The aim of the project is to analyse the compatibility between recycled polyvinyl chloride with Acrylonitrile butadiene styrene and Medium-density fibreboard (MDF). The ABS was obtained from recycled computer CRT monitors, the PVC was obtained from recycled PVC window frames and finally the MDF was obtained from processing down hardwood and softwood to form small fibre particles. The compatibility would be based on making blend which contain percentage ratios of each of the materials used and to determine which blend consists of the best properties.
The blends will be analysed using a variety of techniques to ensure that all possible tests are carries out to determine the physical and mechanical properties. The blends were put through granulation, the haake polydrive and heat compression moulding and were tested using the Melt Flow Indexer (MFI), H10KS (Tensile strength) and placed through the Impact resistance tester (impact modulus).
From the analysis it can be determined which blends are best used for suitable purposes and if the modifiers such as ABS and MDF play an important role in enhancing the properties of PVC in composites.
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Plastics, whether made straight from the crude oil cracking process or the recycling process, are used in day to day life. They can be in the form of bottles for drinks or frames for windows or even bumpers for cars. In the current climate the endless use of plastic materials for packaging and other commercial purposes has brought about a growing concern about what the effects are on the environment when disposing of solid plastic waste. Tonnes and tonnes of waste is generated by plastics whereby some are recyclable and others not. Most plastics have Non-biodegradable properties.
There are a limited number of landfills available to use for the purpose of disposing plastics, so therefore the task of this project is to develop new ways of reusing the plastics. The reusability of the waste plastics all depend on the properties of them. Properties such as tensile stress and strain, thermal stability, degradation and impact strength are all important to analyse. It is also necessary to research whether the properties can be improved and enhanced by either blending other polymers or using additives.
Aims & Objectives
The aim of this experiment is to prepare blends of PVC-ABS and PVC-ABS-CELLULOSE using recyclable PVC, ABS and Cellulose then measure performance properties commonly used by the industry such as:
Melt flow index
The instruments used for the purpose of the project are:
The polydrive is used for blending polymers together to form a composition
Melt Flow Index
The blend is heated and extruded through a small die. The amount released from the die is cut off every 10 minutes and weighed
H10KS Tensile strength Test
The sample is loaded with a variable increasing mass and the resistance to the force is calculated
A Hammer is released against the sample and the resistance to the force is calculated
The materials were obtained from PROCESS PLATICS Ltd Rochdale where the separation of the materials took place. For analysis the Manchester Metropolitan University Laboratory was used where the instruments (above) were present and used following health and safety guidelines. The MDF was obtained from the saw dust of hardwood and softwood. Great care and attention needs to be taken when handling MDF due to the release of Urea-formaldehyde which is and irritation to lungs and the eyes. The ABS was taken from recycled computer fascias and granulated for use.
Polyvinyl Chloride (PVC)
One of the most consumable thermoplastics used is Polyvinyl Chloride (PVC) with the chemical formula [-CH2-CHCl-]n. PVC was first discovered in 1835 and ever since has been analysed and altered so that it is practical enough to be used in production of pipes, window frames and a vast amount of other consumable plastic products. However, these products are not being made from PVC solitary. The production of such consumer products requires addition of modifiers/additives or blends with other polymers which enhance the properties of the PVC to fulfil the purpose of products.
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It is available in two different forms, plasticised and Un-plasticised (U-PVC) which both have a great resistance to weathering conditions therefore used in the manufacture of PVC window frames. Resistance help protect the surface but both are likely to degrade over time and destroy mechanical and physical properties. The reason why U-PVC is used in window frames is due to its toughness and ruggedness. There are a variety of different uses of PVC as it has great physical properties such as flexibility and can be used in the making of flexible products. The life span of recycled PVC is very long depending on the quality of the treatment. Recycled PVC usually consists of more than one individual polymer therefore may lead to a shorter life span.
Stabilisation and degradation
Degradation of polyvinyl chloride is an important problem which can occur once it had been processed. The degradation usually occurs mainly due to heat, light and the general atmospheric environment. When degradation occurs it results in the mechanical properties of the plastic being affected. Properties such as tensile strength, impact resistance and rigidity will be affected, also in every day appliances such as wiring, PVC is used as a insulator to electricity, but may become warn out and convert to a conductor rather than the insulator it is. Change in colour can also occur as well as surface properties being modified.
Degradation of PVC can occur due to weathering conditions which is usually an exposure to UV light from the Sun and water absorption from rainy weather. The water absorption can increase the elasticity of the polymer and the removal of this can result in the polymer turning brittle. The water absorbs in into the surface of the plastic causing minute pores, once the water is removed then the pores still remain but unfilled. The pores create a lack of stability of the plastic. The UV light causes colour change and the chemical bonds in the polymer chain to break which deteriorate the physical and mechanical properties.
