Lamination of polymer layers on a rigid or flexible substrate of various materials is a well known technology and used in numerous technical fields. The polymer material used for the polymer layer can vary depending on the end application of the article comprising the multilayer laminate. For instance ethylene vinyl acetate (EVA) and other thermoplastic based polymers are conventionally used layer materials in lamination processes.
In general, the lamination of layer(s) to a substrate can be performed for instance by 1) so called cast extrusion, wherein at least part of the layers are produced on a premade substrate during the cast extrusion step or 2) by integrating premade substrate and premade layer(s) together under heat and pressure, typically in a vacuum in a laminator equipment.
For instance, lamination is one of the steps also used for producing well known photovoltaic modules, also known as solar cell modules. Photovoltaic (PV) modules produce electricity from light and are used in various kind of applications as well known in the field. The type of the photovoltaic module can vary. The modules have typically a multilayer structure, i.e. several different layer elements which have different functions. The layer elements of the photovoltaic module can vary with respect to layer materials and layer structure. The final photovoltaic module can be rigid or flexible.
The rigid photovoltaic module can for example contain a rigid protective front layer element, such as a glass element, front encapsulation layer element, a photovoltaic element, rear encapsulation layer element, a protective back layer element, which is also called a backsheet layer element and which can be rigid or flexible; and optionally e.g. an aluminium frame.
In flexible modules all the above elements are flexible, whereby the protective front layer element can be e.g. a fluorinated layer made from polyvinylfluoride (PVF) or polyvinylidenefluoride (PVDF) polymer, and the backsheet layer element is typically a polymeric layer element.
The above exemplified layer elements can be monolayer or multilayer elements.
All said terms have a well-known meaning in the art.
The state of the art encapsulation layers in flexible and rigid PV modules are typically made from ethylene vinyl acetate (EVA).
Moreover, there may be adhesive layer(s) between the layers of an element or between the different layer elements.
During the production of the PV module two or more premade elements of the PV module, which elements can be monolayer or multilayer elements are conventionally laminated together in a laminator equipment. Such lamination process normally comprises a step of heating the system, which heating step typically involves evacuation of air from the system, and a step pressurizing the system under heat and vacuum for the lamination to occur between the functionally different layer elements. In case of a PV multilayer element, the multilayer element, e.g. backsheet multilayer element, can be premade completely or partly before lamination to a different functional element, like rear encapsulation element.
Some end applications of laminated articles, like PV modules, bring demanding restrictions to the lamination process. E.g. in case of lamination process of layer elements of a PV module, it is always recommended that the application of pressure should be started only when the encapsulant layer reaches a temperature greater than its melting temperature and also after sufficient time that it is properly melted. This is very important as otherwise applying the pressure on insufficiently molten polymer or very close to its melting temperature will excert large stress on the fragile cells of the photovoltaic element causing their rupture.
In a lamination process using conventional laminator equipments, one very important and critical parameter for article manufacturers, like photovoltaic module manufacturers, is the lamination cycle time. The lamination cycle time has a marked impact on the expansion of production capacity and on the reduction of the production variable costs of a multilayer laminate, like PV module. Therefore there has been a constant attempt to develop various measures along the production value chain of a laminated article, like PV module, which could eventually result in shorter cycle time during lamination step.
One way to reduce the cycle time could be by starting the pressing immediate after the polymer starts melting. However, this approach is not suitable e.g. for EVA based layers, like encapsulant layers, even though EVA melts at temperature below 80° C. This is because EVA to be suitable e.g. as PV encapsulant material must usually have high VA content to get feasible flowability/processability behaviour. The conventional EVA with high VA content has then also very high MFR2 (more than 15 g/10 min). If with such an encapsulant, pressing is started immediately after the EVA melts, large amount of encapsulant will flow out of the system due to its high melt flowability. Therefore EVA needs to be crosslinked simultaneously during the application of pressure, typically by peroxide. Also other thermoplasts are conventionally crosslinked during or before the lamination. Crosslinking of EVA and other thermoplasts can be performed using e.g. irradiation or chemical crosslinker like peroxide or silane condensation catalyst.
When e.g. EVA or other peroxide crosslinkable thermoplastic based encapsulant layer(s) of a PV module is crosslinked during lamination process with a peroxide, it is necessary that lamination temperature is high enough so that peroxide decomposes effectively to initiate the crosslinking reaction and it is also necessary to prolong the lamination time in order to complete the crosslinking reaction. Therefore, even with most effective peroxide, the total lamination time hardly can go below 10 minutes at lamination temperature 150° C. This means e.g. with EVA encapsulant layer, there is certain technical limitation to reduce lamination cycle time beyond certain minimum value. Furthermore, in e.g. peroxide crosslinked encapsulant layers (e.g. EVA), the formed volatiles and reactions products are needed to be expelled from the laminate assemble to ensure non blisters, bubble formation in the laminate, which volatiles removing step, again, increases the lamination cycle time.
A second way of reducing the cycle time is to shorten the pressure holding time as much as possible without sacrificing the module quality with respect to adhesion, bubble formation, etc. Again this approach is not suitable e.g. for EVA or other peroxide crosslinkable thermoplastic based layers that need crosslinking reaction and subsequent removal step of volatiles and/or by-products formed during the crosslink reaction.
As a third attempt to reduce the lamination cycle time of e.g. PV modules, the producers of laminator equipment have tried to design advanced laminator equipments with improved heating process or forced cooling steps, encapsulant layer producers design encapsulant layers (which are typically based on EVA) with faster crosslinking steps, etc. However, in most of these cases the solutions either end up with more expensive equipment need, like advanced laminator equipments or very limited lamination process robustness due to very strict process guideline, like fast cure EVA solution for encapsulant layer.
WO2010124189 of Bemis Associates describes an encapsulation layer based on a blend of terpolymer of ethylene with acrylate and glycidyl methacrylate comonomers together with a heat resistant copolymer (ethylene with glycidyl methacrylate comonomer). The blend may also comprise carrier polymer which is an ethylene polymer modified (copolymerized or grafted) with silane. The formed layer can be crosslinked by irradiation. Also a lamination process has been described, which is stated on p. 7 to be shorter than that of EVA based encapsulant, in one embodiment lamination press cycle times of 1 minute at about 155° C. and about 3 minutes at about 125° C. were given. In the experimental part, page 18 and 19, pressure of about 1 atm and temperature of about 110 to 175° C. for 1 to 15 minutes, preferably about 140 to 160° C. for about 3 to 5 minutes, or alternatively at 1 atm, temperature of about 120 to 140, or of 145 to 155° C., were given, and stated that lower temperatures will require longer times to ensure adequate adhesion. In examples the lamination is effected in a vacuum laminator at 155° C., 1 atm, with 7 min pump time and 8 min press time. EP2144301 of Borealis discloses the possibility to reduce lamination temperature in relation to laminating of crosslinked ethylene copolymer with silane and optionally with acrylate comonomer(s). On p. 7, the temperature, pressure and total lamination time of the lamination process has been indicated. The temperature during the lamination process means the set temperature in the laminator. No specific conditions for different lamination steps are given and, as already said, the benefits for the option for shorter lamination cycle relate to crosslinked encapsulation material.
There is a continuous need to develop further lamination process solutions for producing a multilayer laminates, like photovoltaic modules, to meet the demands required by the multilaminate, like PV module, producers in the further developing lamination industry, such as in the growing PV module industry.