Thermoset polymers contain no set arrangement of chains and as such they can be classified as amorphous i.e. they contain no distinct crystalline structure [3]. Thermoset materials are formed from a chemical reaction of a resin and a hardener or catalyst and this reaction is irreversible and produces a hard and infusible material [4]. Cured thermosets will not become liquid again if heated but above a certain temperature their mechanical properties can change substantially. The temperature at which this change can occur is called the Glass Transition Temperature (Tg) and it varies depending on the particular resin and hardener/catalyst used as well as its degree of cure and whether it was mixed properly. If the temperature of a thermoset material is raised above the Tg, the molecular structure changes from that of a hard crystalline polymer to a more flexible amorphous polymer. At this elevated temperature the properties of the thermoset such as resin modulus (stiffness) drop significantly and as a result the compressive and shear strength of the composite will do the same. Other properties such as water resistance and colour stability also reduce above the resin’s Tg This change can be reversed by cooling the material back down to below the Tg.
The majority of structural parts made of composites are made with three main types of matrix, namely epoxy, vinylester and polyester.
2.3.1. Epoxy Resin
Epoxy refers to a chemical group consisting of an oxygen atom bonded to two carbon atoms that are bonded to other elements. The simplest epoxy is a three-member ring structure known by the term ‘alpha-epoxy’ or ‘1,2-epoxy’ [4]. The idealised chemical structure is shown in Figure 4.
Epoxy has numerous useful properties such as the fac...
... middle of paper ...
...te. The accelerator and catalyst must be added carefully and in controlled amounts in order to control the polymerisation reaction such that the best material properties will be achieved. Adding too much catalyst will cause the resin to harden too quickly while too little catalyst will result in under curing and a loss of strength.
Filler materials are used with polyester resins for a variety of reasons including:
• To reduce the cost of the moulding
• To facilitate the moulding process
• To impart specific properties to the moulding
Up to 50% of the resin weight can be fillers but the addition of these fillers will affect the strength of the composite to varying degrees. The use of fillers can be helpful when casting thick components as pure epoxies generate substantial amounts of heat when curing and the addition of fillers can reduce this exothermic heat output.
There are many different areas that can be looked at in attempting to find ways to reduce micro-leakage of composite fillings.
STRUCTURE Kevlar Aramid Fiber is a synthetic (man-made) material known as Polymer. A polymer is a chain that is made up of many similar molecular groups, better known as ‘monomers’ that are bonded together. The ‘Monomers’ are made up of fourteen Carbon atoms, two Nitrogen atoms, two Oxygen atoms and ten Hydrogen atoms. A single Kevlar polymer chain could possibly have anywhere from one to five million monomers bonded together. A group of polymer chains can be organised together in a fiber.
...an be seen that composite teeth form a high stability bond than PMMA teeth. This is due to the filler content allowing for low shrinkage, increased wear resistance and better cross-linkage with the base. In overall consensus the technique of heat-curing is believed to achieve significantly more polymer cross-linkage than that of self-curing the acrylic resin PMMA base - giving us a stronger base to teeth interface. It should be noted however that both techniques can be used for denture fabrication to achieve a desired result and it is up to the dentist and the technician to determine which one they prefer however, composite teeth bonded to a heat-cured PMMA base works best. Though the tooth and base by themselves may be strong, if the interface between them is not strong, this will result in the overall denture produced being weak independent of material selection.
Polyethylene (PE) is one of the most commonly used polymers which can be identified into two plastic identification codes: 2 for high-density polyethylene (HDPE) and 4 for low density polyethylene (LDPE). Polyethylene is sometimes called polyethene or polythene and is produced by an addition polymerisation reaction. The chemical formula for polyethylene is –(CH2-CH2)n– for both HDPE and LDPE. The formation of the polyethylene chain is created with the monomer ethylene (CH2=CH2).
The aim of this experiment was to investigate the affect of the use of a catalyst and temperature on the rate of reaction while keeping all the other factors that affect the reaction rate constant.
The structure of an epoxy resin present in primers. Taken from "A Perfect Finish" By Gus Palluel, Vol. 3, No.1, September 1993, Philip Allan Publishers Fig. 2 The structure of a polymer often present in a surfacer. Taken from "A Perfect Finish" By Gus Palluel, Vol. 3, No.1, September 1993, Philip Allan Publishers [1] "A Perfect Finish" By Gus Palluel, Vol. 3, No.1, September 1993, Philip Allan Publishers [2] "'Aquabase' Safeguard the environment - without sacrificing the finish" By Graham Wheeler, ICI STEAM Magazine, Issue No. 14, January 1991 [3] Salters Advanced Chemistry - Chemical Storylines
While most of the polyurethanes are thermosetting polymers that don't melt when heated, thermoplastic polyurethanes are also available.
