Polytetrafluoroethylene (PTFE) is a high molecular weight polymer, one of the most versatile plastic materials known and useful for large range of products for applications excluded to other materials.
The most outstanding characteristics are:
- high heat resistance
- high resistance to chemical agents and solvents
- high antiadhesiveness
- high dielectric properties
- low friction coefficient
PTFE is generally considered a thermoplastic polymer; at 327°C, it maintains, a very high viscosity, thus requiring particular transformation techniques for manufacturing of finished and semi-finished goods.
PTFE can be employed at any temperature from -200°C to +260°C.
Properties of PTFE
Table 1 shows the physical properties of molded PTFE, methods for determining these values are listed.
1 Thermal properties
1.1 Thermal Stability
PTFE is one of the most thermally stable plastic material. There are no appreciable decompositions at 260°C, so that PTFE, at this temperature, still possesses the greater part of its properties.
Appreciable decomposition begins at over 400°C.
1.2 Transition points
The arrangement of the PTFE molecules (crystalline structure) varies with the temperature. There are different transition points, with the most important ones being the following: at 19°C corresponding to a modification of some physical properties and that at 327°C which corresponds to the disappearance of the crystalline structure: the PTFE assumes an amorphous aspect conserving its own geometric form.
PTFE linear thermal expansion and expansion coefficient vs. temperature
The liner thermal expansion coefficient varies with the temperature. In addition, because of the orientation caused by the working process, the PTFE pieces are in general anisotropic; in other words, the coefficient of expansion varies also in relation to direction.
1.4 Thermal conductivity
The coefficient of the thermal conductivity of PTFE does not varies with the temperature. It is relatively high, so that PTFE can be considered to be a good insulating material. The mixing of suitable fillers imrpvoes the thermal conductivity (see filled PTFE)
1.5 Specific heat
The specific heat, as well as the heat content (enthalpy) increases with the temperature.
2 Behaviour in presence of foreign agents
2.1 Resistance to chemical agents
PTFE is practically inert against known elements and compounds. It is attacked only by the alkaline metals in the elementary state, by Chlorine trifluoride and by elementary Fluorine at high temperatures and pressures.
2.2 Solvent resistance
PTFE is insoluble in almost all solvents at temperatures up to about 300°C, exercise a certain dissolving effect upon PTFE.
2.3 Resistance to atmospheric agents and light
Test pieces of PTFE, exposed for over twenty years to the most disparate climatic conditions, have not shown any alteration of their characteristic properties.
2.4 Resistance to radiations
High energy radiations tend to cause the breaking of the PTFE molecule, so that the resistance of the product to radiations is rather poor.
2.5 Gas permeability
The permeability of PTFE is similar to other plastic materials. The permeability does not depend, obviously, only on the thickness and pressure, but also on the working techniques.
3 Physical – mechanical properties
3.1 Tensile and compressive properties
These properties are to a large degree influenced by the working processes and the employed powder. PTFE, however, can be used continuously at temperatures up to 260°C, while possessing still a certain compressive plasticity at temperatures near to the absolute Zero.
PTFE tensile strength vs elongation
PTFE is quite flexible and does not break when subjected to stresses of 0,7 N/mm2 according to ASTM D 790. Flexural modulus is about 350 to 650 N/mm2 at room temperature, about 2000 N/mm2 at -80°C, about 200 N/mm2 at 100°C and about 45 N/mm2 at 260°C.
3.3 Impact properties
PTFE possesses very high resilience characteristics at low temperatures (see table 1).
3.4 Plastic memory
If a piece of PTFE is subjected to tensile or compression stresses below the yield point, part of the resulting deformations remain (as permanent deformations) after the discontinuance of the stresses, with the result that certain strains are induced in the piece. If the piece is reheated, these strains tend to release themselves within the piece which resumes its original form. This property of the PTFE is commonly indicated as “plastic memory” and is made use of in different applications.
Also the greater part of the semi-finished products, because of the transformation processes, possesses similar strains, to a certain degree. When it is desired to obtain semi-finished parts dimensionally stable at high temperatures, it is possible to subject the parts to a temperature of 280°C for one hour every 6mm of thickness and then cool them slowly. The parts obtained in this manner are almost completely free from internal strains and are in general known as “conditioned” or “thermostabilised” material.
The hardness Shore D, measured according to the method ASTMD2240, has values comprised between D50 and D60. According DIN 53456 (load13,5 Kg for 30 sec) results in a hardness range between 27 and 32 N/mm2.
PTFE possesses the lowest friction coefficients of all solid materials; between 0.05 and 0.09:
- the static and dynamic friction coefficients are almost equal, so that there is no seizure or stick-slip action
- when increasing the load, the friction coefficient decreases until reaching a stable value
- the friction coefficient increases with the speed
- the friction coefficient remains constant at temperature variations.
The wear depends upon the condition of the other sliding surface and obviously depends upon the speed and loads. The wear is considerably reduced when adding suitable fillers to the PTFE (see filled PTFE).
