Tensile Property Testing

Testing of tensile properties is probably the most widely used short-term mechanical test of all. This is because it is relatively easy to perform, gives reasonably reproducible results and yields a great deal of information. From this test one can obtain not only tensile strength, but also elongation and modulus.

Tensile properties are determined by stretching a test specimen at a constant rate. The resulting stress (or load applied) is measured an recorded as a function of strain (or elongation). The stress and strain are defined mathematically by:

Stress=     Load applied on specimen

                 Cross-section area of specimen

      Strain=    Increase in length of specimen

                                                          Original length of specimen

                                      Modulus=  Stress

                                                      Strain

Tensile strength is the maximum tensile stree which a material is capable of supporting. It is calculated from the maximum load carried during the tensile test and the original cross-sectional area of the specimen. If the tensile strength occurs at the sample's yield point, this stress is designated as the tensile strength at yield. if it occurs at the sample's break point, it is then designted the tensile strength at break 

The percentage of elongation at break is the ratio, expressed in percentage, of the extension (change in gauge length) at break point to the original gauge length multiplied by 100.

Polycaprolactone

Polycaprolactone (PCL) has been thoroughly studied as a substarte for biodegradation (Potts, 1984) and as a matrix in controlled-release systems for drugs (Pitt et al.,1980). Its degradation in vivo is much slower than that of poly (α-hydroxy acid). Thus, it is most suitable for controlled-release devices with long in vivo lifetimes. PCL is generally prepared from the ring-opening polymerization of ε-caprolactone.

PCL was chosen because of its good mechanical properties and its compatibility with many types of polymers, and because it is one of the more hydrophobic of the commercially available biodegradable polymer (Koleske, 1978)

Poly (lactic acid) (PLA)

Poly (lactic acid) (PLA) degrade biologically into acid, a product of the carbohydrate metaolism, and a importance as a substitute for the non-degradable thermoplastics has attracted a lot of attention in recent years.

PLA are of considerable interest as biodegradable polymers in medical applications and also potentially for use as environmentally friendly packaging materials. PLA is a thermoplastic, high strength, high modulus polymer which can be made from annually renewable resources to yield articles for use in either the industrial packaging field or the biocompatible/ bioabsorbable medical device market (Migliaresi et al., 1988).

PLA exists in L and D- form, which are optical isomers. PLA with large amount of L- form isomer is highly crystalline. In general, the crystallinity and biodegradability depend on the content of D0 form isomer (Zhang et al., 1993).

Poly(vinyl alcohol) - PVA

PVA is a biodegradable synthetic polymer, which has excellent strength and flexibility. It has been reported that addition of PVA improves the strength and flexibility of gelatinized starch (Shogren et al., 1998). young (1967) found that films cast from aqueous solutions of amylose or high amylose starch and PVA had higher strength and elongation at break at 23 and 50% humidity than films cast from starch alone.

Otey and Mark (1976) found similar improvement in mechanical properties for films cast from solution mixtures of normal cornstarch and PVA. Shogren et al. (1998b) prepared starch-PVA foam articles by a backing process and found that foam strength, flexibility, and water resistance were markedly improved by addition of 10-30% poly(vinyl alcohol) to starch batters.

Starch

Starch is one of the major components or cereal grains. Commercially important starches come from corn, waxy corn, high amylose corn, wheat, rice, potato, tapioca, and pea. The major sources of commercial starch here in Thailand are tapioca and rice. The starches are readily obtained from these plant source as a fine powder (granular starch) consisting of spherical or ellipsoidal grains ranging in particle size from 3 to 100 μm. Among these, rice starch has among the smallest granule size and potato starch the largest of all starch types. (Kirk and Othmer, 1997)

Starch (David, 1998) is predominantly composed of two polymers of D-glucose, amylose and amylopectin. Amolose is a lightly branched polymer comprised of α-1,4 linked anhydroglucose units and has molecular weights which vary from 10⁵- 10⁶. Amylose from different sources contains, on average, two to eight α-1,6 branch points per molecule. Amylopectin is a highly branched molecule consisting of short (15-45 residues)  α-1,4 oligomers linked by α-1,6 bonds. The overall structure is thought to be tree-like. Molecular weights range from 10⁷- 10⁹ with the average over 10⁹. Whithin the native starch granules, amylopectin is partially crystalline while amylose is amorphous (non-crystalline). The native crystalline structure of amylopectin consists of parallel-stranded double helices with six glucose units per 2.1 nm. rise in each individual strand.

Plasticizers

In most applications in which plasticizers are incorporated into plastics, their purpose is to convert an otherwise hard and rigid plastic to a flexible or semiflexible tough part. In some instances the same or similar conversions are obtainable by copolymerizing certain flexibility imparting monomers or by blending or grafting elastomers with those rigid plastics. Some of them are specifically made for use as polymeric-type plasticizers (Gaechter and Mueller, 1990)

The low-molecular-weight plastizers show the best plasticizing performance but have the disadvantage of bing more prone to migration when in contact with other plastics and of being some what more volatile. The plasticizing power is proportional to the viscosity of the plasticizer or mixture of plasticizers, with the lowest viscosity resulting in the best flexibility.

Evaporation loss of plasticizers render plastics less flexible in time. The plasticizer vapors may cause windshield fogging in automobiles because they condense on the interior glass surfaces.

The effectiveness of any plasticizer in regard to imparted mechanical properties is very much temperature dependent. That is why most plastics are not compounded with plasticizers. At elevated temperature they would become too soft and at the low temperature range they would be too hard and brittle. The cellulostics and polyvinyl chloride represent notable exceptions.

