Fume emissions from laser cutting and hot gas welding of plastics

TWI Bulletin, May - June 1995

Mike Troughton graduated in 1982 from the University of Leeds with a degree in physics and remained there to study for a PhD in the Polymer Physics Department. In 1986 he joined the R & D Centre at BP Chemicals where he was responsible for developing and testing a number of novel plastics products. Mike joined TWI in 1993 as a Senior Research Engineer in the Plastics Joining Group and is responsible for work on the structural integrity of joints in plastics and composites.

Jake Sims joined TWI in 1990 as a Research Engineer in the Plastics Joining Group. Prior to this he worked in industry for four years producing calendered and extruded plastics, and then as a research technologist, where his interest in research led him to study for a Master's Degree in the School of Materials Science, Bath. In 1992 he joined the Membership Group at TWI with responsibility for business development, particularly focusing on France, Italy, the Far East and the UK. At the end of 1994 he joined the Technology Transfer Project team which involves delivering technical support to UK SMEs in Materials Joining Technology.

Peter Ellwood took a first degree in Natural Sciences at the University of Cambridge in 1975. He went on to carry out research in organic chemistry at the University of Sheffield from 1975-1978 and gained an MSc (1977) and PhD (1979). He joined HSE Occupational Medicine and Hygiene Laboratory at Cricklewood in 1978 to undertake research and method development in the field of occupational air monitoring. He was Head of complex substances and fume section 1990-1993. He moved with the Laboratory to Sheffield in 1992 and is currently on secondment to the HSE Research Strategy Unit in Sheffield.

Helen Taylor graduated from the University of Newcastle Upon Tyne with an Honours degree in Chemistry in 1989. In the same year she joined the Occupational Medicine and Hygiene Laboratory of the Health and Safety Executive (HSE) in Cricklewood, London where her work included the analysis of dioxins from various matrices, the identification of the carcinogenic components in bracken spores and the analysis of fume from a number of industrial areas. In 1992 she moved with the Laboratory to Sheffield.

Laser cutting and hot gas welding of plastics are two techniques which are potentially hazardous due to fume emission. Together with the Health and Safety Laboratory of the Health and Safely Executive, fume emissions produced during laser cutting and hot gas welding of various plastics materials have been analysed, Mike Troughton, Jake Sims, Peter Ellwood and Helen Taylor report ...

The use of industrial high power lasers for processing a wide range of materials is increasing. Computer controlled cutting of metals, ceramics, plastics and polymer composites using lasers provides the advantages of accuracy and speed of cutting, absence of tool wear and smooth cut edges. It is estimated that 200-300 lasers are currently used for materials cutting in the UK, and that this number is increasing. There is a wide range of plastics and polymer composite materials which are laser cut for various applications. Examples of laser cutting include the following: rigid sheet material for plastics display signs, woven fabrics for the garment industry and for sailcloth, mouldings (such as body panels and dashboards) for the automotive industry and polymer composites in the electronics and aerospace industries.

Laser cutting of materials with cut-assist gas involves a combination of physical and chemical processes including melting, vaporisation, burning and pyrolysis. ^[1] Together, these processes result in the production of fume during the laser cutting operation. Establishing the chemical nature of this fume when produced during laser cutting of plastics and polymer composites has received little attention so far. It is known that laser cutting of polymethylmethacrylate (PMMA), a material which is used routinely for characterising the laser beam, results in the formation of mainly methyl methacrylate monomer and a small quantity (up to 2%) of polycyclic aromatic hydrocarbons (PAHs), some of which are carcinogens. ^[1] Various studies have indicated that a range of PAHs and other low molecular weight organic compounds are evolved during laser cutting of polyvinylchloride (PVC), PMMA and a number of polymer-based composite materials. ^[2,3,4,5,6]

Hot gas welding is a technique widely used for fabricating and repairing structures for a broad range of applications including tanks for chemicals, fittings for pipe systems and plastics glazing units. It is a manual technique which means that the operator is continuously close to the process, typically with his face within 500mm from the weld and sometimes directly above it. There have been no published studies on the chemical nature of the fume produced during hot gas welding of plastics. However, it is known that the temperatures used for hot gas welding are similar to those encountered during primary processing, eg moulding or extrusion of plastics. Studies carried out to measure pollutants from plastics at typical processing temperatures have shown that organic pollutants are evolved. ^[7]

The work reported here was carried out to acquire qualitative information on the main pollutants produced from laser cutting of a wide variety of plastics and to gather information on fumes from hot gas welding. This preliminary work was restricted to particulate sampling on filters and sampling of volatiles by adsorbent tubes. Thus, for example, no attempt was made to sample permanent gases such as hydrogen chloride from PVC.

Laser cutting

The equipment used for laser cutting was a commercial 1500W continuous wave CO ₂ laser fitted with a conventional coaxial gas cutting nozzle and focusing head with a zinc selenide lens. A moving workpiece system was used in which a motor-driven X-Y table moved the sheet being cut under the stationary laser cutting head. The five plastics materials used in the laser cutting experiments were polyvinylchloride (PVC), polymethylmethacrylate (PMMA), polycarbonate (PC), polyethylene-terephthalate (PET) and glass-reinforced epoxy (epoxy composite). These were all commercially available sheet materials of 3mm thickness. Argon and air were used in separate experiments as the cut-assist gas. The cutting speed was 4 m/min and the laser power set at 750W for all plastics except epoxy composite for which the cutting speed was 2.5 m/min with the laser power set at 1500W.