Polymer-Polyethylene is partially crystalline as well as amorphous because it has crystalline and amorphous regions. Also it has linear chains so this is the simplest structure compared to a branched or network chain. This can be of an advantage to it over other types of materials as its good toughness and elongation makes it very significant in the engineering industry as it can be moulded or extruded into shape...
Polyethylene is a polymer that is made of a long chain of CH2 monomers bonded together. It is one of the most commonly used polymers in everyday items. Grocery bags, soap bottles, children’s toys, and even bullet proof vests are all made from polyethylene.2 This polymer is very versatile and can be used in many materials but this all depends on the way that the polymer is chemically made or enhanced. There are many different types of polyethylene classified by density and branching. 2
There are various materials which can be used on aircrafts. Composite and alloy materials are the primary component used due to the ability to combine certain materials with different strengths together to create a composite or alloy. These have ultimate properties that are needed to create an aircraft to be used safely in application. Composites are used due to the extreme high strength to weight ratio, although using large amounts of metals can be strong, unlike composites this would be too heavy. Composites essentially are designed to create a lighter aircraft, which over a long period of time, would be more cost effective for company running the plane, due to fuel savings.
The elongations-to-break of aged PHB and PHBV at 15°C decreased by 64% and 72%, respectively, after 168 days (Srubar et al., 2012). Ultimate strengths decreased by 28% and 8.9% for PHB and PHBV, respectively. During the aging process, the amorphous regions of PHB and PHBV underwent glassy aging in the rigid amorphous fraction, resulting a significant embrittlement. Over the testing period, PHB and PHBV samples had a maximum increase in modulus of 166% and 178%, and crystallinity of approximately 41% and 58%, respectively (Srubar et al.,
The small molecules which form polymers are called monomers. In other words monomers are the building block of polymers. Hundreds and thousands of monomers combine together to form a large molecule of polymer. A polymer can be a three dimensional network ( repeating units linked together left and right, front and back, up and down)
The word ‘composite’ does not fully explain the wide range and varied compositions of materials that are categorized under it. This paper deals with some of the commonly used composites. Common materials such as metallic, ceramic and polymers consists of substances that can be classified as composites. The steel family, which is considered as the biggest group of material that is used in construction and complex engineering, is composed of composites that are made out of soft metallic matrix and hard ceramic components. [11] Such metallic composites could be shaped as a plate, needle, sphere or be polygonal. Polymeric composites are also found in a similar matrix model where one polymer forms the skeleton and another hard or soft polymer fills the matrix. A good example of this would be wood. Concrete is also a popular matrix composite where the Portland cement provides the matrix phase and the inner filling is made up of sands of different size. Scientists are now able to adjust the composition of these composites to suit the various needs. This adjustment is done by tweaking the microstructure of the materials by changing the state, shape, amount and distribution of the filling, which is also called as the reinforcing phase. Instead of restricting this tweaking process within a certain material class, the idea is being implementing in cross-material classes as well. Ceramics powder can be added to plastics to make hard and fireproof polymers. Ceramic powder when mixed with metals, gives cermets, which are used in tips of metal cutting tools. Another branch of the same idea is the blending of metal filaments, ceramic and polymer into one large bulk to form metal-matrix composites (MMCs), reinforced plastics (RP) and ceramic-matrix composites (CMCs). Such a radical idea of mixing all the three classes of materials has resulted in composites such as carbon fibre reinforced plastic (CFRP), glass reinforced plastic (GRP) and silicon-carbon-fibre-reinforced aluminium. Figure1 shows Comparison between conventional monolithic material and composite material. [17, 18,10and1
At the meantime, the push-out technique is considered as a valuable and useful method for evaluating these goals (23). Various factors are involved in the flexural strength of composites. Reducing the size of the filler material followed by increasing the amount of filler material in the composites lead to an increase in the flexural strength of the composite due to better inter-composite stress transfer and improved mechanical properties of the composite (24-26). In addition, the type of filling material is also effective on flexural strength and the high-strength filling material increases the flexural strength of the composite
There is another method of producing polyester. This method is mainly used by manufacturers to produce polyester. The two basic form of polyester produced are filament yarn and staple fiber2. The filament yarn is a smooth surfaced fabric that are continuous in length and a staple fiber are just yarns cut into short lengths but...