Presentation on theme: "Manufacture of Lower Limb Orthoses in PP PP & EVA PROPERTIES International Committee of the Red Cross Polypropylene Technology."— Presentation transcript:
Manufacture of Lower Limb Orthoses in PP PP & EVA PROPERTIES International Committee of the Red Cross Polypropylene Technology
Polymers/Plastics How do we classified them? Polymers are divided into two distinct groups: Thermoplastics and thermosets. Both thermoplastics and thermosetting resins require heat as well as pressure to shape them into useful articles.
Polymer How do we classified them? Polymers are usually mixed with other materials which alter their properties. For example, small amounts of special chemicals are added to make the material less liable to damage by oxygen in the air, or by strong sun light. Dyes or pigments are added to give colour to the finished articles. Plasticizers, which are usually oily liquids, can be added to make the material softer. Fillers, such as chalk or glass fibre, may be added to make it stiffen
Thermoplastics Mechanical/Physical Particularities The term thermoplastic designates those plastic materials which, when subjected to heat, become plastically shapeable or molten and when cooled anew, regain solidity and can carry load. Processing of thermoplastics, therefore, involves three stages, whatever method is used. The material is first heated enough to soften it, then it is forced into the desired shape, and lastly it is left to cool while it is still held in its new shape. This property allows for easy processing and facilitates recycling.
Thermoplastics Chemical Particularities (Who care you might say) Thermoplastics consist of long-chained, filiform, non- cross-linked molecules which are interwoven in a felt-like way. Since the chains are devoid of cross-linked networks, they can slide past one another, when carrying load and sufficiently heated thus the plastic material can "reshape under heat". There you go. Tonight you will go to bed a little more knowledgeable then you were when you got up this morning.
Thermosets Mechanical/Physical Particularities Thermosets cannot be reformed or remoulded. Once these polymers are formed in a particular shape, that's it, no coming back. It is for life.
Thermosets Chemical Particularities What is the difference - chemically - between these two classes of plastics materials? In thermosetting materials, heating causes strong chemical links to be formed between the long molecules, thus joining them firmly together. (In thermoplastic materials, the long molecules are separate from each other.) This joining together of long chain molecules is called cross linking
Polypropylene Polypropylene is a rather versatile polymer. It does not melt below 160°C (320°F) while polyethylene (low density), a more common plastic (garbage bag, plastic container, etc.), will anneal at around 100°C. Structurally it is a vinyl polymer, and is similar to polyethylene, only that on every other carbon atom in the backbone chain has a methyl group attached to it. Particularities
Polypropylene (PP) and polyethylene (PE) are thermo- softening plastics and not thermo-hardening ones, as are polyester and epoxies. PP is, together with PE, one of the lightest plastics as it will float in water. Although difficult to glue, they can give excellent results with welding, providing that no oxidation takes place. They are referred to as polyolefin, which means oil-like, because of their waxy or oily feel and smell of paraffin when burned. Polypropylene Particularities
PP is more elastic (jumps back to its original shape) and rigid (has very little 'creep'); PE is softer (less elasticity) and has more 'creep'. Gosh I would have be tempt to think the opposite for the elasticity. It is important to define elasticity which, by the way, does not mean flexibility. Polypropylene Particularities
Particularities Pure PP or homopolymer PP does, however, get brittle at low temperatures (i.e. less than 0°C) and for this reason in colder climates we use copolymer, a mixture of propylene and up to 20% ethylene. Welding properties are, we are told, the same for both homopolymer and copolymer. The PP the ICRC started off with was a homopolymer and was first ordered in its natural colour (milky white) but was later ordered in with colour; the colouring is supposed to give a better protection against UV but also gives a better cosmetic result. Polypropylene
- Rather inexpensive. - Has a long shelf life and does not need special air- conditioned room. - It has hardly any limitations as regards transport. (Polyester resin is limited to 5 kg per aeroplane by IATA regulations). -Its use allows a large reduction in the number of different material items needed. -It is easier and quicker to use, less machinery is required -it also allows a more flexible and less rigid socket -It is one of the most easily recyclable plastics Polypropylene Advantages
Advantages -It is rather inexpensive. (Although it price is roughly the same price that one would pay for polyester resin, polyester resin requires many additions such as PVA film, hardener, stockinet, colouring agents, etc. which practically doubles the price). - It has a long shelf life and does not need special air- conditioned room. Providing it is kept out of direct sunlight, it can be stored for many years, even in tropical climates. Polypropylene
Advantages It has hardly any limitations as regards transport. (Polyester resin is limited to 5 kg per aeroplane by IATA regulations as the peroxide used as its hardener is explosive in certain conditions and can burn a hole in aluminium). Polypropylene
