Today's KNOWLEDGE Share: ELASTOMERS (THERMOPLASTIC vs THERMOSET)

 Today's KNOWLEDGE Share:

ELASTOMERS (THERMOPLASTIC vs THERMOSET)

There are times when a thermoplastic elastomer (TPE) cannot match the performance of a thermoset rubber compound, and rubber must be used in a demanding application.

 

I completed a material review in which my client was trying to replace a nitrile rubber (NBR) seal with a thermoplastic elastomer (TPE). They were looking for a material that would offer cost and manufacturing advantages over the thermoset NBR material. The screening tests paralleled the application environment, and included chemical exposure at elevated temperature under conditions of dynamic actuation of the seal.

 

Both thermoset rubber and TPEs are elastomers. As elastomers, both classes of materials exhibit some characteristic properties:

•  Substantial amorphous content

•  Subambient glass transition, which allows substantial segmental molecular motion

•  Relatively low hardness values, meaning they are relatively soft

•  Comparatively low modulus values, meaning that they are flexible

•  High degree of stretch and elongation at break

•     Compared with non-elastomeric materials, are elastic, meaning they exhibit good recovery from stress

 

While both thermoset rubber and TPEs are elastomers, their structure, and as an extension, their performance properties can be dramatically different. Thermoset rubber compounds have a cross-linked molecular structure, with covalent bonds joining the individual polymer chains, essentially forming a molecular network. Conversely, TPEs are thermoplastic in nature, and do not have covalent bonds that join individual polymer chains. Instead, the polymer chains are held together by weak intermolecular forces, such as hydrogen bonding and van der Waals forces.

 

Based upon the difference in molecular structure, the two material classes can exhibit desparate physical properties. It is somewhat difficult to characterize all TPE materials and contrast them against all thermoset rubber materials. There are numerous subcategories within each type of material, and each has its unique characteristics, as well as strengths and weaknesses. However, given the fundamental molecular structure, some differences can be highlighted. The cross-linked structure of thermoset rubber affords them superior performance properties in three general areas compared with TPEs:

 • Superior chemical resistance

 • Higher thermal stability

 • Greater stress recovery, resulting in better compression set - creep and stress relaxation properties

 

In this case, it was concluded through testing and review that a TPE material was not suitable based upon the test and application conditions - specifically the chemical resistance and the stress recovery at elevated temperature. It was recommended that the incumbent NBR rubber compound be maintained in the sealing application.

 

Source:Jeffrey A.Jansen/The Madison Group

Visit MY BLOG http://polymerguru.blogspot.com

#elastomers #tpe #nbr #rubberproducts

Today's KNOWLEDGE Share:AUTOCLAVE CURING PROCESS:

 Today's KNOWLEDGE Share:

AUTOCLAVE CURING PROCESS:

Autoclave curing is a convective heat transfer process used for the curing of FRPs. In autoclave a closed vessel is maintained at a certain temperature and pressure for a definite time depending upon the type of resin under curing. The composite cured by autoclave technique can be prepared by the hand lay-up process or vacuum bagging process using prepregs. After preparation of the laminate stack, the component is placed in the autoclave chamber at a particular temperature and pressure for a particular time. Using certain optimized parameters, the components get cured/solidified. Mostly all types of composite laminates can be cured using cylindrical autoclaves. Large volumetric components of aircraft and wind energy generation wings can be easily cured using autoclave chambers. 

The autoclave curing chamber contains a pressure chamber in which components get cured under the required pressure and heat. The cylindrical shape of the vessel provides for both a flat and a cylindrical body for curing. For proper curing of components, the pressure should be maintained at a sustainable limit. The pressure vessel is made leakproof and the door is properly sealed after closing. The required pressure in the chamber is achieved using an air compressor mounted on the outer body of the chamber. For vacuum bagging, vacuum is maintained by two hose pipes connected to a vacuum compressor. Temperature sensors and thermocouples are placed inside the chamber for detecting temperature. Vessel pressure can be maintained by a safety valve that is mounted on the chamber to release excess pressure above the required level. For heating up the chamber, gas firing and electric heaters are used to complete polymerization/curing of polymer composites. Direct gas firing is mostly preferred for large volumetric components (aircraft and wind turbine blade), although direct heating systems are preferred for small components (automotive components). Large components require high thermal energy to spread over the surface of the components that can be only achieved by the gas firing method, whereas for small components the polymerization of components can be easily achieved by direct heating. The design of the gas firing tube should be done according to prescribed measurements to avoid leakage during operating hours. 

