Combining silicones, adhesives and face stocks is complex. Regardless of how much information is available and how much communication there is between the manufacturer and converter, problems can occur.
Several common problems have been occurring for many years, so there are relatively reliable means for preventing and combating them. This section is devoted to explaining the causes of these common problems. As well as providing suggestions to avoid them or correct them.
Adhesive confusion occurs when the adhesive does not stay on the proper side of the substrate and unwinding defects occur due to the lifting or tearing of the adhesive. Adhesive confusion can be caused by adhesive ooze, non-uniform moisture levels, improper winding tensions, inadequate cure, or not a wide enough release differential on the release liner.
A blister is a disruption in the surface of a substrate, polyolefin coating, adhesive coating, etc. Blisters are generally caused when liquids, such as water or organic solvents, or gases such as air, expand at a rate faster than the overall system is able to dissipate the increased volume and pressure. For instance, if an adhesive has skinned over, blister or bubbles may form in the adhesive as the remaining solvent or water, unable to diffuse rapidly enough through the skin, vaporizes and forms bubbles in the adhesive as a means of pressure relief. In another case, if a two-side polyolefin-coated release liner is heated to an excessively high temperature, the polyolefin will soften, and when the vapor pressure of the water in the paper is high enough, a blister will form in the polyolefin – again as a means of pressure relief.
Blister can be caused by:
- Excessive heat
- Restricted permeation/dissipation of the liquid/vapor components of a system
- Vaporization of solvents or water
Blister can be minimized by:
- Reducing maximum temperatures
- Reducing heating rates
- Perforating the release liner
- Using a more porous liner
When layers of the release liner are fused together making it difficult to unwind the liner without damaging the sheet or using high unwind tension, blocking is usually occurring. Blocking may occur only in parts of the web, such as the edges, or across the entire web and can be caused by a number of different mechanisms.
- Inadequate cure of the coating
- Non-uniform moisture levels
- Excessive winding tensions
- Chemical contamination
- Inadequate release characteristics
Blocking can be prevented by a number of different methods
- Increase the degree of cure
- Reduce ambient moisture to improve storage conditions
- Reduce tension settings by decreasing lay-on roll pressure, adding or increasing winding taper, or reducing torque
Many people also use noise as an indicator for blocking. This method is very subjective and does not necessarily identify blocking because some coatings are inherently “grippy” or ”grabby” by touch.
Delamination is the separation of layers, at the interface, that have been bonded together by mechanical or chemical means. This effect occurs when internal bonding forces are exceeded, frequently causing an expected failure. Delamination can also occur at the interface of polyethylene to paper or adhesive to silicone.
Usually delamination is a function of the material and interface. For example, a polyethylene coated paper could delaminate due to poor surface-treatment before polyethylene extrusion coating. Adhesive delamination could be due to easy release of the silicone.
Delamination is caused by:
- Improper flame-, corona- or chemical surface treatment.
- Poor internal bond
- Poor interply bond between fibers
- Improper release properties of the silicone coating (too easy)
Delamination can be corrected by the following:
- Increasing surface treatment
- Improving internal bond of base sheet
- Increasing release force of the silicone coating
Dimensional stability relates to the ability of a material to resist change when exposed to external forces such as moisture, heat or physical stress. All material exhibits some dimensional change when exposed to these forces and the challenge is to control the degree of change so that the quality of the product is not compromised during processing and use.
For paper, the change due to moisture gain and loss (hygroexpansivity) is the major concern. Changes due to moisture show up as curl, edge waviness or poor layflat characteristics. The dimensional stability of paper is controlled by many factors. Among them are fiber types and percentages, degree of refining, interfiber bonding, filler content, sizing type and amount, fiber orientation, drying process, sheet density and formation. Subsequent converting processes also affect the dimensional stability of the release liner.
Dimensional stability is also a factor with film release liners. However, in this case it is due to thermal stresses. Exposing a film to an excessive temperature, especially under tension, can cause shrinkage, stretching or even severe distortion.
Dimensional stability is influenced by:
- Changes in humidity
- Initial moisture of the paper when produced
- Percentage and types of pulp
- Drying process of paper
- Fiber freeness
- Filler content
- Sheet density and formation
Film and Paper Bagginess
Baggy film/paper occurs when a substrate does not have uniform tension across the width of the web. These areas of low and tight tension are present throughout the sheet because the lengths of incremental bands across the web are not equal.
Bagginess is a property of film or paper that is created in the processing step of the base material. Film can be further processed by annealing to relax and/or minimize the bagginess.