Influence of sliding velocity on Dynamic friction coefficient
4 Electrical properties
PTFE is an excellent insulator and precious dielectric as shown by the relative data reported in table 1, and maintains these characteristics throughout a large range of environmental conditions, temperatures and frequencies.
4.1 Dielectric strength
The dielectric strength of PTFE varies with the thickness and decreases with increasing frequency. It remains practically constant up to 300°C and does not vary even after a prolonged treatment at high temperatures (6 months at 300°C). It depends also upon the transformation processes.
4.2 Dielectric constant and dissipation factor
PTFE has very low dielectric constant and dissipation factors values; these remain unvaried until 300°C, in a frequency field of up to 109 Hz even after a prolonged thermal treatment (6 months at 300°C). The dielectric constant, dissipation factor as well as the volume resistivity and surface resistivity may be considered as being independent from the transformation processes.
PTFE has a good resisstance to the arc. The arc resistance time according to ASTM D 495 is 700 sec..
After a prolonged action there are no signs of surface charing.
4.4 Corona effect resistance
The discharges caused by the corona effect may result in erosions of the PTFE surface which, nevertheless, is indicated as a suitable insulator in case of high potential differences.
5 Surface properties
The molecular configuration of PTFE brings to its surfaces a high anti-adhesiveness. For the same reason these surfaces are hardly wettable, the contact angle with water is about 110° and it is possible to affirm that, beyond a surface tension of 20 dine/cm, the liquid no longer wets the PTFE.
A special etching treatment renders the surfaces bondable and wettable.
The following table shows the way each filler affects physical and mechanical properties.
Combinations of two or more fillers (not considered in the table) allow a large number compounds. Thus the resulting combined properties offer a variety of applications.
|Filler||Property||Most common applications|
|Glass||Enhanced wear resistance.Enhanced chemical resitance (except for alkali and hydrofluoridacid).||Valve seats, seals, bearings, requested to resist sliding and chemicals.Suitable for bearings working at low PV values.|
|Graphite||Extremely low coefficient of friction.Fairly good compressive strength.Good wear resistance.||Bearings for high speed on fairly hard surface.|
|Carbon||Good thermal conductivity.Good resistance to deformation.||Valve seats. Bearings for high speed and when fast dissipation of electrical charges is needed.Elastic bands for unlubricated compressors.|
|Molibdenum disulphide||Enhanced non-stick properties.Low static coefficient of friction.Fairly good resistance to deformation.||Guide bands.Details needing good resistivity.|
|Bronze||Enhanced compressive strength.Good wear resistance and high thermal conductivity.||Anti-extrusion rings.Unlubricated bearings for high speed on not hard surface.|
PTFE ETCHED SHEETS
- Uniform Etching
- A safe and low pollution process
- Clean and contamination free etched surfaces
- Unique etching capabilities
· Thin film
· Effective on irregular surfaces
- Treatment one side or both sides
- Wider range of products in Virgin and Filled P.T.F.E.
· Skived tapes
· Machined parts
Sheets and Skived Tapes
Sheets and skived tapes, etched one side or both sides, are available in the following standard sizes:
- Thickness (mm) : min. 2 max 80
- Width (mm) : 600 x 600
: 1000 x 1000
: 1200 x 1200
- Thickness (mm) : min. 0.02 max 2
- Width (mm) : min. 300 max 1200
Other sizes are available on request.
Rings, bushes, parts and other products as required can be etched either in small or large quantities at low cost, and with uniform quality.
According to end use etching can be carried out partially or on the total surface of the finished product.
QUALITY OF ETCHING
Contact Angle Method
The angle formed by a distilled water droplet on the P.T.F.E. surface is measured.
In Figure 1, two examples of contact angle are illustrated, on an unetched surface (bad wetting), and on a surface (good wetting).
The qualitative relation between contact angle value and degree of etch is illustrated in Figure 2.
Contact angle and etched P.T.F.E surface energy are related by the following formula:
SURF. EN. = 72 + (cos contact angle – 1)
Figure 3. shows contact angle vs. surface energy.
INSTRUCTION FOR USE
To obtain the best results it is necessary to remove grease or other contamination from the etched surface.
To clean the polymer surface use of an ALCOHOL or ACETONE is recommended, afterward making sure it is removed.
WARNING!!! DO NOT USE CHLORINATED SOLVENTS
Etched P.T.F.E. can be bonded to different substrates like metals, rubber, plastics and glass, using conventional adhesives.
Choice of adhesive will depend on the nature of the material to be bonded to P.T.F.E., the characteristics of needed of the bond, and operational conditions of the product (temperature, chemical agents, etc.).
Temperature, humidity, and U.V. light have a negative effect on the etched surface.
It is advisable to store the etched material in a warehouse at room temperature, and low humidity, protecting it from light, direct sun light in particular.
If stored according to the conditions above, the etched products can stay unchanged for a long period. Our experience suggests 10-12 months.