The use of bicomponent fabrics for bonding polypropylene sockets in prostheses
A. G. A. Coombes *
R. B. Lawrence *
R. M. Davies *
A technique has been established for bonding polypropylene sockets to the structural components of prostheses based on the use of bicomponent knitted fabrics which adhere to the surface of polypropylene sockets during thermoforming. The strength of adhesion of fabric bonded polypropylene with polyester resin based microballoon for instance is increased by more than 10 times relative to uncoated polypropylene. This procedure has been approved by the United Kingdom Department of Health for incorporating polypropylene sockets in conventional, laminated plastic patellar tendon bearing (PTB) prostheses. The bonding technique described should find general application for bonding polypropylene during the manufacture of both prosthetic and orthotic devices.
The use of polypropylene for producing sockets for prostheses by vacuum forming techniques is now well established. The Rapidform technique for example (Davies and Russell, 1979) has been used successfully for over six years to produce more than 10,000 sockets for below-knee (BK) PTB prostheses. Patient reaction has been favourable and socket manufacturing times have been shortened significantly. In the Rapidform process a polypropylene injection moulded preform is heated in the oven of a purpose built, semi-automatic, vaccum forming machine (Fig. 1). When sufficiently softened a rectified and foam lined cast of the patient's stump is raised by a ram and pressed into the preform causing the thermoplastic to draw. Vacuum application then pulls the thermoplastic into contact with the cast. After an annealing period the thermoplastic is cooled and the cast removed to leave a fully shaped socket ready for trimming.
Polypropylene sockets have been incorporated satisfactorily in modular prostheses in which the socket is attached to the shank via an alignment device by a single bolt fixing. There is an obvious advantage in fitting the remaining conventional, laminated plastic PTB wearers with polypropylene sockets where limb construction involves bonding the socket to structural limb components. However the adoption of vacuum formed polypropylene sockets for these types of prostheses has been limited by the inability to bond them satisfactorily to other materials such as rigid polyurethane foam or polyester resins. In Britain for instance, the laminated plastic PTB consists of a reinforced polyester resin (GRP) socket bonded to a wooden shank and foot via a microballoon interface. Microballoon is a composite material of hollow phenol formaldehyde spheres and polyester resin. Production of the laminated plastic PTB involves casting a block of microballoon around about the lower third of the socket. The microballoon block functions in the first instance as an interface for attachment of a temporary alignment device e.g. the Berkeley adjustable leg — which is screwed to the microballoon. When optimum alignment has been achieved it is transferred to the definitive prosthesis using a transfer and alignment jig, the space previously occupied by the Berkeley jig being filled with wood. The wood is shaped and hollowed and the microballoon is shaped before the limb is finished by covering with a thin nylon reinforced laminate. A sectioned laminated plastic PTB incorporating a polypropylene socket is shown in Fig. 2.
A good bond between socket and microballoon and in turn between wood and microballoon is essential so that the laminated plastic PTB prosthesis functions as a sound structural unit. Indeed if the modular prosthesis is considered, then it is found that components such as shin tubes and ankle adaptors for instance are mechanically joined and in addition glued to provide the necessary strong jointing. Good bond strengths are established between GRP sockets and microballoon in conventional prostheses but this has not been the case for polypropylene. A polypropylene socket is retained in position in the prosthesis due to the re-entrant shapes present in the socket itself but it is not bonded rigidly to the microballoon and a small degree of relative motion occurs between it and the microballoon as the prosthesis is loaded during use. Although the prosthesis may function satisfactorily, the above condition results in creaking as the patient walks and there is also the possibility of abrasive damage at the interface between socket and microballoon.
Polypropylene is well known for its ease of processing, chemical inertness and good balance of mechanical properties. Of particular relevance to its use for socket manufacture is its excellent flex fatigue resistance. However this polymer is also well known for its reluctance to bond to other materials. In an attempt to overcome this problem so as to incorporate thermoformed sockets in a wider range of prostheses a two-part investigation was undertaken of thermoplastic — microballoon adhesion in general — Part A, and polypropylene (socket) modification so as to achieve bonding with microballoon — Part B.
Part A aimed to identify alternative thermoplastics to polypropylene which would bond satisfactorily with microballoon and also lend themselves to socket manufacture by vacuum forming. A further consideration affecting the choice of socket material is of course its likely performance under service conditions which presents a requirement for durability. Preservation of existing manufacturing practices and the proven service performance of polypropylene sockets supplied the stimulus for Part B of the investigation.
Part A — Thermoplastic-microballoon adhesion
A simple pull-out test was one of the methods used to obtain an indication of the strength of adhesion between microballoon and potential socket materials. Adhesive bonds are good in shear and poor in tension and the pull-out test is biased towards shear bonding conditions. Nonetheless results can be used as a guide for comparing different bonded systems.
