Table of Contents
The Drawing Process
Factors Affecting Drawn Fiber Polymer Properties
Glassy vs. Rubbery State
Drawn fiber is an improved performance monofilament based upon extrusion technology. Due to the added draw-down step during manufacturing to improve performance, Zeus drawn fiber represents a superior monofilament compared to low-cost and more commoditized monofilaments such as polyethylene terephthalate (PET) or nylon.
Drawn fiber is used in a wide range of applications including over-braiding for hoses, medical braiding, weaves as mist eliminators, strengthening for industrial belting applications, instrument and racquet strings, and even bristles for brushes – including tooth brushes.
Drawn fibers can exhibit a wide range of properties based on the particular polymer from which they are made and synthetic process specifics. While chemical attributes lie at the heart of much of the drawn fiber and polymer properties, for lay audiences this article will mostly discuss the macro-level attributes. Properties such as glass transition temperature, melting temperature, and numerous key mechanical traits of drawn fibers and the polymers from which they are created are described in overview.
Crystallinity and polymer chain length and their relation to drawn fiber properties are other attributes that are presented. Together, these characteristics help explain much of the behavior of drawn fiber polymers. This issue of RESINATE will hopefully serve as a gateway to learn the basics describing these fiber-based products and how this information can be used to suit specific application parameters.
An industry term used to describe an enhanced monofilament extrusion that has been stretched, pulled, or drawn down post extrusion is drawn fiber. ‘Drawn fiber’ refers to the characteristic that the fiber has been made thinner due to the drawing process; thus, it has been drawn down in size (diameter).
Monofilament such as this are made from polymer resin materials that are made up of very long polymer chain molecules. In the melt phase, the chains are arranged in a random or disordered state. The extrusion process initiates orientation of the chains in the extrusion direction (Fig. 1 left). Drawing or pulling on the extruded monofilament further orients the polymer chains in the machine direction resulting in more close packing polymer chains within the fiber (Fig. 1 right).
The overall effect of the draw-down process is increased polymer chain density and strength. Therefore, drawn fiber usually possesses superior and often preferable mechanical attributes in comparison to simple extruded monofilament. Stretching or drawing down of the extruded monofilament is a process (Fig. 2). Stretching is done at elevated temperature to facilitate mobility and greater alignment of the polymer chains.
This is achieved by passing the extruded monofilament through heat such as a water bath or oven. Throughout the stretching process, the monofilament fibers are wound onto Godet rolls (also known as stands or draw stands), a series of multiple off-aligned rolls that are used to control stretching by turning at different speeds.
Godet rolls are normally heated to facilitate the stretching process. Godet rolls can be used in an extensive range of combinations, set-ups, and other arrangements to precisely control phases of the drawing process.
Godet rolls also provide a means to control slippage of the monofilaments. Maximum fiber strength is attained in the drawing process when the polymer chains have reached maximum alignment in the same (machine) direction.
Figure 1. Schematic representation of the extrusion process. Left In the melt phase prior to extrusion, polymer chains are randomly oriented. As the melt-state material is forced through the extruder, the polymer chains become more oriented in the extrusion direction. Right Illustration showing the change in polymer chain orientation during the extrusion process. Pre-extrusion, the polymer chains are loosely packed; post-extrusion, the chains are more closely packed and oriented in the extrusion direction.
Figure 2. Representative schematic of the components in a monofilament draw-down line. Friction or pinch rollers are shown in gold. Godet rolls are temperature controlled and can be heated or cooled to give more precise control of the fiber draw-down process. A heat source such as an oven is commonly used between take-off and draw-down rolls. Multiple roll combinations, set-ups, and other arrangements beyond the simplified representation shown here can be used to regulate various aspects of the drawing process.
Many different kinds of polymer resins can produce drawn fiber monofilament. Zeus Industrial Products, Inc., for example, features drawn fiber made from (but not limited to) polyvinylidene difluoride (PVDF), polyfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), ethylene chlorotrifluoroethylene (ECTFE), ethylene tetrafluoroethylene (ETFE), and polyether ether ketone (PEEK) (Fig. 3).