- Its use allows a large reduction in the number of different material items needed, thereby making stock management less complicated. (To make a polyester and wood socket approximately 18 different material items are required: to make a PP socket only 8 different material items are necessary). - It is easier and quicker to use, less machinery is required in order to make components and it also allows a more flexible and less rigid socket compared to the polyester laminated sockets. Polypropylene Other Advantages
-It is one of the most easily recyclable plastics and is considered a non-pollutant. -(Have you try to recycle polyester or acrylic resin? Good luck!) For info: Although PP is the plastic the ICRC mainly use, high density PE (rather than PP) is recommended for bushing as it is a much more suitable material for these, wearing less and sliding better. There was a time when the ICRC projects were making bushing out of recycle PP for the TF knee component. Polypropylene
Disadvantages are few and most of the criticisms made against the use of PP are mainly a result of bad craftsmanship or lack of proper equipment. It must be understood that some of the advantages can become disadvantages if we do not know how to work with the material and work accordingly to its characteristics. Ex. The flexibility of PP compare to metal. Sometime, we have to make reinforcement line to compensate for the flexibility which in many instance is an advantage. Polypropylene Particularities
EVA Particularities EVA foam is an expanded (foamed) copolymer of ethylene and vinyl acetate (EVA). It is the vinyl acetate addition that gives the material its elasticity and bounce. EVA's closed cell structure will allow it to slowly return to its original shape after strong compression. Complete compression will, to a certain extent, provoke a permanent collapse of the closed cells but by choosing a higher density (i.e. less expansion) foam the risk of collapse through complete compression can be reduced.
For info: There is a polyethylene (PE) homopolymer foam (i.e. Plastazote) but if sharply bent at a corner the corner will stay bent and an impressed finger will leave a mark. EVA foam, although more expensive, was therefore considered more suitable. EVA Particularities
Unlike PP and PE, EVA foam allows excellent gluing with contact glue. EVA has long been known in prosthetics under the names Pelite, Tepefom, Multiform, etc. and was mainly used for making soft liners for the sockets of BK prostheses. Did you know that it is not normally the density of the foam that is indicated but rather the expansion? The heaviest quality available has an expansion of 5 (e.g. five times the volume of the material before foaming). EVA Particularities
Particularities For info: Since 1990 the ICRC has been using EVA foam for SACH feet production, covering wooden keels with it and making cushion heels (well over 10,000 feet have been made with EVA foam). The ICRC have also used this material for making cosmetic calves and recently for press vacuum moulding of hands and feet. EVA
- Closed cell structure will not permit moisture penetration and it is therefore less apt to rot or increase in weight. It therefore allows prosthesis with EVA foam cosmetic cover to be worn in the rain or even when showering. - Optimal weight / strength relationship. For the same resistance against compression it has only a fraction of the weight of other foams. EVA Advantages ?
-No toxic reaction to the skin. Non-foamed EVA has a lot of application in the medical field. (Polyurethane (PU) foam however, when not correctly mixed can irritate the skin). - It has a high oxidation resistance and Ultra-Violet (UV) stability. (Some PU foam are very vulnerable to oxidation and to UV. Other kinds are not so vulnerable but will not last long in hot tropical climates). EVA can last for years in hot tropical climates. EVA Advantages ?
- It is well suited for thermo-forming of soft liners for sockets and calves. - It allows excellent Thermo-Press-Moulding (where a new integral skin is made). With this process resistance of the material increases. - Ease of transport as there are no restrictions. (PU has, as one of its components, isocyanides which can cause respiratory distress and be toxic). EVA
-Disadvantages are few, but the price can be an obstacle. Compared to PP it is expensive; compared to PU, due to the high transport costs of the latter, it is not. -Another disadvantage of it use for liner could be the heat factor, as it can be used as a thermo-insulator. EVA Disadvantages?
Do Store the polymer in the dark when it is possible. (All polymer) Make sure the storage space is cool, dry without dust and well ventilated. (Very important for the prosthetic feet but less important for the EVA and not important for the PP.) Check often for sign of deterioration. (More important for the feet, particularly because the rodents love to take a bite at it.) DO & DON'T
Do Do support soft or flexible objects in their normal shape (All). Do handle objects carefully and wear clean cotton gloves if possible. Do wrap objects in uncoloured tissue paper (preferably acid- free tissue) rather than newspaper (Feet). DO & DON'T
Don't Don't expose polymer objects to strong light (All). Don't keep polymers in damp or stuffy places (EVA & Feet). Don't store objects in completely sealed boxes, plastics bags or other wrapping which would restrict ventilation (Feet). Don't clean plastics with solvents or other household cleaners unless their long term effect on polymers is known. Don't allow objects to contact each other (Feet). Please note that most comment for carefulness, whether do or don't, are related to the feet which are made of Polyurethane foam. DO & DON'T