Hot gas welding

Hot gas welding was carried out using a commercial unit fitted with a standard 5mm nozzle. The five plastics materials evaluated were polypropylene (PP), polyamide 6 (PA6), PVC, PMMA and PC. Each material was in the form of 3mm thick sheet with appropriate matching filler rod.

Sampling equipment

Fume sampling was achieved using three types of collection, Fig.1. First, particulate material was collected on pre-extracted glass fibre filters. Second, medium molecular weight volatiles were collected on Tenax adsorption tubes and finally, low molecular weight species were collected on NIOSH charcoal tubes. Each of the samples was connected to a battery-powered air pump.

For the laser cutting experiment a cabinet (1.8 x 1.8 X 1.9m) was constructed around the cutting head and X-Y table. An exhaust port was cut in the top of the cabinet for attachment to fume extraction equipment. The rest of the cabinet was sealed except for a 10mm gap all around the base, in order to allow a source of fresh air during extraction. The fume sampling equipment was placed inside this chamber. Background samples were taken as a control for the laboratory atmosphere. Two sets of cutting experiments were carried out for each material, firstly using air and secondly argon as the cut-assist gas, Fig.2.

For the hot gas welding experiments a blower unit pumped air through a heater and subsequent nozzle on to the welding rod and sheet material. Each material was welded continuously for four hours using the recommended procedure.

Results

Laser cutting

The quantities of particulates collected from above the laser during the laser cutting experiments are shown in Table 1. These are the means of two sampling positions and are corrected to 20 cuts to allow comparison. These results have not been converted to mg/m ³ because sampling was carried out close to the machine rather than near the operator's breathing zone, and also because the experimental setup did not simulate probable factory conditions. By relating the mass of particulate collected to the number of cuts, some idea of relative quantities of particulate produced from similar cut volumes of material can be obtained.

Table 1 Quantities of particulates produced from laser cutting of plastics - mg material collected on filter for 20 cuts.

PMMA produced the least, and the epoxy composite the most, particulate material. The relatively high quantity of particulates produced during laser cutting of the epoxy composite is probably due to the presence of glass fibres in the material. Earlier work has shown that in fume evolved from laser cutting of epoxy glass laminate most of the particles derive from the glass fibres. ^[5] There is some evidence for lower quantities of particulate when changing from argon to air as the cut-assist gas.

The PAHs identified in the cyclohexane soluble extracts of the particulate samples are detailed in Table 2. Changing the cut assist gas affects the PAHs produced. The PAHs identified here differed from those reported elsewhere for laser cutting. For example, Doyle and Kokosa ^[4] report ten PAHs in the fume from both PVC and PMMA. In the present work no PAHs were detected in the fume from PMMA, and only fluorene, pyrene and benzo(a)fluorene from PVC. Ball et al ^[1] report up to 2% of PMMA being converted to PAH.

Table 2 Polycyclic aromatic hydrocarbon extracted from particulates

Analysis of the Tenax samples by gas chromatography/mass spectrometry gave the results shown in Table 3. Only major components have been identified and the results have been simplified by grouping aliphatic alkanes together as a single entry, except for n-hexane which has a lower occupational exposure standard (OES). ^[8] Identifications are based on computer matching of sample mass spectra with Library spectra and have been confirmed by comparison with standards only where indicated in the Table. Table 3 shows that many compounds are produced which have been assigned occupational exposure limits (OELs). Among these, benzene and styrene, found in the fumes from PC, PVC and PET, are subject to maximum exposure limits (MELs). Earlier work on laser cutting of polyesters has shown that between 3% and 7% by weight of material ablated was converted to benzene. ^[1] A number of other aromatic compounds were also identified including toluene, phenol, styrene and naphthalene. PMMA produced methylmethacrylate as the major component, which is consistent with earlier work. ^[1] Laser cutting of epoxy composite resulted in fume which contained mostly substituted alkanes which are not recognised as being particularly hazardous.

Analysis of the charcoal tube samples gave no identifiable products, presumably because of the high dilution factor involved in the extraction.

Table 3 Pollutants from laser cutting of plastics

Hot gas welding

No significant quantities of particulates were collected from the hot gas welding experiments. Similarly, no PAHs were detected in the extracts from the glass fibre filters.

Analysis of the Tenax tube samples by gas chromatography/mass spectrometry gave the results shown in Table 4. As with laser cutting, only major components have been identified and the results have been simplified further by grouping aliphatic alkanes together as a single entry (except for n-hexane which has a lower OES). Again, identifications are based on computer matching of sample mass spectra with library spectra and have been confirmed by comparison with standards only where indicated in the Table. Results are given for three different sets of conditions, namely, with the welding tool at the recommended temperature for each plastics material, and at 15°C above and 15°C below the recommended temperature. Fewer pollutants are identified than for laser cutting, although this could be because of the smaller quantities of fume produced with the resultant difficulties in detection. Several species which have been assigned OELs were identified in the fume, although benzene was not found.

As with laser cutting, analysis of the charcoal tubes gave no identifiable products.

Table 4 Pollutants from hot gas welding of plastics

Conclusions

The fumes from laser cutting of plastics contain particulate and gaseous materials. Benzene and other aromatics are found in the fume from some plastics, as are PAHS.

Hot gas welding produces no measurable quantities of particulate and fewer volatiles than laser cutting.

The work reported here was carried out under laboratory conditions and no attempt was made to simulate worker exposure. Further work is necessary to establish the magnitude of any risk from these processes.

Note: The UK Health and Safety Executive has published the two reports on which this article is based.

• Products evolved during hot gas welding of plastics, Contract Research Report No. 86/1995
• Products evolved during laser cutting of plastics, Contract Research Report No. 87/1995

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References