Background Research

Background research was the key element of this project.  This research would explain what the basic elements of the project were, how those elements are formed and extracted from other materials and most importantly, how to put the resources together in order to get the final deliverable that was desired for this project.  A starting point for this research was breaking down the project into three main components: the epoxy resin, the curing agent, and the fiber.  Once those three categories were agreed upon, basic research on polymers and their uses began.  

A polymer is a compound of high molecular weight derived either by the addition of many smaller molecules or by the condensation of many smaller molecules with the elimination of water or alcohol.  Most of the time people associate polymers with just plastics, but plastics are just one form of polymer.  Natural polymers found in everyday life include rubber and amber, while the range of synthetic polymers includes things like nylon, silicone, Bakelite, neoprene, and polystyrene.  Manufacturers use polymers in the creation of a variety of products, from adhesives and lubricants to implantable devices like orthopedic plates, artificial joints and heart valves.  Polymers can also be used in the production of non-plastic objects such as silicone and paper.  When researching polymers, the term “free radicals” came up quite a bit. Free radicals are molecules with unpaired electrons. In their quest to find another electron, they are very reactive and cause damage to surrounding molecules [6].  Free radicals are used in polymerization to create the polymers and join monomers together into the long chains that are taken and used in epoxy resins.

 

Epoxy resin:

An epoxy resin consists of molecules with epoxide functional groups.  An epoxide functional group consists of a ring of 2 carbon atoms and 1 oxygen atom.  The molecules of the resin can contain one or more of these epoxides groups.  Epoxy resins are usually prepared from the reaction of bisphenol A and epichlorohydrin.  Epoxies can be reacted with a wide range of chemicals like amines, acids, and even other epoxies in the presence of a catalyst.  That reaction is exothermic and causes the epoxy monomers to form bonds with the other chemicals creating a cross-linked nanostructure that gives the polymer its physical, mechanical, and chemical properties.  One important physical property of epoxy resins is the glass transition temperature(T_g).  T_g is the temperature at which the cured resin stops acting like glass(i.e. hard) and begins to act rubbery.  T_g is directly related to the density of cross-linking that occurs during the curing process.  Epoxy resins have various applications as insulating foams, adhesives, corrosion resistant coatings, laminates, binders for concrete, and as a matrix for fibers in composites, like our application[7].

The epoxy resin we are using for this design project is the NC-514 epoxy resin from the Cardolite Company.  NC-514 is a resin containing 2 epoxide groups in its chemical structure that is synthesized from cashew nut oil.  The resin is a derivative of anacardic acid which makes up about 70% of cashew nut shell liquid(CNSL). It has properties similar to that of a traditional bisphenol A resin.  CNSL is good to use as a starting point for epoxies because it is normally a poisonous waste product from processing cashews, and the alternative would be petroleum based epoxy which is nonrenewable.

 

  

Epoxy curing agent:

In order to cure an epoxy group, there are two ways to do that.  The first is known as homo-polymerization, or corrective curing.  In this curing reaction the epoxy molecules react with themselves.  The second way that epoxy resins can be cured is through an addition, or catalytic, curing reaction.  This reaction runs by the small reactions between functional groups and other reactive molecules in the curing agent with the help of the catalyst forming the cross-linked nano structure. [7]  Some of those functional groups are epoxies and acrylics.  Epoxies can be cured with the use of amine groups that open up the epoxy ring and bond to the molecule, or can even display homo-polymerization. This is when one epoxy binds to another in the presence of the proper catalyst.  Acrylics, on the other hand, can be cured utilizing free-radical chemistry with peroxides that cause a chain reaction to go throughout the material fusing all of the molecules at the site of the acrylic groups into one big molecule of high molecular weight.  The cross-linking created from either method of curing is what dictates the properties of various thermosets.  Styrene is a chemical that is often added to thermosetting resins, because it gives better properties and lowers the resin’s viscosity to make it easier to work with.  That is a problem however because Styrene is a known carcinogen, and because of this only a small percentage can be added to a mixture for it to remain safe.  Methacrylated fatty acids (MFA) can be used in place of styrene and is renewable, but MFAs do not give as good properties. Styrene is made up of a benzene ring, as seen in the picture below, which will bind to the acrylic group of the resin. This allows for more bonds to be made and for the cross-linking to be closer, ultimately making the final composite stronger. Whereas the MFA are composed of long carbon chains, as seen below. Because of those long chains the space between the acrylic resin and the other bonding sites increases the distance which inherently weakens the composite.

Fibers:

 

The purpose of fibers in composite materials is to increase the stiffness and strength of the material.  The term for this combination of materials is known as Fiber Reinforced Plastics, or FRP for short.  When a composite is reinforced with a nano fiber, such as glass or carbon, it makes the composite in question far superior to the traditional bulk materials used today.  In the case of glass fibers, the increased strength is due to the process it undergoes.  The process avoids the internal flaws which would normally weaken the glass.  Because of this, the strength and stiffness of the fiber has nearly perfect alignment of the fiber axis with the molecular chains.  Table 1, shown below, give proof of these facts.[8] 

The fiber being utilized for this project is a renewable cellulose fiber known as Biomid.  The fiber is reported to have a density that is close to aramid fiber, a modulus that rivals that of E-glass, a specific tensile strength that has been compared to Innegra Technologies’ eight gram per denier high-modulus polypropylene (HMPP) fiber, and final an appearance that is similar to that of glass fiber.[9]  Biomid is produced in South Korea by a process of dissolving the cellulose and then spinning that solution using a spinneret.  This process, which is similar to how the HMPP fiber is made, produces a cellulose fiber that has a ninety-five percent degree of crystallinity, allowing fiber processing at temperature as high as 360˚ C which converts to 680˚F.  Biomid is a good option over other natural fibers, which begin to degrade at 150˚C/302˚F, because of these qualities.[9]  Biomid fibers are spun which means they can be produced continuously, unlike their discontinuous natural fiber counterparts that are stripped from the stalks of plants.  Because of their ability to be one continuous strand, they do not need to be wrapped into bundles.  The fibers can be used in a parallel sequence which facilitates higher properties and production of a thinner fabric rather than a twisted yarn.  This also reduces resign puddles when the fibers are combined to make a composite which results in a more even resin-to-fiber distribution.[9]  These qualities are what make this Biomid fiber ideal for our composite.

Note:

Before starting any of the actual lab work, a meeting was held with Dr. Palmese, the professor in charge of the lab that all of the research would take place.  He explained how the lab worked, who was in charge of which portion of the lab, and discussed the procedures that our epoxy and composite would undergo in order to yield results that would be worthy of presentation.  From there, a plan was set in place so that all components of this project would be met before the final deliverable was due.  For a detailed description and list of the process of this project, please refer to the Weekly Progress page of this blog by either clicking the tab above or on Weekly Progress.

To see the results from the testing of the epoxy resin and the composite, please click on the Results tab above or on Results.