ABSTRACT
Gum Arabic exudate was collected from Acacia senegal trees around Zaria metropolis, purified in 95% ethanol and its physical and chemical modifications carried out. Physical modification of the gum involved plasticization of the gum with glycerol and ethylene glycol. The chemical method was performed by acid hydrolysis, acetolysis and acetate formation. Appearance of both modifications was observed after three days of drying. Acetic anhydride (AAN), acetolysis (ACT) and ethylene glycol (EGL) modifications became hard and solid, and were ground to powder. Glycerol (GLY) turned very sticky and acid hydrolysis (AHY) turned into a viscous liquid. From characterization of the samples, all modifications were found to be less dense than the pure gum Arabic sample (PGM). AHY sample was found to be more turbid and has the highest conductivity value followed by AAN sample. pH of all samples was found to be below 7.0, indicating acidic nature of the gums. Melting point measurements showed that all test samples have lower melting point values than the pure gum. FTIR and GC-MS spectra of the pure and chemically modified samples were studied. It was found that there were shifts and absorptions at different frequencies, indicating some degree of interaction between the gum and the modifying solvents. The gums were all mixed with PVC at different compositions and cast into films using tetrahydrofuran as solvent. The films produced were subjected to mechanical tests. The AAN sample has the highest modulus at 10, 20, 30 and 40% gum composition. For PGM, the modulus drops from 10-30% gum composition then finally increases. The ACT modification shows decrease in modulus as the percent of gum increases. Modulus of PGM drops at 20, 40, 60 and 80% gum composition. It was found that the tensile strength of the chemical modification formulations reinforced the PVC matrix at 10%/90%, 20%/80% and 30%/70% gum/PVC compositions for AAN and at 10%/90% and 20%/80% gum/PVC compositions for ACT. Though EGL showed increase at 30%/70% composition, its tensile strength is similar to that of the unmodified gum (PGM), decreasing with increasing gum concentration.
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TABLE OF CONTENTS
DECLARATION ……………………………………………………………………………………………………………. ii
CERTIFICATION …………………………………………………………………………………………………………. iii
DEDICATION ………………………………………………………………………………………………………………. iv
AKNOWLEDGEMENTS………………………………………………………………………………………………… v
ABSTRACT ………………………………………………………………………………………………………………….. vi
TABLE OF CONTENTS ……………………………………………………………………………………………….. vii
LIST OF FIGURES ……………………………………………………………………………………………………….. xi
LIST OF TABLES ………………………………………………………………………………………………………… xii
LIST OF PLATES ……………………………………………………………………………………………………….. xiii
LIST OF APPENDICES ……………………………………………………………………………………………….. xiv
ACRONYMS ……………………………………………………………………………………………………………….. xv
CHAPTER ONE …………………………………………………………………………………………………………….. 1
INTRODUCTION ………………………………………………………………………………………………………….. 1
1.1 STRUCTURE OF POLYSACCHARIDES ——————————————————————- 1
1.2 STRUCTURAL MODIFICATION OF POLYSACCHARIDES —————————————- 1
1.2.1 Starch Acetate ————————————————————————————————– 1
1.2.2 Hydroxylethyl ethers —————————————————————————————– 2
1.2.3 Acid hydrolysis ———————————————————————————————— 2
1.2.4 Acetolysis ——————————————————————————————————– 3
1.3 PLASTICIZATION ————————————————————————————————- 3
1.4 BRIEF HISTORY OF PVC ————————————————————————————— 4
1.5 PREPARATION OF VINYL CHLORIDE ——————————————————————– 5
1.6 INDUSTRIAL MANUFACTURE OF PVC —————————————————————– 7
1.6.1 Suspension Polymerization Process———————————————————————– 7
1.6.2 Bulk Polymerization —————————————————————————————— 8
1.6.3 Emulsion Polymerization ———————————————————————————— 9
1.6.4 Solution Polymerization ————————————————————————————- 9
1.7 MECHANISM OF FORMATION —————————————————————————- 10
1.7.1 Initiation ——————————————————————————————————- 10
1.7.2 Propagation ————————————————————————————————— 11
viii
1.7.3 Termination ————————————————————————————————— 11
1.8 PHYSICAL PROPERTIES OF PVC ————————————————————————- 12
1.9 COMPOUNDING OF PVC ————————————————————————————- 13
1.10 ADDITIVES FOR PVC —————————————————————————————— 14
1.10.1 Plasticizers —————————————————————————————————- 14
1.10.2 Stabilizers —————————————————————————————————– 18
1.10.3 Fillers and Reinforcements ——————————————————————————– 20
1.10.4 Lubricants —————————————————————————————————– 20
1.11 PROCESSING OF PVC —————————————————————————————– 21
1.11.1 Extrusion —————————————————————————————————— 21
1.11.2 Molding and Forming ————————————————————————————– 22
1.12 GUM ARABIC —————————————————————————————————– 26
1.12.1 Distribution ————————————————————————————————– 27
1.12.2 Structural Composition ———————————————————————————— 28
1.12.3 Chemistry of Gum arabic ———————————————————————————- 28
1.12.4 Physical Properties —————————————————————————————— 29
1.12.5 Chemical Properties —————————————————————————————- 31
1.12.6 Uses of Gum Arabic —————————————————————————————- 32
1.13 AIMS AND OBJECTVES OF THIS STUDY ————————————————————– 34
1.13.1 Aim of This Study —————————————————————————————– 34
1.13.2 Objectives of this Study ———————————————————————————- 34
CHAPTER TWO ………………………………………………………………………………………………………….. 35
REVIEW OF PAST WORKS …………………………………………………………………………………………. 35
2.1 GUM ARABIC —————————————————————————————————– 35
2.1.1 Pharmacological Properties ——————————————————————————- 38
2.1.2 Toxicity of Gum Arabic ———————————————————————————– 39
2.2 STRUCTURE AND PROPERTIES OF PVC ————————————————————– 41
2.3 EFFECT OF ADDITIVES ON THE STRUCTURE AND PROPERTIES OF PVC ——————- 42
2.4 EFFECT OF PROCESSING ON THE STRUCTURE AND PROPERTIES OF PVC ———– 49
2.5 DEFECTS IN THE STRUCTURE OF PVC —————————————————————- 50
2.6 MODIFICATION OF POLY (VINYL CHLORIDE) —————————————————– 51
2.7 DEGRADATION ————————————————————————————————– 52
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2.8 TOXICOLOGICAL CONCERNS —————————————————————————– 55
2.9 PVC TESTS FOR MECHANICAL AND OTHER RELATED PROPERTIES——————————- 56
CHAPTER THREE ………………………………………………………………………………………………………. 61
MATERIALS AND METHOD ………………………………………………………………………………………. 61
3.1 MATERIALS USED———————————————————————————————- 61
3.2 COLLECTION AND PURIFICATION ———————————————————————- 61
3.3 CHARACTERIZATION—————————————————————————————– 61
3.3.1 Percentage Yield ——————————————————————————————— 61
3.3.2 pH Determination ———————————————————————————————- 62
3.3.3 Viscosity Measurement ———————————————————————————— 62
3.3.4 Moisture Content ——————————————————————————————– 62
3.3.5 Ash Content ————————————————————————————————— 62
3.3.6 Determination of Crude Protein ————————————————————————– 63
3.3.7 Percentage Lipid Determination ————————————————————————- 63
3.3.8 Total Carbohydrate Content —————————————————————————— 64