Bibliographical Reference:

Reinforced Polymer Composites: Processing, Characterization and Post Life Cycle Assessment - Page 82

Source:#managingcomposites/#thenativelab

Today's KNOWLEDGE Share:STERILIZATION METHODS

 Today's KNOWLEDGE Share:

STERILIZATION METHODS:

Plastics are used to manufacture a wide range of medical products, including those used as part of surgical and other invasive medical procedures. According to the Center for Disease Control, approximately 46.5 million of these procedures are performed annually. Each procedure presents a major risk for the introduction of pathogens: bacteria, fungi, parasites, and viruses. Because of this, sterilization and disinfection are important.

Sterilization describes a process that destroys or eliminates all forms of microbial life, including both pathogens and spores, by physical or chemical methods.

Disinfection describes a process that eliminates many or all pathogenic microorganisms, except bacterial spores, on inanimate objects.

Sterilization and disinfection methods include:

· Heat Sterilization: both hot air, and autoclaving (steam)

· Irradiation: both gamma and E-beam radiation

· Chemical Sterilization: both gaseous and liquid products

Each method has advantages and disadvantages based upon many factors including efficacy, equipment and operating costs, time required, and environmental concerns. A major issue with the use of sterilizing methods, is that they all can have a deleterious effect on plastics. Plastic materials can undergo molecular degradation and environmental stress cracking (ESC) when subject to sterilization or disinfection.

This has been illustrated over the last 10 years by the failures of plastic medical devices in contact with aggressive chemical disinfectants, such as those based on quaternary ammonium salts, glutaraldehyde, phenolics, alcohol, and chlorine compounds. The synergistic effects of these chemicals, together with molded-in and assembled-in stress, have produced a rash of failures.

The best way to minimize the chances of premature failure is to do upfront material compatibility testing. A good guideline for this testing is ASTM D543, “Standard Practice for Evaluating the Resistance of Plastics to Chemical Reagents” Practice A involves sterilization of unstressed plastic samples, followed by assessment of weight and dimensional changes, as well as mechanical property changes. This is useful for the assessment of chemical attack and molecular degradation. Practice B of the standard includes exposure of stressed plastic test specimens, followed by inspection for crack formation and testing for mechanical property changes.

This type of chemical compatibility testing should be done early in the product development cycle, before manufacturing tooling and methodologies have been finalized. This allows for material changes with minimal cost implications if incompatibility issues are identified.

Source:The Madison Group

Today's KNOWLEDGE Share:CRTM PROCESS

Today's KNOWLEDGE Share:

CRTM process:

Resin Transfer Moulding (RTM) processes provide for excellent part quality, the relatively large cost of tooling being justified for medium to high production numbers. However, for high fibre content parts from continuous fibre reinforcements, the high resistance to resin flow has proven to be a barrier to the rapid processing required in some industries. The Compression RTM (CRTM, a.k.a. I/CM, RTCM, RI/CM and CTM) process has evolved from RTM to fill this need, by enabling very short mould filling times.

To extend the application of high fibre volume fraction composites into high-level production runs, factors limiting the cycle times for RTM need to be addressed. An RTM cycle is primarily composed of performing, mould filling, resin cure, part demoulding and mould cleaning and preparation. Accurate and repeatable performance is required for the manufacture of quality components, and this stage can be very time-consuming. However, preforming can be carried out independently of moulds used for injection and cure, preform units being prepared in advance using separate equipment. Injection and cure must occur within the mould imparting the final component shape, mould heating and cooling being desirable for the reduction of cycle times.

By working in combination with methods to significantly reduce mould filling times, faster curing resins can be used with more confidence and large improvements made in total cycle time. The filling stage comprises a combination of injection and compression driven flows. Unlike RTM, the mould is not completely closed prior to the initiation of filling. The mould is closed to some predetermined position, which results in lower overall fibre volume fraction, and hence a lower global resin flow resistance offered by the preform. It is also possible to leave a small empty cavity on one side of the preform, further reducing flow resistance during injection. Once the required volume of resin is injected, injection gates are closed, and the mould is closed in a controlled manner. This secondary mould-closing phase drives the resin front through the preform to the vents, and advances the laminate to the final part dimensions and composition. With careful selection of processing parameters, CRTM can realise significant reductions in mould filling times, relative to RTM. However, faster mould filling is typically gained at the expense of larger required mould clamping forces, process design therefore being a complex optimisation problem.

The CRTM process is described in the picture shown, in which (a) is the placement of preform, (b) is the closure to cavity thickness for injection, (c) is injection, (d) is compression driven flow and (e) is cure and demoulding.

Bibliographical Reference:
Manufacturing Techniques for Polymer Matrix Composites - Page 350
Source:#managingcomposites #thenativelab

Today's KNOWLEDGE Share:COMPRESSION LOADING in Plastics

 Today's KNOWLEDGE Share:

COMPRESSION LOADING in Plastics

Compression is the application of a load on opposite surfaces of a structure to cause crushing. In reality, plastics rarely show catastrophic cracking in compression. However, failure may be classified by the degree of distortion from the original shape due to the compressive stress. 

