Mehdi Tavakoli, after gaining a BSc in chemistry, studied for an MSc in polymer science and technology at Aston University. He then completed a PhD in the same field at Aston and graduated in 1981. He continued research into polymer degradation and stabilisation. Prior to joining TWI in 1989 as a principal polymer scientist he worked at Bath University on polymer composite manufacturing processes including RTM, fibre/resin compatibility, stress rupture behaviour and damage mechanisms in phenolic resin composites. Since joining TWI, Mehdi has been working mainly on novel surface preparation techniques, including the application of power beams for adhesive bonding, and mechanisms of degradation in polymer welds and adhesively bonded joints.
The structural performance of bonded components depends largely on the durability of the joints. Mehdi Tavakoli addresses the question 'What affects the durability of polymer welds?'and describes the ageing properties and mechanism of degradation in acrylonitrile-butadiene-styrene (ABS) hot plate welds.
As described in the November/December 1991 issue of the Bulletin, polymeric materials are prone to degradation when exposed to oxygen, heat, light, mechanical forces, or other sources of energy, and irreversible processes can occur leading to loss of useful properties and failure. Thermal and thermal-oxidative degradation may occur when polymers are subjected to elevated temperatures during fabrication or during service.
Once this process is initiated, it continues to develop and can result in discoloration, embrittlement, softening or other undesirable effects leading to deterioration in some properties. The resistance of polymers to degradation is dependent on their structure, composition and service conditions.
Conversion of a thermoplastic based polymer to a finished component normally involves heating it to the liquid state followed by extrusion into a mould or through a die. During these processing operations considerable stresses may be applied to the polymer, and this can result in some of the polymer chains undergoing homolytic cleavage at the carbon-carbon bonds with formation of free radicals.
These active chemical species can initiate the process of degradation by a free radical chain mechanism. During welding operations polymers may undergo similar physical and chemical changes to those occurring during extrusion or moulding procedures. Thermal and residual stresses may build up in polymers during processing operations such as injection moulding or welding.
However, there is a lack of information about the chemical changes which may occur in polymers during welding and effects of this process on long-term durability of bonded components are not known. TWI has been conducting a programme of work to evaluate effects of welding and ageing on a range of polymer welded joints of industrial interest.
In the first part of this work, the effect of ageing on structure and properties of hot plate welds in acrylonitrile-butadienestyrene (ABS) was investigated. ABS is one of the most important two phase commercial polymers. It consists of a continuous rigid phase (styrene-acrylonitrile-copolymer) in which the elastomer phase (polybutadiene) grafted with styrene and acrylonitrile is dispersed.
Because of their attractive properties and relatively low cost compared with other engineering thermoplastics, ABS polymers are now being used in many applications involving severe ageing conditions, such as in automobiles, heater ducts and warm water pipes. Polybutadiene is present in ABS as an impact resistant modifier but it is the prime target for degradative attack.
Experimental
The objective was to study effects of ageing on the mechanical properties and on the chemical and physical structure of hot plate welds of ABS. Samples from ABS parent material were also subjected to similar ageing tests for comparison.
Mechanical properties were determined by tensile testing before and after accelerated ageing in air at 100, 110 and 120°C, and immersion in warm and boiling water. Changes in chemical and physical properties associated with welding and ageing were evaluated using Fourier transform infrared (FTIR) microspectroscopy, dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC).
Air oven ageing was carried out on premachined waisted tensile specimens in accordance with BS 2782: Part 3: method 320C: 1976. Welded specimens were tested transverse to the weld which was located at the midpoint of the gauge length of the specimens.
Results and discussion
The most significant observation was the reduction in cross sectional area which occurred in welded specimens aged in air and water. This was thought to be caused by orientation of the polymer during welding, in an axis perpendicular to the weld line, and to build-up of thermal and residual stresses with their subsequent relaxation during exposure to elevated temperatures. Discoloration was observed on the surfaces of aged parent material and weld, while yellowing occurred during air oven ageing, and whitening occurred during water immersion tests. These changes are illustrated in Fig.1 and 2.
Ageing in air at elevated temperatures, particularly at 110 and 120°C was found to affect ABS welds more than the parent material. Effects of ageing in air at 120°C on ultimate tensile strength and elongation at break of ABS weld and parent material are shown in Fig.3 and 4.
Immersion in warm water and boiling water caused different effects in welds and parent material. The ultimate tensile strength in parent material was almost unaffected, but the elongation at break showed an increase, whereas welds showed loss in both ultimate tensile strength and elongation at break.
Assessment of the characteristic functional groups with FTIR showed that unsaturations in the polybutadiene segment of ABS due to trans-1,4 and vinyl-1,2 decayed with higher ageing time. During ageing, the carbonyl group concentration increased.
This was the case with both parent material and welds, although the losses in unsaturations and increase in carbonyl group concentration were more significant in the welds. The changes in ABS welds during ageing in air at 120°C are shown in Fig.5-7. Functional group changes associated with ageing were consistent with deterioration in tensile properties of welds and parent material. Evaluation of the mechanism of degradation showed that the main target for oxidative attack was the butadiene segment in ABS.
The results obtained by DMTA showed a shift in the position and height of the damping peak (tan δ) of the polybutadiene segment of ABS. The damping peak shifted to higher temperatures and the peak height was reduced with ageing.
DSC showed a decrease in the exothermic oxidation peak temperatures of the polybutadiene segment with ageing time.
Conclusions
Polymeric materials are mainly organic in nature and can react with environmental agents during processing and in service. As a result, degradation occurs which can lead to loss of useful properties and failure of the components.
Polymers used as welded components are often required to perform satisfactorily under severe environmental conditions. Assessment of resistance of polymers to degradation and their environmental durability is a complex subject and requires fundamental knowledge of structure-property relationships for specific applications and service conditions.
Service experience regarding the environmental durability of new polymeric systems, particularly polymer welds, is very limited. Furthermore, there are many new applications of polymers where resistance to aggressive environments, and long-term durability, are required. It is therefore important for polymer scientists and technologists to select and develop polymer systems which are capable of providing adequate service life and are able to withstand the operating environmental conditions.
This requires an assessment of mechanisms of degradation and failure. It also needs development of accelerated ageing tests which closely resemble the service conditions and give data which can be related to environmental durability of polymeric systems in welded components. This can lead to development of methods of selection of best material for the required application, as well as prediction of expected service life.
As a result of increasing need and interest from its member companies, TWI has a programme of work to investigate the ageing properties and durability of welded and adhesively bonded components for specific applications, particularly in pipes, microelectronics, automotive and aerospace applications, and offshore structures. Effects of environmental factors on durability of adhesives and adhesively bonded joints are intended to be the topic of a future Bulletin article.