3.3.9 Internal Energy ————————————————————————————————— 64
3.3.10 Melting Point ———————————————————————————————– 64
3.3.11 Density Measurement ————————————————————————————– 65
3.3.12 Ftir Analysis ————————————————————————————————– 65
3.3.13 GC-MS ——————————————————————————————————— 65
3.4 GUM MODIFICATION —————————————————————————————– 66
3.4.1 Chemical Modification ————————————————————————————- 66
This was performed by acid hydrolysis, acetolysis, and acetate formation. ——————————– 66
3.4.2 Physical Modification ————————————————————————————– 67
3.5 PVC FORMULATION ——————————————————————————————- 67
3.6 FILM CASTING ————————————————————————————————— 67
3.7 MECHANICAL PROPERTY TESTING ——————————————————————– 68
3.8 PERCENT WATER ABSORPTION ————————————————————————- 68
CHAPTER FOUR …………………………………………………………………………………………………………. 70
RESULTS AND DISCUSSION ……………………………………………………………………………………… 70
4.1 PROXIMATE ANALYSIS OF PURE GUM ARABIC ————————————————– 71
4.2 PHYSICOCHEMICAL ANALYSIS OF GUM SAMPLES ——————————————– 72
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4.3 FTIR OF ANALYSIS OF TEST SAMPLES ————————————————————— 78
4.4 GC/MS ————————————————————————————————————— 84
4.4.1 GC-MS of Gum Modifications ————————————————————————— 84
4.5 WATER ABSORPTION —————————————————————————————– 90
4.6 MECHANICAL PROPERTIES ——————————————————————————– 90
4.6.1 Tensile strength ———————————————————————————————- 92
4.6.2 Elastic Modulus ———————————————————————————————- 95
4.6.3 Percentage Elongation of the Samples —————————————————————– 97
CHAPTER FIVE ………………………………………………………………………………………………………….. 99
CONCLUSION …………………………………………………………………………………………………………….. 99
RECOMMENDATION ……………………………………………………………………………………………….. 100
REFERENCES …………………………………………………………………………………………………………… 101
APPENDIX I ……………………………………………………………………………………………………………… 111
APPENDIX II …………………………………………………………………………………………………………….. 116
APPENDIX III ……………………………………………………………………………………………………………. 120
APPENDIX IV……………………………………………………………………………………………………………. 121
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CHAPTER ONE
INTRODUCTION
Polysaccharides have been described as high molecular weight polymers formed by condensation of many monosaccharide units or their derivatives. They have also been defined as polymeric substances, the building blocks of which are monosaccharide. From the foregoing, polysaccharides could be said to be long chain carbohydrate molecules built from some monosaccharides such as glucose, rhamnose, galactose, etc. or their derivatives. Polysaccharides could be classified based on their chemical compositions. In this regard a polysaccharide which yields only one type of monosaccharide on hydrolysis is called homoglycan e.g starch, while those which yield two or more types of monosaccarides are called heteroglycan e.g. gum Arabic (Varki et al, 2008).
1.1 STRUCTURE OF POLYSACCHARIDES
Polysaccharides are normally isolated from their natural environments without much degredation for study. When monosaccharides come together to form polysaccharide, an anomeric hydroxyl group of one monose unit can condense with any hydroxyl group other than that of the former. The few monoses which occur as components of polysaccharides are D-glucose, D-mannose, D-fructose, D-galactose, D-xylose, L-arabinose, D-glucosamine and D-glucuronic acid.
1.2 STRUCTURAL MODIFICATION OF POLYSACCHARIDES
1.2.1 Starch Acetate
Starch acetate derivative has been reportedly prepared by the treatment of starch with acetic anhydride or vinyl acetate (Rutenberg, 1968). The products so formed are low
2
substituted starch esters with good hydrating capacity, flow, filming and viscosity stability. The process by which this reaction occurs is still not clear, it is however possible that the branches are being substituted by the acetate group. This would lead to smaller side chains which might enhance chain flexibility.
1.2.2 Hydroxylethyl ethers
Hydroxylethyl starch ethers are products of reaction of ethylene oxide and starch (Rutenberg, 1968). These ethers have properties similar to starch acetates including flexible film formation. The close resemblance of starch and gum Arabic makes the acetate and ether derivatives formation likely to reduce the brittleness of gum Arabic.
1.2.3 Acid hydrolysis
Acid hydrolysis of polysaccharides normally breaks polysaccharides down to their monomers, dimers and some oligosaccharides. The products of this hydrolysis are low viscous liquids. The acid hydrolysis is believed to fragment the molecules by cleaving bonds other than the (1-6) which is the most resistant to acid hydrolysis (Guthrie 1974; Bochkov, 1979). Acid catalysed hydrolysis of O-glycosides yield corresponding alcohols and some reducing sugars via some protonated intermediates (Bochkov, 1979). This hydrolysis of monosaccharides is a slow reaction with rate constant values in the range of 10-4S-1 to 10-6S-1 at acid concentrations within the range of 0.01M to 0.5M HCl or H2SO4. The rate constant was also shown to increase with increasing acid concentration. It is therefore believed that at very high concentration, the reaction will be very fast.
3
1.2.4 Acetolysis
Acetolysis has been described as the treatment of polysaccharides with a mixture of acetic acid and acetic anhydride in equal weights in the presence of concentrated sulphuric acid. Products of this reaction have their (1-6) link cleaved contrary to acid hydrolysis in which the (1-6) link is very resistant (Govorchenko, 1973). This link is the least resistant to acetolysis (Guthrie, 1974). It is therefore believed that for a polysaccharide which has the (1-6) link as its branching points, the product of such acetolysed polysaccharide will be an almost linear polysaccharide.
1.3 PLASTICIZATION
Plasticization involves incorporation of plasticizer molecules into a polymer. Plasticizer molecules enter between molecules and separate the polymer molecules. Polar plasticizers, compatible with polysaccharides are attracted to polysaccharide molecules by their polar groups. The bulkiness, configuration and polarity of such plasticizers are such that arrangement of polymer molecule is affected to give room for slide chains and bulky side groups, and allow easy slippage of polymer molecules past one another.
PVC can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is used in clothing and upholstery, electrical cable insulation, inflatable products and many other applications in which it would originally have replaced rubber (Titow, 1984). As an amorphous polymer, PVC resin is extensively formulated to produce an extremely large variety of compounds (Blanco, 2000). Also due to its inexpensive nature and flexibility it is used in plastic pressure pipes systems for pipelines in the water industries. The capability of PVC to perform such diverse
4
functions is due to its ability to incorporate various additives to suit the numerous applications (Titow, 1984).
