Properties and applications of Epoxy Moulding Compounds 1
Properties and applications of Epoxy Moulding Compounds
Due to their excellent electrical properties, Epoxy Moulding Compounds are used for numerous applications in the electrical and automotive industry. Based on the ever increasing miniaturisation of parts and enhanced performance requirements, the challenges for materials used in insulating encapsulation system are increasing dramatically. As well as the high mechanical properties, low water take up and good chemical resistance, there is also a demand for higher thermal and dimensional stability.
1. Thermosetting Materials
To better understand the behaviour and the processing demands of thermosetting materials, a short introduction of the general structure of these materials as well as their reaction mechanism can be found below.
The typical properties of thermosetting materials are based on their molecular structure. Contrary to the long chain molecules of thermoplastic materials, thermosetting materials show a 3- dimensional cross-linked structure. The moulded part is formed from a melted low molecular and low viscous substance by cross linking as a result of a chemical reaction inside the mould. The reaction is normally activated by heat and is an irreversible process.
Thermosetting material
meltet cured
Thermoplastic material
amorph crtistallin
Fig. 1 Comparison of thermoplastic and thermosetting materials
Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 2
The curing reaction can be described by 3 different mechanisms, depending on the basic resin:
Polymerisation: i.e. Polyester (UP) Polycondensation: i.e. Phenol (PF), Melamine (MF), Urea (UF ) Polyaddition: i.e. Epoxy (EP)
For the polymerisation (in this case radical polymerisation) double or triple bonds are normally activated by peroxide to enable the molecules to cross link. The polycondensation and polyaddition resins have reactive groups which cross link either with the separation of a volatile substance in the case of a polycondensation mechanism or by simple bonding without any waste products for the polyaddition mechanism.
Independent of the basic resin, all thermosetting materials have a comparable composition: matrix composed of resin and hardener filler, inorganic and/or organic strengthening fibers lubricants and mould release agents other additives pigments
The cross linking of the molecules results in a different temperature/time behaviour compared to thermoplastic materials. For thermoplastic materials, the viscosity drops with increasing temperature. For thermosetting materials, there are 2 competing effects – the reduction of viscosity due to increasing temperature and an increase in viscosity due to the chemical reaction (see figure 2). This behaviour needs to be carefully considered as it strongly influences the processing of thermosetting materials.
Fig. 2 Viscosity of thermosets
Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 3
Due to the chemical cross linking, all thermosets show some common characteristic properties such as:
irreversible reaction i.e. not possible to melt them high strength and dimensional stability even under load at elevated temperature good chemical resistance
Apart from these important basic properties, there are also essential differences which make every type of thermoset suited for specific applications. Below we will discuss those properties that are most relevant for the applications where Epoxy Moulding Compounds are used.
2. Properties of Epoxy Moulding Compounds
Epoxy Moulding Compounds consist of a polymer matrix (Epoxy resins, hardeners and accelerators), which are compounded with fillers, reinforcing materials, pigments, release agents etc. to form granulate.
Fig. 3 Epoxy Moulding Compound (EMC)
Epoxy Moulding Compounds react according to a polyaddition reaction, which means no by- products are formed during moulding (Fig 4). Therefore the same final properties of the moulded part will be reached irrespective of the wall-thickness. When the reaction is finished in the mould no further change of mechanical, electrical or thermal properties takes place. Post curing is not required and the reaction related post shrinkage is virtually zero. This is also one of the reasons for the excellent long-term heat stability of Epoxy Moulding Compounds.
Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 4
W1 W2 W3 wall thickness W1 Fig. 4 Final properties dependent on wall thickness The matrix of Epoxy Moulding Compounds forms a three dimensional network during the reaction, which means the end moulded part is effectively one molecule. This three dimensional network shows uniform properties in all directions of the moulded part. With thermoplastic materials, the long chain molecules are generally aligned causing different behaviours in different directions. Figure 5 shows this influence on the basis of the coefficient of thermal expansion comparing glass fibre reinforced epoxy and PPS. PPS 40 % GF ] EP 20 % GF 6 - 10 80 · H L 1 - T height 60 40 height longitudinal 20 transversal transversal longitudinal 0 Coefficient of linear expansion [ K expansion linear of Coefficient -50 0 100 200 [°C] Fig. 5 Coefficient of linear thermal expansion (TMA) Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 5 Epoxy also gives better performance at welding lines. The epoxy reacts across the welding line which results in the mechanical properties remaining close to the 100% level in this area. With thermoplastic materials the situation is completely different with a decrease in mechanical properties down as low as 30 to 50% level at the welding lines due to the parallel orientation of the molecules in this area. The difference between the physical bond in thermoplastics and the chemical cross linking of thermosets also affects the creep behaviour. Figure 6 clearly shows the different performance of thermoplastics compared to an Epoxy Moulding Compound under load and elevated temperature. The effect is already apparent at room temperature and strengthens with increasing temperature. 8 PA66 30% GF 7 PET 30% GF ] % 6 [ e g n 5 a h PBT 30% GF c n 4 o i s n 3 PPS 40% GF e m i d 2 1 EP 20% GF 100 200 300 time [h] Fig. 6 Creep behaviour of different engineering plastics For the analysis of temperature stability, it is necessary to consider 2 different aspects. Firstly, it is necessary to know the relevant properties at the operating temperature, e.g. the mechanical strength, then it is also important to understand how long these properties are preserved at elevated temperature. Thermal endurance properties are described best by IEC 60216. For this test, the test specimen are stored at different temperatures. The test measures how long the specimen can be stored at the measured temperatures, until the tested property drops to a specified percentage of the initial value. These values are measured at room temperature. Figure 7 shows examples of temperature/time graphs measured on the basis of flexural strength for different Epoxy Moulding Compounds. Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 6 Fig. 7 Endurance vs Temperature (IEC 60216) In reality the properties at the operating temperature are important for an application. Figure 8 shows the shear modulus as a function of temperature. 1.E+10 1.E+09 NU 463 NU 514 NU 4414 NU6110 PW126B 1.E+08 shear modulusshear [Pas] 1.E+07 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 temperature [°C] Fig. 8 Shear modulus of several Epoxy Moulding Compounds Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 7 Figure 8 shows the wide variations possible with epoxy formulations. All Epoxy Moulding Compounds go through a range, where the mechanical properties drop to a lower level. This lower level is maintained until thermal degradation takes place at very high temperatures. The inflexion point of the curve indicates the glass transition temperature (Tg). Variations to the resin and hardener make it possible to create compounds to achieve a wide range of thermal requirements, starting from quite flexible systems (Tg110°C) up to very rigid systems (Tg250°C). The mould shrinkage of Epoxy Moulding Compounds is influenced by the matrix (resin/hardener) as well as the fillers and the reinforcement materials. As Epoxy Moulding Compounds form a 3 dimensional network when they react, the mould shrinkage is a reproducible physical process and directly dependent on the coefficient of linear thermal expansion. The coefficient of thermal expansion for an Epoxy Moulding Compound can be modified to best match the requirements of the material to be encapsulated. Post shrinkage is insignificantly low at 0,01%. mould shrinkage (%) post shrinkage (%) 0 0.5 1.0 1.5 0 0.5 1.0 1.5 2.0 PF UF MF MPF UP DAP EP Fig. 9 Comparison of mould- and post shrinkage of different thermosets The low shrinkage inevitably causes less tension and results in an excellent dimension stability. It is additionally an important requirement to achieve narrow tolerances which are necessary for functional parts, e.g. sealing areas, electrical insulation or substrate for potentiometer. This is also the reason why parts made from Epoxy Moulding Compounds have no shrink marks. This is a big difference compared to thermoplastics, where the orientation of the long-chain molecules results in shrink marks particularly in areas where there are changes in wall thickness. Thermoset molecules have a very low molecular weight during processing compared to thermoplastics. Within the thermoset family, Epoxy Moulding Compounds have the lowest processing viscosities. This is also the reason why Epoxy Moulding Compounds with more than 70% by weight of filler can still be processed with internal mould pressures of less than 50 bar. Epoxy Moulding Compounds can also be processed with a wall thickness of less than 1mm and still maintain the excellent crack resistance. This low viscosity is also one of the requirements to enable the partial or even the complete impregnation of windings (not viable with PA/PBT, and only possible with extremely short injection times with PPS/LCP, i.e. high pressure in the mould cavity), or the encapsulation of electronic components on printed circuit boards under mild conditions to prevent damage to the electronics. Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 8 Fig. 10 Partial impregnation / full impregnation As well as sealing against water and chemicals, impregnation is necessary to enable good heat dissipation and thereby lower operating temperatures. Through the appropriate choice of fillers, it is possible to achieve a thermal conductivity of around 2.0 W/mK with Epoxy Moulding Compounds whilst still maintaining sufficient mechanical properties. This is 4-5 times greater than the values usually reached for thermoplastics. Table 1 compares different properties for Epoxy Moulding Compounds and technical thermoplastics. Properties PA/PBT PPS/LCP EMC Volume Price low middle/high middle Density (g/cm3) 1.5-1.7 1.6-1.7 1.8-2.0 Dimensional stability poor middle very good Shrinkage high high very low Shrink marks yes yes no Creep behaviour (at >100°C) very large middle small Design flexibility very little little very good Crack resistance at poor poor very good weld lines Thermal conductivity poor middle good / very good Heat dissipation very poor poor good / very good Temperature Index (IEC 216) B&F H F&H Water absorption high low low Chemical resistance middle good good Sealing of the windings very poor middle good Table 1 Comparison of the properties between Thermoplastics and EMC Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 9 Processing of Epoxy Moulding Compounds In addition to compression moulding which is not relevant for electronic parts, Epoxy Moulding Compounds can be processed by both transfer moulding and injection moulding. It is important in both cases that the material is plasticised prior to the moulding process, in order to displace air from the granules and to optimise the flow behaviour and the curing time. An exception is the Mutliplunger-technique for very small pellet weights, where the preheating step is omitted. In transfer moulding, this preparation is done externally, either by means of a tabletting device plus HF- preheater or in one step by means of a preplasticiser. In injection moulding the material is plasticised in the injection moulding machine during the process. Epoxy Moulding Compounds Preform Preplastication High-Frequency Preheating Transfer Moulding Transfer Moulding Injection Moulding Productivity Improvement Fig. 10 Processing Technologies Due to the simple handling and high automation, the injection moulding technology is increasing in importance. However in many cases, there are still reservations regarding process reliability in the injection moulding of Epoxy Moulding Compounds. Below are some tips for setting the parameters when processing EMC by injection moulding. The main reason for processing problems results from the concern that the material could react too much in the first part of the cylinder. As a result heat is supplied as late as possible in the cylinder, i.e. set temperature in the first part of the cylinder is low and at the nozzel end relatively high. The large temperature gradient means that a high back pressure is required to ensure the material melts in time. When the material is melted in this way, it is not homogenous and the remaining material in the cylincer continues to receive energy during curing and therefore it cannot be processed with a cushion. The consequence is a lack of control over the compression Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 10 of the part ( quality variations) and high process sensitivity to cycle time fluctuations and process interruptions. If, on the other hand, the entire available cylinder length is used for the plastification by setting higher temperatures on the inlet side, the temperature on the nozzle side only needs to correspond to the desired melt temperature before injection. During the curing time no additional energy is supplied to the cylinder and the back pressure can be reduced to a minimum level. The benefits are: Lower injection pressures Control of the compaction of the moulded part Prevention of material buildup Shorter time to reach optimal production conditions Less wear The advantages with regards to process reliability are clearly shown in figure 11. The advantages of this improved preparation can be seen in terms of the lower processing pressures required even after long interruption times. With the first (inefficient) way of processing, the injection pressure increases by 50% after a 3min interruption and reaches the pressure limit of the machine after a 5min interruption. However, if the temperature profile in the cylinder is optimised, the injection pressure only increases by 20% after a 5min interruption and has only reached the 50% level after a 10min interruption. As a result the process is far more tolerant towards production interruptions. 1500 1300 1100 900 over pressure [bar] pressure over 700 - 500 change 300 old standard with cushion max pressure 100 without interrupt 3 5 10 interruption time [min] Fig. 11 Influence of Interruptions To further improve process reliability, newly developed Epoxy Moulding Compounds have reduced reactivity at plasticising temperatures without sacrificing reactivity at mould temperatures. This is achieved by using an encapsulated catalyst, which is released only at higher temperatures. The advantage can be seen in figures 12 and 13. The spiral flow length based on ASTM D 3123-72 is used to assess the degree of reaction and flow properties. The spiral length is plotted for 1min and 4min interruption times at the plasticising temperature. Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 11 20 18 16 1min 4min 14 12 10 [Inch] 8 6 4 2 0 90° 100° 110° 120° [°C] 130° Fig. 12 Latency behaviour of a product without encapsulated catalyst (measured after different waiting times in a plasticator) The product loses a quarter of its fluidity after a 4 minute interruption at a plasticising temperature of 90°C and it loses more than 50% of its fluidity at 100°C. When the catalyst in the above product is replaced with an encapsulated catalyst (lab sample LMB5634), the fluidity at 90°C after a 4min interruption remains almost completely unchanged. Even after 4 minutes at 110°C there is only about a 35% loss of the initial value. Fig 13 Latency behaviour of LMB 5634 with 100% encapsulated catalyst (measured after different waiting times in a plasticator) Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 12 Table 2 shows a comparison of the processing conditions of Epoxy Moulding Compounds and technical thermoplastics. Processing PA/PBT PPS/LCP EMC Processing technology Injection Injection Injection & Moulding Moulding Transfer Moulding Melting point <300°C <400°C <110°C Mould temperature 80-120°C 120-160°C 160-190°C Injection pressure (barrel) <1500bar <1500bar <800bar Cavity pressure 50-250bar Injection time <10s <5s <20s Cure time (2mm wall thickness) <45s <45s <45s Cure time (5mm wall thickness) <120s <120s <60s Preheating of inserts yes yes yes Reason for preheating filling filling curing Protection of the winding yes very no during injection important Impregnation of the winding no middle very good Complete impregnation of the no no yes winding in vacuum Protection of winding against no middle very good moisture / chemicals Special vents to prevent no yes yes flash formation Material recycling yes yes difficult Table.2 Comparison of processing for thermoplastics and EMC Applications Solenoid X-ray part Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil Properties and applications of Epoxy Moulding Compounds 13 Connector Insulator Speed sensor Speed sensor with integrated PCB Protection of electronics Ignition system Electrical drive Impregnated winding Hans-Fred Buchmann, Duresco GmbH, CH-4108 Witterswil