With use such as window frames, the exposure to UV and constant temperature change would cause the plastic to be more sensitive hence degradation and loss of properties. One of the methods to increase the properties of the plastic is by adding UV stabilisers, or with an addition of another polymer. Taking these matters in to consideration would increase the chances of stability properties becomes stronger and more withstanding to the degradation.
The PVC window frames are brought to the recycling process area in bulk. The frames are a mixture of PVC frames, glass, screws, locking devices, rubber from sealant and metal bars for the internal structure of the frames. The frames have been used in the past so there is also contamination of dirt and sand/cement used when fixing the windows to the buildings.
The steps taken in order to achieve purification of final the PVC product are as follows:
The rubber strip seals are removed from the frames.
The PVC window frames are cut at different angles to remove the metal bars, fixtures and fittings. The exact place of the metal bars is noted on a diagram of each frame. This makes the process of eliminating the metal bars in similar shape and sized frames faster. The metal items can be recycled so that there is minimal waste.
The remainder of PVC frames after the elimination and separation are cut down to size to make it better to handle.
The resized pieces are then granulated in a granulator to decrease the size even further.
The granulated material is then sieved to remove dust particles and other loose contaminants such as sand/cement.
To remove possible contamination from metal, the granulated material could then be placed through a magnetic field so that all the ferrous type metal is removed and just he PVC is extracted. This however must only be done if necessary as it lengthens the whole process.
The material is then ground and sieved once more so that the PVC particles are between 1.5 mm and 15 mm in size.
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To remove the remainder of contaminants and to make the PVC to a size whereby it can be used for the experiment, it is ground again until a particle of 8 mm or less is achieved.
(Acrylonitrile Butadiene Styrene)
One of the most common blends of PVC is with Acrylonitrile Butadiene Styrene (ABS) with the chemical formula (C8H8)xÂ· (C4H6)yÂ·(C3H3N)z). "ABS is an amorphous thermoplastic blend. The recipe is 15-35% acrylnitrile, 5-30% butadiene and 40-60% styrene." ABS is classed as a nitrile rubber generally used to modify the properties of the polymer and act as a plasticiser. The original use for blending nitrile rubbers such as ABS with PVC was to improve the ozone resistance. When blended with PVC the ABS creates properties such as high impact strength at low temperatures (provided by butadiene) and provides the final product with exceptional thermal stability, stiffness and great chemical resistance (Acrylonitrile).
The Nitrile rubber ABS is usually available as either a crumb/grain, powder (highly granulated crumbs) or liquid (highly granulated then heated grains). If the instrument used has a process whereby vigorous mixing is carried out then the crumbs/grains are best to use. If the final product needs to be flexible in such case as extrusion and injection moulding then the liquid form is recommended. The powder can be used for pretty much either of the above processes and is provided as "cross-linked polymer grades".
Another blending method which will be looked at in this project is PVC-ABS-CELLULOSE. Cellulose is usually taken from wood pulp in order to make products like paper. It is necessary to research whether it is possible to combine both PVC and ABS with Cellulose to form a resulting product which has incredible enhanced stiffness properties and also be able to survive against weathering conditions for outdoor use.
ABS is a copolymer made during polymerisation between styrene and acrylonitrile with the involvement of polybutadiene. The Acrylonitrile part of the polymer is formed from the propylene and ammonia and is the synthetic monomer used to produce ABS. The butadiene part of the polymer is a hydrocarbon formed from the process of steam cracking. The ABS polymer contains Nitrile groups that attract to each other making the polymer more stable and wealthy with strength properties when compared with a variety of other polymers.
ABS is part of the group known as the engineering plastics and is used as fabrication for structural engineering. The strength of the polymer is obtained by subjecting it to high amounts of stress. There can only be so much stress applied until failure occurs and the failure point becomes the strength limit. ABS can be tested for toughness by applying a great amount of mass at high speeds to determine how much of an impact it can withstand.
Stabilisation and Degradation
ABS is a copolymer which contains poly(butadiene), poly(styrene) and poly(acrylonitrile). Both the styrene- acrylonitrile and butadiene react in different ways when exposed a variety of weathering conditions. The degradation due to weathering is mainly associated with reactivity and sensitivity of polybutadiene. As a result of the sensitivity of the butadiene rubber, there is an increase in residual unsaturation in the double bond of the butadiene rubber. This results a higher risk to being attacked by the ozone weathering conditions. The double bonds of the rubber that are attacked also ensure the presence of highly reactive hydrogen atoms attached to the carbon atoms which are adjacent to a double bond, can be easily removed in order to create radicals which can begin to produce degradative chain involving the rubber butadiene and the styrene-acrylonitrile. This process will increase the chances of weakness and loss in physical properties to the polymer. The main reason for degradation to occur is the high temperatures which are introduced during the drying of the ABS resins before use for blending and other interactions.