1.4 BRIEF HISTORY OF PVC
The first mention of poly (vinyl chloride) was in 1872 by Baumann when he described formation of a white powder by the action of sunlight on vinyl chloride contained in a sealed tube. The first commercial interest in PVC was shown by the Carbide and Carbon Chemical Corporation, Du Pont and IG Farben who independently filed patents in 1928 (Brydson, 2000). At that stage it was only possible to process homopolymer in the melt state, at temperatures, where high decomposition rates occurred, whereas copolymers could be processed at lower temperatures. Effective so called external plasticization of the PVC homopolymer by incorporating plasticizers was first discovered around 1930. When compounding with dibutyl phthalate and other esters reduction in the softening point PVC occurred, this resulted in rubber-like properties at room temperature (Brydson, 2000). The historical development of PVC is highlighted below according to Titow, (1990). Table 1.1 Historical background of PVC
Year
History
1835
Vinyl chloride monomer was first prepared by Regnault
1872
Baumann discussed the reaction of vinyl halides and acetylene in a sealed tube
1921
Plausen discovered how to polymerize PVC from dry acetylene which made PVC more than a laboratory curiosity
1928-1930
Patent disclosures of Vinyl chloride and Vinyl acrelate copolymers and polyvinyl halides. Introducing of emulsion polymerization to prepared vinyl chloride.
5
1930
Plasticization of PVC by phthalate ester. Introduction in non-rigid vinyl chloride plastics. Suspension polymerization of vinyl chloride in England and Germany. Technical production of chlorinated PVC polymers in Germany and USA.
1942-1943
Commercial production of PVC polymers in England.
Poly(vinyl chloride) (PVC), is the third-most widely-produced plastic, after polyethylene and polypropylene. It is a vinyl polymer made of repeating vinyl groups (ethenyls) having one hydrogen replaced by chlorine. PVC is widely used in construction because it is cheap, durable and easy to assemble. PVC production is expected to exceed 40 million tons by 2016 (Martin, 2008). Generally, the more developed countries are the biggest consumers. In the United States, Canada and Japan, PVC use is about 20kg per-capita (Titow, 1984). In Africa and the Middle East, the use of PVC is about 2kg per- capita. In Eastern Europe, South America, Mexico and Asia, the average use is about 3kg per- capita, resulting in a world average of about 4kg per – capita.
1.5 PREPARATION OF VINYL CHLORIDE
PVC is made from vinyl chloride monomer, known usually by its initials VCM through polymerization (Rossberg et al., 2006; Chanda and Roy, 2006). Some monomers exist in the form of reactive gaseous chemical substances and some of these may cause health hazards when in direct contact with humans. VCM which is the raw material for PVC is a gas at ambient temperature but is usually stored in liquid form under pressure. Ethylene and chlorine are raw materials for PVC also. Upstream industries are those that provide these materials and include producers of basic petrochemicals which supply ethylene and the caustic soda industry which supplies chlorine.
6
By thermal cracking of naphtha or natural gas, the basic petrochemical industry
manufactures ethylene and propylene etc. Naphtha is mainly supplied from the petroleum
refinery industry which uses crude oil as raw material. The caustic soda industry produces
caustic soda, chlorine and hydrogen via electrolysis using industrial grade salt as raw
material.
At first stage in the PVC production process ethylene and chlorine are combined to
produce ethane dichloride.
H2C CH2 + Cl2
Cl
Cl
When heated to 5000C at 15-30atm, ethane dichloride (EDC) decomposes to produce
vinyl chloride and hydrochloric acid.
Cl
Cl
H2C
Cl
+ HCl
Vinyl chloride can also be produced from acetylene and hydrogen chloride using
mercuric chloride catalyst
HC CH + H2C
Cl
HCl
HgCl2
+22.8Kcal/mol
Vinyl chloride is the building block of PVC. It is a gas with a stickly sweet odour
that is easily condensed. It is very toxic (Baird and Collias, 1998).
7
1.6 INDUSTRIAL MANUFACTURE OF PVC
Manufacture is carried out by free radical polymerization, all the four basic polymerization processes are followed viz; suspension, emulsion, mass and solution. The degree of polymerization is controlled by variation of the polymerization temperature. The process is carried out in pressure apparatus because of the low boiling point (-13.9oC) of the monomer, since radical forming agents used require polymerization temperatures of about 50oC. A total of 70-75% of all vinyl chloride is polymerized in suspension or emulsion, where yields of practically 100% are obtained with suspension and approximately 50% with emulsions. To prevent degradation or coagulation of the product, the reaction vessel is made of stainless steel or in glass-lined. Very pure water must be used in suspension polymerizates; the polymerizate is centrifuged and subsequently dried in dry tumblers. The emulsions polymerizate are worked up by either coagulation or spray-drying. In the later case, the product still contains about 5% solid impurity and therefore limited to use as paste (Hans, 1977). The solution technique is used only for specially vinyl coatings, which are generally vinyl chloride monomer copolymers and terpolymers (Reethamma, 2004).
1.6.1 Suspension Polymerization Process
In this technique, the monomer (VCM) is suspended as droplets in aqueous phase, as a result of vigorous agitation of the system (Sunmonu, 1994). The polymerization is divided into two main types, depending on the morphology of the particles that result. In bead polymerization, the polymer is soluble in its monomer and the result is a smooth, translucent bead. In powder polymerization, the polymer is not soluble in its monomer and the resultant bead will be porous and irregular (Kotoulas and Kiparissidas, 2006). In general suspension
8
polymerization is carried out at temperature between 60-85oC under pressure of up to 250psi. Polymerization is stopped by adding an inhibitor and unreacted monomer is flashed off with vacuum. The PVC beads are then filtered in centrifuge, ashed and dried in a flash dryer. The kinetics of the polymerization within an individual bead are similar to those of typical radical polymerization (Kalfas et al., 1993). The polymerization is usually carried to completion (Arshady, 1993).
1.6.2 Bulk Polymerization
The bulk polymerization system is used widely in the manufacture of a radically initiated polymerization reaction involving only the monomer, polymer and possible initiator (Sunmonu, 1994). At high conversion, however, transfer reactions that produce branched polymers can become noticeable. The gel effect can cause overheating which leads to insufficient polymerization control, degradation and possibly discoloration of the polymer. In bulk polymerization, therefore, the polymerization is often terminated at a conversion of 40-60% and the remaining monomer is distilled off. Alternatively, the polymerization can be carried out in two stages. In the first stage, polymerization is taken up to an average conversion in large vessels e.g. in capillaries as thin layers on supports, or falling freely in thin streams. All these methods have the disadvantage that small quantities of non-converted monomer, which can have serious physiological effects, remain in the polymer (Hans, 1977). Polymerization is therefore carried out at higher temperatures like 69-76oC and for 1-15 hours. There after the reaction are pumped into the second stage.
Generally, a series of large horizontal or as more modern process vertical reactants are used in the second stage. More VCM along with initiator, other ingredients and content
9
of the first stage reaction are charged into the reactor. Typical polymerization temperature is 55-600C and time 8 – 10 hours. It is in this stage that molecular weight can be controlled by polymerization temperature and or chain transfer agents. Bulk polymerization process is the simplest process (Saeki and Emura, 2002).
1.6.3 Emulsion Polymerization
The emulsion process is a modification of suspension polymerization in that different emulsifiers are used in slightly greater quantities. Particle size is much smaller for the dispersion resins made this way, 1-2 μm compared to 130-150 μm for the usual suspension resin. The emulsion or slurry is spray-dried because of this small particle size. Otherwise, the process is similar. Some emulsion resins are supplied as water systems or latexes. Manufacture is similar except for the drying stage (Cohan, 1975).