Causes of bagginess are as follows:
- Non-uniform web thickness
- Winding too tight
- Surface winding on a non-uniform thickness
- Improper spreading
- Improper drying
Corrective remedies include the following:
- Reducing caliper variation
- Using a slip shaft mode for rewinding
- Adjusting spreading device to properly obtain uniform web tensions
- Reducing oven temperature gradient
Layflat is generally considered to be the ability of a release liner or a finished construction to resist developing curvature or curl when exposed to humidity changes. This is fundamentally related to the dimensional stability of the release liner. However, other factors such as improperly matched tensions during lamination, shrinkage of the face stock, exposure to excessive temperatures or roll set can cause curl or layflat problems.
Layflat can be optimized by:
- Using release liners of high dimensional stability
- Properly matching tensions during laminating
- Minimizing face stock shrinkage
- Avoiding exposure to excessive temperature
Most paper is produced from a slurry of fiber dispersed in water. Most of the water is removed during manufacture to make paper as we know it. However, paper retains the ability to pick up or lose large amounts of water during additional processing or storage. The absorption of water frequently results in moisture wrinkles, especially on the outer plies of a roll of paper or paper based products.
Clay-coated papers, supercalendered krafts, and polyolefin-coated papers will lose or gain moisture content upon thermal processing or during storage, if packaged improperly. Low moisture-content papers will absorb moisture very quickly and moisture wrinkles on the outer layers are usually the result. Remoisturizing the paper after a thermal processing step, which dries the sheet out, will minimize the tendency to develop moisture wrinkles due to subsequent moisture pickup. Proper packaging with moisture barrier wrapping and proper storage conditions are also very important.
Moisture wrinkles can be caused by absorption of ambient moisture following:
- Excessive drying of the paper during processing
- Exposure to low humidity for extended periods
- Exposure to very high humidity of extended periods
These effects can be minimized or eliminated by the following:
- Remoisturizing rolls after thermal processing
- Controlling storage environment. Wrap rolls in packaging that provides a moisture barrier.
Scratches are physical abrasion of the surface of a release liner caused by contact with a non-uniform area of a material which is harder than the material being damaged. Scratching occurs when line speeds and surface finishes are not matched properly during processing. Consequently, damage occurs to the softer surface which is usually the coating or the substrate. Scratching can also occur when slippage occurs in the machine due to non-uniform tensions throughout the process in the machine or cross direction. Proper roll alignments and tension control is a necessity when processing.
Scratches are caused by:
- Differential in line speed to web speed
- Large rapid changes in surface topography
- Misalignment of idlers and tension setting
Scratches are eliminated by:
- Web speed matching idler speed
- Uniform surface topography
- Uniform web tensions throughout the process
- Good machine alignment
Silicone transfer is the transferring of the coating from one surface to another. This transfer is usually caused by either un-reactive components in the silicone coating or inadequate cure of the silicone. Silicone transfer will normally manifest itself on the back side of a substrate or on the adhesive surface. Transfer can be detected by the change in surface energies across the back side of the substrate. Analytical testing (ESCA) and subsequent adhesion testing against a metal plate are additional ways to identify silicone transfer on the adhesive.
Silicone transfer is caused by:
- Inadequate cure levels of the silicone
- Excessive or inadequate surface treatment levels
- Excessive winding pressure
- Unreactive silicone polymer
The transfer of silicone can be reduced by:
- Increasing cure levels of the silicone
- Determining adequate surface energy levels for coating. Surface energy levels should not be excessive
- Reducing winding tensions in the roll
Static electricity refers to stationary charges that are built up on the surface of a substrate. These changes can be positive or negative.
Static electricity is generated when two different surfaces are separated (triboelectric effect), thereby transferring charges from one surface to the other. Pressure, friction, line speed, humidity, and surface smoothness influence the magnitude of electron generation and transfer.
Roll potentials can routinely exceed 50,000 volt when no static control devices are utilized in the converting process. Corona discharges can occur when the potential level is 4,000 volts or less depending on the distance between charges. Consequently, physical danger exists to operators and equipment if in-line potential is not reduced to acceptable levels during processing. Corona discharges will also affect the wettability of the coating and continuity of the coating on the substrate. In the case of silicone coating, tight release areas will be created.
Static electricity can be controlled by:
- Matching surfaces as closely as possible on the triboelectric series
- Increasing humidity levels (surface moisture)
- Using passive- and air-assisted ionization bars and blowers
- Draping grounded tinsel loosely on the substrate. This is also a very efficient and cost effective method of removing static from rotating rollers.
A wrinkle is defined as a fold over of the web in the transverse or cross direction. Fold over occurs because of poor tension profile and cross direction edge tracking (traction loss). Normally, wrinkles are categorized as hard or soft. The effect of the wrinkle is application dependent.
Major reasons for wrinkles are:
- Insufficient cross or transverse machine tension across the web.
- Loss of traction between the web and idlers
- Poor machine alignment
- Bagginess in base substrates
Corrective actions to remedy the above causes are:
- Applying a spreader roll
- Improving edge guide equipment to keep the web uniform
- Checking machine alignment. Optical alignment may be necessary