With the high levels of atmospheric ozone, the degradation of ABS is rapidly increased. The residual double bonds in polybutadiene react readily with ozone to produce reactive polyatomic anions which eventually decompose producing a cleavage to the polymer chain leaving behind fragments that consist of end groups such aldehydes and carboxylic acids. The affects produced by this process result in the disintegration of the surface and deterioration of the mechanical properties.
The two types of degradation that occur are thermal (heat) and photo (light) degradation. They occur in the form of photo-oxidation and thermo-oxidation degradation. Due to oxidation reactions with the ozone there is an increase in the deformation of surfaces and a decreased performance in mechanical properties. Where photochemical degradation is concerned, the ABS can be protected using UV stabilisers. The UV stabilisers chromophores which absorb incident light which directed towards them, which will mean that any incident light which is directed towards the polymer surface will be absorbed by these UV stabilisers hence terminate the initiation of degradation and once they become in the excited state then release the light in form of heat.
Increase in heat means increase degradation which means loss of properties. One of the methods to increase the heat resistance of ABS is an addition of yet another monomer. This would increase the stability properties whilst also increasing the heat resistance. The additional monomer added to the ABS was vinyl chloride which has fantastic heat resistant and insulating properties when compared to the ABS alone. The main reason for introduction of copolymerisation is to increase the rigidity and stiffness. Taking these steps can increase the glass transition of the final produced polymer.
The reason why PVC is chosen as a blending polymer is that it has similar properties to the high specification and high priced materials, but has a lower price which makes it exceedingly desirable in this sort of application. When PVC is blended with ABS using such machines as the HAAKE Polydrive this results in a material being produced which consists of good impact strength, toughness and inherent flame resistant properties. The PVC/ABS blend is used in a wide range of applications which include the manufacturing of housings for electrical components, household appliances and motor vehicle parts. The flame retardant properties of PVC make it more suitable to use rather than using more expensive materials.
MDF (medium density fibreboard)
Medium density fibreboard is made from hardwood and softwood fibres. MDF has better properties that a variety of other wood fibres. It isn't really used much in the polymer industry as it is impossible to make it react with anything. The main reason for being non reactant is that it is just too dry and lacks fluid. Unless the other reactant contains excess fluid, it won't react. In industry MDF is mixed with adhesive and used as a binder for books. Degradation doesn't really occur with MDF however, there is a possibility that the particles would just set alight if exposed to increased temperature.
As a powdery substance, MDF emits urea-formaldehyde which is known to be harmful to the respiratory process of the human body. It can also cause irritation to the eyes and the lungs. When handling such a substance great care needs to be taken such as wearing goggles and face masks. Hardwood tables are made out of MDF and other products but it needs to be bound with a material which contains sufficient fluid and adhesive properties. When blended with PVC and ABS it increases rigidity and brittleness of the final product. If put through the tensile stress test then it will instantly break rather than go through the elastic flexibility stage.
The Haake Polydrive (Figure 1.1) is an instrument that is used in order to blend polymer samples together. It consists of two rotors which rotate at speeds of up to 70 revolutions per minute (RPM) and is surrounded by three heated plates. The plates can be set to different temperatures, resulting in the polymer blend being affected. In the experiments that were conducted for this project the temperature of each plate was set to 180°C. This aided the PVC and ABS to blend together. Some stages of the experiment required the introduction of MDF. This material is extremely hard to blend as it is relatively dry and has very few binding properties. Research has shown that there is a possibility there could be an increase in fluid properties if MDF was heated, however it is no guaranteed to work (This could be used as a point to further investigate). Before placing the mixture into the chamber a calculation has to be carried out. The chamber can only take 70cm3 there for the amount of sample that can
be analysed is limited. The Maximum amount of Torque 75 Nm-1 was applied to each blend, should this increase, the blending would halt and alarm would sound. If the torque is high then there is a chance that degradation is occurring. The torque determines how much fluid is present in the mixture. If the torque is low then the blending is not likely to occur as observed when the blending between PVC/ABS and MDF whereby the result was not a blend but more of a loose mixture.
Example of calculation
Mixture to consist of a 9:1 ratio of PVC:ABS, Therefore 90% PVC and 10% ABS.
In this calculation the density of both PVC and ABS are needed.