1.6.4 Solution Polymerization
Solution polymerization, as the name implies, refers to a system where the monomer is polymerized in a solvent. In comparison to bulk polymerization, heat control in the polymer mass is made easier by the presence of the solvent (Sunmonu, 2001). The system may be homogeneous with both the monomer and polymer soluble, or it may be heterogeneous in which case the monomer is soluble but the polymer is insoluble. The use of homogeneous solvents is exemplified by polymerizarion in butyl phthalate and in tetrahydrofuran (Krozer and Czlonkowska, 1964).
Solution process has certain advantages over bulk polymerizations. With solvent, heat transfer improves and temperature is more readily controlled. The solvent also allows more efficient agitation and consequently, the local reactions which result in overheating are eliminated. With certain heterogeneous solvent systems, it is possible to precipitate only
10
high molecular weight polymer, leaving low molecular weight material in solution. All these
factors result in a process which produces a more uniform poly (vinyl chloride) which
commands a premium price in today’s market place (Koleske and Wartman, 1995).
1.7 MECHANISM OF FORMATION
Vinyl chloride is a relatively un-reactive monomer, but it responds to free radical
initiators of the usual type to form a high polymer. The steps involved in the polymerization
are initiation, propagation, chain transfer and termination. The end result of these processes
is high molecular weight poly (vinyl chloride), which is quite uniform in a chemical sense
but contain a variety of molecular weight species.
1.7.1 Initiation
There are a series of organic compounds that are readily dissociated at reasonable
temperatures to produce free radicals (Nass, 1976). For the polymerization of vinyl chloride,
the free radical can be generated from hydroperoxides and Azo compounds. The peroxides
decompose by a first-order reaction, giving rise to two radicals, as indicated in the following
equation:
Examples of peroxides used are dilauryl peroxide dicaproxyl peroide, diisopropyl
peroxide and benzoyl peroxide.
O
O
O
O
O
O
+ + C
CO2
When the free radical as produced above is added to the monomer, a new free radical
is produced. Phenyl radical adds on the vinyl chloride monomer in the following manner,
11
C + H2C
Cl
CH
Cl
1.7.2 Propagation
The new free radical produced in initiation adds on to another monomer molecule to
form a new free radical which further adds on another fresh monomer molecule to form
another free radical and so on.
+
H2C
Cl
CH
Cl Cl
CH
Cl
In the course of the reaction, hundreds or even thousands of monomers are added
and incorporated into the polymer chain. These molecules, in principle may be added in one
of two ways, ―head to tail‖ or head to head‖
Analysis revealed head to tail addition accounts for all but a few percent of the
propagation reaction steps.
1.7.3 Termination
Polymerization chains are terminated by deactivating free radical, or the chain may
lease growing when the free radical reacts with the solvent molecule, e.g. CCl4
CH
Cl Cl
+ CCl4
Cl Cl
Cl
+ C
Cl
Cl
Cl
C
Cl
Cl
Cl
+ C
Cl
Cl
Cl
Cl
Cl
Cl Cl
Cl
Cl
12
1.8 PHYSICAL PROPERTIES OF PVC
a. Effect of plasticizer: PVC in combination with plasticizer exhibits lower tensile strength, hardness, and modulus than the corresponding properties of the unplasticized PVC. The plasticization is usually carried out with di-2-ethylhexyl phthalate. However small amounts of the plasticizer actually imparts an increase in the value of these properties.
b. Glass Transition Temperature: The temperature at which an amorphous polymer changes its hard or glassy state to a soft or rubbery material is known as the glass transition temperature Tg. The Tg of unmodified commercial grade poly (vinyl chloride) is 70 – 80oC (Simril, 1947).
c. Melting point: The melting point of PVC is markedly affected by polymerization temperature or by crystallinity. The crystalline melting temperature (Tm) cannot be measured by direct techniques, because polymer decomposition is too rapid at elevated temperatures even in the presence of stabilizers. To measure the melting temperature by another technique, the plasticizer (DOP) is added to the polymer at various concentrations and the stiffness modulus is measured as a function of temperature. On further increase, the crystalline network melts and there is another sharp drop modulus. The melting temperature is taken quite arbitrarily as the temperature at which the plasticized sample had stiffness modules of 10 psi.
d. Differential Thermal Analysis: Differential thermal analysis, DTA, is a technique that uses differences in temperature between an inert reference material and the sample under study, both being heated simultaneously at a uniform rate. The inert material changes in temperature at a constant rate while the sample investigated changes in temperature in a non-uniform manner. Also changes in heat capacity which occur at the glass transition
13
temperature can often be observed. The DTA technique is considered to be qualitative in nature; however, quite useful information can be obtained when it is employed.
e. Crystalline Network in plasticized poly (vinyl Chloride): Plasticized polymer remains tough and strong because of the presence of a non solvated, crystalline network which remains after plasticization. The development of this concept is quite interesting and important from both theoretical and an applied standpoint.
1.9 COMPOUNDING OF PVC
PVC is a polymer which consumes large number of additives during its processing. When the polymer combines with a number of carefully selected additives, it can possess a wide range of properties which enable it to be used in making different products. The myriad of PVC additives are selected on the basis of service requirements. The incorporation of these additives into PVC is generally known as compounding of PVC. Additives commonly used in the compounding process include; plasticizers, heat stabilizers, lubricants, flow promoting agents, fillers, impact modifiers, colourants and flame retardants. Rigid PVC compounds can be prepared by only using stabilizers, lubricants and flow promoters with optional additives of impact modifiers and fillers for opacity. Additives are those materials which are physically dispersed in a polymer. The most important requirement of any additive is that it should be effective for the purpose it is designed to achieve. This effectiveness is usually enhanced by the correct procedure of incorporation of the additive molecules into the polymer matrix. Thus additives used in plastic materials are normally classified according to their specific function rather than on chemical basis (Thomas and Todd, 1971).
14
1.10 ADDITIVES FOR PVC
1.10.1 Plasticizers
Plasticizers are high molecular weight non volatile liquids which solvate and soften the polymer (koleske and Wartman, 1995). It is incorporated into the plastic to increase its workability and flexibility or elongation. A plasticizer may reduce the melt viscosity or lower elastic modulus of the product. Plasticized compound can be processed more easily than the rigid products because the reduced softening points make it possible to use lower temperature 140 – 160oC, compared with 185 – 190oC (Charles et al., 2005). A plasticizer can be usually selected primarily on the basis of the cost, permanence, and low temperature brittleness requirements of the product although other properties are also of importance (Koleske and Wartman, 1995). For a plasticizer to be effective and useful in PVC, it must contain two types of structural components, polar and non-polar. The polar portion of the molecule must be able to bind irreversibly with the PVC polymer, thus softening the PVC, while non-polar portion of the molecule allows PVC interaction to be controlled so it is not so powerful a solvent as to destroy the PVC crystallinity.
