Design Properties for Kynar Resins

KYNAR fluoropolymers are strong and tough as reflected by their tensile properties and impact strengths. Compared to many thermoplastics, KYNAR fluoropolymers have excellent resistance to creep and fatigue, yet in thin sections such as films, filament, and tubing, KYNAR PVDF components are flexible and transparent. As a material of construction for pumps and pipes, KYNAR resins exhibit excellent resistance to abrasion. Where load bearing is important, KYNAR fluoropolymers are rigid and resistant to creep under mechanical stress and load.

All KYNAR resin grades are interchangeably weldable, giving the design professional the option of having combinations of rigid and flexible materials in a given application.

Kynar properties profile.

Linear Expansion

The linear expansion of the low-to-high viscosity KYNAR 700 homopolymer resin series and of KYNAR 460 homopolymer resin grade is shown in Figure 1 for the temperature range -120° to l60°C ( - 184° to 320°F).

Kynar expansion vs. temperature chart.

Thermal Properties

KYNAR resins have excellent thermal stability. The general thermal properties are shown in Table II. Prolonged exposure at 250°C (482°F) in air does not lead to loss of weight. No oxidative or thermal degradation has been detected during continuous exposure of KYNAR resins at 150°C (302 °F) for a period of five years. As Thermogravimetric Analysis (TGA) thermograms indicate, KYNAR homopolymer resins are thermally stable up to 375°C (707°F) when heated in air at the rate of 5°C /min (9°F/min). At temperatures greater than 375°C (707°F), thermal decomposition of KYNAR resins takes place with the evolution of hydrogen fluoride (HF). The melt processing range of KYNAR resins is very broad - from slightly above its melting point of 155 - 170°C (311 - 338°F) to over 300°C (572°F). It is typically processed at temperatures from 190°C (374°F) to 265°C (509°F). In general, KYNAR resins are some of the easiest fluoropolymers to process and can be easily recycled without detriment to physical and mechanical properties. Similar to most thermoplastics, KYNAR resins will discolor and degrade during processing if the processing temperature is too high, the residence time is too long, or the shear rate is too high.

Thermal properties of Kynar and Kynar fluoropolymers.

Strength and Toughness

The ambient temperature tensile strength at yield of 5,000-8,000 psi (35-55 MPa) and the unnotched impact strength of 15-80 ft-lb/in (800-4270 kJ/m) show that all grades of KYNAR homopolymer resin are both strong and tough. These characteristics are retained over a wide range of temperatures, as shown in Figure 2.

Flexural Creep

The long-term resistance of KYNAR resins to flexural creep at elevated temperatures is given in Figure 3. This data indicates that KYNAR resins are suitable for many applications in which load-bearing characteristics are important. Likewise, the short-term flexural creep resistance of KYNAR homopolymer resins reflects superior load-bearing performance.

Tensile Creep

Figure 4 shows the low tensile creep of KYNAR resins when subjected to constant stress of 100 psi (0.69 MPa) over time. The outstanding tensile creep resistance of KYNAR resins over time is maintained even at temperatures as extreme as l40°C (284°F).

Crystallinity

KYNAR resins crystallize in at least three forms designated alpha, beta, and gamma. Normally, KYNAR resins crystallize from the melt pre-dominantly in the alpha form. The degree of crystallinity and the type of crystalline forms present depend upon the KYNAR resin grade and the processing conditions. Rapid cooling (quenching) of the melt impedes crystallization and promotes a smaller crystallite size. Slow cooling or heating below the melting point (annealing) perfects the crystallization process and relaxes stresses. In addition, orientation below the melting point will enhance crystallization and change properties. The crystalline state of KYNAR resin influences its properties; therefore, it is important to select the proper processing conditions to achieve the optimum in physical properties. In general, for long-term performance, KYNAR PVDF fabricated parts should be cooled slowly and/or annealed below the melting point to stabilize the crystalline state of the parts. With increased crystallinity, parts will show higher values for yield strengths, modulus, and hardness.

Kynar resin design properties.