By joining constituent monomers to form long-chain molecules these carbon chain backbone molecules are formed (Fig. 3). As long-chain molecules, sometimes extending hundreds of carbons long, their properties are greatly altered from those of the monomers or even from shorter-chain homologs. This multiplicity of polymer formation together with their chemical structure forms the underpinning governing polymer properties.
Resulting polymer attributes such as crystallinity, degree of polymerization, molecular weight, and molar volume become the basis for the final material properties at the macro level. These fundamentals provide a basis to explain properties of some of the polymers discussed here and used to produce high performance drawn fiber monofilament.
Polymer chain length is a foundational characteristic of these molecules. Chain length is the result of consecutive addition of monomers until the synthesis process is halted. Polymer chain length can generally be controlled through the addition of reagents to terminate the synthetic chemical reaction, during synthesis.
Or, the synthetic reaction can be allowed to terminate on its own upon exhaustion of the reactant monomers. Polymer chain length is a way of relating the average number of repeating units in the polymer material. C60, for example, indicates that the average polymer molecule chain length for a material is 60 carbons long along its carbon backbone. Also, chain length can be inferred by molecular weight (and vice versa).
Bigger molecular weight polymers have more repeating units than their lower molecular weight homologs (Fig. 4). Many commercial polymers are described in this manner using the average molecular weight of the polymer for the polymer mass.
For example, PEG 5000 refers to polyethylene glycol with an average molecular weight of 5000. Polymers such as these are a mixture with certain chain lengths predominating as opposed to a uniform collection of identical molecular weights.
Figure 3. Six polymers and their immediate synthetic precursors used to produce drawn fiber. Top to bottom: Polyvinylidene difluoride (PVDF) is made by combining vinylidene difluoride monomers; ethylene chrlorotrifluoroethylene (ECTFE) is made from ethylene and chlorotrifluoroethylene (CTFE); ethylene tetrafluoroethylene (ETFE) is produced from ethylene and tetrafluoroethylene (TFE); polyether ether ketone (PEEK) is produced from difluorobenzophenone and disodium hydroquinone; perfluorinated alkoxy (PFA) is made from TFE and perfluorinated vinyl ether; and fluorinated ethylene propylene (FEP) is produced from TFE and hexafluoropropylene (HFP). Polymer chain flexibility at the chemical level decreases from top to bottom in this list as shown.
Figure 4. Relation of polymer chain length and molecular weight for the simple hydrocarbon polyethylene (PE). Molecular weight implicitly describes polymer chain length as molecular weight increases with addition of each C2H4– monomer. For PE and similar carbon-backbone analogs, chain length is indicated by the number of carbon atoms shown in the molecular formula.
Polymer chain length is the root of many properties observed at the macro scale for these materials. Chemical nature aside, chain length affects such vital properties as melt temperature (Tm), molecular weight, and tensile strength. These are the aspects which distinguish polymers from small molecules. Longer chain polymers experience greater chain entanglement than their shorter chain homologs (Fig 5).
This greater entanglement contributes to greater chain-to-chain interaction which also translates into increased tensile strength (Fig. 8A). Long-chain polymers (with higher molecular weights) need more energy (such as from heat) to separate the polymer chains and shift the polymer mass from the solid to the melt phase. Therefore, long-chain polymers display higher melt temperatures, greater viscosity, and higher tensile strength than short-chain polymers with lower molecular weights.
Figure 5. Representative simple chain polymers (PE) showing increased potential for chain entanglement with increasing chain length. Longer-chain polymer homologs have an increased likelihood of chain entanglement in their melt phase. Longer-chain polymers also have extended entanglements with other polymer chains in addition to entanglements with themselves.
Crystallinity is another important feature of solid phase polymers. Hardness, density, impact resistance, and melt temperature are some of the properties that are affected by crystallinity. Crystallinity is related to molecular weight and chain length, although it is not directly correlated with molecular weight. More importantly, however, crystallinity is a function of the chemical topology of the molecular chain and conformational limitations stemming from its chemical features. Crystallinity is not uniform.
Crystallinity ranges from entirely amorphous (no crystallinity) to more than 90% crystallinity for solid phase polymers, depending on the polymer (Figs. 3 and 6). Chain flexibility, including rotational capacity along atom-atom bonds, and intermolecular chain interactions, both attractive and repulsive, affect the way polymer chains can arrange themselves in situ leading to crystal formation.