The balance between the polar and non polar portions of the molecules is critical to control its solubilizing effect; if plasticizer is too polar; it can destroy PVC crystallites, and if it is too non polar, compatibility problems may arise. Several theories have been developed to account for the observed characteristics of the plasticization process (Charles et al., 2005). According to the theories plasticization at the molecular level means weakening or rupturing of selected bonds between molecules making the shaping, flexing or moulding of the material possible. For example, water is added to clay to make it possible for pottery. Also complex polymeric substances like animal horns, tortoise shell and simpler
15
natural resins, gums and waxes are commercialized through plasticization. Cellulose nitrate (celluloid) is the earliest resin developed by Schunbein that requires plasticizer for it processing. John and Hyatt developed camphor as plasticizer for cellulose nitrate in application involving thick objects (Nass, 1976). The four general theories proposed to account for the effectiveness of plasticizers on certain resins are: Lubricity theory, Gel theory, free volume theory and Mechanistic theory. i. Lubricity Theory The Lubricity Theory states that plasticizer acts as a lubricant between polymer molecules. It is based on the assumption that rigidity arises from intermolecular friction binding the chains together in a rigid network. When the polymer is heated, these forces are weakened allowing the plasticizer molecules to be inserted between the chains. Once incorporated in to the polymer, plasticizer molecules shield the chains from each other, thus preventing the re-formation of the rigid network. Whilst attractive in its simplicity, this theory does not explain the success of some plasticizers and the failure of others (EPCI, 2010). ii. The Gel theory
This theory considers the plasticized polymer to be neither solid nor liquid but an intermediate state, loosely held together by a three-dimensional network of weak secondary bonding forces. The bonding forces acting between plasticizer and polymer and easily overcome by applied external stresses allowing the plasticized polymer to flex, elongate or compress (Charles et al, 2005). While a certain concentration of plasticizer molecules will provide plasticization by this process, the remainder will act more in accordance with the
16
lubricity theory. Unattached plasticizer molecules will be swelling the gel and facilitating their movement thus imparting flexibity (ECPI, 2010). iii. The Free Volume Theory This extends the lubricity and gel theories and also allows a quantitative assessment of the plasticization process. Free volume is a measure of the internal space available in a polymer for the movement of the polymer chain, which imparts flexibility to the resin. A rigid resin such as unplasticized PVC, possesses very little free volume whereas flexible resins have relatively large amounts of free volume. Therefore, plasticizers act so as to increase the free volume of the resin and also to ensure that free volume is maintained as the resin-plasticizer mixture is cooled down after melting ((EPCI, 2010). iv. The Mechanistic theory In this theory, it is assumed that solvents or plasticizers of different classes are attracted to the resin macro- molecules by forces of different magnitude. This theory can be depicted as having some resemblance to Gel Theory. The mechanistic explanation of plasticization considers the interactions of the plasticizer with the PVC resin macromolecules. It assumes that the plasticizer molecules are not permanently bound to the PVC resin molecules but are free to self-associate and to associate with the polymer molecules at certain sites such as amorphous sites. As these interactions are weak, there is a dynamic exchange process whereby, as one plasticizer molecule becomes attached ata site or center, it is readily dislodged and replaced by another. Different plasticizers yield different plasticization effects because of the differences in the strengths of the plasticizer polymer and plasticizer-plasticizer interactions (Charles et al., 2005).
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1.10.1.1 Types Of Plasticizer
a. General purpose plasticizers (GP)
These plasticizers provide the desired flexibility to PVC along with an overall balance of optimum properties at the lowest cost. They are dialkyl phthalates range from diisoheptyl (DIHP) to diisodesyl (DIDP).
b. Performance plasticizers (PP)
These plasticizers contribute secondary performance properties desired in flexible PVC beyond the GP type, while imposing somewhat high cost. They include specific phthalates and other types of plasticizers with strong polarity. Conversely, low temperature types, such as aliphatic dibasic esters, are less solvating and have higher diffusivity, low volatility requires high molecular weight plasticizer, such as trimellites and polyesters (polymeric).
c. Specialty Plasticizers (SP)
These plasticizers provide properties beyond those typically associated with flexible PVC designed for general purpose or specialty characteristics. This exceptional characteristics are typically a function of specific chemical plasticizer families and may vary as a function of isomeric structure and/or homologous. A few phthalates meet these special requirements. Polyester plasticizers provide low volatility and low diffusivity, along with low smoke (in absence of aromaticity) under fire conditions. Epoxy plasticizers provide adjuvant thermal stability to PVC phthalate and halogenated plasticizers impose even higher cost than PP grades plasticizers.
1.10.1.2 Requisites of PVC Plasticizer
The most important requirement for a plasticizer can be summarized as:
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a. Compatibility; for a material to act as a plasticizer it should be compatible with the polymer.
b. Effectiveness in imparting a desirable property; this is the parameter which expresses the amount of plasticizers required to impart a particular property to the resin.
c. Permanence; this defines the ability of the plasticizers to remain within the vinyl polymer and continue to be effective as a plasticizer under various exposure conditions. Thus, permanence here means ability of the plasticizer system to resist extraction or migration into an adjoining resin.
d. Heat and light stability; plasticizers must have good chemical stability.
1.10.2 Stabilizers
PVC degradation begins by the loss of labile chloride group as HCl and proceeds by a chain reaction generating more HCl and forming polyene sequences, giving rise to colour. The entire process depends upon many factors such as surrounding atmosphere, temperature and molecular weight of the polymer. Once dehydrochlorination of allylic chloride end group starts, new allylic groupings are formed and the reaction can proceed in zipper fashion leading to polyene structures which are largely responsible for discoloration (Baum, 1961). Stabilizers then are chemical additives that prevent, arrest, or at least minimize such changes, and so stabilize the polymer (Kauder, 1989).
At low degree of dehydrochlorination and at a reaction of 180oC, there are average sequence lengths of 5-10 conjugated double bonds. The number of longer sequences decreases continually with maximum length of 25-30 conjugated double bonds. With increasing temperature and degradation, the frequency distribution shifts towards lower
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sequences. At higher rate of conversion, aromatic products such as toluene are formed (straus et al., 1959). As documented in the literature, PVC stabilizers could be classified based on mode of action into the following. 1.10.2.1 Primary Stabilizers These types of stabilizers function by reacting with allylic chlorine atoms, the intermediates in the zipper degradation chain, thereby preventing further dehydrochlorination (Scheme 3) (Fisch and Bacaloglu, 1999; Baltacioglu and Balkose, 1999; Okieimen and Sogbaike, 1995). This process should be faster than the chain propagation itself, requiring a very active nucleophile. However, the reactivity of the nucleophile should not be so high as to react with the secondary chlorine of the PVC chain, a process that rapidly exhausts the stabilizer. To be effective, the stabilizer must be associated by complex formation with the polymer chlorine atoms, which means it should have a Lewis acid character (Fisch and Bacaloglu, 1999).
1.10.2.2 Secondary Stabilizers
This type of stabilizers function by scavenging the HCl/Cl radical generated. HCl is a catalyst for the chain propagation reaction and the initiation step (Gonzalez- Ortiz et al., 2005). Scavenging cannot stop the degradation process completely since it is diffusion controlled. However, HCl scavenging considerably reduces the rate of degradation and avoids the very fast process that eventually causes PVC blackening (Tamer et al., 2005). Stabilization is complicated by the fact that primary stabilizers are strong Lewis acid, reacting with HCl that catalyzes the initiation and propagation of PVC degradation. To
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avoid this, secondary stabilizers should be present to react with HCl to protect the primary stabilizers.
