Real-time particle size and particle shape analysis of EPS during production process
Expandable polystyrene (EPS) foams have been known by the BASF trade name Styropor (or often Styrofoam in English) since the 1950s. Bead-shaped polystyrene granules are a raw material of EPS foams. They are impregnated with a propellant during the manufacturing process. The addition of heat during pre-foaming causes the propellant to evaporate and the compact EPS beads swell up to 20 to 50 times their volume. In the further processing of panels, blocks or moulded parts, the expanded beads are made into the desired shape and welded through foaming in hollow spaces. The finished hard foam has a characteristic appearance that still reveals the original beads. Due to its low density, high moisture resistance, good thermal insulating properties and excellent shock absorption, EPS foams are used as a classic insulation material in construction, as secure packaging material for temperature-sensitive or shock-sensitive goods or for the construction of child seats, helmets and life jackets. The size of the compact EPS raw beads is therefore decisive in the processability and quality of EPS foams. The size of the granules is thus a decisive quality parameter for high-quality and purposefully usable EPS raw beads. This parameter should be reliably controlled during the manufacturing process.
EPS granules are marketed for various processing purposes in certain size fractions with different propellant contents and optional flame protection. The size range of raw beads ranges from a few hundred microns to several millimetres, whereby the individual fractions are narrowly distributed. For example, raw beads in a size range from 200 microns to 400 microns are required for the production of EPS drinking cups with very thin walls. Larger moulded parts, such as blocks or plates, allow coarser granules; however, with clear requirements regarding the size distribution and a predetermined coarse grain that must not be exceeded.
The EPS raw beads are produced in a polymerisation reaction, and are then washed, dried and – depending on the product specification – coated, screened and bottled. For the efficient production of EPS granules, the particle size is therefore a crucial command and control parameter in order to provide reliable product quality and avoid faulty batches, both in the polymerisation and screening.
- Production of defined fractions for the desired end product quality
- Maximisation of the yield of a size fraction
- Avoiding faulty production and extensive rework
- Optimisation of process management
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EPS beads are produced in a polymerisation reaction in which the liquid monomer styrene is heated in water in a vessel with initiators and additives while constantly stirring for several hours. A metastable emulsion of styrene droplets thus forms in water which should be as finely distributed as possible. Under the effect of heat, long chains of or branched polymers gradually build up from the styrene molecules. This way, growing and increasingly solid polystyrene beads are formed from the liquid styrene droplets throughout the polymerisation reaction. Once the target size for the particles is reached, the reaction is stopped, further polymer growth is inhibited and bonding of the beads by adding a stabiliser such as dicalcium phosphate is prevented. Pentane is added to the reactor as the propellant which makes the expandable granules from the polystyrene beads. A final heating of the system ensures complete hardening of the polymer into solid beads which are separated, washed and dried after draining the reactor of the water and then divided into the desired size fractions using screen cuts.
The on-line ultrasonic measuring technology with OPUS is available both for monitoring the droplet size of the finely distributed liquid styrene in the emulsion mixed with water and for monitoring and controlling the polymer growth of the continuously forming polystyrene beads. The ultrasonic measuring finger is incorporated in the reactor for real-time analysis via an adapter flange (AF) through the floor or the wall, meaning that the measurement zone is located directly in the stirred mixture (in-situ). The droplet and particle size, as well as the concentration is measured under process conditions by means of ultrasonic extinction. Around every 2 minutes, the measurement probe delivers a particle size distribution divided into 31 size categories. The characteristic values x10, x50 and x90 can be shown in a Q(t) diagram applied over time thus allowing close monitoring and control in the polymerisation reaction.
The real-time monitoring of the particle size in the reactor makes it possible to both better understand and optimise the process control and to precisely initiate the addition of the stabiliser or propellant. The timely completion of the process reduces the amount of waste (oversize). The OPUS analyses also demonstrate very good comparability to other analysis methods, such as screening, image analysis or laser diffraction which are typically performed on the hardened product.
Application strengths
- In-situ analysis under process conditions
- Measurable in emulsion and suspension
- Usable at high and variable pressures and temperatures
- Optional use in potentially explosive areas (ATEX)
Customer benefits
- Avoiding faulty batches and rework
- Automated process control and process optimisation
- Early detection of possible irregularities in the process
- Increased process reliability
The EPS beads are separated into different product qualities on large screening plants in very close size fractions. In this process, even minimal amounts of coarse material are an indicator of incipient screening damage. This screening damage needs to be reliably detected at an early stage in order to avoid the reprocessing of faulty batches and not to block current production.
The dynamic image analysis achieves a unique sensitivity due to the single grain collection. Even in close distributions, minimal amounts of coarse material are reliably detected at an early stage. The image analysis sensor PICTOS can be integrated into one or more parallel production lines. The representative sampling is performed quasi-continuously with the dynamic sampler TWISTER which leaves the entire pipe cross section of the respective product line on a spiral path. The compact form of the EPS beads promotes easy transport of various TWISTER sampling process to a central PICTOS sensor. This way it is possible to reliably monitor up to 10 product lines in a single multiplex installation using just one image analysis sensor.
PICTOS is capable of repeatedly dissolving the smallest amounts of coarse material and single oversize particles and initiating a timely change of the respective screening deck. The process reliability significantly increases through the constant, timely analysis of the screening output. Faulty batches and rework are avoided and process efficiency therefore significantly increases.
Application strengths
- Quick and easy measurement of particle size on single grain
- High measurement frequency and statistically relevant measurement results
- Powerful evaluation modes for meaningful results
- High sensitivity for detecting minimum coarse material fractions
- Multiplex installation for monitoring multiple product lines with one sensor
Customer benefits
- Ensuring high product quality according to the specifications
- Ensuring tightly distributed particle size fractions
- Early detection of screening damage
- Avoiding faulty batches and rework
- Increased process reliability