The base polyester resin used for the majority of the investigations of polymer/microballoon pull-out strengths was a mixture of two resins — Beetle 8116 and Beetle 8134. Beetle 8134 is a rigid polyester resin and beetle 8116 a flexible resin. The flexible resin is used in combination with rigid resins to give improved flexibility and impact resistance and to prevent crazing when moulding articles with thick cross section. A Beetle resin mix of 60% rigid, 40% flexible was used. Catalyst and accelerator were added prior to the microballoons filler as 1% additions. Microballoon filler was added in the ratio of 7:4 filler to resin by volume. Gel time of unfilled resin is about 37 minutes.
Some of the results of pull-out testing were obtained using a 'Crystic' (471) polyester resin based microballoon. Although the base resins were obtained from different manufacturers a comparison of results should give a valid indication of the effect of different bonding techniques or materials on pull-out strength.
Specimen preparation and test procedure
Three samples of each selected thermoplastic were cut from commercially available sheet. They were unabraded but degreased in soap solution. An array of thermoplastic specimens with parallel gauge lengths and dimensions of 90 x 15 x 3mm were held in position in slits cut into a 25mm thick block of Plastazote foam. Specimens were then lowered carefully into premixed microballoon. After curing, a block of resin approximately 210 x 70 x 50mm deep was obtained containing some 18 specimens. This block was then machined further to give individual thermoplastic specimens embedded in their own block of microballoon which was about 30x 12x50mm in length (Fig. 3, left). The thermoplastic specimens themselves were embedded to various depths usually between 20mm and 35mm, this left a resin free gripping section of between 60 and 70mm. Care was taken to machine the faces of the microballoon blocks parallel to the thermoplastic faces in order to eliminate specimen twist during pullout testing.
The load required to break the adhesion between thermoplastic and microballoon or the maximum load recorded during testing was measured using a Monsanto 2000 Tensometer. Specimens were gripped such that the microballoon block with embedded polymer specimen was mounted horizontally with the tensile axis horizontal. Testing speed in all cases was lOmm/min. The test was allowed to proceed until either specimen fracture or specimen pull-out occurred to the extent of about 5mm. After testing each sample was sectioned to determine the dimensions of the embedded part of the thermoplastic and the figure for original thermoplastic/microballoon contact area was established. Contact area and maximum recorded load enabled a value for pull-out strength to be assigned to each specimen in terms of force per unit area of polymer/resin contact surface.
As background to the investigation of pull-out strengths, tensile tests were carried out on parallel sided specimens of each polymer having the same dimensions as those samples used for pull-out testing. This procedure aimed to elucidate whether or not chemical attack and weakening of the thermoplastic had occurred during the embedding process.
Specimen preparation and test procedure
Three strips of each thermoplastic (unabraded and degreased as noted previously) with length and width of 60mm x 15mm respectively were attached to a sheet of paper using double sided adhesive tape. This was placed in a shallow aluminium tray and premix microballoon of the composition given above was poured over the specimens. After curing the resulting microballoon block with surface mounted samples was machined further to give individual specimens for peel testing (Fig. 3, right). A 3mm diameter hole which had been drilled near the end of each thermoplastic specimen prior to embedding enabled a load to be applied to the specimen via a wire attachment. Peel testing was performed using a Monsanto 2000 Tensometer at a speed of 1Omm/min. The microballoon block being about 25mm wide is located in one of the tensometer grips and the thermoplastic specimen is peeled from it by means of a steel wire fastened to the moving grip. The load required to peel the thermoplastic from the microballoon block was recorded together with the measured width of the specimen to enable a value of peel strength to be assigned to each specimen in terms of force per millimetre width.
The peel test used for evaluating thermoplastic/microballoon adhesion is similar to that described in British Standard 4994:1973, covering the testing of resins, laminates and thermoplastics for vessels and tanks in reinforced plastics.
Results and discussion
Table 1 and Table 2 list the average pull-out strengths and peel strengths respectively for various thermoplastics from microballoon. Also included is an indication of the adhesion failure mode. The values of pull-out and peel strength for unabraded GRP from microballoon are included as a standard for comparison. (Suitable samples were cut from a lay-up over a wooden former having several machined flats.)
