UHMWPE Skived Film

UHMW Polyethylene Film has extremely high abrasion resistance, exceeding the abrasion resistance of steel. Coupled with broad chemical resistance and low coefficient of friction makes UHMW an extremely versatile engineering material for many severe service applications. Slippery like Teflon® Fluoropolymer, but super abrasion & wear resistant. UHMW Polymers have a molecular weight average 10 times that of conventional high density polyethylene resins. Higher molecular weight gives UHMW Polymers its unique combination of characteristics Applications: Inner and outer surfaces for potable water, chemical, fuel and hydraulic hoses, bottom surfaces for skis and snow boards, linings for chutes to decrease friction and wear.

Commercial skived film of ultra-high molecular-weight polyethylene (UHMWPE) with high uniaxial orientation1 was studied during melting and crystallisation with a time-resolution of 30 s in order to identify the crystallisation mechanisms . 

It was found that isotropic crystallisation occurs whenever the melt is heated to 140◦C or above. Oriented crystallisation occurs, if the melt is kept at 138◦C or below. An optimum melt annealing temperature appears to be 136◦C. At this temperature the semicrystalline nanostructure of the original film is completely erased, whereas the orientation memory of the melt is preserved. Moreover, isothermal crystallisation cannot be initiated at a temperature of 110◦C and higher. At a temperature of 105◦C oriented crystallisation starts after 2.5 min. Lamellae with slowly decreasing thickness are growing during an isothermal period of 20 min. 

During the following non-isothermal crystallisation (cooling rate: 20◦C/min) small crystalline blocks with next-neighbour correlation are formed. Thus the crystallisation mechanisms are similar to those found with other polyethylene materials with sufficiently high chain entanglement density studied earlier, except for the considerable undercooling required for the initiation of isothermal crystallisation.

Analysis of the data in real space by means of the multidimensional CDF has been performed. During the melting of the material the average thickness of the crystalline layers remains constant (27 nm), while the long period is strongly increasing from 60 nm to 140 nm. Because the analysis shows that even the original nanostructure is dominated by next-neighbour correlations only this means that the stability of a lamella is monotonously increasing as a function of the distance to its neighbours. While the original structure exhibits extended lamellae, the recrystallised domains are not wider than the distance between them in fibre direction s3.

Applications include lining of conveyor, guide rails, chute liners, chain guides, drawer glides and noise reduction. Excellent abrasion and wear resistance.

Source:https://www.sukoptfe.com/melting-and-application-uhmw-pe-skived-film

Plastic Materials

Plastic Materials

There are a wide variety of resins available to the plastic extrusion industry, but two major categories in particular make up the large majority of resins – thermosets and thermoplastics.

A vast number of plastic materials make up subgroups of thermosets and thermoplastics. These include, but are not exclusive to, many different types of resins that Plastic Extrusion Technologies uses to create products for our clients, such as:

• ABS – a type of Acrylonitrile-Butadiene-Styrene plastic
• PC/ABS – a polycarbonate/ABS composite
• Acrylic – a clear plastic that often resembles glass
• Hytrel – an elastomer with the flexible properties of rubber
• E.V.A. – an elastic polymer that can be processed like a thermoplastic
• Flexible PVC & Rigid PVC – Often used in construction, for tubing or as wire
• Polypropylene – Excellent impact resistance in cold weather applications
• Polyethylene – High and Low density Polyethylene resins are known for their high chemical resistance properties
• TPR/TPO – Thermal Plastic Rubber / Olefin resins used most often in automotive and sealing gasket applications. Excellent cold temperature properties. Also a good alternative to certain thermoset rubbers. Able to be colored.
• Nylon 6 – Resin known for its toughness and resistance to wear
• Noryl – Resin with excellent electrical characteristics. Good for high temperature applications where RPVC fails.
• Polycarbonate – Resin that has excellent impact resistance and clarity

PTFE Absorption Properties

In contrast to metals, plastic and elastomers absorb varying quantities of the materials they contact, especially organic liquids. Absorptivities in PTFE are unusually low, and a chemical reaction between the plastic and the other substances is a rarity (with the few exceptions noted previously). However, when absorption is combined with other effects, this property can influence the serviceability of these resins in a particular chemical environment. For example, if rapid fluctuations in temperature or pressure occur, circumstances may be created that are physically damaging. The wider service temperature range for PTFE resins exposes them to this type of physical damage more frequently that other plastics.

