 

1477 (2002) 

[6] Nakamae K. et all. : Temperature dependence of the elastic modulus of the crystalline regions of PEN, Polymer 36, 1401, 

(1995) 

[7] Wu G., Cucullo J.A.: Structure and property studies of PET/PEN melt blended fibers, Polymer 40, 1011(1999) 

[8] Canadas J. C. et all.: Comparative study of amorphous and partially crystalline PEN, Polymer 41, 2899 (2000) 

[9] Sikkema D.J.: Manmade fibers one hundred years, J. Appl. Polym. Sci. 86, 484 (2002) 

[10] Meloun M., Militký J. and Forina M.: Chemometrics in Analytical Chemistry vol. II, Interactive Model Building and 

Testing on IBM PC, Ellis Horwood, Chichester 1994

linear thermal expansion of pet/pen fibers - Centrum Textil

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?