Slow cooling of the melt phase polymer can increase crystallinity while rapid cooling reduces potential crystal formation. Greater linearity and simple chemical structure of the polymer chains favors crystal formation. On the other hand, complex chain structure and more rigid (less flexible) molecules reduce the capacity for crystal formation. Solid phase polymers such as polystyrene with a high amorphous (less crystalline) character have lower melt temperatures and hardness, however polymers like polyethylene (PE) and polytetrafluoroethylene (PTFE) with greater crystalline (lower amorphous) character have higher melt temperatures and hardness.
Figure 6. Illustration showing amorphous and crystalline regions of a solid phase semi-crystalline polymer material. Amorphous regions are more disordered with randomly oriented polymer chains while crystalline areas are aligned in a regular and ordered arrangement.
Closely tied to the key polymer property of glass transition temperature, Tg, is the degree of crystallinity or amorphous nature of polymers. For solid phase polymer materials that contain both crystalline and amorphous regions, their molecules exist in different environments. Molecules in the amorphous region are highly restricted from movement but can vibrate to a small extent.
This solid phase low temperature and limited vibrational state of the polymer comprises the glassy state of the material and is only relevant to the amorphous region. Glassy state polymers show characteristics similar to crystals in that they are hard, brittle, and rigid.
The polymer chains gain energy as the glassy state is heated and become more mobile with increasing vibration, and gain in disorder. The polymer material then displays a rubbery state and takes on those properties that are characteristically associated with rubber.
At this stage, the polymer still possesses amorphous as well as crystalline regions. The temperature range or zone at which the amorphous region becomes rubbery is known as the glass transition temperature, Tg, and this happens only for the amorphous region.
Crystalline regions of the polymer display a melting point or temperature, Tm, where the solid phase turns to a liquid phase. Semi-crystalline polymers – polymer materials that possess amorphous and crystalline regions – exhibit both a Tg and a Tm. These two temperatures are vital characteristics of the polymer and help to establish its global properties and behavior.
(There are other factors affecting polymer behavior and Tg and Tm including crosslinking of chains, pendant groups, plasticizers, but these shall not be discussed here, and this discussion will be concerned only with those characteristics with respect to the pure polymer).
Figure 7. Change in polymer glass transition temperature, Tg, with change in chain length. Effect of polymer chain mobility is seen in a graphical representation of Tg vs chain length. Tg increases with increasing polymer chain length but exerts less influence on Tg for very extended chains.
Apart from mass, Tg is largely the result of chain mobility. Thus, factors which affect chain mobility must, too, affect Tg. Yet again, chain length plays a vital role as more energy is required to energize – and mobilize – extended chain molecules. Chain length plays a lessening role with respect to Tg, however, as the chain length increases (Fig. 7).
Other critical characteristics surrounding chain mobility, including chain flexibility (at the atom level) and chain interactions (which include both repulsive and attractive forces), affect Tg. Polymer chains with a high degree of chain flexibility can more easily be changed into a rubbery state from the glassy state.
In other words, this change or transition can happen with less input of energy – such as heat – for these molecules. Similarly, chains that exhibit a high degree of attractive forces between them require more energy to be moved from the glassy to rubbery state. Thus, chain flexibility, chain length and chain-chain interactions, play vital roles in determining Tg and the behavior of these polymers.
Most polymer mechanical properties – and particularly those of most popular interest – can largely be grouped as stress-strain relationships. These relationships are unique for each material. Taking tensile strength as a prime example, in its simplest terms, it is the maximum force that a material can support without breaking. For a drawn fiber, this force is applied in a linear direction and is therefore distributed in cross section over the diameter of the fiber. Such force applied through an area is termed stress, σ (or mechanical stress), expressed as force per unit of area: .
Materials can be deformed when stress is applied. This deformation is known as strain, ε. Mathematically, strain is the amount of deformation in the applied force direction divided by the initial length of the material. For example, if the deformation of a material is measured in length: strain = .
Several fundamental characteristics of the fiber can be revealed by graphically plotting stress vs strain (Fig. 8B). Tensile strength shows the applied stress upon fracture of the fiber. Yield strength, on the other hand, shows the point where the linear elastic region of the stress-strain curve ends; this value illustrates where deformation starts to happen. Tensile modulus and toughness can be established using the stress strain curve.