Biodegradable Polymers

Traditional applications of synthetic polymers are mostly based on their relative inertness to biodegradation compared with natural macromolecules such as cellulose and proteins. Concerns about preventing or retarding attack on polymers by bacteria, fungi, insects, rodents, and other animals provided the early incentive for the study of the iodegradation of  polymers. now the use of synthetic polymers has accelerated to the extent that he disposal of the polymer products currently in use, most of them bioresistant, has become increasingly dificult.

One of the important current incentives for the study of biodegradable polymers is their easier disposal. Moreover, biodegadable polymers are useful for applications such as sutures, surgical implants, controlled-release formulations of drugs and agricultural chemicals, agricultural mulch, etc, and interest in them continues to increase.

Foamed Plastics

Foamed plastics, also known as cellular plastics or plastic foams, have been important since primitive man began to use wood, a cellular form of the polymer cellulose. Cellulose is the most abundant of all naturally occurring organic compounds, comprising approximately one-third of all vegetable matter in the world (Ott et al., 1954). Its name is derived from the Latin word cellula, meaning very small cell or room, and most of the polymer does indeed exist in cellular form as in wood, straws, seed husks, etc.

A cellular plastic is defined as a plastic the apparent density of which is decreased substantially by the presence of numerous cells disposed throughout its mass. The gas phase in a cellular polymer is usually distributed in voids or pockets called cells. If these cells are interconnected, the material is termed open-celled. If the cells are discrete and the gas phase of each is independent of that of the other cells , the material is termed closed-cell.

Foaming processes (Ronald and David, 1986) are characterized by techniques that cause tiny bubbles to form within the plastic material such that when the plastic solidifies the bubbles, or at least the holes created by the bubbles, remain, the solidified bubble-containing material thought of as a cellular structure The products made by these processes are referred to as forams or cellular plastics.

Plastic foams can also be classified on the basis of wall stiffness, if the walls are stiff, the foam is called a rigid foam. If the walls collapse when presssed, the foam is called a flexible foam. Bothe open and closed cell foams can either flexible walls or rigid walls,

Plastic foams have some physical characteristics that are valuable for several important applications. The cellular structure means that much of the space in the plastic foam is filled with air or some other gas. The low thermal conductivity of gases means that these foams are very good thermal insulators. Some aplication that utilize this insulating property are hot and cold cups, building insulation slabs, and pipe insulation.

Starch foams

Starch-based composition foams were prepared by baking a mixture of starch and a synthetic biodegradable polymer in a hot mold. This process can be used to prepare a thin-walled object, such as a plate.

Scanning electron micrographs of the cross-sectional view of the foams showed that the cellular size is very dense in the outer layer and less dense in the inner layer. The effects of relative humidity, storage time, the presence of synthetic biodegradable polymers, and the presence of plasticizers on tesnile and flexural properties were investigated.

For all formulations, the amount of relative humidity which gave the maximum value of the ultimate strength of the foams wat 42%, while the storage time which gave the maximum value of the ultimate strength of foams was 2 days.

Addition of the synthetic biodegradable polymers improved the ultimate strength and the elongation at break of the foams. Increasing the plasticizer contents increased the elongation at break at the expense of the ultimate strength. Water absorption and biodegradability of the foams were also studied. Increasing the synthetic biodegradable polymer contents increased the water resisitivity of the foams. Enzymatic degradation tests showed that the foams were degraded by enzyme amylase

What is science?

From wikipedia:

science (from Latin scientia, meaning "knowledge") is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe. In an older and closely related meaning (found, for example, in Aristotle), "science" refers to the body of reliable knowledge itself, of the type that can be logically and rationally explained (see History and philosophy below). Since classical antiquity science as a type of knowledge was closely linked to philosophy. In the early modern era the words "science" and "philosophy" were sometimes used interchangeably in the English language. By the 17th century, natural philosophy (which is today called "natural science") was considered a separate branch of philosophy. However, "science" continued to be used in a broad sense denoting reliable knowledge about a topic, in the same way it is still used in modern terms such as library science or political science.

 From UGA GEOL:

 Science is the concerted human effort to understand, or to understand better, the history of the natural world and how the natural world works, with observable physical evidence as the basis of that understanding1. It is done through observation of natural phenomena, and/or through experimentation that tries to simulate natural processes under controlled conditions. (There are, of course, more definitions of science.)

     Consider some examples. An ecologist observing the territorial behaviors of bluebirds and a geologist examining the distribution of fossils in an outcrop are both scientists making observations in order to find patterns in natural phenomena. They just do it outdoors and thus entertain the general public with their behavior. An astrophysicist photographing distant galaxies and a climatologist sifting data from weather balloons similarly are also scientists making observations, but in more discrete settings.

     The examples above are observational science, but there is also experimental science. A chemist observing the rates of one chemical reaction at a variety of temperatures and a nuclear physicist recording the results of bombardment of a particular kind of matter with neutrons are both scientists performing experiments to see what consistent patterns emerge. A biologist observing the reaction of a particular tissue to various stimulants is likewise experimenting to find patterns of behavior. These folks usually do their work in labs and wear impressive white lab coats, which seems to mean they make more money too.

     The critical commonality is that all these people are making and recording observations of nature, or of simulations of nature, in order to learn more about how nature, in the broadest sense, works. We'll see below that one of their main goals is to show that old ideas (the ideas of scientists a century ago or perhaps just a year ago) are wrong and that, instead, new ideas may better explain nature.