By way of explanation, let us consider the “steam cycle” test described in ATSM standards* for lined pipe. Samples of lined pipe are subjected to 0.8MPa (125 psi) steam, alternating with low pressure cold water, causing very severe thermal and pressure fluctuations indeed. This is repeated for 100 cycles. Steam created a pressure and temperature gradient through the liner causing absorption of a small quantity of steam which condenses to water within the liner wall. On pressure release, or on reintroduction of steam, the entrapped water can expand to vapor causing an original micro pore. The repeated pressure and thermal cycling enlarges the micro pores, ultimately causing visible water-filled blisters within the liner. The ASTM standards note that the blisters do not adversely affect pipe liner performance – the chemical barrier thickness is still intact.

Similarly, in an actual process, the plastic component may absorb process fluids and repeated temperature or pressure cycling can then cause blisters. Such an occurrence may be surprising to one familiar with the extreme chemical inertness of PTFE. This effect is not seen in metals because they fail mainly by corrosion. It is rarely seen in most other plastics and elastomers because temperatures above the boiling point of liquid chemicals are normally beyond their capability. Hence this effect is new to most corrosion engineers and chemical processors, and requires new understanding for proper selection, design, testing, and use of these fluoro-polymers. Further clarification is contained in the section on testing.

There are corrosive measures which reduce the severity of blistering. Thermal insulation of a lined pipe or vessel reduces the temperature gradient in the liner, thereby often preventing condensation and subsequent expansion of absorbed fluids. It also reduced the speed and magnitude of temperature changes, thereby minimizing blistering. Thus, by reducing the resin, insulation can provide a protective measure in many cases. Additional protection can be provided by using operating procedures or devices which limit the rate of process pressure reductions or temperature increases.

Related effects can occur with process materials which may be absorbed and subsequently react, decompose or solidify within the structure of the existing plastic. Prolonged retention of absorbed chemicals can lead to their decomposition within the fluorocarbon component. Absorbed monomers can polymerize similarly. Although unusual, these events can happen, emphasizing the value of in-process testing.

PTFE Molecular Conformation and Crystal Structure

PTFE is a linear chain polymer of great molecular length. The linearity is indicated by an analysis of the infra-red spectrum and by the fact that the powder as produced in the polymerisation reaction is very highly crystalline, with crystalline weight fractions of 0.90 to 0.95 being indicated by density, infra-red and X-ray diffraction measurements. Energy considerations also suggest that branching by chain transfer is unlikely.

The crystal structure and chain conformation have been discussed by Bunn and Howells  and later by others. The crystalline melting point of sintered PTFE is about 327°C (620°F) and of unsintered material 332-346°C (630-655°F) but there are two reversible first order transitions at lower temperatures,19°C and 30°C (66°F and 86°F), which together involve a 1% change in density .Three crystalline phases are observed at atmospheric pressure: phase I (< 19°C; 66°F), phase ll (19-30°C; 66- 86°F) and phase lll ( >30°C; 86°F).

Below the 19°C (66°F) transition, the chain repeat distance is 16.8 Å and the CF2 groups are equally spaced along the chain which is twisted to form a helix on which successive carbon atoms lie, thirteen carbon atoms being involved in a twist of 180°.Between 19 and 30°C (66 and 86°F) the repeat distance increases to 19.5 Å  corresponding to a twist of 15 carbon atoms in 180°. Above 30°C (86°F), further disorder sets in and although the molecular conformation prevailing at lower temperatures is maintained, the chains are displaced or rotated along their long axes by variable amounts which increase as the temperature is raised further. The reason for the helical structure is the necessity to accommodate the large fluorine atoms (van der Waals radius 1.35 Å).The rotation at each chain bond, with the slight opening up of the bond angles to 116°, relieves the overcrowding and permits the shortest F-F distance to be 2.7 Å.

Further studies by various authors have examined the effect of pressure on the room temperature transitions and the melting point. A study of pressures above-atmospheric revealed a 2% increase in density below 19°C (66°F). This fourth crystalline phase has been labelled phase lll by Weir. A triple point exists at about 70°C (158°F) and 4.5 kilobars.The heats of transition were also determined by Yasuda and Araki; dilatometric and calorimetric studies have in addition been reported by other workers.