1.10.3 Fillers and Reinforcements
Fillers are particulate material, such as minerals, diatomaceous earths, and tale, which are added to polymers to reduce cost. Fibrous reinforcements, such as glass and carbon fibers, are added to polymer to increase stiffness and to some degree strength. Both types of materials tend to increase the viscosity of the polymer, especially at low shear rates, resulting in the formation of yield stress. At high shear rates, the effect is less pronounced as the viscosity approaches that of the neat resin. In addition to increase in pressure drops associated with processing these composite materials, they tend to lead wear of the screws, eventually reducing the performance of the extruder (Mascia, 1974). Fillers have been used in plastics industrially worldwide for many decades. The primary purpose for using in thermoplastic matrix is its ability to modify properties (e.g to improve strength and stiffness, scuff resistance, enhance thermal conductivity and electrical properties, dimensional stability etc. (Charles et al, 2005).
1.10.4 Lubricants
A lubricant is a substance that, when added in small quantities, provides a disproportionate decrease in resistance to movement of chains or segments of polymer of at least partly amorphous structure, without disproportionate change in observable properties (Grossman, 1989). The two major classes of lubricants are designated ―internal‖ and ―external‖. It is important to obtain a balance between the external and internal lubricant.
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Lubricants perform several functions (Titow, 1990): reduce frictional forces between resin chains (internal), reduce frictional forces between the resin chains and the metal surface (external) and promote melt flow by reducing melt.
1.11 PROCESSING OF PVC
Resins can be softened by the application of heat processed, solidified and reheated and processed again, unlike thermosetting resin which, once solidified via the process of cross-linking, cannot be softened for reprocessing. Virgin PVC resin is rarely processed; instead, a wide variety of additives are compounded into the resin to improve processing performance and properties. These resins plus appropriate additives are then heated and shaped by flow deformation using a number of processing operations or techniques such as extrusion, injection molding, film blowing, fiber spinning, blow molding, thermoforming, compression molding etc (Mark, 1970).
1.11.1 Extrusion
Both flexible and rigid PVC can be extruded into variety of shapes including profiles, tubes and sheets, as well as wire sheeting by use of a crosshead die. The compound can be processed either from a powder blend or granulated materials. Generally, the granulated material can be handled on extrude of shorter screw length; in the case of plasticized PVC an extruder with the screw 16:1 length –to- diameter (L/D) ratio is satisfactory, whereas if powder blends are being extruded, equipment with screw of 20:1 (L/D) ratio is required for pellets and 25:1 L/D for powder single screw machines are normally used for plasticized compounds. The twin screw extruder has been developed extensively for the extrusion of rigid compounds. The temperature at which extrusion is carried out depends on the type of compounds being processed and can be determined only
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by careful experimentation, to obtain the best physical properties in the finished product. The temperature of the compound as it leaves the die must be maintained at its correct value. The temperature gradient on the extruder must be adjusted so that the head temperature approximates the temperature at which the compound emerges from the break plate (Mark, 1970).
1.11.2 Molding and Forming
1.11.2.1 Injection Molding The molding of PVC has developed rapidly with the availability of the screw type injection molding machines, prior to this, when the ram-type injection machines were standard, only plasticized PVC could be molded, and it requires special attention to prevent hold up of material with consequent degradation. Unplasticised PVC can be handled only on screw type machines, which have the advantage of being to deliver a low-viscosity homogeneous melts into the mold cavity with maximum possible pressure on the melt. The working life of rigid PVC at the temperature required for injection molding necessitates the introduction of the polymer melt into the mold in the shortest possible time once the optimum temperature had been reached, at most this should take only two cycles. This was almost impossible to achieve with a ram machine, but the reciprocating- screw injection molding machine is similar to a single-screw extruder in that a homogeneous melt is obtained by shear forces set up by the rotation of the screw. As the material accumulates at the front of the screw, the latter retracts until a present position is reached at which the volume of the accumulated molten material is sufficient to fill the mold; the screw then stops rotating and moves forwards to inject the melt into the mold. The screw then starts to rotate again and prepares the melt for the next shot.
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The mold cycle can be varied over a wide range by a wide range by adjustment of the speed of rotation of the screw (20 -50rpm); and the times for the various stages of the process. The molten compound a high melt viscosity compared with the majority of moldable thermoplastics. The press during injection of rigid compounds is 12,000 to 15,000 1b/in2, for plasticized compound it is considerably less. 1.11.2.2 Thermoforming This is used primarily for the manufacture of packaging and disposable containers. However, it is also becoming a useful technique in the processing of engineering thermoplastics to produce parts used in the transportation industry. Polymers that are processed by this technique must have sufficient melt strength that on heating they do not sag significantly under their own weight, yet they can be deformed under pressure to take the shape of a mold. Hence, highly crystalline polymers with high melting temperature (Tm) and low molecular weight (Mw) cannot be readily thermoformed.
Thermoplastic sheet is heated usually by means of radiation but sometimes in conjunction with convection cooling to temperatures either just above Tg in the case of amorphous polymer or Tm in the case of semi crystalline polymers. The exact temperature depends on the degree of sag exhibited by the material under its own weight, which is determined by its rheological properties. The sample is then removed from the heating system and brought into position over the mold and forced to take the shape of the mold by applying pressure to the top of the sheet or by generating a vacuum on the underside of the sheet. The forming step occurs in the matter of a second. The sample is maintained in the mold until it is rigid enough to be removed from the mold without altering its shape. For example, in plug-assisted vacuum forming, the heated sheet is forced by a plug into the
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mold with the remainder of the shape being produced by the application of vacuum to the underside of the sheet. However, the sheets can be forced to take on different shapes because each half of the mold can have a different shape. Furthermore, different polymers can be used for each half. The sheets must be held in the mold long enough for bonding to occur. Thermoforming can be divided into four sections: (1) sheet heating without deformation; (2) sheet stretching without significant heat transfer; (3) part cooling in the mold, and (4) post molding operations such as trimming. The time to make a part is primarily determined by step 1 and 3 because these are of the order of minutes. However, the successful functioning of the part is determined by step 2 because the distribution of wall thickness is determines in this step. 1.11.2.2 Blow Molding Blow molding is a manufacturing process widely used to create hollow thin-wall plastic objects such as bottles, cases, containers, and bellows. A typical blow molding process begins with a heated hollow thermoplastic tube, also known as preform or parison. The plastic tube has a hole in one end, allowing compressed air to enter. It is then inflated into the closed chamber of a divided mold to conform to the shape of the mold cavity. The molded plastic will be left to cool and harden. Once released from the mold, the plastic part can be post-processed to have the holes rimmed or residues trimmed. Not all plastics are suitable for blow molding. The most frequently used materials in blow molding are low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), poly (vinyl Chloride) (PVC), polyethylene terephthalate (PET).