ÏPVC= 1.36, ÏABS= 1.06
The density is then divided by the ratio of the polymer
i.e. PVC = 90/1.36 = 66.17 ABS=10/1.06 = 9.43
Overall there is 100% composition therefore,
100/(66.17 + 9.43) = 1.32 this is the overall density
Multiply with 70cm3 (capacity of the chamber) => 1.35 x 70 = 92.59
The Polydrive will only accept an overall of 70% therefore => 92.59 x 0.7 = 64.81g
The result is the total amount of sample the chamber can take. Now apply the ratios 9:1 PVC:ABS
(64.81g /100%) x 90 = 58.329g PVC
64.81g - 58.329g = 6.48g ABS
(For all the calculations please see the appendices)
These amounts need to be placed in the chamber through the funnel and left for 5 minutes (it is important that the ventilation is active to avoid any emissions being ingested which may cause harm to the respiratory functions of the body). After 5 minutes the Machine is deactivated and blend is removed and compressed into a small thin square plate (figure 1.0) using the compression moulding equipment (figure 1.3 & 1.4).
Figure 1.1 The plates which were used to mould the sample in during heat compression moulding
The product can then be used for further analysis. Each of the graphs obtained from the Polydrive show exactly how much torque was needed and how difficult/easy the processes of blending were. (see appendices for Graphs obtained)
Figure 1.1 HAAKE Polydrive
Figure 1.3 Heat Compression Moulding Machine
Figure 1.4 Water cooled Compression Mould cooler
Melt Flow Index (MFI)
Figure 1.5 Melt Flow IndexerMelt Flow Index is used to determine the flow rate (grams) that takes place over a 10 minute interval. Before the timing begins the polymer sample needs to be placed into the extruder which contains a Die approximately 2mm in diameter and 8mm in length. Heat is set usually at around 190Â°C but can be adjusted based on melting point of the samples placed inside. The heat is just to convert the sample from a solid granulated form to a more easily passable fluid form. Once a the melting has taken place, a mass ranging anywhere between 2.16kg and 50kg+ can be applied to help the sample flow through the orifice of the Die. During processing materials such as MDF it is recommended the place a higher mass mainly due to the fact that the small mass doesn't have enough force and the longer the MDF stays inside the extruder the more likely it is to burn and become stuck whilst releasing toxic chemicals such as Urea-Formaldehyde. A fair amount of samples need to be taken from the MFI every 10 minutes to obtain an average reading. The mass of sample taken extruded in 10 minutes needs to be cooled and weighed. The time can be decreased/increased depending on the polymer but must be recalculated to 10 minutes.
In some cases the mass had to be increased tremendously so instead of the recommended 2.16 kg, 21.6kg were being used. Also due to some abnormalities in the binding of the samples and the amount of MDF present, it became increasingly difficult to obtain any results. The samples which consisted of high amounts of MDF had to have a very heavy mass placed on or had to be left for very long times in order to obtain a result. However even providing sufficient time, certain samples didn't come out and became stuck inside and they had to be burnt out. One of the main dangers of using melt flow indexer with samples that contain MDF is that it contains formaldehyde which causes irritation to the lungs when breathed in through the respiratory system.
H10KS Tensile stress
Mechanical strength is a measure of the maximum force required to break a sample by stretching or bending it. All plastic materials when subjected to an increasing load elongate or bend, and eventually break.
Thanks to the H10KS tensile stress instrument the tensile properties of the plastic can be measured. The tensile stress is measured by fixing the sample in the sample holder of the testing machine. The following parameters are set
Load range = 200.0 Newton's
Extension Range = 25.00 millimetres
Speed = 2.0 mm/min
Span = based on calculation
End point = 22.00
Preload = 0 N
The machine is activated and a variable load is applied to the sample. At the start of the process the stress is measured as being a load applied evenly to the cross-sectional area of the sample. Whilst the process continues there is an amount of stress applied to one specific area of the sample which is the centre. Due to this there is an increase in pressure in the centre which leads to elongation. Whilst more and more time goes by the elongation increases to an extent that the sample becomes elastic like. The curve on the graph suddenly begins to degrade and at the point of failure, the graph curve just drops which means that the sample has given in to the pressure. Once the pressure us withdrawn, the sample will automatically return to its original shape. One of the factors that effects how long it takes for the sample to give in is the temperature. If the temperature is cold then the sample will remain stiff and break but if the temperature is warm then the sample acts elastically. All the data is recorded on the computer which shows how much pressure was applied until the sample gave in.
Figure 1.6 Shows the H10KS Tensile Stress Machine
Figure 1.7 shows the area where the sample is placed and the mass loader
The impact test is carried out using an instrument which contains a "hammer like" acting pendulum. This pendulum breaks the sample when a sufficient amount of kinetic energy is applied. The reason why the sample breaks is due to the impact applied in one specific area which the centre of the sample. This is the weakest point as the other two parts of the sample are held against the machine. The resistance that the sample has against the impact is recorded as "impact modulus" and determines the strength of the sample. This technique is used in industry to determine the strength of bricks and other building materials on a larger scale.
Figure 1.8 shows the Impact Modulus tester