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The incoming inspection of adhesive backed copper foil tape is usually completed within a few days after arrival: take a sample, measure the peeling force, check whether the release film is complete, whether the adhesive surface is flat, and if the data falls within the specification range. The entire batch of materials is released for storage.
The materials will be stored in the warehouse for three months. The production line material requisition went online, and the automatic mounting machine began to experience abnormalities: the tape wrinkled at the peeling blade, the peeling angle of the release film was unstable, and some units' adhesive layers were "lifted" by the release film, causing the copper foil position on the substrate to shift. The production line stopped, and the engineer inspected the materials. There were no obvious abnormalities in the appearance. After retesting the peeling force, it was found that the peeling force had changed from "light peeling and opening" upon arrival to "tearing film with adhesive". The problem is not with the copper foil or the adhesive layer formula, but with the interface between the release film and the adhesive layer, which has changed during three months of storage.
The role of release film and the essence of peel force
In the structure of adhesive backed copper foil tape, the release film covers the surface of the adhesive layer to prevent it from sticking to itself or polluting the environment during winding, transportation, and storage. The surface of release film is usually coated with silicone oil or fluorine-containing release agent, forming a low surface energy coating, which makes the adhesion force between the adhesive layer and the release film much lower than that between the adhesive layer and the target substrate.
The peeling force window is not fixed
Peeling force is the quantitative expression of this layer of 'low adhesion'. The peeling force is too low, and the release film may loosen on its own inside the reel, exposing the adhesive surface to air pollution; The peeling force is too high, making it difficult for the release film to peel off during installation, or causing the adhesive layer to be "lifted" from the copper foil during the peeling process, resulting in local transfer of the adhesive layer. The ideal peeling force is' low enough for smooth peeling, high enough to maintain coverage '. But this window is not fixed, it varies over time with the interaction between the release agent and the adhesive layer.
Three mechanisms of peel force aging
The peeling force of release film varies with storage time, mainly due to three mechanisms.
The first one is the migration of release agents
The silicone oil or fluorine-containing release agent on the surface of the release film may migrate towards the adhesive layer during storage. The migration amount depends on the molecular weight, coating amount, and storage temperature of the release agent. Low molecular weight release agents migrate faster, and after migrating to the surface of the adhesive layer, they change the interfacial energy between the adhesive layer and the release film. The direction of the change in peel force depends on the surface energy of the migrating components - some systems have an increase in peel force, while others have a decrease.
The second one is the post curing and cross-linking of the adhesive layer
The crosslinking reaction of pressure-sensitive adhesive may not be completely terminated after coating. During storage, residual cross-linking reactions continue, causing changes in the modulus and surface viscosity of the adhesive layer. The increase in modulus makes it more difficult for the adhesive layer to deform during peeling, resulting in an increase in peeling force; The decrease in surface viscosity weakens the adhesion between the adhesive layer and the release film, resulting in a decrease in peeling force. The net effect depends on the adhesive formula and storage conditions.
The third one is the crystallization or precipitation of thickening resin
Some pressure-sensitive adhesive formulations contain tackifiers, which may crystallize or precipitate onto the surface during long-term storage, altering the adhesive properties of the adhesive layer. The precipitated thickening resin may form a layer of "frost" on the surface of the adhesive layer, reducing the peeling force, or it may interact with the release agent, increasing the peeling force.
The combination of three mechanisms results in a non monotonic time curve for peeling force. Some systems experience an increase in peel strength during the initial storage period, and then decrease after reaching a peak; Some systems continue to rise; Some systems first decrease and then increase. The shape of the curve depends on the combination of release agent, adhesive layer formula, and storage conditions.
How to amplify or suppress aging under storage conditions
Temperature is the accelerator of aging
The rate of change in peeling force accelerates with increasing temperature. The peeling force drift at 25 ° C for three months may reach within a month at 40 ° C. If the warehouse does not have temperature control, the material stored during the high temperature period in summer may have a significantly higher peeling force drift speed than in winter.
The influence of humidity varies depending on the adhesive layer system
The moisture absorption of acrylic adhesive is relatively low, and the influence of humidity on peel strength is limited; Rubber adhesive layers and certain formulations containing hydrophilic groups may experience changes in surface energy and a decrease in peel strength after moisture absorption. For materials stored in humid areas, humidity control needs to be considered together with temperature control.
Winding tension is another variable
The tape is wound with a certain tension in the reel, and the adhesive layer and release film are subjected to continuous pressure. The greater the tension, the stronger the driving force for the migration of the release agent and the deformation of the adhesive layer, and the faster the aging rate. Tightly wound tape is more prone to abnormal peeling force and wrinkling of release film after storage.
Why is the issue of 'invisible' during arrival inspection
The peel strength test for arrival inspection is conducted at the beginning of material storage. The peeling force at this point in time reflects the state of "transportation after leaving the factory", not the state of "production line launch". If the material is stored in the warehouse for three months, the peeling force may have drifted out of the available window, but the data from the arrival inspection is still 'qualified'.
More importantly, the arrival inspection usually only measures the peeling force at one time point, without measuring the trend of peeling force changes. The peeling force of two batches of goods may fall within the specification range, but the peeling force of one batch is stable during storage, while the peeling force of the other batch drifts rapidly during storage. Only measuring a single point cannot distinguish between these two situations.
Selection and inventory judgment based on "type of release film x storage conditions x service life"
Firstly, confirm with the supplier the type of release film and the peel force time data, rather than just confirming the peel force upon arrival
The aging behavior of different release film systems (silicone oil type, fluorinated type, silicon free type) varies. When selecting, the supplier should be required to provide data on the changes in peel strength over storage time (such as 1 month, 3 months, 6 months), as well as recommended storage conditions. If the supplier can only provide data on the arrival order point, there is a lack of basis for inventory period management.
Secondly, set inventory turnover rules based on the usage cycle, rather than general management based on "first in, first out"
If the peeling force aging curve of the material drifts significantly within three months, inventory turnover should be controlled within the aging window. For time sensitive materials, small batch high-frequency procurement can be considered instead of large batch long-term inventory.
Thirdly, control the temperature and winding tension in storage conditions
Controlling the temperature below 25 ° C can significantly slow down the migration of release agents and post curing of the adhesive layer. The winding tension should be within the recommended range of the supplier to avoid over tightening and accelerating the aging process. For warehouses in hot or humid areas during summer, temperature and humidity control should be part of material management, rather than assuming that 'tape is not afraid to be placed'.
Fourthly, establish a "pre launch peeling force sampling" mechanism at the production line end, rather than relying solely on arrival inspection
For materials stored for more than a certain period of time (such as two months), the peel strength should be randomly checked before going online to confirm whether they are still within the available window. The samples for sampling should be taken separately from the outer, middle, and inner layers of the reel, as the tension distribution at different positions of the reel may vary and the degree of timeliness may be different.
Advanced Institute (Shenzhen) Technology Co., LtdThe adhesive backed copper foil tape product line covers silicone oil and fluorine-containing release film systems, providing data on the changes in peel strength over storage time and recommended storage conditions.
The product supports matching the release film system according to the customer's usage cycle and storage environment, helping engineers extend the "qualified delivery" to "available online". For more technical information, please visithttp://www.avanzado.cn/.