The products manufactured by blow molding, although limited to hollow shaped plastics, are widely used in many industrial fields and everyday lives. The following are the products
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that see a lot of blow molded parts: Automobile, Consumer, Electronics, Fuel oil tanks etc (eFunda, 2012). 1.11.2.4 Calendaring This method is used extensively for the production of both plasticized and rigid PVC sheets: it is capable of producing high quality material at very high rates of output. It involves the use of a series of heated rolls which are fed with pre-compounding stock. As the stock passes through consecutive roll nips, a continuous sheet is formed to an appropriate thickness which can then be cooled. Both rigid and plasticized PVC are used for the production of continuous film or sheet up to approximately 1.5mm in thickness (Rossberg et al, 2006). 1.11.2.5 Compression Molding
This method is primarily used to process thermosetting systems and difficult to process thermoplastics, such as fiber filled systems or thermoplastic elastomers. In the case of thermoplastics, for example PVC, a preheated mass of polymer, which may be either a sheet or a pile of pellets or even powder, is placed in the mold. The temperature of the mold is set low enough to cause the polymer to solidify but not so rapidly that it will not flow. Hydraulic pressure is applied to the top or bottom plate pushing the plates together. The molds are designed to prevent the top part of the mold from touching the bottom part, which would squeeze the resin from the mold. The design of a compression molding process consists of four aspects. The first is the selection of the proper amount of material to fill the cavity when the mold valves are closed. The second is determining the minimum time required to heat the blank (empty mold cavity) to the desired processing temperature and the selection of the appropriate heating technique (radiation heating, forced convection, etc). It
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is necessary to make sure that the centre reaches the desired processing temperature without the surface being held at too high of a temperature for too much time. The third is the prediction of the force required to fill the mold. Finally, the temperature mold must be determined, keeping in mind that one wants to cool the part down as rapidly as possible, but too rapid of a cooling rate will prevent the polymer from filling the mold (Charles et al, 2005). Compression molding is the only way of making thick high-quality rigid sheet. A number of sheets of calendered material are pressed together to form a heavy sheet up to one inch thick. The compound used for this processing is usually based on Vinyldiene chloride. The product has excellent clarity and good impact properties. Other techniques such as injection molding are now replacing compression molding (due to improvement in molding machine that allows high pressure to be used).
1.12 GUM ARABIC
A gum, in general, is any water-soluble polysaccharide that is extractable from marine and land plants, or from microorganisms that possess the ability to contribute viscosity or gelling ability to their dispersions (Abu Baker et al., 2007). The most fundamental property of a gum therefore is its water solubility and high viscosity in aqueous dispersions. For this reason, resins, latexes and other hydrophobic gums are not regarded as true gums. Among the advantages of natural gums over their synthetic counterparts are their biocompatibility, low cost, low toxicity (ecofriendliness) and relative widespread availability (Ogaji and Okafor, 2011).
Gum Arabic is obtained from tree of genus; Acacia, subfamily; Mimosodieae, family; Leguminosae (Smolinske, 1992). It is known as gum acacia, a natural gum made of
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hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal. Gum Arabic is a pale to orange-brown solid which brakes along a glassy fracture. The best grades are whole, spheroidal-tear shaped, orange brown, with a matt surface texture. It is a nationally and internationally exported commodity, (Unanaonwi, 2011). Gum export has been the main stay of the Sudanese economy for over 400 years, (Lawal, 1998). It was used at least 4000 years ago when it was shipped as an article of commerce by Egyptian fleets. Ancient Egyptian inscriptions make frequent mention of gum Arabic called ―Kami‖ which was used largely for painting as an adhesive for mineral pigments (Caius and Radha, 1939). The gum is made up of highly branched polysaccharides, and also a complex mixture of saccharides and glycoproteins, which gives it most useful property. It is perfectly edible (Smolinske, 1992). It is very soluble in water and is a neutral or slightly acidic salt of a complex polysaccharide containing Ca2+, Mg2+ and K+ . Gum Arabic is unique in that it is produced by trees only when they are in an unhealthy condition. Healthy trees have not been observed to yield gum. Most authorities believe that the formation of gum exudates is a pathological condition resulting from a microbial infection of the injured tree (Blunt, 1926). Natural factors that tend to lessen the vitality of the tree, such as poor soil, lack of moisture, and hot weather, improve gum yields, others consider the production of gum to be a normal metabolic process in the plant, with the quantity and quality produced being a function of environmental conditions (Malcom, 1936).
1.12.1 Distribution
A tree typical of Sahel climates, Acacia senegal is widespread in the dry regions of tropical Africa, from Senegal and Mauritania, in the west, up to Eritrea and Ethiopia in the
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north-east and down to South Africa, in the south. It is primarily located in between 11o and 16o North latitude. Of the four renowned varieties, senegal is the most widespread and is found in all the regions where Acacia senegal trees are located, except along the West coast of central and southern Africa. Outside Africa, it is also found in Oman, Pakistan and India and has also been introduced in Egypt, Australia, Puerto Rico and the Virgin Islands (ITC, 2008).
1.12.2 Structural Composition
Gum Arabic is best described as heteropolymolecular; a polymer system containing complex polysaccharide, mostly of galactose, arabinose, rhamnose and glucoronic acid, with 2 % proteins as an integral part of its structure (Karamalla, 1998). The gum may have either a variation in monomer or a variation in the mode of linking and branching of the monomer units. Gum Arabic is not very viscous, implying that the gum molecules are essentially globular and close- packed in shape rather than linear. Although the molecular weight reported for gum arabic have been in the 240,000 – 300,000 range, the most recent careful study of the exudates from Acacia senegal has shown an average molecular weight of 600,000 (Thomas, 1975).
1.12.3 Chemistry of Gum arabic
Gum Arabic is a branched-chain, complex polysaccharide, either neutral or slightly acidic, found as a mixed calcium, magnesium and potassium. The chemical composition can vary with its source, the age of the trees from which it was obtained, climatic conditions and soil environment (al-Assat et al, 2005).
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Gum Arabic is a highly heterogenous material, but was separated into three major fractions by hydrophobic affinity chromatography (Randall et al, 1989). Most of the gum (88.4% of total), an arabinogalactin (AG), has a very low protein content (0.35%0 and molecular mass of 3.8 x 105 Da. The second fraction (10.4% of total), an arabinogalactan protein Complex (AGP) contained 11.8% protein and had molecular mass of 1.45 x 106 Da. The third fraction (1.2% of total) referred to as low molecular weight glycoprotein (GP), had a protein content of 47.3 and a molecular mass of 2.5 x 105 Da. The Major amino acids present in the protein of AG and AGP were hydroxyproline, serine and proline, whereas in GP, aspartic acid was the most abundant (Islam et al., 1997). Gum Arabic is primarily indigestible to both humans and animals. It is not degraded in the small intestine, but fermented in the large intestine by microorganisms to short-chain fatty acids, particularly propionic acid (phillips, 1998). Such degradation products are absorbed in the human colon and subsequently utilized energetically in metabolism (Badreldin et al., 2008).