The low value of pull-out strength for polypropylene (0.4 x 106 N/m2) compared to GRP (3xl06xN/m2) a socket material widely used in the construction of conventional prostheses, is immediately apparent. The occurrence of specimen fracture before adhesion failure permits the assignment of only a lower limit to the pull-out strength for acrylic, PVC, ABS and PC in these particular experiments. All the thermoplastics listed show equal or improved adhesion with microballoon relative to polypropylene but various factors tend to decide against them for socket production. Nylon for instance is little used in vacuum forming, one reason being the sharp transition between solid and low viscosity melt which makes thermoforming difficult. Chalk filled polypropylene is used for thermoforming but requires abrading, an additional process step, to expose the filler particles for improved adhesion with microballoon. There is also the possibility that incorporation of mineral fillers results in a decrease in fatigue properties relative to the unfilled thermoplastic (Lang et al, 1982). Satisfactory thermoforming of glass filled materials in general is difficult. Polycarbonate is well known for its susceptibility to stress corrosion cracking (Fenner, 1975) while ABS, PVC and acrvlic are notch sensitive and not noted for their dynamic load bearing characteristics (Bucknall et al, 1972; Marshall, 1982). In addition acrylic (PERSPEX) appears to be attacked and severely weakened by microballoon. Table 3 shows the average failure load recorded during tensile testing for various thermoplastics together with the maximum load recorded at adhesion break or sample failure during pull-out testing. (Sample dimensions are equivalent and they were obtained from the same source i.e. sheet, moulding etc). It should be noted that the failure load of acrylic is reduced by more than 50% after embedding in microballoon which suggests chemical attack on the embedded sample. This observation is particularly relevant to the case of commercially available acrylic sockets and emphasises the importance of applying a layer of reinforcing laminate to the finished socket, rather than using it in the 'self supporting' mode like polypropylene sockets in mechanically fixed modular prostheses.
An interesting result is that the peel strength for unabraded GRP from microballoon is fairly low (Table 2) which tends to support the view that effective bonding between GRP sockets and microballoon during manufacture of conventional limbs is only achieved by coarse abrading of the socket prior to embedding.
Part B — Fabric bonded polypropylene
As noted previously, it would be highly desirable if the use of polypropylene sockets could be extended to 'conventional' prosthesis construction. Their service performance in mechanically fixed prostheses has been proven, the material's fatigue resistance and chemical resistance are excellent and existing socket manufacturing practices could be retained.
Surface modification of polypropylene using commonly available adhesives such as Araldite 2006 (an acrylic based adhesive) can double the pull-out strength of polypropylene from microballoon but bonding is still comparatively poor. The use of filled polypropylene improves the adhesion with microballoon as noted above but this course of action was rejected for socket production on account of the possible deterioration in fatigue properties for chalk filled polypropylene and the difficulty of thermoforming glass filled polypropylene.
The most promising system for improving the adhesion between polypropylene sockets and microballoon was based on the use of bicomponent, knitted fabrics which were bonded to the socket during thermoforming (Fig. 4). A close fitting knitted sleeve containing polypropylene yarn and another yarn such as rayon is positioned over the polypropylene preform — this is also shown in Fig. 4. Both preform and sleeve are clamped using the standard Rapidform procedure then heated and vacuum formed over a rectified plaster cast of the patient's stump as for the preform alone in the Rapidform machine. By this means it was envisaged that the polypropylene component in the sleeve would weld to the polypropylene in the socket during forming and that the second component would bond more readily to microballoon during assembly thereby giving overall socket/microballoon adhesion.
Bicomponent woven fabrics have been used previously to achieve adhesion between two materials which are not readily bonded. Woven glass/PTFE fabrics have been bonded to glass laminates and metal substrates for instance to give a (dry) bearing surface of PTFE (Thomas, 1983).
The essential requirement of the fabric used for bonding polypropylene sockets in this case is extensibility, supplied by the knit construction, to enable it to stretch in contact with the preform during thermoforming.
The following main points arose from an investigation of the vacuum forming behaviour of preform/sleeve combinations.
The standard Rapidform technique described earlier could be used with an adjustment for heating time. Heating time or thermal soak time prior to vacuum forming for preform/sleeve combinations had to be increased by approximately 8 minutes relative to the polypropylene preform alone due to the insulating properties of the sleeve.
Good sleeve preform adhesion was evident after vacuum forming.
Sleeve extension in the draw direction must be greater than 160% to avoid tearing during thermoforming.
Sleeves invariably bridge across socket depressions such as the popliteal and patellar regions due to tension set up in the sleeve during the drawing stage prior to vacuum application. The bridged area is roughened by original contact with the sleeve (Fig. 5). As vacuum is applied to the softened and stretched combination both components are seen to draw into socket depressions such as the patellar area, i.e. bonding is maintained at this stage between sleeve and preform. During the socket annealing stage however the sleeve is observed to spring back from these depressions. Sleeve tension is sufficient to break the bonds between sleeve and preform established during heating and stretching which are probably point welds formed between the polypropylene preform and the polypropylene component of the sleeve. The roughened polypropylene socket texture in the depressions results in part from melted polypropylene fibres from the sleeve remaining attached to the socket. If the bridged area of the sleeve is examined after forming, an open net structure is observed made up mainly of the second sleeve component (Fig. 6). Although total socket/sleeve contact is not obtained, coverage is complete near the socket distal end which is embedded in microballoon prior to alignment. The patellar and popliteal regions are however roughened during socket production which is an advantage for subsequent adhesion to microballoon.