Tensile modulus, E (also referred to as Young’s modulus), relates the stiffness of a material in the tensile direction. This characteristic could be viewed as the material’s resistance to deformation. Tensile modulus is the ratio of stress (σ) to strain (ε): E = . E is determined from the slope of the near-linear region of elasticity of the material from the stress-strain plot (Fig. 8B). This value gives an understanding into the proportional deformation of a material under an applied stress. For a drawn fiber, this property can be viewed as somewhat similar to elongation with respect to the tension direction.
The area under the stress-strain curve is also important as it reveals the toughness of the material (Fig. 8C). Toughness relates the ability of the material to absorb energy before fracture or breaking. The greater the area under the stress-strain curve, the greater the toughness of the material.
After the application of a load or stress to a material and elongation appears to cease, the material may continue to deform though at a much slower rate. This time-dependent continued deformation under constant stress is called creep. This type of viscoelastic deformation is permanent and can be shown through a plot of strain vs time (Fig. 8D).
The plot shows how strain may become constant over time as the polymer material (or fiber) continues to deform. Here, too, chain flexibility and molecular weight play a role. Higher molecular weight polymers and those with less flexible chains result in increased resistance to creep. Therefore, creep should be viewed as a long-term property when considering materials that will be used under significant load for an extended period.
Figure 8. Representative graphs illustrating basic polymer property relationships. (A) Tensile strength increases with increasing molecular weight (MW), but MW has less affect upon strength for extremely long chain polymer materials. (B) Stress-strain curves can be used to determine yield strength, tensile strength, and tensile modulus, E. (C) The area under a stress vs elongation-to-break (strain) curve shows material toughness. (D) Strain vs time graphs for materials under continuous load illustrate viscoelastic creep, a permanent deformation in the material. The relationships depicted in A-D are unique for each polymer material and will have unique graphical representations.
(While there are many more mechanical properties and other graphical representations of the phenomena discussed here, we have chosen to limit our discussion to these basic properties and descriptions as a brief overview of those that are most relevant).
An industry term used for ordinary monofilament that has been stretched and drawn down post extrusion is drawn fiber. This added stage in the creation of the monofilament further orients the polymer chains in the machine direction and results in increased density and tensile strength of the polymer material.
Drawn fiber often holds superior mechanical attributes in comparison to simple extruded monofilament due in a large part to the draw-down step. Fiber characteristics can be controlled during the drawing process making drawn fiber a preferred material in the commercial monofilament fiber market.
Drawn fiber in multiple resins including (but not limited to) PVDF, PFA, FEP, ETFE, ECTFE, and PEEK are offered by Zeus. A main factor affecting drawn fiber traits is polymer chain length. This feature can be controlled during polymer synthesis. Polymer chain length is usually described by the average number of carbons contained in the chains or by the average molecular weight of the polymer chains.
Longer polymer chains increase chain entanglement which in turn affects other critical polymer attributes including tensile strength. Chain length also affects characteristics associated with chain mobility including crystallinity, Tm, and Tg. Longer polymer chains result in higher Tg. Although crystallinity, is more closely associated with the chemical nature and topology of the polymer chains including chain flexibility.
Crystallinity describes the proportion of crystalline and amorphous regions within the polymer material and affects Tm and Tg. Therefore, polymer chain length represents a basis of these properties which are ultimately responsible for the global characteristics of the polymer. In addition to the physical properties of drawn fiber polymers, numerous popular mechanical attributes surrounding stress-strain relationships provide an instructive overview of these materials.
A plot of stress vs strain shows key factors such as yield strength, the point at which it begins to deform, and tensile strength, where the polymer material begins to fracture. The stress-strain curve also shows the tensile modulus of the polymer, or proportional deformation under load; and the area under this curve shows the toughness of the material, or energy that the material can absorb before fracture.
Time is consideration for materials that may be under load for sustained periods. In these instances, polymers may exhibit a permanent deformation known as creep, and this characteristic should be considered. Graphical plots demonstrating these properties are unique to each polymer, and collectively they provide a convenient basis to characterize these materials for optimal practical use.
This information has been sourced, reviewed and adapted from materials provided by Zeus Industrial Products, Inc.
For more information on this source, please visit Zeus Industrial Products, Inc.