This type of failure is commonly seen as:

· Creep when the component distorts and takes a permanent set, such as the flattening of an appliance caster.

· Stress Relaxation when the component loses inference with a mating part, such as a gasket.

A notable exception here are thermoset plastics, which can shatter in compression, and often demonstrate true compressive rupture failure.

More commonly, plastics will continue to compress to a flat disk as the applied stress increases, without fracture. In many instances, the application of compressive loading results in deformation of the part, such as buckling or bending, producing corresponding tensile loads at other locations.

The common responses to compressive loading are:

· Buckling

· Shearing

· Double barreling

· Barreling

· Homogeneous compression

· Instability

Compression of plastics is tested per ASTM D 695. As per the standard, a specimen is placed between two parallel plates on a universal testing machine (UTM). Load is applied in order to move the plates together at a specified rate. The load cell records compressive stress, while displacement is measured from crosshead movement. The test results in a plot of stress versus strain.

The compressive strength of a material is the force per unit area that it can withstand in compression. Plastic resin suppliers generally report compressive yield strength, the stress measured at the point of permanent yield, zero slope, on the stress-strain curve. The analogous test to measure compressive strength in the ISO system is ISO 604.

Typically, the compressive modulus and yield stress are usually greater than the corresponding tensile values.

Source:Jeffrey A. Jansen

Sustainable high-end luggage

 Overcoming the conventional:

Launched in 2015, HORIZN STUDIOS was Europe’s first direct-to-consumer travel brand. Focused on innovative design, convenience, and sustainable solutions, it was the first company in the world to release luggage with a removable charging function.

In 2021, however, HORIZN STUDIOS was looking to really revolutionise luggage design. Despite the challenges of the global pandemic, the company intended to create the world’s most sustainable luxury luggage. Embracing a philosophy of lightweight, high-performance, and sustainable materials, HORIZN STUDIOS sought out a partner that would be able to simultaneously fulfil these demanding criteria.

Natural fibre composite technologies

Already proven in the unrelenting world of motorsport and the equally challenging arena of ultra-high-end furniture, Bcomp’s innovative ampliTex™ and powerRibs™ are ground-breaking carbon-neutral composite reinforcements made entirely from flax fibre.

Cultivated across Europe, flax is an indigenous plant that has been part of the agricultural industry for centuries. With low water and nutrient requirements and little need for pesticides and fertilisers, it is a popular rotational crop with excellent utility – useful for feed, making flax oil, and its fibres can be used in textiles.

ampliTex™ and powerRibs™ make the most of flax’s inherent mechanical properties, creating composite parts with high stiffness, resistance to breakage, torsion, and compression – perfect to form the shell of tough and sturdy luggage. Using appropriate care and processes, Bcomp’s materials also offer a flawless surface finish, suitable for luxury product applications.

Most importantly, ampliTex™ and powerRibs™ are some of the most sustainable composite technologies available today, particularly in the high-performance category. Analysis of past projects has shown that Bcomp’s technologies can provide a material emission reduction of 90% when compared to its most commonly used equivalent, carbon fibre. Overall, they offer an outstanding 80-85% cradle-to-gate emission reduction, while retaining many of the performance benefits.

Unlike other luggage materials that would be sent to landfill at the end of their useful life, the Circle One range has various end-of-life recycling options thanks to Bcomp’s flax fibre composite technologies. ampliTex™ also opens up the possibility of repair, rather than replace, something of great interest to the HORIZN STUDIOS team.

Source:bcomp.ch/JECCOMPOSITES

Today's KNOWLEDGE Share:The first CARBON CONCRETE house

 Today's KNOWLEDGE Share:

The first CARBON CONCRETE house

The Technische Universität Dresden has now reached a new milestone: The world's first carbon concrete house "Cube“ has been completed on the university campus.

Their brand new "carbon concrete" is created by reinforcing concrete with carbon fibers, and for the production of carbon concrete, up to 50,000 of these fibers are woven into a rod or matte-shaped structure, which then serves as reinforcement for the concrete.

In contrast to steel, carbon does not have to be protected from moisture because carbon cannot RUST. As a result, much less concrete has to be used in a carbon concrete building than in steel. With new construction principles that are still under development, up to 80 percent of concrete should be saved in the end. The CO2 emissions that are saved in the production of the building material or through the reduced use of concrete are up to 50 percent.

The Cube makes it clear: carbon concrete opens up completely new design options for a house and for architecture. As visual conspicuity, the carbon concrete house has been given a twisted roof made entirely of carbon concrete. The building material has a high degree of flexibility and is even more stable than reinforced concrete. Another advantage is longevity: a carbon concrete house should be usable for up to 200 years – a reinforced concrete building, on the other hand, only 40 to 80 years.

Source: DasHaus .de/#managingcomposites/#thenativelab