The polypropylene component of the sleeve melts during preheating and wets the second fibre to varying extents resulting in weave locking on cooling. The extent of wetting gauged from microscopical examination appears to increase in the order rayon, nylon, crimplene.
Both fibre types are required in each row of fabric to maintain sleeve cohesion during heating and vacuum forming.
Polypropylene/rayon sleeves were selected for further trials associated with the bonding of polypropylene sockets in conventional prostheses because rayon showed minimal wetting by the melted polypropylene sleeve component. Since it is a non melting fibre, sleeve integrity is maintained during heating.
Pull-out from microballoon
The result of pull-out testing on polypropylene/rayon fabric coated polypropylene from 'Crystic' polyester resin based microballoon is presented in Table 1. Samples were obtained by vacuum forming over a conical wooden former (height 370mm, diameter 130mm, 5° taper) having several machined flats. The details of test method and sample fabrication are as stated earlier. A value of 0.4X106 N/m2 is assumed for the contribution of uncoated polypropylene to the pull-out strength of each sample. Although the base polyester resin differs, the results indicate a marked trend towards increased adhesion with microballoon for fabric coated polypropylene (1.7xl06 N/m2) relative to uncoated samples.
Peel testing of fabric coated polypropylene from microballoon
Peel testing was performed using polypropylene/rayon fabric coated polypropylene specimens to generate further data on the adhesive properties of polypropylene/rayon fabrics. Peel test specimens of length 60mm and width 15mm were cut from vacuum formings produced over a conical wooden former using the Rapidform machine. Preparation of peel test specimens, microballoon composition and method of peel testing were as described above.
The average peel strength expressed in N/mm width of polypropylene/rayon fabric coated polypropylene from microballoon is shown in Table 2 together with the adhesion failure mode. Adhesion failure occurs by gradual debonding of the polypropylene substrate from the fabric. The fabric remains bonded to microballoon. Although the value of peel strength for fabric coated polypropylene (3.9 N/mm) is lower than ABS (4.9 N/mm) for example, load extension curves recorded during peel testing (Fig. 7) give an indication of the higher work of debonding for fabric coated polypropylene relative to ABS. The latter material debonded suddenly and completely on adhesion failure whereas fabric coated polypropylene exhibits a more tenacious bond with microballoon. This characteristic was unique among the materials investigated and is a decided advantage when considering materials for service use in prostheses.
Summary and conclusions
Polypropylene has gained widespread acceptance as a socket material for use in modular prostheses where mechanical fixings are employed to attach the socket to structural components. The problems generally encountered in bonding polypropylene sockets satisfactorily to other materials used in prosthetic construction such as microballoon have been overcome by bonding a blended, knitted fabric to the socket surface during thermoforming by the Rapidform process. These fabrics contain polypropylene fibres and another fibre such as rayon so that during socket forming the polypropylene component bonds to the socket, probably by point welding, while the second fibre bonds more readily to microballoon for instance to give overall socket/microballoon adhesion. Peel testing has demonstrated that the adhesion of fabric coated polypropylene to microballoon is increased by a factor of 13 relative to uncoated polypropylene. Patient trials of fabric coated polypropylene sockets incorporated in conventional, laminated plastic PTB prostheses have been successfully conducted at both Roehampton and Newcastle and the process has been approved for supply by the U.K. Department of Health.
Concurrent investigations have revealed that thermoplastics such as nylon, ABS, acrylic, uPVC and polycarbonate exhibit higher strength of adhesion with microballoon than does polypropylene. These materials however either display inferior forming characteristics, fatigue resistance or chemical resistance relative to polypropylene and have been rejected as replacements in this application.
The technique of bonding bicomponent knitted fabrics to polypropylene to enable subsequent bonding to other materials has potential application throughout the field of prosthetics and orthotics where inherently difficult to-bond thermoplastics such as polyethylene and polypropylene are being used in increasing quantities. Bonding of polypropylene sockets to exoskeletal and modular prostheses provides one example where fabric interfaces can be used to advantage. The problem of bonding the polypropylene keel to the flexible soleplate during manufacture of the ultra-lightweight prostheses (Convery et al, 1984) could also be approached using a variation of the same procedure.
This work was funded by the U.K. Department of Health and Social Security as part of the programme of the Bioengineering Centre.
Visijar Tuckers, 1 Pegasus Road, Croydon Airport, England. (PP, Nylon, Acrylic, uPVC, ABS, PC).
Doeflex Industries Ltd, Holmethorpe Avenue, Redhill, Surrey, England. (Chalk filled PP).
ICI Petrochemicals and Plastics, Bessemer Road, Welwyn Garden City, Hertfordshire, England. (Glass Filled PP).
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