1.12.4 Physical Properties
1.12.4.1 Solubility
Gum Arabic is insoluble in oils and in most organic solvents but usually dissolves completely in hot or cold water, forming a clear, mucilaginous solution. Solutions containing up to 50% gum Arabic can be prepared and the solubility in water increases as the temperature increases. There are certain samples of gum Arabic, usually the exudates collected at the beginning of the dry season that are not completely soluble. They form stringy, mucus like fluid that separates into the phases upon standing. For low-viscosity gum
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Arabic only hot ethylene glycol are effective solvents. It is also soluble in aqueous ethanol up to about 60% of alcohol (Whistler and Bemiller, 1973). 1.12.4.2 Surface tension The addition of gum Arabic to water results in lowering of the surface tension (Banerji, 1952). Temperature has an important effect on the surface tension of a gum Arabic solution. For a 4.066% solution of purified gum Arabic at 30oC, 40oC and 50oC, the surface tension is 61.80, 61.46 and 59.03 dynes /cm respectively. As gum Arabic concentration increases, there is slight increase in surface tension, followed by a general decrease. Addition of electrolytes and especially mineral acid gives lower surface tension. 1.12.4.3 Ageing Age of the gum, i.e., the length of time it has remained attached to the tree after secretion may affect some of its physical properties. The colour of the gum may change from white to brown or dark-brown. The hardness may also change. Gums should be kept in dry places. Dilute gum Arabic solutions of the same temperature and concentration have the same viscosity until the appearance of bacterial growth that usually appears in 36-48 hours after the solution is prepared. 1.12.4.4 Osmotic Pressure
The osmotic pressure of boiled sodium arabate solutions is about 3% higher than that of unboiled solutions. There is a linear relationship between osmotic pressure and concentration at concentrations up to 1%. Above 1% the plot becomes a gradual curve. When sodium chloride is added, the osmotic pressure falls drastically; then the decline flattens out for concentrations above 0.2N sodium chloride. The limiting value of the
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osmotic pressure at any specific gum concentration is independent of the salt concentration (Whistler and Bemiller, 1973).
1.12.5 Chemical Properties
1.12.5.1 Reactivity Gum Arabic as the calcium, magnesium and potassium salts react with many reagents in a manner similar to that of other polysaccharide acid salts. Solutions of gum Arabic will produce precipitates or heavy gels on addition of borax, ferric chloride, basic lead acetate, mercuric nitrate, galatin, potassium silicate, sodium silicate, million’s reagent and stoke’s acid mercuric reagent (mantel, 1947). In general, trivalent metal ion salts will cause precipitation gum Arabic. Dilute (1%) gum Arabic solutions can be can coagulated by the addition of Ruthenium Red (Ru2 (OH)2 Cl4.7NH3.2H20). Hexol nitrate, or Desogen Geigy. In many application, the flocculation or thickening of gum Arabic can be prevented or retarded by the addition of soluble alkali polyphosphate ate concentration of 0.1 – 1.0% (Haller and Frankfurt, 1937). 1.12.5.2 Enzymes present Gum Arabic contains both oxidases and peroxidases that are inactive by heating the gum solution to 80oC or higher for one hour. The proxidase content varies but the enzyme can be inactive without affecting the viscosity by heating to 100oC. Diastase and pectinases have also been reported to be present in gum Arabic. The pectinases will cause precipitation of pectin when both are present in the same medium.
One of the major functional characteristics of gum Arabic is its ability to act as emulsifier for essential oils and flavours. Prolonged heating of gum Arabic solution causes
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protinaceous components to precipitate out of solution, thereby influencing the gums emulsification properties (Williams, 2000).
1.12.6 Uses of Gum Arabic
1.12.6.1 Pharmaceuticals Gum Arabic is used as an effective suspending aid and has been employed to suspend drugs and to prevent the precipitation of heavy materials from solution through the formation of colloidal suspension. It has soothing characteristics which led to its use in pharmaceutical syrup. Gum Arabic syrup is effective in masking the bitter or acid taste of medicament by its protective colloid action. Antiseptic preparations have been made with a mixture of colloidal silver bromide and gum Arabic. Silver Arabic has antiseptic properties that make it suitable as a substitute for silver nitrate and organic silver compounds in the treatment of ophthalmic infection. Gum Arabic has been used as an adhesive or binder for pharmaceutical tablets, such as aluminum acetate tablets and also as an excipient in the manufacture of pills and plasters employ. In addition many types of coatings for pill gum Arabic in the manufacture (Whistler and Bemiller, 1973). 1.12.6.2 Cosmetics In lotions and protective creams, gum Arabic stabilizes emulsions, increases the viscosity, assists in imparting spreading properties, and adds a smooth feeling to the skin. Cosmetic pack has been made using gum Arabic as a binder. Hair fixatives based on vegetable gums have the advantage of vastly improving fixative properties without forming greasy stains on clothing. Gum Arabic mucilage’s have been used in the preparation of their hair dressing (Harry, 1947).
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1.12.6.3 Inks Gum Arabic is a constituent of many special-purpose inks because of its excellent protective colloid properties. It is used to make soluble inks used by textile workers to mark cloth for cutting or sewing operations. The suspension of a pigment in a gum Arabic solution makes a satisfactory ink that can be applied with a brush or pen. Pigments such as titanium dioxide or bronze powder used for gold inks are preferably moistened first with a small amount of ethanol acetone before mixing with the gum solution. A typical white ink is made by mixing a filtered solution of 20 parts of gum Arabic in 160 parts of water with 30 parts of titanium dioxide paint pigment and O.5 part of sodium salicylate preservative (Horn and Sanko, 1953). Inks devised to imitate expensive grains by printing a pattern of wood on metal, paper stone, or inexpensive woods use gum Arabic as a suspension aid. 1.12.6.4 Medicine Intravenous injections of gum Arabic solutions were recommended in 1933 for the treatment of nephritic edema (Osol and Farrar, 1955). A 19th century Ethiopian medical text describes a potion made from an Ethiopian species of Acacia (known as grar) mixed with the root of the tacha, then boiled, as a cure for rabies (Richard, 1990). It has been said that gum Arabic, when used in infusion liquids, is harmful because it absorbs cations and causes an anionic unbalance. Some allergic reactions have occurred with injections of gum Arabic. The symptoms are flushing of the face, coldness of extremities, chills, nausea, vomiting and uticaria. These reactions could be controlled or prevented by administration of epinephrine (Osol and Farrar, 1955).
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1.12.6.4 Foods Gum Arabic has been used as a stabilizer in frozen products such as ice creams, and sherbets, because of its water-absorbing properties. It is widely used in the baking Industry for it viscosity and adhesive property, it is used in glazes and toppings. With the introduction of spray-dried flavours into foods, an extensive use was found for gum Arabic as a fixative for flavours (Whister and Bemiller, 1973). The predominantly herbivorous spider Bagheera kiplingi, which is found in Central America and Mexico, feeds on nubs at the tips of the acacia leaves, known as Beltian bodies, which contain high concentrations of protein. These nubs are produced by the acacia as part of a symbiotic relationship with certain species of ant, which also eat them (Meehah et al, 2008)
1.13 AIMS AND OBJECTVES OF THIS STUDY
1.13.1 Aim of This Study
This work seeks to study the effects of locally produced additives on the structure and properties of PVC, it also intends to investigate improvement related to physical properties, while retaining, if not improving, the volume resistivity, mechanical flexibility and toughness required for a predictable industrial application.
1.13.2 Objectives of this Study
The objectives of this study are to produce and explore the use of locally derived plasticizers (gum Arabic) as an additive in the modification of PVC structure and its resultant properties. The effect of the additive in the structural arrangement will be studied and the properties of the modified PVC will be classed as a possible commercially usable material that could be utilized for end-use application.
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