Materials of Construction (Thermoplastics)

Materials of Construction (Thermoplastics):Teflon® PTFE (Polytetrafluoroethylene),PVDF (Polyvinylidene Fluoride),PP (Polypropylene),CPVC (Chlorinated Polyvinyl Chloride),PVC (Polyvinyl Chloride)

PVC (POLYVINYL CHLORIDE)

The most economical and largest selection of Chemline valves are moulded from PVC. It offers excellent mechanical and chemical resistance properties at low cost. The working temperature range of PVC valves is 0 to 60°C (30 to 140°F).
PVC used for Chemline valves is identified by cell classification number 11564-A as per ASTM Standard D 1784. The suffix “A” refers to the highest chemical resistance rating. Most other PVC valves as well as pipe and fittings have only a “B” chemical resistance rating.
The special PVC “A” compound used in Chemline valves resists attack of most acids, strong alkalais, salts and many other chemicals. High chemical resistance of this material allows its application on aggressive services such as 98% H2 SO4 , dry chlorine and low pressure wet chlorine gas. PVC is attacked by chlorinated hydrocarbons, ketones, esters and some aromatic compounds. It can be used on solutions containing up to 1000 ppm solvents.
Chemline PVC valves are non-toxic. They meet CSA standard B137.0 for toxicity.
They are resistant to damaging effects of sunlight and weathering, thus painting is not necessary.

CPVC (CHLORINATED POLYVINYL CHLORIDE)

CPVC is very similar to PVC in mechanical properties and chemical resistance. It is suitable for applications from 0 to 95°C (30 to 200°F).
The special CPVC compound used for Chemline valves is classified as 23567-A as per ASTM D 1784. The suffix “A” denotes conformance to the highest chemical resistance rating. The compound is non-toxic, conforming to CSA toxicity standard B137.0.
CPVC valves have proven to be an excellent choice for applications at temperatures too high for PVC or when an extra margin of safety is required.

PP (POLYPROPYLENE)

PP is light weight and high in chemical resistance. Valves are suitable for service from -20 to 90°C (-5° to 195°F). PP is unaffected by alkalais, salts, organic solvents and most acids, particularly hydrochloric and phosphoric acid. It is unsuitable on strong acids, chlorinated hydrocarbons, aromatic compounds and high concentrations of free chlorine.
PP is very inert thus popular for high purity applications such as deionized water, etc. The material comes normally opaqued by addition of grey-beige pigment to prevent ultraviolet light penetration. Natural translucent material without pigment will degrade if exposed to UV light (sun light). Chemline offers PP pipe, fittings and valves in pigmented and unpigmented PP, both approved by the FDA for contact with food.

PVDF (POLYVINYLIDENE FLUORIDE)

PVDF is superior to other valve thermoplastics in chemical resistance and abrasion resistance. It has remarkable strength over the largest working temperature range.
The working temperature range of PVDF valves is -40 to 120°C (-40 to 250°F).
PVDF’s impact strength is over twice that of PVC. The valves are extremely durable under mechanical abuse even at -40°F. They also offer the highest abrasion resistance of thermoplastic valves.
PVDF has excellent chemical resistance against halogens such as chlorine and bromine, strong acids such as hydrofluoric and nitric acids, organic solvents and oils. PVDF is not resistant to hot bases.
It is also non toxic and imparts no odours or tastes into the fluid. Our PVDF conforms with USDA Title 21, P121.2593 requirements for contact with food.
Gas permeability of PVDF is extremely low. A patented PVDF gas permeability barrier is available on Type 14 and DV Series Diaphragm Valves. It is a backing to the Teflon® diaphragm and has proven to increase the life of diaphragm valves on chlorine and strong acid services.

TEFLON® PTFE (POLYTETRAFLUOROETHYLENE)

PTFE is almost totally insoluble and chemically inert. It has high temperature resistance. Teflon® PTFE ball seats, because of natural lubricity, require no lubrication. Teflon® PTFE diaphragms and flange gaskets are used in the most severe chemical resistance applications.

Source:https://www.sukoptfe.com/materials-of-construction-thermoplastics